middle devonian olistostromes in the rheno hercynian zone rheinisches schiefergebirge an indication of back arc rifting on the southern shelf of laurussia abstract the rheinisches schiefergebirge in the rheno hercynian zone of the mid european variscides was geographically situated on laurussia s southern shelf during the devonian in its eastern part lahn dill area givetian coarse clastic sediments formerly misinterpreted as transgressional conglomerates are recognised as olistostrome deposits intercalated in basinal sediments that document a phase of rifting on the southern shelf of laurussia the clastic components redeposited as olistolithes and rounded boulders are locally derived rocks of emsian and givetian age the olistostromes show facies patterns comparable with other syn rift deposits the proximal facies is characterised by olistolithes and slideflow debrites overlain by debrites and intercalated with turbidites the distal facies is represented by intercalation of debrites and turbidites both grade upwards into basinal sedimentation based on conodont stratigraphy the redeposition lasted from the middle varcus zone to the disparilis zone during the middle varcus zone footwall uplift raised the sedimentary column of the source area by about 500 m above which brought it above sea level rifting of the laurussian shelf as a result of hinterland transtension and extension caused by the oblique collision of peri gondwana with laurussia provides the most plausible explanation for the development of the olistostromes olistolithes and slideflow debrites in the eastern rheinisches schiefergebirge

u pb and pb pb zircon ages for arc related intrusions of the bolu massif w pontides nw turkey evidence for late precambrian cadomian age we present new u pb and pb pb radiometric age data from two tectono stratigraphic units of the regionally extensive bolu massif in the w pontides fragment n turkey a structurally lower unit sunnice group is cut by small meta granitic intrusions whereas the structurally higher unit com prises meta volcanic rocks ca surtepe fm cut by meta granitic plutons tullukirisand kapokaya plutons u pb single crystal dating of zircons from the kapokaya pluton yielded a concordant cluster with a mean 238 u 206 pb age of 565 3 1 9 ma zircons from the tullukirispluton affected by pb loss gave a 207 pb 206 pb age of 576 6 ma age late precambrian small meta granitic intrusions cutting the sunnice group yielded a less precise 207 pb 206 pb age of 262 19 ma early permian the older ages from the bolu massif confirm the existence of latest precambrian arc magmatism related to subduction of a cadomian ocean we infer that the bolu massif represents a fragment of a cadomian active margin cadomian orogenic units were dispersed as exotic terranes throughout the variscan and tethyan orogens and the bolu massif probably reached its present position prior to latest palaeozoic time our dating results also confirm that nw turkey was affected by hercynian magmatism related to subduction of palaeotethys as inferred for other areas of the pontides

model for the evolution of the avalonian cadomian belt the inception and termination of the ca 670 550 ma avalonian cadomian orogenic cycle are interpreted in terms of the amalgamation and breakup of a late precambrian supercontinent synorogenic arc related volcano sedimentary and granitoid rocks are attributed to oblique southward directed subduction along the margin of the supercontinent after its amalgamation breakup of the supercontinent and inception of the iapetus cycle led to termination of subduction and its replacement by transform activity the resulting closure of late precambrian arc related basins and the development of early cambrian platform successions with acado baltic fauna accompanied the formation of iapetus but took place on the active margin of a preexisting ocean and do not record the evolution of its rifted european flank

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

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

u pb shrimp and nd isotopic data from the western bohemian massif bayerischer wald germany implications for upper vendian and lower ordovician magmatism u pb shrimp dating of zircons of metamagmatites from the bayerischer wald germany reveals a complex evolution of this section of the moldanubian zone exposed in the western bohemian massif of the central european variscan belt in the south western part of the bayerischer wald upper vendian magmatism is constrained by pooled 206pb 238u mean ages of 555 12 549 7 and 549 6 ma from metarhyolites and a metabasite inherited zircon cores were not observed zircon overgrowths yielding pooled 206pb 238u ages of 316 10 and 319 5 ma provide evidence for variscan metamorphic zircon growth cathodoluminescence imaging reveals a two stage metamorphic overprint

palaeogeographical controls on the cambrian trilobite immigration and evolutionary patterns reported in the western gondwana margin abstract southward drifting of the western gondwanan margin during the cambrian has been demonstrated by means of both palaeomagnetic methods and lithological indicators of climate such as carbonates and evaporites recent improvements in biostratigraphical correlations permit an enhanced understanding of the climatic and palaeobiogeographical constraints that controlled the distribution of cambrian benthic communities palaeogeographical and biogeographical reconstructions based on trilobites are reported in this paper in order to test interaction between migration speciation and extinction rates the variability of the documented biogeographical patterns is directly related to species diversity in which wider distribution coincides with transgressive trends and subsequent connection of neighbouring platforms early cambrian trilobite faunas show a high degree of both substrate control and endemicity although transgressions led to parallel shifts in faunal compositions by contrast mid cambrian trilobite faunas are relatively uniform across western gondwana and latest mid and late cambrian associations document influence of an increased similarity with asian trilobite faunas

provenance constraints from detrital zircon u pb ages in the nw iberian massif implications for palaeozoic plate configuration and variscan evolution detrital zircons from pre orogenic upper ordovician to devonian low grade metasedimentary rocks have been dated by laser ablation inductively coupled plasma mass spectrometry la icp ms to 1 constrain the variations of zircon age populations within the autochthonous sequences of nw iberia 2 compare u pb detrital zircon ages with previous data 3 test the hypothesis of the armorica microplate as a peri gondwanan terrane separated from gondwana the similarity in the ages of detrital zircons found in the palaeozoic samples studied here to those published for the upper proterozoic lower cambrian and lower ordovician sediments of nw iberia argues against the separation of armorica and points to a common source area for all of the palaeozoic detrital formations of nw iberia the west african craton and the surrounding pan african belts la icp ms u pb dating of zircons from a synorogenic flysch preserved in klippen in the core of a syncline establishes a maximum depositional age for this deposit as early namurian this age is also a maximum age limit for thrusting and refolding in this part of the iberian massif correlation of the zircon age populations with published ages confirms the link between the emplacement of the allochthonous complexes of nw iberia and synorogenic sedimentation

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

the anarak jandaq and posht e badam metamorphic complexes in central iran new geological data relationships and tectonic implications abstract the anarak jandaq and posht e badam metamorphic complexes occupy the nw part of the central east iranian microcontinent and are juxtaposed with the great kavir block and sanandaj sirjan zone our recent findings redefine the origin of these complexes so far attributed to the precambrian early paleozoic orogenic episodes and now directly related to the tectonic evolution of the paleo tethys ocean this tectonic evolution was initiated by late ordovician early devonian rifting events and terminated in the triassic by the eocimmerian collision event due to the docking of the cimmerian blocks with the asiatic turan block the variscan accretionary complex is a new name we proposed for the most widely distributed metamorphic rocks connected to the anarak and jandaq complexes this accretionary complex exposed from sw of jandaq to the anarak and kabudan areas is a thick and fine grain siliciclastic sequence accompanied by marginal sea ophiolitic remnants including gabbro basalts with a supra subduction geochemical signature new 40ar 39ar ages are obtained as 333 320 ma for the metamorphism of this sequence under greenschist to amphibolite facies moreover the limy intercalations in the volcano sedimentary part of this complex in godar e siah yielded upper devonian tournaisian conodonts the northeastern part of this complex in the jandaq area was intruded by 215 15 ma arc to collisional granite and pegmatites dated by id tims and its metamorphic rocks are characterized by some 40ar 39ar radiometric ages of 163 156 ma the variscan accretionary complex was northwardly accreted to the airekan granitic terrane dated at 549 15 ma later from the late carboniferous to triassic huge amounts of oceanic material were accreted to its southern side and penetrated by several seamounts such as the anarak and kabudan this new period of accretion is supported by the 280 230 ma 40ar 39ar ages for the anarak mild high pressure metamorphic rocks and a 262 ma u pb age for the trondhjemite rhyolite association of that area the triassic bayazeh flysch filled the foreland basin during the final closure of the paleo tethys ocean and was partly deposited and or thrusted onto the cimmerian yazd block the paleo tethys magmatic arc products have been well preserved in the late devonian carboniferous godar e siah intra arc deposits and the triassic nakhlak fore arc succession on the passive margin of the cimmerian block in the yazd region the nearly continuous upper paleozoic platform type deposition was totally interrupted during the middle to late triassic local erosion down to lower paleozoic levels may be related to flexural bulge erosion the platform was finally unconformably covered by liassic continental molassic deposits of the shemshak one of the extensional periods related to neo tethyan back arc rifting in late cretaceous time finally separated parts of the eocimmerian collisional domain from the eurasian turan domain the opening and closing of this new ocean characterized by the nain and sabzevar ophiolitic melanges finally transported the anarak jandaq composite terrane to central iran accompanied by large scale rotation of the central east iranian microcontinent ceim due to many similarities between the posht e badam metamorphic complex and the anarak jandaq composite terrane the former could be part of the latter if it was transported further south during tertiary time

