paleoproterozoic arc and ophiolitic rocks on the northwest margin of the trans hudson orogen saskatchewan canada their contribution to a revised tectonic framework for the orogen abstract a new lithotectonic framework for the northwestern reindeer zone of the trans hudson orogen divides rocks into five northwest to north dipping volcano sedimentary assemblages 1 at the structural base the 1 92 1 87 ga largely sedimentary levesque bay assemblage partly equivalent to former maclean lake gneisses which lies within the confines of the kisseynew domain and is tectonically imbricated with metasedimentary rocks of the the lawrence point assemblage is interpreted as a dismembered supra subduction zone ophiolite high mgo refractory harzburgite group 1 ultramafic rocks at the structural base of the assemblage is geochemically identical to the upper mantle section of selected supra subduction zone ophiolites and mantle tectonites chromite and olivine compositions of the group 1 ultramafic rocks are also comparable to those of ophiolitic harzburgite and mantle tectonite mafic metavolcanic rocks of the assemblage are classified as subalkaline tholeiitic basalts their trace element patterns and hf ta th y nb and la element ratios resemble those of modern back arc basin basalts the reed lake assemblage represents a subduction generated arc complex that was built on top of the lawrence point assemblage its mafic metavolcanic rocks are subalkaline basalts with calc alkaline trends and elevated th and ce concentrations and negative nb anomalies feldspar porphyry dykes intruding the lawrence point and duck lake assemblages constrain timing of lawrence point ophiolite emplacement onto the duck lake assemblage to 1 86 1 84 ga the trace element geochemistry of the dykes suggests continued arc volcanism after ophiolite emplacement mafic metavolcanic rocks of the levesque bay assemblage are geochemically similar to those of the lawrence point assemblage other ultramafic rocks peridotite to pyroxenite are abundant in the lawrence point assemblage but have similar geochemistry to small ultramafic bodies intruding the reed lake duck lake and levesque bay assemblages they represent a separate later phase 1 86 ga of ultramafic plutonism which post dates ophiolite emplacement timing of lawrence point ophiolite emplacement between 1 86 and 1 84 ga and geochemistry of later felsic and mafic ultramafic volcanism suggest that the lawrence point ophiolite and overlying reed lake arc assemblage were not accreted to the hearne craton prior to 1 86 ga but were first accreted to the flin flon glennie complex after 1 86 ga

electrical resistivity imaging of the central trans hudson orogen magnetotelluric mt measurements were made on a profile across the trans hudson orogen in 1992 as part of the lithoprobe transect the present study includes analysis of results from a 300 km long section of the profile in which allocthonous paleoproterozoic juvenile terranes and arc rocks of the western trans hudson orogen have been juxtaposed against the archean sask craton impedance tensor decomposition of data from the 40 mt sites in the area indicates a geoelectric strike of n28 e two dimensional inversion of the data using a non linear conjugate gradient algorithm provided images of the resistivity structure resistivity images reveal that the crust of the sask craton is relatively resistive 2000 m in contrast the rocks of the flin flon belt glennie domain and la ronge domain are mostly relatively conductive 100 1000 m in the east of the study area the images suggest that the tabbernor fault juxtaposes more conductive rocks of the glennie domain in the west against more resistive

a special issue on volcanogenic massive sulfide deposits of the trans hudson orogen preface paleoproterozoic volcanogenic massive sulfide vms deposits are hosted in greenstone belts that formed during assembly of earth s first supercontinent nuna at 2 1 to 1 7 ga these deposits account for a significant component of the world s vms tonnage and thus are an important global exploration target among these paleoproterozoic belts the 1 92 to 1 80 ga trans hudson orogen of canada is the largest and best endowed in laurentia with over 29 deposits totaling 320 mt of cu zn au ag pb ore study of its deposits districts and tectonostratigraphic evolution enables greater understanding of metallogenic and geodynamic processes during the paleoproterozoic we briefly overview the trans hudson orogen its vms deposits and their exploration history and illustrate that various distinct types of volcanogenic massive sulfide deposits form in predictive tectonostratigraphic sequences associated with both pre and postaccretion volcanism we present results from a four year program to investigate the deposits and evolution of the trans hudson orogen highlighting various approaches at district and belt scales to improve exploration models for other structurally complex and covered precambrian terranes

widespread palaeocene volcanism around the northern north atlantic and labrador sea evidence for a large hot early plume head abstract voluminous volcanism took place west of greenland and northwest of the british isles in the palaeocene 6 7 ma before the start of sea floor spreading in the northern north atlantic very hot picritic magmas were produced simultaneously in such widely separated areas that they could not all have been above the central stem of the same mantle plume a single large plume head with temperatures of c 1500 c over a diameter of c 2000 km can explain the observations if surface volcanic activity took place only where the lithosphere was thin the presence of picrites does not indicate proximity to the plume stem

geochemical evidence for the geotectonic setting of early proterozoic metavolcanic sequences in lapland systematic geochemical study of early proterozoic metavolcanic rocks from the northern part of the baltic shield supports the concept that a complex mosaic of continental masses volcanic arcs and marginal basins were welded into one cratonic massif during the svecokarelian orogeny 1900 1750 ma in the north in finnmark and northern norrbotten tholeiitic metabasalts together with thick shallow water clastic metasediments form the main part of the karelian supracrustal sequences which in turn overlie archaean basement the trace element signatures of these metabasalts are comparable to those of post palaeozoic continental tholeiites which erupted in zones of lithosphere attenuation and which suffered contamination by sialic crust it is not however possible to determine by geochemistry alone whether the continental rift basin in question lay in an intraplate or back arc setting metabasalts with a volcanic arc geochemical signature become an increasingly important component of supracrustal sequences as the diffuse southern margin of the karelian continent is approached in the skellefte field an ensialic volcanic arc developed and fed considerable quantities of immature clastic sediment into the deep water norrland geosyncline which may have been fully oceanic in character subduction magmatism continued during the svecokarelian orogeny giving rise to synorogenic metavolcanic suites such as the kiruna porphyries the evidence for marked spatial variation of magmatic activity in the early proterozoic is in contrast to the rather monotonous pattern of greenstone belt development reported from the archaean we believe that this spatial variability is a consequence of geotectonic processes similar to those of modern day plate tectonics and present a plate tectonic model for the early proterozoic development of the baltic shield

a model for the simulation of combined major and trace element liquid lines of descent abstract variations of major and trace element partition coefficients as functions of temperature and composition have severely limited the range of composition for quantitative modeling of the differentiation of natural magmatic systems application of a two lattice melt model greatly reduces compositional dependence for trace elements this allows the temperature dependence of isobaric trace element partitioning to be evaluated independently thus making it possible to model an expanded range of conditions and compositions the first goal of this investigation was to construct a model to simulate major and trace element systematics of natural differentiation processes by combining trace element partitioning with a major element phase equilibrium model nielsen 1985b the second goal was the application of this model to the simulation of magma chamber recharge assimilation eruption and fractional crystallization these simulations indicate that the proportions of minerals fractionating from an open system are not necessarily the same as those in a closed system recharge or assimilation will drive a magma composition off the cotectic along which it is evolving however fractionation will drive the liquid back towards the cotectic in a recharged magma chamber as the recharge flux increases the fractionating mineral proportions will be increasingly biased towards those phases which appear first from the primitive magma the bulk partition coefficient for an open system can be considerably different 5 10 or more for some elements compared to a closed system paired assimilation and fractional crystallization are often cited as the mechanisms responsible for the origin of high al basalts and andesites however the addition of an al rich assimilant into a fractionating basaltic magma will increase the proportion of plagioclase to the extent that al in the derived liquids will be effectively buffered or even decrease the assimilation of forsteritic olivine by reaction keleman and ghiorso 1986 increases the proportions of pyroxene and olivine thus reducing the fe enrichment and al depletion characteristic of low pressure basalt fractionation

carbonate platform evolution in a silurian oceanic island a case study from alaska s alexander terrane abstract silurian limestones exposed in the alexander terrane of alaska are the oldest carbonates of wide distribution in the region and represent the evolution of a shallow marine platform in an island arc setting during waning volcanism and the onset of orogenesis four main stages in carbonate platform development are recognized beginning with calcareous turbidites that accumulated on a volcaniclastic ramp in a slope environment overlying deposits record the formation of a metazoan dominated fringing reef and of backreef sites that were affected by cyclic changes in water depth and sedimentation patterns following the earliest effects of orogenesis and widespread progradation of conglomerates small bioherms and biostromes grew in shallow subtidal settings on the rejuvenated platform co temporaneously a consortium of microbial organisms associated with accessory metazoans principally sphinctozoans created a rimmed carbonate shelf with the construction of stromatolite reefs at the seaward edge of the platform during platform drowning downslope transport from the platform margin resulted in significant accumulations of reefal material as debris flows and slumps along a deep marine slope carbonate sedimentation was terminated within the arc as a result of uplift shoaling and progradation of a clastic wedge during culminating phases of the klakas progeny in the late silurian early devonian this island arc suite is characterized by extraordinarily thick platform and periplatform carbonates sequential evolution of fringing and barrier reefs and patterns of faunal t rnover that differentiate these deposits from coeval carbonates that formed under different tectonic conditions a model for carbonate deposition in island arcs involving high rates of sediment accumulation steep submarine slopes crustal instability and biogeographic isolation contributes new data about carbonate platform development in tectonically active oceanic settings

