拉萨地块中北部白垩纪则弄群火山岩:Slainajap洋南向俯冲的产物?
Cretaceous volcanic rocks of Zenong Group in north-middle Lhusa block: products of southward subducting of the Slainajap ocean?
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摘要: 拉萨地块广泛分布有中生代的岩浆活动,研究它们对于认识特提斯洋的演化和理解整个青藏高原的形成过程有着重要的启示.本文对出露于拉萨地块中北部的则弄群火山岩进行了系统的年代学以及元素地球化学研究.研究的则弄群火山岩主要由玄武安山岩、安山岩和英安岩组成,根据化学成分可将其分为中基性(SiO258%)两个组.中基性岩以低钾和中钾钙碱性岩为主,而中酸性岩则主要位于高钾钙碱性系列;二者微量元素分布特征相似,如均富集Rb、Ba、Th、u等大离子亲石元素(LILE),Nb、Ta、Ti等高场强元素(HFSE)有着明显的负异常,具有明显的弧火山岩成分特征;稀土配分模式均表现为一致的轻稀土富集右倾型,可能反应了其母岩浆的同源性.精确的锆石U-Ph IA-ICPMS定年获得了113.6±1.0Ma年龄.说明研究区则弄群火山岩形成于早白垩世中期.综合前人的研究成果,我们初步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古洋(Slainajap洋)在早白垩世向南俯冲消减的产物.
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