lithostratigraphy of the upper turinian lower chatfieldian upper ordovician foreland succession and a u pb id tims date for the millbrig volcanic ash bed in the ottawa embayment three stages of carbonate platform development are preserved in the upper turinian lower chatfieldian succession of the ottawa group in the ottawa embayment and represent deposition along the late ordovician taconic foreland interior of paleo southern laurentia compared with contemporary stratigraphy in the adjacent northern appalachian southern ontario new york state and western quebec basins the intermediate stage 2 succession which brackets the turinian chatfieldian boundary preserves embayment specific stratigraphic patterns these include i dramatic west to east thickening of the upper turinian watertown formation that defines differential subsidence along the present axis of the embayment ii post watertown base level fall defined by appearance of shoreface siliciclastics iii early chatfieldian marine transgression represented by the proposed l orignal formation that is coeval with but lithologically distinct from the selby formation in the northern appalachian basin and iv platf

crustal extension ag pb zn veins in the xiong ershan district southern north china craton constraints from the shagou deposit the shagou vein type ag pb zn deposit in the xiong ershan district southern margin of the north china craton is hosted within amphibolite facies metamorphic rocks of the late archean to early paleoproterozoic taihua group the ag pb zn veins are localized in ne to nne trending brittle faults and typically display symmetrical zoning consisting of siderite quartz sphalerite galena and quartz calcite from the margin toward the center of each vein ore related hydrothermal alteration is well developed on both sides of the veins dominated by silicification sericitization chloritization and carbonatization sericite separates extracted from a major ag pb zn vein yield a 40 ar 39 ar plateau age of 140 0 1 0 ma 1 and isochron age of 141 1 1 6 ma 1 indicating that mineralization occurred at the beginning of early cretaceous field and textural relationships indicate four hydrothermal stages marked by assemblages of quartz siderite stage i quartz sphalerite ankerite stage ii quartz galena silver minerals ankerite stage iii and quartz calcite stage iv respectively silver minerals are abundant in all veins and are composed of in paragenetic order argentiferous tetrahedrite polybasite jalpaite argentite and native silver these silver minerals commonly occur as replacements of galena chalcopyrite and other sulfides or as fillings of microfractures in sulfides and quartz microthermometric measurements of primary fluid inclusions in quartz carbonates and sphalerite from various hydrothermal stages indicate that ore minerals were deposited at intermediate temperatures 267 157 c from aqueous carbonic to aqueous fluids with moderate salinities 7 2 15 9 wt nacl equiv coexisting galena sphalerite pair yields sulfur isotope equilibrium temperatures of 205 to 267 c consistent with the overall homogenization temperatures of fluid inclusions the microthermometric data also indicate that both fluid mixing and fluid rock interaction were important mechanisms for ore precipitation carbonate minerals siderite ankerite calcite spanning the entire mineralization history have 13 c v pdb values of 5 2 to 1 4 and 18 o v smow of 10 9 to 15 0 corresponding to calculated values for the ore fluids of 6 5 to 1 8 and 1 4 to 5 4 respectively 34 s v cdt values of sulfide minerals pyrite sphalerite galena range from 1 1 to 5 5 consistent with a deep seated sulfur source galena separates have 206 pb 204 pb ratios of 17 472 to 17 813 207 pb 204 pb ratios of 15 411 to 15 498 and 208 pb 204 pb ratios of 38 178 to 38 506 the isotope data together with geological and geochronological evidence favor a primary metamorphic source for sulfur and other components in the ore fluids a synthesis of available data suggests that the shagou deposit is a typical vein type ag pb zn deposit that formed under an extensional geodynamic setting associated with tectonic reactivation of the north china craton during the late mesozoic a time period that is manifested by pervasive magmatism widespread formation of metamorphic core complexes and development of faulted basins throughout the eastern part of the craton metamorphic devolatilization of the meso neoproterozoic marine sedimentary rocks previously subducted beneath the xiong ershan district facilitated by extensive magmatism and elevated heat flow due to lithospheric extension could have provided large amounts of ore fluids responsible for the ag pb zn mineralization the ne to nne trending faults affiliated with the transcrustal machaoying fault may have acted as pathways for the upward migration of deep seated metamorphic fluids mixing of the metamorphically derived fluids with meteoric waters ultimately resulted in deposition of the ag pb zn veins in brittle extensional structures

u th he thermochronology of the ottawa embayment eastern canada the temperature time history of an ancient intracratonic rift basin abstractthe ottawa embayment is an intracratonic rift basin in eastern north america that preserves a unique and eventful history through deep time we report single grain zircon and apatite u th he and biotite 40ar 39ar data from samples collected along a 250 km transect across the crystalline rift flank and through the sedimentary sequences of the embayment the data sets show significant intrasample age dispersions and we investigate the influence of zircon crystallinity and nonideal crystal shapes on he diffusion through the application of raman spectroscopy and x ray microcomputed tomography ultimately we utilize the relationship between radiation damage and helium diffusion to quantify observed age dispersion we integrate our age data from the basement and overlying sedimentary unit of the embayment with regional geology to utilize multisample inverse modeling to reveal viable thermal histories that we then further evaluate by using the forward modeling method to improve our understanding of t

deep crustal ductile extension following thrusting in the southwestern grenville province ontario two lithotectonic assemblages in southern britt domain have different histories of plutonism metamorphism structural development and mafic dyke emplacement these differences are accounted for by postulating that a cryptic grenvillian thrust separates the assemblages amphibolite facies extensional shear along the central britt shear zone cbsz overprinted the thrust obscuring kinematic evidence for its existence the structurally lower bayfield nadeau island assemblage contains orthogneiss suites of disparate age lesser amounts of supracrustal rocks deposited before intrusion of the youngest orthogneiss pre grenvillian and grenvillian metamorphic assemblages and at least three mafic dyke suites the overlying ojibway sand bay assemblage contains only younger orthogneiss with grenvillian metamorphic assemblages volumetrically important supracrustal rocks that are younger than the youngest orthogneiss and lacks cross cutting mafic dykes comparable tectono stratigraphic changes are present acros

textures trace elements and pb isotopes of sulfides from the haopinggou vein deposit southern north china craton implications for discrete au and ag pb zn mineralization the haopinggou deposit in the xiong ershan district southern margin of the north china craton comprises numerous au and ag pb zn veins hosted in metamorphic rocks of the late archean to early paleoproterozoic taihua group two stages of mineralization have been recognized stage 1 pyrite quartz veins and stage 2 pb zn sulfide veins some pyrite quartz veins are surrounded or cut by pb zn sulfide veins others occur as independent veins six generations of pyrite have been identified at haopinggou py1 to py3 in stage 1 and py4 to py6 in stage 2 pyrites from stage 1 are enriched in au as co ni and bi whereas stage 2 pyrites contain higher ag pb zn sn and sb invisible au mostly occurs as lattice bound gold in py2 up to 92 ppm au and py3 up to 127 ppm au and has a close relationship with as native au grains are also present in py3 and likely resulted from mobilization and reprecipitation of the invisible au previously locked in the precursor pyrite this view is supported by extensive plastic deformation in stage 1 pyrite as revealed by electron backscatter diffraction analysis in stage 2 ag is mostly present as lattice bound silver closely associated with sb in galena up to 798 ppm ag a variety of silver minerals are also present as inclusions within galena or as interstitial grains these silver minerals were likely formed via ag cu exchange reaction between tetrahedrite and galena or represent exsolution from galena due to a temperature decrease pb isotopic compositions differ remarkably between stage 1 and stage 2 sulfides indicating different sources of lead pb in stage 2 pb zn sulfide veins is consistent with the haopinggou porphyry close to the veins the field textural compositional and lead isotopic data led us to conclude that the early gold bearing pyrite quartz veins and late silver bearing pb zn sulfide veins likely formed from distinct fluid systems related to discrete mineralization events our study suggests that au and ag pb zn veins hosted in the same metamorphic terrains may not be genetically related to each other

stable isotopic and fluid inclusion indications of large scale hydrothermal paleoflow boiling and fluid mixing in the keno hill ag pb zn district yukon territory canada the keno hill vein system of the central yukon is restricted predominantly to the highly fractured graphitic keno hill quartzite unit of mississippian age hydrothermal mineral zoning is related spatially to a cretaceous granitic pluton which intrudes the quartzite during mineralization the quartzite acted as a district scale aquifer subsequent erosion has exposed a 40 km long vein system from its plutonic roots outward to polymetallic ag pb zn veins and further to assemblages of epithermal character the 18oquartz values from veins near the pluton increase outwards from 10 6 to 20 1 as a result of cooling of the hydrothermal fluids and exchange with the quartzite contours of isotope values outline broad paths of fluid movement within the quartzite proceeding further from the pluton 18oquartz values decrease to 10 5 at the outer edge of the system the presence of meteoric water is indicated here where late stage quartz has 18osmow values as low as 7 l the outward decreasing trend appears to have been established by mixing of isotopically light meteoric water with exchanged fluids that were in isotopic equilibrium with the quartzite fluid inclusions from quartz in the orebodies demonstrate an evolving h2o co2 nacl ch4 system loss of co2 and ch4 during water vaporization coincides with increasing salinity and decreasing temperature resulting from high enthalpy steam loss depressurization during active faulting is the principal mechanism late stage fluids are represented by dilute aqueous inclusions with lower homogenization temperatures quartz from silver rich veins has been shifted to higher 18o values by up to 4 relative to adjacent silver poor veins the result of a minimum 10 25 adiabatic boiling and fractionation dominated by water vaporization and associated cooling graphite initially buffered the hydrothermal fluids to a high co2 content with variable ch4 involvement of organic carbon from the host rocks is indicated by the negative 13cpdb values for the carbonates from 4 0 to 12 9 variations in the carbon isotopes result from fluctuating co2ch4 ratios reflecting the contrasting volatility of the gas pair siderite formed as a late stage product of the boiling event and its formation coincides with a decreasing 18o trend in the water created by the equilibration of graphite and water in replacing exsolved co2 the formation of ch4 during this stage had a reducing effect on the fluid resulting in an increase in 13csiderite values in association with the decreasing 18osiderite values a closed system boiling model together with calculations of water consumption during post boiling co2 and ch4 formation indicates that greater than 50 of the original water in the ore fluid was removed relatively saline mineralizing fluids resulted