case studies on the origin of basalt iii petrogenesis of the mauna ulu eruption kilauea 1969 1971 during the mauna ulu flank eruption on kilauea hawaii the concentrations in the lavas of the minor elements k p na and ti and the incompatible trace elements analyzed by isotope dilution k rb cs ba sr and the ree except yb decreased monotonically and linearly with the time or date of the eruption at the same time the concentrations of the major elements and of yb and the ratios of k rb k cs ba rb 87sr 86sr and 143nd 144nd remained constant most of the scatter in the raw concentration data is removed by a simple correction for olivine plus chromite fractionation previously established by wright et al 1975 these results are explained by simple equilibrium partial melting of a uniform source the degree of melting increased by about 20 of the initial value during the course of the eruption the trace element data are inverted by the method originated by minster and allegre 1978 and simplified by hofmann and feigenson 1983 the source has the following element or isotope ratios k rb 501 7 ba rb 14 0 0 5 rb cs 95 7 rb sr 0 0193 0 0045 0 0090 ce ba cn 1 1 0 1 sr ba cn 1 19 0 30 0 19 87sr 86sr 0 703521 0 000016 and 143nd 144nd 0 512966 0 000008 the ree pattern of the source has a nearly flat or slightly negative slope relative lree enrichment between ce and dy and a strongly positive slope between dy and yb however this relative hree enrichment is poorly constrained by the analytical data is highly model dependent and may not be a true source feature the yb concentration in the source is particularly poorly constrained because it is essentially constant in the melts on the other hand this special feature demonstrates that yb must be buffered by a mineral phase with a high partition coefficient for yb namely garnet the calculated clinopyroxene garnet ratio in the source is roughly equal to one in contrast the source of kohala volcano had previously been found to contain little or no garnet

the pyroxene paradox in morb glasses a signature of picritic parental magmas a persistent conclusion reached in studies of glass in the chilled margins of mid ocean ridge basalts morb is that occult clinopyroxene fractionation is required in order to explain the observed variation in glass compositions1 successful crystal fractipnation models commonly require crystallizing assemblages which are dominated by plagioclase and clinopyroxene rather than the plagioclase and olivine which dominate the phenocryst assemblages the pyroxene paradox arises from the observation that clinopyroxene phenocrysts are uncommon in lavas containing more than 8 5 wt mgo refs 2 3 and the fact that melting experiments on liquids with such compositions indicate that clinopyroxene appears only at 20 40 c below liquidus temperatures4 7 based on a study8 of compositional relationships between glasses and chilled margins of pillow lavas from the baffin bay picrite suite i suggest here that the pyroxene paradox is an illusion which arises in part because of the compositional dispersion of olivine fractionation trends from picritic parental magmas between the limiting cases of equilibrium and fractional crystallization

potential contributions of ancient oceanic islands to evolutionary theory before much was known about the geology of continental margins and the occurrence of oceanic islands in suspect terranes tests of theories related to island biogeography were undertaken mostly by ecologists and in paleontological investigations of non insular biota a significant new source of paleontological data with which to test models for species equilibrium over evolutionary versus ecological time scales and other important problems in evolutionary theory exists within circum oceanic belts where ancient oceanic islands are preserved as allochthonous terranes research on faunas from these areas simplifies attempts to apply the theory of island biogeography to the fossil record and to trace the history of island populations over evolutionary time scales paleontologic data from a paleozoic island arc alexander terrane show significant fluctuations in the taxonomic diversity of brachiopods and thus question one of the main assertions of the dynamic equilibrium model these data also support the con

the baffin bay lavas and the value of picrites as analogues of primary magmas the baffin bay picrites have been the focal point of a controversy concerning the mgo content of primary magmas derived from the upper mantle a sample population of 48 lava chilled margins collected across the baffin bay volcanic succession at the northeastern tip of padloping island exhibits a prominent compositional mode between 14 and 16 weight percent mgo 19 22 mg cation units mg 100 cations the petrography of these samples however requires that the padloping magmas were mixtures of olivine crystals and liquid at their eruption olivine phenocrysts constituted 15 to 30 volume percent of these magmas and retain compositions requiring coexisting liquid compositions with only 10 to 13 5 weight percent mgo 14 18 5 mg however highly magnesian olivine xenocrysts up to fo 93 found in the most magnesian lavas require the former existence of liquids with at least 18 weight percent mgo 24 mg if these xenocrysts represent early cumulates then the primary liquids of the padloping suite must have been at least this mgo rich with temperatures greater than 1 425 c such primary liquids could have evolved by olivine crystallization to a steady state equilibrated crystal liquid mixtures in a shallow reservoir system prior to eruption the compositions of the liquids of these mixtures appear to have been perched at the point of plagioclase saturation at approximately 1 275 c

biogeographic complexity in triassic bivalves of the wallowa terrane northwestern united states oceanic islands not continents provide the best analogues high levels of endemism and complex overlapping biogeographic patterns characterize modern molluscan faunas of the hawaiian islands and the triassic bivalve fauna of the wallowa volcanic arc terrane in hells canyon oregon such biogeographic complexities and high levels of endemism in many modern and mesozoic island settings constrain the use of faunal data as a primary basis for paleogeographic reconstruction of accreted terranes large taxonomically diver samples are required to identify genuine biogeographic patterns in these insular settings selective use of individual species genera or families to reconstruct terrane paleogeography may give misleading results

accretionary tectonics of the north american cordillera continental geology stands on the threshold of a change that is likely to be as fundamental as plate tectonic theory was for marine geology ongoing seismic reflection investigations into the deep crustal structure of north america are verifying that orogenic zones are underlain by low angle faults of regional extent brown et al 1981 the growing body of regional field relations is likewise delineating numerous orogenic sutures that bound discrete crustal fragments paleomagnetic and paleobiogeographic studies are revealing major latitudinal shifts and rotations within and between suture bounded fragments particularly within the north american cordillera such interdisciplinary studies are leading to a consensus that the cordillera has been built by progressive tectonic addition of crustal fragments along the continent edge in mesozoic and early cenozoic time such crustal growth is referred to as accretionary tectonics in this paper we review some of the important concepts in accretionary tectonics discuss the nature of the materials accreted between central alaska and southern california in jurassic and cretaceous time and consider the general relations between cordilleran accretion and the movement of lithospheric plates the concept of continents growing by peripheral accretion through geologic time has long been a topic of great interest with the advent of plate tectonics a number of different mechanisms for crustal accretion have arisen along with mechanisms for crustal attrition accretion mechanisms include the growth of imbricated sedimentary prisms along inner trench walls slicing off of submarine topographic irregularities within subducting plates and collision of continents and volcanic arcs by ocean basin closure tectonic attrition mechanisms include rifting transform faulting and strike slip or underthrust removal of inner trench wall materials coincident with or in place of accretionary prism growth growth of intraorogenic ocean basins by seafloor spreading is an additional important mechanism for creating accretionary materials as well as displacing crustal fragments an important implication of plate kinematic theory is the likelihood for accretionary and attritionary mechanics to operate in series both in time and space along continental margins since attrition by nature leaves little material evidence of having operated one of the major problems confronting cordilleran geologists lies in the recognition of such attrition within the ancient record particularly when interspersed with accretionary events the spectrum of accretion and attrition mechanisms viewed at cm yr 1 plate transport rates over time scales of 100 m y leads one to suspect a highly mobile history for continental margin orogens the serial arrangement of subducting transform and rifting links along the modern cordillera plate juncture system and both serial and parallel arrangements in the western pacific systems show the complex interplay of such mechanisms through space similar arrangements overprinted through time are suggested by the rock assemblages and structural patterns within the cordillera which presently resemble a collage of crustal fragments davis et al 1978 recognition of the structural state of this collage geologic field mapping and geophysical investigations will bring about a new level of understanding in the growth of continental crust and the reading of stratigraphic records within the fragments and future palinspastic restorations will lead to a new level of understanding in paleogeography and earth history the first problem to be considered is the recognition of native north american crust from exotic fragments that have been accreted to its edge

