the role of sediment subduction and crustal growth in hercynian plutonism isotopic and trace element evidence from the sardinia corsica batholith

geodynamic evolution of the sw europe variscides 1 the early evolution of sw europe variscides started by opening of the rheic ocean at 500 ma splitting avalonia from armorica iberia subduction on the se side of rheic generated the paleotethys back arc basin 430 390 ma splitting armorica from iberia with development of porto tomar ferreira do alentejo ptfa dextral transform defining the boundary between continental armorica and finisterra microplate to the w obduction of paleotethys was followed by armorica iberia collision and emplacement of nw iberian allochthonous units at 390 370 ma whereas toward the west of ptfa there was antithetic ophiolite obduction beja acebuches and rheic ophiolites plus finisterra continental slices on top of ossa morena zone with simultaneous development of eclogites and orogenic magmatism under a flake double wedge tectonic regime continued convergence 370 ma proceeded by intracontinental deformation with progressive tightening of the ibero armorican arc through dextral transpression on the cantabrian indentor from iberia to armorica the proposed model is discussed at the light of the driving mechanism of soft plate tectonics

cambro ordovician anatexis and magmatic recycling at the thinned gondwana margin new constraints from the kouřim unit bohemian massif the kou im unit represents one of the largest pre variscan meta igneous complexes in the bohemian massif and a geochronological whole rock geochemical and sr nd isotopic study was conducted to better understand the magmatic evolution of the early paleozoic gondwana margin five orthogneisses yield u pb zircon ages ranging from 492 4 to 484 2 ma two leucogranites give u pb zircon ages of 500 4 and 485 2 ma interpreted to be inherited from their orthogneiss host rock two samples from the metasedimentary host rock are dominated by neoproterozoic cambrian detrital zircons the abundance of zircon inheritance in the orthogneisses and whole rock sr nd isotopic composition imply an origin from relatively mature continental crustal material the subalkaline subaluminous slightly peraluminous and high k calc alkaline arc like geochemical signature of the orthogneisses is interpreted as inherited from the recycled cadomian metasedimentary source and both the magmatic and metasedimentary rocks are correlated with similar occurrences in the adjacent moldanubicum and tepla barrandian unit the late cambrian early ordovician magmatic activity is linked to crustal anatexis which was probably initiated by thermal and gravitational relaxation of the thickened cadomian arc type crust followed by lithospheric thinning assisted by far field forces the extensional event led to the formation of a passive margin associated with the opening of the rheic ocean supplementary material analytical methods petrographic descriptions with photographs of studied samples collections of cl images of representative zircon grains results of la icp ms u pb analyses of zircons and an additional geochemical diagram are available at https doi org 10 6084 m9 figshare c 4649255

review of geochemical variation in lower palaeozoic metabasites from the ne bohemian massif intracratonic rifting and plume ridge interaction

the tectonic frame of the variscan alleghanian orogen in southern europe and northern africa abstract by confronting different geological and geophysical data we attempt to reconstruct the variscan alleghanian orogenic belt with especial emphasis on the links between iberia northwest africa and northeast america the northern iberia transect corresponds to the rifted margin of gondwana inverted during the variscan orogeny and overthrust in its westernmost sector by a pile of allochthonous units some of them with oceanic affinity and witnessing an orogenic suture the southern iberia section reveals two sutures at both boundaries of a terrane closely tied to gondwana namely the ossa morena zone the southern boundary of this zone i e the contact with the south portuguese zone is enhanced by amphibolites with oceanic affinity and probably represents the suture of the rheic ocean the moroccan variscides can be divided into i a western external zone namely the coastal block and the central massif ii an internal zone namely the eastern meseta iii an eastern external zone represented in the anti atlas region and iv the african cratonic foreland since neither ophiolites nor eclogites crop out along this transect of the orogen we consider it to be an incomplete transect lacking the main suture zone rheic ocean stratigraphic and faunal affinities between the moroccan meseta on the one hand and the central iberian western asturian leonese and cantabrian zones of iberia on the other hand suggest that they may well have been part of a common pre orogenic domain at the margin of gondwana on the contrary there are no counterparts in morocco for the ossa morena zone thus the northern iberia palaeogeographic zones would prolong in the moroccan meseta through an eastward arcuate structure while the suture of the rheic ocean would do so offshore morocco in our reconstruction an avalonian spur grand banks indentor which included the caledonian sehoul block in northern morocco and the crust of the south portuguese zone in sw iberia played an important role on the kinematic evolution of the orogen as well as on the final configuration of the suture of the rheic ocean this suture would mould around the avalonian spur being laterally translated to southeastern nova scotia newfoundland

saxothuringian zone in germany and poland differences and common features the classical correlation by kossmat 1927 of the saxothuringian belt of germany with the west sudetes of poland is re evaluated with special reference to the crystalline massifs of munchberg and the gory sowie and the surrounding areas important common features are cambro ordovician magmatism and sedimentation silurian to devonian radiolarian cherts in the low grade to unmetamorphosed units early devonian and older pressure dominated metamorphism followed by rapid uplift the incorporation of high pressure and mantle rocks and early carboniferous mass flow deposits with saxothuringian clasts differing features in the west sudetes are tectonic contacts in and around the gory sowie are mostly subvertical and mainly semi brittle in contrast with subhorizontal and ductile contracts in the munchberg the gory sowie has undergone late re equilibration of the earlier tectonic fabrics an ophiolite juxtaposed with the gory sowie crystallized in devonian time and became only statically metamorphosed as opposed to the regionally metamorphosed cambro ordovician mafic rocks of the saxothuringian belt localized depocenters with upper devonian to lower permian elastic rocks relating to north west trending strike slip faults instead of the north east trending cylindroidal foreland basin in the saxothuringian it is possible that the west sudetes originated as an easterly continuation of the saxothuringian belt but then became involved from the late devonian onwards in a strike slip system parallel with the margin of the east european platform

cambrian ordovician depositional sequences in the middle east a perspective from turkey abstract in southern turkey taurus chain or taurides and southeastern turkey border folds of the arabian plate nearly complete cambrian to ordovician successions are preserved four major sedimentary sequences are defined according to the main transgressive events and the subsequent shelf progradations these sequences allow us to link the turkish mostly distal lower palaeozoic detritals to the more proximal arabian formations in spite of contrasting facies the lower palaeozoic development is outlined through four successive steps defined in southern turkey 1 initiation of a cratonic platform regime terreneuvian i e earliest cambrian depositional sequence 1 with fluviatile conglomerates and red bed sediments associated with volcanics 2 development of a stable marine platform late terreneuvian to early ordovician depositional sequences 2 and 3 including widespread but diachronous middle cambrian carbonates and subsequent storm dominated detritals 3 tectonic instability middle ordovician leading to a new palaeogeography on the gondwana platform with the differentiation of a sag basin bedinan qasim depocenter lower part of depositional sequence 4 and 4 glaciation latest ordovician upper part of depositional sequence 4 identified by glaciomarine deposits forming the hirnantian glacial record this thick 2000 m detrital succession records a first order long term subsidence driven cambrian to ordovician transgressive trend and includes second order sequences reflecting interactions between global eustasy and tectonic instability on a larger scale the identification of recurrent sequence stratigraphic features with similar trends in the cambrian ordovician successions on the arabian platform implies that a former location of the taurus domain cannot be situated in the present day eastern mediterranean area but should lie directly north of the arabian platform this location places southern turkey at the junction between the north african segment of the gondwana siliciclastic shelf to the west present day coordinates and the eastern arabian segment in good agreement with mixed faunal signatures from both areas primary control of the tectonostratigraphic development of southern turkey during the lower palaeozoic may be a forced evolution from arc platform to rift drift transitions cordilleran model that has migrated progressively eastwards from the avalonian domain along the northern gondwana margin to the arabian domain throughout the lower palaeozoic

early palaeozoic rift related magmatism in variscan europe fragmentation of the armorican terrane assemblage early palaeozoic bimodal rift related magmatism is widespread throughout much of the variscides of europe it is traceable from the polish sudetes to nw iberia granitic plutonism generally predates cambro ordovician bimodal magmatism in the n bohemian massif this early palaeozoic granitic plutonism was generated by partial melting of cadomian basement whereas contemporaneous alkali granites with a mantle component are typical of the nw iberian massif silurian devonian mafic magmatism in the n bohemian massif massif central and nw iberian massif is partly preserved as obducted ophiolites compositional diversity displayed by cambro ordovician mafic magmatism can be accounted for by interaction between a spreading centre and an upwelling mantle plume this indicates that combined tensional forces and mantle plume convection assisted the early palaeozoic dispersal of terranes from the n gondwana margin continued fragmentation resulted in development of an archipelago of related terranes separated by a network of seaways and formation of oceanic crust

