nd sr isotopic composition of granulites and constraints on the evolution of the lower continental crust in a statistical study of granulite facies rocks we have analysed 32 samples with ages ranging from 2 900 to 200 myr for sm nd rb and sr and 143nd 144nd and 87sr 86sr ratios the sm nd ratio is found to be constant compared with the highly variable rb sr ratio using the model age formalism we show that in many cases internal differentiation of continental crust follows external differentiation with a large time interval

nd isotope systematics of coarse and fine grained sediments examples from the middle proterozoic belt purcell supergroup sm nd isotopic compositions are reported for 20 fine and 5 coarse grained samples of the belt purcell supergroup of western north america varepsilon nd and sm nd ratios for fine grained belt purcell sediments are remarkably uniform indicating crustal residence ages ranging only from 1 6 to 2 1 ga varepsilon nd and sm nd ratios for coarse grained sediments however are much more varied and contain evidence of sm nd fractionation during sedimentary recycling assuming that this ree fractionation occurred between the time source areas were eroded and when sediment was deposited a new method of calculating crustal residence age intervals is derived for the coarse grained samples which serves to place constraints on the average time since source materials were extracted from the mantle the nd data from both sample groups provide an average age estimate of 2 0 pm 0 3 ga for the middle proterozoic crust from which most of the belt purcell supergroup was derived the isotopic data toge

ndsr isotopic study of proterozoic to triassic sediments from southeastern british columbia abstract sedimentary rocks from the southeastern canadian cordillera varying in age from mid proterozoic to triassic have been analysed for their nd isotopic composition e nd t values range from 1 5 to 17 2 and nd model ages t dm nd vary from 1 00 to 2 32 ga much in excess over their stratigraphic ages the difference between the two ages ranges from 0 43 to 1 93 ga decreasing as the sediments become stratigraphically older nd isotopic compositions of sediments suggest that there are two distinct continental sources of sediments in the cordilleran miogeocline a an older source e nd 0 from 20 3 to 23 8 with apparent crustal residence ages of 2 26 2 32 ga and b a younger source average e nd 0 14 0 with the crustal residence age of about 1 8 ga the older residence ages are interpreted to be due to mixtures of materials from the archean and proterozoic crusts sediments belonging to the laib formation lower cambrian and lower clastic part of the milford group namurian appear to have derived from an archean continental segment with some admixture of sediments from the early proterozoic source the younger crustal source which seems to be the principal supplier of sediments for most of the stratigraphic units aldridge formation middle proterozoic horsethief creek group upper proterozoic hamill and lardeau groups lower cambrian nelway formation middle cambrian active formation ordovician and part of milford group is not the archean canadian shield immediately to the east recent subsurface mapping of the alberta basement beneath the phanerozoic sedimentary cover using u pb zircon dating of igneous lithologies 1 has established the extensive archean and early proterozoic crustal provinces it is proposed here that the early proterozoic province of west central alberta is the most likely source of the easterly derived sedimentary rocks of the cordilleran miogeocline with t cr ages from 1 94 to 1 50 ga on the basis of sedimentologic tectonic and isotopic evidence we suggest that the younger sediments could be to some extent recycled older sediments for the triassic slocan and ymir groups eastern part of quesnel terrane sedimentologic lithologic tectonic and isotopic evidence suggest the source was a western one with about 1 8 ga crustal residence age comparison of the crustal residence ages with stratigraphic ages shows the best fit nd evolution curve for the cordilleran miogeocline approximately coincides with the continental growth curve deduced for the north atlantic province and similar to the growth curve for gondwana during the phanerozoic this curve also passes through the north american composite the levelling off of crustal residence ages for the mid proterozoic to triassic sediments at about 1 8 ga supports the idea of negligible crustal growth since at least 1 7 ga the average crustal residence age of the modern miogeoclinal and cratonic sediments

geochemical mineralogical and isotopic data relating to the origin and tectonic setting of the rossland volcanic rocks southern british columbia bulk rock and mineral chemical and isotopic analyses of rossland volcanic rocks are used to infer the nature of the magma extruded in the nelson rossland area of southern british columbia during th

stability of rock protection on slopes the stability of rock protection on slopes attacked by a flow requires two conditions the first is the resistance to sliding that is provided if the ratio of tangential forces to normal forces on one or several rock layers is less than the tangent of the angle of friction of the layers on their support the second is the resistance to wrenching of the surface layer blocks and is covered by equations derived from the shields criteria this paper proposes a new approach to two factors in the analysis namely 1 for downslope flow the hydrostatic pressure gradient is not vertical but normal to the slope and thus existing equations expressing the effect of slope on the threshold shear stress are modified and 2 a condition of stability that involves the bearing angle of a surface layer block thrusting on neighboring stable blocks that is wider than the commonly used angle of repose

nd sr isotopic constraints on the interactions of the intermontane superterrane with the western edge of north america in the southern canadian cordillera sr and nd isotopic compositions of the late paleozoic metavolcanics and late triassic to early tertiary granitoids from four magmatic episodes in the southern canadian cordillera from the kootenay arc to the fraser fault have been used to i identify the sources of these rocks ii constrain the compressive tectonic history from middle jurassic to paleocene and iii constrain the western boundary of the basement in this region the 215 190 ma old primitive granitoids end 3 1 to 8 7 87sr 86sr 0 7028 0 7043 of the late triassic and early jurassic magmatic episode were emplaced in the paleozoic oceanic crust of quesnellia end 2 9 to 9 3 prior to its obduction over the basement in contrast during the younger magmatic episodes middle late jurassic cretaceous and early tertiary the granitoids from western quesnellia show primitive isotopic compositions and those from eastern quesnellia show eastward increasing crust contaminated compositions the contaminated characters of the middle

