geochemistry of speleothems affected by aragonite to calcite recrystallization potential inheritance from the precursor mineral abstract formerly aragonite speleothems recrystallized to calcite result from solutions subsaturated in aragonite and supersaturated in calcite that infiltrate into the speleothem through the interconnected porosity in most cases the crystal replacement takes place through a thin solution film this diagenetic process can occur under open or semi closed geochemical conditions thus secondary calcite crystals record the composition of the fluid at the time of diagenesis affected by calcite partition coefficients and fractionation factors open system or partly inherit the composition of the primary aragonite semi closed system so whether or not recrystallized aragonite speleothems can record reliable geochemical signals from the time of speleothem primary deposition still is an open debate we studied a stalagmite from eagle cave spain predominantly composed of secondary calcite that replaced aragonite although a core of primary aragonite extending 45 mm along the growth direction was preserved at the base of the sample we obtained mg and sr compositional maps paired u th dating and 18 o and 13 c profiles across the diagenetic front additionally two parallel isotope records were obtained along the speleothem growth direction in the aragonite and calcite sectors our results support that recrystallization of this speleothem took place in open system conditions for 18 o 13 c mg and sr but in semi closed system conditions for u and th the recrystallization of this sample took place during one or several events likely after the younger dryas as a result of climate change influencing drip water composition based on compositional zoned patterns we suggest that the advance of diagenetic fronts in this speleothem had an average rate of 50 45 m yr such recrystallization rate can transform any aragonite speleothem into calcite within a few centuries we suggest that the volume of water interacting with the speleothem at the time of recrystallization is of critical importance for inheritance of different elements the volume of solution is controlled by 1 the discharge of water passing through the sample and 2 the lapse time between aragonite dissolution and calcite precipitation hydrology and hydrochemistry of the interacting solution together with the mineralogy and texture of the speleothem are the essential controls for the diagenesis of the speleothem recrystallization of aragonite speleothems does not follow stratigraphical levels of the sample but occurs along sites with preferential flow paths in any sector of the speleothem in these cases the relationship between age and distance from base is not preserved however alternation of periods of recrystallization with periods of aragonite precipitation causing speleothem accretion can result in recrystallized speleothems with coherent distance from the base age relationship thus early diagenesis of speleothems affected by seasonal or inter annual oscillation of drip waters supersaturated and subsaturated in aragonite may provide best scenario conditions for dating and preservation of paleoenvironmental records along recrystallized speleothems however even in this scenario the variable discharge and the diagenetic rate control the geochemical inheritance from the primary aragonite crystals

provenance of neoproterozoic and early paleozoic siliciclastic rocks of the tepla barrandian unit bohemian massif evidence from u pb detrital zircon ages abstract new la icp ms u pb detrital zircon ages from ediacaran and paleozoic siliciclastic rocks are used to constrain provenance and paleogeographic affinities of the tepla barrandian unit tbu in the centre of the bohemian massif central europe czech republic the samples taken span the period from 635 ma to 385 ma and permit recognition of provenance changes that reflect changes in geotectonic regime detrital zircon age spectra of two ediacaran one lower cambrian and three upper ordovician samples resemble the ages known from the nw african proportion of gondwana particularly the trans saharan belt while three rocks from higher lower cambrian to lowermost ordovician strata contain detritus that may have been derived exclusively from local sources the age spectrum of the devonian rock is a combination of the nw gondwanan and local features these new findings in combination with a wide range of published data are in agreement with a neoproterozoic subduction related setting at the margin of gondwana followed by a cambrian early ordovician rifting stage and an ordovician passive margin setting furthermore the data are in favour of a position of the tbu at the gondwanan margin throughout pre variscan times

high temperature deformation during continental margin subduction exhumation the ultrahigh pressure western gneiss region of norway abstract a new dataset for the high pressure to ultrahigh pressure western gneiss region allows the definition of distinct structural and petrological domains much of the study area is an e dipping homocline with e plunging lineations that exposes progressively deeper more strongly deformed more eclogite rich structural levels westward although eclogites crop out across the wgr scandian deformation is weak and earlier structures are well preserved in the southeastern half of the study area the scandian reworking increases westward culminating in strong scandian fabrics with only isolated pockets of older structures the dominant scandian deformation was coaxial e w stretching the sinistrally sheared more trondelag fault complex and nordfjord mylonitic shear zone bound these rocks to the north and south there was moderate top e amphibolite facies deformation associated with translation of the allochthons over the basement along its eastern edge and the nordfjord sogn detachment zone underwent strong lower amphibolite facies to greenschist facies top w shearing a northwestward increase in exhumation related melting is indicated by leucosomes with hornblende plagioclase and scandian sphene in the western 2 3 of the study area exhumation related amphibolite facies symplectite formation in quartzofeldspathic gneiss postdated most scandian deformation further deformation was restricted to slip along biotite rich foliation planes and minor local folding that the western gneiss region quartzofeldspathic gneiss exhibits a strong gradient in degree of deformation implies that continental crust in general need not undergo pervasive deformation during subduction

a new geological map of the brabant massif belgium a new geological map mostly subcrop of the brabant massif is presented based on a revised lithostratigraphy of the outcrop area and on recently acquired palaeontological and lithological data from boreholes in the concealed area new interpretations of magnetic and gravity data are used to extend the lithostratigraphical units into areas with few or no boreholes a structural model of the western northern and southern parts of the brabant massif is presented the main anticlinal axis plunges towards the west northwest its core comprising upper precambrian to lower cambrian terrigenous rocks which outcrop in the southeast to the north of the main axis younger rocks appear in regular succession but to the southwest this picture is complicated by the occurrence of a second anticlinal structure and by a subparallel magmatic arc

a new look at belgian aeromagnetic and gravity data through image based display and integrated modelling techniques digital processing and image based display techniques have been used to generate contour and shaded relief maps of belgian aeromagnetic data at a scale of 1 300000 for the whole of belgium these highlight the important anomalies and structural trends particularly over the brabant massif north and vertically illuminated shaded relief plots enhanced structural belts trending west east to northwest southeast in the brabant massif and west east to southwest northeast in the core of the ardennes the principal magnetic lineaments have been identified from the shaded relief plots and tentatively correlated to basement structures most short lineaments are correlated with individual folds while the more extensive lineaments are correlated with large scale fault structures magnetic highs within the brabant massif are attributed to folded sediments of the tubize group the magnetic basement in the east of belgium is sinistrally displaced to the north by an inferred deep nnw sse crustal fracture the bouguer anomaly map of belgium identifies the ardennes as a negative area and the brabant massif as a positive area with the exception of a wnw trending gravity low in its western part the southern margin of the brabant massif is defined by a steep gravity gradient coincident with the faille bordiere border fault trial modelling of the gravity and magnetic data carried out along profiles across the brabant and stavelot massifs has identified probable acid igneous intrusions in the western part of the brabant massif and a deep magnetic lower density body underlying the whole ardennes region which is thought to be a distinctive precambrian crustal block

