Geochronology and geochemistry of the Late Cretaceous granitoids in the northern margin of the Sanjiang basin, NE China and its tectonic implications

This paper reports zircon U-Pb dating, geochemical and Hf isotope data of the Late Cretaceous granitoids from the northern margin of the Sanjiang basin, NE China, with the aim of constraining formation time, magma source, and tectonic setting of these granitoids. Zircons from these granitoids are euhedral-subhedral in shape, display fine-scale oscillatory growth zoning, and have Th/U ratios of 0.32 similar to 1.09, indicating a magmatic origin. The dating results indicate that these granitoids formed in the Late Cretaceous (88.9 similar to 95.1Ma) rather than previously believed Late Indo-Chinese epoch. The Late Cretaceous granitoids consist of hornblende-bearing biotite granodiorite, have SiO2 = 63.9% similar to 68.1%, Na2O = 3.03% similar to 425%, K2O = 2.28% similar to 3.27%, Na2O/K2O =1.06 similar to 1.73, A/CNK = 0.94 similar to 1.08. Their REE patterns are right-leaned and characterized by LREE/HREE = 8.13 similar to 15.5, (La/Yb)(N) = 8.61 similar to 1.0, and delta Eu = 0.82 similar to 1.11. These granitoids are relatively enriched in LREE and LILE (such as Rb, Ba, K), and depleted in heavy rare earth elements (HREEs) and high field strength elements (HFSEs) such as Nb, Ta, and Ti, as well as P. The epsilon(Hf) (t) values of zircons from these granitoids range from +4.44 to +9.00, their t(DM2) vary from 583Ma to 875Ma. Taken together, it is suggested that these granitoids chemically belong to metaluminous-weak peraluminous and middle-to high-K calc-alkaline I type ones, and that their primary magma could be derived from partial melting of mafic materials within lower crust. Combined with the coeval igneous rock associations and regional tectonic evolution, we conclude that the Late Cretaceous granitoids occurred in an active continental margin setting related to the subduction of the Paleo-pacific plate beneath the East Asia continent, and this episode of magmatic event would mark the beginning of orthogonal (northwestward) subduction of the Paleo-pacific plate.