age of the brookville gneiss and associated rocks southeastern new brunswick the brookville gneiss is a medium to high grade quartz feldspar biotite and hornblende quartz feldspar biotite gneiss found north and northeast of saint john new brunswick the green head group

volcano monitoring by microgravity and energy budget analysis microgravity monitoring of active volcanoes can provide evidence of sub surface mass and or density changes that precede eruptions if in addition precursory increases in thermal emissions are observed an integrated mechanistic model for volcanic activity may be developed with the potential for forecasting eruptions crater lake volcanoes provide an interesting target for such studies since thermal output can be monitored simply through lake water calotimetry here we summarize 10 years of gravity and thermal data from poas volcano costa rica mass energy balance calculations demonstrate that in the steady state the large thermal inertia of the crater lake acts as a buffer to short term changes in the energy input from the cooling magma feeder pipe since february 1986 it is postulated that there has been gradual emplacement of a shallow magma intrusion associated with vesiculation and gas loss to the surface this follows from unambiguous gravity increases constrained by elevation control that are coincident in time with a period of long term increased energy input to the crater lake progressive reduction of the lake volume by evaporation seepage culminated in an april 1989 ash eruption providing a good documented record of combined gravity and thermodynamic precursors to volcanic activity

a simple magma driven thermal balance model for the formation of volcanogenic massive sulphides abstract a simple model is used to examine the balance of heat water flow and temperature in a long lived single pass hydrothermal system heated by magmatic heat the geometry of the system is based on mid ocean ridge observations cold water percolates downward to the top of a shallow magma chamber is heated by conduction through a thin boundary layer and discharges through a permeable fault zone modelled as a bundle of narrow rough pipes as heat input is increased the model shows a catastrophic temperature transition at an exit water temperature of between 340 and 410 c depending on seafloor pressure which in nature would probably be represented by surging flow and sudden changes in the specific volume of the circulating fluids to generate a 3 million tonne sulphide deposit with 70 efficiency of deposition from water at 350 c within 4000 years would require a heat flux of 110 w m 2 from the magma chamber for a mass flow rate of 140 kg s 1 if the plan area of the reacting part of the system is 2 km 2 the total heat input would be 3 10 19 j under these conditions the thickness of the solid boundary layer between liquid magma and circulating water will be about 10 m and the integrated reactive water rock ratio over the lifetime of the system will be about 30 decreasing the water temperature to 250 c would require a heat flux of only 60 w m 2 but the time to form a 3 million tonne deposit is increased to 33 000 years and the overall heat requirement is more than quadrupled

impact of subglacial geothermal activity on meltwater quality in the jokulsa a solheimasandi system southern iceland the influence of subglacial geothermal activity on the hydrochemistry of the jokulsa a solheimasandi glacial meltwater river south iceland is discussed a radio echosounding and global positioning system survey of south west myrdalsjokull the parent ice cap of the valley glacier solheimajokull establishes the geometry and position of a subglacial caldera a cauldron in the ice cap surface at the basin head is also defined signifying one location of geothermally driven ablation processes background h 2 s concentrations for the jokulsa meltwaters in summer 1989 show that leakage of geothermal fluids into the glacial drainage network takes place throughout the melt season chemical geothermometry na k ratio applied to the bulk meltwaters tentatively suggests that the subglacial geothermal area is a high temperature field with a reservoir temperature of 289 304 c a major event of enhanced geothermal fluid injection was also detected against a background of an apparently warming geothermal reservoir the event began on julian day 205 24 july with a burst of subglacial seismic activity meltwater hydrochemical perturbations followed on day 209 and peaked on day 213 finally leading to a sudden and significant increase in flow on day 214 the hydrochemical excursions were characterized by strong peaks in meltwater h 2 s so 2 4 and total carbonate concentrations transient decreases in ph small increases in ca 2 and mg 2 and sustained increases in electrical conductivity the event may relate to temporary invigoration of the subglacial convective hydrothermal circulation seismic disturbance of patterns of groundwater flow and geothermal fluid recruitment to the subglacial drainage network or a cyclic sweeping out of the geothermal zone by the annual wave of descending groundwater time lags between seismic events and meltwater electrical conductivity responses suggest mean and maximum intraglacial throughflow velocities of 0 032 0 132 ms 1 respectively consistent with a distributed drainage system beneath solheimajokull because increases in flow follow hydrochemical perturbations the potential exists to use meltwater hydrochemistry to forecast geothermally driven flood events in such environments

mantle crust transition zone and origin of wehrlitic magmas evidence from the oman ophiolite abstract abundant exposure and well preserved outcrops in the oman ophiolite allow detailed observation of the transition from mantle to crust in an oceanic environment the mantle crust transition zone shows large lateral variations in thickness and composition but consists essentially of residual dunites and a magmatic component present as clinopyroxene and plagioclase impregnations and gabbro lenses compositionally the transition zone marks a gradational passage from mantle to crust the moho may be placed at the base of the continuous gabbroic crustal section structures both within the transition zone and near the base of the crust are nearly parallel to the moho however the dominant flow mechanisms differ from the upper mantle to the lower crust plastic flow dominates in the upper mantle while viscous flow resulting in strong magmatic fabrics dominates at crustal levels wehrlitic bodies within the crustal gabbro section are intruded from the transition zone into which they are rooted the parent magma of the wehrlitic instrusions is in fact a crystal melt mixture composed of olivine and chromite probably including a component of mantle xenocrysts and xenoliths and of reequilibrated basaltic melt

microstructures and lattice fabrics in the hilti mantle section oman ophiolite evidence for shear localization and melt weakening in the crust mantle transition zone 1 experimental studies have shown that olivine aggregates with 4 melt are significantly weaker than melt free aggregates however questions remain as to the importance of melt weakening in nature in several studies melt weakening has been invoked to explain patterns of mantle flow in the oman ophiolite in this paper we reinvestigate evidence for melt weakening in the hilti mantle section using structural and microstructural methods the average olivine grain size increases with depth below the crust mantle boundary this is related to a change from equigranular to porphyroclastic microstructures at shallow levels to coarse porphyroclastic microstructures at depth a strong foliation and high degree of recrystallization in the upper part of the mantle section are interpreted as the strong imprint of localized deformation at stresses of 4 10 mpa lattice orientation data show that the high strain peridotites have recorded top to the west shear reversed shear senses were found deeper in the section since there is petrographic evidence for melt in the high strain zone strain localization was probably caused by a melt related weakening mechanism high melt contents required for melt weakening suggest that melt accumulated just below the crust mantle boundary we conclude that melt weakening is probably related to enhancement of grain boundary rather than intragranular deformation processes effective viscosities 2 1016 pa s may locally exist in the uppermost mantle beneath ridges if melt weakening occurs although our results agree with those of previous studies in the hilti massif we conclude that both the previously published active flow model and a ridge compression model can account for them

