Rock Deformation,Component Migration and 18O/16O Variations during Mylonitization in the Southern Tan-Lu Fault Belt

This paper discusses the relationship between the volume loss, fluid flow and component variations in the ductile shear zone of the southern Tan-Lu fault belt. The results show that there is a large amount of fluids flowing through the shear zone during mylonitization, accompanied with the loss of volume of rocks and variations of elements and oxygen isotopes. The calculated temperature for mylonitization in different mylonites ranges from 446 to 484℃, corresponding to that of 475 to 500℃ for the wall rocks. The condition of differential stress during mylonization has been obtained between 99 and 210 MPa, whereas the differential stress in the wall rock gneiss is 70-78 MPa. The mylonites are enriched by factors of 1.32-1.87 in elements such as TiO2, P2O5, MnO, Y, Zr and V and depleted in SiO2, Na2O, K2O, Al203, Sr, Rb and light REEs compared to their protolith gneiss. The immobile element enrichments are attributed to enrichments in residual phases such as ilmentite, zircon, apatite and epidote in mylonites and are interpreted as due to volume losses from 15% to 60% in the ductile shear zone. The largest amount of SiO2 loss is 35.76 g/100 g in the ductile shear zone, which shows the fluid infiltration. Modeling calculated results of the fluid/rock ratio for the ductile shear zone range from 196 to 1192 by assuming different degrees of fluid saturation. Oxygen isotope changes of quartz and feldspar and the calculated fluid are corresponding to the variations of differential flow stress in the ductile shear zone. With increasing differential flow stress, the mylonites show a slight decrease of δ^18O in quartz, K-feldspar and fluid.

Mylonitization of Diorites in Jiangshao Fracture Belt, Zhejiang Province,China

Characteristcs of progressive ductile deformation and remnant igneous fabrics indicate that thefoliatal diorites in the middle sector of the Jiangshao fracture belt in Zhuji County, Zhejiang, aremyonites deformed from diorites. The mylonithation occurred about 353 - 370 Ma ago under a condi-tion of 425-475℃ and 3.8-4.6 kb.Therelation between the deformation and metherphism was discussed based on petrographic evidence and mineral chemistry and assemblage. It is suggested that thedeformation was responsible for the deveopment of lowgrad greenschisthaes metamorphism ofmylonitic diorites by introducing fluid phase to aerelerate metamorphic reaction and diffosion.

Mechanical behaviors of mylonitic granite and granitic protomylonite in a deep ductile shear zone

For projects near the tectonic belt,mylonite of varying metamorphic degrees may be present.The matrix proportion of rock reflects its internal microscopic characteristics,thus it is beneficial for engineering geology to study the effect of the matrix proportion on the mechanical properties and rupture behaviors of rock.Samples of mylonitic granite and granitic protomylonite with varying matrix proportions were obtained from a ductile shear zone for a series of uniaxial compression and acoustic emission(AE)tests.The results showed that with the increase in matrix proportion,the average strength and elastic modulus of the samples increased,and the rock sample with the largest matrix proportion exhibited the maximum peak stress of 244.42 MPa,which was 45.86%greater than the average peak stress of the rock samples with the smallest matrix proportions.For the rock samples with larger matrix proportion,their mechanical parameters exhibited greater dispersion and the large-scale appearance of AE events occurred earlier,showing a relatively gradual failure process.These samples had larger accumulated AE parameter values and greater degree of failure.In contrast,for samples with smaller matrix proportions,the large-scale appearance of AE events occurred close to the peak stress,indicating that the occurrence of damage and fractures was centralized and instantaneous.These samples had lower accumulated AE parameter values and fewer cracks after failure.Additionally,for the rock samples with more matrix proportion,the average variance of the b-value was 1.1,which was lower than that of rock samples with the smallest matrix proportion(the average variance of the b-value was 3.7).Furthermore,it can be predicted that under certain stress,the failure depth around a tunnel is generally smaller when the strength of rock samples with larger matrix proportion is greater.

