volcanogenic nutrient fluxes and plant ecosystems in large igneous provinces an example from the columbia river basalt group research from biological and geological sources has highlighted the role of volcanoes in the outgassing of p and thermal fixation and subsequent atmospheric oxidation of no x in volcanic environments the impact of these nutrient fluxes on biological systems has been demonstrated on present day hawai9i and here we consider the impact on the plant communities within a large igneous province lip the miocene columbia river basalt province the youngest lip on earth contains many sedimentary interbeds between the flows of the major extrusive phase and these interbeds preserve variable but often diverse palynofloras by integration of palynofloral analysis with analysis of macronutrient levels in the interbeds it can be suggested that there may have been significantly elevated levels of p within the lava field proximal to the source vents a distribution mirrored by ca and mg evidence for potential volcanogenic eutrophication is restricted to 10 3 year duration interbeds contemporary with eruptive activity elsewhere in the lip the geochemistry and palynology of other interbeds demonstrate nutrient deficiency with a potential route to nutrient sufficiency available from long term symbiotic n fixation elsewhere within the columbia river basalt province this process is short circuited by the imput of felsic ash from the nearby cascades range of volcanoes

a different perspective on glauconite as a chronometer for geologic time scale studies there are advocates of the use of glauconite as a chronometer for geologic time scale studies who insist that if the geohistory of the sample in question is known in detail one can ascertain before hand that the sample will yield a radiometric result that is accurate in the geological sense examples are presented indicating that while some glauconite samples may yield the correct age within their assigned analytical uncertainties when compared to high temperature minerals others clearly do not and are too young by varying degrees the outright acceptance of published results on glauconite can result in erroneous age estimates of various biostratigraphic levels and stage or epoch boundaries

laser probe 40ar 39ar dating studies on sub milligram whole rock basalt samples the age of the steens mountain geomagnetic polarity transition revisited the steens mountain geomagnetic polarity transition is arguably the most comprehensively studied geomagnetic field reversal trapped in igneous rocks over twenty years ago this reversal was dated at 15 5 0 3 ma by the kar method we have now tested the feasibility of carrying out 40ar 39ar dating on 1 mg samples of fine grained whole rock basalt utilizing a laser probe i e on samples 1000 times smaller than have been traditionally analyzed twenty samples taken from lava flows below within and above the reversal n r in steens mountain oregon have been studied by this laser incremental heating technique eighteen of these specimens yield acceptable plateau ages with a weighted mean age of 6 2 0 1 ma standard error on the mean this result is in excellent agreement with the results obtained on two bulk samples of whole rock basalt that average 16 1 0 1 ma an attempt to confirm these ages by fusion of plagioclase laths taken from a few of the individual lava flows was hampered by their low k content 0 2 0 3 our best estimate for the age of six plagioclase laths each 1 mg weight is 16 5 0 5 ma we suggest the techniques utilized herein could be extended to carry out dating of basaltic material of age 1 ma

stratigraphy composition and form of the deccan basalts western ghats india in the western ghats between latitudes 18 20 n and 19 15 n 7000 km2 of deccan basalt have been mapped with the primary objective of establishing a flow stratigraphy as a guide to the volcanic history of the flood basalts using over 70 measured vertical sections major and trace element analyses of nearly 1200 samples and rare earth and87sr 86sr determinations for over 60 samples we divide the basalt into three subgroups and ten formations in this paper we describe the seven principal formations in the area and the most prominent individual flows

a new calibration point for the late miocene section of the geomagnetic polarity time scale 40ar 39ar dating of lava flows from akaroa volcano new zealand recent dating of geomagnetic field reversals over the past 2 ma by both the radiometric and astrochronologic techniques indicates that the k ar derived ages for these reversals are too young we report herein on 40ar 39ar incremental heating study of two geomagnetic field reversals trapped in lavas on akaroa volcano new zealand that demonstrate that the k ar derived ages for field reversals at 10 9 ma are too young by about 7 the new age for the termination of chron 5 a critical tie point on the geomagnetic polarity time scale for late miocene time is 9 67 0 11 ma

thermal response of a moving lithosphere over a mantle heat source the mid ocean bathymetric swells observed around some volcanic island chains can be modeled using the thermal conduction equations of an oceanic lithosphere moving over a heat source located at the base of the plate some previous studies of this problem have suggested that the heat source must lie well within the lithosphere in order to explain the observed bathymetry and surface heat flow these conclusions are based on an isothermal boundary condition at the base of the lithosphere but this choice requires that the heat source is maintained above this boundary and can actually lead to a flux of heat back into the mantle particularly in the vicinity of the heat source if the lower boundary is insulated insofar as the anomalous temperature field is concerned flux from the heat source back into the mantle is prevented resulting in a more rapid uplift of and in an increased heat flow across the seafloor previous solutions have considered only the steady state solutions whereas the present model also evaluates transient solutions for rapidly moving oceanic plates the transient effects can be neglected provided that the heat source has been active for at least 107 years and that the direction of motion and the intensity of the heat source have remained constant in this interval solutions with the heat source located near the base of the conductive layer of the conduction equations for heat flow bathymetry and geoid heights in the spatial domain reproduce the principal observational characteristics of the hawaiian swell at distances away from the immediate origin of the swell and this part of the swell can be used to estimate the thermal conduction model parameters the model does not wholly explain the observations near the origin and the mismatch may reflect a dynamic component in the support of the swell

magnetostratigraphy of the eldorado mountains volcanic complex and the calibration of the early to middle miocene polarity time scale 1 magnetic polarity measurements from a well dated 15 6 13 7 ma volcanic section in southern nevada allow resolution of controversies in the calibration of the middle miocene polarity time scale a sequence of lavas yielding exclusively reversed polarities dated at 15 5 14 9 ma can be confidently identified as c5br and the c5cn ln y reversal is bracketed between 15 67 and 15 48 ma relative to tc 27 92 ma integration with other recent studies shows that polarity chrons c5cr c5abr are now confidently correlated with lava flows dated from 16 9 to 13 4 ma by the 40ar 39ar method

crustal geometry of the abitibi subprovince in light of three dimensional seismic reflector orientations we provide precise estimates of reflector orientations beneath the archean abitibi subprovince using two distinct approaches based on lithoprobe seismic reflection data in the first we use the dip of reflections observed on intersecting profiles to establish the three dimensional orientation of reflective structures in the second the strikes and dips of reflectors are estimated in the crooked parts of seismic reflection profiles by calculating a measure of coherency along the traveltime trajectories defined by a particular azimuth dip depth and medium velocity mid crustal reflectors define two areas with distinctive geometry reflectors beneath the southern abitibi belt are oppositely dipping and convergent at depth providing a v shape aspect to the greenstone rocks other reflectors beneath the northern abitibi belt are in general subparallel dipping at an average of 30 toward the north these north dipping reflectors are partly disrupted by a low reflectivity zone which is attributed to

origin and significance of redistributional secondary porosity abstract redistributional secondary rds porosity is created by the dissolution of framework grains and cements and the simultaneous precipitation of reaction products in the nearby pore space aluminosilicate minerals and in particular feldspars are the usual precursors of rds porosity in sandstones common product minerals are authigenic quartz and clay minerals usually kaolinite this type of secondary porosity is almost ubiquitous in sandstones rds porosity has little effect on the total porosity of reservoir rocks and contrary to popular opinion cannot give rise to higher than expected porosities at depth petrographic criteria alone are an insufficient measure that the rock has undergone leaching resulting in porosity enhancement to demonstrate porosity enhancement due to leaching it is necessary to show that the porosity of the rock is statistically higher than would otherwise be the case for a rock of similar characteristics that has not been leached the dissolution precipitation reactions that give rise to rds arise from the inherent thermodynamic instability of minerals such as the feldspars and other aluminosilicates which formed under magmatic or metamorphic conditions when these minerals dissolve they add aluminium and silica to the pore fluid however the concentration of aluminium and silica in the pore fluid is buffered by clay minerals and quartz respectively during burial diagenesis thus any increase in the concentration of these components leads to the precipitation of an authigenic clay mineral and authigenic quartz carboxylic acids are claimed to have the capacity to complex aluminium and so increase the solubility of aluminium and eliminate the precipitation of a clay phase however they are unlikely to have any major effect on aluminium solubility in natural i e complex pore fluids in fact dissolution will occur in acid neutral or alkaline conditions at a rate that is kinetically controlled reaction rates speed up exponentially with increasing temperature and therefore depth resulting in the destruction of more and more unstable components this increase in the rate of destruction of unstable components at depth can give rise to what appears to be a late leaching event what is seen in fact is the natural culmination of the destruction of thermodynamically unstable phases at an ever increasing rate during burial thus for a rock of a given composition the fraction of the total porosity that is secondary will increase with burial depth until all labile components are consumed the behaviour of mineral mixtures in the presence of a moving pore fluid is an additional contributing factor to the ubiquity of rds porosity where two or more minerals add the same ion to the pore fluid then the steady state concentration of that ion which is reached at some distance down the flow path theoretically an infinite distance is a weighted average of the equilibrium solubilities of that ion with respect to the minerals that contribute the ion thus as the pore fluid approaches a steady state concentration of the ion it will be undersaturated with respect to the least stable minerals and oversaturated with respect to the most stable mineral hence rds porosity must form as a result of the dissolution of the least stable mineral and the precipitation of the most stable mineral although kaolinite is the most common dissolution product of feldspars with increasing temperature its stability field shrinks with respect to other clay minerals such as illite thus it is possible for feldspars and other aluminosilicates to decompose at depth to give illite and authigenic quartz the growth of fibrous illite together with the change from a dominantly primary to a dominantly secondary pore structure can give rise to a rapid loss of permeability with depth where changes in the clay mineral type do not occur the impact of rds porosity on the porosity permeability behaviour of sandstones is minimal up to secondary porosities of about 10 beyond this figure there is at present too little data to make a clear statement

ages of the steens and columbia river flood basalts and their relationship to extension related calc alkalic volcanism in eastern oregon discussion hooper et al 2002 1 present stratigraphic chemical and field relationships for tertiary volcanic rocks of eastern oregon and conclude that 1 the imnaha basalt are 15 5 m y old and 2 the steens basalt ca 16 6 ma represents the earliest eruptions of the columbia river basalt flood

