High-grade metamorphic rocks from Christmas Point in the Napier Complex, East Antarctica

• Main Authors: Yasutaka Yoshimura, Tomoharu Miyamoto, Edward S. Grew, Christopher J. Carson, Daniel J. Dunkley, Yoichi Motoyoshi
• Format: Report
• Language: English
• Published: Department of Natural Environmental Science, Kochi University/Department of Earth and Planetary Science, Kyushu University/Department of Geological Science, University of Maine/Department of Geology and Geophysics, Kline Geology Laboratory, Yale University/School of Geosciences, Edgeworth David Building, University of Sydney/National Institute of Polar Research 2001
• Subjects: UHT metamorphic rocks; UHT metamorphism; Christmas Point; Napier Complex; East Antarctica; Napier; Antarc*; Antarctica; Polar geoscience
• Online Access: https://nipr.repo.nii.ac.jp/?action=repository_uri&item_id=3085; http://id.nii.ac.jp/1291/00003085/; https://nipr.repo.nii.ac.jp/?action=repository_action_common_download&item_id=3085&item_no=1&attribute_id=18&file_no=1

The Christmas Point area is dominated by well-layered gneisses, of which garnet-orthopyroxene gneiss and garnet felsic gneiss are the most abundant types. Orthopyroxene-bearing quartz-rich granulite, layering of which is indistinct to massive, is present as layers within the garnet felsic gneiss. Garnet-amphibole gneiss is present as layers in both the garnet-orthopyroxene gneiss and the garnet felsic gneiss. Orthopyroxene-bearing quartz-rich granulites contain coexisting orthopyroxene, sillimanite and quartz. The occurrence of sapphirine as inclusions in sillimanite implies that metamorphic conditions changed from the stability field of sapphirine+quartz to that of orthopyroxene+sillimanite+quartz. P-T conditions estimated using garnet-orthopyroxene geothermobarometry range from 920 to 1040℃ and 8.8 to 11.5 kbar. Garnet exsolution lamellae present in orthopyroxene from the garnet-orthopyroxene gneiss suggest that a precursor aluminous orthopyroxene was present during the ultrahigh-temperature stage. For the retrograde event, although chemical zoning of major element composition of garnet developed during cooling and reflects high-temperature diffusion, compositional zoning of more slowly diffusing elements, such as Ca, show a complicated pattern caused by partial resorbtion and overgrowth. Garnet rims in all rock types are intergrown with quartz and have relatively high grossular composition. Plagioclase in the garnet-orthopyroxene gneiss and garnet felsic gneiss show reverse zoning. Garnet-amphibole gneiss contains two amphiboles (gedrite and cummingtonite) with different fluorine contents; gedrite occurs as finely crystalline overgrowths surrounding cummingtonite, implying that the retrograde process after peak (ultrahigh-temperature) metamorphism was not simple cooling.