mudstone petrology of the mesoproterozoic unkar group grand canyon u s a provenance weathering and sediment transport on intracratonic rodinia abstract the synthesis of mudstone petrology and interbedded sandstone detrital zircon geochronology provides insights into the provenance weathering regime hydrodynamic effects and diagenesis of the mesoproterozoic unkar group unkar mudstones were derived largely from the grenville orogen go of southwest texas and the adjacent yavapai mazatzal ym and southern granite rhyolite sgr terranes detrital zircon data indicate distinct pulses of go derived sediment during hakatai and dox formation deposition whereas shinumo sediment contains a larger component of ym and sgr material an increase in plagioclase and biotite abundances in the dox further suggest an orogenic pulse bulk chemistry including elevated cr and ni abundances and ree systematics implicate a heterogeneous provenance for unkar sediment best approximated by a mix of granite or granodiorite with some basalt weathering of unkar sediment as determined from mudstone and crystalline source bulk chemical trends is characterized as moderate cia values between 55 and 70 indicative of a temperate climate in an orogenic setting small less than 10 wt amounts of detrital kaolinite and chlorite are consistent with a moderate temperate weathering regime illite the dominant clay mineral in the unkar group is largely the 2m1 polytype and detrital in origin however it is estimated that 7 of the k in the escalante creek member results from k metasomatism and may form up to 15 wt authigenic 1m or 1md illite the clay size fraction and zircon are the chief contributors to the ree budget in unkar mudstones lithostratigraphic variations in the hree distributions can be attributed partially to variable zircon abundance in the silt size fraction a likely function of hydrodynamic sorting age constraints on the duration of unkar group sedimentation range from 30 to 100 my for the 1100 m conformable shinumo dox succession and result in compacted sedimentation rates of between 1 0 and 3 6 cm 103 yr these rates are comparable to mesozoic foreland basin settings

geochemistry of shales from the archean 3 0 ga buhwa greenstone belt zimbabwe implications for provenance and source area weathering abstract phyllites from the archean 3 0 ga buhwa greenstone belt zimbabwe were deposited on a stable cratonic platform analyses of the phyllites generally define a single geochemical group based on major and trace element abundances the phyllites are strongly depleted in cao na2o and sr with respect to average archean upper crust by contrast k2o ba and rb are enriched several times relative to average archean upper crust reflecting basin scale k metasomatism transition metals are somewhat depleted whereas high field strength elements are typically enriched relative to average archean upper crust samples generally have fractionated lree patterns average cen smn 2 4 with small negative eu anomalies average eu eu 0 78 and generally flat hrees average gdn ybn 1 3 such geochemical characteristics suggest that the source dominantly consisted of tonalite with less common occurrences mafic volcanic rocks and granite mixing calculations which massbalance the rees and th sc suggest 70 tonalite 15 mafic volcanic rocks and 15 granite as possible proportions for the source of the phyllites a nearby early archean 3 5 ga continental nucleus preserves similar lithologies and is the probable source intense chemical weathering of the source terrane is indicated by premetasomatized chemical index of alteration values of 95 100 nearly complete depletion of cao and na2o and high al2o3 na2o ratios fine grained sediments of comparable age and presumed tectonic setting elsewhere in southern africa show similar geochemical characteristics implying that source area compositions and weathering intensities were similar these mature platformal deposits suggest a tectonically stable environment where intense chemical weathering took place at 3 0 ga across southern africa

distinguishing climate and time in the soil record mass balance trends in vertisols from the texas coastal prairie mass balance relationships of 10 major elements in a vertisol climosequence and chronosequence formed on alluvial terraces on the texas gulf coastal plain indicate that the soils develop characteristic element translocation patterns in response to climatic forcing and reach mass flux equilibrium within a relatively short period of time vertisols within the climosequence age 35 ka 800 1500 mm mean annual precipitation map approach a net mass flux of 16 3 of parent element contents corresponding to a weathering flux of 1 0 x 10 3 to 1 3 x 10 3 g cm 2 yr 1 when map exceeds 900 mm net mass flux assessments in a vertisol chronosequence 0 4 35 ka 1000 mm map show that this equilibrium is achieved within 5 6 k y below 900 mm map positive net mass fluxes indicate that vertisol profiles are gaining material at a rate of 0 2 x 10 3 g cm 2 yr 1 vertisols forming in drier map areas approach equilibrium with climate by different mechanisms and at different rates compared to their wetter counterparts and may not be as sensitive to millennial scale climate shifts

