西藏中冈底斯带北部早白垩世构造属性：来自则弄群火山岩锆石U-Pb年龄及地球化学的制约

Tectonic attributes of the Northern part of the Middle Gangdise Belt in the Early Cretaceous: evidences of zircon U-Pb dating and lithogeochemistry of volcanic rocks from the Zenong Group

关键词:  早白垩世  火山岩  则弄群  U-Pb年龄  Slainajap洋  冈底斯带

Key words:  early Cretaceous  volcanic Rocks  the Zenong Group  U-Pb dating  Slainajap Ocean  Gangdise

摘要:

中冈底斯带广泛分布则弄群火山岩，其成因机制一直存在争议。本文对出露于来多地区的则弄群火山岩进行了锆石U-Pb定年及全岩地球化学分析，旨在精确限其形成时代，探讨其成因机制。结果表明，来多地区的则弄群火山岩形成于124.0～114.7Ma之间；岩石SiO₂含量在54.53%～78.35%之间，铝饱和指数A/CNK为0.77～3.75，属偏铝质-过铝质岩石，轻稀土富集，负Eu异常明显，相对富集Rb、Th、U等大离子亲石元素，亏损Nb、Ta等高场强元素，岩石具有弧火山岩的特征。综合分析认为，则弄群火山岩可能为班公湖-怒江缝合带南侧的狮泉河-永珠-纳木错-嘉黎蛇绿岩带所代表的Slainajap洋在早白垩世向南俯冲消减的产物。

Abstract:

The genesis of the widely distributed volcanic rocks of the Zenong Group in the Middle Gangdise Belt, Tibet, has been controversial. By carrying out zircon U-Pb dating of the volcanic rocks from the Zenong Group in Laiduo are, the authors believe that the volcanic rocks of the Zenong Group were formed during 124.0～114.7Ma.The geochemical analyses of volcanic rocks from the Zenong Group show that their SiO₂ contents range from 54.53% to 78.35%，and the aluminum saturation index A/CNK is between 0.77 and 3.75, suggesting the volcanic rocks of the Zenong Group belong to the aluminum-peraluminous rocks, enriching in LREEs and LILEs such as Rb、Ba、Th and U, depleting in HFS elements such as Nb and Ta, and with an obvious negative anomaly of Eu. Evidences of the both Zircon U-Pb dating and lithogeochemisty indicate that the Zenong Group may be the product of the southward subduction of the Slainajap ocean which is now in the form of Shiquanhe-Yongzhu-Namucuo-Jiali ophiolite belt during the Early Cretaceous.