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.

摘要

古太平洋大洋板块俯冲在晚中生代引起欧亚大陆东缘广泛的岩浆活动、岩石圈变形和金属矿化已成为广泛共识。然而，古太平洋俯冲的起始时间和程度，特别是古亚洲洋向古太平洋的构造机制转变的时间，以及古太平洋俯冲的程度仍然受到很大限制。针对这些关键问题，我们介绍了华北克拉通北缘侏罗系侵入体的区域地质观测、锆石 U-Pb 定年、全岩地球化学和 Hf 同位素组成，旨在约束其成因和与古太平洋俯冲有关的构造-岩浆事件。年代学和地球化学结果表明，这些侵入体位于177～155 Ma之间，具有明显的地球化学特征。花江沟岩体（HJG）、童家屯岩体（TJT）和豪官屯岩体（HGT）具有高Al₂ O ₃和Sr含量，低MgO、^T Fe ₂ O ₃、Y、Yb 和重稀土元素 (HREEs) 含量，加上高 Sr/Y 值 (34.10–94.36)，显示出埃达克体亲和性。基于这些地球化学特征，我们认为这些埃达克质岩浆（即 HJG、TJT 和 HGT 岩体）是由加厚的下地壳部分熔融产生的，以响应古太平洋板块在北缘北缘之下的早期俯冲。华北克拉通。相比之下，白家沟岩体（BJG）和社山沟岩体（SSG）为高钾钙碱性岩石，Sr/Y比值较低（0.80~17.77），具有I型花岗岩的岩石学和地球化学特征，表明起源于下地壳，具有不同程度的分级结晶。此外，巴克舒岩体（BKS）主要由闪长岩组成，显示出高SiO₂、低MgO、Cr、Ni含量，含Mg ^#39.8-42.9，表明较低的地壳起源。尽管这些侵入体的年龄范围很广，但侏罗纪钙碱性侵入体均表现出典型的活动大陆边缘地球化学特征，如大离子亲石元素（LILEs）和轻稀土元素（LREEs）富集，高场贫化。强度要素（HFSE）。结合华北克拉通北缘同代岩浆岩的时空变化，我们认为古亚洲洋向古太平洋的构造机制转变始于早侏罗世。这些侏罗纪侵入体形成于活跃的大陆边缘环境中，与古太平洋板块向西俯冲有关。重要的，