sedimentology and stratigraphy of thetownsend tuff bed lower old red sandstone in south wales and the welsh borders the townsend tuff bed has a highly distinctive internal stratigraphy which allows it to be recognized at a large number of localities scattered over the welsh borders and south wales it is a group of 3 almost directly superimposed normally graded air fall tuffs that originated in distant but powerful plinian eruptions centre unknown and became spread over extensive but practically featureless coastal mud flats the bed occurs in a barren interval within the early lower old red sandstone between a silurian invertebrate vertebrate fauna below and an already devonian vertebrate assemblage above the base of the devonian system in south wales and the welsh borders is accordingly defined as occurring at a regionally uniform lithological contact within the townsend tuff bed

the metagabbros orthogneisses and paragneisses of the connemara complex western ireland although now largely disappeared within the 400 ma old galway granite the syntectonic connemara metagabbro gneiss complex which crystallized at 490 ma arenig is the largest intrusion into the dalradian rocks it is part of widespread cambro ordovician volcanism and plutonic injection into continental crust in the appalachian caledonide orogenic belt being synchronous in crystallization with the widely obducted 490 ma ophiolites within the orogeny the peridotites and hornblende gabbros were injected broken up and shouldered aside by quartz diorite granodiorite granite and trondhjemite gneisses and it is shown that these orthogneisses are not the in situ differentiation products of the gabbro magma the hornblende in the metagabbros is mainly igneous but is partly metamorphic and replaces olivine fo 75 92 orthopyroxene en 65 92 clinopyroxene wo 48 5 en 50 39 fs 5 14 and plagioclase typically an 80 92 range an 50 96 an aureole at least 10 km wide in the sillimanite zone was imposed and cordierite garnet sillimanite dalradian hornfelses were partially melted at temperatures up to 1000 o c near to the gabbros at 4 5 1 kbar corresponding to c 18 km depth this suggests that the complex forms part of the root of a now eroded batholith of calc alkaline affinities the gabbros corresponding to the transition zone to a basic bottom which is often supposed to underlie many batholiths within the orthogneisses are sheets of paragneiss formed from dalradian metasediments which have been folded up from the floor and down into sides of the magma chamber which process could be illustrative of the means by which basic magma can intimately interact with crustal material the 18 o 16 o d h nd and sr isotope evidence and the existence of inherited zircons in the gneisses indicate that the gabbro and gneiss magmas were significantly contaminated with crustal possibly dalradian material both before and after intrusion with the lowest initial 87 sr 86 sr at 490 ma being 0 708 even in samples far removed from metasediment the metagabbros are tholeiitic to high alumina basalt in character but the gneisses have distinct calc alkaline geochemistry the chemistry igneous hornblende and highly calcic plagioclase match many arc plutons in the western usa the syntectonic nature and the ubiquitous gneissic fabric in the quartz diorites trondhjemites and granites is a consequence of early movements which were probably in part connected with the separation of the connemara massif and which culminated in slicing off the largely inverted complex and carrying it southwards on the mannin thrust which underlies the whole of connemara the complex is thus a fragment of a now disrupted cambro ordovician continental magmatic arc or continent island arc environment which can be traced along the se edge of the laurentian continent

implications for caledonian plate tectonic models of chemical data from volcanic rocks of the british old red sandstone volcanic rocks associated with the continental sediments known as the old red sandstone outcrop widely in northern britain and form a dominantly calc alkaline suite comparable to those developed on modern continental margins rocks rich in ni and cr are unusually abundant and are thought to have incompatible element concentrations little different from those of primary magmas such rocks show pronounced spatial chemical variation with concentrations of sr ba k light rare earth elements and p 2 o 5 and the ratio la y showing up to a six fold increase in a northwestward traverse across scotland in more evolved rocks a similar increase is shown by sr ba and la y but regional comparisons are made difficult by the varying effects of fractional crystallization the chemistry of all old red sandstone volcanic rocks n of the southern uplands fault is consistent with the former existence of a subduction zone that changed in strike from ene in ireland and southern scotland to n in the north sea this orientation in conjunction with almost e w plate motion may explain the great contrast in end silurian deformation styles between britain and scandinavia old red sandstone volcanic and plutonic rocks s of the southern uplands fault are thought to be younger and are perhaps unrelated to subduction activity

