5 late variscan mineralizing systems related to orogenic processes the french massif central abstract the french massif central constitutes an exceptional study area due to the diversity of its metallic deposits its internal position in the variscan belt and the abundance of available geological geophysical and metallogenic data obtained within the geofrance 3d programme the deposits formed towards the end of the orogenic evolution represent the economic products of two distinct mineralizing systems a au sb hydrothermal system and a w sn and rare metals magmatic hydrothermal system which were simultaneously active during a short time span between ca 310 and 300 ma two types of gold deposit can be distinguished on the basis of their depth of emplacement deep seated gold deposits developed under lithostatic to hydrostatic pressure during rapid exhumation and shallow gold deposits emplaced under hydrostatic pressure with no significant uplift deposits of w sn and rare metals were emplaced in the upper crust during final crystallization of specialized magmas after their rapid ascent perhaps enhanced by simultaneous regional uplift the gold bearing systems are associated with a complex network of re activated crustal scale faults initially active during the period between 335 and 315 ma normal motion along the faults coeval with 335 to 315 ma granite migmatite domes played a major role in the 3d distribution of the hydrothermal plumbing system gold and related metals were carried within huge hydrothermal cells which reached ca 100 km by 10 km in area and 30 km in depth in contrast granites rich in magmatophile elements w sn rare metals generated smaller hydrothermal cells 10 km by 10 km in area and

provenance of neoproterozoic and early paleozoic siliciclastic rocks of the tepla barrandian unit bohemian massif evidence from u pb detrital zircon ages abstract new la icp ms u pb detrital zircon ages from ediacaran and paleozoic siliciclastic rocks are used to constrain provenance and paleogeographic affinities of the tepla barrandian unit tbu in the centre of the bohemian massif central europe czech republic the samples taken span the period from 635 ma to 385 ma and permit recognition of provenance changes that reflect changes in geotectonic regime detrital zircon age spectra of two ediacaran one lower cambrian and three upper ordovician samples resemble the ages known from the nw african proportion of gondwana particularly the trans saharan belt while three rocks from higher lower cambrian to lowermost ordovician strata contain detritus that may have been derived exclusively from local sources the age spectrum of the devonian rock is a combination of the nw gondwanan and local features these new findings in combination with a wide range of published data are in agreement with a neoproterozoic subduction related setting at the margin of gondwana followed by a cambrian early ordovician rifting stage and an ordovician passive margin setting furthermore the data are in favour of a position of the tbu at the gondwanan margin throughout pre variscan times

geochemical and isotopic study of a norite eclogite transition in the european variscan belt implications for upb zircon systematics in metabasic rocks abstract three different geochronological techniques u pb conventional multigrain single zircon stepwise pb evaporation and sm nd on minerals were applied to rocks from a norite eclogite transition observed in the west european variscan belt and combined with major and trace element including ree geochemistry the studied rocks belong to a cogenetic tholeiitic suite which originated in an immature back arc basin or within plate magmatic environment the initial ndt 480 ma values around 1 2 imply that the noritic protoliths were emplaced in a geotectonic setting in close association with the continental crust u pb mulfgrain and the sm nd dating of a norite sample indicate an age of 481 ma for the basic magmatism sm nd dating on two whole rock garnet pairs yields an age of 408 ma for the eclogite facies metamorphism these two ages are consistent with the present understanding of the european variscan belt finally the single zircon pb evaporation study of an eclogite sample demonstrates the co existence of several types of zircons each zircon population yields a particular type of geochronological information related to the contrasting behaviours of the u pb isotopic system during the eclogite facies tectonometamorphic event these analytical results suggest that complex and heterogeneous behaviour within the u pb zircon system may also be recorded in numerous other samples this last point raises the question of the significance of results obtained by u pb conventional multigrain dating in metabasic rocks

late visean thermal event in the northern part of the french massif central new 40ar 39ar and rb sr isotopic constraints on the hercynian syn orogenic extension he french massif central belongs to the central metamorphic domain of the european hercynian belt 40ar 39ar mica and amphibole dates have been obtained on magmatic and metamorphic rocks from the northern part of the french massif central sioule and brevenne violay series the 336 to 337 ma dates recorded by a granodiorite the st gervais pluton and its mylonitic border along the ste christine fault and the 337 5 ma muscovite ages from the brevenne series are consistent with the age of wrenching related to the late stage of the hercynian compression the remaining age spectra from the sioule area provide ages ranging from 331 to 335 ma that are interpreted as cooling ages related to rock exhumation after the main stages of metamorphism and deformation in the studied area late visean dates widespread in north massif central correspond to a huge magmatic event characterized by the emplacement of red granites granophyres and sub aerial acidic volcanics known as the tufs anthraciferes formation rb sr isotopic data from volcanic and hypovolcanic rocks from the sioule and montagne bourbonnaise areas place constraints on the source of the magmas simple metapelite crustal melting or a major contribution from depleted mantle sources are ruled out and instead mantle derived magmas were probably involved at least as a heat source during the late visean magmatism exhumation and cooling of metamorphic rocks below 300 c occurred contemporaneously with this late visean magmatism in the geodynamic evolution of the hercynian belt the late visean ca 335 330 ma magmatism corresponds to a major break marking the onset of post collisional extension in the inner zone which had been thickened by collision in latest devonian earliest carboniferous this late visean extension in the inner zone which is synchronous to compression in the northern and southern forelands of the belt appears as a syn orogenic process

