Hafnium–neodymium isotope systematics of the 2.7 Ga Gadwal greenstone terrane, Eastern Dharwar craton, India: Implications for the evolution of the Archean depleted mantle

Abstract

The Neoarchean Gadwal greenstone belt in the eastern Dharwar craton, India, hosts a well preserved metavolcanic sequence that is dominated by tholeiitic and calc-alkaline basalt-andesite-dacite-rhyolite series, which includes boninitic geochemical varieties. Bulk-rock Lu–Hf and Sm–Nd isotope systematics of these apparently arc-related volcanic rocks yield indistinguishable ages of 2.701±0.024Ga and 2.702±0.026Ga, respectively. On the basis of the close spatial association and identical ages of the different rock types we suggest 2.70±0.03Ga as the age of crystallization of the different rock types within the Gadwal metavolcanic sequence. In contrast, bulk-rock Pb–Pb isotope systematics of the same samples yield a significantly younger and less precise age of 2.466Ga (+0.068/−0.110Ga). We tentatively interpret this younger age to represent a metallogenic and crustal reworking event in the Dharwar craton, which disturbed the U–Pb system but not the Lu–Hf or Sm–Nd systems. The Gadwal metavolcanic rocks have positive initial εHf(2.70Ga)=+1.6 to+8.7 and slightly negative to positive εNd(2.70Ga)=−0.1 to+3.0 values, consistent with an origin from a long term depleted source relative to a chondritic reservoir at ∼2.7Ga. Lack of correlation between initial isotopic compositions and major or trace element indices of fractionation and alteration suggest that the observed isotope variability probably reflects compositional variation in the Gadwal source, similar to that observed in modern day island arcs. Two boninitic samples of the Gadwal sequence have εHf∼8.3 and 8.7, and are more radiogenic than average depleted mantle for the time period 3.2 to 2.5Ga (εHf=4 to 6). Early (perhaps Hadean) differentiation events that led to a depleted and heterogeneous mantle are apparent in the Nd and Hf isotope systematics of 3.7–3.8Ga Isua supracrustal rocks. The radiogenic Hf isotopes of the Gadwal boninites and the Hf, Nd isotope systematics of rocks from other locations in the 3.4 to 2.5Ga time period are consistent with the survival of fragments of an early depleted mantle later in the Archean. From ∼2.0Ga to present, the time-integrated 176Lu/177Hf and 147Sm/144Nd of the depleted mantle appears nearly constant and similar to the present day average MORB source. These data indicate that progressive elimination of early (>4.5Ga) formed heterogeneities in the depleted mantle dominated the history of the Archean mantle, and that portions of early depleted reservoirs survived through the Mesoarchean. These results have implications for the mixing scales for the early terrestrial mantle and the timing of the initiation of present day plate tectonics.