鄂尔多斯盆地南缘奥陶纪沉积充填记录的关键时限及其构造意义

Critical timing of sedimentary filling records of Ordovician and its tectonic significance on the southern Ordos Basin

鄂尔多斯盆地南缘上、下古生界呈现明显的角度不整合，标志着该区卷入了加里东造山带变形。本文以盆地内奥陶纪沉积充填记录为线索，利用地层序列中沉积凝灰岩的锆石U-Pb同位素测年，结合豢岭造山带岩体年代学研究成果，探讨了豢岭加里东期构造事件的发生与发展过程。研究表明：1）奥陶系沉积时期，沉积序列经历了海侵至海退的完整旋回，中奥陶世马五期海退序列开始，晚奥陶世背锅山期海水自鄂尔多斯盆地西南缘完全退出；2）晚奥陶世平凉期至背锅山期，地层序列中凝灰岩、滑塌构造、滑塌角砾岩普遍发育，滑塌构造和滑塌角砾岩的直接触发因素是构造活动引发的地震，构造活动性明显加强；3）中奥陶世马五期海退序列的开始，孕育着秦岭洋壳板块开始向北俯冲，时限大约为475-463Ma；4）晚奥陶世平凉期，沉积序列中重力流、滑塌构造和凝灰岩普遍发育，孕育着秦岭洋向北的俯冲碰撞进入了高峰阶段，其时限大约为454-450Ma。

The formation between Upper and Lower Paleozoic shows obvious angular unconformity on the southern Ordos Basin, and constitutes the Caledonian tectonic zone with orogeny in the basin marginal belt. In the paper, depending on sedimentary filling records of Ordovician, using zircon U-Pb dating of tufts, combing with research results of volcanic chronology in the Qingling orogen, we discuss the initiation and development about the tectonic event. The conclusions include the following aspects ： 1） During the Ordovician sedimentation, sedimentary sequences record one complete cycle from transgression to regression, the regression sequences began on the fifth section of the Majiagou Formation of Middle Ordovician, seawater quit completely Ordos Basin on the Beiguoshan stage of Late Ordovician ; 2） From Pingliang term to Beiguoshan stages of Late Ordovician, the tufts and slump structures and limestone breccia developed commonly in the stratigraphic sequence, slump structures and limestone breccia resulted from ancient seismic slumping caused by tectonic movement; 3 ）The regression sequences begin on the fifth section of Majiagou Formation of Middle Ordovician, indicating the initiation age of Early Paleozoic island arcs in northern Qinling, the time is about 475 463 Ma; 4）On the Pingliang stage of Late Ordovician, the tufts, slump structures and limestone breccia developed well in the stratigraphic sequence, indicating the intensive activity age of Early Paleozoic island arcs in northern Qinling, the time is about 454～450 Ma.