general features of the uppermost mantle stratification from long range seismic profiles abstract long range seismic studies made by russian institutions during the last two decades reached depths of 700 800 km on the continent peace nuclear explosions were used as a source and 100 km in the ocean ocean bottom stations recording 500 kg explosions in water comparison of the wave fields of these and other seismic profiles in russia western europe and the south atlantic shows that the classical lithosphere asthenosphere model is too simple to explain the observed data a stratified medium with alternating weak and rigid layers is a more realistic model of the upper mantle one of the layers is globally observed at a depth of 80 to 100 km it is located in the thermal lithosphere beneath old platforms at the bottom of the lithosphere under active tectonic areas and inside the asthenosphere below mid oceanic ridges relatively high velocities 8 4 to 8 5 km s are typical for this boundary both in the old platforms and in the high heat flow oceanic areas a change of structural pattern where the block structure of the uppermost mantle is transformed into a horizontally homogeneous one is typical at that boundary level such features suggest that the boundary separates the brittle and more ductile upper mantle layers and may be considered as the bottom of the global mechanical lithosphere

rapid high temperature formation of large scale rheomorphic structures in the 2 06 ma huckleberry ridge tuff idaho usa in the teton river canyon eastern idaho the ca 2 06 ma 130 m thick huckleberry ridge tuff exhibits large scale rheomorphic fold geometries defined by eutaxitic fabrics parallel to both the primary internal zonation and the basal contact with older strata paleomagnetic data from a large amplitude 150 m northwest trending overturned fold near the failed teton dam indicate folding above maximum magnetization unblocking temperatures 580 c the in situ characteristic remanent magnetization chrm direction is indistinguishable from previous studies of undeformed huckleberry ridge tuff and a fold test is negative k minimized at 100 unfolding anisotropy of magnetic susceptibility data reveal magnetic foliation planes that dip northeast roughly parallel to the axial surface of the fold because deformed and undeformed huckleberry ridge tuff exposures preserve the same anomalous chrm direction large scale rheomorphic structures in the tuff must have formed rapidly at high temperatures shortly after development of compaction fabrics post welding high temperature deformation is consistent with field evidence indicating rapid plastic secondary deformation of much of the tuff prior to devitrification

geochemistry and petrogenesis of early proterozoic amphibolites west central colorado u s a abstract early proterozoic supracrustal rocks exposed in west central colorado include amphibolites intimately associated with felsic volcanics and associated volcaniclastic sediments three successions are recognized the dubois succession 1770 1760 ma is comprised chiefly of amphibolites and felsic volcanics the amphibolites exhibit low concentrations of light ree and hfse and are broadly similar in composition to tholeiites from immature island arcs and in some respects to nmorb the cochetopa succession 1745 1730 ma includes large amounts of felsic volcanics and volcaniclastic sediments together with amphibolites that exhibit variable but slight incompatible element enrichment similar to basalts from evolved island arc systems and associated back arc basins the black canyon succession is coeval with the cochetopa succession and is comprised chiefly of volcaniclastic sediments with minor amphibolite these amphibolites exhibit significant incompatible element enrichment similar to basalts from continental margin arc systems geochemical modeling indicates that the amphibolites evolved by closed system fractional crystallization of depleted for the dubois succession to variably enriched parental basalts for the cochetopa and black canyon successions chiefly at shallow depths incompatible element distributions in the amphibolites indicate that mantle sources changed with time from relatively depleted 1770 1760 ma to variable but slightly enriched 1745 1730 ma a provisional model calls upon the dubois succession forming in an island arc system followed by collision with the archean wyoming province at 1750 ma the cochetopa succession was deposited in an intra arc basin developed within a new continental margin arc and the black canyon succession formed in a back arc basin associated with this arc

cooling degassing and compaction of rhyolitic ash flow tuffs a computational model previous models of degassing cooling and compaction of rhyolitic ash flow deposits are combined in a single computational model that runs on a personal computer the model applies to a broader range of initial and boundary conditions than riehle s earlier model which did not integrate heat and mass flux with compaction and which for compound units was limited to two deposits model temperatures and gas pressures compare well with simple measured examples the results indicate that degassing of volatiles present at deposition occurs within days to a few weeks compaction occurs for weeks to two to three years unless halted by devitrification near emplacement temperatures can persist for tens of years in the interiors of thick deposits even modest rainfall significantly chills the upper parts of ash deposits but compaction in simple cooling units ends before chilling by rainwater influences cooling of the interior of the sheet rainfall does however affect compaction at the boundaries of deposits in compound cooling units because the influx of heat from the overlying unit is inadequate to overcome heat previously lost to vaporization of water three density profiles from the matahina ignimbrite a compound cooling unit are fairly well reproduced by the model despite complexities arising from numerous cooling breaks uncertainties in attempts to correlate in detail among the profiles may be the result of the non uniform distribution of individual deposits regardless it is inferred that model compaction is approximately valid thus the model should be of use in reconstructing the emplacement history of compound ash deposits for inferring the depositional environments of ancient deposits and for assessing how long deposits of modern ash flows are capable of generating phreatic eruptions or secondary ash flows

crustal structures of eastern fennoscandia the area of the eastern fennoscandian shield is quite well covered by deep seismic sounding profiles most of which were shot in the last ten years in general the velocities of the seismic waves in the crust increase continuously with depth but intense reflections also indicate the presence of distinct velocity boundaries the velocities of the refracted waves in the various profiles resemble each other but the depths to the boundaries vary considerably low velocity zones were found in the uppermost crust along most of the profiles the eastern karelia profiles also exhibited large low velocity zones in the lower part of the crust high velocity bodies found in the uppermost crust in eastern fennoscandia are thought to be associated with mafic intrusions and mafic metavolcanic rocks the depth to the moho shows considerable lateral variation and deep depressions in this boundary are thought to be connected with large fracture zones high pn wave velocities of about 8 5 km s are in many cases associated with these depressions

