Two-stage granulite formation in a Proterozoic magmatic arc (Ongole domain of the Eastern Ghats Belt, India): Part 1. Petrology and pressure–temperature evolution

摘要: Abstract The Ongole domain of the Proterozoic Eastern Ghats Belt, India, is dominated by charnockites and enderbites with enclaves migmatitic metapelites. Reaction textures indicate two generations mineral growth. Spinel–hercynite solid solution + quartz-bearing Fe–Al granulites provide critical evidence for an ultrahigh-temperature (UHT) metamorphism. association spinel quartz, now separated garnet sillimanite coronas, suggests that rocks underwent T  > 950 °C peak metamorphism at P  = 6.5–7 kbar, as constrained from – pseudosection. Coarse-grained orthopyroxene in garnet + cordierite-bearing metapelites shows highest Al 2 O 3 content up to 8.2 wt.%, suggesting  = 950–1000 °C. Temperatures estimated mesoperthite plagioclase pairs other geothermometers (  = 900–1000 °C) further support domain. absence cordierite retrograde assemblages points isobaric cooling, giving rise a near heating–cooling trajectory. second generation garnet, occurring thick overgrowths over coarse-grained porphyroblasts, especially charnoenderbites have higher grossular contents than porphyroblasts. Thermobarometry pseudosections they formed pressure but lower temperature (ca. 780 °C, 9.5 kbar) during metamorphic event. Orthopyroxene + sillimanite ± kyanite ± spinel symplectites replacing most likely this presence sillimanite, kyanite well andalusite along biotite same rock marks last stage event characterized cooling just below aluminosilicate triple point. This near-isothermal decompression pressures. UHT low pressures first caused magma emplacement. conditions achieved due crustal thickening collision continental blocks. Near-isothermal ca. 4 kbar subsequent rapid exhumation thickened crust. two-stage evolutionary history fits expected evolution magmatic arc, which can be attributed heat advection arc growth collision.