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THE GEOCHEMICAL EVOLUTION OF
THE CONTINENTAL CRUST
Stuart Ross Taylor
Department of Nuclear Physics
Research School of Physical Sciences
Australian National University, Canberra
Abstract. A survey is given of the dimensions and
composition of the present continental crust. The
abundances of immobile elements in sedimentary
rocks are used to establish upper crustal composition.
The present upper crustal composition is attributed
largely to intracrustal differentiation resulting in the
production of granites senso lato. Underplating of the
crust by ponded basaltic magmas is probably a major
source of heat for intracrustal differentiation. The con-
trast between the present upper crustal composition
and that of the Archean upper crust is emphasized.The nature of the lower crust is examined in the light
of evidence from granulites and xenoliths of lower
crustal origin. It appears that the protoliths of most
granulite facies exposures are more representative of
upper or middle crust and that the lower crust has a
much more basic composition than the exposed upper
crust. There is growing consensus that the crust grows
episodically, and it is concluded that at least 60% of
the crust was emplaced by the late Archean (ca. 2.7
eons, or 2.7 Ga). There appears to be a relationship
between episodes of continental growth and differen-
tiation and supercontinental cycles, probably dating
back at least to the late Archean. However, such
cycles do not explain the contrast in crustal composi-
tions between Archean and post-Archean. Mecha-
Scott M. McLennan 1
Department of Earth and Space Sciences
State University of New York, Stony Brook
nisms for deriving the crust from the mantle are
considered, including the role of present-day plate
tectonics and subduction zones. It is concluded that a
somewhat different tectonic regime operated in the
Archean and was responsible for the growth of much
of the continental crust. Archean tonalites and trond-
hjemites may have resulted from slab melting and/or
from melting of the Archean mantle wedge but at low
pressures and high temperatures analogous to modem
boninites. In contrast, most andesites and subduction-
related rocks, now the main contributors to crustalgrowth, are derived ultimately from the mantle wedge
above subduction zones. The cause of the contrast be-
tween the processes responsible for Archean and post-
Archean crustal growth is attributed to faster subduction
of younger, hotter oceanic crust in the Archean (ulti-
mately due to higher heat flow) compared with subduc-
tion of older, cooler oceanic crust in more recent times.
A brief survey of the causes of continental breakup
reveals that neither plume nor lithospheric stretching is a
totally satisfactory explanation. Speculations are pre-
sented about crustal development before 4000m.y. ago.
The terrestrial continental crust appears to be unique
compared with crusts on other planets and satellites in
the solar system, ultimately a consequence of the abun-
dant free water on the Earth.
INTRODUCTION
The familiar crust of the Earth is of unique impor-
tance because it formed the platform above sea level
onwhich the later stages of evolution occurred that led
to the appearance of Homo sapiens. If the continental
crust had not provided the mountains, broad plains,
and forests that encouraged the development of the
diversity of land-based life, nonmarine life, restricted
to small islands, would have taken a different course;
birds might have become the dominant land dwellers,
as in prehuman New Zealand. It is thus of interest to
IOn sabbatical in 1994 at Abteilung Geochemie, Max-
Planck-Institut fur Chemie, Mainz, Germany.
inquire how the continental crust, which differs so
greatly in composition from the oceanic crust and the
mantle, came into existence, and whether similar
crusts exist on the other terrestrial planets.
The continental crust comprises only about 0.35%
of the mass of the Earth. Although it might thus appear
to be a trivial geochemical reservoir, the crust contains
substantial amounts, exceeding 30% of the bulk Earth
budget, of several of the most incompatible elements,
such as Cs, Rb, K, U, Th, and La. Because the crust
is not easily recycled back into the mantle, it is a major
geochemical reservoir, and its composition is an im-
portant constraint on all geochemical models of bulk
Earth composition and evolution. Any significant tem-
poral change in either the crust's composition or its
mass will have important implications for understand-
Copyright 1995 by the American Geophysical Union.
8755-1209/95/95RG-00262$15.00
Reviews of Geophysics, 33, 2 / May 1995
pages 241-265
Paper number 95RG00262
.241 •
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242 • Taylor and McLennan: CONTINENTAL CRUST GEOCHEMISTRY 33, 2 I REVIEWS OF GEOPHYSICS
ing the evolution of the Earth. Perhaps equally impor-
tant in this context are the details of continental crustal
composition, especially the variations in composition
between upper and lower crust, which provide insights
into the mechanisms of crust formation and differen-
tiation.
The continental crust is especially diverse and het-
erogeneous. Major compositional changes occur,
sometimes on a scale of meters; everyone is familiar
with the complexity displayed on geological maps of
all scales. Arriving at a reliable average composition
might be thought to be difficult, if not impossible.
Nature, however, through the processes of erosion
and sedimentation, has carried out its own overall
sampling of the exposed crust, and this information is
stored within the sedimentary rock record. Some ele-
ments possess the desirable property of having ex-
tremely low concentrations in natural waters and so
are not readily partitioned into fluids during weather-
ing or diagenesis. Such elements, of which the rareearth elements (REE) are a prime example, are thus
transferred quantitatively during erosion and sedimen-
tation from the parent rocks into clastic sediments.
The average composition of the bulk, rather than
the exposed crust, is a far more complicated issue.
Accordingly, there is considerably less of a consensus
about both bulk crustal composition and mechanisms
of crust formation. Constraints from geochemistry,
though model dependent, provide estimates of the
average K, Th, and U content of the bulk crust. In
turn, these data limit models of bulk crustal composi-
tion. The important constraint on K, Th, and U abun-dances in the continental crust relies heavily on the
interpretation of continental heat flowdata, and as will
be noted below, this is proving to be a difficult issue.
Integration of the sedimentary and geophysical
records has helped to resolve the sampling problem
and provided detailed information about the composi-
tion and evolution of the continental crust.
Geological, petrological, geochemical, and geo-
physical observations each produce important con-
straints on the composition and evolution of the con-
tinental crust and combine to allow some broad
generalizations. Thus the composition of the upper
crust cannot persist below about 10 km, so that the
lower crust is in many ways a distinct geochemical
entity. There is much evidence to suggest a fundamen-
tal difference between the crust generated during the
early and middle Archean with that which developed
during and subsequent to the late Archean. The nature
of the earliest crust, though the subject of intense
interest, remains largely a mystery. There has been a
continuing debate over the growth rate of the crust,
the scale of recycling of the crust into the mantle, and
of course its ultimate origin.
Comprehensive reviews of the composition and
geochemical evolution of the continental crust havebeen provided elsewhere [Taylor and McLennan,
1985; Taylor, 1989; McLennan and Taylor, 1991;
McLennan, 1992]. In this paper our purpose is to
provide a general overview of recent developments in
continental crustal geochemistry and to indicate some
of the more promising areas for continuing research.
GLOSSARY
Archean-Proterozoic transition: The time span of
about 3.2 to 2.5 eons (1 eon = 1Ga) that incorporates
the major tectonic stabilization (cratonization) near or
at the end of the Archean. The transition is generally
short-lived on any given craton but is found at different
times in different places. This term is distinguished
from the Archean-Proterozoic boundary that is com-
monly dated at precisely 2.5 eons.
Ce anomaly: Enrichment (positive anomaly) ordepletion (negative anomaly) of cerium compared with
the expected level predicted from a smooth, chondrite-
normalized REE pattern.
ES: European shale composite (a composite of
European shales of Paleozoic age).
Eu anomaly: Enrichment (positive anomaly) or
depletion (negative anomaly) of europium compared
with the expected level predicted from a smooth,
chondrite-normalized REE pattern.
Eu/Eu*: The ratio of measured Eu divided by the
predicted concentration of Eu (Eu*) predicted from a
smooth, chondrite-normalized REE pattern. Eu/Eu*> 1.0 indicates positive Eu anomalies, and Eu/Eu* <1.0 indicates negative Eu anomalies.
First-cycle sediments: Sediments that have been
derived directly from igneous source rocks without an
intervening history of crustal and/or sedimentary re-
cycling.
GdN/YbN: The ratio of chondrite-normalized val-
ues (denoted by subscript N) of Gd and Yb.
HREE: The heavy (or small) rare earth elements,
Gd through Lu.
LaN/YbN: The ratio of chondrite-normalized val-
ues (denoted by subscript N) of La and Yb.
LlLE: Large-ion lithophile elements (e.g., Cs, Rb,
Th, U, LREE).
LREE: The light (or large) rare earth elements, La
through Sm.
Nd model age: The age calculated from the
neodymium isotopic composition of a rock or mineral
when the initial isotopic composition is assumed.
NASC: North American shale composite (a com-
posite of about 40 post-Archean shales and slates,
mainly from North America).
PAAS: Post-Archean average Australian shale
(the average REE abundances in 26 post-Archean
shales from Australia).REE: Rare earth elements. In most geochemical
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33,2/ REVIEWS OF GEOPHYSICS Taylor arid McLennan: CONTINENTAL CRUST GEOCHEMISTRY. 243
TABLE 1. Dimensions of Continental Crust
Mean Mean
Area, Thickness, Volume, Height, Flooded
Continent lrf' km2 km 109 km' m Area, %
Major Continents
Eurasia 62.160 38 2.362 361 25.8
North America 39.793 35 1.393 96 30.6Africa 35.722 24 0.857 244 14.5South America 23~982 42 1.007 149 19.0Antarctica 18.984 33 0.626 -344 78.4Australia 14.163 36 0.510 -244 34.9india 4.648 38 0.177 236 14.9Arabia 4.595 23 0.106 470 10.6New Zealand 3.980 21 0.084 -1568 90.3Central America 1.344 18 0.024 -579 52.8
Submerged Microcontinents
Rockall 0.459 33 0.015 -1702 100Seychelles 0.386 32 0.0.12 -1706 99.9Agulhas 0.i34 12 0.002 -3222 100:0Jan Mayen 0.055 0.002* -1600 99.4
Totals 210.405 7.177t 126 30.6
Adapted from Cogley [1984].
*Value for assllmed crustal thickness between 28 arid 45 km."Sum of individual volumes. Product of total area and average thickness of all continents leads to volume of 7.575 x 109km".
discussions, the term is restricted to the lanthanides
La through Lu, but it also includes Y and Sc.
TDM: Nd model age that assumes an initial Nd
isotopic composition equivalent to a model for the
mantle (various models are used) that has been de-
pleted in LREE for much of its history.
TTG suite: A common suite of Na-rich felsic plu-
tonics (tonalite-trondhjemite-granodiorite) found com-monly in Archean terranes and characterized by,
among other things, depletion in the heavy rare earth
elements. Volcanic equivalents are also found and
generally included in discussions of the TTG suite.
S IZE , STRUCTURE , AND AGE
The continental crust occupies 41.2% of the surface
area of the Earth, or 2.10 x 108 knr'; 71.3% or 1.50 x
108 km2 lies above sea level. There are at least 10
major continental blocks and four submerged micro-continents (Table O . The mean elevation of the conti-
nental crust above sea level is 125m. The elevation of
the area which lies above the 200-m isobath (i.e., the
shelf-slope break) is 690 m. The average elevation of
the continents above the mean seafloor (oceanic crust)
is about 5 km. Average crustal thickness is.36 km but
varies between 10 and SOkm, correlating with the size
of the continental block and the age of the last tectonic
event. The crustal volume is 7.35 x 109 knr', a value
that includes sediment on the ocean floor derived from
the continents and the volumes of the submerged con-
tinental masses. This estimate of volume has an error
of at least ±5% because it depends on the value
adopted for crustal thickness. This is not known with
great accuracy; some estimates of average crustal
thickness are as high as 40 km.
The base of the crust is usually defined as the major
seismic discontinuity, the Mohorovicic discontinuity
or Moho, where compressional wave velocities (Vp)
jump from about 7 to about S km s-1. However, the
Moho is not a sharp boundary everywhere and may be
absent locally. Thus the crust is often defined as ma-
terial with Vp < 7.S km S-1 or seismic shear wave
velocities Vs < 4.3 km s-1. These seismic velocities
are consistent with densities less than 3.1 g em- 3• The
base of the crust is likely to be very complicated in
detail because of the possibility of underplating by
basic or ultramafic material and because of the prob-
able interlayering of high-velocity mantle material with
lower-velocity crustal material.
A midcrustal boundary, the Conrad discontinuity, is
sometimes observed to separate upper and lower crust at
about 10-to 20-km depth. It is commonly discontinuous
and often absent or poorly constrained by seismic data;
especially in shield areas. Sometimes it is gradational
over several kilometers. The failure of superdeep drill
holes to identify many of the discontinuities that were
based on interpretations of the geophysical evidence
remains an intriguing area of investigation.
Crustal density estimates range from 2.7 to 2.9 g
cm-3 and increase with depth on average. This param-
eter is difficult to define precisely because of the mod-
el-dependent nature of the lower crust and because of
the great geological complexity of the crust generally.
A crustal mass of2.06 x 1025 g (±7%) is obtained if an
average density of2.S g cm? is adopted. On the basis
of this calculation, the continental crust forms 0.54%
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244 • Taylor and McLennan: CONTINENTAL CRUST GEOCHEMISTRY
TABLE 2. Crust Formation Age Distributions
33,2/ REVIEWS OF GEOPHYSICS
Nd Model Age Distributions* This Paper
0% Crust by 4.0 Eons 8% Crust by 4.0 Eons
Distribution, Growth Rate, Distribution, Growth Rate,
Age, eons % % eon" % % eon?'
1 . 0 - 0 . 0 1 8 1 8 1 7 1 7
1 . 6 - 1 . 0 1 2 2 0 11 1 8
2 . 6 - 1 . 6 2 2 2 2 2 0 2 0
3 . 4 - 2 . 6 2 6 3 3 2 4 3 0
4 . 0 - 3 . 4 2 2 3 7 2 0 3 3
4 . 5 - 4 . 0 8 1 6
Total 1 0 0 1 0 0
Mean age, eons t 2 . 2 2 . 3
Model
Distribution,
%
Average
Growth rate,% eon"!
108
2 2
5 0
10
1 0
13
2 2
6 2
1 7
100
2 . 4
Data from McCulloch and Bennett [1993]. There is no identified crust greater than 4.0 eons. Two models are presented, one assuming0% crust present at 4.0 eons and a second assuming 8% crust present by 4.0 eons (a value considered by McCulloch and Bennett [1993] tobe consistent with Nd isotopic characteristics of Archean mantle-derived volcanics).
"In each case, crustal growth is assumed to be episodic, consistent with McCulloch and Bennett [1993] and as described in text.
of the mass of the crust-mantle system and only 0.35%
of the mass of the whole Earth.
The continental crust is of great antiquity when
compared with oceanic crust and thus contains most of
the geological record. An understanding of the mean
age of the continents is important for a number of
geochemical and geophysical models but is a difficult
parameter to estimate with confidence. The most robust
approach to date is based on the Sm-Nd isotope system
because in bulk rock samples it is least prone to resetting
during later metamorphism. The most important episode
of fractionation of Sm from Nd takes place at the time ofmantle melting to form continental crust. Hence the
depleted mantle Nd model age (TOM) of crustal igneous
rocks is thought to reflect the age of mantle extraction.
This simple view is complicated by a number of pro-
cesses that may later change SmlNd or underestimate
the importance of older crustal rocks. These processes
include intracrustal melting, metamorphic resetting, and
assimilation of older material. The overall effect is that
estimates of average crustal age based on the Nd isotope
system, while superior to other isotopic approaches, are
still likely to represent a minimum [McLennan, 1988].
On the basis of Nd model age provinces in NorthAmerica, Europe, and Australia (Table 2), an average
age of continental crust is about 2.0-2.3 eons [Nelson
and DePaolo, 1985;Patchett and Arndt, 1986;McCul-
loch, 1987;DePaolo et al., 1991;McCulloch and Ben-
nett, 1993]. These workers conclude that no more than
about 50% of the continental crust was in place by 2.6
eons, a value that we consider too low because it does
not account for the freeboard constraint that crustal
volumes have only increased to a minor degree since
2.5 eons (see below). A mean age of at least 2.4 eons
for the continental crust is obtained if about 60% of the
crust was in place by 2.6 eons (Table 2), a model thatwe favor here.
