discussion on a terrane based nomenclature for the lewisian gneiss complex of nw scotland journal vol 162 2005 pp 175 186 graham park writes the terrane model of kinny et al 2005 see also friend kinny 2001 presents a radically different picture of the lewisian complex from the traditional view of a single piece of archaean crust that has been subsequently modified and augmented in various proterozoic events the authors are to be congratulated on their attempt to rationalize a very confused terminology that has arisen over many decades and for forcing us to rethink our views i have reservations about their re definition of laxfordian but my comments mainly concern the validity of the terrane model as applied to the lewisian and suggestions to refine it see park 2005 i agree that uncritical use of terms such as scourian laxfordian and scourie dyke suite can obscure genuine differences of rock process or event e g see park et al 2002 most of the authors suggested revisions seem sensible with the exception of their proposed restriction of laxfordian to a c 1 74 ga metamorphic event at loch laxford i believe that this term is more useful in its current sense as a general name for the series of events etc that affected the entire lewisian complex after the incorporation of the early palaeoproterozoic material e g the loch maree group and the scourie dykes i have retained the traditional usage below for convenience according to this concept distinction is made between suspect terranes and displaced terranes i e those that are exotic or allochthonous in order to qualify as a displaced terrane a piece of crust should display evidence of significant displacement relative to its neighbour differences in geological history between two adjacent pieces of crust may not in themselves be sufficient evidence of large relative displacements between them transverse or oblique convergence between two continental plates inevitably involves the subduction

the use of the common pb isotope composition of detrital k feldspar grains as a provenance tool and its application to upper carboniferous paleodrainage northern england k feldspar is a common detrital mineral in sandstones it contains little u or th and hence the common pb isotope composition of unaltered k feldspar is that of the source rock pb isotope variations in igneous and metamorphic crustal rocks define broad spatial patterns that make the pb signature of detrital k feldspar grains a useful provenance tool however it is unclear how robust this signal is and to what extent it can be modified by weathering transport burial diagenesis and or exposure to hydrothermal fluids this study reports an evaluation of the technique using two granite arkose pairs from helmsdale northern scotland and shap northwest england where the composition of the detrital grains can be compared to feldspars in the granite from which they were derived the results of this test have informed a pilot study using the pb isotope composition of detrital k feldspar in upper carboniferous fluvial sedimentary rocks of northern england to constrain the sediment routing pb isotope compositions have been measured using an in situ high resolution single grain technique that can distinguish original composition heterogeneity and subsequent alteration within individual sand grains the results for the granite arkose pairs show that the common pb isotope signature of the detrital k feldspar grains matches the in situ feldspars in the source granites the signal is independent of grain size medium grained sand to granules but alteration along a paleo regolith at helmsdale has resulted in perturbations in pb composition and an increase in the errors on the analyses the application of the technique to arkosic carboniferous serpukhovian bashkirian millstone grit group sandstones in the pennine basin reveals two discrete populations of k feldspars that cannot be distinguished petrographically matching of the compositions to potential sourcelands to the north and northeast of the basin suggest a far traveled 500 km source of k feldspar from the archean paleoproterozoic lewisian rocks of northwest scotland or their extension to the north but also an equally important proximal source emanating from the southern uplands belt of scotland in what would have been the lower part of the hinterland drainage basin of the millstone grit

precise u pb ages and geochemistry of palaeoproterozoic mafic dykes from southern west greenland linking the north atlantic and the dharwar cratons palaeoproterozoic magmatism throughout the north atlantic craton nac has produced dyke swarms of diverse orientations and emplacement ages in southern west greenland previously identified swarms include the ca 2500 ma kilarsaarfik dykes ca 2215 ma boninitic norite bn dykes and the abundant ca 2040 ma kangamiut and coeval 2050 2030 ma md3 dykes additional insight into the distribution of swarms is provided by new baddeleyite u pb id tims dates interpreted as emplacement ages for six dykes the geochemistry for a total of 42 samples on 40 dykes including the dated dykes provides some constraints on their petrogenesis two e w trending dykes yield ages of 2365 2 ma and 2374 4 ma representing a hitherto unknown magmatic event we propose the name graedefjord dykes for dykes of this age in southern west greenland one ne sw trending dyke is dated at 2209 5 ma close to a previously published 2214 10 ma age for a n s trending dyke member of bn dykes three dykes yield md3 ages of 2053 2 ma e w trending 2049 6 ma n s trending and 2042 2 ma wnw ese trending ca 2 21 ga old and predominantly ne se and nne sse trending dolerites some of which are high al basalts were derived from a relatively depleted mantle and injected during a boninite dolerite magmatic event together with nearby bn dykes derived from a harzburgitic yet highly lree and lile enriched subcontinental lithospheric mantle sclm ca 2 05 ga kangamiut md3 dolerites were derived from a similar ambient asthenospheric mantle and subsequently experienced crustal assimilation during fractional crystallisation the ca 2370 ma dykes in the nac are coeval with the dharwar giant dyke swarm of the dharwar craton dc and based on this unique age match it is proposed that the dharwar and nac were neighbours in the palaeoproterozoic furthermore both blocks share 222 2 21 ga dykes and matching the trends of 2 2 2 1 and 2 37 ga swarms between the two blocks allows several plausible reconstructions c 2012 published by elsevier b v

the laxford shear zone an end archaean terrane boundary the lewisian gneiss complex of northwestern scotland consists of archaean gneisses variably reworked during the proterozoic it can be divided into three districts a central granulite facies district between districts of amphibolite facies gneiss to the north and south recent work has interpreted these districts in terms of separate terranes initiating a controversy that has implications for how precambrian rocks are understood worldwide the northern district of the lewisian gneiss complex the rhiconich terrane is separated from the central district the assynt terrane by a broad ductile shear zone known as the laxford shear zone this paper reviews the geology of the laxford shear zone clarifying field relationships and discussing other evidence to consider whether or not it does indeed represent a terrane boundary a detailed review of field geochemical and geochronological evidence supports the recognition of the separate assynt and rhiconich terranes mafic dykes the scourie dyke swarm and granitoids of palaeoproterozoic age occur on both sides of the laxford shear zone and thus the terranes were most probably juxtaposed during the late archaean to early palaeoproterozoic inverian event subsequently the less competent more hydrous amphibolite facies gneisses of the rhiconich terrane were affected by later palaeoproterozoic laxfordian deformation and partial melting to a greater extent than the more competent granulite facies gneisses of the assynt terrane

site information and initial results from deep ice drilling on law dome antarctica the aim of deep ice drilling on law dome antarctica has been to exploit the special characteristics of law dome summit i e low temperature and high accumulation near an ice divide to obtain a high resolution ice core for climatic environmental studies of the holocene and the last glacial maximum lgm drilling was completed in february 1993 when basal ice containing small fragments of rock was reached at a depth of 1196 m accurate ice dating obtained by counting annual layers revealed by fine detail 18 o peroxide and electrical conductivity measurements is continuous down to 399 m corresponding to a date of ad 1304 sulphate concentration measurements made around depths where conductivity tracing indicates volcanic fallout allow confirmation of the dating for agung in 1963 and tambora in 1815 or estimates of the eruption date from the ice dating for the kuwae vanuatu eruption 1457 the lower part of the core is dated by extrapolating the layer counting using a simple model of the ice flow at the lgm ice fabric measurements show a large decrease 250 to 14 mm 2 in crystal size and a narrow maximum in c axis verticality the main zone of strong single pole fabrics however is located higher up in a broad zone around 900 m oxygen isotope 18 o measurements show holocene ice down to 1113 m the lgm at 1133 m and warm 18 o about the same as holocene ice near the base of the ice sheet the lgm holocene 18 o shift of 7 0 only 1 larger than for vostok indicates that law dome remained an independent ice cap and was not overridden by the inland ice sheet in the glacial

methanesulphonic acid loss during ice core storage recommendations based on a new diffusion coefficient the loss of methanesulphonic acid msa from stored ice cores can be significant over typical storage times with diffusion to the ice core surface controlling the loss methods for minimizing this loss are discussed and it is shown how measurements can be corrected by calculating the amount of msa lost a revised diffusion coefficient for msa in solid ice 4 1 x 10 13 2 5 x 10 14 m 2 s 1 is derived to improve such msa loss corrections

preface to the special issue of precambrian research on the understanding of gneiss complexes in honour of clark r l friend gneiss complexes are the world s most horrendously complexrocks to understand see cover of this issue yet they form a largeamount of the continents albeit they are normally hidden fromview by sedimentary sequences the last half century has seenenormous advances in understanding these rocks which form notonly a large amount of the continental crust but also hold much of earth s oldest rock record the archaean disciplines medicine and health sciences social and behavioral sciences publication details nutman a p 2014 preface to the special issue of precambrian research on the understanding of gneiss complexes in honour of clark r l friend precambrian research 255 part 3 771 773 this journal article is available at research online http ro uow edu au smhpapers 2672 preface to the special issue of precambrian research on the understanding of gneiss complexes in honour of clark r l friend

