s type granite generation and emplacement during a regional switch from extensional to contractional deformation central iberian zone iberian autochthonous domain variscan orogeny zircon grains extracted from s type granites of the meda escalhao penedono massif central iberian zone variscan orogen constrain the timing of emplacement and provide information about potential magma sources simple and composite zircon grains from three samples of s type granite were analyzed by la icp ms new u pb data indicate that granites crystallized in the bashkirian 318 7 4 8 ma overlapping the proposed age range of ca 321 317 ma of the nearby s type granitic rocks of the carrazeda de anciaes lamego and ucanha vilar massifs the timing of emplacement of such s type granites seems to coincide with the waning stages of activity of a d2 extensional shear zone i e pinhel shear zone developed in metamorphic conditions that reached partial melting and anatexis ca 321 317 ma dykes of two mica granites resembling diatexite migmatite are concordant and discordant to the compositional layering and s2 main foliation of the high grade metamorphic rocks of the pinhel shear zone much of the planar fabric in these dykes was formed during magmatic crystallization and subsequent solid state deformation field relationships suggest contemporaneity between the ca 319 317 ma old magmatism of the study area and the switch from late d2 extensional deformation to early d3 contractional deformation inherited zircon cores are well preserved in these late d2 early d3 s type granite plutons u pb ages of inherited zircon cores range from ca 2576 to ca 421 ma the spectra of inherited cores overlap closely the range of detrital and magmatic zircon grains displayed by the ediacaran to silurian metasedimentary and metaigneous rocks of the iberian autochthonous and parautochthonous domains this is evidence of a genetic relationship between s type granites and the host metamorphic rocks there is no substantial evidence for the addition of mantle derived material in the genesis of these late d2 early d3 s type granitic rocks the end arrays of heterogeneous crustal anatectic melts may be just inherited from the source probably reflecting mixing of a range of crustal materials with different ages and primary isotopic signatures the generation of the bashkirian s type granites has been dominated by continental crust recycling rather than the addition of new material from mantle sources

strain softening induced ductile flow in the sarv thrust sheet scandinavian caledonides abstract the external zone of the scandinavian caledonides is characterized by thrust sheets composed of sedimentary rocks derived from the continent baltica the sarv thrust sheet is unique amongst them because the 4 5 6 km thick flat lying tossasfjallet group clastics were intruded by a sub vertical tholeiitic dike swarm prior to thrusting the sarv thrust sheet is a large horse 100 km long and 80 km wide with a classical ramp flat geometry the original intrusive relationships of the dikes are preserved throughout most of the thrust sheet and deformation is concentrated at the base in a mylonite zone several tens of meters thick in this zone meter thick dikes rotate into parallelism with the basal thrust and concordant mylonites of both lithologies are intercalated on a centimeter to millimeter scale the overall rigid block geometry of the sarv thrust sheet and the intense strain gradient recorded by the rotation of the dike swarm and the concurrent development of mylonites in the thrust zone suggest that thrust emplacement took place by ductile yield and on going pseudoplastic flow continual strain softening localized ductile flow in this zone by means of grain size reduction reaction enhanced ductility and grain boundary sliding processes inferred from the microstructures

tectonic evolution of nw iberia during the paleozoic inferred from the geochemical record of detrital rocks in the cantabrian zone the cantabrian zone of nw iberia preserves a voluminous almost continuous sedimentary sequence that ranges in age from ediacaran to early permian its tectonic setting is controversial and recent hypotheses include i passive margin deposition along the northern margin of gondwana ii an active continental margin or iii the margin of a drifting ribbon continent and iv a combination of the three aforementioned possibilities in this paper we present geochemical data from 21 samples taken in detrital rocks in the cantabrian zone from ediacaran to early permian age the results together with previously published detrital zircon ages from these strata allow a comprehensive analysis of changing tectonic scenarios and provenance through time collectively these data indicate that this portion of iberia was an active margin during the ediacaran that evolved to become part of the passive margin of gondwana at least from ordovician to late devonian times changes in geochemistry zircon age distribution and sm nd isotopes during the carboniferous are attributed to the far field effects of the onset of the collision between gondwana and laurussia and related processes such as recycling of older sedimentary sequences as well as the involvement of the rocks formed during the variscan orogeny and oroclinal buckling latest carboniferous and permian show a more juvenile sm nd and higher values of illite cristallinity that may support the hypothesis of lithospheric foundering after oroclinal buckling