U P P E R CONTINENTAL CRUST
Sedimentary Rocks as Crustal Samples
Goldschmidt [1938] suggested that the mixing and
homogenizing effects of sedimentary processes should
produce uniform rare earth element patterns in sedi-
mentary rocks and that these should reflect the abun-
dances in the upper continental crust. This prediction
of Goldschmidt has proven remarkably accurate, es-
pecially for fine-grained clastic sedimentary rocks
such as shales [Taylor and McLennan, 1985]. The useof elemental abundances in sedimentary rocks as in-
dexes of upper crustal composition is based on the
observation that many elements have extremely low
concentrations in river water and seawater and so are
transported and deposited without effective loss by
clastic sediments, such as shales and sandstones (Fig-
ure O . The most important among these elements are
the REE, Y, Sc, and Th (other insoluble elements,
such as Zr, Hf, and Sn, have similar properties, but the
chemistry of these elements in clastic sediments is
totally dominated by very resistant heavy minerals,
such as zircon and cassiterite, that are readily segre-
gated during sedimentation). Shales and their relativesare volumetrically the most important sedimentary
rock types (constituting about 70% of the sedimentary
mass), and so the abundances of these insoluble ele-
ments in shales can be closely related to that of the
upper crust.
Although some diversity exists in REE patterns,
especially sediments derived directly from volcani-
cally active continental margins, the average REE
pattern is very similar for most Phanerozoic and mod-
ern shales [McLennan et al., 1990, 1993; McLennan
and Taylor, 1991].These include estimates of averages
from three continents: post-Archean Australian aver-age shale (PAAS), European shale composite (ES),
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33, 2/ REVIEWS OF GEOPHYSICS Taylor and McLennan: CONTINENTAL CRUST GEOCHEMISTRY. 245
and North American shale composite (NASC) samples
[see Taylor and McLennan, 1985]. These REE pat-
terns are enriched in the light REE (LREE) relative to
chondritic patterns, which are taken as representative
of the bulk Earth (see Taylor [1992a] for a discussion
of this), and have a depletion in Eu relative to the
neighboring REE, Sm, and Gd. (This relative deple-
tion is usually referred to as a "negative Eu anomaly";
the converse enrichment in Eu is often referred to as a
"positive Eu anomaly. " See glossary for definitions of
LREE, HREE, LaN/YbN, and Eu/Eu*; for REE nor-
malizing factors, see Taylor and McLennan [1985]).
The REE are not readily fractionated in sedimen-
tary processes (except that Ce3+ may oxidize to Ce4+
and separate as the insoluble phosphate in marine
environments, or less frequently as the hydroxide in
terrestrial environments). Thus the REE pattern is
recording a previous signature of igneous events in-
volving partial melting (or crystal fractionation) and
removal of Eu by plagioclase as a residual phase [e.g.,
Taylor and Mcl.ennan, 1988].
Modern sediments mostly display REE patterns
similar to those of the post-Archean shales. The REE
patterns of most suspended particulate matter in some
of the world's major rivers [Goldstein and Jacobsen,
1988] are also very similar to the PAAS, although
slightly enriched in total REE as a result of their finer
grain size (REE are concentrated in the clay fraction
and in heavy minerals).
Location of REEin Sediments
The bulk of the REE elements reside in the fine-grained (clay and silt) fraction (there is no correlation
with specific clay mineralogy) and also in trace miner-
als such as zircon, monazite, and apatite [Cullers et
al., 1979]. There is some indication that significant
fractions of REE in clay minerals may also occur as
microinclusions of resistate minerals such as apatite
[Caggianelli et al., 1992].Sand fractions within a sed-
imentary rock have significantly lower total abun-
dances than finer-grained fractions. Coarse-grained
sedimentary rocks typically have REE patterns essen-
tially parallel to those of shales, but with lower total
abundances than shales, due mainly to the dilutingeffect of quartz.
The overall shape of the patterns (Eu/Eu*, LaNI
YbN)is generally similar for sandstones and shales. Eu
anomalies are similar in all size fractions. The REE
abundances in quartz-rich sedimentary rocks are typ-
ically very low. The potential of heavy minerals to
distort REE patterns in sedimentary rocks is well
recognized. For example, Archean metaquartzites
from the Western Gneiss Terrain, Australia [Taylor et
al., 1986] show enrichments of light REE due to the
presence of monazite. However, these minerals are
only rarely concentrated in amounts sufficient to cause
perceptible effects on the REE patterns. Thus the
resistant mineral zircon, typically enriched in heavy
--------------------8--
(iJ::l
0W0.. 1.000..
: : : >
O J
(ijs:Cf)
0.10
-10 ~ ~ 4 ~ 0
log (Seawater / Upper Crust)
Figure 1. Plot of the concentration ratios of (average shale)/
(upper crust) versus (seawater)/(upper crust) to illustrate the
elements most useful for estimating upper crustal abun-
dances from sedimentary data. Data are taken primarily
from Taylor and McLennan [1985]; however, since some of
their upper crust values are derived directly from shales
(e.g., REE, Th, and Sc), for the purposes of this diagram, the
independently determined values of Shaw et al. [1976] are
used for these elements. With the exception of Co (and
perhaps Ti, for which seawater data are poorly established),
the ratios of (average shale)/(upper crust) vary by a factor of
< 1.5 for elements that do not partition into water during
geological processes (Iseawaterl/tupper crust) <10-5). Com-
plications exist involving redox controls (Fe and Mn) and
heavy mineral concentrations (e.g., Zr), but in general the
abundances of these elements in shales most closely reflect
upper crustal abundances (notably for REE, Th, and Sc). In
contrast, the ratio (average shale)/(upper crust) varies by afactor of >15 for elements that are partitioned into natural
waters (Iseawaterl/tupper crust) >10-3). Such elements are
readily dissolved in natural waters and are concentrated into
chemical sediments (e.g., Ca, Na, and Sr) or are readily
mobilized during weathering, diagenesis, hydrothermal al-
teration, and so forth (e.g., Mg, U, and B).
REE, affects the bulk rock patterns only when zircon
constitutes more than about 0.06% (or Zr abundances
exceed about 300ppm) because every 100ppm ofZr as
zircon adds only about 0.25 times chondri tic levels ofYb (typical bllik rock patterns are about 10-15 times
chondritic levels for that element).
Sedimentary Processes and REEPatterns
Sedimentary processes may affect REE distribu-
tions in sedimentary rocks. Recent studies of granite
weathering indicate that substantial fractionation may
take place within weathering profiles [e.g., Nesbitt,
1979;Banfield and Eggleton, 1989]resulting in, among
other things, significant Ce anomalies associated with
formation of Ce4+ and stable cerium hydroxides. In
most situations, such fractionation does not translate
into unusual sedimentary rock REE patterns because
the weathering profile is well mixed during subsequent
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246 • Taylor and McLennan: CONTINENTAL CRUST GEOCHEMISTRY
erosion and transport [Nesbitt, 1979].Exceptions may
exist. For example, McDaniel et al. [1994]recognized
such a process as affecting Early Proterozoic first-
cycle turbidites derived from granitic rocks. In this
case, Ce anomalies were present, and Nd isotopes
clearly record partial resetting ofthe Sm-Nd system at
about the time of sedimentation.
Sedimentary transport may also result in changing
sedimentary REE patterns due to heavy mineral (no-
tably zircon and monazite) fractionation. Such effects
can be modeled with some confidence [McLennan,
1989]and are most likely to be a significant influence in
clastic sediments such as quartzites with low REE
abundances. Recent studies have also demonstrated
that REE may be significantly affected during certain
conditions of diagenesis [e.g., Ohr et al., 1991;
Milodowski and Zalasiewicz; 1991;Bock et al., 1994].
Redistribution ofREE among newly formed diagenetic
minerals, without significant transport, appears to oc-
cur in most cases [Ohr et al., 1991]. However, inexamples of diagenetic ally altered Ordovician tur-
bidites, there is evidence oflarger-scale transport (and
partial resetting of the Nd isotope system) associated
with formation of REE-bearing minerals such as mon-
azite [Milodowski and Zalasiewicz; 1991;Bock et al.,
1994]. In addition to these examples, reports of heavy
REE (HREE, i.e., Gd-Lu) depletion in continent-de-
rived marine sediment continue to appear but in most
cases may be attributed to analytical difficulties [e.g.,
Sholkovitz; 1988, 1990].
Although such effects appear to be comparatively
rare, they are of some interest. Unusual sedimentaryprocesses mostly result in the production of anoma-
lous REE patterns, such as Ce anomalies [McDaniel et
al., 1994] or severe LREE depletion [Bock et al.,
1994]. In each of these examples, evaluation of the
REE systematics permits constraints to be placed on
the original REE patterns [McDaniel et al., 1994;Bock
et al., 1994]. These results also engender caution in
interpreting the numerous Nd isotope studies of sedi-
mentary rocks in which the geochemical systematics
beyond Sm and Nd are not evaluated.
Loess
About 10% of the Earth's land surface is covered
with Pleistocene loess. The origin of most loess by
aeolian transport from glacial outwash, particularly
during cold dry climatic regimes is well established.
This combination of widespread production of silt-
sized rock flour and its transport by wind over hun-
dreds of kilometers thus provides geochemists with a
natural sampling of comparatively unweathered mate-
rial from the exposed crust. Loesses from North
America, Europe, China, and New Zealand show very
uniform REE abundances (although sometimes diluted
by quartz or carbonate), with Eu depletions equivalent
to those observed in clastic sedimentary rocks, indi-cating that they are providing a reasonable sample of
33,2/ REVIEWS OF GEOPHYSICS
the upper continental crust [Taylor et al., 1983;Liu et
al., 1993]. The similarity of REE patterns for the
widely scattered loess deposits to shale averages (e.g.,
PAAS, ES, and NASC) indicates that loess also pro-
vides the same information on the composition of the
upper crust as that provided by clastic sediments.
There is little evidence that these sediments have been
affected by significant weathering. This demonstrates
that upper crustal provenance is the prime control on
REE patterns of clastic sedimentary rocks rather than
the effects of sedimentary processes.
Composition of the Post-Archean Upper Crust
The composition of the post-Archean upper crust is
well established, with several estimates converging on
a composition close to that of granodiorite. The meth-
ods employed have ranged from extensive sampling
and massive analytical programs involving estimates
of the amounts of various rock types to the use of
sedimentary rocks and loess as natural sampling pro-cesses. Some recent studies employing such ap-
proaches include those of Taylor and McLennan
[1985], Wedepohl [1991], Ronov et al. [1992], and
Condie [1992].
The most remarkable feature of the REE abundance
patterns in post-Archean sedimentary rocks is their
uniformity on a worldwide scale. REE patterns for the
post-Archean Australian average shale are similar to
those of composite shale samples from Europe and
North America. These patterns are all characterized
by light REE enrichment and relatively flatheavy REE
abundances about 10 times chondritic, with a ratheruniform depletion in Eu (Eu/Eu* = 0.65). This unifor-
mity both within and between continents is accord-
ingly interpreted to represent the REE abundances in
the upper continental crust exposed to weathering
(Figure 2). Mass balance calculations involving aver-
ages of the various sedimentary rock types (shales,
sandstones, carbonates, and evaporites) indicate that
while REE patterns in fine-grained sedimentary rocks
are parallel to upper crustal abundances, they proba-
bly overestimate the absolute abundances by about
20% [Taylor and McLennan, 1981].
The abundances of the insoluble elements Th and
Sc also reflect upper crustal abundances in the same
manner as the REE (Figure O . Thorium is typically
concentrated in granitic rocks. Scandium, although
trivalent and a member the same group (III) of the
periodic table as the REE, is a much smaller ion, and
is concentrated in basic rocks, entering early crystal-
lizing pyroxenes. The ThlSc ratio thus forms an excel-
lent index of the relative proportions of granitic and
basic rocks.
From sedimentary REE, Th, and Sc data, it is
possible to extrapolate to obtain the upper crustal
abundances of a number of other elements by using
elemental ratios that either are constant across a widerange of igneous compositions (e.g., K/U) or vary
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33,2/ REVIEWS OF GEOPHYSICS Taylor and McLennan: CONTINENTAL CRUST GEOCHEMISTRY. 247
(/)
Q)
-;::
"0
§100.0
..co
Ea.a.
V24·73
198A·2·2500.0
- -Ea.a.
10.0
Figure 2. Chondrite-normalized REE diagram showing deep-
sea pelagic muds from the Pacific Ocean basin (data from
Ben Othman et al. [1989] and Lin [1992]). Shown for com-
parison are estimates of post-Archean and Archean upper
crust, from Taylor and McLennan [1985]. Note that pelagic
sediments from the Pacific basin, a region surrounded by arc
volcanism, display negative Eu anomalies characteristic of
the upper crust. This signature is largely due to aeolian
sources. The paucity of such patterns in Archean greenstone
belts is considered strong evidence for the limited extent of
stabilized Archean crust that had undergone intracrustal
differentiation resulting in negative Eu anomalies.
systematically with bulk composition (e.g., KlRb).
Thus the abundance of V can be obtained from gen-
erally accepted upper crustal Th/V ratios (3.8) and K
from KIV ratios (10,000). Rb can be obtained from
KlRb (250); Sr from Rb/Sr (0.3) and so on (Table 3).
Table 4 gives the normative mineralogy of the upper
crust, based on its major element composition.
Origin of the Post-Archean Upper Crust
Geochemical balance calculations indicate that theupper crustal composition cannot be representative of
the entire crust. This observation is also consistent
with heat flow and heat production data, although
these do not uniquely separate the relative crustal and
mantle contributions. The dominance in the upper
crust of compositions effectively equivalent to grano-
diorite (including high abundances of heat-producing
elements and other large-ion lithophile elements) and
the ubiquitous occurrence of the PAAS REE patterns
(including negative Eu anomalies) provide major clues
to the derivation of the upper crust.
The Eu depletion in post-Archean sedimentaryrocks provides strong evidence for an intracrustal or-
igin for much of the upper crust. Igneous rocks of
mantle derivation rarely display Eu anomalies, and no
primitive volcanic rock derived directly from the man-
tle exhibits a relative depletion in Eu. The origin of the
marked depletion of Eu typical of chondrite-normal-
ized REE patterns in terrigenous sedimentary rocks is
not due to surficial processes of oxidation or reduc-
tion. Eu is, with rare exceptions, present as thetrivalent ion in sediments [e.g., Sverjensky, 1984], so
that the depletion in Eu is the signature of an earlier
igneous event. Vnder the reducing conditions typical
of magmas, Eu is divalent. The relatively flat and
uniform HREE patterns seen in sedimentary rocks
(GdNNbN = 1.0 to 2.0) suggests that HREE-enriched
minerals, notably garnet and amphibole, are not im-
portant fractionating phases during the formation of
the upper continental crust.
The generally accepted explanation for the origin of
the Eu-depleted, K-rich granites and granodiorites that
now dominate the upper crust is that they form
through intracrustal melting. Thus the most likely ex-
planation for the upper crustal Eu anomaly is that Eu
is left in residual plagioclase in the lower crust. Pla-
gioclase is stable only to a pressure of 10kbar (a depth
of 40 km on the Earth), so that this sink for Eu is
consistent with the scenario of intracrustal melting for
granite origin that is supported by experimental stud-
ies [e.g., Wyllie, 1983]. There are two sources of heat
available to induce intracrustal melting. The first is the
heat generated by radioactive decay of K, V, and Th.
The second source, underplating of the crust by basal-
tic magmas, is less easily evaluated but appears to be
required because the crustal radioactive sources aregenerally considered to be inadequate for the task. As
we discuss in greater detail below, recognition of Eu-
rich residual material in the available lower crustal
samples has proven elusive and represents one of the
major outstanding issues in crustal geochemistry [e.g.,
Rudnick, 1992a].
Sedimentary rocks provide information about how
long this process has been operating. As was discussed
earlier, the REE patterns of loess from widely scat-
tered localities across the globe are uniform, indicating
that the processes producing the upper crust have not
changed over the period represented by the sourceregions of the loesses. However, loess is derived from
source rocks of widely varying age, extending back
into the Proterozoic. The Nd model age of sedimen-
tary rocks is interpreted as the average age from when
the various provenance components were extracted
from the mantle [McCulloch and Wasserburg, 1978].