petrology of ultramafic and mafic xenoliths in picrite of kahoolawe island hawaii a picrite lava 22 wt mgo 35 vol ol along the western shore of the 1 3 1 4 ma kahoolawe tholeiitic shield hawaii contains small xenoliths of harzburgite lherzolite norite and wehrlite the various rock types have textures where either orthopyroxene clinopyroxene or plagioclase is in a poikilitic relationship with olivine the mg s of the olivine orthopyroxene and clinopyroxene in this xenolith suite range between 86 and 82 spinel mg s range from 60 to 49 and plagioclase is an75 80 a 87sr 86sr ratio for one ol norite xenolith is 0 70444 in comparison the host picrite has olivine phenocrysts with an average mg of 86 2 range 87 5 84 5 and a whole rock 87sr 86sr ratio of 0 70426 textural and isotopic information together with mineral compositions indicate that the xenoliths are related to kahoolawe tholeiitic magmatism but are not crystallization products of the magma represented by their host picrite rather the xenoliths are crystalline products of earlier primitive liquids feo mgo ranging 1 to 1 3 at 5 9 kbar in the cumulate environment of a magma reservoir or conduit system the presence of ultramafic xenoliths in picrite but not in typical kahoolawe tholeiitic lava 6 9 wt mgo is consistent with replenishment of reservoirs by dense mg rich magma emplaced beneath resident less dense tholeiitic magma mg rich magmas have proximity to reservoir cumulate zones and are therefore more likely than fractionated residual liquids to entrain fragments of cumulate rock

geoelectric response and crustal electrical conductivity structure of the flin flon belt trans hudson orogen canada a lithoprobe magnetotelluric survey across the palaeoproterozoic trans hudson orogen included 34 sites within the flin flon belt and adjacent geological domains the magnetotelluric impedance tenso

diffusion between olivine and spinel application to geothermometry abstract heating experiments were carried out at 1100 c and 1200 c under controlled oxygen fugacity on olivine spinel assemblages from new caledonia this mineral pair constitutes a perfect experimental material for such studies the result was a marked change in chemical composition of the minor phase olivine it becomes enriched in iron and chromium and the magnesium and nickel contents decrease careful studies of the influence of oxygen fugacity show that though it has a strong effect on the rate of re equilibration it does not affect the equilibrium composition of the phases the stability limits of the mineralogical assemblage used are also examined our results show the possibility of an important subsolidus re equilibration in natural assemblages containing olivine and spinel a new equation for the geothermometer based upon fe 2 mg 2 partitioning between olivine and spinel is proposed and some applications discussed

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

island arc carbonates characterization and recognition in the ancient geologic record abstract carbonates of island arc origin that are preserved in paleozoic mesozoic terranes of the north american cordillera exhibit a distinctive suite of paleontologic and lithologic features and share a fundamental similarity with limestones forming in modern volcanic arcs this study provides the first detailed synthesis of carbonate depositional systems in island arcs and documents primary sedimentary constituents based on facies relationships and faunal communities models are developed that show patterns in the long term evolution of shallow marine organisms and the construction evolution and demise of carbonate platforms in island arcs a suite of criteria is identified that may be used to differentiate island arc carbonates from limestones that accumulated in other platform settings biogeographic isolation prolonged subsidence steep submarine slopes and tectonic instability of volcanic edifices contribute to the development of relatively high levels of species endemism impoverished normal marine faunas complex provincial affinities and relict biotas in limestones that are characterized by exceptionally thick platform and periplatform sequences fringing and barrier reefs at the shelf margin extensive lagoonal deposits and rapid lateral and vertical facies changes although destructive tectonic and geologic processes in island arcs may hinder determining the original size and extent of the carbonate platform and particular facies types may not be represented e g fringing and barrier reefs may be replaced by sand shoals at the platform margin many characteristics have potential value for identifying carbonates of island arc origin in the ancient rock record apart from being associated with calc alkaline volcanic and volcaniclastic assemblages the most valuable suite of features for recognizing island arc carbonates is marine biotas that exhibit elevated levels of endemism and mixed paleobiogeographic affinities extraordinary thicknesses of platform shallow marine and periplatform carbonates and rapid facies changes between volcanic and carbonate rocks and between shallow and deep water limestones recognizing that an ensemble of features is distinctive within island arc carbonates considerably enhances identification of volcanic arcs in the ancient geologic record and thus improves the likelihood of successfully unraveling the complex geologic history of ocean basins

role of olivine cumulates in destabilizing the flanks of hawaiian volcanoes the south flank of kilauea volcano is unstable and has the structure of a huge landslide it is one of at least 17 enormous catastrophic landslides shed from the hawaiian islands mechanisms previously proposed for movement of the south flank invoke slip of the volcanic pile over seafloor sediments slip on a low friction decollement alone cannot explain why the thickest and widest sector of the flank moves more rapidly than the rest or why this section contains a 300 km3 aseismic volume above the seismically defined decollement it is proposed that this aseismic volume adjacent to the caldera in the direction of flank slip consists of olivine cumulates that creep outward pushing the south flank seawards average primary kilauea tholeiitic magma contains about 16 5 wt mgo compared with an average 10 wt mgo for erupted subaerial and submarine basalts this difference requires fractionation of 17 wt 14 vol olivine phenocrysts that accumulate near the base of the magma reservoir where they form cumulates submarine erupted kilauea lavas contain abundant deformed olivine xenocrysts derived from these cumulates deformed dunite formed during the tholeiitic shield stage is also erupted as xenoliths in subsequent alkalic lavas the deformation structures in olivine xenocrysts suggest that the cumulus olivine was densely packed probably with as little as 5 10 vol intercumulus liquid before entrainment of the xenocrysts the olivine cumulates were at magmatic temperatures 1100 c when the xenocrysts were entrained olivine at 1100 c has a rheology similar to ice and the olivine cumulates should flow down and away from the summit of the volcano flow of the olivine cumulates places constant pressure on the unbuttressed seaward flank leading to an extensional region that localizes deep intrusions behind the flank these intrusions add to the seaward push this mechanism ties the source of gravitational instability to the caldera complex and deep rift systems and therefore limits catastrophic sector failure of hawaiian volcanoes to their active growth phase when the core of olivine cumulates is still hot enough to flow

evidence from neodymium isotopes for mantle contributions to phanerozoic crustal genesis in the canadian cordillera sm nd and rb sr isotopic studies of several phanerozoic orogenic belts have shown that much of the crust in these belts is composed of reworked pre existing continental crust in contrast isotopic data collected from two of the largest terranes in the canadian cordilleran orogenic belt the alexander and stikine terranes show that these tectonic fragments are composed of mantle derived continental crust in many respects the style and rate of crustal accretion of these and related terranes appear similar to those of some proterozoic crustal regions

origin of the archean sask craton and its extent within the trans hudson orogen evidence from pb and nd isotopic compositions of basement rocks and post orogenic intrusions u pb zircon ages from the exposed sask craton are 2450 3100 ma from the peter lake domain 2575 2640 ma and from rocks of the trans hudson orogen 1840 1880 ma u pb monazite and zircon ages of post orogenic pegmatites and aplites are 1770 1800 ma common pb and sm nd isotopic compositions of post orogenic intrusions as probes of crust beneath the orogen were compared to sask craton rocks and ca 1850 ma orogenic rocks to infer the origin and subsurface distribution of the sask craton within the internides of the trans hudson orogen results show that post orogenic intrusions within most of the glennie domain and hanson lake block were derived at least in part from archean source materials demonstrating that the sask craton lies beneath paleoproterozoic orogenic rocks present at the surface in contrast common pb and sm nd isotopic compositions from pegmatites and aplites of the la ronge domain are essentially identical with those of the paleoproterozoic orogenic rocks into which they are intruded indicating derivation by partial melting of similar rocks thus if the sask craton extended to the west beneath the la ronge domain it was beneath the zone of melting that produced the post orogenic intrusions making it unlikely that the sask craton is a detached part of the hearne craton many samples from the sask craton have elevated pb 208 pb 204 ratios unlike superior craton or hearne craton rocks suggesting that the sask craton was derived from an exotic source such as the wyoming craton which shares similar elevated pb 208 pb 204 ratios