palaeozoic amalgamation of central europe new results from recent geological and geophysical investigations abstract multidisciplinary studies of geotransects across the north european plain and southern north sea and geological reexamination of the variscides of the north bohemian massif permit a new 3 d reassessment of the relationships between the principal crustal blocks abutting baltica along the trans european suture zone tesz accretion was in three stages cambrian accretion of the bruno silesian lysogory and malopolska terranes end ordovician early silurian accretion of avalonia and early carboniferous accretion of the armorican terrane assemblage ata palaeozoic plume influenced metabasite geochemistry in the bohemian massif explains the progressive rifting away of peri gondwanan crustal blocks before their accretion to baltica geophysical data faunal and provenance information from boreholes and dated small inliers and cores confirm that avalonian crust extends beyond the anglo brabant deformation belt eastwards to northwest poland the location and dip of reflectors along the tesz and beneath the north european plain suggest that avalonian crust overrode the baltica passive margin marked by a high velocity lower crustal layer on shallowly southwest dipping thrust planes forming the heligoland pomerania deformation belt the variscan orocline of southwest poland masks two junctions between the armorican terrane assemblage ata and previously accreted crustal blocks to the east is a dextrally transpressive contact with the bruno silesian and malopolska blocks accreted in the cambrian while to the north is a thrust contact with easternmost avalonia deeply buried beneath younger sedimentary cover in the northeast bohemian and rhenohercynian massifs devonian early variscide deformation dominated by wnw and nw directed thrusting records closure of ordovician devonian seaways between detached islands of the ata and avalonia

sequence stratigraphy of the palaeozoic welsh basin the timing and extent of unconformities in the welsh basin are investigated using rock preservation curves derived from outcrop stratigraphic logs four basin wide unconformities occur focussed in late precambrian late tremadoc pusgillian early ashgill and mid devonian times these bound three megasequences equating with newly defined lithostratigraphic units the dyfed gwynedd and powys supergroups less extensive unconformities bound 18 component sequences the majority of the sequence boundaries reflect a component of tectonic or volcanotectonic activity rather than a pure eustatic sea level change the megasequence boundaries are attributed to late precambrian to early cambrian onset of rifting to form a passive margin tremadoc onset of subduction with intra arc then back arc extension late caradoc end to subduction and late early devonian collisional deformation the megasequences and controlling events can be tentatively matched with other basins on the avalonian margin more generally this study shows that sequence analysis is feasible in onshore basins lacking well and seismic data and that the global eustatic interpretation of sequence stratigraphy is only partially applicable to active margin basins

late ordovician global warming the boda event there is substantial evidence for mid ashgillian global warming before the latest ordovician hirnantian glaciation as shown by the movement of previously lower latitude benthic faunas such as trilobites and brachiopods to progressively higher latitudes and by an increase in endemic faunas at low latitudes this is shown by faunal dynamics on several paleocontinents and endorsed by published evidence of warmer water sediments from gondwana and elsewhere we term this global warming the boda event

boundary lines within petrologic diagrams which use oxides of major and minor elements use of some petrologic diagrams applied to analyses of volcanic rocks is unnecessarily difficult due to lack of data for construction of discriminant lines between rock series coordinates are provided for sufficient points to enable accurate plotting of the boundary lines within seven diagrams viz 1 tas total alkalies na2o k2o vs sio2 2 k2o vs sio2 3 afm 4 jensen 5 ktp k2o tio2 p2o5 6 fms feo mgo vs sio2 and 7 taktip k2o na2o k2o vs tio2p2o5 different versions of these boundaries are collated to indicate their variable position and it is demonstrated that inter laboratory analytical precision suffices to account for almost all of their spread on the tas and k2o vs sio2 diagrams

a comparative analysis of pre silurian crustal building blocks of the northern and the southern appalachian orogen the new york promontory serves as the divide between the northern and southern segments of the appalachian orogen antiquated subdivisions distinct for each segment implied that they had lithotectonic histories that were independent of each other using new lithotectonic subdivisions we compare first order features of the pre silurian orogenic building blocks in order to test the validity of the implication of independent lithotectonic histories for the two segments three lithotectonic divisions termed here the laurentian iapetan and the peri gondwanan realms characterize the entire orogen the laurentian realm composed of native north american rocks is remarkably uniform for the length of the orogen it records the multistage neoproterozoic early paleozoic rift drift history of the appalachian passive margin formation of a taconic seaway and the ultimate demise of both in the middle ordovician the iapetan realm encompasses mainly oceanic and magmatic arc tracts that once lay within the iapetus ocean between laurentia and gondwana in the northern segment the realm is divisible on the basis of stratigraphy and faunal provinciality into peri laurentian and peri gondwanan tracts that were amalgamated in the late ordovician south of new york stratigraphic and faunal controls decrease markedly rock associations are not inconsistent with those of the northern appalachians although second order differences exist exposed exotic crustal blocks of the peri gondwanan realm include ganderia avalonia and meguma in the north and carolinia in the south carolinia most closely resembles ganderia both in early evolution and late ordovician silurian docking to laurentia our comparison indicates that to a first order the pre silurian appalachian orogen developed uniformly starting with complex rifting and a subsequent drift phase to form the appalachian margin followed by the consolidation of iapetan components and ending with accretion of the peri gonwanan ganderia and carolinia this deduction implies that any first order differences between northern and southern segments post date late ordovician consolidation of a large portion of the orogen

effects of drying heating annealing and roasting on carbonate skeletal material with geochemical and diagenetic implications abstract carbonate skeletons subjected to drying heating annealing or roasting at elevated temperatures as part of routine sample preparation for chemical analyses or geochemical experiments differ significantly from skeletal materials as they occur in nature heating of skeletal samples can degrade organic material expel h2o and oh reduce the concentration of some trace elements and change the mineralogy and texture of the material thermal degradation of organics and expulsion of water in inclusions which can occur at temperatures of 100 105 c cause fracturing and pitting of skeletal samples areas of pitting reflect original concentrations of volatile phases within the skeleton coralline aragonites are partially or completely altered to calcite at temperatures of 150 c or higher the degree of alteration varies with temperature and duration of heating and genus of the coral high mg calcites hmcs tend to form calcian dolomite and multiple hmcs of lower mg content on heating but the rate of alteration is related to the taxonomic group rather than the mg content echinoids alter very rapidly dolomite detectable by x ray diffraction formed in clypeaster heated for 6 h at 200 c while the coralline red alga neogoniolithon showed no alteration after heating at 400 c for 23 h mineralogical alteration of coralline aragonites and echinoid hmcs is positively correlated with water loss skeletal carbonates comprise a very diverse and heterogeneous suite of materials and their diversity and heterogeneity are reflected in their responses to heating variations in rate and degree of alteration on heating in many cases between different subsamples of material produced by the same organism make it difficult to obtain a consistent product from heat treatment many workers have used high temperature experiments to model diagenetic processes and the results of this study have implications for diagenetic alteration as well as for laboratory analyses and experiments

lispb delta a lithospheric seismic profile in britain analysis and interpretation of the wales and southern england section abstract the lithospheric seismic profile in britain lispb shot in 1974 included a 310 km profile lispb delta crossing the palaeozoic welsh basin the western extent of the midland microcraton and the cornubian zone of southern england this first comprehensive analysis of these data has produced a sub horizontally layered seismic and associated gravity model that correlates well with surface geology a north south decrease in crustal velocity and density corresponds to the change from avalonian crust into the rheno hercynian zone at the south end of the profile high velocities and densities in the lowest crustal layer beneath north wales are proposed to result from cenozoic and possibly ordovician igneous intrusive rocks the former derived from an upwelling plume associated with the opening of the north atlantic examination of the load distribution throughout the model shows that it is strongly correlated with the earthquake distribution along lispb delta earthquake focal depth also correlates with heat flow a simple heat flow profile has been derived and the seismic velocity model used to constrain crustal heat production values a long wavelength excursion from published data can be explained in terms of an increase in mantle heat flow resulting from a previously identified deep thermal anomaly beneath the irish sea supplementary material normalised synthetic and true amplitude seismic record sections including picks and modelled phases for shots sp04 sp05 sps1 sps2 are available at http www geolsoc org uk sup18450

tonian ediacaran tectonomagmatic evolution of west avalonia and its ediacaran early cambrian interactions with ganderia an example of complex terrane transfer due to arc arc collision