δ13c stratigraphy of a turinian chatfieldian upper ordovician foreland succession ottawa embayment central canada resolving local and inter regional isotope excursions in a tectonically active basin abstract positive 13ccarb excursions are correlated through an upper turinian to lower chatfieldian carbonate platform succession along the axis of the ottawa embayment and into outliers of the northern ottawa bonnechere graben in central canada successive turinian excursions e1 and e2 are lithostratigraphically constrained by erosional surfaces and hosted within the watertown and overlying l orignal formations respectively the latter coeval with the selby formation in the adjacent northern appalachian basin the excursions coincide with periods of regional transgression but geographic patterns of 13c depletion versus enrichment coincide with structurally defined areas of stratigraphically condensed and preferentially thickened formation successions respectively differential subsidence is interpreted to have created bathymetric variation resulting in intrabasinal restriction of seawater exchange between these areas with preferential corg recycling with stratigraphic condensation by early chatfieldian time segmentation of the once regional carbonate platform l orignal formation produced a regional mosaic of low energy muddy carbonate banks rockland formation and a deeper water platform lower hull formation settings subject to fluctuating high to low energy current flow excursion e3 occurs in both successions but 13c enrichment is associated only with the bank top muddy facies this may identify preferential photosynthetic drawdown of 12c across the bank tops due to limited seawater exchange across the bank deeper platform boundaries excursions e1 to e3 and a younger excursion e4 in the hull formation are correlated with varying confidence with excursions across southern laurentia excursion e3 being the local expression of the guttenberg 13c excursion our study supports local modulation of regional if not global 13c excursions arising from structurally controlled changes in oceanography and productivity

geochemistry and metallogeny of ag pb zn veins in the purcell basin british columbia the mesoproterozoic purcell basin in canada contains three different types of veins 1 type 1 pb zn ag veins are composed of fe rich sphalerite pyrrhotite galena freibergite and pyrite with minor quartz gangue 2 type 2 pb ag cu au veins are characterized by galena pyrite freibergite and gold in a quartz gangue and 3 type 3 ag pb zn vein and replacement deposits contain fe poor sphalerite galena pyrite and freibergite in a quartz dolomite gangue that locally replaces host dolostone the sulfur isotope composition of type 1 2 and 3 vein and replacement deposits closely reflects their respective stratigraphic positions type 1 and 2 veins hosted in lower purcell sedimentary rocks have light sulfur isotope compositions 4 8 to 2 5 similar to diagenic pyrite disseminated in clastic sedimentary host rocks type 3 veins and the replacement deposits hosted in upper purcell supergroup rocks have heavy sulfur isotope compositions 8 3 17 2 indicative of thermo chemical reduction of marine sulfate in carbonate host rocks quartz from type 1 pb zn ag veins has 18 o smow values ranging from 7 to 17 whereas type 2 and 3 vein and replacement deposits have 18 o values ranging from 9 to 20 the range of 18 o values for type 1 2 and 3 veins is the result of mixing of metamorphic fluids with a 18 o value higher than 11 5 and an upper crustal fluid with a 18 o lower than 1 2 in equilibrium with the host rocks during proterozoic type 1 and mesozoic cenozoic type 2 and 3 hydrothermal events galena and freibergite lead isotope compositions plot in two groups consistent with previous studies type 1 pb zn ag veins have a nonradiogenic lead signature 206 pb 204 pb 16 322 16 435 and are considered proterozoic in age type 2 pb ag au cu veins and type 3 ag pb zn vein and replacement deposits have a radiogenic lead signature 206 pb 204 pb 17 842 19 347 and are interpreted to be mesozoic cenozoic in age hydrothermal sericite from the type 3 ptarmigan replacement deposit yields a 40 ar 39 ar age of 133 1 0 7 ma indicating a cretaceous age for type 3 deposits type 1 veins are interpreted to be the result of hydrothermal fluids generated during the proterozoic east kootenay orogeny whereas type 2 and 3 veins and replacement deposits are interpreted to be the result of late cretaceous paleocene tectonism

a descriptive model for silver lead zinc veins in clastic metasedimentary terranes a comparative study of selected geologic features from six classic districts the kokanee range and keno hill canada coeur d alne united states freiberg and the harz moun tains germany and pibram czechoslovakia demonstrates that silver lead zinc veins in clastic metasedimentary terranes form a distinct class of mineral deposits which can be typi fied by a descriptive model these silver lead zinc veins have characteristic metal ratios and are comprised of galena sphalerite and a diverse suite of silver and sulfosalt minerals in a gangue of siderite quartz or calcite the veins are typically enclosed by spatially restricted phyllic alteration silver lead zinc veins occur in various tectonic settings the veins are hosted by monotonous sequences ofclastic rocks intruded by gabbroic to granitic plutons and metamorphosed to at least the greenschist facies the veins are late features in the tectonic evolution of an orogen commonly occur near a crustal scale fault and are not genetically related to felsic intrusions mineralization occurs at temperatures near 250 o to 300oc from dilute to saline fluids at an average depth of 6 km precipitation results from district scale mixing of up to three distinct fluids and localized boiling these fluids include a deep seated hydrothermal fluid an upper crustal fluid of ultimate meteoric origin and a late stage meteoric dominated fluid sulfur is derived from the local country rocks whereas carbon is derived from organic and or deep seated sources pb is mainly derived from the local upper crustal country rocks

strontium neodymium and lead isotopic compositions of deep crustal xenoliths from the snake river plain evidence for archean basement abstract xenoliths of intermediate to felsic granulites found in evolved lavas from the snake river plain have been analyzed for nd pb and sr isotopic compositions and related trace element contents overall they exhibit wide ranges in present day values of 87 sr 86 sr 0 70235 0 83011 143 nd 144 nd 0 51023 0 51148 and 206 pb 204 pb 13 43 24 65 the rb sr u pb th pb and sm nd decay schemes have been variably affected by granulite facies metamorphism and possibly by transport in the host magmas however pb pb and sm nd isochron systematics seemingly are preserved in many of the xenoliths and indicate essentially concordant metamorphic ages of about 2 8 ga nd model ages are significantly older ca 3 1 3 4 ga for many of the xenoliths precambrian metasediments exposed along the southern margin of the snake river plain have sm nd systematics similar to those of the xenoliths these results suggest that at least two significant thermal events occurred during archean evolution of the crust in this region a early ca 3 1 3 4 ga additions of mantle derived magmas to the crust and b regional metamorphism at 2 8 ga accompanied at least locally by magmatism concordance between the sm nd and pb pb ages suggests that these isotopic systems are little affected by high grade metamorphism in this case the distribution of xenoliths in snake river plain lavas supports the presence of archean crust beneath much of southern idaho although such rocks rarely are exposed at the surface thus nd and pb isotopic studies of crustal xenoliths can provide a useful means of determining the extent of crustal age provinces where surface exposures are lacking

convective thinning of the lithosphere a model constrained by geoid observations abstract geoid observations in the oceans suggest that lithospheric thinning is effected by small scale convection within hot spot material trapped in dimples a few tens of kilometers deep at the base of the lithosphere significant thinning can occur within 5 ma if the viscosity of the convecting material is 10 16 pa s partial melting can enhance considerably the vigour of convection and the same rate of lithospheric thinning is obtained for a viscosity about five times higher these results are derived from convection models with a realistic temperature dependent rheology using the mean field approximation

long term tectonothermal history of laramide basement from zircon he age eu correlations abstract the long term 1 ga thermal histories of cratons are enigmatic with geologic data providing only limited snapshots of their evolution we use zircon u th he zircon he thermochronology and age composition correlations to understand the proterozoic phanerozoic thermal history of archean wyoming province rocks exposed in the northern laramide ranges of western north america zircon he ages from the wind river range 54 dates and bighorn mountains 32 dates show negative correlations with effective uranium eu a proxy for radiation damage zircon dates from the bighorns are between 960 ma low eu and 20 ma high eu whereas samples from the wind rivers are between 582 ma low eu and 33 ma high eu we applied forward modeling using the zircon radiation damage and annealing model zrdaam to understand this highly variable dataset a long term t t path that is consistent with the available geologic constraints successfully reproduced age eu correlations the best fit to the wind rivers data involves two phases of rapid cooling at 1800 1600 ma and 900 700 ma followed by slower cooling until 525 ma during the phanerozoic these samples were heated to maximum temperatures between 160 and 125 c prior to laramide cooling to 50 c between 60 and 40 ma data from the bighorn mountains were successfully reproduced with a similar thermal history involving cooler phanerozoic temperatures of 115 c and earlier laramide cooling between 85 and 60 ma our results indicate that age eu correlations in zircon he datasets can be applied to extract long term thermal histories that extend beyond the most recent cooling event in addition our results constrain the timing magnitude and rates of cooling experienced by archean wyoming province rocks between recognized deformation events including the 1 ga period represented by the regionally extensive great unconformity

