Petrogenesis of the late Archean Pillow Basalts from the Chitradurga greenstone belt, Western Dharwar Craton (southern India)

The well-preserved exposures of pillow basalts from the late Archean (2.7 Ga) Chitradurga greenstone belt (Western Dharwar Craton) have been studied in detail using petrological, bulk-rock geochemical and isotope data. The pillows are compositionally basalts to basaltic andesites, and constituted of actinolite and plagioclase. The pillows show slight to moderately depleted LREE (La/Sm-N = 0.6-1.03), and nearly flat HREE patterns (Gd/Yb-N = 1.03-1.17) that are comparable with MORBs, but have an overall depletion in REEs relative to the latter. The trace-element patterns of the pillows on an N-MORB normalized multi-element diagram, however, are broadly comparable with that of Island Arc Basalts. The pillows were perhaps generated during the initiation of an intra-oceanic subduction zone where a depleted upper mantle (MORB-source) was metasomatized with slab-derived aqueous fluids. The pillows show a whole-rock Sm-Nd errorchron age of 2433 +/- 400 Ma, which is within error of the previously reported Sm-Nd age of 2747 +/- 15 Ma. The Pb-Pb isochron age (2627 +/- 82 Ma) of the pillows is much closer to the previously reported Sm-Nd age, and it may also indicate the 2.62 Ga thermal event associated with felsic magmatism in the Western Dharwar Craton.