origin of complex upper mantle structures in the southern lewis hills bay of islands ophiolite newfoundland the springers hill area lewis hills bay of islands ophiolite may represent oceanic lithosphere created in close proximity to the nontransform segment of an oceanic fracture zone upper mantle rocks exposed on springers hill were investigated to establish whether their development was affected by the thermal and rheological changes associated with oceanic fracture zones harzburgites of the springers hill area reveal complex structural patterns on a small scale foliations defined by orthopyroxene grains intersect foliations defined by spinel grains at various angles olivine petrofabric work demonstrates that only the spinel foliation is related to the preserved flow plane the orthopyroxene foliation appears to be the result of pull apart of formerly larger grains during high temperature deformation on the larger scale orientation patterns of foliation lineation and dykes suggest that strike slip movement occurred parallel and at high angle to the fracture zone contact at various stages of a com

structure and evolution of the hengill grensdalur volcanic complex iceland geology geophysics and seismic tomography recent geological and geophysical research indicates the presence of three volcanic systems in the hengill grensdalur area and progressive westerly migration of the accretionary plate boundary the hengill system comprises the active hengill central volcano and a nne trending fissure swarm that is the present locus of crustal accretion in the area extinct volcanic systems are identified with the mountain hromundartindur and the grensdalur area and associated nne trending zones the grensdalur system was formerly the locus of accretion in the area a widespread geothermal area encompasses the whole area and is fueled by at least three distinct heat sources associated with the three volcanic systems a tomographic study of the upper 5 km of crust using local earthquakes imaged three bodies with velocities up to 15 higher than the average background velocities and volumes of several tens of cubic kilometers one of these underlies the grensdalur volcano and a second underlies the olkelduhals area within the hromundartindur system these are interpreted as intrusions that are the solidified magma reservoirs of their respective volcanic systems and the heat sources of those parts of the geothermal area above them the third high velocity body underlies the extinct basalt shield husmuli which is not associated with geothermal resources that body is interpreted as a cold intrusion that is the frozen magma conduit that fed the surface eruptive site a low velocity body with a volume of a few cubic kilometers was imaged in the depth range 2 4 km beneath the northern part of the presently active hengill central volcano this volume may contain partial melt and represent the heat source or part of the heat source fueling the hengill field heat balance calculations show that subsurface magmatism continued in the extinct grensdalur volcano long after volcanic activity and crustal accretion migrated from it to the hengill system

the hengill geothermal area iceland variation of temperature gradients deduced from the maximum depth of seismogenesis abstract given a uniform lithology and strain rate and a full seismic data set the maximum depth of earthquakes may be viewed to a first order as an isotherm these conditions are approached at the hengill geothermal area s iceland a dominantly basaltic area the likely strain rate calculated from thermal and tectonic considerations is 10 15 s 1 and temperature measurements from four drill sites within the area indicate average near surface geothermal gradients of up to 150 c km 1 throughout the upper 2 km the temperature at which seismic failure ceases for the strain rates likely at the hengill geothermal area is determined by analogy with oceanic crust and is about 650 50 c the topographies of the top and bottom of the seismogenic layer were mapped using 617 earthquakes located highly accurately by performing a simultaneous inversion for three dimensional structure and hypocentral parameters the thickness of the seismogenic layer is roughly constant and about 3 km a shallow aseismic low velocity volume within the spreading plate boundary that crosses the area occurs above the top of the seismogenic layer and is interpreted as an isolated body of partial melt the base of the seismogenic layer has a maximum depth of about 6 5 km beneath the spreading axis and deepens to about 7 km beneath a transform zone in the south of the area beneath the high temperature part of the geothermal area the maximum depth of earthquakes may be as shallow as 4 km the geothermal gradient below drilling depths in various parts of the area ranges from 84 9 ckm 1 within the low temperature geothermal area of the transform zone to 138 15 ckm 1 below the centre of the high temperature geothermal area shallow maximum depths of earthquakes and therefore high average geothermal gradients tend to correlate with the intensity of the geothermal area and not with the location of the currently active spreading axis

the volcanic stratigraphy and petrogenesis of the oman ophiolite complex the volcanic stratigraphy and trace element geochemistry of the oman ophiolite complex indicate a multistage magmatic origin comprising 1 magmatism due to sea floor spreading in a marginal basin 2 magmatism associated with discrete submarine volcanic centres or seamounts 3 magmatism associated with crustal uplift and rifting and 4 magmatism associated with continent arc collision trace element petrogenetic modelling is used to investigate the nature of the mantle source region and the partial melting and fractional crystallization history for each magmatic event the petrogenetic pathway for the sea floor spreading lavas requires a high degree of melting of a mantle that was depleted in incompatible elements prior to subduction but subsequently selectively enriched in certain elements mostly lil elements and h2o from an underlying subduction zone it also requires magma mixing in an open system magma chamber prior to eruption the seamount lavas were probably derived by a similar degree of partial melting of a similar source but fractional crystallization was restricted to smaller high level magma chambers the rifting lavas were derived from a mantle source that was more depleted than the seamount lavas prior to subduction but which was later modified by a larger subduction zone component the syn collision lavas were however derived from an enriched mantle source which probably underlay the passive continental margin rather than the marginal basin complex results such as these may provide considerable insight into the petrogenetic changes accompanying the transitions from spreading to arc volcanism in a supra subduction zone setting

origin and tectonic significance of the metamorphic rocks associated with the darvel bay ophiolite sabah malaysia banded hornblende gneiss foliated amphibolite hornblende chlorite and siliceous schist form lenses in an 8 km wide belt within the darvel bay ophiolite complex foliation in the belt is generally steep to vertical striking parallel to the trend of the belt and lineations are sub horizontal mineral and geochemical studies show that the metamorphic rocks represent banded and isotropic gabbros plagiogranites doleritic and basaltic dykes basaltic volcanics and cherts formed at a spreading ridge in a supra subduction zone environment which were deformed at high temperatures but low pressures along a transform fault incorporation of supracrustal cherts indicates that the transform extended for hundreds of kilometres between spreading centres garnet pyroxenites and amphibolites found as clasts in miocene volcanic agglomerates formed at high pressures and temperatures are interpreted as derived from a metamorphic sole underlying the complex formed during subduction of ocean crust and the emplacement of the ophiolite complex on sabah