illite and hydrocarbon exploration illite is a general term for the dioctahedral mica like clay mineral common in sedimentary rocks especially shales illite is of interest to the petroleum industry because it can provide a k ar isotope date that constrains the timing of basin heating events it is critical to establish that hydrocarbon formation and migration occurred after the formation of the trap anticline etc that is to hold the oil illite also may precipitate in the pores of sandstone reservoirs impeding fluid flow illite in shales is a mixture of detrital mica and its weathering products with diagenetic illite formed by reaction with pore fluids during burial k ar ages are apparent ages of mixtures of detrital and diagenetic end members and what we need are the ages of the end members themselves this paper describes a methodology based on mineralogy and crystallography for interpreting the k ar ages from illites in sedimentary rocks and for estimating the ages of the end members

depositional history of the middle proterozoic castner marble and basal mundy breccia franklin mountains west texas abstract the ca 1260 ma castner marble is composed of six metamorphosed lithologies stromatolitic limestone cryptalgalaminite massive limestone hornfels mudstone and tuff rhythmite and flat pebble conglomerate on the basis of the associations of these lithologies the castner marble can be divided into three distinct intervals a lower section containing stromatolitic and cryptalgalaminite units a middle section with thick hornfels and massive limestone beds and an upper section of rhythmites interlaminated to thinly interbedded carbonate and hornfels with interbedded flat pebble conglomerate the castner succession was deposited during a transgressive event on a low energy carbonate ramp the contact between the castner marble and the overlying basaltic mundy breccia marks a catastrophic event or series of events in which rhythmite units were disrupted and or displaced along arcuate detachment surfaces and redeposited as lenses of rhythmite megabreccia unconformably overlying intact rhythmite sequences these megabreccia lenses consist of large locally deformed rhythmite blocks in a matrix of muddy flat pebble conglomerate one megabreccia lens contains intervals bearing irregular shaped basalt clasts possible hyaloclastite layers and pod like basalt intrusions this polymictic lens may have been produced by submarine syn eruptive mixing of basalt and sediment local basalt intrusions may be part of the feeder system for this material and also the overlying basaltic mundy breccia the mundy breccia covered the irregular topography created by the megabreccia lenses locally loading and deforming the megabreccia into flame like structures the polymictic carbonate and basalt nature of one lens and presence of load features suggests that little time elapsed between deposition of the castner marble and the mundy breccia and therefore that the mundy breccia was most likely deposited in a submarine setting

fault slip distribution of the 1995 colima jalisco mexico earthquake abstract broadband teleseismic p waves have been analyzed to recover the rupture history of the large m s 7 4 colima jalisco mexico shallow interplate thrust earthquake of 9 october 1995 ground displacement records in the period range of 1 60 sec are inverted using a linear finite fault waveform inversion procedure that allows a variable dislocation duration on a prescribed fault the method is applied using both a narrow fault that simulates a line source with a dislocation window of 50 sec and a wide fault with a possible rise time of up to 20 sec that additionally allows slip updip and downdip from the hypocenter the line source analysis provides a spatio temporal image of the slip distribution consisting of several large sources located northwest of the hypocenter and spanning a range of rupture velocities the two dimensional finite fault inversion allows slip over this rupture velocity range and indicates that the greatest coseismic displacement 3 4 m is located between 70 and 130 km from the hypocenter at depths shallower than about 15 km slip in this shallow region consists of two major sources one of which is delayed by about 10 sec relative to a coherent propagation of rupture along the plate interface these two slip sources account for about one third of the total p wave seismic moment of 8 3 10 27 dyne cm m w 7 9 and may have been responsible for the local tsunami observed along the coast following the earthquake

prediction of some weathering trends of plutonic and volcanic rocks based on thermodynamic and kinetic considerations abstract the exposed crust consists mainly of plagioclase 35 quartz 20 k feldspar 11 volcanic glass 12 biotite 8 and muscovite 5 quartz is a resistate thus feldspars and glass represent approximately 75 percent of the labile minerals the weathering characteristics of these constituents are summarized in the context of thermodynamic mass balance and kinetic considerations experimentally determined release rate constants were used to predict the proportions of ca na and k released by feldspars of plutonic rocks granites to gabbros to weathering solutions the chemical weathering trends of the weathered residues calculated from the kinetic data conform closely to the initial trends observed in some recent weathering profiles demonstrating the accuracy of the predictions since the weathering of feldspars is controlled by processes that should not change through geological time the relative release rates of ca na and k from the feldspars of granitic rocks can be calculated for future and past episodes of continental weathering experimentally determined release rate constants are not available for a wide range of volcanic glass compositions but the limited data indicate that compositional trends are predictable in weathering profiles developed on volcanic rocks the kinetic data available for rhyolitic glasses accurately predict the initial weathering trends observed in a recent rhyolite weathering profile