undepleted mantle composition beneath hawaii compositions of undepleted mauna loa mantle have been calculated assuming derivative residuum relations between a tholeiitic picrite from the 1852 eruption of mauna loa and mantle residua whose compositions are delineated by experimental data and the compositions of the most refractory picrite phases namely mg rich olivine and cr rich spinel the preferred calculated composition based on 40 picrite 60 residuum composed of olivine orthopyroxene and cr spinel has a lower 100 mg mg fe ratio 84 1 than the ratios of pyrolite like undepleted mantle compositions 88 90 and compared with many such compositions it also has significantly higher abundances of tio2 al2o3 cao na2o k2o and p2o5 the higher abundances of incompatibles have implications for the volumes and character of potential derivative melts a calculated mode which assumes that a titaniferous pargasitic amphibole is a component of the undepleted lherzolite indicates that amphibole could be a significant primary phase at temperatures slightly above the lherzolite solidus amphibole would be eliminated on incongruent melting and its breakdown products mainly clinopyroxene plus minor olivine would become part of a re equilibrated anhydrous peridotitic mineral assemblage the lower 100 mg mg fe ratio of the calculated composition would permit the generation of primary basaltic melts with relatively low 100 mg mg fe2 ratios at lower 40 degrees of melting such lower ratios are usually attributed to olivine fractionation of more mg rich primary melts from pyrolite like mantle sources

major element records of variable plume involvement in the north atlantic province tertiary flood basalts major element variations in north atlantic tertiary province primitive early erupted alkaline transitional tholeiite basalts recalculated to a restricted value of mgo give insights into the process of plume related magmatism basalts primitive enough to be crystallizing only olivine were recalculated to a proposed primary magma composition of 15 wt mgo the recalculated dataset shows clear inter element correlations including a strong significant negative correlation between fe and si indicating polybaric melt segregation overlap between basalt compositions and experimental melts from a fertile fe rich low mg number 85 5 peridotite suggests that relative to normal peridotite with mg number 89 the north atlantic basalt source was fe rich linear regression of the experimental data gives apparent pressures of magma segregation of 17 5 37 kbar with intra region variability in the depth derivation from the melt column for each sample thus suggesting that lithospheric thickness lid effect control on magma generation may have been overemphasized in recent studies comparable source composition magma segregation depth and calculated mantle potential temperature 1440 1460 c throughout the province supports the previously suggested plume impact model arriving below east greenland derived from a variably enriched and depleted lower mantle source given the good agreement between conclusions drawn from major element data and previously published results we suggest that restricted mgo recalculated datasets may be usefully applied to study other large igneous provinces

geochemical and mineralogical evidence for the occurrence of at least three distinct magma types in the famous region bulk rock major and trace element variations in selected basalts from the famous area in conjunction with a detailed study of the chemical compositions of phenocryst minerals and associated melt inclusions are used to place constraints on the genetic relationship among the various lava types the distribution of nio in olivine and cr spinel phenocrysts distinguishes the picritic basalts plagioclase phyric basalts and plagioclase pyroxene basalts from the olivine basalts for a given mg mg fe2 atomic ratio of the mineral the nio content of these phenocrysts in the former three basalt types is low relative to that in the phenocrysts in the olivine basalts the zr nb ratio of the lavas similarly distinguishes the olivine basalts from the plagioclase phyric and plagioclase pyroxene basalts and in addition distinguishes the picritic basalts from the other basalt types these differences indicate that the different magma groups could not have been processed through the same magma chamber and preclude any direct inter relationship via open or closed system fractional crystallization

oxygen isotope ratios in olivine from the hawaii scientific drilling project oxygen isotope ratios of olivine in 23 tholeiites from the hawaii scientific drilling project hsdp core 15 from mauna kea 8 from mauna loa and three samples of outcropping subaerial or dredged submarine mauna kea lavas have been measured by laser fluorination the 18 o values are 4 6 5 4 confirming previous observations that some hawaiian lavas are derived from sources with 18 o values lower than typical upper mantle 18 oolivine 5 2 0 2 the mauna kea mauna loa transition marks a shift from 18 o values lower than the mantle average in mauna kea olivines 4 8 to more typical mantle values in mauna loa olivines lavas containing olivines with 18 o values similar to the typical upper mantle are associated with more primitive or less depleted radiogenic isotope characteristics i e with higher 3he 4he 13 ra higher 87 sr 86 sr 0 7036 and lower nd 6 5 and with 206 pb 204 pb ratios less than 18 3 these relationships indicate that the 18 o values of the relatively enriched source components of the hawaiian plume sampled by mauna loa lavas are comparable to or greater than the mantle average this conclusion is supported by 18 o values of olivine from other high 3he 4he islands which are also comparable to the upper mantle average the low 18 o values in hawaiian lavas are derived from a source having more morb like or depleted he nd and sr isotope ratios but more radiogenic pb than is seen in the mauna loa lavas assimilation of 18 o depleted lower oceanic crust from the underlying pacific crust by hot mgo rich parental magmas or melting of older recycled oceanic crust entrained in the hawaiian plume are both possible sources of this 18 o depleted morb like component in hawaiian magmas

intraoceanic tectonics and the development of continental crust 1 92 1 84 ga evolution of the flin flon belt canada the flin flon belt is a complex amisk collage of distinct tectono stratigraphic assemblages that was brought together at a relatively early stage in the tectonic evolution of the paleoproterozoic trans hudson orogen four main tectono stratigraphic assemblage types are recognized isotopically juvenile oceanic arc 1 90 1 88 ga ocean floor ca 1 90 ga oceanic plateau ocean island and isotopically evolved arc 1 92 1 90 ga oceanic arc assemblages include tholeiitic calc alkaline and lesser shoshonitic volcanic and volcaniclastic rocks as well as turbidites and arc rift volcanic rocks the tectono stratigraphic assemblages were juxtaposed in an accretionary complex amisk collage at ca 1 88 1 87 ga probably as a result of arc arc collision collisions accretionary collage structures are largely obliterated by subsequent deformation and metamorphic events but can be inferred where cut by calc alkaline plutons related to a 1 866 1 838 ga arc coeval subaerial volcanism is recorded in ca 1 87 1 85 ga calc alkaline to shoshonitic volcaniclastic sequences unroofing of the accretionary collage development of a paleosol and deposition of continental alluvial fluvial sedimentary rocks occurred ca 1 85 1 84 ga coeval with the waning stages of post accretion arc magmatism stabilization of the flin flon belt by 1 85 1 84 ga as part of a philippines or japan like microcontinent is attributed to both tectonic and magmatic thickening of the lithosphere

lithogeochemistry of volcano plutonic assemblages of the southern hanson lake block and southeastern glennie domain trans hudson orogen evidence for a single island arc complex this paper describes the geology geochemistry and age of two amphibolite facies volcano plutonic assemblages in the southern hanson lake block and southeastern glennie domain of the paleoproterozoic trans hudson orogen of east central saskatchewan the hanson lake assemblage comprises a mixed suite of subaqueous to subaerial dacitic to rhyolitic ca 1875 ma and intercalated minor mafic volcanic rocks overlain by greywackes similarly with modern oceanic island arcs the hanson lake assemblage shows evolution from primitive arc tholeiites to evolved calc alkaline arc rocks it is intruded by younger subvolcanic alkaline porphyries ca 1861 ma synvolcanic granitic plutons ca 1873 ma and the younger hanson lake pluton ca 1844 ma rocks of the northern lights assemblage are stratigraphically equivalent to the lower portion of the hanson lake assemblage and comprise tholeiitic arc pillowed mafic flows and felsic to intermediate volcaniclastic rocks and greywackes which can be traced as far west

nd and sr isotopic characterization of the wrangellia terrane and implications for crustal growth of the canadian cordillera nd and sr isotopic data are reported from samples of most of the major lithologic units of the wrangellia terrane exposed on vancouver island and the queen charlotte islands initial epsilon nd values range from 1 0 to 7 3 and initial 87 sr 86 sr ratios range from 0 70323 to 0 70481 these ratios are similar to those of modern island arcs and demonstrate the unevolved juvenile nature of this terrane these data indicate that wrangellia probably resided in an intra oceanic environment until its accretion to north america nd crustal residence ages for clastic sedimentary rocks of wrangellia mean t cr 0 77 pm 0 33 ga t dm 0 57 pm 0 35 ga are much closer to their stratigraphic ages than is the case for the ages of many other phanerozoic sedimentary rocks thus providing strong evidence for phanerozoic crustal addition to north america early proterozoic crustal components cannot comprise more than about 6 of wrangellia therefore the terrane consists of 94 new mantle d