Excess and Radiogenic Argon in High-Pressure Mylonites of the Tan-Lu Fault Zone, Eastern China

Bulk separates of porphyroclastic phengite, neoformed phengite and their mixtures from the Tan-Lu HP mylonites overprinted on the Sulu UHP rocks were analyzed with the 40Ar/39Ar step heating method. Two samples of the neoformed phengite from ultramylonite give 40Ar/39Ar plateau ages of 209.9±1.8 Ma and 214.3±1.8 Ma, which are interpreted as representing cooling times of the TanLu sinistral faulting, and provide geochronological evidence for the syn-orogenic faulting of the Tan-Lu fault zone. The results show that the phengite formed during the retrograde eclogite-facies mylonitization was not contaminated with excess argon and can be used for dating the deformation. Argon closure in previous K-bearing minerals with excess argon under a retrograde HP dry condition is considered to be the reason for lack of excess argon incorporation in the neoformed phengite. Five porphyroclastic phengite samples yield 40Ar/39Ar plateau ages ranging from 666±12 Ma to 307.1±3.3 Ma, which are interpreted as being contaminated with excess argon. Two mixture samples with plateau ages of 239.4±2.1 Ma and 239.3±2.0 Ma show upward-convex age spectra caused by the mixture of older porphyroclastic phengite with excess argon incorporation and younger neoformed phengite without excess argon incorporation. It is demonstrated that excess argon introduced from the previous UHP metamorphism is still preserved in the pre-existing phengite after the Tan-Lu eclogite-facies mylonitization. The intense deformation under HT and HP conditions cannot erase excess argon in the previous phengite totally due to restricted fluid activities. These porphyroclastic phengite previously contaminated with excess argon cannot be used for dating the later HP deformation. This indicates that deformation under a HP dry condition does not play an important role in removing previous 40Are in phengite.

Macro-and Microstructural, Textural Fabrics and Deformation Mechanism of Calcite Mylonites from Xar Moron-Changchun Dextral Shear Zone, Northeast China

The calcite mylonites in the Xar Moron-Changchun shear zone show a significance dextral shearing characteristics. The asymmetric(σ-structure) calcite/quartz grains or aggregates, asymmetry of calcite c-axes fabric diagrams and the oblique foliation of recrystallized calcite grains correspond to a top-to-E shearing. Mineral deformation behaviors, twin morphology, C-axis EBSD fabrics, and quartz grain size-frequency diagrams demonstrate that the ductile shear zone was developed under conditions of greenschist facies, with the range of deformation temperatures from 200 to 300°C. These subgrains of host grains and surrounding recrystallized grains, strong undulose extinction, and slightly curved grain boundaries are probably results of intracrystalline deformation and dynamic recrystallization implying that the deformation took place within the dislocation-creep regime at shallow crustal levels. The calculated paleo-strain rates are between 10-7.87s-1 and 10-11.49s-1 with differential stresses of 32.63-63.94 MPa lying at the higher bound of typical strain rates in shear zones at crustal levels, and may indicate a relatively rapid deformation. The S-L-calcite tectonites have undergone a component of uplift which led to subhorizontal lifting in an already non-coaxial compressional deformation regime with a bulk pure shear-dominated general shear. This E-W large-scale dextral strike-slip movement is a consequence of the eastward extrusion of the Xing’an-Mongolian Orogenic Belt, and results from far-field forces associated with Late Triassic convergence domains after the final closure of the Paleo-Asian Ocean.

Timing of the Early Paleozoic Yangtze and Cathysian Convergence： Constraint from U-Pb Geochronology of Hydrothermal Zircons from Mafic Mylonite within the Shoucheng-Piaoli Ductile Shear Zone, Northern Guangxi

Objective The northern Guangxi region is in the southwestern part of the Southern China continent,which is located at the junction of the southwest section of the Early Paleozoic Yangtze block and Cathaysian block.A series of NNE-trending ductile shear zones are developed in this region,and these ductile shear zones are mostly previously suggested boundary faults of the Early Paleozoic Yangtze block and Cathaysian block,such as the Shoucheng–Piaoli ductile shear zone in Northern Guangxi （Meng Yuanku et al., 2016; Zhang Xuefeng et al., 2015）.

