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

hydrologic control of carbon cycling and aged carbon discharge in the congo river basin the age of organic material discharged by rivers provides information about its sources and carbon cycling processes within watersheds although elevated ages in fluvially transported organic matter are usually explained by erosion of soils and sedimentary deposits 1 2 it is commonly assumed that mainly young organic material is discharged from flat tropical watersheds due to their extensive plant cover and rapid carbon turnover 3 7 here we present compound specific radiocarbon data of terrigenous organic fractions from a sedimentary archive offshore the congo river in conjunction with molecular markers for methane producing land cover reflecting wetland extent we find that the congo river has been discharging aged organic matter for several thousand years with apparently increasing ages from the mid to the late holocene this suggests that aged organic matter in modern samples is concealed by radiocarbon from atmospheric nuclear weapons testing by comparison to indicators for past rainfall changes we detect a systematic control of organic matter sequestration and release by continental hydrology mediating temporary carbon storage in wetlands as aridification also leads to exposure and rapid remineralization of large amounts of previously stored labile organic matter we infer that this process may cause a profound direct climate feedback that is at present underestimated in carbon cycle assessments

partial melting of metagreywackes part ii compositions of minerals and melts a series of experiments on the fluid absent melting of a quartz rich aluminous metagreywacke has been carried out in this paper we report the chemical composition of the phases present in the experimental charges as determined by electron microprobe this analytical work includes biotite plagioclase orthopyroxene garnet cordierite hercynite staurolite gedrite oxide and glass over the range 100 1000 mpa 780 1025 c biotites are na and mg rich with ti contents increasing with temperature the compositions of plagioclase range from an17 to an35 with a significant orthoclase component and are always different from the starting minerals at high temperature plagioclase crystals correspond to ternary feldspars with or contents in the range 11 20 mol garnets are almandine pyrope grossular spessartine solid solutions with a regular and significant increase of the grossular content with pressure all glasses are silicic sio2 67 6 74 4 wt peraluminous and leucocratic feo mgo 0 9 2 9 wt with a bulk composition close to that of peraluminous leucogranites even for degrees of melting as high as 60 vol with increasing pressure sio2 contents decrease while k2o increases at any pressure the melt compositions are more potassic than the water saturated granitic minima the h2o contents estimated by mass balance are in the range 2 5 5 6 wt these values are higher than those predicted by thermodynamic models modal compositions were estimated by mass balance calculations and by image processing of the sem photographs the positions of the 20 to 70 isotects curves of equal proportion of melt have been located in the pressure temperature space between 100 mpa and 1000 mpa with increasing pressure the isotects shift toward lower temperature between 100 and 200 mpa then bend back toward higher temperature the melting interval increases with pressure the difference in temperature between the 20 and the 70 isotects is 40 c at 100 mpa and 150 c at 800 mpa the position of the isotects is interpreted in terms of both the solubility of water in the melt and the nature of the reactions involved in the melting process a comparison with other partial melting experiments suggests that pelites are the most fertile source rocks above 800 mpa the difference in fertility between pelites and greywackes decreases with decreasing pressure a review of the glass compositions obtained in experimental studies demonstrates that partial melting of fertile rock types in the crust greywackes pelites or orthogneisses produces only peraluminous leucogranites more mafic granitic compositions such as the various types of calk alkaline rocks or mafic s type rocks have never been obtained during partial melting experiments thus only peraluminous leucogranites may correspond to liquids directly formed by partial melting of metasediments other types of granites involve other components or processes such as restite unmixing from the source region and or interaction with mafic mantle derived materials

