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

hydrothermal alteration and fluid circulation related to w au and sb vein mineralization haut allier massif central france w as au and as sb vein mineralizations in the haut allier french massif central are associated with ortho and parametamorphic rocks containing biotite or sillimanite which have been locally intruded by small granitoid bodies hydrothermal alteration is well developed around these veins and has been studied here from a petrographic and a chemical point of view the results are compared with those which were obtained by microthermometric study of fluid inclusions in both mineralized types three superimposed hydrothermal stages are distinguished showing good agreement with the results obtained by other methods in the early stage fluids enriched with co 2 or ch 4 developed a quartz 2m1 muscovite assemblage with 8 paragonitic substitution at temperatures of about 400 degrees c minor chlorite appears replacing biotite near the transition of altered to fresh rocks this early stage is predominant in veins with w as au mineralization in the intermediate stage low salinity aqueous fluids dilute the earlier fluids and control a new alteration with quartz 2m1 phengite or carbonate assemblages at slightly lower temperatures this alteration stage is particularly important in the environment of veins with as sb mineralization in the late stage the last hydrothermal event occurs at a low temperature 200 degrees c and forms a 1m illite kaolinite or quartz assemblage in small alteration veinlets chemically the different phyllosilicates that successively occur in w as au and as sb veins are very similar the differences between these mineralization types can be explained by the proximity of the granitoid bodies to w as au veins and by a greater fluid rock ratio involved in the alteration processes for as sb vein deposition

the northeast brazilian rift system the northeast brazilian rift basins provide important data critical to the understanding of continental rifting processes associated with the opening of the south atlantic these basins represent the locus of intersection of the southern and equatorial branches and some basins yield substantial chronostratigraphic data that constrain the temporal and spatial interaction of the rift phases similar data are not found in its counterpart in africa especially for the neocomian these early cretaceous rift basins of northeast brazil illustrate key three dimensional geometries of intracontinental rift systems mainly controlled by the basement structural framework during the main rift phase syn rift ii neocomian early barremian extensional deformation was distributed over three main rift axes 1 the gabon sergipe alagoas gsa trend 2 the reconcavo tucano jatoba rtj trend and 3 the cariri potiguar cp trend during this phase extensional deformation jumped west from the easternmost basins gsa trend to a series of ne trending intracratonic basins rtj and cp trends characterized by a set of asymmetric half grabens separated by basement highs transfer faults and or accommodation zones these basins are typically a few tens of kilometers wide and trend ne sw roughly perpendicular to the main extension direction during the neocomian preexisting upper crustal weakness zones like the dominantly ne sw trending shear zones of the brazilian pan african orogeny controlled the development of intracrustal listric normal faults internal transverse structures such as transfer faults and accommodation zones were also controlled by the local basement structural framework the megashear zones of pernambuco brazil and ngaundere africa seem to have behaved like a huge accommodation zone accommodating extensional deformation along the rtj gsa trends with simultaneous extension along the cp trend during the late barremian syn rift phase iii a significant change in rifting kinematics occurred when the cp trend was aborted and major rifting initiated at the equatorial branch during the aptian while the equatorial branch and benue trough africa experienced the main rift phase the rtj trend was aborted and the gsa trend developed a transitional phase between the rift and drift stage the gsa trend and the offshore potiguar basin represent the site of continued evolution into passive margin basins following the main rift episode

gold deposits and hydrothermal eruption breccias associated with a maar volcano at wau papua new guinea emplacement of endogenous domes and eruption of pyroclastic flows in the morobe goldfield took place in the interval from 4 to 2 4 m y ago during the later part of this time interval following tilting of the ignimbrite pile a maar was generated at wau the subjacent diatreme is believed to extend downward for about 1 km to a regional dip slip fault zone which hosts deeper level gold mineralization at the nearby ribroaster mine following initial maar formation hydrothermal brecciation and gold deposition were controlled by low angle extensional faults which developed during incipient subsidence of a wedge of poorly supported rock between the regional fault zone and the ring fault delimiting the maar epiclastic sedimentation was widespread in the maar volatile depleted magma reached the surface around the perimeter of the maar during active sedimentation and constructed two principal endogenous domes one dacitic and the other andesitic the dacitic dome dated previously at 2 4 m y gave rise to an apron of crumble breccia along the northwestern edge of the maar accumulation of the intramaar sequence was accompanied and followed by widespread subsidence and slumping which resulted in steep dips and locally overturned beds the regional drainage system eventually breached the crater rim and resulted in deposition of channel conglomerates and fluviolacustrine beds all carrying alluvial gold radiocarbon dating of wood from this unit showed that it accumulated 42 000 years ago the most recent events at wau are believed to be a series of hydrothermal eruptions which constructed a composite breccia apron at koranga crater on the southeastern edge of the maar the most recent eruption is tentatively inferred to have taken place in 1967 modified journal abstract

replacement of primary monazite by apatite allanite epidote coronas in an amphibolite facies granite gneiss from the eastern alps accessory monazite crystals in granites are commonly unstable during amphibolite facies regional metamorphism and typically become mantled by newly formed apatite allanite epidote coronas this distinct textural feature of altered monazite and its growth mechanism were studied in detail using backscattered electron imaging in a sample of metagranite from the tauern window in the eastern alps it appears that the outer rims of the former monazites were replaced directly by an apatite ring with tiny thorite intergrowths in connection with ca supply through metamorphic fluid around the apatite zone a proximal allanite ring and a distal epidote ring developed this concentric corona structure with the monazite core regularly preserved in the center shows that the reaction kinetics were diffusion controlled and relatively slow quantitative electron microprobe analyses suggest that the elements released from monazite breakdown p ree y th u were diluted and redistributed in the newly formed apatite allanite and epidote overgrowth rings and were unable to leave the corona this supports the common hypothesis that these trace elements are highly immobile during metamorphism furthermore microprobe data suggest that the preserved monazite cores lost little possibly none of their radiogenic lead during metamorphism thus metastable monazite grains from orthogneisses appear to be very useful for constraining u th pb protolith ages on the basis of these findings and a review of literature data it seems that monazite stability in amphibolite facies metamorphic rocks depends strongly on lithologic composition while breaking down in granitoids monazite may grow during prograde metamorphism in other rocks such as metapelites

evolution mineralogique et isotopique pb du filon sulfure complexe des borderies massif central francais implications metallogeniques le filon polymetallique sb pb zn ag cu des borderies puy de dome france resulte de la succession de quatre stades de mineralisations possedant chacun ses propres caracteristiques mineralogiques geochimiques et isotopiques pb le premier stade est represente par la berthierite dont l habitus et la composition isotopique du plomb 206pb 204pb 17 76 revele un episode metallogenique ancien malgre une nette evolution geochimique du depot le second stade a mispickel jamesonite i andorite stibine zinkenite etc presente des compositions isotopiques du plomb homogenes 206pb 204pb 17 96 qui demontrent son appartenance a un meme episode metallogenique cet episode est date a 313 5 ma par la methode k ar la source du plomb present dans ces deux stades est a rechercher dans un socle metamorphique de type limousin inconnu a l affleurement dans le district enfin la nature des mineraux du troisieme stade zinkenite ii fuloppite semseyite et les compositions isotopiques du plomb 206pb 204pb 18 21 attestent d une remobilisation des sulfures deja presents dans le filon sous l action d un fluide tardif plombifere ce fluide est a rattacher aux distensions liasiques et emprunte le plomb aux roches du district granite et metamorphites le dernier stade plagionite jamesonite ii zinkenite iii est un simple prolongement du troisieme comme le demontrent la nature des sulfures et les similitudes isotopiques 206pb 204pb 18 21 l etude de filon illustre la complementarite des observations mineralogiques et des analyses isotopiques du plomb sur phases minerales bien identifiees cette double approche est necessaire pour apprehender la source des metaux et les conditions de genese de gites polyphases a parageneses complexes

electron microprobe and la icp ms study of monazite hydrothermal alteration implications for u th pb geochronology and nuclear ceramics this paper presents new data that quantify the response of magmatic monazite to three main types of hydrothermal alteration namely sericitization chloritization and greisenization the samples were taken from three paleozoic granites the st nectaire granite massif central france the carnmenellis granite cornwall england and the skiddaw granite lake district england fluid inclusion thermometry and hydrothermal parageneses indicate that alteration took place at temperatures ranging from 260 to 340 c and salinities from 3 to 18 wt nacl equivalent possible monazite alteration mechanisms found in the course of this study using backscattered scanning electron microscopy bse sem electron microprobe laser raman spectroscopy and laser ablation inductively coupled plasma mass spectrometry la icp ms include cationic substitutions monoclinic to hexagonal structure transition accompanied by chemical exchanges selective th removal dissolution reprecipitation and dissolution with replacement by a different mineral these results show that despite the compositional and crystallographic simplicity of monazite it exhibits varied chemical responses to different mineral fluid interactions elemental and isotopic measurements by la icp ms for the unaltered monazites yield statistically significant 232th 208pb and 238u 206pb magmatic crystallization ages in some cases it was possible to give a reasonable estimate of the age of mineral fluid interaction using the altered parts of the monazites or newly precipitated crystals for other minerals 232th 208pb and 238u 206pb systematics were strongly disturbed by variable inputs and or depletions of u th and pb the data suggest that monazite like nuclear waste forms would be good hosts for tetravalent actinides but may release the lanthanides and actinides with lower and higher valencies especially if the fluids are oxidizing or tend to dissolve monazite