isotope geochemistry of ankerite bearing veins associated with the coeur d alene mining district idaho ankerite bearing veins are common and widespread throughout and adjacent to the coeur d alene mining district oxygen isotope ratios for quartz from the calcite dominant and ankerite dominant veins overlap and range from 12 to 18 per mil and average 16 2 and 15 5 per mil respectively previous studies of quartz from the ag producing veins give average ratios of 14 5 to 14 7 per mil ankerite oxygen and carbon isotope ratios range from 11 0 to 15 8 per mil and from 11 8 to 0 1 per mil respectively all but one of the calcite oxygen and carbon ratios lie in the ranges 13 0 to 16 0 per mil and 6 9 to 2 8 per mil quartz ankerite fractionations lie between 1 1 and 2 9 per mil except for samples from four massive ankerite veins suggesting equilibrium at moderate temperatures perhaps 270 to 320 c quartz calcite oxygen fractionations are scattered as are the quartz ankerite fractionations from the massive ankerite veins and clearly represent disequilibrium the high oxygen ratios suggest that the hydrothermal fluids were strongly buffered prior to entering the veins by isotope exchange with the sedimentary belt supergroup rocks that form the vein walls and perhaps by deeper archean rocks the 40ar 39ar age of fluorphlogopite from a calcite dominant vein is 1014 ma while an ankerite dominant vein yielded a minimum age of 963 ma the latter sample shows partial resetting at 100 to 120 ma these ages fall within the bimodal distribution of hydrothermal ages previously reported for the coeur d alene district one proterozoic age near 1 ga and a younger age in late cretaceous early tertiary time initial 87sr 86sr ratios in the range 0 738 to 0 787 for several of the carbonate minerals require derivation from an old source local belt wall rocks or deeper archean schists and gneisses are likely possibilities

weathering of bi bearing tennantite abstract the tennantite tetrahedrite solid solution series called fahlore cu ag 6cu4 fe zn cu hg cd 2 sb as bi te 4 s se 13 is widespread in many geological environments since it incorporates heavy metals and toxic elements a better understanding of its weathering behaviour and details of its weathering process is important to evaluate environmental risks emerging mainly from the mobilization of as sb bi and local and global metal fluxes in stream waters and oceans in this study weathering of bi rich members of this mineral group was investigated using microscopy empa sem tem la icp ms raman xrd and mla observations reveal a succession of four distinct stages of weathering during stage 1 irregular tubes within fahlore show an assemblage of nm to m sized romeite group minerals tripuhyite crystalline cu oxides and cu sulfides stage 1 textures and secondary sulfides indicate a low redox potential and a low fluid rock ratio typical of cementation zones mass balance calculations show that during this stage all zn and the majority of as and s are released to the weathering fluids bismuth is immobile in this stage and sb and fe are immobile if bi is sufficiently available from fahlore but both are partly released if the fahlore did not contain sufficient bi during stage 2 amorphous and nano crystalline arsenates replace fahlore as weathering fronts such assemblages indicate a higher redox potential than in stage 1 typical for oxidation zones mass balance calculations reveal that zn sb s and partially cu are lost arsenic and bi are immobile stage 3 occurs only locally dissolving former weathering stages and or precipitating amorphous cu arsenates silicates reflecting processes in micro compartments not characteristic for the general weathering process stage 4 is characterized by the formation of crystalline bi cu ba ca or al bearing arsenates and cu carbonates spatially independent of the precursor fahlore copper and as originate from older weathering assemblages whereas many other elements are derived externally ca ba al this stage reflects the increasing importance of the local host rock and gangue mineralogy as it is typical in near surface environments of oxidation zones or on mine dumps where elements are highly mobile and a high fluid rock ratio prevails gossan mineralization the temporal evolution of fahlore weathering textures reflects the transition from fresh ore to cementation zone oxidation zone and gossan assemblages in one hand specimen thus one fahlore grain can record the uplift and erosion and the increasing fluid rock interaction of a weathered ore deposit with time tube like textures similar to the ones observed in stage 1 have been reported from other weathering environments and from a variety of element oxide arsenide and sulfide phases they also were produced during experimental work in ilmenite this is clear evidence that the diffusive process during the first contact with alteration fluids is a basic physical process and that it is not only valid for fahlore the importance of bi to stabilize sb bearing supergene phases and thereby for the immobilization of toxic sb shown in the present work may stimulate further investigations on the environmental immobilization of toxic elements by companion metals during weathering processes

dissolution solution transfer diffusion versus fluid flow and volume loss during deformation metamorphism dissolution and solution transfer during deformation metamorphism are controlled by the partitioning of deformation into progressive shearing and shortening components progressive shearing is readily accommodated by slip on the planar crystal structure of phyllosilicates and graphite without accumulating dislocation density gradients across grain boundaries progressive shortening is accommodated by the cores of most other minerals including sulphides these minerals develop strain and hence dislocation density gradients on their rims due to progressive shearing along grain boundaries these gradients are particularly large when the mineral abuts phyllosilicate or graphite the resulting chemical potential gradients between the core and rim drive dissolution causing removal of the highly strained grain margins removal of dissolved material by solution transfer is aided by the geometry of shearing of phyllosilicates and graphite around other grains in an active anastomosing foliation interlayers and interfaces on boundaries lying at a low angle to the direction of shearing and oriented relative to the sense of shear such that they can open gape by small amounts water present in these interlayer spaces becomes destructured considerably enhancing diffusion rates along the foliation penetrative volume loss especially in deforming metamorphosing pelitic rocks is large at all metamorphic grades increasing and becoming more penetrative with depth to at least the transition into granulite and eclogite facies transference of material by fluid flow from deep to high levels in the earth s crust is precluded because thousands to tens of thousands of rock volumes of fluid are required necessitating continual recirculation of fluid from shallow to deep crustal levels in one large or several small sets of cells unless some extremely large scale form of fluid channelling is possible reassessment of diffusion mechanisms and hence rates during deformation and pervasive foliation generation in large volumes of rock where fluid channeling cannot provide enough fluid indicates that diffusion can proceed with sufficient rapidity that massive recirculation of fluid is no longer required the amount of fluid can be reduced sufficiently to allow large volume losses by a one way flow of fluid to the earth s surface in deforming metamorphosing environments where the fluid pressure equals or exceeds the hydrostatic pressure deformation partitioning controlled dissolution progressively changes the bulk chemistry of a rock containing phyllosilicates or graphite during deformation metamorphism because matrix minerals other than phyllosilicates and graphite are preferentially removed the large size of porphyroblasts if present tends to preserve them from dissolution hence the bulk chemistry operative during subsequent porphyroblast growth can have changed considerably from that operative when the first porphyroblasts grew in rocks in which bedding is still well preserved

late stage anhydrite gypsum siderite dolomite calcite assemblages record the transition from a deep to a shallow hydrothermal system in the schwarzwald mining district sw germany abstract the majority of hydrothermal vein systems of economic interest occur at relatively shallow crustal levels although many of them formed at significantly greater depths their present position is a consequence of uplift and erosion although many aspects of their formation are well constrained the temporal chemical evolution of such systems during uplift and erosion is still poorly understood these vein minerals comprise calcite dolomite ankerite siderite magnesite anhydrite and gypsum forming the last gangue assemblages in jurassic and tertiary sulphide fluorite quartz barite veins of the schwarzwald mining district sw germany mineral textures of samples from nine localities reveal that in these sequences mineral precipitation follows a recurring pattern early calcite is followed by anhydrite or gypsum siderite and or dolomite this succession may repeat up to three times in situ la icp ms u pb age dating of 15 carbonates from three subsequent generations of the late stage vein assemblage yield robust ages between 20 and 0 6 ma each mineral sequence forms in a distinctive period of about 2 5 ma these ages clearly relate these late stage mineral phases to the youngest geological episode of the schwarzwald which is associated with the cenozoic rhine graben rifting and basement uplift based on thermodynamic modelling the formation of the observed mineral assemblages required an deeply sourced mg fe and so4 rich fluid b which was episodically mixed with a shallow crustal hco3 rich fluid a as a consequence of fluid mixing concentrations of mg fe and so4 temporarily increased and initiated the formation of the observed sulphate carbonate mineral sequences this discontinuous large scale vertical fluid mixing was presumably directly related to episodes of active tectonics associated with the cenozoic strike slip regime of the upper rhine graben analogously episodic fluid mixing is a major key in the formation of older jurassic to early tertiary pb zn fluorite quartz barite assemblages in the same specific vein systems albeit involving different fluid compositions late stage hydrothermal 20 70 c vein assemblages reported in this study record the transition from deep 2 km to very shallow 0 1 km crustal conditions as a consequence of successive uplift increasing proportions of shallower and cooler 50 70 c fluids could take part in such mixing processes associated changes in the fluid composition caused the vein mineralogy to change from sulphide quartz fluorite barite to calcite anhydrite gypsum siderite dolomite as the system passively ascended closer to the surface

