西藏冈底斯东部门巴地区晚白垩世埃达克质岩的岩石成因  

Petrogenesis of Late Cretaceous adakite-like rocks in Mamba from the eastern Gangdese,Tibet

作　　者：孟繁一[1] 赵志丹[1] 朱弟成[1] 张亮亮[1] 管琪[1,2] 刘敏[1] 于枫[1] 莫宣学[1] 

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 [2]石家庄经济学院资源学院,石家庄050031

出　　处：《岩石学报》2010年第7期2180-2192,共13页Acta Petrologica Sinica

基　　金：国家自然科学基金重点项目(40830317);国家自然科学基金项目(40973026;40873023);国家"973"规划项目(2009CB421002);教育部新世纪优秀人才支持计划(NCET-10-0711);111计划(B07011);中国地质调查局综合研究项目(1212010610104)联合资助

摘　　要：本文首次报道了西藏冈底斯东部门巴地区晚白垩世花岗闪长岩的岩石学、锆石U-Pb定年和Hf同位素、全岩元素与Sr-Nd同位素地球化学数据。门巴花岗闪长岩锆石U-Pb定年结果为88.3Ma。花岗闪长岩的SiO2含量为66.4%～68.5%,K2O含量为3.1%～4.1%,A/CNK为0.90～0.99,属于高钾钙碱性准铝质岩石。岩石强烈富集轻稀土元素,(La/Yb)N为25.3～46.6,具弱的负Eu异常(δEu=0.83～0.91),高Sr(725×10-6～766×10-6)、高Sr/Y比值(Sr/Y=52.2～65.2),具有埃达克岩的特征,本文称之为门巴埃达克质岩石。门巴埃达克质岩石εNd(t)值为-4.1～-4.3,(87Sr/86Sr)t值为0.70671～0.70672。εHf(t)范围为-3.4～-1.2,Hf同位素地壳模式年龄为tCDM=1.2～1.4Ga。结合门巴埃达克质岩的主、微量元素、同位素以及含有闪长质包体等特征,判断门巴埃达克质岩石主要来源于古老下地壳物质的部分熔融,可能是来源于下地壳物质的熔体与幔源岩浆(如底侵的玄武岩浆)发生混合作用的结果。通过与在南冈底斯发现的埃达克岩的成因及地球动力学背景的对比,本文认为新特提斯洋洋脊俯冲作用不仅为镁铁质下地壳的熔融提供了能量,而且在岩浆形成演化过程中也起到了重要作用。This paper firstly reports a systematic study of petrology,LA-ICPMS U-Pb zircon dating and Hf isotopes,whole rock elements and Sr-Nd isotopic data of the Late Cretaceous Mamba granodiorite in eastern Gangdese.The zircon U-Pb age of Mamba granodiorite is 88.3Ma.The rocks are high-K calc-alkaline metaluminous,with SiO2 ranging from 66.4% to 68.5%,K2O from 3.1% to 4.1%,and A/CNK from 0.90 to 0.99.The rocks show typical adakitic features,with slightly negative Eu anomalies(δEu = 0.83~0.91),strongly enriched in LREE,high(La/Yb)N(25.3~46.6),high Sr contents(725×10-6~766×10-6)and high Sr/Y(52.2~65.2).The Mamba adakites have homogeneous Nd-Sr compositions(εNd(t)=-4.1^-4.3,87Sr/86Sr=0.70671~0.70672).In situ Hf isotopic analyses of zircon by LA-ICPMS yield initial εHf(t)values ranging from-3.4 to-1.2,corresponding to an ancient Hf crustal model ages of 1.2~1.4Ga.Together with the geochemical results,including major,trace elements and isotopes,with that field occurrences that containing dioritic enclaves in the host rock,we interpret the Mamba adakites as the partial melting of ancient mafic lower crust,and later mixed with mantle-derived material.Taking the wide-spreading magmatic rocks in the southern Gangdese Belt into account,we invoke the Neo-Tethyan mid-ocean ridge subduction beneath southern Gangdese belt as a possible model to explain the origin of the adakitic magmatism.

关 键 词：西藏 冈底斯带 门巴 晚白垩世 埃达克岩 地球化学 SR-ND-HF同位素 

分 类 号：P588.122[天文地球—岩石学]