兴蒙造山带岩石圈尺度地电结构与岩浆岩深部信息追踪

The research focuses on the exploration of the geoelectrical structure of the Xing’an Mongolian Orogenic Belt which belongs to Northeast Asia Tectonic Domain on the lithospheric scale and the deep information trace of the magmatite from the southern margin of the Xing’an Mongolian Orogenic Belt, the Da Higgan Mountains and the Xiao Hinggan Mountains.To build the geoelectrical structure of the Xing’an Mongolian Orogenic Belt on the lithospheric scale,the research will take full advantage of the broadband and the long period magnetotelluric data of three profiles,two of which go across the northern margin of the North China Plate,the east part of the Xing’an Mongolian Orogenic Belt while the other one goes across Eergu'Na Xingan Block,the Songliao Basin and Jiamusi Xingkai Massif.Then four important scientific issues will be discussed:1)The existence of signs of the Pan African metamorphic magmatic event in Northeast China and geoelectrical evidences.2)The suture position of the Xing’an Mongolian Orogenic Belt and the North China Plate based on the geoelectrical evidences and the spatial structure and distribution of granite of the southern margin of the Xing’an Mongolian Orogenic Belt and the deep information trace.3) The spatial structure and distribution of granite of the Da Higgan Mountains and the Xiao Hinggan Mountains in Northeast China and the deep information trace.4) Geophysical field description on the lithospheric scale with comprehensive geophysical data while considering the geological evolution of Northeast Asia Tectonic Domain,paying close attention to geoelectrical evidences of the impact ,which comes from the Paleoasian Ocean and the Mongolia Okhotsk ocean closure before the Stage of Yanshan and the subduction of the Pacific Plate after Yanshan period, on the tectonic pattern of Northeast Asia.The research has great significances in the perspective of understanding the connection between the distribution of the shallow magmatite and the structure of the deep and deepening the regional plate kinematics in Northeast Asia.

本项目以东北亚构造域兴蒙造山带岩石圈地电结构和兴蒙造山带南缘、大兴安岭、小兴安岭岩浆岩深部展布为目标，利用2条南北向贯穿华北板块北缘、兴蒙造山带东段及贯穿额尔古纳-兴安地块、松辽盆地、佳木斯-兴凯地块东西向宽频和长周期大地电磁测深资料，建立东北亚构造域地电模型，主要研究四个主要问题：1）中国东北的泛非期变质-岩浆事件的存在标志及地电学证据。2）兴蒙造山带与华北板块碰撞缝合线位置地电学证据及兴蒙造山带南缘花岗岩空间分布结构及深部信息追踪。3）中国东北大兴安岭、小兴安岭岩浆岩空间分布及深部信息追踪。4）综合地球物理资料，围绕东北亚构造域的地质演化，关注燕山期前古亚洲洋及蒙古-鄂霍茨克洋闭合、燕山期后太平洋板块俯冲对东北亚深部构造格局的影响的地电证据，进行岩石圈尺度的地球物理场描述。本项目研究对于认识岩浆岩表层分布与深部结构的联系及深化东北亚区域活动板块理论具有重要的意义。

兴蒙造山带经历了古亚洲洋、蒙古-鄂霍茨克洋和古太平洋构造体制的多重作用，形成了中国东北地区典型的盆岭构造体系，其岩石圈与软流圈系统的改造及耦合过程研究意义重大，也在根本上制约了华北北缘、大兴安岭、小兴安岭等岩浆岩带的形成。项目以大地电磁测深为主，结合深反射地震、区域重磁场研究，揭示了兴蒙造山带南北向及东西向结构差异、岩浆岩深部信息，获得了如下认识及成果：1、建立了大兴安岭以西盆地群、大兴安岭造山带、松辽盆地岩石圈尺度电性结构模型；提出松辽盆地经历了的岩石圈减薄事件，与大兴安岭岩石圈厚度相比，松辽盆地岩石圈厚度减薄了近100km，与东侧张广才岭相比减薄了70km，而与中生代华北地台100km的岩石圈厚度相比，减薄了近50km，其经历了岩石圈伸展期、裂解期、拆沉期和增长期的动力学过程；2、厘定了兴蒙造山带与华北板块缝合线位置地电学证据及岩浆岩深部信息追踪，碰撞带可分为二连-贺根山和索伦-西拉木伦河两个不同时期的汇聚体系，晚泥盆世-晚石炭世早期形成的二连-贺根山汇聚体系由二连-贺根山增生杂岩带、宝力岛弧地体及断裂带组成，深部缝合边界位于二连浩特。而晚二叠-早三叠的索伦-西拉木伦河汇聚体系由二道井子增生杂岩带和温都尔庙增生杂岩带及断裂带组成，深部缝合边界位于苏尼特右旗。3、中亚造山带东段岩石圈内部存在多处低阻异常，地表多对应第四纪火山群或古缝合带，表明这些低阻异常与软流圈上侵有关。软流圈呈现高导特征，南部略薄，电阻率值为10~30Ω•m，北部厚度较大，电阻率值为10~0.1Ω•m，这种电性结构特征体现了中亚造山带东段软流圈南北两侧厚度、部分熔融程度的不均一性。而已有的层析成像结果显示中亚造山带软流圈东西向则呈现东厚西薄的特征。结合区域地质，提出软流圈与岩石圈一样在碰撞造山过程中厚度、部分熔融程度也会随之变化的动力学认识。