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

plate tectonics and the evolution of the british isles this paper is an essay on the rationalism and difficulties of the geological corollaries of plate tectonics with illustrations particularly from the geological history of the british isles there is special emphasis on the mechanics of continental lithosphere in extension when basins are formed and in convergence whereby continental lithosphere thinned in extension is shortened and restacked to form orogenic belts an upper lithosphere stress concentrating elastic lid suffers extremely complex inhomogeneous deformations above a simpler viscoelastic lower lithosphere whose deformation drives strain and displacement in the elastic lid the very nature of plate tectonics as relative motion among a multi plate mosaic of plates provides a rational semi quantitative framework for large scale geological complexity in space and time the caledonian and hercynian history of the british isles may be explained semi quantitatively in terms of plate boundary evolution but it is not possible to define unequivocal solutions the rationalism of plate tectonics is exhibited on a smaller more direct field scale by many of the complexities of ophiolite complexes such as ballantrae scotland that can be explained as the effects of the evolution of ridge ridge transforms and by diachronous deformation and sedimentation in subduction accretion prisms such as the southern uplands deformational and sedimentary patterns in nw europe from triassic times to the present day resulted from a complex interaction of extensional events related to diachronous atlantic opening and compressional events related to sequential alpine shortening phases those widespread events are typical of intracontinental plate boundary systems and create great difficulties for the geologist wishing to decipher relative plate motion from geological sequences because huge areas much larger than the british isles must be analysed and synthesized before much sense can be extracted further difficulties arise from the palinspastic and preservation problems of convergent boundaries orogenic shortening telescopes and partially subducts facies belts most orogenic belts were assembled as a collage of displaced and rotated terrain elements that were subsequently flattened elongated and straightened to an apparent but deceiving simplicity therefore while plate tectonics provides a rational geological framework against which for example british basin evolution can be tested and explained we cannot yet use the geological effects of plate boundary evolution to reconstruct relative plate motion quantitatively

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

caledonian plutonism in britain a summary the caledonian granites of britain are a wide ranging suite of pretectonic and posttectonic tonalite granodiorite granite intrusions which were emplaced in the time interval 650 380 ma ago the volume of magmatic products and the frequency of intrusions increased with time as closure of the iapetus ocean occurred across the caledonide belt in the british isles early pretectonic and posttectonic intrusions older than about 410 ma are distinguished from a later post 410 ma group using geophysical gravity aeromagnetic and heat production and geochemical data the evidence is interpreted in terms of a trend with time from relatively local thermal and crust dominated melting events to a final widespread melting event at deeper levels the latter event produced abundant intrusions with characteristics that resemble the calc alkaline batholiths of modern destructive margins finally we propose that crustal growth during the caledonian period took place by northward lateral accretion of the european continental foreland and by vertical accretion of continental crust on both margins by underplating and by later intrusive magmatism

k rich mantle metasomatism control of localization and initiation of lithospheric strike slip faulting a conceptual model is proposed in which bulk transtension or local transtension during bulk simple shear resulting from mantle anisotropy contrasts or lithosphere rheology contrasts of heterogeneously enriched lithospheric mantle triggers localized k rich magmatism which focuses strain and causes nucleation of lithosphere scale transtensional or strike slip shear zones transtension triggered magmatism is most likely to be located at sites of maximum metasomatism of the lithospheric mantle magma generated fractures propagate upwards nucleating zones of lithospheric weakness which focus shear in narrow transcurrent faults or at basin margins in this way magmatism controls fault timing and location although volcanism will be coeval with fault development and volcanoes will appear fault controlled counterintuitively our model suggests that faults are in a sense volcanocontrolled we suggest that this new transtension k rich magmatism transcurrent faulting association represents a hitherto unrecognized genetic relationship as significant as for example the ocean island magma series

evidence for an early proterozoic crustal province in the north atlantic region published sm nd data for the ketilidian belt of greenland the rockall bank in the north atlantic and the islay inishtrahull block of the british isles are used to suggest the existence of a single early proterozoic crustal province in the north atlantic region with a crustal formation age of c 1 96 ga the three sub provinces also have similar metamorphic histories as documented by sr and pb isotope data they probably represent one or more island arcs which were accreted to the laurentian craton 100 ma later in the 1 85 to 1 75 ga ketilidian laxfordian orogeny a line of suture between the archaean craton and early proterozoic accreted terranes is tentatively proposed