geometric test for late cretaceous paleogene intracontinental transform faulting in the canadian cordillera the tintina trench northern rocky mountain trench tt nrmt fault zone and the fraser river straight creek fr sc fault zone are separate en echelon concentric small circle fault segments of a composite intracontinental transform fault zone more than 2500 km long that cuts diagonally across the canadian cordillera from the outboard part of a tectonic collage of accreted foreign terranes in the south into the north american preaccretionary continental margin in the north most of the 450 km of right hand slip on the tt nrmt fault zone was transformed southward during the late cretaceous and paleocene into oblique convergence in the southern canadian rockies the remainder probably comprising less than 100 km was transformed southwestward during early and middle eocene time via a zone of distributed shear and east west crustal stretching into right hand slip on the en echelon fr sc fault zone these interpretations based on regional systematic mapping of geologic structures are in conflict with interpretations of paleomagnetic measurements that call for more than 1000 km of post mid cretaceous right hand displacement along the general locus of the tt nrmt fault zone involving foreign terranes and the parts of north america to which they had been accreted this paradox must be resolved

north american margin origin of quesnel terrane strata in the southern canadian cordillera inferences from geochemical and nd isotopic characteristics of triassic metasedimentary rocks new geochemical and nd isotopic data from triassic ca 220 ma clastic metasedimentary rocks assigned to the quesnel terrane quesnellia in southeastern british columbia show that detritus was derived from both primitive and evolved sources twenty four samples yield end 2 2 0 values between 9 1 and 5 6 independent of sample location these samples have eu anomalies between 0 6 and 1 0 la n yb n ratios between 3 8 and 11 4 la sc ratios ranging from 0 38 to 4 76 and th sc ratios between 0 09 and 1 62 this range of isotopic and geochemical values overlaps that of samples from upper paleozoic to mesozoic rocks of the north american miogeocline and upper triassic nicola group volcanic rocks of the quesnel terrane the range in nd isotopic and geochemical values supports the interpretation that detritus was derived from both continental north america evolved cratonic source and a volcanic arc primitive source developing on the continental margin these data do not support previous interpretations of the quesnel terrane as an intraoceanic arc the inferred depositional link between north america and the quesnel arc in triassic time is ca 40 ma older than the hypothesized obduction of the quesnel terrane onto the north american continental margin and when combined with existing geological information strongly supports a pericratonic nonexotic origin for the quesnel terrane in southeastern british columbia

a nd isotope investigation of sediments related to crustal development in the british isles the times of original fractionation of the sm and nd component of clastic sediments from a mantle source crustal residence age have been estimated from sm nd model ages calculated relative to a depleted mantle evolution in this way the provenance and evolution of selected precambrian and phanerozoic sediments and metasediments from the british isles have been examined whereas some archaean and early proterozoic sediments have sm nd model ages that are close to their stratigraphic age the phanerozoic sediments analysed have model ages as much as 2 0 ga in excess of their stratigraphic age a more detailed study of lower palaeozoic sediments deposited on the northern margin of the iapetus ocean provides evidence for a marked change of provenance in the ordovician after the deposition of the dalradian supergroup a component with comparatively high143nd 144nd and sm nd ratio presumably basaltic is present in the sediments throughout the accretionary prism crustal residence age estimates average about 1 5 ga for both these lower palaeozoic sediments and modern pelagic clays and collectively fail to provide any evidence for significant continental growth during the phanerozoic

erratum superimposed low grade metamorphism in the mount fisher area southeastern british columbia implications for the east kootenay orogeny middle proterozoic 1500 1350 ma belt purcell strata exposed in the purcell and southwestern rocky mountains were affected by at least three distinct episodes of deformation and regional metamorphism the oldest episode 1300 1350 ma apparently terminated belt purcell sedimentation and involved foiding regional metamorphism and granitic intrusion the second episode 800 900 ma occurred during deposition of the windermere supergroup and involved uplift block faulting and low grade regional metamorphism mesozoic cenozoic metamorphism deformation and plutonism overprinted the results of the earlier deformation and metamorphism illite crystallinity and muscovite polymorph ratios indicate that purcell strata in the mount fisher area are in the lower greenschist to prehnite pumpellyite facies of regional metamorphism in the steeples and fisher blocks this metamorphism is related to structures that formed during the late cretaceous paleocene deformation however in the sand creek block the regional metamorphism is related to the development of a spaced cleavage that is folded by a late cretaceous pdeocene nappe regional considerations suggest that this cleavage formed during the 1300 1350 ma episode of deformation and metamorphism the east kootenay orogeny as currently defined embraces the two older episodes of tectonism it is proposed that the term east kootenay orogeny be restricted to designate the 1300 1350 ma episode and that the term goat river orogeny designate the 800 900 ma episode of tectonism the east kootenay and goat rivet orogenies appearto be correlative with the racklan and hayhook orogenies recognized in the northern canadian cordillera

upper crustal recycling in southern britain evidence from nd and sr isotopes analyses of representative rocks from southern britain are used to examine in detail the evolution of this segment of continental crust the vast majority of late precambrian basement rocks have unradiogenic 87sr 86sr ratios and model tdmnd ages of 1 3 ga although locally the rosslare complex and rushton schists have model tdmnd ages 1 7 ga identifying for the first time an old component in the basement of southern britain following the closure of the iapetus ocean sediments exhibit a progressive increase in model tdmnd ages 1 35 ga in the lower palaeozoic to 1 75 ga in the jurassic this variation demonstrates the southerly transport of detritus tdmnd 2 0 ga from the mixed archaean proterozoic terrains of northern europe and america the granitoids have significantly higher end than those of contemporaneous sediments and thus yield a different nd isotope evolution curve when considered in isolation the majority of rocks analysed have model tdmsr ages in the range 1 2 to 0 46 ga with youngest sediments generally recording the lowest model ages therefore with time the sediments record a striking increase in the ratio of model nd to model sr age the data suggest that the rb sr ratios of the crustal rocks is increased by a factor of approximately 2 every 900 ma implying that on average the british upper crust was reworked by erosion and sedimentation every 600 ma

