A Large Ductile Sinistral Strike-Slip Shear Zone and Its Movement Timing in the South Qilian Mountains, Western China

There is a large ductile shear zone, 2 km wide and more than 3SO km long, in the South Qilian Mountains, western China. It is composed of volcanic, granitic and calcareous mylonites. The microstructures of the ductile shear zone show nearly E-W extending subvertical foliation, horizontal and oblique stretching lineations, shearing sense from sinis-tral to oblique sinistral strike-slip from east to west, "A" type folds and abundant granitic veins. Measured lattice preferred orientations (LPOs) of the mylonitic and recrystallized quartz of the granitic mylonite in the west segment suggest a strong LPO characterized by the dominant slip systems {1010} formed at high temperature (>650℃). K-feldspar of the mylonite shows an 39Ar/40Ar high-temperature plateau age of 243.3±1.3 Ma, and biotite, 250.5±0.5 Ma, which represent the formation age of the ductile shear zone. The 39Ar/40Ar plateau ages of 169.7±0.3 Ma and 160.6±0.1 Ma and the 39Ar/40Ar isochron ages of 166.99±2.37 Ma and 160.6±0.1 Ma of biot     

北祁连南缘右行韧性走滑剪切带的同位素年代学及其地质意义

北祁连南缘右行韧性走滑剪切带位于祁连地块与北祁连俯冲碰撞杂岩带边界 ,长约 80 0km ,走向NWW SEE ,面理向北陡倾 ,中西部宽 5～ 6km ,东部由四条呈帚状撒开的强应变带组成。构造指向及向南东低角度倾伏的拉伸线理揭示出韧性剪切带的右行走滑和转换挤压性质。TIMS法测定的单颗粒锆石U Pb上交点年龄为 96 5～ 95 6Ma ,代表韧性剪切带原岩———基底变质岩的变质时代。糜棱岩中钾长石、黑云母单矿物40 Ar/ 3 9Ar同位素测年结果及与地层和岩浆活动的关系表明韧性剪切带形成于 4 4 0～ 380Ma。北祁连南缘右行韧性走滑剪切带是在祁连加里东造山带形成过程中 ,祁连地块与阿拉善地块间斜向碰撞诱发大规模转换挤压作用的产物。     

The onset of the Tan–Lu fault movement in eastern China: constraints from zircon (SHRIMP) and 40Ar/39Ar dating

In eastern China, the Dabie Shan–Su–Lu orogenic belt has been separated by the Tan–Lu sinistral strike–slip fault. Mylonites are exposed along the strike–slip fault system in the southern segment, and along the eastern margin of the Dabie Shan orogenic belt. The country rocks of the mylonites are retrograde UHP eclogites, gneissic granites, muscovite granites and gneisses. The ductile strike–slip shear zone trends 30–40°N (NE30–40°‐trending) and exhibits stretching lineations and nearly vertical, SE‐dipping foliations. Most of the zircon grains separated from mylonites have a weighted average radiometric age of 233 ± 6–225 ± 6 Myr. These data constrain the onset of the Tan–Lu sinistral strike–slip movement and imply that the Tan–Lu sinistral strike–slip motion developed after retrograde UHP metamorphism. The related phengite within the eclogite rocks on the western side of the Tan–Lu fault, with ⁴⁰Ar/³⁹Ar plateau ages of c. 182–190 Myr, is also deformed and aligned parallel to the almost NE trending stretching lineations. Non‐metamorphosed granites exhibit sinistral strike–slip shearing and indicate that the Tan–Lu fault initially developed after 182–190 Myr. Muscovite collected from the mylonite yields ⁴⁰Ar/³⁹Ar plateau ages of 162 ± 1–156 ± 2 Myr. The zircon SHRIMP age data, the muscovite ⁴⁰Ar/³⁹Ar plateau ages, together with structural and petrological field information support the interpretation that the Tan–Lu strike–slip fault was not related to the Yangtze–north China plates collision, but corresponded to the formation of a NE‐trending tectonic framework in eastern China starting c. 165–160 Ma.     

