the incorporation of pb into zircon abstract the incorporation of pb into zircons grown from pb rich solutions was evaluated using three different approaches 1 high temperature growth of large crystals from pb silicate melts 2 hydrothermal overplating of thin epitaxial layers on substrates of natural zircon and 3 growth of small homogeneously nucleated crystals from aqueous fluids the melt grown zircons 50 400 m were crystallized from pbo sio2 zro2 p2o5 liquid at atmospheric pressure by cooling from 1430 to 1350 c in the p2o5 free system despite 66 wt pbo in the melt these zircons contain 3 atom pb with apparent zircon fluid partition coefficients of 4 2 and 2 6 respectively for pb4 and pb2 in contrast to the case of melt grown zircons available p is excluded from the aqueous epitaxial zircon suggesting that charge balance is accomplished by h instead small 2 5 m zircons grown by cooling aqueous solutions pbo sio2 zro2 p2o5 from 800 c or 900 c contain 0 25 0 5 atom pb 2 4 wt pbo yielding apparent dpb values of 0 2 0 3 available p5 is incorporated in a 2 1 ratio with pb2 suggesting a specific charge balance mechanism 2p5 pb2 2si4 zr4 however pb enters the zircon even when p is unavailable so h may again play a charge balancing role because of the rapid polythermal modes of zircon growth and the high pb content of the experimental systems the apparent partition coefficients should not be viewed as equilibrium values but as qualitative indicators of pb compatibility under various growth circumstances the overall results are consistent with the low but variable levels of non radiogenic common pb in natural zircons the increased compatibility of pb in fluid grown low temperature zircons suggests a possible fingerprint for zircons from hydrothermal and wet metamorphic rocks i e high concentrations of common pb

40ar 39ar dating of shear zones in the variscan basement of greater kabylia algeria evidence of an eo alpine event at 128 ma hauterivian barremian boundary geodynamic consequences a ductile shearing event is recognized in gneisses and barite veins of the crystalline internal massifs of the alpine maghrebides in algeria dating of shearing movement is carried out by 40ar 39ar laser probe experiments performed on synchronous neoformed micas the proposed age at 128 ma is constrained by a two component mixing model for the saddle shaped age spectra on phengites and by a plateau age at 128 3 0 3 ma on muscovite this hauterivian barremian boundary age records a strong reworking of the alboran plate input of detritals in the flysch basin and surrounding areas it can be correlated with the j magnetic anomaly in the atlantic and with the distensive events in the pyrenees and external alps therefore isotopic ages 80 130 ma obtained on the alpine mediterranean basement could reflect interferences between this event at 128 ma and late alpine stages

petrogenesis of hercynian leucogranites from the southern armorican massif contribution of ree and isotopic sr nd pb and o geochemical data to the study of source rock characteristics and ages abstract ree geochemistry and isotopic pb sr nd o compositions have been studied in up to nine separate leucogranite massifs from three structurally adjacent zones in south brittany ages decrease from 340 ma in the north to 300 ma in the south whereas initial 87 sr 86 sr ratios increase from 0 706 to 0 710 0 717 ree abundances are relatively low la n 20 80 chondrites with a strong enrichment of lree relative to hree la n lu n 13 4 26 6 pb isotope ratios are homogeneous for a given massif but heterogeneous among massifs and plot very close to a normal terrestrial lead growth curve at 300 ma ago 18 o 16 o ratios are high and relatively uniform at 10 7 to 12 8 mean 11 9 u pb studies on zircon indicate the presence of an inherited radiogenic pb component and that the sediment dominated source region had been metamorphosed during the siluro devonian ligerian negativee nd t values imply that the source region had been enriched in lree prior to the melting episode based on the combined data large scale partial melting of crustal material dominantly of sedimentary origin is proposed the model age of the source rocks is not very old 600 700 to 1200 ma the pb and sr isotopic variations observed among different massifs is probably the result of age differences in the source materials of the different leucogranites

cenozoic collision of the lesser antilles arc and continental south america and the origin of the el pilar fault it is proposed that the cenozoic tectonic record of the southern lesser antilles arc and northeastern continental south america can be explained by ongoing right oblique collision between the arc and continent the collision has proceeded by the transport of the leading edge of the arc across the slope and outer shelf of a former north facing passive margin of the south american continent the overriding began in the study region near the gulf of cariaco in eastern venezuela in late eocene or oligocene time and has migrated with a generally se vector suturing has occurred between the arc and continent after the attainment of a critical distance of overlap today s point of suturing lies in the paria peninsula east of there overriding continues major tectonic elements engaged in or created by the collision are the southern lesser antilles magmatic arc forearc basin the araya tobago terrane a south american foreland thrust and fold belt and a foreland basin the araya tobago terrane is thought to consist of sediments of south american provenance that were accreted to the lesser antilles forearc during its transit of an ocean basin and the continental slope and outer shelf the emplacement of the magmatic arc and the araya tobago terrane caused tectonic imbrication of shelf strata to propagate ahead of the arc front as a foreland thrust and fold belt tectonic loading of the shelf also caused subsidence of a major foreland basin on the continentward side of the thrust belt it is proposed the el pilar fault exists between the gulf of cariaco and the paria peninsula as an active right slip fault but not east of paria it is not a throughgoing transform fault between the south american and caribbean plates the el pilar fault exists where the overlapping arc and the continent are sutured and takes up a suture parallel component of convergence between arc and continent the eastern tip of the fault propagates east with the point of suturing reconstructions of the cenozoic collision of the lesser antilles arc and the south american continent suggest that the arc lay somewhat north and west of its present position in the eocene this conclusion differs from that of plate reconstructions that assume that the arc was the leading edge of a caribbean plate that has moved east from pacific longitudes since the eocene

evolution of southern caribbean plate boundary vicinity of trinidad and tobago two processes oblique collision of an island arc and strike slip motion parallel to the south american coast have been suggested as dominant in the tectonic evolution of the southeastern corner of the caribbean field work in the el pilar fault zone of trinidad and interpretation of offshore seismic data lead us to conclude that the dominant process was strike slip motion on five or more major fault systems within a 250 km wide east west trending plate boundary zone extending north to south from grenada to the mouth of the orinoco river during the last 30 m y movements in this fault zone 1 diverted the orinoco river eastward 2 juxtaposed blocks with diverse stratigraphic characteristics and 3 generated secondary extensional compressional and strike slip str ctures including anticlines of northeast to north northeast trend of which the tobago arch the central range of trinidad and the deformed mud diapirs off the southeastern coast of trinidad are the most prominent

boussinesq modeling of wave induced hydrodynamics in coastal wetlands in this paper an improved formulation of the vegetation drag force applicable for the fully non linear boussinesq equations and based on the use of the depth varying higher order expansion of the horizontal velocity in the quadratic vegetation drag law has been presented the model uses the same numerical schemes as funwave tvd but is based on the cactus framework the model is validated for wave height and setup against laboratory experiments with and without vegetation cover the wave attenuation results using the improved formulation were compared with those using the first order reference velocity as well as with analytical solutions using linear wave theory the analytical solution using the depth varying velocity predicted by the linear wave theory was shown to match the model results with the fully expanded velocity approach very well for all wave cases except under near emergent and emergent conditions when the ratio of stem height to water depth is greater than 0 75 and when the ursell ur number is less than 5 simulations during peak storm waves during hurricane isaac showed that vegetation is very effective in reducing setup on platforms and in reducing the wave energy within the first few hundred meters

migration of brines in the basement rocks of the athabasca basin through microfracture networks p patch u deposit canada abstract the unconformity type uranium deposits of the athabasca basin saskatchewan canada are hosted near the unconformity between a middle proterozoic intracratonic sedimentary basin and an archean to paleo proterozoic metamorphic and plutonic basement these deposits which are considered to be the richest u deposits in the world are the result of massive basinal fluid migrations in the basement rocks this study shows that basinal brines have strongly penetrated into the basement not only through faults and major pathways but also by way of dense networks of microfractures which favoured the percolation of fluids down to considerable depths hundred metres below the unconformity and their chemical modification salinity increase by interaction with basement lithologies these processes are one of the major causes of uranium mobility within the basement rocks and the formation of unconformity type mineralization microfracture networks which opened during the basinal brine stage ca 1600 1400 ma are interpreted as sets of mode i cracks corresponding to a specific stage of deformation and occur as fluid inclusion planes after healing the stress field at that stage 1 n130 150 e subvertical partly reopened the earlier microcrack networks 1 n80 110 e and n130 150 e subvertical issued from the trans hudson orogeny late retrograde metamorphic stage ca 1795 1720 ma the circulation of the two types of fluids carbonic and brines occurs thus at two distinct events trans hudson orogeny late retrograde metamorphism for carbonic fluids and maximal burial diagenesis for brines but the same main microfissure geometry was used by the fluids this demonstrates the existence of a similar stress field direction acting before and after the basin formation moreover the brine circulations in the basement acted in a wider volume than the clay rich alteration halo surrounding the u ores generally considered as the main envelope of fluid percolation outside the fault systems the data on the chemistry of the fluids and on the geometry of their migration at various scales emphasise the fundamental role of the basement in the chemical evolution of highly saline brines linked to unconformity related uranium mineralization in the athabasca basin

multiple crust reworking in the french armorican variscan belt implication for the genesis of uranium fertile leucogranites muscovite peraluminous granites mpgs form by partial melting of the continental crust and can be related to metalliferous deposits such as tin tungsten and uranium u metal enrichment in mpgs commonly results from fractional crystallization but the metal contents of the source play a major role for their fertility between ca 320 and 300 ma late carboniferous the french armorican variscan belt was intruded by numerous u fertile mpgs that contain inherited zircon grains with a wide range of ages from archean to carboniferous u pb and hf isotopic data of zircon grains from brioverian to carboniferous sediments cambrian to early carboniferous granitoids and late carboniferous mpgs indicate that the crust of the armorican massif is made up by detritus mainly derived from the west african craton 3500 1600 ma tdm 3 8 2 3 ga grenvillian belt 1200 900 ma tdm 2 7 1 2 ga and avalonian cadomian belt 800 550 ma tdm 2 5 0 8 ga and that the crust was affected by magmatic events at 510 470 ma tdm 1 6 0 6 ga 410 330 ma tdm 1 6 1 ga and 320 300 ma furthermore they reveal that the late carboniferous mpgs were mainly formed by partial melting of brioverian sediments with cambro ordovician and devonian carboniferous granitoids which are all genetically linked with each other and characterized by th u 4 the new data suggest that the u fertile mpgs result from multiple reworking of u rich brioverian sediments deposited ca 550 ma ago on the northern margin of gondwana and partially molten during several paleozoic events causing a successive increase in u content in the middle upper crust

mixing of metamorphic and surficial fluids during the uplift of the hercynian upper crust consequences for gold deposition a detailed geochemical study of fluids from representative quartz sealed faults hosting late hercynian gold concentrations shows that fluids percolating the mineralised faults had two main distinct reservoirs one was a quite shallow and the other rather deep seated both fluids have lost a great part of their original geochemical signature through interactions with host metamorphic formations early fluids present during the primary sealing of the faults by quartz are considered to have effectively equilibrated with the metamorphic pile and then predominantly flowed upwards along the faults they are characterised by ch4 co2 h2o ratios rather typical of fluids equilibrated with graphite and moderate to medium chlorinities with a high br cl ratio the striking feature of the gold bearing quartz is that gold is not synchronous within any quartz deposition and appears located in late microfractures and associated with pb bi sb sulphosalts and sulphides these late stages are characterised by fluids whose salinities decrease to very low values indicating their progressive dilution by waters of more surficial origin in the fault system the long lived activity of the fault favoured the connection between two distinct fluid reservoirs at a critical time during the basement uplift the fluids evolved through two main driving mechanisms which were responsible for the au deposition i decrease in temperature accompanying decompression from supra lithostatic to hydrostatic conditions which yielded in some instances volatile unmixing in the faulted systems ii mixing of the resulting fluids with waters entering the hydrological systems from shallower reservoirs in addition to dilution and fluid mixing which are favourable factors for decreasing the gold solubility the presence of microfractured sulphides could have enhanced gold precipitation through electrochemical processes d 2002 elsevier science b v all rights reserved

probability distribution of wave heights attenuated by salt marsh vegetation during tropical cyclone abstract zero crossing wave heights obtained from the field measurement of random waves propagating through salt marsh vegetation spartina alterniflora during a tropical storm were analyzed to examine their probability distribution wave data significant wave heights up to 0 4 m in 0 8 m depth were collected over a two day period along a 28 m transect using three pressure transducers sampling at 10 hz wave height distribution was observed to deviate from the rayleigh distribution the observed probability densities of the larger wave heights were reduced significantly by vegetation producing wave heights lower than those predicted by the rayleigh distribution assuming rayleigh distributed wave heights for the incident waves to the vegetation patch existing vegetation induced wave attenuation formulations are used to derive a special form of two parameter weibull distribution for wave heights in the inundated wetland the scale parameter of the distribution is theoretically shown to be a function of the shape parameter which agrees with the measurements effectively reducing the proposed distribution to a one parameter type the derived distribution depends on the local parameters only and fits well to the observed distribution of wave heights attenuated by vegetation empirical relationships are developed to estimate the shape parameter from the local wave parameters

