Early Jurassic mafic magmatism in the Lesser Xing'an–Zhangguangcai Range, NE China, and its tectonic implications: Constraints from zircon U–Pb chronology and geochemistry

Abstract

LA-ICP-MS zircon U–Pb dating and geochemical data have been obtained from five representative mafic–ultramafic intrusions in the Lesser Xing'an–Zhangguangcai Range, NE China, with the aim of improving our understanding of the Mesozoic tectonic evolution in the region, and in particular, determining the time of initiation of the circum-Pacific tectonic system. The selected zircons exhibit striped absorption in cathodoluminescence (CL) images and have high Th/U ratios (0.20–3.16), indicating a magmatic origin. The zircon U–Pb dates indicate that most of these magmatic zircons (other than a few relics that were captured and entrained in the magma) formed in the late Early Jurassic (186–182Ma), and not as previously supposed in the Middle Caledonian. The five mafic–ultramafic plutons are composed of olivine-gabbro, hornblendite, gabbro, hornblende-gabbro, and gabbro–diorite. The olivine-gabbro and hornblendite display cumulate textures, implying that fractional crystallization of olivine and plagioclase took place during magma evolution. These mafic–ultramafic igneous rocks have SiO2=37.3%–55.7%, MgO=3.05%–13.3%, Al2O3=11.8%–23.8%, Mg#=42–69 [Mg#=100Mg/(Mg+Fe2+total)], and δEu=0.88–1.32, and they display three types of rare earth element (REE) distribution patterns: right-slipped, flat patterns, or dome-like. The trace element spider diagrams show that the rocks are enriched in large ion lithophile elements (LILEs) such as Ba, K, and Sr, and depleted in high field strength elements (HFSEs) such as Nb, Ta, Zr, and Hf. The zircons have εHf (186–182Ma)=+2.7 to +12.0, and TDM1=366–732Ma. The geochemical data indicate that the Early Jurassic mafic magma originated in an extensional environment from the partial melting of a depleted mantle wedge that had been metasomatized by fluids released from a fossil subducted slab. These data, combined with information on the spatial variation of coeval igneous rocks, indicate that the formation of the Early Jurassic mafic–ultramafic rocks in the Lesser Xing'an–Zhangguangcai Range was related to the subduction of the Paleo-Pacific Plate beneath the Eurasian continent, and this event would mark the beginning of the circum-Pacific tectonic system.