late cadomian crustal tilting and cambrian transtension in the tepla barrandian unit bohemian massif central european variscides the cadomian basement of the tepla barrandian unit is characterized by a classic barrovian type metamorphism the degree of which increases considerably towards the west reaching amphibolite facies conditions in the domaz lice crystalline complex dcc the number and volume of plutons also increases towards the west the emplacement ages of the te s ovice granite and the mrac nice trondhjemite have been determined at 521 7 2 ma and 523 4 5 ma respectively applying conventional u pb analyses of zircons pervasive high temperature prolate fabrics and north northwest dipping dextral oblique slip shear zones within the mrac nice trondhjemite suggest a synkinematic melt emplacement within a lower cambrian transtensional setting transtension is probably related to early stage rifting that introduced the separation of the tepla barrandian unit as part of armorica from gondwana structural and petrological data of the country rocks show that the barrovian type metamorphism and two deformation stages d1 with unknown kinematics and d2 top to the north shearing are older than the melt emplacement and thus can be attributed to the cadomian orogeny the intrusion depth of both plutons is nearly the same ca 7 km although the degree of barrovian type metamorphism differs significantly within the country rocks this suggests late cadomian eastward tilting of the metamorphic isograd planes the weak post plutonic lower greenschist to subgreenschist facies folding and thrusting result from variscan northwest southeast compression

the anatomy of a gondwanan terrane the neoproterozoic ordovician basement of the pre alpine sebes lotru composite terrane south carpathians romania abstract the sebes lotru terrane in the south carpathians mountain range comprises a lower neoproterozoic metamorphic unit lotru and an upper ordovician metamorphic unit cump na that were juxtaposed during the variscan orogeny two orthogneisses from the lotru metamorphic unit yield u pb la icp ms zircon crystallization ages of 549 3 3 8 ma and 587 5 3 ma respectively two orthogneisses from the cump na metamorphic unit yield zircon crystallization ages of 458 9 3 5 ma and 466 0 4 2 ma respectively high u zircons from two other orthogneisses from the cump na metamorphic unit have ages ranging from 400 ma to 320 ma which are interpreted to reflect protracted zircon recrystallization during the regionally significant variscan collisional event detrital zircons from a metasedimentary gneiss in the cump na metamorphic unit have ages ranging from 0 5 ga to 2 8 ga the 0 5 ga age constrains the maximum sediment deposition age to be late cambrian the source most compatible with the range of precambrian detrital ages in the sebes lotru terrane is northeastern gondwana the sebes lotru terrane was part of a continental subduction collision system as a lower plate after about 400 ma and reached peak metamorphic conditions between 350 and 320 ma e g medaris et al 2003 a cross cutting granite vein has a zircon u pb crystallization age of 321 5 3 1 ma which constrains minimum age of ductile deformation during the variscan collision in this region the trace of the rheic suture within the south carpathians is located between the ordovician upper part of the sebes lotru terrane and the dr gsan pre alpine terrane of the danubian domain

the armorica microplate fact or fiction critical review of the concept and contradictory palaeobiogeographical data abstract the problem of an armorica microplate detached from gondwana and having had during part of the palaeozoic an independent latitudinal evolution is reconsidered in terms of a critical review of the palaeomagnetic data that were the very roots of this concept and through an alternative approach based on palaeoclimatic and palaeobiogeographical data palaeomagnetic data for the silurian and the devonian of the south european regions supposedly constituting the armorica microplate remain rare and ambiguous those from gondwana are more numerous but contradictory enough to give rise to diverging models regarding the latitudinal evolution of this continent consequently the reality of an armorica microplate cannot be considered as established on the contrary lithological indicators of palaeoclimate and palaeobiogeographical data are in total harmony and indicate that in actual fact the southern european regions remained permanently closely connected with gondwana of which they composed the northern margin although repeatedly maintained for more than 20 years the concept of an armorica microplate can thus be considered a fiction this conclusion should lead to the dismissal of geodynamical models proposed for the variscan belt to which this concept was integrated and are contradicted by inescapable palaeobiogeographical constraints

origin of the rheic ocean rifting along a neoproterozoic suture the rheic ocean is widely believed to have formed in the late cambrian early ordovician as a result of the drift of peri gondwanan terranes such as avalonia and carolina from the northern margin of gondwana and to have been consumed in the devonian carboniferous by continent continent collision during the formation of pangea other peri gondwanan terranes e g armorica ossa morena northwest iberia saxo thuringia moldanubia remained along the gondwanan margin at the time of rheic ocean formation differences in the neoproterozoic histories of these peri gondwanan terranes suggest the location of the rheic ocean rift may have been inherited from neoproterozoic lithospheric structures formed by the accretion and dispersal of peri gondwanan terranes along the northern gondwanan margin prior to rheic ocean opening avalonia and carolina have sm nd isotopic characteristics indicative of recycling of a juvenile ca 1 ga source and they were accreted to the northern gondwanan margin prior to voluminous late neoproterozoic arc magmatism in contrast sm nd isotopic characteristics of most other peri gondwanan terranes closely match those of eburnian basement suggesting they reflect recycling of ancient 2 ga west african crust the basements of terranes initially rifted from gondwana to form the rheic ocean were those that had previously accreted during neoproterozoic orogenesis suggesting the rift was located near the suture between the accreted terranes and cratonic northern gondwana opening of the rheic ocean coincided with the onset of subduction beneath the laurentian margin in its predecessor the iapetus ocean suggesting geodynamic linkages between the destruction of the iapetus ocean and the creation of the rheic ocean

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

birth of a volcanic passive margin in cambrian time rift paleogeography of the ossa morena zone sw spain abstract mapping at 1 10000 scale of both north n and south s flanks of the variscan monesterio antiform in the ossa morena zone southern spain in association with detailed petrographic and major element analyses of magmatic rocks has revealed the building of a passive but volcanically active oceanic margin in cambrian time this margin was located in northern gondwana and represents the beginning of an extensional tectonic regime that concluded with the opening of the rheic ocean at time of eruption middle cambrian the n flank was located on the northern coastline of a marine basin in the process of passive rifting the vents mainly located in its western part were subaerial continental and or shallow marine on the s flank eruptions and deposits were shallow marine occurring as lava flows and pillows interstratified within shallow marine sediments on both flanks intrusives were emplaced after the extrusives lateral thickness variations of the sedimentary deposits testify to the building during four distinct periods of a rifted passive margin with a shifting locus of volcanic activity over time the composition of magmas was bimodal the mafic magmas are silica saturated alkaline to sub alkaline and somewhat ti rich perhaps originating from partial melting of a subcontinental mantle that was less metasomatized with time the youngest magmatism with an affinity close to that of morb likely had an asthenospheric source the alkaline felsic products are diverse and may have originated from a very heterogeneous continental crust in each set partial melting relationships prevail suggesting that the melts rose along open fractures as soon as they formed in the context of lithospheric thinning it is possible that mantle and crustal melting was triggered by decoupling between rheologically different materials

multi phase reactivations and inversions of paleozoic mesozoic extensional basins during the wilson cycle case studies from the north sea uk and the northern apennines italy

gondwana break up and the northern margin of the saxothuringian belt variscides of central europe at the northwestern edge of the hercynian bohemian massif saxothuringian belt new u pb zircon age data from rift related magmatic rocks indicate that the initiation of gondwana break up in this area started during the middle to upper cambrian magmatic rocks from a bimodal morb to within plate volcanic sequence in the vesser area are dated between ca 517 and 501 ma the volcaniclastic sequences analysed exhibit basal layers of conglomerates and mature sandstones which can be correlated with a widespread gondwana derived onlap horizon of an uppermost cambrian tremadocian age that links the vesser area with the saxothuringian continental basin the association of the vesser rocks with the saxothuringian terrane as part of the armorican terrane assemblage is further demonstrated by a coeval magmatic development and by identical detrital components which are derived from a common cadomian basement white mica with a ca 539 ma k ar minimum age and inherited zircon signatures the vesser unit situated between the nw margin of the saxothuringian zone and the mid german crystalline zone probably represents a n facing remnant of an ocean continent transition of the or within the armorican terrane assemblage and involves sections of the early break up process at the peri gondwanan shelf south of the rheic ocean

