碎屑颗粒裂变径迹热年代学在沉积物源区剥露历史分析中的应用

沉积盆地中未遭受热重置的碎屑颗粒裂变径迹(FT)热年代学正成为山前冲断带蚀源区抬升-剥露过程及其与相邻盆地沉积作用关系研究的重要方法.对于未热重置碎屑岩样品的磷灰石(或锆石)单矿物颗粒裂变径迹年龄(FTGA)数据,采用高斯或二项式拟合方法可以获得样品中不同组分碎屑颗粒矿物的FT峰值年龄或称蚀源区抬升一剥露事件的FT封闭(冷却)年龄.已有的统计分析结果表明,抬升冷却矿物的FT封闭年龄(tc)与其剥露搬运至相邻盆地的沉积年龄(td)之间具有较好的线性关系(tc=A+Btd),二者的滞后时间(△t=tc-td≈tc-te,te为抬升冷却的碎屑颗粒矿物剥露到近地表的剥露年龄)构筑了蚀源区抬升冷却矿物FT封闭深度(Zc)与抬升剥露速率(E)之间的定量关系(E=Zc/△t)和不同碎屑颗粒的滞后时间与其沉积年龄之间相关变化的统计预测模型△t=A+(B-1)td.显然,滞后时间越短,造山带蚀源区的抬升剥露速率越大、物源供应越充分,山前带沉积一沉降作用趋于增强,反之亦然.

Application of Detrital Grain Thermochronology on the Exhumation History of the Source Areas

Detrital grain fission-track thermochronology is a promising method for analyzing the exhumation history of the detrital source areas and the process of mountain building and basin deposition.For an unreseted sample from a sedimentary basin,the zircon or apatite fission track grain-age(FTGA) populations and their related FTGA peaks can be obtained by Gaussian or binomial peak-fitting.Then,the lag time(Δt),defined as the difference between the peak age tc and the corresponding depositional age td,can provide a measure of the exhumation,such as the exhumation rate E=Zc/Δt where Zc is the FT closure depth.Changes in tc with td can be approximated by a linear relationship,tc=A+Btd where A and B are fit parameters.Moreover,conversion to lag time gives Δt=A+(B-1)td which shows that A is the present lag time and(B-1) is the change in lag time with depositional age.It is inferred that the upsection trends in lag time can be represented by B,with B<1 associated with orogenic construction,B=1 with steady state,and B>1 with decay.Therefore,a decrease trend in lag time indicates an increase of the orogenic exhumation rate along with an intensive tectonic activity and a sufficient deposits supply inducing a rapid subsidence of the adjacent foreland basin,and vice versa.