continuous deformation ductile thrusting and backfolding of cover and basement in the sierra de guadarrama hercynian orogen of central spain abstract hercynian deformation has affected two different crustal ensembles a crystalline basement and a metasedimentary cover in the sierra de guadarrama the rheological basement is a medium to high grade metamorphic complex composed mostly of granitic orthogneisses with minor amounts of sediments the cover consists of a thick sequence of low to high grade metasediments with minor gneissic bodies ranging in age from late proterozoic to devonian the deformation in the cover was analysed mainly in the eastern part of the sierra de guadarrama whereas the crystalline basement was analysed in the segovia area to the west both cover and basement deformation started with a stage d 1 of continuous penetrative deformation the deformational regime has a strong component of subhorizontal simple shear with east vergence a second phase d 2 of shear deformation gave rise to a broad subhorizontal shear zone and a thrust in the cover and to narrow ductile shear zones in the basement the transition between the two first phases of deformation seems to have been gradual the third phase d 3 is a generalized backfolding episode which induced an intense and penetrative strain in broad areas of both ensembles finally large open folds with steep axial surfaces developed in two mutually perpendicular systems giving rise to dome and basin interference patterns and some subvertical ductile shear zones and faults were also created the structural evolution of the basement and cover points to a thicker skinned model of deformation giving rise to an important crustal thickening a salient feature of the model arose during the second phase when a thick detachment level developed in the lower part of the cover the generalized existence of this level may account for the scarcity of basement outcrops in the spanish hercynian belt

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

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

a sequence of pan african and hercynian events recorded in zircons from an orthogneiss from the hercynian belt of western central iberia an ion microprobe u pb study sensitive high resolution ion microprobe u pb dating shows that a biotite orthogneiss from the hercynian belt of western central iberia contains 1000 300 ma zircon older 1000 570 ma ages within this range represent inherited detrital material among which four age components may be recognized 980 ma 830 ma 616 10 2s ma and 582 5 2s ma this inherited zircon commonly forms cores that are surrounded by rims yielding late pan african ages identical to those found in slender prismatic and some stubbier bi pyramidal euhedral crystals this is the predominant type of zircon with an average age of 546 3 2s ma thought to have been formed during the main magmatic crystallization stage of the granitic protolith of the gneiss local deuteric replacements of magmatic zircon yield a virtually identical average age of 547 5 2s ma suggesting rapid magmatic cooling typical of shallow intrusive settings many zircon crystals have very thin low th u rims with an age of 315 ma suggested to represent the gneissification of the granitic rock during the hercynian orogeny the abundance in the gneiss body of al rich restitic material and inherited detrital zircon suggests that the granitic magma was formed by anatectic melting of a metasedimentary source rock complex the age of the youngest inherited detrital zircon constrains the sedimentation age of the youngest parts of the anatectic source rock complex to the late neoproterozoic 582 5 ma and leaves a maximum period of 40 myr for metamorphism and anatexis of the source rock complex and migration and intrusion of the granitic magma among the inherited detrital zircon the 616 10 2s ma and 582 5 2s ma ages are by far the most abundant and might be derived from the west african craton where such ages are common for main stage pan african complexes the older 980 ma and 830 ma inherited zircon ages are absent or rare in the west african craton and a derivation from the amazon craton grenvillian sunsas orogens where such ages are common is an alternative

the transition from pangea amalgamation to fragmentation constraints from detrital zircon geochronology on west iberia paleogeography and sediment sources abstract detrital zircon u pb geochronology data from late carboniferous to triassic clastic sedimentary rocks in sw iberia were used to investigate the regional paleogeography during the transition from pangea amalgamation to break up the major u pb zircon age peaks are middle devonian to carboniferous 390 300 ma cambrian ordovician 530 440 ma cryogenian ediacaran 750 540 ma stenian tonian 1 2 0 9 ga and paleoproterozoic 2 3 1 8 ga rapid exhumation of variscan crystalline rocks at the contact between the south portuguese zone and ossa morena zone explains the abundance of late paleozoic ages in the upper carboniferous lower permian continental successions the u pb zircon data constrain the maximum depositional age of the santa susana basin to c 304 ma and the viar basin to c 297 ma the triassic sequences despite being c 100 ma younger than the variscan tectonothermal events contain low proportions of late paleozoic zircon the major peaks in all zircon spectra closely resemble those found in the adjacent basement rocks indicating small source areas mainly located near the rift shoulders longer travelled fluvial systems are postulated for the eastern portions of the algarve basin which was closer to the westward advancing tethys ocean than the rift basins of west iberia sequences that contain significant proportions of 1 2 0 9 ga zircon are probably recycled from post collisional carboniferous permian continental deposits that were more extensive than those found today

