Contributions to Contrib. Mineral. Petrol. 65, 433-442 (1978) Mineralogy and Petrology by Springer-Verlag 1978 Igneous Pyroxenes From Metamorphosed Anorthosite Massifs* S.R. Bohlen and E.J. Essene Department of Geology and Mineralogy, The University of Michigan, Ann Arbor, Michigan, U.S.A. Abstract. Anorthosites, mangerites and charnockites between the pyroxenes are both sensitive to temperature. from metamorphosed anorthosite massifs (the Adi- The pyroxene thermometers have been applied to igne- rondacks and elsewhere) commonly contain coarsely ous and metamorphic rocks with varying degrees of suc- exsolved pyroxenes with substantial amounts of cess. Binns (1962) used natural pyroxene pairs to de- exsolved orthopyroxene (in clinopyroxene) and clino- termine a pyroxene solvus for Australian granulites. pyroxene (in orthopyroxene). Electron microprobe Davidson (1968) attempted to infer metamorphic tem- reintegration of such pyroxenes yields compositions peratures from another granulite terranes relative to which indicate that pigeonite and subcatcic augite Binns' (1962) data. Davidson (1968) also demon- coexisted before metamorphic reequilibration. Equili- strated that KD(Fe2+/Mg) 1 changes by up to 0.2 with bration temperatures of 1100~ 100~ for anortho- the pyroxene composition and offered a crystal-chem- site and 1000 ~ C_ 100~ for mangerites and char- ical explanation for this secondary effect. nockites are inferred from the solvus of Ross and Because of the many difficulties in determining Huebner (1975). These temperatures constrain mini- equilibration temperatures in rocks from independent mum magmatic thermal conditions and suggest that thermometers, fieled calibration of the pyroxene solvus the magmas were relatively dry. Exsolution lamellae is exceedingly difficult. Consequently many workers of coarse pyroxenes and small equant coexisting py- have devoted much effort to experimental calibration roxenes (with little or no exsolution) yield tempera- of the system. Lindsley et al.. (1974) investigated the tures of < 750 ~ C, consistent with equilibration during pyroxene system at 810~ and attempted reversed granulite facies metamorphism. Relict igneous tex- equilibria. Reversals were not achieved due to slug- tures and compositions persisted through the meta- gish reaction rates. Many other workers, including morphic event due to dry P(H20)~P(solid) meta- Davis and Boyd (1966), Lindsley and Munoz (1969), morphic conditions. The reintegrated pyroxene com- Grover et al. (1972), Smith (1972), Lindsley et al., positions provide a window through the metamor- (1973), Lindsley etal. (1974), Nehru and Wyllie phism and yield constraints on the pre-metamorphic (1974), Warner and Luth (1974) and Mori and Green igneous events. (1975) have experimentally investigated portions of the pyroxene quadrilateral. Most of the experiments by these investigators were attempted above 1000 ~ C and at elevated pressure to promote reaction rates. Introduction Extrapolation to lower temperatures involves sub- stantial uncertainty. Because of difficulties with low Despite the lack of extensive experimental data within temperature extrapolations as well as problems of the pyroxene quadrilateral the compositions of coex- order-disorder and additional components, the pyrox- isting Ca-rich and Ca-poor pyroxenes have often been ene thermometer is poorly calibrated for natural sys- used to estimate equilibration temperatures. The tems at temperatures below 1000 ~ C. Consequently, distribution coefficients Ca/(Mg + Fe z + ) and Fe z +/Mg it is of limited value in metamorphic terranes. The pyroxene thermometer has been applied with greater * Contribution No. 340 from the Mineralogical Laboratory, De- partment of Geologyand Mineralogy, The University of Michigan, 1 The KD is here defined as the ratio of Fe2 +/Mg between coex- Ann Arbor, Michigan, 48109, U.S.A. isting Ca-poor and Ca-rich pyroxenes 0010-7999/78/0065/0433/$2.00 434 S.R. Bohlen and E.J. Essene: Igneous Pyroxenes From Metamorphosed Anorthosites success in igneous rocks. Recently Wood and Banno contain, at most, minor fine exsolution lamellae or (1973) have proposed a thermodynamic model to ac- no exsolution products at all (Fig. 1). These pyroxenes count for additional components in natural pyroxene are often found growing around the coarse pyroxenes pairs, and Wood (1975) has applied it to granulites as small equant granules. Also, they are frequently at South Harris, Scotland. As yet, their thermometer found intergrown with granulated plagioclase, quartz based upon coexisting pyroxenes has not been care- and a variety of other phases with textures typical fully evaluated in areas where temperatures have been of metamorphic rocks. Generally these pyroxenes are obtained using independent