geochemical character of silurian volcanism in sw ireland the dunquin group mid silurian of dingle southwest ireland is a sequence of basaltic to rhyolitic lavas and pyroclastic rocks interbedded with shallow to non marine sediments that records one of only four known outbreaks of silurian volcanicity south of the iapetus suture in the british isles new geochemical petrographic and field evidence indicate that the volcanic sequences exposed in the dunquin inlier and the islands of beginish and inishvickillane could have formed by fractionational crystallization in a layered repeatedly replenished basaltic magma chamber the site of which is probably coincident with an intense aeromagnetic anomaly to the southwest of inishvickillane the volcanic suite is subalkaline transitional between calc alkaline and tholeiitic and is non bimodal high lile hfse ratios and high lree concentrations suggest that the parent basaltic magmas were generated at a destructive continental margin although subduction may have ceased before eruption

post acadian sediment recycling in the devonian old red sandstone of southern ireland abstract the upper devonian munster basin of southern ireland has traditionally been viewed as a post orogenic molasse deposit that was sourced from the caledonides of central ireland and subsequently deformed by the end carboniferous variscan orogenic event the basin fill is composed of super mature quartz arenite sandstone that clearly represents a second cycle of deposition the source of this detritus is now recognized as lower devonian dingle basin red bed sequences to the north this genetic link is based on the degree of similarity in the detrital mica chemistry in both of these units micas plot in identical fields and define the same trends in addition the two sequences show increased textural and chemical maturity up sequence and define indistinguishable 40 ar 39 ar age ranges for the detrital mica grains partial resetting of the ar ages can be attributed to elevated heat flow in the region caused by munster basin extension and subsequent variscan deformation the combined evidence from southwest ireland therefore points to a caledonian or possibly taconian primary source area that initially shed detritus into the lower devonian dingle basin which was subsequently recycled into the upper devonian munster basin following mid devonian acadian basin inversion

conductive incubation and the origin of dome and keel structure in archean granite greenstone terrains a model based on the eastern pilbara craton western australia 1 the archean east pilbara granite greenstone terrane epggt of the pilbara craton in western australia preserves a prolonged record of voluminous mafic dominated volcanism and felsic plutonism commencing at circa 3515 ma and ultimately resulting in the development of spectacular dome and keel structures early crustal growth was dominated by basaltic magmatism and the regional development of a 12 18 km thick greenstone sequence by circa 3335 ma overlying widespread 3470 3430 ma tonalite trondhjemite granodiorite ttg suites emplaced into the middle crust stratigraphic and structural relationships imply that voluminous granitoid plutonism and crustal scale dome and keel formation in the southeastern epggt was not initiated until circa 3325 ma but proceeded rapidly over the interval 3325 3308 ma this scenario is consistent with a conductive incubation period of 10 100 myr duration following the burial of radiogenic granitic crust beneath the accumulated greenstone pile for heat production parameters appropriate to the 3515 3308 ma epggt we show that the burial of 3470 3430 ma granitoid rich crust beneath the 3335 ma euro basalt and its precursors led to long term crustal temperature increases of 170 234 c at 35 km depth with a corresponding reduction in effective viscosity of 2 5 orders of magnitude in combination with the greenstone over granitoid density inversion the changes in effective viscosity of the mid to deep crust due to the burial of heat sources may have provided the crucial trigger for the initiation of dome and keel formation similarly cooling and strengthening of the crust during syndoming exhumation of the heat production potentially terminated large scale dome and keel amplification

digital mapping and three dimensional model building of the ben nevis igneous complex southwest highlands scotland new insights into the emplacement and preservation of postorogenic magmatism abstractcaledonian postorogenic magmatic processes are not well understood new digital mapping and three dimensional modeling of the end caledonian late silurian to early devonian ben nevis igne