geochemistry of footwall alteration associated with the early proterozoic united verde massive sulfide deposit arizona three zones of progressively increasing alteration occur in the deception rhyolite associated with the united verde massive sulfide deposit of early proterozoic age near jerome arizona zone 1 most intense alteration is comprised chiefly of chlorite and exhibits apparent losses of sio 2 na 2 o cao and k 2 o enrichments in mgo fe 2 o sub 3 total 1 al 2 o 3 volatile constituents chiefly h 2 o and relatively unchanged tio 2 which is used to monitor changes in other major elements sio 2 and k 2 o losses are most prominent in zone 1 quartz chlorite and sericite are the dominant secondary minerals in zones 2 and 3 these zones also show major losses of na 2 o and cao and variable but small enrichments in mgo fe 2 o sub 3 total 1 sio 2 and locally in k 2 o zone 1 is characterized by major decreases in k rb sr and ba and increases in cs u th mn sc co rare earth elements and high field strength elements zr nb y ta hf increases in cu zn and pb occur in all zones but are greatest in zone 1 with the exception of light rare earth elements nb and y in zone 1 ratios of high field strength elements viz ti zr ta hf hf th and rare earth element distribution patterns are relatively unchanged during alteration and provide a means of identifying protoliths alteration at jerome is complex involving both submarine diagenetic hydrothermal and possibly metamorphic processes zone 1 formed at the highest temperatures and at high water rock ratios the geochemical character and lithologic association of the volcanic host rocks are consistent with a back are basin tectonic setting for the united verde deposit 1 fe 2 o sub 3 total total fe expressed as fe 2 o 3

gallium germanium indium and other trace and minor elements in sphalerite as a function of deposit type a meta analysis abstract while a significant amount of analytical data on trace and minor element concentrations in sphalerite has been collected over the last six decades no meta analysis of this data has ever been conducted in this study the results of such an analysis are presented while the study focusses on ga ge and in data for six other elements ag cd co cu fe and mn was also included the results show that there are systematic statistically significant differences in the mean concentrations of fe ga ge in and mn in sphalerite from different deposit types while cd and cu concentrations show no such differences and ag and co concentrations are only significantly different for vein type deposits a principal component analysis demonstrates that the differences between deposit types are approximately one dimensional being expressible in terms of a single number this number correlates strongly with the homogenisation temperature of fluid inclusions r 2 0 82 p 16 it may be expressed as follows pc 1 ln c ga 0 22 c ge 0 22 c fe 0 37 c mn 0 20 c in 0 11 with ga ge in and mn concentrations in ppm and fe concentration in wt the relationship is sufficiently strong to be used as a geothermometer ggimfis the empirical relationship between pc 1 and the homogenisation temperature t is t c 54 4 7 3 pc 1 208 10 our results indicate a strong control of sphalerite chemistry by fluid temperature particularly for the concentrations of ga r 2 0 40 ge r 2 0 65 fe r 2 0 30 and mn r 2 0 60 and to a lesser degree in r 2 0 10 the concentrations of ag cd co and cu appear to be independent of temperature as a consequence of the strong temperature control on pc 1 metamorphic overprinting of pb zn deposits even by lower greenschist facies events may lead to significant changes in sphalerite composition namely a relative decrease in ga and ge concentrations and increase in fe mn and to a lesser degree in concentrations the closure temperature of sphalerite in regional metamorphic events appears to be around 310 50 c such that higher grade events will not be reflected in its composition factors other than temperature such as differences in fluid salinity or source rock composition do not appear to be responsible for differences between deposit types but rather appear to cause differences between individual deposits particularly the cu activity in ore forming systems appears to have a strong influence on in concentrations in sphalerite the observed trends in sphalerite compositions provide a useful tool for future studies of different types of pb zn deposits as well as for mineral exploration they should be particularly relevant for the identification of new resources of ga ge and in

geochemical modeling of the zoned footwall alteration pipe hellyer volcanic hosted massive sulfide deposit western tasmania australia the mineralogy of hydrothermal alteration assemblages siliceous core chlorite zone sericite zone in the footwall pipe of the hellyer volcanic hosted massive sulfide vhms deposit is controlled mainly by the temperature ph and redox state of the hydrothermal fluids based on the results of our numerical modeling the zoned alteration system could have formed by a combination of water rock interaction and cooling with a siliceous core predicted to form at the center of the conduit passing outward to a chlorite dominated zone a sericite rich zone and finally grading out to unaltered andesite these mineralogical changes relate to a gradual evolution of fluid chemistry water rock ratio ph and temperature from the core to the margin of the system initial temperatures were probably close to 350 degrees c and the hydrothermal fluids were reduced h2s dominated at low temperatures 200 degrees c a peripheral alteration zone consisting mostly of k feldspar chlorite sericite and hematite is predicted to form this low temperature zone has not been recognized at hellyer but elevated k2o contents and local k feldspar development have been noted at other vhms deposits suggesting that this zone might form at low temperatures and low water rock ratios in the footwall of vhms deposits if the existence of barren outer k feldspar alteration zones is confirmed the findings could have important implications for exploration specifically with regards to distinguishing ore related alteration zones from barren alteration systems

interpretation of geoid anomalies in the contact zone between the east european craton and the palaeozoic platform i estimation of effects of density inhomogeneities in the crust on geoid undulations summary in our paper we compile a detailed 3 d velocity model of the crust for the territory of poland and investigate the effect of a 3 d density distribution in the crust on geoid undulations the major tectonic feature in the study area is the trans european suture zone tesz separating the precambrian east european craton from younger tectonic units of palaeozoic europe high resolution seismic models of the crust and upper mantle from recent controlled source experiments were interpolated into a 3 d p wave velocity model for the whole poland the model was converted to a density model assuming different equations connecting seismic velocities to densities for crustal rocks sediments and the uppermost mantle the crustal density model was used to calculate synthetic geoid undulations for the territory of poland and to estimate influence of separate layers topography sediments crust and uppermost mantle on the geoid to calculate the gravity effect of a large scale 3 d density model we developed a code based on a point mass algorithm as well as prisms algorithm that uses some easy to implement optimizations the synthetic geoid undulations were compared to an existing gravimetric quasi geoid solution the modelling showed that the difference in crustal structure between the east european platform and palaeozoic platform explains significant part of the observed geoid undulations in the study area the remained residual could be related to density inhomogeneities in the upper mantle

