diagenetic and environmental control of the clay mineralogy organic matter and stable isotopes c o of jurassic pliensbachian lowermost toarcian sediments of the rodiles section asturian basin northern spain abstract clay mineralogical and geochemical analyses 18ocarb and paired carbonate and organic matter carbon isotope completed by the characterization of organic matter have been performed on the pliensbachian sediments exposed on the rodiles section asturian basin northern spain the objectives were to precise the pliensbachian climate evolution at a relatively low paleolatitude of the tethyan domain by comparison with previously published high resolution clay mineralogical and geochemical data from the nw tethyan domain e g paris basin and cardigan bay basin west wales the clay mineral assemblages are dominantly composed of illite and illite smectite mixed layers i s associated with minor proportions of chlorite and kaolinite smectites are not identified what may result either to their absence at the time of sedimentation or to their progressive illitization due to the burial diagenesis rock eval data show that organic matter reaching more than 5 in black shale horizons is thermally mature with tmax comprised between 444 and 454 c indicating that the section is in the oil window and that the burial temperature reached or exceeded 80 90 c the low 18obulk values comprised between 3 and 6 clearly results from a diagenetic influence thermal effect due to burial but their fluctuations compared with 18o measured on well preserved belemnites guards show that the overall trend is however preserved by comparison with higher latitude sedimentary basins the most striking feature is the very small proportions of kaolinite generally less than 5 while the proportions of this mineral reach 40 at higher paleolatitudes except at the pliensbachian toarcian transition where the proportion of kaolinite increases significantly clay minerals likely originated from the erosion of the iberian massif located to the south of the asturian basin in a semi arid climatic belt this may explain the scarcity of kaolinite by comparison with sedimentary basins located at higher latitude influenced by more humid climate the increasing proportions of kaolinite by the end of the pliensbachian suggest southward expansion of the humid belt as a consequence of cooler temperatures and the establishment of steeper latitudinal gradients of temperature likely resulting from the formation of restricted polar ice cap the main 13c on both 13cinorg and 13corg pliensbachian excursions including the negative excursion of the sinemurian pliensbachian boundary event spbe and the late pliensbachian positive excursion are well recognized

holocene relative sea level changes and coastal vegetation history at kentra moss argyll northwest scotland abstract a late holocene fall in relative sea level in northwest scotland from ca 1 3mm yr 1 to ca 1 0 mm yr 1 is interpreted from lithostratigraphic biostratigraphic chronostratigraphic and numerical analyses of fossil tidal marsh and acidic peat bog communities elevated by isostatic uplift pollen diatom and stratigraphic data from contemporary depositional environments are used to define the indicative range 0 2 m and reference water level mean high water of spring tides or highest astronomical tide of thirteen dated sea level index points no holocene intertidal sediments are recorded above 7 7 m od and all sea level index points are younger than ca 4 kyr b p in parts of kentra moss beyond the limit of holocene intertidal clastic sedimentation raised bog communities were established by at least 8 3 kyr b p these age and altitude parameters differ from those interpolated for the main postglacial shoreline but support a regional model in which isostatic uplift continues at present in the kentra moss area

updated geochronology and isotope geochemistry of the vila de cruces ophiolite a case study of a peri gondwanan back arc ophiolite

isotope geochemistry and revised geochronology of the purrido ophiolite cabo ortegal complex nw iberian massif devonian magmatism with mixed sources and involved mesoproterozoic basement abstract in the purrido ophiolite cabo ortegal complex new u pb zircon dating of the amphibolite g03 8 by laser ablation inductively coupled plasma mass spectrometry confirms the existence of a dominant mesoproterozoic zircon population with a refined age of 1155 14 ma however the u pb zircon dating of two more amphibolite samples by sensitive high resolution ion microprobe has provided new ages of 395 3 ma and 395 2 ma respectively interpreted as the crystallization age hf isotope data for zircon show that most of the devonian zircons crystallized from a juvenile depleted mantle source the mesoproterozoic zircons have relatively juvenile hf isotopic composition reflecting some influence of an older component a few devonian zircon crystals show evidence of mixing with an older component represented by the mesoproterozoic zircons the whole rock sm nd isotope data indicate an important heterogeneity in the composition of the purrido amphibolites only compatible with the generation of their protoliths from two sources we interpret these puzzling data as resulting from the mixing of a devonian mantle derived magma with a mesoproterozoic basement these new data provide new perspectives in the interpretation of the most common ophiolites across the variscan suture in europe supplementary material analytical methods zircon u th pb shrimp analytical data zircon u th pb la icp ms analytical data zircon lu hf la mc icpms analytical data and whole rock sm nd isotope data are available at http www geolsoc org uk sup18449

high pressure metamorphism in the early variscan subduction complex of the sw iberian massif abstract several units exposed in the boundary area of the ossa morena zone omz and the south portuguese zone spz preserve petrographic and thermobarometric evidence for an early metamorphic episode m1 developed under a high p low to intermediate t gradient related to early variscan subduction in the sw iberian massif in the omz these are the cubito moura unit p min 9 2 kbar and t 395 45 c blueschists from its basal melange p min 12 4 kbar and t 310 11 c and the underlying fuenteheridos group p 10 9 0 4 kbar and t 449 31 c the equivalent units in the spz are the la minilla formation p 8 7 0 4 kbar and t 388 16 c and the lawsonite pseudomorphs bearing pulo do lobo formation all these units formed part of an approximately ne verging orogenic wedge present coordinates developed by the accretion of subducted slabs of the outermost margin of gondwana and other elements of the rheic ocean realm from at least the middle devonian to the lowermost tournaisian high pressure rocks were subsequently emplaced on more internal zones of the omz that only experienced a younger high t low to intermediate p metamorphism m2 this high t event was coeval with magmatic activity from the uppermost devonian to the middle mississippian probably as a consequence of transtensional lithospheric thinning and or delamination of the lower crust and mantle lithosphere of the gondwana margin pre late devonian synorogenic sedimentation in forearc and back arc basins of the subduction complex evolved to a late devonian middle mississippian foreland basin system in early collisional stages finally a new middle late mississippian fold and thrust belt with opposite sw vergence and new foreland basins developed during late collisional stages

