青海省同仁地区早白垩世大陆裂谷环境火山岩的确定及意义

The Determination and Significance of Lower Cretaceous Continental Rift Environment Volcanic Rock in Tongren Area,Qinghai Province

西秦岭西端在同仁地区沿近南北向断裂分布一套典型的灰紫色陆相火山岩,岩性组合为灰紫色块层状-杏仁状橄榄玄武岩、安山玄武岩、火山角砾岩夹灰紫色厚层状复成分砾岩、灰紫色—青灰色中厚层状含砾粗砂岩和灰紫色薄层状泥岩,火山岩以碱性系列为主,其形成时代为早白垩世。其地球化学显示轻稀土富集,Eu具轻度正异常,不相容元素富集,岩石具有较高的87Sr/86Sr、143Nd/144Nd、εNd和206Pb/204Pb特点。岩石的εNd均值为7.69,εSr均值为-10.7,其物源与典型的EM或HIMU型洋岛玄武岩明显不同,物源受部分大陆壳的混染,岩浆源成分属于EM与HIMU幔源间混合成因。综合分析其为大陆板块内部拉张环境下的岩浆活动产物,是形成于伸展构造背景下类似于大陆裂谷环境的火山岩。

A set of classical continental facies volcanic rock with grey purple are located on western margin of Western Qinling, distributing alone near North and South proximity fracture zone. IC s lithologic as- semblage is grey purple block layered- amygdaloidal olivine basalt, andesite basalt, lava breccias, and clamps some grey purple thick-bedded polymictic conglomerate, grit stone, grey purple or caesious straat- iculate mud stone. The volcanic rock are close to alkaline series. Its formation epoch was Lower Creta- ceous. IC s geochemistry character display LREE-rich right-oriental pattern with weak Eu normal anoma- ly, general enrichment in incompatible elements. The rock has higher ratios of 8ZSr/86 Sr,143 Nd/TM Nd. εNd and 206Ph/204Ph, Its average value of εNd was 7, 69, average value of εSr was --10.7. Its provenance is different from EM type or HIMU type OIB, the rocks are obviously contaminated by crustal materials. The source region exhibits a mixture between EM and HIMU. By comprehensive analysis, it was the product of igneous magma activity in the tensile environment in the inner of the continental plate. The rock formed in similar continental rift in the spread tectonic background.