Geochronology and geochemistry of Late Triassic bimodal igneous rocks at the eastern margin of the Songnen-Zhangguangcai Range Massif, Northeast China: petrogenesis and tectonic implications

摘要

New zircon laser ablation inductively coupled plasma mass spectrometry and secondary ion mass spectroscopy U-Pb ages, and Hf isotope and whole-rock geochemical data are reported for Mesozoic igneous rocks from the eastern margin of the Songnen-Zhangguangcai Range Massif, Northeast China, in order to document the petrogenesis of the igneous rocks and reconstruct the early Mesozoic tectonic setting of the region. Zircons from five representative igneous rocks are euhedral-subhedral and display oscillatory growth zoning or striped absorption in cathodoluminescence images, suggesting a magmatic origin. The dating results indicate that granite, gabbro, and rhyolite from the eastern Songnen-Zhangguangcai Range Massif formed during Late Triassic (204-211Ma). The Late Triassic granitoids and rhyolites have an affinity to A-type granites or rhyolites. Their zircon epsilon(Hf)(t) values and Hf two-stage model ages range from -3.8 to +3.8 and from 999 to 1485Ma, respectively, indicating that their primary melts were derived from the partial melting of the Meso-Proterozoic crust. The geochemistry of coeval gabbros, which reflects primary magma composition, shows a significant large ion lithophile element (e.g. Ba and Sr) enrichment and high field strength element (i.e. Zr, Hf, Nb, Ta, and Ti) depletion. Based on zircon epsilon(Hf)(t) values (-4.2 to +2.8) and Hf single-stage model ages (746-1031Ma), we conclude that the mafic magma is the product of partial melting of lithospheric mantle that was metasomatically enriched by fluids derived from the subducted oceanic crust. The Late Triassic magmatism along the eastern margin of the Eurasian continent has bimodal magma compositions, indicating an extensional setting after the final closure of the Palaeo-Asian Ocean rather than being related to subduction of the Palaeo-Pacific Plate beneath the Eurasian continent. The occurrence of Late Triassic igneous rocks on the eastern side of the Mudanjiang Fault suggests that this fault does not represent the suture zone between the Songnen-Zhangguangcai Range and Jiamusi massifs.