The geometry of ductile conjugate fold systems in the Ardnamurchan Moine,Scotland

Four ductile phases of superimposed deformation are regionally mappable in the Ardnamurchan Moine in the Western Highlands of Scotland. These deformations are recognized on the basis of their individual geometrical characteristics and are named, in ascending order, the Kentra phase (D₁), the Ardtoe phase (D₂), the Glenmore phase (D₃), and the Claish phase (D₄). Non-systematic earlier structures (pre-D₁), possibly coeval with the climactic metamorphism, are locally observable. Folds and fabrics produced by the D₂, D₁, and D₄ deformations, however, developed as conjugate sets. There is a significant spatial variation in the distribution and intensity of each of the D₁-D₄ phases. The regional deformation of the Moine schists is notably inhomogeneous due to the nature of the D₁-D₄ incremental strains. In the areas affected by large finite strains, pervasive, normally composite LS-fabrics are developed. In the areas that persistently suffered negligible strain increments, right-way-up rocks commonly display several mineral foliations lying at various angles to bedding. The conjugate systems of folds and fabrics appear to be related to the evolution of a ductile shear belt in Ardnamurchan.     

Structural evolution within the Luning–Fencemaker fold-thrust belt, Nevada: progression from back-arc basin closure to intra-arc shortening

The Luning–Fencemaker fold-thrust belt (LFTB) of central Nevada reflects major Mesozoic shortening in the western US Cordillera, and involved contractional deformation in Triassic and lower Jurassic back-arc basinal strata. Structural analyses in the Santa Rosa Range, in the northern LFTB, provide new insight into the evolution of this belt. Four phases of deformation are recognized in the Santa Rosa Range. D₁ involved tight to isoclinal folding, cleavage development under low-grade metamorphic conditions, and reverse faulting. This deformation phase reflects NW–SE shortening of 55–70% in the Early and/or Middle Jurassic. D₂ structures are similar in orientation to D₁ but involved much less overall strain and are well developed only to the southeast. D₂ appears to be related to thrusting along the eastern margin of the LFTB in the Middle and/or Late Jurassic. D₃ deformation reflects very minor shortening (<5%) in a subvertical direction, and is tentatively interpreted to reflect stresses generated during initial intrusion of mid-Cretaceous plutons in the area. D₄ deformation demonstrably occurred synchronously with emplacement of Cretaceous granitoids dated at 102 Ma (U–Pb zircon) based on syntectonic relations between D₄ structures and thermal metamorphism associated with intrusion, and an upgrade in D₄ strain in the thermally softened metamorphic aureoles of the intrusions. This last phase of deformation reflects minor regional NE–SW shortening, coupled with localized strain associated with pluton emplacement.Formation of the LFTB structural province was accomplished during the D₁ and D₂ phases of deformation, and most shortening occurred during the D₁ event. This Jurassic deformation led to structural closure of the back-arc basin by top-to-the-SE tectonic transport and development of a largely ductile fold-thrust belt. Subsequent deformation (D₃ and D₄) is >50 m.y. younger and unrelated to development of the LFTB. The younger deformation reflects a combination of minor regional shortening, interpreted to be related to the Cretaceous Sevier orogeny, plus localized shortening related to emplacement of Cretaceous intrusions.     

Episodicity of stress state in an overriding plate: Evidence from the Yalu River Fault Zone,East China

The ca. 700-km-long Yalu River Fault Zone (YRFZ) in East China, adjacent to the Pacific Ocean, underwent a polyphase evolution during the Cretaceous when it controlled the development of rift basins interrupted by several shortening events. The East China continent lies in an overriding plate with respect to the subducting Paleo-Pacific Plate during the Cretaceous. The YRFZ is ideal for studying the episodicity of stress state in the overriding plate. To constrain the polyphase evolution of the YRFZ, structural observations, fault-slip data measurements and LA–ICP–MS zircon U–Pb dating on Cretaceous volcanic rocks and sandstones were undertaken in this study. The first deformation (D₁) is characterized by sinistral strike-slip shear in the earliest Cretaceous. The D₂ event is featured by normal faulting deformation along the fault zone, which led to development of rift basins during the rest of the Early Cretaceous. Sinistral faulting (D₃) developed again in the earliest Late Cretaceous, followed by dextral normal faulting (D₄) and rift basin development during the rest of the Late Cretaceous, and finally reverse dextral faulting (D₅) at the end of the Cretaceous. The fault-slip data show that compressional directions during D₁, D₃ and D₅ faulting events are N–S, N–S and E–W respectively. Extensional directions during D₂ and D₄ faulting events are NW–SE and N–S. The zircon U–Pb ages indicate that the Early Cretaceous basins (D₂ event) controlled by the YRFZ were active between 131 and 100 Ma, and the Late Cretaceous basins (D₄ event) were active between 97 and 70 Ma. These U–Pb ages, together with previous geochronological data, show that the D₁ and D₃ episodes of compression each lasted 3 Ma, D₂ extension lasted 31 Ma, and D₄ extension 27 Ma. These data indicate an episodicity in the stress state with longer periods of extension and shorter periods of compression. A slab-driven model with relatively long periods of low-velocity subduction alternating with shorter periods of high-velocity subduction could account for the episodicity of stress state in the overriding plate from D₁ to D₅.     

