Geochronology, petrogenesis and tectonic implications of Jurassic granites in the Liaodong Peninsula, NE China

Abstract

Mesozoic igneous rocks are widespread throughout eastern China, but precise geochronological and petrogenetic constraints were previously lacking. Ten samples from the Liaodong Peninsula in northeastern China were chosen for zircon U–Pb SHRIMP and laser ablation ICP-MS dating. The magmatic ages range from 179 ± 3 to 156 ± 3 Ma. Data compilation indicates that contemporaneous granitic magmatism is widespread throughout eastern China, establishing the Jurassic as an important period of igneous activity in eastern China. Petrographically, these granites can be divided into three groups that underwent a complex history of crystal fractionation. Two end-members of granodioritic and monzogranitic magma are identified. The granodioritic rocks, having lower (⁸⁷Sr / ⁸⁶Sr)_i and higher ɛ_Nd(t) values than those of the monzogranitic rocks, came from the partial melting of juvenile crust, whereas the monzogranitic rocks came from partial melting of the Precambrian basement. It is proposed that Pacific plate subduction resulted in crustal thickening and subsequent lithospheric delamination which resulted in the upwelling of asthenospheric mantle and formation of juvenile crust by underplating of mantle-derived magma in the lower crust. A subsequent underplating and heating event from the asthenosphere partially melted the overlying pre-existing underplated mafic rocks and ancient crust, leading to the formation of granodioritic and monzogranitic magmas. This thickening by subduction could have been a necessary precursor for limited delamination in the Jurassic and more extensive delamination in the Early Cretaceous.