crustal reworking at nanga parbat pakistan metamorphic consequences of thermal mechanical coupling facilitated by erosion within the syntaxial bends of the india asia collision the himalaya terminate abruptly in a pair of metamorphic massifs nanga parbat in the west and namche barwa in the east are actively deforming antiformal domes which expose quaternary metamorphic rocks and granites the massifs are transected by major himalayan rivers indus and tsangpo and are loci of deep and rapid exhumation on the basis of velocity and attenuation tomography and microseismic magnetotelluric geochronological petrological structural and geomorphic data we have collected at nanga parbat we propose a model in which this intense metamorphic and structural reworking of crustal lithosphere is a consequence of strain focusing caused by significant erosion within deep gorges cut by the indus and tsangpo as these rivers turn sharply toward the foreland and exit their host syntaxes the localization of this phenomenon at the terminations of the himalayan arc owes its origin to both regional and local feedbacks between erosion and tectonics

precision of seafloor gravity and pressure measurements for reservoir monitoring changeswithgravityovertimehaveproventobevaluable forinferringsubsurfacedensitychangesassociatedwithproduction from oil and natural gas reservoirs such inferences allowthemonitoringofmovingfluidfrontsinareservoirand provideanopportunitytooptimizeproductionoverthelifeof the reservoir our group began making time lapse seafloor gravityandpressuremeasurementsin1998 todate wehave surveyed sixfields offshore norway we have made three repeatsurveysatonefieldandonerepeatsurveyatanother we incorporated a land gravity sensor into a remotely operated seafloor housing three such relative gravity sensors mountedinasingleframearecarriedbyaremotelyoperatedvehicle rov to concrete benchmarks permanently placed on the seafloor reference benchmarks sited outside the reservoir boundaries are assumed to provide stable fiducial points typicalsurveyslastfromafewdaystoafewweeksandcover from8to80benchmarks withmultipleobservationsofeach in our earliest surveys we obtained an intrasurvey repeatability of approximately 20 gal but recently we have been achieving 3 gal repeatability in gravity and approximately 5 mminbenchmarkdepthdeducedfromsimultaneouslyrecorded ambient seawater pressure we attribute the improved precision to several operational factors including the use of multiple gravity sensors frequent benchmark reoccupation precise relocation and orientation of the sensors repeated calibrations on land and minimization of vibrational and thermal perturbations to the sensors we believe that high precision time lapse gravity monitoring can be used to trackchangesintheheightofagas watercontactinaflooded reservoir withaprecisionofafewmeters

timing and tectonic setting of grenvillian metamorphism constraints from a transect along georgian bay ontario systematic mapping of a transect along the well exposed shores of georgian bay ontario combined with the preliminary results of structural analysis geochronology and metamorphic petrology places some constraints on the geological setting of high grade metamorphism in this part of the central gneiss belt correlations within and between map units gneiss associations have allowed us to recognize five tectonic units that differ in various aspects of their lithology metamorphic and plutonic history and structural style the lowest unit which forms the footwall to a regional decollement locally preserves relic pre grenvillian granulite facies assemblages reworked under amphibolite facies conditions during the grenvillian orogeny tectonic units above the decollement apparently lack the early granulite facies metamorphism out of sequence thrusting in the south produced a duplex like structure two distinct stages of grenvillian metamorphism are apparent the earlier stage c 1160 1120 ma produced granulite facies assemblages in the parry sound domain and upper amphibolite facies assemblages in the parry island thrust sheet the later stage c 1040 1020 ma involved widespread dominantly upper amphibolite facies metamorphism within and beneath the duplex deformation and metamorphism recently reported from south and east of the parry sound domain at c 1100 1040 ma have not yet been documented along the georgian bay transect the data suggest that early convergence was followed by a period of crustal thickening in the orogenic core south east of the transect area with further advance to the north west during and after the waning stages of this deformation

nature and distribution of deep crustal reservoirs in the southwestern part of the baltic shield evidence from nd sr and pb isotope data on late sveconorwegian granites late sveconorwegian postorogenic granites c 1 0 0 93 ga make up a voluminous and widespread suite of intrusions across south norway from radiogenic isotope data three groups of late sveconorwegian granites can be distinguished 1 granite with more than 150 ppm sr 87 rb 86 sr 87 sr 86 sr 0 93 ga nd 87 rb 86 sr 5 87 sr 86 sr 0 93 ga 0 710 and e nd 87 sr 86 sr 0 93 ga nd 0 granite plutons belonging to group 1 normal sr concentration granite occur all over south norway and include the largest batholiths ostfold fla herefoss granite plutons of group 2 low sr concentration granites are restricted to north central telemark rjukan rift and are associated with c 1 5 ga rjukan group rhyolite group 3 is represented by one intrusion only but still suggests input of mantle derived magma or the presence of young mantle derived rocks in the deep crust in the region at c 0 93 ga the group 1 granites are similar in sr and nd characteristics to some of the older 1 05 ga sveconorwegian granitic intrusions augen gneisses in the region in terms of radiogenic isotope systematics but pb isotopes suggest that the magmas did not form by simple remelting of augen gneiss the nd sr and pb isotopic systematics of the late sveconorwegian granites indicate mixing between a depleted mantle derived component and two or more major components with an extended crustal history one of the crustal end members is present throughout south norway and has an isotopic signature similar to older granitic rocks of the trans scandinavian igneous belt tib the other crustal end member is indistinguishable from rjukan group rhyolites and slightly younger intrusions associated with these and is restricted to areas within the mid proterozoic rjukan rift the available data suggest that the deep continental crust of south norway both east and west of the permian oslo rift is an integral part of the baltic shield with a common history back to 1 7 1 9 ga that is to the end of the svecofennian orogeny and the tib magmatism

the farsund intrusion sw norway a marker of late sveconorwegian grenvillian tectonism emplaced along a newly defined major shear zone abstract the ca 930 ma farsund intrusion sw norway belongs to a series of 0 99 0 92 ga post collisional plutons from the sveconorwegian grenvillian orogen it is made of two rock facies charnockite and quartz mangerite and subordinate quartz monzonite and quartz monzodiorite that show mingling relationships as shown elsewhere these two facies belong respectively to the two suites of a type affinity recognized in the sveconorwegian post collisional magmatism of southern norway namely the amc and hbg suites a structural study of the farsund intrusion based on the anisotropy of magnetic susceptibility ams technique is presented here the ams is controlled by the shape preferred orientation of low ti titanomagnetite grains and it can be used as a proxy for the global petrofabric the ams data when combined with micro to macrostructural observations unfold the occurrence of a steeply dipping shear zone straddling the ne border of the pluton and characterized by a likely strike slip component of shearing this high strain zone is roughly coincident with the boundary between the outcrop domains of the amc and hbg suites and was formed or more probably reactivated during the gravitational collapse of the sveconorwegian orogen in an extensional possibly transtensional tectonic regime it would have controlled the ascent and emplacement of the farsund intrusion and materializes as well the structural weakness that would have channelled the magmas of the neighbouring and coeval rogaland anorthosite province it is also suggested that vertical gravity driven movements were recorded in the farsund intrusion and its close surroundings they would have been induced in a very hot environment akin to that prevailing in the precambrian ultra hot orogens but linked in the present case to the emplacement of anorthosites and penecontemporaneous igneous bodies including the farsund intrusion

sedimentary recycling in arc magmas geochemical and u pb hf o constraints on the mesoproterozoic suldal arc sw norway the hardangervidda rogaland block within southwest norway is host to 1 52 to 1 48 ga continental building and variable reworking during the 1 1 to 0 9 ga sveconorwegian orogeny due to the lack of geochronological and geochemical data the timing and tectonic setting of early mesoproterozoic magmatism has long been ambiguous this paper presents zircon u pb hf o isotope data combined with whole rock geochemistry to address the age and petrogenesis of basement units within the suldal region located in the centre of the hardangervidda rogaland block the basement comprises variably deformed grey gneisses and granitoids that petrologically and geochemically resemble mature volcanic arc lithologies u pb ages confirm that magmatism occurred from 1 521 to 1 485 ma and conspicuously lack any xenocrystic inheritance of distinctly older crust hafnium isotope data range from ehf initial 1 to 11 suggesting a rather juvenile magmatic source but with possible involvement of late palaeoproterozoic crust oxygen isotope data range from mantle like 18o 5 to elevated 10 suggesting involvement of low temperature altered material e g supracrustal rocks in the magma source the hf o isotope array is compatible with mixing between mantle derived material with young low temperature altered material oceanic crust sediments and older low temperature altered material continent derived sediments this combined with a lack of xenoliths and xenocrysts exposed older crust afc trends and s type geochemistry all point to mixing within a deep crustal magma generation zone a proposed model comprises accretion of altered oceanic crust and the overlying sediments to a pre existing continental margin underthrusting to the magma generation zone and remobilisation during arc magmatism the geodynamic setting for this arc magmatism is comparable with that seen in the phanerozoic e g the sierra nevada and coast range batholiths with compositions in the suldal sector reaching those of average upper continental crust as within these younger examples factors that drive magmatism towards the composition of the average continental crust include the addition of sedimentary material to magma source regions and delamination of cumulate material underthrusting of sedimentary materials and their subsequent involvement in arc magmatism is perhaps a more widespread mechanism involved in continental growth than is currently recognised finally the suldal arc magmatism represents a significant juvenile crustal addition to sw fennoscandia

water magma interaction some theory and experiments on peperite formation experiments using molten thermite as a magma analog produce peperite when the melt interacts with wet sand these experiments also show explosive behavior developing strombolian and surtseyan like bursts the results demonstrate that the application of fuel coolant interaction fci theory is appropriate for interpretation of peperites the theory described includes discussion of the importance of mass interaction ratios of wet sediment and magma rs which determine thermal equilibrium temperature limits and contact interface dynamics the dynamics of the interface between magma and wet sediments involves heat transfer over a wide range of rates from passive quenching to explosive fragmentation a vapor film layer develops at the interface and acts both as an insulating barrier promoting passive quenching as well as a potential energy reservoir that can cause magma fragmentation mingling of the magma with wet sediments and explosive quenching when the vapor film becomes unstable an important parameter in determining the behavior of the vapor film is the value of rs which controls whether heat can be convectively removed from the layer as more is being added from its contact with magma if rs 1 for fully saturated sediments there is enough water in the sediments to make convective heat flow effective in quenching the magma but below that value there is the potential that the vapor film will be unstable producing highly dynamic phenomena including explosive fragmentation at values of rs 0 1 there is insufficient water to allow the escalation of explosive fragmentation

