tension gash like back arc basin opening and its control on subduction rollback inferred from tertiary faulting in sardinia 1 the tertiary reactivation of the paleozoic basement in ne sardinia is shown to reflect transtensional back arc tectonics related to the western mediterranean subduction rollback during the algero provencal basin opening the left lateral nuoro fault which is the prevailing structure in central sardinia operated as radial transfer between the west sardinian rift and northern tyrrhenian basin subsequently it contributed to a tension gash like opening of the southern tyrrhenian basin as conjugate fault to a right lateral fault set in se sardinia resulting antithetic rotations with respect to the arc are revealed by reoriented dikes in the basement the tension gash like opening of the southern tyrrhenian basin determined the gulf shape of the east sardinian margin and the arc shape of its calabrian counterpart this structural control on the southern tyrrhenian rollback scenario suggests that tension gash like opening of back arc basins represents an intrinsic aspect of subduction rollback

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

variscan orogeny in corsica new structural and geochronological insights and its place in the variscan geodynamic framework in western corsica remnants of pre batholitic lithological and metamorphic assemblages are preserved as km scale septa enclosed within lower carboniferous to early permian plutons two groups of septa were recognized 1 the argentella and agriates tenda fragments correspond to neoproterozoic rocks deformed and metamorphosed during the cadomian panafrican orogeny and 2 the zicavo porto vecchio solenzara fautea belgodere topiti and vignola fragments consist of variscan metamorphic rocks the lithological content and the main ductile deformation events for each septum are presented in the zicavo porto vecchio and topiti septa a top to the sw ductile shearing d1 event coeval with an amphibolite facies metamorphism is responsible for crustal thickening at ca 360 ma this main event was preceded by eclogite and granulite facies metamorphic events preserved as restites within migmatites dated at ca 345 330 ma a top to the se ductile shearing d2 event coeval with the crustal melting accommodated the exhumation of the d1 event in contrast the belgodere segment is peculiar as it exhibits a top to the e vergence although retrogressed high pressure rocks are also recognized the pre permian fragments are arranged in four nw se striking stripes that define a sw ne zoning with 1 a western domain in topiti vignola zicavo porto vecchio and solenzara fautea 2 a neoproterozoic basement with its unconformable early paleozoic sedimentary cover in argentella 3 an eastern metamorphic domain in belgodere 4 another neoproterozoic basement with its upper paleozoic sedimentary cover in agriates tenda the argentella basement is separated from the western and eastern domains by two sutures s1 and s2 the variscan corsica represents the eastern part of the sardinia corsica maures segment the comparison of this segment with other variscan domains allows us to propose some possible correlations we argue that the western domain argentella belgodere and agriates tenda domains can be compared with the southern variscan belt exposed in french massif central southern massif armoricain armorica microblock leon block respectively

late hercynian shear zones in sardinia abstract in the past few years two main late hercynian shear zones have been identified in sardinia the mount grighini shear zone in the centre of the island and the posada valley shear zone in the north both are dextral wrench shear zones affecting the hercynian basement during the late carboniferous and characterized by a simple shear deformation with an evolution from cataclastic to ultramylonitic rocks the meso and microstructural deformation features are discussed here on the basis of these two maps of the different deformative zones within the shear zones have been prepared and both displacements have been calculated an attempt has also been made to correlate with other late hercynian shear zones in continental europe on the basis of common features and geological constraints

ca distribution between almandine rich garnet and plagioclase in pelitic and psammitic schists from the metamorphic basement of north eastern sardinia garnet and plagioclase pairs from fourteen selected samples from garnet to sillimanite zones collected along a ns traverse throughout the metamorphic basement of ne sardinia have been analyzed by microprobe

