河南舞阳凹陷底部火山岩的发现及其锆石年代学和Hf同位素地球化学研究

Zircon U-Pb Dating and Hf Isotope Compositions of the Volcanic Rocks from the Bottom of the Wuyang Sag, Henan Province

河南省中部的舞阳凹陷长期被认为是一新生代盐湖盆地。它位于华北克拉通南缘,基底由新太古代–古生代地层组成,盆地沉积物为新生代碎屑–化学岩系。最新钻探发现盆地南缘发育一套火山–沉积建造,火山岩与红色砂砾岩互层,总厚度达一千多米。作者对粗面岩(WY-1)和粗安质火山角砾岩(WY-2)开展了岩性组合、区域地层对比划分以及LA-ICP-MS锆石U-Pb定年及Hf同位素研究。锆石多具有生长振荡环带,个别锆石显示核-边结构。WY-1样品中25颗锆石的^(206)Pb/^(238)U加权平均年龄为129.1±1.0 Ma(MSWD=0.87);WY-2样品中4颗锆石的^(206)Pb/^(238)U加权平均年龄为132.5±9.8 Ma(MSWD=5.0),两者的年龄比较接近,表明火山岩形成于133 Ma左右,属中生代早白垩世,盆地地层并非前人认为的全部属于新生代。藉此,我们认为舞阳凹陷乃至周口盆地的发育始于早白垩世或更早,是扬子–华北古板块碰撞之后秦岭造山带岩石圈伸展的结果。WY-2样品中其他13颗继承锆石或继承核的^(207)Pb/^(206)Pb年龄介于1033~2931 Ma,与华北克拉通南缘主要岩性的时代一致。火山岩中的早白垩世锆石ε_(Hf)(t)为-21.82^-19.10,t_(DM2)变化于2.39~2.56 Ga之间,与太华超群中部地层年龄和特征吻合,表明火山岩主要源自华北克拉通南缘深部早前寒武纪岩石的部分熔融作用。

The Wuyang Sag in central Henan province is an important salt-bearing basin located in the southern margin of the North China Craton, which is considered to be formed in Cenozoic. The basement of the basin is composed of the Neoarchean-Paleoproterozoic Taihua Supergroup metamorphic rocks, and the cover has been considered to be a Cenozoic sedimentary sequence in previous studies. However, in the ongoing drilling work at the southern margin of the Wuyang Sag, a volcanic succession with thickness of 1000 m, intercalated with sandstones, conglomerates and limestones, was recently discovered. The volcanic rocks mainly include trachyandesitic to trachytic lava, tuffs and breccia. We conducted zircon LA-ICP-MS U-Pb dating combined with Hf-isotope analysis of the trachyandesitic breccia and trachyte samples collected from the drill holes. Zircon grains exhibit obvious oscillating zoning texture, and a few show core-rim structures. Twenty-five analyses of sample WY-1 yield a weighted mean age of 129.1±1.0 Ma（MSWD=0.87）, while four analyses of sample WY-2 yield a weighted mean age of 132.5±9.8 Ma（MSWD=5.0）. These ages are identical within errors and strongly suggest that the volcanic rocks were formed at 129~133 Ma. We thus conclude that the basins on the southern margin of the North China Craton began to sink since Early Cretaceous or somewhat earlier, coeval to post-collisional lithospheric extension of the Qinling Orogen suturing the Yangtze and North China cratons. Inherited zircon grains or cores yield 207^Pb/206^ Pb ages of 1033~2931 Ma, according with the ages of the main lithologies in the southern margin of the North China Craton. Zircons of Early Cretaceous yield εHft values of-21.82 to-19.10, and tDM2（Hf） ages of 2.39 to 2.56 Ga, which are similar to the ages and features of the middle Taihua Supergroup. This implies that the Cretaceous volcanic rocks mainly originated from the partial melting of the Early Precambrian rocks at the depth of the southern margin of the North China Craton.