formation and age of sphalerite mineralization in carbonate rocks of bajocian age in the swiss jura mountains evidence of mesozoic hydrothermal activity a combination of petrographic and geochemical techniques was applied to better constrain the origin and evolution of the fluid systems responsible for the formation of disseminated cd rich up to 0 6 wt sphalerite zns mineralization in the northeastern part of the jura mountains switzerland the rb sr ages of sphalerite samples indicate that a main phase of sphalerite formation occurred near the boundary between the late middle and early late jurassic at around 162 ma the negative 34s values 22 3 to 5 3 suggest that biogenic sulfide sulfur was involved in zns precipitation the strontium isotope composition is more radiogenic than that of contemporaneous seawater reflecting the interaction of mineralizing fluids with silicate rocks lead isotope signatures are very uniform 206pb 204pb 18 63 18 67 207pb 204pb 15 63 15 64 208pb 204pb 38 51 38 63 indicating an isotopically well homogenized fluid system the basement rocks underlying the jurassic strata are considered to be the main source of metals for the sphalerite mineralization the migration of deep sourced hydrothermal saline metal bearing fluids into the bajocian host carbonates containing sedimentary reduced sulfur resulted in the precipitation of sulfides the period of sphalerite formation near the middle late jurassic boundary is characterized by enhanced tectonic and hydrothermal activity in europe related to the opening of the central atlantic and tectonic thermal subsidence during spreading of the alpine tethys our study provides evidence that the bajocian carbonate rocks in the jura mountains area were affected by the circulation of deep sourced metal bearing hydrothermal fluids in response to these continent wide tectonothermal events the presence of sphalerite mineralization and associated geochemical anomalies in zn and cd contents in carbonate rocks may also be used to trace basement features

oxygen and hydrogen isotope stratigraphy of the rustenburg layered suite bushveld complex constraints on crustal contamination new do values for plagioclase pyroxene and olivine and limited whole rock dd values are presented for samples from the rustenburg layered suite of the bushveld complex south africa in combination with existing data these provide a much more complete composite o isotope stratigraphy for the intrusion throughout the layered suite mineral do values indicate that the magmas from which they crystallized had do values that were about 7 1 that is 1 4 higher than expected for mantle derived magmas suggesting extensive crustal contamination more limited h isotope data suggest that the oh present within whole rocks regardless of the degree of alteration is of magmatic origin and not an alteration phenomenon there appears to be no systematic change in do value with stratigraphic height and this requires the contamination to have taken place in a staging chamber before emplacement of the magma s into the present chamber large amounts 30 40 of contamination by the lower to middle crust are needed to explain these do values which is in general agreement with previous estimates based on srand nd isotope data alternatively smaller amounts of contamination 20 by sedimentary rocks or their partial melts represented by the country rock can explain the data but it is not apparent how such material could have been present at the depth of the staging chamber in the lower to middle crust

deformation and metamorphism associated with crustal rifting the permian to liassic evolution of the lake lugano lake como area southern alps abstract in connection with movements between europe and adria the southern alps underwent extension leading to the formation of the middle jurassic south alpine passive continental margin as a result of mainly s vergent alpine thrusting and folding a substantially preserved mesozoic crustal section ranging from the surface to a paleo depth of ca 15 km is exposed in the lugano lake como area tectonic processes associated with permo mesozoic extension can thus be investigated at different crustal levels rifting began with a thermal anomaly during which rocks at middle crustal levels were sheared at ca 650 750 c deformation was distributed on a several km wide zone and late kinematic pegmatites were emplaced at this stage temperatures then decreased and beginning in the norian extension was accommodated by a major e dipping normal fault the lugano val grande normal fault the fault which can be followed down to a paleo depth of ca 12 km was listric and flattened to 20 at ca 7 9 km depth in the upper 8 10 km deformation was only brittle whereas at deeper crustal levels greenschist mylonites were formed with continued normal faulting the colder hanging wall cooled the fault zone and greenschist mylonites were progressively replaced by lower temperature ultramylonites and cataclasites the thickness of the fault zone varies from some tens of meters in the upper 5 6 km to several hundred meters at deeper levels deformation was thus discrete at a crustal scale

the south tibetan detachment system in the rongbuk valley everest region deformation features and geological implications abstract structural and microstructural investigations into the south tibetan detachment system in the rongbuk valley southern tibet pointed to the presence of a complex extensional setting with the development of two major fault zones with movement in a top to ne direction the lower one is an at least 1000 m wide ductile shear zone that brings into contact the overlying north col formation subject to low grade metamorphism and the high grade micaschist and gneiss of the higher himalayan crystallines characterized by polymetamorphism from barrovian to low pressure high temperature conditions the upper low angle fault zone is characterized by a metric thick level of cataclasites and brings into contact the slightly metamorphosed ordovician limestones and the north col formation the lower ductile shear zone is thought to accommodate at least 5 km of vertical displacement whereas the upper low angle normal fault zone is thought to accommodate at least 1600 m of vertical displacement the lower ductile shear zone is thus to be considered the main extensional feature of the stds in the rongbuk valley

la icp ms depth profile analysis of apatite protocol and implications for u th he thermochronometry abstract heterogeneous concentrations of producing nuclides in apatite have been recognized through a variety of methods the presence of zonation in apatite complicates both traditional ejection corrections and diffusive models both of which operate under the assumption of homogeneous concentrations in this work we develop a method for measuring radial concentration profiles of 238 u and 232 th in apatite by laser ablation icp ms depth profiling we then focus on one application of this method removing bias introduced by applying inappropriate ejection corrections formal treatment of laser ablation icp ms depth profile calibration for apatite includes construction and calibration of matrix matched standards and quantification of rates of elemental fractionation from this we conclude that matrix matched standards provide more robust monitors of fractionation rate and concentrations than doped silicate glass standards we apply laser ablation icp ms depth profiling to apatites from three unknown populations and small intact crystals of durango fluorapatite accurate and reproducible durango apatite dates suggest that prolonged exposure to laser drilling does not impact cooling ages intracrystalline concentrations vary by at least a factor of 2 in the majority of the samples analyzed but concentration variation only exceeds 5x in 5 grains and 10x in 1 out of the 63 grains analyzed modeling of synthetic concentration profiles suggests that for concentration variations of 2x and 10x individual homogeneous versus zonation dependent ejection corrections could lead to age bias of 5 and 20 respectively however models based on measured concentration profiles only generated biases exceeding 5 in 13 of the 63 cases modeled application of zonation dependent ejection corrections did not significantly reduce the age dispersion present in any of the populations studied this suggests that factors beyond homogeneous ejection corrections are the dominant source of overdispersion in apatite u th he cooling ages

trapped liquid paleo porosity and formation time scale of a chromitite ortho pyroxenite cumulate section bushveld south africa to evaluate compaction and interstitial melt expulsion during cumulate formation a 20 m cumulate section including the ug2 and ug3 chromitites from a 264 m drill core through the upper critical zone of the bushveld complex south africa has been studied the cumulates in the studied section are as follows 3 m plagioclase pyroxenite to pyroxenite pegmatoid footwall pyroxenite at the lower contact to ug2 0 7 m ug2 chromitite 6 8 m pyroxenite 0 24 m ug3 chromitite 2 0 m plagioclase rich pyroxenite changing locally to norite the two 5 cm leader stringers ug3a and ug3b and 7 m total of olivine pyroxenites grading into plagioclase pyroxenites all pyroxenites are dominated by orthopyroxene opx and the cumulate sequence is topped by mottled anorthosite grading into norite stratigraphic concentrations of major and trace elements of 52 bulk rock samples were determined bulk rock mg numbers are 0 79 0 81 throughout the silicate cumulate units and 0 40 0 46 in the chromitite layers the stratigraphic distribution of six incompatible trace elements k rb ba cs zr and th has been used to determine the amount of trapped liquid ftl or paleo porosity in the cumulate rocks final porosities volume fractions based on averages from the six trace elements are 0 06 0 33 in the pyroxenites in chromitite layers trapped melt fractions of 0 12 0 36 are calculated from incompatible trace element concentrations but bulk sio2 concentrations and x ray tomography yield 0 04 0 17 higher porosities hence the bulk silicate fraction in the chromitites may not necessarily correspond to the trapped liquid fraction as poikilitic opx was crystallizing while the silicate melt still equilibrated using a previously derived experiment based model for compaction time scales gravitationally driven chemical compaction in the ug2 ug3 pyroxenite section is calculated to occur within 1 10 years this time frame corresponds to the times necessary to cool a 20 m layer by 10 50 c the temperature interval argued to encompass the liquidus and almost complete solidification compaction within a decade can in fact easily develop the paleo porosities indirectly observed today and is probably stopped by crystallization of the interstitial liquid contrary to previous assertions melt expulsion from the cumulate pile does not hinder compaction calculated permeabilities would allow for the migration of an order of magnitude higher amount of melt than has to be expelled from the 20 m pile of cumulate the pegmatoid zones in the chromitite footwalls enriched in incompatible trace elements are consistent with a collection of interstitial melts expelling from the underlying compacting pyroxenites their entrapment below the chromitite layers suggests that these act as permeability barriers this is in part due to their finer grain size compared with the pyroxenites but is mainly due to the crystallization of large poikilitic opx during compaction

filling the bushveld complex magma chamber lateral expansion roof and floor interaction magmatic unconformities and the formation of giant chromitite pge and ti v magnetitite deposits his mind was like a soup dish wide and shallow irving stone on william jennings bryan a compilation of the sr isotopic stratigraphy of the bushveld complex shows that the evolution of the magma chamber occurred in two major stages during the lower open system integration stage lower critical and lower main zone there were numerous influxes of magma of contrasting isotopic composition with concomitant mixing crystallisation and deposition of cumulates larger influxes correspond to the boundaries of the zones and sub zones and are marked by sustained isotopic shifts major changes in mineral assemblages and development of unconformities during the upper closed system differentiation stage upper main zone and upper zone there were no major magma additions other than that which initiated the upper zone and the thick magma layers evolved by fractional crystallisation the lower and lower critical zones are restricted to a belt that runs from steelpoort and burgersfort in the northeast to rustenburg and northam in the west and an outlier of the lower and lower critical zone up to the lg4 chromitite layer in the far western extension north of zeerust it is only in these areas that thick harzburgite and pyroxenite layers are developed and where chromitites of the lower critical zone occur these chromitites include the economically important c 1 m thick lg6 and mg1 layers exposed around both the eastern and western lobes of the bushveld complex the upper critical zone has a greater lateral extent than the lower critical zone and overlies but also onlaps the floor rocks to the south of the steelpoort area the source of the magmas also appears to have been towards the south as the mg chromitite layers degrade and thin northward whereas the lg layers are very well represented in the north and degrade southward sr and os isotope data indicate that the major chromitite layers including the lg6 mg1 and ug2 originated in a similar way extremely abrupt and stratigraphically restricted increases in the sr isotope ratio imply that there was massive contamination of intruding melt which hit the roof of the chamber and incorporated floating granophyric liquid which forced the precipitation of chromite kruger 1999 kinnaird et al 2002 therefore each chromitite layer represents the point at which the magma chamber expanded and eroded and deformed its floor nevertheless this was achieved by in situ contamination by roof rock melt of the intruding critical zone liquids that had an orthopyroxenitic to noritic lineage the main zone is present in the eastern and western lobes of the bushveld complex where it overlies the critical zone and onlaps the floor rocks to the south and the north where it is also the basal zone in the northern lobe the new magma first intruded the northern lobe north of the thabazimbi murchison lineament interacted with the floor rocks incorporated sulphur and precipitated the platreef along the floor rock contact before flowing south into the main chamber this exceptionally large influx of new magma then eroded an unconformity on the critical zone cumulate pile and initiated the main zone in the main chamber by precipitating the merensky reef on the unconformity the upper zone magma flowed into the chamber from the southern bethal lobe as well as the tml this gigantic influx eroded the main zone rocks and caused very large scale unconformable relationships clearly evident as the gap areas in the western bushveld complex the base of this influx which is also coincident with the pyroxenite marker and a troctolitic layer in the northern lobe is the petrological and stratigraphic base of the upper zone sr isotope data show that all the pge rich ores including chromitites are related to influxes of magma and are thus related to the expansion and filling of the magma chamber dominantly by lateral expansion with associated transgressive disconformities onto the floor rocks coincident with major zone changes these positions in the stratigraphy are marked by abrupt changes in lithology and erosional features over which succeeding lithologies are draped the outcrop patterns and the concordance of geochemical isotopic and mineralogical stratigraphy indicate that during crystallisation the bushveld complex was a wide and shallow lobate sill like sheet and the rock strata and mineral deposits are quasi continuous over the whole intrusion

