the age and origin of felsic intrusions of the thetford mines ophiolite quebec the thetford mines ophiolite in southern quebec was obducted in early ordovician time during the closing of the proto atlantic the tectonized peridotite lower unit of the ophiolite is intruded by felsic dikes and pods including isolated lenses of massive rodingite small bodies of strongly deformed diorite and younger less deformed quartz monzonite these intrusions are found only near the base of the ophiolite do not intrude the surrounding country rock and are rootless for these reasons they are considered to have been emplaced in the ophiolite before it reached its present location the younger group of intrusions consists of biotite muscovite quartz monzonite and leuco quartz monzonite analyzed samples have high k2o contents high k2o 100 na2o k2o ratios and high initial strontium ratios 0 7171 0 7179 indicating that the magma source region was continental and that these felsic rocks formed by partial melting of continental sediments whole rock and mineral isochron ages suggest that

helium and neon isotopes in the imnaha basalt columbia river basalt group evidence for a yellowstone plume source abstract helium neon and argon isotopic compositions were measured in two flows of the columbia river flood basalt the imnaha basalt has a 3 he 4 he ratio of 11 4 times atmospheric and 20 ne 22 ne and 21 ne 22 ne ratios characteristic of a plume component the measured 3 he 4 he is a lower limit due to possible preferential 3 he loss and or addition of radiogenic 4 he a wanapum basalt flow erupted approximately 2 ma later in the waning stages of volcanism has more morb like noble gases the he nd and sr isotopic compositions of these lavas suggest that the columbia river basalts were derived from the yellowstone plume head which contained both high helium plume material and entrained depleted mantle as the eruptions progressed the plume component in the melting region was gradually diluted or replaced

the primordial noble gases in the earth a key constraint on earth evolution models abstract we made an extensive compilation of noble gas data obtained from morb oceanic island basalts oib and diamonds on the basis of this data base we discuss the primordial noble gases in the earth and its implications on mantle evolution we conclude that in the contrary to a common assumption the primordial ne ar kr and xe in the earth are distinctly different from the solar noble gases the primordial noble gases were derived from the solar noble gas through rayleigh distillation like mass dependent fractionation the primordial ar kr xe are identical with the atmospheric components but ne observed in the mantle derived materials differs significantly from the atmospheric ne we suggest that ne was further fractionated in the atmosphere while the abundances of ar kr and xe in the mantle derived materials are well correlated to each other he and ne in morb is totally uncorrelated with the heavier noble gases this indicates that he and ne in morb is decoupled from the heavier noble gases in the mantle source he ne ar systematics indicate that he was open to the morb and oib mantle sources and ne in the morb mantle source while he is excess in the morb mantle source it is deficient in the oib mantle source relative to the closed system values however it is not known whether this apparent complementary characteristic corresponds to some fundamental mantle dynamics or accidental coincidence

origin of the columbia plateau and snake river plain deflection of the yellowstone plume previous models relating the snake river plain and columbia river basalt northwest united states to the yellowstone plume have not taken into account the effects of the subducted farallon and juan de fuca plates not only did subduction shield the surface from mantle plume activity but it deflected the plume in the direction of plate motion a variety of geophysical and geologic evidence suggests that the plume penetrated the subducted slab at 17 5 ma causing eruption of the columbia river basalt the plate boundary then migrated over the plume and the yellowstone plume has recovered to a more nearly vertical conduit forming the snake river plain and yellowstone plateau

ultramafic and mafic rocks from the garret transform fault near 13 30 s on the east pacific rise igneous petrology serpentinized peridotites metagabbros and fresh basaltic rocks were dredged from the garret transform fault near 13 s on the east pacific rise two dredge hauls taken on the northern wall from about 3100 3600 m consisted of aphyric and picritic basalts while peridotites gabbros and moderately phyric basalts were recovered in a single dredge located near the deepest part 4616 4820 m of the transform valley well preserved igneous textures and mineralogical assemblages enable us to differentiate between tectonites including harzburgites and clinopyroxene bearing harzburgites and cumulates consisting of plagioclase dunite troctolites olivine gabbros and ilmenite gabbros the harzburgites are likely to represent deep seated peridotites left after extraction of basaltic melt during upper mantle partial melting of a lherzolite the ultramafic cumulates underwent some deformation and show textural and mineralogical evidences which suggest formation at a minimum depth corresponding to the very lower crust it is also inferred that the composition of the coexisting liquid along with early cumulus crystals has a ratio mg mg fe2 of 0 7 high cao na2o ratios and a low ni content about 150 ppm when compared to similar rock types from atlantic fracture zones subsequent uplift during rifting of the oceanic lithosphere enhanced plastic deformation subsolidus recrystallization and retrogressive metamorphism of the gabbros and the ultramafics

