tectonothermal evolution of the badajoz cordoba shear zone sw iberia characteristics and 40ar 39ar mineral age constraints field characteristics indicate that the badajoz cordoba shear zone has had a long polyphase tectonothermal evolution 40ar 39ar cooling ages recorded within central ductile sectors of the badajoz cordoba shear zone range from ca 370 to 360 ma amphibole and 340 to 330 ma muscovite these are interpreted to date post metamorphic cooling through contrasting temperatures required for intracrystalline retention of argon following a regionally significant variscan tectonothermal overprint these results combined with field relationships indicate that the badajoz cordoba shear zone experienced at least 15 km of uplift relative to adjacent areas during late paleozoic sinistral transpression the data also indicate that individual structural units within the shear zone experienced contrasting uplift histories both in total amount and rate the 40ar 39ar cooling ages indicate complete variscan rejuvenation of all older intracrystalline argon systems within central sectors of the badajoz cordoba shear zone

provenance of upper precambrian lower cambrian shales in the central iberian zone spain evidence from a chemical and isotopic study geochemical and isotopic sr nd o data are presented for upper precambrian and lower cambrian shale sequences from the central iberian zone spain in order to define geochemical discriminants between them and to assess the nature of the source regions supplying these basins elemental data especially th sc ratios coupled with endt values of 2 5 suggest that the upper precambrian rocks contain a greater proportion of a much younger more mafic component than the lower cambrian sequence which has high th sc and endt of 7 3 coherent trends between elemental and o isotope data imply a homogeneous recycled source as the dominant provenance for the upper precambrian shales whereas a lack of such coherence for the lower cambrian sediments suggests a more heterogeneous provenance the sedimentological geochemical and isotopic data are consistent with the upper precambrian succession being derived ultimately from a polycyclic mixture of young pre cadomian mafic sources and older 2 ga crust in contrast the lower cambrian sequence was mainly derived from old more felsic crust which would have contributed increasingly greater proportions of material as the younger sources were progressively removed by erosion

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

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

the missing rheic ocean magmatic arcs provenance analysis of late paleozoic sedimentary clastic rocks of sw iberia abstract early carboniferous turbiditic sedimentary rocks in synorogenic basins located on both sides of the rheic suture in sw iberia were studied for provenance analysis an enigmatic feature of this suture which resulted from closure of the rheic ocean with the amalgamation of pangea in the late carboniferous is that there are no recognizable mid to late devonian subduction related magmatic rocks which should have been generated during the process of subduction on either side of it u pb la icp ms geochronology of detrital zircons from early carboniferous turbidites in the vicinity of the rheic suture in sw iberia where it separates the ossa morena zone with gondwana continental basement to the north from the south portuguese zone with unknown meguma continental basement to the south reveals the abundance of mid to late devonian 51 81 and early carboniferous 13 25 ages the cabrela and mertola turbidites of the ossa morena and south portuguese zones respectively are largely devoid of older zircons differing from the age spectra of detrital zircons in the oldest late devonian strata in the underlying south portuguese zone which contain abundant cambrian and neoproterozoic ages mid to late devonian zircons in the cabrela formation age cluster at c 391 ma eifelian givetian transition and mertola formation age clusters at c 369 ma and at c 387 ma famennian and givetian respectively are attributable to a source terrane made up of magmatic rocks with a simple geological history lacking both multiple tectonic events and older continental basement the terrane capable of sourcing sediments dispersed on both sides of the suture is interpreted to have been completely removed by erosion in sw iberia given that closure of the rheic ocean required subduction of its oceanic lithosphere and the absence of significant arc magmatism on either side of the rheic suture we suggest 1 the source of the zircons in the sw iberia basins was a short lived rheic ocean magmatic arc and 2 given the lack of older zircons in the sw iberia basins this short lived arc was probably developed in an intra oceanic environment

mineralisations pb zn sous inconformite des series de plates formes carbonatees exemple du gisement de treves gard france relations entre dolomitisations dissolutions et mineralisations le gisement zn pb de treves est situe dans le lias de la bordure orientale des causses sud est du massif central francais les concentrations principales constituent des amas stratoides portes par le lotharingien inferieur elles sont aureolees par des mineralisations fissurales ou disseminees reparties de l hettangien superieur a la base du domerien dans une serie entierement dolomitisee l analyse comparee de la serie calcaire peu transformee en bordure du gisement et de son equivalent lateral dolomitise remanie par dissolution et mineralise nous a permis de retracer l evolution paleographique et sedimentologique de l enveloppe des concentrations ainsi que le cheminement parallele des metaux cette reconstitution est resumee schematiquement par la figure 18 l etude gitologique montre que les phenomenes de dolomitisation dissolution et mineralisation s operent en relation avec une emersion qui se situe a la limite lotharingien inferieur lotharingien superieur et se developpent en milieu margino littoral de type mangrove les premiers indices sulfures apparaissent precocement dans les calcarenites du lotharingien inferieur avant leur dolomitisation puis vraisemblablement durant celle ci la mise en place des concentrations succede a la dolomitisation et s effectue parallelement au developpement de dissolutions l etude gitologique nous conduit en outre a envisager que le halo d indices qui cerne la mineralisation principale resulte de la remobilisation partielle de celle ci lors de recristallisations ulterieures en dolosparite blanche l etude de la distribution de pb zn cu et ba montre que le niveau porteur des concentrations principales succede a des formations deficitaires en zn pb et ba en interpretant ce deficit en termes de retenue a la source preconcentration continentale ou de mise en reserve dans les eaux connees on peut envisager une reutilisation ulterieure du stock metal correspondant l etude geochimique montre egalement que la dolomitisation s accompagne d un lessivage de zn ba et pb et n affecte pas cu les quantites mises en jeu ainsi que la concordance entre les elements lessives et la nature des accumulations qui succedent au lessivage nous ont amenes a opter pour un modele genetique dans lequel les metaux liberes par la dolomitisation contribuent pour une part importante aux concentrations principales

first lead isotopic data for cinnabar in the almaden district spain implications for the genesis of the mercury deposits the almaden district constitutes the largest and probably the most intriguing mercury concentration in the world two types of mineralization are recognized 1 stratabound of lower silurian age well represented by the large almaden deposit and 2 fully discordant mercury deposits of minor importance in terms of size and exemplified by the deposit of las cuevas the latter ones can be found at different positions along the almaden stratigraphic column both types of deposits are always associated with the so called frailesca rocks diatremes of alkali basaltic composition this paper reports the first lead isotope compositions of cinnabar in the district whole samples and stepwise leaching cinnabar aliquots display relatively homogeneous isotopic compositions 206pb 204pb 18 112 18 460 207pb 204pb 15 635 15 705 208pb 204pb 38 531 38 826 taken together with jebrak et al s 2002 pyrite lead isotope results the new cinnabar isotopic data define a steep array trend on the 207pb 204pb 206pb 204pb diagram indicating a mixed contribution of lead and probably mercury from different sedimentary sources in the almaden basin the almaden hg deposits are related to a contemporaneous mafic magmatism that might have provided part of the mercury hydrothermal leaching of organic matter from sedimentary rocks and formation of hg organic complexes enhanced metal solubility promoting transport from and within the volcanic units

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

sm nd isotopic systematics as tectonic tracers an example from west avalonia in the canadian appalachians abstract the tectonothermal evolution of west avalonia in mainland nova scotia appalachian orogen provides an excellent example of the application of sm nd isotopic analyses to tectonic studies regional syntheses indicate that west avalonia originated in the neoproterozoic as one of a number of terranes collectively known as peri gondwanan terranes along the gondwanan margin these terranes are characterized by late neoproterozoic voluminous arc related magmatism west avalonia became detached from gondwana in the early ordovician was accreted to laurentia by the silurian and was deformed by post accretionary strike slip tectonics in the late devonian and carboniferous associated with the amalgamation of pangea all stages of this evolution can be recognized in the sm nd isotopic signature in various neoproterozoic and paleozoic volcanic and sedimentary successions in mainland nova scotia crustally derived felsic volcanic rocks of late neoproterozoic ca 615 ma early cambrian and early silurian ages define an envelope that provides gross constraints on the sm nd isotopic composition of the underlying basement source e nd values range from 0 78 to 2 92 in the neoproterozoic rocks from 2 0 to 5 0 in the cambrian rocks and from 0 1 to 2 5 in the silurian rocks t dm model ages for each of these suites are remarkably similar they typically range from 0 9 to 1 2 ga suggesting that the sm nd isotopic composition of these suites was profoundly influenced by repeated melting of the same basement source this sm nd envelope provides a reference by which other suites may be compared and interpreted for example turbidites in a late neoproterozoic volcanic arc basin have sm nd compositions that lie within this envelope suggesting that they are derived from the coeval volcanic rocks that flank the basin the genetic significance of the model ages can be investigated by the analysis of felsic and metasedimentary rocks that pre date the main arc phase two felsic complexes have compositions that lie within the envelope and have similar model ages the presence of this signature in early avalonian rocks suggests that the model ages represent a genuine tectonothermal event at ca 1 0 to 1 2 ga that formed juvenile proto avalonian crust by implication the main arc phase in avalonia recycled this crust sm nd isotopic analyses of the metasedimentary sequences are incompatible with previous interpretations of deposition in a passive margin setting that pre dated avalonian arc activity instead the data indicate derivation from two distinct sources ancient gondwanan basement and juvenile avalonian arc suggesting deposition in a western pacific type back arc environment this re interpretation has important implications for neoproterozoic reconstructions the sm nd isotopic composition of west avalonian basement contrasts with that of other peri gondwanan terranes for example similarly aged felsic rocks in the cadomian terrane of nw france are characterized by more negative e nd values and older t dm model ages 1 0 1 9 ga these data together with detrital u pb data suggest that west avalonia was developed along the amazonian margin whereas cadomia was developed along the west african margin of gondwana sm nd isotopic analyses of early silurian clastic rocks in mainland nova scotia show fundamental differences with unconformably underlying neoproterozoic and early paleozoic avalonian successions these data have e nd values ranging from 5 7 to 6 1 with t dm model ages of about 1 4 to 1 7 ga indicating that the clastic rocks cannot have been derived from underlying successions a variety of geologic data and regional constraints suggests that they may have been derived from the caledonide orogen of western europe thereby providing a minimum age for the accretion of west avalonia to laurentia baltica sm nd isotopic analyses from late devonian early carboniferous continental clastic rocks deposited along the southern flank of avalonia in mainland nova scotia also show little influence from avalonian rocks and are compatible with derivation from the meguma terrane immediately to the south these data are consistent with regional synthesis which indicate uplift of the meguma terrane relative to west avalonia during dextral transpression