origin of the ug2 chromitite layer bushveld complex chromitite layers are common in large mafic layered intrusions a widely accepted hypothesis holds that the chromitites formed as a consequence of injection and mixing of a chemically relatively primitive magma into a chamber occupied by more evolved magma this forces supersaturation of the mixture in chromite which upon crystallization accumulates on the magma chamber floor to form a nearly monomineralic layer to evaluate this and other genetic hypotheses to explain the chromitite layers of the bushveld complex we have conducted a detailed study of the silicate rich layers immediately above and below the ug2 chromitite and another chromitite layer lower in the stratigraphic section at the top of the lower critical zone the ug2 chromitite is well known because it is enriched in the platinum group elements and extends for nearly the entire 400 km strike length of the eastern and western limbs of the bushveld complex where we have studied the sequence in the central sector of the eastern bushveld the ug2 chromitite is embedded in a massive 25 m thick plagioclase pyroxenite consisting of 60 70 vol granular cumulus orthopyroxene with interstitial plagioclase clinopyroxene and accessory phases throughout the entire pyroxenite layer orthopyroxene exhibits no stratigraphic variations in major or minor elements mg number1 4 79 3 81 1 however the 6 m of pyroxenite below the chromitite footwall pyroxenite is petrographically distinct from the 17 m of hanging wall pyroxenite among the differences are 1 phlogopite k feldspar and quartz are ubiquitous and locally abundant in the footwall pyroxenite but generally absent in the hanging wall pyroxenite and 2 plagioclase in the footwall pyroxenite is distinctly more sodic and potassic than that in the hanging wall pyroxenite an45 60 vs an70 75 the lower critical zone chromitite is also hosted by orthopyroxenite but in this case the rocks above and below the chromitite are texturally and compositionally identical for the ug2 we interpret the interstitial assemblage of the footwall pyroxenite to represent either interstitial melt that formed in situ by fractional crystallization or chemically evolved melt that infiltrated from below in either case the melt was trapped in the footwall pyroxenite because the overlying ug2 chromitite was less permeable if this interpretation is correct the footwall and hanging wall pyroxenites were essentially identical when they initially formed however all the models of chromitite formation that call on mixing of magmas of different compositions or on other processes that result in changes in the chemical or physical conditions attendant on the magma predict that the rocks immediately above and below the chromitite layers should be different this leads us to propose that the bushveld chromitites formed by injection of new batches of magma with a composition similar to the resident magma but carrying a suspended load of chromite crystals the model is supported by the common observation of phenocrysts including those of chromite in lavas and hypabyssal rocks and by chromite abundances in lavas and peridotite sills associated with the bushveld complex indicating that geologically reasonable amounts of magma can account for even the massive 70 cm thick ug2 chromitite the model requires some crystallization to have occurred in a deeper chamber for which there is ample geochemical evidence

settling and compaction of chromite cumulates employing a centrifuging piston cylinder and application to layered mafic intrusions the time scales and mechanics of gravitationally driven crystal settling and compaction is investigated through high temperature 1 280 1 500 c centrifuge assisted experiments on a chromite basalt melt system at 100 1 500g 0 5 gpa subsequently the feasibility of this process for the formation of dense chromite cumulate layers in large layered mafic intrusions lmis is assessed centrifugation leads to a single cumulate layer formed at the gravitational bottom of the capsule the experimentally observed mechanical settling velocity of a suspension of 24 vol chromite is calculated to be about half 0 53 of the stokes settling velocity with a sedimentation exponent n of 2 35 3 gravitational settling leads to an orthocumulate layer with a porosity of 0 52 all porosities as fraction formation times for such a layer from a magma with initial chromite contents of 0 1 1 vol are 140 3 5 days equal to a growth rate of 0 007 0 3 m day for grain sizes of 1 2 mm more compacted chromite layers form with increasing centrifugation time and acceleration through chemical compaction an increase of grain contact areas and grain sizes together with a decrease in porosity is best explained by pressure dissolution at grain contacts reprecipitation and grain growth into the intergranular space and a concomitant expulsion of intergranular melt the relation between the porosity in the cumulate pile and effective pressure integrated over time h a t is best fit with a logarithmic function in fact confirming that a pressure dissolution reprecipitation process is the dominant mechanism of compaction the experimentally derived equation allows calculating compaction times 70 80 chromite at the bottom of a 1 m thick chromite layer are reached after 9 250 years whereas equivalent compaction times are 0 2 0 9 years for olivine both for 2 mm grain size the experiments allow to determine the bulk viscosities of chromite and olivine cumulates to be of magnitude 109 pa s much lower than previously reported as long as melt escape from the compacting cumulate remains homogeneous fluidization does not play any role however channelized melt flow may lead to suspension and upward movement of cumulate crystals in lmis chromitite layers are typically part of a sequence with layers of mafic minerals compaction occurs under the additional weight of the overlying layers and can be achieved in a few years to decades

incorporating 3 d parent nuclide zonation for apatite 4he 3he thermochronometry an example from the appalachian mountains the ability to constrain km scale exhumation with apatite 4he 3he thermochronometry is well established and the technique has been applied to a range of tectonic and geomorphic problems however multiple sources of uncertainty in specific crystal characteristics limit the applicability of the method especially when geologic problems require identifying small perturbations in a cooling path here we present new 4he 3he thermochronometric data from the appalachian mountains which indicate significant parent nuclide zonation in an apatite crystal using la icpms measurements of u and th in the same crystal we design a 3 d model of the crystal to explore the effects of intra crystal variability in radiation damage accumulation we describe a numerical approach to solve the 3 d production diffusion equation using our numerical model and a previously determined time temperature path for this part of the appalachians we find excellent agreement between predicted and observed 4he 3he spectra our results confirm this time temperature path and highlight that for complex u and th zonation patterns 3 d numerical models are required to infer an accurate time temperature history in addition our results provide independent and novel evidence for a radiation damage control on diffusivity the ability to exploit intra crystal differences in 4he diffusivity i e temperature sensitivity greatly increases the potential to infer complex thermal histories

environmental memory and a possible seasonal bias in the stable isotope composition of u th he dated goethite from the canadian arctic abstract goethite ax 2 from axel heiberg island 80 n on the margin of the arctic ocean is the dominant mineral in a sample of petrified eocene wood but u th and he measurements suggest that the goethite feooh crystallized in the latest miocene pliocene ca 5 5 to 2 8 ma measured d and 18 o values of ax 2 are 221 6 and 9 6 0 5 respectively the inferred d and 18 o values of the ancient water were about 139 and 18 6 respectively with a calculated temperature of crystallization of 3 5 c which compares with the modern summer j j a temperature of 3 c and contrasts with a modern mat of 19 c published results from various biological proxies on nearby ellesmere island indicate a pliocene 4 ma mat of either 6 or 0 4 c and corresponding seasonal amplitudes of about 18 or 13 c a conductive heat flow model suggests that a temperature of 3 c could represent goethite crystallization at depths of 100 200 cm for mat 6 c or 250 450 cm for mat 0 4 c over seasonally restricted intervals of time the 18 o value of the ax 2 water 18 6 is more positive than the modern j j a precipitation 22 in combination the paleotemperatures and 18 o values of ancient waters from ax 2 and published results from three eocene or pliocene proxy sites on axel heiberg and ellesmere islands are consistent with a warm season bias in those isotopic proxies the results are also consistent with higher proportions of j j a precipitation in the annual total if so this emphasizes the importance of seasonality at high latitudes even in times of warmer global climates and suggests that the arctic hydrologic cycle as expressed in the seasonal distribution and isotopic composition of precipitation perhaps modified by a warmer arctic ocean differed from modern the 13 c value of the fe co 3 oh component in the ax 2 goethite is 6 6 which is much more positive than expected if crystallizing goethite incorporated co 2 derived primarily from oxidation of relict eocene wood with 13 c values of about 24 this apparent paradox may be resolved if the goethite is a product of oxidation of 13 c rich siderite which had previously replaced wood in an eocene methanogenic burial environment thus the goethite retains a carbon isotope memory of a diagenetic eocene event but a d and 18 o record of the latest miocene pliocene arctic climate

halogen variations in the paleoproterozoic layered mafic ultramafic intrusions of east kimberley western australia implications for platinum group element mineralization a number of paleoproterozoic layered mafic ultramafic intrusions in the central part of the halls creek orogen of east kimberley western australia have been explored for platinum group elements chromium nickel copper cobalt and gold here we report on the halogen geochemistry of apatite and biotite in a number of these intrusions interstitial apatite is ubiquitous in these intrusions and in most samples tends to be relatively enriched in f and oh end members and relatively poor in cl 20 mole fluorapatite occurs in the more evolved igneous rocks and in samples from the margins of the intrusions that apparently have been contaminated by metamorphic country rock cl f ratios generally increase with bulk rock molar mg mg fe ratios as observed in other intrusions only a few samples show cl enrichment as high as that seen in the stillwater and bushveld complexes beneath the major stratabound platinum group element deposits the most cl rich compositions observed occur in the upper part of the springvale intrusion where it is associated with troctolite and in a single sample from the mcintosh intrusion for the former intrusion it is suggested that volatiles migrating out of the lower part of the mafic stratigraphy stabilized olivine at the expense of pyroxene associated biotite tends to contain little no more than 10 mole cl or f it is concluded that the east kimberley intrusions contained a low to moderate volatile component that during the combined processes of crystallization degassing and fractionation of interstitial halogen bearing minerals produced a late mobile interstitial silicate liquid or volatile rich fluid phase of variable cl f content that in turn caused most of the observed variations within any given intrusion the exceptions include the margins of the intrusions that appear to have been affected by country rocks either during emplacement assimilation or during later metamorphism apatite s generally low cl f ratio the lack of primary amphibole and the high background sulfur concentrations of the east kimberley intrusions suggest that these magmas were relatively volatile poor the development of high grade platinum group element enriched horizons by late stage hydrothermal processes that could have mobilized significant amounts of the platinum group elements and sulfur is considered to be unlikely

slab breakoff a model for syncollisional magmatism and tectonics in the alps slab breakoff is the buoyancy driven detachment of subducted oceanic lithosphere from the light continental lithosphere that follows it during continental collision in a recent paper davies and von blanckenburg 1994 have assessed the physical conditions leading to breakoff by quantitative thermomechanical modeling and have predicted various consequences in the evolution of mountain belts breakoff will lead to heating of the overriding lithospheric mantle by upwelling asthenosphere melting of its enriched layers and thus to bimodal magmatism breakoff will also lead to thermal weakening of the subducted crustal lithosphere thereby allowing buoyant rise of released crustal slices from mantle depths in this paper we present a test of this model in the tertiary evolution of the european alps in the alps both basaltic and granitoid magmatism occur between 42 and 25 ma following the closure of oceanic basins by subduction and continental collision the granitoids are now well established to result from mixing of basalt with assimilated continental crust to identify the tectonically crucial origin of the partial mantle melts we have compiled all published geochemical and isotopic data of numerous mafic dykes occurring throughout the whole alpine arc their trace element and isotopic composition suggests that they have been formed by low degree melting of the mechanically stable lithospheric mantle we see no evidence for melting of asthenospheric mantle it was thus not decompressed to depths shallower than 50 km once initiated rapid lateral migration of slab breakoff will result in a linear trace of magmatism in locally thermal weakened crust this explains why all alpine magmatic rocks intruded almost synchronously along a strike slip fault the periadriatic lineament a compilation of ages from penninic high pressure rocks subducted to depths of up to 100 km shows that subduction took place at circa 55 40 ma followed by uplift at 40 35 ma from the short time interval between their uplift and the onset of magmatism we infer that both processes have been induced by the breakoff the slab breakoff model fulfills its predictions in the case of the alps and therefore supports the assumptions made in the theoretical model on a geological basis we believe that the characteristic association of magmatic activity with the return of high pressure rocks to the surface allows the identification of this process in the earth s mountain belts