redox effect on the partitioning of nickel in olivine abstract the fractionation of nickel by olivine in the lower zone of the kiglapait intrusion in company with the olivine compositions compared to estimated liquid compositions places stringent limits on the value of the olivine liquid partition coefficient for ni estimated at 4 0 0 8 at the base of the lower zone data from a strong ni mgo correlation in mauna loa picrites also require a low value of d estimated at 5 8 0 4 these values are much lower than usually considered normal for mafic magmas and the effect is ascribed to the low oxygen fugacity of the magmas estimated independently for the kiglapait case to be near the wustite magnetite buffer at this redox state a significant amount in this case about half of the ni in the melt is dissolved as ni 0 which inflates the liquid concentration without entering the olivine because the analyzed values reflect only total ni rather than the species the partition coefficient decreases with reduction as shown by recent experimental studies the mauna loa data imply a reduced condition for that magma as well knowledge of the redox regime governing the partition coefficient is vitally important in modelling fractional crystallization and in discussing early planetary history conversely knowledge of the ni fractionation in a magma series can specify the redox regime of the magma as at mauna loa

galapagos hot spot spreading center system 1 spatial petrological and geochemical variations 83 w 101 w we report on the petrology and geochemistry of basalts dredged at 40 50 km intervals along the galapagos spreading center between 83 sup 0 w and 101 sup 0 w 40 stations emphasis is on spatial variations of whole rock major elements rare earths trace metals of the first transition series and the nature of phenocryst assemblages and their abundances these results provide new constraints on the nature and scale of mantle source heterogeneities melting conditions thermal field and dynamics of crustal formation of the region we suggest that ridge segments outside the high magnetic amplitude zone are at a steady state as a result of passive seafloor spreading basalts from these segments are apparently derived from an asthenosphere relatively uniformally depleted in incompatible elements which appears of worldwide extent we reject vogt and deboer s 1976 model invoking damming at fracture zones of subaxial asthenosphere flow of crystal slushes and increasing fractional crystallization down the flow line because this model would not explain the gradients in ree observed about the galapagos platform our preferred model combines the mantle plume binary mixing model of schilling 1973 with the concept of recurring rift propagation proposed by hey 1977a we further propose that pulsating mantle plume flux perhaps inmore the form of a chain of blobs may initiate the development of new rifts and their propagation the present position of the tips of such new propagating rifts locate the wave fronts of such pulsating mantle plume flow a two million year period is suggested for the last 4 m y from wilson and hey s 1979 information rigorous testing of our preferred model is possible less

geochemistry of the archean kam group yellowknife greenstone belt slave province canada abstract the geochemistry and isotope systematics of archean greenstone belts provide important constraints on the origin of the volcanic rocks and tectonic models for the evolution of archean cratons the kam group is a 10 km thick pile of submarine tholeiitic mafic and subordinate felsic volcanic rocks erupted between 2712 and 2701 ma that forms the bulk of the yellowknife greenstone belt in the dominantly granite metasedimentary slave province mafic rocks range from normal mid ocean range basalt like basalts to slightly light rare earth element enriched lree enriched but nb depleted basaltic andesites and andesites whereas dacitic to rhyodacitic felsic rocks are strongly lree enriched and highly depleted in nb the documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcomma

a coral reef in the telkwa range british columbia the earliest jurassic example an end triassic mass extinction profoundly affected reef ecosystems that flourished in the late triassic tethys seaway the collapse of late triassic coral sponge reefs was followed by an early jurassic hettangian sinemurian perturbation interval with a near global absence of reefs and sharp reductions in diversity a jurassic sinemurian reef in the hazelton group of central british columbia appears to fill the gap its paleoecology and composition show it to be the first large framework example in north america and perhaps the world it demonstrates that the reef building triassic coral phacelostylophyllum survived the extinction event and was constructing reefs in early jurassic time during a global reef eclipse the reef is a 48 m thick bioherm that grew within the island arc complex of stikinia following a decrease in volcanism reef building began with bivalves growing upon water lain tuffs the reef was dominated by large dendroid phaceloid corals primarily phacelostylophyllum rugosum a spe

three dimensional collisional structure of the trans hudson orogen canada abstract the three dimensional structure of the eastern trans hudson orogen tho part of a paleoproterozoic continent continent collision zone in central north america is revealed through a network of lithoprobe seismic reflection profiles the seismic images are interpreted to delineate a series of stacked thrust sheets essentially confined to the crust e w profiles show strong e dipping reflections extending throughout the crust while n s profiles record events outlining antiformal and synformal structures this allows the identification of decollements that may have localized along pre existing structures e g possible basin bounding extensional faults and at major rheological boundaries e g basement cover contact upper middle crust transition the present topographic surface displays oblique crustal sections with 10 15 km of structural relief generated during post collisional intracontinental transpression of tho as a result of crustal scale folding and faulting

dispersed remnants of a northeast pacific fringing arc upper paleozoic terranes of permian mccloud faunal affinity western u s two fragmentary subparallel belts of terranes within the western north american cordillera contain upper paleozoic rocks and are characterized by contrasting lithotectonic assemblages and contrasting permian faunal affinity these two belts are a volcanic arc related successions of permian mccloud faunal affinity mccloud belt and b subduction related accretionary complexes of permian tethy an faunal affinity cache creek belt this paper supports the hypothesis that the fragmentary terranes of the mccloud belt once constituted parts of a northeast pacific fringing arc system and in constrast to some earlier interpretations concludes that the volcanic arc evolved above an eastward dipping subduction zone the absolute distance between this arc and western north america during the late paleozoic cannot be constrained however there is little evidence to suggest closure of a major 10 km ocean basin or protracted periods of westward dipping subduction parts of devonian to permian volcanic island arc sequences of the western u s cordillera are represented in the northern sierra eastern klamath bilk creek grindstone and chilliwack terranes these scattered volcanic arc remnants share several fundamental characteristics 1 the sequences were constructed across continental affinity basement assemblages 2 they underwent similar tectonic evolution during late paleozoic time such as coeval pulses in volcanism and related depositional histories 3 they contain early permian mccloud type fauna of distinctive biogeographic affinity 4 mccloud belt terranes are spatially and possibly genetically related to westward lying accretionary complexes of the cache creek belt which contain fragments of upper triassic blueschist and permian limestone blocks bearing tethyan permian fusulinids and corals based on the presence of distinctive early permian mccloud fauna the island arc remnants discussed in this paper are referred to as the mccloud belt the similarities between mccloud belt terranes suggest that they were once parts of a long lived late paleozoic volcanic arc complex the stratigraphy of these terranes records the tectonic evolution of the volcanic arc volcanism within the paleozoic arc began as early as the early devonian and was succeeded or accompanied by carbonate and epiclastic sedimentation during the middle and late devonian an influx of detritus derived from sedimentary and metasedimentary rocks during the late devonian and mississippian records the uplift and erosion of arc basement sequences an event that was at least locally accompanied by ongoing volcanism a pulse in volcanism during the pennsylvanian and coeval uplift led to the establishment of shallow marine and deltaic volcanogenic and carbonate sedimentation and locally subaerial exposure which persisted into the early early permian elsewhere during the early permian shallow water carbonate deposition was established with only minor influx of volcanogenic debris during the late permian and locally as early as late early permian a major pulse in volcanism occurred and is now represented by voluminous sequences of volcanogenic debris and lavas most of these sequences are overlain by upper triassic carbonate platformal sequences arc volcanism was recorded at least locally until the middle jurassic paleomagnetic data are used to constrain a schematic pre cretaceous reconstruction of these volcanic arc sequences and their associated subduction assemblages the inferred paleogeography of island arc sequences and the adjacent accretionary terranes suggests that volcanism occurred above an east dipping subduction zone with a long lived paleogeographic relation to north america the current structural position of these terranes is largely the result of mesozoic and locally younger deformation

experimentally determined mineral melt partition coefficients for sc y and ree for olivine orthopyroxene pigeonite magnetite and ilmenite our current lack of understanding of the partitioning behavior of sc y and the ree rare earth elements can be attributed directly to the lack of a sufficiently large or chemically diverse experimental data set to address this problem we conducted a series of experiments using several different natural composition lavas doped with the elements of interest as starting compositions microprobe analyses of orthopyroxene pigeonite olivine magnetite ilmenite and co existing glasses in the experimental charges were used to calculate expressions that describe ree partitioning as a function of a variety of system parameters using expressions that represent mineral melt reactions versus element ratio distribution coefficients it is possible to calculate terms that express low ca pyroxene melt partitioning behavior and are independent of both pyroxene and melt composition compositional variations suggest that sc substitution in olivine involves either a paired substitution with al or more commonly with vacancies the partitioning of sc is dependent both on melt composition and temperature our experimentally determined olivine melt ree ds partition coefficients are similar to but slightly higher than those reported by mckay 1986 and support their conclusions that olivines are strongly lree depleted y and ree mineral melt partition coefficients for magnetite range from 0 003 for la to 0 02 for lu ilmenite partition coefficients range from 0 007 for la to 0 029 for lu these experimental values are two orders of magnitude lower than many of the published values determined by phenocryst matrix separation techniques

