the fazenda nova gold deposit goias magmatic arc late neoproterozoic intrusion related auriferous mineralization controlled by intracontinental strike slip faulting abstract the fazenda nova gold deposit with an inferred resource of 650 koz at 4 0 g t au cut off grade of 2 0 g t au consists of stockwork au as mineralization hosted in late neoproterozoic dikes in the goias magmatic arc central brazil the dikes which host the mineralization consist of a set of post collisional ilmenite bearing gabbros dolerites trachyandesite porphyry trachyandesite and trachyte porphyry known as the bacil ndia intrusions u pb dating indicates at least two pulses of dike intrusions the first pulse consists of small volume dikes with ages varying from 593 5 to 590 5 ma and the second pulse comprises a large volume of dikes with ages from 574 7 to 572 5 ma three veining or hydrothermal alteration stages are identified in the fazenda nova deposit the first is the early or main stage which comprises the pervasive biotitization of mafic minerals and a stockwork of quartz associated with a disseminated sulfide assemblage of arsenopyrite pyrrhotite stibnite in addition to hydrothermal siderite scheelite rutile and apatite the most common sulfide in this stage is a fine grained acicular arsenopyrite hosting inclusions of gold galena and pyrite grains the temperature estimated for acicular arsenopyrite pyrrhotite and gold is 340 c based on the arsenopyrite geothermometry the intermediate stage is represented by a stockwork of calcite quartz veins associated with hydrothermal sericite chlorite tourmaline epidote and titanite and a sulfide assemblage of pyrrhotite arsenopyrite pyrite the arsenopyrite at this stage displays a tabular texture with no inclusions of sulfides or gold the crystallization temperature of the tabular arsenopyrite pyrite was estimated to be 305 c the c o isotope data from the calcite veins of the second stage provided 13c values ranging from 9 68 to 11 57 and 18o values range from 12 87 to 13 84 these values are equivalent to those of hydrothermal mantle magmatic carbonates the late stage is represented by veining and brecciation infilled by monotonous calcite ankerite with cockade and colloform vein textures and no gold mineralization the dikes and gold mineralization present a strong relationship with the crustal scale strike slip bacil ndia fault this is a second order crustal scale fault that branches out of the major crustal scale moipora novo brazil shear zone mnbsz along a nnw direction the bacil ndia fault played an important role in channeling melts and mineralizing hydrothermal fluids to the upper crust the major activity of this fault occurred at 572 ma with the emplacement of the second pulse of dikes and most likely the development of hydrothermal stages in the fazenda nova deposit the collectively evidence allows fazenda nova to be classified as an epizonal reduced intrusion related deposit following that i the mineralization developed in a post collisional tectonic setting above previously metasomatized subcontinental lithospheric mantle ii the dikes crystallized from metaluminous magmas with a primary oxidation state which formed the ilmenite series intrusions iii the ore deposition had relatively high temperatures 340 300 c and vein textures formed only at shallow depths

from passive margin to continental collision geochemical and isotopic constraints for e morb and oib like magmatism during the neoproterozoic evolution of the southeast brasilia belt abstract metamorphic rocks derived from mafic rocks can be found in metamorphic belts and their geochemical and isotopic characteristics may be valuable sources of information for interpretation of ancient geologic magmatic scenarios the southern brasilia belt between the amazonian and sao francisco cratons in central brazil exhibits different amphibolite rocks that were emplaced into verissimo sequence and araxa group metasedimentary sequences during the neoproterozoic the obtained results indicate that the studied amphibolite rocks from pires do rio catalao area represent three magmatic events associated with three magmatic environments 1 a passive margin development at 979 ma a forearc back arc basin at 819 ma and a continental collision at 651 ma the first magmatic event related to continental extension and passive margin development is represented by the emplacement at 979 17 ma of e morb low k tholeiitic basalts with positive end t values between 4 4 and 2 and tdm between 1 26 and 1 82 ga related with the verissimo metavolcanosedimentary sequence the zircon populations include a high amount of metamorphic inherited grains the second magmatic event related with an forearc back arc extension is represented by e morb basalts with negative nb ta anomalies and slightly positive end t values of 0 78 to 2 71 cogenetic with the araxa group an oib like basalt sample is also related to this stage and has a u pb age of 819 7 6 3 ma end t values of 1 82 to 2 57 ehf t in zircon around zero and tdm ages from 1 03 to 1 18 ga the third event is represented by syn to late tectonic oib like basaltic gabbroic magmatism at 651 6 ma with positive end t value of 2 57 and younger tdm age at 1 18 ga that can be associated to regional continental collision and closure of the neoproterozoic ocean also provenance studies into metasedimentary rocks support that passive and active margins could be successively superposed during the main thrusting tectonic event in the brasiliano orogeny but slices of different stratigraphic units preserve their depositional isotopic signature these findings suggest that the occurrence of main mafic magmatic events over a timespan as long as the entire evolution of the brasilia belt 1 0 0 6 ma might be a key to individualize superposed tectonic basins in central brazil

mid neoproterozoic ridge subduction and magmatic evolution in the northeastern margin of the indochina block evidence from geochronology and geochemistry of calc alkaline plutons abstract the mid neoproterozoic medium to high k calc alkaline magmatic rocks in the northeastern margin of the indochina block sw china provide important insights into the relationship of the indochina block with the gondwana supercontinent here we report zircon la icp ms u pb data from the early and late stage plutons which yield weighted mean 206pb 238u ages of 765 ma and 732 739 ma suggesting mid neoproterozoic emplacement the zircon ehf t values show a range of 3 2 to 2 4 average 0 1 0 9 with tdmc of 1510 to 1870 ma for the early plutons and 5 4 to 5 1 average 2 1 to 3 9 with tdmc of 1366 to 1985 ma for late plutons both groups show similar geochemical characteristics including high mg enrichment of lile and lree slight negative eu anomalies and strongly negative nb ta and ti anomalies with all the samples falling within the continental island arc field in tectonic discrimination diagrams these features suggest that the early and late stage magmas were produced by the mixing of mantle derived magma and crust derived magma in different proportion within an active continental margin in subduction related continental arc tectonic setting the linear zoning and roughly parallel distribution of the two generations of intrusions with a hiatus of 20 ma might suggest an episode of ridge subduction with asthenosphere upwelling through the slab window that generated the second phase of plutons

laser ablation coupled with icp ms applied to u pb zircon geochronology a review of recent advances abstract improvements in the technology of laser ablation and icp ms instruments make la mc icpms a rapid precise and accurate method for u pb zircon geochronology in this review we describe the main stages of the evolution of this in situ approach from the early 1990s to the present time some key points have been progressively improved the crater size has been reduced to achieve real in situ measurements the laser wavelength has been reduced as well as the duration of each pulse in order to lower inter element fractionation the blank from the gas has to be lowered as far as possible double focusing instruments and magnetic field sectors allow flat topped peaks required for precise isotope ratio measurement to be obtained the use of a multi ion counting system significantly improves the sensitivity of the method and the static mode of integration favours the precision of measurement of the transient signal originating from a noisy laser ablated particle beam combining the use of a 213 nm uv laser and a mc icpms equipped with a multi ion counting system operating in static mode the common precisions achieved for the key ratios 207 pb 206 pb and 206 pb 238 u are better than 1 and 3 2 respectively including error propagation associated with standard normalization until now the use of a zircon standard has remained necessary to ensure the accuracy of the calculated age a strategy for common pb correction is proposed according to the age of the zircon and according to the th u ratio of the grains after recording sixteen to twenty spot analyses the precision usually achieved on the age is about 1 and even significantly better for proterozoic samples in order to show the performance achieved by modern la mc icpms geochronology we tested four zircon samples covering a wide age range from 290 to 2440 ma these new age determinations can be compared in term of precision and accuracy since they have already been dated by reference methods id tims and shrimp further developments in the technology of ion counters equipping modern mc icpms and in laser systems will certainly be applied to a large field of geochronology studies in the near future as an alternative to sims for in situ age determination