the moldanubian thrust zone a terrane boundary in the central european variscides refined based on lithostratigraphy and u pb zircon geochronology abstract the zircon age populations of metavolcano sedimentary successions in the orlica nieznik dome osd stare m sto belt smb and velke vrbno dome the sudetes poland and czech republic have been used to refine the location of the moldanubian thrust zone mtz which is a boundary between the saxothuringian terrane of gondwana descent and the brunovistulian terrane being a promontory of laurussia in the northern continuation of the mtz a set of multiply activated regional scale thrusts developed and brought into contact rocks of different ages and geological histories metarhyolites in the orlica nieznik dome and the stare m sto belt have similar geochemistry and u pb isotopic zircon records which is taken in favour of their coeval formation and common saxothuringian affinity felsic metavolcanic rocks from the osd and from the thrust bounded upper and lower units of the smb yielded protolith ages of 500 3 ma and 493 4 ma to 498 5 ma respectively which indicates that metavolcano sedimentary successions in the osd and smb were deposited in late cambrian times structurally below these rocks there are the highly sheared brousek quartzites with detrital zircons that yielded a maximum depositional age of 530 ma the mylonitic quartzites accommodate deformation induced by tectonic transport on the east n znerov thrust which is interpreted as the easternmost margin of saxothuringia this fault separates palaeozoic rocks of the saxothuringian terrane from neoproterozoic bimodal volcanogenic succession in the velke vrbno dome of brunovistulia dated at 558 ma the late variscan thermal events in the early carboniferous left imprints in the form of u rich rims around the zircons of the metavolcanic rocks from the stare m sto belt and only a very minor overprint in the zircons from the velke vrbno dome and orlica nieznik dome

late cambrian ordovician magmatic arc type volcanism in the southern gemericum basement western carpathians slovakia u pb shrimp data from zircons the low grade crystalline basement of the southern gemeric unit inner western carpathians consists mainly of siliciclastic deep water turbidite sequence associated with a huge mass of redeposited rhyolite dacite volcanics and their volcaniclastics in situ u pb shrimp zircon dating applied to metavolcanic rocks contributed to specification of the southern gemeric crystalline basement stratigraphy the gelnica group zircon populations from ten samples of three lithostratigraphic units of the gelnica group rock complex have been analysed the zircons presented significant magmatic zoning part of the magmatic zircons contained inherited older cores the vlachovo formation gives the 494 1 6 ma zircon average age the bystr potok formation gives 465 8 1 5 ma and the drnava formation 463 9 1 7 ma average age the geochemical data are compatible with an orogenic geodynamic setting dominant rhyolite dacite volcanites belong to the peraluminous calc alkaline high k magmatic series the age data let us presume two phase longlasting magmatic activity starting in the late cambrian and continuing to the middle late ordovician thus the studied late cambrian volcanic sequence of the gelnica group reflects the active continental margin setting the second middle ordovician volcanic phase documents an assumed rifting stage accompanied by a strong volcanic activity with inherited features of the late cambrian magmatic arc the inherited cores of magmatic zircons have proterozoic 600 760 1 000 1 400 2 000 2 700 ma and archean 3 099 3 164 ma ages

the margins of laurussia in central and southeast europe and southwest asia the continuation of the laurussian margin into central and southeast europe and southwest asia is defined by the rheic suture which becomes harder to trace to the southeast because of complex post variscan strike slip displacements and tectonic stacking linked to the cimmerian and alpine cycles the uncertain extent of the neoproterozoic baikalian orogeny fringing baltica ambiguous early paleozoic paleobiogeographic data and strike slip activity complicate the distinction of the tornquist suture from the transeuropean suture zone there is growing evidence for the incorporation of the brunovistulian malopolska and west moesian terranes into the late neoproterozoic cambrian baltican margin the istanbul zonguldak terrane along with the bitesh and east moesia terranes may have been part of the avalonian terrane assemblage although an arabian nubian shield or baltica provenance cannot be excluded the terranes of the armorica terrane assemblage bordering the rheic suture include the outer carpathian tatra getic and balkan terranes in southeast europe and sakarya and the anatolide tauride terrane in turkey further east the rheic suture may be present in the caucasus in all probability between the greater caucasus and the scythian platform or alternatively between the greater caucasus and transcaucasia alborz in northern iran is interpreted as a distal part of transcaucasia the location of the rheic suture in the southeast part of the turan domain of central asia is poorly constrained

uhp metamorphism recorded by coesite bearing metapelite in the east kunlun orogen nw china the east kunlun orogen eko is the nw part of the central china orogenic belt which records the evolutionary history of the proto and palaeo tethys oceans from the cambrian to the triassic an early palaeozoic eclogite belt has been recognized in recent years which extends discontinuously for 500 km as three eclogite bearing terranes in this study we report an integrated study of zircon grains from mica schists accompanying the eclogites in terms of mineral inclusions u pb age systematics and p t conditions the presence of coesite is identified as inclusions within the metamorphic domain of zircons which provides unambiguous evidence for subducted terrigenous clastic rocks of the proto tethys ocean exhumed from coesite forming depths u pb dating of the metamorphic zircons yields a concordia age of 426 5 0 88 ma which is likely to be the time of ultrahigh pressure metamorphism in the kehete terrane p t calculations suggest that metapelite may have experienced a clockwise p t path with peak p t conditions of 685 41 c and 28 kbar and equilibrated at 482 566 c and 5 6 8 9 kbar during subsequent exhumation the high pressure ultrahigh pressure hp uhp metamorphic belt within the eko may have formed by collision between the qaidam block and the south kunlun block as a consequence of the closure of the proto tethys ocean

age and geochemistry of coeval felsic volcanism and plutonism in the palaeoproterozoic ndembera group of southwestern tanzania constraints from shrimp u pb zircon and sm nd data abstract the ndembera metavolcanic rocks represent a continuum of compositions ranging from intermediate to more siliceous calc alkaline trachyandesites dacites trachytes rhyolites which have been intruded by largely metaluminous calc alkaline i type granites in the palaeoproterozoic usagaran belt of southwestern tanzania shrimp u pb zircon age data show that the ndembera metavolcanic rocks were extruded at 1871 30 ma an event that was largely coeval with the emplacement of cross cutting microcline biotite rich granites at 1896 29 ma despite some compositional differences both the granites and metavolcanic rocks share similar geochemical features including very coherent ree patterns characterized by enrichment of the lree relative to the hree la ybcn 13 6 32 8 average 23 9 for metavolcanic rocks and la ybcn 10 8 38 2 average 21 8 for granites negative eu anomalies eu eu 0 45 1 01 average 0 83 for volcanic rocks and eu eu 0 48 0 70 average 0 60 for granites negative ta ti and nb anomalies nb lapm 0 14 0 29 average 0 20 for volcanic rocks and nb lapm 0 01 0 37 average 0 16 for granites the metavolcanic rocks have nd 1871 ma values of 0 03 to 4 89 and tdm ages of 2243 2714 which are broadly similar to those of the granites nd 1896 ma values of 3 56 to 4 59 and tdm ages of 2469 2646 ma the geochemical similarities between the ndembera volcanic rocks and granites coupled with their similar emplacement age suggest that the two rock suites were derived by the same processes in the same tectonic setting from an isotopically similar source these geochemical features coupled with the paucity of mafic rocks suggest the derivation of the pluto volcanic suite by crustal anatexis of basic meta igneous rocks mixed with a minor metasedimentary component in an intra continental setting the more siliceous compositions in the suite may have formed by subsequent fractionation involving hornblende plagioclase biotite titanite and apatite as indicated by the negative correlation between sio2 and mgo fe2o3 al2o3 cao tio2 and p2o5 low sr contents and negative eu and ti anomalies the 1 87 ga coeval felsic volcanic and granitic magmatic event documented in the ndembera group is also a feature of the nearby ubendian belt to the west pointing to a palaeoproterozoic regional thermal event that occurred in southwestern and western tanzania

zircon u pb geochronology of ordovician magmatism in the polycyclic ruitor massif internal w alps among the middle penninic basements of the internal nw alps the ruitor massif shows the best preserved remnants of pre permian metamorphic rocks their barrovian type mineral associations are somewhat masked by the greenschist to blueschist alpine metamorphism of tertiary age four ruitor gneisses have been analysed showing geochemical characters of granitoids from orogenic zones zircon morphology also suggests magmatic protoliths and a crustal source some of the morphological zircon types suggest anatectic granites the first u pb ages on zircon for this massif have been obtained concurrently through conventional multigrain and ion microprobe dating two metavolcanic rocks at 471 5 and 468 22 ma could be slightly older than the porphyritic augen gneisses at 465 11 and 460 7 ma regional data from the other internal basement massifs suggest that the variscan event is poorly recorded except in ruitor type units ruitor and sapey gneisses belonged to the same unit nappe des pontis which was affected by a 480 450 ma event including volcanism and anatexis and ended with a late calc alkaline granite emplacement at 460 450 ma the distribution of variscan basement units roughly parallels alpine zonation

