Petrology of granulites from Anakapalle-evidence for Proterozoic decompression in the Eastern Ghats, India

Abstract

The granulite complex at Anakapalle, which was metamorphosed at ~1000 Ma, comprises orthopyroxene granulites, leptynite, khondalite, mafic granulites, calc-silicate rock, spinel granulites, and two types of sapphirine granulites-one quartz-bearing and migmatitic and the other devoid of quartz and massive. Reaction textures in conjunction with mineral-chemical data suggest several continuous and discontinuous equilibria in these rocks. In orthopyroxene granulites, dehydration-melting of biotite in the presence of quartz occurred according to the reaction biotite+quartz= garnet (Py₃₇)+K-feldspar+orthopyroxene + liquid. Later, this garnet broke down by the reaction garnet (Py₃₇)+quartz = orthopyroxene + plagioclase. Subsequently, coronal garnet (Py₃₀) and quartz were produced by the same reaction but proceeding in the opposite direction. In spinel granulites, garnet (Py₄₂) and sillimanite were produced by the breakdown of spinel in the presence of quartz. In the two types of sapphirine granulites, garnet with variable pyrope content broke down according to the reaction garnet = sapphirine + sillimanite + orthopyroxene. The highest pyrope content (59 mol %) was noted in garnets from quartz-free sapphirine granulites compared with the quartz-bearing one (53 mol % pyrope). The calculated positions of the mineral reactions and diserete P-T points obtained by thermobarometry define a retrograde P-T trajectory during which a steep decompression of ~1.5 kbar from P-T_max of 8 kbar and 900 °C was followed by near-isobaric cooling of ~300 °C. During this decompression, garnet with variable pyrope contents in different rocks broke down on intersection with various divariant equilibria. Near-isobaric cooling resulted in the formation of coronal garnet around second-generation orthopyroxene and plagioclase replacing earlier porphyroblastic garnet in orthopyroxene granulites. It has been argued that the deduced P-T trajectory originated in an extensional regime involving either a crust of near-normal thickness of a slightly overthickened crust owing to magmatic underaccretion.