西秦岭造山带印支期凤县岩体成因及其构造意义  

Petrogenesis and Tectonic Implications of the Indosinian Fengxian Pluton,Western Qinling Orogeny

作　　者：倪春雨[1] 姜耀辉[1] NI Chunyu;JIANG Yaohui(State Key Laboratory for Mineral Deposits Research,School of Earth Sciences and Engineering,Nanjing University,Nanjing 210023,China)

机构地区：[1]内生金属矿床成矿机制研究国家重点实验室,南京大学地球科学与工程学院,南京210023

出　　处：《高校地质学报》2023年第4期559-570,共12页Geological Journal of China Universities

基　　金：国家自然科学基金项目(41872054)资助。

摘　　要：文章对西秦岭造山带东段印支期凤县岩体开展了岩相学,锆石SHRIMP U-Pb年代学及原位Hf同位素地球化学和全岩主微量及Sr-Nd同位素地球化学研究,并探讨了岩石成因及其构造意义。结果表明,凤县岩体由花岗闪长岩和石英二长闪长岩组成,侵位于中三叠世(~237.5 Ma);岩石SiO_(2)含量为61.7~66.7 wt%,属准铝质岩石和高钾钙碱性系列,并具有较高的Mg^(#)值(0.53~0.58),富集轻稀土,无Eu负异常,同时富集大离子亲石元素,亏损高场强元素;岩石显示埃达克岩地球化学特征:高Sr(474×10^(-6)~629×10^(-6)>400×10^(-6)),低Y (6.85×10^(-6)~7.60×10^(-6)<18×10^(-6))和Yb(0.57×10^(-6)~0.72×10^(-6)<1.9×10^(-6)),高的Sr/Y和La/Yb比值;全岩初始^(87)Sr/^(86)Sr比值为0.7072~0.7077,ε_(Nd)(t)值为-4.2~-5.8,锆石原位ε_(Hf)(t)值为-1.1~-4.3。通过综合研究认为凤县花岗质岩石形成于阿尼玛卿—勉略洋洋壳北向俯冲环境,是由俯冲洋壳上覆沉积物发生部分熔融并伴随熔体上侵过程中与地幔楔相互作用而形成的;凤县花岗岩成因暗示阿尼玛卿—勉略洋洋壳在237.5 Ma之前已经向西秦岭之下俯冲,西秦岭东段至少在中三叠世仍处于陆缘弧环境。This paper reports systematically petrological and SHRIMP zircon U-Pb dating,whole-rock geochemical and Sr-Nd-Hf isotopic research for the Fengxian Pluton.We discussed the magmatic evolution,source characteristics,tectonic and geodynamic settings of these plutons.Our study shows that:1.The Fengxian pluton is composed of granodiorite and quartz monzonite,invaded in the Middle Triassic(~237.5 Ma).The SiO_(2) content of the rock is 61.7~66.7 wt%,belonging to the high potassium calc-alkaline series,with relatively high MgO content(1.49-2.43 wt%)and Mg^(#)value(0.53-0.58),belonging to the high-K calc-alkaline series.They are enriched in light rare earth elements(LREE)and lack marked negative Eu anomalies.They are also enriched in LILE and depleted in HFSE.Their high Sr(474×10^(-6)^(-6)29×10^(-6))low Y(6.85×10^(-6)-7.60×10^(-6))and Yb(0.57×10^(-6)-0.72×10^(-6))value are similar to those of adakites.They showε_(Nd)(t)values of-4.2 to-5.8,initial ^(87)Sr/^(86)Sr ratios of 0.7072-0.7077 andε_(Hf)(t)values of-1.1 to-4.3.We believe that the Fengxian pluton is mainly derived from subducted sediments in the continental margin environment of the northward subduction of the Paleo-Tethys oceanic slab.The origin of the Fengxian pluton suggests that the Animaqing-Mianlue oceanic crust had subducted under the West Qinling before 237.5 Ma,and the eastern segment of the West Qinling remains a continental arc till the Middle Triassic.

关 键 词：西秦岭 印支期 花岗岩 俯冲沉积物 古特提斯洋 

分 类 号：P588.115[天文地球—岩石学]