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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。
As an important Meso-Cenozoic arc-arc magmatic belt, the Gangdese has always been the hottest geological research field in the Qinghai-Tibet Plateau. However, the study on the nature of Late Paleozoic volcanic rocks and their tectonic setting is still lacking. Based on the data of regional geological survey, this paper systematically studied the stratigraphy and geochemistry of the Carboniferous Volcano-Sedimentary Rock Series in the Gangdese, and measured the key sections. The volcanic rocks were enriched in trace elements, Sr and Nd , Pb isotope geochemical test. The research shows that the unconformity or the abrupt surface of lithology and sedimentary facies generally exists between the Carboniferous and the underlying Devonian or Precambrian, and represents an important tectonic transformation surface. Carboniferous sedimentary environment roughly twice the coastal - deep sea or deep water slope - coastal sedimentary evolution cycle, deep sea - deep water slope sedimentary or ice-sea brecciated slate associated with two volcanic activities. Carboniferous volcanic rocks are mainly AnShan basalt and dacite, rhyolite, slightly bimodal volcanic rocks. Compared with the typical MORB and island-arc basalts, the composition of Anshan basalts has the characteristics of low MgO content, high content of TiO2, Al2O3 and P2O5. The REE and trace elements are LREE and LILE-rich distributional patterns, which are similar to continental tholeiite basalts . The REE and trace element geochemical characteristics of the Carboniferous acidic volcanic rocks are similar to those of the in-situ rhyolite. Geochemical tracing and stratigraphic studies show that the Carboniferous of the Gangdese zone is a Gondwanaland marginal rifting environment under extensional background. Volcanic rocks have typical Dupal anomalies with complex mixing, involving primitive mantle, enrichment of mantle EMII and crustal composition, indicating that there occurred the process of subduction of Gondwana continental crust and recirculation into ancient mantle . Some special characteristics of basalt components may be related to the mixing of source areas.