the influence of second scale convection on the thickness of continental lithosphere and crust abstract the effect of sublithospheric convection on the thickness and structure of the continental lithosphere is studied in numerical models assuming different rheologies newtonian non newtonian and temperature and pressure dependent heat fluxes and heating modes bottom versus internal heating the stagnant regions near the top of the models are identified with the lithosphere we distinguish a seismic and thermal lithosphere controlled by temperature an elastic lithosphere controlled by viscosity and a mechanical lithosphere controlled by strain rate strong lateral thickness undulations with a thick lithosphere above the downwelling regions and a thin lithosphere above the upwelling regions develop the thickness of the elastic lithosphere is about two thirds of the mechanical lithosphere while the ratio of the elastic to the thermal lithosphere varies between 0 4 and 0 8 the time dependence of some models with internal heating and a surface heat flow of 20 mw m 2 is characterized by long periods of the order of 1 ga close to steady state and short periods 100 ma of changing cell patterns and lithospheric thicknesses models showing thick lithospheric roots suggest that the mantle beneath some old shields may be associated with cold slowly downwelling convective flow rather than being firmly attached to the lithosphere the missing gravity and topography signals from such regions can be explained by elastic bending of the lithosphere and by dynamic adjustment of the moho observed lithospheric thickness variations in europe and their minimum to maximum spacing agree well with the models suggesting sublithospheric convection to be the cause compared to other observed features thickness undulations of the lithosphere seem to provide the strongest indications for sublithospherical convection relatively low viscosities in the lower crust may lead to a long term decoupling of surface topography from moho topography and mantle dynamics the moho depth is capable of adjusting to mantle stresses within relaxation times of the order of 100 ma while surface topography may flatten out such a mechanism may be important in regions with a deep moho such as fennoscandia or with a shallow moho such as the pannonian basin neither region showing a pronounced surface topography

topography and geoid undulations caused by small scale convection beneath continental lithosphere of variable elastic thickness summary the effect of subcontinental upper mantle convection on topography gravity and geoid undulation is studied with special regard to the effective elastic thickness of the continental lithosphere hel we designed a numerical model of the upper mantle lithosphere system consisting of a constant viscosity fluid overlayed by a partly elastic plate the boundary tractions stresses at the top of the convecting mantle model were applied to the lithosphere model for which we used the thin plate approximation the total thickness of the lithosphere was kept constant to 150 km while the elastic thickness was varied between 0 and 150 km the aspect ratio of the convecting mantle was varied between 0 7 and 3 and the rayleigh number between 104 and 106 our calculations demonstrate the effect of a thick elastic lithosphere on the surface observables peak to peak averaged amplitude of topography geoid undulation and gravity are strongly reduced and their spectra are low pass filtered for an aspect ratio less than 1 and a very thick elastic lithosphere topography tends to zero and while geoid undulation and gravity may even become negative above the up welling plume the spectral relation of geoid or gravity to topography admittance is nearly unaffected for hie 70km for aie 70km admittance values for all wavelengths generally decrease with increasing elastic thickness of the plate and may become negative geoid admittance for the basic mode increases with larger aspect ratio while gravity admittance for hie 100 km slightly decreases associating higher rayleigh numbers of the convecting fluid with lower viscosity an increase in rayleigh number leads to considerable decrease of the surface observables however admittance values are only slightly affected and decrease approximately linearly with increasing rayleigh number comparison of our results with observations of the topography spectrum and the free air gravity admittance for north america suggests that large aspect ratio cells beneath continents are unlikely however convection cells of aspect ratio 1 or less underlying an elastic lithosphere of at least 100 km thickness is in agreement with data the rayleigh number should be as high as 106

anisotropy of magnetic susceptibility studies of depositional processes in the campanian ignimbrite italy the late pleistocene trachytic campanian ignimbrite underlies much of the campanian plain near naples italy and occurs in valleys in the mountainous area surrounding the plain out to about 80 km from its source the campi flegrei caldera at sites within 15 km of the campi flegrei anisotropy of magnetic susceptibility ams principal directions indicate that in the absence of significant topography deposition came from a flow moving in a roughly radial direction ams studies of the more distal ignimbrite reveal downhill and or downvalley flow directions prior to deposition even where these directions are at high angles to a generally radial transport direction from the vent on the flanks of roccamonfina volcano flow was directly downhill as if the source of the ignimbrite was the summit of the volcano in most localities the ignimbrite consists of a single massive deposit in a few localities in the apennine mountains however the confluence of multiple drainage systems off mountains resulted in multiple local flow units that cannot be correlated between valleys a detailed study of the ignimbrite in the flat titerno river valley near massa shows that the ams fabrics are not due to late stage creeping during deposition or compaction well defined but non parallel ams fabrics from vertical and lateral sections in the massa area are best explained by the merging of gravity currents flowing down the valley and steep valley sides to form a single aggradational deposit clast compositions and ams axes at mondragone indicate that the pyroclastic flow encountered the monte massico massif and was partially blocked so that flow during deposition was toward the campi flegrei similar ams data from sites along the edge of the campanian plain indicate back flow off the first ridge of the apennine mountains reached at least 5 km from their base the campanian ignimbrite was deposited from a density stratified pyroclastic flow the depositional system consisted of the lower denser portion of the current and was controlled by topography the grouping of the ams axes is interpreted to indicate that deposition occurred under laminar flow conditions

