Arc-Related Pyroxenites Derived from a Long-Lived Neoarchean Subduction System at the Southwestern Margin of the Cuddapah Basin: Geodynamic Implications for the Evolution of the Eastern Dharwar Craton, Southern India

Mineralogy and bulk-rock geochemistry (including Sr and Nd isotopes) of two ultramafic (pyroxenite) intrusions at Gurramkonda, situated adjacent to the southwestern margin of the Paleo-Mesoproterozoic Cuddapah Basin within the Eastern Dharwar Craton, southern India, are presented. The pyroxenites have a characteristic cumulate texture, with a predominance of pyroxene (augite) and amphibole (hornblende and actinolite), whereas chromite is present mostly as inclusions in pyroxene. Mineral chemistry of these ultramafics is akin to that of arc cumulate rocks, particularly those associated with the Alaskan-type complexes, formed in suprasubduction zones. High Mg#'s (72-74), coupled with high bulk-rock Ni (830-1130 ppm) and Cr (1110-1,360 ppm), strikingly correspond to those of continental arc cumulates. The enrichment of large ion lithophile elements over high field strength elements, conspicuous negative Nb-Ta, Zr-Hf, and Ti anomalies, and chondrite-normalized light rare earth element patterns highlight their arc affinity. Fluid-related modification of the mantle source is apparent from the various trace-element ratios. Negative epsilon Nd-2.15Ga values (-0.93 to -2.90) also support contribution of an enriched-mantle reservoir in their genesis. Partial melting of this fertile mantle source has been attributed to the Paleoproterozic lithospheric extension with heat contributions from a mantle plume or convective asthenosphere. Ultramafic intrusions of arc-related geochemical affinity thus constitute rare, but genetically important, occurrences within the Eastern Dharwar Craton. We propose that, together with the recently studied calc-alkaline lamprophyres occurring immediately toward the western margin of the Cuddapah Basin, the Gurramkonda pyroxenites are manifestations of a long-lived and major subduction system of Neoarchean age in the Eastern Dharwar Craton, southern India.