u pb chronometry of polymetamorphic high pressure granulites an example from the allochthonous terranes of the nw iberian variscan belt

contrasting isotopic sources sm nd of late ediacaran series in the iberian massif implications for the central iberian ossa morena boundary abstract in the iberian massif westernmost sector of the variscan orogen the nature of the boundary between the central iberian zone and the ossa morena complex has been largely discussed peri gondwanan ediacaran series similar in age c 565 ma but different in composition occur at both sides of that boundary the lower alcudian series central iberian zone is located to the north while the serie negra ossa morena complex can be found to the south of the boundary major and trace element compositions and sm nd isotopic systematics of siliciclastic rocks from both series suggest a common active margin setting for their deposition the serie negra likely occupying a more external fore arc position within the arc system relative to the lower alcudian series which would have been deposited within a large back arc domain tdm ages for these two series are notably different yielding values of 1421 2040 ma and 1256 1334 ma for the serie negra and lower alcudian respectively these values suggest higher input from old cratonic sources in the ossa morena complex with probable participation of the west african craton which would be less prominent in the central iberia zone the whole data set indicate deposition of the ediacaran series in separated basins probably far away from each other along across the margin of gondwana current juxtaposition of these basins is explained by variscan and probably also pre variscan tectonics similar conclusions could be also be extended to other similar boundaries in the variscan orogen

the late devonian variscan suture of the iberian massif a correlation of high pressure belts in nw and sw iberia correlation of three high p and low to intermediate t belts from nw and sw iberia allows recognition of a single terrane affected by late devonian continental subduction during the variscan orogeny this terrane is located in the lower part of the allochthonous complexes of nw iberia and in a lower position in most of the ossa morena zone that therefore emerges as another allochthonous complex of iberia the ossa morena complex together with the allochthonous complexes of nw iberia form a huge rootless nappe that was transported towards the variscan foreland during final assembly of pangea

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

time space distribution of silicic plutonism in a gneiss dome of the iberian variscan belt the evora massif ossa morena zone portugal abstract in the iberian variscan belt polyphasic deformation has been recognized as comprising an early phase of crustal thickening followed by an intermediate phase of crustal extension and doming and a later phase of shortening the evora massif is a gneiss dome of the westernmost domains of the ossa morena zone sw iberia which provides a remarkable insight into the late paleozoic deep crustal structure of the variscan continental crust of northern gondwana in this study we bring new structural and geochronological u pb data for the northern hanging wall of the evora massif we describe the existence of low dipping d2 extensional shear zones associated with buchan type metamorphism m2 this enables three tectono metamorphic units to be distinguished the lower gneiss unit the intermediate schist unit and the upper slate unit d2 m2 structures experienced subhorizontal shortening d3 and were transposed by low plunging folding thrusting and strike slip faulting zircon grains extracted from pavia quartz feldspathic gneiss of the lower gneiss unit yielded a crystallization age of ca 521 ma cambrian stage 2 3 which establishes a correlation with tectono metamorphic units of the footwall and southern hanging wall of the evora massif u pb zircon dating of divor foliated quartz diorite 339 7 ma malarranha weakly foliated biotite rich granite 322 3 ma and undeformed porphyritic granite of the pavia pluton 314 4 ma constrain the timing of emplacement of granitic magmas synchronously with doming carboniferous magmatism initiated with doming me1 ca 343 335 ma continued through d2 m2 me2 ca 328 319 ma and lasted until the waning stage of crustal extension me3 ca 317 313 ma the evora massif gneiss dome probably formed as result of the combined effect of gravitational collapse of the thickened crust and buoyancy driven gravitational instability developed in the partially molten continental crust influenced by the transfer of heat from rising mantle derived i e dioritic gabbroic magmas rocks found in the footwall of the evora massif

correlation of the nappe stack in the ibero armorican arc across the bay of biscay a joint french spanish project a correlation between allochthonous units exposed in the nw iberian massif and the southern armorican massif is carried out based on lithological associations structural position age and geochemistry of protoliths and tectonometamorphic evolution the units on both sides of the bay of biscay are grouped into upper middle and lower allochthons whereas an underlying allochthonous thrust sheet identified in both massifs is referred to as the parautochthon the lower allochthon represents a fragment of the outermost edge of gondwana that underwent continental subduction shortly after the closure of a palaeozoic ocean which in turn is represented by the middle allochthon the latter consists of supra subduction ophiolites and metasedimentary sequences alternating with basic mid ocean ridge basalt morb type volcanics with inheritances suggesting the proximity of a continental domain seafloor spreading began at the cambro ordovician boundary and oceanic crust was still formed during the late devonian covering the lifetime of the rheic ocean which is possibly represented by the middle allochthon the opening of the oceanic domain was related to pulling apart the peri gondwanan continental magmatic arc which is represented by the upper allochthon