α emitting mineral inclusions in apatite their effect on u th he ages and how to reduce it u th rich mineral inclusions in apatite are often held responsible for erroneously old u th he ages because they produce parentless he three aspects associated with this problem are discussed here first simple dimensional considerations indicate that for small mineral inclusions the parentless helium problem might not be as serious as generally thought for example a mineral inclusion that is 10 the length width and height of its host apatite needs to be a thousand times more concentrated in u and th to produce an equal amount of he therefore single isolated inclusions smaller than a few m are unlikely to contribute significant helium for larger or more abundant inclusions the parentless helium problem can be solved by dissolution of the apatite and its inclusions in hot hf second besides creating parentless helium inclusions also complicate ejection corrections mathematical exploration of this latter problem for spherical geometries reveals that for randomly distributed inclusions the probability distribution of single grain ages is predicted to have a sharp mode at the mean age with tails towards younger and older ages multiple grain measurements will yield accurate and precise age estimates if 10 or more randomly distributed emitting mineral inclusions are present in a sample third thermal modeling indicates that mineral inclusions have a non trivial but minor 5 c effect on the closure temperature these predictions were tested on apatites from rapidly cooled migmatites of naxos greece which contain abundant u rich zircon inclusions thirty seven samples were subjected to two kinds of treatment the pooled age i e the synthetic multi grain age computed from a number of single grain analyses of four inclusion free samples 13 apatites prepared in hno3 is 10 9 ma close to apatite and zircon fission track ages from the same rock u th he ages of 14 inclusion bearing samples dissolved in hno3 range between 9 and 45 ma with a pooled age of 22 6 ma the ages of 19 hf treated samples range between 5 and 16 ma with 10 of 14 single grain samples between 9 and 13 ma and a pooled age of 10 9 ma these observations agree with the theoretical predictions and support the addition of hf treated apatite u th he dating to the thermochronological toolbox

the thermal history of the new york basement determined from 40ar 39ar k feldspar studies constraints on the thermochronologic evolution of the new york crystalline basement are mainly restricted to the high temperature early cooling history following 1 1 ga grenville metamorphism u pb methods and the late cooling history fission track methods the thermal history for the one billion years between these intervals is herein assessed using 40ar 39ar hornblende muscovite biotite and k feldspar thermochronology hornblende preferred ages suggest cooling below 450 500 c between ca 900 and 950 ma total gas ages of biotites range from 746 1060 ma and their age spectra complexity is probably related to younger thermal events and or excess argon a single muscovite analysis yields a preferred age of 854 ma argon isotope analyses of k feldspar provide a quantitative evaluation of the thermal history between 175 and 350 c multi diffusion domain thermochronology on highly variable k feldspar results yield internally consistent thermal histories and along with geologic constraints suggest thermal maxima of 275 350 c at ca 700 470 450 and 300 ma combining all of the k feldspar analyses provides an internally consistent thermal history for the region and allows for the following inferences reheating at 700 ma associated with late precambrian rifting during the formation of the iapetus ocean apparently affected the entire adirondack region with the highest temperatures occurring in the eastern part of new york local reheating in the ordovician is inferred to have affected the eastern adirondack mountains and probably resulted from the combined effects of burial during the early paleozoic emplacement of taconic thrust sheets and migration of hot fluids along normal faults during the taconic orogeny high paleotemperatures for the devonian section of eastern new york are related to maximum burial of the basement during the carboniferous at ca 300 ma

geochemistry and u pb zircon age of comenditic metafelsites of the tibbit hill formation quebec appalachians metafelsites in waterloo area quebec represent the only known silicic volcanic rocks in the predominantly basaltic tibbit hill formation low grade metamorphism accompanied by hydration and albitization has converted the felsic volcanic rocks mainly to muscovite quartz albite schists the volcanic parent of these metafelsites was formed partly as lava flows and partly as tuffs the principal compositional type was a comendite a component of intermediate rocks is also present but its extent is undetermined and probably minor u pb zircon studies of the metafelsites have yielded a reliable age of this early cambrian age is probably representative of the age of the tibbit hill formation as a whole the tibbit hill formation accumulated at one of the clearest examples of a rrr rift rift rift triple junction the sutton mountains triple junction of the continental rift system formed as a prelude to the opening of the iapetus ocean its volcanic rocks are products of the youngest major episode of rift rela

comment on geochronology of calcite filled joints southeast canada insight into late cretaceous deformation of eastern north america jennifer spalding david a schneider and julie brown

siderite mineralization of the gemericum superunit western carpathians slovakia review and a revised genetic model abstract the gemericum is a segment of the variscan orogen subsequently deformed by the alpine carpathian orogeny the unit contains abundant siderite sulphide and quartz antimony veins together with stratabound siderite replacement deposits in limestones and stratiform sulphide mineralization in volcano sedimentary sequences the siderite sulphide veins and siderite replacement deposits of the gemericum represent one of the largest accumulations of siderite in the world with about 160 million tonnes of mineable feco 3 more than 1200 steeply dipping hydrothermal veins are arranged in a regional tectonic and compositional pattern reflecting the distribution of regional metamorphic zones siderite sulphide veins are typically contained in low grade chlorite zone sedimentary volcano sedimentary or volcanic lower and upper paleozoic rocks quartz antimony veins are hosted by higher grade units biotite zone siderite sulphide veins are dominated by early siderite followed by a complex set of stages including quartz sulphide chalcopyrite tetrahedrite barite tourmaline quartz and sulphide remobilization stages the temporal evolution of these stages is difficult to study because of the widespread and repeated tectonic processes within vein replacement and recrystallization siderite sulphide veins show considerable vertical up to 1200 m and lateral up to 15 km extent and a thickness typically reaching several metres carbonate replacement siderite deposits of the gemericum are hosted by a silurian limestone belt and are similar to stratabound siderite deposits of the eastern alps e g erzberg austria based on a review of geological petrological and geochronological data for the gemericum and extensive stable and radiogenic isotope data and fluid inclusion data on hydrothermal minerals the siderite sulphide veins and siderite replacement deposits are classified as metamorphogenic in a broad sense the deposits were formed during several stages of regional crustal scale fluid flow isotope s c sr pb fingerprinting identifies the metamorphosed rock complexes of the gemericum as a source of most components of hydrothermal fluids fluid inclusion and stable isotope data evidence the participation of several contrasting fluid types and the existence of contrasting p t conditions during vein evolution a high 1 8 o medium to high salinity h 2 o type fluid is the most important component during siderite deposition whereas h 2 o co 2 type fluid inclusion containing dense liquid co 2 and corresponding to minimal pressures between 1 and 3 kbar were found in a younger tourmaline quartz stage younger quartz ankerite siderite sulphide stages are characterized by high salinity 17 to 35 wt nacl equivalent and low temperature t h 90 to 180 c h 2 o type fluids the vein deposits are interpreted as a result of multistage hydrothermal circulation with variscan and alpine mineralization phases based on available indirect data the most important mineralization phase was related to regional fluid flow during the uplift of a variscan metamorphic core complex producing siderite sulphide barite mineralization while tourmaline quartz stage and sulphide remobilization stages are related to alpine processes two phases of vein evolution are evident from two groups of 87 sr 86 sr isotope ratios of sr rich rb poor hydrothermal minerals 0 71042 0 71541 in older barite and 0 7190 0 7220 in late stage celestine and strontianite

metamorphic origin of the coeur d alene base and precious metal veins in the belt basin idaho and montana fluid inclusions geochronology and field studies show that the world class base and precious metal veins of the coeur d alene district idaho and montana are the product of deformation coupled with regional metamorphism of the belt basin around 850 ma fluid inclusion data show that the mineralizing solutions were complex co2 ch4 cnhm n2 h2o nacl fluids that changed in composition from ch4 cnhm rich fluids in the older zn rich veins to co2 rich fluids in the younger ag rich veins ore deposition occurred at about 250 to 350 c and at pressures in excess of 1 kbar

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

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

hydrochronology of a proposed deep geological repository for low and intermediate level nuclear waste in southern ontario from u pb dating of secondary minerals response to silurian and cretaceous events u pb ages have been measured on secondary dolomite and silica cements in cambrian sandstone at the base of an 840 m thick sub horizontally bedded sedimentary sequence beneath the bruce nuclear site

evidence for proterozoic and late cretaceous early tertiary ore forming events in the coeur d alene district idaho and montana new 40 ar 39 ar age spectra on sericite and lead isotope data on tetrahedrite siderite galena bournonite and stibnite together with previously published isotopic geochemical and geologic studies provide evidence for two major vein forming events in the coeur d alene district and surrounding areas of the belt basin the data suggest that the zinc and lead rich veins e g bunker hill and star morning mines formed in the proterozoic 1 0 ga whereas the silver rich veins e g silver belt mines antimony veins e g u s antimony mine and gold bearing quartz veins murry subdistrict formed in late cretaceous to early tetitary time

low temperature thermal history and landscape development of the eastern adirondack mountains new york constraints from apatite fission track thermochronology and apatite u th he dating the adirondack mountains in northern new york state form an elongate domal exposure of mainly high grade metamor phic tectonites in a mountainous setting with topographic relief of 1 km the origin of the adirondack mountains and this relief has long been enigmatic since the adirondacks presently lie within an intracratonic setting inboard of the north american passive mar gin and far from any active plate boundaries through the application of apatite fi ssion track aft thermochronology and apatite u th he ahe dating within the eastern adirondack mountains this study provides constraints on both the thermal and ero sional effects of mesozoic passage near a hot spot and the timing of relief development aft thermochronology and ahe dating record relatively stable thermal conditions within the eastern adirondacks from the middle jurassic into the early cretaceous during the early cretaceous ca 130 120 ma the region underwent heating as sociated with progressive movement near the great meteor hotspot resulting in the temporary establishment of an elevated geo thermal gradient following regional heat ing cooling rates increased considerably ca 105 95 ma likely due to both thermal doming producing an increase in erosion rate and the relaxation of isotherms after passage near the hotspot the regional distribution of aft ages across the eastern adirondacks reveals no systematic age gradient from core to periph ery which would be expected under con ditions of persistent high relief during the decay of a crustal root over many tens to hun dreds of millions of years instead thermo chronological data suggest that the present relief developed during the late cretaceous cenozoic through plateau dissection during periodic base level changes

gravity and magnetic anomalies of the sutton mountains region quebec and vermont expressions of rift volcanics related to the opening of iapetus prominent nearly coincident positive gravity and magnetic anomalies occur in the sutton mountains region centered about 100 km east of montreal quebec several lines of evidence indicate that t

mesozoic hotspot epeirogeny in eastern north america geologic data especially conodont paleotemperatures imply that southeastern canada and new england have been elevated at least 4 km relative to the central appalachian region apatite fission track ages suggest that the uplift and consequent erosion occurred in cretaceous early tertiary time consistent with the formation of a broad swell as north america moved northwestward over the great meteor hotspot