structural ams and geochronological study of a laccolith emplaced during late variscan orogenic extension the rocles pluton se french massif central in the southern french massif central the rocles leucogranite of variscan age consists of three petrographic facies textural analysis shows that they experienced the same subsolidus deformation new chemical u th pb dating on monazite yielded 324 4 ma and 325 5 ma ages for muscovite rich and biotite rich facies respectively ams study results agree with petrostructural observations the magnetic planar and linear fabrics which correspond to the preferred orientation of biotite and muscovite are consistent with the foliation and lineation defined by the preferred mineral orientation this fabric developed during pluton emplacement the accordance of this granite foliation with that observed in the host rock suggests that the rocles pluton is a laccolith but its present geometry resulted from post emplacement southward tilting due to the uplift of the late carboniferous velay dome restoration of the primary geometry of the pluton and its country rocks to a flat lying attitude places the granite lineation close to the trend measured in other plutons of the area this restoration further supports the interpretation of the rocles laccolith as a pluton emplaced along a tectonic contact reactivated during the late orogenic collapse of the variscan belt

conventional and in situ geochronology of the tepla crystalline unit bohemian massif implications for the processes involving monazite formation the tepla crystalline unit tcu western bohemian massif proves highly suitable for studying the effects of differential metamorphic reworking on the u th pb systematics in monazite as the overprint of variscan regional metamorphism onto high grade cadomian paragneisses intensifies progressively towards the northwest although variably hampered by scarcity small size and low uranium contents of monazite isotope dilution thermal ionisation mass spectrometry of monazite from paragneisses from the garnet staurolite and kyanite zones of the tcu gives a narrow 206pb 238u age range from 387 to 382 ma for variscan peak metamorphism these data are supported by 382 373 ma monazite ages derived from electron microprobe analyses inheritance of older components in grains from the central tcu imply major resetting of pre variscan monazite around 380 ma possibly due to widespread garnet growth during variscan metamorphism which led to the consumption of pre variscan high y monazite and subsequent growth of new low y monazite concordant 498 494 ma monazite ages in a migmatitic paragneiss close to the adjacent marianske lazn complex mlc grew in response to metagabbro emplacement in the mlc from 503 to 496 ma and not during either cadomian or variscan regional metamorphism backscatter imaging and electron microprobe analyses reveal that discordant monazite of the migmatite comprises a mix of various age domains that range from ca 540 to 380 ma combined evidence presented here suggests that instead of pb loss by volume diffusion the apparent resetting of the u th pb systematics in monazite rather involves new crystal growth or regrowth by recrystallisation and dissolution reprecipitation

vertical extrusion and horizontal channel flow of orogenic lower crust key exhumation mechanisms in large hot orogens a large database of structural geochronological and petrological data combined with a bouguer anomaly map is used to develop a two stage exhumation model of deep seated rocks in the eastern sector of the variscan belt an early sub vertical fabric developed in the orogenic lower and middle crust during intracrustal folding followed by the vertical extrusion of the lower crustal rocks these events were responsible for exhumation of the orogenic lower crust from depths equivalent to 18 20 kbar to depths equivalent to 8 10 kbar and for coeval burial of upper crustal rocks to depths equivalent to 8 9 kbar following the folding and vertical extrusion event sub horizontal fabrics developed at medium to low pressure in the orogenic lower and middle crust during vertical shortening fabrics that record the early vertical extrusion originated between 350 and 340 ma during building of an orogenic root in response to se directed saxothuringian continental subduction fabrics that record the later sub horizontal exhumation event relate to an eastern promontory of the brunia continent indenting into the rheologically weaker rocks of the orogenic root indentation initiated thrusting or flow of the orogenic crust over the brunia continent in a north directed sub horizontal channel this sub horizontal flow operated between 330 and 325 ma and was responsible for a heterogeneous mixing of blocks and boudins of lower and middle crustal rocks and for their progressive thermal re equilibration the erosion depth as well as the degree of reworking decreases from south to north pointing to an outflow of lower crustal material to the surface which was subsequently eroded and deposited in a foreland basin indentation by the brunia continental promontory was highly noncoaxial with respect to the se oriented saxothuringian continental subduction in the early visean suggesting a major switch of plate configuration during the middle to late visean

tectonics and plate tectonics model for the variscan belt of europe abstract a plate tectonics model is presented to explain the tectonometamorphic characteristics of the european variscides after the closing of two oceanic domains by two sided subduction 500 420 ma and obduction 420 380 ma collision of the european and african continental plates occurred we propose that the subsequent complex intracontinental deformation 380 290 ma is the result of a double subduction of the continental lithosphere accompanied by crust mantle decollement this mechanism explains the progressive crustal thickening and migration of the deformation through time from the sutures toward the external parts of the variscan belt accounting for this model and for the relationships between the european variscides and the other paleozoic peri atlantic belts caledonides appalachian mauritanides and morocco we infer the relative positions of africa america and europe between the silurian and the permian

metamorphic evolution of the pre hercynian basement of the schwarzwald federal republic of germany abstract the high grade gneisses and migmatites of the central schwarzwald display a polyphase metamorphic history with a dominating l p h t metamorphism numerous small intercalations of eclogites or eclogitic amphibolites ultramafics both garnet and spinel bearing peridotites and pyroxenites and granulitic gneisses together with mineral relics in the high grade gneisses are indicative of h p and m p metamorphic events predating the l p h t metamorphism the widespread occurrences of these rocks their petrological evolution and their relationship to their gneissic country rocks strongly indicate that the metamorphic history of at least some of the gneiss complex is identical to that of the h and m p intercalations late hercynian low grade metamorphism is related to retrograde processes associated with the uplift history and with brittle deformation of the basement

comment on deep structure recent deformation and analog modeling of the gulf of cadiz accretionary wedge implications for the 1755 lisbon earthquake by gutscher et al 2009 abstract gutscher et al 2009b address a very interesting and unsolved problem which is the continuation of the azores gibraltar plate boundary to the e of the gorringe bank this comment comprises three parts 1 short introduction to the geologic setting 2 comment on the analogue modelling the relationship of the setup with the prototype and the relationship of the experimental results with the prototype 3 short discussion of proposed subduction to the e

