Paleozoic magmatic events in the Strandja Massif,NW Turkey

The Strandja massif consists of metamorphic basement intruded by large Early Permian plutons of the Kirklareli type and overlain by Triassic metasedimentary cover. Together with its continuation in Bulgaria this massif forms an important link between the Pontides and the orogenic belts of Europe. Various types of orthogneisses constitute a significant part of the metamorphic basement however these rocks have until now escaped a particular study and therefore the Paleozoic history of the massif is essentially unknown. In this study these rocks are classified and mapped as hornblende-biotite, biotite-muscovite, and leucocratic orthogneisses. Their modal compositions correspond to quartz diorite, tonalite, granodiorite and trondhjemite. Geochemical data suggest a calc-alkaline trend of differentiation and metaluminous character of the parent magmas. Isotopic dating using the single zircon evaporation method has shown that magmatic ages of these orthogneisses cluster within a short time interval between 312±2 and 315 ±5 Ma in the Carboniferous. At the same time inherited ages of magmatic zircons in these rocks record a long lived magmatic activity between 340 and 650 Ma. We infer that the Carboniferous orthogneisses were formed in a magmatic arc that evolved atop of a mature continental basement. Previously established ([1, 2]) Early Permian magmatic event has been confirmed by additional age determinations constraining it at 257±6 Ma. Tectonic setting of this episode is also interpreted as subduction related taking into consideration its geochemical features and relationships with surrounding tectonic units.     

Discovery of high-pressure granulite-facies metamorphism in northern Vietnam: Constraints on the Permo-Triassic Indochinese continental collision tectonics

High-pressure mafic granulites containing granoblastic garnet, quartz, and minor hornblende have been found from the Song Ma Suture zone in northern Vietnam, regarded as a microcontinental boundary between the South China and Indochina blocks. Fine-grained symplectite formed during the decompression stage is developed in the granulite and is divided into orthopyroxene + plagioclase and orthopyroxene + clinopyroxene + plagioclase ± hornblende. The former replaces garnet and the latter is regarded as a breakdown of sodic clinopyroxene. Detailed observation and careful data selection revealed that the high-Mg and low-Ca garnet should be in equilibrium with the precursor sodic clinopyroxene, and the pair indicates high-temperature and -pressure conditions (910–930 °C at 1.9–2.0 GPa). Although we could not obtain quantitative age data from the high-pressure granulite, the U–Th–Pb age (233 ± 5 Ma) of pelitic gneiss strongly suggests a Middle to Early Triassic metamorphic event. If the age indicates the timing of the high-pressure granulite-facies metamorphism, it might be related to a continental collision setting by following crustal subduction. According to the metamorphic signatures, north to central Vietnam may be regarded as an orogenic belt formed by the micro-continental collision between the South China and Indochina cratons.     

Timing of metamorphism and deformation in the Swat valley,northern Pakistan:Insight into garnet-monazite HREE partitioning

New metamorphic petrology and geochronology from the Loe Sar dome in the Swat region of northern Pakistan place refined constraints on the pressure, temperature and timing of metamorphism and deformation in that part of the Himalayan orogen. Thermodynamic modelling and monazite petrochronology indicate that metamorphism in the area followed a prograde evolution from ～525 ± 25 ℃and 6 士 0.5 kbar to ～610 ± 25 ℃ and 9 士 0.5 kbar, between ca. 39 Ma and 28 Ma. Partitioning of heavy rare earth elements between garnet rims and 30-28 Ma monazite are interpreted to indicate coeval crystallization at peak conditions. Microtextural relationships indicate that garnet rim growth post-dated the development of the main foliation in the area. The regional foliation is folded about large-scale N-S trending fold axes and overprinting E-W trending folds to form km-scale domal culminations. The textural relationships observed indicate that final dome development must be younger than the 30-28 Ma monazite that grew with garnet rims post-regional foliation development, but pre-doming-related deformation. This new timing constraint helps resolve discrepancy between previous interpretations,which have alternately suggested that N-S trending regional folds must be either pre-or post-early Oligocene. Finally, when combined with existing hornblende and white mica cooling ages, these new data indicate that the study area was exhumed rapidly following peak metamorphism.     

