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

implications of pre eruptive magmatic histories of zircons for u pb geochronology of silicic extrusions zircons in silicic magmas begin to crystallize 10 s to 100 s of thousands of years ka prior to their eruption u pb and u th dating for zircons from 32 young extrusions indicate a range of mean pre eruption ages from 33 to 247 ka with a median of 92 ka explosive large volume widespread eruptions show a narrower range and a younger median of 70 ka these results indicate that the difference between initial zircon growth and eruption age measured by other e g 40ar 39ar techniques is in excess of the achievable analytical uncertainty for eruptions 30 million years ma and likely for eruptions of up to 300 ma protracted pre eruption zircon growth is well established by an increasing number of studies on magma process time scales but this source of eruption age bias has been largely ignored in chronostratigraphic applications of high precision u pb data until intrinsic properties of zircons or their host deposits enable a priori estimates of pre eruption age bias it is prudent to expand the uncertainties attending all u pb zircon ages interpreted as deposition ages in e g chronostratigraphic studies by 0 200 ka

proto avalonia a 1 2 1 0 ga tectonothermal event and constraints for the evolution of rodinia the neoproterozoic evolution of avalonia is thought to have been geodynamically linked to the amalgamation and dispersal of rodinia similar sm nd isotopic signatures for different periods of arc activity suggest that avalonian basement or proto avalonia was generated in a series of primitive oceanic island arcs between 1 2 and 1 0 ga because this interval coincides with the amalgamation of rodinia proto avalonia is inferred to have been located in a panthalassa like peri rodinian ocean an early 760 660 ma phase of avalonian arc activity is attributed to renewed subduction in the peri rodinian ocean following the breakup of rodinia which caused the accretion of avalonian terranes to the gondwanan margin by ca 650 ma further subduction along the margin occurred outboard of these terranes and resulted in the onset of main phase avalonian volcanism at 630 ma the diachronous cessation of arc magmatism is attributed to ridge trench collision and the generation of a continental transform the geodynamic link between avalonia and rodinia is analogous to that between the mesozoic dispersal of pangea and the tectonothermal evolution of western north america this event also resulted in the accretion of outboard terranes and in arc related magmatism that is currently being terminated in a diachronous manner by ridge collision and the generation of the san andreas transform the model implies that the neoproterozoic evolution of avalonia and other peri gondwanan terranes provide important constraints on the tectonic history of a large portion of the rodinian continental margin

topographic control of wind driven circulation in the northern adriatic 1 two synoptic wind driven situations of the circulation in the northern adriatic were studied using the princeton ocean model over the northern adriatic sea napom in both situations the basin was driven by a relatively steady wind 8 m s along and across the basin over three days in the first situation 28 30 october 2008 the sse southern jugo or scirocco wind blew along the basin and in the second 19 21 march 2009 the ene bora wind blew across the basin cyclonic turn was present in the first situation while the cyclonic branch of a known double gyre circulation north of the strip of wind minima was evident in the second we show that during the jugo the model does not confirm quantitatively the simple topographic control of a wind driven circulation suitable for elongated basins while qualitatively the model meets expectations with downwind transport in shallow areas close to shorelines and upwind transport in places with greater depths during the bora wind however the wind driven circulation in the gulf of trieste is well explained by this topographic control 82 of the flux through the transect

self induced fracture generation in zircon the mineral zircon has particular geological significance because of its use in the age determination of rocks and as a structural analogue phase for radioactive waste forms despite its widespread utility however radiation damage of the crystal structure metamictization causes a volume expansion of the crystal lattice and the generation of internal stresses which can induce fractures in the crystal a model has been developed which describes the state of stress in n concentric spherical shells each of which may have different material properties the case of a three shelled composite sphere has been examined explicitly in combination with fracture mechanics theory the model predicts that spatially distinct radial and or concentric fracture sets can be produced and such fracture patterns accurately describe the distribution and types of self induced fractures observed in natural zircons in general the formation of such microfractures in zircon is a function of the degree of metamictization shell thickness and the confining pressure the model predicts that the maximum depths in the earth s crust at which radial versus concentric fractures can form in a crystal are different suggesting that the types of fractures found in a suite of zircons from a particular rock might be potential indicators of the depth of burial because metamictization induced fractures may serve as potential pathways for the rapid leaching of various elements from the zircon crystal this may also have important implications in interpreting the u pb ages of fractured zircons or in evaluating the suitability of related crystalline phases as hosts for nuclear waste

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

cadomian metasediments and ordovician sandstone from corsica detrital zircon u pb hf constrains on their provenance and paleogeography the island of corsica which belongs to the southern variscan realm was detached from southern france in the tertiary alongside alpine and variscan edifices it carries outliers of neoproterozoic metasediments and lower paleozoic siliciclastics including ordovician sandstone and conglomerates as well as hirnantian diamictites in its nw sector we investigated the u pb hf of detrital zircons of basement metasediments and overlying ordovician sandstone and channeling conglomerate to constrain their provenance the timing of their deposition and to deduce the late ediacaran to ordovician paleogeography the youngest detrital zircons in the metasediments are 0 55 0 53 ga indicating their maximum age of deposition is late ediacaran to early cambrian thus classifying the corsica basement metasediments as cadomian the u pb analyses revealed that a preponderance of the detrital zircons in the basement micaschist and quartzite portray neoproterozoic ages concentrating between 0 55 and 0 65 ga this is partly consistent with derivation from pan african terranes of north africa yet the presence of detrital zircons younger than 0 6 ga indicates significant input from cadomian magmatic arcs that resided within or at the margin of the cadomian basin in which the metasediments of corsica were deposited ehf values of the ediacaran zircons varies between samples indicating the provenance comprised both juvenile arcs and magmatic arcs that involved various degrees of mixing with old crustal components the hf tdm ages of many of the ediacaran aged zircons point to a plausible involvement of meso paleoproterozoic crust in the generation of these cadomian arcs the presence of small but fairly distinguished populations of mesoproterozoic aged 1 0 1 6 ga as well as 2 0 2 2 ga and 2 4 2 6 ga detrital zircons in the cadomian metasediments farther indicates the presence of such crust in the provenance although mesoproterozoic detrital zircons are usually considered the hallmark of avalonian terranes the presence of hirnantian glacial sediments at the corsican sequence indicates it resided in the vicinity of gondwana we therefore postulate that the pre neoproterozoic zircons have sourced from exotic crustal vestiges that were entrained and accreted within the cadomian realm itself the transition into the overlying ordovician sandstone and conglomerate marks a major change in the provenance possibly pointing to lateral motions along the strike of the peripheral cadomian domain the youngest concordant detrital zircon in the ordovician ciuttone sandstone yielded an age of 0 48 ga the detrital zircon ages define an overwhelming concentration at 0 55 ga indicating the source of the ordovician sandstone was cut off from the gondwana hinterland and that sand was exclusively derived from a latest ediacaran arc in view of the sharp detrital zircon age peak the corsica ordovician sandstone cannot be straightforwardly correlated with armorican sandstone because the detrital zircon spectra of the latter are generally broader indicating derivation from various sectors of the north gondwana crust ehf t values of the 0 55 ga zircons are mostly positive and the corresponding tdm ages at 0 7 1 2 ga indicating derivation from a juvenile island arc while tdm ages of this type are common for late ediacaran avalonian rocks the presence of hirnantian diamictite in corsica further substantiates that the aforementioned 0 55 ga island arc evolved in the peripheral cadomian realm as a whole the u pb hf zircon data from the corsica sequences reveal the presence of juvenile cadomian arcs alongside cadomian arcs that recycled ancient pre neoproterozoic crust along strike variations of this type are known from the japanese islands in line with the peripheral cadomian orogeny being an ancient analog of a western pacific type plate boundary