tracing the cambro ordovician ferrosilicic to calc alkaline magmatic association in iberia by in situ u pb shrimp zircon geochronology gredos massif spanish central system batholith abstract u pb geochronological study of zircons from nodular granites and qtz diorites comprising part of variscan high grade metamorphic complexes in gredos massif spanish central system batholith points out the significant presence of cambro ordovician protoliths among the variscan migmatitic rocks that host the late carboniferous intrusive granitoids indeed the studied zone was affected by two contrasted tectono magmatic episodes carboniferous variscan and cambro ordovician three main characteristics denote a close relation between the cambro ordovician protholiths of the prado de las pozas high grade metamorphic complex strongly reworked during the variscan orogeny and other cambro ordovician igneous domains in the central iberian zone of the iberian massif 1 geochemical features show the ferrosilicic signature of nodular granites they plot very close to the average analysis of the metavolcanic rocks of the ollo de sapo formation iberia qtz diorites present typical calc alkaline signatures and are geochemically similar to intermediate cordilleran granitoids 2 both qtz diorite and nodular granite samples yield a significant population of cambro ordovician ages ranging between 483 and 473 ma and between 487 and 457 ma respectively besides 3 the abundance of zircon inheritance observed on nodular granites matches the significant component of inheritance reported on cambro ordovician metagranites and metavolcanic rocks of central and nw iberia the spatial and temporal coincidence of both peraluminous and intermediate granitoids and specifically in nodular granites and qtz diorite enclaves of the prado de las pozas high grade complex is conducive to a common petrogenetic context for the formation of both magmatic types tectonic and geochemical characteristics describe the activity of a cambro ordovician arc back arc tectonic setting associated with the subduction of the iapetus tornquist ocean and the birth of the rheic ocean the extensional setting is favorable for the generation emplacement and fast rise of subduction related cold diapirs supported by the presence of typical calc alkaline cordilleran granitoids contemporary with ferrosilicic volcanism

brunswick subduction complex in the canadian appalachians record of the late ordovician to late silurian collision between laurentia and the gander margin of avalon the brunswick subduction complex in the new brunswick part of the canadian appalachians records the late ordovician to late silurian collision between laurentia and the gander margin of avalon the brunswick complex is anomalously well preserved compared with equivalent rocks and structures elsewhere owing to its unique position in the deepest part of the quebec reentrant of the laurentian margin this part of the margin experienced less underthrusting and exhumation and overprinting by orogen parallel faulting than the adjacent promontories where collision started earlier the early southeast to east vergent thrust related structures represent a progressive d1 deformation that formed in response to northwestward subduction of the previously extended gander margin and subsequent tectonic unroofing of the subduction complex the original shallow northwestward dipping envelope to s1 was deformed in the late silurian by d2 upright folds and associated shear zones into a steep belt during terminal collision the d2 structures probably formed in response to sinistral transpression together d1 and d2 indicate that convergence was oblique and sinistral most rocks incorporated in the subduction complex formed in the tetagouche back arc basin that evolved from rifting of an arenig magmatic arc built on the gander margin into a wide marginal basin subduction was initiated in the late ordovician 455 ma in the back arc basin following collision of the middle ordovician popelogan arc with laurentia in the caradoc shortly after its opening in the late arenig 473 ma the closure history of the iapetus ocean involved more than one subduction zone and arc continent collision and rivals the southwestern pacific ocean in its complexities

assessing bulk assimilation in cordierite bearing granitoids from the central system batholith spain experimental geochemical and geochronological constraints

late precambrian series and structures in the navalpino variscan anticline central iberian peninsula the navalpino anticline is a major variscan structure in the central iberian zone of spain three lithological groups are defined in the pre ordovician rocks of this anticline the rifean or lower vendian extremeno dome group is unconformably overlain by the upper vendian ibor navalpino group this latter group presents two different facies separated by a nw se trending synsedimentary fault the lower cambrian valdelacasa group unconformably overlies both the extremeno dome and the ibor navalpino groups

