西藏冈底斯带石炭纪陆缘裂陷作用：火山岩和地层学证据

冈底斯作为重要的中、新生代岛弧岩浆岩带,历来是青藏高原最热门的地质研究领域,但是对晚古生代火山岩的性质及其形成的构造背景仍缺乏研究。本文在区域地质调查资料的基础上,对冈底斯带石炭纪火山-沉积岩系进行了系统的地层学和岩石地球化学研究,测量了重点剖面,对火山岩进行了常量、微量元素和Sr、Nd、Pb同位素地球化学测试。研究表明,石炭系与下伏泥盆系或前寒武系之间普遍存在不整合或岩性、沉积相的突变面,代表重要的构造转换面。石炭纪的沉积环境大致有两次滨海-深海或深水斜坡-滨海沉积演化旋回,深海-深水斜坡沉积或冰海相含砾板岩与两次火山活动伴生。石炭纪火山岩主要为安山玄武岩和英安岩、流纹岩类,略具双峰式火山岩特点。安山玄武岩类的成分与典型MORB和岛弧玄武岩相比,具有MgO含量低,TiO2、Al2O3、P2O5含量高等特点,稀土和微量元素为LREE和LILE富集型分配模式,与大陆拉斑玄武岩相似。石炭纪酸性火山岩的稀土和微量元素地球化学特征与陆内流纹岩相似。岩石地球化学示踪和地层学研究表明,冈底斯带石炭纪为伸展背景下的冈瓦纳陆缘裂陷环境。火山岩的源区地幔具有典型的Dupal异常,发生过复杂的混合作用,涉及到原始地幔、富集地幔EMII和地壳成分等,说明发生过冈瓦纳古陆壳俯冲、再循环进入古老地幔等过程。玄武岩类成分的某些特殊性,可能与源区混合作用有关。

Carboniferous Marginal Rifting in Gangdese: Volcanic Rocks and Stratigraphic Constraints, Xizang (Tibet), China

As a Mesozoic and Cenozoic magmatic arc, the Gangdese zone is an important field of Tibetan geologic studies. Yet the late-Paleozoic volcanic rocks and their tectonic settings however, still remain unstudied. Based on new data of geologic survey, the authors carried out stratigraphical and geochemical studies on the Carboniferous volcano-stratigraphic sequences in the Gangdese zone. We measured key geologic sections and analysed volcanic rock samples for major and trace elements and Sr, Nd and Pb isotopes. The boundary between Carboniferous and Devonian or Precambrian rocks is an important tectonic transitional surface in the Gangdese, which represented by uncomformity or sharp contacts between different lithologies and lithofacies. There are two sedimentary cycles in the Carboniferous from littoral zone, abyssal slope to littoral zone, with two phases of volcanic eruption accompanying the abyssal slope deposits and pebbly slate of icy sea facies. Major volcanic rock types of Carboniferous times are andesitic basalt, dacite, rhyolite, characterized by a bimodal volcanic suite. Compared with typical MORB and arc basalt, the andesitic basalt has lower content of MgO and higher contents of TiO2, Al2O3, and P2O5 with REE and trace elemental normalized patterns similar to those of continental flood basalts. The silicic volcanic rocks of Carboniferous times has similar REE and trace elemental geochemistry as continental rhyolite. Geochemical and stratigraphical studies suggest that the tectonic setting of the Gangdese region is depressional rifting to the northern margin of Gondwanaland. The mantle source of volcanic rocks, with typical Dupal anomaly, experienced complex mixing process involving primitive mantle, EMII and crust rocks, which suggest subduction of older crust of Gondwanaland and then it was recycled into the mantle. Some particular chemical features of the Carboniferous basaltic rocks may relate to source rock mixing.