The samples studied by Taylor et al. [1983] came from
China, Europe, New Zealand, and North America,
with Nd-depleted mantle model ages TDM ranging from
1060 to 1700 Ma. In fact, such sedimentary REE pat-
terns likely exist for sedimentary rocks with Nd model
ages older than about 2 eons [e.g., McLennan andHemming, 1992]. This variation in average prove-
nance age, coupled with their uniform chemistry, in-
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TABLE 3. Chemical Composition of the Continental Crust
Upper Bulk Lower Average Archean ArcheanContinental Continental Continental Andesitic Upper Bulk
Element Crust Crust Crust Crust Crust Crust
Li, ppm 2 0 1 3 1 1 1 0
Be, ppm 3 . 0 1 . 5 1 . 0 1 . 5
B, ppm 1 5 1 0 8 . 3
Na, wt. % 2 . 8 9 2 . 3 0 2 . 0 8 2 . 6 0 2 . 4 5 2 . 2 3
Mg, wt. % 1 . 3 3 3 . 2 0 3 . 8 0 2 . 1 1 2 . 8 3 3 . 5 6
AI, wt. % 8 . 0 4 8 . 4 1 8 . 5 2 9 . 5 0 8 . 1 0 8 . 0 4
Si, wt. % 3 0 . 8 2 6 . 8 2 5 . 4 2 7 . 1 2 8 . 1 2 6 . 6
P, ppm 7 0 0
K, wt. % 2 . 8 0 0 . 9 1 0 . 2 8 1 . 2 5 1 . 5 0 0 . 7 5
Ca, wt. % 3 . 0 0 5 . 2 9 6 . 0 7 5 . 3 6 4 . 4 3 5 . 2 2
Sc, ppm 1 1 3 0 3 6 3 0 1 4 3 0
Ti, wt. % 0 . 3 0 0 . 5 4 0 . 6 0 0 . 4 8 0 . 5 0 0 . 6 0
V, ppm 6 0 2 3 0 2 8 5 1 7 5 1 9 5 2 4 5
Cr, ppm 3 5 1 8 5 2 3 5 5 5 1 8 0 2 3 0
Mn, ppm 6 0 0 1 4 0 0 1 7 0 0 1 1 0 0 1 4 0 0 1 5 0 0
Fe, wt. % 3 . 5 0 7 . 0 7 8 . 2 4 5 . 8 3 6 . 2 2 7 . 4 6
Co, ppm 1 0 2 9 3 5 2 5 2 5 3 0
Ni, ppm 2 0 1 0 5 1 3 5 3 0 1 0 5 1 3 0
Cu, ppm 2 5 7 5 9 0 6 0 8 0
Zn, ppm 71 8 0 8 3
Ga, ppm 1 7 1 8 1 8 1 8
Ge, ppm 1 . 6 1 . 6 1 . 6
As, ppm 1 . 5 1 . 0 0 . 8
Se, ppm 5 0 5 0 5 0
Rb, ppm 1 1 2 3 2 5 . 3 4 2 5 0 2 8
Sr, ppm 3 5 0 2 6 0 2 3 0 4 0 0 2 4 0 2 1 5
Y,ppm 2 2 2 0 1 9 22 1 8 1 9
Zr, ppm 1 9 0 1 0 0 7 0 1 0 0 1 2 5 1 0 0
Nb, ppm 2 5 1 1 6 1 1
Mo, ppm 1 . 5 1 . 0 0 . 8
Pd, ppb 0 . 5 1 1
Ag, ppb 5 0 8 0 9 0
Cd, ppb 9 8 9 8 9 8
In, ppb5 0 5 0 5 0
Sn, ppm 5 . 5 2 . 5 1 . 5
Sb, ppm 0 . 2 0 . 2 0 . 2
Cs, ppm 3 . 7 1 . 0 0 . 1 1.7Ba, ppm 5 5 0 2 5 0 1 5 0 3 5 0 2 6 5 2 2 0
La, ppm 3 0 1 6 1 1 1 9 2 0 1 5
Ce, ppm 64 3 3 2 3 3 8 4 2 3 1
Pr, ppm 7 . 1 3 . 9 2 . 8 4 . 3 4 . 9 3 . 7
Nd, ppm 2 6 1 6 1 2 . 7 1 6 2 0 1 6
Sm, ppm 4 . 5 3 . 5 3 . 1 7 3 . 7 4 . 0 3 . 4
Eu, ppm 0 . 8 8 1.1 1 . 1 7 1 . 1 1 . 2 1.1Gd, ppm 3 . 8 3 . 3 3 . 1 3 3 . 6 3 . 4 3 . 2
Tb, ppm 0 . 6 4 0 . 6 0 0 . 5 9 0 . 6 4 0 . 5 7 0 . 5 9
Dy, ppm 3 . 5 3 . 7 3 . 6 3 . 7 3 . 4 3 . 6
Ho, ppm 0 . 8 0 0 . 7 8 0 . 7 7 0 . 8 2 0 . 7 4 0 . 7 7
Er, ppm 2 . 3 2 . 2 2 . 2 2 . 3 2 . 1 2 . 2Tm, ppm 0 . 3 3 0 . 3 2 0 . 3 2 0 . 4 3 2 0 . 3 0 0 . 3 2
Yb, ppm 2 . 2 2 . 2 2 . 2 2 . 2 2 . 0 2 . 2
Lu, ppm 0 . 3 2 0 . 3 0 0 . 2 9 0 . 3 0 0 . 3 1 0 . 3 3
Hf, ppm 5 . 8 3 . 0 2 . 1 3 . 0 3 3
Ta, ppm 2 . 2 1 . 0 0 . 6
W,ppm 2 . 0 1 . 0 0 . 7
Re,ppb 0 . 4 0 . 4 0 . 4
Os, ppb 0 . 0 5 0 . 0 5 0 . 0 5
Ir, ppb 0 . 0 2 0 . 1 0 . 1 3
Au, ppb 1 . 8 3 . 0 3 . 4
TI, ppb 7 5 0 3 6 0 2 3 0
Pb, ppm 2 0 8 . 0 4 . 0 1 0
Bi, ppb 1 2 7 6 0 3 8
Th, ppm 1 0 . 7 3 . 5 1 . 0 6 2 . 5 5 . 7 2 . 9
V,ppm 2 . 8 0 . 9 1 0 . 2 8 1 . 0 1 . 5 0 . 7 5
Adapted from Taylor and McLennan [1985], with minor alterations [see Esser and Turekian, 1993; Jochum et al., 1993].
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33,2/ REVIEWS OF GEOPHYSICS Taylor and McLennan: CONTINENTAL CRUST GEOCHEMISTRY. 249
TABLE 4. Normative Mineralogy of Continental Crust
Bulk Lower
Upper Crust, Crust,
Crust, % % %
Quartz(Q) 16.1 6.6 3.7Orthoclase (Or) 20.1 6.5 2.0
Albite (Ab) 33.0 26.2 23.7Anorthite (An) 13.9 26.2 30.4Diopside (Di) 5.8 8.7 9.8Hypersthene (Hy) 10.0 24.1 28.6Ilmenite (II) 1.0 1.7 1.9Total 99.9 100.0 100.1
dicates that the upper continental crust was both uni-
form in composition and produced by similar
processes over periods extending back at least 2 billion
years. Accordingly, the processes producing the upper
crustal composition which is being sampled during the
formation of sedimentary rocks have remained the
same well back into the Proterozoic.
Archean-Proterozoic Transition
Compared with post-Archean PAAS-type REE pat-
terns, abrupt changes are observed in clastic sedimen-
tary rocks over time intervals from 3.2 to 2.5 eons in
South Africa [McLennan et al., 1983] and inWestern
Australia [Taylor and McLennan, 1985]. In the early
Proterozoic (2.5-2.2 eons) Huronian sedimentary suc-
cession of Canada, McLennan et al, [1979] found a
progressive change from REE patterns lacking Eu
depletions at the base, to typical post-Archean pat-terns at the top. Similar changes in sedimentary REE
patterns have been noted in Early Proterozoic succes-
sions of the Pine Creek Geosyncline and Hamersley
Basin, Australia, and the Slave Province of Canada
(see review by Taylor and McLennan [1985]). This
change was interpreted to reflect an episodic change in
upper crustal composition from a more primitive state
and related to a massive emplacement of K-rich gra-
nitic rocks, depleted in Eu, toward the close of the
Archean. This is generally referred to as "cratoniza-
tion," the process that produces massive intracrustal
melting to produce granites, transfers heat-producingelements to the upper crust, and generally "stabilizes"
the crust [e.g., Pollack, 1986]. (The development of a
thick lithospheric keel during crustal growth also is
likely to contribute to the inherent stability of cratons.)
It was nonsynchronous over the globe and extended
over several hundred million years. These events mark
the development of extensive fully stabilized continen-
tal crust.
The Archean-Proterozoic transition thus marks a
major change both in the volume of crust, and in
intracrustal differentiation. The crustal processes re-
sponsible for these changes took place during the late
Archean and are recorded in Early Proterozoic sedi-
mentary rocks. The presence of large masses of un-
subductable continental crust changed the tectonic
regime from the multiplate Archean crust and pro-
duced the present linear (e.g., South America) or arc-
uate (e.g., western Pacific arcs) subduction zones.
Many geological events correlate with a major
change at the Archean-Proterozoic boundary. The
widespread occurrence of uranium deposits in basal
Proterozoic sediments can be attributed to the enrich-
ment of the upper crust in incompatible elements due
to intracrustal melting. The dramatic increase in 87Sr
in marine carbonates from that period [e.g., Veizer,
1983] is similarly due to an upper crustal enrichment in
87Rbin the K-rich granites that came to dominate the
upper crust (and a reduction of mantle flux of Sr
resulting from a cooling Earth). It is also interesting to
note the great proliferation of stromatolites after 2.5
eons, which reflects the widespread occurrence of
stable continental marine shelves that formed as a
consequence of events around the Archean-Protero-
zoic boundary (the stromatolites declined early in the
Phanerozoic owing to the emergence of metazoan
grazing species [e.g., Schidlowski, 1992]). The prolif-
eration of banded iron formations, a chemical sedi-
ment that fills the sedimentary environment niche of
carbonate rocks in later geological periods, also can be
related in part to the development of stable shelves
during the late Archean-Early Proterozoic [e.g.,
McLennan, 1980]. The first supercontinent likely
formed at this time [e.g., Nance et al., 1986; Hoffman,
1992; Unrug, 1992], and major climatic changes also
appear to correlate with the Archean-Proterozoic tran-
sition [e.g., Young, 1991].
POST-ARCHEAN LOWER CRUST
Seismic reflection profiling has revealed the very
complex nature of the lower crust, which appears to be
at least as diverse as the upper crust. It is much more
difficult to arrive at a representative composition for
the lower crust than for the upper crust, owing to its
inaccessibility and to the absence of some sampling
technique, such as is provided for the upper crust by
sediments, that might provide a meaningful average.Two types of samples, xenoliths arid granulite facies
rocks, are available, both providing enigmatic infor-
mation [Rudnick and Presper, 1990].
Granulite facies regions are often interpreted as
samples of the lower crust. Archean granulite terranes
commonly possess positive Eu anomalies; however,
these are typically found in the more evolved compo-
sitions rather than in mafic material that could repre-
sent residues after partial melting. Positive Eu anom-
alies are less frequent in post-Archean granulite
terranes [Rudnick and Presper, 1990] and in any case,
such terranes appear, on compositional grounds, to be
mostly upper crust, depressed in Himalayan-type col-
lisions or formed in association with rifts, hot spots,
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250 • Taylor and McLennan: CONTINENTAL CRUST GEOCHEMISTRY 33, 2/ REVIEWS OF GEOPHYSICS
and volcanic arcs, and thus not a good model on which
to base lower crustal compositions [e.g., Bohlen, 1987;
Ellis, 1987;Bohlen and Mezger, 1989;Mezger, 1992].
Indeed, Mezger [1992] suggested that most regional
granulites represent the transitional region between
upper and lower crust.
Xenoliths, recording pressure-temperature (P- T)conditions indicative of derivation from the lower
crust, are frequently found in volcanic pipes. Unlike
the granulite facies regions, they are commonly much
more basic in composition and frequently show a rel-
ative enrichment in Eu. They appear to be related to
episodes of underplating by basaltic magmas, and in
most cases the positive Eu anomaly is best related to
the accumulation of cumulate phases rather than to
simple residues from partial melts [e.g., Rudnick and
Taylor, 1987;Rudnick, 1992b].
Current understanding of the petrogenesis of the
majority of granitic rocks and the ubiquitous presence
of negative Eu anomalies in sedimentary rocks indi-
cate that intracrustal partial melting must be a funda-
mental process governing the composition and chem-
ical structure of the continental crust. Although
complementary positive Eu anomalies are commonly
present in the available lower crustal samples (granu-
lite terranes and xenoliths), they typically cannot be
readily characterized as residual material left behind
after intracrustal partial melting.
Accordingly, a fundamental paradox exists in
crustal geochemistry. Upper crustal rocks (granites,
sediments) appear to bear clear evidence that intrac-
rustal partial melting is the dominant process causingcrustal differentiation, but the predicted residues of
such processes are surprisingly uncommon in lower
crustal samples. Paradoxes, of course, arise not from
the operations of nature, but from our imperfect un-
derstanding of them. Rudnick [1992a]has attempted to
resolve this paradox by suggesting a multistage model
whereby basaltic underplating supplies heat to melt
the lower crust (providing melts with negative Eu
anomalies) and creates an environment for the accu-
mulation of crystal cumulates (with positive Eu anom-
alies). Other possible explanations include the follow-
ing: (1) crustal melting takes place at lowertemperatures than commonly thought (i.e., under wet
conditions), and restites are present in lower-grade
(amphibolite) terranes, or (2) neither granulite terranes
nor xenoliths are entirely representative of lower
crustal material, and areas representing residues after
partial melting are not typically sampled at surface by
natural processes. In any case, the solution of this
paradox is a critical step in furthering our understand-
ing of crustal geochemistry.
In summary, the lower crust appears to be essen-
tially the residue left after extraction of the granodior-
itic upper crust together with additions from under-
plating by basaltic magmas. It is thus likely to be very
complex in detail, an example of which appears to be
the Ivrea Zone in northern Italy [Voshage et al., 1990].
On at least a local scale, delamination and sinking of
heavy residual material may also be expected, adding
to the complexity and to the variable nature of the
Moho [Kempton and Harmon, 1992].Estimates of the
average composition and normative mineralogy are
presented in Tables 3 and 4.
THE ARCHEAN CRUST
Archean Upper Crust
The sedimentary record again provides us with a
sampling of the Archean crust and reveals that the
composition of the Archean upper crust stands in
marked contrast to that of the post-Archean crust
(Table 3). The most notable difference is shown by the
REE patterns in the Archean sedimentary rocks,
which in general show less enrichment in the LREE
and no Eu anomalies (Figure 3), relative to those of the
post-Archean crust. These differences inREE patterns
between Archean and post-Archean clastic sediments
have become a crucial observation for models of the
evolution of the continental crust and have been doc-
umented in many studies (see summary of Taylor and
McLennan [1985]).
Most studies have dealt with the relatively well
preserved low-grade sedimentary rocks of the ubiqui-
tous Archean greenstone belts in which turbidites are
the most abundant type of sedimentary rock. There is
often a general superficial resemblance to the REE
patterns of island arc volcanic rocks such as andesites.However, this similarity is merely the result of deri-
vation largely from a mixture of the ubiquitous bi-
modal suite of basaltic and felsic igneous rocks (to-
nalites, trondhjemites, granodiorites, or the "TTG
suite," and their volcanic equivalents) which charac-
terize many Archean terranes.
There is, in detail, an extreme variability of REE
patterns in Archean sediments in contrast to the gen-
erally uniform post-Archean REE patterns. Although
many Archean sediments have intermediate patterns
reminiscent of andesites, both very steep and flat pat-
terns are locally abundant [McLennan and Taylor,1984]. The steep patterns occur in first-cycle sedi-
ments derived directly from tonalites, trondhjemites,
and granodiorites (TTG igneous suites), whereas the
flat patterns are derived from basaltic precursors to
first-cycle sediments. Both REE patterns come from
the Archean "bimodal igneous suite," which largely
dominates the Archean upper crust.
This distinction between the REE patterns of Ar-
chean and post-Archean sediments has led to some
controversy. Thus some workers [e.g., Gibbs et al.,
1986]have argued that this distinction is without age
significance and is the result of differing tectonic set-
tings, implying that post-Archean greywackes should
be identical to those of Archean age. However, these
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33, 2 I REVIEWS OF GEOPHYSICS Taylor and McLennan: CONTINENTAL CRUST GEOCHEMISTRY. 251
1.2
-I e
::l1.0
W
-0.80.6
1.2
Post-Archean:'"
l1i: e
i~
: : - - - - - 8f - - - - - - < w £ v : - ' Q ' , , - - - - - - - -
. , " " ://////"~ I
> ~ / » » ~ / : ; .: /~ :'>7-~;~;~~<'/""'" : Cratonic
! Sediments
0.6
0.4 '-'-' .J...O- ~ 0.4 ..........
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 0.5
GdN / YbN
1.0 2.0 2.5
GdN
/ YbN
3.0.5 3.5 4.0
Figure 3. Plot of Eu/Eu* versus GdNlYbN for sedimentary rocks. Three fields are shown: Archean and
post-Archean sedimentary rocks deposited in volcanically active tectonic settings and "cratonic" sedi-
mentary rocks of all ages. Adapted fromMcLennan and Taylor [1991].Each field incorporates at least 90%
of the available data. Post-Archean active margin sedimentary rocks almost completely overlap the fieldof cratonic sediments and thus are significantly more depleted in Eu (lower Eu/Eu*) than are Archean
sedimentary rocks of comparable tectonic association. Archean sedimentary rocks commonly possess
heavy REE depletion (high Gd/Yb), whereas post-Archean sediments very rarely display such features.