eclogites with a short lived granulite facies overprint in the moldanubian zone czech republic petrology geochemistry and diffusion modelling of garnet zoning the petrology and geochemistry of a newly discovered suite of high pressure garnet clinopyroxene bearing rocks from the monotonous series of the moldanubian zone of the bohemian massif southwest czech republic have been investigated three types common eclogites quartz kyanite clino zoisite eclogites and garnet hornblende clinopyroxenites are distinguished by petrography and geochemistry all underwent a significant degree of partial breakdown under granulite and amphibolite facies conditions during exhumation important features include the growth of orthopyroxene in breakdown domains after garnet and omphacite and anorthite spinel corundum exceedingly peraluminous sapphirine replacing kyanite garnet zoning and inclusion patterns support a prograde evolution from low pressures for at least some of the samples the post eclogite stage granulite facies overprint indicates that high temperatures prevailed during exhumation but preservation of zoning in some garnets and the results of diffusion modelling suggest that this overprint took place over a very short time scale the geochemical and petrological results allow characteristic differences to be recognized between these eclogites and metabasites found in other tectonic units of south bohemia and consequently the assigning of all high pressure rocks to a single now disrupted tectonic unit is a gross simplification that seriously misrepresents the tectono metamorphic history of the region

a mantle derived bimodal suite in the hercynian belt nd isotope and trace element evidence for a subduction related rift origin of the late devonian brevenne metavolcanics massif central france in the brevenne series ne massif central a low grade bimodal association of metabasalts and metarhyolites is exposed together with intrusive trondhjemite bodies zircon u pb dating constrains their magmatic emplacement at 366 5 ma and 358 1 ma respectively the metabasalts are characterized by a distinct enrichment in incompatible elements e g th and lree and positive ndi from 5 to 8 combined isotope and trace element systematics rule out crustal contamination of mafic melts as a suitable cause of the lile large ion lithophile element enrichment rather a mixing process between a component similar to mid ocean ridge basalts and an enriched end member with ndi 5 is suggested an enriched mantle source of ocean island basalt affinity is precluded by the relative depletion of high field strength elements especially nb which shows negative anomalies in chondrite normalized patterns on the contrary a subduction related origin for the lile enrichment would be more consistent it may be inferred that arc like melts enriched in th and lree light rare earth elements and depleted in nb with ndi 5 were produced through partial melting of a depleted mantle source to which a small amount of crustally derived component had been added the metarhyolites are enriched in lile and have a close genetic relationship with the metabasalts as evidenced by their high ndi from 4 7 to 6 8 although the chemical evidence remains ambiguous it is suggested that fractional crystallization accompanied by subordinate assimilation is the petrogenetic process most consistent with the data the trondhjemites are isotopically distinct from the metarhyolites their ndi values from 1 0 to 2 2 reflect an important contribution of continental crust to their genesis and disprove their inferred cogenetism with the felsic volcanics a review of modern environments in which such bimodal suites are exposed shows that settings involving incipient rifting of a volcanic arc fringing a continental margin or built upon young thin continental crust might provide suitable analogues geodynamic reconstructions are complicated by subsequent tectonic events which disrupted the initial patterns and by mesozoic cenozoic sedimentary cover however this subduction related magmatism enlarges the growing body of evidence for southward subduction processes until the late devonian during the evolution of the northern flank of the european variscides as a general implication it is suggested that the combined use of the sm nd system with incompatible elements relatively resistant during alteration and low grade metamorphism ree th zr nb may provide diagnostic criteria for recognizing the tectonic setting of bimodal metaigneous suites in ancient orogenic belts

constraining genesis and geotectonic setting of metavolcanic complexes a multidisciplinary study of the devonian vrbno group hrubý jesenik mts czech republic the low grade metavolcanic volcanosedimentary complex of the devonian vrbno group silesicum ne bohemian massif czech republic occurs in two ne sw trending belts separated by tectonic slices of cadomian metagranitic paraautochton 1 the basic intermediate lavas of the calc alkaline western volcanic belt came from a moderately depleted mantle left varepsilon text nd 370 sim 3 right rare rhyolites 374 0 1 7 ma 2 la icp ms u pb zrn were derived most likely from immature crust or by extensive fractionation of primary basaltic melts the rock association is interpreted as a vestige of a deeply dissected continental arc 2 the eastern volcanic belt consists mainly of nearly contemporaneous 371 0 1 4 ma felsic alkaline lavas with high hfse contents as well as high ga al and fe mg ratios typical of within plate igneous setting the petrology and nd sr isotopic data point to a high t anatexis of a young metagranitic crust resembling the cadomian brunovistulian basement in a back arc setting the attenuated brunovistulian lithosphere could have partially melted by the heat provided by the upwelling asthenosphere and or underplating basic magma 3 finally the region was penetrated by numerous subalkaline morb emorb like dolerite sheets a hallmark of the considerable crustal thinning

chronology of late paleozoic tectonothermal activity in the southeastern bohemian massif austria moldanubian and moravo silesian zones 40ar 39ar mineral age controls abstract new 40ar 39ar hornblende and muscovite ages from the moldanubian and moravian zones of the southeastern bohemian massif indicate rapid cooling associated with continental underplating during early carboniferous plate collision and closure of the moravo silesian foredeep 40ar 39ar ages have been determined for hornblende concentrates from 1 amphibolite from the moldanubian nappe complex including the dobra gneiss rehberg amphibolite and loosdorf complex all part of the variegated unit 2 micaschist structurally overlying the bittesch gneiss and 3 amphibolite within the bittesch gneiss moravian zone the five hornblende concentrates display variably discordant 40ar 39ar age spectra because of contamination with extraneous argon components however 36ar 40ar vs 39ar 40ar isotope correlations are well defined for the five hornblende concentrates mswd the hornblende and muscovite ages are interpreted to date post variscan metamorphic cooling through ca 500 c and ca 400 c respectively no record of pre variscan thermal events is preserved in either the hornblende or muscovite argon systems however the widespread occurrence of extraneous argon components suggest a pre variscan stage of tectonothermal development the near concordancy of the hornblende and muscovite cooling ages suggest relatively rapid cooling through the contrasting argon closure temperatures and indicate that relatively high crustal levels had likely been attained during the early carboniferous

