crustal scale mass transfer geotherm and topography at convergent plate boundaries end member cases for the fate of the crust at convergent lithospheric plate boundaries accommodated by subduction are considered combining i plate kinematics slab retreat vs slab advance and ii mechanical coupling decoupling within the subducting slab slab advance is accommodated by thickening of the overriding lithospheric mantle whereas slab retreat is associated with thinning crust mantle mechanical coupling results in continental subduction whereas decoupling leads to continental accretion the thermal and topographic evolutions of the associated orogenic belts are functions of the crust mantle thickness ratio resulting from the combined effects of crust mantle coupling decoupling and slab advance retreat crustal accretion associated with slab advance results in crustal thickening and contributes to an increase in radioactive heat production and buoyancy which is impeded by concomitant thickening of the conductive and dense lithospheric mantle in contrast crustal accretion combined with slab retreat results in thickening simultaneous with thinning of the lithospheric mantle this peculiar situation is the most favourable for generating a high temperature and buoyant orogenic belt

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

experimental melting of biotite plagioclase quartz muscovite assemblages and implications for crustal melting abstract in order to understand the role of mica rich rocks as a source of granite magmas a series of melting experiments was performed on two different starting materials the first composition is a model biotite gneiss consisting of 30 wt biotite 30 wt plagioclase and 40 wt quartz the second composition is a model two mica pelites consisting of 15 wt biotite 15 wt muscovite 30 wt plagioclase and 40 wt quartz experiments were performed under vapor absent conditions at 1 0 gpa and between 750 o and 950oc with only biotite in the starting material the volume of melt is always less than 15 vol below 900oc and reaches 25 vol at 950oc in experiments that involve both biotite and muscovite in the starting material the melt proportion increases up to 28 vol at 825oc and reaches 60 vol at 950oc for the biotite plagioclase quartz bpq assemblage the solidus is located at 800oc at 1 0 gpa the melting reaction produces a metaluminous granitic liquid and leaves a residuum consisting of garnet biotite orthopyroxene plagioclase quartz in addition the experiments show that at 1 0 gpa biotite can be stable above 950oc with both micas in the starting material bpqm the solidus at 1 0 gpa is located at 750oc the melting reactions produce a peraluminous granitic liquid and leave a residuum of garnet sillimanite biotite quartz plagioclase kfeldspar in experiments below 900oc at 950oc the residuum consists of garnet orthopyroxene biotite plagioclase the melt fraction is determined by the proportions of the hydrous phases and of the amount of feldspar relative to quartz mineral modes of the source rocks particularly the amount of quartz are at least as important as the amount of available h20 in controlling the melt fraction generated during crustal anatexis

geochemical and isotopic study of a norite eclogite transition in the european variscan belt implications for upb zircon systematics in metabasic rocks abstract three different geochronological techniques u pb conventional multigrain single zircon stepwise pb evaporation and sm nd on minerals were applied to rocks from a norite eclogite transition observed in the west european variscan belt and combined with major and trace element including ree geochemistry the studied rocks belong to a cogenetic tholeiitic suite which originated in an immature back arc basin or within plate magmatic environment the initial ndt 480 ma values around 1 2 imply that the noritic protoliths were emplaced in a geotectonic setting in close association with the continental crust u pb mulfgrain and the sm nd dating of a norite sample indicate an age of 481 ma for the basic magmatism sm nd dating on two whole rock garnet pairs yields an age of 408 ma for the eclogite facies metamorphism these two ages are consistent with the present understanding of the european variscan belt finally the single zircon pb evaporation study of an eclogite sample demonstrates the co existence of several types of zircons each zircon population yields a particular type of geochronological information related to the contrasting behaviours of the u pb isotopic system during the eclogite facies tectonometamorphic event these analytical results suggest that complex and heterogeneous behaviour within the u pb zircon system may also be recorded in numerous other samples this last point raises the question of the significance of results obtained by u pb conventional multigrain dating in metabasic rocks

collapse of the thickened variscan crust in the french massif central mont pilat extensional shear zone and st etienne late carboniferous basin abstract a large scale extensional shear zone is described from the mont pilat area in the northeastern part of the french massif central this shear zone is about 2 km thick and is characterized by well developed n dipping foliation and stretching lineations deformation occurred under conditions of retrograde metamorphism from high temperature conditions to greenschist facies it is superimposed on a composite foliation related to the earlier variscan 380 350 ma compressional events associated with southward emplacement of nappes and hp mt metamorphism regional n s lineations and shear criteria observed at a regional scale indicate a northward displacement of the hanging wall during extension large strains locally recorded in thick mylonitic zones suggest that displacement was substantial some tens of kilometres late superimposed extensional brittle deformation and pseudotachylites show that northward displacement on normal faults continued at a shallower depth a large granitic pluton granite du velay intruded late in the the variscan orogeny was associated with extension some small leucogranitic syntectonic melts were emplaced within the shear zone and are strongly deformed geochronological rb sr data on these leucogranite bodies give an age of 320 ma and 40 ar 39 ar dating of synkinematic biotites from gneissose rocks in the shear zone yields a plateau age of 313 ma these data show that extensional ductile deformation and magmatism were still active during the latest late carboniferous thus extension occurred immediately after the well known late palaeozoic collision type compressional tectonics which had significantly thickened the crust we propose that the extensional tectonism described here is due to the gravitational collapse of the initially thick variscan crust the stephanian intracontinental sedimentary basin of st etienne was formed along the hanging wall of the normal shear zone during the late stages of this post orogenic crustal extension

late visean thermal event in the northern part of the french massif central new 40ar 39ar and rb sr isotopic constraints on the hercynian syn orogenic extension he french massif central belongs to the central metamorphic domain of the european hercynian belt 40ar 39ar mica and amphibole dates have been obtained on magmatic and metamorphic rocks from the northern part of the french massif central sioule and brevenne violay series the 336 to 337 ma dates recorded by a granodiorite the st gervais pluton and its mylonitic border along the ste christine fault and the 337 5 ma muscovite ages from the brevenne series are consistent with the age of wrenching related to the late stage of the hercynian compression the remaining age spectra from the sioule area provide ages ranging from 331 to 335 ma that are interpreted as cooling ages related to rock exhumation after the main stages of metamorphism and deformation in the studied area late visean dates widespread in north massif central correspond to a huge magmatic event characterized by the emplacement of red granites granophyres and sub aerial acidic volcanics known as the tufs anthraciferes formation rb sr isotopic data from volcanic and hypovolcanic rocks from the sioule and montagne bourbonnaise areas place constraints on the source of the magmas simple metapelite crustal melting or a major contribution from depleted mantle sources are ruled out and instead mantle derived magmas were probably involved at least as a heat source during the late visean magmatism exhumation and cooling of metamorphic rocks below 300 c occurred contemporaneously with this late visean magmatism in the geodynamic evolution of the hercynian belt the late visean ca 335 330 ma magmatism corresponds to a major break marking the onset of post collisional extension in the inner zone which had been thickened by collision in latest devonian earliest carboniferous this late visean extension in the inner zone which is synchronous to compression in the northern and southern forelands of the belt appears as a syn orogenic process

dehydration melting of amphibolite at 10 kbar the effects of temperature and time we have simulated the dehydration melting of a natural low k calcic amphibolite 67 4 hornblende 32 5 anorthite in piston cylinder experiments at 10 kbar and 750 1000 c for 1 9 days the solidus temperature is lower than 750 c garnet appears at 850 c the overall reaction is hb p l cpx al hb ca hb ga opx three stages of reaction are 1 melting dominated by the growth of clinopyroxene and garnet with little change in composition of liquid or garnet 2 a reversal of this reaction between 875 c and 900 c with decreases in the amounts of liquid and garnet and 3 a large increase in liquid along with the loss of hornblende and decrease of plagioclase while clinopyroxene and garnet increase garnet is enriched in pyrope and zoned from fe cores to mg edges range 3 mol pyrope liquid composition is enriched first in an to 950 c and then in ab the liquids are more calcic and aluminous than natural tonalites which is attributed to the plagioclase composition an90 the formation of peraluminous liquid from the metaluminous amphibolite is caused by anorthite not h2o saturated conditions the results are consistent with an amphibolite phase diagram with relatively high solidus temperatures in the garnet absent field 900 1000 c but with a solidus backbend at 7 9 kbar coincident with the garnet in boundary hornblende breakdown due to garnet formation in a closed system must make h2o available for h2o undersaturated melting right down to the h2o saturated solidus below 700 c which defines a large low temperature pt area where hydrous granitoid melts can be generated with residual garnet and hornblende

slab detachment during continental collision influence of crustal rheology and interaction with lithospheric delamination abstract collision between continents can lead to the subduction of continental material if the crust remains coupled to the downgoing slab a large buoyancy force is generated this force slows down convergence and promotes slab detachment if the crust resists to subduction it may decouple from the downgoing slab and be subjected to buoyant extrusion we employ two dimensional thermo mechanical modelling to study the importance of crustal rheology on the evolution of subduction collision systems we propose simple quantifications of the mechanical decoupling between lithospheric levels and the potential for buoyant extrusion of the crust the modelling results indicate that a variable crustal rheological structure results in slab detachment delamination or the combination of both mechanisms a strong crust provides coupling at the moho low and remains coherent during subduction low it promotes deep subduction of the crust 180 km and slab detachment exhumation occurs in coherent manners via eduction and thrusting slab detachment triggers the development of topography 4 5 km close to the suture a contrasting style of collision occurs using a weak crustal rheology mechanical decoupling at the moho high promotes the extrusion of the crust high disabling slab detachment ongoing shortening leads to buckling of the crust and development of topography on the lower plate collisions involving rheologically layered crust allow decoupling at mid crustal depths this structure favours both the extrusion of upper crust and the subduction of the lower crust such collisions are successively affected by delamination and slab detachment topography develops together with the buoyant extrusion of crust onto the foreland and is further amplified by slab detachment our results suggest that the occurrence of both delamination apennines and slab detachment himalayas in orogens may indicate differences in the initial crustal structure of subducting continental plates in these regions

