Zircon U–Pb geochronology and Lu–Hf isotopes from the Kolar greenstone belt, Dharwar Craton, India: Implications for crustal evolution in an ocean-trench-continent transect

Abstract

The Dharwar Craton in southern India is composed of Neoarchean greenstone successions in association with banded iron formations (BIFs), felsic volcanics and voluminous calc-alkaline to potassic plutons intruding tonalite–trondhjemite–granodiorite (TTG) basement rocks. In this study, we investigate a suite of BIFs (pelagic sediments), amphibolite (oceanic basalts), quartz mica schist (trench sediments from continental source), and porphyritic granite with mafic magmatic enclave (magma mixing and mingling in continental magmatic arc), together representing an ocean-trench-continent transect within a Neoarchean subduction–accretion–collision zone in the Kolar greenstone belt of the Eastern Dharwar Craton. The morphology, internal structure and high Th/U values of zircon grains from these rocks suggest magmatic crystallization, closely followed by metamorphism. The magmatic zircons in the BIFs show upper intercept ages of 2719±31Ma to 2698±50Ma and weighted mean 207Pb/206Pb mean ages of 2718±26Ma to 2696±35Ma marking the time of deposition of the pelagic sediments with which the ocean floor basalts were intercalated, and from which the magmatic zircons where incorporated within the pelagic sediments. The ca. 2.5Ga ages from the younger group of zircons in the BIFs correspond to the timing of metamorphism. Zircons from the amphibolite shows upper intercept age of 2581±31Ma representing the crystallization age of the protolith basalts. Zircons from the porphyritic granite yield upper intercept age of 2576±10Ma and weighted mean 207Pb/206Pb mean age of 2572±13Ma suggesting the timing of emplacement of this rock, which is comparable to the ages of magmatic zircons in the amphibolite. Magmatic zircons in the dioritic enclave show upper intercept age of 2564±13Ma and weighted mean 207Pb/206Pb age of 2560±10Ma. The younger population of zircons in this rock show upper intercept age of 2492±33Ma. Inherited zircons in some of these rocks show older 207Pb/206Pb ages of 2707±21Ma and 2666±22Ma. The zircon εHf(t) values are dominantly positive (up to 5.1), although some grains show negative values. The crustal residence ages suggest Mesoarchean to Neoarchean juvenile and reworked sources consistent with vertical and lateral accretion in a continental arc setting. Our data trace crustal growth during Neoarchean at 2.72–2.56Ga in Eastern Dharwar along an active convergent margin. The multiple events of crustal growth recorded in our study are comparable with similar features in some of the other cratonic nuclei elsewhere on the globe, and suggest that the late Archean marks an important period of continent building.