THE AMERICAN MINERALOGIST, VOL 55, NOVEMBER_DECEMBER, 1970

THE ANORTHOSITE-CHARNOCKITE SUITE OF ROCKSOF ROARING BROOK VALLEY IN THE EASTERN

ADIRONDACKS (MARCY MASSIF)

Drnr pB Waenr, Department of Geol,ogy, Syracuse(Jnioersity, Syracuse, Neu Yorh 13210.

Assrnecr

Within the bounds of the Marcy anorthosite massif, fresh and extensive rock exposures

in the valley of Roaring Brook displal' a great variety of rock rypes ranging from hyper-

sthenite and anorthosite to charnockite and leptynite. With the exception of xenolithicmetasediments all rocks are considered members of one difierentiation series. Consanguinityis indicated by tra.nsitional field relations between rock t1pes, and by the complete modal

and chemical gradation between members of the suite.Transgressive relationships demonstrate that anorthosite and hypersthenite are the

older, and the K-feldspar and quartz-rich rocks the younger members of the series. Tex-

tures indicate that hypersthenite, anorthosite, and leuconorite are orthocumulates.

Orthopyroxene in hypersthenite, and plagioclase in anorthosite and leuconorite are theprincipal cumulus minerals. Igneous lamination is a common structure in leuconorite;

rhythmic layering is observed in jotunite. Hypersthenite, which occurs in a dike intersecting

the igneous iamination, apparently intruded leuconorite when both rocks were in the state

of a crystal mush.Although all rocks have undergone orogenic deformation and granulite-facies meta-

morphism, the rocks poor in K-feldspar and quartz are little affected. Foliation developed

only in part of the area, in jotunitic, farsunditic and charnockitic rocks. The development

of garnet and metamorphic recrystallization occurred in rocks with an Fe:Mg ratio above

a value of about 1.3.

INrnonucrtoN

On the 29th of June 1963 heavy rains caused large-scale slides in thecirque of Giant Mountain, and muddy torrents swept through the valleyof the Roaring Brook. The freshly exposed rock in the valley' floor and onthe slides are a petrologist's paradise. Located well within the bounds ofthe Marcy anorthosite massif (Fig. 1), the outcropping rocks are sur-prisingly varied in composition and texture. Although all rocks haveundergone metamorphism and many are recrystall ized, few are deformed,and magmatic textures and transgressive relationships can be observedin detail.

Since 1963 the writer and his students have studied outcrops and rocksamples of the valley as a research project in petrography courses. Theaccumulated petrographic knowledge is summarized here so that it mayserve as a guide for petrologic fieldtrips in the Roaring Brook valley.

LrtnorocrBs

The rock section exposed in the valley is 3.5 km long, and runs fromthe base of the cascade at 1400 ft altitude to the top of Giant Mountain

2064 DIRK DE WAARD

Frc. 1. Outline of the Marcy anorthosite massif after the Geologic Map of New YorkState (Broughton et al, 1962) , showing the location of the Roaring Brook section.

at 4627 ft (Fig. 2 and 3). The section can be roughly divided into threelithologically difierent parts. The western part, between 1400 and2000 ft, is composed of coarse to medium-grained rocks of anorthositicand leuconoriticr composition. Textures vary; the mafic constituents maybe dispersed between plagioclase crystals, or form poikilitic clots inplagioclase rock, or they may increase in amount to form an ophitictexture. Locally, xenolithic masses of anorthosite occur in the leucon-orite. Elsewhere, the two rock types appear to grade into each other.One important feature in this part of the valley is a dike of hypersth-enite, which is followed by the brook between 1600 and 1900 ft. It is atIeast 875 m long and between 1 and 2 m wide, with offshoots and veinsparalleling it.

At about 2000 ft there is an abrupt change in lithologies: beyond awell-exposed sharp contact the rock is medium to fine grained, containsbetween 10 and 40 percent mafic minerals, and K feldspar and quartz areinvariably present, but in variable amounts. This part of the valley, be-

r The term leuconorite is used here as an abbreviation of leucocratic norite, which is anorite containing between 5 and 25 percent mafic minerdls, too much to be called anortho-site. See Figure 5 for rock terms and classification.

