Initiation of left-lateral deformation along the Ailao Shan–Red River shear zone: new microstructural,textural, and geochronological constraints from the Diancang Shan metamorphic massif,SW Yunnan,China

The Diancang Shan metamorphic massif, the northwestern extension of the Ailao Shan Massif, is a typical metamorphic complex situated along the NW–SE-trending Ailao Shan–Red River shear zone. Diancang Shan granitic and amphibolitic mylonites collected from sheared high-grade metamorphic rocks were studied using petrographic and electron-backscatter diffraction techniques. Sensitive high-resolution ion microprobe U–Pb dating of zircon grains from the granitic mylonites constrains the timing of shearing. Macro- and microstructural and textural analysis reveals intense plastic deformation of feldspar, quartz, and amphibole under amphibolite-facies conditions, all consistently document left-lateral shearing. Porphyritic monzogranitic mylonite within the shear zone possesses evidence supporting a sequential, progressive process from crystallization during magma emplacement, through submagmatic flow to solid-state plastic deformation. We suggest that the early-kinematic pluton subsequently underwent strong left-lateral strike–slip shearing. The development of complex textures of quartz, feldspar, and amphibole from the granitic and amphibolitic mylonites apparently records successive variation of conditions attending coherent, solid-state high-temperature ductile deformation during regional left-lateral shearing. All magmatic zircons from the mylonitized porphyritic monzogranite give U–Pb ages of 30.95 ± 0.61 million years for the crystallization of the granite. This age provides the timing of onset of left-lateral shearing along the Ailao Shan–Red River shear zone in the Diancang Shan high-grade metamorphic massif.     

中国南天山萨恨托亥-大山口成矿带韧性剪切带与金成矿作用

以南天山中段萨恨托亥-大山口成矿带内控矿韧性剪切带为例,对韧性剪切带的金成矿作用进行了初步探讨.通过对地质体的构造变形特点、变形演化过程的分析表明,韧性剪切带的构造属性控制了金矿的产状及规模,金矿化阶段与韧性剪切带的变形演化过程密切相关.矿化类型、矿化强度及矿化方式受韧性剪切带发展阶段制约,剪切带内物质组分迁移变化揭示出韧性剪切带与金在剪切带内的迁移富集、沉淀成矿的内在联系.韧性剪切带成矿作用是南天山成矿带中段重要的金矿成矿作用.     

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.      

江西武功山东区大型韧性剪切带的显微构造特征

武功山东区存在一条大型韧性剪切带。鞘褶皱倒向以及旋转变形构造(如S-C面理组构、旋转碎斑系、雪球构造和粒内显微破裂构造等)显示此剪切带为由南向北逆冲推覆性质。砾石、黄铁矿还原斑和石英斑晶的有限应变分析表明剪切带西段和东段岩石分别以收缩型椭球和压扁型椭球变形为特征。剪切带的主要变形时代是早古生代,可能与早古生代华夏陆块和扬子陆块之间的碰撞造山作用有关。     

Grain-size reduction of feldspars by fracturing and neocrystallization in a low-grade granitic mylonite and its rheological effect

Feldspar grain-size reduction occurred due to the fracturing of plagioclase and K-feldspar, myrmekite formation and neocrystallization of albitic plagioclase along shear fractures of K-feldspar porphyroclasts in the leucocratic granitic rocks from the Yecheon shear zone of South Korea that was deformed under a middle greenschist-facies condition. The neocrystallization of albitic plagioclase was induced by strain energy adjacent to the shear fractures and by chemical free energy due to the compositional disequilibrium between infiltrating Na-rich fluid and host K-feldspar. With increasing deformation from protomylonite to mylonite, alternating layers of feldspar, quartz and muscovite developed. The fine-grained feldspar-rich layers were deformed dominantly by granular flow, while quartz ribbons were deformed by dislocation creep. With layer development and a more distributed strain in the mylonite, lower stresses in the quartz-rich layers resulted in a larger size of dynamically recrystallized quartz grains than that of the protomylonite.     

