paleo and neoproterozoic magmatic and tectonometamorphic evolution of the isla cristalina de rivera nico perez terrane uruguay the isla cristalina de rivera crystalline complex in northeastern uruguay underwent a multistage magmatic and metamorphic evolution based on shrimp u pb zircon th u pb monazite chime epma method and k ar age data from key units several events can be recognized 1 multistage magmatism at 2 171 2 114 ma recorded on zircon of the granulitic orthogneisses and their 2 093 2 077 ma overgrowths 2 a distinct amphibolite facies metamorphism at 1 980 ma recorded by monazite 3 greenschist facies reworking and shearing at ca 606 ma monazite and k ar on muscovite along the rivera shear zone and finally 4 intrusion of the post tectonic sobresaliente and las flores granites at around 585 ma lithological similarities geographic proximity and coeval magmatic and metamorphic events indicate a similar tectonometamorphic evolution for the isla cristalina de rivera the valentines block in uruguay and the santa maria chico granulitic complex in southern brazil since at least 2 1 ga

lewisian crustal history from in situ sims mineral chronometry and related metamorphic textures abstract a combination of detailed petrography and in situ sims 207 pb 206 pb micro chronometry of monazite has been carried out on samples from the lewisian high grade metamorphic terrain northwest scotland two episodes of granulite facies metamorphism have been recognized the thermal maxima of which are estimated at or before 2760 12 ma 2 and at 2526 8 ma respectively pegmatites at scourie crystallised at 2480 ma the amphibolite facies metamorphism of the main laxfordian event is estimated at 1752 12 ma and the formation of kyanite and staurolite is associated with this event the oldest ages of 2760 ma have been obtained from micro inclusions of monazite in garnet it appears that garnet which has a high closure temperature for the u pb system plays an important role in the behaviour of the monazite u pb system it serves to shield monazite from reaction with the other phases and to armour monazine from diffusive pb loss or from resetting

late archaean terrane accretion in the godthab region southern west greenland tectono stratigraphic terranes fault bounded blocks of the earth s crust characterized by a geological history distinct from that of adjacent terranes1 are now widely recognized in phanerozoic and proterozoic orogenic belts and their present configurations are thought to result from plate tectonic processes similar to those in operation today2 4 there has been much debate about whether such processes operated in archaean times5 6 here we present new data from the archaean high grade gneiss complex of the godthab now nuuk region of southern west greenland which show that discrete tectonic units recognized in the south of the region7 can be more widely identified at least four terranes are recognized each of which evolved separately until their juxtaposition in the late archaean this suggests that at least some archaean high grade gneiss complexes may resemble more recent orogenic belts formed by plate tectonic processes

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

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

petrogenesis and chronology of lunar meteorite northwest africa 4472 a kreepy regolith breccia from the moon northwest africa nwa 4472 is a polymict lunar regolith meteorite the sample is kreep rich high concentrations of potassium rare earth elements and phosphorus and comprises a heterogeneous array of lithic and mineral fragments these clasts and mineral fragments were sourced from a range of lunar rock types including the lunar high magnesian suite the high alkali suite kreep basalts mare basalts and a variety of impact crater environments the kreep rich nature of nwa 4472 indicates that the sample was ejected from regolith on the nearside of the moon in the procellarum kreep terrane and we have used lunar prospector gamma ray remote sensing data to show that the meteorite is most similar to and most likely sourced from regoliths adjacent to the imbrium impact basin u pb and pb pb age dates of nwa 4472 phosphate phases reveal that the breccia has sampled pre nectarian 4 35 ga rocks related to early episodes of kreep driven magmatism some younger phosphate u pb and pb pb age dates are likely indicative of impact resetting events at 3 9 4 ga consistent with the suggested timing of basin formation on the moon our study also shows that nwa 4472 has sampled impact melts and glass with an alkali depleted incompatible trace element rich high sc low rb th ratios low k compositional signature not related to typical apollo high k kreep or that sampled by kreepy lunar meteorite sayh al uhaymir sau 169 this provides evidence that there are numerous sources of kreep rich protoliths on the moon

