【摘要】 仁布蛇绿构造混杂带位于雅鲁藏布江缝合带中段之最东部,其间混杂伴生有大量较为新鲜的辉绿岩构造岩块。为了确定辉绿岩的构造属性,探讨其成因,对该辉绿岩进行了岩相学、矿物学和全岩地球化学研究。结果表明辉绿岩SiO2偏低,全碱含量高,Ti和P含量也较高,相对贫Ca,Mg#平均55,属碱性系列;稀土元素总量高,L(a Y/bN)=9.30～10.9,为轻稀土元素强烈富集的分布模式,无Eu异常;大离子亲石元素和高场强元素明显富集,微量元素的原始地幔标准化蛛网图显示,随元素不相容性降低,其原始地幔标准化值逐渐降低;辉绿岩和另一套与其同源的玄武岩具有较高的87Sr/86Sr比值,Nεd(t)平均为5.4,显示亏损地幔D(M)向富集地幔Ⅱ(EMⅡ)的演化趋势;显然具有典型的洋岛玄武岩(OIB)的特征。辉绿岩的主元素和相容元素C(r、Co和Ni等)较均一,仅显示轻度结晶分异,不活泼的不相容元素(Th、Ta、Nb、H f、Zr、Ti、Y和REE等)、特征微量元素比值以及SrN-d同位素组成表明,辉绿岩几乎没有遭受地壳混染,可能源于地幔柱尾部的含石榴子石和尖晶石的橄榄岩相软流圈地幔的低程度部分熔融,并类似于Kerguelen洋岛碱性玄武岩。认为该洋岛碱性辉绿岩代表了藏南特提斯洋内热点的产物,成因上与印度洋90E°洋隆洋岛链可能类似,暗示印度洋可能继承了特提斯洋的物质组成。更多还原

【Abstract】 The Renbu ophiolitic mélange zone locates in the easternmost middle section of the Yarlung Zangbo suture zone, and there are lots of fresh diabase tectonic block outcrops in the mélange zone. In order to acquire the tectonic property of the diabase and discuss its petrogenesis, the petrography, mineralogy and whole rock geochemistry are carefully studied. The results show that the diabase samples have relatively low SiO2 content and high total alkalis, and are rich in Ti and P, but poor in Ca, with an average Mg#of 55. All samples belong to alkaline series. The rare earth element (REE) abundances are higher than that of chondrite and all samples show the enrichment distribution patterns of light rare earth elements (LREE), with (La/Yb)N of 9.30～10.9 and no Eu anomaly, indicating LREE clearly being fractionated from the HREE. Large ion lithophile elements (LILE) and high field strength elements (HFSE) are all enriched, and the spider diagram indicates that the normative value of incompatible elements to primitive mantle increases with their incompatibility . Sr-Nd isotopic ratios of the diabase and its congenetic basalts display a evo lution trend from depleted mantle (DM) to enriched mantle Ⅱ(EM Ⅱ). Obviously, the geochemical characteristics of the diabase are similar to those of typical o ceanic island basalts (OIB). In addition, the major elements and compatible elem ents (Cr, Co and Ni, etc.) are uniform wholly, and show very low crystallization differentiation. Immobile incompatible elements (including Th, Ta, Nb, Hf, Zr, Ti, Y and REE), typical trace element ratios and SrNd isotopic compositions su ggest that the diabase almost has no crustal contamination and probably derived from a late-stage outpouring of relatively low-degree partial melts of primiti ve garnet and spinel iherzolite because of mantle plume tails, and is similar to the oceanic islands alkalic basalts of Kerguelen. Therefore, the Renbu OIAB-ty pe diabase probably represents the product from the hotspots in the southern Xiz ang Tethys, moreover, because it is similar to the 90°E Ridge hotspots of the I ndian Ocean in origin, it is believed that the Indian Ocean mantle has inherited some characteristics of the Tethyan Ocean.更多还原