muscovite recrystallization and saddle shaped 40 ar 39 ar age spectra example from the blond granite massif central france muscovites from the blond granite west french massif central were dated by the 40ar 39ar single grain method the 40ar 39ar ages obtained vary from 305 5 0 3 ma to 311 3 0 5 ma and most of the age spectra are slightly saddle shaped the analyzed muscovites show phengitic recrystallization under optical microscope observations sem images and electron microprobe chemical analyses it is proposed that the saddle shaped age spectra result from a partial recrystallization which produced three different isotopic reservoirs in the analyzed white mica single grains domains of early muscovite domains of neocrystallized muscovite formed by phengitic and al fe substitutions and low activation energy sites

resetting monazite ages during fluid related alteration abstract fluid related coupled dissolution reprecipitation processes can modify the composition of monazite and other orthophosphate minerals at temperatures well below diffusional closure temperatures it is critical to evaluate the effects of alteration processes on th u pb dates especially because incomplete removal of pb during alteration could lead to spurious dates either older or younger than the actual age of the monazite formation experimental alteration of monazite at 450 mpa and 450 c has resulted in the production of altered domains in the vicinity of the grain rim as well as in the vicinity of inclusions and microfractures without any obvious structural control within grains the composition of the altered domains is relatively constant from domain to domain and from grain to grain thorium was reduced from ca 7 wt tho 2 to ca 2 wt uranium and pb were almost entirely removed from the altered domains while unaltered areas of the monazite grains as well as the original monazite grains before the experiment yield very consistent dates of ca 350 ma dates from the altered domains are consistently near zero the implication is that fluid aided alteration of monazite for the conditions of this experiment has resulted in a near complete resetting of the monazite age at temperatures well below the closure temperature because alteration textures in monazite are relatively common in nature careful analysis of such textures offers a means of recognizing and potentially dating fluid rock interactions

fluid infiltration and volume change during mid crustal mylonitization of proterozoic granite king island tasmania the development of 10 30 m wide mylonite zones at mid crustal depths in late proterozoic granitoids on king island tasmania was associated with pervasive infiltration of low 18o fluids 5 to 7 on the scale of the shear zones syndeformational fluid rock interaction produced substantial differences in mineral composition and bulk rock chemistry among several adjacent shear zones which are hosted by the same granite in a shear zone at cape wickham with a normal slip component changes in whole rock chemistry between granite and mylonites indicate a gain of ca and losses of k and na during deformation which was nearly isovolumetric notable losses of k rb and si occurred during partial retrograde alteration of mylonites near the western margin of this shear zone the alteration suggests a component of up temperature fluid flow in contrast 3 km to the south east in a strike slip shear zone at disappointment bay complete albitization of plagioclase was associated with na gain and ca loss deformation also involved losses of mg and fe up to 60 volume gain occurred during the formation of closely spaced mesoscopic to microscopic quartz veins during mylonitization the substantial silica gain in this as well as in two mylonite zones further south east is interpreted to have been associated with upward flow of aqueous fluids along these shear zones on the basis of a gradient reaction model minimum time integrated fluid fluxes of 106 m3 m2 are estimated for the disappointment bay west shear zone

late collisional granites in the variscan erzgebirge germany the late collisional erzgebirge granites 325 318 ma were introduction emplaced at shallow crustal levels in the variscan metamorphic the erzgebirge is a ne sw trending antiformal structure basement shortly after large scale extension caused by orogenic exposing variscan crystalline basement rocks at the northcollapse these granites comprise mildly peraluminous transitional ern margin of the bohemian massif the region is famous i s types and strongly peraluminous s type rocks which can be among economic geologists for its extensive and highly subdivided into three major groups low f biotite granites low f variable metallic ore deposits which have been mined two mica granites and high f high p2o5 li mica granites the since the middle ages e g stemprok seltmann 1994 highest degree of differentiation is reached in the li mica granites tischendorf forster 1994 and for the classic studies which exhibit strongly elevated concentrations of p f li rb cs in the fields of economic geology mining and metallurgy ta sn w and u but very low ti mg co ni sr ba y zr chemistry and mineralogy e g werner 1791 breithaupt hf th and rare earth elements crystal melt fractionation is the 1849 agricola 1974 dominant process controlling the bulk composition of all groups of as in many of the other variscan provinces in central granites however metasomatic processes involving late stage residual europe granitic rocks make up a large proportion of the melts and high t orthomagmatic fluids became increasingly more exposed rocks in the erzgebirge and a great variety of important in highly evolved units and have modified the abundances compositional and textural types occur fig 1 knowof mobile elements p f li rb cs ba sr in the li mica ledge of the processes involved in the generation and granites particularly isotopic and geochemical characteristics suggest evolution of these silicic magmas is essential for underthat the three granite groups cannot be derived from a common standing heat and mass transport during the variscan precursor magma their discrete compositions are source related orogeny and also for the formation of metallic ore and are attributed to melting of quartzo feldspathic and pelitic deposits several lines of evidence indicate that early crustal lithologies in different proportions granites are common in high temperature sn w deposits are associated with the the central european variscides but the erzgebirge is unusual for emplacement of highly evolved volatile rich granitic the predominance of evolved li mica granites associated with melts tischendorf 1986 forster tischendorf 1992 economically important sn w and u deposits the abundance of stemprok 1993 the role of the granites in formation of li mica granites is attributed to a combination of favourable factors later permian u pb zn and younger ore mineralization 1 low degrees of anatectic melting of crustal protoliths 2 wide f ba is controversial but good evidence exists that the distribution of fertile lithologies rich in large ion lithophile elements vein type u deposits owe their uranium to leaching from and ore elements 3 extended magmatic differentiation by crystal the uraninite rich granites e g tischendorf forster melt fractionation and subsequent autometasomatism 1994

giant uranium deposits formed from exceptionally uranium rich acidic brines the athabasca basin canada is home to some of the world s largest uranium deposits analysis of preserved ore forming fluids and experimental measurement of uranium solubility in analogous solutions show that the giant deposits could have formed relatively rapidly from extremely uranium rich brines under acidic conditions

a preliminary paleomagnetic pole for mid cretaceous rocks from tobago further evidence for large clockwise rotations in the caribbean south american plate boundary zone abstract the island of tobago occupies the eastern end of the central igneous belt of the caribbean south american plate boundary zone volcaniclastic sediment of the albian 100 ma tobago volcanic group and dikes of similar age within it were sampled in two homoclinal sections with different attitudes the mean of virtual geomagnetic poles for 12 sites 25 4 n 24 1 e a95 4 2 is well defined pre tilting and apparently reliable yet is far removed from a similar age reference pole for south america five other paleomagnetic studies of cretaceous rocks from widely separated localities farther west in the plate boundary zone yield pole positions that are remarkably similar to the tobago pole poles obtained from the guajira peninsula of colombia the islands of aruba and bonaire and the caribbean mountains of venezuela are among those that agree with the tobago result the paleolatitudes for study areas within the plate boundary zone are consistent with an origin on or near south america yet the poles throughout the zone are rotated roughly 90 dextral relative motion between the caribbean and south american plates was probably responsible for the rotation

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

fast cooling following a late triassic metamorphic and magmatic pulse implications for the tectonic evolution of the korean collision belt we discuss the evolution of korea in the context of a relatively short lived tectonically induced magmatic and metamorphic pulse that affected large portions of the crust of the peninsula s southern part during the late triassic recent 40ar 39ar single grain laser step heating dates imply a prolonged metamorphic recrystallization between 243 and 220 ma which occurred in distinct phases that were not coeval throughout the peninsula we obtained identical plateau ages between 231 4 0 8 and 228 9 0 8 ma 1 85 95 39ar release on single grains of detrital muscovite from jurassic sandstones gimpo group a literature review shows that the ages of detrital muscovites are identical to 1 concordant 40ar 39ar ages of biotite 228 ma and amphibole 230 ma in amphibolites of the deokjeongri gneiss formation and the weolhyeonri complex pointing to very rapid cooling of 100 150 c ma and 2 231 229 ma muscovite from the low grade metamorphic mid paleozoic turbidites of the taean formation the efficiency of cooling is further underlined by the near coincidence of these 40ar 39ar ages with 243 229 ma average 234 6 ma zircon u pb ages in the gyeonggi massif and the hongseong belt in the literature it is argued that the late triassic magmatic and metamorphic pulse is superimposed on an earlier tectono metamorphic event possibly related to collision indicated by 1 243 237 ma muscovite ages or age components in age spectra and 2 two generations of folds and associated tectonic foliations truncated by 229 5 ma old syenites and earlier mafic dykes the late triassic thermal pulse could have been the result of post collisional delamination of the lower crust and uppermost mantle and or oceanic slab break off which is also suggested by almost coeval widespread mantle sourced mg rich potassic magmatism continuing ductile deformation is shown by mylonitization of late triassic magmatic rocks an 220 ma muscovite age may be related to this

in situ oxygen isotope analysis of monazite as a monitor of fluid infiltration during contact metamorphism birch creek pluton aureole white mountains eastern california monazite from the hydrothermal aureole of the cretaceous two mica birch creek plu ton in the white mountains of eastern california records the infiltration of magmatic fluids into the metasedimentary early cambrian deep spring formation monazite in the birch creek pluton displays concentric euhedral magmatic zoning 18 o 8 7 0 2 and th pb magmatic ages of 78 0 0 7 ma the middle deep spring formation 0 5 km east of the contact underwent moderate to low temperature alteration by f rich mag matic fluids monazite displays patchy zoning but has similar 18 o values 8 7 0 4 and th pb ages 78 3 1 6 ma to monazite in the birch creek pluton in contrast monazite from the upper deep spring formation 0 6 km west of the contact and outside the mapped hydrothermal zone shows concentric zoning 18 o 5 2 0 3 and partially reset detrital ages from 583 to 1069 ma deep spring formation monazite within the hydrothermal alteration zone dissolved and reprecipitated during magmatic fluid infiltra tion whereas monazite outside the zone was unaffected in contrast deep spring zircon within the hydrothermal alteration zone preserved its magmatic zoning and cambrian precambrian u pb ages zircon can reliably date events prior to hydrothermal activity whereas monazite being more susceptible to alteration by fluids is useful for mapping the extent and timing of fluid infiltration events

two stage partial melting during the variscan extensional tectonics montagne noire france one of the striking features that characterise the late stages of the variscan orogeny is the development of gneiss and migmatite domes as well a s extensional late carboniferous and permian sedimentary basins it remains a matter of debate whether the formation of domes was related to the well documented late orogenic extension or to the contractional tectonics that preceded migmatization and magmatism are expected to predate extension if the domes are compression related regional anticlines but they must both precede and be contemporaneous with extension if they are extensional core complexes in the montagne noire area s outhern french massif central where migmatization magmatism and the deformation framework are well documented the age of the extensional event was unequivocally constrained to 300 0 ma therefore dating migmatization in this area is a key point for discriminating between the two hypotheses and understanding the late palaeozoic evolution of this part of the variscan belt for this purpose a migmatite and an associated anatectic granite from the montagne noire dome were dated by la icp ms u th pb on zircon and monazite and laser probe 40 ar 39 ar k ar on muscovite although zircon did not record any variscan age unequivocally related to compression 380 330ma two age groups were identified from the monazite crystals a first event at ca 319 ma u th pb on monazite is interpreted as a first stage of migmatization and as the emplacement age of the granite respectively a second event at ca 298 295 ma recorded by monazite u th pb and by the muscovite 40 ar 39 ar system in the migmatite and in th e granite could be interpreted as a fluid induced event probably related to a second melting event identified through the syn extensional emplacement of the nearby montalet leucogranite ca 295 ma ago the age s of these two events post date the variscan compression and agree with an overall extensional context for the development of the montagne noire dome shaped massif comparison of these results with published chemical epma dating of monazite from the same rocks demonstrates that the type of statistical treatment applied to epma data is crucial in order to resolve different monazite age populations