spatial and temporal distribution of the orogenic gold deposits in the late palaeozoic variscides and southern tianshan how orogenic are they abstract a principal uncertainty in models of orogenic ore deposits concerns their ages relative to orogenic processes the yardstick of the relation has resided loosely in the peak of metamorphism age estimates in the variscides and tianshan indicate that most orogenic ore deposits were formed in the course of the late carboniferous to middle triassic with a peak between 305 and 280 ma their locations settings and ages suggest an association with coeval lithosphere scale strike slip deformation which played a role in the destruction of the orogenic system following crustal shortening this destruction tends to invalidate the orogenic association of the orogenic ore deposits observation of a mantle signature in some of these ore deposits and in coeval magmatic rocks suggests that the strike slip belts tapped into sub crustal levels here lithological composition had been modified by subduction during the stage of construction consequently any orogenic element in the ore deposit models is apart from location only an element of inheritance of subduction modified lithosphere features acquired during construction of the orogen these features may however well have been crucial for the formation of the ore deposits in view of the translithospheric extent of the strike slip belts fluids which contributed to the ore deposits may have come from at least the entire in part metasomatized lithosphere column the ore deposits could probably not have been formed without these deep reaching strike slip systems a model is proposed for the geological setting of the orogenic gold deposits in the variscides and the southern tianshan it consists of plutono metamorphic elements in the mantle in the lower and in the middle crust within a transpressional to transtensional tectonic framework of translithospheric faults and associated domes and pull apart basins with shallow marine to terrestrial sediments and interlayered felsic and mafic volcanics the thermal engines of these systems were probably in localized upwellings of the asthenosphere prompted and controlled by the lithosphere scale deformation at the time at an even larger scale oroclinal bending of the old orogenic backbone may have played yet another role in the localization of the ore deposits the thesis of specific localized tectono thermal engines is at odds with the original proposals of a relation between orogenic ore deposits and regional metamorphism it suggests that the late palaeozoic gold antimony mercury ore deposits in the variscides and the southern tianshan are more akin to the intrusion related ore systems

early palaeozoic palaeobiogeography of the variscan regions abstract the palaeobiogeographic method appears to be an attractive alternative for reconstruction of the palaeogeography of the variscan regions during the palaeozoic the evolution of faunal assemblages and climatic characteristics from the cambrian up to the end of the devonian demonstrates that baltica and north gondwana represent two major and independent palaeogeographic entities separated by a mid european rheic ocean which opened as early as the cambrian and closed progressively from the devonian the south armorican ocean whose closure generated the so called ligerian orogen is regarded as a branch of the rheic ocean both a tornquist s sea separating baltica from its western extension avalonia during the ordovician and a proto tethys ocean extending between southwestern europe and north africa during the devonian are excluded from our reconstructions

fluid rock interaction processes related to hydrothermal vein type mineralization in the siegerland district germany implications from inorganic and organic alteration patterns altered wallrocks of vein type pb zn sb mineralization siegerland district rheinisches schiefergebirge have been investigated by a combination of inorganic and organic geochemical methods including major and trace element analysis vitrinite reflectance measurements c isotope and elemental analysis of kerogen alteration features of the siliciclastic pelitic psammitic lower devonian wallrocks are increased k na ratios significant desilicification and relative immobility of a number of elements notably al ti zr cr v wallrock kerogens display elevated vitrinite reflectance values decrease in h c atomic ratios coupled with increase in s c atomic ratios and heavier c isotope compositions compared to the unaltered precursor sedimentary rocks interaction processes between the hydrothermal fluids and the respective wallrocks related to injection of high temperature silica undersaturated solutions are dominated by quartz dissolution coupled with sericitization reactions heat transfer due to fluid infiltration convection and wallrock reactions caused fluid cooling which promoted the sequential deposition of quartz and stibnite sulphosalts within the vein systems hydrocarbons detected in ore assemblages of pb zn and sb mineralization were most probably derived from the lower devonian very low grade meta sedimentary rocks high maturity levels and pronounced typical organic alteration patterns indicate that thermochemical so2 4 reduction tsr played an important role in precipitation of metal sulphides the present study demonstrates that a combination of inorganic and organic investigations on fluid rock interaction processes is particularly useful for deciphering precipitation mechanisms of base metal sulphides

oxide and sulphide isograds along a late archean deep crustal profile in tamil nadu south india oxide sulphide fe mg silicate and titanite ilmenite textures as well as their mineral compositions have been studied in felsic and intermediate orthogneisses across an amphibolite north to granulite facies south traverse of lower archean crust tamil nadu south india titanite is limited to the amphibolite facies terrane where it rims ilmenite or occurs as independent grains pyrite is widespread throughout the traverse increasing in abundance with increasing metamorphic grade pyrrhotite is confined to the high grade granulites ilmenite is widespread throughout the traverse increasing in abundance with increasing metamorphic grade and occurring primarily as hemo ilmenite in the high grade granulite facies rocks magnetite is widespread throughout the traverse and is commonly associated with ilmenite it decreases in abundance with increasing metamorphic grade in the granulite facies zone reaction rims of magnetite quartz occur along fe mg silicate grain boundaries magnetite also commonly rims or is associated with pyrite both types of reaction rims represent an oxidation effect resulting from the partial subsolidus reduction of the hematite component in ilmenite to magnetite this is confirmed by the presence of composite three oxide grains consisting of hematite magnetite and ilmenite magnetite and magnetite pyrite micro veins along silicate grain boundaries formed over a wide range of post peak metamorphic temperatures and pressures ranging from high grade so2 to low grade h2s dominated conditions oxygen fugacities estimated from the orthopyroxene magnetite quartz orthopyroxene hematite quartz and magnetite hematite buffers average 2 5 log units above qfm it is proposed that the trends in mineral assemblages textures and composition are the result of an external infiltrating concentrated brine containing an oxidizing component such as caso4 during high grade metamorphism later acted upon by prograde and retrograde mineral reactions that do not involve an externally derived fluid phase

uranium rich accessory minerals in the peraluminous and perphosphorous belvis de monroy pluton iberian variscan belt the strongly peraluminous perphosphorous 0 85 wt p2o5 and low ca granites from the belvis de monroy pluton contain the most u rich monazite ce and xenotime known in igneous rocks along with these accessory minerals p rich zircon occurs reaching uncommon compositions particularly in the more fractionated units of this zoned pluton monazite displays a wide compositional variation of uo2 23 13 wt and tho2 19 58 wt positively correlated with ca si p y and ree xenotime shows a high uo2 content 2 37 13 34 wt with parallel increases of lree ca and si zircon contains comparatively much lower uo2 1 53 wt but high p2o5 14 91 wt al2o3 6 96 wt feo 2 93 wt and cao 2 24 wt contents the main mechanism of incorporating large u and th amounts in studied monazite and u in xenotime is the cheralite type th u 4 ca2 2 y ree 3 substitution zircon requires several coupled mechanisms to charge balance the p substitution resulting in non stoichiometric compositions with low analytical totals compositional variations in the studied accessory phases indicate that the substitution mechanisms during crystal growth depend on the availability of non formula elements the strong p rich character of the studied granites increases monazite crystallization triggering a progressive impoverishment in th and lree in the residual melts and consequently increasing extraordinarily the u content in monazite and xenotime this is in marked contrast to other peraluminous i type or p poor s type granite series the p rich and low ca peraluminous melt inhibits uraninite crystallization so contributing to the u availability for monazite and xenotime

experimental determination of ree distributions between monazite and xenotime potential for temperature calibrated geochronology solid solutions of the ree y orthophosphates monazite and xenotime were synthesized hydrothermally between 300 c and 800 c at 2 and 5 kbar using ree y oxides and h3po4 as starting materials the ree bulk composition was similar to those found in metapelitic rocks all experiments produced two solid phases monoclinic monazite and tetragonal xenotime in volumetric ratios similar to relative abundances in metapelites ree concentrations of coexisting monazite xenotime pairs were determined by emp analysis and ree distribution coefficients dree xreemonazite xreexenotime were derived for the entire ree y suite values of dree depend strongly on ionic radii of the rees and on temperature the log dree vs ionic radius curve shows a nearly linear decrease from la to lu and comprises almost five orders of magnitude from dla 102 5 to dlu 10 2 between 450 and 800 c most dree values vary continuously by a factor of about 2 to 3 dnd decreases from 48 to 16 and dsm from 9 to 5 dtb increases from about 0 2 to about 0 4 ddy from 0 1 to 0 2 and dy from 0 06 to 0 12 along this temperature range experimentally determined values of dree are in close agreement with those derived from natural monazite xenotime pairs equilibrated between 400 and 700 c ree partitioning between monazite and xenotime is appropriate for geothermometry the most useful application is that equilibrium and disequilibrium partitioning of ree in metamorphic rocks are easily recognized in the case of equilibrium distributions u pb chronometry of single monazites or xenotimes provides a direct link to their growth conditions and information on temperature time paths equilibrium distributions of sm and nd expressed in molar fractions result in kd xsmmonazitexndxenotime xsmxenotimexndmonazite 0 2 to 0 3 from 600 to 800 c and sm nd chronometry should provide reasonable two mineral isochrons for such monazite xenotime pairs

