北祁连冷龙岭蛇绿岩地质地球化学特征及构造意义

在北祁连造山带冷龙岭地区新发现一条蛇绿岩带,该蛇绿岩从下向上由地幔橄榄岩、辉长辉绿岩、玄武岩、硅质岩组成.岩石地球化学特征表明,玄武岩可分类高Ti和低Ti两类,其中高Ti玄武岩具有LREE富集的稀土配分型式,富集K、Rb、Ba、Th、Nb、Ta等不相容元素,呈现隆起型(驼峰式)分布型式,显示了典型的洋岛火山岩地球化学特征,为板内岩浆作用的产物.低Ti玄武岩具有LREE亏损,类似N-MORB的稀土配分模式,同时又具有相对于N-MORB富集的大离子亲石元素,亏损Nb、Ta等高场强元素的岛弧火山岩的地球化学特征,代表了弧后盆地环境岩石组合.蛇绿岩岩石组合和岩石地球化学特征显示该蛇绿岩套形成于弧后盆地环境,是早奥陶世北祁连洋的残留.

Geochemistry of Lenglongling Ophiolite,North Qilian,and Its Tectonic Implications

This paper reports a new ophiolite belt found in the Lenlongling area of north Qilian. From bottom to top, the rock belt is composed of mantle peridotites, gabbros, basalts and silicon rocks. Lithogeochemical characteristics indicate that the basalt can be divided into two groups according to its content of Ti. The high-Ti basalt show LREE enrichment patterns in chondrite-normalized REE diagram, and also display enrichment of incompatible elements such as K, Rb, Ba, Th, Nb, Ta in an uplift(hump) tendency when normalized to N-MORB. With typical geochemical characteristics of OIB, the rock may be formed in an intra-plate magma environment. In contrast, the low-Ti basalt has geochemical characteristics of island arc volcanic rock, showing deficiency of LREE similar to N-MORB, enrichment of the LILE and depletion of HFSE including Nb, Ta etc., which represent the back-arc basin setting. The rock assemblage and lithogeochemistry suggest that the Lenglongling ophiolite belt is formed in the back-arc basin and is the residual of the north Qilian ocean in Early Ordovician.