川西同德麻粒岩地球化学特征及其地质意义

LITHOGEOCHEMICAL FEATURES OF GRANULITE IN TONGDE AREA OF WESTERN SICHUAN PROVINCE AND THEIR IMPLICATIONS

在扬子陆块西缘攀枝花～西昌（攀西）地区，中基性二辉麻粒岩被认为是变质程度最高的结晶基底岩石。同德麻粒岩岩石地球化学的研究表明，常量元素中富Al，以及Ba、Sr、Ce、LREE的富集和Nb、Ta、Y、Yb、HREE亏损的微量元素分布特征，不具有板内玄武岩的地球化学性质，与活动大陆边缘构造环境有关。Rb-Th和Nb-Ta亏损的共存，指示了同德麻粒岩的原始岩浆曾受到古老下地壳物质的混染。常量元素中TiO2的富集，与麻粒岩的角闪岩相退变质作用过程有关。结合麻粒岩的锆石U-Pb测年结果认为，同德麻粒岩的原岩形成于新元古代（844±12Ma）扬子陆块西缘活动大陆边缘的岛弧环境。

The intermediate - basic two - pyroxene granulites have been considered as the crystalline basement rocks with the oldest tonnation age and the highest metamorphic grade in Panzhihua - Xichang area, the western margin of Yangtze block. Major elements of Tongde granulites are characteristiced with Al enrichment and trace elements are enriched in Ba, Sr, Ce, LREE and relatively depleted in Nb,Ta, Y, Yb, HREE. The Tongde granulites were probably formed at an active continental margin without geochemistry of WPB. The depletion of Rb - Th and Nh - Ta indicates that the primitive mag,na of the Tongde granulites was contaminated by the lower crust. The enrichment of Ti in major elements was probably associated with the amphibolite - facies retrograded metamorphism of the Tongde granulites. Based on the above- mentioned features of geochemistry and U -Pb data,the author considers that the Tongde granulites were formed in the island - arc environment of an active continental margin at Neoproterozoic （844 ±12 Ma）,to the western border of Yangtze block.