rapid extension in an eocene volcanic arc structure and paleogeography of an intra arc half graben in central idaho a study of extension volcanism and sedimentation in the middle eocene panther creek half graben in central idaho shows that it formed rapidly during an episode of voluminous volcanism the east southeast tilted panther creek half graben developed across the northeast edge of the largest cauldron complex of the challis volcanic field and along the northeast trending trans challis fault zone two normal fault systems bound the east side of the half graben one fault system strikes northeast parallel to the trans challis fault zone and the other strikes north to northwest the geometry of the basin fill deposits shows that movement on these two normal fault systems was synchronous and that both faults controlled the development of the panther creek half graben strikes of the synextension volcanic and sedimentary rocks are similar throughout the half graben whereas dips decrease incrementally upsection from as much as 60 to less than 10 previous k ar dates and a new 40 ar 39 ar plateau date from the youngest widespread tuff in the basin suggest that most of basin formation spanned 3 m y between about 47 7 ma and 44 5 ma as much as 6 5 km of volcanic and sedimentary rocks were deposited during that time although rates of extension and subsidence were very high intense volcanic activity continually filled the basin with ash flow tuffs outpacing subsidence and sedimentation until the end of basin development after the abrupt end of challis volcanism locally derived pebble to boulder conglomerate and massive reworked ash accumulated in the half graben these sedimentary rocks make up a small part of the basin fill in the panther creek half graben and were derived mainly from proterozoic metasedimentary rocks uplifted in the footwall of the basin the east southeast tilt of the sedimentary rocks their provenance and coarse grain size and the presence of a gravity slide block derived from tilted volcanic rocks in the hanging wall attest to continued tectonism during conglomerate deposition provenance data from the sedimentary rocks imply that the highland in the footwall of the panther creek half graben was never thickly blanketed by synextension volcanic rocks despite intense volcanic activity analysis of the panther creek half graben and other intra arc rift basins supports previous interpretations that relative rates of volcanism and subsidence control the proportion of volcanic rocks deposited in intra arc rifts

origin of the copper cobalt deposits of the zambian copperbelt an epigenetic view from nchanga the zambian copperbelt is arguably the most significantly mineralized neoproterozoic basin on earth preserving a truly spectacular scale of mineralization in excess of 1 109 t of ore at 2 7 copper has been extracted to date and there are also major cobalt accumulations the origin of these deposits has been hotly debated for more than six decades yet the driving forces that generated this system are poorly understood in particular the relationships between tectonics paleo fluid circulation and ore deposition we present new field and isotopic data for the nchanga deposits in which the bulk of the mineralization is hosted by shale capped feldspathic arenites and arkoses that have undergone recrystallization and hydrothermal alteration within a host rock package controlled by low angle thrust faults by using in situ laser combustion we show for the first time that the range of 34s for copper cobalt ore sulfides 1 to 18 cannot have the same source as diagenetic pyrite 1 to 17 we suggest a new epigenetic model for the formation of these spectacular nchanga orebodies that involves the introduction of metal and sulfate bearing hydrothermal fluids into quartzofeldspathic units during basin inversion with sulfide derived from thermochemical reduction of the sulfate near the site of deposition

role of plutonism in low pressure metamorphic belt formation wickham and oxburgh1 recently proposed that low pressure high temperature low p high t metamorphism in the eastern pyrenees and possibly all low p high t metamorphic belts resulted from anomalously high mantle heat flow brought about by rifting their model is largely constrained by the presence of nearby synmetamorphic rift related sedimentary rocks and the interpretation that the migmatites and granites are the product of in situ melting in the presence of an anomalously steep geotherm here we present an alternative model in which low p high t metamorphism pro grade reactions at pressures near or below the al2sio5 triple point results from contact effects near sill like igneous intrusions at intermediate crustal levels low p high t conditions can be achieved through this process in regions of continent continent collision with normal mantle heat flux as well as in zones of extension our model is based on studies of the low p high t metamorphic terrane in the new england appalachians

variscan tectonics of central europe postaccretionary intraplate deformation of weak continental lithosphere the area occupied by the variscan belt of central europe forms part of gondwana derived microplates essentially avalonia and armorica that according to paleomagnetic data juxtaposed baltica and laurentia from the upper ordovician to the lower devonian the actual structure of this area is a puzzle of rather small fault bounded crustal blocks different in p t t histories which was created by later tectonic processes at around 360 to 320 ma according to a common view and by contrast to paleomagnetic data the structural record of fault bounded blocks is interpreted in terms of mid devonian to lower carboniferous amalgamation of narrow continental plates represented by the fault bounded blocks themselves however this is not supported in this article instead it will be shown that fault systems and fault bounded blocks were created by intraplate deformation of a weak domain of gondwana derived continental lithosphere significantly after its accretion deformation was partitioned into newly formed deep seated strike slip and kinematically connected reverse or normal detachment systems large translation magnitudes caused juxtaposition of crustal areas of variable thickness until they were emplaced along the fault systems the various categories of fault bounded complexes record different tectonic events category 1 metamorphic complexes represent deep seated parts of thickened crust some time before unroofing on the detachment systems between about 340 360 and 320 ma however the record of subduction and extreme crustal thickening during earlier stages of the collision history was erased to a high degree and is only preserved by mineral and structural relics strike slip and detachment systems propagated across and dismembered tectonic boundaries formed during these early events category 2 metamorphic complexes and the barrandian basin recording predominant pre late devonian metamorphic or stratigraphic events represent the brittle upper crustal parts during continuing metamorphism in the category 1 complexes weakly or unmetamorphosed lower paleozoic category 3 basins represent areas of thinned continental crust and record subsidence before and to some degree during activity of detachment systems the complex overall kinematic pattern of block bounding strike slip and detachment systems indicates compression and extension acting simultaneously this probably reflects the effects of continuing oblique plate convergence rotation and wsw extrusion of fault bounded blocks in addition to heating and extensional flow of previously thickened crustal areas

unpairing metamorphic belts p t paths and a tectonic model for the ryoke belt southwest japan southwest japan is divided into outer and inner zones by the median tectonic line mtl a major transcurrent fault the outer zone is composed of the sambagawa high pressure intermediate or high p t type metamorphism chichibu and shimanto belts in the inner zone the ryoke belt andalusite sillimanite or low p t type metamorphism was developed mainly within a jurassic accretionary complex this spatial relationship between high p t type and low p t type metamorphic belts led miyashiro to the idea that metamorphic belts were developed as paired systems textural relationships and petrogenetically significant mineral assemblages in pelites from the ryoke belt imply peak p t conditions of 5 kbar and up to 850 c in migmatitic garnet cordierite rocks from the highest grade metamorphic zone it is likely that the thermal anomaly responsible for metamorphism of the ryoke belt was related to a segment of the farallon izanagi ridge as it subducted under the eastern margin of the asian continent during the cretaceous the sequence of mineral assemblages developed in pelites implies a metamorphic field gradient with shallow dp dt slope inferred to have been generated by a nested set of hairpin like clockwise p t paths these p t paths are characterized by limited prograde thickening minor decompression at peak t and near isobaric cooling features that may be typical of p t paths in low p t type metamorphic belts caused by ridge subduction a ridge subduction model for the ryoke belt implies that juxtaposition of the high p t metamorphic rocks of the sambagawa belt against it was a result of terrane amalgamation belt parallel ductile stretching recorded as syn metamorphic predominantly constrictional strain in both ryoke and sambagawa belt rocks and substantial sinistral displacement on the mtl are consistent with left lateral oblique convergence diachroneity in fast cooling of the ryoke belt is implied by extant thermochronological data and is inferred to relate to progressive sw to ne docking of the sambagawa belt thus an alternative interpretation of paired metamorphic belts in japan is that they represent laterally contemporaneous terranes rather than outboard and inboard components of a trench arc paired system amalgamation of laterally contemporaneous terranes during large translations of forearcs along continental margins may explain other examples of paired metamorphic belts in the geological record

gas bubble transport and emissions for shallow peat from a northern peatland the role of pressure changes and peat structure gas bubbles are an important pathway for methane release from peatlands the mechanisms controlling gas bubble transport and emissions in peat remain uncertain the effects of hydrostatic pressure and peat structure on the dynamics of gas bubbles in shallow peat were therefore tested in laboratory experiments a peat monolith was retrieved from a raised bog and maintained in a saturated state three distinct layers were identified from noninvasive permittivity measurements supported by soil physical properties porosity bulk density phase i of the experiment involved monitoring for the accumulation of gas bubbles under steady pressure and temperature conditions the data showed evidence for gas bubbles being impeded by a shallow semiconfining layer at depths between 10 and 15 cm visible gas bubbles observed on the side of the sample box were recorded over time to estimate changes in the vertical distribution of volumetric gas content porosity estimates derived using the complex refraction index model crim suggest that gas bubbles enlarge the pore space when the exerted pressure is high enough phase ii involved triggering release of trapped bubbles by repeatedly increasing and decreasing hydrostatic pressure in an oversaturated condition comparison of changes in pressure head and methane density in the head space confirmed that the increasing buoyancy force during drops in pressure is more important for triggering ebullition than increasing mobility during increases in pressure our findings demonstrate the importance of changes in hydrostatic pressure on bubble size and variations in resistance of the peat fabric in regulating methane releases from peatlands

u th pb and rb sr geochronology of middle proterozoic granite and augen gneiss salmon river mountains east central idaho coarsely porphyritic granite and augen gneiss in east central idaho have been dated by the u th pb zircon method when plotted on concordia diagrams the results give nonlinear data arrays that indicate the presence of at least one component of inherited radiogenic lead as well as postcrystallization probably recent episodic lead loss nevertheless a systematic relationship between zircon size and convergence of the 207 pb 206 pb ages for the 100 and 100 150 mesh fractions from several samples are interpreted to provide a valid age of about 1 370 m y for these plutons a single sample which is free of inheritance for all size fractions confirms this conclusion rb sr whole rock samples are highly enriched in radiogenic strontium but do not define an isochron probably because the system did not remain closed following crystallization of the plutons intrusive contacts of the granitic rocks indicate that deformation and regional metamorphism of the host yellowjacket formation occurred prior to 1370 ma rather than prior to 1500 ma as previously suggested by others this refinement of the geochronology 1 removes the necessity that the yellowjacket formation and overlying proterozoic metasedimentary rocks of central and east central idaho be pre belt in age 2 permits the traditional age correlation of the east central idaho metasedimentary sequence yellowjacket through lawson creek formations with the belt supergroup and 3 suggests that a re evaluation is in order for paleogeographic models of central idaho especially the proposed topographic high known as the salmon river arch

