testing the reliability of discrimination diagrams for determining the tectonic depositional environment of ancient sedimentary basins the lower palaeozoic rocks of the west of ireland caledonides provide a novel way to test the reliability of discrimination diagrams in sedimentary rocks a comparison between the geochemical signature of the tuff bands and the signature from the adjacent sedimentary rocks reveals that a combination of multi element diagrams and discrimination diagrams is best suited to identify the tectonic environment of deposition the tuff bands from the ordovician and silurian formations all carry an active continental margin signature only three of the sedimentary discrimination diagrams assigned more than 50 of the samples to this setting the results from the discrimination diagrams indicate an active margin setting for the ordovician mweelrea and rosroe formations and the silurian formations whilst the derrylea and sheeffry sedimentary samples have a passive margin signature overall the results show that in the case where felsic volcanism occurred on a quartz rich source region the active continental margin field on many established discrimination diagrams is not reliable in identifying this setting however the study highlights one of the key strengths of discrimination diagrams in their ability to be used as a lever to reveal significant information about the sediment source as in this case to identify sediment that has been transported from different tectonic settings and deposited in one sedimentary basin

very low grade metamorphism of sedimentary rocks of the meliata unit western carpathians slovakia implications of phyllosilicate characteristics the meliata unit represents a melange like accretionary wedge containing blueschist facies tectonic blocks and slices in a triassic and jurassic sedimentary matrix the blueschist facies rocks are tectonic remnants of the subducted parts of the meliata hallstatt branch of the tethys the phyllosilicate assemblages in very low grade metapelites represent metastable disequilibrium stages which the assemblages have reached during reaction progress therefore temperature and pressure values of low t metamorphism of the sedimentary series and the late stages of decompressional cooling of blueschist facies rocks obtained by phyllosilicate crystallinity chlorite thermometric and white k mica geobarometric methods can be regarded as semiquantitative estimates however results of chlorite white mica thermobarometry suggest that local equilibrium was approached at a microscopic scale for deciphering the age relations of prograde and retrograde events k ar isotope geochronological methods were applied the sedimentary series and related basalts of the meliata unit experienced high t anchizonal prograde regional metamorphism the temperature and pressure of which can vary between ca 280 and 350 c and ca 2 5 and 5 kbar white k mica b geobarometry suggests possible minimal pressures of ca 1 5 to 3 kbar the mylonitic retrogression of blueschist facies phyllites is characterised by 340 c and 4 kbar minimal p the low t prograde metamorphism was synchronous with the retrograde metamorphism of the blueschists the ages of these two events may be between ca 150 and 120 ma culminating most probably at around 140 145 ma thus the upper jurassic lowermost cretaceous very low grade metamorphism of the meliata unit is younger than the subduction related 160 155 ma blueschist facies event and definitely older than the cretaceous 100 90 ma metamorphism of the footwall gemer palaeozoic

low angle extensional faulting and the emplacement of the connemara dalradian ireland a breccia mapped at the contact of the connemara dalradian and the unconformably overlying silurian succession of north galway shows evidence of having formed from strains developed at the brittle ductile deformation transition this breccia was surficially exposed just prior to the onset of an upper llandovery marine transgression the geometry of the breccia outcrop is consistent with its having developed along a regional low angle extensional fault two possible regimes of formation are proposed in the first the breccia represents the remnant of a significant low angle strike slip extensional fault along which the connemara dalradian was laterally emplaced in the second the extensional faulting may be a slightly later event related to the rapid uplift of the connemara massif both models involve rapid footwall uplift an event already inferred from metamorphic criteria the fault may be compared to major extensional detachments such as those in the basin and range province of the united states however this is the first description of an early caledonian fault of this nature and may have important consequences for future tectonic studies in this and other orogen s

displacement based seismic design of a shallow strip footing positioned near the edge of a rock slope abstract this paper presents a displacement based dynamic design of shallow strip footings located close to the edge of homogeneous and isotropic rock slopes the analysis is conducted using the generalized tangential technique in which the hoek brown strength is replaced by an optimal tangential mohr coulomb domain within the framework of the kinematic approach of limit analysis theory in order to better understand the influence of these factors including geometrical features rock strength parameters and seismic loading a parametric study is carried out an expression is derived and used to obtain a least upper bound solution for the bearing capacity of a footing on a rock slope considering a kinematically admissible log spiral failure mechanism in addition a similar expression of the horizontal yield seismic coefficient is derived for a given load acting on the foundation the assessment of earthquake induced permanent displacement is performed by a simplified procedure this requires the knowledge of a ground motion parameter that can be evaluated by site specific seismic hazard analyses or ground motion prediction equations and that considers the actual sliding surface the calculated displacement should not be greater than a selected limiting tolerable displacement for the specific case an example is presented illustrating the application of the design procedure for the bridge foundations

