relations between thrust belt evolution grain scale deformation and metamorphic processes cape smith belt northern canada abstract an important step towards understanding how continental thrust belts evolve is to study the relationship between the map scale structures and the associated grain scale deformation mechanisms and metamorphic processes this relationship is examined in the early proterozoic cape smith thrust belt in the context of the belt s p t t history and fluid rock interactions overprinting deformation metamorphism relationships allow the thrust belt s evolution to be described in terms of four events 1 1 piggyback thrusting above a basal decollement localized at the basement cover contact 2 2 greenschist to amphibolite grade metamorphism and development of a ductile basal shear zone 3 3 re imbrication out of sequence faulting of the internal zone at pre to syn thermal peak conditions 4 4 post thermal peak out of sequence thrusting and retrograde metamorphism out of sequence thrusting was accommodated by the development of ductile shear zones in general relatively broad ductile shear zones developed adjacent to thrusts during prograde metamorphism while relatively narrow shear zones developed adjacent to thrusts during retrograde metamorphism the width of the shear zones was controlled by factors such as the competition between softening and hardening processes prograde shear zones and the extent of fluid influx retrograde shear zones four processes were primarily responsible for accomplishing penetrative deformation and accommodating thrust faulting during the belt s history 1 1 partitioning of strain into pre existing weak zones or into relatively weak layers in a compositionally stratified sequence 2 2 growth and or rotation of metamorphic products into orientations of maximum resolved shear stress 3 3 transitions in dominant deformation mechanism from brittle or semi brittle to crystal plastic 4 4 retrograde metamorphism involving fluid influx and hydration reactions producing weaker products than reactants

composition and structure of the central aleutian island arc from arc parallel wide angle seismic data new results from wide angle seismic data collected parallel to the central aleutian island arc require an intermediate to mafic composition for the middle crust and a mafic to ultramafic composition for the lower crust and yield lateral velocity variations that correspond to arc segmentation and trends in major element geochemistry the 3 d ray tracing 2 5 d inversion of this sparse wide angle data set which incorporates independent phase interpretations and new constraints on shallow velocity structure produces a faster and smoother result than a previously published velocity model middle crustal velocities of 6 5 7 3 km s over depths of 10 20 km indicate an andesitic to basaltic composition high lower crustal velocities of 7 3 7 7 km s over depths of 20 35 km are interpreted as ultramafic mafic cumulates and or garnet granulites the total crustal thickness is 35 37 km this result indicates that the aleutian island arc has higher velocities and thus more mafic compositions than average continental crust implying that significant modifications would be required for this arc to be a suitable building block for continental crust lateral variations in average crustal velocity below 10 km roughly correspond to trends in major element geochemistry of primitive mg 0 6 lavas the highest lower crustal velocities and presumably most mafic material are detected in the center of an arc segment between unmak and unalaska islands implying that arc segmentation exerts control over crustal composition

upb la mc icp ms dating of rutile new reference materials and applications to sedimentary provenance in response to the general lack of sufficiently abundant and high quality rutile usingle bondpb reference materials for in situ geochronology we have characterised two new potential rutile 1 8 ga reference materials sugluk 4 and pca s207 from granulite facies belts of the canadian shield namely the northern cape smith belt of quebec and the snowbird tectonic zone sasatchewan characterisation includes id tims and la icp ms usingle bondpb dating imaging and trace element analysis we compare these materials with existing rutiles used already r19 and r10 luvizotto et al 2009 and zack et al 2011 and show that the measured usingle bondpb compositions i e including any common pb of our rutiles are considerably more homogeneous this makes possible a usingle bondpb normalisation procedure not reliant upon a common pb correction that results in a significant decrease in the uncertainty contribution from the common pb correction and better reproducibility of reference materials and unknowns for provenance analysis and other applications the reproducibility is 2 4 2rsd for 206pb 238u and 207pb 206pb only slightly greater than long term data for zircon reference materials we show in a rutile provenance study from young orogens bhutan himalaya and canadian cordillera that the sensitivity of our analytical set up allows dating of 90 of rutiles in a sediment using a 50 m laser ablation spotsize within samples containing rutile as young as 10 20 ma and obviates the requirement for u concentration pre screening thus reducing or eliminating rutile selection bias unsuccessful analyses are due to poor quality rutiles with predominant common pb 207pb signal below detection or u content below 1 2 ppm we have used the 207pb method using the tera wasserburg diagram to correct for substantial common pb in very young and or very low u rutiles rather than developing an on line correction since rutile ages reflect mainly the time of cooling rutile is a sensitive recorder of metamorphic thermochronological information and therefore is an excellent complement to detrital zircon u207pb data the contrast between zircon and rutile signatures in himalayan samples with rutile as young as 10 ma is shown to be very dramatic most zircons from the same sample are 480 ma with only a few grains or metamorphic rims reflecting miocene metamorphism as such rutile provides complementary information about the thermal events within the source regions of the grains rutile usingle bondpb dating is an underexploited provenance method with wide applicability to sedimentary provenance studies

age constraints on the paleoproterozoic tectonometamorphic history of the committee bay region western churchill province canada evidence from zircon and in situ monazite shrimp geochronology in situ u pb sensitive high resolution ion microprobe shrimp analyses of monazite from upper amphibolite facies paragneiss of the committee bay supracrustal belt central rae domain canada reveal three age populations ca 2350 1850 and 1780 ma the ca 1850 ma age also corresponds to growth of low th u zircon as indicated by u pb shrimp analyses of zircon separates from melanosome and leucosome the contextual advantage of the in situ monazite analysis allows evaluation of the geochronological data in terms of the regional structural and metamorphic evolution the region is dominated by a northeast striking s2 s1 fabric axial planar to tight northwest vergent f2 folds early garnet is enveloped by this biotite sillimanite cordierite s2 fabric garneti hosts ca 1850 ma monazite inclusions with ca 2350 ma cores placing a maximum age on garneti growth and s2 development d2 metamorphic conditions progressed through 3 5 kbar 1 kbar 100 mpa and 600 c to near peak conditions of 5 kbar and 675 c a minimum age for s2 is provided by unstrained ca 1820 ma monzogranite that locally and regionally truncates s2 dominantly ca 1780 ma matrix monazite is interpreted to date post s2 garnetii and cordierite which record 5 kbar and 675 c these data indicate that the committee bay region experienced penetrative d2 tectonometamorphism at ca 1850 1820 ma with a subsequent static overprint the absence of a ca 1 85 ga plutonic suite in the region suggests that low pressure metamorphism was a response to thick skinned crustal thickening initiated at ca 1870 ma the new data highlight the importance of paleoproterozoic reworking of the central rae domain in the hinterland of the trans hudson orogen

the lithospheric root beneath hudson bay canada from rayleigh wave dispersion no clear seismological distinction between archean and proterozoic mantle abstract we characterise the nature and thickness of the seismological lithosphere beneath hudson bay using dispersion analysis of fundamental mode rayleigh waves along two station paths crossing the region hudson bay is largely underlain by the paleoproterozoic trans hudson orogen and is surrounded by archean cratons this study region thus provides an opportunity to investigate whether fundamental differences exist in the nature of the subcontinental lithosphere between archean and proterozoic terranes dispersion measurements for hudson bay carried out in the 15 220 s period range show seismic phase velocities consistently higher than those derived from global reference models in addition the phase velocities are higher than those of the canadian shield average and of many other cratonic regions worldwide monte carlo modelling is used to estimate 1d shear wave velocity profiles for each of the dispersion curves the models are characterised by a high velocity lid interpreted as the seismological expression of the subcontinental lithosphere beneath which velocities decrease gradually toward global reference values we find lithospheric thicknesses in the range 185 260 km beneath the hudson bay region with maximum lithospheric shear wave velocity perturbations of 4 5 7 3 above global reference values by treating the geotherm as a free parameter the average dispersion curve for hudson bay can be fit using laboratory based mineral physics relations with simple compositional models including constant composition and layered mantle models low mantle temperatures corresponding to surface heat flow values between 37 and 40 mw m 2 are required to match the observed data a layered model based on the archean slave craton is favoured as it provides a lithospheric root that is close to the isopycnic state i e neutrally buoyant for all depths in common with studies of other continental shields particularly fennoscandia and central australia the velocity structure modelled for the hudson bay region suggests that there is no systematic difference between the velocity structure and thickness of archean and proterozoic seismological lithosphere this result may have important implications for models of lithospheric stabilisation and evolution between 2 8 and 1 8 ga

