Zircon U-Pb dating and geological significance of the bentonites from the Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation in western Daba Mountains
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摘要: 扬子板块北缘大巴山地区上奥陶统-下志留统地层中斑脱岩较发育。笔者对大巴山西段陕西紫阳麻柳和四川万源皮窝乡上奥陶统五峰组-下志留统龙马溪组剖面的斑脱岩进行采样,开展了高精度锆石U-Pb测年,首次在该地区获得了445.1±3.5Ma和446.1±7.2Ma的锆石U-Pb年龄,限定了五峰组-龙马溪组地层沉积年龄,为扬子板块北缘大巴山地区奥陶系-志留系界线附近火山喷发事件、地层年代学研究提供了依据。本文所获得的年龄数据与秦岭-大别山造山带奥陶纪岩浆弧形成时间同步,略晚于华北克拉通西南缘奥陶系斑脱岩(449.0~465.8Ma),其火山活动可能与古秦岭洋壳向北的俯冲有关,火山凝灰质可能源自沿古秦岭洋盆北缘的火山弧喷发。中奥陶世晚期至早志留世早期,多幕次的高频火山喷发事件影响了当时海洋化学条件、碳循环波动、气候变冷和生物辐射脉动,造成了晚奥陶世末期的冰川启动和生物集群绝灭。Abstract: Abundant bentonites are well developed in the Late Ordovician-Early Silurian strata in western Daba Mountains, northern Yangtze block. The bentonite samples for this study are collected from the Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation sections in Maliu, Ziyang, Shaanxi and Piwo, Wanyuan, Sichuan in western Daba Mountains, and give the zircon U-Pb ages of 445.1 ±3.5 Ma and 446.1 ±7.2 Ma, respectively. These ages represent those of the deposition of the Wufeng and Longmaxi Formations, and provide data support for volcanic eruption events and stratigraphic chronology between the Ordovician and Silurian boundary in these areas. They are also synchronous with the formation time of the Ordovician magmatic arcs in the Qinling-Dabieshan orogen, and slightly later than the ages (449.0-465.8 Ma)of the Ordovician bentonites from the southwestern margin of the North China craton. The volcanic activity in this period may be associated with the northward subduction of the Palaeo-Qinling oceanic crust, and the volcanic tuffs may be originated from the volcanic arc eruption along the northern margin of the Palaeo-Qinling Ocean. The multi-episodic high-frequency volcanic eruption during the later Middle Ordovician to the earlier Early Silurian would influence in that time on many aspects such as the changes of oceanic chemical conditions, oscillation of carbon cycling, climatic colling and pulsation of biological radiation, and resulted in the initiation of glaciers and biological massive extinction at the end of the Late Ordovician.
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