SOME TRENDS IN THE STUDY OF CARBONATE SEDIMENTS IN THE FEDERAL REPUBLIC OF GERMANY

A stratigraphically coherent blueschist terrane near Aksu in northwestern China is unconformably overlain by unmetamorphosed sedimentary rocks of Sinian age (～600 to 800 Ma). The pre-Sinian metamorphic rocks, termed the Aksu Group, were derived from shales, sandstones, basaltic volcanic rocks, and minor cherty sediments. They have undergone multi-stage deformation and transitional blueschist/greenschist-facies metamorphism, and consist of strongly foliated chlorite-stilpnomelane-graphite schist, stilpnomelane-phengite psammitic schist, greenschist, blueschist, and minor quartzite, metachert, and meta-ironstone. Metamorphic minerals of basaltic blueschists include crossitic amphibole, epidote, chlorite, albite, quartz, and actinolite. Mineral parageneses and compositions of sodic amphibole suggest blueschist facies recrystallization at about 4 to 6 kbar and 300 to 400° C. Many thin diabasic dikes cut the Aksu Group; they are characterized by high alkali, TiO₂, and P₂O₅ contents and possess geochemical characteristics of within-plate basalts; some of these diabasic rocks contain sodic clinopyroxene and amphibole as primary phases and have minor pumpellyite, albite, epidote, chlorite, and calcite as the prehnite/pumpellyite-facies metamorphic assemblage. This prehnite/pumpellyite-facies overprint did not affect the host rocks of the blueschist-facies lithologies.

K-Ar and Rb-Sr ages of phengite and whole rocks from pelitic schists are ～690 to 728 Ma, and a ⁴⁰Ar/³⁹Ar age of crossite from the blueschist is 754 Ma. The basal conglomerate of the overlying Sinian to Eocambrian sedimentary succession contains clasts of both the blueschist and cross-cutting dike rocks, clearly demonstrating that conditions required for blueschist-facies metamorphism were attained and ceased at least 700 Ma. The northward-increasing metamorphic grade of the small blueschist terrane may reflect northward subduction of an accretionary complex beyond the northern edge of the Tarim craton. Abundant subparallel diabasic dikes indicate a subsequent period of Pre-Sinian rifting and diabasic intrusion along the northern margin of Tarim; a Sinian siliciclastic and carbonate sequence was deposited unconformably atop the Aksu Group and associated diabase dikes.     

Polyphase greenschist-facies reactivation of the Dent Blanche Basal Thrust (Western Alps) during progressive Alpine orogeny

This study assesses the significance, geometry, and kinematics of greenschist-facies deformation along the Dent Blanche Basal Thrust (DBBT), a major tectonic contact in the Internal Western Alps of Switzerland and Italy. The DBBT separates continental units of the Dent Blanche nappe, the structurally highest unit in the Western Alps, from underlying Piemont-Ligurian ophiolites. Mylonites and deformation structures along the contact provide a record of its retrograde greenschist-facies evolution after earlier high-pressure metamorphism. A first phase of foreland-directed, reverse-sense, top-(N)W shearing (D1) occurred between ca. 43 and 39 Ma, related to exhumation of the Dent Blanche nappe from high-pressure conditions. It led to the formation of mylonitic fabrics under high- to medium-grade greenschist-facies conditions along the entire DBBT. A phase of ductile normal-sense top-SE shearing (D2) at ca. 38–37 Ma was mainly localized within underlying ophiolitic units and only partly affected the DBBT. Another phase of ductile deformation (D3) under medium- to low-grade greenschist-facies conditions at ca. 36–35 Ma occurred in response to underthrusting of European continental margin units and resulted in the updoming of the nappe stack. Especially the southeastern DBBT was characterized by bulk top-NW shearing, partly conjugate top-NW/top-SE shearing, and resulting orogen-perpendicular crustal extension. Subsequently, the DBBT was affected by a phase of orogen-perpendicular shortening (D4) and formation of folds and crenulations at ca. 34–33 Ma due to increasing compressional tectonics. Finally, a phase of semi-ductile to brittle normal-sense top-NW and conjugate shearing (D5) from ca. 32 Ma onwards particularly affected the southeastern segment and indicates exhumation of the DBBT through the ductile–brittle transition. This was followed by brittle NW–SE extensional deformation. This study suggests that the DBBT experienced a polyphase deformation and reactivation history under decreasing greenschist-facies metamorphic conditions during which different segments of this major shear zone were variably affected.     

