藏东雅鲁藏布江大拐弯蛇绿岩变基性岩类岩石地球化学再研究

Geochemical study on metamorphosed mafic rocks in ophiolitic zone in the Yarlung Zangpo Great Bend,eastern Tibet,China.

在1:25万区域地质填图和专题研究的基础上,再次对喜马拉雅东构造结地区雅鲁藏布江蛇绿混杂带中变基性岩的常量元素、微量元素和Sr-Nd-Pb同位素地球化学特征进行了深入研究,进一步阐明了SSZ型蛇绿岩的成因。变基性岩类可分为具有玻安岩类、IAT和OIB特征的3类。玻安岩类为LREE呈略亏损—平坦型,稀土元素总量低,∑REE为7.56×10-6～20.89×10-6,富相容元素Cr、Ni和常量成分MgO、Al2O3,形成于弧前环境。IAT类为本带的主要类型,在蜘蛛图上有一定的Nb、Ta亏损,REE为大致平坦型,稀土元素总量低,∑REE为37.84×10-6～55.75×10-6。OIB类为LREE较富集型,稀土元素总量高,∑REE为102.09×10-6～240.95×10-6,与IAT和MORB相比,具较高的La、Nb、P、Ti含量,而相容元素Cr、Ni等含量较低。Sr、Nd、Pb同位素组成反映PREMA(或BSE)和EMI提供了源区,暗示未分异的原始地幔和下地壳参与形成了原始岩浆。推测新特提斯洋盆演化经历了较早的地幔柱、洋岛阶段,形成了MORB-OIB组合;较晚期为洋内弧阶段,形成了SSZ型蛇绿岩和IAT组合。

Based on regional geological mapping at 1∶250000 and thematic studies,we carried out geochemical studies on the metamorphosed mafic rocks,for major,trace elements and Sr-Nd-Pb isotope geochemistry of the Yarlung Zangpo ophiolitic zone in the area of the eastern Himalayan Syntaxis,to further clarify the SSZ-type ophiolite petrogenesis.The metamorphosed mafic rocks can be divided into boninite,IAT and OIB types.The boninite type samples have slightly depleted in LREE or flat-type pattern,with low total REE content（∑REE=7.56×10-6~20.89×10-6）.They have enriched Cr,Ni,MgO and Al2O3 contents and originated from fore-arc setting.The IAT samples,as the main type in this zone have Nb-Ta depletion in the spider diagram and flat-type REE pattern with low REE content（ΣREE=37.84×10-6~55.75×10-6）.The OIB samples are LREE-enriched type with higher REE content（ΣREE=102.09×10-6~240.95×10-6）.Compared with the IAT and MORB,they show higher La,Nb,P,Ti content,but lower Cr and Ni content.Sr,Nd and Pb isotopic composition suggest that PREMA（or BSE） and EMI provided magma source,suggesting that primitive mantle and lower crust are involved in the original magma forming.We proposed a two-stage hypothesized model for Neotethyan evolution.The Neotethyan evolution involves a plume and oceanic island as the early stage that formed MORB-OIB combination.The late stage formed intra-oceanic arc and resulted in SSZ-type ophiolites and IAT combination.