litho stratigraphic effect on variscan fluid flow within the prague synform barrandian evidence based on c o sr isotopes and fluid inclusions the palaeozoic sedimentary sequence of the prague synform ordovician devonian in the centre of the bohemian massif underwent variscan deformation and thermal overprint events variscan veins widespread throughout the sedimentary strata have precipitated from syntectonic aqueous and hydrocarbon rich fluids homogenization temperatures of aqueous inclusions increased from 70 up to 226 degc in the cambrian rocks underlying the prague synform seawater modified due to intensive water rock interaction was the main fluid component fluid flow was limited and restricted to the lithostratigraphic compartments forming a rock buffered system stable isotopic modelling c o and final interpretations of the confined hydrostratigraphic fluid migration was supported by the 87sr 86sr ratio in veins and wall rocks siliciclastic cambrian and ordovician rocks and the associated intersecting veins yielded similar isotopic signatures and consequently the fluid migration is restricted to layer and formation scales gradually downwards increasing fluid temperature and compositional changes reflect burial at oil window conditions an open fluid system could be expected in proximity of major faults of the prague syncline and at the top of the sedimentary sequence

the genesis of archaean chromitites from the nuasahi and sukinda massifs in the singhbhum craton india abstract the chromitite deposits of the nuasahi and sukinda massifs are part of layered ultramafic bodies which occur within archaean low grade metamorphic rocks of the iron ore group iog in the singhbhum craton of the indian shield the chromitite seams are interlayered with dunite and associated with orthopyroxenite detailed electron microprobe study reveals very high mg numbers 0 66 0 82 and cr numbers 0 75 0 87 for chromite in massive chromitite from the seams high fo content fo 92 95 for olivine in dunite and high en content en 89 94 for orthopyroxene in orthopyroxenite our study suggests that the original igneous compositions for these minerals are preserved in the respective monomineralic rocks and can be used to evaluate the primary magmatic petrogenesis of the rocks the primitive compositions of the minerals in monomineralic rocks suggest high degree of partial melting of the source mantle during the mesoarchaean the parental magma from which the massive chromitites crystallized was of siliceous high mg basaltic or boninitic compositions similar to the compositions of spatially associated chromite bearing siliceous high mg basalts of the iron ore group the parental magma may have generated due to the interaction of a depleted mantle with a fluid enriched melt possibly derived in response to the dehydration of a subducting slab monomineralic chromitite layers formed either in response to mixing of magmas or due to the suppression of silicate mineral crystallization in response to elevated h 2 o concentrations in the parent magmas the parental magmas that produced chromitites may have intruded into the volcano sedimentary greenstone belts of a supra subduction zone setting

diagenetic evolution of the carnian wetterstein platforms of the eastern alps the carbonate platforms of the wetterstein formation of the eastern alps drau range and northern calcareous alps show a distinct facies zonation of reefs and lagoons while some lagoonal areas were episodically emerged and formed lagoonal islands others remained permanently flooded the scale of near surface meteoric or marine diagenesis was related to this lagoonal topography at shallow burial depth cementation was dominated by altered marine solutions which additionally caused recrystallization of metastable constituents of the sediment and earlier marine cements high magnesian calcite aragonite connected with a carbon and oxygen isotopic change to more negative values deeper burial cementation shows a succession with two types of saddle dolomite and three types of blocky calcite carbon and oxygen isotopic values of these cements show a trend towards more negative values from the first to the last generation in the following succession clear saddle dolomite zoned blocky calcite cloudy saddle dolomite post corrosion blocky calcite replacive blocky calcite fluid inclusion studies of the carbonate cements are interpreted to indicate a deeper burial temperature development that first increases from 175 to 317 c followed by a temperature decrease to 163 260 c and subsequent increase up to 316 c whereby the samples of the drau range always show the lowest values calculations of the isotopic composition of the water from which the carbonate cements were precipitated yielded positive 18o values from 6 66 to 17 81 o smow which are characteristic for formation and or metamorphic waters also the isotopic compositions of the palaeofluids probably changed during deeper burial diagenesis following the temperature development

relationship between fluid flow and faulting in the alpine realm austria germany italy abstract five carbonate cement generations in faults and associated fractures which precipitated during two major cementation phases were studied in permian to tertiary sedimentary rocks in the drau range in the southern alps and in different nappes of the northern calcareous alps nca the first major cementation phase is represented by clear saddle dolomite csd that can be further subdivided by cathodoluminescence cl into five cl zones a non carbonate zone consisting of chlinochlorite chlorite zone is intercalated between cl zones ii and iii cl zones i iv occur in rocks of permian to early jurassic age while cl zone v is also present in upper jurassic strata the second major cementation phase characterized by another saddle dolomite and three generations of blocky calcite is of miocene age dedolomitized csd which occurs in a synsedimentary normal fault of early jurassic age contains a zone of pyrite the trace element composition of this pyrite zone and of an upper hettangian fe mn crust marmorea crust were compared with the trace element composition of the chlorite zone similar trace element patterns within all three non carbonate zones suggest the same fluid source for these zones during upper hettangian time this fluid could be of hydrothermal origin as indicated by the geochemical characteristics of the fe mn crust hydrothermal activities started during the beginning of rifting in the alpine realm in the late triassic early jurassic fluid inclusion data reveal variable homogenization temperatures for the five cl zones in the clear saddle dolomite formation temperatures between 146 and 201 c were calculated considering the overburden pressure during the formation of the cl zones calculated equilibrium oxygen isotopic fluid compositions indicate 18 o values between 4 0 and 9 0 smow for the fluid from which the different saddle dolomite zones precipitated the origin of the fluid can be related to brines present in the hinterland vindelician high bohemian massif the stratigraphic occurrence of csd and its paragenetic relationship to the non carbonate zones enables a more exact dating of the cl zones cl zones i and ii precipitated between norian and late hettangian time and cl zones iii and iv are of early sinemurian age the formation period of cl zone v is late jurassic to early cretaceous the distribution of the carbonate cements was investigated within different fault sets and in associated subparallel fractures in the nca i e the allgau lechtal and inntal nappe which show a polyphase deformation history csd occurs in normal faults n s trending high angular faults and wnw trending strike slip faults indicating minimum an early jurassic age for these faults the only carbonate cements that are present within ne trending sinistral strike slip faults are of the second major cementation phase implicating a miocene age for these faults fibrous shaped crystals of the first calcite cement of the second major cementation phase preferentially occur within wnw trending strike slip faults reactivation of these faults as conjugated shear pairs in connection with the formation of ne trending strike slip faults is assumed

fluid participation in deep fault zones evidence from geological geochemical and 18o 16o relations fluid incursion into fault zones and their deeper level counterparts brittle ductile shear zones is examined in a number of different crustal environments at the grenville front translation was accommodated along two mylonite zones and an associated boundary fault the high mz ii and low temperature mz i mylonite zones formed at 580 to 640 c and 430 to 490 c respectively in the presence of fluids of metamorphic orgin indigenous to the immediate rocks a population of posttectonic quartz veins occupying brittle fractures were precipitated from fluids with extremely negative 18o at 200 to 300 c the water may have been derived from downwards penetration into fault zones of low 18o precipitation on a mountain range induced by continental collision with uplift accommodated at deep levels by the mylonite zones coupled with rebound on the boundary faults at lagoa real brazil archaean gneisses overlie proterozoic sediments along thrust surfaces and contain brittle ductile shear zones locally occupied by uranium deposits following deformation at 500 to 540 c in the presence of metamorphic fluids and under conditions of low water to rock ratio shear zones underwent local intense oxidation and desilicification all minerals undergo a shift of 10 indicating discharge of meteoric water recharged formation brines in the underlying proterozoic sediments up through the archaean gneisses during overthrusting about 1000 km3 of solutions passed through these structures at yellow knife a series of large scale shear zones developed by brittle ductile mechanisms involving volume dilation with migration of 5 wt volatiles into the shear zone from surrounding metabasalts this early deformation involved no departures in redox state or whole rock 18o from background states of fe2 fe 0 72 and 7 to 7 5 respectively attesting to conditions of low water rock shear zones subsequently acted as high permeability conduits for pulsed discharge of 9 km3 of reduced metamorphic hydrothermal fluids at 360 450 c the west bay fault a late major transcurrent structure contains massive bull quartz that grew at 200 300 c from fluids of 2 to 6 possibly formation brines in general flow regimes in these fault and shear zones follow a sequence from conditions of high temperature and pressure with locally derived fluids at low water to rock ratios during initiation of the structures to high fluxes of reduced formation or metamorphic fluids along conduits as the structures propagate and intersect hydrothermal reservoirs later in the tectonic evolution and at shallower crustal levels there was incursion of oxidizing fluids from near surface reservoirs into the faults

crustally contaminated komatiite primary source of the chromitites and marginal lower and critical zone magmas in a staging chamber beneath the bushveld complex in the lowermost rocks of the rustenburg layered suite bushveld complex levels of total cr 2 o 3 are up to ten times higher than its putative solubility 0 16 cr 2 o 3 in the magmas that many investigators have accepted as parental to the dunites harzburgites pyroxenites and norites of the lower lower critical and upper critical zones various existing models fail to explain this chromium paradox in so far as they may explain why chromite is precipitated but do not account for its abundance in many cumulates simple mixing calculations and quantitative adiabat 1ph runs support our model in which a heterogeneous liquid crystals system was evolved in a staging chamber beneath the complex by mixing of komatiitic melts with 20 to 30 of crustal melts at depths appropriate to pressures between 10 and 4 5 kbars we contend that crystal settling and the effects of elutriation during magma ascent had generated a high sio 2 cap with modal olivine orthopyroxene and chromite increasing with depth in the staging chamber the magma from the high sio 2 cap of the staging chamber was first injected into the largely sedimentary transvaal supergroup generating what is now termed the marginal zone slurries of crystals plus liquid from the spasmodically rejuvenated main body of the staging chamber were then injected into the shallower bushveld chamber to form the lower and critical zones chromite within the slurries introduced in this way could account for their high gross cr content frequent reversals of normal evolutionary trends evident in the cumulates are attributable to the episodic nature of the influxes from the staging chamber the lithologically diverse nature of the floors upon which the chromitite layers accumulated peridotites pyroxenites norites and anorthosites was therefore more a function of the timing of emplacement of the cr rich slurries generated in the deeper reservoir than of specific evolutionary processes within the bushveld chamber itself

the halogen geochemistry of the bushveld complex republic of south africa implications for chalcophile element distribution in the lower and critical zones solidification of the interstitial liquid the stratigraphic distribution halogen bearing minerals especially apatite are minor but ubiof s cu and the pge in the critical zone cannot readily be quitous phases throughout the bushveld complex interstitial apatite explained either by precipitation of sulfide as a cotectic phase or as is near end member chlorapatite below the merensky reef lower a function of trapped liquid abundance evidence from potholes and and critical zones and has increasingly fluorian compositions with the pge rich driekop pipe of the bushveld complex imply that increasing structural height above the reef main and upper zones migrating cl rich fluids mobilized the base and precious metal cl f variations in biotite are more limited owing to crystal chemical sulfides we suggest that the distribution of sulfide minerals and controls on halogen substitution but are also consistent with a the chalcophile elements in the lower and critical zones reflects a decrease in the cl f ratio with structural height in the complex general process of vapor refining and chromatographic separation of a detailed section of the upper lower zone to the critical zone is these elements during the evolution and migration of a metalliferous characterized by an upward decrease in sulfide mode from 0 01 cl rich fluid phase 0 1 to trace 0 001 cu tends to correlate with other incompatible elements in most samples whereas the platinum group elements pge can behave independently particularly in the critical zone the decrease in the cl f ratio of apatite in the

th pb and u pb dating of ore stage calcite and paleozoic fluid flow thorium 232 lead 208 and uranium 238 lead 206 radiometric ages for ore stage calcite show that mississippi valley type mvt ore deposits can form in distinct tectonic settings an age of 251 11 million years for the jumbo mine in kansas united states is in agreement with other ages for mvt deposits in the midcontinent of north america the similarity of ages of these deposits supports the concept that they formed in response to fluid flow during the late paleozoic alleghenian ouachita orogeny an age of 351 15 million years for twelve mile bore and bloodwood kapok deposits in australia indicates that these mvt ores were deposited in a rifting environment