initial hf isotope compositions in magmatic zircon from early proterozoic rocks from the gawler craton australia a test for zircon model ages abstract we evaluate the validity of age information obtained from two stage hf model ages in zircon multiple isotope investigations of minerals from the kiana granite of the archean sleafordian complex from the southern gawler craton australia reveal a complex geologic evolution of the suite rubidium sr mineral ages define a tectono thermal event at 1673 4 4 2 ma associated with the kimban orogeny id tims u pb ages from single zircons yield an upper concordia intercept at 2348 21 ma which marks the time of magmatic emplacement whole rock nd model ages yield crustal residence ages of 2 8 ga for each single zircon grain the lu hf isotope systematic was determined by solution mc icpms two stage hf model ages for the magmatic zircons vary by as much as 500 myrs depending on the choice of lu hf for the source of the host rock putting further emphasis on the need to constrain the source lu hf of a rock used for model age calculations an average lu hf for mafic crust was assumed for the source reservoir of the rocks to calculate its hf isotope evolution through time which yields zircon model ages of 3 5 ga that coincide with the initial growth episode postulated for the gondwana supercontinent it is demonstrated that two stage zircon model ages deduced from initial hf isotopes can provide important age information for the source of igneous rocks and consequently on important crustal formation events even when conventional geochronologic investigations fail e g due to thermo tectonic overprints or mixing

on the prediction of channel heads in a complex alpine terrain using gridded elevation data 1 threshold conditions for channel initiation are evaluated by using gridded elevation data derived from a lidar survey a reliable algorithm for the determination of surface flow paths and field observations of channel heads for a study area located in the eastern italian alps these threshold conditions are determined by considering the channel heads observed across a portion of the study area and computing the related values of 1 drainage area a 2 area slope function as2 with s being the local slope and 3 strahler order of surface flow paths extracted from gridded elevation data attention is focused on the dependence of the obtained threshold values on the size of grid cells involved and on the ability of the identified threshold conditions to provide reliable predictions of channel heads across the entire study area the results indicate that the threshold values of a as2 and are all significantly dependent on grid cell size and the uncertainty in the determination of threshold values of is significantly smaller than that affecting the determination of threshold values of a and as2 the comparison between predicted and observed channel heads indicates that the considered methods display variable reliability and sensitivity over different drainage basins and grid cell sizes with a general tendency to predict more channel heads than can be observed in the field acceptable predictions are normally obtained where channel heads are formed essentially by surface runoff more comprehensive methods seem however to be needed to predict channel heads affected by groundwater seeping upward

simultaneous temperature and velocity measurements of the internal wave field in the corsican channel eastern ligurian sea during a study of circulation in the eastern ligurian sea current and temperature were measured close to the sill of the corsican channel between corsica and the italian mainland in march april 1987 internal wave activity at the interface between the modified atlantic water and the levantine intermediate water was observed the statistical description of the internal wave field shows some deviations from garret munk theory the fine structure has a remarkable influence on the vertical and horizontal scales which have values smaller than those of theoretical prediction the mean velocity together with the topographic constriction causes significant anisotropy of the internal wave field the polarization effect is predominantly along channel while near bottom energy intensifications were not observed

observations of the near surface internal wavefield abstract during january 1977 a cruise was conducted off the california coast on the research platform flip repeated temperature profiling devices were used to sense the internal wavefield in the top 400 m of the sea from a sequence of 8192 profiles vertical velocity spectra and vertical coherence were calculated near surface coherence was found to increase with increasing frequency between local inertial and vaisala frequencies below 200 m the coherence was approximately constant with frequency the near surface change in the vertical coherence patterns results from the selective attenuation of the longer vertical wavelengths as the surface is approached from frequency depth changes in the near surface coherence variations of the internal wave spectral form can be inferred in spite of the fact that the deep vertical coherence remains constant this near surface effect is not so apparent in data from horizontal velocity sensors as only the vertical component of motion is constrained to vanish at t

tightening up ne brazil and nw africa connections new u pb lu hf zircon data of a complete plate tectonic cycle in the dahomey belt of the west gondwana orogen in togo and benin the dahomey belt in togo and benin is an important segment of the larger west gondwana orogen here we review the geodynamic evolution of the dahomey belt and discuss new u pb and lu hf zircon data in light of similar data previously acquired on the geologically related northern borborema province in ne brazil eighteen samples from different tectonic settings and regions within the belt were collected for zircon isotopic investigation passive margin deposits of the atacora structural unit and lower units of the volta basin have detrital zircon signatures compatible with the flanking west africa craton the arc related magmatism resulted from the east dipping subduction of the goias pharusian oceanic lithosphere and is represented by a variety of granitoids emplaced in the benino nigerian shield between 670 and 610 ma these granitoids were mainly sourced from crustal reservoirs with subordinate juvenile input detrital zircon ages from syn orogenic deposits in benino nigerian shield suggest that arc development could have started as early as 780 ma the main period of melting in the internal part of the belt the benino nigerian shield is related to crustal thickening and occurs only ca 30 m y after initiation of the continental collision marked by the ca 610 ma ultra high pressure uhp metamorphism recorded at lato hills foreland development represented by the upper units of the volta basin developed soon after continental collision and persisted with the development of the west verging thrust front synchronously with the main period of crustal melting due to collision at ca 580 ma the subvertical transbrasiliano lineament in south america that corresponds to the kandi lineament in africa provides a present day fit between nw africa and ne brazil restoration of the movement of the transbrasiliano kandi lineament strike slip plate boundary places the dahomey belt and borborema province ne brazil along the same section of the west gondwana orogen this configuration would explain some of the misfits previously discussed in the literature and aligns the uhp eclogites in togo and ne brazil

coevolution of nonlinear trends in vegetation soils and topography with elevation and slope aspect a case study in the sky islands of southern arizona 1 feedbacks among vegetation dynamics pedogenesis and topographic development affect the critical zone the living filter for earth s hydrologic biogeochemical and rock sediment cycles assessing the importance of such feedbacks which may be particularly pronounced in water limited systems remains a fundamental interdisciplinary challenge the sky islands of southern arizona offer an unusually well defined natural experiment involving such feedbacks because mean annual precipitation varies by a factor of five over distances of approximately 10 km in areas of similar rock type granite and tectonic history here we compile high resolution spatially distributed data for effective energy and mass transfer eemt the energy available to drive bedrock weathering above ground biomass soil thickness hillslope scale topographic relief and drainage density in two such mountain ranges santa catalina scm pinaleno pm strong correlations exist among vegetation soil topography variables which vary nonlinearly with elevation such that warm dry low elevation portions of these ranges are characterized by relatively low above ground biomass thin soils minimal soil organic matter steep slopes and high drainage densities conversely cooler wetter higher elevations have systematically higher biomass thicker organic rich soils gentler slopes and lower drainage densities to test if eco pedo geomorphic feedbacks drive this pattern we developed a landscape evolution model that couples pedogenesis and topographic development over geologic time scales with rates explicitly dependent on vegetation density the model self organizes into states similar to those observed in scm and pm our results highlight the potential importance of eco pedo geomorphic feedbacks mediated by soil thickness in water limited systems

