the tectonothermal evolution of the cadomian belt of northern brittany france a neoproterozoic volcanic arc abstract the neoproterozoic cadomian belt northern brittany france results from the accretion of crustal blocks that record distinctive p t deformation paths in addition abundant gabbro dioritic to granitic intrusions during and after deformation influence the thermal pattern and are used as time markers as a whole the deformation history of the cadomian belt is characterised by a migration in time of the active zones which become younger towards the southeast relics of eocadomian rocks reveal that arc development took place at about 660 640 ma abundant calc alkaline magmatism intruding strongly deformed palaeoproterozoic orthogneisses characterise the northernmost unit of the cadomian belt i e the tregor unit which cooled before 600 ma the lanvollon basin a short lived intra arc rift containing abundant mafic and felsic volcanics followed by greywackes opened at about 600 580 ma the subsequent tectonothermal history of the cadomian belt is divided into two major episodes the first episode is related to the shortening of the intra arc rift and its southwestward thrusting over the guingamp unit this led to crustal thickening inducing partial melting in the guingamp unit exhumation of the latter rocks is accommodated by late extensional shearing along the boundary between the saint brieuc and guingamp units cooling of the saint brieuc unit is dated at about 570 560 ma the second episode is dominated by sinistral wrenching mainly localised in the eastern part of the cadomian belt at the boundary between the domnonean i e the arc domain and mancellian possibly a fore arc basin domains several shear zones developed during this episode some of them being associated with minor granitic intrusions dated around 550 ma the sinistral shear located southeast of the arc domain is reworking or crosscutting earlier high temperature fabrics within the saint brieuc unit in addition the vertical component of this shearing episode contributes significantly to the exhumation of the saint malo migmatites near the proterozoic cambrian boundary cordierite biotite granitoids intruded at shallow depths the fougeres unit their emplacement outlasting deformation of the sedimentary sequences minor stocks of muscovite leucogranites of cambrian age are possible equivalents of the abundant volcanism in an adjacent nw se trending rift this marks a major change in the kinematic framework with predominance of extension finally minor reactivation of the cadomian structures occurred during the carboniferous associated with the variscan orogeny

a precise late neoproterozoic u pb zircon age for the syntectonic perelle quartz diorite guernsey channel islands uk a precise u pb zircon age of 611 4 2 0 1 1 ma has been determined for the foliated quartz diorite at perelle bay guernsey uk channel islands the age is based on the analysis of single zircons half of a single grain and groups of two and three zircons the age is nearly 100 ma younger than that suggested by other workers based on 207 pb 206 pb dates of large multi grain fractions of zircon and thus rules out this intrusion as the earliest marker of the onset of cadomian magmatism in the north armorican massif a 611 ma crystallization age for the perelle quartz diorite is consistent with previous detailed fabric development studies that suggested magmatic fabrics within the pluton are largely overprinted by similarly oriented solid state deformation fabrics resulting from syntectonic emplacement during cadomian orogenesis the u pb age is approximately 8 15 ma older than 40 ar 39 ar hornblende dates for the pluton consistent with the emplacement of the intrusion followed by slow cooling in a region of moderate to high ambient crustal temperatures these new data emphasize the fact that periods of pluton emplacement are considerably briefer than periods of deformation and thus syntectonic intrusions do not date regional deformational events but only mark events within a period of regional tectonism the ability to analyse single zircons and selected portions of single zircons using ultra low blank u pb analytical techniques documents the feasibility of determining precise emplacement ages of other neoproterozoic granitoids that contain a significant percentage of xenocrystic zircon

deformation of partially molten granite a review and comparison of experimental and natural case studies experimental and natural investigations of partially molten granite are compared and reviewed experiments suggest that deformation of partially molten granite with low viscosity melt 104 pa s exhibits a rheological critical melt percentage rcmp in case of high viscosity melts 108 pa s however the relationship between melt fraction and log viscosity of the partially molten granite may be linear considerations about viscosity rheological thresholds and segregation of natural melts suggest that low viscosity melt experiments simulate natural conditions more realistically therefore an rcmp is to be expected under natural conditions both diffusion creep and dislocation creep may occur under natural conditions whereas cataclastic flow is only observed under experimental conditions a melt induced transition from dislocation creep to diffusion creep occurs under experimental and natural conditions melt topology is controlled by the magnitude of differential stress under experimental conditions if differential stress is higher than 100 150 mpa melt pockets are elongate and oriented at a low angle to the maximum compressive stress in contrast in nature melt pockets tend to be oriented subparallel to the foliation plane i e presumably at a high angle to the maximum compressive stress