is apatite u th zonation information necessary for accurate interpretation of apatite u th he thermochronometry data abstract new u th and ce concentration maps were acquired by la icpms for 70 apatites from 18 cratonic basement samples from the canadian shield to characterize the nature and variability in apatite u th zonation all apatites are zoned in effective u concentration eu with 80 exhibiting modest zoning that varies from a factor of 1 2 to 2 4 zonation patterns include those with a general eu decrease from core to rim 25 a core to rim eu increase 35 and patchy or irregularly shaped eu distributions 40 most samples consist of individual apatites with markedly different and opposing eu profiles cathodoluminescence cl images were obtained for 258 apatites in 25 samples comparison of eu and cl patterns reveals no consistent relationships suggesting that cl is not a reliable proxy for apatite eu zonation we explore the implications of eu zonation for the interpretation of apatite u th he ahe data by comparing the age predictions for representative and endmember apatite eu profiles with those for unzoned apatites we focus on thermal histories that should magnify eu zonation effects including slow monotonic cooling extended he partial retention zone heprz residence and protracted reheating and cooling application of incorrect ejection correction f t factors different he concentration gradients and variable intracrystalline retentivity due to heterogeneous radiation damage may cause the ahe dates for zoned and unzoned apatites to differ in our dataset the magnitude of the f t effect is by simulating zoned apatite suites from representative and endmember samples we find that eu zoning adds modest age dispersion to most samples the commonly opposing eu profiles for individual apatites in most samples have different effects on the predicted age and partially cancel each other out to reduce the predicted age difference at the sample level higher predicted age differences between zoned and unzoned samples generally correlate with greater sample scatter the age deviations generally fall within the 15 uncertainty range that we commonly apply for sample ahe dates except in unusual circumstances it appears unlikely that the unzoned apatite assumption will lead to misinterpretation of ahe datasets thus precluding the need to routinely acquire apatite eu zonation information as part of ahe dating studies

conditions for hydrofracture and the fluid permeability of accretionary wedges abstract the dewatering of fine grained sediment piles in accretionary wedges is controlled by zones of active deformation episodic hydrofracture in these zones enhances permeability enabling fluid flow to be faster than the speed of underthrusting here i investigate the mechanics of simultaneous shear failure and hydrofracture in muddy rocks as a function of the faulting mode the physical properties of model rock types are chosen to represent uncemented mud or cherty marly mudstones that have undergone significant diagenesis during thrust faulting hydrofracture always requires pore fluid pressures in excess of the lithostatic load whereas shallow level wrenching or normal faulting permits hydrofracture at hydrostatic pressures significantly lower than lithostatic load all three faulting modes are expected to occur in an accretionary wedge attempting to dynamically maintain its critical taper the model computations indicate that in the upper 80 120 m of a section of soft mudstone hydrofracture inevitably accompanies wrench or normal faulting hard mudstones inevitably hydrofracture to depths of up to 1400 m during normal faulting these differences in mechanical behaviour have profound consequences for fluid venting capabilities of accretionary wedges they influence hydrocarbon trapping and play a role in the localisation of mud diapirs and mud volcanism

variations of sulfur isotopes trace element compositions and cathodoluminescence of mississippi valley type pb zn ores from the drau range eastern alps slovenia austria implications for ore deposition on a regional versus microscale the sulfur isotope composition of sulfide ore minerals of the drau range varies considerably down to centimeter scale variations both on a regional and a local scale broadly scattering 34s values ranging between 2 and 25 per mil with a mean around 17 per mil are probably related to both local bacteriogenic and thermal reduction of late triassic marine sulfate the 34s is typically more depleted in ore deposited in subtidal carbonate rocks than in the economic zn and pb ore preferably deposited within intratidal carbonate host rock the 34s 32s ratio increases on the order of fes2 pbs zns which reflects disequilibrium and low temperature formation the strong depletion of 34s in botryoidal sphalerite ranging between 24 and 32 per mil appears to be correlated with the crystal mode wurtzite sphalerite fibers and the enrichment of the trace elements fe as tl and ge sphalerite generations of specific cathodoluminescense color and closely related trace element composition are correlated district wide a decrease of the trace elements fe as tl and ge in the sphalerite generations is coeval with a minor depletion of 34s in the range of 3 and 12 per mil for 34s values in several hand specimens variations in 34s values and trace element zonation on a regional to local scale indicates mixing of local sulfur sources with a regional metal rich brine however distinct peaks in some ore deposits and more positive 34s values of between 5 and 10 per mil are also compatible with derivation of the sulfur from basement rocks the enrichment of the trace element cu in orange luminescing sphalerite negatively correlated with fe as tl and ge and therefore strongly suggests leaching of copper from late permian to early triassic continental red beds frequent evaporites in this stratigraphic section might have supplied sulfur of relatively heavy isotopic composition to the brine hence some sulfur may also have been transported with a metal rich brine

extensional detachment faulting in the evolution of a tethys passive continental margin eastern alps switzerland the austroalpine nappes of switzerland represent an exhumed and tectonically imbricated segment of the passive continental margin of the jurassic tethys within one of these nappes err nappe part of an upper crustal extensional detachment is preserved indicating that the thinning of the crust was achieved by non uniform extension the mesozoic age of the detachment is shown by comparison between its associated cataclasites and identical cataclasites that are found as redeposited components in middle jurassic sedimentary breccias this low angle detachment within the basement was kinematically linked to synsedimentary high angle normal faults at the surface the area of the err nappe belongs in terms of jurassic paleogeography to the most distal part of the continental margin where both low and high angle normal faults dipped oceanward that is west to northwest in present day coordinates in the more proximal part of the margin however the high angle normal faults dipped eastward toward the continent ammonite stratigraphy within the sediment prisms adjacent to the faults gives evidence for an early jurassic age of the faulting in the proximal part of the margin whereas in the distal part faulting occurred during latest early to middle jurassic time we therefore propose that the jurassic extension of the crust which finally led to the opening of the piemont ligurian ocean was achieved by two fault systems which differ in geometry fault orientation and age these fault systems were composed of a basal low angle detachment with high angle normal faults above the orientation of the older eastward dipping detachment was prone for reactivation during early alpine crustal shortening the present day thrust contact between the two major tectonic units of the area the lower austroalpine and the central austroalpine nappe complex therefore might correspond to an eastward dipping jurassic low angle normal fault

fluid flow along a strike slip fault at the toe of the oregon accretionary prism implications for the geometry of frontal accretion sediment offscraping and accretion takes place predominantly by landward vergent thrusting along most of the oregon accretionary prism north of latitude 44 50 n this indicates very low basal shear stress probably due to near lithostatic fluid pressure on the decollement horizon recent multichannel seismic and sidescan sonar studies have demonstrated the existence of an oblique basement involved left lateral strike slip fault termed the wecoma fault it cuts abyssal plain sediments and oceanic basement of the subducting plate where the wecoma fault intersects the wedge thrust front at 45 09 n accretion to the wedge occurs through an anomalous seaward vergent thrust linear fault parallel erosional gullies which trend obliquely across the slope on the seaward flank of the frontal anticline in the accretionary wedge are interpreted as marking the surface traces of splays of the wecoma fault data collected during six alvin dives on and around these gullies demonstrate that fluid expulsion from the deforming sediments is preferentially concentrated along the fault traces evidence includes the presence of chemosynthetic biological communities pervasive veining and fracturing of the rock and extensive carbonate cementation all forming preferentially within the gullies this strike slip fault apparently acts as a high angle fluid escape conduit channeling fluids from deep within the wedge to the surface we propose that this fluid conduit causes the local inversion of thrusting to seaward vergence by allowing the escape of highly overpressured fluids from the incipient decollement horizon the consequent increase in basal effective normal stress favors the development of a seaward vergent thrust the discovery of this strike slip fault acting as an efficient fluid conduit to the surface of the wedge emphasizes the generally unappreciated importance of strike slip and normal faults in dewatering accretionary prisms such dewatering features may significantly influence the development of large scale structures and they have broad applicability to other tectonic settings including collision zones and continental fold and thrust belts

the south tibetan detachment system facilitates ultra rapid cooling of granulite facies rocks in sikkim himalaya 1 the eastern himalaya is characterized by a region of granulites and local granulitized eclogites that have been exhumed via isothermal decompression from lower crustal depths during the india asia collision spatially most of these regions are proximal to the south tibetan detachment system an orogen parallel normal sense detachment system that operated during the miocene suggesting that it played a role in their exhumation here we use geo and thermochronological methods to study the deformation and cooling history of footwall rocks of the south tibetan detachment system in northern sikkim india these data demonstrate that the south tibetan detachment system was active in sikkim between 23 6 and 13 ma and that footwall rocks cooled rapidly from 700 to 120 c between 15 13 ma while active the south tibetan detachment system exhumed rocks from mid crustal depths but an additional heat source such as strain heating advected melt and or crustal thinning is required to explain the observed isothermal decompression cessation of movement on the south tibetan detachment system produced rapid cooling of the footwall as isotherms relaxed a regional comparison of temperature time data for the eastern south tibetan detachment system indicates a lack of synchronicity between the sa er sikkim yadong section and the nw bhutan section to accommodate this requires either strike slip tear faulting or local out of sequence thrusting in the younger segment of the orogen

effect of alpha damage annealing on apatite u th he thermochronology recent data on he diffusion challenge the temperature sensitivity of apatite u th he thermochronology the damage induced by recoil of u and th decay series during emission of particles recoil damage has been proposed to modify he diffusion properties through time however we propose that annealing of these irradiation defects may be an important phenomenon and may be significant in case of slowly cooled or reheated basement rocks to test this hypothesis we developed a quantitative model including an explicit treatment of recoil damage annealing and their effect on he diffusion kinetics and calibrate it against literature data our model is based on two hypotheses 1 helium is in equilibrium between an apatite crystal and its defects and 2 alpha recoil damage annealing can be described analogously to fission track annealing this model has been embedded into a monte carlo simulation of helium production ejection diffusion and applied to data from the french massif central a complex slowly cooled terrain with burial reheating where the thermal history has been constrained by previous fission track ft data including ft length distributions u th he ages are close to the ft ages from the same samples and are generally reproducible among replicates but some samples present he age dispersion that is not correlated with crystal size our model reproduces the massif central data very well except for three samples where he ages are older than corresponding ft ages we show that annealing of irradiation damage has an important impact on retentivity of helium and that the he content he is only a rough approximation of the damage level in particular our results show that independence of he ages on crystal sizes in case of reheated samples is a clear indication of the higher he retentivity induced by recoil defects and that an explicit treatment of defect annealing is required for a correct interpretation of u th he ages in such a case more generally a correlation with the crystal size can bring information on the thermal path only if the age of defects well represented by the fission track age is available due to the dependence of the partial retention zone on damages conversely in case of rapid cooling or for samples having low u and th contents damage effects can be ignored without significant effects on he ages

simultaneous miocene extension and shortening in the himalayan orogen the south tibetan detachment system separates the high grade metamorphic core of the himalayan orogen from its weakly metamorphosed suprastructure it is thought to have developed in response to differences in gravitational potential energy produced by crustal thickening across the mountain front geochronologic data from the rongbuk valley north of qomolangma mount everest in southern tibet demonstrate that at least one segment of the detachment system was active between 19 and 22 million years ago an interval characterized by large scale crustal thickening at lower structural levels these data suggest that decoupling between an extending upper crust and a converging lower crust was an important aspect of himalayan tectonics in miocene time

