【作者】 刘忠；

【导师】 曹淑云；

【作者基本信息】 中国地质大学 ， 构造地质学， 2022， 博士

【摘要】 大陆造山带深部地壳构造-热演化历史及剥露过程是固体地球科学研究前缘科学问题。解译造山带中的构造岩浆活动、深部地壳岩石的宏观、微观构造特征、变形-变质作用以及精细年代学综合分析是研究工作的热点和难点。滇东南老君山地区位于华夏地块、扬子地块和印支陆块所夹持的特殊大地构造位置,出露一套不同变质-变形岩系和花岗质侵入岩体,且发育丰富的锡钨等多金属矿床,具有重要研究意义。本文选取滇东南老君山片麻岩穹窿为研究对象,在进行野外详细观测与室内试验分析的基础上,对其构造单元、变形-变质作用、年代学及形成与剥露过程开展了深入的研究。主要取得了以下几点认识:1、厘定了区域构造单元。老君山地区既呈片麻岩穹窿状构造特征,也发育有变质核杂岩的三个主要构造单元,即以深变质-变形岩石及花岗岩为主体的下盘/核部,弧形状的拆离断层系和上盘/沉积盖层。核部出露的花岗质混合岩、片麻岩和云母片岩出现高温晶质塑性变形的特征。这些岩石单元的面、线理产状呈现出由下盘指向上盘（呈弧形状指向沉积盖层）倾斜的分布规律。由核部向主拆离断层靠近,依次出现初糜棱岩、糜棱岩以及超糜棱岩,岩石的变形程度逐渐增强。穿过主拆离断层,向沉积盖层方向,出现断层碎裂岩和断层角砾岩,构成了拆离断层带的岩石序列。2、反演了片麻岩穹窿构造剥露晚期的区域应力场,并对分布特征及演化规律进行分析。结果表明,区域内存在以挤压与伸展应力为主的两种不同性质的应力场,且以伸展应力场为主。这些伸展应力场围绕片麻岩穹窿核部分别在东、北东、北、北西、西和南西方向出现规律性方位变化,其方向均由核部指向沉积盖层,呈现放射状展布的特征,反映片麻岩穹窿剥露晚期的构造特征。3、确定了区域早期高温变质-变形,并叠加低温脆-韧性转换构造和流体作用。核部出露的混合岩、斜长角闪岩和花岗片麻岩保留典型的高温矿物组合和塑性变形构造特征,糜棱岩中发育矿物拉伸线理,形成典型的L型和S-L型构造岩。剥露过程中还叠加了脆-韧性转换构造,发育热擦痕线理。绿泥石集合体呈现出细粒化,具有塑性流动特征,与强烈塑性变形的石英和云母共存,表明区域经历了韧-脆性转换下的叠加退变质-变形。在剥露过程中流体作用强烈,伴随新生矿物如绿泥石、绿帘石、白云母、石英等的出现,是热液流体作用的直接证据。4、约束了区域剥露过程中的变质-变形条件。利用斜长石与角闪石矿物对温压计获得温压范围为0.79～0.56 GPa、710～620℃,为高角闪岩相变质条件。变形矿物EBSD组构分析表明,核部角闪石发育（100）[001]滑移系,指示高温（角闪岩相）塑性变形环境。石英发育多种滑移系,出现以柱面<c>和柱面<a>滑移系为主的高温-中高温变形机制和以菱面<a>与底面<a>滑移系为主的中低温变形机制。方解石在靠近拆离断层出现<0001>（11-20）滑移系的高温变形,远离拆离断层则出现（01-18）（11-20）滑移系的低温变形特征。利用花岗片麻岩中出现蠕英结构进行二长石温度估算,获得了蠕英结构出现的变形温度范围510～400°C。拆离断层带内的糜棱岩中石英动态重结晶的分形维数范围1.1148～1.1577,差异应力范围为14.12～15.92 Mpa,应变速率变化范围为5.089×10^-17～9.566×10^-17s^-1,反映出拆离断层带内的糜棱岩具有中高温-高温-中高温变形的变化特征。5、确定了老君山地区花岗片麻岩和浅色花岗岩的形成时限。获得445～420 Ma的岩浆锆石U-Pb年龄,代表了加里东期老君山地区的岩浆活动时限,是老君山片麻岩穹窿中花岗片麻岩原岩的主形成期。核部出露浅色花岗岩属于高钾钙碱性系列S型花岗岩,锆石U-Pb年龄为416～411 Ma,可能代表了区域地壳加厚深熔时限。斜长角闪岩中锆石变质边部的U-Pb测年结果为241～220Ma,为印支期构造高温变质与高温缩短变形的时限。6、提出老君山片麻岩穹窿的剥露模型。结合前人热年代学数据,综合限定老君山片麻岩穹窿的形成与剥露演化过程。本文认为加里东早期（445～420 Ma）为NE-SW向区域性伸展作用与岩浆侵位,之后经历了挤压碰撞造山及地壳加厚和深熔作用（420～410 Ma）。印支期（241～230 Ma）,在NE-SW向挤压作用下,新元古代变质岩组由南西向北东方向逆冲,核部花岗岩发生变形变质,沉积盖层受挤压形成区域性褶皱。之后,出现挤压到伸展构造的转换及区域拆离断层的发育,研究区域持续剥露。燕山期（144～80 Ma）持续的伸展作用导致花岗岩体侵位（92～70 Ma）,促使穹窿构造快速剥露,主拆离断层面受岩体侵位影响,发生弧形弯曲。新生代阶段（33～21 Ma）受红河-哀牢山走滑断裂带的控制和影响,产生显著的左行旋扭作用并形成弧形状旋转性拆离断层/系,最终导致穹窿快速剥露至地表。更多还原

【Abstract】 The tectono-thermal evolution and exhumation process of the lower crust in the continental orogenic belt are the frontier scientific issues of solid geoscience.Therefore,the interpretation of tectono-magmatic activities in orogenic belts,macro and micro structural characteristics of lower crust rocks,deformation-metamorphism and precise chronology are focus and difficulties for the research work.Laojunshan is located in a special tectonic position sandwiched by Cathaysian,Yangtze and Indosinian block in southeast Yunnan.It exposes a set of metamorphic-deformation rocks and granitic plutons,and develops rich tin tungsten and other polymetallic deposits.This paper selects Laojunshan gneiss dome as the research object,based on detailed field observation and analysis,makes an in-depth study on its structural unit,deformation-metamorphism,chronology,formation and exhumation process.This paper mainly obtains the following results:1.The composition of tectonic units is determined.It is determined that Laojunshan area has the structural characteristics of gneiss dome,combined with the three