the cambrian to devonian odyssey of the brabant massif within avalonia a review with new zircon ages geochemistry sm nd isotopes stratigraphy and palaeogeography abstract this study provides an up to date and comprehensive review of the early palaeozoic evolution of the brabant massif belonging to the anglo brabant deformation belt situated at the southeastern side of avalonia microplate it is the only well known part of the northern passive margin of the rheic ocean the cambrian silurian sedimentary pile is 13 km thick with 9 km for the cambrian only the unraveling of this continuous registration reflects the successive rifting and drifting of avalonia from the gondwana mainland followed by soft collisional processes with baltica and finally the formation of laurussia based on recently established detailed stratigraphy sedimentology and basin development on u pb la icp ms analyses of igneous and detrital zircon grains along with geochemical data including sm nd isotopes a new geodynamic and palaeogeographic evolution is proposed brabant megasequence 1 lower cambrian to lowermost ordovician 9 km thick represents an embayment of the peri gondwanan rift from which the rheic ocean has evolved detrital zircon ages demonstrate that the brabant is a typical peri gondwanan terrane with a major pan african neoproterozoic age and a mixed west african and amazonian source palaeoproterozoic archaean and some mesoproterozoic age the transition towards the avalonia drifting is marked by an unconformity and a short volcanic episode the northward drift of avalonia towards baltica is recorded by the megasequence 2 middle to upper ordovician 1 3 km thick the source for mesoproterozoic zircons vanished as the result of the rheic ocean opening and the isolation from amazonian sources the transition to megasequence 3 is marked by a drastic change in palaeobathymetry and an important sub volcanic episode during a tectonic instability period 460 430 ma reflecting the avalonia baltica soft docking as also shown by the reappearance of mesoproterozoic detrital zircons typical of baltica unradiogenic nd isotope signature end 4 5 and tdm model ages 1 3 1 7 ga for brabant magmatic rocks indicate an old recycled component megasequence 3 uppermost ordovician to lowermost devonian 3 5 km thick includes the onset of a silurian foreland basin that reflects the tectonic inversion of the core of the massif brabantian orogeny in response to the baltica avalonia laurentia collision finally the comparison with the strikingly similar cambrian successions of the harlech dome wales avalonia and the meguma terrane nova scotia peri gondwana allows the construction of a new early cambrian palaeogeographic model for the whole avalonia microplate in which the meguma terrane is included

comment on the late devonian variscan suture of the iberian massif a correlation of high pressure belts in nw and sw iberia tectonophysics 654 96 100 by r fernandez and r arenas abstract a vast variscan nappe spreading from the allochthonous complexes of nw iberia to the ossa morena zone in sw iberia has been proposed by diez fernandez and arenas 2015 based on the correlation of different outcrops of high pressure and morb type rocks this interpretation conflicts with well known geological and geophysical data regarding the sw iberian variscides which point to i major kinematic and geometric inconsistencies come up when interpreting the ossa morena zone as an allochthonous unit ii the outcrops of high pressure and morb type rocks belong to two different sutures rather than a single one and iii sw iberia is characterized by transpressional deformation related to a very oblique left lateral continental collision while nw iberia features a rather frontal collision

relationships between syn orogenic sedimentation and nappe emplacement in the hinterland of the variscan belt in nw iberia deduced from detrital zircons flysch type syn orogenic deposits of carboniferous age occur in relation to the emplacement of a large allochthonous nappe stack in the variscan belt of nw iberia new u pb age populations of detrital zircons obtained using laser ablation inductively coupled plasma mass spectrometry la icp ms are considered together with others from previously dated samples to establish the relationships between sedimentation and thrusting the age populations of four syn orogenic formations are compared with those of the pre orogenic sequence in the autochthon and parautochthon representing the gondwanan passive margin and in the allochthon formed by peri gondwanan and oceanic terranes in addition a new structural study has been carried out to understand the relationships between the syn orogenic deposits and the development of variscan structures the aims are to identify the sources of sediments and to establish the relationship between variscan structural evolution and syn orogenic sedimentation development of a forebulge outwards from the allochthonous front deduced from the structural study suggests the existence of depocentres that hosted the syn orogenic sediments together with the trend shown by the more recent zircons in each formation that are younger towards the external zones the data suggest that sedimentation occurred in progressively migrating depocentres formed in front of the allochthonous wedge during its emplacement the zircon age populations point to the allochthon as the main source of detritus for the syn orogenic basins with perhaps a limited participation of the parautochthon and autochthon in the younger formations

crustal seismic structure beneath portugal and southern galicia western iberia and the role of variscan inheritance abstract the crustal structure in western iberia is the result of a complex geodynamic history most of the surface is covered by rocks dating to the variscan orogeny the coastal ranges dominated by mesozoic structures and mesocenozoic basins covering partially the mainland in this study we present the results of a local earthquake tomographic study performed to image in depth this complex crustal structure down to 24 km depth the obtained tomograms present a good correlation with the surface geology in general with higher vp velocities in the north and lower vp velocities in the south the heterogeneity observed on the surface geology of the galicia tras os montes zone is well marked being a relatively thin layer over the smoother structure of the centro iberia zone ciz the ciz crustal block confined between the porto tomar ferreira do alentejo and the manteigas braganca faults having higher vp values enhancing the contrast to the lusitanian basin to the west the ossa morena zone corresponds to the unit presenting the greater heterogeneity in both vp and vp vs models showing also with a complex transition to the south portuguese zone and a relatively smooth transition to the centro iberian zone unexpectedly the south portuguese zone present an east west velocity variation with no apparent correspondence with the surface geology the transition to the coastal mesocenozoic basins is clearly marked the lower tagus valley corresponding to a 4 km thick low velocity region the relocation inland recorded seismicity in the period 2000 2014 allows cleansing some of the alignments and establish their correlation with some of the active structures in portugal the model features and seismicity pattern reveal the strong role played by the variscan heritage