geologic section through the samail ophiolite and associated rocks along a muscat ibra transect southeastern oman mountains regional mapping at a 1 60 000 scale of a 30 km strip from the gulf of oman muscat across the oman mountains 130 km to the south provides the geologic setting for the 95 m y ibra section of the samail ophiolite where best preserved the ibra ophiolite section is an 8 km thick section of oceanic consisting of 0 5 km of pillow lavas 1 2 1 6 km of sheeted diabase dike complex 0 2 1 0 km of high level noncumulate gabbro and 3 0 5 0 km of cumulate gabbro that is underlain by tectonite peridotite 9 12 km thick the ibra section is found on the southward dipping limb of the sayah hatat antiform the tectonite peridotite represents uniformly depleted harzburgite and dunite that have been deformed by high temperature low stress asthenospheric flow discordant dunites within the tectonite peridotite appear to represent either flow crystallization products from primary picritic liquids or reaction products of these liquids with the harzburgite the structural base of the tectonite peridotite is overprinted by a high stress low temperature deformation that can be related to its oceanic detachment the layered gabbros are predominantly olivine clinopyroxene plagioclase cumulates and orthopyroxene does not occur as a cumulus phase occurrence of cumulate wehrlites and picrites at high stratigraphic levels within the layered gabbros is evidence that the gabbroic section crystallized predominantly from the bottom upward in a periodically replenished magma chamber high level gabbro represents remnants of crystallization at the roof of the magma chamber and intrudes most overlying diabase dikes both the diabase dike complex and pillow lavas are hydrothermally altered and alteration and metamorphism increase downward zeolite to epidote amphibolite facies in spite of pervasive alteration relict primary mineralogy and bulk chemistry suggest that the diabase dikes and pillow lavas are cogenetic with the underlying gabbros the present day samail thrust surface truncates ophiolite stratigraphy and puts the samail ophiolite on top of unmetamorphosed hawasina melange the last motion on this surface was probably no older than maestrichtian 70 65 m y garnet amphibolites exposed as remnants of earlier thrusting 90 m y record initial ophiolite detachment at a 14 20 km depth within the tethyan oceanic lithosphere the hawasina group which underlays the samail ophiolite is a block melange where exposed near muscat in the north and it grades into an imbricated broken formation at the southern limit of the map area the hawasina group is thrust over permian to late cretaceous shelf carbonates that represent autochthonous arabian continental shelf deposits recent post miocene collapse and dome structures such as the ibra dome have complicated ophiolite stratigraphy south of jabal dimh our geologic studies strongly indicate the samail ophiolite represents a large coherent slab of transported oceanic lithosphere formed at a late cretaceous spreading center in the tethyan sea

chemical transport in geothermal systems in iceland evidence from hydrothermal alteration abstract this study focuses on the chemical changes in basaltic rocks in fossil low and high temperature hydrothermal systems in iceland the method used takes into account the amount of dilution caused by vesicle and vein fillings in the rocks the amount of dilution allows a calculation of the primary concentration of the immobile element zr and by multiplying the composition of the altered rock by the ratio of zr protolith zr altered rock one can compute the mass addition caused by the dilution of the void fillings and also make a direct comparison with the likely protoliths from the same areas the samples were divided into three groups two from tertiary fossil high temperature systems hafnarfjall geitafell and the third group from a low temperature zeolite altered plateau basalt succession the results show that hydrothermally altered rocks are enriched in si al fe mg and mn and that na k and ca are mobile but show either depletion or enrichment the elements that are immobile include zr y nb and probably ti the two high temperature systems show quite similar chemical alteration trends an observation which may apply to icelandic fresh water high temperature systems in general the geochemical data show that the major changes in the altered rocks from icelandic geothermal systems may be attributed to addition of elements during deposition of pore filling alteration minerals a comparison with seawater dominated basalt hosted hydrothermal systems shows much greater mass flux within the seawater systems even though both systems have similar alteration assemblages the secondary mineral assemblages seem to be controlled predominantly by the thermal stability of the alteration phases and secondarily by the composition of the hydrothermal fluids

segmentation at a fossil spreading axis the plutonic sequence of the wadi haymiliyah area haylayn block sumail nappe oman abstract wadi haymiliyah in the central part of the haylayn block offers a splendid continuous section across the crustal sequence of the sumail ophiolite the plutonic sequence of this area presents several specific features of great interest for the understanding of the structure and evolution of this ophiolite the petrological moho and the whole plutonic sequence display a subvertical attitude parallel to the mean dyke complex orientation high temperature folding occurred in the mantle sequence and in the lower crust whereas the upper crust remained unfolded and behaved as a rigid slab during this deformation resulting in a tectonic decoupling between lower and upper crust in the gabbro cumulate sequence a remarkable large scale curvature of the magmatic structures layering and lamination planes whose axis is totally discordant with the regional folds axes is interpreted as a primary magmatic feature indicating the southern ending of a magma chamber the lower crust about 2 km thick consists of a primary layered gabbroic sequence consisting of rhythmic mineral graded layers from several metres to several hundred metres thick syndepositional structures such as oblique stratification microslumps eroding mini channels and small normal faults are widespread the mineralogical composition modal variations and cryptic evolution of the whole layered sequence are described both at the megascale and on the simple cyclic scale the data support a model of crystallization of the layered sequence by open system fractionation at the floor and sides of a periodically replenished steady state magma chamber a unique feature of this cumulate sequence is the ubiquitous presence of orthopyroxene throughout the whole cumulate pile this is very unusual in the sumail crust with the exception of the wadi rajmi section in the fizh block two huge plagioclase free dunite wehrlite laccoliths about 300 m thick and numerous ultramafic sills and mushroom like bodies intrude the base of the layered sequence these ultramafic intrusions exhibit in places an internal layering and dunitic differentiation products they have sharp sinuous and lobate contacts with the gabbros which were still hot and probably not entirely consolidated at the moment of the intrusion the mineral compositions of these intrusions are much more primitive than those of the adjacent gabbros higher in the plutonic section the layered gabbros are overlain by 1 km of fine grained non layered planar laminated gabbronorites characterized by the abundance of pleochroic cumulus hypersthene the disappearance of olivine and the development of fe ti oxides and amphibole as post cumulus phases numerous occurrences of concordant coarse grained layered gabbros occur in this unit which is characterized also by a dense net of highly residual dioritic to tonalitic veins resulting in a huge magmatic breccia in the uppermost few hundred metres where the gabbronorites are brecciated by the acidic rocks hydrothermal veins are also extremely abundant in this unit which coincides with the core of the large scale curvature this unit comprising unusually fractionated rocks is interpreted as resulting from the evolution of the haymiliyah magma chamber towards a closed system fractionation due to the cutting off of source supply our interpretation of both structural and petrological data may be summarized by the following major points 1 1 the haymailiyah magma chamber was located at the southern tip of a spreading cell and was cut off from the magma supply at the end of its evolution either vertically when accretion slowed down or horizontally when it became isolated from the central part of the spreading segment we suggest that orthopyroxene bearing cumulates should be looked for all along the sumail nappe and used as markers of segmentation 2 2 early high temperature compressional deformation took place in both the mantle and crust before the intraoceanic detachment of the ophiolite the development of this compression regime activated filter pressing processes and in particular helped with the extraction of wehrlitic magmas from the upper mantle and their ascent to the uppermost levels of the crust

