the emplacement of geochemically distinct groups of rhyolites during the evolution of the lower rhyolitic tuff formation caldera ordovician north wales u k rhyolites in the vicinity of snowdon north wales are intimately associated with the evolution of the lower rhyolitic tuff formation lrtf caldera of ordovician caradoc age they occur as deep seated dykes sills and small stocks shallow level intrusive domes and domes extruded within a predominantly shallow marine environment extrusion occurred during three main phases indicating the episodic availability of rhyolite magma the rhyolites can be divided on their trace element ratios e g nb zr into five main groups extrusive representatives indicate that each group correlates strongly with a single phase of rhyolite extrusion within each group the distribution and variation of intrusive form with stratigraphic level suggests that geochemically similar rocks were emplaced at approximately the same time consequently the groups represent discrete magma compositions tapped from the evolving snowdon subvolcanic magma system differences in distribution of the groups reflect changes in structural controls of emplacement before and after development of the lrtf caldera

copper lead zinc and cobalt mineralization in the english lake district classification conditions of formation and genesis the copper lead zinc and minor cobalt mineralization of the english lake district has been classified on the basis of mineralogy spatial distribution and geological setting of the veins and age of mineralization the two major types of mineralization are 1 chalcopyrite pyrite arsenopyrite of lower devonian caledonian age and 2 galena sphalerite of early carboniferous age when the former is closely associated with granite intrusions more complex mineral assemblages are found involving tungsten or molybdenum bearing phases also distinguished from these two major types are later assemblages which probably result from their alteration a later upper carboniferous permian baryte mineralization is also present minor graphite and stibnite deposits and the major haematite mineralization found in the area are incorporated into the classification but are not discussed since they have been dealt with by other authors using data from fluid inclusions and experimental data on particular mineral assemblages occurring in the major types of mineralization estimates have been made of the temperatures and compositions of the ore forming fluids the lower devonian veins formed at temperatures in the range of 350 200 c with typical a s 2 values of 10 10 l0 16 atm and a o 2 of 10 25 10 45 atm or even less the solutions were probably brines of low to moderate salinity 5 10 equiv wt nacl the lower carboniferous veins formed at temperatures in the region of 110 130 c from highly saline brines 23 equiv wt nacl theories regarding the origins of the mineralization are also discussed the distribution of lower devonian veins shows a clear relationship to the underlying composite granite batholith the lower carboniferous galena sphalerite veins show no such relationship but tensional tectonic activity in the early carboniferous may have provided a mechanism for the opening of fissures the source of the mineralizing fluids forming the lower devonian veins may have been the borrowdale volcanic group with contributions from fluids associated with the granites which are also likely to have provided a source of heat the high salinities of the fluids producing the lower carboniferous veins suggest that seawater may have been involved in leaching the metals from basement rocks

evidence for a widespread mafic cover sequence and its implications for continental growth in the northeastern superior province abstract archean greenstone belts of the northeastern superior province nesp were emplaced over a 160 ma period 2 88 2 72 ga spanning a major episode of crustal reworking in which early tonalite trondhjemite plutonism evolved to dominant granite granodiorite and pyroxene bearing felsic plutonism the numerous greenstone belts that crop up across the craton contain lavas belonging to three mafic volcanic suites 1 mg tholeiites that have chemical compositions typical of many archean basalts with 4 10 wt mgo 9 15 wt fe2o3t 0 4 1 2 wt tio2 and la sm ratios between 1 and 3 2 fe tholeiites that have similar mgo contents and la sm ratios but markedly higher fe2o3t 11 20 wt tio2 1 0 2 6 wt and gd yb ratios that may reflect derivation from a distinct garnet bearing mantle source 3 light rare earth element lree enriched rocks that have the chemical characteristics typical of calc alkaline mafic magmas with higher sio2 al2o3 and la sm ratios but lower fe2o3t and tio2 than the tholeiitic suites at a given mgo content the presence of unconformities within single greenstone belts numerous inheritance ages in volcanic rocks replicating those of older volcanic plutonic assemblages and the recognition of widespread geographically separate yet coeval 2 78 ga extrusive assemblages containing the distinctive fe tholeiites do not support a plate tectonic model for a large portion of the nesp the correlations in composition and ages of greenstone belts are more consistent with a model in which they represent the remnants of an extensive autochthonous mafic cover sequence nd isotopic evidence for the presence of evolved crust in some syn volcanic felsic plutons and volcaniclastic rocks imply that this mafic cover sequence was erupted onto an older granitoid crust the amalgamation of two isotopically distinct terranes at ca 2 76 2 74 ga created a proto cratonic mass that consisted of a composite tonalite trondhjemite greenstone crust underplating by mafic mantle melts and widespread insulation after 2 75 ga increased temperatures sufficiently to produce extensive melting of this crust to generate pyroxene bearing felsic rocks and granodiorite to granite partial melts the isotopically enriched character of mantle derived magmas after 2 75 ga may reflect more extensive contamination by a felsic crust affected by widespread partial melting furthermore a decrease of nb th ratios in the lree enriched mafic rocks younger than 2 75 ga reflects a change in the nature of the crustal contaminant from tonalite trondhjemite prior to 2 75 ga to granite granodiorite afterwards

late archean quetico accretionary complex superior province canada the 1200 km long quetico subprovince consists of monotonous metagraywacke with derived migmatite and granite in thrust and or transcurrent fault contact with the adjacent wabigoon and wawa metavolcanic subprovinces imbrication of sedimentary wedges derived from volcanic arcs to the north and south produced a 10 100 km wide stratigraphically north facing pile thermal relaxation in the 20 m y following accretion of the southern arc resulted in melting of the accretionary pile to produce peraluminous granite and associated low pressure metamorphism

new caledonia obducted peridotite nappe offshore extent and implications for obduction and postobduction processes one of the largest ophiolitic peridotite masses in the world covers a quarter of the island of grande terre new caledonia the peridotite nappe was obducted during the eocene is weakly deformed and corresponds to the highest of a structurally simple pile of thrust nappes we present new marine seismic data that allows us to track the offshore continuation of the peridotite nappe along strike for a distance of more than 500 km south of new caledonia and to image its pre syn and post obduction sedimentary records offshore the peridotite nappe underlies a c 150 km wide and 2 km deep basin flat topped horsts of peridotite are clearly bounded by major normal faults in contrast faults are obscure onland to the east the peridotite nappe roots along the eastern margin of the felicite ridge new name a c 300 x 25 km dome shaped ridge which we interpret as being the southern extension of the hp lt metamorphic core complex observed in new caledonia two alternative tectonic models address the relative timing and relationships between peridotite nappe emplacement uplift of a metamorphic core complex and extensional tectonics these models provide new ideas for the understanding the formation of the eastern margin of the zealandia continent our results contribute to an understanding of how oceanic mantle is emplaced onto continental margins

distribution of rb ga and cs in agricultural land soils at european continental scale gemas implications for weathering conditions and provenance abstract agricultural soil ap horizon 0 20 cm samples were collected from a large part of europe 33 countries 5 6 million km2 as part of the gemas geochemical mapping of agricultural and grazing land soil soil mapping project the soil data have been used to provide a general view of element mobility and source rocks at the continental scale either by reference to average crustal abundances or to normalised patterns of element mobility during weathering processes the survey area includes a diverse group of soil parent materials with varying geological history a wide range of climate zones and landscapes total concentrations of rb ga and cs in european soil were determined by xrf x ray fluorescence spectrometry in addition hot aqua regia available element concentrations were analysed by inductively coupled plasma mass spectrometry icp ms their spatial distribution patterns are shown in geochemical maps the spatial distribution of rb ga and cs in ap soil is quite comparable for the aqua regia and xrf results the cs spatial pattern reflects best the large difference between trace element concentrations in northern europe with predominantly low concentrations and southern europe with approximately two times higher values the maximum extent of the last glaciation is visible as a clear concentration break on the map lithology of the underlying bedrock is inferred as the major source of geochemical anomalies in search for characterising the soil parent material and degree of weathering the soil data were examined for ga and cs using two methods first the estimated degree of extractability dei of an element in a sample was calculated by dividing aqua regia available concentrations by the total concentrations obtained by xrf analysis for al na ca k rb ga and cs the different dei were investigated for 10 geological parent material subgroups alkaline rocks granite calcareous rocks basalt mafic rocks unclassified lithologies precambrian granitic gneiss bedrocks loess organic soils schist and soil developed on coarse grained sandy deposits the role of clay as main carrier of ga and cs phases was inferred for soils developed on several types of bedrock such as granite gneiss and alkaline rocks additionally an affinity of cs for loess and carbonate rocks has been observed

kibaran a type granitoids and mafic rocks generated by two mantle sources in a late orogenic setting burundi in the mesoproterozoic northeastern kibaran belt of burundi central africa two distinct late kibaran magmatic suites coexist both including a type granitoids they are located along the boundary zone between the kibaran mobile belt western internal domain and the archaean tanzanian craton overlain by mesoproterozoic foreland deposits eastern external domain intense deformation high temperature metamorphism and intrusion of abundant peraluminous anatectic crustal granites occur only in the former domain whereas the mesoproterozoic sedimentary cover of the latter is much less or even nearly undeformed nor metamorphosed the first late kibaran magmatism 350 km long kabanga musongati with an emplacement age of 1275 ma u pb on zircon is mainly composed of mafic and ultramafic layered rocks with subordinate a type acidic differentiates moderately enriched in incompatible elements initial isotopic ratios srlr 0 708 nd 8 indicate an old continental lithospheric mantle origin the emplacement of these late kibaran magmatic rocks was controlled by late lateral shear possibly contemporaneous with the latest intrusions of the kibaran peraluminous synkinematic granites of the western internal domain 1330 1260 ma the second late kibaran magmatism 40 km long gitega makebuko and bukirasazi alignment with an emplacement age of 1249 s ma u pb on zircon is limited in volume it is mainly granitic in composition a type can be strongly enriched in incompatible elements and comprises both syenites and mafic rocks initial isotopic ratios srlr 0 702 na 4 5 to 1 4 point to an oib type asthenospheric lower continental lithospheric mantle origin with only slight contamination by the lower crust during differentiation this group was also intruded during the late lateral shear in both groups liquid lines of descent can be reconstructed although some of the rocks have been strongly albitized this indicates that the granites are produced by differentiation of less evolved magmas and not by crustal anatexis upwelling of the asthenosphere along the tanzanian craton can generate by adiabatic pressure release the oibtype basic melts and provide the heat necessary to melt the continental lithospheric mantle sources this mechanism assigns a major role to a lithosphere scale late kibaran shear event occurring at the end of the regional compressive deformation between two rheologically contrasted domains ascent of the asthenosphere continental lithospheric mantle delamination and late orogenic extensional collapse of the western internal domain are suggested as a possible geodynamic model for the entire northeastern kibaran belt additional work is however necessary to test this model finally our results indicate that in the northeastern kibaran belt the kibaran orogeny ended at 1250 ma despite various reactivation events occurring later e g at 1137 ma

nd and sr isotopic study of crustal and mantle inclusions from the sierra nevada and implications for batholith petrogenesis sm nd and rb sr systematics have been examined for eight upper mantle and lower crustal xenoliths the xenolith bearing trachyandesite and the granodiorite intruded by the trachyandesite in the sierra nevada batholith isotopic heterogeneity is shown by the xenoliths with 143 nd 144 nd ranging from 0 51169 to 0 51306 and 87 sr 86 sr ranging from 0 7031 to 0 7333 these ranges are similar to those of the plutonic rocks of the batholith this similarity suggests that the xenoliths believed to be representative of mantle and crustal reservoirs may approximate the diverse source regions from which the varied granitic rocks of the batholith were derived and that the sr nd isotopic correlation seen in granitic rocks may not be due to simple mixing of two end member components the subgranitic crust in this part of the sierra nevada apparently consists of metamorphosed sedimentary and igneous rocks which are highly variable isotopically the mantle beneath the sierra nevada is also isotopically heterogeneous both vertically and laterally

crustal evolution of the western minto block northern superior province canada abstract geochemical and nd isotope data are presented for suites of meso to late archean 2 7 2 9 ga granitoid rocks from the western minto block of the northeast canadian superior province the samples provide coverage of the inukjuak and tikkerutuk domains and include biotite amphibole and or orthopyroxene bearing tonalites granodiorites and granites from tonalite trondhjemite granodiorite ttg and monzodiorite suites both suites are characterized by strong light rare earth element ree and large ion lithophile lil element enrichment variably negative to positive eu anomalies and moderate heavy ree depletion high la yb and sr y ratios with correspondingly low yb and y concentrations in both suites likely reflect parental melts that left residual garnet correlations between eu anomalies and fractionation indices suggest that compositional variations in both suites reflect plagioclase accumulation in tonalite samples and plagioclase and fe ti oxide fractionation among granitic samples the nd isotope compositions range from nd values of 2 to 10 the largest range found among granitoids of the minto block and indicate the presence of a crustal contaminant of ca 3 8 ga in age there is a decrease in nd values from east to west across the studied region and a measurable decrease across tikkerutuk and inukjuak domain boundary this suggests increasing addition of an older crustal component in the inukjuak domain possibly as a result of recycling of early archean crust from the nearby ca 3 8 ga nuvvuagittuq sequence