the palaeozoic variscan oceans revisited abstract geological evidence supported by biogeographical data and in accord with palaeomagnetic constraints indicates that one ocean models for the variscides should be discarded and confirms instead the existence of three gondwana derived microcontinents which were involved in the variscan collision avalonia north armorica franconia and thuringia subdivided by a failed vesser rift and south armorica central iberia armorica bohemia all divided by small oceans in addition parts of south eastern europe including adria and apulia are combined here under the new name of palaeo adria which was also peri gondwanan in the early palaeozoic oceanic separations were formed by the break up of the northern gondwana margin from the late cambrian onwards most of the oceans or seaways remained narrow but much like the alpine cenozoic oceans gave birth to orogenic belts with hp uhp metamorphism and extensive allochthons the saxo thuringian ocean between north and south armorica and the galicia moldanubian ocean between south armorica and palaeo adria only the rheic ocean between avalonia and peri gondwana was wide enough to be unambiguously recorded by biogeography and palaeomagnetism and its north western arm closed before or during the emsian in europe ridge subduction under the northernmost part of armorica in the emsian created the narrow and short lived rheno hercynian ocean it is that ocean and not the rheic whose opening and closure controlled the evolution of the rheno hercynian foldbelt in south west iberia south west england germany and moravia czech republic devonian magmatism and sedimentation set within belts of early variscan deformation and metamorphism are probably strike slip related the first arrival of flysch on the forelands and or the age of deformation of foreland sequences constrains the sequential closure of the variscan seaways galicia moldanubian in the givetian saxo thuringian in the early famennian rheno hercynian in the tournaisian additional mid to late devonian and partly early carboniferous magmatism and extension in the rheno hercynian saxo thuringian and galicia moldanubian basins overlapped with variscan geodynamics as strictly defined the early carboniferous episode was the start of episodic anorogenic heating which lasted until the permian and probably relates to tethys rifting

records of precambrian early palaeozoic volcanic and sedimentary processes in the central european variscides a review of shrimp zircon data from the kaczawa succession sudetes sw poland abstract the early pre orogenic stages of evolution in the variscan belt i e rifting processes opening of sedimentary basins and associated igneous activities are often obscure because many successions have yielded little or no biostratigraphic data have a strong metamorphic overprint and are tectonically deformed and dislocated the increasing application of shrimp zircon dating has provided useful constraints on magmatic and metamorphic processes helped locate probable source areas for detritus within sedimentary successions and facilitated large scale palaeogeographic correlations this methodology has recently thrown considerable light on the age and relationships of the previously poorly constrained rock units of the kaczawa complex in the polish west sudetes thus recent shrimp studies in the kaczawa mountains have yielded early ordovician ages of the initial rift type bimodal volcanic suites at the bottom part of the kaczawa succession c 503 ma for metarhyodacites of crustal derivation and c 485 ma for alkaline metatrachytes of mantle signature these dates provide a firm temporal constraint on the initial rift magmatism interpreted as related to the continental break up of the northern peripheries of gondwana new shrimp data from metavolcaniclastic and metasedimentary rocks of the kaczawa complex have yielded results that have provided significantly changed interpretations on their age and relationships for instance a siliciclastic sequence interpreted as belonging to the lower part of the kaczawa complex the gackowa sandstones and seemingly sourced using an array of geochemical and mineralogical evidence from nearby early ordovician volcanic rocks has surprisingly yielded zircon ages not younger than precambrian and thus this unit has tentatively been reinterpreted as a possible correlative of the neoproterozoic lusatian graywackes felsic metavolcaniclastic rocks embedded in the carbonate succession of the wojcieszow limestone have yielded shrimp zircon ages of c 497 ma thus supporting a late cambrian early ordovician age for this shallow water limestone sequence which had been proposed to be late ordovician silurian or younger in age based on controversial foraminifera evidence and more recently assigned to the mid cambrian based on newly found archaeocyathid fragments and age spectra of zircon populations in the metamudstone sequence of the radzimowice slates of thus far enigmatic stratigraphic position indicate that these deposits may have accumulated during early palaeozoic rifting with source areas similar to those known from parts of gondwana e g nw africa overall the new shrimp zircon studies both in metavolcanic and metasedimentary sequences have considerably illuminated their affinities and should provide encouragement for further application of this technique in this region in general the results provide support and detail for interpretation of this rock complex as reflecting the early stages of development of the volcanic and sedimentary processes operating along north gondwana during latest precambrian and early palaeozoic times

ordovician passive continental margin magmatism in the central european variscides u pb zircon data from the se part of the karkonosze izera massif sudetes sw poland approximately 500 ma old orthogneisses are widespread in the eastern part of the variscan belt and are commonly interpreted to have intruded mica schist series of assumed neoproterozoic age new shrimp zircon ages of quartzofeldspathic metavolcanogenic rocks of the mica schist series in the eastern part of the karkonosze izera massif sw poland indicate that they are late cambrian early ordovician rather than neoproterozoic in age based on the zircon age spectra distributed mainly between ca 500 and 660 ma with a few proterozoic inherited minimum ages of ca 970 and 1 825 ma younger zircon dates dispersed between ca 412 and 464 ma are interpreted as a result of pb loss likely caused by subsequent metamorphism consequently the felsic metavolcanogenic rocks appear to be roughly contemporaneous with the intrusion of ca 500 ma old orthogneiss protoliths with the pooled concordia age of 487 8 ma interpreted as the best approximation of the protolith intrusive age field relationships petrological and geochemical features of the felsic and mafic rocks studied support a model in which the accompanying mica schist series are not the original country rocks to the ca 500 ma granite intrusions and indicate that their recent close proximity is the result of tectonic juxtaposition however both the mica schists enclosing the bimodal metavolcanic rocks and the orthogneisses are interpreted to represent a cambro ordovician passive continental margin sequence being part of the saxothuringian domain they are tectonically overlain to the east by hp t metamorphic units comprising morb type metaigneous rocks and delineating a tectonic suture separating the saxothuringian block in the west from an assumed continental block tepla barrandian to the south east

early palaeozoic palaeobiogeography of the variscan regions abstract the palaeobiogeographic method appears to be an attractive alternative for reconstruction of the palaeogeography of the variscan regions during the palaeozoic the evolution of faunal assemblages and climatic characteristics from the cambrian up to the end of the devonian demonstrates that baltica and north gondwana represent two major and independent palaeogeographic entities separated by a mid european rheic ocean which opened as early as the cambrian and closed progressively from the devonian the south armorican ocean whose closure generated the so called ligerian orogen is regarded as a branch of the rheic ocean both a tornquist s sea separating baltica from its western extension avalonia during the ordovician and a proto tethys ocean extending between southwestern europe and north africa during the devonian are excluded from our reconstructions

first record of a large callixylon trunk from the late devonian of gondwana a 5 m long permineralized trunk from the upper kellwasser member of southern morocco represents the first record of a large trunk of an identifiable species of callixylon c erianum from gondwana this occurrence constitutes the most reliable evidence based on plant megafossils for a floral connection between laurussia and gondwana in late devonian times and for a proximity of these continents in famennian times the potential of this trunk for studies of the architecture and growth patterns of the earliest trees with a gymnospermous type of arborescent habit is discussed

geochemistry and tectonic significance of metabasic suites in the gory sowie block sw poland gneisses of the gory sowie block of probable late precambrian early cambrian age contain numerous small metabasite bodies which may have originated as dykes or sills many metabasites have phase assemblages and textures which show that they suvered early granulite facies and later amphibolite facies metamorphism although some exhibit no evidence for the earlier event trace element geochemistry enabled four metabasite groups to be distinguished a a dominant meta tholeiite group sowie group probably incorporating more than one set of intrusive rocks b meta tholeiites characterized by higher nb y high nb y group c ti poor metabasites myslecin group that exhibit strongly depleted hfse d alkali metabasalts wloki group sowie group meta tholeiites were also divided according to their dominant host lithology granitoid gneiss migmatitic paragneiss or gneiss containing relict granulite facies rocks those in granitoid gneiss are compositionally restricted although they fall within the same compositional range as other sowie meta tholeiites and may comprise both early metabasites and a later intrusive group postdating granite emplacement both the latter metabasites and the granitoid gneiss lack phase assemblages indicative of granulite facies metamorphism this suggests that both were emplaced after the granulite facies metamorphic event but before or during amphibolite facies metamorphism an early ordovician emplacement age for syn metamorphic granitoid gneiss shows that the gory sowie block underwent an ordovician early caledonian metamorphic event following metabasite emplacement or was at this grade during the ordovician nonetheless the chemistry of all the metabasite groups and their geological setting is consistent with emplacement either through continental crust undergoing extension at a passive continental margin or during the formation of an intracratonic basin before subsequent metamorphism recorded a local compressional event