radar imaging of glaciovolcanic stratigraphy mount wrangell caldera alaska interpretation model and results glaciological measurements and an airborne radar sounding survey of the glacier lying in mount wrangell caldera raise many questions concerning the glacier thermal regime and volcanic history of mount wrangell an interpretation model has been developed that allows the depth variation of temperature heat flux pressure density ice velocity depositional age and thermal and dielectric properties to be calculated some predictions of the interpretation model are that the basal ice melting rate is 0 64m yr 1 and the volcanic heat flux is 7 0 w m 2 by using the interpretation model to calculate two way travel time and propagation losses radar sounding traces can be transformed to give estimates of the variation of power reflection coefficient as a function of depth and depositional age prominent internal reflecting zones are located at depths of approximately 59 91 m 150 m 203 m and 230 m we attribute these internal reflectors to buried horizons of acidic ice possibly intermixed with volcanic ash that were deposited during past eruptions of mount wrangell the depositional dates corresponding to these reflecting zones are 1950 1924 1853 1770 and 1728 a d and we use this information to propose a volcanic history for mount wrangell

variations in the thermal output of the subglacial grimsvotn caldera iceland a 69 year record 1922 1991 of heat release has been obtained from the subglacial geothermal system of the grimsvotn caldera within the vatnajokull ice cap the data were derived from in situ measurements of the volume of meltwater accumulated in the caldera lake subtracting climatically induced melting the overall fluctuations in the heat flux are closely related to volcanic activity and are dominated by a main pulse of 11 600 mw caused by a major eruption in 1938 gradually declining to 1600 mw in 1976 1982 heat extracted from the roof of a magma chamber with the aid of hydrothermal convection may have given a basic contribution to the heat flux of 1500 to 2000 mw an upper bound the variable part of the heat flux from 0 to 10 000 mw was released from magma erupted at the base of the glacier and from shallow intrusions the total heat released over the period 1922 to 1991 was 8 1 1 6 1018j equivalent to the energy released by the solidification and cooling of 2 1 0 4 km of basaltic magma the contribution to the total heat flux was 45 max from a magma chamber 35 min from shallow intrusions and 20 from eruptions this implies that magma at the roof of a chamber solidified and cooled at the rate of 1 2 1 6 107 m a 1 or about 1 km over the last 69 years heat release at grimsvotn was probably more intense in the 19th century when volcanic eruptions were more frequent

late miocene valley confined subglacial volcanism in northern alexander island antarctic peninsula isolated late miocene volcanogenic sequences in northern alexander island antarctic peninsula form an unusual cogenetic association of volcaniclastic sandy gravelly lithofacies including tillites and volcanic lava hyalocalstite lithofacies using simple lithofacies analysis and theoretical considerations of hydrodynamic effects of subglacial eruptions valley confined volcanic activity beneth thin wet based ice is suggested the alexander island successions are complete enough to be regarded as model sequences for this uncommonly recorded type of eruptive depositional activity the sedimentary lithofacies represent resedimented tuffs and meltout or flow tills which were probably deposited in subglacial ice tunnels eroded or enlarged by volcanically heated meltwater the volcanic lithofacies formed by the interaction of hot magma with the ice tunnel walls generating abundant meltwater and water saturatedsediments resulting in the formation of heterogeneous masses of lava and hyaloclastite there is no obvious sequence organisation in the sedimentary sections this is probably due to a complex interplay of eruption related and environmental hydrodynamic factors affecting the relative proportions of water and entrained sediment

ramifications of the skin effect for crater lake heat budget analysis abstract mass energy and chemical balance analysis for volcanic crater lakes offers a means to interpret magmatic hydrothermal processes in addition to water temperature several meteorological parameters including windspeed rainfall solar irradiation air temperature and humidity need to be evaluated in order to balance heat fluxes conventional contact thermometry yields bulk water temperatures however these are typically a few degrees or tenths of a degree higher than the water surface temperatures which should be used in several bulk aerodynamic and other formulae for heat flux calculations the magnitude of overestimation of derived evaporative flux that can arise from ignoring this skin effect is shown here to depend on bulk water temperature air temperature and other meteorological factors field observations obtained at kawah ijen indonesia with a lake bulk temperature of 43 1 c suggest a bulk skin temperature difference of around 1 5 c leading to an overestimation of evaporative and sensible heat fluxes of 10 if the skin effect is overlooked this error can be minimised by assuming a value for the bulk skin temperature difference or eliminated by ensuring that evaporation formulae are parameterised with the temperatures skin or bulk for which they were originally defined

ice volcano interaction of the 1996 gjalp subglacial eruption vatnajokull iceland volcanic eruptions under glaciers can cause dangerous floods and lahars1 2 3 and create hyaloclastite fragmented glassy rock mountains4 5 6 7 8 but processes such as the rate of heat transfer between ice and magma edifice formation and the response of the surrounding glacier are poorly understood because of the lack of data here we present observations from the fissure eruption at vatnajokull ice cap iceland in october 1996 in the 13 days of the eruption 3 km3 of ice were melted and the erupted magma fragmented into glass forming a hyaloclastite ridge 6 7 km long and 200 300 m high under 500 750 m of ice meltwater of temperatures of 15 20 c flowed along a narrow channel at the glacier bed into the grimsvotn subglacial lake for five weeks before draining in a sudden flood or jokulhlaup subsidence and crevassing of the ice cap occurred over the eruptive fissure and the meltwater path whereas elsewhere the glacier surface remained intact suggesting that subglacial eruptions do not trigger widespread basal sliding in warm based glaciers