40ar39ar thermochronological constraints on the structural evolution of the mesoproterozoic natal metamorphic province se africa abstract ten 40 ar 39 ar age spectra are presented for hornblende grains separated from mylonitized and weakly deformed amphibolites from the mesoproterozoic 1 2 1 0 ga natal metamorphic province nmp since the proposal of the sweat hypothesis the nmp has become a crucial area in which to study grenville aged locally termed namaquan accretion in this part of rodinia gondwana unlike many segments of the world s 1 1 ga belts the nmp is particularly suited to such studies since it is devoid of a high temperature pan african 500 ma overprint in this study we have dated hornblendes from within the earliest d1 ne directed thrust dominated structural domains of the belt the d1 domains are characterized by ne directed recumbent fold and thrust nappes and a pervasive generally s to sw dipping metamorphic foliation with down dip stretching lineations the new data constrain the minimum age of this early event which includes obduction of the northernmost tugela terrane onto the kaapvaal craton to 1135 9 ma in the d1 domains of the mzumbe terrane to the south cooling to below 550 c had only been attained by 1005 ma we also dated amphibolites from the later d2 structural domains which are characterized by sub vertical sinistral mylonite belts with sub horizontal to oblique stretching lineations our data show that d2 oblique shearing commenced at 1050 1035 ma in the mzumbe terrane and only terminated at 980 ma the youngest movements also included reactivation of major d1 structures such as the melville thrust mzumbe margate terrane boundary at 990 ma finally indications of minor pan african resetting was detected during the first 5 of the ar ar spectra from mylonites of the lilani matigulu shear zone tugela mzumbe terrane boundary indicating the polyphase nature of this major structure it has been suggested that the d1 event records the early ne sw directed arc continent collision history of the belt whilst the d2 event represents a continuation of essentially the same convergence vectors after extensive crustal thickening the new data place important time constraints on the major tectonic events in the nmp which appear comparable in many ways to other parts of the global grenvillian orogen

magnetite in ci chondrites on the basis of 939 measurements of the ci chondrites their magnetite contents have been determined to be alais 8 9 0 9 ivuna 11 1 0 3 orgueil 11 0 0 4 revelstoke 9 4 and tonk 8 5 standard statistical analysis of the data indicates the existence of two subgroups one comprised of ivuna and orgueil the other of alais revelstoke and tonk while we are not proposing a change in classification it is clear that distinctive differences exist between the two subgroups the precision of the method is 3 so that the standard deviations listed reflect the extent of actual variation in magnetite distribution the wide range from 6 to nearly 50 fe3o4 in previous estimates of the magnetite content of orgueil is due to experimental error not to a real heterogeneity

early partitioning of quebec microcontinent formation in the paleoproterozoic the distribution of allochthonous versus parautochthonous carbonate platforms combined with the timing of initial continental rift magmatism versus the timing of subsequent rifting of the continental margin provides tectonostratigraphic evidence for the formation and isolation of the oldest documented microcontinent perhaps as a consequence of the impingement of a mantle plume onto a cratonic margin during the paleoproterozoic initial rifting of the archean nucleus of north america in eastern canada is constrained to have been diachronous between 2 17 and 2 03 ga renewed rifting of a segment of the continental margin 292 to 120 m y later was accompanied by the emplacement of ultramafic layered sills the accumulation of komatiitic and alkalic basalts the deposition of banded iron formations and the isolation of a microcontinent and its 1 93 ga continental shelf succession subsequently accreted to the telescoped continental margin during collisional orogenesis

footwall involvement during arc continent collision ungava orogen northern canada geological evidence and theoretical models are presented to illustrate deformation paths followed by continental crust during underthrusting subduction and collision fieldwork in an early proterozoic collisional orogen indicates that a thrust belt and its autochthonous footwall basement are obliquely exposed in a crustal cross section with significant structural relief the collisional orogen records an evolution in the style and extent of involvement of the archaean footwall from 1 underthrusting without evidence of internal deformation 2 limited thrusting and ductile deformation of footwall rocks to 3 folding of the entire thrust belt and its footwall into cuspate lobate long wavelength structures the structural evolution occurred during hydration of basement rocks coeval with collision related metamorphism lithospheric strength profiles constrained by simple numerical heat conduction models suggest that the observed deformation path for the footwall crust was controlled by weakening due to heating of basement rocks and by changes in the relative strengths of different crustal layers the profiles predict a very weak lower crust and upper mantle at the time of footwall involved folding suggesting that the lower crust and upper mantle deformed by ductile flow concomitant with buckling of the stronger upper crust

structural evolution of the cape smith thrust belt and the role of out of sequence faulting in the thickening of mountain belts out of sequence thrusting represents a mechanism capable of heterogeneous internal deformation in thrust belts and was largely responsible for internal thickening in the early proterozoic cape smith belt the south verging thrust belt is marked by a complex history of accretion internal deformation thickening induced metamorphism uplift and erosion an overall chronology for thrust belt evolution is built around the timing of thermal peak metamorphism prethermal peak deformation includes 1 initial piggyback stacking of basalt dominated thrust sheets above a basal decollement localized at the basement cover contact 2 greenschist to amphibolite grade metamorphism accompanied by development of a ductile shear zone at the base of the thrust belt and 3 out of sequence thrusting precipitating the metamorphic thermal peak both above and below these faults synthermal peak out of sequence thrusting was responsible for accretion of autochthonous crystalline basement slices and eventual abandonment of the external basal decollement postthermal peak deformation is characterized by the continued development of out of sequence thrusts which appear to young toward the hinterland north internal thickening by development of flattening cleavages or folding appears to be regionally insignificant when compared to that accomplished by the out of sequence faults this suggests that 1 an eroding and accreting thrust wedge may be able to maintain its critical shape by out of sequence thrusting and 2 this style of internal deformation is favored in belts dominated by relatively strong material e g basalt as in the cape smith belt

syn tectonic magmatism and the development of compositional layering ungava orogen northern quebec canada abstract layer and foliation parallel emplacement of granitic veins was an important process in the regional development of compositional layering in now exhumed middle crustal sections of both archean and paleoproterozoic age in the northern ungava peninsula quebec canada in the archean superior province diorite and tonalite plutons were penetratively deformed and metamorphosed at granulite facies conditions coeval with voluminous granitic magmatism the paleoproterozoic narsajuaq arc contains evidence for contemporaneous magmatism transpressional deformation and granulite facies metamorphism prior to its collision with the superior province basement in both plutonic domains regional compositional layering is defined by 1 metre to kilometre scale alternation of generally well foliated tonalite and quartz diorite bodies and 2 centimetre to kilometre scale variably deformed granitic veins and sheets that lie parallel to layering tectonic foliation in the host rocks syn tectonic intrusion of a substantial portion of the veins along extension fractures sub parallel to layering foliation i e at high angle to the regional shortening direction is interpreted to have occurred due to the combination of a strong anisotropy and high magma pressures compositional layering generated and or enhanced by this process may contribute to the overall seismic reflectivity of the middle and lower crust

structural geology across a proposed archaean terrane boundary in the eastern yilgarn craton western australia abstract the eastern goldfields province of the archaean yilgarn craton western australia can be divided into several nw se trending terranes with different stratigraphic signature at least two main phases of penetrative ductile deformation affect the terranes d1 is a phase of nappe type transport mainly to the north along gently dipping ductile shear zones it is interpreted as a phase of regional extension contemporaneous with intrusion of massive granitoid plutons and possibly contemporaneous with deposition of felsic volcanics and metasediments in the greenstone belts terrane boundaries were probably formed during d1 d2 gave rise to steep nw se to n s trending foliations and upright folds d2 is strongly partitioned over the area for strain and vorticity the pattern of partitioning seems to be dependent on the position of granitoid plutons which give rise to development of d2 strain shadows d1 structures have been preserved mainly in these strain shadows terrane boundaries are transected by boundaries of d2 structural domains minor intrusion of granitoids continued during and after d2 d1 is possibly associated with arrival of a mantle plume below the yilgarn craton and d2 may be a phase of regional shortening in a thinned lithosphere weakened by the thermal event