the cambrian to devonian odyssey of the brabant massif within avalonia a review with new zircon ages geochemistry sm nd isotopes stratigraphy and palaeogeography abstract this study provides an up to date and comprehensive review of the early palaeozoic evolution of the brabant massif belonging to the anglo brabant deformation belt situated at the southeastern side of avalonia microplate it is the only well known part of the northern passive margin of the rheic ocean the cambrian silurian sedimentary pile is 13 km thick with 9 km for the cambrian only the unraveling of this continuous registration reflects the successive rifting and drifting of avalonia from the gondwana mainland followed by soft collisional processes with baltica and finally the formation of laurussia based on recently established detailed stratigraphy sedimentology and basin development on u pb la icp ms analyses of igneous and detrital zircon grains along with geochemical data including sm nd isotopes a new geodynamic and palaeogeographic evolution is proposed brabant megasequence 1 lower cambrian to lowermost ordovician 9 km thick represents an embayment of the peri gondwanan rift from which the rheic ocean has evolved detrital zircon ages demonstrate that the brabant is a typical peri gondwanan terrane with a major pan african neoproterozoic age and a mixed west african and amazonian source palaeoproterozoic archaean and some mesoproterozoic age the transition towards the avalonia drifting is marked by an unconformity and a short volcanic episode the northward drift of avalonia towards baltica is recorded by the megasequence 2 middle to upper ordovician 1 3 km thick the source for mesoproterozoic zircons vanished as the result of the rheic ocean opening and the isolation from amazonian sources the transition to megasequence 3 is marked by a drastic change in palaeobathymetry and an important sub volcanic episode during a tectonic instability period 460 430 ma reflecting the avalonia baltica soft docking as also shown by the reappearance of mesoproterozoic detrital zircons typical of baltica unradiogenic nd isotope signature end 4 5 and tdm model ages 1 3 1 7 ga for brabant magmatic rocks indicate an old recycled component megasequence 3 uppermost ordovician to lowermost devonian 3 5 km thick includes the onset of a silurian foreland basin that reflects the tectonic inversion of the core of the massif brabantian orogeny in response to the baltica avalonia laurentia collision finally the comparison with the strikingly similar cambrian successions of the harlech dome wales avalonia and the meguma terrane nova scotia peri gondwana allows the construction of a new early cambrian palaeogeographic model for the whole avalonia microplate in which the meguma terrane is included

ganderia laurentia collision in the caledonides of great britain and ireland during terrane convergence an influx of clastic sediment from an upper plate onto a lower plate is an early indication of terrane juxtaposition in the caledonides of great britain and ireland units accreted to laurentia during the early palaeozoic era include peri gondwanan terrane assemblages that earlier separated from west gondwana however the southern uplands terrane contains detrital zircon populations apparently derived entirely from laurentia characterized by a large asymmetric mesoproterozoic peak and a scarcity of zircon at 600 ma and 2 1 ga in contrast cambrian and ordovician rocks from the lake district and the leinster massif of ireland show abundant grains with these ages together with a range of mesoproterozoic zircon these characteristics are shared with the monian terrane of anglesey and with ganderia in the appalachians indicating probable derivation from amazonia in west gondwana silurian sandstones from the lake district show an influx of laurentia derived zircon and lack the peri gondwanan signal this indicates that in the caledonides ganderia was not accreted to the laurentian margin until c 430 ma in contrast to the ordovician accretion of ganderian fragments recorded in the appalachians suggesting that the configuration of the closing iapetus ocean varied significantly along the strike of the orogen supplementary material details of sample coordinates analytical procedure and u pb detrital zircon analytical data are available at www geolsoc org uk sup18739

aragonite calcite relationships in speleothems grotte de clamouse france environment fabrics and carbonate geochemistry abstract in grotte de clamouse france aragonite forms in a variety of crystal habits whose properties reflect the conditions of formation prolonged degassing and evaporation yield needle aragonite which is more enriched in 18o and 13c than aragonite ray crystals which form near isotopic equilibrium at present aragonite ray crystals form at the tops of stalagmites at very low discharge 0 00035 ml min and when fluid mg ca ratio is 1 1 temperature and evaporation do not seem to have a significant role in their formation the presence of aragonite in stalagmites should be indicative of a decrease in drip rate related to either dry climate conditions or local hydrology fossil aragonite was in part replaced by calcite in a time frame 13c signal and u content and preserved aragonite relicts up to 16 weight the isotope signal of different aragonite habits may reflect conditions of formation rather than climate parameters the real extent of aragonite to calcite transformation may be underestimated when replacement calcite inherits both textural and chemical properties of the precursor

stable isotope composition alteration produced by the aragonite to calcite transformation in speleothems and implications for paleoclimate reconstructions abstract aragonite a mineralogical constituent of speleothems in cave environments is unstable and susceptible to inversion to calcite a diagenetic process that involves changes in the mineralogy texture and geochemistry of speleothems however the exact alterations of stable isotope compositions during such diagenesis have not been fully investigated in this study two aragonite stalagmites sn3 and sn15 from the shennong cave southeast china were found partially inverted to calcite as determined by x ray diffraction xrd and scanning electron microscopy sem analyses and thin section inspections under microscope the fiber relics and textural ghosts of aragonite preserved in coarse and equant mosaic calcite crystals clearly indicate that the calcite in these two stalagmites were inverted from aragonite the stable isotope compositions 13 c and 18 o given in per mil versus vpdb standard of primary aragonite and secondary calcite were analyzed and compared along both growth layers and growth axes the results show that along growth layers differences of 13 c values between aragonite and calcite are negligible 0 1 0 2 whereas differences of 18 o values between aragonite and calcite are significant 0 63 0 87 comparisons along growth axes show similar results i e differences of 13 c values are negligible 0 06 0 22 whereas differences of 18 o values are significant 0 85 0 29 most likely the aragonite in sn3 and sn15 were internally inverted by interactions of trace calcite crystallites and pore water within intercrystalline pore spaces by a dissolution reprecipitation process occurring in trapped pore water in the case of the inversion of aragonite to calcite in speleothems such as that observed in sn3 and sn15 the 13 c values could be used in paleoclimate and paleoenvironment reconstructions because they are inherited from those of primary aragonite although the 18 o values might be cross calibrated to those of primary aragonite if the aragonite calcite fractionation offset is known e g 0 85 0 29 in this study however the 18 o values of secondary calcite should be used with caution in such reconstructions as the 18 o offset value is not consistently invariable

nature and origin of organic matter in carbonates from speleothems marine cements and coral skeletons organic matter in speleothem calcite marine carbonate cements and aragonitic coral skeletons was studied to determine its location molecular structure functionality and effect on mineral growth sem analyses showed that inorganically precipitated carbonates incorporate during growth adsorbed organic matter between submicroscopic subunits of the crystals whereas biologically secreted carbonates incorporate biogenic tissue between the crystals molecular fluorescence spectroscopy indicated that low molecular weight fulvic acids are the most important constituents of the organic matter the fulvic acids are probably derived from soils speleothem calcite dissolved organic matter marine carbonates and biological decay products aragonitic coral skeleton

timing of magmatism foreland basin development metamorphism and inversion in the anglo brabant fold belt new data implying crustal activation of eastern avalonia along the anglo brabant fold belt are presented late ordovician subduction related magmatism in east anglia and the brabant massif coupled with accelerated subsidence in the anglia basin and in the brabant massif during silurian time indicate a foreland basin development final collision resulted in folding cleavage development and thrusting during the mid lochkovian to mid eifelian in the southeast of the anglo brabant fold belt acadian deformation produced basin inversion and the regional antiformal structure of the brabant massif the uplift inferred from the sedimentology petrography and reworked palynomorphs in the lower devonian of the dinant synclinorium is confirmed by illite crystallinity studies the tectonic model discussed implies the presence of two subduction zones in the eastern part of eastern avalonia one along the anglo brabant fold belt and another under the north sea in the prolongation of the north german polish caledonides