experimental resetting of the u th pb systems in monazite abraded fragments 200 400 m of a large chemically homogeneous and non metamict brazilian monazite crystal characterised by a concordant u pb ages of 474 1 ma 208pb 206pb 19 5 were hydrothermally treated at varying temperatures with solutions of different compositions product monazites were analysed with scanning electron microscope sem electron microprobe emp secondary ion mass spectrometer sims and isotope dilution thermal ionisation mass spectrometer id tims experiments with pure water over a temperature range of 800 1200 c at 700 mpa and durations ranging from 5 to 60 days showed that even at 1200 c any dissolution and recrystallization of new monazite is confined to the outermost surface of the grain neither pb diffusion at the emp scale nor significant discordancy were observed we performed experiments at 800 and 1000 c for different durations using different fluid compositions at quartz saturation a 10 wt cacl2 fluid a 10 wt srcl2 fluid a 10 wt nacl fluid and a fluid containing nbs 982 pb standard 208pb 206pb 1 for all runs emp traverses revealed no pb diffusion profiles significant overgrowths of newly formed monazite are documented by sem analyses they occurred only in the 1000 c experiments when either cacl2 or pb bearing fluids were present in the cacl2 experiment two zones could be distinguished within the crystal a core possessing the initial monazite composition and a rim consisting of newly formed monazite produced by dissolution precipitation which was enriched in ca and pb free id tims dating of single grains treated with srcl2 and cacl2 solutions at 1000 c are significantly discordant experiments employing the nbs pb standard produced sub concordant monazite for which the 207pb 206pb apparent age has become older than prior to the experiment 544 ma at 800 c and 495 ma at 1000 c the newly grown monazite rim had obviously incorporated pb from the fluid none of our reaction products contained a detectable diffusion profile the only resetting mechanism we detected involved dissolution precipitation the extent of the dissolution precipitation process depends on fluid composition and is a more efficient mechanism than diffusion for controlling the resetting of monazite in natural rocks

pre grenvillian evolution and grenvillian overprinting of the parautochthonous belt in key harbour ontario u pb and field constraints the parautochthonous belt in the region of key harbour ontario is composed of early proterozoic migmatitic para and orthogneiss and mid proterozoic granitoids which were reworked during the grenville orogeny grenvillian deformation is localized into anastomosing arrays of high strain shear zones enclosing elongate bands and lozenges of rock subjected to lower and near coaxial strain crosscutting relationships preserved in the low strain domains document two pre grenvillian plutonic and tectonometamorphic events which are bracketed in age by u pb zircon geochronology a 1694 ma leucogranite intrudes and provides a minimum age for high metamorphic grade gneisses formed during an earlier tectonometamorphic event d1 m1 the leucogranite was intruded by mafic dykes deformed and metamorphosed at uppermost amphibolite facies during d2 m2 before the emplacement of mid proterozoic granitoids at ca 1450 ma following the emplacement of gabbro dykes and pods at ca 1238 ma the area was overprinted by

a flow law for dislocation creep of quartz aggregates determined with the molten salt cell we have used the molten salt cell to conduct an experimental study on the rheology of a natural quartzite containing 0 15 wt water co axial deformation experiments were conducted at constant piston displacement rates approximating constant strain rates at low strain the strengths of our natural quartzite measured in the molten salt cell are approximately half those measured at the same conditions in solid media because unlike solid confining media molten salt does not contribute to the strength of the sample it reduces the friction on the moving piston and it allows clear identification of the hit point we have limited the experimental conditions to those required for dislocation creep and have used only steady state flow stresses measured during climb accommodated dislocation creep to calculate the flow law parameters two flow laws were determined one for samples containing minor amounts of melt 1 2 and one for melt free samples in both cases the power law stress exponent n is 4 0 0 9 which is greater than that previously reported in flow laws for dislocation creep of quartz aggregates determined in solid media the activation energy q is 137 34 kj mol 1 for samples with melt and 223 56 kj mol 1 for those without within the range of previously determined values for quartz aggregates containing 0 1 wt water the pre exponential term a is 1 1 10 4 2 mpa ns 1 for samples without melt and 1 8 10 8 2 mpa ns 1 for those with melt the lower strengths measured in the molten salt cell indicate that previous piezometer relations for quartz experimentally determined in solid media are not correct extrapolation of the flow law for melt free aggregates to natural strain rates predicts higher strengths than most previous quartz flow laws however accurate extrapolation requires determining the dependence of flow stress on fh2o and or ah

the sudbury dyke swarm and its bearing on the tectonic development of the grenville front ontario canada abstract in the southern province ontario southeast striking diabase dykes of the 1235 ma sudbury continental swarm intrude folded huronian strata and paleo and mesoproterozoic plutons the dykes are truncated at the grenville front southeast of which irregularly oriented metadiabase dykes correlative in age and chemical composition with the sudbury swarm cut gneissic fabrics in the grenville province some irregularities in dyke trend including marked deflections in strike at and southeast of the grenville front mylonite zone are primary features resulting from dyke propagation across pre existing structure and an associated change in paleostress field however other irregularities in particular the markedly sinuous traces of many dykes analogous to classic buckle fold forms are attributable to superimposed grenvillian deformation the deformation history of the dykes is constrained by a distinctive sequence of macroscopic and microscopic structures early deformation related to northwest southeast compression and accompanying overthrusting resulted in layer parallel shortening and buckling of the dykes in a regime with elements of both pure and simple shear a process accommodated internally by alignment bending and kinking of plagioclase and other primary minerals coeval metamorphism manifested by reaction coronas around primary olivine and fe ti oxide outlasted early deformation a later period of deformation resulted in tightening of the buckle folds superseded by faulting in response to further strain accumulation the equivalent microscopic scale structures are late fractures and brittle ductile microfaults which post date plastic strain a cause and effect relationship whereby pre metamorphic microstructures strongly influenced the location and development of syn to post metamorphic microstructures suggests that the two phases of dyke deformation represent more or less a continuum separated only by a period of decreased strain rate coinciding with the peak of metamorphism in relation to existing models for the tectonic development of the northwestern grenville orogen the progression of dyke structures and their relationship to metamorphism are best explained by three principal stages 1 at 1035 ma an early stage of penetrative shortening and overthrusting correlated with buckling of the dykes and their depression to lower crustal levels 2 an intermediate stage commencing some time before 1020 ma when compressive stresses waned enabling thermal relaxation and ductile extension took place in the interior of the orogen initiating passive uplift of the dykes 3 at 1010 980 ma a stage of renewed thrusting during which the dykes were further deformed as they were uplifted across the ductilebrittle transition documenting the final advance of the grenville orogen toward its foreland

sveconorwegian massif type anorthosites and related granitoids result from post collisional melting of a continental arc root article i nfo two magmatic suites were emplaced during the post collisional evolution of the sveconorwegian orogeny an anorthosite mangerite charnockite suite amc suite and an hornblende and biotite bearing granitoids suite hbg suite the amc suite is exclusively located in the westernmost and warmest part of the orogen in granulite facies gneisses whereas the hbg suite intruded in the rest of the orogen but not in the granulite domain new u pb zircon geochronological data confirm previous age determinations 970 932 ma hbg suite and 933 916 ma amc suite the mafic facies of the two post collisional magmatic suites have similar geochemical compositions but the hbg differentiation trend displays higher cao sr u and th as well as lower k2o and feot mgo than the amc differentiation trend the hbg suite is hydrous and has a broadly higher fo2 whereas the amc suite is anhydrous the inferred parent magmas of both suites have overlapping initial sr nd and pb isotopic compositions with increasing differentiation the two trends point towards two different crustal contaminants together with the recent recognition of a major crustal shear zone located just east of the amc suite this difference suggests that the suites were emplaced in two different lithotectonic units using published experimental constraints and geochemical modeling we suggest that the mafic facies of both suites were produced by partial melting of lower crustal sources which were previously underplated probably during the evolution of a long lasting convergent margin later these lower crustal sources were modified by the regional granulite facies metamorphism 1 035 to 0 97 ga prevailing in the westernmost part of the orogen thus producing an anhydrous lower crustal source for the amc suite accordingly we conclude that the sveconorwegian massif type anorthosites result from partial melting of the continental arc root this process if accepted for other amcg anorthosite mangerite charnockite granite complexes was possible in the proterozoic because of a sufficiently high temperature but not in the archean because in subduction zones the main transfer to the crust was then felsic tonalites granodiorites and not basaltic we thus further suggest that the onset of massif type anorthosites at the beginning of the proterozoic may mark the time when plate tectonics began to operate in a similar way as today

docking of the central metasedimentary belt to laurentia in geon 12 evidence from the 1 17 1 16 ga chevreuil intrusive suite and host gneisses quebec the chevreuil intrusive suite 1 17 1 16 ga represents a chronological field marker of regional extent that intruded the central metasedimentary belt in the western grenville province of quebec after peak metamorphism style and site of magma emplacement and extent of deformation of chevreuil plutons and dykes permit unravelling of the early grenvillian evolution of the belt with respect to cratonal north america the suite comprises a series of vertically layered gabbro stocks and monzonite diorite gabbro sheet intrusions and a swarm of microdiorite dykes that cut across gneisses the dykes display systematic variations in extent of deformation across the belt we targeted u pb geochronology on gneisses within the identified strain windows they preserve the record of a ca 1 20 ga high pressure temperature p t metamorphic event the sheet intrusions define magmatic corridors all along and concordant with the western northern and eastern tectonic boundaries of the belt the concordant and elonga

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

a seafloor and sea surface gravity survey of axial volcano seafloor and sea surface gravity measurements are used to model the internal density structure of axial volcano seafloor measurements made at 53 sites within and adjacent to the axial volcano summit caldera provide constraints on the fine scale density structure shipboard gravity measurements made along 540 km of track line above axial volcano and adjacent portions of the juan de fuca ridge provide constraints on the density over a broader region and on the isostatic compensation the seafloor gravity anomalies give an average density of 2 7 g cm 3 for the uppermost portion of axial volcano the sea surface gravity anomalies yield a local compensation parameter of 23 significantly less than expected for a volcanic edifice built on zero age lithosphere three dimensional ideal body models of the seafloor gravity measurements suggest that low density material with a density contrast of at least 0 15 g cm 3 may be located underneath the summit caldera the data are consistent with low density material at shallow depths near the southern portion of the caldera dipping downward to the north the correlation of shallow low density material and surface expressions of recent volcanic activity fresh lavas and high temperature hydrothermal venting suggests a zone of highly porous crust seminorm minimization modeling of the surface gravity measurements also suggest a low density region under the central portion of axial volcano the presence of low density material beneath axial caldera suggests a partially molten magma chamber at depth

thickening the altiplano crust by gravity driven crustal channel flow 1 in the central andes crustal thickness is not well correlated to upper crustal shortening only little shortening is documented in the upper crust of the 60 65 km thick altiplano plateau whose thickening and uplift were delayed with respect to earlier and greater thickening in the adjacent western and eastern cordilleras because crustal thickness variations induce horizontal stress gradients and cause crustal flow a thickness dependent channel flow is modeled here and applied to the central andes in situ thickening is assumed for both cordilleras while the altiplano crustal thickening is generated by lateral flow from these overthickened adjacent domains a 8 1018 pa s viscosity channel is predicted for crustal thicknesses exceeding 45 50 km to match the estimated topographic evolution of the central andes thickening in the cordilleras was sufficient to generate a flow of 6 109 m3 per unit length toward the initially 30 35 km thick altiplano