ANORTHOSITE-CHARNOCKITIIIN THEADIRONDACKS 2065

tween 2000 and about 2700 ft, is characterized, by the occurrence ofjotunite, farsundite, and charnockite. All of these rocks are metamor-phosed and garnet bearing. Some are foliated, but many are granuliteswithout distinct orientation. Locally, and especially between 2000 and22A0 ft, xenoliths of various size and composition occur. The inclusionsconsist of fine-grained, finely layered diopside-microperthite metasedi-ment (first reported by Kemp, 1921,35), and of anorthositic and leu-conoritic rocks represented in the western part of the valley. The xeno-liths are embedded in a mafic granulite of jotunitic composition in whichrelict magmatic textures and structures can be recognized. Between2250 and 2450 tt the rocks are foliated, and compositions range fromjotunite with plagioclase augen to charnockite with K-feldspar augen.

A third set of l i thologies is encountered in the eastern part of the valley,from about 21AA ft (unfortunately, outcrops are scarce here) to the top ofGiant Mountain. The rocks are predominantly coarse to medium-grainedanorthosite and leuconorite, similar to the rocks in the western part ofthe valley, but here dikes and bodies of jotunite, farsundite, and char-nockite cut across the anorthositic and leuconoritic rocks. The number ofdikes varies locally; more were encountered on the Bottle Slide than onthe Tulip Slide. The upper portion of the Bottle Slide displays blockstructure of leuconorite with the usual anorthositic parts and masses,intersected by dikes of garnet-bearing charnockite. The top of GiantMountain consists of a crosscutting body of farsundite. As in the middlepart of the valley, these transgressive rocks are now metamorphic rocksof granulitic texture and composition. The most acidic type, leptynite,contains garnet as virtually the only mafic mineral.

TUB Rocx Sune

With the exception of the xenolithic metasediments all rocks of Roar-ing Brook valley are considered more or less metamorphosed and de-formed members of a differentiation series. A close genetic relationship isindicated by the gradual transitions observed between rock types in thefield. The order of differentiation is considered to be largely reflected bythe sequence of rocks in Table 1. The table shows modal and chemicalcompositions of 16 rock samples which are representative for the varia-tion among the rocks occurring in the valley. The sample locations areshown in Figure 2. Sample numbers reflect altitude in feet.

The rocks in the table are arranged in order of decreasing value of themolecular normative ratio (Abf An)/(Q*Or*Abf An). The sequenceshows a general increase in K-feldspar and quartz content of the rock,and a general decrease of the An percentage of plagioclase and the Enpercentage of orthopyroxene. Trends in the series expressed by cation

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IINOR?' IIOSIT]i-CIIARNOCKITL, IN THE A DIRON DACKS 2067

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2068 DIRK DIJ WAARI)

Frc. 2. Map of the Roaring Brook Valley showing sample locations and the extent ofrock exposure on the valley floor (stippled). The Bottle Slide is west of sample locationt1250N, and the Tulip Slide is east of location 3400T. Topography based upon U.S. Geologi-cal Survey topographic 15 minute quadrangle maps, sheets Mount Marcy, N. Y., andElizabethtown, N.Y.

ratios are further demonstrated in variation diagram Figure 4 in whichthe samples are arranged in the same order. Crosscutting contacts ob-served in the valley between the members of the suite indicate that arelationship of decreasing age exists, which is largell 'correlative with theorder shown in the table.

The different behavior of the members of the rock suite to metamor-phism and deformation hampers the use of a consistent rock terminology.Some of the rocks are not affected by these processes, and they may betermed as igneous cumulates. Other members of the rock suite are foli-ated and completely recrystall ized. In this paper, as in previous ones(de Waard, 1969; de Waard and Romey, 1969a, b), use is made of theterms jotunite, f arsundite and charnockite, which ref er to orthopyroxene-bearing magmatic or metamorphic rocks, chemically the equivalents ofmonzodiorite, adamellite and granite, respectivell ' . The classificationof these rocks is shown in Figure 5, which also demonstrates the serialnature of the rock suite as a function of the proportions of leucocraticminerals. Part of the rocks having granitic bulk composition are free oforthopyroxene and therefore cannot be called charnockite. They are herenamed leptynite a term used for a felsic granulite essentially consistingol quartz, feldspars and garnet.

ANORTIIOSITE-CHARNOCKITN IN THE ADIRONDACKS

Frc. 3. Geologic map and section of Roaring Brook valley

TBxrunBsThree groups of textures can be distinguished in the rocks of the Roar-

ing Brook valley: igneous textures, unoriented metamorphic textures,and oriented metamorphic textures.

