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Mineralogy of Mafic and Fe.Ti Oxide-Rich Differentiates of The American Mineralogist, Volume 67, pages 14J7, 1982 Mineralogy of mafic and Fe.Ti oxide-rich differentiatesof the Marcy anorthositemassif, Adirondacks, New york LBwrs D. Asuwnl Lunar and Plnnetary Institute 3303 NASA Road l, Houston, Texas 77058 Abstract Rocks enriched in mafic silicates, Fe-Ti oxides and apatite form a minor but ubiquitous facies of nearly all Proterozoic massif-typeanorthosite complexes.In the Marcy massif of the Adirondack highlands,New York, the mafic rocks have two modesof occurrence: l) as conformable segregationsinterpreted as cumulate layers in the border zones, and 2) as dikes throughout the massif, interpreted as having crystallized from residual liquids extracted at varying stagesduring differentiation. Primary mineral compositions in these rocks are commonly preserved,despite the effectsof subsolidusrecrystallization in the high pressure granulite facies. In the mafic rock suite, primary compositions of mafic silicates reveal an iron enrichment trend which is broadly similar to that of basic layered intrusions, but suggestive of somewhat higher crystallization temperatures. Textural relationships suggest the following crystallization sequence: plagioclase, pigeonite + augite, hemo-ilmenite * magnetite,apatite. Fe-rich olivine replacedpigeonite in the latest- stage residual liquids. The mineralogy and field relationships of these mafic rocks are consistent with their origin as diferentiates from the same melts which produced the anorthosites. This conclusion agrees with recent geochemical data that suggest an independentorigi-n for the spatially associatedmangerite-charnockite suite. Introduction whose associationwith anorthositemassifs is well For the past several decades,the controversy known (Rose, 1969).Itis thesemafic rocks, and not over the origin of massif-type anorthosites has the mangerite-charnockites,which complementthe centered around the question of the consanguinity anorthosites,as demonstratedby REE geochemical of anorthositic rocks and spatially associatedsuites relationships(Ashwal and Seifert, 1980).The pur- of orthopyroxene-bearinggranitic rocks (charnock- pose of this paper is to describe the field relation- ite-mangerite series). Recent trace element geo- ships and mineralogical characteristics of mafic chemical studies focusing on, but not limited to, rocks associatedwith the Marcy anorthosite massif rare earth elements (REE) (Philpotts et al., 1966; in the Adirondack highlands of northern New York Greenet al., 1969,1972;Seifert et a|.,1977;Seifert, State, and to emphasize the importance of these 1978; Simmons and Hanson, 1978; Ashwal and rocks in understandingthe fractionation histories of Seifert, 1980)demonstrate that the acidic rocks do this and other massif-typeanorthosite complexes. not represent differentiates from the melts which producedthe anorthosites.This was recognizedas Field relations of mafic faciesof anorthosite early as 1939by A. F. Buddingtonon the basis of In the Marcy massif,the most felsic anorthosites field relationships (summarized by Buddington, are found in the central regions, and more mafic 1969). varieties occur near the margins. Becauseof the In addition to huge volumes of pure anorthosite, domical form of the massif, the mafic border facies the typical massif anorthosite suite contains minor overlies the anorthositic core zone (Buddington, facies with larger proportions of mafic silicates, Fe- 1939,1960,1969). In someplaces there is a system- Ti oxides and apatite. Extreme concentrations of atic increase in mafic silicates and oxide minerals these minerals give rise to minor ultramafic rocks, from coarse, core zone anorthosite (Marcy facies) as well as the ilmenite-magnetite ore deposits structurally upwards to finer-grained, border zone 0003-004)v82t0r02-0014$02.00 14 ASHWAL : OXIDE.RICH DIFFERENTIATES, MARCY ANORTHO SITE t5 /u"30'N /u"15'N 4"OO'N 71"30'W 1'51 Ptedominontly leucogobbro ond ltons"rir"-rhornockire series .t (<ontocts @ J. :.'i. leuGonorite doshed wngte tnlelleol : Porogneiss,morble, quortzite ond othet melosedimsntqry ro<ks Pr.do.inontly onoithosits 7/t4- I (in(ludesminor gobbrc & ohphibolite) Mofi< or ultromoli< <umulote loyer O 5 l0 15 Miles ,/ ,/ moti, an - " O 5 lO 15 2O Kilometers )( fe-fi oxide deposir Fig. I Geologic map of the Marcy anorthosite massif, Adirondacks, New York, showing samplelocations. Base map taken from lsachsenand Fisher (1970). gabbroicanorthositer (Whiteface facies) (Budding- known, of these mafic layers and dikes are shown ton, 1939;Davis, 1971).Elsewhere, the border on a geologicmap of the Marcy massifin Figure 1.2 zonescontain local mafic to ultramafic layers (color In the border zones,local ultramaficlayers com- index > 40%) which are conformable to foliation