从西南天山超高压变质带多期构造变形看天山古生代构造演化

通过对西南天山阿克雅孜和木扎尔特地区高压-超高压变质带构造几何学和岩石变形相关运动学的详细剖析,厘定出高压-超高压变质岩石及其相关围岩的构造单元。详细研究表明,研究区可划分为三个构造单元:北部单元、中部单元和南部单元。确定了每个构造单元的构造几何学特征及各个构造单元之间的相互关系。通过分析岩石变形特征和叠加关系,确定了岩石所记录多期变形的运动学特征。根据研究区的多期构造变形特点,建立了阿克雅孜和木扎尔特河地区构造演化序列。共划分出四期构造可识别的事件(E1-E4),分别代表了E1:高压-超高压岩石折返过程;E2:高压-超高压岩石造山带的早期改造过程;E3:北部构造事件对高压-超高压造山带影响;E4:走滑构造对高压-超高压造山带的叠加。沿造山带系列构造分析表明,西南天山高压-超高压带中发育的四期构造事件沿中天山北缘具有很好的一致性,各期构造事件也有一定的横向可对比性。在此基础上通过对多期变形事件的构造背景的探讨,建立了整个天山在古生代的构造拼合过程,揭示我国西部洋壳相关的深俯冲造山带形成过程和参与深俯冲作用(超)高压变质岩的变形变质历史。     

大别山东北缘桐城高压-超高压变质带的构造解析及其对郯庐断裂带的制约

通过对郯庐断裂带南段桐城地区高压-超高压变质带详细的岩石学和构造学研究,将研究区从空间结构上划分为三个构造单元:上部低温-高压单元、中部中温-高压单元和下部超高压单元。根据研究区多期构造变形分析,共识别出了五期有区域构造地质学含义的事件(D_1-D_5):D_1代表高压-超高压变质岩中-晚三叠世同碰撞早期折返过程;D_2表征了高压-超高压变质岩晚三叠世同碰撞晚期折返过程;D_3记录了早白垩世中大别变质核杂岩的形成,也即整个中国东部晚中生代大规模伸展构造在研究区的表现;D_4可能标志着郯庐断裂走滑构造对高压-超高压造山带的叠加;D_5表现为脆性正断作用,控制了晚白垩世-古近纪潜山半地堑盆地的形成。这些结果表明了研究区所经历构造演化的复杂性,其构造几何形态很难用郯庐断裂左行平移南大别超高压变质岩来解释,也不支持桐城地区存在巨大走滑作用的证据。     

Superposed deformation and inherited structures in an ancient dilational step-over zone: Post-mortem of the Rengali Province,India

In the eastern Indian shield, a dextral strike-slip system juxtaposed the Archaean Singhbhum Province against the Proterozoic Eastern Ghats Belt at ∼490–470 Ma. Two WNW–ESE trending strands of the strike-slip system enclose a multiply deformed (D₁ to D₃) intervening domain called the Rengali Province, with D₃ representing dextral shearing. In a granulite lens within the province, an early fabric (S_gr) was deformed by an amphibolite facies D₁–D₂ deformation continuum in the late Archaean time, forming cylindrical folds. In the surrounding quartzofeldspathic gneisses, quartzites and mica schists of the province, superimposition of syn-D₃ shortening on D₁-D₂ folds generated complex non-cylindrical geometries; the granulites escaped D₃ strain. Microstructures in the province-bounding shear zones confirm that D₃ deformation was associated with mylonitization, dynamic recrystallization and greenschist facies metamorphism. In the quartzites, syn-D₃ folds can be correlated with rotation of D₁–D₂ structures through the shortening zone of bounding dextral shears. Since the province-bounding shears form a step-over zone, the structural complexity within the Rengali Province arises from superposition of syn-D₃ shortening structures on initially asympathetically oriented inherited cylindrical D₁-D₂ folds. Hydrous fluid channeling causing greenschist facies metamorphism and quartz vein emplacement accompanied D₃ as the step-over zone was dilational in nature.     