The igneous textures are not deformed because the rock reacted rigidlyto orogenic stress, and the rock is not recrystall ized because the mineralassemblage was stable under metamorphic conditions. Most of the ig-neous textures are those of orthocumulates or mesocumulates, containingpore material amd strongly zoned plagioclase. The hypersthenite is anorthopyroxene orthocumulate with pore material consisting of zonedlabradorite, qtartz, K-feldspar, biotite, hornblende, clinopyroxene, ap-atite, and ore. Part of the plagioclase is scapolit ized, presumably by thetrapped residual l iquid.

The sequence of crystallization is reversed in anorthosite and leucon-orite, where plagioclase is the cumulus mineral and pyroxenes the prin-cipal pore material. The large, commonly subparallel plagioclase crystalsare compositionally zoned (see Table 1). The Iess calcic rim matches incomposition that of the smaller, surrounding plagioclase grains. De-formation in the rock is evidenced by the bent and broken nature of theIarge plagioclase crystals. The undeformed pore material demonstrates,however, that crushing and granulation of plagioclase occurred before

2070

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DIRK Dli WAIIRI)

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Fro. 4. Variation diagram for rocks of the anorthosite-charnockite suite o{ Roaring

Brook valley (circles) showing trends in the variation of characteristic cation ratios plotted

against the molecular normative ratios (Ab*An)/(Q*OrtAb*An). Ratios of published

rock analyses from about 30 km further to the nort-hwest in the Marcy massif (Saranac

Lake area) have been added (dots) for comparison (Buddington, 1953, Table 3,no.1,2,3,

4, 5, 6,8, Table 5, no. 1, 2, 3,4,6,7,8; Buddington, 1939, Table 32, no. 114)

INORTITOSITIi.CHARNOCK]7']' IN THE ADIROND'4.CKS 2071

tqu,o

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Frc. 5. Volumetric modal ratios of quartz, K feldspar, and plagioclase (circles), and the

molecular normative ratios of Q, or, and AbtAn (dots) of the analyzed rocks of Roaring

Brook valley. The molecular normative ratios of rocks of the Saranac Lake area have been

added for comparison. The terminology used (de waard, 1969) is based on the subdivision

of the triangular quartz-K-feldspar-plagioclase field of modal compositions. Amounts of

perthite components have been estimated and treated as K-feldspar and plagioclase in the

classification.

complete consolidation, i.e., in the state of a crystal mush. The texture

of the pyroxenes is poikilitic, interstitial poikilitic, or sub-poikilitic. Also

hornblende, K feldspar, and quartz occur as sub-poikilitic pore material

in the plagioclase orthocumulates.Unoriented metamorphic textures occur in rocks in which the igneous

mineral assemblage was in disequilibrium during metamorphism, and the

rock recrystall ized to a stable assemblage. No foliation developed, either

because the rock reacted rigidly to orogenic stress) or because it was situ-

ated in a sheltered position during orogenesis' Texturally and compo-

sitionally, most of these rocks are granulites.

The transformation of the igneous mineral assemblage to a metamor-

phic assemblage involves the production of garnet, clinopyroxene' and

qtartz at the expense of orthopyroxene and the anorthite component of

plagioclase (de Waard, 1965a, b). The reaction appears controlled by

the Fe-Mg ratio of the rock, occurring only when this exceeds a value

of about 1.3. Corona textures in some of the rocks attest to equilibration

on a local scale. In other rocks recr.vstallization produced an equigranular

2072 DIRK DE WAARD

mosaic of the stable assemblage. Jotunitic rocks commonly contain relictigneous plagioclase crystals, in which garnet developed in the rim, pro-ducing intense compositional zoning in the plagioclase (see Table 1). Themore acidic rocks tend to be more thoroughly recrystallized to equigran-ular granulites such as the leptynite.

Oriented metamorphic textures have developed in rocks of the middlepart of the valley. Here, orogenesis caused planar and linear textures ingranulitic rock types which appear unaffected by deformation elsewherein the valley. Lenses and thin layers of granulated mafic and felsic min-erals define the foliation, as does the orientation of plagioclase and K-feldspar augen which presumably are igneous relicts.

SrnucrunBs

Most of the structures in the Roaring Brook valley are of pre-orogenic,igneous origin; in part of the valley the early structures are blurred orobliterated by orogenic deformation and recrystallization. fgneous struc-tures include igneous lamination, rhythmic layering, flow structure, con-formable and discordant contacts, xenoliths, block structure, and dikes.

