青藏高原南部与北部新生代高镁钾质岩地球化学对比：南北地幔源区差异  

Geochemical comparison of the Cenozoic high-MgO-potassic volcanic rocks between northern and southern of Tibetan Plateau:difference of the both mantle sources.

作　　者：陈建林[1] 许继峰[1] 康志强[1,2] 王保弟[1,2] 

机构地区：[1]中国科学院广州地球化学研究所同位素年代学和地球化学实验室,广州510640 [2]中国科学院研究生院,北京100039

出　　处：《岩石学报》2008年第2期211-224,共14页Acta Petrologica Sinica

基　　金：中国科学院知识创新工程重要方向性项目(KZCX3-SW-143);973项目(2002CB412602);国家杰出青年基金项目(40425003)资助

摘　　要：青藏高原广泛分布有一些新生代高镁钾质岩(MgO≥6%,K_2O/Na_2O>1),通常认为它们应起源于地幔源区,虽然它们均有着富钾、富集大离子亲石元素和亏损高场强元素的共同特征,但青藏高原南部和北部的这些新生代火山岩在形成时代和地球化学特征方面均有明显的不同。青藏高原南部和北部新生代火山岩在形成时间和空间上是不连续的。青藏高原南部和北部的高镁钾质岩可能均源于与古俯冲环境相联系的富集地幔源区,但它们的地幔源区矿物组成和形成深度却是不同的;高原南北高镁钾质岩明显不同的组成可能是因它们源区的壳源物质的来源或含量不同所致,而高原南部比北部高镁钾质岩明显低的^(143)Nd/^(144)Nd但高的^(87)Sr/^(86)Sr同位素比值特征,暗示后者起源于一个相对亏损的(含较少沉积物组分的)富集地幔源区。虽然对流减薄和陆内俯冲模式分别可以解释青藏高原抬升和与其共生的新生代火山岩的部分现象,但从总体上看北向俯冲的印度岩石圈的多次断离和南向俯冲的亚欧板块相结合的模式可能是解释新生代以来青藏高原的抬升和与其共生的火山岩最有效的形成机制。Cenozoic high-MgO-potassic volcanic rocks （ MgO ≥ 6 wt. %, K2O/Na2O 〉 1 ） are widely distributed in the Tibetan plateau, which rocks are generally believed to be derived from a mantle source. These volcanic rocks have some common chemical characteristics such as enrich-K, LILE and depleted-HFSE, but the rocks from northern area of the Tibetan Plateau show geochemical characteristics clearly different from those from southern area. The distribution of Cenozoic volcanic rocks in the Tibetan plateau is not continuous in spatial-temporal. The high-MgO-potassic volcanic rocks in both areas were probably derived from an enriched mantle which is related to metasomatism during earlier subduction events, but mineral association and depths of their mantle sources are different. The geochemical differences of both south and north Tibetan mantle sources are probably caused by the different source composition or/and amount of crustal materials involved into their sources. The high-MgO-potassic volcanic rocks in south have lower ^143Nd/^144Nd but higher ^87Sr/^86Sr isotopic ratio relative to those in north of the Tibetan Plateau, which suggests the latter was sourced from a relatively depleted enriched-mantle than the former. Although the convective removal and southern-norhtern-dipping continental subduetion models may explain some geological phenomena of uplift and distribution of the Cenozoic volcanic rocks in the Tibetan plateau, as a whole it seems more likely for the combination of both the multiple slab break-off of northern subduetion of the India continent and southward-dipping subduetion of the Asian continent on the northern margin of the Tibetan plateau, which can better explain the uplift process and origin of Cenozoic volcanic rocks on the Tibetan plateau.

关 键 词：新生代 高镁钾质岩 地幔源区 青藏高原南部与北部 

分 类 号：P588.144[天文地球—岩石学]