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

oxygen isotope map of the giant metamorphic hydrothermal system around the northern part of the idaho batholith u s a abstract determinations of 18o values from 100 outcrops of belt supergroup wallace fm metasedimentary rocks in the idaho panhandle reveal a regular regional pattern that was produced by pervasive fluid infiltration and isotopic exchange low grade argillites at large distances 60 km from the idaho batholith have high 18o values 15 compatible with their probable primary values pelitic rocks with anomalously low 18o values of 8 7 to 12 7 occur in the following zones 1 in a 5000 km2 zone of schist and gneiss peripheral to the idaho batholith generally coincident with high grade sillimanite bearing assemblages 2 in high grade metasedimentary roof pendants within the idaho batholith 3 peripheral to small cretaceous stocks and 4 within and near the scapolite bearing zone south west of st regis on 18o 18o plots data from coexisting minerals define trends with unit slopes indicating that the reductions in 18o occurred under high grade metamorphic conditions this metamorphism culminated in the emplacement of the idaho batholith probably as a consequence of profound crustal thickening associated with the cretaceous accretion of the wallowa seven devils arc terranes with north america the huge low 18o region is bounded by a steep 18o gradient 0 1 0 5 km that occurs in low grade rocks along and near the lewis and clark line well below the biotite isograd this boundary zone may be analogous to but is not nearly as sharp as those of meteoric hydrothermal systems in many regions the important ore deposits of the coeur d alene district are located in this peripheral zone suggesting that the metamorphic hydrothermal system may have been intimately involved in their formation in addition the metamorphic hydrotermal system in idaho is larger deeper and higher in temperature than typical meteoric hydrothermal systems and it involved fluids with much higher 18o values that were probably dominantly derived from formation waters accordingly this system produced rocks with 18o values similar to those of the idaho batholith and mineral assemblages that approach isotopic equilibrium under high temperature conditions

accounting for long alpha particle stopping distances in u th sm he geochronology 3d modeling of diffusion zoning implantation and abrasion in apatite u th he thermochronology the helium distribution in a crystal is a function of the simultaneous processes of radiogenic production thermally activated volume diffusion and the ejection of he caused by long alpha stopping distances these processes are further complicated by zonation of u th and sm within the grain and implantation of 4he from neighboring u th sm bearing minerals we use a refined version of the 3d monte carlo diffusion code of gautheron and tassan got 2010 to simulate the interplay between ejection and diffusion with or without zonation ejection and abrasion for a suite of thermal histories we examine the phenomenon of over correction produced by the alpha ejection correction parameter ft or fzac for homogeneous or heterogeneous eu repartition by comparing the raw measured and ft or fzac corrected ages for a number of scenarios to the ejection free age aef which we define as the age that would be obtained if alpha ejection had not occurred or equivalently if the stopping distance was zero we show that the use of ft or fzac corrected ages generally reproduces the ejection free age to within typical u th he uncertainties 8 even for zoned apatites we then quantify the effect of alpha implantation on u th he ages showing that implantation from a single external source with modest relative u or th enrichment can generate as much as 50 excess he for more extreme cases where an apatite is surrounded by multiple external sources the measured age can be 300 of that determined from an isolated crystal while abrasion of the outer 20 25 m can significantly reduce the age dispersion for rapidly cooled samples slowly cooled samples can still retain 10 30 excess he the removal of the rim of the crystal reduces the thermal information from very low temperatures 40 c and introduces additional technical complications and biases and should therefore be used with caution overall we demonstrate that although zonation and implantation may not be routinely determined we now have the 3d modeling capability to fully investigate and constrain the causes of age dispersion within a sample leading to significant improvement in our ability to interpret u th he data

anorogenic granites magmatic underplating and the origin of intracratonic basins in a non extensional setting abstract we present an active rift model for cratonic basin formation with crustal kinematics governed by basaltic underplating a case study of a deeply eroded continental rift belt in south finland shows close association of anorogenic intrusives and remnants of cratonic basins lower crustal thinning took place during anorogenic magmatism whereas the upper crust experienced vertical block faulting in a laterally neutral deformation facies combined analyses of field data petrological constraints and geophysical evidence allow the reconstruction of the kinematic scenario of crustal thinning this model involves the formation of an incipient crustal dome by asthenospheric upwelling associated with basaltic underplating and migration of induced lower crustal partial melts to upper crustal levels in this model light lower crustal diorite is replaced by a denser mafic layer the consequences for the post magmatic crustal history are evaluated by numerical modelling of the effects of crustal densification subsidence occurs in the absence of crustal extension suggesting that the magmatic underplate is the key element for the formation of an intracratonic basin subsidence curves for this model show an incipient nearly linear fast downwarp related to crystallization of gabbro at the moho followed by slow contraction induced by solid state cooling the predicted subsidence is similar to the pattern of the vertical movement induced by passive extension in the proposed model of continental rifting the effects of extension and underplating largely cancel out concurrent crustal extension will cause the removal of a magmatic underplate by promoting basaltic extrusion the model provides a consistent explanation for a number of key aspects of cratonic basins including their circular shape crustal configuration and evolution and their spatial distribution on continents

cumulate xenolith in oahu hawaii implications for deep magma chambers and hawaiian volcanism the maximum depth at which large 1000 km3 terrestrial mafic magma chambers can form has generally been thought to be the moho which occurs at a mean depth of about 35 kilometers beneath the continents and 8 kilometers beneath ocean basins however the presence of layers of cumulus magnesium rich spinel and olivine and intercumulus garnet in an unusual mantle xenolith from oahu hawaii suggests that this rock is a fragment of a large magma chamber that formed at a depth of about 90 kilometers hawaiian shield building magmas may pond and fractionate in such magma chambers before continuing their ascent this depth is at or near the base of the 90 million year old lithosphere beneath oahu thus rejuvenated stage alkalic magmas containing mantle xenoliths evidently also originate below the lithosphere

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

genesis of the hardat tolgoi ag pb zn deposit inner mongolia northeast china constraints from geology fluid inclusions and c o s pb isotope systematics abstract the hardat tolgoi ag pb zn deposit 4 3 mt at 3 15 pb zn and 26 4 g t ag is located in the erenhot east ujimqin polymetallic metallogenic belt on the western part of the great xing an range inner mongolia northeast china the ag pb zn mineralization mainly occurs as vein type ores hosted in argillaceous siltstone feldspar quartz sandstone with silty slate interlayers of the aobaotinghundi formation of lower devonian the hydrothermal ore forming process can be divided into three stages i e quartz pyrite arsenopyrite stage i quartz polymetallic sulfides stage ii and quartz calcite fluorite stage iii four types of fluid inclusions have been identified in the quartz bearing veins of each stage i e liquid rich two phase l type single liquid phase pw type vapor rich two phase v type and pure vapor phase pv type inclusions the fluid inclusions of stage i to iii yield homogenization temperatures of 285 407 c 202 317 c and 144 239 c with salinities of 1 2 9 2 wt nacl equiv 0 9 5 0 wt nacl equiv and 0 2 2 6 wt nacl equiv respectively the ore forming fluids at hardat tolgoi are characterized by moderate temperatures low salinities and low densities and dominated by the h2o nacl system carbon and oxygen isotope compositions of calcite demonstrate that the ore forming fluids come from a primary magmatic hydrothermal system with subsequent involvement of meteoric water at a later stage sulfur isotope analyses of sulfides yield 34sv cdt values of 1 8 to 8 7 with calculated 34sh2s values of 1 4 to 9 4 the 206pb 204pb 207pb 204pb and 208pb 204pb ratios of sulfides range from 18 141 to 18 468 15 521 to 15 617 and 38 040 to 38 377 respectively similar to those of granites from the hardat tolgoi deposit both sulfur and lead isotope data indicate that the ore forming metals were most likely derived from the magmatic reservoirs and sedimentary country rocks the available fluid inclusion results and isotope studies indicate that the pahse separation temperature decrease and fluid mixing resulted in large scale ag pb zn ore formation at hardat tolgoi

geologic transect across the grenville orogen of ontario and new york revised cross sections of the western grenville province incorporate new geologic results and reprocessed seismic reflection data the geology is presented in terms of three tectonic elements 1 pre grenvillian laurentia and its margin with ca 1740 and 1450 ma continental arc plutons and associated supracrustal rocks 2 composite arc belt of allochthonous 1300 1250 ma volcanic arcs and sedimentary rocks and 3 frontenac adirondack belt characterized by supracrustal and granitoid rocks and anorthosites of uncertain affinity that may represent a distinctive part of the composite arc belt or an offshore micro continent rocks of the composite arc and frontenac adirondack belts were amalgamated with each other by ca 1160 ma were then thrust over laurentia during ca 1080 1035 ma and ca 1010 980 ma phases of convergence and were dissected and exhumed by 1040 ma normal faults penetrative deformation was restricted to that part of the pre grenvillian laurentian margin that lies to the southeast of the grenville front and parts of the accreted composite arc and frontenac adirondack belts the laurentian rocks in the grenville province are bounded to the northwest and southeast by southeast dipping ductile thrust and or normal shear zones the composite arc and frontenac adirondack belts to the southeast are bounded by ductile and brittle ductile thrust and or normal faults that separate domains with contrasting cooling histories despite a long pre grenvillian tectonic and plutonic history the present crustal architecture and much of the seismic reflectivity were acquired during 1080 980 ma phases of compression and extension