archaean cratonization and deformation in the northern superior province canada an evaluation of plate tectonic versus vertical tectonic models abstract the archaean minto block northeastern superior province is dominated by tonalite trondhjemite enderbite pyroxene tonalite granodiorite and granite with subordinate mafic rocks and supracrustal belts the plutons have been interpreted as the batholithic roots of andean type plate margins and intra oceanic arcs existing horizontal tectonic models propose that penetrative recrystallization and transposition of older fabrics during terrane assembly at 2 77 and 2 69 ga produced a n nw tectonic grain in the douglas harbour domain northeastern minto block tonalite and trondhjemite dominate the faribault thury complex 2 87 2 73 ga and enderbite constitutes 50 100 km scale ovoid massifs troie and qimussinguat complexes 2 74 2 73 ga magmatic muscovite and epidote in tonalite trondhjemite have corroded edges against quartz plagioclase suggesting resorption during ascent of crystal charged magma foliation maps and air photo interpretation show the common development of 2 10 km scale ovoid structures throughout the douglas harbour domain outcrop and thin section scale structures imply that many plutons experienced a phase of syn magmatic deformation typically followed by high temperature sub magmatic overprints thermobarometric data for plutons indicate near solidus recrystallization at 4 6 kbar pressures the common preservation of syn magmatic fabrics in plutons of different ages seems incompatible with the origin of these fabrics through superimposed regional orogenesis the broad uniformity of intrusion ages and lithologies throughout the minto block and the rarity of shallowly dipping planar fabrics also seem inconsistent with accretion of disparate older terranes each of which should preserve distinct histories a possible alternative explanation for these features is provided by vertical tectonic models whereby buoyant felsic magmas ascended as crystal slurries while dense supracrustal rocks and solidified felsic intrusions emplaced into them subsided as cold fingers 10 20 km scale instabilities shear between upwelling and downwelling limbs would have concentrated in the weak intrusions generating steeply plunging syn magmatic fabrics and producing ductile overprints in solidified rocks

isotopic and geochemical evidence for crust mantle interaction during late archaean crustal growth abstract the charnockites and enderbites of the north marginal zone of the limpopo belt in southern zimbabwe represent a suite of dry magmas of tonalite trondhjemite granodiorite ttg affinity which crystallised directly in the lower crust they were emplaced between 2 6 and 2 7 ga and were derived from a source which showed remarkable lead isotope homogeneity unlike many other lower crustal rocks they have low k rb and low k u ratios and have not experienced depletion of their heat producing elements th u k and rb and thus represent an unusual suite of granulites in this study we have combined the constraints of trace element and isotope geochemistry to characterise the nature of the protolith to the nmz enderbites and charnockites we use a combination of high values calculated lead and strontium isotope initial ratios and old nd model ages to show that the enderbites and chamockites were derived from a protolith which was in part formed more than 500 ma before magma genesis we argue from the very high values that this protolith constituted two separate components an old high low sm nd crustal component and a younger low high sm nd mantle derived component both of which were homogenised by extensive mixing prior to 2 7 ga the presence of an older crustal component contrasts with other archaean ttg suites worldwide which show little evidence for a crustal prehistory the process of mantle crust interaction during late archaean crustal growth in southern zimbabwe invites comparison with modern convergent margins such as the andes

the mesozoic marginal basin of central peru the west peruvian trough1 2 extends some 1 300 km from north of trujillo to about 150 km short of arequipa fig 1 it received upwards of 9 000 m of material during the mesozoic and within the cretaceous rocks there is a well defined west to east facies change whereby thick volcaniclastic turbidites and pillow lavas the eugeosyncline of cobbing3 thin eastward to be replaced by a mixed clastic shelf sequence the miogeosyncline we report here chemical analyses of the volcanic rocks and data on the association of cherts volcaniclastic turbidites dyke and sill complexes and large gabbro masses as well as geophysical evidence which suggests that the eugeosyncline is an extensional marginal basin similar to that seen in rocks of the same age in southern chile4 the coastal batholith is thought to have been emplaced within this marginal basin during the upper cretaceous5 and the evolution of this ensialic new crust therefore has important implications for the growth of the continental margin of south america as well as for batholith genesis

elemental evidence for crustal contamination of mantle derived caledonian syenite by metasediment anatexis and magma mixing abstract this paper presents elemental evidence that the mafic unit of the caledonian glen dessarry syenite northern scotland u k has been variably contaminated with material derived from the surrounding moine metasediments uncontaminated portions are shown to have trace element abundances and ratios similar to those of the central leucosyenite and island arc high k or shoshonitic magmas from present day oceanic settings by comparison contaminated samples display grossly disturbed trace element patterns including higher u th zr hf nb ta and heavy rare earths lower ba and sr and negative eu anomalies these features are matched in the elemental systematics of the moinian envelope but bulk incorporation of excessive amounts of metasediment would be required to reproduce the observed perturbations in especially the high field strength elements xenoliths within the contaminated mela syenites and parts of the adjacent country rock show petrographic evidence of melting since the source composition moine metasediment and residual mineralogy xenoliths are known simple geochemical models of melting and subsequent mixing with uncontaminated melasyenite melt have been computed the results provide close approximations to the observed geochemistry of the contaminated melasyenites particularly for the ree s moreover the residues of this hypothetical process have similar ree patterns to several samples of metasediment from immediately adjacent to the syenite contact nevertheless contemporaneous crystal liquid separation is required to explain the full range of elemental data contamination by this mechanism of melting followed by magma mixing is not immediately apparent in the major element relationships of the two melasyenite groups this seems to be the result of the approximate colinearity of the established melasyenite crystal fractionation path and the proposed granitic contaminant and may be a more general problem in the recognition of in situ contamination with metasedimentary melts in plutons whose fractionation itself proceeds towards a broadly granitic terminus

a review of the superior province of the canadian shield a product of archean accretion abstract the superior province consists of northern and southern high grade gneiss subprovinces and a central region of alternating granite greenstone and metasedimentary belts subprovinces are commonly fault bounded and display contrasting lithological assemblages metamorphic and structural styles geophysical characteristics and ages they are comparable to phanerozoic suspect terranes the middle and late archean rocks of the superior province are of mantle and juvenile crustal origin with little evidence of inheritance of components older than ca 3 1 ga high grade gneiss subprovinces are mainly deeper level equivalents of the supracrustal rich belts but one minnesota river valley is part of an older ca 3 6 ga terrane that was juxtaposed late in the tectonic history granite greenstone terranes are characterized by low grade volcano sedimentary sequences tholeiitic komatiitic submarine lava plain tholeiitic calc alkalic submarine to subaerial central complexes early platform type quartz arenite stromatolitic carbonate late timiskaming type shoshonitic alkalic volcanic fluvial sediment that range in age from ca 3 0 ga to 2 7 ga the volcanics represent oceanic island arc and continental arc volcanism the quartz arenite carbonate sequences deposition on relatively stable early formed ca 3 0 ga crust and the timiskaming type sequences deposition in pull apart basins developed along strike slip faults late archean ca 2 75 2 70 ga greenstone sequences occur throughout the superior province middle archean ca 3 0 2 8 ga sequences are common in northern and central superior province rare or absent in the south volcanism was accompanied by mafic to trondhjemitic plutonism the metasedimentary belts consist of variably metamorphosed volcanogenic turbidites with detrital zircons ranging in age from ca 3 0 ga to 2 7 ga the metasediments were deposited after major volcanism and probably represent accretionary prisms late archean orogenesis accompanied and followed by calc alkalic plutonism youngs southward from about 2 73 2 72 ga to 2 70 2 68 ga it involved large scale north south compression and dextral transpression and displays thin to thick skinned transitions the contemporaneity of events along the lengths of the belts and their southward younging coupled with the evidence of major compression are consistent with subduction driven oblique accretion of oceanic and continental volcanic arcs accretionary sedimentary wedges older microcontinental fragments etc in a convergent margin setting analogous to parts of the pacific rim differences eg abundance of komatiites and tonalitic plutonic suites are attributable to a hotter archean mantle

archean continental assembly in the southeastern superior province of canada between 1988 and 1993 seismic reflection and refraction surveys were acquired across the medium to high grade opatica plutonic gneiss belt the low grade abitibi greenstone belt and the pontiac metasedimentary belt all of which form part of the late archean superior province shallowly north dipping reflections define a structural style consistent with the northward underthrusting and accretion over about 30 ma of various exotic terranes against a backstop provided by the opatica belt this rapid southward growth of the archean protocraton was driven by at least one north dipping subduction zone as revealed by north dipping reflections that extend to 65 km depth in the upper mantle below the opatica belt in contrast to the mainly orthogneissic opatica and pontiac belts the midcrust of the abitibi belt comprises metasedimentary and igneous rocks plus imbricated units of unknown affinity relict midcrustal accretionary complexes of substantial size whieh are indicative of primary suture zones are interpreted near the northern and southern limits of the abitibi belt an interpreted basal decollement and significantly older ages in the north suggest that the upper crustal greenstone rocks are allochthonous evidence of large scale extension appears to be confined to the southern volcanic zone of the abitibi which developed into a half graben as the original suture zone was reactivated in extension unusually high seismic p wave velocities 7 5 8 2 kms 1 are present in the lower 8 km of the abitibi crust and they correlate well with a downward reduction in seismic reflectivity attributable to late modification of the deepest part of the crust crustal xenolith studies suggest that this process may be linked to early proterozoic magmatism

overthrust emplacement of new caledonia ophiolite geophysical evidence geophysical studies support inferences from outcrop geology that during the late eocene an ophiolite sheet exposed on new caledonia was thrust southward over a rock complex consisting of sedimentary volcanic and metamorphic rocks that range in age from pre permian to eocene the outcropping ultramafic complex consists of a layered sequence approximately 3000 m thick including harzburgite dunite wehrlite serpentinite and gabbro the absence of pillow basalts and sheeted dikes on land suggests that during or after overthrusting these units were removed either by thrust faulting or by erosion seismic refraction profiles collected over the adjacent loyalty basin show that the lower crust and upper mantle are characterized by velocities of 4 7 5 8 6 8 and 8 1 8 4 km s suggesting the presence of a complete oceanic section that includes sediment basalt gabbro and peridotite the lower crust is probably 6 5 km thick beneath the loyalty basin a seismic reflection profile that extends across the loyalty basin is interpreted to show the oceanic crust rising toward new caledonia we suggest that the ophiolite on new caledonia is continuous with this rising oceanic crust studies of gravity anomalies observed both on land and offshore indicate the presence of a short wavelength high amplitude 180 mgal asymmetrical free air gravity anomaly along the northeastern coast of new caledonia a gravity anomaly profile calculated for a geological model characterized by a 10 km thick slab of oceanic crust and mantle material extending continuously from the ophiolite on new caledonia to the oceanic crust of the loyalty basin successfully matches the observed gravity data we interpret the extension of the high gravity anomaly over the whole length of the eastern lagoon as evidence for the lateral extension of the root zone of the ultramafic complex a tentative geodynamic reconstruction suggests that the proto loyalty basin crust formed a marginal basin along the eastern margin of gondwanaland prior to upper cretaceous time after the opening of the new caledonia basin during upper cretaceous to early paleocene time a subduction zone developed along the western margin of new caledonia while this zone was active the loyalty basin lithosphere overthrust new caledonia in a southeasterly direction and locally rotated clockwise this compressional tectonic event was terminated by upper eocene time

the system tonalite h_2o at 15 kbar and the genesis of calc alkaline magmas experimental phase relations and well characterized mineral and melt compositions have been determined for a sierra nevada tonalite andesite with 2 5 to 10 wt h 2o added at 15 kbar and 850 to 1100 c all results are from experiments conducted in au capsules minimal fe loss to capsule at oxygen fugacities measured to be near ni nio with increasing water content the liquidus mineral changes from clinopyroxene at 3 wt h20 to garnet followed within 50 c by clinopyroxene at 3 to 9 wt h 2o to hornblende at water contents greater than 9 wt microprobe analyses of glasses quenched from within the tonalite melting interval show an enrichment in ca relative to mg fe where fe represents total fe as fe 2 with increasing sio 2 instead of the typical calcalkaline trend of approximately constant ca mg fe from basalt through rhyolite the results document the importance of garnet and clinopyroxene in andesitic to dacitic compositions at deep crustal to uppermost mantle pressures and they indicate that it is not possible to directly produce magmas of calc alkaline composition at 15 kbar and the investigated range of water contents by either 1 partial melting of tonalitic to gabbroic eclogitic lower crust or 2 crystal fractionation from melts of andesitic to basaltic composition melting or fractionation at lower pressures or with lower water contents may produce calc alkaline liquids as plagioclase replaces garnet as the major al bearing phase cale alkaline rocks showing evidence of garnet liquid equilibration in their ree patterns must also have undergone lower pressure fractionation in order to explain their major element compositions