evidence of a paleoproterozoic basement in the moroccan variscan belt rehamna massif western meseta abstract for the first time an eburnian magmatic event has been identified in the rehamna massif moroccan variscan belt western meseta located north of the south meseta fault the best estimate of the crystallization age of rhyolitic porphyry is given by a weighted mean 207pb 206pb age of 2050 6 3 ma rhyacian orosirian the new u pb age obtained for rhyolitic porphyry from the rehamna massif is of great relevance for improving geological knowledge about the boundaries of the wac because i it overlaps the older eburnian magmatic event described in the anti atlas belt and the icartian magmatic event of the european variscan belt ii this suggests that exists paleoproterozoic basement in the western meseta a hundred kilometers further to the north of the south meseta fault as old continental crust slivers preserved in the cadomian and variscan belts and iii this means that the cambrian transgression in the western meseta probably took place based on a more complex structural architecture affecting the precambrian basement composed not only of ediacaran rocks as has been suggested in the literature but also with paleoproterozoic rocks ca 2 05 ga as discovered in this study

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

comparison of model simulated and observed currents in the central adriatic during dart 1 numerical simulations of the adriatic sea were performed with the navy coastal ocean model ncom during the dynamics of the adriatic in real time dart experiments conducted between october 2005 and september 2006 grid resolution was 1 km model forcing included tides surface fluxes from the aire limitee adaptation dynamique developpement international aladin atmospheric model relaxation to a daily satellite sea surface temperature analysis extensive river and runoff discharges and open boundary conditions south of otranto specified from a global model currents predicted by the model were compared with currents from 12 acoustic doppler current profiler adcp moorings located along a line between the gargano peninsula italy and split croatia the nontidal comparisons were performed with detided currents correlations between the model and adcp currents were highest in the western adriatic current wac which flows southeastward along the italian coast lowest correlations were in the interior of the adriatic likely because of instability processes correlations between the aladin winds and the model and adcp currents at the mooring locations were also highest in the wac for november 2005 through august 2006 the model and adcp mean wac transports were 0 321 and 0 304 sv respectively with a temporal correlation of 0 79 comparison of current variance showed best agreement near the italian and croatian coasts in the interior the ncom current variance compared fairly well with that of the adcps for november january but decreased significantly relative to the adcps for february august spectral analyses indicate most of the difference in variance to be at periods exceeding 2 days

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

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

regional differences in pb isotopic compositions of feldspars in plutonic rocks of the northern appalachian mountains u s a and canada a geochemical method of terrane correlation feldspar pb isotopic compositions from paleozoic tonalitic to leucogranitic caledonian plutons from the northern appalachian mountains of the u s a and canada range from nonradiogenic to radiogenic relative to the evolution curve of stacey and kramers 1975 pb isotopic values in the feldspars are generally independent of modal and major and trace element contents of the plutons plutons with the least radiogenic pb isotopic signatures northern group have intruded allochthonous rocks closest to the north american craton and plutons with the most radiogenic pb southern group have intruded avalonian basement feldspars from plutons in the central group have intermediate pb isotopic compositions between the northern and southern groups and do not define an isotopically unique group the predominant source of the felsic magmas was the continental lithosphere as indicated by the general pb isotopic similarity of the plutonic feldspars with either grenville or chain lakes basement in the case of the northern group and with avalonian basement rocks in the case of the southern group the source region for plutons in the central group may have contained a mixture of grenvillian and avalonian components plutons in the northern appalachians are as a group more evolved in their pb isotopic compositions relative to caledonian plutons in the british isles

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

precadomian relicts in the armorican massif their age and role in the evolution of the western and central european cadomian hercynian belt abstract well dated precambrian is mostly developed in the north of the armorican massif in which area voluminous cadomian magmatism is dated at between 650 and 550 ma much older relicts occur at cap de la hague in guernsey and in the tregor and are also found in the northern continental margin of the iberian peninsula in all these occurrences whole rock systems have been opened so that the true ages cannot be determined by the rb sr whole rock isochron method four u pb zircon ages are between 1 8 and 2 ga in guernsey icart orthogneisses in tregor port beni trebeurden morguignen orthogneisses there is no evidence from strontium isotopes that these isolated and scattered relicts have a wide extension or that such ancient continental crust played an important role in magma genesis from 650 ma to 270 ma ago on the contrary the evolution of initial 87sr 86sr ratios with time shows that the observed mid and west european continental crust is probably not older than 700 ma the increase of the initial 87sr 86sr ratios of the magmas with time suggests that after its formation in cadomian times this segment of continental crust evolved virtually as a closed system and hercynian magmatism arose principally from re melting of relatively young sialic components

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

the hydrothermal stability of zircon preliminary experimental and isotopic studies experimental investigations of the stability of the u pb isotopic system in nonmetamict zircons show that appreciable losses of pb and u can be induced at amphibolite grade conditions 400 c to 600 c 4 to 6 kb in 2 m nacl and 2 hno sub 3 solutions the severity of u loss and to a lesser extent pb loss varies with solution composition in this case the 2 m nacl solution induced more pb and u loss than the 2 hno sub 3 solution at the same p t conditions scanning electron microscopy of the run products also revealed a range of corrosion related surface features which suggests that some of the observed trends in pb and u loss must be attributed to zircon dissolution backscattered electron bse imaging of the run products further suggests that partial homogenization of chemical zoning patterns occurred during the experiments microprobe analyses of treated and untreated grain show that both populations have a similar range of hf contents thus the apparent loss of sharp well defined zoning features is most likely due to small scale smearing out of formerly sharp chemical gradients and is perhaps related to the annealing of lattice defects caused by alpha recoil damage thus experimentally induced u pb isotopicmore discordance in zircon is a complex function of zircon stability and annealing effects less