disturbance versus preservation of u th pb ages in monazite during fluid rock interaction textural chemical and isotopic in situ study in microgranites velay dome france monazite is extensively used to date crustal processes and is usually considered to be resistant to diffusive pb loss nevertheless fluid assisted recrystallisation is known to be capable of resetting the monazite chronometer this study focuses on chemical and isotopic disturbances in monazite grains from two microgranite intrusions in the french central massif charron and montasset petrologic data and oxygen isotopes suggest that both intrusions have interacted with alkali bearing hydrothermal magmatic fluids in the charron intrusion regardless of their textural location monazite grains are sub euhedral and cover a large domain of compositions u pb chronometers yield a lower intercept age of 297 4 ma an inherited component at 320 ma is responsible for the scattering of the u th pb ages the montasset intrusion was later affected by an additional f rich crustal fluid with crystallisation of ca ree fluorocarbonates fluorite calcite and chloritisation pristine monazite is chemically homogeneous and displays 208pb 232th and 206pb 238u concordant ages at 307 2 ma by contrast groundmass monazite shows dissolution recrystallisation features associated with apatite and thorite precipitation th silicate and strong chemical reequilibration 208pb 232th ages are disturbed and range between 270 and 690 ma showing that the th pb ratio is highly fractionated during the interaction with fluids apparent u pb ages are older due to common pb incorporation yielding a lower intercept age at 312 10 ma the age of the pristine monazite these results show that f rich fluids are responsible for th mobility and incorporation of excess pb which thus strongly disturbed the u th pb chronometers in the monazite

the moldanubian zone in the french massif central vosges schwarzwald and bohemian massif revisited differences and similarities in order to portray the main differences and similarities between the northeastern variscan segments french massif central fmc vosges black forest and bohemian massif bm we review their crustal scale architectures the specific rock associations and lithotectonic sequences as well as the ages of the main magmatic and metamorphic events this review demonstrates significant differences between the moldanubian domains in thebmand the fmc on this basis we propose distinguishing between the eastern and western moldanubian zones while the vosges black forest mountains are an intermediate section between the bm and the fmc the observed differences are the result of first the presence in the french segment of an early large scale accretionary system prior to the main variscan collision and second the duration of saxothuringian armorican subduction which generated long lived magmatic arc and back arc systems in the bohemian segment while the magmatic activity in the fmc was comparably short lived

late visean hidden basins in the internal zones of the variscan belt u pb zircon evidence from the french massif central u pb zircon ages from volcanic rocks interbedded in two upper carboniferous basins of the variscan massif central france document the presence of an unexpected late visean sedimentary sequence zircons extracted from a fireclay at the base of the sedimentary pile in the bosmoreau basin provide an age of 332 4 ma identical to the 333 2 ma age obtained from a rhyolitic lava sampled in the decazeville basin in addition both volcanic rocks contain inherited or xenocrystic zircons reflecting the ages of the basement and or source components the bosmoreau fireclay contains a limited amount of xenocrystic zircons having ages of ca 390 400 ma whereas inheritance is much more abundant in the decazeville rhyolite which includes a mesoproterozoic source component 1 0 ga the ca 330 ma ages demonstrate that these two basins with mainly stephanian sedimentary infill were actually initiated during the late visean and are the first known upper crustal evidence of the visean westphalian extensional event recognized previously only in midcrustal plutonic bodies moreover the location of these basins in the hanging wall blocks of two major fault systems i e the argentat and sillon houiller faults suggests a genetic link between fault motion and basin opening as early as the late visean these results indicate that magmatism and associated volcanism block movement along major crustal scale faults erosion and basin opening were contemporaneous and shed new light on the visean westphalian synconvergent extension that was previously understood only as a deep seated event

is there a remnant variscan subducted slab in the mantle beneath the paris basin implications for the late variscan lithospheric delamination process and the paris basin formation abstract the paris basin northern france is a late paleozoic mesozoic intracratonic basin that settled upon the collapsed variscan collisional belt the lithospheric roots of the variscan orogenic system below the paris basin have been investigated using a european scale p wave velocity tomographic model tomography points out the existence of a significant high velocity anomaly in the upper mantle below the western part of the basin at 150 200 km depth the anomaly extends with a nw se trend along the buried northern france trace of the northern variscan suture zone i e the bray segment of the upper carboniferous lizard rhenohercynian lrh suture moreover the high velocity anomaly is spatially correlated with the prominent paris basin magnetic anomaly its downdip extent reaches depths greater than 200 km below the southern margin of the paris basin as suggested in previous tomographic studies below ancient suture zones these data argue for such anomaly being the remnant of a variscan subducted slab that escaped the extensive late orogenic delamination process affecting the lithospheric roots by late carboniferous early permian times and that was preserved stable over 300 ma at the base of the lithosphere on a general geodynamical perspective these results provide a new insight into the long term evolution of subducted lithosphere into the mantle in the case of the western european variscan orogenic belt they suggest that the subduction of the lrh slab below the previously thickened variscan crust and its final detachment from the orogenic root have played an important role in the collapse of the belt inducing thermal erosion and extension of the overriding lithosphere the spatial evolution of late orogenic extension across the belt and of subsequent thermal subsidence in the paris basin is suggested to result from the heterogeneous delamination of the lithospheric roots along strike and from the resultant pattern of asthenospheric rise

from sediment to granite timescales of anatexis in the upper crust abstract granite formation is the culmination of a sequence of events initiated by prograde heating of the protolith and followed by formation of a grain boundary melt melt segregation into a vein network ascent of the melt through the network and finally crystallisation of the melt experimental constraints on the formation of a crustal melt from the incongruent melting of muscovite combined with geochemical studies of anatexis in the high himalaya allow the timescales required for each of these processes to be assessed discordant temperatures determined from monazite and zircon thermometry for himalayan anatectic granites indicate that at least for some intrusives the melt was undersaturated in lree implying that melts have probably been extracted in less than 10 ka experimental studies suggest that some himalayan melts are also undersaturated in zr implying segregation may have occurred within 100 years such short timescales confirm that deformation driven mechanisms are important in extracting these melts from their source the transport distances of himalayan granitic melts of 10 km may be achieved by the ascent of magma through dykes in about 1 day at such rates even the largest granite could theoretically be emplaced in 10 years crystallisation of himalayan melts involves much longer periods if emplaced as thin sheets 100 m wide a timescale of 500 years is required compared with 30 ka for single stage intrusion of the larger laccoliths for composite sheet complexes magma crystallisation rather than melt ascent comprises the rate determining step on the emplacement of the intrusion the overall timescales of melt segregation and emplacement for many orogenic granites are therefore less than 10 ka and possibly less than 1 ka in contrast the timescale required for prograde heating of the protolith is 1 ma since the melt production rate is determined by heat flow into the protolith and not by reaction kinetics for any geologically significant period we conclude that heat flow determined by both the mechanism of heating and the thermal diffusivities of crustal rocks provides the overall rate determining step of crustal anatexis

vertical extrusion and horizontal channel flow of orogenic lower crust key exhumation mechanisms in large hot orogens a large database of structural geochronological and petrological data combined with a bouguer anomaly map is used to develop a two stage exhumation model of deep seated rocks in the eastern sector of the variscan belt an early sub vertical fabric developed in the orogenic lower and middle crust during intracrustal folding followed by the vertical extrusion of the lower crustal rocks these events were responsible for exhumation of the orogenic lower crust from depths equivalent to 18 20 kbar to depths equivalent to 8 10 kbar and for coeval burial of upper crustal rocks to depths equivalent to 8 9 kbar following the folding and vertical extrusion event sub horizontal fabrics developed at medium to low pressure in the orogenic lower and middle crust during vertical shortening fabrics that record the early vertical extrusion originated between 350 and 340 ma during building of an orogenic root in response to se directed saxothuringian continental subduction fabrics that record the later sub horizontal exhumation event relate to an eastern promontory of the brunia continent indenting into the rheologically weaker rocks of the orogenic root indentation initiated thrusting or flow of the orogenic crust over the brunia continent in a north directed sub horizontal channel this sub horizontal flow operated between 330 and 325 ma and was responsible for a heterogeneous mixing of blocks and boudins of lower and middle crustal rocks and for their progressive thermal re equilibration the erosion depth as well as the degree of reworking decreases from south to north pointing to an outflow of lower crustal material to the surface which was subsequently eroded and deposited in a foreland basin indentation by the brunia continental promontory was highly noncoaxial with respect to the se oriented saxothuringian continental subduction in the early visean suggesting a major switch of plate configuration during the middle to late visean

dehydration melting of tonalites part ii composition of melts and solids dehydration melting of tonalitic compositions phlogopite or biotite plagioclase quartz assemblages is investigated within a temperature range of 700 1000 c and pressure range of 2 15 kbar the solid reaction products in the case of the phlogopite plagioclase an45 quartz starting material are enstatite clinopyroxene and potassium feldspar with amphiboles occurring occasionally at 12 kbar zoisite is observed below 800 c and garnet at 900 c the reaction products of dehydration melting of the biotite ann50 plagioclase an45 quartz assemblage are melt orthopyroxene clinopyroxene amphibole and potassium feldspar at pressures 8 kbar and temperatures below 800 c epidote is also formed almandine rich garnet appears above 10 kbar at temperatures 750 c the composition of melts is granitic to granodioritic hence showing the importance of dehydration melting of tonalites for the formation of granitic melts and granulitic restites at pressure temperature conditions within the continental crust the melt compositions plot close to the cotectic line dividing the liquidus surfaces between quartz and potassium feldspar in the haplogranite system at 5 kbar and ah2o 1 the composition of the melts changes with the composition of the starting material temperature and pressure with increasing temperature the melt becomes enriched in al2o3 and feo mgo potash in the melt is highest just when biotite disappears the amount of cao decreases up to 900 c at 5 kbar whereas at higher temperatures it increases as amphibole clinopyroxene and more an component dissolve in the melt the na2o content of the melt increases slightly with increase in temperature the composition of the melt at temperatures 900 c approaches that of the starting assemblage the melt fraction varies with composition and proportion of hydrous phases in the starting composition as well as temperature and pressure with increasing modal biotite from 20 to 30 wt the melt proportion increases from 19 8 to 22 3 vol 850 c and 5 kbar with increasing temperature from 800 to 950 c at 5 kbar the increase in melt fraction is from 11 to 25 8 vol the effect of pressure on the melt fraction is observed to be relatively small and the melt proportion in the same assemblage decreases at 850 c from 19 8 vol at 5 kbar to 15 3 vol at 15 kbar selected experiments were reversed at 2 and 5 kbar to demonstrate that near equilibrium compositions were obtained in runs of longer duration

