Pyroxene exsolution in mafic granulites from the Grove Mountains, East Antarctica: constraints on Pan-African metamorphic conditions

摘要

Exsolution textures were observed in clinopyroxenes in mafic granulites from the Grove Mountains, East Antarctica. The first generation of exsolution occurs only in subcalcic augite megacrysts, in which coarse orthopyroxene lamellae exsolved parallel to the (100) plane. Electron microprobe reintegration of such augites yields a pre-metamorphic igneous temperature of ca. 970 ℃. The second generation of exsolution develops in both igneous and metamorphic clinopyroxenes. In metamorphic clinopyroxenes, two sets of very narrow pigeonite lamellae, subsequently inverted to orthopyroxene, exsolved parallel to (100) and (001) planes, respectively with 0°and 113°deviation to c-axis of the host. A peak metamorphic temperature of ca. 850 ℃ was obtained from the reintegrated compositions of exsolved clinopyroxene, and an exsolution temperature of 740-770 ℃ from the orientations of exsolved lamellae. The preservation of augite megacrysts and appearance of equilibrium assemblages suggests that mafic granulites from the Grove Mountains were formed during a single episode of high-grade metamorphism. Temperature estimates fro original pyroxene crystallization and pigeonite exsolution from clinopyroxene host provide a good constraint on the peak conditions for this metamorphic event. In contrast with the Prydz Bay area, it is proposed that the Pan-African metamorphism in East Antarctica had reached a higher temperature of ca. 850 ℃, an later underwent a slow cooling process, which probably resulted from the magmatic underplating of the lower crust.