内蒙古大石寨-霍林郭勒地区晚石炭世-二叠纪陆内伸展和陆内造山过程

Late Carboniferous-Permian intracontinental extension and orogeny in Dashizhai-Holin Gol area,Inner Mongolia

俯冲和挤压过程将形成具有加厚地壳的岛弧带或造山带,而伸展过程则形成具有减薄地壳的伸展盆地,因此可以通过地壳厚度推测岩石组合形成时的大地构造背景,并揭示它代表的深部地球动力学过程。兴蒙造山带东部大石寨地区以著名的大石寨组火山岩为特征,其岩浆活动的性质、形成过程和构造背景一直备受争议,其中,该套岩石的构造背景的认识存在岛弧和陆内裂谷两种主要观点。本文根据岩性组合及年代学特征,将大石寨地区主要岩石组合从下到上分为晚石炭世火山岩、早二叠世寿山沟组和大石寨组、中二叠世哲斯组,并利用大石寨-霍林郭勒地区的火山岩和碎屑岩锆石的微量元素及火山岩的全岩微量元素数据,估算了晚石炭世-二叠纪地壳厚度的变化趋势。结果表明,360Ma到320Ma时期发生地壳加厚,320~300Ma地壳从加厚转为减薄;而在300~280Ma时期,地壳厚度减薄最明显且厚度最小。综合岩浆活动、沉积环境和地壳厚度变化曲线等特征,可将大石寨-霍林郭勒地区晚石炭世到二叠纪的构造演化分为4个阶段:第一阶段(360~320Ma),碰撞产生的挤压背景导致区域性隆升和早-中古生代造山带物质的堆叠,使地壳厚度增大,导致幔源岩浆上侵,引起部分熔融作用,形成以侵入岩为特征的地壳垂向增生;第二阶段(320~300Ma),由于碰撞后伸展使得地壳处于从加厚到减薄的转换过程,发育与伸展相关的岩浆活动;第三阶段为300~280Ma,软流圈上涌造成地壳发生强烈伸展,导致地壳厚度明显减薄和大规模岩浆活动,以大石寨组岩浆活动进入高峰期为标志。该时期大规模岩浆活动和裂谷沉积特征与地壳厚度减薄的地球动力学背景高度吻合,从而揭示大石寨-霍林郭勒地区早-中二叠世处于地壳伸展而非俯冲-碰撞过程。第四阶段为280~260Ma,由于蒙古-鄂霍茨克造山带和大别-秦岭中央造山带的远距离效应造成地壳加厚,形成陆内造山带。

Subduction and extrusion processes will form island arcs or orogenic belts with a thickened crust,whereas extension processes will form extensional basins with a thinned crust,and thus,the crustal thickness can be used to predict the tectonic backgrounds during the formation of rock assemblages and reveal the related deep geodynamic processes.Dashizhai-Huolin Gol area in the eastern Xing an-Mongolia orogenic belt is characterized by the famous volcanic rocks of the Dashizhai Formation,whose magmatic activity,formation process and tectonic setting have always been controversial,among which two main views about its tectonic setting were proposed,i.e.,the island arc or the intracontinental rift.Based on study of lithologic assemblages and geochronology,the main rock units in the Dashizhai area are divided from bottom to top into the Late Carboniferous volcanic rocks,Early Permian Shoushangou Formation and Dashizhai Formation,and Middle Permian Zhesi Formation.Using trace element data of the zircons from volcanic and clastic rocks and whole-rock data of volcanic rocks in the Dashizhai-Huolin Gol area,the variation trends of crust thickness during the Late Carboniferous to Permian are estimated.The results indicate that the crust thickened from 360Ma to 320Ma,but changed to thinning from 320Ma to 300Ma,and then obviously accelerated its thinning speed and reached to the thinnest during 300Ma to 280Ma.Based on the characteristics of magmatic activity,sedimentary environment and crustal thickness curve,the tectonic evolution from Carboniferous to Permian in the Dashizhai-Holin Gol area can be divided into four stages:The first stage(360~320Ma),a regional uplift and stacking of Early-Middle Paleozoic orogenic belt happened because of the collisional,which resulted in an accumulation of crustal thickness and the mantle magma upward invading accompanied with partial melting and vertical crustal accretion as characterized by intrusive rocks.The second stage(320~300Ma),the crust changed from thickening to thinning due to post-collision extension as the magmatic activity related to extension.The third stage(300~280Ma),the upwelling of asthenosphere caused strong extension of the crust,resulting in significant thinning of the crust thickness and unusual magmatic activity,which was marked by the magmatic activity peak of the Dashizhai Formation.The large-scale magmatic activity and rift-related sedimentary characteristics of this period are highly consistent with the geodynamic background of crustal thinning,which reveals that there was a process of intracontinent crustal extension rather than subduction-collision during the Early to Middle Permian in the Dashizhai-Holin Gol area.The fourth stage(280~260Ma),the crust thickened due to the remote tectonic effect of the Mongolian-Okhotsk orogenic belt and the Dabie-Qinling central orogenic belt,and the intracontinental orogenic belt was formed.