西藏阿里札达韧性剪切带特征及其X光岩组分析

文中简述了西藏阿里札达盆地的地质背景、区域地层和札达韧性剪切带的基本特征。采用X射线衍射法对札达韧性剪切带中的石英、方解石和白云母等三种矿物,进行了X光岩组分析,确定了韧性变形岩石的组构特征、韧性剪切带的属性和变形岩石的应变类型,以及韧性剪切带形成时的温压条件。研究表明,韧性变形岩石均具不对称组构,反映韧性带属于南盘(下盘)俯冲型韧性剪切带,韧性变形是在高温、高压、低应变速率条件下发生的,处于>10km的地壳深度,岩石应变类型以压扁应变为主。     

滇西地区韧性剪切带岩石特征及其类型

滇西地区的哀牢山、点苍山、崇山及高黎贡山等变质带,都具有韧性剪切带的岩石特征,即以糜棱岩为主、不同类型的韧性变形岩石。本文仅就这些变质带发育的塑性流变及剪切应变特征和变形岩石的大量存在,说明这些变质带都是韧性剪切带。韧性变形岩石除糜棱岩类外,还有千糜岩类和构造片岩类。     

桐柏杂岩北界剪切带的构造变形特征研究

桐柏杂岩北界剪切带是桐柏—大别造山带发育的一条大型的左行走滑韧性剪切带,发育糜棱岩、超糜棱岩以及强直片麻岩等强变形岩石。通过对剪切带内岩石的矿物共生组合、剪切带的位移量计算、石英的分维数计算、石英颗粒的差异应力以及应变速率的估算等多种方法分析结果显示:桐柏杂岩北界剪切带的位移量为38.3~41.1km;动态重结晶石英颗粒边界统计数据的相关系数R均大于0.96,动态重结晶颗粒边界具有统计意义上的自相似性;分维数为1.207~1.302之间,从剪切带的核部向边部,颗粒粒径和分维值的变化是逐渐变大的。石英的分维值反映出石英的动态重结晶与剪切带密切相关,变质变形由剪切带核部向边部逐渐减弱。桐柏杂岩北界剪切带的差异应力值为23.583~46.983 MPa之间,应变速率为1.70×10~(-11)~9.74×10~(-11)s~(-1)。通过对比剪切带内动态重结晶石英颗粒的粒径、差异应力和应变速率可以发现,差异应力与应变速率表现出一定的正相关性,都随着从剪切带的核部到边部慢慢变小;而石英的粒径则与差异应力和应变速率表现出负相关性,从剪切带核部到边部,随着石英粒径的变大,差异应力和应变速率变小。差异应力越大,导致石英颗粒的受力强度越大,变形越强,表现出来的就是应变速率越强。从显微构造变形角度来研究认识该剪切带,对揭示桐柏—大别造山带的构造演化以及构造模型的建立有着重要的构造意义。     

早前寒武纪多期变质变形区主期韧性剪切带

早前寒武纪多期变质变形区的韧性剪切带，根据其与变形变质的关系可分为（１）早期主变形变质作用形成的韧性剪切带，简称主期韧性剪切带；（２）晚期叠加变形变质作用形成的韧性剪切带，简称叠加韧性剪切带。主期韧性剪切带糜棱岩，退变质和新生面理等标志，其高应变特征主要通过岩石中片理和线理发育程度，矿物定向程度，矿的粒度及应变大小表现出来。由于主期声望生剪切带内的片理和线理及矿物组合与围岩中的相同，因此相同岩石的     

海南岛戈枕脆、韧性叠加剪切带基本特征及其成矿意义

戈枕脆、韧性叠加剪切带为一条呈北东向横贯海南岛土外山金矿区的窄长线性强应变带,具有显著的塑性流变、剪切应变及递进演化的特点。带内岩石表现出强烈的韧性变形和糜棱岩化及叠加的碎袭岩化,以及绿片岩相退化变质作用,形成一套由不同时代不同类型岩石的糜棱岩、碎袭岩所组成,呈似透镜状平行产出的剪切带。其中较显著的是产生一套类似混合岩外貌的长英质糜棱岩系列岩石。戈枕脆、韧性叠加剪切带的发现和确定对研究土外山金矿床的成因以及进一步找矿具有十分重要的意义。     

