大兴安岭北段伸展隆升样式:来自科洛-嘎拉山韧性变形带的证据

The extensional uplift style of north part of the Da Hinggan Mountains: Evidences from ductile deformation zone of Keluo-Galashan

大兴安岭地区出露的系列古老变质变形岩石,为研究大兴安岭隆升机制和兴蒙造山带演化提供了一个独特的视角。大兴安岭北段科洛新开岭群和嘎拉山落马湖群内发育典型向SE的伸展滑脱构造,宏微观构造和石英EBSD组构均指示岩石变形发生在地壳浅部的低绿片岩相,是中低温(350～450℃)条件下缓慢变形的的结果。科洛地区变形代表一种拉伸近似等于压扁的应力状态,变形岩石多为L=S构造岩,较少发育b型线理,多片理化和不对称褶皱等;而嘎拉山地区的应变强度更大,拉伸作用更强,变形岩石以LS和L=S构造岩为主,发育大量叶理、a型及b型线理、鞘褶皱。运动学和流变学特征表明,越远离主伸展区应变强度越弱,应变趋于平面应变,差异应力减小,应变类型纯剪组分增多。结合新近锆石年代学数据,认为新开岭岩群和落马湖群的原岩多形成于早石炭世兴安地块和松嫩地块汇聚拼贴时期的花岗质岩石侵位,早白垩世伴随着古太平洋板块的持续NW斜向俯冲,大兴安岭东部长距离俯冲的古太平洋板片发生断离,造成地幔物质上涌,大规模强烈的岩浆侵位,大规模的强烈的韧性变形和伸展构造发育,并导致大兴安岭深部基底快速隆起以及两侧断陷盆地的发展。

Series 0f ancient metamorphosed and deformed rocks exposed in the Da Hinggan Mountains provide a unique perspective for understanding the uplift mechanism of the Da Hinggan Mountains and the evolution of the Xing＇an Mongolian orogenic belt. The typical SE-extensional and detachment structures developed within the Xinkailing Group in Keluo and the Luomahu Group in Galashan located in the northern segment of the Da Hinggan Mountains. Their micro-macro-structures and quartz EBSD fabrics all indicate that the deformation occurred in the low-greenschist facies with slow strain rates under low-middle temperatures（ ranging from 350℃ to450℃）. The deformation in Keluo area represents a constriction strain,approximately equal to the flattening,with smaller differential stress and more pure shear component. In this region,the deformed rocks are mostly foliated,featured by L-S tectonites with less atype lineations and more asymmetric folds. The strain intensity is greater in the Galashan area and the tensile effect is stronger in comparison,where deformed rocks are dominated by LS and L = S tectonites,and a large number of foliation,a-type and b-type lineations and sheath folds are developed. The newly published zircon geochronology studies show that the original rocks of most of the Xinkailing Group and Luomahu Group are formed during the convergence of the Xing ＇an and Songnen blocks in the Early Carboniferous. In the Early Cretaceous,it is accompanied by the continuous NW oblique subduction of the Paleo-Pacific plate,the long-distance subducted Paleo-Pacific slabs in the eastern part of the Da Hinggan Mountains break-off and caused mantle convection,resulting in upwelling of mantle materials and massive magmatic emplacement. This has resulted in the rapid uplift of the Da Hinggan Mountains,large-scale strong ductile deformation and extension structures,and the development of rift basins on both sides.