Evidence of Crustal Growth During Archean/Proterozoic Transition India

Convergent margin processes during Archean–Proterozoic transition in southern India: Geochemistry and zircon U–Pb geochronology of gold-bearing amphibolites, associated metagabbros, and TTG gneisses from Nilambur

Authors:

Shaji et al

Abstract:

The northern domain of the Southern Granulite Terrane (SGT) in India comprises a collage of Mesoarchean to Neoarchean crustal blocks which preserve important imprints of crustal growth during the early history of the Earth. Here we investigate the zircon U–Pb geochronology and geochemistry of a suite of amphibolites, metagabbros, TTG gneisses and charnockites from the Nilambur region, which forms part of the Wynad Gold Belt in the SGT. Magmatic zircons from three amphibolites yield weighted mean 207Pb/206Pb ages of 2661 ± 59 Ma, 2499 ± 19 Ma, 2570 ± 19 Ma and 2542 ± 49 Ma, closely followed by metamorphism at ca. 2.45 Ga. Zircons from the TTG gneisses show protolith emplacement ages of 2619 ± 21 Ma and the overgrowth rims define an age of 2524 ± 6 Ma. Zircons from the metagabbro show spot ages between 2576 Ma and 2717 Ma and a weighted mean 206Pb/207Pb age of 2644 ± 30 Ma, with metamorphism at 2503 ± 28 Ma. The U–Pb data suggest prominent magmatic and metamorphic events during the Archean–Proterozoic transition.

Geochemical features of the Nilambur rock suite suggest that the amphibolites, metagabbros, and TTG gneisses could be related to a common basaltic protolith. Their data are consistent with formation in primitive arc magmatic settings with MORB-like components in the source followed by magmatic differentiation. The amphibolites and metagabbros show negative Nb–Ta, Zr–Hf and Ti anomalies, typical of subduction-related intraoceanic tholeiitic arc basalt. Their low Th/Ce ratios (less than 0 .1 ) preclude any significant contribution from subducted sediments. The rocks are characterized by marked enrichment in LILE and LREE, and show relative depletion of HFSE. Most of the samples show a flat or slightly LREE enriched patterns, with P, Ti, Th and Nb depletion. The results are consistent with magma derivation from MORB-like mantle wedge, without metasomatism by LILE–LREE-rich fluids derived through the dehydration of the subducted slab. The data obtained in this study, and those from recent studies suggest that the crustal blocks adjacent to the southern margin of the Dharwar Carton in Peninsular India preserve important evidence for active convergent margin tectonics during Archean–Proterozoic transition associated with the generation and emplacement of subduction-related arc magmas and continental growth.