南盘江盆地中部西林断隆南翼中-下三叠统沉积学特征及其大地构造意义

Sedimentary characteristics of the Early-Middle Triassic on the south flank of the the Xilin faulted block in the Nanpanjiang basin and its tectonic implications

印支造山带对华南地质演化具有重要影响,南盘江盆地作为华南与印支造山带相关的最大的盆地,其构造演化过程备受关注,然而对于盆地由伸展到挤压的构造转换时限还存在争议。西林县位于南盘江盆地的中部,古生界和中-下三叠统出露良好,是进行沉积大地构造学研究的理想地区。为进一步明确南盘江盆地的构造演化过程,本文选取南盘江盆地中部西林县一带中-下三叠统进行了详细的沉积相分布和演化、古水流以及碎屑组成分析。结果表明,空间上,下三叠统自下而上自北向南依次出露:(1)泥岩、泥灰岩、砂屑灰岩和凝灰岩构成的潮坪-泻湖相沉积;(2)砾岩、含砾砂岩、细砂岩和粉砂岩构成的块体搬运沉积(MTD,Mass-transport deposit)与浊积扇相沉积互层;(3)中粗砂岩、粉砂岩、泥岩组成的浊积扇相沉积,构成向上水体急剧加深的沉积序列。中三叠统为一套浊积扇相沉积,由粗砂岩、细砂岩、粉砂岩和泥岩组成。古水流和碎屑组成分析结果显示,西林县一带下三叠统碎屑物主要来自北侧西林-隆林一带的碳酸盐岩台地,而中三叠统碎屑除来自北侧相邻碳酸盐岩台地外,还可能来自江南造山带、康滇古陆、云开地块和峨眉山玄武岩。沉积相的垂向变化记录了西林一带由古生代孤立碳酸盐岩台地向三叠纪半深海浊积岩盆地的演化过程,而早三叠世MTD是盆地张开过程中的直接沉积记录,以上地质事实暗示了南盘江盆地在早三叠世受控于区域伸展体制,西林一带的古生代孤立碳酸盐岩台地则是伸展过程中形成的断隆。盆地内的基性岩墙侵位(258~248Ma)以及西林一带早三叠世晶屑沉凝灰岩的锆石U-Pb年代学分析结果(249.4±1.2Ma)表明南盘江盆地在晚二叠世至早三叠世处于伸展构造背景之下,中三叠统为半深海浊积岩沉积则表明这一伸展过程至少持续至中三叠世。由于在时间上和空间上,南盘江盆地的张开都与古特提斯分支洋盆的俯冲消减有关,因而我们认为南盘江盆地也是晚二叠世到中三叠世古特提斯洋岩石圈俯冲体系的一部分。由于南盘江盆地及其邻近区缺失上三叠统沉积,这可能说明古特提斯洋俯冲结束于中三叠世。

The Indosinian orogeny plays an important role in the tectonic evolution of the South China block（ SCB）. As the biggest basin associated with the Indosinian orogeny on the southern margin of the SCB,the Nanpanjiang basin draws great attention in the geological community. Although many advances have been achieved,the time of the tectonic regime transition from extension to compression of the Nanpanjiang basin remains in dispute. In the central Nanpanjiang basin,the Paleozoic and Triassic strata are well outcropped around the Xilin faulted block,which provides an ideal studying object on revealing the basin evolution. In order to reveal the tectonic evolution of the Nanpanjiang basin,we have conducted a detailed sedimentary faces analysis,paleo-current measurement and frame-work clast statistics in the south flank of the Xilin faulted block. Our detailed field investigations show that the Lower Triassic on the south flank of the Xilin faulted block is composed of three subsequences which outcrop one by one in a southward younger order：（ 1） Tidal flat and lagoon deposits consist of mudstone,marlstone and calcarenite with tuff layers;（ 2） Mass-transport deposits（ MTD）alternated with turbidite fan deposits include conglomerate,pebbly sandstone,fine sandstone and siltstone; and（ 3） Turbidite fans deposits include medium-coarse sandstone,siltstone and mudstone. The Middle Triassic consists of a sequence of coarse sandstone,fine sandstone,siltstone and mudstone which is interpreted to be a turbidite deposition. The facies＇ evolution indicates a deepening sediment environment during the Early to Middle Triassic. The paleocurrent and detrital composition analysis indicate that the detritus of the Lower Triassic in the Xilin area were mainly sourced from the Xilin fault block,while the Middle Triassic deposits in the basin were from composed provenances including the Jiangnan orogenic belt,the Kangdian massif,the Yunkai massif,the Emeishan basalt and the faulted blocks. The sediment facies＇ and provenance evolutions varying with time documented a sedimentary environmental evolution of the Nanpanjiang basin from the isolated carbonate platform to the half-deep sea turbidite basin. The Early Triassic MTD is the direct sedimentary records of the basin opening. The geological facts acquired in this study suggest that the Nanpanjiang basin was controlled by a regional extension regime in the Early Triassic,and the isolated carbonate platform with the Paleozoic strata is faulted blocks formed during the basin opening. In accord with this study,the mafic dikes（ ca. 258 - 248 Ma） inside the basin intruding intounderlying strata of the Lower Triassic and tuff interlayers（ ca. 249. 4 ± 1. 2 Ma） in the Lower Triassic in the Xilin area also suggest an extension regime coeval with the basin opening during the Late Permian and the Early Triassic. The Middle Triassic turbidite fans are dominant deposits in the Nanpanjiang basin,which indicates the basin extension continued to the Middle Triassic. The opening of the Nanpanjiang basin is highly coincident with the closure of a branch of the Paleotethys Ocean in temporal and spatial,indicating that the basin was the part of subduction system of the Paleoethys oceanic lithosphere during the Late Permian to Middle Triassic. The absence of the Upper Triassic in the Napanjiang basin and adjacent areas indicates that the basin extension or subduction of the Paleotethys oceanic lithosphere terminated after the Middle Triassic.