transpressional collision structures in the upper crust the fold thrust belt of the northern calcareous alps abstract two major structural events characterize the tectonic evolution of the northern calcareous alps nca 1 late early cretaceous to late eocene nw directed dextral transpressional stacking of nappes as an expression of the formation of the austroalpine orogenic wedge and 2 miocene sinistral wrenching due to eastward lateral extrusion of crustal wedges along the central eastern alps three first order detachment horizons are defined by differences of competence within areas of facies transition preexisting normal faults control the thrust architecture transpressional contraction in the nca is indicated by a nw directed thrusting oblique to both the long axis of the nca and the edge of the orogenic foreland b the occurrence of en echelon arrays of thrusts and folds laterally displaced by dextral strike slip faults the thrusts and strike slip faults are kinematically connected to each other and dissect the nca into rhomboidal blocks c ne directed extension parallel to fold and ramp axes and the internal strike of the nca and d clockwise rotation 30 of the entire nca around a vertical axis kinematic and dynamic analysis of mesoscale fault striae data related to transpressional stacking indicates a nw trend of contraction directions and in general a ne trend of the extension directions parallel to fold axes and branch lines of thrusts before rotation the average contraction direction was probably parallel to the generally w directed shear direction recorded in the crystal plastically deformed central part of the eastern alps three balanced cross sections across the nca yield a minimum of 54 65 total shortening

carbonate cements indicate channeled fluid flow along a zone of vertical faults at the deformation front of the cascadia accretionary wedge northwest u s coast to understand the relation between fluid seeps and structures sedimentary rocks were collected with the dsrv alvin from a vertical fault zone that transects the deformation front of the cascadia accretionary wedge the rocks contained diagenetic carbonate cement that was precipitated from fluids expelled during accretion carbon oxygen and strontium isotope data are consistent with a fluid source at 2 km depth most carbon isotopes range from 1 to 25 pdb peedee belemnitel standard consistent with a thermogenic methane source oxygen isotopes show extreme 18 o depletions 4 to 13 pdb that are consistent with precipitation from fluids with temperatures as high as 100 c 87 sr 86 sr values of 0 70975 to 0 71279 may be due to strontium in fluids derived from clay rich parts of the stratigraphic section the ubiquity of carbonate precipitates and the isotope data indicate that the vertical fault zone is an efficient conduit for fluid dewatering from deep levels of the accretionary wedge

highly siderophile and strongly chalcophile elements in magmatic ore deposits an ore deposit by definition must be economically viable that is to say it must contain sufficient material at high enough grade to make it possible to mine and process it at a profit bates and jackson 1987 this requires the elements to be collected and concentrated by some phase and for them to be deposited close to the surface of the earth at the oxygen fugacities found in the crust native fe is not normally stable and thus the highly siderophile elements defined as ru rh pd re os ir pt and au cannot behave as siderophile elements except in rare cases such as on disko island klock et al 1986 where the magma is sufficiently reduced for native fe to be present however if mafic magmas become saturated in a base metal sulfide liquid the highly siderophile elements behave as highly chalcophile elements table 1 thus these elements are generally found in association with base metal sulfide minerals which crystallized from a magmatic sulfide liquid namely pyrrhotite pentlandite chalcopyrite cubanite pyrite an exception to this is au although au is strongly chalcophile and is produced as a by product from many platinum group element pge deposits table 2 most primary au deposits consist of native au groves et al 1998 these will not be discussed in this chapter there are many pge deposits i e accumulations of pge minerals and base metal sulfides containing pge bates and jackson 1987 around the world but most of these do not constitute pge ore deposits because they are either too small or their grade is too low or other political or infrastructure factors prevent the economic exploitation of the deposit bates and jackson 1987 for the purpose of this work we have defined pge ore deposits as those which have significant production 2 of the annual world

influence of crystal size on apatite u th he thermochronology an example from the bighorn mountains wyoming near surface tectonic and geomorphic processes involve cooling of rocks through low temperatures 50 200 c because rates of helium diffusion in apatite titanite and zircon are sensitive to temperature variations in this range uranium thorium helium thermochronometry u th he dating is well suited to establishing the timing and rates of these processes in the geologic record however because fractional loss of he is controlled by crystal size such that larger crystals retain a larger fraction of radiogenic he u th he ages must vary not only with thermal history but also with crystal size here we present crystal size correlated he ages from co existing apatites from the bighorn mountains wyoming that range from 100 to 350 ma these correlations are a sensitive indicator of the rock s thermal history in a temperature range below the system s nominal closure temperature t c 70 c for apatite and are consistent with a thermal history involving residence in the upper 2 3 km of crust since the precambrian with maximum temperatures of 65 80 c just prior to laramide orogenic exhumation the influence of crystal size on he ages will be most apparent in rocks where temperatures have been in the range of partial he retention for long periods of time 30 70 c for 10 7 years in such cases accurate interpretation of u th he ages must incorporate the effect of crystal size and this method may provide insights to thermal histories of rocks in previously inaccessible low temperature ranges

melt generation and fluid flow in the thermal aureole of the bushveld complex granite sheets emplaced into the migmatite zone of the eastern contact aureole of the bushveld complex resulted from fluid enhanced incongruent biotite melting of the underlying silverton formation shales during prograde metamorphism ba concentrations are extreme in both the sheets 1000 ppm and the hornfels 800 ppm into which they have been emplaced we conclude that a ba rich hydrothermal fluid induced melting in the aureole and that fluid transport of ba2 and to a lesser extent sr2 and eu2 persisted in the melt zones under subsolidus conditions sr isotope systematics from high ba localities define an errorchron of 2161 106 ma with an initial 87sr 86sr ratio of 0 705 0 001 metasedimentary rocks unaffected by fluid infiltration were homogenized at the same time but with an increased initial ratio suggesting that whereas isotope homogenization was achieved between outcrops permeated by fluids there is no evidence of regional homogenization oxygen isotope compositions of psammitic metasediments in the aureole are uncorrelated with distance from the contact suggesting the infiltrating fluid equilibrated isotopically with the metasediments their elevated 18o values 11 3 12 1 are consistent with a fluid source from devolatilization of the sedimentary lithologies textural analysis at both outcrop and thin section scale of arkose and psammites in the aureole lakenvalei and magaliesberg formations shows that the extent of melting was highly heterogeneous even on the grain scale and resulted from heterogeneously distributed infiltration of aqueous fluid on dilatant cracks and grain boundaries cathodoluminescence imaging of quartz shows a marked difference in the amount of fine structure with that in the melted rocks having uniform luminescence in contrast to that in rocks containing little or no melt which preserve textures inherited from the regionally metamorphosed protolith simple finite difference thermal modelling of the aureole suggests that the width of the melt zone 500 m is inconsistent with conductive heat transfer and hence that the thermal structure has been modified by fluid advection

apatite u th he thermochronometry using a radiation damage accumulation and annealing model helium diffusion from apatite is a sensitive function of the volume fraction of radiation damage to the crystal a quantity that varies over the lifetime of the apatite using recently published laboratory data we develop and investigate a new kinetic model the radiation damage accumulation and annealing model rdaam that adopts the effective fission track density as a proxy for accumulated radiation damage this proxy incorporates creation of crystal damage proportional to production from u and th decay and the elimination of that damage governed by the kinetics of fission track annealing the rdaam is a version of the helium trapping model hetm shuster d l flowers r m and farley k a 2006 the influence of natural radiation damage on helium diffusion kinetics in apatite earth planet sci lett 249 148 161 calibrated by helium diffusion data in natural and partially annealed apatites the chief limitation of the hetm now addressed by rdaam is its use of he concentration as the radiation damage proxy for circumstances in which radiation damage and he are not accumulated and lost proportionately from the crystal by incorporating the rdaam into the hefty computer program we explore its implications for apatite u th he thermochronometry we show how u th he dates predicted from the model are sensitive to both effective u concentration eu and details of the temperature history the rdaam predicts an effective he closure temperature of 62 c for a 28 ppm eu apatite of 60 m radius that experienced a 10 c ma monotonic cooling rate this is 8 c lower than the 70 c effective closure temperature predicted using commonly assumed durango diffusion kinetics use of the rdaam is most important for accurate interpretation of u th he data for apatite suites that experienced moderate to slow monotonic cooling 1 0 1 c ma prolonged residence in the helium partial retention zone or a duration at temperatures appropriate for radiation damage accumulation followed by reheating and partial helium loss under common circumstances the rdaam predicts u th he dates that are older sometimes much older than corresponding fission track dates nonlinear positive correlations between apatite u th he date and eu in apatites subjected to the same temperature history are a diagnostic signature of the rdaam for many but not all thermal histories observed date eu correlations in four different localities can be explained with the rdaam using geologically reasonable thermal histories consistent with independent fission track datasets the existence of date eu correlations not only supports a radiation damage based kinetic model but can significantly limit the range of acceptable time temperature paths that account for the data in contrast these datasets are inexplicable using the durango diffusion model the rdaam helps reconcile enigmatic data in which apatite u th he dates are older than expected using the durango model when compared with thermal histories based on apatite fission track data or other geological constraints it also has the potential to explain at least some cases in which u th he dates are actually older than the corresponding fission track dates

re os isotope and pd ru variations in chromitites from the critical zone bushveld complex south africa abstract rhenium osmium isotope measurements of selected chromitites from the critical zone of the bushveld igneous complex reveal that a significant component of radiogenic os was present in the magma when it formed 2 06 ga ago measured 187 os 188 os ratios increase with stratigraphic height and individual layers have similar values across the complex initial 187 os 188 os ratios for chromitites from the western bushveld decrease from 0 140 at the lg to 0 129 at the mg2 chromitite increasing to 0 153 at the ug2 chromitite the inflection at mg2 mg3 is the boundary between the lower and upper critical zone consistent with previous mineralogical and geochemical studies indicating a shift in magma composition at this point chromite separates have os values of 12 35 at 2 06 ga and are too radiogenic for a purely mantle derivation assimilation of 5 mafic granulitic lower crust in a tholeiitic parent magma can explain the radiogenic values pd ru ratios for the intergranular and chromite separate components of each layer differ by orders of magnitude but relative shifts from layer to layer are similar combined with the re os data the results suggest that pge were concentrated into chromitites by magmatic or early deuteric processes that were specific to each layer post layering upward mobilization of pge by hydrothermal processes probably did not play a significant role in pge enrichment

natural age dispersion arising from the analysis of broken crystals part i theoretical basis and implications for the apatite u th he thermochronometer abstract over the last decade major progress has been made in developing both the theoretical and practical aspects of apatite u th he thermochronometry and it is now standard practice and generally seen as best practice to analyse single grain aliquots these individual prismatic crystals are often broken and are fragments of larger crystals that have broken during mineral separation along the weak basal cleavage in apatite this is clearly indicated by the common occurrence of only 1 or no clear crystal terminations present on separated apatite grains and evidence of freshly broken ends when grains are viewed using a scanning electron microscope this matters because if the 4 he distribution within the whole grain is not homogeneous because of partial loss due to thermal diffusion for example then the fragments will all yield ages different from each other and from the whole grain age here we use a numerical model with a finite cylinder geometry to approximate 4 he ingrowth and thermal diffusion within hexagonal prismatic apatite crystals this is used to quantify the amount and patterns of inherent natural age dispersion that arises from analysing broken crystals a series of systematic numerical experiments were conducted to explore and quantify the pattern and behaviour of this source of dispersion using a set of 5 simple thermal histories that represent a range of plausible geological scenarios in addition some more complex numerical experiments were run to investigate the pattern and behaviour of grain dispersion seen in several real data sets the results indicate that natural dispersion of a set of single fragment ages defined as the range divided by the mean arising from fragmentation alone varies from c 7 even for rapid c 10 c ma monotonic cooling to over 50 for protracted complex histories that cause significant diffusional loss of 4 he the magnitude of dispersion arising from fragmentation scales with the grain cylindrical radius and is of a similar magnitude to dispersion expected from differences in absolute grain size alone spherical equivalent radii of 40 150 m this source of dispersion is significant compared with typical analytical uncertainties on individual grain analyses c 6 and standard deviations on multiple grain analyses from a single sample c 10 20 where there is a significant difference in the u and th concentration of individual grains eu the effect of radiation damage accumulation on 4 he diffusivity assessed using the rdaam model of flowers et al 2009 is the primary cause of dispersion for samples that have experienced a protracted thermal history and can cause dispersion in excess of 100 for realistic ranges of eu concentration i e 5 100 ppm expected natural dispersion arising from the combined effects of reasonable variations in grain size radii 40 125 m eu concentration 5 150 ppm and fragmentation would typically exceed 100 for complex thermal histories in addition to adding a significant component of natural dispersion to analyses the effect of fragmentation also acts to decouple and corrupt expected correlations between grain ages and absolute grain size and to a lesser extent between grain age and effective uranium concentration eu considering fragmentation explicitly as a source of dispersion and analysing how the different sources of natural dispersion all interact with each other provides a quantitative framework for understanding patterns of dispersion that otherwise appear chaotic an important outcome of these numerical experiments is that they demonstrate that the pattern of age dispersion arising from fragmentation mimics the pattern of 4 he distribution within the whole grains thus providing an important source of information about the thermal history of the sample we suggest that if the primary focus of a study is to extract the thermal history information from u th he analyses then sampling and analytical strategies should aim to maximise the natural dispersion of grain ages not minimise it and should aim to analyse circa 20 30 grains from each sample the key observations and conclusions drawn here are directly applicable to other thermochronometers such as the apatite rutile and titanite u pb systems where the diffusion domain is approximated by the physical grain size