geomagnetic fluctuations during a polarity transition the extensive roza member of the columbia river basalt group washington state has intermediate paleomagnetic directions bracketed by underlying normal and overlying reverse polarity flows a consistent paleomagnetic direction was measured at 11 widely distributed outcrops the average direction has a declination of 189 and an inclination of 5 with greater variation in the inclination rietman 1966 in this study the roza member was sampled in two pasco basin drillcores where it is a single cooling unit and its thickness exceeds 50 m excellent core recovery allowed uniform and dense sampling of the drillcores during its protracted cooling the roza flow in the drillcores recorded part of a 15 5 ma geomagnetic polarity transition the inclination has symmetric quasicyclic intraflow variation while the declination is nearly constant consistent with the results from the outcrops thermal models of the cooling flow provide the timing for remanence acquisition the inclination is inferred to have progressed from 0 to 15 and back to 3 over a period of 15 to 60 years at rates of 1 6 to 0 5 yr because the geomagnetic intensity was probably weak during the transition these apparently high rates of change are not significantly different from present day secular variation these results agree with the hypothesis that normal secular variation persists through geomagnetic transitions the iow amplitude quasicyclical fluctuations of the field over tens of years recorded by roza suggest that the geomagnetic field reverses in discrete steps and that more than 15 60 years were required to complete this reversal

mesozoic igneous activity in the maranha o province northern brazil 40ar 39ar evidence for separate episodes of basaltic magmatism low and high ti basaltic rocks of the maranha o province in northern brazil appear to be related to the opening of the atlantic ocean 40ar 39ar dating was carried out on powdered whole rock basalts both washed and unwashed in dilute nitric acid to document this relationship acid treatment does not alter the argon retention properties of retentive phases but is effective in removing alteration phases high ca k degassed at low laboratory temperatures our results identify that at least two periods of volcanic activity formed the maranha o province low ti basalts from the western sector are 200 190 ma and are related to the opening of the north atlantic high ti basalts from the eastern sector are 129 124 ma slightly younger than compositionally similar rocks in the serra geral flood basalt province of southern brazil and are related to the opening of the south atlantic other flood basalts such as the columbia river deccan rajmahal ferrar and siberian provinces were each predominantly formed in 1 2 ma the maranha o basalts remnants of separate igneous events spanning 70 ma should not be classed as a single flood basalt province the temporal complexities noted in the maranha o basaltic rocks appear to be related to the tracks of hotspots that led to the rifting of the atlantic ocean two sets of mesozoic basaltic rocks occur in southern africa india and south america resulting from events leading to the fragmentation and dispersal of sections of gondwanaland in each sub continent the two provinces were formed by events separated by 50 70 ma of these pairs only the maranha o and serra geral show geochemical similarities indicating compositionally distinct upper mantle source regions under sections of brazil separated by 3000 km

the timing of prograde metamorphism in the pontiac subprovince superior craton implications for archean geodynamics and gold mineralization abstract the pontiac subprovince is located in the superior province south of the abitibi subprovince the metasedimentary rocks of the pontiac group are characterized by a barrovian metamorphic gradient increasing from north to south from biotite through garnet to staurolite zone conditions the pontiac subprovince has been interpreted as an accretionary wedge or alternatively as an exotic terrane that was tectonically docked to the abitibi subprovince during archean subduction however few studies have attempted to characterize its pressure temperature time path to test this hypothesis and little is known about the timing relationship between regional metamorphism and the gold mineralization an e w lepidoblastic ductile micaceous foliation defines the regional fabric related to the second episode of deformation garnet and staurolite porphyroblasts are interpreted to be late to post kinematic to the regional fabric locally cordierite or andalusite porphyroblasts fully pseudomorphed by a polymineralic assemblage of muscovite feldspar and quartz are wrapped by the main foliation indicating that a potential low pressure high temperature metamorphism m1 preceded the main episode of deformation and barrovian metamorphism m2 pressure and temperature p t forward thermodynamic modelling on a water saturated pseudosection yielded peak conditions of 550 600 c and 5 6 kbar during a prograde clockwise p t path for m2 lu hf dating of garnet from three different locations within the staurolite zone yielded a weighted average age of 2657 5 4 4 ma 95 confidence level that is inferred to be representative of these conditions the age of garnet growth calls into question the previous tectonic interpretation of the abitibi and pontiac subprovinces and is coherent with the new rafted ribbon continent model of bedard 2018 because the canadian malartic gold mineralization is older or synchronous with the garnet growth episode in the staurolite zone south of the ore body this garnet age also provides an estimate of the minimum age of the mineralization event previously dated at 2664 11 ma with re os on molybdenite

a detachment fold model for fault zones in the late archean abitibi greenstone belt abstract the abitibi belt is one of the largest and most extensively studied late archean greenstone belts the structural geology of the abitibi belt consists of one generation of upright to slightly overturned doubly plunging first order folds with half wavelengths of 20 60 km and e w striking steeply dipping fault zones that are parallel to the fold limbs two of the main fault zones are continuous for hundreds of kilometers previous tectonic models for the abitibi belt interpret the fault zones to have formed as extensional growth faults bounding a volcanic sedimentary basin which were reactivated as thrusts during subsequent crustal shortening other models propose that the fault zones represent tectonic sutures implying that the abitibi belt is a collage of exotic terranes however distinct geological terranes have not been geologically demonstrated we propose a new detachment fold model for the deformational history of the southern abitibi belt in ontario that explains the formation of the fault zones during the single well documented folding event that deformed the entire region the internal structure of the fault zones documented here with emphasis on the porcupine destor fault zone consists of isoclinally folded strongly schistose highly metamorphosed rock cross cut by numerous fault segments we interpret that the upper crust greenstones was folded above a proposed detachment in the lower part of the volcanic stratigraphy the fault zones would be in essence highly evolved detachment anticlines ultramafic metavolcanic rock that crops out within the fault zones would represent material from the detachment horizon that was emplaced in the cores of the detachment anticlines the numerous segments that make up the mapped fault zones would be linked faults that formed within the isoclinal detachment anticlines to accommodate folding of the rheologically complex greenstones the detachment fold model is compared to the results of analogue experiments designed to investigate crustal scale folding using viscous and frictional materials detachment folds are produced in the brittle upper crustal analogue on the limbs of folds formed in the ductile middle and lower crust analogues the experimentally produced structures scale to the structures in the study area and indicate the detachment fold model for the southern abitibi is mechanically viable

archean continental assembly in the southeastern superior province of canada between 1988 and 1993 seismic reflection and refraction surveys were acquired across the medium to high grade opatica plutonic gneiss belt the low grade abitibi greenstone belt and the pontiac metasedimentary belt all of which form part of the late archean superior province shallowly north dipping reflections define a structural style consistent with the northward underthrusting and accretion over about 30 ma of various exotic terranes against a backstop provided by the opatica belt this rapid southward growth of the archean protocraton was driven by at least one north dipping subduction zone as revealed by north dipping reflections that extend to 65 km depth in the upper mantle below the opatica belt in contrast to the mainly orthogneissic opatica and pontiac belts the midcrust of the abitibi belt comprises metasedimentary and igneous rocks plus imbricated units of unknown affinity relict midcrustal accretionary complexes of substantial size whieh are indicative of primary suture zones are interpreted near the northern and southern limits of the abitibi belt an interpreted basal decollement and significantly older ages in the north suggest that the upper crustal greenstone rocks are allochthonous evidence of large scale extension appears to be confined to the southern volcanic zone of the abitibi which developed into a half graben as the original suture zone was reactivated in extension unusually high seismic p wave velocities 7 5 8 2 kms 1 are present in the lower 8 km of the abitibi crust and they correlate well with a downward reduction in seismic reflectivity attributable to late modification of the deepest part of the crust crustal xenolith studies suggest that this process may be linked to early proterozoic magmatism

fine tuning the radiometrically derived geomagnetic polarity time scale gpts for 0 10 ma new 40ar 39ar step heating ages for geomagnetic field reversals at 3 2 and 4 5 ma along with previous data at 1 2 10 and 16 ma are used to derive a geomagnetic polarity time scale for 0 10 ma these ages for chron boundaries are fitted to the relative widths of the polarity intervals on the ocean floor using a cubic spline technique the interpolated ages of chrons for 0 6 ma are in good agreement with the astronomical time scale and differ considerably from earlier attempts based in part on k ar dates significant differences between the interpolated ages and the astronomical time scale are noted for the onset of the olduvai and reunion sub chrons there is better agreement if 40ar 39ar data obtained from direct dating of these chrons is utilized this time scale is estimated to be accurate to 1 for 0 6 ma for 6 10 ma the lack of a tie point limits the accuracy to about 2

new 40ar 39ar dating of the grande ronde lavas columbia river basalts usa implications for duration of flood basalt eruption episodes by discussion abstract barry et al 2010 presented 40 ar 39 ar ages to argue for a very limited duration 0 4 m y for the eruption of the grande ronde lavas critical examination of their data sets shows 1 a number of their plateau ages are statistically invalid and 2 many of their groundmass matrix samples were altered and did not yield valid estimates of the time of crystallization in general their ages err on the young side reflecting partial 40 ar loss due to alteration and there is no compelling evidence therein to argue for very limited duration for the grande ronde basalt the imnaha grande ronde and wanapum basalt erupted within 0 7 m y starting at 16 4 ma by contrast the ages of barry et al 2010 span the interval 16 6 to 14 7 ma and in some instances are out of stratigraphic sequence

constraints on the development of secondary porosity in the subsurface re evaluation of processes abstract secondary porosity formed by the dissolution of grains and cements is of great economic importance in both clastic and carbonate reservoirs a review of the commonly proposed mechanisms for the production of secondary porosity indicates that meteoric water penetration and mixing corrosion may be able to create significant volumes of secondary porosity in the shallow subsurface leaching by acidic fluids generated by solution of co 2 or monocarboxylic acids both derived during the thermal maturation of organic matter or by acidic fluids created by clay mineral reactions in shales are the most commonly proposed mechanisms by which secondary porosity is said to form in the deep subsurface however these mechanisms suffer from great difficulties in producing significant volumes of secondary porosity once mass balance and fluid migration problems are taken into account it is highly likely that the correct mechanism for the creation of economically significant amounts of secondary porosity in the deep subsurface is yet to be found