experimentally derived high pressure cumulates from hydrous arc magmas and consequences for the seismic velocity structure of lower arc crust 1 this contribution explores the consequences of experimentally derived ultramafic plutonic rocks for estimates of continental crust composition the formation of high pressure cumulates from primitive to evolved hydrous arc magmas was investigated in the range 0 8 1 5 gpa and 1350 800 c to provide constraints on solid assemblages that form the roots of island arc crust at pressures exceeding 0 8 gpa 25km 40 60 of ultramafic cumulates are produced to obtain andesite to dacite compositions that are typical for evolved island arc volcanic and plutonic rocks calculated cumulate densities are comparable or higher than upper mantle values while seismic velocities vp vary continuously between 7 3 and 8 km s resulting in a broad crust mantle transition the moho represents a plagioclase saturation boundary our results are consistent with seismic imaging from continental margins and island arcs revealing that beneath an active volcanic front a sharp geophysical moho signature is sometimes absent

geology u pb and pb pb geochronology of the lake harbour area southern baffin island implications for the paleoproterozoic tectonic evolution of northeastern laurentia geological and geochronological results of an investigation of the paleoproterozoic siliciclastic and carbonate supracrustal rocks of the lake harbour group lhg and surrounding tonalitic gneisses on southern baffin island are presented conventional u pb geochronology of monazite from rocks of the lhg suggest that penetrative deformation of these rocks occurred prior to or during peak metamorphic conditions at ca 1845 1840 ma conventional u pb zircon results indicate that much of the tonalitic gneiss ranges in age from to 1827 the tonalitic gneisses and lake harbour group units were tectonically imbricated by ca 1805 ma and are part of a southwest verging thrust belt interpreted from regional considerations to represent the northern continuation of the ungava orogen the present results indicate that current tectonic models for the evolution of northeastern laurentia that involve a dominantly archean southeastern rae province require revision it is proposed that much of the metaigneous mate

anatomy of a terrane boundary an integrated structural geographic information system and remote sensing study of the late paleozoic avalon meguma terrane boundary mainland nova scotia canada remote sensing and geographic information system analysis complimented by geological mapping have resulted in a new interpretation of the late carboniferous evolution of the avalon meguma terrane boundary known as the minas fault zone in the canadian appalachian orogen various images including optical radar and shaded relief elevation have been integrated with magnetic and gravity data to compliment mapping in the vicinity of the exposed terrane boundary in mainland nova scotia throughout much of the region the style of deformation is typical of dextral motion along the east west chedabucto fault the most prominent structure in the minas fault zone lineament analysis of the shaded relief elevation and radar images has identified an important lineament trending east northeast which corresponds to the axial trace of folds that rotate clockwise into parallelism with the chedabucto fault however in eastern mainland nova scotia the shaded relief and geophysical images together with field data s

u pb geochronology of the eastern hall peninsula southern baffin island canada a northern link between the archean of west greenland and the paleoproterozoic torngat orogen of northern labrador abstract a corridor east of iqaluit that crosses several regionally significant lithologic boundaries has been mapped and sampled for geochronology zircon monazite and titanite were analysed by the isotope dilution u pb method to characterize igneous and metamorphic events and pb pb ages of detrital zircons were determined by laser ablation microprobe icp ms tonalitic gneisses at the eastern end of the corridor are archean with emplacement ages of 2920 8 6 2848 3 2844 6 5 and 2797 27 15 ma these samples record variable tectonothermal overprints at ca 2 77 ga and between 1844 and 1736 ma and may be related to similar rocks in west greenland a sequence of quartzites and semipelites associated with the archean gneisses contains exclusively archean detrital zircons and was metamorphosed at 1 88 ga in the central part of the corridor semipelitic and psammitic rocks contain almost exclusively paleoproterozoic detritus deposited after 1 93 0 05 ga and are interpreted as the northern continuation of the tasiuyak gneiss of the torngat orogen at the western end of the corridor monzogranites of the cumberland batholith intruded at 1869 9 3 1857 5 3 and 1850 3 2 ma the present data suggest that the north south nagssugtoqidian collision

large scale fluid infiltration metasomatism and re equilibration of archaean basement granulites during palaeoproterozoic thrust belt construction ungava orogen canada abstract the metamorphic history of the archaean superior province crystalline basement in the palaeoproterozoic ungava orogen attests to the importance of structural and geohydrological controls on a retrograde amphibolite granulite transition two distinct metamorphic suites separated in age by nearly one billion years are recognized in extensively exposed tonalitic to dioritic metaplutonic gneisses the older suite comprises c 2 7 ga granulite facies assemblages orthopyroxene clinopyroxene hornblende plagioclase ilmenite biotite quartz that record moderate pressures 5 kbar and high temperatures 800 c a younger c 1 8 ga suite resulted from amphibolitization of the granulites and is characterized by regionally extensive amphibolite facies mineral zones that broadly parallel the basal decollement of the overlying proterozoic cape smith thrust belt deformation mineral growth relationships in the amphibolitized basement indicate that extensive hydration and re equilibration of the archaean granulites occurred during thrust belt deformation the transition from granulite facies to amphibolite facies assemblages is characterized by the growth of garnet hornblende quartz cummingtonite coronas between plagioclase and orthopyroxene clinopyroxene as well as titanite coronas on ilmenite multi equilibrium thermobarometry on the coronitic assemblages documents re equilibration of the granulitic gneiss to 7 7 kbar at 644 c in the south and 9 8 kbar at 700 c in the north the variably deformed amphibolite facies domain sandwiched between the coronitic garnet zone and the basal decollement is marked by significant metasomatic changes in major element concentrations within tonalite these changes are compatible with equilibrium flow of an aqueous chloride fluid down a temperature gradient the source of fluids for basement hydration metasomatism is interpreted to be dehydrating clastic rocks in the overlying thrust belt with fluid flow probably focused along the basal decollement

terrane accretion in the internal zone of the ungava orogen northern quebec part 1 tectonostratigraphic assemblages and their tectonic implications the tectonostratigraphic record of the ungava orogen contains evidence for the interaction of divergent transform and convergent plate boundaries over a 0 2 ga period in the early proterozoic three principal tectonic domains are recognized 1 autochthonous basement plutonic and supracrustal rocks of archean age 2 autochthonous and allochthonous sedimentary and volcanic units associated with a ca 1 99 1 92 ga rift to drift margin and 3 suspect crustal components of a ca 2 00 ga ophiolite and ca 1 90 1 83 ga island arc terrane domain 1 is the stratigraphic or structural basement to domains 2 and 3 field relationships indicate that domain 2 units were accumulated on or near domain 1 whereas no tectonostratigraphic linkages are known between domain 3 and the others accumulation of continent derived sedimentary rocks continental tholeiites komatiitic basalts and mafic lavas equivalent to modern normal mid oceanic ridge basalts n morb s in domain 2 is interpreted as recording continental rifting leading to the possible formation of oceanic crust pillowed basalts sheeted dykes and cumulate rocks suggest that the crustal portion of an ophiolite is preserved within domain 3 the ophiolite is interpreted as a suspect fragment of oceanic crust which may represent an earlier rifted portion of domain 2 possibly juxtaposed by transform plate boundary motions within domain 3 calc alkaline volcanic and plutonic units and sequences of volcanogenic and siliciclastic sedimentary rocks are thought to indicate the development of a magmatic arc and fore arc basin and thus the establishment of a convergent plate boundary the most internal lithotectonic packages in domain 3 are predominantly crystalline thrust sheets containing i an older intrusive suite interpreted as the plutonic foundation of the magmatic arc and ii younger plutons interpreted as the result of a reversal in subduction polarity the reversal may have followed attempted subduction of domains 1 and 2 beneath the upper plate arc assemblage of domain 3

