北京千家店地区侏罗系后城组磷灰石裂变径迹分析及其地质意义

Apatite fission track analysis of Upper Jurassic Houcheng Formation at Qianjiadian area,Beijing and its geological significance

运用裂变径迹分析方法,探讨分析了千家店地区侏罗系后城组地层的构造热演化特征.千家店地区后城组上段三个磷灰石样品,AFT年龄集中在85.7～76.0Ma,小于其相应的地层年龄;平均封闭径迹长度为9.4～10.8μm,小于初始径迹长度(16.3±0.9μm),呈非对称的单峰态分布,标准偏差为2.1～2.5.后城组下段的三个AFT样品,AFT年龄集中在82.6～62.4Ma,小于其相应的地层年龄,也小于上段层位的AFT年龄;平均封闭径迹长度仅为7.2～7.7μm,远小于初始径迹长度(16.3±0.9μm),其中YQ-07样品的封闭径迹长度呈似双峰态分布,标准偏差达到3.1;显然,侏罗系样品经历了明显的中度退火行为,最大温度可能接近于90℃.AFT年龄和封闭径迹长度的规律性变化主要是由于埋深不同引起的温度差异造成的.裂变径迹热历史模拟结果表明,沉积物自进入盆地充填埋藏一直到115Ma左右,盆地沉积物达到最大埋深3000多米,盆地温度达到最大值90℃多,这一过程沉积速率达到66.7m/Ma.115Ma之后盆地处于相对稳定期,没有明显的温度波动,直到6Ma左右温度以11.7℃/Ma的速度突然下降,表明侏罗系地层遭受剥蚀,迅速上升、快速冷却直至地表,剥露速率超过了500m/Ma.

Apatite fission track analysis is made to reconstruct the tectonic-thermal history of Jurassic basin at Qianjiadian area, Beijing. AFT ages of the three samples located at upper Houcheng Formation are concentrated in 85. 7 - 76.0 Ma, smaller than corresponding stratigraphic age; the mean confined track length is 9.4- 10. 8 μm, smaller than the initial confined track length （16.3±0. 9 μm）, showing a single peak state of skewed length distribution, and the standard deviation is 2. 1 - 2. 5; AFT ages of the three samples located at lower Houcheng Formation are concentrated in 82.6-62.4 Ma, also smaller than corresponding stratigraphic age; the mean confined track length is only 7.2-7.7 μm, far shorter than the initial track length （16.3 ± 0.9 μm）, where the length of the mean confined track of YQ-07 shows proximal bimodal distribution, with the standard deviation of 3.1. Clearly, these six samples experienced moderate annealing, and the maximum temperature may be close to 90 ℃. The regularity of AFT ages and the mean confined track length was mainly caused by the temperature difference due to changes in depth. Fission track thermal history modelling results show that the basin sediments reached the maximum depth of more than 3000 meters, and the basin temperature reached maximum of more than 90 degrees since sediments filling into the basin until about 115Ma, with deposition rate of about 66.7 m/Ma during the process. After 115 Ma the basin was in a relatively stable period, and there was no significant temperature fluctuation until late Miocene. The temperature suddenly dropped with a rate around 11.7 ℃/Ma in about 6 Ma, and the Jurassic sediments were exhumated rapidly to the surface, with denudation rates of more than 500 m/Ma.