40ar 39ar geochronological evidence for multiple postmetamorphic hydrothermal events focused along faults in the southern abitibi greenstone belt thermal histories in the abitibi and pontiac subprovinces vary for three general domains 1 zones of impermeable rock with cooling rates of 8 c ma 2 zones of impermeable rock that are spatially associated with monzogranite plutons with apparent cooling rates of 1 2 cima and 3 permeable high strain zones with apparent cooling rates of 2594 ma from the volcanogenic massive sulphide related sericite alteration with the coarser mica yielding the younger date based on radiometric dating impermeable rocks were affected weakly by postmetamorphic hydrothermal events minerals in zones of higher permeability were reset during the intrusion of monzogranite plutons highly permeable zones such as the larder lake cadillac and porcupine destor faults were overprinted repeatedly this produces a drawn out series of dates from minerals with varying closure temperatures radiometric ages from fault zones or ore deposits cannot be used to interpret the cooling history and evolution of the abitibi belt as a whole

crustal and lithospheric scale structures of the precambrian superior grenville margin abstract the lithosphere beneath the margin of the archean superior and proterozoic grenville provinces was investigated with a northwest southeast oriented 650 km long profile of 40 magnetotelluric stations dominant geoelectric strike azimuths of n45 e and n85 e were defined for the crust and the lithospheric mantle respectively a 2 d isotropic resistivity model derived using the crustal strike images resistive laurentian margin rocks dipping southeast to the base of the crust bounded to the northwest by the grenville front and to the southeast by the central metasedimentary belt boundary zone the observation is in contrast to conductive mid to lower crust elsewhere in the region a 2 d isotropic resistivity model determined using the lithospheric mantle strike azimuth reveals an extremely resistive region in the upper 100 km of the mantle lithosphere of the northern grenville province the geometry of this body which includes a well defined base and southeast dip suggests that it is superior lithosphere a sub vertical conductor located approximately 50 km along strike from the mesozoic kirkland lake and cobalt kimberlite fields is interpreted to be due to re fertilization of an older mantle scar the resistivity model includes a horizontal conductor at 160 km depth beneath the southern superior province that is possibly the resistivity signature of the lithospheric asthenospheric boundary

crustal evolution along a seismic section across the grenville province western quebec results of deep seismic reflection survey along a 375 km long transect of the grenville province in western quebec are combined with a review of geological observations and published isotopic ages the seismic profile offers a remarkably clear image of the crust mantle boundary and a good definition of the various crustal blocks crust about 44 km thick beneath the grenville front zone thins abruptly to ca 36 km southeastward perhaps the result of extension on southeast dipping surfaces extending to the moho other zones of relatively thin crust although less pronounced occur where proterozoic crust overlies archean crust and beneath the morin anorthosite complex the thickest crust is found at the extreme southeast of the transect east of the morin anorthosite from northwest to southeast three main crustal subdivisions are 1 deformed archean rocks with southeast dipping reflectors in the grenville front zone 2 an archean parautochthon with northwest dipping reflectors extending to the lower

40ar 39ar dating of the brunhes matuyama geomagnetic field reversal magnetostratigraphic studies are widely used in conjunction with the geomagnetic polarity time scale gpts to date events in the range 0 to 5 million years ago a critical tie point on the gpts is the potassium argon age of the most recent brunhes matuyama geomagnetic field reversal astronomical values for the forcing frequencies observed in the oxygen isotope record in ocean drilling project site 677 suggest that the age of this last reversal is 780 ka thousand years ago whereas the potassium argon based estimate is 730 ka results from 4039 ar incremental heating studies on a series of lavas from maui that straddle the brunhes matuyama reversal give an age of 783 11 ka in agreement with the astronomically derived value the astronomically based technique appears to be a viable tool for dating young sedimentary sequences

postmineralization faults in the beaufor gold deposit abitibi greenstone belt canada geometry origin and tectonic implications for the val d or mining district the beaufor mine is a mesothermal vein type gold deposit hosted by the bourlamaque pluton a synvolcanic intrusion crosscutting archean volcanic rocks of the abitibi greenstone belt in the val d or mining district the auriferous system is related to south dipping reverse shear zones and consists of up to 2 m wide sheared and extensional sulfide rich quartz veins the auriferous veins system is crosscut by two generations of faults east west trending south dipping 60 brittle ductile reverse faults e g the dike 01 central fault system and subvertical to steeply north dipping brittle faults e g the beaufor fault system the dike 01 central fault system presents structural fabrics kinematic and hydrothermal characteristics similar to those of auriferous shear zones both are interpreted as master faults and secondary structures respectively with the dike 01 central fault system acting as a channelway for the updip draining of ore fluids that were mainly trapped in low angle synthetic shear zones incremental deformation and progressive decrease of bulk fluid pressure led to strain localization and faulting of auriferous quartz veins by the dike 01 central fault system the beaufor fault system crosscuts all structures of the beaufor deposit it consists of a series of oblique reverse dextral faults that lack evidence for pervasive hydrothermal alteration the main structure of that system is the east west trending north dipping beaufor fault which is associated with northeast southwest trending synthetic i e west shear faults and north south trending antithetic faults i e post ore faults fault zones of the beaufor mine are reminiscent of large scale tectonics in the val d or mining camp auriferous quartz tourmaline vein deposits are commonly associated with east west trending high angle reverse shear zones and hydrothermal events coeval with or slightly postdating peak metamorphism and regional deformation several fault zones of the val d or area preserve evidence for both reverse shearing and dextral strike slip faulting structures recorded in the beaufor mine suggest that reverse and dextral trancurrent faulting is genetically unrelated and probably resulted from sequential faulting rather than transpression

block and shear zone architecture of the minnesota river valley subprovince implications for late archean accretionary tectonics the minnesota river valley subprovince of the superior province is an archean gneiss terrane composed internally of four crustal blocks bounded by three zones of east northeast trending linear geophysical anomalies two of the block bounding zones are verified regional scale shears the geological nature of the third boundary has not been established potential field geophysical models portray the boundary zones as moderately north dipping surfaces or thin slabs similar in strike and dip to the morris fault segment of the great lakes tectonic zone at the north margin of the subprovince the central two blocks of the subprovince morton and montevideo are predominantly high grade quartzofeldspathic gneiss some as old as 3 6 ga and late tectonic granite the northern and southern blocks benson and jeffers respectively are judged to contain less gneiss than the central blocks and a larger diversity of syntectonic and late tectonic plutons a belt of moderately metamorphosed mafic and ultramafic rocks h

thrusting magmatic intraplating and metamorphic core complex development in the archaean belleterre angliers greenstone belt superior province quebec canada abstract the belleterre angliers greenstone belt in the pontiac subprovince is the youngest and southernmost of the greenstone belts in the archaean superior province of the canadian shield the greenstone belt has been disrupted into three fragments from north to south the baby belleterre and lac des bois groups respectively the belleterre angliers greenstone belt has sheared contacts with younger pontiac metasediments and older tonalites the pontiac metasediments dip beneath the baby group and are tectonically interlayered with the tonalites thus the belleterre angliers belt is interpreted to be a thrust sheet 6 km thick in the north thinner in the south above a mid crustal duplex consisting of metasedimentary and tonalite imbricates the thrust sheet was transported from the north but age and geochemical constraints rule out derivation from the abitibi subprovince it probably roots in a band of ultramafic rocks that mark a major south vergent thrust in the northern pontiac subprovince after emplacement the belleterre angliers sheet and the adjacent pontiac metasediments were deformed by a system of linked extensional shears and transfer faults that border dome or antiformal areas in a style similar to phanerozoic metamorphic core complexes the orientation of shear zones and foliations indicates that the regional far field stresses were compressional both before and after extensional shearing since the system of extensional shears is intruded by a suite of monzodiorite granodiorite syenodiorite and syenite plutons with high mg number it is concluded that crustal inflation caused by the injection intraplating of large volumes of magma perturbed the regional compressive stress field and facilitated extensional faulting

a geomagnetic polarity time scale for the period 0 17 ma based on 40ar 39ar plateau ages for selected field reversals utilizing 40ar 39ar plateau ages for selected field reversals at 1 2 10 16 and 34 ma a geomagnetic polarity time scale gpts is constructed following the technique of cande and kent 1992 this differs markedly from previous gpts especially in middle late miocene time the ages for the chrons in post miocene time are in agreement with values derived by the astrochronological technique refinements in this gpts await acquisition of 40ar 39ar plateau ages for field reversals in the time frames 3 8 ma and 11 15 ma

pressure seals their diagenetic banding patterns abstract pressure seals are economically significant geological phenomena since they play an important role in deep natural gas entrapment identified in basins worldwide they offer a new frontier for exploration of natural gas reservoirs below 10 000 ft 3000 m pressure seals are low permeability envelopes which enclose abnormally pressured reservoirs there are three different types of seals basal lateral and top basal seals which define the bottom of abnormal pressure compartments usually follow a stratigraphic horizon lateral seals are generally associated with faults top planar seals may parallel or cut across time stratigraphic boundaries they may be developed in any lithology the southeastern portion of the anadarko basin displays a layered sequence of abnormally pressured fluid compartments at depths ranging from 3 000 to 13 500 ft 900 to 4150 m these compartments are separated from each other by pressure seals in mcclain county a top pressure seal separating two abnormally pressured compartments is located between 10 000 ft 3000 m to 12 000 ft 3650 m within the simpson group ordovician the seal zone is characterized by alternating cemented and porous intervals silica and carbonate cements comprise a diagenetic laminated and banded pattern the compositional and textural heterogeneity within this structure evolved through diagenetic processes active during the seal evolution

recalibration of the geomagnetic polarity timescale the quest for improved time control to estimate rates of geologic and paleoenvironmental processes is central to the earth sciences until the middle of this century geochronology was based solely on macrofossil evolution providing the largely unmodified definition of geologic stages in the last few decades with impetus from the deep sea drilling project dsdp and the ocean drilling program odp microfossil biostratigraphy has become increasingly important although new radiometric techniques have resulted in improved absolute age precision the direct correlation of absolute ages to biozonations and hence to geologic stage boundaries remains poor this is due to the difficulty of finding materials suitable for radiometric dating which can be directly correlated to biozonations the bridge between biozonations and absolute ages and the interpolation between absolute ages are best accomplished particularly for cenozoic and late mesozoic time through the geomagnetic polarity timescale gpts the template for late jurassic to quaternary geomagnetic polarity is derived from oceanic magnetic anomalies as demonstrated by heirtzler et al 1968 the gpts can be constructed using oceanic magnetic anomaly record s assuming constant seafloor spreading rate s and interpolating between available radiometric ages in the last quadrennium there have been outstanding developments in radiometric dating and astrochronology which have greatly improved the absolute age control of the gpts