the volcanogenetic significance of garnet bearing minor intrusions within the borrowdale volcanic group eskdale area cumbria a number of garnetiferous minor intrusions have been mapped within the borrowdale volcanic group they underlie garnetiferous extrusive volcanic rocks which occur toward the top of a sequence of ignimbrite and lava the airy s bridge formation which is the product of a major caldera forming eruptive episode garnet and whole rock geochemistry indicate that most of the intrusions are indistinguishable from garnetiferous dacite forming the final eruptive unit of the airy s bridge formation a co magmatic link is therefore postulated one of the intrusions which intrudes the airy s bridge formation is distinct and may be related to the later eskdale pluton it is suggested that following the emplacement of ignimbrites forming the basal half of the airy s bridge formation caldera collapse partially sealed a fissure conduit system and degassed garnet bearing magma was intruded as dykes and sills and locally extruded as a post explosive lava dome it is also postulated that garnet crystallized in a high level magma chamber p

stitching together the ungava orogen northern quebec geochronological tims and icp ms and geochemical constraints on late magmatic events late magmatic activity in the ungava orogen of northern quebec is manifest as granitic dykes and small rare plutons that crosscut all tectono stratigraphic elements of the orogen conventional u pb geochronology thermal ionization mass spectrometry tims on one particularly important pluton that cuts all these domains the lac duquet monzogranite indicates its age of emplacement at 1742 2 1 3 ma this undeformed and nonmetamorphosed pluton postdates the youngest structures in the orogen d4 folds thereby constraining the timing of the latest deformation to 1742 ma laser ablation inductively coupled plasma mass spectrometry icp ms on zircons from the same sample identified a large range in 207pb 206pb ages of inherited grains from 1 7 to 3 2 ga corresponding to the ages of the host rocks for the pluton this high k peraluminous monzogranite pluton contains moderate to high concentrations of large ion lithophile elements and fractionated and enriched light rare earth elements similar in compo

petrogenetic modelling of in situ fractional crystallization in the zoned loch doon pluton scotland the late caledonian loch doon granitic intrusion ranges in composition from hypersthene diorite at the margin through quartz diorite granodiorite and granite to cordierite microgranite at its core petrogenetic modelling of trace element variations and least squares analysis of major elements indicate that two distinct magmas are involved each magma controlled by crystallization of plag opx cpx bio late stage rocks related to the second magma include the cordierite microgranites and aplites which are interpreted as the final residue which crystallized rapidly after a build up and loss of volatiles analyses of whole rocks and minerals for ree s and other elements of moderate high ionic potential indicate that these elements are strongly controlled by minor phase crystallization apatite zircon sphene and allanite are dominant at intermediate compositions but other accessory minerals such as monazite and xenotime may also become important at acid compositions it is probable that within each magma the mechanisms of crystal settling and filter pressing operated the former being initially dominant and the latter becoming more important with increasing degree of fractional crystallization

u pb ages 3 8 2 7 ga and nd isotope data from the newly identified eoarchean nuvvuagittuq supracrustal belt superior craton canada we report new u pb ages and sm nd isotope data for an eoarchean volcano sedimentary sequence in the northeast superior province of northern quebec canada the nuvvuagittuq supracrustal sequence is characterized by mafi c amphibolite rocks with rare felsic schists accompanied by ultramafi c sills oxide rich and quartz rich iron formations and possible conglomeratic units with peak metamorphism having reached upper amphibolite facies the sequence crops out as an open shallow south plunging synform with tonalites in the center and around the exterior margins of the fold structure a 3817 16 ma age from a felsic schist is interpreted to represent a maximum age for the sequence whereas a 3661 4 ma age from the enclosing tonalites suggests a minimum age detrital zircons from a conglomeratic unit yielded similar old ages of ca 3 78 ga younger ages of ca 3 66 and 3 3 ga from a conglomeratic unit and a felsic schist and 2 7 ga ages from an amphibolite gabbro and a pegmatite dike underline the polymetamorphic history of the sequence initial nd isotope compositions end 3 8 0 5 to 2 3 for mafi c amphibolites and felsic schists are indicative of a moderately depleted mantle sm nd analyses of a 2 7 ga pegmatite dike and the ca 3 66 ga tonalites both yield nd depleted mantle model ages tdm of ca 3 9 ga indicating that they were derived from a crust of juvenile eoarchean origin we propose that fragments of paleoarchean to eoarchean terranes that rim the superior province represent remnants of a larger terrane s that was rifted and dismembered prior to the formation of the 3 0 2 7 ga superior craton

contrasting types of late archean plutonic rocks in northwestern ontario implications for crustal evolution in the superior province abstract the late archean aulneau and lount lake batholiths have contrasting geochemical characteristics that are interpreted to be a consequence of derivation from different sources the aulneau batholith which was intruded into low grade metavolcanic rocks of the wabigoon subprovince is characterized by high na2o and low k2o rb and y together with enriched light rare earth elements lree near chondritic heavy rare earth elements hree negligible eu anomalies and mantle type initial sr and nd isotopic compositions these geochemical characteristics are consistent with derivation from the partial melting of metamorphosed tholeiitic basalt in contrast the lount lake batholith which was intruded into older 2 83 3 17 ga tonalitic plutonic rocks of the winnipeg river subprovince has relatively high k2o rb y and ree low na2o and prominent eu anomalies the initial sr and nd isotopic compositions of the lount lake batholith are consistent with a crustal source at least 200 400 ma older than the crystallization age of the batholith these geochemical characteristics indicate that the lount lake batholith originated from the partial melting of deeper level equivalents of the tonalitic metaplutonic rocks 2 83 3 17 ga which they intrude these results suggest that reworked 3 0 ga crust may be extensive in the winnipeg river subprovince and that the geochemical and isotopic signature of late archean plutonic rocks may be a useful reconnaissance tool in the identification of areas of extensive older crust the results can be interpreted within a uniformitarian model diachronous ensimatic magmatic arcs represented by the winnipeg river and western wabigoon subprovinces developed independently collision of these arcs at 2707 8 ma marks the cessation of subduction and related magmatic activity and the initiation of high grade metamorphism and melting in tectonically thickened portions of the crust

crustal magnetization and long wavelength aeromagnetic anomalies of the minto block quebec measured susceptibilities of 3500 samples from a 500 500 km area of the minto block superior province quebec have an average value of 0 03 si equivalent to induced magnetizations of 1 5 a m charnockitic lithologies igneous orthopyroxene bearing granodiorite granite diorite that constitute large parts of the minto region yield the highest susceptibilities the variations in susceptibility show a good correlation with the variations in the observed magnetic field including those at long wavelengths 100 km suggesting that charnockitic and related granitoid rocks could be responsible for the long wavelength anomalies modeling of a magnetic profile perpendicular to the strike of mapped lithotectonic subprovinces in the minto block supports this contention charnockitic lithologies are not usually considered a possible sources of long wavelength anomalies mafic rock types are generally favored the dominant minto lithologies are more felsic but are more efficient in the production of magnetite from the available iron

the age and nature of the basement of southern britain evidence from sr and pb isotopes in granites the isotopic study of granitic intrusions in southern britain provides an indication of the age and geochemical nature of the south british basement which is otherwise generally obscured from observation by sedimentary cover this paper reports pb isotopic compositions of feldspars in granites from sw england lundy mountsorrel and shap in addition to age and initial 87 sr 86 sr data on two of the granites a review of published rb sr age and isotope data for southern britain and consideration of the new pb data using the plumbotectonics model of doe zartman 1979 indicate that the basement to southern britain has an age of 800 ma with an upper limit of 1200 ma the granites studied in this work have substantial components of crustally derived sr and pb

sm nd garnet age for the ordovician borrowdale volcanic group english lake district garnets separated from andesites and dacites of the calc alkaline borrowdale volcanic group are highly enriched in heavy rare earth elements hree relative to their host rocks d gt yb from 9 in andesite to 37 in dacite sm nd ratios and d gt lree show a wide range probably resulting from small quantities of an ree rich impurity in the separates the four garnet host rock pairs analysed yield sm nd ages identical within analytical error of a weighted mean of 457 4 ma the borrowdale group overlies upper llanvirn sediments and is unconformably overlain by mid caradoc the age of 457 ma is considered to be the best estimate available for the age of the llandeilo as the samples analysed are from the lower part of the group the age suggests that a lower palaeozoic timescale intermediate between those of gale et al 1979 and mckerrow et al 1980 might be the most appropriate

dislocation formation and albitization in alkali feldspars from the shap granite orthoclase rich alkali feldspars in the lower devonian shap granite northwest england contain two generations of albite rich feldspar these have partially replaced earlier exsolution microtextures consisting of albite lamellae coarse semicoherent albite films and fine coherent albite platelets in tweed orthoclase the earlier generation of replacive albiterich feldspar ab90an9ori occurs together with orthoclase rich feldspar ab10or90 i n veins that crosscut exsolution microtextures throughout grain interiors this episode of recrystallization was mediated by magmatic fluids at 410 8c estimated from two feldspar geothermometry and was driven by stored elastic strain energy which was relatively homogeneously distributed throughout the microtextures the later generation of replacive albite rich feldspar which is restricted to grain margins and is compositionally pure ab 99 was produced by magmatic hydrothermal fluids at 370 8c this generation of albite rich feldspar does not crosscut exsolution microtextures and has selectively replaced volumes of highly elastically strained feldspar surrounding edge dislocations along semicoherent albite films marked differences in controls of the localization of the two generations of replacive albite rich feldspar by pre existing exsolution microtextures indicate that significant numbers of edge dislocations developed along albite films after the first phase of fluid feldspar interaction and associated albitization but before the second phase this relation indicates that edge dislocations formed between 410 and 370 8c these observations have important implications for understanding the factors that control the interaction of alkali feldspars with fluids both in cooling igneous rocks and in clastic sedimentary rocks during diagenesis

biomechanical and biochemical weathering of lichen encrusted granite textural controls on organic mineral interactions and deposition of silica rich layers the crustose lichen rhizocarpon geographicum weathers the lower devonian shap granite by both biomechanical and biochemical means biomechanical weathering is mediated by fungal hyphae that penetrate into the rock via intergranular boundaries at 0 002 0 003 mm year 1 once inside the granite hyphae exploit intragranular pores along cleavage and fracture planes in biotite alkali and plagioclase feldspar grains of biotite exposed at the lichen granite interface have been fragmented by biomechanical action in 122 years after an extended period of biomechanical weathering of 10 kyr outcrop surfaces sub mm sized fragments of biotite and plagioclase feldspar abound in lower parts of the lichen s thallus grains of biomechanically weathered biotite show the clearest evidence for biochemical weathering typically k and fe are leached from the biotite but other cations may also be removed leaving a silica dominated relic the silica rich remains of biotite and possibly also feldspar have been redistributed along the lichen granite interface forming a silica rich layer this silica rich material also cements fractures and pores beneath the interface alkali feldspars from outcrop surfaces have been weathered chemically as indicated by etch pits on cleavage surfaces and fractures but it is not obvious that lichen was involved in the dissolution the etch pits were probably formed during earlier phases of non biochemical weathering that widened intergranular and intragranular pores facilitating subsequent access of fungal hyphae into the granite despite evidence for biomechanical and biochemical weathering it is unlikely that colonisation of granite surfaces by r geographicum at shap significantly enhances their weathering rate relative to bare rock surfaces the lichen may actually retard weathering by protecting the surface from frost action binding fragmented mineral grains and depositing protective silica rich layers

metamorphic zircon trace elements and neoarchean metamorphism in the ca 3 75 ga nuvvuagittuq supracrustal belt quebec canada abstract the nuvvuagittuq supracrustal belt nsb in northwestern quebec ranks as one of the oldest granitoid gneiss complexes thus far discovered emplacement ages for intrusive trondhjemitic dikes that transect amphibolites and quartz magnetite schists at the porpoise cove outcrops of the nsb cluster at 3 75 ga as with all other pre 3 7 ga terranes thus far documented the nsb has been metamorphosed and multiply deformed in a combined approach to explore the thermal history of the nsb conventional garnet biotite and plagioclase hornblende geothermometry was coupled with zircon u th pb depth profiles ree partitioning and ti in zircon thermometry to show that neoarchean metamorphism in the belt reached upper amphibolite facies conditions 640 c this metamorphism resulted in some cases in thick zircon overgrowths on older cores and likely corresponds to the amalgamation of terranes in the northeast superior province and the initiation of widespread igneous activity in the vicinity of the nsb an earlier resolvable metamorphic episode from a zircon depth profile recorded at 3 62 ga probably corresponds to the intrusion of the granitoids which envelope the nsb

