provenance of the greater himalayan sequence evidence from mafic granulites and amphibolites in nw bhutan abstract mafic granulites and amphibolites in the masang kang area of nw bhutan himalaya have been investigated for their geochemical and isotopic characteristics in order to determine their protolith history bulk rock major and trace element geochemistry indicate that the rocks were originally tholeiitic and alkali basalts with minor ultramafics u pb zircon sims data suggest an age of 1742 39 ma for mafic magmatism the age corrected e nd 1742 values of the rocks are highly variable ranging from high positive 8 4 to negative 3 3 the positive value suggests a primitive magma source similar to that of rift related tholeiites we suggest that the rocks of the masang kang suite were produced during a major late paleoproterozoic thermal event that caused the mobilization and enrichment of the sub continental lithospheric mantle beneath the north indian margin the geochemical signature of these rift related metabasic rocks may have been produced during an earlier episode of oceanic underplating or subduction from which the fluid required to mobilize and enrich the overlying sub lithospheric mantle may have been derived though their occurrence is rare paleoproterozoic igneous rocks within the greater himalayan sequence ghs in addition to sources identified throughout the lhs may have contributed to the detrital zircon population that form the 1 7 1 9 ga peak in the age spectra of the lesser himalayan sequence lhs in addition the coeval paleoproterozoic magmatism in both lhs and ghs suggests that the two lithotectonic units may have belonged to the same continental plate at that time period

the caldwell group lavas of southern quebec morb like tholeiites associated with the opening of iapetus ocean the caldwell group belongs to the internal nappe domain of the humber zone and consists of basaltic lavas quartzo feldspathic sandstones and mudslates the lavas are clinopyroxene plagioclase olivine phyric tholeiites and are typically altered to epidote chlorite carbonate and or hematite rich secondary assemblages in most cases the high field strength elements do not appear to have been perturbed by the alteration and preserve magmatic signatures most caldwell basalts exhibit coupled major and trace element variations compatible with low to medium pressure 10 kbar where 1 kbar 100 mpa fractional crystallization paleotectonic discriminants imply an ocean floor or normal mid ocean ridge basalt n morb affinity most basalts have flat n morb normalized profiles except for the highly incompatible elements ba th nb which show slight relative enrichment melting models suggest that most of these lavas formed by about 20 melting from a mantle slightly less depleted than fertil

ferro andesites in the grande ronde basalt their composition and significance in studies of the origin of the columbia river basalt group flows from the grande ronde basalt of the columbia river basalt group include several with the chemical composition of ferro andesites these flows have sio2 55 mgo 4 and also have higher fe mg than the average value for grande ronde basalt they are also distinctly richer in cs rb k ba la ce th and u than the remainder of the grande ronde basalt flows and possess small negative eu anomalies their pb isotopic compositions define a mixing line with a negative slope on a 206pb 204pb versus 207pb 204pb plot their nd isotopic compositions lie between 143nd 144nd 0 51252 and 0 51264 and their sr isotopic compositions lie between 87sr 86sr 0 7055 and 0 7060 these values define the enriched end of the columbia river basalt group spectrum on a sr nd epsilon diagram excluding the saddle mountains basalt these ferro andesite flows also form a compositional end member of the columbia river basalt group and are sufficiently distinctive to warrant special consideration we compare them with

columbia river flood basalts from a centralized crustal magmatic system the columbia river basalt group in the northwestern united states derived from flood basalt eruptions that occurred 16 million years ago exhibits variability in geography and trace element geochemistry that has led to a number of proposed magma origins however the geochemical variability can be explained by a relatively simple model in which magma is derived from a mantle plume that assimilated continental crust in a centralized magma system the columbia river basalt group in the northwestern united states1 comprising about 230 000 cubic kilometres of rock exhibits unusual patterns in lava distribution geochemistry and its apparent relationship to regional tectonics consequently there is little consensus on the origin of its magmas2 3 4 5 6 7 8 9 10 11 12 here we examine the isotopic ratios of sr nd pb and os and trace element abundances in columbia river basalts the results suggest that most of the lava was produced when magma derived from a mantle plume assimilated continental crust in a central magma chamber system located at the boundary between the north american craton and the accreted terranes of idaho and oregon other related basalts are the product of mixing between the mantle plume and different types of regional upper mantle magma was then transported over a wide region by an extensive network of dykes a process that has been identified in other flood basalt provinces as well13 interactions of the plume with surrounding upper mantle and of mantle derived magmas with regional crust provide a relatively simple6 7 9 14 15 model to explain the more unusual features of the main phase columbia river basalts