Igneous lamination is well developed in the leuconorite and anortho-site of the western part of the valley. Tabular plagioclase crystals, about5 cm long, have parallel or subparallel orientation, forming a planarstructure which dips steeply to the east. Rhythmic layering is observedparticularly in jotunite, e.g., at location 3050T. At this location there areabout 20 layers exposed, 5 to 30 cm thick, dipping 25o to the west. Eachlayer shows a gradual decrease in the proportion of mafic minerals up-ward, as in density-graded bedding. Sample 3050T in Table 1 is takenfrom the leucocratic portion of a layer; the lower part of the layer is analmost ultramafic, garnet-clinopyroxene rock.

Between 1500 and 1900 ft the hypersthenite dike cuts across theigneous lamination of the leuconorite and anorthosite, seeminqly dem-onstrating that hypersthenite is the younger rock. Cation ratios in Table1 and Figure 4 indicate that both the hypersthenite and the plagioclaserock are early crystallization products in the anorthosite-charnockiteseries. Along the borders of the dike some mixing appears to have takenplace between the two rock types. Here pyroxene and plagioclase occur inall proportions. In places the hypersthenite dike appears broken and in-truded in its turn by plagioclase-rich material. These phenomena seem toindicate that both the intruding and the intruded rock were crystal mush,solid enough for the plagioclase mush to break along an almost straightsurface, and plastic enough for the hypersthene mush to intrude thefissure.

At 2000 ft the igneous lamination of the plagioclase cumulates is inter-

ANORTHOSITE-CHARNOCKITETN THE ADIRONDACKS 2073

rupted by a discordant contact with jotunite which apparently intrudedanorthosite and Ieuconorite as a magma. A flow structure, conformablewith the contact plane, is indicated by schlieren of mafic minerals in thejotunite and by parallelism of platy metasedimentary xenoliths.

Xenoliths of various sizes and compositions are common in placeswhere the rock has not been affected by orogenic deformation. Anortho-site blocks and masses, with sharp contacts and commonly with a maficborder, occur in many places in leuconorite. Blocks of anorthosite and ofleuconorite (e.g., sample 2060N3) are found in jotunite. Dikes and in-trusive masses of jotunite occur in anorthosite and leuconorite. Farsun-dite, charnockite and leptynite form dikes and intrusive bodies in an-orthosite, leuconorite and jotunite. In some places abundant dikes forma reticulate dtructure of the intruding, and a block structure of the in-truded rock type.

Foliation and lineation are locally superimposed on the pre-orogenicstructures, which are gradually obliterated between 2190 and 2230 It.Metasedimentary xenoliths are still faintly visible at 2230 ft. Between2200 and 2900 It the foliation forms an arch with low-angle dips. Thelineation, which is in the foliation plane and plunges about 20" to thenorthwest, parallels the fold axis of the arch.

CoNcr-usror.rs

The problem, whether the variation in rock types of Roaring Brookvalley is the result of a single occurrence of magmatic differentiation asadvocated by Bowen (1917), Balk (1931), Buddington (1931), Crosby(1968), and de Waard and Romey (1969), or is the result of two magmaticevents, separated in time, as proposed by Buddington (1936, 1939, and1969), cannot be solved conclusively from the observations in the field.However, the mutual relationships between the rock types, the completegradation of members within the rock suite, and the consistent chemicaltrends shown in variation diagrams, speak for a differentiation processinvolving only one magmatic event. If all rocks belong to one rock suite,then there are three end members in the differentiation series; plagio-clase cumulate and hypersthene cumulate are the two early products,and charnockite or granite is the final product of differentiation.

A hypersthene dike is a very unusual phenomenon in anorthositemassifs. Ultramafic rocks are not known to occur associated with anor-thosite bodies, though they do occur locally as clots of mafic mineralsin anorthosite or leuconorite. Models for the formation of anorthositemassifs from a basaltic parent magma require an ultramafic differentiate.The absence of masses of ultramafic rocks in or below anorthosite massifs,which was confirmed in the Adirondacks by geophysical data, indicates

2074 DIRK DE WAARD

a possible diorit ic or granodiorit ic parent magma, or a basaltic magmaaltered in composition by assimilation or anatexis. The presence of thehypersthenite dike in the valley indicates that ultramafics are indeed anearly differentiation product during the formation of anorthosite bodies,but the rarity of such occurrences makes it unlikely that the quantity ofultramafi,c rock produced was very large. There are no indications as towhether the hypersthenite cumulate was formed above, below, or as alayer in the plagioclase cumulate before intrusion.