hotspots mantle plumes flood basalts and true polar wander persistent long lived stationary sites of excessive mantle melting are called hotspots hotspots leave volcanic trails on lithospheric plates passing across them the global constellation of fixed hotspots thus forms a convenient frame of reference for plate motions through the orientations and age distributions of volcanic trails left by these melting anomalies hotspots appear to be maintained by whole mantle convection in the form of upward flow through narrow plumes evidence suggests that plumes are deflected little by horizontal flow of the upper mantle mantle plumes are largely thermal features and arise from a thermal boundary layer most likely the mantle layer just above the core mantle boundary experiments and theory show that gravitational instability drives flow beginning with the formation of diapirs such a diapir will grow as it rises fed by flow through the trailing conduit and entrainment of surrounding mantle the structure thus develops a large spherical plume head and a long narrow tail on arrival at the base of the lithosphere the plume head flattens and melts by decompression producing enormous quantities of magma which erupt in a short period these are flood basalt events that have occurred on continents and in ocean basins and that signal the beginning of major hotspot tracks the plume supported hotspot reference frame is fixed in the steady state convective flow of the mantle and is independent of the core generated axial dipole paleomagnetic reference frame comparison of plate motions measured in the two frames reveals small but systematic differences that indicate whole mantle motion relative to the earth s spin axis this is termed true polar wander and has amounted to some 12 since early tertiary time the direction and magnitude of true polar wander have varied sporadically through the mesozoic probably in response to major changes in plate motions particularly subduction zone location that change the planet s moments of inertia

komatiites an early precambrian phenomenon using the presently accepted definition komatiites are ultramafic volcanic rocks characterised by bladed and or skeletal olivine texture spinifex texture where textural evidence is absent the geochemical definition is equivocal and as a result there is considerable ambiguity particularly among the basaltic types if the added control of al2o3 tio2 25 is added to the definition this eliminates rocks of the boninite group and also the low ti basalts found in some ophiolite complexes the bulk of komatiites occur in the archaean and the most obvious explanation is that the eruption of high up to 30 mgo melts was controlled by an enhanced geothermal gradient there is some evidence to suggest that the high cao al2o3 ratio characteristic of some komatiites e g barberton is found only in those greenstone belts older than 3 0 b y

tectono magmatic evolution of the west philippine mariana region and the origin of boninites re examination of published data on the tectomagmatic evolution of the west philippine mariana region indicate that arc magmatism and back arc extensional pulses are not synchronous but are largely asynchronous arc volcanism ceases within a few million years of the development of a back arc basin and recommences oceanward on a new arc during the final stages in the development of the back arc basin following karig s model we believe that ascent beneath the arc axial chain of a line of diapirs of morb source mantle interferes with processes of arc magma generation and arc magmatism wanes partial melting of the diapirs produces voluminous morb type tholeiites which split the arc and form the crust of a widening back arc basin we show for both the west philippine mariana region and occurrences of boninite like lavas in ophiolites that boninites appear to be erupted after arc magmatism and immediately before eruption of morb type lavas passage of a diapir of hot morb source mantle through refractory sub arc peridotite which has been enriched in lile and h2o will result in limited partial melting of the hydrous peridotite and the restricted production of boninitic magmas these are more likely to erupt modified or only slightly modified by crystal fractionation through the thin forearc crust continued ascent of the morb source mantle diapir will result in it partially melting to yield morb tholeiites which erupt shortly after and in considerably more voluminous amounts than the boninites finally we suggest that forearcs regions are likely to be incorporated into foldbelts and that boninite or low ti lava bearing ophiolites may characterize such regions