a re evaluation of a laxfordian terrane boundary in the lewisian complex of south harris nw scotland the recent reinterpretation of the lewisian complex as a series of amalgamated terranes represents a significant advance in our understanding of these rocks however on the outer hebrides a key observation that a supposed post 1675 ma terrane boundary is based on is flawed specifically on south harris a group of c 1675 ma granite pegmatites has previously been described as occurring only to the ne of and terminating abruptly within a supposed terrane bounding shear zone the langavat shear zone which thus post dates the pegmatites new mapping indicates that granite pegmatites occur on both sides of and throughout the langavat shear zone and that within most of this shear zone the pegmatites are not strongly deformed inconsistent with them predating this shear zone furthermore new u pb geochronological data suggest that the pegmatites sw of the supposed boundary were also intruded around 1675 ma similar to those in the ne and supporting the field evidence that the pegmatites continue through the langavat shear zone without interruption two phases of post pegmatite deformation are identified but these relate to relatively minor reactivation of the langavat shear zone not the initiation of this major shear zone which predates the pegmatites

kinematic and thermal constraints on the reactivation of the outer hebrides fault zone nw scotland the outer hebrides fault zone is a major easterly dipping reactivated shear zone which displaces lewisian gneiss of the laurentian craton nw scotland despite a number of detailed field studies the fault zone remains poorly understood with regard to both its age of inception and precise conditions of reactivation the island of scalpay in the northern portion of the outer hebrides fault zone provides exceptional exposures through a variety of reactivated fault rock types and therefore represents an ideal location to investigate fault zone evolution via fluid inclusion studies of syntectonic quartz veins this fluid inclusion study constrains reactivation temperatures more precisely than hitherto possible with top to the nw ductile thrusting occurring at 500 30 c subsequent phyllonitization is associated with oblique sinistral top to the ne strike slip at 230 20 c followed by a discrete system of top to the ne se extensional detachments at 150 20 c other recent fluid inclusion studies in the southern portion of the outer hebrides fault zone constrain phyllonitization associated with top to the e displacement to 370 20 c with subsequent top to the ne extensional detachments operating at 150 210 c thus late stage extensional detachment systems record consistent conditions of reactivation along the strike length of the outer hebrides fault zone however our results also clearly emphasize that conditions of earlier fault zone reactivation and phyllonitization were highly heterogeneous between the northern and southern portions thus suggesting a spatial and temporal variation in the deformation and or fluid flux system

proposal for a terrane based nomenclature for the lewisian gneiss complex of nw scotland the current nomenclature for the lewisian gneiss complex has evolved from lithological and structural correlations made prior to any dating initial flawed geochronological studies gave some names an apparent chronological standing but as work advanced fitting events into a coherent regional framework became increasingly difficult modern dating studies have shown that the lewisian gneiss complex was progressively assembled from disparate blocks of archaean continental crust and juvenile proterozoic arcs which satisfy the definition of terranes each terrane had its own separate accretionary and metamorphic history followed by a common history once juxtaposed against other terranes based on a new compilation of modern geochronology allied to the many detailed structural and metamorphic studies this paper proposes a new systematic terminology for the lewisian gneiss complex that is more applicable to this new tectonic framework

new light on the construction evolution and correlation of the langavat belt lewisian complex outer hebrides scotland field petrographic and geochronological evidence for an early proterozoic imbricate zone the south harris complex is a domain of largely palaeoproterozoic rocks within the late archaean tonalite trondhjemite granodiorite ttg dominated lewisian gneisses of the outer hebrides nw scotland the complex is distinguished by a high proportion of metasedimentary rocks and distinctive meta igneous units in part representing the remnants of a continental volcanic arc the langavat belt defines the ne border zone of the south harris complex separating the latter from the archaean gneisses further ne and has been repeatedly interpreted as a discrete supracrustal unit however detailed mapping and petrographic analysis reveals that up to 60 of the belt may be composed of highly deformed felsic orthogneiss the metasedimentary rocks that are present have a late archaean zircon provenance signature implying that they are younger than and possibly derived from the ttg gneisses to the ne previous work on zircon populations in the leverburgh metasediments towards the southern flank of the south harris complex indicates very different provenance and we repudiate correlation of the two sequences the disposition of units of metasediment amphibolite and felsic orthogneiss is not a primary pre tectonic feature and is very unlikely to be the result of folding although direct evidence in the form of early fabrics has been wiped out by later penetrative ductile shearing and metamorphic annealing we propose that the langavat belt was assembled as a zone of thrust imbrication during early proterozoic contraction after imbrication ductile shearing occurred in two distinct periods separated by intrusion of granite pegmatites dated at c 1660 ma

the palaeoproterozoic anatomy of the lewisian complex nw scotland evidence for two laxfordian tectonothermal cycles a new structural examination of palaeoproterozoic high p granulites on south harris nw scotland when integrated with previous geochronological structural and metamorphic studies on key areas of the lewisian complex suggests the existence of two distinct tectonothermal cycles within the palaeoproterozoic laxfordian event which on south harris are separated by a 100 myr hiatus in deformation the older cycle from c 1 91 to 1 85 ga records the development of an active continental margin on the archaean gneisses that dominate the complex and the subsequent onset of continent continent collision this represents the continuation of the nagssugtoqidian orogen of greenland evidence for this is concentrated in allochthonous slivers of the former active continental margin displaced during the younger cycle the younger cycle around 1 75 1 65 ga began with thrust related crustal thickening that initiated regionally extensive amphibolite facies metamorphism and ductile deformation which dominates the preserved laxfordian deformation history this may be the peripheral expression of the accretion of the malin block to the sw of the lewisian and represents the lateral continuation of the labradorian ketilidian orogen of north america supplementary material additional figures are available at http www geolsoc org uk sup18863

new pieces to the archaean terrane jigsaw puzzle in the nuuk region southern west greenland steps in transforming a simple insight into a complex regional tectonothermal model in the south of the nuuk region of west greenland our 1980s mapping recognized four archaean gneiss terranes f ringehavn tre brodre tasiusarsuaq and akia terranes with different protolith ages and separate early tectonothermal histories later in the archaean these were juxtaposed and then experienced the same 2700 2500 ma tectonothermal events here we abandon extrapolation of only these four terranes across the whole region and distinguish two new terranes in the ne the northernmost isukasia terrane previously regarded as the northernmost exposure of the f ringehavn terrane consists of palaeoarchaean rocks 3600 ma tectonically bounded on its south by the 3075 2960 ma kapisilik terrane these were juxtaposed and metamorphosed together by 2950 ma the previously recognized f ringehavn terrane to the sw is another separate entity of palaeoarchaean rocks that was juxtaposed with adjacent terranes only after c 2800 ma hence in an increasingly complex regional model there were several mid to neoarchaean terrane assembly events with superimposed orogenies from c 2950 ma until after 2700 ma although the f ringehavn and isukasia terranes were incorporated into the later archaean terrane collage at different times they might be fragments from a larger palaeoarchaean complex rifted apart from c 3500 ma onwards

new u pb age constraints for the laxford shear zone nw scotland evidence for tectono magmatic processes associated with the formation of a paleoproterozoic supercontinent the lewisian gneiss complex in north west scotland is a part of the extensive network of archaean cratonic areas around the margins of the north atlantic it is considered to be made up of a number of terranes with differing protolith ages which have been affected by a range of different metamorphic events a major shear zone the laxford shear zone forms the boundary between two of these terranes new dates presented here allow us to constrain the timing of terrane assembly related to the formation of palaeoproterozoic supercontinents early deformation along the laxford shear zone and primary accretion of the two terranes occurred during the inverian event at c 2480 ma this was followed by extension and the intrusion of the mafic scourie dykes subsequently renewed silicic magmatic activity occurred at c 1880 ma producing major granite sheets considered to have formed as part of a continental arc a further collisional event began at c 1790 ma and was followed by slow exhumation and cooling this laxfordian event caused widespread crustal melting metamorphism and deformation and is considered to represent the final assembly of the lewisian gneiss complex within the major supercontinent of columbia or nuna

geochemical and petrographic variations in pseudotachylyte along the outer hebrides fault zone scotland the islands of the outer hebrides scotland host a c 200 km long fault zone that was active between 1100 and 250 ma during which time a belt of fault rock several kilometres wide was created the fault zone hosts crush breccias phyllonite mylonite and pseudotachylyte the latter unit is often found in major fault zones and within meteorite impact structures although pseudotachylyte is widely believed to be a product of frictional melting within a fault zone there is still controversy over the specific mechanisms of formation pseudotachylyte was collected from the southern block of the outer hebrides fault zone a geochemical mixing model was applied to reconstruct the geochemistry of the pseudotachylyte matrix using the locally available lithologies the results show that the pseudotachylyte could have been derived from the local country rock although some mixing was necessary to provide accurate reconstructions suggesting a degree of mixing between melts derived from different protoliths petrographic observations identified additional characteristics of melt transportation including transported clasts aligned fragments melt segregation and injection of the melt into tensile fractures the latter probably served as the transport mechanism drawing melt into the low pressure tensile structures that developed adjacent to the melt forming zones within the fault plane supplementary material sample site co ordinates and electron microprobe settings are available at https doi org 10 6084 m9 figshare c 4567931

