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

Spinel granulites, with or without sapphirine, occur as lenses in garn etiferous quartzofeldspathic gneisses (leptynites) near Gokavaram in t he Eastern Ghats Belt, India. Spinel granulites are mineralogically he terogeneous and six mineral associations occur in closely spaced domai ns. These are (I) spinel-quartz-cordierite, (II) uartz-cordierite-garn et-orthopyroxene-sillimanite, (III) spinel-cordierite-orthopyroxene-si llimanite, (IV) spinel-quartz-sapphirine-sillimanite-garnet, (V) spine l-quartz-sapphirine-garnet and (IV) rhombohedral (Fe-Ti) oxide-cordier ite-orthopyroxene-sillimanite. Common to all the associations are a po rphyroblastic garnet (containing an internal schistosity defined by bi otite, sillimanite and quartz), perthite and plagioclase. Spinel conta ins variable amounts of exsolved magnetite and is distinctly Zn rich i n the sapphirine-absent associations. X(Mg) in the coexisting phases d ecreases in the order ite-sapphirine-orthopyroxene-spinel-garnet-(Fe-T i) oxides. Textural criteria and compositional characteristics of the phases document several retrograde mineral reactions which occurred su bsequent 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 --> cor dierite + orthopyroxene, (4) garnet + quartz + sillimanite --> cordier ite, (5) spinel + cordierite --> orthopyroxene + sillimanite, (6) spin el + sillimanite + quartz --> sapphirine, (7) spinel + sapphirine + qu artz --> garnet + sillimanite, and (8) spinel + quartz --> sapphirine + garnet. A partial petrogenetic grid for the system FeO-MgO-Al2O3-SiO 2-K2O-H2O at high f(O2) has been constructed and the effects of ZnO an d Fe2O3 on this grid have been explored. Combining available experimen tal and natural occurrence data, the high f(O2) invariant points in th e partial grid have been located in P-T space. Geothermobarometric dat a and consideration of the deduced mineral reactions in the petrogenet ic grid show that the spinel granulites evolved through an anticlockwi se P-T trajectory reaching peak metamorphic conditions >9 kbar and 950 degrees C, followed by near-isobaric cooling (dT/ dP = 150 degrees C/ kbar). This was superimposed by an event of near-isothermal decompress ion (dT/dP = 15 degrees C/kbar). The studied spinel granulites, theref ore, preserve relic prograde mineral associations and reaction texture s despite being metamorphosed at very high temperatures and bear evide nce of polymetamorphism.