【作者】 吴素娟；

【导师】 胡健民；

【作者基本信息】 中国地质科学院 ， 构造地质学， 2014， 博士

【摘要】 巴彦乌拉山-狼山断裂带是阿拉善地块东北缘北东走向的一条大型断裂带,延伸长度约为500公里。长期的研究表明这条断裂带经历了长期、复杂的的演化历史,也是记录阿拉善地块东北缘和华北克拉通地球动力学演变的重要地点。该断裂带的前寒武纪变质基底广泛出露,古生代之后的构造保存较好。然而,关于该断裂带的早期的变质变形历史及起源,特别是它与阿拉善地块和华北克拉通之间的关系,虽然在国际地学界得到了广泛的关注,但是在认识上却一直存在着很大的分歧与争论。阿拉善东缘前寒武纪变质基底是研究阿拉善与华北克拉通之间关系的关键点。本文对采自巴彦乌拉山变质杂岩中的角闪斜长片麻岩、黑云斜长片麻岩、花岗质片麻岩进行了变质作用研究。野外产状和变质矿物组合显示,巴彦乌拉山变质杂岩组至少经历了两期退变质事件,即以斜长石+角闪石为代表的角闪岩相,和以绿泥石+绿帘石取代角闪石为代表的绿片岩相。根据角闪石-斜长石地质温度计估算,巴彦乌拉山地区角闪斜长片麻岩退变质作用温压环境为温度为733～755℃,压力为0.26～0.48GPa。运用LA-ICP-MS锆石U-Pb定年方法对巴彦乌拉山变质杂岩中主要岩石类型进行了定年,结果表明其原岩结晶年龄为2.30～2.24Ga,时代为古元古代。两期变质作用的时代分别为1.94～1.91Ga和1.88～1.86Ga。本文对划分阿拉善地块和华北克拉通的区域边界巴彦乌拉山-狼山大型断裂带进行了详细的研究,确定了剪切带组成、几何学特征、运动学特征及显微构造特征,同时运用不同方法的同位素年代学方法确定了剪切变形时代。研究表明,巴彦乌拉山-狼山断裂带存在有两期韧性剪切变形叠加。早期韧性剪切糜棱面理走向集中在NE-NNE向,面理倾角较陡,矿物拉伸线理近水平。另一期的韧性剪切糜棱面理的优势走向为NE向,部分可见近EW向,面理倾角较陡,拉伸线理NE-SW向或NEE-SWW向,部分可见EW向,倾角较小。S-C组构、残斑拖尾、云母鱼构造等和石英组构指示这些糜棱岩分别形成于断裂带的左旋和右旋两期剪切过程中。通过糜棱岩中新生矿物组合、石英的变形特征估算分别获得了巴彦乌拉山-狼山断裂带的变形温度约为500-600℃。石英具有3种滑移机制即：底面滑移、菱面滑移和柱面滑移,其中以柱面<a>滑移的变形机制占有明显的优势。岩石的变形机制主要是位错滑移系蠕变和快速重结晶-恢复。在狼山西南段糜棱岩中选取了白云母进行40Ar/39Ar年代学研究,获得了356.7±2.5Ma和379±4.7Ma的白云母年龄值,样品的形成温度高于白云母的封闭温度,记录了巴彦乌拉山-狼山剪切带较早一期剪切活动的冷却时间。在巴彦乌拉山中段和狼山西南段的同构造花岗岩中获得了330.8±2.8Ma的岩浆锆石U-Pb年龄,在后期侵入的花岗岩脉体中分别获得了269.8±1.8Ma和280.5±1.5Ma的年龄。分析认为巴彦乌拉山-狼山在-350Ma前后经历了一次构造运动,且在270-280Ma前后,该地区经历了一次岩浆热事件。更多还原

【Abstract】 The Bayanwulashan-Langshan fault zone is a major fault zone trending NE in the northeastern margin of the Alxa Block and extends about500km. Long-term studies suggest that the large fault zone experienced complicated evolution history and recorded geodynamic processes of the Alxa Block and the North China Craton(NCC). The Precambrian metamorphic basement is widely exposed in the Alxa Block, and the structures from late Paleozoic are conserved well. However, though lots of international geologists paid more attention to the strike-slip history and origin of the Bayanwulashan-Langshan area, especially the relation between it and the NCC, but their viewpoints about that were different. Some workers considered the fault zone as a eastern part of the Alxa arc tectonic belt and was formed by the compression from the Central Asian Orogenic Belt. Some workers suggest that the fault zone is the northeast boundary fault and originated as a result of the collision between Alxa Block and the NCC in Devonian.This paper show detailed studies on the physical components, structural deformation characteristics, geometry characteristics, kinematics characteristics and different isotope geochronology research of the Bayanwulashan-Langshan fault zone, the predominate boundary of diving Alxa Block and the NCC.This paper show metamorphism research on Bayanwulshan Metamorphic Complex. Multiple stage metamorphisms have been identified within the metamorphic basement rocks based on the mineral assemblage of plagioclase and amphibole, the chemical variations of amphiboles, and the replacement of amphibole by chlorite and epidote. We speculate that the metamorphic basement rocks underwent retrograde metamorphic episodes from amphibolites facies to greenschist facies and overprinted the preexisting petrogenesis information. Based on newly-formed mineral assembles and deformation behaviors of quartz in mylonites, these works show that deformation temperatures of Bayanwulashan-Langshan faulting ale separately500-600℃. Thermodynamic computations show that the metamorphic conditions of the amphibole plagioclase gneisses are719-755℃at0.26-0.48Gpa. The LA-ICP-MS zircon U-Pb ages reveal that the primary magmatic activities of the metamorphic basement rocks were at ca.2.30-2.24Ga and the two metamorphic events were at ca.1.95-1.91Ga and ca.l.88-1.85Ga respectively.According to lots of field works-two phases of strike-slip ductile shear belts were found in the Bayanwulashan-Langshan fault zone. Steep mylonite foliation in the sinistral shear belts strikes NE-NNE, its stretching lineation is gentle. Steep mylonite foliation in the dextral shear belts strikes NE or EW. Its stretching lineation is NE-SW or NEE-SWW. Such microstructures as S-C fabrics, rotated feldspar porphyroblasts, and mica fish and quartz c-axis fabrics all indicate two different shear sense. LPO of quartz exhibit (0001)<a>,{1101}<a> and{1010}<a> slip systems, indicating a NE-directed shear at middle-high temepreatures. Rock deformation mechanisms are mainly the creep of the dislocation glide system and fast recrystallization-recovery This work selected muscovite from mylonites in southwestern segment of Langshan fault zone, for40Ar/39Ar dating. In this study, muscovite40Ar/39Ar ages of356.7±2.5Ma and379±4.7Ma were obtained.The formation temperatus of all these mylonite are more than the closure temperature of muscovite, so these ages are cooling ages of the Langshan ductile shear zone,and the sinistral shear must have occurred after these time. U-Pb zircon dating results indicate that the magmatic age of the syn-tectonic granite is330.8±2.8Ma, and the age of late granite dikes are269.8±1.8Ma and280.5±1.5Ma, espectively.更多还原