fluid transfers in a carbonate gaseous aquifer through the local tectonic and geodynamic history abstract a multidisciplinary approach was developed to analyse the impact of the various tectonic joints on water and gas transfers in the carbonate aquifer of quezac southern massif central france microtectonic measurements confirmed the local tectonic history and the structural map was completed by a morphostructural approach using detailed photointerpretation of both aerial photographs and a digital elevation model the past fluid transfers were discussed on the basis of field observations and the possible geodynamic context the present fluid transfers observed in the field were related to four main groups of tectonic joints based on a tectonic and morphostructural approach the history of tectonic constraints geodynamic context and fluid transfers was then reconstructed it highlighted the major role of the north south tectonic joints for water flows followed by east west joints because of their longest karstification history joints with nw se and ne sw orientations which were more recently karstified appeared to have less influence on water transfers the weak gas production at the surface of north south faults was explained by its dispersion owing to their intense karstification on the basis of the innovative multidisciplinary approach we propose a model for the water and gas preferential flow paths taking the local tectonic and geodynamic context into account

the importance of along margin terrane transport in northern gondwana insights from detrital zircon parentage in neoproterozoic rocks from iberia and brittany abstract detrital zircons from late neoproterozoic rocks of the peri gondwanan cadomian belt of sw iberia and north armorican domain of brittany record neoproterozoic ca 860 550 ma palaeoproterozoic ca 2300 1800 and archaean ca 3300 2600 ma u pb ages the absence of mesoproterozoic zircons suggests that these terranes evolved in a peri w african realm this is in contrast to other western european terranes that preserve mesoproterozoic zircons and are likely to have evolved in a peri amazonian realm such a contrast in detrital zircon populations coupled with the presence of mesoproterozoic zircons in the ordovician armorican quartzite deposited in a peri african platform is interpreted to record along margin terrane transport the change in provenance suggests that subduction was replaced by transform faults that juxtaposed amazonia derived terranes against w africa derived terranes to form the avalonia and armorica microcontinents subsequent extension along the margin resulted in the birth of the rheic ocean and the outboard drift of avalonia

petrogenesis of graphitic cherts in the armorican segment of the cadomian orogenic belt nw france graphitic cherts are interbedded within terrigenous sediments in the cadomian orogenic belt of end proterozoic age in the armorican massif nw france the graphitic cherts are of two types massive cherts essentially composed of quartz sio2 96 and with rare sedimentary structures laminated cherts containing up to 3ae4 al2o3 and 92 98 sio2 sedimentary structures observed in the laminated cherts are indicative of a restricted hypersaline tidal or supratidal environment the origins of both types of chert are to be found in the diagenetic processes of silification of terrigenous and mixed terrigenousevaporitic facies these processes which could be mediated by the presence of organic matter were controlled by the migration of the freshwater saltwater mixing zone during periods of relative sea level change the proposed diagenetic origin for the cherts places a number of constraints on their use in the establishment of stratigraphic correlations

the galicia ossa morena zone proposal for a new zone of the iberian massif variscan implications abstract correlation of a group of allochthonous terranes referred to as basal ophiolitic and upper units exposed in the nw and sw of the iberian massif is used to propose a new geotectonic zone in the southern branch of the variscan orogen the galicia ossa morena zone recent advances in sw iberia identify most of the former ossa morena zone as another allochthonous complex of the iberian massif the ossa morena complex equivalent to the cabo ortegal ordenes malpica tui braganca and morais complexes described in nw iberia the new geotectonic zone and its counterparts along the rest of the variscan orogen constitute an internal variscan zone with ophiolites and units affected by high p metamorphism the galicia ossa morena zone includes a variscan suture and pieces of continental crust bearing the imprint of ediacaran cambrian events related to the activity of peri gondwanan magmatic arcs cadomian orogenesis in the iberian massif the general structure of this geotectonic zone represents a duplication of the gondwanan platform the outboard sections being juxtaposed on top of domains located closer to the mainland before amalgamation this interpretation offers an explanation that overcomes some issues regarding the differences between the stratigraphic and paleontological record of the central and southern sections of the iberian massif also equivalent structural relationships between other major geotectonic domains of the rest of the variscan orogen are consistent with our interpretation and allow suspecting similar configurations along strike of the orogen a number of issues may be put forward in this respect that potentially open new lines of thinking about the architecture of the variscan orogen

geochemistry and tectonostratigraphy of the basal allochthonous units of sw iberia evora massif portugal keys to the reconstruction of pre pangean paleogeography in southern europe abstract the basal allochthonous units of nw and sw iberia are members of an intra gondwana suture zone that spreads across the iberian massif and was formed during the collision of gondwana and laurussia in the late paleozoic this suture zone is made of allochthonous terranes and is currently preserved as a tectonically dismembered ensemble a multi proxy analysis is applied to the basal allochthonous units of iberia to test their affinity and potential usage for tracing a suture zone a comparison of the lithostratigraphy tectonometamorphic evolution geochronology and geochemical characteristics of the ediacaran series of these units reveals striking affinities they derive from rather similar immature sedimentary successions deposited along the same continental margin and in relation to a cadomian magmatic arc sm nd systematics indicates that the isotopic sources are among the oldest of the iberian massif ca 2 15 1 5 ga suggesting a very strong contribution from the west african craton these ediacaran series were affected by high p and low to medium t metamorphism blueschist to eclogite facies during the late devonian ca 370 ma they occur below allochthonous ophiolitic sequences and on top of autochthonous or parautochthonous domains lacking of high p and low to medium t devonian metamorphism i e tectonically sandwiched between lithosphere scale thrusts the combination of all these characteristics makes these particular ediacaran series different from the rest of the terranes of the iberian massif such singularity could be useful for tracing more occurrences of the same suture zone along the variscan orogen particularly in cases where its preservation and recognition may be cryptic it also contributes to improve the paleogeographic reconstruction of the margin of gondwana during the ediacaran

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

space and time in the tectonic evolution of the northwestern iberian massif implications for the variscan belt recent advances in geochemical studies of igneous rocks isotopic age data for magmatism and metamorphism quantitative pressure temperature p t estimates of metamorphic evolution and structural geology in the northwestern iberian massif are integrated into a synthesis of the tectonic evolution that places the autochthonous

rifting and arc related early paleozoic volcanism along the north gondwana margin geochemical and geological evidence from sardinia italy abstractthree series of volcanic rocks accumulated during the cambrian to silurian in the metasediment dominated variscan basement of sardinia they provide a record of the changing geodynamic setting of the north gondwana margin between upper cambrian and earliest silurian a continuous upper cambrian lower ordovician succession of felsic submarine and subaerial rocks dominantly transitional alkaline in character ca 492 480 ma is present throughout the variscan nappes trace element data together with nd isotope data that point to a depleted mantle source indicate an ensialic environment a middle ordovician ca 465 ma calc alkaline bimodal suite restricted to the external variscan nappes overlies the sardic unconformity negative ndi values 3 03 to 5 75 indicate that the suite is a product of arc volcanism from a variably enriched mantle a late ordovician early silurian ca 440 ma volcano sedimentary cycle consists of an alkalic mafic suite in a post caradocian transgressive sequence

radiogenic isotopes for deciphering terrigenous input provenance in the western mediterranean abstract radiogenic isotopic signatures in marine sediments can be used to trace terrigenous source areas and transport mechanisms which are in turn related to climate variability to date most of the published studies using this approach have been focused on eastern mediterranean sediments in contrast we study here the terrigenous input provenance in the westernmost mediterranean alboran sea basin by using radiogenic isotope proxies and nd model ages in a marine record spanning the last 20 ka nd sr and pb isotopes obtained from carbonate free samples from the

genesis and evolution of a syn orogenic basin in transpression insights from petrography geochemistry and sm nd systematics in the variscan pedroches basin mississippian sw iberia abstract the pedroches basin is an outstanding syn orogenic variscan mississippian depocenter located in the vicinity of and overstepping onto the neoproterozoic cadomian suture between the ossa morena and central iberian zones of the sw iberian massif its formation appears to be related to transtensional processes at a major releasing bend in the reactivated suture during sinistral escape of the ossa morena zone from the zone of frontal variscan collision between northern gondwana and laurussia subsequent basin inversion resulted in compartmentalization and internal deformation and was probably related to transpression at a restraining bend along the reactivated suture badajoz cordoba shear zone basin fill consists of both igneous and metasedimentary rocks generally deposited in a shallow marine storm dominated platform environment by using new petrographic geochemical and sm nd isotopic data we attempt to characterize 1 the sources and geological evolution of the pedroches basin fill and 2 the processes involved in basin development and sediment supply and dispersal sampling was carried out in both sedimentary and igneous rocks belonging to three major structural units within the central part of the basin guadiato unit guadalbarbo unit and south pedroches valley unit the combined nd isotopic and geochemical data from the pedroches mississippian metasedimentary rocks indicate an upper continental crust provenance and a high degree of sedimentary recycling of these materials throughout the basin i e they are derived from old recycled upper crust without any significant juvenile component despite the presence of interbedded basaltic rocks the igneous rocks in turn correspond to submarine basalts and trachy andesitic basalts plus some hybrid rocks including crust derived components concerning the mafic rocks major element compositions reveal a tholeiitic and transitional tholeiitic character for the guadalbarbo unit rocks morb type and an alkaline affinity for those of the guadiato unit trace element analyses along with sm nd isotopic ratios suggest two kinds of provenance sources for the pedroches basin syn sedimentary igneous rocks 1 an important crust contaminated and or old enriched mantle source for the guadiato unit and the pedroches batholith and 2 a time integrated depleted mantle source for the guadalbarbo rocks in which no significant interaction with continental crust components is discernible these latter rocks suggest that a strong attenuation or even almost complete tectonic and or thermal erosion of the continental lithosphere was reached at least locally in this region

