Petrogenesis of late Jurassic Mufushan high-Mg diorites and late Mesozoic tectonic evolution of the eastern South China Block

Abstract

Abundant Mesozoic magmatic rocks are widely distributed in southeastern China, forming several remarkable magmatic-metallogenetic belts. However, the geodynamic mechanism for the Late Jurassic-Early Cretaceous magmatic processes and their relation to the rich polymetallic deposits of the region remain highly controversial. Here, we report zircon U-Pb ages, geochemistry, and Sr-Nd-Hf-Pb isotopic data for the high-Mg diorites in the Mufushan region, northeastern Hunan Province. The high-Mg diorites were emplaced in the late Jurassic (U-Pb ages of ∼151–155 Ma), contemporaneous with the surrounding granitic rocks which have ages ranging from 140 to 150 Ma. These high-Mg diorites are characterized by the enrichment of large-ion lithophile elements (LILEs, e.g. Ba, Pb, K and Rb) and depletion of high field strength elements (HFSEs, e.g., Nb, Ta, Ti), displaying typical geochemical features of continental-arc basalts (CAB). The relatively high Mg# values (71–76, average of 72), Cr contents (546–1562 ppm, average of 944 ppm) and Ni contents (211–519 ppm, average of 335 ppm) suggest that their derivation was likely from a relatively primitive magma, accompanied with weak crustal assimilation or fractional crystallization.The high-Mg diorites display isotopic characteristics similar to those of enriched mantle-derived mafic rocks, such as low 87Sr/86Sr(i) (0.70692–0.70789), εNd(t) and εHf(t) ranging from −2.4 to −4.3 and −3.8 to + 1.1 respectively, with single-stage Nd model ages (TDM1) of 1.3–1.5 Ga. In combination with a compilation of coeval magmatic rocks in eastern South China, a significant addition of ancient lithospheric mantle materials was likely involved in the Mesozoic magma genesis. The decreasing radiogenic Nd isotope compositions and negative Hf signatures suggest an increasing contribution of enriched components to the magma source from the Late Jurassic to Early Cretaceous. Our results suggest that the Mufushan high-Mg diorites were derived from mantle-derived magma, followed by the incorporation of slab-derived materials into mantle and subsequent sediment melting. Integrated with previous work, the widespread Late Jurassic-Early Cretaceous magmatic rocks in eastern South China formed within a continental back-arc setting that was formed by subduction-induced convection in the mantle wedge. The subsequent roll-back of subducted Paleo-Pacific slab (Izanagi plate) triggered extensive mantle upwelling and melt-mantle interaction, accounting for the production of contemporaneous felsic rocks and related polymetallic deposits in regions.