Plate movements, ductile deformation and geochronology of the Sanbagawa belt, SW Japan: tectonic significance of 89–88 Ma Lu–Hf eclogite ages

Prograde P–T paths and thermal modelling suggest metamorphism in the Sanbagawa belt represents unusually warm conditions for subduction-type metamorphic belts, and these likely reflect conditions of a convergent margin a few million years before the arrival of an active spreading ridge. Radiometric age data and kinematic indicators of ductile deformation suggest the Sanbagawa belt formed in a Cretaceous convergent margin associated with a plate movement vector that had a large sinistral oblique component with respect to the belt, the East Asian margin. Plate reconstructions for the Cretaceous to Tertiary for this region show that the only plausible plate compatible with such motion at this time is the Izanagi plate. These reconstructions also show that progressively younger sections of the Izanagi plate were subducted beneath eastern Asia, i.e. a spreading ridge approached, until 85–83 Ma when the Izanagi Plate ceased to exist as an independent plate. The major reorganization of plates and associated movements around this time is likely to be the age of major interaction between the ridge and convergent margin. The ridge-approach model for the Sanbagawa metamorphism, therefore, predicts that peak metamorphism is a few million years older than this age range. New Lu–Hf dating of eclogite in the Sanbagawa belt gives ages of 89–88 Ma, in excellent agreement with the prediction. Combining this estimate for the peak age of metamorphism with published P–T-t results implies vertical exhumation rates of greater than 2.5 cm yr⁻¹. This high rate of exhumation can explain the lack of a significant thermal overprint in the Sanbagawa belt during subduction of the ridge.     

Multiple episodes of tectonothermal processes in the Eastern Ghats granulite belt

A suite of rocks from Borra Carbonate Granulite Complex (BCGC) in the Eastern Ghats granulite belt displays superposed structures and overprinted mineral assemblages that reveal multiple episodes of tectonothermal reworking of the complex under granulite facies condition. Five distinct episodes of deformation (D₁, D₂, D₃, D₄ and D₅) and four phases of metamorphism (M₁, M₂, M₃ and M₄) are recorded. The signature of the earliest tectonothermal event, D₁ is a gneissic foliation (S₁) denned by segregation of peak granulite facies mineral assemblages corresponding to prograde M₁ metamorphism. M₂ metamorphic overprint represents an episode of near-isobaric cooling of the complex under a static condition. D₂ represents an episode of ductile deformation manifested by isoclinal folding (F₂) and associated extensional structures, within a broad framework of coaxial bulk deformation. The present study reveals that D₂ took place subsequent to M₂ - Subsequent deformation, D₃, produced F₃ folds and also deformations of boudins formed during D₂. M₃, which is synchronous with F₃, represents a near isothermal decompression of the BCGC. This was followed by a weak structural readjustment (D₄), producing E-W cross folds. The latter was not, however, associated with any recognizable petrological reworking. In the terminal events, deformation (D₅) and mineral reactions (M₄) were localized along narrow intersecting shear zones. The latter acted as channelways for carbonic and still later hydrous fluid infiltration. The available thermobarometric data from BCGC and other areas of the Eastern Ghats belt reveal that reworking during M₂ and M₃ ensued in a thermally perturbed regime. The high thermal regime might also have persisted during carbonic fluid infiltration related to terminal reworking (M₄).     