geochronological studies of proterozoic crustal evolution in the northern outer hebrides abstract new data on the proterozoic thermal and tectonic crustal history of the lewisian complex have been determined using sm nd and 40 ar 39 ar isotopic analyses igneous assemblages from two mafic ultramafic masses indicate crystallization at 2 54 and 2 14 ga from magmas with initial nd isotopic composition lower than predicted depleted mantle at those times sm nd isotope systematics t dm 2 45 ga in two samples of metasediment from the leverburgh belt imply that they contain a significant post archaean component and cannot therefore be part of an archaean stratigraphic sequence sm nd mineral isotope data on three samples confirm the proterozoic age of high pressure granulite facies metamorphism in the south harris igneous complex the most precise estimate is 1 827 0 016 ga on a metagabbro which was intruded after deformation of the host anorthosite under granulite facies conditions localized granulite facies assemblages in younger basic bodies further north are significantly younger at 1 63 ga whole rock sm nd data on two samples from the 1 7 ga granite injection complex of south harris indicate nd between 11 5 and 13 7 at the time of intrusion providing a strong indication that they were produced by remelting of pre existing crust similar to the exposed grey gneisses which had values close to 12 at that time 40 ar 39 ar data from 13 hornblendes generally show complex argon release patterns with indications of excess argon in most samples flat sections of the spectra range in age between 1 63 and 1 78 ga previous rb sr data on biotite showed a distinct break in age pattern across the langavat belt there is no indication of such a break in the hornblende data and it is inferred that despite variations in the timing of peak metamorphism the whole of the northern outer hebrides cooled through hornblende closure temperatures in the interval 1 7 1 6 ga

provenance and magmatic tectonic setting of campanian aged volcaniclastic sandstones of the kannaviou formation in western cyprus evidence for a south neotethyan continental margin volcanic arc abstract regionally developed late cretaceous subduction related magmatism in the eastern mediterranean records progressive closure of the southern neotethys in western cyprus the late cretaceous c 90 ma troodos ophiolite is depositionally overlain by volcaniclastic sandstones up to 750 m thick that are dominated by redeposited pyroclastic fallout interbedded with both non calcareous and calcareous radiolarian bearing mudstones the sands were mainly deposited by channelised mass flow processes and to a lesser extent by turbidity currents in a deep water forearc basin the succession was folded and locally thrust deformed related to latest cretaceous emplacement of adjacent mesozoic continental margin and oceanic units mamonia complex sims u pb analysis of euhedral to subhedral magmatic zircon crystals yielded a weighted mean 206pb 238u age of 80 1 1 1 ma campanian sandstone petrography including compositional framework q12f21l67 suggests sediment supply from a volcanic arc subordinate continentally derived detritus suggests a continental margin arc setting for the mafic to felsic fallout ash sims analysis of little altered volcanic glass indicates high th nb and th la ratios that suggest the involvement of continental crust and or subducted terrigenous sediments in magma genesis whole rock chondrite normalised ree patterns have comparable trends to modern and ancient forearc basin volcaniclastic sands and sandstones e g northwest pacific region trace element chemistry although scattered towards the continental island arc fields on some geochemical diagrams e g th sc zr mainly suggests an oceanic island arc source for the kannaviou formation sandstones with variable enrichments in v cr ni sc depletion in nb ta and relatively low trace element ratios e g la co th co la sc th sc an explanation for this apparent discrepancy is that the volcaniclastic sandstones were derived from a relatively primitive arc constructed on previously depleted rifted thinned neotethyan continental crust the inferred continental margin arc developed during early stage northward subduction of the southern neotethys beneath a tauride microcontinent to the north large volumes of volcanic ash were derived from continental margin arc volcanism probably in the vicinity of the kyrenia range in northern cyprus