magma chamber profiles from the bay of islands ophiolite complex models of the oceanic lithosphere derived from constraints imposed by the geology of ophiolite complexes require a magma chamber to produce the cumulate and other plutonic portions of the oceanic crust and upper mantle evidence from the bay of islands ophiolite complex indicates that layered plutonic rocks have not formed solely due to differential crystal settling to the horizontal floor of a magma chamber but have formed largely by heterogeneous nucleation and in situ crystallization along gently inclined as well as steeply inclined bounding surfaces of a large magma reservoir coherent well defined and systematically oriented large scale arcuate patterns described by mesoscopic igneous layering have been identified in the plutonic sections of two ophiolite massifs in the bay of islands complex these layering patterns seem to outline the shape of a large continuously evolving steady state magma chamber that extended along a ridge axis and ended at a transform fault intersection

fracture analysis of a volcanogenic massive sulfide related hydrothermal cracking zone upper bell river complex matagami quebec application of permeability tensor theory located in the matagami mining district of the abitibi greenstone belt quebec the bell river complex is a 5 km thick tholeiitic layered gabbro and or anorthosite body 2724 6 2 5 1 9 mau pb which likely acted as the heat source that drove hydrothermal convection and volcanogenic massive sulfide vms hydrothermal mineralization in the area abundant fractures and veins crosscutting the western lobe of the bell river complex formed over a range of temperatures from 250 to 700 c the 250 to 400 c assemblage quartz epidote sericite chlorite plagioclase is the most widespread and occurs as orthogonal anastomosing and random vein sets vein densities average between 15 and 25 veins per m2 locally reaching as high as 40 to 60 veins per m2 these veins typically 1 to 3 mm wide are interpreted to represent thermal cracking associated with hydrothermal fluid mineralization in the district furthermore they crosscut earlier higher temperature pyroxene plagioclase 600 c and magnetite rich 300 600 c veins 1 10 mm wide densities commonly 0 5 veins per m2 detailed field measurements of quartz epidote vein geometries coupled with permeability tensor theory have produced a first order approximation of the maximum model permeability structure veins unfilled of the hydrothermal cracking zone representative district wide values indicate maximum model bulk permeabilities of 10 10 to 10 8 m2 for the hydrothermal cracking zone similar to permeabilities calculated for the fractured sheeted dike complexes of the semail and troodos ophiolites however a high flow zone located within the central parts of the hydrothermal cracking zone is characterized by a maximum model permeability of 10 7 m2 locally 1 m2 the high flow zone reaches maximum model permeability values as high as 10 6 to 10 5 m2 where two or more veins occur with apertures in excess of 2 cm mapping has shown that the hydrothermal cracking zone is confined to an 350 m thick interval located within a strongly layered gabbro and or anorthosite horizon of the layered zone upper bell river complex the base of this interval is 1 000 m below the top of the bell river complex furthermore in and around the town of matagami the dominant orientation of quartz epidote veins parallels the orientation of the layering 110 15 this parallelism is further reflected in the orientation of the calculated quartz epidote vein permeability tensors 94 30 1 n 71 the parallelism suggests that heterogeneities associated with the layer contacts provided planes of low tensile strength along which the hydrothermal cracks preferentially developed such an interpretation further explains why the hydrothermal cracking zone appears to be restricted to the strongly layered zone there are many more low tensile strength planes zones of quartz epidote veining approximately orthogonal to layering are also present the orthogonal veins are usually less continuous and commonly truncated but not crosscut by layer parallel veins the orthogonal subset of veins is interpreted to represent short pathways that allowed fluids to travel between adjacent layer parallel veins however locally longer layering orthogonal quartz epidote veins showing offsets of 10 to 30 cm normal to the layering of the bell river complex may represent initial conduits that allowed fluids to travel into overlying stratigraphy and through to the paleosea floor although volumetrically small in comparison with the permeability structure of the bulk sea floor the significantly higher permeability of the hydrothermal cracking zone indicates its importance in controlling the flow paths and fluxes of fluids deep within the hydrothermal system

seismological constraints on the gabbro eclogite transition in subducted oceanic crust abstract dispersion of seismic waves travelling along the strike of the subducted pacific plate beneath tonga kermadec shows evidence for a high velocity layer of crustal thickness extending to a depth of at least 450 km the opposite dispersion with low frequencies arriving first is observed in japan and has been taken together with inferences from converted phases as evidence of a thin low velocity layer both layers have in previous studies been interpreted as subducted oceanic crust which has been transformed to eclogite but in tonga kermadec the eclogite must have high seismic velocity and beneath japan it must have a low seismic velocity an apparent contradiction therefore exists here we find theoretical estimates of the seismic velocities of eclogite and normal mantle using the data and equations of duffy and anderson and a variety of mineral assemblages and bulk compositions we find a range of both low and high seismic velocities the high end of the range depending mainly on the contents of jadeite pyrope and grossular it is therefore possible in principle to explain both the high velocity layer beneath tonga kermadec and the low velocity layer beneath japan as being consistent with an eclogitic crust provided they have significantly different compositions however we can find no clear reason for such a difference between that part of the pacific plate currently subducting beneath japan and that beneath tonga kermadec

penetration of water into hot rock boundaries of magma at grimsvotn the grimsvotn geothermal area is located within one of the most active caldera volcanoes in iceland in the interior of the ice cap vatnajokull at an altitude of 1 400 m fig 1 its setting oilers unique conditions for calorimetric measurement of the heat release from the subglacial geothermal system the melting of ice due to the geothermal activity creates a depression in the surface of the ice cap ice and water are diverted towards the depression from a 300 km2 drainage basin the meltwater accumulates in a 30 km2 subglacial lake periodic outbursts of water jokulhlaups drain the lake subglacially down to the skeidararsandur plain we propose here that penetration of water into hot rock is the primary reason for the intense heat release 5 000 mw thermal of the subglacial grimsvotn geothermal area injection of water into boundary rocks of magma should be considered as a method of heat exploitation

low pressure cordierite bearing migmatites from kelly s mountain nova scotia the kelly s mountain gneiss complex of cape breton island nova scotia is a migmatitic paragneiss dominated by biotite and cordierite bearing assemblages metamorphic grade throughout the complex is in the upper amphibolite facies with garnet absent and only retrograde muscovite present in the high grade core of the complex the reaction biotite andalusite quartz cordierite k feldspar sillimanite ilmenite h2o is preserved the pelitic migmatites contain cordierite and k feldspar rich leucosomes and biotite rich melanosomes minor clinopyroxene bearing amphibolite in the complex does not show migmatitic textures the migmatites are interpreted as in situ peraluminous partial melts on the basis of phase relations and textural criteria retrograde metamorphism under conditions of high fluid pressure locally produced muscovite after k feldspar and muscovite green biotite chlorite after cordierite in paragneiss and sphene after ilmenite in amphibolite peak metamorphic conditions of 1 3 5 kb and 580 700 c are estimated the high geothermal gradient inferred from these conditions was probably caused by the intrusion of diorites associated with the gneiss complex the kelly s mountain complex represents a rare example of migmatites formed in the low pressure facies series and illustrates some of the reactions involving melting in high grade pelitic rocks

