晋东北盆地群侏罗纪沉积充填过程及其构造、古气候背景

The transition time of Jurassic and Cretaceous is an important orogenic activity period in the course of the geological history evolution on the central and eastern of China, tectonic-topography-magma-climate -environment, all these have undergone a series of mutation, but the original time of the orogenic activity and the mutual relationship between the various geological events are also controversial. In the study, as a guidance on modern sedimentology and continental dynamics theory, on the basis of basin analysis, we will choose the Jurassic basin group of Jurassic sedimentary sequence complete and structural deformation weak as the study area, and deduce the paleotectonic and the paleoclimatic background of Jurassic on the central and eastern of China by the changes of basin sedimentary sequence and clastic material composition, and built the mutual coupling relationship between sedimentary-tectonic-geomorphology-climate-environment. On the study methods, we will adopt the principle of combining the regional macro contrast with micro anatomy on important section, and establish the regional isochronous interface, and divide sedimentary cycle, and find the key horizon about sedimentary sequence conversion , formation color changes , detrital material composition changes , heavy mineral assemblages and content changes , the geochemical elements characteristic changes, the changes of the relative content and combination type about clay mineral. and analyze the correlation of various parameters on time series and the contrast on the region. On the basis of zircon U-Pb isotopic dating about volcanic tuff and clastic, we will determine the key time of the various parameters. By the study of basin sedimentary and regional tectonic deformation, the conclusion may provide the evidence for the original time of tectonic events and development process and interrelation of geological events on the trasition time of Jurassic and Cretaceous.

侏罗纪/白垩纪之交是中国中东部造山活动的重要时期，发生了构造、岩浆、成矿、地貌、气候等一系列地质事件，但对其启动时限及相互关系的认识一直存在着争议。本次研究以盆地分析为基础，选择侏罗纪沉积序列完整、构造变形微弱的晋东北盆地群作为研究区域，力图通过盆地内碎屑物质成分变化来演绎侏罗纪构造及古气候背景，建立起沉积-构造-地貌-气候-环境之间的耦合关系，确立侏罗纪隆升剥蚀过程及关键时限。区域对比与重点剖面研究相结合，建立三维等时地层年代格架。在层序单元的控制下，寻找沉积序列、地层颜色、碎屑物质成分、重矿物组合及含量、地球化学元素、粘土矿物组合类型及含量等相关参数变化的关键点及耦合性。利用火山碎屑岩、凝灰质夹层、火山岩及碎屑锆石同位素测年，确定其关键时限。盆地沉积与区域构造研究相结合，建立典型的汇-源系统，为中国中东部侏罗纪/白垩纪之交重大构造事件的启动时限、发展过程及其相互耦合关系提供重要的证据。

通过研究将华北克拉通北缘侏罗纪造山运动的过程划分为起始期（168 Ma±）、强烈活动期（160.6±0.55 Ma）及高峰期（159 Ma±）。各阶段划分的具体证据如下：（1）中侏罗统云岗组底部砾岩沉积时期是侏罗纪造山运动的起始阶段。该时期沉积序列由早期的湖进序列向晚期的湖退序列转换（河流相→湖泊、沼泽相→河流、三角洲相），预示着区域应力场由早期的拉张向晚期的挤压过渡，代表了源区地势抬升与造山作用的开始；中侏罗统云岗组底部砾岩可以用其下的大同组含煤地层做为等时界面，进行全区追踪对比，作为区域性的沉积间断，暗示着物源区存在强烈的构造事件；在碎屑岩QmFLt端元成分三角投影图中，下侏罗统永定庄组和中侏罗统大同组投影点落于基底抬升区域，而中侏罗统云岗组投影点都落于石英质再旋回造山带之中，预示着源区造山作用的开始；重矿物组合由早期以稳定的锆石为主过渡为相对稳定的石榴子石，主量元素中的CaO、MgO、CO2、Fe2O3+FeO含量以及Fe2O3/FeO比值发生了突变，稀土元素球粒陨石标准化曲线出现类似于玄武岩的异常变化，预示着源区隆升剥蚀作用的开始；通过下侏罗统永定庄组（J1y）火山碎屑岩的年龄（179.2±0.79 Ma）和中侏罗统云岗组顶部凝灰质碳酸盐岩年龄（160.6±0.55 Ma）的制约，推测中侏罗统云岗组底部砾岩沉积时间大约为168 Ma左右，这个时间代表了侏罗纪构造活动（燕山运动）的起始时限。（2）中侏罗统云岗组顶部凝灰质泥晶碳酸盐岩沉积之后，侏罗纪造山运动进入了强烈活动阶段。地层序列的颜色由早期的灰绿色突变为紫红色，预示着区域气候环境的突变；在碎屑岩QmFLt端元成分三角投影图中，出现了部分火山弧的投影点，说明源区构造活动加剧；重矿物组合以早期的石榴子石为主过渡为石榴子石+赤褐铁矿+少量的不稳定矿物（尖晶石、褐帘石、绿帘石、辉石、角闪石）的组合，碎屑锆石中出现大量的自形程度较好、同期的岩浆锆石，预示着源区进入了快速剥蚀、快速堆积、近源物质迅速注入的强烈活动阶段；根据中侏罗统顶部凝灰质碳酸盐岩年龄的制约，具体时限大约为160.6±0.55 Ma。（3）中侏罗统天池河组（J2t）沉积之后，侏罗纪造山运动进入了高峰期。华北克拉通整体上缺失了晚侏罗世沉积，仅在造山带前缘发育砾岩和火山岩沉积，根据沉积速率的推测，具体时限大约在159.0 Ma左右。