evidence for a lost ocean in variscan terranes of the western massif central france plate tectonics is now commonly invoked to explain the variscan orogeny1 3 but only the presence of ophiolites can provide real evidence for an early variscan ocean none has been described so far in south western europe with the possible exception of a few scattered lenses4 within the low grade moeche unit in cabo ortegal spain and the chamrousse ophiolite5 6 french alps which displays no clear relationships to documented variscan terranes to fill this gap eclogites with mid ocean ridge basalt morb compositions have been tentatively ascribed to an oceanic origin7 and various serpentinites and amphibolites have been inferred to derive from ophiolites1 3 but many geologists are still reluctant to admit that the variscan orogeny resulted from the closure of a true ocean here we describe the remains of an ophiolite found in high grade crystalline rocks in the european variscan fold belt in limousin western french massif central this ophiolite comprises diopside bearing harzburgites depleted harzburgites dunites wehrlites and morb type gabbros and has structural petrological and geochemical characteristics typical of oceanic lithosphere thereby providing evidence for a lower palaeozoic ocean

evolution of nappes in the eastern margin of the bohemian massif a kinematic interpretation the entire pile of nappes in the eastern margin of the bohemian massif is characterized by two stages of variscan nappe emplacement each exhibiting a different kinematic and metamorphic evolution

deducing the ancestry of terranes shrimp evidence for south america derived gondwana fragments in central europe we present here an example of how the sensitive high resolution ion microprobe shrimp zircon dating method can provide a terrane specific geochronological fingerprint for a rock and thus help to reveal major tectonic boundaries within orogens this method applied to inherited zircons in a ca 580 ma metagranitoid rock from the eastern bohemian massif has provided for the first time in the central european variscan basement unequivocal evidence for mesoproterozoic and late paleoproterozoic geologic events ca 1 2 ga 1 5 ga and 1 65 1 8 ga the recognition of such zircon ages has important consequences because it implies that parts of the precambrian section of variscan central europe were originally derived from a grenvillian cratonic province as opposed to the common assumption of an african connection a comparison with previously published shrimp data suggests however that these mesoproterozoic and late paleoproterozoic zircon ages may be restricted to the moravo silesian unit in the eastern variscides whereas the saxothuringian and moldanubian zones appear to contain a typical north african i e neoproterozoic plus eburnian inherited zircon age spectrum this finding supports new tectonic concepts according to which variscan europe is composed of a number of completely unrelated terranes with extremely different paleogeographic origins the moravo silesian unit can be best interpreted as a peri gondwana terrane which was situated in the realm of the amazonian cratonic province by the late precambrian comparable to the avalonian terranes of north america and the united kingdom

the rand granite in the southern schwarzwald and its geodynamic significance in the variscan belt of sw germany the rand granite is a heterogeneous metamorphosed granitoid rock complex with numerous wallrock inclusions situated in the moldanubian zone at the southern margin of the central schwarzwald gneiss complex it is a largely mylonitized elongated body and is thrust over the badenweiler lenzkirch zone forming a nappe with shear zones along its northern and southern boundaries it comprises meta granites meta trondhjemites and biotite augen gneisses derived from monzogranites to granodiorites mineral behaviour indicates that the magmatic body has been deformed under upper greenschist facies conditions nappe thrusting which also affected the south schwarzwald gneiss complex occurred in visean time during high temperature low pressure metamorphism kinematic indicators in the mylonites document e to ese directed nappe transport highly transpressive relative to the trend of the nappe boundaries and the foliation the trondhjemites formed at 351 5 4 ma predating the variscan ht metamorphism they have initial end values of 6 6 to 6 7 and relatively low initial 87sr 86sr ratios 0 7042 to 0 7063 indicating a predominant mantle origin the granites and protoliths of the biotite augen gneisses probably crystallised between 436 and 377 ma suggested by u pb zircon model ages they are different from the trondhjemites with low initial end values 4 7 to 3 3 and higher initial 87sr 86sr ratios 0 7068 0 7077 indicating that large part of the rand granite originated from anatexis of continental crust internal structure of zircons from the rand granite reveals mixing of magmas derived from both mantle and crust sources these new data support an interpretation that the rand granite developed along an active continental margin and therefore represents a possible root of a variscan magmatic arc

thick skinned versus thin skinned thrusting rheology controlled thrust propagation in the variscan collisional belt the southeastern bohemian massif czech republic austria the variscan nappe assembly within the southeastern bohemian massif includes 1 crystalline nappes which were transported from initial granulite to amphibolite facies conditions to uppermost crustal levels 2 nappe assembly of footwall units under very low to low grade metamorphic conditions and 3 frontal thin skinned imbrication of flysch sequences which were deposited within a deep and narrow foreland basin during loading of the lower plate thick skinned thrusting of hinterland tectonic units resulted in an inverted metamorphic zonation and contrasts with the thin skinned tectonic character of foreland units 40ar 39ar hornblende and muscovite plateau ages from hanging wall units reflect complete rejuvenation of intracrystalline argon isotopic systems during late variscan thrusting between 340 ma and 325 ma the 40ar 39ar mineral ages together with previously published 40ar 39ar and sm nd data record systematically decreasing variscan mineral ages structurally downward into footwall units this is explained by rapid exhumation and cooling synchronously with thrusting along crustal scale ramps during foreland directed nappe propagation the location of decollement zones within specific structural levels was apparently controlled by crustal scale rheological con trasts late variscan cooling ages contrast with cadomian 40ar 39ar hornblende and muscovite ages recorded within the structural basement immediately below the major variscan thrust plane where intracrystalline isotopic systems were only partially rejuvenated during variscan thrusting these record a sequence of cadomian magmatic and metamorphic events in foreland units including 1 pre intrusive metamorphism and deformation under amphibolite facies metamorphic conditions 610 ma 2 greisen formation during a cadomian collisional event 600 ma and 3 intrusion of calcalkaline granitoid bodies with i type affinities 590 ma