imaging a mantle plume beneath the french massif central abstract teleseismic tomography is now widely recognised as a very powerful tool for studying the velocity structure of the lithosphere asthenosphere system on both local and regional scales a prime objective is to identify velocity perturbations and to constrain their location and intensity here we use the arrival times of p waves from teleseismic events recorded by a dense network of mobile and permanent stations within the french massif central to derive a 3d p velocity model for an area of 425 km 300 km combined with knowledge of the basement geology of the area and the timing and geochemical characteristics of tertiary quaternary volcanic activity we consider that the velocity model provides unequivocal evidence for the ascent of a thermally and chemically anomalous mantle plume beneath the massif central during tertiary times our model is consistent with regional bouger gravity anomalies documented patterns of neogene recent uplift and deformation fabrics within lithosphere derived mantle xenoliths exhumed by the volcanics

controls on uranium and thorium behaviour in ocean floor hydrothermal systems examples from the pindos ophiolite greece abstract a suite of hydrothermally altered basaltic rocks from the jurassic pindos ophiolite of greece was analysed for its u and th content samples taken from different levels within the ophiolite display variable degrees of hydrothermal alteration ranging from low water rock ratio recharge zone alteration to high water rock ratio discharge zone alteration most samples analysed exhibit constant u th ratios regardless of the degree of alteration however neither of these two elements correlate with any other immobile element zr y ti ree these results suggest that both u and th have been mobile during high temperature ocean floor hydrothermal alteration and indicate a common probably mineralogical control for both elements the chemical composition of hydrothermal fluids from modern tectonic settings equivalent to that of the pindos ophiolite east pacific rise guaymas basin was used to predict theoretical aqueous speciation and solubility of u and th during water rock interaction in these environments the results of this treatment agree with the above conclusions drawn from rock studies and indicate that oxide mineral phases uraninite thorianite control u and th solubility during hydrothermal alteration at oceanic spreading environments

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

timing styles and kinematics of cambro ordovician extension in the tepla barrandian unit bohemian massif and its bearing on the opening of the rheic ocean this paper describes late cambrian dikes and early ordovician volcano sedimentary successions of the prague basin bohemian massif to discuss the timing and kinematics of breakup of the northern margin of gondwana andesitic dikes indicate minor e w crustal extension in the late cambrian whereas the tremadocian to dapingian lithofacies distribution and linear array of depocenters suggest opening of this rheic ocean rift related basin during nw se pure shear dominated extension this kinematic change was associated with the onset of basic submarine volcanism presumably resulting from decompression mantle melting as the amount of extension increased we conclude from these inferences and from a comparison with other avalonian cadomian terranes that the rifting along the northern gondwana margin was a two stage process involving one major pulse of terrane detachment in the early cambrian and one in the early ordovician while the geodynamic cause for the former phase remains unclear but still may include effects of cadomian subduction roll back slab break off isostatic rebound or mantle plume the incipient stage of the latter phase may have been triggered by the onset of subduction of the iapetus ocean at around 510 ma followed by advanced extension broadly coeval tremadocian to darriwilian in large portions of the avalonian cadomian belt unequal amounts of extension resulted in the separation and drift of some terranes while other portions of the belt remained adjacent to gondwana

upper neoproterozoic lower cambrian sedimentary successions in the central iberian zone spain sequence stratigraphy petrology and chemostratigraphy implications for other european zones o3 tio2 zr and nb abundances in shales from all the units strongly suggest a gradual compositional change within this sedimentary succession together with the petrological data the chemical results do not reveal any obvious coeval volcanic contribution to the sediments on the basis of the chemical data a comparison is made with other european zones containing detrital sediments composed of reworked crustal components

geochronological constraints on the evolution of a suture the ossa morena central iberian contact variscan belt south west iberian peninsula one of the main tectonic boundaries of the variscan belt in the iberian peninsula is the ossa morena central iberian contact this contact is marked by a highly deformed unit central unit which recorded an initial high pressure high temperature metamorphic evolution rb sr whole rock isotopic data from three gneissic bodies cropping out in the central unit yield two late proterozoic ages 690 134 and 632 103 ma and an early palaeozoic age 495 13 ma which we interpret as protolith ages the two late proterozoic orthogneisses show initial 87sr 86sr ratios typical of mantle derived materials or those with significant mantle participation 87sr 86sr 0 709 these new radiometric data together with ages previously published and the structural evolution of the central unit lead to the conclusions that 1 there are magmatic protoliths of late proterozoic and early palaeozoic ages 2 the metamorphic evolution of this area including the high pressure event belongs to the variscan orogenic cycle 3 the deformations observed affect the rocks of the entire central unit accordingly they are post ordovician i e variscan and 4 consequently the ossa morena central iberian contact is interpreted here as a variscan suture

geochemical and isotopic smnd provenance of ediacaran cambrian metasedimentary series from the iberian massif paleoreconstruction of the north gondwana margin the geochemical and isotopic smnd features of the ediacaran cambrian siliciclastic series from the central iberian zone and the iberian allochthonous domains indicate a significant paleogeographic change at the northern margin of gondwana associated with the development of the avalonian cadomian arc during the neoproterozoic the opening of a peri gondwanan back arc basin and its subsequent widening during the cambrian led to a progressive separation of the sedimentary basins from the main focus of the probably waning cadomian magmatic arc activity this work presents a paleogeographic reconstruction of the distribution of the iberian terranes along the north gondwanan margin during ediacaran and cambrian times based on the geochemical and nd isotope data from the iberian allochthonous and autochthonous domains these results suggest a location close to the west africa craton for both the allochthonous and autochthonous terranes but they would be distributed laterally along the northern margin of gondwana occupying arc or continental ward positions within a wide back arc basin in this context the peri gondwanan volcanic arc acted as the main supplier of the abundant juvenile material as reflected in positive values of end and young tdm model ages 720 1215 ma of the middle cambrian siliciclastic series from nw iberia upper allochthonous units on the other hand the rather equivalent geochemical and isotopic features of the sedimentary series of the nw and sw iberia basal allochthonous units suggest a common paleolocation for those series yet within the back arc basin but closer to the mainland and older isotope sources the sedimentary series of these terranes present the oldest tdm ages 1499 2156 ma clearly greater than the tdm ages that characterize coeval sedimentary series of the iberian autochthonous domain 1256 1334 ma this finding allows us to place iberian basal allochthonous units very close to the west africa craton from where they received a dominant contribution of old crustal materials whereas the iberian autochthonous domain occupied a more eastern paleoposition closer to the sahara metacraton

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

petrography and geochemistry of the magnesites and dolostones of the ediacaran ibor group 635 to 542 ma western spain evidences of their hydrothermal origin the ediacaran deposits between 635 and 542 ma of the central zone of the iberian massif consist of alternating siliciclastic and carbonate beds these carbonates are dolostones and magnesites which are interpreted to have been formed by the replacement of primary peritidal limestones through petrographic and geochemical analyses we recognize different types of dolomites d1 to d4 and magnesites m1 and m2 despite distinct petrographic features of the four types of dolomite their oxygen and carbon isotopes overlap with 18o values ranging from 15 45 to 17 51 smow and 13c from 0 13 to 3 21 pdb sr isotope values for d1 and d2 range from 0 7028 to 0 7091 magnesites m1 and m2 show oxygen values higher than 17 87 0 and 13c values show the same variability as for the dolomites d3 and d4 oxygen isotope values are between 18 91 and 19 61 and the carbon isotope values range are similar to the other diagenetic phases sr isotope values for the magnesites and late dolomites d3 and d4 are 0 7095 to 0 7104 being higher than those of the d1 and d2 dolomites d1 is a relatively early dolomite phase formed by the replacement of fine grained peritidal limestones the coarser crystal size of d2 which shows similar geochemical features as d1 suggests formation by dolomitization of coarser grained limestones the replacement of d1 and d2 by m1 and m2 advanced along stylolites fractures and bedding planes this replacement is interpreted to have occurred by hydrothermal fluids which is suggested by the presence of talc and forsterite d3 a coarse dolomite completely destroyed any previous texture and d4 dolomite cement post dates magnesite formation interactions of hydrothermal fluids with the prior carbonates reset the oxygen isotopes of the earlier dolomite the study of these magnesites and related dolostones may offer new insights into the model of formation of sparry magnesites hosted by mixed siliciclastic carbonate platform deposits the establishment of the factors and mechanism that control the diagenetic evolution of these carbonates has a great importance in order to understand and predicts porosity and permeability variations of rocks formed under similar geological conditions