calculated mineral equilibria in the pelite system kfmash k 2 o feo mgo al 2 o 3 sio 2 h 2 o

a model for low pressure facies metamorphism during crustal thickening low pressure metamorphic facies i e high t p ratios are widespread in a wide range of tectonic settings explanations offered for the occurrence of these facies include extensional and or magmatic models however these fail to explain that the low p facies metamorphism is commonly coeval with a phase of pervasive crustal thickening with t p ratios increasing during or slightly lagging behind the thickening we propose an alternative explanation based on the approximate synchroneity of crustal thickening and erosion thinning of the mantle lithosphere

measuring physical and hydraulic properties of peat from x ray tomography abstract organic soils cover a large percentage of the canadian land mass in the form of peatlands 14 and other organic terrains including arctic and alpine tundra and taiga 25 sphagnum mosses are the dominant peat forming species in all of these terrains understanding the processes by which this material conducts retains and redistributes moisture and energy is crucial to the successful development of physically based methods of measuring and modelling their hydrological response sphagnum peat was sampled from scotty creek in the wetland dominated zone of discontinuous permafrost near fort simpson northwest territories canada physical properties of the peat samples were measured from standard methods and x ray tomographic images of sub samples were obtained at 5 levels of volumetric moisture content between 0 4 and 0 7 the volumetric moisture content measured using the tomographic images closely approximated the gravimetric measurements properties of the pore spaces were measured in both two and three dimensions the active porosities measured in 2d and 3d were very similar and matched closely with values reported in the literature specific surface area 3d however was larger than the specific perimeter 2d the 2d analysis suggests a relatively gradual transition from small to large pores while in 3d the distribution is dominated by a single large pore space whose volume and surface area are 3 orders of magnitude larger than the next largest pore and 99 of the total inter particle pore volume this single large pore space contains the flow network this study elucidated the volume and configuration of this network for discrete ranges of soil water pressure that typically occur in the field the flow network diminished from 17 6 to 9 0 of a sample volume as its volumetric moisture content lowered from 0 65 to 0 42 this volume loss was accommodated by the thinning and disaggregation of moisture films although as the sample lost moisture the flow network maintained a relatively even spatial distribution

structural controls on tertiary orogenic gold mineralization during initiation of a mountain belt new zealand two types of structurally controlled hydrothermal mineralization have occurred during folding of fissile schist in southern new zealand fold related mineralization and normal fault related mineralization both types have the same mineralogy and textures and are dominated by quartz ankerite veins and silicified breccias with ankeritic alteration most mineralized zones are thin centimetre scale although host schist is commonly impregnated with ankerite up to 20 m away thick up to 5 m wide mineralized zones are generally gold bearing and contain pyrite and arsenopyrite with stibnite pods locally some of these auriferous zones have been extensively mined historically despite rugged topography and difficult access mineralization occurred during regional tectonic compression in the initial stages of development of the southern alps mountain belt at the pacific australian plate boundary in the miocene most of the gold bearing deposits occur in east to south east striking normal faults that cut across mesoscopic folds in a belt that coincides with the southern termination of a regional scale north trending antiform mineralized zones have similar structural control and relative timing to a nearby swarm of miocene lamprophyre dykes and carbonatites limited stable isotopic data c and o and trace element geochemistry suggest that there was probably no genetic link between the igneous activity and gold mineralization however these two types of fluid flow have been controlled by the same tectonically created crustal plumbing system this miocene hydrothermal activity and gold deposition demonstrates that orogenic mesothermal mineralization can occur during the inception of an orogenic belt not just in the latter stages as is commonly believed these miocene structures have been preserved in the orogen because the locus of uplift has moved northwards so the early formed gold deposits have not yet been structurally overprinted or eroded

reconnaissance lead isotope characteristics of the blackbird deposit implications for the age and origin of cobalt copper mineralization in the idaho cobalt belt united states strata bound cobalt copper deposits occur in the metasedimentary rocks of the apple creek formation mesoproterozoic in a linear belt called the idaho cobalt belt near salmon idaho we report the first set of lead isotope measurements on silicate rocks and sulfide ores and new sulfur isotope data on sulfide ores the pb isotope compositions of cobalt copper co cu ores 206pb 204pb 30 8 40 4 207pb 204pb 16 8 17 6 208pb 204pb 49 7 63 9 are very radiogenic much more radiogenic than any known sedimentary exhalative sedex deposits the data plot well beyond the crustal growth model curves a feature shared by the mississippi valley type mvt deposits which is characteristic of an upper crustal source it is unlikely that the ore lead could have come from any mafic igneous source of mantle origin proterozoic moyie sill and its equivalent or the tertiary challis volcanic group the cretaceous felsic igneous rocks of the region the idaho batholith and related rocks also have much lower pb isotope ratios than the co cu ores the pb isotope ratios of proterozoic crystalline rocks partially overlap pb isotope ratios of the apple creek formation but would have been much less radiogenic than the ores at an assumed mineralization age of about 1400 ma only the host apple creek formation is known to have appropriate pb isotope compositions 206pb 204pb 26 8 86 7 207pb 204pb 16 3 21 1 208pb 204pb 47 9 64 8 to be the source of co cu ores leaching of metals from the host sedimentary sequence by hydrothermal fluids and subsequent deposition of ores at chemically and structurally favorable sites could have resulted in the formation of co cu deposits in the idaho cobalt belt

origins of mineral deposits belt purcell basin united states and canada an introduction the fill of the mesoproterozoic belt purcell basin which straddles the united states canada border within the rocky mountains of western north america fig 1 consists of marine and nonmarine clastic and carbonate strata 15 to 20 km thick three giant metal producing ore deposits or districts account for the bulk of the known metal endowment within the bounds of the belt purcell basin 1 the syndepositional sullivan pb zn ag deposit in southern british columbia total production pb 8 4 million tonnes mt zn 7 9 mt ag 0 0093 mt lydon 2000 2 the mesothermal pb zn ag veins of the coeur d alene district in northern idaho total production pb 7 5 mt zn 3 0 mt ag 0 052 mt long 1998 post 1997 data from usgs annual minerals yearbooks and 3 the cretaceous porphyry copper deposit and associated polymetallic veins in the butte district in montana total resource cu 35 mt zn 4 6 mt ag 0 044 mt long et al 1998 the sullivan mine closed in 2001 after more than 92 years of production mining of 26 major vein deposits in the coeur d alene district began in the 1880s and peaked about 1950 production in the coeur d alene district continues today from the galena and lucky friday mines the latter closed for 2012 to refurbish the mile deep vertical access shaft mining at butte began in 1875 with copper production peaking in 1917 mining continues today in the eastern upfaulted portion of the butte porphyry copper deposit at the continental mine fig 1 locations of major ore deposits within the belt purcell basin area along with other ore deposits and mineralized trends that are discussed in this issue deposit locations are plotted on a geologic map showing the distribution of five stratigraphic units within the belt purcell basin in the united states and canada lower belt formations include the prichard altyn appekunny chamberlain newland lahood

pressure temperature evolution of blueschist facies rocks from sifnos greece and implications for the exhumation of high pressure rocks in the central aegean the blueschist and greenschist units on the island of sifnos cyclades were affected by eocene high pressure hp metamorphism using conventional geothermobarometry the hp peak metamorphic stage was determined at 550 600 c and 20 kbar close to the blueschist and the eclogite facies transition the retrograde p t paths are inferred with phase diagrams pseudosections based on a quantitative petrogenetic grid in the model system na2o cao feo mgo al2o3 sio2 h2o reveal coeval decompression and cooling for both the blueschist and the greenschist unit the conditions of the metamorphic peak and those of the retrograde stages conform to a similar metamorphic gradient of 10 12 c km 1 for both units the retrograde overprint can be assigned to low pressure blueschist to hp greenschist facies conditions this result cannot be reconciled with the prograde barrovian type event which affected parts of the cyclades during the oligocene to miocene instead the retrograde overprint is interpreted in terms of exhumation directly after the hp stage without a separate metamorphic event constraints on the exhumation mechanism are given by decompression cooling paths which can be explained by exhumation in a fore arc setting during on going subduction and associated crustal shortening back arc extension is only responsible for the final stage of exhumation of the hp units

stepping stones and pitfalls in the determination of an anticlockwise p t t deformation path the low p high t cooma complex australia low pressure high temperature low p high t metamorphic rocks of the cooma complex southeastern australia show evidence of an anticlockwise pressure temperature time deformation p t t d path similar to those of some other low p high t metamorphic areas of australia prograde paths are reasonably well constrained in cordierite andalusite schists cordierite k feldspar gneisses and andalusite k feldspar gneisses these paths are inferred to be convex to the temperature axis involving increase in pressure with increase in temperature evidence of the retrograde path is inconclusive but is consistent with approximately isobaric cooling as are available isotopic data on the cooma granodiorite which indicate initially rapid cooling following attainment of peak temperatures the retrograde path is inconsistent with either a clockwise p t t d path involving rapid or even moderate decompression immediately post dating the peak of metamorphism or a path in which the retrograde component simply reverses the prograde component because both these paths should cross reactions forming cordierite from aluminosilicate for which no evidence has been observed determination of the deformational metamorphic history of the complex is not straightfoward and depends on careful examination of critical samples evidence necessary for successful elucidation of the prograde and part of the retrograde deformational metamorphic history in the cooma complex includes 1 sequentially grown porphyroblasts that can be timed relative to surrounding foliations 2 partial replacement microstructures providing relative timing of metamorphic reactions that cannot be timed relative to foliation development 3 a tectonic marker foliation s4 at cooma that allows correlation of foliations from one location to another and 4 single samples containing all of the foliations and all generations of porphyroblast growth within a single metamorphic zone the latest two or three foliations involve low strain accumulation allowing relative timing relationships between foliations and porphyroblasts to be more clearly determined sequential porphyroblast growth and foliation development in the cordierite andalusite schists is examined for situations involving rotation and non rotation of porphyroblasts relative to geographically fixed coordinates although the number of foliations developed varies in the rotational situation depending on the deformation history proposed the sequential order of porphyroblast growths does not differ from the non rotational situation thus whether or not porphyroblasts rotated in the cooma rocks the sequence of reactions and therefore p t t paths inferred from the relative timing of porphyroblast growths remain the same for the deformational histories evaluated

