青藏高原西部蛇绿岩类型:岩石学与地球化学证据

对青藏高原西部地区的班公湖蛇绿岩、狮泉河蛇绿岩、雅鲁藏布江西段蛇绿岩和普兰—当穷蛇绿岩带中代表性岩体的地质学、岩石化学、稀土元素、微量元素、Pb、Sr同位素地球化学研究表明,青藏高原西部地区4条蛇绿岩中的地幔橄榄岩主要为方辉橄榄岩和少量纯橄岩,岩石化学成分具有富镁、贫铝、钙、碱的特点;论述了地幔橄榄岩轻稀土元素富集是由于先经历了较强的部分熔融,后经历了俯冲消减过程中的流体交代的二次过程;微量元素中大离子亲石元素Rb、不活动元素Nb、Zr、Hf和放射性生热元素Th等元素的丰度较高,以及Ti、Sm、Y、Yb等强不相容元素亏损的特点,与交代地幔岩特征类似;Pb、Sr同位素组成具有明显的壳源组分混入的特点,说明青藏高原西部的蛇绿岩曾受洋壳俯冲消减过程中的流体交代作用,蛇绿岩产于SSZ构造环境。对比青藏高原东部、三江、西昆仑地区以及形成于典型的SSZ环境的Troodos蛇绿岩中的地幔橄榄岩,就岩石化学富MgO、轻稀土元素富集而言,它们具有与青藏高原西部基本一致的地质地球化学特征,结合与俯冲岩浆作用有关的玻安岩和埃达克岩产出,说明可能包括三江、西昆仑库地在内的青藏高原不同时代蛇绿岩都主要形成于俯冲消减环境,属于SSZ型蛇绿岩。

Ophiolite types in western Qinghai-Tibetan plateau-Evidences from petrology and geochemistry

The study of geology, geochemistry, rare earth elements, trace element, and Pb, Sr isotope in representative ophiolite rock of four ophiolite belts in western Qinghai-Tibetan plateau indicates that the mantle peridotites in ophiolites mainly consist of the harzburgite and a little dunite. These rocks are characterized by high abundances of magnesium oxide (MgO), and low abundances of aluminum, calcium and alkali oxides. The enrichment of the light rare earth elements in mantle peridotites can be explained by assuming that the mantle peridotites had been subjected to at least two geological processes:1) relatively high degree of partial melting, and 2)the metasomatism by the fluids released during subduction of oceanic crust. The abundances of trace elements in mantle peridotites are characterized by the low Sr, Ti and Y and relative high Rb, Nb, Zr, Hf and Th , similar to the metasomatic pyrolite. The isotopic composition of Sr and Pb suggests interaction with a crust-derived source. All evidence indicates that the four ophiolite belts located in the western Qinghai-Tibetan plateau had undergone the metasomatism by fluids released during the process of oceanic crust subduction. It is suggested that the beets were formed in a supra subduction zone(SSZ) setting. Mantle peridotites in ophiolite belts in eastern Qinghai-Tibetan plateau, Sanjiang, west Kunlun, and Troodos in Cyprus which are considered as having formed in SSZ settings, the geochemical characteristics (enrichment in MgO and LREE), are essentially the same as those of the ophiolites in western Qinghai-Tibetan plateau. Considering the Tibetan ophiolites(Qinghai,Sanjiang and Kunlun) with the occurrence of boninite and adakite rocks which are associated with the subduction magmatism, it is suggested that these ophiolites formed in different times in the tectonic setting of supra subduction zone.