filling sequence in late paleozoic continental basins a chimera of climate change a new light shed given by the graissessac lodeve basin se france abstract the characteristics of sedimentary deposits in continental basins especially those controlled by extension are often used to infer global climatic changes we question the relevance of such an approach because the stratigraphic pattern of those basins are mostly similar whatever their latitudes paleolatitudes i e their general climatic conditions namely the transition from 1 an ancestral drainage network to 2 deep lake with a dominance of black and grey shale deposits 3 shallow oxic lake with a dominance of red pelites deposits and to eventually 4 a new drainage network such transitions observed in late palaeozoic continental basins throughout laurasia and gondwana are generally interpreted as evidence of global change from a wet climate to an arid climate from the sedimentological and structural analysis of a late palaeozoic basin the graissessac lodeve basin in the south of the variscan french massif central we proposed a new tectono stratigraphic sequence of this basin from the late carboniferous to the early trias we estimate the paleo area from 295 450 km 2 to 880 1150 km 2 of the basin and we deduce a subsidence curve from the late carboniferous to the late permian from 600 200 m my to 100 10 m my we conclude that the graissessac lodeve basin stratigraphic pattern corresponds to a classical continental rift sequence controlled by tectonics in order to assess a possible control of the stratigraphic sequence by the climate we have reconstructed the paleo depth of the lake with 1 the morphometric analysis of the wave ripples and 2 with an estimation of the lacustrine slope considering the tectonic origin of the lake we find that the maximum lacustrine depth has evolved from a range of 80 300 m during the late carboniferous to 40 45 m during the late permian from our estimates of the different configurations of the basin we show that the volume of the water in the lake has remained nearly constant between 10 and 30 km 3 from the late carboniferous 300 my to the late permian 250 my eventually we conclude that the sedimentological indicators of shallowing and episodic drying observed from the late carboniferous to late permian in this basin and probably in all the pangea basins of the same type are the consequence of the basin enlargement by tectonics and its sedimentary filling rather than climatic change

preservation of sub microscopic structural relicts in micas from the gran paradiso massif western alps implications for 40ar 39ar geochronology abstract permian igneous biotite and white mica that were re heated at 500 550 c in the middle late eocene were investigated by laser step heating and in situ 40ar 39ar techniques in conjunction with scanning electron microscopy sem electron microprobe emp and transmission electron microscopy tem to assess the influence of short lived metamorphism on k ar systematics large intra and inter grain age variations ranging from the triassic to the late eocene are primarily related to the extent of metamorphic re equilibration brown biotite porphyroclasts from weakly re equilibrated samples are characterized by core to rim zoning in major element composition mineral structure and 40ar 39ar ratios titanium concentration and fe mg fe ratios decrease from crystal cores towards the rims which are compositionally indistinguishable from green biotite aggregates in metamorphic coronas compositional changes are coupled with structural modifications towards the crystal edges where the disordered igneous 1m 2m1 stacks are replaced topotactically by highly ordered metamorphic 1m polytypes apparent ages up to 66 ma were determined in crystal cores by in situ laser probe analyses while ages as young as 45 ma are typical of crystal rims step heating experiments of these zoned biotites yielded a discordant saddle shaped age spectrum with a concordant central segment yielding an error weighted mean age of 44 9 0 3 ma biotite porphyroclasts from a more re equilibrated specimen gave markedly different results with uniform major elements compositional profiles homogeneous argon distribution throughout the crystals and a flat age spectrum at 36 5 0 3 ma tem investigations of these optically homogeneous porphyroclasts revealed that the original igneous structure had been replaced by highly ordered 1m polytypes during alpine metamorphism white mica yielded comparable results with spot ages up to 218 ma in domains characterized by high na na k ratios and low si and ti content representing the best preserved igneous relicts significantly younger apparent ages down to 44 39 ma are typical of domains affected by partial metamorphic re equilibration at high pressure conditions as indicated by increasing si and ti contents or by exhumation related exsolution of amoeboidal quartz these results indicate that different intra grain and inter grain ar concentration patterns arise primarily from the relative amount and distribution of relict igneous and metamorphic crystal chemical domains core to rim age gradients in biotite result from inward directed recrystallization from the crystal edges eventually leading to flat age profiles as metamorphic replacement of pre existing porphyroclasts runs to completion this study calls for caution when estimating thermal histories from apparent age gradients or from age vs grain size correlations unless the presence of pre metamorphic mineral relicts can be ruled out

dating microstructures by the ar 40 ar 39 step heating technique deformation pressure temperature time history of the penninic units of the western alps abstract white micas from the penninic units of the urtier valley in the western alps have been dated by the 40 ar 39 ar step heating method a strong correlation is observed between measured ages mica composition and deformation fabrics samples characterized by a single mineral reservoir yielded relatively flat age spectra allowing to estimate an age of 44 4 0 4 ma for the formation of phengite bearing fabrics in eclogite facies conditions s hp and an age of 42 0 0 4 ma for the microstructurally younger muscovite bearing shear fabrics which formed in greenschist facies conditions sz gr1 several micaschist samples are characterized by the overprint of the phengite bearing s hp fabric by a younger muscovite bearing fabric f u that is axial planar to tight upright folds that post date the sz gr1 shearing event bse images reveal that the different generations of syn kinematic micas are not exclusively tied to their respective fabric but muscovite is commonly found also in the s hp microlithons while relict phengite is often transposed into the new f u axial planar fabric all the samples containing these two generations of white micas yielded convex upward spectra which following considerations based on the 36 ar 40 ar vs 40 ar 39 ar plots have been interpreted to indicate mixing of two argon reservoirs of different ages related to the older phengite and the younger muscovite visible with optical and electron microscopy this method allowed to estimate a maximum age of ca 36 5 0 5 ma for the folding of the shear related structures the ages estimated for s hp and sz gr1 in the study area are consistent with those obtained in other parts of the penninic units with other geochronological techniques therefore the post crystallization loss of argon from the phengite associated with s hp and the muscovite defining sz gr1 is negligible despite evidence indicating that conditions of t 480 510 c were reached during the later upright folding event

metal mobility during hydrothermal breakdown of fe ti oxides insights from sb au mineralizing event variscan armorican massif france hydrothermal alteration related to sb au mineralization is widespread in the variscan armorican massif but mineral replacement reactions are not well characterized in particular the hydrothermal breakdown of ilmenite titanohematite based on petrography electron probe micro analyzer and laser ablation inductively coupled plasma mass spectrometer analyses we document mineralogical change at rock and mineral scale and the redistribution of sb and others trace elements during the recrystallization of ilmenite titanohematite to hydrothermal rutile hydrothermal alteration is mainly potassic with associated carbonation the replacement mechanism is interpreted to be an interface coupled dissolution reprecipitation process results show that mn zn co ni sn mo and u are released during hydrothermal alteration whereas sb and w are incorporated in newly formed hydrothermal rutile from the hydrothermal fluid furthermore the concentration of sb evolves through time suggesting a change in fluid composition likely related to an enrichment of fluid in sb during rutile crystallization considering that fe ti oxides breakdown during hydrothermal alteration is common within epithermal and mesothermal orogenic au sb mineralizing systems results report in this study yield important constraints about metal mobility and exchanges in hydrothermal gold systems

regional scale cretaceous albitization in the pyrenees evidence from in situ u th pb dating of monazite titanite and zircon abstract albitization is a common metasomatic process active in various geodynamic contexts in the northern pyrenees there are several occurrences of albitites but until now only one occurrence has been dated 117 ma ar ar dating this paper presents new u th pb ages for several albitite occurrences throughout the pyrenees to test whether they are contemporaneous and if so to specify the regional extent of the albitization event ages obtained from large euhedral titanite and monazite grains from distinct albitites are 110 8 and 98 2 ma respectively the zircon u th pb isotopic system did not record this cretaceous metasomatic event even when grains were selected in metasomatically zr enriched rocks or in hydrothermal structures millimetre sized veins cross cutting granitoids we argue that the total time span of 20 ma recorded by albitites corresponds to a long lived hydrothermal system that was active during the rotation of iberia around europe along the north pyrenean fault because albitization and talc mineralization have the same spatial and temporal distribution in the pyrenees we argue that these two metasomatic phenomena are two independent records of this single regional scale long lived hydrothermal system

on the loss of 40ar from muscovite during polymetamorphism abstract recent laser studies have documented 40ar concentration gradients in micas that correspond to age variations of tens to hundreds of millions of years a fundamental problem to interpretation of such variations is to evaluate whether they represent loss via lattice diffusion or alternatively loss during chemical reactions and deformation new data of this study and comparisons with previous work suggest that muscovite is far less prone to lose accumulated 40ar via lattice diffusion during overprinting events than has been inferred previously kyanite muscovite assemblages in proterozoic schist along the eastern flank of the green mountain massif central vermont appalachians formed during proterozoic metamorphism at ca 1 1 ga and were remetamorphosed in the greenschist facies during both taconian ca 465 ma and acadian ca 390 ma events the overprinting events resulted in deformation of earlier kyanite and muscovite growth of chemically and isotopically distinct rims on earlier muscovite and growth of neoblastic muscovite and chloritoid with each event at inferred minimum temperatures of ca 425 c laser spot analyses of the grenville muscovite range from 988 11 ma in the centers of porphyroclasts to 397 7 ma along rims ages younger than 750 ma occur only on the deformed edges of porphyroclasts and in highly deformed areas within crystals parts of early formed muscovite that escaped late deformation and chemical reaction seem remarkably retentive of accumulated 40ar laser spot analyses along the outer 100 m of an undeformed growth face yield ages close to the timing of proterozoic metamorphism and cooling the present study suggests episodic heating events are ineffective for producing measurable diffusive argon loss along the edges of large muscovite crystals ca 1 mm in diameter particularly in comparison with deformation and reaction mechanisms and are unlikely to result in complete 40 ar 39 ar age resetting of such crystals the results of this study further emphasize that the argon diffusivity in muscovite is lower than commonly considered in previous work

the effects of retrograde reactions and of diffusion on 40ar 39ar ages of micas the effects of metamorphic reactions occurring during decompression were explored to understand their influence on the ar ar ages of micas monometamorphic metasediments from the lepontine alps switzerland reached lower amphibolite facies during the barrovian metamorphism related to the collision between european and african adria continental plates mineral assemblages typically composed of garnet plagioclase biotite muscovite and paragonite or margarite were screened for petrological equilibrium to focus on samples that record a minimum degree of retrogression x ray diffraction data indicate that some mineral separates prepared for ar ar stepwise heating analysis are monomineralic whereas others are composed of two white micas muscovite with paragonite or margarite or biotite and chlorite in monomineralic samples ar ar and ar ar proportional to ca k and cl k ratios did not change and the resulting ages can be interpreted unambiguously in mineral separates containing two white micas ca k and cl k ratios were variable reflecting non simultaneous laboratory degassing of the two heterochemical ar reservoirs these ratios were used to identify each ar reservoir and to unravel the age in a chlorite margarite biotite calcschist equilibrated near 5608c and 0 65 gpa biotite margarite and muscovite all yield ages around 18ma at slightly higher grade 560 5808c 0 8 0 9 gpa the assemblage muscovite paragonite plagioclase is in equilibrium and remains stable during retrogression in this case muscovite and paragonite yield indistinguishable ages around 16 5ma above 5908c paragonite was mostly consumed to form plagioclase 45908c whereby the relict mica yields an age up to 5 6 ma younger than muscovite this partial or total resetting of the ar clock in paragonite is interpreted to reflect plagioclase growth during decompression where biotite is present within this same assemblage it systematically yields a younger age than muscovite by 0 5 2 ma however these biotites all show small amounts of retrograde chlorite formation we conclude that even very minor chloritization of biotite is apparently a more effective process than temperature in resetting the ar clock as is the formation of plagioclase from paragonite decomposition multi equilibrium thermobarometry is an excellent means to ensure that equilibrium in investigated samples is preserved and this helps to obtain geologically meaningful metamorphic ages however even samples passing such equilibrium tests may still show retrograde effects that affect the ar retention of micas a more robust interpretation of such ar ar results may require use of a second geochronometer such as u pb on monazite

excess argon in k ar and ar ar geochronology the k ar and ar ar dating techniques occasionally produce anomalously old ages attributed to excess argon and such data is often rejected as not offering any insight into the age thermal history or geochemistry of the rock however improvements in the quantification of argon geochemistry now provide a framework to model excess argon in both open and closed systems solubility data for argon in hydrous fluids melts and emerging data for minerals can be used to understand the behaviour of excess argon and provide valuable insights into the environment in which the samples cooled to their argon retention or closure temperature treating excess argon as a trace element also throws light on its behaviour in minerals above the closure temperature in deeply buried dry systems such as eclogites blueschists granulites and even in the lithospheric mantle extremely low partition coefficients between k feldspar and hydrous fluid phases predict lower excess argon susceptibility than micas and this is observed in fluid poor systems variation of partition coefficients can lead to excess argon in fluids being introduced into minerals or removed from minerals as grain boundary fluids change during flow through a rock however excess argon can also be introduced or removed from minerals by varying temperature without the need for fluid flow high mineral melt and mineral fluid partition coefficients are also the reason why excess argon is often concentrated in inclusions within minerals partition coefficients between minerals and hydrous fluids as low as 10 6 lead fluid inclusions to dominate the radiogenic argon budget particularly in low potassium minerals melt inclusions are less dominant but become critical in dating younger samples