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

mineral chemistry of ultramafic and mafic plutonic rocks of the appalachian ophiolites quebec canada abstract this paper presents the first detailed mineralogical record of ophiolitic plutonic rocks in the thetford mines asbestos area quebec the plutonic rocks form the lower 5 5 km section of the 8 km thick thetford mines ophiolite the ophiolitic cumulate sequence is in tectonic contact with underlying thick plastically deformed harzburgitic peridotite now largely serpentinized the mineralogy of this peridotite is a typical example of a very depleted ultramafic rock since the minerals are poor in hygromagmaphile elements and enriched in the high partition coefficient elements ni cr and mg the cumulates form a rock suite varying from the bottom to the top of the sequence as follows dunite chromitite wehrlite clinopyroxenite hornblende gabbro olivine from the very base of the sequence is highly magnesian fo91 94 and ni rich nio 0 30 0 41 wt chromite grains are characterized by high cr 0 60 0 87 and varying mg 0 30 0 62 related to host rock composition and higher fe3 than harzburgite spinels the tio2 content is usually very low calculations using mineral chemistry data suggest that the primitive liquid which was in equilibrium with early fractionated minerals was highly magnesian ni and cr rich and conversely tio2 and alkali poor this composition is comparable to that of the boninitic lavas of the lower volcanic group overlying the gabbroic rocks by taking into account subsolidus re equilibration it is possible to recalculate an average primitive magma composition the genesis of this liquid results from a 20 30 partial melting of a source depleted in hygromagmaphile elements it is concluded that both magmatic processes and source composition are responsible for the genesis of low tio2 magnesian liquids and that these conditions may not be restricted to a single geodynamic environment paleogeographic reconstructions of appalachian terranes in cambrian ordovician times the occurrence of boninitic lavas and low ti cumulate sequence constrain the formation of the thetford mines ophiolite in a marginal basin or the frontal arc region of an island arc system it is stressed that geodynamic significance of such liquids must be considered in relation to a geological context in order to ascertain their real implication

amphibolite associated with the thetford mines ophiolite complex at belmina ridge quebec amphibolite in the quebec appalachians at belmina ridge 45 58 n 71 28 w constitutes a steeply dipping allochthonous sheet as much as 800 m thick between the obducted thetford mines ophiolite complex to the east and rocks of the allochthonous caldwell group to the west the amphibolite which structurally and chemically is unlike amphibolite in the adjacent caldwell is fresh coarse to fine grained and massive to layered it is composed chiefly of edenitic to pargasitic amphibole pyrope and grossular rich almandine salite and epidote with small amounts of quartz plagioclase and accessory minerals metamorphism ranged from at the base of the sheet to t 780 c at the top near the tectonic contact with rocks of the superjacent ophiolite complex under a barrovian facies series with ptotal between 5 and 7 kbars 500 700 mpa the amphibolite is interpreted to have been derived from ocean floor basalt that was metamorphosed under a 20 km thick westward directed obducted allochthon of oceanic l

a deep low velocity body under the yellowstone caldera wyoming delineation using teleseismic p wave residuals and tectonic interpretation in previously published papers iyer 1975 iyer and stewart 1977 iyer 1979 evidence was presented for the presence of a large body of anomalous material with compressional velocities considerably lower than normal from the surrounding rock in the crust and upper mantle under yellowstone national park wyoming in this paper we give a detailed analysis of p residual data from the 26 element telemetered seismic array and three groups of portable stations operated by the u s geological survey in the yellowstone region using a variety of analytical techniques we delineate the shape of the low velocity body determine the velocity structure inside the body and interpret the nature of the material that constitutes the body we will also outline some tectonic implications of our model the spectacular surface geothermal phenomena for which yellowstone is famous result from deep seated volcanism the volcanic evolution of yellowstone is discussed by christiansen and blank 1972 and summarized by eaton and others 1975 briefly the geothermal manifestations in yellowstone are results of three cycles of quaternary volcanism each culminating in a devastating pyroclastic eruption and resulting in large collapse calderas and predominantly rhyolitic flows the oldest cycle ended about 1 9 m y ago forming a caldera from island park to perhaps the central part of the present yellowstone caldera the second cycle climaxed about 1 2 m y ago and was confined to the island park caldera the third cycle began soon after the second and is responsible for the 75 by 45 km yellowstone caldera as it is outlined today this caldera is the result of a catastrophic eruption approximately 600 000 yr ago the most recent episode of volcanism probably began about 150 000 yr ago in the western half of the caldera near old faithful geyser and the youngest flows in yellowstone about 70 000 yr old are probably associated with this episode

the petrochemistry of ophiolite gabbroic complexes a key for the classification of ophiolites into low ti and high ti types ophiolites have been divided into two groups high ti and low ti types these can be discriminated by studying the fractionation trends of both gabbroic complexes this work and lavas and dykes 16 particularly in the tio2 m i diagram the first type typically shows morb like magmas whereas in the second the magma types have a spectrum of composition from mid ocean ridge basalts to island arc tholeiites and boninite like magmas often occur high ti ophiolites are petrologically and geochemically similar to major oceanic and ensialic back arc basin crusts as well as oceanic crust generated during the intermediate and late stage opening of intraoceanic back arc basins parental magmas and fractionation processes of low ti ophiolites fit with an hypothesis of their formation in the early stage of opening of intraoceanic back arc basins