caledonian strike slip terrane accretion in w ireland insights from very low grade metamorphism illite chlorite crystallinity and b0 parameter analysis of pelites with detrital white micas in the clew bay galway bay segment of the irish caledonides indicates that b0 data from whole rock and 2 m fractions generally show differences smaller than the errors of the method irrespective of 001 illite crystallinity values probably due to metamorphic recrystallization intermediate pressure metamorphism of the ordovician silurian clew bay group indicates slow subduction allowing partial thermal re equilibration before exhumation in contrast the croagh patrick group laurentian shelf sediments underwent high pressure alteration suggesting rapid subduction exhumation synchronous with strike slip faulting the murrisk group which underwent high intermediate pressure metamorphism in an ordovician back arc forms a separate terrane to the croagh patrick group to the north and also to the ordovician lough nafooey and tourmakeady groups and rosroe formation in the south in which low intermediate pressure alteration occurred these together with the silurian north galway group may have undergone heating due to movement over or deposition on the hot gowlaun detachment as the connemara dalradian was exhumed the south connemara group also underwent a high pressure alteration consistent with its inferred subduction environment evidence of contact alteration due to known or inferred buried late to post caledonian granitoid plutons has been found in the clew bay louisburg clare island croagh patrick murrisk and south connemara groups these show evidence of lower pressure alteration than the surrounding country rocks

geochemical evolution of arc magmatism during arc continent collision south mayo ireland the lough nafooey arc in the western irish caledonides collided with laurentia during the early ordovician this event is recorded in the stratigraphy of the south mayo trough the preserved forearc basin of this system lavas at the base of the oldest lough nafooey group show intraoceanic arc composition end t decreases and light rare earth elements rees become more enriched upsection in the lough nafooey group ca 495 481 ma reflecting early collision with laurentia the subsequent tourmakeady group ca 481 470 ma is rhyolitic light ree enriched and has strongly negative end t these rocks were erupted during the grampian orogeny the rosroe and mweelrea formations 470 ma show wide scatter of la sm and nb zr values suggesting mixed mantle sources this correlates with rapid exhumation of the adjacent connemara metamorphic terrane the chemical evolution of the arc supports models of collision orogeny and tectonic unroofing within 15 m y and shows that genesis of magmas more enriched than continental crust can occur during arc continent collision clarifying the potential role of arc volcanism in continental crust formation

ophiolitic melange separates ortho and para tectonic caledonides in western ireland the nature of the contact between the orthotectonic late pre cambrian to cambrian and the paratectonic ordovician and silurian caledonides has long been a source of discussion in western ireland this junction occurs in a complex zone which crops out along the southern shores of clew bay county mayo where dalradian middle cambrian and ordovician arenig rocks are separated by a pre silurian rock unit termed the deer park complex1 fig 1 the complex contains both meta sedimentary and meta igneous rocks and has been variously interpreted as representing a pre caledonian basement ridge1 an imbricate thrust zone2 related to the development of a major fault system the fair head clew bay line3 or an arenig ophiolite4 we report here geological geochemical and geophy sical studies on the deer park complex which indicate that it is a northerly dipping melange unit containing the components of a dismembered ophiolite these findings imply a source of ophiolitic rocks to the north possibly within the orthotectonic caledonides

the ordovician evolution of the south mayo trough western ireland the south mayo trough a broad synclinorium containing a 10 km sequence of early ordovician turbidites passing up into shallow water sandstones is interpreted as a forearc basin with an ophiolitic basement that formed a backstop to an accretionary prism on the northern edge of a north facing arc the arc collided with the laurentian margin in early llanvirn times to deform the dalradian supergroup rifted margin clastic sequence prior to the late llandovery sinistral transcurrent faulting transposed the connemara dalradian terrane to its present position on the outboard ocean ward side of the south mayo trough the south mayo trough was fed principally by arc and ophiolite derived elastics during the arenig with an increasing metamorphic component during the llanvirn the late ordovician transpressional docking of connemara provided the southerly source for high grade metamorphic clasts in the deny veeny formation

