Age and geochemistry of Silurian gabbroic rocks in the Tongbai orogen, central China: Implications for the geodynamic evolution of the North Qinling arc–back-arc system

Abstract

The tectonic properties of the Erlangping unit and the subduction polarity of oceanic basins in the North Qinling–Tongbai orogen have been the focus of debate for more than twenty years. The resolution of these controversies hinges on the refined constraints on the location and nature of subduction-related magmatism. In this contribution, we provide a combined study of zircon U–Pb ages, major and trace element geochemistry, and Sr–Nd–Hf isotope compositions of gabbroic rocks from the Erlangping unit in the Tongbai orogen. U–Pb dating of zircons from two gabbroic diorite samples yielded two weighted mean ages of 443±3 and 440±4Ma. The Erlangping gabbroic rocks are tholeiitic in composition with variable MgO (4.48–8.17%), and low K2O (0.25–1.01%) and TiO2 (0.60–1.79%) contents. They are characterized by enrichment of LILEs, depletion of HFSEs, and highly positive whole-rock εNd(t) (3.76–5.68) and zircon εHf(t) (12.0–12.1) values. The geochemical features indicate that the Erlangping gabbroic rocks were derived from a depleted lithospheric mantle that had been exposed to metasomatism by slab-derived fluid shortly before their generation. Their occurrence, coupled with coeval granitoids with highly positive εNd(t) values, manifest significant crustal growth in the North Qinling–Tongbai orogen during the late Ordovician–Silurian. In addition, the Erlangping gabbroic rocks have low Sr/Y (7.32–26.2) and (La/Yb)N (1.34–4.07) ratios, but high HREEs, Y and Sc contents. This implies that their mantle source had no residual garnet, and thus partial melting of the lithospheric mantle occurred under relatively low-pressure conditions. The generation of the Erlangping gabbroic rocks might result from incipient rifting of the Erlangping back-arc basin induced by the northward subduction of the Paleotethyan Shangdan Ocean. Taking into account the ca. 490Ma collision between the Erlangping arc and North Qinling microcontinent, we suggested that the evolution of the North Qinling belt from an arc to a back-arc realm involved a prolonged period of underplating of basaltic magma, granulite-facies metamorphism, and anatexis of precursor rocks. Consequently, the Qinling–Tongbai orogenic belt provides a key place for better understanding the Phanerozoic crustal growth and differentiation.