鄂尔多斯盆地延长组长7段凝灰岩锆石U-Pb年龄、Hf同位素组成特征及其地质意义

Zircon U-Pb ages and Hf isotope characterization and their geological significance of Chang 7 tuff of Yanchang Formation in Ordos basin

鄂尔多斯盆地延长组长7优质烃源层中广泛分布了薄层和纹层状凝灰岩,凝灰岩中锆石U-Pb同位素年代学研究表明,长7段凝灰岩结晶年龄为234Ma左右。对比勉略缝合带、南秦岭的花岗岩的年龄,三叠系延长组凝灰岩应该形成于俯冲阶段。锆石的Hf同位素分析表明延长组长7段凝灰岩主要来自地壳岩石部分熔融,并且有一定的地幔物质加入。Hf同位素二阶段模式年龄大部分集中在1.1-1.3Ga,其中以1.2Ga为主峰,表明延长组凝灰岩主要来自元古代增生的地壳物质,在物质组成和时代上可能类似于南秦岭基底中的耀岭河群基性火山岩和郧西群酸性的混合。

Thin-layered and laminated tuff was extensively developed in the high grade hydrocarbon source rock in Chang 7 member of the Yanchang Formation in Ordos basin. LA-ICP-MS zircon U-Pb data show Chang 7 tuff crystallized in the 234Ma. Compared with the ages of the Mianlue suture zone and South Qinling granite,Triassic tuff from the Yanchang Formation should be formed in the subduction stage. Zircon Hf isotope composition suggests that Chang 7 tuff mainly resulted from a partial melting process of crustalderived granitic magma. The two-stage Hf-isotope model ages of all three tuffs mostly range from 1. 1Ga to 1. 3Ga,and peak at 1. 2Ga. This indicates that Chang 7 tuff mainly originated from the proliferation crustal material in Neoproterozoic,as for the material composition and age,whose provenance probably are the mixture between Yaolinghe Group basic volcanic rocks and Yunxi Group acid volcanic rocks of South Qinling basement.