东疆哈尔里克变质地带变质作用特征及形成构造环境研究

本文通过对哈尔里克变质地带的变质岩石学、变质矿物及矿物包裹关系的研究，确定该变质带的形成曾经历了晚古生代中、晚石炭世的高温－低压型的区域热变质作和晚石炭世末期中温－中压型的断裂区域变质作用，且后者叠加改造了前者。前者温度为922℃～780 ℃，后者可划分出四个递增变质带，温压为450℃～670℃和2.3～4.1kb，这一研究反映丁哈尔里克泥盆纪火山岛弧经过了由拉伸作用到碰撞挤压造山引发的A型俯冲作用演变过程，并产生了上述相应的变质现象。     

Diachronous deformation and a strain gradient beneath the Selkirk allochthon,northern Monashee complex,southeastern Canadian Cordillera

Structural relationships of granitoid rocks dated by the U–Pb method indicate that deformation was diachronous and a strain gradient exists in a 6-km-thick section beneath the Selkirk allochthon, in the northern Monashee complex, one of the deepest structural exposures in the southern Canadian Cordillera. At high structural levels, immediately beneath a crustal-scale thrust zone that transported the allochthon eastward, a metasedimentary-dominated cover sequence was strongly affected by kilometre-scale east-verging isoclinal folds (F₁) and outcrop-scale folds (F₂) that are associated with the dominant foliation and lineation. The F₂ folding occurred, at least in part, after 58 Ma and ceased by 55 Ma. In deeper levels of the cover sequence and the underlying orthogneiss-dominated basement, F₂ folding occurred, at least in part, after 52 Ma and ceased by 49 Ma. Proterozoic dykes in the basement were locally weakly affected by D₂. These new findings require that: (i) D₂ compression youngs structurally downward, synchronous with the thermal peak of metamorphism; (ii) D₂ in deeper levels is synchronous with extension above the complex that was partly responsible for its exhumation; and (iii) a D₂ strain gradient lies between strongly deformed cover rocks and weakly D₂-deformed basement rocks. We propose a model in which rocks that were tectonised at different places and times within the orogen were juxtaposed, likely during east-verging kilometre-scale F₁ folding and shearing along the isocline limbs (a similar model was previously proposed to explain a pattern of downward younging thermal peak ages and an inverted metamorphic sequence in higher rocks). The rapid downward decrease in deformation intensity suggests that the lower limit of significant Cordilleran strain lies in the exposed basement. Cessation of deformation at this level is attributed to the fact that the basement attained elevated temperatures and began straining when the Cordilleran tectonic regime changed from compressional to extensional.     

Precambrian tectonic evolution of eastern India: A model of converging microplates

The Precambrian formations of the Singhbhum and Chotanagpur region of the Indian Peninsular Shield are tectonically classified and their implications in the context of plate tectonics are reviewed on the basis of the stratigraphic, structural, petrologic, geochemical, geophysical and geochronologic data that have accumulated through extensive research in the region in recent years. It is shown that the essential elements in tectonic settings, geological facies and structural and metamorphic characters of the Singhbhum orogenic belt and the reactivated Chotanagpur plateau are elegantly interpretable in terms of interaction of two converging microplates, named here as the Singhbhum and Chotanagpur plates. A detailed correlation of the tectonic evolution with the different stages of a proposed model of plate motions is attempted in the paper.The study reveals three cycles of plate motions with intervening periods of “quiescence”. During the first cycle (2000-1600 Ma), the Singhbhum plate moved northward and collided with the Chotanagpur plate: this led to the tectonic emplacement of the Dalma ophiolite belt and development of the F₁ folds and thrusts and M₁ metamorphism. During the second cycle (1550-1170 Ma), a clockwise rotation of the Singhbhum plate towards the NE generated the F₂ folds and a transcurrent sinistral shear zone. Obduction of the continental lithosphere of this plate occurred during the third cycle (1000-850 Ma) as a result of its continued impingement on the Chotanagpur plate in the NNW direction; this is documented by the evolution of the F₃ folds, M₃ metamorphism and the Singhbhum thrust zone. The “quiescence” periods allowed time for isostatic readjustments, viz., uplifts, intrusions of basic dyke swarms, erosion and paralic sedimentation.     