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

birth and demise of the rheic ocean magmatic arc s combined u pb and hf isotope analyses in detrital zircon from sw iberia siliciclastic strata paleozoic continental reconstructions indicate that subduction of rheic oceanic lithosphere led to collision between laurussia and gondwana which was a major event in the formation of the ouachita appalachian variscan orogenic belt and the amalgamation of pangea however arc systems which record rheic ocean subduction are poorly preserved the preservation of devonian detrital zircon in late devonian early carboniferous siliciclastic rocks of sw iberia rather than arc related igneous rocks indicates that direct evidence of the arc system may have been largely destroyed by erosion here we report in situ detrital zircon u pb isotopic analyses of late devonian early carboniferous siliciclastic rocks from the pulo do lobo zone which is a reworked late paleozoic suture zone located between laurussia and gondwana detrital zircon age spectra from the pulo do lobo zone frasnian formations show striking similarities revealing a wide range of ages dominated by neoproterozoic and paleoproterozoic grains sourced from rocks typical of peri gondwanan terranes such as avalonia the meguma terrane and the ossa morena zone pulo do lobo rocks also include representative populations of mesoproterozoic and early silurian zircons that are typical of avalonia and the meguma terrane which are absent in the ossa morena zone the famennian tournaisian formations from the pulo do lobo zone however contain more abundant middle late devonian zircon indicating the contribution from a previously unrecognized source probably related to the rheic ocean magmatic arc s the middle late devonian to early carboniferous zircon ages from the siliciclastic rocks of sw iberia south portuguese pulo do lobo and ossa morena zones have a wide range in ehft values 8 2 to 8 3 indicating the likely crystallization from magmas formed in a convergent setting the missing rheic ocean arc was probably built on a meguma avalonia type basement we propose for the pulo do lobo zone that the frasnian sedimentation occurred through the opening of a back arc basin formed along the laurussian active margin during rheic ocean subduction as has been recently proposed for the rhenohercynian zone in central europe detrital zircon ages in the frasnian siliciclastic rocks indicate provenance in the meguma terrane avalonia and devonian rheic ocean arc s as a result of back arc basin inversion the frasnian formations underwent deformation metamorphism and denudation and were unconformably overlain by famennian to visean siliciclastic strata including the phyllite quartzite formation of the south portuguese zone the latest devonian early carboniferous detritus were probably shed to the pulo do lobo zone represa and santa iria formations by recycling of devonian siliciclastic rocks from the south portuguese zone meguma terrane and from a new distinct source with baltica laurentia derivation preserved in the horta da torre formation and alajar melange

tectonic evolution of variscan iberia gondwana laurussia collision revisited an integrated interpretation of the late paleozoic structural and geochronological record of the iberian massif is presented and discussed under the perspective of a gondwana laurussia collision giving way to the variscan orogen compressional and extensional structures developed during the building of the variscan orogenic crust of iberia are linked together into major tectonic events operating at lithosphere scale a review of the tectonometamorphic and magmatic evolution of the iberian massif reveals backs and forths in the overall convergence between gondwana and laurussia during the amalgamation of pangea in late paleozoic times stages dominated by lithosphere compression are characterized by subduction both oceanic and continental development of magmatic arcs over and under thrusting of continental lithosphere and folding variscan convergence resulted in the eventual transference of a large allochthonous set of peri gondwanan terranes the iberian allochthon onto the gondwana mainland the iberian allochthon bears the imprint of previous interaction between gondwana and laurussia including their juxtaposition after the closure of the rheic ocean in lower devonian times stages governed by lithosphere extension are featured by the opening of two short lived oceanic basins that dissected previous variscan orogenic crust first in the lower middle devonian following the closure of the rheic ocean and then in the early carboniferous following the emplacement of the peri gondwanan allochthon an additional major intra orogenic extensional event in the early middle carboniferous dismembered the iberian allochthon into individual thrust stacks separated by extensional faults and domes lateral tectonics played an important role through the variscan orogenesis especially during the creation of new tectonic blocks separated by intracontinental strike slip shear zones in the late stages of continental convergence

tectonic setting of cadomian low pressure metamorphism in the central ossa morena zone iberian massif sw spain abstract the upper proterozoic serie negra a monotonous pre and syntectonic slate greywacke series including volcanic episodes crops out widely in the central area of the ossa morena zone sw iberian massif iberian peninsula this unit was affected by complex late precambrian tectonothermal events the cadomian orogeny during which two phases of deformation d1 and d2 are recognized in the 610 585 ma time span the second deformational episode d2 was the most prominent and was related to progressive low pressure high temperature metamorphism lp ht reaching the highest grade in the lower structural levels where thermal evolution led to extensive migmatization and gave rise to the generation of different kinds of anatectic granitoids in this work we present field petrographic and analytical data dealing with the metamorphic evolution of a cadomian lp ht terrane from the central ossa morena zone iberian massif sw spain whose tectonic setting is unclear so far the tectonic setting proposed herein for cadomian d2 lp ht events is discussed in terms of its relationship to plate convergence or to extensional tectonic regimes modelling of the p t t trajectories undergone by a crust moderately overthickened by the thrusting of thinned sialic crust slices reproduces well the observed p t t aths if moderate or low uplift rate and high geothermal gradients are assumed for the uplift history numerical simulations combined with structural and metamorphic evidence support a compressional tectonic setting as responsible for the observed facies series distribution and deformation metamorphism relationships furthermore the regional geodynamic context coeval high pressure metamorphism in neighboring areas of the ossa morena zone and extensive calc alkaline magmatism is consistent with such a tectonic setting a correlation is tentatively proposed here between compressional cadomian deformations in the ossa morena zone upper proterozoic docking of avalonian terranes in ne america and cadomian orogenesis in the northern armorican massif

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

modelling of the erosion in the rhone valley during the messinian crisis france abstract a geometric reconstruction of the present day messinian rhone valley shows two slope breaks after taking into account the post messinian tectonics and messinian isostatic compensation due to deep valley incision and mediterranean sea unloading these two slope breaks were still visible on the reconstructed original messinian rhone profile one is located on the cevennes fault system and the other one is downstream of st marie the slope of the ante messinian profile was approximately 0 05 the messinian rhone profile was simulated using a numerical erosional model based on mass conservation the messinian profile cannot be explained only by variations of erosional processes such as lithology or changes in discharge and one has to consider that the messinian crisis happened in two phases the profile of the rhone was in steady state just after the first messinian phase of lowering the amplitude of the sea level lowering is around 600 m for the first phase whereas the amplitude of the second one is 1300 1700 m the isostatic deformation during the messinian crisis due to bedrock erosion and sea unloading produced important changes in the mediterranean dynamic

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

tectonic evolution of the careon ophiolite northwest spain a remnant of oceanic lithosphere in the variscan belt abstract analysis of the careon unit in the ordenes complex northwest iberian massif has supplied relevant data concerning the existence of a paleozoic oceanic lithosphere probably related to the rheic realm and the early subduction related events that were obscured along much of the variscan belt by subsequent collision tectonics the ophiolite consists of serpentinized harzburgite and dunite in the lower section and a crustal section made up of coarse grained and pegmatitic gabbros an early devonian zircon age 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

presence of palaeoproterozoic and archean components in the granulite facies rocks of central iberia the hf isotopic evidence abstract hf isotope analyses in previously dated zircon crystals from four variscan granulite facies rocks from central iberia have been studied to examine crustal evolution processes two samples are from exposed migmatite terranes the other two are granulite xenoliths from lower crustal levels hf model ages show that lower crustal granulites display a bigger contribution of archean components than exposed high grade rocks 14 of the zircon population instead of 6 reinforcing ideas of preservation of ancient crustal material in the lower crust the distribution of age hf isotope data from zircon suggests two juvenile crust formation events in central iberia the main one during the neoproterozoic 540 620 ma and a possible second mesoproterozoic event around 1 0 1 2 ga magmatism appears in the central iberian zone exclusively related to that younger juvenile addition hf data of granulite zircons agrees with recent palaeogeographical reconstruction models which present central iberia as a peri gondwanan terrane located between the west african craton and the arabian shield far from the amazonia microplate

metasedimentary and igneous xenoliths from tallante betic cordillera spain inferences on crust mantle interactions and clues for post collisional volcanism magma sources abstract the deep seated xenolith association exhumed in the pliocenic volcano of tallante betic cordillera spain includes protogranular mantle peridotites felsic metasedimentary crustal rocks as well as cumulus igneous rocks such as norites and amphibole phlogopite clinopyroxenites the whole xenolith suite equilibrated at the same pressure 0 7 0 9 gpa representing the local crust mantle boundary moho characterized by extreme lithological heterogeneity this heterogeneity resulted from orogenic processes that induced the juxtaposition of crustal rocks variably depleted in fusible components within mantle domains including metasomes as it is commonly observed in orogenic mantle massifs of the mediterranean area in this contribution we report new mineral compositions of igneous parageneses recorded in these xenoliths and we present sr nd isotope data on both igneous and metasedimentary xenoliths that integrate those from the literature sr nd isotopes coherently indicate a restitic character of the metasedimentary xenoliths which according to model ages were affected by partial melting in paleozoic times sr nd isotopic errorchrons on the igneous xenoliths on the other hand qualitatively indicate tertiary ages which are corroborated by u pb zircon datings of one norite xenolith and two composite xenoliths having zircon bearing norite veinlets the new data are discussed proposing that moho lithologies of tallante could provide significant source compositions for the genesis of the neogene volcanics of the betic area which included calcalkaline lavas as well as more potassic products such as lamproites

