西藏那曲南措金铜矿区酸性侵入岩锆石U-Pb年龄、地球化学特征及地质意义

对西藏那曲南措金铜矿区内与成矿关系密切的花岗闪长岩和花岗闪长斑岩运用LAICP-MS锆石U-Pb测年与岩石地球化学的分析方法确定其形成时代、岩浆源区性质及成岩构造背景。结果显示,岩石样品的加权平均年龄分别为159.2 Ma±0.69 Ma和161.5 Ma±1.5 Ma,形成于中—晚侏罗世。地球化学方面,岩石属于钙碱性系列,A/CNK介于0.83~0.98,Mg#值为53~59,具有高Sr(525×10~(-6)~901×10~(-6))、低Y(12.05×10~(-6)~13.72×10~(-6))、Yb(1.16×10~(-6)~1.44×10~(-6))、富Na(Na2O/K2O值为2.07~2.43)的特征,LREE富集,Eu弱正异常,判定岩石属于O型埃达克岩类。岩石轻重稀土元素分馏明显,总体相对富集大离子亲石元素(Rb、K、Ba)和轻稀土元素(Sr、Zr、Hf),相对亏损高场强元素(Ta、Nb、Ti);无明显的Eu负异常,显示了与火山弧岩浆微量元素地球化学性质的亲缘性。岩石的Nb/Ta值为10.45~18.79,Zr/Hf值为32.76~36.90,判定岩浆起源于洋壳部分熔融并有少量地幔物质混入。结合前人研究结果认为,岩石形成于班公湖—怒江洋盆板片向北俯冲的活动大陆边缘环境。

Zircon U-Pb ages and geochemical characteristics of the acidic intrusive rocksin Nancuo Au-Cu deposit,Nakchu,Tibet,and its geological significance

LA-ICP-MS zircon U-Pb dating and lithogeochemical analysis of the granodiorite and granodiorite porphyry that are closely related to the deposit genesis in Nancuo Au-Cu deposit,Nakchu,Tibet,are conducted to determine their geochronology,magma sources and tectonic settings.The zircon dating results show that granodiorite and granodiorite porphyry were formed in Middle-Late Jurassic (159.2 Ma±0.69 Ma and 161.5 Ma±1.5 Ma).Geochemically,these rocks have A/CNK=0.83-0.98,Mg#=53-59,belonging to the calc-alkaline series.The rocks are considered O-type adakitic rocks with high Sr (525×10-6-901×10-6),low Y(12.05×10-6-13.72×10-6) and Yb (1.16×10-6-1.44×10-6),Na enrichment(Na2O/K2O value of2.07-2.43),LREE enrichment and minor positive Eu anomalies.They have high degree of fractionation,generally with LILEs (Rb,K and Ba) and LREEs (Sr,Zr and Hf) enriched,HFSEs (Ta,Nb and Ti) relatively depleted.No evident negative Eu anomalies are shown,which indicated its affinity to the geochemical characteristics of the microelements in volcanic arc magma.The values of Nb/Ta and Zr/Hf are 10.45-18.79 and 32.76-36.90,suggesting a mixed source from partial melting of oceanic crust and depleted mantle.The paper,combining the study with the predecessors',thinks that the rocks were formed in the active continental margin of Bangonghu-Nujiang oceanic crust northward subduction.