Syn-collisional transform faulting of the Tan-Lu fault zone,East China

Origin of the continental-scale Tan-Lu fault zone (TLFZ), East China, remains controversial. About 550 km sinistral offset of the Dabie orogenic belt (DOB) and Sulu orogenic belt (SOB) is shown along the NE-NNE-striking TLFZ. Syn-collisional, sinistral ductile shear belts in the TLFZ have been identified. Thirteen phengite bulk separates from the mylonites were dated by the ⁴⁰Ar/³⁹Ar method. They gave cooling ages of the 198–181 Ma for the shear belts along the eastern margin of the DOB and 221–210 Ma from the western margin of the SOB. Distribution of the foreland basin deposits suggests that sinistral offset of the DOB and SOB by the TLFZ took place prior to deposition of the Upper Triassic strata. The marginal structures around the DOB and SOB support syn-collisional faulting, and indicate anticlockwise rotation of the DOB during the displacement. The folding and thrust faulting related to crustal subduction, coeval with the Tan-Lu faulting, is older than the foreland basin deposition related to the orogenic exhumation. Several lines of evidence demonstrate that the TLFZ was developed as a syn-collisional transform fault during latest Middle to earliest Late Triassic time when the DOB and SOB experienced crustal subduction of the South China Block (SCB). Eastward increase of the crustal subduction rates is believed to be responsible for the sinistral transform faulting.     

Short-lived polyphase deformation during crustal thickening and exhumation of a collisional orogen (Ribeira Belt,Brazil)

The Ribeira Belt (Brazil) is a Neoproterozoic collisional-related feature that was located in a south-central position in West Gondwana. We present quantitative data on finite strain, flow vorticity and deformation temperatures for the Curitiba Terrane, a major segment of the southern Ribeira Belt. Six deformation phases (D₁-D₆) related with crustal thickening and exhumation were recognized. D₁ and D₂-related microstructures are preserved exclusively within porphyroblasts, in part grown during stages of high-pressure (∼9–12 kbar) isobaric heating after crustal thickening. D₃ phase was active from peak metamorphism attained in contrasting crustal levels (810–400 °C), to the early stage of exhumation (500–400 °C), as indicated by petrological, microstructural and quartz c-axis fabric evidence. Kinematic vorticity results indicate that the SL₃ mylonitic fabric resulted from a simple shear-dominated deformation related with westward thrusting. North-verging overturned D₄ folds with E-W-trending subhorizontal axes derived from a pure shear-dominated deformation. Regional D₅ open folds with subvertical axes and NNE-SSW-trending traces were produced by indentation tectonics. D₆ phase comprises retrograde orogen-parallel transcurrent shear zones related with scape tectonics. Geochronological data indicate that D₃-D₆ phases occurred between 584 and 580 Ma, suggesting a fast exhumation rate of ∼8 mm/year for the deepest rocks from the southern Ribeira Belt.     

Jadeite–garnet glaucophane schists in the Bizan area,Sambagawa metamorphic belt,eastern Shikoku,Japan: significance and extent of eclogite facies metamorphism

Eclogite facies metamorphic rocks have been discovered from the Bizan area of eastern Shikoku, Sambagawa metamorphic belt. The eclogitic jadeite–garnet glaucophane schists occur as lenticular or sheet‐like bodies in the pelitic schist matrix, with the peak mineral assemblage of garnet + glaucophane + jadeite + phengite + quartz. The jadeitic clinopyroxene (X_Jd 0.46–0.75) is found exclusively as inclusions in porphyroblastic garnet. The eclogite metamorphism is characterized by prograde development from epidote–blueschist to eclogite facies. Metamorphic P–T conditions estimated using pseudosection modelling are 580–600 °C and 18–20 kbar for eclogite facies. Compared with common mafic eclogites, the jadeite–garnet glaucophane schists have low CaO (4.4–4.5 wt%) and MgO (2.1–2.3 wt%) bulk‐rock compositions. The P–T– pseudosections show that low X_Ca bulk‐rock compositions favour the appearance of jadeite instead of omphacite under eclogite facies conditions. This is a unique example of low X_Ca bulk‐rock composition triggered to form jadeite at eclogite facies conditions. Two significant types of eclogitic metamorphism have been distinguished in the Sambagawa metamorphic belt, that is, a low‐T type and subsequent high‐T type eclogitic metamorphic events. The jadeite–garnet glaucophane schists experienced low‐T type eclogite facies metamorphism, and the P–T path is similar to lawsonite‐bearing eclogites recently reported from the Kotsu area in eastern Shikoku. During subduction of the oceanic plate (Izanagi plate), the hangingwall cooled gradually, and the geothermal gradient along the subduction zone progressively decreased and formed low‐T type eclogitic metamorphic rocks. A subsequent warm subduction event associated with an approaching spreading ridge caused the high‐T type eclogitic metamorphism within a single subduction zone.     