Kinematics and deformation of central and southwestern Greece from historical triangulation data and implications for the active tectonics of the Aegean

Changes in the coordinates of 38 stations, 32 of which were common to three historical triangulation surveys (1889–1905, 1927–1930 and 1950–1970) in central Greece were computed based on conventional adjustment techniques and the assumption of a nearly fixed baseline length. Some of the calculated coordinate changes are significant against the a posteriori network adjustment errors and are likely to indicate tectonic motions consistent with those deduced from geological and geophysical data and other geodetic studies (comparison of the first survey network with GPS data).

More explicitly, the historical geodetic data confirm the rotational character of the deformation in the area, but they show that the strain pattern is likely to have changed after 1930: between 1890 and 1930 strain was changing smoothly, as if no strain discontinuities existed, while the Peloponnesus was under contraction from the northeast, probably reflecting accommodation of strain from the arc. Significant left-lateral shear in the gulf of Corinth and N-S extension in the whole of the study area were observed only after 1930. Estimates of strain are consistent with those deduced from comparison of historical triangulation and GPS data, corrected for the scale error that exists in the terrestrial geodetic data in Greece, and estimates of strain from seismological data. The change in the kinematic pattern after 1930 seems to be confirmed by changes in the shear strain computed directly from changes in observed angles between stations and from changes in the seismicity rates; such changes, although not unusual in the geological record, are not easy to explain, but may reflect elastic rebound effects in area with an extremely complicated tectonic fabric or hysteresis in the accommodation of E-W compression from the arc by N-S stretching.     

Seismotectonics of the northern Aegean area

Data concerning the focal mechanism and the spatial distribution of earthquakes have been used to investigate the active tectonics of the northern Aegean and the surrounding area.A thrust region, which includes the northernmost part of the Aegean and at least part of the Marmara Sea, has been defined. An amphitheatrical Benioff zone dipping towards the thrust region from south, east and probably from west, at a mean angle of about 30°, has been detected.The thrust region is surrounded by a region of normal faulting. An eastward progression of the seismic activity in this normal faulting region between 1954 and 1971 has been observed.A correspondence between the earthquake occurrence in the thrust and normal faulting regions has also been observed. Each large shock produced by tensional mechanism in the region of normal faulting is preceded or followed by one or more shocks of compressional mechanism in the thrust region.The focal mechanism, the distribution of the earthquake foci with intermediate focal depth, as well as some magnetic and gravimetric observations can be interpreted by assuming that dense oceanic crust sinks in the northern part of this area and that the adjacent lithosphere moves by segmentation to fill the void with the consequence of producing tensile stresses associated with normal faulting. Such a mechanism of lithospheric interaction suggests that accretion probably takes place in this area.     

Tertiary plate tectonics and high-pressure metamorphism in New Caledonia

The sialic basement of New Caledonia is a Permian-Jurassic greywacke sequence which was folded and metamorphosed to prehnite-pumpellyite or low-grade greenschist facies by the Late Jurassic. Succeeding Cretaceous-Eocene sediments unconformably overlie this basement and extend outwards onto oceanic crust. Tertiary tectonism occurred in three distinct phases.

1. (1) During the Late Eocene a nappe of peridotite was obducted onto southern New Caledonia from northeast to southwest, but without causing significant metamorphism in the underlying sialic rocks.

2. (2) Oligocene compressive thrust tectonics in the northern part of the island accompanied a major east-west subduction zone, at least 30 km wide, which is identified by an imbricate system of tectonically intruded melanges and by development of lawsonite-bearing assemblages in adjacent country rocks; this high-pressure mineralogy constituted a primary metamorphism for the Cretaceous-Eocene sedimentary pile, but was overprinted on the Mesozoic prehnite-pumpellyite metagreywackes.