slab breakoff a model for caledonian late granite syn collisional magmatism in the orthotectonic metamorphic zone of scotland and donegal ireland abstract none of the existing models for calc alkaline late granite siluro devonian genesis in the metamorphic caledonian orogenic belt of ireland and scotland fully explains their spatial age or chemical character a consistent model must involve the closure of iapetus ocean where slab breakoff is a natural consequence of attempted subduction of continental crust expected outcome is a long linear belt of high k calc alkaline magmas some with characteristic trace element signatures specifically high ba sr and zr other features include the critical magmatic association of coeval appinite and granite rapid uplift erosion and the low grade regional metamorphism in the southern uplands the linear heat pulse on breakoff is spatially intensity and time limited producing small volume melts emplaced as separated plutons over a short time span magmatism in the caledonian metamorphic belt is accurately accounted for by slab breakoff on collision of baltica with the scoto greenland margin during the scandian orogeny following iapetus ocean closure the two chemically isotopically and areally distinctive suites in the metamorphic belt in scotland viz the argyll and cairngorm suites can be modelled by reference to the donegal granites where sequential partial melting of new lamprophyric underplated crust then shallower old crust as heat conduction moved up through the crust on slab breakoff produced magmas characteristic of the two suites

a tectonic model for the emplacement of the main donegal granite nw ireland the main donegal granite is shown to lie in a major late stage ne sw trending transcurrent sinistral shear zone in the caledonides of nw ireland the zone which is bent both laterally and upwards around the pluton has displacements which diminish sw along its length from a maximum of approximately 20 km at the ne end of the pluton the granite has also been deformed by this sinistral shear but it has a strain component of its own the nature and development of which indicate that the pluton was accommodated rather than forcefully emplaced in the shear zone strain variations between granite and wall rock and an early plagioclase fabric associated with the petrographic banding of the granite suggest that intrusion and deformation were more or less synchronous at around 400 ma it is suggested that unconsolidated granite in the shear zone locally created high strain rates and instability which caused the zone to bend split lengthwise and progressively create an internal low pressure zone into which the main granite was emplaced the wide variety of emplacement styles of the other radiometrically synchronous plutons of the batholith can be attributed to the progressive development of the shear zone thorr and fanad in tension between the zone and a complimentary structure to the n ardara and toories in local compression at the shear zone termination rosses in tension on the outer arc of the bending zone and trawenagh bay as displaced main granite magma in a similar tensional setting during the final stages of deformation

paleozoic evolution of the armorica plate on the basis of paleomagnetic data new paleomagnetic data support the contention that armorica and gondwana formed a coherent block until ordovician time however by early devonian time armorica collided with the north american northern european assembly to form the old red continent and the collision itself was responsible for the acadian orogeny not for the earlier taconic orogeny as postulated previously the carboniferous collision of gondwana with the old red continent subsequently formed pangaea and produced the appalachian hercynian orogeny

basement geology and tectonic evolution of ireland as deduced from pb isotopes in ireland lead isotope data collected on carboniferous hosted zn pb deposits can and has been interpreted to indicate that lead and other metals were derived from the underlying basement rocks in order to test this hypothesis whole rock lead isotope measurements were made on greywackes from the longford down inlier which represent the along strike continuation of the northern part of the caledonian basement underlying the base metal deposits the lead isotope data define a linear array on a 207 pb 204 pb versus 206 pb 204 pb plot with end points at 207 pb 204 pb 15 542 206 pb 204 pb 18 456 and 207 pb 204 pb 15 461 206 pb 204 pb 17 973 no evidence of lead isotope overprinting by the mineralizing event could be found in the samples studied when the lead isotope data are corrected for addition of radiogenic lead since the time of base metal mineralization c 350 ma ago there is a 1 1 correlation between the lead isotopic composition of galenas in the base metal deposits and the longford down grewyackes these results support the hypothesis that lead and other metals were derived from the greywackes underlying the base metal mineralization on a 208 pb 204 pb versus 206 pb 204 pb plot neither whole rock greywacke nor galena lead isotope data define a linear array the 208 pb 204 pb 206 pb 204 pb trend is best explained by tectonic or sedimentological mixing of three isotopically distinct lead isotope end members these end members are 1 old unradiogenic basement in nw ireland lewisian complex precursor 2 ordovician arc material from nw ireland and 3 ordovician arc material form se ireland isotopic modelling of these end members shows that the present day mixing line seen in the ordovician and silurian sediments can be calculated back to values inferred for the ordovician arc end members using the same values of as were used to back calculate the lead isotopic composition of the greywackes to the time of mineralization this information is further confirmation of the sediment mixing model presented a tectonic model developed from these and established geological data suggest a late ordovician closure of the iapetus ocean and that the silurian greywackes are a by product of eroded ordovician arc material from nw and se of the iapetus suture