Cratonic sedimentary rocks of all ages appear generally similar, with significant Eu depletion, but the
extent of such terranes appears to have been much less in the Archean. Overall, these and other
geochemical data indicate that the Archean and post-Archean upper continental crust differed in compo-
sition, that the nature ofmantle sources ofcontinental crust (or the processes of crust formation) may have
differed, and that the mechanisms of crustal differentiation (notably the role of intracrustal melting) also
differed.
younger sediments display the crucial signatures of Eu
depletion in all post-Archean tectonic environments(e.g., Figure 3), except those of forearc basins of
oceanic island arcs. Many Archean greywackes ap-
pear to have formed in tectonic settings such as back
arc, continental arc, trailing edge, and foreland basins
and are clearly petrographically distinct from their
modern counterparts in being plagioclase-rich (to-
nalitic sources) but having relatively few andesitic
rock fragments [McLennan, 1984].Finally, the general
evidence that Archean turbidites are derived from the
"bimodal basaltic-TI'G suite" rather than from "is-
land arc andesites" renders the supposed analogy be-
tween conditions in the Archean and modern arc en-
vironments superficial.
Others [e.g., Condie, 1992] appear to restrict the
usage of "upper crust" to cratonic settings only and
argue that by this definition the Archean and post-
Archean upper crustal compositions are indistinguish-
able. By this interpretation, Archean greenstone belt
sediments are not considered to be part of continental
crust, even though they are mostly developed on gra-
nitic terranes and are not Archean oceanic crust
[Bickle et al., 1994].We find this definition unsatisfac-
tory because for modern environments, it would ex-
clude volcanically active settings, such as Japan, the
southwest Pacific islands, the Andes, Indonesia, andmuch of New Zealand from being considered part of
the continental crust (also note, for example, that
sediments derived from many of these terranes indeedpossess negative Eu anomalies [e.g., McLennan et al.,
1990]). There is no dispute that the upper crust ex-
posed in cratons of any age is likely to be of compa-
rable composition and possess the distinctive negative
Eu anomaly [Taylor et al., 1986] (see below). In any
case, making such arcane distinctions reduces the dis-
cussion to a question of semantics rather than address-
ing the problems of how the Earth actually evolves.
Apart from the greenstone terrane sediments, small
amounts of Archean crust are preserved in high-grade
metamorphic terranes. Studies of such terranes from
Greenland [Boak et al., 1982;McLennan et al., 1984;
Jacobsen and Dymek, 1988], India [Naqvi et al.,
1983],Montana-Wyoming [Mueller et al., 1982;Gibbs
et al., 1986],Canadian Shield [Sawyer, 1986;Taylor et
al., 1986], Western Gneiss Terrain, Australia, and
Limpopo Belt, South Africa [Taylor et al., 1986;Bo-
ryta and Condie, 1990] have indicated that these re-
gions commonly represent a tectonic environment dis-
tinct from that of the greenstone belts. Typical REE
patterns fall into two groups. One group (e.g., Ka-
puskasing [Taylor et al., 1986]and Quetico Belt, Can-
ada [Sawyer, 1986])are highly metamorphosed equiv-
alents ofArchean greenstone belt sediments. A second
group (Wyoming-Montana [Gibbs et al., 1986] andWestern Gneiss Terrain [Taylor et al., 1986])contains
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252 • Taylor and McLennan: CONTINENTAL CRUST GEOCHEMISTRY
many samples that are indistinguishable from typical
post-Archean patterns and represent deposition on a
stable shelf environment, most probably occurring on
small, stable minicratons.
Both groups of REE patterns are found in close
proximity in Archean high-grade terranes in India,
Greenland, and the Limpopo Belt. This diversity ofREE patterns demonstrates that the sediments were
derived from highly localized sources, as is the case
for many Archean sediments. In such regions, small
areas of stable crust (minicratons) must exist in close
association with greenstone belts, but the cratons are
not contributing significant amounts of erosional de-
bris to the greenstone belt sedimentary basins. Be-
cause of the tendency of the greenstone belt environ-
ments to undergo destruction by erosion and recycling
[Veizer and Jansen, 1985], the high-grade belts are
preferentially preserved.
The limited extent of these Archean terranes with
"post-Archean" REE signatures is indicated by the
absence in the greenstone belt sediments of such sig-
natures, despite the close association of greenstone
belts with "granitic" terranes [Bickle et al., 1994].
This is striking evidence for the limited extent of "cra-
tonic" terranes in the Archean because wind-borne
dust should have been widespread in that epoch owing
to the absence of land-based vegetation. Thus any
granitic terrane with a PAAS REE signature should
have contributed the characteristic signature of Eu
depletion to the greenstone belt sediments. The ab-
sence of a detectable signature of Eu depletion means
that such high-grade terranes represented less than10% of the exposed Archean crust [Taylor et al.,
1986].Most of the crust as sampled by the greenstone
belt terranes was derived from areas where the bi-
modal suite of basalt and TTG dominated the land area
being eroded to supply the sediments.
Another observation of importance when discuss-
ing the cause of the difference between Archean and
post-Archean sedimentary REE patterns is that in
modern environments where volcanic provenances
should dominate, such as arcs or Pacific deep-sea
environments, there is ubiquitous evidence for contri-
butions of upper crustal material with negative Eu
anomalies [e.g., Ben Othmann et al., 1989;McLennan
et al., 1990;Lin, 1992](see Figure 2). The isolation of
small Archean cratons enabled the survival of distinct
suites of sedimentary rocks with much variation in
REE patterns, in contrast to the situation in post-
Archean time.
A probable scenario for the Archean crust com-
prises many small, fast spreading plates with numer-
ous but relatively short volcanic arcs. Volcanism as-
sociated with plumes is expected to be common,
producing numerous Icelandic-type islands. Regions
of the crust that had undergone intracrustal melting
(generating upper crustal negative Eu anomalies) and
stabilization to form cratonic regions were restricted in
33,2/ REVIEWS OF GEOPHYSICS
number and of limited extent, perhaps only slightly
greater in size than the present extent of early Archean
terranes such as inWest Greenland-Labrador and the
Minnesota River Valley.
Origin of the Archean Crust
There appears to have been only minor intracrustalmelting in the Archean. This was localized as shown
by the limited extent of PAAS-type REE patterns.
Thus there is not that distinction in the Archean crust
that is observed in the post-Archean crust, which is
highly stratified as a result of extensive intracrustal
partial melting. The steep REE patterns of the Ar-
chean TTG suites are indicative of either (1) garnet in
the residue during partial melting [e.g., Martin, 1986,
1993]or (2) partial melting of mantle sources already
possessing the signature of LREE enrichment [e.g.,
Stern et al., 1989;Evans and Hanson, 1995].
In the latter case, the ultimate origin of the HREE
depletion is likely also due to fractionation of a mineral
such as garnet, but in an earlier event. Because resid-
ual garnet is stable in mafic-ultramafic systems only at
depths below about 40 km, an origin by melting at
mantle depths for the TTG suite is indicated. The
Archean crust thus appeared to form as a mixture of
piled-up basalt-komatiite and tonalite-trondhjemite in-
trusions and extrusives. In the upper crust, sedimen-
tary data suggest that the ratio of basalt to TTG was
about equal. A ratio of 2 basalt/1 TTG suite for the
Archean bulk crust is suggested from a variety of
geochemical and geophysical constraints [Taylor and
McLennan, 1985].In our models the Archean crust be-comes more basic with depth, which we attribute to a
higher ratio of basalt to TTG suites with depth. This
would be expected from simple density considerations.
Production of the tonalite-trondhjemite suite may
be explained by rapid subduction of warm basaltic
crust [Martin, 1986; Defant and Drummond, 1990,
1993; Drummond and Defant, 1990; Abbott et al.,
1994]. This process still operates today in regions
(e.g., southern Andes) where rapid subduction of
young hot oceanic crust occurs; the slab reaches melt-
ing temperatures before complete dehydration occurs,
so that hornblende and garnet are present during melt-
ing of the subducting slab. Similarly, the generation of
the high-Mg andesite (boninite) suite is also considered
to be possibly related to subduction of young, high-
temperature lithosphere, resulting in mantle melting at
low pressure [Crawford et al., 1989].
B U L K CRUSTAL COMPOSITION
It is important to realize that the sedimentary rock
data provide information only on that portion of the
crust exposed to weathering and erosion. Simple mass
balance calculations show that a 36-km-thick crust
with K, Th, and U abundances equal to that of the
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33, 2/ REVIEWS OF GEOPHYSICS Taylor and McLennan: CONTINENTAL CRUST GEOCHEMISTRY. 253
present upper continental crust would require about
80-90% of the entire Earth's complement of these
elements to be present in the continental crust. Thus
the upper crust is not representative of the entire
36-km thickness of the continental crust. Evidence
from heat flow data shows that the upper crust (about
10km thick) is strongly enriched in the heat-producing
elements (K, U, and Th).
There are two primary sources of heat: radioactive
decay, principally from 4oK, 238U, 235U, and 232Th,
contributing up to 80%of the terrestrial heat flow, and
other sources, such as secular cooling of the Earth and
core crystallization, responsible for at least 20%.
Some estimates place the radiogenic component of
terrestrial heat flow as low as 50%, and the relative
importance of the various sources of the Earth's heat
remains an important outstanding question. The
present average heat flow from continental areas is
about 50 mW m-2 compared with 100mWm-2 for the
oceans and 84mWm-2 for the whole Earth. However,it is difficult to assign precise values to the contribu-
tions from crust and underlying mantle. Temperatures
at the base of the crust are estimated as 300°-400°C in
shield areas, 500°-600°C in young orogenic areas, and
?650o-750°C in rift regions [e.g., McLennan, 1992].
This heat is derived from three main sources. One is
the so-called "reduced" heat flow, which is heat loss
from the deep interior transmitted mainly by conduc-
tion across the crust-mantle boundary. The second is
heat loss associated with tectonic activity within the
lithosphere. This component decays rapidly over pe-
riods of about 400 m.y. and is unimportant except inregions that experienced Phanerozoic tectonic activ-
ity. The third component is heat generated by radio-
active decay within the lithosphere; this is dominated
by upper crustal heat production. The estimation of
this component and the relative amounts of heat pro-
duced in the crust and mantle are currently under
debate [e.g., Nyblade and Pollack, 1993], and the
interpretation of the heat flow data is not without
problems. Until these problems are resolved, the heat
flow data place only loose constraints on bulk crustal
composition. Previous estimates based on a crustal
heat flow component of 23 mW m-2 were used by
Taylor and McLennan [1985] to arrive at the bulk
crustal composition given in Table 3. These are gen-
erally consistent with the geochemical balance con-
straints that limit the amount of incompatible elements
that can be concentrated in the crust. Until some new
constraints appear from geophysical or geochemical
evidence, and until the nature of the subcontinental
lithosphere is resolved, these values appear to us to
provide the best current estimates.
There is a well-established difference between the
heat flow in Archean and later Precambrian terranes
[e.g., Morgan, 1984;Pollack, 1980,1986;Nyblade and
Pollack, 1993],even when the effects of a tectonic heatcontribution are removed for younger crust. There
appears to be a steep offset in the data at the Archean-
Proterozoic boundary. This is not due to deeper ero-
sion of the Archean crust, removing a surficial hot
layer, because erosional levels are not significantly
deeper in Archean terranes [Watson, 1976].This dif-
ference in heat flow is probably not due to a difference
in composition between the Archean and later crusts
but is attributed by Nyblade and Pollack [1993] to
result mostly from an increase in the thickness of the
subcrustal lithosphere under Archean cratons. This
acts to deflect mantle heat flow around the Archean
cratons, resulting in a lower surface heat flow. Origi-
nally, these differences were attributed to differing
crustal abundances between Archean and post-Archean
stabilized crust [Morgan, 1984; Taylor and McLennan,
1985].Itappears that the principal controllingfactor may
be not the crustal abundances of the heat-producing
elements but the thickness of the subcrustal lithosphere
[e.g., Nyblade and Pollack, 1993].
Thus there may be little difference in bulk compo-sition between the Archean and present bulk crust
(that incorporates a 75%Archean component) despite
a significant difference in the upper crustal composi-
tions between Archean and post-Archean time. The
bulk crustal compositions listed by Taylor and McLen-
nan [1985] (see also Table 3) in fact are quite similar
(present values of 0.91% K, 3.5 ppm Th, and 0.91 ppm
U compared with bulk Archean crustal values of
0.75% K, 2.9 ppm Th, and 0.75 ppm U). These differ-
ences are probably within the error limits.
Nevertheless, there are some significant distinc-
tions between Archean and post-Archean crustal com-positions that have important implications for crust-
mantle evolution. For example, HREE depletion
(GdN/YbN > 2.0) is a common feature in Archean
crustal rocks but is virtually absent from rocks formed
during the post-Archean. The implications for crustal
evolution are discussed below. In our models, most of
the crust is generated in the Archean, with lesser
additions from later island arc volcanism, to make up
the present crust.
THE ORIGIN OF THE CRUST
The geological record indicates that there is a basic
distinction between the igneous activity that contrib-
uted to the formation of the continental crust in the
Archean and post-Archean epochs (Figure 4). The
basic cause of this difference is higher heat flowduring
the Archean. This results in the subduction of young
hot oceanic lithosphere during the Archean. There are
a number of possible fates for this subducted material.
In most models, such basaltic crust reaches melting
temperatures before dehydration has occurred. Partial
melting occurs under these conditions, leaving a horn-
blende-garnet residue [e.g., Arth and Hanson, 1975;Martin, 1986]resulting in production of the TTG suite
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254. Taylor and McLennan: CONTINENTAL CRUST GEOCHEMISTRY 33, 2/ REVIEWS OF GEOPHYSICS
Post-Archean Growth of New Continental Crust
Andesite volcano Sedimentary
Sea level
Trench
t
FromMid-ocean ridge-
Plume headof basaltunderplatedb e ne a th c ru s t?
Mantle
Growth of New Continental Crust during the Archean
--
Felsic volcano
First cycle sediments
Sea level
25
Oceanic crust
Tonalite (sodium granite)
intrusion
Mantle km
Zone of mantle melting?
50
'---J--'-_- Zone of dehydration and/or partialMelt ing of downgoing slab
Figure 4. Schematic models illustrating the difference between Archean and post-Archean crustal devel-
opment at subduction zones. During the Archean, subducting oceanic crust was younger and hotter on
average, and the pressure-temperature conditions of dehydration/melting of the subducting slab and
overlying mantle differed considerably.
with steep REE patterns and either a zero or an occa-
sional positive Eu anomaly.
Recently, modeling of REE and other elements inundeformed posttectonic TTG suite rocks has led to
suggestions that they may be derived directly as man-
tle melts that have been enriched in a large-ion litho-
phile element (LILE) component shortly before«100-200 m.y.) formation [e.g., Shirey and Hanson,
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33,2/ REVIEWS OF GEOPHYSICS Taylor and McLennan: CONTINENTAL CRUST GEOCHEMISTRY. 255
1984; Stern and Hanson, 1991; Evans and Hanson,
1995]. In this model, two observations are particularly
significant.
First, in cogenetic suites, HREE are commonly
subparallel (i.e., uniform Gd/Yb ratios) over a wide
range of Si02 content. Such an observation is incon-
sistent with garnet's being a significant residual phasebecause even minor differences in the amount of re-
sidual garnet would lead to highly variable levels of
HREE depletion (i.e., variable GdlYb ratios).
Second, the Mg/(Mg + Fe) ratios in some suites of
TTG rocks are uniformly higher than those generated
by melting of basaltic compositions under a wide va-
riety of experimental conditions and compositions
[Evans and Hanson, 1995]. Thus melting of basalt
does not appear to be capable of generating the appro-
priate compositions for at least some of the TTG
suites.
Evans and Hanson [1995] have noted some similar-
ities in the compositions of high-Mg (most primitive)
TTG rocks (part of the so-called Sanukitoid Suite
[Shirey and Hanson, 1984]) in the southwestern Supe-
rior Province and modern boninites (high-Mg andes-
ites) in island arc environments. Generation of such
material appears to require low-pressure and high-
temperature conditions. In the Archean, such condi-
tions could be achieved by mantle melting associated
with dehydration and/or partial melting of subducted
young and hot oceanic lithosphere [Martin, 1986; De-
fant and Drummond, 1990, 1993; Drummond and De-
fant, 1990].
