羌塘西北部松西地区新生代火山岩岩石地球化学特征及成因讨论

羌塘西北部松西地区新生代火山岩由安山岩、英安岩和晚期火山颈相流纹斑岩3种岩石类型组成,属于钙碱性-高钾钙碱性岩石系列.岩石富集大离子亲石元素和LREE,相对亏损高场强元素,Nb、Ta、Ti负异常,反映源岩具有壳源特征,基性端员的SiO2含量＜53%,表明松西地区玄武安山岩不可能完全由陆壳直接局部熔融产生,应该有少量基性的地幔物质加入.岩浆Eu负异常不明显,说明岩浆来源于加厚陆壳中下部,是印度板块与欧亚板块发生长期碰撞挤压导致青藏高原北部包括羌塘地区的陆壳缩短和加厚、拉萨地块大陆岩石圈的北向俯冲作用以及羌塘陆块之下上涌的软流层物质的底侵作用,引发增厚下地壳发生部分熔融形成的.

Geochemistry and genesis of Cenozoic volcanic rocks in Songxi area, northwestern Qiangtang, Tibet

Cenozoic volcanic rocks are distributed in Songxi area of northwestern Qiangtang within northern Tibet, a marine volcanic rock association composed of andesite, dacite and rhyolite belongs to the calc-alkaline-high K calc-alkaline series. The rocks have high large-ion lithophile elements LIL and LREE, low HFSE, and relatively depleted Nb, Ta, Ti, reflecting features of terrigenous magma. SiO2 contents of basic volcanic rocks are less than 53%, suggesting that basaltic andesite in Songxi area was not entirely produced by direct partial melting from the continental crust generation, and that the formation of basaltic andesite must have had the participation of a small amount of mantle materials. Eu negative anomalies are not obvious, implying that magma was derived from the thickening of the lower continental crust resulting from the collision and long-term squeeze between the Indian plate and the Eurasian plate, which led to the shortening and thickening of the continental crust in the northern part of Qinghai-Tibet plateau, including Qiangtang area. The northward subduction of the Lhasa block of continental lithosphere and the underplating of upwelling asthenosphere materials in the Qiangtang landmass triggered off the formation of the thickened lower crust through partial melting.