cordilleria revisited with a three dimensional model for cretaceous tectonics in british columbia the cordilleria microcontinent theory for the tectonics of british columbia in the cretaceous and early tertiary and for the origin of the canadian rocky mountains and related features is reviewed new isotopic geological and paleomagnetic data support the hypothesis that the amalgamated superterranes of british columbia and alaska were not attached to the mainland of north america in the early cretaceous and were probably well south of their present relative position consideration of that evidence together with sedimentological evidence from the albian leads to a slightly earlier best estimate 115 ma for the time of initial collision in the yukon new geophysical data for southeastern british columbia and idaho lead to a comparison with the iapetus suture in northern england it is suggested that the cordilleria suture dips west from the present purcell thrust and related structures and should cut the moho a new model for regional deformation in se british columbia is proposed involving a flake

nd evidence for proterozoic crustal development in the belt purcell supergroup a knowledge of the chemical and isotopic characteristics of the continents as they existed in the past is essential to an understanding of their evolution1 7 vestiges of these characteristics remain in the sedimentary record for example the character and crustal residence age4 of ancient continental crust has been deduced from the sm nd isotopic patterns of clastic sediments that have survived to the present day4 6 8 we report here sm nd isotopic measurements from the belt purcell supergroup of western north america which form the first detailed isotopic investigation of a major proterozoic sedimentary succession the belt purcell has many features also seen in the detrital load of the world s major contemporary rivers including exceptionally homogeneous 147sm 144nd and 143nd 144nd ratios and a small range of crustal residence ages mantle additions to the continental crust seem to have been relatively minor since the middle proterozoic

northward translation of baja british columbia along the late cretaceous to paleocene margin of western north america paleomagnetic data suggest that a large part of british columbia and southern alaska was at the latitude of baja california mexico in early late cretaceous time this allochthonous block baja bc includes the insular talkeetna in alaska and intermontane composite terranes the miniterranes between them and possibly part of the omineca metamorphic belt assuming that baja bc was adjacent to north america by middle cretaceous an analysis using plate kinematics of the pacific basin produces a model of northward movement of baja bc which is compatible with the known geology of the reconstructed margin of western north america baja bc moved north 2400 km as part of the kuia plate along a transform margin with north america from 85 to 66 ma this time interval is marked by the cessation of igneous activity in the sierra nevada development of wrench faults and basins along coastal california and initiation of laramide style tectonics to the east within baja bc diminished igneous activity to the west was coeval with possible wrench tectonic basins along its eastern margin by the end of the cretaceous baja bc was positioned from oregon to northern british columbia between 66 and 56 ma baja bc became attached to north america in a dextral transpressive stage during which the kula north america plate boundary expanded to encompass the whole block with a series of northwest trending strike slip faults a fold and thrust belt in the east and large scale uplifts and the formation of an incipient volcanic arc on the west southern alaska was the site of convergence during the whole of the period of northward translation of baja bc a late cretaceous subduction zone in alaska evolved into a major compressional belt as the basin and miniterranes on the northern margin of baja bc collided with interior alaska the collision culminated in the paleocene with formation of the mckinley granites and coeval dextral slip on the denali fault system

analysis of the wallowa baker terrane boundary implications for tectonic accretion in the blue mountains province northeastern oregon the baker terrane exposed in the blue mountains province of northeastern oregon is a long lived ancient late paleozoic early mesozoic accretionary complex with an associated forearc this composite terrane lies between the partially coeval wallowa and olds ferry island arc terranes the northern margin of the baker terrane is a broad zone 25 km wide of fault bounded imbricated slabs and slices of meta igneous and metasedimentary rocks faulted into chert argillite melange of the elkhorn ridge argillite metaplutonic rocks within tectonic units in this zone crystallized between 231 and 226 ma and have low initial 87 sr 86 sr ratios 0 7033 0 7034 and positive initial e nd values 7 7 to 8 5 in contrast siliceous argillites from the chert argillite melange have initial 87 sr 86 sr values ranging from 0 7073 to 0 7094 and initial e nd values between 4 7 and 7 8 we interpret this broad imbricate fault zone as a fundamental tectonic boundary that separates the distal wallowa island arc terrane from the baker accretionary complex terrane we propose that this terrane boundary is an example of a broad zone of imbrication made up of slabs and slices of arc crust tectonically mixed within an accretionary complex providing an on land ancient analogue to the actualistic arc arc collisional zone developed along the margins of the molucca sea of the central equatorial indo pacific region

proterozoic mantle under quesnellia variably reset rb sr mineral isochrons in ultramafic nodules carried up in cenozoic volcanic vents of the southern omineca belt handpicked acid washed diopside orthopyroxene and olivine separates from 14 lherzolite nodules from four localities in southern british columbia have been analyzed for 87rb 86sr and 87sr 86sr the host lavas of late cenozoic to pleistocene age are nepheline normative alkali basalt and basanite with sr and nd isotopic ratios 0 7025 0 7029 0 51310 0 51326 close to those of mid ocean ridge basalt the present day sr isotopic compositions of nodule diopsides 9 239 ppm sr are 0 7021 0 7041 similar to those of the host rocks orthopyroxene and olivine however have 87rb 86sr ratios of 0 1 to 1 0 and much more radiogenic sr 87sr 86sr commonly 0 708 0 720 and in several cases the three minerals form linear arrays in 87rb 86sr versus 87sr 86sr plots if interpreted as mineral isochrons these give dates from 99 to 1989 ma many cluster around 440 770 ma four nodules from two localities west kettle river and big timothy mountain give dates of 1201 1989 ma maximum dates for the other localities