The Archean crust is interpreted as being derivedfrom mixtures of the two dominant ("bimodal") igne-
ous lithologies in the Archean: basalts and Na-rich
igneous rocks such as tonalites, trondhjemites, and
granodiorites, or the TTG suite (and their volcanic
equivalents). Rarely, REE patterns with Eu depletion
similar to PAAS are observed in the sedimentary
record, but they appear to be restricted to cratonic
sediments preserved in some high-grade metamorphic
terranes. These are interpreted as being derived from
minicratons [Taylor et al., 1986], forerunners of the
massive cratonic development in the late Archean.
A massive increase in the growth rate of the conti-
nental crust occurred OVer an extended period be-
tween 3.2 and 2.6 eons (with the exact age differing for
individual cratonic regions). This is well documented
by the Nd isotopic evidence [e.g., McCulloch and
Wasserburg, 1978; Chase and Patchett, 1988; Galer
and Goldstein, 1991; McCulloch and Bennett, 1994].
Toward the end of the late Archean, massive intra-
crustal melting of newly formed late Archean crust
produced an upper crust dominated by K-rich grano-
diorites and granites, typically showing a relative de-
pletion in Eu. This change is reflected in the REE
patterns observed in the clastic sediments. Archean-
type REE patterns are swamped, and the upper crustassumed its present composition.
There is a little-understood relationship between
the growth of continents and the intracrustal melting
that shortly followed. A significant observation is that
such intracrustal melting mostly occurs within 50-100
million years of the derivation of new crust from the
mantle [e.g., Moorbath, 1977]. Perhaps the litho-
spheric keel beneath older cratons deflects mantle heatflow or rising mantle plumes toward such areas. The
development of extensive crustal areas may produce
an insulating trap for mantle-derived magmas to accu-
mulate at their base and so provide the necessary heat
for crustal melting. Another factor that may be impor-
tant in providing additional heat is the thermal pertur-
bation associated with the continental collisions that
ultimately must terminate the subduction of the oce-
anic plate.
As global heat flowdiminished in the late Archean,
the number of plates became fewer, and modern-style
plate tectonics became dominant. The result was that
the oceanic crust was subducted on a much longer
timescale and so on average was both older and colder
by the time it reached the subduction zone [e.g., Da-
vies, 1992]. Under these conditions it was dehydrated
at higher pressures during subduction, becoming re-
fractory before reaching the melting temperatures of
the basaltic crust. This older oceanic crust thus returns
to the mantle to greater depth without being remelted.
The fluids resulting from dehydration ascend into the
overlying mantle wedge, where they induce melting,
and result in the production of the present-day calc-
alkaline suite [Martin, 1986; Defant and Drummond,
1990, 1993; Drummond and Defant, 1990]. These con-ditions, which result in the addition of material of
andesitic composition to the crust, appear to have
persisted since the early Proterozoic.
The Freeboard Constraint
Flat-lying Proterozoic sediments provide evidence
that for the past 2 billion years, sea level has been
within about 1 km of the present level. This is strong
evidence that the volumes of continents relative to
oceans have been similar over this period [e.g., Wise,
1974; Armstrong, 1981; Kasting and Holm, 1992].
However, the sedimentary record in the Archean is
much too fragmentary to make freeboard a restriction
on crustal volume. Although the constraints from the
freeboard record are now considered considerably less
stringent than was previously thought [McLennan and
Taylor, 1982; Schubert and Reymer, 1985; McLennan,
1988;Armstrong, 1991; Galer, 1991], the general argu-
ments appear valid at least back into the Early Pro-
terozoic, or about halfway through the geological
record. Thus although the freeboard observation has
been one of the cornerstones of the no-growth early
crust school, it is valid for only the latter part of the
geological record and provides no constraints of
crustal volume before the Proterozoic.The freeboard constraint is accommodated in most
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256. Taylor and McLennan: CONTINENTAL CRUST GEOCHEMISTRY
models by proposing that there have been relatively
minor additions to the continents since the Early Pro-
terozoic and that most crustal growth occurred by the
late Archean. It is noteworthy, however, that a num-
ber of models requiring significant crustal growth at
2.1-1.8 eons, such that the amount of crust present at
2.5 eons was less than 50% (in some models as little as
30-40%), have not come to terms with this constraint.
Andesite Models
In early estimates of crustal composition [e.g., Tay-
lor, 1967, 1977]it was assumed that the bulk crust was
equivalent in composition to that of average island arc
volcanic rocks. This formed the basis for the so-called
"andesite" model. Island arc volcanism is the princi-
pal mechanism at the present day by which material is
derived from the mantle and accreted to form the
continental crust (see McCulloch and Gamble [1991]
andHawkesworth et al. [1991, 1993]for recent reviews
of island arc magmatism). The model has major prob-
lems in that it contributes too much potassium (i.e.,
the composition generates too much heat, although
note the difficulties with interpretation of heat flow
discussed above) and other incompatible elements and
too little nickel and chromium to account for the ob-
served crustal abundances [Taylor and McLennan,
1985].Furthermore, the major growth of the continen-
tal crust appears to have occurred episodically, mostly
in the late Archean where andesites are comparatively
scarce and the TTG suite was the chief nonbasaltic
product of igneous activity. In contrast, the produc-
tion of andesites in island arcs has probably beenoperating in its present mode only for about the past 2
billion years. This process has produced only about
25-40% of the crust.
The overall crustal bulk composition in our models
was calculated from a 75/25 mixture of the Archean
bimodal and the post-Archean andesitic compositions
[Taylor and McLennan, 1985], and as the crustal
growth curve is further refined, these proportions will
also require adjustment. In this model, any addition of
material by basaltic underplating is not taken into
account.
Plumes and Basaltic Underplating
There is growing appreciation that there may be
additional mechanisms for the growth of continents,
mainly by basaltic underplating at convergent margins
[e.g., Kempton and Harmon, 1992]and by magmatism
and underplating largely associated with mantle
plumes [Hill et al., 1992;Campbell and Griffiths, 1992;
Hill, 1993]. The volumetric scale of such processes
and the implications for crustal composition are much
less understood.
Basaltic underplating at convergent margins or as-
sociated with plume activity has been suggested as an
important mechanism to explain the chemistry andmetamorphic history of lower crustal xenoliths [e.g.,
33,2/ REVIEWS OF GEOPHYSICS
Kempton and Harmon, 1992;Rudnick, 1990, 1992a].
Rudnick [1990] suggested that as much as 20% (or
more) of the crust in the Ivrea Zone [Voshage et al.,
1990]was derived from basaltic underplating, in this
case associated with convergent margin magmatism.
On the other hand, underplating may not be a signifi-
cant process at times substantially after the major
episode of crustal growth [Wendlandt et al., 1993].
Estimates of crustal additions derived directly from
magmatism associated with continental intraplate
mantle plumes or obducted ocean plateaus are re-
stricted to no more than a few percent [Hill et al.,
1992].
Although there are a number of caveats, if basaltic
underplating is an important process for the growth of
the continents, then all models of bulk crustal compo-
sition may have to be reevaluated. It has long been
argued on petrological grounds that the average com-
position of island arcs is more mafic than andesite
[e.g., Arculus, 1981;Ellam and Hawkesworth, 1988;Pearcy et al., 1990]. Recent evidence from the
geochemistry of turbidites derived from the lzu-Bonin
Arc adds some support to this suggestion [Gill et al.,
1995].Incorporation of basaltic material at the base of
the arc crust would enhance such a model. The major
difficulty with a basaltic continental crust is that it is
difficult to differentiate such material in order to gen-
erate sufficient granite to provide for the well-con-
strained upper crustal compositions. At least one quar-
ter of crustal volume, is needed to provide for the
granitic composition observed in the upper crust. If
such basaltic material is calc-alkaline in nature, theremay be barely sufficient concentrations of heat-pro-
ducing elements to be consistent with the observed
heat flow data [Basaltic Volcanism Study Project,
1981].
Various models have been proposed to avoid such
dilemmas. One is to maintain an intermediate overall
composition by delaminating the more ultramafic com-
ponents at the base of the crust and resorbing them
into the mantle [e.g., Turcotte, 1989; Pearcy et al.,
1990; Kay and Kay, 1991]. Such mechanisms, while
worthy of study, are at this stage rather ad hoc. Ifthe
crust does indeed evolve through such convoluted
mechanisms, then estimates of its bulk composition
would be largely unconstrained.
One end-member proposes that because basalts are
the primary silicate melt from the mantle, and because
continents are ultimately derived from the mantle,
their bulk composition must be basaltic. The answer to
this paradox is of course that the siliceous composition
of the continental crust is mostly a consequence of
recycling and remelting of the primary basalts follow-
ing subduction. It is this process, apparently unique to
the Earth, that has resulted in the production of the
fractionated continental crust. The lack of such recy-
cling on other planets and satellites has precluded thedevelopment of continental crust on those bodies.
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33, 2/ REVIEWS OF GEOPHYSICS Taylor and McLennan: CONTINENTAL CRUST GEOCHEMISTRY. 257
Subcrustal lithosphere
A crucial problem, closely related to crustal
geochemistry, is the nature of the subcrustal litho-
sphere. Are there deep roots to the continents? If so,
what is their relationship to the overlying crust? There
is good evidence that fast seismic P wave velocities
extend to depths of 220 km. Lateral variations in S
wave velocities extend much deeper, to 400 km. Thus
there appear to be deep keels to the continental re-
gions, which are relatively cold and appear to be re-
fractory in composition [Jordan, 1981, 1988;Boyd and
Gurney, 1986].One interpretation of the heat flowdata
[Nyblade and Pollack, 1993;Bickle, 1986]is consistent
with a much thicker lithosphere beneath Archean cra-
tons than beneath those of younger age. This view has
received considerable support from studies of inclu-
sions in diamonds that indicate that low temperatures
have persisted beneath shields for up to 3000 m.y.
[Richardson et al., 1984;Walker et al., 1989;Pearson
et al., 1995]. The general interpretation from the seis-mic, petrological and isotopic evidence is that these
deep roots represent refractory, lower-density mate-
rial, consistent with their being the residue remaining
from the extraction of a Fe-rich partial melt, leaving
behind an Mg-enriched zone of lower density.
Whether these roots are residual from the extraction of
basaltic oceanic crust or are connected with the ex-
traction of continental crust is an interesting but
largely unresolved question.
T H E N AT U RE O F C R US T AL G R OW TH
Two major hypotheses have been advanced to ex-
plain the evolution of the continental crust. The first
proposes that the present mass of the crust formed
very early in Earth history and has been recycled
through the mantle in steadily decreasing fashion such
that new additions are balanced by losses, resulting in
a steady state system [Armstrong, 1968, 1981, 1991].
The second proposes that the crust grew throughout
geological time, with variations from a steady rate to
growth in major episodic pulses [e.g., Moorbath, 1977;
Taylor and McLennan, 1981]. The REE abundances
shed considerable light on these problems. As was
noted above, there is a major dichotomy in the sedi-
mentary record at about the Archean-Proterozoic tran-
sition (ca. 2.5 eons), and this has been correlated with
an earlier major pulse of crustal growth during the late
Archean.
There is growing evidence (e.g., from lOBeand Pb
isotopes) that sediments have been recycled into the
mantle and are important in controlling the composi-
tion of island arc and perhaps intraplate volcanics (see
reviews by McLennan [1988]and White [1989]). The
evidence from lOBe and various other geological,
geochemical, and isotopic constraints limit the amountof subducted sediments to a few percent. However,
those models that propose massive recycling of the
crust through the mantle, on a scale large enough to
maintain a steady state crustal mass, encounter vari-
ous difficulties. Data from long-lived radiogenic iso-
topes for mantle-derived rocks provide no indepen-
dent constraints on this problem, a point long made by
Armstrong [1968, 1981, 1991]. One constraint, from
the mass-age distribution and Nd isotopic characteris-
tics of sedimentary rocks, is that during various sedi-
mentary recycling processes the sedimentary mass
receives only about 15%new material from the mantle
[Veizer and Jansen, 1985;McLennan, 1988].Thus the
mass of sediment available for subduction is <1.6 x
1015g yr " (about 0.5 krrr' yr-I of crust), Such an
amount likely provides insufficient material to support
a steady state crustal mass [McLennan, 1988].
Two observations inform us of the limited extent of
very early "granitic" crusts. First, the small expo-
sures of early Archean potassic granites do not con-
tribute a negative Eu signature to the greenstone beltsediments. There was no land vegetation in the Ar-
chean [Cloud, 1988]. Hence there must have been
wide exposures of bare rock, with the consequent
formation and wind transport of dust. Present-day
mid-ocean sediments show the telltale signature of Eu
depletion of the upper continental crust derived largely
by wind transport [Ben Othman et al., 1989;Lin, 1992]
(Figure 2). This signature of Eu depletion is missing
from virtually all Archean sediments, including those
from greenstone belts (Figure 3) and from many high-
grade terranes, except for the local cratonic areas
discussed, for example by Taylor et al. [1986].Accord-ingly, most of the exposed crustal rocks from which
the Archean sediments were derived were basalts and
the TTG suite, rather than the now familiar granites
and granodiorites that constitute the present exposed
upper crust of the continents.
The second crucial observation is that of Stephen-
son and Patchett [1990],who failed to find evidence of
extensive pre-3.5-eon recycled zircon populations that
would have been indicative of large areas of old gra-
nitic crust. In this study they analyzed zircons, mainly
from quartzites, from the Canadian Shield and the
Wyoming, North Atlantic (Labrador, Greenland,
Scotland) and Kaapvaal (Southern Africa) cratons.
Zircon, being a highly durable mineral, survives
through many cycles of weathering, erosion, and dep-
osition of sediments. It also appears to be aerodynam-
ically suited for large-scale wind transport, as is wit-
nessed by enrichments seen in loess deposits [Taylor
et al., 1983] and the presence of substantial exotic
zircon in ancient soils [Brimhall et al., 1990].Stephen-
son and Patchett found that the age of the zircon
populations, dated by the 176Lu_176Hfechnique, in
early Archean quartzites, was usually the same age as
the terrane in which they are found and that there is a
scarcity or absence of zircons of significantly olderage. If there was an early sialic crust, then a popula-
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258 • Taylor and McLennan: CONTINENTAL CRUST GEOCHEMISTRY 33, 2/ REVIEWS OF GEOPHYSICS
tion of ancient zircons derived from it by erosion
should have survived and been recycled into younger
Archean sediments. A. P. Nutman (personal commu-
nication, 1994) finds no older zircon populations in
3.8-eon metasediments from Isua, Greenland, using
U-Pb dating by ion microprobe.
Armstrong[1991]attempted to refute this argument
by noting a similarity between the sedimentary zircon
ages and the average age of crust (at the time of
sedimentation) predicted by Armstrong's [1981]
steady state model. The difficulty with this is that
sedimentary rocks are not derived solely from contem-
poraneous "average" crust but instead are typically
derived substantially from older, cannibalistically re-
cycled sedimentary rocks [e.g., Veizer and Jansen,
1985] and would be expected to have components
older than "average" crust. Thus younger Proterozoic
sediments, in contrast, commonly contain a significant
population of zircons derived from rocks of late Ar-
chean age. The conclusion from this study is that old
Archean sialic continental crust was rare and not sub-
stantially more extensive than the few remnants that
are currently exposed.
The efficient averaging carried out by the processes
of sedimentation and the buffering effects of cannibal-
istic sedimentary recycling probably conceals small-
scale variations in the rate of continental growth and
perhaps even in upper crustal composition. Thus there
is considerable evidence for a period of growth at
about 1800m.y. ago that is not clearly reflected in the
sedimentary geochemical record. The major change in
upper crustal composition in the late Archean and therecycling of sediments effectively swamp out the ef-
fects of later, smaller episodes. Detailed sedimento-
logical, geochemical and isotopic studies should un-
cover these effects [e.g., McLennan et al., 1995].The
situation was different in the early Archean. Small
cratonic areas were isolated, resulting in the preserva-
tion of individual episodes of cratonization, now re-
flected in the local variations in REE patterns.
Nd model ages are now well established as a means
of estimating crustal formation ages [McCulloch and
Wasserburg, 1978;DePaolo, 1981].During extraction
of material from the mantle and its incorporation into
the continental crust, major chemical fractionation of
Sm and Nd occurs. Intracrustal melting events lead to
further fractionation of Sm and Nd, although this gen-
erally follows crust formation within 100-200 m.y. and
is comparatively minor. The Nd model ages of sedi-
mentary rocks in general give the average time of
formation of the various sources of the sediments
rather than the age of deposition. This assumes that
Sm and Nd are not fractionated during sedimentation
or diagenesis (see above).
Evidence regarding possible changes in upper
crustal composition during the post-Archean can be
obtained from the Nd model ages of sediments, asdiscussed earlier (see section on loess). For post-Ar-
chean sediments these range from about 2.5 to 1.0
eons and represent the average age of upper crustal
material which was the source of these sediments.