the ordovician of the condroz inlier belgium trilobites from the southeastern margin of avalonia abstract during the early palaeozoic belgium was located in the eastern part of the microcontinent of avalonia which moved from high to low latitudes during the course of the ordovician in the belgian condroz inlier bande de sambre et meuse ordovician trilobites have been reported from four formations the early llanvirn huy formation the caradoc sart bernard and oxhe formations and the ashgill fosses formation these assemblages are some of the easternmost occurrences of trilobites in avalonia and they include aspects that are particularly interesting from a palaeobiogeographical standpoint the trilobite faunas of the huy and sart bernard formations are revised in detail while the others are discussed more briefly having been redescribed in recent years the huy formation belongs to a biofacies characteristic of the outer shelf upper slope and its faunas are typically avalonian almost identical to those recorded from the skiddaw group of northern england and from the ebbe and remscheid anticlines in the rhenish massif germany pricyclopyge binodosa which is inferred to be mesopelagic is common to all these areas and also occurs in bohemia and armorica placoparia cambriensis has a remarkably wide distribution in the early llanvirn and has been recorded from a range of biofacies across avalonia armorica morocco and bohemia it is suggested that this species may have been epipelagic re examination of the trilobites from the sart bernard formation has confirmed that all are conspecific with or compare closely with bohemian taxa the trilobite assemblages of the oxhe formation are typical eastern avalonian faunas while the ashgill fosses formation assemblage has much in common with those of other parts of armorica as well as with baltica and bohemia attesting to the increasing proximity of avalonia baltica and the prague basin during the latest ordovician

large scale slumping deduced from structural and sedimentary features in the lower palaeozoic anglo brabant fold belt belgium distinguishing slump folds from tectonic folds in poorly exposed areas can be difficult especially when the scale of the slump folds exceeds outcrop scale in the southeastern part of the single phase deformed lower palaeozoic anglo brabant fold belt a comparison of cleavage fold relationships and stratigraphic polarity shows that a 200 m thick interval of middle caradoc fine grained turbidites in the core of a large synform was overturned prior to tectonic deformation this overturning is attributed to large scale slumping which was most likely a result of middle caradoc seismic activity the exposed portion of the large slump sheet contains only a few small slump folds and intraformational breccias making up less than 5 of the exposed thickness if the beds were not overturned large scale slumping would never be suspected and the small slump folds would probably be interpreted as localized features in an overall stable sedimentary pile this may explain why so few ancient large scale slides and slumps have been reported the small amount of internal deformation makes them very difficult to recognize especially when dealing with poorly exposed areas as such large ancient slides and slumps may be more common than suggested by the geological literature

a pre hirnantian late ordovician interval of global cooling the boda event re assessed facies analysis suggests that the early mid ashgill late ordovician boda event of fortey and cocks fortey r a cocks l r m 2005 late ordovician global warming the boda event geology 33 5 405 408 was an episode of global cooling not of warming as they proposed limestones are widely developed at this level from mid latitude to high latitude commonly interrupting thick ordovician successions of fine clastic sediments bryozoan carbonate mud mounds in a laterally extensive cool water carbonate belt along the high palaeolatitude margin of w gondwana are associated with glacial diamictites and grew during glacial lowstand with sea ice prior to the hirnantian glaciation from high to mid palaeolatitudes contemporaneous cool water carbonates with mud mounds including stromatactis both in gondwana and in baltica developed in mid outer ramp settings during regression in the dominantly clastic welsh basin of avalonia phosphatic facies on an iapetus facing carbonate ramp indicate upwelling cold waters down ramp yet up ramp coralliferous algal limestones suggest warm surface waters we propose that the late ordovician cool water carbonates developed in response to global cooling and expansion of gondwanan terrestrial and sea ice that lowered eustatic sea level and on the w gondwanan shelf reduced or shut down fluvial clastic input previously poorly circulated salinity stratified oceans that had restricted benthic dispersal in mid low latitudes were replaced by thermohaline ocean circulation that oxygenated bottom waters and dispersed benthic faunas an earlier caradoc carbonate interval in low mid palaeolatitudes that shares many facies characteristics with the ashgill cool water limestones and correlates with a widely recognised isotope excursion is interpreted as indicating an earlier global cooling episode

trace element and nd isotopes in shales as indexes of provenance and crustal growth the early paleozoic from the brabant massif belgium abstract the cambrian ordovician shales from the brabant massif show an increase in th 6 7 17 7 ppm and la 16 6 71 2 ppm contents in lree hree ratio la n yb n 3 8 13 9 and in tnd values 3 3 to 8 9 with younger age of the sedimentary rocks this evolution is related to a change in the dominant source rock from basic metavolcanics in the early cambrian to granite gneiss schist rocks in the early ordovician the ta content appears to be controlled by the provenance area as well the modification in the origin of detritus is not reflected in the abundance of sc cr and v but it is slightly recorded in the co and ni concentrations tnd responds more sensitively to source changes than sm nd ratios and crustal residence ages and it is confirmed to be a more powerful parameter to detect new addition of mantle material to the crust the evolution of tnd with stratigraphic age tnd vs t for the brabant shales parallels the trends for coeval clastic rocks from brittany france and southern britain michard et al 1985 davies et al 1985 and is considered to be a characteristic of the armorica plate the tnd vs t path of sediments deriving from crystalline rocks first cycle sediments turns out to be a convenient function to delineate the boundaries of fossil lithospheric plates on the contrary such first cycle sediments are not very useful to investigate the long term continental growth because their tnd vs t path represents a second order pattern episodic evolution that disturbs the first order trend of the sedimentary nd isotope record secular evolution

effect of aragonite to calcite transformation on the geochemistry and dating accuracy of speleothems an example from castanar cave spain aragonite speleothems are increasingly being used as high resolution climate proxies however aragonite is unstable and susceptible to diagenetic transformation into calcite leading to mineralogical textural and geochemical alterations that have not been fully investigated to provide some insights to these modifications this study combines stable isotope geochemistry and u series dating with petrological observations and empa elemental analyses to characterize a stalagmite from castanar cave caceres spain that shows primary aragonite diagenetically altered and with reverse ages the diagenetic processes include recrystallization to calcite micritization and dissolution characterization and interpretation of the chemistry and nature of the waters from which the aragonite precipitated and of the ones that subsequently formed the calcite indicates that both polymorphs formed from the same fluids these fluids hydrochemically evolved from an aragonite mode to a calcite mode along a diagenetic path within the sample the results show that during transformation the system stayed close in one of the stages and open in the other with the same textural result reverse ages are found either in the recrystallized areas or in the primary aragonite caused by two different u remobilization mechanisms u leaching due to flowing waters on the surface and u loss during the transformation process these results highlight the complex behavior of speleothems during diagenesis and the impact on the geochemical information of the primary and secondary phases and thus the importance of petrological and geochemical characterization of speleothems for paleoclimatic studies

reply to the discussion of the reply by r l romer and u kroner on geochemical signature of ordovician mn rich sedimentary rocks on the avalonian shelf 1 1appears in the canadian journal of earth sciences 49 11 1372 1377 doi 10 1139 e2012 049 romer and kroner 2012 stated the core of the harlech dome i in part is made up of ordovician deposits and ii contains tectonic repetitions in their comment schofield et al this issue d

