四川乐山震旦系灯影组火山碎屑岩锆石LA-ICP-MSU-Pb定年及盆地裂陷演化讨论

LA-ICP-MS U-Pb Zircon Ages of Volcaniclastic Beds of the Third Member of the Sinian( Ediacaran) Dengying Formation in Leshan,Sichuan,and a Discussion on the Rift Evolution in the Basin

四川盆地中部晚震旦(埃迪卡拉)世—早寒武世克拉通内裂陷与安岳震旦(埃迪卡拉)系—寒武系特大气田的形成关系密切,年代学研究对认识克拉通内裂陷构造意义重大。对四川盆地乐山先锋剖面灯影组三段火山碎屑岩进行锆石LA-ICP-MS U-Pb定年,研究结果表明四川盆地灯影组三段火山碎屑岩的锆石U-Pb年龄为(539.6±1.4)Ma,年代学结果表明四川盆地灯三段与灯四段属于寒武系。基于灯影组底界(551.1 Ma)、含火山碎屑岩的灯三段(539.6 Ma)、含小壳化石的麦地坪组(535.2 Ma)和下寒武统Ni—Mo多金属硫化物富集层(521.0 Ma)四个等时面构建了克拉通内裂陷演化的时间框架。在此基础上认为四川盆地中部晚震旦(埃迪卡拉)世—早寒武世隆坳构造的形成主要受边界断裂控制,断层活动使裂陷内水体加深,少量灯四段含硅质白云岩沉积后,麦地坪组与筇竹寺组开始沉积,麦地坪组与灯四段的界线在裂陷内与裂陷两侧并不等时,裂陷内麦地坪组、筇竹寺组与裂陷两侧灯影组四段为同期异相沉积。

The late-Sinian（Ediacaran） to early-Cambrian intracratonic rift in central Sichuan basin has a close relationship with the formation of Anyue Sinian（Ediacaran）—Cambrian giant gas field. geochronology study of the Dengying Formation is significant in understanding the evolution of the intracratonic rift. LA-ICP-MS U-Pb analysis of volcanic ash beds in the Third Member of the Dengying Formation at the Xianfeng Section, Sichuan, Southwest China yield a highly reliable Concordia age of 539.6±1.4 Ma（MSWD=0.22）, suggesting a lower placement of the Precambrian—Cambrian boundary in the Sichuan Basin. We eatablished a evolution time frame of this intracratonic rift based on four isochronous strata, the bottom of the Dengying Formation（551.1 Ma）, the Third Member of Dengying Formation（539.6 Ma）, the Maidiping Formation（535.2 Ma） and the Ni—Mo sulfide enriched layer in the lower Cambrian（521 Ma）. The boundary of the Maidiping Formation and the Dengying Formation inside and outside the rift is not synchronous. According to those, we propose that the formation of the late Sinian（Ediacaran） to early-Cambrian intracratonic rift in central Sichuan Basin was mainly controlled by the boundary fault. Due to the deep-water depositional environment in the rift, the Maidiping Formation began to deposit rapidly after the deposition of the thin Third and Fourth Member of the Dengying Formation, the Maidiping Formation and the Qiongzusi Formation inside the rift and the Fourth Member of the Dengying Formation outside the rift are contemporary heteropical sediments.