However, post-Archean sediments have very uniform
REE patterns and other key geochemical features
(e.g., Th/Sc) despite the wide range of provenance
age. Minor compositional changes in the upper crust
may be masked to the sedimentary record by the
buffering effects of cannibalistic sedimentary recycling
[McLennan, 1988;McLennan and Hemming, 1992].
Nevertheless, within these uncertainties, this indicates
that the processes forming the upper crust must have
been uniform over that interval and that any additions
to the upper crust during the post-Archean could not
have differed significantly in composition.
In summary, the crust continues to grow at present
by island arc volcanism and related magmatism, fol-
lowed by episodes of intracrustal melting (e.g., eastern
Australia). There has been no significant change in this
process since the Archean-Proterozoic boundary that
is detectable in the sedimentary REE record. The
growth of the continental crust has proceeded in an
episodic fashion throughout geological time [e.g.,
Moorbath, 1977, 1978; Taylor and McLennan, 1981,
1985], with a major increase in the growth rate in the
late Archean (Figure 5).
Supercontinental Cycles
There is a growing consensus that periodically the
continental masses unite to form one or two supercon-
tinents [e.g., Nance et al., 1986;Hoffman, 1988, 1992;
Unrug, 1992; Anderson, 1994]. These episodes arefollowed by times, such as the present, when the
continents are fragmented and dispersed. The recog-
nized periods of supercontinental accumulations
[Nance et al., 1986; Young, 1991; Unrug, 1992] are
during the Late Paleozoic (Pangea; 0.3 eons), Late
Proterozoic (0.9 eons), possibly at about 1.8 eons (for
North America/Baltica at least) and perhaps during the
late Archean (ca. 2.7 eons). During these episodes, sea
level appears to be generally low (i.e., lowstands of
first-order sea level fluctuations) probably due to the
maturing of ocean basins associated with relatively
minor plate motion. Among other things, this conti-
nental configuration appears to correlate with major
climatic fluctuations [e.g., Young, 1991] and funda-
mental biological changes such as the radiation of
stromatolites and mass extinctions.
Apart from the geographical configuration of conti-
nents, supercontinental cycles also likely have an im-
portant connection with crustal evolution [McLennan
and Taylor, 1991].For example, supercontinental cy-
cles may explain the apparent periodicity of crust-
forming events during the post-Archean. Apart from
the early Archean, these episodes of crustal growth,
generally taken to have occurred at about 3.8-3.5,
2.9-2.7,2.0-1.7, 1.3-1.1, and 0.5-0.3 eons [e.g., Me-Culloch and Bennett, 1993], appear to correlate rea-
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33, 2/ REVIEWS OF GEOPHYSICS Taylor and McLennan: CONTINENTAL CRUST GEOCHEMISTRY. 259
IntenseMeteoriteBombardment
High T / Low P Modern Island arc regime< "" ;:>-
Subduction regime10 0
4.0 3.0 2.0
A ge (A e)
Q)
80E~(5
>60
C ii
-J)
~. . .40 o
20
0
0.0
Major episodes of
crustal growth
Major supercontinents In place
1.0
Figure 5. Schematic model for the growth and evolution of the continental crust. The actual values of crust
present at any given time are not well constrained; however, a value of 50% crust by about 2.5 eons is
likely a minimum value to satisfy freeboard constraints. Although major global episodes of crustal growth
and differentiation are well documented during the late Archean and at about 2.1-1.7 eons, it is less clearif crustal growth is episodic on a global scale during younger times. In general, there appears to be a
relationship between crustal growth episodes and assembly phases of supercontinents.
sonably well with supercontinental assembly phases.
During supercontinental assembly, subduction of old
oceanic crust would be at a maximum, leading to an
increase in arc volcanism [Nance et al., 1986; Zhong
and Gurnis, 1993]. In addition, the continental colli-
sions that terminated the assembly phases would
promote crustal thickening, intracrustal melting and
differentiation, granulite facies metamorphism, conti-
nental underplating, and, accordingly, preservation of
such crust in the geological record.
On the other hand, the strong contrast in crustal
geochemistry seen at the Archean-Proterozoic transi-
tion does not appear to be related simply to the accu-
mulation of a late Archean supercontinent [McLennan
and Taylor, 1991]. Comparable changes in composi-
tion are not seen during younger cycles. The massive
increase in crustal growth in the later Archean is most
probably related to higher mantle heat production.
Intracrustal differentiation of this newly formed crust
to form stabilized continents resulted in a widespread
"granitic" upper crust (as opposed to localized earlyArchean differentiated cratons) possessing the charac-
teristic negative Eu anomaly, enrichment in heat-pro-
ducing elements, and other geochemical characteris-
tics. Subsequent episodes of crustal growth appear to
have followed a similar pattern because they have
produced little change in upper crustal compositions
[e.g., McLennan and Hemming, 1992].
Causes of Continental Breakup
Another interesting question, although somewhat
peripheral to this review, is the cause of continental
breakup [e.g., Duncan and Turcotte, 1994]. Because
such rifting is frequently accompanied by vast out-
pourings of basaltic lavas, a major debate has arisen
about whether such basaltic magmatism is the cause of
the breakup or merely a consequence of decompres-
sion melting of the mantle following the thinning of the
overlying crust [e.g., Storey et al., 1992]. Related to
this question are the role that deep mantle plumes play
in this process [Hill, 1993] and the influence of super-
continental cycles in controlling mantle convection
patterns [Gurnis, 1988; Zhong and Gurnis, 1993]. Do
mantle plumes initiate splitting of the crust by doming?
These chicken and egg arguments are extensively dis-
cussed by Storey et al. [1992]. Thus White [1992]
distinguishes various types of rifting that occur during
continental breakup. The extreme cases may be called
cold and hot rifts, respectively. During cold rifting
there is very little volcanism, and the continental rift is
"stretched like gum." When it breaks, this allows the
formation of a new oceanic spreading center. Hot rifts,
in contrast, form as a consequence of massive volca-
nism originating from mantle plumes. Very large vol-
umes (several kilometers thick) of lava are extruded.Thus mantle plumes, although not the cause of exten-
sion, may provide a trigger in the shape of additional
heat that boosts the splitting and breakup of the con-
tinents. Perhaps the most difficult aspect for plume-
related models is the linear nature of most rifting and
of many large igneous provinces, such as the 3000-km-
long Ferrar- Tasman Province.
T H E EARLIEST PRE-3.9 EON CRUSTS
The oldest preserved terrestrial rock is the 3.96-eon
Acasta Gneiss, in the Northwest Territories of Canada
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260 • Taylor and McLennan: CONTINENTAL CRUST GEOCHEMISTRY 33, 2/ REVIEWS OF GEOPHYSICS
[Bowring et al., 1990]. What happened in the 500-m.y.
gap between the formation of the Earth and that of the
Acasta Gneiss? In order to place this question in an
appropriate perspective, it is necessary to consider theproblems encompassing both the formation of this
planet and some associated events.
The gaseous parts of the solar nebula were dis-persed very quickly on timescales of a few million or
less years following To (4.56 eons) by early violent
solar activity [Strom et al., 1989]. The Earth then
accreted from a hierarchy of rocky planetesimals in a
gas-free environment [Wetherill, 1986, Taylor, 1992a,
1993a] between 10 and 100 m.y. after To. The Moon
was probably formed during the final stages of plane-tary accretion by the collision with the Earth of a body
0.15 Earth masses, somewhat larger than Mars [Benz
et al., 1989],with derivation of the Moon from the rocky
mantle of the impactor [Newsom and Taylor, 1989].
High temperatures are a consequence of the formation of
the Earth from planetesimals, and total melting of theEarth seems unavoidable. Indeed the lunar-forming
event was sufficiently energetic to have accomplished
this on its own [Benz and Cameron, 1990].
In an initially molten Earth, separation of the me-
tallic core is effectively coeval with accretion. The
subsequent crystallization history of the terrestrial
mantle is a matter of intense geochemical debate. A
number of things did not happen. The crystallization of
large bodies of molten rock, such as the Skaergaard or
Stillwater layered intrusions, might lead one to expect
the production of zones of differing mineralogy whose
elemental and isotopic fingerprints would inform us ofthis early event in the mantle. This was the course ofevents on the Moon. However, no terrestrial an-
orthositic crust, analogous to the lunar crust, formed
for various reasons: the Earth is less well endowed
with Ca and Al than the Moon, so that plagioclase willnot be an early crystallizing phase from a terrestrial
magma ocean; the massive Eu anomalies produced, as
on the Moon, by such an event, are missing; plagio-
case floated on the bone-dry lunar magma ocean but
would have sunk in the wet terrestrial magma; plagio-
clase is stable on the Earth only to 40 km, below whichit will react to form garnet. In addition, there is no sign
of a reservoir of primitive 87Sr/86Srvalues from such a
feldspathic crust [Taylor, 1982, 1992b].
The molten mantle was so large that conventional
terrestrial analogies, derived from layered intrusions
or the Moon, which is 2 orders of magnitude smaller,
are inadequate to describe the solidification of a mol-
ten terrestrial mantle. Probably, the mantle crystal-
lized without producing separate mineral zones [Tonks
and Melosh, 1990].
No good evidence exists for that enduring geologi-cal myth of a primordial world-encircling crust of
"sial" or granite. Such models originated through false
analogies between the production of a silicic residuumduring crystallization of basaltic magmas and conditions
in an early molten Earth, and a lack of appreciation of
the difficulties of producing granite. The absence of ev-
idence for such a crust is discussed earlier. A primitive"sialic" crust is refuted, among other evidence, by the
absence of an old zircon population in younger sedi-
ments [Stephenson and Patchett, 1990].
The conditions for the production of massive granitic
crusts are probably unique to the Earth and require three
or more stages of derivation from a primitive mantle
composition. The Earth has transformed less than 0.5%
of its volume to continental crust of intermediate com-
position and less than 0.2% of its volume into granitic
continental crust (i.e., upper continental crust) in over 4
billion years, so that the process is inefficient. The high-
land feldspathic crust of the Moon, about 12% of lunar
volume, formed in contrast during crystallization from a
magma ocean within a few million years.Regardless of how the terrestrial mantle crystal-
lized, there is extensive evidence for some probably
localized early mantle differentiation. Rocks derivedfrom the upper mantle from 3900 m.y. ago onward
come from a source with high Sm/Nd ratios. Although
the evidence refers specifically to Sm and Nd, by
inference the other LREE and other incompatible el-ements (e.g., K, Rb, Cs, Ba, Th, U, Zr, and Hf) were
also depleted in parts of the Archean mantle at a very
early stage [Galer and Goldstein, 1991]. This depletion
must have occurred well before the time of formation
of the oldest surface rocks. Although many Archean
rocks are derived from such a LREE-depleted (Sm/
Nd-enriched) reservoir, this may merely indicate that
during the Archean the mantle depletion in LREE wasrestricted to a small mantle reservoir (that presumablygrew in approximate proportion to crustal growth) and
not a planet-wide affair [McCulloch and Bennett, 1993,
1994]. After all, caution is warranted, since Archean
rocks now cover only a trivial percentage of the sur-face of the Earth and so provide only limited informa-
tion on the state of the bulk mantle.
Apparently, portions of the upper mantle under-
went partial melting, which led to an increase in the
Sm/Nd ratio in the mantle as the LREE were extracted
into the melt. Extraction of an early crust is an obviousway to carry this out, but early granitic or anorthositic
crusts are unlikely candidates as noted earlier. Basalts,
being the primary melt from silicate planetary mantles,
are the most likely candidates for an early crust.
However, the basic problem is that if this primor-
dial early basaltic crust ever existed, it seems to have
vanished into the mantle without providing the finger-
print of its enriched LREE signature to rocks subse-
quently derived from that region. No Archean or later
rocks have been derived from such a primitive crust
(which would resemble an alkali basalt and be en-riched in LREE, with low Sm/Nd ratios [e.g., Galer
and Goldstein, 1991]).
Another possibility is that the earliest crust wascomposed of komatiite [Campbell and Jarvis, 1984;
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33, 2/ REVIEWS OF GEOPHYSICS Taylor and McLennan: CONTINENTAL CRUST GEOCHEMISTRY. 261
Chase and Patchett, 1988].Such high-Mg melts might be
produced under the early high-temperature conditions inthe Hadean (pre-4.0 eons [Cloud, 1988])Earth. Such a
crust would be too refractory to melt during subduction,
so it could have been recycled into the deep mantlewithout the telltale signature of LREE and incompatible
element enrichment subsequently reappearing in surfacerocks. However, komatiitic rocks are produced by highdegrees of partial melting and are unlikely to be enriched
in incompatible elements to the degree required.
The probable explanation is that only small regions
of the early Archean mantle underwent partial melting
and that the early LILE-enriched mafic crust was
buried deep in the mantle and is no longer accessible.
Any crust existing before 4.0 eons was probably
highly disrupted and perhaps destroyed by the ongoing
meteoritic bombardment which was endemic through-
out the solar system down to about 3.9 or 3.8 eons.
Major basin-forming (>300 km diameter) events
ceased on the Moon only at that time. In the interval
between about 4.4 and 3.8 eons, at least 200 such
major collisions occurred on the Earth. This evidence
may need to be reevaluated if most of the basin-
forming events on the Moon were due to a late spike or
"cataclysm" in the cratering record around 3.8-4.0
eons [Grieve and Parmentier, 1985; Ryder, 1990].
U NIQ UE NE SS O F T E RR EST R IAL C O NT IN EN TAL
C R U ST IN T H E S O L AR SY ST E M
The formation of crusts distinct from the bulk plan-etary composition is a common planetary phenome-
non. However, no crusts similar to the continental
crust of the Earth appear to have formed on the other
terrestrial planets or on the icy satellites of the giant
planets. On the inner planets these crusts are basaltic,
which are the typical partial melts derived from the
Fe-Mg silicate mantles. On the satellites of the giant
planets, crusts involving water, methane, or ammonia
ices form by melting of ice-rock mixtures. All such
crusts have been termed secondary [Taylor, 1989] be-
cause they form as a result of melting in planetary
interiors subsequent to planetary formation.The Moon presents a special case of a primary crust
[Taylor, 1982] composed essentially of anorthosite,
which floated on an anhydrous magma ocean. This
was a consequence of total melting at the time offormation of the Moon. There is no evidence of a
similar crust forming on the Earth, as was noted ear-
lier, nor any sign of a lunar granitic crust. The largest
"lunar granites" are tiny «2 g) fragments of siliceous
differentiates [Taylor, 1982].
Venus is the closest planet in size and composition
to the Earth, and accordingly might be expected to
have had a rather similar geological history. However,
both planets show major differences [e.g., Kaula,
1990; Taylor, 1993b] and the Venusian surface appears
to be mostly basalt, perhaps with a few scattered
"pancakes" possibly of more siliceous differentiates
[e.g., Basilevsky et al., 1990]. The high-standing re-
gions (e.g., Ishtar Terra, Aphrodite Terra) appear to
consist of crumpled-up basaltic crust. The planet is
apparently unable to subduct its basaltic crust, possi-
bly because of the greater depth of the basalt-eclogitetransition (70-80 km) than on Earth (40 km), a conse-
quence of the slightly smaller size of Venus. The
planet appears to have choked itself on basalt without
being able to produce more differentiated rocks.
The northern crust of Mars appears to be basaltic,
consistent with the geomorphic evidence for basaltic
plains and volcanoes. The two Viking landers, 4000
km apart in the northern hemisphere, gave similar
basaltic compositions for the material analyzed. The
fine material analyzed by the Viking landers will con-
tain a significant component from the ancient cratered
terrane that dominates the southern hemisphere,
where most of the global dust storms originate. How-
ever, no component more siliceous than basalt appears
in the Viking compositions. Accordingly, there is no
evidence for a granitic terrane [Taylor, 1992a].
There is little evidence about the crust of Mercury,
except that it has a reflectance spectrum similar to thatof the Apollo 16 highlands [Vilas et al., 1988, p. 71]
and hence is unlikely to be granitic. Enigmatic plains
units may be basaltic, but differ in albedo from the
lunar maria.
The significant feature about the Earth, in contrastto the other terrestrial planets, appears to be the pres-
ence of liquid water at the surface, coupled with platetectonics and subduction, that enables recycling of
subducted basaltic crust through the mantle. It is this
process that permits the slow production of the conti-
nental crust [e.g., Campbell and Taylor, 1983]. In
other planets the absence of subduction leads to the
persistence of barren basaltic plains such as we ob-
serve on Venus and Mars.