distribution and geochronology of oregon plateau u s a flood basalt volcanism the steens basalt revisited the timing and petrogenesis of mid miocene flood basalt volcanism in the northwest united states has been extensively addressed yet the chemical characteristics and temporal details of the steens basalt exposed on the oregon plateau are poorly defined steens basalt volcanism has generally been accepted to have occurred at 16 6 ma coeval and or just prior to the onset of columbia river basalt group volcanism to the north new major and trace element analyses and nine 40 ar 39 ar ages ranging from 15 51 0 28 to 16 58 0 18 ma were obtained on oregon plateau flood basalt lava flows from stratigraphic sections in close proximity to steens mountain additionally new 40 ar 39 ar ages were obtained on the uppermost and thirty first lava flow down from the top of the 1 km section of steens basalt exposed at steens mountain and yield eruption ages of 16 59 0 10 and 16 55 0 10 ma respectively field relations between these basalt sections suggest that multiple eruptive centers were present in the vicinity of steens mountain the chemical and chronologic data presented in this study illustrate that flood basalts with the steens basalt chemical signature erupted across the southern oregon plateau over a much greater timespan than what is typically quoted for the steens mountain type section these data suggest that the main volume of steens basalt volcanism erupted over at least an 1 m y duration from eruptive loci in the vicinity of steens mountain while likely much less voluminous volcanism and lithospheric input of mafic magma appears to have occurred for n2 ma across the oregon plateau these new geochemical and geochronological constraints verify a common temporal link between steens malheur gorge region and columbia river basalt group crbg volcanism this direct temporal link requires that petrogenetic and tectonic models of mid miocene northwestern u s flood basalt volcanism recognize that the northern columbia plateau and southern oregon plateau portions of this mid miocene basalt province were erupting simultaneously during portions of the regional event 2007 elsevier b v all rights reserved

concordant sea floor spreading rates obtained from geochronology astrochronology and space geodesy reversals of the earth s magnetic field allied to sea floor spreading have left their imprint in the form of a magnetic tape recorder on the sea floor it is demonstrated that the geomagnetic polarity time scale for the past few million years derived utilizing a radiometric dates of magnetic field reversals trapped in subareal igneous rocks and b as derived from the milankovitch forcing frequencies and their imprint in the sedimentary environment are in good agreement further sea floor spreading rates calculated from these results are in substantial agreement with the rate of plate movement averaged over the last decade as obtained by space geodetic measurements since the principles underlying these dating techniques are entirely different it lends credence to the results obtained for sea floor spreading arguably the most fundamental process in plate tectonics

new tie points for the geomagnetic polarity time scale during the middle miocene from the mogan group on gran canaria and ocean drilling program leg 157 site 953 a thick sequence of volcaniclastic sediments drilled at site 953 during ocean drilling program odp leg 157 northeast of gran canaria canary islands contains an almost complete magneto stratigraphy back to the shield stage of the island 14 8 ma ago onshore a sequence of reversals has been identified and dated in 19 dominantly peralkaline rhyolitic ignimbrites one rhyolitic and one basaltic lava flow of the mogan group 13 35 13 95 ma which overlie basalt flows of the island s shield stage 14 ma the magneto stratigraphy of the ignimbrites onshore has been correlated with the marine magneto stratigraphy at site 953 determined in syn ignimbritic volcaniclastic turbidites which were deposited practically synchronously immediately following the entry of the parent pyroclastic flows into the sea around the circumference of the island the four polarity intervals recorded in the sequence of the mogan group ignimbrites correspond to c5acr c5acn c5adr and c5adn single crystal 40ar 39ar age determinations of the ignimbrites bracketing the polarity changes gave the following ages and uncertainties for the reversals c5ad t 13 95 0 07 ma c5ac o 13 89 0 08 ma and c5ac t 13 47 0 09 ma the newly dated polarity changes fit and refine the miocene age model proposed in the global polarity time scale

comments on deccan flood basalts at the cretaceous tertiary boundary by v courtillot j besse d vandamme r montigny j j jaeger and h cappetta the age of the flood basalts of the deccan area and the length of the interval during which the outpourings took place are not known precisely the authors have tried to date those basalts and estimate how long the volcanic activity lasted on the basis of new paleomagnetic paleontological and geochronological data of the deccan effusives they came to the preliminary conclusion that the volcanic activity may have lasted less than 1 ma and that the majority of the lavas poured out during chron 29r unfortunately the authors were not familiar with our paper on paleomagnetism and radiomettic age determinations of deccan trap lavas 2 in our paper we presented the paleomagnetic data derived from 223 individual lava flows from 16 sections all over the deccan area and gave the k ar age determinations for 13 selected unaltered lava flows we agree that the enormous volume of deccan extrusives poured out in a relatively short interval and that the volcanic activity was probably concerned with the spreading in the central indian ocean in late cretaceous and early tertiary time

magma supply and storage at kilauea volcano hawaii 1956 1983 abstract shallow crustal magma reservoirs beneath the summit of kilauea volcano and within its rift zones are linked in such a way that the magma supply to each can be estimated from the rate of ground deformation at the volcano s summit our model builds on the well documented pattern of summit inflation as magma accumulates in a shallow summit reservoir followed by deflation as magma is discharged to the surface or into the rift zones magma supply to the summit reservoir is thus proportional to summit uplift and supply to the rift zones is proportional to summit subsidence the average proportionality constant is 0 33 10 6 m 3 rad this model yields minimum supply estimates because it does not account for magma which escapes detection by moving passively through the summit reservoir or directly into the rift zones calculations suggest that magma was supplied to kilauea during july 1956 april 1983 at a minimum average rate of 7 2 10 6 m3 month roughly 35 of the net supply was extruded the rest remains stored within the volcano s east rift zone 55 and southwest rift zone 10 periods of relatively rapid supply were associated with the large kapoho eruption in 1960 and the sustained mauna ulu eruptions in 1969 1971 and 1972 1974 bursts of harmonic tremor from the mantle beneath kilauea were also unusually energetic during 1968 1975 suggesting a close link between kilauea s deep magma supply region and shallow storage reservoirs it remains unclear whether pulses in magma supply from depth give rise to corresponding increases in shallow supply or if instead unloading of a delicately balanced magma transport system during large eruptions or intrusions triggers more rapid ascent from a relatively constant mantle source

gold bearing fault zones related to late archean orogenic folding of upper and middle crust in the abitibi granite greenstone belt ontario abstract this work investigates the structures of the abitibi granite greenstone belt ontario with a focus on regional scale folds and on regional scale auriferous fault zones the folds and the fault zones resulted from late archean orogenesis the analysis is based on studies of surface geology and on modelling of the gravity field amphibolite grade orthogneiss was involved in the regional folding that also deformed the overlying greenstones folding resulted in uplift of the orthogneiss that preserves the metamorphic mineral assemblage ferropargasite garnet biotite andesine quartz geothermobarometric calculations indicate recrystallization of a metatonalite sample under peak paleopressures of 7 0 1 2 to 7 9 1 1 kb providing evidence that folding involved crust to depths of at least 18 25 km the regional bouger anomaly map reveals previously unrecognized first order folds interpretation of the bouger anomaly map also confirms that granitic batholiths are hosted by anticlines a wavelet based inversion method was used to determine the locations orientations and continuations at depth of the maxima of density contrasts the maximum density contrasts correspond to steeply dipping geological contacts on the limbs of the open to tight regional scale folds regional scale fault zones only have prominent gravity signatures where they correspond to sheared rocks on the limbs of folds modelled gravity profiles were used to produce profiles that visualize the shapes of batholiths and greenstone bodies within the present day upper crust the granitic batholiths have tabular shapes which are locally folded with preserved main body thicknesses of 1 3 km the greenstones form keels corresponding to synclines some of which reach 5 6 km depth the results of this study confirm that the principal deformational response of the abitibi granite greenstone belt to late archean collisional tectonics was large half wavelength 10 40 km folding of the upper and middle crust our results and interpretations also strongly suggest the regional scale fault zones which are loci for orogenic gold deposits are fold related structures they represent quasi continuous lineaments of shearing and faulting on the limbs of regional folds and locally in the cores of detachment anticlines as shown in a previous paper

conditions and timing of metamorphism in the southern abitibi greenstone belt quebec regional metamorphism ranging in grade from the subgreenschist facies to the greenschist amphibolite facies transition affects all archean supracrustal rocks 2677 ma in the rouyn noranda area contact metamorphic minerals associated with the posttectonic preissac lacorne batholith 2643 ma show no evidence of a regional retrograde event accordingly the age of regional metamorphism can be bracketed between 2677 and 2643 ma three reaction isograds were mapped in subgreenschist facies metabasites dividing the low grade rocks into three metamorphic zones the pumpellyite actinolite zone the prehnite pumpellyite zone and the prehnite epidote zone in addition the pumpellyite actinolite epidote quartz bathograd corresponding to a pressure of approximately 200 mpa occurs on both sides of the porcupine destor fault low pressure regional metamorphism is also indicated both by the occurrence of an actinolite oligoclase zone and the persistence of pre regional metamorphic andalusite the coincidence

