Low-temperature Thermochronological Study On The Exhumation Process Of Cretaceous Putos In Yanqing-fengning Area Of The Western Yanshan Belt

The western part of the Yanshan Tectonic Belt has experienced complex magmatic activity and tectonic evolution since the Mesozoic.The evolution process of magma in the lithosphere of the earth and the transformation of geodynamic model are directly reflected in the the change of surface elevation.The recovery of thermal history and the calculation of the denuded thickness of the target area by using low-temperature thermochronology are helpful to restore the ancient elevation of the basin mountain system.The quantitative analysis of denudation intensity of basin mountain system in the target area and the restoration of the late Mesozoic paleogeomorphological pattern in the western part of the Yanshan Belt are beneficial to the further study of the geodynamic model of multi-plate convergence and collision process of China.It lays a foundation for exploring the accumulation and preservation of coal,oil,natural gas and other exogenous mineral resources in eastern China,as well as the paleoclimate evolution model in East Asia.The scientific problem to be solved in this study is whether there is a unified rapid uplift(exhumation)in the Yanqing-Fengning area of the western Yanshan Belt at the turn of the early and late Cretaceous(about 100Ma).Three Cretaceous plutons(Fangshan pluton,Dahaituo pluton and Dengzhazi pluton)covering the target area from south to north are selected for sampling.Zircon(U-Th)/ He dating method combined with apatite(U-Th)/ He and apatite fission track dating method are used to test the samples.The thermal history inversed by HeFTy software combined with elevation-age method and mineral pair method are used to analyze the age data in order to restore the thermal evolution history and determine the tectonic stage in Yanqing-Fengning area of the western Yanshan Belt since the late Mesozoic.The AFT age,AHe age and ZHe age of one sample taken from the Fangshan pluton are 60.8±6.4Ma,35.1±1.5Ma and 77.2±2.8Ma,respectively.Three samples of the Dahaituo pluton were tested.The AFT age of one sample is 124.2±14.5Ma,the AHe age is 32.2±1.2Ma and the ZHe age is 96.6±3Ma.The ZHe ages of the other two samples are 93.6±3.4Ma and 89.4±3.3Ma,respectively.There is no significant difference in ZHe ages between the 3 samples.Three samples of the Dengzhazi pluton were tested.The AHe age of one sample was 96.7±4.2Ma,and the ZHe age is 116.9 ±4.2Ma.The ZHe ages of the other two samples are 93.4±2.7Ma and 147.3±5.3Ma,respectively.The distribution of ZHe ages is extensive.By using thermochronology,three rapid uplift events are identified in the Fangshan pluton,which occurred in 124-100 Ma,77-74 Ma,36-28 Ma,and the total denudation amount is 13.24 km since 124 Ma.Two rapid uplift events occurring in 117-100 Ma and 46-19 Ma are identified in the Dahaituo pluton.Since 117 Ma,the denudation amount has a total of 11.68 km.Two rapid uplift events are identified in the pluton of the Dengzhazi,which occurred in the range of 123-109 Ma and 101-88 Ma,respectively.The total denudation is 5.8km since 123 Ma.Two large-scale rapid uplift events occurred in Yanqing-Fengning area of the western Yanshan Belt.The first phase of regional uplift is in 124-100 Ma and the second phase of southern and central uplift is in 46-19 Ma.