timing of anatexis in the eastern adirondack highlands implications for tectonic evolution during ca 1050 ma ottawan orogenesis u pb sensitive high resolution ion microprobe shrimp analyses of zircons from migmatitic metapelites in the eastern adirondack highlands demonstrate that substantial anatexis took place ca 1050 ma during the late but still high grade ca 1090 1050 ma ottawan orogeny these results contrast with data from metapelites of the southwestern adirondack highlands and adirondack lowlands which indicate that anatexis occurred ca 1200 1160 ma during the shawinigan orogeny and subsequent ca 1155 ma anorthosite mangerite charnockite granite amcg magmatism ca 1180 1150 ma zircons from this western regime do not contain ca 1050 ma ottawan metamorphic overgrowths the absence of ca 1050 ma ottawan anatexis and overgrowths in the adirondack lowlands is explained by the observation that consistent with titanite cooling ages post 1150 ma temperatures did not exceed 700 c in the southwestern adirondack highlands the absence of ca 1050 ma anatexis can be accounted for by earlier dehydration of metapelites during ca 1180 1150 ma shawinigan amcg metamorphism the occurrence of ca 1050 ma anatexis and formation of metamorphic zircons in the eastern adirondacks cannot be explained by higher temperatures because geothermometry indicates that all of the adirondack highlands reached a nearly uniform 750 800 c during this period accordingly the occurrence of ca 1050 ma ottawan anatexis in the eastern regime is interpreted to be the result of 1 infl uxes of hydrous fl uids at ca 1050 ma or 2 decompression melting during late extensional exhumation a recently recognized low angle late ottawan ca 1050 ma fault system may have provided both fl uid conduits and footwall decompression the sense of displacement along the shear zone has not yet been unequivocally determined but preliminary investigations of kinematic indicators demonstrate normal displacement thus this structure may be an analogue of the ca 1050 ma northwest dipping carthage colton zone normal fault system that juxtaposed the adirondack lowlands against the adirondack highlands in this context these fault zones form a quasi symmetrical core complex or gneiss dome within which the high grade core of the adirondack highlands was tectonically exhumed a similar east dipping along strike normal fault in quebec tawachiche shear zone exhumed the eastern morin and lac taureau terranes at ca 1050 ma and may merge with the eastern adirondack shear zone described here

40ar 39ar ages for plutons of the monteregian hills quebec evidence for a single episode of cretaceous magmatism 40ar 39ar data for biotite and amphibole from six plutonic complexes of the monteregian petrographic province in quebec canada are presented generally uncomplicated and concordant incremental heating spectra are observed although there is evidence for minor excess 40ar at several localities biotites and amphiboles are concordant consistent with rapid cooling of the high level complexes ages for different phases of individual complexes are analytically indistinguishable and suggest that the various principal lithologies are cogenetic the 40ar 39ar ages of six monteregian complexes st hilaire rougemont johnson shefford brome megantic all fall within the very restricted range of 124 1 m y it is suggested that all of the monteregian plutons with the exception of oka were formed within a short interval 1 2 m y during the cretaceous consistent with paleomagmetic results the new ages along with geochemical characteristics support derivation of the alkaline magmas by a fixed mantle hot spot which also produced broadly similar complexes in new england and the chain of new england seamounts

the permeability controlled accumulation of primary magma in a planetary mantle abstract the initial accumulation of magma within a partially molten planetary mantle occurs in three stages within a mantle with evenly dispersed minerals the first melt is formed as small drops at grain corners and edges by increasing degree of partial melting the drops of interstitial liquid become interconnected whereafter the partially molten mantle becomes permeable at the onset of permeability the interstitial melt begins to accumulate in horizontal layers of melt situated tens of metres apart these layers form the source of magma for the magma chambers that supply magma during eruptions

regional scale mid jurassic to late cretaceous unroofing from the adirondack mountains through central new england based on apatite fission track and u th he thermochronology abstract apatite fission track aft ages of 178 to 70 ma for 108 samples from the adirondack mountains and eastern new york state vermont western massachusetts and connecticut and western new hampshire indicate that widespread unroofing occurred throughout the region during the middle jurassic to late cretaceous additionally u th he ages of 167 to 85 ma were determined on nine of those samples and support cooling rates of 1 8 2 2 c m yr from 100 to 65 c during the middle jurassic to late cretaceous assuming a geothermal gradient of 25 c km unroofing rates for the region were estimated to be on average 0 07 0 08 km m yr the aft age discontinuities between the high peaks region and the fault dissected southeastern adirondack mountains and along the ammonoosuc fault in the connecticut river valley between vermont and new hampshire suggest differential unroofing that may have been accommodated by fault reactivation syn post late cretaceous time after the youngest aft ages 82 and 70 ma resp

extraction of melt from veined mantle source regions during eruptions abstract harzburgite is the predominant type of peridotite in the mantle sections of ophiolite complexes the residual character of harzburgite suggests that the magma extraction is efficient and that harzburgite form a residual layer above a subsequent mantle source this residual layer form an upper lid for subsequent magma accumulation a theoretical estimate of the depletion rate of a veined mantle source region show that it is possible for a veined source region to become nearly completely depleted for melt within a year this period of time is comparable to the durations of abyssal and subaerial volcanic eruption episodes which varies from a few months to a few years

the permeability controlled accumulation of primary magma the initial accumulation of primary magma occurs just after the mantle has become permeable the accumulation is caused by the compaction of the residuum which either may be controlled by the rate of creep or by the rate of flow of the interstitial melt experimental results suggest that the rate of compaction is controlled by the permeability and a model for the accumulation process is worked out on this basis the compaction causes the formation of a lower compaction boundary and an upper layer of melt the ascending mantle of plumes and convection currents will form layers of melt situated 20 100 m apart the type of partial melting for this accumulation is critical melting

thermal evolution and physics of melt extraction on the ureilite parent body we develop a physical model of the thermal history of the ureilite parent body upb that numerically tracks the history of its heating hydration dehydration partial melting and smelting as a function of its formation time and the initial values of its composition formation temperature and water ice content petrologic and chemical data from the main group non polymict ureilite meteorites which sample the interior of the upb between depths corresponding to pressures in the range 3 10 mpa are used to constrain the model we find that to achieve the 30 melting inferred for ureilites from all sampled depths the upb must have had a radius between 80 and 130 km and must have accreted about 0 55 ma after cai formation melting began in the body at 1 ma after cai and the time at which 30 melting was reached varied with depth in the asteroid but was always between 4 5 and 5 8 ma after cai the total rate at which melt was produced in the upb varied from more than 100 m 3 s 1 in the very early stages of melting at 1 ma after cai to 5m 3 s 1 between 2 and 3 ma after cai decreasing to extremely small values as the end of melting was approached beyond 5 ma although the initial period of high melt

olivine crystallographic control and anisotropic melt distribution in ultramafic partial melts in the upper mantle physical properties permeability electrical conductivity and seismic velocity are highly dependent on the melt geometry both flat and curved melt interfaces exist in olivine basalt partial melts to further understand anisotropic wetting behavior of partial melting within the mantle crystal orientation studies have been conducted to determine the miller indices of flat interfaces wetted by the melt in olivine basalt partial melt samples the samples were equilibrated at pressures of 1 2 gpa and temperatures of 1350 1400 c for 142 549 hours crystal orientation studies show that lower miller indices of crystals are associated with wetted flat melt interfaces and melt films specifically 010 110 and 021 olivine faces in these studies were dominantly wetted indicating that they may be potentially large contributors to permeability and seismic anisotropy in the upper mantle where partial melts occur with high degrees of preferred crystallographic alignment

tertiary meteoric hydrothermal systems and their relation to ore deposition northwestern united states and southern british columbia tertiary meteoric hydrothermal systems have altered the rocks exposed over more than 5 of the land surface of the northwestern united states and southern british columbia including at least 25 000 km 2 in idaho the systems typically involved convective circulation of fluid derived from ordinary meteoric groundwaters around crystallizing calc alkaline epizonal plutons emplaced into coeval volcanic cover rocks these individual systems had widely ranging lifetimes of 10 3 to 10 6 years and operated locally throughout the cenozoic although the most profound development of such activity occurred during eocene time individual systems varied in size from a few tens of square kilometers yankee fork idaho or less to several thousand square kilometers sawtooth and castro ring zones idaho typically regional propylitization aacompanied the fluid circulation although the higher temperature alteration assemblages were developed locally as were intense alteration effects e g silicification sericitization etc near some veins and in mining districts a significant amount probably 25 50 of the mineral production and potential in the region is closely related to tertiary meteoric hydrothermal systems oxygen and hydrogen isotopic data clearly demonstrate the close geologic association of meteoric hydrothermal systems and mineralization in 1 the paleocene cu zn pb mn main stage mineralization at butte montana 2 numerous eocene epithermal deposits principally valued for au and ag but also including significant deposits of cu pb zn f sb etc as at republic washington and in several mining districts in the idaho batholith and the challis volcanic field 3 several eocene skarn deposits valued for w ima idaho and cu mackay idaho 4 important lead silver vein and replacement deposits of tertiary bluebell british columbia and of probable cretaceous and early tertiary age wood river idaho 5 several potentially economic mo be and u bearing eocene porphyry plutons and 6 miocene epithermal deposits most prominently the au and ag bearing veins at silver city and delamar idaho the hg deposits at the mcdermitt caldera nevada and oregon and at weiser idaho and au deposits in the western cascade range and lake county oregon a close spatial association has been demonstrated between ore deposits and rocks having anomalous 18 o values and low d values the most important deposits are associated with relatively small generally 5 300 km 2 zones of low 18 o values and they are particularly closely linked with zones of very steep 18 o 16 o gradients in the altered rocks these associations hold much promise for the use of 18 o and d contour maps in future exploration efforts

