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Chapter 11: Diversification ofChapter 11: Diversification of
MagmasMagmas
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Magmatic DifferentiationMagmatic Differentiation
Any process by which a magma is able toAny process by which a magma is able todiversify and produce a magma or rock ofdiversify and produce a magma or rock of
different compositiondifferent composition
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Magmatic DifferentiationMagmatic Differentiation
Two essential processesTwo essential processes
1. Creates a compositional difference1. Creates a compositional difference in one or morein one or more
phases phases
. !reserves. !reserves the chemical difference bythe chemical difference by segregatingsegregating "or"or
fractionatingfractionating# the chemically distinct portions# the chemically distinct portions
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!artial Melting!artial Melting
$eparation of a partially melted li%uid from$eparation of a partially melted li%uid from
the solid residuethe solid residue
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&ffects of removing li%uid at various stages of melting&ffects of removing li%uid at various stages of melting &utectic systems&utectic systems
'irst melt'irst melt alwaysalways ( eutectic composition( eutectic composition Ma)or Ma)or element composition of eutectic melt iselement composition of eutectic melt is
constant until one of the source mineral phases isconstant until one of the source mineral phases is
consumedconsumed "trace elements differ#"trace elements differ# *nce a phase is consumed+ the ne,t increment of melt*nce a phase is consumed+ the ne,t increment of melt
will be different - and Twill be different - and T
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$eparation of a partially melted li%uid from$eparation of a partially melted li%uid fromthe solid residue re%uires a critical melt the solid residue re%uires a critical melt
$ufficient melt must be produced for it to$ufficient melt must be produced for it to
'orm a continuous+ interconnected film'orm a continuous+ interconnected film
/ave enough interior volume that it is not/ave enough interior volume that it is not
all of it is adsorbed to the crystal surfacesall of it is adsorbed to the crystal surfaces
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The ability to form an interconnected film is dependentThe ability to form an interconnected film is dependent
upon theupon the dihedral angle "dihedral angle "θθ## a property of the melta property of the melt
Figure 11-1Figure 11-1 Illustration of the dihedralIllustration of the dihedral
angle (angle (
) of melt droplets that typically) of melt droplets that typically
form at multiple grain junctions.form at multiple grain junctions. AfterAfter
Hunter (198)Hunter (198) In I. !arsons (ed.)" #riginsIn I. !arsons (ed.)" #rigins
of Igneous $ayering. %eidel" &ordrecht"of Igneous $ayering. %eidel" &ordrecht"
pp. '-*'.pp. '-*'.
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0ravitational effects "0ravitational effects " buoyant buoyant li%uid#li%uid# 'ilter pressing'ilter pressing+ or+ or compaction+compaction+ ofof crystal mushcrystal mush
$hear $hear the 2CM! drops considerablythe 2CM! drops considerably 2CM! varies with2CM! varies with
TT
viscosityviscosity --
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Dominant mechanism by which mostDominant mechanism by which most
magmas+ once formed+ differentiate3magmas+ once formed+ differentiate3
Crystal 'ractionationCrystal 'ractionation
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0ravity settling0ravity settling
The differential motion of crystals and li%uidThe differential motion of crystals and li%uidunder the influence of gravity due to theirunder the influence of gravity due to their
differences in densitydifferences in density
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0ravity settling0ravity settling
Cool pointCool point aa →→ olivine layer at base of plutonolivine layer at base of plutonif first olivine sinksif first olivine sinks
4e,t get ol5cp, layer 4e,t get ol5cp, layer
finally get ol5cp,5plagfinally get ol5cp,5plag
Cumulate te,ture:Cumulate te,ture:
Mutually touchingMutually touching phenocrysts with phenocrysts with
interstitial crystalli6edinterstitial crystalli6ed
residual meltresidual melt
Figure -+. After ,oen
(191)" A. . /ci." and
0orse (199')" ,asalts
and !hase &iagrams.
rieger !u2lishers.
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Figure 11-+Figure 11-+ 3ariation diagram using 0g# as the a2scissa for la4as associated ith the 199 ilauea eruption in Haaii.3ariation diagram using 0g# as the a2scissa for la4as associated ith the 199 ilauea eruption in Haaii.
