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

Abstract

The Ongole domain of the Proterozoic Eastern Ghats Belt, India, is dominated by charnockites and enderbites with enclaves of migmatitic metapelites. Reaction textures indicate two generations of mineral growth. Spinel–hercynite solid solution+quartz-bearing Fe–Al granulites provide critical evidence for an ultrahigh-temperature (UHT) metamorphism. The association of spinel and quartz, now separated by garnet and sillimanite coronas, suggests that the rocks underwent T>950°C peak metamorphism at P=6.5–7kbar, as constrained from P–T pseudosection. Coarse-grained orthopyroxene in garnet+cordierite-bearing metapelites shows the highest Al2O3 content up to 8.2wt.%, suggesting T=950–1000°C. Temperatures estimated from mesoperthite and plagioclase pairs and other geothermometers (T=900–1000°C) further support ultrahigh-temperature metamorphism in the Ongole domain. The absence of cordierite in retrograde mineral assemblages points to isobaric cooling, giving rise to a near isobaric heating–cooling trajectory. The second generation of garnet, occurring as thick overgrowths over coarse-grained garnet porphyroblasts, in metapelites and especially in charnoenderbites have higher grossular contents than the porphyroblasts. Thermobarometry and P–T pseudosections indicate that they formed at a higher pressure but lower temperature (ca. 780°C, 9.5kbar) during a second metamorphic event. Orthopyroxene+sillimanite±kyanite±spinel symplectites replacing cordierite is most likely formed during this second metamorphic event. The presence of sillimanite, kyanite as well as andalusite along with coarse-grained retrograde biotite in the same rock marks the last stage of the second metamorphic event characterized by isobaric cooling just below the aluminosilicate triple point. This further suggests near-isothermal decompression from higher pressures. UHT metamorphism at low pressures during the first metamorphic event is most likely caused by magma emplacement. The higher pressure but lower temperature conditions achieved during the second metamorphic event is due to crustal thickening during collision of continental blocks. Near-isothermal decompression to ca. 4kbar points to subsequent rapid exhumation of the over thickened crust. The two-stage evolutionary history of the Ongole domain fits well to the expected evolution of a magmatic arc, in which the first can be attributed to magmatic heat advection during arc growth and the second to crustal thickening during collision.