thermometers. considerably smaller than the coarsely exsolved py- Despite the many difficulties in experimental cali- roxenes, although there is some overlap in size. Pyrox- bration of the system, coexisting ortho- and clinopy- enes containing little or no exsolution are not roxenes can be used qualitatively to distinguish the restricted to rocks with coarsely exsolved pyroxenes, equilibration temperatures in different rocks which but are ubiquitous in the granulite facies of the Adi- equlibrated at substantially different temperatures. rondacks, east and southeast of the orthopyroxene For instance, the system can be used to distinguish isograd mapped by Engel and Engel (1962). pyroxene pairs which equilibrated at igneous tempera- In addition to the pyroxenes described above, an- tures from those which equilibrated at metamorphic orthositic rocks in the Elizabethtown (Emslie, written temperatures. Semi-quantitative data of this type may communication), Mount Marcy, and Long Lake qua- prove valuable in constraining models on the se- drangles also contain two types of pyroxene mega- quence of events in a given metamorphic terrane. crysts. One occurs as bronze to khaki-colored ortho- pyroxene crystals up to 8 cm in maximum dimension. These contain substantial amounts of exsolved plagio- Adirondack Pyroxenes clase (An 85) and magnetite. This megacryst type is found in other anorthosite massifs and has been Coexisting ortho- and clinopyroxenes are ubiquitous investigated by Emslie (1975) and Morse (1975). in the garnet-granulite facies terrane of the Adiron- Though Emslie (1975) has occasionally found coexist- dack Highlands in Upper New York State. During ing ortho- and clinopyroxenes with exsolved plagio- a continuing study of regional metamorphism in the clase in the Morin complex of Canada, only orthopy- Adirondacks several texturally distinct pyroxenes have roxenes with exsolved plagioclase have been found been observed in metamorphosed igneous rocks. A so far in the Adirondacks. The second type of pyrox- large variety of rock types ranging in composition ene megacryst occurs as very large (up to 0.5 m) black from anorthosite and anorthositic gabbros to manger- crystals in a typical anorthosite matrix. They are or- ites and charnockites often contain coarse (up to thopyroxenes with minor exsolved clinopyroxene. 3 cm) clino- and orthopyroxene phenocrysts which They differ from other Adirondack pyroxenes not contain coarse (up to 25 g wide) exsolution lamellae only in morphology, but also in their chemistry; they (Fig. 1). In anorthosite these pyroxenes are typically are substantially more magnesium-rich than other Adi- intergrown with plagioclase in ophitic or subophitic rondack pyroxenes. Sample localities of rocks con- textures. In mangerites and charnockites they are taining pyroxenes which are discussed in this paper usually microphenocrysts. Coarsely exsolved ortho- are shown in Figure 2. Sample locality descriptions rhombic and monoclinic pyroxenes commonly coexist are compiled in Appendix I. in many anorthosites, mangerites, and charnockites. Substantial amounts of exsolved augite are contained in orthopyroxenes and hypersthene in clinopyroxene Analytical Procedure (Fig. 1). Although the coarse low calcium pyroxene is now an orthopyroxene host, average analysis of Quantitative electron microprobe analyses were obtained in order these pyroxene grains (Table 1) yield 7-10 mole % to interpret the significance of the.several Adirondack pyroxene Wo component. This indicates that they were once types. Major and minor element analyses were obtained using an pigeonites which have subsequently changed to a mix- ARL-EMX electron microprobe analyzer with wavelength disper- sive PET, LiF, and TAP crystal spectrometers. An accelerating ture of ortho- and clinopyroxene. Similarly, reinte- potential of 15 keV and an emission current of 150 # A were stan- grated coarse clinopyroxene analyses (Table 1) clearly dard operating conditions. Sample current was typically .005- indicate that the grains were very subcalcic augites .008 # A. Beamcurrent was digitized with counting times of 25-35 s before a low-calcium phase exsolved from them. Very for each point analyzed. For homogeneousunknowns spectrometer small amounts of Fe-oxide, typically ilmenite, occur data were taken on 5 different points on a single grain and were averaged to obtain an analysis. Obtaining average analyses of as thin "exsolution" lamellae in both ortho- and cli- coarsely exsolvedpyroxenes is discussed in detail below. Natural, nopyroxene. In the same rocks with the exsolved py- well-analyzed almandine-rich garnet (Ingamells, unpublished), A1- roxenes are equant ortho- and clinopyroxenes which rich clinopyroxene(Irving, written comm.), Fe-rich kaersutite