Petrogenesis and geodynamic settings of the Devonian adakites and A-type granites in the North Qinling orogen

Abstract

The Devonian adakites and A-type granites developed in the eastern section of the North Qinling (NQ) orogen in Central China records the evolution of the East Qinling orogen. In this study, petrological, zircon UPb and LuHf isotopic, whole rock element and SrNd isotopic analyses were carried out to investigate the origin and geodynamic setting of the Sikeshu and Shenxianya plutons in NQ. The Sikeshu two-mica granites formed at 415.7 ± 2.3 Ma possess geochemical characteristics similar to those of adakites, while the Shenxianya monzogranites formed at 386.4 ± 1.8 Ma exhibit A-type granite characteristics. Both plutons show enriched SrNd isotopic characteristics and display enrichment and depletion of the zircon Hf isotope. Results show that the Sikeshu two-mica granites were derived by partial melting of a combined source from Erlangping mafic rocks and the Qinling Group, while the Shenxianya monzogranites were derived by partial melting of lower crust granulite. In the late Late Silurian, the mantle-derived magma produced by the slab failure underplated the thickened lower crust, forming the Sikeshu two-mica granites with adakitic features. The NQ underwent a transition from compression to extension during the late Late Silurian-Early Devonian following the closure of the Shandfan Ocean. The mantle-derived magma formed by delamination underplated the lower crustal granulites, generating the Shenxianya A-type granite.