evolution of geochemistry and depositional settings of early palaeozoic siliciclastics of the barrandian tepla barrandian unit bohemian massif czech republic cambrian and ordovician middle devonian sequences of two successive early palaeozoic basins of the barrandian unconformably overlie cadomian basement in the bohemian massif nw interior tepla barrandian unit which is the easternmost peri gondwanan remnant within the variscides correlation of stratigraphy and geochemistry of the early palaeozoic siliciclastic rocks elucidated sediment provenances sandstones of the middle cambrian p ibram jince basin were derived from a cadomian neoproterozoic island arc the source area of the ordovician shallow marine siliciclastics of the successor prague basin is a dissected cadomian orogen late cambrian acid volcanics of the barrandian and cambrian meta granitoids emplaced in the w part of the tepla barrandian cadomian basement are also discernible in these sediments old sedimentary component increased during the ordovician early llandovery siliciclastic rocks show characteristics of an abruptly weakened supply of terrigenous material and an elevated proportion of synsedimentary basic volcanics as a result of silurian transgression emsian siliciclastics intercalated in the late silurian to early devonian limestone suite presumably comprise an addition of coeval basic ultrabasic volcaniclastics middle devonian flysch like siliciclastics indicate reappearance of cadomian source near the barrandian during early variscan convergences of armorican microplates that preceeded accretion of the tepla barrandian unit within the bohemian massif terrane mosaic

magmatic and metamorphic evolution of metagabbros in the munchberg massif n e bavaria the central gneiss complex of the munchberg massif consists of the liegendserie at the base and the hangendserie at the top metagabbros are found in the liegendserie eclogites occur in the hangendserie new isotope data revive the discussion whether a genetic relationship exists between metagabbros and eclogites in the munchberg massif it is possible to relate the two types of rocks to the same protoliths by variable degrees of crustal contamination and magmatic accumulation therefore a common magmatic origin may be assumed both the metagabbros and the eclogites were affected by amphibolite facies metamorphism the amphibolitization of the metagabbros was a prograde metamorphic event increasing temperature is indicated by inverse zonation of recrystallized plagioclase and increasing mg fe ratios in garnet from core to rim geothermobarometry yields a temperature of 600 c and a pressure of about 11 kbar for the peak of metamorphism in contrast the eclogites underwent a first high pressure stage at a minimum pressure of 14 kbar and a temperature estimated at 600 c and were subsequently overprinted under amphibolite facies conditions at 10 kbar 700 c a common magmatic origin of metagabbros liegendserie and eclogites hangendserie of the munchberg massif can no longer be discarded however the converging p t t paths reflect a different geodynamic evolution of the liegendserie and the hangendserie after magmatism and before amphibolite facies metamorphism

the early palaeozoic geography of europe in the early ordovician three main continents are recognized by faunas in europe laurentia northwest of the closed suture of the iapetus ocean which was tropical baltica north of the general area of the trans european suture zone which was temperate and gondwana south of the tesz which was high latitude as the ordovician progressed various terranes separated and drifted away from the gondwana supercontinent at different times namely avalonia which was probably originally part of gondwana near south america iberia armorica perunica bohemia and various alpine fragments each terrane developed progressively different faunas as time went by avalonia collided with baltica near the end of ordovician confirmed by faunas tectonics and palaeomagnetism and subsequently avalonia baltica collided with laurentia to form laurussia from the mid silurian to the early devonian the ibero armorica perunica and alpine terranes did not join the european collage until the devonian the modern sites of the terrane boundaries bear little direct relationships to the original palaeogeographical boundaries in the early palaeozoic

estimation of the crustal bulk properties beneath mainland portugal from p wave teleseismic receiver functions in this work we present results from teleseismic p wave receiver functions prfs obtained in portugal western iberia a dense seismic station deployment conducted between 2010 and 2012 in the scope of the wilas project and covering the entire country allowed the most spatially extensive probing on the bulk crustal seismic properties of portugal up to date the application of the h stacking algorithm to the prfs enabled us to estimate the crustal thickness h and the average crustal ratio of the p and s waves velocities v p v s for the region observations of moho conversions indicate that this interface is relatively smooth with the crustal thickness ranging between 24 and 34 km with an average of 30 km the highest v p v s values are found on the mesozoic cenozoic crust beneath the western and southern coastal domain of portugal whereas the lowest values correspond to palaeozoic crust underlying the remaining part of the subject area an average v p v s is found to be 1 72 ranging 1 63 1 86 across the study area indicating a predominantly felsic composition overall we systematically observe a decrease of v p v s with increasing crustal thickness taken as a whole our results indicate a clear distinction between the geological zones of the variscan iberian massif in portugal the overall shape of the anomalies conditioned by the shape of the ibero armorican arc and associated late paleozoic suture zones and the meso cenozoic basin associated with atlantic rifting stages thickened crust 30 34 km across the studied region may be inherited from continental collision during the paleozoic variscan orogeny an anomalous crustal thinning to around 28 km is observed beneath the central part of the central iberian zone and the eastern part of south portuguese zone

u pb ages from sw poland evidence for a caledonian suture zone between baltica and gondwana faunal and palaeomagnetic evidence suggests the existence of a c 3000 km wide tornquist sea between gondwana and baltica in early ordovician times which narrowed to 50 2 ma an unmetamorphosed ophiolite gives a 420 20 2 ma age whilst in two other terranes amphibolite facies island arc lavas have fossiliferous lower ludlow 424 415 ma sequences all are unconformably overlain by unmetamorphosed upper devonian conglomerates a sinistral transpressive regime is observed in regionally extensive mylonite zones titanite and zircon ages 338 2 3 and 339 4 ma record variscan magmatism the data suggest considerable narrowing of the tornquist sea during the ordovician continuing ocean floor and island arc activity in the silurian and final sinistral transpressive closure by the mid devonian

ordovician magmatism in the levezou massif french massif central tectonic and geodynamic implications new u pb dating on zircon yielded ca 470 ma ages for the granitoids from the levezou massif in the southern french massif central these new ages do not support the previous interpretation of these granitoids as syn tectonic intrusions emplaced during the late devonian early carboniferous thrusting the geochemical and isotopic nature of this magmatism is linked to a major magmatic ordovician event recorded throughout the european variscan belt and related to extreme thinning of continental margins during a rifting event or a back arc extension the comparable isotopic signatures of these granitoids on each side of the eclogite bearing leptyno amphibolitic complex in the levezou massif together with the fact that they were emplaced at the same time strongly suggest that these granitoids were originally part of a single unit tectonically duplicated by either isoclinal folding or thrusting during the variscan tectonics

palaeogeographical and palaeoecological aspects of the cambro ordovician radiation of echinoderms in gondwanan africa and peri gondwanan europe abstract ecology and tempo of the lower palaeozoic radiation of echinoderms are discussed in this paper based on comparison of the diversity patterns observed in cambro ordovician faunas from laurentia and the northern gondwana margin the cambrian agronomic revolution triggered a global radiation of echinoderms with the progressive disappearance of biomat related lifestyles and the colonisation of new environments both in laurentia and on the northern gondwana margin soft substrate echinoderm assemblages related to cold and or deep environments were dominated by blastozoans and stylophorans these assemblages show a pattern of continuous diversification from the middle cambrian to the middle ordovician the major radiation of crinoids in the lower to middle ordovician of laurentia is only a local diversification correlated with the development of hardgrounds in warm shallow environments no comparable major ordovician radiation event occurred in the cooler environments of the northern gondwana margin

palaeozoic terranes and their lithospheric boundaries within the trans european suture zone tesz a review the trans european suture zone tesz is a broad and complex zone of terrane accretion separating ancient lithosphere of the baltic shield and east european craton eec from the younger lithosphere of western and southern europe there is debate about the number of terranes involved and their origins the most significant terrane boundaries originally oceanic sutures are poorly exposed and are defined using faunal provinciality prominent geophysical lineaments may represent sub terrane boundaries within composite terranes the avalonia composite terrane was amalgamated to baltica in latest ician time the provenance of the malopolska and moravo silesian terranes whose neoproterozoic basement is suspect with respect to baltica but with which they nonetheless share faunal affinities is still debated the extent of the variscide rhenohercynian zone defines the southern limit of the early palaeozoic accreted terranes terranes within the bohemian massif exhibit hp metamorphism recording subduction driven collision prior to incorporation in the tesz collage itself associated with a hp event in romania the saxothuringian zone and rheic suture are directly juxtaposed with the eec and the crustal structure of the tesz has been much modified by post palaeozoic tectonism deep seismic reflection data where available show that the oceanic sutures are frequently associated with inclined reflectivity zones irz in the lithospheric mantle here inferred to mark relict subduction zones interpreting the age of subduction is complicated by offset of originally coplanar mantle and crustal segments of the irz resulting from early orogenic lithospheric delamination at the moho interface and or post orogenic reordering processes the mantle irz appear to indicate a n directed polarity for early palaeozoic subduction apparently incompatible with the geological evidence which indicates subduction of the tornquist sea beneath avalonia and a large amount of underthrusting of avalonia by baltica possible explanations for these apparently incompatible observations are evaluated