isobaric cooling and anti clockwise p t paths in the variscan odenwald crystalline complex germany mineral assemblages in al2o3 rich sio2 and k2o poor metapelitic rocks from the western odenwald crystalline complex variscan mid german crystalline rise southern germany include corundum spinel cordierite sillimanite garnet and staurolite quartz is absent from almost all samples therefore the applicability of conventional geothermobarometry is very limited or even impossible detailed petrographic investigation on selected samples permits inference of the sequence of appearance and disappearance of several mineral assemblages the recognition of such partial re equilibration stages and their associated mineral assemblages together with mineral stabilities predicted from kfmash pseudosections enables the determination of the pressure temperature p t trajectories experienced by these rocks during the variscan metamorphism the rocks were metamorphosed under low p high t conditions and underwent an anti clockwise p t evolution a pressure increase from about 2 kbar to 4 0 5 kbar was accompanied by heating peak metamorphic conditions were reached at pressures of 4 0 5 kbar and temperatures of at least 640 c probably even higher the retrograde evolution is characterised by near isobaric cooling from 640 c to approximately 550 c the rocks underwent the anti clockwise evolution in a subduction related magmatic arc setting the close spatial association of the low p high t rocks with recently discovered metabasic eclogites in the eastern part of the odenwald crystalline complex may indicate a fossil paired metamorphic belt in the central european variscides

iron and chlorine as guides to stratiform cu co au deposits idaho cobalt belt usa the cu co au deposits of the idaho cobalt belt are in lithostratigraphic zones of the middle proterozoic yellowjacket formation characterized by distinctive chemical and mineralogical compositions including high concentrations of fe 15 30 wt percent fe2o3 cl 0 1 1 10 wt percent and magnetite or biotite 50 vol percent the cu co au deposits of the blackbird mine are stratabound in fe silicate facies rocks that are rich in biotite fe and cl but stratigraphically equivalent rocks farther than 10 km from ore deposits have similar compositions a lower lithostratigraphic zone containing magnetite and small cu co au deposits extends for more than 40 km the fe rich strata are probably exhalative units related to mafic volcanism and submarine hot springs but the origin of the high cl concentrations is less clear former chlorine rich pore fluids are suggested by the presence of supersaline fluid inclusions by cl rich biotite and scapolite as much as 1 87 percent cl in fe rich biotite and by high cl concentrations in rock samples chlorine is enriched in specific strata and in zones characterized by soft sediment deformation thus probably was introduced during sedimentation or diagenesis unlike some metasedimentary rocks containing scapolite and high cl the yellowjacket formation lacks evidence for evaporitic strata that could have been a source of cl more likely the cl reflects a submarine brine that carried fe k and base metals strata containing anomalous fe k cl are considered to be a guide to sub basins favorable for the occurrence of stratiform base metal deposits

re os systematics and geochemistry of cobaltite coass in the idaho cobalt belt belt purcell basin usa evidence for middle mesoproterozoic sediment hosted co cu sulfide mineralization with grenvillian and cretaceous remobilization we report the first study of the re os systematics of cobaltite coass using disseminated grains and massive sulfides from samples of two breccia type and two stratabound deposits in the co cu au idaho cobalt belt icb lemhi subbasin to the belt purcell basin idaho usa using a 185re 190os spike solution magnetic and non magnetic fractions of cobaltite mineral separates give reproducible re os analytical data for aliquot sizes of 150 to 200 mg cobaltite from the icb has highly radiogenic 187os 188os ratios 17 45 and high 187re 188os ratios 600 1800 but low re and total os contents ca 0 4 4 ppb and 14 64 ppt respectively containing 30 to 74 radiogenic 187os cobaltite from the icb is amenable to re os age determination using the isochron regression approach re os data for disseminated cobaltite mineralization in a quartz tourmaline breccia from the haynes stellite deposit yield a model 1 isochron age of 1349 76 ma 2 n 4 mean squared weighted deviation mswd 2 1 initial 187os 188os ratio 4 7 2 2 this middle mesoproterozoic age is preserved despite a possible metamorphic overprint or a pulse of metamorphic hydrothermal remobilization of pre existing cobaltite that formed along fold cleavages during the ca 1190 1006 ma grenvillian orogeny this phase of remobilization is tentatively identified by a model 3 isochron age of 1132 240 ma 2 n 7 mswd 9 3 initial 187os 188os ratio of 9 0 2 9 for cobaltite in the quartz tourmaline breccia from the idaho zone in the blackbird mine all mesoproterozoic cobaltite mineralization in the district was affected by greenschist to lower amphibolite facies garnet zone metamorphism during the late jurassic to late cretaceous cordilleran orogeny however the fine to coarse grained massive cobaltite mineralization from the shear zone hosted chicago zone blackbird mine is the only studied deposit that has severely disturbed re os systematics with evidence for a linear trend of mixing with metamorphic fluids the new re os ages and extremely high initial 187os 188os ratios of cobaltite reported here favor a magmatic hydrothermal genetic model for a multi stage ree y co cu au mineralization occurring at ca 1370 to 1349 ma and related to the emplacement of the big deer creek granite pluton at ca 1377 ma in our model deposition of paragenetically early xenotime and gadolinite was followed by an influx of mesoproterozoic evaporitic brines and magmatic hydrothermal fluids containing metals and reduced sulfur derived from mafic and oceanic island arc archean to paleoproterozoic rocks in the laurentian basement cobaltite mineralization occurred upon cooling of these fluids at an inferred temperature of 300 c or below

the effect of pore structure on ebullition from peat the controls on methane ch4 bubbling ebullition from peatlands are uncertain but evidence suggests that physical factors related to gas transport and storage within the peat matrix are important variability in peat pore size and the permeability of layers within peat can produce ebullition that ranges from steady to erratic in time and can affect the degree to which ch4 bubbles bypass consumption by methanotrophic bacteria and enter the atmosphere here we investigate the role of peat structure on ebullition in structurally different peats using a physical model that replicates bubble production using air injection into peat we find that the frequency distributions of number of ebullition events per time and the magnitude of bubble loss from the physical model were similar in shape to ebullition from peatlands and incubated peats this indicates that the physical model could be a valid proxy for naturally occurring ebullition from peat for the first time data on bubble sizes from peat were collected to conceptualize ebullition and we find that peat structure affects bubble sizes using a new method to measure peat macro structure we collected evidence that supports the hypothesis that structural differences in peat determine if bubble release is steady or erratic and extreme collected pore size data suggests that erratic ebullition occurs when large amounts of gas stored at depth easily move through shallower layers of open peat in contrast steady ebullition occurs when dense shallower layers of peat regulate the flow of gas emitted from peat

high ree and y concentrations in co cu au ores of the blackbird district idaho analysis of 11 samples of strata bound co cu au ore from the blackbird district in idaho shows previously unknown high concentrations of rare earth elements ree and y averaging 0 53 wt percent ree y oxides scanning electron microscopy indicates ree and y residence in monazite xenotime and allanite that form complex intergrowths with cobaltite suggesting coeval co and ree y mineralization during the mesoproterozoic occurrence of high ree and y concentrations in the blackbird ores together with previously documented saline rich fluid inclusions and cl rich biotite suggest that these are not volcanogenic massive sulfide or sedimentary exhalative deposits but instead are iron oxide copper gold iocg deposits other strata bound co deposits of proterozoic age in the north american cordillera and elsewhere in the world may have potential for ree and y resources iocg deposits with abundant light ree should also be evaluated for possible unrecognized heavy ree and y mineralization

surface deformations as indicators of deep ebullition fluxes in a large northern peatland 1 peatlands deform elastically during precipitation cycles by small 3 cm oscillations in surface elevation in contrast we used a global positioning system network to measure larger oscillations that exceeded 20 cm over periods of 4 12 hours during two seasonal droughts at a bog and fen site in northern minnesota the second summer drought also triggered 19 depressuring cycles in an overpressured stratum under the bog site the synchronicity between the largest surface deformations and the depressuring cycles indicates that both phenomena are produced by the episodic release of large volumes of gas from deep semi elastic compartments confined by dense wood layers we calculate that the three largest surface deformations were associated with the release of 136 g ch4 m 2 which exceeds by an order of magnitude the annual average chamber fluxes measured at this site ebullition of gas from the deep peat may therefore be a large and previously unrecognized source of radiocarbon depleted methane emissions from northern peatlands

sequential porphyroblast growth during progressive deformation and low p high t lpht metamorphism cooma complex australia the use of microstructural analysis to better understand deformation and metamorphic histories abstract detailed microstructural analysis has been conducted in the cooma complex which is one of the most thoroughly studied areas structurally and metamorphically in australia this was done to demonstrate the usefulness of microstructural analysis for better understanding deformation and metamorphic histories and the results add significant new information about deformation and metamorphism of this area microstructural relationships in the schists and gneisses indicate that four coaxially superposed foliations formed with alternating subhorizontal s 1 and s 3 and subvertical s 2 and s 4 orientations and the meso and macrostructural relationships are easily understood within this deformational framework three stages of sequential porphyroblast growth involve cordierite and andalusite in the schists and cordierite andalusite and k feldspar in the gneisses the porphyroblasts overgrew and preserved progressive stages in the structural development of these rocks with each period of growth occurring during a different deformation event d 2 d 3 and d 4 the sequential timing of these porphyroblasts is consistent with kfmash reaction curve topology and indicates a prograde p t t path convex to the temperature axis combining this with isobaric cooling inferred from previous studies gives an anti clockwise p t t path subvertical and subhorizontal foliations in porphyroblasts contrast with the variable orientations of the same foliations measured in the field however careful field work and microstructural analysis demonstrate that matrix foliation rotation rather than porphyroblast rotation relative to geographic coordinates is largely responsible for the contrasting observations even where bedding has been overturned after porphyroblast growth