paleomagnetism and u pb geochronology of diabase dyke swarms of minto block superior province quebec canada in the first collaborative study of paleomagnetism and precise u pb geochronology in the minto block of the superior province mafic dyke swarms with three widely divergent paleomagnetic signatures and isotopic ages have been identified the 2505 2 ma ptarmigan dykes trend north to northeast and have a virtual geomagnetic pole at 42 s 220 e similar to that of 2473 2446 ma matachewan dykes of the southern superior province the ca 2230 ma maguire dykes trend west to northwest and yield a paleopole at 9 s 267 e similar to those for 2216 8 4 ma senneterre dykes and 2217 2210 ma nipissing sills of the southern superior and southern provinces respectively the 2209 1 ma klotz dykes trend west northwest but do not carry a consistent magnetization direction finally 1998 2 ma minto dykes of west northwest to northwest trend identical in age to the 1998 ma 2 ma purtuniq ophiolite of the cape smith belt have a paleopole at 38 n 174 e the similarity of paleopoles for the ca 2 23 2 21 ga maguire

magmatic zoning in apatite a monitor of porosity and permeability change in granites apatites from the shap granite northern england are strongly zoned reflecting multiple generations of growth and dissolution such chemical zoning is most readily displayed in cathodoluminescence images and correlates well with trace element variation determined using la icp ms analyses the zoned apatites provide a detailed record of the changing scales of permeability during progressive crystallisation within the magma chamber early periods of apatite growth are preserved within cores and represent both early growth within a magma chamber dominated by vigorous mixing processes and inherited grains with significantly different chemistries the main phase of apatite growth within the magma was strongly controlled by the presence of adjacent biotite phenocrysts and is characterised by fine scale oscillatory zoning followed by the growth of a thin rim of relatively uniform composition the chemical evolution of the later phases of apatite growth and the stratigraphy of the zoning appear to record late stage crystallisation within progressively more isolated interstitial melt pockets

geochemical variation and magmatic cyclicity within an ordovician continental arc volcanic field the lower borrowdale volcanic group english lake district abstract the origin differentiation and temporal evolution of magmas have been examined from the ordovician lower borrowdale volcanic group bvg a calc alkaline plateau andesite pile of continental arc affinity the basalt dacite compositional range can be modelled by poam type crystal fractionation of primitive melts allied with minor crustal assimilation derived from enriched mantle modified by fluids from the subduction zone detailed sampling of sections through 2 3 km of stratigraphy reveals localised and successive highly systematic magmatic evolutionary trends or cycles cycles in which magma becomes less differentiated with time can be viewed as episodes of rapid eruption of a compositionally zoned chamber possibly due to magma recharge by contrast increasing differentiation with time implies reduced recharge and eruption rates whereby fractionation processes dominate the lack of evidence for compositional change over time represents a balance between recharge eruption and differentiation processes successive cycles within a local sequence can be related in many cases to discrete batches of magma that ascend into sub volcanic chambers and undergo fractionation the differences between contemporaneous sequences confirms the current view of the lower bvg as a multicentred volcanic field it was deposited in a subsiding extensional volcano tectonic rift zone consistent with episodes of rapid upward magma flux with the eruption locally of primitive high mg ni and cr basaltic lavas

paleoproterozoic 1 90 1 86 ga arc volcanism in the flin flon belt trans hudson orogen canada geochemical and isotopic nd sr data are reported on paleoproterozoic 1904 1864 ma maficintermediate 1870 ma collision and subsequent intra arc deformation

a diffusive ion flux of non marine origin in cumbrian lake sediments implications for element budgets in catchments abstract diffusion controlled concentration gradients of many elements exist in the sediments of derwentwater lake cumbria northern england which indicate hydraulic connection with a reservoir of high salinity water a widely distributed source of saline groundwater beneath cumbria with a composition that is compatible with anomalous surface water chemistry is demonstrated element and stable isotope characteristics indicate that the most likely source of enhanced salinity is from water rock interaction at depth involving the lake district granite batholith and surrounding meta sediments these results demonstrate the permeability and reactivity of basement rocks and suggest also that chloride cannot be regarded as conservative in water balance studies

the d entrecasteaux zone southwest pacific a petrological and geochronological reappraisal abstract the d entrecasteaux zone southwest pacific is an arched submarine horst and graben structure which extends from the northern end of the new caledonia ridge to the western border of the new hebrides island arc a review of the bathymetry seismic reflection data paleomagnetism gravimetry seismology and dsdp data available for this area is combined with a study of basaltic samples dredged along the horsts of this regional fracture zone these basalts show strong petrographic and chemical affinities with morb their fissiontrack ages range between 56 ma paleocene eocene boundary and 36 ma eocene oligocene boundary it is suggested that the d entrecasteaux zone represents the northern arcuate extension of the northeast dipping eocene subduction obduction zone located along the new caledonia loyalty islands ridge while its present morphology appeared from post obduction extensional movements resulting in a progressive uplift of basaltic ocean floor since middle miocene times

giant granulite terranes of northeastern superior province the ashuanipi complex and minto block the ashuanipi complex and minto block of the superior province are large regions that have been classified as high grade gneiss terranes on the basis of the presence of orthopyroxene bearing units like the granite greenstone and metasedimentary subprovinces of the southern superior province the two terranes consist predominantly of intrusive rocks but are distinguished by their primary magmatic orthopyroxene both high grade and gneiss are misnomers because granulite facies gneisses are only sparingly present and the regions consist dominantly of massive unmetamorphosed plutonic rock the ashuanipi complex consists of a deformed metamorphosed package of metasedimentary rocks and primitive early tonalite cut by widespread orthopyroxene garnet granodiorite diatexite as well as plutons of tonalite granite and syenite based on its lithological and chronological similarity and on strike position the complex appears to be the continuation of metasedimentary subprovinces such as the quetico its evolution involved deposition of immature greywacke in an accretionary prism early arc tonalitic magmatism and deformation followed by widespread intracrustal magmatism in the period 2700 2670 ma both metamorphic and igneous rocks record equilibration under granulite facies conditions 700 835 c 0 35 0 65 gpa a 0 3 and indicate exposure levels of 20 km the minto block at the latitude of leaf river consists of several north northwest trending domains of similar scale and diversity to the east trending subprovinces of the southern superior province the central goudalie domain is dominantly amphibolite facies tonalitic rocks including some with ages 3 ga with small belts of volcanic and sedimentary origin lake minto domain contains poorly preserved supracrustal remnants in a plutonic complex comprising hornblende granodiorite clinopyroxene orthopyroxene granodiorite orthopyroxene biotite diatexite and granite the hornblende granodiorite suite constitutes most of the utsalik and tikkerutuk domains and is thought to represent continental arc magmatism on the basis of their distinct aeromagnetic and lithological character two additional domains are evident north of the leaf river area the inukjuak domain in the west and the douglas harbour domain in the east the northerly grain of the minto block appears to have been established in situ with respect to the easterly belts of the southern superior province i e no large scale block rotation during the same interval of time 3 0 2 7 ga modification of the tectonic framework for the superior province is required to explain minto arc magmatism in the interval 2730 2690 ma ago a continental magmatic arc built the berens river and bienville subprovinces and minto block on the southern and eastern edges respectively of a northern protocratonic foundation in the same period primitive volcanic arcs and accretionary prisms developed outboard on oceanic crust and were accreted to form a southern tectonic regime

archean magmatism and deformation were not products of plate tectonics abstract the granite and greenstone terrains that dominate upper crust formed from about 3 6 to about 2 6 ga and record magmatic and tectonic processes very different from those of a younger time they indicate heat loss by the archean earth primarily by voluminous magmatism from a mantle much hotter than that of the present plate tectonic processes were not then operating the distinctive array of petrologic structural and stratigraphic features that characterize phanerozoic convergent plate systems ophiolites magmatic arcs accretionary wedges fore arc basins etc have no viable analogues in archean terrains purported archean plate tectonic indicators consist merely of rock types that even superficially resemble actual phanerozoic indicators only when considered in isolation from their association and structure archean ultramafic and mafic volcanic rocks neither resemble ophiolitic rocks in petrology nor occur in ophiolite type successions they often depositionally overlie felsic basement rocks and often overlie and are intercalated with sedimentary and felsic volcanic rocks and they require a mantle about 200 c hotter than now archean graywackes are coherent strata derived from nearby volcanic rocks late in the histories of their regions and they lack the setting and disruption that characterize modern accretionary wedges the lithologic structural and stratigraphic assemblages that typify proterozoic and phanerozoic rifted and reassembled margins also lack archean analogues and no evidence has been found for archean rifting rotation and reassembly of continental plates conversely characteristic archean assemblages lack modern equivalents in any tectonic setting ultramafic lavas erupted at liquidus temperatures are voluminous granite and greenstone terrains have no modern analogues greenstone belts are typically anastomosing networks of upright synforms formed by crowding aside by and sinking between large rising diapiric elliptical composite batholiths the batholiths include both the products of new crustal melts and variably remobilized mid crustal gneisses the greenstone belts are defined by late deformation of regionally semiconcordant volcanic and sedimentary successions and are not relics of successively formed volcanic chains little deformation generally preceded the diapirism and metamorphism was primarily of contact type the regionally uniform areal density and accordant crustal level of the diapiric batholiths their contacts primarily against the oldest strata of the synforms their general age 10 20 million years younger than most of the flanking stratiform rocks and considerations of high archean radiogenic heat productivity all fit the explanation that the batholiths were mobilized by partial melting of hydrous lower crust by radiogenic heating diapirism was accompanied by modest regional orthogonal shortening and extension of the hot upper crust producing the orientations of the batholiths rise of the batholiths greatly increased the petrologic fractionation of the crust and the concentration of radionuclides high in it resulting in cooling of the deeper crust and subjacent mantle and thus cratonization the upper crust containing the granite and greenstone aggregates was decoupled from the gneissic middle crust which underwent flattening and extension subparallel to the elongation of the shallow batholiths this deep deformation may have been driven by flow of dense restites toward delamination loci from which they sank into the mantle the early earth was probably wholly molten the surface of the earth like that of the moon must have been wholly recycled by impacts before 3 9 ga and heavily modified by them until 3 8 ga zircons as old as 4 2 ga have been found as clastic grains in much younger archean quartzites and polycyclic migmatites last partly melted and reconstituted under hydrous conditions only after 3 6 ga contain relict zircons as old as 4 0 ga the lithologies of the early earth protoliths in which these zircons formed have not been established but impact melts and breccias must be represented and magma ocean fractionates may be the nature of the transition in tectonic style into the granite and greenstone mode is unknown plate tectonic rifting and convergence were operating by about 2 0 ga and were in an essentially modern mode by about 0 8 ga the nature of the transition from the granite and greenstone mode at about 2 6 ga to plate mode by about 2 0 ga has yet to be defined the change may have been facilitated by the increasing content of water and carbon dioxide in the mantle as dense but hydrated delaminated archean crust sank into it

strain localization and fluid infiltration in the mantle wedge during subduction initiation evidence from the base of the new caledonia ophiolite despite decades of petrological and geochemical studies the nature and setting of obducted ophiolites remain controversial the influence of supra subduction zone environments on pre existing oceanic lithosphere is yet to assess and the processes leading to subduction obduction initiation are still poorly constrained our study documents successive influx of slab derived fluids and progressive strain localization within the upper mantle in a supra subduction environment during the first few my of the subduction history we focus on strongly sheared mafic amphibolites intruding peridotites near the mantle crust transition of the new caledonia obducted ophiolite and 50 to 100 m above the basal thrust contact of the ophiolite these m to hm long and several m thick shear bands are interpreted as inherited small scale intrusions of mafic melts probably dikes or sills which were derived from a moderately refractory mantle source refertilized by supra subduction zone fluids 40ar 39ar age constraints on pargasite at ca 90 ma suggest that they could be inherited from the former pacific west dipping subduction secondary deformation of these mafic intrusions is intimately associated to three major stages of fluid infiltration 1 the first stage of deformation and metasomatism is marked by syn kinematic growth of ca amphibole at 700 800 c and 3 5 kbar with a distinctive supra subduction zone signature and controlled later channelization of aqueous fluids 40ar 39ar dating on magnesio hornblende indicates that this deformation episode occurred at ca 55 ma coincident with east dipping subduction initiation 2 the main metasomatic stage characterized by the development of a phlogopite rich matrix wrapping peridotites and amphibolite boudins points to the percolation of alkali rich aqueous fluids at still high temperature 650 750 c 3 the last low temperature 600 c metasomatic stage results in the formation of deformed veinlets containing talc chlorite and serpentine