evolution of the rheic ocean the rheic ocean which separated laurussia from gondwana following the closure of iapetus is arguably the most important ocean of the palaeozoic its suture extends from mexico to turkey and its closure produced the climactic variscan alleghanian ouachita orogeny that assembled the supercontinent pangaea following protracted cambrian rifting that represented a continuum from neoproterozoic orogenic processes the rheic ocean opened in the early ordovician with the separation of several neoproterozoic arc terranes from the continental margin of northern gondwana separation occurred along the line of a former neoproterozoic suture following the onset of subduction in the outboard iapetus ocean the timing of rift drift transition and drive for subsequent spreading was likely governed by slab pull accounting for the rapid rate 8 10 cm yr at which the rheic ocean widened during the ordovician the ocean broadened at the expense of iapetus and attained its greatest width 4000 km in the silurian by which time baltica had sutured to laurentia and the neoproterozoic arc terranes had accreted to laurussia closing iapetus in the process closure of the rheic ocean began in the devonian and was facilitated by northward subduction beneath southern baltica and southward subduction beneath northwest gondwana closure was largely complete by the mississippian as gondwana and laurussia sutured to build pangaea north africa colliding with southern europe to create the variscan orogen in the devonian carboniferous and west africa and south america suturing to north america to form the alleghanian and ouachita orogens respectively during the carboniferous permian the rheic ocean consequently plays a dominant role in the basement geology of southern europe in the appalachian ouachita orogeny of north america and in the palaeozoic sedimentary structural and tectonothermal record from middle america to the middle east with its closure the ocean brought about the assembly of pangaea and brought the palaeozoic era to an end

formation and maintenance of a forced pool riffle couplet following loading of large wood abstract pool riffle maintenance has been documented in numerous studies but it has been almost impossible to characterize detailed natural pool riffle formation mechanisms because of the lack of baseline data prior to pool establishment in 2013 a study was conducted on the blackledge river in connecticut to document the formation of a new pool riffle couplet on a section of river that had previously been studied from 1999 to 2001 in 2001 the study reach contained a scour hole with a residual depth of 0 08 0 09 m downstream of a 1930s paired deflector with no identifiable riffle immediately downstream at this time a large severely undercut hemlock tree was noted along the left bank sometime between fall 2001 and 2004 the tree fell perpendicular to flow across the channel and formed a large wood lw jam and new pool riffle couplet several meters downstream of the old scour hole pool spacing along the reach decreased from 4 47 bankfull widths bfw in 1999 to 3 83 bfw after the new pool riffle couplet formed the new pool has a residual depth the water depth of the streambed depression below the elevation of the immediate downstream hydraulic control of 1 36 0 075 to 1 59 0 075 m which resulted from a combination of 1 32 0 09 m or less of incision below the old scour hole 95 6 or less of the depth increase and up to 0 18 0 09 m of downstream deposition and associated backwater formation 13 2 or less of the depth increase to assess dynamic stability of the pool riffle couplet over several flood cycles surficial fine sediment and organic material along the reach were quantified the 23 m long pool stores 25 7 of the surficial fine grained sediments and 15 4 of organic material along a 214 m long reach that includes one additional artificially created pool an adjacent 50 m long secondary channel impacted by the lw jam stores 65 3 of the surficial fine grained sediments and 54 8 of organic material along the full reach

the linkage between velocity patterns and sediment entrainment in a forced pool and riffle unit a field based project was initiated in order to characterize velocities and sediment entrainment in a forced pool and riffle sequence three dimensional velocities and turbulence intensities were measured with an acoustic doppler velocimeter at 222 different points at three similar flows that averaged approximately 4 35 m 3 s 1 within a large pool riffle unit on north saint vrain creek colorado sediment sorting patterns were observed with the introduction of 500 tracer particles painted according to initial seeding location tracer particles moved sporadically during a 113 day period in response to the annual snowmelt peak flow which reached a maximum level of 14 8 m 3 s 1 velocity data indicate high instantaneous velocities and turbulence levels in the centre of pools patterns of sediment deposition support the notion that stream competence is higher in the pool than the downstream riffle flow convergence around a large channel constriction appears to play a major role in multiple processes that include helical flow development and sediment routing and backwater development with low velocities and turbulence levels above the constriction that may locally limit sediment supply jet flow flow separation vortex scour and turbulence generation enhance scour in the centre of pools ultimately multiple processes appear to play some role in maintenance of this forced pool and the associated riffle copyright 2008 john wiley and sons ltd

durbachites vaugnerites a geodynamic marker in the central european variscan orogen durbachites vaugnerites are k mg rich magmatic rocks derived from an enriched mantle source observed throughout the european variscan basement their present day geographical distribution does not reveal any obvious plate tectonic context published geochronological data show that most durbachites vaugnerites formed around 335 340 ma plotted in a visean plate tectonic reconstruction the occurrences of durbachites vaugnerites are concentrated in a hotspot like cluster in the galatian superterrane featuring a distinctive regional magmatic province reviewing the existing local studies on variscan durbachite vaugnerite rocks we interpret their extensive appearance in the visean in terms of two factors i long term mantle enrichment above early variscan subduction systems and ii melting of this enriched subcontinental mantle source during the variscan collision stage due to thermal anomalies below the galatian superterrane possibly created by slab windows and and or the sinking of the subducted rheic slab into the mantle the tectonic reorganization of europe in the late palaeozoic and during the alpine orogeny has torn apart and blurred this marked domain of durbachites vaugnerites

the characteristics of turbulence in a shear zone downstream of a channel constriction in a coarse grained forced pool a field based project was initiated to characterize the influence of varying discharge and bed topography along a pool exit slope on the strength of turbulence generated by vortex shedding velocities were measured with an adv downstream of a boulder constriction within a shear zone of a large pool measurements were repeated for four flows that varied from 20 to 50 of bankfull discharge an additional longitudinal profile was conducted along the pool thalweg plots of velocity demonstrate high near bed velocities and turbulence levels in the pools turbulence levels were highest closer to the constriction in the shear zone turbulent kinetic energy decayed in the downstream direction in response to the bed topography and at lower discharges instantaneous velocities are large enough to temporarily lift pebbles at this depth it appears that instantaneous forces created by vortex shedding may play an important role in scouring pools downstream of constrictions the turbulence may also respond to changes in bed topography in a manner that encourages feedbacks among bed topography characteristics of turbulence and sediment transport

geochemical constraints on restite composition and unmixing in the velay anatectic granite french massif central the main anatectic granite of the velay complex is unique among major french massif central hercynian granitoids in that rather than having an entirely lower crustal source it formed by mixing between partial melts of the meta igneous lower crust and upper crustal country rock schists and orthogneisses the geochemical variations in the velay main anatectic granites cannot however be explained by mixing alone as their compositions range to lower sio2 with higher al2o3 fe2o3 and tio2 and lower na2o and cao than either end member in mixing the variations are interpreted as being due to the presence of up to 35 restite in minimum melts of country rock compositions primary restites form equilibrium assemblages represented by biotite ilmenite and surmicaceous enclaves which consist of biotite apatite zircon and almandine the main anatectic granites more rarely contain schist and gneiss enclaves quartz resisters and plagioclase restites secondary restites are mainly represented by cordierite and possibly k feldspar which formed by recrystallisation of primary biotite rich restites the unique characteristics of the velay main anatectic granites are likely to be due in part to its late formation close to the end of the hercynian orogeny the metasedimentary lower crust may have become too refractory to yield large volumes of melt following partial melting to form the other major massif central granitoids the heat necessary for partial melting at higher crustal levels was transferred from the lower crust by the intrusion of i type granites and low volume diorites from the mantle upper crustal anatexis was mainly controlled by muscovite breakdown reactions 830 to 850 c and the liberation of water due to the recrystallisation of biotite to cordierite the temperatures necessary for biotite breakdown were only achieved locally and resulted in the formation of high lree granites

tectonics and plate tectonics model for the variscan belt of europe abstract a plate tectonics model is presented to explain the tectonometamorphic characteristics of the european variscides after the closing of two oceanic domains by two sided subduction 500 420 ma and obduction 420 380 ma collision of the european and african continental plates occurred we propose that the subsequent complex intracontinental deformation 380 290 ma is the result of a double subduction of the continental lithosphere accompanied by crust mantle decollement this mechanism explains the progressive crustal thickening and migration of the deformation through time from the sutures toward the external parts of the variscan belt accounting for this model and for the relationships between the european variscides and the other paleozoic peri atlantic belts caledonides appalachian mauritanides and morocco we infer the relative positions of africa america and europe between the silurian and the permian