ripple marks in quartz arenites of the hurwitz group northwest territories canada evidence for sedimentation in a vast early proterozoic shallow fresh water lake abstract supermature quartz arenites of the whiterock member deposited in the early proterozoic intracratonic hurwitz basin have a maximum thickness of 400 m and a minimum areal extent of 100 000 km2 the unit is unusual in that it is virtually devoid of other rock types and displays sedimentary structures that consist almost entirely of short spaced ripple marks and parallel stratification both formed by oscillatory flow the ripples have spacings that are consistently small average

constitutive mass balance relations between chemical composition volume density porosity and strain in metasomatic hydrochemical systems results on weathering and pedogenesis relations characterizing the chemical physical and mechanical changes resulting from metasomatic hydrochemical processes are developed using mass balance models which formally link chemical composition to bulk density mineral density volumetric properties porosity and amount of deformation strain rigorous analysis of aqueous solute transport effects is then made possible in a variety of porous media flow environments including chemical weathering pedogenesis soil formation diagenesis ore deposition and enrichment and metamorphism application of these linear constitutive relations to chemical weathering profiles shows that immobile and locally mobile chemical elements with masses conserved on the scale of soil profiles can be accurately identified from analysis of appropriate data arrays and then used as natural geochemical tracers to infer the nature and extent of hydrochemical weathering processes and volume changes during pedogenesis assumptions commonly made in the past about the supposed immobility of certain elements e g ti and zr become unnecessary quantitative differentiation between the effects of residual and supergene fractionation is then easily made these methods are applied to ni rich laterites developed by weathering of ultramafic rocks showing that during ordinary residual enrichment ni is concentrated by as much as 4 protolith peridotite concentrations this occurs simply by silicate mineral dissolution and removal of chemical elements other than ni e g mg with a corresponding reduction in saprolite density and increase in bulk porosity without significant deformation in contrast laterites with mineable concentrations of ni which are similarly undeformed such as the nickel mountain mine in riddle oregon have experienced in addition to residual enrichment strong supergene enrichment by fractionation of ore elements between a leached zone from which ni is extracted and a complementary enriched zone positioned farther along the direction of ground water flow soil forming processes in podzol chronosequences developed on sandy beach terraces of the mendocino coast of california involved soil column collapse of 60 percent by dissolution of silicate minerals in the albic horizon of al and fe leaching and 70 percent dilation expansion in the overlying organic rich layer by root growth the amount of erosion based upon paleosurface reconstructions using the excess mass of fe al pb ga and cu in the zone of supergene enrichment spodic horizon below the ground water table indicates that subsurface erosion by dissolutional collapse is three times that of surficial erosion finally using published chemical data for ti zr and cr on major bauxite deposits in australia where erosion rates are thought to be low we infer that there may have been major amounts of dissolutional collapse to explain the upwards increase of detrital zircon and rutile in weathering profiles

beyond whole rock geochemistry of shales the importance of assessing mineralogic controls for revealing tectonic discriminants of multiple sediment sources for the ouachita mountain flysch deposits the origin of the ouachita mountains has been the focus of significant debate for decades considerable confusion also exists concerning the provenance of the carboniferous flysch of the ouachitas trace element geochemistry of shales from the stanley group delineates the provenance of the sediments and provides clues to the plate tectonic evolution of the southern continental margin during mississippian time th sc and cr th ratios indicate a cratonic source for the majority of the stanley group sedimentary rocks however in several samples low th sc ratios and high cr th ratios suggest a contribution from a mafic source using element ratio diagrams all of the samples plot along a curve consistent with a two component mixing model consisting of a dominant felsic and a subordinate mafic source the heavy mineral fraction of these shales sequester many of the trace elements used in whole rock studies monazite is ubiquitous in trace amounts and is the probable site for much of the rare earth elements in the whole rock the occurrence of monazite almost exclusively in sialic igneous rocks implies that sm nd isotopic signatures are not sensitive to sediment input from more mafic sources in some stanley shale samples chromite and mn oxides were identified and positively identify an oceanic crustal component as a source of stanley group sediment the results of this study emphasize the importance of determining the mineralogic sites of trace elements and realization of specific mineralogic contributions from mafic or sialic tectonic provenances