a cryptic caledonian flysch terrane in scotland a substantial part of the lower old red sandstone crawton group in the north east midland valley of scotland has a first cycle provenance from the south and east i e from a source block within the midland valley from the clast assemblages in proximal conglomerates it is clear that this provenance comprised a flysch terrane cut by granodioritic and tonalitic plutons angular clasts of lithic arenite and metagreywacke derived from this cryptic flysch sequence show it to be distinct from a metasedimentary sequence which flanked the lower old red sandstone basin to the north and north east the basin must overlie or lie adjacent to a boundary between two quite different sedimentary assemblages petrographic examination and chemical analysis of the clasts show that the southerly cryptic succession includes an important mafic ultramanc provenance implying that ophiolitic source rocks may have originally contributed to it sr and nd isotopic compositions for the clasts establish that the midland valley flysch is isotopically unrelated to either southern uplands shales and greywackes or dalradian metasedimentary rocks the clasts have lower sr contents than either suite and sr isotopic compositions 87 sr 86 sr in the range 0 7099 0 7176 at 450 ma which are more radiogenic than the southern uplands but less radiogenic than the dalradian crustal residence ages t dm have a narrow range of 1 76 1 94 ga the mean of 1 87 0 06 ga compares with a mean residence age of 1 49 0 21 ga for southern uplands greywackes and shales mixing calculations demonstrate that the incorporation of young ophiolite derived basaltic detritus into the caledonian sedimentary mass can explain properties of the nd data previously attributed to a bimodal proterozoic provenance the distinct compositions of the southern uplands and midland valley flysch sediments suggest they originally tapped different parts of the laurentian margin and have subsequently been brought together structurally as displaced fore arc slivers the diverse isotopic compositions of outboard flysch terranes has implications for the nature of the sedimentary component incorporated in inboard magma suites

tertiary calc alkaline magmatism associated with lithospheric extension in the pacific northwest the long standing assumption that calc alkaline magmatism requires contemporaneous subduction is critically reviewed two of many widely spread fields of tertiary calc alkaline magmatism in the pacific northwest one eocene and one miocene provide clear examples of the association between calc alkaline magmatism and lithospheric extension we suggest that the calc alkaline magmatism of these fields is better explained by decompression melting of the subcontinental lithospheric crust and mantle enriched by previous subduction events than by contemporaneous eocene subduction this model proposes that extension will cause calc alkaline dominated magmatism in areas of continental crust with a history of previous subduction as along the western margin of north america currently popular models for the tectonic magmatic evolution of the basin and range province in which subduction driven calc alkaline magmatism is followed by extension related bimodal magmatism may need modification much more of this magmatism may result from extension in the pacific northwest we suggest that subduction beneath the idaho batholith ceased at the end of the cretaceous blocked by the accreted terranes to be reactivated in its present position beneath the cascades during the eocene the various isolated eocene volcanic fields spread across the pacific northwest were due to lithospheric extension not to the presence of a subducting plate immediately below

a dynamical model of lithosphere extension and sedimentary basin formation convection in the upper mantle can give rise to normal stresses on the base of the continental lithosphere that may cause localized elevation of its surface and extensional deviatoric stresses this possibility is investigated assuming homogeneous biaxial strain in a continental lithosphere whose strength is governed by a combination of brittle failure and power law creep in the crust and by brittle power law or dorn law behavior in the mantle for a given geotherm and for rheologies based on current laboratory data the lithosphere responds to convection induced uplift in one of three ways depending primarily on the magnitude of the driving stresses these regimes are as follows 1 when the driving stress exceeds a threshold value the resulting strain rates are high enough greater than about 3 10 16 s 1 that the lithosphere strains almost isothermally its effective rheology is strain thinning and it undergoes accelerating extension the magnitude of the dynamic uplift required to produce the threshold value of the driving stress depends on the geotherm characterized here by the moho temperature and varies from about 1 km for a moho temperature of 700 c to around 3 km for a moho temperature of 550 c an uncertainty of at least 100 c is attached to these temperatures owing to the inaccuracies in extrapolating laboratory data to geological strain rates 2 at stress levels due to less than about half of the critical uplift extensional strain is negligible 3 at intermediate stress levels moderate uplift produces extension that is bounded by strength increases due to cooling of the ductile portion of the lithosphere during extension