ACKNOWLEDGMENTS. We are grateful to Owen
Evans, Gil Hanson, Klaus Mezger, and Roberta Rudnick for
helpful discussions. Useful reviews were provided by Alan
Chave, Bill Curry, Roberta Rudnick, Steve Shirey, and Tom
Torgersen. S.M.M. was supported by the National Science
Foundation under grant EAR-8957784. He is also grateful for
the support from the Deutscher Akademischer Austausch-
dienst and the Max-Planck-Institut fur Chemie(Abteilung
Geochemie) during the final stages of preparing the manuscript.
R E F E R E N C E S
Abbott, D., L. Burgess, J. Longhi, and W. H. F. Smith, Anempirical thermal history of the Earth's upper mantle, J.
Geophys. Res., 99, 13,835-13,850, 1994.Anderson, D. L., Superplumes or supercontinents?, Geol-
ogy, 22, 39-42, 1994.
5/14/2018 The Geoghemical Evolution of the Continental Crustal - slidepdf.com
http://slidepdf.com/reader/full/the-geoghemical-evolution-of-the-continental-crustal 22/25
262 • Taylor and McLennan: CONTINENTAL CRUST GEOCHEMISTRY
Arculus, R. J., Island arc magmatism in relation to the
evolution of the crust and mantle, Tectonophysics, 75,
113-133, 1981.
Armstrong, R. L., A model for Sr and Pb isotope evolution
in a dynamic Earth, Rev. Geophys., 6, 175-199, 1968.
Armstrong, R. L., Radiogenic isotopes: The case for crustal
recycling on a near-steady-state no-continental-growth
Earth, Philos. Trans. R. Soc. London A, 301, 443-472,1981.
Armstrong, R. L., The persistent myth of crustal growth,
Aust. J. Earth Sci., 38, 613-630, 1991.
Arth, J. G., and G. N. Hanson, Geochemistry and origin of
the early Precambrian crust of northeastern Minnesota,
Geochim. Cosmochim. Acta, 39,325-362, 1975.
Banfield, J. F., and R. A. Eggleton, Apatite replacement and
rare earth mobilization, fractionation, and fixation during
weathering, Clays Clay Miner., 37, 113-127, 1989.
Basaltic Volcanism Study Project, Basaltic Volcanismon the
TerrestrialPlanets, 1286pp., Pergamon, New York, 1981.
Basilevsky, A. T., J. W. Head, G. H. Pettengill, and R. S.
Saunders (Eds.), Geology and tectonics of Venus, Earth,
Moon Planets, 50/51, 3-589, 1990.
Ben Othman, D., W. M. White, and J. Patchett, Thegeochemistry ofmarine sediments, island arc magmagen-
esis, and crust-mantle recycling, Earth Planet. Sci. Lett.,
94, 1-21, 1989.
Benz, W., and A. G. W. Cameron, Terrestrial effects of the
giant impact, in Origin of the Earth, edited by H. E.
Newsom and J. H. Jones, pp. 61-67, Oxford University
Press, New York, 1990.
Benz, W., A. G. W. Cameron, and H. J. Melosh, The origin
of the Moon and the single impact hypothesis, Icarus, 81,
113-131,1989.
Bickle, M. J., Implications of melting for stabilisation of the
lithosphere and heat loss in the Archean, Earth Planet.
Sci. Lett., 80, 314-324, 1986.
Bickle, M. J., E. G. Nisbet, and A. Martin, Archean green-
stone belts are not oceanic crust, J. Geol., 102, 121-138,1994.
Boak, J. L., R. F. Dymek, and L. P. Gromet, Early crustal
evolution: Constraints from variable REE patterns in
metasedimentary rocks from the 3800 Ma Isua su-
pracrustal belt, West Greenland (abstract), Lunar Planet.
Sci., 13, 51-52, 1982.
Bock, B., S. M. McLennan, and G. N. Hanson, REE redis-
tribution and its effects on the Nd isotope system in the
Austin Glen Member of the Normanskill Formation, New
York, Geochim. Cosmochim. Acta, 23,5245-5253, 1994.
Bohlen, S. R., Pressure-temperature-time paths and a tec-
tonic model for the evolution of granulites, J. Geol., 85,
617-632, 1987.
Bohlen, S. R., and K. Mezger, Origin of granulite terrains
and the formation of the lowermost continental crust,Science, 244, 326-329, 1989.
Boryta, M. D., and K. C. Condie, Geochemistry and origin
of the Archean Beit Bridge Complex, Limpopo Belt,
South Africa, J. Geol. Soc. London, 147, 229-239, 1990.
Bowring, S. A., T. B. Housh, and C. E. Isachsen, The Acasta
Gneisses: Remnants of the Earth's earliest crust, in Origin
of the Earth, edited by H. E. Newsom and J. H. Jones, pp.
319-343, Oxford University Press, New York, 1990.
Boyd, F. R., and J. J. Gurney, Diamonds and the African
lithosphere, Science, 232, 472-477, 1986.
Brimhall, G. H. 0., O. A. Chadwick, C. J. Lewis, W.
Compston, I. S. Williams, K. J. Danti, W. E. Dietrich,
M. E. Power, D. Hendricks, and J. Bratt, Deformational
mass transport and invasive processes in soil evolution,
Science, 255,695-702, 1990.
Caggianelli,A., S. Fiore, G. Mongelli, and A. Salvemini,REE
33,2/ REVIEWS OF GEOPHYSICS
distribution in the clay fraction of pelites from the southern
Apennines, Italy, Chem. Geol., 99, 253-263, 1992.
Campbell, I.H., and R. W. Griffiths, The changing nature of
mantle hotspots through time: Implications for the chem-
ical evolution of the mantle, J. Geol., 100,497-523,1992.
Campbell, I.H., and G. T. Jarvis, Mantle convection and early
crustal evolution, PrecambrianRes., 26, 15-56, 1984.
Campbell, I. H., and S. R. Taylor, No water, no gran-
ites-No oceans, no continents, Geophys. Res. Lett., 10,
1061-1064, 1983.
Chase, C. G., and P. J. Patchett, Stored mafic/ultramafic
crust and early Archean mantle depletion, Earth Planet.
Sci. Lett., 91, 66-72, 1988.
Cloud, P., Oasis in Space, 508pp., Norton, New York, 1988.
Cogley, J. G., Continental margins and the extent and num-
ber of continents, Rev. Geophys., 22, 101-122, 1984.
Condie, K. C. Chemical composition and evolution of the
upper continental crust: Contrasting results from surface
samples and shales, Chem. Geol., 104, 1-37, 1992.
Crawford, A. J., T. J. Falloon, and D. H. Green, Classifi-
cation, petrogenesis and tectonic setting of boninites, in
Boninites and Related Rocks, edited by A. J. Crawford,
Unwin, London, 1989.Cullers, R. L., S. Chaudhuri, N. Kilbane, andR. Koch, Rare
earths in size fractions and sedimentary rocks of Penn-
sylvanian-Permian age from the mid-continent of the
U.S.A., Geochim. Cosmochim. Acta, 43,1285-1302,1979.
Davies, G. F., On the emergence of plate tectonics, Geol-
ogy, 20, 963-966, 1992.
Defant, M. J., and M. S. Drummond, Derivation of some
modern arc magmas by melting of young subducted litho-
sphere, Nature, 347, 662-665, 1990.
Defant, M. J., and M. S. Drummond, Mount St. Helens:
Potential example of the partial melting of subducted
lithosphere in a volcanic arc, Geology, 21,547-550, 1993.
DePaolo, D. J., Neodymium isotopes in the Colorado Front
Ranges and crust-mantle evolution in the Proterozoic,
Nature, 291, 193-196, 1981.DePaolo, D. J., A. M. Linn, and G. Schubert, The continen-
tal crustal age distribution: Methods of determining man-
tle separation ages from Sm-Nd isotopic data and appli-
cation to the southwestern United States, J. Geophys.
Res., 96, 2071-2088, 1991.
Drummond, M. S., and M. J. Defant, A model for trondh-
jemite-tonalite-dacite genesis and crustal growth via slab
melting: Archean to modern comparisons, J. Geophys.
Res., 95, 21,503-21,521, 1990.
Duncan, C. C., and D. L. Turcotte, On the breakup and
coalescence of continents, Geology, 22, 103-106, 1994.
Ellam, R. M., and C. J. Hawkesworth, Is average continen-
tal crust generted at subduction zones?, Geology, 16,
314-317,1988.
Ellis, D. J., Origin and evolution of graulites in normal andthickened crust, Geology, 15, 167-170, 1987.
Esser, B. K., and K. K. Turekian, The osmium isotopic
composition of the continental crust, Geochim. Cosmo-
chim. Acta, 57, 3093-3104, 1993.
Evans, O. C., and G. N. Hanson, Post-kinematic Archean
tonalites, trondhjemites, and granodiorites of the S.W.
Superior Province: Derivation through direct mantle
melting, Tectonic Evolution of Greenstone Belts, edited
by L. Ashwal and M. deWit, Oxford University Press,
New York, in press, 1995.
Galer, S. J. G., Interrelationships between continental free-
board, tectonics and mantle temperature, Earth Planet.
Sci. Lett., 105, 214-228, 1991.
Galer, S. J. G., and S. L. Goldstein, Early mantle differen-
tiation and its thermal consequences, Geochim. Cosmo-chim. Acta, 55,227-239, 1991.
5/14/2018 The Geoghemical Evolution of the Continental Crustal - slidepdf.com
http://slidepdf.com/reader/full/the-geoghemical-evolution-of-the-continental-crustal 23/25
33,2/ REVIEWS OF GEOPHYSICS Taylor and McLennan: CONTINENTAL CRUST GEOCHEMISTRY. 263
Gibbs, A. K., C. W. Montgomery, P. A. O'Day, and E. A.
Erslev, The Archean-Proterozoic transition: Evidence
from the geochemistry of metasedimentary rocks of Guy-
ana andMontana, Geochim. Cosmochim. Acta, 50, 2125-2141, 1986.
Gill, J. B., R. N. Hiscott, and P. Vidal, Turbidite geochem-
istry and evolution of the Izu-Bonin Arc and continents,
Lithos, in press, 1995.
Goldschmidt, V. M., Geochemische verteilungsgesetze der
Elemente IX, Skr. Nor. Vidensk. Akad. Kl. 1 Mat.
Naturvidensk, Kl., no. 4, 1-148, 1938.
Goldstein, S. J., and S. B. Jacobsen, Rare earth elements in
river waters, Earth Planet. Sci. Lett., 89,35-47, 1988.
Grieve, R. A. F., and E. M. Parmentier, Considerations of
large scale impact and the early Earth, LPI Tech. Rep.
85-01, pp. 23-24, Lunar and Planet. Inst., Houston, Tex.,1985.
Gurnis, M., Large scale mantle convection and the aggrega-
tion and dispersal of supercontinents, Nature, 332, 695-
699, 1988.
Hawkesworth, C. J., J. M. Hergt, R. M. Ellam, and F.
McDermott, Element fluxes associated with subduction
related magmatism, Phi/os. Trans. R. Soc. London A,335, 393-405, 1991.
Hawkesworth, C. J., x. Galagher, J. M. Hergt, and F.
McDermott, Mantle and slab contributions in arc mag-
mas, Annu. Rev. Earth Planet. Sci., 21, 175-204, 1993.
Hill, R. I., Mantle plumes and continental tectonics, Lithos,
30, 193-206, 1993.Hill, R. I., I. H. Campbell, G. F. Davies, and R. W. Griffiths,
Mantle plumes and continental tectonics, Science, 256,
186-193, 1992.
Hoffman, P. F., United plates of America, The birth of a
craton,Annu. Rev. Earth Planet. Sci., 16, 543-603,1988.
Hoffman, P. F., Supercontinents, in Encyclopedia of Earth
System Science, vol. 4, pp. 323-328, Academic, San
Diego, Calif., 1992.
Jacobsen, S. B., and R. F. Dymek, Nd and Sr isotopesystematics of clastic sediments from Isua, West Green-
land: Identification of pre-3.8 Ga differentiated crustal
components,l. Geophys. Res., 93,338-354, 1988.
Jochum, K. P., A. W. Hofmann, and H. M. Seufert, Tin in
mantle-derived rocks: Constraints on Earth evolution,
Geochim. Cosmochim. Acta, 57, 3585-3595, 1993.
jordan, T. H., Continents as a chemical boundary layer,
Phi/os. Trans. R. Soc. London A, 301, 359-373, 1981.Jordan, T. H., Structure and formation of the continental
tectcisphere, 1. Petrol., 11-37, 1988.
Kasting, J. F., and N. G. Holm, What determines the vol-
ume of the oceans, Earth Planet. Sci. Lett., 109, 507-515,1992.
Kaula, W. M., Venus: A contrast in evolution to the Earth,
Science, 247, 1191-1196, 1990.
Kay, S. M., and R. W. Kay, Creation and destruction of
lower ccintinental crust, Geol. Rundsch., 80, 259-278,1991.
Kempton, P. D., and R. S. Harmon, Oxygen isotope evi-
dence for large-scale hybridization of the lower crust
during magmatic underplating, Geochim. Cosmochim.
Acta, 56, 971-986, 1992.
Lin, P.-N., Trace element and isotopic characteristics of
western Pacific pelagic sediments: Implications for the
petrogenesis of Mariana Arc magmas, Geochim. Cosmo-
chim. Acta, 56, 1641-1654, 1992.
Liu, C.-Q., A. Masuda, A. Okada, S. Yabuki, J. Zhang, and
Z-L. Fan, A geochemical study of loess and desert sand
in northern China: Implications for continental crustweathering and composition, Chern. Geol., 106, 359-374,1993.
Martin, H., Effect of steeper Archean thermal gradient on
geochemistry of subduction-zone magmas, Geology, 14,753-756, 1986.
Martin, H., The mechanisms of petrogenesis of the Ar-
chaean continental crust-Comparison with modern pro-
cesses, Lithos, 30, 373-388, 1993.
McCulloch, M. T., Sm-Nd isotopic constraints on the evo-
lution of Precambrian crust in the Australian continent, in
Proterozoic Lithospheric Evolution, Geodyn. Ser., vol.
17, edited by A. Kroner, 115-130, AGU, Washington,
D. C., 1987.
McCulloch, M. T., and V. C. Bennett, Evolution ofthe early
Earth: Constraints from 143Nd_142Ndsotopic systemat-
ics, Lithos, 30,237-255,1993.McCulloch, M. T., and V. C. Bennett, Progressive growth of
the Earth's continental crust and depleted mantle:
Geochemical constraints, Geochim. Cosmochim. Acta,
58, 4717-4738, 1994.
McCulloch, M. T., and J. A. Gamble, Geochemical and
geodynamical constraints on subduction zone magma-
tism, Earth Planet. Sci. Lett., 102, 358-374, 1991.
McCulloch, M. T., and G. J. Wasserburg, Sm-Nd and Rb-Sr
chronology of continental crust formation, Science, 200,1003-1011, 1978.
McDaniel, D. K., S. R. Hemming, S. M. McLennan, and
G. N. Hanson, Resetting of Nd isotopes and redistribu-
tion of REE during sedimentary processes: The Early
Proterozoic Chelmsford Formation, Sudbury Basin, On-
tario, Geochim. Cosmochim. Acta, 58,931-941, 1994.
McLennan, S. M., Timing and relationships among Precam-
brian crustal and atmospheric evolution and banded iron-
formations, in Biogeochemistry of Ancient and Modern
Environments, edited by P. A. Trudinger, M. R. Walter,
and B. J. Ralph, pp. 173-182, Springer-Verlag, New
York, 1980.
McLennan, S. M., Petrological characteristics of Archean
graywackes,l. Sediment. Petrol., 54, 889-898, 1984.
McLennan, S. M., Recycling of the continental crust, PureAppl. Geophys., 128, 683-724,1988.
McLennan, S. M., Rare earth elements in sedimentary
rocks: Influence of provenance and sedimentary pro-
cesses, Rev. Mineral., 21, 169-200, 1989.
McLennan, S. M., Continental crust, in Encyclopedia of
Earth System Science, vol. 1, pp. 581-592, Academic,
San Diego, Calif., 1992.
McLennan, S. M., and S. R. Hemming, Samarium/neodym-
nium elemental and isotopic systematics in sedimentary
rocks, Geochim. Cosmochim. Acta, 56, 887-898, 1992.
McLennan, S. M., and S. R. Taylor, Geochemical con-
straints on the growth of continental crust, 1. Geol., 90,347-361, 1982.
McLennan, S. M., and S. R. Taylor, Archaean sedimentary
rocks and their relation to the composition of the Ar-
chaean continental crust, inArchaean Geochemistry, ed-
ited by A. Kroner et al., pp. 47-72, Springer-Verlag, New
York, 1984.
McLennan, S. M., and S. R. Taylor, Sedimentary rocks and
crustal evolution: Tectonic setting and secular trends, 1.Geol., 99, 1-21, 1991.