inherited arc signature in ediacaran and early cambrian basins of the ossa morena zone iberian massif portugal paleogeographic link with european and north african cadomian correlatives abstract geochemical data from clastic rocks of the ossa morena zone iberian massif show that the main source for the ediacaran and the early cambrian sediments was a recycled cadomian magmatic arc along the northern gondwana margin the geodynamic scenario for this segment of the avalonian cadomian active margin is considered in terms of three main stages 1 the 570 540 ma evolution of an active continental margin evolving oblique collision with accretion of oceanic crust a continental magmatic arc and the development of related marginal basins 2 the ediacaran early cambrian transition 540 520 ma coeval with important orogenic magmatism and the formation of transtensional basins with detritus derived from remnants of the magmatic arc and 3 gondwana fragmentation with the formation of early cambrian 520 510 ma shallow water platforms in transtensional grabens accompanied by rift related magmatism these processes are comparable to similar cadomian successions in other regions of gondwanan europe and northwest africa ediacaran and early cambrian basins preserved in the ossa morena zone portugal and spain the north armorican cadomian belt france the saxo thuringian zone germany the western meseta and the western high atlas morocco share a similar geotectonic evolution probably situated in the same paleogeographic west african peri gondwanan region of the avalonian cadomian active margin

samarium neodymium elemental and isotopic systematics in sedimentary rocks abstract trace element sm nd th sc and nd isotopic data for sedimentary rocks are examined in order to evaluate 1 1 possible secular trends in the composition of the sedimentary mass and upper continental crust 2 2 the role of tectonic setting in controlling the chemical and nd isotopic composition of the sedimentary mass when plotted against either sedimentation age or average provenance age determined mainly from nd model ages sm nd in sedimentary rocks decreases abruptly from about 0 21 to 0 19 at the archean proterozoic transition consistent with a change towards a more felsic upper crustal composition the change is only somewhat less pronounced for turbidite sediments than for those of continental affinity a trend toward increasing sm nd approaching or exceeding archean values is observed for phanerozoic turbidites this latter change is interpreted as indicating undifferentiated young crust as a component in the sediment which has been lost from the older post archean record due to sedimentary and igneous intracrustal recycling processes when plotted against sedimentation age th sc ratios exhibit a trend directly comparable to that of sm nd in this case th sc increases abruptly at the archean proterozoic boundary from about 0 5 to 1 0 and then decreases to about 0 7 during the phanerozoic nd isotopic composition of modern sediments is affected by tectonic association with those deposited at active margin settings tending to have higher nd reflecting younger provenance and higher and more scattered sm nd reflecting less homogenized and differentiated provenance it is likely that these distinctions can be traced back for about 1 2 ga but with the current data base are not readily resolved in older sedimentary rocks the degree to which the preserved sedimentary mass is open to new additions from the mantle either through replacement during sedimentary recycling or sediment mass growth is highly variable but averages about 15 25 during the phanerozoic

transfer faults and pull apart model in the rhinegraben from analysis of multisource data from analysis of gravimetric data comparisons of levelings seismic profiling paleostress analysis and seismicity together with remotely sensed and morphological data the rhinegraben may be divided into several rhomb shaped basins separated by ene trending basement highs these data express neotectonic movements and slight spatial differences in the subsidence rate these basins are asymmetrical on one side they are bounded by a large several kilometers of vertical throw normal fault zone on the opposite side there is a megaflexure cut by smaller normal faults shaping tilted blocks since the middle miocene extensional directions are subparallel to the ene trending basement highs that consequently may be interpreted as strike slip fault zones of transfer type across the erstein basement high there is a reversal in the rift asymmetry major bordering fault on the western side to the south on the eastern side to the north when considered in the context of the west european rift the rhinegraben is interpreted in terms of a pull apart depression formed during the horizontal movement of blocks guided by large ene trending transcurrent faults the normal faults that are curved shaped in map view may mark the boundaries between areas where the maximum component 1 of the paleostress field developed in pull apart patterns was changing from horizontal to vertical

u pb evidence for a polyorogenic evolution of the hp ht units of the nw iberian massif a isotope dilution thermal ionisation mass spectrometry u pb geochronological study was carried out on the high pressure and high temperature units hp ht units overlyingthe oceanic suture in the allochthonous complexes of the nw iberian variscan belt the rocks investigated are seven granulite to eclogitefacies paragneisses and one leucosome within mafic highpressure granulites in the ordenes and cabo ortegal complexes of nw spain u pb datingof zircon monazite titanite and rutile reveal the presence of a pervasive early ordovician metamorphic event at ca 500 480ma and a later early devonian event at ca 400 380 ma the u pb ages in conjunction with petrological and structural data indicate that the high pressure event recorded by these rocks is early ordovician in age monazite ages in the paragneisses suggest that peak metamorphic conditions were reached at ca 500 485ma subsequently the rock ensemble underwent exhumation accompanied by partial meltingan d zircon growth at ca 485 470ma meltingof mafic granulites was coeval with this latter episode as indicated by zircon crystallisation age in the leucosomes dated at ca 486 ma based on these data and on the general features of magmatism and metamorphic evolution it is proposed that this process took place at a convergent plate boundary within a peri gondwanan oceanic domain monazite titanite and rutile data in some of the samples studied show evidence of a second metamorphic episode that took place between ca 400 and 380 ma with a peak at ca 390 385ma this early devonian event at variance with the previous one was not pervasive but rather was localised in areas of intense variscan tectonothermal reworking it is claimed that this later metamorphic event was recorded by the u pb system in areas where monazite and titanite growth was enhanced by fluid circulation in highly strained rocks variscan shear zones accordingto previous structural studies and ar ar datingof fabrics this early devonian episode took place as the hp ht units were deformed and thrusted upon the ophiolitic units in the early stages of the variscan collision

the continental margin of the mesozoic tethys in the western alps abstract most of the structural units of the western alps were derived from the european continental margin of the ligurian ocean a segment of the mesozoic tethys ocean their mesozoic palaeotectonic and stratigraphic evolution bears witness of the following main stages 1 deposition of the triassic platform carbonates essentially prerift but nevertheless bearing the imprint of some extensional movements 2 the liassic middle jurassic rifting corresponds to the creation of a horst and graben system with especially tilted blocks and hence a shoal and basin palaeogeography but without a preliminary doming event 3 the late jurassic early cretaceous opening and spreading of the ligurian ocean began with a general collapse of the continental margin thermal subsidence latest middle jurassic and earliest late jurassic in this paper emphasis is given to the refting derived structures more especially to the major tilted crustal blocks a few tens of kilometres wide which can be either reconstructed or directly observed the rifting stage lasted roughly 40 m y with the alternation of extensional tectonic phases and of relatively quiet periods during the extensional phases movements along fault planes and related episodes of tilting were followed by sudden and rapid episodes of subsidence the latter may be interpreted as resulting from the isostatic and thermal readjustment that follows a phase of stretching of the crust and of the lithosphere the tectonic evolution of the margin continued but decreased during the late jurassic early cretaceous oceanspreading stage tectonic activity resumed in the late cretaceous this probably resulting from the beginning of contraction of both the ocean and the continental margin leading progressively to the continental collision in the tertiary

u pb geochronology and hf nd isotope compositions of the oldest neoproterozoic crust within the cadomian orogen new evidence for a unique juvenile terrane new u pb dates combined with nd and hf isotopic data from rocks within the port morvan area of the baie de st brieuc region of brittany identify a unique portion of the neoproterozoic cadomia terrane two gneisses near port morvan yielded u pb dates of 754 6 0 8 ma and 746 0 0 9 ma ages that are more than 130 myr older than the oldest units formed during the main phase of early cadomian magmatism two trondhjemite boulders from the monogenetic facies of the cesson conglomerate yielded identical ages of 665 2 0 5 ma and 665 5 0 7 ma and a cobble from the polygenetic facies yields a 207pb 206pb date of 637 2 ma individual detrital zircons from a sandstone associated with the cesson conglomerates yield concordant u pb dates ranging from 650 3 ma to 624 1 0 6 ma initial nd values for the rocks in this region range from 5 0 to 6 6 indicative of a substantial input from depleted mantle initial hf values determined on zircons from these neoproterozoic rocks including the detrital zircons range from 6 7 to 14 5 consistent with the nd isotopic results maximum initial hf values for two 2 ga icartian gneisses considered basement to cadomia average 8 4 and 8 7 in contrast to the results of the port morvan rocks 616 608 ma syn tectonic intrusions from normandy and the british channel islands all have negative initial nd values 10 4 to 8 3 consistent with significant contamination by ancient crust such as the 2 ga gneisses the oldest arc related magmas should have interacted most extensively with cadomian basement buffering younger mantle derived magmas that were generated in subsequent magmatic episodes the rocks within the port morvan region are thus inconsistent as examples of the earliest cadomian intrusions as they show no evidence of interaction with 2 ga basement instead the older ages and mantle like isotopic composition of these rocks suggest they are part of an independent terrane that formed prior to and independently from the cadomian arc possible terrane scale structural boundaries have recently been identified including the newly recognized port morvan thrust fault and the nw dipping main cadomian thrust

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

the late ordovician diamictic formations from southwestern europe north gondwana glaciomarine deposits abstract this paper presents a review and a discussion of the clast bearing formations diamicts which occur in the uppermost part of the ordovician succession of numerous regions in southwestern europe lithology facies stratigraphic setting and age are discussed the strong similarities of these formations with contemporaneous glaciomarine deposits of the northern and northwestern margins of the african ice sheet and the late ordovician paleogeographic pattern based upon paleomagnetic data lead to the conclusion that the south european clast bearing formations are glaciomarine deposits supplied with clasts by floating icebergs released from the african ice sheet the proposed paleogeographic model confirms the close connection between southwestern europe and gondwana during early palaeozoic times

