Petrological and geochemical characterization of Proterozoic ophiolitic mélange, Nellore-Khammam schist belt, SE India

The ophiolitic mélange occurs close to a major fault within the metavolcanics of Kanigiri area in the western margin of the Khammam-Nellore schist belt, Andhra Pradesh, India. The unique feature of Kanigiri mélange is the occurrence of ophiolitic bodies of various sizes, most of which display petrological and geochemical characteristics typical of supra-subduction zone oceanic crust. The ophiolitic mélange is composed mainly of blocks of variably deformed and metamorphosed pillow basalts, serpentinized ultramafic rocks, meta-gabbros, cherts and pelagic sediments in a pervasive scaly argillite matrix. The mélange has a tectonic contact with a granite intrusion and numerous thrust faults cut across the mélange exposures. Primary magmatic features such as drainage cavities in pillow basalts, magmatic layering in gabbros, and clinopyroxene cumulates in ultramafic rocks are well preserved in low-strain domains. Mafic components enclosed in the mélange are characterized by low pressure secondary mineral assemblages retrograded from amphibolite down to greenschist facies as commonly observed in subducted and exhumed orogens. All amphiboles in the studied fragments are calcic and cluster in the range from winchite, actinolite to hornblende. Amphibole compositions are characterized by Al2O3 (2.4-13.8%), FeO (5-13%), K2O (0.04-0.7%) and XMg (0.67-0.77). The metabasalt and wherlite samples exhibit light rare earth element (LREE) - enriched patterns and negative high field strength element (HFSE: Zr and Ti) consistent with a suprasubduction zone geodynamic setting. A notable feature of both metabasalt and wherlite samples is their pronounced negative Ce anomaly and a positive Eu anomaly as observed in a number of modern island arc suites. As the petrological features and the geochemical characteristics of the remnant fragments we analyzed in this study are reminiscent of their Phanerozoic ophiolite counterparts, we suggest that these rocks were derived from a Proterozoic oceanic plate belonging to a suprasubduction ophiolite assemblage. Subduction zone geochemical signatures and ophiolite-like crustal lithologies in the study region are consistent with the accretion of juvenile oceanic fragments to the pre-existing continental blocks through a forearc-continent collision.