leaching and differential recrystallization of metamict zircon under experimental hydrothermal conditions we report results of hydrothermal experiments on four alluvial zircons from sri lanka which cover a wide range of radiation damage at 450 c and 1 3 kbar for 744 h with 2 m cacl2 solution as reactive fluid after the hydrothermal treatment the most metamict samples show micrometer thick reaction rims which surround apparently unreacted zircon as revealed by cathodoluminescence cl and nomarski differential interference contrast ndic images these rims have sharp curved and transgressive boundaries with unreacted zircon and are in some cases spread out along cracks the thickness of reaction rims increases with increasing cumulated dosage of the starting materials the reaction rims are strongly enriched in ca up to 7000 ppm and a water species and depleted in radiogenic pb zr and si as revealed by electron microprobe analyses a significant th loss from the reaction rims was detected in the case of the most metamict sample whereas u remained in the structure ft infrared spectrometry and x ray diffraction measurements revealed that the bulk run products were recrystallized using micro raman spectrometry we were able to demonstrate that differential recrystallization took place the reaction rims are strongly recrystallized whereas the unreacted grain interiors underwent only minor recrystallization recrystallization of the rims is accompanied by an enhancement of the integral cl intensity it is suggested that recrystallization in the reaction rims was catalyzed by water infiltration and ion exchange and prevented significant congruent zircon dissolution under the given experimental conditions previous zircon studies have shown that 1 a transgressive morphology 2 a reduced th u ratio and 3 an enhanced cl emission are also characteristics of rims in zircons from high grade metamorphic rocks based on these similarities between natural and experimentally produced rims it is suggested that leaching catalyzed recrystallization is an important alteration process in zircon under wet geological conditions and can account for many complex core rim structures found in natural zircons furthermore the strong enrichment of ca in the reaction rims supports previous assumptions that high ca concentrations in natural zircons are of secondary origin it is suggested that lower u pb concordia intercept ages obtained from single phase zircons with high ca contents date a leaching event

crustal evolution of the hercynian belt of western europe evidence from lower crustal granulitic xenoliths french massif central abstract granulite facies lower crustal xenoliths scavenged by the tertiary basaltic pipes of bournac and roche pointue massif central france have been analysed for major and trace elements and radiogenic sr nd isotope ratios they display wide variations in major elements rare earth elements and isotopes 87 sr 86 sr 0 7041 0 7216 nd 11 9 to 1 5 which tend to correlate with the three different rock types encountered basic and acidic meta igneous and metasediments the basic liquids are of calc alkaline affinity and cumulates are abundant there is a gap in silica content between the basic and acid meta igneous xenoliths the isotopic compositions suggest mixing of old pre existing continental crust represented by the metasediments and mantle derived basic melts which represent juvenile addition to the crust during the hercynian orogeny the basic meta igneous rocks have been contaminated during their emplacement into the crust whereas the surrounding metasediments have suffered partial fusion and contamination by the same liquid producing the hercynian granitoids which have nd and sr initial ratios identical to those of the lower crustal xenoliths the residue from melting may be represented by the acid meta igneous rocks some of which have positive eu anomalies these results indicate that an important basaltic underplating event occurred during the evolution of hercynian orogenic belt although the exact timing of the underplating requires further constraint

migmatites granites and orogeny flow modes of partially molten rocks and magmas associated with melt solid segregation in orogenic belts abstract this paper presents a model for the genesis of migmatites and granites during orogenic evolution based on the analysis of several phanerozoic crustal segments and a review of the physical properties of partially molten rocks and magmas this model inventories the modes of bulk flow of partially molten rocks and magmas and the mechanisms of melt solid segregation for each mode partial melting of rocks is associated with a strength decrease of two to three orders of magnitude leading to strain partitioning expressed by channel flow driven either by forces related to plate tectonics vertical channel flow or by the gravity force associated with lateral variations of crustal thicknesses horizontal channel flow although the structural characteristics of migmatites indicate that deformation plays a role in melt migration the emplacement of laccoliths of leucogranites above migmatites attests to the efficiency of the buoyancy force another decrease in apparent strength of about ten orders of magnitude is associated with the loss of continuity of the solid framework marking the transition from partially molten rocks to magmas the breakdown of the solid framework also allows for settling of the solid in suspension increasing the buoyancy of the remaining magma accordingly domes cored by diatexites former heterogeneous magmas and mantled by metatexites former partially molten rocks are interpreted as gravitational instabilities driven by the relative buoyancy of the magma and permitted by the weakness of the partially molten rocks this model provides a first order framework to elucidate the development of a crustal scale horizontal layering during the evolution of orogenic belts that are affected by partial melting in this case the middle crust is dominated by migmatites with domes cored by diatexites and mantled by metatexites that correspond to a partially molten and magmatic zone respectively the granitic dikes and sills and the associated laccoliths of leucogranites correspond to an intrusive zone overlying the partially molten zone the refractory lower crust is potentially in part formed by accumulation of solids segregated from the overlying heterogeneous magmas

phase equilibria of the lyngdal granodiorite norway implications for the origin of metaluminous ferroan granitoids the proterozoic 950 ma lyngdal granodiorite of southern norway belongs to a series of hornblende biotite metaluminous ferroan granitoids hbg suite coeval with the post collisional rogaland anorthosite mangerite charnockite amc suite this granitoid massif shares many geochemical characteristics with rapakivi granitoids yet granodiorites dominate over granites to constrain both crystallization p t fo2 h 2o in melt and magma generation conditions we performed crystallization experiments on two samples of the lyngdal granodiorite with 60 and 65 wt sio2 at 4 2 kbar mainly at fo2 of nno nickel nickel oxide to nno 1 and under fluid saturated conditions with various h2o co2 ratios for each temperature comparison between experimental phase equilibria and the mineral assemblage in the lyngdal granodiorite indicates that it crystallized between 4 and 2 kbar from a magma with 5 6 wt h2 oa t an fo2 of nno to nno 1 these oxidized and wet conditions sharply contrast with the dry and reduced conditions inferred for the petrogenesis of the amc suite and many other rapakivi granites worldwide the high liquidus temperature and h2o content of the lyngdal granodiorite imply that it is not a primary magma produced by the partial melting of the crust but is derived by the fractionation of a mafic magma lyngdal type magmas appear to have volcanic equivalents in the geological record in particular our results show that oxidized high silica rhyolites such as the bishop tuff could be derived via fractionation of oxidized intermediate magmas and do not necessarily represent primary crustal melts this study underlines the great variability of crystallization conditions from anhydrous to hydrous and reduced to oxidized and petrogenetic processes among the metaluminous ferroan magmas of intermediate compositions granodiorites quartz mangerites quartz latites suggesting that there is not a single model to explain these rocks

a mantle derived bimodal suite in the hercynian belt nd isotope and trace element evidence for a subduction related rift origin of the late devonian brevenne metavolcanics massif central france in the brevenne series ne massif central a low grade bimodal association of metabasalts and metarhyolites is exposed together with intrusive trondhjemite bodies zircon u pb dating constrains their magmatic emplacement at 366 5 ma and 358 1 ma respectively the metabasalts are characterized by a distinct enrichment in incompatible elements e g th and lree and positive ndi from 5 to 8 combined isotope and trace element systematics rule out crustal contamination of mafic melts as a suitable cause of the lile large ion lithophile element enrichment rather a mixing process between a component similar to mid ocean ridge basalts and an enriched end member with ndi 5 is suggested an enriched mantle source of ocean island basalt affinity is precluded by the relative depletion of high field strength elements especially nb which shows negative anomalies in chondrite normalized patterns on the contrary a subduction related origin for the lile enrichment would be more consistent it may be inferred that arc like melts enriched in th and lree light rare earth elements and depleted in nb with ndi 5 were produced through partial melting of a depleted mantle source to which a small amount of crustally derived component had been added the metarhyolites are enriched in lile and have a close genetic relationship with the metabasalts as evidenced by their high ndi from 4 7 to 6 8 although the chemical evidence remains ambiguous it is suggested that fractional crystallization accompanied by subordinate assimilation is the petrogenetic process most consistent with the data the trondhjemites are isotopically distinct from the metarhyolites their ndi values from 1 0 to 2 2 reflect an important contribution of continental crust to their genesis and disprove their inferred cogenetism with the felsic volcanics a review of modern environments in which such bimodal suites are exposed shows that settings involving incipient rifting of a volcanic arc fringing a continental margin or built upon young thin continental crust might provide suitable analogues geodynamic reconstructions are complicated by subsequent tectonic events which disrupted the initial patterns and by mesozoic cenozoic sedimentary cover however this subduction related magmatism enlarges the growing body of evidence for southward subduction processes until the late devonian during the evolution of the northern flank of the european variscides as a general implication it is suggested that the combined use of the sm nd system with incompatible elements relatively resistant during alteration and low grade metamorphism ree th zr nb may provide diagnostic criteria for recognizing the tectonic setting of bimodal metaigneous suites in ancient orogenic belts

sediment hosted micro disseminated gold mineralization constrained by basin paleo topographic highs in the youjiang basin south china abstract the youjiang basin is a devonian triassic rift basin on the southern margin of the yangtze craton in south china strong syndepositional faulting defined the basin and range style paleo topography that further developed into isolated carbonate platforms surrounded by siliciclastic filled depressions finally thick triassic siliciclastic deposits covered the platforms completely in the youjiang basin numerous sediment hosted micro disseminated gold smg deposits occur mainly in permian triassic chert and siliciclastic rocks smg ores are often auriferous sedimentary rocks with relatively low sulfide contents and moderate to weak alteration similar to carlin type gold ores in north america smg ores in the youjiang basin are characterized by low temperature mineral assemblages of pyrite arsenopyrite realgar stibnite cinnabar marcasite chalcedony and carbonate most of the smg deposits are remarkably distributed around the carbonate platforms accordingly there are platform proximal and platform distal smg deposits platform proximal smg deposits often occur in the facies transition zone between the underlying platform carbonate rocks and the overlying siliciclastic rocks with an unconformity often a paleo karst surface in between in the ores and hostrocks there are abundant synsedimentary syndiagenetic fabrics such as lamination convolute bedding slump texture soft sediment deformation etc indicating submarine hydrothermal deposition and syndepositional faulting numerous fluid escape and liquefaction fabrics imply strong fluid migration during sediment basin evolution such large scale geological and fabric evidence implies that smg ores were formed during basin evolution probably in connection with basinal fluids it is well known that basinal fluids especially sediment sourced fluids will migrate generally 1 upwards 2 towards basin margins or basin topographic highs 3 and from thicker towards thinner deposits during basin evolution the isolated carbonate platform as a basin paleo high and related syndepositional fault system together with the unconformity related facies succession may have controlled the migration pathway of ore forming basinal fluids and subsequently determined the location of smg deposits in the youjiang basin unlike carlin type gold deposits smg mineralization in the youjiang basin may represent an integral aspect of the dynamic evolution of extensional basins along divergent continental margins