chemostratigraphy of the neoproterozoic alona bay lavas ontario a basal sequence of flood basalt lavas associated with the neoproterozoic midcontinent rift system crops out in alona bay along the southeastern shore of lake superior in ontario the alona bay lava succession is about 1200 m thick and lies just north of the well studied contemporaneous mamainse point formation detailed chemostratigraphy of the alona bay lavas suggests they are grossly correlative with the basal portion of the mamainse point formation for instance like the basal part of mamainse point formation the alona bay section contains numerous high mgo lavas and can be subdivided into 4 5 groups with distinct chemical characteristics chemical variations within the alona bay groups are largely the result of fractional crystallization likely at moderate pressures one small group of alona bay lavas also carries the compositional imprint of crustal contamination the remaining inter group chemical distinctions at alona bay are the consequence of temporal changes in partial melting and source ch

advances in borehole imaging technology and applications abstract borehole imaging devices are currently available for both wireline and logging while drilling lwd environments for use in both openhole and cased wells most wireline and all lwd devices provide indirect images that are derived from a high density grid of electrical or ultrasonic acoustic measurements optical techniques such as television and photography provide direct images of the borehole imaging devices consist of 1 a rotating sensor acoustic or electrical that scans the borehole with each revolution 2 multiple azimuthally placed sensors or 3 a downhole television or photographic camera advances in digital acquisition and processing permit real time display and evaluation of images of the open borehole or the inside of casing the resolution and borehole coverage of microelectrode devices has increased with newer designs that allow additional measurement sensors per pad additional pad segments on a four arm tool and the introduction of six arm imaging tools in nonconductive borehole fluids a four arm microinduction device can provide crude images macroelectrode imaging tools laterolog type wireline provide lower resolution electrical images from either an azimuthal arrangement of sensors around a mandrel wireline or from azimuthally sensitive sensors logging while drilling that scan the borehole during rotation of the bottomhole assembly in acoustic imaging recent devices incorporate design changes that provide improved image resolution and operate in a wider range of borehole conditions ultrasonic transducers now use lower transmitting frequencies and spherical focusing and these tools also offer surface selectable acquisition parameters acoustic and microresistivity imaging techniques measure different physical properties and these data are complementary one recently introduced wireline service combines microresistivity and acoustic imaging devices in a single tool for simultaneous single pass acquisition recent advances in cable and sonde design permit downhole video in a wider range of petroleum environments in flowing wells and in tubing openhole image interpretation involves qualitative visual identification and quantitative characterization of strike and dip on planar features lithology bedding fractures and vugs when core is available features identified on log derived images can be correlated or calibrated to it or when core is absent images may serve as a substitute borehole images are useful in formation evaluation when calibrated sedimentology stratigraphy and structural analysis current applications include core orientation reconstructing missing core identification and characterization of fractures and depositional features e g bedforms reservoir architecture bedding grain size porosity permeability net sand and net pay counts faults folds and fractures hydrocarbon water geothermal wells and in rock engineering evaluation of borehole stress borehole stability directional placement of wellbore for optimal drainage fractured reservoirs evaluation of fracture stimulations and perforation control cased hole applications include cement evaluation casing inspection detection of fluid entry identifying production blockages and assisting in fishing operations

a late paleoproterozoic 1 80 ga subduction related mafic igneous suite from lomza ne poland the lomza orthoamphibolites in the crystalline basement of ne poland dated at 1802 9 ma by shrimp zircon u pb are characterized by high incompatible and ree element contents these features the low nb and the position of the rock compositions on la y nb zr nb zr and zr nb y tectonomagmatic discrimination diagrams suggest that the metavolcanics formed in a subduction setting possibly within a continental margin island arc the likely extension of the belt of paleoproterozoic mafic magmatism from latvia and lithuania into ne poland provides convincing evidence for the existence of an active plate margin in that region at that time