late cenozoic uplift of denali and its relation to relative plate motion and fault morphology apatite fission track analysis of samples that cover a 4 kilometer vertical section from the western flank of denali mount mckinley north america9s highest mountain suggests that the mountain massif was formed by rapid uplift 1 kilometer per million years beginning 6 million years ago ma uplift was a result of the morphology of the denali fault and a change in motion of the pacific plate with respect to north america at 5 ma which created opposing tangential vectors of relative movement along the fault and forced the intervening crustal blocks upward

characterization of the rhyolite of bodie hills and 40ar 39ar intercalibration with ar mineral standards abstract the rhyolite of bodie hills california is characterized compositionally and the geochronology of selected phases is studied sanidine bhs from the rhyolite is well suited as a 40ar 39ar reference material with high k ca and radiogenic yield intercalibration with ga1550 biotite from the dromedary igneous complex new south wales australia yields an age of 9 7946 0 0031 ma for bhs relative to an age of 98 79 0 54 ma for ga1550 and a calibration factor rbhs ga1550 of 0 096719 0 000032 bhs is also intercalibrated with sanidines of the taylor creek rhyolite tcs rtcs bhs of 2 90874 0 00067 fish canyon tuff fcs rfcs bhs of 2 88339 0 00088 and rhyolite of alder creek acs racs bhs of 0 12028 0 00024 these calibration factors yield ages of 28 344 0 011 ma 28 099 0 013 ma and 1 1809 0 0024 ma for tcs fcs and acs respectively relative to ga1550 full propagation of errors increases these uncertainties and that of bhs to 0 9 of their ages calibration of bhs using the astronomically tuned age of fcs determined by kuiper et al 2008 yields an age of 9 8295 0 0036 ma stepwise heating of bhs reveals the same small progressive increase in age across the age spectrum reported for fcs acs and other potassium feldspars e g foland and xu 1990 phillips et al 2017 this increase is consistent with mass fractionation of argon during step heating and favors use of single step fusion ages of all sanidines for monitor and calibration purposes zircons from the rhyolite of bodie hills are strongly zoned in u and th and u pb geochronologic analyses suggest multiple generations of zircon growth the youngest ages indicate a crystallization age of 9 97 0 08 ma 2 arguably similar to the 40ar 39ar results when residence time is considered but previous episodes of zircon growth began at least 400 ka prior to eruption precise u th he thermochronology of these zircons is difficult because crystal to crystal variations in the magnitude and complexity of their u th zoning complicates accurate alpha ejection calculations replicate conventional single crystal u th he ages are more widely dispersed than predicted by analytical uncertainties but laser ablation u th he ages are more reproducible with an inverse variance weighted mean of 9 71 0 58 ma

deciphering fracturing and fluid migration histories in northern appalachian basin an integrated structural and geochemical analysis of mineralized faults and fractures revealed that the most widespread fracture systems in the northern appalachian basin resulted from two periods of post alleghenian deformation fluid inclusion homogenization measurements on calcite samples from pre syn and post kinematic structural settings bracketed the temperature ranges predominantly a function of depth of burial for each period of fracturing the first period of deformation produced northwest west northwest and east northeast fractures and related faults that experienced recurrent strike slip movements during a cooling event from 176 to 120 c 349 to 248 f this cooling episode was associated with the waning of an elevated thermal grad ent attained during dewatering caused by late paleozoic burial metamorphism at depths of 3 to 3 5 km 9 800 to 11 500 ft later normal faulting reopened the earlier strike slip fault zones during subsequent cooling of the system from 112 c 234 f to below 76 c 169 f this second cooling stage resulted from the 1 to 2 km 3 300 to 6 600 ft of uplift and erosion that occurred throughout the eastern united states during the cretaceous the ubiquitous occurrence of carbonate silicate and sulfide epigenetic mineralization in many environments allows for widespread application of the dating techniques demonstrated in this study basic to this approach is the analysis of mineral assemblages deposited during increases in hydrothermal circulation resulting from enhanc d permeability associated with periods of faulting and fracturing the presence of different mineral assemblages representing a range of ages and crystallization environments on various generations of brittle structures provides a means for developing detailed faulting and circulation histories and the determination of approximate as well as precise relative ages for each event

deformation of leucogranites of the crystalline main central sheet in southern tibet china abstract towards the top of the main central sheet in southern tibet china leucogranites intrude metamorphic rocks related to rocks of the main central thrust in the himalayas our study shows that the leucogranites suffered a northward shear deformation the significance of this event is discussed in terms of gravity driven decollement of the 10 000 m thick tethyan sedimentary pile over its crystalline basement

current strain accumulation in the hinterland of the northwest himalaya constrained by landscape analyses basin wide denudation rates and low temperature thermochronology abstract rupture associated with the 25 april 2015 m w 7 8 gorkha nepal earthquake highlighted our incomplete understanding of the structural architecture and seismic cycle processes that lead to himalayan mountain building in central nepal in this paper we investigate the style and kinematics of active mountain building in the himalayan hinterland of northwest india approximately 400 km to the west of the hypocenter of the nepal earthquake via a combination of landscape metrics and long ma and short term ka erosion rate estimates from low temperature thermochronometry and basin wide denudation rate estimates from 10 be concentrations we focus our analysis on the area straddling the pt 2 the physiographic transition between the lesser and high himalaya that has yielded important insights into the nature of hinterland deformation across much of the himalaya our results from northwest india reveal a distinctive pt 2 that separates a lesser himalaya region with moderate relief 1000 m and relatively slow erosion 2 has persisted in the same relative position since at least the past 1 5 ma we interpret these observations to suggest that strain accumulation in this hinterland region throughout at least the past 1 5 ma has been accomplished both by crustal thickening via duplexing and overthrusting along transient emergent faults despite the 400 km distance between them similar spatiotemporal patterns of erosion and deformation observed in northwest india and central nepal suggest both regions experience similar styles of active strain accumulation and both are susceptible to large seismic events

u th he geochronology of single zircon grains of known tertiary eruption age we performed a systematic study of u th he ages of volcanic zircons of known eruption age to establish a reliable protocol for zircon he dating and to validate the resulting ages using laser he extraction and dissolution by lithium borate flux melting we analyzed many aliquots of single zircon grains separated from four rapidly cooled rocks fish canyon tuff accepted age 27 8 0 7 ma buluk tuff 16 3 0 2 ma tardree rhyolite 58 4 0 7 ma and utaosa rhyolite 2 52 0 02 ma he ages of fish canyon tuff buluk tuff and utaosa rhyolite zircons are in good agreement with their reference ages mean emission corrected ages were 28 6 1 4 1 16 1 0 8 and 2 61 0 18 ma respectively however the zircon he ages of tardree rhyolite are irreproducible and consistently too old with a mean he age of 78 8 7 0 ma spontaneous fission track densities of tardree rhyolite zircons indicate that about 80 of these zircons are strongly zoned in u with high track density i e high u content in the core and low density on the rim this causes an overestimation of the ejection correction that likely explains the erratic and excessively high he ages from these zircons assessment of the degree and style of u zonation within a zircon population is thus prudent before application of the u th he method step heating experiments indicate that for most zircons he extraction is 99 complete after 1 h at 1300 c however for as yet unknown reasons occasional grains seem to retain substantial amounts of he under these conditions

structural and compositional constraints on the emplacement of the bushveld complex south africa abstract despite a plethora of petrological studies the emplacement mechanics of the world s largest layered intrusion the 2 05 ga bushveld complex are still poorly understood early models considered the intrusion to comprise separate lopolithic intrusions or even concentric cone sheets but recently overwhelming support for a sill like intrusional form has emerged examination of the contact aureole reveals three groups of emplacement related structures interfinger deformation zones and bridges formed between intruding and dilating magma fingers preserved in both the western and eastern parts of the complex sub bushveld complex intrusions that reflect finger like emplacement into the underlying contact aureole and diapiric domes that characterize the eastern contact aureole formed by diapiric amplification of initial interfinger deformation zones associated with the earliest mafic ultramafic pulse of the bushveld complex most of these structures exhibit a strong nw se preferred orientation while longitudinal terminations and divergence of the sub bushveld intrusions away from their source horizons indicate magma emplacement towards the se this emplacement direction is supported by petrological data lateral lithological facies variations and mineral chemical variations within specific horizons the thickest and most chemically primitive accumulations of the lower zone of the complex are found adjacent to and thin towards the ne and se away from the thabazimbi murchison lineament tml a crustal scale lineament that has undergone polyphase reactivation since at least 2 7 ga the bushveld complex was most likely fed by a feeder dyke that utilised the tml spreading laterally from the dyke axis to form its current sill like geometry the stress field at 2 05 ga was suitably oriented to allow for dilation of the ene trending tml suggesting the kaapvaal craton was subject to a component of nw se extension during this time nw se directed collision of the kaapvaal and zimbabwe cratons is recorded in the limpopo belt which would suggest emplacement under conditions of far field extensional stress within a back arc setting this is corroborated by petrological data that supports a significant component of subducted crust in the bushveld magmas

halogens of bushveld complex south africa δ37cl and cl f evidence for hydration melting of the source region in a back arc setting abstract the bushveld complex is one of only a few layered intrusions anomalously enriched in chlorine this study presents the first chlorine stable isotope analyses of bushveld and associated rocks designed to constrain the source of bushveld cl most samples overlap the range of values for pristine morb and altered seafloor samples 37 cl 4 relative to sea water this contrasts with distal metasedimentary floor rocks 37 cl 0 with little evidence of contamination away from the marginal rocks the cl isotope composition and high cl f ratio is therefore a fundamental characteristic of the lower and critical zone magmas and not derived from infiltration from the country rock these results the high cl f ratio and the boninitic arc like character of bushveld magma are consistent with hydration partial melting of a depleted source in a subduction related setting with cl supplied either by the dehydration of a subducted slab with a heavy cl signature or an overridden wet mantle hotspot

contrasting origin of palaeofluids in a strike slip fault system calcite cements in a dextral strike slip fault system in the variscan front complex belgium have been investigated to determine the origin of the fluids migrating along the fault system generation of non ferroan and ferroan calcites occurs along the fault plane a right stepover between two faults is occupied by a breccia it formed due to repeated hydraulic implosions within a dilational jog ferroan calcites cement the breccia the general geological setting and the microthermometric data tt 50 c indicate that calcite precipitation along the faults took place at shallow depth however the geochemistry and origin of the fluids from which the calcites along the fault plane and in the breccia precipitated were different the non ferroan calcites along the fault plane precipitated from a highly saline fluid 18 1 to 25 3 eq wt cacl2 at a temperature around 50 c secondary inclusions in trails in these calcites demonstrate that similar highly saline fluids reached temperatures of at least 130 c the latter migrated from a depth of over 4 km implying the fluid was tapped from the lower palaeozic siliciclastic basement the ferroan calcites along the strike slip fault plane also formed from highly saline fluids but which interacted with upper palaeozoic strata before migration to shallow depth in contrast primary fluid inclusions in the ferroan calcites in the breccia indicate cementation occurred from a low salinity fluid 1 7 to 8 6 eq wt nacl at a temperature of 50 c or less this fluid likely originated from shallow depth the contrasting origin of the fluids indicate different flow regimes have been intersected by the faults during their strike slip reactivation

