Sediment contribution in post-collisional high Ba-Sr magmatism: Evidence from the Xijing pluton in the Alxa block, NW China

摘要

High Ba-Sr granitoids occur in a number of tectonic settings, and variable petrogenetic models have been proposed. Those from post-collisional settings are rarely studied and are the focus of this study. Zircon U-Pb geochronology and whole-rock elemental and Sr-Nd-Hf isotopic data are presented for a suite of high Ba-Sr granitoids from the southern margin of the Alxa block, NW China. U-Pb zircon dating shows that the Xijing high Ba-Sr granitoids were emplaced in two periods at similar to 440 Ma and similar to 410 Ma, and they are characterized by elevated Ba and Sr contents. The older similar to 440 Ma high Ba-Sr granitoids range from monzodiorite to quartz monzonite, and the younger similar to 410 Ma high Ba-Sr granites tend to be more evolved. Both suites have similar enriched Sr-Nd-Hf isotopic ratios, low Ba/Th and Sr/Th ratios, and high Th contents and Th/Ce ratios indicative of a sediment contribution. This is potentially linked to previous subduction of the North Qilian ocean slab. Major and trace element calculations suggest that the older similar to 440 Ma granitoids experienced up to similar to 60% fractionation at 4-6 kbar with a crystallizing assemblage dominated by amphibole and plagioclase. The younger -410 Ma granites could in turn have been formed by a further 80% fractional crystallization at lower pressures (1-2 kbar). Trace element and Sr-Nd isotopic modeling suggest incorporation of similar to 5% slab-derived sediment into mantle magma source of these high Ba-Sr granitoids. We note that post-collisional granitoids with high Ba and Sr characteristics may also reflect a sediment contribution in their source region, and this may be a key aspect of why such granites plot in the field of postcollisional granites in the Rb vs Y + Nb diagram. Some sanukitoids with high Ba and Sr contents in the late Archean also appear to reflect sediment subduction and they hence may represent early stages of crustal recycling. (C) 2019 International Association for Gon