partial convective overturn of archaean crust in the east pilbara craton western australia driving mechanisms and tectonic implications strain varies systematically from weakly developed outward dipping s tectonites in the 3320 3310 10 ma mount edgar batholith to intensely deformed subvertical l tectonites in greenstones of the warrawoona syncline a consistent greenstone down batholith up sense of shear is recorded in batholithic domal margins and adjacent high grade supracrustal rims lineations converge to a central zone of subvertical extension zone of sinking along the synclinal axis at domal margins early kinematic granitoid sheets and intrusive diatexites are subconcordant to a well developed dome parallel schistosity but late to post kinematic intrusives are discordant high level plutons all granitoids are the same age within analytical error these syn doming features conform with structural tests for diapirs and differ from those expected during metamorphic core complex formation and cross folding diapirism is part of a larger process involving partial convective overturn of the crust based on strain patterns and kinematic criteria we argue that deformation was initiated by sinking of greenstones which rapidly subsided the domes then rose passively as accommodation structures ongoing doming d4 partitioned strain to the southern rim of the batholith and uplifted a wedge of the warrawoona syncline keel we consider that crustal overturn occurred in response to mantle plume activity eruption of a 5 10 km thick 3325 ma mafic ultramafic greenstone pile onto an older granite greenstone terrain created a negatively buoyant crust but convective overturn occurred some 5 20 ma later triggered by widespread anatexis thermal softening convective overturn may have been a common archaean process occurring in response to a hotter mantle and represents an end member deformation mechanism that includes thrust accretion of lithotectonic assemblages in other archaean terranes such as the superior province of canada

palaeomagnetism and radiometric dating of the british tertiary igneous province muck and eigg summary the lavas dykes and other intrusive bodies of muck and eigg have been sampled for palaeomagnetic purposes all the bodies are reversely magnetized except for four dykes of muck which are normally magnetized the lavas show variations of direction of magnetism believed to be a record of secular variation the radiometric dates support the geological evidence that the lavas of eigg and muck predate parts of the rhum central igneous complex the lavas of rhum and canna and some of those of skye the combined evidence shows that most of the activity took place during an r n r n sequence between 63 and 58 ma the younger r n sequence is the same as the first part of the r n r sequence found in mull but the sgurr pitchstone of eigg and parts of the eastern redhills of skye belong to later reversed intervals and correspond to chron 26r and chron 26 the igneous activity occurred over a 10 myr period from about 63 ma and can probably be associated with chrons 27r to 23r of the harland et al time scale

oblique archean subduction accretion and exhumation of an oceanic arc during dextral transpression southern volcanic zone abitibi subprovince canada abstract the archean abitibi subprovince divided into northern nvz and southern svz volcanic zones is characterized by the crustal scale destor porcupine manneville fault zone dpmfz which links the svz and nvz and the cadillac larder lake fault zone cllfz which separates the abitibi and pontiac subprovinces the dpmfz and cllfz represent major deformation zones that record over 60 million years of archean deformation the dextral transpressional phase in the nvz which represents incipient svz deformation occurred between 2700 and 2692 ma and is characterized by southeast se trending dextral faults e g macamic fault synorogenic flysch sedimentation is focussed at the interface between the svz nvz and pontiac abitibi subprovinces the se trending parfouru fault linking the two major trench fault systems is interpreted as a reactivated leaky transform fault and is associated with synorogenic flysch sedimentation the early monzodioritic suite of the preissac lacorne batholith was related to dpmfz thrusting ca 2692 2690 ma the change from early thrusting to transcurrent motion is recorded in the duparquet strike slip basin which formed between 2690 and 2680 ma along the dpmfz and the granada pull apart basin which evolved between 2680 and 2670 ma along the cllfz the timing of these basins indicates a diachronous evolution renewed thrusting affected the granada basin but not the northern duparquet strike slip basin so that a southward migrating deformation front is inferred late exhumation resulted in extension along both fault zones and was responsible for the juxtaposition of medium and low grade metamorphic rocks the monzogranitic suite of the lamotte pluton which occurred between 2660 and 2642 ma is linked to exhumation final dextral transpression post dating exhumation produced shearing and folding the svz of the abitibi subprovince shows the salient attributes of modern oblique orogenic collisions with alternating phases of thrusting and strike slip movement along crustal scale faults during dextral transpression

moho reflectivity patterns a comparison of canadian lithoprobe transects abstract the complex nature of the continental moho may be strongly influenced by the tectonic and magmatic history of the lithosphere observations of moho variability from near vertical incidence seismic reflection surveys suggest that it may be possible to correlate moho reflectivity and lithospheric evolution as an initial step in attempting to determine such correlations we qualitatively compare seismic reflection data from canadian lithoprobe transects that sample regions of exceptionally diverse age and tectonic history the seven selected datasets were collected using consistent acquisition parameters facilitating effective and meaningful comparisons among transects the moho reflections exhibit considerable variability however in virtually all recent transects a distinct moho is observed even in regions where lower crust is considered to have stabilized in the early proterozoic or archean this suggests not only that relationships between moho reflectivity and tectonics can be extended into the archean but that tectonic age is poorly correlated with moho reflectivity the qualitative comparisons do not clearly link a specific tectonic process with a distinctive reflection pattern however well defined moho reflections are associated with lower crustal deformational processes including compression transpression and extension this indicates that ductile shear and the rheologies of the lower crust moho transition and upper mantle play important roles in influencing reflective character indistinct moho reflectivity may be associated with regions that have experienced little ductile deformation or with processes that have disrupted reflective laminations however the majority of incoherent moho reflections appear related to signal degradation associated with data acquisition the study demonstrates that the qualitative characteristics of moho reflectivity are difficult to correlate with tectonic history and thus use as an interpretive tool isolating quantitative reflection characteristics of the moho from distortions imposed by the overlying crust is a challenging problem but appears necessary to further understanding of the crust mantle transition

comment to distribution and geochronology of the oregon plateau u s a flood basalt volcanism the steens basalt revisited by m e brueseke et al j volcanol geotherm res 161 2007 187 214 abstract the 40 ar 39 ar stepheating results of brueseke et al 2007 are critically evaluated firstly for possible alteration of the samples dated steps passing the alteration index test baksi 2007a can then be used to derive plateau ages which must satisfy rigorous statistical tests of the eleven samples studied by brueseke et al 2007 only four yield plateau values that reflect eruption ages these suggest steens basalt was erupted within a relatively short period of time 0 2 m y considerably less than the 1 m y suggested by the authors the seeming difference between the plateau ages for the steens mountain basalts obtained by these authors and two earlier attempts is resolved by a proper adjustment of ages at various laboratories for interlaboratory calibration differences and by rejecting ages from altered samples the age of the important steens mountain geomagnetic polarity transition is 16 59 0 03 ma

the age of the steens reversal and the columbia river basalt group abstract the columbia river basalt group crbg eruptions have a well defined relative magnetostratigraphy but have not been definitively correlated to the geomagnetic polarity time scale 40 ar 39 ar ages are presented from lavas erupted in the r 0 through n 1 magnetozones of the crbg and in the transition between r 0 and n 0 four ages from transitionally magnetized lava flows at steens mountain catlow peak and poker jim ridge with a weighted mean age 16 58 0 10 ma 1 and the more precise age 16 654 0 025 ma of the normally magnetized oregon canyon tuff at the top of the catlow peak section show that the oldest crbg magnetozone r 0 correlates with the c5cr chron bayesian statistical analysis applied to data from four flows at catlow peak using the mean age of the steens reversal gives a best and preferred age of the steens reversal of 16 73 0 13 0 08 ma 95 confidence depending on the geomagnetic polarity time scale model the eruption rate from n 0 through r 2 0 34 0 45 ma in the middle and the bulk of the crbg emplacement averaged 0 30 0 41 km 3 a and peaked at a rate 1 1 2 to 4 1 2 times higher during r 2

u pb geochronology of archean metasedimentary rocks in the pontiac and abitibi subprovinces quebec constraints on timing provenance and regional tectonics abstract u pb isotopic ages of single detrital zircons have been obtained from metasandstones in the northern part of the pontiac subprovince and from the cadillac kewagama and lac caste groups within the southern abitibi subprovince youngest detrital zircon ages from these sequences are 2685 2686 2687 and 2693 ma respectively the lac fourniere pluton which intrudes pontiac group metasedimentary rocks is dated at 2682 1 ma a similar age was previously measured from a pluton that cuts the kewagama group these data bracket deposition of greywacke in the pontiac and abitibi subprovinces to 2685 3 ma in contrast the youngest zircon found in a volcaniclastic unit interbedded with timiskaming group metawacke is 2673 2 ma while the age of a pluton that intrudes the timiskaming is 2672 1 ma thus the timiskaming group is distinctly younger than the other metasedimentary rocks the normalized probability distributions of detrital zircon ages from the pontiac and the abitibi metasedimentary rocks as well as of dated igneous rocks from the southern abitibi subprovince all show that their major zircon populations formed in the age range 2685 2710 ma the distributions for the abitibi metaturbidites and dated igneous rocks are very similar suggesting that metasedimentary rocks in the abitibi belt were mostly derived from the surrounding granite greenstone terrain the largest peak of the pontiac distribution is a few million years younger than that from the abitibi metasedimentary rocks but there is a correlation between major peaks and both groups show similar age ranges for older pre abitibi components 2750 2850 and 3020 3030 ma deposition of the ca 2685 ma sediments was virtually syn deformational based on previous field geochemical and isotopic studies as well as the detrital zircon age data the pontiac group may represent a foreland basin that may have developed from an accretionary prism the source of much of the sediment may have been an uplifted extinct continental arc to the north the arc was mostly eroded into the sediment basin and perhaps tectonically overridden by a southward advancing nappe complex comprised of its back arc basin now partly preserved in the southern volcanic zone of the abitibi subprovince metaturbidites exposed between metavolcanic assemblages within the greenstone belt may represent overturned unconformities at the structural base of the greenstones these were brought up by high angle faults which also localized deposition of non marine timiskaming type sediments it is tentatively suggested that pontiac type metasedimentary rocks extend most of the way beneath the abitibi subprovince the oldest part of the basin emerging as the quetico group in the western superior province

estimation of lava extrusion and magma production rates for two flood basalt provinces utilizing the statistical approach of cox and dairymple 1967 it is shown that the geomagnetic polarity time scale of harland et al 1982 may contain errors of 0 85 1 0 m y for middle late miocene times a revised time scale based on the work of mcdougall et al 1984 is correlated with the magnetostratigarphic units of the columbia river basalts united states lava extrusion rates reached a peak of 275 km3 kyr for a period of 0 5 m y around 15 8 ma during the columbia volcanic episode it is suggested that even higher rates of lava extrusion may have occurred in the deccan province central india 65 ma a simple energy budget calculation indicates that the reunion hotspot could provide enough heat to melt the total volume of rock needed to produce the deccan traps during the 5 m y it would have taken the subcontinent to traverse the hotspot

