Origin of high Sr/Y-type granitic magmatism in the southwestern of the Alxa Block, Northwest China

The petrogenesis of high Sr/Y-type magmas is still open to debate. Usually, such magmas could result from melting under high-pressure settings (>12 kbar). In this paper, we gave an example that some high Sr/Y-type magmas could originate from melting of crustal materials at pressure of 10-12 kbar. We carried out a study of petrogenesis for Devonian high Sr/Y granites from the Beidashan batholith (397-411 Ma), southwestern Alxa Block, Northwest China. The Beidashan granites have SiO2 (69.21-74.60 wt.%) and Al2O3 (14.01-16.20 wt.%) with A/CNK ratios of 0.99-1.08. According to their trace element compositions and whole-rock zirconium saturation temperatures (T-Zr), the Beidashan granites can be divided into two groups: Group I ((Dy/Yb)(N) = 1.2-3.0, Eu/Eu* = 0.77-13, T-Zr = 761-856 degrees C), resulted from fluid-absent partial melting of mafic to intermediate crustal materials leaving garnet residuum at pressure of similar to 10 kbar, and Group II ((Dy/Yb)(N) = 0.76-2.16, Eu/Eu* = 1.7-5.3, T-Zr = 651-785 degrees C), formed by fluid-present melting of mafic to intermediate crustal materials with residual amphibole in the source at pressure of similar to 10 kbar. Both Group I and Group II show high Sr/Y and (La/Yb)(N) features. They show I-sr = 0.7134-0.7180, epsilon(Nd)(t) = 6.61 to 9.71, T-2DM = 1.7-1.9 Ga; epsilon(Hf)(t) = -5.6 to -9.9 and T-DM(C) = 1.7-2.0 Ga, indicating that the Beidashan high Sr/Y granites were derived from melting of crustal basement materials. Our results suggest that some high Sr/Y-type granites formed under relatively lower pressure conditions (similar to 10-12 kbar), and they could not be an indicative of partial melting of thickened crust. (C) 2016 Elsevier B.V. All rights reserved.