hydrothermal alteration and mineralization in the neves corvo volcanic hosted massive sulfide deposit portugal i geology mineralogy and geochemistry the neves corvo volcanic hosted massive sulfide deposit is one of the largest 300 mt and richest known deposits of the iberian pyrite belt the deposit is unique because of its extremely high cu and sn grades 45 mt at 6 cu plus 4 5 mt at 12 cu and 2 2 sn more than 300 000 metric tons t of tin metal is contained in several types of tin mineralization that grade from 500 ppm to 60 percent sn the deposit consists of five orebodies including corvo the focus of the present study the volcanic volcaniclastic and sedimentary lithofacies that directly host the neves corvo mineralization consist of a rhyolitic dome cryptodome hyaloclastite complex facies interpretation indicates that volcanic activity occurred at moderate water depth below storm wave base in variably subsiding basins of different orders several syn and postore modification events included gravity driven mass transport processes and subsequent low angle thrusting and asymmetric detachment folding which shaped the present deposit the ore geology and geochemistry and the textural mineralogical and geochemical aspects of the ore forming process in the corvo orebody collectively suggest that ore formation resulted from a long lived hydrothermal system including two main mineralizing events an early stage of stringer and massive cassiterite deposition and a subsequent episode of massive sulfide ore generation two spatially independent stockworks fed these genetically related mineralizing systems the early deposition of stringer and massive cassiterite ores took place almost exclusively in the tin corridor a structural alignment bounded by synvolcanic faults textural analysis and geochemical data both indicate minimal fluid rock interaction during ascent of the tin bearing fluid the massive and semimassive cassiterite ores at corvo most likely formed by direct venting into seawater and or emplacement in water laden unlithified sediments and early hydrothermal products that mantled the footwall volcanic sequence stringer and massive cassiterite precipitation is thought to have been a product of a short lived episode in the overall ore forming system the vein filling mineral assemblage in the cassiterite stockwork quartz cassiterite pyrite donbassite kaolinite chamosite coupled with whole rock and mineral geochemical data indicate that the tin bearing fluid had a low ph and relatively high temperature 380 c the massive sulfide related hydrothermal alteration in corvo is essentially strata bound the permeability contrasts in the uppermost portion of the footwall succession played an important role in controlling the fluid flow the hydrothermal alteration is zoned and consists of an inner chlorite donbassite quartz sulfide sericite core that grades outward into successive enveloping k sericite quartz sulfide and na sericite quartz sulfide halos textural evidence from the mine to the microscopic scale and mass balance considerations indicate that extensive silicate replacement in the coherent volcanic rocks of the footwall sequence and disseminated replacement mineralization in the uppermost volcaniclastic and or sedimentary units were major mechanisms of massive sulfide deposition at corvo alteration mineralogy and geochemistry indicate that the neves corvo ore fluids were hotter and more acidic than typical ore forming fluids of the iberian pyrite belt the geochemical characteristics of the tin and copper mineralization also differ from the ores of typical deposits of the iberian pyrite belt and imply the involvement of additional metal sources possibly magmatic in the ore forming system copper enrichment due to mechanical and fluid assisted tectonometamorphic remobilization processes accounts for the formation of extremely high grade ore shoots in the deposit

anisotropy of magnetic susceptibility in relation to source vents of ignimbrites empirical observations the anisotropy of magnetic susceptibility ams of young undeformed ashflow deposits e g bishop tuff california bandlier tuff new mexico show alignment of the maximum axis of magnetic susceptibility kmax with inferred flow lines the sense of flow is deduced from the predominant plunge bias of kmax axes these principles are discussed in relation to four deformed settings of ashflow deposits in the great basin of the western united states the magnetic susceptibility data provide valuable but not always unambiguous insight into probable source areas various complications arise in interpreting the ams data in terms of flow directions but these can be reduced by integration with geologic observations

a mechanism for crustal thinning without lateral extension in some continental regions thin crust is found without observing significant indications of extensional tectonics we argue that in such regions upwelling mantle flows may give rise to sublithospheric loading stresses which may be able to squeeze out the ductile lower crust but leave the upper crust and sub crustal lithosphere essentially unstretched this model allows us to constrain the viscosity of the lower crust if the amount and time scale of crustal thinning is observed in any subsidence basin we applied our model to the pannonian basin hungary and found lower crustal viscosities between 1020 and 1021 pa s comparing this viscosity with laboratory data of crustal rocks results in lower crustal temperatures of about 600 to 800 c which are in good agreement with geothermal models our model might also help to understand the appearance of a very thick crust and lithosphere without any pronounced topography as it is found in fennoscandia

rheomorphism of welded tuffs abstract peralkaline welded tuffs from the islands of gran canaria canary islands and pantelleria italy show abundant evidence for post depositional flow it is demonstrated that rheomorphism or secondary mass flowage can occur in welded tuffs of ignimbrite and air fall origin the presence of a linear fabric is taken as the diagnostic criterion for the recognition of the process deposition on a slope is an essential condition for the development of rheomorphism after compaction and welding internal structures produced during rheomorphic flow can be studied by the methods of structural geology and show similar dispositions to comparable features in sedimentary slump sheets it is shown that secondary flowage can occur in welded tuffs emplaced on gentle slopes provided that the apparent viscosity of the magma is sufficiently low compositional factors favor the development of rheomorphism in densely welded tuffs of peralkaline type

isostatic topography and crustal depth corrections for the fennoscandian geoid abstract the medium wavelength geoid wavelengths the result of this study demonstrate that crustal thickness variations are important for the interpretation of the fennoscandian geoid or gravity however submoho deviatoric stresses might become very high about 200 mpa if the entire anomaly is assumed to be supported by lateral density variations at less than 80 km depth this implies that the pronounced negative anomaly is additionally maintained by dynamical flow inside the mantle

petrogenetic evolution of felsic volcanic sequences associated with phanerozoic volcanic hosted massive sulphide systems the role of extensional geodynamics abstract felsic volcanic hosted massive sulphide deposits form in many different geotectonic environments usually associated with episodes of extension however empirical geochemical links between felsic volcanism geotectonic environment and extensional geodynamics have not been adequately documented so as to distill the fundamental relationships between these features and formation of volcanic hosted massive sulphide vhms mineralization which is important considering most vhms deposits are hosted in felsic volcanic bearing sequences using felsic geochemical data from six different vhms mineralized phanerozoic geotectonic settings it seems that the felsic geochemical data are consistent with the overall geotectonic interpretations for these areas which in turn helps in stratigraphic and geotectonic interpretations for detailed vhms exploration in extensional geodynamic settings bimodal volcanism is common compared to the continuum of compositions observed under compressional to neutral high stress regimes in calc alkaline arc building environments the empirical geochemical metallogenic vhms relationships evident in the archean felsic volcanic rocks in particular the low zr y n yb n 2 o conditions in different source areas and geotectonic environments they seem to be the predominant controls on the compositional evolution of felsic magmas in extensional settings because of the limited degree of assimilation and fractional crystallization afc affecting their compositions with rapid intrusion into the uppermost crust to positions were they achieve neutral buoyancy in the volcanic to subvolcanic environment within plate felsic or a type anorogenic type and anomalous ocean ridge or type felsic magmas that have tholeiitic affinities relatively low alkali elements are common within parts of the volcanic sequences hosting massive sulphide deposits reflecting in part a relationship to extension the high contents of high field strength elements zr ga lree hree y nb etc and other lines of evidence indicate they are relatively high temperature relatively reduced magmas probably derived from previously dehydrated granulitic or melted oceanic simatic or continental sialic crust low p granulites or amphibolites but also in some cases possibly mantle rocks such that they are significantly water undersaturated during melting i e a h 2 o 1 reduced viscosities typical of high temperature melts enable rapid intrusion to higher levels diking vs diapirism higher rates of heat advection from mantle mafic magmas into the thickened oceanic and or continental crust during lithospheric extension are needed to facilitate melting of these rock to form a type and or or type partial melts tholeiitic felsic this usually results in bimodal magmatism as magma mixing is not favored in extensional settings especially at high magma production intrusion and extrusion rates i e high heat flow this would inhibit ferromagnesian phase in particular hornblende and magnetite fractionation especially at low f o 2 in the felsic magmas that would deplete the melt in y hree as well as ore related elements particularly zn which is 2 to 3 times higher in a type and or type rocks compared to other magma types therefore the empirical felsic igneous geochemical metallogenic relationships observed by others low zr y n yb n

