长白山与大兴安岭晚中生代以来剥露作用的对比研究

The Changbai Mountains and the Dahinggan Mountains are the most important and northeast to north-northeast-trending orogens in Northeastern(NE) China,which are roughly parallel to each other. The widely exposed Jurassic and Early Cretaceous granitoids in the Changbai Mountains and the Dahinggan Mountains suggest that the overlying rocks of these granitoids have largely been unroofed or denuded. The total exhumation since the Late Mesozoic is on kilometer scale. Systematical thermochronological study and comparison between these two mountain chains may provide firm constraints on their exhumation history, which is of critical importance for us to understand the geological evolution of NE China and its geodynamic background. Therefore, we plan to carry out fission-track (FT) and (U-Th)/He thermochronology analyses based on the detailed field work, so as to explore the exhumation mechanism and the exhumation processes in the Changbai Mountains and the Dahinggan Mountains since the Late Mesozoic. The cooling histories of the samples will be obtained using HeFTy software with apatite FT data, constrained by the zircon FT age, and zircon and apatite (U-Th)/He ages, while the exhumation histories of rocks are calculated based on the inferred paleotemperature gradient in the NE China. Consequently,the exhumation histories of the rocks constrain the exhumation processes and geodynamical setting of the Changbai Mountains and the Dahinggan Mountains. By doing these, we are aiming at better constraints on the relationships between the exhumation process of the Changbai Mountains and the Dahinggan Mountains and the sedimentation of the Songliao Basin since the Late Mesozoic.

长白山与大兴安岭是中国东北最重要的两条造山带，两者大致平行，呈北东-北北东向展布。区内侏罗纪和早白垩世花岗岩广泛出露地表，表明它们晚中生代以来经历了数公里的剥露作用。对长白山和大兴安岭进行系统的热年代学研究可以限定两者的剥露史，进而对比研究两者晚中生代以来剥露作用的相似性和差异性，可以加深对东北地区地质演化及其动力学背景的理解。故本项目拟在详细的野外地质考察的基础上，运用裂变径迹和(U-Th)／He 热年代学分析，结合已有资料，研究长白山和大兴安岭晚中生代以来剥露作用的机制和过程。利用磷灰石裂变径迹数据，借助HeFTy 软件模拟样品热历史，并用锆石裂变径迹年龄以及锆石与磷灰石（U-Th)／He 年龄作为约束条件，获得岩体的冷却曲线，进而依照推断的古地温梯度计算其剥露史，从而约束长白山和大兴安岭晚中生代以来的剥露过程及动力学背景。最后探讨两者晚中生代以来剥露作用与松辽盆地沉积作用之间的关系。

通过此项研究取得如下主要研究成果：.1. 详细的野外考察，查明长白山和大兴安岭晚中生代-新生代剥露作用的机制，主要是断块差异隆升，而在长白山的局部地区存在推覆抬升。.2. 长白山的AFT年龄在41.2±2.7~70.2±2.4Ma之间，平均径迹长度为12.0 ± 2.2 ~ 13.1 ± 1.4 μm； ZFT年龄在88.6±3.6~108.1±3.5Ma之间，AHe年龄在34.5±1.5~113.2±0.32Ma之间，ZHe年龄在79.8±1.6~143.3±0.99Ma之间，与大兴安岭的热年代学年龄大致一致。.3. 热史模拟所得长白山岩体的冷却和推断的剥露史也是幕式的。.4. 根据延吉地区西南部天佛指山的样品高程对应的磷灰石和锆石裂变径迹年龄，采用最小二乘法，获得的剥露速率分别为49 m/Ma 和 73 m/Ma。比以前假定稳定的 35ºC/km 地温梯度，根据热模拟结果计算获得晚白垩世 ~190m/Ma 的剥露速率要小。但延吉盆地早龙井期的地热流高，相邻的松辽盆地晚白垩世地温梯度也高于 35ºC/km，因此，天佛指山，乃至延吉地区晚白垩世的地温梯度应高于 35ºC/km。.5. 长白山剥露过程主要是太平洋板块俯冲动力学背景下的区块差异抬升及推覆抬升，而大兴安岭剥露过程是在太平洋板块俯冲和陆内造山共同作用背景下的区块差异抬升。