西藏朗县蛇绿混杂岩的地球化学特征及地质意义

Geochemical characteristics and tectonic significance of ophiolite mélange from the Langxian County of Southeastern Tibet,China

在岩石学研究的基础上,对西藏东南部朗县蛇绿混杂岩的岩石地球化学进行了系统研究。朗县蛇绿混杂岩的变质橄榄岩和变辉绿岩具有低Na2O+K2O,且Na2OK2O,低P2O5以及MgO和TiO2变化较大等特点;稀土配分模式为轻稀土略微亏损或富集的平坦型;大离子亲石元素Rb和Sr强烈亏损,高场强元素变化较大。变玄武岩的TiO2,P2O5与N-MORB十分接近;稀土配分模式为轻稀土略微亏损或富集的平坦型,拉索村和白露村-朗县附近地区玄武岩具有E-MORB特征,而里龙地区的玄武岩具有N-MORB特点;与典型的洋中脊玄武岩比较,大离子亲石元素和高场强元素的丰度值普遍偏高,拉索地区的大离子亲石元素比较接近N-MORB,里龙地区的高场强元素十分接近N-MORB。体现具有典型洋中脊玄武岩的特征但又不完全相同,微量元素分布形式非常类似于阿曼蛇绿岩的特征,表明朗县蛇绿混杂岩可能形成于大洋中脊到岛弧的过渡环境。

The ophiolite mélange belt is an important part of the Yalu Tsangpo suture in Langxian County of southeastern Tibet,China.There are two ophiolite mélange belts.One is the Yumai ophiolite mélange belt while the other the Langxian ophiolite mélange belt.This paper discusses the Langxian ophiolite mélange belt.It is made up of meta-olivinfels,epidiabase and meta-basalt,and occurrs within the segments of ophiolite suits.This paper analyses petrological geochemistry of the ophiolite mélange systematically on the basis of petrology.The meta-olivinfels and epidiabase of the Langxian ophiolite mélange are characterized by low Na2O＋K2O content（Na2OK2O）,low P2O5 content and greater variation of MgO and TiO2.Their REE patterns are flat-type LREE with a little depletion or enrichment.The characteristics of trace-elements are depleted in LILEs（Rb,Sr）and varied in HFSEs.TiO2and P2O5contents of meta-basalts are quietly close to N-MORB and their REE patterns are flat-type LREE with a little depletion or enrichment.The meta-basalts in the Lasuo and Bailu districts have E-MORB characteristics,while the meta-basalts in the Lilong district have NMORB characteristics.Compared with the typical N-MORB,abundance is high in LILEs and HFSEs elements,but it is the same with LILEs in Lasuo district and the same with HFSEs in Lilong district.Different from the typical N-MORB,the distribution characteristics of trace elements of meta-basalts resemble Oman ophiolite quietly,which shows the meta-basalts in this district formed in the transient setting of mid-ocean ridge and island-arc.