High-grade calcareous metasediments from the Sawtooth Metamorphic Complex,Idaho, USA: evidence for passive margin strata and polymetamorphism within the Idaho batholith

ABSTRACT

The Sawtooth Metamorphic Complex (SMC) of central Idaho contains exposures of metasupracrustal rocks, in a crucial location between the Archaean Wyoming craton to the east and the adjacent Mesozoic terranes to the west, that provide constraints on Precambrian crustal evolution in the northwestern United States. Mineral textures, whole-rock geochemistry, and thermobarometry of calc-silicate gneisses record multiple stages of crustal evolution, including protolith deposition, burial and multiple metamorphic and deformational overprints. Whole-rock signatures are consistent with derivation from post-Archaean, continental sources that have undergone sedimentary maturation and recycling typical of clastic sedimentation in passive-margin environments and with a metamorphosed sequence of calcareous sandstones and marls, containing varying proportions of clay and quartzo-feldspathic detritus. The sandstone-to-marl continuum may reflect a shallow-to-deep water transition in the depositional environment of the calc-silicate protoliths. Two metamorphic events and three deformational events are preserved. The assemblage clinopyroxene + quartz + plagioclase + K-feldspar + rutile represents peak metamorphic conditions (M1), estimated at 750–775°C with relatively oxidized metamorphic fluids, during development of D1a foliation. Deformation twinning in clinopyroxene records a widespread, high strain D1b event at high temperature. A second static thermal event (M2), associated with randomly oriented amphiboles overprinting M1 and D1b features, records conditions of ~550–725°C and H₂O-rich fluids. Late-stage, brittle–ductile D2 deformation is characterized by mylonitic lenses of quartz, fractures within M1 clinopyroxenes that crosscut D1b deformation twins, and localized fracturing of M2 amphiboles. Geochemical and mineral chemical signatures of SMC calc-silicates preserve fingerprints of the original protolith through burial to mid-crustal conditions and high-grade metamorphism and suggest crustal thickening to about 20 km. The SMC may reflect rocks at depth beneath the Idaho batholith. These data, indicative of newly recognized post-Archaean terrain with passive-margin, continental sediments suggest that a continuous Cordilleran passive margin sequence may have extended along the western edge of Laurentia.     

Variscan deformation,microstructural zonation and extensional exhumation of the Moravian Cadomian basement

Abstract

The Cadomian Dyje Batholith, in the foot–wall of the Variscan Moravian nappe pile, has been involved in Variscan ductile deformation. The Cadomian Brunovistulian rocks were obliquely underthrusted during Carboniferous dextral transpression.

Strain intensity is inversely proportional to the distance from the contact of the Variscan thrust front. The microstructures of deformed granodiorites and quartz–diorites show a characteristic zonality marked by relatively high temperature flow in the west (550–580 °C) characterized by dynamic recrystallization of feldspars and grain boundary migration recrystallization of quartz. The size of quartz grains decreases with decreasing strain towards the east. At the easternmost part of the autochthonous Dyje massif, fracturing of feldspar and subgrain rotation recrystallization of quartz predominate. Flow stress estimates calculated from recrystallized quartz grain size show a regional increase of stress intensity from the highly strained margin towards the less deformed core of the Dyje massif. This microstructural zonation is oblique with respect to the major thrust boundary and corresponds roughly to metamorphic isogrades. The microstructural zonation reflects underthrusting of the Brunovistulian domain below the Moldanubian nappe.

The main ductile tectonic event D₁ is followed by a retrogressive brittle–ductile and brittle deformation D₂. D₂ results in the development of shear zones and faults superimposed on the D₁ mylonite fabric. D₂ is related to extension oblique to the D₁ fabric, associated with detachment and the westward movement of the Moravian nappes. © Elsevier, Paris     

Experimental deformation of sintered albite above and below the order-disorder transition

Abstract

Purified albite powder (44-53 μm) has been sintered to form an albite polycrystal suitable for deformation studies close to the melting temperature. Experiments have been carried out in Griggs solid medium deformation apparatus at 800, 1020 °C and 700 MPa pressure in a dehydrating pyrophyllite confining medium at constant strain rates of 10^?4, 10^?5, 10^?6 and 10^?7/s. At 800 °C the samples were brittle-ductile whereas at 1020 °C they were ductile with a rheology well described by a power law with a stress exponent of 3. The transition from brittle-ductile to ductile also coincided with the order-disorder or low-high albite transition, as indicated by the marked increase in mechanical twinning on the albite law at high temperature. At 1020 °C high dislocation densities (10^?10 - 10^?11/cm² and mechanical twinning characterised the original high albite grains, whereas fine recrystallised grains ( < 5 цm) had low dislocation densities (10⁷ - 10⁸/cm²) and often contained polysynthetic albite and pericline (M-type) twins. It is suggested that the recrystallized grains were monalbite (monoclinic) under test conditions which have inverted to high albite (triclinic) and in so doing produced M-twins, and that the recrystallization mechanism involved grain-boundary bulging to nucleate new high angle boundaries. The implications of the order-disorder transition for twinning and grain boundary migration are discussed and it is suggested that the data cannot be simply extrapolated to natural deformation in the low albite field.     