AAfter 0urata and %ichter" 1955 (as modified 2y ,est" 198+)fter 0urata and %ichter" 1955 (as modified 2y ,est" 198+)
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$toke7s 8aw$toke7s 8aw
99 ( the settling velocity "cmsec#( the settling velocity "cmsec#
gg ( the acceleration due to gravity ";<= cmsec( the acceleration due to gravity ";<= cmsec
##r r ( the( the radiusradius of a spherical particle "cm#of a spherical particle "cm#
ρρss ( the density of the solid spherical particle "gcm( the density of the solid spherical particle "gcm>>##
ρρll
( the density of the li%uid "gcm( the density of the li%uid "gcm>>
##
ηη ( the viscosity of the li%uid "1 ccm sec ( 1 poise#( the viscosity of the li%uid "1 ccm sec ( 1 poise#
9 gr " #;
= −ρ ρ
η s l
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*livine in basalt*livine in basalt
*livine "*livine "ρρss ( >.> gcm( >.> gcm>>++ r ( =.1 cmr ( =.1 cm##
?asaltic li%uid "?asaltic li%uid "ρρll ( .@ gcm( .@ gcm>>++ ηη ( 1=== poise#( 1=== poise#
9 ( B;<=B=.19 ( B;<=B=.1 ">.>.@#;B1=== (">.>.@#;B1=== ( =.==1> cmsec=.==1> cmsec
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2hyolitic2hyolitic meltmelt ηη ( 1=( 1=CC poise and poise and ρρll ( .> gcm( .> gcm>>
hornblendehornblende crystal "crystal "ρρss ( >. gcm( >. gcm>>++ r ( =.1 cmr ( =.1 cm##
9 ( , 1=9 ( , 1=CC cmsec+ orcmsec+ or @ cmyear @ cmyear
feldsparsfeldspars ""ρρll ( . gcm( . gcm>>##
9 ( cmyear 9 ( cmyear (( == m in the 1=== m in the 1=DD yearsyears that a stock might coolthat a stock might cool
Ef =. cm in radius "Ef =. cm in radius "1 cm diameter 1 cm diameter # settle at# settle at =.@=.@
metersyear metersyear + or @. km in 1=+ or @. km in 1=DD
year cooling of stock year cooling of stock
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$tokes7 8aw is overly simplified$tokes7 8aw is overly simplified
1. Crystals are not spherical1. Crystals are not spherical
.. *nly basaltic magmas very near their li%uidus*nly basaltic magmas very near their li%uidus
temperatures behave as 4ewtonian fluidstemperatures behave as 4ewtonian fluids
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Many silicic magmas approach the ternary eutecticMany silicic magmas approach the ternary eutectic
&ither&ither fractional crystalli6ationfractional crystalli6ation does take place or theydoes take place or they
areare minimum "eutectic# meltsminimum "eutectic# melts
Figure 11-Figure 11- !osition of the H+#-saturated!osition of the H+#-saturated
ternary eutectic in the al2ite-orthoclase-ternary eutectic in the al2ite-orthoclase-
silica system at 4arious pressures. 6hesilica system at 4arious pressures. 6he
shaded portion represents theshaded portion represents the
composition of most granites. Includedcomposition of most granites. Included
are the compositions of the 6uolumneare the compositions of the 6uolumne
Intrusi4e /eries (Figure '-+)" ith theIntrusi4e /eries (Figure '-+)" ith the
arro shoing the direction of the trendarro shoing the direction of the trend
from early to late magma 2atches.from early to late magma 2atches.
7perimental data from yllie7perimental data from yllie et al et al . (195).. (195).
From inter (+**1) An Introduction toFrom inter (+**1) An Introduction to
Igneous and 0etamorphic !etrology.Igneous and 0etamorphic !etrology.
!rentice Hall!rentice Hall
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!olybaric!olybaric 'ractional Crystalli6ation'ractional Crystalli6ation
1. $tability of phases changes "hi! garnet...#1. $tability of phases changes "hi! garnet...#
. $hift of the eutectic point with pressure will. $hift of the eutectic point with pressure will
cause the %uantity of the li%uidus phases to varycause the %uantity of the li%uidus phases to vary
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Ol
Low-P
Pyx
Hi-P/igh! "red tieline#/igh! "red tieline#
has li% F olhas li% F ol
8ow! "yellow tie8ow! "yellow tie
line# has ol F li%uidline# has ol F li%uid
&,pansion of olivine field at low pressure causes&,pansion of olivine field at low pressure causes
an increase in the %uantity of crystalli6ed olivinean increase in the %uantity of crystalli6ed olivine
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Two other mechanisms that facilitate theTwo other mechanisms that facilitate the
separation of crystals and li%uidseparation of crystals and li%uid
1. Compaction1. Compaction
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Two other mechanisms that facilitate theTwo other mechanisms that facilitate the
separation of crystals and li%uidseparation of crystals and li%uid
. 'low segregation. 'low segregation
Figures 11-' and 11-Figures 11-' and 11- &re4er and ohnston (198). %oyal&re4er and ohnston (198). %oyal
/oc. 7din2urgh 6rans." 5" '9-'99./oc. 7din2urgh 6rans." 5" '9-'99.