Characteristics of Compositional Migration in Mylonites from the Ductile Shear Zones of the Southern Tancheng-Lujiang Fault Belt, Eastern Anhui Province

On the basis of field geology, three typical ductile shear zones in the southern part of the Tancheng-Lujiang fault belt have been chosen for a detailed study. Altogether ten samples of the tectonites have been collected for this study. The paper is focused on a comprehensive study of the tectonites in the medium-lower horizons of the ductile shear zones. The mineral compositions of the rocks are analyzed with EPMA and some typical whole-rock samples analyzed by chemical and ICP methods. Based on the comprehensive study of the characteristics of the deformation, the mineral assemblages and the changes of chemical composition of the bulk rocks, this paper presents a discussion on the relationship between the volume loss, the fluid flow and compositional changes during mylonitization of the ductile shear zones in this region. Our study shows that there are a large amount of fluids flowing through the shear zones during the process of mylonization, accompanied by the loss of rock volume and migration of elements and components. Modelling calculation results under different saturation conditions of fluids show that the maximum volume loss of the tectonites is about 60% relative to their protolith, while the fluid/rock ratio ranges from 10 to 103 in different ductile shear zones.

Identification of the Early Jurassic mylonitic granitic pluton and tectonic implications in Namling area,southern Tibet

A number of studies revealed that the Gangdese magmatic belt of southern Tibet was closely related to the northward subduction of the Neo-Tethys oceanic lithosphere and Indo-Asian collision.However,pre-Cretaceous magmatism is still poorly constrained in the Gangdese magmatic belt,southern Tibet.Here,we conducted systematically geochronology and geochemistry studies on a newly-identified granitic pluton in the middle Gangdese magmatic belt(Namling area),southern Tibet.Zircon SHRIMPⅡU-Pb dating for one representative sample gives a weighted age of 184.2±1.8 Ma(MSWD=±1.11),corresponding to emplacement and crystallization age of the granitic pluton in the Early Jurassic(Pliensbachian).High SiO2(68.9-72.1 wt.%)contents and intermediate Mg#values(35-38)together suggest that the newly-identified granitic pluton was probably formed by partial melting of crustal material with minor injection of mantle-derived magma,precluding an origin from melting of metasedimentary rocks that are characterized by low Mg#and high zirconδ^18O values(>8‰).Geochemically,the newly-identified granitic pluton belongs to typical I-type granitic affinity,whereas this is inconsistent with aluminium saturation index(ASI=A/CNK ratios)and geochemical signatures.This suggests that zircon oxygen isotopes(4.30‰-5.28‰)and mineral features(lacking Al-rich minerals)are reliable indicators for discriminating granitic origin.Significantly depleted whole-rock Sr-Nd-Hf isotopic compositions and zirconεHf(t)values indicate that the granitic pluton was derived from partial melting of depleted arc-type lavas.In addition,the granitic pluton shows zirconδ^18O values ranging from 4.30‰to 5.28‰(with a mean value of 4.77‰)that are consistent with mantle-derived zircon values(5.3‰±0.6‰)within the uncertainties,indicating that the granitic pluton might have experienced weak short-living high-temperature hydrous fluid-rock interaction.Combined with the Sr-Nd-Hf-O isotopes and geochemical signatures,we propose that the newly-identified granitic pluton was originated from partial melting of depleted mafic lower crust,and experienced only negligible wall-rock contamination during ascent.Integrated with published data,we also propose that the initial subduction of the Neo-Tethys oceanic lithosphere occurred no later than the Pliensbachian of the Early Jurassic.

The Gondwana Orogeny in northern North Patagonian Massif:Evidences from the Caita Có granite, La Se?a and Pangaré mylonites,Argentina

Structural analyses in the northern part of the North Patagonia Massif, in the foliated Caita Co granite and in La Sefia and Pangare mylonites, indicate that the pluton was intruded as a sheet-like body into an opening pull-apart structure during the Gondwana Orogeny. Geochronological studies in the massif indicate a first, lower to middle Permian stage of regional deformation, related to movements during indentation tectonics, with emplacement of foliated granites in the western and central areas of the North Patagonian Massif. Between the upper Permian and lower Triassic, evidence indicates emplacement of undeformed granitic bodies in the central part of the North Patagonian Massif. A second pulse of deformation between the middle and upper Triassic is related to the emplacement of the Caita CO granite, the development of mylonitic belts, and the opening of the Los Menucos Basin. During this pulse of deformation, compression direction was from the eastern quadrant.