methane rich fluid inclusions in the proterozoic zn pb ag deposit at dugald river nw queensland potential as an exploration guide abstract calculated isochores determined from microthermometry and laser raman microprobe analysis for ch 4 inclusions hosted in quartz grains from 3 different ore types in the dugald river zn pb ag deposit yield fluid trapping p t conditions of 300 350 c and 0 8 2 3 kbar for post depositional metamorphic conditions of sulfide mineralization the consistent relationship between fluid composition and metamorphic grade of the host rocks suggests that ch 4 has been formed by reaction of graphite with aqueous fluids during metamorphic retrogression even though fluids responsible for mineralization are generally similar to those for barren quartz veins in chemistry and microthermometry the substantial contents of ch 4 and co 2 in the dugald river lode inclusions contrast sharply with co 2 alone in barren vein inclusions the presence of ch 4 rich inclusions in the mineralization at dugald river and other similar deposits and their absence in the barren veins in a regional context can be regarded as a possible exploration guide for defining target areas hosting mineralization of this style

microbial community signature in lake coeur d alene association of environmental variables and toxic heavy metal phases national science foundation under grant no 0628258 defense university research instrumentation program durip contract number w911nf0510255 msu thermal biology institute from the nasa exobiology program project nag5 8807

eocene plutonic rocks in south central idaho large areas previously thought to be underlain by cretaceous granodiorite of the idaho batholith are now known to be underlain by plutonic rocks of the eocene challis magmatic episode 50 44 ma recent mapping conducted in south central idaho as part of the u s geological survey s conterminous united states mineral assessment program cusmap indicates that roughly 30 of the exposed plutonic rocks are part of this younger event the eocene plutonic rocks are subdivided into a pink granite suite consisting of varieties of biotite granite and a quartz monzodiorite suite which includes hornblende biotite granite granodiorite quartz monzodiorite diorite and gabbro the members of each suite form discrete epizonal stocks and some exposures of pink granite are of batholithic size silica content of the quartz monzodiorite suite ranges from 48 to 70 the rocks vary considerably in texture and mineralogy the most mafic rocks of the quartz monzodiorite suite are cumulates these are layered intrusive phases containing pyroxene olivine and plagioclase other mafic phases include hornblende and pyroxene rich diorites which contain plagioclase feldspar as an interstitial phase and are cumulates in the sense that they probably represent rocks that were enriched in early formed mafic minerals by some fractionation process stocks of the pink granite suite are more compositionally restricted and range from 70 to 77 in sio2 content the quartz monzodiorite suite appears to be older than the pink granite suite but age data are limited field relations initial 87sr86sr ratios cao content and mgo mgo feo ratios demonstrate that the two suites are not related by fractional crystallization rather the respective magmas probably originated from different sources large ranges in mgo sc cr and co content in the quartz monzodiorite suite can be explained by crystal liquid fractionation of olivine pyroxene and hornblende but variations in composition of parental magmas are also likely large variations of ba content in the pink granite suite are probably a result of removal or accumulation of variable amounts of k feldspar mixing between the two eocene suites has not been extensive but some varieties of hybrid pink granite were generated that contain mafic inclusions and have less than 72 sio2 the pink granite has geochemical affinities with a type granite but unlike typical a type granite it is not uniformly enriched in the highly charged cations such as zr nb y and ce by analogy to other a type granites the pink granite suite probably originated from partial melting of lower crustal rocks under high temperature vapor absent conditions

oxygen carbon and strontium isotope geochemistry of the sunshine mine coeur d alene mining district idaho the pb ag ores of the sunshine mine located in the coeur d alene mining district of northern idaho occur within steeply dipping tabular west northwest trending veins of siderite gangue that crosscut the overturned northern limb of the big creek anticline the delta 13 c pdb 9 9 to 6 4 and delta 18 o smow 13 1 17 7 values of the siderites vary in a proportion of about 1 2 but siderites with delta 18 o or 16 exhibit a slope close to the 1 4 trend expected for a system with variable temperature whereas no correlation is evident for siderites with delta 18 o 16 the delta 18 o values of crosscutting quartz veins 13 4 15 5 and the surrounding wall rocks 10 7 15 5 are several per mil too low to be in isotopic equilibrium with the siderite at the temperature of approximately 325 degrees c determined from fluid inclusion studies at this temperature the calculated delta 18 o values for the precipitating fluids range from 7 8 to 12 4 for the siderite and 6 9 to 8 9 for the quartz the high delta 18 o values calculated for these fluids reflect their exchange with the high 18 o metasedimentary rocks of the belt supergroup in higher temperature fluid source regions detailed sample traverses across two 1 to 3 m wide siderite veins show that 1 some veins are composite containing materials deposited at different times or in different places which were juxtaposed by subsequent vein parallel faulting 2 siderite in structurally simple veins is most enriched in 13 c and 18 o adjacent to the walls resulting in u shaped spatial trends and 3 the delta 18 o values of metasedimentary wallrocks are substantially lower than those of the siderite in the veins and increase adjacent to them these small scale variations account for the lack of a simple relationship between delta 18 o and depth in the vein systems these data also indicate that the vein fluids were hotter and considerably higher in 18 o than the fluids in equilibrium with adjacent wall rock the progressive 18 o decrease from the margins to the centers of the siderite veins further decreasing into the early crosscutting quartz veins probably represents a progressive increase in the temperature of deposition but could also represent variations in fluid source in an evolving system the dispersion of delta 12 c values for samples with delta 18 o 16 per mil may be the result of late stage explosive pressure release and co 2 effervescence during fluid decompression when fissure filling was nearly complete the structural simplicity of the veins and the primary hydrothermal character of the stable isotope variations argue against postdepositional deformation and metamorphism stable isotope and strontium isotope data support the interpretation that ore deposition occurred during the late cretaceous or early tertiary as the result of metamorphic hydrothermal processes associated with regional compression and the formation of major granitic batholiths

chemical and pb isotope composition of phenocrysts from bentonites constrains the chronostratigraphy around the cretaceous paleogene boundary in the hell creek region montana an excellent record of environmental and paleobiological change around the cretaceous paleogene boundary is preserved in the hell creek and fort union formations in the western williston basin of northeastern montana these records are present in fluvial deposits whose lateral discontinuity hampers long distance correlation geochronology has been focused on bentonite beds that are often present in lignites to better identify unique bentonites for correlation across the region the chemical and pb isotopic composition of feldspar and titanite has been measured on 46 samples many of these samples have been dated by 40ar 39ar the combination of chemical and isotopic compositions of phenocrysts has enabled the identification of several unique bentonite beds in particular three horizons located at and above the cretaceous paleogene boundary can now be traced based on their unique compositions across the region clarifying previously ambiguous stratigraphic relationships other bentonites show unusual features such as pb isotope variations consistent with magma mixing or assimilation that will make them easy to recognize in future studies this technique is limited in some cases by more than one bentonite having compositions that cannot be distinguished or bentonites with abundant xenocrysts the pb isotopes are consistent with a derivation from the bitterroot batholith whose age range overlaps that of the tephra these data provide an improved stratigraphic framework for the hell creek region and provide a basis for more focused tephrostratigraphic work and more generally demonstrate that the combination of mineral chemistry and pb isotope compositions is an effective technique for tephra correlation

axial and off axial heterogeneity of basaltic rocks from the east pacific rise at 12 35 n 12 51 n and 11 26 n 11 30 n surface ship operations and 40 submersible dives have provided a large amount of field observations and rock samples from two segments of the epr at 12 35 n 12 51 n and at 11 26 n 11 30 n and from four adjacent seamounts centered at less than 18 km from the rise axis the basaltic samples show a great variety of morphological features and a diversity in the proportions of early formed mineral phases and chemical composition there is no correlation between lava flow morphology geological settings and compositions based on mineral and compatible element composition these basalts were classified into three types the least evolved olivine basalts and highly phyric plagioclase basalts type a have high fo87 89 high mg 66 70 and high ni 100 contents the most evolved plagioclase olivine basalts with or without pyroxene type b and c have relatively low fo78 86 low mg 55 65 and low ni 60 100 ppm contents simple crystal fractionation 3 18 accounts for some of the observed compositional range however based on their incompatible element contents the above basalts are divided into three groups the various types a to c are found in each group depleted basalts with low k ti 0 04 0 15 ratios low zr 100 ppm and low nb 0 4 ppm contents undepleted basalts with the highest k ti 0 25 0 46 zr 150 170 ppm and nb 8 16 ppm contents and the transitional basalts having compositions between these two extremes these three basaltic groups occur within limited portions of the axial graben and its limbs in areas of less than 10 km in length and less than 3 4 km in width near 12 43 n 12 51 n on the epr similar volcanic diversity in a more limited area 3 km2 is observed on off axial seamounts and other constructional features centered up to 6 and 18 km from the rise axis depleted and undepleted melts are believed to have been formed during sequential batch melting of a composite mantle the transitional basalts probably result from a mixing of depleted and undepleted liquids which have undergone variable degrees of crystal fractionation two distinct mantle sources are distinguishable by their incompatible element contents and their susceptibility to melting we suggest that a lherzolitic mantle with minerals having a variable behavior under partial melting constitutes this type of composite mantle indeed clinopyroxene enriched in incompatible elements will be consumed at an earlier stage of melting than the olivine plus orthopyroxene eruption of compositionally distinct magma batches in close proximity on axial and off axial structures implies periods of quiescence between successive magmatic stages and cycles