crustal structure of the transpressional variscan orogen of sw iberia sw iberia deep seismic reflection profile iberseis 1 iberseis a 303 km long 20 s deep seismic reflection profile was acquired across the variscan belt in sw iberian peninsula the acquisition parameters were designed to obtain a high resolution crustal scale image of this orogen the seismic profile samples three major tectonic terranes the south portuguese zone the ossa morena zone and the central iberian zone which were accreted in late paleozoic times these terranes show a distinctive seismic signature as do the sutures separating them late strike slip movements through crustal wedges are apparent in the seismic image and have strongly modified the geometry of sutures the upper crust appears to be decoupled from the lower crust all along the seismic line but some deformation has been accommodated at deeper levels a sill like structure is imaged in the middle crust as a 1 2 s thick and 175 km long high amplitude conspicuous reflective band it is interpreted as a great intrusion of mafic magma in a midcrustal decollement taking into account surface geological data and the revealed crustal architecture a tectonic evolution is proposed for sw iberia which includes transpressional collision interacting during early carboniferous with a mantle plume the moho can be identified along the entire transect as subhorizontal and located at 10 to 11 s indicating a 30 35 km average crustal thickness its seismic signature changes laterally being very reflective beneath the south portuguese zone and the central iberian zone but discontinuous and diffuse below the ossa morena zone

provenance implications of th u pb electron microprobe ages from detrital monazite in the carboniferous upper silesia coal basin poland abstract this paper reports the results of chime chemical th u pb isochron method dating of detrital monazites from carboniferous sandstones in the upper silesia coal basin uscb a total of 4739 spots on 863 monazite grains were analyzed from samples of sandstone derived from six stratigraphic units in the sedimentary sequence age distributions were identified in detrital monazites from the uscb sequence and correlated with specific dated domains in potential source areas most monazites in all samples yielded ca 300 320 ma variscan ages however eo variscan caledonian and cadomian ages were also obtained the predominant ages are comparable to reported ages of certain tectonostratigraphic domains in the polyorogenic bohemian massif bm which suggests that various crystalline lithologies in the bm were the dominant sources of uscb sediments

evolution of the northern santa cruz mountains by advection of crust past a san andreas fault bend the late quaternary marine terraces near santa cruz california reflect uplift associated with the nearby restraining bend on the san andreas fault excellent correspondence of the coseismic vertical displacement field caused by the 17 october 1989 magnitude 7 1 loma prieta earthquake and the present elevations of these terraces allows calculation of maximum long term uplift rates 1 to 2 kilometers west of the san andreas fault of 0 8 millimeters per year over several million years this uplift in concert with the right lateral translation of the resulting topography and with continual attack by geomorphic processes can account for the general topography of the northern santa cruz mountains

a structural model for the western central sudetes a deformed stack of variscan thrust sheets a model based on new structural and geochemical data is presented it unifies the structural history of the izera rudawy janowickie and kaczawa complexes with the fore sudetic block despite their current separation by the intra sudetic and marginal sudetic faults above the granitoid izera kowary and w droze gneisses at the base of the structural sequence the ductile kowary shear zone marks the basal decollement of the wierzawa thrust sheet comprising often highly strained metasediments associated with enriched tholeiitic and alkaline metabasites above it the ductile kaczorow shear zone corresponding to the main mylonitic zone within the leszczyniec shear zone in the rudawy janowickie complex marks the base of the dobromierz thrust sheet characterized by voluminous morb like meta tholeiites and minor metasediments in the higher parts of the rudawy janowickie and kaczawa complexes and the pyszczynska hill area of the fore sudetic block in the east the sleza ophiolite and the gory sowie block override the entire nappe stack kinematic fabrics in the major and related shear zones indicate d 1 compressional transport towards the northwest followed by minor d 2 extensional movements the thrust stack was deformed during d 3 by southwest verging folds was subsequently intruded by post orogenic granites and later disrupted by the intra sudetic marginal sudetic and associated faults

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

geochemistry of early palaeozoic amphibolites from the orlica śnieznik dome bohemian massif petrogenesis and palaeotectonic aspects orthoamphibolites within and marginal to the orlica nieznik dome in the eastern bohemian massif are associated with a series of proterozoic lower palaeozoic supracrustal meta sediments of the stronie nove m sto and stare m sto formations massive and variably foliated amphibolites range from common epidote plagioclase blue green green hornblende varieties to assemblages with diopside garnet and brown amphibole all the amphibolites have a basaltic composition and are divided into three main chemical groups which are intimately associated in the field main series tholeiites low ti tholeiites and alkali basalts all groups exhibit a common enrichment in lil elements nb ta relative to high field strength elements which reflects contamination by upper continental crust and or pelagic sediment compositions the gross chemistry of the tholeiitesapproaches that for morb although the geological environment and nature of crustal contamination suggests that the basaltic precursors were probably emplaced in a rifted ensialic basinal environment if the amphibolites are representative of an early phase of the lower palaeozoic fragmentation of gondwanaland then rifting did not fully develop ocean crust the wider significance of the chemical interpretation implies that some of the variation from variscan proterozoic palaeozoic metabasite suites that is ascribed to a complex of different eruptive settings could be accounted for by variable contamination and or source composition

the magmatic evolution at the moroccan outboard of the west african craton between the late neoproterozoic and the early palaeozoic

palaeozoic evolution of the variscan vosges mountains a geological synthesis of the palaeozoic vosges mountains ne france is presented using existing observations and new data the geodynamic evolution involves 1 early palaeozoic sedimentation and magmatism 2 late devonian subduction triggering back arc spreading 3 early lower carboniferous continental subduction continent continent collision and polyphase deformation and metamorphism of the orogenic root and 4 late lower carboniferous orogenic collapse driven by thermal weakening of the middle crust the evolution is integrated within the framework of the european variscan belt the northern vosges comprise sediments of rhenohercynian affinity separated from tepla barrandian metasediments by a lower carboniferous magmatic arc the latter is correlated with the mid german crystalline rise and is ascribed to the south directed subduction of the rhenohercynian basin the saxothuringian moldanubian suture is thought to be obliterated by the magmatic arc while the lalaye lubine fault is interpreted as the tepla barrandian moldanubian boundary the central vosges are paralleled with the moldanubian domain of the bohemian massif where identical lithologies record the devonian carboniferous se directed subduction of the saxothuringian passive margin below the moldanubian upper plate the southern vosges represent the upper moldanubian crust and are linked to the southern black forest the presence of an oceanic domain to the south of the vosges black forest remains unclear

palaeozoic terrane amalgamation in central europe a ree and sm nd isotope study of the pre variscan basement ne bohemian massif

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

baltica a synopsis of vendian permian palaeomagnetic data and their palaeotectonic implications torsvlk t h smethurst m a van der voo r trench a abrahamsen n and halvorsen e 1992 baltlca a synopsis of vendlan permmn palaeomagnetic data and their palaeotectonlc implications earth sci rev 33 133 152 in light of recent additions to the palaeozoic palaeo magnetlc data base particularly for the ordovlclan era a revised apparent polar wander apw path for baltica has been constructed following a rigorous synthesis of all late precambrlan perm lan data the apw path is characterized by two prominent loops firstly a late precambrlan camb rlan loop probably relating to a rifting event and secondly a younger loop relating to a mid silurian scandlan colhsion event these features imply major change in plate tectonic reconfiguratlon baltlca probably represented an individual continental unit m early palaeozoic times and was positioned m high southerly latitudes in an inverted geographic orientation in such a reconstruction baltlca was separated from the northern margin of gondwana by the tornqulst sea and from laurentla by the iapetus ocean the tornqulst zone is thus interpreted as a passive or dextral transform margin during the early palaeozoic while undergoing counter clockwise rotations up to 1 6 ma baltica drifted northward through most ol the palaeozoic except for a short period of southerly movement in late silurian early devonian times after colhslon with laurentla rapid movements in latitude up to 9 cm yr are noted m late precambrlan earl y palaeozoic times and slgmficant decrease in velocities throughout palaeozoic time probably reflect the progressive amalgamation of a larger continent by early devonian euramerlca and permian pangea times the tornqulst sea had a principal component of palaeo east west orientation hence it is difficult to be precise in the timing of when micro continents such as eastern avalonla and the european masslfs ultimately collided along the southwestern margin of baltica these micro continents are considered to have been peripheral to gondwana m high southerly latitudes during the early ordovlclan eastern avalonla clearly had rifted off gondwana by llanvlrn llanded o times and may have collided with baltlca during late ordovlclan times although the present available sdunan palaeomagnetlc data from eastern avalonia may suggest colhslon in late silurian times across the lapetus facing margin of baltica laurentla was s tuated in equatorml to southerly latitudes during most of the lower palaeozoic these continents collided in mid silurian times l e a first collision between southwestern norway and greenland scotla nd which gave rise to the early scandlan orogeny 425 ma in southwestern norway possible followed by a later but less dramatic scandlan event in northern norway at around 410 ma since baltlca was geographically reverted m early palaeozoic times the colhsional margin could not have been a margin that once rifted off laurentm as assumed in a number of plate tectonic models