zircon and titanite recording 1 5 million years of magma accretion crystallization and initial cooling in a composite pluton southern adamello batholith northern italy abstract the southern part of the adamello batholith the so called re di castello unit is an example of a composite pluton ranging from gabbro to granodiorite in composition u pb dating of single zircon crystals from four tonalitic to granodioritic lithologies reveals that zircon crystallization is protracted in all studied lithologies showing apparent durations of growth between 90 and 700 ka the youngest zircons crystallized near the solidus and yield identical or slightly older ages than titanite the formation of these autocrystic zircons is considered to approximate the age of emplacement of the melt and its final crystallization in contrast to antecrystic zircons present in the same sample which had formed earlier in the magmatic column or were derived from re mobilized earlier magma the autocryst derived emplacement ages range from 42 43 0 09 ma to 40 90 0 05 ma recording 1 5 ma of intrusion and crystallization history we anticipate that extended periods of zircon crystallization may be common in silicic rocks whereas the zircons from residual melts from initially undersaturated mafic liquids should yield far more precise emplacement ages within our present analytical uncertainties of 0 1 0 2 in 206 pb 238 u age decreasing th u ratios of dated zircons within one melt batch document the depletion of the residual melt portion in th due to the contemporaneous crystallization of titanite preliminary hf isotopic compositions of the dated zircon grains suggest that the early stage melts of the southern re di castello unit represent hybrid melts with an important crustal component e hf between 2 8 and 3 0 subsequently emplaced melts are more juvenile at e hf values at 6 4 to 8 9 and may thus reflect the addition of large volumes of mafic melt to the magmatic system

long term seafloor tidal gravity and pressure observations in the north sea testing and validation of a theoretical tidal model seafloor gravity and pressure measurements for 4d reservoir monitoring require precise models of the time varying tidal signals current seafloor instrumentation can resolve 0 003 mgal in time lapse gravity differences and 0 05 kpa 5 mm in pressure to verify model accuracy a seafloor gravimeter and pressure gauge were operated continuously for 446 days next to the troll a gas platform in the north sea 60 64227 north 3 72417 east at a depth of 303 m the seafloor gravity and pressure time series were filtered and corrected with estimates from the tidal model which predicts the solid earth tide ocean loading and direct gravitational attraction of the varying water level the rms difference between the observed tidal gravity signal and the prediction is about 0 0013 mgal during periods when there are no surface storms a slight difference is observed for the direct attraction of the water overhead as computed from the tidal prediction versus that computed from direct seafloor pressure measuremen

1 05 1 01 ga sveconorwegian metamorphism and deformation of the supracrustal sequence at saesvatn south norway re os dating of cu mo mineral occurrences

orogeny and orography the effects of erosion on the structure of mountain belts a numerical model of the coupled processes of tectonic deformation and surface erosion in convergent orogens is developed to investigate the nature of the interaction between these processes crustal deformation is calculated by a two dimensional finite element model of deformation in response to subduction and accretion of continental crust erosion operates on the uplifted surface of this model through fluvial incision which is taken to be proportional to stream power the relative importance of the tectonic and erosion processes is given by a dimensionless erosion number relating convergence velocity rock erodibility and precipitation rate this number determines the time required for a system to reach steady state and the final topographic shape and size of a mountain belt fundamental characteristics of the model orogens include asymmetric topography with shallower slopes facing the subducting plate and an asymmetric pattern of exhumation with the deepest levels of exhumation opposite to subduction these characteristics are modified when the regional climate exhibits a dominant wind direction and orographically enhanced precipitation on one side of the mountain belt the two possible cases are dominant wind in the direction of motion of the subducting plate and dominant wind direction in the opposite direction of the subducting plate velocity models of the former case predict a broad zone of exhumation with maximum exhumation in the orogen interior models of the latter case predict a focused zone of exhumation at the margin of the orogen and at high erosion number a reversal in the topographic asymmetry natural examples of these two cases are presented the southern alps of new zealand exhibits the climate and exhumation asymmetry characteristic of wind in the direction opposite to motion of the subducting plate the asymmetry of topography suggests that erosion is not efficient enough to have reversed the topographic asymmetry the contrasting example of dominant wind in the direction of subduction motion is provided by the olympic mountains of washington state in this case exhumation of deep levels of the cascadia accretionary wedge shows a broad domal pattern consistent with the observed orographic precipitation

a new sea floor gravimeter a new reservoir management application uses precise time lapse gravity measurements on the sea floor to detect seawater infiltration in offshore natural gas fields during production reservoir models for the north sea troll field predict gravity changes as large as 0 060 mgal within a 3 5 year period we have constructed and deployed a new instrument the rovdog remotely operated vehicle deployed deep ocean gravimeter system for this application because the measurements must be relocated accurately within 3 cm we required a gravimeter which could be handled by an rov and placed atop sea floor benchmarks we have built an instrument based upon the scintrex cg 3m land gravimeter motorized gimbals level the gravimeter sensor within a watertight pressure case precision quartz pressure gauges provide depth information a shipboard operator remotely controls the instrument and monitors the data the system error budget considers both instrumental and field measurement uncertainties the instrument prototy

on the formation of eruption columns following explosive mixing of magma and surface water when magma vents into the sea or a crater lake the ensuing magma water interaction can affect the style of eruption dramatically if the mass of surface water incorporated into the erupting material is small 108 kg s the height progressively decreases with surface water content this occurs when the magma and surface water begin to constitute a significant fraction of the mass at the top of the column so that an increasing fraction of the initial magmatic thermal energy is converted to the surface water rather than the entrained air the transitions in eruption style which result from changes in the mass of surface water mixing with the magma may account for observations of both buoyant plumes and wet surge during the eruptions of taal in 1965 and miyake jima in 1983 and for the changes in the eruptive activity at surtsey in 1963 1964 as the access of seawater to the vent became more restricted we also present calculations which suggest that the accretionary lapilli which are often found in wet flow deposits may result from condensation of vapor in both the cold wet collapsing fountains and in the flows themselves

molybdenite re os dating of biotite dehydration melting in the rogaland high temperature granulites s norway abstract the ability of the re os system in molybdenite to record and preserve the age of granulite facies metamorphism in polymetamorphic belts is tested using the orsdalen w mo district rogaland s norway a low pressure high temperature granulite facies domain displaying osumilite and pigeonite isograds is exposed around the 931 2 ma rogaland anorthosite complex available u pb monazite and zircon data show that a 0 93 ga contact metamorphism overprints a 1 03 0 97 ga regional sveconorwegian metamorphism in the gneiss basement molybdenite and scheelite in the orsdalen district occur in orthopyroxene bearing leucocratic veins parallel to the regional foliation the veins are interpreted as migmatic leucosomes formed by fluid absent incongruent melting of the biotite rich host rock above ca 800 c producing a granulite facies orthopyroxene garnet residual assemblage molybdenite is interpreted as a product of the melting reaction crystallized from trace amounts of mo released from biotite in the host rock during partial melting four re os analyses of molybdenite from three samples representing two mines yield an isochron age of 973 4 ma the isochroneity of the data indicates that the precipitation of molybdenite and the partial melting event are recorded on the district scale the re os system in molybdenite was not affected by subsequent 0 93 ga contact metamorphism corresponding to formation of garnet quartz coronitic textures around molybdenite and other minerals in the deposits the results indicate that granulite facies conditions prevailed at 973 4 ma near the end of a protracted event of regional metamorphism 1 03 0 97 ga biotite dehydration melting recorded in orsdalen took place at a pressure of ca 5 5 kbar orthopyroxene garnet plagioclase quartz thermobarometry possibly in association with regional decompression the study shows that granulite facies metamorphism 0 97 ga took place before intrusion of massif type anorthosites in rogaland 0 93 ga the study suggests that the high temperature osumilite bearing assemblage may be related to the 0 97 ga event and that massif type anorthosites may not be a cause for the high temperature thermal anomaly in the crust but a late expression of it

metamorphic constraints on the tectonic evolution of the allochthonous monocyclic belt of the grenville province western quebec the tectono metamorphic evolution of grenvillian monocyclic terranes in western quebec has been constrained along a northwest southeast transect from reservoir cabonga to the morin anorthosite by t

synconvergent ductile flow in variable strength continental crust numerical models with application to the western grenville orogen 1 we present results from numerical models for a convergent orogen with laterally variable lower crustal strength representing a simplified orogenic system in which a strong craton flanked by progressively weaker terranes collides with another continent with progressive convergence crustal thickening and thermal relaxation lower crust becomes decoupled from upper and middle crust forming a ductile orogenic infrastructure beneath a stronger superstructure collision with strong external crust results in uplift and expulsion of ductile nappes from the orogenic core creating allochthonous terranes overlying a lower crustal indentor the extent of transport and exhumation of lower crustal nappes over the indentor reflects the amount of convergence and the erosion rate the western grenville orogen displays across strike variations in age tectonic history and protolith association suggesting a systematic variation in precollision crustal strength the laurentian craton margin and accreted terranes were variably reworked at synorogenic depths of 25 35 km during the ottawan orogeny deformation propagated from younger monocyclic rocks in the southeast into older polycyclic rocks flanking the craton on the northwest a comparison between numerical model results and crustal scale cross sections from the georgian bay and montreal val d or transects shows close correspondence between crustal structure and model geometry this indicates that the models produce geologically realistic results and provides a context for interpreting the tectonic evolution of the western grenville orogen contrasts between the results of homogeneous channel flow models and the present ductile nappe models suggest that the effects of different styles of ductile flow can be distinguished in the geologic record

monazite as a metamorphic chronometer south of the grenville front western quebec monazite was utilized as a chronometer to examine the effects of high grade metamorphism across the parautochthonous belt and allochthon boundary thrust of the grenville province in western quebec this study in addition to previous geo chronological studies indicates an archean component in the gneisses which is consistent with the presence of more than one set of peak metamorphic conditions single grain monazite analyses from metasedimentary gneisses from four locations within the parautochthonous belt yielded grenvillian u pb dates of 1000 5 to 1006 2 ma the location farthest to the northwest 45 km southeast of the grenville front included monazite with a distinct archean signature southeast of this point an archean signature was not detected in the monazite at 70 km southeast of the grenville front monazite yielded two discrete ages of 1005 2 and 1020 3 ma xenotime from one location indicated that the closure temperature of this mineral may be equivalent to that of monazite 725 25

crustal channel flows 1 numerical models with applications to the tectonics of the himalayan tibetan orogen 1 plane strain thermal mechanical numerical models are used to examine the development of midcrustal channel flows in large hot orogens in the models radioactive self heating reduces the viscosity of tectonically thickened crust and increases its susceptibility to large scale horizontal flow channels can be exhumed and exposed by denudation focused on the high relief transition between plateau and foreland we interpret the himalaya to have evolved in this manner channel flows are poorly developed if the channel has a ductile rheology based on wet quartz flow laws and well developed if there is an additional reduction in viscosity to 1019 pa s this reduction occurs from 700 c to 750 c in the models and is attributed to a small percentage of in situ partial melt melt weakening model ht1 provides an internally consistent explanation for the tectonic evolution of many features of the himalayan tibetan orogenic system erosional exhumation exposes the migmatitic channel equivalent to the greater himalayan sequence ghs between coeval normal and thrust sense ductile shear zones corresponding to the south tibetan detachment and the main central thrust systems outward flow of unstable upper crust rotates these shears to low dip angles in the model both the ghs and the lesser himalayan sequence are derived from indian crust with the latter from much farther south similar models exhibit a range of tectonic styles including the formation of domes resembling north himalayan gneiss domes model results are relatively insensitive to channel heterogeneities and to variations in the behavior of the mantle lithosphere beneath the model plateau

