藏东洛隆地区早白垩世冻错岩体岩石成因及构造意义

PETROGENESIS AND TECTONIC SIGNIFICANCE OF THE EARLY CRETACEOUS DONGCUO PLUTON IN LUOLONG AREA,EASTERN TIBET

北拉萨地体广泛发育白垩纪岩浆岩,但是对其岩石成因及其动力学背景仍存在争议。本次研究选取西藏洛隆地区冻错岩体为研究对象,在野外地质调查基础上进行了岩石学、岩石地球化学、锆石U-Pb年代学和Hf同位素研究。结果表明:冻错岩体主要为花岗闪长岩和二长花岗岩,且发育暗色闪长质包体;锆石U-Pb测年结果显示花岗闪长岩(127.6±1.0 Ma,129.9±0.9 Ma)和二长花岗岩(131.7±1.1 Ma)的形成时代为早白垩世;花岗闪长岩具有负的ε_(Hf)(t)值(-12.9~-10.3)和古老的二阶段模式年龄(1809~1977 Ma);岩体花岗岩类的SiO_(2),Al_(2)O_(3)和全碱(Na_(2)O+K_(2)O)质量分数分别为67.01%~72.35%,14.37%~16.55%和6.49%~8.11%,里特曼指数(σ)为1.52~2.30,铝饱和指数A/CNK值为0.90~1.12,属准铝质-弱过铝质Ⅰ型花岗岩类。闪长质包体SiO_(2),Al_(2)O_(3)和全碱(Na_(2)O+K_(2)O)质量分数分别为56.61%~58.12%,17.01%~17.43%和5.27%~5.90%。稀土和微量元素组成显示花岗闪长岩、二长花岗岩和闪长质包体均富集La,Ce,Pr,Nd 4种轻稀土元素和Rb,Th,Pb等大离子亲石元素,亏损Er,Tm,Yb,Lu 4种重稀土元素和Nb,Ta,Ti等高场强元素。综合区域地质调查研究成果,早白垩世冻错岩体为班公湖-怒江洋向南俯冲的岩浆记录,花岗闪长岩和二长花岗岩由古老地壳部分熔融形成,暗色闪长质包体则为壳幔岩浆混合作用的产物。

On the basis of field geological investigation,this paper studies the petrology,petrogeochemistry,zircon U-Pb chronology and Hf isotopes of Cretaceous magmatic rocks developed in the northern Lhasa terrane,in an attempt to solve the controversial problems of rock genesis and dynamic background.The results show that the Dongcuo pluton is mainly composed of granodiorite and monzogranite with some dark diorite inclusions.Zircon U-Pb dating shows that the granodiorite(127.6±1.0 Ma,129.9±0.9 Ma)and monzogranite(131.7±1.1 Ma)were formed in the Early Cretaceous.Geochemical analysis shows that the granodiorite has a negative ε_(Hf)(t)value(-12.9 to-10.3)and an ancient two-stage model age(1809 to 1977 Ma).The contents of w(SiO_(2)),w(Al_(2)O_3)and total alkali w(Na_(2)O+K_(2)O)in granites are 67.01% to 72.35%,14.37% to 16.55% and 6.49% to 8.11%,respectively.The Ritman index(σ)is 1.52 to 2.30,and the aluminum saturation index A/CNK is 0.90 to 1.12,indicating that the Dongcuo pluton belongs to quasi-weak peraluminous Ⅰ type granites.The contents of w(SiO_(2)),w(Al_(2)O_3)and total alkali w(Na_(2)O+K_(2)O)of diorite inclusions are 56.61% to 58.12%,17.01% to 17.43% and 5.27% to 5.90%,respectively.Composition of rare earth and trace elements shows that the Dongcuo granodiorite,monzogranite and diorite inclusions are enriched in light rare earth elements(La,Ce,Pr,Nd)and large ion-lithic elements(Rb,Th,Pb),while deficient in heavy rare earth elements(Er,Tm,Yb,Lu)and high field strength elements(Nb,Ta,Ti).According to the results of regional geological survey,the early Cretaceous Dongcuo pluton serves as the magmatic record of the southward subduction of Bangonghu-Nujiang Ocean,the granodiorite and monzogranite were formed by partial melting of the ancient crust,and the dark diorite inclusions are the products of crust-mantle magmatic mixing.