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

geochemistry and nd isotope signature of the collserola range palaeozoic succesion ne iberia gondwana heritage and pre mesozoic geodynamic evolution the collserola range includes a representative stratigraphic succession of the palaeozoic of the central part of the catalan coastal ranges ranging from cambro ordovician to carboniferous times in this paper we present an up to date review of the stratigraphy and structure of the palaeozoic of the collserola range and provide geochemical and sm nd isotope data to constrain the pre mesozoic crustal evolution of this sector of the variscan belt geochemical compositions indicate that the palaeozoic siliciclastic rocks of the collserola range were fed by a relatively mature heterogeneous source of sediment comprising quartz rich sediments to intermediate igneous rocks the siliciclastic rocks of the collserola range show great geochemical affinity with the turbidites of passive margins the sm nd signature of the siliciclastic rocks is compatible with those of the palaeozoic and late proterozoic fine grained siliciclastic rocks of the neighbouring terrains of sw europe there is a small decrease of the end t with decreasing age of sedimentation from the cambro ordovician to the carboniferous suggesting an increase of the amount of more juvenile material the presence of small volumes of alkaline basaltic rocks provides evidence for the input of juvenile material in the early palaeozoic basin and suggests that an extensional tectonic regime prevailed during the cambro ordovician sedimentation from a geodynamic point of view the overall analysis of the data implies that the palaeozoic rocks of the catalan coastal ranges were part of the northern gondwana passive margin before the closure of the rheic ocean and the subsequent variscan orogeny

assembly of the armorica microplate a strike slip terrane delivery evidence from u pb ages of detrital zircons abstract detrital zircons separated from the ordovician armorican quartzite in the foreland of the variscan belt of iberia were dated by laser ablation inductively coupled plasma mass spectrometry the age groups found correspond to proterozoic and archean crust formation events the proterozoic events reflected by zircon ages are avalonian cadomian pan african 550 800 ma grenvillean sunsas 900 1300 ma and eburnian transamazonian icartian 1800 2200 ma archean zircon ages are comprised between 2500 and 2800 ma central amazonian liberian events the presence of mesoproterozoic grenvillean zircons in the armorican quartzite which was deposited in a shallow platform bordering the nw african craton north gondwana margin suggests that the nw iberian terrane drifted from the south american margin where it was located during the neoproterozoic toward africa the drift of the iberian block likely achieved via transform faults was arrested by its docking and amalgamation with the peri nw africa

evolution of the western european continental crust implications from nd and pb isotopes in iberian sediments abstract this paper presents smz sbnd nd and geochemical data on fine grained sediments of the iberian massif spain together with new and published uz sbnd pb results of selected sedimentary rocks profiles from two major zones cantabrian and central iberian zone have been investigated both profiles exhibit almost complete stratigraphic successions from uppermost precambrian to lower carboniferous paleozoic shallow depth platform sediments are essentially unmetamorphosed the precambrian syn orogenic succession underwent a metamorphic overprint up to biotite zone 147 sm 144 nd ratios range from 0 0988 to 0 1457 and nd contents from 6 to 112 ppm nd today values vary from 16 5 to 7 6 pb contents from 1 7 to 97 ppm samples with high nt t 3 are found to be low in 2 330ma t dm are 1 92 ga cantabrian zone and 1 96 ga central iberian zone as juvenile cadomian crust is present in the iberian massif the oldest components have to be significantly older than 2 ga within the european variscides comparably old t dm were so far only described from the armorican massif france an active margin of the gondwana continent is the most probable provenance of the uppermost precambrian strata as indicated by their cadomian age syn orogenic facies and juvenile components precambrian as well as samples from the precambrian cambrian boundary gave nt t values 4 8 significantly higher than average paleozoic rocks contributions of juvenile cadomian crust to the syn orogenic sediments must be in the order of 30 50 to fit the nd and pb data there is no evidence for any addition of mafic material according to major or trace elements or to sm nd ratios lower cambrian samples are already below 7 3 in nd t thus compared to the total pre existing crust contribution of juvenile crust was very small nd t values of almost all samples ranging in stratigraphic age from 515 to 410 ma are strikingly negative 12 9 to 10 1 and thus indicate the absence of newly differentiated continental crust in the provenance s only the tremadocian of the southern central iberian zone gave high nt t values 4 0 to 1 0 again there is no evidence for addition of mafic material to the strata as precambrian and tremadocian nt t peaks are separated by the lowermost cambrian of nt t 7 juvenile crust may have been added related to the pre ordovician unconformity sardic phase during the middle paleozoic 410 330 ma nt t values increase from 12 to 7 indicating a continuous contribution of juvenile continental crust of 20 as major terranes of the iberian massif were not amalgamated until late paleozoic it can be assumed that the respective juvenile crust was formed within the regime of one or several small subduction zones nd isotope data of this study compared with published results of brittany france suggest that the medio north armoricancentro iberian domain persisted from uppermost precambrian to the variscan