tectonothermal history of the western lachlan fold belt australia insights from white mica studies detailed b lattice parameter and illite crystallinity ic studies of k white micas in slates from the stawell and ballarat bendigo zones sz bbz in the western lachlan fold belt of victoria australia reveal a metamorphic pattern characterized by regional metamorphism associated with crustal thickening and younger contact metamorphism accompanied by deformation the ic data indicate that rocks regionally metamorphosed prior to the intrusion of the early and late devonian granitoids vary in grade from epizonal greenschist facies to diagenetic zeolite facies and that most are of epizonal to anchizonal prehnite pumpellyite facies grade in the bbz a decrease in grade from west to east occurs across fault zones ic values show little change indicating that limited vertical displacement has occurred this is in accord with the thin skinned deformation model proposed for the western lachlan fold belt the b lattice parameters x 9 022 a n 137 s n 0 009 indicate baric conditions intermediate between those of new hampshire p al 2 sio 5 triple point and otago intermediate p thus a moderately low geother mal gradient existed 450 430 ma ago when these rocks were deformed k d fe mg actinolite fe mg chlorite values 0 52 0 70 obtained from coexisting actinolite and chlorite in metabasites from fault zones support the moderately high p c 4 kbar metamorphism suggested by the b cell parameter values the metamorphic conditions indicated by these data are contrary to the low p high t conditions pro posed by previous authors who inferred an intimate association between deformation granitoid intrusion and gold mineralization the b lattice parameter of white micas in slates adjacent to early devonian c 400 ma granitoids with schist bearing aureoles in the north eastern part of the bbz x 9 002 a n 27 s n 0 007 indicate pressures in the order of c 2 5 kbar which are in accord with those obtained from andalusite cordierite and zoisite garnet bearing assemblages observed in the higher grade metapelitic and calcareous rocks this contrasts with the higher pressure c 4 kbar existing during regional metamor phism and implies that c 6 5 8 km of metasedimentary rocks in the bbz were removed before the emplacement of the early devonian granitoids metamorphic assemblages in hornfelses associated with late devonian granitoids indicate a further 5 6 km of metasediment were removed in the next 40 ma prior to their emplacement this study shows the value of white mica studies in elucidating the tectono thermal history of a low grade metamorphic terrane dominated by metapelitic rocks

strength of jointed rock masses jointed rock masses comprise interlocking angular par ticles or blocks of hard brittle material separated by discontinuity surfaces which may or may not be coated with weaker materials the strength of such rock masses depends on the strength of the intact pieces and on their freedom of movement which in turn depends on the number orientation spacing and shear strength of the discontinuities a complete understanding of this problem presents formidable theoretical and experi mental problems and hence simplifying assumptions are required in order to provide a reasonable basis for estimating the strength of jointed rock masses for engineering design purposes this paper summarizes some of the basic information upon which such simplifying assumptions can be made a simple empirical failure criterion is presented and its application in engineering design is illustrated by means of a number of practical examples des masses jointives de rochers comprennent des particules angulaires enchevetrees ou des

the sedimentology of the south connemara group western ireland a possible ordovician trench fill sequence the stratigraphy of the ordovician south connemara group is described the group is approximately 3 km thick and includes sedimentary and volcanic rocks the sedimentary elements of the succession include conglomerates sandstones shales cherts and minor limestone breccias the lettermullen formation exhibits the thickest continuous sedimentary sequence in the group it is divided into five facies a conglomerate facies thick sandstone facies medium sandstone facies sandstone siltstone facies and chert facies the sedimentary features of these rocks include channelling and grading and are described in terms of bouma units where applicable the coarse elements of the formation are thought to represent deposition in or proximal to submarine channels feeding a more distal depositional site represented by the finer elements igneous rocks of the group include pillowed and non pillowed lavas massive amphibolites and minor tuffs often in conformable contact with the sediments petrography of conglomerate clasts and sandstones indicates derivation from a continental source area palaeocurrent analysis reveals a primary easterly mode with a subsidiary northerly mode it is suggested that the group exhibits some characteristics of trench fill sequences and the succession may be related to similar environments within the southern uplands accretionary wedge

shear strength and stress strain behaviour of athabasca oil sand at elevated temperatures and pressures the results of a series of triaxial compression tests on undisturbed samples of athabasca oil sand at elevated temperatures ranging from 20 to 200 c are summarized the material tested had experienced gradual unloading and depressurization as a result of erosion in the saline creek valley near fort mcmurray more deeply buried oil sands are known to contain much higher concentrations of dissolved hydrocarbon gases in the pore fluids the measured shear strength of athabasca oil sand did not change significantly as a result of the increased temperatures that were applied the strength of athabasca oil sand at 20 200 c was found to be greater than comparable shear strengths reported for dense ottawa sand at 20 c although heating to 200 c had little effect on shear strength it is recognized that pore pressure generation during undrained heating may cause substantial reduction of the available shearing resistance particularly in gas rich oil sands the experimental data were used to investigate the