variscan collisional magmatism and deformation in nw iberia constraints from u pb geochronology of granitoids u pb geochronology of variscan granitoid rocks from the west asturian leonese zone of the nw iberian belt documents the episodic nature of magmatism in this section of the western european variscides each magmatic episode is characterized by granitoids with distinct features and has a duration on the scale of several millions of years the ages of these granitoids place new constraints on the age and duration of magmatic and tectonic events that are consistent with previous structural studies and proposed models for the tectonic evolution and migration of deformation in the nw iberian variscan belt granitoid rocks in this zone belong to two main magmatic episodes syn and post tectonic relative to the variscan orogeny and are broadly representative of the granitoid types found in the nw iberian variscan belt the syntectonic association is formed by i tonalite granodiorite monzogranite intrusions emplaced synchronously with the main phase of crustal deformation d 2 at c 325 ma and ii younger leucogranite intrusions emplaced synchronously with syn convergence extensional structures at c 320 310 ma the post tectonic association is composed of i volumetrically dominant tonalite granodiorite monzogranite intrusions and associated minor mafic intermediate rocks emplaced at c 295 290 ma and ii scarce leucogranite intrusions emplaced at c 290 285 ma

flood and vortex scour of the channel bed of the prosna river and their depth range flood and vortex scour were observed in the prosna channel central poland in the years 1980 1985 flood scour increases the thickness of the reworked channel deposits by 30 per cent to 66 per cent in relation to the thickness determined by the elevation of the normal bed and the bankfull stage vortex scour can increase the thickness by as much as 90 95 per cent knowledge of these properties of contemporary alluvium allows correct palaeohydrological and stratigraphic interpretation of the alluvial fills of valley floors

granite magma formation transport and emplacement in the earth s crust the origin of granites was once a question solely for petrologists and geochemists but in recent years a consensus has emerged that recognizes the essential role of deformation in the segregation transport and emplacement of silica rich melts in the continental crust accepted petrological models are being questioned either because they require unrealistic rheological behaviours of rocks and magmas or because they do not satisfactorily explain the available structural or geophysical data provided flow is continuous mechanical considerations suggest that far from being geologically sluggish granite magmatism is a rapid dynamic process operating at timescales of or 100 000 years irrespective of tectonic setting

datation u pb sur zircons de l eclogite de la borie haut allier france et consequences sur l evolution ante hercynienne de l europe occidentale a crystallization age of 432 10 20 ma has been determined for the la borie eclogite haut allier france by u pb dating on zircons this age is yielded by the upper intercept with the concordia curve in agreement with recent dates obtained in the southern part of the massif central in brittany and in vendee these data indicate that the high pressure high temperature metamorphic phase representative of the pre variscan evolution of the western europe took place at the silurian ordovician boundary this high pressure high temperature metamorphism could be significant of subduction processes more or less synchronous preceding the main continental collision responsible of the variscan belt s s

protracted coeval crust and mantle melting during variscan late orogenic evolution u pb dating in the eastern french massif central the late stages of the variscan orogeny are characterized by middle to lower crustal melting and intrusion of voluminous granitoids throughout the belt which makes it akin to hot orogens these processes resulted in the development of large granite migmatite complexes the largest of which being the 305 300 ma old velay dome in the eastern french massif central fmc this area also hosts a wide range of late variscan plutonic rocks that can be subdivided into four groups i cordierite bearing peraluminous granites cpg ii muscovite bearing peraluminous granites mpg iii k feldspar porphyritic calc alkaline granitoids kcg and iv mg k rich monzo diorites and lamprophyres vaugnerites new results of la sf icp ms u pb zircon and monazite dating on 33 samples from all groups indicate that both granites and mafic rocks emplaced together over a long period of 40 million years throughout the carboniferous as shown by intrusion ages between 337 4 1 0 and 298 9 1 8 ma for the granitoids and between 335 7 2 1 and 299 1 1 3 ma for the vaugnerites low zircon saturation temperatures and abundant inherited zircons with predominant late ediacaran to early cambrian ages indicate that the cpg and mpg formed through muscovite or biotite dehydration melting of ortho and paragneisses from the lower gneiss unit the kcg and vaugnerites contain very few inherited zircons if any suggesting higher magma temperatures and consistent with a metasomatized lithospheric mantle source for the vaugnerites the kcg can be explained by interactions between the cpg mpg and the vaugnerites or extensive differentiation of the latter the new dataset provides clear evidence that the eastern fmc was affected by a long lived magmatic episode characterized by coeval melting of both crustal and mantle sources this feature is suggested here to result from a lithospheric scale thermal anomaly triggered by the removal of the lithospheric mantle root the spatial distribution of the dated samples points to a progressive southward delamination of the lithospheric mantle perhaps in response to rollback following continental subduction or to retro delamination owing to the retreat of a south verging subduction zone

iberian late variscan granitoids some considerations on crustal sources and the significance of mantle extraction ages a suite of post tectonic granitoids mostly peraluminous broadly i type granodiorites and monzogranites and mafic rocks from nwiberia with crystallization ages between ca 309 and 290 ma has been investigated for sm nd isotopes and inherited zircon content in order to constrain the nature of their source rocks end values at 300 ma vary from 0 2 to 5 9 and tdm values range from 1 01 to 1 58 ga inherited xenocrystic zircons yielded ages ranging from 458 to 676 ma with 90 of data between 490 and 646 ma corresponding to neoproterozoic mostly ediacaran cambrian andordovician ages only three highlydiscordant analyses yielded ages older than 650 ma based on the data reported herein and relevant data fromthe literaturewe contend that post tectonic granitoids of the iberian variscan belt with exception of the scarce anatectic s type granitoids were derived mostly from metaigneous lower crustal sources which in turn were ultimately derived from a subcontinental lithospheric mantle enriched between ca 0 9 and 1 1 ga i type granitoids and mantle derived mafic rocks both underwent varying degrees of contamination by ametasedimentary lower crust depleted in pre 650 ma zircon through previousmelting episodes with a time integrated sm nd evolution different to that of the metaigneous lower crust participation of this metasedimentary crust in the genesis of these granitoids may account for nd isotopic variability and nd model ages well in excess of 1 2 ga

sources of hercynian granitoids from the french massif central inferences from nd isotopes and consequences for crustal evolution the french massif central is characterized by a great abundance and variety of granitoids emplaced during hercynian times 360 280 ma distinct nd i and 87sr86sr i are not associated with particular petrographic types however lower nd i values 8 5 occur among the younger leucogranites from the western part of the massif intermediate nd i 6 correlate with aluminous monzogranites and higher values 6 to 3 with calc alkaline granodiorites in the eastern massif central nd i are homogeneous at the regional scale in the western massif central the leucogranites and the monzogranite granodiorites display different trends of nd i with emplacement ages from these results extreme models involving a depleted mantle or an old archean or early proterozoic source may be ruled out the trend of increasing nd i with decreasing age observed in the monzogranite granodiorite group suggests either an increasing contribution of a mantle component or the progressive melting of more mafic more refractory crustal sources or both the reverse trend displayed by the leucogranites might be explained in terms of the upward migration of partial melting across an isotopically zoned metasedimentary pile the large amount of granites may be the result of favourable processes during the collision tectonics combined with the unusual abundance of fertile materials enriched in water and heat producing elements in the pre carboniferous crust

precambrian protoliths and early paleozoic magmatism in the french massif central u pb data and the north gondwana connection in the west european variscan belt abstract u pb geochronological data were collected on zircon by la mc icp ms on orthogneiss and paragneiss from the limousin area in the french massif central fmc in order to investigate the connection between the west european variscan belt and the northern margin of gondwana fifteen samples were collected in the four main tectonostratigraphic units of the fmc namely the para autochthonous unit the lower gneiss unit lgu the upper gneiss unit ugu and the thiviers payzac unit orthogneiss yield intrusion age between 521 7 and 446 6 ma considering all the results from both magmatic and metasedimentary samples two peaks at 531 and 473 ma are recognized rifting processes taking place along the north gondwana margin during the lower paleozoic went on until the end of the ordovician as suggested by the magmatic event recorded around 450 ma several maximum depositional ages were ascertained in the metasedimentary formations of the fmc as determined by the youngest detected detrital zircon crystal ranging from 604 16 ma for metasediments of the para autochthonous unit 573 12 ma for the lgu 564 9 ma for the thiviers payzac unit and 523 4 ma in the ugu minimum depositional ages are given by magmatic emplacement ages obtained in the crosscutting orthogneiss there is some evidence for a decrease of this maximum age upwards in the tectonostratigraphy detrital zircon in metasedimentary formations and inherited zircon in orthogneiss display a wide spectrum of ages with significant peaks at around 590 ma and 560 ma archean paleoproterozoic and neoproterozoic detrital zircons suggest a west african craton source of the sedimentation the large amount of neoproterozoic and lower paleozoic ages obtained in this study suggests that these periods played a significant role in the continental crustal growth history of western europe

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

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

delamination vs break off the fate of continental collision 1 the fate of a convergent continental margin is investigated we perform a set of 2d numerical models to study how and why continental collision can evolve in different scenarios since the rheology of continental lithosphere has a major control on the dynamics of subduction we explore a range of different lithosphere and lower crust viscosity values to understand their sensitivity on the possible scenarios we find that with a rheologically layered crust both delamination and break off are feasible we identify three modes 1 slab detachment in which the lithospheric mantle and the crust are strongly coupled subduction slows down and the slab eventually breaks 2 delamination of the lithospheric mantle that separates from the crust and continue to subduct and 3 an intermediate mode where the lithospheric mantle and the crust remain partially coupled resulting in an initial stage of delamination followed by the slow down and cessation of subduction

