中生代合肥盆地南部的沉积过程与大别山变质地体的剥露

Mesozoic sedimentation of the southern Hefei basin and exhumation of the Dabieshan metamorphic terrane.

合肥盆地南部的构造-沉积演化历史可划分出两个不同阶段,即侏罗纪伸展断陷和盆地向南扩展阶段和早白垩世盆地南缘火山喷发和盆地向北退缩阶段。合肥盆地自早侏罗世开始形成,强烈的断陷-沉积作用发生在中、晚侏罗世。盆地边缘沉积主要由冲积扇与辫状河体系组成,明显受边缘正断层控制,并且随断层向南迁移,盆地也不断向南扩展。盆地主体沉积以河流-湖泊体系为特征。古流向恢复结果证明盆地沉积物来自于大别山变质地体。下侏罗统防虎山组中含柯石英包体的三叠纪变质锆石的发现表明,超高压岩石在早侏罗世就已经剥露到地表。凤凰台组中榴辉岩砾石的出现指使大别山在晚侏罗世经历了强烈的抬升和剥蚀。合肥盆地南部在早白垩世时开始抬升,并发生强烈的火山喷发,盆地沉积范围向北明显迁移。合肥盆地二阶段式构造-沉积演化过程反映,大别山及邻区的构造体制在侏罗纪末发生了明显的变化。我们认为大别山变质地体在侏罗纪时期可能是通过构造挤出的方式折返到地表的,这种挤出构造过程一方面导致大别山变质地体的前缘(南缘)发育逆冲推覆构造和形成前陆盆地,另一方面也同时造成其后缘(北缘)发生伸展拆离和产生断陷盆地。早白垩世时期大别山所经历的区域性地壳伸展和强烈的岩浆活动可能与深部岩石圈的拆沉和软流圈热物质的上涌有关。

Mesozoic tectono-sedimentary evolution of the southern Hefei basin can be divided into two distinct stages. The first stage is featured by extensional subsidence and southward expansion of the basin in Jurassic time, while the second stage is characterized by vigorous volcanic eruptions in the southern margin of the basin and northward migration of main depositional area. The Hefei basin was initiated in the Early Jurassic, but the Mid Late Jurassic saw a period of pronounced subsidence. Alluvial fan and braided fluvial systems dominated proximal areas of the basin, and were evidently controlled by basin-border faults. The basin expanded to the south as a result of the south-directed migration of the normal faulting. Fluvial lacustrine sedimentation were prevalent in the main area of the basin. Sediments were demonstrated to have come from the Dabieshan metamorphic terrane, as indicated by the restored paleo-current directions. Detrital Triassic metamorphic zircons with coesite inclusions have recently been recognized in Lower Jurassic Fanghushan Formation, thereby suggesting that the ultrahigh-pressure metamorphic rocks must have been exhumed to the surface in the Early Jurassic. Occurrence of eclogite gravels in the Fenghuangtai Formation indicates that the Dabieshan terrane experienced marked denudation in the Late Jurassic. The southern Hefei basin underwent the uplift in association of vigorous volcanism, and basin depositional area was shifted far to the north. The two-stage evolution of the Hefei basin is indicative of a change of tectonic settings in the Dabieshan terrane and adjacent regions. It is proposed that the ultrahigh-pressure rocks might have been brought up to the surface by means of laterally upward extrusion. The extrusion model not only explains Jurassic extensional subsidence of the Hefei basin to the north of the Dabie Shan terrane, but also accounts for coeval development of fold-thrust system of the southern Dabie Shan and formation of foreland basin to the south. Regional crustal extension and striking magmatism within the Dabieshan terrane and surrounding areas are considered the consequence of the upwelling of asthenospheric hot materials, presumably triggered by the foundering of subducted lithospheric slab.