gondwana derived microcontinents the constituents of the variscan and alpine collisional orogens abstract the european variscan and alpine mountain chains are collisional orogens and are built up of pre variscan building blocks which in most cases originated at the gondwana margin such pre variscan elements were part of a pre ordovician archipelago like continental ribbon in the former eastern prolongation of avalonia and their present day distribution resulted from juxtaposition through variscan and or alpine tectonic evolution the well known nomenclatures applied to these mountain chains are the mirror of variscan resp alpine organization it is the aim of this paper to present a terminology taking into account their pre variscan evolution at the gondwana margin they may contain relics of volcanic islands with pieces of cadomian crust relics of volcanic arc settings and accretionary wedges which were separated from gondwana by initial stages of rheic ocean opening after a short lived ordovician orogenic event and amalgamation of these elements at the gondwanan margin the still continuing gondwana directed subduction triggered the formation of ordovician al rich granitoids and the latest ordovician opening of palaeo tethys an example from the alps external massifs illustrates the gradual reworking of gondwana derived pre variscan elements during the variscan and alpine tertiary orogenic cycles

folding and granite emplacement inferred from structural strain tem and gravimetric analyses the case study of the tulle antiform sw french massif central abstract in the south limousin french massif central the tulle anticline is a kilometre scale upright open fold which re folds an already foliated variscan stack of nappes three main granodioritic plutons associated with numerous folded dykes and boudinaged sills crop out in the core of the tulle anticline microstructural study including quartz c axis petrofabrics and transmission electron microscope tem analysis allows us to recognize the timing of deformation during pluton emplacement and cooling the earliest fabric is a pre full crystallization deformation the second stage corresponds to a high temperature ca 650 c solid state deformation the last deformation is responsible for the c axis preferred orientation at low temperature several methods of strain analysis applied in various mineral phases show that pluton emplacement is coeval to a constrictional strain this strain is characterized by a nw se maximum stretching axis x parallel to the tulle anticline axis and an intermediate ne sw trending shortening axis y also responsible for large open folding of the host rocks gravimetric modelling allows us to determine a laccolithic shape for the plutons hot stokes diapirism is ruled out and the hypothesis of granitic magma channelling through dykes is preferred a laccolith emplacement model based on a syn folding decollement level is proposed

the age of the kagenfels granite northern vosges and its bearing on the intrusion scheme of late variscan granitoids in the saxothuringian part of the vosges france a first series of variscan plutonic rocks diorites to granites has been intruded by several younger granites rocks of both the older generations have been cross cut by the late orogenic kagenfels granite the averages of the hitherto published mineral ages of the earlier rock generations are 331 and 334 ma respectively whereas rb sr and k ar dates around 290 ma have been reported for the kagenfels granite because of the unlikely large age hiatus a redetermination of the intrusion age of the kagenfels granite formation appeared to be irrevocable the newly obtained mineral ages on the kagenfels granite k ar and 40ar 39ar biotite ages as well as single zircon radiogenic 207pb 206pb data 331 5 ma are about 40 ma older than the previous results they are interpreted as giving the time of emplacement of the kagenfels granite during the latest viscan the mineral ages of the earlier plutonic rocks in this part of the variscan orogeny in all probability are not significantly different from their ages of intrusion therefore the age concordance of all three granitoid generations constrains a rather narrow time interval of orogenic magmatism close to the lower upper carboniferous boundary

fabric and kinematic study of the bites orthogneiss southwestern moravia result of large scale northeastward shearing parallel to the moldanubian moravian boundary abstract the body of the bites orthogneiss bo in southwestern moravia was affected by ductile shear deformation of probably late variscan age north to northeast directed shearing is indicated by the orientation of sheath fold axes s c fabric strong asymmetrical c axis preferred orientation and ne dipping shear bands deformational structures stretching lineation mylonitic foliation and sheath folds associated with thrust wrench tectonics are connected with lower amphibolite to green schist facies metamorphism a strain study shows complex fabric development from the core to the boundaries of the bo thrust sheet fabrics in the core of the bo body are characterized by oblate strain ellipsoids low strain intensity and quartz c axis fabrics that can be approximated to small circle girdles centred around the z axis of finite strain strain ellipsoids near the boundaries of the bo thrust sheet are mostly prolate with high strain intensity and quartz diagrams show single or incomplete crossed girdles highly asymmetrical with respect to the structural framework the coaxial strain history in the core and the noncoaxial history which prevails at the boundaries of the bo thrust sheet are interpreted as a result of different strain paths in the framework of a crustal scale shear zone

thermobarometry and geotectonic significance of high pressure granulites examples from the moldanubian zone of the bohemian massif in lower austria

thermobarometry and granite genesis the hercynian low p high t velay anatectic dome french massif central the velay dome french massif central offers a quasi continuous section across an anatectic domain comprising low to high grade schists gneisses and granites two main tectonometamorphic events and their related generation of granitic material were recognized in addition to a major barrrovian tangential event d2 attributed to intracontinental collision tectonics i a medium to low p high t event d3 which gave rise to migmatites and syntectonic monzonitic granites and granodiorites and ii a widespread melting event d4 which led to the generation of migmatities the velay granite and post anatectic granites thermobarometry on samples collected from both the metamorphic envelope and the granitic core distinguishes two distinct geotherms i a first associated with the d3 event characterized by p 5 kbar t 750 c and water present melting biotite remains stable which led to large scale migmatization but minor amount of granites ii a second associated with the d4 event and characterized by vapour absent melting p 4 5 kbar t 760 850 c which gave rise to the velay granites and late migmatitic granites the temperature increase during the d4 event is attributed to the intrusion of hot mafic magmas within the crust the time integrated features of the different granitic rocks in the velay dome can be directly related to ah2o in the source region and illustrate the progressive dehydration of a middle to lower crustal segment over 60 ma

