西藏嘉黎断裂带凯蒙蛇绿岩的地球化学特征及构造意义

GEOCHEMISTRY AND ITS TECTONIC SIGNIFICANCE OF KAIMENG OPHIOLITES IN JIALI FAULT BELT,TIBET

蛇绿岩主要分布于嘉黎断裂内的凯蒙沟山脊上,沿嘉黎断裂带呈东西向线状分布,组成蛇绿岩的岩石类型主要有纯橄岩、方辉橄榄岩、单辉橄榄岩、橄辉岩、橄长岩和辉长岩等。蛇绿岩具有贫硅、贫碱、低钛,而富铁、镁的特点。稀土总量很低,ΣREE值变化于0.415μg^3.273μg/g之间,其中方辉橄榄岩和橄长岩的轻稀土略有富集,而单辉橄榄岩和辉长岩的轻稀土略有亏损,分配曲线基本呈近乎平坦的“W”型,具明显的正铕异常。微量元素在原始地幔标准化配分形式上具有明显的R b,B a,S r低度富集和N b相对亏损,预示着该蛇绿岩形成于消减作用下的岛弧环境。凯蒙蛇绿岩的存在揭示了嘉黎断裂带为一条重要的缝合带,它与班公湖-怒江缝合带的同时存在进一步证明了中特提斯洋(T3—J1)具多岛洋盆的构造格局。

Kaimeng ophiolites locate in Jiali fault belt and expose along Kaimeng ridge with a E- W trend. The ophiolites mainly consist of harzhurgite, clinopyroxene peridotite, pyrolite, troctolite and gabbo. Kaimeng ophiolites are low content in Si,K2O＋Na2O and Ti and high content in Mg and Fe,low ∑REE（∑REE=0. 415 × 10^-6-3. 273×10^-6） and enrichment of ∑LREE in the harzburgite and troctolites,and depletion of ∑LREE in the clinopyroxene peridotite and gabboes. The ∑REE patterns display almost a flat-model with ＂W＂ type, showing obvious Eu positive anomaly. In the curves of the trace element distribution patterns normalized by the primitive mantle, Kaimeng ophiolites exhibit Rb ,Ba,Sr enrichment and Nb,Ta depletion, showing that the ophiolites were formed in the island arc circumstance. The existence of Jiali suture and Bangongcuo-Nujiang suture at the same time proved that the Meso Tethyan ocean （T3-J2） was an archipelatic ocean.