lead isotopes and the sources of the columbia river basalt group a detailed study of 208pb 204pb 207pb 204pb and 206pb 204pb ratios suggests that the number of pb isotopic reservoirs required for the columbia river basalt group crbg as currently defined must be increased from the presently accepted four to at least six the identities of the six reservoirs are two of mid ocean ridge basalt morb slightly contaminated with sediment rl and r2 rl being cascades pb one a probable plume component r3 two of unspecified mantle material but probably of rl type contaminated with local crustal pb r4 and r5 and the sixth a complex enriched reservoir of 2150 ma age r6 rl is an end member for all crbg except the saddle mountains basalts r2 is a second end member for the picture gorge basalt and appears to be unique to it both rl and r2 are related to the juan de fuca subduction zone and the back arc spreading associated with it r3 is the principal source for most of the wanapum basalt as well as for most of the chemically evolved portions of the grande ronde basalt volumetric and tectonic considerations require that one source of the crbg be a plume or at least a nonlocal crustal or uppermost mantle source and r3 is the ideal candidate for that role r4 is the second source for the imnaha basalt and r5 is the source for the 206pb rich varieties of the grande ronde basalt r4 and r5 are identified with contamination by local phanerozoic crust because of their similarity to local crustal pb and because the rocks which contain them also have cu contents correlated with their pb isotopic compositions r6 is the parent for pb in all the saddle mountain basalts this last source appears to have been homogeneous 2150 m y ago but has since split into a number of separate discrete pockets each with its own characteristic pb isotopic signature some of these appear in individual saddle mountain basalt flows while other sources have mixed with r4 to produce individual saddle mountain basalt flows of the ice harbor set r6 is similar in all its isotopic properties to the precambrian basement that is exposed to the immediate north of the columbia river basalt group the overall model for columbia river basalt group genesis is termed plume reinforced back arc magmatism

petrogenesis of the cretaceous cassiar batholith yukon british columbia canada implications for magmatism in the north american cordilleran interior the ca 100 ma cassiar batholith is the single largest plutonic body in the hinterland of the canadian cordillera and is part of widespread middle cretaceous to eocene magmatism that occurred in the omineca crystalline belt the cassiar batholith is dominated by muscovite biotite granite and biotite muscovite granodiorite along with subordinate biotite hornblende granodiorite quartz monzodiorite and quartz monzonite the batholith has a wide range of strontium neodymium and oxygen isotope compositions 87 sr 86 sr i 0 706 to 0 734 nd 2 7 to 17 1 t dm 2000 to 980 ma 18 o 7 6 to 11 6 but the batholith9s lead isotope compositions are limited 206 pb 204 pb 19 14 to 19 24 207 pb 204 pb 15 70 to 15 72 208 pb 204 pb 39 14 to 39 26 these data require that the cassiar magmas have been generated from at least two distinct sources an isotopically evolved felsic source and an isotopically more primitive source of mafic to intermediate composition we propose that the felsic source comprised mesoproterozoic to neoproterozoic metasedimentary rocks and paleoproterozoic basement rocks of the north american cratonic margin determining the mafic source is more problematic but most likely it was proterozoic basement rocks of mafic to intermediate composition such as have been reported from various parts of the omineca crystalline belt the cassiar batholith is lithologically geochemically and isotopically similar to other major batholiths in the cordilleran interior including the batholiths of southeastern british columbia and the idaho batholith we interpret these similarities to reflect melting of common crustal source materials namely proterozoic basement and cover rocks of western laurentia the cassiar batholith and other cordilleran interior granitoids are distinct from granitoids generated in known subduction zone environments in terms of their mineralogical geochemical and isotopic characteristics these differences reflect a predominantly or exclusively crustal source region for the cordilleran interior granitoids in contrast subduction zone magmatism contains a significant mantle input even in cases where subduction takes place beneath thick continental crust these results strongly suggest that cordilleran interior granitoids did not form directly from a subduction setting rather an intracontinental collisional setting seems more appropriate in which magma generation occurred in response to crustal thickening followed in some cases by extension

petrogenesis and significance of late caledonian granitoid magmatism in western norway obduction of the late ordovician solund stavfjord ophiolite complex 443 3 ma west norwegian caledonides involved generation and high level emplacement of granitic and granodioritic dikes and plutons initial 87sr 86sr ratios in the granites are low 0 7042 0 7059 suggesting either a mantle component or a rb poor crustal source initial end end t ranges from 0 8 to 8 8 indicating that the granites represent recycling of old crustal rocks which is supported by precambrian inheritance in zircons from two of the studied granites i argue that the rb sr and the sm nd isotope systems are decoupled in the sense that the sr and the nd isotopes derive their dominant signals from two different sources a mantle source and a crustal source respectively the granites are metaluminous to peraluminous and typically have high sr ba and na2o k2o ratios sio2 contents range from 66 to 74 wt ree abundances are highly variable the la contents range from 80 to 200 times chondrite and are inversely correlated with the contents of sio2 the concentration of nd in the granites decreases asymptotically with decreasing end t suggesting fractional crystallization of accessory phases and assimilation of continental crust this argument is supported by the presence of partly dismembered xenoliths in the granites with end t values that are significantly lower than end t values in the host granite the following models are suggested for the granites when the ophiolite complex obducted an outboard subduction zone approached the continental margin and subduction related magmas accumulated beneath the continental margin and probably intruded the overlying eugeosynclinal deposits the mantle derived magmas most likely evolved to granitoid composition by assimilation of these eugeosynclinal sediments and by fractional crystallization of amphibole feldspar sphene and allanite alternatively but less likely the heat content of the mantle derived magmas caused extensive melting of immature graywackes and calc alkaline volcaniclastic rocks in the deepest portions of the eugeosyncline either way during ascent the compositions of the granitic melts were modified by fractional crystallization of lree rich phases and by assimilation of continental metasediments