testing ocean tide models in the nordic seas with tidal gravity observations summary tidal gravity observations at ny alesund on spitzbergen made with a lacoste romberg gravimeter with electrostatic feedback have been used to assess the accuracy of ocean tide models in the nordic seas eight ocean tide models have been used to compute the ocean tide loading at ny alesund the station is very close to the coast and therefore experiences a significant direct gravitational attraction of the nearby tidal water mass for the bay adjacent to the station the model values have been replaced by the tide gauge values and a special effort has been made to fit the models correctly to the coastline the ocean tide models for m2 and o1 have also been compared with observations made over 30 years ago at longyearbyen spitzbergen with askania gravimeters the comparisons of the tidal gravity observations with computed ocean tide loading show that for the harmonic m2 the phase lag of the ocean tide model of schwiderski and the local tide model of gjevik et al are wrong by 15 and 5 in the norwegian basin for the harmonic s2 the models fes94 1 got00 2 and schwiderski give a poor fit to the observed tidal loading at ny alesund because of phase lags that are too large in the norwegian basin by 10 fes94 1 and got00 2 also have s2 amplitudes that are 5 cm too large in the norwegian basin for n2 fes94 1 fes95 2 got00 2 and schwiderski have phase lags that are 10 too large in the norwegian basin and fes94 1 and 95 2 also have n2 amplitudes that are 2 4 cm too large in that region overall for these three harmonics the global model fes99 gives the best fit to the tidal gravity observations at ny alesund and this agreement is even closer than that obtained using regional arctic ocean tide models

transect across the northwestern grenville orogen georgian bay ontario polystage convergence and extension in the lower orogenic crust the grenville orogenic cycle between 1190 and 980 ma involved accretion of magmatic arcs and or continental terranes to the laurentian craton a transect across the western central gneiss belt georgian bay ontario which crosses the boundary between parautochthonous and allochthonous units at an inferred orogenic depth of 20 30 km offers some insights on the thermal and mechanical behavior of the lower crust during the development of the grenville orogen prior to grenvillian metamorphism this part of laurentia consisted largely of meso proterozoic 1450 ma granitoid orthogneisses granulites and subordinate mafic and supracrustal rocks grenvillian convergence along the transect began with transport of the previously deformed and metamorphosed 1160 ma parry sound domain over the craton sometime between 1120 ma and 1080 ma this stage of transport was followed by out of sequence thrusting and further convergence along successively deeper foreland propagating ductile thrust zones a major episode of extension at 1020 ma resulted in southeast directed transport of allochthonous rocks along the midcrustal shawanaga shear zone the final stage of convergence involved deformation and metamorphism in the grenville front tectonic zone at 1000 980 ma peak metamorphism along most of the transect at 1065 1045 ma followed initial transport of allochthonous rocks over the craton by 15 35 m y regional cooling which postdated peak metamorphism by 70 m y was probably delayed by the combined effects of late stage extension and convergence transport of allochthons at least 100 km over the craton was accomplished along a weak migmatitic decollement further propagation of the orogen into the craton followed partial melting and weakening of parautochthonous rocks below this decollement extensional deformation was associated with distributed ductile flow the formation of regional transverse folds with axes parallel to the stretching direction and reactivation of the allochthon parautochthon thrust boundary as an extensional decollement the extensional lower crustal flow was likely the primary cause of the subhorizontal attitude of many structures and seismic reflectors in this part of the central gneiss belt

disappearance of a crater lake implications for potential explosivity at soufriere volcano st vincent lesser antilles soufriere volcano in st vincent west indies is one of the most active volcanoes in the eastern caribbean with at least six eruptions since 1718 ad the latest of which occurred in 1979 prior to the 1979 eruption the active crater hosted deep water lakes during periods of repose which were always replenished within a few years after the eruptions in 1979 the crater was filled with 108 m3 of fragmental material and despite constant precipitation has remained virtually dry ever since with the exception of a small shallow pond a resistivity survey was conducted in july 2006 to investigate groundwater occurrence in the crater results from the resistivity data inversion on several 2 d profiles show a shallow horizontal conductor across the crater floor consistent with a water saturated aquifer they also show that the post 1979 pond currently present in the crater lake is in fact an outcropping part of the groundwater water reservoir the reservoir water table is 28 m above the pre 1979 lake level and reflects mass equilibrium in the system where constant seepage underground balances the meteoric recharge we suggest that the groundwater body extends at depth to the bottom of the pre 1979 crater lake either due to a significant structural discontinuity or because of a reduction of permeability at depth the estimated maximum volume of water stored underground is 10 30 106 m3 and energy considerations indicate that 2 4 7 3 1010 kg of magma would potentially be sufficient to vaporise the whole groundwater body this amount of magma represents only 13 41 of the mass erupted during the last eruption in 1979 which was the smallest of the past 3 eruptions 1902 1971 72 1979 since explosive phreatic or phreatomagmatic eruptions at soufriere seem to be linked to magma water interaction within confined space the results from this survey suggests that phreatic or phreatomagmatic activity is a distinct possibility during future magma intrusion in the summit area despite the apparent disappearance of water in the summit crater

timing of the yuchiling giant porphyry mo system and implications for ore genesis the yuchiling mo deposit is a recently discovered giant porphyry system in the east qinling mo belt china its apparent causative intrusion i e the yuchiling granite porphyry is the youngest intrusion phase 4 of the heyu multiphase granite batholith which was emplaced between 143 and 135 ma new robust constraints on the formation of the yuchiling porphyry mo system are provided by combined zircon u pb biotite 40ar 39ar and molybdenite re os dating zircon grains from the mo mineralized granite porphyry yield weighted 206pb 238u age of 134 0 1 4 ma n 19 2 error mswd 0 30 magmatic biotite from the same sample yield a 40ar 39ar plateau age of 135 1 1 4 ma 2 error and an inverse isochron age of 135 6 2 0 ma n 7 2 error mswd 10 8 which are effectively coincident with the zircon u pb age within analytical error three pulses of mineralization can be deduced from the molybdenite re os ages namely 141 137 and 134 ma which agree well with the zircon u pb ages of granitic phases 1 2 and the yuchiling porphyry phase 4 respectively these well constrained temporal correlations indicate that mo mineralization was caused by pulses of granitic magmatism and that the ore forming magmatic hydrothermal activity responsible for the yuchiling porphyry mo system lasted about 8 ma the yuchiling mo deposit represents a unique style of porphyry mo system formed in a post collision setting and associated with f rich high k calc alkaline intrusions which differ from convergent margin associated porphyry mo deposits

crustal architecture and tectonic assembly of the central gneiss belt southwestern grenville province canada a new interpretation the central gneiss belt southwestern grenville province is characterized by parautochthonous crust in the north and allochthonous lithotectonic domains in the south despite nearly two decades of study the basal decollement to allochthonous domains transported from the southeast known as the allochthon boundary thrust has not been precisely located throughout much of the belt between lake nipissing and georgian bay where its surface trace is known it separates 1 24 ga sudbury metadiabase in the footwall from eclogite remnants and 1 17 1 15 ga coronitic olivine metagabbro confined to its hanging wall on the premise that this relationship can be used to trace the allochthon boundary thrust elsewhere in the central gneiss belt we have sought to extend the known distribution of these mafic rock types making use of field petrographic and geochemical criteria to identify them new occurrences of all three mafic types are identified in a region extending from south of lake nipissing to western quebec

erratum metamorphic constraints on the evolution of the gneisses from the parautochthonous and autochthonous polycyclic belts grenville province western quebec the tectono metamorphic history of polycyclic grey gneisses located in the central grenville province of western quebec has been constrained along a transect perpendicular to the length of the gr

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

ductile extrusion of the higher himalayan crystalline in bhutan evidence from quartz microfabrics quartz textures measured from deformed quartz tectonites within the lesser himalaya and higher himalaya crystalline of bhutan show similar patterns orientation and distribution of the quartz crystallographic axes were used to confirm the regional shear sense the asymmetry of c axis and a axis patterns consistently indicates top to the south shearing the obliquity of the texture and the inferred finite strain plane strain to moderately constrictional suggest the strain regime had a combination of rotational and irrotational strain path in most of the samples from the bhutan himalaya the inferred deformation mechanisms suggest moderate to high temperature conditions of deformation that produced the observed crystallographic preferred orientation much higher temperature of deformation is indicated in the quartz veins from a leucogranite the observed ductile deformation is pervasively developed in the rocks throughout the investigated area the intensity of deformation increases only slightly in the vicinity of the main central thrust simultaneous southward shearing within a large part of the higher himalaya crystalline near and above the main central thrust and normal faulting across the south tibetan detachment is explained by the tectonically induced extrusion of a ductily deforming wedge the process of extrusive flow suggested here can be approximated quantitatively by channel flow models that have been used to describe subduction zone processes channel flow accounts for some observed phenomena in the himalayan orogen such as inverted metamorphic sequences near the main central thrust not related to an inversion of isotherms and the syntectonic emplacement of leucogranites into the extruding wedge locally leading to an inversion of isotherms due to heat advection

mechanism of the 1979 explosive eruption of soufriere volcano st vincent abstract the vulcanian explosive phase of the soufriere of st vincent in 1979 occurred when magma penetrated the water logged 1971 1972 lava island in the center of the crater lake the 13 day period of explosive and volcanoseismic activity ceased when the crater lake had dried up and subsequent activity was restricted to quiet lava extrusion the explosive phase produced 15 5 105m3 dense rock equivalent d r e of new magma and 22 106m3 d r e of lithic material from the 1971 1972 island whereas the following extrusive phase produced a dome of volume 47 106m6 considerations of the energetics of the explosive eruption indicate that the available heat content of the new magma 5 8 1016j was not sufficient to account for the observed phenomena including evaporation of the 25 106m3 crater lake and that additional 8 1016 j of heat remaining in the 1971 1972 lava mass are required to drive the eruption plumes field relations the high proportion of lithic material and the energy relations of the eruption are consistent with a phreato magmatic eruption mechanism involving a three way interaction of the 1971 1972 lava mass crater lake water and magma as a consequence of net veining and thermally induced hydraulic fracturing of the water logged 1971 1972 lava island by new magma in 1979

nloadf a program for computing ocean tide loading the loading of the earth by the ocean tides produces several kinds of signals which can be measured by geodetic technique in order to compute these most accurately a combination of global and local models of the ocean tides may be needed the program nloadf convolves the green functions for loading with ocean tide models using a station centered grid with fixed dimensions making it easy to combine different ocean models without overlap in the convolution the program computes all the quantities of interest gravity displacement tilt and strain and includes the case where measurements are made beneath the surface of the ocean

tectonomagmatic evolution of the sveconorwegian orogen recorded in the chemical and isotopic compositions of 1070 920 ma granitoids abstract the sveconorwegian province in southern norway and sweden hosts at least four granitoid suites representing apparently continuous magmatism at the sw margin of the fennoscandian shield between 1070 and 920 ma this study presents a compilation of published and new zircon la icp ms u pb geochronology whole rock and zircon geochemistry and sm nd isotope data for the granitoid suites and demonstrates the granitoids ability to record changes in the tectonomagmatic evolution of this orogenic province the sirdal magmatic belt smb ca 1070 1010 ma represents the earliest magmatism west in the province followed by two hornblende biotite granitoid suites hbg ca 1000 920 ma and the fla iddefjord bohus suite fib ca 925 ma in central and eastern parts of the province respectively the smb and the hbg bodies located outside of the smb referred to as hbgout are chemically similar whereas the hbg bodies located in the same region as the smb referred to as hbgin are more ferroan enriched in incompatible elements and have higher zircon saturation temperatures isotopically the smb and both hbg suites fall on an evolutionary trend from widespread 1 5 ga crust in the region suggesting this was the dominant crustal contribution to magmatism the fib suite is more peraluminous rich in inherited zircon and has isotopic compositions suggesting a more evolved source than both the hbg suites and the smb trace element modelling shows that the smb and hbgout suites could have formed by 50 partial melting of 1 5 ga crust whereas 5 10 remelting of the dehydrated and depleted smb residue accounts for the geochemical composition of the hbgin suite the available data suggest a scenario where the 1 5 ga lower crust underwent melting due to long lived mafic underplating giving rise to the smb suite after ca 1000 ma regional scale extension may have led to more widespread mafic underplating causing remelting of the residue following smb melt extraction forming the hbgin suite with lower crustal melting farther east forming the hbgout suite changes in melt composition over this 150 myr time interval may thus be ascribed to an evolving melt source rather than fundamental changes in tectonic regime deep continental subduction at ca 990 ma east in the orogen provided an isotopically evolved crustal source for the fib suite the data underline the difference in tectonic processes across the orogen with long lived high temperatures in the western and central parts and colder high pressure events in the eastern parts of the orogen