滇西南团梁子岩组分异石英脉锆石U-Pb年龄——晚三叠世澜沧江构造带区域变质变形的时代制约

在滇西南澜沧江构造带东侧、扬子板块西缘中元古代团梁子岩组含有大量的平行于区域面理(S2)的构造热液石英脉,利用LA-ICP-MS对3件石英脉和1件绿片岩中的锆石进行~(206)U/~(238)Pb测年,获得3组明显的组合年龄:395～461Ma、240～260Ma和222～228Ma,大部分集中于222～228Ma。对比研究表明,区域上2期变质变形(M_1D_1、M_2D_2)与获得的锆石年龄有较好的对应性,早期的变质变形(M_1D_1)形成于早古生代(395～461Ma)原特提斯洋盆向东俯冲阶段;晚期的2期变质变形(M_(2a)D_(2a),M_(2b)D_(2b))发生于晚古生代—中生代早期(240～260Ma)古特提斯洋盆向东俯冲阶段和晚三叠世早期(222～228Ma)古特提斯洋盆闭合阶段。晚三叠世早期变质变形(M_(2b)D_(2b))构造热液发生在临沧花岗岩侵位和弧陆碰撞型忙怀组火山岩(229～235Ma)之后,早于小定西组/芒汇河组拉伸期火山岩(210～222Ma),是古特提斯洋与扬子陆块碰撞后的应力松弛阶段俯冲岩片快速折返的证据,同时也反映了古特提斯洋盆在晚三叠世早期之前已经关闭。     

Hercynian subduction-related processes within the metamorphic continental crust in Calabria (southern Italy)

Linking the deformation history of mylonitized continental rocks to the progress of devolatilization reactions that trigger reaction softening is critical for the understanding of crustal scale processes. We have analysed the field geometries and microstructures of deformed rocks within the southern Hercynian belt in Calabria, as well as modelled the pressure–temperature–deformation (P–T–d) trajectory of a main ductile shear zone that tectonically coupled the deeper crustal Mammola Paragneiss Unit with the upper crustal Stilo–Pazzano Phyllite Unit. P–T modelling of the mylonitic Mammola Paragneiss Unit was performed through calculation of phase equilibrium diagrams with the software thermocalc in the MnNCKFMASHTO model system. The prograde P–T–d trajectory is based on the zoning profiles of garnet porphyroblasts and their mineral inclusions, primarily barroisite and epidote. P–T modelling shows that peak metamorphic conditions of ~0.9 GPa and 585°C were reached during a D_n-1 under-thrusting event. The following exhumation during the D_n mylonitic event, and contact metamorphism during D_n+1 and D_n+2 folding events, have also been modelled because they are essential to restore the previous tectono-metamorphic history. The exhumation trajectory was modelled down to 0.3 GPa with temperatures of 440–460°C, under fluid-deficient conditions, as well as the final late Carboniferous contact metamorphism up to T_max of 680–720°C. The prograde path shows clear evidence for thermal buffering during garnet growth at the expense of chlorite, with a heating-dominated stage after chlorite breakdown. Subsequently, a rheological change associated with epidote breakdown (i.e. reaction softening) occurred, highlighted by a net steepening of the P/T trajectory towards the pressure peak. On the basis of the barroisite inclusions within garnet porphyroblasts as well as the ‘hairpin’ shape of the reconstructed P–T–d path (before contact metamorphism), we infer that the unusual low T/P gradient for the Hercynian crust exposed in the Mammola Paragneiss Unit records its involvement in the Palaeotethys–Gondwana subduction beneath Laurussia during D_n-1 under-thrusting. We present a new palaeotectonic interpretation along the southern Hercynian belt in Calabria during the Upper Mississippian–Lower Pennsylvanian, that is consistent with previous geochronology studies.     