in situ u pb zircon ages for early ordovician magmatism in the eastern pyrenees france the canigou orthogneisses u pb age determinations with an ion probe on zircons within orthogneisses from the canigou massif eastern pyrenees give an emplacement age for their granitic protolith of 475 10 ma arenigian this invalidates the previous interpretation of basement cover relationships between the orthogneisses and the overlying lower cambrian canaveilles metasedimentary series and corroborates the intrusive character of the orthogneisses which is further supported by field evidence the data also define two groups of inherited zircons paleo meso proterozoic and cadomian in addition to partial lead loss during the hercynian late carboniferous high temperature low pressure metamorphic event these results are in agreement with ages reported from other orthogneissic bodies across the entire variscan chain of western europe and indicate the existence of a widespread cambro ordovician magmatic event

early ordovician orogenic event in galicia nw spain evidence from u pb ages in the uppermost unit of the ordenes complex new u pb data zircon monazite and rutile obtained from rocks of the uppermost allochthon in the variscan belt of nw spain indicate that the hangingwall to the suture includes an allochthonous unit with a pre variscan tectonothermal evolution this evolution is characterised by an early ordovician 498 500 ma bimodal magmatism followed almost immediately 493 498 ma by a barrovian style metamorphism up to the granulite facies the metamorphism subsequent to the igneous intrusions requires convergence and crustal thickening in order to generate the barrovian facies pattern shown by the lithologies of the uppermost allochthon the almost coeval magmatism and metamorphism and the chemistry of the metabasites suggest an accretionary complex probably related to a volcanic arc as the most probable setting for the origin of the uppermost unit the implications of the existence of early ordovician convergent plate boundaries are discussed in the context of gondwana laurentia avalon interactions

peri amazonian avalonian type and ganderian type terranes in the south carpathians romania the danubian domain basement abstract the danubian domain basement of the south carpathians romania comprises two neoproterozoic continental crustal fragments the dr gsan and lainici p ius terranes which were sutured by the closure of an intervening oceanic domain the tisovi a terrane magmatic and detrital zircons extracted from an orthogneiss four granitoid plutons two metasedimentary units and a liassic sandstone were dated by zircon u pb la icp ms the f ge el augen gneiss from the dr gsan terrane basement yielded an age of 803 2 4 4 ma the oldest well constrained crystallization age reported from the romanian carpathians basement the tismana usi a novaci and olte granitoid plutons which intrude the lainici p ius terrane basement yielded ages of 600 5 4 4 591 0 3 5 592 7 4 9 and 588 2 9 ma respectively the tismana granitoid age of 600 ma and the youngest detrital zircon ages of 637 622 ma from a metaquartzite within the lainici paius terrane constrain the deposition of the metaquartzite protolith to ca 620 600 ma the 803 ma age represents an old pan african age whereas the younger neoproterozoic ages suggest pan african cadomian thermotectonic events detrital and inherited zircon ages within the dr gsan and lainici paius terranes attest to a peri amazonian avalonian type provenance for the dr gsan terrane and possibly a ganderian type provenance for the lainici p ius terrane the lainici p ius terrane rifted off gondwana before the dr gsan terrane both terranes were attached to moesia during the early paleozoic

the basement of the mount athos peninsula northern greece insights from geochemistry and zircon ages the mount athos peninsula is situated in the south easternmost part of the chalkidiki peninsula in northern greece it belongs to the serbo macedonian massif smm a large basement massif within the internal hellenides the south eastern part of the mount athos peninsula is built by fine grained banded biotite gneisses and migmatites forming a domal structure the southern tip of the peninsula which also comprises mount athos itself is built by limestone marble and low grade metamorphic rocks of the chortiatis unit the northern part and the majority of the western shore of the mount athos peninsula are composed of highly deformed rocks belonging to a tectonic melange termed the athos volvi suture zone avz which separates two major basement units the vertiskos terrane in the west and the kerdillion unit in the east the rock types in this melange range from metasediments marbles and gneisses to amphibolites eclogites and peridotites the gneisses are tectonic slivers of the adjacent basement complexes the melange zone and the gneisses were intruded by granites ierissos ouranoupolis and gregoriou the ouranoupolis intrusion obscures the contact between the melange and the gneisses the granites are only slightly deformed and therefore postdate the accretionary event that assembled the units and created the melange pb pb and u pb shrimp dating of igneous zircons of the gneisses and granites of the eastern athos peninsula in conjunction with geochemical and isotopic analyses are used to put athos into the context of a regional tectonic model the ages form three clusters the basement age is indicated by two samples that yielded permo carboniferous u pb ages of 292 6 2 9 ma and 299 4 3 5 ma the main magmatic event of the granitoids now forming the gneiss dome is dated by pb pb ages between 140 0 2 6 ma and 155 7 5 1 ma with a mean of 144 7 2 4 ma a within error identical age of 146 6 2 3 ma was obtained by the u pb shrimp method this late jurassic age is also known from the kerdillion unit and the rhodope terrane the rather undeformed granites are interpreted as piercing plutons the small granite stocks sampled have late cretaceous to early tertiary ages of 66 8 0 8 ma and 68 0 1 0 ma u pb shrimp 62 8 3 9 ma pb pb the main accretionary event was according to these data in the late jurassic since all younger rocks show little or no deformation the age distribution together with the geochemical and isotopic signature and the lithology indicates that the eastern part of the mount athos peninsula is part of a large scale gneiss dome also building the kerdillion unit of the eastern smm and the rhodope massif this finding extends the area of this dome significantly to the south and indicates that the tectonic boundary between the smm and the rhodope massif lies within the avz

neoproterozoic cambrian synsedimentary magmatism in the central iberian zone spain geology petrology and geodynamic significance petrographic geochemical and field studies in low grade metamorphic areas ciudad rodrigo hurdes sierra de gata domain crhsg central western spain show that neoproterozoic lowermost cambrian series in the central iberian zone ciz record two kinds of provenance sources including 1 detrital material derived from recycled orogens and 2 a cadomian coeval juvenile contribution that governs their isotopic signature evidence of magmatism contemporaneous with neoproterozoic cambrian sedimentation is provided by the presence of coherent massive volcanic rocks metabasalts metaandesites and metarhyolites volcaniclastic shales sandstones conglomerates and breccias the appearance of volcanogenic lithic fragments and crystals mixed in different proportions with siliciclastic constituents and also present within calcareous components in the sedimentary succession reinforces this evidence although most of the selected volcanic and volcaniclastic samples appear to show tholeiitic affinity some of them display calc alkaline affinity different trace element ratios such as sm nd nb yb and ta yb suggest a magmatic evolution in the same tectonic setting the geochemical results reported here support the existence of an active geodynamic setting as a direct contributor to the synsedimentary and magmatic content of the neoproterozoic lowermost cambrian successions in the ciz in particular the relatively high end t values and the high range of f sm nd ratios are consistent with an active margin during the neoproterozoic early cambrian the existence of tectonic activity is also confirmed by the presence of synsedimentary deformation and volcanic rocks all of these traits favour a geodynamic model in which the iberian cadomian segment represented in the ciz would have been part of an active northern margin of gondwana with an associated magmatic arc and related basins during neoproterozoic lower cambrian times a proposed link between the ossa morena and the central iberian zones might account for late cadomian pull apart basins developed on both sides of the magmatic arc sharing the same scenario and involving similar magmatic activity during the neoproterozoic cambrian transition

pb ages of detrital zircons in relation to geodynamic evolution paleozoic of the northern gemericum western carpathians slovakia abstract new u pb sensitive high resolution ion microprobe shrimp detrital zircon ages from early paleozoic to permian siliciclastic metasediments are used to constrain provenance and paleogeographic affinity of the northern gemericum unit ngu in the western carpathians central europe the samples span the time period from 480 ma to 260 ma and permit the recognition of provenance changes that reflect the realm s geodynamic evolution the detrital zircon age spectrum of one early paleozoic sample resembles the neoproterozoic and archean ages of the cadomian belt 550 650 ma and the west african craton provenances 1 8 2 7 ga the origin of the early paleozoic ngu rakovec basin is inferred to be the rifting as followed by back arc spreading which occurred at the cambrian ordovician boundary ca 480 ma as continental pieces started to split off from the gondwanan margin the deformation and closure of the early paleozoic basin is coincident with the 355 ma peak of the post tournaisian detrital zircon age assemblage the detrital zircon age spectra of two post mississippian and two post artinskian samples indicate high exhumation and erosion rates in the variscan collision belt pennsylvanian permian sedimentary troughs formed between 318 and 275 ma record contemporaneous unroofing of the variscan collision belt and magmatic activity during post orogenic transtensional tectonics zircon age spectra reflect these three paleozoic 480 419 ma 340 380 ma 320 275 ma tectono thermal events and permit the recognition of provenances both of which supporting the interpreted changes in geotectonic regime from ordovician rifting to variscan compression and the subsequent extension the zircon age data constrain the provenance linkage of the low grade ngu basement to the cadomian fragments within the west european variscides