the pelican thrust zone basal detachment between the archean sask craton and paleoproterozoic flin flon glennie complex western trans hudson orogen the pelican thrust zone is a 3 7 km wide recrystallized mylonite zone along which paleoproterozoic arc volcano plutonic and derived sedimentary rocks of the flin flon glennie complex were thrust over an archean package jan lake complex consisting of ca 3 1 ga calc alkaline orthogneisses pelitic migmatites and a ca 2 45 ga tholeiitic charnockite norite intrusive suite a regional northeast plunging stretching lineation and a variety of kinematic indicators imply southwesterly transport matching that of other coeval shallower crustal level structures observed throughout the northern flin flon domain and southern flank of the kisseynew domain e g sturgeon weir and annabel lake shear zones subsequent east side up displacement on the tabbernor fault together with domal fold interference has exposed the pelican window and mantling mylonite zone where it has been seismically profiled along lithoprobe transects tectonic windows also expose archean rocks in the glennie domain to the west where similar southwest verging recrystallized mylonite zones have been documented together these zones record collision and underplating of the flin flon glennie complex by the archean sask craton during prolonged protocontinent continent collision over a minimum 1826 1805 ma interval resume la zone de chevauchement pelican est une zone de mylonite recristallisee d une largeur de 3 7 km le long de laquelle des roches volcano plutoniques d un arc datant du paleoproterozoique et des roches sedimentaires derivees du complexe de flin flon glennie ont ete chevauchees par dessus un ensemble archeen le complexe de jan lake qui est compose d orthogneiss calco alcalins et de migmatites pelitiques gees d environ 3 1 ga ainsi que d une suite intrusive charnockite norite tholeiitique gee d environ 2 45 ga une lineation regionale d etirement a pendage nord est et divers indicateurs cinematiques correspondent a un transport vers le sud ouest rejoignant celui d autres structures contemporaines de niveaux crustaux moins profonds observees a travers tout le domaine de flin flon nord et le flanc sud du domaine kisseynew p ex les zones de cisaillement de sturgeon weir et d annabel lake un deplacement subsequent le cote est vers le haut le long de la faille de tabbernor avec une interference de plis de dome a fait affleurer la fenetre pelican et la zone mylonitique qui la recouvre elle a ete determinee par des profils sismiques le long des transects lithoprobe des fenetres tectoniques permettent aussi l affleurement de roches archeennes du do maine de glennie vers l ouest la ou ont ete documentees des zones semblables de mylonites recristallisees a pendage sud ouest ensemble ces zones enregistrent la collision et le sous placage du complexe de flin flon glennie par le craton archeen sask au cours d une collision prolongee entre le protocontinent et le continent au cours d un intervalle d au moins 1826 a 1805 ma

early tertiary basalts from the labrador sea floor and davis strait region fine to medium grained phyric and aphyric basalt samples from odp leg 105 site 647a in the labrador sea show little evidence of alteration chemically these rocks are low potassium 0 01 0 09 wt k 2 o olivine to quartz normative tholeiites that compare closely with the very depleted terrestrial paleocene volcanic rocks in the davis strait region of baffin island and west greenland however differences exist in the sr nd isotope systematics of the two suites the labrador sea samples have e nd values 9 3 indicative of a more depleted source and are higher in 87 sr 86 sr 0 7040 relative to the davis strait basalts e nd 2 54 to 8 97 mean 87 sr 86 sr 0 7034 the higher 87 sr 86 sr in the labrador sea samples may reflect seawater exchange despite no petrographic evidence for significant alteration the labrador sea and early davis strait basalts may have been derived from a similar depleted mantle source composition however the later davis strait magmas were generated from a different mantle none of the baffin island west greenland or labrador sea samples show unequivocal geochemical evidence for contamination with continental crust

komatiites and picrites evidence that the plume source is depleted high temperature magmas such as picrites and komatiites are candidates for the material in the hot axial region of mantle plumes in current plume theories these magmas should have an enriched ocean island basalt oib type chemistry high mgo magmas however are often more like depleted mid ocean ridge basalts morb in trace element and isotopic chemistry this is a paradox for deep mantle plume or primitive mantle theories but is expected if hotspots and morb share a common depleted source and the transition to oib chemistry involves shallow level cooling crystal fractionation and contamination ancient komatiites appear to have come from the same deep depleted reservoir as modern picrites and morb

late triassic rifting and jurassic cretaceous passive margin development of the southern neotethys evidence from the adiyaman area se turkey evidence of rifting and continental break up to form the s neotethys is found within the volcanic sedimentary kocali complex this is a folded thrust imbricated succession that includes lavas volcaniclastic sediments pelagic carbonates radiolarites and manganiferous deposits interbedded ribbon cherts contain radiolarians of late triassic to late jurassic age the lower part of the succession of mid late triassic age tarasa formation is dominated by enriched mid ocean ridge basalt e morb the overlying late triassic to mid jurassic interval konak formation is characterised by intercalations of ocean island basalt and e morb taking account of structural position the basalts erupted within the outer part of a continent ocean transition zone continental break up probably occurred during the late triassic carnian norian early to mid jurassic lavas and volcaniclastic sediments record volcanism probably after continental break up in addition the karadut complex is a broken formation that is located at a relatively low structural position just above the arabian foreland pelagic carbonates redeposited carbonates and radiolarites predominate radiolarians are dated as early to mid jurassic and late cretaceous in age the pelagic carbonates include planktic foraminifera of late cretaceous age the karadut complex resulted from the accumulation of calcareous gravity flows pelagic carbonate and radiolarites in a relatively proximal base of slope setting after continental break up morb and ophiolitic rocks formed within the s neotethys further north tectonic emplacement onto the arabian platform took place by earliest maastrichtian time regional interpretation is facilitated by comparisons with examples of triassic rifting and continental break up in the eastern mediterranean region and elsewhere

kisseynew metasedimentary gneiss belt trans hudson orogen canada back arc origin and collisional inversion the kisseynew metasedimentary gneiss belt initially developed in a back arc setting the ages of detrital zircons and crosscutting plutons constrain turbidite sedimentation to 1 855 1 841 ga postdating early deformation and metamorphism associated with arc arc and arc continent collisions affecting much of the trans hudson orogen these turbidites are interpreted to have been deposited in a back arc basin behind a retreating subduction boundary the kisseynew domain and its southern flank are marked by 1 84 1 83 ga magmatic rocks that may be related to subduction of back arc basin oceanic crust collapse of the kisseynew basin by ductile fold and thrust deformation began during 1 84 1 83 ga magmatism and continued through peak metamorphism at ca 1 820 1 805 ga from authors

crystallization of titaniferous chromite magnesian ilmenite and armalcolite in tholeiitic suites in the karoo igneous province titaniferous chromite up to 8 wt tio2 and magnesian ilmenite up to 10 wt mgo coexist at the base of the differentiated tholeiitic mount ayliff intrusion in the karoo province of southern africa suggesting that the original magma was tio2 rich picritic lavas with 3 tio2 from the lebombo monocline of the karoo province also contain microphenocrysts of magnesian ilmenite up to 6 wt mgo and armalcolite up to 7 wt mgo these oxide mineral associations and compositions are atypical of tholeiitic magmas in which chromite usually has less than 1 wt tio2 ilmenite less than 3 wt mgo and armalcolite is rarely a primary mineral experiments have been conducted at one atmosphere pressure on a range of compositions to determine the effect of tio2 on the crystallization and composition of chromite ilmenite and armalcolite the results indicate that increasing the tio2 content of picritic magmas increases the tio2 content of the spinel mainly at the expense of al2o3 whereas cr2o3 is not affected spinel compositions in the mount ayliff intrusion with over 45 wt cr2o3 less than 10 wt al2o3 and 8 wt tio2 were duplicated in experiments on a picrite at temperatures of about 1 200 c at the ni nio buffer increasing fo2 from fayalite magnetite quartz to ni nio buffer is shown to increase the crystallization temperature of armalcolite and to decrease that of ilmenite the total feo content of the liquid has little influence on the crystallization temperature of these phases the tio2 content of the liquid when either ilmenite or armalcolite crystallizes varies inversely with sio2 content the mgo content of the liquid at which ilmenite or armalcolite crystallizes depends upon the tio2 content of the starting composition with naturally occurring and experimetally determined saturation being demonstrated for liquids with 5 wt mgo and 5 5 wt tio2 the partition coefficent for mgo between armalcolite or ilmenite and liquid is about 1 5 observed magnesian armalcolite and ilmenite compositions in picrite lavas both minerals and in the mount ayliff intrusion ilmenite only are consistent with crystallization from a tio2 rich magma with approximately 5 wt mgo the fe23 tio5 component of armalcolite in the picrite lavas matches those formed experimentally at temperatures of 1 150 1 110 c and fo2 of the ni nio to ni nio 1 log unit similarities also exist between the compositions of chromite ilmenite and armalcolite and liquid fraction ation trends of some hawaiian high tio2 lavas and the experimental studies presented here