crustal growth through successive arc magmatism reconnaissance u pb shrimp data from the northeastern superior province canada abstract north of the la grande subprovince in northern quebec the superior province is dominated by plutonic rocks of the bienville subprovince and minto block which have been further subdivided into domains on the basis of reconnaissance geological mapping and geochronology and projected along strike using aeromagnetic anomalies to test the validity of the extrapolations u pb ages were obtained through shrimp analyses of zircon from 18 small archival samples selected from a suite of 2000 specimens collected during a helicopter reconnaissance survey conducted in the 1960s the results reveal a history of successive arc magmatism between 3 07 and 2 70 ga followed by metamorphism emplacement of crustally derived granites 2 702 2 685 ga and late hydrothermal events ca 2 65 ga in the northeastern minto block the douglas harbour domain previously extrapolated from dated localities to the northeast is substantiated through an age of 2 87 ga the older tonalitic crust is punctured by 2 734 2 725 ga granodioritic plutons of the leaf river suite that locally carry 3 01 ga inherited zircon the northern utsalik domain is made up of pyroxene and hornblende bearing leaf river plutons with consistent 2 725 2 723 ga ages and local inheritance of 2 82 and 2 77 ga however the southern extension of utsalik domain contains pyroxene bearing granodiorites both older 2 762 ga and younger 2 692 ga than the leaf river suite a tonalitic rock from the southern goudalie domain with an age of 2 833 ga supports a linkage to 2 84 2 83 ga units of the qalluviartuuq domain along strike to the north tikkerutuk domain represents a north trending 50 km wide calc alkaline magmatic arc with ages between 2 71 and 2 70 ga that extends southward into the bienville subprovince on the basis of aeromagnetic anomalies and geochronology zircon inheritance ages indicate that the tikkerutuk arc was built upon antecedent arcs of 2 84 2 77 and 2 725 ga the inukjuak domain in the west consists mainly of granite dated in one locality at 2 72 ga

paleo to mesoarchean basement recycling and terrane definition in the northeastern superior province quebec canada abstract the northeastern superior province nesp is a 500 km 700 km segment of plutonic archean crust generated by 1 2 ga of tectonomagmatic activity the nesp exhibits the classical plutonic series of archean terranes low k tonalite trondhjemite granodiorite ttg and high k granodiorite granite monzogranite ggm with unusually large volumes of px ttgs mostly enderbites and subordinate charnockites volcanic rocks constitute t 0 45 to 5 03 mafic to felsic volcanic rocks and mafic to ultramafic plutonic rocks t 0 40 to 6 90 these radiogenic signatures as well as inherited zircon cores record recycling of mesoarchean 3 8 2 9 ga crust either through direct melting or by coupled assimilation fractional crystallization processes in comparison the riviere arnaud terrane is underlain by younger t values 2 34 to 0 67 indicating either a juvenile character or recycling of fairly juvenile late mesoarchean early neoarchean crust overall geochemical and isotopic data of the late neoarchean

the emplacement of the peridotite nappe of new caledonia and its bearing on the tectonics of obduction the peridotite nappe of new caledonia is one of the few ophiolites worldwide that escaped collisional orogeny after obduction here we describe the deformation associated with serpentinization in two klippes of the nappe in northwestern new caledonia the klippes are flat lying and involve s sw vergent reverse slip shear zones which are true compressional structures in origin further northeast the nappe is folded in association with the development of a steep schistosity in low grade metasediments this difference in structural style indicates that the peridotite nappe experienced compression at greater depths toward its root zone suggesting a push from the rear mechanism of emplacement this supports the view that the nappe has been emplaced through horizontal contraction sustained by plate convergence we establish a crustal scale cross section at the end of the obduction event before neogene extension this involves a large fold nappe of high pressure rocks bounded from below by a major thrust furthermore we show that obduction in new caledonia occurred through dextral oblique convergence oblique convergence probably resulted from the initial obliquity between the subduction trench and the continental ribbon that became incorporated in it this obliquity can solve the paradox of the peridotite nappe seemingly being emplaced at the same time the high pressure rocks were exhumed oblique convergence together with focused erosional denudation on the northeastern flank of the island led to exhumation of the metamorphic rocks in a steep fold nappe rising through the rear part of the orogen

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

provenance of heinrich layers in core v28 82 northeastern atlantic 40ar 39ar ages of ice rafted hornblende pb isotopes in feldspar grains and nd sr pb isotopes in the fine sediment fraction abstract several correlatable layers of sediment rich in ice rafted grains have been documented in the north atlantic the most notable within the last glacial cycle are the heinrich layers layers extremely rich in ice rafted detritus and generally barren of foraminifera within the north atlantic ice rafted detritus ird belt the view of these layers is that they represent events where great armadas of icebergs were launched into the north atlantic the importance of the heinrich layers lies in their connection with abrupt climate change in the north atlantic and perhaps globally there is a growing number of published provenance studies of the heinrich layers in the north atlantic based on a variety of methods however there is little overlap of methods applied to the same samples in this contribution we present a multi component provenance study of heinrich layers h1 h2 h4 and h5 from core v28 82 in the eastern north atlantic our results indicate that virtually the entire inventory of terrigenous clastic detritus in heinrich layers h2 h4 and h5 came from ancient continental sources surrounding the labrador sea although heinrich layer h1 is similar in many respects it appears to have some significant differences relative to the other three

plutonism oblique subduction and continental growth an example from the mesozoic of california major episodes of mesozoic plutonism in california correlate with periods of oblique subduction and trench parallel transport of western california along intrabatholithic faults major episodes of plutonism occurred in the late jurassic during left oblique convergence and in the mid cretaceous during right oblique convergence in contrast a conspicuous lull in plutonism but continuation of volcanism in the earliest cretaceous coincides with a time when the north america farallon convergence vector although large in magnitude was oriented perpendicular to the trench this correlation suggests that plutonism is facilitated by strike slip faulting within the batholithic belt one explanation which helps to solve the plutonic room problem is that plutons are passively emplaced at releasing bends in the strike slip faults and volume is conserved by thrusting at the trench if this correlation is generally applicable then it implies that mid crustal plutonism is limited beneath areas in which the convergence vector is subperpendicular to the trench continental growth in such areas may occur dominantly by volcanism

crustal growth and late precambrian early palaeozoic plate tectonic evolution of england and wales comparison of the basement rocks of britain reveals major contrasts to the n and s of the iapetus suture solway shannon line north of the suture polymetamorphic high grade basement schists and gneisses and igneous rocks date from about 2900 ma to c 400 ma the latter age corresponding to closure of the iapetus ocean by contrast s of the suture in england and wales the basement is dominantly low grade metamorphic rock together with intrusive and extrusive igneous and sedimentary rocks there is little geological or geochemical evidence for the presence of basement older than c 900 ma below this area and the predominant expression of crustal growth in england and wales is indicated by radiometric ages between c 700 ma and 450 ma reflecting late precambrian palaeozoic magmatism and metamorphism palaeomagnetic data from scotland demonstrate that the crust in northern britain developed as an integral part of the laurentian shield during mid proterozoic times 1600 1000 ma the palaeomagnetic record from the pre ordovician rocks of england and wales shows that this area was unrelated to the laurentian shield prior to closure of the iapetus ocean the apparent polar wander a p w path is that of a distinct microplate involved in rapid growth of iapetus during early cambrian times and emplaced in its present configuration adjacent to northern britain as a result of subduction of the ocean basin between cambrian and silurian times we conclude that whereas the basement to the north of the iapetus suture formed an essentially continental unit from 2900 to 400 ma the basement of england and wales is a result of crustal growth from about 900 ma to 400 ma and formed by accretion of island arcs associated accretionary prisms and fore arc basin sediments within the iapetus ocean

pb and nd isotopic study of two archean komatiitic flows from alexo ontario isotopic analysis of two archean komatiitic flows from alexo ontario gives a pb pb isochron age of 2690 15 ma and a sm nd isochron age of 2752 87 ma these ages agree well with u pb zircon ages from underlying and overlying volcanics the variations in element ratios that define the isochrons were not produced during crystallization of the lavas the spread in u pb was caused by submarine alteration soon after eruption and the spread in sm nd resulted from a differences in the composition of the residue of melting and b contamination of the upper komatiite flow through thermal erosion of the lower flow the 147sm 144nd ratio of uncontaminated komatiite is 0 25 which reflects the depleted nature of its mantle source the th u ratio of about 3 4 is probably also representative of depleted mantle the initial nd of 2 44 0 51 indicates that the mantle depletion took place long before magma formation

comparative reos smnd and rbsr isotope and trace element systematics for archean komatiite flows from munro township abitibi belt ontario abstract a series of komatiite flows from munro township ontario have re and os concentrations that range from 0 545 to 1 27 ppb and 1 00 to 2 20 ppb respectively the187re 186os in the suite range from 14 05 to 52 37 and the187os 186os range from 1 49 to 3 22 giving a crystallization age of2726 93ma and an of initial187os 186os of0 873 0 035 this age is in good agreement with the crystallization ages that have been reported for these rocks using other isotopic systems the re os isochron indicates that the system was resistant to the low grade metamorphism that altered the rocks the initial os isotopic composition is consistent with the derivation of the melts from a mantle source s that originated with a chrondritic 187os 186os i and evolved with an approximately chondriticre os sm nd data for the same suite cluster about anend value at 2 72 ga of 2 to 3 rb sr isotope data indicate significant mobilization of these elements by the greenschist grade metamorphism that has altered the rocks comparisons of re and os concentrations with those of mgo feo ni cr and s suggests that re was incompatible d 1 os concentrations were likely controlled by the presence or precipitation of trace metallic or sulfide phases both during melting and crystallization and not by the modally predominant silicate minerals

a three dimensional perspective on the evolution of archaean crust lithoprobe seismic reflection images in the southwestern superior province in 1990 1991 the lithoprobe project completed 450 km of seismic reflection profiles across the late archaean crust of the southwestern superior province the results define a broad three fold division of crust upper crust in the abitibi greenstone belt is non reflective and is a 6 8 km veneer of volcanic and plutonic supracrustal rocks whereas in the sediment gneiss dominated pontiac subprovince upper crust comprises shallow northwest dipping turbidite sequences mid crust in both the abitibi and the pontiac subprovinces is interpreted as imbricate sequences of metasedimentary and metaplutonic rocks lower crust in both subprovinces has a horizontal layer parallel strycture which may represent interleaved mafic intermediate gneisses the seismic signature of the northern abitibi greenstone belt may be represented in an exposed 25 km crustal section in the kapuskasing stuctural zone preliminary tectonic models based on the seismic data are consistent with a plate tectonic scenario involving oblique subduction and imbrication of sedimentary plutonic and volcanic sequences the northern abitibi supracrustal sequences either represent an allochthon or overlie an allochthonous underthrust metasedimentary and plutonic sequence which may be equivalent to a metasedimentary subprovince such as the pontiac or quetico seismic velocities have yet to be defined however crustal thicknesses are relatively constant at 35 40 km the thinnest crust is adjacent to the grenville front where moho is very well defined

the geochemistry metasomatism and petrogenesis of the granites of the english lake district new geochemical data for the carrock threlkeld ennerdale shap skiddaw and eskdale granites of the lake district are presented and discussed with particular reference to the metasomatism and petrogenesis of the intrusions the caledonian granites of the lake district have more associated hydrothermal activity and mineralization than their equivalents north of the iapetus suture in scotland pervasive high temperature metasomatism which affects the shap skiddaw and eskdale intrusions is accompanied by remobilization of the large ionic lithophile elements k rb sr u and li and b although high field strength elements including the rare earth elements remain unaffected except near greisen and mineral veins the threlkeld intrusion appears to have suffered loss of a ba and sr during a low temperature event however there is no consistent relationship between granite composition hydrothermal activity and mineralization which could be used to support the granite cupola model for mineralization in the province rather the episodic mineralization which affects the lake district may be related more to the broad geothermal field consequent upon the emplacement of the deeper lake district batholith identified from geophysical data despite the variable metasomatic perturbations of their primary chemistry three distinct groups of intrusions can be recognized on geochemical grounds the carrock granophyre is a product of tholeiitic fractionation and the threlkeld and ennerdale bodies are more typical calc alkaline intrusions whereas the eskdale shap and skiddaw plutons show a progressive evolution towards more complex geochemical patterns suggesting decreasing hornblende but increasing plagioclase control of fractionating processes the culmination of this trend may have been the emplacement of a large high heat production granite beneath the lake district at the end of the caledonian orogeny the suggestion made on geophysical grounds that the eskdale intrusion is a cupola of the lake district batholith cannot be simply reconciled with geochemical modelling none of the lake district granites has geochemical or isotopic characteristics which are convincingly s type i e formed by partial melting of a sedimentary protolith and a model of subcrustal magmagenesis beneath an evolving arc or continental margin appears to be more appropriate