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

geochemistry of the upper neoproterozoic and lower cambrian siliciclastic rocks and u pb dating on detrital zircons in the central iberian zone spain sandstones and shales from the upper neoproterozoic un succession in the central iberian zone ciz show parallel ree patterns and relatively restricted and similar ranges and average values of some element ratios such as al2o3 tio2 ti nb eu eu la yb n gd yb n and th u this remarkable geochemical homogeneity for related medium and fine grained rocks is unusual and strongly suggests a recycled source area however the lower cambrian lc equivalent rocks are in general terms geochemically less mature more heterogeneous and more fractionated their average ree patterns are practically coincident probably as a consequence of ree redistribution related to the reworking of sediments during a stage of sea level fall in lower cambrian times ti and zr abundances chemical index of alteration cia values and element ratios such as al2o3 tio2 k rb ti nb and rb zr can be used for discriminating purposes between the un and lc siliciclastic rocks detrital zircons from a un and a lc sandstone display morphological differences however u pb data are discordant and cannot be interpreted in a straightforward manner nevertheless age data are compatible with a model in which the continent of gondwana would have supplied zircons showing a bimodal age distribution in variable proportions to the respective sandstone units

zircon ages and nd isotopic and chemical compositions of orthogneisses from the black forest germany evidence for a cambrian magmatic arc single zircon u pb dating combined with 207pb 206pb ages obtained by the evaporation method constrains the emplacement of tonalitic trondhjemitic and granodioritic orthogneisses of the moldanubian zone in the black forest between 500 and 510 ma two detrital zircon populations of 1 9 and 1 6 ga indicate early middle proterozoic material in the former setting of the basement the initial end values range from 0 1 to 3 4 and mean crustal residence ages of 1 0 1 4 ga are consistent with involvement of early middle proterozoic crust and a subordinate juvenile component probably originating from subduction related melting of the mantle the orthogneisses have fractionated ree patterns and slightly higher k2o na2o ratios than typical low k tonalite trondhjemite granite suites the chemical data are interpreted as evidence for melting of amphibolite and contributions from evolved crust the emplacement of the orthogneisses was superceded by a high temperature metamorphic event at 480 ma which we interpret as a result of lithospheric thinning in a marginal basin behind a cambrian magmatic arc

are the oroclines of the variscan belt related to late variscan strike slip tectonics terra nova 23 241 247 2011 abstract early variscan structures and magnetic anomalies delineate four oroclines along the variscan belt the arcs bend the lithostratigraphic zones of the autochthon and the allochthonous terranes which include ophiolites of the rheic suture and units of gondwanan derivation subducted on both sides of the rheic ocean a transcurrent model dominated by right lateral motion explains the genesis and evolution of the oroclines and a simple plate tectonics scenario links late variscan dextral strike slip movements in the variscides to a broad intracontinental shear zone kinematically equivalent to a transform fault terra nova 00 1 7 2011

lower paleozoic extensional tectonics in the limit between the west asturian leonese and central iberian zones of the variscan fold belt in nw spain the limit between the west asturian leonese and central iberian zones in the nw of the iberian peninsula is described in the area of caurel truchas provinces of orense and leon from the distribution of the sedimentary formations and the variscan structures it is inferred that the transition between the two zones was a fault system which was active during at least the ordovician and lower silurian the faults were supposedly normal though they had probably some strike slip component and gave rise to a half graben in which a syn rift sequence was deposited the latter underwent inversion at the onset of the variscan tectonism producing a ramp anticline syncline pair that forms the more important folds in the area the varying geometry of these folds is interpreted partly as being due to the existence of previous en echelon folds related to the strike slip component of the faults and partly as a result of the variable intensity of the shear strain undergone during the first variscan phase

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

contact metamorphism in middle ordovician arc rocks sw sardinia italy new paleogeographic constraints abstract in the early cambrian bithia formation in the variscan foreland of sardinia a middle ordovician granitic intrusion 478 457 ma is hosted by marly metasedimentary rocks that were affected by high temperature ht metamorphism a detailed structural petrographical transect was conducted through the granitic intrusion and its host rocks field data and relationships between ht low pressure lp mineral assemblages in the metasedimentary rocks grt wo ves in carbonate lenses and and in pelite demonstrate that the study area was affected by a polyphase ht overprint i t 520 620 c at xco2 0 1 p 0 2 0 4 gpa and ii t 600 670 c at xco2 0 1 p 0 2 0 4 gpa that pre dates the variscan tectonic metamorphic and igneous phases in the canigo or canigou massif eastern pyrenees the somail massif montagne noire and the ruitor massif internal massifs nw alps middle ordovician orthogneiss with relict igneous textures are deciphered despite being overprinted by variscan amphibolite to granulite facies metamorphism and subsequent alpine low grade metamorphism comparisons of associated igneous and metasedimentary rocks in the sardinia foreland with the high grade metamorphic complex in the variscan axial zone and the canigou massif indicate a convergent middle ordovician evolution that was overprinted by ht variscan metamorphism

huaiyuan epeirogeny shaping ordovician stratigraphy and sedimentation on the north china platform abstract ordovician conodont studies have revealed a depositional hiatus extending from the late floian to early darriwilian on the north china platform recognition of this widespread gap entails revision of the original concept of the huaiyuan epeirogeny with definition of two distinct regional tectonic events event 1 which initiated this hiatus and a subsequent event 2 that was responsible for terminating early palaeozoic sedimentation in the region from the late katian the timing of these two events partly coincides with widely recognized eustatic sea level falls and separates ordovician sedimentation into two episodes that can be broadly correlated with eustatic sea level rises in combination with these sea level changes the huaiyuan epeirogeny played a decisive role in shaping and controlling ordovician sedimentation and sequence stratigraphic architecture on the north china platform lower ordovician carbonates were deposited during an apparent regression decreasing accommodation space resulting from rapid sediment accumulation exceeding the overall rate of basement subsidence and eustatic sea level rise sedimentation ceased in the middle to late floian when basement uplift commenced in the south and extended northward to affect the entire platform the diachronous top surface of the lower ordovician succession reflects extensive erosion that is most pronounced in the south and southwest parts of the platform where the disconformity surface cut down into tremadocian or even upper cambrian strata deposition of the younger sequence darriwilian to katian was the result of the interplay between rejuvenated basement subsidence and the late middle ordovician eustatic sea level rise event 2 which was initially coupled with eustatic sea level fall induced by the end ordovician glaciations terminated ordovician deposition in the region with the top of the ordovician marked by an unconformity representing a hiatus of some 122 ma extending from latest ordovician to latest mississippian

temperature and bulk composition control on the growth of monazite and zircon during low pressure anatexis mount stafford central australia the formation age and trace element composition of zircon and monazite were investigated across the prograde low pressure metamorphic sequence at mount stafford central australia three pairs of inter layered metapelites and metapsammites were sampled in migmatites from amphibolite facies t 600 c to granulite facies conditions t 800 c sensitive high resolution ion microprobe u pb dating on metamorphic zircon rims and on monazite indicates that granulite facies metamorphism occurred between 1795 and 1805ma the intrusion of an associated granite was coeval with metamorphism at 1802 3ma and is unlikely to be the heat source for the prograde metamorphism metamorphic growth of zircon started at t 750 c well above the pelite solidus zircon is more abundant in the metapelites which experienced higher degrees of partial melting compared with the associated metapsammites in contrast monazite growth initiated under sub solidus prograde conditions at granulite facies conditions two distinct metamorphic domains were observed in monazite textural observations petrology and the trace element composition of monazite and garnet provide evidence that the first metamorphic monazite domain grew prior to garnet during prograde conditions and the second in equilibrium with garnet and zircon close to the metamorphic peak ages from sub solidus prograde and peak metamorphic monazite and zircon are not distinguishable within error indicating that heating took place in less than 20 myr

late ordovician glaciation in southern turkey abstract we present a new survey of several palaeozoic sections in both the taurus range and the border folds that documents typical glacial features including a glacial pavement and striated dropstones halevikdere formation and demonstrates the former presence of an ice sheet in southern and south eastern turkey evidence for the late ashgill hirnantian age of this episode is provided by macro and microfossils found within the glacial formation the extension of ice related deposits into the northernmost part of the arabian platform mardin region implies a much wider distribution of the ordovician ice sheet than was previously believed and strongly suggests that southern turkey lay close to egypt during the lower palaeozoic

new data on the neoproterozoic cambrian geotectonic setting of the tepla barrandian volcano sedimentary successions geochemistry u pb zircon ages and provenance bohemian massif czech republic the tepla barrandian unit tbu of the bohemian massif was a part of the avalonian cadomian belt at the northern margin of gondwana during neoproterozoic and early cambrian times new detrital zircon ages and geochemical compositions of late neoproterozoic siliciclastic sediments confirm a deposition of the volcano sedimentary successions of the tbu in a back arc basin a change in the geotectonic regime from convergence to transtension was completed by the time of the precambrian cambrian boundary the accumulation of around 2 500 m lower cambrian continental siliciclastics in a basin and range type setting was accompanied by magmatism which shows within plate features in a few cases but is predominantly derived from anatectic melts displaying the inherited island arc signature of their cadomian source rocks the geochemistry of clastic sediments suggests a deposition in a rift or strike slip related basin respectively a marine transgression during middle cambrian times indicates markedly thinned crust after the cadomian orogeny upper cambrian magmatism is represented by 1 500 m of subaerial andesites and rhyolites demonstrating several geochemical characteristics of an intra plate setting zircons from a rhyolite give a u pb shrimp age of 499 4 ma the cambrian sedimentary and magmatic succession of the tbu records the beginning of an important rifting event at the northern margin of gondwana