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

late archaean early proterozoic source ages of zircons in rocks from the paleozoic orogen of western galicia nw spain abstract u pb data are reported for nine suites of zircons and three monazites from the paleozoic orogen in western galicia nw spain one paragneiss and six orthogneisses from the early paleozoic basement and two carboniferous ca 310 ma old intrusions of two mica granite new whole rock rb sr analyses along with earlier data indicate an age of ca 470 440 ma ordovician for the emplacement of the granitic precursors of the orthogneisses monazite from the paragneiss also yields an u pb age of ca 470 ma for all nine investigated suites of zircons the u pb systematics signal the presence of a minor proportion of precambrian zircon the zircon data from two orthogneisses and the paragneiss display roughly linear arrays with upper intercepts between about 3 0 ga and 2 0 ga and lower intercepts between ca 480 and 460 ma the former are interpreted as approaching the age of the old zircon component and the latter as reflecting either the time of crystallization of new zircons from the magma orthogneisses or the time of radiogenic lead loss from the old zircons paragneiss the suites of zircons from all other investigated orthogneisses suffered isotopic disturbance posterior to the granite emplacement 470 440 ma ago in most cases leading to false discordias without geochronological significance similarly the zircons of the two investigated two mica granites do not produce meaningful discordias because of post paleozoic disturbance the monazite u pb systems of the latter granites indicate sub recent lead loss from the high initial 87 sr 86 sr ratios an involvement of precambrian continental crust material is evident in the generation of the early paleozoic suite of granites while the zircon u pb data give evidence of the presence of about 3 0 2 0 ga old late archaean early proterozoic components in the source material zircons from the oldest sedimentary rocks in the area now present as catazonal paragneisses and a likely source for the granites likewise reveal a provenance age of 3 0 2 0 ga the late archaean early proterozoic source rocks in western galicia probably formed part of an archaean continental crust underlying much of western europe but mostly modified beyond recognition by younger events

mesozoic extensional history of the southern tethyan continental margin in the se aegean the islands of rhodes alimnia and symi are situated at the eastern end of the hellenic arc well exposed on these islands are a series of nappes composed of mesozoic early tertiary principally carbonate sequences which formed the sedimentary cover of the southern tethyan margin and were stacked during alpine deformation in the late palaeogene the stratigraphy of these sequences is described and interpreted as preserving a transition from a carbonate platform through slope to basin the sequences formed part of the cover of a passive continental margin but this margin differed from typical modern margins in recording several episodes of extension with no clear evidence of ocean crust formation in the se aegean three important episodes of extension are recorded a platform basin topography was initiated by mid late triassic rifting late liassic extension led to platform collapse and development of carbonate slopes and renewed late cretaceous extension further reduced the size of carbonate platforms the late cretaceous extension was probably linked to the formation of ocean crust further east in cyprus and turkey sedimentological evidence indicates that the slope was eastward facing and the platform basin margin was orientated approximately north south this evidence is not consistent with previously proposed palaeogeographic reconstructions of the region based on tracing the isopic zones of the hellenides through the arc a new two dimensional reconstruction of the form of the southern tethyan margin in the se aegean is proposed

