北黄海盆地上侏罗统–下白垩统火山岩形成时代:锆石LA-ICP-MSU-Pb定年证据

Zircon U-Pb Geochronology of Early Jurassic-Late Cretaceous Volcanic Rocks in North Yellow Sea Basin

北黄海盆地X钻井岩心揭示了中生界两套碎屑岩系：下部的暗色岩系和上部的红色岩系。暗色岩系底部和顶部存在两层火山岩。下层火山岩的岩石学鉴定为粗面英安岩，化学成分位于英安岩–流纹岩分区，靠近粗面英安岩分区。上层火山岩的岩石学鉴定为碎屑碎斑英安岩，化学成分位于英安岩分区。下层和上层火山岩锆石的阴极发光图像显示出自形–半自形柱状的晶形和韵律环带结构，下层粗面英安岩中22颗锆石的206Pb/238U年龄介于141~151 Ma之间，其加权平均年龄为145±2 Ma。上层碎屑碎斑英安岩中18颗锆石的206Pb/238U年龄介于139~149 Ma之间，其加权平均年龄为141±2 Ma。反映X井火山岩属于晚侏罗世晚期–早白垩世早期，因此这套暗色岩系应为晚侏罗世–早白垩世地层。对比山东半岛中生代岩浆作用的年代学和地球化学资料发现，北黄海盆地与胶莱盆地火成岩可能具有相同的动力学机制，形成于与俯冲有关的火山弧或弧后伸展环境背景。

The X-drilling cores of the North Yellow Sea basin reveal two sets of Mesozoic clastic rocks, which are the dark rocks in lower part and the red rocks in upper part, respectively. There are two layers of volcanic rocks at the bottom and the upper part of the dark rock unit. The volcanic rocks at the bottom part are trachytic dacite while rocks at the upper part are clastic dacite. The zircon grains from the upper and lower units of volcanic rocks are euhedral-subhedral columnar crystals and show oscillatory zoning on cathodoluminescence images. 22 tests of zircons in the trachytic dacite from the bottom part yield an age of 141–151 Ma, with weighted mean 206Pb/238U age of 145±2 Ma. Whereas 18 tests of zircons from the sample at the upper part give206Pb/238U ages around 139–149 Ma with weighted mean206Pb/238U age of 141±2 Ma, which implies that the X well volcanic rocks belong to Late Jurassic-Early Cretaceous. Comparing with the age and geochemical characteristics of the Mesozoic igneous rocks in Shandong peninsula, we suggest that the igneous rocks from both the North Yellow Sea basin and Jiaolai basin were formed under same dynamic setting, i.e., the subduction related volcano arc and back-arc extension.