metallogenesis of the french part of the variscan orogen part ii time space relationships between u au and snw ore deposition and geodynamic events mineralogical and upb data abstract mineralogical geochemical structural and isotopic u pb studies were carried out on representative uranium and gold deposits from the french part of the variscan orogen results are compared with complementary data from the literature in order to propose a reconstruction of the metallogenesis in the variscan terranes with a particular emphasis on u deposits the data show that the early metal concentrations or pre enrichments in the terranes are affected by successive metallogenic stages which caused significant metal transport and deposition thus the significant geodynamic events which affected the variscan terranes were responsible for multistage metal deposition or alteration of the early concentrations and for the superimposed features of the metal deposits u pb geochronological results complemented by a compilation of geochronological data on u and pb zn ba f deposits show that three major stages of u deposition took place in western europe in the periods 325 300 ma 290 260 ma and 190 170 ma in addition small uranium mineralization events have been dated at 425 ma 90 110 ma and as tertiary about 40 ma the permo stephanian stage is associated with late brittle deformation associated with major shear zone movements and constitutes the major stage of u concentration the jurassic phase has a prominent part in the hydrothermal remobilization of uranium and in f ba pb zn deposition throughout western europe especially in the eastern part of the massif central other stages correspond to subordinate uranium migration within variscan terranes during earlier or later geodynamic phases the silurian metamorphism in vendee and the mesozoic to tertiary tectonic movements related to the pyrenean or alpine orogenies in the south eastern part of the massif central and in the alps data available on au and sn w as sb deposits have been considered for comparison with the u metallogenesis stages related to the deposition of such metals display significant similarities with those related to uranium deposition they occurred mostly during visean to permian events related to the intrusion of the late visean sn w and late carboniferous sn w li f granitoids and to the late brittle deformation stages which affected the hercynian basement in the vicinity of major shear zones au as sb however the significant discrepancies between the physical chemical conditions of solubilization transport and deposition of the metal led to distinct deposit location independently of the metal source location within the terranes

continental crust formation seen through the sr and nd isotope systematics of s type granites in the hercynian belt of western france the isotopic compositions of sr nd and pb in leucogranites which are intercorrelated bernard griffiths et al 1985 1 may be explained by the mixing of ancient basement 1800 ma with juvenile crust late precambrian or early palaeozoic this hypothesis does not involve the existence of mid proterozoic crust as apparently indicated by thetdm model ages of the leucogranites ranging between 1600 and 1100 ma the nd isotopes reveal the crustal reworking while sr isotopes mainly record juvenile crust formation this paradox is explained by the geochemical heterogeneity of the sources involved

petrochronology and hygrochronology of tectono metamorphic events abstract u th pb petrochronology is based on the incontrovertible fact that the diffusion of radiogenic pb is negligibly small relative to retrograde reaction rates multi element maps demonstrate that patchy textures tightly correspond to u th pb age variations requiring that fluid induced dissolution reprecipitation is the principal cause of pb mobility attempts to model intracrystalline core rim pb zonations as diffusive transport are not legitimate unless genuine bell shaped diffusion profiles in minerals can be documented which happens only exceptionally monazite and zircon intra grain age maps confirm that coupled dissolution reprecipitation and retrogression reactions assisted by fluids control th u pb ages not temperature the chemical zonations observed in many th u bearing mineral chronometers e g monazite allanite xenotime zircon provide petrological constraints linking petrology with textures and the isotope record allows reconstructing entire segments of the p t a x d t history of a rock and its geodynamic environment the dearth of mathematically sound diffusion profiles equally applies to the isotope records of micas and feldspars the tight link between petrology microtextures chemical composition and geochronology also pertains to rb sr and k ar overdetermined multi mineral rb sr isochrons with excess scatter and spatially resolved stepwise release 39ar 40ar results demonstrate ubiquitous correspondence between relict phases and isotopic inheritance many rock forming minerals are highly retentive of sr and ar unless they are obliterated by retrograde reactions the rates of dissolution in fluid controlled reactions are several orders of magnitude faster at upper and mid crustal levels than diffusive reequilibration rates thus as a rule rb sr and k ar chronometers date their own formation accurately establishing p t paths of monometamorphic rocks requires assessing petrologic equilibrium using multivariate thermodynamic software dating complex parageneses of polymetamorphic unequilibrated rocks requires labor intensive disentangling by i qualitative identification of relicts retrogression reactions and chemically open systems by imaging techniques e g cathodoluminescence element maps etc ii microchemical analyses at the m scale quantifying heterochemical disequilibrium phases and assigning them to a p t a x segment iii spatially resolved stepwise release relating the chemical signature of the analyzed mineral to its age k ar and rb sr usually provide a different perspective on the p t evolution of a rock than does th u pb as k rb rich minerals phyllosilicates and especially feldspars mostly form later and react dissolve faster in the retrograde path than u rich accessory phases the present paper reviews these general principles by means of well understood examples both successful and unsuccessful in matching the independently known external constraints

tectonic record magmatic history and hydrothermal alteration in the hercynian guerande leucogranite armorican massif france abstract the guerande peraluminous leucogranite was emplaced at the end of the carboniferous in the southern part of the armorican massif at the scale of the intrusion this granite displays structural heterogeneities with a weak deformation in the southwestern part whereas the northwestern part is marked by the occurrence of s c and mylonitic extensional fabrics quartz veins and pegmatite dykes orientations as well as lineations directions in the granite and its country rocks demonstrate both e w and n s stretching therefore during its emplacement in an extensional tectonic regime the syntectonic guerande granite has probably experienced some partitioning of the deformation the southwestern part is characterized by a muscovite biotite assemblage the presence of restites and migmatitic enclaves and a low abundance of quartz veins compared to pegmatite dykes in contrast the northwestern part is characterized by a muscovite tourmaline assemblage evidence of albitization and gresenization and a larger amount of quartz veins the southwestern part is thus interpreted as the feeding zone of the intrusion whereas the northwestern part corresponds to its apical zone the granite samples display continuous compositional evolutions in the range of 69 8 75 3 wt sio 2 high initial 87 sr 86 sr ratios and low e nd t values suggest that the peraluminous guerande granite a cnk 1 1 was formed by partial melting of metasedimentary formations magmatic evolution was controlled primarily by fractional crystallization of k feldspar biotite and plagioclase an20 the samples from the apical zone show evidence of secondary muscovitization they are also characterized by a high content in incompatible elements such as cs and sn as well as low nb ta and k rb ratios the apical zone of the guerande granite underwent a pervasive hydrothermal alteration during or soon after its emplacement u th pb dating on zircon and monazite revealed that the guerande granite was emplaced 309 7 1 3 ma ago and that a late magmatic activity synchronous with hydrothermal circulation occurred at ca 303 ma these new structural petrological and geochronological data presented for the guerande leucogranite highlight the interplay between the emplacement in an extensional tectonic regime magmatic differentiation and hydrothermal alteration and provide a general background for the understanding of the processes controlling some mineralization in the western european hercynian belt

close spatial relationship between plutons and shear zones this paper explores the relationship between granitic plutons and transcurrent shear zones using examples from the borborema province northeastern brazil syntectonic plutons of this province generally have en cornue shapes and were emplaced preferentially within or at the margins of wide regional scale transcurrent shear zones particularly where shear zones intersect major lithological boundaries it is argued that the plutons shapes and their closeness to shear zones are indicative of a causal relationship between granite plutons and shear zones with the support of numerical models it is further argued that pluton emplacement in transcurrent terranes is controlled by low mean pressure magma traps at the shoulders of shear zones resulting from strain incompatibility caused by regional variations in rheological properties

palaeozoic oolitic ironstone of the french armorican massif a chemical and structural trap for orogenic base metal as sb au mineralisation during hercynian strike slip deformation in the saint aubin des ch teaux quarry armorican hercynian belt western france an epigenetic hydrothermal alteration affects an oolitic ironstone layer intercalated within the lower ordovician gres armoricain formation the hydrothermal overprint produced pervasive and massive sulphidation with stratoid pyritised lenticular bodies within the oolitic ironstone layer these sulphide lenses are spatially associated with strike slip faults and extend laterally from them after the massive sulphidation stage fe as stage 1 subsequent fracturing allowed the deposition of base metals stage 2 and pb sb au stage 3 parageneses in veins the dominant brittle structures are vertical extension veins conjugate shear veins and strike slip faults of various orders all these structures are filled with the same paragenetic sequence deformation analysis allows the identification of structures that developed incrementally via right lateral simple shear compatible with bulk strain affecting the central armorican domain each increment corresponds to a fracture set filled with specific parageneses successive hydrothermal pulses reflect clockwise rotation of the horizontal shortening direction geothermometry on chlorite and arsenopyrite shows an input of hot hydrothermal fluids maximum of 390 350 c during the main sulphide stage 1 the subsequent stages present a marked temperature drop 300 275 c lead isotopes suggest that the lead source is similar for all hydrothermal stages and corresponds to the underlying neo proterozoic basement lead isotope data relative ages of deformation and comparison with neighbouring deposits suggest that large scale fluid pulses occurred during the whole hercynian orogeny rather than pulses restricted to the late hercynian period the vicinity of the hercynian internal domain appears as a key control for deformation and fluid flow in the oolitic ironstones which acted as a chemical and structural trap for the hydrothermal fluids the epigenetic mineralisation of saint aubin des ch teaux appears to be very similar to epigenetic sulphidation described in banded iron formation hosted gold deposits

isotopic systematics o h c sr nd of superimposed barren and u bearing hydrothermal systems in a hercynian granite massif central france abstract the hercynian 325 ma granitic complex of saint sylvestre experienced two hydrothermal episodes the first one leading to the formation of a typical alteration facies called episyenite mainly characterized by the dissolution of granitic quartz the second one leading to the deposition at 280 ma of a major u deposit the present study documents the isotopic characteristics of these hydrothermal events 18 o 16 o d h and 13 c 12 c systematics demonstrate that barren episyenites were generated during hydrothermal interaction of large amounts of mainly meteoric water with the granite the ore depositing fluid equilibrated in a sedimentary formation before entering the granite episyenitization was dated at 301 2 ma which is 25 ma later than the granite emplacement the rb sr isochron relationship implies isotopic equilibrium within the fluid phase and between the fluid and the host rocks which confirms the high water rock ratio inferred from o h isotope systematics a coupled nd and sr isotope investigation demonstrates that at least nd was reworked from the granite in the episyenite genesis but sr and possibly nd had an exotic origin consistent with the inferred sedimentary origin during the ore forming stage the present study emphasizes the fact that spatially superimposed hydrothermal events in a granitic environment can be 1 disconnected from the magmatic history of the granite itself and 2 mutually disconnected from chronological and chemical points of view

tobago west indies a fragment of a mesozoic oceanic island arc petrochemical evidence the pre cenozoic rocks of tobago west indies represent an exceptional cross section through a portion of a composite mesozoic arc system other parts of which may be found in the villa de cura klippe the aruba blanquilla island chain and perhaps in the western cordillera of colombia and ecuador the geochemical data from tobago presented in this paper provide a basis from which a regional view of the petrologic character of the accreted mesozoic arc of the southern caribbean mountain system can be deduced the mesozoic rocks of tobago can be divided into four main groups the north coast schist the ultramafic to mafic plutonic complex the tobago volcanic group and late mafic dykes rocks of the north coast schist served as wallrock for the plutonic volcanic dyke complex which evolved during the mid cretaceous geochemical and nd and sr isotopic data presented demonstrate the oceanic island arc character of all the pre cenozoic rocks of the island all suites span the calc alkaline and tholeiitic regions on discrimination diagrams rare earth element ree patterns for the north coast schist tobago volcanic group and plutonic suite are essentially identical and are moderately lree enriched la yb 1 1 2 7 the dykes exhibit the most unevolved ree patterns and are interpreted as most closely representative of parental liquid compositions initial nd 6 6 to 9 4 and 87 sr 86 sr 0 70303 0 70364 for all samples except north coast schist and tobago volcanic group argillites are similar in value and overall range to other oceanic island arcs limited isotopic data from argillite samples from tobago suggest an important change in the continental derived sediment contribution during mesozoic arc growth nd isotopic data from north coast schist siliceous argillite indicate virtually no continental detritus in this neocomian sediment while nd isotopic data from the tobago volcanic group argillite suggest a definite continental contribution during albian arc growth

