西藏吉定蛇绿岩地球化学特征及其构造指示意义

吉定蛇绿岩位于雅鲁藏布江蛇绿岩带的中段,是该带保存较好的蛇绿岩之一,通过对该岩体的研究及与附近蛇绿岩剖面的对比有助于恢复早白垩世雅鲁藏布江蛇绿岩带的演化过程。吉定蛇绿岩包括玄武岩、辉绿岩、堆晶岩及地幔橄榄岩四个岩石单元。壳层岩石岩浆结晶顺序为:橄榄石→单斜辉石→斜长石,代表湿岩浆系统分异。吉定蛇绿岩壳层熔岩(玄武岩和辉绿岩)Ti O2含量为0.87%~1.45%,平均1.1%,与印度洋N-MORB玻璃(1.19%)相似。REE配分模式具有明显的LREE亏损特征,稀土配分模式与典型的大洋中脊玄武岩相似。但其微量元素蛛网图上表现为富集LILE,而亏损HFSE,并具有较高LILE/HFSE比值特征,与俯冲带上的(SSZ)蛇绿岩相似。蛇绿岩熔岩在岩石地球化学上表现出既亲MORB,又具部分IAB的特征。结合区域上大竹卡、得几等蛇绿岩岩石及地球化学资料对比分析,提出吉定蛇绿岩形成于在洋内俯冲带上发育起来的弧后盆地,并提出日喀则地区早白垩世洋壳演化的解释模式:雅鲁藏布江中段蛇绿岩至少包含三种组分特征的蛇绿岩体,其代表性剖面分别是吉定,得村和大竹卡,分别形成于近俯冲带的弧后盆地、弧前盆地和弧后盆地,这些洋壳共同组成早白垩世时期的与特提斯洋俯冲带斜交的一条分段发育的洋中脊。

Geochemistry of the Jiding ophiolite in SW Tibet and its tectonic implications

The Jiding ophiolite lies in the middle segment of the Yarlung-Zangbo ophiolite zone. It consists of basalts, diabases, mafic and ultramafic cumulates and mantle peridotites. Crystallization sequence of the cumulate rocks is olivine to clinopyroxene, plagioclase, representing crystal fractionation in a wet magma system. The basalts and diabases of the Jinding ophiolite have 0.87%~1.45% TiO₂ with an average of 1.10%, equivalent to the glasses of the Indian Mid-ocean ridge basalts. The REE patterns of lavas are depleted in LREE, approximate to those of N-MORB. However, relative to the N-MORB, these lavas are enriched in LILE, and depleted in HFSE, resembling supra-subduction zone (SSZ) ophiolites. They have an affinity of MORB, and partially of IAB. The Jiding ophiolite may represent remnants of back-arc oceanic basin above a north-dipping intra-subduction zone. Ophiolites in the middle segment of the Yarlung-Zangbo ophiolite zone are composed of three types of lavas that are represented by Jiding, Deji and Dazhuka. These lavas have formed in back-arc oceanic basins close to the subduction zone, forearc basin and back-arc oceanic basin, respectively. All three types of oceanic crust consist up divided-segment Mid-ocean ridges which subducted obliquely in Cretaceous.