large scale rheomorphic shear deformation in miocene peralkaline ignimbrite e gran canaria the single ignimbrite cooling unit e average thickness 28 m volume ca 30 km3 forms the uppermost member of the miocene upper mogan formation on gran canaria it is strongly chemically zoned from basal first erupted comendite peralkaline rhyolite to late erupted trachyte and apart from an upper trachytic zone it is densely welded e was emplaced onto a surface inclined ca 2 5 from the source caldera detailed mapping of key sections up to 300 m long exposed in barranco walls ca 10 km from the caldera margin reveals structures that are interpreted to have been produced by rheomorphic deformation of the ignimbrite along shear zones the shear zones formed within the lower viscosity comenditic tuff extensional structures include mega boudinage and decapitated sequences and compression resulted in sequence repitition by overthrusting mechanisms traditionally thought to be important during rheomorphic deformation of welded tuffs compaction lateral creep folding vertical density driven diapirism cannot account for these features which reflect lateral post compactional rheomorphic movement locally in excess of 800 m we suggest the following sequence of events emplacement of the several flow units compaction with little lateral movement rheomorphic deformation during and after compaction layers of secondary porosity developed within the comenditic tuff possibly where upward escape of gas was prevented by overlying relatively impermeable layers of densely compacted ignimbrite these structurally weak layers of high porosity subsequently acted as shear zones

the gravity field along the central segment of the egt abstract we give an overview of the gravity field of central europe based on a compilation of bouguer maps available by mid 1989 a generalized gravity map corrected for moho effects is presented as well as several filtered maps of the second derivative additionally sediment stripped bouguer maps are used in the case of the northwest german basin and for northeastern franconia the north german line the erzgebirge minimum the kraichgau high and the regensburg gottingen low are treated in detail and a number of geologic interpretations are discussed

dependence of the duration of geomagnetic polarity reversals on site latitude an important constraint on the processes governing the geodynamo the flow in the outer core responsible for generating earth s magnetic field is the duration of geomagnetic polarity reversals that is how long it takes for earth s magnetic field to reverse1 it is generally accepted that earth s magnetic field strength drops to low levels during polarity reversals and the field direction progresses through a 180 change while the field is weak1 the time it takes for this process to happen however remains uncertain with estimates ranging from a few thousand up to 28 000 years here i present an analysis of the available sediment records of the four most recent polarity reversals these records yield an average estimate of about 7 000 years for the time it takes for the directional change to occur the variation about this mean duration is not random but instead varies with site latitude with shorter durations observed at low latitude sites and longer durations observed at mid to high latitude sites such variation of duration with site latitude is predicted by simple geometrical reversal models in which non dipole fields are allowed to persist while the axial dipole decays through zero and then builds in the opposite direction and provides a constraint on numerical dynamo models

time scales of compaction in volcanic systems we used 12 high temperature deformation experiments on fabricated cores of rhyolite ash to explore the effect of porosity on the rheology of pyroclastic deposits during deformation the cores of ash accommodate strain mainly by shortening and reduction of porosity porosity loss creates a strain dependent rheology expressed by a marked and continuous increase in effective viscosity the effective viscosity graphic 1 of pyroclastic materials is predicted as a function of porosity graphic 2 and melt viscosity graphic 3 o by graphic 4 the optimal value for the parameter based on the experimental data is 0 78 0 15 these experiments provide constraints on the time scales of compaction and flow processes in volcanic systems under variable conditions i e temperature load stress our results indicate that welding and compaction processes in pyroclastic deposits e g ignimbrites conduits can occur on time scales of tens of minutes to hours at these time scales welding is fully decoupled from cooling of the deposits and may be coupled to depositional processes within volcanic conduits welding processes operate on fragmented magma rapidly enough to eliminate porosity in a matter of hours thereby sealing off permeability and supporting cyclical vulcanian style explosive eruptions 1 embed inline graphic 1 gif 2 embed inline graphic 2 gif 3 embed inline graphic 3 gif 4 embed inline graphic 4 gif

revised ages for tuffs of the yellowstone plateau volcanic field assignment of the huckleberry ridge tuff to a new geomagnetic polarity event 40 ar 39 ar ages were determined on the three major ash flow tuffs of the yellowstone plateau volcanic field in the region of yellowstone national park in order to improve the precision of previously determined ages total fusion and incremental heating ages of sanidine yielded the following mean ages huckleberry ridge tuff 2 059 0 004 ma mesa falls tuff 1 285 0 004 ma and lava creek tuff 0 639 0 002 ma the huckleberry ridge tuff has a transitional magnetic direction and has previously been related to the reunion normal polarity subchron dating of the reunion event has been reviewed and its ages have been normalized to a common value for mineral standards the age of the huckleberry ridge tuff is significantly younger than lava flows of the reunion event on re union island supporting other evidence for a normal polarity event younger than the reunion event

reply to discussion on geochemistry and geodynamic setting of volcanic and plutonic rocks associated with early archaean volcanogenic massive sulfide mineralisation pilbara craton vearncombe s e kerrich r 1999 precambrian research 98 243 270 by c w brauhart p morant