main structural units with metamorphic core complex,that is,the footwall/core include deep metamorphic deformed rocks and granite pluton,arc-shaped detachment fault system and hanging wall/sedimentary cover.Granitic mylonite,gneiss and mica schist exposed in the core of the dome show the high-temperature crystalline plastic deformation,the foliation and lineation show a distribution regulation inclined from footwall to hanging wall（arc shape to surrounding sedimentary cover）.In the detachment fault,protomylonite,mylonite and ultramylonite appear successively from the core to the detachment fault,and the rock deformation degree increases gradually.Through the detachment fault,cataclastic rock and breccia appear in the direction of sedimentary cover,forming the sequence of rocks in the detachment fault zone.2.The distribution characteristics and evolution regulation of stress field in the late stage of gneiss dome are inversed.Through the analysis of the stress field orientation,the results show that there have two different stress fields dominated by compressive and extensional stress,and the extensional one is domain by all stress fields.These extensional stress fields change regularly around the core of gneiss dome in the directions of east,northeast,north,northwest,west and southwest respectively.Their directions are from the core to the sedimentary cover,showing the characteristics of radial style.It reflects the structural morphological characteristics of the late stage of gneiss dome exhumation.3.The early high-temperature metamorphism-deformation superimposed low-temperature brittle ductile transition and fluid activity are determined.The migmatite,amphibolite and granitic gneiss exposed in the core of dome retain typical high-temperature mineral assemblages and plastic deformation structural characteristics.High-temperature crystalline plastic deformation,and mineral stretching lineation is developed in mylonite,L and S-L type tectonite are formed.It is also found that the brittle-ductile transition is superimposed in exhumation process,and the thermal lineation is developed.Chlorite aggregate rotates and elongates along the edge of feldspar porphyroblasts or cracks,showing fine-grained chlorite with plastic flow.It coexists with quartz and mica with strong plastic deformation,indicating that the region has experienced superimposed retrograde metamorphism deformation under the ductile-brittle transition.In the process of exhumation,its fluid activity is intense,accompanied by the emergence of new minerals such as chlorite,epidote,muscovite and quartz,which are obvious evidence of hydrothermal fluid activity.4.The metamorphic-deformation conditions in the process of exhumation are finely constrained.The metamorphic P-T condition obtained by the thermobarometer of plagioclase-amphibole is 0.79-0.56 GPa and 710-620°C,indicating that the metamorphic degree in the area has reached high amphibolite facies.Through mineral deformation fabric analysis（EBSD）,it is obtained that amphibole（100）[001]slip system is developed,indicating the high-temperature（amphibolite facies）plastic deformation.Quartz