Melt‐Fluxed Melting of the Heterogeneously Mixed Lower Arc Crust: A Case Study from the Qinling Orogenic Belt, Central China

Abstract

AbstractAdakitic rocks with high MgO content and lower crust affinity at convergent margins provide important constraints on arc crust evolution and regional tectonics. An integrated study on the only adakitic pluton with high MgO content and its mafic microgranular enclaves (MMEs) in the Early Paleozoic North Qinling arc terrane was conducted in this paper. Zircons from 2 host rock samples yielded identical weighted mean ages of 428 Ma. The MMEs contain two types of zircons: type 1 zircons share similar morphology, chemical composition, and indistinguishable age with those from the host rock; type 2 zircons yielded an older age of 450 Ma. The similarity in zircon Hf isotopic composition for these zircons indicates a cognate origin of the MMEs. However, the lower Ce4+/Ce3+, higher crystallization temperature and the LREE overabundance of type 2 zircons indicate that mafic intrusions in the lower arc crust were subjected to a post‐magmatic event at ∼450 Ma. Samples from the Tangzang pluton are characterized by typical adakitic geochemical features, and high MgO (Mg#: 50–52), Cr, Ni, and Ba contents. Considering their similar isotopic compositions with the Neoproterozoic metabasites in the North Qinling terrane, we argue that the Tangzang pluton was generated by crustal anatexis of two sources with contributions from the metasomatized mantle wedge. The mantle connection was achieved by aqueous melt‐fluxed melting of a “hot zone” in the lower arc crust. This implies that andesitic continental crust with Mg# of ∼0.5 may also be associated with fluxed‐melting of the lower arc crust.