Metamorphic History of Sapphirine- and Relict-Kyanite-bearing Mg-Al Rich Rocks at Hatimunda Hill near Jumagarh, 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 Ghats belt. Peak mineral assemblages in these rocks included phlogopite, bronzite, kyanite (now reliet) and quartz/corundum. Sapphirine typically occurs as corona around corundum or in chisters along with corundum. The sapphirine is slightly more aluminous than the MgO:Al₂O₃:SiO₂ ratio of 7:9:3. Textural criteria and phase analysis in the model system MAS indicate the follwing reactions: Opx+Sil+Qtz+Crd; Opx+Sil=Crd+Crn; and Opx+Crn=Spr+Crd. A qualitative petregenetic grid shows that these reactions may be crossed in sequence along a decompressional P-T trajectory. The rocks preserve textural evidence of formation of sapphirine, at least in part, from phlogopite and sillimanite. P-T conditions for peak metamorphism were 700-800° C and > 8kbar based on the stability field of mineral assemblages. Texturally, early kyanite and late sillimanite indicate that the decompression is possibly part of a clockwise P-T path. This interpretation is supported by the occurrence of Mg-rich garnet (X_prp + 0.47) in the amphibole-dominated Mg-Al rich schist of the neighbouring Jilindara area.