late quaternary sedimentation in the paraiba basin northeastern brazil landform sea level and tectonics in eastern south america passive margin abstract late quaternary deposits in the northeastern brazil have been scarcely investigated despite their relevance to the discussion of the post rift evolution of the south american passive margin within the context of landform sea level and tectonic deformation sedimentological stratigraphic and morphological characterization of these deposits referred as post barreiras sediments led to their distinction from underlying early middle miocene strata based on optically stimulated luminescence osl dating two sedimentary units pb1 and pb2 were recognized and related to the time intervals between 74 8 9 3 and 30 8 6 9 ka and 8 8 0 9 and 1 8 0 2 ka respectively unit pb1 consists of indurated sandstones and breccias either with massive bedding or complex types of soft sediment deformation structures generated by contemporaneous seismic activity unit pb2 is composed of massive sands or sands related to structures developed by dissipation of dunes the present work focusing on the post barreiras sediments discusses landform sea level and tectonics of the eastern south american passive margin during the latest quaternary non deposition and sub aerial exposure related to the tortonian worldwide low sea level combined with tectonic quiescence followed the miocene transgression tectonic deformation in the latest pleistocene created space to accommodate unit pb1 in downthrown faulted blocks and perhaps also synclines produced by strike slip deformation although deposition of this unit was simultaneous with the progressive fall in sea level that followed the last interglacial maximum punctuated rises combined with land subsidence led to marine deposition close to the modern coastline renewed subsidence in the holocene gave rise to accommodation of the post barreiras sediments most of unit pb2 was deposited during the holocene transgression but it is not composed of marine sediments which suggests either an insignificant rise in relative sea level or aeolian reworking of thin transgressive sands the data presented here lead to a review of the evolution of the south american passive margin based on assumptions of uniform sedimentation and undeformed planation surfaces over a wide coastal area of the northeastern brazil

formation of al rich titanite catisio4o caalsio4oh reaction rims on ilmenite in metamorphic rocks as a function of fh2o and fo2 abstract reaction rims of titanite on ilmenite are described in samples from four terranes of amphibolite facies metapelites and amphibolites namely the tamil nadu area southern india the val strona area of the ivrea verbano zone northern italy the bamble sector southern norway and the northwestern austroalpine otztal complex the titanite rims and hence the stability of titanite catisio4o and al oh titanite i e vuaganatite hypothetical end member caalsio4oh are discussed in the light of fh2o and fo2 buffered equilibria involving clinopyroxene amphibole biotite ilmenite magnetite and quartz in the systems cao feo fe2o3 tio2 sio2 h2o o2 cftsh and cao feo fe2o3 al2o3 sio2 h2o o2 cfash present in each of the examples textural evidence suggests that titanite reaction rims on ilmenite in rocks from tamil nadu val strona and the bamble sector originated most likely due to hydration reactions such as clinopyroxene ilmenite quartz h2o amphibole titanite and oxidation reactions such as amphibole ilmenite o2 titanite magnetite quartz h2o during amphibolite facies metamorphism or as in the case of the otztal complex during a subsequent greenschist facies overprint overstepping of these reactions requires fh2o and fo2 to be high for titanite formation which is also in accordance with equilibria involving al oh titanite this study shows that in addition to p t bulk rock composition and composition of the coexisting fluid fo2 and fh2o also play an important role in the formation of al bearing titanite during amphibolite and greenschist facies metamorphism

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

soft sediment deformation structures interpreted as seismites in the uppermost aptian to lowermost albian transgressive deposits of the chihuahua basin mexico several levels of soft sediment deformation structures s s d s cut by synsedimentary normal faults have been observed in the transition beds between the las vigas and la virgen formations cretaceous in the northeastern part of the chihuahua basin in mexico these structures consisted of four kinds of motifs floating breccias flame like structures large pillow structures and wavy structures they are restricted to five stratigraphic levels 1 5 and surrounded by undeformed beds in fluvio lacustrine and tidal deposits and can be traced over a distance of several hundred meters this deformation is interpreted to have resulted from the combined effects of liquidization and shear stress in soft sediments due to local earthquakes in the area which could have been generated during the rifting stage of the chihuahua basin new constraints placed on the age of the las vigas formation bracketed by late aptian charophytes at the bottom and colomiellids of late aptian to earliest albian age at the top suggest that this synrift tectonism lasted at least until the end of the aptian

sediment deformation in miocene and post miocene strata northeastern brazil evidence for paleoseismicity in a passive margin abstract stratigraphic intervals characterized by varied and complex styles of soft sediment deformation structures are well preserved in miocene and late pleistocene to holocene deposits of a sedimentary basin located in northeastern brazil the miocene strata represented by the barreiras formation record only brittle structures including numerous faults and fractures with straight and high angle dipping planes that are often filled with sands derived from overlying beds folds consisting of broad anticlines and synclines are also present in this unit the late pleistocene to holocene deposits named post barreiras sediments contain an indurated sandy package with a large variety of ductile and brittle deformation structures i e massive sandstones with isolated sand fragments and breccias undulatory strata sand dykes and diapirs sinks and bowls pebbly pockets plunged sediment mixtures fitted sand masses cone shaped cracks fault grading and sedimentary enclaves these features confined to sharp based stratigraphic horizons that progressively grade downward into undisturbed deposits are related to seismic shocks of high surface wave magnitude i e ms 5 or 6 amalgamated seismites suggest that previously formed seismites were affected by subsequent seismic wave propagation seismic waves caused by activity along one or most likely several tectonic structures would have propagated throughout the depositional environment producing laterally extensive seismites the close proximity to earthquake epicenters would have promoted pervasive re sedimentation due to pore overpressure resulting high volumes of massive sandstones and breccia the similarity between deposits with correlatable strata from many other areas along the brazilian coast allows raise the hypothesis that the seismic episodes might have affected sedimentation patterns in a large i e extension of several hundreds of kilometers geographic area thus the modern seismicity recorded along northeastern brazil was recurrent during the quaternary and perhaps also in the pliocene the estimated high magnitude of the seismic events and the great regional extent of the affected area demonstrate that the brazilian coast experienced tectonic stress through the last geological episodes of its evolution which would have favored sediment accumulation and penecontemporaneous re sedimentation this geological context is unexpected in a passive margin inducing to revisit the debate on how active is a passive margin

early devonian trace fossils in marine to non marine redbeds in podolia ukraine palaeoenvironmental implications abstract lochkovian redbeds in the dnester formation in podolia ukraine record an upward transition from marginal marine to fluvial conditions a 60 m thick section through transitional strata reveals a regressive stack of three facies associations laid down in offshore lagoon inshore lagoon or bay and a fluvial estuarine plain the offshore lagoonal association shows green to variegated alternation of fossiliferous bioturbated limestones quartz arenites and shales numerous coquinas contain schizohaline ostracods vermiform gastropods lingula charophyte gyrogonites all mixed with fragments of marine forms such as articulate brachiopods tabulates goniatites and leiospheres trace fossil assemblages contain abundant skolithos spirophyton teichichnus and simple tubular forms the inshore lagoon or bay association is a largely non fossiliferous locally bioturbated red succession of interbedded mudstones and sandstones which are arranged in coarsening up units capped by thin channelized sandstones the presence of lockeia monomorphichnus rusophycus and skolithos suggests that the coarsening up units could have originated as small bayhead deltas and washover fan channel complexes a new ichnospecies monomorphichnus podolicus described here occurs also in these deposits in the overlying fluvial estuarine association the only trace fossil recorded is skolithos which occurs in dense clusters along upper levels of cross bedded ribbon shaped channel fills until now piperock has been considered as an ichnofabric of the skolithos ichnofacies that is typical of sandy beaches bars and spits whereas the findings presented here expand landwards the environmental range of skolithos piperocks into more intracoastal marine influenced settings

the la belliere gold and antimony district french armorican massif a two stage evolution model controlled by variscan strike slip tectonic the la belliere gold district in the ligerian domain of the armorican massif is the third one of france in term of production it shows gold bearing quartz veins neighboring with a complex network of sb veins hosted in the neoproterozoic metasedimentary basement of the mauges nappe new structural data mineralogical textural and geochemical assays have been carried out to clarify the gold and sb veins relationships our results favor of two step hydrothermal event with an early deep seated gold event followed by a late shallower sb event the gold bearing quartz veins are hosted by n60 e to n90 e sinistral strike slip faults these veins exhibit a four stage paragenesis 1 early barren quartz filling 2 arsenopyrite pyrite with minor scheelite assemblage 3 gold and base metal sulfides crystallized after a ductile brittle deformation stage and 4 pyrite carbonate coeval with a late brittle stage these results support a continuous evolution model from a deep emplacement level 12 to 9 km to a shallow one 7 to 5 km with a brutal change in the p t conditions that triggered the deposition of gold stage 2 controlled by a seismic valve mechanism on the contrary sb veins correspond to open space vein types hosted in a n20 w to n20 e conjugated strike slip fault network arranged in a riedel shear model controlled by n130 140 e regional dextral strike slip shear zone associated to the south armorican shear zone system a two stage evolution is recognized the first one consists of early pyrite arsenopyrite with microcrystalline quartz followed by a late stibnite stage formed after geodic quartz deposition quartz texture argues for a shallow emplacement 5 km of veins controlled by a suction pump mechanism chronologically the gold vein opening complies with a ne sw maximum shortening direction different from the late variscan stress field responsible for the dextral armorican shear zone during late carboniferous and constrains an indirect dating thus a visean age is proposed for the formation of gold veins the overall features of gold deposits are close to those of mesozonal orogenic gold deposits in europe conversely sb bearing veins are generated by a n s shortening during the late carboniferous the sb event distinct in time from the earlier gold one might correspond to a different expression of the so called epizonal type in the orogenic gold model it is proposed that the late carboniferous or 300 event is not the unique metallogenic period for gold deposition in the french variscan domain our study highlights the role of the sinistral strike slip shearing of possible visean age in the gold metallogenesis of the ligerian domain