late archaean tectonics in the faeringehavn tre brodre area south of buksefjorden southern west greenland the faeringehavn tre brodre area consists of three distinct terranes tectonically jux taposed by a previously unrecognized event contacts between the terranes are mylonitic shear zones truncating lithological units in adjacent terranes the terrenes are 1 faeringehavn terrane largely composed of early archaean amitsoq gneiss cut by younger archaean granitic gneiss defined here as the satut gneiss 2 tre brodre terrane comprising mid archaean malene supracrustal rocks anorthosite complex and polyphase nuk gneisses 3 tasiusarsuaq terrane largely comprising mid archaean nuk gneisses affected by c 2800 ma granulite facies metamorphism the tasiusarsuaq terrane is structurally above both the faeringehavn and tre brodre terranes which at c 2800 ma experienced a lower grade of metamorphism juxtaposition of the terranes took place between 2800 and 2500 ma and involved thrusting and crustal thickening subsequent re equilibration involved folding steeply inclined shear belts intrusion of synkinematic granitoids under amphibolite facies conditions and retrogression of granulite facies assemblages this thrusting post dates the 2800 ma granulite facies metamorphism it is younger and distinct from the thrusting postulated to explain the intercalation of the early archaean amitsoq gneisses and the mid archaean malene supracrustal rocks associated with intrusion of the nuk gneiss precursors described from godthabsfjord the tectonic breaks provide for the first time marker horizons which can be used to assess the amount and type of late archaean deformation in the southern godthabsfjord region

metasomatism and the crystallization of zircon megacrysts in archaean peridotites from the lewisian complex nw scotland zircon megacrysts are locally abundant in 1 40 cm thick orthopyroxenite veins within peridotite host rocks in the archaean lewisian gneiss complex from nw scotland the veins formed by metasomatic interaction between the ultramafic host and si rich melts are derived from partial melting of the adjacent granulite facies orthogneisses the interaction produced abundant orthopyroxene and within the thicker veins phlogopite pargasite and feldspathic bearing assemblages two generations of zircon are present with up to 1 cm megacrystic zircon and a later smaller equant population located around the megacryst margins patterns of zoning rare earth element abundance and oxygen isotopic compositions indicate that the megacrysts crystallized from crustal melts whereas the equant zircon represents new neocryst growth and partial replacement of the megacryst zircon within the ultramafic host both zircon types have u pb ages of ca 2464 ma broadly contemporaneous with granulite facies events in the adjacent gneisses zircon megacrysts locally form 10 of the assemblage and may be associated to zones of localized nucleation or physically concentrated during movement of the siliceous melts their unusual size is linked to the suppression of zircon nucleation and increased zr solubility in the si undersaturated melts the metasomatism between crustal melts and peridotite may represent an analog for processes in the mantle wedge above subducting slabs as such the crystallization of abundant zircon in ultramafic host rocks has implications for geochemistry of melts generated in the mantle and the widely reported depletion of high field strength elements in arc magmas

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

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

an example of reverse discordance during ion microprobe zircon dating an artifact of enhanced ion yields from a radiogenic labile pb abstract sample 74459 contains 204pb rich zircons which when analyzed by ion microprobe yield u pb ages that are exceptionally older than their 207pb 206pb ages the 64 ion microprobe measurements define an age of 2681 6 ma 1 for the zircons with individual analyses giving dilution analyses were conducted on 12 single grains previously analyzed by shrimp the idtims measurements yielded radiogenic 238u 206pb ratios between 2 0 and 8 5 whereas the 34 ion microprobe analyses on the same 12 grains gave systematically lower values between 0 6 and 2 5 the discrepancy between the shrimp and idtims data has been traced to the labile pb by plotting the 204pb vs 206pb concentrations observed by the ion microprobe an unusually low 204pb 206pb ratio of 0 02 has been defined for the labile component apparently the secondary ion emission yield of the labile pb was significantly greater than that of in situ radiogenic pb thereby invalidating shrimp s interelement calibration and leading to inaccurate u pb ages these data provide important insights into the geochemistry of labile pb in 74459 the labile pb was locally derived and was well mixed at the kilogram scale prior to becoming housed within the zircon grains high resolution tem data suggest that the labile pb is contained within amorphous microdomains which are intimately associated with crystalline zircon another feature of the labile pb is its ubiquitous presence in the zircons whereas it was not taken by the coexisting feldspar this labile component has an age of 0 ma