the mara rosa arch in the tocantins province further evidence for neoproterozoic crustal accretion in central brazil the mara rosa volcano sedimentary sequence consists of several nne trending belts of metavolcanic metabasalts to metarhyolites and metasedimentary rocks micaschists quartzites cherts exposed over large areas of the tocantins province a neoproterozoic brasiliano orogenic region in the central part of brazil the supracrustal belts are separated by terrains dominated by metatonalites and metadiorites and intruded by several post orogenic granitic and dioritic bodies these volcano sedimentary and associated metaplutonic rock associations have been previously interpreted as a typical granite greenstone terrain of the archaean basement in this paper we present geochronological and isotopic data for rocks of the metavolcanic metaplutonic rock association samples of a felsic metavolcanic rock from the posse gold mine and a metatonalite yielded u pb zircon ages of 862 8 ma and 856 7 13 ma respectively these are interpreted as crystallization ages of the igneous protoliths titanites from the same metavolcanic rock sample yielded a concordant recrystallization age of 632 4 ma sm nd isotopes for these rocks indicate primitive compositions with nd t of 4 6 and 3 7 and tdm model ages of 1 0 ga a syn tectonic dioritic intrusion has an u pb crystallization age of 630 6 ma and nd t of 1 9 and tdm of 1 0 ga additional sm nd analyses on metasedimentary rocks and post orogenic granites of the mara rosa region yield tdm ages between 1 2 and 1 0 ga the isotopic data combined with preliminary trace element results suggest that the protoliths of the metavolcanic and metaplutonic rocks investigated were formed in an island arc system off the coast of the sao francisco congo continent at around 860 ma recrystallization and deformational ages of 630 ma may represent the timing of final ocean closure and continental collision the rock associations structural pattern geochronological and isotopic characteristics of the rocks from the mara rosa region are very similar to those of neoproterozoic arc terrains exposed 300 km to the southwest in the arenspolis area proving the regional importance of the neoproterozoic crustal accretion event in the central part of brazil

the goias massif implications for a pre columbia 2 2 2 0 ga continent wide amalgamation cycle in central brazil abstract the goias massif is a collage of archean paleoproterozoic terranes that crop out in the central brazil northern brasilia belt and acted as the continental margin onto which neoproterozoic brasiliano orogenic events took place most belts in the massif formed from 2 2 to 2 0 ga a widespread age bracket in the sao francisco congo craton pericratonic areas and other terranes associated with the formation of the atlantica supercontinent however the genesis of the goias massif has always been a matter of debate due to a sharp crustal thickness contrast marked on surface by the trace of the rio maranhao thrust and widely hypothesized as a suture zone the hypothesis that this suture would have been formed when a microcontinent was accreted to the rest of the massif during the neoproterozoic brasiliano orogeny is tested in this paper through mapping and zircon geochronology of rocks along the rio maranhao thrust our study indicates that units contain paleoproterozoic ages have similar geological features to other goias massif 2 2 2 0 ga terranes and show no evidence of syn collisional neoproterozoic magmatism we therefore reinterpret the available data to propose instead that the amalgamation of the massif occurred fully in the paleoproterozoic first the gravimetric contrast marked by the rio maranhao thrust represents in fact a neoproterozoic lower crust delamination event that affected both the brasiliano orogen and the terrane it was accreted against the campinorte arc second serra da mesa suite mesoproterozoic granites intruded both sides of the rio maranhao thrust and thus indicate the two domains were amalgamated prior to the mesoproterozoic finally mesoproterozoic volcanic and plutonic rocks follow the trace of the rio maranhao thrust suggesting a reactivated intracontinental structure rather than an original product of brasiliano collision the goias massif also show important similarities to the sao francisco craton a orogen building and magmatism from 2 2 to 2 0 ga b metamorphic peaks from 2 11 to 2 08 ga c zircon age peaks at 1 76 ga 1 58 ga and 1 2 ga largely attributed to crustal extension events and d contiguous seismic signature from massif to craton these common features allied to our reassessment of central brazil geochronology and the nature of central brazil sharp crustal thickness contrast indicate that the goias massif was fully assembled during a 2 2 2 0 ga orogeny as part of the western border of the sao francisco congo paleocontinent a comprehensive understanding of this orogeny is vital to test the validity of atlantica and to infer the landmasses later involved in the assembly of columbia

u pb and hf isotope study on detrital zircons from the paranoa group brasilia belt brazil constraints on depositional age at mesoproterozoic neoproterozoic transition and tectono magmatic events in the sao francisco craton abstract the eastern portion of the brasilia belt part of the neoproterozoic tocantins province in central brazil is mainly constituted by metasedimentary rocks that partially represent the evolution of a passive margin developed during the meso neoproterozoic in the western margin of the sao francisco paleocontinent new in situ u pb and lu hf isotope analyses on detrital zircons by la mc icp ms were obtained for clastic sedimentary rocks of the paranoa group considered to be the oldest unit of the this sedimentary sequence the u pb data define a maximum depositional age of 1 54 ga and a minimum depositional age of 1 04 ga the latter obtained on diagenetic xenotime overgrowth on zircon grains combined u pb and lu hf data permitted to reconstruct the tectonomagmatic evolution of the potential source areas identified in the sao francisco craton two zircon populations at 1 54 and 1 78 ga with hf t values from 5 1 to 5 5 are possibly related with the mesoproterozoic extensional magmatism registered in the arai and espinhaco rifts located in the brasilia belt and in the sao francisco craton respectively the most important paleoproterozoic zircon population at 2 0 2 25 ga shows positive hf t 0 to 8 and paleoproterozoic tdm values indicating a continuous long period of magmatism with generation of juvenile crust during the paleoproterozoic two minor zircon populations at 2 1 and 2 15 ga showing negative hf t values 0 to 10 1 and tdm values between 2 65 and 3 22 ga may represent arc related magmatism with reworking of archean crust during the evolution of an active margin of the sao francisco craton a third minor zircons population at 2 05 ga with strongly negative hf t values between 8 6 and 17 7 and tdm values between 2 93 and 3 46 ga may represent a collisional magmatism that characterized the final stage of an active margin evolution with reworking of early archean crust several minor zircon populations of archean ages 2 7 2 85 3 1 and 3 42 ga derived from the archean nuclei in the interior of the sao francisco craton suggest the presence of different magmatic pulses that repeatedly reworked a juvenile paleoarchean crust the oldest event of generation of juvenile crust is registered by zircons with u pb age of 3 42 ga positive hf t values of 3 0 and tdm of 3 54 ga

new constraints on the jurassic cretaceous boundary in the high andes using high precision u pb data abstract the jurassic cretaceous j k boundary is poorly constrained and is the only phanerozoic system boundary that lacks an internationally accepted reference stratigraphic section gssp precise radio isotopic u pb data are unavailable for the earliest stage of the cretaceous the berriasian the age of the jurassic cretaceous boundary was based on several assumptions including the relative duration of ammonite zones the constant spreading rates of magnetic anomalies and the extrapolation of rb sr or k ar isotopic data this paper discusses a site in an andean basin of western gondwana showing the j k boundary interval with geographically widespread nannofossil markers which are here uniquely combined with precise radiometric dates the recent finding of a sequence of marine black shales in the high andes of argentina interbedded with ash fall tuffs provides important constraints on this boundary this succession bears calcareous nannofossils and ammonites which allow correlation with well established tethyan floras and faunas in the northern hemisphere the tithonian berriasian transition in the andes was recognized on the basis of ammonite zones and nannofossil bioevents for the first time in the southern hemisphere the new ages obtained are 137 9 0 9 ma by sensitive high resolution ion microprobe shrimp and 139 55 0 09 0 18 ma by chemical abrasion isotope dilution thermal ionization mass spectrometry tims near the base of the berriasian these new ages can be interpreted in two different ways the first alternative would indicate that the present geological time table is correct and the fossil levels should be late berriasian the second alternative is that the j k boundary is 5 ma younger than the recently published geological time scale the authors support the last alternative and propose that the j k boundary should be close to 140 ma