a triassic sponge from vancouver island possible holdover from the cambrian tiny cap shaped fossils recovered from silicified residues in upper triassic rocks of vancouver island wrangellia are identified as a calcareous sponge nucha vancouverensis n sp nov the genus nucha pickett and jell was first reported from middle cambrian rocks of australia the triassic example constitutes the first occurrence outside the cambrian system and the continent of australia the absence of representatives in rocks between triassic and cambrian age suggests that it is a long range refugium taxon the occurrence of such a wide ranging holdover organism reinforces the role of refugia in preferential survivorship over long periods of geologic time other holdover taxa are known from volcanic displaced terranes of western american cordillera and their study may help clarify neglected aspects of survival and evolution

greenstones in the mino paleozoic mesozoic terrane of the east takayama area central japan evidence for magmatism evolution from normal ridge to plume volcanism abstract greenstones of early permian age occur extensively in the mino terrane of the east takayama area greenstones are mainly tholeiitic basalt and subordinately alkalic basalt tholeiitic basalt comprises a lava sequence about 1000 m thick and tectonically contacts alkalic basalt fusuline fossils in associated limestones show that alkalic basalt is slightly younger than tholeiitic basalt chrome spinel occurs locally in tholeiitic and alkalic basalts it is occasionally aluminous and similar to spinel in tholeiitic and alkalic basalts of oceanic ridge regions basalts in the lower portion of the tholeiite sequence are depleted in tio2 zr and nb and are characterized by extremely low nb y ratios the lower basalts appear to be normal mid oceanic ridge basalt n morb the middle upper basalts are enriched in tio2 zr and nb and show nb y ratios that are high relative to zr y they are similar to icelandic tholeiite basalt chemically transitional from n morb to icelandic basalt occurs in the l

magma solidification processes beneath kilauea volcano hawaii a quantitative textural and geochemical study of the 1969 1974 mauna ulu lavas sents a late magma component formed in the summit magma storage region our results are consistent with the hypothesis that the components of the mu magmas followed two different routes the deformed olivine bearing magma moved along the deep basal

chrome spinels in gabbro wehrlite intrusions of the pechenga area kola peninsula russia emphasis on alteration features abstract chrome spinels are common accessory minerals of the pechenga ultramafic rocks in nw russia varying in metamorphic grade from sub greenschist to amphibolite facies magmatic chrome spinels can be grouped into two main types spinel 1 which crystallized first is represented by al ti chromite it occurs mainly as inclusions in olivine and rather less commonly enclosed within interstitial minerals spinel 2 crystallized after spinel 1 and has composition varying from ti rich chromite to ti rich chrome magnetite spinel 2 occurs in the interstitial spaces between olivine crystals and enclosed within intercumulus silicates zoned crystals with spinel 1 in the core overgrown by spinel 2 are also common cr poor titanomagnetite occurring in pyroxenites and gabbro apparently represents further evolution of spinel 2 phases metamorphic chrome spinels are represented by ferritchromit and cr magnetite which are the dominant chrome spinel phases in the amphibolite facies rocks the continuous trend of the magmatic chrome spinels composition is indicative of their intensive post cumulus alteration by reaction with evolving intercumulus liquid this reaction led to depletion of the earlier crystallized chrome spinels in mg cr and al and their enrichment in ti and fe 3 oxidation exsolution of ilmenite from ti rich chrome spinels is another common process of post crystallization alterations serpentinization of olivine led to formation of magnetite which overgrew and cut chrome spinel grains low temperature hydrothermal processes in particular talc carbonate alterations could cause a local loss of ti and further redistribution of mg mn and zn prograde metamorphic reactions led to formation of ferritchromit and magnetite replacing primary chrome spinels another common process affecting ti rich chrome spinels is a prograde metamorphic recrystallization of exsolved ilmenite which changes from submicron lamellae through a regular grid of coarse ilmenite crystals and further to their irregular blebs and rims metamorphism of epidote amphibolite and amphibolite facies has led to the liberation of most of the elements metamorphically liberated cr was captured by antigorite

geochemistry of 1 9 ga morb and oib like basalts from the amisk collage flin flon belt canada evidence for an intra oceanic origin subaqueously erupted basalts that occur in kilometre scale allochthons within the 1 9 ga flin flon belt canada appear to have been generated at oceanic ridges and possibly oceanic plateaus remote from archean cratons the ocean floor basalts fall into two categories 1 n type resembling n morbs and mariana type back arc basin basalts depleted to flat ree patterns high zrnb variable thnb and initial nd 3 3 to 5 4 2 e type resembling transitional and plume morbs slightly enriched ree patterns lower zrnb initial nd 3 1 to 4 5 in the largest and best studied allochthon the elbow athapapuskow assemblage mixing between depleted n morb and enriched oib sources or melts coupled with variable addition of a subduction lile component can explain the chemical variations in the basalts zircon u pb dates of 1904 ma for a syn volcanic diabase sill and 1901 6 5 ma for a gabbro peridotite cumulate complex demonstrate that crystallization of the ocean floor basalts overlapped with in part eruption of the tectonically juxtaposed 1 90 1 88 ga arc volcanic rocks the elbow athapapuskow allochthon is interpreted as back arc basin crust that developed simultaneously with flin flon arc magmatism subaerially erupted basalts that chemically resemble tholeiitic oibs 8 14 mgo relative hree depletion initial nd 2 2 to 3 4 occur in tectonic contact with the elbow athapapuskow assemblage the oibs may have been generated by deeper garnet residue melting of enriched mantle tapped during extension in the elbow athapapuskow back arc basin and were possibly erupted onto a remnant arc deeper mantle melting is also indicated by the presence of the lree enriched oceanic plateau like basalts of the sandy bay assemblage the back arc 01b and plateau volcanic assemblages were jux taposed against ca 1 9 ga arc assemblages in a philippines like intraoceanic accretionary complex by 1 87 ga

proterozoic collisional tectonism in the trans hudson orogen saskatchewan isotopic and structural data from the juvenile reindeer zone of the trans hudson orogen northern saskatchewan indicate a pre 1 85 ga thermotectonic event possibly reflecting arc continent collision followed by a more extensive nappe forming ca 1 83 1 80 ga thermotectonism during terminal continent continent collision preliminary data from the adjacent ensialic cree lake zone suggest high grade reworking of archean crust by the pre 1 85 ga event in the rae province to the west high grade metamorphism and reworking of archean crust occurred about 2 0 ga and may be related to the formation of the coeval taltson magmatic zone

high magnesium primary magmas from haleakala volcano east maui hawaii petrography nickel and major element constraints abstract for any given volcanic field the compositions of primary melts provide important constraints on models of magmatic processes and volcanic eruptions in this paper based on petrography olivine and bulk rock compositions two tholeiitic picrites samples c122 and c123 from haleakala volcano east maui are evaluated as possible primary melts sample c122 bulk rock mgo 16 6 has a high apparent mg fe exchange coefficient kd between olivine phenocrysts and bulk rock 0 6 however major elements and ni mass balance calculations show that the olivines in c122 are in equilibrium with the residual melt matrix after closed system equilibrium fractionation of 25 wt olivine therefore the mg fe ratio ca content and ni content of c122 are consistent with the hypothesis that the bulk composition of c122 is close to a primary melt formed by partial melting of a mantle containing olivine with composition around fo89 to fo91 the uniform composition and small size mostly 0 2 0 3 mm of the olivine and the glass patches in the matrix suggest fast ascent and rapid cooling at shallow depth for c122 on the contrary sample c123 which has an apparent kd between the most mafic olivine megacrysts and the bulk rock close to the equilibrium value 0 27 the multiple planar subgrain boundaries in most of the olivine crystals indicate that it may not be a primary melt unless the deformed olivines are generated at magmatic condition as phenocrysts if the deformed subgrain boundary texture in olivine could indeed be generated at magmatic condition then the wide compositional range of olivine crystals in c123 fo74 to fo91 suggests multi stage crystallization over a wide range of cooling temperatures the compositions of the two picrites and a differentiated basalt which does not contain xenocrysts suggest that the haleakala tholeiites are derived from primary melts with at least 16 17 wt mgo lavas with such high mgo content are rare in haleakala and other hawaiian volcanoes therefore most hawaiian tholeiites must have undergone extensive fractionation histories