a description of coads surface meteorological fields and the implied sverdrup transports for the atlantic ocean from 30 s to 60 n abstract using coads data spanning 1947 1988 we describe the regional nature of the atlantic ocean wind driven circulation between 30 8 and 60 n and its annual and interannual variability the sverdrup streamfunction defines the circulation gyres our focus is on three central gyres the northern hemisphere anticyclonic subtropical gyre the cyclonic tropical gyre just north of the equator and the clockwise equatorial gyre straddling the equator this rendition of the sverdrup streamfunction computed with constant drag coefficient and air density compares favorably with that from other climatologies in the straits of florida analyses suggest that differences between the annual cycle in sverdrup transport and observations may be due to regional winds farther north in the tropical gyre the sverdrup circulation argues against a continuous western boundary current transporting water from the equatorial region into the caribbean in boreal winter bringing to question the mechanisms for the known interh

post depositional movement of methanesulphonic acid at law dome antarctica and the influence of accumulation rate a series of ice cores from sites with different snow accumulation rates across law dome east antarctica was investigated for methanesulphonic acid msa movement the precipitation at these sites up to 35 km apart is influenced by the same air masses the principal difference being the accumulation rate at the low accumulation rate w20k site 0 17 m ice equivalent msa was completely relocated from the summer to winter layer moderate movement was observed at the intermediate accumulation rate site 0 7 m ice equivalent dome summit south dss while there was no evidence of movement at the high accumulation rate de08 site 1 4 m ice equivalent the main dss record of msa covered the epoch ad 1727 2000 and was used to investigate temporal post depositional changes co deposition of msa and sea salt ions was observed in the surface layers outside of the main summer msa peak which complicates interpretation of these peaks as evidence of movement in deeper layers a seasonal study of the 273 year dss record revealed msa migration predominantly from summer into autumn in the up core direction but this migration was suppressed during the tambora 1815 and unknown 1809 volcanic eruption period and enhanced during an epoch 1770 1800 with high summer nitrate levels a complex interaction between the gradients in nss sulphate nitrate and sea salts which are influenced by accumulation rate is believed to control the rate and extent of movement of msa

a coastal transect of mcmurdo dry valleys antarctica snow and firn marine and terrestrial influences on glaciochemistry samples of snow and firn from accumulation zones on clark commonwealth blue and victoria upper glaciers in the mcmurdo dry valleys 77 788 s 161 1648 e antarctica are evaluated chemically and isotopically to determine the relative importance of local site specific factors vs regional scale influences in defining glaciochemistry spatial variation in snow and firn chemistry confirms documented trends within individual valleys regarding major ion deposition relative to elevation and to distance from the coast sodium and methylsulfonate ms for example follow a decreasing gradient with distance from the coast along the axis of victoria valley 350 119 gl 1 for na 33 14 gl 1 for ms a similar pattern exists between commonwealth and newall glaciers in the asgaard range when comparing major ion concentrations e g na m s c a 2 or trace metals e g al fe among different valleys however site specific exposures to marine and local terrestrial chemical sources play a dominant role because chemical signals at all sites respond to particulates with varying mixtures of marine and terrestrial sources each of these influences on site glaciochemistry must be considered when drawing temporal climate inferences on regional scales

comment to paper timing of magmatism and metamorphism in the gruinard bay area of the lewisian gneiss complex comparison with the assynt terrane and implications for terrane accretion by g j love p d kinny and c r l friend contr mineral petrol 2004 146 620 636

field and petrographic evidence for partial melting of ttg gneisses from the central region of the mainland lewisian complex nw scotland the central region of the mainland lewisian complex is dominated by granulite facies tonalite trondhjemite granodiorite ttg gneisses that are highly depleted in some mobile trace elements cs rb th and u relative to amphibolite facies ttg gneisses elsewhere in the lewisian complex and to the average composition of ttg gneisses worldwide over almost half a century of research there has been vigorous debate as to the origin of this depletion in particular with respect to the role of partial melting and melt loss here we provide field and petrographic evidence that ttg gneisses across the central region partially melted during granulite facies badcallian metamorphism partial melting occurred largely by fluid absent incongruent reactions consuming plagioclase quartz hornblende and biotite to produce melt and peritectic clino and orthopyroxene the preservation of dry granulite facies assemblages requires loss of melt consistent with the presence of an interconnected network of leucosomes and larger felsic sheets that probably record segregation and transfer of melt to higher crustal levels regardless of whether or not partial melting and melt loss can explain fully the unusual geochemical signature of the central region ttg gneisses these fundamental processes did occur supplementary material figure s1 showing additional field photographs of boulders from poll eorna is available at www geolsoc org uk sup18580

a critical analysis of the structure and tectonic significance of rift oblique lineaments transfer zones in the mesozoic cenozoic succession of the faroe shetland basin ne atlantic margin abstract nw se trending rift oblique lineaments transfer zones9 occur along the length of the ne atlantic margin previous workers have suggested that these lineaments played an important role in providing conduits and or barriers to sedimentation during the cretaceous and palaeocene it has also been suggested that they were active as discrete basin wide strike slip faults this study uses a well calibrated 3d seismic survey of regional extent to critically assess the structural and stratigraphic evidence for three rift oblique lineaments in the uk sector of the faroe shetland basin victory clair and judd lineaments structures previously attributed to basin wide strike slip deformation can be more simply explained as igneous intrusions hydrothermal vent complexes gas chimneys and or faults that transfer extensional strain between en echelon rift segments there is little evidence to suggest that activity along discrete basin wide lineaments controlled palaeocene sedimentation within the faroe shetland basin rather sediment transport and deposition at this time are likely to have been controlled by along and across strike variations in the magnitude of thermal subsidence which in turn reflect the 3d nature of the underlying mesozoic rift architecture

timing of magmatism and metamorphism in the gruinard bay area of the lewisian gneiss complex comparison with the assynt terrane and implications for terrane accretion reply u pb zircon data from selected gneiss samples provide estimates of the timing of igneous protolith emplacement and granulite facies metamorphism in the gruinard bay badcall bay and lochinver areas of the mainland lewisian gneiss complex of north west scotland zircons from the gneisses contain complex zoning patterns making them amenable to shrimp ion microprobe analysis with inherited zircon distinguishable from magmatic growth and metamorphic recrystallisation from gruinard bay a tonalitic granulite yielded a protolith age of 2 825 8 ma for oscillatory zoned zircon cores with recrystallised low u rims preserving an age of 2 733 12 ma an inherited 2 905 15 ma age component was recognised in a sample of trondhjemitic gneiss the protolith of which is estimated to have been emplaced at 2 858 11 ma low u rims yielding an age of 2 729 20 ma are also present which together with the rim age from the granulite are considered to date granulite facies metamorphism in the gruinard region for comparison with the gruinard bay gneisses a trondhjemitic partial melt and trondhjemite sheet were analysed from the granulite facies rocks at badcall bay in the assynt terrane age data do not show definitive relationships with zoning patterns in the zircons and show a spread along concordia typical of previous ion microprobe age data from the scourie gneisses both samples do however show a significant isotopic disturbance at 2 480 ma the most recent estimate for the badcallian metamorphism protolith ages for components of the assynt terrane range between ca 2 900 and 3 030 ma and appear to be older than the 2 860 2 825 ma gneiss protoliths from the gruinard bay area the timing of granulite facies metamorphism also appears to be different between the two areas suggesting that the two granulite regions of the mainland may not be related as previously thought instead they are interpreted in terms of a terrane accretion model whereby the two areas evolved as separate blocks prior to their juxtaposition during the palaeoproterozoic the boundary between the two terranes is thought to lie to the south of lochinver corresponding with the strathan line because of the observed 2 480 ma age signature to the north and 2 730 ma signature to the south by defining a new mainland terrane the gruinard terrane the number of mainland blocks which includes the assynt central rhiconich northern and southern terranes has been expanded

discussion a unifying model for the torridon group early neoproterozoic nw scotland product of post grenvillian extensional collapse by ge williams and j foden the torridonian succession in nw scotland is a well exposed sequence of sedimentary rocks that record the early neoproterozoic geological evolution of laurentia an excellent review by stewart 2002 summarised the observations on the three main torridonian units the stoer sleat and torridon groups the torridon group deposited between 1000 950 ma was initially interpreted as a rift basin stewart 1982 and williams 2001 but a number of studies rainbird et al 2001 kinnaird et al 2007 and krabbendam et al 2008 have now concluded that it represents a foreland basin positioned in front of the grenville orogen 1200 960 ma in a recent paper in earth science reviews williams and foden 2011 propose instead that the torridon group was deposited in an extensional collapse basin we question this conclusion and take this opportunity to highlight aspects of their new interpretation that we find equivocal