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

natural nuclear fission reactors time constraints for occurrence and their relation to uranium and manganese deposits and to the evolution of the atmosphere abstract knowledge of the formation conditions of francevillian uranium and manganese ore deposits as well as natural fission reactors sheds light on the early evolution of the atmosphere between 1950 and 2150 ma ago the model explaining the formation of the oklo uranium deposits suggests that at the time of sediment deposition in the franceville basin 2150 million years ago the oxygen deficient atmosphere would have inhibited uranium dissolution dissolution of uranium was only possible during later diagenesis approximately 1950 ma reduction reactions in the presence of hydrocarbons allowed precipitation of dissolved uranium to u4 forming deposits with high enough uranium contents to trigger subsequent nuclear fission reactions such a model is in agreement with earlier suggestions that oxygen contents in atmosphere increased during a transition phase some 2450 2100 ma ago the manganese deposits were formed before the uranium deposits during the deposition of the black shales and very early diagenesis and thus at a time when oxygen content in atmosphere was very low carbon isotopes data of organic matter show decrease of 13c upward in the francevillian series 20 to 46 pdb reflecting the high ch4 and low o2 contents in the atmosphere during sediment deposition this favoured anoxic conditions during deposition of the basinal fb black shales and likewise the migration of mn over long distances the manganese precipitated first as mn oxides at the shallow edges of the franceville basin in photic zones where photosynthetic organisms flourished mn oxides were then reduced in the black shales forming mn carbonates when conditions became more reducing during transgression episodes and or the first stages of burial in the black shales reducing conditions prevailed until recent weathering allowing the good preservation of organic matter and the mn deposits the present day alteration is responsible for the dissolution of mn carbonates and precipitation of mn oxides at the water table to form the high grade mn ore 45 50 mn development of photosynthesizing organisms a volcanic source of the mn and favourable palaeogeography of the francevillian basins are all important parameters for the formation of the mn deposits for the occurrence of the natural nuclear reactors the age of 2 0 ga is the main parameter that controls the abundance of fissile 235u and the critical mass before 2 0 ga the 235u 238u ratio was sufficiently high for fission reactions to occur but conditions favourable for forming high grade uranium ores were not achieved then after 2 0 ga the increase of oxygen in the atmosphere commonly led to the formation of high grade uranium ores in which the 235u 238u ratio was too low to support criticality

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

heat flow and heat production in brittany western france new heat flow determinations in brittany western france allow mapping with a good coverage 1 point per 1000 km2 over that region a high heat flow zone oblique to the present geological structures trends from northern central brittany to the south and then turns to the ese towards the massif central heat production a and heat flow q are tested against a linear relationship the data do not fit a strict linear relationship but the points are distributed within an elongated cluster in which a linear regression can be approximated yielding the following equation q 26 6 3 5 mw m2 15 6 0 6 km a several tests of significance corroborate the linear cluster and do not alter the computed parameters the mapping of the residual values shows a geographical distribution which can be correlated with filtered gravimetric and magnetic data those residuals reflect local variations in heat production distribution or in the heat transport mechanism the high value of the slope 15 6 km computed during the linear regression reflects the influence of the upper to middle crust this value is too elevated to represent the thickness of the granitic intrusions which are known to be shallow rooted 4 6 km from gravity data interpretation compared to adjacent hercynian countries the data lie within the range of those of central and southwestern england

geochemical constraints of the petrogenesis of the o okiep koperberg suite and granitic plutons in namaqualand south africa a crustal source in namaquan grenville times the namaquan grenville orogeny late mesoproterozoic in the o okiep district is characterized by two tectono magmatic episodes the o okiepian episode 1210 1180 ma with the intrusion of batholitic granites and the klondikean episode 1040 1020 ma which includes the intrusion of the copper bearing koperberg suite and the rietberg granite this study focuses on the geochemistry major and trace elements sr and nd isotopes of intrusive rocks of the o okiep terrane to provide better constraints on their source rock characteristics and on their petrogenesis the o okiepian granites belong to the k rich granite kindred with shoshonitic affinities the concordia granite results from dehydration melting of a pelitic source the rietberg granite shows geochemical similarities with post collisional magmatic series the anorthosites and related rocks of the koperberg suite are cumulates their ree distribution is controlled by their apatite content a new rock type jotunite has been identified in the koperberg suite it is analogous to the rogaland chilled jotunite a characteristic which gives strong evidence that the koperberg rocks belong to the massif type anorthosite suite inversion modelling of plagioclase ree compositions from anorthosite permits the reconstruction of melt compositions and places constraints on the melting process and on the characteristics of the source rocks the occurrence of jotunite in the koperberg suite is strong additional evidence for a crustal source because jotunite is produced by remelting of gabbronorite under dry conditions the various intrusions in the koperberg suite show distinct isotopic signatures which resulted from isotopic heterogeneities of the crustal source and from minor contamination with the country rocks the characteristic negative end 1030 ma values 5t o 11 can be explained by remelting at 1030 ma of a 1900 ma old oceanic crust protolith with an enriched morb ree distribution the large range in sr initial ratios 0 709 0 748 may reflect hydrothermal alteration of the oceanic crust a process which may also explain the cu enrichment the koperberg intrusions were produced by forceful injection of cumulate crystal mush which were differentiated in deeper magma chambers or in conduits

