Intracontinental magmatism and its chronology, geochemistry, Lu-Hf isotopic characteristics—a case from Mesozoic composite pluton in the North China Craton

The influence, extent, intensity, and spatiotemporal evolution of the subduction of the paleo-Pacific Plate on Eastern China during the Middle Mesozoic remains unknown. The Shanxi province, located in the central part of the North China Craton, has found several Mesozoic igneous rocks as typical intracontinental magmatism. Here, systematic zircon LA-ICP-MS U-Pb dating, whole rock geochemistry, and in-situ zircon Lu-Hf isotopic analysis were conducted on different lithologies from the outcrop of the composite pluton to study how these Mesozoic rocks formed and whether they are directly or indirectly related to the subduction of the paleo-Pacific Plate. The results show that the composite pluton has a high silica, potassium-calcareous alkalinity, and aluminum content, and it is classified as an I/S granite. The composite pluton is enriched in large ionic lithophilic elements such as Rb and K and depleted in high field strength elements such as Nb and Ta. The chondrite-normalized rare earth element patterns exhibit a right skewed distribution of light rare earth element enrichment and heavy rare earth element depletion. The overall formation of the composite pluton occurred in four different periods. At 162±1 Ma, the quartz-porphyry originated from the lower paleo-crust and was subjected to water-rich fluids. At 152±1 Ma, the porphyritic biotitic granite originated from the metamorphic composite pluton in the middle and upper crust, which were formed by the breakdown and dehydration of mica as a result of decompression melting. At 142±1 Ma, the quartz diorite formed in the thickened lower crust, with contribution of mantle material during the formation process, and was affected by subduction fluids. At 136±2 Ma, the granite formed via the activation of magma from remelted metamorphic sedimentary rock in the upper-middle crust and mixed with magma originating from the younger crust. In addition to the regional geological background, it is believed that the magmatism indicates that the central part of the North China Craton underwent a transition from compression to extension from the Late Jurassic to the Early Cretaceous, and this transition was related to the subduction of the paleo-Pacific Plate as it began to subduct under eastern Asia in the Early Jurassic, and then Cretaceous extensional state due to the retreat of the Pacific Ocean.