狮泉河蛇绿混杂岩带早侏罗世辉长岩锆石年代学及地质意义

Geochronology and Geological Significance of Early Jurassic Gabbro in Shiquanhe Ophiolite Melange Belt

西藏狮泉河-申扎-嘉黎蛇绿混杂岩带紧邻班公湖-怒江蛇绿混杂岩带南侧,出露有较为完整的蛇绿岩岩石组合。嘎铃辉长岩出露于拉梅拉山口地区,野外实测剖面显示其具有明显的混杂增生特征。样品富集Ba、U、K、Pb、Sr等大离子亲石元素,亏损Nb、P等高场强元素,稀土配分模式属于轻稀土元素弱亏损的总体平坦型,判断其可能形成于俯冲背景下的弧后裂谷环境。LA-ICP-MS锆石U-Pb年龄为(189±1)Ma。结合区域构造背景,认为狮泉河蛇绿岩代表的特提斯洋可能是一个早侏罗世开始俯冲,早白垩世闭合的具有独立演化历程的局限性洋盆。

The Shiquanhe-Shenzha-Jiali ophiolite melange belt in Tibet is adjacent to the south of the Bangong Co-Nujiang ophiolite melange belt. The results show that it has a relatively complete ophiolite rock assemblage. The Galin gabbro is exposed in the lameila pass area. And the field profile shows that it has obvious hybrid accretion characteristics. The sample is enriched with large ionic lithophile elements such as Ba, U, K, Pb, Sr and deficit in high field strength elements such as Nb, P. The rare earth element(REE) pattern shows light REE weak losses, heavy REE flat. It indicates that it may be formed in the backarc rift environment under subduction background. LA-ICP-MS zircon U-Pb ages are(189±1)Ma. Combined with the regional tectonic background, it is considered that the tethys ocean represented by Shiquanhe ophiolite may be a restricted ocean with independent evolution process which started subduction in Early Jurassic and closed in Early cretaceous.