鄂尔多斯盆地南部凝灰岩地质特征及其对构造体制转换的制约

Geogical characteristics of tuffs and its constraints on the tectonic regimes transformation in the southern Ordos Basin

凝灰岩是火山活动的产物,沉积盆地中的凝灰岩层往往具等时性特点,是探索沉积盆地与周邻造山带构造演化的重要纽带。鄂尔多斯盆地显生宙火山活动不发育,但早古生代晚期和晚三叠世延长期地层中广泛分布凝灰岩夹层,且主要分布在盆地南部,暗示其与周邻造山带火山活动之间联系密切。前人对这2期凝灰岩分别开展了探讨,但对为何分布于盆地南部、具有怎样的成因联系、构造意义等缺乏综合研究与探讨。基于前人成果,对上述2期凝灰岩的展布特征、源岩判别、发育时限、区域构造背景等进行了系统研究。结果显示,鄂尔多斯盆地2期凝灰岩均发育于深水—较深水环境,且由南到北厚度逐渐减薄;元素地球化学判别显示出中—酸性岩浆性质,与火山岛弧喷发成因联系密切,主要发育时限分别集中于晚奥陶世桑比期—凯迪期(449~453 Ma)和中晚三叠世拉丁期—卡尼期(227~242 Ma),在时空上与鄂尔多斯盆地演化转折关键时期联系密切。结合区域构造分析认为,上述2期凝灰岩是秦岭造山带加里东期商丹洋盆和印支期勉略洋盆俯冲闭合过程中火山活动事件在相邻盆地的响应和记录,秦岭造山带构造演化对鄂尔多斯盆地南部早古生代海相盆地消亡和中生代内陆大型湖盆发育具有重要的控制作用。上述认识对深入探讨鄂尔多斯盆地南部和秦岭造山带显生宙期间长时间尺度盆山演化具有重要的指导意义。

Tuff is the product of volcanic activity.Tuff layers in sedimentary basins are often isochronous,which is an important link for exploring the tectonic evolution of sedimentary basins and adjacent orogenic belts.The Phanerozoic magmatism in the Ordos Basin is very limited.However,in the late Early Paleozoic and Late Triassic Yanchang periods,there are many layers of tuff interlayers widely distributed in the southern basin.It indicates a close relationship between magmatic activities in adjacent orogenic belts.Predecessors have conducted a certain discussion on each of the two periods of tuff,but there is a lack of overall comprehensive research and analysis on why tuff is distributed in the southern part of the basin,and what genetic connections and tectonic significance it has.Based on previous research results,this paper systematically analyzes the distribution characteristics,source rock discrimination,development time and regional tectonic background of the above-mentioned two stage tuffs.The results show that the tuffs in the Ordos Basin are developed in deep water environments,gradually thinning from south to north.At the same time,geochemical discrimination reveals that they are all intermediate and acidic magmas,which is closely related to the origin of volcanic island arc eruption.The development time are respectively concentrated in the Late Ordovician Sandbian-Katian period(449~453 Ma)and the Late Triassic Ladinian-Carnian period(227~242 Ma),which are closely related to the key period of the Ordos Basin evolution in time and space.Combined with regional tectonics analysis,it is believed that the above-mentioned two stages of tuff development is the responses and records of volcanic events in adjacent basins during the subduction and closure of Caledonian Shangdan Ocean and Indosinian Mianlue Ocean in the Qinling Orogenic Belt.It shows that the tectonic evolution of the Qinling Orogenic Belt has an obvious control on the early Paleozoic marine basin disappearance and the development of the Mesozoic large intracontinental lake basin in the southern Ordos Basin.The above understanding has certain guiding significance for in-depth exploration of the long-term basin-mountain evolution and petroleum exploration during the Phanerozoic in the southern Ordos Basin and the Qinling Orogenic Belt.