Geochronology and geochemistry of the Late Triassic intrusive rocks in the Liaodong Peninsula, NE China: Petrogenesis and implications for early Mesozoic tectonic evolution

Abstract

ABSTRACT New whole-rock major and trace element, Sr-Nd isotopic, Zircon U-Pb, and Hf isotopic data for the Late Triassic intermediate-felsic intrusive rocks in the Liaodong Peninsula in the northeastern part of the North China Craton (NCC) were obtained in order to investigate their petrogenesis and provide insights into the Late Triassic tectonic evolution of the eastern segment of the NCC. The laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb ages indicate that they were emplaced at 230–219 Ma. Based on the petrology, petrography, and geochemistry, we divided these intrusions into dioritic rocks and granitic rocks. The dioritic rocks are mainly high-K calk-alkaline to shoshonitic biotite diorite and biotite amphibolite diorite. They are metaluminous, have high MgO contents at low to intermediate silica contents, are depleted in heavy rare earth elements (HREEs) and high field-strength elements (HFSEs), are enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs), and have εNd(t) values of −16.99 to −9.78 and εHf(t) values of −15.2 to −6.9, indicating that they were mainly derived from the lithospheric mantle, possibly with a small contribution from ancient continental crust. The granitic rocks mainly include biotite monzogranite and monzogranite. They have low MgO, Yb, and Y concentrations and high Sr/Y, La/Yb, and silica contents. They are high-K calk-alkaline to shoshonitic and metaluminous to weakly peraluminous adakitic I-type granites. They are enriched in LREEs and LILEs, are depleted in HREEs and HFSEs, and have εNd(t) values of −16.95 to −14.21 and εHf(t) values of −18.4 to −10.8, indicating that they were mainly derived from partial melting of a thickened ancient lower continental crust. Combined with the spatial and temporal distributions of the Late Triassic rocks, we conclude that these Late Triassic rocks were formed in an extensional environment resulting from slab break-off of the subducted Yangtze Craton beneath the North China Craton, which also resulted in lithospheric thinning of the North China Craton.