u pb geochronology of the syn orogenic knaben molybdenum deposits sveconorwegian orogen norway paired isotope dilution thermal ionization mass spectrometry id tims and secondary ion mass spectrometry sims zircon u pb data elucidate geochronological relations in the historically important knaben molybdenum mining district sveconorwegian orogen south norway this polyphase district provided c 8 5 mt of ore with a grade of 0 2 it consists of mineralized quartz veins silica rich gneiss pegmatites and aplites associated with a heterogeneous locally sulphide bearing amphibolites facies gneiss called knaben gneiss and hosted in a regional scale monotonous commonly weakly foliated granitic gneiss an augen gneiss at the knaben i deposit yields a 1257 6 ma magmatic zircon age dating the pre sveconorwegian protolith of the knaben gneiss mineralized and non mineralized granitic gneiss samples at the knaben ii and kvina deposits contain some 1488 1164 ma inherited zircon and yield consistent intrusion ages of 1032 4 1034 6 and 1036 6 ma this age links magmatism in the district to the regional 1050 1020 ma sirdal i type granite suite corresponding to voluminous crustal melting during the sveconorwegian orogeny a high u low th u zircon rim is present in all samples it defines several age clusters between 1039 6 and 1009 7 ma peaking at c 1016 ma and overlapping with a monazite age of 1013 5 ma the rim records protracted hydrothermal activity which started during the main magmatic event and outlasted it this process was coeval with regional high grade sveconorwegian metamorphism molybdenum deposition probably started during this event when silica rich mineralizing fluids or hydrous magmas were released from granite magma batches an analogy between the knaben district and shallow short lived porphyry mo deposits is inappropriate

rapid least squares inversion of apparent resistivity pseudosections by a quasi newton method1 a fast inversion technique for the interpretation of data from resistivity tomography surveys has been developed for operation on a microcomputer this technique is based on the smoothness constrained least squares method and it produces a two dimensional subsurface model from the apparent resistivity pseudosection in the first iteration a homogeneous earth model is used as the starting model for which the apparent resistivity partial derivative values can be calculated analytically for subsequent iterations a quasi newton method is used to estimate the partial derivatives which reduces the computer time and memory space required by about eight and twelve times respectively compared to the conventional least squares method tests with a variety of computer models and data from field surveys show that this technique is insensitive to random noise and converges rapidly this technique takes about one minute to invert a single data set on an 80486dx microcomputer

new tectonic divisions of the grenville province southeast canadian shield on the basis of geological geophysical and geochronological data the grenville province has been divided into three first order longitudinal belts the parautochthonous belt pb allochthonous polycyclic belt apb and allochthonous monocyclic belt amb these are set apart by three first order tectonic boundaries the grenville front gf allochthon boundary thrust abt and monocyclic belt boundary zone mbbz the belts are subdivided into terranes based on internal lithological character the gf separates the archean to proterozoic foreland northwest of the orogen from reworked equivalents to the southeast continuous at the scale of the orogen its main characteristic is that of a crustal scale contraction fault the pb although less clearly identified along the length of the orogen in most places represents upgraded and tectonically reworked rocks of the adjacent foreland the boundary between the pb and the apb to the southeast the abt is most clearly delineated in the eastern half of the province it is the locus of major crustal delamination along which high grade mostly middle proterozoic polycyclic terranes were tectonically transported northwest toward and onto the pb the amb comprises two separate areas underlain by the wakeham supergroup and what is currently known as the grenville supergroup respectively its basal contact the mbbz is a decollement zone of variable kinematic significance between older polycyclic rocks and tectonically overlying monocyclic rocks this first order zonation implies a tectonic polarity to the grenville province superimposed on which are second order features evident from contrasting tectonic styles and radiometric ages these characteristics are consistent with a diachronous or oblique collisional model for the grenville orogen

role of melt during deformation in the deep crust deformation in the deep crust is strongly influenced by the presence of melt injected melt or magma weakens the crust because strain will tend to localize where melt is present the amount of strain a pluton may accommodate is dependent on the length of time it takes for a pluton to crystallize and the strain rate for plutons that intrude into rocks which are near the solidus temperature of the melt crystallization times can be quite long 1myr partial melting of deep crustal rocks can lead to melt enhanced embrittlement this occurs because the volume change for most melting reactions is positive therefore when the rate of melt production outpaces the rate at which melt can leave the system the melt pressure increases eventually the melt pressure may become sufficiently high that the melting rocks behave in a brittle fashion and fracture conjugate sets of dilatant shear fractures filled with melt occur in migmatite from the central gneiss belt canada this suggests that melt enhanced embrittlement occurred in these rocks an expression which relates the magnitude of differential stress to the angle between conjugate dilatant shear fractures is derived assuming that migmatite has a small tensile strength differential stresses are 20 mpa in migmatitic rocks at the time melt enhanced embrittlement occurs the occurrence of melt enhanced embrittlement shows that a switch in deformation mechanism from plastic flow to cataclasis is possible in the deep crust during melting furthermore repeated episodes of melt enhanced embrittlement in migmatitic rocks may be an efficient mechanism for extracting melt from partially melted terrains

low rhenium molybdenite by metamorphism in northern sweden recognition genesis and global implications abstract re os dating of molybdenite is an accurate means to date intrusions and intrusion related ore deposits using the model age or isochron approach but molybdenite has a new niche in the greenschist to granulite facies metamorphic environment re os ages for metamorphic molybdenite may be used to construct regional metamorphic histories age significance and accuracy are established by analyzing multiple molybdenite separates extracted from single petrographically characterized molybdenite occurrences in this study twelve geologically distinct molybdenite bearing samples from two small mo districts in northern sweden trace a 150 m y paleoproterozoic svecofennian metamorphic history from 1900 to 1750 ma these data reveal a little known widespread and protracted late svecofennian anatexis in northern sweden the kataberget mo cu f deposit is located in the moskosel granite batholith north of the economically renown skellefte district four different molybdenite samples from outcrop at kataberget indicate an intrusion age of 1895 6 ma with the formation of later pegmatite aplite at 1875 6 ma the allebuoda bjorntjarn and munka mo w deposits in the rappen district are represented by three outcrop and five drill core samples of molybdenite bearing aplite pegmatite granite these two deposits were previously described as intrusion related climax type mo mineralization re os ages for molybdenites from these deposits range from 1865 to 1750 ma and significantly re concentrations are markedly low extending to the sub ppm level age agreement within the deposits is conspicuously lacking whereas with one exception age agreement within any single sample geologic occurrence as established by analysis of additional molybdenite separates is very good these data together with fundamental geologic observations discussed in this paper suggest that mo w mineralization in northern sweden is not intrusion related but the local product of episodic melting of archean paleoproterozoic supracrustal gneisses related to the svecofennian orogeny petrographic traverses across the boundary between widespread foliation parallel units of aplitic to pegmatitic pink granite and hosting biotite gneiss directly capture the process of ore formation dehydration breakdown of zircon rich biotite aligned with the foliation in the gneiss is accompanied by formation of new pristine post deformational biotite plus sulfides oxides hydrothermal zircon and fluorite all associated with microcline dominant leucosomes this process has profound implication for the traditional leucogranite intrusion related genesis attributed to the broad classification of mo w sn base and precious metal mineralization e g south mountain batholith nova scotia okiep namaqualand south africa mactung yukon pogo liese tintina alaska carajas and goias rio tocantins brazil new england batholith nsw australia bergslagen sweden nevoria western australia alpeinerscharte austria erzgebirge germany sardinia corsica batholith in addition to biotite metallogenic contributions e g mo w sn u bi cu pb zn fe ni co au ag te as sb ree in various combinations may also be controlled by breakdown of amphibole in effect the trace element composition of dehydrating or recrystallizing components in a gneissic rock essentially defines the local and district metallogenic suite in the absence of focusing structures e g shear zones sheeted vein development this process will generally form small and disconnected subeconomic deposits with erratic and unpredictable grades low re content in associated molybdenite is a key indicator for a subeconomic origin by local melting of biotite gneiss mo w or muscovite schist sn w

crustal structure of the grenville front and adjacent terranes under the auspices of the great lakes international multidisciplinary program on crustal evolution approximately 320 km of deep seismic reflection data were collected in lake huron along a profile that extends east from the manitoulin terrane across the grenville front to the interior of the grenville orogen the manitoulin terrane is characterized by a series of gently east dipping reflections at about 20 km depth that separate a highly reflective lower crustal layer from a markedly less reflective upper layer imaged by strong reflections at the western end of a spectacular band of moderately east dipping reflections the grenville front clearly truncates manitoulin terrane structures to the west these data are interpreted in terms of a speculative two stage model involving 1 creation of a major decollement during northward collision of an allochthonous terrane with the southern superior cratonic margin 1 83 1 89 ga penokean orogeny and 2 northwest directed stacking of microterranes at the southeastern margin of the craton followed by crust penetrating ductile imbrication under high pressure high temperature conditions leading to the ramping of deeply buried rocks to the near surface 1 0 1 3 ga grenvillian orogeny

linking growth episodes of zircon and metamorphic textures to zircon chemistry an example from the ultrahigh temperature granulites of rogaland sw norway in situ u th pb analyses by ion microprobe on zircon in intact textural relationships are combined with backscatter and cathodoluminescence imaging and trace element analyses to provide evidence for growth episodes of zircon this approach helps a to unravel the polymetamorphic history of aluminous migmatitic and granitoid gneisses of the regional contact aureole around the rogaland anorthosite norite intrusive complex and b to constrain the age of m 2 ultrahigh temperature uht metamorphism and the subsequent retrograde m 3 event all samples yield magmatic inherited zircon of c 1035 ma some an additional group at c 1050 ma this suggests that loss of pb by volume diffusion in non metamict zircon is not an important factor even under extreme crustal conditions furthermore the identical inheritance patterns in aluminous garnet cordierite osumilite bearing migmatites and orthogneisses indicate a metasomatic igneous instead of a sedimentary protolith for the migmatite results for the m 1 metamorphic event at c 1000 ma bp are consistent in all samples including those from outside the orthopyroxene in isograd the latter do not show evidence for zircon growth during the m 2 metamorphic episode zircon intergrown with or included within m 2 metamorphic minerals magnetite spinel orthopyroxene give an age of 927 7 ma 2 n 20 the youngest observed results are found in zircon outside m 2 minerals some overgrown by m 3 mineral assemblages late garnet coronas garnet quartz and orthopyroxene garnet symplectites and yield a slightly younger pooled age of 908 9 ma 2 n 6 these textures are relative time markers for the crystallization of zircon overgrowths during discrete stages of the uht event these youngest age groups are consistent with the emplacement age of the rogaland intrusive complex and the last magmatic activity tellnes dyke intrusion respectively this is direct and conclusive evidence for uht metamorphism in the regional aureole being caused by the intrusions and corrects earlier notions that the events are not linked trace element behaviour of zircon tb u and y content has been tracked through time in the samples and shows variations both within and between samples this heterogeneous behaviour at all scales appears to be common in metamorphic rocks and precludes the use of rules of thumb in the interpretation of zircon chemistry but chemical tracers are useful for recognition of zircon growth or recrystallization during metamorphism