Fluid evolution during burial and Variscan deformation in the Lower Devonian rocks of the High-Ardenne slate belt (Belgium): sources and causes of high-salinity and C–O–H–N fluids

Fluid inclusions in quartz veins of the High-Ardenne slate belt have preserved remnants of prograde and retrograde metamorphic fluids. These fluids were examined by petrography, microthermometry and Raman analysis to define the chemical and spatial evolution of the fluids that circulated through the metamorphic area of the High-Ardenne slate belt. The earliest fluid type was a mixed aqueous/gaseous fluid (H₂O–NaCl–CO₂–(CH₄–N₂)) occurring in growth zones and as isolated fluid inclusions in both the epizonal and anchizonal part of the metamorphic area. In the central part of the metamorphic area (epizone), in addition to this mixed aqueous/gaseous fluid, primary and isolated fluid inclusions are also filled with a purely gaseous fluid (CO₂–N₂–CH₄). During the Variscan orogeny, the chemical composition of gaseous fluids circulating through the Lower Devonian rocks in the epizonal part of the slate belt, evolved from an earlier CO₂–CH₄–N₂ composition to a later composition enriched in N₂. Finally, a late, Variscan aqueous fluid system with a H₂O–NaCl composition migrated through the Lower Devonian rocks. This latest type of fluid can be observed in and outside the epizonal metamorphic part of the High-Ardenne slate belt. The chemical composition of the fluids throughout the metamorphic area, shows a direct correlation with the metamorphic grade of the host rock. In general, the proportion of non-polar species (i.e. CO₂, CH₄, N₂) with respect to water and the proportion of non-polar species other than CO₂ increase with increasing metamorphic grade within the slate belt. In addition to this spatial evolution of the fluids, the temporal evolution of the gaseous fluids is indicative for a gradual maturation due to metamorphism in the central part of the basin. In addition to the maturity of the metamorphic fluids, the salinity of the aqueous fluids also shows a link with the metamorphic grade of the host-rock. For the earliest and latest fluid inclusions in the anchizonal part of the High-Ardenne slate belt the salinity varies respectively between 0 and 3.5 eq.wt% NaCl and between 0 and 2.7 eq.wt% NaCl, while in the epizonal part the salinity varies between 0.6 and 17 eq.wt% NaCl and between 3 and 10.6 eq.wt% for the earliest and latest aqueous fluid inclusions, respectively. Although high salinity fluids are often attributed to the original sedimentary setting, the increasing salinity of the fluids that circulated through the Lower Devonian rocks in the High-Ardenne slate belt can be directly attributed to regional metamorphism. More specifically the salinity of the primary fluid inclusions is related to hydrolysis reactions of Cl-bearing minerals during prograde metamorphism, while the salinity of the secondary fluid inclusions is rather related to hydration reactions during retrograde metamorphism. The temporal and spatial distribution of the fluids in the High-Ardenne slate belt are indicative for a closed fluid flow system present in the Lower Devonian rocks during burial and Variscan deformation, where fluids were in thermal and chemical equilibrium with the host rock. Such a closed fluid flow system is confirmed by stable isotope study of the veins and their adjacent host rock for which uniform δ¹⁸0 values of both the veins and their host rock demonstrate a rock-buffered fluid flow system.     

Intrafolial folds and associated structures in a progressive strain environment of Darjeeling-Sikkim Himalaya

The Dating rocks and Darjeeling gneisses, which constitute the Sikkim dome in eastern Himalaya, as well as the Gondwana and Buxa rocks of ‘Rangit Window’, disclose strikingly similar sequences of deformation and metamorphism. The structures in all the rocks belong to two generations. The structures of early generation are long-limbed, tight near-isoclinal folds which are often intrafolial and rootless. These intrafolial folds are associated with co-planar tight folds with variably oriented axes and sheath folds with arcuate hinges. Penetrative axial plane cleavage and mineral lineation are related structures; transposition of bedding is remarkable. This early phase of deformation (D ₁) is accompanied by constructive metamorphism. The structures of later generation are open, asymmetrical or polyclinal; a crenulation cleavage or discrete fracture may occur. The structures of early generation are distorted by folds of later generation and recrystallized minerals are cataclastically deformed. Recrystallization is meagre or absent during the later phase of deformation (D ₂). The present discussion is on structures of early generation and strain environment during theD ₁ phase of deformation. The concentration of intrafolial folds in the vicinity of ductile shear zones and decollement or detachment surface (often described as ‘thrust’) may be considered in this context. The rocks of Darjeeling-Sikkim Himalaya display minor structures other than intrafolial folds and variably oriented co-planar folds. The state of finite strain in the rocks, as observed from features like flattened grains and pebbles, ptygmatic folds and boudinaged folds indicate combination of flattening and constrictional type strain. The significance of the intrafolial folds in the same rocks is discussed to probe the environment of strain during progressive deformation (D ₁).     