extensional tectonics in the central iberian peninsula during the variscan to alpine transition abstract the passage from the variscan cycle to the early alpine framework in the central part of the iberian peninsula can be explained in terms of a transitional process involving four clearly differentiated tectonic episodes 1 1 a first variscan compressional stage vi middle devonian to early carboniferous dominated by compressional conditions leading to the building up of the orogenic edifice the stress regime was relevant to what might be called variscan type compression e w oriented this stage was characterized by major himalayan type tectonics with frontal nappes thrusts overturned folds lateral transcurrent ramps and localized anatectic magmatism minor synorogenic extension and plutonism was also recorded during this stage in the tormes granitic dome 2 2 a second variscan stage v2 early to middle carboniferous was characterized by increasing extensional conditions leading to widespread plutonism adamellites granodiorites wanning compressional conditions were restricted to the eastern and southern realms of central iberia the eastern part of the spanish central system and the toledo mountains 3 3 a third stage here defined as late variscan lv developed from middle carboniferous to early permian as a result of n s late orogenic extension this episode is relevant to detachment tectonics and the gravitational collapse of the variscan orogenic edifice under combined simple pure shear conditions plutonism granites and leucogranites was still of major importance early permian andesitic to dacitic volcanism and sedimentary basins developed within the eastern part of the spanish central system 4 4 a fourth stage here defined as early alpine ea early permian to triassic marks the onset of the alpine framework this stage was characterized by what might be called an early alpine type regional stress regime i e e w extension and n s compression within a simple shear model and resulted in the configuration of the iberian peninsula into two contrasted realms a western inherited variscan block and an eastern alpine block subjected to post orogenic extension elements developed during this event include n s high angle normal faults nw se and ne sw conjugate strike slip faulting and asymmetric rifting involving listric low angle detachments

format and philosophy for collecting compiling and reporting microprobe monazite ages microprobe monazite dating has been increasingly used to constrain the timing of deformation and metamorphism because of the potential to date very small monazite domains down to 5 m or less in structural and petrologic context this paper presents an analytical strategy presentation format and error considerations for microprobe monazite dating the strategy involves high resolution compositional mapping to delineate compositional domains within monazite crystals then for each compositional domain a series of th u and pb analyses are made and a single date and error are calculated the number of analyses in each domain is determined by the desired statistical precision of the date results from several monazite grains are typically combined and along with textural relationships are used to build an argument that the dates constrain the age of a deformation or metamorphic event the total error involves three components short term random error dominated by counting statistical uncertainty short term systematic error uncertainty in background correction conductive coating variation and calibration and long term systematic error uncertainty in standard composition mass absorption factors decay constants etc in homogeneous compositional domains short term random errors 2 of less than 10 m y can be obtained from five to ten analyses however short term systematic error mainly background estimation uncertainty would typically result in a doubling of the magnitude of random error microprobe dates are presented as a single gaussian probability distribution for each domain along with representative compositional maps it is recommended that a consistency standard be analyzed during each analytical session and the results be reported along with those from the unknown this proposed strategy and format are compatible with those of other geochronological techniques they incorporate analytical limitations associated with trace as opposed to major element microprobe analysis and will allow better comparisons to be made between labs and between different geochronological techniques

variation of coastal dynamics during the last 7000 years recorded in beach ridge plains associated with river mouths example from the central brazilian coast abstract the reconstruction of relative sea level changes during the last 7000 years has shown that the central brazilian coast was been subjected to submergence until approximately 5100 yr b p however the emergence after 5100 yr b p was interrupted by two important fluctuations with an amplitude of approximately 2 3 m and a duration of 200 300 yr this evolutionary history which is quite different from that of several other regions in the world played an essential role in the development of the central brazilian coastal plains whether or not they are associated with important river mouths the periods of submergence characterized by erosional phases introduced noticeable changes in the geometry of the coastal deposits the periods of emergence gave rise to sandy terraces covered by beach ridges whose orientation is determined by the longshore drift elsewhere on a low sandy coast the direction of longshore sand transport depends on the orientation of the swell in a coast subjected to various swell patterns the efficient swells are defined as those which determine the resulting longshore transport direction such swells are not necessarily the most prevalent for instance along the central brazilian coast the southern sector waves in spite of their infrequent occurrences are much more powerful than the northern sector waves and therefore generate a longshore sand transport from south to north when fossil beach ridges are present as is the case on the central brazilian coast their geometry reflects the past directions of longshore sand transport this makes it possible to determine the provenance of past efficient swells and to establish the past wind patterns a detailed study of the beach ridge geometry of some sections of the central brazilian coast showed a sequence of reversals of the longshore drift during the last 5100 yr with a duration ranging from 10 to 100 yr these reversals represent changes in the direction of effective waves which determine the longshore transportation of sediments and consequently indicate changes in the wind pattern

fluid inclusion study of the bournac polymetallic sb as pb zn fe cu vein deposit montagne noire france mineralogical studies demonstrate that the hercynian polymetallic antimony rich deposit of bournac can be described by four stages of ore deposition and one of partial ore remobilization fluid inclusion data permit calculation of the composition and temperature of the fluids associated with each stage of hydrothermal mineralization and concomitant wall rock alteration stages i and ii fe as and zn are represented by moderate salinity h2o co2 nacl inclusions which correlate closely with early carbonate deposition stage iii fluids which are responsible for the deposition of pb sb ores are characterized by low salinity h2o nacl inclusions during the final stage of mineralization iv corresponding to the main phase of stibnite deposition abundant aqueous inclusions confirm the continued involvement of low salinity fluids and the intense development of potassic clays and secondary silica in the wall rocks homogenization temperatures suggest that the whole cycle of mineralization took place during a gradual decrease in fluid temperature of 380 140 c stibnite deposition is restricted to the interval of 230 140 c thus confirming an essentially epithermal environment stage v partial remobilization is distinguished by the presence of high salinity cacl2 rich inclusions which are tentatively related to triassic barite mineralization in the region and therefore postdate the bournac antimony ores homogenization temperatures for this stage range 140 60 c

paleosurfaces from northeastern amazonia as a key for reconstructing paleolandscapes and understanding weathering products the characterization and mapping of paleoweathering surfaces as well as their associated deposits throughout the northeastern amazonia can be a useful tool for inter basin stratigraphic correlations and for establishing the evolution of landscapes and climates through time this type of investigation has led to the recognition of five weathering surfaces developed from the upper cretaceous through the quaternary designated here as s1 to s5 surface s1 occurs at the top of late cretaceous deposits represented by the ipixuna cujupe succession as well as on older precambrian and paleozoic rocks surface s2 marks the top of an indurated massive semi flint1 unit for cases in which surfaces s1 and s2 are amalgamated a thick lateritic paleosol displaying hexagonal branched concretions is present surface s3 occurs at the top of the pirabas and lower portion of the barreiras formations of late oligocene early miocene age and it is also associated with paleosol but has no lateritization when surface s3 is amalgamated with surfaces s1 and s2 bauxite is present in association with the underlying laterites surface s4 bounds the top of the middle to upper barreiras formation of middle miocene age as well as correlatable deposits known as the belterra clay this surface is marked by a lateritic paleosol having columnar concretions surface s5 occurs between a lower unit consisting of massive light red sands of the plio pleistocene post barreiras 1 sediments and an upper unit of yellow eolian sands of the holocene post barreiras 2 sediments the paleoweathering surfaces present in the northeastern amazonia enable the reconstruction of several depositional episodes represented by thin only few meters thick sedimentary units from the late cretaceous to the quaternary the creation of new accommodation space on subsiding areas gave rise to sediment accumulation during transgressions intervening weathering surfaces formed during significant falls in relative sea level associated with tectonically stable episodes the overprinting of weathering surfaces formed during different times and thus under different climatic conditions seems to have been crucial for the development of the various weathering products observed throughout the region which include economically important laterites bauxites semi flint beds and kaolin deposits this paper shows that the genesis of these deposits was controlled by the complex interplay of tectonics climate sediment deposition and erosion resulting in a complex evolution of paleolandscapes