molybdenum isotope variations in molybdenite vapor transport and rayleigh fractionation of mo molybdenum isotopes in 20 molybdenite samples dated by the re os method and representing a range of geologic settings show mass dependent fractionation spanning 0 63 per atomic mass unit amu previous mo isotope data for molybdenite reveal variations in fractionation of 0 5 amu interpretation of these data is hampered however by limited sample numbers in each study lack of a common standard for interlaboratory comparison and limited range of geologic settings here we show that mo isotope compositions of molybdenites do not correlate with crystallization temperature age geographic distribution or geologic conditions rather rayleigh distillation may explain variations of as much as 0 34 amu in a single molybdenite occurrence exceeding the proposed variability in average continental crust vapor transport and rapid precipitation of mo in propagating fractures may account for isotope fractionation of mo and perhaps other metals at very small scales if so the average isotopic composition of mo at each molybdenite occurrence may be representative of bulk crust our results suggest that the isotopic composition of mo delivered to the oceans is uniform geographically and through geologic time

geochronology of eclogite facies metamorphism in the sveconorwegian province of sw sweden abstract decompressed eclogites in the sveconorwegian province sw sweden have been dated using u pb geochronology zircons are common as inclusions in garnet and kyanite and other minerals in the decompressed eclogites titanite inclusions are found exclusively in the core of garnets the mineral inclusions and the chemical zoning of the garnets suggest inital growth under prograde amphibolite facies conditions followed by eclogite facies metamorphism and subsequent decompression through the high pressure granulite and upper amphibolite facies the zircon and titanite thus formed prior to the eclogite stage of the p t path the age of the eclogite forming event was determined by ion probe dating of zircon inclusions in garnets the obtained age of 972 14 ma is the maximum age of the eclogitisation the age of the titanite inclusions in garnet is 945 4 ma this age is similar to other u pb ages of titanite in the region which suggest that the titanite has been isotopically reset and that the age reflects cooling the mode of occurrence textural relationships and the chemical homogeneity suggest that the zircons formed from zr released from magmatitic fe ti oxides and possibly amphiboles during breakdown of magmatic minerals at the onset of the sveconorwegian metamorphism spot analyses of complex zircons from a granitic dyke in the eclogite yielded an age of 1403 15 ma for magmatic cores and an age of 963 22 ma for metamorphic rims the older age is a minimum age of the eclogite protolith and correspond to the age of a generation of granites in the region the rim age is within error identical to the age of eclogite metamorphism the eclogite metamorphism in sw sweden is younger than its grenvillian counterparts in scotland canada and usa

genesis of intermediate igneous rocks at the end of the sveconorwegian grenvillian orogeny s norway and their contribution to intracrustal differentiation abundant ferroan metaluminous granitoids 970 950 ma emplaced at the end of the sveconorwegian collisional orogeny 1130 900 ma are dominated by intermediate to silicic compositions with rare mafic facies both 73 fractional crystallization of an amphibole bearing gabbroic cumulate substracted from the parent mafic composition and 30 non modal batch melting of an amphibolitic source equivalent in composition to the mafic facies produce a monzodioritic liquid with appropriate trace element composition a better fit is obtained for the partial melting process both processes could have occurred simultaneously to produce mafic cumulates and restites as there is no evidence for large volumes of dense mafic rocks in the sveconorwegian upper crust these dense mafic rocks were probably produced in the lower crust formation of these granitoids thus contributed to the vertical stratification of the proterozoic continental crust and also to the transfer of water from the lower crust to the surface

what controls chemical variation in granitic magmas abstract consideration of the models that have been applied to explain the chemical variations within granitic rock suites shows that most are inadequate to account for the main variations this stems from a variety of model deficiencies ranging from physical or energetic inadequacies to incompatibility with the chemical data or internal inconsistency between models based on for example isotope or trace element data and major element data we contend that any model that fails any of these tests of internal consistency cannot be considered further thus although we can point to examples in which many of the traditionally accepted mechanisms have played secondary roles in producing variation there presently remains but one viable choice for the primary mechanism by which most granitic magmas acquire compositions beyond the range defined by the compositions of crustal melts that primary mechanism is peritectic assemblage entrainment pae we infer that once a partial melt has formed in a crustal protolith it may segregate from its complementary solid residue carrying small crystals of the peritectic phase assemblage formed in the melting reaction and that the ratios of individual peritectic minerals in the entrained assemblage remains fixed in the ratio decreed by the stoichiometry of the melting reaction for those elements with low solubilities in granitic melts pae in varying degrees accompanied by co entrainment of accessory minerals is responsible for most of the primary elemental variation in granitic magmas in contrast the concentrations of elements with high solubilities in silicic melts reflect the protolith compositions in a simple and direct way the source is the primary control on granite magma chemistry it dictates what is available to dissolve in the melt and what will be formed as the entrainable peritectic assemblage the apparent complexity in granitic rock suites is largely a consequence of these processes in the source all other mechanisms contribute only as a secondary overlay

improved accuracy of u pb zircon dating by selection of more concordant fractions using a high gradient magnetic separation technique abstract a loop of soft iron wire or a paper clip or a ferromagnetic grid mounted between the poles of an electromagnet picks up and allows further magnetic separation of zircons previously found to be non magnetic on a frantz isodynamic separator tests on previously analysed samples indicate that most such zircons that are fairly discordant say 10 can be picked up and isolated from associated grains that are more concordant tests on new samples indicate that even when most grains can be picked up the last few percent of the sample contain less uranium and are more concordant than the bulk sample the degree of discordance is the dominant factor affecting the uncertainty of u pb zircon ages both because of the error amplification in projections and because the assumption of a simple two stage system may not be valid only by eliminating or reducing discordance can errors approaching the uncertainty in a single analysis say 2 m y for 2700 m y rocks be achieved rutile normally concentrated with zircon as non magnetic has been successfully removed from a small amount of low uranium zircon using the high intensity separation technique

sharbot lake terrane and its relationships to frontenac terrane central metasedimentary belt grenville province new insights from u pb geochronology the sharbot lake terrane of the central metasedimentary belt in ontario comprises marbles minor tholeiitic basalts and siliciclastic rocks metamorphosed at low to moderate degrees and is intruded by several suites of mafic to felsic plutons gabbroic and associated monzogranitic phases of the lavant complex in northwestern parts of the terrane yield ages of 1224 2 ma for zircon and monazite and 1221 1211 ma for titanite the wolf grove structure a gneissic domain in the northeastern part of the area was migmatized and intruded by granites at 1168 ma prior to its tectonic juxtaposition on amphibolites and marbles of the sharbot lake terrane correlative events recorded by gneiss quartzite and marble from carleton place farther east stress the strong affinity of the wolf grove structure with rocks of the frontenac terrane the 1156 2 ma posttectonic maberly stock intrudes rocks near the boundary zone between the frontenac and sharbot lake terranes suggesting that the two terranes were linked by

molybdenite re os dating constrains gravitational collapse of the sveconorwegian orogen sw scandinavia abstract re os dating of molybdenite from small deposits is used to define crustal domains exhibiting ductile versus brittle behaviour during gravitational collapse of the sveconorwegian orogen in sw scandinavia a 1019 3 ma planar quartz vein defines a minimum age for brittle behaviour in central telemark in rogaland vest agder molybdenite associated with deformed quartz and pegmatite veins formed between 982 3 and 947 3 ma in the amphibolite facies domain three deposits and between 953 3 and 931 3 ma west of the clinopyroxene in isograd two deposits in the vicinity of the 0 93 0 92 ga rogaland anorthosite complex the data constrain the last increment of ductile deformation to be younger than 0 95 and 0 93 ga in these two metamorphic zones respectively molybdenite is the product of an equilibrium between biotite oxide and sulfide minerals and a fluid or hydrated melt phase after the peak of 1 03 0 97 ga regional metamorphism molybdenite precipitation is locally episodic a model for gravitational collapse of the sveconorwegian orogen controlled by lithospheric extension after 0 97 ga is proposed in the west of the orogen the rogaland vest agder sector is interpreted as a large shallow gneiss dome formed slowly in two stages in a warm and structurally weak crust the first stage at 0 96 0 93 ga was associated with intrusion of the post collisional hornblende biotite granite suite the second stage at 0 93 0 92 ga restricted to the southwesternmost area was associated with intrusion of the anorthosite mangerite charnockite suite most of the central part of the orogen was already situated in the brittle upper crust well before 0 97 ga and did not undergo significant exhumation during collapse in the east of the orogen situated against the colder cratonic foreland exhumation of high grade rocks of the eastern segment occurred between 0 97 and 0 95 ga and included preservation of high pressure rocks but no plutonism

last gasp of the grenville orogeny thermochronology of the grenville front tectonic zone near killarney ontario we present u pb titanite zircon and 40 ar 39 ar hornblende mica k feldspar data from a transect across the western part of the grenville front tectonic zone gftz near killarney ontario high grade metamorphic assemblages 1450 ma in this part of the gftz pre date the grenvillian orogeny and were primarily exhumed with little or no metamorphic overprinting by grenvillian deformation the titanite and zircon data form a discordant array with an upper intercept of 1454 pm 8 ma and a lower intercept of 978 pm 13 ma these data are interpreted in terms of partial lead loss during a short lived thermal event that increased in intensity from west to east across the transect 40 ar 39 ar data from hornblende indicate cooling through 450 c at 993 979 ma multiple diffusion domain models for the interpretation of discordant k feldspar spectra indicate cooling through 365 340 c at 990 960 ma and muscovite data indicate cooling through 320 c at 930 ma biotite data are not easily

crustal channel flows 2 numerical models with implications for metamorphism in the himalayan tibetan orogen 1 results from a thermal mechanical model ht1 that includes midcrustal channel flow are compatible with many features of the himalayan tibetan system radioactive self heating and rheological weakening of thickened model orogenic crust lead to the formation of a hot low viscosity midcrustal channel and a broad plateau channel material corresponding to the greater himalayan sequence ghs flows outward from beneath the plateau in response to topographically induced differential pressure at the plateau flank it is exhumed by focused surface denudation and juxtaposed with cooler newly accreted material corresponding to the lesser himalayan sequence lhs the model channel is bounded by coeval thrust and normal sense ductile shear zones interpreted to represent the main central thrust mct zone and south tibetan detachment system respectively inverted metamorphism associated with the model mct zone results from distributed ductile shear along the mct and extrusion of the hot channel a variety of model p t t path styles resembling those observed in the ghs and lhs are produced for points traveling through contrasting tectonic regimes that coexist in different parts of the model predicted times of peak metamorphism cooling and erosion of metamorphic facies are generally compatible with observations although model ghs cooling ages are too young the times of m1 and m2 metamorphic events observed in the ghs correspond to model times of maximum burial and maximum heating respectively the results highlight the need to integrate tectonics and metamorphism in continental collision models and demonstrate the importance of lateral transport of both heat and material in large hot orogens

