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Orientation of Pigeonite Exsolution Lamellae in Metamorphic Augite: Correlation with Composition and Calculated Optimal Phase Bo

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Orientation of Pigeonite Exsolution Lamellae in Metamorphic Augite: Correlation with Composition and Calculated Optimal Phase Bo American Mineralogist, Volume 60, pages 9-28, 1975 Orientationof PigeoniteExsolution Lamellae in Metamorphic Augite:Correlation with Compositionand Calculated OptimalPhase Boundaries Howano W. Jerre. Perrn RosrNsoN.Ronrnr J. Tnacv, Departmentol Geology,Uniaersity of Massachusetts, Amherst, Massachusetts0 1002 eNn Mnrcor,u Ross U.S.Geological Suraey, Reston, Virginia 22092 Abstract Optical examination of metamorphic augites coexisting with orthopyroxenes ranging in composition from Fs,u to Fs"s from the Adirondacks, the Hudson Highlands, and the Cortlandt Complex, New York, and the Belchertown Complex, Massachusetts, shows three sets of exsolution lamellae. X-ray single crystal photographs show these are orthopyroxene lamellae on (1@), and pigeonite lamellae, termed "0Ot" and "l00", oriented on irrational planes near (001) and (100). Optically observed angles of the phase boundaries of "001" and "100" lamellae with respect to the c axis of host augite vary with iron-magnesium ratio determined from electron probe analyses of 29 pyroxenes, supplemented by 76 measurements of gamma index of refraction. The angles are largest in magnesian specimensand are in near agreement with angles of optimal phase boundaries calculated from measured lattice parameters of host and one or both sets of pigeonite lamellae. Compositional control of lamellae orientation is related to compositional control of lattice parameters, which appear not to have changed significantly since exsolution in the range 800-500'C. Introduction and (100) directionsof the host, as had beengen- (Poldervaart and fless, 1951). Many igneous and metamorphic rocks contain erally accepted establishedsolely by pyroxeneswhich have unmixed during cooling. The Thesesignificant relations were petrographic of grain mounts by exsolved pyroxenes form as lamellae which have microscopicstudy betweenthe two promi- been generally described in the past :rs being noting that the obtuseangle was 122" rather than the orientedparallel to the (100) or (001) lattice nent sets of lamellae requiredby two setsof lamellaeparallel planesof the host (Poldervaartand Hess, 1951). 105-106" (001) (100) Metamorphic augites containing sets of exsolution to and of augite. a study of these exsolution phe- lamellae in three different orientations were first Subsequently, nomena in more detail by Robinson, Jaffe, Ross, describedby Jaffe and Jaffe (1973)1 from their Klein (197l)L establishedby singlecrystal X-ray occurrence in augite-orthopyroxenegranulites and and photographsthat the lamellae parallel to (100) of amphibolitesassociated with Precambriancrystalline augite host were orthopyroxene. They found gneissesof the Monroe quadranglein the Hudson the that the two more prominent setsof lamellae,desig- Highlandsof New York (Fig. 1). These authors nated as "001" and "100" lamellae,were indeed noted that only one of the three sets of lamellae pigeonite' with phaseboundaries, separating lamel- was parallel to a crystallographicaxis, the c axis, of the host augite,whereas the two more prominent 'The definitions of pigeonite and augite used here are sets of pigeonitewere not parallel to the (0Ol) based partly on the work of Ross, Huebner, and Dowty (1973). Pigeonite is defined as a monoclinic pyroxene con- 'The paper by Jaffe and Jaffe (1973) was submitted for taining less than 20 mole percent CaSiOs component (Wo ( publication in 1969, but not published until 1973 because 20). The FeSiO" component can vary from zero to 100 of a freeze on funds at the N.Y. State Museum and Science mole percent, although for the end-membersonly, we would Service. It thus preceded the study and publication of the use the names clinoenstatite (En'.) and clinoferrosilite paper by Robinson,Jafte, Ross,and Klein (1971). (Fs"-). Pigeonite can be unambiguously identified by single 10 TAFFE, ROBINSON, TRACY,,{ND ROSS of Bollmann, as applied to exsolution in feldspars by Bollmannand Nissen(1968). Soon thereafter, a study of glain immersion nrountsrevealed that the exsolgtion phenomenadis- covered in the two-pyroxenegranulites of the Hud- son Highlands, described above, are even more widespread in two-pyroxene-bearinganorthosites, granulites, and charnockitic gneissesin the Mount Marcy quadrangle of the Adirondacks. Detailed areasurementsof the gamma indices of refraction of orthopyroxeRescoexisting with these clinopy- roxenes indicated a wide range of iron-magnesium 1sfi6s-n36ely, 100 (Fe2** Fe3. * Mn)/(Mg + Fe2* * Fe8* * Mn)-fronr 40 to 95 (Tabtre1, Fig. 2). Pyroxene assemblagesricher in magnesium were obtainedfrom the BElchertownfnfiusive Com- plex of central Massachusetts(Fig. 