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9olatile Transport9olatile Transport
1. 9apor released by heating of hydrated or carbonated1. 9apor released by heating of hydrated or carbonated
wall rockswall rocks
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9olatile Transport9olatile Transport
. As a volatilebearing. As a volatilebearing
"but undersaturated#"but undersaturated#magma rises andmagma rises and
pressure is reduced+ the pressure is reduced+ the
magma may eventuallymagma may eventually
become saturated in the become saturated in thevapor+ and a free vaporvapor+ and a free vapor
phase will be released phase will be released
Figure -++. From ,urnham and &a4is (19'). A /ci."
+'" 9*+-9'*.
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>. 8atestage fractional crystalli6ation>. 8atestage fractional crystalli6ation
'ractional crystalli6ation enriches late melt in'ractional crystalli6ation enriches late melt in
incompatible+ 8E8+ and nonlithophile elementsincompatible+ 8E8+ and nonlithophile elements
Many concentrate further in the vapor Many concentrate further in the vapor
!articularly enriched with resurgent boiling "melt!articularly enriched with resurgent boiling "melt
already evolved when vapor phase released#already evolved when vapor phase released# 0et a silicatesaturated vapor 5 a vaporsaturated0et a silicatesaturated vapor 5 a vaporsaturated
late derivative silicate li%uidlate derivative silicate li%uid
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9olatile release raises li%uidus temperature9olatile release raises li%uidus temperature →→
porphyritic te,ture porphyritic te,ture
May increase ! fracture the roof rocksMay increase ! fracture the roof rocks 9apor and melt escape along fractures as dikes9apor and melt escape along fractures as dikes
$ilicate melt$ilicate melt →→ %uart6 and feldspar %uart6 and feldspar
→→ small dikes ofsmall dikes of apliteaplite
9apor phase9apor phase →→ dikes or pods ofdikes or pods of pegmatite pegmatite
C i ibl lC i ibl l
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Concentrate incompatible elementsConcentrate incompatible elements
Comple,: varied mineralogyComple,: varied mineralogy
May displayMay display concentric 6onationconcentric 6onation
Figure 11-5Figure 11-5 /ections of three :oned fluid-phase deposits (not at the same scale)./ections of three :oned fluid-phase deposits (not at the same scale). a.a. 0iarolitic pod in granite (se4eral cm0iarolitic pod in granite (se4eral cm
across).across). 2.2. Asymmetric :oned pegmatite di;e ith aplitic 2ase (se4eral tens of cm across).Asymmetric :oned pegmatite di;e ith aplitic 2ase (se4eral tens of cm across). c.c. Asymmetric :oned pegmatiteAsymmetric :oned pegmatite
ith granitoid outer portion (se4eral meters across).ith granitoid outer portion (se4eral meters across). From ahns and ,urnham (1959).From ahns and ,urnham (1959). Econ. Geol.Econ. Geol." 5'" 8'-85'." 5'" 8'-85'.
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< cm tourmaline crystals< cm tourmaline crystals
from pegmatitefrom pegmatite
mm gold from a mm gold from ahydrothermal deposithydrothermal deposit
8i id E i ibili
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8i%uid immiscibility in the 'o$i*8i%uid immiscibility in the 'o$i* systemsystem8i%uid Emmiscibility8i%uid Emmiscibility
Figure 5-1+. Iso2aric 6-< phasediagram of the system Fo-/ilica at *.1
0!a. After ,oen and Anderson (191')
and =rieg (19+). Amer. . /ci.
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The effect of addingThe effect of adding
alkalis+ alumina+ etc. isalkalis+ alumina+ etc. is
toto eliminate the solvuseliminate the solvus
completelycompletely
Figure -'. Iso2aric
diagram illustrating the
cotectic and peritectic
cur4es in the system
forsterite-anorthite-silica
at *.1 0!a. After Anderson(191) A. . /ci." and Ir4ine
(19) >I ?ear2. '.
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2enewed interest when 2oedder "1;1# discovered a2enewed interest when 2oedder "1;1# discovered a
second immiscibility gap in the ironrich 'a8c$i*second immiscibility gap in the ironrich 'a8c$i*
systemsystem
Figure 11-. 6o immisci2ilitygaps in the system fayalite-
leucite-silica (after %oedder"
199). ?oder (ed.)" 6he
74olution of the Igneous
%oc;s. !rinceton @ni4ersity
!ress. pp. 1-8. !rojected
into the simplified system are
the compositions of natural
immisci2le silicate pairdroplets from interstitial Fe-
rich tholeiitic glasses
(!hilpotts" 198+). >ontri2.