in situ u pb zircon geochronology of neogene garnet bearing lavas from slovakia carpatho pannonian region central europe abstract zircons armored in garnet from four neogene lavas of slovakia three andesites and one dacite were dated in situ using excimer laser ablation icp ms for the andesites the 66 analytical points yield a lower intercept age of 13 3 0 1 ma mswd 1 2 which is older than the age of 12 4 0 2 ma recorded for the dacite u pb ages obtained in this study are significantly younger than the lower badenian ages k ar and fission track dating coupled with biostratigraphic correlations classically assumed for the garnet bearing lavas of the western carpathians with a minimum discrepancy of 3 ma accessorily a composite zircon with an inherited core reveals that some pan african crustal component is present at depth garnet bearing andesites and dacites potentially bear critical information about melt generation in the lower crust and possible recycling of pre existing crust into igneous rocks practically the problem is to assess the nature of the melt from which the garnet actually grew for this purpose zircon grains included into garnet may help to clarify the beginning of the magma history it seems likely that zircon was not grown from the bulk dacitic or andesitic melt but rather from a specific rhyolitic batch later mixing and consecutive change in magma chemistry explain that free zircons in the melt were almost totally dissolved whereas zircons shielded into garnets were preserved finally owing to their miocene age the zircon grains armored into garnet preclude any restitic origin for their host and definitely demonstrate the magmatic origin of garnet

polyphase zircon in ultrahigh temperature granulites rogaland sw norway constraints for pb diffusion in zircon shrimp u pb ages have been obtained for zircon in granitic gneisses from the aureole of the rogaland anorthosite norite intrusive complex both from the ultrahigh temperature uht 900 c pigeonite in zone and from outside the hypersthene in isograd magmatic and metamorphic segments of composite zircon were characterised on the basis of electron backscattered electron and cathodoluminescence images plus trace element analysis a sample from outside the uht zone has magmatic cores with an age of 1034 7 ma 2 n 8 and 1052 5 ma 1 n 1 overgrown by m1 metamorphic rims giving ages between 1020 7 and 1007 5 ma in contrast samples from the uht zone exhibit four major age groups 1 magmatic cores yielding ages over 1500 ma 2 magmatic cores giving ages of 1034 13 ma 2 n 4 and 1056 10 ma 1 n 1 3 metamorphic overgrowths ranging in age between 1017 6 ma and 992 7 ma 1 corresponding to the regional m1 sveconorwegian granulite facies metamorphism and 4 overgrowths corresponding to m2 uht contact metamorphism giving values of 922 14 ma 2 n 6 recrystallized areas in zircon from both areas define a further age group at 974 13 ma 2 n 4 this study presents the first evidence from rogaland for new growth of zircon resulting from uht contact metamorphism more importantly it shows the survival of magmatic and regional metamorphic zircon relics in rocks that experienced a thermal overprint of c 950 c for at least 1 myr magmatic and different metamorphic zones in the same zircon are sharply bounded and preserve original crystallization age information a result inconsistent with some experimental data on pb diffusion in zircon which predict measurable pb diffusion under such conditions the implication is that resetting of zircon ages by diffusion during m2 was negligible in these dry granulite facies rocks imaging and th u y systematics indicate that the main processes affecting zircon were dissolution reprecipitation in a closed system and solid state recrystallization during and soon after m1

the relationship between ductile shear zone and mineralization in the jiufeng sn deposit northern guangxi south china evidence from structural analysis and cassiterite u pb dating abstract the jiufeng sn deposit is located in the outer contact zone southwest of the neoproterozoic motianling granitic pluton in northern guangxi south china the deposit is characterized by a nne striking 30 200 m wide ductile shear zone named the jiufeng ductile shear zone majority of the sn orebodies including four large ones are hosted by the shear zone field and microstructural observations electron backscatter diffraction measurements and magnetic fabric analyses show that the shear zone is characterized by sinistral thrust shearing the shear zone has also slightly modified the sn orebodies quartz in the shear zone exhibits core mantle structure and undulose extinction quartz has experienced significant sub grain rotation and grain boundary migration recrystallization lattice preferred orientation analysis reveals primarily basal rhomb and prism slips cassiterite grains in the shear zone however demonstrate brittle deformation therefore the deformation temperature during the ductile shearing is estimated lower than 650 c and below u pb closure temperature of cassiterite as such the u pb isotopic composition of the cassiterite was not disturbed by the shearing and the u pb age of the cassiterite could be used for the age of the sn mineralization la icp ms u pb dating of cassiterite grains selected from a cassiterite bearing tourmalinite sample performed in this study yielded a tera wasserburg u pb lower intercept age of 832 3 5 3 ma this age is consistent within error with that of the neighboring neoproterozoic motianling granitic pluton which suggests a close genetic relationship between the sn mineralization and the granitic magmatism studies show that the jiufeng ductile shear zone was produced by a widespread caledonian or kwangsian tectonic event in south china indicating that the shearing was much later than the sn mineralization and has no generic relationship to the sn deposit

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

petrology and geochemistry of mafic granulite xenoliths from the lahtojoki kimberlite pipe eastern finland abstract the lahtojoki kimberlite pipe in kaavi eastern finland contains lower crustal mafic granulite xenoliths with a mineral assemblage cpx amph pl grt opx bt the tweequ thermobarometry indicates crystallization at ca 800 900 c and 0 75 1 25 gpa which corresponds with crustal depths of ca 22 38 km reaction textures and results of thermobarometry in some xenoliths indicate post peak decompression with cooling chemical composition of the xenoliths suggest that their protoliths crystallized from basaltic melts with some k enrichment their ree patterns are slightly lree enriched some xenoliths having small negative eu anomalies the u pb ion probe dating of zircons from two samples yielded variable ages for zircons even in the same xenolith between ca 2 6 and 1 7 ga these ages correspond with main late archaean and palaeoproterozoic orogenic events in the fennoscandian shield the sm nd garnet clinopyroxene isochron age from one dated sample is 1 6 ga which represents either a cooling or reheating of the lower crust by rapakivi magmatism because the petrology geochemistry and ages of xenoliths do not correlate well with the nearest exposed archaean mafic granulites it is evident that the present lower crust underlying the archaean rocks has a considerable palaeoproterozoic component

pb u and th diffusion in natural zircon zircon zrsio4 is one of the most widely used minerals for determining the age origin and thermal history of rocks by u th pb geochronology but the parameters describing the solid state volume diffusion rates of these elements in natural zircon which are themselves important for establishing the limits on the applicability of zircon for geochronological studies have remained poorly quantified1 this is because of the measurement difficulties associated with the low p p m concentrations and low diffusion rates of these elements in natural zircon and the chemical and physical heterogeneity present in most crystals here we present direct measurements of the uranium thorium and lead loss from a thermally treated gem quality natural zircon and show that lead diffuses much faster than uranium or thorium we find that the u th pb isotopic system in natural zircon will typically have a closure temperature greater than 900 c which explains why zircon is apparently such a robust geochrometer and is capable of remaining isotopically closed through extended periods of high grade metamorphism and partial melting of the host rock