structure of the sheeted dike complex of the samail ophiolite near ibra oman the sheeted dike complex is a regionally mappable geologic unit with a consistent position in the samail ophiolite stratigraphy discontinuous exposures of the complex in the ibra area of the southeast oman mountains provide structural data that allow reconstruction of a paleo spreading ridge axis of 347 for the region and suggests a maximum spreading width of 275 km for the ophiolite contact relations within the well exposed al ahmadi section indicate that the complex was formed by multiple parallel intrusion over a narrow zone at the paleo ridge axis split dikes are abundant facing directions indicate a spreading ridge southwest of the present outcrop area in conflict with other spreading axis direction indicators

seismic constraints on magma chambers at hekla and torfajokull volcanoes iceland hekla and torfajokull are active volcanoes at a rift transform junction in south iceland despite their location next to each other they are physically and geologically very different hekla is an elongate stratovolcano built mainly of basaltic andesite torfajokull is a prominent rhyolitic centre with a 12 km diameter caldera and extensive geothermal activity the scope of this study is to examine the propagation of body waves of local earthquakes across the hekla torfajokull area and look for volumes of anomalous s wave attenuation which can be evidence of magma chambers so far the magma chamber under hekla has been modelled with various geophysical means and its depth has been estimated to be 5 9 km a data set of 118 local earthquakes providing 663 seismic rays scanning hekla and torfajokull was used in this study the major part 650 seismograms did not show evidence for s wave attenuation under these volcanoes only six seismograms had clear signs of s wave attenuation and seven seismograms were uncertain cases the data set samples hekla well at depths of 8 14 km and south part of it also at 4 8 km and 14 16 km western torfajokull is sampled well at depths of 4 14 km eastern and southern torfajokull at 6 12 km conclusions cannot be drawn regarding the existence of magma beyond these depth ranges also magma volumes of smaller dimensions than about 800 m cannot be detected with this method if a considerable molten volume exists under hekla it must be located either above 4 km or below 14 km the former possibility seems unlikely because hekla lacks geothermal activity and persistent seismicity usually taken as expressions of a shallow magma chamber an aseismic volume with a diameter of 4 km at the depth of 8 km in the west part of torfajokull has been inferred in earlier studies and interpreted as evidence for a cooling magma chamber our results indicate that this volume cannot be molten to a great extent because s waves travelling through it are not attenuated intense geothermal activity and low frequency earthquakes are possibly signs of magma in the south part of torfajokull but a magma chamber was not detected there in the areas sampled by this study

the biostratigraphy of sediments in the volcanic unit of the samail ophiolite sedimentary rocks of pelagic and ocean ridge affinity interbedded with or overlying the volcanic sequence at the top of the oman ophiolite contain datable radiolarian faunas that indicate 1 that the lavas were erupted in an oceanic environment during a narrow time interval ranging from the early cenomanian to the early turonian and 2 that the age progression within this volcanic sequence matches its development from a ridge axis to an off axis setting emplacement of the ophiolite has been dated as late campanian early maestrichtian glennie et al 1974 thus the time gap between formation and emplacement was very narrow

structural implications of some radiometric ages of igneous rocks in southeastern new england u th pb zircon rb sr whole rock and k ar hornblende isotopic ages reveal two major episodes of igneous activity in southeastern new england one in late proterozoic z time 600 650 m y ago and the other in devonian to ordovician time 375 450 m y ago the dated rocks are discussed in the context of several individual structural provinces that evolved more or less independently of each other prior to middle to late paleozoic assembly these provinces generally are bounded by major faults and or abrupt changes in metamorphic grade and from east to west include i the milford dedham zone ii the putnam nashoba zone iii the merrimack synclinorium and iv the bronson hill zone from the present and other recently published studies reasonably accurate ages representative of the two major episodes can now be assigned to many of the igneous rocks other units remain less well constrained but probably may be placed into one or the other of the general age groupings case by case examination of the geochronologic data makes us cautious in accepting some apparent fine structure age resolution within each grouping until factors such as inherited zircon and metamorphic overprinting can be better evaluated with the above mentioned qualifications in mind igneous rock ages summarized according to structural province are as follows i milford dedham zone a late proterozoic z dedham granite and related rocks 630 15 m y 595 15 m y 612 646 m y milford granite 630 15 m y 591 50 m y hope valley alaskite gneiss and scituate granite gneiss ages uncertain unnamed granite near assonet 600 650 m y b devonian to ordovician alkalic rocks including quincy cape ann and peabody granites wenham monzonite and nahant gabbro ii putnam nashoba zone a late proterozoic z no plutonic rocks identified but may include some metavolcanic rocks b devonian to ordovician sharpners pond diorite 430 5 m y andover granite 408 22 m y 450 22 m y muscovite granite near west berlin age uncertain preston gabbro 424 5 m y ill merrimack synclinorium a late proterozoic z granitic orthogneiss unit of the massabesic gneiss complex 600 620 m y metavolcanic unit of the massabesic gneiss complex 646 m y or older b devonian to ordovician fitchburg complex at maiden hill 402 11 m y fitchburg complex at rollstone quarry 390 15 m y newburyport complex 450 15 m y ayer granite 433 5 m y chelmsford granite age uncertain muscovite biotite granite at millstone hill 372 7 m y canterbury gneisses 395 10 m y 392 9 m y iv bronson hill zone a late proterozoic z dry hill gneiss of the pelham dome 600 m y b devonian to ordovician monson gneiss 440 10 m y these distinctive age patterns together with contrasts in petrography and texture of the igneous rocks in stratigraphy of the country lock and in metamorphic grade accentuate the separate geologic development of each structural province no correlation involving preassembly geology should be attempted across province boundaries without due consideration of the exotic character of each province

