西秦岭甘南早子沟石英闪长玢岩岩石成因及其地质意义  

Petrogenesis of the quartz diorite porphyry in the Zaozigou,West Qinling and its geological implications

作　　者：薛宪法 邱昆峰[1] 何登洋 黄雅琪 朱锐 于皓丞 姜桂鹏 XUE XianFa;QIU KunFeng;HE DengYang;HUANG YaQi;ZHU Rui;YU HaoCheng;JIANG GuiPeng(School of Earth Science and Resources,China University of Geosciences,Beijing 100083,China;Gansu Hezuo Zaozigou Gold Company,Zhaojin Mining Industry Co.,Ltd.,Hezuo 747000,China)

机构地区：[1]中国地质大学(北京)地球科学与资源学院,北京100083 [2]甘肃省合作早子沟金矿有限责任公司,招金矿业股份有限公司,合作747000

出　　处：《岩石学报》2023年第2期463-483,共21页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(42072087、42261134535);高等学校学科创新引智计划(BP0719021);矿床地球化学国家重点实验室开放研究(202202);北京市科技新星计划课题(Z201100006820097)联合资助。

摘　　要：西秦岭造山带位于华北板块和华南板块之间,是我国中央造山带的重要组成部分,记录了东亚古特提斯洋东北支的演化历史,对于探讨华北板块和华南板块的碰撞过程具有重要意义。西秦岭广泛发育的早-中三叠世火成岩与勉略洋壳向北俯冲有关,但勉略洋壳的闭合时限仍有争议。因此本文对采自早子沟地区的石英闪长玢岩进行锆石U-Pb年代学、Lu-Hf同位素和全岩主微量地球化学分析,查明早子沟石英闪长玢岩成因及其构造演化背景,并进一步约束勉略洋壳闭合时间。锆石LA-ICP-MS U-Pb定年结果表明,石英闪长玢岩侵位年龄为~232Ma,为中三叠世岩浆活动的产物。石英闪长玢岩含有较高的SiO_(2)(56.18%~68.26%),K_(2)O为1.82%~3.70%,A/CNK值为0.73~1.07,是准铝质-弱过铝质的钙碱性-高钾钙碱性岩石。其富钾的特征,与中高钾基性下地壳部分熔融产生的熔体成分一致。轻重稀土分异明显,(La/Yb)N比值为8.10~61.2,富集大离子亲石元素(K、Sr、Ba、Cs),亏损高场强元素Nb、Ta,具有弱负铕异常(δEu=0.65~0.93),显示了典型的大陆地壳岩石特征。石英闪长玢岩较高的Mg^(#)值(47.57~70.35)意味着有一定幔源物质的加入。ε_(Hf)(t)在-12.9~-7.2之间,对应的二阶段Hf模式年龄(t2DM)为1.70~2.10Ga,表明岩浆来自于古元古代下地壳的重熔。本文认为石英闪长玢岩形成于勉略洋壳向北俯冲背景下的活动大陆边缘环境,具有碰撞前花岗质岩石特征,即勉略洋壳在~232Ma仍处在俯冲阶段。The West Qinling Orogenic Belt,located between the North China Block and the South China Block,is one of the most important gold regions in China.It records the evolution of the northeastern branch of the Paleo-Tethys in East Asia,which is of great significance for understanding the collision of the North China Block and the South China Block.Most of the Early-Middle Triassic igneous rocks are associated with the subduction process of the Mianlue Ocean,however,its closure time remains unclear.In present study,the zircon U-Pb geochronology,major-and trace-element geochemistry,and Lu-Hf isotopic analyses are employed to investigate the petrogenesis and geodynamic setting of the quartz diorite porphyry from the Zaozigou,and to constrain the closure time of the Mianlue Ocean.Zircon LA-ICP-MS U-Pb age suggests the quartz diorite porphyry intruded at ca.232Ma,as the products of Middle Triassic magmatism.The quartz diorite porphyry is characterized by relatively high SiO_(2)values(56.18%~68.26%),high K_(2)O contents(1.82%~3.70%)and intermediate A/CNK values(0.73~1.07).Their metaluminous to weakly peraluminous and potassium-rich characteristics reveal that the Triassic granitoids are generated by partial melting of a medium-to high-K mafic lower crust.Moreover,they are enriched in LREEs,depleted in HREEs,and characterized by negative Eu anomalies in the chondritenormalized REE patterns,which is typical of continental crust rocks.The high Mg^(#)values(47.57~70.35)of the quartz diorite porphyry indicate the involvement of lithospheric mantle materials and/or magma mixing.The relatively wide ranges ofε_(Hf)(t)from-12.9 to-7.12 and two-stage Hf model ages(1.70~2.10Ga)imply that the magma originated from partial melting of the PaleoProterozoic lower crust.We conclude that the quartz diorite porphyry was formed in the active continental margin,with characteristics of pre-collision granitic rock,and formed under the background of northward subduction of the Mianlue ocean crust,supporting that the Mianlue oceanic crust closed a

关 键 词：锆石U-PB年代学 LU-HF同位素 全岩地球化学 早子沟石英闪长玢岩 西秦岭造山带 

分 类 号：P588.134[天文地球—岩石学] P597.3[天文地球—地质学]