西藏尼木渐新世花岗岩中的岩浆混合作用:对岩石成因及陆壳增生的启示

Magma Mixing in Oligocene Granites from Nyemo,Tibet:Implications for Petrogenesis and Continental Crust Accretion

尼木渐新世黑云母二长花岗岩中含有丰富的形态各异的暗色镁铁质微粒包体。本文报道了寄主岩和镁铁质包体的年代学、元素地球化学及Sr-Nd-Hf同位素组成,据此阐明了岩石的成因,并探讨了岩浆混合作用在成岩中的意义及其对陆壳增生的启示。锆石LA-ICP-MS U-Pb定年结果表明,寄主岩和镁铁质包体的成岩年龄在误差范围内基本一致,均为约30Ma,说明它们同时形成。元素地球化学组成上,寄主黑云母二长花岗岩为高钾钙碱性、准铝质,富集Rb、Th、U等大离子亲石元素,亏损Nb、Ta、Ti、P等高场强元素,且具有高Sr、低Y和Yb含量,Sr/Y比值高,缺乏明显铕负异常,表现出埃达克质岩石的特征。镁铁质包体贫硅,富铁、镁,具有与寄主岩相似的稀土与微量元素分布模式。二者的全岩Sr、Nd同位素和锆石Hf同位素组成也相近[寄主岩:(87 Sr/86 Sr)i=0.7057～0.7064,εNd(t)=-1.45～0.35,εHf(t)=1.21～7.34;镁铁质包体:(87Sr/86Sr)i=0.7058～0.7064,εNd(t)=-2.23～-1.57,εHf(t)=2.40～7.04]。综合分析表明,尼木渐新世黑云母二长花岗岩应主要起源于碰撞加厚的新生镁铁质下地壳的部分熔融,镁铁质包体应为幔源玄武质岩浆与其诱发加厚地壳熔融形成的埃达克质岩浆经不均匀混合作用的产物。结合对冈底斯带岩浆岩已有资料的全面分析,表明幔源玄武质岩浆的底侵及其诱发的岩浆混合作用既是冈底斯岩基形成的主要方式,也是导致青藏高原陆壳增生的重要途径。

The Oligocene biotite monzogranites in Neymo of Tibet contain abundant melanocratic mafic microgranular enclaves(MMEs)with different shapes.This paper presents geochronology,elemental geochemistry and Sr-Nd-Hf isotopic compositions for the host granites and mafic enclaves,with the aims to elucidate the origin of magma mixing and discuss its implications for petrogenesis and continental crust accretion.Zircon LA-ICP-MS U-Pb dating yields the crystallization ages of about 30 Ma for both the host granites and the M MES,which are within analytical errors,suggesting that the host rocks and MMEs were coeval.Geochemically,the host biotite monzogranites are high-K calc-alkaline,metaluminous,and enriched in large ion lithophile elements(LILEs,e.g.,Rb,Th and U),and depleted in high field strength elements(HFSEs,e.g.,Nb,Ta,Ti and P).High Sr,low Y and Yb concentrations,high Sr/Y ratios and indistinct Eu anomalies show geochemical signatures of adakitic rocks.The MMEs are relatively depleted in Si and enriched in Fe and Mg,and have similar distribution pattern of trace elements and rare earth elements(REE)to the host granitic rocks.The host biotite monzogranites and MMEs also have close whole-rock Sr-Nd and zircon Hf isotopic compositions(host rocks:(87 Sr/86 Sr)i=0.7057~0.7064,εNd(t)=-1.45~0.35,εHf(t)=1.21~7.34;MMEs:(87 Sr/86 Sr)i=0.7058~0.7064,εNd(t)=-2.23^-1.57,εHf(t)=2.40~7.04).The integrated petrology and elemental and isotopic compositions suggest that the host biotite monzogranite originated mainly from partial melting of thickened juvenile mafic lower crust,and the MMEs were produced by inhomogeneous magma mixing between mantle-derived basaltic magmas and the adakitic parental magmas of the host granitic rocks.In combination with the comprehensive analyses of the available data for the magmatic rocks along the Gangdese belt,it can concluded that underplating of mantle-derived basaltic magmas and their induced magma mixing is not only the main mechanism for the formation of Gangdese batholith,but also an important way resulting in continental accretion of the Tibetan Plateau.