郯庐断裂带张八岭隆起北段的左旋走滑挤压变形及其~(40)Ar/~(39)Ar定年

郯庐断裂带张八岭隆起北段,自西向东分别出露北北东向的韧性滑脱变形带、脆-韧性过渡带和脆性的前陆褶断带。韧性滑脱变形带内的张八岭群片岩,广泛发生了低绿片相背景下的糜棱岩化。其中呈现为平缓的糜棱面理和近南北向的矿物拉伸线理。显微构造及石英C轴组构分析显示,该韧性滑脱带一致为上盘向南的运动。该带以东依次变为上盘向南南东→南东的逆冲活动,总体上为左旋走滑挤压变形带。张八岭群所在的韧性变形带为深部陡立走滑构造与浅部脆性构造之间的滑脱变形带,其间的差异走滑变形,造成了该滑脱层在总体向北运动中出现上盘向南的剪切变形。对6处张八岭群片岩中15个不同粒级白云母的40Ar/39Ar定年指示,变形发生在(236.2±0.5)~(238.0±0.4)Ma的中三叠世晚期。这表明郯庐断裂带的左行平移发生在华北与华南板块碰撞的深俯冲阶段,起源于陆内转换断层。     

Geochronology of Gold Deposits and Its Implication for Metallogenesis in the Fengxian-Lixian Area, Qinling Orogenic Belt, China

Abstract: A series of super large‐scale and large‐scale Pb and Zn, and Au deposits are distributed in the Qinling orogenic belt, China. Gold deposits were generally ascribed to Carlin‐type originated from circular meteoric water. Visible and coarse‐grained gold (up to over 3mm in grain size) was recently identified in some gold deposits in the Fengxian‐Lixian area, Qinling. Au‐bearing quartz lodes related to magmatism were discovered in the Xiaogouli gold deposit. Two types of Au‐bearing quartz veins, i.e., NW‐trending quartz veins and NE‐trending quartz veins cutting strata are widely present in the Baguamiao gold deposit. Both are spatially associated with each other. The former is generally snake–like, S‐shape or zigzag, which was resulted from plastic deformation by ductile shearing, being generally cut by the latter. The latter is generally linear with widely developed bleaching alteration zones in its adjacent wall rocks, which symbolizes the superimposition of brittle deformation and filling and metasomatism of magmatic hydrothermal solution in ductile shear zones after uplifting of the shear zones near the surface. The NW‐trending quartz veins contain Au of lower than 3ppm. The NE‐trending quartz veins contain Au of more than 3 ppm, so that NE‐trending quartz veins and the adjoining altered rocks are important ores. The NW‐trending Au–bearing quartz vein was dated as 210.61.26 to 232.581.59 Ma by ⁴⁰Ar–³⁹Ar method, i.e., late Indosinian epoch (Triassic). The NE‐trending Au–bearing quartz vein was dated as 131.910.89 to 197.451.13 Ma by ⁴⁰Ar–³⁹Ar method, i.e., Yanshanian epoch (Jurassic). The ⁴⁰Ar–³⁹Ar age of the NW‐trending Au–bearing quartz veins represents the age of the ductile shear formation. The isotope data of the NE‐trending quartz veins indicate that gold mineralization was closely related to Indosinian and Yanshanian granite intrusives not only in time and space, but also in origin.     

阿拉善地块晚奥陶世—石炭纪的构造演化历史——来自北大山地区多期岩浆-变质-变形事件的约束

阿拉善地块西部的北大山地区识别出两期韧性变形构造,早期的韧性变形以从南(南东)向北(北西)逆冲为特征,晚期的韧性变形以近东西向左行剪切为特征。利用LA-ICP-MS锆石U-Pb测年方法,在记录早期韧性变形的片麻岩中获得锆石的变质年龄为422±1 Ma,代表晚志留世变质事件;在记录晚期韧性变形的2件二长花岗岩中获得锆石结晶年龄分别为326.2±1.2 Ma和323.8±2.6 Ma,代表早石炭世岩浆活动。结合前人研究成果,发现阿拉善地块西部北大山地区的两期韧性变形特征、变形时代与阿拉善地块东部巴彦乌拉山-狼山地区相似,同时阿拉善地块东、西部晚奥陶世-石炭纪多期岩浆活动的岩石类型、期次、地化及Nd同位素等特征也非常相似。以上特征表明,阿拉善地块东、西部经历了相同的构造演化历史,形成一条发育在早前寒武纪变质基底之上、呈近东西向弧形展布的晚奥陶世-石炭纪构造-岩浆岩带(西起桃花拉山,经北大山、诺尔公-巴彦乌拉山,东到狼山),其成因与古亚洲洋的俯冲造山事件密切相关。     