timing and role of the maranhao river thrust in the evolution of the neoproterozoic brasilia belt and tocantins province central brazil abstract a low angle thrust fault places high pt granulites hangingwall of the internal zone of the neoproterozoic brasilia belt tocantins province central brazil in contact with a lower grade footwall external zone comprised of nappes of distal passive margin and back arc basin related supracrustals the footwall units were emplaced at 750 ma onto proximal sedimentary rocks paranoa group of the sao francisco paleo continent passive margin the high pt belt is comprised of 645 630 ma granulite facies paragneiss and orthogneiss and mafic ultramafic complexes that include three major layered intrusions and metavolcanic rocks granulitized at 750 ma these complexes occur within lower grade metasedimentary rocks in the hangingwall of the maranhao river thrust which forms the internal zone external zone boundary fault to the north of the pirineus zone of high strain detailed lithostructural studies carried out in maranhao river thrust hangingwall and footwall metasedimentary rocks between the niquel ndia and barro alto complexes and also to the east of these indicate the same lithotypes and sm nd isotopic signatures and the same d1 d2 progressive deformation and greenschist facies metamorphism additionally footwall metasedimentary rocks exclusively display a post d2 deformation indicating that the maranhao river thrust propagated through upper crustal rocks of the paranoa group relatively late during the tectonic evolution of the belt fault propagation was a consequence of intraplate underthrusting during granulite exhumation the results allow for a better tectonic understanding of the brasilia belt and the tocantins province as well as explaining the presence of the pirineus zone of high strain

devonian a type granitic magmatism on the northern margin of the north china craton shrimp u pb zircon dating and hf isotopes of the hongshan granite at chifeng inner mongolia china abstract we report first data for the devonian hongshan a type granite along the northern margin of the precambrian north china craton the granite is rich in silica sio2 76 07 77 62 wt and alkalis na2o k2o 8 45 8 78 wt but poor in calcium cao 0 28 0 70 wt and magnesium mgo 0 08 0 20 wt chondrite normalized ree patterns for the samples from the hongshan granite show significant negative eu anomalies eu eu 0 13 0 19 they contain high rb ga y nb and zn but low ba and sr contents and in the multi element spider diagrams normalized to primitive mantle they show a similar pattern to that of a type granites such as negative ba nb ta sr p and ti anomalies geochemical plots also show an affinity with a type granites and suggest a post orogenic setting a2 type granite shrimp u pb zircon dating yielded a concordant weighted mean 206pb 238u age of 387 4 ma which defines a devonian magmatic event the zircons have negative ehf t of 8 4 to 11 5 and give old hf model ages of tdm2 2548 2820 ma mean 2667 ma most 176hf 177hf and ehf values display hf isotope evolutional line of the lower continental crust these results suggest that the hongshan granite originated from melting of archaean lower crust beneath the north china craton in a post orogenic extensional setting during the devonian

a hafnium isotopic record of magmatic arcs and continental growth in the iapetus ocean the contrasting evolution of ganderia and the peri laurentian margin abstract we test the sensitivity of combined u pb hf isotopic analyses of detrital and magmatic zircon to document complex processes in accretionary orogens using ganderia from the canadian appalachian orogen as an example ganderia hosted a long lived magmatic arc that began at least by 650 ma and continued until 450 ma during which time it drifted northward from the amazonian margin its isotopic record suggests arc activity began earlier possibly at 750 ma proto ganderia within an oceanic domain the u pb hf zircon isotopic array suggests that ganderia formed on a sliver of paleoproterozoic mesoproterozoic basement tdmc 2 2 1 1 ga a transition towards more evolved hf isotopic compositions between 650 and 600 ma is interpreted to coincide with its accretion to the amazonian margin of gondwana consistent with other geological evidence accretion was followed by increasing amounts of juvenile crustal inputs between 550 and 500 ma which is interpreted to reflect subduction roll back and the onset of drift from the gondwanan margin trench retreat culminated in the separation of the ganderia arc and formation of the exclusively oceanic 510 450 ma penobscot popelogan victoria arcs on ganderia accretion of ganderia to the active notre dame subzone along the laurentian margin occurred in the late ordovician and is marked by the abrupt termination of ganderian arc magmatism the notre dame arc evolved along the laurentian margin between 515 and 425 ma and was built on paleoproterozoic archean t dm c 2 9 2 5 ga crust supra subduction zone magmatism occurred at 490 460 ma during the closure of the taconic seaway and was dominated by the recycling of laurentian crust with progressive mixing of evolved and juvenile arc magmas over time the preservation of very evolved notre dame arc zircon in ganderian overstep sequences confirms the arrival of the leading edge of ganderia to laurentia by 450 ma the hafnium isotopic method is proven here to be a powerful tool when used in conjunction with geological constraints and is able to identify the changing nature of magmatism during the evolution of complex accretionary systems

uplift histories of africa and australia from linear inverse modeling of drainage inventories this is the final version it first appeared at http onlinelibrary wiley com wol1 doi 10 1002 2014jf003297 abstract

detection of transience in eroding landscapes past variations in climate and tectonics have led to spatially and temporally varying erosion rates across many landscapes in this contribution i examine methods for detecting and quantifying the nature and timing of transience in eroding landscapes at a single location cosmogenic nuclides can detect the instantaneous removal of material or acceleration of erosion rates over millennial timescales using paired nuclides detection is possible only if one of the nuclides has a significantly shorter half life than the other currently the only practical way of doing this is to use cosmogenic in situ carbon 14 14c alongside a longer lived nuclide such as beryllium 10 10be hillslope information can complement or be used in lieu of cosmogenic information in soil mantled landscapes increased erosion rates can be detected for millennia after the increase by comparing relief and ridgetop curvature this technique will work as long as the final erosion rate is greater than twice the initial rate on a landscape scale transience may be detected based upon disequilibria in channel profiles or ridgetops but transience can be sensitive to the nature of transient forcing where forcing is periodic landscapes display differing behavior if forcing is driven by changes in base level lowering rates versus changes in the efficiency of either channel or hillslope erosion e g driven by climate change oscillations in base level lowering lead to basin averaged erosion rates that reflect a long term average erosion rate despite strong spatial heterogeneity in local erosion rates this averaging is reflected in 10be concentrations in stream sediments changes in hillslope sediment transport coefficients can lead to large fluctuations in basin averaged erosion rates which again are reflected in 10be concentrations the variability of erosion rates in landscapes where both the sediment transport and channel erodibility coefficients vary is dominated by changes to the hillslope transport coefficient copyright 2016 john wiley sons ltd

u pb age and hf isotope geochemistry of zircon from felsic volcanic rocks of the paleoproterozoic aillik group makkovik province labrador abstract u pb zircon shrimp geochronology and lu hf zircon la mc icpms isotope geochemistry are used to constrain the ages of paleoproterozoic felsic magmatism and their crustal sources in the aillik group makkovik province of labrador this information is important for understanding the extent of crustal reworking vs juvenile crustal growth in subduction related magmatism during the paleoproterozoic and improves framework knowledge for uranium exploration in the central mineral belt of labrador u pb shrimp zircon geochronology from four felsic tuff samples collected from two different areas of the aillik group yield magmatic 207pb 206pb ages of 1852 7 ma 2 1854 7 ma 2 1861 7 ma 2 and 1862 7 ma 2 the ages fall within the range of previous u pb zircon dates reported for felsic volcanic rocks of the aillik group altogether these dates indicate that felsic volcanism spanned at least 30 m y and was perhaps most intense during the last 10 m y of that interval a foliated syn deformational monzogranite in the aillik group cross lake granite yields a 207pb 206pb zircon date of 1805 6 ma 2 which indicates that development of a pervasive planar fabric associated with northwestward thrusting of the aillik group continued after this time initial e h f s t values for magmatic zircons from the four felsic tuffs 4 8 to 1 9 1 7 to 2 2 11 8 to 3 8 9 0 to 4 6 and monzogranite 4 8 to 1 5 exhibit significant variability which indicates heterogeneous sources most of the zircon grains have negative initial e h f s t values and calculated crust formation hf isotope tdm model ages point to substantial contributions from melts of ancient crust ranging in age from ca 2200 to 2900 m y and clustering at ca 2500 m y inherited zircon grains in the felsic tuffs and monzogranite mainly have u pb ages of ca 1880 1920 ma though others are older up to ca 2743 ma hf isotope tdm model ages for the inherited zircons are similar to those of the magmatic zircons the hf isotope results indicate that subduction related felsic magmatism in the makkovik orogen involved significant crustal reworking 80 during formation of the aillik group