论韧性剪切带研究及其地质意义

系统研究韧性剪切带变形岩在天然强剪切应力作用条件下常量元素迁移机制及活化转移的应力排序、微量元素迁移的动力控制、稀土元素配分变化和变形矿物晶体化学变异的应力制约等构成了当代韧性剪切带研究的前沿课题,也是当前糜棱岩岩石地球化学研究难点和精华所在。其研究成果将对动力成岩（成矿）机理的认识有重要的突破,具有重要的理论意义和潜在应用价值。对韧性剪切带及其变形岩石的研究现状和研究意义进行系统的综述,提出了未来韧性剪切带及其糜棱岩的研究方向和目标: ①系统研究糜棱岩中主要造岩矿物组合及其变形特征,计算剪切变形岩石的应力—应变参数,搞清韧性剪切带所处的应力应变环境;②系统研究韧性剪切带岩石在天然分强剪切应力作用条件下常量元素迁移机制及活化转移的应力排序问题;③系统研究剪切变形作用过程中岩石化学组成的微量和稀土元素变化,讨论强变形条件下岩石中微量元素活化和迁移规律,深入探讨微量元素迁移的动力控制,包括稀土元素配分变化的应力制约以及应变矿物晶格化学变化行为及其对其寄主的变形岩石元素（组分）在应变过程中迁移变化的制约和影响;④从理论上探讨天然强剪切应变条件下岩石中组分活化、转移与应力（应变）的因果联系,为深入探讨韧性剪切带动力成岩（成矿）作用提供理论的科学依据,为探讨中、下地壳中韧性剪切带的形成和演化提供科学依据（如韧性剪切带金的富集）,同时为韧性剪切变形作用条件下成岩、成矿地球化学作用提供理论和实验依据;⑤现代分析技术如激光同位素原位分析以及激光ICP MASS分析技术对研究变形域内的岩石（矿物）的元素和同位素的活化迁移规律,对深刻揭示糜棱岩化过程中的元素活化迁移机制提供更高质量的地球化学证据具有重要的作用。     

早前寒武纪多期变质变形区主期韧性剪切带

早前寒武纪多期变质变形区的韧性剪切带，根据其与变形变质的关系可分为（１）早期主变形变质作用形成的韧性剪切带，简称主期韧性剪切带；（２）晚期叠加变形变质作用形成的韧性剪切带，简称叠加韧性剪切带。主期韧性剪切带不发育糜棱岩、退变质和新生面理等标志，其高应变特征主要通过岩石中片理和线理发育程度、矿物定向程度、矿物粒度及应变大小表现出来。由于主期韧性剪切带内的片理和线理及矿物组合与围岩中的相同，因此相同岩石的应变强度比较是识别它的有效途径。比较拟引入反映岩石变形强度的物理参数ｎ（ＸＺ面上平行Ｘ和垂直Ｘ两个方向上单位长度矿物颗粒数之比）、Ｓ（单位体积内矿物颗粒表面积）和Ｒ＿ｓ（矿物形态应变）作为比较参数。     

舒兰韧性剪切带应变分析及石英动态重结晶颗粒分形特征与流变参数估算

舒兰北东向韧性剪切带位于佳木斯-伊通断裂带(佳-伊断裂带)中南段, 剪切带内糜棱岩具有明显左行走滑特征, 片麻理产状近NNE向.糜棱岩中长石有限应变Flinn图解判别岩石类型为L-S型构造岩, 属拉长型应变.石英C轴EBSD组构分析表明, 石英组构以中低温菱面为主, 滑移系为{0001} < 110>.剪切带内糜棱岩的剪应变为0.44, 不同方法计算所得运动学涡度值均大于0.95, 指示剪切变形以简单剪切为主.综合矿物变形温度计、石英C轴EBSD组构、石英的粒度-频数图及Kruhl温度计综合估计该韧性剪切带变形机制以位错蠕变机制为主, 变质相为低绿片岩相, 发生韧性变形和糜棱岩化温度范围在400~500 ℃之间.糜棱岩内石英动态重结晶新晶粒边界普遍具有锯齿状或港湾状结构, 利用分形方法对其重结晶新晶边界研究表明, 这些晶粒边界具有自相似性, 表现出分形特征, 分形维数值为1.195~1.220.根据石英重结晶粒径估算差应力值为24.35~27.59 MPa, 代表了舒兰韧性剪切带糜棱岩化作用过程的差异应力下限.使用不同实验方法估算、比较和分析了该剪切带古应变速率, 认为该速率应为10-12.00~10-13.18 s-1, 与区域性应变速率10-13.00~10-15.00 s-1对比, 说明舒兰韧性剪切带的应变速率与世界上大多数韧性剪切带中的糜棱岩应变速率一致, 是缓慢变形的结果, 其形成可能与早白垩世伊泽纳崎板块向欧亚大陆俯冲发生转向有关.      