tidal marsh stratigraphy sea level change and large earthquakes i a 5000 year record in washington u s a abstract many of the estuaries of the pacific northwest of the u s a and canada contain stratigraphic sequences typified by alternating peat mud couplets recent studies in this region interpret such couplets as the product of repeated large magnitude s or 9 earthquakes on the cascadia subduction zone the resultant pattern of land level movements is described by a model the earthquake deformation cycle of coseismic land subsidence followed by land uplift during interseismic strain accumulation however peat mud couplets similar to those recorded in the pacific northwest are found on other less tectonically active temperate latitude coasts such as northwest europe and the atlantic coast of the u s a where they have been interpreted as the product of non seismic coastal processes in this paper we apply the methods and scientific framework common to sea level investigations in northwest europe to a sequence of peat mud couplets recorded in the lower johns river an estuary in southern washington to provide a test of the earthquake deformation cycle stratigraphic investigations of the intertidal sediments along the lower johns river using lithological pollen diatom and foraminiferal data show evidence for eight coastal submergence events during the last 5000 years to evaluate the earthquake deformation cycle we assess the lateral extent of peat mud couplets the synchroneity of submergence the presence of tsunami deposits accompanying submergence and the suddenness and amount of submergence each submergence is shown to be accompanied by changes in coastal sedimentation broadly commensurate with those predicted by the earthquake deformation cycle demonstrating the continued intermittent seismic activity of the cascadia subduction zone throughout the mid and late holocene quantitative analyses of contemporary and fossil biostratigraphic data using twinspan and detrended correspondence analysis enable us to estimate the magnitude of submergence accompanying each peat mud couplet one event was accompanied by submergence of about 1 5 m or more four events by intermediate submergence of about 1 0 5 m and a further three events by submergence of

the onset of the assembly of pangaea in nw iberia constraints on the kinematics of continental subduction excellent exposures of high pressure rocks developed in a variscan continental subduction system outcrop in nw iberia the kinematic criteria provided by the high pressure metamorphic fabrics can be used to infer tectonic flow within the deep sections of this system the dominant trend of the ductile flow is oblique to that of the orogenic belt indicating oblique continental subduction its azimuth a few tens of degrees clockwise relative to the orogenic trend suggests dextral transpression between gondwana and laurussia during continental subduction that took place at the upper devonian and provides a consistent kinematic reference for the earliest assembly of pangaea in nw iberia

pangea b and the late paleozoic ice age abstract the late paleozoic ice age lpia was the penultimate major glaciation of the phanerozoic published compilations indicate it occurred in two main phases one centered in the late carboniferous 315 ma and the other in the early permian 295 ma before waning over the rest of the early permian and into the middle permian 290 ma to 275 ma and culminating with the final demise of alpine style ice sheets in eastern australia in the late permian 260 to 255 ma recent global climate modeling has drawn attention to silicate weathering co2 consumption of an initially high greater variscan edifice residing within a static pangea a configuration as the leading cause of reduction of atmospheric co2 concentrations below glaciation thresholds here we show that the best available and least biased paleomagnetic reference poles place the collision between laurasia and gondwana that produced the greater variscan orogen in a more dynamic position within a pangea b configuration that had about 30 more continental area in the prime equatorial humid belt for weathering and which drifted northward into the tropical arid belt as it transformed to pangea a by the late permian the presence of widespread equatorial coal basins with euramerica flora in the footprint of the greater variscan orogen during the late carboniferous is more compatible with a heterogeneous horst and graben morphology characterized by uplifted crystalline massifs acting as loci of intense silicate weathering co2 consumption and supplying sediment for proximal basins as venues of organic carbon burial than a contiguous high mountain plateau as assumed in recent climate modeling of the lpia and its demise the culminating phase of the lpia occurred at about 275 ma with the transformation from pangea b to pangea a and the attendant reduction of continental area in the equatorial humid belt as well as with continued northward drift that placed what remained of the greater variscan orogen into the zechstein arid belt in the late permian by which time the geologic landscape was largely blanketed with siliciclastics the resulting warming from reduced silicate weathering and thus increasing pco2 was interrupted at 260 ma with a cooling trend that coincided with emplacement of the emeishan large igneous province on the equatorial south china craton as well as the drift of the cimmerian continental blocks through the equatorial humid belt due to opening of the neo tethys a return to ice age conditions from the increase in silicate weathering uptake of co2 was avoided by drift of the emeishan large igneous province out of the equatorial belt that in conjunction with massive outgassing from emplacement of the siberian traps in high latitudes at the end of the permian 252 ma helped steer the climate system to sustained non glacial conditions

tectonic evolution of the boundary between the central iberian and ossa morena zones variscan belt southwest spain three tectonometamorphic units can be differentiated in the boundary between the central iberian and ossa morena zones of the iberian massif variscan belt the three units have been named the northern unit central unit and southern unit the northern unit corresponds to the border of the central iberian zone it evolved at low temperature and intermediate or low pressure metamorphic conditions and was affected by top to the se ductile shearing the central unit placed under the northern unit preserves high pressure silurian eclogitic assemblages retrograded by high to medium temperature and finally low temperature ductile shearing with top to the nw sense of movement oblique left lateral the central unit is superposed on the southern unit the latter corresponds to the border of the ossa morena zone and underwent low pressure intermediate temperature metamorphism synchronous with right lateral top to the se ductile shearing the envisaged tectonic evolution is as follows after a stage of lower paleozoic rifting subduction of the central unit under the northern unit took place in silurian times as a result of the crustal thickening a gravitational instability developed giving way to left lateral extensional shearing that affected the entire central unit the combined action of oblique thrusting at the front of the wedge and oblique extensional shearing at the rear caused the exhumation of the high pressure rocks of the central unit this tectonic evolution reveals that the boundary between the central iberian and ossa morena zones is a suture of the variscan belt

