Monticellite Marble at Cascade Mountain. Adirondack Mountains. New York
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AmericanMineralogist, Volume 63, pages 991-999, 1978 Monticellitemarble at CascadeMountain. Adirondack Mountains. New York Rosrnr J. Tnecyl Departmentof GeologicalSciences, Haruard Uniuersity Camb ridge, M assachuset ts 02 I 38 Howeno W. JerneeNo Pernn RosrNsoN Departmentof Geology,Uniuersity of Massachusetts A mherst,M assachuset ts 0I 003 Abstract An Adirondackmarble contains the assemblagecalcite-diopside-forsterite-monticellite, with traceamounts of zincianspinel, titanian andradite, idocrase, and sphalerite. Microprobe analysesindicate that the monticelliteand forsterite both haveMg/(Mg+ Fe)greater than 0.9, and that monticelliteis more iron-richthan coexistingforsterite. Monticellite host grains containrare microscopic exsolution lamellae of forsterite,a featurenoted in only oneother terrestrialmonticellite occurrence. Diopside is stronglyzoned and containsup to 6 weight percentAlrOr and substantial ferric iron, indicating a fassaiticcharacter typical ofcalc-silicate clinopyroxenes. Distributionof Feand Mg amongthe major silicate minerals indicates that the monticellite- producingreaction must be an Fe-Mg continuousreaction combining the end-member reactlon, MgrSiOn* CaMgSirO.* 2 CaCO, : 3 CaMgSiO4+ 2CO2 with two Fe-Mg exchangeequilibria: 2 CaMgSiOn* Fe2SiO.: 2 CaFeSiO,* Mg,SiOn CaMgSirO.* CaFeSiO.: CaFeSirOu* CaMgSiO. Calculationssuggest that Fe doesnot lower the reactiontemperature significantly. The occurrenceof monticellite-freemarble in the sameoutcrops and variablemineral composi- tions in the calcite-diopside-forsterite-monticelliteassemblage implies that theserocks formed in an aCO, gradientwith aCO, probablydecreasing from centerto edgeof the marblelayer. P-T-XCO, relationsfor the iron-freereaction have been calculated, and suggestthat forma- tion of monticellitein theabove reaction, under presumed conditions of Adirondackregional metamorphismof 8-10kbar and about700'-800oC, requires either great dilution of CO, by HzOin metamorphicfluid, or low aCO,under fluid-absent conditions. A possiblealternative explanationsuggests earlier contact metamorphismat low pressureslollowed by a later Grenville-agehigh-pressure regional metamorphismin which the pre-existinghigh-grade mineralassemblages were not substantiallyaffected. Introduction documentedoccurrence in metamorphicrocks in the Marbleon CascadeMountain in the Adirondacks easternUnited Statesof monticellite2.which is usu- nearKeene, New York, containsthe assemblagecal- ally restrictedto contactmetamorphic environments cite-diopside-forsterite-monticellite.This is the only 'zChoquette(1960, Table 2) listsmonticellite in themctdes of two quartz-bearingmarbles of the CockeysvilleFormation in Mary- I Presentaddress: Department of Geologyand Geophysics,Yale land.G W. Fisher(personal communication , 1974) has examined University,New Haven,Connecticut 06520. thesethin sectionsand believesthe mineralis misidentified. 0003-004x/78/09I 0-099l$02.00 99I 992 TRACY ET AL.: MONTICELLITE MARBLE as at Skye (Tilley, l95l), Crestmore (Burnham, lessin thin section.It occursin roundedisolated 1959),and West Texas (Joesten,1976). Monticellite, grainsup to 2 or 3 mm. In handspecimen, forsterite Ca(Mg,Fe)SiOn,is an olivine which typically forms in is colorlessto pale greenwhere fresh, and dark gray marbles only in the highest range of metamorphic where partially serpentinized.Monticellite is pale temperatures,above about 900'C at low pressures, brown. Both are colorlessin thin sectionand havea and in rare alkalic and ultramaficrocks and carbona- similar size and isolatedoccurrence as diopside. tites (Bowen, 1922; Eckermann, 1948;Sahama and Thereare rare smallgrains of pale greenspinel and Hytdnen, 1957). The occurrence of monticellite at sphalerite.Both idocraseand andraditic garnet occur CascadeMountain is even more striking in that the as sporadicthin rims on diopsideor monticellite mineral might have been produced during Pre- grainsagainst calcite. Diopside contains inclusions of cambrian regional metamorphismof the Adirondack spinel,and monticellitecontains inclusions of both massif. A further interesting feature of this occur- diopsideand spinel.Forsterite appears to be freeof renceis the presenceof forsteriteexsolution lamellae inclusions. in monticellite hosts.Though monticellite and forste- All phasesin threeselected samples were analyzed rite (MgrSiOa) are known to have a limited mutual by electronmicroprobe. Analyses were made on the miscibility (Warner and Luth, 1973),exsolution fea- MAC microprobeat M.I.T., with operatingcondi- tures have been previously reported for only one tionsof 15kV acceleratingpotential and samplecur- terrestrial occurrence of this mineral pair, in calc- rent of about .025microamps. Data werecorrected silicate blocks included in norite of the Bushveld usingthe methodof Benceand Albee (1968) and the Complex (Willemse and Bensch,1964). correctionfactors of Albeeand Ray (1970). Chemical The monticellite-bearingmarble crops out on the datafor monticellite,forsterite, and diopsideare pre- north flank of CascadeMountain in the Mt. Marcy sentedin Table l. Thesephases and othersare dis- quadrangle, which is being remapped by Jaffe (Jaffe cussedbelow. and Jaffe, in preparation; Jaffe et al., 1977). It is Forsteriteand monticellite involved in an isoclinal recumbentfold with adjacent layers of syenitic gneiss and bordering anorthositic Magnesianolivine rangesin compositionfrom gneiss.It is not clear whether or not the marble was Fo89to Fo94.The mostiron-rich olivine was found intruded by the adjacentgneisses, but it appearsthat in the monticellite-freesample, Ca-22F. There is also all these rock units have been folded and metamor- a significantamount of iron in themonticellite (Table phosed in the Precambrian Grenville (1100 m.y.) 1) andthe ratio Fel(Fe+Mg) is higherin monticellite event that affected the Adirondacks and adjacent than in coexistingforsterite. Terrestrial occurrences rocks to the north and west. Some details of the of monticellite+ forsteriteare very rare-less than a geology of the outcrop area of the marble may be dozenhave been found in the literature,of whichthe found in Baillieul (1976). majorityare igneous.No dataexist on compositions In this paper we describesome of the mineral as- of thesepairs. However, the Fe-Mg fractionationin semblagesand texturesin samplesof the marble, and the CascadeMountain occurrenceis similarto that presentmicroprobe analysesof the silicateand oxide found for coexistingfayalitic olivine and magnesian minerals. The petrology of the monticellite-produc- kirschsteinitein Angra dos Reismeteorite (Prinz et ing reaction is discussed,and the implications it has al.,1977)and in the Sharpschondrite (Dodd, l97l). for the composition and behavior of the metamor- Thus, the preferentialpartitioning of iron into the phic fluid are considered.Finally, we note the con- high-calciumphase is the reverseof fractionation straints that this and other Adirondack rocks place found in most pairs of coexistinglow- and high- on pressure,temperature, and activity of fluid species calciumpyroxenes. during metamorphism. One explanation for this monticellite-forsterite Fe-Mg fractionationmay be found in the distribu- Petrography and mineral chemistry tion of cationsbetween M(l) and M(2) sitesin the Mineral assemblagesidentified in different speci- two olivines.Since monticellite M(2) is essentially mens of the marble include: calcite-forsterite-diop- filled by Ca, all Fe and Mg must be in M(l). On the side-magnetite + spinel; calcite-forsterite-mon- otherhand, in forsteritethere is abundantMg in both ticellite-diopside-spinel-andradite; and calcite- M(l) and M(2). Even if forsteriteand monticellite forsterite-m onticellite-diopside-spinel-idocrase. containedthe samenumber of atomsof Fe per for- Diopside is pale greenin hand specimenand color- mula unit, the FelMB ratio of monticellitewould be TRACY ET AL,,' MONTICELLITE MARBLE 993 higher. But there also appearsto be an absolutepref- larger and more distorted than those in forsterite. erenceof Fe for monticellite, since it contains more While M(2) in monticellite is much larger and more Fe atoms per 4 oxygenformula (Table l). Brown and distorted than M(2) in forsterite, monticellite M( I ) is Prewitt (1973) and Smyth (1975) argue that Fe-Mg larger but only marginally more distorted Ihan M(l) ordering in olivinesis due to the greaterdistortion of in forsterite.Whether this relative distortion of M(l) theM(l) site(Robinson et al., l97l) which causesFe sites is enough to be effective is arguable, but the to prefer M(l). By extension of this argument, Fe increasedsize of monticellite M(l) alone might be might be preferentially partitioned into monticellite enough to causeFe to prefer it. M(l), becausethe monticellite octahedral sites are An important feature of the monticellite is the presenceof rare microscopic exsolution lamellae of forsterite (Fig. l), which are up to about l0 microns Table l. Microprobe analysesof minerals thick and are apparently oriented along the (010) ea-22t Ca-22 CL-3a planesof the monticellitehost. The forsteritecompo- Fo Fo Mo Fo Mo sition of these lamellae has been established by sio2 40.79 42.t| 37.93 42.42 36.83 electron probe analysis of several larger lamellae, TiO2 0.00 0.00 0.00 0.00 0, 0r which have the approximate composition orzo3 0.00 0.00 0.00 0.00 0,04 Cao.orMgr.rr Feo.orM no.orSir.ooOo. This is the secondre- Cr 203 0.00 0.00 0.00 0.00 0.06 FeO 5. 09 2.85 3.34 3.49 4.47 ported occurrenceof exsolution in terrestrialolivine MnO 0.58 0.05 0.19 0. 1r