Geochemical and Mineralogical Studies of the Mylonite Xenoliths and Monzogranite Rocks at Wadi Abu Rusheid,South Eastern Desert,Egypt:Insights on the Genesis of Mineralization

Abu Rusheid area lie in the south of the Eastern Desert of Egypt and comprises(1)ophiolitic mélange,consisting of ultramafic rocks and layered metagabbros in metasedimentary matrix(2)cataclastic group,consisting of protomylonites,mylonites,ultramylonites and silicified ultramylonites(3)monzogranites(4)pegmatite pockets,quartz veins and postgranite dykes.Focus on the monzogranites and the xenoliths of mylonite rocks from the geochemical and mineralogical points of view introduces a new view about the genesis of the related mineralization.Geochemically,the monzogranites have a metaluminous character and were crystallized under moderate water-vapor pressure around 3 kb and temperatures of 750–800°C.The monzogranites are altered along strike-slip faults exhibiting propylitic,with slightly sodic metasomatism and record high radioactive measurements.The average uranium and thorium(U and Th)contents in fresh monzogranites,mylonite xenolith and altered monzogranites are(7.3,21.20),(40.36,94.82),(60.34 and 347.88 ppm),respectively.These high radioactivities are attributed to the presence of kasolite,uranothorite,cerite,fluorite,zircon,apatite and columbite.The mylonite xenolith is higher in radioactivity than the surrounding fresh monzogranites,reflecting U and Th enrichment before emplacement of the monzogranites,then the latter were subjected to right lateral strike-slip faulting with producing hydrothermal solution rich in Th and U.

亚临界水环境下川西糜棱岩水-热腐蚀效应研究

位于青藏高原东部的川西构造带分布有高温地热资源,但同时也是地震活动多发区。为探究地热流体对断层岩石的腐蚀损伤,本文对川西金河-箐河断裂带上的糜棱岩开展100~350℃亚临界水环境(100~374℃,饱和蒸汽压0.1~22.1MPa)下的水-岩相互作用试验,通过电感耦合等离子体质谱仪(ICP-MS)和低温氮气吸附实验(LTNA)分析了反应后溶液成分和岩石孔隙结构随温度的变化规律。研究结果表明,川西糜棱岩Si的溶解速率在100~350℃范围内与水-岩作用温度呈正相关,350℃时溶解速率是100℃的10.97倍,而Na、Ca、K、Al和Mg等金属元素的溶解速率远低于Si。在长期水-热腐蚀下,糜棱岩Si元素的大量析出和硅酸盐矿物的分解会破坏岩石的孔隙结构,伴随微孔、中孔的萌生和发育,糜棱岩总孔体积由25℃时的0.00541cm^(3)·g^(-1)升高到350℃的0.00659cm^(3)·g^(-1),且比表面积随热处理温度的升高呈上升趋势,这会加强水与糜棱岩的接触,从而导致岩石的进一步劣化。本项研究结果为探究糜棱岩在亚临界水环境下的水-热腐蚀机制和地热流体影响断层稳定性提供了一定的见解。