Structure and kinematics of the Jungfrau syncline, Faflertal (Valais, Alps), and its regional significance

The formation and structural evolution of the Jungfrau syncline is described, based on excellent outcrops occurring in the Lötschental, in the Central Alps of Switzerland. The quality of the outcrops allows us to demonstrate that the External Massifs of the Swiss Alps have developed due to internal folding. The Jungfrau syncline, which separates the autochtonous Gastern dome from the Aar massif basement gneiss folds, is composed of slivers of basement rocks with their Mesozoic sedimentary cover. In the Inner Faflertal, a side valley of the Lötschental, the 200 m thick syncline comprises four units, the Gastern massif with a reduced Mesozoic sedimentary cover in a normal stratigraphic succession, two units of overturned basement rocks with their Mesozoic sedimentary cover, and the overturned lower limb of the Tschingelhorn gneiss fold of the Aar massif with lenses of its sedimentary cover. Stratigraphy shows that the lower units, related to the Gastern massif, are condensed and that the upper units, deposited farther away from a Gastern paleo-high, form a more complete sequence, linked to the Doldenhorn Meso-Cenozoic basin fill. The integration of these local observations with published regional data leads to the following model. On the northern margin of the Doldenhorn basin, at the northern fringe of the Alpine Tethys, the pre-Triassic crystalline basement and its Mesozoic sedimentary cover were folded by ductile deformation at temperatures above 300 °C and in the presence of high fluid pressures, as the Helvetic and Penninic nappes were overthrusted towards the northwest during the main Alpine deformation phase. The viscosity contrast between the basement gneisses and the sediments caused the formation of large basement anticlines and tight sedimentary synclines (mullion-type structures). The edges of basement blocks bounded by pre-cursor SE-dipping normal faults at the northwestern border of the Doldenhorn basin were deformed by simple shear, creating overturned slices of crystalline rocks with their sedimentary cover in what now forms the Jungfrau syncline. The localisation of ductile deformation in the vicinity of pre-existing SE-dipping faults is thought to have been helped by the circulation of fluids along the faults; these fluids would have been released from the Mesozoic sediments by metamorphic dehydration reactions accompanied by creep and dynamic recrystallisation of quartz at temperatures above 300 °C. Quantification of the deformation suggests a strain ellipsoid with a ratio (1+ e₁ / 1+ e₃) of approximately 1000. The Jungfrau syncline was deformed by more brittle NW-directed shear creating well-developed shear band cleavages at a late stage, after cooling by uplift and erosion. It is suggested that the external massifs of the Alps are basement gneiss folds created at temperatures of 300 °C by detachment through ductile deformation of the upper crust of the European plate as it was underthrusted below the Adriatic plate.     

Petrology of a portion of the Eastern Peninsular Ranges mylonite zone,Southern California

Peraluminous and metaluminous plutonic rocks of the Peninsular Ranges batholith near Borrego Springs in southern California were mylonitized in the large shear zone known as the eastern Peninsular Ranges mylonite zone (EPRMZ). Accompanying mylonitization in this portion of the EPRMZ was metamorphism at intermediate-low-pressure amphibolite-facies conditions. Deformation in the zone overlapped in time with Cretaceous intrusion of the batholith. In the San Ysidro Mountain — Pinyon Ridge area, four north-south trending zones of differing intensity of deformation have been defined; from east to west the degree and style of deformation gradually change from undeformed or weakly deformed rocks to strongly mylonitized rocks. Electron microprobe analysis shows that recrystallized hornblende, biotite, and plagioclase are variable in composition, probably reflecting a range of metamorphic conditions accompanying deformation. Comparison of mineral compositions with those from mafic schists of Vermont suggests conditions ranged from andalusite-staurolite through sillimanite-muscovite grades as defined for pelitic rocks. Stability of muscovite+quartz in mylonite assemblages and lack of remelting of granitic rocks indicate that temperature did not exceed about 650° C during mylonitization and lithostatic pressure did not exceed about 5 kbar. Over time, any given rock volume experienced a range of temperature, lithostatic pressure, and perhaps fluid pressure and differential stress. Mineral reactions in the zone involved hydration, requiring introduction of water. The possibility of large-scale migration of K and Fe is suggested by whole-rock chemical data. Brittle and ductile deformation features are closely associated in one part of the EPRMZ. The combined evidence suggests the presence of a pore fluid with fluid pressure close to lithostatic pressure. Short periods of low fluid pressure and possible high differential stress cannot be ruled out.     