polyphase low grade metamorphism of the ingleton group northern england uk a case study of metamorphic inversion in a mudrock succession a series of boreholes in horton quarry northwest yorkshire horton in ribblesdale inlier penetrated mudstones and slates belonging to the austwick formation windermere supergroup overlying laminated mudstones of the ingleton group illite ic and chlorite chc crystallinity measurements indicate a metamorphic inversion between the two groups of mudrocks the windermere supergroup mudrocks are mostly in the high anchizone or epizone whereas the ingletonian samples are lower grade in terms of ic and are mostly deep diagenetic zone or low anchizone hence younger strata at higher grades rest on older strata at lower grades creating a metamorphic inversion ingletonian slates exposed at pecca falls on the river twiss show epizonal and anchizonal ic values and greywacke samples from ingleton quarry contain pumpellyite this suggests that grade in the ingletonian may increase to the nw from the horton to ingleton inliers k white mica b cell dimensions show further differences between the ingleton group and the windermere supergroup the ingletonian samples are characterized by low b cell values 8 989 9 035 mean 9 007 a whereas the windermere supergroup has higher values in the range 9 022 9 034 mean 9 027 a the windermere supergroup values are similar to those recorded from the windermere supergroup of the southern lake district and lower palaeozoic rocks from the scottish southern uplands and are consistent with metamorphism in a low heat flow convergent geotectonic setting the ingletonian b cell values suggest metamorphism in a higher heat flow setting most likely an extensional basin the metamorphic inversion at horton and differences in k white mica b cell dimensions suggest that the ingleton group and windermere supergroup strata evolved in different geotectonic settings and record two separate metamorphic events the discovery of the metamorphic inversion at horton provides further evidence in favour of an ordovician rather than neoproterozoic depositional age for the controversial ingleton group

implications of new microfloral evidence from the clew bay complex for silurian relationships in the western irish caledonides the supposed laurentian margin in western ireland is separated from an ordovician arc related basin by the clew bay complex and two silurian successions informally termed the croagh patrick silurian and the louisburgh silurian whose relationships are enigmatic palynomorphs tubular structures and sheets of cuticle from turbidites within the clew bay complex indicate a silurian wenlock age in the light of these new data and reappraisal of contacts the croagh patrick silurian is now considered to be in tectonic contact with the clew bay complex the louisburgh silurian may also be in tectonic contact with parts of the clew bay complex and unconformable on other parts but is probably younger than previously thought the silurian age of elements of the clew bay complex and the relationship of the complex with other silurian successions in western ireland indicate that significant deformation occurred in this part of the caledonides during mid to end silurian times and imply that significant silurian devonian terrane movement took place

basin thermal history favours an accretionary origin for the southern uplands terrane scottish caledonides there is general agreement that the siluro ordovician southern uplands suspect terrane is an imbricate thrust belt formed adjacent to the laurentian continental margin during closure of the iapetus ocean however there are conflicting models for its tectonosedimentary evolution especially for the initial depositional environment was it a convergent forearc trench with subsequent incorporation of strata into a suprasubduction accretionary prism or was it a back arc or continental margin extensional basin subsequently caught up in a foreland fold and thrust belt to address this issue the different thermal histories of the contrasting convergent and extensional sedimentary basin models have been investigated palaeogeothermal conditions in the southern uplands terrane assessed from regional metamorphic patterns clay mineral assemblages and k white mica composition rule out extension and support deposition in a low heat flow plate convergent setting there is no evidence for a change from extensional to convergent basin regimes from north to south as required in some models instead throughout the terrane there is a close temporal and spatial relationship between deposition burial metamorphism and tectonism that is elsewhere typical of accretion

oroclinal bending in the caledonides of western ireland new palaeomagnetic data from the caledonides of western ireland indicate that the silurian rocks of south mayo underwent oroclinal bending following folding in siluro devonian time bending was accommodated on faults cutting the silurian sequence and was driven by strike slip motion across the antrim galway line a recently recognized major curvilinear lineament we propose that the silurian rocks in the east of the region suffered up to 30 clockwise rotation increasing towards the west to approximately 70 this newly inferred differential rotation of the silurian rocks about a sub vertical axis implies that the underlying dalradian rocks of the connemara massif to the south must also have undergone clockwise rotation in late silurian early devonian time this is consistent with published palaeomagnetic data from the connemara gabbro and accounts for the swing in strike of the caledonian orogen in this part of western ireland from this we deduce that the swing in strike of the caledonian rocks elsewhere on the west coast of ireland might have a similar explanation