interpretation of anisotropy of magnetic susceptibility fabric of ignimbrites in terms of kinematic and sedimentological mechanisms an anatolian case study abstract a study of the neogene low aspect ratio kizilkaya ignimbrite in the calc alkaline central anatolian volcanic province turkey has been carried out using the anisotropy of magnetic susceptibility ams technique on 46 sites irm hysteresis loops and curie temperature analysis indicate that magnetite is the main carrier of the magnetic signal in the ignimbrite the shape of the ams ellipsoids varies widely from site to site and the anisotropy is commonly low we review the data processing used in the literature on the ams of ignimbrites to calculate principal directions or to evaluate the quality of the results and we develop various simple techniques data filtering and contouring procedures in order to more accurately define the axis or symmetry of the ams fabric there is no simple relationship between the shape of the ams ellipsoids and the shape of the petrofabric ellipsoids similarly there is no direct relationship between magnetic and kinematic axes our results show that the source of the measured magnetic signal in these rocks is a complex result of various contributions including shape anisotropy of free and fixed magnetic crystals shape ams crystallization of magnetic grains on all surfaces and discontinuities during cooling of the ignimbrite distribution anisotropy and alteration by hydrothermal and meteoric water particular attention is paid to the interpretation of the ams stereoplots departing from the standard fabric of sedimentary rocks formed under unidirectional flow in order to explain these non standard ams fabrics we discuss their possible origin 1 in analytical and mineralogical terms instrumental artifact pollution by xenoclasts mineralogical inversion of the susceptibility axes secondary mimetic fabric and 2 in terms of sedimentological mechanisms rheology depositional processes in the present state of knowledge we conclude that due to the physico chemical complexity of the magnetic source the ams signal should be used and interpreted with care when attempting to decipher the sedimentological or rheological processes occurring in moving pyroclastic flows on the other hand ams has it greatest potential when used in conjunction with other techniques and when a large number of sampling sites and specimens is used

geochemistry and geodynamic setting of volcanic and plutonic rocks associated with early archaean volcanogenic massive sulphide mineralization pilbara craton abstract early archaean volcanic rocks and syn volcanic granites at strelley east pilbara craton western australia were formed during an extensional tectonic phase associated with an active convergent continental margin the volcanic sequence comprises a bimodal suite of fractionated sub alkalic tholeiitic basalts andesites and dacites siliceous laminites and volcanogenic massive sulphide vms deposits in the footwall and hangingwall calc alkaline dacites the footwall rocks are characterized by th lapm 1 variably fractionated lree and relatively high y and yb 33 and 3 ppm respectively negative normalized anomalies of nb p ti are consistent with derivation via slab dehydration fusing the mantle wedge the co magmatic and syn volcanic strelley granite is enriched in lile relative to hfse and has distinctively low al2o3 11 14 and la yb n 4 10 suggesting a relatively mature and cool subducted oceanic lithosphere as for the majority of phanerozoic arc granitoids and convergent margin magma compositions the chemical composition of the footwall basaltic to dacitic rocks and the sub volcanic strelley granite display signatures highly prospective for vms mineralization large ve eu anomalies low zr y ratios high level magmatic fractionation comparable with the noranda fiiia type suite the hangingwall dacites have more pronounced hree fractionation than footwall counterparts and lower y and yb the chemistry of the footwall volcanic rocks and granite reflect typical phanerozoic conditions of steep subduction hydrous metasomatism in the mantle wedge and fractionation similar characteristics are also evident in plutonic volcanic suites associated with vms deposits in the late archaean as at noranda canada early extension with nascent back arc development coupled with steep subduction is considered critical to the development of vms deposits

the mineral chemistry of hydrothermally altered and metamorphosed wall rocks at the stollberg fe pb zn mn ag deposit bergslagen sweden the c 1 9 ga old stollberg sulphide and mnrich skarn iron ores and sulphide ores in bergslagen south central sweden are hosted by hydrothermally altered and metamorphosed felsic volcanic and volcaniclastic rocks the ores are underlain by comformable alteration zones characterized by albite gedrite quartz and biotite muscovite plagioclase k feldspar quartz garnet assemblages the present mineralogies are interpreted as medium grade metamorphic equivalents to the original alteration mineral assemblages pt conditions during prograde regional metamorphism are semiquantatively determined to be 510 to 560 c at approximately 3 kbar with increasing modal content of gedrite and biotite in the alteration zones the mg fe ratios and xmg s in octahedral positions of these minerals also increase in the gedrite bearing strata whole rock mg fe ratios remain constant whereas in the biotite rich unit the wholerock mg fe trend is parallel to that of the biotites

geochemical and detrital mode evidence for two sources of early proterozoic sedimentary rocks from the tonto basin supergroup central arizona the tonto basin supergroup includes up to 6 5 km of early proterozoic sedimentary and volcanic rocks that were deposited in a relatively short period of time at about 1 7 ga in central arizona moderate correlations of rare earth elements ree and ti with al2o3 and ree distributions in detrital sediments of this supergroup suggest that these elements are contained chiefly in clay mica and or zircon fractions ree distributions including negative eu anomalies in most tonto basin sediments are similar to those in phanerozoic shales weak to moderate correlations of fe sc ni and co to al2o3 also suggest a clay mica control of these elements detrital modes and geochemical characteristics of sediments indicate two dominant sources for sedimentary rocks of the tonto basin supergroup a granitoid source and a volcanic source the granitoid source was important during deposition of the upper part of the succession the mazatzal group as shown by increases in k2o al2o3 and th in pelites with stratigraphic height and increases in zr and hf and decreases in eu eu cr and ni in in pelites of the maverick shale sediment provenance characteristics and paleocurrent indicators are consistent with deposition of the supergroup in a continental margin back arc basin the granitoid sediment source appears to have been the north american craton on the north and the volcanic source a more local source from an arc on the south