sm nd isotope characteristics of late cadomian granite magmatism in northern france and the channel islands sm nd isotopic studies of granites within the late precambrian cadomian orogenic belt of the north armorican massif northwestern france and channel islands reveal differences between arc related granite magmatism in outboard terranes and intracrustal granite magmatism in inboard terranes late cadomian c 570 ma arc related granitoids exhibit a range of e nd 2 to 6 and nd model ages t dm 1 0 1 3 ga reflecting variable contamination between late precambrian mantle derived magmas and ancient c 2 0 ga continental crust the contamination did not involve exposed granitic icartian basement to anygreat degree a more likely contaminant being unexposed lower crust of intermediate to acidic granulitic composition or early cadomian plutons which were themselves contaminated by lower crust voluminous granites of the mancellian region c 550 540 ma share common isotopic characteristics e nd 4 to 7 t dm 1 5 1 7 ga with migmatites and anatectic granites produced by partial melting of metasedimentary sequences within the st malo region consistent with a common source

ordovician passive continental margin magmatism in the central european variscides u pb zircon data from the se part of the karkonosze izera massif sudetes sw poland approximately 500 ma old orthogneisses are widespread in the eastern part of the variscan belt and are commonly interpreted to have intruded mica schist series of assumed neoproterozoic age new shrimp zircon ages of quartzofeldspathic metavolcanogenic rocks of the mica schist series in the eastern part of the karkonosze izera massif sw poland indicate that they are late cambrian early ordovician rather than neoproterozoic in age based on the zircon age spectra distributed mainly between ca 500 and 660 ma with a few proterozoic inherited minimum ages of ca 970 and 1 825 ma younger zircon dates dispersed between ca 412 and 464 ma are interpreted as a result of pb loss likely caused by subsequent metamorphism consequently the felsic metavolcanogenic rocks appear to be roughly contemporaneous with the intrusion of ca 500 ma old orthogneiss protoliths with the pooled concordia age of 487 8 ma interpreted as the best approximation of the protolith intrusive age field relationships petrological and geochemical features of the felsic and mafic rocks studied support a model in which the accompanying mica schist series are not the original country rocks to the ca 500 ma granite intrusions and indicate that their recent close proximity is the result of tectonic juxtaposition however both the mica schists enclosing the bimodal metavolcanic rocks and the orthogneisses are interpreted to represent a cambro ordovician passive continental margin sequence being part of the saxothuringian domain they are tectonically overlain to the east by hp t metamorphic units comprising morb type metaigneous rocks and delineating a tectonic suture separating the saxothuringian block in the west from an assumed continental block tepla barrandian to the south east

records of precambrian early palaeozoic volcanic and sedimentary processes in the central european variscides a review of shrimp zircon data from the kaczawa succession sudetes sw poland abstract the early pre orogenic stages of evolution in the variscan belt i e rifting processes opening of sedimentary basins and associated igneous activities are often obscure because many successions have yielded little or no biostratigraphic data have a strong metamorphic overprint and are tectonically deformed and dislocated the increasing application of shrimp zircon dating has provided useful constraints on magmatic and metamorphic processes helped locate probable source areas for detritus within sedimentary successions and facilitated large scale palaeogeographic correlations this methodology has recently thrown considerable light on the age and relationships of the previously poorly constrained rock units of the kaczawa complex in the polish west sudetes thus recent shrimp studies in the kaczawa mountains have yielded early ordovician ages of the initial rift type bimodal volcanic suites at the bottom part of the kaczawa succession c 503 ma for metarhyodacites of crustal derivation and c 485 ma for alkaline metatrachytes of mantle signature these dates provide a firm temporal constraint on the initial rift magmatism interpreted as related to the continental break up of the northern peripheries of gondwana new shrimp data from metavolcaniclastic and metasedimentary rocks of the kaczawa complex have yielded results that have provided significantly changed interpretations on their age and relationships for instance a siliciclastic sequence interpreted as belonging to the lower part of the kaczawa complex the gackowa sandstones and seemingly sourced using an array of geochemical and mineralogical evidence from nearby early ordovician volcanic rocks has surprisingly yielded zircon ages not younger than precambrian and thus this unit has tentatively been reinterpreted as a possible correlative of the neoproterozoic lusatian graywackes felsic metavolcaniclastic rocks embedded in the carbonate succession of the wojcieszow limestone have yielded shrimp zircon ages of c 497 ma thus supporting a late cambrian early ordovician age for this shallow water limestone sequence which had been proposed to be late ordovician silurian or younger in age based on controversial foraminifera evidence and more recently assigned to the mid cambrian based on newly found archaeocyathid fragments and age spectra of zircon populations in the metamudstone sequence of the radzimowice slates of thus far enigmatic stratigraphic position indicate that these deposits may have accumulated during early palaeozoic rifting with source areas similar to those known from parts of gondwana e g nw africa overall the new shrimp zircon studies both in metavolcanic and metasedimentary sequences have considerably illuminated their affinities and should provide encouragement for further application of this technique in this region in general the results provide support and detail for interpretation of this rock complex as reflecting the early stages of development of the volcanic and sedimentary processes operating along north gondwana during latest precambrian and early palaeozoic times