0ineral. !etrol." 8*" +*1-+18.
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$ome &,amples$ome &,amples
8ate silicarich immiscible droplets in 'erich8ate silicarich immiscible droplets in 'erichtholeiitic basalts "as in 2oedder#tholeiitic basalts "as in 2oedder#
$ulfidesilicate immiscibility "massive sulfide$ulfidesilicate immiscibility "massive sulfide
deposits#deposits# Carbonatitenephelinite systems "Chapter 1;#Carbonatitenephelinite systems "Chapter 1;#
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Tests for immiscible origin ofTests for immiscible origin of
associated rock pairsassociated rock pairs
1. The magmas must be immiscible when1. The magmas must be immiscible when
heated e,perimentally+ or they must plot onheated e,perimentally+ or they must plot on
the boundaries of a known immiscibilitythe boundaries of a known immiscibility
gap+ as in 'ig. 11gap+ as in 'ig. 11
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. Emmiscible li%uids are in e%uilibrium with. Emmiscible li%uids are in e%uilibrium with
each other+ and thus they must be ineach other+ and thus they must be in
e%uilibrium with the same mineralse%uilibrium with the same minerals
Tests for immiscible origin ofTests for immiscible origin of
associated rock pairsassociated rock pairs
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Compositional Convection andCompositional Convection and In In
SituSitu Differentiation !rocessesDifferentiation !rocesses In-situ: crystals don7t sinkmove
Typically involves
Diffusion
Convective separation of li%uid and crystals
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The $oret &ffect andThe $oret &ffect and
Thermogravitational DiffusionThermogravitational Diffusion
Thermal diffusionThermal diffusion+ or the+ or the $oret effect$oret effect
/eavy/eavy elementsmolecules migrate toward theelementsmolecules migrate toward the
colder colder end andend and lighter lighter ones to theones to the hotter hotter end ofend ofthe gradientthe gradient
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Galker and De8ong "1;<# sub)ected two basalts toGalker and De8ong "1;<# sub)ected two basalts to
thermal gradients of nearly =thermal gradients of nearly =ooCmm "H#Cmm "H#
'ound that:'ound that: $amples reached a steady$amples reached a steady
state in a few daysstate in a few days
/eavier elements/eavier elements →→ coolercoolerend and the lighterend and the lighter →→ hothot
endend
The chemical concentrationThe chemical concentrationis similar to that e,pectedis similar to that e,pected
from fractionalfrom fractional
crystalli6ationcrystalli6ation Figure -'. After al;er"
&. >. and /. 7. &e$ong
(198+). >ontri2. 0ineral.!etrol." 9" +1-+'*.
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Thermogravitational diffusionThermogravitational diffusion
$table and persistent stagnant boundary layers$table and persistent stagnant boundary layers
have been shown to occur near the top andhave been shown to occur near the top and
sides of magma chamberssides of magma chambers
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/ildreth "1;;# =. Ma/ildreth "1;;# =. Ma ?ishop Tuff ?ishop Tuff at 8ongat 8ong
9alley+ California9alley+ California
9ertical9ertical compositionalcompositional variation in the stratifiedvariation in the stratifiedtuff tuff
Thermal gradient in chamber Thermal gradient in chamber
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ModelModel
Figure 11-11. /chematic section through a rhyolitic magma cham2er undergoing con4ection-aided in-situ differentiation. After Hildreth (199). =eol. /oc. Amer. /pecial !aper" 18*" '-.
