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

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.

学界普遍认为，晚中生代时期古太平洋（Paleo-Pacific）大洋板块的俯冲作用，引发了欧亚大陆东缘广泛的岩浆作用、岩石圈变形与金属成矿作用。然而，古太平洋大洋板块俯冲的起始时间与影响范围，尤其是构造体制从古亚洲洋（Paleo-Asian Ocean）向古太平洋转换的时间节点，以及古太平洋大洋板块俯冲的波及范围，仍缺乏精确约束。为解决这些关键科学问题，我们针对华北克拉通（North China Craton）北缘的侏罗纪侵入岩开展了区域地质观测、锆石U-Pb定年、全岩地球化学分析与Hf同位素组成测试，旨在明确其岩石成因，以及与古太平洋大洋板块俯冲相关的构造-岩浆事件。年代学与地球化学结果显示，这些侵入岩的侵位时代介于177 Ma至155 Ma之间，且具有独特的地球化学特征。花涧沟岩体（Huajianggou pluton, HJG）、佟家屯岩体（Tongjiatun pluton, TJT）与郝官屯岩体（Haoguantun pluton, HGT）具有高Al₂O₃与Sr含量、低MgO、全铁（TFe₂O₃）、Y、Yb及重稀土元素（heavy rare earth elements, HREEs）含量的特征，同时伴随较高的Sr/Y比值（34.10~94.36），表现出埃达克质岩的地球化学属性。基于上述地球化学特征，我们认为这些埃达克质岩浆（即HJG、TJT与HGT岩体）形成于华北克拉通北缘下方古太平洋大洋板块早期俯冲引发的加厚下地壳的部分熔融。与之相反，白家沟岩体（Baijiagou pluton, BJG）与社山沟岩体（Sheshangou pluton, SSG）为高钾钙碱性岩石，具有较低的Sr/Y比值（0.80~17.77），且呈现I型花岗岩的岩石学与地球化学特征，指示其起源于下地壳，并经历了不同程度的分离结晶作用。此外，巴克树岩体（Bakeshu pluton, BKS）主要由闪长岩组成，具有高SiO₂、低MgO、Cr与Ni含量的特征，镁指数（Mg#）为39.8~42.9，同样指示下地壳起源。尽管这些侵入岩的形成时代存在一定差异，但所有侏罗纪钙碱性侵入岩均表现出活动大陆边缘的典型地球化学特征：如富集大离子亲石元素（large ion lithophile elements, LILEs）与轻稀土元素（light rare earth elements, LREEs），且亏损高场强元素（high field strength elements, HFSEs）。结合华北克拉通北缘同期岩浆岩的时空分布特征，我们认为构造体制从古亚洲洋向古太平洋的转换起始于早侏罗世。这些侏罗纪侵入岩形成于活动大陆边缘环境，与古太平洋大洋板块向西俯冲作用相关。尤为重要的是，中国东北地区与俯冲作用相关的早侏罗世钙碱性岩浆岩，构成了一条北东向的岩浆弧带，这一特征最合理的解释为古太平洋大洋板块向西俯冲引发的大陆岩浆作用。