P-T-X relationships deduced from corona textures in sapphirine-spinel-quartz assemblages from paderu, southern India

Abstract

The sapphirine (Sa)-spinel (Sp)-quartz (Qz)-bearing rocks from Paderu occur as lenticular enclaves within the Precambrian khondalitecharnockite terrane of southern India. In addition these rocks contain orthopyroxene (Opx), sillimanite (Sill), garnet (Gt), cordierite (Cd), biotite, potash feldspar (Kf), plagioclase, and symplectites of Cd-Kf-Qz-Opx. The symplectites may have formed from the breakdown of osumilite. Grain contacts of sapphirine and spinel with quartz are rarely observed and the incompatibility with quartz during later stages is displayed by the development of several types of polymineralic reaction coronas. The coronas in the different rock types A, B, etc. are (minerals listed from core to rim of corona): (A-1) sapphirine-bearing rock type without spinel: Sa-Sill-Opx, Sa-Sill-Cd, Sa-Cd-Opx (A-2) sapphirine and spinel-bearing: Sp-Sa-Sill-Opx-Qz, Sp-Sa-Sill, Sp-Sa-Opx, Sp-Sill-Opx, Sp-Sa-Sill-Gt-Qz, Sa-Sill-Opx, Sp-Sa-Sill-Opx, Sa-Sill-Opx-Gt, Sp-Sa-Opx-Gt, Sp-Sa-Sill-Gt; and (B) spinel-bearing but sapphirine free: Sp-Sill-Opx, Sp-Sill-Gt, Sp-Cd. Commonly the coronas in the rock type A 2 and B also contain ilmeno-hematite±corundum in the core in association with spinel. These rock types also provide textural evidence for later crystallization of Cd, Cd + Sa, and Gt + Qz from Opx+Sill±Qz and Gt+Sill+Qz. Sapphirine is aluminous (near 7(Mg, Fe²⁺)O.9(Al, Fe³⁺)₂O₃.3SiO₂) and contains up to 12.2 wt. per cent iron as FeO. Orthopyroxene is also aluminous, containing up to 10.4 wt. per cent Al₂O₃. Sapphirine and spinel have relatively high contents of Fe₂O₃. X_Mg in the Fe-Mg minerals increases from rock type B to A2 to A1. A sequence of reactions has been deduced from coronas and other reaction textures, and from the phase compatibility relations in the FeO-MgO-Al₂O₃-SiO₂-H₂O system. The P-T-X relationships from geothermobarometry and petrogenetic grids, viz. μ_Fe2O3 vs. μ_FeO and μ_H2O vs. μ_Fe2O3, suggest: (1) a retrograde, mildly decompressive trajectory from 900±60°C/65±0.7 kb (core) to 760±50°C/5 ± 0.6 kb (rim); and (2) the observed mineralogy of the coronas and reactions deduced from them are dependent on the relative FeO, Fe₂O₃, and H₂O contents of the rocks (μ_FeO3, μ_Fe2O3), and μ_H2O).