paleoclimatic control on the composition of the paleoproterozoic serpent formation huronian supergroup canada a greenhouse to icehouse transition unlike other feldspathic arenites in the 12 km thick huronian supergroup strata of the 250 350 m thick serpent formation contain abundant plagioclase deposited in a distal alluvial setting the serpent formation consists of arenites with minor shale and siltstone the plagioclase abundance could reflect unroofing of a unique source relative to other huronian units or could be due to less intense chemical weathering major trace and rare earth element ree geochemistry of the serpent formation are used to evaluate these two possibilities serpent formation source rocks form part of the archean superior province which mainly consists of tonalitic granitic and supracrustal assemblages and their metamorphosed equivalents in al2o3cao na2ok2o acnk space shales define a linear array which indicates that the unweathered source contained plagioclase and k feldspar in the ratio of 5 1 this particular array indicates that serpent fluvial systems tapped a variably weathered carapace developed on the superior province rather than purely fresh bedrock a modelled mixture of 80 tonalite and 20 granite group rocks reproduces ree and trace element characteristics of the serpent formation sandstones and shales average eu eu 0 87 lan smn 4 4 gdnybn 1 8 and th sc 1 65 mass balance considerations indicate that less than 5 supracrustal group rocks can be added at the expense of the granite group a unique source for serpent detritus is rejected because underlying and overlying units have approximately the same provenance composition plagioclase k feldspar ratios in serpent sandstones range from 2 1 to 1 2 which achieves mass balance among source mudstone and sandstone compositions when plotted in acnk space achievement of mass balance implies that sandstones and mudstones were derived from the same weathering profiles these findings indicate that plagioclase preservation in the serpent formation is related to less intense compared with other huronian arenites paleoweathering conditions probably a harbinger of the widespread continental glaciation recorded in the overlying gowganda formation

another look at rare earth elements in shales linear correlation and mass balance considerations indicate that clays are more important than zircon or other heavy minerals in hosting both light and heavy ree in cratonic shales considering the uniformity of the eu anomaly and ree distributions in cratonic shales it seems likely that detritus is thoroughly mixed during weathering and sediment transport and that the ree patterns in shales reflect the average ree pattern of the sources regardless of the ratio of clays to heavy minerals in the shales shales suspended river loads and near shore marine sediments all have broadly similar ree patterns and reported hree depletions in the latter two groups may reflect incomplete dissolution of heavy minerals during sample preparation ree distributions in shales and suspended river loads determined by inna are similar to those in average upper continent crust and ree distributions in shale averages nasc and paas seem to be reasonably representative of the average composition of post archean upper continental crust

okak bay amt data set case study lessons in dimensionality and scale electromagnetic em exploration for base metals using the natural source audio magnetotelluric amt technique has increased significantly during the last five years due to enhancements in all aspects of amt and to the demand for imaging deeper than conventional controlled source em methods however regional currents induced by natural sources can be problematic in certain situations and the appropriate interpretational dimensionality must be known herein we demonstrate that a two dimensional 2d interpretation is valid for a defined frequency band but that the effects of large scale three dimensional 3d structures must be considered at lower frequencies using an amt dataset from an area located north of voisey s bay labrador canada we analyse the responses to determine the appropriate dimensionality and to test them for internal consistency maps of the distortion corrected data identify the lateral extent of connected conducting mineralization intersected by a drilling program one dimension

icp ms a powerful tool for high precision trace element analysis in earth sciences evidence from analysis of selected u s g s reference samples abstract inductively coupled plasma mass spectrometry icp ms is a powerful analytical technique with considerable application to geochemistry in this paper we demonstrate the power of icp ms as a tool in petrogenetic studies data for 28 elements in seven u s g s basalt and andesite reference samples agv 1 bcr 1 bhvo 1 bir 1 dnc 1 w 1 and w 2 are presented accuracy and precision standard deviation for 26 of these elements is either excellent a detailed description of the icp ms technique we have developed is given a simple hf hno 3 dissolution in screwtop teflon bombs was used for sample preparation icp ms instrumental sensitivity was measured by external solutions with surrogate calibration for 4 elements matrix correction was made by use of standard addition naturally occurring internal standards rb y ce and pb were used to correct for instrumental drift between sample and spiked sample measurements we report data for the elements li cs rb sr ba zr hf nb ta y sc u th pb and all 14 of the ree in addition the procedure acquired data for be mo w tl and bi many elements in this array are crucial for the evaluation of petrogenetic models and source tracing herein lies the real power of icp ms one technique one dissolution small sample size good limits of detection good to excellent accuracy and good precision for many geochemically important elements

