藏南米拉山地区中新世埃达克质岩石的年代学、地球化学和成因  

Geochronology,geochemistry and petrogenesis of Miocene adakitic rocks in Milashan,Southern Tibet

作　　者：孙浩哲 赵志丹[1] 朱弟成[1] 刘栋[1] 王青[1] 唐演 齐宁远 董国臣[1] SUN HaoZhe;ZHAO ZhiDan;ZHU DiCheng;LIU Dong;WANG Qing;TANG Yan;QI NingYuan;DONG GuoChen(State Key Laboratory of Geological Processes and Mineral Resources,School of Earth Science and Mineral Resources,China University of Geosciences,Beijing 100083,China)

机构地区：[1]中国地质大学地质过程与矿产资源国家重点实验室,中国地质大学地球科学与资源学院,北京100083

出　　处：《岩石学报》2021年第11期3479-3500,共22页Acta Petrologica Sinica

基　　金：第二次青藏高原综合科学考察研究项目(2019QZKK0702);国家自然科学基金项目(42073035、9206220026);国家重点研发计划项目(2016YFC0600304)联合资助.

摘　　要：中新世是青藏高原隆升、增厚的重要时期,并且在这一时期内拉萨地块广泛发育碰撞后岩浆岩。本文对南拉萨地块米拉山地区的钙碱性钾质火山岩进行了锆石U-Pb年代学、Lu-Hf同位素和全岩主量、微量元素的测定与系统研究。米拉山中新世火山岩为粗面英安岩、英安岩和流纹岩(SiO 2=59.89%~71.78%)。锆石U-Pb定年结果为16.1±0.2Ma~20.4±0.3Ma,表明其喷发时代为中新世。岩石具有较高的Al 2O 3含量(13.54%~16.31%),低MgO(0.46%~1.95%)、高Sr(388×10^(-6)~804×10^(-6))、低Y(6.55×10^(-6)~11.20×10^(-6))和Yb(0.70×10^(-6)~1.07×10^(-6))的特征,具有较高的Sr/Y值(51~80)、低相容元素(Cr=4.26×10^(-6)~32.53×10^(-6),Ni=4.16×10^(-6)~25.75×10^(-6))和弱Eu负异常。岩石具有轻稀土元素和Rb、Th、U、K等元素富集、重稀土元素和高场强元素Nb、Ta、Ti亏损的特征。米拉山中新世火山岩显示出埃达克质岩石的地球化学特征,可能来自于的镁铁质加厚下地壳的部分熔融,推测下地壳源区是石榴石角闪岩。锆石εHf(t)值为+2.2~+7.8,表明源区为新生地壳物质,有俯冲板片熔体加入。米拉山中新世火山岩的喷发时代与米拉山断裂活动时间一致,二者可能同为拉萨地块岩石圈拆沉的结果。Miocene is an important period for the uplifting and thickening of the Tibetan Plateau,and meanwhile,post-collisional igneous rocks are widely developed.The zircon U-Pb dating and Lu-Hf isotopes,and whole rock major and trace element compositions were analyzed on the calc-alkaline potassic volcanic rocks in the Milashan area,Southern Lhasa terrane.Milashan Miocene volcanic rocks are trachydacite,dacite and rhyolite,with intermediate to acid composition and SiO_(2) ranging from 59.89%to 71.87%.The zircon U-Pb dating yielded ages from 16.1±0.2Ma to 20.4±0.3Ma.All samples exhibit high Al_(2)O_(3)(13.54%~16.31%),low MgO(0.46%~1.95%),high Sr(388×10^(-6)~804×10^(-6)),low Y(6.55×10^(-6)~11.20×10^(-6))and Yb(0.70×10^(-6)~1.07×10^(-6)),high Sr/Y ratios,low compatible element concentration(Cr=4.26×10^(-6)~32.53×10^(-6),Ni=4.16×10^(-6)~25.75×10^(-6))and slight negative Eu anomaly.The samples are characterized by enrichment of light rare earth elements and large ion lithophile element(LILEs,Rb,Th,U,K),and depletion of heavy rare earth elements and high field strength element(HFSEs,Nb,Ta,Ti).These Miocene volcanic rock samples show adakitic affinity,and may have been formed by partial melting of mafic thickened lower crust.The source region is probably garnet amphibolite.Lu-Hf isotopic determination yielded positive zirconεHf(t)values ranging from+2.2 to+7.8,indicating that the source was juvenile crust with the addition of subducted slab melt.Milashan Miocene volcanic rocks were formed at the same time as the action of Milashan fault,which probably suggests that both Milashan Miocene volcanic rocks and Milashan fault are caused by the delamination of Lhasa terrane lithosphere.

关 键 词：南拉萨地块 米拉山地区 中新世 钙碱性钾质火山岩 埃达克质岩石 

分 类 号：P588.14[天文地球—岩石学] P597.3[天文地球—地质学]