a petrologic perspective of kīlauea volcano s summit magma reservoir two hundred years of magmatic history are documented by the lavas and tephra sampled from k olauea s historical summit eruptions this paper presents detailed petrographic and geochemical data for a comprehensive suite of samples erupted within or near k olauea s summit caldera since the 17th century our results elucidate the range of magmatic processes that operate within the volcano s summit magma reservoir and document two compositional trends that span nearly the entire known range for the volcano prior to the 1924 summit crater collapse a trend of increasing incompatible element and cao and decreasing sio2 abundances at a constant mgo prevailed thereafter the trend reversed direction and has persisted for the rest of the 20th century including during the current puau a oa o eruption the rapid and systematic nature of these temporal geochemical variations indicates that the summit reservoir is a single relatively small body rather than a plexus of dikes and sills olivine fractionation is the dominant petrologic process within this reservoir petrographic observations and olivine and whole rock geochemical data suggest that the summit reservoir has a crown of aphyric more evolved low density magma differentiation within this crown involving clinopyroxene and plagioclase is more extensive than previously recognized in k olauea summit lavas the effects of crystal fractionation are superimposed upon an evolving hybrid magma composition produced by mixing new mantle derived magmas with more fractionated reservoir magma frequent eruptions of these hybrid reservoir magmas document the rapid variation in parental magma composition these compositional variations correlate with magma supply rate both are thought to be influenced by the degree of melting of small scale source heterogeneities within the hawaiian plume however k olauea s source compositions and partial melting processes have varied only within a narrow range over the past 350 kyr

petrology of gabbroic xenoliths in 1960 kilauea basalt crystalline remnants of prior 1955 magmatism the 1960 kapoho lavas of kilauea s east rift zone contain 1 10 cm xenoliths of olivine gabbro olivine gabbro norite and gabbro norite textures are poikilitic ol sp cpx in pl and intergranular cpx pl ol opx poikilitic xenoliths which we interpret as cumulates have the most primitive mineral compositions fo82 5 cpx mg 86 5 and an80 5 many granular xenoliths ol and noritic gabbro contain abundant vesicular glass that gives them intersertal hyaloophitic and overall open textures to suggest that they represent mush and crust of a magma crystallization environment their phase compositions are more evolved fo80 70 cpx mg 82 75 and an73 63 than those of the poikilitic xenoliths associated glass is basaltic but evolved mgo 5 wt tio2 3 7 5 8 wt the gabbroic xenolith mineral compositions fit existing fractional crystallization models that relate the origins of various kilauea lavas to one another feo mgo crystal liquid partitioning is consistent with the poikilitic ol gabbro assemblage forming as a crystallization product from kilauea summit magma with 8 wt mgo that was parental to evolved lavas on the east rift zone for example least squares calculations link summit magmas to early 1955 rift zone lavas 5 wt mgo through 28 34 crystallization of the ol sp cpx pl that comprise the poikilitic ol gabbros the other ol gabbro assemblages and the olivine gabbro norite assemblages crystallized from evolved liquids such as represented by the early 1955 and late 1955 lavas 6 5 wt mgo of the east rift zone the eruption of 1960 kapoho magmas then scoured the rift zone reservoir system to entrain portions of cumulate and solidification zones that had coated reservoir margins during crystallization of prior east rift zone magmas

volcanism around k t boundary time its role in an impact scenario for k t extinction events abstract a widespread debate on the cretaceous tertiary k t boundary 65 1 ma extinctions has largely revolved around an impact or a volcanic cause evidence for both events is strengthened by recent work impact triggered deccan volcanism or independent impact and mantle plume events have been suggested this review examines evidence for both events and suggests that they overlapped in time but with globally uneven effects the combined effects produced generally inhospitable k t environments leading to mass extinctions although some life forms flourished afterwards volcanic contributions to the k t boundary environment probably include increased platinum group element pge and acid rain fall out particularly from large scale mantle upwellings e g deccan cameroon and coral sea events a previously proposed post k t greenland starting plume apparently already existed in the cretaceous k t features such as the iridium ir anomaly most likely include longer term volcanic as well as sudden meteoritic impact contributions volcanic contributions were greatest from southern hemisphere sources whereas meteorite impacts featured in the northern hemisphere

the relationship between e and n type magmas in the baffin bay lavas the picritic lavas of baffin island represent one of the most primitive phanerozoic volcanic suites in the world with mgo contents ranging from 22 wt 29 mg cation unit mg 100 cations for olivine rich lavas to 11 wt 16 mg for olivine poor lavas two magma types can be recognized on the basis of trace element and isotopic geochemistry n type magma which dominates the high mgo lavas has depleted lree patterns la sm n 0 6 0 7 typical of n morb k ti 0 5 and 87sr 86sr ranging between 0 7032 0 7039 these two magma types are however virtually indistinguishable in terms of major clements and many other trace elements the e and n type samples are intermixed throughout the volcanic succession indicating that both types of magma erupted contemporaneously although the compositional spectrum observed for major and highly incompatible elements is consistent with olivine fractionation crystal fractionation cannot account for the difference in the lree between e type and n type lavas crustal contamination involving a lower crust composition cannot reproduce the more magnesian e type lavas and can only repoduce the lowest la sm n ratios of the e type lavas if high degrees of assimilation 50 have occurred partial melting models can reasonably account for the distinct la sm n ratios and the similar zr y values of the two magma types but fail to reproduce the observed abundances or ree sr y and zr compositionally different mantle sources are required to explain the two distinct magma types observed in baffin island a model in which the mantle source is a mixture of enriched plume material and depleted entrained mantle in the head of a mantle plume may explain the contemporaneous eruption of n and e type magmas in the baffin bay picritic suite the baffin island e type lavas are less enriched in sr y and zr contents and have lower zr y for similar la sm n than the other e type lavas of the northern north atlantic region

a mesosiderite rock from northern siberia russia not a meteorite a possible mesosiderite meteorite was found in the area of the putorana plateau noril sk district siberia russia although this rock resembles a mesosiderite in its hand sample aspect and in having ni bearing iron metal it is not a meteorite this inference is based on the lack of a fusion crust the lack of cosmogenic nuclides oxygen with terrestrial isotope ratios and several mineral chemical criteria most likely the rock is from the iron metal bearing basalts of the siberian trap basalt sequence which are mined for their base and platinum group metals mesosiderite imposters like this may be recognized by 1 the presence of cu metal in hand sample or as microscopic blebs in the low ni metal kamacite 2 the absence of high ni metal taenite and 3 the presence of iron carbide cohenite enclosing the kamacite even if these macroscopic tests are inconclusive isotopic and mineral chemical tests will also distinguish rocks like this from mesosiderites

picrites as parental magma of morb type tholeiites recent experimental work on peridotites and basalts1 3 and a reappraisal of ophiolite complexes4 supports the concept that basalts are derived from picritic parents5 7 the occurrence of a density minimum in the liquid line of descent8 10 explains why mid ocean ridge tholeiites fall in a restricted compositional range and parental picrites can only be found in the exceptional case when eruption is not yet controlled by a steady state magma chamber7 11 we present here a preliminary account of a natural example of such a case from the caribbean

the petrology of some picrites from mauna loa and kilauea volcanoes hawaii the mineralogy and chemistry of picrites from mauna loa and kilauea have been investigated to evaluate for hawaiian tholeiitic picrites the contrasting genetic models which have been proposed for these mg rich volcanics namely products of direct crystallization of high mg melts 20 25 mgo or the result of accumulation of olivine phenocrysts into less mg rich melts genetic interpretations rely heavily on mg fe partitioning relations between olivines and their picrite hosts although the 100 mg mg fe2 ratios m of picrites are wide ranging m 73 6 82 9 for fe2o3 feo 0 15 with mgo as high as 27 8 the average 100 mg mg fe ratios mg of the cores of olivine phenocrysts megacrysts show only restricted compositional variation mg 87 2 89 0 successive olivine generations are progressively more fe rich olivine liquid mg fe partitioning data and the mn and ni abundances in olivine phenocrysts collectively indicate that they were precipitated by mg rich basaltic melts with 12 14 mgo spinel compositions range from liquidus magnesiochromites occurring mainly as inclusions in olivine phenocrysts to groundmass titanomagnetites which crystallized at nearsolidus temperatures the cr2o3 contents and m values of liquidus magnesiochromites suggest that their parent melts were neither mg rich picritic mgo 20 nor relatively mg poor basaltic types