Archean Polymetamorphism in the Central Dharwar Craton, Southern India

Abstract

Petrochronological investigations of granulite-facies metapelitic rocks from the eastern contact of the Closepet granite in the Central Dharwar Craton (CDC), southern India, provide new pressure–temperature–age (P–T–t) constraints on two seemingly discrete Archean metamorphic events during the Neoarchean and late Paleoarchean eras. Phase equilibrium modelling and conventional thermobarometry coupled with in situ monazite and garnet geochronology constrain Neoarchean (ultrahigh-temperature; UHT) metamorphic peak conditions to ~930 °C and ~6.7 kbar at c. 2.63 Ga, then cooling and limited decompression to ~5.7 kbar at ≤810 °C. Monazite inclusions in garnet least affected by Neoarchean recrystallisation have distinct positive Eu anomalies and yield ages of c. 3.2 Ga, whereas garnet cores interpreted to have grown at the same time have slightly younger apparent ages of c. 3.1 Ga. We interpret this age mismatch to be the result of extensive resorption of garnet during Neoarchean UHT metamorphism, which led to widespread modification of the initial Lu–Hf systematics in garnet to produce younger apparent ages. The effect of retention and inward intracrystalline diffusion of Lu on the isotopic composition of garnet is most pronounced close to the resorbed grain margins and decreases towards the core, as reflected by progressively younger apparent single-spot garnet dates from core to rim. Despite extensive overprinting of the sample at c. 2.63 Ga, the trace element composition of Paleoarchean monazite indicates growth in equilibrium with garnet but in the absence of feldspar, which is predicted to occur over a broad stability range at P ≥ 8 kbar and T ≤ 700 °C. Such P–T conditions are uncommon in the metamorphic rock record prior to the Neoarchean, but are typical of Barrovian-type metamorphism, which is considered to be an expression of accretionary-to-collisional orogenesis. Rocks of similar age and metamorphic grade have been reported from the core of the Western Dharwar Craton (WDC) and may reflect regional subduction at the margins of the CDC and WDC since the late Paleoarchean. Neoarchean UHT metamorphism in the Dharwar Craton is coeval with a cluster of other UHT occurrences at 2.7–2.6 Ga, indicating the existence of globally elevated thermal gradients at that time.