西藏珠穆朗玛峰北坡地区三叠纪磁性地层研究

Triassic magnetostratigraphy of the north slope of Mount Qomolangma, Tibet

本文对在西藏南部珠穆朗玛峰北坡土隆地区三叠纪地层剖面中采集的728件古地磁定向标本进行了较详细的研究。样品采集是在实测剖面之后进行的。野外样品采集使用国产GSD-1型手提式轻便取样钻机,钻取的岩心直径为24～25mm,长度一般为30～60mm,用磁罗盘定向器定向取心。采样间距一般控制在0.5～3m。在重要地层界线上、下部位进行加密采样,样品间距为0.1～0.2m。分析结果表明,珠穆朗玛峰北坡地区三叠系岩石的天然剩余磁化强度属中偏弱。样品的天然剩磁强度为1.9×10-2A/m～9.4×10-1A/m,特征剩磁强度为3.9×10-2A/m～5.0×10-1A/m。大多数样品在退磁测试到550～600℃时,样品剩余磁化强度已衰减了90%～100%,表明岩样中的磁性载体基本上为磁铁矿。研究表明,在土隆群中获得了12个正向极性和11个反向极性带,这在中生代早期是一个中等频率的正、反向混合极性巨带,命名为土隆混合极性巨带,与ILLAVARA(依拉瓦拉)混合极性巨带相吻合。曲龙共巴组和德日荣组中仅测出2个小的反向极性带,其余样品均为正向极性,笔者将该地层单元命名为土隆正向极性巨带,与GRAHAM正极性巨带相当。德日荣组上部为反向极性带。据此厘定了该区三叠系下统、中统和上统的界线。珠穆朗玛峰地区三叠纪磁性地层的深入研究,为该区三叠系多重地层划分与对比研究,建立珠穆玛朗玛峰地区三叠系古地磁样品数据库和三叠系磁性地层极性柱,为进一步深入研究冈瓦纳大陆的演化、古特提斯洋的形成、消亡历史和青藏高原隆升机制提供科学依据。

A total of 728 oriented paleomagnetic samples have been collected from Triassic sedimentary strata in the Tulong area on the north slope of Mount Qomolangma （Everest） in southern Tibet. All the sampling was carried out with a portable GDS-1 Type air-powered core drill after measuring the section. The cores were oriented with a magnetic compass and an inchnometer. Samples were commonly taken at 0.5-3 m intervals, and the intervals reached 0.1-0.2 m near some important geological boundaries. The cores taken were 24-25 mm in diameter and 30-60 mm in length. The results of analysis show that the natural remanent magnetization of the sedimentary rocks in the study area is medium, generally reaching 1.9×10^-2-9.4×10^-1 A/m and their characteristic remanent magnetization reaches 3.9×10^-2-5.0×10^-1 A/m. Thermal demagnetization study indicates that when samples are thermally demagnetized at 550-600℃2 the remanent magnetization of most samples is reduced by 90-100%, which shows that the carrier of this magnet in the rock is mainly magnetite. Magnetostratigraphic study indicates that there are 12 normal polarity zones and 11 reversed polarity zones in the Tulong Group, which form an Early Mesozoic mixed normal and reversed magnetic polarity superzone with an intermediate frequency, called the Tulong mixed polarity superzone polarity superzone. It coincides with the Illavara mixed polarity superzone. Only two small reversed polarity zones were found in the Kyunglung Goinba and Derirong formations, while the other samples all show normal polarity zones. In this paper this stratigraphic unit is named the Tulong normal polarity superzone, which is equivalent to the GRAHAM normal polarity superzone. The upper part of the Derirong Formation is a reversed polarity zone. Based on these studies, the boundaries of the Lower, Middle and Upper Triassic are determined. The intensive study of the Triassic magnetostratigraphy on the north slope of the Mount Qomolangma area has provided a scientific basis for the multiple stratigraphic division and correlation of the Triassic and establishment of a Triassic paleomagnetic database and Triassic magnetostratigraphic polarity column in the Mount Qomolangma area, as well as the in-depth study of the evolution of the supercontinent Gondwana, Formation and disappearance of the Paleo-Tethys and uplift mechanism of the Qinghai-Tibet Plateau.