3. (3) Post-Oligocene transcurrent faulting along a northwest-southeast line (the sillon) parallel to the west coast caused at least 150 km of dextral offset of the southwest frontal margin of the Eocene ultramafic nappe.

At the present time, the tectonics of the southwest Pacific are related to a series of opposite facing subduction (Benioff) zones connected by transform faults extending from New Britain-Solomon Islands south through the New Hebrides to New Zealand and marking the boundary between the Australian and Pacific plates. Available geologic data from this region suggest that a similar geometry existed during the Tertiary and that the microcontinents of New Guinea, New Caledonia and New Zealand all lay along the former plate boundary which has since migrated north and east by a complex process of sea-floor spreading behind the active island arcs.     

Stress and deformation patterns in the Aegean region

The Aegean region constitutes the overriding plate of the Africa–Eurasia convergent plate system, in the eastern Mediterranean. To explain the fault kinematics and tectonic forces that controlled rift evolution in the Aegean area, we present fault-slip data from about 900 faults, and summarise the structural analyses of five key structural “provinces”. Five regional tectonic maps are used as the basis for a new stress map for the Aegean region and for discussions on regional geodynamics.Since the Late Miocene, the central Aegean has been affected by WNW- and NE-trending faults which transfer the motion of the Anatolian plate to the southwest, synchronous with arc-normal pull acting on the boundary of the Aegean plate. At the same time, the Hellenic Peninsula has suffered moderate extension by NW-trending grabens formed due to collapse of the Hellenic mountain chain.During intense extension in the southern Aegean in the Plio-Quaternary the arcuate shape of the Hellenic Trench was established. Arc-normal pull in the Aegean plate margin, combined with transform resistive forces along the Hellenic subduction gave rise to widespread strike-slip and oblique-normal faults in the eastern segment and moderate oblique extension in the western segment of the arc. To the north, subduction involves more continental crust and consequently the push of subduction is transmitted to the overriding plate (Hellenic Peninsula), resulting in the formation of NE-trending grabens. WNW-trending grabens in this area are considered to have propagated westward from the Aegean Sea to the Ionian Sea during Plio-Quaternary times, probably acting as pull-apart structures between stable Europe and the rapidly extending southern Aegean area.     

苏北榴辉岩变质变形模式与苏胶造山带的隆升机制

苏北榴辉岩经历５期以上变质变形作用,其中至少有两期以上为高压变质变形作用,ｐ—ｔ轨迹呈顺时针方向旋转。榴辉岩形成后,随着地体的反弹、推覆,于印支期末迅速回返到中地壳,再经燕山期—喜山期区域隆升和拉伸折返,最终剥露于现代侵蚀面。苏胶造山带至少经历了晋宁期、印支期和燕山期３次以上的造陆或造山运动     

No significant Alpine tectonic overprint on the Cimmerian Strandja Massif (SE Bulgaria and NW Turkey)

We provide the first comprehensive picture of the thermochronometric evolution of the Cimmerian Strandja metamorphic massif of SE Bulgaria and NW Turkey, concluding that the bulk of the massif has escaped significant Alpine-age deformation. Following Late Jurassic heating, the central part of the massif underwent a Kimmeridgian-Berriasian phase of relatively rapid cooling followed by very slow cooling in Cretaceous-to-Early Eocene times. These results are consistent with a Late Jurassic–Early Cretaceous Neocimmerian (palaeo-Alpine) phase of north-verging thrust imbrication and regional metamorphism, followed by slow cooling/exhumation driven by erosion. From a thermochronometric viewpoint, the bulk of the Cimmerian Strandja orogen was largely unaffected by the compressional stress related to the closure of the Vardar–?zmir–Ankara oceanic domain(s) to the south, contrary to the adjacent Rhodopes. Evidence of Alpine-age deformation is recorded only in the northern sector of the Strandja massif, where both basement and sedimentary rocks underwent cooling/exhumation associated with an important phase of shortening of the East Balkan fold-and-thrust belt starting in the Middle–Late Eocene. Such shortening focused in the former Srednogorie rift zone because this area had been rheologically weakened by Late Cretaceous extension.     