high precision u pb dating of complex zircon from the lewisian gneiss complex of scotland using an incremental ca id tims approach abstract a novel approach of thermally annealing and sequentially partially dissolving single zircon grains prior to high precision isotope dilution thermal ionization mass spectrometry id tims is presented this technique is applied to complex zircon from the precambrian lewisian gneiss complex of scotland up to six partial dissolutions were conducted at incrementally higher temperatures and analysed at each successive step id tims analyses reveal the portions of zircon affected by the lowest temperature partial dissolution step have suffered pb loss successively higher temperature partial dissolution steps yield a series of analyses from the younger domains followed by mixing trajectories with older components presumably from the inner domains specifically for a partially retrogressed granulite tonalite gneiss from the central block assynt high grade metamorphic zircon ages of c 2500 ma and c 2700 ma are resolved with a protolith age of c 2860 ma also recognised this unequivocally demonstrates two separate episodes of high grade metamorphism affected rocks from this region the c 2700 ma age provides a minimum age constraint on the highest pressure event known from archean crustal rocks using this technique of pseudo spatial resolution coupled with high precision analysis it is possible to recognise discrete pb loss and multiple stages of zircon growth or isotopic resetting within single grains to within 0 1 0 2 error 2 on individual 207pb 206pb ages this method has relevance to u pb zircon geochronology where conventional micro beam techniques are unable to resolve between separate ages within single grains

ion concentrations in ice wedges an innovative approach to reconstruct past climate variability abstract for atmospheric gases and aerosols snow and ice provide a useful archive of paleoenvironmental history in the northern hemisphere this archive has been largely limited to greenland ice but promising pioneering work has been done recently using chronology of an ice wedge in barrow northern alaska here we investigate past aerosols at the same ice wedge sampling site reconstructing the sea ice fluctuations in the adjacent beaufort sea during the bolling allerod ba and younger dryas yd periods we confirm the integrity of methanesulfonate ion ms concentrations in the biws as marine proxies and then find that the ice wedge has a high ms concentration through the beginning coldest yd periods the high ms concentration indicates that even during the coldest yd periods 12 900 12 700 yrbp the near shore region in the alaskan beaufort sea near barrow may not have been completely filled by permanent sea ice

multiple post svecofennian 1750 1560 ma pegmatite dykes in archaean palaeoproterozoic rocks of the west troms basement complex north norway geological significance and regional implications abstract the west troms basement complex north norway consist of meosarchaean neoarchaean and palaeoproterozoic crust which likely represent the continuation of the karelian craton of the fennoscandian shield beneath and across the caledonian nappes the complex is transected by several meta sedimentary volcanic belts with ages from ca 2 7 to 1 9 ga and linear ductile shear zones that record polyphase deformation after emplacement of several late svecofennian plutonic complexes 1 80 1 79 ga the astridal supracrustal belt of the senja shear zone was intruded by various generations of granite pegmatite dykes which yield zircon and titanite u pb ages ranging between 1758 and 1562 ma with evidence of a weak caledonian overprint at about 400 ma that formed or reset some but not all titanites the age range indicates that formation of the pegmatites occurred after termination of the main svecofennian orogeny 1 92 1 79 ga of the karelian craton comparison within the larger scale context of the fennoscandian shield suggests that emplacement of these pegmatites may have been caused by late stage reactivation of intracratonic shear zones possibly reflecting distal effects of deeper magmatic processes such as intrusion of coeval aged rapakivi granite suites ca 1 65 1 5 ga in finland and or the mesoproterozoic gothian orogeny and magmatism affecting the southwestern parts of fennoscandia the mesoproterozoic ages however also correspond to late laxfordian ca 1 7 1 6 ga tectonic events in the lewisian basement on the laurentian margin of scotland suggesting a potential early link between these basement complexes

3d characterization of fracture systems using terrestrial laser scanning an example from the lewisian basement of nw scotland fractured gneiss lithologies form a basement cored high the rona ridge in the faroe shetland basin basement structures are known to play an important role in the petroleum system for the overlying giant clair field an onshore analogue exposure in the lewisian gneiss complex at kinlochbervie in nw scotland provides an example of a hanging wall damage zone of a large basement hosted normal fault in this study we used remote sensing 2d outcrop line sample methods 1d and a virtual outcrop created by terrestrial laser scanning methods 3d to characterize spatial variations of the fracture systems spacing distributions from 1d line samples collected from exposures and pseudo wells constructed through the virtual outcrop show power law distributions the virtual outcrop data enable us to extend the scale invariant description from 1 to 3 orders of magnitude we developed a novel box counting workflow to provide an assessment of 2 and 3d variations in the fracture properties fracture density and fractal dimension are elevated whereas the number of intersections is decreased within a 220 m wide volume adjacent to the fault we discuss how the methods and results from this study can aid the development of analogue for basement reservoirs in the offshore uk continental shelf

migration of methane sulphonate in antarctic firn and ice we investigate the seasonal relationship of the sulphur bearing anions methane sulphonate msa and non sea salt sulphate nssso42 in sections of firn and ice cores from the antarctic peninsula and weddell sea region of antarctica in cores from dolleman island and berkner island msa has clearly migrated from the summer snow layer where it is initially deposited to become concentrated in the winter layer a similar behavior is evident in a core from the dyer plateau though in deeper layers cores from gomez nunatak and beethoven peninsula show little evidence of relocation of msa though migration at greater depth in the ice sheet cannot be ruled out in contrast in all these cores non sea salt sulphate remains in the summer layer from comparisons between the ice core characteristics and the migration behavior at these sites we conclude that the movement of msa does not occur via percolation and refreezing of meltwater simple concentration driven diffusion is also not a factor as the msa peaks are sharp in the winter layer the data presented indicate that the movement of msa in firn is likely to be linked to the concentration of other ionic species in the snowpack and to the snow accumulation rate a possible mechanism for the migration of msa in the snowpack is via an initial diffusion in the liquid or vapor phase which is halted by trapping in the winter layer when the msa forms a salt with a cation

velocity and temperature observations during the seasonal response of the equatorial atlantic experiment at 0 28 w continuous 2 7 year time series of upper ocean currents and temperature at 0 28 w collected during the seasonal response of the equatorial atlantic experiment are presented the thermocline underwent distinctive annual cycles in response to forcing by the surface wind stress and the core of the equatorial undercurrent tracked the thermocline while the vertical position of the thermocline and the undercurrent appear to be simply related the transport of the undercurrent did not show a replicating annual cycle and the speed at the undercurrent core remained relatively constant near surface flow when averaged over the 2 7 years was not statistically different from zero and a westward flowing south equatorial current was rarely established despite strong easterly wind stress in all 3 years cross equatorial flow was also zero in the mean at this location

laboratory study of the migration of methane sulphonate in firn two 10cm lengths of firn core from the dyer plateau 700 3l s 65 0l w antarctic peninsula were used to carry out a laboratory experiment to investigate the migration of methane sulphonate the anion of methane sulphonie acid msa in natural fim each length was cut vertically into tour pieces and a dopant solution containing cl no3 so4 2 f and msa pipetted onto the top of three sections the fourth being kept as a blank the doped sections were stored vertically with the doped end uppermost for r months at a range of temperatures nominally 5 10 and 22 c before subsampling at 1em resolution and analysis by ion chromatography the two firn lengths were treated identically and the results were consistent profiles of the doped fim sections showed that cl no3 so4 2 remained in the uppermost subsample although the no3 concentrations were variable compared to the blank the yo profile shows slightly elevated concentrations in the second sample down compared to the blank at temperatures of l0 c and above the msa showed higher concentrations in the second third and fourth samples down at 10 c and above which indicates that some percolated downwards from its original position at the top of the core this experiment shows that msa is mobile in warm fim even over a short period of time we propose that the mechanism tor the mobility of msa in natural fim is via liquid msa drainage though we cannot yet discount vapour phase transport

the mean and annual cycle of upper layer temperature fields in relation to sverdrup dynamics within the gyres of the atlantic ocean using 28 years of expendable bathythermograph data 1967 1994 we describe the mean and annual cycle of the upper ocean temperature fields in the atlantic from 30 s to 50 n in the context of the basin scale wind driven gyres sverdrup stream function field which provide a framework for describing the oceanographic measurements we examine the circulation field implied by the temperature distributions which are used as a proxy for the field of mass similarities between the temperature and stream function fields increase with depth in the lower to subthermocline depths of the tropical and equatorial gyres the zonal currents form a closed circulation a southeastward boundary current is suggested near and below 150 m that provides closure for the tropical gyre and the equatorial gyre axis is southward of that suggested by the stream function field higher in the water column the north equatorial countercurrent necc may be a surface manifestation of the north equatorial undercurrent neuc where the latter can be interpreted as the southern limb of the tropical gyre because there are large vertical shears in the tropics the equatorial gyre is not clearly indicated in the vertically integrated temperature field but appears below about 200 m here the south equatorial undercurrent seuc can be interpreted as the eastward flowing northern limb of the equatorial gyre and is opposite in direction to the westward flowing south equatorial current above both the neuc and seuc are analagous to currents in the pacific that are governed by non sverdrup dynamics despite the shortcomings of the data the mean annual cycle appears to be relatively stable and we have discounted the possibility that in regions where it represents a significant percentage of the total variance it is changing slowly over the 28 years of record the wind forcing fields which undergo large meridional movements 5 6 of latitude during their annual cycle with some exceptions have essentially no counterpart in gyre movements between their seasonal extremes most of the variability associated with the annual cycle is confined to the upper 300 m greatest variability where ranges exceed 6 c occurs in the northwestern atlantic in late winter and early spring during this time of year south of the gulf stream and below about 100 m water temperatures exhibit a systematic phase lag with depth the next largest area of variability where ranges can also exceed 6 c resides in the tropical western basin between the equator and 10 n just below 100 m in the eastern basin ranges decrease and shoal additionally the phase fields are consistent with the intensification and relaxation of the tropical ridge trough system where the necc disappears in march in the west but the necc neuc complex is strongest in september