revised age calibration points for the geomagnetic polarity time scale two of the seven cenozoic age calibration points in the widely used cande and kent s geomagnetic polarity time scale have recently been revised the rest of the five calibration points are revised here based on recently available radioisotopic dates and magnetostratigraphic data ages of the magnetic reversals are recalculated using the revised calibration points differences between the present time scale and cande and kent s revised time scale are generally less than 0 3 m y the largest up to 1 3 m y being in the early miocene

new 40ar 39ar dating of the grande ronde lavas columbia river basalts usa implications for duration of flood basalt eruption episodes abstract grande ronde basalt grb lavas represent the most voluminous eruptive pulse of the columbia river snake river yellowstone hotspot volcanism with an estimated eruptive volume of 150 000 km 3 grb lavas form at least 66 of the total volume of the columbia river basalt group new 40 ar 39 ar dates for grb lavas reveal they were emplaced within a maximum period of 0 42 0 18 my a well documented stratigraphy indicates at least 110 grb flow fields or individual eruptions and on this basis suggests an average inter eruption hiatus of less than 4000 years isotopic age dating cannot resolve time gaps between grb eruptions and it is difficult to otherwise form a picture of the durations of eruptions because of non uniform weathering in the top of flow fields and a general paucity of sediments between gr lavas where sediment has formed on top of grb lavas it varies in thickness from zero to 20 30 cm of silty to fine sandy material with occasional diatomaceous sediment individual grb eruptions varied considerably in volume but many were greater than 1000 km 3 in size most probably eruptive events were not equally spaced in time some eruptions may have followed short periods of volcanic repose perhaps 10 2 to 10 3 of years whilst others could have been considerably longer many 1000 s to 10 4 years recent improvements in age dating for other continental flood basalt cfb lava sequences have yielded estimates of total eruptive durations of less than 1 my for high volume pulses of lava production the grb appears to be a similar example where the main pulse occupied a brief period even allowing for moderate to long duration pahoehoe flow field production the amount of time the system spends in active lava producing mode is small less than c 2 6 based on eruption durations of approximately 10 000 years compared to the duration of the entire eruptive pulse of c 420 000 years a review of available 40 ar 39 ar data for the major voluminous phases of the columbia river basalt group suggests that activity of the steens basalt imnaha basalt grb may have at times been simultaneous with obvious implications for climatic effects resolving intervals between successive eruptions during cfb province construction and durations of main eruptive pulses remains vital to determining the environmental impact of these huge eruptions

expanding the size of multi parameter metasomatic footprints in gold exploration utilization of mafic dykes in the canadian malartic district quebec canada ore forming hydrothermal fluids react differently with different country rocks but few studies have applied this knowledge with the specific goal of expanding the size of hydrothermal footprints in mineral exploration to develop this concept 122 metamorphosed mafic dykes from the world class canadian malartic gold district 18 6 moz au were sampled and analyzed for mineralogy physical properties and lithogeochemistry partial and total digestion the mafic dykes intrude mainly metasedimentary rocks post date d1 deformation and cross cut early d2 quartz monzodiorite intrusions but they were deformed and altered during d2 deformation mineralization and metamorphism they can be subdivided into three groups group 1 dykes are least altered characterized by amphibole rich regional metamorphic assemblages and distributed throughout the pontiac subprovince group 2 and 3 dykes underwent ore related hydrothermal biotite calcite pyrite alteration and are associated with density and volume adjusted concentration gains of over 100 in au w te c s ag cs mo cu k rb se u pb ba f bi sn they define a metasomatic footprint up to 6 km away from the deposit toward the se and up to 2 km away from the deposit toward the sw fifty eight variables that define halos around the canadian malartic deposit have been identified and integrated using principal component analysis pc1 explains 30 of the variance separates least mobile elements from ore related elements and is interpreted to reflect the ore forming alteration process pc2 and pc3 represent igneous processes pc4 highlights the calcite pyrite and biotite alteration spatial variations of the modal abundances of amphibole biotite calcite and pyrite are the simplest expression of the metasomatic footprint and they can easily be documented during exploration at the camp scale

evidence for errors in the geomagnetic polarity time scale at 17 15 ma 40ar 39ar dating of basalts from the pacific northwest usa 40ar 39ar incremental heating studies have been carried out on whole rock basalt samples from the columbia river basalt crb and the steens basalt to obtain crystallization ages with a standard error 1 these high precision accurate ages are utilized with the known magnetostratigraphy of the crb to test the accuracy of the early to middle miocene section of the geomagnetic polarity time scales currently in use we conclude that these time scales underestimate the ages of various chrons within this time range by 0 9 m y

constraints on the geodynamic evolution of the southern superior province u pb la icp ms analysis of detrital zircon in successor basins of the archean abitibi and pontiac subprovinces of ontario and quebec canada abstract to investigate the tectonic development of the southern superior province new u pb laser ablation inductively coupled mass spectrometry analysis of detrital zircon was conducted on graywacke samples from successor basins of the abitibi and pontiac subprovinces the three successor basin successions investigated are the 2690 2685 ma porcupine assemblage and the 2679 2669 ma timiskaming assemblage of the abitibi subprovince and the 2682 ma sedimentary rocks of the pontiac subprovince predominately neoarchean zircon grains 80 95 of the total zircon population reflect local sources in the abitibi and pontiac subprovinces the successor basins were deposited within 100 ma of the majority of the detrital zircon grains primary crystallization ages which is consistent with patterns observed at modern convergent margin settings porcupine assemblage samples contain 5 mesoarchean zircon while pontiac subprovince and timiskaming assemblage samples contain 18 and 13 respectively no local mesoarchean sources exist in the abitibi or pontiac subprovinces suggesting input from an adjacent hinterland the higher proportion of mesoarchean zircon in the timiskaming assemblage relative to the porcupine assemblage suggests that detritus from the hinterland was more prevalent during the later stages of collision probably as a result of progressive uplift and denudation of the hinterland the high proportion of mesoarchean zircon in the pontiac subprovince sedimentary rocks may reflect an additional mesoarchean source comparison to previously published geochronology data for the superior province indicates that the hinterland was to the northwest and is in part composed of the winnipeg river marmion and opatica subprovinces patterns of contractional deformation successor basin formation and felsic plutonism progressively young to the south in the southern superior province and are interpreted in terms of accretionary processes detrital transport across subprovince or terrane boundaries progressive hinterland emergence and foreland directed propagation of the deformational front recorded by successor basins is similar to that observed in modern style collisional or accretionary orogens

volatile fluxes during flood basalt eruptions and potential effects on the global environment a deccan perspective abstract we examine the role that flood basalt eruptions may have played during times of mass extinction through the release of volcanic gases continental flood basalt provinces have formed by numerous eruptions over a short period of geologic time characteristically a few million years within this period a short lived climactic phase that lasts about 1 ma typically emplaces a large proportion of the lava volume this phase consists of a series of huge eruptions each yielding 10 3 10 4 km 3 of magma each eruption lasted on the order of a decade or more and built an immense p hoehoe dominated lava flow field by eruptive activity along fissures tens to hundreds of km long high fire fountains emanating from vents along the fissures at times sustained eruption columns that lofted gas and ash into the upper troposphere and lower stratosphere while the lava flows covered huge areas the combination of large eruption magnitudes maintained high effusion rates during eruptions and the repeated nature of the characteristic large scale eruptive activity occurs in earth history only during periods of flood basalt volcanism based on recent analogs and determination of volatile contents of ancient flood basalt lavas we estimate that individual eruptions were capable of releasing 10 000 tg of so 2 resulting in atmospheric loadings of 1000 tg a 1 during a sustained decade long eruptive event we apply this model of flood basalt volcanism to estimate the potential mass of co 2 and so 2 released during formation of the 65 ma deccan province the deccan lava pile contains the record of hundreds of enormous p hoehoe flow fields erupted within a period of about 1 ma consequently atmospheric perturbations associated with so 2 emissions from just one of these long lasting eruptions were likely to have been severe and constantly augmented over a decade or longer by contrast the amounts of co 2 released would have been small compared with the mass already present in the atmosphere and thus much more limited in effect individual eruptions were followed by hiatuses of hundreds to thousands of years during which the gas contributions to the atmosphere would be recycled it is clear that the nature and potential atmospheric impact of a series of huge volume repeated long term degassing events requires further investigation in conjunction with appropriate climate models

structural setting for canadian malartic style of gold mineralization in the pontiac subprovince south of the cadillac larder lake deformation zone quebec canada abstract the structural setting of the pontiac subprovince in the vicinity of the world class canadian malartic gold deposit has been revisited by combining and reinterpreting airborne geophysical surveys together with a century of structural observations felsic intermediate intrusive bodies are a key component of this deposit defining the regional and local favorable structural setting for intrusive rock emplacement within the clastic meta sedimentary rocks of the pontiac group south of the cadillac larder lake deformation zone may further assist gold exploration in similar tectonic environment three structural domains are interpreted in the area based on the geometry of the bedding folds and structural fabrics related to the three major phases of deformation during these events four phases of magmatism and one metamorphic episode occurred the north domain which hosts the canadian malartic deposit is characterized by highly variable bedding orientations produced by the interference patterns of isoclinal f 1 folds overprinted by open to tight steeply dipping f 2 folds the bedding in this domain is cut by a penetrative s 2 biotite foliation which possibly built on rheological changes enhanced by metasomatism in the footprint of the canadian malartic deposit by contrast the central and south domains display homogeneous bedding orientations cross cut by a discrete s 2 biotite foliation and syn to late d 2 metamorphic porphyroblasts in all domains the l 2 stretching lineation consistently plunges at about 60 degrees toward the east d 3 is a minor deformation event in the pontiac subprovince which possibly correlates with late dextral transcurrent movement along the cadillac larder lake deformation zone intrusive bodies were emplaced throughout the first and second deformation events phase 1 monzonite quartz monzodiorite and granodiorite plutons ca 2683 2680 ma intruded into consolidated pontiac sedimentary rocks during d 1 phase 2 quartz monzodiorite bodies ca 2679 2676 ma predominantly formed in the north domain along f 1 fold axial surfaces prior to or at the onset of d 2 phase 3 basic dykes ca 2675 2673 ma cross cut earlier felsic intermediate intrusions across all domains and subsequently developed an s 2 metamorphic hornblende foliation phase 4 magmatism ca 2672 2662 ma is related to the decelles batholith s type granite and pegmatite which is interpreted to be contemporaneous with the peak of regional metamorphism the decelles batholith may project at depth underneath the canadian malartic deposit and could be associated with magmatic hydrothermal mineralizing fluids in the canadian malartic deposit gold mineralization at canadian malartic is spatially located on the contact of phase 2 quartz monzodiorite bodies the proximity to the cadillac larder lake deformation zone combined with the rheological contrast between steeply dipping clastic meta sedimentary rocks and quartz monzodiorite intrusions favored the protracted failure of the contacts between these two rock masses thus forming favorable conduits for hydrothermal fluids e g the sladen fault zone such specific rheological behavior is demonstrated by domains of structural complexity emphasized by the variance of the bedding dip these structurally complex zones systematically host gold mineralization in the canadian malartic district of the pontiac subprovince proximal to the cadillac larder lake deformation zone