genesis of hydrothermal silver antimony sulfide veins of the braunsdorf sector as part of the classic freiberg silver mining district germany the peripheral regions of the freiberg vein type silver mining district comprise several sub districts of which braunsdorf was among the richest in terms of ag grade historically 114 t about 3 9 million ounces of ag were produced from the neue hoffnung gottes mine near braunsdorf the neuer segen gottes stehender is a sigmoidally shaped nne ssw trending vein which varies significantly in thickness 0 05 to 3 m and extends over about 2 6 km strike length at the surface the vein infill is marked by a pb zn cu fe sulfide quartz stage 1 and an abundant ag sb sulfide sulfosalt quartz carbonate assemblage stage 2 to develop a sound genetic understanding of the polymetallic mineralisation in the braunsdorf sub district we conducted detailed textural analyses of ore and gangue minerals fluid inclusion analyses electron microprobe analyses and thermodynamic computations in order to characterise the ore fluids and ore forming processes the early stage pb zn cu fe sulfide mineralisation stage 1 is related to fluids with low salinities 0 5 4 eq w nacl and formed at temperatures 300 c microthermometric data related to the ag sb sulfide sulfosalt assemblage stage 2 show similar salinites compared to ore stage 1 but have significantly lower homogenisation temperatures in the range of 180 280 c based on fluid inclusion data cooling can be regarded as the major ore forming process reaction path model calculations for cooling of fluids with different initial ph values 4 5 5 and 7 reproduce the observed mineral assemblages very well and predict spatial zonation of the ag sb and sb sulfide minerals that are in excellent agreement with field observations we conclude that ag rich zones may well occur below sb rich zones in hydrothermal vein type systems similar to those of the freiberg district this relationship may be of potential use for exploration targeting

titaniferous magnetite ilmenite thermometry and titaniferous magnetite ilmenite orthopyroxene quartz oxygen barometry in granulite facies gneisses bamble sector se norway implications for the role of high grade co 2 rich fluids during granulite genesis oxygen fugacities have been estimated for a wide distribution of samples from the granulite facies terrane region c of the bamble sector se norway using both the titaniferous magnetite ilmenite and orthopyroxene titaniferous magnetite quartz oxygen barometers these oxygen fugacities are estimated using temperatures calculated from the titaniferous magnetite ilmenite thermometer of ghiorso and sack 1991 and are both internally consistent with each other as well with the thermometer in samples for which the estimated temperature is high the two oxygen barometers show good agreement whereas agreement is poor for low temperature samples in these low temperature samples oxygen fugacities estimated from titaniferous magnetite ilmenite are considerably less than those estimated from orthopyroxene titaniferous magnetite quartz an increase in this discrepancy with decrease in temperature appears to reflect preferential resetting of the hematite component in the ilmenite grains without significant alteration of the more numerous titaniferous magnetite grains this is due in part to greater re equilibration of the ilmenite grains during retrograde interoxide resetting between the ilmenite grains and the titaniferous magnetite grains the mean temperature for the non reset samples 791 17 c 1 is in good agreement with temperatures obtained from garnet orthopyroxene kd exchange thermometry in the same region 785 795 c 1 harlov 1992 2000a most non reset oxygen fugacities range from log10 f o2 14 to 11 8 or approximately 0 5 1 5 log units above quartz fayalite magnetite at 7 5 kbar both these temperatures and the range of oxygen fugacities are in good agreement with those estimated using the titaniferous magnetite ilmenite thermometer oxygen barometer of andersen et al 1991 the quilp equilibrium quartz ulvospinel ilmenite pyroxene is used to project self consistent equilibrium temperatures and oxygen fugacities for samples reset due to hematite loss from the ilmenite grains these projected temperatures and oxygen fugacities agree reasonably well with the non reset samples the mean projected quilp temperature is 823 6 c 1 this result supports the conclusion that low titaniferous magnetite ilmenite temperatures down to 489 c and accompanying low oxygen fugacities are the result of hematite loss from the ilmenite grains non reset oxygen fugacities lie approximately 1 5 log10 units above the upper graphite stability curve indicating that the stable c o h fluid phase interacting with these gneisses whether regionally or locally was co2 this is borne out by the presence of numerous co2 rich fluid inclusions in these rocks

from thickening to extension in the variscan belt kinematic evidence from sardinia italy the variscan nappe stack of se sardinia originated as a result of several stages of nappe imbrication during the lower carboniferous phases of the variscan orogeny the crustal shortening caused regional ssw and w directed thrusting greenschist facies metamorphism and open to isoclinal polyphase folding the final stage of shortening produced large scale antiforms and synforms post collisional deformation resulted in inversion of earlier thrusts as normal faults development of low angle normal faults and refolding of earlier foliation and thrust planes by asymmetric folds with subhorizontal axial planes facing directions of these latest folds are directed horizontally outward from the hinge zones of main antiforms suggesting that they cannot be regarded as parasitic folds of the latest thickening phase but instead are the consequence of vertical shortening during gravitational collapse of dome like km scale antiforms leading to denudation of antiformal culminations

antimony deposits in the variscan armorican belt a link with mafic intrusives the variscan armorican belt was the world leading producer of antimony at the beginning of the twentieth century however geological controls on the deposits remain unconstrained here we illustrate the field setting of mineralisations and perform a statistical analysis using geographical information system software geophysical and geological data the analysis shows that sb deposits are spatially correlated with high density and magnetic zones this may reflect the presence of mafic ultramafic bodies at depth a suggestion that is further supported by numerous occurrences of doleritic dykes throughout the region future antimony prospecting should focus on the high density and high magnetic zones which comprise about 18 of the armorican massif links between sb deposits and mafic intrusions are recognised elsewhere in the west european variscan belt emphasising that these may be key for antimony prospecting in deformation belts

collapse of a hercynian tibetan plateau into a late palaeozoic european basin and range province the large width the importance of crustal shortening and the likelihood of thick crust in the late palaeozoic hercynian or variscan belt of southern europe suggest a tectonic evolution analogous to the evolution of the tibetan plateau and its neighbouring himalaya1 and of the altiplano and other high plateaux of the andes here we extend dewey and burke s analogy1 of the hercynian chain with tibet to include the subsequent stage of pervasive normal faulting and crustal extension and thinning which characterize the active tectonics of tibet parts of the high andes and the basin and range province of western north america it appears that normal faulting and rifting also characterize the late palaeozoic tectonic activity of the hercynian chain

liquefaction induced structures in quaternary alluvial gravels and gravelly sediments ne brazil abstract we have identified numerous well preserved elutriation and fluidization structures probably induced by liquefaction in quaternary gravels and gravelly sediments of braided fluvial channel deposits in the rio grande do norte and ceara states northeastern brazil they show evidence of upward directed water escape after sediment deposition and before sediment compaction among the several types of structures observed the most frequent are pillars pockets and dikes these structures range in width from a few centimeters to as much as 4 m and in height from 60 cm to 4 m dikes pillars and pockets are systematically associated clastic dikes vented large quantities of sand to the upper layers or the surface pebbles and cobbles from the host rock sank into the dikes and formed pillars and pockets pockets form the root part pillars form the intermediate part and dike the upper part of the composite structure the morphology of the structures in sectional and plan views indicates a 3d geometry composed of a tabular dike and pillar that present a downward continuous transition to a bowl shaped pocket this stratigraphy of liquefaction features is different from that usually presented in the current literature field data suggest that both the location and the geometry of the features were controlled by sedimentary properties rather than joints and small faults the size and abundance of these features suggest that they were formed by great events rather than localized mechanisms field evidence also indicates that these features are the product of fluidization and elutriation and may have been induced by liquefaction processes associated with seismic shaking a nonseismic origin related to elutriation processes however cannot be ruled out for some of the features

the variscan post collisional volcanism in late carboniferous permian sequences of ligurian alps southern alps and sardinia italy a synthesis abstract in the considered wide sector of the west mediterranean southern europe the collisional phase of the variscan orogeny during late carboniferous and permian times was followed by magmatic intrusive and effusive activity and sedimentation into intracontinental alluvial to lacustrine basins originated by wrench to normal fault systems the first volcanic cycle generally late carboniferous early permian in age is represented by early calc alkaline andesites and rhyolites in variable amounts and by following large volume of rhyolites and by dacites both andesites and rhyolites show k normal and high k calc alkaline character the origin of the liquids of the first cycle is ascribed to partial melting processes at the mantle crust interface telescoped within a thickened crust the melting is considered as the consequence of thermal re equilibration following isostatic disequilibrium and the subsequent collapse of the orogenic belt the ascent of liquids occurred in a trans tensional regime the second magmatic cycle is represented by alkaline magmatism and exhibits typical anorogenic features consistent with a rifting regime this event was no more related with the collapse of the variscan belt but rather to the post variscan global re organization of plates that evolved during late triassic times to the neo tethyan rifting in both cycles important differences in timing areal distribution and outpoured volumes arise

new advances in trace element geochemistry of ore minerals and accessory phases the trace element composition of ore minerals is of fundamental importance to the understanding of mineralizing processes in situ analyses of ore minerals by empa sims and la icp ms have become essential approaches to many geochemical studies progress in micro beam techniques allows the examination at any location in the sample with unprecedented detail

