西南天山阔克萨彦岭地区巴雷公地幔橄榄岩成因及其地质意义

Petrogenesis and Geological Implication of Serpentinized Peridotites in the Kokshal Segment,South Tianshan

巴雷公蛇绿混杂岩位于西南天山阔克萨彦岭地区,代表了南天山古生代洋盆的洋壳残片.通过其中的地幔橄榄岩进行详细的地球化学研究,分析其岩石成因及其形成环境.主量和微量元素特征显示巴雷公蛇纹石化橄榄岩Ti,Al含量低,Cr(1604～3863),Ni(1719×10-6～2375×10-6)和Mg#值(0.90～0.92)高;样品的稀土强烈亏损(ΣREE=0.28×10-6～0.35×10-6,Yb=0.1～0.11×CI),呈宽缓的U型REE配分模式;原始地幔标准化蛛网图显示,强烈富集Rb、U和Sr,从LILE(Cs、Rb、Ba、U、K和Sr)到HREE,元素含量自左向右随不相容性的逐渐降低,呈右倾标准化微量元素配分型式;橄榄岩熔融程度在20%～25%之间,为俯冲带氧化条件下的熔融残留物.上述特征显示了俯冲板片流体与俯冲带上地幔楔物质的相互作用,具有俯冲带型地幔橄榄岩特征.结合区域新获得的资料,指示了南天山早古生代洋盆为一成熟的大洋,早古生代曾发生过洋内俯冲作用.巴雷公蛇纹石化橄榄岩可能产生在南天山古生代洋内俯冲作用中的大洋岛弧的弧前地幔楔部位.

South Tianshan is a prominent intracontinental collisional orogenic belt and a key to understand the central Asia tectonic evolution. The serpentinized peridotites from Baleigong ophiolitic mélange was emplaeed in the southwestern Kokshal area, which represents lithosphere remnants of the Early Paleozoic Southwestern Tianshan Ocean. The serpentinized peridotites are characterized by low Ti, Al contents and high Cr （1 604-3 863）, Ni （1 719 × 10^-6 -2 375× 10^-6）, Mg^# （0.90-0.92）. The right slopping PUM-normalized trace element patterns and the U-shape chondrite-normalized REE pattern reveal that these rocks cannot be residues of simple melt extraction. They can be modeled as restites after 20%-30% partial melting from already depleted mantle in a supra-subduction zone （SSZ） geotectonic setting, in which a constant flux of a slab-derived fliud component added to the mantle wedge during melting. Combined with recent studies, it is concluded that the Paleozoic Southwestern Tianshan Ocean was a mature one that had even experienced a subduction event in Early Paleozoic. The serpentinized peridotites were formed at the fore-arc of the intra-oceanic subduction zone.