paleoclimatology and provenance of the glaciogenic gowganda formation paleoproterozoic ontario canada a chemostratigraphic approach the gowganda formation dated as about 2 3 ga is widely but not universally interpreted as glaciogenic and our chemostratigraphic study supports such an interpretation the bulk compositions of diamictite matrix materials and associated argillites are considered to have resulted from the mixture of two detrital components one is glacial flour similar in composition to the unweathered archean upper crust whereas the other is chemically weathered detritus resembling average proterozoic shale in composition the first component has a chemical index of alteration cia value of 50 reflecting the abundance of feldspar and dearth of aluminous clay minerals the second component has a cia value of 70 and was produced primarily in weathering profiles by chemical weathering typical of temperate climatic regimes diamictite matrix materials 92 samples have an average cia value of 57 the argillites have higher values the average is 62 97 samples the lower cia values of the diamictites reflect a higher proportion of glacial flour suggesting a stronger influence of frigid climatic conditions than that which prevailed during deposition of the argillites systematic trends in cia values are developed across diamictite argillite boundaries and within a thick argillite between two major diamictite units these values reflect climatic variations with maximum amelioration during deposition of the middle section of the argillite rare earth element data and th sc and ti al ratios from the diamictite matrix materials all suggest a provenance that included a large supracrustal component 45 together with lesser amounts of tonalite trondhjemite granite and granite the proportion of supracrustal materials such as volcanic rocks and shales may have been exaggerated by selective comminution during glacial transport the matrix materials of the gowganda diamictites are severely depleted in ca and proportionally enriched in na relative to the source the anorthitic component of plagioclase has been selectively replaced by an albitic component during diagenesis and metamorphism a chemostratigraphic approach provides high resolution data for reconstruction of paleoclimatic conditions and valuable information concerning provenance and metasomatic processes

thermochronology of the grenville orogeny in west texas abstract the carrizo mountain group cmg comprises the westernmost exposures of the metamorphic core of the grenville orogen in texas unlike the correlative llano uplift 500 km to the east the cmg underwent one phase of medium p t metamorphism peak t was attained during d2 in all cmg exposures the peak metamorphic paleotemperatures decrease to the northwest in the overall direction of tectonic transport from staurolite to biotite grade garnet in the southeast carrizo mountains grew isothermally and probably during decompression i e part of a clockwise p t loop at ca 520 c in between biotite grade d1 and d3 events hornblende in this region cooled below closure t during d2 at 1057 6 ma within error of muscovite closure in the northwest carrizo mountains at 1059 2 ma hornblende in higher grade exposures to the southeast cooled through closure t as late as 1002 7 ma muscovite in the garnet grade and higher exposures closed to ar diffusion at 992 4 ma to 981 3 ma biotite and partially recrystallized hornblende from the northwest carrizo mountains cooled through closure in this same time period we interpret these data as signifying 1 ar loss due to peak metamorphism in hornblende southeast carrizo mountains and muscovite northwest carrizo mountains at ca 1059 ma 2 post tmax cooling through 500 c in the higher grade outcrops until ca 1000 ma and 3 cooling of all cmg exposures through 300 c due to uplift along the streeruwitz thrust and erosion from 1000 to 980 ma these ages significantly post date known tectonism in the llano uplift and extend the duration of grenville orogenesis in texas by ca 115 million years the 1000 980 ma time period is a time of thrusting in frontal regions of the grenville province of canada and the northeastern us and shearing in the natal province south africa suggesting ultimate closure of the grenville orogen and supporting recent rodinia reconstructions