subsidence analysis of the cordilleran miogeocline implications for timing of late proterozoic rifting and amount of extension tectonic subsidence of the cordilleran miogeocline in the western united states during late proterozoic through early paleozoic time is best explained by rifting and subsequent thermal contraction of the lithosphere the tectonic component of subsidence can be calculated by removing the isostatic effects of sediment and water loading the resultant subsidence curves indicate that rifting began about 600 m y ago and that extension of the lithosphere during rifting was in excess of a factor of 1 3

estimation of flow through and over armored slopes models developed for the transient routing of intense rainfall on small armored slopes depend on expressions for flow resistance below and above the surface of the armored layer a series of 52 experiments are conducted in two flumes to collect data on the basic relationships of flow resistance crushed rock of 26 155 mm median diameter is placed on embankments with slopes ranging from 0 01 to 0 2 and subjected to steady flow in the flumes flow through the rock layer is found to be approximately proportional to the rock layer porosity the square root of the slope and the mean diameter of the rock and weakly proportional to the stage the portion of flow through the filter layer generally cannot be neglected flow resistance decreases and conveyance greatly increases when the stage exceeds the depth of the rock layer a model for flow both beneath and above the rock surface on an armored slope is synthesized from formulas developed for flow in rubble dams and darcy weisbach formulas developed for flow

seepage induced mass wasting in coarse soil slopes an expression is derived for the critical shear stress of surface flow on a noncohesive soil slope subjected to seepage induced mass wasting i e erosion caused by a successive mobilization of flake aggregates of surficial grains the expression is supported with data from experiments with stone fill subjected to throughflow in current theories for hill slope erosion of riprap protection on slopes buoyancy is assumed to be a constant vector directed vertically or the seepage force is omitted as a variable however the present analysis shows that for many conditions buoyancy should be described in accordance with the dynamic pressure distribution and the seepage force should be included as a variable in the theory furthermore a parameter denoted minimum stable volume per unit width of stone fill is introduced as a measure of global stability of the stone fill along with seepage height on the downstream face and grain size experimental results indicate that the minimum stable volume decreases with increase in grain size and for the materials investigated is about 20 lower for crushed rock than for gravel

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

the role of metamorphic decarbonation reactions in returning strontium to the silicate sediment mass the strontium isotope evolution of the earth s silicate sediments indicates that they must interact with a strontium reservoir whose 87sr 86sr ratio is relatively low the sr isotope budget for these sediments can be balanced if the sr lost to solution by weathering is replaced by sr from the oceans through diagenesis and from carbonate sediments through metamorphic decarbonation reactions because of the latter connection the 87sr 86sr ratio of silicate sediments can constrain the contribution of metamorphism to the earth atmosphere c02 flux

source terranes for proterozoic sedimentary rocks in southern british columbia nd isotopic and petrographic evidence quartz arenites of the fort steele formation the lowest exposed stratigraphic unit of the purcell supergroup have nd crustal residence times tcr of 2 61 2 63 ga these overlap the low end of the tcr range of the polymetamorphic kenoran hudsonian basement of southern alberta and southwestern saskatchewan the tcr values of overlying fine grained purcell rocks average 2 0 ga near lake windermere the basal toby conglomerates of the windermere supergroup apparently derived from underlying purcell strata have tcr values of 1 98 and 2 03 ga stratigraphically higher argillite and arkoses range from 2 35 to 2 54 ga this higher range indicates contributions from a source terrane different from the main body of the purcell supergroup our interpretation of events in the east kootenay area is that early purcell sedimentation fort steele formation was derived from reworking of the crystalline basement of the alberta shelf as the beltian sea transgressed as the basin deepened it was filled by fine grained

nd sr isotope systematics of nicola group volcanic rocks quesnel terrane volcanic rocks of the nicola group belong to an arc built on the western margin of the quesnel terrane in the late triassic to early jurassic low k high k compositional types define a rb sr isochron of 222 15 ma with initial 87sr 86sr 0 70367 2 the corresponding nd isotopic compositions of these samples end 222 ma 5 1 to 7 8 fall within the range for early mesozoic island arcs a comparable range of end 222 ma 5 0 to 7 9 in picrite shoshonite samples precludes generation of increasingly potassic magmas by progressive metasomatism of the mantle wedge alone source region heterogeneity possibly imparted by changes in the composition of subducted slab components or interaction with amphibole or phlogopite in the source remnant of an earlier permian arc on the quesnel terrane is required to account for geochemical differences between these rock suites crustal contamination is severely limited from the high end values such that continental basement now underlying the quesnel terrane

