Retrograde pressure-temperature path for spinel-bearing metapelites in Rayagada, Eastern Ghats, India

Abstract

Two assemblages have been identified in spinel-bearing metapelites from Rayagada, in the Eastern Ghats Belt, India: the first, an iron-rich assemblage, is characterized by iron-rich spinel and garnet; the second, 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 assemblage 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 ∼950°C and 8.7-9.0 kbar for the iron-rich, and ∼800°C and 8 kbar for the iron-poor assemblage. Thermobarometric results and observed phase relations, using the model system FeO-MgO-Al2O3-SiO2 (FMAS) indicate near-isobaric cooling from ∼950° to ∼800°C and subsequent decompression from ∼8 to ∼6.5 kbar. The corona-forming event in the iron-rich assemblage has been correlated with the dominant second phase of deformation (D2), but that in the iron-poor assemblage apparently postdates D2.