kinematic constraints on buckling a lithospheric scale orocline along the northern margin of gondwana a geologic synthesis abstract the paleozoic variscan orogeny was a large scale collisional event involving amalgamation of multiple continents and micro continents existing data suggests oroclinal buckling of an originally near linear convergent margin during the last stages of variscan deformation in the late paleozoic closure of the rheic ocean resulted in e w shortening present day coordinates in the carboniferous producing a near linear n s trending east verging belt subsequent n s shortening near the carb permian boundary resulted in oroclinal buckling this late stage orogenic event remains an enigmatic part of final pangea amalgamation the present day arc curvature of the variscan has inspired many tectonic models with little agreement between them while there is general consensus that two separate phases of deformation occurred various models consider that curvature was caused by dextral transpression around a gondwana indentor strike slip wrench tectonics or a change in tectonic transport direction due to changing stress fields more recent models explain the curvature as an orocline with potentially two opposite facing bends caused by secondary rotations deciphering the kinematic history of curved orogens is difficult and requires establishment of two deformation phases an initial compressive phase that forms a relatively linear belt and a second phase that causes vertical axis rotation of the orogenic limbs historically the most robust technique to accurately quantify vertical axis rotation in curved orogens is paleomagnetic analysis but recently other types of data including fracture geochemical petrologic paleo current and calcite twin data have been used to corroborate secondary buckling a review of existing and new variscan data from iberia is presented that argues for secondary buckling of an originally linear orogenic system together these data constrain oroclinal buckling of the cantabrian orocline to have occurred in about 10 ma during the latest carboniferous which agrees well with recent geodynamical models and structural data that relate oroclinal buckling with lithospheric delamination in the variscan

early ordovician continental break up in variscan europe ndsr isotope and trace element evidence from bimodal igneous associations of the southern massif central france abstract the high grade marvejols group and the low grade albigeois cevennes sedimentary sequence contain bimodal igneous rocks of early ordovician age which are representative of a widespread thermal event in the european variscides comparison of their nd sr isotope and trace element characteristics provides additional evidence for their origin in an ensialic extensional setting as an alternative to the back arc model proposed by previous authors we propose that these associations record a continental break up episode unrelated to contemporaneous subduction in this model the widespread early paleozoic bimodal magmatism marks the birth of a mid european oceanic arm we propose that the break up was controlled by both transtensional processes and mantle plume activity high grade associations such as the marvejols group and low grade bimodal associations such as the albigeois cevennes might be regarded as representing opposite rifted passive margins the northern marvejols type margin was involved in a late ordovician silurian subduction like process with hp ht metamorphism in contrast the southern albigeois cevennes margin remained largely unaffected possibly as part of the overriding plate of the subduction zone pseudo calc alkaline signatures unrelated to subduction processes may occur in magmatic rocks associated with continental break up episodes in this case negative nb anomalies are produced by the addition of crustal components enriched in th and lree relative to nb this inferred alternative origin of nb anomalies has important bearing on the paleogeodynamic settings based on geochemical data

geochemical characterization and isotopic age of caradocian magmatism in the northeastern iberian peninsula insights into the late ordovician evolution of the northern gondwana margin this paper presents new geochronological geochemical and isotopic data for late ordovician magmatism 455 2 ma from the variscan massifs in the northeastern iberian peninsula this magmatism is mainly composed of aluminous and metaluminous plutonic rocks and a sequence of calc alkaline ignimbrites and epiclastic volcanic rocks which were deformed and metamorphosed during the variscan orogeny the metaplutonic rocks are located in the lower part of a neoproterozoic early cambrian metasedimentary sequence in the canigo massif eastern pyrenees aluminous orthogneisses have isotopic signatures that indicate a crustal origin whereas metaluminous orthogneisses originate from a mixture of juvenile and crustal melts the metavolcanic rocks define a calc alkaline ignimbritic sequence of crustal origin located in the lower part of the late ordovician sequence in the les gavarres massif catalan coastal ranges tdm ages for all the studied late ordovician rocks range from mesoproterozoic to paleoproterozoic 1 3 ga 1 5 ga 1 7 ga and 2 2 ga suggesting a neoproterozoic crust with paleoproterozoic components and an arc signature as the most probable source for the crustal melts the isotopic similarities between the studied samples and other early ordovician magmatic rocks of the central and western iberian massif suggest repeated melt extractions from a common basement as the source for the early and late ordovician magmatism in the iberian realm however the studied massifs are more closely linked to other massifs from the mediterranean realm the most probable tectonic setting for the emplacement of the late ordovician volcanic and plutonic bodies is that of an extensional regime which postdates a previous ordovician contractional event and the opening of the rheic ocean

cyanobacterial mats from silurian black radiolarian cherts phototrophic life at the edge of darkness abstract remains of silicified microbial mats composed of benthic colonial coccoid cyanobacteria similar to modern entophysalidaceans and or pleurocapsaleans have been identified in lower silurian black radiolarian cherts from central and southwestern poland contrary to widespread views ascribing the genesis of such deposits to permanently anoxic deep water marine environments the abundance of benthic mats of phototrophic cyanobacteria suggests that the water mat interface must have been located at moderate depth most probably close to the limit of light penetration dysphotic zone depending on ambient sulfide levels the mats could intermittently perform anoxygenic psi or oxygenic psii photosynthesis thriving under anoxic oxic or dysoxic microaerophilic conditions the open marine offshore character of these cherts is consistent with their paleooceanographic location and with the presence of remains of such planktonic organisms as acritarchs radiolarians chitinozoans and graptolites entrapped by the cyanobacterial mats

rapid extensional unroofing of a granite migmatite dome with relics of high pressure rocks the podolsko complex bohemian massif the podolsko complex bohemian massif is a high grade dome that is exposed along the suprastructure infrastructure boundary of the variscan orogen and records snapshots of its protracted evolution the dome is cored by leucocratic migmatites and anatectic granites that enclose relics of high to ultrahigh pressure rocks and is mantled by biotite migmatites and paragneisses whose degree of anatexis decreases outwards our new u pb zircon ages indicate that the leucocratic migmatites were derived from early ordovician c 480 ma felsic igneous crust the same age is inferred for melting the proto source of the metapelitic migmatites the relics of high to ultrahigh pressure rocks suggest that at least some portions of the complex witnessed an early variscan subduction to mantle depths followed by high temperature anatexis and syntectonic growth of the podolsko dome in the middle crust at c 340 339 ma subsequently the dome exhumation was accommodated by crustal scale extensional detachments similar c 340 ma ages have also been reported from other segments of the variscan belt yet the significance of this tectonothermal event remains uncertain here we conclude that the 340 ma age post dates the high pressure metamorphism the high temperatures required to cause the observed isotopic resetting and new growth of zircon were probably caused by heat input from the underlying mantle and finally the extensional unroofing of the complex requires a minimum throw of about 8 10 km we use this as an argument for significant early carboniferous palaeotopography in the interior of the variscan orogen

geochemistry and tectonic implications of two ordovician bimodal igneous complexes southern french massif central two bimodal meta igneous complexes from the southern massif central provide important tectonic constraints on the initial stage of the palaeozoic orogen the metabasites may be subdivided into two groups according to the distribution of the large ion lithophile lil and high field strength hfs elements n type morbs with low lil hfs ratios and subduction type basalts with high lil hfs ratios these associations suggest a complex tectonic setting such as an extensional zone or a back arc basin above a subduction zone the great volume of felsic rocks with low incompatible trace element contents indicates partial melting of an underlying continental crust the association of terrigeneous sediments with n type morbs subduction type basalts and high silica peraluminous rhyolites argue against an intra oceanic setting and suggest that the area was one of ensialic crustal tension during middle ordovician the two complexes are identified as remnants of ensialic back arc basins rather than relicts of major oceanic sutures such marginal basins could have been located in a mainly continental domain such as the armorican microplate

the amphibolites from the ossa morena central iberian variscan suture southwestern iberian massif geochemistry and tectonic interpretation abstract basic rocks included in the ossa morena central iberian variscan suture of sw iberia have been studied in order to decipher the pre orogenic evolution along this major boundary these rocks appear as garnet free amphibolites and garnet bearing amphibolites previous geochronological data on these basic rocks suggest the existence of two different protolith ages upper precambrian and lower palaeozoic ordovician upper precambrian amphibolites form small size up to several metres thick lense dike or elongated shaped bodies while lower palaeozoic ones form elongated bodies with thicknesses up to several hectometres the chemical compositions of the amphibolites enable us to separate cumulate rocks from those representing more basaltic magmas as for the originally basaltic rocks their geochemical characteristics indicate a shallow melting with different proportions of at least three end members namely i a morb like asthenospheric source ii a more enriched plume derived or subcontinental lithosphere mantle source and iii a continental crust component moreover upper precambrian and lower palaeozoic amphibolites have different geochemical affinities in the former group we envisage an advanced intra continental rifting situation or an intra continental back arc setting related to the cadomian orogeny while for the latter we propose an environment of oceanic crust formation predating variscan subduction and collision the recognition of these remains of oceanic affinity basic rocks along the omz ciz boundary allows us to propose that some sort of ocean may well have existed between the two zones during early palaeozoic times and that this ocean was presumably closed at a later stage probably during the devonian by oceanic subduction under the ciz crust