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

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

interpreting magmatic fabric patterns in plutons abstract most plutons have widespread magmatic fabrics the interpretation of which remains controversial we propose a method to constrain likely causes of fabric patterns the application of which indicates the following 1 preserved fabric patterns often form after chamber construction and only rarely provide information about ascent or emplacement 2 fabrics are poor recorders of total strain and are easily reset preserving only the last increment of strain during crystallization 3 in magmatic systems mechanically decoupled from host rocks patterns may result from strain during internally driven flow filter pressing or porous flow in relatively static chambers or by final increments of strain during emplacement 4 with greater emplacement depths fabric patterns increasingly reflect strain caused by regional deformation and 5 given that magmatic fabrics are easily reset and reflect only the last increment of strain of comparatively weak materials they may provide a relatively direct record of paleostress in orogenic belts

the record of changing hematite and goethite accumulation over the last 22 ma on the chinese loess plateau from magnetic measurements and diffuse reflectance spectroscopy

u pb geochronological constraints on the tectonothermal evolution of the paleoproterozoic basement of cadomia la hague nw france abstract the la hague region of nw france provides some of the best exposures of paleoproterozoic basement in the northernmost segment of the cadomian orogen one of several neoproterozoic arc fragments in the circum atlantic region the basement rocks which include orthogneiss metasediments and metabsites contain a comparable series of structures with a common metamorphic history thus the basement units are regarded to have undergone the same broad tectonothermal evolution the metasedimentary succession includes migmatites that preserve relict kyanite overprinted by sillimanite associated with the development of garnet bearing leucosome biotite dehydration at a metamorphic peak of 700 800 c and 8 9 kbar is considered responsible for the anatexis evident in both the metasediments and orthogneiss units a u pb zircon crystallization age of 2061 2 7 ma for the culeron orthogneiss a member of the metamorphic suite confirms the paleoproterozoic protolith age idioblastic rims to 2 06 ga zircons from the culeron orthogneiss yield shrimp u pb dates of c 618 ma providing the first geochronological constraints on the likely age of high grade metamorphism and anatexis of the paleoproterozoic basement in cadomia foliated c 615 610 ma intrusions that intrude the metamorphic units record magmatic to solid state fabrics that parallel metamorphic fabrics within the basement consistent with deformation and metamorphism being late neoproterozoic in age

late quaternary climatic changes revealed by luminescence dating mineral magnetism and diffuse reflectance spectroscopy of river terrace palaeosols a new form of geoproxy data for the southern african interior lyons j r tooth s duller g 2014 late quaternary climatic changes revealed by luminescence dating mineral magnetism and diffuse reflectance spectroscopy of river terrace palaeosols a new form of geoproxy data for the southern african interior quaternary science reviews 95 43 59

u pb geochronological constraints on the timing of plutonism volcanism and sedimentation jersey channel islands uk uranium lead dates for single and small multiple grain zircon separates are presented for the main geological units from jersey british channel islands a turbiditic sandstone from the jersey shale formation brioverian supergroup contains detrital zircons of a variety of ages ranging from c 2 6 to 0 6 ga the youngest zircons yielded nearly concordant u pb dates of 586 7 3 0 ma and 587 1 2 7 ma providing a maximum depositional age for the unit zircons from overlying volcanic rocks anne port rhyolite give an upper intercept date of 582 8 3 2 7 ma considered to be the age of eruption the undeformed sw jersey granite which intrudes and locally thermally metamorphoses the folded jersey shale formation is dated at 580 2 3 1 6 ma these dates constrain tightly the timing of deposition of the jersey shale formation and the age of deformation on jersey and indicate a dynamic arc environment during the late neoproterozoic zircons from the nw jersey granite define an upper intercept date of 482 7 1 6 ma similar to the previously reported 40 ar 39 ar hornblende date of c 473 ma confirming a renewed pulse of magmatism during the ordovician in this sector of cadomia