the age and origin of the garnet amphibolite underlying the thetford mines ophiolite quebec the thetford mines ophiolite in southern quebec is partially underlain by an 800 m thick amphibolite aureole rocks in the metamorphic unit grade inward towards the base of the ophiolite from chlorite muscovite schist epidote amphibolite and garnet amphibolite to clinopyroxene bearing garnet amphibolite closest to the fault contact with the basal tectonized peridotite unit of the ophiolite the polydeformed metamorphic unit pinches out north and south of belmina ridge and is discordant with the overlying tectonized peridotite analyses of nine samples show a range of compositions but the low k2o p2o5 rb zr and sr suggest that the amphibolite protolith was tholeiitic basalt probably midocean ridge tholeiite microprobe mineral compositions determined for coexisting clinopyroxene garnet amphibole and garnet amphibole suggest that amphibolites near the peridotite contact equilibrated at about 850 c and that the temperatures decreased to about 380 c 700 m from the contact geobarometers based on coex

enrichment of basalt and mixing of dacite in the rootzone of a large rhyolite chamber inclusions and pumices from the rattlesnake tuff oregon a variety of cognate basalt to basaltic andesite inclusions and dacite pumices occur in the 7 ma rattlesnake tuff of eastern oregon the tuff represents 280 km3 of high silica rhyolite magma zoned from highly differentiated rhyolite near the roof to less evolved rhyolite at deeper levels the mafic inclusions provide a window into the processes acting beneath a large silicic chamber quenched basaltic andesite inclusions are substantially enriched in incompatible trace elements compared to regional primitive high alumina olivine tholeiite haot lavas but continuous chemical and mineralogical trends indicate a genetic relationship between them basaltic andesite evolved from primitive basalt mainly through protracted crystal fractionation and multiple cycles 10 of mafic recharge which enriched incompatible elements while maintaining a mafic bulk composition the crystal fractionation history is partially preserved in the mineralogy of crystal rich inclusions olivine plagioclase clinopyroxene and the recharge history is supported by the presence of mafic inclusions containing olivines of fo80 small amounts of assimilation 2 of high silica rhyolite magma improves the calculated fit between observed and modeled enrichments in basaltic andesite and reduces the number of fractionation and recharge cycles needed the composition of dacite pumices is consistent with mixing of equal proportions of basaltic andesite and least evolved high silica rhyolite in support of the mixing model most dacite pumices have a bimodal mineral assemblage with crystals of rhyolitic and basaltic parentage equilibrium dacite phenocrysts are rare dacites are mainly the product of mingling of basaltic andesite and rhyolite before or during eruption and to a lesser extent of equilibration between the two the rattlesnake magma column illustrates the feedback between mafic and silicic magmas that drives differentiation in both low density rhyolite traps basalts and induces extensive fractionation and recharge that causes incompatible element enrichment relative to the primitive input the basaltic root zone in turn thermally maintains the rhyolitic magma chamber and promotes compositional zonation

mise en place et petrologie du granite associe au complexe ophiolitique de thetford mines quebec sheets of granite are confined to shear zones in the tectonite peridotite unit of the thetford mines ophiolite complex their orientation is parallel to the thrust plane at the base of the complex the granite which is calc alkaline and rich in potassium does not belong to the ophiolite sequence we show that it was tectonically incorporated within the partially serpentinized peridotite during thrusting and emplacement of the complex during this process the granite was strongly deformed and recrystallized at low pressure 500 bar 50 mpa and low temperature 500 c or less which gave rise to the mineral assemblage orthoclase albite hydromuscovite and hydrothermal biotite this episode was followed by a rodingite alteration characterized by the formation of grossular which was associated with the development of economic chrysotile within the enclosing peridotite the k ar ages of biotite muscovite and feldspar from the granite cluster around 450 ma these cooling ages suggest that the emplace

characteristic mineralogy of arc related cumulate gabbros implications for the tectonic setting of gabbroic plutons and for andesite genesis several mineralogically distinct suites of gabbroic cumulates occur as xenoliths in basalt and andesite from active arc volcanos and in plutons exposed in deeply eroded arcs calcic an85 100 plagioclase and moderately fe rich fo60 80 olivine commonly occur together in arc cumulate gabbros but not in cumulate gabbros from mid ocean ridges ocean islands or tholeiitic layered intrusions compositions of coexisting olivine and plagioclase in gabbro cumulates can establish the tectonic setting of ancient volcano plutonic complexes in deeply eroded arcs gabbroic cumulates typically occur in zoned plutons that also contain gabbros diorites and quartz diorites of basaltic to andesitic composition in many cases it is clear that the cumulate gabbros are chemically and physically related to the gabbro diorite basalt andesite fractionation trends seen in the zoned plutons zoned plutons in deeply eroded arcs provide physical evidence that crystal fractionation plays an important role in andesite petrogenesis