magmatic activity at the silurian devonian boundary in the brunovistulia and malopolska terranes s poland possible link with the rheic ocean closure and the onset of the rheno hercynian basin the age of granophyric diorite from the sosnowiec ig 1 borehole brunovistulia terrane was studied by means of u pb single grain zircon analysis performed on a shrimp sensitive high resolution ion microprobe iie device the isotope ages and provenance of zircons from the emsian tuffs cropping out in the southern part of the holy cross mountains malopolska terrane were also investigated using the same method the age of the diorite intrusion 420 2 ma is comparable with the combined ar ar magnetostratigraphic age of the bardo diabase intrusion from the northern part of the malopolska terrane these intrusions were emplaced during the same event of regional tectonic extension associated with the rheic ocean closure and the onset of processes creating the rheno hecynian basin near the silurian devonian boundary a negative nb anomaly characteristic of both intrusions could be linked with the subduction of the rheic oceanic crust under the se margin of the old red continent emsian magmatic activity in the distant rheno hercynian zone provided several tuff layers in the northern part of the malopolska terrane as can be inferred from zircon ages these tuffs were derived from mafic eruptions that cut sedimentary rocks containing detrital zircons transported from baltica this interpretation fits the existing models of development of the rheno hercynian basin in the emsian

the galicia ossa morena zone proposal for a new zone of the iberian massif variscan implications abstract correlation of a group of allochthonous terranes referred to as basal ophiolitic and upper units exposed in the nw and sw of the iberian massif is used to propose a new geotectonic zone in the southern branch of the variscan orogen the galicia ossa morena zone recent advances in sw iberia identify most of the former ossa morena zone as another allochthonous complex of the iberian massif the ossa morena complex equivalent to the cabo ortegal ordenes malpica tui braganca and morais complexes described in nw iberia the new geotectonic zone and its counterparts along the rest of the variscan orogen constitute an internal variscan zone with ophiolites and units affected by high p metamorphism the galicia ossa morena zone includes a variscan suture and pieces of continental crust bearing the imprint of ediacaran cambrian events related to the activity of peri gondwanan magmatic arcs cadomian orogenesis in the iberian massif the general structure of this geotectonic zone represents a duplication of the gondwanan platform the outboard sections being juxtaposed on top of domains located closer to the mainland before amalgamation this interpretation offers an explanation that overcomes some issues regarding the differences between the stratigraphic and paleontological record of the central and southern sections of the iberian massif also equivalent structural relationships between other major geotectonic domains of the rest of the variscan orogen are consistent with our interpretation and allow suspecting similar configurations along strike of the orogen a number of issues may be put forward in this respect that potentially open new lines of thinking about the architecture of the variscan orogen

diversity of neomorphic fabrics in new zealand plio pleistocene cool water limestones insights into aragonite alteration pathways and controls abstract many ancient cool water carbonates include little or no evidence for aragonite being present in their primary sediment composition and consequently that they inherited a low diagenetic potential however despite differences in skeletal assemblages age and depositional settings plio pleistocene cool water limestones from eastern north island of new zealand are characterized by locally abundant aragonite remains typically c 20 range 60 either directly as variably altered skeletons or indirectly as biomolds oversized pores and detrital material resulting from collapse of dissolved shells specifically five types of aragonite preservation are evident a skeletons with original aragonite b skeletons with original aragonite but with leaching cavities c skeletons with aragonite relics encased in either dirty or pale brown neomorphic calcite spar d skeletons neomorphically altered to calcite preserving relic shell structures and e dissolved out skeletons leaving an empty or calcite cement filled cavity all these types of preservation including transitions between them can occur within the same sample these observations motivate a conceptual model for the timing and complex pathways of neomorphism in these plio pleistocene cool water limestones related to their diagenetic evolution from shallow marine to burial to uplift related meteoric environments in this the aragonite alteration pathways suggest that the different neomorphic fabrics formed at any stage during the diagenetic history of the host carbonates and are likely not unequivocal indicators of specific diagenetic environments instead differing alteration pathways can potentially produce similar fabrics the observations are consistent with the interpretation that shell microstructures controlling the nature of the intraskeletal neomorphic front and the degree of openness of the diagenetic system were of greater importance in determining the neomorphic fabrics and relic preservation of aragonite than was the general diagenetic environment be it marine meteoric or burial also important were external controls linked to the types of depositional settings that dictated the composition of carbonate factories the morpho tectonic evolution of the host basin and the influence of relative sea level fluctuations on sediment production accumulation and condensation these results illustrate that factors controlling ultimate preservation of carbonate biofacies are multifarious and complex moreover they suggest that the importance of aragonitic components could be underestimated for many ancient cool water carbonate deposits thereby compromising estimation of original skeletal assemblages depositional environments and sediment budgets

coticules of the belgian type area stavelot venn massif limy turbidites within the nascent rheic oceanic basin abstract coticule is the name originating in belgium for a fine grained metasedimentary yellowish rock mainly composed of quartz spessartine and mica which is repeatedly interstratified in the hematite rich purple slates of the lower ordovician of the ardenne region it was described for the first time in the nineteenth century in the stavelot venn massif ardenne which is accordingly the type area for this peculiar lithology since then numerous occurrences were described all over the world in metamorphic rocks of various grades and ages the exact nature of the belgian coticule protolith remained hypothetical although intensively discussed for a long time in the literature after an extensive review of all the stratigraphical sedimentological and geochemical results and new field geochemical major and trace elements and nd isotopic investigations we propose a significantly improved and sustained genetic model the thin coticule layers were deposited offshore in a deep oceanic basin as limy mud turbidites during the early floian c 477 ma early ordovician density currents came from the north from the shelf which bordered the emergent brabant massif at that time and flowed down the slope to the deep basin plain the purple shales enclosing the coticule layers represent the normal pelagic sedimentation in the basin these shales are exceptionally rich in fe and mn because of the hydrothermal activity of the nearby and young rheic ocean ridge indeed the rheic ocean opened in the early ordovician c 482 ma during diagenesis the strongly oxidizing depositional environment favoured the mobility of mn 2 as opposed to fe 3 that remained insoluble and immobile this allowed for the migration of divalent manganese from the pelagic shales and for replacing calcium in the turbiditic carbonate fraction to form rhodochrosite later epizonal metamorphism transformed the clay quartz rhodochrosite paragenesis of the protolith into the muscovite quartz spessartine paragenesis of the coticule hence in the type area coticule genesis needed a peculiar environment including a continental shelf with limy mud deposits a continental slope generating periodical turbidites and a nearby oceanic ridge here the nascent rheic ocean delivering hydrothermal iron and manganese

comparing the ability of soil taxonomy 2014 and wrb 2015 to distinguish lithologic discontinuity and an abrupt textural change in major soils of iran abstract a considerable increase in clay content within a very short distance and significant change of particle size distribution or mineralogy are diagnostic soil properties defined as abrupt textural change and lithologic discontinuity in the most soil classification systems i e soil taxonomy st and world reference base for soil resources wrb this study investigates the ability of the latest versions of st 2014 and wrb 2015 to classify soils that have an abrupt textural change and lithologic discontinuity six study sites were selected from different regions north middle north central south southeast and west of iran one representative pedon was selected described and sampled in each study site the physicochemical properties of the different genetic horizons of each pedon were determined and the soils were classified according to the st 2014 and wrb 2015 systems in the st system the abrupt textural change characteristic was only used for some of the great groups or subgroups furthermore the abruptic qualifier was not considered for all of the reference soil groups rsgs in the wrb system although the presence of lithologic discontinuity at taxon name was totally neglected by the st system the wrb system showed this property with the raptic qualifier this seems to be a limitation for both of the soil classification systems thus the management of soils with abrupt textural change and lithologic discontinuity still faces an important challenge to overcome this problem and to harmonize both the classification systems defining new great groups such as palegypsids and new subgroups including lithic calcixerepts calcic palexerolls abruptic argigypsids calcic argigypsids calcic paleustalfs and the raptic subgroup for all taxa in the st system is highly recommended the addition of abruptic qualifier for the rsgs such as chernozems calcisols and gypsisols seems necessary in the wrb system