crustal growth and late precambrian early palaeozoic plate tectonic evolution of england and wales comparison of the basement rocks of britain reveals major contrasts to the n and s of the iapetus suture solway shannon line north of the suture polymetamorphic high grade basement schists and gneisses and igneous rocks date from about 2900 ma to c 400 ma the latter age corresponding to closure of the iapetus ocean by contrast s of the suture in england and wales the basement is dominantly low grade metamorphic rock together with intrusive and extrusive igneous and sedimentary rocks there is little geological or geochemical evidence for the presence of basement older than c 900 ma below this area and the predominant expression of crustal growth in england and wales is indicated by radiometric ages between c 700 ma and 450 ma reflecting late precambrian palaeozoic magmatism and metamorphism palaeomagnetic data from scotland demonstrate that the crust in northern britain developed as an integral part of the laurentian shield during mid proterozoic times 1600 1000 ma the palaeomagnetic record from the pre ordovician rocks of england and wales shows that this area was unrelated to the laurentian shield prior to closure of the iapetus ocean the apparent polar wander a p w path is that of a distinct microplate involved in rapid growth of iapetus during early cambrian times and emplaced in its present configuration adjacent to northern britain as a result of subduction of the ocean basin between cambrian and silurian times we conclude that whereas the basement to the north of the iapetus suture formed an essentially continental unit from 2900 to 400 ma the basement of england and wales is a result of crustal growth from about 900 ma to 400 ma and formed by accretion of island arcs associated accretionary prisms and fore arc basin sediments within the iapetus ocean

benue trough and benue chain it is suggested that the benue trough may have been initiated by the extension of the equatorial oceanic fracture zones onto the continent during the early stages of the separation of africa and south america a transcurrent movement along deep seated faults of the basement is responsible for the structure of the trough after a period of extension characterized by the sedimentary infilling of the trough and igneous activity the trough is turned into a chain through a short phase of compression a discrete pattern of en echelon folds and the strain pattern deduced from the analysis of large scale and microfracturing correspond to a n s to n160 e direction of compression coupled to a sinistral transcurrent movement along a ne sw direction the difference in age of the folding phase in the northern and southern parts of the chain indicates a diachronous nature for this tectonic event in relation to the wrench regime the tertiary evolution of the trough marked by a tensional regime resulting in an e w extension is to be related to the cameroon volcanic line

mesoscale activity in the comoros basin from satellite altimetry and a high resolution ocean circulation model recently it has been shown that anticyclonic eddies are generated in the comoros basin contesting the long held notion of a single large anticyclonic cell the comoros gyre limited knowledge exists about the mesoscale activity within the basin a potential key source of variability for the mozambique channel and subsequently the agulhas current in this paper an automated eddy tracking scheme applied to satellite altimetry data and a high resolution model simulation is used to determine the characteristics of the anticyclonic eddies generated in the comoros basin the generation and characteristics of cyclonic eddies are also investigated the eddy tracking scheme revealed that anticyclonic eddies are primarily generated west of the tip of madagascar due to barotropic instabilities whereas cyclonic eddies are mainly generated along the northwest coast of madagascar as a result of baroclinic instabilities anticyclonic eddies with a mean lifespan of about 3 months reside in the basin for half their lifespan before propagating into the mozambique channel on the other hand the majority of cyclonic eddies with a similar mean lifespan dissipate within the basin initially the anticyclones with translation speeds of 6 8 km d 1 and mean radii of 80 100 km follow the trajectory of the north east madagascar current and turn south upon reaching the african coast the cyclonic eddies tend to be smaller 60 km and have slower translation speeds 2 5 3 5 km d 1 than their anticyclonic counterparts