petrogenesis of neoarchean metavolcanic rocks in changyukou northwestern hebei implications for the transition stage from a compressional to an extensional regime for the north china craton abstract the 2 5 ga metavolcanic rocks in changyukou northwestern hebei can be classified into three groups based on major and trace elements high mg basalts tholeiitic basalts and the calc alkaline series basaltic andesites andesites and dacites rhyolites both high mg basalts and tholeiitic basalts have negative anomalies of nb zr ti and heavy rare earth elements hree as well as enrichments of sr k pb ba and light rare earth elements lree and show typical subduction zone affinities the petrogenesis of high mg basalts can be ascribed to high degree partial melting of an enriched mantle source in the spinel stability field that was previously enriched in large ion lithophile elements lile and lree by slab derived hydrous fluids melts supercritical fluids as well as the subsequent magma mixing processes of different sources at different source depths with little or no influence of polybaric fractional crystallization the flat hree of tholeiitic basalts indicates they may also originate from the spinel stability field but from obviously shallower depths than the source of high mg basalts they may form at a later stage of the subduction process when rapid slab rollback leads to extension and seafloor spreading in the upper plate we obtain the compositions of the archean lower crust of the north china craton based on the archean wutai jining section by compiling the average tonalite trondhjemite granodiorite ttg components average mafic granulite components and average sedimentary rock components the modeling results show that the generation of high al basalts basaltic andesites and andesites can be attributed to assimilation by high mg basalts primary basalts of relatively high al 2 o 3 thickened lower crust and the subsequent crystallization of prevailing mafic mineral phases while al 2 o 3 rich plagioclase crystallization is suppressed under high pressure and nearly water saturated conditions dacites and rhyolites may be the result of further fractional crystallization of basaltic andesites high al basalts and andesites mixing of magmas at various stages along the fractionation course of basaltic andesites high al basalts toward rhyolites promotes the trend of the calc alkaline series to reconcile the 2 55 to 2 5 ga ttgs derived from overthickened crust the 2 51 to 2 50 ga calc alkaline volcanic rocks derived from thickened crust tholeiitic basalts representing low pressure and an extensional tectonic setting 2493 ma leucosyenogranites derived from overthickened crust 2437 ma biotite monzogranites derived from slightly thinner crust than leucosyenogranites but still thickened as well as the clockwise hybrid itd and ibc p t paths of the hp granulites and widespread extension and rifting setting within the ncc from 2300 ma we propose a model of an evolving subduction process among them the composition of the 2 5 ga changyukou volcanic rocks and potassic granites as well as the clockwise hybrid itd and ibc p t paths of the hp granulites may reveal that the tectonic setting in northwest hebei was in a transition stage from a subduction related compressional regime to an extensional regime related to plate rollback

critical role of water in the formation of continental crust continental arcs are the sites of production of continental crust but the origin of these magmatic systems is not well understood although a number of processes have been suggested to be important the role of water migrating from slab to surface during subduction has been underappreciated directly below the moho hot approximately 1 100 c hydrous basaltic magmas fractionate as they cool to the regional geotherm at 750 to 800 c ultimately solidifying as mafic underplates cooling and fractionation cause water to exsolve and ascend triggering fluid fluxed melting of overlying mafic underplates and other crust melting of prior mafic underplates buffers temperatures and generates the voluminous juvenile low k magmas of cordilleran batholiths these granitoid magmas comprise a low temperature slurry of melt and residue and recrystallize into silicic mush during adiabatic ascent such hydrous mushes are intermittently infused by hotter more mafic magmas which hybridize and facilitate ascent and potentially eruption fluid fluxed melting overcomes many of the general petrological and geochemical problems associated with models dominated by fractional crystallization the role of water during repeated episodes of mafic underplating is critical to generate the juvenile granitoid infrastructure of the continents migration of water from the slab to the surface during subduction is highlighted as a key process in the formation of continental crust

trans hudson orogen of north america and himalaya karakoram tibetan orogen of asia structural and thermal characteristics of the lower and upper plates 1 the trans hudson orogen tho of north america and the himalaya karakoram tibetan orogen hkto of asia preserve a paleoproterozoic and cenozoic record respectively of continent continent collision that is notably similar in scale duration and character in tho the tectonothermal evolution of the lower plate involves 1 early thin skinned thrusting and barrovian metamorphism 2 out of sequence thrusting and high t metamorphism and 3 fluid localized reequilibration anatexis and leucogranite formation the crustal evolution of the indian lower plate in hkto involves 1 early subduction of continental crust to ultrahigh pressure uhp eclogite depths 2 regional barrovian metamorphism and 3 widespread high t metamorphism anatexis and leucogranite formation the shallow depths of the high t metamorphism in hkto are consistent with early to mid miocene ductile flow of an indian lower plate midcrustal channel from beneath the southern tibetan plateau to the greater himalaya melt weakening of the lower plate in tho is not observed at a similar scale probably due to the paucity of pelitic lithologies tectonothermal events in the upper plate of both orogens include precollisional accretion of crustal blocks emplacement of andean type plutonic suites and high t metamorphism syncollisional to postcollisional events include emplacement of garnet biotite muscovite leucogranites anatectic granites and sporadic metamorphism up to 90 myr following the onset of collision in tho comparing the type and duration of tectonothermal events for tho and hkto supports the notion of tectonic uniformitarianism for at least the later half of dated earth history and highlights the complementary nature of the rock record in an older exhumed orogen compared to one undergoing present day orogenesis

precambrian moho offset and tectonic stability of the east european platform from the urseis deep seismic profile deep seismic reflection data from the 1995 urals seismic experiment and integrated studies urseis transect of the southern urals document a distinct 5 km vertical offset of the moho beneath the uralian foreland here named the makarovo fault zone this offset does not disrupt the overlying riphean sedimentary section nor does it correlate with any major surface lineaments or known phanerozoic subduction features it is thus interpreted to be an archean or early proterozoic fault zone possibly a terrane boundary that disrupts an even older moho most important is the implication that this structure and the moho it offsets were not significantly modified by two subsequent cycles of continental collision that affected this region the baikalian 700 570 ma and the late paleozoic uralian 350 230 ma orogenic phases similarly the pronounced reflectivity of the moho beneath the uralian foreland must also be a precambrian signature not a young feature as would be implied by certain recent thermo rheological and reflectivity models that associate a reflective lower crust and moho with areas that underwent pronounced synorogenic or postorogenic collapse the makarovo fault zone provides yet another demonstration that relicts of ancient tectonic processes can survive for long periods of time in the lower crust and that the moho can be a key marker horizon in delineating the tectonic evolution of the lithosphere

review of crustal architecture and evolution in the ungava peninsula baffin island area connection to the lithoprobe ecsoot transect within the trans hudson orogen in northern quebec and southern baffin island parautochthonous archean basement and paleoproterozoic cover as well as allochthonous accreted paleoproterozoic units are exposed in a series of antiformal culminations and complementary synclinoria the parautochthonous rocks of the superior province margin and two assemblages of accreted paleoproterozoic units ungava and meta incognita crustal terranes define a basic tripartite crustal architecture which is characterized by polyphase deformation and metamorphic histories early structures and mineral assemblages are recognized in all three crustal components and shown to be temporally distinct the earlier structures and assemblages are overprinted by younger elements and mineral assemblages that are related to at least three regional contraction episodes and one thermal event that are common to all crustal components of the orogen close correlation of regional aeromagnetic data in northern quebec and southern baffin island with the three crustal components offers a first order approximation of the distribution of basement lithological units across hudson strait continuation of the aeromagnetic domains beneath ungava bay to northeastern quebec and northern labrador allows for the southward extrapolation of the tripartite trans hudson orogen crustal architecture into the lithoprobe eastern canadian shield onshore offshore transect area

u pb geochronology of detrital zircons in metasedimentary rocks from southern baffin island implications for the paleoproterozoic tectonic evolution of northeastern laurentia a geochronological investigation of metasedimentary rocks from southern baffin island using the geological survey of canada shrimp ii sensitive high resolution ion microprobe has characterized the ages of detrital zircon populations to determine their provenance bracket timing of deposition and distinguish potentially distinct sequences of rocks four lithologically and structurally distinct metasedimentary packages have been identified each appears to have been derived from a different source region in the structurally lowest package all analysed zircons are archean and 90 have ages between 2 83 and 2 63 ga these rocks are interpreted as the northernmost exposures of the paleoproterozoic povungnituk group of the cape smith belt northern quebec with detritus derived from the superior craton occupying the intermediate structural levels the most abundant supracrustal rocks on southern baffin island are siliciclastic and carbonate units of the lake harbour group and the tasiuyak paragneiss