a hafnium isotopic record of magmatic arcs and continental growth in the iapetus ocean the contrasting evolution of ganderia and the peri laurentian margin abstract we test the sensitivity of combined u pb hf isotopic analyses of detrital and magmatic zircon to document complex processes in accretionary orogens using ganderia from the canadian appalachian orogen as an example ganderia hosted a long lived magmatic arc that began at least by 650 ma and continued until 450 ma during which time it drifted northward from the amazonian margin its isotopic record suggests arc activity began earlier possibly at 750 ma proto ganderia within an oceanic domain the u pb hf zircon isotopic array suggests that ganderia formed on a sliver of paleoproterozoic mesoproterozoic basement tdmc 2 2 1 1 ga a transition towards more evolved hf isotopic compositions between 650 and 600 ma is interpreted to coincide with its accretion to the amazonian margin of gondwana consistent with other geological evidence accretion was followed by increasing amounts of juvenile crustal inputs between 550 and 500 ma which is interpreted to reflect subduction roll back and the onset of drift from the gondwanan margin trench retreat culminated in the separation of the ganderia arc and formation of the exclusively oceanic 510 450 ma penobscot popelogan victoria arcs on ganderia accretion of ganderia to the active notre dame subzone along the laurentian margin occurred in the late ordovician and is marked by the abrupt termination of ganderian arc magmatism the notre dame arc evolved along the laurentian margin between 515 and 425 ma and was built on paleoproterozoic archean t dm c 2 9 2 5 ga crust supra subduction zone magmatism occurred at 490 460 ma during the closure of the taconic seaway and was dominated by the recycling of laurentian crust with progressive mixing of evolved and juvenile arc magmas over time the preservation of very evolved notre dame arc zircon in ganderian overstep sequences confirms the arrival of the leading edge of ganderia to laurentia by 450 ma the hafnium isotopic method is proven here to be a powerful tool when used in conjunction with geological constraints and is able to identify the changing nature of magmatism during the evolution of complex accretionary systems

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

outline of upper precambrian and lower paleozoic evolution of the iberian peninsula according to u pb dating of zircons in the northwestern part of the iberian peninsula recent geological studies have defined the following stratigraphic sequence from bottom to top basal orthogneiss of miranda do douro is overlain by a complex of schist and greywacke which have been assigned to the cambrian or late precambrian these are followed by the ollo de sapo gneisses of volcanic sedimentary origin which in turn are overlain by the fossiliferous layers of the lower ordovician the schist and greywacke complex and the lower part of the ollo de sapo formation are transected by granitic dikes viana del bollo orthogneiss all these rocks were folded faulted metamorphosed and intruded by granite plutons during variscan times the emplacement of the miranda do douro and viana del bollo orthogneisses have been respectively dated at 618 9 ma and 465 10 ma by the u pb method on zircons these data yield lower and upper limits for the deposition of the ollo de sapo formation and demonstrate the occurrence of a precambrian basement pan african or cadomian orogeny in the variscan belt in the northwestern part of the iberian peninsula the emplacement of viana del bollo orthogneiss could be genetically related to the main phase of crustal thinning 470 520 ma characteristic of the prevariscan evolution of western europe typology and u pb data of the ollo de sapo zircons indicate the occurrence of inherited old precambrian zircons and yield an age of 325 3 ma for the variscan main thermal event in the area considered four possible evolutionary models for the u pb system of the ollo de sapo zircons are discussed

ediacaran palaeozoic tectonic evolution of the ossa morena and central iberian zones sw iberia as revealed by sm nd isotope systematics abstract sm nd isotopic analyses of palaeozoic sedimentary and igneous rocks in the southwest iberian massif western end of the european variscan belt are presented in order to unravel its complex poly orogenic evolution during the closure of the rheic ocean and the amalgamation of pangea the gondwanan margin in southwest iberia sw iberia is subdivided into the ossa morena and central iberian zones separated by the badajoz cordoba shear zone which represents a cryptic suture zone between these terranes the relationships between these terranes and between units preserved within the suture zone e g the sierra albarrana group during the palaeozoic and neoproterozoic are controversial sm nd isotopic studies of representative sedimentary sequences covering the entire pre variscan record of the ossa morena and central iberian zones show very similar characteristics from the uppermost ediacaran onwards these data indicate that their accretion to one another must have been completed by the late neoproterozoic ediacarin that time an event assigned to cadomian orogeny and that they never separated substantially from each other since that time the sm nd isotopic composition of the sierra albarrana group metasedimentary rocks is similar to that of the pre cadomian sequences of the ossa morena zone serie negra suggesting derivation from a common source the common provenance of the palaeozoic sequences in the two zones is identical to that of the pre cadomian serie negra of the ossa morena zone which in accordance with the data presented herein and published u pb zircon data indicates a west african affinity