recycled metaigneous crustal sources for s and i type variscan granitoids from the spanish central system batholith constraints from hf isotope zircon composition abstract the spanish central system scs is one of the largest granite batholiths in the european variscan belt zircons from five granitic intrusions from the eastern scs have been separated and analysed for hf isotopes by laser ablation mc icpms in order to evaluate the nature of granitic magma source two samples are cordierite bearing s type granites the rest are amphibole bearing i type monzogranites hf isotope composition of zircons from these granites defines a narrow range of negative hf values 1 1 to 5 8 typical of crustal protoliths the within sample variation of zircon hf isotope composition is slightly above the analytical uncertainty 1 5 units suggesting the mixing of different magma batches during pluton assembly the s and i type granites display the same range of initial hf values that is in agreement with previous data showing that both scs granite types have similar sr nd o pb isotope signatures these isotopic data suggest that they derived from similar sources while the variation in peraluminosity could be related to different partial melting conditions the presence of a lower ordovician inherited zircon population 478 462 ma at least in i type granites suggests the involvement of cambro ordovician orthogneisses as components in the source of some scs granites inherited zircon population of similar age has also been identified in the scs lower crustal granulites the hf isotope composition of the analysed granite zircons is within that of zircons from the above granulites moreover some evidence of a juvenile input at 560 595 ma is recorded by hf isotope data in zircons from both the scs granites and granulite xenoliths hf isotope signature in conjunction with other chemical isotopic sr nd o pb and geochronological evidences suggests that scs granites were probably derived from metaigneous lower crustal sources the variscan magmatism in central spain is dominantly a crustal reworking event

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

geochemical and isotopic characteristics of blueschist facies rocks from the ile de groix armorican massif northwest france abstract nine samples of blueschist facies metabasic rocks and four samples of associated metasedimentary lithologies were analysed for ree contents and nd isotopic composition sampling includes the main rock types exposed on groix concentrating on those metabasic rocks with least dispersed rb sr systems the ree abundance patterns appear not to have suffered significant alteration since emplacement of the protolith magma despite subsequent blueschist facies metamorphism the data when integrated into a wider geochemical data base allow the identification and characterization of both tholeiitic and alkaline magma types in the original igneous suite which was differentiated from depleted mantle sources the basalts probably originated in an oceanic context some sediments are predominantly volcaniclastic in which a continental crustal component may be recognized other sediments are closer in composition to continental crust with variable additions of tholeiitic or alkali basalt debris the volcanic sedimentary assemblage on groix is suggestive of ocean island magmatic activity associated with a sedimentary sequence derived from a nearby continent

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

a model for monazite melt equilibrium and application to the generation of granitic magmas abstract in order to model the behaviour of zr hf ti p u th y and the rare earth elements ree in the continental crust the parameters which control the equilibria between accessory minerals and granitic melts must be determined this paper proposes equations for the equilibrium between monazite and ca poor felsic melts in a first step all the light ree are considered as a single component and an equation describing monazite solubility is deduced from the available experimental data in a second step the fractionation of ree by monazite is investigated on the basis of a natural example of coexisting monazite and obsidian from macusani peru the solubility of monazite can be used as a thermometer based on the total ree content of natural magmatic rocks application of ree thermometry to various peraluminous rock series suggests that it is a reliable tool for determining temperatures of magmatic processes the equation governing ree fractionation by monazite is used to discuss genetic relationships between a melt and a presumed parental rock using the composition of the monazite in the parental rocks and the ree pattern of the daughter rock this method has been applied successfully to high himalayan manaslu and tibetan slab granites it is suggested that detailed petrographical work on monazite including electron microprobe analyses and sem imaging of its internal structures combined with the model presented here can be a useful tool for deciphering the genesis and the differentiation of peraluminous granitic magmas in the continental crust