the preservation of methanesulphonic acid in frozen ice core samples ice core records of methanesulphonic acid msa provide a potentially powerful tool for producing proxy records of sea ice a critical but poorly understood component of the earth s climate system however msa is able to diffuse through solid ice and here we examine the effect of two different methods of frozen storage on the preservation of msa in archived ice core samples re analysis of archived ice sticks confirms that msa diffuses out of ice cores archived in this manner despite msa losses of up to 39 after 7 years storage the ice sticks studied here preserve much of the variability of the original msa record suggesting that useful proxy records can be obtained from archived ice sticks furthermore re analysis of ice core samples that had been refrozen into discrete bottled samples for storage demonstrates that it is possible to archive ice samples in a way that prevents msa loss in this case accurate records of msa variability and concentration were preserved even over storage periods of 15 years this has important implications for the storage of ice cores and subsequent determination of msa and demonstrates that ice storage history needs to be considered when interpreting msa records

mesoarchaean collision of kapisilik terrane 3070ma juvenile arc rocks and 3600ma isukasia terrane continental crust greenland abstract the mesoarchaean kapisilik and eoarchaean isukasia terranes in the nuuk region of southern west greenland were tectonically juxtaposed in the archaean the north of the isukasia terrane is distal from the kapisilik terrane and has only rare growth of 2690 ma metamorphic zircon and no 2980 2950 ma metamorphic zircon the southern part of the isukasia terrane lies between two 2690 ma shear zones and has locally preserved high pressure granulite facies assemblages and widespread growth of 2980 2950 ma metamorphic zircon and also sporadic growth of 2690 ma metamorphic zircon within this southern part of the isukasia terrane there is a folded klippe of mylonitised mesoarchaean detrital meta sedimentary rocks carrying 3600 and 3070 ma detrital zircons mafic and ultramafic rocks with 2970 ma metamorphic zircon overgrowths south of the isukasia terrane is the kapisilik terrane containing 3070 ma arc related volcanic rocks gabbro anorthosites and meta tonalites intruded by 2970 2960 ma granites zircons of an ivis rtoq supracrustal belt 3075 ma intermediate volcanic rock have initial hf values of 2 to 5 thus are juvenile crustal additions 3070 ma tonalites along the northern edge of the kapisilik terrane have whole rock positive initial nd values and thus are also juvenile crustal additions in contrast igneous zircons in 2960 ma granites intruded into juvenile 3075 ma supracrustal rocks of the kapisilik terrane have initial hf values of 5 to 10 and must have involved the partial melting of 3600 ma isukasia terrane rocks the integrated structural and zircon u th pb hf isotopic data show that at 2980 2950 ma the kapisilik terrane juvenile arc components collided with and over rid the isukasia terrane the southern edge of the isukasia terrane came to lie in the deep crust under the ivis rtoq supracrustal belt and melted at 2970 2960 ma to produce granites these granites derived from ancient crust rose into the upper crust where they intruded the overlying allochthonous juvenile 3075 ma ivis rtoq supracrustal belt arc assemblages the southern edge of the isukasia terrane is interpreted as an interior nappe of eoarchaean basement rocks interfolded with a klippe of mesoarchaean metasedimentary and mafic ultramafic rocks both of which are affected by 2980 2950 ma metamorphism the mixed eoarchaean mesoarchaean detrital provenance suggests that the klippe could be dismembered components of an accretionary prism or forearc crust the northern part of the isukasia terrane is interpreted as foreland free of 2980 2950 ma high grade metamorphic overprint this shows that the isukasia terrane is not a coherent block but contains ancient rocks that are parautochthonous or allochthonous to each other with contrasting later metamorphic history at 2690 ma the crustal architecture arisen from mesoarchaean collision between an older continental block and an island arc was reworked along intra crustal shear zones coeval with amphibolite facies metamorphism this reworking followed on from major terrane assembly at 2710 2700 ma in the southern part of the nuuk region when the eoarchaean faeringehavn terrane was juxtaposed with 2840 2825 ma arc rocks thus the 2980 2950 ma assembly of the isukasia and kapisilik terranes is distinct from the later 2710 2700 ma terrane assembly further south in the nuuk region

archaean ttgs as sources of younger granitic magmas melting of sodic metatonalites at 0 6 1 2 gpa two natural low k2o na2o ttg tonalitic gneisses one hornblende bearing and the other biotite bearing were partially melted at 0 8 1 2 gpa fluid absent the chief melting reactions involve the breakdown of the biotite and hornblende the hornblende tonalite is slightly less fertile than the biotite tonalite but melt volumes reach around 30 at 1 000 c this contrasts with results of most previous work on more potassic ttgs which generally showed much lower fertility though commonly producing more potassic melts garnet is formed in biotite bearing tonalitic protoliths at p 0 8 gpa and at 1 0 gpa in hornblende bearing tonalitic protoliths all fluid absent experiments produced peraluminous granitic to granodioritic melts typically with sio2 70 wt for the biotite tonalite increasing t formed progressively more melt with progressively lower k2o na2o however the compositions of melts from the hornblende tonalite do not vary significantly with t with increasing p melts from the biotite tonalite become less potassic due to the increasing thermal stability of biotite for the hornblende tonalite again there is no consistent trend fluid absent melting of sodic ttgs produces melts with insufficient k2o to model the magmas that formed the voluminous late potassic granites that are common in archaean terranes reconnaissance fluid present experiments at 0 6 gpa imply that h2o saturated partial melting of ttgs is also not a viable process for producing magmas that formed these granites the protoliths for these must have been more potassic and less silicic nevertheless at granulite facies conditions sodic ttgs will produce significant quantities of broadly leucogranodioritic melt that will be more potassic than the protoliths upward abstraction of this melt would result in some lile depletion of the terrane younger k rich magmatism is unlikely to represent recycling of ttg crust on its own and it seems most likely that evolved crustal rocks and or highly enriched mantle must be involved

the nature and significance of the faroe shetland terrane linking archaean basement blocks across the north atlantic abstract core samples of the continental basement rocks that underlie the eastern faroe shetland basin fsb and its inshore margins west of shetland reveal a suite of predominantly granodioritic to granitic orthogneisses including ttg together with lesser volumes of foliated granitoids and subordinate dioritic to mafic gneisses and amphibolites a small area of lithologically similar gneisses also crops out onshore at north roe uyea west of the caledonian front in shetland in the core samples coarse grained gneissose textures and mineralogies consistent with upper amphibolite facies metamorphism are overprinted by a weak but ubiquitous static greenschist facies retrogression later structures include widespread epidote quartz veining and local developments of mylonite cataclasite pseudotachylite and phyllonite regions associated with the rona ridge oil fields e g clair lancaster also preserve extensive brittle fracturing and associated low temperature hydrothermal mineralization quartz adularia calcite pyrite chalcopyrite with significant fracture hosted hydrocarbons new and published u pb zircon analyses from the gneisses offshore give a relatively narrow range of neoarchaean protolith ages ca 2 7 2 8 ga spread over a geographic area of 60 000 km2 west of shetland detrital zircon data from overlying triassic cretaceous sedimentary sequences in the fsb suggest an equivalent limited range of neoarchaean source materials hf isotopic data indicate involvement of palaeo to mesoarchaean crustal sources overall our findings suggest that a major phase of neoarchaean crustal formation and associated high grade metamorphism dominates basement rocks in the region they are similar in age and lithology to the protoliths of the nearest onshore lewisian complex of nw scotland rhiconich terrane however they lack geochronological or textural evidence for the widespread proterozoic laxfordian events ca 1 7 1 8 ga which are widespread in scotland this suggests that the precambrian rocks west of shetland the faroe shetland terrane can be correlated with the archaean rocks of the central greenland rae craton and that a northern limit of proterozoic reworking lies just offshore from the north coast of scotland

new sims u pb zircon ages from the langavat belt south harris nw scotland implications for the lewisian terrane model secondary ionization mass spectrometry sims u pb dating of zircon has been applied to rocks of the lewisian complex of south harris outer hebrides nw scotland yielding insights into the complex geological evolution of this region and enhancing our understanding of terranes in the lewisian gneiss complex results show that tonalite trondhjemite granodiorite magmatism occurred in the tarbert terrane between c 2850 and 2830 ma equivalent to similar rocks in other parts of the outer hebrides magmatism was accompanied or closely followed by high grade metamorphism at c 2830 ma the first reported occurrence of such an event at this time these rocks were affected by a subsequent high grade metamorphic event at c 2730 ma which has correlatives both in the outer hebrides and in the mainland lewisian gruinard terrane these data suggest that the terranes forming the outer hebrides and parts of the mainland lewisian were once contiguous and were fragmented by post 2730 ma events the c 1700 1660 ma ages from felsic sheets and pegmatites within the langavat shear zone may record the reassembly of these crustal fragments

