合肥盆地中生代地层时代与源区的碎屑锆石证据

合肥盆地位于大别造山带北侧、郯庐断裂带西侧,其发育过程与这两大构造带演化密切相关。本次工作对合肥盆地南部与东部出露的中生代砂岩与火山岩进行了锆石年代学研究,从而限定了各组地层的沉积时代,确定了火山岩喷发时间,指示了沉积物的源区。这些年代学数据表明,合肥盆地南部的中生代碎屑岩自下而上分别为下侏罗统防虎山组、中侏罗统圆筒山组或三尖铺组、下白垩统凤凰台组与周公山组(或黑石渡组)与上白垩统戚家桥组,其间缺失上侏罗统。盆地东部白垩系自下而上为下白垩统朱巷组与响导铺组和上白垩统张桥组。该盆地出露的毛坦厂组或白大畈组火山岩喷发时代皆为早白垩世(130～120 Ma)。盆地南部的下——中侏罗统及白垩系源区皆为大别造山带,分别对应该造山带的后造山隆升与造山后伸展隆升。而盆地东部白垩系的源区始终为东侧的张八岭隆起带,后者属于郯庐断裂带伸展活动中的上升盘。

Detrital Zircon Evidence for Depositional Time and Provenance of Mesozoic Sediments in the Hefei Basin

Objectives: The Hefei Basin is situated to the north of the Dabie Orogen and west of the Tan-Lu Fault Zone. Its evolution is closely related to the two major tectonic belts. However, depositional time and provenance of Mesoizoic strata in the basin remain controversial. This work aims at constraining ages of the Mesozoic strata and their provenance. Methods: LA-ICP-MS U-Pb dating on detrital zircon was conducted on Mesozoic sandstones in the Hefei Basin. Their youngest ages and age spectra were used to constrtain depositional time and provenance of the dated sandstones from different formations. The zircon dating was also made on volcanic rocks to determine their eruption time. Results: The dating results show that the Mesozoic clastic rocks in the southern Hefei Basin include the Lower Jurassic Fanghushan Formation, the Middle Jurassic Yuantongshan or Sanjianpu formation, the Lower Cretaceous Fenghuangtai or Zhougongshan Formation and Upper Cretaceous Qijiaqiao Formation from the bottom to the top, with the absence of Upper Jurassic strata. The exposed Cretaceous clastic rocks in the eastern Hefei Basin are the Lower Cretaceous Zhuxiang or Xiangdaopu Formatiom and Upper Cretaceous Zhangqiao Formation from the bottom to the top. The zircon data indicate eruption time of Early Cretaceous (130~120 Ma) for the exposed Maotanchang or Baidafeng volcanic rocks in the basin. Conclusions: The detrital zircon demonstrate that the Lower to Middle Jurassic and Cretaceous clastic rocks in the southern basin were derived from the Dabie Orogen as a result of the post orogenic exhumation and late-orogenic extensional uplifting respectively. The provenance of the Cretaceous clastic rocks in the eastern Hefei Basin is the Zhangbaling uplifting belt which is a rising wall of the Tan-Lu Fault Zone during its normal faulting.