Palaeoproterozoic granitic magmatism in the northern segment of the Jiao-Liao-Ji Belt: implications for orogenesis along the Eastern Block of the North China Craton

As the northern segment of the Jiao-Liao-Ji Belt (JLJB), the Palaeoproterozoic Liaoji Belt is a key region for deciphering the formation and evolution of the North China Craton (NCC). In this study, we present the geochronology, geochemical, and isotopic studies on the monzogranitic gneiss, which is one of the major lithotectonic elements of the Liaoji Belt. LA-ICP-MS zircon U–Pb dating reveals that the studied monzogranitic gneisses were formed in the period of 2213–2178 Ma. They are in tectonic contact with the Palaeoproterozoic volcano-sedimentary rocks in the field. The monzogranitic gneisses belong to the high-K calc-alkaline series, and are metaluminous to peraluminous. They have 10,000 Ga/Al ratios of 2.63–3.14 with an average of 2.90, and are thus classified as aluminous A-type granites. Their εNd(t) values vary from −3.4 to +2.5, indicating heterogeneous source region. The monzogranitic gneisses are characterized by enrichment in LREE and LILE (e.g. Rb, Ba, Th, and K) and depletion in HREE and HFSE (such as Nb, Ta, and Ti), and are typical to magmatism in active continental margins formed in a subduction-related tectonic setting. Taking into account their A-type affinity and regional geological data, we suggest that the monzogranitic gneisses were most probably generated in a local extensional back-arc environment during subduction.

作为胶辽吉带（Jiao-Liao-Ji Belt, JLJB）的北段，古元古代辽吉带是解读华北克拉通（North China Craton, NCC）形成与演化的关键区域。本研究针对辽吉带主要岩石构造单元之一的二长花岗质片麻岩，开展了年代学、地球化学与同位素研究。LA-ICP-MS锆石U–Pb定年结果显示，研究区二长花岗质片麻岩的形成时代为2213~2178 Ma。野外地质调查表明，该类片麻岩与古元古代火山沉积岩呈构造接触关系。二长花岗质片麻岩属于高钾钙碱性系列，为准铝质至过铝质岩石；其10000 Ga/Al比值介于2.63~3.14之间，平均值为2.90，因此被归为铝质A型花岗岩。该类岩石的εNd(t)值变化范围为-3.4至+2.5，指示其源区具有不均一性。二长花岗质片麻岩以轻稀土元素（LREE）、大离子亲石元素（LILE，如Rb、Ba、Th、K）富集，重稀土元素（HREE）与高场强元素（HFSE，如Nb、Ta、Ti）亏损为特征，属于俯冲相关构造背景下活动大陆边缘的典型岩浆岩组合。结合其A型花岗岩亲缘性与区域地质资料，我们认为该二长花岗质片麻岩最可能形成于俯冲过程中局部伸展的弧后环境。