timing and duration of melting in the mid orogenic crust constraints from u pb shrimp data muskoka and shawanaga domains grenville province ontario the central gneiss belt in the grenville province ontario exposes metaplutonic rocks orthogneisses and minor paragneisses that were deformed and metamorphosed at crustal depths of 20 35 km during the mesoproterozoic grenvillian orogeny we present sensitive high resolution ion microprobe shrimp u pb zircon data from eight samples of migmatitic orthogneiss granite and pegmatite from the muskoka and shawanaga domains that constrain the age and duration of partial melting in the mid orogenic crust our results support earlier interpretations that the protoliths to these migmatitic orthogneisses formed at ca 1450 ma emplacement and crystallization of granite and pegmatite in the shawanaga domain took place at ca 1089 ma apparently coevally with deformation and high grade metamorphism leucosomes in the muskoka and shawanaga domains yield ages of 1067 and 1047 ma respectively interpreted as the ages of melt crystallization the geochronological data and field observations suggest that melt was pr

crustal scale shortening and extension across the grenville province of western quebec a deep seismic reflection survey shot in 1993 crosses almost the full width of the grenville province in western quebec the seismic transect provides a very clear image of the crust mantle boundary and the most precise definition to date of the various grenvillian terranes the crust is around 44 km thick beneath the grenville front but thins rapidly to 36 km some 60 km to the southeast it is also notable that the greatest crustal thickness of 50 km occurs at the southeast end of the transect far from the inferred location of the main grenvillian collision the grenville front zone in which nw directed thrusting at about 1 ga was followed by se directed extension is defined by discontinuous se dipping reflections which extend down to the moho to the southeast the overlying migmatitic archean parautochthon almost half of the transect is characterised instead by nw dipping reflectors extending into the lower crust these reflectors are in turn truncated to the southeast by a 12 km thick zone of intense se dipping reflections the baskatong crustal ramp the base of the allochthonous terranes allochthon boundary thrust is likely located at this ramp which flattens out at around 30 km depth into the base of the relatively transparent intermediate crust a highly reflective upper crustal deck corresponding to rocks of the mont laurier terrane was thrust over the baskatong crustal ramp and is represented further to the northwest by the klippelike cabonga allochthon the synformal transparent morin anorthosite charnockite complex belongs to the same upper crustal level ramp anticlines and the overriding basal thrust of the upper allochthons demonstrate the nw directed propagation of tectonic transport during the grenvillian orogeny involving deformation and displacement along the baskatong ramp with a relay into the grenville front zone postaccretional extension appears to have been primarily accommodated along the latter two crustal discontinuities thinning the crust immediately south of the grenville front and affecting the crust up to 350 km away from the front

fluid mediated partial alteration in monazite the role of coupled dissolution reprecipitation in element redistribution and mass transfer monazite ce lree th u ca p si o4 with complex zoning in th and other elements is commonly observed in metamorphic and igneous rocks the hypothesis that this alteration is a product of fluid mediated element mass transfer has been tested in the piston cylinder press caf2 assembly cylindrical graphite oven at 1 000 mpa and 900 c and in cold seal autoclaves on a hydrothermal line at 500 mpa and 600 c experiments included a relatively homogeneous monazite ce 7 8 wt tho2 from a heavy mineral sand plus a series of alkali bearing fluids including 2n naoh 2n koh and na2si2o5 h2o experiments were conducted using bse imaging emp analysis and both tem and hrtem a subset of monazite grains from each experiment show evidence of partial alteration in the form of areas enriched in th si with sharp curvilinear compositional boundaries extending from the grain rim into the monazite interior these thsio4 enriched textures are similar to those commonly seen in natural examples of metasomatised monazite in both magmatic and metamorphic rocks in the na2si2o5 h2o experiments scarce inclusions of britholite formed in the altered monazite the altered monazite is also characterised by strong depletion in pb ca and y thorium and si mobility coupled with the formation of britholite inclusions during partial alteration in the monazite grain is considered to be the product of fluid aided coupled dissolution reprecipitation as opposed to solid state diffusion since other fluids including nacl and kcl brines do not result in the formation of these textures the experimental replication of thsio4 enriched areas in the monazite strongly suggests that similar textures in monazite observed in nature are fluid induced specifically by alkali bearing fluids if true complex metasomatically induced textures in monazite could yield information concerning the nature of the fluid responsible for their formation as well as allow for the dating of the metasomatic event presuming that all the original radiogenic pb has been removed

refinement of the time space evolution of the giant mio pliocene rio blanco los bronces porphyry cu mo cluster central chile new u pb shrimp ii and re os geochronology and 40ar 39ar thermochronology data representing one of the largest known estimated 5 gt at 1 cu and 0 02 mo porphyry system the rio blanco los bronces deposit incorporates at least five hypabyssal intrusive and hydrothermal centres extending for about 5 km from the rio blanco and los bronces mines in the north through the don luis mine to the sur sur mine la americana and los sulfatos in the south the new geochronology data which now include data on different molybdenite vein types confirm the u pb ages of the pre mineralisation intrusions but slightly increase their age range from 8 8 to 8 2 ma the distinct magmatic pulses of the mineralisation associated porphyritic intrusives late porphyries indicate an age interval instead of the previously suggested individual ages the quartz monzonite porphyry ranges from 7 7 to 6 1 ma sur sur 5 74 0 13 ma the feldspar porphyry shows an interval from 5 8 to 5 2 ma and the don luis porphyry from 5 2 to 5 0 ma the new re os data on distinct molybdenite vein types confirm the protracted history of cu mo mineralisation inferred previously the vein development occurred at least from 5 94 to 4 50 ma indicating a time span of 1 5 ma for the hydrothermal activity hydrothermal minerals dated by the 40ar 39ar method are generally too young to record the age of early high temperature mineralisation the majority of the 40ar 39ar data in the rio blanco porphyry cluster record reheating by either the youngest member of the late porphyry suite or the post mineralisation dacite or rhyolite plug formations at around 4 9 4 7 ma

kinematical and rheological evolution of a crustal scale ductile thrust zone central metasedimentary belt grenville orogen ontario reply the central metasedimentary belt boundary thrust zone is a 10 km thick 200 km long stack of crystalline thrust sheets enclosed by an anastomosing network of ductile thrust zones formed at mid

explosive activity of the last 1000 years at la soufriere st vincent lesser antilles abstract the products of explosive activity of la soufriere volcano on the island of st vincent over the last 1000 years are described dates for the different eruptions were determined using information from contemporary accounts fieldwork and radiocarbon dating scoria flow type pyroclastic density currents pdcs dominate the products of both the historical eruptions 1979 1902 03 1718 1812 ce and prehistoric eruptions 1580 and 1440 ce with subordinate fallout components associated with several eruptions radiocarbon dating shows that these six eruptions define a crude cyclicity with repose periods ranging between 77 and 140 years and systematically decreasing in more recent times two prehistoric eruptions in 1440 and 1580 ce respectively both produced magmatic lapilli fallout and pdcs and were fed by slightly more evolved magmas than the historical eruptions the eruptions in 1902 and 1812 ce had ash rich possible phreatomagmatic activity at their onset the iconic 1902 03 ce eruption generated radial distributed pdcs which were responsible for the deaths of 1500 people however only small remnants of these deposits remain and the original distribution cannot be determined from the preserved geology which has important implications for hazard studies petrochemical work has shown that magmas involved in the explosive eruptions were quite narrow in compositional range mainly comprising basaltic andesites the 1902 03 eruption involved a late stage basaltic component in march 1903 however activity in the last 1000 years generated notably more homogeneous magmas with a narrower range than the older eruptive periods previously reported in the literature suggesting a significant variation in the magmatic reservoir feeding system with time

geochemical and geo electrical study of mud pools at the mutnovsky volcano south kamchatka russia behavior of elements structures of feeding channels and a model of origin this study presents data on the geochemical composition of boiling mud pools at the mutnovsky volcano the physicochemical characteristics of the pools and the concentrations of major minor and trace elements in pool solutions vary widely a comparison of the geochemical compositions of host rocks and solutions indicates that leaching from rocks is not the only source of chemicals in thermal solutions geophysical studies reveal the inner structure of thermal fields which reflect the shapes of the underground reservoirs and feed channels using geophysical methods electrical resistivity tomography and frequency domain investigations it was shown that the vertical structure and complex geochemical zonation of the feed channels leads to a high contrast in the compositions of the mud solutions these findings answer questions about the origin and composition of surface manifestations to elucidate the mechanisms of solution formation an attempt was made to describe the magmatic fluid evolution and the resulting mixing of waters by physical and mathematical models the model illustrates fluid migration from a magma chamber to the surface it is shown that the formation of brines corresponding to the mud pool composition is possible during secondary boiling

time of metamorphism beneath the central metasedimentary belt boundary thrust zone grenville orogen ontario accretion at 1080 ma new u pb zircon and titanite data from the muskoka domain grenville province ontario provide protolith and metamorphic ages for the southwestern central gneiss belt discordant analyses from a migmatitic orthogneiss and its leucosome form a linear array with an upper intercept age of 1457 6 ma and a lower intercept age of 1064 18 ma u pb analyses on metamorphic zircon from an amphibolite yield a set of concordant analyses with an average 207pb 206pb age of 1079 3 ma a weakly migmatitic granitoid rock and a transecting charnockitic vein in the immediate footwall of the central metasedimentary belt boundary thrust zone yielded a discordant array of analyses wth an upper intercept age of 1394 13 ma and a lower intercept age of 1066 8 ma the charnockitic vein yielded concordant zircon ages of 1077 2 ma the upper intercept ages are interpreted in terms of protolith crystallization and the concordant and lower intercept ages as grenviilian high grade metamorphism and associated anatexis we h

electrical resistivity tomography study of taal volcano hydrothermal system philippines taal volcano 311 m in altitude is located in the philippines 14 n 121 e and since 1572 has erupted 33 times causing more than 2 000 casualties during the most violent eruptions in march 2010 the shallow structures in areas where present day surface activity takes place were investigated by dc resistivity surveys electrical resistivity tomography ert lines were performed above the two identified hydrothermal areas located on the northern flank of the volcano and in the main crater respectively due to rough topography deep valleys and dense vegetation most measurements were collected using a remote method based on a laboratory made equipment this allowed retrieval of information down to a depth of 250 m erts results detail the outlines of the two geothermal fields defined by previous self potential co2 soil degassing ground temperature and magnetic mapping harada et al japan acad sci 81 261 266 2005 zlotnicki et al bull volcanol 71 29 49 2009a phys chem earth 34 294 408 2009b hydrothermal fluids originate mainly from inside the northern part of the main crater at a depth greater than the bottom of the crater lake and flow upward to the ground surface furthermore water from the main crater lake infiltrates inside the surrounding geological formations the hydrothermal fluids outlined by gas releases and high temperatures cross the crater rim and interact with the northern geothermal field located outside the main crater