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

new evidence of blueschist facies rocks and their geotectonic implication for variscan suture s in the bohemian massif blueschist facies rocks exposed within consolidated continental blocks provide some of the best evidence for the existence of previous suture s they usually occur as lenses or layers embedded within greenschist or amphibolite facies rocks and indicate reequilibration at medium to low pressure conditions in the bohemian massif a few occurrences of blueschists have been reported and here new evidence of high pressure hp metamorphism in various lithologies is presented that suggests a larger extent of blueschist facies rocks along the northern border of this massif an earlier blueschist facies metamorphism is documented by inclusions of glaucophane in garnet epidote and titanite from metabasites along with zoned white mica having a phengitic core and a muscovite rim in metapelites and orthogneisses the estimated p t conditions obtained using pseudosections and mineral isopleths correspond to blueschist and low temperature eclogite facies conditions 1 1 2 0 gpa at 350 550 c together with medium temperature eclogites from different units in the bohemian massif they indicate a geothermal gradient of 8 10 c km 1 which is typical for cool subduction radiometric dating on phengite from metapelites confirms an early palaeozoic cooling age of c 360 ma for this hp metamorphic event the presence of blueschist facies rocks their p t relations and age constraint together with those from eclogite facies rocks allows us to locate the variscan suture which straddles the se margin of saxothuringian zone from erzgebirge to sudetes and its possible continuation to the moldanubian zone where eclogite facies and uhpm rocks are abundant

age constraints to the relationships between magmatism metamorphism and tectonism in the aracena metamorphic belt southern spain the aracena metamorphic belt amb southwest iberian peninsula is characterized by the following geological elements a a high temperature low pressure ht lp metamorphic belt a few kilometres wide and more than 200 km long b a linear belt of oceanic amphibolites with a low pressure inverted metamorphic gradient c crustal scale ductile shear zones and d mafic noritic intrusions of high mg andesite boninite composition the relationships between these elements led to the proposal of a model of ridge subduction for this sector of the hercynian belt of europe this interpretation is supported by the age relationships displayed between the main rock units considered representative of the main tectonic and petrological processes responsible for the geological elements mentioned previously the results of a geochronological study ar ar rb sr and sm nd clearly support a late paleozoic tectonic evolution at an active continental margin the time evolution of the metamorphism in the oceanic domain ranging from 342 6 0 6 ma in the west to 328 4 1 2 ma in the east over a distance of 70 km along the metamorphic belt support a tectonic model of triple junction migration responsible for the creation at depth of a slab free window with decisive consequences for the thermal evolution of the region the origin of the linear metamorphic belt of ht lp regime may be explained by the migration along a continental margin of a punctual thermal anomaly induced by the creation of a triple junction at the continental margin

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

unfolding the arc the use of pre orogenic constraints to assess the evolution of the variscan belt in western europe abstract we present a pre orogenic early paleozoic palinspastic reconstruction of the northern gondwana margin that was subsequently involved in the late paleozoic variscan orogeny in central and western europe our reconstruction is based on two pre orogenic data sets the age and distribution of cambrian ordovician magmatism and the detrital zircon age signature of late neoproterozoic early paleozoic clastic rocks we obtain this reconstruction by unfolding the ibero armorican arc and by restoring the movement of the large scale dextral strike slip faults that transect the different tectono stratigraphic units our results favour an irregular shape for this part of the northern gondwana margin with a n s central segment linking two e w oriented segments the proposed reconstruction and the structural restoration of the main features of variscan deformation is in accordance with some aspects of previously proposed structural models such as the curved geometry of the gondwanan margin required by the indentor model for continental collision the role played by the large strike slip faults in dispersing formerly juxtaposed units and the regional scale oroclinal folding of part of this margin during late carboniferous early permian times the combined use of the pre orogenic geological constraints and palinspastic restoration is a useful approach that may provide a foundation for continual refinement of reconstructions as more data become available

the variscan french massif central a new addition to the ultra high pressure metamorphic club exhumation processes and geodynamic consequences abstract this paper presents the first documentation of coesite bearing eclogites in the eastern french massif central monts du lyonnais unit and discusses the exhumation processes for these very high pressure rocks a combination of mineralogical and geochronological datasets allows us to quantify a depth time path and related exhumation rates high pressure metamorphism is constrained to 400 420 ma minimum 28 kbar or ca 90 km by 360 380 ma the rocks were exhumed to 30 km depth these kinetic results conform to the geological constraints extracted from the tectonic and sedimentary record of the eastern french massif central these multidisciplinary approaches provide new information on paleozoic orogeny and allow us to discuss the relative roles of subduction and collision in exhumation of very high pressure rocks we suggest that a significant amount of exhumation of these rocks occurred during subduction prior to continental collision continental collision itself was responsible only for the final stages of exhumation under a transpressive regime

100 myr cycles of oceanic lithosphere generation in peri gondwana neoproterozoic devonian ophiolites from the nw african iberian margin of gondwana and the variscan orogen

two stage collision exploring the birth of pangea in the variscan terranes abstract the variscan suture exposed in nw iberia contains a stack of terranes including two allochthonous units with continental affinity and gondwanan provenance upper and basal units separated by an ophiolite belt where the most common units show protolith ages at c 395 ma recent lu hf zircon data obtained from these ophiolites indicate interaction between the gabbroic magmas and old continental crust hence the ophiolites could not have originated in a deep ocean basin associated with a mature mid ocean ridge or intraoceanic subduction the tectonothermal evolution of the continental terranes bounding the suture zone records two consecutive events of deep subduction the upper units record an initial high p ultra high p metamorphic event that occurred before 400 390 ma while the basal units were affected by a second high p low to intermediate t metamorphic event dated at c 370 ma continental subduction affected the most external margin of gondwana and developed in a setting of dextral convergence with laurussia development of the two high p events alternated with the opening of an ephemeral oceanic basin probably of pull apart type in early devonian times this ephemeral oceanic domain is suggested as the setting for the protoliths of the most common ophiolites involved in the variscan suture current ideas for the assembly of pangea advocate a single collisional event between gondwana and laurussia in the carboniferous however the new evidence from the allochthonous terranes of the variscan belt suggests a more complex scenario for the assembly of the supercontinent with an interaction between the colliding continental margins that started earlier and lasted longer than previously considered based on modern analogs of continental interaction the development of complex collisions as here suggested for gondwana and laurussia during the assembly of pangea could have been the norm rather than the exception throughout earth history

tectonometamorphic evolution of the aracena metamorphic belt sw spain resulting from ridge trench interaction during variscan plate convergence 1 the aracena metamorphic belt is a high temperature low pressure ht lp band located at the southern end of the european variscan chain it marks a suture between armorica and avalonia this belt is characterized by 1 the presence of mid ocean ridge basalt derived metabasites that were affected by an inverted ht lp metamorphism related to sw verging thrusting whose thermal peak diachronously migrated eastward 2 the occurrence of a ht lp metamorphism related to an extensional event affecting continental rocks belonging to the ossa morena zone with peak temperatures that are 150 c higher than that recorded in the oceanic metabasites and 3 the presence of near trench magmatism with high mg andesite composition these and other characteristics can be interpreted in terms of a ridge trench trench triple junction that migrated along the armorican margin during variscan oblique convergence