permian volcanism in the central western carpathians slovakia basin and range type rifting in the southern laurussian margin the 1 500 to 2 000 m thick permian volcano sedimentary malu ina formation of the uppermost nappe of the central western carpathians a segment of the alpine carpathian orogenic belt occurs in several fault blocks distributed across slovakia this unit is a part of a post variscan overstep suite that followed accretion of the gothic terranes to laurussia it consists of three upward fining megacycles of semi arid arid fluvial lacustrine clastic redbeds and local dolomites and evaporites abundant intercalated volcanic rocks are predominantly mafic lava flows volcaniclastic rocks and dykes are subordinate felsic rocks are represented by rare volcaniclastics and dykes compositionally the mafic rocks are rift related continental tholeiites with enriched light ree patterns having la yb n ratios between 2 and 5 5 and with mantle normalized patterns characterized by negative nb ta anomalies the rocks were derived from subcontinental lithospheric mantle and were affected by crustal contamination it is inferred that the volcanism of the malu ina formation formed in a basin and range tectonic setting in which rifting followed collision of the palaeo tethys ridge with the trench bordering southern laurussia this model can be applied to other permian volcanic suites of rift basins in the eastern alps and carpathians over a strike length of about 1 000 km which indicates the width of the slab window

tectonic controls on magma genesis and evolution in the northwestern united states abstract field chronologic chemical and isotopic data for late cenozoic basaltic rocks from the northwestern united states illustrate the relationship between crustal structure and tectonic forces in controlling the genesis and evolution of continental volcanism in the northwestern u s the first major episode of basaltic volcanism was triggered by crustal rifting in a back arc environment east of the westward migrating volcanic arc created by the subduction of the juan de fuca plate beneath the north american plate rifting and volcanism were concentrated by pre existing zones of crustal weakness associated with boundaries between the old archean core of the continent and newly accreted terranes basalts erupted during this time columbia river steens mountain show evidence of significant fractionation histories including contamination by crust of varying age depending on the crustal structure at the eruption site presumably this reflects ponding and stagnation of primary magmas in the crust or at the crust mantle interface due to their encounter with thick crust not yet extended and still containing its low density easily fusible component continued rifting of this crust and modification of its composition through extraction of rhyolitic partial melts and deposition of the fractionation products from primary basaltic melts coupled with a shift in stress orientation roughly 10 5 ma ago allowed relatively unfractionated and uncontaminated magmas to begin reaching the surface in the western part of the region oregon plateau these magmas tapped a mantle source similar to that which produced most of the ocean island basalts of the northern hemisphere to the east snake river plain however the mantle sampled by basaltic volcanism has isotopic characteristics suggesting it has preserved a record of incompatible element enrichment processes associated with the formation of the overlying archean crustal section some 2 6 ga ago

geochemistry of the miocene mount noel volcanic complex british columbia and comparison with the columbia river basalt the 19 6 ma mount noel volcanic complex mnvc is located about 150 km east of the garibaldi volcanic belt and adjacent to the chilcotin plateau basalts in southwestern british columbia the mnvc unconformably overlies jurassic to cretaceous metasedimentary rocks and lower cretaceous coast plutonic rocks the complex consists of flat lying columnar jointed basalt flows along with volcaniclastic debris flows and minor pyroclastic rocks basaltic flows have olivine fo 60 to fo 85 and plagioclase an 60 to an 80 phenocrysts with clinopyroxene microphenocrysts major trace and rare earth element analyses of the mnvc indicate the flows can be divided into two groups the largest group comprises tholeiitic to transitional basalt whereas a smaller subset is alkaline basalt rare earth element patterns are slightly enriched in the light rare elements in the transitional group and more strongly enriched in the alkaline basalts in tectonic discriminant diagrams all basalts plot in the within plate fields or in overlap areas typical of continental flood basalts alkaline basalts found only at the base of the thick sequence of tholeiitic to transitional basalts are characterized by high p 2 o 5 sr and ba contents the mnvc tholeiitic to transitional basalts are comparable to the chilcotin group basalts and to the lowermost basalts of the columbia river group the origin of mnvc basalts is best interpreted as volcanism behind the pemberton volcanic arc