asymmetric exhumation of the mount everest region implications for the tectono topographic evolution of the himalaya the tectonic and topographic history of the himalaya tibet orogenic system remains controversial with several competing models that predict different exhumation histories here we present new low temperature thermochronological data from the mount everest region which combined with thermal kinematic landscape evolution modeling indicate asymmetric exhumation of mount everest consistent with a scenario in which the southern edge of the tibetan plateau was located 100 km farther south during the mid miocene northward plateau retreat was caused by erosional incision during the pliocene our results suggest that the south tibetan detachment was a localized structure and that no coupling between precipitation and erosion is required for miocene exhumation of greater himalayan sequence rocks on mount everest

thermochronologic constraints on the slip history of the south tibetan detachment system in the everest region southern tibet highlights the south tibetan detachment system played a major role in himalayan evolution near mt everest the detachment system accommodated large displacements under both brittle and ductile conditions rapid cooling of footwall rocks reflected tectonic denudation by brittle slip from ca 15 6 to at least 13 0 ma thermal kinematic modeling suggests displacement on the detachment to be at least 61 km abstract north dipping low angle normal faults of the south tibetan detachment system stds are tectonically important features of the himalayan tibetan orogenic system the stds is best exposed in the n s trending rongbuk valley in southern tibet where the primary strand of the system the qomolangma detachment can be traced down dip from the summit of everest for a distance of over 30 km the metamorphic discontinuity across this detachment implies a large net displacement with previous studies suggesting 200 km of slip here we refine those estimates through thermal kinematic modeling of new u th he and 40ar 39ar data from deformed footwall leucogranites while previous studies focused on the early ductile history of deformation along the detachment our data provide new insights regarding the brittle ductile to brittle slip history thermal modeling results generated with the program qtqt indicate rapid monotonic cooling from muscovite 40ar 39ar closure ca 15 4 14 4 ma at ca 490 c to zircon u th he closure ca 14 3 11 0 ma at ca 200 c followed by slower cooling to apatite u th he closure at ca 9 8 ma at ca 70 c although previous work has suggested that ductile slip on the detachment lasted only until ca 15 6 ma thermal kinematic modeling of our new data suggests that rapid ca 3 4 km ma tectonic exhumation by brittle ductile to brittle fault slip continued to at least ca 13 0 ma much lower modeled exhumation rates 0 5 km ma after ca 13 ma are interpreted to reflect erosional denudation rather than tectonic exhumation projection of fault related exhumation rates backward through time suggests total slip of ca 61 to 289 km on the qomolangma detachment with slightly more than a third of that slip occurring under brittle ductile to brittle conditions

ultramafic rocks in the centre of the vredefort structure south africa geochemical affinity to bushveld rocks abstract the geochemistry and mineral chemistry of ultramafic rocks in the centre of the vredefort structure have been reinterpreted a detailed examination of previously published major element trace and ree analyses as well as mineral analyses strongly suggests a magmatic origin in contrast to previously postulated tectonic emplacement of mantle material a strong geochemical affinity to ultramafic rocks from the bushveld complex is evident the magmatic nature of the ultramafic rocks in the centre of the vredefort structure has implications for the discussion of the metamorphic history of the vredefprt structure which shows evidence of unusually high and as yet not satisfactorily explained geothermal gradients during peak metamorphism

cr and sr keys to parental magmas and processes in the bushveld complex south africa abstract large layered intrusions are almost certainly periodically replenished during their protracted cooling and crystallization the exact composition s of the replenishing magma s in the case of the bushveld complex south africa has been debated mainly on the basis of major element composition and likely crystallization sequences the intrusion is dominated by orthopyroxene and plagioclase and so their cr and sr contents and likely partition coefficient values can be used to re investigate the appropriateness of the various proposed parental magmas one magma type with about 12 mgo 1000 ppm cr and 180 ppm sr can explain the genesis of the entire lower and critical zones a number of other magma compositions proposed to produce the critical zone fail to match these trace element constraints by being too poor in cr a fundamentally different magma type was added at the base of the main zone but none of the proposed compositions is consistent with the trace element requirements specifically the cr contents are higher than predicted from pyroxene compositions a further geological constraint is demonstrated from a consideration of the cr budget at this level there is an abrupt decrease from about 0 4 to 0 1 cr 2 o 3 in orthopyroxene across this critical zone main zone transition no realistic proportions of mixing between the residual magma at the top of the critical zone and any proposed added magma composition can have produced a composition that could have crystallized these low cr orthopyroxenes instead it is suggested that the resident magma from the upper critical zone was expelled from the chamber possibly as sills into the country rocks during influx of a dense differentiated magma near the level of the pyroxenite marker in the main zone there is further addition of a ferrobasaltic magma with 6 mgo 111 ppm cr and 350 ppm sr that is consistent with the geochemical requirements

are bushveld u type parent magmas boninites or contaminated komatiites three distinct categories of magmas bushveld u type parent magmas boninites and siliceous high magnesium basalts from archaean greenstone belts share the distictive geochemical characteristics of high mgo 9 19 low tio2 less than 1 and high sio2 greater than 52 boninites are generally thought to form by hydrous melting of metasomatized previously depleted upper mantle while siliceous high magnesium basalts shmb in greenstone belts have recently been recognized as the products of combined fractionation and crustal contamination of komatiites both these mechanisms can apparently give rise to similar end products and both mechanisms have been proposed for the petrogenesis of bushveld u type magmas

tectonics and structural zonation of southern tibet china the yarlung zangbo suture zone is composed of several thrust sheets with distinct stratigraphies states of deformation and metamorphic grades successive deformations have been produced by persistent north south shortening since obduction of the ophiolites by late cretaceous palaeocene times a possible decoupling of the crust may be associated with the intracontinental thrust system which has formed in the subducted indian plate this article emphasizes the role of shear deformations in the building of a collision belt

sr and pb isotopic disequilibrium between coexisting plagioclase and orthopyroxene in the bushveld complex south africa microdrilling and progressive leaching evidence for sub liquidus contamination within a crystal mush progressive leaching of plagioclase for sr iso topes and microdrilling for sr and pb isotopes from grains of plagioclase and orthopyroxene from the critical zone and the lower zone indicates that these minerals are not in isotopic equilibrium leaching suggests critical zone plagioclase either lost rb or had a more radiogenic sri rim relative to the core whereas plagioclase from an upper zone sample is isotopically homogeneous for sri microdrilling analyses of plagioclase from the lower and critical zones consistently have a higher initial 87 sr 86 sr sri and a less radiogenic modeled 238 u 204 pb composition l2 than coexisting ortho pyroxene the range of calculated sri for plagioclase and orthopyroxene is 0 70506 0 70662 34 and 0 70290 0 70654 36 respectively the average difference in sri between mineral pairs was 0 00095 the range of calculated l2 for plagioclase and orthopyroxene is 9 42 10 30 average 9 7 and 9 83 15 75 average 10 1 respectively the range of measured 208 pb 206 pb for plagioclase and orthopyroxene is 34 757 36 439 33 and 36 669 41 845 85 respectively one orthopyroxenite without evidence for more than one population of crystal size distribution nonetheless had sri 0 70654 36 with calculated l2 of 10 32 for larger grains as compared with sri 0 70290 32 and calculated l2 of 9 97 for smaller grain size fractions isotopic results from this study demonstrate that whole rock isotopic data may not provide the appropriate level of detail necessary to address some processes in the bushveld complex however systematic changes have the potential to elucidate the timing of contamination with regard to other processes crystal aging compaction driven recrystallization and mineral exsolution occurring within a slowly cooled crystal liquid vapor mush system

trace element and platinum group element distributions and the genesis of the merensky reef western bushveld complex south africa the merensky reef of the bushveld complex is one of the world s largest resources of platinum group elements pge however mechanisms for its formation remain poorly understood and many contradictory theories have been proposed we present precise compositional data major elements trace elements and platinum group elements pge for 370 samples from four borehole core sections of the merensky reef in one area of the western bushveld complex trace element patterns incompatible elements and rare earth elements exhibit systematic variations including small scale cyclic changes indicative of the presence of cumulus crystals and intercumulus liquid derived from different magmas ratios of highly incompatible elements for the different sections are intermediate to those of the proposed parental magmas critical zone and main zone types that gave rise to the bushveld complex mingling but not complete mixing of different magmas is suggested to have occurred during the formation of the merensky reef the trace element patterns are indicative of transient associations between distinct magma layers the porosity of the cumulates is shown to affect significantly the distribution of sulphides and pge a genetic link is made between the thickness of the merensky pyroxenite the total pge and sulphide content petrological and textural features and the trace element signatures in the sections studied the rare earth elements reveal the important role of plagioclase in the formation of the merensky pyroxenite and the distribution of sulphide

the bushveld complex south africa formation of platinum palladium chrome and vanadium rich layers via hydrodynamic sorting of a mobilized cumulate slurry in a large relatively slowly cooling subsiding magma chamber platinum group element pge deposits in the bushveld complex and other layered intrusions form when large incompletely solidified magma chambers undergo central subsidence in response to crustal loading resulting in slumping of semi consolidated cumulate slurries to the centres of the intrusions and hydrodynamic unmixing of the slurries to form dense layers enriched in sulfides oxides olivine and pyroxene and less dense layers enriched in plagioclase the most economic pge cr and v reefs form in large multiple replenished intrusions because these cool relatively slowly and their central portions subside prior to termination of magmatism and complete cumulate solidification the depth of emplacement has to be relatively shallow as otherwise ductile crust would not be able to flex and collapse in smaller intrusions cooling rates are faster subsidence is less pronounced and where it occurs the cumulate may be largely solidified resulting in insignificant mush mobility and mineral sorting layering is thus less pronounced and less regular and continuous and the grades of the reefs are lower but the reefs can be relatively thicker an additional factor controlling the pge cr and v prospectivity of intrusions is their location within cratons intra cratonic environments offer more stable emplacement conditions that are more amenable to the formation of large layered igneous bodies furthermore intrusions sited within cratons are more readily preserved because cratons are underlain by thick buoyant keels of harzburgite that prevent plate tectonic recycling and destruction of crust

geochemical constraints on the petrogenesis of the kamiskotia gabbroic complex and related basalts western abitibi subprovince ontario canada barrie c t gorton m p naldrett a j and hart t r 1991 geochemical constraints on the petrogenesis of the kamiskotia gabbroic complex and related basalts western abitibi subprovince ontario canada precambrian res 50 173 199 the kamiskotia gabbroic complex kgc is a large 170 km 2 late archean synvolcanic intrusion located in the western abitibi subprovince canada it is divided into four stratigraphic units a lower zone lz comprised of adcumulus and mesocumulus peridotites troctolites and gabbro norites mg number 86 73 a middle zone mz comprised of mesocumulus gabbro norites mg number 74 60 mesocumulus and orthocumulus gabbro norites and ferroan gabbro norites of the upper zone uz mg number 64 37 which are overlain and partly along strike with granophyric rocks of intermediate to felsic composition the cumulates are characterized by fiat ree patterns with lan ybr 0 4 2 6 and range from 0 6 2 chondrites for olivine bearing lz adcumulates to 10 25 chondrites for upper uz meso to orthocumulates europium anomalies are strongly positive for the lz and and diminish upsection to the upper uz which has slight negative eu anomalies chilled rocks are found in mixed magma and contact chill outcrops and have a mg number of 54 58 lan ybn 0 8 1 2 and slight negative eu anomalies phase relationships and mass balance calculations using the ree indicate that lz cumulates may be derived from the average chill composition similarly upper uz cumulates may be derived from evolved basalts which overlie the kgc assimilation fractional crystallization modeling indicates that the evolved basalts may be derived from the average chill composition by 70 80 fractionation at low pressures or lower degrees of fractionation accompanied by recharge of the average chill composition into the system the basalt compositions do not reflect significant assimilation of nearby felsic rocks or average archean crust during fractionation primitive kgc liquids are estimated by adding an average olivine bearing adcumulate composition to the kgc chill composition in equal proportions to obtain a liquid with an mg number of 70 the ree content of such a liquid is consistent with a derivation by 34 partial melting of a chemically primitive mantle neodymium isotope ratios for kgc cumulates mineral separates and overlying volcanic rocks plot on a 2710 ma isochron with nd t 2 5 0 8 in close agreement with u pb zircon ages for the kgc and kamiskotia felsic volcanic rocks at 2707 2 ma and 2705 2 ma respectively the initial value is identical to the isotopically depleted morb iike abitibi mantle signature at 2 7 ga and provides supporting evidence for fractionation dominated supracrustal processes with no detectable assimilation of an isotopically enriched crustal component kamiskotia mafic magmatism is similar to more recent rift related systems represented by the skaergaard intrusion east greenland and differentiated tholeiitic volcanic fields in eastern iceland and along the galapagos spreading center

