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

Abstract

The petrogenesis of high Sr/Y-type magmas is still open to debate. Usually, such magmas could result from melting under high-pressure settings (>12kbar). In this paper, we gave an example that some high Sr/Y-type magmas could originate from melting of crustal materials at pressure of 10–12kbar. We carried out a study of petrogenesis for Devonian high Sr/Y granites from the Beidashan batholith (397–411Ma), southwestern Alxa Block, Northwest China. The Beidashan granites have SiO2 (69.21–74.60wt.%) and Al2O3 (14.01–16.20wt.%) with A/CNK ratios of 0.99–1.08. According to their trace element compositions and whole-rock zirconium saturation temperatures (TZr), the Beidashan granites can be divided into two groups: Group I ((Dy/Yb)N=1.2–3.0, Eu/Eu*=0.77–1.3, TZr=761–856°C), resulted from fluid-absent partial melting of mafic to intermediate crustal materials leaving garnet residuum at pressure of ~12kbar, and Group II ((Dy/Yb)N=0.76–2.16, Eu/Eu*=1.7–5.3, TZr=651–785°C), formed by fluid-present melting of mafic to intermediate crustal materials with residual amphibole in the source at pressure of ~10kbar. Both Group I and Group II show high Sr/Y and (La/Yb)N features. They show ISr=0.7134–0.7180, εNd(t)=−6.61 to −9.71, T2DM=1.7–1.9Ga; εHf(t)=−5.6 to −9.9 and TDMC=1.7–2.0Ga, 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 (~10–12kbar), and they could not be an indicative of partial melting of thickened crust.