剪切带的流体－岩石相互作用

作为大陆岩石圈中的应变局部化带，剪切带中一般都渗透着大量流体。流体的来源与剪切带所处的构造背景、流变域和水文条件有关，而剪切带中流体的流动则受岩石的渗透率、孔隙度、孔隙性质、流体的扩散和渗透能力、环境的温压条件、应力或载荷的梯度等因素所制约。剪切带中流体的成分、通量及赋存状态或流动方式，直接影响着岩石的流变。由应变局部化及力学失稳所引起的化学不平衡和由流体与岩石的相互作用，使剪切带岩石的矿物成分和化学成分发生调整，其变异程度取决于原岩的性质、剪切的温压条件和流体的成分及通量等。由于流体的渗透流动和流体与岩石的相互作用使剪切带的体积有所变化，体积变化过程是一种自组织行为。较大的体积亏损，意味着剪切带中渗透过大量的流体，这对剪切带的流变行为、化学行为和成矿作用都有深刻的影响。     

剪切带的流体－岩石相互作用

作为大陆岩石圈中的应变局部化带，剪切带中一般都渗透着大量流体。流体的来源与剪切带所处的构造背景、流变域和水文条件有关，而剪切带中流体的流动则受岩石的渗透率、孔隙度、孔隙性质、流体的扩散和渗透能力、环境的温压条件、应力或载荷的梯度等因素所制约。剪切带中流体的成分、通量及赋存状态或流动方式，直接影响着岩石的流变。由应变局部化及力学失稳所引起的化学不平衡和由流体与岩石的相互作用，使剪切带岩石的矿物成分和化学成分发生调整，其变异程度取决于原岩的性质、剪切的温压条件和流体的成分及通量等。由于流体的渗透流动和流体与岩石的相互作用使剪切带的体积有所变化，体积变化过程是一种自组织行为。较大的体积亏损，意味着剪切带中渗透过大量的流体，这对剪切带的流变行为、化学行为和成矿作用都有深刻的影响。     

Strain distribution within a km-scale,mid-crustal shear zone: The Kuckaus Mylonite Zone,Namibia

The subvertical Kuckaus Mylonite Zone (KMZ) is a km-wide, crustal-scale, Proterozoic, dextral strike-slip shear zone in the Aus granulite terrain, SW Namibia. The KMZ was active under retrograde, amphibolite to greenschist facies conditions, and deformed felsic (and minor mafic) gneisses which had previously experienced granulite facies metamorphism during the Namaqua Orogeny. Lenses of pre- to syn-tectonic leucogranite bodies are also deformed in the shear zone. Pre-KMZ deformation (D₁) is preserved as moderately dipping gneissic foliations and tightly folded migmatitic layering. Shear strain within the KMZ is heterogeneous, and the shear zone comprises anastomosing high strain ultramylonite zones wrapping around less deformed to nearly undeformed lozenges. Strain is localized along the edge of leucogranites and between gneissic lozenges preserving D₁ migmatitic foliations. Strain localization appears controlled by pre-existing foliations, grain size, and compositional anisotropy between leucogranite and granulite. The local presence of retrograde minerals indicate that fluid infiltration occurred in places, but most ultramylonite in the KMZ is free of retrograde minerals. In particular, rock composition and D₁ fabric heterogeneity are highlighted as major contributors to the strain distribution in time and space, with deformation localization along planes of rheological contrast and along pre-existing foliations. Therefore, the spatial distribution of strain in crustal-scale ductile shear zones may be highly dependent on lithology and the orientation of pre-existing fabric elements. In addition, foliation development and grain size reduction in high strain zones further localizes strain during progressive shear, maintaining the anastomosing shear zone network established by the pre-existing heterogeneity.     