fe mg mixing in cordierite constraints from natural data and implications for cordierite garnet geothermometry in granulites from chemicaldata on 108 pairs of naturally occurring garnet and cordierite the interchange energy of fe mg mixing in cordierite wo and the free energy change of the exchange equilibrium feralrsiro t t zmgraiosiro y mgralrsiro yzferalosiro in the temperature range of 700 850 c have been extracted using standard methods of linear regression analysis the input thermochemical parameters for multicomponent camg fe mn garnet solid solutions are based on the updated and revised values after ganguly and saxena 1984 e g wr 2500 cal w w 3000 cal calculated on the basis of one third formula unit on the basis of the derived values at 710 760 800 and 850 c fe mg mixing in cordierite has been approximated to a symmetric regular solution the fe mg substitution shows positive deviation from ideality in cordierite with tzi 1 9 34 4 54kj mol of divalent cation the values derived for the free energy of mixing at xi d xi ds 0 5 are 3 26 kj at 700 c and 4 39 kl at 900 c the ag value in joules for the above equilibrium can be expressed as 15079 8 552 incorporating these parameters the geothermometric expression for the exchange equilibrium is

structural and u pb geochronological evidence for 1 47 ga rifting in the belt basin western montana structural mapping and u pb dates from mafic sills in the perma culmination of west central montana constrain the timing of a major rifting event in the early history of the belt basin magmatic zircon from the plains and paradise mafic sills within the prichard formation yielded u pb ages of 1469 2 5 and 1457 2 ma respectively the older sill invaded unconsolidated wet sediments at a depth of less than 1 6 km and its emplacement is coeval with widespread soft sediment folding mud diapirism and bathymetric collapse of the basin between members e and f of the prichard formation the youngest sill intruded near the base of the exposed section after significant accumulation of sediment had occurred in the basin correlation of these data with recent geochronologic results from the moyie sills in british columbia and purcell lava in montana suggests that the rifting and subsidence event was basin wide and that at least two thirds of the deposition of the belt supergroup occurred between 1470 and 14

metamorphic reactions in dry and aluminous granulites a perple_x p t pseudosection analysis of the influence of effective reaction volume in high mg al metapelites monophase sapphirine corona occur around spinel corundum aggregates in monomineralic cordierite layers and bi phase orthopyroxene sillimanite aggregates replace locally warped sapphirine in polygonized cordierite aggregates p t phase topologies computed perple x software using compositions of cordierite rich layers that host the reaction textures did not match the assemblages for the discontinuous reactions spinel corundum cordierite sapphirine and sapphirine cordierite orthopyroxene sillimanite instead the reaction assemblages were reproduced using p t pseudosection analysis for micro domain reaction volumes estimated from compositions of product phases in the volume proportion they occur the results are consistent with known phase relations deduced using schreinmakers p t grids apparently the compositions of cordierite rich layers that hosted the reaction textures were inadequate chemical proxies for determining p t x relations of phase boundary controlled reactions influenced by compositions of the nearest neighbor minerals in the proportion they react effective composition and not in the proportion they existed in the layer bulk rock in other words p t x phase topologies and reconstructed p t paths in dry and aluminous rocks may be best understood by thermodynamic modeling of reactions using effective reaction volume compositions rather than the bulk composition of the rock or the mineralogical layer that host the reaction textures

metasomatic albitites and related biotite rich schists from a low pressure polymetamorphic terrane snake creek anticline mount isa inlier north eastern australia microstructures and p t d paths rocks of the snake creek anticline are mainly pelitic schists psammitic schists and quartzites that were metamorphosed during multiple high t low p events extending from d1 to d5 with the metamorphic peak occurring late to post d3 albitites are widespread but are concentrated in five areas they are typically fine to medium grained and consist of albite with or without combinations of quartz biotite staurolite cordierite garnet andalusite sillimanite kyanite gedrite and tourmaline from the presence or absence of albite inclusions in porphyroblasts the albitites are interpreted as forming early in the d3 event as a result of infiltration of external fluids psammitic schists and quartzites were preferentially altered but pelitic schists were also albitized in localities where the alteration was more extreme with the replacement of muscovite total and the replacement of quartz and biotite variable structural controls on albitization include fracturing and syn d3 shear zones in fold hinges biotite schists with abundant porphyroblasts combinations of staurolite garnet andalusite and cordierite occur adjacent to albitites and it is argued that they formed by the addition of fe and mg sourced from the albitites in several albitite rich areas cordierite grew early in d3 and was partly or entirely replaced during or after d3 by combinations of biotite andalusite tourmaline staurolite and sillimanite a postulated p t d path involved an increase in pressure with or without a decrease in temperature subsequent to early d3 albitization followed by an increase in temperature up to the metamorphic peak late d3 to early d4 the metamorphism was contemporary in part with the emplacement of the williams batholith c 1550 1500 ma which probably supplied the na rich fluids

emplacement of synkinematic plutons in the variscan odenwald germany controlled by transtensional tectonics in the odenwald area sw germany an intermediate level of variscan magmatic arc is exposed between 360 and 335 ma the neunkirchen intrusive complex intruded into an active ne sw trending zone of ductile divergent strike shear after the thermal peak of metamorphism but before the emplacement of the magmatic rocks shear zones developed in the wall rocks the microfabrics of these zones infer plastic deformation of feldspar hornblende and quartz and an increasing degree of postkinematic annealing towards the intrusive complex during emplacement and solidification the magmatic rocks experienced bulk strain the intrusive complex locally crosscut the shear zones of the wall rocks but show magmatic and also mylonitic foliations which parallel these zones a change from plastic to brittle behavior of feldspar and amphibole and a varying degree of postkinematic annealing of quartz suggest that in the intrusive complex strike shear continued during cooling in wall rocks and plutonic rocks kinematic indicators imply sinistral displacement in the wall rocks adjacent to the intrusive complex strike shear was combined with normal displacement this suggests that a component of horizontal extension which was associated with strike shear transtension facilitated ascend and emplacement of the magma transtension occurred at the same time when the schwarzwald vosges area in the south of the odenwald was subjected to nw se compression transtension was probably initiated by lateral escape of crustal fragments during overall convergent plate movements transtension is also supposed to be associated with significant uplift of metamorphic rocks

peat hydraulic conductivity in cold regions and its relation to pore size and geometry subsurface flow through peat plays a critical role in the hydrology of organic covered permafrost terrains which occupy a large part the continental arctic sub arctic and boreal regions hillslope drainage in these terrains occurs predominantly through the active flow zone between the relatively impermeable frost table and the water table above it the hydraulic conductivity profile within this zone controls the subsurface drainage of snowmelt and storm water peat hydraulic conductivity profiles were examined at three sites in north western canada each representing a widely occurring organic covered permafrost terrain type three independent measures of saturated hydraulic conductivity were used tracer tests constant head well permeameter tests and laboratory measurements of undisturbed samples at all three sites the conductivity profiles contained very high values 10 1000 m d 1 within the top ca 0 1 m where the peat is only lightly decomposed a large reduction with increasing depth below the ground surface in the transition zone and relatively low values in a narrow range 0 5 5 m d 1 below ca 0 2 m depth where the peat is in an advanced state of decomposition digital image analysis of resin impregnated peat samples showed that hydraulic conductivity is essentially controlled by pore hydraulic radius the strong dependence of hydraulic conductivity on hydraulic radius implies that peat soils subjected to similar degrees of decomposition and compaction have a similar hydraulic conductivity regardless of the location this explains the similarity of the depth conductivity profiles among all three terrain types copyright 2008 john wiley sons ltd

strata bound fe co cu au bi y ree deposits of the idaho cobalt belt multistage hydrothermal mineralization in a magmatic related iron oxide copper gold system mineralogical and geochemical studies of strata bound fe co cu au bi y rare earth element ree deposits of the idaho cobalt belt in east central idaho provide evidence of multistage epigenetic mineralization by magmatic hydrothermal processes in an iron oxide copper gold iocg system deposits of the idaho cobalt belt comprise three types 1 strata bound sulfide lenses in the blackbird district which are cobaltite and less commonly chalcopyrite rich with locally abundant gold native bismuth bismuthinite xenotime allanite monazite and the be rich silicate gadolinite y with sparse uraninite stannite and bi tellurides in a gangue of quartz chlorite biotite muscovite garnet tourmaline chloritoid and or siderite with locally abundant fluorapatite or magnetite 2 discordant tourmalinized breccias in the blackbird district that in places have concentrations of cobaltite chalcopyrite gold and xenotime and 3 strata bound magnetite rich lenses in the iron creek area which contain cobaltiferous pyrite and locally sparse chalcopyrite or xenotime most sulfide rich deposits in the blackbird district are enclosed by strata bound lenses composed mainly of cl rich fe biotite some deposits have quartz rich envelopes whole rock analyses of 48 co and or cu rich samples show high concentrations of au up to 26 8 ppm bi up to 9 16 wt y up to 0 83 wt rees up to 2 56 wt ni up to 6 780 ppm and be up to 1 180 ppm with locally elevated u up to 124 ppm and sn up to 133 ppm zn and pb contents are uniformly low 821 and 61 ppm respectively varimax factor analysis of bulk compositions of these samples reveals geochemically distinct element groupings that reflect statistical associations of monazite allanite and xenotime biotite and gold detrital minerals chalcopyrite and sparse stannite quartz and cobaltite with sparse selenides and tellurides significantly cu is statistically separate from co and as consistent with the general lack of abundant chalcopyrite in cobaltite rich samples paragenetic relations determined by scanning electron microscopy indicate that the earliest y ree be mineralization preceded deposition of co cu au and bi allanite xenotime and gadolinite y commonly occur as intergrowths with and inclusions in cobaltite the opposite texture is rare monazite in contrast forms a poikiloblastic matrix to cobaltite and thin rims on allanite and xenotime reflecting a later metamorphic paragenesis allanite and xenotime also show evidence of late dissolution and reprecipitation forming discordant rims on older anhedral allanite and xenotime and separate euhedral crystals of each mineral textural data suggest extensive deformation of the deposits by folding and shearing and by pervasive recrystallization all during cretaceous metamorphism sensitive high resolution ion microprobe u pb geochronology by aleinikoff et al 2012 supports these paragenetic interpretations documenting contemporaneous mesoproterozoic growth of early xenotime and crystallization of megacrystic a type granite on the northern border of the district these ages are used together with mineralogical and geochemical data from the present study to support an epigenetic iocg model for fe co cu au bi y ree deposits of the idaho cobalt belt a sulfide facies variant of iocg deposits is proposed for the blackbird district in which reducing hydrothermal conditions favored deposition of sulfide minerals over iron oxides this new model includes mesoproterozoic vein mineralization and related fe cl metasomatism that formed the biotite rich lenses a predominantly felsic magmatic source for metals in the deposits given their local abundance of y rees and be and a major sedimentary component in the hydrothermal fluids based on independent sulfur isotope and boron isotope data for sulfides and ore related tourmaline respectively