Polyphase Variscan tectonics and metamorphism along a segment of the Saxothuringian-Moldanubian boundary: The Baden-Baden Zone,northern Schwarzwald (F.R.G.)

The Baden-Baden Zone (BBZ) separates polymetamorphic gneisses of the Central Schwarzwald Gneiss Complex (CSGC) in the south from accreted crustal slices of different structural and metamorphic evolution in the north. From SSE to NNW, the following slices (units) can be distinguished: (A) gneisses and amphibolites metamorphosed at low pressures and temperatures of the upper amphibolite facies; (B) kyanite-garnet mica schists formed at pressures around 0.9 GPa and at temperatures between 630 and 670°C; (C) Paleozoic metasediments and metabasites metamorphosed under low pressures and temperatures (greenschist facies), and locally imbricated with mylonitic two-mica granites (S-type).Variscan convergence caused mylonitic, oblique, rightlateral deformation of Paleozoic metasediments and metabasites along NE-SW trending, SE dipping shear planes, accompanied by the intrusion of subsequently sheared porphyric two-mica granites (S-type). In unit C strain and metamorphic grade increase from NW to SE. The rocks of units A and B do not show the NE-SW trending mylonitic foliations found in rocks of unit C. Cataclastic strike-slip imbrication of mica-schists (B) and gneisses (A) was followed by the intrusion of a hornblende-biotite granite (I-type; unit D) into the low-grade metamorphics of unit C during the late Lower Carboniferous. Later on, this granite and the porphyric two-mica granites of unit D were subjected to ductile left-lateral strike-slip faulting along E-W to ENE-WSW-trending, steeply S-dipping shear zones. Geometry and kinematics of these shear zones suggest exhumation of the northern block (units C and D) relative to the southern block (units A and B) and thus point to the onset of extension. Synkinematic mineral assemblages in the northern block point to an exhumation from greenschist facies to surface conditions.South of the BBZ, two-mica granites intruded during Namurian times and subsequently were rapidly exhumed. Pebbles of these granites occur in an ENE-WSW-trending molasse basin. Continental red bed sedimentation and rhyolitic volcanism in this basin lasted from Stephanian to late Lower Permian times.

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Zusammenfassung In der Zone von Baden-Baden grenzen die polymetamorphen Gneise der südlich gelegenen Zentralschwarzwälder Gneismasse gegen akkretionierte Krustenspäne von unterschiedlicher struktureller und metamorpher Entwicklung. Von SSE nach NNW lassen sich folgende Krustenspäne (Einheiten) unterscheiden: (A) Gneise und Amphibolite, die bei niederen Drücken und amphibolitfaziellen Temperaturen kristallisierten; (B) Disthen-Granat-Glimmerschiefer und -Quarzite, die bei Drücken um 0.9 GPa und Temperaturen zwischen 630 und 670°C gebildet wurden; (C) Paläozoische Metasedimente und Metabasite, die bei geringen Drücken unter grünschieferfaziellen Temperaturen rekristallisierten und lokal mit mylonitischen Zweiglimmer-Graniten verschuppt sind.In den paläozoischen Metasedimenten und Metabasiten (C) bewirkte die variszische Konvergenz eine dextrale, mylonitische Deformation entlang von NE-SW streichenden, SE einfallenden Scherflächen. Damit verbunden war die Intrusion und darauffolgende Zerscherung porphyrischer Zweiglimmer-Granite (S-Typ). Metamorphosegrad und Deformationsintensität nehmen von NNW nach SSE zu. In den Einheiten (A) und (B) fehlt die NE-SW streichende mylonitische Foliation. Die Akkretion dieser Einheiten erfolgte wahrscheinlich entlang von E-W und NE-SW streichenden Blattverschiebungen im kataklastischen Niveau.Im späten Unterkarbon intrudierte ein porphyrischer Hornblende-Biotit-Granit (I-Typ; Einheit D) in die niedergradigen Metamorphite der Einheit (C) und bewirkte eine Kontaktmetamorphose. Dieser Hornblende-Biotit-Granit wurde später lokal entlang von E-W bis ENE-WSW streichenden, steil nach S einfallenden Scherbändern mylonitisch deformiert. Geometrie und Kinematik dieser späten Scherzonen deuten auf eine sinistrale, blattverschiebende Heraushebung des nördlichen Bereichs mit den vorher akkretionierten Abfolgen (C) und dem Hornblende-Biotit-Granit (D) gegenüber dem südlichen, ehemals tektonisch hangenden Gneisen (A) und Glimmerschiefern (B) hin und belegen damit eine beginnende Extension. Synkinematische Paragenesen zeigen eine Heraushebung von grünschieferfaziellen bis zu oberflächennahen Bedingungen an.Südlich der Zone von Baden-Baden intrudierten im Namur Zweiglimmer-Granite. Sie wurden schnell herausgehoben und erodiert. Der Abtragungsschutt wurde ab dem Stephan in einem ENE-WSW streichenden Molassebecken abgelagert. Absenkung und Ablagerung kontinentaler Sedimente und saurer Vulkanite in diesem Becken dauerte bis zum Ende des unteren Perms.

