拉萨地体南缘错布拉果花岗岩体锆石U-Pb年龄、地球化学特征及成岩动力学背景

本次研究文对拉萨地体南缘错布拉果岩体进行了两件锆石U-Pb年代学和全岩地球化学分析,揭示了错布拉果花岗岩类型、岩浆源区与演化,并讨论了区域成岩动力学背景。分析结果显示,错布拉果岩体二长花岗岩LA-ICP-MS锆石U-Pb定年结果为61 Ma^59 Ma,与区域上冈底斯南缘岩浆弧为同一时期。在地球化学组成上,错布拉果花岗岩w(SiO2)70.09%~72.64%,具有高w(Al2O3)14.40%~15.99%,低w(TiO2)0.08%~0.24%,w(MgO)(0.41%~0.76%),w(Fe2O3T)6.82%~29.9%,w(P2O5)0.07%~0.12%和w(CaO)1.06%~1.75%的特征;属于高钾钙碱性-钾玄岩系列。错布拉果花岗岩轻稀土元素(LREE)含量介于133.69×10^-6~226.64×10^-6之间,重稀土元素(HREE)含量介于17.36×10^-6~32.11×10^-6之间,LREE/HREE介于5.05~7.83之间,富集轻稀土(LREE)和Rb,K,U等大离子亲石元素(LILE),亏损P,Nb,Ta等高场强元素(HFSE),具有弧型岩浆岩的地球化学组成。另外,错布拉果花岗岩体的铝饱和指数(A/CNK=1.06~1.11)属于弱过铝质的Ⅰ型花岗岩。综合分析表明,错布拉果花岗岩是在印度-亚洲板块碰撞的开始,俯冲下插的新特提斯洋板块由于重力作用造成与大陆板块脱离,引起软流圈上涌,导致岩石圈地幔下地壳底部发生部分熔融从而形成母岩浆,最终在侵位与成岩后期主要经历分离结晶作用。

ZIRCON U-Pb AGE,GEOCHEMICAL CHARACTERISTICS AND DIAGENETIC DYNAMIC BACKGROUND OF THE CUOBULAGUO GRANITES IN THE SOUTHERN MARGIN OF LHASA TERRANE

Zircon U-Pb chronology and whole-rock geochemical analysis are used to study the lithological characteristics and magmatic source area of the Cuobulaguo granite in the southern margin of the Lhasa platform and discuss its regional diagenetic dynamics background and evolution. LA-ICP-MS zircon U-Pb dating for the monzonitic granite yields an age of 61 Ma^59 Ma,similar to the formation age of igneous rocks in the southern margin of the Gangdese.Geochemical analyses for the Cuobulaguo granite reveal that the rock is rich in w(SiO2) 70.09%~72.64%,with high content of w(Al2O3)14.40%~15.99%,poor in w(TiO2) 0.08%~0.24%,w(MgO)(0.41%~0.76%),w(Fe2O3T) 6.82%~29.9%,w(P2O5)0.07%~0.12% and ω(CaO)1.06%~1.75%,characteristic of high potassium calc-alkaline-potassium series rocks.The content of LREE of the skeletal granite is between 133.69×10^-6~226.64×10^-6,and the content of HREE is in the range of 17.36×10^-6 to 32.11×10^-6 with the LREE/HREE ratio ranging from 5.05~to 7.83.The granite is enriched in light rare earth(LREE) and large ion lithophile elements(LILE),such as Rb,K,U,etc.,and depleted in high field strength elements(HFSE),such as P,Nb and Ta,and has the same geochemical compositions as arc magmatic rocks. The value of aluminum saturation index(A/CNK=1.06~1.11) of the Cuobulaguo granite implies that it belongs to the weakly-aluminous I-type granite. Through comprehensive analysis,it is considered that as the initiation of collision between the Indian and Asian plates,the oceanic plate subducted by the subduction was separated from the continental plate due to gravity,causing the upwelling of asthenosphere and resulting in partial melting of the lithospheric mantle,which was intruded to the bottom of the lower crust and triggered partial melting of the lower crust and eventually formed granitic magma.