西藏西南部达巴-休古嘎布蛇绿岩带中地幔橄榄岩的成因

The Origin of Mantle Peridotites in the Daba-Xiugugabu Ophiolite Belt,SW Tibet

本文报道了西藏西南部达巴-休古嘎布蛇绿岩带中橄榄岩的矿物化学资料。橄榄岩中主要造岩矿物化学成分的分析研究表明,该区橄榄岩为残余地幔成因,但它们不是地幔简单熔化的残余物。尖晶石中Cr#及辉石中Ti的广泛变化表明它们具有复杂的熔融历史及地幔交代作用的印记,其形成过程可能经历了两种构造环境的转变。早期在MORB环境下形成低Cr#(尖晶石)橄榄岩;其后由于洋内俯冲作用,早先形成的低熔橄榄岩被消减到岛弧之下再度发生熔融形成高Cr#(尖晶石)橄榄岩。从而,在古大洋消失之后形成的碰撞带上同时保存了MORB型和SSZ型两类蛇绿岩。

This paper reports the mineral chemical data of mantle peridotites in the Daba-Xiugugabu ophiolite Belt, SW Tibet. In the relationship between Cr^# of spinel and Fo of olivine, the peridotites fall within the OSMA and it indicate they are the residual mantle peridotite, but not residue of a simple partial melting. The variations of Cr^# in spinel and Ti in pyroxene suggest that the Daba-Xiugugabu peridotite had a history of complex melting and an impress of mantle metasomatism. The processes of formation of the peridorites possibly include a transition from a tectonic setting to another. The lower Cr^# of spinel in peridotite can be explained by partial melting and MORB extraction in mid-ocean stage. Subsequently, the peridotites with low melting degree were subducted beneath the arc area and re-melted, and peridotites with higher Cr^# of spinel formed. Therefore, two of MORB-type and SSZ-type ophiolite crop out along collision belt formed after the closing of the old ocean.