Crustal evolution in the western margin of the Nilgiri Block, southern India: Insights from zircon U–Pb and Lu–Hf data on Neoarchean magmatic suite

Abstract

Extensive magmatism along convergent margin settings marks the late Neoarchean tectonics in the northern segment of the Southern Granulite Terrane (SGT) in Peninsular India. Here we investigate a suite of magmatic rocks (tonalite–trondhjemite–granodiorite [TTG], charnockite and amphibolite) together with accreted banded iron formation (BIF) from the western margin of the Nilgiri Block. The petrologic features of these rocks including the presence of primary amphiboles in all cases suggest crystallization from calc-alkaline magmas, typical of those derived from slab melting. The zircon grains in all the rock types investigated in this study show clear magmatic features including well-crystallized prismatic form, oscillatory zoning and high Th/U values. Zircons from two TTG gneiss samples show identical 207Pb/206Pb mean ages of 2521±13Ma and 2522±17Ma. Those from amphibolites display 207Pb/206Pb mean ages of 2590±13Ma and 2470±17Ma, with the cores of some grains preserving 2.6Ga ages. Zircon grains from the charnockite yield 207Pb/206Pb mean age of 2601±25Ma whereas a single grain from the BIF shows 207Pb/206Pb age of 2493±17Ma. The age data converge to indicate late Neoarchean magmatism between ca. 2.6 and 2.5Ga. The Lu–Hf isotope data on zircons from the rocks show positive εHf(t) values ranging from 2.3 to 9.3. The remarkably consistent and positive εHf(t) values suggest magma derivation from juvenile components, with no significant crustal participation. The mean TDMC values of zircons fall between 2674 and 2815Ma, with the oldest at 2913Ma, suggesting Meso- to Neoarchean juvenile components in the magma source. Our data indicate major crustal building events in the late Archean similar to those reported globally from other regions. The magmatism was a manifestation of melt generation along multiple subduction zones that assembled continental blocks into coherent cratonic architecture in southern India.