Structure,Timing, and Mechanism of the Pliocene and Late Miocene Uplift Process of the Ailao Shan-Diancang Shan,SE Tibet,China

The uplift of the Ailao Shan-Diancang Shan(ASDS) along the Ailao Shan-Red River(ASRR) shear zone is an important geological event in the southeastern margin of Qinghai-Tibet Plateau tectonic domain in the Late Cenozoic,and it preserves important information on the structures,exhumational history and tectonic evolution of the ASRR shear zone.The uplift structural mode and uplift timing of the ASDS is currently an important scientific topic for understanding the ASDS formation and late stage movements and evolution of the ASRR shear zone.The formation of the ASDS has been widely considered to be the consequence of the strike-slip movements of the ASRR shear zone.However,the shaping of geomorphic units is generally direct results of the latest tectonic activities.In this study,we investigated the timing and uplift structural mechanism of the ASDS and provided the following lines of supportive evidence.Firstly,the primary tectonic foliation of the ASDS shows significant characteristic variations,with steeply dipping tectonic foliation developed on the east side of the ASDS and the relatively horizontal foliation on the west side.Secondly,from northeast to southwest direction,the deformation and metamorphism gradually weakened and this zone can be further divided into three different metamorphic degree belts.Thirdly,the contact relationship between the ASDS and the Chuxiong basin-Erhai lake is a normal fault contact which can be found on the east side of the ASDS.40Ar/39 Argeochronology suggests that the Diancang Shan had experienced a fast cooling event during 3–4 Ma.The apatite fission track testing method gives the age of 6.6–10.7 Ma in the Diancang Shan and 4.6–8.4 Ma in the Ailao Shan,respectively.Therefore the uplift of the ASDS can be explained by tilted block mode in which the east side was uplifted much higher than the west side,and it is not main reason of the shearing movements of the ASRR shear zone.The most recent uplift stages of the ASDS happened in the Pliocene(3–4 Ma) and Late Miocene(6–10 Ma).     

西昆仑库地韧性剪切带的40Ar/39Ar年龄

西昆仑库地以南有一套变质变形较强的岩系,前人依照区域对比关系将其划为前寒武的古老基底。对西昆仑早期构造演化的论述均基于该观点,但没有提供确凿的同位素年代学证据。笔者通过野外观察、室内研究,确认库地以南的变质变形岩系是大型韧性推覆剪切作用的产物。通过对新生变质矿物角闪石和黑云母单矿物的⁴⁰Ar/³⁹Ar年龄分析,确定剪切变质年龄为426-451Ma,说明库地的变质变形岩系是形成于早古生代晚期的一条大型韧性剪切带,这对于解释西昆仑的早期构造演化具有重要意义。     

华北克拉通中部带中段古元古代构造格架与主要变形事件研究

古元古代是华北克拉通构造演化的关键时期,中部带的性质是近年来华北克拉通构造演化研究争论的焦点之一。文中根据古元古代韧性剪切带重新划分了中部带北段构造格架,即由近EW向朱家坊韧性剪切带、NEE向王家会韧性剪切带和NE向龙泉关韧性剪切带分割恒山、繁峙、五台和阜平地块。这些地块基底具有不同的前寒武纪变质杂岩组成、构造变形特征和构造演化历史,一般都具多期变质变形叠加的特征。研究表明华北克拉通中部构造带北段在古元古代末发生3次重要的构造变形事件：繁峙地块由SW向NE方向的构造挤入,挤入过程中导致五台地块变形并向南东方向逆冲(D1);以阜平地块为核部的大规模伸展抬升,阜平变质杂岩抬升至近地表,阜平地块与五台地块之间形成一系列低角度伸展型韧性剪切带(D2);最后一次构造事件是NE向拉伸导致的大量NW向基性岩墙群侵位(D3)。     

Triassic high-strain shear zones in Hainan Island (South China) and their implications on the amalgamation of the Indochina and South China Blocks: Kinematic and Ar/Ar geochronological constraints