geological constraints on the paleozoic tectonic evolution of tectonostratigraphic terranes in the iberian massif abstract the presence of oceanic paleozoic sequences in the iberian variscan fold belt is used to identify and separate suspected exotic terranes from a reference iberian autochthon terrane which constitutes the greater part of it in this paper the relevant data characterizing the pre accretionary evolution of the paleozoic iberian terranes are presented as well as the available evidence concerning the accretionary process itself emphasis is placed on the analysis of the sedimentary igneous and structural expression of the major events which affected the iberian terranes before during and after their amalgamation the main conclusions of such an analysis may be summarized as follows 1 a recognition of rift and passive margin stages in all the iberian massif terranes during the early paleozoic 2 b late paleozoic accretion of the exotic terranes to the iberian autochthon via a long and complex process variscan orogeny which involved initial subduction underneath the latter and eventual obduction followed by oblique collision of a promontory the iberian autochthon northwestern margin with a northwestern continental block this latter process could account for the large scale crustal imbrication shown by the northern branch of the orogen coeval with predominant strike slip faulting in the southern branch as well as with the formation of the typical arcuate shape of this orogenic belt major present uncertainties relate to the palinspastic provenance of continental suspect terranes and their correlation with similar units in other circum atlantic paleozoic orogens a discussion and speculations on this subject are dealt with in the final section of this paper

tectono stratigraphy and evolution of the mesozoic pindos ophiolite and related units northwestern greece the northwest pindos mountains expose mesozoic and tertiary thrust sheets which include the jurassic pindos ophiolite composed of ultramafic and mafic oceanic crustal rocks the tectono stratigraphy of these units is i mainly ultramafic ophiolitic thrust sheet and basal metamorphic sole dramala complex and loumnitsa unit ii late cretaceous platformal limestones orliakas group iii dismembered intrusive and extrusive ophiolitic rocks aspropotamos complex iv tectonic melange and olistostromes dominated by triassic jurassic volcanic and sedimtntary rocks avdella melange v coherent thrust sheets of late jurassic late cretaceous deep water sediments dio dendra group vi early tertiary flysch pindos flysch immobile trace element studies indicate that triassic and jurassic extrusives of the volcanic sedimentary melange formed mainly at within plate and or mid ocean ridge settings by contrast structurally overlying ophiolitic extrusives include boninitic volcanics and island arc tholeiites of supra subduction zone origin initial ophiolite displacement c 165 ma is recorded in metamorphic sole formation including basal peridotite mylonite amphibolites and greenschists the sole rocks have morb and wpb chemical affinities in our favoured model the pindos ophiolite formed above a mid jurassic westerly dipping intra oceanic subduction zone accompanied by accretion to form melange followed by collision and emplacement with the pelagonian margin final suturing of the pindos ocean however did not take place until early tertiary times

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

faunal evidence for oceanic separations in the palaeozoic of britain the faunal criteria for the recognition of the relative separation of old continents are 1 the contrast between shallow not deep water faunas off opposite shorelines 2 the recognition of deeper water facies surrounding the edges of continents and 3 the recognition of the disposition of ancient climatic belts these criteria are defined and used to assess the relative position of the various parts of what is now britain in the palaeozoic the iapetus ocean is well characterized by faunal differences from the cambrian until the lower ordovician but from caradoc times onwards these differences dwindle until by the latest silurian only the distribution of the ostracods separates the two sides of the ocean on faunal grounds in the early ordovician s british faunas are comparable with those from bohemia france and elsewhere to the s indicating connection with gondwanaland and these faunas differ from those in the baltic area a distribution which we attribute to a true ocean tornquist9s sea between n and s europe tornquist9s sea probably closed in the upper ordovician and from ashgill times onwards the s british and baltic faunas remained similar during the middle and late silurian there is evidence of increasing faunal distinction between britain the baltic area and north america on the one hand and southern europe and gondwanaland on the other indicating an opening rheic ocean which appears to have persisted until late carboniferous times