苏鲁地体南部南岗-高公岛韧性剪切带的变形记录、组分迁移和体积亏损

南岗-高公岛韧性剪切带是一条自南东向北西斜向逆冲的岩片边界性构造带,糜棱岩化作用强烈。韧性变形过程中,岩石中的主要矿物长石、黑云母等发生了不同程度的分解或蚀变并导致组分迁移。其中,活动组分的迁移是流体渗滤作用引起的,不活动组分的变异是体积亏损造成的。通过对韧性剪切带内不同变形程度岩石主要地球化学组分的对比分析,确立等比线斜率,计算出初糜棱岩的体积亏损率约为17%,糜棱岩的体积亏损率约为27%;质量平衡计算结果及等比线图表明韧性剪切作用导致SiO2流失量最大,其次为Al2O3,FeO、CaO、Na2O等都有不同程度的丢失,表明它们具有较强的活动性,MgO位于等比线上方,属于带入组分。糜棱岩中的石英或长英质条带和团块是长石的钠黝帘石化、绢云母化和黑云母的绿泥石化等导致SiO2、Al2O3、Na2O、CaO、FeO等活动组分从岩石中析出迁移形成的。这种岩石体积亏损和组分迁移是部分难溶组分富集的重要机制,对研究韧性剪切带中长英质条带和部分贵金属矿床的形成机制具有重要的指导作用。     

Water-rock interactions and chemical compositional variations during ductile deformation of the NW-striking shear zone in the Jiapigou gold belt,China

Two kinds of mylonite series rocks, felsic and mafic, have been recognized in the NW-striking shear zone of the Jiapigou gold belt. During ductile deformation, a large amount of fluid interacted intensively with the mylonite series rocks: plagioclases were sericitized and theAn values declined rapidly, finally all of them were transformed to albites; dark minerals were gradually replaced by chlorites (mostly ripidolite). Meanwhile, large-scale and extensive carbonation also took place, and the carbonatization minerals varied from calcite to dolomite and ankerite with the development of deformation. The δ¹³C values of the carbonates are −3.0‰ – −5.6‰ suggesting a deep source of carbon. The ductile deformation is nearly an iso-volume one (f _v≈1). With the enhancement of shear deformation, SiO₂ in the two mylonite series rocks was depleted, while volatile components suchs as CO₂ and H₂O, and some ore-forming elements such as Au and S were obviously enriched. But it is noted that the enrichment of Au in both the mylonite series rocks did not reach the paygrade of gold. The released SiO₂ from water-rock interactions occurred in the form of colloids and absorbed gold in the fluid. When brittle structures were formed locally in the ductile shear zone, the ore-forming fluids migrated to the structures along microfractures, and preciptated auriferous quartz because of reduction of pressure and temperature. Fluid inclusion study shows that the temperature and pressure of the ore-forming fluids are 245–292°C and 95.4–131.7 MPa respectively; the salinity is 12.88–16.33wt% NaCl; the fluid-phase is rich in Ca²⁺, K⁺, Na⁺, Mg²⁺, F⁻ and Cl⁻, while the gaseous phases are rich in CO₂ and CH₄. The δD and δ¹⁸O, values of the ore-forming fluid are −84.48‰ – −91.73‰ and −0.247‰ – +2.715‰ respectively, suggesting that the fluid is composed predominantly of meteoric water. This project is financially supported by the National Natural Science Foundation of China (No. 9488010).     

