合肥盆地圆筒山组物源分析及其地质意义:来自碎屑锆石的证据

Provenance analysis of the Yuantongshan Formation in the Hefei Basin and its geological signature:evidences from detrital zircon dating

合肥盆地位于华北板块东南缘,形成于华南、华北板块碰撞过程中。合肥盆地形成时表现为大别造山带向北逆冲形成的前陆挠曲盆地,早白垩世在区域伸展背景下转变为断陷盆地。中侏罗统圆筒山组是合肥盆地前陆挠曲阶段的沉积地层之一,主要表现为湖泊相沉积,与下伏的防虎山组典型的河流相沉积明显不同。为了获得圆筒山组更详细的物源信息,对肥西地区出露的圆筒山组紫红色粉砂岩开展了碎屑锆石LA-ICP MS U-Pb定年。定年结果显示,两个粉砂岩样品均获得了约2.0Ga和约770 Ma两个主要峰值以及约2.4Ga次要峰值。该特征与扬子板块锆石年龄分布特征几乎完全一致,指示圆筒山组物源应来自扬子板块。考虑到盆地地层的物源不应来自其周边隆起区分水岭的另一侧,因而推测圆筒山组物源应来自张八岭隆起中侏罗世时地表出露岩石。在燕山运动A幕影响下,下扬子地区发生逆冲-褶皱活动,张八岭隆起发生明显隆升,上部岩石被剥蚀殆尽,仅保留现今出露的新元古代张八岭群及肥东杂岩,被剥蚀的岩石搬运沉积于合肥盆地内,形成圆筒山组。

The Hefei Basin in southeastern North China block was formed during the collision between the South and North China blocks.It transformed from a foreland basin,developed by northward thrusting of the Dabie Orogen,into a fault subsidence basin in an extensional tectonic setting in the Early Cretaceous.The Middle Jurassic Yuantongshan Formation in the Hefei foreland strata features lacustrine facies,which is very different from fluvial facies within the Lower Jurassic Fanghushan Formation.In order to obtain detailed provenance information,we performed detrital zircon LA-ICP MS U-Pb dating of purple siltstone from the Yuantongshan Formation,Feixi district.Zircon dating results of two collected samples revealed two major age peaks at^2.0 Ga and^770 Ma and one minor peak at^2.4 Ga.Histograms of detrital zircon ages for the Yuantongshan Formation and Yangtze block are almost identical,suggesting that the later is provenance of the former.Considering that provenance of strata in a basin should not originate from other side of a watershed,provenance of the Yuantongshan Formation,therefore,is likely rocks cropping out in the Zhangbaling Uplift in the Middle Jurassic.In the early period of the Yanshanian movement,a fold-and-thrust belt occurred in the Lower Yangtze region.The Zhangbaling Uplift raised significantly at that time,causing surface rocks to erode while only the Zhangbaling Group and Feidong Complex preserved.Materials eroded from the Zhangbaling Uplift were transported to and deposited in the Hefei Basin to form the Yuantongshan Formation.