A kinematic and geochronological study has been carried out on the Triassic high-strain shear zones in Hainan Island, the southern South China Block. There are WNW- and NE-trending high-strain shear zones with greenschist- to amphibolite-facies metamorphism in this island. Kinematic indicators suggest a dextral top-to-the-NNE thrust shearing for the WNW-trending high-strain shear zones and a sinistral top-to-the-SE thrust shearing for the NE-trending shear zones. The quartz c-axis orientations of mylonitic rocks exhibit the domination of basal slip and some activation of a rhombohedra gliding system. The timing of shearing for these shear zones has been constrained by the ⁴⁰Ar/³⁹Ar dating analyses of synkinematic minerals. Middle Triassic (242–250 Ma) and late Triassic–early Jurassic (190–230 Ma) have been identified for the WNW- and NE-trending shear zones, respectively. A synthesis of these kinematic and thermogeochronological data points to a two-stage tectonic model for Hainan Island, that is, top-to-the-NNE oblique thrusting at 240–250 Ma followed by top-to-the-SE oblique thrusting at 190–230 Ma. In combination with the available data from the southern South China and Indochina Blocks, it is inferred that South Hainan and North Hainan have affinity to the Indochina and South China Blocks, respectively. The tectonic boundary between South Hainan and North Hainan lies roughly along the WNW-trending Changjiang–Qionghai tectonic zone probably linking to the Song Ma and Ailaoshan zones. The middle Triassic structural pattern of Hainan Island is spatially and temporally compatible with those of the South China and Indochina Blocks, and thus might be a derivation from the amalgamation of the Indochina with South China Blocks in response to the closure of the Paleotethys Ocean and subsequent subduction/collision.     

阿拉善地块中元古代诺尔公群的构造变形特征

阿拉善地块作为华北板块的重要组成部分及关键的构造单元,经历了不同时期构造热事件的叠加作用。本文选取阿拉善地块巴音诺尔公地区出露的中元古代诺尔公群进行研究,总结了这些前寒武纪岩石的几何学、运动学及显微构造特征,并对其构造格架、变形期次进行了初步划分。研究表明,中元古代诺尔公群内部至少存在两期变形,早期变形表现为原始层理的片理化过程,形成了区域走向近NE-SW向的片理;而晚期变形则表现为顺片理走向的右行走滑剪切作用。结合前人已有研究成果及年代学资料,认为中元古代诺尔公群的内部变形主要受到石炭纪-二叠纪查干础鲁弧后盆地向阿拉善微陆块之下俯冲作用的控制,俯冲作用造成了NW-SE向的挤压,从而发生了早期片理化,而随后顺俯冲带走向的走滑作用则调节了挤压造成的缩短。     

内蒙古呼和浩特变质核杂岩韧性拆离带40Ar-39Ar定年及其构造含义

晚中生代,内蒙古大青山依次经历晚侏罗世盘羊山逆冲推覆、早白垩世呼和浩特变质核杂岩伸展、早白垩世大青山逆冲推覆断层及早白垩世以来高角度正断层复杂构造演化。其中,呼和浩特变质核杂岩韧性剪切带的冷却时间和抬升机制的制约尚不明确。本文在野外考察和显微构造分析基础上,采用逐步加热40Ar-39Ar定年法对韧性剪切带内不同单矿物的冷却年龄进行了测定。角闪石、白云母、黑云母和钾长石单矿物40Ar-39Ar冷却年龄处于120~116Ma之间。结合已有年龄数据及单矿物封闭温度,构建了韧性剪切带的冷却曲线。结果表明,韧性剪切带在122~115Ma期间存在一个明显的快速冷却过程。这一阶段快速冷却是与变质核杂岩拆离断层相关核部杂岩拆离折返作为大青山逆冲推覆断层上盘抬升的结果。     

青藏高原东北缘宗务隆韧性剪切带40Ar/39Ar年代学及对印支期造山时限的制约

青藏高原东北缘的宗务隆构造带位于柴达木地块与南祁连地块之间,位置极为重要,其中发育的韧性剪切带变形特征和形成时代对于理解该构造带的构造属性具有重要的制约意义。详细的野外构造解析,显微构造解析与年代学研究表明,宗务隆剪切带发育走向NWW-SEE向糜棱面理,其上发育NWW-SEE向缓倾的拉伸线理,指示该剪切带逆冲-走滑剪切的特征。宏观尺度上可见由于剪切作用形成的不对称褶皱、旋转碎斑、构造透镜体及褶劈理等变形形迹;显微镜下可观察到云母鱼、S-C组构、σ型残斑及石英动态重结晶、拔丝构造等变形现象,指示该韧性剪切变形的温度在300~400℃。对剪切带同构造变形的白云母和黑云母进行了⁴⁰Ar/³⁹Ar同位素年代学分析,2个样品的坪年龄分别为(245.8±1.7)Ma、(238.5±2.6)Ma,指示了该剪切变形发生在早—中三叠世期间。结合对区域地质、岩石学等资料的综合分析,该期韧性剪切变形年龄代表了宗务隆构造带印支期造山作用的时间,这期造山活动可能与宗务隆有限洋盆闭合后,南祁连地块与欧龙布鲁克地块的斜向碰撞有关。     

