藏北乌兰乌拉湖地区第三纪陆相红层古地磁研究的初步结果及地质意义

Magnetostratigraphic Studies of Tertiary Continental Redbeds in Wulanwula Lake Area of Northern Tibetan Plateau and Their Geologic Significance

过对乌兰乌拉湖地区第三纪风火山群上部岩性段共计 15 6件样品的古地磁采样和室内退磁研究 ,揭示出一组高温特征剩磁分量 ,在 95 %置信度下通过正倒转检验和正褶皱检验 ,说明这组高温分量很可能代表岩石形成时的原生剩磁。采样剖面获得的磁性地层显示多个正、反极性带交替出现的图案 ,参考晚期侵入红层中浅成岩脉钾 氩法同位素年龄对陆相红层沉积顶界的限定 ,可对比国际标准地磁年代表的C2 3n至C2 0n极性带 ,沉积时限为 5 2～ 4 2Ma ,相当于始新世中期。根据野外调查及区域地层对比 ,本文提出风火山群是一个跨区分布的穿时性岩石地层单元 ,它的沉积充填历史与印度 亚洲大陆碰撞是同步发生的。

A total of 156 individual oriented samples were collected from Tertiary continental red beds in the upper lithologic unit of the Fenghuoshan Group within Wulanwula Lake area, northern Tibetan plateau. Rock magnetic measurement and demagnetization analysis were performed at the paleomagnetism laboratories of the University of California, Santa Cruz, USA. Most samples exhibited two components of magnetization in thermal demagnetization experiments. The most stable high-unblocking temperature components were isolated and passed positive reversal and fold tests at the 95% confidence level. The polarity patterns from the red bed sequence show that a reliable magnetostratigraphy can be established. Its normal polarity intervals are correlated with C23n-C20n of the geomagnetic polarity time scale (GPTS). The age of measured Fenghuoshan Group section is paleomagnetically dated as 52～42 Ma (Middle Eocene) and defined by K-Ar isotopic dating of discrete intrusion dikes. This suggests that the Fenghuoshan Group is a diachronic lithostratigraphic unit across paleo-suture zone and its deposition occurred synchronously with the Indian and Eurasian continental collision.