extension of the yellowstone plateau eastern snake river plain and owyhee plateau formation of the late cenozoic volcanic province comprising the owyhee plateau eastern snake river plain and yellowstone plateau has been accompanied by east northeast directed crustal extension a new vector of 45 mm yr n56 degree e for the migration of silicic volcanism across the volcanic province is calculated if migration of volcanism reflects west southwest continental drift over a mantle plume a zone of crustal extension must separate the volcanic province from the more slowly moving north american craton space time relations of basin fill in the adjacent basin and range province provide evidence for a zone of extension about 125 km wide coincident with and east of coeval silicic volcanism since 16 ma the zone of extension has migrated along with silicic volcanism maintaining its position between the province and the unextended craton

late paleozoic gabbroic rocks of the bridge river accretionary complex southwestern british columbia geology and geochemistry gabbroic bodies in the bralorne gold bridge area of southwestern british columbia are associated with the oceanic bridge river complex of the western canadian cordillera one of the suspect terranes accreted to north america in the jurassic the gabbros are locally cut by tonalites and are structurally interleaved with ultramafic rocks phyllites graphitic cherts and carbonate lenses that comprise the lower part of the bridge river complex their late carboniferous crystallization age overlaps the depositional age of affiliated supracrustal rocks mississippian jurassic some of which have been metamorphosed to blueschist facies compositionally the gabbros resemble mafic plutonic rocks of ophiolitic complexes and gabbroic rocks of the nearby shulaps range they display some affinity to oceanic island arc tholeiitic suites the bralorne and shulaps gabbros include cumulates and appear to have been derived from a single light ree depleted peridotitic source by melting and subsequent fractional crystallization accumulation of various combinations of plagioclase pyroxenes and olivine the tonalites are compositionally distinct from typical ophiolitic plagiogranites but might be related to the associated gabbros the gabbroic bodies occur within tectonic slivers derived from the oceanic crust that floored a deep ocean basin that existed during the late paleozoic and early mesozoic the bridge river complex comprises fragments of oceanic crust that were tectonically incorporated into an east verging accretionary prism during a middle late triassic to jurassic collisional event

geological and geochemical evidence for variable magmatism and tectonics in the southern canadian cordillera paleozoic to jurassic suites greenwood southern british columbia the paleozoic and early mesozoic rocks of the greenwood mining camp in southern british columbia are a part of the quesnel terrane in the eastern part of the intermontane belt of the canadian cordillera upper paleozoic rocks include the knob hill group composed of oceanic tholeiitic basalts with la yb n 0 4 1 2 associated with deep ocean sedimentary rocks and serpentinites the attwood group that comprises island arc tholeiites with la yb n 1 4 and positive end values clastic sedimentary rocks and limestones and a unit of oceanic gabbros with la yb n 0 5 these lithologically defined units occur as tectonically emplaced slivers of oceanic crust probably produced during the closure of the slide mountain basin during the permian they are unconformably overlain by middle triassic calc alkaline volcanic and sedimentary rocks of the brooklyn group the brooklyn group volcanic rocks have characteristics of mature island arc rocks including la yb n 2 5 4 5 and positive end values the paleozoic r

isotopic constraints on the origin of appalachian mafic complexes isotopic analyses of modern oceanic basalts and ophiolites have shown that both modern and ancient oceanic crust have a characteristic nd and sr isotopic signature indicative of derivation from a depleted mantle reservoir it also appears that the nd isotopic system is not appreciably disturbed by metamorphism these isotopic characteristics have been extended to the pt sal kings kaweah and josephine ophiolites of california we have used these characteristics in an attempt to identify pieces of proto atlantic oceanic crust among the mafic and ultramafic rocks of the appalachians sm nd mineral isochrons for the baltimore mafic complex maryland bmc yield an age of 490 20 my which we interpret as the igneous crystallization age bmc whole rock samples do not define isochrons and have initial isotopic compositions of 6 4 nd t 2 2 51 sr t ll5 nd t and sr t are anti correlated this is not the signature of depleted mantle and oceanic crust hut is similar to old continental crust we propose that the bmc is a mafic continental intrusion possibly subduction related which was contaminated with old continental crust during emplacement whole rock samples from the thetford mines complex qe tmc do not define isochrons and have 1 5 nd t 4 2 2 6 sr t 114 these data do not in any way reflect the signature of normal oceanic crust these results are in contrast with geologic relationships that show the tmc to have the characteristics of an ophiolite complex the tmc is chemically and isotopically similar to a class of other ophiolites that have affinities to modern honinites the chunky gal amphibolite north carolina lake chatuge complex north carolina and hazen s notch amphibolite vermont were found to have a depleted mantle signature with 5 nd t 8 and may he fragments of oceanic crust the webster addie body north carolina has nd t 1 sr t 30 and is not isotopically similar to oceanic crust or the other north carolina mafic bodies analyzed from these isotopic results we infer that appalachian mafic rocks have diverse origins some are continental intrusives bmc others are probably fragments of oceanic crust vermont and north carolina amphiholites future models for the development of the appalachians must allow for these various origins the possibility that some onhiolites are not normal oceanic crust but have an origin in a partially continental setting or as anomalous oceanic crust may require further attention

