Nb-Ta systematics of Kohistan and Gangdese arc lower crust: Implications for continental crust formation

Abstract

• Kohistan and Gangdese (garnet) hornblendites and/or garnet gabbros are of cumulate origins. • Amphiboles in these lower crust cumulates have high Nb/Ta but low Nb and Ta concentrations. • Amphibole fractionation will not change Nb/Ta ratio of arc magmas significantly. • Rutile likely plays an important role in Nb/Ta fractionation in continental crust formation. Earth’s continental crust has a lower Nb/Ta ratio than that of its building blocks –basaltic melts from the mantle. This Nb/Ta mass imbalance implies the existence of a complementary high Nb/Ta reservoir in deep crust. Amphibole-rich arc cumulates have been proposed to be one such candidate due to the preferential partitioning of Nb over Ta into amphibole. To test this proposal, we carried out in-situ analyses of amphibole and rutile in the lower crustal rocks from Kohistan arc and Gangdese arc. Petrological observations and geochemical results suggest that these rocks in both Kohistan arc and Gangdese arc are of magmatic origins and formed by crystal accumulation, i.e. of cumulate. The Kohistan garnet-bearing hornblendite cumulates were formed at 952–1009 ℃; the garnet-free hornblendite cumulates in Kohistan arc and Gangdese arc were formed at 941–992 ℃ and 955–1027 ℃, respectively. We find that, although the amphibole of the Kohistan and Gangdese cumulates have high Nb/Ta ratios (average 18.2 and 17.0, respectively), their Nb and Ta concentrations are far lower than mantle-derived volcanics (Nb < 1.1 ppm, Ta < 0.11 ppm). We show that, with such low concentrations of Nb and Ta, amphibole fractionation will not change arc magma Nb/Ta ratios significantly. Instead, the rutile in Kohistan cumulates has high Nb/Ta ratios and high Nb-Ta concentrations. The calculation shows that rutile may play a more important role in Nb/Ta fractionation during arc magma differentiation in thickened arc crust. Our study provides a mineralogical perspective to understand the formation of continental crust, and improves our understanding how Nb or Ta becomes enriched in arc setting.