the exhumation history of collision related mineralizing systems in tibet insights from thermal studies of the sharang and yaguila deposits central lhasa abstract the large newly discovered sharang porphyry mo deposit and nearby yaguila skarn pb zn ag mo deposit reside in the central lhasa terrane northern gangdese metallogenic belt tibet multiple mineral chronometers zircon u pb sericite 40 ar 39 ar and zircon and apatite u th he reveal that ore forming porphyritic intrusions experienced rapid cooling 100 c ma during a monotonic magmatic hydrothermal evolution the magmatic hydrothermal ore forming event at sharang lasted 6 0 myr 1 8 myr for cooling from 900 to 350 c and 4 0 myr for cooling from 350 to 200 c whereas cooling was more prolonged during ore formation at yaguila 1 8 myr from 900 to 500 c and a maximum of 16 myr from 900 to 350 c all porphyritic intrusions in the ore district experienced exhumation at a rate of 0 07 0 09 mm yr apatite he ages between 37 and 30 ma combined with previous studies this work implies that uplift of the eastern section of the lhasa terrane expanded from central lhasa 37 30 ma to southern lhasa 15 12 ma at an increasing exhumation rate all available geochronologic data reveal that magmatic hydrothermal exhumation activities in the sharang yaguila ore district occurred within four periods of magmatism with related mineralization significant porphyry type mo mineralization was associated with late cretaceous eocene felsic porphyritic intrusions in the central lhasa terrane resulting from neotethyan oceanic subduction and india asia continental collision

recent geodynamic characteristics in the arabian eurasian and indian eurasian collision region by active fault data abstract a part of the alpine himalayan orogenic belt is studied limited by 30 104 e and by 26 46 n to the west of 64 e and 26 56 n to the east of 64 e a technique is proposed to calculate the field of the recent late pleistocene and holocene deformation rate tensors in the upper crust 15 to 20 km by active fault data a hydrodynamic model of the medium is used for the calculation monotonous fault segments as long as 10 15 km are taken as the elementary cells of the medium the cell square the length multiplied by the depth multiplied by the displacement vector magnitude gives the geometric moment similar components of the local tensors of the geometric moments are summarized within the windows into which the region is divided finally parameters of the axes of principal rates of deformation are calculated and their directions and magnitudes are mapped this tectonic deformation is compared with the seismotectonic one calculated by focal mechanisms of earthquakes the compiled maps show a concentration of high deformation rates in the plate boundary zones especially in front of the southern plate syntaxes and a smaller concentration in some microplate boundary zones a predominance of the n s trending shortening and of strike slip type of deformation the rates of shortening and lengthening are usually almost equal to each other and differ essentially the shortening rate is higher than the lengthening only in specific sites these are areas of intense recent detachment or late quaternary volcanism it seems that the double axis lengthening extension is suitable for these processes

the rand granite in the southern schwarzwald and its geodynamic significance in the variscan belt of sw germany the rand granite is a heterogeneous metamorphosed granitoid rock complex with numerous wallrock inclusions situated in the moldanubian zone at the southern margin of the central schwarzwald gneiss complex it is a largely mylonitized elongated body and is thrust over the badenweiler lenzkirch zone forming a nappe with shear zones along its northern and southern boundaries it comprises meta granites meta trondhjemites and biotite augen gneisses derived from monzogranites to granodiorites mineral behaviour indicates that the magmatic body has been deformed under upper greenschist facies conditions nappe thrusting which also affected the south schwarzwald gneiss complex occurred in visean time during high temperature low pressure metamorphism kinematic indicators in the mylonites document e to ese directed nappe transport highly transpressive relative to the trend of the nappe boundaries and the foliation the trondhjemites formed at 351 5 4 ma predating the variscan ht metamorphism they have initial end values of 6 6 to 6 7 and relatively low initial 87sr 86sr ratios 0 7042 to 0 7063 indicating a predominant mantle origin the granites and protoliths of the biotite augen gneisses probably crystallised between 436 and 377 ma suggested by u pb zircon model ages they are different from the trondhjemites with low initial end values 4 7 to 3 3 and higher initial 87sr 86sr ratios 0 7068 0 7077 indicating that large part of the rand granite originated from anatexis of continental crust internal structure of zircons from the rand granite reveals mixing of magmas derived from both mantle and crust sources these new data support an interpretation that the rand granite developed along an active continental margin and therefore represents a possible root of a variscan magmatic arc

aerial observations of the yugoslavian bora abstract the first aircraft observations of the bora in yugoslavia were accomplished during the alpex project in 1982 data from all five alpex bora flights have been analyzed in a comparative study of bora structure although the bora varies considerably in depth in the strength of the incoming low level flow and in the direction of the winds aloft several common features are evident these include upstream descent and acceleration beginning where the mountains rise an approximate coincidence between the depth of the uppermost descending streamline and the wind reversal level upstream when a reversal exists a decoupling of the flow aloft associated with a splitting of the inversion and the formation of a thick mixed layer downstream a narrow region of intense turbulence and an ascending jet just downstream of the plunging bora the bora structure is similar in many respects to the boulder windstorm internal hydraulic theory taking into account the decoupling effect of the intermediate layer a

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

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

tidal currents in the adriatic as measured by surface drifters 1 velocities of surface drifters are analyzed to study tidal currents throughout the adriatic sea spectral and harmonic analyses indicate that the m2 s2 and k1 constituents dominate maps of tidal characteristics show that m2 and s2 are rectilinear currents reversing tides aligned with the main axis of the adriatic basin with maximum amplitude 7 cm s for m2 and 4 cm s for s2 in the northern area off the istrian peninsula near the northern coast semidiurnal tidal currents decrease in amplitude and rotate in the counterclockwise sense near the po river delta m2 s2 motions rotate in the counterclockwise clockwise sense s2 rotation is also counterclockwise near the northeastern coast m2 phases increase from about 130 on the eastern croatian coast to 190 on the western italian side s2 phases range from 150 to 200 in the middle and southern adriatic the semidiurnal tides are small 1 cm s the diurnal tidal currents k1 are strong across the basin at the levels of monte conero and the gargano peninsula with speed larger than 5 cm s and mainly clockwise rotation and also in coastal areas e g on the albanian shelf and close to the otranto channel phases increase from the east to the west coasts by as much as 150 these new results compare satisfactorily with previous observations and numerical simulations although tidal amplitudes are under estimated with respect to mooring measurements they extend for the first time the description of the adriatic tidal currents to the entire basin based on direct velocity observations