neoproterozoic chuar group 800 742 ma grand canyon a record of cyclic marine deposition during global cooling and supercontinent rifting abstract chuar group sediments were deposited in a marine cratonic basin synchronously with marine deposition in other basins of western north america and australia the top of the chuar group is 742 ma thus this succession offers a potentially important record of global climatic and tectonic changes during the mid neoproterozoic a critical time leading to possible global glaciation during supercontinent rifting chuar group cycles and sequences may record glacioeustatic fluctuations during the climatic transition into the sturtian ice age 750 700 ma as well as extension related to the dispersal of rodinia the mid neoproterozoic chuar group 1600 m thick is predominantly composed of mudrock with subordinate meter scale beds of dolomite and sandstone facies analysis leads to the interpretation of a wave and tidal influenced marine depositional system diagnostic marine features include 1 marine fossils and high local pyrite content in the mudrock facies 2 mudcracked mud draped symmetric ripples and reverse flow indicators in the sandstone facies 3 facies associations between all facies and 4 no unequivocal terrestrial deposits chuar facies stack into 320 dolomite and sandstone capped meter scale cycles 1 20 m thick and non cyclic intervals of uniform mudrock facies 20 150 m thick nearly all cycles have mudrock bases indicating subtidal water depths dolomite capped cycles shallow to peritidal environments peritidal cycles some indicating subaerial exposure of varying degrees exposure cycles sandstone capped cycles shallow to peritidal environments with no evidence of prolonged exposure peritidal cycles non cyclic intervals represent the deepest water depths 10s of meters peritidal cycles and non cyclic intervals dominate the lower and middle chuar group in the upper chuar group the percentage of exposure cycles increases as does the thickness of individual cycles and non cyclic intervals and the degree of subaerial exposure in the cycle caps correlation of cycles across 10 km shows that many cycles are laterally continuous and some vary in thickness some thickness changes are coincident with local and regional extensional structures these cycle trends reflect a resolvable mixture of tectonic and climatic controls cycles are interpreted to be high frequency and glacioeustatically controlled based on comparison of cycle character and thickness to phanerozoic examples and lateral continuity of many cycles respectively cycles in the lower and middle chuar group show characteristics suggestive of low amplitude sea level changes similar to meter scale cycles from global greenhouse climates in the phanerozoic upper chuar group cycles show characteristics suggestive of moderate amplitude sea level changes similar to phanerozoic greenhouse icehouse transition climates indicating an increase in continental ice volume variability in cycle thickness and lateral pinching of some cycle caps is controlled by variable tectonic subsidence evidenced by thickening trends coincident with active rift related structures and also by variability in sediment supply the presence of dolomite caps in this otherwise siliciclastic succession is interpreted to be due largely to rapid changes in climate related to short term glacioeustatically controlled sea level changes four crude lithostratigraphic sequences 150 775 m thick are defined based on dolomite poor to dolomite rich stratigraphic intervals long term changes in cycle cap lithology may be driven by long term changes in climate similar to short term glacioeustatic controls on meter scale cycle cap lithology large scale thickness variability in these sequences is due to local differential tectonic subsidence

k uptake by modern estuarine sediments during early marine diagenesis mississippi delta plain louisiana u s a abstract marine sediment porewaters are commonly depleted in k relative to conservative mixing trends but the mineralogical sink for k in the sediment has not been well characterized results are presented from a geochemical study of surface waters and porewaters and associated muddy sediments in the mississippi delta plain estuary spanning the salinity gradient from 0 12 evidence of k depletion in sediment porewaters is integrated with scanning transmission and analytical electron microscopy stem aem analyses of clay components to determine possible mineralogical sinks for k in the sediments conservative mixing between the freshwater influx from the atchafalaya mississippi river systems and gulf of mexico seawater controls the surface water major element composition porewaters from the most saline site however are depleted in k by up to 33 relative to overlying water implying uptake of k by the sediment textural characterization of the sediment at the same location by tem indicates that it is dominated by aggregates of highly disordered smectite rich clay material which is mixed with a small component of more ordered illite rich material aem compositions obtained on the most smectite like component in these aggregates indicates that the average k contents increase by up to 1 0 wt k2o relative to similar smectite rich clay in freshwater sediments ca2 concentrations in smectite rich clay decrease concomitantly whereas na concentrations are similar between the sites x ray diffraction xrd analyses confirm that smectite rich clay from the freshwater site has a larger interlayer spacing than smectite rich clay from the brackish water site consistent with exchange of smaller hydrated interlayer k na for larger ca2 mg2 aem data indicate that the discrete illite rich component also takes up k the integrated porewater and sediment analytical data imply that uptake of k by smectite rich clay is a rapid process decadal occurring at the earliest stages of diagenesis in estuarine environments upon exposure to brackish water our study shows that k uptake in marine sediments may not require the formation of authigenic minerals instead exchange of k for other interlayer cations in smectite rich clay and weathered illite may provide a substantial sink for k in the marine environment

material sources chemical weathering and physical denudation in the chubut river basin patagonia argentina implications for andean rivers abstract the chubut is a medium size 42 000 km2 river basin that drains the arid to semiarid patagonian seaboard and pours its waters into the southwestern atlantic ocean ca lat 43 20 documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape arcmin end document s long 65 04 documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2