breaking probabilities for dominant surface waves on water of finite constant depth this paper extends our previous study of the breaking probability of dominant deep water gravity surface waves into the finite water depth environment it reports a unified behavior of the mean breaking statistics once the effects of finite water depth are taken into account the shallow water wave data that form the basis of this study were acquired at a field experiment site at lake george new south wales australia the breaking events were detected through visual observation of videotaped records of the wave field in combination with acoustic signatures of the breaking waves from a collocated hydrophone following banner et al 2000 we argue that when constant finite depth bottom influence is operative nonlinear hydrodynamical effects associated with energy and momentum fluxes within deforming wave groups remain the primary determinant of breaking onset this underpins our proposed finite depth water parameterization for the environmental dependence of dominant wave breaking probability given by the average number of breakers passing a fixed point per dominant wave period the additional influence of bottom interaction with the wind drift current shear and wind forcing are also included in our finite constant depth formulation this is a natural extension of our recently proposed deep water dependence and reduces to it as the significant wave height becomes much smaller than the water depth in common with the deep water case we propose that there exists a threshold of the significant peak steepness below which negligible dominant wave breaking occurs the available data show encouraging agreement with our proposed dependence with a correlation coefficient approaching 0 9

oceanic crust of the grenada basin in the southern lesser antilles arc platform seismic refraction data permit the southern lesser antilles arc and surrounding regions to be divided by the velocity of their basement we propose that high velocity basement of the arc platform beneath the grenadine islands and below a part of the tobago trough forearc basin is oceanic and continuous and was originally connected with oceanic crust of the grenada basin low velocity basements of the tobago terrane and the arc platform from st vincent north lie south and north respectively of the high velocity basement of the arc platform an oceanic origin of this high velocity crust in the grenadines is argued to be more plausible than an origin as unroofed lower arc crust the segment of probable oceanic crust in the arc platform was greatly uplifted during development of the present island arc mainly in late neogene time relative to the grenada basin and tobago trough accepting the proposition of shallow oceanic crust in the grenadines early middle eocene and possibly older pillow basalts of mayreau the oldest rock unit of the southern lesser antilles arc platform may be an exposure of such basement major and minor element compositions of mayreau basalt are indicative of a spreading rather than arc origin the stratigraphy of the pillow basalts indicates extrusion in an open marine environment distant or shielded from sources of arc or continental sediment followed by a period of pelagic sedimentation above the carbonate compensation depth the eocene basalt and pelagic cover formed a relatively deep floor of a marine basin in which arc derived turbidites and pelagic sediments accumulated over the succeeding 25 30 ma such basalts thus indicate a probable spreading origin of the grenada basin and an age of cessation of spreading in the region of mayreau in eocene time the configuration of the eocene basin and the direction of spreading however are unknowns regional structural relationships imply the spreading was probably backarc an origin also hinted at by ce zr ratios of mayreau basalt although the aves ridge probably was the remnant arc the existence and position of an early middle eocene frontal arc are unresolved the early paleogene tectonic architecture of the southeastern caribbean differed substantially from that of the neogene

primary uranium sources for sedimentary hosted uranium deposits in ne china insight from basement igneous rocks of the erlian basin carboniferous permian triassic and jurassic igneous basement rocks around the erlian basin in northeast china have been investigated through detailed mineralogical whole rock geochemistry geochronological data and sm nd isotope studies carboniferous permian biotite granites and volcanic rocks belong to a calc alkaline association and were emplaced during the late carboniferous early permian 313 1 286 2 ma these rocks are characterised by positive end t 3 3 5 3 and fairly young t dm model ages 485 726 ma suggesting a dominant derivation from partial melting of earlier emplaced juvenile source rocks triassic biotite granites belong to a high k calc alkaline association and were emplaced during the middle triassic 243 3 233 2 ma their negative end t 2 to 0 1 and higher t dm model ages 703 893 ma suggest a contribution from precambrian crust during the magma generation processes leading to a strong enrichment in k and incompatible elements such as th and u highly fractionated magmas crystallised in u rich biotite up to 21 ppm u and two mica granites in biotite granite the major u bearing minerals are uranothorite and allanite they are strongly metamict and the major part of their uranium 90 has been released from the mineral structure and was available for leaching mass balance calculations show that the triassic biotite granites may have at least liberated 14 000 t u km3 and thus correspond to a major primary uranium source for the u deposits hosted in the erlian basin

fluid induced disturbance of the monazite th pb chronometer in situ dating and element mapping in pegmatites from the rhodope greece bulgaria abstract the monazite th pb chronometer is widely used in a variety of geological contexts but may be affected by interaction with fluids in the central part of the rhodope massif greece bulgaria synfolial pegmatite veins were emplaced within two major synmetamorphic shear zones in the chepelare shear zone czs which displays no evidence of post metamorphic fluid flow consistent monazite th pb ages of 36 ma are interpreted as dating vein emplacement higher in the metamorphic pile zircon and monazite give u th pb ages of 42 ma for vein emplacement in the nestos shear zone nsz pegmatites show evidence of post emplacement interaction with fluids during greenschist facies ductile deformation evidence for this includes the precipitation of calcite in microscale tension gashes within feldspar porphyroclasts and the oxygen isotope disequilibrium between quartz and feldspar aqueous carbonic fluids deriving from carbonate lithologies in the footwall of the nsz are identifiable in the pegmatites by the high 18 o value of quartz up to 21 8 and the presence of calcium in calcite veinlets in these samples some of the monazite grains show large intragrain scattering of th pb age spanning up to 12 ma with intergrain scattering reaching 16 ma from 39 to 55 ma within individual grains age domains correlate with chemical heterogeneities and some show a characteristic ca excess these chemical and isotopic alterations are interpreted to be caused by interaction with fluids derived from carbonate lithologies complementary u pb data on zircons from the nsz pegmatites yield mostly mesozoic ages related to an older metamorphic cycle and an age of 48 ma for one grain with typical magmatic zoning the th pb ages of 49 3 1 6 to 54 9 1 7 ma probably relate to the emplacement of the pegmatites the spread of younger th pb ages from 38 6 1 1 to 46 2 1 6 ma probably reflects the period of fluid circulation during progressive cooling to greenschist facies conditions or variable perturbations of the monazite isotope system during fluid assisted greenschist facies deformation this is consistent with 39 ar 40 ar mica ages of 32 to 34 ma interpreted as a tighter constraint for the timing of the greenschist facies metamorphism this study illustrates the capacity of monazite to record distinct events in a single rock and highlights the need for identifying the potential involvement of fluids in order to interpret monazite ages

cenozoic plate tectonic history of the northern venezuela trinidad area geological and geophysical data coupled with recent plate tectonic reconstructions suggest that the cenozoic geologic history of the northern venezuela trinidad area has been dominated by strike slip displacement of discrete crustal blocks allochthonous terranes within the area include metavolcanic rocks of the cretaceous villa de cura group and metamorphic rocks of the precambrian to cretaceous cordillera de la costa a relatively competent crustal block margarita block is defined by an outline around the metamorphic basement of margarita island the araya paria peninsula the northern range of trinidad and tobago island reconstruction of the margarita block to its original position requires at least partial closure of the falcon basin closure of the bonaire and cariaco basins and restoration of about 50 km of motion on both the oca and bocono faults post middle eocene eastward translation of the caribbean plate caused eastward motion of the margarita block a minor change in relative plate motion during the late oligocene or early miocene produced a right step in the moron fault forming the cariaco pull apart basin and el pilar fault zone maximum offset on el pilar fault is estimated to be no more than 125 km though displacement along the entire fault zone may have been greater transpressional stresses between the caribbean plate and northern south america caused folding of the serrania del interior of venezuela and the central range of trinidad eastward migration of transpressional stresses at the southeastern corner of the caribbean south american plate boundary is being accommodated by formation of oblique thrusts transpressive anticlines and downwarping of the crust bouguer gravity data suggest that jurassic aged atlantic oceanic crust is being depressed as the caribbean plate expands into the demerara plateau area this study suggests that the faults and transtensional transpressional compressional structures identified in this study are the result of stresses produced during the large eastward translation of the caribbean plate since the paleocene and are not the product of a shear couple

quaternary geomorphological evolution of a granitic shore platform constrained by in situ 10 be concentrations penmarc h sw brittany france abstract the penmarc h granitic coastal domain sw brittany western france offers the opportunity to address the little investigated issue of erosional processes in a granitic coastal context from land to sea the granitic coast is composed of 1 an aerial marine terrace 10 be analyses supplies new constraints on the weathering and denudation processes recorded by the penmarc h platform in quaternary times the role of at least two denudation events is emphasized i a 1 8 m thick level of granitic material is assumed to have been removed by erosion during the mis5 sea level highstand both in the offshore domain and in the low part of the marine terrace ii an additional 0 6 m thick level of the granitic substratum was later removed during the last holocene transgression mainly in the marine domain the lack of any morphological step between the subaerial marine terrace and the shore platform is here attributed to the combined effect of multiple factors including i a quaternary tectonic quiescence ii a major eustatic transgression mis5 shaping both the shore platform and the low part of the marine terrace iii the mechanically strong behaviour of the granitic basement with regard to erosion and iv the moderate influence of the holocene sea level stillstand since 6 ka on the shore platform thanks to 10 be content analysis a range of vertical erosion rates of ca 3 6 m ma 1 is obtained for the onshore domain showing no discernable difference between the intertidal and aerial erosion rates from that result it is thus determined that the long term development of the penmarc h granitic shore platform was mainly driven by marine erosive processes during the mis5 and holocene marine transgressions whereas subaerial erosive processes prevailed after the mis5 event

age discordance and mineralogy observations of discordant ages meaning that an age given by one mineral geochronometer is different from the age given by another geochronometer from the same rock began in the early days of geochronology in the late 1950s and 1960s discordant u pb zircon ages were unquestioningly attributed to pb diffusion at high temperature later the mineralogical properties and the petrogenesis of the zircon crystals being dated was recognized as a key factor in obtaining concordant u pb ages advances in analytical methods allowed the analysis of smaller and smaller zircon multigrain fractions then the analysis of individual grains and even pieces of grains with higher degrees of concordancy further advances allowed a higher analytical precision a clearer perception of accuracy and a better statistical resolution of age discordance as for understanding the cause s of discordance belief revision followed the coupling of imaging cathodoluminescence cl and back scattered electrons bse to in situ dating by secondary ion mass spectrometry sims or by laser ablation inductively coupled plasma mass spectrometry la icpms discordant zircon and other accessory minerals e g monazite apatite etc often consist of young rims accreted onto into older cores age gradients are sharp and no pb diffusion gradients are observed as u pb discordance in crystalline non radiation damaged grains is caused by diachronous heterochemical mineral generations interpretations of mineral ages based on the exclusive role of diffusion are superseded and closure temperatures of zircon and monazite are irrelevant in geological reality other isotopic systems rb sr k ar were believed since the 1960s to be similarly controlled by the diffusivity of radiogenic daughters when zircon and monazite discordance were recognized as zone accretion reaction with sharp boundaries that showed little or no high temperature diffusive re equilibration the other chronometric systems were left behind and interpretations of mineral ages based on the exclusive role of diffusion survived the evidence from textural petrologic imaging cl bse and element mapping by electron probe microanalyzer epma or high spatial resolution sims or la icpms provides the decisive constraints all microcline and mica geochronometers that have been characterized in detail document patchy textures and evidence for mineral replacement reactions it is important not to confuse causes and effects heterochemical microstructures are not the cause of ar and sr loss rather they follow it argon and sr loss by dissolution of the older mineral generation occurs first heterochemical textures form later when the replacive assemblage recrystallizes heterochemical mineral generations are identified and dated by their ca cl k systematics in 39ar 40ar replacive reactions adding or removing cl such as e g sericite overgrowths on k feldspar retrograde muscovite intergrowths with phengite etc are detected by cl k vs ar k isotope correlation diagrams calcium poor reaction products such as e g young biotite intergrown with older amphibole adularia replacing microcline etc can be easily identified by ca k vs ar k diagrams supported by epma analyses mixed mineral generations are observed to be the cause of discordant staircase shaped age spectra while step heating of crystals with age gradients produces concordant plateaus age gradients are therefore unrelated to staircase age spectra there is a profound analogy between the u pb rb sr and k ar systems lead and ar diffusion rates are both much slower than mineral replacement rates for all t 750 c patchy retrogression textures are always associated with heterochemical signatures u th ratios ree patterns ca cl k ratios as a rule single generation minerals with low amounts of radiation damage give concordant ages whereas discordance is caused by mixtures of heterochemical resolvably diachronous mineral generations in petrologic disequilibrium this can also include sub grains that have accumulated significant amounts of radiation damage for accurate geochronology the petrologic characterization with the appropriate technique s of the minerals to be dated and the petrologic context at large are as essential as the mass spectrometric analyses

fluid inclusions related to uranium deposits a review fluid inclusions have been observed and studied in rocks of most types of uranium deposits except surflcial ones such as calcretes and the roll type in sandstone temperatures of formation range up to nearly 400 c but most deposits were formed at more moderate temperatures of 90 250 c it is evident from the relatively small number of studies however that the ore forming process occurred at deeper levels than previously thought the chemistry of the fluids ranges from nearly pure water to very dense brines and co 2 is a common component in some cases gas analyses allow determination of f o 2 of the fluid a key parameter for uranium transport and deposition