dynamic mingling of magma and liquefied sediments abstract hydrodynamic mingling of magma and liquefied sediments is generally accepted to represent the key process in the formation of some peperites experimental studies on simulant liquids and calculations based on recent empirical findings in the field of polymer research were undertaken to investigate the effectiveness of this process these studies show that for a wide range of shear rates a laminar flow behaviour of the system magma liquefied sediment can be expected i e turbulent mingling is not a realistic scenario formation of peperitic fabrics with grain sizes on a cm scale can hydrodynamically be explained under realistic intrusion velocities in addition to the hydrodynamics cooling processes must also be considered during peperite formation calculations of the cooling history of the juvenile magmatic component under realistic cooling conditions demonstrate a significant limitation of the hydrodynamic mingling time the consequence for peperite formation under normal intrusion conditions is that domains of magmatic melt and domains of liquefied sediment smaller than 10 cm are not probable however as smaller domain sizes peperitic grain sizes exist in nature we conclude that either those peperites represent the products of explosive events or that hydrodynamic mingling was accompanied by additional fragmentation processes

crustal structure of the mid atlantic ridge south of the kane fracture zone from seafloor and sea surface gravity data seafloor and sea surface gravity data are inverted together to construct a model for the near axis crustal structure of a slow spreading ridge the seafloor data set offers two main advantages it allows us to recover shorter wavelengths signal and to constrain the value of a potential field at two different levels the model we propose here would not have been derived from sea surface data alone it is based on a dense sea surface gravity coverage and on 121 sea bottom gravity measurements collected in the mid atlantic ridge at kane mark area during the hydrosnake 1988 and gravinaute 1993 cruises the primary goal of the seafloor surveys was to test for the presence of a magma reservoir beneath the axial neovolcanic ridge first a forward two dimensional 2 d model of the crustal structure across the axis is fit to observed gravity anomalies using constraints from geological and structural observations bouguer anomalies computed from sea bottom measurements and downward continuation of sea surface measurements both constrain the forward modeling this forward model is the starting point of a 2 d monte carlo inversion of seafloor and sea surface data in addition to the crustal thickness variations along axis our data document the amplitude variations of the crustal thickness and or its density in the across axis direction the model resulting from our inversion exhibits several features of the crustal structure in the mark area 1 the presence of a low density ap 300 50 kg m 3 body beneath the neovolcanic ridge is suggested and could correspond to a magma chamber or more probably to a highly hydrothermally fissured zone 2 both long and short wavelength gravity signals exhibit a difference between the western and eastern sides of the axial domain the mean value and the amplitude of bouguer anomalies are higher on the western part this difference suggests that axial processes in this area are very asymmetric 3 abyssal hills are not associated with a single gravity signature for instance on the west side of the axis one of the explored hills has no bouguer anomaly and is interpreted as a neovolcanic ridge whereas the others are associated with a shifted bouguer anomaly high and are interpreted as having thinner magmatic crust 4 the last feature of the crustal fabric we document here is the asymmetric emplacement of some deep rocks outcrops in the mark area we find that pink hill a topographic high where serpentinized peridotites are outcropping is much more serpentinized on its east flank toward the axial valley than on its west flank alteration occurring mainly by fluid circulation through faulted zones the asymmetric serpentinization suggests that deep origin rocks have outcropped by means of a main fault zone and are not emplaced by diapirism

crustal volume balance during the india eurasia collision and altitude of the tibetan plateau a working hypothesis a working hypothesis is developed for the two stages of evolution of the tibetan plateau during the continental collision between india and eurasia the first phase was dominated by crustal shortening and thickening in what is now tibet while indochina was expelled southeastward away from eurasia at the end of this phase at 17 ma the estimated tibetan crustal thickness was 51 km the second phase has been dominated by thickening of tibet by the influx of crustal material from india with no associated crustal shortening the crustal thickening that is inferred to have occurred by this process is only feasible because leakage of crust from beneath the plateau is restricted by neighboring archean cratons that block lower crustal flow this leakage to the east and south from tibet to beneath yunnan is estimated as 20 30 of the influx at present but was much smaller at the start of this second phase of deformation it is suggested that at 11 ma the force exerted against the tibetan upper crust by this flowing material first exceeded the threshold that this brittle layer could resist and tibet began to extend eastward any satisfactory working hypothesis for the tibetan plateau must explain its dimensions its 17 ma ending of shortening and 11 ma start of extension and volcanism the eastward slip sense on its active normal and strike slip faults and the ratios of their displacements to present day slip rates which imply that their slip rates have gradually increased this working hypothesis accounts for these features and predicts a smooth and continuing increase in altitude

analyses of a 426 day record of seafloor gravity and pressure time series in the north sea continuous gravity observations of ocean and solid tides are usually done with land based gravimeters in this study we analyze a 426 day record of time varying gravity acquired by an ocean bottom scintrex spring gravimeter between august 2005 and november 2006 at the troll a site located in the north sea at a depth of 303 m sea bottom pressure changes were also recorded in parallel with a paroscientific quartz pressure sensor from these data we show a comparison of the noise level of the seafloor gravimeter with respect to two standard land based relative gravimeters a scintrex cg5 and a gwr superconducting gravimeter that were recording at the j9 gravimetric observatory of strasbourg france we also compare the analyzed gravity records with the predicted solid and oceanic tides the oceanic tides recorded by the seafloor barometer are also analyzed and compared to the predicted ones using fes2014b ocean model observed diurnal and semi diurnal components are in good agreement with fes2014b predictions smallest constituents reflect some differences that may be attributed to non linearity occurring at the troll a site using the barotropic tugo m dynamic model of sea level response to ecmwf atmospheric pressure and winds forcing we show a good agreement with the detided ocean bottom pressure residuals about 4 hpa of standard deviation of remaining sea bottom pressure are however not explained by the tugo m dynamic model

coronitic metagabbro and eclogite from the grenville province of western quebec interpretation of u pb geochronology and metamorphism we present new u pb and metamorphic data on high pressure coronitic metagabbros from three distinct structural settings in the parautochthonous belt of the grenville province in western quebec intrusive ages are i for metagabbro close to the grenville front correlative with the sudbury dykes defined in ontario ii for an eclogitized lens at the base of the highest structural level sl4 a new age for mafic magmatism in the western grenville and iii for metagabbro from sl4 interpreted as correlative with metagabbros from the algonquin and shawanaga domains in ontario metamorphism in all cases is grenvillian with the best constrained age of 1069 3 ma for the metagabbro of sl4 metamorphic grade increases from the grenville front to the south the mafic rocks preserve relict igneous textures overprinted by garnet clinopyroxene that developed as coronas and or pseudomorphs after igneous phases the highest grade metagabbros contain omphacite and some lack primary plagioclase therefore b

40ar 39ar thermochronology along a western quebec transect of the grenville province canada ar ar thermochronology on 24 hornblendes 3 biotites 2 muscovites and 2 k feldspars collected along a 400 km long nw se geotraverse through the grenville province in western quebec is employed to provide time constraints on the intermediate and low temperature stages of cooling of part of the grenville orogen in the grenville front zone the c 1000 ma time of exhumation previously established from thermobarometric and isotopic studies is supported by the hornblende age data presented here from 60 km to 160 km se of the front reworked archaean migmatites of the parautochthonous reservoir dozois terrane rdt 1004 ma old metamorphic monazites contain hornblendes with 972 950 ma cooling ages assuming metamorphic geotherms between 25 and 30 c km 1 calculated cooling and unroofing rates are about 6 c ma 1 and 0 33 km ma 1 in the p t range 725 c 800 mpa and 450 c 400 mpa hornblendes from monocyclic rocks of the mont laurier and morin terranes mlt and mt monazite ages c 1165 ma give ages of about 1040 and 1010 ma respectively calculation of cooling unroofing rates from peak metamorphic conditions in this area is hampered by thermal perturbations associated with the still poorly dated grenville collision which took place approximately between 1060 and 1020 ma cooling ages of c 900 ma for muscovite and biotite and 860 810 ma for k feldspar show that cooling rates decreased to around 1 5 c ma 1 under retrograde greenschist facies conditions in the mlt on a time vs distance diagram the hornblende data define several distinct age ranges suggesting that each terrane had a characteristic thermal history thus cooling was diachronous and probably non homogeneous throughout this segment of the grenville orogen the time lag between the cooling history of the parautochthon 972 950 ma and the allochthons 1040 1010 ma is compatible with an earlier pre 1040 ma peak of metamorphism in the allochthons the reservoir cabonga allochthon was transported toward the nnw from its probable root zone in the mlt during the 1060 1020 ma grenvillian collision as a partially cooled slab the remobilization of the archaean parautochthon is attributed to this collision in the grenville front zone slightly older cooling ages and cooling rates initially faster than in the remaining part of the parautochthon are probably as a result of rapid tectonic exhumation shortly after collision the minor delay 20 30 ma in unroofing of the mt compared to the adjacent mlt is most likely related to post 1040 ma extensional displacement along the labelle shear zone in terranes like those described above where metamorphism is diachronous determination of cooling rates and the history of exhumation may be meaningless without a firm control on the regional structure however identification of contrasting cooling histories contributes to unravelling the independent movement of terranes

precise u pb ages for grenvillian and pre grenvillian thrusting of proterozoic and archean metamorphic assemblages in the grenville front tectonic zone canada while the position of the grenville front is placed at a structural feature associated with the onset of high grade rocks when approached from the north the time of first metamorphism in these rocks is in general pre grenvillian and highly variable at killarney amphibolite to granulite grade rocks were metamorphosed lineated and foliated to their present front parallel configuration mainly by 1453 6 ma north of north bay amphibolite grade migmatites have been subjected to a pre 2650 ma metamorphism granulite grade gneisses south of the grenville front near val d or record a 2640 ma metamorphic event south of chibougamau mafic enclaves in tonalite south of the front were converted to migmatitic amphibolite at about 2680 to 2690 ma in central labrador a 1650 ma foliated granite is present south of the grenville front and at least some of the banding is this old at the atlantic coast granulite facies metamorphism in front parallel gneisses predates a 1790 1662 ma deformation in contrast each of these areas shares a near common age of second metamorphism that is variable in degree and accompanies nw directed thrusting or crustal loading near killarney on georgian bay titanite records an episode of 977 2 ma partial resetting the degree of which increases with distance into the grenville province here grenvillian deformation where present is manifest as feldspar porphyroclastic fabric with strong se plunging lineations near north bay archean pegmatites in the grenville front zone were similarly deformed at 988 10 ma and strongly rodded ductile amphibolite underwent late melting to produce postlineation tonalite in which new zircon tips grew at 987 2 ma a 991 4 ma cooling age for titanite in this melt is analytically identical to the zircon growth age hence rapid cooling occurred south of chibougamau new zircons grew in high pressure coronite in response to tectonic loading at 995 6 ma and in central labrador melt pods that developed in 1650 granites in response to thrusting record a zircon new growth age of 995 2 ma titanite in the granitic host rock has an analytically identical age of 992 4 ma again indicating rapid cooling this second event is recorded by 28 titanite and zircon analyses for samples up to 225 km apart that record a 1651 5 ma to 988 5 ma two age mixing line grenville resetting at about 1037 19 ma in the grenville front zone on the labrador coast is indicated but major thrusting is pre 1662 ma u pb isotopic results indicate that the grenville front zone is characterized along its length by partially reset u pb systems high pressure assemblages local melting and rapid cooling crustal loading and shearing with lead loss related to mechanically induced recrystallization and local heat that was rapidly terminated by rebound and cooling are required