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

near isothermal conditions in the middle and lower crust induced by melt migration simple one dimensional thermal models that include the effects of melt migration have been developed the models show that long lived plutonism results in a quasi steady state geotherm with a rapid temperature increase in the upper crust and nearly isothermal conditions in the middle and lower crust the thermal structure of the crust strongly influences deformation metamorphism and plutonism1 2 3 models for the geothermal gradient in stable crust predict a steady increase of temperature with depth this thermal structure however is incompatible with observations from high temperature metamorphic terranes exhumed in orogens1 4 5 6 global compilations7 of peak conditions in high temperature metamorphic terranes define relatively narrow ranges of peak temperatures over a wide range in pressure for both isothermal decompression and isobaric cooling paths here we develop simple one dimensional thermal models that include the effects of melt migration these models show that long lived plutonism results in a quasi steady state geotherm with a rapid temperature increase in the upper crust and nearly isothermal conditions in the middle and lower crust the models also predict that the upward advection of heat by melt generates granulite facies metamorphism and widespread andalusite sillimanite metamorphism in the upper crust once the quasi steady state thermal profile is reached the middle and lower crust are greatly weakened due to high temperatures and anatectic conditions thus setting the stage for gravitational collapse8 exhumation and isothermal decompression after the onset of plutonism near isothermal conditions in the middle and lower crust result from the thermal buffering effect of dehydration melting reactions that in part control the shape of the geotherm

geochemistry and tectonic implications of two ordovician bimodal igneous complexes southern french massif central two bimodal meta igneous complexes from the southern massif central provide important tectonic constraints on the initial stage of the palaeozoic orogen the metabasites may be subdivided into two groups according to the distribution of the large ion lithophile lil and high field strength hfs elements n type morbs with low lil hfs ratios and subduction type basalts with high lil hfs ratios these associations suggest a complex tectonic setting such as an extensional zone or a back arc basin above a subduction zone the great volume of felsic rocks with low incompatible trace element contents indicates partial melting of an underlying continental crust the association of terrigeneous sediments with n type morbs subduction type basalts and high silica peraluminous rhyolites argue against an intra oceanic setting and suggest that the area was one of ensialic crustal tension during middle ordovician the two complexes are identified as remnants of ensialic back arc basins rather than relicts of major oceanic sutures such marginal basins could have been located in a mainly continental domain such as the armorican microplate

geochemical constraints on the genesis of hercynian two mica leucogranites from the massif central france abstract late hercynian two mica leucogranites dated at 340 280 3a old occur across the entire massif central france and form dykes and stocks up to 130 km in length they are peraluminous molecular al 2 o 3 na 2 o k 2 o cao asi 1 16 1 25 and have low initial nd 8 2 to 6 1 indicating derivation from the metasedimentary lower crust as sampled in crustal xenoliths nd 340 8 6 to 4 3 nd 280 9 0 to 4 8 their high sio 2 70 78 wt p 2 o 5 up to 0 5 rb and ga and low ba sr and ree are mainly a result of partial melting processes rather than fractionation partial melting was initiated by underplating of the lower crust by mantle derived basic magmas between 360 and 270 ma and was intensified by granulite facies metamorphism between 300 and 280 ma from this model there is little similarity between the genesis of two mica leucogranite in the massif central and that in the himalaya with which it has previously been compared

crust mantle relationships in the french variscan chain the example of the southern monts du lyonnais unit eastern french massif central garnet lherzolite from the lyonnais area eastern french massif central occurs as several lenses elongated within the regional foliation of garnet biotite sillimanite gneisses within the peridotites a mylonitic foliation can be observed which clearly is oblique to the regional foliation of the surrounding gneisses petrological and thermobarometric studies emphasize a tectonometamorphic re equilibration for both crustal and mantle rocks characterized by a prograde metamorphic stage followed by retrograde evolution during the burial stage interpreted as lithospheric subduction the peridotites underwent their mylonitic deformation under high pressure conditions 23 30 kbar in contrast the paragneisses have suffered their deformation during the retromorphic evolution under mesozonal conditions 6 8 kbar 700 c our thermobarometric investigations allow us to interpret the granulitic ultramafic association from the monts du lyonnais area as a lithospheric section buried into a palaeozoic subduction zone laminated during continental collision and uplifted by erosion processes

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

thermobarometry and granite genesis the hercynian low p high t velay anatectic dome french massif central the velay dome french massif central offers a quasi continuous section across an anatectic domain comprising low to high grade schists gneisses and granites two main tectonometamorphic events and their related generation of granitic material were recognized in addition to a major barrrovian tangential event d2 attributed to intracontinental collision tectonics i a medium to low p high t event d3 which gave rise to migmatites and syntectonic monzonitic granites and granodiorites and ii a widespread melting event d4 which led to the generation of migmatities the velay granite and post anatectic granites thermobarometry on samples collected from both the metamorphic envelope and the granitic core distinguishes two distinct geotherms i a first associated with the d3 event characterized by p 5 kbar t 750 c and water present melting biotite remains stable which led to large scale migmatization but minor amount of granites ii a second associated with the d4 event and characterized by vapour absent melting p 4 5 kbar t 760 850 c which gave rise to the velay granites and late migmatitic granites the temperature increase during the d4 event is attributed to the intrusion of hot mafic magmas within the crust the time integrated features of the different granitic rocks in the velay dome can be directly related to ah2o in the source region and illustrate the progressive dehydration of a middle to lower crustal segment over 60 ma

hercynian polyphase tectonics in the northeast french massif central the closure of the brevenne devonian dinantian rift field and laboratory structural studies show that the devonian dinantian units of the northeast french massif central experienced a complex and contrasting tectonic metamorphic evolution during the hercynian orogeny the structural analysis of the pre middle visean brevenne violay beaujolais rocks in the loire area shows a polyphase tectonic evolution associated with greenschist to amphibolite facies metamorphism the first event d1 probably occurred in early tournaisian or latest devonian times it is responsible for the flat lying regional foliation and the nw se to n s trending lineation it is well observed in the violay group and corresponds to the nw vergent emplacement of the late devonian units upon their gneissic basement represented by the affoux gneisses the second event d2 is responsible for the ne sw to e w trending lineation to the south d2 deformation is locally reworked by the grand chemin dextral wrench fault around 345 350 ma ago this polyphase deformation is also found in several devonian dinantian areas of the ne massif central but not in morvan this tectonics corresponds to the tournaisian closure by northward thrusting and subsequent intracontinental deformation of the oceanic brevenne violay beaujolais rift which opened in devonian times in a back arc setting

shrimp dating of zircons from high grade metasediments of the schwarzwald sw germany and implications for the evolution of the moldanubian basement four metasedimentary zircon populations from different tectonometamorphic units of the central and the northern schwarzwald variscan belt sw germany were investigated using sem cathodoluminescence and shrimp dating despite partially strong modifications of primary internal morphologies during variscan metamorphism at amphibolite 750 c 0 4 0 6 gpa and granulite facies conditions 950 1 000 c 1 4 1 8 gpa many grains show well preserved protolith ages the detritus indicates a northern gondwana origin and different palaeozoic episodes of sediment deposition and consolidation two of the studied sediments were deposited in cambrian early ordovician times and consolidated in positions close to northern gondwana late ordovician and rare devonian detritus from sediments of two other tectonometamorphic units indicates much later sedimentation close to the leading edge of gondwana or a terrane assemblage during northern drift towards laurussia subsolidus growth of new zircon due to variscan granulite facies metamorphism of one of the tectonometamorphic units is precisely dated at 335 2 ma

modelling diverse processes in the petrogenesis of a composite batholith the central bohemian pluton central european hercynides the multiple intrusions making up the central bohemian pluton in the central european hercynides have petrographic and geochemical features consistent with the presence of four main granitoid suites major element trace element and sr nd isotopic compositions are used to model their petrogenesis partial melting of metabasic locks or of a chur like mantle source are interpreted to have produced melts parental to the most primitive calc alkaline sazava suite interaction of basic with more acidic magmas followed by extensive amphibole plagioclase dominated fractionation accounts for the production of trondhjemites alternatively the trondhjemites correspond to small degree melts of a metabasic source afc assimilation fractional crystallization modelling with a paragneiss as a contaminant and increasing d nd values simulates the characteristics of the blatna suite closed system fractionation of strongly enriched mantle derived magmas or their interaction with leucogranitic melts is deduced for the petrogenesis of the shoshonitic certovo bremeno suite partial melting of a metasedimentary source followed by k feldspar dominated fractionation accounts for the granites of rite ricany suite the progression from relatively primitive calc alkaline granitoids towards evolved k rich calc alkaline and shoshonitic rocks is interpreted to reflect the increasing enriched mantle input in the petrogenesis of the later suites the evidence for hercynian subduction is equivocal and the mantle enrichment could have been significantly older

lower crustal rejuvenation and growth during post thickening collapse insights from a crustal cross section through a variscan metamorphic core complex the variscan lower crust of europe acquired its main features seismic layering intrusion of mantle derived magmas granulite grade metamorphism when the upper crust underwent post thickening extension through gravitational collapse we suggest that post thickening collapse is a geodynamic process that contributes to the growth and differentiation of continental lithosphere in collapsed mountain belts

geochemical comparison between minettes and kersantites from the western european hercynian orogen trace element and pbsrnd isotope constraints on their origin trace element and isotopic characteristics of late carboniferous to early permian minettes and kersantites have been determined these lamprophyres have been sampled throughout the western european hercynian orogen from brittany to the west to schwarzwald to the east in spite of sharp petrological differences reflected by mineralogy and major element geochemistry minettes and kersantites exhibit close identity with respect to trace element and isotopic features these features comprise enrichment in incompatible elements highcs rb and lowce pb ratios ta and ti relative depletion high abundance in transition elements and highni mg ratios pb isotope ratios are undistinguishable from those measured on hercynian continental crust initial143nd 144nd ratios are between0 5120 ei 5 and0 5122 ei 1 for minettes and kersantites whereas initial87sr 86sr ratios vary between 0 7055 0 710 for minettes and 0 707 0 708 for kersantites no simple mixing relations are visible on rbsr and smnd isochron diagrams the exceptional homogeneity of these geochemical characteristics along a 1000 km traverse does not allow for an hypothesis of enrichment through upper level assimilation and thus leads to propose that these rocks originated through melting of a mantle enriched by recycling of crustal material