distribution and significance of extraneous argon in uhp eclogite sulu terrain china insight from in situ 40ar 39ar uv laser ablation analysis in situ 40ar 39ar uv laser ablation analyses of garnet omphacite epidote phengite barroisite and symplectite in a sample of uhp eclogite from qinglongshan sulu terrain china reveal extremely heterogeneous argon isotopic ratios results from the 220 240 ma eclogite indicate a significant and variable extraneous argon signature that is restricted to the early formed phengite and epidote and later formed barroisite individual compositionally zoned phengite grains record a large variation in 40ar 39ar dates 300 1100 ma that are positively correlated with celadonite content these data are consistent with retrograde chemical exchange and argon isotopic repartitioning in response to decreasing partial pressures of argon during decompression consistent with the observation of atmospheric argon ratios in garnet omphacite and symplectite these data support an interpretation that the extraneous ar was incorporated early in the crystallization history of the eclogite and not from a late infiltrating fluid the ultimate source of the extraneous argon remains speculative however on the basis of supporting stable oxygen and hydrogen isotope data from this locality which support a closed isotopic system it is inferred to have been inherited from the protolith of the uhp eclogite

thermochronologic data from tobago west indies constraints on the cooling and accretion history of mesozoic oceanic arc rocks in the southern caribbean fission track thermochronologic studies on mid cretaceous dioritic and tonalitic rocks from tobago west indies provide important temporal constraints on post magmatic mesozoic cooling as well as the final phase of accretion of these oceanic arc plutonic rocks onto the continental margin of northern south america zircon fission track data indicate that the plutonic rocks of tobago cooled through the zircon closure isotherm 250 200 c at about 103 ma apatite ages and track length analysis suggest rapid cooling through the apatite annealing zone 120 60 c at about 45 ma furthermore the track lengths in apatites from these plutonic rocks are all greater than 14 microns which implies that these rocks have not been above 50 c since 45 ma if a geothermal gradient of 25 c km is inferred the analyzed samples have been at depths of less than 2 km since 45 ma tobago has occupied an upper crustal structural position since the mid cretaceous and significant final uplift of these mesozoic oceanic arc rocks occurred in the mid eocene the tectonothermal history of tobago since the mid eocene has not involved deep burial or a high temperature thermal flux this history apparently has chiefly involved translation of the tobago terrane along the caribbean south america plate boundary zone at upper crustal levels

oxygen isotope exchange kinetics of mineral pairs in closed and open systems applications to problems of hydrothermal alteration of igneous rocks and precambrian iron formations the systematics of stable isotope exchange between minerals and fluids are examined in the context of modal mineralogical variations and mass balance considerations both in closed and in open systems on mineral pair 18 o plots samples from terranes that have exchanged with large amounts of fluid typically map out steep positively sloped non equilibrium arrays analytical models are derived to explain these effects these models allow for different exchange rates between the various minerals and the external fluids as well as different fluid fluxes the steep arrays are adequately modelled by calculated isochron lines that involve the whole family of possible exchange trajectories these isochrons have initially steep near vertical positive slopes that rotate toward a 45 equilibrium slope as the exchange process proceeds to completion the actual data point array is thus analogous to the hand of an isotopic clock that measures the duration of the hydrothermal episode the dimensionless ratio of the volumetric fluid flux to the kinetic rate parameter u k determines the shape of each individual exchange trajectory in a fluid buffered system u k 1 the solutions to the equations 1 are independent of the mole fractions of the solid phases 2 correspond to taylor s open system water rock equation and 3 yield straight line isochrons that have slopes that approach 1 f where f is the fraction reacted of the more sluggishly exchanging mineral the isochrons for this simple exchange model are closely congruent with the isochrons calculated for all of the more complex models thereby simplifying the application of theory to actual hydrothermal systems in nature in all of the models an order of magnitude of time in units of kt separates steep non equilibrium arrays e g slope 10 from arrays approaching an equilibrium slope of unity on a diagram because we know the approximate lifetimes of many hydrothermal systems from geologic and heat balance constraints we can utilize the 18 o 16 o data on natural mineral assemblages to calculate the kinetic rate constants k s and the effective diffusion constants d s for mineral h 2o exchange these calculated values k qtz 10 14 k feld 10 13 10 12 agree with experimental determinations of such constants in nature once the driving force or energy source for the external infiltrating fluid phase is removed the disequilibrium mineral pair arrays will either 1 remain frozen in their existing state if the temperatures are low enough or 2 re equilibrate along specific closed system exchange vectors determined solely by the temperature path and the mineral modal proportions thus modal mineralogical information is a particularly important parameter in both the open and closed system scenarios and should in general always be reported in stable isotopic studies of mineral assemblages these concepts are applied to an analysis of 18 o 16 o systematics of gabbros plagioclase clinopyroxene and plagioclase amphibole pairs granitic plutons quartz feldspar pairs and precambrian siliceous iron formations quartz magnetite pairs in all these examples striking regularities are observed on and plots but we point out that plots have many advantages over their equivalent diagrams as the latter are more susceptible to misinterpretation using the equations developed in this study these regularities can be interpreted to give semiquantitative information on the exchange histories of these rocks subsequent to their formation in particular we present a new interpretation indicating that precambrian cherty iron formations have in general undergone a complex fluid exchange history in which the iron oxide magnetite precursor has exchanged much faster with low temperature 400 c fluids than has the relatively inert quartz

surface wave attenuation by vegetation with the stem root and canopy abstract to experimentally study the performance of the stem root and canopy on wave attenuation by vegetation a total of 112 laboratory experiments were conducted with seven vegetation models four vegetation densities five wave heights and four water depths wave period was assumed constant limiting the understanding of wave attenuation phenomena by vegetation frictional energy losses from sidewall and bottom were measured and removed by comparing with non vegetated tests experimental results show that wave attenuation by vegetation with stems roots and canopies varied with wave conditions due to vertically non uniform physical vegetation properties the root significantly impacted wave dissipation when the relative root height was greater than 0 167 higher wave attenuation occurred when the still water level was located at the canopy geometrical centroid the transmission coefficients of vegetation of the stem root and canopy ranged from 0 21 to 0 83 an empirical formula for predicting transmission coefficients of vegetation with stems roots and canopies was obtained based on experimental data the wave energy dissipated by vegetation was estimated in terms of drag coefficients calculated by analytical models of dalrymple et al 1984 and kobayashi et al 1993 a semi empirical analytical equation was proposed on the basis of experimental data for predicting wave attenuation by vegetation of the stem root and canopy

the granite hosted gold deposit of moulin de cheni saint yrieix district massif central france petrographic structural fluid inclusion and oxygen isotope constraints the moulin de cheni orogenic gold deposit is the only granite hosted deposit of the saint yrieix district french massif central it occurs in 338 1 5 ma old peraluminous leucogranites and is characterized by intense microfracturing and bleaching of the granite in relation to pervasive sulfide crystallization formation of quartz veins and gold deposition occurred in two successive stages an early mesozonal stage of quartz sulfide fe as s deposition usually devoid of gold and a late epizonal stage of base metal and gold deposition both stages postdate peak metamorphism and granite intrusion the genesis of the deposit is the result of four successive fluid events 1 percolation of aqueous carbonic metamorphic fluids under an assumed lithostatic regime of 400 450 c at a maximum depth of 13 km 2 formation of the main quartz lodes with coeval k alteration and introduction of as and s from aqueous carbonic fluids percolating along regional faults arsenopyrite and pyrite deposition was linked to the alteration of fe silicates into k feldspar and phengite at near constant iron content in the bulk granite temperature was similar to that of the preceding stage but pressure decreased to 100 50 mpa suggesting rapid uplift of the basement up to 7 5 km depth 3 the resulting extensional tectonic leads to the deposition of gold boulangerite galena and sphalerite in brecciated arsenopyrite and pyrite from aqueous fluids during a mixing process temperature and salinity decrease from 280 to 140 c and 8 1 wt eq nacl to 1 6 wt eq nacl respectively 4 sealing of the late fault system by barren comb quartz which precipitated from dilute meteoric aqueous fluids 1 6 wt eq nacl to 0 9 wt eq nacl under hydrostatic conditions at 200 150 c

an extended episode of early mesoproterozoic metamorphic fluid flow in the reynolds range central australia the products of metamorphic fluid flow are preserved in zones within the marbles and metamorphosed semipelites of the upper calcsilicate unit in the granulite portion of the late palaeoproterozoic reynolds range group northern arunta block central australia the zones of retrogression characterized by minerals such as wollastonite grossular and clinohumite local resetting of oxygen isotopic compositions and local major element metasomatism were channelways for water rich fluids derived from granulite facies metapelites u th pb isotopic ages measured by the shrimp ion microprobe on zircon and monazite from a granulite facies semipelite an early semiconcordant aluminous quartz rich fluid flow segregation and a late discordant quartz rich segregation record some of the extended thermal history of the area zircon cores from the semipelite show its likely protolith to be an igneous rock 1812 11 ma old itself derived from a source containing zircon as old as 2 2 ga low th u overgrowths on the zircon grew during granulite facies metamorphism at 1594 6 ma monazite cooled to its blocking temperature at 1576 8 ma zircon cores from the semiconcordant segregation are dominantly 2 3 ga old indicating that the source of the fluids was not the particular metamorphosed semipelite studied two generations of low th u overgrowths on the zircon give indistinguishable ages for the older and younger of 1589 8 and 1582 8 ma respectively the monazite age is the same 1576 12 ma zircon from the late discordant segregation gave 1568 4 ma fluid flow occurred for at least 18 3 ma and ended 26 3 ma after the peak of metamorphism suggesting a very slow cooling rate of 3 c ma 1 the last regional high grade metamorphism in the reynolds range occurred at 1 6 ga not 1 78 ga as previously thought the high grade event at 1 78 ga is a separate event that affected only the basement to the reynolds range group

wave damping in reed field measurements and mathematical modeling wave damping in vegetation in shallow lakes reduces resuspension and thereby improves the light climate and decreases nutrient recycling in this study wave transformation in reed phragmites australis was measured in a shallow lake theoretical models of wave height decay based on linear wave theory and transformation of the probability density function pdf using a wave by wave approach were developed and compared to the collected data field data showed an average decrease in wave height of 4 5 m sup 1sup within the first 5 14 m of the vegetation incident root mean square wave height was 1 8 cm a species specific drag coefficient cd was found to be about 9 most probable range 3 25 cd showed little correlation with a reynolds number or a keulegan carpenter number the pdf for the wave heights did not change significantly but for longer distances into the vegetation and higher waves it tended to be more similar to the developed transformed distribution than to a rayleigh distribution relationships developed in this study can be employed for management purposes to reduce resuspension and erosion less

volume self diffusion of oxygen in biotite muscovite and phlogopite micas the kinetics of oxygen self diffusion have been measured in natural samples of biotite muscovite and phlogopite micas under hydrothermal conditions using both bulk exchange and profiling analyses all experiments were run at 1000 bars 100 mpa water pressure except those designed to measure water pressure dependence which ranged from 200 to 2000 bars the arrhenius relations obtained for layer parallel diffusive transport in the three micas by bulk exchange experiments fit to an infinite cylinder model are mineral biotite muscovite phlogopite 34 2 39 5 42 3 9 1 10 6 7 7 10 5 1 4 10 4 t c 500 800 512 700 600 900 the results of ion microprobe sims depth profiling perpendicular to the layers parallel to c axis yield diffusion coefficients 3 to 4 orders of magnitude lower than for transport parallel to the layers a single layer parallel step profiling experiment yields a diffusion coefficient identical within uncertainty to those obtained from bulk exchange experiments a slight dependence of diffusion rate on water pressure may obtain in phlogopite however the difference is approximately within the uncertainty of the measurement diffusion rates are similar within about a half an order of magnitude for a range of biotite composition from annite4 to annite63 as well as for muscovite comparison of oxygen diffusion rates in biotite from this study with ar k and rb diffusion from published data supports the idea that water is the oxygen bearing species in hydrothermal diffusion experiments the implication of these results for natural systems is that the micas especially biotite will continue to exchange oxygen isotopes via volume diffusion down to temperatures as low as 300 c the rapid diffusion kinetics may explain biotite oxygen isotope disequilibrium which is commonly observed even in rocks where other mineral phases preserve equilibrium compositions full size table table options view in workspace download as csv