very low grade metamorphic sr isotopic resettings of magmatic rocks and minerals evidence for a late givetian strike slip division of the brabant massif belgium rb sr isochron ages recorded by the ashgillian ignimbrites from deerlijk and harelbeke and the caradocian quartz diorites from quenast and bierghes significantly post date their emplacement at quenast two mineral isochrons biotite wr plagioclase 372 13 ma k feldspar wr epidote 375 14 ma reveal that the brabantian branch of the caledonian belts was affected by a post siegenian pre tournaisian event this overprint turns out to be contemporaneous with the development of the oudenaarde bierghes fault zone which is dated to 373 11 ma and 379 13 ma on foliated rocks from bierghes deerlijk and harelbeke this c 375 ma cluster of events is interpreted as representing late givetian stresses developed along the oudenaarde bierghes fault zone for which the balance of geological data favours strike slip movements since this compressive phase of deformation is posterior to the givetian opening of the borain trough the brabant massif appears to have been affected by successive crustal extension and shortening events in the middle devonian petrological evidence demonstrates the very low grade character of the metamorphism in the metasediments on both sides of the oudenaarde bierghes fault zone from this we deduce that sr isotopic homogenization of whole rock ignimbrite and the loss of radiogenic sr from biotite and k feldspar could be operative even at temperatures below 250 c

truncation and translation of appalachian promontories mid paleozoic strike slip tectonics and basin initiation accreted terranes of the appalachian iapetan and peri gondwanan realms display structural trends that are mainly concordant with promontories and embayments in the laurentian margin indicating that during accretion the shape of the continental margin acted as a template around which accreted terranes were molded in north carolina and new foundland post accretion transcurrent motion appears to be recorded by displaced outboard portions of promontories no longer concordant with those in laurentia a bend in structural trends confined to the peri gondwanan realm at the north carolina south carolina state line is interpreted to represent the dextrally displaced outboard portion of the virginia promontory in newfoundland the hermitage flexure is interpreted as a dextrally displaced laurentian promontory that originated along strike to the northeast in both places promontories were truncated and dextrally translated for 220 250 km by a devonian mississippian orogen parallel transcurrent system which may well have extended for the length of the eastern laurentian margin south of the nearby st lawrence promontory extreme thinning of appalachian crust beneath the maritimes basin is consistent with extension at a releasing stepover estimates of extension are consistent with those obtained from promontory offsets

structural analysis of the adjacent acadian and variscan fold belts in belgium and northern france from geophysical and geological evidence abstract two palaeozoic fold belts are juxtaposed in belgium and northern france the northern belt of the brabant massif traditionally considered a part of the european caledonides was folded during the acadian orogeny in the early devonian although it is largely concealed gravity and magnetic maps give a general picture of the existing structures a curved gravity trend is interpreted as an arc shaped succession of granitic batholiths structures north and south of this arc are completely different the northern subdomain containing magnetic lower cambrian metasediments shows multiple thrust faults towards the south over the granites whereas the southern subdomain without magnetic rocks is moulded around the granitic core the southern variscan belt is characterized in general by thrusting with a north nw and ne vergence during the variscan orogeny the brabant massif acted as an obstacle to the general transport of the ardennes massif to the n nw the oblique convergence with the brabant massif caused differential displacement towards the northwest the western areas having travelled farther as seen from a pattern of wnw oriented strike slip faults

relationships between colour and diagenesis in the aragonite calcite speleothems in basajaun etxea cave spain abstract basajaun etxea cave north spain contains a wide morphological and colour variety of speleothems most of them are composed of aragonite but calcite speleothems are also found their most common colour is white but there are also pink green and turquoise speleothems in different areas of the cave mg rich dissolution waters from the cave s crystalline dolostone and magnesite host rock favour aragonite precipitation and drive important diagenetic changes in this paper we will discuss how diagenesis modifies speleothem texture mineralogy and geochemistry causing significant changes in the colour we also discuss how speleothems that have undergone diagenesis may also be useful indictors of paleoclimatic conditions our work shows that diagenesis changes the colour of the speleothems because of the mobility of the chromophore elements during this process along with this elemental alteration comes the loss of the primary isotopic signals of speleothems detailed petrological studies of speleothems should precede their analysis for palaeoclimatic reconstruction so that influence of areas affected by diagenesis can be eliminated in addition diagenetic signals also contain valuable information on the changes of the waters within the cave and the overall evolution of speleothems and the cave itself

variation in the diagenetic response of aragonite archives to hydrothermal alteration abstract diagenesis of carbonate minerals is ubiquitous throughout the geologic record alteration is initiated immediately after deposition or takes place in the endo and exoskeletons as early as during the lifetime of a given carbonate secreting biota and can continue throughout the burial history of carbonate sediments and rocks variations in the diagenetic response of carbonate archives pose challenges for the reconstruction of past environmental conditions based on proxy data this paper comparatively assesses alteration features of different aragonitic materials by experimentally induced diagenesis a multitude of factors lead to different reactivity and responses of a given aragonite archive and provide insight to the interpretation of diagenetically altered material in the rock record chosen materials used in this study include relatively organic rich samples such as coral skeletons and bivalve shells and organic lean abiotic carbonates such as speleothems and aragonite single crystals obtained datasets include distributions of elements organics carbon and oxygen isotope ratios and crystallographic features observed variations in diagenetic responses include mineralogy of the diagenetic phases rate and extent of mineral transformation distribution of foreign ions in the crystal lattice primarily sr mg and s and the number of specific processes and products along diagenetic pathways alteration is shown to be primarily controlled by the initial diagenetic susceptibility of the sample including porosimetry and structural characteristics concentrations of organic material and primary amounts of trace elements in the carbonate such as mg structural characteristics lead to initial internally fluid or rock buffered conditions with low porosity and permeability resulting in a greater effect of internal fluids and organics differences in the amount of organic content and internal fluids affect transformation rates secondary mineralogy and isotope equilibria samples with relatively high porosity high mineral transformation rates and high primary mg ca may preferentially form secondary aragonite during fast equilibration with the diagenetic environment our results suggest that the degree and nature of diagenetic alteration of aragonite materials are strongly controlled by the micro to nano scale internal architecture governing the availability and transfer of aqueous fluids results of this study provide significant implications for the interpretation of diagenetic signals in carbonate archives and have direct significance for the mechanistic understanding of carbonate diagenesis and paleo environmental conditions

the alkaline peralkaline granitic post collisional tin zebane dyke swarm pan african tuareg shield algeria prevalent mantle signature and late agpaitic differentiation abstract the tin zebane dyke swarm was emplaced at the end of the pan african orogeny along a mega shear zone separating two contrasting terranes of the tuareg shield it is located along the western boundary of the archaean in ouzzal rigid terrane but inside the adjacent tassendjanet terrane strongly remobilized at the end of the precambrian the tin zebane swarm was emplaced during post collisional sinistral movements along the shear zone at 592 2 5 8 ma 19wr rb sr isochron it is a dyke on dyke system consisting of dykes and stocks of gabbros and dykes of metaluminous and peralkaline granites all rock types have sr and nd isotopic initial ratios sr i 0 7028 and e nd 6 2 typical of a depleted mantle source similar to the prevalent mantle prema at that period no crustal contamination occurred in the genesis of the tin zebane swarm even the samples showing evidence of fluid interaction essentially alkali mobility have the same isotopic signature the peralkaline granites have peculiar geochemical characteristics that mimic subduction related granites this geochemical signature is interpreted in terms of extensive differentiation effects due to late cumulates comprising aegirine zircon titanite allanite and possibly fergusonite separated from the liquid in the swarm itself due to magmatic flow turbulence the tin zebane dyke swarm is thus of paramount importance for constraining the differentiation of mantle products to generate highly evolved alkaline granites without continental crust participation in a post collisional setting