conglomerats polygeniques ophiolitiques anciens eboulis de talus de fond oceanique field observations on the polygenetic conglomerates associated with the ophiolitic complexes in the eastern townships are comparable to those on the common slope facies in the present day ocean bot

the snake river plain experiment revisited relationships between a farallon plate fragment and the transition zone we reconsider the analysis of the 93 passcal oregon srp experiment data the techniques applied to the processing of pds waves which are p to s conversions at seismic discontinuities at depth d in the receiver region differ from those employed by dueker sheehan 1997 we perform global move out corrections and migration taking advantage of the alignment of seismicity along the great circle described by the array south america and the kuril and aleutians islands we do not detect a deep signature of a mantle plume the 410 km discontinuity exhibits a fragmented aspect which prevents us from reliably estimating the transition zone thickness the 660 km discontinuity is clearly more visible and largely deflected in a large part of the profile we associate this anomalous behaviour and the noisy aspect of the discontinuities with the presence of a farallon plate fragment

the saint daniel melange evolution of an accretionary complex in the dunnage terrane of the quebec appalachians the saint daniel melange is part of a series of melanges located along the baie verte brompton line in the northern appalachians this line marks the suture between rocks of oceanic affinities and those of the ancient passive margin of north america with which they collided during the taconian middle to late ordovician orogeny the saint daniel melange contains a wide variety of lithologies including well bedded to dismembered sedimentary sequences pebbly mudstone olistostromes and slivers of igneous and metamorphic rocks black shales with interbeds of green shale calcareous siltstone or sandstone are the dominant units they exhibit various stages of melange formation such as those present in shallow parts of an accretionary complex units of oceanic origin include sediments derived from the forearc basin and slivers of an ophiolite and of a magmatic arc units derived from sediments of the passive margin of north america are also present the ratio between these various lithologies changes greatly within the melange on a kilometric scale along strike the saint daniel melange is a structural complex in which the various units were assigned a sequential order mimicking a stratigraphic order the saint daniel melange is interpreted as the relict of an accretionary complex because of its actual structural position within the northern appalachians and because all its lithologies and their structural fabric can be found in modern accretionary complexes

amphibolitized sheared gabbros from ophiolites as indicators of the evolution of the oceanic crust bay of islands newfoundland abstract in the bay of islands ophiolite complex both the basal part of the cumulate gabbro sequence and the underlying peridotites have been plastically deformed higher in the series the cumulate gabbros are undeformed except in localized shear zones slightly to strongly oblique to the layering the resulting shear bands are narrow and composed either of mylonitic amphibole bearing gabbros or of highly foliated amphibolites the amphibole composition and nature and the chemistry of coexisting minerals combined with structural evidence imply that a first episode of shearing subhorizontal in the crust occurred at decreasing temperatures ranging from 750 to 450 c this shearing probably represent the response of the lithosphere to stresses also responsible of asthenospheric flow in the upper mantle a second episode of shearing oblique to the first one and probably related to cooling of the crust away from the ridge occurred at lower temperatures ranging from 500 to 300 c the surrounding gabbros have undergone static hydrothermal metamorphism at temperatures decreasing from 650 to 450 c all metamorphic events occurred in hydrous conditions in the vicinity of the ridge axis