quartz veins with associated sb pb ag au mineralization in the schwarzwald sw germany a record of metamorphic cooling tectonic rifting and element remobilization processes in the variscan belt a combination of textural observations from 38 permian sb pb ag au bearing quartz veins in the schwarzwald sw germany with isotopic fluid inclusion and geochemical data allows to refine genetic models for this common type of mineralization to understand the origin of mineralogical diversity and the correlation with large scale tectonic events textures record four main mineralization stages i fe as sb au ii pb zn cu iii pb sb and iv ag sb stage i sulfides dominated by pyrite and arsenopyrite formed due to cooling of low salinity metamorphic fluids during the permian with maximum homogenization temperatures of 400 c invisible gold in arsenopyrite pyrite marcasite and stibnite was remobilized and locally precipitated as electrum at the end of stage i minerals of stages ii and iii comprise a rich diversity of sb bearing sulfosalts including e g bournonite zinkenite and jamesonite this assemblage formed during jurassic times due to mixing of high salinity mid and upper crustal fluids up to 27 wt nacl cacl2 with homogenization temperatures between 50 and 250 c stage iii is marked by a rich variety of pb sb sulfosalts its local abundance is directly related to the presence of stage i mineralization and its extent of remobilization during a significant influx of pb in the jurassic cretaceous the formation of the upper rhine graben during the tertiary reactivated especially ne sw oriented veins percolating ag rich fluids reacted with earlier stage iii pb sb sulfosalts forming ag rich minerals stage iv such as miargyrite pyrargyrite and stephanite the transition from metamorphic fluids to basinal saline e g pb bearing brines over hundreds of millions of years as is shown in this study and present in other variscan occurrences indicates a possibly typical poly stage characteristic of sb deposits worldwide which has however not been investigated in detail so far

antimoniferous vein type mineralization of the berga antiform eastern thuringia germany a fluid inclusion study abstract the antimoniferous vein mineralization in the schleiz and greiz area eastern thuringia are bound to ne sw striking variscan fault zones along the margins of the berga antiform the sb pb zn ores form massive sulfide sulfosalt quartz carbonate veins breccias or stockworks within ordovician and silurian metasedimentary units devonian metabasalts and metagranitoids mineralogical investigations from several sb mines and hard rock quarries near schleiz and greiz germany show that the sb pb zn sulfides are texturally in close association with quartz and carbonates stibnite is the major sb bearing mineral minerals of the sb pb zn sulfide assemblage replace an earlier zn fe as sulfide quartz assemblage quartz i named the ore stage i mineralization stibnite and quartz ii overgrow these early sulfides accounting for the ore stage ii lead dominated sb sulfides e g zinkenite plagionite semseyite represent the ore stage iii phase followed by sphalerite ii galena and quartz iii accounting for ore stage iv fluid inclusion studies on ore and gangue minerals of the different ore stages give insights into the composition and temperature of the mineralizing fluid s primary fluid inclusion assemblages in ore stage ii stibnite and co genetic quartz ii show variable vapor fractions within fluid inclusion assemblages heterogeneous trapping of co2 at constant salinities of fluid inclusions of the same assemblage indicates the influx of a buoyant pure co2 fluid into an aqueous fluid phase the presented data demonstrates the first example of co2 influx into an aqueous fluid identified by fluid inclusion analyses during this ore stage ii a h2o co2 nacl fluid with eq w nacl of 1 to 2 7 at 220 226 c prevails in contrast fluid inclusions in ore stage iii minerals boulangerite contain a homogeneously trapped h2o nacl cacl2 fluid with eq w nacl up to 8 1 at temperatures 146 c ore stage iv minerals sphalerite ii quartz iii contain a homogeneously trapped h2o nacl cacl2 fluid with eq w nacl of 20 and homogenization into the liquid phase at around 120 c fluid inclusion data suggest p t conditions at the onset of mineralization at around 220 c at 0 70 to 0 75 kbar fluid pressure the mechanisms of sb ore precipitation were induced by the influx of co2 fluid cooling quartz ii stibnite boulangerite and late stage mixing with a high salinity fluid resulting in the precipitation of sphalerite ii galena and quartz iii the unique opportunity to analyze fluid inclusions in many different subsequently precipitating ore and gangue minerals enables the reconstruction of the temporal evolution of the sb veins the distinct change of the ore forming processes from the influx of co2 into an aqueous fluid and cooling ore stage i ii to mixing ore stage iii iv as the dominant precipitation mechanisms are also accompanied by a drastic change in the mineralogy from sb sulfides to pb zn sulfides late orogenic granitic intrusions served as a potential heat metal and sulfur source variscan wrench and thrust tectonics and associated metamorphism account for the ore stage i mineralization hydrothermal sb pb zn assemblages ore stages ii iii and iv precipitated during the latest part of the variscan orogeny most likely around the carboniferous permian boundary although fluid composition and formation temperatures of the greiz and schleiz mineralization are on the first view quite similar to those of other variscan sb mineralization the observation of co2 influx and fluid mixing with a high salinity fluid is unique for european sb occurrences

sulphur isotope geochemistry of black shale hosted antimony mineralization arnsberg northern rhenish massif germany implications for late stage fluid flow during the variscan orogeny vein type and bedding concordant mesothermal 180 410 c stibnite sulphosalt mineralization at arnsberg ne rhenish massif germany is hosted by carboniferous pyrite rich black shales and siliceous limestones a detailed sulphur isotope study of the stibnite sulphosalt mineralization and pyrite from a variety of regional host rock lithologies has been carried out using an in situ laser combustion technique the 34 s values of stibnite of various textural types are distinctly negative and lie in a narrow range between 23 9 and 17 1 mean 20 1 in contrast regional sedimentary diagenetic pyrites display a large variation of their 34 s values between 45 4 and 9 3 there is little evidence for significant modification of the hydrothermal fluid during deposition and the s isotope signatures suggest that the sulphur of the stibnite mineralization was not locally derived the 34 s values of pyrite in givetian shales display a significantly narrower range of 28 2 to 7 5 and their mean composition 17 1 is close to the 34 s values of the arnsberg stibnite deposits considering the temperature dependent isotopic fractionation between stibnite and reduced sulfur species the 34 s values of the mineralizing fluid 16 8 200 c and the givetian rock source are essentially identical therefore we propose a model of leaching and isotopic homogenization of sulphur from the middle devonian shales and a subsequent northward migration of these fluids the fluids were trapped in permeability controlled positions within anticlinal zones where fluid cooling induced deposition of stibnite and sulphosalts

fluid inclusions and ore precipitation mechanism in the giant xikuangshan mesothermal antimony deposit south china conventional and infrared microthermometric constraints abstract the giant xikuangshan antimony deposit located in the xiangzhong central hunan basin south china is the largest antimony deposit in the world stibnite is the predominant ore mineral and gangue minerals consist of quartz calcite fluorite and barite the ore mineral assemblages include early stage quartz stibnite fluorite quartz stibnite and barite quartz stibnite and late stage calcite stibnite in spite of numerous research works on the xikuangshan deposit during the past several decades the signatures of the ore forming fluids and the ore precipitation mechanisms are poorly constrained in order to characterize the fluids responsible for antimony mineralization and ascertain the genetic processes in the xikuangshan ore district conventional and infrared microthermometric measurements have been performed in this study on fluid inclusions hosted in transparent minerals quartz fluorite and barite and opaque stibnite respectively four types of fluid inclusions were identified for the early stage mineralization based on their phase assemblages including type i pure liquid inclusions type ii two phase liquid rich aqueous inclusions type iii two phase vapor rich aqueous inclusions and type iv pure vapor inclusions stibnite only contains type i and type ii inclusions while all types of fluid inclusions are common in the gangue minerals the homogenization temperatures and salinities of fluid inclusions in stibnite and associated gangue minerals in this ore district display significant differences fluid inclusions in stibnite exhibit relatively low homogenization temperatures 112 323 c and higher salinities 0 2 15 4 wt nacl equiv than inclusions in coexisting gangue minerals 119 366 c 0 2 4 2 wt nacl equiv usually about 50 c less than the intergrown gangue minerals according to the petrography of fluid inclusions and the relationship between their homogenization temperature and salinity it was concluded that stibnite and gangue minerals in the xikuangshan ore district are not co genetic instead they were deposited from two distinctly different hydrothermal solutions fluid mixing was responsible for stibnite precipitation but the precipitation of gangue minerals resulted from fluid boiling due to an abrupt drop in pressure the xikuangshan deposit is a typical example of mesothermal deposits rather than an epithermal deposit as previously considered

reply to the comment by f tornos et al we welcome the discussion of our paper by tornos et al the epithermal character of the hiendelaencina veins might have been an assumption in the early to mid 1980s however this early idea has been reaffirmed after many years of research involving fieldwork and mineralogical sulphur isotopes and fluid inclusions studies the same applies to the alleged extensional frame a tectonic episode now well documented not only in central spain spanish central system doblas 1987 doblas et al 1988 doblas 1991 but in france french central massif menard and molnar 1988 malavieille et al 1990 munoz et al 1992 the deposits are hosted by metamorphic rocks and the nearest volcanic outcrops to hiendelaencina are those of atienza andesites some 12 km northward this is the reason why the relationships between the atienza volcanics and the hiendelaencina veins were initially regarded as obscure these stephanian permian volcanic outcrops are only local evidence of the late variscan magmatism which in the case of hiendelaencina remained concealed it is evident that the geologic environments of hiendelaencina and atienza are very different see discussion p 88 of the paper as a direct consequence of this the local structural conditions led to contrasted expressions of the late variscan magmatism i e subaereal at atienza and hypabyssal at hiendelaencina