end silurian modifications of ordovician terranes in western ireland the ordovician elements in the caledonides of western ireland may be divided into the clew bay south mayo north connemara and south connemara sub zones evidence suggests the presence of a number of proximal suspect terranes and the remnants of one or possibly two arc complexes palaeontological and sedimentary evidence suggests an allochthonous relationship between the silurian rocks of croagh patrick and north galway these successions may therefore not represent a single sedimentary overlap to the ordovician suspect terranes end silurian deformation was the result of transpression which was responsible in part for the present configuration of the area s geology deep seated shear zones with fuchsite and gold mineralization were foci for lateral translation concomitant with southward directed thrusting northward directed back thrusts and variable degrees of metamorphism being of higher grade in the north the end silurian event is intermediate between the high grade metamorphism in parts of the appalachians and the non metamorphic silurian sequences of scotland

persistence of an eddy signature in the central tyrrhenian basin during july and december 2005 two basin scale hydrographic cruises were performed in the tyrrhenian basin to monitor hydrographic conditions after the significant changes observed during previous decades a detailed investigation of the surface layer suggested the presence of a mesoscale structure in the central basin off naples its principal feature is a subsurface cold lens of western intermediate water a water mass produced in the northwestern mediterranean during the winter season the lens which appears as an anticyclonic eddy was detected for the first time in july five months later an eddy signature was detected in the same position with the same hydrographic characteristics the positive altimetric anomaly in correspondence with the observed structure suggests that the anticyclonic circulation lasted for a long period and its persistence was related to the presence of a seamount in the region for the first time the importance of isolated topography in influencing the interior tyrrhenian circulation is highlighted the vavilov seamount jointly with the weak mean current seems to be responsible for the persistence of eddy structures in the region the topographic effect is discussed on the basis of the eddy properties and theoretical findings

distribution and activity of bacteria and archaea in the different water masses of the tyrrhenian sea abstract this study examines the abundance of the bacteria crenarchaeota and euryarchaeota and bulk activities phosphatase and aminopeptidase activities heterotrophic prokaryotic production and dark co2 fixation in the major water masses of the tyrrhenian sea from surface to bottom modified atlantic water maw levantine intermediate water liw and tyrrhenian deep water tdw in july and december 2005 data from the catalyzed reporter deposition coupled with fluorescence in situ hybridization card fish analyses indicate that the percentage of bacteria was always higher than the percentage of crenarchaeota and euryarchaeota throughout the water column while the percentage of euryarchaeota was relatively homogeneous 10 through the water column the percentage of crenarchaeota increased with depth from 5 to 14 in july and from 7 to 17 in december in maw and tdw respectively regarding differences between july and december 2005 the percentage of bacteria in the maw was lower in july than in december 25 versus 43 respectively while quite constant 40 in the tdw the pattern of phosphatase and aminopeptidase activity varied according to the stations considered but both ectoenzyme activities showed higher maximum velocity rates in july than in december in the deep sea waters particularly specific activity of phosphatase in the deep sea waters tdw was 7 times higher median value than in surface waters maw prokaryotic production aminopeptidase and phosphatase activity measurements were always higher under in situ pressure conditions than after decompression for the first time the measurement of the dark co2 fixation was investigated under in situ pressure conditions and its decompressed counterparts these data give new information to understanding the role of prokaryotes bacteria and archaea in biogeochemical cycles of the meso and batypelagic waters of the oceans

the role of straits and channels in understanding the characteristics of mediterranean circulation abstract straits in the mediterranean sea form an important network from which one can determine the characteristics of the water exchange between all the constituent sub basins this includes the definition of water masses and water transport and their time variability from 1994 all the major straits in the mediterranean sea gibraltar sicily otranto balearic sea straits cretan arc straits and corsica were subject to long term observations as part of various research projects besides adding new elements to the knowledge of internal strait conditions the data sets collected allow us to propose a fairly consistent representation of the mediterranean circulation and budgets in key points within the basin the amplitude of the annual water transport measured at these straits was about 1 sv and it appears to be modulated by a significant low frequency and seasonal variability for the first time a seasonal component was identified at gibraltar thus raising new questions on the actual state of the mediterranean also the very likely existence of a significant interannual component was documented in the corsica channel this component was found to be related to the interannual variability of the north atlantic oscillation the observations in the cretan arc straits have provided a more comprehensive representation of the recent changes in the eastern mediterranean thermohaline cell it is noteworthy that the effects of these changes have been observed both in the otranto and sicily straits and are now affecting the adjacent sea regions the presence of a stream of modified atlantic water in the balearic sea channels indicates that part of the atlantic inflow may be diverted directly into the northern region of the western mediterranean finally data gathered in the sardinia channel indicate that the central mediterranean region plays a critical role in controlling exchanges between the eastern and the western mediterranean while it is emphasized that the tyrrhenian sea area plays a role in strongly modifying some of the water masses that contribute to the large scale basin circulation their mixing creates new water types which modify the currently known pattern and composition of the mediterranean circulation