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

a review of the relationships between granitoid types their origins and their geodynamic environments abstract granitoids are divided into several types according to their mineral assemblages their field and petrographical features and their chemical and isotopic characteristics this typology complements most of the recent classifications because it is not based solely on chemical and isotopic criteria but also on the field petrographical and mineralogical criteria it thus has the advantage of distinguishing the various granitoid types in the field in most cases the proposed classification shares many similarities with the twenty most used genetic classifications of granitoids both types of peraluminous granitoids are of crustal origin the tholeiitic alkaline and peralkaline granitoids are of mantle origin and both types of calc alkaline granitoids are of mixed origin and involve both crustal and mantle materials each granitoid type is generated and emplaced in a very specific tectonic setting each stage of the wilson cycle is characterised by typical associations of granitoids well typed and precisely dated granitoids can then complement structural approaches and indicate on the geodynamic environment with reference to some case studies the use of granitoids as tracers of the geodynamic evolution is also proposed and discussed

petrogenesis of the concordia granite gneiss and its relation to wmo mineralization in western namaqualand south africa abstract the 1 1 ga concordia granite gneiss cgg is part of the late to postorogenic spektakel suite in the western namaqualand metamorphic complex south africa it intruded synkinematically with respect to the main d2 deformation event into lower to middle crustal rocks and granite emplacement was more or less coeval with the peak of granulite facies metamorphism 800 c 5 kbar several genetically related rock types megacrystic garnet bearing granite minor aplitic leucogranites and pegmatites are distinguished all varieties are sio 2 rich 69 79 wt peraluminous granites and show subalkaline monzonitic magma characteristics geochemical differences in whole rock chemistry between megacrystic granite and aplitic leucogranites e g lower al 2 o 3 mgo cao ba zr higher k 2 o rb nb w rb sr ga al and the decrease of e g cao mgo fe 2 o 3 ba zr th u with increasing sio 2 in the megacrystic granites as well as the variation in fe mn of magmatic garnets are best explained with crystal fractionation processes fractional crystallization of plagioclase produced potassium and silica rich residual melts characterised by very high rb sr rb ba u th mn fe ratios and higher concentration of w cu and zn crystal fractionation processes also resulted in a relative lree depletion and hree enrichment megacrystic granite la lu cn 8 87 31 67 aplitic leucogranite and pegmatites la lu cn 0 71 1 44 and evolution of pronounced negative eu anomalies the crystallization sequence near solidus crystallization of biotite prior to alkali feldspar suggests that the cgg magmas were h 2 o undersaturated over a long period of their evolution water saturation during late stage crystallization is however indicated by coarse late stage eutectic mineral textures pegmatites and w mo bearing siliceous rocks furthermore the economic potential of the cgg is supported by its geochemical signature e g high u th contents which is similar to evolved high heat production hhp granites the granitic magmas are attributed to partial melting of peraluminous crustal source rocks and are tentatively interpreted as fractionated s type granites the w mo deposits represent vein type and pegmatitic deposits genetically related to a deep seated granitic system