韧性剪切带的剪切作用类型和韧性减薄量

韧性剪切带组构的演化和剪切作用类型受到许多研究者的关注。运用极莫尔圆法、有限应变法、刚性颗粒法、石英光轴组构结合有限应变测量法、拖尾形态法、剪切带内变形脉体(岩墙)法、碎斑法等方法可以估算剪切带变形过程中的运动学涡度,进而判别剪切带中单剪切组分与纯剪切组分的相对含量。自然界的剪切带一般介于单剪与纯剪之间,运动学涡度Wk介于0~1之间,表明韧性剪切带在变形过程中发生了垂直于剪切带边界(Z轴)方向的韧性减薄。剪切带变形过程中的韧性减薄量可依据有限应变测量与运动学涡度估算求得,也可依据剪切带内的石香肠(布丁)构造求解,还可依据构建极莫尔圆求解。以华北克拉通北缘的楼子店变质核杂岩及其韧性剪切带,以及希腊西奈山的Chelmos剪切带为例,介绍估算韧性剪切带韧性减薄的方法,这种韧性减薄是对大规模岩石圈减薄的有益补充和完善。研究结果表明,定量估算与变质核杂岩相关的韧性剪切带的剪切作用类型是分析变质核杂岩形成机制的有效途径和方法。     

Thermomechanics of an extensional shear zone,Raft River metamorphic core complex,NW Utah

A detailed structural and microstructural analysis of the Miocene Raft River detachment shear zone (NW Utah) provides insight into the thermomechanical evolution of the continental crust during extension associated with the exhumation of metamorphic core complexes. Combined microstructural, electron backscattered diffraction, strain, and vorticity analysis of the very well exposed quartzite mylonite show an increase in intensity of the rock fabrics from west to east, along the transport direction, compatible with observed finite strain markers and a model of ``necking'' of the shear zone. Microstructural evidence (quartz microstructures and deformation lamellae) suggests that the detachment shear zone evolved at its peak strength, close to the dislocation creep/exponential creep transition, where meteoric fluids played an important role on strain hardening, embrittlement, and eventually seismic failure.Empirically calibrated paleopiezometers based on quartz recrystallized grain size and deformation lamellae spacing show very similar results, indicate that the shear zone developed under stress ranging from 40 MPa to 60 MPa. Using a quartzite dislocation creep flow law we further estimate that the detachment shear zone quartzite mylonite developed at a strain rates between 10⁻¹² and 10⁻¹⁴ s⁻¹. We suggest that a compressed geothermal gradient across this detachment, which was produced by a combination of ductile shearing, heat advection, and cooling by meteoric fluids, may have triggered mechanical instabilities and strongly influenced the rheology of the detachment shear zone.     

Fabric development as the key for forming ductile shear zones and enabling plate tectonics

Lithospheric deformation on Earth is localized under both brittle and ductile deformation conditions. As high-temperature ductile rheologies are fundamentally strain-rate hardening, the formation of localized ductile shear zones must involve a structural or rheological change or a change in deformation conditions such as an increase in temperature. In this contribution, I develop a localization potential that quantifies the weakening associated with these changes. The localization potential corresponds to the increase in strain rate resulting from that change under constant stress conditions. I provide analytical expressions for the localization potential associated with a temperature increase, grain size reduction, an increase in water fugacity, melt content, or the abundance of a weak mineral phase. I show that these processes cannot localize deformation from a mantle convection scale (10³ km) to a ductile shear zone scale (1 km). To achieve this, is it necessary to invoke a structural transition whereby the weak phase in a rock forms interconnected layers. This process is efficient only if one phase is much weaker than the others or if the weakest phase has a highly non-linear rheology. Micas, melt, and fine-grained aggregates – unless dry rheologies are used – have the necessary characteristics. As none of these phases is expected to be present in the dry lithosphere of Venus, this concept can explain why Venus, unlike the Earth, does not display a global network of plate boundaries. The diffuse plate boundary in the Central Indian Ocean may be as yet non-localized because serpentinization has not reached the ductile levels of the lithosphere.     

Mylonitic microfabrics from the rocks of MCT zone in Alakhnanda valley,Garhwal Himalaya

MCT Zone of Alakhnanda valley is a major ductile shear zone in Garhwal Himalaya, which is characterised by different types of mylonite rocks. On the basis of grain size and the percentage of matrix in the rock, zones comprising protomylonite, augen mylonite, mylonite and ultramylonite have been identified. The study of microstructures, grain size and crystallographic preferred orientation of quartz c-axis fabric reveals that the rocks of the MCT zone were deformed by a combination of intracrystalline creep (power law creep) and grain boundary migration (sliding super plasticity).     