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

mid and late holocene sea level changes in roudsea marsh northwest england a diatom biostratigraphical investigation diatoms vascular vegetation sediment data and water salinity from contemporary intertidal environ ments in roudsea marsh northwest england are used to define altitudinal relationships between diatom assem blages and reference water levels comparison of these data with the high resolution diatom biostratigraphy of a fossil marsh sequence allows identification of fluctuations of relative sea level in the mid and late holo cene two oscillations in relative sea level are recorded a rise in sea level 0 6 m led to the inundation of the site after 6000 bp and was followed by a fall in sea level c 1 m around 4700 bp a second rise in sea level 0 5 m occurred before 2420 bp and was followed by a relatively stable or smaller rise of sea level during the last millennium the use of diatom groups based on marsh altitude was successful

re interpreting the devonian ophiolites involved in the variscan suture u pb and lu hf zircon data of the moeche ophiolite cabo ortegal complex nw iberia new u pb zircon data of a mylonitic greenschist from the moeche ophiolite one of the mafic units involved in the variscan suture in the cabo ortegal complex nw of the iberian massif yielded an age of 400 3 ma consequently this unit can be considered one of the devonian ophiolites the most extended group of oceanic units in the variscan belt the mafic rocks show transitional compositions between n morb and island arc tholeiites although lu hf isotope signatures of its zircons clearly indicate contribution from an old continental source ehf values in the analysed zircons are negative generally below ehf 5 and thence they are not compatible with their generation from a juvenile mantle source accordingly the igneous protoliths were generated in a setting where juvenile mafic magmas interacted with an old continental crust the devonian ophiolites from the variscan suture have been repeatedly interpreted as remnants of the rheic ocean however the presence of a continental source in the origin of the mafic rocks of the moeche ophiolite allows discarding an intraoceanic setting for their generation at least for the nw iberian counterparts the tectonic setting for the devonian ophiolites of nw iberia is very likely represented by an ephemeral oceanic basin opened within a continental realm herein the real rheic ocean suture could only be located west of the terrane represented by the upper units of the allochthonous complexes apparently that suture is not represented in nw iberia

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

allochthonous terranes involved in the variscan suture of nw iberia a review of their origin and tectonothermal evolution abstract nw iberia includes a rather complete section of a variscan suture where different terranes with continental or oceanic affinities appear with clear structural relationships three groups of terranes namely upper ophiolitic and basal units and a frontal tectonic melange appear in galicia in cabo ortegal ordenes and malpica tui complexes they constitute a huge allochthonous pile thrust over the iberian parautochthonous and autochthonous domains which represent the section of the gondwanan margin that escaped continental subduction during the variscan cycle considering the allochthonous character of the nappe pile and the strong deformation associated to the variscan collision there are problems to identify the original tectonic setting of the terranes and thence it is difficult to reconstruct the paleogeographic context during the variscan and pre variscan times in detail key features to perform any model for the variscan convergence should consider the existence of two different high p metamorphic events dated at c 400 and 370 ma respectively separated in time for the generation of mafic ultramafic sequences at c 395 ma which constitute the most common ophiolites described in the variscan suture some dynamic models developed in nw iberia have important problems to explain the observed tectonothermal evolution especially the older high p metamorphic event and the exhumation of deeply subducted transitional type sections on the other hand the recently discovered participation of an older continental crust in the generation of different protoliths of the middle devonian ophiolites makes difficult the interpretation of these ophiolites in relation to open wide oceanic domains this paper describes the distribution structure lithologies geochronology and chemical composition of the terranes involved in the variscan suture of nw spain the scope of this description ranges from detailed regional aspects to the discussion of the development of the variscan orogen in the context of the assembly of pangea a two stage collisional model affecting a wide gondwanan platform may explain most of the evidences in nw iberia the generation of a long pull apart basin probably occurred after the first collision where the devonian ophiolites were formed these ophiolites have been in general described in the context of the rheic ocean but according to the new existing data they do not seem related to this ocean but rather they were formed after its closure

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

reply to comment on the late devonian variscan suture of the iberian massif a correlation of high pressure belts in nw and sw iberia the existence of a large allochthonous nappe pile emplaced onto the hinterland of the iberian massif and covering most of the basement of nw and sw iberia proposed by diez fernandez and arenas has been called into question on the basis of structural geophysical and regional data from sw iberia the discussion provided here is based on a revision of the fundamentals of the variscan geology of nw iberia and on a more detailed presentation of the geology of sw iberia our counterarguments reveal the reasoning of simancas et al against the verisimilitude of the large allochthonous pile as inconclusive incomplete or erroneous in some cases therefore we maintain that our structural correlation and its implications remain fully valid for the case of the iberian massif

40ar 39ar laserprobe dating of mylonitic fabrics in a polyorogenic terrane of nw iberia the tectonothermal evolution of a polyorogenic terrane in the variscan belt of nw spain has been constrained by 40 ar 39 ar laserprobe incremental heating experiments on mylonitic fabrics developed in major structures transitional levels between hp ht and ip upper units in the ordenes complex where metamorphic and structural records demonstrate two cycles of burial and exhumation were selected for dating two groups of ages have been defined 1 silurian early devonian obtained from mylonites of the fornas extensional detachment here considered as the minimum age for the start of tectonic exhumation of the hp ht units and an upper age limit for the hp ht event itself 2 early to mid devonian from structures related to the variscan convergence in the area which include top to the east thrusts and extensional detachments a single younger carboniferous age obtained from the uppermost allochthonous sequences possibly reflects the final stages of emplacement of the allochthonous complexes our data indicate a polyorogenic character for a part of the iberian allochthonous complexes including variscan sensu stricto and early variscan convergence as well as an older early palaeozoic cycle

