Chromite-silicate chemistry of the Neoarchean Sittampundi Complex, southern India: Implications for subduction-related arc magmatism

The Neoarchean layered anorthositic complex at Sittampundi in southern India is known for its chromitite layers that are mostly associated with anorthosite (An(90-100)). The chromitites contain FeAl-rich chromites concentrated in layers between amphibole-rich layers with a dominant mineralogy of amphibole-spinel-plagiocase+/-sapphirine. The chromite-rich layers contain only amphibole and plagioclase. Mineral compositions illustrated by X-ray composition maps and profiles show subtle chemical differences. The chrome spinels are of refractory grade with Cr2O3 and Al2O3 contents varying between 34-40 wt.% and 23-28 wt.%. The chromite compositions are noticeably different from those in layered igneous intrusions of the Bushveld-Stillwater type. The existence of original highly calcic plagioclase, FeAl-rich chromite, and magmatic amphibole is consistent with derivation from a parental magma of hydrous tholeiitic composition that was most likely generated in a supra-subduction zone arc setting. In terms of mineralogy and field relations, the Sittampundi chromitites are remarkably similar to anorthosite-hosted chromitites in the Neoarchean Fiskensset anorthositic complex, Greenland. We propose that the Sittampundi chromitites formed by partial melting of unusually aluminous harzburgite in a hydrated mantle wedge above a subduction zone. This melting process produced hydrous, aluminous basalt, which fractionated at depth to give rise to a variety of high-alumina basalt compositions from which the anorthositic complex with its cumulate chromite-rich and amphibole-rich layers formed within the magma chamber of a supra-subduction zone arc. (C) 2011 Elsevier B.V. All rights reserved

Study of calc-silicate rocks of Hammer-Head Syncline from southern Sandmata Complex, northwestern India: implications on existence of an Archaean protolith

Existence of an Archaean protolith is suggested in present study from an ensemble of rocks named as Sandmata Complex from northwestern India which have a debatable stratigraphic status of Archaean vs. Proterozoic. Rocks of the Sandmata Complex are represented by a highly metamorphosed volcano-sedimentary complex with multiple cycles of deformation. The manifold tectono-thermal events have obscured the pristine character of the protoliths. In this work we present geochemical features of calc-silicate protolith that show consistent Archaean affinity in the Hammer-Head Syncline (HHS) from southern part of the Sandmata Complex. Notable geochemical characteristics of calc-silicate metasediments in the HHS include high Th/U, high Cr concentrations, high La/Th, moderate La/Yb, and weak positive Eu anomaly. Carbon and oxygen stable isotope compositions of these carbonate metasediments vary between -3.0 and -0.3‰ (δ13Ccarb),- 11.6 and -35.0 (δ13Corg) and -19.1 and -13.4‰, (δ18O) respectively. These geochemical observations are in conjunction with the recently published Neoarchaean ages from the HHS and the proximal Hooke syncline