Modeling of corundum breakdown textures in anorthosite Sittampundi layered magmatic complex, South India.

Abstract

The highly calcic anorthosite (An>95) from the Sittampundi Layered Complex (SLC) develops corundum, spinel and sapphirine. The SLC is a magmatic-layered complex consisting of an interlayered sequence of anorthosite, chromite-bearing clinopyroxenite, and chromite-free mafic-ultramafic rocks. The members of the SLC were emplaced at 2.8–2.9 Ga and were subsequently metamorphosed at ~ 2.48 Ga under granulite-facies conditions (12 kbar, 800 C) with a steep decompressive retrograde P-T-path (7 kbar, 730 C). The studied anorthosite is milky white and highly recrystallized imparting a saccharoidal appearance. It contains laterally discontinuous dark bands of amphibole. Locally, millimeter- to centimeter-thick discontinuous layers of corundum are found in the anorthosite.Corundum is always separated from amphibole by successive coronae of green spinel (proximal to corundum) and plagioclase (toward the amphibole-plagioclase matrix). Sapphirine forms around spinel making the contact between anorthite and corundum highly irregular. The textures indicate the following sequence of mineral growth: plagioclasematrix → corundum; corundum + amphibole → plagioclasecorona+ spinel; and spinel + corundum → coronitic sapphirine. Topological constraints in parts of the Na2O–CaO–MgO–Al2O3–SiO2–H2O (NCMASH) system suggest that aqueous fluid(s) permeated the rock and the assemblage corundum + amphibole + anorthite ± clinozoisite was stabilized during high-pressure (HP) metamorphism (11 ± 2 kbar, 750 ± 50 °C). Constraints of the NCMASH topology also suggest that coronitic plagioclase and spinel formed at the expense of corundum + amphibole during a steeply decompressive retrograde P–T path (7–8 kbar and 750 ± 50 °C) in an open system. Chemographic projections in the MAS system, combined with textural modeling studies support the view that the peraluminous sapphirine formed from due to silica and Mg metasomatism of the precursor spinel + corundum, on the steeply decompressive retrograde P–T path. Extremely channelized fluid flow and the positive solid volume change of the stoichiometrically balanced sapphirine forming reaction explains the localized growth of sapphirine.