Sapphirine and corundum bearing ultrahigh temperature rocks from the Palghat-Cauvery Shear System, southern India

Abstract

We report a new occurrence of sapphirine- and corundum-bearing silica-deficient Mg-Al rocks from Paramati at the northern margin of the Palghat-Cauvery Shear System which define the Archaean-Proterozoic collisional boundary in southern India. Sapphirine here occurs as medium-grained euhedral to subhedral crystals enclosed within plagioclase and contains inclusions of spinel and sillimanite, and is locally intergrown with corundum, suggesting the reaction Spl + Sil → Spr + Crn. Temperatures estimated from sapphirine-spinel equilibria lie around 930-950°C, suggesting ultrahigh-temperature peak metamorphic conditions. Gedrite is also a common rock forming mineral in these rocks, and coexists with cordierite, corundum, and sillimanite. As gedrite is totally surrounded by cordierite, we infer a prograde reaction Ged + Sil → Crn + Crd + V. Lack of orthopyroxene in our samples suggests a decompressional P-T history following the prograde event. Our results suggest that the whole of northern Madurai Block as well as parts of the Palghat-Cauvery Shear System experienced ultrahigh temperature metamorphism followed by exhumation along a clockwise P-T path.