zircon typology combined with smnd whole rock isotope analysis to study brioverian sediments from the armorican massif the identification of source materials contributing to mature terrigenous clastic deposits is made difficult due to a lack of useful discriminant criteria sm nd isotope analysis can provide some indication of the present of mantle derived constituents in clastic rocks however since this method is based on whole rock samples it is only possible to obtain the averaged composition of the different source materials involved moreover sm nd isotope systematics can be strongly influenced by the presence of heavy minerals rich in rare earth elements and or displaying isotopic ratios very different to the sedimentary host in this manner 0 1 of zircon or 0 01 of monazite from an extraneous source is sufficient to modify significatively the sm nd signature of the whole rock the typological study of zircon populations is an extremely valuable tool especially since this mineral is highly resistant and exhibits a morphology controlled by the physical and chemical conditions under which it crystallized zircons that have been reworked in a sedimentary deposit display typologies that make it possible to identify the different igneous rock types present in the source region moreover the typological study can be associated with a single grain pb pb dating this twofold approach was applied to brioverian sedimentary rocks neoproterozoic iii to early palaeozoic in age from the central brittany domain armorican massif nw france it is proposed that these deposits inherited a major component from juvenile crustal materials end 540 1 4 to 6 3 reflecting the presence of igneous precursors with a dominantly mantle derived origin zircon sub types s19 s20 s24 s25 etc to which was added another component of crustal anatectic affinity zircon sub types s2 s6 s7 etc two igneous source regions are identified on the basis of the present study the anatectic granitic massifs of the mancellian batholith 540 ma along with a coeval ignimbritic suite of end proterozoic eocambrian age and the syn and post tectonic dioritic massifs 590 to 530 ma apart from these main sources of clastic supply it is very likely that sporadic contributions arose from basic to intermediate volcanic activity of mantle origin linked to the cadomian orogeny and from the icartian basement

inferring protoliths of high grade metamorphic gneisses of the erzgebirge using zirconology geochemistry and comparison with lower grade rocks from lusatia saxothuringia germany protoliths of highly metamorphosed gneisses from the erzgebirge are deduced from the morphology age and chemistry of zircons as well as from whole rock geochemistry and are compared with lower grade rocks of lusatia gneisses with similar structural appearance and or geochemical pattern may have quite different protoliths the oldest rocks in the erzgebirge are paragneisses representing meta greywackes and meta conglomerates the youngest group of zircon of meta greywackes that did not undergo pb loss represents the youngest igneous component for source rocks about 575 ma similar ages and zircon morphology reflect the subordinate formation of new zircon grains or only zircon rims in the augengneiss from barenstein and wolkenstein which probably represent metamorphic equivalents to lower cambrian two mica granodiorites from lusatia bulk rock chemistry intense fracturing and high u and th concentrations of zircons suggest deformation induced and fluid enhanced recrystallisation of zircon grains temperatures during tectonic overprinting too low to reset zircon ages indicate mid or upper crustal levels for shearing recorded in these augengneisses lower cambrian 540 ma granodiorites are widespread in lusatia but are exclusively represented by the freiberg gneiss dome in the eastern erzgebirge ordovician protolith ages were recorded by zircons from the augengneisses of the reitzenhain catherine dome and the schwarzenberg dome western erzgebirge documenting significant regional differences between the eastern and the western erzgebirge 540 vs 490 ma in the western erzgebirge most meta volcanic rocks muscovite gneisses and meta granites mainly red augengneisses yield ordovician zircon ages whereas in the eastern part similar rocks mainly recorded lower cambrian protolith ages zircon overprinting was highest within discrete tectonic zones where the combination of fluid infiltration and deformation induced variable degrees of recrystallisation and formation of a new augengneiss structure variable degrees of pb loss caused age shifts that do not correspond to changes in zircon morphology but may be associated with u and th enrichments major changes in bulk rock composition appear to be restricted to discrete zones and to u hp nappes whereas gneisses with a mp mt metamorphic overprint basically show no geochemical modifications

crustal generation of the himalayan leucogranites abstract detailed studies of the himalayan two mica leucogranites such as the manaslu pluton indicate that they have very uniform mineralogical petrological and structural characteristics one can relate their occurrence to the thickest zones of the underlying tibetan slab in these zones migmatization attains its greatest development and vertical extension the granite is emplaced at first along the main disharmonic plane above the main central thrust mct at the top of the tibetan slab infrastructure ductile deformation of the granite is variable the granite being syn to late kinematic with regard to the functioning of the mct major elements are very homogeneous except for na and k implying that p t conditions of melting were relatively uniform the melted material was of a similar composition over a vast volume and the percentage of melting was small 10 15 trace elements are highly variable some are characteristic for very evolved material ta rb cs u or show the link with the tibetan slab ba sr whilst others are problematic th ree ree and th abundances being much less in the granite than in the tibetan slab imply that they have been extracted during one of the main stages of formation possibly by monazite radiogenic pb sr nd and stable o isotopes are consistent with the origin of the granite from the tibetan slab however the heterogeneous sr isotopic ratios make age dating difficult and imply poor mixing or little fluid interaction during its evolution h isotope data indicate that magmatic compositions of the main body of the manaslu granite have been preserved late or post magmatic alterations are extremely local in the main pluton to the north another belt of two mica granites occurs whose characteristics are very similar to the high himalaya belt they were probably generated in a similar way during this recent intracontinental evolution