new u pb ages for early ordovician magmatism in central spain felsic orthogneisses occur widely in the ollo de sapo domain of the central iberian zone most of which were previously considered to represent a precambrian basement to the palaeozoic sequences of this domain however new u pb dating across the berzosa riaza shear zone sierra de guadarrama central spain indicates an early ordovician age for the most representative types of orthogneisses in the eastern part of the ollo de sapo domain dated rocks include the volcaniclastic cardoso gneiss 480 2 ma from the low medium grade hanging wall the riaza gneiss 468 8 16 ma mylonitic granite in the berzosa riaza shear zone three types of leucogneiss buitrago gneiss 488 8 10 ma megacrystic granite 482 11 14 ma aplitic vein and 482 8 9 ma gneissic leucogranite and the la morcuera granitic augen gneiss 477 4 ma in the high grade footwall the new age of the cardoso gneiss brackets to the mid late arenig the so called early ordovician sardic unconformity characteristic of the central iberian zone these new ages suggest that the broadly coeval volcanism and plutonism were closely associated with the sardic events and that these orthogneisses were part of a felsic magmatic belt which extended along the ollo de sapo domain of the central iberian zone this magmatic belt is interpreted to have been active during the early ordovician break up of the peri gondwanan margin of the iapetus ocean

present trends and the future of zircon in geochronology laser ablation icpms in situ u th pb geochronology was born some two decades ago with the introduction and development of high resolution secondary ion mass spectrometry sims or shrimp sensitive high mass resolution ion microprobe compston et al 1984 williams 1998 compston 1999 davis et al this volume ireland and williams this volume this technique clearly demonstrated the existence of age heterogeneities within the single crystals of zircon and other accessory phases and therefore the need for high spatial resolution tens to hundreds of cubic micrometers geochronological data in situ dating by ion probe is capable of achieving an analytical precision that is only an order of magnitude worse than the conventional isotope dilution thermal ionization mass spectrometry id tims dating technique it has the advantage however of more readily identifying concordant portions of grains does not require chemical treatment of samples prior to the analysis is essentially nondestructive and can achieve greater sample throughputs a major obstacle to the wider use of ion probe dating has always been the high cost of instrumentation and hence relative scarcity of suitably equipped geologic laboratories laser ablation inductively coupled plasma mass spectrometry la icpms emerged in 1985 and rapidly became an important analytical tool for trace element determinations in geological samples jackson et al 1992 it was soon realized that the large variations in radiogenic pb and pb u isotopic ratios found in nature could be resolved by icpms techniques and when coupled to a laser icpms could be used as a dating tool similar to the ion probe the pioneering work of feng et al 1993 fryer et al 1993 hirata and nesbitt 1995 and jackson et al 1996 illustrated the potential usefulness of laser sampling for in situ dating by icpms particularly well however these studies and others that followed also revealed the major

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

geochronology of the medium to high grade metamorphic units of the peloritani mts sicily the peloritani mountains are a fragment of an orogen variably attributed to the alpine or hercynian orogeny on the basis of 39ar 40ar u pb and rb sr dating the main metamorphism of the two medium high grade metamorphic units the mela and aspromonte units and most of the thrusting responsible for stacking the orogenic edifice are seen to be hercynian the main thrusting of the aspromonte unit over the lower grade units took place at 301 2 ma brittle deformation during tertiary reactivation of hercynian thrust planes did not generate any rejuvenation of white micas in the studied sector our dataset shows a great complexity and we propose to unravel it by considering different levels of information to first order the mela and aspromonte units differ in their metamorphic paths and their geochronological evolution the mela unit shows generally younger ages carboniferous than the aspromonte unit and unlike the latter was extensively retrogressed in greenschist facies the aspromonte unit is itself geochronologically heterogeneous proterozoic ages are preserved both in titanite and in amphibole relics of one tectonic subunit devonian to carboniferous amphibole ages are found in different other subunits tertiary overprint is minor and spatially limited we propose to consider the chronologically heterogeneous subunits as accreted pre hercynian terranes amalgamated late during the hercynian orogeny micas in both units give scattered mesozoic 39ar 40ar and rb sr ages with evidence for heterochemical mica generations we interpret them as a result of widespread hydrothermal circulation event s tertiary overprint is generally absent with the exception of a small area near messina where biotite and muscovite underwent a complex recrystallisation history in the interval between 48 and 61 ma