u pb geochronology of reactivated archean basement and of hudsonian metamorphism in the northern labrador trough the labrador trough is a proterozoic orogenic belt bordering the eastern margin of the archean superior province the volcano sedimentary sequences of the trough were deformed and metamorphosed during the hudsonian orogeny ca 1800 ma the eastern basement present as domal inliers was remobilized during this orogeny whereas the western basement remained unaffected in this study we present u pb ages of the western and eastern basement of the proterozoic labrador trough orogen between leaf river and koksoak river and of the northernmost extension of the depas batholithic complex farther east from the western basement the leaf bay granodiorite has an age of 2721 3 ma zircon from the gneiss domes of the eastern basement the ages are as follows lac moyer 2883 6 ma zircon 1793 5 ma monazite and 1746 ma minimum age rutile lac boulder an upper intercept age of 2868 8 ma and a lower intercept age of 1783 11 ma zircon titanite regression and an age of 1740 5 ma rutile lac olmst

ferropicrite driven reworking of the ungava craton and the genesis of neoarchean pyroxene granitoids article history voluminous pyroxene bearing intermediate to felsic plutons were emplaced during a 20 50 million year long spatially extensive neoarchean igneous event that culminated in the cratonization of north america s 500 km wide ungava craton the crystallization ages of pyroxene bearing plutons coincide with the emplacement of numerous ca 2 72 2 70 ga fe rich ultramafic mafic intrusions of the qullinaaraaluk suite q suite that are scattered across the disparate domains of the ungava craton a high proportion of relatively sodic pyroxene bearing granitoids with intermediate silica contents fall in a compositional gap between the q suite and pyroxene free granitoids suggesting that the pyroxene granitoids may be formed by the simultaneous fractional crystallization and assimilation of older tonalitic and trondhjemitic tt crust by the q suite magmas we estimate that pyroxene granitoids containing 65 wt sio2 may reflect 40 50 wt contamination of mantle derived picritic magma by trondhjemitic melts of the pre 2 74 ga ttg crust the craton wide occurrence of the q suite intrusions and pyroxene granitoids suggests that underplating by ferropicritic magmas played a key role in the cratonization of the ungava craton at the end of archean a major contribution of mantle derived magmas to the petrogenesis of the ca 2 74 2 70 ga pyroxene granitoids is consistent with the proposed global generation of voluminous juvenile continental crust ca 2 7 ga

internal differentiation of the archean continental crust fluid controlled partial melting of granulites and ttg amphibolite associations in central finland fault bound blocks of granulite and enderbite occur within upper amphibolite facies migmatitic tonalitic trondhjemitic granodioritic ttg gneisses of the iisalmi block of central finland these units record reworking and partial melting of different levels of the archean crust during a major tectonothermal event at 2 6 2 7 ga anhydrous mineral assemblages and tonalitic melts in the granulites formed as a result of hydrous phase breakdown melting reactions involving amphibole at peak metamorphic conditions of 8 11 kbar and 750 900 c a nominally fluid absent melting regime in the granulites is supported by the presence of carbonic fluid inclusions the geochemical signature of light rare earth element lree depleted mafic granulites can be modelled by 10 30 wt partial melting of an amphibolite source rock leaving a garnet bearing residue the degree of melting in intermediate granulites is inferred to be less than 10 wt and was restricted by the availability of quartz pressure temperature estimates for the ttg gneisses are significantly lower than for the granulites at 660 770 c and 5 6 kbar based on the p t conditions melting of the ttg gneisses is inferred to have occurred at the wet solidus in the presence of an h 2 o rich fluid a hydrous mineralogy abundant aqueous fluid inclusions and the absence of carbonic inclusions in the gneisses are in accordance with a water fluxed melting regime low ree contents and strong positive eu anomalies in most leucosomes irrespective of the host rock composition suggest that the leucosomes are not melt compositions but represent plagioclase quartz assemblages that crystallized early from felsic melts furthermore similar plagioclase compositions in leucosomes and adjacent mesosomes are not a migmatite paradox as both record equilibration with the same melt phase percolating along grain boundaries

petrogenesis of archaean trondhjemites tonalites and granodiorites from eastern finland major and trace element geochemistry

accessory minerals and the geochemical evolution of crustal magmatic systems a summary and prospectus of experimental approaches it has recently been recognized that accessory minerals in continental rocks may contain a substantial fraction of the whole rock complement of trace elements and geochemically important isotopes this observation is in itself interesting but its importance is magnified many fold when one considers the possible consequences to the geochemical rearrangement that accompanies crustal melting if for example an accessory phase is insoluble in early formed partial melts it may effectively retain in the residue many incompatible elements e g ree and daughter isotopes e g radiogenic pb that would otherwise be partitioned into the melt in addition should an unmelted accessory mineral e g zircon be entrained in a departing melt fraction it may carry with it radiogenic components pb nd143 hf176 characteristic of the source region the effectiveness of phases such as zircon apatite sphene allanite epidote and monazite in the roles noted above can only be gauged from knowledge of the three fundamental accessory phase parameters i e 1 the solubilities of the accessory minerals in crustal melts 2 the equilibrium mineral liquid partition coefficients for the trace element and isotopes of interest and 3 the diffusivities that govern the rates at which equilibrium will be approached in general these parameters can only be measured in laboratory experiments and relatively few data have been accumulated to date it does seem clear however that apatite generally is residual insoluble during crustal melting and zircon can be depending on melting conditions and abundance of zircon in the source petrographic and geochemical studies indicate that monazite allanite and epidote may also be highly insoluble in some cases but the specific circumstances that promote insolubility remain unknown the few experimentally determined partition coefficients now available for apatite and zircon agree qualitatively with natural rock data on mineral separates however preliminary diffusion measurements and inferences from natural systems suggest that trace element and isotopic equilibrium between undissolved accessories and crustal melts is in fact rarely approached ironically it is this very disequilibrium in the form of preserved diffusion profiles that may afford the most information on time temperature histories of melting events continued experimental interest in the fundamental accessory phase parameters will eventually provide the tools that will enable rigorous assessment not only of the timing and duration of crustal melting episodes but also of the extent of melting and re distribution of trace elements and radiogenic isotopes

gravity profiles across the samail ophiolite oman four hundred and sixty gravity stations have been established over the samail ophiolite and adjacent units exposed in the oman mountains the bouguer anomaly pattern follows more or less parallel to ophiolite exposures along the coastal areas the bouguer anomaly values range from 20 to 140 mgal and increase northward four gravity profiles across the ophiolite are correlated with known geology and structure the main gravity profile extending 325 km northward from ibra to muscat and into the gulf of oman shows two positive bouguer anomalies 85 over the southern folded ophiolite nappe and 110 mgal near the shoreline at muscat that are associated with two nappes of tabular form the nappes are nearly continuous and there is no evidence that they are rooted in the mantle the gravity high across the southern nappe is centered over the axis of the synform maximum nappe thickness its position over the gabbro peridotite contact rather than over the peridotite suggests extensive serpentinization of the peridotite at the southern end of the profile in the ibra dome area the negative bouguer anomaly 10 to 15 mgal is interpreted as an infracambrian salt piercement similar to those in the zagros fold belt geologic modeling of the main gravity profile provides a good correlation in terms of known thicknesses of the mappable ophiolite units the nature of the gravity anomaly across the continental margin indicates a transition from 42 km thick continental crust to a thinner 16 18 km oceanic crust the two gravity profiles in the north oman region show a doublet of positive residual bouguer anomalies 50 to 90 mgal the anomalies correlate well with imbricated ophiolite exposures a result of low angle thrusting caused by strong compressive forces in the absence of imbricate thrusting the gravity profile in the central region has only one large positive residual bouguer anomaly 130 mgal the anomalies of all the four profiles are further analyzed to elucidate the nappe transport distances

anomalously thin crust in oceanic fracture zones new seismic constraints from the kane fracture zone three detailed and carefully positioned seismic refraction experiments have recently been carried out along the kane fracture zone near 24 n 44 w in order to place better geophysical constraints on the extent and origin of the anomalously thin crust reported from parts of this and other large atlantic fracture zones these three experiments when combined with earlier studies result in reversed structural control along about a 300 km long segment of the kane fracture zone extending from the ridge transform intersection out onto crust about 25 m y old an analysis of refraction lines obtained along the fracture zone trough by using both travel time modeling and delay time function techniques reveals large variations in crustal thickness and or velocity however along most of the eastern fracture zone trough upper mantle type velocities are found at depths of only 2 3 km below the seafloor less than half the typical depth to moho in the ocean basins this anomalous fracture zone crust is generally characterized by unusually low compressional wave velocities relatively high velocity gradients throughout most of the thinned crustal section about 1 2 s 1 and a distinct crust mantle boundary the very thin crust appears to be confined to the deepest parts of the kane fracture zone trough 10 km wide although a more gradual crustal thinning may extend up to several tens of kilometers from the fracture zone a specially designed delay time experiment at the intersection of the kane fracture zone and the mid atlantic ridge also revealed the presence of a thinned crustal section of a relatively uniform thickness about 3 km except beneath the nodal basin where the crust may be less than 1 km thick the anomalous crust in the intersection area appears to extend at least part way down the median valley we interpret this anomalous fracture zone crust as a primary accretionary feature that forms as a result of a restricted magma supply near ridge transform intersections although tectonic processes may also play a part at least locally in thinning the crust our results support a ribbon model of the gross seismic structure of the ocean basins in which bands of relatively homogeneous so called normal crust 50 100 km wide extend away from spreading centers each formed at a discrete ridge crest segment and each separated by the thin crust underlying the fracture zone trough

secular variation in greenstone sequence development emphasizing superior province canada abstract evidence from the superior province of the canadian shield suggests the existence of several distinct lithostratigraphic associations in greenstone belts 1 quartz arenite and carbonate bearing sequences 2 mafic ultramafic volcanic sequences 3 mafic to felsic volcanic cycles and 4 timiskaming type sequences of fluviatile sediments and calc alkaline to alkaline volcanics recently recognized quartz arenite bearing sequences in the sachigo and wabigoon subprovinces are from base to top quartz arenite stromatolite bearing carbonate oxide facies iron formation and komatiite or tholeiite primary structures in the quartz arenites indicate shallow water deposition and represent mature sedimentation granitoid provenance of these sediments is postulated based upon petrography of coarse arenites and clasts of associated conglomerates the few dated superior province quartz arenite bearing sequences are 2 85 ga the shallow water origin of the primary structures and biota and the areal extent of the quartz arenite and carbonate sequences invite comparison with platform sediments at passive margins platform sequences unconformably overlie 3 ga granitoid rocks and older mafic volcanics where original lower contacts are preserved in two localities mafic volcanic sequences contain sheet flows iron formations and deep water argillites based on physical volcanology and sedimentology these sequences are compared to modern deep water mafic plain and hawaiian volcanism they generally underlie mafic to felsic volcanic cycles and overlie platform sequences mafic sequences are 2740 2700 ma in age mafic to felsic volcanic cycles consist of deep water tholeiitic flows overlain by shallow water to subaerial intermediate to felsic pyroclastics and flows the facies style of volcanism petrography and geochemistry of the mafic to felsic cycles most closely resemble the characteristics of crust in continental magmatic arc systems this type of lithostratigraphic association varies in age from 2775 to 2700 ma in the superior province with minor exceptions timiskaming type sequences consist of fluviatile conglomerate immature arenites and wackes with subaqueous to subaerial volcanics of variable chemistry longitudinal and lateral variation within fault bounded basins is extreme and the sequences have generally been affected by only late stages of deformation the combination of rock types facies distribution the fault bounded nature of the repositories and the locus of sedimentation along zones of transcurrent displacement suggest timiskaming sequences are best compared with modern pull apart basins superior province timiskaming sequences are within the superior province platform sequences are the earliest postorogenic lithostratigraphic unit in greenstone belts followed by mafic plain sequences arc volcanics and pull apart basins we suggest that platform sequences based upon limited evidence are preceded by an orogeny and that pull apart basins represent a subsequent orogeny the lithostratigraphic units above have distinct metallogenic associations 1 platform sequences based upon compilation of several shields may contain detrital au and u and oxide facies iron formations associated felsic volcanics contain no recognized volcanogenic cu zn deposits in the superior province the following metallogenic associations are noted 2 mafic plain sequences contain komatiite flow related ni deposits cu ni and platinum group deposits are associated with flows and coeval intrusions 3 arc volcanic sequences contain volcanogenic cu zn sulphide deposits and scarce deposits of porphyry type cu mo and 4 timiskaming sequences and related plutonism are related to lode au

u pb chronology and duration of late ordovician magmatism in the english lake district u pb zircon ages are presented from igneous rocks in the english lake district for which a wide range of ages had been obtained previously by other methods the little stand and crinkle tuffs from the borrowdale volcanic group have ages of 451 6 1 4 and 452 8 0 7 ma 2 respectively the threlkeld microgranite 451 0 1 1 ma 2 and the carrock microgranite 452 4 4 1 ma 2 these dates are identical to the previously published dates for the ennerdale and eskdale granites and together are considered to be a good estimate for both the age and length of caradoc magmatism in the lake district this is the first of three magmatic events recognized in the lake district the other two occurring during ashgill and early devonian times regional comparisons with the leinster terrane in se ireland and the largely concealed english midlands basement emphasize the magnitude and significance of the caradoc event along the eastern avalonia margin

