滇东北昭通地区发现峨眉山玄武岩沉积夹层:峨眉山玄武岩幕式喷发新证据

    The discovery of sedimentary intercalations in Emeishan basalt in the Zhaotong area, Northeastern Yunnan: the new evidence of episodic eruption

    • 摘要: 首次在滇东北昭通地区峨眉山玄武岩中识别出3套沉积夹层,显示峨眉山玄武岩主喷发期内至少存在3次较长的喷发间歇期,以沉积夹层为亚旋回划分标志,将研究区峨眉山玄武岩划分为4个喷发亚旋回,沉积夹层的存在为峨眉山玄武岩幕式喷发提供了新的关键性证据。研究区峨眉山玄武岩第三亚旋回和第四亚旋回之间的沉积夹层沉积厚度大、沉积特征明显,本次工作选取其中的玄武质细砂岩进行了LA-ICP-MS测年,获得锆石U-Pb年龄为261.6 ± 0.6 Ma,代表了昭通地区峨眉山玄武岩主喷发期的时间,同时限定了沉积夹层的沉积时间下限。峨眉山玄武岩的喷发中心和火山机构展布可能受到区域上深大断裂的影响和控制,喷发形式为多期次多点位喷发,而沉积夹层发育于玄武岩多点喷发形成的小型山间凹陷汇水盆地。

       

      Abstract: Three sets of sedimentary intercalations were systematically identified in Emeishan basalt in the Zhaotong area, Northeastern Yunnan for the first time, indicating that there were at least three long eruption intermissions during the main eruption period of Emeishan basalt.Taking the sedimentary intercalations as the subcycle division mark, the Emeishan basalt in the study area could be divided into four eruption subcycles.The existence of sedimentary intercalation provides a new key evidence for the episodic eruption of Emeishan basalt.The sedimentary intercalations between the third and fourth subcycles of Emeishan basalt in the study area have large sedimentary thickness and obvious sedimentary characteristics.In this work, the basaltic fine sandstone was selected for LA-ICP-MS dating, and the zircon U-Pb age was 261.6 ± 0.6 Ma.This age represents the time of the main eruption period of Emeishan basalt in the Zhaotong area, and defines the lower limit of the sedimentation time of the sedimentary intercalation.The eruption center and volcanic mechanism distribution of Emeishan basalt may be influenced and controlled by the deep faults in the region.The eruption form is multi-stage and multi-point eruption, while the sedimentary intercalations are developed in a small intermountain depression catchment basin formed by multi-point eruption of basalt.

       

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