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Résumé Dans la zone de Baden-Baden (BBZ) les gneiss de la Forêt Noire Centrale (CSGC) sont bordés vers le nord par des unités lithologiques accrétionnées, qui se caractérisent par des évolutions tectono-métamorphiques différentes. Du SSE au NNW on distingue ainsi au moins trois segments cristallins sur la base de critères pétrographiques: (A) des gneiss et des amphibolites formés à basse pression et aux températures du faciés supérieur des amphibolites; (B) des micaschistes à distène et grenat formés à une pression d'environ 0.9 GPa et une température de 630° à 670 °C; (C) des métasédiments d'âge paléozoïque et des métabasites de basse pression et basse température (faciès des schistes verts) mêlés localement à des écailles mylonitiques de granites à deux micas (type S).Dans cette unité (C) la convergence varisque est à l'origine d'une déformation mylonitique associée à des cisaillements transcurrents dextres orientés NE-SW et pentes vers le SE. L'intrusion syntectonique des granites marque cette phase principale. L'intensité de la déformation et du métamorphisme augmente du NNW au SSE. Les gneiss (A) et les micaschistes (B) ne montrent pas cette déformation intense et se caractérisent par un écaillage cataclastique lors de leur accrétion. Cette phase a été suivie à la fin du Carbonifère inférieur par l'intrusion d'un granite à hornblende-biotite (type I) avec métamorphisme de contact dans les séries peu métamorphiques (unité C). Ce granite est finalement affecté par des cisaillements ductiles dans des décrochements sénestres orientés NE-SW et fortement pentés vers le SE. L'analyse géométrique et cinématique de ces structures suggère l'exhumation de l'ensemble des granites et des séries épizonales septentrionales par rapport aux unités accrétionnées et initialement sus-jacentes (A et B) méridionales et témoigne ainsi d'un début d'extension crustale. Des paragenèses, syncinématiques des mouvements extensifs, attestent de conditions «schistes verts» à superficielles.Au sud de la zone de Baden-Baden des granites à deux micas intrudés au Namurien sont finalement rapidement exhumés et soumis à l'érosion. On en trouve des galets dans le bassin molassique allongé ENE-WSW qui s'est formé dès le Stéphanien. La subsidence de ce domaine a persisté jusqu'au Permien, accompagnée d'une sédimentation continentale et d'un volcanisme acide intermittent.