eclogites in the french palaeozoic orogen geodynamic significance abstract eclogites are widespread in the devonian internal zone of the french palaeozoic orogen they occur in different tectonic settings either in high pressure low temperature or in high pressure high temperature environments they are tectonically incorporated into several major allochthonous units as demonstrated either by their blastomylonitic texture or by their being in evident disequilibrium with their country rocks the protoliths of the presently well identified eclogite bearing series present different typical associations depending on their present location the igneous series include within plate basalts normal and transitional morbs and even calc alkaline rocks they were erupted mostly during the palaeozoic although they sometimes contain inherited precambrian zircons further some of them may be remnants of old precambrian oceanic crust most of the eclogitic parageneses were generated during the earliest silurian tectonometamorphic climax either in low temperature or in high temperature conditions later they followed various retrograde courses in still geographically distinct low temperature or high temperature associations the basic rocks in the leptynolitic amphibolitic complex and particularly the eclogites thus provide sufficiently clear evidence to support a new geodynamic scenario implying one or several subordinate narrow tensional zones in a thinned but well developed continental crust rather than in a true oceanic domain

the variscan belt in central europe main structures geodynamic implications open questions abstract the variscan belt of europe originated from the confrontation of northern europe and gondwana with intervening pre variscan blocks cadomian and older armorica tepla barrandean moravo silesian though a strike slip component cannot be excluded geological evidence suggests a subduction collision process newly discovered features include basaltic rocks with morb affinities high pressure metamorphic rocks trench deposits and large scale allochthonism which also affects the pre variscan basement and has often led to an inversion of the metamorphic isogrades large scale thrusting is evident at all crustal levels and is also observable by geophysical methods the structure of the variscan crust is best described as horizontal tectonic layering late ordovician through carboniferous convergence closed one rhenohercynian and one saxo thuringian basin to the north and at least one mediterranean basin to the south of the pre variscan blocks subduction occurred from the north as well as from the south down under the dorsal core region the exact timing and the driving mechanism of this bilateral activity remains a problem likewise uncertain are the exact limination of the pre variscan blocks and the existence and width of oceanic areas in the saxothuringian zone and in the south facing part of the moldanubian zone there is good evidence of early palaeozoic rifting and of late ordovician through carboniferous plate convergence palaeomagnetic and palaeoclimatic constraints however suggest only a limited extent of oceanic basins in these areas while they appear to allow a wider ocean at the rhenohercynian saxothuringian boundary which is relatively unobtrusive from the geological point of view research during the past decade has produced a wealth of information on the variscan fold belt in europe the resulting geodynamic models however are extremely diverging evidently it is not the time yet for definitive statements the present paper intends to review the state of knowledge to deduce such basic items of a geodynamic model as appear reasonably well documented and to point out the most urgent open questions stress will be laid on central europe which offers a complete cross section through the northern flank of the variscan belt and where we have carried out most of our studies during the last years

quartz trace element composition by la icp ms as proxy for granite differentiation hydrothermal episodes and related mineralization the beauvoir granite echassieres district france despite quartz being a ubiquitous mineral in the earth s crust only little is known about its trace chemical composition and whether and how this is influenced by the medium from which it precipitated using in situla icp ms analysis we investigated the trace element composition of several types of quartz from a magmatic hydrothermal system in the echassieres district of the french massif central this system consists of the colettes granite the beauvoir rare element granite a w bearing stockwork and dispersed mineralized veins all of the elements tested li be b na al ti ge rb sr nb sn sb ta w were detected in quartz from at least one sample a rigorous multivariate statistical approach permitted to assign a specific chemical signature to each of the quartz groups that were recognized by petrographic observations differences among these groups were interpreted to reflect different sources or genetic evolutionary trends within magmatic and hydrothermally derived quartz groups for example the evolution of al rb ge li and ti could be related to genetic processes such as the degree of magma differentiation and crystallization temperatures quartz from the w stockwork anterior to the beauvoir leucogranite clearly stands out from that related to the beauvoir mineralization system i e the greisen facies and proximal ta nb sn w mineralized veins it was also possible to demonstrate a similarity in composition notably the sb contents between quartz from the greisen and from distal stibnite bearing veins in addition elevated values of sb sn as well as li and rb in quartz correspond to similarly high values of these elements in the rock indicating that quartz trace chemistry can provide useful information for the mineralization potential of its host at least for these elements on the other hand nb ta and w are less reliable as they occur mostly at very low levels in quartz the highest values corresponding to samples that were not particularly enriched in these metals a comparison of our quartz data with those from a similar granitic complex at cinovec czech republic breiter et al 2017a shows a similar trend for the ge ti ratio reflecting magmatic differentiation conversely the al ti ratio values differ in the two complexes the al content of quartz from the various cinovec granites being systematically lower than in quartz from echassieres nevertheless caution must be taken when analyzing quartz for its trace element chemistry as processes occurring during i e trapping of fluid inclusions and formation of zoning and after crystal growth i e diffusion and alteration can introduce heterogeneities thus affecting the original chemistry

antimony quartz and antimony gold quartz veins from northern portugal abstract antimony and pb sb quartz veins from the braganca district portugal are mainly hosted by silurian phyllites antimony au quartz veins from the durico beira region are mainly hosted by a cambrian schist metagraywacke complex as well as ordovician phyllites and quartzites the deposits were mostly exploited in the late 19th century mineralogical characteristics and chemical compositions of individual ore minerals are similar in the two areas first and second generations of arsenopyrite precipitated at 390 and 300 c respectively berthierite and stibnite are the most abundant sb bearing minerals and precipitated between 225 and 128 c native antimony at

late miocene earliest pliocene glaciation in southern argentina implications for global ice sheet history abstract in south america at latitude 47 s till was deposited sometime between 7 m y and 4 6 m y ago at a time when the local climate was colder than today s during this same interval glaciers extended to sea level in southeast alaska and widespread cooling of the ocean surface in middle latitudes worldwide marine regression and change in the oxygen isotopic composition of ocean water occurred from the last three events major late miocene expansion of the antarctic ice sheet has been inferred on the assumption that the history of north atlantic ice rafting precludes the existence of northern hemisphere ice sheets until 3 m y ago this is disputed first because precipitation in antarctica would probably have decreased at temperatures below today s second because the antarctic ice sheet cannot expand appreciably until buildup of northern hemisphere ice sheets has lowered sea level third because virtually no late miocene sediments are present at the labrador sea dsdp sites that are critical to the reconstruction of north atlantic ice rafting history and fourth because the scale of late miocene glaciation in alaska is at least permissive for simultaneous buildup of ice at similar latitudes further east if global ice sheet volume at the end of the miocene was greater than it is today by an amount that would have lowered sea level by several tens of meters the excess ice did not accumulate in antarctica but in the northern hemisphere chiefly in north america away from the atlantic coast

multi stage fluid incursion in the palaeozoic basement hosted saint salvy ore deposit nw montagne noire southern france abstract the saint salvy vein hosted zn ge deposit occurs in an e w fault system which flanks the southern margin of the late variscan sidobre granite and cross cuts cambrian black shales of the palaeozoic basement comprehensive mineralogical and geochemical studies of vein samples have revealed four mineralizing events m1 m4 related to late and post variscan tectonic events a further late stage event may be related to weathering m1 skarn deposits and m2 patchily mineralized quartz veinlets are associated with granite emplacement quartz contains low salinity h2o co2 nacl dominated fluids 6wt nacl equiv of relatively high temperature 300 580 c trapped under moderate to high pressure estimated m1 fluid d and calculated fluid 18o plot within the metamorphic water field there appears to be no involvement of magmatic fluids by contrast m3 barren quartz and m4 zinciferous economic mineralization stages have h2o co2 nacl cacl2 fluid inclusions with high salinities 23 25 wt nacl equiv and low temperatures 80 140 c which were trapped under low pressure conditions the high salinity and nacl cacl2 content of both m3 and m4 indicates that their parent fluids leached evaporitic salts m3 fluids are meteoric water dominated falling close to the meteoric water line d and 18o averaging 64 and 8 respectively m4 fluids have highly distinctive d averaging 101 and calculated fluid 18o varying from 1 2to 7 1 the unusually low d composition of m4 suggests the involvement of organic fluids in which h is derived directly or indirectly from organic matter the relatively high 18o of m4 fluids indicates that considerable isotopic exchange with sedimentary material took place displacing the 18o from the meteoric water line the data imply interaction of meteoric waters with evaporite and hydrocarbon bearing sedimentary sequences most probably the adjacent aquitain basin the main economic mineralization m4 stage took place during a tensional event probably coincident with the lias dogger transition calculated 34sh2s of m4 sulphide 5 4to 8 2 is almost identical to 34s of local cambrian sulphides 4 7to 9 4 suggesting a genetic link abundant siderite associated with m4 sphalerite has 13c ranging from 2 6to 4 4 indicating that carbon was sourced from sedimentary carbonate mobilized by or equilibrated with the hydrothermal fluid late stage sulphides exhibit extraordinary and highly distinctive 34s sphalerite has extremely low 34s 42 5to 50 5 whereas pyrite has an extraordinary large range from 33 2 to 74 3 closed system sulphate reduction is held to be responsible for the extremely high 34s whereas more open system reduction produces the very low values the coincidence of isotopically low 13c 7 6to 11 9 for co genetic calcite suggests the involvement of organic matter in the reduction process