496 my age of plagiogranites in the chamrousse ophiolite complex external crystalline massifs in the french alps evidence of a lower paleozoic oceanization genuine ophiolitic series are extremely rare in the variscan orogenic belt and the chamrousse formation appears as an exception the main lithological and geochemical characters of the ophiolite are summarized and a special attention is paid to the field relationships and the magmatic filiation of the leucocratic rocks the time evolution of the mafic magmas is considered to express the transition of attenuated continental lithosphere to oceanic lithosphere the silicic rocks represents the final products from a multisequence fractional crystallization of a tholeiitic lil enriched magma and their compositional diversity results from the local increase of the h2o partial pressure during syn accretion tectonics dating the plagiogranites using theup b method on zircons also give the date of the oceanization the resulting age of496 6ma is closely comparable 1 to the accretion age of ophiolites from peri atlantic paleozoic belts and 2 to the protolith crystallization age of the leptyno amphibolitic groups lags from western europe but these relics ophiolites and lags of the cambro ordovician distensional event later undergo very different tectonometamorphic evolution the distinct post magmatic evolutions may be related to their rather distinct original geodynamic significance and to their primitive location with respect to the subsequent zonation of the hercynian belt

age determination of ordovician magmatism in ne sardinia and its bearing on variscan basement evolution a newly discovered pre variscan cover basement relationship in ne sardinia is defined by differences in structural and metamorphic histories the oldest metamorphic fabrics in the basement are absent from a series of metaplutons that are inferred to be comagmatic with metavolcanic rocks in the cover cover and basement are further distinguished by contrasting metapelite compositions superimposed metamorphism is represented by pre variscan garnet and variscan plagioclase porphyroblasts that appear in the field as a mixed garnet albite zone in situ u pb zircon dating of meta igneous rocks from cover and basement by laser ablation inductively coupled plasma mass spectrometry gives ages of 458 7 ma for the tanaunella orthogneiss 456 14 ma for the lode orthogneiss and 474 13 ma for the lula porphyroid the internal sardinian variscides thus incorporate a mid ordovician magmatic belt developed upon an older basement correlation is proposed with a tectonostratigraphic domain that is at present exposed in the alps within the mid ordovician peri gondwanan realm the internal sardinian variscides appear to have developed along an active margin whereas the foreland correlates with the ibero aquitaine domain and is related to the evolution of a passive margin during rifting and detachment of avalonia to form the rheic ocean

transcurrent continental tectonics model for the ossa morena zone neoproterozoic paleozoic evolution sw iberian massif portugal the ossa morena zone sw iberian massif was affected by continuous orogen parallel transcurrent continental tectonics from the neoproterozoic to the carboniferous times involving transtension tt and transpression tp processes that co existed together occurred separately in neighbouring regions by the means of strain partitioning or even worked diachronically a first stage of transpression tp1 took place during the late neoproterozoic lower cambrian as a result of cadomian arc continent collisional processes structures generated by transtension tt1 from cambrian to lower devonian were related to strong lithosphere stretching responsible for the development of basins controlled by major detachments tilting rifting and important tectono thermal diachronic processes denudation phenomena and inhibition of sedimentation related with thermal uplift asthenosphere upwelling and consequent subsidence caused by isostatic equilibrium involving generalized transgressions were processes responsible for major unconformities the variscan tp2 tt2 episodes that followed diachronically tp1 tt1 by maintaining the orogen parallel transport direction were concomitant with syntectonic deposition of continental basins in the omz and foreland basins in the spz tt2 local transtension and tectonic exhumation of deep crustal rocks along major shear zones favoured the opening of tectonic troughs filled up by sediments and volcanism tp2 shortening have generated fold axes parallel to the orogen strike and composite dissymmetric flower structures

ocean floor hydrothermal metamorphism in the limousin ophiolites western french massif central evidence of a rare preserved variscan oceanic marker the limousin ophiolite is located at the suture zone between two major thrust sheets in the western french massif central this ophiolitic section comprises mantle harzburgite mantle dunite wehrlites troctolites and layered gabbros it has recorded a static metamorphic event transforming the gabbros into undeformed amphibolites and the magmatic ultramafites into serpentinites and or pargasite bearing chloritites with various thermobarometric methods it is possible to show that the different varieties of amphibole have registered low p c 0 2 gpa conditions with temperature ranging from high t late magmatic conditions to greenschist zeolite metamorphic facies the abundance of undeformed metamorphic rocks which is typical of the lower oceanic crust the occurrence of ca al mg metasomatism illustrated by the growth of ca al silicates in veins or replacing the primary magmatic minerals the p t conditions of the metamorphism and the numerous similarities with oceanic crustal rocks from ocean drilling program and worldwide ophiolites are the main arguments for an ocean floor hydrothermal metamorphism in the vicinity of a palaeo ridge among the west european variscan ophiolites the limousin ophiolites constitute an extremely rare occurrence that has not been involved in any hp subduction related or mp orogenic metamorphism as observed in other ophiolite occurrences i e france spain and germany

the greater caucasus a galatian or hanseatic terrane comment on the formation of pangea by g m stampfli c hochard c verard c wilhem and j von raumer tectonophysics 593 2013 1 19 abstract a set of new global plate tectonic reconstructions sheds light on the evolution of the earth s oceans continents and individual terranes in the palaeozoic triassic the greater caucasus is interpreted as a hanseatic terrane that separated from laurussia in the devonian and then collided with the gondwana derived galatian superterrane however this contrasts with the hypothesis according to which the greater caucasus terrane is galatian in its nature the reasons for the interpretation of the greater caucasus as a hanseatic block are unclear and this interpretation does not explain some observations further research is necessary in order to understand the plate tectonic position of the greater caucasus terrane in the palaeozoic

an alternative plate tectonic model for the palaeozoic early mesozoic palaeotethyan evolution of southeast asia northern thailand burma abstract an alternative model for the geodynamic evolution of southeast asia is proposed and inserted in a modern plate tectonic model the reconstruction methodology is based on dynamic plate boundaries constrained by data such as spreading rates and subduction velocities in this way it differs from classical continental drift models proposed so far the different interpretations about the location of the palaeotethys suture in thailand are revised the tertiary mae yuam fault is seen as the emplacement of the suture east of the suture we identify an indochina derived terrane for which we keep the name shan thai formerly used to identify the cimmerian block present in southeast asia now called sibumasu this nomenclatural choice was made on the basis of the geographic location of the terrane eastern shan states in burma and central thailand and in order not to introduce new confusing terminology the closure of the eastern palaeotethys is related to a southward subduction of the ocean that triggered the eastern neotethys to open as a back arc due to the presence of late carboniferous early permian arc magmatism in mergui burma and in the lhasa block south tibet and to the absence of arc magmatism of the same age east of the suture in order to explain the presence of carboniferous early permian and permo triassic volcanic arcs in cambodia upper triassic magmatism in eastern vietnam and lower permian middle permian arc volcanites in western sumatra we introduce the orang laut terranes concept these terranes were detached from indochina and south china during back arc opening of the poko song ma system due to the westward subduction of the palaeopacific this also explains the location of the cathaysian west sumatra block to the west of the cimmerian sibumasu block

geodynamic evolution of a cadomian arc region the northern ossa morena zone iberian massif abstract ductile deformation and polyphase metamorphism in the ossa morena zone of the iberian massif are related to two major tectonothermal episodes of cadomian late neoproterozoic to early cambrian and variscan age middle to late paleozoic the available petrological structural and geochronological data suggest that a number of tectono metamorphic and magmatic episodes occurred during the 620 480 ma interval that would comprise a complete cadomian wilson cycle the geodynamic scenario was that of an andean type continental margin an evolutionary model is presented for this orogeny comprising stages of volcanic arc generation crustal thickening back arc extension tectonic inversion and cratonization a correlation with comparable areas from pre mesozoic massifs elsewhere in europe is proposed in particular with the armorican massif of northern france