geochemical investigation of a neoproterozoic glacial unit the mineral fork formation in the wasatch range utah the mineral fork formation is an isolated unit of neoproterozoic glaciogenic rocks in the wasatch range in northwestern utah geochemical analyses of diamictite and mudstone samples were carried out to document its major and trace element geochemistry comparison of the lower and upper parts of the formation which have been ascribed to different tectonic and sedimentological settings revealed no striking differences but the analyzed samples show a gradual upward increase in cao mgo fe 2 o 3 mno and p 2 o 5 and a decrease in sio 2 tio 2 al 2 o 3 ratios that suggest a provenance throughout the formation with an average composition slightly more mafic than average upper crust the mudstones are more k rich than the diamictites calculation of a chemical index of alteration cia is complicated by the presence of carbonate minerals after making corrections for carbonate content cia values range from 65 to 70 these values are higher than those from paleoproterozoic glaciogenic deposits some mudstones in the upper part of the formation are fe rich 15 fe 2 o 3 approaching the composition of iron formations the ferruginous nature supports previous correlation with fe rich glaciogenic rocks of the rapitan group in the northern canadian cordillera trace elements including rare earth elements suggest that the materials in the mineral fork formation were largely derived from a post archean source most of the widespread glaciogenic rocks of the neoproterozoic were derived from older sedimentary materials whereas the deposits of the earlier great glacial episode in the paleoproterozoic contain a much higher proportion of crystalline materials

tectonic evolution of paleoproterozoic rocks in the grand canyon insights into middle crustal processes paleoproterozoic supracrustal rocks of the upper granite gorge are divided into three mappable units the rama brahma and vishnu schists here collectively named the granite gorge metamorphic suite the brahma schist consists of mafic to intermediate composition metavolcanic rocks that have yielded an age of 1750 ma the rama schist consists of felsic metavolcanic rocks that have yielded an age of 1742 ma on the basis of the presence of relict pillow structures interlayering of metavolcanic units and the large volumes of metavolcanic rocks the combined volcanic package is interpreted to be metamorphosed island arc related submarine volcanic rocks the metavolcanic units are locally overlain by the metamorphosed arc basin submarine sedimentary rocks of the vishnu schist the term zoroaster plutonic complex previously used for all paleoproterozoic and mesoproterozoic plutonic rocks in the grand canyon is abandoned in favor of use of names for individual plutons plutons are classified into two genetically and temporally separate intrusive suites 1 1740 1713 ma plutons that are typically concordant to foliation range in composition from gabbro to granodiorite have calc alkalic chemistry and are interpreted to be arc related plutons and 2 1698 1662 ma granite plutons stocks and pegmatite and aplite dikes that have textures and intrusive geometries indicating they were emplaced synchronously with peak metamorphism and northwest southeast shortening the dominant tectonic fabric is a northeast striking subvertical variably developed foliation s 2 that is axial planar to plunging open to isoclinal upright asymmetric folds f 2 of an early penetrative apparently initially northwest striking foliation s 1 f 2 folds have wavelengths of kilometres and cause repetition of rock units across the transect partitioning of deformation in space and perhaps in time was strongly influenced by crustal heterogeneity for example plutons pegmatite networks and fold hinge zones acted as buttresses and s 2 high strain domains and shear zones are localized at the margins of these features generally northeast striking s 2 related shear zones segment the transect into blocks shear zones last moved late during shortening and postdate the metamorphic peak one of these the crystal shear zone may be a fundamental crustal province boundary paleoproterozoic rocks in the grand canyon record the accretion of 1750 1713 ma island arc rocks and their assembly to laurentia via dynamic interaction of deformation metamorphism and magmatism over 20 m y changes in styles of deformation from f 1 related thrusting and penetrative s 1 fabric development to f 2 related upright folding and variable s 2 fabric development coincided with changes in styles of magmatism the 1740 1730 ma arc related mafic to intermediate composition plutons preserve strong s 1 foliation the 1717 1713 ma arc related plutons preserve weak s 1 tectonic layering and locally strong s 2 strain the 1698 1685 ma granitic plutonism locally preceded s 2 and f 2 strain and was regionally synchronous with peak s 2 and f 2 strain 1700 1685 ma and peak metamorphism 1706 1697 ma more limited deformation and plutonism continued until at least 1662 ma

