Reaction textures in a suite of spinel granulites from the Eastern Ghats Belt, India: Evidence for polymetamorphism, a partial petrogenetic grid in the system KFMASH and the Roles of ZnO and Fe₂O₃

Abstract

Spinel granulites, with or without sapphirine, occur as lenses in garnetiferous quartzofeldspathic gneisses (leptynites) near Gokavaram in the Eastern Ghats Belt, India. Spinel granulites are mineralogically heterogeneous and six mineral associations occur in closely spaced domains. These are (I) spinel-quartz-cordierite, (II) spinel-quartz-cordierite-garnet-orthopyroxene-sillimanite, (III) spinel-cordierite-orthopyroxene-sillimanite, (IV) spinel-quartz-sapphirine-sillimanite-garnet, (V) spinel-quartz-sapphirine-garnet and (IV) rhombohedral (Fe-Ti) oxide-cordierite-orthopyroxene-sillimanite. Common to all the associations are a porphyroblastic garnet (containing an internal schistosify defined by biotite, sillimanite and quartz), perthite and plagioclase. Spinel contains variable amounts of exsolved magnetite and is distinctly Zn rich in the sapphirine-absent associations. X_Mg in the coexisting phases decreases in the order cordierite-biotite-sapphirine-orthopyroxene-spinel-garnet-(Fe-Ti) oxides. Textural criteria and compositional characteristics of the phases document several retrograde mineral reactions which occurred subsequent to prograde dehydration melting reactions involving biotite, sillimanite, quartz, plagioclase and spinel. The following retrograde mineral reactions are deduced: (1) spinel + quartz → cordierite, (2) spinel + quartz → garnet + sillimanite, (3) garnet + quartz → cordierite + orthopyroxene, (4) garnet + quartz + sillimanite → cordierite, (5) spinel + cordierite → orthopyroxene + sillimanite, (6) spinel + sillimanite + quartz → sapphirine, (7) spinel + sapphirine + quartz → garnet + sillimanite, and (8) spinel + quartz sapphirine + garnet. A partial petrogenetic grid for the system FeO-MgO-Al₂O₃-SiO₂-K₂O-H₂O at high fo₂, has been constructed and the effects of ZnO and Fe₂O₃ on this grid have been explored Combining available experimental and natural occurrence data, the high fo₂ invariant points in the partial grid have been located in P-T space. Geothermobarometric data and consideration of the deduced mineral reactions in the petrogenetic grid show that the spinel granulites evolved through an anticlockwise P-T trajectory reaching peak metamorphic conditions >9 kbar and 950 °C, followed by near-isobaric cooling (dT/dP = 150 °C/kbar). This was superimposed by an event of near-isothermal decompression (dT/dP = 15 °C/kbar). The studied spinel granulites, therefore, preserve relic prograde mineral associations and reaction textures despite being metamorphosed at very high temperatures, and bear evidence of polymetamorphism.