three dimensional microstructure and numerical calculation of elastic properties of alpine snow with a focus on weak layers abstract the microstructure and stratigraphy of a snowpack determine its physical behaviour weaklayers or weak interfaces buried under a slab are prerequisites for the formation of dry snow slabavalanches and a precise characterization of weak layers or interfaces is essential to assess stability yettheir exact geometry and micromechanical properties are poorly known we cast weak layers and theiradjacent layers in the field during two winters and reconstructed their three dimensional microstructureusing x ray microcomputer tomography the high resolution of 10 20 m m allowed us to study snowstratigraphy at the microstructural scale we quantified the microstructural variability for 32centimetre sized layered samples and we calculated young s modulus and poisson s ratio bytomography based finite element simulations layers in a sample could therefore be differentiatednot only by a change in morphology or microstructure but also by a change in mechanical properties we found a logarithmic correlation of young s modulus with density for two different density ranges consistent with previous studies by calculating the relative microstructural changes within our samples we showed that a large change could indicate a potential weak layer but only when the weak layer andboth adjacent layers i e the sandwich were considered keywords

rainfall dependent transformations of iron oxides in a tropical saprolite transect of hainan island south china spectral and magnetic measurements 1 the iron oxide content of soils and sediments controlled by weathering and pedogenesis is generally considered a reasonable indicator of climate previous studies in temperate zones have established a positive correlation in aerobic soils between ferrimagnets and low to moderate rainfall the correlation seems to be reversed under extreme climates with high rainfall here we present a transect of saprolitic soils from hainan island south china with high rainfall 1440 2020 mm yr little temperature variation 23 24 c and extreme weathering along this transect we observed that both hematite concentration and magnetic susceptibility decrease with increasing rainfall whereas goethite concentration displays a large increase however there is no systematic trend in the total amount of iron oxides related to chemical weathering intensity along the transect goethite is the favored mineral phase of iron oxide with increasing rainfall and accumulates at the expense of hematite and maghemite through the dominance of rainfall driven processes these pedogenic processes coincide with the fundamentals of previous nonmonotonic models of hematite magnetic susceptibility and rainfall control of an inflection point this study also verifies a common genetic relationship between hematite and pedogenic ferrimagnets across a wide climate range a conceptual model considering both rainfall and temperature is proposed to help interpret the mechanism of ferrimagnet formation and changes in the rainfall inflection point

time span of plutonism fabric development and cooling in a neoproterozoic magmatic arc segment u pb age constraints from syn tectonic plutons sark channel islands uk abstract new u pb zircon and titanite dates from syn tectonic plutons on the british channel island of sark constrain the time span of plutonism fabric development and cooling in this part of the neoproterozoic cadomian magmatic arc the tintageu leucogneiss is a mylonitic unit that was dated previously at 615 6 4 2 2 3 ma the port du moulin quartz diorite which intruded the tintageu unit contains a high strain solid state deformation fabric that is less intense than but parallel to fabrics in the leucogneiss and yields a u pb zircon date of 613 5 2 3 1 5 ma the little sark quartz diorite also displays solid state deformation fabrics in addition to relict magmatic textures and yields a u pb zircon date of 611 4 2 1 1 3 ma the north sark granodiorite is largely penetratively undeformed exhibits mainly magmatic fabrics and textures and has a u pb zircon date of 608 7 1 1 1 0 ma two fractions of titanite from each intrusion are essentially concordant and are identical within error with mean dates of 606 5 0 4 ma port du moulin quartz diorite 606 2 0 6 ma little sark quartz diorite 606 4 0 6 ma north sark granodiorite the new u pb data in combination with previous u pb and 40 ar 39 ar data and previous field studies confirm the syn tectonic nature of the sark plutons and quantify the time span ca 7 m y required for intrusion and sufficient crystallization of each body to record incremental strain during waning deformation titanite u pb and hornblende 40 ar 39 ar dates mark final cooling about 2 m y after intrusion of the last pluton

