中国东部扬子板块同华北板块在中──晚三叠世拼接的沉积学证据

Sedimentologic Evidence on the Docking of Yangtse Plate and North China Plate in Middle-Late Triassic Period in East China

下扬子区中－晚三叠世黄马青群为湖相沉积，其边缘相带发育完整，而前黄马青期海相沉积的西北缘相带缺失，说明由于扬子板块同华北板块的拼接作用导致了黄马青期盆地的新生与前黄马青期盆地的构造侵蚀。黄马青群底部产出一系列扇砾岩，标志着在板块拼接早期，下扬子区地壳已开始受到构造变形。黄马青群砂岩中含有多硅白云母、变质岩及构造岩岩屑，砂岩中所含变质型重矿物组合与构成大别地块及张八岭隆起的宿松群及张八岭群同源。砂岩的地球化学特征还显示，其沉积盆地属于走滑盆地，盆地同具有很厚地壳的基底隆起带相邻。

The Middle-Late Triassic Huanmaqing Group in the Lower Yangtse area is characteristic of lacustrine factes deposits. The Huangmaqing sedimentary basin, extending in NE-direction and developing perfect northwestern peripheral factes deposits, are bounded on the northwest by the NNE-striking Tanlu fault zone, a great strike-slip fault zone in East China. However,there develops a marine basin extending in the same direction and in the same space. The evolution of the marine basin was lasting from Late Devonian to the end of Early-Middle Triassic.Significantly, there did not exist northwestern peripheral factes deposits of the marine basin.They used to be appear, but have already disappeared in this area just since Middle-Late Triassic. It is obvious that the occurrence of the Huanmaqing basin and the tectonical erosion of the pre-Huangmaqing basin are controlled by the activity of the Tanlu fault zone, a transform-type boundary following the docking of Yangtse Plate and North China Plate in Early-Middle Triassic. Therefore, the formation of Huangmaqing basin must be closely related to the docking event of these two plates not only in time but also in space.Sedimentary conglomerates occur at the base of the Huangmaqing Group, extending discontinuously along boundary faults of the basin, as a series of subaqueous deposit fans, and overlying always unconformably on underlying strata. This shows that the earth crust of this area began to suffer from significantly tectonical deformation with folding and faulting, and established a base for the formation of the Huangmaqing basin during the early stage of plate docking.Sandstones, as a main portion of the Huangmaqing Group are composed of a large number of elastics derived from metamorphic rocks and tectonites. Detrital minerals derived from a high pressure metamorphic zone, such as phengite, corindon etc. are also discovered in the sandstones. These facts mean that Dabie Massive and Zhangbalin Uplift composed of old metamorphic rocks and located respectively at both sides of the Tanlu fault zone are the main source areas of the Huanmaqing deposits. In fact, the characteristics of heavy mineral association contained in the sandstones are coincident with that of the rocks composing the Susong formation in Dabie Massive and the Zhangbalin formation in Zhangbalin Uplift.Research on geochemistry of rare earth elements, rare elements and main elements in the sandstones shows that the Huangmaqing basin genetically belongs to a strike-slip basin related to an uplifted basement with thick earth curst. The discrimination result is completely coincident with the geological setting of the lower Yangtse area and the above-mentioned geological facts.