Metamorphic history of sapphirine- and relict-kyanite-bearing Mg-Al rich rocks at Hatimunda Hill near Junagarh, Kalahandi District, Orissa

Cordierite-phlogopite dominated Mg-AL rich rocks occur as small enclaves at the Hatimunda hill near Junagarh in the Orissa sector of the Eastern Chats belt. Peak mineral assemblages in these rocks included phlogopite, bronzit e, kyanite (now relict) and quartz/corundum. Sapphirine typically occurs as corona around corundum or in clusters along with corundum. The sapphirine is slightly more aluminous than the MgO:AL(2)O(3):SiO2 ratio of 7:9:3. Text ural criteria and phase analysis in the model system MAS indicate the follo wing reactions: Opx + Sil + Qtz = Crd; Opx + Sil = Crd + Crn; and Opx + Cm = Spr + Crd. A qualitative petrogenetic grid shows that these reactions may be crossed in sequence along a decompressional P-T trajectory. The rocks p reserve textural evidence of formation of sapphirine, at least in part, fro m phlogopite and sillimanite. P-T conditions for peak metamorphism were 700 -800 degreesC and >8 kbar based on the stability field of mineral assemblag es. Texturally, early kyanite and late sillimanite indicate that the decomp ression is possibly part of a clockwise P-T path. This interpretation is su pported by the occurrence of Mg-rich garnet (X-Prp = 0.47) in the amphibole -dominated Mp-Al rich schist of the neighbouring Jilindara area.