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

u pb baddeleyite ages for the scourie dyke swarm scotland evidence for two distinct intrusion events precise and accurate radiometric ages for continental mafic dyke swarms are a prerequisite for global correlation of mafic magmatic events calibration of apparent polar wander paths and deciphering mechanisms of dyke emplacement precambrian dyke swarms such as the scourie dyke swarm in north west scotland are invaluable time markers so that a precise and accurate knowledge of their emplacement age is critical when unravelling the complex geological evolution of many terrains precise dating is often difficult however because magma interaction with country rock and subsequent metamorphic events can severely perturb some isotopic systems recent advances in u pb geochronology1 3 combined with the discovery that some mafic dykes contain trace amounts of uranium bearing minerals such as baddeleyite zro2 and or zircon have made it possible to obtain u pb ages for these rocks with a precision typically on the order of 1 2 myr4 5 some of the first detailed u pb studies of precambrian mafic dyke swarms5 6 have shown that large volumes of mafic magma 100 000 km3 were emplaced into the continental crust in surprisingly short periods of time 2 myr here we report the results from a u pb study of three members of the scourie dyke swarm and find at least two periods of dyke emplacement at 2 418 and 1 992 myr bp we speculate on a potential global correlation of early proterozoic mafic magmatism and hence on the origin of dyke swarms

geochronological constraints on paleoproterozoic crustal evolution and regional correlations of the northern outer hebridean lewisian complex scotland abstract ion microprobe u th pb geochronological data are presented for four samples from paleoproterozoic belts in the lewisian of the northern outer hebrides north west scotland two of these samples a tonalite sheet associated with the south harris igneous complex and a psammite from the leverburgh metasupracrustal belt south harris yield zircons with a dominant ca 1 87 ga age these are interpreted as the igneous crystallisation age for the tonalite and the source rock for the psammite and their age concordance suggests that the latter was developed in an arc basin sequence derived largely from contemporaneous igneous rocks and buried during collision which resulted in documented 1 83 ga high grade metamorphism a diorite from the paleoproterozoic shear zone at the northern tip of lewis has a probable 2 7 2 8 ga protolith age although its zircons have strongly been affected by pb loss during later events culminating in development of low th u overgrowths at ca 1 86 ga zircons from a tonalite from berneray in the sound of harris yield an archean crystallisation age of ca 2 83 ga with no indication of later disturbance thus providing a southern limit to the region affected by paleoproterozoic tectonothermal events the paleoproterozoic arc in south harris represents a major tectonic boundary active margin in the lewisian of the outer hebrides possibly correlated with the laxford or gairloch shear zones of the mainland lewisian contrasts in the flanking region geology and geochronology possibly reflecting lateral heterogeneities may be introduced by major thrusts and or extensional faults e g the outer isles fault developed between the shear zones on a broader regional scale evidence for a magmatic arc in the lewisian is consistent with the tectonic style of other ca 1 9 ga paleoproterozoic collisional orogens throughout laurentia fennoscandia suggesting a reappraisal of the formerly proposed intracratonic evolution of the lewisian at this time