u pb data on zircons for the thermal peak of metamorphism in the variscan odenwald germany u pb data are presented for zircons from metamorphic rocks of the crystalline odenwald that comprises part of the mid german crystalline rise the odenwald can be divided into four principal domains with different magmatic and metamorphic histories on the basis of published structural and petrological data and the presented ages in the eastern boll stein odenwald the metasedimentary rocks contain zircons derived from basement rocks that exceed 2 2 ga in age the intrusion of granitic magmas into these metasedimentary rocks took place before or close to the end of the thermal peak of metamorphism at 375 5 ma ago rapid uplift of the bollstein odenwald occurred at about 320 ma bp later than in the bergstrasser odenwald the western bergstrasser odenwald can be subdivided by geochronological petrological and especially structural data into a northern a central and a southern part in the northern part basic magmatism has an age of 362 7 ma the peak of thermal metamorphism in the southernmost odenwald was accompanied by partial melting of metasedimentary rocks and the formation of new zircons with ages between 332 and 342 ma probable peak metamorphic conditions revealed by two samples from the heterogeneous central part occurred at about 336 ma i e in the same time span as in the southernmost part but later than in the bollstein area and the northern odenwald rapid uplift started earlier in the central and southern part and 10 20 ma earlier than in the bollstein odenwald cooling from 500 to 300 c occurred in the northern part in the time interval between the elevation of the other parts of the bergstraesser odenwald and the bollstein odenwald there is no indication for any thermal equilibrium between 2 25 ga and 375 ma recorded in the zircons of both areas

the significance of cordierite hypersthene assemblages from the beitbridge region of the central limpopo belt evidence for rapid decompression in the archean

baltica and gondwana derived sediments in the mid german crystalline rise central europe implications for the closure of the rheic ocean abstract combined u pb and lu hf isotope analysis of detrital zircons reveals that the mid german crystalline rise mgcr contains palaeozoic sediments of distinct provenance in close proximity zircons from a metapelite of the brotterode group were derived from a gondwanan source whereas zircons from the rogis quartzite of the ruhla group point to provenance from baltica brotterode zircons mostly yield ages of 500 720 ma and 2 0 2 1 ga and show an age gap between 1 0 and 1 7 ga which is characteristic of gondwana peri gondwanan detrital zircon populations combined u pb and hf isotope data suggest intense crustal recycling at 2 6 2 9 ga 1 8 2 1 ga and 570 720 ma as well as formation of juvenile crust during the latter two periods in contrast zircons of the rogis quartzite mostly yield ages between 0 9 and 1 8 ga which are typical of southwestern baltica minor age populations at 435 466 and 550 660 ma indicate that the quartzite is younger than late ordovician early silurian and that the baltica derived sediment is contaminated by minor peri gondwanan perhaps avalonia derived detritus minor age populations at 2 0 2 56 2 67 and 2 86 ga additionally point to a svecofennian and or karelian source u pb and hf isotope data imply that the baltica derived zircons were produced mainly through melting of juvenile palaeo to mesoproterozoic crust the data presented here indicate that the mgcr hosts the rheic suture between gondwana and balonia and that this suture zone was closed in a complex sequence of stages from the late silurian onwards

controls on methane released through ebullition in peatlands affected by permafrost degradation permafrost thaw in peat plateaus leads to the flooding of surface soils and the formation of collapse scar bogs which have the potential to be large emitters of methane ch4 from surface peat as well as deeper previously frozen permafrost carbon c we used a network of bubble traps permanently installed 20cm and 60cm beneath the moss surface to examine controls on ebullition from three collapse bogs in interior alaska overall ebullition was dominated by episodic events that were associated with changes in atmospheric pressure and ebullition was mainly a surface process regulated by both seasonal ice dynamics and plant phenology the majority 90 of ebullition occurred in surface peat layers with little bubble production in deeper peat during periods of peak plant biomass bubbles contained acetate derived ch4 dominated 90 by modern c fixed from the atmosphere following permafrost thaw post senescence the contribution of ch4 derived from thawing permafrost c was more variable and accounted for up to 22 on average 7 in the most recently thawed site thus the formation of thermokarst features resulting from permafrost thaw in peatlands stimulates ebullition and ch4 release both by creating flooded surface conditions conducive to ch4 production and bubbling as well as by exposing thawing permafrost c to mineralization 2014 american geophysical union all rights reserved

pb isotope constraints on the age of sulfide ore deposition and u pb age of late uraninite veining at the musoshi stratiform copper deposit central africa copper belt zaire the musoshi copper deposit is a typical ore shale type deposit located centrally in the zaire zambian copper belt cu fe sulfides are disseminated in a 20 m thick laminated black siltstone unit the ore shale which occurs above a thick approximately 1 km sequence of oxidized arkosic sediments of the roan group at the base of the katangan sedimentary basin uraninite occurs in veins which cut the ore shale and extend from major hydrothermal alteration zones in the footwall rutile accompanying this veining was dated by richards et al 1988 at 514 or 2 m y 2sigma in the present study nine analyses of five uraninite samples give a u pb age of 514 or 3 m y 2sigma in exact agreement with the rutile age information concerning the age and source of the copper mineralization at musoshi has been sought by study of pb isotopes in the cu fe sulfides and other minerals leach residue analyses of most ore samples show poor correlations on 207 pb 204 pb vs 206 pb 204 pb diagrams and scatter in a band these samples typically display mature recrystallized textures and may contain thin concordant sulfide veinlets with coarse secondary biotite quartz and carbonate however analyses of three samples from near the center of the ore shale which show less evidence for recrystallization and contain no veinlets form a wellcorrelated line mswd 2 54 15 analyses corresponding to a two stage model age of 645 or 15 m y 2sigma data from the other samples scatter below this line on the pb pb diagram and toward the secondary isochron obtained from the hydrothermal rutiles similar phenomena are observed on a 208 pb 204 pb vs 206 pb 204 pb diagram where addition of u to the disturbed samples is clearly demonstrated we conclude that the samples record disturbance of pb isotopes in a simple two stage system by the late uranium depositing hydrothermal event at 514 m y the significance of the 645 m y fractionation event is not clear however at least two interpretations may be given that the event reflects primary cu fe sulfide ore deposition or that pb isotopes were homogenized at this time in a preexisting orebody by some process which also introduced u and th into the deposit we are not able at present to distinguish between these two alternatives but whichever is correct it is clear that copper ore deposition occurred at or before 645 m y and is thus unrelated to the later 514 m y hydrothermal event a high integrated u pb source ratio mu 10 2 obtained from the 645 m y sulfide isochron suggests an evolved crustal source for pb in the orebody and rules out a significant contribution from mafic igneous rocks if cu and pb are assumed to share a common source these results do not support derivation of the metals from mantle derived materials at musoshi this u pb ratio contrasts strongly with a value of mu 9 7 calculated from the rutile analyses for the late hydrothermal overprint and further confirms that this later event was not involved in the formation of the copper deposit other than in coarsening and locally concentrating the ore the high temperature 397 degrees or 5 degrees c fluids associated with this event were probably derived from deep levels in the crust and appear to have been expelled during a late pulse of activity at the close of the lufilian orogeny

gas bubble size estimation in peat soils from em wave scattering observed with ground penetrating radar the size of biogenic gas bubbles in peatlands is believed to regulate ebullition of carbon gases to the atmosphere the measurement of electromagnetic em wave travel times using ground penetrating radar gpr is a proven field scale method for indirect estimation of volumetric gas content however there is also the possibility that information on the size of the gas bubbles can be determined from the analysis of the spectral content of gpr signals as scattering attenuation possesses a frequency dependence for bubbles smaller than the em wavelength rayleigh type scattering theoretical modeling shows that gpr data acquired with typical antenna frequencies are likely to be affected by bubble size in peat soils analysis of gpr data from two recent studies on peat monoliths where biogenic gas production was documented produced results consistent with the model predictions using the approach zero offset cross borehole gpr data in a northern peatland suggest that large bubble clusters i e 0 05 m radius occur in peat these findings broaden the utility of gpr for providing information on biogenic gas dynamics in peatlands

a petrogenetic grid for pelitic schists in the system sio 2 al 2 o 3 feo mgo k 2 o h 2 o a quantitative petrogenetic grid for pelitic schists in the system kfmash that includes the phases garnet chlorite biotite chloritoid cordierite staurolite talc kyanite andalusite sillimanite and pyrophyllite with quartz h2o and muscovite or k feldspar in excess is presented the grid is based on thermodynamic data of berman et al 1985 and berman 1988 for endmember kfash and kmash equilibria and natural fe mg partitioning for the kfmash system calculation of p t slopes and the change in fe fe mg along reactions in the kfmash system were made using the gibbs method in addition the effect on the grid of mno and cao is evaluated quantitatively the resulting grid is consistent with typical buchan and barrovian parageneses at medium to high grades at low grades the grid predicts an extensive stability field for the paragenesis chloritoid biotite which arises because of the unusual facing of the reaction chloritoid biotite quartz h2o garnet chlorite muscovite which proceeds to the right with increasing t in the kfmash system however the reaction proceeds to the left with increasing t in the mnkfash system so the assemblage chloritoid biotite is restricted to bulk compositions with high fe fe mg mn typical metapelites will therefore contain garnet chlorite at low grades rather than chloritoid biotite

transformation of a magmatic arc and an orogenic root during oblique collision and it s consequences for the evolution of the european variscides mid german crystalline rise the architecture of the european variscides has been subdivided by kossmat 1927 into paleogeographically coherent units which are presently interpreted as former plate fragments the mid german crystalline rise mgcr separates two fragments rhenohercynian and saxothuringian belts at the site of an inferred plate boundary and reequilibrated orogenic root the commonly favoured model interprets the mgcr as the magmatic arc on saxothuringian crust above a south dipping subduction zone in upper devonian and carboniferous times data from the mgcr the kinematic evolution of the mid european variscides and first order volume balancing suggest a reinterpretation of this unit which challenges classical views on the mgcr as well as on the subdivision of variscan architecture the mgcr is composed of two rock groups with different tectonic identity a lower carboniferous low pressure high temperature magmatic arc association on lower paleozoic basement rests tectonically on a stack of medium pressuremedium temperature rocks of inferred rhenohercynian origin the latter were tectonically accreted to the base of the overriding plate by tectonic underplating the entire process was controlled by oblique convergence this led to regional partitioning of the plate kinematic vector into contractional domains lower rhenohercynian plate and back arc area of the upper saxothuringian plate bulk heterogeneous plate margin parallel extensional domains mgcr and plate margin parallel wrench domains mgcr boundaries during this process material was continually transferred from the lower plate to the upper plate uplifted and exhumed by net crustal extension the concomitant removal of parts of the former arc and the entire orogenic root necessitates a reappraisal of variscan architecture and evolution