geochemical evidence for progressive rift related early palaeozoic volcanism in the western sudetes the volcanigenic rocks of the kaczawa mts western sudetes poland in the eastern variscides show changing geological and geochemical evolution during early palaeozoic time the lower part of the succession cambrian ordovician has three components 1 shallow marine to subaerial metabasalts associated with limestones and volcaniclastics the lavas are dominantly of a transitional tholeiitic alkaline type and their trace element patterns typically represent a rift related environment they pass laterally and upwards into more depleted basalts resembling enriched morb with nd isotopic characteristics indicating contamination by continental crust 2 interlayered rhyodacitic lavas and volcaniclastics which show negative nd values suggesting formation of the original magma by crustal melting 3 an overlying alkaline bimodal suite of lavas and volcaniclastic rocks as well as alkaline metabasites of shallow intrusive character the geochemistry of the latter resembles oceanic island volcanics but they may well have been emplaced in the same evolving rift environment the upper part of the kaczawa succession ordovician silurian in age comprises a thick and monotonous sequence of deep marine pillowed and massive metabasalts associated with black shales and cherts these lavas exhibit morb trace element characteristics with minor evidence of crustal contamination during this stage of rifting true oceanic crust probably formed it is thus suggested that the studied part of the kaczawa succession developed in a progressively evolving rift initially within an ensialic environment and finally reaching the stage of a basin underlain by oceanic type crust together with similar cambrian ordovician volcanic sedimentary associations widely distributed in western europe from portugal through france and germany they represent a record of the early palaeozoic rifting in the northern periphery of gondwana

variscan exhumation of a subducted paleozoic continental margin the basal units of the ordenes complex galicia nw spain a structural and metamorphic study was carried out in the basal units of the ordenes complex in spain thought to represent a subducted part of the paleozoic margin of gondwana according to their metamorphic evolution this part of the margin was subducted at the onset of the variscan orogeny becoming part of an accretionary complex developed below a colliding element built previously variations in the pt conditions of the first high pressure metamorphic event along the units indicate a polarity of the subduction to the west subsequent underthrusting of more continental material blocked the subduction and triggered the ascent and exhumation of the basal units whereas the convergence continued recumbent folds and thrusts developed along with successive normal detachments compressional and extensional structures were synchronous or alternated in time and together induced the thinning and tapering of the orogenic wedge and its lateral spreading the unroofing took place locally under an inverted temperature gradient caused by a detachment which carried a part of the hot mantle wedge above the subduction zone over the subducted units

recognition of the trans european suture zone tesz by the palaeobiogeographical distribution pattern of early to middle ordovician acritarchs

the south western branch of the variscan belt evidence from morocco abstract this work is based on the compilation and re evaluation of the most significant data either personal or from the literature concerning the moroccan variscides the latter constitute the only moderately disturbed or even undisturbed part of the south western branch of the variscan belt facing directly nw gondwana they include two orogenic segments namely the northern mauritanides and the meseta domain exposed in the saharan and atlas meseta regions respectively and a foreland belt cropping out essentially in the anti atlas the eastward thrust units of saharan morocco oulad dlim mostly originate from the west african craton wac border in an area of thin palaeozoic sedimentation thin skinned fold thrust foreland arcs develop progressively northward zemmour at the expense of the increasingly thick palaeozoic series whereas thick skinned deformation characterizes the inverted proximal paleomargin in the anti atlas domain as suggested by the meseta and anti atlas stratigraphic similarities the meseta domain corresponds to a collage of moderately displaced thinned crustal blocks from the distal gondwana paleomargin variscan deformation is dominated by nw verging thrusts and metamorphism developed in the thickened tectonic prism in relation with crustal anatexis at depth the meseta anti atlas boundary is a major ene trending transpressional dextral fault referred to as the south meseta fault smf discussing the correlations between the variscan segments of morocco and sw iberia allows us to suggest that a latitudinal transform zone similar to the smf separated these segments during the late palaeozoic subduction of the rheic ocean crust would have been directed se ward along both the iberian and moroccan meseta and nw ward south of the smf i e along the wac

vestige of an early cambrian incipient oceanic crust incorporated in the variscan orogen letovice complex bohemian massif abstract the letovice complex composed of metamorphosed enriched mid ocean ridge basalts with trondhjemitic sheets as well as gabbroic and ultrabasic bodies is exposed within an important variscan suture separating the moldanubian domain to the west from the brunovistulicum to the east in the eastern part of the bohemian massif czech republic a polyphase tectonic evolution of the study area was connected with underthrusting and subsequent exhumation of the former oceanic realm which resulted in three deformation phases d 1 d 3 new u pb laser ablation inductively coupled plasma mass spectrometry dating of magmatic zircons from amphibolite and trondhjemite yielded statistically identical concordia ages of 530 6 ma and 529 7 ma 2 respectively these are interpreted as dating intrusions of the early cambrian protoliths of the studied rocks based on this age as well as whole rock geochemical and nd isotopic data the letovice complex is interpreted as a vestige of an incipient oceanic basin developing on attenuated crust the most likely geotectonic setting was a post cadomian extensional regime at the northern margin of gondwana that marked the onset of its break up supplementary material photomicrographs of microstructures sample locations descriptions of analytical techniques and selected electron microprobe analyses of the main rock forming minerals are available at http www geolsoc org uk sup18424

thermo mechanical role of a cambro ordovician paleorift during the variscan collision the ne margin of the bohemian massif abstract the stare m sto sm belt ne margin of the bohemian massif represents a preserved example of an intracontinental cambro ordovician rift that has been shortened during the variscan orogeny the rifted sequence consists of gabbroic intrusions and a leptyno amphibolite complex of cambro ordovician protolith age the latter suffered medium pressure granulite facies cambro ordovician metamorphism associated with extension in a continental protorift the variscan tectonometamorphic event is manifested by convergent orogenesis with syn convergent intrusion of a carboniferous tonalitic sill high temperature medium pressure compressional deformation of gabbros and high temperature metamorphism of peridotites numerical thermal and rheological modelling suggests that the sm paleorift after 140 ma of cooling represented a stiff heterogeneity between an easterly situated cadomian continent and a westerly lying variscan orogenic root zone carboniferous underplating of the paleorift by tonalitic magma was probably responsible for the thermal weakening of both the upper mantle and the mafic lower crust and the subsequent uplift of the remaining rigid part of the cambro ordovician crust all the units were later deformed in transpressional heterogeneous greenschist facies regime at a supracrustal level

thermal evolution of the kisseynew sedimentary gneiss belt manitoba metamorphism at an early proterozoic accretionary margin

u pb zircon ages and structural development of metagranitoids of the tepla crystalline complex evidence for pervasive cambrian plutonism within the bohemian massif czech republic u pb zircon dating of three metagranitoids situated within a tilted crustal section at the northwestern border of the tepla barrandian unit tepla crystalline complex tcc yields similar cambrian ages the u pb data of zircons of the tepla orthogneiss define an upper intercept age of 513 7 6 ma the 207pb 206pb ages of 516 10 and 511 10 ma of nearly concordant zircons of the hanov orthogneiss and the lestkov granite are interpreted to be close to the formation age of the granitoid protolith similar to the cambrian granitoids of the southwestern part of the tepla barrandian unit doma lice crystalline complex dcc the middle cambrian emplacement of the tcc granitoids postdates cadomian deformation and metamorphism of the upper proterozoic country rocks but predates variscan tectonometamorphic imprints structural data as well as sedimentological criteria suggest a dextral transtensional setting during the cambrian plutonism related to the early paleozoic break up of northern gondwana due to strong variscan crustal tilting the degree of variscan tectonometamorphic overprint is strikingly different in the dated granitoids it is lowest in the weakly or undeformed lestkov granite located in the greenschist facies domain the tepla orthogneiss in the north underwent pervasive top to nw mylonitic shearing under amphibolite facies conditions there is no indication for a resetting of the u pb isotopic system of the tepla orthogneiss zircons that could be attributed to this imprint radiation damages accumulated until recent have probably caused lead loss

petrogenesis of early paleozoic peralkaline rhyolites from the macedo de cavaleiros region ne portugal the petrology and geochemistry of variscan peralkaline gneisses from ne portugal discovered in 1962 is presented major and trace element data for the oversaturated sequence of the macedo de cavaleiros area depict some specific geochemical patterns high fe2o3 t even higher than in the ethiopian main rift high ree concentrations and small eu anomalies field and petrographical observations also suggest that liquid immiscibility has played an important petrogenetic role in the crystallization trends the alternating pyroxenefeldsparhematite rich bands observed in outcrop are interpreted as resulting from fluctuating fo2 conditions petrological and geochemical data show that these pantelleritic and comenditic rocks are the end members of an alkaline transitional basaltic sequence by which they are overlain an uppermost sequence of oceanic tholeiitic basalts completes the picture of an evolving rift environment where mantle processes generate crustal thinning and eventually oceanic crust