modeling magma flow and cooling in dikes implications for emplacement of columbia river flood basalts 1 the columbia river flood basalts include some of the world s largest individual lava flows most of which were fed by the chief joseph dike swarm the majority of dikes are chilled against their wall rock however rare dikes caused their wall rock to undergo partial melting these partial melt zones record the thermal history of magma flow and cooling in the dike and consequently the emplacement history of the flow it fed here we examine two dimensional thermal models of basalt injection flow and cooling in a 10 m thick dike constrained by the field example of the maxwell lake dike a likely feeder to the large volume wapshilla ridge unit of the grande ronde basalt two types of models were developed static conduction simulations and advective transport simulations static conduction simulation results confirm that instantaneous injection and stagnation of a single dike did not produce wall rock melt repeated injection generated wall rock melt zones comparable to those observed yet the regular texture across the dike and its wall rock is inconsistent with repeated brittle injection instead advective flow in the dike for 3 4 years best reproduced the field example using this result we estimate that maximum eruption rates for wapshilla ridge flows ranged from 3 to 5 km3 d 1 local eruption rates were likely lower minimum 0 1 0 8 km3 d 1 as advective modeling results suggest that other fissure segments as yet unidentified fed the same flow consequently the maxwell lake dike probably represents an upper crustal 2 km exposure of a long lived point source within the columbia river flood basalts

the yellowstone hotspot in space and time nd and hf isotopes in silicic magmas abstract over the course of its 16 m y history the yellowstone hotspot has produced silicic magmas exhibiting systematic and often sympathetic variations in isotopic and chemical composition temperature and frequency of eruption nd and hf isotopic ratios vary systematically from initial eruptions at 16 ma contemporaneous with basaltic volcanism in eastern oregon and washington to the present day yellowstone volcanic plateau nd and hf isotopic ratios co vary and span the range of most terrestrial samples reflecting mixing of mantle and crustal sources earliest erupted silicic magmas were hot in excess of 1050 c relatively less evolved and have isotopic ratios within the range of contemporaneous columbia river flood basalts the transit of the hotspot across the lithospheric boundary between the western accreted oceanic terrain and the precambrian craton at 15 ma is marked by shifts in e nd from 4 to 11 and in e hf from 10 to 10 the duration of the transit yields a crustal magma source diameter of 70 km in the interval from 14 to 9 ma e nd systematically increases from 11 to 7 recording a minimum increase in the mantle component from 5 to 30 the mantle component could be twice as great depending upon the isotopic composition of crust and mantle reservoirs in this same interval peak temperatures of 1000 c occurred at 9 ma the last 8 m y are characterized by less frequent eruption of lower temperature 830 900 c and more compositionally evolved magmas

phase relations and liquid lines of descent in hydrous ferrobasalt implications for the skaergaard intrusion and columbia river flood basalts crystallization experiments using a hydrous ferrobasalt as starting material conducted at 200mpa 940 12008c at a wide range of water activities 0 1 1 and redox conditions qfm 3 to qfm 4 where qfm is the quartz fayalite magnetite oxygen buffer show that h2o influences significantly the differentiation history of ferrobasaltic magmas a combination of our data with published experiments on dry ferrobasalt at 1atm provides an extensive experimental database for modeling and quantifying crystallization and differentiation processes within a typical fe rich tholeiitic system under both dry and hydrous conditions the addition of h2o decreases liquidus temperatures and changes significantly the proportions temperature range and sequence of crystallizing mineral phases the dry liquidus is at about 11708c whereas the liquidus for h2o saturated melts is at 10608c the main phases crystallizing from h2o bearing ferrobasalt at the investigated conditions are olivine ol clinopyroxene cpx plagioclase pl magnetite mt hematite hm ilmenite ilm and amphibole am the phase assemblage is similar to that of the dry system except for the presence of hm at extremely oxidizing conditions and am at low temperatures 59508c and h2o saturated conditions the important observation made in this study is that the stability of fe ti oxides and in particular mt as well as the simultaneous coprecipitation of mtand ilm are almost independent of the activity of h2o ah2o in the system whereas the liquidus temperatures of the silicate minerals are dramatically depressed by increasing ah2o the stabilities of oxides are controlled mainly by the redox conditions prevailing in the system the most pronounced effect of ah2o on the liquidus temperatures of silicates is observed for pl which shows a considerable delay in crystallization with progressive magma differentiation early crystallization of fe ti oxides in h2o bearing ferrobasaltic compositions precludes any significant fe enrichment during differentiation as fe enrichment is a characteristic feature of the skaergaard intrusion it implies that the skaergaard parental magma did not contain considerable amounts of water on the other hand our experiments indicate that the differentiation of some ferrobasaltic series from the columbia river flood basalt province might have occurred in magmatic systems containing significant amounts of volatiles 0 5 3 wt h2o