geochemical evolution of an ordovician island arc south mayo ireland early ordovician volcanic rocks exposed in the south mayo region of western ireland document the history of a volcanic arc complex produced following the initiation of south dipping subduction within the iapetus ocean in the late cambrian or early ordovician lavas of the lough nafooey group tremadoc arenig show an eruption history marked by an initial period of tholeiitic basaltic volcanism followed by a shift to increasing silica compositions of calc alkaline affinity subsequent volcanism of the tourmakeady group arenig shows a mixture of rhyolitic and andesitic compositions which are in turn succeeded by tuffs of the rosroe and mweelrea formations of the murrisk group llanvirn these are also rhyolitic and andesitic but are generally more siliceous the overall increase in silica contents may reflect increasing fractional crystallization due to crustal thickening trace element studies of the basaltic members of the lough nafooey group show a strong subduction zone influence while absolute levels of immobile trace and rare earth elements decrease upsection relative depletion of the most incompatible elements decreases over the same time period these trends suggest decreasing degrees of partial melting of a progressively more depleted source mantle upsection the basaltic bohaun volcanic formation tremadoc arenig shows greater depletion in its incompatible trace elements than the lough nafooey group so indicating a more depleted mantle source comparison with modern arc systems suggests that the bohaun volcanic formation was probably generated shortly after the initiation of subduction and was erupted in a more trenchward position than the lough nafooey group i e in the forearc this confirms a south dipping polarity to the subduction zone and suggests an age of initiation during the late cambrian or earliest ordovician

metamorphic rocks of an intermediate facies series juxtaposed at the start boundary southwest england x ray diffraction analysis of metamorphic rocks from the start peninsula shows that the dominant mineral assemblage in the start schists is quartz muscovite chlorite paragonite albite the assemblage in the devonian phyllites to the north of the start boundary is quartz muscovite chlorite and the crystallinity of the white mica there is indicative of greenschist fades the b 0 parameter of white micas indicates that the metamorphism was of an intermediate facies series barrovian type but of a lower pressure variety to the north of the start boundary juxtaposed against a higher pressure variety to the south no evidence exists of the high pressure metamorphism needed to support tectonic models in which the start boundary is identified as a suture representing a variscan subduction margin

atypical gold mineralization in an orogenic setting the bohaun deposit western irish caledonides the bohaun gold deposit in county galway western ireland is located in the caledonian orogenic belt proximal to a major accretionary boundary the mineralization is hosted in a normal extensional fault representing a zone of repeated extension and vein infilling of open cavities within clastic sedimentary rocks that have only reached the late diagenetic zone of low grade metamorphism the mineralization consists of undeformed veins stockworks and breccias quartz dominates all the veins and commonly shows multiple growth stages and a variety of classic epithermal textures including comb plumose and banded indicative of low confining pressures the veins have a simple mineralogy made up of quartz sericite chlorite with minor sulfides hematite rare visible gold with a high silver content up to 41 wt and late barite and dolomite vein quartz is associated with two fluids compositions type i with moderate to high salinity 8 0 23 6 wt nacl equiv and type ii with low to moderate salinity 0 0 7 8 wt nacl equiv co2 contents in both fluid types are 3 5 wt percent the majority of fluid inclusion homogenization temperatures range from 126 to 257 c and geologically constrained depth estimates 5 km indicate maximum fluid inclusion trapping temperatures between 175 and 245 c the geodynamic setting of bohaun which is located in a deformed continental margin of an allochthonous terrane is typical of orogenic gold deposits however an orogenic association for the bohaun mineralization is discounted on the basis of fluid inclusion evidence atypical vein textures gold grain composition and undeformed veins while vein textures locally resemble those found in epithermal quartz adularia mineralization there is no obvious magmatic association and alteration is restricted to the veins and immediate host rocks the origin of the gold at bohaun remains enigmatic however we favor an association with widespread carboniferous or later mineralizing fluids and a model which involves transport and deposition of gold from a basinal brine type fluid in a similar fashion to red bed au pd mineralization bohaun provides further evidence that low temperature brines may be capable of generating economic grade gold mineralization independent of other fluids