variscan syncollisional extension in the iberian massif structural metamorphic and geochronological evidence from the somosierra sector of the sierra de guadarrama central iberian zone spain abstract the rocks affected by the variscan orogeny in the sierra de guadarrama central iberian zone can be separated into two domains a high grade western domain and a medium to low grade eastern domain these domains are separated by the berzosa riaza shear zone a new integrated analysis of structural metamorphic and u pb monazite age data indicates that the berzosa riaza shear zone is a major d2 extensional structure associated with oblique crustal scale extensional shearing and important decompression low p high t metamorphism associated with the d2 event overprints earlier barrovian metamorphic assemblages formed during the variscan d1 compressional event the kilometre scale berzosa riaza shear zone exhibits a s c mylonitic front at its base and a crenulation front at its top with successive fabric development under different p t conditions indicating crustal thinning textural relationships and u pb dating of monazite indicate that the main d2 extensional event took place between 337 2 and 326 3 ma early mid carboniferous a late d2 extensional episode overprinted the major d2 ductile shear zone along narrow low grade oblique normal detachment zones kinematic indicators in both d2 and late d2 fabrics indicate a displacement of the upper structural levels to the southeast field relationships microstructures and qualitatively derived p t t d paths suggest that the early mid carboniferous syncollisional extension in the sierra de guadarrama was associated with the major extensional collapse of the variscan hinterland central iberian zone and was contemporaneous with the earliest thrusting in the foreland

crustal melting and recycling geochronology and sources of variscan syn kinematic anatectic granitoids of the tormes dome central iberian zone a u pb la icp ms study in this study we report u pb laser ablation icp ms zircon and id tims monazite ages for peraluminous granitoid plutons biotite muscovite cordierite sillimanite in the tormes dome one of the gneiss cored domes located in the central iberian zone of the variscan belt of northern spain textural domains in zircon interpreted to represent the magmatic crystallization of the granitoids and one monazite fraction in the ledesma pluton yielded ages around 320 ma in agreement with other geochronological studies in the region this age is interpreted to date the timing of decompression crustal melting driven by the extensional collapse of the orogenic belt in this domain of the variscan chain of western europe in addition there are several populations of inherited xenocrystic zircon 1 carboniferous zircon crystals ca 345 ma as well as one of the monazite fractions in the coarse grained facies of the ledesma pluton that also yielded an age of ca 343 ma 2 devonian silurian zircon xenocrysts with scattered ages between ca 390 and 432 ma 3 middle cambrian ordovician ca 450 511 ma 4 ediacaran cryogenian zircon ages ca 540 840 ma 5 mesoproterozoic to archaean zircon 900 2700 ma the abundance of carboniferous inherited zircon shows that crustal recycling cannibalization may often happen at a fast pace in orogenic scenarios with only short lapses of quiescence in our case study it seems plausible that a crustal layer of ca 340 ma granitoids migmatites was recycled partially or totally only 15 20 my after its emplacement

u pb ages of zircons from meta igneous and meta sedimentary rocks of the sierra de guadarrama implications for the central iberian crustal evolution u pb data of zircons from various gneisses of the eastern sierra de guadarrama central spain make it possible to reconstruct the pre hercynian and hercynian history of this area 1 upper intercept ages 2400 ma for meta igneous and 2000 ma for meta sedimentary rocks of discordias defined by data of anhedral zircon fractions document the existence of early proterozoic crust 2 lower intercept minimum ages of anhedral zircon fractions show radiogenic lead loss at about 540 ma due to a thermal event leading to volcanism and emplacement of granitoid rocks into precambrian crust growth of euhedral zircons is possibly related to this event 3 lower intercept minimum ages of about 380 ma defined by anhedral zircon fractions in meta sedimentary rocks prove an early hercynian event according to present knowledge on the metamorphism in the sierra de guadarrama this event could be explained in terms of a barrow type medium pressure metamorphism the inferred cadomian igneous event relates the geological history of central spain with that of western africa to the south and brittany to the north furthermore similarities of the crustal evolution in the area studied and other internal zones of the hercynian belt moldanubian zone french central massiv are confirmed the early hercynian event for the first time affected all the rocks of the area together the pre hercynian evolution of the two complexes is different and the present association of the basement rocks may be explained by thrusting or in terms of early hercynian nappe transport

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

provenance of braided alluvial deposits of the thari formation rhodes se aegean evidence for major erosion of an ophiolite bearing thrust sheet

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