Abstract
Anorthosites, mangerites and charnockites from metamorphosed anorthosite massifs (the Adirondacks and elsewhere) commonly contain coarsely exsolved pyroxenes with substantial amounts of exsolved orthopyroxene (in clinopyroxene) and clinopyroxene (in orthopyroxene). Electron microprobe reintegration of such pyroxenes yields compositions which indicate that pigeonite and subcalcic augite coexisted before metamorphic reequilibration. Equilibration temperatures of 1100 °±100 °C for anorthosite and 1000 °C±100 °C for mangerites and charnockites are inferred from the solvus of Ross and Huebner (1975). These temperatures constrain minimum magmatic thermal conditions and suggest that the magmas were relatively dry. Exsolution lamellae of coarse pyroxenes and small equant coexisting pyroxenes (with little or no exsolution) yield temperatures of ≦ 750 °C, consistent with equilibration during granulite facies metamorphism. Relict igneous textures and compositions persisted through the metamorphic event due to dry P(H2O) ≪P(solid) metamorphic conditions. The reintegrated pyroxene compositions provide a window through the metamorphism and yield constraints on the pre-metamorphic igneous events.
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Contribution No. 340 from the Mineralogical Laboratory, Department of Geology and Mineralogy, The University of Michigan, Ann Arbor, Michigan, 48109, U.S.A.
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Bohlen, S.R., Essene, E.J. Igneous pyroxenes from metamorphosed anorthosite massifs. Contr. Mineral. and Petrol. 65, 433–442 (1978). https://doi.org/10.1007/BF00372290
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DOI: https://doi.org/10.1007/BF00372290