If . in conclusion. a tentative evolution of the anorthosite massif needsto be given in order to generate discussions on the outcrops of RoaringBrook valley, the following sequence of events appears consistent withthe observations. From a parent magma a l itt le hypersthene cumulatedifferentiated on the floor and much plagioclase cumulate near the roofof the magma chamber. Both cumulates contained interstit ial l iquid ofgranitic composition, and ultimate consolidation of the cumulates couldonly take place after a sufficient drop in temperature. The two cumulateswere separated by magma whose composition was changing, with de-creasing density, as crystall ization progressed. Finally the density becameIess than that of the plagioclase cumulate, which settled in large chunks.The igneous Iamination became ti lted, and a fissure developed in whichthe bottom deposit of hypersthene penetrated as a crystal mush. Whilethe plagioclase cumulate sank, the residual magma rose, intruded in andabove the plagioclase cumulate, and incorporated, as it flowed, xenolithsof anorthosite, Ieuconorite. and metasedimentary rock from the roof ofthe magma chamber. This short and simplified development of an anor-thosite massif is essentially the same as the one proposed by Morse(1e6e) .

AcrNowmncnlmrls

Contributions from the following students are here acknowledged: C. E. Bartberger,

Sara J. Clement, B K. Dugolinsky, C. O. Irrank, P. M. Gardner, R. A Isaac, G. C. Kelley,

J. H. Kent, T D. Kurtz, W. A. Newman, Linda J. Pattison, L. B. Smith, J. E. Taggart,

and V. K. Vere. C. C. Rubins assisted with the detailed field work. Dr. E. P. Wheeler II

and Dr S A. Morse kindly read the manuscript and suggested improvements. The writer'sresearch on contact relations of anorthosite massifs is financially supported'by National

Science Foundation Research Grant GA-1401.

RnrunnNcts

Bax, R. (1931) Structural geology of the Adirondack anorthosite, a structural study

ANORTHOSITE-CHARNOCKITI;IN THEADIRONDACKS 2075

BuoorNcroN, A. F. (1931) The Adirondack magmatic sl.el:i '. J . Geo|.39,240,263.- (1936) Origin of anorthosite in the Adirondacks and in general. Amer. Geophys.

Union Trans. 1936, 255-256.- (1939) Adirondack igneous rocks and their metamorphism. Geol. Soc. Amer. Mem.

/. t-JJ+.- (1953) Geology of the Saranac quadrangle, New York. N.Y. Statb Mtu. BuJl.346,

1-100.- (1969) Adirondack anorthositic series. iV.I{. Stote Mus. Sci. Sen. Mem. 18,215-231.Cnosnv, P. (1968) Igneous difierentiation of the Adirondack anorthosit series. 23rd, Int.

Geol,. Congr., Prague, 1968, Rep Proc.21 3748Dtrn, W. A., R A. Horvm, axo J. ZussrraN (1963) Roch l'ormi,ng Mi.nerals. VoI.2, Chain

Sili.cates. lohnWiley and Sons Inc., New York, N .Y .,l-379.DE WAARD, D. (1965a) The occurrence of garnet in the granulite-facies terrane of the

Adirondack highlands. J. P etr olo gy 6, 165-191.- (1965b) A proposed subdivision of the granulite facies. Amer. J . \ci.263, 455-461.- (1969) The anorthosite problem: the problem of the anorthosite-charnockite suite

of rocks. N. Y. State Mus. Sci.. Sen:. Mem. 18r 71 97..tlto W. D. Rounv (1969a) Chemical and petrologic trends in the anorthosite-

charnockite series of the Snowl' Mountain mzrssif, Adirondack highlands. Amer.Mineral.54, 529-538.

---, AND - (1969b) Petrogenetic relationships in the anorthosite-charnockiteseries of Snowy Mountain dome, south-central Adirondacks N. Y State Mw. Sci.Seyl Mem.18, 307-315.

Konr, J. F. (1921) Geology of the Mount Marcy quadrangle, Essex County, Neiv York.N.Y. S tate M us. Billl. 229-23A, 7-86.

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