new shrimp u pb zircon ages and chime monazite ages from south harris granulites lewisian complex nw scotland implications for two stages of zircon formation during palaeoproterozoic uht metamorphism abstract sensitive high resolution ion microprobe shrimp u pb zircon age dating was applied to several types of granulites and gneisses from the lewisian complex in south harris nw scotland to clarify the timing of palaeoproterozoic tectono thermal events for comparison monazite was dated using the chemical th u total pb isochron method chime the obtained u pb zircon age data were evaluated under the constrains of the modal amount of constituent minerals and whole rock compositions of analysed samples the zircon internal texture based on th u ratios and the monazite chime ages zircons in three samples characterised by zoned overgrowth rims sector zoned grains and structureless high u cores yielded concordant ages of approximately 1900 ma 1909 3 ma 1899 10 and 1888 15 ma respectively the age of 1899 10 ma obtained from melanocratic granulite garnet and kyanite sillimanite 76 91 vol is interpreted to represent the timing of zircon crystallisation prior to the formation of restitic garnet and kyanite sillimanite therefore the age of 1900 ma is proposed to represent the timing of early metamorphism close to the thermal peak m1 in the leucocratic gneiss sample 93921 7 the age population at 1868 35 ma obtained from low u and low th overgrowth rims represents a second stage of zircon growth from crystallising anatectic melt m3 whereas the high u cores represent the timing of zircon growth and recrystallisation prior to anatexis monazites from two leucocratic gneisses yield a relatively narrow range of chime ages 1858 7 ma and 1840 7 ma these ages probably represent the timing of zircon and monazite crystallisation from partial melt and resetting of the u th pb system these results suggest that the timing of zircon formation was controlled by metamorphic processes such as restite formation and crystallisation from partial melt

equatorial atlantic velocity and temperature observations february november 1981 abstract upper ocean velocity and temperature measurements were obtained in the central equatorial atlantic using surface moored current meters from february to november 1981 distinct seasonal variations were observed in the zonal momentum and temperature on the equator of both the surface south equatorial current sec and the subsurface equatorial undercurrent euc after initially intensifying during boreal spring the sec halted abruptly in early summer and the signature associated with this change progressed at the observational depth of 100 m both speed and temperature increased during early summer with eastward progression and then decreased in fall higher frequency motions were also modulated by the seasonal cycle notable were the 2 3 week time scale oscillations of the meridional velocity component which appeared most energetically during the summer and fall seasons oscillations generally tended to be anisotropic and inhomogeneous vertical advection of zonal momentum was an important term i

tropical instability waves at 0 n 23 w in the atlantic a case study using pilot research moored array in the tropical atlantic pirata mooring data temperature salinity velocity and wind from a mooring at 0 n 23 w are used along with satellite data for sea surface temperature and sea level to examine the contribution of tropical instability waves tiws to the energy and heat balance of the equatorial atlantic mixed layer the tiws appear as periodic 20 30 day fluctuations of currents temperature and salinity which intensify beginning in june and peak in late boreal summer the intensification occurs in phase with strengthening of the southeasterly trade winds and the seasonal appearance of the equatorial tongue of cold mixed layer temperatures in 2002 these waves which warm the mixed layer by 0 35 c during summer months are maintained by both barotropic and baroclinic conversions that are of comparable size salinity fluctuations previously neglected increase the magnitude of baroclinic energy conversion

high spatial resolution 40ar 39ar dating of pseudotachylites geochronological evidence for multiple phases of faulting within basement gneisses of the outer hebrides uk abstract pseudotachylites within the neoarchaean to palaeoproterozoic basement of the lewisian gneiss complex have been analysed using the high spatial resolution 40 ar 39 ar uv laserprobe technique it has been possible to measure 40 ar 39 ar age and apparent9 age variations that would be beyond resolution by lower spatial resolution or bulk 40 ar 39 ar dating methods we report a range of complexities including significantly younger narrow 40 ar 39 ar ages for pseudotachylite forming events in lewis the c 1900 ma event may relate to early stages of laxfordian reworking in barra and south uist a 1300 1200 ma event is recorded whereas in barra south and north uist a second event is recorded at c 700 ma which is likely to be associated with nw and west directed thrusting and could represent far field effects of tectonic events recorded at this time east of the moine thrust in nw scotland supplementary material 40 ar 39 ar data are available at http www geolsoc org uk sup18372

vertical structure of the seasonal cycle in the central equatorial atlantic ocean xbt sections from 1980 to 1988 abstract a set of temperature profiles from expendable bathythermographs collected from 1980 to april 1988 along two ship routes transecting the equatorial atlantic from 11 n to 11 s is analyzed to inter the vertical structure of the annual variability of the temperature and the currents in the upper ocean during the average seasonal cycle the vertical isotherm displacements occur earlier below 300 meters than near the surface at most locations within 4 degrees of the equator at the equator the amplitude of the displacements does not decrease with depth in the upper 500 meters this still holds down to 700 meters but there are less data at these depths the lead of the deeper isotherm displacements with respect to those in the upper thermocline implies that there is a contribution to the pressure forces from these layers that is not in phase with the contribution of the upper thermocline this also suggests that the energy source of the seasonal variability is close to the surface dynamic height and

the nature and age of mesoproterozoic strike slip faulting based on re os geochronology of syntectonic copper mineralization assynt terrane nw scotland in ancient basement regions such as the lewisian complex nw scotland the ages of brittle deformation events are commonly poorly constrained due to a lack of datable fills an array of nw se sinistral and antithetic e w dextral faults related to a regionally recognized episode of brittle shearing cut neoarchaean gneisses and c 2 25 ga quartz pyrite veins close to the trace of the unexposed regional scale nw se fault copper iron mineralisation occurs at an intersection between an antithetic dextral fault and an older c 2 25 ga quartz vein optical microscopy sem and xrd analyses reveal an array of intergrown co genetic copper iron sulphides hematite and barite complex mm thick zoned alteration rims rich in epidote occur at contacts between the sulphides and gneisses rhenium osmium copper iron sulphide geochronology yields an age of c 1 55 ga for the hydrothermal mineralization event associated with faulting fault movements demonstrably overlap with mineralisation based on the asymmetric fibrous growth forms of these minerals within local dextral shears which acted as local channelways for mineralizing fluids during and after faulting we tentatively propose that this regionally recognised strike slip faulting previously termed the late laxfordian should be referred to as the assyntian in order to distinguish it from kinematically distinct laxfordian events

magnesium methanesulfonate salt found in the dome fuji antarctica ice core using micro raman spectroscopy we identified the chemical forms of methanesulfonate salt particles in reference samples of the dome fuji antarctica ice core we found only ch3so3 2mgnh2o among methanesulfonate salts and this salt particle is most prevalent in the last glacial maximum lgm ice we suggest that during the lgm ch3so3 2mgnh2o may have formed in the atmosphere through the chemical reaction of ch3so3h with sea salts but probably not in the firn and ice due to the neutralization of acid in lgm ice of inland antarctica

triassic sand supply to the slyne basin offshore western ireland new insights from a multi proxy provenance approach to assess provenance data and derive basin infill models with a higher level of certainty it is vital to understand how minerals used in sedimentary provenance behave in the sedimentary system a multi proxy approach helps ensure rigour in how provenance data are interpreted u pb geochronology of refractory zircon and relatively less stable apatite and pb isotopic analysis of labile k feldspar are employed in this study to reassess the provenance of triassic sandstones in the slyne basin offshore western ireland this approach aims at reducing the potential bias in each method producing a complementary dataset u pb zircon and apatite geochronology yields archean paleoproterozoic ages caledonian ages and previously unrecognized permo triassic ages pb isotopic analysis of k feldspar does not identify the permo triassic as a source the detection of permo triassic aged detritus may suggest that volcanism was more widespread than previously recognized these results support the hypothesis of a triassic drainage divide between the basins offshore western ireland and those onshore britain and in the irish sea sand supply was dominantly from the north with significant input from the flanks of the basin and previously unrecorded sources such as permian aged rocks playing an important role the provenance signal is consistent and homogenized throughout the sampled sequence indicating that the drainage system was long lived supplementary material details of laboratory and sample preparation data tables and summary pie charts for each sample are available at https doi org 10 6084 m9 figshare c 4571096

a review of sea ice proxy information from polar ice cores sea ice plays an important role in earth s climate system the lack of direct indications of past sea ice coverage however means that there is limited knowledge of the sensitivity and rate at which sea ice dynamics are involved in amplifying climate changes as such there is a need to develop new proxy records for reconstructing past sea ice conditions here we review the advances that have been made in using chemical tracers preserved in ice cores to determine past changes in sea ice cover around antarctica ice core records of sea salt concentration show promise for revealing patterns of sea ice extent particularly over glacial interglacial time scales in the coldest climates however the sea salt signal appears to lose sensitivity and further work is required to determine how this proxy can be developed into a quantitative sea ice indicator methane sulphonic acid msa in near coastal ice cores has been used to reconstruct quantified changes and interannual variability in sea ice extent over shorter time scales spanning the last 160 years and has potential to be extended to produce records of antarctic sea ice changes throughout the holocene however the msa ice core proxy also requires careful site assessment and interpretation alongside other palaeoclimate indicators to ensure reconstructions are not biased by non sea ice factors and we summarise some recommended strategies for the further development of sea ice histories from ice core msa for both proxies the limited information about the production and transfer of chemical markers from the sea ice zone to the antarctic ice sheets remains an issue that requires further multidisciplinary study despite some exploratory and statistical work the application of either proxy as an indicator of sea ice change in the arctic also remains largely unknown as information about these new ice core proxies builds so too does the potential to develop a more comprehensive understanding of past changes in sea ice and its role in both long and short term climate changes