mantle and crustal granites genetic classification of orogenic granites and the nature of their enclaves abstract the nature of enclaves must be taken into account in a genetic classification of granites granites containing only restites are different from those also containing basic magmatic enclaves microgranular enclaves if the former are of crustal origin the latter certainly include a mantle component rather than a distinction between s type granites of sedimentary origin and i type granites of igneous origin a distinction is made between c type granites crustal and m type granites mantle or mixed i e crustal mantle the c type granites can be subdivided into cs type anatexis of sedimentary rocks and ci type granites anatexis of igneous rocks in the french massif central the variscan granites belong to these different varieties leucogranites are typical c type granites whereas granodiorites and monzogranites are m type granites including various amounts of mantle component

electron microprobe dating as a tool for determining the closure of th u pb systems in migmatitic monazites high spatial resolution dating of monazite by the electron probe microanalyzer epma enables systematic and detailed studies of small minerals like zircon monazite records the complex history undergone by the host rocks recent improvements in the statistical treatment of many in situ data now make it possible to decipher the related thermal events and so obtain reliable and precise ages our work shows that a significant number of individual spot analyses is required to reach such precise information i e more than 30 40 data using the examples of monazites from three migmatites and one granite we show how to select the most efficient method of age calculation according to the u and th geochemistry of the grains or grain domains that we are trying to date three situations may be met 1 monazites exhibiting significant th u ratio variation 2 monazites exhibiting a fairly constant th u ratio but significant u th heterogeneity and 3 monazites of constant u and th concentrations for the first case a precise mean age can be calculated using a method of data reduction in the th pb f u pb diagram whereby a precision of 5 10 ma 2 is commonly achieved for the second case an isochron age can be calculated according to the pb f th method with a common precision of around 20 ma 2 whereas for the third case a simple weighted average age can be calculated using these approaches coupled with a back scattered electron image study we demonstrate that inheritance is probably as common for monazite as for zircon in addition the combination of high spatial resolution and precise age determination show the limited extent of pb diffusion in monazite finally an example from a migmatite from southern french guiana demonstrates the especially robust behavior of the th u pb system in monazite this system remains closed during late migmatization and during the subsequent zircon crystallization and zircon overgrowth of protolith zircons the monazite yielded exactly the same age as the protolith zircons

geodynamic models of mature continental collision evolution of an orogen from lithospheric subduction to continental retreat delamination 1 the behavior of deep lithospheric processes in continental collision are frequently speculated on but poorly understood in this study we build on previous work to explore the styles of continental lithosphere deformation during mature collision where 1800 km of convergence has been accommodated by horizontal shortening we conducted a suite of numerical geodynamic experiments that test the sensitivity of mature continent collision to varying mantle lithosphere density mantle lithosphere yield stress lower crustal strength and to the presence of phase change related density changes in the lower crust i e eclogitization of mafic lower crust the models suggest that the early stages of collision are accommodated by subduction of lower crust and mantle lithosphere along a discrete shear zone beneath the overriding plate following this initial stage of subduction the subducting lower crust and mantle lithosphere can retreat from the collision zone permitting the sub lithospheric mantle to upwell and intrude the overriding plate as a result the lower crust and mantle lithosphere of the overriding plate delaminate from the overlying crust with local isostatic adjustment subduction and delamination driven crustal processes plateau like uplift occurs the sub crustal evolution also causes bands of syn convergent crustal extension to develop in models with a rheologically weaker lower crust surface crustal response to the deep lithosphere dynamics becomes more diffuse as an example the numerical experiments satisfy a number of surface observables of the himalayan tibetan orogen namely 1 the current general mantle lithosphere architecture as defined by seismic analyses 2 the long wavelength plateau uplift of the western tibetan plateau 3 the surface heat flow measurements in india the tethyan himalaya qiangtang and in qaidam basin and 4 the anomalous syn convergent extension in the southern portion of the orogen

the formation of pangea abstract the making of pangea is the result of large scale amalgamation of continents and micro continents which started at the end of the neoproterozoic with the formation of gondwana as pieces were added to gondwana on its south american antarctica and australia side ribbon like micro continents were detached from its african and south chinese side cadomia in the late neoproterozoic avalonia and hunia in the ordovician galatia in the devonian and cimmeria in the permian cadomia was re accreted to gondwana but the other ribbon continents were accreted to baltica north china laurussia or laurasia finding the origin of these numerous terranes is a major geological challenge recently a global plate tectonic model was developed together with a large geological geodynamic database at the lausanne university covering the last 600 ma of the earth s history special attention was given to the placing of gondwana derived terranes in their original position using all possible constraints we propose here a solution for the variscan terranes another paper deals with the altaids the galatian super terrane was detached from gondwana in the devonian during the opening of paleotethys and was quickly separated into four sub terranes that started to by pass each other the leading terranes collided at the end of the devonian with the hanseatic terrane detached from laurussia in the carboniferous gondwana started to impinge onto the amalgamated terranes creating the variscan chain and the pangean super continent east of spain paleotethys remained opened until the triassic subducting northward under laurasia roll back of the paleotethyan slab triggered the collapse of most of the european variscan orogen which was replaced by series of permian rifts some of them becoming oceanized back arc basins during the triassic major force changes at the pangean plate limits at the end of the triassic provoked its break up through the opening of the proto caribbean central atlantic alpine tethys oceanic seaways

u pb dating of an early paleozoic bimodal magmatism in the french massif central and of its further metamorphic evolution in the marvejols area southern french massif central the gneissic marvejols supergroup is overthrust on the metasedimentary serie du lot deposited in part prior to 540 ma the allochtonous terranes are characterized by the occurrence of a leptyno amphibolitic group a complex association of mafic and felsic rocks of igneous and sedimentary derivation a 480 10 ma age has been obtained by u pb dating of zircons for the crystallization of both mafic and felsic meta igneous rocks these rocks were emplaced during an important extensional tectonics relics of eclogites pyrigarnites coronite gabbros and hp trondhjemites are clear evidence for a further hp ht event dated at 415 6 ma on zircons from a hp trondhjemite subsequently the marvejols supergroup underwent an amphibolite facies metamorphism with incipient mobilization dated at 345 10 ma

geochronological and geochemical characterization of magmatic hydrothermal events within the southern variscan external domain cevennes area france geochronological mineralogical and geochemical analyses have been focussed on the mont lozere borne plutonic complex and surrounding rocks cevennes french massif central in which b w sn and as au sb mineralization is encountered two main results are highlighted 1 the existence of a 301 306 ma magmato hydrothermal event unrelated to the emplacement of the pont de montvert borne plutonic body at 316 ma 2 the magmatic and hydrothermal features are strongly associated both in time and in space thus demonstrating an intimate connection between mineralizing processes and magmatism in this part of the french massif central we also show that mineralization and associated hydrothermal occurrences do not correspond to a simple and single geochemical signature and that a contamination model must be invoked in order to account for the complexity of isotopic results this study demonstrates that the application of the o and h isotopic signatures as tracers of the source and nature of fluids in an orogenic context requires some specific care finally a model of the tectonic magmatic hydrothermal evolution of the study area is suggested in which we discuss two alternative scenarios the first one implies the existence of two different hydrothermal mineralizing events bo w sn and as au sb ones the second one suggests the same source for all hydrothermal and mineralized structures

sr nd al36 1997 99 122 hercynian granodiorites and monzogranites massif central france variations in sr nd and pb initial isotope ratios of three large hercynian granitoid massifs of the french massif central are consistent with formation of the granitoids by partial melting of the lower crust the nature of the lower crust of the massif central is known from the granulite facies lower crustal xenoliths brought to the surface by tertiary alkaline volcanics studies of the xenoliths have revealed that during the hercynian orogeny 360 290 ma ago the pre existing metasedimentary and acid metaigneous lower crust was intruded by basic magmas causing anatexis the mantle derived basic magmas mixed with melts from the pre existing lower crust forming the hercynian granitoids the peraluminous gueret and margeride granitoids which are very similar in their major and trace element compositions have 87sr 86sri 0 7089 to 0 7121 and ndi 7 5 to 3 9 values which indicate that they have a high sedimentary component considered to have been derived from lower crustal metasedimentary rocks however the granitoids differ significantly in their initial pb isotope compositions with the gueret granodiorites being less radiogenic indicating the presence of a slightly different crustal source component for the gueret massif in contrast monzogranites from the millevaches massif are less peraluminous and have higher la ybn and lower 87sr 86sri values 0 7064 to 0 7109 these features together with the relatively radiogenic pb isotope compositions of the millevaches granitoids suggest derivation from the acid metaigneous lower crust

formation of riffle pool sequences field evidence for an autogenetic process abstract riffle pool sequences are the characteristic reach scale bedforms of mixed and gravel bedded rivers while most research has been directed to the quantitative description of these sequences and to their maintenance comparatively little attention has been given to the processes responsible for their formation in this paper field results from quasi continuous velocity records obtained with an array of electro magnetic current meters are analysed with respect to a model of macro scale structure in the turbulent velocity field of natural channels this model has been suggested as the most plausible basis for riffle pool formation the results demonstrate the occurrence of a variety of coherent flow structures some of which scale on channel dimensions as predicted by the model but which also suggest that some revision of the association between flow structure and riffle pool formation is necessary it is suggested that riffle pool units are initiated with the generation of roller eddies upstream and downstream from a major flow obstacle corresponding to engineering criteria for scour around bridge piers the obstacle persists long enough to fix the flow pattern and hence account for significant modification of channel form but is ultimately removed as part of the process of bed modification via the extension of scour three distinct stages in the process are involved local scour of a single pool creates deposition downstream which then generates the next downstream flow irregularity a riffle pool sequence is thus created autogenetically as the summation of a sequence of irregularities each unit of which is formed and maintained only by flow dynamics operating at the local scale the process is essentially both deterministic and statistical in character it is further suggested that spatial differences in the near bed turbulence field arising from incipient riffle pool topography themselves create differences in surface sediment entrainment which enhance and maintain the sequence in a form process feedback mechanism similar reasoning might possibly be extended to the entire range of bedform sizes