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

sm nd dating of fluorite from the worldclass montroc fluorite deposit southern massif central france a three point sm nd isotope isochron on fluorite from the very large montroc fluorite vein deposit southern massif central france defines an age of 111 13 ma initial end of 8 6 and initial 87sr 86sr of 0 71245 suggest an upper crustal source of the hydrothermal system in agreement with earlier work on fluid inclusions which indicated a basinal brine origin the mid cretaceous age of 111 ma suggests the albian aptian transition as the most likely period for large scale fluid circulation during a regional extensional tectonic event related to the opening of the north atlantic ocean

age and characteristics of the loma del aire unit sw iberia implications for the regional correlation of the ossa morena zone abstract finding of ca 620 ma old zircons in a volcanic rock of the loma del aire unit one of many structural divisions in the ossa morena zone some years ago attracted much attention to this unit which would contain if proven the oldest rocks so far dated not only in ossa morena but also in the entire iberian massif in this paper new field petrographic whole rock geochemical and sm nd isotope data as well as new tims u pb zircon ages are presented which collectively allow a much better characterization and dating as cambrian of the rock sequence in the loma del aire unit the previously found ca 620 ma old zircon population is now interpreted as xenocrysts incorporated in the cambrian magma our data together with recently published provenance studies bear important implications concerning the nature and evolution of the underlying lithospheric basement of the ossa morena zone which shares many characteristics with the neoproterozoic peri west african craton arc systems currently exposed in the precambrian inliers of the moroccan anti atlas the north armorican zone of brittany normandy and the channel islands cadomian arc and the saxo thuringian and tepla barrandian zones of the bohemian massif a correlation of the ossa morena zone with these arc systems in the neoproterozoic and early paleozoic is thus proposed

accretionary history and crustal evolution of the variscan belt in western europe abstract the variscan belt of western europe is part of a large intra paleozoic belt extending on both sides of the atlantic from the ouachitas in the us and the mauritanides in west africa to the bohemian massif in czechoslovakia and poland the belt was constructed between 500 and 250 ma as a result of convergence and collision between two main continents laurentia baltica and africa after the closure of various oceanic basins iapetus rheic and galicia massif central now recorded as discrete in part cryptic sutures which form the roots of large crystalline nappes in which are found dismembered remnants of oceanic crust and mantle the accretionary history of the variscan belt in western europe is related to the progressive closure of probably two oceanic basins the rheic in the north and another the galicia massif central in the south by respectively southward and northward intraoceanic subduction followed by obduction and intracontinental lithospheric subduction the result is a broad 700 800 km fan like divergent belt which must be considered as a classical obduction collision orogen with a long intracontinental history of about 100 150 ma its arcuate shape is probably due to the northwestward impingement of a promontory of the african continent into laurentia baltica

metamorphic and deformational imprint of cambrian lower ordovician rifting in the ossa morena zone iberian massif spain abstract the high temperature metamorphism recorded in the valuengo and monesterio areas constitutes a rare occurrence in the ossa morena zone of southwest iberia where low grade metamorphism dominates the metamorphism of the valuengo area has been previously considered either cadomian or variscan in age whereas that of monesterio has been interpreted as a cadomian imprint however these areas share important metamorphic and structural features that point towards a common tectonometamorphic evolution the metamorphism of the valuengo and monesterio areas affects late proterozoic and early cambrian rocks and is syn kinematic with a top to the north mylonitic foliation which was subsequently deformed by early variscan folds and thrusts the u pb zircon age 480 7 ma we have obtained for an undeformed granite of the valuengo area is consistent with our geological observations constraining the age of the metamorphism we propose that this high temperature metamorphic imprint along a nw se ductile extensional shear zone is related to the crustal extension that occurred in the ossa morena zone during the cambro ordovician rifting in the same way the tectonothermal effect of the preorogenic rifting stage may have been wrongly attributed to orogenic processes in other regions as well as in this one

early cambrian granitoids of north gondwana margin in the transition from a convergent setting to intra continental rifting ossa morena zone sw iberia two distinct cambrian magmatic pulses are recognized in the ossa morena zone sw iberia an early rift er and a main rift related event this cambrian magmatism is related to intra continental rifting of north gondwana that is thought to have culminated in the opening of the rheic ocean in lower ordovician times new data of whole rock geochemistry 19 samples sm nd sr isotopes 4 samples and id tims u pb zircon geochronology 1 sample of the early cambrian er plutonic rocks of the ossa morena zone are presented in this contribution the er granitoids barreiros barquete calera salvatierra de los barros and tablada granitoid massifs are mostly peraluminous granites the sm nd isotopic data show moderate negative endt values ranging from 3 5 to 0 1 and tdm ages greatly in excess of emplacement ages most er granitoids are crustal melts however a subset of samples shows a transitional anorogenic alkaline tendency together with more primitive isotopic signatures documenting the participation of lower crust or mantle derived sources and suggesting a local transient advanced stage of rifting the barreiros granitoid is intrusive into the ediacaran basement of the ossa morena zone serie negra succession and has yielded a crystallization age of 524 7 0 8 ma consistent with other ages of er magmatic pulse this age 1 constrains the age of the metamorphism developed in the ediacaran back arc basins before the intrusion of granites and 2 defines the time of the transition from the ediacaran convergent setting to the lower cambrian intra continental rifting in north gondwana

neoproterozoic palaeogeography of the cadomia and avalon terranes constraints from detrital zircon u pb ages detrital zircons from three neoproterozoic sandstone units from the cadomia terrane of northern france and the channel islands yield ages in three broad groups late neoproterozoic 650 600 ma early palaeoproterozoic 2 4 2 0 ga and archaean 2 5 ga the lack of mesoproterozoic zircon crystals combined with the high abundance of grains between 2 20 and 2 00 ga corresponds closely to the ages of exposed rocks in the west africa craton and thus it is suggested that cadomia was in close proximity to west africa by c 580 ma in contrast the main age groups of detrital zircon from the neoproterozoic avalon terrane are mesoproterozoic and there is a distinct gap of ages between 2 40 and 2 05 ga these significant differences suggest that the two terranes were in different locations relative to major gondwanan cratons in latest neoproterozoic time

regional significance of kilometric scale north east vergent recumbent folds associated with east to south east directed shear on the southern border of the central iberian zone hornachos oliva region variscan belt iberian peninsula on the southern border of the central iberian zone there are two sectors with different styles of deformation to the south west in the hornachos sector large scale recumbent folds associated with ductile shearing can be seen this shearing is characterized by a direction of movement parallel to the fold axes and can be correlated for 150 km along strike the k values of the strain ellipsoid range from 0 8 to 2 0 stretching in the x direction parallel to the recumbent fold axes is more than 100 to the north east in the oliva sector first phase folds are upright and the strain intensity is lower than in the hornachos sector metamorphic geometric and kinematic considerations lead us to conclude that the shearing in the hornachos sector is better explained as conjugate to a main shear zone along which the southern border of the central iberian zone is moved onto the ossa morena zone this main thrust is at present obliterated by a left lateral extensional shear zone that affects a high pressure exotic unit located between the central iberian and the ossa morena zones this high pressure unit constitutes a suture of the variscan belt in the iberian peninsula

the structure of a major suture zone in the sw iberian massif the ossa morena central iberian contact abstract we have investigated the stratigraphy structure and metamorphism of the boundary between the ossa morena zone omz and the central iberian zone ciz two significant continental portions of the variscan iberian massif the omz ciz contact is marked by a strongly deformed and metamorphosed nw se trending narrow band namely the central unit in which partially retrogressed eclogites are included during the middle late devonian the ciz overthrust the omz and in the footwall km scale recumbent folds and thrusts developed with decoupling and underplating of the lower crust at the same time in the hanging wall there took place intense though localized back folding and back shearing in the early carboniferous a transtensional tectonic regime sank the overthrust block resulting in the exhumation of eclogites these eclogites probably came from the underthrust omz lower crust and they are at present included in the suture zone central unit of this continental collision the extension is responsible for the origin of a basin and bimodal magmatism on the southern border of the ciz a late episode of folding and fracturing significantly contributed to the final complex picture of this suture boundary

the mean life of continents estimates of continent recycling rates from nd and hf isotopic data and implications for mantle structure an analysis of the initial neodymium and hafnium isotopic compositions of modern and ancient mantle derived rocks shows that the divergence of the mantle 143nd 144nd and 176hf 177hf ratios from the chondritic ratios over the past 2 billion years has been much less than would be expected based on the sm nd and lu hf ratios of the mantle source of mid ocean ridge basalt the most likely explanation is that rock material from the continents has been recycled semi continuously back into the mantle over this time period the required rate of return of continents to the mantle calculated from a simple transport model is about 0 35 0 15 of the continental mass per billion years 2 5 km year this result is consistent with the hypothesis that the mass of the continents has been approximately constant since the late archean and implies that recycled continental material must be an important component of the upper mantle

u pb geochronology and sm nd isotopic composition of proterozoic gneisses channel islands uk gneissic units of the channel islands united kingdom have traditionally been considered to be palaeoproterozoic basement to the late neoproterozoic cadomia terrane based on lithological correlations and imprecise or ambiguous isotopic data a new precise u pb date of 2061 2 ma based on analyses of single zircons for the icart granitic orthogneiss of guernsey confirms a palaeoproterozoic age of its igneous protolith the nd depleted mantle model age t dm of this gneiss is 2220 ma only slightly older than its crystallization age indicating that it represents juvenile crust other gneisses from guernsey have similar t dm ages ranging from 2210 to 2370 ma suggesting that they are also exposures of juvenile palaeoproterozoic crust a component from a penetratively deformed orthogneiss from sark previously correlated with icartian gneisses on guernsey yields a u pb zircon date of 616 4 2 ma this crystallization age demonstrates that the protolith was a cadomian intrusion and that penetrative deformation and amphibolite facies metamorphism in the northern channel islands were cadomian in age the use of gneissic features as a means of invoking the antiquity of many other undated gneissic units in the region is thus considered unreliable the presence of 2170 7 ma zircon xenocrysts within the sark orthogneiss combined with its initial e nd 615 9 5 are strong evidence that pre cadomian basement was a significant source component and that documenting the extent and age of pre cadomian basement may be possible by indirect geochemical methods