magnetite morphologies in the essebi and haripura cm chondrites many of the unusual magnetite morphologies previously observed only in the ci chondrites have been seen in the cm chondrites essebi and haripura these include heavily textured spheroids which are possibly spherulites found both singly and in groups of up to a dozen and ranging in size from 4 to 10 m in diameter collections of microcrystals varying from 0 3 to 6 m in diameter spheres of 1 to 6 m in diameter layered nearly circular plates forming a spheroidal plaquette 7 5 m in diameter clusters of variable sized nodules 1 5 to 6 m with concave features framboidal magnetite and finally collections of less distinctive globules

isotope and rare earth element evidence for a late archaean terrane boundary in the southeastern pilbara craton western australia abstract rb sr sm nd and ree data have been obtained from granitic rocks exposed in archaean granite greenstone terrain in the southeastern part of the pilbara craton samples were collected from the kurrana batholith the cooninia billinooka and sylvania inliers and from the bottom of a borehole drilled through the hamersley basin succession in the fortescue valley almost all the samples have 87 rb 86 sr 144 nd and sm nd model ages tdm in the range 3 1 to 3 3 ga these characteristics together with ree patterns differ from published data for granitic rocks of the northern pilbara granite greenstone terrain that have similar geological histories rb sr isotopic systematics have been variably disturbed with data arrays on isochron plots all indicating ages younger than the accepted age of the base of the hamersley basin succession 2 76 ga this is attributed to pervasive burial metamorphism beneath the hamersley basin and to later hydrothermal events some rock units appear to be younger than their northern pilbara equivalents a shear zone separating the mosquito creek synclinorium from the kurrana batholith has the characteristics of a suture zone and is postulated to be the boundary between two distinct terranes that joined to form the pilbara craton between 3 0 and 2 76 ga

terrane accretion in the internal zone of the ungava orogen northern quebec part 2 structural and metamorphic history the tectonic history of the early proterozoic ungava orogen is marked by structural metamorphic episodes that both predate and postdate a collision between a magmatic arc terrane and the northern c

ductile shear zones related to crustal shortening and domain boundary evolution in the central fennoscandian shield the paleoproterozoic part of the fennoscandian shield is composed of crustal components formed in different tectonic settings and generally separated by well defined shear

late devonian palinspastic reconstruction of the avalon meguma terrane boundary implications for terrane accretion and basin development in the appalachian orogen abstract a late devonian early carboniferous palinspastic reconstruction of atlantic canada restoring subsequent motion along the avalon meguma terrane boundary and other major faults facilitates re examination of the spatial distribution of important accretionary and post accretionary events in the northern appalachian orogen the reconstruction suggests a genetic relationship between 415 and 360 ma tectonothermal activity in both these terranes and the collision between laurentia and gondwana in a zone of anomalously intense shortening opposite the st lawrence promontory in the appalachian orogen erosional products of this collision are an order of magnitude greater than the thin ca 1 km veneer of latest devonian early carboniferous clastic rocks that were deposited in intermontane basins across the orogen this missing detritus may be found in the thick deposits of the catskill clastic wedge in the central appalachians

paleozoic and mesozoic rocks of stikinia exposed in northwestern british columbia implications for correlations in the northern cordillera stikinia is a tectonostratigraphic terrane in the canadian cordillera that formed in a volcanic arc environment during paleozoic and mesozoic time until now the paleozoic rocks that form a semicontinuous belt along its western margin stikine assemblage were only recognized in a restricted area in northern british columbia between the stikine and taku river areas in contrast mesozoic stikinia rocks form an almost continuous belt that extends much farther to the north leading some authors to question the nature of the unexposed paleozoic basement north of the taku river area the following correlations have significant implications for tectonic reconstructions of the northern cordillera because they suggest that stikinia9s paleozoic volcanic sedimentary basement is more widespread than previously thought on the basis of similar rock types and lithologic associations six new u pb zircon dates and the common intrusive relationship with 184 195 ma plutons the stikine assemblage is correlated with the boundary ranges suite a metamorphosed paleozoic volcanic assemblage exposed in the tagish lake area north of the taku river and south of the yukon british columbia border the recognition of the boundary ranges suite and the jurassic plutons that intruded it tagish lake suite as part of stikinia has implications for the age and character of the stikinia tracy arm terrane boundary because the boundary ranges and tagish lake suites form the footwall of a major middle jurassic shear zone that carried the continental margin like rocks of the tracy arm terrane in its hanging wall this correlation also implies that the late paleozoic basement to the mesozoic stikinia arc is not a continental margin assemblage at least as far north as the british columbia yukon border and possibly farther the boundary ranges suite and therefore the stikine assemblage are also tentatively correlated with parts of the yukon tanana terrane in yukon aishihik lake area parts of the taku terrane in southeast alaska and undivided metamorphic rocks in west central british columbia differences in the isotopic signatures of these rocks may reflect along strike changes in the character of the basement rocks of the late paleozoic stikinia volcanic arc

petrological and geochemical characteristics of a paleoproterozoic magmatic arc narsajuaq terrane ungava orogen canada and comparisons to superior province granitoids abstract the narsajuaq terrane is a major component of the paleoproterozoic ungava trans hudson orogen of northern quebec and is a well exposed mid crustal block interpreted to be the plutonic roots of a magmatic arc the narsajuaq terrane is comprised of a series of hornblende pyroxene bearing plutons and supracrustal rocks which were accreted to the archean superior province basement during the paleoproterozoic on the basis of geological field relationships and geochronological data the plutonic rocks of the narsajuaq terrane are divided into four main groups 1 the cape smith suite 1898 1839 ma consists of various deformed and undeformed plutons that are found intrusive into the cape smith belt lithologies 2 the gneissic suite which ranges in age from 1863 1844 ma and consists of a well layered sequence of diorite and tonalite intruded by and interlayered with variably deformed and metamorphosed granitic veins 3 the younger suite consisting of discrete kilometre size bodies of metamorphosed plutons diorite monzodiorite tonalite granite intrusive into the gneissic suite ranging in age from 1836 1821 ma and 4 granitic sheets of the anatectic suite which intruded the narsajuaq terrane ca 1800 ma a series of metamorphosed sedimentary rocks predominantly semi pelites of the sugluk group are also found in the narsajuaq terrane and contain zircons of both archean and paleoproterozoic age plutons of the gneissic and younger suites are predominantly calc alkaline and metaluminous to mildly peraluminous and contain variable but generally moderate to high concentrations of the large ion lithophile elements and light rare earth elements rees pronounced negative nb and ti anomalies and fractionated rees comparable to modern arc granites a range in isotopic compositions is noted for the suites with the gneissic and younger suites having initial end and 87sr 86sr i values ranging from juvenile 4 0 and 0 7020 respectively to enriched 10 7 and 0 7048 the anatectic suite granites range to extremely enriched isotopic compositions with initial end up to 18 5 and 87sr 86sr i 0 7064 the cape smith suite has a much more restricted isotopic composition which is predominantly juvenile initial end 3 2to 1 5 and 87sr 86sr i 0 7020 to 0 7024 and suggests a minor role for crustal contamination primarily via sediment subduction the isotopic signature of the narsajuaq terrane plutons indicate that at least three sources were involved in their petrogenesis depleted mantle average 2 8 ga superior province crust and older 3 1 ga superior province crust when compared to plutonic suites from the ne and se superior province minto block abitibi pontiac and opatica belts the narsajuaq terrane suites define a similar chronology and evolution indicating a commonality and dominance of arc accretionary processes for crustal formation in the precambrian shield of canada higher trace element abundances in the narsajuaq terrane plutons compared to the archean samples may be related to different subduction zone fluxes which may reflect different bulk compositions of subducted oceanic sediments during the proterozoic

matrix mineralogy of the orgueil ci carbonaceous chondrite a petrographic and transmission electron microscopic study of the orgueil ci carbonaceous chondrite shows that the matrix consists mainly of fe bearing mg rich serpentine and smectite saponite as well as a poorly crystallized fe rich material that contains minor variable s and ni the fe rich material is probably ferrihydrite a ferric hydroxide the ferrihydrite occurs in small particles 80 a in diameter and the s and ni are probably adsorbed on its surfaces the serpentine and saponite occur in poorly crystallized fine crystallites fine phyllosilicates that are intimately associated with the ferrihydrite coarsely crystallized phyllosilicates coarse phyllosilicates occur in clusters relatively free of ferrihydrite the ferrihydrite is a major matrix constitutent and probably accounts for most of the fe3 as well as the superparamagnetic fraction in the orgueil matrix the textures suggest that the fine phyllosilicates resulted from alteration of the coarse phyllosilicates the alteration probably involved a substantial amount of water and coincided with the period of sulfate vein formation during the aqueous alteration magnetite grains disaggregated from framboids and fe ni sulfides were replaced by ferrihydrite simultaneously aqueous solutions reacted with the coarse phyllosilicates and precipitated ferrihydrite producing intimate pervasive mixtures with the fine phyllosilicates the matrix mineralogy of the orgueil chondrite suggests that orgueil experienced considerably different alteration conditions from the cm chondrites the mineralogical and petrological differences between ci and cm chondrites can not simply be explained as the result of different degrees of aqueous alteration