controls on cenozoic exhumation of the tethyan himalaya from fission track thermochronology and detrital zircon u pb geochronology in the gyirong basin area southern tibet the gyirong basin southern tibet contains the record of miocene pliocene exhumation drainage development and sedimentation along the northern flank of the himalaya the tectonic controls on basin formation and their potential link to the south tibetan detachment system stds are not well understood we use detrital zircon zft and apatite aft fission track analysis together with detrital zircon u pb dating to decipher the provenance of gyirong basin sediments and the exhumation history of the source areas results are presented for nine detrital samples of gyirong basin sediments aft zft and u pb two modern river sediment samples zft and aft and six bedrock samples zft from transect across the gyirong fault bounding the basin to the east the combination of detrital zircon u pb and fission track data demonstrates that the gyirong basin sediments were sourced locally from the tethyan sedimentary sequence this provenance pattern indicates that deposition was controlled by the gyirong fault active since 10 ma whose vertical throw was probably 5000 m rather than being controlled by normal faults associated with the stds the detrital thermochronology data contain two prominent age groups at 37 41 and 15 18 ma suggesting rapid exhumation at these times a 15 18 ma phase of rapid exhumation has been recorded widely in both southern tibet and the himalaya a possible interpretation for such a major regional exhumation event might be detachment of the subducting indian plate slab during the middle miocene inducing dynamic uplift of the indian plate overriding its own slab

effects of inherited cores and magmatic overgrowths on zircon u th he ages and age eu trends from greater himalayan sequence rocks mount everest region tibet previous constraints on the timing and rate of exhumation of the footwall of the south tibetan detachment system stds north of mount everest suggest rapid miocene cooling from 700 c to 120 c between 14 17 ma however 25 new single grain zircon he ages from leucogranites intruding greater himalayan sequence rocks in the footwall of the stds are between 9 9 and 15 ma with weighted mean ages between 10 and 12 ma zircon grains exhibit a positive correlation between age and effective uranium eu laser ablation zircon u pb geochronology detailed sem observations and laser ablation depth profiling of these zircons reveal low eu 0 5 2 5 ga inherited cores overgrown by high eu 17 22 ma rims this intragranular zonation produces ages as much as 32 too young when a standard alpha ejection correction assuming uniform eu distribution is applied modeling of the effects of varying rim thickness and rim eu concentration on the bulk grain eu and alpha ejection correction suggests that zonation also exerts the primary control on the form of the age eu correlation observed application of grain specific zonation dependent age corrections to our data yields zircon he ages between 14 and 17 ma in agreement with aft and 40ar 39ar ages growth of magmatic rims followed by cooling to 120 c within 1 6 million years supports rapid tectonic exhumation associated with slip along the stds in the miocene this study highlights the importance of characterizing parent nuclide zonation in zircon he studies which seek to understand the timing of exhumation along exhumed crustal blocks

early precambrian crustal development changing styles of mafic magmatism the work of sutton watson 1951 was one of the cornerstones in the interpretation of the evolution of early precambrian archaean and early proterozoic gneiss complexes and in the documentation of mafic dyke swarms that intruded recently accreted archaean cratons since then the concept of geological terranes and terrane accretion has been extended back to the archaean and different events can be distinguished by increasingly precise isotopic dating techniques there is growing evidence that with certain clear provisos the principle of uniformitarianism can be applied as equally to the early precambrian as it can through the rest of the geological record in terms of mafic magmatism there are similarities between some ancient and modern suites both intrusive and extrusive but there are also some major differences such as the presence of komatiites in the archaean and the formation of large scale layered intrusions and extensive dyke swarms at the end of the late archaean and in the early proterozoic the mantle has evolved continuously with the progressive extraction of mafic magmas but allowing for this continental tholeiites appear to have changed little with time ancient mid ocean ridge basalts are elusive because of their poor preservation record the geochemistry of mafic volcanics occurring in many greenstone belts commonly suggests a continental contamination component and that these volcanic rocks are most closely akin to those from present day arc systems the relative abundance of early precambrian intrusive mafic suites parallels the crustal growth curve reflecting the global crustal accretion differentiation superevent towards the close of the archaean bimodal tholeiitic noritic mafic magmatism associated with many of these intrusive suites stems from this rapid turnover in the mantle mafic crust continental crust system this in turn gives a possible link between the respective evolutions of archaean to recent komatiitic noritic and boninitic suites

proterozoic c type eclogites hosting unusual tife cr cu mineralization in northeastern brazil abstract mafic to ultramafic rocks occur at bodoco about 640 km west of recife cofolded with early proterozoic gneisses and migmatites close to the tectonic contact with middle to late proterozoic metasedimentary rocks of the salgueiro cachoeirinha group the jadeite content of clinopyroxenes the fe mg partitioning between clinopyroxene and garnet the si activity in phengite and the na content of amphiboles in retrograde symplectites in gabbroic members suggest a progressive metamorphic evolution up to c type eclogites at 13 kbar 525 c followed by a retrograde path of medium p t ratio to the greenschist facies the final event transformed parts of these mafic rocks into banded amphibole gneisses and migmatites the metagabbroic members contain up to 12 rutile metapyroxenitic members bear two types of massive ores a cumulatic cr ti spinel bands and b network ilmenomagnetite the cr ti spinel is unusual in that it has a composition between ulvospinel magnetite and chromite and has a zonation of cr mg enrichment from core to rim which may be explained by peripheral ti loss during high pressure metamorphism several ti occurrences in the floresta area 400 km west of recife including the 20 mt riacho da posse deposit show striking similarities with the bodoco occurrences 1 massive ores with high ti fe ratios related to ultramafic rocks olivine cumulates 2 gabbroic intercalations composed of only garnet and amphibole with the same mineral compositions recorded in bodoco lacking plagioclase but rich in rutile and 3 migmatitic gneissic wall rocks in the vicinity of the contact with younger metasedimentary rocks a similar tectono stratigraphic setting a few km from the contact with the late proterozoic macururo group and high pressure metamorphic host rocks mafic granulites has been described in the literature for the serrote da lage deposit of massive titanomagnetite with a 60 mt interstitial copper mineralization the only other eclogite occurrence recorded in brazil at pouso alegre state of minas gerais has an identical geological setting chemistry of the metagabbroic rocks of bodoco and floresta indicate an oceanic tholeiitic picritic character while the rare earth element patterns of the ultramafic rocks suggest a modified ocean floor peridotite nature positive co ni and very high cu pd ratios in the sulphides also suggest an oceanic origin all these features with exception of the high ti fe ratio support an ophiolitic character of the mafic rocks a detailed geochronological study may help to determine whether the mafic ultramafic rocks represent relics of a true oceanic crust of the same age as the salgueiro cachoeirinha group obducted onto older continental crust on an active late proterozoic continental margin or if the proximity to the contact between basement and metasedimentary rocks is coincidental

platinum group element segregation and mineralization in a noritic ring complex formed from proterozoic siliceous high magnesium basalt magmas in the vestfold hills antarctica

incomplete retention of radiation damage in zircon from sri lanka a suite of 18 zircon gemstones from placers in the highland southwestern complex sri lanka were subjected to a comprehensive study of their radiation damages and ages the investigation included x ray diffraction raman and pl spectroscopy electron microprobe pixe and hrtem analysis as well as u th he and shrimp u th pb age determinations zircon samples described in this study are virtually homogeneous they cover the range from slightly metamict to nearly amorphous generally concordant u th pb ages averaging 555 11 ma were obtained late ordovician zircon u th he ages scattering around 443 9 ma correspond reasonably well with previously determined biotite rb sr ages for rocks from the hswc slightly to moderately metamict zircon has retained the radiogenic he whereas only strongly radiation damaged zircon calculated total fluences exceeding 3 5 10 18 events g has experienced significant he loss when compared to unannealed zircon from other localities sri lanka zircon is about half as metamict as would correspond to complete damage accumulation over a 555 m y lasting self irradiation period suggesting significant annealing of the structural radiation damage insufficient consideration of this has often resulted in significant underestimation of radiation effects in zircon we suggest to estimate effective doses for sri lanka zircon by multiplying total fluences which were calculated using the zircon u th pb age by a correction factor of 0 55 this conversion may be applied to literature data as well because all gem zircon samples from sri lanka this work and previous studies seem to reveal the same general trends of property changes depending on the radiation damage the use of effective doses for sri lanka zircon contributes to more reliable quantitative estimates of radiation effects and makes possible direct comparison between natural and synthetic radiation damaged zircon

effects of u th rich grain boundary phases on apatite helium ages abstract single grain apatite u th sm he ages he ages from non detrital samples sometimes show larger dispersions than can be explained by known sources of age dispersion such as grain size radiation damage parent zonation fracturing or intracrystalline inclusions we present observations and model age bias effects of an additional source of apatite he age dispersion observed in some cases u th rich grain boundary phases gbps precipitated on apatite crystal surfaces apatite grains from several samples with dispersed apatite he ages are coated or partially coated by reddish orange gbps rich in fe u and th these gbps are heterogeneous in thickness grain coverage and composition and have effective u concentrations eu up to 1000 ppm these phases may have large effects on the bulk eu and 4he compositions of apatite grains and can produce significant he age biases the direction and magnitude of this age bias is primarily a function of four factors 1 the host grain s size 2 the gbps thickness and eu relative to the host grain 3 the timing of gbp formation relative to the cooling age of the host grain and 4 whether the gbps are preserved and analyzed with the grain during he dating some of the most severe effects occur when gbps formed before or near the time of the apatite cooling age but were lost just prior to analysis for example during mineral separation in this case gbps of commonly observed thicknesses 1 10 m with eu 2 10 times that of the host apatite grain implant enough 4he into typical sized apatite grains to produce hundreds of percent positive age biases in contrast when u th rich gbps are preserved and analyzed with apatite grains the resulting he ages can be negatively biased though the heterogeneity variable preservation and ambiguous formation ages of gbps preclude a general quantitative and practical solution to this problem our model demonstrates that observed age dispersions in some samples are consistent with the effects of u th rich grain boundary phases and illustrates the conditions under which and by how much they will bias he ages

crustal evolution of island arc ultramafic magma galmoenan pyroxenite dunite plutonic complex koryak highland far east russia alaskan type platinum bearing plutons and potassium enriched mafic to ultramafic volcanic rocks are temporally and spatially associated within the late cretaceous paleocene achalvayam valaginskii intra oceanic palaeo arc system allochthonousy present in the kogak highland and kamchatka peninsula far east russia the compositions of the parental magmas to the alaskan type complexes are estimated using the galmoenan platonic complex as an example this complex composed of dunites pyroxenites and minor gabbros is the largest 20 km3 in the system and the best studied owing to associated platinum placer deposits the compositions of the principal mineral phases in the galmoenan intrusive rocks olivine fo 79 92 clinopyroxene 1 3 5 wt al2o3 0 1 0 5 wt tio2 and cr spinel 5 15 wt al2o3 and 0 3 0 7 wt tio2 are typical of liquidus assemblages in primitive island arc magmas in intra oceanic settings and closely resemble the mineral compositions in the achaivayam valaginskii ultramafic volcanic rocks the temporal and spatial association of intrusive and extrusive units and the similarity of their mineral compositions suggest that both suites were formed from similar parental magmas ne composition of the parental magma for the galmoenan platonic rocks is estimated using previously reported data for the achaivayam valaginskii ultramafic volcanic rocks and phenocryst hosted melt inclusions quantitative simulation of crystallization of the parental magma in the galmoenan magma chamber shows that the compositions of the cumulate units are best modelled by fractional cryallization with periodic magma replenishment the model calculations reproduce well the observed mineral assemblages and the trace element abundances in clinopyroxene based upon the estimated composition of the parental magmas and their mantle source we consider that fluxing of a highly refractory mantle wedge similar to the source of boninites by chlorine rich aqueous fluids is primarily responsible for both high degrees of partial melting and the geochemical characteristics of the magmas including their enrichment in platinum group elements