comparison of isotopic and petrographic provenance indicators in sediments from tertiary continental basins of new mexico abstract isotopic variations of nd in sedimentary rocks provide a new tool for provenance determination we have compared the isotopic technique with classic petrographic indicators of provenance in sediments from two tertiary basins of the rio grande rift north central new mexico the siliciclastic sediments in these basins were derived from varying proportions of precambrian basement and oligocene volcanic rocks which are isotopically and petrographically distinct in the analyzed sediments a good correspondence exists between petrographic and isotopic provenance indicators the data also suggest that the isotopic tracer is more sensitive to minor detrital input from a second source than is the framework petrology the similar sm nd ratios for all analyzed sediments and the identical isot pic values of interlayered mudstone and sandstone indicate that no significant fractionation of sm and nd occurs during the formation of the sediments thus the nd model ages of the sediments preserve the crust formation age of the source the sr isotopic and concentration data show trends generally similar to the nd data but they do not indicate the sedimentary provenance as clearly this ambiguity is a consequence of strong rb sr source heterogeneities and rb sr fractionation during the sedimentary cycle

sediment transport formula for steep channels a description is given and results presented of experiments to investigate the sediment capacity of stream flows on flume slopes of up to 20 a comparison of the results with values predicted by the meyer peter mueller equation shows that the latter equation seriously underestimates sediment transport capacity on slopes steeper than 3 mainly because of deficiencies in the form resistance factor a new equation is proposed giving the transport capacity for materials with grain sizes larger than 0 4 mm 0 016 in the equation is calibrated by both the original meyer peter data and new steep flume experiments and modified to incorporate the effect of nonuniform sediment grain size distributions with flow depths calculated by the manning strickler equation the new equation still gives excellent estimates of sediment capacity

stratospheric aerosol clouds due to very large volcanic eruptions of the early twentieth century effective particle sizes and conversion from pyrheliometric to visual optical depth spectral and pyrheliometric measurements of atmospheric transmission were made by staff members of the astrophysical observatory of the smithsonian institution at washington d c and at mount wilson california during the years 1901 1920 these valuable data are analyzed here with the help of a new inversion method to derive the effective or area weighted radii of stratospheric aerosols formed after three very large volcanic eruptions in this period after the great eruptions of katmai 1912 and santa maria 1902 reff remained close to 0 3 m for at least two years this near constancy of reff has been duplicated in modern times by the aerosols from el chichon 1982 following ksudach s 1907 eruption reff grew from 0 2 0 3 m to 0 4 0 5 m in about 1 year pinatubo s 1991 aerosols grew similarly generally speaking the pyrheliometric optical depth perturbation as measured for the three early eruptions is not equal to the visual optical depth perturbation but in fact the latter is larger by a factor of about 1 6 at least for reff 0 25 0 45 m wider physical implications of the present results are discussed

nd evidence for extensive archean basement in the western churchill province canada to determine the extent of reworked archean crust in the western churchill province we have examined sm nd crustal residence ages of basement cores in southern alberta and southwestern saskatchewan along with crustal residence ages calculated for other sm nd data available from the churchill province the deep drill hole samples from the interior platform give sm nd crustal residence ages that average 2 8 ga granulites from northeastern alberta and composite gneisses from northern saskatchewan also provide archean crustal residence ages these data demonstrate that the presence of reworked archean crust in the western churchill province is more widespread than has been confirmed previously we suggest that sm nd crustal residence ages are valuable estimates of crust formation times especially in orogenic areas where other isotope systems have been disturbed

rare earth element distributions among minerals in a granodiorite and their petrogenetic implications a study of the distribution of lanthanide rare earths in a granodiorite from the eastern peninsular ranges batholith southern california reveals that a large fraction of the ree in this rock resides in the accessory phases sphene and allanite the minerals plagioclase alkali feldspar biotite epidote and apatite each contribute approximately 1 or less of each ree to the whole rock with the exception of eu for which plagioclase contributes 7 sphene and allanite together contain 80 to 95 of each ree each of these phases is zoned in ree concentration with substantial decreases from core to margin textural observations argue for relatively early saturation and precipitation of sphene and allanite in the magma ree zoning trends in sphene and allanite and unexpectedly low ree concentrations in largely later crystallizing minerals such as feldspar indicate that the precipitation of sphene and allanite significantly reduced ree concentrations in residual melts these results illustrate the potential that sphene and allanite have for controlling the behavior of ree in granitic magmas available information collectively suggest that the sampled granodiorite existed as a complete melt that the ree contained in the assemblage of phases were derived by direct crystallization from the melt and that the melt behaved essentially as a closed system once crystallization of the phases now present began close correspondences between the major and trace element chemistries of the granodiorite and phenocryst poor lavas from similar tectonic settings support these conclusions the ree pattern of the granodiorite melt appears to have originated at depth and is characteristic of its source regions and derivation mechanism the high liquidus temperature of a granodiorite melt 1000 c indicates the importance of mantle derived components within the sources of batholithic magmas in the peninsular ranges

