南鄂尔多斯盆地上石炭统本溪组碎屑锆石物源示踪及对古地理重建的约束

Due to the remoulding effect of the strong tide on the sediments that formed by the pre-existing source-sink system, the conventional indicators of provenance of the Upper Carboniferous Benxi Formation, South Ordos Basin, were nearly destroyed. This may be why most of hypotheses on the provenance of Benxi Formation ignore the south source. The clues of south source (present orientation) and strong tidal effect were found in our pre-feasibility study, which indicate that the first straum of the Upper Paleozoic is very important for constraining the tectonic palaeogeography amang North China plate, South Qinling microplate and North Qinling microplate. This project intends to carry out observation of field outcrops and core intervals, and the related petrology, mineralogy, chronology, and geochemistry tests, focusing on the key scientific questions how to trace the source sediments of Benxi Formation disturbed by tides. Using the effective method of tracing complex source by detrital zircons, U-Pb geochronology, Lu-Hf isotopes, and trace element analysis of detrital zircon would be carried out to reveal the tectonic setting of the source area and the relationship between these plates, and define the onset and transition of the south Late Paleozoic source. Then we will try to discover more reliable and residual markers of paleo-current, analyze their symbiotic relationship with the tidal flow, and reveal two sedimentary parameters for tidal development of paleo-water-depth and paleotidal difference by the Moore content of Co element and the thickness of the middle tidal flat respectively. Finally, combining all the results, the source-sink system will be reconstructed, and the paleogeographic pattern will be rebuilt in the study area and its adjacent area.

由于强潮汐对先存源-汇系统沉积物的改造和再分配效应，南鄂尔多斯盆地上石炭统本溪组的常规物源分析指标几乎破坏殆尽，造成大多数研究忽略了南物源（现今方位）的存在。申请人前期研究发现了南源-汇系统的残存线索，表明该上古生界第一套地层记录了华北板块与秦岭微板块在晚石炭世关键期的构造古地理信息。本项目拟开展露头和岩心观测及相关的岩石学、年代学、地球化学测试，回答被潮汐扰动的本溪组物源这一关键科学问题。利用碎屑锆石单矿物这一示踪复杂物源的有效介质，开展锆石U-Pb年代学、Lu-Hf同位素和微量元素分析，揭示源区构造背景及板块构造关系，限定晚古生代南物源的起始时间及变迁。发掘更多残存的三角洲和潮汐环境标志及其共生关系，通过Co元素法和中潮坪厚度法，进一步论证古水深和古潮差这两项潮汐发育的重要条件参数。联合分析物源和沉积环境，恢复源-汇系统及其时空演变，为研究区及相关板块的构造演化模式及古地理重建提供约。

鄂尔多斯盆地上石炭统本溪组是覆盖在华北地块(NCB)中奥陶统之上的最早地层，有大约150Ma的沉积间断。通过采用地层、岩相学、锆石U-Pb年代和Hf同位素综合分析方法，探讨了其沉积充填成因及其与北兴蒙造山带和南秦岭造山带演化碰撞的关系。研究结果揭示本溪组不只是有与北兴蒙造山带相关的北部物源，还具有来自南缘秦岭造山带的物源。本溪组的两个层序组成正好对应于莫斯科期至格舍尔早期的长周冰期-间冰期旋回，响应了全球海平面和风化作用的系统变化。沉积等厚和沉积相的时空分布表明，盆地南缘和北缘均发育潮汐改造三角洲，古潮差大概在3.54-9.04米之间，古水深具有从西向东增加的趋势，平均24.77米。碎屑锆石年龄谱、微量元素谱和εHf(t)值表明，形成南部三角洲的硅碎屑来自秦岭造山带，而北部三角洲则来自兴安-蒙古造山带。上古生界本溪组的源汇演化记录了华北板块南、北造山带的逐步演化，可能是地壳横向增生边缘向华北地块垂向造山的转换期。特别地，北部物源区的性质为华北板块内部的边缘隆升造山带。