the upper ocean internal wave field influence of the surface and mixed layer a model is developed to calculate the upper ocean internal wave spectrum as modified by the surface boundary and mixed layer the garrett munk spectrum is assumed to describe the deep ocean wave field the main effect of the surface and mixed layer is to align the vertical structure of the waves forming vertically standing waves locally this contrasts with the assumption of random alignment in the garrett munk model model spectra and coherences are calculated for idealized buoyancy frequency profiles and compared with the garrett munk model measurements in the upper 200 m from the mixed layer dynamics experiment in the northeast pacific in 1983 are also compared with the model results the most striking success of the model is predicting the observed high coherence and 180 phase difference across the mixed layer of horizontal velocity

cosmocalc an excel add in for cosmogenic nuclide calculations as dating methods using terrestrial cosmogenic nuclides tcn become more popular the need arises for a general purpose and easy to use data reduction software the cosmocalc excel add in calculates tcn production rate scaling factors using lal stone dunai and desilets methods topographic snow and self shielding factors and exposure ages erosion rates and burial ages and visualizes the results on banana style plots it uses an internally consistent tcn production equation that is based on the quadruple exponential approach of granger and smith 2000 cosmocalc was designed to be as user friendly as possible although the user interface is extremely simple the program is also very flexible and nearly all default parameter values can be changed to facilitate the comparison of different scaling factors a set of converter tools is provided allowing the user to easily convert cut off rigidities to magnetic inclinations elevations to atmospheric depths and so forth because it is important to use a consistent set of scaling factors for the sample measurements and the production rate calibration sites cosmocalc defines the production rates implicitly as a function of the original tcn concentrations of the calibration site the program is best suited for be 10 al 26 he 3 and ne 21 calculations although basic functionality for cl 36 and c 14 is also provided cosmocalc can be downloaded along with a set of test data from http cosmocalc googlepages com

theory of acoustic measurement of internal wave strength as a function of depth horizontal position and time high frequency 1 cpd variations in travel time of acoustic transmissions over ocean mesoscale distances are known to be dominated by the effects of internal wave displacements of the sound speed stratification flatte et al 1979 flatte 1983a variations in the difference in travel time between transmissions in opposite directions along the same path reciprocal transmissions are dominated by internal wave currents munk et al 1981 we investigate the usefulness of a two mooring acoustic system for determining the statistical variances of internal wave displacements and currents as a function of depth geographical position and time we find that statistical fluctuations in the internal wave field itself prevent recovery of range dependent information between the two moorings however range averaged information can be obtained about mean energy level and about vertical energy migration we find that uncertainties in the buoyancy and sound speed profiles do not significantly affect the usefulness of the method

the juchatengo complex an upper level ophiolite assemblage of late paleozoic age in oaxaca southern mexico the juchatengo complex jc suite is located between the proterozoic oaxacan complex to the north and the xolapa complex to the south and was amalgamated by late paleozoic magmatism it consists of mafic and sedimentary rocks that have oceanic affinities with internal pseudostratigraphic structural and metamorphic characteristics which resemble a typical upper level ophiolite assemblage new u pb zircon and previous hornblende k ar analyses yield ages of ca 291 313 ma u pb for plagiogranites and ca 282 277 ma for tonalites intruding the entire sequence including pelagic sediments at the top with a maximum deposition age of ca 278 ma and noteworthy local provenance these data constrain the age of the jc to the late pennsylvanian early permian period hf isotopic analyses obtained from zircons in the jc plagiogranite and tonalite show that they come from a similar primitive mantle source 176hf 177hf 0 282539 0 283091 hf t 3 2 to 15 0 hf t values from near 0 to 2 8 in the tonalites indicate a contribution from the continental crust trace elements and ree patterns in whole rock and zircons point to a primitive mantle source for differentiated mafic plagiogranite dykes and tonalitic plutons geochronological and geochemical data address the generation of new oceanic crust above the subduction zone probably in a backarc setting in this tectonic scenario the jc ophiolite originated due to the convergence of the paleo pacific plate below the already integrated oaxacan and acatlan complexes in western pangea the dextral displacement places the deformation in a transtensional regime during the late paleozoic age

deep sea floor observations of typhoon driven enhanced ocean turbulence abstract the impact of large atmospheric disturbances on deep benthic communities is not well known quantitatively observations are scarce but may reveal specific processes leading to turbulent disturbances here we present high resolution deep ocean observations to study potential turbulent mixing by a large atmospheric disturbance we deployed an array of 100 hz sampling rate geophysical broadband ocean bottom seismometers obss on the seafloor within the footprint of this array we also deployed an oceanographic 0 5 hz sampling rate vertical temperature sensor string covering the water phase between 7 and 207 m above the seafloor at about 3000 m depth off eastern taiwan between june 2017 and april 2018 in september 2017 all instruments recorded category 4 cyclone typhoon talim s passage northeast of the array one day ahead of the cyclone s closest approach when the cyclone s eye was more than 1 000 km away for 10 days a group of near inertial motions appeared most clearly in temperature the group contained the largest inertial amplitudes in the ten month time series and which led to turbulence dissipation rates o 10 7 m2 s 3 the observation reflects the importance of barotropic response to a cyclone and the propagation of inertio gravity waves in weak density stratification in addition to internal tides these waves drove turbulent overturns larger than 200 m that were concurrently recorded by obss the turbulent signals were neither due to seismic activity nor to ocean surface wave action cyclones can generate not only microseisms and earth hums as well as turbulence in the water column producing additional ground motions quantified turbulence processes may help constrain models on sediment resuspension and its effect on deep sea benthic life

detrital zircon u pb geochronology and whole rock nd isotope constraints on sediment provenance in the neoproterozoic sergipano orogen brazil from early passive margins to late foreland basins abstract shrimp u pb detrital zircon geochronology and depleted mantle nd model ages of clastic rocks were combined to understand the sediment provenance in the neoproterozoic sergipano belt the sergipano is the main orogenic belt between the borborema province and the sao francisco craton eastern south america it is divisible into several lithostratigraphic domains from north to south caninde poco redondo maranco macurure vaza barris and est ncia nd model ages tdm and detrital zircon u pb shrimp geochronology indicate that the protoliths of clastic metasedimentary rocks from the maranco and macurure domains were mostly derived from eroded late mesoproterozoic to early neoproterozoic rocks 1000 900 ma whereas detritus of similar rocks from the caninde domain came from a younger source ca 700 ma and 1000 ma samples from the vaza barris domain show the greatest scatter of both tdm and zircon ages amongst all domains but with important contributions from proterozoic sources 690 1050 ma and ca 2100 ma and less from archaean sources the est ncia domain samples have zircon population peaks at 570 ma 600 ma and 920 980 ma with a few older grains one diamictite contains only ca 2150 ma zircon grains our preliminary results support a model in which sediments of the maranco and macurure domains were deposited on a continental margin of the ancient borborema plate before its collision with the sao francisco craton the caninde domain is likely to be an aborted neoproterozoic rift assemblage within the southern part of the borborema plate pernambuco alagoas massif the basal units of the vaza barris and est ncia domains have clast sources from the sao francisco craton and are best interpreted as passive margin sediments however the uppermost units of the est ncia and vaza barris domains come from foreland basins formed during collision of borborema plate with the sao francisco craton

