北山地区月牙山-洗肠井蛇绿岩的地球化学特征及形成环境

摘要

位于北山中带的月牙山-洗肠井蛇绿岩带是北山地区出露最好的蛇绿岩带之一,枕状玄武岩和堆晶辉长岩表现出轻稀土元素亏损-平坦的分配模式,(La/Yb)N=0.47～1.62,类似N-MORB；而相对于N-MORB则又富集大离子亲石元素,亏损Nb、Ta等高场强元素,与典型岛弧火山岩相似；即基性岩类同时具有类似岛弧火山岩和洋中脊火山岩的地球化学特征,与产出于弧后盆地的新疆库尔提蛇绿岩的基性岩及现代弧后盆地(Mariana)相似,根据其地球化学特征进行构造环境判别,基本反映出弧后盆地火山岩的特征.另外样品的ENd(t)为较高的正值(6.11～8.17),表明其源区应为亏损地幔.结合研究区的沉积建造特征可以判断,月牙山-洗肠井蛇绿岩应形成于弧后盆地,与其北部的斜山-东七一山火山弧构成塔里木板块北缘的“弧-盆”体系.%The YueyashairXichangjing ophiolite, located at the middle section of the Beishan area,is one of the well exposed ophiolitic belts in the Beishan area. The pillow basalt and cumulate gabbro are characterized by depletion of LREE, flat distribution pattern, and(La/Yb)N = 0. 47-- 1. 62, similar to those of N-MORB; and by enrichment of LILEs, and depletion of HFSEs such as Nb and Ta, different from those of N-MORB, but similar to those by typical island-arc volcanic rocks, I. e. the mafic rocks have both the MORB-like and arc-like geochemical features. These features are similar to those of mafic rocks occurring in the Kuerti ophiolite which formed in the back-arc basin. Their geochemical features reflect a tectonic setting of back arc basin in which volcanic rock formed. The End(t) of other samples are positive and high (6. 11 --8. 17),which implies that the source area is the depleted mantle. Combined with the geochemical and sedimentary characteristics, it can be concluded that the Yueyashan-Xichangjing ophiolite may form in a back-arc basin, which, along with Xieshan-Dongqiyi volcanic arc in the north of the basin, constituted an "arc-basin" system in the Tarim plate.