hercynian polyphase tectonics in the northeast french massif central the closure of the brevenne devonian dinantian rift field and laboratory structural studies show that the devonian dinantian units of the northeast french massif central experienced a complex and contrasting tectonic metamorphic evolution during the hercynian orogeny the structural analysis of the pre middle visean brevenne violay beaujolais rocks in the loire area shows a polyphase tectonic evolution associated with greenschist to amphibolite facies metamorphism the first event d1 probably occurred in early tournaisian or latest devonian times it is responsible for the flat lying regional foliation and the nw se to n s trending lineation it is well observed in the violay group and corresponds to the nw vergent emplacement of the late devonian units upon their gneissic basement represented by the affoux gneisses the second event d2 is responsible for the ne sw to e w trending lineation to the south d2 deformation is locally reworked by the grand chemin dextral wrench fault around 345 350 ma ago this polyphase deformation is also found in several devonian dinantian areas of the ne massif central but not in morvan this tectonics corresponds to the tournaisian closure by northward thrusting and subsequent intracontinental deformation of the oceanic brevenne violay beaujolais rift which opened in devonian times in a back arc setting

modelling diverse processes in the petrogenesis of a composite batholith the central bohemian pluton central european hercynides the multiple intrusions making up the central bohemian pluton in the central european hercynides have petrographic and geochemical features consistent with the presence of four main granitoid suites major element trace element and sr nd isotopic compositions are used to model their petrogenesis partial melting of metabasic locks or of a chur like mantle source are interpreted to have produced melts parental to the most primitive calc alkaline sazava suite interaction of basic with more acidic magmas followed by extensive amphibole plagioclase dominated fractionation accounts for the production of trondhjemites alternatively the trondhjemites correspond to small degree melts of a metabasic source afc assimilation fractional crystallization modelling with a paragneiss as a contaminant and increasing d nd values simulates the characteristics of the blatna suite closed system fractionation of strongly enriched mantle derived magmas or their interaction with leucogranitic melts is deduced for the petrogenesis of the shoshonitic certovo bremeno suite partial melting of a metasedimentary source followed by k feldspar dominated fractionation accounts for the granites of rite ricany suite the progression from relatively primitive calc alkaline granitoids towards evolved k rich calc alkaline and shoshonitic rocks is interpreted to reflect the increasing enriched mantle input in the petrogenesis of the later suites the evidence for hercynian subduction is equivocal and the mantle enrichment could have been significantly older

geochronology and geochemistry of eclogites from the marianske lazně complex czech republic implications for variscan orogenesis the marianske lazn complex mlc is located in the bohemian massif along the north western margin of the tepla barrandian microplate and consists of metagabbro amphibolite and eclogite with subordinate amounts of serpentinite felsic gneiss and calcsilicate rocks the mlc is interpreted as a metaophiolite complex that marks the suture zone between the saxothuringian rocks to the north west and the tepla barrandian microplate to the south east sm nd geochronology of garnet omphacite pairs from two eclogite samples yields ages of 377 7 and 367 4 ma samples of eclogite and amphibolite do not define a whole rock sm nd isochron even though there is a large range in sm nd ratio implying that the suite of samples may not be cogenetic eclogites do not have correlated nd values and initial 87sr 86sr ratios five of the eight eclogite samples have high nd values 10 2 to 7 1 consistent with derivation from a morb like source but variable 87sr 86sr ratios 0 7033 to 0 7059 which probably reflect hydrothermal seawater alteration three other eclogite samples have lower nd values 5 4 to 0 8 and widely variable 87sr 86sr ratios 0 7033 to 0 7096 such low nd values are inconsistent with derivation from a morb source and may reflect a subduction or oceanic island basalt component in their source the mlc is an important petrotectonic element in the bohemian massif providing evidence for cambro ordovician formation of oceanic crust and interaction with seawater late devonian frasnian famennian high and medium pressure metamorphism related to closure of a saxothuringian ocean basin early carboniferous visean thrusting of the tepla terrane over saxothuringian rocks and late visean extension

a high resolution fluid inclusion δ18o record from a stalagmite in sw france modern calibration and comparison with multiple proxies abstract the 18 o of speleothem fluid inclusions can provide information about local climate and the atmospheric circulation if the fate of oxygen isotopes from precipitation to the stalagmite is well understood at villars cave sw france the present day relationship between the 18 o of precipitation and the 18 o of cave drip water has been determined by previous monitoring studies in this article we examine the link between drip water and fluid inclusions in speleothems from the same cave and investigate whether the fossil water in the stalagmites provides a record of the isotopic composition of precipitation replicated fluid inclusion isotope measurements d and 18 o from two modern calcite samples which had precipitated under the monitored stalactites are in agreement with the corresponding contemporaneous drip waters the drip water in turn represents average pluri annual precipitation this gives strong evidence that the isotopic composition of the infiltrating water is preserved in the pores of speleothem calcite thus providing a record of average precipitation 18 o at the site furthermore we measured isotope ratios in fluid inclusions in the stalagmite vil stm1 a 2300 year old sample with a high growth rate up to 0 6 mm yr 1 and a high water content which allows a high resolution reconstruction of the drip water isotopic composition covering the last two millennia the stalagmite is well dated by 13 u th ages and the counting of annual growth layers the fluid inclusion 18 o varies by 2 with high 18 o values in medieval times and low values during the little ice age indicating a temperature influence on precipitation 18 o however temperature change alone cannot account for the full range of variability observed other proxies measured in the same stalagmite indicate that the cave environment has undergone substantial changes during this period stable isotopes and trace element concentrations in calcite show large variations with amplitudes similar to those found in other villars speleothems during large climatic shifts such as dansgaard oeschger events or glacial interglacial transitions in the context of the last 2000 years the proxy variability cannot be attributed to climate alone but is likely influenced by anthropogenic deforestation and the associated changes in carbon dynamics and hydrology these changes might also have an influence on the isotope ratios in drip water relative to precipitation e g through an altered evaporation transpiration ratio nevertheless despite some common trends in all proxy records the marked shifts in the other proxies are not seen in the fluid inclusion stable isotopes which gives evidence that the relationship between precipitation and drip water 18 o in the past was similar to present day conditions and not much affected by the anthropogenic impact a replication of the fluid inclusion isotope record using stalagmites from the same region will be necessary to determine whether local cave processes or large scale temperature and atmospheric circulation dominate the fluid inclusion 18 o signal and in how far the fluid inclusions can provide a direct record of precipitation 18 o