sm nd and u pb zircon isotopic constraints on the provenance of sediments from the amadeus basin central australia evidence for ree fractionation abstract the amadeus basin of central australia is a late proterozoic to late palaeozoic ensialic depositional basin located between two proterozoic basement blocks of different nd crustal formation ages the older arunta block t dm nd 2 0 to 2 2 ga u pb zircon ages 1 5 1 9 ga to the north and the younger musgrave block t dm nd 1 7 to 1 9 ga u pb zircon ages 1 0 1 7 ga to the south initial nd isotopic compositions of the amadeus basin sediments generally fall into the region defined by the evolutionary trajectories of these two basement blocks indicating that the sediments are dominated by essentially two source components the older arunta block 147 sm 144 nd 0 114 and nd 0 20 4 and younger musgrave block 147 sm 144 nd 0 118 and nd 0 14 7 and or their equivalents stratigraphically higher sediments plot more closely to the evolutionary trajectory of the musgrave block this result together with the average provenance ages indicates the proportion of material from the musgrave block increases as the sediments become younger the sm nd isotopic data preclude a substantial involvement of archaean sources u pb zircon ion probe analyses of the late proterozoic heavitree quartzite and the late cambrian goyder formation are consistent with the sm nd isotopic constraints most of the zircons from the basal heavitree quartzite give concordant u pb zircon ages of 1500 to 1900 ma consistent with derivation from an arunta type source whilst zircons from the younger goyder sandstone give six discrete u pb zircon age groups of 511 20 615 15 960 109 1190 54 1633 24 and 1878 48 ma indicative of a dominantly musgrave type source with a minor contribution from the arunta block or reworked preexisting amadeus basin sediments the two youngest u pb zircon ages probably indicate some contribution from late proterozoic cambrian volcanics in central australia or a previously unrecognised younger local source in the basement blocks the 511 20 ma constrains the maximum depositional age of the sediment the amadeus basin sediments have a surprisingly wide range of 147sm 144nd ratios 0 077 0 136 irrespective of grain size and rock types in addition the calculated t dm nd values for samples from the same stratigraphic unit are well correlated with 147sm 144nd ratios with samples of different stratigraphic units forming sub parallel arrays these phenomena are interpreted in terms of ree fractionation and preferential sorting of preexisting ree rich phases during sedimentary recycling and deposition this process can lead to either an increase or a decrease in 147sm 144nd ratio of a sediment relative to its source and therefore an erroneous estimate of its t dm nd provenance age a theoretical model is developed which accounts for the observed correlation between t dm nd and 147sm 144nd ratios for suites of stratigraphically related sediments and allows a reliable estimate to be made of the provenance age of individual stratigraphic units assuming the average 147sm 144nd ratio of the source is known or can be estimated the provenance age for each stratigraphic unit of the amadeus basin has been calculated using this method the sm nd isotopic systematics in the amadeus basin sediments suggest that 1 1 ree fractionation during sedimentary recycling can in some cases be an important factor and needs to be considered 2 2 the provenance of the amadeus basin sediments was controlled by local source regions and cannot be used to infer large scale continental averages hence some caution must be evoked in using sm nd isotopic constraints from restricted sampling for more generalised models of crustal evolution

the provenance and crustal residence ages of british sediments in relation to palaeogeographic reconstructions 24 smnd isotope analyses of fine grained phanerozoic and modern clastic sediments from britain and quebec are presented in combination with published data they have been used to calculate crustal residence ages tcr and to assess the provenance of the british sedimentary mass sediments now preserved on either side of the suture formed by the closure of the iapetus ocean ca 400 ma ago were derived from isotopically distinct source regions sediments north of the suture are characterised by1 7 tcr 2 8 ga whereas those to the south exhibit a smaller range and an average value of 1 6 ga the northern and southern source regions were most probably laurentia and gondwana respectively it seems likely that a third source perhaps baltica provided the lower palaeozoic southern uplands succession sediments deposited in southern britain after the closure of iapetus were derived mainly from the recycling of older sediments the tectonic rearrangements which occurred during the phanerozoic are not reflected in the smnd isotopic structure of the southern british sedimentary mass suggesting that the caledonian and hercynian orogenies and even the grenville orogeny involved minimal accretion of new mantle derived material into the british and adjacent continental crust smnd analyses of fine grained clastic sediments provide a powerful sedimentological tool for elucidating palaeogeography clastic source areas sediment recycling and maturity and some aspects of sediment transport

samarium neodymium elemental and isotopic systematics in sedimentary rocks abstract trace element sm nd th sc and nd isotopic data for sedimentary rocks are examined in order to evaluate 1 1 possible secular trends in the composition of the sedimentary mass and upper continental crust 2 2 the role of tectonic setting in controlling the chemical and nd isotopic composition of the sedimentary mass when plotted against either sedimentation age or average provenance age determined mainly from nd model ages sm nd in sedimentary rocks decreases abruptly from about 0 21 to 0 19 at the archean proterozoic transition consistent with a change towards a more felsic upper crustal composition the change is only somewhat less pronounced for turbidite sediments than for those of continental affinity a trend toward increasing sm nd approaching or exceeding archean values is observed for phanerozoic turbidites this latter change is interpreted as indicating undifferentiated young crust as a component in the sediment which has been lost from the older post archean record due to sedimentary and igneous intracrustal recycling processes when plotted against sedimentation age th sc ratios exhibit a trend directly comparable to that of sm nd in this case th sc increases abruptly at the archean proterozoic boundary from about 0 5 to 1 0 and then decreases to about 0 7 during the phanerozoic nd isotopic composition of modern sediments is affected by tectonic association with those deposited at active margin settings tending to have higher nd reflecting younger provenance and higher and more scattered sm nd reflecting less homogenized and differentiated provenance it is likely that these distinctions can be traced back for about 1 2 ga but with the current data base are not readily resolved in older sedimentary rocks the degree to which the preserved sedimentary mass is open to new additions from the mantle either through replacement during sedimentary recycling or sediment mass growth is highly variable but averages about 15 25 during the phanerozoic

decoupled evolution of nd and sr isotopes in the continental crust and the mantle nd and sr isotopes evolve independently in clastic sedimentary rocks and the continental crust as a whole because sr is recycled preferentially from the continents to the mantle as a result the differentiation age of the mantle inferred from sr isotopes and the mean rb sr age of the continental crust are younger than their true ages