rb sr and 40ar 39ar ages in blueschists from the ile de groix armorican massif france implications for closure mechanisms in isotopic systems rb sr and 40ar 39ar data mainly ranging from 345 5 to 361 5 ma and from 348 4 1 4 to 364 6 1 4 ma respectively have been obtained on phengite from the hp lt rocks from the ile de groix armorican massif france which represents one of the few occurrences of variscan blueschists petrological data show two blueschist facies units recording distinct peak p t conditions about 16 20 kbar 500 c and 12 16 kbar 450 c in the upper and lower units respectively followed by a nearly isothermal decompression in the greenschist facies rb sr and 40ar 39ar results defining two groups of ages mainly around 345 353 ma and 358 366 ma are consistent in well preserved blueschist facies rocks as well as in strongly overprinted samples from both units one sample excepted there is no evidence of inherited or excess argon and resetting of the isotopic systems oldest rb sr and 40ar 39ar ages are found in rocks with well preserved blueschist facies parageneses while younger ages are obtained on samples with a strong greenschist facies overprint this suggests that the closure of the k ar system is diachronous in samples which record the same p t evolution a closure of the k ar clock at around 450 500 c in phengite from rocks left nearly undeformed after the blueschist facies metamorphism is in agreement with petrological geochemical and structural data thus in these samples the oldest rb sr and 40ar 39ar ages around 360 370 ma represent the age of the hp metamorphism in the overprinted samples later closure of the k ar and rb sr clocks is the result of retrogression and continuing ductile deformation the small age difference between the oldest and the youngest rb sr and 40ar 39ar ages around 10 20 ma argues in favour of a relatively fast cooling

reconstructing fluid flow events in lower triassic sandstones of the eastern paris basin by elemental tracing and isotopic dating of nanometric illite crystals lower to middle triassic sandstones from eastern paris basin were buried to a maximum depth of 2500 m at a paleo temperature of about 100 c they contain extensive amounts of authigenic platy and filamentous illite particles similar to those reported in reservoirs generally buried at 3000 to 5000 m and subjected to temperatures of 120 to 150 c to evaluate this unexpected occurrence such sandstones were collected from drill cores between 1825 and 2000 m depth and nanometric sized sub fractions were separated the illite crystals were identified by xrd observed by sem and tem analyzed for their major trace rare earth elements and oxygen isotope compositions and dated by k ar and rb sr illite particles display varied growth features in the rock pore space and on authigenic quartz and adularia that they postdate tem eds crystal chemical in situ data show that the illite lath fiber and platelet morphologies correspond at least to two populations with varied interlayer charges between 0 7 and 0 9 in the former and between 0 8 and 1 0 in the latter the fe fe mg ratio being higher in the platelets except for the deeper conglomerate the paas normalized ree patterns of the illite crystals are bell shaped enriched in middle rees ca carbonates and ca phosphates were detected together with illite in the separates these soluble components yield 87sr 86sr ratios that are not strictly in chemical equilibrium with the illite crystals suggesting successive fluids flows with different chemical compositions the k ar data of finer 0 05 m illite separates confirm two crystallization events at 179 4 4 5 and 149 4 2 5 ma during the early and late jurassic the slightly coarser fractions contain also earlier crystallized or detrital k bearing minerals characterized by lower 18o values the 18o of the finest authigenic illite separates tends to decrease slightly with depth from 18 2 0 2 to 16 3 0 2 suggesting different but contemporaneous crystallization conditions deeper in the section the illite platelets and filaments crystallized in changing physical chemical crystallization conditions induced by fluids flows through the host rock pore system these flow events were probably driven by repetitive rifting episodes of the north atlantic ocean although located several hundreds kilometers away from eastern paris basin and or by fracturing events in the nearby basement of the vosges massif complex relationships between geodynamical events thermal anomalies and advective fluids confirm that remote tectonic activities can impact quiescent basins even if located far from tectono thermal activities by discrete and long distance fluid flows