McLennan, S. M., B. J. Fryer, and G. M. Young, Rare earth
elements in Huronian (Lower Proterozoic) sedimentary
rocks: Composition and evolution of the post-Kenoran
upper crust, Geochim. Cosmochim. Acta, 43, 375-388,
1979.
McLennan, S. M., S. R. Taylor, and A. Kroner, Geochem-
ical evolution of Archean shales from South Africa, I,
The Swaziland and Pongola Supergroups, PrecambrianRes., 22,93-124, 1983.
McLennan, S. M., S. R. Taylor, and V. R. McGregor,
5/14/2018 The Geoghemical Evolution of the Continental Crustal - slidepdf.com
http://slidepdf.com/reader/full/the-geoghemical-evolution-of-the-continental-crustal 24/25
264 • Taylor and McLennan: CONTINENTAL CRUST GEOCHEMISTRY
Geochemistry of Archean metasedimentary rocks fromWest Greenland, Geochim. Cosmochim. Acta, 48, 1-21,1984.
McLennan, S. M., S. R. Taylor, M. T. McCulloch, and J. B.Maynard, Geochemical and Nd-Sr isotopic compositionof deep-sea turbidites: Crustal evolution and plate tec-tonic associations, Geochim. Cosmochim. Acta, 54,
2015-2050, 1990.
McLennan, S. M., S. R. Hemming, D. K. McDaniel, andG. N. Hanson, Geochemical approaches to sedimenta-tion, provenance and tectonics, Spec. Pap. Geol. Soc.
Am., 284, 21-40, 1993.McLennan, S. M., S. R. Hemming, S. R. Taylor, and K. A.
Eriksson, Early Proterozoic crustal evolution: Geochem-ical and Nd-Pb isotopic evidence from metasedimentaryrocks, southwestern North America, Geochim. Cosmo-
chim. Acta, in press, 1995.Mezger, K., Temporal evolution of regional granulite ter-
rains: Implications for the formation of lowermost conti-nental crust, in Continental Lower Crust, edited by D. M.Fountain, R. Arculus, and R. M. Kay, pp. 447-474,Elsevier, New York, 1992.
Milodowski, A. E., and J. A. Zalasiewicz, Redistribution ofrare earth elements during diagenesis of turbidite/hemi-pelagite mudrock sequences of Llandovery age from cen-tral Wales, in Developments in sedimentary provenancestudies, edited by A. C. Morton et aI., Geol. Soc. Spec.Pap. London, 57, lOl-124, 1991.
Moorbath, S;, Ages, isotopes and evolution of the Precam-brian continental crust, Chem. Geol., 20, 151-187, 1977.
Moorbath, S., Age and isotope evidence for the evolution ofthe continental crust, Philos. Trans. R. Soc. London A.,
288, 401-413, 1978.Morgan, P., The thermal structure and thermal evolution of
the continental lithosphere, Phys. Chem. Earth, 15, 107-193, 1984.
Mueller, P. A., J. L. Wooden, and D. R. Bowes, Precam-
brian evolution of the Beartooth Mountains, Montana-Wyoming, USA, Rev. Bras. Geocience., 12, 215-222,1982.
Nance, R. D., T. R. Worsley, and J. B. Moody, Post Ar-chean biogeochemical cycles and long-term episodicity intectonic processes, Geology, 14, 514-518, 1986.
Naqvi, S. M., K. C. Condie, and P. Allen, Geochemistry ofsome unusual Early Archean sediments from DharwarCraton, India, Precambrian Res., 22,215-144, 1983.
Nelson, B. K., and D. J. DePaolo, Rapid production ofcontinental crust 1.7 to 1.9 b.y. ago: Nd isotopic evidencefrom the basement of the North American mid-continent,Bull. Geol. Soc. Am., 96, 746-754,1985.
Nesbitt, H. W., Mobility and fractionation of rare earthelements during weathering of a granodiorite, Nature,
279, 206-210, 1979.
Newsom, H. E., and J. H. Jones (Eds.), Origin of theEarth,pp. 59-227, Oxford University Press, New York, 1990.
Newsom, H. E., and S. R. Taylor, Geochemical implicationsof the formation of the Moon by a single giant impact,Nature, 338, 29-34, 1989.
Nyblade, A. A., and H. N. Pollack, A global analysis of heatflow from Precambrian terrains: Implications for the ther-mal structure of Archean and Proterozoic lithosphere, J.Geophys. Res., 98, 12,207-12,218, 1993.
Ohr, M., A. N. Halliday, and D. R. Peacor, Sr and Ndisotopic evidence for punctuated clay diagenesis, TexasGulf Coast, Earth Planet. Sci. Lett., 105, 110-126, 1991.
Patchett, P. J., and N. T. Arndt, Nd isotopes and tectonicsof 1.9-1. 7 Ga crustal genesis, Earth Planet. Sci. Lett., 78,329-338, 1986.
Pearcy, L. G., S. M. DeBari, and N. H. Sleep, Mass balancecalculations for two sections of island arc crust and im-
33,2/ REVIEWS OF GEOPHYSICS
plications for the formation of continents, Earth Planet.
Sci. Lett., 96,427-442, 1990.Pearson, D. G., R. W. Carlson, S. B. Shirey, F. R. Boyd,
and P. H. Nixon, The stabilisation of Archean litho-spheric mantle: A Re-Os isotope study of peridotite xeno-liths from the Kaapvaal and Siberian cratons, Earth
Planet. Sci. Lett., in press, 1995.Pollack, H. N., The heat flow from the Earth: A review, in
Mechanisms of Continental Drift, edited by P. A. Davisand S. K. Runcorn, pp. 183-192, Academic, San Diego,
Calif., 1980.Pollack, H. N., Cratonization and thermal evolution of the
mantle, Earth Planet. Sci. Lett., 80, 175-182, 1986.Richardson, S. H., J. J. Gurney, A. J. Erlank, and J. W.
Harris, Origin of diamonds in old enriched mantle, Na-
ture, 310, 198-202, 1984.Ronov, A. B., N. V. Bredanova, and A. A. Migdisov,
General trends in the evolution of the chemical composi-tion of sedimentary and magmatic rocks of the continen-tal Earth crust, Sov. Sci. Rev., Sect. G., Geology, 1,
1-37, 1992.Rudnick, R. L., Coritinental crust: Growing from below,
Nature, 347, 711-712, 1990.Rudnick, R. L., Restites, Eu anomalies, and the lower con-tinental crust, Geochim. Cosmochim. Acta, 56, 963-970,1992a.
Rudnick, R. L., Xenoliths-Samples of the lower continen-tal crust, in Continental Lower Crust, edited by D. M.Fountain, R. Arculus, and R. M. Kay, pp. 269-316,Elsevier, New York, 1992b.
Rudnick, R. L., and T. Presper, Geochemistry of interme-diate to high-pressure granulites, in Granulites and
Crustal Evolution, edited by D. Vielzeuf and P. Vidal, pp.523-550, Kluwer Academic, Hingham, Mass., 1990.
Rudnick, R. L., and S. R. Taylor, The composition andpetrogenesis of the lower crust: A xenolith study, J.Geophys. Res., 92,13,981-14,005,1987.
Ryder, G., Lunar samples, lunar accretion, and the earlybombardment of the Moon, Eos Trans. AGU, 71(10), 313,322-323, 1990.
Sawyer, E. W., The influence of source rock type, chemicalweathering, and sorting on the geochemistry of clasticsediments from the Quetico Metasedimentary Belt, Su-perior Province, Canada, Chem. Geol., 55, 77-95, 1986.
Schidlowski, M., Early organic evolution and mineral andenergy resources, in Early Organic Evolution: Implica-
tions for Mineral and Energy Resources, edited by M.Schidlowski, pp. 1-6, Springer-Verlag, New York, 1992.
Schubert, G., and A. P. S. Reymer, Continental volume andfreeboard through geological time, Nature, 316,336-339,1985.
Shaw, D. M., J. Dostal, and R. R. Keays, Additional esti-
mates of continental surface Precambrian shield compo-sition in Canada, Geochim. Cosmochim. Acta, 40, 73-83,1976.
Shirey, S. B., and G. N. Hanson, Mantle-derived Archaeanmonzodiorites and trachyandesites, Nature, 310, 222-224, 1984.
Sholkovitz, E. R., Rare earth elements in the sediments ofthe North Atlantic Ocean, Amazon delta, and East ChinaSea: Reinterpretation of terrigenous input to the oceans,Am. J. Sci., 288,236-281, 1988.
Sholkovitz, E. R., Rare-earth elements in marine sedimentsand geochemical standards, Chem. Geol., 88, 333-347,1990.
Stephenson, R. K., and P. J. Patchett, Implications for theevolution of the continental crust from Hf isotope sys-
tematics of Archean detrital zircons, Geochim. Cosmo-
chim. Acta, 54,1683-1697, 1990.
5/14/2018 The Geoghemical Evolution of the Continental Crustal - slidepdf.com
http://slidepdf.com/reader/full/the-geoghemical-evolution-of-the-continental-crustal 25/25
33,2/ REVIEWSOF GEOPHYSICS Taylor and McLennan: CONTINENTAL CRUST GEOCHEMISTRY. 265
Stem, R. A., and G. N. Hanson, Archean high-Mggranodio-
rite: A derivative of light rare earth element-enriched
monzodiorite ofmantle origin,J.Petrol., 32, 201-238, 1991.
Stem, R. A., G. N. Hanson, and S. B. Shirey, Peterogenesis of
mantle-derived, LlLE-enriched Archean monzodiorites
and trachyandesites (sanukitoids) in southwestern Superior
Province, Can. J. Earth Sci., 26, 1688-1712, 1989.
Storey, B. C., T. Alabaste, and P. J. Pankhurst, Magmatism
and the causes of continental breakup, Geol. Soc. Spec.
Publ. London, 68, 404 pp., 1992.
Strom, S. E., S. Edwards, and K. M. Strom, Constraints on
the properties and environment of primitive stellar nebu-
lae from the astrophysical record provided by young
stellar objects, in The Formation and Evolution of Plan-
etary Systems, edited by H. A. Weaver and L.Danly, pp.
91-109, Cambridge University Press, New York, 1989.
Sverjensky, D., Europium redox equilibrium in aqueous
solutions, Earth Planet. Sci. Lett., 67, 70-78, 1984.
Taylor, S. R., The origin and growth of continents, Tectono-
physics, 4,17-34, 1967.
Taylor, S. R., Island arc models and the composition of the
continental crust, in Island Arcs, Deep Sea Trenches, and
Back-Arc Basins, Maurice Ewing Ser., vol. 1, edited by
M. Talwani and W. C. Pitman III, pp. 325-335, AGU,
Washington, D. C., 1977.
Taylor, S. R., Planetary Science: A Lunar Perspective, 481
pp., Lunar and Planetary Institute, Houston, Tex., 1982.
Taylor, S. R., Growth of planetary crusts, Tectonophysics,
161, 147-156, 1989.
Taylor, S. R., Solar System Evolution: A New Perspective,
307 pp., Cambridge University Press, New York, 1992a.
Taylor, S. R., Origin of the Earth, in Understanding The
Earth, edited by G. Brown, C. Hawkesworth, and C.
Wilson, pp. 25-43, Cambridge University Press, New
York, 1992b.
Taylor, S. R., Early accretional history of the Earth and the
Moon-forming event, Lithos, 30, 207-221, 1993a.
Taylor, S. R., Is Venus a twin planet to Earth? New datafrom Magellan, Mitt. Oesterr. Mineral. Gesell., 138, 33-
43, 1993b.
Taylor, S. R., and S. M. McLennan, The composition and
evolution of the continental crust: Rare earth element
evidence from sedimentary rocks, Phi/os. Trans. R. Soc.
London A, 301, 381-399, 1981.
Taylor, S. R., and S. M. McLennan, The Continental Crust:
Its Composition and Evolution, Blackwell Scientific, 312
pp., Boston, Mass., 1985.
Taylor, S. R., and S. M. McLennan, The significance of the
rare earths in geochemistry and cosmochemistry, in
Handbook on the Physics and Chemistry of Rare Earths,
edited by K. A. Gschneider and L. Eyring, pp. 485-578,
Elsevier, New York, 1988.
Taylor, S. R., S. M. McLennan, and M. T. McCulloch,Geochemistry of loess, continental crustal composition
and crustal model ages, Geochim. Cosmochim. Acta, 47,
1897-1905, 1983.
Taylor, S. R., R. L. Rudnick, S. M. McLennan, and K. A.
Eriksson, Rare earth element patterns in Archean high-
grade metasediments and their tectonic significance,
Geochim. Cosmochim. Acta, 50, 2267-2279, 1986.
Tonks, W. B., and H. J. Melosh, The physics ofcrystal settling
and suspension in a turbulent magma ocean, inOriginof the
Earth, edited by H. E. Newsom and J. H. Jones, pp.
151-174,Oxford University Press, New York, 1990.
Turcotte, D. L., Geophysical processes influencing the
lower continental crust, in Properties and Processes of
Earth's Lower Crust, Geophys. Monogr. Ser., vol. 51,
edited by R. F. Mereu, S. Mueller, and D. M. Fountain,pp. 321-329, AGU, Washington, D. C., 1989.
Unrug, R., The supercontinent cycle and Gondwanaland
assembly: Component cratons and the timing of suturing
events, J. Geodyn., 16, 215-240, 1992.
Veizer, J., Trace elements and isotopes in sedimentary car-
bonates, Rev. Mineral., 11, 265-299, 1983.
Veizer, J., and S. L. Jansen, Basement and sedimentary
recycling, 2, Time dimension to global tectonics, J.Geol.,
93,625-643, 1985.
Vilas, F., C. R. Chapman, and M. S. Matthews, Mercury,
794 pp., University of Arizona Press, Tucson, 1988.
Voshage, H., A. W. Hoffman, M. Mazzucchelli, G. Rival-
enti, S. Sinigoi, I. Raczek, and G. Demarchi, Isotopic
evidence from the Ivrea Zone for a hybrid lower crust
formed by magmatic underplating, Nature, 347, 731-736,
1990.
Walker, R. J., R. W. Carlson, S. B. Shirey, and F. R. Boyd,
Os, Sr, Nd and Pb isotopic systematics of southern Afri-
can peridotite xenoliths: Implications for the chemical
evolution of subcontinental mantle, Geochim. Cosmo-
chim. Acta, 53, 1583-1595, 1989.
Watson, J., Verticalmovements inProterozoic structural prov-
inces, Philos. Trans. R. Soc. London A, 280, 629-640,
1976.Wedepohl, K.H., Chemical composition and fractionation of
the continental crust, Geol. Rundsch., 80, 207-223, 1991.
Wedlandt, E., D. J. DePaolo, and W. S. Baldridge, Nd and
Sr isotope chronology of Colorado Plateau lithosphere:
Implications for magmatic and tectonic underplating of
the continental crust, Earth Planet. Sci. Lett., 116, 23-
43, 1993.
Wetherill, G. W., Accumulation of the terrestrial planets and
implications concerning lunar origin, in Origin of the
Moon, edited by W. K. Hartmann et aI., pp. 519-550,
Lunar and Planetary Institute, Houston, Tex., 1986.
White, R. S., Magmatism during and after Continental
breakup, in Magmatism and the Causes of Continental
Breakup, edited by B. C. Storey, T. Alabaster, and R. J.
Pankhurst, Geol. Soc. Spec. Publ. London, 68, 1-16,1992.
White, W. M., Geochemical evidence for crust-to-mantle
recycling in subduction zones, in Crust/Mantle Recycling
at Convergence Zones, edited by S. R. Hart and L.
Gullen, pp. 43-58, Kluwer Academic, Hingham, Mass.,
1989.
Wise, D. U., Continental margins, freeboard and the vol-
umes of continents and oceans through geological time, in
The Geology of Continental Margins, edited by C. A.
Burk and C. L. Drake, pp. 45-58, Springer-Verlag, New
York, 1974.
Wyllie, P. J., Experimental and thermal constraints on the
deep seated parentage of some granitoid magmas in sub-
duction zones, inMigmatites and Metamorphism, edited
by M. P. Atherton and C. D. Gribble, pp. 37-51, Shiva,Cambridge, Mass., 1983.
Young, G. M., The geologic record of glaciation: Relevance
to the climatic history of Earth, Geosci. Can., 18, 100-
108, 1991.
Zhong, S., and M. Gurnis, Dynamic feedback between a
continentlike raft and thermal convection, J. Geophys.
Res., 98, 12,219-12,232, 1993.
S. M. McLennan, Department of Earth and Space Sci-
ences, State University of New York, Stony Brook, NY
11794-2100.
S. R. Taylor, Department of Nuclear Physics, Research
School of Physical Sciences, Australian National Univer-
sity, Canberra, A.C.T. 2601, Australia.