Petrogenesis and tectonic implications of the Early Paleozoic intermediate and mafic intrusions in the South Qinling Belt, Central China: Constraints from geochemistry, zircon U–Pb geochronology and Hf isotopes

Abstract

The characteristics and tectonic implications of the Early Paleozoic alkaline magmatic belt in the South Qinling Belt, which was originally part of the northern Yangtze Block prior the Devonian, have remained elusive. Whether this magmatic belt is related to rifting of the passive continental margin, to back-arc extension in the active continental margin, or to mantle plume activity is debated. Understanding the origin and geodynamic significance of this magmatic belt can provide new constraints on the Early Paleozoic tectonic evolution of the northern Yangtze Block. Here we present zircon U-Pb data from a suite of nepheline syenite, quartz syenite, diabase, and gabbro from the northern margin of the Yangtze Block which show an age range of ca. 435–440Ma. The εHf(t) values of the intermediate rocks up to 16.59 suggest magma generation from depleted mantle sources and new crustal growth. Geochemically, the syenites showing high total alkali contents and are enriched in LREE, LILE (Rb, Ba, and K), and HFSE (Th, U, Nb, Ta, Zr, and Hf), with depletion in Sr, P, and Ti. The intermediate and mafic magmatic rocks were generated through magmas sourced from the subcontinental lithospheric mantle metasomatized by asthenospheric mantle and underwent fractional crystallization without significant crustal contamination. The magmatic suite represents a significant phase of crustal extension in the northern margin of the Yangtze Block.