冀北三叠系二马营组分布于下板城—平泉一带，主要为一套河流相红色砂泥岩组合，时代原定为中三叠世。通过1∶5万区域地质调查，首次在二马营组上部发现凝灰岩夹层，并进行了LA-ICP-MS锆石U-Pb定年，获得其206Pb/238U年龄加权平均值为234.2±2.6Ma(MSWD=3.2)，综合古生物化石、后期脉岩、地层接触关系等依据，将二马营组沉积时限重新厘定为中—晚三叠世。结合区域地质资料，认为冀北三叠系二马营组凝灰岩夹层的形成可能与西伯利亚板块与华北板块碰撞造山有关，该凝灰岩层作为有效的地质事件标志层，记录了当时构造背景下华北陆块北缘的一次火山活动。%The Ermaying Formation, previously assigned to Middle Triassic, is distributed in Xiabancheng-Pingquan area and mainly composed of fluvial red sandstone and mudstone rocks. During 1∶50000 regional survey, the authors discovered for the first time a layer of rhyo-tuffite in the upper part of the Ermaying Formation. Zircon LA-ICP-MS U-Pb dating of the tuff sample yielded an age of 234.2±2.6Ma (MSWD=3.2). In combination with fossils, intrusive rocks and lithology, the authors hold that a more appropri⁃ate sedimentary time of the Ermaying Formation in northern Hebei may be Middle to Late Triassic. Furthermore, the appearance of tuff interlayer, as the response to a volcanic events, might be related to the collision between the Siberia Block and the North China Block during Middle to Late Triassic.
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