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

proterozoic anticlockwise p t path of the lewisian complex of south harris outer hebrides nw scotland the leverburgh belt and south harris igneous complex in south harris northwest scotland experienced high pressure granulite facies metamorphism during the palaeoproterozoic the metamorphic history has been determined from the following mineral textures and compositions observed in samples of pelitic quartzofeldspathic and mafic gneisses especially in pelitic gneisses from the leverburgh belt 1 some coarse grained garnet in the pelitic gneiss includes biotite and quartz in the inner core sillimanite in the outer core and is overgrown by kyanite at the rims 2 garnet in the pelitic gneiss shows a progressive increase in grossular content from outer core to rims 3 the alvi aliv ratio of clinopyroxene from mafic gneiss increases from core to rim 4 retrograde reaction coronas of cordierite and hercynite cordierite are formed between garnet and kyanite and orthopyroxene cordierite and orthopyroxene plagioclase reaction coronas develop between garnet and quartz 5 a p t path is deduced from inclusion assemblages in garnet and from staurolite breakdown reactions to produce garnet sillimanite and garnet sillimanite hercynite with increasing temperature and 6 in sheared and foliated rocks hydrous minerals such as biotite muscovite and hornblende form a foliation modifying pre existing textures the inferred metamorphic history of the leverburgh belt is divided into four stages as follows m1 prograde metamorphism with increasing temperature m2 prograde metamorphism with increasing pressure m3 retrograde decompressional metamorphism with decreasing pressure and temperature and m4 retrograde metamorphism accompanied by shearing peak p t conditions of the m2 stage are 800 30 c 13 14 kbar pressure increasing from m1 to m2 suggests thrusting of continental crust over the south harris belt during continent continent collision the inferred p t path and tectonic history of the south harris belt are different from those of the lewisian of the mainland

devensian late glacial and early flandrian environmental history of the vale of pickering yorkshire england pollen sedimentological and charcoal particle analyses are presented from devensian late glacial and early to mid flandrian deposits from a former lake in the vale of pickering yorkshire the combined analytical methods provide evidence for a short lived climatic deterioration towards the end of the late glacial interstadial followed by a brief recovery prior to the loch lomond stadial this deterioration may be correlated with one of the pre younger dryas cooling periods identified not only in other pollen sequences from britain and europe but from such diverse sources as foraminifera from the norwegian sea and electrical conductivity measurements from the greenland ice sheet loss on ignition and magnetic susceptibility data suggest that the loch lomond stadial was characterised by an initial prolonged temperature decline followed by a sudden more severe downturn resulting in particularly intense solifluction radiocarbon accelerator dating of the early flandrian marl deposits illustrates the problem of age determination in calcareous lakes and an estimate of the magnitude of hard water error is obtained the local population expansion of alnus glutinosa is dated to 7640 85 yr bp but there is possible evidence for a late glacial presence of the tree the significance of which is discussed in relation to other sites in east yorkshire 1996 john wiley sons ltd

polymetamorphic evolution of the lewisian complex nw scotland as recorded by u pb isotopic compositions of zircon titanite and rutile u pb isotopic relations in zircon and titanite of granulite and amphibolite gneisses in the lewisian complex and bordering laxford front reveal complex discordance patterns indicating multiple late archean and early proterozoic crystallization overgrowth and pb loss events the earliest stages in the evolution of the complex remain poorly resolved zircon ages of 2710 ma date high grade metamorphism and magmatism probably related to tectonic and magmatic accretion in a continental arc setting a distinct event at 2490 2480 ma possibly initiated by metamorphism and deformation at high grade conditions caused amphibolitization of the granulites and emplacement of granitic pegmatites this event can be correlated with development of inverian shear zones and formation of granitoid layers along the laxford front the emplacement of a younger generation of granitoid sheets during the laxfordian event fromed hydrothermal titanite at 1754 ma in gneisses south of the laxford bridge and partially reset older titanite at scourie growth of secondary titanite and rutile also occurred during subsequent low grade metamorphism at 1690 1670 ma