the northern samail ophiolite an oxygen isotope microprobe and field study geological petrological and oxygen isotopic data are presented for 228 whole rock and mineral samples collected from a 100 20 km area of the northern samail ophiolite in oman most of these samples are from three detailed profiles through the pillow lavas sheeted dikes and layered gabbros of this laterally heterogeneous fragment of cretaceous oceanic crust down to and across the petrologic moho the profiles encompass a range of petrologic and tectonic styles and each profile exhibits distinctive 18 16 o variations compared to one another and to mid ocean ridge basalts as a result of pervasive seawater hydrothermal interaction that varied in intensity along strike in the ophiolite in general 18 o depletions are observed in the layered gabbros and 18 o enrichments in most of the sheeted dikes and pillow lavas similar to results previously observed in the southern part of the ophiolite ibra area where 18 o depletions within the gabbroic section are quantitatively balanced by 18 o enrichments in the shallower parts of the oceanic crust the wadi hilti profile selected as an example of relatively intact crust differs from ibra in having more uniform and slightly higher 18 o in the gabbros 5 4 to 6 3 as well as in containing more hydrous alteration minerals amphibole epidote chlorite and prehnite the profiles in the wadi kanut shafan and wadi rajmi sections are much more complex and reveal the impact of off axis intrusions and deep crustal shearing plagiogranite wehrlite intrusions in the shafan kanut area superimposed a local hydrothermal aureole on the ophiolite evident in dikes highly depleted in 18 o quartz sulfide veins abundant epidote thullite and chlorite in shallower rocks and low temperature hydrous alteration of deeper gabbroic rocks the latter produced an overall increase in whole rock 18 o 6 2 to 6 9 such late stage intrusions are found throughout the northern half of the samail ophiolite the wadi rajmi area which is a possible fossil transform or propagating rift represents the most complex of the three profiles it also contains the most abundant highly deformed and hydrothermally altered rocks together with the deepest and largest zone of 18 o depletion yet found in any ophiolite locally 18 o 2 0 conduits for large volumes of high temperature hydrothermal fluids were provided by fractures now occupied by low 18 o gabbro pegmatites and low 18 o dikes material balance estimates for the regional samples and from the various transects through the ophiolite give crustal bulk 18 o averages 5 9 to 6 3 that are within sampling error almost identical to the average morb basalt value of about 5 8 if both vertical and lateral crustal heterogeneities are integrated into a three dimensional model this supports and amplifies the conclusion of earlier workers that the 18 o of seawater is buffered and controlled by hydrothermal interaction with oceanic crust as long as the cumulative effects both spatial and temporal of all seafloor magmatic hydrothermal processes are considered the very slight out of balance enrichment of the integrated crustal average 18 o compared to morb may be explained by the ubiquitous mineralogical and isotopic evidence for a late low temperature alteration event in the basal gabbros these effects are prominent in the vicinity of the petrologic moho and may indicate exchange with low temperature aqueous fluids during or after detachment of the obducted slab

temperature flow statistics and thermomechanics of low temperature geothermal systems in iceland abstract a survey of low temperature geothermal activity in iceland shows that there is a strong positive correlation between the temperature and the mass flow of the systems the relation is very clearly displayed by the thermal systems in central northern iceland subdividing the temperature interval from 20 dgc to 100 c into 8 intervals of equal length and plotting the average system flow in each interval against the average temperature of the interval we find that the mass flow increases with the temperature at the lower end this relation is the result of a conductive cooling of the thermal water as it ascends to the surface at temperatures above 50 c convective effects appear to be the dominant cause a very simple lumped element analysis of the low temperature systems results in reasonable estimates of important flow parameters the convective downward migration of fracture spaces along the walls of mafic dikes appears to be a dominant thermomechanical process in the development of the low temperature systems in northern iceland

possible correlation of precambrian rocks of newport rhode island with those of anglesey wales the bedded succession in newport rhode island consists of the metavolcanic price9s neck formation and the older metasedimentary newport formation the newport formation consists of turbidites sandstone and conglomerate slate phyllite bedded ash and olistostromes the price9s neck formation consists of agglomeratic breccia conglomerate coarse to fine tuff and graded tuffite merging into tuffaceous sedimentary rock both sequences have been affected by two principal fold generating deformations the price9s neck formation was first folded into upright to overturned folds f 1 whereas the newport formation was folded into overturned to recumbent folds in the price9s neck formation the f 1 folds trend east west to north northeast south southwest whereas the f 2 folds trend approximately east west in the main outcrop area the f 1 folds in the newport formation are dominantly north south trending are strongly overturned toward the east and in the west are recumbent and subisoclinal to isoclinal the second deformation produced a low lying cleavage and relatively small folds that verge toward the east the difference in the style of folding is attributed to ductility contrast between the rocks of the two stratigraphic sequences the newport granite which intrudes the price9s neck and newport formations has a metamorphic aureole the granite was emplaced prior to the f 2 movements but after the f 1 movements the succession therefore is late precambrian in age because the granite is about 600 m y old the deformation of the sediments may be attributed to the avalonian orogeny that is manifested in anglesey wales and northern france as the cadomian there are strong similarities between anglesey and newport in lithologies stratigraphic succession and plutonic history

oceanic ridge crest processes oceanic ridge crest processes constitute a vigorous frontier of multidisciplinary research with global ramifications in the earth atmospheric and biologic sciences this deep ocean frontier is gradually opening in response to investigations which are elucidating the role of these processes in the lithosphere hydrosphere atmosphere and biosphere and the application of advanced instrumental and interpretive techniques which are increasing the quality and quantity of data these techniques include crustal drilling with borehole logging deep sea drilling project reports high resolution bathymetry sea beam etc side scan sonar seamarc i and ii gloria etc imagery with video and photography multichannel seismics determination of water column properties ctd with rosette sampler etc and submersible applications

the evolution of the avalonian plate abstract the avalon platform of newfoundland is the type locality of the avalonian plate that is characterized by late precambrian volcanic and clastic rocks marine at the bottom and terrestrial at the top all are intruded by 650 550 ma granites late precambrian rocks in the southern british isles are similar in both areas northeastern parts of the belt are affected by deformation and metamorphism that have orogenic aspects the orogeny is referred to as the avalonian in newfoundland and monian in britain to the southeast of the avalonian rocks in europe lies another late precambrian cadomian orogenic belt of rocks as a part of the pan african orogenic belt the cadomian and avalonian belts extend into north africa morocco since these belts are ophiolite bearing it is suggested that the plate was bordered by oceanic domains it is therefore proposed that a microcontinental volcanic arc that was generated in the late precambrian was orogenically deformed by collision with north america europe and africa and survived as a coherent terrain until the late carboniferous

stable isotopic aspects of fluid flow in the krafla namafjall and theistareykir geothermal systems of northeast iceland abstract water and steam condensates from the krafla namafjall and theistareykir geothermal systems have been studied using hydrogen and oxygen stable isotope ratios to gain more information on thermal fluid flow the effect of rifting on individual fields and the provenance of recharge to these fields at krafla there are minor differences between individual wellfields but recharge is essentially local only one well shows signs of a change in composition over a 6 year production period but a local volcanic eruption in 1984 affected fumarole steam compositions the nearby namafjall field has isotopic compositions suggesting that recharge is ultimately derived from a long distance to the south perhaps by a combination of surface and groundwater flow some local spring and shallow well compositions have been affected by the post 1975 rifting episode the undrilled theistareykir field has an inflow estimated to be of similar isotopic composition to namafjall some fumaroles have an anomalously depleted 18o composition and rayleigh condensation is postulated to explain them