in monly have sharp lower contacts and are gradation- host gabbroicanorthosites. Mafic rocks also occur al upwardsinto more feldspathicrocks. These are as dikesthroughout the massif,as discussedbelow. most easily interpretedas resultingfrom processes The distributionand structuralrelationships, where involving gravitational settling and accumulation of mafic silicatesand Fe-Ti oxide minerals. The layers 'Rocks are classifiedon the basis of color index (volume 7o range in thickness from 1 cm to 1 m and are often mafic minerals), and the mineralogicalmodifiers gabbroic, nori- tic, or troctolitic are useddepending on whether the predominant 2Details mafic phase is high-Ca pyroxene, low-Ca pyroxene, or olivine, of sample locations may be obtained by ordering respectively(Buddington, 1939;Streckeisen, 1976). The modifier Document AM-82-182 from the Business Office, Mineralogical oxide-rich is added when modal ilmenite f maenetite exceeds Societyof America, 2000Florida Avenue, N.W., Washington, lOVo. D.C. 20009.Please remit $1.00in advancefor the microfiche. l6 ASHWAL : OXIDE.RICH DIFFERENTIATES, MARCY ANORTH)SITE rhythmically repeated several times on the scale of Conformable occurrences of mafic rocks individual outcrops. The attitudes of the layers are Modes of conformable mafic facies rocks from in most placesconsistent with the domical shapeof the Marcy massif are given in Table 1. There is the massifs(Fig. 1), althoughtheir lateral extent is generally a progressivedecrease in mafic and oxide difficult to estimatedue to the paucity of exposures. minerals with increasing stratigraphic height above There are also mafic rocks which occur as sharp- the basalcontact of a cumulate layer; the scale over walleddikes of variedcomposition, crosscutting the which this variation in modal mineralogy takes anorthositic foliation at high angles. These dikes place ranges from centimeters to meters. Cumulus may have crystallized from residual liquids extract- phasesinclude augite,inverted pigeonite,ilmenite, ed at various stages of differentiation of the anor- magnetite,and, lesscommonly, Fe-rich olivine and thosite suite (Ashwal, 1978a,b). In the Marcy apatite. The primary mineralogies and textures of massif, mafic dikes are not spatially restricted to the theserocks, however,have been affected to varying border facies, but may be found even in the central degreesby subsolidusrecrystallization, which has regions(Fie. 1). The dikes are generally less than resulted in the production of metamorphic garnet 7 m thick, although Buddington (1939,p. 50-51) and hornblende,and in the reconstitutionofcoarse- described a mafic gabbro dike some 60 m wide in ly exsolvedpyroxenes into smallerdiscrete grains the Carthage anorthosite massif in the western with different compositions. Zones of garnet Adirondacks. 1-5 mm thick commonly occur betweenthe basalcon- tacts of mafic layers and underlying leucogabbros Mineral constitution and petrography or leuconorites. Although metamorphic garnet, hornblende, and pyroxenes constitrrte nearly 20Vo Primary minerals of the Marcy anorthosite com- of some samples,other rocks appearto have com- plex includeplagioclase, high-Ca pyroxene, low-Ca pletely retained their primary mineralogiesand tex- pyroxene, olivine, ferrian ilmenite, titaniferous tures (Table 1, sample SA-3D). In general, it is magnetite, and apatite. Metamorphic minerals in- possible to see through the overprinting effects of clude garnet, hornblende, biotite and quartz, in subsolidusrecrystallization, as discussedbelow. addition to recrystallized equivalentsof the primary The color indices of the lower, most mafic parts phases,suggesting equilibration in the high pressure of the layers vary between approximately 60Voand granulitefacies (deWaard,1965). 100%(Table l). The layers generally lie on and are Table l. Point counted modes of conformable mafic rich rocks from the Marcv massif sA-3D SA-3A SA-5 SL-IIY SIJ-IIA SA-15A SA-I5B Mtr-568 MM-56A SL-2IA Sr,-21 Plagioclase L.2r 63.7 66. S 2.4 75.6* 42.6* 73.5* 20.9 74,O* 19,3* 51.5* 27.L* qq)1 Inverted pigeonite I 4.1 tr 7.L 3.2 L.7 cr Orthopyroxene 3.2 2.4 7.! 3.4 4.8 2.L t.u J-b 0.9 1.9 Clinopyroxene t ,:t 52.3 13.4 19.0 II.4 2A.9 II.2 48.6 t5.5 9.5 Ollvine _n IIrenite 25.6 I.1 0.9 t8.9 4.5 ]5.3 4 .2 4.3 1,, - 8.4 Magnetite 13.9** 0.6 0.I 4.2 0.3 I I nt I.O O.7 L6.2 f"' 3.3 Apatite - 0.2 0.5 - u.b q Garnet - 11 2 ? - 2.2 10.5 4.1 l. r L5.2 18.O Hornblende v.z 9.0 L4.7 0.5 Sulfides 0.5 - 0.I I.O O.2 Biotite tr o.2 0.4 0.5 ScatrpLite _ 0.3 Other secondaries**r -tr 0.5 0.5 100.0 100.0 100.0 r00.0 Ioo.0 100.0 100.0 100.0 r00.0 100.0 loo.0 100.0 Color index 94.8 36.3 91.2 24.4 57.4 25.5 79.1 26.0 80.7 48.0 72.9 oxide- leuco- Ieuco- oxide- Ieuco- oxide- leuco- oxide- leuco- oxide- oxide- oxide- rich norite norite rich norite rich
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