baltica and gondwana derived sediments in the mid german crystalline rise central europe implications for the closure of the rheic ocean abstract combined u pb and lu hf isotope analysis of detrital zircons reveals that the mid german crystalline rise mgcr contains palaeozoic sediments of distinct provenance in close proximity zircons from a metapelite of the brotterode group were derived from a gondwanan source whereas zircons from the rogis quartzite of the ruhla group point to provenance from baltica brotterode zircons mostly yield ages of 500 720 ma and 2 0 2 1 ga and show an age gap between 1 0 and 1 7 ga which is characteristic of gondwana peri gondwanan detrital zircon populations combined u pb and hf isotope data suggest intense crustal recycling at 2 6 2 9 ga 1 8 2 1 ga and 570 720 ma as well as formation of juvenile crust during the latter two periods in contrast zircons of the rogis quartzite mostly yield ages between 0 9 and 1 8 ga which are typical of southwestern baltica minor age populations at 435 466 and 550 660 ma indicate that the quartzite is younger than late ordovician early silurian and that the baltica derived sediment is contaminated by minor peri gondwanan perhaps avalonia derived detritus minor age populations at 2 0 2 56 2 67 and 2 86 ga additionally point to a svecofennian and or karelian source u pb and hf isotope data imply that the baltica derived zircons were produced mainly through melting of juvenile palaeo to mesoproterozoic crust the data presented here indicate that the mgcr hosts the rheic suture between gondwana and balonia and that this suture zone was closed in a complex sequence of stages from the late silurian onwards

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

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

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

lidar offshore structural mapping and u pb zircon monazite dating of variscan strain in the leon metamorphic domain nw brittany an exceptional structural picture of the immerged variscan basement offshore the leon metamorphic domain is supplied by high resolution lidar and echosounder data recorded in the molene archipelago western brittany france various types of fabrics are identified and from in situ rock sample analyses further combined with field structural data are interpreted on a lineament trajectory map as the trace of magmatic and tectonic structures our onshore offshore study leads us to propose a two phase kinematic model that emphasizes the role of a strike slip duplex in an ew trending relay zone linking the north armorican and pierres noires ductile shear zones nasz pnsz dextral shearing occurred within a transtensional setting synchronously with magmatic intrusions st renan granite and an offshore gabbro diorite complex dated at 314 320 ma by new u th pb ages it post dated an early regional foliation related to top to the ne ductile transpressional shearing our study emphasizes the key role of strike slip tectonics in the nw part of the armorica variscan belt

the late variscan ht lp metamorphic event in nw and central iberia relationships to crustal thickening extension orocline development and crustal evolution

cadomian tectonics in northern brittany a contribution of 3 d crustal scale modelling abstract both available and newly acquired geological and geophysical data processed in the framework of the armor project have been used to construct a 3 d geometric and geological model of the cadomian domain of northern brittany the major results of the data processing are presented and discussed relative to a composite profile transecting the studied domain and constructed from the surface geology reflection seismics magnetotelluric soundings and gravimetric data they are then extrapolated at regional scale through a series of cross sections compiled from the surface geology and gravimetric data and calibrated onto the composite profile the 3 d crustal scale models were computed according to two procedures the first using a strim matra datavision modeler defines various interfaces through correlation between adjacent cross sections and is well adapted to the 3 d representation of geological structures the second based on voronoi diagrams leads to a volumetric model of the geological units and is a powerful tool for calculating 3 d geophysical responses of the model when the physical properties of the rock units are known the tectonic implications of the computed 3 d crustal scale geometry of the cadomian domain are then discussed in terms of volcanic arc thrusting with special emphasis being placed on the obliquity of the shortening direction by reference to the thrust fault trend finally the data are used to demonstrate the reactivation of some cadomian faults during the deposition of ordovician sandstones and following the emplacement of the paleozoic dolerite dyke swarm of northern brittany