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8angmuir Model8angmuir Model Thermal gradient atThermal gradient at
wall and capwall and cap →→ variation in variation in
crystalli6edcrystalli6ed
CompositionalCompositional
convectionconvection →→
evolved magmasevolved magmas
from boundaryfrom boundary
layer to cap "or mi,layer to cap "or mi,
into interior#into interior#
Figure 11-1+Figure 11-1+ Formation of 2oundary layersFormation of 2oundary layers
along the alls and top of a magmaalong the alls and top of a magma
cham2er.cham2er. From inter (+**1) AnFrom inter (+**1) An
Introduction to Igneous and 0etamorphicIntroduction to Igneous and 0etamorphic
!etrology. !rentice Hall!etrology. !rentice Hall
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Magma Mi,ingMagma Mi,ing
&nd member mi,ing&nd member mi,ing for a suite of rocksfor a suite of rocks 9ariation on /arkertype diagrams9ariation on /arkertype diagrams
should lie on a straight lineshould lie on a straight line between the between the
two most e,treme compositionstwo most e,treme compositions
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Figure 11-+Figure 11-+ 3ariation diagram using 0g# as the a2scissa for la4as associated ith the 199 ilauea eruption in Haaii.3ariation diagram using 0g# as the a2scissa for la4as associated ith the 199 ilauea eruption in Haaii.AAfter 0urata and %ichter" 1955 (as modified 2y ,est" 198+)fter 0urata and %ichter" 1955 (as modified 2y ,est" 198+)
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?asalt pillows?asalt pillows
accumulating at the bottomaccumulating at the bottom
of a in granitic magmaof a in granitic magma
chamber+ 9inalhavenchamber+ 9inalhaven
Esland+ MaineEsland+ Maine
Comingled basalt2hyoliteComingled basalt2hyolite
Mt. Mc8oughlin+ *regonMt. Mc8oughlin+ *regon
Figure 11-8Figure 11-8 From inter (+**1) AnFrom inter (+**1) An
Introduction to Igneous andIntroduction to Igneous and
0etamorphic !etrology. !rentice Hall0etamorphic !etrology. !rentice Hall
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AssimilationAssimilation
Encorporation of wall rocks "diffusion+Encorporation of wall rocks "diffusion+,enoliths#,enoliths#
Assimilation by melting is limited byAssimilation by melting is limited by
the heat available in the magmathe heat available in the magma
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Ione meltingIone melting
Crystalli6ing igneous material at theCrystalli6ing igneous material at the base e%uivalent to the amount melted base e%uivalent to the amount melted
at the topat the top
Transfer heat by convectionTransfer heat by convection
D i d i i il iD t ti d i i il ti
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Detecting and assessing assimilationDetecting and assessing assimilation
EsotopesEsotopes are generally the bestare generally the best
Continental crust becomes progressively enrichedContinental crust becomes progressively enrichedinin <C<C$r$r<@<@$r and depleted in$r and depleted in 1D>1D> 4d 4d1DD1DD 4d 4d
Figure 9-1.Figure 9-1. 7stimated %2 and /r7stimated %2 and /r
isotopic e4olution ofisotopic e4olution of
the 7arths upperthe 7arths upper
mantle" assuming amantle" assuming a
large-scale meltinglarge-scale melting
e4ent producinge4ent producing
granitic-typegranitic-type
continental roc;s atcontinental roc;s at
.* =a 2.p After.* =a 2.p Afterilson (1989).ilson (1989).
IgneousIgneous
!etrogenesis. @nin!etrogenesis. @nin
HymanBluer.HymanBluer.
D t ti d i i il tiD t ti d i i il ti
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;1><
J → >D
J → =@
!b "λ ( 1.1 , 1=1=
a1
#; >AJ → =C!b "λ ( ;.<< , 1=1= a1#
;> >Th → =<!b "λ ( .; , 1=11 a1#
JTh!b system as an indicator of continentalJTh!b system as an indicator of continental
contamination is particularly usefulcontamination is particularly useful
All are incompatible 8E8 elements+ so theyAll are incompatible 8E8 elements+ so they
concentrate strongly into the continental crustconcentrate strongly into the continental crust
Detecting and assessing assimilationDetecting and assessing assimilation
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Mi,ed !rocessesMi,ed !rocesses May be more than coincidence: twoMay be more than coincidence: two
processes may operate in con)unction processes may operate in con)unction
"cooperation3#"cooperation3#
A'C: '- supplies the necessary heatA'C: '- supplies the necessary heatfor assimilationfor assimilation
'ractional crystalli6ation 5 recharge of'ractional crystalli6ation 5 recharge of
more primitive magmamore primitive magma
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Tectonic-Igneous AssociationsTectonic-Igneous Associations
Associations on a larger scale than theAssociations on a larger scale than the
petrogenetic provinces petrogenetic provinces
An attempt to address global patternsAn attempt to address global patterns
of igneous activity by groupingof igneous activity by grouping
provinces based upon similarities in provinces based upon similarities in
occurrence and genesisoccurrence and genesis
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Mid*cean 2idge 9olcanismMid*cean 2idge 9olcanism
*cean Entraplate "Esland# volcanism*cean Entraplate "Esland# volcanism
Continental !lateau ?asaltsContinental !lateau ?asalts
$ubductionrelated volcanism and plutonism$ubductionrelated volcanism and plutonism
Esland ArcsEsland Arcs
Continental ArcsContinental Arcs
0ranites "not a true TE Association#0ranites "not a true TE Association#
Mostly alkaline igneous processes of stableMostly alkaline igneous processes of stable
craton interiorscraton interiors
Anorthosite MassifsAnorthosite Massifs
Tectonic-Igneous AssociationsTectonic-Igneous Associations