kinematics of the alpine west carpathian orogen and palaeogeographic implications we present the euler rotations of the plates involved in the alpine west carpathian orogen the rotations are defined to a large extent by the magnetic anomalies of the atlantic ocean a few extra rotations occurred during two collisions and a mid cretaceous event at variance with earlier reconstructions we additionally control the rotations by the orientation of palaeomagnetic declinations the plate rotations are integrated into a model illustrated by palaeogeographical maps special features of the model are 1 subdivision of the northern margin of adria into the two plates pelso and austroalpine west carpathia on the basis of palaeomagnetic data 2 eohellenic obduction of meliata units onto the eastern margins of pelso and austroalpine west carpathia from the tethys side 3 first eoalpine collision of the marginal plates of adria with tisza far off the west european plate margin 4 a 80 90 rotation of austroalpine west carpathia during the eoalpine collision 5 subdivision of the neoalpine collision into a palaeogene stage of predominantly strong se nw shortening and a neogene stage of predominantly lateral extrusion westward and eastward in principle the maps show quantitative ocean spreading subduction and plate rotations however possible modifications of the model discussed in this paper limit the quantitative evaluation

late jurassic to early cretaceous sedimentary tectonic development in the chengde basin yanshan fold thrust belt north china craton abstract the chengde basin is located in the central part of the yanshan fold thrust belt in the northern north china craton the sediments in the upper jurassic to lower cretaceous tuchengzi formation in the chengde basin provide a detrital record of basin dynamics and uplift of the basin margins during that time we analyzed the sedimentary facies paleocurrents and provenance of the tuchengzi formation in the chengde basin for the period of the late jurassic and early cretaceous shortening in the yanshan fold thrust belt four sedimentary facies associations have been identified in the tuchengzi formation corresponding to proximal fan mid fan distal alluvial fan and fluvial facies the transport and distribution of the upper jurassic to lower cretaceous sediments in the chengde basin was controlled by the faults bounding the basin paleocurrent indicators and provenance data of conglomerate clasts reveal that the sediments of the tuchengzi formation in the northern part of the chengde basin were delivered from source regions to the north of the basin the early sediments of the tuchengzi formation in the southern part of the basin comprise a suite of fluvial deposits similar to the fluvial sediments in the northern part of the basin and their paleocurrent data and the compositions of conglomerate clasts also suggest a northern source however the subsequent sedimentation in the tuchengzi formation in the southern part of the basin changed markedly to proximal fan facies with sediments being derived from the south of the basin according to the paleocurrent data and conglomerate clast lithology the sandaohe sheet which is located in the southeast limb of the chengde syncline is not a klippe formed as a result of long distance northward thrusting but an autochthonous pop up tectonic wedge generated by n s shortening during the early cretaceous sedimentation of the tuchengzi formation the sedimentation ended before the onset of the early cretaceous volcanic eruption recorded by the zhangjiakou formation which unconformably overlies the tuchengzi formation

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

the east variscan shear zone geochronological constraints from the capo ferro area ne sardinia italy abstract the exhumation of the variscan high grade metamorphic complex of sardinia was driven by large scale asymmetric shear zones which reached middle crustal levels at 325 1 3 u pb monazite ma exhumation was accompanied by heating and partial melting the migmatization process was followed by the emplacement of syntectonic granites dated with u pb on zircon at 318 3 ma and 317 2 ma which produced contact metamorphism at 315 1 3 ma u pb monazite in mylonitic silurian orthogneisses the nw se dextral shear zone of the capo ferro area ne sardinia is affected by partial melting by granite emplacement and shows remarkable petrotectonic analogies to and can thus be related to the east variscan shear zone a major intra continental shear zone which was active in the southern european variscides from 325 ma to 315 ma and that has been precisely dated with the u pb monazite system the emplacement of migmatitic gneisses along large shear zones within this time interval is not restricted to sardinia but has also been observed in southern corsica in the french massif central in the western alps and in central spain this pattern of shear zones is related to a complex geodynamic scenario linked to the oblique convergence of the gondwana and laurussia plates during the middle upper carboniferous