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

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

ediacaran to cambrian oceanic rocks of the gondwana margin and their tectonic interpretation in tectonic maps of variscan europe allochthonous pieces of cadomian basement clearly stand out with their predominant metabasic to ultrabasic elements the so called exotic terranes with ophiolites most of these domains are observed in basements of the central iberian allochthone the south armorican domain the nappe structures of the french massif central the saxothuringian zone and the bohemian massif similar relics can be recognized in many alpine basement areas and correlations with supposedly more autochthonous basements such as the ossa morena zone and the central iberian basement can be envisaged all of these relics are thought to represent the interrupted trace of a former continuous or discontinuous structure characterized by the presence of ocean derived proto rheic rock suites these can be interpreted as pieces of former magmatic arcs of ediacaran to cambrian age accreted to the gondwana margin which later were scattered as allochthonous units during the variscan plate tectonic processes the presence of similar rock suites of ordovician age in the alpine realm is explained by the accretion of exotic china derived basements and their collision with the gondwana margin during the opening of the rheic ocean

a single devonian subduction event for the hp ht metamorphism of the cabo ortegal complex within the iberian massif abstract ion microprobe data are reported for zircons from ultramafic mafic and metasedimentary high pressure hp rocks of the cabo ortegal allochthonous complex galicia nw spain the new data suggest that the protoliths of eclogites morb type and mg and al rich and mafic granulites have formed in early ordovician times 495 11 469 9 and 478 12 ma respectively involving oceanic and or continental mantle sources we have found no evidence for a hp metamorphic event at 480 ma and for the previously proposed second hp event in the silurian instead a single subduction induced pt t loop with a climax around 390 ma early devonian can be deduced from the relevant domains of zircons from the following rocks two different types of eclogites two hp paragneisses one garnet pyroxenite and one spinel harzburgite rapid cooling and exhumation minimum rates of ca 90 c ma and ca 5 8 km ma are derived for the time immediately after the peak conditions after cooling to ca 500 c slower cooling and exhumation rates ca 15 18 c ma and ca 0 18 km ma are probably artefacts due to an episode of reheating at ca 350 ma this interpretation is supported by the scatter of various mineral ages and the presence of synkinematic granitoids a subduction related acadian early devonian hp metamorphism followed by an early carboniferous collision can thus be demonstrated for the first time also for the iberian part of the european variscides

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 bazar ophiolite of nw iberia a relic of the iapetus tornquistocean in the variscan suture the bazar ophiolite one of the ophiolitic units involved in the variscan suture of nw iberia is mainly formed by metagabbroic high t amphibolites with n morb affinity the ophiolite appears accreted under an arc derived upper terrane affected by intermediate p granulite facies metamorphism dated at 496 484 ma u th pb geochronology and lu yb hf isotope geochemistry of zircons allow recognizing two growth stages the first occurred during crystallization of the gabbroic protolith and has been dated at 495 2 ma whereas the second one interpreted as dating the high t metamorphism yielded an age of 475 2 ma the chronology of the bazar ophiolite and its accretionary history suggest that this unit is a relic of the cambrian ocean located to the north of gondwana the iapetus tornquist ocean accreted to a dissected arc during or before the early stages in the opening of the rheic ocean

holocene evolution of relative sea level and local mean high water spring tides in belgium a first assessment abstract holocene age depth data from the belgian coastal plain basal peat data particularly are assessed in terms of local water and tide levels basal peat formation was generally induced by sea level rise effects of local seepage were limited and significant river gradient effects were absent no relation was found between the timing of basal peat inception and substrate permeability in general edaphic moisture conditions were unfavourable for organic accumulation prior to sea level related rise of the groundwater table error envelopes for the minimum level of local mean high water spring tides and for upper and extreme lower relative mean sea level limits during the last 9400 solar years 8400 yrs b p are presented these indicate that the rate of relative sea level rise showed marked retardations at ca 7500 7000 yrs cal b p 6650 6100 yrs b p and ca 5500 5000 yrs cal b p 4400 yrs b p a eustatic explanation appears to be plausible particularly for the former prior to ca 4000 yrs cal b p ca 3600 yrs b p the relative sea level rise in belgium differs noticeably from that established for the western netherlands with both diverging progressively back in time this implies that considerable differential crustal movement occurred between the two areas in the early holocene

magmatism and early variscan continental subduction in the northern gondwana margin recorded in zircons from the basal units of galicia nw spain in situ uranium lead dating la sf icpms and sims and lu hf isotope analyses la mc icp ms of zircon from eclogite facies rocks from the basal units of the variscan belt in galicia constrain their magmatic and metamorphic evolution and give some clues about the nature and origin of the involved basement the samples studied are two felsic gneisses two eclogites and one eclogitic gneiss of intermediate composition metatonalite oscillatory zoned zircon cores from the felsic samples gave a main clustering of u pb ages at 493 2 and 494 2 ma and some older ages that represent inherited cores zircon grains from the intermediate and one of the mafi c rocks show no inherited cores and yielded ages of 494 3 and 498 6 ma respectively interpreted as time of protolith crystallization variably developed homogeneous zircon rims in one felsic gneiss yielded an age of 372 3 ma and very tiny zircons of one eclogite gave 350 2 ma both of which we interpret as metamorphic ages the new age data demonstrate that the calc alkaline magmatic suite described in the basal unit is ca 20 ma older than the alkaline to peralkaline plutonic suite of the same unit dated at 472 2 ma rodriguez et al 2007 and thus probably represents a distinct geologic event overgrowth rims are interpreted as metamorphic on the basis of their lu hf and th u ratios the 372 3 age is considered as dating the high pressure high p metamorphism and is essentially in agreement with previous ar ar and rb sr data this high p metamorphism marks the initial early variscan subduction of the gondwana margin the inherited zircon ages and hf isotopic composition of zircons point to a considerable input of crustal material with west african craton provenance to the felsic magma