new perspectives on the order and style of granite emplacement in the galway batholith western ireland the late caledonian galway batholith is cut by two major faults which divide it into three separate areas the western central and eastern blocks the upthrown and more deeply eroded central block is bounded by these faults in the west by the north northeast trending shannawona fault and in the east by the north northwest trending barna fault we present new granite field relations from part of the central block inveran sector which are fundamental in establishing the order and style of emplacement for the granites of the central block and the batholith as a whole unequivocal field evidence from the inveran sector indicates upward movement of early central block granites which then became the solid roof rocks to subsequent intrusions in the case of the knock granite these earlier intrusions were block stoped we use this field evidence to review the geology of the central block in a 200 km 2 area that incorporates the previously mapped costelloe and spiddal areas sharp intrusive contacts are a predominant feature of this sector of the central block and are in marked contrast to the gradational contacts recorded elsewhere in the batholith whereas juxtaposition of plutons in the western block occurred as the granites were partly crystallized the central block reveals earlier deeper level granites that were consolidated by the time they were intruded by late stage higher level granites

interlaboratory standardization and calibration of day mineral crystallinity and crystallite size data clay mineral crystallinity and crystallite domain size data determined by x ray diffraction xrd are methods extensively used in the characterization of very low grade metamorphic conditions however the lack of sufficient interlaboratory standardization has made comparisons between different research groups unreliable due to significant variations in numerical results obtained a consequence of the different machine conditions measurement methods and sample preparations used during analysis a calibration approach to the standardization of data using rock chip standards is presented which allows data sets produced by different research groups to be directly and quantitatively compared a standardized scale the crystallinity index standard cis is proposed with illite crystallinity anchizonal boundary limits of 0 25d 2 and 0 42d 2 and equivalent illite crystallite sizes of 52 and 23 nm respectively determined by the warren averbach method calibrating both old and new data will enable more reliable comparisons between similar and contrasting geological environments and should improve the accuracy and reliability of correlations made between xrd data and other indicators of very low grade metamorphism hence increasing the value of such clay mineral studies

terrane amalgamation in the clew bay region west of ireland the caledonides of the west of ireland provide a well exposed and well mapped example of an oblique collision zone the east northeast trending deer park and achill beg fault system is a crustal scale ductile sinistral strike slip duplex of late ordovician age imbricating late precambrian granulite facies lower crustal rocks near eclogite facies supracrustal rocks up to amphibolite facies dalradian metasedimentary rocks and greenschist facies cambro ordovician rocks this fault system is correlated with a pre devonian component of the highland boundary fault system in southern scotland in the clew bay area the high pressure low temperature facies metamorphic rocks in tectonic contact with greenschist facies cambro ordovician rocks are together interpreted as an accretionary prism complex related to northwestward directed subduction both of these are allocthonous terrains with respect to the dalradian terrane to the north north west mayo to the south the cambro ordovician rocks docked with a probable dalradian block containing ultramafic intrusives deer park complex during the late ordovician the deer park complex and south mayo trough linked earlier during the arenig silurian and lower middle devonian redbed successions sit unconformably on the metamorphic rocks deposition and deformation of these cover rocks was controlled by oblique strike slip movements on the leek fault whose strike swings from west northwest to north northeast following earlier basement trends as it is traced eastwards from clew bay the leek fault system may be correlated with the leannan fault of northwest donegal a splay of the great glen fault system of central scotland east of clew bay this sinistral shear generated local dilation on the more northerly trending bend of the leek fault lower and middle old red sandstone redbeds were developed here the west northwest trend of the leek fault in clew bay acted as a compressional bend during these sinistral movements and transpressional southwest directed thrusting developed in silurian rocks post middle old red sandstone pre late tournaisian dextral displacement on the leek fault reversed this pattern with transtension in clew bay allowing intrusion of small carbonated peridotite bodies into silurian rocks and easterly directed thrusting of middle old red sandstone rocks east of the bay on the transpressional north south bend a tectonic model for the region is presented here this model involves a northwestward directed subduction system 150 to 750 km of arenig sinistral strike slip movement and eastwards insertion of the connemara block with formation of the ordovician south mayo trough as a pull apart basin subsequently a further 130 to 650 km eastward displacement of rocks took place south of the deer park fault in later ordovician times the magnitudes of these estimates are directly proportional to an assumed maximum wavelength of 1500 km for promontories on the original laurentian margin and using the current juxtaposition of terranes a minimum wavelength of 300 km is inferred