multistage accretion and exhumation of the continental crust ivrea crustal section italy and switzerland the ivrea crustal section exposes in map view all levels of the southern alpine continental crust from ultramafic mafic and felsic granulite facies rocks of the deep crust ivrea verbano zone through medium grade basement rocks strona ceneri zone and val colla zone to unmetamorphosed permo mesozoic sediments the oldest part of the crustal section is preserved in the medium grade basement units which are interpreted to be the overprinted remains of an ordovician 440 480 ma magmatic arc or forearc complex during variscan subduction this arc was technically underplated by a carboniferous accretion subduction complex 320 355 ma containing metasediments and slivers of rheic oceanic crust presently found in the ivrea verbano zone during the late stages of variscan convergence 290 320 ma lithospheric delamination triggered magmatic underplating and lead to polyphase deformation under amphibolite to granulite facies conditions this was broadly coeval with extensional exhumation and erosion of the variscan overprinted ordovician crust presently exposed in the strona ceneri and val colla zones post variscan transtensional tectonics 270 290 ma were associated with renewed magmatic underplating mylonitic shearing and incipient exhumation of the lower crust in the ivrea verbano zone this coincided with the formation of elongate basins filled with volcanoclastic sediments in the upper crust early mesozoic tethyan rifting of the southern alpine crust 180 230 ma reduced crustal thickness to 10 km or less in the lower crust most of this thinning was accommodated by granulite to retrograde greenschist facies mylonitic shearing the lower crust was exhumed along a large noncoaxial mylonitic shear zone that was linked to asymmetrical rift basins in the upper crust the composite structure resulting from this complex evolution is probably typical of thinned late variscan continental crust on the passive margins of western europe alpine faulting and folding 20 50 ma fragmented the crustal section the originally deepest levels of the crustal section in the ivrea verbano zone as well as some segments of the basement cover contact were steepened whereas other parts of the crustal section particularly the strona ceneri zone underwent only minor to moderate alpine rotation

unconformities related to the jurassic cretaceous synrift post rift transition of the northern north sea in the jurassic cretaceous north sea basin the synrift sequence is separated from the post rift sequence by the base cretaceous or late cimmerian unconformity the unconformity covers almost the entire basin has a distinct character in seismic reflection data and wireline logs and hence is easily identified and correlated making it the most important marker horizon in the area the unconformity displays great local complexity in many localities and great variability on a regional scale from one locality to another thus the unconformity is classified as a nonconformity a disconformity and an angular unconformity we suggest that these variations basically reflect different structural position within the basin so that the short wavelength variation is dominated by local structural development e g the rotational history of a fault block whereas the long wavelength variation reflects basin scale tectonic thermal and isostatic processes the merging of this unconformity with younger erosional surfaces its complex configuration and polychronous character makes the general term base cretaceous unconformity inadequate thus the term northern north sea unconformity complex is used here

