Sm [sbnd]Nd isotopic systematics of Enderby Land granulites and evidence for the redistribution of Sm and Nd during metamorphism

Abstract

Measurements of 143Nd/ 144Nd and 147Sm/ 144Nd are reported for whole rocks and mineral separates from granulites of the Napier Complex at Fyfe Hills. Charnockites, leuconorites and gabbros yield a whole rock Sm Nd isochron age of 3060 ± 160m.y. and an initial 143Nd/ 144Nd ratio of 0.50776 ± 10 (ɛ Nd(3060m.y.) = −2.0 ± 1.8) . The negative ɛ Nd value and the presence of geologically induced dispersion in the data suggest that the isochron age does not represent the time of primary crystallization of the complex but instead indicates a time of later redistribution of Sm and Nd and partial re-equilibration of 143Nd/ 144Nd ratios. This probably occurred during the upper granulite facies metamorphism which has also been dated at ∼ 3100m.y. by Rb Sr and U Pb zircon studies [1]. Coexisting clinopyroxene, apatite and total rock fractions in two adjacent samples define an approximately linear array corresponding to an age of 2300 ± 300 m.y. This array indicates that redistribution of Sm and Nd and re-equilibration of 143Nd/ 144Nd ratios occurred on an intermineral scale during the upper amphibolite to lower granulite facies metamorphism at ∼ 2450m.y. Due to the resetting of the Sm Nd system on both whole rock and mineral scales, the primary crystallization age of the igneous protolith is not well constrained by the present data, although it is clearly 3100m.y. If it is assumed that the complex was derived initially from a depleted mantle reservoir (ɛ Nd(T) ⋍ 2) , evolution of the negative ɛ Nd value of −2.0 with the observed Sm/Nd ratios requires a prehistory of ∼ 380m.y. This implies a primary age of ∼ 3480m.y. However, substantially older primary ages can be inferred if the source reservoirs had ɛ Nd(T) > 2 and/or substantial reductions in the Sm/Nd ratio occurred in whole rocks during the granulite facies metamorphism at 3100 m.y. Such an inferred reduction in the Sm/Nd ratio may have been the result of preferential loss of Sm relative to Nd, or introduction of a low Sm/Nd fluid with ɛ Nd ≥ 0 during granulite facies metamorphism.