seismic constraints on the crustal structure within the vema fracture zone results from recent studies of the in well surveyed areas such as the famous area in kane and oceanographer fracture zones suggest that the central north atlantic ramberg et al 1977 significant variations in crustal thickness are transform faults are found every 50 to 100 km associated with parts of these fracture zones making them one of the most common tectonic with total crustal thicknesses in places less than features in the ocean basins morphologically half the thickness of normal oceanic crust the fracture zones on slowly spreading ridges display extent of this anomalous crust and its origin are many similar characteristics deep closed con still very poorly constrained by available geo tour depressions are usually present at ridge physical data here we report results from a 75 axis transform intersections sleep and biehler km long refraction line shot in late 1977 over 1970 the axis of the transform is typically three hawaii institute of geophysics ocean bottom defined by a deep linear trough bounded by escarpments rising eimom uuyuu withe ve valley near its western intersection with the mid transform valley floor a diverse suite of atlantic ridge 10o53 n 43o39 w we show that the apparent crustal thickness beneath this por tion of the vema transform valley is about 5 km not substantially different from crustal thick nesses typically associated with oceanic crust however the vema fracture zone crust is clearly anomalous and similar in many respects to crust reported from the kane and oceanographer fracture zones it is characterized by low compressional wave velocities a relatively uniform velocity gradient throughout most of the crust and the absence of a typical layer 3 refractor this anomalous crust is interpreted as intensely fractured hydrothermally altered and serpenti nized crustal and upper mantle rocks overlain by locally thick accumulations of basaltic rubble from the adjacent fracture zone walls the large variation in moho depths found in these large offset slowly slipping transforms is attributed to differences in the depth and extent of serpen tnnization of ultramafics lying beneath a thin and highly fractured basaltic and gabbroic layer one important consequence of the relatively low seismic velocities and densities inferred for the crust in these fracture zones is that transform valleys are not in local isostatic equilibrium with the surrounding seafloor

isotopic study of the manaslu granite himalaya nepal inferences on the age and source of himalayan leucogranites a detailed isotopic study of the manaslu leucogranite was carried out a u pb age of 25 ma and a whole rock rb sr age isochron of 18 ma were obtained suggesting that the magmatic activity lasted at least 7 ma initial sr isotopic ratios are very high 0 740 to 0 760 and initial nd isotopic ratios are low nd in 13 to 16 and they show the existence of large isotopic variations even at the metre scale these are not the result of perturbations by fluids but rather they reflect the initial isotopic heterogeneity of the source material which has not been obliterated by magmatic processes e g fusion mixing by convection these results also support the crustal origin of this leucogranite the tibetan slab paragneisses whose sr and nd isotopic ratios are very similar to those of the granite at an age of 20 ma are the most probable parental material nd model ages for both the leucogranite and the gneisses are in the range 1 5 2 ga a model of formation of the manaslu granite by coalescence of different batches of magma is in agreement with the present data

coupled sm nd and u pb systematics in late caledonian granites and the basement under northern britain the british late caledonian granites display a close correspondence between inherited u pb systematics in zircons and initial end indicating that both were controlled by the age of a major component of continental crust the data confirm and refine previous models of a basement comprising young crust in the south which includes arc and oceanic crust scrapings and progressively older continental crust northwestwards across scotland

allochthonous 2 78 ga oceanic plateau slivers in a 2 72 ga continental arc sequence vizien greenstone belt northeastern superior province canada embedded within the vast granitoid terrane of the minto block of northeastern superior province are late archean greenstone belts of the goudalie domain that preserve a long lived record of continent ocean interaction the vizien greenstone belt is one such belt and it contains four fault bounded structural panels the 2786 ma mafic ultramafic sequence is an allochthonous package of pillowed basaltic andesite komatiite and volcaniclastic rocks cut by peridotite and gabbro sills the mafic rocks are lree depleted tholeiites which have primitive mantle prim normalized abundances of th la nb and a range of nd values from 0 1 to 1 7 the 2722 ma lac serindac bimodal subaerial tholeiitic volcanic sequence contains andesite locally with tonalite xenoliths basalt gabbro sills lenses of quartz rich sedimentary rocks and a thick upper rhyolite sequence the lac serindac tholeiites are lree enriched have prim normalized th la nb high zr to 300 ppm and ti contents and low nd values from 0 8 in basalt to 1 4 in rhyolite the 2718 ma basement cover sequence comprises 2 94 ga tonalitic gneiss unconformably overlain by clastic sediments and a thin upper sequence of 2700 ma gabbro siliceous high mg basalt shmb and andesite the shmb are characterised by lree depletion and nd values of 2 6 whereas the andesite is lree enriched and has nd values of 0 3 the 2786 ma mafic ultramafic sequence is interpreted as a sliver of plume related oceanic plateau crust the 2724 lac lintelle sequence represents a continental arc formed on the eastern protocraton the 2722 ma lac serindac volcanic sequence represents late continental rift deposits the various 2 8 2 7 ga supracrustal sequences were accreted deformed and metamorphosed to mid amphibolite facies during late stage assembly of the minto block between 2 718 and 2 693 ga

u pb chronology of the ennerdale and eskdale intrusions supports sub volcanic relationships with the borrowdale volcanic group ordovician english lake district u pb analyses of zircons from the ennerdale granitic intrusion and eskdale granite english lake district give ages of 452 4 ma and 450 3 ma respectively these data confirm the sub volcanic association of these intrusions and show that previously published rb sr whole rock and k ar mineral ages do not correspond to the emplacement and cooling of these igneous bodies the new ages show that only two phases of acid magmatism can now be linked to the exposed parts of the lake district batholith the caradoc late ordovician phase was caused by the subductionrelated processes which gave rise to volcanism and was followed by a separate early devonian phase of granite emplacement

evolution of mantle derived augite hypersthene granodiorites by crystal liquid fractionation barrington tops batholith eastern australia the barrington tops batholith btb is a shallow level intrusive complex of permian age 262 ma in zone a of the new england fold belt northern new south wales it comprises a comagmatic suite of early pyroxene plagioclase phyric quartz diorite stocks and dykes 10 by area augite hypersthene granodiorite 75 and aplite less voluminous hornblende biotite granodiorite 15 appears to be related to the augite hypersthene granodiorite by the addition of minor amounts of an incompatible element rich component the augite hypersthene granodiorite plutons formed by fractional crystallisation of a dry 2 0 high temperature 1000 c liquid major element modelling using analysed phenocryst compositions closely reproduces the geochemical differentiation trends observed combined petrographic and mineral chemistry evidence indicates that parental liquids were of quartz dioritic and more mafic compositions probably mantle derived low initial 87 sr 86 sr 0 7036 0 7039 and high nd values 5 6 to 7 8 suggest these were produced from a geochemically depleted mantle source the btb may be regarded as an end member of the spectrum of calc alkaline granitic rocks that are formed by the mixing of variably fractionated melts of upper mantle origin with lower continental crust material it is suggested that lower crustal mafic and ultramafic xenoliths found m mesozoic to cenozoic basalts represent cumulates complementary to the more evolved mantle derived component of calc alkaline magmatism crystal liquid fractionation aided by mixing of evolved mantle derived magma with crustal material appears to be the most important process giving rise to the diverse geochemical and isotopic continuum displayed by calc alkaline magmatism m orogemc terrains

timing of magmatism and deformation in the kamiskotia kidd creek area western abitibi subprovince canada abstract the kamiskotia kidd creek area comprises well preserved granitoid greenstone belt lithologies of the western most abitibi subprovince similar in composition and structural style to other late archean granitoid greenstone terranes e g kalgoorlie norseman western australia belingwe zimbabwe slave province canada it contains the kidd creek volcanogenic massive sulfide deposit and lesser deposits of the kamiskotia district and lode gold deposits associated with the destor porcupine fault zone near the prolific porcupine gold district the magmatic and tectonic history for the area is provided through detailed structural mapping and high precision u pb geochronology field observations support the following sequence of magmatic and structural events in the kamiskotia area 1 formation of older mafic and intermediate volcanic rocks followed by the emplacement of the kamiskotia gabbroic complex kgc and cogenetic massive sulfide bearing bimodal volcanic rocks kv also accompanied by coeval intrusion of granitoid a into the crystallizing kgc 2 regional bulk rotation about a sub horizontal axis of this stratigraphy to form a ne facing monocline 3 emplacement of three large granitoid intrusions imparting their respective contact strain aureoles on the stratigraphy 4 regional sub horizontal bulk north south shortening that formed a non penetrative east west flattening fabric with an element of 5 late dextral transpression that formed high strain zones related to western destor porcupine fault zone dpfz within less competent volcanic units and 6 emplacement of alkalic dikes associated with the late stages of transpressive deformation u pb ages help substantiate this chronology and aid in the correlation of kamiskotia lithologies across the southern abitibi subprovince zircon ages for the kgc and a kamiskotia rhyolite are 2707 2 ma and 2705 2 ma respectively these ages are coeval with the reid rhyolite with a zircon age of 2706 2 ma but are significantly younger than the kidd creek rhyolite and the prosser rhyolite with zircon ages of 2717 2 ma data refined by the addition of an abraded concordant analysis and 2716 6 3 ma respectively two of the granitoid intrusions have zircon ages of 2696 1 5 and 2694 4 ma the older granitoid intrusion has a titanite population with a 207 pb 206 pb minimum age of 2692 5 ma which may represent a protracted cooling history or a metamorphic event after the setting of the zircon u pb system two zircon fractions from the younger granitoid intrusion exhibit inheritance with 207 pb 206 pb minimum ages up to 2926 ma indicating the presence of significantly older crust in this region timing of north south compression and late transpression may be constrained by a u pb garnet titanite age on a late to post tectonic alkalic dike at 2687 3 ma

adakites the key to understanding lile depletion in granulites abstract for more than 20 years it has been argued that granulites of the lower continental crust are depleted in fluid mobile elements such as cs rb u and th either because they were removed by melting of the lower crust or through dehydration we argue that there is little evidence for a simple relationship between granulite facies metamorphism and element depletion and propose that for felsic orthogneisses the depletion may be a primary feature of crust generation processes modern adakites show a range of fluid mobile element ratios and adakites from the austral volcanic zone avz chile show fluid mobile element ratios very similar to those found in lower crustal felsic granulites this suite of adakites has a strong slab derived component we propose that modern adakites of the type found in the austral volcanic zone are analogous in their genesis to lower crustal felsic granulites and that their mafic precursors were already depleted in fluid mobile trace elements prior to melting an incremental melting model in which fluid mobile elements are released from an eclogitic slab prior to partial melting can explain the fluid mobile element behaviour of both modern adakites and lower crustal felsic granulites

structural constraints on the tectonic evolution of a late archean greenstone belt in the northeastern superior province northern quebec canada abstract a detailed structural analysis shows that the vizien greenstone belt of northeastern superior province records five generations g1 to g5 of ductile deformation as well as brittle faulting g1 is indicated by the local presence of a pre g2 foliation and is interpreted to be related to thrusting g2 is responsible for the main penetrative foliation s2 axial planar to tight to isoclinal f2 folds and subparallel to major boundaries between lithotectonic panels it is concentrated in a thrust with sense of shear opposite to that interpreted for g1 f3 and f4 folds are open to tight with nnw sse and e w trending axial surfaces respectively they warp the s2 foliation and major panel boundaries and dominate the map pattern g5 is associated with dextral transcurrent movement along a nnw sse trending shear zone through understanding of its geometry and deformation history the vizien belt has been unfolded into its much simpler pre g3 geometry this greatly helps in the investigation of relationships between the different panels proposed terranes their tectonic settings and the kinematics of panel amalgamation the structural study in concert with geological and geochemical work leads to a tectonic model for the evolution of the greenstone belt which involves subduction of oceanic lithosphere and continental arc volcanism at 2724 ma intra arc rifting at 2722 ma and collision between 2718 and 2693 ma with associated obduction of oceanic plateau crust formation of melange and deposition in a possible foreland basin the g1 and g2 deformations are interpreted to be related to synthetic thrusting and back thrusting respectively during the collision

the ammonium content of caledonian granites the caledonian granites of the british isles have nh 4 contents in the range of 0 167 parts per million high contents 60 ppm are confined to muscovite rich granites occurring principally in a restricted area namely se ireland and the isle of man their magmas are inferred to have formed by the partial melting of a sedimentary protolith rich in organic matter there is no apparent correlation between nh 4 content and initial 87 sr 86 sr ratios

