Geochronology and geochemistry of Late Cretaceous–Paleocene granitoids in the Sikhote-Alin Orogenic Belt: Petrogenesis and implications for the oblique subduction of the paleo-Pacific plate

Abstract

We present zircon U–Pb ages, major and trace element analyses, and zircon Hf isotope data on the Late Cretaceous–Paleocene granitoids at the southern end of the Sikhote-Alin Orogenic Belt of the Russian Far East. These data are used to discuss the petrogenesis of the granitoids in the context of the paleo-Pacific subduction beneath the eastern Eurasia. Zircons from four granitoid samples give emplacement ages of ~56, ~83, ~91, and ~92Ma. These granitoids with high SiO2 (73.43–76.76wt%) are metaluminous to weakly peraluminous (A/CNK=0.97–1.03) and belong to the high-K calc-alkaline series (K2O=3.75–4.95wt%). They are all enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs), and relatively depleted in high field strength elements (HFSEs) with striking depletion also in Ba, Sr, P and Eu. They are petrographically and geochemically consistent with being of I-type granitoids. The zircons have εHf (t) values (−0.8 to +7.6) higher than whole-rock εHf (t) values predicted from whole-rock εNd (t) (−4.1 to +0.5) in the literature. All these observations suggest that primary magmas parental to these granitoids were likely to have derived from partial melting of a juvenile lower crust accompanied by assimilation with ancient mature crust during magma ascent and evolution. A recent study illustrates that the collision of an exotic terrane carried by the paleo-Pacific plate with the continental China at ~100Ma accreted the basement of the Chinese continental shelf (beneath East and South China Seas), and resulted in a new plate boundary of transform nature between the NNW moving paleo-Pacific plate and the eastern margin of the shelf. Our new data and analysis of existing data support this hypothesis, but we hypothesize that this transform becomes transpressional in its northern segment with oblique subduction of the paleo-Pacific plate beneath northeastern Asia as manifested by the Late Cretaceous-Paleocene granitoids in the Russian Far East-NE China (97–56Ma) and South Korea-SW Japan (96–71Ma). These tectonic and magmatic events took place prior to the opening of the Huanghai, Bohai and Japan seas, which are of rift-origin from continental China much later. Our study emphasizes the necessity and significance of plate tectonics reconstruction in the greater northwestern Pacific region since the Cretaceous.