timing of post tectonic cadomian magmatism on guernsey channel islands evidence from 40ar 39ar mineral ages the channel island of guernsey exposes pre and post tectonic calc alkaline plutonic rocks attributed to late precambrian cadomian subduction activity 40 ar 39 ar mineral cooling ages presented here clarify the age and time span of post tectonic intrusions amphibole from the bon repos meladiorite yields an isotope correlation age of 605 3 4 6 ma indicating emplacement during late stages of the tectonothermal event recorded within its host rocks igneous hornblendes from the northern igneous complex record consistent isotope correlation ages st peter port gabbro 570 2 1 1 ma bordeaux diorite complex 569 0 1 1 and 565 3 2 2 ma and a comagmatic enclave within cobo granite 570 2 1 5 ma hornblende and biotite from contact metamorphosed host rocks adjacent to the cobo granite yield isotope correlation ages of 569 6 1 1 and 563 8 3 3 ma respectively these ages together with field data demonstrate that the various members of the northern igneous complex were emplaced contemporaneously at c 570 ma significantly earlier than a c 500 ma rb sr whole rock isochron age previously reported for the cobo granite the 40 ar 39 ar ages indicate that post tectonic magmatism on guernsey was broadly coeval with post metamorphic cooling around the baie de st brieuc on the adjacent french mainland and occurred c 30 ma prior to peak cadomian deformation and metamorphism in the st malo area

significance of a hiatus in down temperature fabric development within syn tectonic quartz diorite complexes channel islands uk detailed fabric development histories have been deduced for neoproterozoic foliated quartz diorite complexes of the channel islands uk the plutons were emplaced during regional d2 deformation magmatic fabrics formed at low crystal contents pre rheologically critical melt percent are only locally preserved due to overprinting by solid state deformation which occurred at moderate temperatures 400 550 c this is demonstrated by brittle microcracking of plagioclase and amphibole with only local ductile bending and dynamic recrystallization marginal recrystallization and myrmekite development on alkali feldspar ductile deformation of quartz to form types 2 3 ribbons quartz recrystallization by sub grain rotation associated with a slip but not c slip biotite recrystallization within micro shears and variable amphibole recrystallization the deformation histories display a distinct division between magmatic state and moderate temperature solid state deformation features with little evidence for intervening high temperature solid state fabrics this contrasts with criteria widely considered diagnostic of syn tectonic plutons which envisage development of a continuum of magmatic through high temperature solid state deformation fabrics however we consider the plutons syn tectonic and attribute the hiatus to contrasting strain and pluton cooling rates length and nature of deformation events and host rock ambient temperatures intermediate magmas emplaced at moderate to shallow levels in the crust may be more prone to develop such apparently punctuated deformation histories than sio 2 richer granitoids

theory of metamorphism of dry snow the growth of ice particles in dry seasonal snow is caused by vapor diffusion among particles due to temperature gradients imposed on the snow cover the diffusion is calculated by using the potential field solutions for electrostatically charged particles the stereography of snow is represented by using a log normal distribution function for a geometrical enhancement factor defined here reasonable crystal growth rates and supersaturations are found the transition between the growth of highly faceted crystals and the growth of highly rounded crystals is determined by the critical supersaturation for the onset of dislocation aided growth thermal convection and the continuous movement of vapor around the particles due to the imposed temperature gradient are accommodated in the theory although both have little effect relative to the interparticle diffusion the growth of a layer of faceted crystals just below semi permeable crusts is explained by showing that crusts can cause the local supersaturation to exceed the critical value faceted crystals are shown to grow most rapidly in the lower warmer portions of the snow cover because of temperature effects on growth rate

the impact of accumulation rate on anisotropy and air permeability of polar firn at a high accumulation site the first three dimensional properties of polar firn obtained by x ray microtomography are used to study the microstructure of snow on a 15 m deep firn core from west antarctica the snow is found to undergo coarsening down to approximately 2 5 m depth before grain growth and densification become the prevalent mechanisms of microstructure change in contrast to previous assumptions distinct anisotropy of the ice and pore geometry is observed throughout the profile with a maximum at 2 5 m depth the air permeability and the degree of anisotropy vary with depth and can be linked to short term changes in accumulation rate via the residence time for which a certain snow layer stays in the uppermost 2 5 m patterns of the degree of anisotropy and air permeability of buried polar firn are relative indicators of past accumulation rates