coupled ocean atmosphere nested modeling of the adriatic sea during winter and spring 2001 1 realistic simulations of the adriatic sea for over 125 days are conducted using the navy coastal ocean model with atmospheric forcing provided by the coupled ocean atmosphere mesoscale prediction system coamps is a registered trademark of the naval research laboratory coamps in two separate simulations of the adriatic a nested 2 km resolution ocean model is forced by the inner 4 km and outer 36 km nests of the atmospheric model two meteorological stations and two acoustic doppler current profiler observation sites are used to evaluate modeled atmosphere and ocean velocity fields for 28 january 4 june 2001 modeled observed correlations of atmospheric 10 m velocity are greater than 0 85 for both resolution models oceanic 5 and 25 m current fluctuations from both simulations generally match the magnitude and orientation of the observations the 4 km resolution atmospheric model is differentiated from the 36 km resolution model by its ability to resolve the small scale flow structures of the bora wind and by its better agreement with observed wind velocity statistics the ocean simulation forced by the 4 km resolution model is distinguished from the one forced by the 36 km resolution model by its ability to reproduce the expected double gyre circulation in the northern adriatic and by its ability to better capture the magnitude and shape of the observed depth dependent velocity correlation with wind at the deeper site though the 36 km forced ocean model agrees better with many observed velocity statistics the 4 km forced ocean model produces the highest correlations with observations exceeding 0 78 at subsurface depths that are most strongly correlated with winds

40ar 39ar thermochronology of paleoproterozoic granitoids of northeast burkina faso west african craton implications for regional tectonics abstract the greenstone belts of northeast burkina faso in the west african craton wac comprise birimian meta sedimentary and mafic ultramafic complexes that are intruded by granitoids and late dolerite dykes the granitoid greenstone belt assemblages underwent at least two phases of deformation that have resulted in their principal nw and ne trends we performed a detailed 40ar 39ar dating together with zircon u pb ages in order to unravel the thermo chronological history of the studied granitoids our previous zircon u pb data indicated that the granitoids were emplaced between 2138 7 and 2181 7 ma this period is well known for crustal growth in the wac the emplacement of granitoids was concomitant with d1 or the tangaean event 2130 2170 ma amphibole and micas yield two 40ar 39ar age groups the first group 2046 12 2132 13 ma constrained the d2 deformation event and the second group 1918 24 to 1984 12 ma constrained the d3 or wabo tempelse event considering closure temperatures of the isotopic systems the u pb ages and the present 40ar 39ar dating results suggest that after emplacement the granitoids cooled to 315 c at ca 2020 ma with a mean cooling rate of 6 5 c ma corresponding to a mean uplift rate of 0 65 km ma the additional k feldspar 40ar 39ar data indicate a later slow cooling stage with a rate of 0 54 c ma corresponding to a mean uplift rate of 0 054 km ma subsequent to the end of the major crustal growth in the wac at 2000 ma

possible southwestward extrusion of the ordos block in the late paleoproterozoic constraints from kinematic and geochronologic analysis of peripheral ductile shear zones abstract the paleoproterozoic is a pivotal period of the tectonic evolution of the north china craton ncc various models have been postulated by previous researchers through the study of petrology and metamorphism but the lack of structural analyses has hampered further understanding of the tectonic evolution of the ncc a series of well exposed ductile shear zones developed on the northern northwestern and eastern edges of the ordos block in the western part of the ncc we carried out detailed field studies along with isotopic dating focused on the kinematics and geochronology of the ductile shear zones on the north margin of the ordos block the wulashan daqingshan ductile shear zone is characterized by an e w trending shearing foliation that steeply dips 75 80 to the north or south with subhorizontal stretching lineation plunging 10 15 toward the east or west and dextral strike slip shearing kinematics on the west margin of the ordos block is the zongbieli ductile shear zone with top to the nw thrusting kinematics on the east margin of the ordos block is the xueling ductile shear zone with top to the se thrusting accompanied by sinistral strike slip shearing kinematics the 40ar 39ar ages of deformed minerals from mylonitic high grade metamorphic rocks and la icp ms u pb dating results of zircons from syn tectonic anatectic granites reflect that the ductile shear zones surrounding the ordos block deformed between 1 85 and 1 81 ga the similar deformation ages indicate that the ductile shear zones might be genetically correlated the geometry kinematics and geochronology characteristics of the wulashan daqingshan ductile shear zone on the north margin the zongbieli ductile shear zone on the west margin and the xueling ductile shear zone combined with the zhujiafang sinistral ductile shear zone on the east margin defined the possible southwestward extrusion of the ordos block in the late paleoproterozoic which might have resulted from the westward subduction collision of the eastern block under the western block at approximately 1 85 ga and which is correlated with the amalgamation of the columbia supercontinent

wave breaking induced surface wakes and jets observed during a bora event 1 an observational and modeling study of a bora event that occurred during the field phase of the mesoscale alpine programme is presented research aircraft in situ measurements and airborne remote sensing observations indicate the presence of strong low level wave breaking and alternating surface wakes and jets along the croatian coastline over the adriatic sea the observed features are well captured by a high resolution coamps simulation analysis of the observations and modeling results indicate that the long extending wakes above the boundary layer are induced by dissipation associated with the low level wave breaking which locally tends to accelerate the boundary layer flow beneath the breaking farther downstream of the high peaks a hydraulic jump occurs in the boundary layer which creates surface wakes downstream of lower terrain passes the boundary layer flow stays strong resembling supercritical flow

simulation of the northern adriatic circulation during winter 2003 1 numerical simulations of the adriatic sea were conducted with the navy coastal ocean model during the adriatic circulation experiment in the fall and winter of 2002 2003 and results were compared with observations the ocean model used a 1 km resolution grid over the entire adriatic sea model forcing included atmospheric fluxes from the coupled ocean atmosphere mesoscale prediction system coamps tides boundary conditions from a global model and freshwater river and runoff inflows model tidal elevation showed good agreement with international hydrographic organization station data and model tidal currents showed good agreement with tidal currents determined from acoustic doppler current profiler adcp measurements detided model currents showed good agreement with adcp currents with rms errors along the principal variance axes ranging from 6 to 12 9 cm s and correlations ranging from 0 16 to 0 81 correlations between model and adcp currents along the minor variance axes were generally low comparison of the model simulated temperature and salinity profiles during january and february with conductivity temperature depth measurements indicated that the model captured some of the spatial structure of the observed fields the model response to several bora wind events in january and february showed a recurring pattern of cyclonic and anticyclonic gyres that generally agreed with observations and reflected the pattern of wind stress curl from the coamps wind stress forcing during strong bora forcing two large cyclonic circulation gyres form in the northern adriatic with a smaller anticyclonic circulation between them near the istrian peninsula additionally a couple of large meanders frequently occur within the eastern adriatic current southeast of kvarner bay and these meanders sometimes close to form small gyres