nitrogen isotope record of fluid rock interactions in the skiddaw aureole and granite english lake district the skiddaw granite and its contact metamorphic aureole in the english lake district provide an excellent opportunity to test the capability of the n isotope system to trace devolatilization and large scale transfer of crustal fluids in the aureole skiddaw group metasedimentary rocks with relatively uniform lithology and major element compositions show a dramatic decrease in n content toward the granite contact from or 800 ppm at distances 2 5 km from the contact to 1 5 km these rocks have extremely uniform delta 15 n air near 3 7 per mil whereas closer to the contact or 1 km delta 15 n is shifted to higher values up to 8 7 per mil the coupled decreases in n content and increases in delta 15 n are compatible with the removal of n having low delta 15 n in fluids during continuous prograde devolatilization reactions involving the break down of white mica and the stabilization of biotite cordierite and andalusite bearing assemblages in the same metasedimentary rocks the lack of obvious trends in major element concentrations including sio 2 tio 2 sio 2 al 2 o 3 and the ratios of other major oxides to tio 2 and al 2 o 3 with distance from the granitic contact is consistent with minimal change in major element composition during the contact metamorphism ratios of whole rock n b rb and ba concentrations to whole rock k 2 o content are believed to reflect the differing fluid mica partitioning and involving varying relative proportions of white mica and biotite of these trace elements during devolatilization reactions greisenized skiddaw granite from a borehole is enriched in n range of 17 225 ppm for whole rocks and white mica separates relative to the unaltered granite whole rock 30 ppm and has delta 15 n of 1 0 to 4 8 per mil the n concentrations and delta 15 n of the wall rocks and greisenized granites combined with c isotopic data carbonate and carbonaceous matter for the same rocks are consistent with the mobilization of fluids having low delta 15 n and delta 13 c pdb values from the devolatilized aureole into the cooling intrusive body such transport is consistent with the predictions of recent theoretical models of late stage hydrothermal evolution in cooling intrusive systems

comment on crustal contributions to arc magmatism in the andes of central chile by w hildreth and s moorbath in their paper crustal contributions to arc magmatism in the andes of central chile hildreth and moorbath conclude that the stratovolcanos forming the volcanic front at the northern end between 33 37 5 s of the southern volcanic zone svz 33 46 s of the andes show along arc changes in isotopic and elemental ratios that demonstrate a northward increase in crustal contributions to magma genesis stern et al 1984a b futa and stern 1988 and stern 1988 previously reached this same conclusion by comparing the chemistry of three of the northernmost volcanos in the svz between 33 34 s with those from south of 36 s however the processes by which crustal components are incorporated in andean magmas and specifically what is causing crustal components to be incorporated in increasing amounts in the magmas erupted at the northern end of the svz remain controversial with regard to andean volcanism in general hildreth and moorbath suggest that although the principal magma source is presumed to lie within the mantle wedge fluxed by a slab derived contribution the base level isotopic and chemical values for each volcano are established by blending of subcrustal and deep crustal magmas in zones of melting assimilation storage and homogenization mash at the mantle crust transition they further suggest that the along arc changes in isotopic and elemental ratios observed at the northern end of the svz are due to changing composition age and most significantly increased proportional contribution of the lowermost crust in conjunction with increased depth to the mash zone as the crust thickens northward below the volcanic front in contrast stern et al 1984a futa and stern 1988 and stern 1988 1991 argue that the distinctive petrochemical characteristics of the volcanoes at the northernmost end of the svz reflect a northward change in the composition and amount of continental components subducted into the mantle source region hildreth and moorbath s preference for intra crustal upper plate influence on the along arc variations in magma chemistry at the northern end of the svz appears to be based on both geological and geochemical arguments that i find in many cases debatable i discuss these separately below followed by a brief comment on the relative effectiveness of lower crustal mash versus mantle source region contamination processes in explaining along arc chemical variations observed in andean magmas

the geochemical characteristics of granitoids in contrasting arcs and comments on magma sources geochemical variations in modern subduction related igneous suites with respect to arc maturity in time and space are illustrated using data for both volcanic suites basalt andesite and dacite and intrusive granitoid suites diorite tonalite granodiorite and granite from circum pacific arcs using trace element data we suggest that two groups of processes control the natural variation in the parental magmas of both suites a subduction zone enrichment of lithospheric mantle locally coupled with crustal assimilation allied with fractional crystallization afc in zones of thickened crust all of which yield magmas with enhanced concentrations of the large ion lithophile lil elements k rb th u lrees etc b with increasing distance from the active trench contributions from within plate sub continental lithosphere producing mantle derived magmas with enhanced levels of high field strength hfs elements among which nb ta hf and y are particularly distinctive thus even for the evolved granitoids of intrusive arc series ratios of hfs lil elements not significantly affected by crystal fractionation such as ta nb k rb la may throw some light on the origin of mafic intermediate precursor magmas in terms of these elements we suggest the following groupings for interpreting the tectonic associations of granitoid suites 1 primitive calcic arc granitoids with low lil and hfs element abundances 2 normal calc alkaline continental arc granitoids with enhanced lil element abundances and low hfs lil ratios 3 mature alkali calcic arc granitoids with high levels of lil and hfs elements and higher hfs lil ratios 4 back arc anorogenic alkaline granitoids with the highest levels of hfs elements

the nagercoil charnockite a magnesian calcic to calc alkalic granitoid dehydrated during a granulite facies metamorphic event the paleoproterozoic nagercoil charnockite massif in southern india consists of garnet absent and garnet bearing varieties with garnet occurring only in those areas where the charnockite incorporates supracrustal lithologies there is a late neoproterozoic cambrian granulite facies metamorphic overprint on the charnockite massif this study examines whether orthopyroxene in the charnockites crystallized during the original magmatic stage or formed during the later high grade metamorphic event the textural association of orthopyroxene and relic garnet grains and reaction intergrowths of garnet and or biotite with orthopyroxene similar to those reported in dehydration zones within the gneisses surrounding the charnockite massif show that orthopyroxene in the charnockite massif formed during the late high grade metamorphic event from dehydration reactions and is not part of the original igneous assemblage metasomatic textures such as thin k feldspar rims together with the presence of co2 rich fluid inclusions in these rocks suggest that co2 metasomatism widely prevalent in the gneisses surrounding the massif probably resulted in the low water activity necessary to form orthopyroxene during the metamorphic overprint an oxygen isotope traverse along a metapelite enclave charnockite contact within the massif shows a distinct variation in d 18 o values with the charnockite away from the contact having the lowest d 18 o value 8 9o similar to that of typical metaluminous granitic rocks the charnockite at the contact shows an intermediate d 18 o value 10 3o and the surrounding metapelite has the highest d 18 o value 12o the intermediate d 18 o value indicates partial assimilation of metapelite with a high d 18 o value into the charnockite with a low d 18 o value both the garnet absent and garnet bearing charnockites have similar compositional ranges to tonalite granodiorite granite ttg suites irrespective of whether garnet is present or not the sr contents of the charnockites show a bimodal distribution a high sr group which is magnesian calcic to calc alkalic metaluminous showing ttg affinities and a low sr group which is dominantly ferroan calcic to calc alkalic metaluminous showing similarities to the orthopyroxene bearing dehydration zones petrogenetic characterization suggests that the protoliths of the nagercoil charnockites were high temperature melts of hydrous basalt

geochemical evolution of the minto block a 2 7 ga continental magmatic arc built on the superior proto craton abstract a 400 km long by 100 km wide geological transect across the minto block of northeastern superior province reveals five plutonic suites and minor amphibolite to granulite grade supracrustal remnants the oldest regionally significant plutonic rocks include 3 0 0 1 ga foliated to gneissic tonalite characterized by high na k ratios high al contents light ree enrichment and initial gend 1 8 the leaf river plutonic suite is the most widespread with crystallization ages of about 2725 5 ma across the transect the suite comprises massive to foliated pyroxene and hornblende bearing granodiorite tonalite granite diorite and minor gabbro pyroxenite and syn plutonic mafic dykes the rocks are i type calc alkaline have variable na k ratios moderate to high ba and sr contents and ree patterns with variable light ree enrichment depletion of heavy rees with increasing silica and neutral negative or positive eu anomalies initial nd values for the suite range from 0 5 to 1 3 mean 0 4 and initial 87 sr 86 sr ratios isr are 0 7020 the diatexite plutonic suite 2713 ma occurs within a 150 km wide zone and consists of orthopyroxene garnet granodiorite containing 25 50 inclusions of granulite grade mafic supracrustal and tonalitic inclusions the variable major and trace element chemistry of the suite with generally high ni and cr contents reflects heterogeneous source materials and the presence of non liquid components initial nd values are from 0 1 to 1 7 monzogranite and orthopyroxene granite dykes plugs and plutonic masses both 2690 ma comprise the last important plutonic suites both biotite monzogranite and orthopyroxene granite are light ree enriched but the former have negative eu anomalies and the latter positive initial nd values range from 0 7 to 3 2 for monzogranite isr 0 7016 to 0 7067 and 0 7 to 0 2 for orthopyroxene granite isr 0 701 to 0 702 the initial nd and isr values of the 2 7 ga plutonic suites are lower and higher respectively than estimatesf of the contemporaneous depleted mantle suggesting significant and wide scale reworking of older tight ree enriched high rb sr lithosphere early tonalite orthogneiss from goudalie domain may be derived by partial melting of mafic crust at high pressures the lithological and geochemical diversity of the 2725 ma leaf river plutonic suite indicate a range of parental magma compositions and petrogenetic processes but fractional crystallization of mantle derived basaltic melts which had assimilated small amounts of significantly older lower crust may have been the dominant mechanism crustal inclusion charged granodioritic diatexite 2713 ma was probably derived by partial melting of heterogeneous lower crustal materials under relatively dry conditions late biotite monzogranite was generated by partial melting of older sialic crust and orthopyroxene granite by deeper melting or fractional crystallization of more juvenile source materials with garnet as a residual mineral we infer from the presence of coeval mafic dykes in all plutonic suites that heat for melting was ultimately derived from intrusion of basaltic magmas at depth the 2 7 ga magmatism in the minto block occurred during a period of active production and accretion of juvenile oceanic terranes in the southern superior province the northwest directed subduction regime inferred from the southern superior province arc terranes may also have been responsible for the magmatism that acted on 3 ga lithosphere in the northern superior province melting within the mantle wedge beneath the proto cratonic lithosphere and in the lower crust occurred within a tectonic setting similar to that of modem continental magmatic arcs

a combined geochronological approach to investigating long lived granite magmatism the shap granite uk abstract with the advent of more precise dating methods it has become apparent that zircon dates from granite plutons frequently indicate older emplacement ages than other dating methods here we attempt to reconcile a number of dating methods from the c 5 km2 caledonian shap granite northern england the results reveal a more complex and protracted evolution than indicated by application of any single dating method zircon u pb dates give a weighted mean age of 415 6 1 4 2 ma a mafic enclave dated at 412 2 2 ma revised rb sr feldspar age from davidson et al 2005 contains resorbed k feldspar and zircon crystals scavenged from the host crystal mush these ages are at odds with field relations in the thermal aureole that suggest final emplacement at approximately 404 ma or later during acadian deformation previously reported re os ages on molybdenites associated with magmatic fluids have given ages of 405 2 1 8 2 ma selby et al 2008 and confirm the overlap of at least some magmatic activity with acadian deformation a similar emplacement age is supported by rb sr whole rock mineral and biotite k ar dates when adjusted for revised decay constants 402 3 ma and 401 7 ma respectively wadge et al 1978 the lower closure temperatures of these systems relative to the u pb system in zircon means that they are more likely to record the timing of final granite emplacement these data suggest that most zircons grew before final granite emplacement by about 10 ma on average we suggest that the majority of zircon crystals record pre emplacement magmatic activity within a deeper part of the system mafic enclaves and their scavenged cargo of crystals record the assembly of a mid crustal batholith where crystals remained at least locally mobile at 412 ma gravity data support the existence of an extensive 1500 km2 intrusive body originally at about 15 km depth beneath shap this batholith is likely to have remained below the granite solidus for much of its existence due to conductive heat loss but episodic influxes of silicic magma between c 412 and 405 ma are thought to have enabled periods of rejuvenation these influxes are recorded by complex compositional zoning patterns within k feldspar megacrysts the shap granite itself is likely to represent a rejuvenated crystal slurry emplaced as a cylindrical cupola above the main magma body during acadian transpression this study highlights the importance of integrating different dating techniques and that final emplacement of granites can only be indicated by the youngest zircon ages

crystallization of the shap granite evidence from zoned k feldspar megacrysts textural chemical and sr isotopic studies of feldspars from the subsolvus shap granite northern england demonstrate that a number of magma mixing events have dominated the evolution of this pluton k feldspar megacrysts are phenocrysts formed in the magma chamber they contain a number of ba rich zones that developed during periods of slight dissolution and regrowth linked to the hybridization of the granite by the intrusion of basic magmas diorite enclaves represent the relicts of these magmas and these also contain k feldspar megacrysts which show evidence of major dissolution they are xenocrysts picked up from the host granite and incorporated in the basic magma increasing h 2 o contents during fractional crystallization caused a late switch from growth of megacrysts to finer grained k feldspars in the matrix the chemically and isotopically zoned k feldspar megacrysts preserve an exceptional record of the evolution of the magma and the zones also had a significant influence on the development of exsolution microtextures during cooling