a metamorphosed proterozoic carbonaceous shale hosted co ni cu deposit at kalumbila kabompo dome the copperbelt ore shale in northwestern zambia the kalumbila co ni cu prospect 12 15 s 25 20 e is located 300 km west of the zambian copperbelt on the southeastern flank of the kabompo dome in the middle unit of the lufilian arc kalumbila contains a drill indicated resource of 3 04 mt at 0 67 wt percent ni 0 2 wt percent co and 0 11 wt percent cu using a 0 08 wt percent co and 0 3 wt percent ni cutoff with an average thickness of 12 75 m ni co cu mineralization occurs as strata bound disseminated to finely laminated locally semimassive to locally discordant chalcopyrite pyrrhotite violarite and siegenite within a finely laminated highly carbonaceous and pyritic metapelite 300 500 m thick surface rock exposure in this low relief terrain of woodland and seasonal swamp is 1 percent and geophysics magnetic and electromagnetic surveys is an essential exploration tool detrital zircons in the kalumbila metasediments shrimp u pb ages of 2004 1884 ma are derived from the underlying paleoproterozoic granitic gneiss such as that exposed in the adjacent kabompo basement dome the metasedimentary sequence grades upward from siliciclastic rocks mica schists to the carbonaceous metapelite which hosts the co ni cu these units are overlain by dolomitic and mg rich schists possible meta evaporites which are in turn succeeded by calc silicate rocks and a deformed diamictite this sequence is broadly similar to the stratigraphy in the zambian copperbelt and suggests that the kalumbila carbonaceous metapelite may be the chronostratigraphic equivalent of the katangan roan ore shale a significant exploration target kalumbila occurs within a moderately 40 north northwest dipping band of wamikumbi formation carbonaceous metapelite on the flank of a recumbent south southeast verging f1 or f2 back fold during the lufilian orogeny peak metamorphic conditions at f1 were of amphibolite facies grade 650 c 13 kbars both monazites and rutiles within the kalumbila metasediments have metamorphic characteristics and late orogenic lufilian shrimp u pb ages 548 6 7 6 ma and 531 21 ma respectively sulfides in the metasediments display metamorphic textures and therefore predate the peak of lufilian tectonism and thermal metamorphism a suite of spatially associated apparently late lufilian gabbros intrude the metasedimentary rocks these gabbros are unmineralized but the thermal effects of their emplacement may have modified ore distribution a genetic model in which synsedimentary pyrite in a carbonaceous pelite was partially replaced by syndiagenetic or metamorphic co ni cu bearing fluids of unknown origin is favored

cobalt in the zambian copperbelt abstract a review of available information concerning the regional distribution of cobalt in the zambian copperbelt is presented which emphasises the limitation of this metal to a narrow possibly tectonically controlled zone on the southwestern flanks of the kafue anticline the stratigraphic location of cobalt bearing mineralisation is also examined though the majority of the known deposits are within the ore shale horizon cobalt has been found from the basement lower roan unconformity to the top of the lower roan and in one case high in upper roan dolomites the behaviour of cobalt relative to copper can best be summarised by considering arenite hosted deposits separately from those in the ore shale it is shown that though the latter appear to contain cobalt within the confines of the copper orebody a marked vertical antipathetic relationship exists the arenite hosted deposits on the other hand tend to display a marked lateral differentiation of the metals in that the cobalt has apparently migrated or been precipitated at or beyond the fringes of the copper orebody compositional variations within the cobalt bearing phases are described which are attributed to corresponding variations in host rock lithology primary porosity the availability of sedimentary or diagenetic sulphate and in particular to the activity of reduced sulphur a genetic model is thus presented which envisages influx of hot hydrothermal solutions into sediments during early diagenesis cooling of these solutions by mixing with connate waters together with the differing solubilities of the various metals results in the precipitation of the metals in a zonal fashion variable bacterial and possibly inorganic reduction of interstitial sulphate and consumption of the reduced sulphur controls not only the quantity but also the composition of the sulphides precipitated in the past the original source of the metals now contained in the orebodies has been dismissed as somewhat irrelevant but generally they were seen as having been derived from erosion of the basement granites or contained mineralisation a case is made that such a source is not compatible with the presence of high cobalt concentrations and that derivation from a differentiated basic magma is a more satisfactory explanation of the facts crustal rifting is the required mechanism for high level penetration of metal enriched hydrothermal fluids

structural controls and evolution of gold silver and ree bearing copper cobalt ore deposits blackbird district east central idaho epigenetic origins the cu co au ag ni ree ore deposits of the blackbird district east central idaho have previously been classified as besshi type vms sedex and iocg deposits within an intact stratigraphic section new studies indicate that across the district mineralization was introduced into the country rocks as a series of structurally controlled vein and alteration systems quartz rich and biotite rich veins and alteration zones and minor albite and siderite veinlets maintain consistent order and sulfide mineral associations across the district both early and late quartz veins contain chalcopyrite and pyrite whereas intermediate stage tourmaline biotite veins host the cobaltite barren early and late albite and late carbonate generally siderite form veins or are included in the quartz veins ree minerals principally monazite allanite and xenotime are associated with both tourmaline biotite and late quartz veins the veins are in mineralized intervals along axial planar cleavage intrafolial foliation and shears mineralized intervals are hosted by a variety of metasedimentary rocks including three phyllitic units of mesoproterozoic age and two schistose units all of these units are s tectonites in the footwall of a regional thrust fault specifically the district lies within an oblique thrust ramp containing a series of structural horses three domains in a duplex system the deposits span the three domains and are hosted by metamorphic rocks that range from lower amphibolite facies in the structurally upper domain to lower middle greenschist facies in the lower domain an inverted metamorphic sequence early quartz and biotite veins were introduced during progressive folding and prolonged peak metamorphic conditions and they underwent late tectonic retrograde recrystallization and metamorphic mineral growth to the same extent as the country rocks in each domain where little subsequent deformation occurred early veins are discordant to bedding but where folding was polyphase and fabrics are penetrative mineralized zones are concordant with metamorphic compositional layering late quartz veins in the zones are associated with retrograde minerals and textures and are only locally deformed 40 ar 39 ar dating of unoriented muscovite from the selvage of a late quartz vein yields a late cretaceous age of about 83 ma the time of retrograde metamorphism associated with introduction of late quartz veins textural data at all scales indicate that the host sites for veins and the tectonic evolution of both host rocks and mineral deposits were kinematically linked to late cretaceous regional thrust faulting heat fluids and conduits for generation and circulation of fluids were part of the regional crustal thickening the faulting also juxtaposed metaevaporite layers in the mesoproterozoic yellowjacket formation over blackbird district host rocks we conclude that this facilitated chemical exchange between juxtaposed units resulting in leaching of critical elements cl k b na from metaevaporites to produce brines scavenging of metals co cu etc from rocks in the region and finally concentrating metals in the lower plate ramp structures although the ultimate source of the metals remains undetermined the present cu co au ag ni ree blackbird ore deposits formed during late cretaceous compressional deformation

atmospheric pressure drives changes in the vertical distribution of biogenic free phase gas in a northern peatland 1 atmospheric pressure patm is known to regulate methane emissions from northern peatlands however recent conceptual models differ in how gas production and release occurs in shallow defined here as less than 1 m depth versus intermediate to deep peat soils i e more than 1 m depth we used ground penetrating radar gpr measurements to non invasively estimate the vertical distribution of free phase gas and the dependence of this distribution on atmospheric pressure in a northern peatland variations in the travel time of the electromagnetic wave to three interfaces in the peat column were used in conjunction with deformation rod data and a petrophysical model relating electromagnetic wave velocity to free phase gas content to model changes in the vertical distribution of free phase gas over time we found a negative linear relation between changes in free phase gas content and changes in patm for shallow peat soils and a positive linear relation for deeper soils our results suggest that 1 free phase gas content confined in deep peat soils is larger and less variable to changes in patm than gas in shallow intermediate peat soils 2 increases in patm result in gas release from shallow peat soils into the atmosphere i e rapid ebullition and 3 decreases in patm result in upward gas movement from intermediate layers to replenish shallow layers our results suggest that changes in patm drive changes in the vertical distribution of free phase gas in peat soil and regulate methane ebullition from peat soils to the atmosphere our data shows a relationship between free phase gas and depth that may be due to changes in peat properties or increasing water pressure with depth

a calculated petrogenetic grid for the system k2o feo mgo al2o3 sio2 h2o with particular reference to contact metamorphosed pelites a petrogenetic grid is presented for the system kfmash k o feo mgo a1 0 sio h o including biotite muscovite k feldspar chlorite chloritoid staurolite cordierite garnet ortho amphibole orthopyroxene spinel andalusite sillimanite kyanite quartz and corundum with h o in excess which was calculated using the computer program thermocalc and the powell and holland internally consistent thermodynamic dataset by removing the normal constraint of having quartz in excess both quartz bearing and quartz absent equilibria are shown quartz absent equilibria are particularly relevant at high t and low p conditions because of their common occurrence at these conditions the calculated mineral assemblage and mineral compositional variations in terms of femg and fe mg siall exchange vectors are broadly compatible with observations on natural rocks particularly when non kfmash components are taken into account