geochronology of high pressure mafic granulite dykes in sw sweden tracking the p t t path of metamorphism using hf isotopes in zircon and baddeleyite although the u pb zircon chronometer has been widely used for dating metamorphism in moderate to high grade rocks it is generally difficult to link the u pb age of zircon to specific metamorphic reactions in this study the initial hf isotopic composition of secondary zircon is compared with the evolution of hf isotopic composition of the bulk sample back projected from the measured value through time this approach may enhance the interpretation of radiometric ages performed on metamorphic mineral assemblages here u pb sm nd and lu hf geochronology and thermobarometry have been integrated and applied to two metamorphosed diabase dykes in the sveconorwegian orogen sw sweden the dykes are located 5 km east of the nne trending gota alv deformation zone in the idefjorden terrane and trend parallel to this zone the lunden dyke is recrystallized into a coronitic granulite facies assemblage u pb isotopic analyses of baddeleyite in this dyke indicate an emplacement age of 1300 ma thermobarometric techniques applied to garnet and omphacitic clinopyroxene coronas indicate high pressure metamorphism at c 15 kbar and c 740 c growth of polycrystalline zircon at the expense of baddeleyite occurred at 1046 6 ma identical hf isotopic composition of polycrystalline zircon and baddeleyite shows that the baddeleyite to zircon transition took place before hf equilibration among the other metamorphic minerals and hence the c 1046 ma age of polycrystalline zircon sets an upper age limit of metamorphism of this sample the haregarden dyke is recrystallized into a granoblastic transitional upper amphibolite to granulite facies assemblage the estimated p t conditions are c 10 kbar and c 700 c analyses of small 30 m clear and round zircon in this sample yield a concordia u pb age of 1026 4 ma which is indistinguishable from the lu hf and sm nd mineral isochron ages of 1027 9 ma and 1022 34 ma this type of secondary zircon plots at the lower end of the lu hf isochron and indicates simultaneous growth with garnet at c 1026 ma a time when hf isotopic equilibrium among minerals must have been reached

active faults in eurasia general remarks abstract the database and map of active faults of eurasia 1 5 000 000 have been compiled according to the ilp project ii 2 world map of major active faults the map shows the complicated distribution of active faulting the major faults group in broad mobile belts between relatively weakly faulted parts of the main plates the faults form the plates microplates and crustal blocks boundaries there and are accompanied by deformation of the boundary areas using active fault data for the seismic hazard assessment estimation of maximum possible magnitudes of earthquakes in fault zones it is necessary to take into account the total neotectonic activity of the territory and average rate and sense of motion on the fault because trenching gives a possibility to determine only vertical earthquake offset on a fault it is necessary to make corrections for strike slip faults to estimate the earthquake magnitude by using this parameter

introduction to special section recent advances in oceanography and marine meteorology of the adriatic sea 1 owing to its strategic location rich fisheries and natural resources the adriatic sea possesses a long history of scientific exploration cushman roisin et al 2001 provides a comprehensive review the two volumes of this adriatic sea special section contribute to this rich literature by collecting results from a recent period of intense research activity during 2002 2003 the combined efforts of several large multi disciplinary international programs brought an exceptional array of observational and numerical resources to bear on contemporary questions in adriatic research supported by the u s office of naval research nato the croatian ministry of science and technology and the italian ministry of the environment and ministry of universities and research scientists from several countries pursued interrelated collaborative research programs broad ranging studies included investigations of circulation at kilometer to basin scales physical and biological response to bora wind forcing sediment transport and mucilage formation extensive numerical efforts contributed to the success of many of these programs providing interpretation and understanding and in some cases extensions into short term forecasting

model based directed drifter launches in the adriatic sea results from the dart experiment abstract a high resolution numerical model of the adriatic sea is used to predict lagrangian coherent structure boundaries quantified by finite size lyapunov exponents fsle for flow features in the region of the gargano peninsula during the course of the dynamics of the adriatic in real time dart observational program fsle fields computed from two day model forecasts of the surface velocity indicate distinct regions of high relative drifter dispersion model predictions of such regions located on available ship tracks were used to direct the launching of pairs of surface drifters on three days during march 2006 with the goal of maximizing coverage of the sampling area for two of the three launches the observed trajectories separated at locations and along directions closely approximated by those predicted from the model fsle fields the third case acted as an inadvertent control experiment model predictions at release time showed minimal fsle structure at the launch locations and the observed drifter pair advected in a coherent fashion for two days while there are considerable differences between individual drifter observations and trajectory envelopes computed from ensembles of synthetic drifters the experiment confirms the model s ability to approximate the location and shape of energetic flow features controlling the near time fate of quasi lagrangian particles overall the combined use of fsles with realistic coastal circulation models appears to be a promising avenue to aid real time directed drifter launches in observational programs

u pb age constraints on the stratigraphy and tectonic history of the avalon terrane new brunswick canada six new u pb ages for avalonian plutonic and volcanic rocks from the eastern caledonia highlands southern new brunswick indicate that magmatic activity occurred between 625 600 ma ago with another probably separate major episode at ca 550 ma this gap in igneous activity is consistent with differences in the metamorphic and deformational character of rocks from the two episodes the 625 600 ma activity was calc alkalic and probably occurred in an andean type magmatic arc it correlates with most volcanism and plutonism in the type avalon terrane of newfoundland the ca 550 ma activity formed bimodal volcanic rocks and high level plutons probably in an extensional setting u pb ages of xenocrystic zircons from avalonian igneous rocks indicate both archean and proterozoic components at depth in avalonian basement an observation consistent with pb isotopic signatures which suggest an old radiogenic crustal source the african craton for the magmatic products

relationship of late quaternary tectonics and volcanism in the khanarassar active fault zone the armenian upland the syunik rhombus like structure in the khanarassar active dextral fault zone of armenia is a typical pull apart basin formed between terminal parts of two adjacent en echelon fault segments some component of subsidence associated with the faults of the structure is found between the en echelon segments nevertheless the dextral component continues to be predominant even on the boundaries of the pull apart basin the late pleistocene and holocene lava volcanoes of the basin are also associated with those faults that have a component of extension the relative ages of fault displacements and volcanic eruptions have been identified by the mutual correlation of lavas moraines and topographic features and by archaeological and radiocarbon dating according to the interpretation of rupturing and volcanism major earthquakes and volcanic eruptions appear inter related and three pulses of such activity during the earlier and middle holocene have been identified