variscan very low grade metamorphism in southwest england a diastathermal and thrust related origin abstract regional metamorphism in the external variscides of southwest england varied from diagenetic level to greenschist facies there is a fundamental difference in the metamorphic character between the northern and southern regions of the area in the north m1 metamorphism is of a sedimentary burial character associated with high heat flow whilst to the south it is related to tectonic burial during thrust thickening processes with lower geothermal gradients this pattern appears to be related to the character of basin development and its subsequent tectonic evolution the northern region has features that accord with a diastathermal extensional origin for the very low grade metamorphism whilst in the southern region the very low grade metamorphism is linked to thrusting as a consequence of variscan compression the tintagel high strain zone presents an anomaly in this regional pattern where an m2 metamorphic phase is attributed to localized d2 thrust stacking along the southern margin of the culm basin there is no extensive overprint of the regional metamorphic pattern by the contact aureoles surrounding the granite plutons of the region however there is a noticeable coincidence between the areas of regional epizone grade and the extent of the geophysically defined subsurface limit of the granite batholith excluding the north devon area this link is attributed largely to the late stage structural up doming of the higher grade areas over the roof of the batholith

transition from diagenesis to metamorphism in extensional and collision settings areas in which there is a transition from diagenesis to metamorphism occur in a variety of tectonic settings some of which are of an extensional character rather than the collision setting normally associated with regional metamorphism in collision settings the diagenetic to metamorphic transition is typically followed by a continuum into the highest grades of metamorphism the baric type is usually of an intermediate pressure variety and metamorphism occurs at a late stage in the tectonic cycle in extensional settings such as marginal basins a restricted facies series is developed that ranges from diagenetic levels up to a low greenschist facies maximum the metamorphism is normally of a low pressure type and develops early in the tectonic cycle these characteristics suggest that it is possible to subdivide broadly the field of low grade regional metamorphism into compressional orogenic and extensional settings

short paper the silurian of clew bay ireland part of the midland valley of scotland a new agnathan dominated biota found near louisburgh co mayo in the west of ireland is correlated with post llandovery biotas of the midland valley of scotland mvs it is argued that this biotic similarity implies a 500 km elongate aquatic connection between the two areas an alternative hypothesis that the fault bounded irish outcrop is a displaced terrane originally in closer contiguity with the mvs inliers is rejected this results from a detailed structural analysis of the bounding fault emlagh thrust which constrains the post llandovery strike slip motion to less than 25 km

use of the lattice parameter b 0 of dioctahedral illite muscovite for the characterization of p t gradients of incipient metamorphism the p t gradient or baric type of low grade metamorphic terranes can be characterized on the basis of cumulative frequency curves of the lattice parameter b0 of dioctahedral micas sassi and scolari 1974 which essentially reflects celadonite content and increases with pressure the method is here applied to illite muscovite of sub greenschist incipient metamorphism and low greenschist facies terranes from the marginal zones of the swedish caledonides swiss alps and venezuelan andes where the grade of metamorphism was determined on the basis of illite crystallinity

incipient metamorphism in the lower palaeozoic marginal basin of wales a diagenctic through anchizone to epizone transition is demonstrated in pelitic rocks of the lower palaeozoic marginal basin of wales by examination of variations in phyllo silicate mineralogy illite crystallinity and bo parameter of white micas this transition represents a temperature range from 150 c to 400 c and the metamorphism is of a low pressure facies series type with a geothermal gradient of 40 ckm 1 variations in grade can be correlated largely with the original basin and shelf form suggesting a depth related metamorphism however in areas closer to the site of caledonian plate collision an increasingly syn tectonic metamorphic event is apparent correlation of pelite data with metabasite assemblages is variable the most consistent relationship being between epizone crystallinity values andepidote actinolite greenschist facies assemblages diagenetic clay mineral assemblages are found associated with prehnite pumpellyite assemblages in metabasites and it is suggested that the latter represent non buffered and therefore non diagnostic assemblages

flow modification and shelf slope exchange induced by a submarine canyon off the northeast spanish coast we present results from a study of the modifications induced by the short timescale response of a shelf break baroclinic jet to the presence of a downstream canyon the field site was the canyon of palamos located in the northeastern edge of spain to determine the short time scale variability of the area three intensive surveys were carried out at approximately 10 day intervals data indicate that the jet s location usually over the slope is subjected to significant onshore offshore displacements analysis of the data has shown the importance of a small scale temporal variability induced by river runoff and the climatology of the area this variability generates on off shore displacements of the jet which interacts with the head mouth of the canyon the different interactions with the canyon induced by these displacements produce different regimes of shelf slope exchange in the canyon domain that strongly modify the biomass patterns in the area