mid ordovician u pb ages of porphyroids in the peloritan mountains ne sicily palaeogeographical implications for the evolution of the alboran microplate petrographic geochemical and geochronological work on a metavolcanic suite interbedded in a metapelitic sequence of the palaeozoic basement in the peloritan mountains sicily is used to contribute to a better understanding of the pre variscan geodynamic evolution of the former alboran microplate major and trace element data show that the volcanic suite consists of two main groups an intermediate unit of andesites and dacites with volcanic arc affinity and a felsic porphyroid unit of rhyolites with peraluminous composition isotope dilution thermal ionization mass spectrometry u pb analyses for single and multigrain zircon fractions from four samples of the felsic porphyroids yield results that are tightly clustered on or near concordia giving caradoc ages in the range of 456 452 ma the consistent morphological characteristics and oscillatory zoning of the zircons indicate single stage magmatic crystallization xenocrystic zircon grains with ages of 1150 and 2015 ma indicate that these rocks were developed upon a cratonic basement with grenvillian and transamazonian components hence supporting a peri gondwanan origin from the combined evidence we conclude that this portion of the alboran microplate formed as a mid ordovician volcanic arc in a sector of the peri gondwanan terranes that was affected by a tectonothermal caradoc event

neoproterozoic to early cambrian history of an active plate margin in the tepla barrandian unit a correlation of u pb isotopic dilution tims ages bohemia czech republic abstract the tepla barrandian unit tbu of the bohemian massif shared a common geological history throughout the neoproterozoic and cambrian with the avalonian cadomian terranes the neoproterozoic evolution of an active plate margin in the tepla barrandian is similar to avalonian rocks in newfoundland whereas the cambrian transtension and related calc alkaline plutons are reminiscent of the cadomian ossa morena zone and the armorican massif in western europe the neoproterozoic evolution of the tepla barrandian unit fits well with that of the lausitz area saxothuringian unit but is significantly distinct from the history of the moravo silesian unit the oldest volcanic activity in the bohemian massif is dated at 609 17 19 ma u pb upper intercept subduction related volcanic rocks have been dated from 585 7 to 568 3 ma lower intercept rhyolite boulders which pre dates the age of sedimentation of the cadomian flysch st chovice group accretion uplift and erosion of the volcanic arc is documented by the neoproterozoic dob is conglomerate of the upper part of the flysch the intrusion age of 541 7 8 ma from the zgorzelec granodiorite is interpreted as a minimum age of the neoproterozoic sequence the neoproterozoic crust was tilted and subsequently early cambrian intrusions dated at 522 2 ma t sovice granite 524 3 ma vsepadly granodiorite 523 3 ma smr ovice tonalite 523 1 ma smr ovice gabbro and 524 0 8 ma orlovice gabbro were emplaced into transtensive shear zones

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

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

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

u pb zircon dating and nature of metavolcanics and metarkoses from the monte grighini unit new insights on late ordovician magmatism in the variscan belt in sardinia italy in the external units of the sardinian variscides nappe zone volcanic and volcanoclastic successions of middle ordovician age follow lower paleozoic calc alkaline magmatism developed at the northern gondwana margin we present geochemical and zircon u pb isotopic data for the truzzulla formation a low to medium grade metamorphic volcanic volcanoclastic succession belonging to the monte grighini unit the deepest unit in the nappe zone geochemical and radiometric data allow us to define a late ordovician katian magmatic volcanic event of calc alkaline affinity these new data in conjunction with previously published data indicate that in the sardinian variscides the age of lower paleozoic andean type calc alkaline magmatism spans from middle to late ordovician moreover the age distribution of calc alkaline volcanics and volcanoclastic rocks in the nappe zone is consistent with a diachronous development of middle late ordovician andean type magmatic arc through the portion of the northern gondwanian margin now represented by the sardinian variscides this reconstruction of the sardinian variscides reflects the complex magmatic and tectonic evolution of the northern margin of gondwana in the lower paleozoic

life in near anoxic conditions a case study of the ichnology and infaunal ecology of silurian graptolitic black shales from sardinia italy abstract a trace fossil assemblage from the silurian llandovery black shales of the genna muxerru formation is reported for the first time the ichnofossil record is abundant well preserved and comprises alcyonidiopsis cochlichnus phymatoderma planolites large morphotype planolites rough morphotype and a small chondritid sedimentological and ichnological evidence indicates that the bioturbated black shales were deposited in outer shelf or deeper settings with severe depletion in oxygen the seafloor provided a high food supply for the infauna the palaeoenvironment of the genna muxerru formation is an ancient analogue of modern oxygen minimum zones burrow fills are apparently massive in macroscopic view whereas observations under the petrographic and stereoscopic microscope show that a lifestyle dominated i e that of pellet filling sediment feeders as such the genna muxerru formation provides not only a new ethologic reference for low oxygen settings but also a methodological benchmark for ichnological analysis of black shales bioturbation in black shales may be commoner than expected but its recognition requires microscopic observation

the timing of metamorphism in the odenwald spessart basement mid german crystalline zone new in situ electron microprobe monazite and white mica 40ar 39ar step heating ages support the proposition that the odenwald spessart basement mid german crystalline zone consists of at least two distinct crustal terranes that experienced different geological histories prior to their juxtaposition the monazite ages constrain tectonothermal events at 430 43 ma 349 14 ma 331 16 ma and 317 12 ma 316 4 ma and the 40ar 39ar analyses provide white mica ages of 322 3 ma and 324 3 ma granulite facies metamorphism occurred in the western odenwald at c 430 and 349 ma and amphibolite facies metamorphism affected the eastern odenwald and the central spessart basements between c 324 and 316 ma we interpret these data to indicate that the otzberg michelbach fault zone which separates the eastern odenwald spessart basement from the western odenwald basement is part of the rheic suture which marks the position of a major variscan plate boundary separating gondwana and avalonia derived crustal terranes the age of the carboniferous granulite facies event in the western odenwald overlaps with the minimum age of eclogite facies metamorphism in the adjacent eastern odenwald the granulite and eclogite facies rocks experienced contrasting pressure temperature paths but occur in close spatial proximity being separated by the rheic suture as high pressure and high temperature metamorphisms are of similar age we interpret the odenwald spessart basement as a paired metamorphic belt and propose that the adjacent high pressure and high temperature rocks were metamorphosed in the same subduction zone system juxtaposition of these rocks occurred during the final stages of the variscan orogeny along the rheic suture

the formation of pangea abstract the making of pangea is the result of large scale amalgamation of continents and micro continents which started at the end of the neoproterozoic with the formation of gondwana as pieces were added to gondwana on its south american antarctica and australia side ribbon like micro continents were detached from its african and south chinese side cadomia in the late neoproterozoic avalonia and hunia in the ordovician galatia in the devonian and cimmeria in the permian cadomia was re accreted to gondwana but the other ribbon continents were accreted to baltica north china laurussia or laurasia finding the origin of these numerous terranes is a major geological challenge recently a global plate tectonic model was developed together with a large geological geodynamic database at the lausanne university covering the last 600 ma of the earth s history special attention was given to the placing of gondwana derived terranes in their original position using all possible constraints we propose here a solution for the variscan terranes another paper deals with the altaids the galatian super terrane was detached from gondwana in the devonian during the opening of paleotethys and was quickly separated into four sub terranes that started to by pass each other the leading terranes collided at the end of the devonian with the hanseatic terrane detached from laurussia in the carboniferous gondwana started to impinge onto the amalgamated terranes creating the variscan chain and the pangean super continent east of spain paleotethys remained opened until the triassic subducting northward under laurasia roll back of the paleotethyan slab triggered the collapse of most of the european variscan orogen which was replaced by series of permian rifts some of them becoming oceanized back arc basins during the triassic major force changes at the pangean plate limits at the end of the triassic provoked its break up through the opening of the proto caribbean central atlantic alpine tethys oceanic seaways

the birth of the rheic ocean early palaeozoic subsidence patterns and subsequent tectonic plate scenarios abstract new plate tectonic reconstructions of the gondwana margin suggest that the location of gondwana derived terranes should not only be guided by the models but should also consider the possible detrital input from some asian blocks hunia supposed to have been located along the cambrian gondwana margin and accreted in the silurian to the north chinese block consequently the gondwana margin has to be subdivided into a more western domain where the future avalonian blocks will be separated from gondwana by the opening rheic ocean whereas in its eastern continuation hosting the future basement areas of central europe different periods of crustal extension should be distinguished instead of applying a rather cylindrical model it is supposed that crustal extension follows a much more complex pattern where local back arcs or intra continental rifts are involved guided by the age data of magmatic rocks and the pattern of subsidence curves the following extensional events can be distinguished during the early to middle cambrian a back arc setting guided the evolution at the gondwana margin contemporaneous intra continental rift basins developed at other places related to a general post pan african extensional phase affecting africa upper cambrian formation of oceanic crust is manifested in the chamrousse area and may have lateral cryptic relics preserved in other places this is regarded as the oceanisation of some marginal basins in a context of back arc rifting these basins were closed in a mid ordovician tectonic phase related to the subduction of buoyant material mid ocean ridge since the early ordovician a new phase of extension is observed accompanied by a large scale volcanic activity erosion of the rift shoulders generated detritus armorican quartzite and the rift basins collected detrital zircons from a wide hinterland this phase heralded the opening of palaeotethys but it failed due to the silurian collision eo variscan phase of an intra oceanic arc with the gondwana margin during this time period at the eastern wing of the gondwana margin begins the drift of the future hunia microcontinents through the opening of an eastern prolongation of the already existing rheic ocean the passive margin of the remaining gondwana was composed of the galatian superterranes constituents of the future variscan basement areas remaining under the influence of crustal extension they will start their drift to laurussia since the earliest devonian during the opening of the palaeotethys ocean