oxygen isotopes in separated components of ci and cm meteorites oxygen isotopic compositions have been determined for whole rock and separated components from the ci chondrites alais ivuna and orgueil and from the atypical cm chondrites bells and essebi the composition of ci matrix is in agreement with previous measurements and is systematically different from that of cm matrix probably due to warmer and wetter conditions of aqueous alteration for ci matrix although bells matrix is like that of other cm chondrites the matrix of essebi falls in the ci field essebi also contains abundant magnetite like cis and unlike other cms the isotopic compositions of magnetite in alais ivuna orgueil and essebi define a trend with 17o of 1 3 to 1 8 distinctly out of isotopic equilibrium with the phyllosilicates in each meteorite the magnetite serves as an isotopic tracer for the fluid which brought about aqueous alteration and oxidation and thus provides additional evidence for a gaseous reservoir in the solar nebula which was more 16o depleted than the earth

sandstone petrographic evidence and the chugach prince william terrane boundary in southern alaska the contact between the upper cretaceous valdez group and the paleocene and eocene orca group has been inferred to be the boundary between the chugach and the prince william tectonostratigraphic terranes sandstone petrographic data from the prince william sound area show no compositional discontinuity across this contact these data are best explained by considering the valdez and orca groups to be part of a single terrane a thick flysch sequence derived primarily from a progressively unroofing magmatic arc with increasing input from subduction complex sources through time

seismic imaging of a convergent continental margin and plateau in the central andes andean continental research project 1996 ancorp 96 1 a 400 km long seismic reflection profile andean continental research project 1996 ancorp 96 and integrated geophysical experiments wide angle seismology passive seismology gravity and magnetotelluric depth sounding across the central andes 21 s observed subduction of the nazca plate under the south american continent an east dipping reflector nazca reflector is linked to the down going oceanic crust and shows increasing downdip intensity before gradual breakdown below 80 km we interpret parts of the nazca reflector as a fluid trap located at the front of recent hydration and shearing of the mantle the fluids being supplied by dehydration of the oceanic plate patches of bright quebrada blanca bright spot to more diffuse reflectivity underlie the plateau domain at 15 30 km depth this reflectivity is associated with a low velocity zone p to s wave conversions the upper limits of high conductivity and high vp vs ratios and to the occurrence of neogene volcanic rocks at surface we interpret this feature as evidence of widespread partial melting of the plateau crust causing decoupling of the upper and lower crust during neogene shortening and plateau growth the imaging properties of the continental moho beneath the andes indicate a broad transitional character of the crust mantle boundary owing to active processes like hydration of mantle rocks in the cooler parts of the plate margin system magmatic underplating and intraplating under and into the lowermost crust mechanical instability at moho etc hence all first order features appear to be related to fluid assisted processes in a subduction setting

upper and lower plate juxtaposition deformation and metamorphism during crustal convergence trans hudson orogen quebec baffin segment canada abstract the lower plate of the quebec baffin segment of paleoproterozoic trans hudson orogen comprises parautochthonous archean crystalline basement of the superior province 3 22 2 74 ga and imbricated paleoproterozoic rift margin strata 2 04 1 92 ga the collisional upper plate contains 1 allochthonous ophiolitic units 2 00 ga 2 a continental arc terrane narsajuaq arc 1 86 1 82 ga 3 shelf sequence strata ca 1 86 ga and basement orthogneisses ca 1 95 ga 4 foreland basin deposits 1 86 ga and 5 a batholith which intrudes the shelf and foreland basin rocks cumberland batholith 1 86 1 85 ga lower and upper plate assemblages were juxtaposed during a 1 82 1 79 ga accretion collision event and deformed into regional scale folds during two post collisional folding episodes ca 1 76 1 74 ga as a result of the structural relief generated by the post collisional folding and subsequent exhumation all orogenic elements can be directly examined along a 400 km across strike transect in the external zone northern quebec lower plate rift margin units were internally imbricated cape smith belt and translated southward across the superior province basement paleoproterozoic amphibolite facies mineral growth is syn to post thrusting tectonic imbrication has resulted in a structural inversion of the accreted i e upper plate ophiolite stratigraphy north of the cape smith belt mid crustal plutonic units of the narsajuaq arc form an upper plate crystalline wedge whose sole thrust is localized near the thrust belt s basal decollement within narsajuaq arc syn magmatic granulite facies assemblages are retrogressed to amphibolite facies assemblages in proximity to major accretion collision faults in the internal zone southern baffin island a tripartite orogen scale tectonostratigraphy is exposed on the flanks of the post collisional fold structures at the lowest structural level lower plate superior province orthogneisses are imbricated with paleoproterozoic rift margin units and mineral growth is syn thrusting at intermediate structural level upper plate orthogneisses are correlated with the narsajuaq arc in northern quebec at the highest structural level a north facing shelf sequence is structurally overlain by an assemblage of clastic rocks interpreted as a foreland basin sequence dominantly monzogranite plutons of the cumberland batholith are restricted to the upper structural level and indicate pre emplacement imbrication of tectonostratigraphic units within both upper and lower plates granulite facies assemblages are retrogressed to upper amphibolite assemblages in proximity to 1 82 1 79 ga accretion collision structures exposures of superior province basement across the orogen document a progressive increase in the extent of reworking during accretion of upper plate units from a narrow shear zone in the external zone to km scale imbrication and more pervasive shear deformation in the internal zone a minimum of 160 km of basement underthrusting is constrained by the structural overlap of upper plate allochthons on lower plate units basement involvement during post collision shortening is manifest as thick skinned folding indicating coupling across the collisional basal decollement

purtuniq ophiolite cape smith belt northern quebec canada a reconstructed section of early proterozoic oceanic crust the crustal part of a dismembered ophiolite is preserved in the structurally highest thrust sheets of the early proterozoic cape smith belt in northern quebec canada the rocks of the ophiolite represent two magmatic suites each with a mantle source that is nd isotopically distinct the older suite comprises pillowed and massive volcanic rocks sheeted dikes dominantly gabbroic layered cumulate rocks and rare plagiogranite sills and dikes in the volcanic sequence this 5 km thick tholeiitic suite is ca 1998 ma and is compositionally and petrographically similar to rocks formed at modern oceanic spreading ridges the younger suite 4 km comprises sheeted mafic dikes and layered ultramafic to mafic cumulate rocks that are similar to tholeiitic rocks found in modern plume generated oceanic islands such as hawaii the composite section is in excess of 9 km although the total thickness of the purtuniq ophiolite may not be typical of that for oceanic crust formed at early proterozoic spreading ridges elsewhere it does suggest that ancient ridge formed crust was at least as thick as that in modern oceans

squeezing water from a stone h2o in nominally anhydrous minerals from granulite xenoliths and deep hydrous fractional crystallization although 10 of earth s water resides within continents ho distribution throughout the continental lithosphere and partitioning of ho in nominally anhydrous minerals nams remain poorly co

mylonite to megabreccia tracking fault events within a transcurrent terrane boundary in nova scotia canada transcurrent terrane boundaries commonly evolve into long lived faults that preserve little evidence for early docking events a remarkable exception is exposed at clarke head along the appalachian meguma terrane boundary in nova scotia canada where a late carboniferous fault megabreccia contains devonian 369 ma u pb zircon granulite grade mylonite fractured by veins filled with visean amphibole ca 335 ma ar ar this fractured mylonite was later mixed with early carboniferous sedimentary rocks during megabrecciation ca 315 310 ma these three fault events are reflected in the tectonostratigraphic record devonian ca 370 360 ma transpressional terrane docking ramped meguma up against avalonia and shed clastic detritus across the fault system the visean brittle deformation recorded by the amphibole veins was coeval with marine regression at surface the late namurian megabrecciation event similarly produced unconformity followed by renewed nonmarine clastic sedimentation the clarke head megabreccia therefore preserves an episodic late paleozoic fault history spanning some 55 m y during convergence between laurentia and gondwana and the assembly of the pangean supercontinent