广东河台金矿田糜棱岩磁组构对剪切带变形与成矿的约束

河台金矿田是广东省内规模最大的金矿,由于金矿床都产于糜棱岩带中,因而被认为是典型的与剪切带有关的金矿。然而,对于Au矿化与剪切带变形条件之间的关系目前仍不十分清楚。高村和云西是河台金矿田两个代表性的金矿床,分别受ML11和ML9两条剪切带的控制。本文在对两条剪切带中的含矿和无矿糜棱岩的变形特征进行野外调查和室内岩(矿)相学观察的基础上,分别开展岩石磁学和磁化率各向异性(AMS)特征研究。糜棱岩x-T曲线显示了磁黄铁矿的居里温度,结合磁滞回线、等温剩磁(IRM)以及矿相学特征,表明河台金矿田糜棱岩几乎不含磁铁矿,载磁矿物主要为黑云母和磁黄铁矿,但是磁黄铁矿的贡献要远大于黑云母。两条糜棱岩带中不含矿糜棱岩的平均磁化率K_(m)<500×10^(-6)(SI)、磁化率各向异性度P_(j)、椭球体形状参数T、磁面理F和磁线理L没有明显差别。并且形成两条剪切带的主应力方向也近于一致,最大主应力方向σ_(1)(K_(3))为近水平NNW-SSE向,最小主应力σ_(3)(K_(1))为NEE-SWW向,倾伏角较缓。运动学涡度W_(k)值也近似,在0.12~0.65之间,小于0.71。上述特征暗示两条含金剪切带都是在印支期NNW-SSE向近水平的强烈挤压作用下形成的,其中发育的糜棱岩为S型和SL型构造岩,具有强烈压扁的特征,并且在剪切变形过程中纯剪切作用所占的比重要明显大于简单剪切。然而,两条剪切带中含矿糜棱岩与无矿糜棱岩的AMS特征有明显的区别。含矿糜棱岩的K_(1)近于直立,且K_(m)>500×10^(-6)(SI)和P_(j)值都明显大于无矿糜棱岩。因此,糜棱岩的变形强度与含矿性具有一定的正相关关系,而在燕山期叠加的脆韧性变形使得最大磁化率主轴(即最小主应力轴)变陡,更有利于热液的运移和最终成矿。

广东省德庆县长泽金矿中硫化物特征与金富集规律浅析

广东省德庆县长泽金矿是位于广东省河台金矿田内的蚀变糜棱岩型金矿床。该矿床当前处于普查阶段,研究总结工作较少,尤其是对糜棱岩中与金品位关系最为密切的硫化物特征分析较为有限。本次为总结矿区硫化物特征与金含量的关系,通过对采集不同金含量糜棱岩进行光片鉴定和化学分析,研究不同金含量糜棱岩中硫化物的分布、种类、含量、大小等特征差异,总结规律,指导后期找矿。

糜棱岩化过程中矿物变形温度计

对有效确定中—低温下糜棱岩变形温度一直以来都没有比较理想的方法,而在研究韧性剪切带过程中对其变形温度的确定又常是必不可少的。根据近年来国际上对天然石英、长石、方解石等矿物变形的研究成果,总结了利用矿物变形指示变形温度的方法。在不同的温度条件下,长石与石英的变形方式具有阶段性,其变形与动态重结晶型式与温度具有明显的对应关系。石英变形中的滑移系及其C轴组构图主要受变形温度的控制。低温变形中的方解石e双晶纹形态也与温度呈密切的相关性。观测这些矿物变形的显微构造,可以很好地估计韧性剪切带糜棱岩化过程中的变形温度。

论糜棱岩的分类

糜棱岩是在构造应力作用下发生明显塑性变形、具有明显的运动指向标志、集中呈带状产出的构造岩。根据实用性和目的性建立庆棱岩的描述性和成因两个分类系列。在描述性分类中，将糜棱岩分为细粒糜棱岩、中粒糜棱岩和粗粒糜棱岩３个类型。各类型再进一步分为均匀、均匀碎斑和非均匀碎斑３个亚类。在成因分类中，将糜棱岩分为低温脆一韧性糜棱岩、中温韧性糜棱岩和高温超韧性糜棱岩３个类型，再分为低温石英脆一韧性糜棱岩、中温石英韧性糜棱岩、中温石英一斜长有韧性糜棱岩及高温斜长石一钾长石超韧性糜棱岩、高温钾长石一辉石超韧性糜棱岩、高温辉石－橄榄石超韧性糜棱岩６个亚类。

越城岭花岗岩体西侧滑脱型韧性剪切带的发现及其形成的构造体制

越城岭花岗岩体西侧片麻状花岗岩带主体是一条由韧性剪切作用形成的复合糜棱岩带,包括早期高绿片岩相条件下形成的糜棱片麻岩和晚期低绿片岩相条件下形成的糜棱岩。高绿片岩相韧性剪切带和低绿片岩相韧性剪切带具有基本一致的运动学性质,为滑脱型韧性剪切带,总体为在向NWW缓倾的糜棱面理上向SW方向滑动,并在平面上表现为左旋效应。根据构造对比研究,认为越城岭西侧滑脱型韧性剪切带形成于后造山阶段的伸展构造体制。

