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系统研究了西藏雅鲁藏布江蛇绿岩带中部日喀则地区德村、吉丁和昂仁蛇绿岩中基性岩石的元素与 Sr-Nd-Pb 同位素地球化学特征。这些基性岩石,包括玄武岩、辉长岩和辉绿岩,属于低钾拉斑玄武岩系列,球粒陨石标准化稀土元素分配模式为轻稀土元素亏损的 N-MORB 型,(La/Yh)_N=0.31～0.65(除样品 DC993为1.17)。在原始地幔标准化微量元素图上,亏损高度不相容元素,与 N-MORB 配分模式一致。相对于 Th,无 Nb、Ta的亏损,显示样品不是产于 SSZ 环境。经构造环境图解判别,样品落入了 N-MORB 区域内;这些元素成分特征表明样品具有洋中脊环境或成熟的弧后盆地环境属性。Sr、Nd 和 Pb同位素组成特征表明特提斯地幔源区以 DM(亏损地幔)为主,同时存在少量 EMⅡ(富集地幔类型Ⅱ)、Sr,Nd 和 Pb 同位素组成特征还表明特提斯地幔域具有印度洋 MORB 型的 Sr-Nd-Pb 同位素组成特征。本文的结果进一步支持 Zhang et al.(2005)的研究结果,现今印度洋不仅在地理位置上占据了曾经是特提斯洋的大部分,而且它的地幔域还继承了曾经特提斯的地幔域的地球化学特征。
The elemental and Sr-Nd-Pb isotopic geochemical characteristics of the basic rocks in the Dejian, Jiding and Angren ophiolites in the Shigatse area in the central part of the Yarlung Zangbo ophiolite belt in Tibet are systematically studied. These basic rocks, including basalts, gabbro and diabase, belong to the series of low-potassium tholeiite basalts. The normalized distribution patterns of chondrite melatonites are LREE-depleted N-MORB type, (La / Yh) _N = 0.31 to 0.65 (except for sample DC993, 1.17). Loss of highly incompatible elements on the trace map of the original mantle normalized, consistent with the pattern of N-MORB partitioning. Loss of Nb vs. Ta relative to Th shows that the sample is not produced in the SSZ environment. Based on the tectonic setting, the samples fall into the N-MORB region. The characteristics of these elemental compositions indicate that the samples have mid-ocean ridge environment or mature back-arc basin environmental attributes. The characteristics of Sr, Nd and Pb isotopic compositions indicate that the Tethyan mantle source is dominated by DM (depleted mantle) with a small amount of EM II (enriched mantle type II), and the Sr, Nd and Pb isotopic compositions also indicate that the Tethyan mantle The domain has the MOR-type Sr-Nd-Pb isotopic compositions of the Indian Ocean. Our results further support the findings of Zhang et al. (2005) that nowadays the Indian Ocean occupies most of the area that used to be the Tethyan not only geographically, but also inherits the former Tethyan mantle Geochemical characteristics.