energy based binary segmentation of snow microtomographic images x ray microtomography has become an essential tool for investigating the mechanical and physical properties of snow which are tied to its microstructure to allow a quantitative characterization of the microstructure the grayscale x ray attenuation coefficient image has to be segmented into a binary ice pore image this step called binary segmentation is crucial and affects all subsequent analysis and modeling common segmentation methods are based on thresholding in practice these methods present some drawbacks and often require time consuming manual post processing here we present a binary segmentation algorithm based on the minimization of a segmentation energy this energy is composed of a data fidelity term and a regularization term penalizing large interface area which is of particular interest for snow where sintering naturally tends to reduce the surface energy the accuracy of the method is demonstrated on a synthetic image the method is then successfully applied on microtomographic images of snow and compared to the threshold based segmentation the main advantage of the presented approach is that it benefits from local spatial information moreover the effective resolution of the segmented image is clearly defined and can be chosen a priori

precise u pb zircon ages from alderney channel islands growing evidence for discrete neoproterozoic magmatic episodes in northern cadomia the northernmost exposures of rocks formed during the late neoproterozoic cadomian orogeny in the channel islands northern france region occur on alderney the island mainly comprises foliated quartz diorite once considered to be 2 ga pre cadomian basement and an undeformed basic to intermediate plutonic complex a precise age of 610 2 ma based on u pb analyses of single and small groups of zircons for the foliated fort tourgis quartz diorite demonstrates that the oldest rocks were emplaced and deformed during a cadomian magmatic event the age is virtually identical to ages from similar foliated syntectonic quartz diorite bodies on the islands of guernsey and sark and at la hague north normandy indicating that this magmatic and deformational event was regional in extent discordant zircon xenocrysts define an upper intercept age of c 2 ga indicating the presence of palaeoproterozoic basement at depth single zircons from the undeformed bibette head granodiorite give a precise u pb age of 572 1 ma this age is closely similar to that for the emplacement of the northern igneous complex of guernsey the emerging data indicate that cadomian magmatism in the northern channel islands region was not a protracted continuum but occurred during two distinct short lived events separated by c 30 40 my

multiple archaean to early palaeozoic events of the northern gondwana margin witnessed by detrital zircons from the radzimowice slates kaczawa complex central european variscides sims dating of detrital zircons from the stratigraphically enigmatic radzimowice slates of the kaczawa mountains sudetes sw poland near the eastern termination of the european variscides has yielded age populations of 1 493 512 ma corresponding to late cambrian to early ordovician magmatism and constraining a maximum depositional age 2 between 550 and 650 ma reflecting input from diverse cadomian sources and 3 older inherited components ranging to c 3 3 ga with age spectra similar to those from gondwanan north africa the new data show that the radzimowice slates cannot form a proterozoic base to the kaczawa mountains succession as suggested by earlier models but was deposited at the earliest as an extensional basin fill during a relatively late stage of the break up of this part of northern gondwana

timing of cadomian deformation and magmatism within la hague nw france the la hague peninsula normandy nw france contains the most northerly components of the late neoproterozoic cadomian orogeny u pb dates for single grain and small multiple grain zircon separates from carefully selected plutons provide geochronological constraints on the timing of deformation and magmatism within the region the syntectonic jardeheu quartz diorite and omonville granodiorite were emplaced and contemporaneously deformed during the development of a greenschist facies fabric pervasive across the peninsula u pb zircon dates of 610 1 ma for the jardeheu quartz diorite and 608 2 ma for the omonville granodiorite are similar to a previously reported sensitive high resolution ion microprobe u pb date for metamorphism of the basement and indicate that a regionally significant period of metamorphism and magmatism occurred between 620 and 608 ma an emplacement age of 573 9 1 3 0 9 ma for the syntectonic thiebot granodiorite places limits on the timing of deformation along the thiebot shear zone zircons from the undeformed st martin monzonite give a precise u pb age of 581 3 1 1 ma the new dates point to reactivation of magmatism and localized greenschist facies deformation between 582 and 573 ma

