Transition in tectonic regime from the Paleo-Asian Ocean to Paleo-Pacific Ocean: constraints from the Jurassic adakitic and I-type granites, and calc-alkaline diorites at the northern margin of the North China Craton

Abstract

ABSTRACT There is a broad consensus that subduction of the Paleo-Pacific oceanic plate has caused extensive magmatism, lithospheric deformation, and metal mineralization in the eastern margin of the Eurasian continent during the Late Mesozoic. However, the onset and extent of subduction of the Paleo-Pacific Ocean, especially the timing of the transition in tectonic regime from the Paleo-Asian Ocean to Paleo-Pacific, and extent of the subduction of the Paleo-Pacific Ocean remain poorly constrained. To address these key issues, we present regional geological observations, zircon U–Pb dating, whole-rock geochemistry, and Hf isotopic compositions of the Jurassic intrusions at the northern margin of the North China Craton, with the aim of constraining their petrogenesis and the tectono-magmatic events related to subduction of the Paleo-Pacific Ocean. The geochronological and geochemical results indicate that these intrusions were emplaced between 177 and 155 Ma, with distinct geochemical features. The Huajianggou pluton (HJG), Tongjiatun pluton (TJT), and Haoguantun pluton (HGT) are characterized by high Al2O3 and Sr contents, and low MgO, TFe2O3, Y, Yb, and heavy rare earth elements (HREEs) contents, coupled with high Sr/Y values (34.10–94.36), showing adakitic affinities. Based on these geochemical characteristics, we suggest that these adakitic magmas (i.e. the HJG, TJT, and HGT plutons) were produced by partial melting of thickened lower crust in response to earlier subduction of the Paleo-Pacific oceanic plate beneath the northern margin of the North China Craton. In contrast, the Baijiagou pluton (BJG) and Sheshangou pluton (SSG) are high-K calc-alkaline rocks with low Sr/Y ratios (0.80–17.77), and they exhibit petrological and geochemical characteristics of I-type granites, indicating an origin from the lower crust, with variable degrees of fractional crystallization. In addition, the Bakeshu pluton (BKS) is mainly comprised of diorite that displays high SiO2, low MgO, Cr, and Ni contents, with Mg# of 39.8–42.9, indicating a lower crustal origin. Despite the range in ages of these intrusions, all the Jurassic calc-alkaline intrusions show typical geochemical characteristics of active continental margins, such as enrichment in large ion lithophile elements (LILEs) and light rare earth elements (LREEs), and depletion in high field strength elements (HFSEs). Combined with spatial-temporal variations of coeval magmatic rocks along the northern margin of the North China Craton, we suggest that the transition in tectonic regime from the Paleo-Asian Ocean to Paleo-Pacific Ocean was initiated in the Early Jurassic. These Jurassic intrusions formed in an active continental margin setting, related to westward subduction of the Paleo-Pacific oceanic plate. Importantly, the Early Jurassic subduction-related calc-alkaline magmatic rocks in northeast (NE) China constitute a NE-trending magmatic arc belt, which is best explained as continental magmatism triggered by the westward subduction of the Paleo-Pacific oceanic plate.