develops a variety of slip systems,with high-medium temperature deformation mechanism dominated by cylindrical<c>and<a>slip systems,and superimposes the medium and low temperature deformation dominated by rhomb<a>and basal<a>slip systems.Calcite shows high temperature deformation of<0001>（11-20）slip system near the detachment fault;Far away from the detachment fault,there is a（01-18）（11-20）slip system,which is characterized by low temperature deformation.The temperature of feldspar is calculated by using the myrmekite in granitic gneiss,and the deformation temperature range is 510-400°C.The fractal dimension（D）of quartz dynamic recrystallization in mylonite in the detachment fault zone ranges from 1.1148 to 1.1577,the differential stress range is 14.12-15.92 Mpa,and the strain rate varies from 5.089-9566×10^-17s^-1,which reflects that mylonite in the detachment fault zone has the variation characteristics of medium-high temperature deformation.5.Determine the main formation stage and tectonic-magmatic activity time of the gneiss unit in the core of Laojunshan dome.The magmatic zircon U-Pb ages of445-420 Ma are obtained for the core granitic gneiss,represents the magmatic activity time of Laojunshan area in Caledonian and is the main formation stage of granitic gneiss protolith in Laojunshan gneiss dome.A set of leucogranite are also exposed in the core,and Zircon U-Pb ages is 416-411 Ma.These belong to S-type granites and represents the regional crustal thickening and anatectic melting time.The U-Pb dating results of metamorphic zircons in amphibolite is 241.5±1.4 Ma,which can restrict the stage of Indosinian tectonic high-temperature metamorphism and shortening deformation to 241-220 Ma.6.A exhumation model of Laojunshan gneiss dome is proposed.Based on the formation and exhumation evolution process,it is considered that the early Caledonian（445-420 Ma）was NE-SW extension,the granitic magma was emplaced.After that,the region experienced compression collision orogeny,crustal thickening and melting,developed high-K calc alkaline series S-type granite（420-410 Ma）,and the surface sedimentation was interrupted to form a regional unconformity.In the Indosinian（241-230 Ma）,under the NE-SW direction compression,the Neoproterozoic metamorphic rock groups were thrust from southwest to northeast,and the Laojunshan granitic gneiss was formed.The sedimentary cover was compressed to form regional folds,and the dome began to form.After that,the tectonic transformation from compression to extension.Stepped normal faults are developed in the sedimentary cover,and the area is continuously exposed.The continuous extension in the Yanshanian（144-80 Ma）led to the emplacement of granite pluton in Laojunshan area（92.4-70.8 Ma）,which prompted the rapid exhumation of dome.The main detachment fault was affected by the emplacement of rock mass and curved.Controlled and influenced by the Ailaoshan-Red River strike slip fault zone in the Cenozoic（33-21Ma）,the dome produced significant sinistral rotation and torsion,forming arc-shaped rotational detachment faults/systems,resulting in the exhumation of the dome to the surface.更多还原