testing the accuracy of the geomagnetic polarity time scale gpts at 2 5 ma utilizing40ar 39ar incremental heating data on whole rock basalts abstract recent40ar 39ar dating and astrochronologic calculations indicate that the k ar derived gpts underestimates the ages of geomagnetic field reversals in the interval 2 0 ma we report tests to check the accuracy of k ar data utilized to date the reunion event 2 1 ma and part of the early pliocene portion 4 6 ma of the gpts 40ar 39ar incremental heating studies on whole rock basalts from the island of reunion yield an age of2 14 0 03 ma for the reunion event somewhat older than the earlier k ar value of2 07 0 02 ma on the same set of lava flows study of a single specimen from the newer volcanics australia indicates that the earlier k ar date was accurate but the new40ar 39ar results yield a plateau age 4 56 ma with a standard error of 0 6 as compared to the k ar value 3 the improved precision and accuracy resulting from such40ar 39ar incremental heating studies will play a key role in calibrating more precisely the gpts for the interval 10 0 ma

hydrothermal alteration mineralogy and geochemistry of the archean world class canadian malartic disseminated stockwork gold deposit southern abitibi greenstone belt quebec canada

variations in the velocities of the major plates since the late cretaceous abstract we present models for the relative and absolute velocities of the major plates from 80 m y ago to the present as tests of several alternative plate reconstructions and as a guide in the evaluation of the hypothesis that pronounced changes in plate speeds or directions arise directly from changes in the configuration of plate boundary forces digitized plate boundaries were rotated backwards in time using published rotations to determine positions at 5 m y intervals back to the late cretaceous the oceanic plate boundaries were then corrected so as to properly match magnetic anomalies relative plate velocities were obtained by differencing finite rotations absolute velocities were determined by assuming that no net torque is exerted on the lithosphere rms absolute speeds of each plate were calculated as functions of time as a conceptually simple measure of plate velocities and accelerations we have tested several possible reconstructions for the antarctic and pacific plates proposed to explain the failure of simple rigid antarctic and or pacific plate models to match paleomagnetic data hotspot tracks and equatorial crossings defined by pelagic sediment deposition we find that 1 1 plate reconstruction models with either two antarctic or two pacific plates in the early tertiary are distinguishable from models with single antarctic and pacific plates principally on the basis of faster speeds for the kula plate and the northern pacific greater areas for the kula and farallon plates and faster subduction at the aleutians and along the west coast of south america for that time 2 2 oceanic plates as a group have moved faster than those with continental lithosphere only since 50 m y ago active subduction appears to be a sufficient condition for rapid plate motion regardless of the fractional plate area comprised of continental lithosphere 3 3 the absolute motion of india in the late cretaceous was anomalously fast 15 20 cm yr compared to the speeds of the present plates 4 4 the rms velocities of a number of plates and of the lithosphere as a whole are much higher in the late cretaceous than through most of the cenozoic 5 5 the predicted apparent polar wander for north america is in fairly good agreement with paleomagnetic data with the slight discrepancy attributable to a net contribution to the torque balance from plate boundary forces

diagenetic alteration of the wingate formation possible indications of hydrocarbon microseepage lisbon valley utah at most localities in southeastern utah the wingate formation is a sandstone that exhibits generally uniform mineralogy and appearance along the southwest dipping flank of the lisbon valley anticline however substantial differences occur in the spatial distribution of authigenic kaolinite and hematite that permit differentiation of three outcrop areas the northern and southern portions of this flank of the anticline are white bleached the bulk of the remaining centrally located wingate outcrops are uniformly red unbleached because of hematite clay coatings on constituent grains bleached rocks lack these grain coatings in addition most of the quartz grains in the bleached rocks have overgrowths compared to the other areas the northern bleached rocks have three to five times more authigenic kaolinite half the porosity twice as many altered feldspars and one third as much mixed layer clay both northern and southern bleached rocks contain hematite pseudomorphs after pyrite and the norther

experimental simulation of plagioclase diagenesis at p t conditions of 3 5 km burial depth dissolution of plagioclase under the physical conditions at shallow to intermediate burial depths is a prime candidate for secondary porosity generation in feldspathic siliciclastic sediments the diagenetic behavior of granular aggregates of plagioclase feldspar and quartz has been investigated by experimentation performed in a bridgeman type pressure vessel the experiments each of two weeks duration simulated pressure temperature conditions approximating 3 5 km burial depth by using a double acting pore fluid reservoir solutions of various chemistries were cycled through samples composed of oligoclase or labradorite feldspar and quartz 90 10 wt respectively

crustal genesis on the oregon plateau the areal and temporal distribution and the chemical and isotopic characteristics of cenozoic volcanic rocks from the oregon plateau reflect the changing tectonic conditions affecting this area over the last 20 m y following subduction zone related calc alkaline volcanism flood basalt eruptions occurred in the time interval 18 14 ma this primarily basaltic volcanism is interpreted to be the result of a major shift in spreading orientation of the juan de fuca ridge that resulted in changes in the geometry of subduction against the pacific northwest the compositions of the mid miocene flood basalts show signs of extensive crystal fractionation in some cases accompanied by crustal contamination roughly 11 m y ago basalt eruptions showed an increasing tendency toward smaller volumes and less evolved compositions this implies that as extension in the oregon plateau proceeded the crust became less of an impediment to the passage of relatively unfractionated mantle melts initial sr nd and pb isotopic compositions of all the basalts show more correlation with the geographic position of eruption than with the composition of the lava primary control over the isotope systematics appears to be due to changes in the age and chemical characteristics of the crust and mantle throughout this area these observations suggest that an area of some 130 000 km2 of crust and basalt depleted subcontinental mantle has been added to northwestern north america over the last 20 m y the intensity of basalt volcanism as manifest in the columbia river group and in the oregon plateau does not appear to reflect the presence of a hot spot beneath the area both the areal and temporal distribution of the lavas and their geochemical characteristics are more easily reconciled with the suggestion that this crustal section grew by back arc spreading in a manner analogous to the formation of the marginal basins of the western pacific this spreading initiated along the western border of the archean wyoming craton with the location of mantle ascent melt production and extraction controlled by the changing geometry of subduction the existence of a zone of lithospheric weakness and the presence of a thick low density subcontinental mantle keel existing beneath the archean craton in this area

synthesis of the structural evolution and associated gold mineralization of the cadillac fault abitibi canada abstract the cadillac fault zone cfz is a 250 km long east trending crustal discontinuity associated with numerous world class gold deposits in the archean abitibi subprovince in quebec the fault follows the boundary between the abitibi and the pontiac subprovinces and can be divided into four segments 1 rouyn 2 joannes 3 malartic and 4 val d or each displays different attitudes and structural styles the malartic segment strikes se whereas the other segments are e trending abrupt changes in strike or smaller scale faults represent limits between the segments structural styles include four generations of foliation with numerous shear sense indicators and folding specifically related to each foliation structural features can be correlated from one segment to another but display clear variations in attitude or expression a stretching lineation signature is definitely associated with each segment in the malartic segment the predominant e trending schistosity displays an angular relationship with the contact between groups and bedding suggesting that the se trending architecture of this segment was already emplaced prior to ductile deformation the evidence suggests a model where the cfz originates from several independent structures inherited from volcanism prior to timiskaming deformation and abitibi pontiac accretion the proto cfz was then linked up during the timiskaming deposition forming high strain zones inherited structures form the frame on which each deformation event was overprinted with different responses depending on segmentation the structural styles of each segment can be integrated in a structural evolution model for the cfz the n s shortening led to the development of a dominantly e striking penetrative foliation and folding during the subsequent extensional stage the cfz was reactivated with normal kinematics during the pontiac uplift a third deformation event is represented by the local development of nw trending cleavage axial planar to the s shaped minor folds finally a late nw se shortening event produced a dextral strike slip movement with ne striking vertical foliation and development of z shaped folds on bedding surfaces and anterior fabrics linkage of the cfz had a major impact on hydrothermal activity in the southern abitibi as it channeled fluids along large scale pathways distribution of gold bearing intrusions can also be related to segmentation and inherited structures of the cfz this association is expressed by gold mineralization styles present in several longitudinal fields reflecting fault segmentation

regional correlation of grande ronde basalt flows columbia river basalt group washington oregon and idaho the tholeiitic flood basalts of the columbia river basalt group of middle and late miocene age cover more than 200 000 km2 in washington oregon and idaho the most voluminous formation of the group the grande ronde basalt erupted for 2 m y from north northwest trending fissure systems concentrated in southeast washington and adjacent oregon and idaho four magnetostratigraphic units designated r1 n1 r2 and n2 from oldest to youngest are recognized on the basis of polarity in the grande ronde and provide the broad stratigraphic framework for the formation in this study major element chemistry and relative stratigraphic position within the polarity intervals are used to identify and correlate individual flows and sequences of flows within the grande ronde basalt on a regional scale systematic examination of more than 350 analyses from 47 stratigraphic sections show that most flows fall into one of five major chemical groupings which are distinguished primarily by small but significant variations in mgo tio2 and p2o5 content in addition four minor chemical types local to the eastern part of the province have been identified feeder dikes of each chemical type have also been located flows or packets of flows of each chemical type can be correlated between field sections to define specific chemical stratigraphic subunits these subunits consist of several flows collectively 30 150 m thick subunits of most chemical types are repeated at irregular intervals throughout the formation no progressive chemical trend occurs within the grande ronde many of the chemical stratigraphic subunits extend to the margins of the province although most are confined to the source region in eastern washington although the total number of subunits is less in the west away from the fissure systems the total thicknesses of the n2 and r2 magnetostratigraphic units are each as thick or thicker than the corresponding units in eastern washington the greatest thicknesses occur in the central part of the province within the pasco basin the distribution of basalt relative to the location of vents as well as the relative east west thicknesses suggests that basalt flowed hundreds of kilometres westward during the most voluminous grande ronde eruptions ponding against the irregular margin of the cascade range and being diverted through the ancestral columbia gorge toward the washington oregon coast between these huge sheetflood events smaller eruptions blanketed areas within the source region and ongoing regional subsidence created a shallow westward draining basin in the center of the province