elongated prismatic magnetite crystals in alh84001 carbonate globules potential martian magnetofossils using transmission electron microscopy tem we have analyzed magnetite fe 3o 4 crystals acid extracted from carbonate globules in martian meteorite alh84001 we studied 594 magnetites from alh84001 and grouped them into three populations on the basis of morphology 389 were irregularly shaped 164 were elongated prisms and 41 were whisker like as a possible terrestrial analog for the alh84001 elongated prisms we compared these magnetites with those produced by the terrestrial magnetotactic bacteria strain mv 1 by tem again we examined 206 magnetites recovered from strain mv 1 cells natural darwinian selection in terrestrial magnetotactic bacteria appears to have resulted in the formation of intracellular magnetite crystals having the physical and chemical properties that optimize their magnetic moment in this study we describe six properties of magnetite produced by biologically controlled mechanisms e g magnetotactic bacteria properties that collectively are not observed in any known population of inorganic magnetites these criteria can be used to distinguish one of the modes of origin for magnetites from samples with complex or unknown histories of the alh84001 magnetites that we have examined the elongated prismatic magnetite particles 27 of the total are indistinguishable from the mv 1 magnetites in five of these six characteristics observed for biogenically controlled mineralization of magnetite crystals

lithospheric structure in the southern canadian cordillera from a network of seismic refraction lines lithospheric velocity structure and its relationship to regional tectonics and development of the southern canadian cordillera are derived from a synthesis of interpretations from nine in line seismic refraction wide angle reflection profiles and broadside data recorded during the lithoprobe southern cordillera refraction experiment score and other refraction experiments across southern british columbia and one profile in northwestern washington consistency of the score two dimensional models at their intersection positions is achieved through application of a simultaneous inversion of all relevant traveltime data the cross sectional and map presentations demonstrate the strong degree of three dimensional heterogeneity within the crust and upper mantle a first order characteristic is the continuous increase in crustal velocities westward from the foreland belt to the insular belt the variations do not correlate with the morphogeological belts they do correspond with large scale geological and or

upb geochronology of the western cape smith belt canada new insights on the age of initial rifting and arc magmatism abstract the cape smith belt of northern quebec is the foreland to hinterland transition of the ungava orogen a component of the paleoproterozoic trans hudson orogen of north america u pb ages for rift and volcanic arc related units from the cape smith belt are presented and used to evaluate models of its tectonic evolution a gabbroic sill intrusive into the rift related sediments near the base of the povungnituk group is 2038 2 4 ma old indicating that rifting of the superior province was initiated prior to that time 80 m y earlier than previously reported a granodiorite intrusive into the volcanic rocks of the povungnituk group is dated at 1991 2 ma indicating that magmatism associated with the rifting event was active at that time 30 m y earlier than formerly thought the age of parent group arc related magmatism ranges from 1874 3 4 ma the age of a diorite intrusion to 1860 2 ma the age of another diorite intrusion and of a rhyolite a granodiorite intrusive in the parent group yields an age of 1845 6 ma magmatism of the parent group is thought to be related to northward dipping subduction of the superior province margin below the watts group a gabbroic intrusion in the 2000 ma old watts group is dated at 1858 2 ma these ages are consistent with a tectonic model in which the rocks of the cape smith belt formed in a single continental rift to oceanic progression followed by subduction which led to an arc continent collision a comparison of u pb ages for similar events in different sectors of the trans hudson orogen in canada reveals that early events are diachronous and range from 2169 ma to 2038 ma the main collisional events are coeval in the ungava and new quebec orogens and in some of the western sectors at 1830 1860 ma younger ages of 1800 1820 ma are obtained for the late collisional stages throughout the trans hudson orogen

inferring crustal structure in the aleutian island arc from a sparse wide angle seismic data set compressional seismic travel times from a relatively sparse wide angle data set hold key information on the structure of a 800 km long section of the central aleutian arc since the source and receiver locations form a swath along the arc crest that is similar to50 km wide we trace rays in 3 d for a collection of 8336 seismic refraction and reflection arrivals we investigate variations in seismic velocity structure parallel to the aleutian arc assuming that our result represents average crustal structure across the arc we explore seismic velocity models that consist of three crustal layers that exhibit smooth variations in structure in the 2 d vertical plane we consider the influence of additional constraints and model parameterization in our search for a plausible model for aleutian arc crust a tomographic inversion with static corrections for island stations reduces the data variance of a 1 d starting model by 91 our best model has seismic velocities of 6 0 6 5 km s in the upper crust 6 5 7 3 km s in the middle crust and 7 3 7 7 km s in the lower crust and a total crustal thickness of 35 37 1 km a resolution analysis shows that features having a horizontal scale less than 20 km cannot be imaged but at horizontal length scales of similar to50 km most model features are well resolved the study indicates that the aleutian island arc crust is thick compared to other island arcs and strongly stratified and that only the upper 60 of the arc crust has seismic velocities that are comparable to average seismic velocities in continental crust

a geochemical study of magmatism across a major terrane boundary sr and nd isotopes in proterozoic granitoids of the southern laramie range wyoming proterozoic granitoids in the laramie range of southeastern wyoming crop out on both sides of an inferred suture between the archean wyoming craton and proterozoic terranes to the south these rocks offer an unusual opportunity to test the various hypotheses for the geometry of the suture no difference in major or trace element composition is observed across the suture nor is there a large difference in isotopic composition nd and sr isotopic data indicate that only a small proportion of archean material has contributed to this granitoid suite even in two samples that intrude archean gneiss the isotopic data therefore require a dominantly proterozoic lower crust both beneath the proterozoic rocks and beneath the margin of the archean block this observation is inconsistent with models in which the suture between the archean and proterozoic blocks dips to the south this inconsistency can be reconciled by magmatic underplating or mechanical underthrusting of proterozoic crust beneath the archean block

origin of magnetite in oxidized cv chondrites in situ measurement of oxygen isotope compositions of allende magnetite and olivine magnetite in the oxidized cv chondrite allende mainly occurs as spherical nodules in porphyritic olivine po chondrules where it is associated with ni rich metal and or sulfides to help constrain the origin of the magnetite we measured oxygen isotopic compositions of magnetite and coexisting olivine grains in po chondrules of allende by an in situ ion microprobe technique five magnetite nodules form a relatively tight cluster in oxygen isotopic composition with delta 18o values from 4 8 to 7 1 and delta 17o values from 2 9 to 6 3 seven coexisting olivine grains have oxygen isotopic compositions from 0 9 to 6 3 in delta 18o and from 4 6 to 7 9 in delta 17o the delta 17o values of the magnetite and coexisting olivine do not overlap they range from 0 4 to 2 6 and from 4 0 to 5 7 respectively thus the magnetite is not in isotopic equilibrium with the olivine in po chondrules implying that it formed after the chondrule formation the delta 17o of the magnetite is somewhat more negative than estimates for the ambient solar nebula gas we infer that the magnetite formed on the parent asteroid by oxidation of metal by h2o which had previously experienced minor o isotope exchange with fine grained silicates

constraints on pb closure temperature in titanite based on rocks from the ungava orogen canada implications for u pb geochronology and p t t path determinations on the basis of multiequilibrium metamorphic pressure temperature p t estimates we have determined that 660 700 c is a reasonable minimum estimate of the closure t of pb diffusion in titanite a temperature significantly higher than previously suggested activity corrected reactions that produce titanite define narrow t stability fields 40 c that correspond closely to metamorphic p t determinations derived for coronitic garnet clinopyroxene hornblende samples growth of titanite occurring between 1789 and 1814 ma in the underthrust archean basement of the paleoproterozoic ungava orogen northern quebec is associated with retrograde metamorphism during underthrusting of the archean basement approx 1 b y after formation of the original granulitic assemblages