les gisements uraniferes de la presqu ile guerandaise sud bretagne approche structurale et metallogenique les gisements et indices d uranium de la presqu ile guerandaise bretagne meridionale sont comme en vendee lies spatialement a un leucogranite a muscovite appartenant a l ensemble important de granites localises au voisinage des zones de cisaillement liees a la tectogenese hercynienne les mineralisations consistent en un remplissage filonien presque exclusivement de pechblende avec phases sulfurees subordonnees les etudes structurales montrent que les mineralisations se sont developpees dans un reseau ouvert de paraclases qui resultent d une phase tectonique bien definie celle ci consiste en des cisaillements senestres est ouest regionaux developpant la fracturation preferentielle de certains niveaux metamorphiques encaissant septentrional du granite ou enclaves metamorphiques l influence des facteurs mecaniques semble primordiale dans le depot de l uranium

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

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

genesis of the two main types of peraluminous granitoids two mica monzogranites to leucogranites and biotite rich cordierite bearing tonalites to monzogranites form two distinct groups of peraluminous granitoids they can be distinguished by mineral and rock associations and by the variation of their peraluminosity during differentiation except for the rare muscovite bearing granitoids produced by extreme fractionation or local contamination of metaluminous magmas the majority of peraluminous granitoids are produced by partial melting of crustal rocks production of either muscovite bearing granites or biotite rich cordierite bearing granitoids does not depend only on the nature of the sources but is also controlled by the physical parameters of partial melting and consequently by the way anatexis of a thickened crust is enhanced biotite rich cordierite bearing granitoids are generated where mantle derived magmas are injected into or have underplated crustal rocks two mica granites are generated where thickened crust is affected by major crustal shears or thrusts correlations between differentiation and peraluminosity indicate the dominant role of either restite unmixing or fractional crystallization in the production and evolution of the various types of peraluminous granitoids

petrogenesis of late variscan high k alkali calcic granitoids and calc alkalic lamprophyres the aber ildut north ouessant complex armorican massif france the aber ildut north ouessant variscan granitoid complex in the armorican massif is an example of high k alkali calcic zoned pluton c 304 ma in age a first magmatic batch intruded through a northern ew trending sinistral transcurrent shear zone before injecting southwards as a huge horizontal zoned sill with moderately peraluminous muscovite free granitoids in the north and strongly peraluminous muscovite bearing leucogranites to the south the second magmatic stage resulted in the intrusion of a large two mica leucogranitic body from a root zone along the same shear zone prior to the end of crystallization of the first injection finally ultrapotassic dykes including calc alkalic lamprophyres leucominettes intrude the complex the strongly peraluminous granites are interpreted as pure melting products of crustal clay rich pelitic material all the other petrographic types including leucominettes are thought to result from mixing of crustal melts and mantle derived mafic liquids highly silicic tourmaline bearing leucogranites are significantly affected by a tetrad effect in relation with ree complexing behavior aber ildut north ouessant granitoids probably formed during the variscan late orogenic stage associated with exhumation and lithospheric thinning at low pressure and by advective heating of a 50 km thick crust from hot metasomatized asthenosphere

meta igneous origin of hercynian peraluminous granites in n w french massif central implications for crustal history reconstructions seventy samples of hercynian peraluminous granites gueret millevaches and saint sylvestre massifs and metamorphic units of the limousin area were analysed for rb sr and sm nd the source rocks of the peraluminous granites can be found in the metamorphic rocks of limousin among them meta igneous rocks were largely predominent over meta sedimentary rocks in the source of the three granites millevaches and gueret granites were generated by the partial melting of rocks comprising meta volcanics and meta sediments whereas the saint sylvestre granite was produced exclusively by the melting of late precambrian granites this leads to confusing t dm nd values the confusion being amplified by the segregation of monazite during the petrogenetic evolution of the peraluminous granites which leads to dramatic fractionation in sm nd ratios the data of the present study tend to demonstrate that peraluminous granites do not give a good representation of isotopic mean crustal estimates late precambrian time seems however to have been a period of extensive crustal generation in western europe

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