the emplacement history of a remarkable heterogeneous chemically zoned rheomorphic and locally lava like ignimbrite tl on gran canaria abstract ignimbrite tl on gran canaria is a complex compositionally zoned rheomorphic tuff that locally exhibits features previously considered to be diagnostic of lavas it is made up of two locally overlapping lobes of ignimbrite that were emplaced during a single eruptive episode the eastern lobe is high grade with rheomorphic zones and localised patches that are lava like the western lobe is extremely high grade more extensively lava like and welded to its top surface both parts are zoned with a basal comendite rich zone grading up through a mixed zone into an upper trachyte rich zone lithic contents and the relative proportions of comendite and trachyte pyroclasts vary with height each comendite rich zone is vitroclastic whereas each trachyte rich zone is partly lava like with local gradations into vitroclastic ignimbrite mixed zones are intermediate in character and locally show compositional banding gradational zoning in massive ignimbrite best seen in lower strain zones and welding fabrics that are pervasively lineated and oblique to bedding suggest that deposition was sustained agglutination was rapid and rheomorphic deformation began during the sustained deposition the viscosity and porosity of the agglutinate varied with height because successively deposited pyroclast populations varied in grainsize composition and temperature the hot agglutinate continued to compact and shear downslope after the density currents had dissipated causing further rheomorphic folding thrusting attenuation and autobrecciation the western lobe locally overlies the partly welded top of the eastern lobe in part because it advanced rheomorphically across it for at least 300 m hot state loading and auto intrusion occurred due to unstable density layering in the chemically zoned agglutinate deformation behaviour changed during cooling and degassing and because of heat transfer between juxtaposed agglutinates and localised retention of dissolved volatiles where there was an overlying impermeable cap

magnetic fabric welding texture and strain fabric in the nuraxi tuff sardinia italy anisotropy of magnetic susceptibility ams has been used to interpret flow directions in ignimbrites but no study has demonstrated that the ams fabric corresponds to the flow fabric in this paper we show that the ams and strain fabric coincide in a high grade ignimbrite the nuraxi tuff a miocene rhyolitic ignimbrite displaying a wide variability of rheomorphic features and a well defined magnetic fabric natural remanent magnetization nrm data indicate that the magnetization of the tuff is homogeneous and was acquired at high temperatures by ti magnetite crystals comparison between the magnetic fabric and the deformation features along a representative section shows that ams and anisotropy of isothermal remanent magnetization airm fabric are coaxial with and reproduce the shape of the strain ellipsoid magnetic tests and scanning electron microscopy observations indicate that the fabric is due to trails of micrometer size pseudo single domain magnetically interacting magnetite crystals microlites formed along discontinuities such as shard rims and vesicle walls mimicking the petrofabric of the tuff the fabric was thus acquired after deposition before late rheomorphic processes and accurately mimics homogeneous deformation features of the shards during welding processes and mass flow

interpretation of geoid anomalies in the contact zone between the east european craton and the palaeozoic platform ii modelling of density in the lithospheric mantle summary we present analysis of lateral variations of density in the upper mantle in the area of contact of the precambrian east european craton eec and the palaeozoic platform pp in poland obtained by analysis of the gravimetric geoid undulations a precise 3 d density model of the crust in the study area down to a depth of 50 km discussed in the first part of this paper majdanski et al in press did not explain all features of the observed geoid this suggests that these features can be due to density inhomogeneities in the upper mantle to estimate them we performed inversion of a residual between the observed geoid and undulations caused by the 3 d density distribution in the crust basing on the assumption of local isostatic compensation and pratt hayford isostasy model the density distribution in the upper mantle was parametrized as a 40 km thick layer located above the assumed compensation depth of 140 km and subdivided into irregular blocks the boundaries of the blocks were defined according to boundaries of major tectonic units in the study area and position and shape of the most pronounced anomalies in the residual geoid a series of sensitivity tests calculated for such density heterogeneities in the upper mantle showed that they can produce geoid undulations of the order of several metres the density values in each unit were taken as model parameters for the inversion procedure and inverse problem was solved using global optimization with constraints the density variations in the upper mantle in the final model correlate well with the surface heat flow this suggests that these variations can be due to diversity in mantle temperature the trans european suture zone tesz which is a major suture separating the eec from the pp is not observed as a distinct unit in the mantle instead our study suggests continuation of the lithosphere of the eec beneath the pp and confirms subdivision of the tesz into terranes with distinctly different evolution a simple calculation shows that condition of local isostatic equilibrium is almost achieved at the assumed compensation depth with the exception of several small scale units however different loads and mass deficiencies in the lithosphere are compensated at different depth levels this proves that the lithosphere is alternation of mechanically strong and weak layers both in the crust and upper mantle and that different loads can be compensated at different depth however the total compensation of all lithospheric loads occurs in the upper mantle

on the interpretation of geoid anomalies in europe with special regard to the egt profiles abstract we have studied medium wavelength geoid anomalies over europe in particular along three main egt profiles fennolora profile central profile through west germany alpine profile across the alps and the po basin in relation to the lithospheric structure for a general interpretation of the whole european area we used a simple model prescribing topography moho depth undulations and appropriate densities and applying the concept of pratt isostasy with this simple model we could explain most of the larger features in the geoid while in northern europe the crustal thickness is crucial for central and southern europe it is the combination of both topography and moho depth we found a geoid gradient of 4 06 m km for topography and 0 58 m km for moho undulations with a compensation depth of 60 km to be the best to explain the observations however also a compensation depth of 27 km for topography and 80 km for crustal thickness undulations fits the data the negative anomalies over the north sea and the po basin become even more pronounced if crustal effects are considered which means they can not be explained adequately by crustal thickness of constant density but need additional thermal or lithological effects to locally reduce the densities a remaining negative anomaly in fennoscandia of about 4 m might reflect the effect of post glacial return flow but can also be explained by increased crustal density compared to central europe along the fennolora profile the geoid undulations can be explained by the combined effect of the increased thickness of the crust and lithosphere lateral density variations in each layer are not necessary to explain the geoid observation and upper and lower crustal density is high the density decrease at the lithospheric base is small along the central profile the effect of topography together with the low density sediments in the northern german lowland already explains most of the geoid anomaly the existence of a large low velocity body in the middle crust which might be a trapped granitic intrusion is weakly supported by the geoid observation a clear decrease in density at the transition to the asthenosphere fits the geoid observation the short but structurally most complicated alpine profile is difficult to interpret geoid observations can well be explained by the effect of low compensated topography and sedimentary layers this means that isostasy in the alps demands considerable lateral intra crustal density variations and can not be mainly attributed to the moho deflection however isostasy can well be incomplete across the alps two seismically detected crustal bodies penetrating into the upper mantle are best compatible with the geoid observation if their density is around 3 g cm 3 which agrees with an eclogitic composition of these bodies

