合肥盆地构造热演化的裂变径迹证据

Fission track evidence for the tectonic-thermal history of the Hefei basin

运用裂变径迹分析方法,探讨分析了合肥盆地中新生代的构造热演化特征.上白垩统和古近系下段样品的磷灰石裂变径迹(AFl)数据主体表现为靠近部分退火带顶部温度(±65℃)有轻度退火,由此估算晚白垩世至古近纪早期合肥盆地断陷阶段的古地温梯度接近38℃/km,高于盆地现今地温梯度(27.5℃/km).下白垩统、侏罗系及二叠系样品的AFT年龄(97.5～2.5Ma)和锆石裂变径迹(ZFT)年龄(118～104Ma)均明显小于其相应的地层年龄,AFT年龄-深度分布呈现冷却型曲线形态,且由古部分退火带、冷却带或前完全退火带及其深部的今部分退火带组成,指示早白垩世的一次构造热事件和其随后的抬升冷却过程.基于AFT曲线的温度分带模式和流体包裹体测温数据的综合约束,推算合肥盆地早白垩世走滑压陷阶段的古地温梯度接近67℃/km.径迹年龄分布、AFT曲线拐点年龄和区域抬升剥蚀时间的对比分析结果表明,合肥盆地在早白垩世构造热事件之后的104Ma以来总体处于抬升冷却过程,后期快速抬升冷却事件主要发生在±55Ma.

Fission track analyses to Permian, Jurassic, Cretaceous and Paleogene core samples are made to reconstruct the tectonic-thermal history of Hefei basin. The apatite fission track （AFT） data of the Upper Cretaceous and Lower Paleogene samples show a slight annealing close to the upper-boundary of partial annealing zone （PAZ） at a paleo-temperature of about ± 65%. The estimated geothermal-gradient of the Hefei extensional-faulted basin during the Late Cretaceous to Early Paleogene is about 38 ℃/km, which is higher than the present-day value of 27.5%/km. The AFT ages of 97.5 -2.5 Ma and the zircon fission track （ZFT） ages of 118- 104 Ma from the Lower Cretaceous to Permian samples are younger than the strata ages. The curve of AFT age vs. depth shows a cooling pattern composed of a paleo-PAZ, a cooling zone and a present-day PAZ. These results indicate a tectonic-thermal event during the Early Cretaceous and then an uplift-cooling process in the Hefei basin. Based on the temperature-dividing model of the AFT curve with temperature constraints from fluid inclusions, the estimated geothermal-gradient of the Hefei wrench-compressive basin during the Early Cretaceous was about 67℃/km. The fission track ages and the AFT curve break age, as weii as the regional uplift-erosive time, reveal that the uplift-cooling process following the tectonic-thermal event has developed since 104 Ma and a later rapid uplift-cooling event occurred mainly around ± 55Ma.