archaean to proterozoic crustal evolution in the central zone of the limpopo belt south africa botswana constraints from combined u pb and lu hf isotope analyses of zircon a combined set of u pb and lu hf in situ laser ablation icp mc ms zircon analyses were obtained from orthogneisses and granitoids in the central zone of the limpopo belt which comprises the beit bridge and mahalapye complexes the results indicate that by combining the two isotope systems primary magmatic zircon domains can be distinguished from those formed during later metamorphic events even if the distinct zircon domains underwent multiple pb loss and the texture age relationships as obtained by cathodoluminescence images and u pb analyses are ambiguous furthermore the applied technique allows distinction of zircon grains formed in juvenile magmas from those generated by melting of older continental crust or affected by substantial crustal contamination the combined u pb and lu hf data reveal that the sand river gneiss suite of the beit bridge complex was emplaced at 3283 8ma and formed from melting of an older archaean crust which was derived from a depleted mantle source at around 3 65 ga the hafnium model age tdm is significantly older than those obtained from zircons from numerous neoarchaean granitoids of the beit bridge complex comprising the singelele gneiss 2647 12ma the bulai granite 2612 7ma the regina gneiss 2649 9ma and two samples of the zanzibar gneiss 2613 6ma these granitoids show initial ehf t values between 0 5 and 7 1 which correspond to initialtdm between 3 46 and 3 01ga these variable tdminitial and ehf t initial values are interpreted to be the result of different mixtures of reworked 3 65 ga palaeoarchaean crust with juvenile magmas extracted from the depleted mantle during the neoarchaean at 2 65 ga this conclusion is supported by results obtained from the mahalapye complex which was affected by migmatization and granite intrusions during the palaeoproterozoic at 2 02 2 06 ga the mokgware granite 2019 9ma contains zircon xenocrysts with pb pb ages of 2 52 2 65 ga and 2 93 ga and hafnium model ages of 3 0 3 4 ga indicating that this granite is derived from remelting of archaean crust in contrast uniformtdm hf initial ages of 2 61 2 67 ga obtained from a diorite gneiss 2061 6ma of the mahalapye complex indicate that its protolith may have been formed from remelting of a neoarchaean juvenile crust variable ehf t initial values from 3 7 to 6 3 of zircon cores 2711 11ma in an adjacent leucosome also support a model of mixing of juvenile mantle derived matter with older crust in the neoarchaean

deep conductivity structure beneath the northern brasilia belt central brazil evidence for a neoproterozoic arc continent collision abstract the brasilia belt in central brazil experienced a broad range of tectonic events including several periods of compressional and extensional deformations during the proterozoic magnetotelluric data were collected at 27 sites along a profile crossing the northern part of the belt strike analysis and distortion decomposition indicated that most of the data are two dimensional 2d and fit a regional geoelectric strike angle of n20e parallel to the tectonic grain 2d conductivity model is constrained by available gravity and seismic refraction data and reveals the cumulative effects of the multiple tectonomagmatic episodes that affected the area in the past on current crustal and upper mantle structure the results show that the resistive lithospheric root of the sao francisco cratonic plate is restricted to the easternmost part of the profile and does not extend westward beneath the brasilia belt the external zone of the fold belt composed of slightly metamorphosed rocks at the surface presents crustal conductors that can be tentatively interpreted as regions with fossil fluid flows at mid crust depth beneath a gold mineralization zone and as a very shallow mineralized zone within the metamorphic rocks the external zone is separated to the west from the goias massif by a strong conductivity contrast indicative of a suture zone along the rio maranhao fault system crustal conductivity structure under the niquel ndia complex is distinct from what is observed in other parts of the goias massif in agreement with the proposal of the complex being an allochthonous block in spite of the large tectonothermal events that affected the goias magmatic arc there are no significant conductive anomalies at crustal depths in its eastern side the most prominent feature detected in the geoelectric model is a huge high conductivity anomaly below the entire goias massif in the central part of the belt mainly concentrated at the uppermost mantle but also affecting mid to lower crustal depths it can be interpreted as associated with thermal events either related to mesoproterozoic and or neoproterozoic continental rifting processes or to delamination and detachment processes at the end of the evolution of the belt in the neoproterozoic effects related to oceanic lithosphere subduction during the collision and accretion of the goias massif and goias magmatic arc to the western passive margin of the sao francisco cratonic plate in the neoproterozoic may alternatively explain the anomaly

bottom pressure observations of deep sea internal hydrostatic and non hydrostatic motions in the ocean sloping bottom topography is important for the generation and dissipation of internal waves here the transition of such waves to turbulence is demonstrated using an accurate bottom pressure sensor that was moored with an acoustic doppler current profiler and high resolution thermistor string on the sloping side of the ocean guyot great meteor seamount water depth 549 m the site is dominated by the passage of strong frontal bores moving upslope once or twice every tidal period with a trail of high frequency internal waves the bore amplitude and precise timing of bore passage vary every tide a bore induces mainly non hydrostatic pressure while the trailing waves induce mainly internal hydrostatic pressure these separate internal wave pressure terms are independently estimated using current and temperature data respectively in the bottom pressure time series the passage of a bore is barely visible but the trailing high frequency internal waves are a bore is obscured by higher frequency pressure variations up to similar to 4 x 10 3 cpd approximate to 80n cpd cycles per day n the large scale buoyancy frequency these motions dominate the turbulent state of internal tides above a sloping bottom in contrast with previous bottom pressure observations in other areas infra gravity surface waves contribute little to these pressure variations in the same frequency range here such waves do not incur observed pressure this is verified in a consistency test for large reynolds number turbulence using high resolution temperature data the high frequency quasi turbulent internal motions are visible in detailed temperature and acoustic echo images revealing a nearly permanently wave turbulent tide going up and down the bottom slope over the entire observational period the spectral slope and variance of bottom pressure are equivalent to internal hydrostatic pressure due to internal waves in the lower 100 m above the bottom by non hydrostatic pressure due to high frequency internal waves and large scale overturning the observations suggest a transition between large scale internal waves small scale internal tidal waves residing on thin similar to 1 m stratified layers and turbulence

pb diffusion in zircon abstract chemical diffusion of ti under anhydrous conditions at 1 atm and under fluid present elevated pressure 1 1 1 2 gpa conditions has been measured in natural zircon the source of diffusant for 1 atm experiments was a zro2 tio2 zircon mixture with experiments run in crimped pt capsules diffusion experiments done in the presence of h2o co2 fluid were run in a piston cylinder apparatus using a source of ground tio2 zrsio4 and sio2 with oxalic acid added to produce h2o co2 vapor and partially melt the solid source material yielding an assemblage of rutile zircon melt vapor nuclear reaction analysis nra with the resonant nuclear reaction 48ti p 49v was used to measure diffusion profiles for both sets of experiments the following arrhenius parameters were obtained for ti diffusion normal to cover the temperature range 1350 1550 c at 1 atm d ti 3 33 10 2 exp 754 56 kj mol 1 r t m 2 s 1 ti diffusivities were found to be similar for experiments run under fluid present conditions a fit to all of the data yields the arrhenius relation d 1 34 102 exp 741 46 kj mol 1 rt m2 s 1 these data suggest that zircon should be extremely retentive of ti chemical signatures indicating that the recently developed ti in zircon crystallization geothermometer watson e b harrison t m 2005 zircon thermometer reveals minimum melting conditions on earliest earth science 308 841 844 will be quite robust in preserving temperatures of zircon crystallization titanium diffuses somewhat faster in zircon than larger tetravalent cations u th and hf but considerably more slowly than pb the ree and oxygen hence ti crystallization temperatures may be retained under circumstances when radiometric ages or other types of geochemical information are lost

the pacific gondwana margin in the late neoproterozoic early paleozoic detrital zircon u pb ages from metasediments in northwest argentina reveal their maximum age provenance and tectonic setting abstract u pb detrital zircon ages are reported from puncoviscana formation late neoproterozoic early cambrian and meson group late cambrian greywackes of northwest argentina to constrain provenance and depositional environment the new data are combined with previously published detrital zircon ages to show that puncoviscana formation age patterns contain two broad groups late mesoproterozoic early neoproterozoic 1150 850 ma and late neoproterozoic early cambrian 650 520 ma with their relative proportions varying inversely with youngest component age the 1150 850 ma age components are dominant in greywackes with oldest late neoproterozoic components 600 ma the former diminish considerably when late neoproteozoic components become dominant and younger to 520 ma a northernmost greywacke sample from purmamarca jujuy is distinctive whilst its zircon age pattern partly resembles other puncoviscana formation samples it contains no cambrian late neoproterozoic ages the youngest ages being early neoproterozoic this may reflect an early neoproterozoic passive margin depocentre for the formation or an older early neoproterozoic succession within it which may predate the brasiliano orogeny in brazil the youngest age components c 520 ma in a greywacke from rancagua cachi salta province dominate an almost unimodal pattern suggestive of contemporary volcanic sources at a late early cambrian depocentre detrital zircon age patterns of the meson group lizoite formation have major cambrian latest neoproterozoic components resembling those of the puncoviscana formation but its mesoproterozoic component is diminished and there are no significant age components of this age small youngest components at c 500 ma suggest a maximum late cambrian stratigraphic age the puncoviscana formation detrital zircon patterns suggest a provenance in a continental hinterland having a stabilised extensive late mesoproterozoic orogen with minor paleoproterozoic and archean precursors and a more variable late neoproterozoic orogen containing an evolving sequence of less extensive subcomponents a direct relationship with the brazilian shield is suggested with sediment supplies originating within active margin orogens of the interior and collisional orogens at the suture between african and south american cratons but ultimate deposition in passive margin environments of western gondwanaland