contrasting styles of taconian eastern acadian and western acadian metamorphism central and western new england the two major early to middle palaeozoic tectonic metamorphic events in the northern appalachians were the taconian middle to late ordovician in central to western areas and the acadian late silurian to early middle devonian in eastern to west central areas this paper presents a model for the acadian orogenic event which separates the acadian metamorphic realm into eastern and western belts based on distinctively different styles we propose that the acadian metamorphism in the east was the delayed consequence of taconian back arc lithospheric modification east of the taconian island arc thick accumulations of late ordovician and silurian sediments coupled with plutons rising along a magmatic arc produced crustal thermal conditions appropriate for anomalously high t low p metamorphism accompanied by major crustal anatexis in this zone upward melt migration was coupled with subsequent e w crustal shortening possibly due to outboard collision with the avalon terrane to produce mechanical conditions that favoured formation of fold and thrust nappes and resultant tectonic thickening to the west and probably to the east as well the basis for the distinction between the eastern and western acadian events lies in the contrasting styles of metamorphism accompanying each evidence for contrasting metamorphic styles consists of 1 estimated metamorphic field gradients mfgs based on thermobarometric studies and 2 petrological evidence for contrasting p t trajectories west of the acadian metamorphic front the taconian zone has an mfg in which peak temperatures of 400 600 c were reached at pressures of about 4 6 kbar with both p and t increasing to the east near its western edge the western acadian metamorphic overprint has a similar mfg to the taconian and is mainly discriminated by 40ar 39ar dating and microtextural evidence east of this narrow zone the western acadian overprint is characterized by progressively higher temperatures 600 725 c and pressures 6 5 10 kbar or more to the east yielding an overall mfg that lies along or slightly above the kyanite sillimanite boundary on a p t diagram there is little or no plutonism accompanying western acadian metamorphism in contrast thermobarometry in the eastern acadian east of the bronson hill belt yields high t intermediate p conditions for the highest grade rocks known in new england t 650 750 c p 4 5 6 5 kbar for granulite facies assemblages which apparently formed along an anticlockwise p t path the bronson hill belt lies geographically between the eastern and western acadian zones and shows transitional petrological behaviour anomalously high temperatures at intermediate pressures but a clockwise path with decompression cooling radiometric dating indicates peak taconian conditions may have been achieved as early as 475 ma in the taconian hinterland and as late as 445 ma in the taconian foreland including the taconic allochthons eastern acadian magmatism may have started as early as 425 ma and most nappe stage deformation and metamorphism in the eastern acadian zone appears to have ended by about 410 ma tectonic thickening in the western acadian including the western counterparts of the nappe stage deformation documented in the eastern acadian must pre date attainment of peak metamorphic conditions dated at 395 385 ma dome stage deformation clearly post dates peak metamorphism and deforms metamorphic isograds the end of western acadian deformation is well constrained by 370 375 ma radiometric ages of late pegmatites and granitoids which cross cut all structures

moderate overthickening of thinned sialic crust and the origin of granitic magmatism and regional metamorphism in low p high t terranes formation of low pressure p high temperature t metamorphic terranes by homogeneous shortening and thickening of tectonically thinned sialic crust and overlying sedimentary basins at geologically reasonable strain rates avoids the shortcomings of hypotheses linking the characteristically high geothermal gradients to doubling of normal thickness crust crustal extension alone intrusion of granitoids or extensive basic magmatism on the basis of geological and p t constraints from the archean slave province the p t histories of three points in a hypothetical block of crust are presented in p t p distance and p time diagrams the relations between the rock that ends up at the surface base of the basin base of the crust isotherms and depth determine the type of metamorphism and the degree of partial melting required to produce abundant granitoids this simple qualitative framework can account for the characteristic features of low p high t metamorphic terranes and reconcile those who believe granites are the cause of the metamorphism with those who favor the contrary

geodynamic evolution of the european variscan fold belt palaeomagnetic and geological constraints the variscan fold belt of europe resulted from the collision of africa baltica laurentia and the intervening microplates in early paleozoic times over the past few years many geological palaeobiogeographic and palaeomagnetic studies have led to significant improvements in our understanding of this orogenic belt whereas it is now fairly well established that avalonia drifted from the northern margin of gondwana in early ordovician times and collided with baltica in the late ordovician early silurian the nature of the gondwana derived armorican microplate is more enigmatic geological and new palaeomagnetic data suggest armorica comprises an assemblage of terranes or microblocks palaeobiogeographic data indicate that these terranes had similar drift histories and the rheic ocean separating avalonia from the armorican terrane assemblage closed in late silurian early devonian times an early to mid devonian phase of extensional tectonics along this suture zone resulted in formation of the relatively narrow rhenohercynian basin which closed progressively between the late devonian and early carboniferous in this contribution we review the constraints provided by palaeomagnetic data compare these with geological and palaeobiogeographic evidence and present a sequence of palaeogeographic reconstructions for these circum atlantic plates and microplates from ordovician through to devonian times

sulfide bearing veinlets throughout the stratiform mineralization of the central african copperbelt temporal and genetic implications the central african copperbelt of zambia and democratic republic of congo along with its broad time equivalents in botswana and namibia the kalahari copperbelt is the world s largest sediment hosted stratiform copper province the economically dominant stratiform copper cobalt or silver mineralization accepted by many as at least partly early diagenetic in origin is ubiquitously accompanied by sulfide bearing quartz carbonate veins and veinlets traditionally this veining is considered to be the product of either metamorphic lateral secretion or discrete late diagenetic to synorogenic mineralization events this view is challenged because identical sulfide assemblages and textures characterize both the disseminated and accompanying veinlet mineralization throughout the zoned sulfide assemblages that constitute many of the stratiform orebodies a situation that is taken to imply contemporaneity of the two styles consequently during the mineralization the siltstone sandstone or dolomite host rocks must have been sufficiently competent to undergo widespread brittle fracturing implying that the disseminated mineralization cannot have been introduced by passive fluid infiltration during early diagenesis this conclusion is further supported by the local occurrence of minor disseminated and veinlet mineralization hosted by mafic sills and dikes both the disseminated and veinlet mineralization styles lack ductile deformation features in many congolese deposits but are variably metamorphosed and deformed in some deposits in zambia botswana and namibia in conjunction these observations suggest that the stratiform copper orebodies were the results of massive saline fluid expulsion by hydraulic fracturing after the mafic magmatism 765 715 ma most probably spanning peak damara lufilian metamorphism and ductile deformation 530 ma and continuing in places until at least 500 ma thus basin inversion contractional tectonism and associated uplift and exhumation and not the earlier extension and rifting seem more likely to have been the ultimate drivers for the fluid mobilization and expulsion

new evidence for an old idea geochronological constraints for a paired metamorphic belt in the central european variscides abstract new geochronological data reveal a prolonged tectonothermal evolution of the variscan odenwald spessart basement being part of the mid german crystalline zone in central europe we report the results from i secondary ion mass spectrometry sims u pb dating of zircon rutile and monazite ii sims zircon oxygen isotope analyses iii laser ablation multicollector inductively coupled plasma mass spectrometry la mc icpms zircon lu hf isotope analyses and iv la icpms zircon and rutile trace element data for a suite of metamorphic rocks five amphibolite and eclogite facies mafic meta igneous rocks and one granulite facies paragneiss the protoliths of the mafic rocks formed from juvenile as well as depleted mantle sources in distinct tectonic environments at different times magmatism took place at a divergent oceanic margin possibly in a back arc setting at 460 ma in an intraoceanic basin at ca 445 ma and at a continental margin at 329 ma regardless of lithology zircon in eclogite amphibolite and high temperature paragneiss provide almost identical carboniferous ages of 333 7 4 1 ma eclogite 329 1 1 8 to 328 4 8 9 ma amphibolite and 334 0 2 0 ma paragneiss respectively rutile yielded ages of 328 6 4 7 and 321 4 7 0 ma in eclogite and amphibolite and monazite in high temperature paragneiss grew at 330 1 2 4 ma all ages are quoted at the 2 level the data constrain coeval high pressure eclogite and high temperature granulite facies metamorphism of the odenwald spessart basement at ca 330 ma amphibolite facies conditions were attained shortly afterwards the lower plate eclogite formed in a fossil subduction zone and the upper plate high temperature low pressure rocks are the remains of an eroded carboniferous magmatic arc the close proximity of tectonically juxtaposed units of such radically different metamorphic conditions and thermal gradients is characteristic for a paired metamorphic belt sensu miyashiro 1961 thus the odenwald spessart basement represents the first recognised paired metamorphic belt in the european variscides

shallow horizontal groundwater flow in peatlands is reduced by bacteriogenic gas production 1 we tested the extent to which horizontal hydraulic conductivity kh below the water table in near surface peats is affected by the accumulation of biogenic gas bubbles we found 1 immediately after water table rise the peat is not saturated levels of initial undersaturation were between 1 and 9 percent 2 the production of biogenic gas bubbles below the water table causes large reductions typically more than a factor of two in kh

age of intrusion and rapid cooling of the frankenstein gabbro odenwald sw germany evidenced by40ar 39ar and single zircon207pb 206pb measurements the frankenstein gabbro complex in the northern odenwald fr germany is one of the hercynian plutonic bodies of the saxothuringian zone in the variscan belt 87sr 86sr isotopic investigations on pyroxene 40ar 39ar age determinations on hornblendes plagioclases biotites and pyroxenes and207pb 206pb dating on single zircon crystals have been carried out

the timing of metamorphism in the odenwald spessart basement mid german crystalline zone new in situ electron microprobe monazite and white mica 40ar 39ar step heating ages support the proposition that the odenwald spessart basement mid german crystalline zone consists of at least two distinct crustal terranes that experienced different geological histories prior to their juxtaposition the monazite ages constrain tectonothermal events at 430 43 ma 349 14 ma 331 16 ma and 317 12 ma 316 4 ma and the 40ar 39ar analyses provide white mica ages of 322 3 ma and 324 3 ma granulite facies metamorphism occurred in the western odenwald at c 430 and 349 ma and amphibolite facies metamorphism affected the eastern odenwald and the central spessart basements between c 324 and 316 ma we interpret these data to indicate that the otzberg michelbach fault zone which separates the eastern odenwald spessart basement from the western odenwald basement is part of the rheic suture which marks the position of a major variscan plate boundary separating gondwana and avalonia derived crustal terranes the age of the carboniferous granulite facies event in the western odenwald overlaps with the minimum age of eclogite facies metamorphism in the adjacent eastern odenwald the granulite and eclogite facies rocks experienced contrasting pressure temperature paths but occur in close spatial proximity being separated by the rheic suture as high pressure and high temperature metamorphisms are of similar age we interpret the odenwald spessart basement as a paired metamorphic belt and propose that the adjacent high pressure and high temperature rocks were metamorphosed in the same subduction zone system juxtaposition of these rocks occurred during the final stages of the variscan orogeny along the rheic suture