the plate tectonic setting of nw britain and ireland in late cambrian and early ordovician times deposition of dalradian sediments in a stretching ensialic basin peripheral to iapetus continued into cambrian times late in the history of dalradian deposition a southern continental landmass was removed by strike slip motions and a subduction zone then developed beneath the dalradian sediments deformation and moderately high pressure metamorphism of dalradian rocks occurred and was followed by thermal relaxation and development of a volcanic arc with magmatic centres in connemara by tremadoc times and in ne scotland by the arenig the s mayo trough was a back arc basin that opened shortly after the initiation of the arc the oldest volcanic rocks seen there being the extrusive equivalents of the basic intrusions of connemara dalradian rocks outcropping in ordovician times over the highlands and probably extending southwards are a possible source for high pressure metamorphic clasts at ballantrae while clasts of arc volcanics may have come from n of the highland boundary fault plate models that involve a separate arc in the midland valley are rejected because the tectonic elements are impossibly small by comparison with modern convergent plate margins the arc rooted in dalradian rocks ceased to be active at the end of the llanvirn as a result of a further change in plate motions and there are considerable spatial problems in locating an arc coupled to the southern uplands accretionary prism after llanvirn times

faunal evidence for oceanic separations in the palaeozoic of britain the faunal criteria for the recognition of the relative separation of old continents are 1 the contrast between shallow not deep water faunas off opposite shorelines 2 the recognition of deeper water facies surrounding the edges of continents and 3 the recognition of the disposition of ancient climatic belts these criteria are defined and used to assess the relative position of the various parts of what is now britain in the palaeozoic the iapetus ocean is well characterized by faunal differences from the cambrian until the lower ordovician but from caradoc times onwards these differences dwindle until by the latest silurian only the distribution of the ostracods separates the two sides of the ocean on faunal grounds in the early ordovician s british faunas are comparable with those from bohemia france and elsewhere to the s indicating connection with gondwanaland and these faunas differ from those in the baltic area a distribution which we attribute to a true ocean tornquist9s sea between n and s europe tornquist9s sea probably closed in the upper ordovician and from ashgill times onwards the s british and baltic faunas remained similar during the middle and late silurian there is evidence of increasing faunal distinction between britain the baltic area and north america on the one hand and southern europe and gondwanaland on the other indicating an opening rheic ocean which appears to have persisted until late carboniferous times

coupled sm nd and u pb systematics in late caledonian granites and the basement under northern britain the british late caledonian granites display a close correspondence between inherited u pb systematics in zircons and initial end indicating that both were controlled by the age of a major component of continental crust the data confirm and refine previous models of a basement comprising young crust in the south which includes arc and oceanic crust scrapings and progressively older continental crust northwestwards across scotland

suspect terranes and accretionary history of the appalachian orogen the north american connection or the grenvillian continental crust that separated from eastern north america during the initiation of the appalachian orogenic cycle is poorly defined within the present orogen a variety of terranes occur outboard of the north american miogeocline all structurally uncoupled and therefore suspect stratigraphic analysis indicates that appalachian accretion progressed from the miogeocline outward the boundaries of the earliest accreted western terranes are marked by melange and ophiolite complexes later boundaries between eastern terranes are steep mylonitic zones and brittle faults accretionary events defined by stratigraphic analysis correspond to times of major deformation plutonism and metamorphism in the history of the orogen

