The American journal of science

Abstract

Whole-rock major and trace element geochemistry together with zircon U–Pb ages and Lu–Hf isotope compositions are reported for a Late Triassic intrusive complex in the Jidong region, Jiamusi–Khanka Block, NE China. Zircon U–Pb dating yields ages between 211 and 208 Ma for enclaves of microgranular diorite and quartz diorite, and between 211 and 209 Ma for the host granitoids. These ages correlate with a previously established intensive Late Triassic magmatic event along the eastern Asian margin. Field observations, together with petrographic features, geochemistry, and zircon Hf isotope data, preclude simple crystal fractionation or restite unmixing as a genetic link for the various rock types within the intrusive complex. The syenogranite suite has high SiO2 (75.5–76.3 wt.%) and low MgO (0.15–0.19 wt.%), and yields enriched LILE and LREE patterns. Most of the zircons in the syenogranites have two-stage model ages of 766 and 1461 Ma, together with positive εHf(t) values of + 0.6 to + 9.1. These results indicate that the granitoid magmas were generated by partial melting of Meso- to Neoproterozoic lower crust. The gabbro suite has a restricted range of SiO2 (46.1–51.9 wt.%) together with high Mg# values (49–70) and high concentrations of Ni, Co, and Cr. Zircons from two diorite samples have single-stage Hf model ages of 557–787 Ma and εHf(t) values of + 1.9 to + 8.3 that are consistent with the coeval gabbros previously studied in the Jidong region. These features, together with the observation that all the gabbros are enriched in LREE and LILE, suggest that the mafic magmas were derived from melting of depleted Neoproterozoic lithospheric mantle that had been metasomatized by slab-derived fluids. It is concluded that the dominant igneous suites within the Late Triassic intrusive complex formed by mingling/mixing of felsic and mafic magmas. The geochemical data, combined with regional geological investigations, indicate that the Late Triassic intrusive complex formed during lithospheric extension caused by slab break-off of the Yuejinshan oceanic plate that was subducting westward beneath the Jiamusi–Khanka Block.