new evidence for protolith ages of lewisian granulites northwest scotland the granulite facies scourian rocks in the central region of the lewisian complex of northwest scotland play an important role in the understanding of the development of mid archean high grade gneiss complexes whereas the later largely proterozoic history of the scourian has been clarified in a detailed u pb study unraveling the archean ages of the protoliths using conventional isotope techniques has proved impossible in order to advance our understanding of the deep crustal processes responsible for these granulites it is essential to establish the age s of accretion of the protoliths and their subsequent tectonothermal history a combined cathodoluminescence cl and sensitive high mass resolution ion microprobe shrimp single zircon study has revealed a hitherto unrecognized morphological complexity in zircons from the type localities of the granulites use of cl allowed identification of relict oscillatory igneous zoning metamorphic overgrowths and irregular areas of recrystallization from the shrimp data an age of ca 2960 ma is inferred for the gneiss protoliths which were altered considerably during an important metamorphic event ca 2490 ma

u pb zircon systematics at gruinard bay northwest scotland implications for the early orogenic evolution of the lewisian complex the gruinard bay area of the mainland lewisian complex comprises a metamorphosed suite of archaean trondhjemites and minor granites enclosing remnants of older tonalitic gneiss and mafic to ultramafic enclaves the u pb zircon dating yields ages of 2731 14 ma and 2728 2 ma for two trondhjemite and 2732 4 ma for one granite sample also revealing the presence of large amounts of inherited xenocrystic zircons although the region has been pervasively overprinted by retrogressive events in amphibolite to greenschist facies the textural relations between biotite hornblende quartz and titaniferous minerals indicate that these minerals are pseudomorphs of pyroxene and high ti amphibole formed in hornblende granulite facies structural relations link this metamorphism to a steep northeast trending fabric coeval with the intrusion of the trondhjemites dated at 2730 ma dating of zircon in amphibolite and tonalite enclaves yields complex internal isotopic relations with apparent ages ranging from 2825 to 2740 ma this age range reflects new growth during the 2730 ma metamorphic metasomatic events superimposed on older zircon phases which include combinations of xenocrystic cores and magmatic and or metamorphic growth phases whose mode of formation cannot clearly be resolved by imaging techniques e g cathodoluminescence alone a pegmatitic vein that escaped the d3 strain and related isotopic disturbances yields a precise age of 2792 2 ma which constrains to some degree the earliest orogenic events in the area age relationships displayed in the central block at scourie badcall and in the gruinard bay area indicate that petrogenetic events in both areas were comparable about 2800 ma and that both areas underwent trondhjemitic magmatism about 2730 ma in contrast at gruinard bay there is no isotopic evidence for a period of high grade metamorphism and magmatism at 2490 2480 ma that drastically affected the scourie block indicating that at this stage the two regions occupied different levels of the crust

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

u pb isotopic evidence for the accretion of different crustal blocks to form the lewisian complex of northwest scotland single zircon and titanite u pb shrimp data presented for tonalite trondhjemite granodiorite ttg suite gneisses and an ultramafic rock from the northern and central regions of the lewisian complex of northwest scotland show that protolith ages of tonalitic gneisses in the northern region 2800 2840 ma are significantly younger than those in the central region 2960 3030 ma further evidence of a major 2490 2480 ma metamorphic event in the central region is documented by a metamorphic zircon associated with a granulite facies ultramafic body a dioritic gneiss from the northern region has also been dated at c 2680 ma the northern region therefore does not comprise reworked central region rocks and consequently the old models for the evolution of the lewisian which were based upon this concept need replacing it is instead proposed that two distinct crustal blocks now the northern and central regions were tectonically juxtaposed along a boundary corresponding to the laxford front juxtaposition would appear to have occurred in proterozoic times as it must have postdated the 2490 2480 ma inverian metamorphism recorded only in the central region and the emplacement of granite sheets restricted to the northern side of the boundary the first recorded event common to both regions is resetting of titanite ages associated with c 1750 ma laxfordian amphibolite facies metamorphism zircon inheritance in rocks of both regions is scarce within one zircon from the northern region a c 3550 ma core was found this represents the oldest known material from the region