tectonic rotation within the british paratectonic caledonides and early palaeozoic location of the orogen magnetizations of late ordovician caradoc early ashgill age in the paratectonic caledonides of wales and the lake district are directed westerly and northerly respectively they identify c 55 of relative rotation of probable acadian age southward subduction of iapetus ocean crust occurred in ordovician times beneath a latitudinally oriented orogen sited in mid southerly latitudes with the borrowdale and snowdonia volcanic provinces forming parallel fore arc and back arc lineaments respectively mid ordovician to mid devonian palaeomagnetic poles from the orogen correlate with contemporaneous gondwana data to identify a former location sited west of the south american perimeter and not near to north africa as assumed in previous reconstructions

arcuate fold and cleavage patterns in the southeastern part of the anglo brabant fold belt belgium tectonic implications abstract the brabant massif belgium is situated at the southeastern extremity of the anglo brabant fold belt the predominantly concealed eastern branch of the british belgian caledonides in contrast to the other parts of this caledonian slate belt the deformation history in the brabant massif has to date never been constrained properly in this paper a synthesis is presented of the structural observations in the limited outcrop area of the brabant massif in particular with respect to cleavage and fold trajectories and mutual cleavage fold relationships arcuate fold and cleavage patterns as well as the presence of cleavage transected folds are demonstrated similar to the other parts of the belt moreover the arcuate trajectories are considered to reflect the trace of the caledonian deformation front once obscured by the variscan ardenne allochthon combined with interpretations of the aeromagnetic and bouguer anomaly maps an overall kinematic model is proposed for the acadian deformation event in the brabant massif with the proposed model calling upon processes of indentation by promontories and lateral escape of lithostructural domains as well as on the control exerted by rigid granitoid basement blocks the strong similarities with the other parts of the british belgian caledonides become apparent by defining an overall shortening direction and proposing a trace for the caledonian deformation front in this southeastern extremity of the anglo brabant fold belt an integrated tectonic image is obtained for the entire british belgian caledonides

diagenesis of mollusc aragonite and the role of fluid reservoirs abstract biogenic carbonate minerals are widely used as archives in paleoenvironmental research providing substantial information for past depositional and diagenetic regimes however nearly all biogenic carbonates undergo post mortem diagenetic alteration to variable degrees diagenetic features are essentially caused by complex fluid solid interaction including recrystallization and neomorphosis of shell architecture and related geochemical resetting a common conception is that a given primary shell texture is replaced by a secondary fabric via micro scale dissolution reprecipitation reactions that may reach geochemical and or isotopic equilibrium with the diagenetic fluid here we document that the process of petrographic and geochemical alteration of a biogenic carbonate archive progresses stepwise and the re equilibration processes can be separated in space and time more specifically attached and bound aqueous fluids within the shell organic matter and fluid inclusions likely play a crucial role in the early stages of alteration within a relatively rock buffered system degradation of organics was observed via decreased sulfur concentration and decreased fluorescence in this early stage of alteration the conversion of micro scale aragonite biominerals to metastable diagenetic calcite meso crystals is documented by raman spectroscopy electron backscatter diffraction ebsd and a decrease in strontium concentrations further stabilization to larger neomorphosed calcite crystals resulted from re equilibration with external fluids and was evidenced by ebsd oxygen isotope data and further reduction of strontium spatial chemical and isotope data were used to decipher the impact of fluid availability and transfer behavior i e carbonate buffered versus diagenetic fluid buffered and isotope and element exchange behavior and analyzed in context to textural alteration features these data suggest that during diagenesis the evolution to more sustainable diagenetically induced fabrics is accompanied by individual reaction steps traced by elemental and isotopic signatures kinetics and thus degree of diagenetic alteration of biominerals at a given exposure to physicochemical alteration conditions is initially strongly controlled by the micro to nano scale internal architecture governing the availability and transfer of aqueous fluids results shown here have significance for those concerned with biogenic carbonate archives and have wider implications for a mechanistic understanding of carbonate diagenesis

replacement of aragonite by calcite in sediments from the san cassiano formation italy abstract the fossils of the patch reefs of the san cassiano formation triassic northern italy still preserve aragonite in their shell the explanation for the persistence of the metastable polymorph of caco3 has been believed to be the lowering of the porosity and permeability of the sediment by early cementation and the presence of marls scherer 1977 the transmission electron microscope tem adds to the understanding of the replacement mechanism recrystallization occurs primarily along crystal defects suggesting that the kinetics of the replacement are controlled mainly by the microstructure of aragonite at least under conditions of slow diagenetic alteration the replacive calcite in the san cassiano fossils and the calcite crystals of the cement show a pervasive modulated microstructure when observed with the tem this can be attributed either to compositional variations calcite contains mg and fe to stacking disorder or to positional disorder of anions we believe that these modulations account mainly for the metastability of the san cassiano calcite this study further demonstrates that transmission electron microscopy is a powerful technique in investigations of sedimentary rocks and their diagenesis

pre mesozoic basement rocks offshore nova scotia canada new constraints on the accretion history of the meguma terrane basement samples from offshore wells and geophysical data show that the meguma terrane forms the basement of mesozoic cenozoic sedimentary basins as far as 200 km offshore nova scotia the data provide constraints on avalon meguma terrane interaction during middle to late paleozoic events in the appalachian orogen the basement consists of metasedimentary rocks intruded by late paleozoic granitoid plutons principally magnetite bearing granodiorites which contrast with more peraluminous ilmenite bearing devonian granodiorite and granite onshore offshore tonalites have a subduction related trace element signature that is similar to the signature in minor devonian gabbro and tonalite intrusions on land nd sr and pb isotopes suggest three sources for meguma terrane plutons 1 2 ga basement possibly saharan shield and sediments derived therefrom this basement may be present beneath the southwestern scotian shelf 2 late proterozoic underthrust avalonian basement and basement autochthonous to the meguma terrane and 3 melts from mantle last remobilized in the late proterozoic pan african orogeny seismic reflection profiles show meguma terrane thrust northward over a wedge of avalon basement extending 30 km south of the surface suture there is no evidence that the devonian tonalites are directly related to subduction the lack of either ophiolites or paleozoic pelagic sediment suggests that convergence of the avalon and meguma terranes was achieved principally by strike slip motion resulting from low angle continent continent convergence with only a minor thrust component the widespread devonian carboniferous igneous activity of the amalgamated meguma and avalon terranes was a consequence of regional extension which resulted in decompression melting of upper mantle and production of mafic magmas variably mixed with crustal material

cambrian successions of the meguma terrane nova scotia and harlech dome north wales dispersed fragments of a peri gondwanan basin abstract the meguma terrane of nova scotia and the harlech dome of north wales preserve similar sedimentary successions of cambrian age both successions comprise a thick succession of early cambrian sandstone turbidites overlain by early to middle cambrian alternating mud rich and sand rich units in which manganese is concentrated in two stratigraphic intervals above these both successions comprise anoxic organic rich turbidites shallowing upward into paler more bioturbated tremadocian mudstone with rhabdinopora within the limited constraints of the available biostratigraphic and geochronological data major changes in environment occurred synchronously in the two successions both successions show much greater similarity to each other than to adjacent successions on avalonia9 a detrital zircon analysis from the rhinog formation low in the harlech dome succession reveals distinct clusters of ages around 537 ma and 2 0 2 1 ga a close similarity to analyses from the meguma terrane suggests proximity between the two terranes on the margin of gondwana during the cambrian period we suggest the term megumia for the palaeogeographical domain that included the two successions which was dispersed during subsequent appalachian and caledonian movements these observations suggest that megumia may have separated terranes previously regarded as parts of avalonia supplementary material u pb detrital zircon analytical data are available at http www geolsoc org uk sup18431