back arc convection model for columbia river basalt genesis abstract genesis of the columbia river basalt group crbg may be accounted for by the progressive thermal erosion of an attenuated heterogenous subcontinental lithosphere scl by back arc convection behind the cascade arc assembly of the source components of the crbg began during the early mesozoic when slices of former oceanic lithosphere were isolated below terranes accreted to the continental margin subsequent lithospheric shearing accompanying northward translation of the terranes allowed the injection of small degree partial melts from the asthenosphere which solidified in the scl as pyroxenite veins the scl source of the crbg underwent a further episode of shearing accompanied by attenuation during the eocene and oligocene when it was caught between contrasting right lateral terrane translation and e w extension regimes in the northern cordillera and basin and range province respectively the attenuation resulted in the raising of the geotherm close to that of the pyroxenite and iherzolite solidi melting was triggered by back arc convection behind the cascade arc the pyroxenite was the first component mobilised constituting the principal source of the imnaha formation continued back arc convection resulted in the failure of the scl and the generation of the voluminous grande ronde formation from pyroxenite and the surrounding scl the change to wanapum volcanism marks the stage of exhaustion of in situ sources and their replacement with a combination of proterozoic scl from the east and asthenospheric material by back arc convection the saddle mountains formation was derived entirely from proterozoic scl small magma volumes resulted in a susceptibility to local contamination such that the distinct isotopic arrays of the saddle mountains formation comprise three mixing lines and do not have any age significance

comment and reply on age and origin of ballantrae ophiolite and its significance to the caledonian orogeny and the ordovician time scale reply

uplift rupture and rollback of the farallon slab reflected in volcanic perturbations along the yellowstone adakite hot spot track field geochemical and geochronological data show that the southern segment of the ancestral cascades arc advanced into the oregon back arc region from 30 to 20 ma we attribute this event to thermal uplift of the farallon slab by the yellowstone mantle plume with heat diffusion decompression and the release of volatiles promoting high k calc alkaline volcanism throughout the back arc region the greatest degree of heating is expressed at the surface by a broad ene trending zone of adakites and related rocks generated by melting of oceanic crust from the farallon slab a hiatus in eruptive activity began at ca 22 20 ma but ended abruptly at 16 7 ma with renewed volcanism from slab rupture occurring in two separate regions the eastern rupture resulted in the extrusion of steens basalt during the ascent and melting of a dry mantle plume source contaminated with depleted mantle the contemporaneous western rupture resulted in renewed subduction melting of a wet mantle source and the rejuvenation of high k calc alkaline volcanism near the nevada california border at 16 7 ma here the initiation of slab rollback is evident in the westward migration of arc volcanism at 7 8 km ma today the uplifted slab is largely missing beneath the oregon back arc region replaced instead by a seismic hole which is bound on the south by the adakite hotspot track we attribute slab destruction to thermal uplift and mechanical dislocation that culminated in rapid tearing of the slab from 17 15 ma and possible foundering and sinking of slab segments from 16 10 ma

palaeozoic within plate volcanic rocks in nova scotia canada reinterpreted isotopic constraints on magmatic source and palaeocontinental reconstructions palaeozoic volcanism in the avalon terrane of northern nova scotia occurred during three time intervals cambrian early ordovician late ordovician early silurian and middle late devonian in the meguma terrane of southern nova scotia palaeozoic volcanism is limited to the middle ordovician geochemical data show that most of these volcanic rocks are bimodal within plate suites initial e nd signa tures range from 5 4 to 1 9 in the rhyolites and 6 8 to 2 7 in the basalts a difference attributable to the absence or presence respectively of a significant crustal component the data and regional tectonic settings of the avalon and meguma terranes suggest that the volcanism was generated in three different within plate settings 1 cambrian early ordovician volcanism related to thermal decay of late proterozoic arc magma tism during transtensional deformation 2 middle ordovician early silurian volcanism during sinistral tele scoping between laurentia and gondwana where extensional bends in the appalachians produced rifting and 3 devonian volcanism resulting from lithospheric delamination during dextral transpression and tele scoping in each setting active faults served as conduits for the magmas nd isotopic data indicate that the source of the palaeozoic felsic volcanic rocks is isotopically indistinguishable beneath southern and northern nova scotia and did not substantially change with time this crustal source appears to have separated from the mantle during the proterozoic a conclusion consistent with the hypothesis that the palaeozoic rocks in nova scotia were deposited upon a late proterozoic oceanic cratonic volcanic arc terrane the nd data when combined with published faunal palaeomagnetic and u pb isotopic data suggest that the avalon terrane was peripheral to gondwana off northwestern south america during neoproterozoic and early palaeozoic times