hydrothermal alteration of magmatic zircon related to nacl rich brines diffusion reaction and dissolution reprecipitation processes magmatic zircon from altered gabbros adjacent to the 1 07 ga lala fe cu deposit sw china was modified by nacl rich brines related to the fe cu mineralization the modified zircon grains are composed mostly of both inclusion free and porous domains and some grains also have overgrowth rims the inclusion free domains commonly overgrown by the porous domains are roughly homogeneous under bse imaging but display oscillatory or sector zoning under cl imaging in contrast the porous domains are distinctly mosaic like under cl imaging and contain abundant pores and mineral inclusions such as thorite xenotime ree rich phases actinolite albite biotite and calcite the inclusion free domains have concentrations of non formula elements for example al ca and fe much higher than the magmatic zircon and are thus interpreted to be products of interaction between magmatic zircon possibly metamictized and the fluids via a diffusion reaction process however these resultant domains have retained the th and u contents ree patterns u pb ages and hf isotopes of the precursor magmatic zircon on the other hand the porous domains have stoichiometric end member compositions that is lowered th u ree y and p and given that they are porous and inclusion rich we proposed that they have formed from the precursor element rich inclusion free domains or magmatic zircon via a fluid induced dissolution reprecipitation process it is notable that the porous domains retained the 177hf 176hf ratios of the precursors but possess modified and meaningless u pb ages instead a meaningful u pb age 1006 62 ma similar to the timing of the lala deposit is well recorded by the overgrowth rims with distinctly high lrees la 50 700 ppm and elevated hf isotopic ratios our new results reveal that in the presence of the nacl rich brines the nature of the precursor magmatic zircon that is high element budgets and or metamictization plays a key role on hydrothermal alteration of zircon

adcp observations of the western adriatic slope current during winter of 2001 abstract data from an acoustic doppler current profiler adcp mooring maintained during the winter of 2001 in 57 m of water near senigallia italy are used to describe the winter conditions of the western adriatic current in the northern adriatic sea wind and water temperature measurements from the acqua alta tower in 19 m water depth off venice and winds from a 4 km resolution reanalysis model coamps are used to interpret the adcp data especially with respect to currents generated by bora winds at the adcp site mean depth average current for the winter deployment period was 10 4 cm s toward 140 roughly aligned with local bathymetric contours both mean currents and fluctuations about the means were highly barotropic during the winter southeastward along shore mean currents increased slightly 1 8 cm s with depth between 7 and 47 m following a nearly linear trend offshore cross slope mean currents only increased 2 0 cm s deep in the water column between 35 and 51 m current variability was dominated by four bursts of currents in which flow toward the southeast greatly increased these bursts followed bora winds the timing of which was identified by measured winds at the acqua alta tower depth averaged de tided currents exceeded 30 cm s during all four events and reached 45 cm s during one event correlations between these along shore currents and simulated wind stress from the coamps model were calculated over the whole north adriatic the cross covariance between currents and wind stress had a spatial pattern similar to the bora wind itself reaching maxima along the northern adriatic coast off the southern tip of istria and near ilovik island 14 5 n 44 4 e wind stress in these regions had stronger correlation with currents at the mooring than did wind stress anywhere near the mooring bottom water at the mooring was too warm for it to have been north adriatic dense water except perhaps for a brief pulse of cold water associated with the third current burst on april 1st

vertical structure of bottom ekman tidal flows observations theory and modeling from the northern adriatic 1 from september 2002 to may 2003 fifteen bottom mounted acoustic doppler current profilers measured currents of the northern adriatic basin tidal fluctuations at all seven of the major adriatic frequencies were synthesized from a response tidal analysis of these measurements most observed tidal current ellipses were nearly reversing but near the bottom tidal current ellipses all shortened and broadened semidiurnal currents led upper water column currents and diurnal tidal current ellipse orientations rotated counterclockwise toward the bottom theoretical solutions for a tidally forced bottom ekman layer with vertical eddy viscosity of the form az z k were least squares fit to the observations average values were 3 10 4 m s and k 5 10 4 m2 s the value of k was important in matching tidal orientation and phase changes and a nonzero was important in matching tidal amplitude changes the navy coastal ocean model ncom and the quoddy model were also compared to the observations the average rms errors for the bottom ekman layer were 0 22 cm s for the best fit theory 0 35 cm s for ncom and 0 36 cm s for quoddy az structures from ncom and quoddy show that time variation in az is relatively unimportant for adriatic tides the bottom shear stresses from theory were larger in magnitude than those from the bottom drag formulations in ncom and quoddy

the oceanographic tower acqua alta activity and prediction of sea states at venice abstract the oceanographic tower acqua alta was constructed in early 1970 in the adriatic sea some 20 km from venice the main objective was to obtain information about the meteomarine conditions off the venice lagoon where the old town is threatened by ever more severe floodings the paper describes the tower and its instrumentation and summarizes the experience and results of activities relevant to coastal engineering this encompasses modelling of wind field storm surge and wind generated waves various types of models have been applied to and validated for the adriatic sea and in particular for the coastline of the venice lagoon this includes application of long term statistics of extreme wave heights specific studies of the influence of bottom friction and wind gustiness on waves are presented as well as measurements of kinematics of wind generated waves further a short discussion of white capping is included finally some information about the management of the tower is given

geochronological constraints on late precambrian intrusion metamorphism and tectonism in the anti atlas mountains the bou azzer inlier anti atlas morocco is a critical element for understanding gondwanan geology because it exposes rocks formed during the paleosuturing of the gondwanan margin with peri gondwanan arc terranes numerous intrusions within the inlier allow specific tectonic events associated with the tectonic evolution of the gondwanan margin to be bracketed detailed examination of plutons within the inlier reveal down temperature magmatic to solid state fabrics and contact relationships indicative of emplacement during oblique collision between the margin and one or more arc terranes u pb geochronological data for the ao t abdulla diorite ma and 653 8 1 6 bou offroh granodiorite ma provide a limit to the onset of collision and regional metamorphism a 653 0 1 3 precise age of ma for the ousdrat quartz diorite provides an indication of the lower limit of the collision 640 8 1 4 in the region tectonothermal activity in the anti atlas at ca 650 ma correlates with similar periods of tectonism in the peri gondwanan terranes of avalonia and cadomia and points to the emergence of continent wide tectonism in western gondwana as early as 650 ma

four zircon ages from one rock the history of a 3930 ma old granulite from mount sones enderby land antarctica ion microprobe u th pb analyses of zircons from a granulite grade orthogneiss from mount sones enderby land antarctica record the ages of four principal events in the history of the gneiss three of which already have been recognized through previous isotopic dating of other samples the structure of the zircons indicates at least four different stages of growth the several zircon ages were obtained by grouping the analyses according to the stage they represented in the observed stratigraphic succession of growth and thereby defining separate u pb discordance patterns for each stage the stratigraphically oldest zircon rare discrete cores is indistinguishable in age from the most common euhedrally zoned zircon both crystallized when the tonalitic precursor of the orthogneiss was emplaced into the crust 3927 10 ma ago making the orthogneiss currently the oldest known terrestrial rock the outer parts of most grains and some whole grains recrystallized at 2948 31 17 ma during or immediately after possibly 100 ma of high granulite grade metamorphism the recrystallized zircon was isotopically disturbed by tectonism associated with reactivation of the southern margin of the napier complex at 1000 ma in the intervening time at 2479 23 ma the cores and zoned zircon suffered a major isotopic disturbance involving movement of radiogenic pb which left most of the crystals with radiogenic pb deficiencies but produced local radiogenic pb excesses in others a new generation of zircon characterized by very high th u and low u grew at that time that event deformation and possibly a minor rise in temperature produced widespread perturbations of other isotopic systems throughout the napier complex