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The role of the fluid phase during regional metamorphism and deformation

Evidence from rock microstructures, mass transfer and isotopic exchange indicates that substantial quantities of aqueous fluids are involved in low- and medium-grade regional metamorphism. Similar conclusions are drawn from many retrograde environments, whereas high-grade metamorphic fluids may be melt dominated. The mobile fluids play essential roles in metamorphic reactions, mass transport and deformation processes. These processes are linked by the mechanical consequences of metamorphic fluid pressures (P_f) generally being greater than or equal to the minimum principal compressive stress. Under such conditions metamorphic porosity comprises grain boundary tubules and bubbles together with continuously generated (and healed) microfractures. Deformation results in significant interconnected porosity and hence enhanced permeability. Lithologically and structurally controlled permeability variations may cause effective fluid channelling.
Simple Rayleigh-Darcy modelling of a uniformly permeable, crustal slab shows that convective instability of metamorphic fluid is expected at the permeabilities suggested for the high P_f metamorphic conditions. Complex, large-scale convective cells operating in overpressured, but capped systems may provide a satisfactory explanation for the large fluid/rock ratios and extensive mass transport demonstrated for many low- and medium-grade metamorphic environments. Such large-scale fluid circulation may have important consequences for heat transfer in and the thermal evolution of metamorphic belts.     

Phanerozoic polyorogenic deformation in southern Jiuling Massif,northern South China block: Constraints from structural analysis and geochronology

The structure of the Jiuling Massif has been investigated in order to delineate the polyorogenic deformation and discuss its geodynamic evolution and orogenic mechanisms. Detailed structural analysis indicates that the D₁ event is characterized by top-to-the NNW ductile shearing with pervasive foliation, and mineral and stretching lineation developed in the entire region. Compared with the D₁ deformation, D₂ structures are localized in ductile shear zones with subvertical foliation and subhorizontal E–W trending lineation, indicating a dextral ductile shearing. The D₃ event, marked by folds and thrusts mainly in a brittle domain, modified the D₁ structures by asymmetrical folds. The dominant D₄ structures are gravitational folds and normal faults, corresponding to a later extension. Our new geochronological data suggest that the D₁ event occurred between 465 and 380 Ma with D₂ dextral shearing at the end of this Early Paleozoic orogen, and the D₃ event has been constrained at 245–215 Ma. The final uplift of the Jiuling Massif by the D₄ event can be correlated with the Late Mesozoic extension across the eastern South China block. Along with previous studies in the South China block, the structural pattern of the Jiuling Massif elucidates the influence of the Early Paleozoic and Early Mesozoic intracontinental belts triggered by repeated reactivation of the Jiangshan–Shaoxing Fault. Combined with deformation to the south, the Early Paleozoic belt shows a positive flower pattern, with opposing kinematics, rooted in the Jiangshan–Shaoxing Fault. During the Early Mesozoic, a general intracontinental belt was developed with uniform kinematics in both the Jiuling Massif and the Xuefengshan Belt, possibly resulted from the far-field effect of the Triassic NW-directed Paleo-Pacific subduction.     

Pre-Cadomian to late-Variscan odyssey of the eastern Massif Central,France: Formation of the West European crust in a nutshell

The East Massif Central (EMC), France, is part of the internal zone of the Variscan belt where late Carboniferous crustal melting and orogenic collapse have largely obliterated the pre- to early-Variscan geological record. Nevertheless, parts of this history can be reconstructed by using in-situ U-Th-Pb-Lu-Hf isotopic data of texturally well-defined zircon grains from different lithological units. All the main rock units commonly described in the EMC are present in the area of Tournon and include meta-sedimentary and meta-igneous rocks of the Upper Gneiss Unit (UGU) and of the Lower Gneiss Unit (LGU), as well as cross-cutting Variscan granitoid dikes and a heterogeneous granite coring the major Velay dome. Herein we demonstrate that the UGU and the LGU have markedly distinct zircon records. The results of this study are consistent with deposition of the protoliths of the paragneisses within a back-arc basin that was located adjacent to the Arabian-Nubian shield and/or the Saharan Metacraton during the late Ediacaran and collected detritus from the Gondwana continent. At ~ 545 Ma some of these sedimentary rocks were affected by a first melting event that formed the protoliths of the LGU orthogneisses, those of which subsequently remelted at ca. 308 Ma to form the Velay granite-migmatite dome. Protoliths of the UGU result mainly from a bimodal rift-related magmatism at ~ 480 Ma, corresponding to melting of the Ediacaran sediments and depleted mantle. Zircon rims from the UGU additionally provide evidence for a metamorphic/migmatitic overprint during the Lower Carboniferous (~ 350–340 Ma). Finally, several generations of granite dikes of which inherited zircons display characteristics of both the UGU and the LGU were protractedly emplaced from ~ 322 Ma to ~ 308 Ma, the youngest of which being coeval with the formation of the Velay dome. Our data further show that the vast majority of crustal material ultimately involved in the Variscan orogeny, which forms the present-day basement in the EMC, was derived from a sedimentary mixture of various components from the Gondwana continent deposited in Ediacaran times, with no evidence for the involvement of an older autochthonous crust.     