u pb geochronology of two augen gneiss terranes idaho new data and tectonic implications u pb isotopic analyses of cogenetic zircon suites revise the ages of two augen gneiss terranes in central and northern idaho the augen gneiss of priest river yields an upper intercept concordia age for only moderately discordant zircons of 1576 13 ma there is no evidence of an inherited xenocrystic lead component because this unit probably is in basement beneath belt purcell strata it provides a maximum age for belt and purcell sedimentation however high grade metamorphism and structural complexity permit several tectonic interpretations all of which should be treated with caution pending further detailed studies zircons from the augen gneiss of red river contain inherited radiogenic lead but by analogy with lithologically and isotopically similar augen gneiss and porphyritic granite near salmon and shoup idaho an age of about 1370 ma is suggested however the uncertainty associated with this value is unknown and could be large intrusion preceded or was synkinematic with at least part of the

proterozoic pb isotope evolution in the belt purcell basin constraints from syngenetic and epigenetic sulfide deposits sediment hosted massive sulfide and ag pb zn veins are two major deposit types which contain large concentrations of lead in the middle proterozoic belt purcell basin new pb isotope data on the sullivan sediment hosted massive sulfide deposit and the st eugene ag pb zn vein deposit are combined with existing data to provide a comprehensive overview of proterozoic pb isotope evolution in the belt purcell basin the pb isotope ratios for sulfides in the sullivan deposit form a linear array the sullivan array bedded sulfides plot within this array and no systematic change in pb isotope ratios with stratigraphic position time is observed the sullivan array results from the mixing of leads from the upper crustal and or orogen pb reservoirs represented by metasedimentary rocks of the lower aldridge formation in the footwall of the sullivan deposit with perhaps leads derived from the precambrian basement of the belt purcell basin a zone within the massive sulfides of the sullivan deposit has been altered to chlorite pyrite calcite the center of the alteration zone is comprised of massive pyrite calcite which has more radiogenic pb isotope compositions the sullivan alteration the sullivan alteration apparently formed from the same pb reservoirs as the sullivan array perhaps as a product of the east kootenay orogeny 1370 ma after a period of up to 108 m y which allowed radiogenic pb growth ag pb zn occurrences in the belt purcell basin form an array in pb isotope space the coeur d alene array which is not collinear with the sullivan array it is characterized by pb isotope compositions less radiogenic than those of the sullivan array the coeur d alene array is a mixture of leads derived from the upper crustal and mantle pb reservoirs it is proposed that mantle lead was introduced into the belt purcell basin at 1468 ma by the mafic moyie sills leads in sediment hosted massive sulfide deposits in middle aldridge rocks and leads of the coeur d alene array in epigenetic veins formed after intrusion of the moyie sills but before east kootenay orogeny metamorphism

using shrimp zircon dating to unravel tectonothermal events in arc environments the early palaeozoic arc of nw iberia revisited dating of zircon cores and rims from granulites developed in a shear zone provides insights into the complex relationship between magmatism and metamorphism in the deep roots of arc environments the granulites belong to the uppermost allochthonous terrane of the nw iberian massif which forms part of a cambro ordovician magmatic arc developed in the peri gondwanan realm the obtained zircon ages confirm that voluminous calc alkaline magmatism peaked around 500 ma and was shortly followed by granulite facies metamorphism accompanied by deformation at c 480 ma giving a time framework for crustal heating regional metamorphism deformation and partial melting the main processes that control the tectonothermal evolution of arc systems traces of this arc can be discontinuously followed in different massifs throughout the european variscan belt and we propose that the uppermost allochthonous units of the nw iberian massif together with the related terranes in europe constitute an independent and coherent terrane that drifted away from northern gondwana prior to the variscan collisional orogenesis

provenance of ore metals in base and precious metal deposits of central idaho as inferred from lead isotopes sulfide ores containing principally argentiferous galena sphalerite and tetrahedrite occur both in paleozoic sedimentary rocks and in late cretaceous granites along the eastern margin of the idaho batholith in central idaho two genetic models have been proposed for the sediment hosted deposits one involving vein and replacement styles of mineralization during the cretaceous and tertiary and the other representing initial paleozoic sedimentary exhalative sedex style of mineralization and subsequent remobilization during the cretaceous and tertiary there is also uncertainty about the sources of ore components studies based on fluid inclusions and stable isotopes suggest a shallow crustal source whereas the pb isotope studies favor sources deeper than the exposed crustal rocks we present new lead isotope data on 16 sulfide ore samples and 24 whole rock and mineral separate samples and compare them with data from the literature to assess the possible sources of base and precious metals in the deposits of central idaho two potential ore metal sources are recognized a sedimentary source and an igneous source deposits that derived their metals from a sedimentary source are characterized by radiogenic pb and are subdivided into three types triumph 206 pb 204 pb 19 4 20 1 207 pb 204 pb 15 7 15 9 and 208 pb 204 pb 39 0 39 9 minnimoore 206 pb 204 pb 19 9 20 6 207 pb 204 pb 15 8 16 0 and 208 pb 204 pb 40 1 41 3 and pacific 206 pb 204 pb 20 4 21 4 207 pb 204 pb 15 7 15 8 and 208 pb 204 pb 40 8 41 1 the pb isotope compositions of sediment hosted deposits can be explained by the mixing of pb derived from paleozoic sedimentary rocks and early proterozoic crystalline rocks or by the mixing of pb derived from the cretaceous granite and early proterozoic crystalline rocks alternatively the pb in sediment hosted deposits may represent a mixture of pb derived from all three sources in varying proportions none of these scenarios requires pb from deep sources the results of this study and prior fluid inclusion and stable isotope studies support a genetic model involving shallow crustal sources for metals and sulfur mobilized by meteoric water dominated hydrothermal systems deposits in which the metals were derived from an igneous source are subdivided into carrietown and non carrietown types carrietown type ores have relatively low uranogenic pb isotope ratios 206 pb 204 pb 17 3 19 1 and 207 pb 204 pb 15 4 15 7 and variable thorogenic pb isotope ratios 208 pb 204 pb 38 9 41 0 the non carrietown type ores are isotopically similar to their host granites and have higher uranogenic pb isotope ratios 206 pb 204 pb 19 3 20 6 207 pb 204 pb 15 7 15 8 and 208 pb 204 pb 39 3 40 1 compared to the carrietown type ores we propose that the two groups are dominated by pb sources from different depths the carrietown type ores from middle crustal sources uranium depleted and the non carrietown type ores from upper crustal sources enriched in both u and th a magmatic hydrothermal origin for both types of ores is supported by their granitic host rocks hydrothermal alteration of intrusive granite and surrounding rocks high temperatures of ore formation and similar radiometric ages for the granite and mineralized veins

genesis of the ores of the zambian copperbelt abstract the theories for the origin of the ores of the copperbelt of zambia and zaire are reviewed following an account of the historical development of ideas from epigenetic through syngenetic to syngenetic diagenetic and ultimately to neohydrothermalist theories models for the genesis of these deposits are critically assessed in the light of the data currently available this examination is conducted with particular regard to the questions of the sources of copper and cobalt the timing of mineralizing events and the physicochemical conditions of metal sulphide precipitation although accepting that the origin of these ores remains controversial it is concluded that the most likely sources of both copper and cobalt are the rocks of the basement complex that metals were probably transported both in surface and ground waters and that the main mineralizing event took place during early diagenesis with sulphide precipitation occurring via bacterial reduction of seawater sulphate the precipitation of sulphides probably occurred at temperatures around 20 60 c and at near neutral ph with minor fluctuations in eh and variations in the solubilities of the metals involved causing mineral zonation the burial and compaction of these sediments probably resulted in the expulsion of metal rich fluids and formation of sulphides in the immediate footwall and hanging wall of the main ore horizon with continued burial resulting in the formation of sulphide bearing pods and veins of lateral secretion origin tectonic deformation and metamorphism in the lufilian orogeny although resulting in a set of cross cutting high temperature mineralized veins appears not to have resulted in significant remobilization of the sulphides

pressure temperature time paths of regional metamorphism i heat transfer during the evolution of regions of thickened continental crust the development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional scale thermal regimes these are the total radiogenic heat supply within the thickened crust the supply of heat from the mantle the thermal conductivity of the medium and the length and time scales of erosion of the continental crust the orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening during which little temperature change occurs in individual rocks followed by a lengthier phase of erosion at the end of which the crust is at its original thickness the principal features of pressure temperature time ptt paths followed by rocks in this environment are a period of thermal relaxation during which the temperature rises towards the higher geotherm that would be supported by the thickened crust followed by a period of cooling as the rock approaches the cold land surface the temperature increase that occurs is governed by the degree of thickening of the crust its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary for much of the parameter range considered the heating phase encompasses a considerable portion of the exhumation decompression part of the ptt path in addition to the detailed calculation of ptt paths we present an idealized model of the thickening and exhumation process which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path an important feature of these ptt paths is that most of them lie within 50 c of the maximum temperature attained for one third or more of the total duration of their burial and uplift and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm a companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes 1984 oxford university press

provenance of upper devonian clastic meta sediments of the bollstein odenwald mid german crystalline zone variscides detrital zircons separated from paragneisses of the northern bollstein odenwald mid german crystalline zone variscides yielded cambrian to upper devonian u pb ages the age of the youngest detrital zircon population 371 3 ma and the lower carboniferous metamorphic overprint indicate an upper devonian depositional age of the paragneiss protolith the age spectra of detrital zircons suggest the latter to be derived from 1 ordovician igneous rocks of the saxothuringian zone 2 a late cadomian magmatic arc tepla barrandian unit and 3 a silurian devonian magmatic arc cadomian igneous activity is also documented by cambrian zircon cores in lower to middle devonian detrital zircons meso and paleoproterozoic detrital zircons which are typical for the old red continent and the rhenohercynian zone are entirely lacking the restricted palaeozoic detrital zircon age spectrum is attributed to a silurian devonian intra arc or trench setting both the lack of mesoproterozoic detrital zircons and the striking similarity of the u pb ages of the detrital zircon obtained from the bollstein odenwald with u pb ages from crystalline rocks of the saxothuringian basement rules out that the bollstein odenwald is forming a tectonic window rhenohercynian lower plate inside the mid german crystalline zone saxothuringian upper plate