a cryptic record of magma mixing in diorites revealed by high precision sims oxygen isotope analysis of zircons high precision in situ ion microprobe sims oxygen isotope analysis of zircons from two diorite intrusions associated with the late caledonian lochnagar pluton in scotland has revealed large differences in the degree of heterogeneity in zircon 18o between the diorites zircon crystals from the cul nan gad diorite cng show a unimodal distribution of oxygen isotope values 18o 6 0 0 6 2 and no or only minor grain scale variation those from the allt darrarie diorite ad1 show a large range in 18o and an apparent bimodal distribution with modes of 6 6 0 4 and 7 3 0 4 variations of up to 1 2 occur between and within grains both an increase and decrease in 18o with zircon growth has been observed the 18o composition of growing zircon can only change if open system processes affect the magma composition i e if material of contrasting 18o composition is added to the magma the variability in ad1 is interpreted to represent a cryptic record of magma mixing a deep crustal hot zone is a likely site for generation of the dioritic magmas which developed by mixing of residual melts and crustal partial melts or by melting of mafic lower crustal rocks the overall small number of zircons with mantle like 18o values 5 3 0 6 2 in the lochnagar diorites is largely the product of crustal differentiation rather than crustal growth the 18o of quartz from the cng and ad1 diorites shows only minor variation cng 10 9 0 5 2 ad1 11 7 0 6 2 within single populations with no evidence of mixing quartz zircon isotopic disequilibrium is consistent with later crystallisation of quartz from late magmatic fluids and in case of the ad1 diorite after the inferred magma mixing from a homogenised higher 18o melt high precision sims oxygen isotope analysis of zircon provides a new approach to identifying and resolving previously undetected early stage magma mixing and constraining the compositions and origins of the component magmas a combination of zircon quartz and whole rock data has proven to be a powerful tool in reconstructing the petrogenetic evolution of diorite from early crystallisation to late alteration

the geochemical characteristics of granitoids in contrasting arcs and comments on magma sources geochemical variations in modern subduction related igneous suites with respect to arc maturity in time and space are illustrated using data for both volcanic suites basalt andesite and dacite and intrusive granitoid suites diorite tonalite granodiorite and granite from circum pacific arcs using trace element data we suggest that two groups of processes control the natural variation in the parental magmas of both suites a subduction zone enrichment of lithospheric mantle locally coupled with crustal assimilation allied with fractional crystallization afc in zones of thickened crust all of which yield magmas with enhanced concentrations of the large ion lithophile lil elements k rb th u lrees etc b with increasing distance from the active trench contributions from within plate sub continental lithosphere producing mantle derived magmas with enhanced levels of high field strength hfs elements among which nb ta hf and y are particularly distinctive thus even for the evolved granitoids of intrusive arc series ratios of hfs lil elements not significantly affected by crystal fractionation such as ta nb k rb la may throw some light on the origin of mafic intermediate precursor magmas in terms of these elements we suggest the following groupings for interpreting the tectonic associations of granitoid suites 1 primitive calcic arc granitoids with low lil and hfs element abundances 2 normal calc alkaline continental arc granitoids with enhanced lil element abundances and low hfs lil ratios 3 mature alkali calcic arc granitoids with high levels of lil and hfs elements and higher hfs lil ratios 4 back arc anorogenic alkaline granitoids with the highest levels of hfs elements

the iapetus suture in the british isles the position of the iapetus ocean suture can be traced between ordovician fossil localities in the southern uplands and the lake district which contain respectively distinct north american and european faunas the southern uplands contains north american caradoc faunas in the northern belt and is considered to have been accreted onto the laurentian north american margin between the llandeilo and the end of the wenlock deep seismic reflection profiling shows that a surface probably parallel to the suture zone dips down to the northwest from the solway line in eastern ireland the surface trace of the suture coincides with the navan fault which separates the longford down massif from ordovician sediments containing european faunas the navan silvermines fault may represent the surface expression of the suture but it is masked by wenlock turbidites which appear to have crossed over the plate boundary northward subduction of continental crust below the margin of laurentia during the late silurian may explain the influx of turbidite fans from the north or north west onto depressed crust to the south of the suture termination of subduction coincides with the widespread acadian orogeny in the emsian at 395 5 ma

the nature of the variscan basement in southeast england evidence from integrated potential field modelling the variscides of southeast england are buried beneath post carboniferous cover interpretations of the basement are based mainly on deep boreholes geophysical signatures from the basement are contained within the regional gravity and magnetic data a gravity stripping exercise has been undertaken to remove the gravitational effect of the post variscan cover to generate a residual gravity map this map is interpreted along with integrated potential field modelling along four long interconnected profiles and compared with a revised pre permian subcrop map the magnetic evidence suggests that precambrian magnetic basement of the midlands microcraton has been buried southwards by north vergent variscan thusting over the foreland north of the variscan front short wavelength anomalies superimposed upon this deep precambrian source are due to shallower silurian and carboniferous volcanic rocks many residual gravity lows within the rhenohercynian zone may be related to thick low density devonian basins in the english channel a change in geophysical signature occurs north of the portland wight fault coinciding with phyllites in the basement models are presented in which the english channel magnetic anomalies originate within the pre permian basement comparisons with anomalies in the southwestern approaches suggest that the portland wight thrust is a terrane boundary possibly a subduction related suture implying southerly directed variscan subduction