conventional and ion microprobe u pb dating of detrital zircons of the tentudia group serie negra sw spain implications for zircon systematics stratigraphy tectonics and the precambrian cambrian boundary conventional multi grain and ion microprobe dating of detrital zircons from a very low grade metagraywacke of the tentudia group upper part of the serie negra ossa morena zone sw spain reveals an uppermost vendian age for the deposition of the metagraywacke the youngest detrital zircon grain provides a maximum depositional age of about 565 ma thus these data contradict earlier middle to upper riphean ca 1350 850 ma estimates on the age of the tentudia group and favour a precambrian cambrian boundary falling into the range of 540 to 530 ma the presence of about 20 of pan african detrital zircons ranging from about 700 to 550 ma indicates the derivation from gondwana from the upper intercept ages of the fan shaped data field defined by conventionally determined zircon fractions it can be deduced that 2 1 ga old zircons as well as archean zircons existed in the provenance s of the serie negra sediments this mixing of crustal components of different ages is in line with the nd crustal residence age of 1 9 ga the latter value as well as other model ages of the iberian massif indicates unusually high amounts of ancient crust to be present in the strata this is different to other meta sediments of the european hercynides and suggests that the iberian strata of uppermost precambrian age may contain the detritus of more internal older parts of gondwana than other european strata of comparable ages geochemical data on the analysed sample and further metagraywackes of the tentudia group argue for a deposition in an arc environment such a scenario would conform with the syn to post orogenic shallow marine deposition of the studied sediments furthermore an upper time limit for the pre lower cambrian deformational history including two phases of regional deformation is given by the maximum age of deposition implying a very short time interval for deposition and deformation of the tentudia group concerning the u pb systematics of detrital zircon fractions it is probable that numerous previously published conventional multi grain zircon data on meta sedimentary rocks of the european hercynides readily can be explained by the presence of up to 20 of pan african detrital zircons and later phanerozoic lead loss during metamorphic transformation of the sedimentary protoliths moreover this implies that such metasediments originated from post pan african sedimentary precursors

variscan accretionary complex of northwest iberia terrane correlation and succession of tectonothermal events the allochthonous terranes of northwest iberia can be correlated with specific paleogeographic realms of the continental masses and intervening oceans involved in the variscan collision assuming that the existing ophiolites represent the suture formed by the closure of the rheic ocean the units in the footwall to the suture correspond to the outer edge of the gondwana continental margin and the units in the hanging wall are interpreted as fragments of the conjugate margin represented by the meguma terrane this correlation establishes a precise link between circum atlantic terranes and makes it possible to draw a relatively simple scenario of the successive tectonothermal events recorded following the amalgamation of avalon to laurentia the remaining outboard terranes underwent a progressive accretion to this continent that ended with the collision between laurentia and gondwana

provenance of the pre devonian sediments of england and wales sm nd isotopic evidence nd values and clast petrography of the longmyndian sediments of shropshire indicate a provenance dominated by juvenile high nd t uriconian volcanic sources but with an increasing detrital contribution from older low nd t metamorphic sources up section by contrast most lower palaeozoic strata of england and wales have low nd t values typical of sediments derived from long established continental crust the sm nd isotopic evidence suggests that avalonia received sediment from an adjacent large continent in the cambrian to lower ordovician and in the silurian ordovician and silurian volcanicity supplied juvenile detritus only during and shortly after active volcanism

short paper cadomian terrane tectonics and magmatism in the armorican massif the north armorican composite terrane nw france is a collage of displaced terranes which result from the amalgamation of cadomian continental arcs and marginal basin complexes by sinistral transpression along a continental margin above a subduction zone early cadomian arc activity occurred at c 700 650 ma but terrane accretion did not occur until c 540 ma and post tectonic magmatism persisted well into the palaeozoic cadomian events thus span a considerably greater period than previously supposed

contrasting chemical and isotopic signatures from neoproterozoic metasedimentary rocks in the central iberian zone spain of pre variscan europe implications for terrane analysis and early ordovician magmatic belts neoproterozoic metasedimentary units from central spain and the northern central iberian zone ciz show distinctive geochemical signatures with respect to the schist greywacke complex which is dominant in the southwestern part of the ciz significant differences occur in major and trace element composition and initial nd isotope ratios which cannot be explained in terms of metamorphic grade and or anatexis in some layers of these metasedimentary sequences nd540 values from metapelites of central spain range from 5 6 to 10 4 whereas those from the schist greywacke complex span a narrower and higher range of 0 8 to 3 1 this difference suggests different source regions the presence of two contrasting early ordovician magmatic belts in both subzones reinforces the idea of different precambrian basement terranes in the ciz these data are crucial for pre variscan paleogeographic reconstructions of the westernmost european terranes and for checking the validity of current subdivisions of the iberian massif

the upper neoproterozoic lower cambrian of the central iberian zone spain chemical and isotopic sm nd evidence that the sedimentary succession records an inverted stratigraphy of its source the upper neoproterozoic lower cambrian sedimentary succession in the central iberian zone has recently been divided into 12 sedimentary units i to xii units i to iv are of late neoproterozoic age and units v to xii are early cambrian throughout the stratigraphic section shales with similar ranges of sio2 and al2o3 have tio2 and zr contents that are coupled and vary gradually from the bottom unit i tio2 1 0 1 1 zr 245 287 ppm to the top unit xii tio2 0 74 0 86 zr 141 192 ppm these two parameters clearly distinguish upper neoproterozoic from lower cambrian compositions as do certain elemental ratios al2o3 tio2 rb zr nb ti which vary from low values at the bottom of the section to high at the top however other elements the rare earth elements rees y do not fit this general trend with heavy rees and y showing extreme concentrations in some lower cambrian units which also have negative ce anomalies units i to iii and units xi to xii have uniform tdm neodymium model ages 1 1 and 1 7 ga respectively but the units between these two groups have erratic values between 1 4 ga unit vii and 3 4 ga unit viii thus the bottom and top units have neodymium isotope ratios that indicate the presence of two source compositions a composition that records a juvenile contribution younger than 1 1 ga and a composition mainly derived from an old basement some samples from lower cambrian units have disturbed ree abundances and neodymium isotope ratios that are attributed to extensive chemical alteration that resulted in ree and y redistribution this alteration probably affected the material now preserved as unit iv during a period of relative sea level fall at the end of the late neoproterozoic both elemental and isotopic results support the suggestion that the upper neoproterozoic lower cambrian stratigraphic section records an inverse stratigraphy of a source hinterland composed of a cover sequence of relatively juvenile crustal materials underlain by an older basement through time the deeper crust gradually increased its contribution to the sediments in the basin from late neoproterozoic to lower cambrian times these results are compatible with data from many other european zones

structural analysis of an accretionary prism in a continental collisional setting the late paleozoic pulo do lobo zone southern iberia abstract models concerning the tectonic evolution of accretionary complexes typically relate outcrop scale to plate scale multiphase deformation as a smooth variation of strain on all scales however at oblique convergent margins regional scale brittle faults in the shallow crust are commonly parallel to the main orogenic grain these faults impose a strong structural anisotropy and can subsequently control deformation at subordinate scales as a result finite strain in each domain may not record local kinematics consistent with the overall orogenic scale motion implying that structural data must be analyzed selectively from a large area in order to relate outcrop scale kinematics to global plate scale dynamics field mapping and preliminary structural analysis of the late devonian pulo do lobo pdl formation and suspect exotic south portuguese zone spz in southern iberia indicate tectonic juxtaposition of diverse deposits such as foreland basin flysch sedimentary and tectonic melange and passive margin sediments showing an overall geometry consistent with an accretionary wedge setting variations in finite strain lithology and regional structure were used as proxies for defining tectonic domains for structural analysis numerous local kinematic indicators within the pdl suggest a complex regional deformation with several enigmatic features that can be explained by sequential compartmentalization of strain during the development of the imbricate stack followed by late stage bulk strain imposed across the entire complex structural data produced by local strain partitioning reveals kinematic indicators which contradict the overall regional structural style e g spatial juxtaposition of sinistral and dextral fabrics when viewed at larger scales i e regional scale however these data indicate that significant sinistral strike slip movement occurred in conjunction with both an extension and shortening outcrop scale deformation in polydeformed domains is controlled by local conditions resulting from brittle deformation coeval with orogenic scale bulk strain the entire pulo do lobo zone is dominated by a pervasive late stage vertical to sub vertical e w cleavage axial planar to chevron folds which overprint earlier deformation in the older passive margin units this overprinting suggests that in the late stages of the evolution of the accretionary complex bulk strain was imposed over the entire complex as a result of internal locking of the accretionary complex and reduced strain rates during the waning stages of collision between gondwana and laurussia stereographic analysis of fabric elements from each distinct tectonic domain together with regional geological constraints support this hypothesis and are indicative of progressive deformation imposed on the pdl during the variscan orogeny

dating of alteration episodes related to mercury mineralization in the almaden district spain the almadin mercury deposits represent one of the largest geochemical anomalies on earth but the mode and timing of their formation remains a matter of controversy there are two main possibilities hydrothermal solutions associated with alkali basalt volcanism in the silurian and devonian or regional hydrothermal activity during later hercynian metamorphism although these models can be distinguished by determining the age of ore deposition no suitable isotopic method had been applied to the problem prior to this study we report here on vacuum encapsulated and more traditional laser ar ar dating performed on illite concentrates and cr rich micas that are intimately associated with mercury mineraiization in both devonian and silurian host rocks illite is associated with a later stage of hg mineralization cinnabar kaolinite pyrophyllite chlorite which locally replaces an earlier episode associated with carbonates and cr mica the dating results from illite separates suggest that the later stages of hg mineralization or remobilization occurred about 360 ma which is about 20 ma younger than any mercury hosting rocks in the district and coincides with the onset of regional deformation during the hercynian orogeny cr mica ages are variable and range from 365 ma to 427 ma this age range spans the period between the deposition of the criadero quartzite the oldest sedimentary unit hosting mercury and the later episode of mineralization this suggests that mineralization started at least by the mid to lower silurian and the younger argon ages for cr mica could represent partial to nearly total argon loss caused by the hercynian metamorphic event it appears that both postulated styles of mercury mineralization occurred separated in time by up to 80 million years