provenance of the vazante group new u pb sm nd lu hf isotopic data and implications for the tectonic evolution of the neoproterozoic brasilia belt abstract the vazante group show varied u pb provenance patterns along the basin zircon ages range from 936 to 3409 ma but paleo and mesoproterozoic terrains constitute the main sources of the original sediments the youngest population 930 ma establishes the maximum depositional age of the group sm nd t dm data show the predominance of paleoproterozoic ages 1 90 2 08 ga and also indicate some input from younger sources in rocks of the lapa formation 1 67 to 2 0 ga in the upper part of the group whereas rocks of the serra do garrote formation present the oldest model ages 2 03 to 2 76 ga hf isotopic compositions of the detrital zircons indicate that they were derived mainly from recycled paleoproterozoic crust with a minor mesoproteroic juvenile component terranes within the sao francisco craton represent the main sources of detrital sediments of this group and reinforce the interpretation that it may be a passive margin sequence developed along the western margin of the original continent however the origin of mesoproterozoic grains remains uncertain slightly younger sm nd model ages in the lapa formation however are not entirely consistent with derivation solely from the craton and may indicate contribution from younger sources such as the neoproterozoic goias magmatic arc

early mesozoic southern mexico amazonian connection based on u pb ages from detrital zircons the la mora paleo river in the mixteca terrane and its paleogeographic and tectonic implications abstract the la mora formation is the oldest mesozoic floodplain succession in the mixteca terrane of southern mexico the presence of amazonian detrital zircons in the la mora formation and in the overlying volcanic diquiyu unit indicates a major fluvial system that drained the mixteca terrane the la mora paleo river crossed the oaxaquia microcontinent and the mixteca terrane prior to the breakup of pangea during late triassic early jurassic time when the acatlan oaxaquia block was part of the northwestern portion of the amazonian craton detrital zircons in the la mora formation have ages between 3307 31 and 210 12 ma which suggest that they originated in amazonia and southern mexico 33 2 of the detrital zircons have ages that are found only in amazonian sources whereas 66 7 of the zircons may be associated with either the amazonian craton the andean basement or southern mexico we propose that the la mora fluvial system drained the amazon basin in a westward direction with its mouth in central pangea and that it most likely fed the toliman submarine fan the inferred location of the acatlan oaxaquia tectonic block at the time of the la mora fluvial system implies that the basement of southern mexico experienced a right lateral displacement of at least 2300 km with respect to south america during the mesozoic

neoproterozoic crustal accretion in central brazil recent geochronological studies have demonstrated the existence of several neoproterozoic orthogneiss and metavolcanic are suites ca 900 to 600 ma in the southern part of the tocantins province a major neoproterozoic orogenic area in central brazil exposed between the amazon and sao francisco cratons nd and sr isotopic characteristics of these metamorphic suites are primitive initial 87 sr 87 sr ratios vary from 0 7024 to 0 7042 and initial nd values are positive varying between 0 2 and 6 9 the most primitive initial sr and nd isotopic compositions are very similar to model depleted mantle compositions at the time of formation of the original magmas the mantle like characteristics of these are suites in western goias contradict previous ensialic evolution models for the tocantins province and reveal that crustal accretion processes were important in large areas of central brazil during neoproterozoic time

in situ zircon u pb and lu hf isotope systematic on magmatic rocks insights on the crustal evolution of the neoproterozoic goias magmatic arc brasilia belt central brazil abstract the mara rosa arc constitutes the northern portion of the goias magmatic arc an important tectonic component of the brasilia belt that represents the result of the convergence of the amazonian and san francisco cratons between 900 and 600 ma new in situ zircon u pb and lu hf isotope data by la mc icp ms on representative magmatic rocks permitted to characterize three neoproterozoic magmatic events in the mara rosa arc zircons from a metavolcanic rock yielded a u pb crystallization age of 916 5 ma with positive ehf values 8 to 12 suggesting that the host magma derived from a depleted mantle two orthogneisses showed heterogeneous zircon populations the youngest zircons from the two samples gave u pb crystallization ages of 792 8 ma and 811 7 ma respectively the abundant inherited zircons with meso to paleoproterozoic u pb ages showed for both samples negative ehf and old tdm values suggesting an important contribution of paleoproterozoic to archean crust in the petrogenesis of the host magmas zircons from post orogenic qz diorite yielded a crystallization age of 638 4 ma with a variation in ehf values 1 to 8 suggesting that the host magma involved both a juvenile and a recycled older crustal components

early to late ediacaran conglomeratic wedges from a complete foreland basin cycle in the southwest sao francisco craton bambui group brazil abstract stratigraphic isotopic and geochronological data from two late neoproterozoic aged conglomerate wedges in the southwest sao francisco craton support the interpretation that the ediacaran bambui group in east central brazil was deposited in a foreland basin the sambura formation forms the base of the bambui group in the southwestern part of the bambui basin and was deposited synchronously with the brasilia orogeny it is interpreted to be a sedimentary product of a retrogradational coastal alluvial fan system deposited in an underfilled flexural foredeep during the early stages of foreland basin development sometime between 630 and 560 ma the basal sete lagoas formation carbonates were deposited towards the cratonic margin on the forebulge which provided an ideal environment for carbonate production the lateral relationship between the sambura and sete lagoas formations further implies that an unconformity was generated by foreland flexure and that this unconformity separates an early ediacaran phase of the foreland basin from a late ediacaran phase the lagoa formosa formation was deposited in the latter phase after peak orogenesis with a provenance that includes post orogenic granites and zircons as young as 560 ma it records a prograding turbidite fan system in the lagoa formosa formation that was deposited during orogenic unroofing and basin wide shallowing in a filled stage of the foreland basin a shift from highly enriched 13 c values towards global like carbon isotopes values in carbonates within the lagoa formation in conjunction with the occurrence of banded iron formation may suggest deposition in a basin with anoxic and ferruginous deep waters in the bambui basin in the latest ediacaran

detrital zircon provenance from the neuquen basin south central andes cretaceous geodynamic evolution and sedimentary response in a retroarc foreland basin the surface response in terms of drainage pattern changes to the cretaceous geodynamic reorganization of the andean subduction zone between 36 s and 41 s is reconstructed through the geochronology based provenance study of alluvial detrital zircons the age spectra obtained by 500 spot u pb ages record an eastward provenance of detritus coming from the foreland during the early cretaceous backarc extensional stage followed by westward sourced clastics coming from the cordillera during the cenomanian this drainage pattern reversal fits the regional unconformity in the sedimentary record that is linked to the geodynamic reorganization of the continental margin from an extensional to a compressional regime forcing the neuquen basin to evolve from a retroarc to a foreland stage after this inversion the clastic systems progressively returned to be mainly fed by the foreland due to the uplift of the peripheral bulge as a consequence of the late cretaceous thrust front migration this tectonic evolution of the neuquen basin and the related response of the drainage pattern are thought to be the surface expression of the dip decrease of the benioff subduction zone