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

cao mgo al 2 o 3 sio 2 h 2 o to 35 kb amphibole talc and zoisite dehydration and melting reactions in the silica excess part of the system and their possible significance in subduction zones amphibolite melting and magma fractionation

zircon as a metamorphic timekeeper a case study from the caledonides of central norway abstract during metamorphism inherited or detrital zircon grains commonly serve as a template onto which new generations of metamorphic zircon precipitates charting successive episodes in the thermal history of rocks in addition chemical modification of pre metamorphic zircon can leave an imprint in the u pb systematics we present a case study from a sample of migmatitic amphibole biotite gneiss from central norway that contains zircon grains that chart much of the history of an entire orogenic episode not only through generation of new zircon but also resetting of the u pb system via radiogenic pb loss the investigated gneiss from the helgeland nappe complex within the uppermost allochthon of the scandinavian caledonides records a prolonged metamorphic evolution phase equilibria modelling implies peak metamorphic conditions of 7 5 8 0 kbar and 650 675 c consistent with low degree partial melting and the presence of small volumes of leucosome laser ablation split stream analysis that simultaneously measures u pb ree and lu hf in the same volume of zircon yields a range of analyses that spread along concordia a zircon population between 485 and 443 ma is characterized by faded and blurred oscillatory zoned domains that have gained nonformula elements these analyses are interpreted to indicate the presence of reactive solutions during this time period that led to leaching and mobility of pb another texturally distinct zircon component yields an age of 478 4 ma has typical igneous ree zircon profiles and records low apparent alpha doses interpreted to reflect new zircon growth during migmitization a further population of zircon rims and discrete grains yields an age of 453 2 ma interpreted as a later episode of zircon growth during high grade metamorphism a final set of zircon overgrowths yield an age of 439 3 ma also interpreted to reflect new metamorphic growth many zircon growth phases in this rock have similar ree patterns implying broadly isochemical metamorphism consistent with the evidence for only limited partial melting furthermore the hf isotopic signature in the analyzed zircon grains suggests that there was no release of radiogenic hf during metamorphism the metamorphic zircon in the studied rock records several episodes of growth and or alteration over a period of 40 ma that was related to protracted destruction of the iapetus ocean in the context of regional data the new results suggest that the helgeland nappe complex which is generally considered to be exotic to baltica underwent geographically widespread long lived ordovician high temperature metamorphism this record of metamorphism is distinct from that recorded by tectonostratigraphically lower units which are conventionally regarded to have been derived from baltica the results shed light on the assembly of the scandinavian caledonides and show that zircon from individual samples may be used to unravel complex orogenic histories

the adriatic sea general circulation part ii baroclinic circulation structure abstract in the second part of the paper dedicated to the adriatic sea general circulation the horizontal structure of the hydrographic parameters and dissolved oxygen fields is described on a seasonal timescale maps of temperature and salinity climatological fields reveal the enhanced seasonal variability of the adriatic sea which at the surface is associated with the major dilution effects of river runoff the density and derived dynamic height fields show for the first time the baroclinic geostrophic structure of the general circulation winter is dominated by compensation effects between temperature and salinity fronts along the western coastline the resulting baroclinic circulation is weak and suggests the presence of barotropic current components not accessible by the dataset spring and summer seasons have the smallest spatial scales in the temperature and salinity fields and stronger subbasin scale gyres and current systems which have been classified in a schematic representation of the circu

palaeozoic polymetamorphism in the north qinling orogenic belt central china insights from petrology and in situ titanite and zircon u pb geochronology abstract the qinling orogenic belt experienced multiple phases of orogenesis during the palaeozoic unraveling the timing and p t conditions of these events is the key to understanding the convergence processes between the south china and the north china blocks the songshugou complex located in the southern part of the north qinling orogenic belt has registered multistage metamorphism in palaeozoic and thus potentially provides insights into the tectonic evolution of the qinling orogenic belt in this study three metabasic rocks a garnet pyroxenite a garnet amphibolite and a gneissic amphibolite from the songshugou complex were selected for petrological study and zircon and titanite u pb dating our results show that the metabasic rocks experienced three metamorphic events during the palaeozoic the first metamorphic event m1 is characterized by high pressure conditions two zircon grains in equilibrium with garnet and in absence of plagioclase were recognized from the garnet pyroxenite sample they yielded ti in zircon temperatures of 660 851 c at 12 0 kbar and a weighted mean age of 498 15 ma providing the constraints on the temperature and timing of prograde or peak metamorphism m1 1 zircons that are inequilibrium with garnet from the garnet pyroxenite and the garnet amphibolite gave u pb ages of 494 9 ma and 484 4 ma and ti in zircon temperatures of 793 33 c and 738 18 c respectively thus these zircons were formed on the retrograde amphibolite facies conditions at 8 0 kbar m1 2 titanite inclusions were found in actinolite cores of zoned amphibole from the garnet amphibolite they yielded a u pb age of 470 ma and zr in titanite temperature of 676 23 c at pressure of 7 0 kbar suggesting that the amphibolite facies retrogression perhaps persisted to 470 ma weakly zoned zircons from the garnet amphibolite and inclusion free titanites from the garnet pyroxenite gave consistent u pb ages of 418 5 ma and 423 10 ma and ti in zircon temperature of 742 26 c and zr in titanite temperature of 764 18 c at 7 0 kbar respectively it is suggested that a heating event m2 is registered by a subsequent phase of amphibolite facies metamorphism the ilmenite bearing titanite crystals from the garnet pyroxenite yielded a u pb age of 352 4 ma recording a late thermal event m3 on the basis of combined petrological and geochronological results we propose a revised tectonic model for the north qinling orogeny in palaeozoic the high pressure granulites were formed by the northward subduction of the shangdan oceanic slab and the arc continent collision at ca 500 ma their exhumation happened at ca 494 484 ma as a result of slab breakoff subsequent amphibolite facies metamorphism dated at ca 440 420 ma are coeval with the widespread magmatism in the north qinling terrane which are likely caused by the reinitiation northward subducted of shangdan oceanic slab at ca 350 ma the north qinling terrane was likely affected by another thermal overprinting event