早古生代阿拉善地块与华北地块之间的关系:来自阿拉善东缘中奥陶统碎屑锆石的信息

阿拉善东缘奥陶纪地层位于鄂尔多斯(华北地块)与北祁连早古生代造山带之间的过渡地区,该区的构造背景一直是长期争论的问题,它涉及到阿拉善地块是否与华北地块相连、奥陶系的物源以及"贺兰拗拉槽"是否存在等问题。分布于阿拉善地块东缘的中奥陶统米钵山组的碎屑锆石LA-ICP-MSU-Pb年龄测试表明,样品中数量最多的锆石年龄为900～950Ma,Alxa-1的峰值年龄为916Ma,Alxa-2的峰值年龄为953Ma,次者在494～623Ma之间,这个区间内存在多个峰值,如Alxa-1存在505Ma和588Ma两个主要峰值,Alxa-2则存在494Ma、517Ma、623Ma等几个峰值。在2.5Ga左右两个样品都存在一个弱的峰值,Alxa-1峰值为2517Ma,而Alxa-2峰值为2552Ma和2670Ma。除此之外,两个样品都有个别大于3.0Ga的成分,Alxa-1样品中最年轻的锆石为451±8Ma,Alxa-2样品则为483±4Ma。这些年龄以及沉积特征表明:(1)传统认为的奥陶纪"贺兰拗拉槽"并不存在,鄂尔多斯西南缘地区以及阿拉善东部地区当时属于北祁连早古生代周缘前陆盆地系统;(2)早古生代主要物源来自北祁连造山带,新元古代物源来自阿拉善地块;(3)鄂尔多斯西缘整个米钵山组的锆石年龄分布及其变化,指示出北祁连造山带(岛弧)逐渐靠近阿拉善地块,其间洋盆逐渐消失的过程;(4)阿拉善地块基底与华北有明显差别,阿拉善地块明显受到新元古代和古生代构造热事件的影响,两者可能是在中奥陶世或之后才拼贴在一起。     

An 40Ar–39Ar investigation of the Mertz Glacier area (George V Land,Antarctica): Implications for the Ross Orogen–East Antarctic Craton relationship and Gondwana reconstructions

George V Land (Antarctica) includes the boundary between Late Archean–Paleoproterozoic metamorphic terrains of the East Antarctic craton and the intrusive and metasedimentary rocks of the Early Paleozoic Ross–Delamerian Orogen. This therefore represents a key region for understanding the tectono-metamorphic evolution of the East Antarctic Craton and the Ross Orogen and for defining their structural relationship in East Antarctica, with potential implications for Gondwana reconstructions. In the East Antarctic Craton the outcrops closest to the Ross orogenic belt form the Mertz Shear Zone, a prominent ductile shear zone up to 5 km wide. Its deformation fabric includes a series of progressive, overprinting shear structures developed under different metamorphic conditions: from an early medium-P granulite-facies metamorphism, through amphibolite-facies to late greenschist-facies conditions. ⁴⁰Ar–³⁹Ar laserprobe data on biotite in mylonitic rocks from the Mertz Shear Zone indicate that the minimum age for ductile deformation under greenschist-facies conditions is 1502 ± 9 Ma and reveal no evidence of reactivation processes linked to the Ross Orogeny. ⁴⁰Ar–³⁹Ar laserprobe data on amphibole, although plagued by excess argon, suggest the presence of a ∼1.7 Ga old phase of regional-scale retrogression under amphibolite-facies conditions. Results support the correlation between the East Antarctic Craton in the Mertz Glacier area and the Sleaford Complex of the Gawler Craton in southern Australia, and suggest that the Mertz Shear Zone may be considered a correlative of the Kalinjala Shear Zone. An erratic immature metasandstone collected east of Ninnis Glacier (∼180 km east of the Mertz Glacier) and petrographically similar to metasedimentary rocks enclosed as xenoliths in Cambro–Ordovician granites cropping out along the western side of Ninnis Glacier, yielded detrital white-mica ⁴⁰Ar–³⁹Ar ages from ∼530 to 640 Ma and a minimum age of 518 ± 5 Ma. This pattern compares remarkably well with those previously obtained for the Kanmantoo Group from the Adelaide Rift Complex of southern Australia, thereby suggesting that the segment of the Ross Orogen exposed east of the Mertz Glacier may represent a continuation of the eastern part of the Delamerian Orogen.     

