中扬子区巴东组凝灰岩层的锆石U-Pb年龄约束  

Geochronological constraints on tuffs of the Badong Formation along the north margin of the Middle Yangtze region

作　　者：马千里 柴嵘[1,2] 杜远生 杨江海[1,2] 戴贤铎 MA Qianli;CHAI Rong;DU Yuansheng;YANG Jianghai;DAI Xianduo(State Key Laboratory of Biogeology and Environment Geology,China University of Geosciences,Wuhan,430074;School of Earth Sciences,China University of Geosciences,Wuhan,430074)

机构地区：[1]生物地质与环境地质国家重点实验室,中国地质大学,武汉430074 [2]中国地质大学(武汉)地球科学学院,武汉430074

出　　处：《地质学报》2019年第11期2785-2796,共12页Acta Geologica Sinica

基　　金：国家自然科学基金项目（编号41672106）资助的成果

摘　　要：印支运动深刻地改变了华南大陆和东亚的古地理格局,导致了大规模的海陆变迁。在扬子北缘发育有以紫红色碎屑岩为主的巴东组及相关地层,代表了从碳酸盐岩为主的海相沉积到碎屑岩为主的陆相沉积的转变。巴东组可划分为三段,即一、三段的紫红色粉砂岩、泥岩夹二段灰岩泥灰岩。由于巴东组三段的化石稀少,其年代归属一直存有争议。本文运用LA-ICP-MS定年方法,对新发现于鄂西秭归地区巴东组二段的凝灰岩夹层进行了锆石U-Pb定年。结果表明,来自于巴东组二段中部和顶部的两件凝灰岩样品分别形成于241.7±1.7Ma和237.5±2Ma。结合凝灰岩层的就位年龄和生物化石记录,巴东组地层时代被限定为中三叠世安尼期至晚三叠世卡尼期。凝灰岩中锆石颗粒多呈长柱状,无磨圆,发育明显的振荡环带,且具有与造山型岩浆锆石一致的微量元素化学特征。以巴东组灰岩为代表的"最高海相层"穿时地分布于鄂东南至川西一线,反映了上、中扬子区由中三叠世安尼期晚期至晚三叠世卡尼期海水大规模向西退去的海陆变迁特征。The Indosinian Movement resulting in the large-scale sea-continent transition deeply changed the paleogeograpical framework of the South China Contient and East Asia. The Badong Formation and related strata, which consist of carmine clastic rocks, are well developed in the north margin of the Yangtze block and respresent a transition process from marine carbonate-dominated marine sedimentation to clastic rock-dominated continental deposit. The Badong Formation can be divided into three members, with member 2 marlstone sandwhiched by member 1 siltstone and member 3 mudstone. Because there are few fossils found in the third member of the Baodong Formation, its stratigraphic age has been controversial. This study carried out LA-ICP-MS dating for zircons from the tuff in the 2 nd Member of the Baodong Formation in the Zigui area. The reults show that two tuff samples from the middle and top sections of the 2 nd Member in the Badong Formation formed at 241.7±1.7 Ma和237.5±2 Ma, respectively. The stratigraphic age of the Badong Formation, combined with emplacement age and fossil record of tuff layer, has been constrainted between the Middle Triassic Anisian and the Late Triassic Carnian. Zircons in the Badong tuffs occur in elongated crystals without rounding, show obvious oscillatory zones and the similar trace elements characteristic as orogenic magmatic zicons. The top marine stratum, which is represented by the Badong Formation limestone, is distributed mainly along southeastern Hubei to western Sichuan provinces, reflecting the sea-land changing of the Upper-Middle Yangtze regions due to westward regression of seawater during the period from Middle Triassic Ainsian Stage to Late Triassic Carnian Stage.

关 键 词：凝灰岩 巴东组 中扬子 印支运动 海陆变迁 

分 类 号：P61[天文地球—矿床学]