大别造山带折返剥露历史:来自合肥盆地南缘中生界变质岩碎屑的证据

在合肥盆地南缘广泛分布的中生界内,砾石主要由变质岩碎屑组成。碎屑在地层中分布及垂向上的变化,为重塑造山带折返剥露历史提供了证据。防虎山组和三尖铺组底部砾岩碎屑组合为:石英片岩+云母片岩+石墨片岩+千枚岩+石英岩+脉石英+片麻岩(局部),表明在早侏罗世晚期Pliensbachian期(距今195Ma)之前,佛子岭群和卢镇关群已经折返到地表并遭受剥露。早白垩世凤凰台组、毛坦厂组和周公山组中砾岩碎屑组合为:片岩+石英岩+片麻岩+混合岩+榴辉岩+角闪岩+斜长岩+花岗岩+大理岩等,榴辉岩以及其它基性岩的微量元素特征表明它们可能是一个在早白垩世以前(距今135Ma)折返到地表并遭受完全剥露,而现在已经从造山带消失的超高压构造地层单元。大别造山带出露的超高压变质带(大别杂岩)在北缘中生代地层中目前尚未发现有可靠的沉积记录,推测它们的大规模折返和剥露可能在新生代,并持续到现在。据此认为大别造山带大规模的折返剥露分为3个阶段:早侏罗世之前、晚侏罗世—早白垩世早期和新生代。

EXHUMATION AND DENUDATION OF THE DABIE OROGENIC BELT: CONSTRAINTS FROM METAMORPHIC CLASTS OF MESOZOIC SEDIMENTARY ROCKS IN THE SOUTHERN MARGIN OF HEFEI BASIN

The conglomerates, composed of metamorphic detritals, extensively developed in the Mesozoic strata in the south margin of Hefei Basin. The clasts'variety in the strata and sections provided important constraints on the types of source rocks and exhumation and unroofing history of the Dabie orogenic belt. The clasts of conglomerates of Jurassic Sanjianpu and Fanghushan Formations were dominated by metamorphic rocks of greenschist facies and amphibolite facies. The clast associations could match with metamorphic rocks of Foziling and Luzhenguan Groups, which implied that they might be source lands of the Jurassic sediments. The Fuziling and Luzhenguan Groups constituted the basement of the south margin of the basin, so that they had outcropped prior to late period of Early Jurassic (Pliensbachian stage, 195 Ma ago) and suffered erosion. The clasts, occurred in the conglomerates of Early Cretaceous Fenghuangtai, Maotanchang and Zhougongshan Formations, consisted of schist, gneiss, migmatite, eclogite, ultrabasic rock, granite, marble and so on. The clasts'features failed to support that they originated from the UHP terrains preserved in the Dabie orogenic belt, in despite of the eclogite clasts belonging to UHP group. They might belong to a UHP terrain that exhumated to surface prior to Early Cretaceous and suffered erosion, and had disappeared at present. The clasts in Mesozoic strata in the south margin of Hefei Basin lacked a petrographic linkage with the UHP terrains preserved in the Dabie orogenic belt, suggesting that it was in Cenozoic that they came to exhumate and denudate on scale and continue to now. So, three stages for exhumation and denudation history in the Dabie orogenic belt could be distinguished: in prior to Early Jurassic, Late Jurassic-Early Cretaceous and Cenozoic.