Metasomatic alteration of chromite from parts of the late Archaean Sittampundi Layered Magmatic Complex (SLC), Tamil Nadu,India

Chromite deposits associated with layered anorthosite complexes in the Archaean high-grade terranes are rare in the world. The late Archaean Sittampundi Layered Magmatic Complex, Tamil Nadu, India is one of the few such deposits in the world where layers of Fe-Al rich chromites are associated with extremely calcic (An>95) anorthosite. ‘Frozen in’ magmatic mineralogy of the chromitite and the enclosing anorthosite suggest successive crystallization of chromite + clinopyroxene and chromite + clinopyroxene + anorthite from a hydrous Al-rich basaltic melt that was emplaced in a suprasubduction zone setting. Intense deformation and upper amphibolite facies metamorphism at ∼2.45 Ga converted the magmatic assemblages to hitherto unreported hornblende + gedrite + Mg-Al rich spinel ± chlorite bearing assemblages. During metamorphic reconstitution, chromite was pseudomorphically replaced by green spinel in the domains rich in secondary amphiboles. Mass-balance calculation and algebraic analyses of the observed mineralogy suggest that a number of chemical species including chromium became mobile during the formation of spinel pseudomorph in response to infiltration driven metamorphism. Aluminium became mobile in the length scale of chromite grain but remained immobile in the length scale of a thin section.