rheic ocean ophiolitic remnants in southern iberia questioned by shrimp u pb zircon ages on the beja acebuches amphibolites 1 the rheic ocean was a major oceanic domain between avalonia and gondwana in ordovician silurian times most of the paleozoic plate reconstructions assume that the rheic ocean suture lies within southern iberia coinciding with the contact between the south portuguese zone and the ossa morena zone this paper reports four sensitive high resolution ion micro probe shrimp u pb zircon ages from mid ocean ridge basalt morb featured rocks of the beja acebuches amphibolite unit which crops out along the boundary between the ossa morena and the south portuguese zone and is considered its most conspicuous suture unit the obtained ages range from 332 3 to 340 4 ma corresponding to the crystallization of the mafic protoliths these early carboniferous ages for the beja acebuches amphibolites imply that this unit can no longer be viewed as an ophiolite belonging to the rheic ocean suture since this oceanic domain was presumably closed in devonian times tectonic reconstructions joining in a single suture line the beja acebuches amphibolite unit in southern iberia to either the devonian lizard ophiolite in southern england or the root zone of the devonian ordovician ophiolitic units in northwest iberia must be therefore reconsidered because of the age difference we interpret the beja acebuches amphibolite unit to represent a narrow and very ephemeral realm of oceanic like crust that opened in early carboniferous times after total consumption of the rheic ocean we suggest that a mantle plume underneath southern iberia in early carboniferous times is the most plausible large scale geodynamic scenario for the formation of these morb featured rocks

using shrimp zircon dating to unravel tectonothermal events in arc environments the early palaeozoic arc of nw iberia revisited dating of zircon cores and rims from granulites developed in a shear zone provides insights into the complex relationship between magmatism and metamorphism in the deep roots of arc environments the granulites belong to the uppermost allochthonous terrane of the nw iberian massif which forms part of a cambro ordovician magmatic arc developed in the peri gondwanan realm the obtained zircon ages confirm that voluminous calc alkaline magmatism peaked around 500 ma and was shortly followed by granulite facies metamorphism accompanied by deformation at c 480 ma giving a time framework for crustal heating regional metamorphism deformation and partial melting the main processes that control the tectonothermal evolution of arc systems traces of this arc can be discontinuously followed in different massifs throughout the european variscan belt and we propose that the uppermost allochthonous units of the nw iberian massif together with the related terranes in europe constitute an independent and coherent terrane that drifted away from northern gondwana prior to the variscan collisional orogenesis

introduction to the special issue on spatial modelling and analysis of ore forming processes in mineral exploration targeting abstract ore deposits are diverse with much of their diversity attributable to the complex interplay of ore forming processes with a variety of geological environments over a range of scales and both in space and time this diversity makes it difficult for geoscientists to accurately predict the location of undiscovered ore deposits improving our understanding of the processes that are critical to ore deposit formation would help us to hone our predictive capabilities however this task is difficult to achieve as we cannot observe these genetic processes first hand and different parameters and ingredients are important at different scales modelling offers a means of simulating and analysing ore forming processes and their mappable expressions this knowledge can then be used to build a predictive model by translating key process components into spatial proxies that can be mapped or recognized in mineral exploration data modelling and analysis of ore forming processes are therefore critical for the future success of mineral exploration currently underutilized in exploration targeting the application of statistical and mathematical concepts can help steer geoscientists towards a better understanding of the complex geological processes critical in the formation of mineral deposits and ultimately improved exploration success rates this editorial article presents a brief introduction to the main concepts that support a collection of articles published in a virtual special issue vsi of ore geology reviews entitled spatial modelling and analysis of ore forming processes in mineral exploration targeting the articles examine three critical themes 1 translating the expressions of ore forming processes and critical parameters of mineral systems into mappable spatial proxies 2 identifying mineral deposit footprints through geochemical and geophysical data analysis and 3 targeting and improving the discovery chance of mineral deposits by way of spatial data analysis

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

from opening to subduction of an oceanic domain constrained by la icp ms u pb zircon dating variscan belt southern armorican massif france in the variscan belt of western europe the lifetime and evolution of the oceanic domain is poorly constrained by sparse outdated and unreliable multigrain id tims u pb zircon dating in this article we present a complete in situ la icp ms dataset of about 300 u pb zircon analyses obtained on most of the ophiolitic and eclogitic outcrops of southern brittany comprising new dating of previously published zircon populations and newly discovered rock samples in situ dating and cathodo luminescence imaging of each zircon grain yields new absolute time constraints on the evolution of the galicia moldanubian ocean the new results confirm that the opening of this oceanic domain is well defined at about 490 ma in contrast the generally quoted 400 410 ma age for the high pressure event related to the subduction of the oceanic crust is definitely not recorded in the zircons of the eclogites in light of these new data we propose that the obduction of oceanic rocks occurred at about 370 380 ma while the high pressure event is recorded at 355 ma in only a few zircon grains of some eclogite samples additionally this large scale dating project demonstrates that the zircons from eclogites do not systematically recrystallise during the high pressure event and consequently their u pb systems do not record that metamorphism systematically these zircons rather preserve the isotopic memory of the magmatic crystallization of their igneous protolith another example of an eclogite sample from the french massif central illustrates the frequent mistake in the interpretation of the ages of the early hydrothermal alteration of zircons in the oceanic crust versus partial or complete recrystallization during eclogite facies metamorphism

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

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