New petrologic evidence for Hercynian metamorphism in the Venn-Stavelot Massif,Belgium

The basal quartzite (grit) of the Gedinne that overlies unconformably the Salmian of Ordovician age was found to contain a pebble of the typical garnetiferous Mn-rich coticules occurring in the Salmian. However, the metamorphic mineral assemblage Mn-rich garnet-chlorite-muscovite present in the pebble was also formed in the pelitic matrix of the quartzite itself and must, therefore, be due to a post-Gedinnian, Hercynian event of metamorphism. The garnets of the pebble and the matrix are of identical composition and free from chemical zoning, but they are richer in Fe than the spessartine of in situ coticules. This seems to indicate that the pebble did not contain any garnet at the time of its deposition thus ruling out any noteworthy degree of pre-Gedinne, i. e. Caledonian metamorphism. The temperature of the Hercynian metamorphism was about 400?–450? C and represents the highest degree of metamorphism found in rocks exposed in the central and northern parts of the Rhenish Mass.     

Fluids, tectonics and crustal deformation

In the plate tectonic process, lithosphere creation at ocean ridges and its cooling leads to volatile fixation in the oceanic crust. The outer 10 km or so of all crust contains abundant water in pores and fractures and variable amounts of volatiles in minerals. When surface rocks are buried by tectonic processes, fluids must be released and modify the mechanical properties. In the subduction process hydrated oceanic crust may be decoupled from the remaining oceanic lithosphere. At depth rising aqueous fluids or melts lead to a complex series of mass-energy transfer processes which may decouple continental crust near the Moho. Continental crust if subducted, may also be decoupled from its lithosphere by degassing. Fluid release processes which create gas-solid mixtures beneath impermeable cover create low-strength systems subject to facile deformation, hydraulic fracture processes and diapiric phenomena.     

煤岩构造变形与动力变质作用

煤岩是一种对温度、压力等地质环境因素十分敏感的有机岩,地质演化过程中的各种构造-热事件必然导致煤岩发生一系列物理与化学结构的变化,并形成不同类型的构造煤。在构造应力作用下,煤岩不仅发生脆性和韧性变形,而且还产生不同程度的动力变质作用。因而,关于煤岩构造变形与动力变质作用的研究不仅具有重要的科学意义,而且在煤层气资源评价以及煤与瓦斯突出危险性预测方面也具有重要的实际意义。文中在已有研究成果基础上,通过对构造煤系列Ro,max、XRD和NMR(CP/MAS+TOSS)等测试和实验方法的对比研究,深入分析了煤岩不同构造变形和动力变质特征,进一步探讨了构造应力下煤岩动力变质作用的机理。研究成果表明,在构造应力作用下,煤岩脆性变形主要是通过破裂面上快速机械摩擦转化为热能而引起煤岩化学结构与其成分的改变;而韧性变形煤主要是通过局部区域应变能的积累而引起煤岩化学结构的破坏,从而发生不同机制的动力变质作用。     

Dynamics of shallow late-Variscan gold mineralization: the Le Châtelet Au-arsenopyrite quartz veins,Massif Central,France