radiometric discrimination of pre variscan amphibolites in the ediacaran serie negra ossa morena zone sw iberia new results on the geochronology of metabasites hosted by the neoproterozoic late ediacaran serie negra of the ossa morena zone iberian massif are presented these rocks record a protracted subduction at least since the latest cryogenian followed in a continuum by early cambrian rifting the serie negra country rocks are continental crust forearc segments of the thinned continental margin of gondwana the metabasite suite exhibits e and n morb as well as volcanic arc signatures and discloses the existence of a diverse late ediacaran early cambrian magmatism across the ossa morena zone protoliths of the e morb metabasites are clearly ediacaran they exhibit geochemical characteristics typical of younger island arcs notably of their forearc zones affected by extension soon after the initiation of subduction radiometric dating of n morb metabasites shows that these rocks are significatively younger than their country rocks whose age has been stratigraphically and paleontologically constrained as ediacaran they record early cambrian rifting developed in a continuum after a protracted subduction metabasite petrological zonation permits to reconstruct a single ediacaran subduction zone located to the s of the current omz and dipping towards the n in present day geographical coordinates

variscan intracontinental deformation the coimbra cordoba shear zone sw iberian peninsula the coimbra cordoba shear zone is a major lineament roughly parallel to the nw se variscan structures of the southern iberian peninsula inside the shear zone evidence for a non coaxial strain path exists from structural microstructural and lattice preferred orientation data on quartz plane strain is suggested by the study of microstructures in the xy and yz sections in all formations occurring in the shear zone with increasing strain a mylonitic series develops the deformation contemporaneous with epizonal to mesozonal metamorphism is responsible for 1 1 the regional subvertical foliation oblique to the shear zone 2 2 the well defined subhorizontal lineation striking nw se 3 3 the discrete planes of shear roughly parallel to the shear zone the data are interpreted as resulting from a left lateral slip motion with a displacement of at least 72 km in the light of plate tectonics it is inferred that the coimbra cordoba lineament could have been a suture zone which evolved into an intracontinental left lateral shear zone during the variscan orogeny of the ibero armorican arc

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

geochemistry and paleotectonic setting of ediacaran metabasites from the ossa morena zone sw iberia new results on the geochemistry of neoproterozoic late ediacaran metabasites of the ossa morena zone omz iberian massif are presented the metabasite suite exhibits n and e morb signatures as well as volcanic arc signatures the three amphibolite groups are discerned on the basis of major and trace element contents and the nd isotope relations help unravel the existence of a diverse magmatism during late ediacaran times across the omz n morb serie negra metabasites are distributed in the southern and mostly the central omz monesterio antiform calc alkaline metabasites are exclusively constrained to the coimbra cordoba sector of the northern omz finally e morb type metabasites are widespread across the entire omz in present day active geodynamic settings such magma types are typical of younger island arcs notably of forearc zones that are affected by extension soon after the initiation of subduction the new geochemical data permit us to postulate a petrological zonation congruent with a n dipping subduction zone located to the s of the current omz during the late ediacaran we argue that the metabasite host units represent continental shallow crustal forearc segments of a convergent margin the volcanic arc edifice would have developed during the latest ediacaran to early cambrian times and has been preserved often without major orogenic reworking in the central and northern omz

cadomian subduction collision and variscan transpression in the badajoz cordoba shear belt southwest spain abstract the tectonothermal evolution of the badajoz cordoba shear belt southwest iberian massif spain involved the build up of a cadomian upper proterozoic lower paleozoic subduction collision orogenic wedge complex made up of five stacked tectonic slices and its deformation under transpression during hercynian late paleozoic intracontinental shearing episodes cadomian metamorphism in the high grade gneissic units of the wedge complex is characterized by peak metamorphic conditions of 720 c 8 kbar for the lowermost tectonic slice higuera de llerena mylonite gneiss and 670 c 10 kbar in the overlying gneissic slice blastomylonitic migmatic gneisses the p t conditions recorded by eclogites structurally overlying the previous gneisses enhance a scheme of inverted metamorphism since these eclogites equilibrated at 685 700 c 15 kbar during peak metamorphic conditions after the high pressure event the highest temperatures attained by the eclogites correspond to a period of nearly isothermal decompression with a drastic decrease in pressure later on the retrogressed eclogites experienced a pressure increase before they began to reequilibrate under progressively lower pandt conditions similar to those recorded by the retrogression of gneisses such a p increase in eclogites might be related to a period of underthrusting below continental crust responsible for the amalgamation of the different tectonic units of the badajoz cordoba shear belt the present results unravel that the models concerned with the geotectonic evolution of the ibero armorican arc and the respective correlations among hercynian and pre hercynian cadomian for our aims terranes across the north atlantic must be reconsidered

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

tectonic analysis of fault slip data sets using data that include the direction and the sense of motion on individual fault surfaces determined by slickenside lineations it is possible to reconstruct reduced stress tensors that correspond to the orientation of stress axes and to the ratio 2 3 1 3 between principal stress values 1 2 3 compression being positive no assumption is made concerning the orientation of fault planes relative to stress axes so that reactived faults are taken into account as well as newly created ones qualitative and quantitative methods for analysis of fault slip data were developed during the last 10 years the practical limitation of the methods and the necessity for critical field observations are emphasized these methods can be applied to focal mechanisms of earthquakes a more complex analysis of heterogeneous data sets involving an iterative separation of different stress systems is also discussed this analysis enables one to distinguish successive faulting events careful qualitative study in the field is in all cases essential

rift related volcanism predating the birth of the rheic ocean ossa morena zone sw iberia abstract two very different periods of magma emplacement in the crust of the ossa morena zone early and main events in sw iberia have been previously interpreted to record a cambrian early ordovician rifting event that is thought to have culminated in the opening of the rheic ocean during the early ordovician new stratigraphic petrographic geochemical and sm nd isotope data from cambrian volcanic rocks included in six key low grade sections in both portugal and spain considerably improve our understanding of these events these data 1 confirm the existence of two rift related magmatic events in the cambrian of the ossa morena zone 2 demonstrate that the early rift related event was associated with migmatite and core complex formation in the mid upper crust and is represented by felsic peraluminous rocks the parent magmas of which were derived mainly from crustal sources and 3 show the main rift related event to be represented by a bimodal association of felsic and mafic rocks with minor amounts of intermediate rocks some of the mafic rocks show n morb affinity whereas others have oib or e morb affinities suggesting different heterogeneous mantle sources depleted and enriched asthenospheric and lithospheric plume like and non plume like the acid and intermediate rocks appear to represent hybrid mixtures of crust and mantle derived magmas this new data supports the hypothesis that the onset of rifting was associated with a process of oblique ridge trench collision we interpret the significant differences between the early and main events as reflecting the evolution from a wide rift stage with passive extension mainly accommodated by lower crust flow in a high heat flow setting to a narrow rift stage with active extension characterized by extension rates that outpaced thermal diffusion rates

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

adakitic like magmatism in western ossa morena zone portugal geochemical and isotopic constraints of the pavia pluton abstract granitic rocks are a major component of the earth s continental crust and occur in a wide variety of tectonic settings their chemical and isotopic characterization is crucial to the recognition of the potential sources and mechanisms involved in their generation in this study we present the first whole rock chemical and isotopic sr nd o data for the pavia pluton 328 317 ma located near the western border of the ossa morena zone evora massif portugal major and trace element geochemistry suggests that the different granitic phases composing this intrusive body enclaves granites s l and crosscutting dikes represent independent magma pulses and the majority is similar to ttgs and adakites the little sr nd o isotopic variation with 87 sr 86 sr 328 0 70428 0 70560 end 328 ranging between 3 4 and 0 4 and 18 o varying from 5 6 to 8 4 implies an isotopically similar protolith for all phases the most viable mechanism for the generation of the pavia pluton adakitic like magmatism is assimilation fractional crystallization of a mantle derived magma this mechanism was also invoked to explain the genesis of other plutons within the evora massif but they have a distinct chemistry typical arc calc alkaline rocks the chemical differences between them and the pavia pluton granitic rocks are interpreted as the result of lower degrees of crustal assimilation and higher degrees of contamination of mantle derived magmas by the sinking slab after subduction blocking and subsequent slab break off

variscan collisional magmatism and deformation in nw iberia constraints from u pb geochronology of granitoids u pb geochronology of variscan granitoid rocks from the west asturian leonese zone of the nw iberian belt documents the episodic nature of magmatism in this section of the western european variscides each magmatic episode is characterized by granitoids with distinct features and has a duration on the scale of several millions of years the ages of these granitoids place new constraints on the age and duration of magmatic and tectonic events that are consistent with previous structural studies and proposed models for the tectonic evolution and migration of deformation in the nw iberian variscan belt granitoid rocks in this zone belong to two main magmatic episodes syn and post tectonic relative to the variscan orogeny and are broadly representative of the granitoid types found in the nw iberian variscan belt the syntectonic association is formed by i tonalite granodiorite monzogranite intrusions emplaced synchronously with the main phase of crustal deformation d 2 at c 325 ma and ii younger leucogranite intrusions emplaced synchronously with syn convergence extensional structures at c 320 310 ma the post tectonic association is composed of i volumetrically dominant tonalite granodiorite monzogranite intrusions and associated minor mafic intermediate rocks emplaced at c 295 290 ma and ii scarce leucogranite intrusions emplaced at c 290 285 ma