藏南南木林地区闪长岩脉的发现及其地质意义  

Discovery and geological significance of diorite dykes in the Namling area,southern Tibet

作　　者：黄永高 秦宇龙[2] 康志强[1] 韩飞[2] 李应栩 冯佐海[1] 李光明[3] 贾小川[2] 杨学俊[2] Huang Yonggao;Qin Yulong;Kang Zhiqiang;Han Fei;Li Yingxu;Feng Zuohai;Li Guangming;Jia Xiaochuan;Yang Xuejun(Collaborative Innovation Center for Exploration of Nonferrous Metal Deposits and Efficient Utilization of Resources by the Province and Ministry,Guilin University of Technology,Guilin,Guangxi 541004,China;Evaluation and Utilization of Strategic Rare Metals and Rare Earth Resource Key Laboratory of Sichuan Province&Sichuan Geological Survey,Chengdu,Sichuan 610081,China;Chengdu Center,China Geological Survey,Chengdu,Sichuan 610081,China)

机构地区：[1]桂林理工大学,有色金属矿产勘查与资源高效利用省部共建协同创新中心,广西桂林541004 [2]四川省地质调查院,稀有稀土战略资源评价与利用四川省重点实验室,四川成都610081 [3]中国地质调查局成都地质调查中心,四川成都610081

出　　处：《地质学报》2023年第5期1434-1446,共13页Acta Geologica Sinica

基　　金：国家自然科学基金项目(编号42173048,41863005);国家重点研发计划项目(编号2020YFA0714804);中国地质调查局项目(编号DD20190147);广西研究生教育创新计划项目(编号YCBZ2021065)联合资助的成果。

摘　　要：在拉萨地体中南部南木林地区发现了侵入于晚古生代盆地中的闪长岩脉,并对其开展了岩石学、锆石U-Pb年代学和全岩主微量元素地球化学研究,以探讨其岩石成因和构造背景。野外地质调查和岩石学研究表明,闪长岩脉以岩枝或岩株状顺层侵入于晚古生代地层中,以闪长岩、石英闪长岩和辉石闪长岩为主。LA-ICP-MS锆石U-Pb测年获得58.31±0.39 Ma、60.53±0.96 Ma、61.75±0.44 Ma、65.02±0.31 Ma和68.71±0.92 Ma五组年代学数据,显示其为晚白垩世晚期到古新世岩浆活动的产物。岩石具有较低含量的SiO_(2)(49.58%~55.61%),较高含量的Al_(2)O_(3)(16.56%~17.38%)、MgO(3.09%~5.48%)和Mg^(#)值(45~55),属于高钾钙碱性—钾玄质系列准铝质岩石。稀土元素标准化配分型式呈右倾式,稀土元素总含量低,轻重稀土分异中等,具有微弱的Eu异常(δEu=0.85~0.95),微量元素蛛网图显示富集大离子亲石元素(Rb、Ba、K)和轻稀土元素,亏损高场强元素(Nb、Ta、Ti),与俯冲带岩浆岩地球化学特征相似。锆石结晶温度平均为728℃,表明其岩浆经历了在水近饱和条件下发生的熔融过程。综合研究认为,南木林地区的闪长岩脉可能形成于新特提斯洋北向俯冲消减过程中,是俯冲沉积物部分熔融形成的熔体交代上覆地幔楔的产物。Diorite dykes intrusive in the Late Paleozoic basin were discovered in the Namling area in the middle part of Gangdese belt.Petrology,zircon U-Pb geochronology,major and trace element geochemistry studies were carried out to explore their petrogenesis and tectonic setting.Field geological survey and petrological studies show that diorite dykes are dominated by diorite,quartz diorite and pyroxene diorite,occurring as apophyses or stock,intruded into the Late Paleozoic strata.LA-ICP-MS zircon U-Pb dating obtained five groups of ages,58.31±0.39 Ma,60.53±0.96 Ma,61.75±0.44 Ma,65.02±0.31 Ma and 68.71±0.92 Ma,indicating that they were the product of magmatic activity from the late Late Cretaceous to the Paleocene.These dykes have relatively low content of SiO_(3)(49.58%~55.61%),high content Al_(2)O_(3)(16.56%~17.38%),MgO(3.09%~5.48%)and Mg^(#)(45~55),which belong to the series of high-potassic calc-alkalic to shoshonite and metaluminous.All samples are enriched in LREEs and depleted in HREEs with distinct right-dipping distribution and weak Eu anomalies(δEu=0.85~0.95).Their trace elements are relatively enriched in LREEs and large-ion lithophile elements such as Rb,Ba and K,depleted in high field strength elements,such as Nb,Ta and Ti,similar to those igneous rocks that form in subduction zone.The average temperature of zircon crystallization is 728℃indicating that the magma had experienced melting process under the condition of nearly complete water saturation.Considering the temporal and spatial distribution of the Late Cretaceous to Paleocene magmatic rocks along the middle Gangdese,it is proposed that the northward subduction of Neo-Tethys was responsible for the diorite dykes in the Namling area.The rock is interpreted as resulting from partial melting of subduction sediments metasomatized with the overlying mantle wedge.

关 键 词：闪长岩 锆石U-PB定年 地球化学 南木林 拉萨地体 

分 类 号：P597.3[天文地球—地球化学] P588.122[天文地球—地质学]