Late Jurassic terrane collision in the northwestern margin of Gondwana (Cajamarca Complex,eastern flank of the Central Cordillera,Colombia)

Medium-grade metabasites and metapelites from the Cajamarca Complex (Central Cordillera of Colombia) are in fault contact with the Jurassic Ibague batholith and show a penetrative foliation, locally mylonitic, suggesting intense dynamic–thermal metamorphism. The amphibolites are composed of calcic amphibole + epidote + plagioclase + quartz plus rutile + titanite + apatite + carbonate as accessory phases. Chlorite and albite appear as retrograde replacements. The metapelites are mainly composed of phengite + quartz + garnet + chlorite, plus epidote + albite + apatite + titanite + haematite as accessory phases. Bulk geochemistry of the amphibolites indicates basaltic protoliths with a mid-ocean ridge basalt (MORB) signature, although enrichment in the mobile large-ion lithophile elements compared to MORB suggests pre- and/or syn-metamorphic alteration by fluids. Peak pressure–temperature determinations for both types of rocks are similar, ranging 550–580°C and 8 kbar (approximately 26 km depth and an apparent geothermal gradient of 22°C/km). ⁴⁰Ar-³⁹Ar dating of amphibole from two amphibolite samples and one phengitic mica from a pelitic schist yielded plateau ages of 146.5 ± 1.1 Ma and 157.8 ± 0.6 Ma, and 157.5 ± 0.4 Ma, respectively. These Late Jurassic ages contrast with previously published (Permian)Triassic ages of metamorphism in the Cajamarca Complex. Taken together, our data indicate tectonic-driven burial of oceanic supracrustal sequences down to mid-crustal depths during Late Jurassic times and are best explained as the result of terrane collision-related metamorphism and deformation in a fore-arc/volcanic-arc environment of the active western margin of Gondwana rather than as a result of Jurassic thermal–metamorphic resetting of a (Permian)Triassic metamorphic sequence during intrusion of the Jurassic Ibague batholith. Our results represent the first report of Jurassic terrane collision tectonics involving supracrustal oceanic rocks in the northwestern margin of Gondwana in Colombia.     

定结地区韧性剪切带变形特征与糜棱岩研究

定结地区位于喜马拉雅造山带中段 ,发育有多方向、多尺度、多层次、多期次的韧性剪切带。在剪切带中 ,各种韧性变形组构极为丰富 ,表明剪切带岩石的变形主要为韧性变形机制所致。变形岩石类型为花岗质糜棱岩、长英质糜棱岩和硅质糜棱岩 ;由于岩石受糜棱岩化作用程度的不同 ,在韧性剪切带中发育糜棱岩化岩石、初糜棱岩、糜棱岩及超糜棱岩 ;剪切带岩石的变形温度为 2 0 8～ 5 5 9℃。     

A seismogenic zone in the deep crust indicated by pseudotachylytes and ultramylonites in granulite-facies rocks of Calabria (Southern Italy)

Pseudotachylyte veins frequently associated with mylonites and ultramylonites occur within migmatitic paragneisses, metamonzodiorites, as well as felsic and mafic granulites at the base of the section of the Hercynian lower crust exposed in Calabria (Southern Italy). The crustal section is tectonically superposed on lower grade units. Ultramylonites and pseudotachylytes are particularly well developed in migmatitic paragneisses, whereas sparse fault-related pseudotachylytes and thin mylonite/ultramylonite bands occur in granulite-facies rocks. The presence of sillimanite and clinopyroxene in ultramylonites and mylonites indicates that relatively high-temperature conditions preceded the formation of pseudotachylytes. We have analysed pseudotachylytes from different rock types to ascertain their deep crustal origin and to better understand the relationships between brittle and ductile processes during deformation of the deeper crust. Different protoliths were selected to test how lithology controls pseudotachylyte composition and textures. In migmatites and felsic granulites, euhedral or cauliflower-shaped garnets directly crystallized from pseudotachylyte melts of near andesitic composition. This indicates that pseudotachylytes originated at deep crustal conditions (>0.75 GPa). In mafic protoliths, quenched needle-to-feather-shaped high-alumina orthopyroxene occurs in contact with newly crystallized plagioclase. The pyroxene crystallizes in garnet-free and garnet-bearing veins. The simultaneous growth of orthopyroxene and plagioclase as well as almandine, suggests lower crustal origin, with pressures in excess of 0.85 GPa. The existence of melts of different composition in the same vein indicates the stepwise, non-equilibrium conditions of frictional melting. Melt formed and intruded into pre-existing anisotropies. In mafic granulites, brittle faulting is localized in a previously formed thin high-temperature mylonite bands. migmatitic gneisses are deformed into ultramylonite domains characterized by s-c fabric. Small grain size and fluids lowered the effective stress on the c planes favouring a seismic event and the consequent melt generation. Microstructures and ductile deformation of pseudotachylytes suggest continuous ductile flow punctuated by episodes of high-strain rate, leading to seismic events and melting.     