a reappraisal of the lewisian gneiss complex geochronological evidence for its tectonic assembly from disparate terranes in the proterozoic new u pb single zircon geochronology undertaken on tonalitic gneisses granite sheets migmatites and metasediments from the lewisian gneiss complex on the mainland and the northern part of the outer hebrides nw scotland have been used to test the correlation of so called laxfordian events across the complex from the outer hebrides to the mainland and the current model for the evolution of the complex as a whole the study has revealed that the granite sheets originated in two quite different melting events those on the mainland at loch laxford are ca 1 855 ma old whereas those on harris and lewis with which they are presently correlated are ca 1 675 ma old grey gneisses associated with granites on the south side of loch laxford are confirmed to belong to the northern region a migmatitic grey gneiss on harris has given a protolith age of ca 3 125 ma the currently oldest recognised in the complex detrital zircons in the leverburgh and langavat belts range in age from 2 780 to 1 880 ma and unequivocally demonstrate deposition in the palaeoproterozoic the granulite facies metamorphism in this block is dated from zircon overgrowths at ca 1 880 ma the laxford shear zone which separates the northern and central regions is interpreted to have evolved post 1 860 ma during amphibolite facies metamorphism accompanying deformation which took place at ca 1 740 ma in both regions on harris the langavat finsbay shear zone developed after 1 675 ma when a ca 1 880 ma granulite facies proterozoic arc was juxtaposed against amphibolite facies archaean rocks to the north therefore the shear zones which bound tectonic blocks in the lewisian complex evolved at different times and can be interpreted as terrane boundaries the new data confirm that the lewisian complex was not constructed from one contiguous piece of archaean crust reworked in the proterozoic but was progressively assembled from several discrete terranes during the proterozoic accordingly the former regional divisions of the lewisian complex are here renamed as follows on the mainland the northern region is called the rhiconich terrane and the central region the assynt terrane on the outer hebrides the archaean gneisses of lewis and the northern part of harris comprise the tarbert terrane whereas the newly accreted proterozoic blocks are called the roineabhal terrane in harris and the niss terrane in the north on lewis wider correlations show that the geology of the outer hebrides has more in common with east greenland than mainland scotland on the eastern side of the minch fault

two stages of sapphirine formation during prograde and retrograde metamorphism in the palaeoproterozoic lewisian complex in south harris nw scotland

the geology of the uk hatton rockall margin abstract seismic potential field sea bed sample and shallow drilling data have been used to investigate the geology of the uk part of the rockall plateau most of the rockall plateau is underlain by early proterozoic rockall bank islay terrane metamorphic basement although george bligh bank and the north eastern end of hatton bank may be underlain at depth by lewisian rocks the basement crops out on rockall bank but occurs at a much deeper level beneath hatton bank and hatton basin the widespread palaeogene lavas of rockall plateau have been sourced from large central igneous complexes rockall swithin sandastre mammal lyonesse and sandarro local volcanoes and fissures some of the lavas may also have been derived from the incipient rift axis west of hatton bank at the time of continental break up differences in seismic facies allow the extent of the lavas from the large centres to be mapped where the lavas are absent beneath parts of hatton bank either due to non emplacement or subsequent erosion during the cenozoic seismic data image the underlying mesozoic and older rocks here bgs boreholes 99 1 and 99 2a have proved mid cretaceous albian shales and sandstones respectively beneath a thin cenozoic cover the mesozoic and older rocks appear to be quite extensive and are folded and faulted mesozoic strata are not observed on rockall bank in uk waters the post lava cenozoic succession of rockall plateau can be sub divided into three megasequences separated by two unconformities which by analogy with rockall basin have been designated c30 intra late eocene and c10 intra early pliocene the ages can be corroborated by dsdp odp boreholes in the hatton basin the cenozoic succession contains eocene prograding fan sequences contourite deposits on the flanks of the banks and shallow faulting some of which affects the present day sea bed structurally rockall high appears mainly as a large basement block whereas hatton high is more complex exhibiting half grabens inversion structures and reverse fault geometries due to cenozoic tectonism the northern end of hatton high is a large anticline with its axis aligned ene wsw the structural differences between the highs may be due to their locations with rockall high exhibiting fewer tectonic effects due to its more landward intraplate setting further away from plate margin deformation

1 99 ga mafic magmatism in the rona terrane of the lewisian gneiss complex in scotland abstract the scourie dyke swarm has long been important to unravelling the geological history of the lewisian gneiss complex lgc and the north atlantic craton recent dating has documented that the majority of those dykes were emplaced between c 2418 2375 ma here we show that a quartz dolerite dyke in the rona terrane of the lgc has a u pb zircon age of 1989 08 4 3 0 99 ma this is the first mafic dyke to be dated from the rona terrane the southernmost of those proposed for the terrane model of the lgc our new age also overlaps with the c 1992 ma age of a previously dated olivine gabbro dyke in the northern region of the lgc and shows that the lgc contains at least three and possibly four temporally discrete episodes of palaeoproterozoic mafic magmatism the scourie dyke swarm sensu stricto at c 2 4 ga a suite of younger dykes at c 1 99 ga referred to here as the strathan dyke swarm and the c 1 99 1 90 ga mafic rocks that are part of the palaeoproterozoic loch maree group a c 2 04 ga dyke in the assynt terrain may be part of the younger suite of dykes or perhaps records a temporally separate event crucially our new age data demonstrate that suites of mafic dykes emplaced across the mainland lgc are similar in age and supports the correlation of structural and metamorphic features across that complex

re evaluating ambiguous age relationships in archean cratons implications for the origin of ultramafic mafic complexes in the lewisian gneiss complex abstract archean ultramafic mafic complexes have been the focus of important and often contentious geological and geodynamic interpretations however their age relative to the other components of archean cratons are often poorly constrained introducing significant ambiguity when interpreting their origin and geodynamic significance the lewisian gneiss complex lgc of the northwest scottish mainland a high grade tonalite trondhjemite granodiorite ttg terrane that forms part of the north atlantic craton nac contains a number of ultramafic mafic complexes whose origin and geodynamic significance have remained enigmatic since they were first described previous studies have interpreted these complexes as representing a wide range of geological environments from oceanic crust to the sagducted remnants of archean greenstone belts these interpretations which are often critically dependent upon the ages of the complexes relative to the surrounding rocks have disparate implications for archean geodynamic regimes in the nac and globally most previous authors have inferred that the ultramafic mafic complexes of the lgc pre date the ttg magmas this fundamental age relationship is re evaluated in this investigation through re mapping of the geodh nan sgadan complex where tonalitic gneiss reportedly cross cuts mafic rocks and new mapping of the 7 km2 ben strome complex the largest ultramafic mafic complex in the lgc alongside detailed petrography and spinel mineral chemistry this new study reveals that despite their close proximity in the lgc 12 km the ben strome and geodh nan sgadan complexes are petrogenetically unrelated indicating that the lgc and thus nac records multiple temporally and or petrogenetically distinct phases of ultramafic mafic archean magmatism that has been masked by subsequent high grade metamorphism moreover field observations and spinel mineral chemistry demonstrate that the ben strome complex represents a layered intrusion that was emplaced into a ttg dominated crust further to representing a significant re evaluation of the lgc s magmatic evolution these findings have important implications for the methodologies utilised in deciphering the origin of archean ultramafic mafic complexes globally where material suitable for dating is often unavailable and field relationships are commonly ambiguous

basement reservoir plumbing fracture aperture length and topology analysis of the lewisian complex nw scotland upfaulted ridges of neoarchean crystalline basement rocks formed in the faeroe shetland basin as a consequence of mesozoic rift processes and are an active target for oil exploration we carried out a comprehensive fault and fracture attribute study on the extensive exposures of geologically equivalent crystalline basement rocks onshore in nw scotland lewisian gneiss complex as an analogue for the offshore oil and gas reservoirs of the uplifted rona ridge basement high our analysis shows a power law distribution for fracture sizes aperture and length with random to clustered spacing and high connectivity indices regional variations between the scottish mainland and the outer hebrides are recognized that compare directly with variations observed along the rona ridge in the faeroe shetland basin here we develop a model for the scaling properties of the fracture systems in which variations in the aperture attributes are a function of the depth of erosion beneath the top basement unconformity more generally the combination of size spatial and connectivity attributes we found in these basement highs demonstrates that they can form highly effective well plumbed reservoir systems in their own right supplementary material additional methods and results are available at https doi org 10 6084 m9 figshare c 5017139 thematic collection this article is part of the the geology of fractured reservoirs collection available at https www lyellcollection org cc the geology of fractured reservoirs

antarctic aerosol and snowfall chemistry implications for deep antarctic ice core chemistry in this paper we first summarise major findings of recent atmospheric studies of nitrogen and sulphur species present in the boundary layer of coastal antarctic regions we then discuss the implications of such atmospheric data for the interpretation of nitrate ammonium methanesulphonate and sulphate records in deep ice cores extracted from central antarctica in terms of past atmospheric chemistry changes