origin of microgranular enclaves in granitoids equivocal sr nd evidence from hercynian rocks in the massif central france we examined the rb sr and sm nd systematics of three different high level to subvolcanic granitoid plutons and their microgranular enclaves the host granitoids are adamellites and grano diorites while the enclaves are mostly microtonalites with igneous textures the enclaves are rounded in shape and vary from 1 m across contacts with the enclosing granitoids are sharp and mostly nonlobate the microtonalitic enclaves are similar in mineralogy to their hosts initial 87sr 86sr are all 0 7070 and initial end is 3 0 to 6 6 for both the enclaves and their hosts this precludes a major input of an unmodified mantle derived component at the time of magma genesis nd isotopic contrasts between enclaves and hosts are 1 e nd unit at most these data indicate broadly cogenetic origins for enclave host systems though some enclaves have slightly less crustal signatures than the enclosing granitoids there are several possible interpretations 1 there may have been nearly complete isotopic homogenization between enclaves and host magmas derived from different starting parents 2 the enclaves and granitoids could all have purely crustal origins enclaves might represent magmas derived from slightly less evolved protoliths or parental magma or they could be concentrations of early formed crystals with interstitial melt 3 basaltic magma from the depleted mantle could have thoroughly mixed with a large volume of crustal melt in the lower crust 4 there could have been a significant input of melt derived from an enriched mantle source several of these mechanisms may have played roles in the genesis of the granitoids and their enclaves nd isotopes do not always allow unequivocal recognition of mantle components in granitoid magmatism we argue that enclaves of this type are probably not products of major mixing processes relevant to granitoid protoliths and large scale crustal evolution they commonly result from second order mingling of broadly cogenetic magma fractions that may have a variety of different origins

electron microprobe monazite geochronology of magmatic events examples from variscan migmatites and granitoids massif central france u th pb dating of monazite with the electron probe microanalyser epma is increasingly documented as a reliable geochronological method offering high spatial resolution this method has been applied on monazite from the cevennes migmatites and granitoids from the southeast of the french massif central measurements were performed on separated grains after systematic back scattered electron bse imaging monazites from migmatites record two main ages i a protolith age of about 550 543 ma obtained on inherited cores and ii a migmatization event between 329 5 and 323 3 ma recorded by monazite rims and all other monogenetic grains monazite from the peraluminous rocles pluton yields a 318 3 ma age finally three granite dykes are dated at 333 6 318 5 and 311 5 ma the older dyke is the most deformed of them and is interpreted as linked to the migmatization event the two other dykes are geochronologically petrologically and structurally coeval with the rocles pluton the data constrain the timing of crustal melting following variscan thickening in the northern cevennes area migmatization of ordovician protoliths took place at 329 323 ma and was shortly followed by intrusion of leucogranite at 318 311 ma the study shows that epma dating of monazite can be successfully used to resolve a close succession of regional melting events

fluid absent melting behavior of a two mica metapelite experimental constraints on the origin of black hills granite we have studied experimentally the vapor absent melting behavior introduction of a proterozoic metapelite hp 60 1 from the black hills south peraluminous leucogranites such as the miocene granites dakota to assess whether the high do tourmaline bearing granites of the himalaya and tourmaline granites of the harney of the harney peak suite resulted from muscovite dominated depeak suite in south dakota are widely considered to be hydration melting at 10 kbar samples were sealed with no added the product of muscovite dehydration melting yet few h2o in gold capsules then run in a 0 5 inch 1 27 cm pistonexperimental studies of muscovite dehydration melting cylinder apparatus over the temperature interval 812 975 c reactions have been performed because the low temexperiments were conducted at 10 kbar to allow comparison with peratures of the reactions hinder the approach to equiother metapelite melting studies mass balance calculations imply librium in addition melt fractions generated are typically the melting reaction muscovite biotite plagioclase quartz low resulting in small melt pockets which are very difficult melt alkali feldspar aluminosilicate garnet until to analyze with the electron microprobe in this study muscovite is exhausted 812 c then the reaction biotite we assess the possible genetic relationship between the plagioclase quartz alkali feldspar aluminosilicate tourmaline bearing granites of the harney peak suite melt garnet the inferred muscovite reaction is completed below and the aluminous metasediments these granites intrude 812 c and produces 1 2 wt melt melt production of the although the attempt to investigate the muscovite decontinuous biotite reaction increases steadily with rising temperature hydration melting reaction was hindered by the surto 32 wt in our highest temperature run the low melt prisingly low melt productivity we were able to infer a productivity of the muscovite reaction at 10 kbar suggests that balanced reaction illuminating the dominantly peritectic melting at lower pressures may be necessary to generate mobile melt nature of fluid absent muscovite breakdown under the fractions by muscovite dominated reactions in our starting material conditions investigated in addition melts of hp 60 1 are considerably more mafic than isotopic and trace element studies indicate a genetic the tourmaline bearing granites with average combined tio2 relationship between s type granites and aluminous metamgo feo contents of 1 85 wt and 0 84 wt respectively sediments in the black hills of south dakota where the in particular melts of hp 60 1 are enriched in ti and would harney peak granite hpg intruded proterozoic and probably stabilize biotite rather than tourmaline upon cooling archean schists and metagreywackes undergoing regional arguing against hp 60 1 as a potential source rock for the metamorphism the timing and mode of emplacement of tourmaline granites the hpg and the deformation and regional metamorphic

palaeozoic granitoids from the french massif central age and origin studied by 87rb87sr system abstract according to geochronological results granitoids of the french massif central fall into two main chronological groups 1 a cambro silurian group 545 410 ma occurring all over the massif central later metamorphosed into orthogneisses during the devonian metamorphism 400 370 ma 2 an upper devonian carboniferous group 360 280 ma including non or weakly deformed granites the oldest 360 340 ma localized in the north of the massif central and the youngest 340 285 ma outcropping all over the massif geochemistry of rb sr and isotopic evolution diagram give the following results strontium initial ratios for orthogneisses increase with time from 0 704 545 ma to 0 710 410 ma excluding a purely mantellic origin of the magmas strontium initial ratios for granites show that their magmas cannot result from a remelting of orthogneisses the isotopic evolution of the leucogranitic family defines a trend originating 370 ma i e during the main metamorphic phase the source rocks of the early palaeozoic granitoids contain old zircons 1 5 2 0 ga probably originating from an earlier continental crust but the evidence of this crust is not pointed out by strontium isotopes

pool fills a window to palaeoflood history and response in bedrock confined rivers channel scale sedimentary units associated with bedrock controlled riffle pool morphology are examined in detail along sandy creek gorge an ephemeral stream in arid south eastern central australia pool fills comprise cut and fill assemblages of poorly sorted sediments ranging in texture from muds to boulders five unit types are defined based on particle size sedimentary structures geometry and bounding surface character 1 coarse grained bar platform 2 fine grained bar supraplatform 3 fine grained pool fill 4 fine grained bench and 5 modern pool fill the last coarse grained unit currently lining the pools suggests an altered sedimentation style over the post settlement period post ad 1860s situated at bedrock valley constrictions pool fills are compared with other sedimentary units associated with recirculating currents eddy bars and slackwater deposits but only the fine grained bench units reflect eddy recirculation the pool fills are principally forced bars associated with bedrock controlled or forced riffle pool morphology a late holocene palaeoflood history is proposed based on radiocarbon ages from the pool fills multiple phases of cut and fill activity were preceded by a superflood 3400 1900 years ago that eroded the pool fills to bedrock the resilience of the pool fills was illustrated by the passage of a 1 in 100 year flood in 1992 which caused only minor erosion the presence of pool fills may provide a window to past phases of river activity that cannot be extracted from either historical records observations or palaeoflood slackwater sediment analyses the formation and sedimentary preservation potential of these landforms reflect a combination of hydraulic and structural influences but the occurrence of high magnitude floods exerts the dominant control

the velay dome french massif central melt generation and granite emplacement during orogenic evolution this paper is a synthesis of available data on the velay dome that include both small and large scale lithologic and structural mapping strain analysis isotope geochemistry geochronology and pressure temperature estimates the velay dome one of the largest granite migmatite domes of the variscan belt formed during orogenic collapse at around 300 ma its study allows an assessment of the thermal and geodynamic context leading to voluminous crustal anatexis of the variscan orogenic crust a first melting stage developed in connection with south verging thrust zones during the early carboniferous leading to a crustal thickening estimated at 20 km minimum the involvement of fertile lithologies and the intrusion of plutons of deep origin contributed to the development of water saturated melts the volume of biotite granite extracted from melt during this period was limited the second phase of melting corresponded to generalized melting of gneiss achieved by biotite dehydration melting reactions and accompanied by the generation of cordierite bearing granites at this stage crustal scale detachment faults were activeandpartiallyobliteratedtheearlierstructures thenewstructureswereprogressivelytiltedtotheverticalatthemarginofthe velay dome due to the southward and lateral ballooning of the granitic dome the reconstructed p t path indicate that the large volume ofmelt produced was a consequence ofa significant increase in temperature at the onset of biotite dehydration melting at the base of the crust this melting event is coeval with granulite facies metamorphism associated to underplating of mantle derived magmas as suggested by the geochemical signature of late paleozoic lower crustal xenoliths sampled by cenozoic volcanoes and withtheisotopicsignatureofthelategraniticintrusions accordingly itisproposedthatasthenosphericupwellingwasresponsible for the temperature increase favoring melting of hydrous minerals d 2001 published by elsevier science b v