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

early palaeozoic initial rift volcanism in the central european variscides the kaczawa mountains sudetes sw poland evidence from sims dating of zircons early palaeozoic volcanic suites are widespread throughout the variscan belt and have commonly been ascribed to incipient rifting along the northern periphery of gondwana in cambrian to ordovician times their distribution across europe defines the present day extent of gondwana derived terranes and constrains the timing of their separation from the gondwanan margin the kaczawa mountains in the west sudetes at the eastern termination of the variscan belt include bimodal rift related rocks but their protolith age and hence their significance have been highly uncertain we have applied secondary ionization mass spectrometry zircon geochronology to a metarhyodacite and a metatrachyte from this suite yielding ages of 502 4 2 6 ma and 485 7 1 6 ma respectively this constrains the initial rift magmatism to c 500 485 ma in this part of the european variscides the rift related early palaeozoic volcanism thus seems to have been broadly synchronous throughout the peri gondwanan terranes of europe

the cadomian active margin north armorican massif france a segment of the north atlantic panafrican belt abstract within the european variscan belt the armorican massif northwestern france contains an isolated fragment of the north atlantic panafrican belt the cadomian block mostly unaffected by variscan deformation this block is composed of neoproterozoic rocks ranging in age from 620 to 540 ma brioverian locally overlying a composite basement of icartian 2000 ma or pentevrian 750 ma age the cadomian tectonics the last magmatic manifestations of which are dated at around 540 ma were sealed by a cover of early cambrian rocks in normandy and early ordovician rocks in brittany five units are distinguished in this cadomian block tregor saint brieuc guingamp saint malo and fougeres although the units are bound by major faults the overall coherence of their different protolithic characteristics suggests that they can be fitted into a simple geodynamic model the tregor unit represented by the north tregor batholith of crustal affinity and its icartian host rock probably reflects an early cadomian evolution the saint brieuc unit essentially made up of magmatic rocks of juvenile affinity includes two assemblages juxtaposed by the cadomian tectonics 1 a volcanosedimentary succession formed in a typical island arc setting reflecting subduction around 610 ma and 2 a volcanic plutonic complex emplaced around 590 ma in an extensional active continental margin of pentevrian age the guingamp unit predominantly migmatitic includes a tectonic slice of deep origin the extrusion of which accompanied the exhumation of the migmatites it may represent the trace of a possible suture the saint malo unit is composed of brioverian detrital rocks the significance of which is uncertain passive continental margin or accretionary prism it is structured around a late migmatitic dome 540 ma the fougeres unit corresponds to the outer domain of the cadomian orogeny it was only weakly deformed by cadomian tectonics and is intruded by the large mancellian batholith of crustal origin dated at 540 ma together the structural units are interpreted in the framework of an evolving active margin juxtaposing continental margin marginal basin and volcanic arc before being shortened during the cadomian convergent tectonics

tectonomagmatism in continental arcs evidence from the sark arc complex abstract the island of sark channel islands uk exposes syntectonic plutons and country rock gneisses within a precambrian cadomian continental arc this sark arc complex records sequential pulses of magmatism over a period of 7 ma ca 616 609 ma the earliest intrusion ca 616 ma was a composite sill that shows an ultramafic base overlain by a magma mingled net vein complex subsequently deformed at near solidus temperatures into the amphibolitic and tonalitic tintageu banded gneisses the deformation was synchronous with d 2 deformation of the paragneissic envelope with both intrusion and country rock showing flat top to the south ls fabrics later plutonism injected three homogeneous quartz diorite granodiorite sheets the creux moulin pluton 150 250 m ca 614 ma the little sark pluton 700 m 611 ma and the northern pluton 500 m 609 ma similar but thinner sheets in the south derrible hogsback dixcart and west port es saies brecqhou are interpreted as offshoots from the creux moulin pluton and little sark pluton respectively all these plutons show the same ls fabric seen in the older gneisses with rare magmatic fabrics and common solid state fabrics recording syntectonic crystallisation and cooling the cooling rate increased rapidly with decreasing crystallisation age 9 ma for the oldest intrusion to cool to lower amphibolite conditions 7 8 ma for the creux moulin pluton 5 6 ma for the little sark pluton and 2 the initiation of the d 2 event is suggested to have been a response to the intrusion of the tintageu magma which promoted a rapid increase in strain rate 10 14 s 1 that focussed extensional deformation into the sark area the increased rates of extension allowed ingress of the subsequent quartz diorite granodiorite sheets although strain rate slowly declined as the whole complex cooled during exhumation the regional architecture of syntectonic cadomian arc complexes includes flat lying sark type and steep guernsey type domains produced synchronously in shear zone networks induced by oblique subduction a pattern seen in other continental arcs such as that running from alaska to california