major early thrusting as a control on the palaeoproterozoic evolution of the lewisian complex evidence from the outer hebrides nw scotland new structural metamorphic and geochronological data suggest that the lewisian complex of the outer hebrides can be interpreted as a proterozoic orogen in which a thrust sheet of juvenile arc material was driven over archaean gneisses at an early stage incubation and loading of the underlying archaean gneisses triggered prograde metamorphism and extensive ductile deformation with a strong gravitational flattening component combined with top to the wnw transport large shear zones developed at this time are of local importance but are not fundamental structural boundaries the recognition of early orogenic thrusting followed by extensive ductile deformation shows a close resemblance to the sequence of events recognized within the nagssugtoqidian orogen with which the lewisian complex is correlated as in the nagssugtoqidian orogen younger penetrative ductile deformation often masks the major structural boundaries formed by older thrusts supplementary material detailed structural and lithological maps of the langavat belt and geochronological data and analytical methods description are available at www geolsoc org uk sup18515

structural characteristics and re os dating of quartz pyrite veins in the lewisian gneiss complex nw scotland evidence of an early paleoproterozoic hydrothermal regime during terrane amalgamation in the archaean basement rocks of the assynt and gruinard terranes of the mainland lewisian complex in nw scotland a regional suite of quartz pyrite veins cross cut regional palaeoproterozoic badcallian ca 2700 ma inverian ca 2480 ma fabrics and associated scourie dykes the quartz veins are overprinted by amphibolite greenschist facies laxfordian deformation fabrics ca 1760 ma and later brittle faults the hydrothermal mineral veins comprise a multimodal system of tensile hybrid hydraulic fractures which are inferred to have formed during a regional phase of nw se extension the almost orthogonal orientation of the quartz veins ne sw to the scourie dykes nw se are incompatible and must result from distinct paleostress regimes suggesting they are related to different tectonic events this hypothesis is supported by rhenium osmium dating of pyrite that yields an age of 2249 77 ma placing the vein hosted mineralisation event after the oldest published dates for the scourie dykes 2420 ma but before the youngest ages 1990 ma sulphur isotope analysis suggests that the sulphur associated with the pyrite is isotopically indistinguishable from primitive mantle the presence of the ca 2250 ma quartz pyrite veins in both the assynt and gruinard terranes confirms that these crustal units were amalgamated during or prior to inverian deformation the absence of the veins in the rhiconich terrane is consistent with the suggestion that it was not finally amalgamated to the assynt terrane until the laxfordian

structure of the palaeoproterozoic nagssugtoqidian orogen south east greenland model for the tectonic evolution abstract the palaeoproterozoic nagssugtoqidian orogen extends over 250 km along the east coast of greenland around the settlement of tasiilaq the orogen includes archaean rocks from the adjoining rae craton to the north and the north atlantic craton to the south and palaeoproterozoic rocks the rae craton consists of orthogneiss and amphibolite included in the schweizerland and kuummiut terranes and is tectonically overlain in the kuummiut terrane by ca 2100 2200 ma units that include marble meta pelite and psammite and amphibolite assigned to the helheim and kuummiut units the kuummiut terrane was probably subducted underneath the se trending ca 1885 ma ammassalik intrusive complex which has a high temperature metamorphic halo and is characterised by a change from sinistral faulting to pure shear deformation the southern isortoq terrane consists of medium pressure amphibolite facies bimodal meta volcanic and the rocks of the kuummiut terrane were tectonically imbricated in ene verging structures during ca 1870 ma high pressure metamorphism this was followed by ne sw convergence and close to orthogonal extrusion in the weakened crust which is characterised by partial melting during decompression the rocks of the isortoq terrane were imbricated in a se vergent thrust and ramp system either during oblique subduction of the kuummiut terrane or an earlier tectonic stage elsewhere ne sw compression as in the northern terranes formed sw vergent thrust systems and folds this was most likely caused by a change in the regional stress field during collision between ca 1870 and 1820 ma in the north the schweizerland terrane was juxtaposed to the kuummiut terrane in southeasterly direction causing refolding of earlier structures in the lower amphibolite facies this hinteland type of deformation was possibly related to tectonism in western greenland the latest recognised deformation event was during ca 1740 1680 ma associated with ne sw extension which is interpreted as orogenic collapse the complex structural evolution of the orogen was caused by oblique convergence during wsw directed subduction the convergence of irregularly shaped cratons and the change of the regional stress field from ene wsw to nw se during progressive collisional tectonics between the two archaean cratons

the mesoproterozoic stac fada proximal ejecta blanket nw scotland constraints on crater location from field observations anisotropy of magnetic susceptibility petrography and geochemistry the stac fada member of the mesoproterozoic stoer group torridon supergroup in nw scotland is a proximal ejecta blanket surrounding an unidentified asteroid impact crater a combination of field observations of the ejecta deposit and underlying strata the geographical distribution of terrane identified basement clasts found embedded in the impactite and anisotropy of magnetic susceptibility of the impact melt rocks at different locations can constrain the crater location to be about 15 20 km wnw of enard bay and thus buried under mesozoic sediments in the minch syncompressional structures within the suevite at stattic point give a clear indication of a southeasterly direction of mass motion the signatures of two different terranes within the lewisian gneiss help identify the origin of clasts found in the impactite at three locations these clasts are unshocked and interpreted as having been swept up by the density current post impact their geographical distribution provides an important clue to ejecta emplacement pathways crossing the assynt and gruinard terranes anisotropy of magnetic susceptibility is used to measure flow direction in pyroclastic density current deposits and is applied here to derive a direction of motion for the impactoclastic density current it provides good agreement with the other independent methods

geochronology and geochemistry of the enderbite series in the lapland granulite belt generation tectonic setting and correlation of the belt1nordsim publication no 322 the lapland granulite belt of the northeastern fennoscandian shield is dominated by blastomylonitic migmatitic metagreywackes and a deformed structurally conformable norite enderbite series the b

precise u pb baddeleyite ages of mafic dykes and intrusions in southern west greenland and implications for a possible reconstruction with the superior craton the archaean block of southern greenland constitutes the core of the north atlantic craton nac and is host to a large number of precambrian mafic intrusions and dyke swarms many of which are regionally extensive but poorly dated for southern west greenland we present a u pb zircon age of 2990 13 ma for the amikoq mafic ultramafic layered intrusion fiskefjord area and four baddeleyite u pb ages of precambrian dolerite dykes specifically a dyke located se of ameralik fjord is dated at 2499 2 ma similar to a previously reported ar 40 ar 39 age of a dyke in the kangamiut area for these and related intrusions of ca 2 5 ga age in southern west greenland we propose the name kilaarsarfik dykes three wnw trending dykes of the md3 swarm yield ages of 2050 2 ma 2041 3 ma and 2029 3 ma a similar u pb baddeleyite age of 2045 2 ma is also presented for a se trending dolerite iglusuataliksuak dyke in the nain province the rifted western block of the nac in labrador we speculate that the md3 dykes and age equivalent nne trending kangamiut dykes of southern west greenland together with the iglusuataliksuak dyke after closure of the labrador sea represent components of a single areally extensive radiating swarm that signaled the arrival of a mantle plume centred on what is presently the western margin of the north atlantic craton comparison of the magmatic barcodes from the nain and greenland portions of the north atlantic craton with the established record from the north eastern superior craton shows matches at 2500 ma 2214 ma 2050 2030 ma and 1960 1950 ma we use these new age constraints together with orientations of the dyke swarms to offer a preliminary reconstruction of the north atlantic craton near the north eastern margin of the superior craton during the latest archaean and early palaeoproterozoic possibly with the core zone craton of eastern canada intervening c 2010 elsevier b v all rights reserved less

the initiation and early tectonic significance of the outer hebrides fault zone scotland the outer hebrides fault zone is a major ese dipping detachment exposed within base ment gneisses of the archaean palaeoproterozoic lewisian complex northwest scotland the fault zone exhibits a long lived displacement history and was active during proterozoic end silurian carboniferous and mesozoic times the earliest deformation event preserved onshore was associated with top to the nw ductile thrusting a previous study proposed that thrust related protomylonitic and mylonitic fabrics are cross cut by amphibolites younger basics and laxfordian granite and pegmatite sheets this evidence was used to suggest that ductile thrusting occurred during the palaeoproterozoic inverian event at c 2500 ma our observations demonstrate however that mylonitic fabrics within the ductile thrust zone are superimposed on all components of the gneiss complex including amphibolites and laxfordian intrusions it therefore follows that the outer hebrides fault zone cannot be older than c 1685 ma the age of recently dated laxfordian granites in the outer hebrides geochronological studies have shown that the basement blocks of the northern outer hebrides and scottish mainland have different geological histories and were amalgamated dur ing proterozoic times at or after c 1700 ma we propose that early ductile thrusting along the outer hebrides fault zone formed part of this amalgamation process leading to burial and reheating of the footwall gneisses in lewis and north harris this would account for the c 1100 ma thermal event recorded by previous workers and implies that ductile thrusting along the outer hebrides fault zone is of grenvillian age