geomagnetic excursions reflect an aborted polarity state abstract geomagnetic excursions represent short episodes of a few thousand years at most during which the field considerably exceeds its normal range of variability during a polarity state paleomagnetic records have now been obtained with extremely high temporal resolution which have improved our knowledge of these short events we have compiled the most detailed records of excursions that had occurred during the brunhes and matuyama chrons we show that virtual geomagnetic poles vgps of at least one record of each event are able to reach the opposite polarity in the next step we have computed different simulations of excursions during which the dipole progressively vanishes before growing back without reversing this scenario produces very few reversed directions which are only visible at some latitudes we infer that it is impossible to reach the ratio of reversed to intermediate vgps present in the paleomagnetic records if the excursions were not associated with a short period of reversed dipole field therefore excursions should be regarded as two successive reversals bracketing an aborted polarity interval we propose that the same underlying mechanisms prevail in both situations excursions or reversals and that below a certain strength the field reaches an unstable position which preludes either the achievement of a reversal or its return to the former polarity

lithophile element systematics of archean greenstone belt au ag vein deposits implications for source processes reply potassic alteration domains of greenstone belt lode gold deposits are characterized by systematic partitioning between lithophile elements k rb and ba are generally co enriched and linearly correlated over almost three orders of magnitude in abundance where k rb 220 400 and k ba 30 85 k cs 104 and k tl 2 104 are also correlated though more weakly lithium abundances and rb sr ratios are erratic in altered rocks these interelement trends collectively are present in deposits variously hosted by ultramafic mafic or felsic volcanic rocks and sediments or granitoids magmatic processes involving crystal fractionation of biotite k feldspar and plagioclase generate trends to systematically diminished k rb 50 k li k cs and k tl but enhanced k ba 8 103 and rb sr in most late stage differentiates such late stage trends are the rule in magmatophile deposits including the archean cadillac molybdenite deposit phanerozoic cu and mo porphyry deposits sn w greisens and most

rare earth elements and hydrothermal ore formation processes abstract the geochemical behaviour of ree is influenced by nearly all important hydrothermal ore formation processes including fluid rock interactions fluid precipitations adsorption and scavenging onto particles and changes in fluid temperature pressure ph eh alkalinity and ligand concentration destabilization of ree complexes in response to these physicochemical changes and possible chemical crystallographic controls determine the concentration and distribution of ree within hydrothermal minerals mineraloids and amorphous phases alteration assemblages of intrusive related mineralisations may exhibit a wide range of ree distributions and ree fractionation trends the ree distribution within these hydrothermally altered lithologies will depend on the ree concentration in the rock and the fluid the partitioning behaviour of the ree between the rock phases and the fluid and the types of alteration reactions which take place reactive rocks such as carbonate rich lithologies of skarn deposits may gain or loose significant amounts of ree during fluid rock interactions ree geochemical investigations combined with fluid inclusion and stable isotope studies may point to the origin of alteration products and the source s of ore fluids ancient massive sulphide ores commonly exhibit anomalous eu concentrations similar to many recent submarine hydrothermal precipitates the recognition of hydrothermal fluid seawater sedimentary or volcanically derived ree distributions within ores chemical sediments and hydrothermally altered lithologies may help to constrain genetic modelling of massive sulphide deposits care should be taken with the interpretation of ree distributions from metamorphosed hydrothermal ore deposits becase metamorphic and diagenetic alteration reactions may change the ree distributions mobility of ree may occur during diagenesis of carbonate rich rocks and during the development of shear zones and migmatites in contrast very large fluid rock ratios are necessary to cause significant changes in ree patterns of silicate rich rocks during diagenesis and regional metamorphism

the teton fault wyoming topographic signature neotectonics and mechanisms of deformation we integrated geophysical and geological methods to evaluate the structural evolution of the active teton normal fault wyoming and its role in the development of the dramatic topography of teton range and jackson hole the teton fault bounds the precipitous eastern front of the teton range and is marked by large well preserved postglacial fault scarps that extend for 55 km along the range front comparison of variations in surface offsets with the topographic expression of the teton range crest and drainage divide and the overall structure of the range suggests that the effects of postglacial faulting cannot be discriminated from the influence of pre extensional structures and differential erosion on the footwall topography in contrast the effects of multiple scarp forming normal faulting earthquakes are expressed by the anomalous drainage pattern and westward tilt of the hanging wall jackson hole toward the teton fault kinematic boundary element fault models suggest that the westward tilt of the valley floor is the product of 110 125 m of displacement on a 45 75 e dipping teton fault in the past 25 000 75 000 years comparisons with historic normal faulting earthquake displacements imply that this range of displacement corresponds to 10 50 m 7 scarp forming earthquakes a total throw of 2 5 to 3 5 km across the teton fault is suggested by inverse ray tracing and forward gravity models these models also suggest that laramide age structures have been offset across the teton fault and obscure its geophysical signature but also continue to influence the structural and topographic expression of the footwall and hanging wall blocks paleomagnetic analyses of the 2 0 ma huckleberry ridge tuff suggest that the overall westward tilt of the teton range is a result of about 10 of west side down tilt across the teton fault since tuff emplacement this suggests that much if not all of the throw across the teton fault has accumulated in the past 2 m y complex demagnetization and rock magnetic behavior and local emplacement of the huckleberry ridge tuff on preexisting topography preclude determination of the amount or variations in throw along strike of the teton fault from the paleomagnetic data

the direction correction tilt test an all purpose tilt fold test for paleomagnetic studies abstract the tilt or fold test in paleomagnetism is used to infer whether paleomagnetic remanence was acquired before or after tectonic tilting while several tilt test formulations have been proposed none fully satisfies the requirements of statistical validity applicability to all bedding geometries and ease of use this paper introduces the direction correction dc test which examines the relationship between the paleomagnetic site mean directions and their corresponding bedding tilt corrections the dc test is similar to mcfadden s correlation test because both test whether or not the site directions contain information about the bedding tilts however the dc test gives greater weight to sites with greater counter bedding the dc test is similar to watson s numerical tilt tests because they determine the degree of untilting which gives optimal concentration of site directions however the dc test uses analytical rather than numerical methods graphical output aids the researcher in recognizing problem sites using both real and simulated data the dc test is demonstrated to be more discriminating than other tilt test formulations for all bedding geometries the simulations show that the power of the tilt test is inversely proportional to the 95 confidence interval 95 of the overall mean as a rule of thumb paleomagnetists should attempt to sample sufficient sites to obtain an 95 less than 1 6 of the bedding attitude difference