dynamics of magma emplacement in centrifuge models of continental extension with implications for flank volcanism a set of experimental small scale models of continental extension investigated the emplacement of orthogonal and oblique magma chambers initially underplated at the base of the crust the models are driven by a centrifugal body force which simulates the role of gravity in nature the models represent crustal conditions analogous to relatively mature continental rifts and consider both symmetric and asymmetric extension the experimental results suggest that magma emplacement is controlled by the interactions between tectonics and rheology of the crustal layers in particular extension is mainly accommodated by lateral flow and ductile doming in the viscous layer simulating the lower crust and by listric normal faults in the overlying brittle layer associated with the ductile domal uplift stretching of the continental crust induces a reactive migration of viscous layers into the footwall of major normal faults where magma accumulation at the core of the domes takes place this behavior suggests that widespread magmatism is expected to localize in the footwall of major normal faults a situation that is often observed in core complex structures although the models presented here are intended to simulate magma emplacement during extension of a two layer brittle ductile system lateral flow and migration of magma initially underplated at the base of the crust beneath narrow rifts e g the ethiopian rift may also provide a similar mechanism and a possible explanation for the occurrence of important volcanoes on the plateaus flanking the rift zones in the suggested model such volcanoes are related to large basaltic magma reservoirs located at the base of the crust to accommodate the space vacated by lateral flow and thinning of the ductile crust the similarity in structure of the models with natural examples of continental extension modes such as rift systems and core complexes may suggest a close similarity of dynamic processes

a miocene subcontinental plume in the pacific northwest geochemical evidence most of the recent discussions on the number and kinds of components which can be distinguished in the columbia river basalt group crbg magmas have used arguments developed from isotopic evidence in this paper we consider relative contents of excluded trace elements interpreted by means of spidergrams and abundance ratios and compare data from the crbg and related lavas with those of selected oceanic basalts although many recent models for crbg petrogeneses do not include a plume component spidergrams for the american bar flows of the imnaha basalt of the crbg have broad humps at ba th and nb ta this feature suggests that an enriched mantle plume component is present in the american bar magmas an inference which is weakly confirmed by 87sr 86sr data this plume may have been identical to that presently beneath yellowstone the inferred hot spot track lies almost directly east west and requires a vector of motion of the pacific northwest part of the north american plate significantly different from that inferred by minster and jordan 1978 31 our proposed vector for the north american plate agrees however with that suggested by the anahim belt a volcanic lineament in southern british columbia that has been proposed as a hot spot track this proposed vector requires alterations in current models for pacific and north american plate movement during the cenozoic

chemical discrimination of columbia river basalt flows 1 the homogeneity and distinctive chemical composition of individual columbia river basalt flows are illustrated in a series of chemical plots and the procedure for flow identification is outlined the unique characterization of many individual eruptions permits units to be mapped from feeder dike to flow margins across the columbia plateau for hundreds of kilometers and so provides critical constraints on the physical and chemical evolution of this continental flood basalt province

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

the eocene oligocene magmatic hiatus in the south central canadian cordillera a capture of the kula plate by the pacific plate the tertiary paleogene and neogene geological record in south central canadian cordillera is dominated by the 350 400 km wide lower eocene volcanic arc and the overlying miocene recent back arc lavas that are separated by a hiatus in magmatic activity between 48 and 24 ma in the black dome area 240 km north of vancouver the eocene volcanic rocks are mainly continental margin calc alkaline andesite and dacite resulting from the melting of a juvenile mafic source at the base of the crust in contrast the miocene volcanic rocks resemble continental flood basalts both eocene and miocene rocks from the black dome volcanic complex have high positive end values 7 2 to 7 4 and 6 4 to 7 6 respectively and low initial sr isotopic ratios 0 702 516 0 703 528 and 0 703 376 0 703 392 respectively comparable to modern oceanic basalts the onset of the hiatus in magmatism at 48 ma coincides with capture of the kula plate by the pacific plate resulting in a change in convergence direction with the

a clinopyroxene basalt geothermobarometry perspective of columbia plateau nw usa miocene magmatism this article describes investigations into the pre eruptive evolution of columbia river basalt group magmas to constrain pressures and temperatures of ascending magmas and plumbing conditions