the nature of orogenic crust in the central andes 1 the central andes 16 22 s are part of an active continental margin mountain belt and the result of shortening of the weak western edge of south america between the strong lithospheres of the subducting nazca plate and the underthrusting brazilian shield we have combined receiver function and surface wave dispersion results from the banjo seda project with other geophysical studies to characterize the nature of the continental crust and mantle lithospheric structure the major results are as follows 1 the crust supporting the high elevations is thick and has a felsic to intermediate bulk composition 2 the relatively strong brazilian lithosphere is underthrusting as far west 65 5 w as the high elevations of the western part of the eastern cordillera ec but does not underthrust the entire altiplano 3 the subcrustal lithosphere is delaminating piecemeal under the altiplano ec boundary but is not completely removed beneath the central altiplano the altiplano crust is characterized by a brittle upper crust decoupled from a very weak lower crust that is dominated by ductile deformation leading to lower crustal flow and flat topography in contrast in the high relief inland sloping regions of the ec and sub andean zone the upper crust is still strongly coupled across the basal thrust of the fold thrust belt to the underthrusting brazilian shield lithosphere subcrustal shortening between the altiplano and brazilian lithosphere appears to be accommodated by delamination near the altiplano ec boundary our study suggests that orogenic reworking may be an important part of the felsification of continental crust

evolution of regional metamorphism in the cape smith thrust belt northern quebec canada interaction of tectonic and thermal processes syn metamorphic re imbrication of the internal part of thrust belts can result in distinct pressure temperature time deformation p t t d pathways for different structural metamorphic domains in the early proterozoic cape smith thrust belt canada an external piggyback sequence thrusting domain is characterized by thermal peak metamorphism occurring after deformation in contrast thermal peak metamorphism in an internal domain occurred during re imbrication by out of sequence thrusting the interactions of tectonic and thermal processes have been studied using three methods i qualitative evaluation of the timing between mineral growth and deformation ii analytical p t paths from growth zoned garnet porphyroblasts and iii numerical modelling of vertical heat conduction derived p t t d pathways suggest that uplift in the external domain resulted in part from erosion and isostatic unloading in contrast paths for the internal domain indicate that the out of sequence portion of the thrust belt may have experienced faster unroofing relative to the external domain this is attributed to thickening by out of sequence thrusting and possibly to extensional faulting at now eroded higher structural levels observations on the timing of metamorphism coupled with numerical modelling suggest that the thermal peak metamorphism documented in the external domain is a consequence of the emplacement of the out of sequence thrusts stack in the internal portion of the thrust belt

terrane boundaries within trans hudson orogen quebec baffin segment canada changing structural and metamorphic character from foreland to hinterland within the trans hudson orogen in northern quebec and southern baffin island parautochthonous archean basement and paleoproterozoic cover as well as allochthonous accreted paleoproterozoic units are exposed in a series of antiformal culminations and complementary synclinoria the parautochthonous rocks and two assemblages of accreted paleoproterozoic units crustal terranes are separated by major tectonic boundaries and associated deformation zones field petrological and isotopic data document an increase in the width of the deformation zones and a concomitant increase in p t conditions of metamorphism from orogenic foreland to hinterland tectonostratigraphic and geochronological criteria allow footwall and hangingwall units to be consistently distinguished in spite of the changing structural and metamorphic aspect of the terrane boundaries across the width of the orogen

seismo and neotectonics in finnmark kola peninsula and the southern barents sea part 1 geological and neotectonic framework abstract as a background for assessing the seismo and neotectonics of the finnmark kola peninsula region emphasis has been directed towards the lineament framework involving a careful examination of digital landsat tm imagery integrated with a variety of data sets ranging from geophysics and digital elevation models to microtectonics based on a detailed field structural control of many of the multiphase lineaments in this 70 000 km2 area of the northeastern part of the fennoscandian shield as well as microscopic investigation of the fault mylonites and breccias five principal lineament sets were distinguished of the nw se polyphasal megafractures the murmansk shear zone carries geological and isotopic evidence of an archaean tectonothermal history and demarcates an important terrane boundary lineaments irrespective of their trends were classified according to their significance in the regional geological framework some lineaments are clearly associated with gravity and aeromagnetic anomalies and probably extend to deep crustal levels quite a few of the prominent faults within lineament zones were reactivated in post glacial times in one area in finnmark core drilling through a se dipping post glacial reverse fault has confirmed its listric shape at depth earthquake focal mechanisms for this same fault are presented in the second part of this dual contribution

adakite like volcanism of ecuador lower crust magmatic evolution and recycling in the northern andes of ecuador a broad quaternary volcanic arc with significant across arc geochemical changes sits upon continental crust consisting of accreted oceanic and continental terranes quaternary volcanic centers occur from west to east along the western cordillera frontal arc in the inter andean depression and along the eastern cordillera main arc and in the sub andean zone back arc the adakite like signatures of the frontal and main arc volcanoes have been interpreted either as the result of slab melting plus subsequent slab melt mantle interactions or of lower crustal melting fractional crystallization and assimilation processes in this paper we present petrographic geochemical and isotopic sr nd pb data on dominantly andesitic to dacitic volcanic rocks as well as crustal xenolith and cumulate samples from five volcanic centers pululagua pichincha ilalo chacana sumaco forming a nw se transect at about 0 latitude and encompassing the frontal pululagua pichincha main ilalo chacana and back arc sumaco chains all rocks display typical subduction related geochemical signatures such as nb and ta negative anomalies and lile enrichment they show a relative depletion of fluid mobile elements and a general increase in incompatible elements from the front to the back arc suggesting derivation from progressively lower degrees of partial melting of the mantle wedge induced by decreasing amounts of fluids released from the slab we observe widespread petrographic evidence of interaction of primary melts with mafic xenoliths as well as with clinopyroxene and or amphibole bearing cumulates and of magma mixing at all frontal and main arc volcanic centers within each volcanic center rocks display correlations between evolution indices and radiogenic isotopes although absolute variations of radiogenic isotopes are small and their values are overall rather primitive e g end 1 5 to 6 87sr 86sr 0 7040 0 70435 rare earth element patterns are characterized by variably fractionated light to heavy ree la ybn 5 7 34 and by the absence of eu negative anomalies suggesting evolution of these rocks with limited plagioclase fractionation we interpret the petrographic geochemical and isotopic data as indicating open system evolution at all volcanic centers characterized by fractional crystallization and magma mixing processes at different lower to mid crustal levels as well as by assimilation of mafic lower crust and or its partial melts thus we propose that the adakite like signatures of ecuadorian rocks e g high sr y and la yb values are primarily the result of lower to mid crustal processing of mantle derived melts rather than of slab melts and slab melt mantle interactions the isotopic signatures of the least evolved adakite like rocks of the active and recent volcanoes are the same as those of tertiary normal calc alkaline magmatic rocks of ecuador suggesting that the source of the magma did not change through time what changed was the depth of magmatic evolution probably as a consequence of increased compression induced by the stronger coupling between the subducting and overriding plates associated with subduction of the aseismic carnegie ridge

multiple sources for archean granitoids in the yalgoo area yilgarn craton western australia geochemical and isotopic evidence abstract in the yalgoo area of the north central yilgarn craton three small granitoid bodies named the lady lydia llg yalgoo east yeg and yalgoo north granitoids yng intruded 2 8 ga mafic supracrustal rocks they form two suites i granodiorites from both the llg and yeg with strongly fractionated ree patterns high lree contents low hree contents weakly positive eu anomalies and of archean ttg affinity and ii biotite monzogranites from the yng with very high lree contents and prominent negative eu anomalies all the granitoids are calc alkaline with the granodiorites recording ages of 2763 2752 ma and the biotite monzogranites recording ages of 2636 2634 ma the ree and hfse characteristics of the granodiorites favor an archean subduction environment abundant xenocrystic zircons 3 00 2 94 ga in the granodiorites and their lu hf isotopic characteristics imply a long history of crustal recycling in the north central yilgarn the lu hf isotopes indicate that the 2 76 ga granodiorites and xenocrysts were formed from a composite source ranging in age from 3 5 to 3 0 ga increased crustal thickening due to convergence after cessation of subduction caused melting in the upper mid crustal levels with generation of parental melt at 2 64 ga to source the biotite monzogranites the latter are characterized by strongly ve ehf t 10 to 12 and largely eoarchean tdm2 hf model ages 3 85 3 6 ga significantly they provide evidence of eoarchean crustal material in this part of the yilgarn craton