pre variscan variscan and early alpine thermo tectonic history of the north eastern bohemian massif an 40 ar 39 ar study the orlica snieznik and jesenik mountains correspond to three main domes from west to east the snieznik keprnfk and desna domes they are composed of a basement of autochthonous gneisses a thick series of blastomylonites and a supposed para autochthonous or allochthonous metamorphic pre devonian to devonian cover their broad direction is nne ssw 40ar 39ar radiometric measurements allow three main groups of ages to be defined 1 300 310 ma represented in the keprnik and desna domes this age is interpretated following the constraints on the age of the metamorphism which is linked with the extensional process occurring during the westphalian 2 320 340 ma represented mainly in the snieznik dome but not in the keprnfk massif the nappe structure of orlik vysoka hole in the northern area of the desna dome also exhibits this age which is interpretated as reflecting the period of the major variscan barrowian metamorphism which accompanied the compressional process it is only represented in the zones where the extensional process was not strong enough to result in a complete overprinting 3 340 440 ma corresponding to a very strictly defined area in the eastern rim of the desna dome occupied by ultramylonites and mylonites these ages obtained on muscovites result from an incomplete resetting of the minerals developed during the cooling of a granitic protolith and mylonitized during the extensional process a laser probe analysis confirms the extreme inhomogeneity of the ages of the muscovites and their different resetting from one grain to another the late alpine overprinting is more discrete but can be deciphered through the low extraction temperatures with ages between 80 and 120 ma these ages can be compared with alpine ages in the close western carpathians

hot contacts of garnet peridotites in middle upper crustal levels new constraints on the nature of the late variscan high t low p event in the moldanubian central vosges ne france p t paths based on parageneses in the immediate vicinity of former high temperature contact zones between mantle peridotites and granulitic country rocks of the central vosges ne france were derived by applying several conventional thermometers and thermobarometric calculations with an internally consistent dataset the results indicate that former garnet peridotites and garnet spinel peridotites were welded together with crustal rocks at depths corresponding to 1 1 2 gpa the temperature of the crustal rocks was about 650 700 c at this stage whereas values of 1100 c garnet peridotites and 800 900 c garnet spinel peridotites were calculated for the ultramafic rocks after emplacement of the mantle rocks exhumation of the lower crust took place to a depth corresponding to 0 2 0 3 gpa the temperatures of the incorporated peridotite slices were still high 900 1000 c at this stage this is indicated by the presence of high t low p parageneses c 800 c 0 2 0 3 gpa in a small 1 10 m contact aureole around a former garnet peridotite crustal rocks distant to the peridotites equilibrated in the same pressure range at lower temperature 650 700 c high cooling rates 102 103 c ma 1 were calculated for a garnet biotite rock inclusion in the peridotites and for the crustal rocks at the contact by applying garnet biotite diffusion modelling minimum rates of 0 75 7 5 cm a 1 are required for vertical ascent of rock units 30 km vertical distance derived from the crust mantle boundary resulting in a late variscan 340 ma high t low p event

u pb dating of an early paleozoic bimodal magmatism in the french massif central and of its further metamorphic evolution in the marvejols area southern french massif central the gneissic marvejols supergroup is overthrust on the metasedimentary serie du lot deposited in part prior to 540 ma the allochtonous terranes are characterized by the occurrence of a leptyno amphibolitic group a complex association of mafic and felsic rocks of igneous and sedimentary derivation a 480 10 ma age has been obtained by u pb dating of zircons for the crystallization of both mafic and felsic meta igneous rocks these rocks were emplaced during an important extensional tectonics relics of eclogites pyrigarnites coronite gabbros and hp trondhjemites are clear evidence for a further hp ht event dated at 415 6 ma on zircons from a hp trondhjemite subsequently the marvejols supergroup underwent an amphibolite facies metamorphism with incipient mobilization dated at 345 10 ma

variscan oceans ages origins and geodynamic implications inferred from geochemical and radiometric data abstract a review of the magmatic associations providing evidence for the existence of extensional regimes during the palaeozoic evolution of variscan europe is presented the available geochronological data allow recognition of two major rifting events one in the early ordovician and the other in the middle late devonian scarce relicts of ophiolitic complexes indicate that both extensional episodes reached a true ocean spreading stage it is suggested that the variscan belt resulted from a complex two stage collision process through successive suturing in the silurian and the early carboniferous of these oceanic domains the superposition of two distinct opening closure cycles involving geographically separate oceanic basins may partly account for the unusual width of the variscan orogen

