内蒙狼山地区中生代构造演化及年代学特征

TECTONIC EVOLUTION AND CHRONOLOGY CONSTRAINS OF LANGSHAN REGION IN INNER MONGOLIA AUTONOMOUS

通过U-Pb、Ar-Ar同位素定年,对狼山中生代不同时期构造运动演化进行年代学厘定,可分为3期:1)中三叠世:随着古亚洲洋的闭合,西伯利亚板块向南与华北板块挤压碰撞,在狼山地区形成了挤压造山和相伴随的大量中酸性岩浆侵入,同期或者后期,狼山地区发育了韧性剪切作用。2)中—晚侏罗世:狼山地区发育有NE向的走滑断层和韧性剪切带,与中国东部地区NE向构造格局形成具有同时性,这可能与鄂尔多斯地块中—晚侏罗世时的逆时针旋转和古太平洋板块向西俯冲的远程效应有关。3)晚侏罗—早白垩世:狼山地区广泛发育了一套由NW向SE的逆冲推覆构造,将结晶基底推覆于白垩纪地层之上。这可能受到蒙古—鄂霍茨克海闭合后,西伯利亚板块向南与华北板块碰撞影响,从而导致了狼山地区NW—SE向的地壳挤压缩短。

During Mesozoic time, the Langshan structural belt has experienced a complex intraplate deformation and sedimentation, mainly developed with NE-trending strike-slip motion and SE-vergent thrusting, where that the thrusting deformation transformed the earlier strike-slip faults. In the ductile shear zones, granitic intrusion was extensively suffered by NE-, NNE-trending cleavage deformation. During these deformation stages, muscovite and biotite developed as synkinematic minerals elongated parallel to the stretching lineations and deformational surfaces. U-Pb zircon TMS dating and muscovite, biotite 40Ar/39Ar dating yielded sequenced age constraints： 1） 246-235Ma, Middle Triassic： with the closure of Paleo-Asian Oceanic Sea, the Siberian plate was coming into collision with the North China Craton, and the Lang Shan was intruded with a series of intermediateacidity granite. 2）Middle to Late Jurassic： NE-trending strike-slip motion happened in the mid- upper in the Lang Shan area, the biotite＇s Ar-Ar isotope ages are yielded as at 173-178Ma. This NE-trending strike-slipping may be related to the far-field stress-release of the west Pacific plate subduction,which resulted in the Ordos block＇s counter-clockwise rotation at the same time. 3）The most prominent structure on the cross- sections is a moderate to low-angle, SE-vergent and NW-dip thrust faults that juxtaposes crystalline units against a variety of footwall strata. We dated biotite and got another group of the Ar-Ar isotope ages at 121-116Ma. The SE-vergent compression maybe represent the final closure of the Mongol-Okhotsk Ocean, and the southward moving of the Siberian Block. This deformation stage can be compared to the ESE-ward crustal thinning of eastern China during the Early Cretaceous.