wustite in a hydrothermal silver lead zinc vein lucky friday mine coeur d alene mining district u s a the coeur d alene mining district of northern idaho is one of the major ag pb zn producers in the world it contains ag pb zn replacement veins in clastic burial metamorphic rocks of the middle proterozoic belt supergroup electron microprobe analyses and raman spectroscopy of samples from the gold hunter vein of the lucky friday mine revealed the presence of the rare iron oxide wustite fe1 xo coexisting with stable magnetite an assemblage of galena pyrite and siderite is associated with the two iron oxides although these minerals do not share phase boundaries with wustite and their coexistence is therefore thermodynamically precluded wustite is stable under high temperature conditions above 570 c at 1 bar increasing pressure can lower this temperature significantly however the veins in the coeur d alene district formed well outside the wustite stability field and represent a low temperature 250 350 c low pressure 1 3 kbars and low f o2 environment to form wustite at temperatures around 250 350 c a pressure increase of at least 70 kbars would be required this corresponds to burial of around 250 km which is geologically unrealistic similarly to form wustite the temperature must have exceeded 570 c which exceeds the maximum temperatures of vein formation by at least 200 c a temperature increase of that scale is not reflected in the mineral assemblage of the veins or the host rocks which both contain siderite we conclude that wustite formed as a metastable phase at 250 350 c and pressures below 100 bars because of the low f o2 conditions in the coeur d alene district

radiation damage and uranium concentration in zircon as assessed by raman spectroscopy and neutron irradiation radiation damage of natural and synthetic zircon grains is evaluated by raman spectroscopy to understand annealing and stability of fission tracks analyses focus on a suite of 338 paleozoic detrital zircon grains from metamorphosed strata in the hellenic forearc that were variably annealed by a miocene thermal event as well as a suite of 97 synthetic zircon grains the raman wavenumber shift of v 3 si0 4 reveals that radiation damage and damage distribution in this suite mainly depends on uranium concentration in zircon with similar uranium concentration the raman wavenumber shift allows for the determination of radiation damage in different crystals which is a function of effective accumulation time nine detrital zircons grains with moderate radiation damage were stepwise annealed at 1000 and 1400 c which resulted in progressive removal of radiation damage revealed in an increase of 3 si0 4 peak positions for a partly reset sample that was brought to temperatures of 350 c in a geologic setting hellenic forearc we use the raman measurements and uranium determination to estimate a zircon damage discrimination factor z rdd which is our attempt to estimate only radiation damage in single grains by accounting for affects of the uranium atom in the raman wavenumber this discrimination allows for a separation of zircon fission track zft ages of single ages based on grains that have a low track retention high damage fully reset grain thus refining the age determination of cooling in a rock that shows variable resetting

circum iapetus paleogeography of the precambrian cambrian transition with a new paleomagnetic constraint from laurentia abstract the paleogeography of the precambrian cambrian transition is still poorly known but is fundamental for understanding late neoproterozoic climate extremes and the spatial associations of the rapidly evolving ediacaran and cambrian fauna this period has also been proposed to host unusual geodynamic events such as rapid plate motions and or the bulk tumbling of the earth with respect to its spin axis called true polar wander tpw new paleomagnetic and 40ar 39ar geochronologic results have been obtained from a pair of shallowly emplaced syenitic intrusions in western quebec the mont rigaud and the chatham grenville stocks are related to a failed rift arm of the iapetus ocean both intrusions bear hornblendes which provide overlapping 40ar 39ar plateau ages of 533 2 1 1 ma mont rigaud and 531 4 3 4 ma chatham grenville corresponding to early cambrian nemakit daldynian time characteristic paleomagnetic reversed directions from both intrusions are easterly and shallow d 89 3 i 30 1 95 6 2 k 38 8 n 15 sites and the mont rigaud intrusion also carries antipodal normal polarity directions the combined result yields a north paleopole at location 11 9 s 184 6 e dp 3 8 dm 6 9 placing the montreal region at 16 4 s paleolatitude in the early cambrian in good agreement with late ediacaran and middle to late cambrian paleomagnetic results from laurentia laurentia likely resided at low southerly paleolatitudes throughout the cambrian ruling out a major inertial interchange tpw event for early to middle cambrian time a comparison of late ediacaran to middle cambrian paleomagnetic results from laurentia baltica siberia and elements of west gondwana implies that by early cambrian time a large paleolatitudinal gap existed between laurentia and its presumed conjugate rift margins thus placing a wide iapetus ocean between them the position of baltica within the circum iapetus paleogeography is contentious due to differing interpretations of the few paleomagnetic results during ediacaran time laurentia underwent large apparent polar wander that is manifested as motion from low latitude at 615 ma to the south polar region at 590 575 ma and then rapidly back to low latitude by 565 550 ma the large and perhaps rapid apparent polar wander of laurentia and other continents in the ediacaran remains enigmatic and controversial

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

constraining compositions of hydrothermal fluids in equilibrium with polymetallic ore forming sulfide assemblages to provide an improved understanding of the genesis of high temperature ag pb zn ore deposits we have obtained revised estimates for the thermodynamic properties for cu ag 10 fe zn 2sb4s13 fahlore endmember components calculations of aqueous solution mineral equilibria 250 300 c for fahlore bearing assemblages from ag pb zn ore deposits from the coeur d alene idaho united states and keno hill yukon territory canada mining districts demonstrate consistency between predicted and observed mineral assemblages and hydrothermal fluids based on these new estimates these calculations confirm that much of the ag mined from the coeur d alene district was originally present in galena they also confirm that sphalerite was typically undersaturated in fluids crystallizing fe rich fahlores in the coeur d alene ag pb zn ores and constrain the parameters of the fluids equilibrated with these ores to within narrow limits for the keno hill deposits these calculations demonstrate that the early high temperature 300 310 c fluids 0 07 0 7 in pb2s2 agsbs2 galena or their compositionally equivalent sulfosalt assemblages diaphorite pb2ag3sb3s8 miargyrite agsb pb2 s2 or diaphorite pb2ag3sb3s8 frieslebenite pbagsbs3 were deposited initially in ag rich fahlore bearing ores these minerals were replaced by ag poor galena and epithermal ag minerals that include acanthite stephanite and polybasite

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

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

age and origin of base and precious metal veins of the coeur d alene mining district idaho ore bearing quartz carbonate veins of the coeur d alene mining district yield 87sr 86sr ratios of 0 74 to 1 60 for low rb sr carbonate gangue minerals similar to current ranges measured in middle proterozoic high rb sr rocks of the belt supergroup stable isotope and fluid inclusion studies establish a genetic relationship between vein formation and metamorphic hydrothermal systems of the region these extraordinary 87sr 86sr ratios require accumulation of radiogenic 87sr in a high rb sr system over an extended period prior to incorporation of sr into the hydrothermal veins evaluation of the age and composition of potential sources of highly radiogenic sr indicates that the ore bearing veins of the coeur d alene district formed during the cretaceous from components scavenged from rocks of the belt supergroup the primary host rocks of the district proterozoic pb isotope ratios observed in galena from many coeur d alene veins were established when pb separated from uranium during deposition or diagenesis of the belt supergroup at 1400 to 1500 ma possibly as disseminated syngenetic deposits k ar and rb sr apparent ages and 18o values of belt supergroup rocks decrease from the coeur d alene district toward the idaho and kaniksu batholiths approximately normal to the trends of metamorphic isograds fold axes foliation and the major reverse faults of the district isoclinal folding thrust faulting high temperature metamorphism granitic plutonism and regional scale metamorphic hydrothermal activity is documented in the region between 140 and 45 ma representing the only such combination of events in the coeur d alene region subsequent to about 1300 ma the sr and oxygen results and geologic evidence favor formation of the ore bearing carbonate veins by fluids related to a complex metamorphic hydrothermal system during the cretaceous pb with proterozoic isotopic compositions was probably mobilized and incorporated like other metals into the hydrothermal veins during this event the ore bearing veins were sheared and displaced during early tertiary northwest trending dextral strike slip faulting along the osburn fault and related structures of the lewis and clark line

magma accumulation in the ascending mantle summary the most important factors which control the accumulation of primary magma in the mantle are 1 the degree of partial melting required for permeability which is suggested as10 20 2 the creep properties of the partially molten mantle the type of creep probably being nabarro herring creep 3 the type of flow of the ascending mantle material the flow may be parallel or divergent dependent on the depth 4 the depth of extension of the feeder dykes depending on the tensional stress distribution with depth and the rheological properties of the mantle it is shown that the partially molten mantle becomes stratified into layers of magma and residuum the layers of residuum being one or several metres thick the stratified mantle constitutes the primary source of magma a dispersed source system is formed in ascending convection currents while a large magma chamber is formed in ascending plumes the small diffusion rates imply that the melting is of a fractional type rather than batch melting