RETROGRADE PRESSURE-TEMPERATURE PATH FOR SPINEL-BEARING METAPELITES IN RAYAGADA, EASTERN GHATS, INDIA

Two assemblages have been identified in spinel-bearing metapelites fro m Rayagada, in the Eastern Ghats Belt, India: the first, an iron-rich assemblage, is characterized by iron-rich spinel and garnet; the secon d, iron-poor assemblage, by iron-poor spinel and garnet, together with cordierite and hematite. Garnet and sillimanite coronas around spinel show elongation parallel to the dominant fabric in the iron-rich asse mblage but not in the iron-poor one. Both textures suggest the corona forming retrograde reaction spl + qtz = grt + sil. The P-T conditions for these. corona-forming reactions are estimated at similar to 950 de grees C and 8.7-9.0 kbar for the iron-rich, and similar to 800 degrees C and 8 kbar for the iron-poor assemblage. Thermobarometric results a nd observed phase relations, using the model system FeO-MgO-Al2O3-SiO2 (FMAS) indicate near-isobaric cooling from similar to 950 degrees to similar to 800 degrees C and subsequent decompression from similar to 8 to similar to 6.5 kbar. The corona-forming event in the iron-rich as semblage has been correlated with the dominant second phase of deforma tion (D-2), but that in the iron-poor assemblage apparently postdates D-2.