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

depositional and tectonic setting of the paleoproterozoic lower aillik group makkovik province canada evolution of a passive margin foredeep sequence based on petrochemistry and u pb tims and lam icp ms geochronology abstract the paleoproterozoic lower aillik group is a deformed metasedimentary metavolcanic succession located in the makkovik province of labrador eastern canada the group is situated near the boundary between reworked archaean gneiss of the nain north atlantic craton and juvenile paleoproterozoic crust that was both tectonically accreted and formed on or adjacent to this craton during the ca 1 9 1 78 ma makkovikian orogeny the lower aillik group is structurally underlain by archaean gneiss and structurally overlain by ca 1860 1807 ma bimodal dominantly felsic volcanic and volcaniclastic rocks of the upper aillik group we present geochemical data from metavolcanic rocks and u pb geochronological data from several units of the lower aillik group in order to address the depositional and tectonic history of this group u pb data were obtained using both thermal ionization mass spectrometry tims and laser ablation microprobe inductively coupled plasma mass spectrometry lam icp ms two quartzite units near the structural base of the lower aillik group contain detrital zircons only of archaean age and are interpreted to have been deposited on the nain craton during post 2235 ma rifting and initiation of a passive continental margin overlying mafic metavolcanic rocks contain thin horizons of intermediate tuff one of which is dated at 2178 4 ma this relatively old age and an inferred stratigraphic relationship with underlying sedimentary units suggest that the volcanic rocks represent transitional oceanic crust consistent with their geochemical similarity to tholeiitic rifted margin sequences of mesozoic age in eastern north america a package of interlayered psammitic and semipelitic metasedimentary rocks that appears to stratigraphically overlie the mafic volcanic unit is dominated by paleoproterozoic detrital zircons but also contains archaean grains this package was deposited after 2013 ma the age of the youngest concordant zircon the u pb data imply a minimum 165 m y time gap between mafic volcanism and sedimentation and are consistent with deposition of the psammite semipelite unit in an evolving foredeep that heralded the approach of a paleoproterozoic arc terrane accretion of this terrane to the nain cratonic margin at ca 1 9 ga initiated the makkovikian orogeny although the lower aillik group is highly deformed and may contain internal tectonic boundaries or be incomplete the u pb and geochemical data allow quantitative assessment of a prolonged rift drift basin closure cycle that characterized the early paleoproterozoic evolution of the southern nain cratonic margin

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

volume loss from lower peyto glacier alberta canada between 1966 and 2010 mass loss from mountain glaciers contributes to sea level rise and reduces freshwater availability in glacier fed river basins with negative effects on hydropower generation agriculture and the health of aquatic ecosystems in this study we determine the volume of lower peyto glacier alberta canada from ground penetrating radar surveys in 2008 10 and compare our volume estimate with previous estimates from 1966 and 1984 the long term record of mass balance estimates on peyto glacier highlights peyto s importance as an index glacier for the region we calculate a mean volume of 3 39 0 30 107 m3 for the glacier snout for the period 2008 10 glacier volume decreased linearly from 1966 to 2010 if this trend persists the glacier snout will disappear by 2019 and peyto glacier will have retreated by 1 km our results agree with modelling studies which suggest that peyto glacier and other nearby glaciers along the eastern slopes of the canadian rocky mountains will likely lose 80 90 of their present day volume by 2100

a petrogenetic grid for pelitic schists in the system sio 2 al 2 o 3 feo mgo k 2 o h 2 o a quantitative petrogenetic grid for pelitic schists in the system kfmash that includes the phases garnet chlorite biotite chloritoid cordierite staurolite talc kyanite andalusite sillimanite and pyrophyllite with quartz h2o and muscovite or k feldspar in excess is presented the grid is based on thermodynamic data of berman et al 1985 and berman 1988 for endmember kfash and kmash equilibria and natural fe mg partitioning for the kfmash system calculation of p t slopes and the change in fe fe mg along reactions in the kfmash system were made using the gibbs method in addition the effect on the grid of mno and cao is evaluated quantitatively the resulting grid is consistent with typical buchan and barrovian parageneses at medium to high grades at low grades the grid predicts an extensive stability field for the paragenesis chloritoid biotite which arises because of the unusual facing of the reaction chloritoid biotite quartz h2o garnet chlorite muscovite which proceeds to the right with increasing t in the kfmash system however the reaction proceeds to the left with increasing t in the mnkfash system so the assemblage chloritoid biotite is restricted to bulk compositions with high fe fe mg mn typical metapelites will therefore contain garnet chlorite at low grades rather than chloritoid biotite