anatexis during high pressure crustal metamorphism evidence from garnet whole rock ree relationships and zircon rutile ti zr thermometry in leucogranulites from the bohemian massif in situ mineral garnet zircon trace element and garnet rock ree distribution data obtained on leucogranulites from the bohemian massif are consistent with an origin of these now strongly deformed rocks as migmatitic leucogranites formed through dehydration melting of muscovite bearing protoliths at high pressures during the variscan orogeny partial melting at p t conditions of 900 9408c and 1 6 1 8 gpa consumed mica and feldspar to produce peritectic kyanite and high ca 20 24 mol grossular high eu garnet along with leucogranitic melt dree grt rock values calculated from this garnet and its host rock are consistent with garnet melt equilibrium at these p t conditions ternary feldspar and rutile 2000 3200 ppm zr crystallized from the leucogranite melts in the migmatites at minimum temperatures of 4880 9208c these formed along with apatite prior to and accompanying the crystallization of garnet rims and the majority of the garnet that occurs in the leucogranulites this garnet is typified by strong depletion in eu and dree grt rock values consistent with garnet melt equilibrium only after the fractionation of apatite 50 3 wt and ternary feldspar 5 15 wt zircons in one leucogranulite are dominated by oscillatory zoned grains that yield pre variscan u pb ages 495 and 433 ma and crystallized from ti in zircon thermometry at 700 8108c in their original host magmas these xenocrystic grains are potentially preserved because of the short timespan of the high p t variscan event at 340 ma and the moderate amounts of leucogranitic partial melt present in the rocks estimated as at least 30 wt melt from zr mass balance the production of new zircon associated with the variscan event in this leucogranulite is limited to weakly zoned zircon rims characterized by flat heavy ree patterns that approach ree equilibrium with garnet rims this post peak zircon was formed at minimum p t conditions of 8408c and 0 8 1 0 gpa along the decompression cooling path defined by previous work for the bohemian massif granulites the garnet whole rock ree relationships observation of eu depletion between early and later garnet growth phases and zr ti thermometric estimates indicating temperatures of48408c for this evolution cannot be explained by the alternative interpretation of the leucogranulites as reworked and recrystallized pre variscan low p granitoids but support a model in which the bohemian leucogranulites were high p variscan age migmatites containing significant amounts of melt

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

ventilation of the glacial deep pacific ocean measurements of the age difference between coexisting benthic and planktic foraminifera from western equatorial pacific deep sea cores suggest that during peak glacial time the radiocarbon age of water at 2 kilometers depth was no greater than that of today these results make unlikely suggestions that a slowdown in deep ocean ventilation was responsible for a sizable fraction of the increase of the ratio of carbon 14 14c to carbon in the atmosphere and surface ocean during glacial time comparison of 14c ages for coexisting wood and planktic foraminifera from the same site suggests that the atmosphere to surface ocean 14c to c ratio difference was not substantially different from today s

geochronological constraints on the evolution of the southern dom feliciano belt uruguay abstract new u pb sensitive high resolution ion microprobe k ar and ar ar data from the southernmost dom feliciano belt allow the identification of four major events orthogneisses from the punta del este terrane indicate a magmatic episode at c 770 ma and high grade metamorphism at 641 17 ma granitoid emplacement at 627 23 ma was roughly coeval with peak metamorphism volcaniclastic rocks of the las ventanas formation dated at 573 11 ma can be correlated with the peripheral foreland basin 571 8 ma transpression and coeval high k calc alkaline magmatism is recorded in the maldonado granite dated at 564 7 ma the following events are postulated 1 magmatism at 850 750 ma related to rifting 2 metamorphism and granite emplacement at 650 600 ma 3 molasse sequences and foreland basins at c 573 ma 4 late magmatism at 580 560 ma associated with transpression the data indicate that 1 the punta del este terrane could be a portion of the coastal terrane of the kaoko belt 2 granitoid emplacement at 650 600 ma in the punta del este and nico perez terranes favours westward subduction and 3 widespread post collisional synkinematic magmatism occurred in the dom feliciano and kaoko belts between 580 and 550 ma supplementary material analytical methods and data are available at http www geolsoc org uk sup18369