Structural evolution of a nappe complex,southern Vanoise massif,French Penninic Alps

The Arpont-Parrachée region in the southern Vanoise massif comprises a stack of minor fold- and thrust-nappes that were emplaced during subduction and closure of the Piémont ocean basin in Late Cretaceous to Eocene time. The stack includes the Arpont nappe, composed mainly of pre-Permian schist metamorphosed to blueschist facies early in the Alpine history, and several sheets of Permian to Eocene metasedimentary rocks. Nappe formation, recumbent folding, and associated ductile deformation postdated the high-pressure metamorphic peak, and probably involved translation to the northwest. The rocks were then refolded by large- and small-scale folds trending roughly E-W. These deformational events were accompanied by a decrease in metamorphic pressure, indicating uplift. They were followed by regional greenschist-facies metamorphism, which caused breakdown of high-pressure parageneses, annealing of microstructures, and widespread growth of albite porphyroblasts. The entire nappe pile was then refolded by large- and small-scale folds overturned towards the southeast. Reorientation of small-scale structures with increasing strain by this event indicates a large component of ESE-directed shear, which culminated in the formation of anastomosing ductile shear-zones.     

Orogenic Gold Mineralization in the Qolqoleh Deposit, Northwestern Iran

The Qolqoleh gold deposit is located in the northwestern part of the Sanandai‐Sirjan Zone, northwest of Iran. Gold mineralization in the Qolqoleh deposit is almost entirely confined to a series of steeply dipping ductile–brittle shear zones generated during Late Cretaceous–Tertiary continental collision between the Afro‐Arabian and the Iranian microcontinent. The host rocks are Mesozoic volcano‐sedimentary sequences consisting of felsic to mafic metavolcanics, which are metamorphosed to greenschist facies, sericite and chlorite schists. The gold orebodies were found within strong ductile deformation to late brittle deformation. Ore‐controlling structure is NE–SW‐trending oblique thrust with vergence toward south ductile–brittle shear zone. The highly strained host rocks show a combination of mylonitic and cataclastic microstructures, including crystal–plastic deformation and grain size reduction by recrystalization of quartz and mica. The gold orebodies are composed of Au‐bearing highly deformed and altered mylonitic host rocks and cross‐cutting Au‐ and sulfide‐bearing quartz veins. Approximately half of the mineralization is in the form of dissemination in the mylonite and the remainder was clearly emplaced as a result of brittle deformation in quartz–sulfide microfractures, microveins and veins. Only low volumes of gold concentration was introduced during ductile deformation, whereas, during the evident brittle deformation phase, competence contrasts allowed fracturing to focus on the quartz–sericite domain boundaries of the mylonitic foliation, thus permitting the introduction of auriferous fluid to create disseminated and cross‐cutting Au‐quartz veins. According to mineral assemblages and alteration intensity, hydrothermal alteration could be divided into three zones: silicification and sulfidation zone (major ore body); sericite and carbonate alteration zone; and sericite–chlorite alteration zone that may be taken to imply wall‐rock interaction with near neutral fluids (pH 5–6). Silicified and sulfide alteration zone is observed in the inner parts of alteration zones. High gold grades belong to silicified highly deformed mylonitic and ultramylonitic domains and silicified sulfide‐bearing microveins. Based on paragenetic relationships, three main stages of mineralization are recognized in the Qolqoleh gold deposit. Stage I encompasses deposition of large volumes of milky quartz and pyrite. Stage II includes gray and buck quartz, pyrite and minor calcite, sphalerite, subordinate chalcopyrite and gold ores. Stage III consists of comb quartz and calcite, magnetite, sphalerite, chalcopyrite, arsenopyrite, pyrrhotite and gold ores. Studies on regional geology, ore geology and ore‐forming stages have proved that the Qolqoleh deposit was formed in the compression–extension stage during the Late Cretaceous–Tertiary continental collision in a ductile–brittle shear zone, and is characterized by orogenic gold deposits.     

碎屑质韧性动力变质岩结构构造分类

根据新疆觉罗塔格碎屑质韧性动力变质岩的形变相变方式、构造、结构组成和粒度结构,将碎屑质韧性动力变质岩划分为６大类：片岩类、板状变粒岩类、糜棱岩类、压扁拉长岩类和布丁岩类,讨论了碎屑质韧性动力变质岩的组合与命名。