Evolution of Archaean crust in the Dharwar craton: The Nd isotope record

Abstract

A synthesis of the available Nd isotope data from the Dharwar craton puts important constrains on antiquity of crust, extent of basement terranes, and events marking juvenile crustal addition and crustal recycling. The craton is divided into two blocks, namely eastern Dharwar craton (EDC) and western Dharwar craton (WDC). Nd model ages trace crust extraction as far as 3.5Ga back in both the blocks, although rock record of such antiquity is yet to be found in the craton. In WDC 3.35–3.0Ga is the most significant period of juvenile crustal addition as well as crustal recycling. Significantly depleted mantle (ɛ=+1.5 to +6.4) existed below the WDC as early as 3.35Ga, which was probably refertilized in a later event. From the resultant chondritic to slightly enriched mantle (ɛ=−0.3 to +0.4) juvenile mafic crust was added to the WDC during 2.9–2.6Ga. A widespread crustal recycling event at ∼2.6Ga marks the last major event in this block. In EDC 3.3–3.0Ga granitoids occur as vestiges and show Nd model ages higher than crystallizations ages suggesting recycling of Palaeoarchaean crust. During 2.7–2.5Ga extensive juvenile magmatism took place in EDC from variably depleted mantle (ɛ=+1.4 to +5.6) which is later than the global 2.7Ga peak of crustal growth. Attendant crustal recycling destroyed most of the earlier crust. Distribution of Nd model ages suggests, in contrast to general believe, that Palaeoarchaean crust was not only abundant in the WDC but also widespread in an area extending from the WDC–EDC boundary to further 60–150km east up to the Hungund-Ramagiri-Kolar (HRK) belt. This belt appears to be a major crustal boundary along which terranes of different Nd isotope signatures were amalgamated during the late Neoarchaean by horizontal accretion. Major Neoarchaean crustal recycling events documented in the Dharwar craton are missing from Nd isotope record of the Eastern Block (EB) of the North China Craton. This fact does not support correlation between the two cratons as suggested by some authors.