lochkovian earliest devonian transgressions and regressions along the tethyan margin of gondwana a review of lithostratigraphical data abstract global sea level rise and consequent regional transgressions are hypothesized at the beginning of the devonian a brief review of lithostratigraphical data available from three regions of the tethyan margin of gondwana namely northern africa arabia and the tethys himalaya suggests that all of them were characterized by a regressive setting in the lochkovian in contrast data from some terranes of the greater galatian superterrane provide evidence of early lochkovian transgression the regressive setting documented on the tethyan margin of gondwana differs from the transgressive regressive cycles of euramerica and the norm of global sea level rise followed by a highstand only regional tectonic processes including those linked with dynamic topography could explain these differences these processes may have been associated with syn rift uplift preceding the opening of the palaeo tethys ocean mantle uplift at the supercontinent margin or the eo variscan orogenic phase

provenance analysis of the paleozoic sequences of the northern gondwana margin in nw iberia passive margin to variscan collision and orocline development abstract the cantabrian zone of nw iberia preserves a voluminous almost continuous sedimentary sequence that ranges from neoproterozoic to early permian in age its tectonic setting is controversial and recent hypotheses include i passive margin deposition along the northern margin of gondwana or ii an active continental margin or iii a drifting ribbon continent in this paper we present detrital zircon u pb laser ablation age data from 13 samples taken in detrital rocks from the cantabrian zone sequence ranging from early silurian to early permian in depositional age the obtained results together with previously published detrital zircon ages from ediacaran ordovician strata allow a comprehensive analysis of changing provenance through time collectively these data indicate that this portion of iberia was part of the passive margin of gondwana at least from ordovician to late devonian times zircon populations in all samples show strong similarities with the sahara craton and with zircons found in libya suggesting that nw iberia occupied a paleoposition close to those regions of present day northern africa during this time interval changes in provenance in the late devonian are attributed to the onset of the collision between gondwana and laurussia additionally the middle carboniferous to permian samples record populations consistent with the recycling of older sedimentary sequences and exhumation of the igneous rocks formed before and during the variscan orogeny late devonian to permian samples yield zircon populations that reflect topographic changes produced during the variscan orogeny and development of the lithospheric scale oroclinal buckling

neoproterozoic to early palaeozoic magmatic evolution in the gondwana derived austroalpine basement to the south of the tauern window eastern alps in the austroalpine basement to the south of the tauern window distinct suites of metabasites occur with orthogneisses in pre early ordovician units tholeiitic and alkaline within plate basalt type metabasites are associated with acid meta porphyroids in the post early ordovician thurntaler phyllite group according to their correlated trace element abundances metabasite zircons crystallized with their host rocks protolith pb pb zircon ages whole rock ta yb th yb and oxygen sr nd isotope data define two principal evolution lines an older evolution at elevated th yb typical of subduction related magmatism started by 590 ma n morb type and 550 530 ma volcanic arc basalt type basic suites which mainly involved depleted mantle sources it finished with mainly crustal source 470 450 ma acid magmatites an other evolution line by tholeiitic and 430 ma alkaline within plate basalt type suites in both pre and post early ordovician units is characterized by an intraplate mantle metasomatism and enrichment trend along multicomponent sources these magmatic evolution lines can be related to a plate tectonic scenario that involved terranes in a progressively mature neoproterozoic to ordovician active margin and a subsequent palaeo tethys passive margin along the north gondwanan periphery

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

a two stage exhumation in western french massif central new geochronological evidences of syn collisional extension abstract systematic 40ar 39ar dating of granitoids and surrounding metamorphic rocks has been performed in the limousin area w of french massif central in order to provide new constrains concerning its late orogenic thermal and tectonic evolution the same 40ar 39ar age of ca 335 338 ma is found on the present day outcrop surface for most of the limousin metamorphic rocks hence indicating that the entire massif was uniformly cooled down at this period these ages are very close to the 40ar 39ar ages proposed in the sioule and brevenne areas in the literature but also to the coupled tectonic and magmatic activity in the south limousin these results help to document the existence of two exhumation stages each one associated with an extensional stage a first exhumation stage has been active at ca 335 340 ma available data indicate that this extension and associated exhumation and cooling occurred over a distance of more than 200 km in the internal zones of the belt whereas compression was still active in the northern and southern parts of the belt a second exhumation stage may be related to the late orogenic extension and collapse of the belt at ca 310 305 ma and is characterized by the uplift of the st sylvestre complex by unroofing of the overlying metamorphic and granitoidic formations

non random distribution of euler poles is plate tectonics subject to rotational effects terra nova 24 467 476 2012 abstract a full global physically coherent geodynamical model over 600 ma has been developed at the university of lausanne during the past 20 years based on the paleodyn database compiling information of geodynamical interest one of the outstanding results presented herein is the distribution of euler poles axis around which tectonic plates are moving on the earth s surface from one reconstruction to another which are not randomly distributed but smeared out along a weakly developed girdle the scattering reflects the tendency of plates to follow a random evolution however the distribution along a preferential plane best fitted plane suggests a long term influence on plate motion the fact that the poles predominantly lie within the inter tropical zone together with the computed net rotation throughout the phanerozoic indicating a westward drift suggest a long term influence of rotational effects

the vila de cruces ophiolite a remnant of the early rheic ocean in the variscan suture of galicia northwest iberian massif abstract the vila de cruces ophiolite is one of the ophiolitic units involved in the variscan suture of the northwest iberian massif this ophiolite consists of a tectonically repeated succession of greenschist facies volcanic rocks common alternations of metasediments of pelitic or siliceous character and scarce orthogneisses metagabbros and serpentinites the protolith age of a granitic orthogneiss that intruded the mafic rocks is dated at documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape 497 pm 4 end document ma u pb in

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

ordovician and silurian protolith ages of metamorphosed clastic sedimentary rocks from the southern schwarzwald sw germany a palynological study and its bearing on the early palaeozoic geotectonic evolution sedimentation ages of metamorphosed clastic sedimentary rocks in the southern schwarzwald were determined by associations of palynomorphs in the northern subunit of the badenweiler lenzkirch zone two lithostratigraphic assemblages could be discerned in low grade metamorphic units by their facies and age thus revealing a more complex internal structure of this zone than previously assumed lower ordovician metagreywackes and metapelites were discerned from silurian metasiltstones in the cataclastically overprinted metasiltstones and phyllites of the southern subunit of the badenweiler lenzkirch zone only poorly preserved microfossil remains could be detected these show that the sedimentation ages must be ordovician or younger but still probably early palaeozoic high grade metapelitic rocks of the south schwarzwald gneiss complex contain chitinozoans in lenses and layers of schists that are rich in biotite and graphite they yielded mid silurian ages and show that this crystalline complex does not represent an older basement unit but was the result of marine sedimentation at that time the new age determinations have a bearing on geodynamic reconstructions of the internal variscides in early palaeozoic time they show that sedimentation in the oceanic realm of the badenweiler lenzkirch zone or its margins did not occur before the ordovician after transformation of the northern passive into an active continental margin younger greywackes not older than middle devonian received detritus from a volcanic arc forming above the subduction zone

rifting during separation of eastern avalonia from gondwana evidence from subsidence analysis subsidence curves for cambrian ordovician sequences from the anglo welsh segment of the paleocontinent of avalonia reveal two periods of regionally enhanced basement subsidence early cambrian 545 518 ma and late cambrian to early tremadocian 505 490 ma the earlier event may record transtension following the avalonian cadomian orogeny the second event may be a transtensional precursor to the late tremadocian volcanic arc on eastern avalonia however paleomagnetic faunal volcanic and sedimentary evidence suggests that the main separation of eastern avalonia from gondwana occurred after middle arenigian time rifting during separation is probably recorded by localized middle arenigian to llanvirnian 480 462 ma subsidence along the welsh basin margin but rifting must have occurred mainly on the now obscured southern margin of the avalonian continent pronounced caradocian 462 449 ma subsidence is associated with back arc rifting after separation from gondwana