the newer granite problem a geotectonic view the siluro devonian suite of granitic plutons in the british caledonides known as the newer granites together with their associated extrusive rocks represent one of the most extensively researched examples of calc alkaline magmatism apparently related to orogeny although recent chemical studies have credibly interpreted some of the scottish intrusions and volcanic rocks as part of a continental margin magmatic arc generated by the subduction of lapetus oceanic lithosphere beneath laurentia insurmountable problems of distribution and timing arise when attempts are made to relate the magmatic activity as a whole to a traditional two plate collision model for the orogeny newer granite magmatism is here discussed in the context of more mobilistic models for the post grampian evolution of the british caledonides which involve e w closure between laurentia and baltica terminated by collision in the silurian followed by the northward accretion of gondwana derived terranes in the early devonian the former produced the main caledonian tectonometamorphism of the northern highlands of scotland the latter the late caledonian deformation of the slate belts in the paratectonic caledonides these models imply much more complex convergence geometries which can in principle account for the whole newer granite suite as a series of subduction generated magmatic arcs overlapping in space and time the model proposed involves three late caledonian magmatic arcs in addition to the ordovician borrowdale arc which is not considered in this paper one is related to laurentia baltica convergence with westward subduction beneath the scottish sector of the laurentian margin in the ordovician and early silurian which generated the early members of the newer granite suite in the highlands a second is related to northward silurian early devonian subduction at the solway line which generated the younger newer granites and volcanic rocks north of the highland border and a third related to northward accretion of the armorican terrane in early devonian time produced intrusive and extrusive magmatism as far south as southeast ireland and the english midlands

strike slip terranes and a model for the evolution of the british and irish caledonides evidence is presented that many of the major strike faults in the british and irish caledonides were active as sinistral strike slip zones in the end silurian to pre mid devonian period some such as the highland boundary fault moved in this way at an earlier stage in the ordovician these data allow the caledonian rocks lying between the laurentian miogeocline whose basement is represented by the lewisian moine and possibly the dalradian and the gondwanaland miogeocline midland platform and welsh basin to be re analysed as a group of disorganized terranes which originated to the southwest in north america and southwest europe africa prior to the silurian the highland border terrane and northern belt terrane are interpreted as duplicated pieces of a mid ordovician sequence which was a back are to northwest subduction the midland valley terrane is interpreted as a slice of laurentian foreland onto which ophiolites were obducted in the lower ordovician but which became the basement of a continental margin arc to northwest subduction in the mid ordovician the cockburnland terrane is inferred to be part of the same arc repeated and then broken up and dispersed by continuing strike slip the connemara terrane is regarded as an allochthonous piece of the dalradian miogeocline and the south mayo terrane as a remnant of an early ordovician arc and fore arc which in mid ordovician times became a back arc marginal basin to northwest subduction the lake district wexford terrane is part of an arc to southeast subduction under gondwanaland whose activity climaxed in the mid ordovician the central terrane is interpreted as a silurian overstep assemblage which blankets the junction between laurentian and gondwanaland derived oceanic terranes and therefore iapetus is regarded as an ordovician ocean which closed prior to the silurian the model suggests that at the end of the silurian a clockwise rotating gondwanaland having carried laurentia into collision with baltica broke free and created a major sinistral strike slip zone which disrupted the ordovician palaeogeography in the british isles north american sector of iapetus

british paleomagnetism iapetus ocean and the great glen fault new paleomagnetic data for britain establish the width of the iapetus ocean as 1000 km in mid ordovician time but do not yet allow precise estimates of the rate of its subsequent closure the new lower paleozoic data from northern scotland are similar to data from the rest of britain but are not compatible with north america data on the bullard et al reassembly if this discrepancy is correct lines of major transcurrent dislocation have yet to be identified either in the north atlantic continental shelves and margins or in eastern north america and the great glen fault was not the main locus of offset between britain and north america the tectonic complexities associated with this configuration require further evaluation