early palaeozoic crustal melting in an extensional setting petrological and sm nd evidence from the izera granite gneisses polish sudetes the izera block in the west sudetes which is composed of granites gneisses and transitional granite gneisses and minor mica schists is one of the largest outcrops of early palaeozoic ca 500 ma metagranitoid rocks in the basement units of the variscides of central europe the izera granites show s type features magmatic cordierite relict garnet and sillimanite lack of mafic enclaves and absence of coexisting tonalites and diorites the paucity of pegmatites indicates that the granitic magma was relatively dry the s type character of these granites is further supported by their peraluminous character a cnk 1 0 1 63 high content of normative corundum up to 3 5 and relatively high 87sr 86sr initial ratio the chemical variation of these rocks was controlled by the fractional crystallization of plagioclase cao sr eu eu biotite and cordierite al2o3 mgo feo zircon zr hf and monazite ree initial end values range from 5 2 to 6 9 mean 5 9 sd 0 6 these largely negative end values imply that the granitic magmas emplaced ca 500 ma were extracted from a source reservoir that was strongly enriched in lree i e with low sm nd ratio on a time integrated basis the relatively consistent depleted mantle model ages 1 730 2 175 ma mean 1 890 ma is in agreement with the earlier reported presence of ca 2 1 ga old inherited pb component in zircon from the closely related rumburk granite this points to an old early proterozoic crustal residence age of the inferred metasedimentary protoliths of the izera granitoids with only minor contribution to their protoliths of juvenile components of late proterozoic early palaeozoic age although the izera granites show some trace element features reminiscent of syn collisional or post collisional granitoids they more likely belong to the broad anorogenic class our data corroborate some previous interpretations that granite generation was connected with the early palaeozoic rifting of the passive margin of the saxothuringian block well documented in the region by bimodal volcanic suites of similar age kaczawa unit eastern and southern envelope of the karkonosze izera block in this scenario granite magmatism and bimodal volcanism would represent two broadly concomitant effects of a single major event of lithospheric break up at the northern edge of gondwana

failed silurian continental rifting at the nw margin of gondwana evidence from basaltic volcanism of the prague basin tepla barrandian unit bohemian massif the silurian volcanic rocks of the prague basin represent within plate transitional alkali to tholeiitic basalts which erupted in a continental rift setting through the thick cadomian crust of the tepla barrandian unit bohemian massif despite the variable often intense alteration resulting in post magmatic replacement of the basalt mass due to carbonatization the geochemical signatures of silurian basalts are still sufficiently preserved to constrain primary magmatic processes and geotectonic setting the studied interval of silurian volcanic activity ranges from wenlock homerian 431 ma to late ludlow gorstian 425 ma with a distinct peak at the wenlock ludlow boundary 428 ma trace element characteristics unambiguously indicate partial melting of a garnet peridotite mantle source wenlock basalts are similar to alkaline oib with depleted radiogenic nd signature compared to ludlow basalts which are rather tholeiitic emorb like with enriched radiogenic nd signature the correlation of petrogenetically significant trace element ratios with nd isotopic compositions points to a mixing of partial melts of an isotopically heterogeneous possibly two component mantle source during the wenlock ludlow melting lava eruptions were accompanied by intrusions of doleritic basalt and meimechite sills the latter represent olivine rich cumulates of basaltic magmas of probably predominantly ludlow age meimechites with dolerites and to a lesser extent some lavas were subject to alteration due to wall rock fluid interaction the trigger for the wenlock to ludlow 431 425 ma extension and related volcanism in the prague basin is related to far field forces namely slab pull regime due to progressive closure of the iapetus ocean the main stage of the baltica laurentia collision then caused the prague basin rift failure at ca 425 ma that has never reached an oceanic stage

geochemistry and 40ar 39ar geochronology of the mafic metavolcanic rocks from the rýchory mountains complex west sudetes bohemian massif palaeotectonic significance the mafic metavolcanic rocks blueschists and greenschists of the r chory mountains crystalline complex west sudetes experienced sea floor hydrothermal alteration spilitization prior to regional metamorphism the metabasite geochemistry namely trace element and ree abundances indicates that the protolith was comparable in composition with 1 tholeiitic to transitional ocean floor basalts and 2 transitional and alkaline intra oceanic island basalts two main metamorphic events affected the r chory mountains metabasites in an earlier high pressure low temperature metamorphic event the rocks experienced blueschist facies metamorphism the results of 40 ar 39 ar geochronology studies on phengites from the mafic blueschists date the end of the earlier metamorphism to 360 ma the greenschist metamorphic overprint followed around 340 ma the elongated bodies of mafic metavolcanic rocks are situated within the prominent nne ssw leszczyniec shear zone following the trend of the r chory mountains and the rudawy janowickie mountains both the geochemical affinities and the blueschist facies metamorphism of the metabasites suggest that this shear zone evolved from the variscan suture dividing western and central terranes of the west sudetes according to the radiometric age for the end of the high pressure low temperature metamorphism the terranes accreted during the famennian a considerable time span between the formation of the metabasite protolith and the blueschist metamorphism may indicate long lasting subduction of a large oceanic plate between gondwana and laurussia possibly accompanied by terrane accretion prior to the variscan orogeny

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

the central sudetic ophiolites sw poland petrogenetic issues geochronology and palaeotectonic implications abstract the central sudetic ophiolites cso including the largest and best studied leza ophiolite are relatively well preserved complete palaeozoic ophiolitic sequences at the ne margin of the bohemian massif near the eastern edge of the variscan belt in central europe despite a detailed study and a well documented morb type affinity the geochemical complexity of these ophiolites has only recently been appreciated allowing construction of petrogenetic models we review here petrological and geochemical data indicating a contrast between the plutonic and volcanic members of the leza ophiolite this contrast was previously interpreted in terms of a model of two stage incremental melting of a depleted asthenospheric source producing basaltic magmas and subsequent melts crystallizing as gabbros however we argue that the geochemical differences between the plutonic and volcanic members can be explained by differentiation and cumulation processes of broadly similar parental magmas preliminary interpretation of new shrimp zircon age data from plutonic and volcanic members are consistent with earlier u pb zircon ages and provide evidence for magmatic crystallization of the leza ophiolite at c 400 ma the cso interpreted as traces of the rheic ocean are key for reconstructing the early to mid palaeozoic rifting processes distinct similarities exist with time equivalent ophiolitic units in the western variscan segment of the nw iberian massif the cso represent c 400 ma oceanic crust fragments and their structural position delineates likely major variscan tectonic sutures and tectonic mosaic domains within the variscan accretionary prism

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 palaeozoic evolution in the alps from gondwana to pangea more than 50 of the alps expose fragments of palaeozoic basement which were assembled during the alpine orogeny although the tectonic and metamorphic history of the basement units can be compared to that of the variscan crust in the alpine foreland most of the basement pieces of the alps do not represent the direct southern continuation of variscan structural elements evident in the massif central the vosges black forest or the bohemian massif the basement units of the alps all originated at the gondwana margin they were derived from a precambrian volcanic arc suture fringing the northern margin of gondwana from which they were rifted during the cambrian ordovician and silurian a short lived ordovician orogenic event interrupted the general rifting tendency at the gondwana active margin after the ordovician the different blocks drifted from the gondwana margin to their pangea position colliding either parallel to armorica with laurussia or with originally peri gondwanan blocks assembled presently in armorica from the devonian onwards many basement subunits underwent the complex evolution of apparently oblique collision and nappe stacking docking started in the external massifs the penninic and lower and middle austroalpine units in approximately devonian early carboniferous times followed by the upper austroalpine and the south alpine domains in the visean and the namurian times respectively wrenching is probably the best mechanism to explain all syn and post collisional phenomena since the visean followed by post orogenic collapse and extension it explains the occurrence of strike slip faults at different crustal levels the formation of sedimentary troughs as well as the extrusion and intrusion of crustal and mantle derived magmas and allows for contemporaneous rapid uplift of lower crustal levels and their erosion from the stephanian onwards all regions were deeply eroded by large river systems

geochemical and geochronological cross section of the deep variscan crust the cabo ortegal high pressure nappe northwestern spain the high pressure rocks of the cabo ortegal nappe and related massifs are represented by several tectonometamorphic units which underwent separate histories before regional overthrusting and amphibolite facies recrystallization at 380 ma the occurrence of final cooling is recorded by 40ar 39ar and rb sr mica ages of up to 350 ma u pb zircon dating and sm nd and rb sr isotope data together with ree and major and trace element geochemistry provide constraints on the timing of earlier tectonothermal events and the nature of the protoliths the earliest tectonothermal event occurred at 490 480 ma and corresponds to the emplacement of lree enriched magmas of probable calc alkaline affinity and their metamorphism in the granulite facies this occurred at approximately the same time as the eclogite facies metamorphism of morb like protholiths associated with basic to intermediate granulites together this suggests formation of syn accretion granulites in an active margin setting a second high grade metamorphic event is recorded at 420 ma in metasedimentary formations associated with eclogite like assemblages garnet omphacite plagioclase this event is possibly related to a collisional plate tectonic setting in a back arc basin the age and origin of the associated ultrabasic rocks remains unclear but a late pyroxenite vein has yielded an emplacement age of 390 ma