subduction flip during iapetus ocean closure and baltica laurentia collision scandinavian caledonides evidence is presented here from the northern scandinavian caledonides for development of an extensional basin of ashgill to mid llandovery age along the baltoscandian margin immediately prior to baltica laurentia collision u pb multigrain and ion microprobe zircon dating of plagiogranites in the halti igneous complex complement previous baddeleyite and zircon dating of a dolerite dyke and zircon dating of anatectic granite they demonstrate that this dunite troctolite gabbro sheeted dyke complex ranges in age from c 445 to 435 ma the dolerite dykes intruded and melted arkoses of inferred neoproterozoic age this evidence taken together with previous documentation of ophiolites solund stavfjord ophiolite like associations sulitjelma igneous complex and several other mafic suites e g rana artfjallet of ashgill to llandovery age further south in the northern scandinavian caledonides implies that scandian collisional orogeny along this nearly 2000 km long mountain belt was immediately preceeded by development of short lived marginal basins the latter developed during the final closure of the iapetus ocean and are inferred to be of back arc origin some perhaps all related to e dipping subduction collision of the continents at c 435 ma is inferred to have induced a flip in subduction polarity leading to underthrusting of laurentia by baltica

late pan african tectonics marking the transition from subduction related calc alkaline magmatism to within plate alkaline granitoids adrar des iforas mali boullier a m liegeois j p black r fabre j sauvage m and bertrand j m 1986 late pan african tectonics marking the transition from subduction related talc alkaline magmatism to within plate alkaline granitoids adrar des iforas mali in b johnson and a w bally editors intraplate deformation characteristics processes and causes tectonophysics 132 233 246 the pan african trans saharan belt resulting from oceanic closure and oblique collision between the west african craton and the tuareg nigerian shields around 600 ma ago displays in the iforas a rapid switch from subduction and collision related talc alkaline magmatism to typical a type granitoids 560 540 ma the transition is clearly seen in associated spectacular acid dyke swarms early talc alkaline e w trending sets being cut by alkaline n s trending arrays isotopic evidence shows that the change in chemistry involves a different more primitive mantle source structural observations indicate that the transition from talc alkaline to alkaline magmatism is related to reversals in the stress field during and after collision in the model proposed the first major reversal from d with ei ca 13s0n lloon to ds with ui ca 50 n 60 n is thought to have been responsible for the slicing of the subducted plate thereby allowing the rise to shallow depth beneath the continental lithosphere of asthenospheric mantle believed to be the source of the alkaline magmatism the tapping of this new source occurred between ds and d4 events contemporaneous with a rotation of the maximum compressive stress back from 60 n to 105 n

the tornquist sea and baltica avalonia docking early ordovician late arenig limestones from the sw margin of baltica scania bomholm have multicomponent magnetic signatures but high unblocking components predating folding and the corresponding palaeomagnetic pole latitude 19degreesn longitude 051degreese compares well with arenig reference poles from baltica collectively the arenig poles demonstrate a midsoutherly latitudinal position for baltica then separated from avalonia by the tomquist sea tornquist sea closure and the baltica avalonia convergence history are evidenced from faunal mixing and increased resemblance in palaeomagnetically determined palaeolatitudes for avalonia and baltica during the mid late ordovician by the caradoc avalonia had drifted to palaeolatitudes compatible with those of sw baltica and subduction beneath eastern avalonia was taking place we propose that explosive vents associated with this subduction and related to andean type magmatism in avalonia were the source for the gigantic mid caradoc c 455 ma ash fall in baltica i e the kinnekulle bentonite avalonia was located south of the subtropical high during most of the ordovician and this would have provided an optimum palaeoposition to supply baltica with large ash falls governed by westerly winds in scama we observe a persistent palaeomagnetic overprint of late ordovician ashgill age pole latitude 4degreess longitude 012degreese the remagnetisation was probably spurred by tectonic derived fluids since burial alone is inadequate to explain this remagnetisation event this is the first record of a late ordovician event in scania but it is comparable with the shelveian event in avalonia low grade metamorphism in the north sea basement of ne germany 440 450 ma and sheds new light on the baltica avalonia docking c 2002 elsevier science b v all rights reserved less

age of detrital zircon and titanite in the meguma group southern nova scotia canada clues to the origin of the meguma terrane abstract precise u pb isotopic analysis of seventeen selected high quality laboratory abraded zircon single grains seven multigrain fractions of similar material and four paramagnetic titanite fractions record the ages of the various detrital grain types from two turbidite samples collected from the top and about 5 km below the top of the goldenville formation of the meguma group the results show that the source of both sediments contained 600 ma and 2000 ma old volcanic plutonic components whereas a third 3000 ma old component occurs exclusively in the upper sample and the youngest zircons and titanite are found in the lower sample lead lead ages of bulk fractions at 2300 ma and two nearly concordant single grains at 2040 ma and 2140 ma upper sample together with 2157 ma bulk fraction and 2060 ma and 2080 ma single grain fractions lower sample indicate that the 2000 ma component dominates the pink purple euhedral grain types detrital titanite gives ages of 605 ma upper sample and 552 ma lower sample with the latter sample containing a minor 1975 ma component colourless zircons record this same age difference the lower sample contained the youngest concordant single zircon grain 566 ma and 580 605 ma bulk grain fraction even though two concordant single grains have ages of 632 and 643 ma estimated bulk fraction ages for certain zircon grain types from the upper sample fall in the range of 605 to 675 ma these data suggest that while the youngest rocks in the source region were removed by erosion during the early stages of deposition of the goldenville formation the oldest source rocks were not unroofed until later in the depositional history the 566 8 ma age of the youngest concordant zircon from the stratigraphically lower sample provides a maximum age for the deposition of this grain with the base of the cambrian placed between 545 and 530 ma the goldenville formation may straddle the precambrian cambrian boundary graptolites from the immediately overlying halifax formation are of early ordovician tremadocian age the lower sample contains spherical zircons that may have been derived from mafic granulite with an estimated age of 2000 2200 ma with this one possible exception all of the zircons analyzed appear to be igneous in origin and all are relatively euhedral indicating that rocks of the ages referred to above probably outcrop in the source area anomalous discordance patterns imply that part of the 2960 ma area was metamorphosed by the 2000 ma event and that both the 2960 and 2000 ma regions were metamorphosed during the 600 ma event the complete lack of detritus with ages between 680 and 2040 ma virtually eliminates a northern north american or baltic shield provenance however the available data are not sufficient to distinguish between a west african or guyana shield source

rifting during separation of eastern avalonia from gondwana evidence from subsidence analysis subsidence curves for cambrian ordovician sequences from the anglo welsh segment of the paleocontinent of avalonia reveal two periods of regionally enhanced basement subsidence early cambrian 545 518 ma and late cambrian to early tremadocian 505 490 ma the earlier event may record transtension following the avalonian cadomian orogeny the second event may be a transtensional precursor to the late tremadocian volcanic arc on eastern avalonia however paleomagnetic faunal volcanic and sedimentary evidence suggests that the main separation of eastern avalonia from gondwana occurred after middle arenigian time rifting during separation is probably recorded by localized middle arenigian to llanvirnian 480 462 ma subsidence along the welsh basin margin but rifting must have occurred mainly on the now obscured southern margin of the avalonian continent pronounced caradocian 462 449 ma subsidence is associated with back arc rifting after separation from gondwana