Neoarchean continental growth through arc magmatism in the Nilgiri Block, southern India

Abstract

The formation and growth of continental crust in the Archean have been evaluated through models of subduction–accretion and mantle plume. The Nilgiri Block in southern India exposes exhumed Neoarchean lower crust, uplifted to heights of ∼2500m above sea level along the north western margin of the Peninsula. Major lithologies in this block include charnockite with or without garnet, anorthosite-gabbro suite, pyroxenite, amphibolite and hornblende-biotite gneiss (TTG). All these rock types are closely associated as an arc magmatic suite, with diffuse boundaries and coeval nature. The charnockite and hornblende-biotite gneisses (TTG) show SiO2 content varying from 64 to 73wt.%. The hornblende-biotite gneisses (TTG) are high-Al type with Al2O3 >15wt.% whereas the charnockites show Al2O3 <15wt.%. The composition of charnockite is mainly magnesian and calcic to calc-alkaline. The mafic-ultramafic rocks show composition close to that of tholeiitic series. The low values of K2O (<3wt.%), (K/Rb)/K2O (<500), Zr/Ti, and trace element ratios like (La/Yb)n/(Sr/Y), (Y/Nb), (Y+Nb)/Rb, (Y+Ta)/Rb, Yb/Ta indicate a volcanic arc signature for these rocks. The geochemical signature is consistent with arc magmatic rocks generated through oceanic plate subduction. The primitive mantle normalized trace element patterns of these rocks display enrichment in large ion lithophile elements (LILE) and comparable high field strength elements (HFSE) in charnockite and hornblende-biotite gneisses (TTG) consistent with subduction-related origin. Primitive mantle normalized REE pattern displays an enrichment in LREE in the charnockite and hornblende-biotite gneisses (TTG) as compared to a flat pattern for the mafic rocks. The chondrite normalized REE patterns of zircons of all the rock types reveal cores with high HREE formed at ca. 2700Ma and rims with low HREE formed at 2500–2450Ma. Log-transformed La/Th–Nb/Th–Sm/Th–Yb/Th discrimination diagram for the mafic and ultramafic rocks from Nilgiri displays a transition from mid-oceanic ridge basalt (MORB) to island arc basalt (IAB) suggesting a MORB source. The U–Pb zircon data from the charnockites, mafic granulites and hornblende-biotite gneisses (TTG) presented in our study show that the magma generation during subduction and accretion events in this block occurred at 2700–2500Ma. Together with the recent report on Neoarchean supra-subduction zone ophiolite suite at its southern margin, the Nilgiri Block provides one of the best examples for continental growth through vertical stacking and lateral accretion in a subduction environment during the Neoarchean.