sm nd in marine carbonates and phosphates implications for nd isotopes in seawater and crustal ages this study explores the possibility of establishing nd isotopic variations in seawater over geologic time calcite aragonite and apatite are examined as possible phases recording seawater values of nd modern biogenic and inorganically precipitated calcite and aragonite from marine environments were found to have nd concentrations of from 0 2 to 70 ppb showing that primary marine caco 3 contains little ree and that nd ca is not greatly enhanced relative to seawater during carbonate precipitation very young marine limestone and dolomite containing no continental detritus have 200 ppb nd all the carbonates are lree enriched 0 16 sm nd 0 45 modern and very young atlantic and pacific carbonates have nd in the range of shallow atlantic and pacific seawater respectively implying that they derive their ree from local seawater the nd in well preserved carbonate fossils is 4 10 4 ppb much greater than in their modern counterparts but like the high values found for carbonates in other studies we believe the high ree contents at the 500 ppb level in some detritusfree carbonates are due to ree rich fe hydroxide in on the carbonate in favorable cases such material may record seawater nd values however introduction of extraneous ree may obscure the original isotopic composition of pure caco 3 because of its very low intrinsic primary ree abundance modern biogenic apatite is also shown to have very low ree content 150 ppb nd but appears to quickly scavenge ree from seawater inorganically precipitated apatite from phosphorites has high concentrations of seawater derived ree young phosphorite apatite from the atlantic and pacific oceans has nd in the range of the seawater from these oceans older apatite samples of similar age from different localities bordering common oceans record similar values of nd t sedimentary apatite has sr t values in good agreement with the curves for 87 sr 86 sr of seawater as a function of time individual conodonts from a single formation yield the same sr t and nd t other workers have shown that sedimentary apatite preserves seawater ree patterns these characteristics suggest that sedimentary apatite can be used to determine nd t in ancient seawater the seawater values so inferred range between 1 7 and 8 9 over the last 700 my and lie in the range of modern seawater showing no evidence for drastic changes high values of seawater nd t in the triassic and latest precambrian may correlate with the breakup of large continental landmasses the initial nd t 15 0 of a 2 ae old phosphorite implies the presence of 1 5 ae old continental crust at 2 ae ago the approach outlined here can be used to constrain the age of the exposed crust as a function of time

eocene shoshonitic mafic dykes intruding the monashee complex british columbia a petrogenetic relationship with the kamloops group volcanic sequence the newly named three valley suite tvs kersantite lamprophyre to shoshonitic mafic dykes of the monashee complex are inferred to be hypabyssal feeder dykes to an alkaline to calc alkaline volcanic suite related to the kamloops group these dykes were emplaced in a subvertical north trending orientation coincident with inferred eocene crustal extension 50 0 ma based on the flat 40ar 39ar step heating plateau of contact metamorphic muscovite on the margin of a tvs dyke these weakly altered mafic dykes are fine grained with phenocrysts 0 5 2 0 mm of phlogopite augite amphibole and olivine pseudomorphed by clays rare labradorite and both primary and secondary carbonates set in a fine grained groundmass of similar mineralogy consistent with their classification as plagioclase bearing potassic diorite to kersantite lamprophyre the dykes are weakly silica undersaturated and alkalic 2 8 wt k2o 7 7 wt mgo with high large ion lithophile element contents 300 times primitive mantle and ele

the growth of the continent through geological time studied by nd isotope analysis of shales abstract we determined the nd isotopic composition and the sm nd ratios in a series of australian shales ranging from 0 2 gy to 3 3 gy the first result of this study is the constancy of the sm nd ratio in these shales as in granitoids secondly the initial 143nd 144nd ratio gives a regular curve decreasing through geological time both results confirm that shales are representative samples of the continental crust when insoluble elements like ree are studied we calculated their nd model ages of crustal differentiation the model ages regularly decrease with the stratigraphic ages and after 2 gy the curve flattens and tends to an asymptotic value at around 1 8 gy the significance of the shales is that they represent a mixture of continental materials and we consider the model age of such a mixture directly linked with the mean age of the continental portion feeding the sedimentary basin from these results we deduce a quantitative model of the growth curve of the continental source of the shales taking into account the effects of erosion which selectively sample recent mountains relative to shield areas we propose that the results obtained here are representative of the whole gondwana continent having studied the case of the australian shales we try to extend our study first we applied our quantitative approach to the recent results obtained by o nions et al on north atlantic provinces the continental growth curves obtained by our inversion procedure are quite distinct from the australian shales showing the regional character of shales with these two cases studied we try to compare our results with the already developed model for continental growth the consideration of the surface conservative versus surface non conservative models clearly show the non uniqueness of the geological interpretation of the growth curve on the other hand we have calculated for each case the recycling rate versus geological time by comparing the growth curve with the hurley rand province age curve such recycling increases with time in agreement with the data obtained on nd and sr initial ratios on granitoids

geochemistry and sr nd isotopic composition of eocene lamphrophyre dykes southeastern british columbia mineral compositions geochemical analyses and sr nd isotopic compositions are reported for alkaline and calc alkaline lamprophyres collected along the southern margin of the valhalla complex southeastern british columbia the lamprophyres were emplaced during eocene extension and lithospheric thinning associated with tectonic denudation of the valhalla complex sio2 contents range from 44 4 51 6 wt k2o from 1 3 3 7 wt and volatile contents h2o co2 so3 from 0 8 4 6 wt mgo and cr contents are 9 5 7 6 wt and 540 130 ppm respectively for samples with mg s between 0 69 and 0 65 chrondrite normalized rare earth element patterns are strongly fractionated with cen 120 375 and ybn 8 4 12 7 alkaline lamprophyres contain biotite kaersutite calcic plagioclase and exhibit a limited range in initial 87sr 86sr 0 7051 0 7057 initial end 3 7 to 4 3 and tdm 766 796 ma calc alkaline lamprophyres contain f rich phlogopite and sodic plagioclase and exhibit a wider range in initial 8