Petrography and textural modeling from a suite of sapphirine-cordierite-corundum-sillimanite bearing orthoamphibolite from parts of the southern granulite terrane, India

Abstract

The Granulitic Terrane of South India exposes diverse variety of late mafic and ultramafic enclaves, hosted within late Archaean felsic orthogneiss. These Archean rocks are dissected by a number of crustal scale shear zones collectively named as Cauvery Shear System (CSS). The eastern part of CSS exposes intensely deformed mafic and felsic granulites that are extensively altered to amphibolite facies ensembles. This study focuses on one of such highly retrogressed rock-units near Sevitthurangampatti that is dominated by ortho-amphibole (gedrite) ± kyanite/sillimanite and contains rare and exotic mineral assemblages of sapphirine+cordierite+corundum. Microscopic study reveals that symplectic intergrowth of these three minerals forms corona and pseudomorphs over sillimanite. Petrographic study aided with mineral chemical analysis suggests a two stage reaction that led to the formation of sapphirine-cordierite symplectite and corundum-cordierite successively, at the expense of gedrite and sillimanite. Textural modeling of the pseudomorph and corona suggest the following balanced reactions with several mobile elements like Mg, Fe, Na and SiO2 were responsible for the formation of these assemblages. (1) 5.80 Gedrite + 30.69 Sillimanite + 3.32 Mg2+ = 2.04 Sapphirine + 13.67 Cordierite + 1.00 Na+ + 2.30 Fe2+ + 5.80 H2O (2) 130.10 Sillimanite + 12.38 Sapphirine + 1.00 Na++ = 147.30 Corundum + 20.00 Cordierite + 48.09 SiO2 + 1.09 Fe2+ Brine-rich aqueous fluids can potentially play an important role in this element transfer process. Calculation of petrogenetic grid in MASH system constrains a decompressive P-T path from 760-880°C and 6.5-8.5 kbar. At least a part of the CSS experienced infiltration-induced retrogression at granulite grade conditions during the Pan-African time.