calculation of oxygen isotope fractionation in hydroxyl bearing silicates abstract the modified increment method has been applied to the calculation of oxygen isotope fractionation factors for hydroxyl bearing silicate minerals the order of 18o enrichment obtained in common rock forming minerals is pyrophyllite kaolinite tourmaline talc prehnite topaz illite phengite lepidolite muscovite staurolite epidote glaucophane serpentine chlorite tremolite hornblende phlogopite biotite humite norbergite ilvaite hydroxyl bearing silicates are enriched in 18o relative to hydroxyl groups but depleted in 18o relative to anhydrous counterparts three sets of self consistent fractionation factors between quartz and the hydroxyl bearing silicate minerals between calcite and the silicate minerals and between the silicate minerals and water have been calculated for a temperature range of 0 1200 c the fractionation factors calculated for mineral pairs are applicable to isotopic geothermometry in igneous metamorphic and sedimentary petrology they can be used as a test of isotopic equilibrium or disequilibrium in natural mineral assemblages over all temperature ranges of geological interest the difference in oxygen isotope composition between the hydroxyl bearing mineral and the oh group is quantitatively demonstrated to be temperature dependent and therefore can be used as a single mineral geothermometer

four phase tectonostratigraphic development of the southern caribbean abstract although the stratigraphy of northern south america and the southern caribbean has been studied in detail in many areas a clear understanding of the sequence of tectonic events and their relation to regional stratigraphy has not been forthcoming since northern south america collided with southern north america in the late palaeozoic we identify four discrete tectonic phases from the stratigraphy of the area that is now the southern caribbean these phases derived from a synthesis of regional stratigraphy were 1 panama arc collistion with colombia in the late miocene early pliocene which drove a complex triangular wedge bounded by strike slip faults northward into the caribbean 2 right lateral strike slip faulting associated with post eocene eastern motion of the caribbean ocean floor into the gap between the separating americas 3 island arc development and collision associated with the entry of the present caribbean ocean floor an area of buoyant ocean crust into the gap between the separating americas and 4 continental rifting and passive margin development in the late jurassic early cretaceous associated with the separation of north and south america approximately along the palaeozoic suture zone overprinting of younger structures on inherited structures from previous phases is responsible for the overall complexity of the region

wave height distributions on shallow foreshores wave height distributions on shallow foreshores deviate from those in deep water due to the effects of the restricted depth to height ratio and of wave breaking laboratory data of wave heights on shallow foreshores of different slopes have been analysed to determine these effects and to derive generalised empirical parameterisations a model distribution is proposed consisting of a rayleigh distribution or a weibull distribution with exponent equal to 2 for the lower heights and a weibull with a higher exponent for the higher wave heights the parameters of this distribution have been estimated from the data and expressed in terms of local wave energy depth and bottom slope yielding a predictive model that is found to be significantly more accurate than existing expressions

selective heat producing element enrichment in a crustal segment of the mid european variscan chain abstract heat production has been calculated from u th and k content measurements on selected rock from the armorican massif western france geographical geochemical and age criteria have been introduced during the selection in order to obtain a representative sampling of the outcropping formations this procedure allows the delineation of a high heat production zone with values nearly twice the average upper crustal heat production ones the high heat production zone laterally ends through a high gradient belt indicating an upper crustal origin this zone cross cuts the geological structures and encompasses various granitoid plutons of different geochemical typologies and ages however among the granites belonging to the same geochemical type only those plutons located in this zone are enriched in radioelements those located outside with the exception of a few massifs present normal radiogenic element concentrations the enrichment affects all intruded granitoids or orthogneisses irrespective of their age when geographical features are restored prior to the late carboniferous shear deformation then the high heat production zone no longer appears as fortuitous it shapes a region intruded by numerous granitoids and structured during the early proterozoic we therefore propose that the high heat production zone reflects a previous selective enrichment of this crustal segment related to the late cambrian plutons

thermochronology of the highest central asian massifs khan tengri pobedi se kyrgyztan evidence for late miocene ca 8 ma reactivation of permian faults and insights into building the tian shan abstract geological study of the khan tengri and pobedi massifs highlights two phases of compressional fault displacements a late permian triassic displacement phase is highlighted 1 by biotite 40ar 39ar ages of 265 256 ma suggestive of cooling of pobedi mid crustal granulites during 8 km of top to the north motion of the pobedi thrust 2 by 40ar 39ar dating of syn kinematic phengite at 249 248 ma which suggests a crystallization age during top to the south motion of the khan tengri thrust shear zone 3 by apatite helium ahe ages of 280 240 ma on the crystalline basement below the mesozoic peneplain which gives insight into the final exhumation stage cenozoic reactivation of the pobedi thrust is indicated by ahe thermochronology with a mean age of 8 3 2 5 ma in agreement with 3 km exhumation and 4 2 km of top to the north motion in the late miocene along a north south transect of the west tian shan range from issyk kul lake to tarim basin compiled thermochronological data outline out of sequence deformation beginning with the activation of north directed crustal scale faults at 22 15 ma along the northern margin of the north tian shan followed by 20 10 ma motion at the boundary between the middle and the south tian shan and ending with the

restoration of lithosphere scale wrenching from integrated structural and tomographic data hercynian belt of western france tomographic images of the mantle below the hercynian belt of brittany display strong anomalies of p wave traveltimes that likely represent variations in composition and whose trends change above and below 130 km a quantitative strain analysis of surface geological structures has shown that the crust in the area was affected by regional scale distributed simple shear the change of trends of tomographic anomalies below and above 130 km is largely removed by extending the simple shear model down to 130 km from this we infer lithospheric scale wrenching and therefore deep seated horizontal decoupling possibly at the lithosphere asthenosphere boundary simple shear restoration further provides arguments for the occurrence of a steeply dipping slab geological data support its interpretation as a remnant of oceanic lithosphere subducted prior to the hercynian collision beyond regional implications the study underlines the interest of combining geology and tomography for the understanding of lithosphere deformation

hydrothermal alteration of monazite in the precambrian crystalline basement of the athabasca basin saskatchewan canada implications for the formation of unconformity related uranium deposits microanalytical studies of basement rocks below the athabasca sandstone basin indicate that monazite is the dominant uranium bearing mineral in the study area drill core samples of hydrothermally altered basement show that monazite is commonly altered to a th silicate phase and uranium has been significantly mobilized on average 75 of the uranium bound to monazite is leached out during monazite alteration in contrast no substantial mobilization of uranium from detrital minerals e g zircon has yet been observed in the athabasca sandstones it is suggested that hydrothermal alteration of granitic rocks especially potassic pegmatoids and potassic orthogneisses of the sub athabasca basement represents the most important uranium source for the unconformity type deposits

metallogenesis in the french part of the variscan orogen part i u preconcentrations in pre variscan and variscan formations a comparison with sn w and au abstract in the french part of the variscan orogen the pre variscan metamorphic rocks and the variscan granites resulting from pure crustal recycling are enriched in uranium and tin data on tungsten and gold are too scarce for a reasonable estimation of the average concentrations of these elements to be obtained the spatial distribution of uranium preconcentrations is transverse to the main variscan structural domains and the distribution of the sn w mineralizations prompts the same conclusions u sn w and au metal enrichment in crustal domains in europe seems to result from late proterozoic to early palaeozoic basic and acid magmatism for uranium at least two main stages of partial melting early palaeozoic and late carboniferous lead to enrichment of the metal in peraluminous leucogranites these leucogranites represent the main source of late variscan uranium and tin within the peraluminous granitic complexes the main metal source is related to the late intrusion of small granitic cupolas strongly enriched in metals in the case of uranium the efficiency of the hydrothermal remobilization depends on the proportion of metal located in uraninite which is easily leachable by hydrothermal solutions the emplacement of these cupolas is structurally controlled in well defined areas by large shear zones these shear zones are continuously active during the successive emplacement of the granitic magmas most of the hydrothermal uranium mineralization is related to the permian reactivation of the shear zones in a brittle stage these zones then channel the hydrothermal fluids involved in the last metal concentration step leading to the formation of the orebodies the efficiency of metal remobilization is dependent on a close spatial relationship between magmatic structures enriched in uranium and brittle structures channeling the hydrothermal fluids

giant quartz vein formation and high elevation meteoric fluid infiltration into the south armorican shear zone geological fluid inclusion and stable isotope evidence giant quartz veins associated with the south armorican shear zone record important fluid circulation during the hercynian period regional scale mapping of veins allows two groups of veins to be identified on the basis of their geometric relationship with the south armorican shear zone veins in the first group are parallel to the shear zone whereas those in the second group developed in a direction oblique to it the former probably record infiltration of fluids along permeable pathways in highly deformed zones the latter may represent crustal scale tension gashes in the regional context most quartz veins have 18 o values between 10 and 16 indicating a mid crustal origin for the fluids microthermometry on fluid inclusions from euhedral quartz indicates that late fluids were mostly aqueous with very low salinity 0 1 7 wt eq and with homogenization temperatures ranging between 150 and 270 c together with very low 18 o values of some euhedral quartz down to 2 these features argue for a surface origin corresponding 18 o fluid values estimated near 11 are probably related to the high palaeo elevation of meteoric precipitation scarce but significant h 2 o co 2 fluid inclusions in euhedral quartz indicate a metamorphic contribution these were probably sourced from the exhumed metamorphic basement in the southern part of the region

direct evidence for the source of uranium in the baiyanghe deposit from accessory mineral alteration in the yangzhuang granite porphyry xinjiang province northwest china

uranium metallogenesis of the peraluminous leucogranite from the pontivy rostrenen magmatic complex french armorican variscan belt the result of long term oxidized hydrothermal alteration during strike slip deformation in the french armorican variscan belt most of the economically significant hydrothermal u deposits are spatially associated with peraluminous leucogranites emplaced along the south armorican shear zone sasz a dextral lithospheric scale wrench fault that recorded ductile deformation from ca 315 to 300 ma in the pontivy rostrenen complex a composite intrusion the u mineralization is spatially associated with brittle structures related to deformation along the sasz in contrast to monzogranite and quartz monzodiorite 3 3 the leucogranite samples are characterized by highly variable u contents 3 to 27 ppm and th u ratios 0 1 to 5 suggesting that the crystallization of magmatic uranium oxide in the more evolved facies was followed by uranium oxide leaching during hydrothermal alteration and or surface weathering u pb dating of uranium oxides from the deposits reveals that they mostly formed between ca 300 and 270 ma in monzogranite and quartz monzodiorite apatite grains display magmatic textures and provide u pb ages of ca 315 ma reflecting the time of emplacement of the intrusions in contrast apatite grains from the leucogranite display textural geochemical and geochronological evidences for interaction with u rich oxidized hydrothermal fluids contemporaneously with u mineralizing events from 300 to 270 ma infiltration of surface derived oxidized fluids leached magmatic uranium oxide from fertile leucogranite and formed u deposits this phenomenon was sustained by brittle deformation and by the persistence of thermal anomalies associated with u rich granitic bodies

comparison of 40ar 39ar conventional and laser dating of biotites from the north tregor batholith bulk samples and single grains of biotites from the north tregor batholite armorican massif france were analysed by the 40ar39ar method apparently pure single biotite grains analysed by the step heating procedure with the laser probe displayed plateau ages or slightly disturbed age spectra one strongly chloritized single biotite grain gave a very disturbed age spectrum with younger ages the bulk samples of biotites of the same rocks displayed disturbed age spectra characterized by younger low temperature ages and a hump at intermediate temperature steps these disturbances are explained by 39ar recoil from the biotite into the chlorite which is clearly evidenced by hrtem observations and not by excess 40ar the more concordant results obtained on single biotites are probably due to a more careful selection of the minerals in most cases we obtain good concordance of the ages 600 610 ma measured on the apparently pure single biotite grains and the high temperature ages of both the bulk samples of amphiboles and biotites from the same rocks

tectonic evolution of the caribbean the development of the caribbean is discussed in terms of modern tectonic theory the nature of the site on which the caribbean formed is examined and the development of the rifted margins of the caribbean is described constraints on caribbean evolution from the relative motions of north and south america are briefly examined and the caribbean oceanic plateau is discussed the great island arc system of the caribbean is addressed in detail emphasizing the way the great arc of the caribbean was segmented the role of central america in caribbean history is briefly considered

the relationship between tectono metamorphic evolution and argon isotope records in white mica constraints from in situ 40ar 39ar laser analysis of the variscan basement of sardinia the basement of sardinia represents a nearly complete section of a segment of the variscan belt that experienced a polyphase tectonometamorphic evolution and barrovian metamorphism this basement is well suited to investigate the relationship between tectono metamorphic evolution and argon isotope records in white mica micaschists from the garnet zone maximum t of up to 520 560 c contain two texturally and chemically resolvable generations of white mica 1 deformed celadonite rich flakes defining a relict s1 foliation preserved within the main s2 foliation or enclosed in rotated albite porphyroblasts 2 celadonite poor white micas aligned along the main s2 foliation the s1 foliation developed earlier and at a deeper crustal level with respect to that at which the thermal peak was reached from the staurolite zone t of up to 590 625 c to the sillimanite k feldspar zone white mica is nearly uniform in composition muscovite and is predominantly aligned along the s2 foliation or is of later crystallization in situ ar ar laser analyses of white mica yielded ages of 340 315ma in the garnet zone and 320 300ma in the staurolite and sillimanite k feldspar zones results highlight a close link between argon isotope records in white mica and both textures and structure forming major elements the oldest ages were detected in samples where the earlier syn d1 white mica generation did not texturally and chemically re equilibrate at upper crustal levels this study suggests that the white micas that escaped recrystallization retain argon isotope records of an earlier metamorphic stage that survived a later event characterized by temperatures higher than 500 c