Petrology, Metamorphic Process and Genesis of The Dabie--Sulu Eclogite Belt, Eastern--Central China

The Qinling-Dabie-Sulu high-pressure and ultra-high pressure metamorphic belt wasformed by subduction and collision between the North China and Yangtze plates. The study ofthe eclogite belt is very important in understanding the evolution of the Qinling Dabie orogen. Inthe present paper the geology, petrology, minerology and chronology of the eclogites in the Dabieand Sulu areas are described. The principal conclusions of this work are as follows: (1) Based up-on the field occurrence and the P-T conditions of the eclogites, two types of eclogite can be dis-tinguished: Type 1—the low-temperature and high-pressure eclogite in the mid-late Proterozoicmetamorphic series, and Type 2—the ultra-high pressure eclogite in the late Archaean to earlyProterozoic metamorphic complex. In the Dabie area, the ultra-high-pressure eclogite,high-pressure eclogite and epidote-blueschist units are nearly parallel to each other and stretchintermittently from north to south. (2) The P-T conditions of the high-pressure eclogites and ul-tra-high pressure eclogites have been estimated. The former are formed at 450-550℃ and1.4-1.6 GPa; while the latter at 650-870℃ and >2.7-2.9 GPa in the Dabie area and at820-1000℃ and >2.8-3.1 GPa in the Sulu area. The metamorphic temperatures of the eclogitesincrease progressively from west to east. (3) The ultra-high pressure eclogites were subjected to 5stages of metamorphism: pre-eclogite epidote amphibolite facies, peak coesite eclogite facies,post-eclogite amphibolite facies, epidote-blueschist facies or epidote amphibolite facies andgreenschist facies. The general features of the PTt path of the ultra-high pressure eclogite are:clockwise pattern, progressive metamorphism being a process of slow increasing temperature andrapid increasing pressure, and the retrogressive section with nearly isothermal decompression atthe early stage, isobaric cooling at the middle stage and nearly isothermal decompression at thelate stage. (4) At least two stages of high-pressure metamorphism occurred in the orogenic belt:the high-pressure eclogite and ultra-high pressure eclogite were formed by the subduction of theoceanic crust northward beneath the North China plate or the Dabie block during theCaledonian; while the epidote-blueschist belt came into being by subdution and collision be-tween the two continental plates during the Indosinian. (5) Due to the continuous sequentialsubduction of the cold plate, the ultra high-presssure metamorphic rocks were uplifted to thecrust by the underplating processes. They can be preserved just because of the "frozen effect" re-sulting from the continuous subduction of the cold plate. (6) The carbonates, such as magnesite,breunnerite, aragonite and dolomite, and the H_2O-bearing minerals, such as phengite, epidoteand zoisite, were stable during the high-pressure and/or ultra-high pressure metamorphism.     

Systematic changes in orientation of linear mylonitic fabrics: An example of strain partitioning during transpressional deformation in north Golpaygan,Sanandaj–Sirjan zone,Iran

Kilometer-scale, shallowly dipping, NW-striking top-to-the NE reverse and dextral strike-slip shear zones occur in metamorphic rocks of north Golpaygan. These metamorphic rocks are exposed at the NE margin of the central part of the Sanandaj–Sirjan zone in the hinterland of the Zagros orogen. NW-striking top-to-the NE normal shear zones were also found in a small part of the study area. Structural evidence of three deformation stages were found. Pre-mylonitization metamorphic mineral growth happened during D₁. The main mylonitization event was during the D₂ deformational event, following coaxial refolding, synchronous to retrograde metamorphism of amphibolite to greenschist facies in the Late Cretaceous–Paleocene, and before D₃ folding and related mylonitization. We documented the systematic changes in the orientations of D₂ linear fabrics especially stretching lineations and superimposition relations of structures. It is concluded that the dextral strike-slip and dip-slip shear zones were coeval kinematic domains of partitioned dextral transpression. The shallowly dipping reverse and strike-slip shear zones are compatible with partitioning in a very inclined transpressional model. Fabric relations reflect that the top-to-the NE normal shear zones were not produced during deformation partitioning of inclined dextral transpression. The Late Cretaceous–Paleocene strain partitioning was followed by later N–S shortening and NE-extension in the north Golpaygan area.