Revisiting the origin of the Carboniferous Oytag pluton in West Kunlun orogenic belt, northwest China

Abstract

The Oytag pluton in the West Kunlun orogenic belt of China represents the rare exposed Carboniferous granitoids associated with the early-stage evolution of the Paleo-Tethys Ocean, and its origin is controversial. New zircon U-Pb data suggest an emplacement age of 327–324 Ma for trondhjemite, which slightly predated tonalite (∼314 Ma). These results, along with available published ages, indicate several episodes of magmatism to build the Oytag pluton. Both trondhjemite and tonalite have positive whole-rock Nd isotopic compositions (ε Nd (t) = +4.56 to +8.01 for trondhjemite; ε Nd (t) = +4.68 to +6.15 for tonalite), which positively correlate with whole-rock MgO values. Given the presence of Ordovician inherited zircon grains, variations of whole-rock Nd isotopic compositions are attributed to addition of <10% crustal contaminants during assimilation and fractional crystallization (AFC) processes. Interestingly, trondhjemite and tonalite follow distinct AFC trends, consistent with their different ages. Zircons from two trondhjemite samples have overlapping U-Pb ages but show distinctly different trace elements and internal structures (sector-zoned vs. oscillatory-zoned). The sector-zoned zircons have a less evolved signature of lower Hf contents, higher Eu/Eu* and Th/U ratios than the oscillatory-zoned ones, indicating significant crystal fractionation processes for their parental melts. The more variable Hf contents of the oscillatory-zoned zircons than those of the sector-zoned ones reflect a more dynamic environment where they crystallized. We propose that the sector-zoned zircons were crystallized from locked, interstitial melt in a mush and the oscillatory-zoned zircons from extracted melt. In situ crystal-liquid segregation in a mush chamber thereby could be an efficient approach to the differentiation of Oytag trondhjemite. On the other hand, zircons from tonalite have similar Hf contents to the sector-zoned zircons from trondhjemite, but display slightly higher Eu/Eu* ratios and much higher Th/U ratios. This indicates that tonalite should have experienced much different AFC processes from trondhjemite. Both the episodic magmatism feature and the distinct differentiation processes argue for rises of different batches of magmatism to generate trondhjemite and tonalite in Oytag. Along with the large variations of Hf isotopes (5–6 epsilon units) in zircons, we propose a partial melting of juvenile mafic crust model for the Oytag pluton. Heat from upwelling of asthenosphere may have triggered the partial melting of wet, mafic underplates during the opening of a Paleo-Tethyan back-arc basin. • The Oytag pluton comprises predominant trondhjemite with minor tonalite. • Oytag trondhjemites (327–324 Ma) formed earlier than tonalites (∼314 Ma). • The Oytag pluton was built by different batches of magmas via partial melting.