simultaneous resetting of the muscovite k ar and monazite u pb geochronometers a story of fluids terra nova 23 390 398 2011 abstract although water is ubiquitous in the continental crust its effect on geochronometers through mineral reequilibration is rarely taken into account herein we present 40ar 39ar analyses on muscovite and u pb isotopic data on zircon and monazite from a variscan syn tectonic granite from western france both the k ar in the muscovite and u pb in the monazite isotopic systems were hydrothermally reset whereas the u pb radiogenic system in most of the zircons was unaffected and dates the granite emplacement age titanium chemical maps obtained on muscovites from various dated samples display a spectacular overprinting of their magmatic zoning resulting from increasing fluid rock interaction these results reiterate the need to combine geochronological data with petrological mineralogical and geochemical studies to accurately interpret ages obtained in this type of geodynamical settings

a two stage exhumation in western french massif central new geochronological evidences of syn collisional extension abstract systematic 40ar 39ar dating of granitoids and surrounding metamorphic rocks has been performed in the limousin area w of french massif central in order to provide new constrains concerning its late orogenic thermal and tectonic evolution the same 40ar 39ar age of ca 335 338 ma is found on the present day outcrop surface for most of the limousin metamorphic rocks hence indicating that the entire massif was uniformly cooled down at this period these ages are very close to the 40ar 39ar ages proposed in the sioule and brevenne areas in the literature but also to the coupled tectonic and magmatic activity in the south limousin these results help to document the existence of two exhumation stages each one associated with an extensional stage a first exhumation stage has been active at ca 335 340 ma available data indicate that this extension and associated exhumation and cooling occurred over a distance of more than 200 km in the internal zones of the belt whereas compression was still active in the northern and southern parts of the belt a second exhumation stage may be related to the late orogenic extension and collapse of the belt at ca 310 305 ma and is characterized by the uplift of the st sylvestre complex by unroofing of the overlying metamorphic and granitoidic formations

uranium mobilization from the variscan questembert syntectonic granite during fluid rock interaction at depth leaching of uranium fertile granites represents a major source of uranium as uraninite is easily dissolved in oxygenated aqueous solutions this phenomenon is well documented at surface conditions but remains poorly documented for granites at depth in this study we propose that surface derived oxidized hydrothermal fluids leached uranium from uraninite in the questembert peraluminous granite at temperatures greater than 70 to 160 c this variscan synkinematic granite is characterized by widespread and pervasive development of vertical and permeable c s structures these structures likely facilitated the infiltration of oxidized hydrothermal fluids from the surface their circulation at depth and the subsequent fluid rock interaction in the granite published oxygen isotope data shows that it has undergone subsolidus fluid rock interaction dated between 312 and 303 ma by 40ar 39ar analyses on muscovite these interactions were responsible for the concomitant decrease of the feldspar 18o values together with uranium leaching mass balance calculations suggest that this hydrothermal event could have liberated several hundred thousand tonnes of uranium from the questembert granite the liberated uranium may have been dispersed in early permian intramountainous basins and therefore disseminated over large areas this study emphasizes that the efficiency of uranium leaching may be directly related to the extent of subvertical structure development in granites emplaced along strike slip shear zones which allow for downward infiltration of oxidized surface derived fluids a specific and systematic sampling is required to better constrain the proposed model

variations of wave parameter statistics as influenced by water depth in coastal and inner shelf areas abstract wave height pressure and orbital velocity statistics as influenced by the factor of water depth are analyzed on the basis of five data sets collected in situ from two intertidal sites with a mean water depth of 0 8 2 7 m and three inner shelf sites water depths 14 6 27 6 m acoustic doppler velocimeters advs were used to measure instantaneous wave pressure and current velocity at 0 20 0 35 m above sea bed the zero crossing wave analysis method was applied to analyze the wave data to determine the wave pressure amplitude p and orbital velocity amplitude u of individual waves together with the frequency of occurrence distributions of p and u which are compared with the three commonly used classic rayleigh modified rayleigh and weibull distributions it is found that the distribution of p is almost identical to that of u for each of the five study sites the distributions of p and u for the intertidal flat sites fit the weibull distribution better than the classic or modified rayleigh ones indicating a shallowness effect which is associated with finite banded wind wave spectra and possible wave breaking or near bed turbulence on the other hand the distributions of p and u measured at the three inner shelf sites agree almost equally with the three distributions with increasing water depth the distributions of p and u are shown to reduce from the two parameter weibull distribution at the shallow intertidal sites to the classic rayleigh at the deep water sites for coastal engineering applications an empirical formula is proposed to estimate more accurately the significant wave height in intermediate coastal waters based on the surface water elevation data

heavy mineral record of andean uplift and changing sediment sources across the ne margin of south america a case study from trinidad and barbados the heavy mineral compositions of sandstones in trinidad and barbados record the onset of andean related erosion and a reduction of craton derived sediments into ne south america the changing provenance was deduced by comparing heavy mineral assemblages interpreted from ancient sandstones with associations recognized in modern sands that can be reasonably correlated to existing tectonic domains the impact of the andean orogeny across the margin was to introduce a suite of minerals characteristic of low temperature metamorphism that today is prevalent adjacent to the caribbean mountain belt and differs from the zircon rich assemblage produced within cratonic plains twenty one paleocene late pliocene sandstone samples from trinidad revealed systematic changes in mineral diversity and maturity that recorded this provenance transition and suggests andean erosion during deposition of the late oligocene nariva formation similar to palaeogene sandstones of trinidad four eocene scotland formation samples from barbados support craton derivation but with additional evidence of minor andean input probably due to the proximity of the scotland formation delivery systems to an earlier uplift episode by the late miocene most of the sediments delivered into trinidad basins were supplied from the andean orogeny as suggested by the relative abundance of minerals of this affinity the heavy mineral records of trinidad and barbados are similar to that described across northern south america from both modern and ancient environments that collectively mark the uplift of the andean mountain belt with its strong influence on drainage patterns and reservoir provenance along this sector of the continental margin

5 late variscan mineralizing systems related to orogenic processes the french massif central abstract the french massif central constitutes an exceptional study area due to the diversity of its metallic deposits its internal position in the variscan belt and the abundance of available geological geophysical and metallogenic data obtained within the geofrance 3d programme the deposits formed towards the end of the orogenic evolution represent the economic products of two distinct mineralizing systems a au sb hydrothermal system and a w sn and rare metals magmatic hydrothermal system which were simultaneously active during a short time span between ca 310 and 300 ma two types of gold deposit can be distinguished on the basis of their depth of emplacement deep seated gold deposits developed under lithostatic to hydrostatic pressure during rapid exhumation and shallow gold deposits emplaced under hydrostatic pressure with no significant uplift deposits of w sn and rare metals were emplaced in the upper crust during final crystallization of specialized magmas after their rapid ascent perhaps enhanced by simultaneous regional uplift the gold bearing systems are associated with a complex network of re activated crustal scale faults initially active during the period between 335 and 315 ma normal motion along the faults coeval with 335 to 315 ma granite migmatite domes played a major role in the 3d distribution of the hydrothermal plumbing system gold and related metals were carried within huge hydrothermal cells which reached ca 100 km by 10 km in area and 30 km in depth in contrast granites rich in magmatophile elements w sn rare metals generated smaller hydrothermal cells 10 km by 10 km in area and

geochronological framework of the xiadian gold deposit in the jiaodong province china implications for the timing of gold mineralization abstract the xiadian gold deposit located in the zhaoyuan laizhou belt of the jiaodong gold province is a typical jiaojia type gold deposit which is characterized by disseminated and stockwork ores enclosed by hydrothermally altered wall rocks the exact age and the genesis of the gold deposit remain controversial here we present precise in situ monazite u pb dating to constrain the age of the gold mineralization which we integrate with published geochronological work on the associated geological events to understand the genesis of the gold deposit the linglong granite represents the earliest magmatic event at xiadian and yielded a zircon la icpms u pb age of 159 5 0 9 ma 2 mswd 0 63 subsequent geologic events were recorded by minor amounts of quartz pyrite molybdenite veins which are dated at 124 8 2 1 ma 2 mswd 0 01 by molybdenite re os dating in a granitic pegmatite the formation of such veins was close to the emplacement of adjacent guojialing granodiorite 130 ma and thus may be considered as a product of post magmatic hydrothermal activity it is proposed that the molybdenum bearing hydrothermal fluids acted as a prelude to gold mineralization and participated in the formation of the latter ore fluids gold mineralization took place at 120 0 1 4 ma 2 mswd 0 59 determined by la icpms u pb dating on hydrothermal monazite from quartz polymetallic sulfide veins prior to and post mineralization voluminous hydrothermally altered porphyritic diorite and fresh quartz diorite porphyry dykes were emplaced which yielded u pb ages of 121 3 1 4 ma 2 mswd 0 28 and 115 8 1 9 ma 2 mswd 0 71 respectively based on these geochronological data we have established a comprehensive geochronologic framework which suggests that the genesis of the xiadian deposit might be related to the craton destruction and lithosphere thinning in the north china craton

a plate kinematic framework for models of caribbean evolution abstract we define the former relative positions and motions of the plates whose motions have controlled the geological evolution of the caribbean region newly determined poles of rotation defining the approximate spreading histories of the central north and the south atlantic oceans are given for the late jurassic early cretaceous anomaly sequence of the central north atlantic we have used previously published definitions of fracture zone traces and magnetic anomaly picks redetermining the pole positions and angular rotations for various isochrons on an evans and sutherland interactive graphics system for magnetic anomalies younger than the cretaceous quiet period in both oceans we 1 used seasat altimeter data to help define fracture zone traces and 2 identified and used marine magnetic anomalies to determine the positions of spreading isochrons along the flowlines indicated by the fracture zones by the finite difference method the relative paleopositions and the relative motion history between north and south america were computed this analysis defines the size and shape and the rate at which the size and shape changed of the interplate region between north and south america since the middle jurassic thus a plate kinematic framework is provided for the larger plates pertaining to the caribbean region in which can be derived more detailed scenarios for gulf of mexico and caribbean evolution north and south america diverged to approximately their present relative positions from late triassic to early campanian about 84 m y ago time this is the period during which the gulf of mexico and a proto caribbean seaway were formed since the campanian only minor relative motion has occurred from early campanian through to middle eocene times south america diverged only another 200 km and since the middle eocene minor n s convergence has occurred these very minor post early campanian motions have probably been accommodated by imperfect shear and compression along the atlantic fracture zones to the east of the lesser antilles and along the northern and southern borders of the caribbean plate accordingly it is suggested that from campanian time to the present the relative motions between the north and south american plates have had only minor effects on the structural development of the caribbean region primarily using the data of engebretson et al the convergence history of pacific plates with north america was calculated for two points near the western caribbean by completing finite difference solutions the convergence history of the pacific plates with the caribbean and south american plates can be approximated the direction and rate of convergence of the pacific plates with the americas may have controlled the style of subduction and possible microplate migration along the north american south american and western caribbean boundaries that define the eastern pacific plate margin

ophiolite obduction in the quebec appalachians canada 40 ar 39 ar age constraints and evidence for syn tectonic erosion and sedimentation 1 2 3 detailed field work conducted in the dunnage zone of the quebec appalachians is herein combined with 40 ar 39 ar dating on a series of ophiolitic massifs crosscutting granites and associated metamorphic rocks occurring along the baie verte brompton line the taconian suture between laurentia and lower paleozoic peri laurentian oceanic terranes studied massifs are the lac brompton ophiolite and the riviere des plante ultramafic complex in southern quebec and the nadeau ophiolitic melange in the gaspe peninsula our work suggests that these massifs form remnants of eroded ophiolitic nappes which are unconformably overlain by the saint daniel and riviere port daniel melanges and correlate with the thetford mines and mont albert ophiolitic complexes our 40 ar 39 ar data and compiled regional age constraints indicate that ophiolite obduction was diachronous along the strike of the orogen the timing of obduction and melange formation varies according to the irregular geometry of the early paleozoic laurentian margin with earlier collision occurring along or at the margins of promontories obduction was initiated with the formation of infraophio litic metamorphic soles between ca 479 and 472 ma in southern quebec and the nadeau ophiolitic melange and pos sibly as late as ca 470 466 ma for the mont albert complex these sole rocks were later exhumed and translated onto the laurentian margin with the overlying ophiolites between 475 and 460 457 ma the uplifting and erosion of the orogenic wedge during the waning stages of obduction has resulted in the sedimentation of olistostromal melanges and onlapping flysch units above the ophiolitic nappes as well as foredeep flysch successions during the latest arenig to earliest caradoc