the structures of two ophiolite massifs bay of islands newfoundland a model for the oceanic crust and upper mantle abstract for a detailed analysis of their main structures accumulation planes foliation planes and lineations the peridotite gabbro units of table mountain and blow me down mountain bay of islands newfoundland have been divided into several structural units each one being characterized by specific microstructures of deformation or accumulation a fishbone pattern with the accumulation plane in the mafic cumulates dipping toward the ridge and the foliation plane in the ultramafic tectonites dipping away from it is postulated for the structure of the lithosphere formed at a spreading center the tectonic structures in the peridotites are related to asthenospheric flow in the uppermost mantle and the direction of the ridge is inferred from the lineation trend in the peridotites the structures in the cumulates are related to subsidence of the magma chamber floor during spreading at the base of the massifs the tectonic structures in the mafic metamorphic rocks are parallel to those observed the latter have been deformed at moderate temperatures under high stresses and display mylonitic textures at their contact with the mafic tectonites from these superimposed textures we infer a kinematic model for the oceanic thrusting of the lithosphere the direction of thrusting is given by the lineation trend and its sense by the analysis of shear all these data are combined into a paleogeographic scheme for the newfoundland caledonides

evaluation of upper mantle microstructures in the table mountain massif bay of islands ophiolite within the undisrupted 6 km thick mantle section of the table mountain massif bay of islands ophiolite newfoundland several distinct peridotite microstructures are recognized in the uppermost mantle section very high temperature t 1200 high strain microstructures with evidence for syntectonic melt infiltration are preserved remarkable among these is an orthopyroxene impregnated dunite at an intermediate level of the mantle section peridotites have been affected by a weak lower temperature t 1000 1100 c overprint they are underlain by a thin sliver of low strain high temperature peridotites all these peridotites are interpreted to have formed during spreading related strain lower temperature t 1000 c high stress microstructures with high accumulated strain are locally preserved at intermediate levels of the mantle section and are regionally present at its base they culminate in metasomatic ultramafic ultramylonites and are related to detachment of the ophiolite with the exception of the basal ultramylonitic peridotites the activated olivine slip system was 0kl 100 in the highest temperature peridotites a weak preference for the activation of distinct slip planes 010 001 011 was noted based on piezometry and olivine flow laws strain rate estimates for the basal detachment fabrics are 10 12 s 1 this strain rate is consistent with the observed width of detachment overprint in the ophiolite and reasonable plate tectonic convergence rates strain rate estimates for the very high temperature peridotites are even higher recent data about grain growth rates in olivine indicate that in order to preserve the peridotite microstructures olivine grain growth must have been highly impeded olivine flow law data suggest that even peridotites with lower temperature microstructures t 900 950 c could form within the rheological asthenosphere the detachment or spreading related origin of peridotites with a lower temperature overprint may thus only be concluded from the regional geological context

mixing characteristics and compositional differences in mantle derived melts beneath spreading axes evidence from cyclically layered rocks in the ophiolite of north oman within the ophiolite of north oman interlayered gabbros and peridotites form a unit up to 3 5 km thick between pervasively foliated harzburgites and dunites of mantle origin and isotropic gabbros and plagiogranites spatial and chemical relationships within this sequence require the existence of a large reservoir 20 km wide 3 5 km high of tholeiitic magma that was periodically replenished by mantle derived picritic to tholeiitic melts during the spreading process cyclicity in the appearance of minerals during cooling of the magma is marked by the repetition of stratigraphies in the layered rocks within these cyclic units basal peridotites with magnesian olivines and pyroxenes are abruptly overlain by gabbros with less magnesian mafic phases this abrupt discontinuity is explained by density stratification set up at the base of the chamber whenever a picritic melt influx occurs by virtue of its greater density than tholeiitic magma sparks et al 1979 any picritic magma emplaced into the base of the tholeiitic magma reservoir would spread out along the floor of the chamber with only limited mixing taking place across the liquid liquid interface cooling of this basal layer by contact with the cooler overlying magma would result in the precipitation of olivine chrome spinel pyroxene from the picritic liquid this process would continue until the now more evolved input liquids were incorporated into the tholeiitic magma resevoir the precipitation of a gabbroic layer would then take place until a new magma input occurred to repeat the cycle variable thicknesses of the cyclic units different liquid lines of descent and various entry points within the liquid line of descent represented by the lowermost layered rocks reveal that the input liquids varied in volume primary composition and the extent and nature of prefractionation in the mantle liquids expelled from the chamber as lavas and dykes are shown to be more fractionated than liquids precipitating the layered rocks and are considered to have undergone additional fractionation within a zone of underplated gabbro and plagiogranite beneath the dyke swarm the existence of a large magma reservoir supports earlier interpretations based on age tilton et al 1981 and geochemical data pearce 1980 that the oman ophiolite is an analogue for fast spreading ridges

mantle flow at oceanic spreading centers models derived from ophiolites abstract the internal structures of eleven ophiolite complexes have been structurally investigated the attitude of the foliation and the lineation in the peridotites is related to asthenospheric flow in the uppermost mantle at an oceanic spreading center from the orientation of these structures with respect to the paleohorizontal plane two oceanic situations are distinguished the horizontally spreading asthenosphere illustrated by the table mountain massif newfoundland is characterized by 1 a homogeneous regular and sub horizontal plastic flow pattern over large distances 2 b flat paleo mono and absence of large scale folding 3 c rare economical chromite deposits and depleted mantle the diapirically spreading asthenosphere illustrated by the acoje type philippines displays 1 a inhomogeneous and sub vertical plastic flow pattern 2 b deformed paleo moho and large scale folds 3 c important economical chromite deposits and less depleted mantle presumably due to secondary magmatic impregnation this leads to vogt s model of asthenospheric flow beneath an oceanic ridge the direction of which being equated with the dike swarm in this model the horizontally spreading asthenosphere is derived from the diapirically spreading one in the following way the vertical mantle flow at the diapir center diverges horizontally in every direction including that parallel to the ridge at about 50 km from the diapir center the mantle flow becomes progressively perpendicular to the ridge due to the damming effect of transform faults an important economical consequence of this model is that large chromite deposits are expected preferentially in the diapiric type of ophiolite because chromite is preferentially precipitated at the diapir center from pristine magma