白云鄂博矿区北部韧性剪切带特征及其构造意义

在白云鄂博主矿北西约6.5km处的查干楚鲁一带,有一条近东西向延伸的断层带,前人在该带中部识别出了蛇绿岩混杂堆积。本文作者在蛇绿岩东部的大理岩质糜棱岩中发现鞘褶皱、拉伸线理及旋转碎斑系等变形构造,在大理岩周围发现石英岩型宏观压力影构造,在变质石英砂岩中识别出褶劈理以及方解石脉等相关构造,在这条东西向断裂带北边发现典型的黄铁矿型压力影。根据上述特征判断,查干楚鲁一带为一条近东西向延伸的韧性剪切带,同时具有NEE—SWW向逆左旋走滑的特征。该带揭示出蒙古洋板块与华北板块在晚古生代汇聚拼合的基本格局。     

扬子地块与南秦岭造山带的盆山系统与构造耦合

本文重新厘定了扬子地块西北缘晚古生代至早中生代沉积盆地的原型,在综合分析南秦岭造山带和勉略缝合带形成规律的基础上,对于南秦岭造山带与扬子地块北缘的拼合演化历史以及盆山耦合关系进行了研究。指出在晚二叠世晚期(长兴组沉积上段)和早三叠世早期(飞仙关组沉积下段)发生点式碰撞,在两个不同的大地构造单元之间形成了与碰撞相关的裂谷盆地群(包括开江-梁平裂谷、城口-鄂西裂谷和东部的当阳裂谷等),碰撞裂谷群的持续演化时间为5~6Ma,这一阶段典型的沉积标志为水下早期阶段形成的海相磨拉石层序。至早三叠世的嘉陵江二段沉积时期,两个不同地块的持续拼合导致大巴山和米苍山地区与周缘前陆盆地相关的古冲断带的形成,该阶段在缝合带接触部位发育角度不整合和河流相沉积,扬子地块其余大部仍然是保持连续的海相碳酸盐岩沉积。晚三叠世南秦岭造山带与扬子北缘之间的残余大洋消失,为整体闭合的碰撞后期阶段,沉积了须家河组开始的陆相碎屑岩系,大巴山和米苍山地区进入到了以陆相磨拉石为主的前陆盆地阶段,在扬子北缘形成了神农架-黄陵隆起和米苍山隆起。晚三叠世以后大巴山和米苍山地区进入了比较复杂的后期改造阶段,产生了多期的收缩性构造活动,包括以形成区域性的假整合和小角度不整合为特征的晚侏罗世-早白垩世早期(J₃-K₁)的低幅度活动期;以大巴山和米苍山冲断带的强烈改造为主,形成薄皮冲断构造系统的早白垩世晚期变形和以形成大巴山弧形冲断带和米苍山基底卷入的冲断带为特征的新生代晚期变形。     

Microstructure,metamorphism, thermochronology and P-T-t-deformation history of the Port aux Basques gneisses,south-west Newfoundland,Canada*