The Le Châtelet gold deposit is unique among the mined deposits of the French Variscan basement in that the gold is not free but caught up in the arsenopyrite crystal lattice. Microstructural analysis shows that the Au-arsenopyrite quartz veins are hosted by north-trending dextral strike-slip faults resulting from NNE-SSW compression during the early Stephanian (around 300 Ma). Textural study indicates that vein emplacement was due to sequential dynamics related to spasmodic strike-slip movements along these brittle structures (seismic regime). The classic sequence begins with the slow crystallization of sulphide-free comb quartz followed by a co-deposition of microcrystalline quartz and needles of Au-arsenopyrite; the rapid crystallization of the Au-arsenopyrite-bearing microcrystalline quartz was due to a sudden drop in fluid pressure. The veins correspond to dilational jogs formed by a suction pump mechanism during a late phase of the Variscan orogeny; they were trapped in migmatite about 1 km below surface.     

Polyphase wrench tectonics in the southern french Massif Central: kinematic inferences from pre- and syntectonic granitoids

In the Variscan French Massif Central, the South Limousin area consists of low- to medium-grade metamorphic rocks intruded by two granitic bodies. The structural and textural analyses of these plutons undertaken in parallel with the structural analysis of their host rocks allow us to characterize and to date different stages in the tectonic evolution of this area. This study shows that the South Limousin area experienced successivelly two strike-slip events along two geographically distinct shear zones, from north to south the left-lateral Estivaux and the right-lateral South Limousin strike-slip faults, respectively. These ductile faults subdivide the South Limousin into three structural units, from north to south they are the Upper Gneiss unit, Thiviers-Payzac unit .and Génis unit. The two granitic bodies intrude the Thiviers-Payzac unit only. The younger Estivaux granite is a syntectonic pluton which emplaced during left-lateral wrenching. ⁴⁰Ar/³⁹Ar dates from biotites indicate an Early Carboniferous age (346 ± 3 Ma). The older granite is a pretectonic body. It is the Ordovician Saut du Saumon augen orthogneiss in which detailed structural analyses show the polyphase nature of the solid-state deformation. Our microtectonic data indicate that the right-lateral motions overprint the left-lateral ones and produce apparently symmetrical fabrics.     

Tectonic controls on metamorphism, partial melting, and intrusion: timing and duration of regional metamorphism and magmatism in the Ni de Massif, Turkey

Mafic high-pressure granulite, eclogite and pyroxenite xenoliths have been collected from a Mesozoic volcaniclastic diatreme in Xinyang, near south margin of the Sino-Korean Craton (SKC). The high-pressure granulite xenoliths are mainly composed of fine-grained granoblasts of Grt+Cpx+Pl+Hbl±Kfs±Q±Ilm with relict porphyritic mineral assemblage of Grt+Cpx±Pl±Rt. P–T estimation indicates that the granoblastic assemblage crystallized at 765–890 °C and 1.25–1.59 GPa, corresponding to crustal depths of ca. 41–52 km with a geotherm of 75–80 mW/m². Calculated seismic velocities (V_p) of high-pressure granulites range from 7.04 to 7.56 km/s and densities (D) from 3.05 to 3.30 g/cm³. These high-pressure granulite xenoliths have different petrographic and geochemical features from the Archean mafic granulites. Elevated geotherm and petrographic evidence imply that the lithosphere of this craton was thermally disturbed in the Mesozoic prior to eruption of the host diatreme. These samples have sub-alkaline basaltic compositions, equivalent to olivine– and quartz–tholeiite. REE patterns are flat to variably LREE-enriched (La_N/Yb_N=0.98–9.47) without Eu anomaly (Eu/Eu*=0.95–1.11). They possess 48–127 ppm Ni and 2–20 ppm Nb with Nb/U and La/Nb ratios of 13–54 and 0.93–4.75, respectively, suggesting that these high-pressure granulites may be products of mantle-derived magma underplated and contaminated at the base of the lower crust. This study also implies that up to 10 km Mesozoic lowermost crust was delaminated prior to eruption of the Cenozoic basalts on the craton.