polyphase magmatic pulses along the northern gondwana margin u pb zircon geochronology from gneiss domes of the pyrenees abstract alkaline granitic dikes intruding the metasedimentary mantle and orthogneiss cores of the aston and hospitalet domes of the axial zone of the pyrenees are subjects of a laser ablation icp ms u pb zircon geochronology study the age spectra recorded by detrital magmatic xenocrystic and inherited zircons reveal a more complex nearly continuous paleozoic magmatic history of the variscan basement of the pyrenees than previously known inherited and detrital zircons of mesoarchean paleoproterozoic to ediacaran ages attest to the peri gondwana location of the cambrian sediments that later form the metamorphic core of the variscan pyrenees the youngest magmatic zircon ages fall into the late carboniferous and earliest permian ranging from ca 306 297 ma and represent the emplacement ages of the dikes and small granite intrusions the age spectra of magmatic xenocrystic zircons contain several maxima middle 475 465 ma and late ordovician 455 445 ma early 415 402 ma and late devonian 385 383 ma early 356 351 ma and middle carboniferous ca 328 ma middle ordovician and middle carboniferous ages are obtained from xenocrystic zircons that were assimilated from the rocks the dikes intruded the aston and hospitalet orthogneisses and the soulcem granite the presence of early mid carboniferous magmatic zircons in several samples lends further support to a wide spread early variscan magmatic activity in the central pyrenees the other age peaks do not have equivalent igneous or metaigneous rocks in the central axial zone but are thought to be present in the pyrenean crust not exposed and yet to be identified the diversity of ordovician devonian and carboniferous up to permian magmatic ages indicates polyphase emplacement of intrusive bodies during pre variscan and variscan orogenies the source of the heat for the devonian to early mid carboniferous magmatic activity remains elusive and may involve intracontinental subduction zone lithospheric scale shearing or a mantle plume tuzo

new late middle to early late ordovician u pb zircon ages of extension related felsic volcanic rocks in the eastern pyrenees ne iberia tectonic implications pre variscan basement rocks from the pyrenees provide evidence of several magmatic episodes with complex geodynamic histories from late neoproterozoic to palaeozoic times one of the most significant episodes consisting of several granitic and granodioritic bodies and volcanic rocks mostly pyroclastic in nature dates from the late ordovician period in the eastern pyrenees this magmatism is well represented in the ribes de freser and nuria areas here the nuria orthogneiss and the ribes granophyre both dated at c 457 460 ma seem to form a calc alkaline plutonic suite emplaced at different crustal levels the presence of numerous pyroclastic deposits and lavas interbedded with upper ordovician sandbian lower katian formerly caradoc sediments intruded by the ribes granophyre suggests that this magmatic episode also generated significant volcanism moreover the area hosts an important volume of rhyolitic ignimbrites and andesitic lavas affected by alpine deformation these volcanic rocks were previously attributed to late variscan volcanism extensively represented in other areas of the pyrenees here we present the first five laser ablation u pb zircon dates for this ignimbritic succession and two new ages for the ribes granophyre the ages of the ignimbrites overlapping within error are all 460 ma suggesting a genetic relationship between the plutonic and volcanic rocks and indicating that the sandbian katian magmatism is much more voluminous than reported in previous studies and possibly includes mega eruptions linked to the formation of collapse calderas

multiple early paleozoic volcanic events at the northern gondwana margin u pb age evidence from the southern variscan branch sardinia italy abstract in sardinia one of the southernmost remains of the european variscan belt a crustal section through different gondwanan paleodomains is largely preserved laser ablation icp ms u pb ages on undoubtedly igneous sites of the zircons were determined on the lower palaeozoic volcanic rocks constrained by defined field relationships thus evidencing three subsequent volcanic events i intermediate and felsic 491 7 3 5 ma 479 9 2 1 concordia ages transitional volcanic rocks embedded within a cambro ordovician terrigenous succession that occurs with continuity in external and inner nappes bounded to the top by the sardic unconformity ii this cambrian lower ordovician succession is cut by calc alkalic rhyodacites which yielded a concordia age of 465 4 1 4 ma confirming their pertinence to the huge bimodal mid ordovician arc volcanism commonly interpreted as the widespread marker of the rheic ocean subduction iii alkalic metaepiclastites in the external nappe within the post caradocian transgressive sequence dated at 440 1 7 ma likely related to rifting and collapse of the mid ordovician volcanic arc in the reshaped lower palaeozoic stratigraphy of sardinia the timing of the early steps of the variscan wilson cycle can be inferred

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