The Chemical Composition Of Clastic White Micas And The LA-ICPMS U-Pb Dating Of Zircon For The Mesozoic Sediments In The Eastern Jianghan Basin,China,and Its Significance For Their Source

Jianghan Basin is located between Qinling-Dabie and Jiangnan Orogenic belt. It is considered production of collision orogen stage during Mesozoic time. Voluminous sedimentary materials were transited into the ambient basins and deposited there during the process of continent-continent collision and the return of orogen. These sedimentary rocks provide an important insight into the sedimentary mechanism and information on the oerogen. The detrital white mica, as a stable and common composition in clastic rocks, can give some information on their source. Phengite, as one of the most characteristic minerals in HP-UHP metamorphic rocks which is generated commonly in collisional orogen, may reveal an important information on the process that the HP-UHP metamorphic rocks returned to the surface in the orogen. Furthermore, the detrital zircon is not only the important component of clastic sediment but distributes widely in various clastic rocks, whose U-Pb dating can provide an important information on the age of sedimentary rocks, accordingly, in combination with the natures of geo-tectonic and outcrop of the volcanism or the intrusion in the studied region, we may find out the information on the source of sediments deposited in different times. In this paper, we carried out the study on the compositions of white micas and the LA-ICPMS U-Pb dating of zircon from the Mesozoic clastic rocks in Eastern Jianghan Basin. The major conclusions are summarized as below:(1) The upper-Triassic white micas characterized by low Si (<3.2 Si atoms pfu ), indicates that the HP-UHP rocks in Dabie orogen improbably cropped out to the surface at that time.(2) Since the early Jurassic, the majority of phengites (Si>3.3 atoms pfu) of clastic whit micas started to occur in the sandstone. To the Middle Jurassic, both low-Si and high-Si white micas occurred abundantly in the clastic rocks. Such features suggested that the high-pressure and ultrahigh-pressure rocks of the Dabie orogenic belt were not exposed to the surface and carried down to deposit in the Jianghan basin until the early Jurassic.(3) The HP-UHP metamorphic rocks were the important materials contributed to the sediments in the Jianghan Basin and other ambient Basins during the early Jurassic.The majority of phengites occurring in the clastic sediments in the lower Yangtze regions and the Hefei Basin in the middle to late Jurassic, small amount in Jianghan Basin, imply that despite the HP-UHP metamorphic rocks were cropped out on the surface, the HP-UHP metamorphic rocks contributed mainly to sediments in the Hefei Basin in the north part of Dabie orogen and the lower Yangtze Basins in the eastern Dabie orogen, only small volume into the Jianghan Basin.(4) From the late Triassic to the middle upper Jurassic, the clastic rocks depositing in the Basin mainly derived from the North China Block. The relatively small amounts originned from the Yangtze Block. Though the information on the age of subducted collision or the return of HP-UHP metamorphic rocks to surface had been recorded in the late Triassic sediments, the HP-UHP metamorphic rocks had been voluminously cropped out on the surface and transmitted into the embient Basins to deposit during the middle late Jurassic.(5) The source of sedimentary materials deposited in the Jianghan basin had varied since the middle Jurassic. If the Yangtze Block and the North China Block had collided together to become a mountain during these times, some rocks from the basement of North China Block would occur in the Jianghan Basin. Otherwise, it could be the case that continent-continent collision occurred but did not become a mountain during those times.