asynchronous records of brunhes matuyama reversal in marine sediments and chinese loess review and discussion abstract geomagnetic polarity reversals are generally considered to occur synchronously around the world and are commonly used as time markers to correlate events between different sediment archives they also play an indispensable role for understanding the mechanisms which generate the earth s geomagnetic field nevertheless in the case of the most recent reversal the brunhes matuyama b m reversal 780 ka comparison of paleomagnetic studies in chinese loess paleosol sequences versus marine cores reveal a marked discrepancy in timing of this event this timing offset has led to uncertainties in correlating the chinese loess paleosol sequences with other global climate records this paper briefly reviews the history of study on the b m reversal both in marine and chinese loess sediments and reviews several contentious explanations of the aforementioned b m boundary offset finally the most recent progress of paleogeomagnetic field variation studies using cosmogenic 10 be in chinese loess is introduced with the purpose of suggesting a potential way to address this b m boundary timing offset issue

crustal structure and kinematic framework of the northwestern pontiac subprovince quebec an integrated structural and geophysical study structural mapping gravity and magnetic modelling and interpretation of a deep seismic profile in the northwestern pontiac subprovince outline the crustal structure and early structural developme

migration of hydrocarbons and secondary porosity in sandstones a unified hypothesis concerning generation and migration of hydrocarbons and development of secondary porosity in sandstones is presented carbonic acid is considered to be the primary reagent responsible for dissolution in the development of secondary porosity in sandstones the degree to which reservoir quality is enhanced by development of secondary porosity may be proportional to the amount of hyrocarbons generated as well as to the amount of unstable constituents during diagenesis in some cases formation of secondary porosity may be responsible not only for development of reservoir but also for a significant element of entrapment by the partial but areally and stratigraphically selective dissolution of sandstone in the migration path of hydrocarbons jmt

the use of lithogeochemistry in delineating hydrothermal fluid pathways and vectoring towards gold mineralization in the malartic district quebec abstract the world class oxidized intrusion related canadian malartic gold deposit with reserves estimated at 5 56 moz au grading 1 10 g t au and a total geological endowment of 16 3 moz au is one of the largest gold deposits in the archean superior province of canada the gold mineralization is hosted predominantly by pontiac group metasedimentary rocks piche group metavolcanic rocks and quartz monzodiorite to granodiorite porphyritic intrusions the ore takes the form of a low grade envelope of disseminated pyrite 0 35 to 1 g t au grading inwards into higher grade 1 g t au stockwork and breccia zones hydrothermal alteration in the metasedimentary rocks is zonally distributed around the fluid pathways proximal alteration is characterized by a microcline albite quartz replacement type assemblage with lesser phlogopite calcite fe dolomite pyrite and rutile the distal alteration assemblage comprises biotite microcline albite phengite quartz calcite pyrite and rutile in this study we assess the magnitude and distribution of fluid rock interaction in the metasedimentary rocks of the malartic district the metaturbidites are separated into four lithotypes based on grain size to reduce the effects of primary depositional processes on mass change calculations despite the variability in protolith composition the metasedimentary rocks define a geochemically consistent cogenetic sequence the results of the mass transfer calculations indicate progressive gains in co2 s k2o and loi as well as au te w ag as be sb bi mo and pb from background to distal and proximal alteration zones relative to the least altered samples molar element ratio analysis alkali aluminum indicates an increase in alkali metasomatism k and na adjacent to the main hydrothermal fluid pathways which is manifested by the progressive stabilization of microcline and albite at the expense of oligoclase biotite and white mica ore associated pathfinder elements delineate broad enrichment patterns around the deposit and are used to understand hydrothermal fluid circulation in the malartic district a statistical approach based on a comparison of the mass change results with the background composition provides robust constraints on the magnitude and extent of the lithogeochemical haloes generally the alteration forms envelopes that extend along the s2 fabric with the largest lithogeochemical anomalies e g au w te and ag reaching up to 10 km in length and 2 km in width the results of this study demonstrate that whole rock lithogeochemistry can provide a valuable tool with which to define vectors toward gold mineralization in a regional exploration context

critical evaluation of the age of the deccan traps india implications for flood basalt volcanism and faunal extinctions attempts to evaluate the possible link between flood basalt volcanism and the faunal extinctions observed at the k t boundary must rely on precise knowledge of the amount of such volcanism that took place 65 ma the current geochronological data do not permit close resolution of either the timing or the duration of deccan volcanism the age of the bulk of the material is placed in the range 70 60 ma further resolution of this hypothesis awaits acquisition of high quality 40 ar 39 ar plateau ages on key sections of the deccan traps

hawaiian xenolith populations magma supply rates and development of magma chambers hawaiian volcanoes pass through a sequence of four eruptive stages characterized by distinct lava types magma supply rates and xenolith populations magma supply rates are low in the earliest and two latest alkalic stages and high in the tholeiitic second stage magma storage reservoirs develop at shallow and intermediate depths as the magma supply rate increases during the earliest stage magma in these reservoirs solidifies as the supply rate declines during the alkalic third stage these magma storage reservoirs function as hydraulic filters and remove dense xenoliths that the ascending magma has entrained during the earliest and latest stages no magma storage zone exists and mantle xenoliths of lherzolite are carried to the surface in primitive alkalic lava during the tholeiitic second stage magma storage reservoirs develop and persist both at the base of the ocean crust and 3 7 km below the caldera only xenoliths of shallow origin are carried to the surface by differentiated lava during the alkalic third stage magma in the shallow subcaldera reservoir solidifies and crustal xenoliths including oceanic crustal rocks are carried to the surface in lava that fractionates in an intermediate depth reservoir worldwide xenolith populations in tholeiitic and alkalic lava may reflect the presence or absence of subvolcanic magma storage reservoirs

predicting rock type and detecting hydrothermal alteration using machine learning and petrophysical properties of the canadian malartic ore and host rocks pontiac subprovince quebec canada abstract the canadian malartic deposit is a world class intrusion related archean gold deposit hosted in the pontiac subprovince superior province in quebec canada laboratory petrophysical properties measurements were performed on 824 rock samples collected from the various rock types observed within the ore body and peripheral host rocks the various rock types present in the malartic district mainly meta sedimentary rocks felsic intermediate intrusive rocks and mafic dykes have contrasting grain densities and magnetic susceptibilities using support vector machines it is shown that these two physical properties can be used to predict the rock type of a sample with an average precision and recall rate of 89 within the meta sedimentary rocks class variations in magnetic susceptibility are due to the changes in mineralogy associated with hydrothermal alteration these are caused by the destruction of iron bearing silicate minerals and magnetite in unaltered rocks 10 4 to 10 3 si to form pyrite carbonates k feldspar and fe depleted hydrothermal biotite in altered rocks 10 5 to 10 4 si within the felsic intermediate intrusive rocks grain densities below 2 7 g cm3 and magnetic susceptibilities in the 10 6 to 10 4 si range yield the highest probabilities that a rock has been submitted to carbonate and pyrite alteration however magnetic susceptibility and grain density of these intrusive rocks are also dependent on their fe2o3 al2o3 and tio2 al2o3 ratios which are not related to the hydrothermal alteration footprint but are rather due to distinct protolith compositions within the mafic dykes grain density is the best indicator of hydrothermal alteration unaltered mafic dykes 2 95 to 3 10 g cm3 are mostly composed of amphibole whereas altered mafic dykes 2 70 to 2 95 g cm3 have reduced amphibole contents and higher abundances of carbonates pyrite quartz and biotite alteration the support vector machine classifier is extended to predict if meta sedimentary rocks felsic intermediate intrusive rocks and mafic dykes have undergone hydrothermal alteration with average f1 scores of 73 69 and 93 respectively in altered meta sedimentary rocks the integration of grain density and magnetic susceptibility allows the identification of altered but unmineralized samples the classifier is further extended to predict if the gold content of meta sedimentary rocks is above or below threshold values of 0 01 0 1 and 1 ppm with average f1 scores of 83 80 and 76 respectively using conceptual models of the rock physical properties at the malartic district scale it is shown that ground magnetic surveys are the most promising geophysical tool for early stage greenfield exploration of this type of deposit however depending on the scale at which the surveys are conducted magnetic susceptibility contrasts between the various investigated rock types can overshadow the specific signatures of hydrothermally altered rocks this in part explains why past airborne geophysical exploration campaigns for this type of deposit in the malartic district were inconclusive finally the machine learning process used in this case study can be applied in advanced exploration stages during which drilling and either subsequent laboratory petrophysical analyses of core samples or downhole geophysical surveys produce large amounts of data that can be used to train prediction models

crustal velocity models for the archean abitibi greenstone belt from seismic refraction data we present velocity models for two seismic wide angle refraction profiles across the archean abitibi greenstone belt and the pontiac subprovince the seismic profiles are 210 and 220 km long traveltime inversion and amplitude forward modelling were used to obtain two dimensional velocity structure and interface geometry the main features of the velocity models include 1 three crustal layers 2 variable velocities 5 6 6 4 km s in the upper crust 0 12 km with the higher velocities generally associated with mafic metavolcanics and the lower velocities with metasediments and granitic plutons 3 a relatively uniform middle crust 12 30 km with velocities ranging from 6 4 to 6 6 km s 4 a velocity increase of 0 3 km s across the middle crust lower crust boundary 5 a lower crust 30 40 km with velocities increasing from 6 9 km s at the top to 7 3 km s at the base 6 an average upper mantle velocity of 8 15 km s 7 depth to moho of about 40 km in the north central abitibi belt decreasi