Zircon U-Pb chronology, geochemistry of the previously "Pre-Sinian" intrusive rocks in Dandong area of southeastern Liaoning Province, and its tectonic implications

LA-ICP-MS zircon U-Pb ages and geochemical data are presented for the previously believed Pre-Sinian intrusive rocks from the Dandong area of southeastern Liaoning Province, with the aim of determining their petrogenesis and the regional tectonic evolution. Detailed petrological studies have shown that the four-selected intrusive rocks are monzogranite, hornblende biotite monzogranite granite granite, biotite monzogranite granite and amphibole gabbro, respectively. All zircons are euhedral-subhedral in shape, display striped absorption and fine-scale oscillatory growth zoning in cathodoluminesence (CL) images, have high Th/U ratios (0.53 similar to 2.40), and show depletion in light rare earth elements (LREEs), relative enrichment in heavy rare earth elements (HREEs), negative Eu and positive Ce anomalies in chondrite-normalized REE patterns, indicating a typical magmatic origin. Zircon U-Pb dating results show that they were formed in the Early Cretaceous (ca. 126Ma). Petrologically and geochemically, the Early Cretaceous intrusive rocks are typical of bimodal magmatism. The granitoids have SiO2 = 66.99% similar to 73.13%, K2O/Na2O = 1.01 similar to 1.43, MgO = 0.45% similar to 2.61%, Mg-# = 29 similar to 56, Al2O3 = 13.97% similar to 14.92%, and display enrichment in LREEs and LILEs and relative depletion in HREEs and HFSEs, suggesting that they have the features of A-type granites and originate by partial melting of lower crust. On the other hand, the amphibole gabbro has SiO2 = 50.22%, MgO = 10.61%, Mg-# = 69, high Cr (196 x 10(-6), Co (44.8 x 10(-6) Ni (73.4 x 10(-6)) contents, is enriched in LREEs and LILEs, and is relatively depleted in HREEs and HFSEs (such as Nb, Ta and Ti), indicating the magma could be derived from the partial melting of the lithospheric mantle modified by subducted slab-derived fluids. Together with the regional geological data, it is proposed that the Early Cretaceous intrusive rocks from the Dandong area of southeast Liaoning Province could form in an extensional environment, similar to a back-arc basin, which be possibly related to the NW subduction of the Paleo-Pacific plate beneath the Eurasian plate.