dissolution rates and vadose zone drainage from strontium isotope measurements of groundwater in the pasco basin wa unconfined aquifer abstract strontium isotope ratios 87sr 86sr measured in groundwater samples from 273 wells in the pasco basin unconfined aquifer below the us department of energy hanford site show large and systematic variations that provide constraints on groundwater recharge weathering rates for saturated zone dissolution of the aquifer host rocks communication between unconfined and deeper confined aquifers and vadose zone groundwater interaction the impact of millions of cubic meters of wastewater discharged to the vadose zone 103 105 times higher than ambient drainage shows up strikingly on maps of groundwater 87sr 86sr extensive access through the many groundwater monitoring wells at the site allows for an unprecedented opportunity to evaluate the strontium geochemistry of a major aquifer hosted primarily in unconsolidated sediments and relate it to both long term properties and recent disturbances groundwater 87sr 86sr increases systematically from 0 707 to 0 712 from west to east across the hanford site in the general direction of groundwater flow as a result of addition of sr from the weathering of aquifer sediments and from diffuse drainage through the pasco basin vadose zone the lower 87sr 86sr groundwater reflects recharge waters that have acquired sr from columbia river basalts based on a steady state model of sr reactive transport and drainage there is an average natural drainage flux of 0 1 4 mm yr near the western margin of the hanford site and ambient drainage may be up to 30 mm yr in the center of the site assuming an average bulk rock weathering rate of 10 7 5 g g yr the weathering rates cannot be higher than those assumed by more than a factor of 2 3 in order to match sr isotope data

the chon aike province of patagonia and related rocks in west antarctica a silicic large igneous province the field occurrence age classification and geochemistry of the mesozoic volcanic rocks of patagonia and west antarctica are reviewed using published and new information dominated by rhyolitic ignimbrites which form a bimodal association with minor mafic and intermediate lavas these constitute one of the largest silicic igneous provinces known equivalent in size to many mafic lips diachronism is recognized between the early middle jurassic volcanism of eastern patagonia marifil and chon aike formations and the middle jurassic earliest cretaceous volcanism of the andean cordillera el quemado ibanez and tobifera formations this is accompanied by a change in geochemical characteristics from relatively high zr and nb types in the east to subalkaline arc related rocks in the west although the predominance of rhyolites remains a constant factor all of the associated mafic rocks are well fractionated compared to direct mantle derivatives petrogenetic models favour partial melting of immature lower crust as a result of the intrusion of basaltic magmas possibly with some hybridisation of the liquids and subsequent fractionation by crystal settling or solidification and remelting the formation of large amounts of intracrustal silicic melt acted as a density barrier against the further rise of mafic magmas which are thus rare in the province

astoria fan sediments dsdp site 174 cascadia basin hf nd pb constraints on provenance and outburst flooding abstract the astoria submarine fan located off the coast of washington and oregon has grown throughout the pleistocene from continental input delivered by the columbia river drainage system enormous floods from the sudden release of glacial lake water occurred periodically during the pleistocene carrying vast amounts of sediment to the pacific ocean deep sea drilling project dsdp site 174 located on the southern distal edge of the astoria fan is composed of 879 m of terrigenous sediment the section is divided into two major units separated by a distinct seismic discontinuity an upper turbidite fan unit unit i and an underlying finer grained unit unit ii both units have overlapping ranges of nd and hf isotope compositions with the majority of samples having e nd values of 7 1 to 15 2 and e hf values 6 2 to 20 0 the most notable exception is the uppermost sample in the section which is identical to modern columbia river sediment nd depleted mantle model ages for the site range from 2 0 to 1 2 ga and are consistent with derivation from cratonic proterozoic source regions rather than cenozoic and mesozoic terranes proximal to the washington oregon coast the astoria fan sediments have significantly less radiogenic nd and hf isotopic compositions than present day columbia river sediment e nd 3 to 4 goldstein s j jacobsen s b 1987 nd and sr isotopic systematics of river water suspended material implications for crustal evolution earth planet sci lett 87 249 265 and suggest that outburst flooding tapping proterozoic source regions was the dominant sediment transport mechanism in the genesis and construction of the astoria fan pb isotopes form a highly linear 207 pb 204 pb 206 pb 204 pb array and indicate the sediments are a binary mixture of two disparate sources with isotopic compositions similar to proterozoic belt supergroup metasediments and columbia river basalts the combined major trace and isotopic data argue that outburst flooding was responsible for depositing the majority top 630 m of the sediment in the astoria fan