early palaeozoic initial rift volcanism in the central european variscides the kaczawa mountains sudetes sw poland evidence from sims dating of zircons early palaeozoic volcanic suites are widespread throughout the variscan belt and have commonly been ascribed to incipient rifting along the northern periphery of gondwana in cambrian to ordovician times their distribution across europe defines the present day extent of gondwana derived terranes and constrains the timing of their separation from the gondwanan margin the kaczawa mountains in the west sudetes at the eastern termination of the variscan belt include bimodal rift related rocks but their protolith age and hence their significance have been highly uncertain we have applied secondary ionization mass spectrometry zircon geochronology to a metarhyodacite and a metatrachyte from this suite yielding ages of 502 4 2 6 ma and 485 7 1 6 ma respectively this constrains the initial rift magmatism to c 500 485 ma in this part of the european variscides the rift related early palaeozoic volcanism thus seems to have been broadly synchronous throughout the peri gondwanan terranes of europe

geochemistry of bimodal amphibolitic felsic gneiss complexes from eastern massif central france high grade basic and acidic meta igneous rocks are widespread in the bimodal amphibolitic felsic gneiss complexes which are characteristic formations of the middle allochthonous unit from eastern and southern french massif central the metabasites from the lyonnais and doux complexes are chemically diverse and range from n morb type tholeiitic to transitional types the two populations are not related by fractional crystallization or crustal contamination processes and their chemical characteristics reflect differences in their mantle sources an ensialic setting is supported by the crustally derived character of some of the associated felsic rocks but the presence of n morb type metabasites argues for an extensional environment this bimodal association compares well with the magmatism of rifted continental margins and may reflect a transitional stage between continental rifting and oceanic crust formation during the cambro ordovician spreading event

late cambrian ordovician northeastern gondwanan terranes in the basement of the apuseni mountains romania abstract the pre alpine basement of the apuseni mountains romanian carpathians comprises three terranes somes baia de aries and biharia metaigneous rocks within these terranes record late cambrian ordovician u pb zircon crystallization ages two samples from an orthogneiss layer in the somes terrane yielded an age of 459 8 2 7 ma in the baia de aries terrane an augen gneiss yielded an age of 470 8 3 8 ma a porphyroid yielded an age of 467 8 3 8 ma and two metagranites yielded ages of 467 8 4 7 ma and 467 1 3 9 ma respectively three samples from a metagranitoid outcrop located in the lower part of the biharia terrane yielded an age of 495 0 2 1 ma three other samples from a metabasite layer situated lithostratigraphically several hundred metres above the metagranitoid yielded an age of 477 8 3 2 ma detrital u pb zircon ages from a paragneiss interlayered within the baia de aries terrane scatter between 548 and 3518 ma detrital zircon ages together with the inherited ages recorded by magmatic zircon populations suggest a northeastern gondwanan provenance for the somes baia de aries and biharia terranes adjacent to the arabian nubian shield lead loss in some zircons is interpreted to have taken place during the variscan orogeny supplementary material coordinates for samples from the apuseni mountains palaeozoic palaeogeographical models of gondwana laurentia and baltica to the formation of pangaea sample preparation and analytical procedures sample description and age measurements of zircon grains are available at http www geolsoc org uk sup18425

the palaeogeographic position of central iberia in gondwana during the ordovician evidence from zircon chronology and nd isotopes terra nova 22 341 346 2010 abstract the abnormally elevated abundance of inherited zircon locally up to 80 90 of zircon grains contain pre magmatic cores in the cambro ordovician magmatic rocks of central iberia made possible an estimation of the distribution of ages of their source rocks the comparison of inherited u pb ages and whole rock nd model ages with those of three main north african neoproterozoic terranes reveals that the iberian magmas can only have been generated from a crust similar to that of the east african orogen west of the arabian nubian shield such crust is currently found in the three precambrian inliers of the western desert of egypt east of the archaean terranes of gebel kamel palaeontological evidence also indicates that the ordovician fauna of central iberia is similar to that of the region between eastern algeria and arabia therefore we conclude that during the ordovician iberia was not attached to north west africa near morocco as is usually assumed but instead to the east african orogen in northern egypt

highly depleted oceanic lithosphere in the rheic ocean implications for paleozoic plate reconstructions the rheic ocean formed at ca 500 ma when several peri gondwanan terranes e g avalonia and carolinia drifted from the northern margin of gondwana and were consumed during the late carboniferous collision between laurussia and gondwana a key event in the formation of pangea several mafic complexes ranging in age from ca 400 330 ma preserve many of the lithotectonic and or chemical characteristics of ophiolites they are characterized by anomalously high end values that are typically either between or above the widely accepted model depleted mantle curves these data indicate derivation from a highly depleted hd mantle and imply that i the mantle source of these complexes displays time integrated depletion in nd relative to sm and ii depletion is the result of an earlier melting event in the mantle from which basalt was extracted the extent of mantle depletion indicates that this melting event occurred in the neoproterozoic possibly up to 500 million years before the rheic ocean formed if so the mantle lithosphere that gave rise to the rheic ocean mafic complexes must have been captured from an adjacent older oceanic tract the transfer of this captured lithosphere to the upper plate enabled it to become preferentially preserved possible mesozoic cenozoic analogues include the capture of the caribbean plate or the scotia plate from the pacific to the atlantic oceanic realm our model implies that virtually all of the oceanic lithosphere generated during the opening phase of the rheic ocean was consumed by subduction during laurentia gondwana convergence

deciphering the variscan tectonothermal overprint and deformation partitioning in the cadomian basement of the tepla barrandian unit bohemian massif the tepla barrandian unit tbu has long been considered as a simply bivergent supracrustal median massif above the saxothuringian subduction zone in the variscan orogenic belt this contribution reveals a much more complex style of the variscan tectonometamorphic overprint and resulting architecture of the neoproterozoic basement of the tbu for the first time we describe the crustal scale ne sw trending dextral transpressional krakovec shear zone ksz that intersects the tbu and thrusts its higher grade northwestern portion severely reworked by variscan deformation over a southeastern very low grade portion with well preserved cadomian structures and only brittle variscan deformation the age of movements along the ksz is inferred as late devonian 380 370 ma on the basis of structural microstructural and anisotropy of magnetic susceptibility data from the ksz we propose a new synthetic model for the deformation partitioning in the tepla barrandian upper crust in response to the late devonian to early carboniferous subduction and underthrusting of the saxothuringan lithosphere we conclude that the saxothuringian tepla barrandian convergence was nearly frontal during 380 346 ma and was partitioned into pure shear dominated domains that accommodated orogen perpendicular shortening alternating with orogen parallel high strain domains that accommodated dextral transpression or bilateral extrusion the synconvergent shortening of the tbu was terminated by a rapid gravity driven collapse of the thickened lithosphere at 346 337 ma followed by or partly simultaneous with dextral strike slip along the baltica margin parallel zones driven by the westward movement of gondwana from approximately 345 ma onwards

geochemistry and tectonic environment of ordovician meta igneous rocks in the rudawy janowickie complex sw poland a bimodal suite of ordovician metavolcanic rocks in the rudawy janowickie mountains and the lasocki ridge sudetic region sw poland is dominated by voluminous epidote amphibolites and greenschists derived from basalt dolerite and gabbro precursors less abundant felsic components comprise both mylonitized granitic sheets that intrude the mafic rocks and minor interlaminated volcanic rocks ranging from andesite to rhyolite in composition these rocks are collectively referred to as the rudawy janowickie complex in chemical terms the mafic volcanic rocks define three north south trending provinces western central and eastern which are separated tectonically by broad major mylonite zones the western province contains all the alkali basalts in the study area and minor tholeiites all exhibiting within plate features specifically high zr y 4 and enriched chondrite normalized incompatible element patterns the central province contains two fractionated tholeiitic suites leszczyniec and okraj groups with low zr y ratios 2 0 3 5 depleted to flat ree patterns and incompatible element compositions ranging from normal morb to slightly enriched morb the eastern province has some features in common with the central province notably metabasalts with normalized flat ree patterns although it is also characterized by metagabbro bodies with enriched light ree patterns la yb n 4 7 but depleted high field strength element contents in tectonic terms the rudawy janowickie complex is interpreted as a former intracratonic rift floored by oceanic crust and bounded to the west by an attenuated continental margin existing data do not allow a definitive conclusion about the exact nature of the eastern margin of the basin it might in part reflect the presence of a distant subduction zone

rare earth elements ree minerals in the silius fluorite vein system sardinia italy publisher elsevier articletitle rare earth elements ree minerals in the silius fluorite vein system sardinia italy journaltitle ore geology reviews articlelink http dx doi org 10 1016 j oregeorev 2015 11 016 content type article copyright copyright 2015 elsevier b v all rights reserved