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

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.It's lithologic assemblage 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 straaticulate mud stone.The volcanic rock are close to alkaline series.Its formation epoch was Lower Cretaceous.It's geochemistry character display LREE-rich right-oriental pattern with weak Eu normal anomaly,general enrichment in incompatible elements.The rock has higher ratios of ⁸⁷Sr/⁸⁶Sr、¹⁴³Nd/¹⁴⁴Nd、εNd and ²⁰⁶Pb/²⁰⁴Pb.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.