Abstract The Port aux Basques gneisses comprise three lithostratigraphic units separated by major fault zones: the Grand Bay Complex; the Port aux Basques Complex; and the Harbor le Cou Group. A similar regionally developed polyphase history of penetrative deformation characterizes each of these units. Thickening during D1 produced rare recumbent folds (F1) and an axial planar schistosity (S1), overprinted by D2 recumbent folds (F2), and transposed during development of a locally penetrative, differentiated crenulation cleavage (S2). In western sectors of the area, D2 was associated with NW-directed reverse shearing. The NE-trending structural grain reflects D3 transpression, partitioned into dextral transcurrent movement along major shear zones and development of upright-to-steeply inclined, periclinal folds (F3) and a variably penetrative schistosity (S3). Amphibolite facies metamorphism increases in grade from west to east across the area. Microstructures, including porphyroblast-matrix foliation relations and internal textural unconformities in garnet, indicate episodic porphyroblast nucleation and growth, which reflect a prograde traverse sequentially across univariant reactions during syntectonic metamorphism. Garnet, kyanite and staurolite porphyroblasts are wrapped by the S2 foliation, but each may contain trails of inclusions that define S1; commonly these trails preserve early stages of S2 crenulation cleavage development. Progressive and sequential reaction out of kyanite, staurolite and muscovite in favour of sillimanite, garnet, biotite and K-feldspar, and the development of an increasing volume of anatectic migmatite in south-eastern sectors of the area record syn- to late-D2 peak metamorphic conditions. Microstructural relationships and petrogenetic grid considerations indicate clockwise trajectories in P-T space for units of the Port aux Basques gneisses. Peak metamorphic conditions are estimated to have been 620–650° C at ≤8kbar in the west and 700–750° C at ≤8 kbar in the east. Titanite from an upper amphibolite facies calc-silicate gneiss yields U-Pb ages of c. 420 Ma, interpreted to date cooling shortly after the thermal peak in these gneisses. Variable D3 strain was associated with some recrystallization of hornblende and micas. ⁴⁰Ar/³⁹Ar hornblende plateau isotope correlation ages range from c. 419 to c. 393 Ma, from east to west across the area, and are interpreted to record cooling through c. 500° C coeval with or soon after D3 deformation. The range in ages may record the effects of heterogeneous D3 deformation and differential uplift from south-east to north-west associated with displacement on major shear zones. ⁴⁰Ar/³⁹Ar muscovite plateau ages cluster at c. 390 Ma, and date cooling through c. 375° C during regional exhumation. Cooling rates are moderate to fast and may indicate a component of tectonic exhumation. The Port aux Basques gneisses are a product of Silurian collisional tectonics. The higher grade of metamorphism in comparison with adjacent areas of the Canadian Appalachians is interpreted to reflect greater thickening due to juxtaposition of the St Lawrence promontory (Laurentian margin) with the Cabot promontory (Avalonian margin) during closure of the Iapetus Ocean.     

Biotite and muscovite <Superscript>40</Superscript>Ar–<Superscript>39</Superscript>Ar geochronological constraints on the post-Svecofennian tectonothermal evolution,Forsmark site,central Sweden

In order to characterize the post-Svecofennian tectonothermal evolution of the Fennoscandian Shield, ⁴⁰Ar–³⁹Ar biotite and some ⁴⁰Ar–³⁹Ar muscovite geochronological data are reported from a total of 30 surface outcrop and 1,000 m long borehole samples at Forsmark, central Sweden. The 13 surface samples were collected across 3 branches of a major WNW to NW trending system of deformation zones, whereas the boreholes were drilled within a tectonic lens, in between two of these zones. The ⁴⁰Ar–³⁹Ar biotite ages indicate that the present erosion surface, in central Sweden, cooled below c. 300°C at 1.73–1.66 Ga, and that the rocks could have accommodated strain in a brittle manner between 1.8 and 1.7 Ga. The variation in surface ages is suggested to be due to fault along the large WNW to NW trending deformation zones, following the establishment of a sub-Cambrian peneplain. The minor variation of ages within a single crustal block may be due to disturbance along ENE to NNE trending fracture zones. Possible cooling paths, derived from ⁴⁰Ar–³⁹Ar hornblende, muscovite and biotite ages, were calculated for the time interval from 1.80 to 1.67 Ga, when the area cooled from c. 500 to 300°C. Cooling rates of 1.9–4°C/m.y. have been attained. Between 1.68 and 1.64 Ga, uplift rates of c. 22 m/m.y. were calculated from borehole ⁴⁰Ar–³⁹Ar biotite data. Tectonothermal histories, inferred from the combined cooling and uplift rates, are related to simple cooling after the Svecofennian orogeny, to crustal movement in response to far-field effects of c. 1.7 Ga orogenic activities further to the west or to a combination of these possibilities.     

陕西小秦岭华阳川韧性剪切带的特征 及其区域构造意义

陕西小秦岭华阳川韧性剪切带发育在新太古界太华群之中,野外调研和显微构造观察结果表明,该韧性剪切带是由构造片岩、眼球状片麻岩组成的深层次韧性剪切带,具有逆冲兼左行走滑的斜冲特征。对韧性剪切带构造片岩黑云母进行^40Ar／^39Ar同位素定年,获得坪年龄为419M&#177;0．6Ma,反等时线年龄为417Ma&#177;0．8Ma。认为华阳川韧性剪切带及其相应的小秦岭区域主导构造变形是发生于419Ma左右的秦岭加里东事件的结果。