the biards sb au bearing shear zone massif central france an indicator of crustal scale transcurrent tectonics guiding late variscan collapse structural textural mineralogic microthermometric and isotopic data obtained on the biards sb au district located in the south limousin area massif central france provide new evidence for the tectonic setting of stibnite and gold deposition during the late orogenic variscan collapse in western europe a variety of geologic and geophysical data define a major crustal scale left lateral strike slip shear zone that acted as a trap for sb au mineralization during these movements fault fill veins and then extensional veins of limited extent were created along rheologic heterogeneities that focused brittle deformation geometry and orientation of both vein sets indicate two main episodes of emplacement of ore related to a progressive wrench movement in response to an counterclockwise rotation of the regional stress axes both veins sets are filled by quartz and sulfides in three stages an early arsenopyrite pyrite stage with fine grained quartz a sphalerite pb sb sulfosalt native gold stage with comb quartz and a late stibnite berthierite stage with comb quartz combined fluid inclusion and stable isotope studies on quartz muscovite and sulfides reveal that the three stages result from the continuous evolution of a deep seated hydrothermal system in which the latest stages mixed with meteoric water these data argue for cooling and a pressure drop during ore deposition and indicate a shallow level of emplacement for sb au veins a comparison of the biards deposits with other upper paleozoic gold and antimony deposits in the massif central the western variscides and elsewhere in the world illustrates their common association with crustal scale strike slip shear zones the upper carboniferous age 305 300 ma of the sb au deposits indicates that they formed during the early stages of the postthickening extension of the variscan lithosphere sb au bearing strike slip shear zones resulted from the late variscan crustal collapse guided by the very large scale dextral shearing between central europe northern africa and the northern appalachians and associated with the opening of the paleo tethys and the closing of the theic ocean during the late paleozoic gondwana laurasia translation

the massive stibnite veins of the french palaeozoic basement a metallogenic marker of late variscan brittle extension the stibnite ore deposits of the french palaeozoic basement are spatially related to major late variscan strike slip faults they occur as small discontinuous veins mostly hosted in epizonal or catazonal siliciclastics stibnite crystallizes in the final stage of a polymetallic paragenesis from an antimony bearing solution which deposits a first discrete fe as assemblage at 300 400 c and 0 5 0 8 kbar experimental data have shown that antimony solubility drastically decreases on cooling characterization of the late variscan antimony bearing fluids enables us to predict the temperature range 270 150 c under which stibnite was precipitated since the latter coincides with the fluid inclusion homogenization temperature range the pressure at which stibnite crystallized can be estimated from vapour pressure data to have been around 0 1 kbar extensional brittle shear zones developed at the end of the late variscan orogeny probably at the westphalian stephanian boundary drained the hydrothermal fluids near to the surface c 1000 m reaching the critical p t conditions for stibnite crystallization

late hercynian polymetallic vein type base metal mineralization in the iberian pyrite belt fluid inclusion and stable isotope geochemistry s o h cl late variscan vein type mineralization in the iberian pyrite belt related to the rejuvenation of pre existing fractures during late variscan extensional tectonism comprises pyrite chalcopyrite quartz galena sphalerite quartz stibnite arsenopyrite quartz pyrite quartz cassiterite scheelite fluorite galena sphalerite chalcopyrite and quartz manganese oxide mineral assemblages studies of fluid inclusions in quartz stibnite and barite as well as the sulfur isotopic compositions of stibnite galena and barite from three occurrences in the central part of the iberian pyrite belt reveal compelling evidence for there having been different sources of sulfur and depositional conditions quartz stibnite mineralization formed at temperatures of about 200 c from fluids which had undergone two phase separation during ascent antimony and sulfide are most probably derived by alteration of a deeper lying volcanic hosted massive sulfide mineralization as indicated by 34s signatures from 1 45 to 2 74 sub critical phase separation of the fluid caused extreme fractionation of chlorine isotopes 37cl between 1 8 and 3 2 which correlates with a fractionation of the cl br ratios the source of another high salinity fluid trapped in inclusions in late stage quartz from quartz stibnite veins remains unclear by contrast quartz galena veins derived sulfide and metals by alteration of a sedimentary source most likely shale hosted massive sulfides the 34s values in galena from the two study sites vary between 15 42 and 19 04 barite which is associated with galena has significantly different 34s values 0 2 to 6 44 and is assumed to have formed by mixing of the ascending fluids with meteoric water

stibnite microstructures as imprints of fault movement the natural deformation of stibnite in relation to the kinematic history of a regional fault is resolved using microstructural analysis of massive stibnite along the biards fault in the massif central france stibnite underwent intra crystalline deformation associated with a strong linear and planar anisotropy lepidoblastic textures undulatory extinction of oriented grains growth of antimony between these grains and a colour change of stibnite from white to black result from dynamic recrystallisation a range of asymmetric microstructures indicate non coaxial deformation and a left lateral sense of shear related to the movement of the fault zone the computed displacement corresponds to the geological offsets measured along the fault we conclude that stibnite has microstructurally recorded the bulk kinematics of its host fault zone

rapid sea level changes at the close of the last interglacial substage 5e recorded in bahamian island geology rapid and abrupt relative sea level changes within the last interglacial substage 5e are recorded in the island geology of the tectonically stable bahamas from 132 to 118 ka reef growth reached a maximum elevation near 2 m as indicated by fossil reef elevation across the platform whereas bioeroded notches are incised in coastal cliffs as high as 6 m the end of the interval is characterized by voluminous eolianites exhibiting palm tree and frond impressions it is inferred that sea level for most of the interval remained near 2 m restraining reef growth and that the notch at 6 m represents a rapid and brief excursion just before the close of the substage the subsequent fall must have been rapid in order to leave the notch profile intact and mobilize windward lagoon ooids into dunes before cementation could anchor them in order to explain the rapid rise to 6 m glacial surging is invoked the subsequent fall also rapid may be a consequence of the surge flooding high latitudes and providing enough moisture to initiate reglaciation and drawdown

lead isotope systematics of vein type antimony mineralization rheinisches schiefergebirge germany a case history of complex reaction and remobilization processes mesothermal vein type sb mineralization in the rheinisches schiefergebirge germany is characterized by two different mineralization styles which are 1 extensional quartz stibnite vein systems and 2 cu pb sb sulphosalt assemblages in overprinted pre existing pb zn veins a detailed pb isotope study of 52 representative samples from both mineralization types indicates distinct compositional patterns cu pb sb sulphosalts meneghinite boulangerite bournonite formed by reaction remobilization are characterized by pb isotope compositions 206pb 204pb 18 179 18 223 which are identical to the precursor galena 206pb 204pb 18 168 18 223 the pb isotope composition of sulphosalt minerals in these vein systems was controlled by lead inherited from pre existing galena stibnite and pb sulphosalts zinkenite semseyite plagionite formed in quartz stibnite vein systems display pb isotope ratios 206pb 204pb 18 250 18 354 which are more radiogenic than galenas from variscan pb zn ore veins 206pb 204pb 18 162 18 303 detailed small scale investigation of thrust zones hosting pb zn ores and crosscutting quartz ankerite fissure veins ramsbeck deposit indicates that the pb isotope compositions of recrystallized galena and remobilized phases boulangerite semseyite bournonite are arranged along a linear trend this is interpreted as mixing between primary galena with 206pb 204pb 18 206 and overprinting hydrothermal fluids with a more radiogenic composition 206pb 204pb 18 354 expressed by intermediate compositions 206pb 204pb 18 256 18 334 of newly formed sulphosalts the pb isotope systematics of the vein type sb mineralization is in accordance with a model of pb extraction from similar crustal sources palaeozoic sedimentary sequences at different times

the 1986 1988 intraplate earthquake sequence near joao câmara northeast brazil evolution of seismicity abstract the 1986 1988 intraplate earthquake sequence near joao c mara 5 5 s 35 7 w has been among the most significant in northeast brazil this century some 14 000 events were recorded in the 18 months from august 1986 including fifteen above magnitude 4 0 and one of 5 1 4000 buildings were damaged a local network of four portable seismographs was operated during most of the period augmented by a nine station telemetric network between may 1987 and april 1988 the temporal and spatial distribution of this seismicity is examined and determination of unusually good hypocentres is made possible by the simple velocity structure good signal transmission and impulsive p and s arrivals on most seismograms activity has been closely confined to a 28 km linear zone with a strike of 040 except for some small events along a parallel zone displaced laterally by 12 km neither zone has been related to any surface feature some unusual characteristics have raised concern about possible future seismicity the largest event occurred four months after the onset of activity and after several weeks of relative quiescence a 5 km intermediate section of the inferred fault has so far experienced only a few very small events

sedimentary facies and stratigraphy of a last interglacial coastal barrier in south brazil shoreline sedimentation during the last interglacial highstand on the rio grande do sul coastal plain southern brazil is represented by a sandy barrier that extends almost uninterrupted for hundreds of kilometers along the coast facies analyses of high quality exposures in several sandpits situated in the northern part of the coastal plain provided an opportunity to examine the stratigraphic nature of these deposits based on distinct assemblages of physical and biogenic sedimentary structures the vertical sequence of facies reveals a regressive prograded barrier with vegetated coastal dune sands overlying beach and shallow marine sediments progradation was probably promoted by abundant supplies of nearshore sand during the highstand and early fall of sea level forced regression associated with the last interglacial period around 125 ka as indicated by elevations of ophiomorpha burrows the maximum level reached by the sea was around 7 m above the present sea level paleocurrent data suggest that in the studied area shoreline orientation hydrodynamics and sediment transport during the last interglacial highstand were similar to present day conditions