boninitic and low ti subduction related lavas from intraoceanic arc backarc systems and low ti ophiolites a reappraisal of their petrogenesis and original tectonic setting a comprehensive review of boninitic and other low ti subduction related lavas from modern intraoceanic arc backarc systems and of low ti ophiolites shows a complete compositional range from boninite to island arc basalts transitional low ti types distinguishable from the main magma types are well represented and defined here as 1 transitional boninites mainly from low ti ophiolites and 2 low ti island arc basalts major compatible and incompatible elements are clearly decoupled in all these low ti subduction related magmas in general their petrogenesis reflects a high degree of hydrous but not necessarily water saturated melting at pressures lower than 15 20 kbar of severely depleted mantle sources enriched in lile and h2o which are residual after the earlier extraction of basalt s as the residues after the extraction of low ti subduction related magmas very likely have a dunitic clinopyroxenefree harzburgitic composition it is proposed that the cao al2o3 ratios of the primary melts reflect those of their mantle sources particularly for the boninites and transitional boninites in this model the source of boninites sensu stricto would show a variable degree of depletion from clinopyroxene bearing to clinopyroxene free harzburgite whereas those of transitional boninites low ti island arc basalts would have clinopyroxene bearing harzburgite clinopyroxene poor lherzolite compositions the metasomatizing components involved in the genesis of low ti island arc basalts have a very close affinity with those involved with the sources of normal island arc tholeiites the common eruptive setting indicates that these low ti lavas represent extreme magma types of an otherwise normal island arc tectonomagmatic regime the petrogenesis of tertiary boninites and transitional boninites and the comparable magma types of the low ti ophiolites require at least these components 1 a variably depleted mantle source probably a subarc mantle residual after island arc basalt extraction 2 a hydrous fluid probably derived from dehydration of the subducted altered oceanic crust which carries most of sr ba k and rb in the subarc source and 3 a lree zr and nb enriched component with a low 143nd 144nd ratio probably derived from an ocean island type source it is proposed that this third component also carried significant amounts of rb and k which totally obliterated the ba spike typical of island arc tholeiites iats and produced the distinctive rb spike typical of the boninite transitional boninite magmas this component is hypothetically identified in liquids produced by partial melting of small scale large magnitude k richerite phlogopite bearing heterogeneities within the hot rising asthenospheric diapirs required in the model of crawford et al 1981 for the genesis boninite transitional boninite magmas geochemical thermal and geological constraints are best explained in a model which restricts the genesis of boninite transitional boninite magmas to the early stages of intraoceanic interarc spreading and to important rifting phases of the arc and or early interarc basin structures connected with hot rising asthenospheric mantle diapirs

coupled variations in helium isotopes and fluid chemistry shoshone geyser basin yellowstone national park abstract early studies of 3he 4he variations in geothermal systems have generally attributed these fluctuations to either differences in the source of the magmatic 3he rich helium or to local differences in the deep flux of magmatic 3he rich helium kennedy et al 1987 however show that near surface processes such as boiling and dilution may also drastically affect 3 he 4 he ratios of geothermal vapors helium isotope ratios were determined for several hot springs at shoshone geyser basin of yellowstone national park for this study along with other noble gas data stable isotope data and water and gas chemistry data for each spring were also compiled the water chemistry indicates that there is one deep hot thermal water in the area which is mixing with dilute meteoric water that has entered the system at depth spring hco3 concentrations correlate with 3 he 4 he values as in nearby lower geyser basin this correlation is attributed to variable amounts of deep dilution of thermal waters with a relatively cool water that inhibits boiling at depth thus preventing the loss of co2 and therefore hco3 and magmatic he in the most diluted samples oxygen and hydrogen isotope data also support a boiling and dilution model but to produce the observed fractionations the boiling event would have to be extensive with steam loss at the surface whereas the boiling that affected the helium isotope ratios was probably a small scale event with steam loss at depth it is possible that deep boiling occurred in the basin and that small amounts of steam escaped along fractures at about 500 m below the surface while all subsequently produced steam was lost near or at the surface

mantle contributions to crustal thickening during continental collision evidence from cenozoic igneous rocks in southern tibet the tibetan crust is twice as thick as average continental crust crustal compression and shortening as a result of indian asian collision isoftenconsideredtobetheprimarycauseforthecrustalthickening inthispaper weshowthatmagmaticcontributionisalsoimportant we come to this conclusion by documenting the paleogene linzizong volcanic succession lvs its coeval granitoid batholiths and the miocene adakitic rocks along the gangdese magmatic belt in southern tibet it has been widely accepted that the indian asian collision proceeded from a soft phase at 65 70 ma to a hard phase at 45 40 ma followed by continued post collisional convergence to the present in response to the collision and post collision convergence are a series of tectono magmatic events recorded in the gangdese magmaticbelt theseinclude 1 thesyn collisionallvsvolcanism 65 40ma andtheemplacementofsoutherngangdesebatholiths a peakageof 50ma 2 aperiod 40mato25ma thatismagmaticallyquiescent yettectonicallydominatedbyactivecompressionand crustal shortening and 3 the emplacements of post collisional adakitic rocks 25 12 ma potassic ultrapotassic volcanics 25 10 ma and peraluminous muscovite bearing granites between 24 and 18 ma these three major events contribute in different ways to the crustal thickness phase i formation of the lower juvenile crust from 65 ma to 50 ma with crustal thickening largely concentrated at 50 40maviainputofmantle derivedmagmas phaseii crustalthickeningbytectonicshorteningat 40 25ma andphaseiii retaining crustal thickness but thinning of the lithospheric mantle since 25 ma in response to crustal extension and upwelling and lateral flow of asthenosphericmantle weemphasizethatcollision inducedcrustalthickeningtookplacemainlyintheperiodof 50 40maand 25ma i e the period between the late stage of the lvs volcanism and the beginning of the adakitic rock emplacement most of the lvs rocks and the collision related granitoids in southern gangdese have end 0 attesting to the significance of mantle input most likely through remelting of mantle derived basaltic rocks including the subducted neo tethyan ocean crust the petrologic and geochemical characteristics of the miocene potassic adakitic rocks support the idea that the lower portion of the thickened tibetan crust is mafic and is genetically associated with the earlier lvs magmatism we estimate that the mantle material input contributed about 30 of the total thickness of the present day tibetan crust by assuming a pre collision crustal thickness of 35 km then the tectonic contribution would be about 20 km