petrogenesis of mid proterozoic granitic magmas examples from central and west texas abstract circa 1 1 ga granitic magmatism in texas was manifested as two compositional groups 1 the 1 12 ga red bluff granitic suite in west texas and 2 1 12 1 07 ga granites of the llano uplift of central texas both suites share some characteristics typical of anorogenic granites e g potassium and iron rich bulk compositions fe rich hydrous silicates emplacement conditions involving low oxygen fugacities and water contents and exhibit similar isotopic characteristics however rock associations mineral chemistries and trace element compositions of the two suites are distinct and no single petrogenetic model for the two suites is possible the red bluff granitic suite includes cogenetic syenites quartz syenites and granites transitional ferrobasaltic dikes are also present in contrast syenitic and mafic rocks are not associated with the llano granites the llano granites contain biotite and calcic amphibole with lower fe fe mg ratios compared to those occurring in the red bluff rocks alkali amphiboles e g arfvedsonite occur in the red bluff granites but not in the llano granites the red bluff granitoids are characterized by high feo t mgo ratios high na2o k2o high concentrations of hfse and rare earth elements ree and other features typical of a type within plate granites e g the pikes peak batholith ppb the llano granites are geochemically distinct with generally higher p2o5 and sr lower na2o feo t mgo zr y and ree and much lower ta and nb nd isotopic data overlap between the two granite suites and have juvenile signatures however trace element data suggest different petrogeneses for the two suites the red bluff suite is interpreted as having a direct derivation from mantle sources via extended fractional crystallization of basaltic parental magmas with minor crustal assimilation the llano granites appear to represent anatectic melts derived from slightly older juvenile crustal sources some melts underwent fractional crystallization controlled by feldspar and accessory minerals the petrology and geochemistry of 1 1 ga granites in texas indicate that they should not be considered as part of a single anorogenic magmatic event the red bluff granitic suite was emplaced into a shelf sequence north of the grenville front within a broad zone characterized by mild extension in contrast llano granites are late stage intrusions emplaced into multiply deformed and metamorphosed crust south of the grenville front during or after waning stages of grenville orogenesis

rare earth elements in weathering profiles and sediments of minnesota implications for provenance studies abstract the relative abundance of rare earth elements in sediments has been suggested as a tool for determining their source rocks this correlation requires that weathering erosion and sedimentation do not alter the ree abundances or do so in a predictable manner we find that the rare earth elements are mobilized and fractionated by weathering and that sediments derived from the weathered materials can display modifications of the original pattern of rare earth elements of some due to grain size sorting of the weathered material however the ree distribution pattern of the provenance terrane can be recognized in the sediments

high resolution δ13c stratigraphy of the chuar group ca 770 742 ma grand canyon implications for mid neoproterozoic climate change a high resolution c isotope record based on 13 c org from organic rich shales and 13 c carb from dolomites in the ca 770 742 ma chuar group provides important new data for evaluating the signifi cance of large magnitude c isotope anomalies in neoproterozoic climate change three successive large magnitude isotopic excursions 8 15 are interpreted to represent primary seawater values based on a series of diagenetic tests and they are not associated with evidence of signifi cant long term 10 6 10 7 m y sea level change nor glaciomarine deposits intrabasinal correlation of 13 c org values suggests that most chuar shales record primary values and is consistent with previously reported h c ratios of 0 49 indicating that chuar shales experienced minimal thermal alteration although some chuar dolomites reveal early diagenetic alteration their 13 c dol values typically fall near those of coeval least altered dolomites or organic rich shales relative to dolomite values the chuar carbon record is interpreted to refl ect predominantly primary organic carbon 13 c values and contains suf

grenville magmatism in west texas petrology and geochemistry of the red bluff granitic suite zone of regional extension as suggested by recent geophysical and the red bluff granitic suite rbgs underlies part of the franklin petrological studies mountains in west texas it was emplaced into a middle proterozoic shelf sequence at 1120 35 ma the suite is predominantly metaluminous it consists of early granitic to quartz syenitic sills a main mass of alkali feldspar granite small bodies of alkali feldspar syenite leucogranitic dikes and late stage peralkaline

chromium and nickel in shale of the taconic foreland a case study for the provenance of fine grained sediments with an ultramafic source abstract to test the suitability of using shale geochemistry as a provenance indicator of a source with an ultramafic component we used i c p mass spectrometry to analyze 130 samples from seven localities from newfoundland to new york these samples represent mud from the foreland basin produced during the collision of the taconic are with north america in the ordovician the sandstones in the flysch contain detrital chromite and other detritus that indicate that the source contained ultramafic rocks presumably from ophiolites emplaced during collision this setting is an ideal test case for examining the spatial and temporal variation in cr and ni geochemistry of shale derived from ophiolite bearing highlands shale samples with high concentrations of cr and ni have a cr ni ratio of 1 4 which is approximately the cr ni ratio for ultramafic rocks 1 6 suggesting only minor geochemical partitioning but sandstones have a cr ni ratio of 3 0 suggesting significant sedimentary fractionation analyses of samples taken upsection in a single stratigraphic section suggest that proximity to the source influences cr and ni concentrations a decrease in cr and ni concentrations through time suggests that uplift and erosion of the thrust complex non ophiolitic diluted the ultramafic signal we attribute significant along strike variation in cr and ni concentrations to the relative proportion of ultramafic rocks in the source region this case study shows that cr and ni geoch mistry of shale from basin strata can be used to determine the lateral and temporal variability of ultramafic rocks in an active orogenic setting