辽西兴城—台里韧性剪切带北段变形特征

辽西兴城—台里地区发育系列花岗质岩石,强烈构造变形特征均显示其具有韧性剪切带的特点。对剪切带北段进行详细宏微观构造解析,结合岩石变形强度差异性分析、有限应变测量、石英C轴EBSD测试以及古差异应力值估算等研究,结果表明剪切带内花岗质片麻岩和眼球状花岗质片麻岩具有NEE向左行剪切变形特征,变形岩石为S-L构造岩,应变类型属于平面应变,古差异应力值介于30~40 MPa之间。长石-石英矿物温度计以及石英C轴EBSD组构指示剪切带以中低温变形为主,温度在400℃~500℃,属绿片岩相变质,具中-低温韧性剪切带特征。韧性剪切带内普遍存在变形分解现象,弱变形带内岩石残斑含量较高,眼球状构造和S-C组构较为发育;强变形带岩石残斑含量较低,剪切面理较为发育,糜棱面理发育较弱或者不发育。     

延边地区古洞河韧性剪切带变形特征及变形时代

古洞河韧性剪切带位于兴蒙造山带东段南缘,其变形特征和时间对探讨延边地区晚古生代-中生代构造演化具有重要意义.鉴于此,对该韧性剪切带进行了锆石U-Pb年代学和显微构造解析,以期限定韧性剪切带变形时间和特征.研究结果显示,古洞河韧性剪切带具有多期变形的特征,晚期变形糜棱叶理倾向为西,倾角较缓,线理倾伏向为南西,具有由南西向北东方向右行逆冲的特征.其分形维数值为1.159~1.214,Flinn参数K值为0.19~0.31,Kruhl温度计显示变形温度为450~550℃,石英动力重结晶粒径估算的差应力值为16.83~20.09 MPa,古应变速率为10-12~10-14 s-1.古洞河韧性剪切带花岗质糜棱岩锆石U-Pb年龄为192±2 Ma,晚期变形时代应为早侏罗世,形成应与早侏罗世古太平洋板块向欧亚大陆俯冲有关.      

Volume strain, strain type and flow path in a narrow shear zone

This study explores the state of finite strain and changes in the mean kinematic vorticity number, grain size, whole-rock chemistry and mineralogy across an upper amphibolite-facies shear zone in a metadiorite, northern Malawi, east-central Africa. P–T conditions during shear-zone formation and deformation were approximately 700–750^?°C and 5–7?kbar and are slightly less than P–T conditions for the regional peak of metamorphism. The major rock-forming minerals, plagioclase, hornblende, biotite, and quartz, were deformed by crystal-plastic processes accompanied by, except for hornblende, dynamic recrystallization. The modal abundance of all four major rock-forming minerals shows no systematic change from the country rock into and across the shear zone, indicating that shear-zone development was not associated with retrograde mineral reactions. The grain size of the major rock-forming minerals decreases within the shear zone. Plagioclase and hornblende, which occur as porphyroblasts outside the shear zone, exhibit a bimodal grain-size distribution within the shear zone. Quartz has a unimodal grain-size distribution in the shear zone. Major and trace element chemistry does not change systematically across the shear zone, implying no volume change in the mylonite. Matrix strain data for plagioclase and hornblende by the Fry method and fabric strain as deduced from R_f/φ analysis of plagioclase and quartz grains demonstrate a slightly constrictional strain type (K≈1.5) across the shear zone. The quantitative finite-strain data for the different residual minerals as obtained by unlike methods show no systematic variation, but recrystallized plagioclase grains record higher strain than the residual grains. The mean kinematic vorticity number changes from approximately 0.3 outside to approximately 0.8 within the shear zone, indicating that the bulk deformation path deviated from progressive simple shear. The estimates for finite strain and the degree of noncoaxiality account for approximately 50% of thinning normal to the shear zone.     

桐柏-大别南缘殷店-马垅韧性剪切带的变形及年代学分析

殷店-马垅（简称殷马）韧性剪切带是桐柏-大别造山带南缘的一条重要的边界断裂,对其精细的构造变形研究将有助于进一步认识桐柏-大别造山带的演化方式和过程。本文主要对殷店和马垅两个地区进行了详细的剖面观测,利用有限应变测量,计算出殷马剪切带中岩石应变椭球长短轴的平均轴率为1.69,靠近剪切带的岩石应变量大,向北远离剪切带,应变量减小;通过构造应力场分析查明剪切带的古应力状态,最大压应力σ₁为北北东-南南西向,对应最大拉应力σ₃为北西西-南东东向。岩石的有限应变分析和应力场状态,反映出板块间的碰撞挤压作用力在平行剪切带和垂直剪切带方向上存在应力分解。剪切带内糜棱岩中的锆石U-Pb定年结果显示,其年龄为172～137 Ma,据此得出殷马剪切活动时代主要为中侏罗世-早白垩世。