The role and significance of juvenile sediments in the formation of A-type granites, West Junggar oceanic arc (NW China): Zircon Hf-O isotopic perspectives

Abstract

Oceanic arc subduction systems are the loci of substantial recycling of oceanic crust and production of juvenile arc crust that differentiates to more evolved felsic crust. Inevitably, some juvenile sediments are subducted with the oceanic crust. However, distinguishing the incorporation of juvenile sediments in oceanic arcs is not always straightforward, because they may not measurably shift many geochemical signatures, such as Sr and Nd isotopes, of oceanic arcs. Nevertheless, combined zircon U-Pb, Hf, and O isotope data can provide a powerful tool to decipher sedimentary flux into oceanic arc magmas, and here we report a case study for the late Paleozoic A-type granites from the West Junggar oceanic arc in the southern Central Asian Orogenic Belt. These plutons contain hastingsite and iron biotite diagnostic minerals and have high alkali, FeOT/MgO, Zr, and Ga/Al, but possess low CaO contents, and strongly negative Eu, Sr, and Ba anomalies, demonstrating their close affinity with A-type granites. Zircon U-Pb analyses indicate that these A-type granites emplaced in the Late Carboniferous to Early Permian (ca. 307−298 Ma). Their high zircon εHf(t) values (+12.4 to +15.5), suggest that the magmas were derived from a mantle or juvenile crustal source. However, their δ18Ozrn (+7.2‰ to +11.9‰) values are significantly higher than that of the mantle, and modeling using Hf-O isotope and rare earth element data indicate the assimilation of sedimentary materials at a proportion of ∼50%. Our data suggest that juvenile sediments (e.g., greywacke) played an important role in the formation of the studied A-type granites. The re-melting of sedimentary material induced by the late Carboniferous ridge subduction can promote the transition from an intra-oceanic arc to continental crust. Our results show that the subduction and re-melting of juvenile sediments in oceanic arc systems could be an important mechanism for the maturation of oceanic arc crust.