云南兰坪-思茅盆地含钾盐及其下伏地层磁性地层和碎屑锆石U-Pb年代学研究

The Lanping-Simao and Sakhon Nakhon basins are the ancient marine potash-bearing basins located in the east part of the Tethys tectonic domain. However, the potash resources between the basins differ greatly, and the age, relationship and mechanism of the potash formation remain controversial. Reconstructing the paleogeography of the Lanping-Simao and Sakhon Nakhon basins is of great significance to reveal their relationships during the deposition of the potash-bearing strata and the constraints on the potash formation. The applicant conducted detailed magnetostratigraphic and paleomagnetic analyses in the earlier stage, which preliminarily indicated that the potash-bearing strata of the two basins might have deposited in a unified basin. However, the magnetostratigraphic age of the potash-bearing strata Mengyejing Formation was not eventually tamped, due to some problems, like the absence of other precise or macro absolute chronological constraints from the Jiangcheng section in the Lanping-Simao Basin. Besides, the sedimentological researches on the basin relationship is relatively insufficient. The detrital zircon U-Pb geochronology has a unique advantage in constraining the lower limit of the depositional age of the sediments and reestablishing the paleogeographic environment. Therefore, this project plans to carry out further magnetostratigraphic and detrital zircon U-Pb geochronologic studies on the Mengyejing Formation and its underlying strata in the Jiangcheng section, hoping to establish the accurate depositional ages and obtain the detrital zircon age spectra of these sediments and then make comparative analyses with those from the Sakhon Nakhon Basin. Finally this project aims to restore the Cretaceous paleogeographic environment between the two basins and the surrounding blocks based on the integrated study of the multi-geological evidence.

兰坪-思茅盆地与沙空那空盆地是位于特提斯域东段的海相含钾盐盆地，但二者资源量相差悬殊，其形成时代和关系及成因争议很大。重建这两个盆地含钾盐地层沉积时期的古地理，对于揭示二者的关系及其对成盐成钾的约束等具有重要的理论和现实意义。申请人前期通过开展磁性地层和古地磁学分析，从大地构造角度初步认为两个盆地的含钾盐地层沉积于统一的泛盆地。但由于兰坪-思茅盆地江城剖面含钾盐地层勐野井组缺乏其它直接的年代学控制等问题，并未夯实该地层年代。此外对盆地关系的沉积学研究不足。碎屑锆石U-Pb年代学对于提供地层下限沉积年龄和重建古地理环境具有独特作用。因此，本项目拟针对江城剖面勐野井组及其下伏地层开展进一步的磁性地层和碎屑锆石U-Pb年代学研究，以期获得这些地层的准确沉积年代和碎屑锆石年龄谱，并与沙空那空盆地的研究结果进行对比分析，最后通过多地质证据综合集成研究重建白垩纪两个盆地及其与周边主要陆块的古地理环境。

思茅盆地与呵叻盆地是位于特提斯构造域东段的古代海相含钾盐盆地。建立两个盆地含钾盐地层准确的年代学，重建盆地古地理环境，对于揭示盆地关系及其对成盐成钾的制约和认识思茅盆地勐野井钾盐矿成矿规律具有重要的科学意义。本项目针对思茅盆地江城剖面勐野井组及其下伏地层开展了磁性地层与碎屑锆石U-Pb年代学相结合的研究，并在多地质证据综合集成分析基础上，重建了白垩纪两个盆地及其与周边主要陆块的古地理环境。获得以下主要研究结果：（1）思茅盆地江城剖面勐野井组、曼岗组和景星组的沉积年代分别为>112 Ma-~63 Ma（早白垩世晚期至晚白垩世）、~121 Ma-~112 Ma（早白垩世早阿普特期-早阿尔布期）、~142.5 Ma-~121 Ma（早白垩世中贝里阿斯期-早阿普特期）。（2）对比分析显示，思茅盆地与邻区呵叻盆地含钾盐地层年代仅部分重叠，前者至少要比后者早开始沉积20Ma，呵叻盆地含钾地层相当于思茅盆地的中上部。（3）老挝万象盆地班蓬坎剖面和班农本剖面的年代为>92 Ma-123 Ma（位于白垩纪超静磁带C34n内部），通过衔接他曲钻孔ZK2893的研究结果，建立了呵叻盆地早白垩世晚期-晚白垩世（123 Ma->63.5 Ma）的磁性地层年代序列。（4）基于构造、物源、古气候及年代学等地质证据的综合集成分析表明，思茅盆地与呵叻盆地在白垩纪可能属于一个统一的泛盆地，推测~85 Ma呵叻盆地大型钾盐矿沉积时段也可能是思茅盆地另一个重要潜在成钾时段。（5）两个盆地碎屑锆石年龄谱序列可以划分为三个演化阶段，即早白垩世早期-阿普特期、阿普特期-晚白垩世早期和晚白垩世晚期，结合剖面岩性和沉积环境变化，可能指示了二者在白垩纪期间的物源演化。