广宁深层次推覆构造前缘陡倾带变形构造研究

河台含金韧性剪切带是广宁深层次推覆构造前缘的一个陡倾带，它又是继推覆剪切之后再行右旋走滑剪切的复合带。首先解析了河台带中推覆与走滑两期构造变形的存在，并研究了带中的变形特征和运动规律及估算了几个主要糜棱岩带的水平走滑距离。通过显微镜和透射电镜，还研究了糜棱岩的显微、超显微构造，估算了变形的差异应力和应变速率。

中天山地块南北两缘的韧性剪切带

本文详细描述了天山中段沿乌鲁木齐—库尔勒公路后峡—乌瓦门段的构造变形特点。综合室内外观察研究结果 ,在中天山地块南、北缘 ,及内部的乌拉斯台地区鉴别出三条规模较大的韧性剪切带 :中天山北缘剪切带前人已有研究 ,出露宽达 10 km的右行斜冲剪切带 ,见大量糜棱岩、超糜棱岩 ;乌拉斯台剪切带中的糜棱岩宽超过 2 0 0 m,具有左行走滑运动特点 ;中天山地块南缘剪切带宽度达 5 km,内部发育大量闪长质糜棱岩、超糜棱岩 ,运动方式为左行斜冲。中天山块体南北两侧的脆性边界断层与糜棱岩带变形中心基本重合。这些剪切带具有相当大的走滑位移量 ,指示天山各地质单元间目前的空间关系可能是走滑拼接的结果。研究天山造山带内剪切带的展布、运动方式、总变形量 ,对于恢复各地质单元的初始位置。

郯庐断裂带肥东段走滑运动的^(40)Ar/^(39)Ar法定年

郯庐断裂带在华北与华南板块碰撞之后是否发生过大规模左行平移及其准确的时间,仍然是存在着争议的重要问题。郯庐断裂肥东段地表出露了大规模的、北北东走向的韧性剪切带。野外构造、显微构造与石英C轴组构分析皆指示为左旋走滑韧性剪切带。糜棱岩中变形矿物组合与矿物变形行为指示该韧性剪切带形成于中—高绿片岩相环境。通过对该段走滑糜棱岩中角闪石与黑云母的40Ar/39Ar年代学研究,本次工作中获得了一个角闪石(N14)的40Ar/39Ar坪年龄为143.3±1.3Ma(早白垩世初),据变形温度判断其代表了该断裂带左行平移中的变形年龄。该糜棱岩(N14)中新生的黑云母40Ar/39Ar坪年龄为134.1±0.6Ma,而同一采场中另一糜棱岩(N13)中新生黑云母40Ar/39Ar坪年龄为130.3±0.6Ma,皆指示了左行平移活动的冷却年龄。另外4处走滑糜棱岩中黑云母的40Ar/39Ar坪年龄分别为137.2±0.8Ma(N47)、135.6±0.6Ma(N17)、124.8±0.7Ma(N21)和125.9±0.4Ma(N22),也都属于冷却年龄,反映了该走滑韧性剪带内的不均匀冷却现象。由此变形年龄与冷却年龄可以判断该断裂带的左行平移持续时间不超过6Ma左右。由N14糜棱岩中的角闪石与黑云母坪年龄得到该处的平均冷却速率为21.7℃/Ma,属于较快速的冷却。本次40Ar/39Ar测年结果。

浙西南八都群变火山岩系及变晶糜棱岩的同位素年龄及其构造意义

浙西南八都群变火山岩系的Ｓｍ－Ｎｄ全岩等时线参考年龄为２０１４±４３Ｍａ。其中变质玄武岩具有扁平的ＲＥＥ模型。这说明在早元古，华夏古陆张裂并发生一次陆壳增生事件。对该区与具有逆掩断层性质的Ｍ１大型韧性剪切带有关的含石榴石变晶糜棱岩给出１５１８±１６Ｍａ和１００１±１４Ｍａ的Ｓｍ－Ｎｄ和Ｒｂ－Ｓｒ石榴石十全岩等时线年龄。前者指示原含石榴石变质岩的变质时代；后者指示的是糜棱岩化时代。它们可能分别与中元古时华夏古陆的再次张裂，及晚元古时龙泉群沉积盆地闭合有关。