stratigraphy and geochemistry of the lac brompton ophiolite canada evidence for extensive forearc magmatism and mantle exhumation in the southern quebec ophiolite belt three ophiolites are currently recognized in the southern quebec appalachians from south to north the mont orford ophiolite dominated by tholeiitic to transitional alkaline basalt and diabase a

upper mantle origin of the yellowstone hotspot fundamental features of the geology and tectonic setting of the northeast propagating yellowstone hotspot are not explained by a simple deep mantle plume hypothesis and within that framework must be attributed to coincidence or be explained by auxiliary hypotheses these features include the persistence of basaltic magmatism along the hotspot track the origin of the hotspot during a regional middle miocene tectonic reorganization a similar and coeval zone of northwestward magmatic propagation the occurrence of both zones of magmatic propagation along a first order tectonic boundary and control of the hotspot track by preexisting structures seismic imaging provides no evidence for and several contraindications of a vertically extensive plume like structure beneath yellowstone or a broad trailing plume head beneath the eastern snake river plain the high helium isotope ratios observed at yellowstone and other hotspots are commonly assumed to arise from the lower mantle but upper mantle processes can explain the observations the available evidence thus renders an upper mantle origin for the yellowstone system the preferred model there is no evidence that the system extends deeper than 200 km and some evidence that it does not a model whereby the yellowstone system reflects feedback between upper mantle convection and regional lithospheric tectonics is able to explain the observations better than a deep mantle plume hypothesis

the volcanic and intrusive rocks of the quebec appalachian ophiolites canada and their island arc setting abstract the early to middle ordovician quebec appalachian ophiolites contain three geochemically distinct groups of extrusive and intrusive rocks the predominant group consists of ni and cr rich olivine clinopyroxene phyric lavas and dikes grading in composition from basalt and andesite to rhyodacite these rocks are geochemically similar to arc related boinites they are closely associated at the base of the extrusive sequence with aphyric lavas and dikes which have geochemical affinities with volcanic arc basalts the third group is restricted to a single locality mount orford at the southwestern end of the ophiolite belt it consists of alkaline diabases occurring in interdigitation with arc basaltic diabases to form a 1000 m thick sheeted sill complex the thetford mines ophiolite is interpreted to be typical of ocean crust created by fore arc rifting in a subduction zone environment the mount orford ophiolite with its alkaline rocks is considered to be remnant of the back arc section of the same island arc system

structural evolution of the thetford mines ophiolite complex canada implications for the southern quebec ophiolitic belt 1 the thetford mines ophiolite complex tmoc preserves a complete ophiolitic sequence and occupies the hanging wall of a major se dipping normal fault the saint joseph fault preobduction synobduction and postobduction structures can be recognized in the tmoc ns trending preobduction paleonormal faults are parallel to ultramafic minor intrusions and to sheeted dykes recording extension related to seafloor spreading in a pericontinental suprasubduction zone basin wnw trending synobduction synmetamorphic fabrics are found toward the base of the tmoc and in the underlying continental margin rocks but are absent in the upper part of the tmoc and overlying sedimentary rocks these ordovician taconian structures record the development of a dynamothermal aureole immediately below the mantle margin contact and emplacement of the young ophiolite onto the continental margin postobduction structures include late silurian early devonian se verging backthrusts and back folds that inverted the tmoc and middle devonian acadian nw verging folds and reverse faults the tectonic history established for the tmoc is consistent with that of the adjacent laurentian margin and can be applied to the southern quebec ophiolitic belt as a whole the structural synthesis of the ophiolitic belt complemented with new observations and our compilation of stratigraphical geochemical geochronological and petrological data suggests that the southern quebec ophiolites may represent the remnants of the obduction of a single large slab of suprasubduction oceanic lithosphere extending for over a 100 km of strike length