1; Emerson, 1898, 1917; Guthrie and Robinson,1967; Guthrie, 1,972; Hall, L973) and the Cortlandt ultramafic complex of southeasternNew York (Fig. 1; Shand, 1942;Tracy, l97O). Mea$ure(nentsof the gamma EXPLANATION index of retraction of orthopyroxenesfrom these rocks indicated iron-magnesiumratios of 15 to 35 lF.n Mesozoicond CenozoicCpver (Table 1). Thus pyroxene pairs were available for study in which the orthopyroxenesrange in com- T;-l Belchertown,Corllondl Complexes position from Fsrs to Fse5(Fig. 2). In all cases these orthopyroxenescoexisting with augite con- tain augite exsolutionlamellae parallel to (100). D'Kl Anorthosite Although of different primary origins, all of the pyroxene pairs under discussion are believed to ffi.,:iln Precombrion have equilibrated under generally similar meta- Ftc. l. Generalized geologic map slrowing locations from morphic conditions of high tqmperature, inter- which coexisting pyroxene$ \ilere obtained: (1) Monroe mediate pressure, and low humidity equivalent to quadrangle, Hudson (2) quad- Highlands; Mpunt Marcy the granulite facies.Hudson Highlands and Adiron- rangle, Adirondacks; (3) Belphertown Compkx; (4) East- ern part of the Cortlandt Complex. dack specimensappear to have been involved in Precambriangranulite facies metamorphismof re- gional extent, although the Adirondack specimens lae host, parallel from that are not oriented to a also contain some relict featuresof earlier plutonic specific rational plane lattice of the host. These events.The specimensfrom the BelchertownCom- authors also demonstrated that the orientation of plex came from a Devonian batholith. Subsequent exsolution lamellae pigeonite of in augitenand also to syntectonic emplacernent,the batholith under- that of monoclinic amphibolesin each other, was in went hydration during continued regional metamor- accordancewith the optimal phaseboundary theory of composition Wog (En, Fs)* clinohypersthene. Yet crystal X-ray diffraction by ( I ) its space group symmetry pigeonito on cooling may unmix enough augite to give it Hh/c at room temperature, and (2) by having a B angle this low calcium content. of greater than 107.5' (usually 108-109") at room tem- Augite is defined as a elinopyroxene having greater than perature. The names clinoenstatite, glinobrEnzite, clino- 20 mole percent CaSiO' (Wo ) 2O) aad Fs content from hypersthene, and clinoferrosilite are not used for naturally zero to E0 mole perc€nt. Augite is unambiguously identified occurring pyroxenes because there is no consensuson the by having space group symmetry C2/c ar:.d a p angle of range of Fs or Wo content to be attached to those names. less than 107.5' (usually 105.5-106.5"). The names diop Some would prefer to call a clinopyroxene of cornposition side and hedenbergite are reserved for the augite end- 'bigeonite" Wo$ (En, Fs)* but would call a clinopyroxene members CaMgSLOo and CaFeSLOorespectively. PIGEONITE EXSOLUTION LAMELLAE IN METAMORPHIC AUGITE 11 HE EN t5 Frc. 2. Compositions of co-existing pyroxenes from the Hudson Highlands (open circles), Adirondacks (closed circles), Belchertown Complex (open squares), and Cortlandt Complex (closed squares). All compositions were determined by electron probe, except for thosemarked by the smaller closed circles, which were determined optically. Solid tie lines'indicate specimens for which X-ray single crystal data have been obtained; long-dashed tie lines, only optical data on exsolution lamellae. Short-dashed lines indicate trend of zoning in angites 447,T65, and A21. Stippled pattern indicates limits of mutual solid solution of synthetic Ca-Mg-Fe augite and orthopyroxene at 810"C as determined by Lindsley, King, and Turnock (1974). phism that left the central core with the relict stageof the microscope.According to Deer, Howie, granulite facies mineralogy of an orthopyroxene- and Zussman (1963), each atomic percent of augiterronzodiorite. The interior of the easternend (Fe'z.* Fe3** Mn) leadsto an increaseof O.@125 of the Ordovician Cortlandt Intrusive Complex in the gamma index of refraction of orthopyroxenes seemsalso to have preserved essentiallygranulite from both igneous and metamorphic parageneses. facies assemblagesalthough undergoing recrystalliza- This correlation incorporates data for orthopy- tion during later regional metamorphism (Tracy, roxenes from plutonic rocks (Hess, 1952, 1960), 1970). Further detail concerningthe specimensand from volcanic rocks (Kuno, 1954), and from high- their geologic setting is given in the Appendix. grade metamorphic rocks (Muir and Tilley, 1958; Howie, 1955), all of which are plottedon Figure 3. Optical Properties and Composition of Pyroxenes Electron probe and optical data obtained during The compositionsof coexistingpyroxenes in 38 this study show lessscatter and a new curve relating specimenswere estimatedby measurementof indices iron-mapesium ratios with gamma indices of re- of refraction
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