the quartz diorites of limousin elemental and isotopic evidence for devono carboniferous subduction in the hercynian belt of the french massif central a chain of late devonian early carboniferous intrusions on the western margin of the french massif central the limousin tonalite belt consists of quartz diorites which are unusual among french hercynian granitoids in their low silica contents 53 68 wt sio2 and abundant hornblende they represent the earliest and most primitive plutonism post dating the main phase of hercynian metamorphism and have chemical characteristics strongly resembling andesitic lavas of calc alkaline affinity they differ from other massif central hercynian granitoids in their low 87sr86sr360 0 7047 0 7059 and high nd360 0 7 to 1 8 their initial pb composition was rather homogeneous 206pb204pb360 18 15 18 38 207pb204pb360 15 57 15 62 208pb204pb360 38 06 38 26 and their 207pb204pbi values are significantly lower than those of other hercynian granitoids the limousin quartz diorites are considered to have originated from an enriched mantle with characteristics modified by subduction of oceanic crust and terrigenous sediments the primitive quartz dioritic magmas were then contaminated in the lower continental crust by an afc process involving acid metaigneous or metasedimentary lithologies the occurrence of such plutons in the earliest phase of hercynian plutonism in the massif central is probably related to minor subduction at an active continental margin

electron microprobe monazite geochronology of magmatic events examples from variscan migmatites and granitoids massif central france u th pb dating of monazite with the electron probe microanalyser epma is increasingly documented as a reliable geochronological method offering high spatial resolution this method has been applied on monazite from the cevennes migmatites and granitoids from the southeast of the french massif central measurements were performed on separated grains after systematic back scattered electron bse imaging monazites from migmatites record two main ages i a protolith age of about 550 543 ma obtained on inherited cores and ii a migmatization event between 329 5 and 323 3 ma recorded by monazite rims and all other monogenetic grains monazite from the peraluminous rocles pluton yields a 318 3 ma age finally three granite dykes are dated at 333 6 318 5 and 311 5 ma the older dyke is the most deformed of them and is interpreted as linked to the migmatization event the two other dykes are geochronologically petrologically and structurally coeval with the rocles pluton the data constrain the timing of crustal melting following variscan thickening in the northern cevennes area migmatization of ordovician protoliths took place at 329 323 ma and was shortly followed by intrusion of leucogranite at 318 311 ma the study shows that epma dating of monazite can be successfully used to resolve a close succession of regional melting events

tectonometamorphic evolution of the bohemian massif evidence from high pressure metamorphic rocks the variscan orogenic belt of which the bohemian massif is a part is typically recognized for its characteristic low pressure high temperature metamorphism and a large volume of granites however there are also bodies of high pressure rocks eclogites garnet peridotites and high pressure granulites which are small in size but widely distributed throughtout the massif initially the high pressure rocks were considered to be relicts of a much older orogenic event but the increasing data derived from isotopic and geochronological investigations show that many of these rocks have palaeozoic protoliths metamorphic ages from the high pressure rocks define no single event instead a number of discrete clusters of ages are found between about 430 ma and the time of the dominant low pressure event at around 320 330 ma

chemical non isotopic and isotopic dating of phanerozoic zircon a case study of durbachite from the třebic pluton bohemian massif abstract although chemical non isotopic u th pb dating of monazite by electron microprobe is well established dating of zircon by this method is still controversial relatively u rich zircon up to 2500 ppm of u from the t ebic pluton provides a test case for the technique zircon grains are commonly partially or completely metamict and therefore susceptible to pb loss and common pb contamination using ca and k contents as indicators of damaged or contaminated zircon a chime chemical u th total pb method age of 347 8 18 ma was obtained from data with 0 0062 wt of pbo this age was confirmed by isotopic u pb shrimp dating 341 6 2 8 ma these results are in agreement with previous estimates for the timing of durbachitic intrusions into the root of the variscan orogen and from the variscan basement incorporated into western alps between 343 and 332 ma

the moldanubian zone in the french massif central vosges schwarzwald and bohemian massif revisited differences and similarities in order to portray the main differences and similarities between the northeastern variscan segments french massif central fmc vosges black forest and bohemian massif bm we review their crustal scale architectures the specific rock associations and lithotectonic sequences as well as the ages of the main magmatic and metamorphic events this review demonstrates significant differences between the moldanubian domains in thebmand the fmc on this basis we propose distinguishing between the eastern and western moldanubian zones while the vosges black forest mountains are an intermediate section between the bm and the fmc the observed differences are the result of first the presence in the french segment of an early large scale accretionary system prior to the main variscan collision and second the duration of saxothuringian armorican subduction which generated long lived magmatic arc and back arc systems in the bohemian segment while the magmatic activity in the fmc was comparably short lived

model of syn convergent extrusion of orogenic lower crust in the core of the variscan belt implications for exhumation of high pressure rocks in large hot orogens reflection seismic section field structural analysis and gravimetric modelling of orogenic lower crust in the core of a carboniferous orogenic root reveal details of the polyphase process of exhumation subvertical amphibolite facies fabrics strike parallel to former plate margins that collided in the nw the fabrics are developed in both mid crustal and lower crustal high pressure granulite units as a result of intensive nw se intraroot horizontal shortening driven probably by the west directed collision in granulites the steep fabrics originated as a result of extrusion of orogenic lower crust in a 20 km wide vertical ascent channel from lower crustal depths at 350 340 ma the large granulite bodies preserve older granulite facies fabrics documenting a two stage evolution during the exhumation process surface exposures of granulites coincide with the absence of subhorizontal seismic reflectors at depth suggesting preservation of the 20 km wide subvertical tabular structure reaching moho depths horizontal seismic reflectors surrounding the vertical channel structure corroborate a dominant flat migmatitic fabric developed in all tectonic units this structural pattern is interpreted in terms of subhorizontal spreading of partially molten orogenic lower crust in mid crustal levels 765 c and 0 76 gpa at 342 337 ma large massifs of extruded and progressively dismembered felsic granulites disturbed mid crustal fabrics in the surrounding horizontally flowing partially molten crust the horizontal mid crustal flow resulted in collapse of the supra crustal tepla barrandian unit interpreted as the orogenic lid along a large scale crustal detachment above the extruded lower crustal dome the presence of felsic granulites at the bottom of the orogenic root is considered to be a key factor controlling the exhumation of orogenic lower crust in large hot orogens