importance of the lu hf isotopic system in studies of planetary chronology and chemical evolution the 176lu 176hf isotope method and its applications in earth sciences are discussed greater fractionation of lu hf than sm nd in planetary magmatic processes makes 176hf177hf a powerful geochemical tracer in general proportional variations of 176hf177hf exceed those of 143ndl44nd by factors of 1 5 3 in terrestrial and lunar materials lu hf studies therefore have a major contribution to make in understanding of terrestrial and other planetary evolution through time and this is the principal importance of lu hf new data on basalts from oceanic islands show unequivocally that whereas considerable divergences occur in 176hf177hf 87sr86sr and 143ndl44nd 87sr86sr diagrams 176hf177hf and 143nd144nd display a single linear isotopic variation in the suboceanic mantle these discordant 87sr86sr relationships may allow with the acquisition of further hf nd sr isotopic data a distinction between processes such as mantle metasomatism influence of seawater altered material in the magma source or recycling of sediments into the mantle in order to evaluate the hf nd isotopic correlation in terms of mantle fractionation history there is a need for measurements of hf distribution coefficients between silicate minerals and liquids and specifically for a knowledge of hf behavior in relation to rareearth elements for studying ancient terrestrial hf isotopic variations the best quality hf isotope data are obtained from granitoid rocks or zircons new data show that very u pb discordant zircons may have upwardly biased 176hf177hf but that at least concordant to slightly discordant zircons appear to be reliable carriers of initial 176hf177hf until the controls on addition of radiogenic hf to zircon are understood combined zircon whole rock studies are recommended lu hf has been demonstrated as a viable tool for dating of ancient terrestrial and extraterrestrial samples but because it offers little advantage over existing methods is unlikely to find wide application in pure chronological studies

upper ocean internal waves in the marginal ice zone of the northeastern greenland sea oceanographic measurements obtained in the northeastern greenland sea fram strait region were studied in order to characterize internal wave activity in the marginal ice zone miz of the arctic ocean experiments were performed with an array of four horizontally separated current meters and a ctd instrument in 1984 as part of the marginal ice zone experiment spectra of horizontal and vertical motion show that within the frequency range from the inertial frequency to 3 cph the miz internal wave field is similar in spectral shape and energy content to the wave field found beneath arctic pack ice above 3 cph the miz spectra contain more energy than expected based upon both beneath ice measurements and the garrett munk model spectra elevated energy levels at high frequencies are shown to be caused by intermittent increases in internal wave activity occurring on time scales from 2 to 12 hours phase differences between pairs of time series computed during the passage of several packets of internal waves are used to estimate horizontal wave number vectors a comparison of the calculated wave number vectors to a dispersion relation derived from a simple normal mode model of internal waves suggests that the wave packets may be adequately described as being comprised of lowest mode horizontally propagating internal waves the interaction of ice bottom topography with a sharp elevated pycnocline is explored as a possible source of the intermittent increases in wave activity spectra of vertical displacement and velocity computed from time series of both temperature and density are presented in order to show the influence of intrusive activity in the miz differences in both spectral shape and energy content found at frequencies below 2 5 cph emphasize the need for careful interpretation of temperature based measurements of vertical motion obtained over long periods of time in the miz

trace element homogeneity from micron to atomic scale implication for the suitability of the zircon gj 1 as a trace element reference material the quality of a chemical reference material relies on the fact that the composition of the material is homogeneous across all scales a series of different techniques have been used to evaluate the trace element homogeneity of the gj 1 reference zircon from the micron to atomic scale cathodoluminescence imaging was conducted along with quantitative crystallographic orientation analysis and trace element analysis using laser ablation inductively coupled plasma mass spectrometry la icpms the nanometre scale homogeneity was evaluated by analysing five mineral tips using atom probe tomography which provides atomic scale three dimensional chemical reconstructions with unprecedented spatial resolution results show that the gj 1 reference zircon is homogeneous at all scales both structurally and chemically crystallographic orientation data confirms that this gem quality zircon has no detectable internal crystallographic orientation changes such as crystal plastic deformation features or cracks no mineral inclusions were found atom probe tomography shows that there is a lack of any chemical clustering or other modes of spatially defined elemental accumulation or depletion for the most abundant trace elements such as y yb and hf this finding is supported by la icpms data revealing homogeneity within the analytical precision trace elements of significant abundance include p yb y u and hf with contents of 30 6 65 2 238 5 284 14 and 6681 57 ppm respectively hence the gj 1 zircon used as a reference zircon for upb and hf isotopic studies is also a suitable zircon reference material for trace element analyses

nd isotopic study of the neoproterozoic mara rosa arc central brazil implications for the evolution of the brasilia belt abstract the goias magmatic arc is a 0 9 0 64 ga island arc underlying large areas of the western part of the brasilia belt in central brazil it is exposed in two separate sections known as the arenopolis arc in the south and the mara rosa arc in the north it is made of island arc volcanic sedimentary sequences forming narrow nne to nnw belts separated from each other by tonalitic granodioritic granitic orthogneisses all rocks are metamorphosed under upper greenschist to amphibolite facies conditions but have kept major geochemical and isotopic features indicating the juvenile nature of the igneous protoliths the meta igneous and metasedimentary rocks are intruded by a number of late to post orogenic granites e nd 850 ranging from 1 5 to 4 6 with t dm values between 1 0 and 1 1 ga metasedimentary rocks in the mara rosa supracrustal belts display geochemical features suggesting that they represent first cycle immature sediments which preserve chemical characteristics of their igneous sources their nd isotopic characteristics support this interpretation t dm model ages for detrital metasediments are mostly in the interval between 0 9 and 1 2 ga indicating that they are a product of erosion of the arc rocks the absence of older contributions to these sediments seems to indicate an intraoceanic setting this is also suggested by the nd isotopic composition of the 0 8 ga old tholeiitic amphibolites which display positive initial e nd 4 9 indicating derivation from a depleted mantle source syn orogenic deformed granites forming small intrusions within the supracrustal belts have t dm ages ranging from 0 77 to 2 54 ga the older model age values indicate the presence of hidden fragments of ancient crust which were probably tectonically interleaved with younger rocks during accretion of the arc the post orogenic granites are moderately peraluminous and the nd model ages between 1 0 and 1 3 ga indicate that their parental magmas were predominantly re melts of the arc rocks their peraluminous nature reflects the incorporation of substantial amounts of arc sediments in the original magma garnet whole rock sm nd ages for the metasediments may be interpreted to refer to two metamorphic events an older event at 0 76 0 73 ga and a younger event at 604 610 ma mineral chemistry indicate that the older episode was characterized by higher p t and the younger by lower p t the older metamorphic event is interpreted preliminarily as the result of the accretion of island arc terranes to the western edge of the orogen and the younger event as a result of final ocean closure and continental collision between the amazon and sao francisco continents the goias arc formation and accretion is partially coeval with rodinia break up suggesting that the western margin of the sao francisco congo was peripheral in that supercontinent and faced a wide oceanic basin during most of the neoproterozoic

solutions of the stream power equation and application to the evolution of river longitudinal profiles 1 erosion by bedrock river channels is commonly modeled with the stream power equation we present a two part approach to solving this nonlinear equation analytically and explore the implications for evolving river profiles first a method for non dimensionalizing the stream power equation transforms river profiles in steady state with respect to uniform uplift into a straight line in dimensionless distance elevation space second a method that tracks the upstream migration of slope patches which are mathematical entities that carry information about downstream river states provides a basis for constructing analytical solutions slope patch analysis explains why the transient morphology of dimensionless river profiles differs fundamentally if the exponent on channel slope n is less than or greater than one and why only concave up migrating knickpoints persist when n 1 at migrating knickpoints slope patches and the information they carry are lost a phenomenon that fundamentally limits the potential for reconstructing tectonic histories from bedrock river profiles stationary knickpoints which can arise from spatially varying uplift rates differ from migrating knickpoints in that slope patches and the information they carry are not lost counterparts to migrating knickpoints called stretch zones are created when closely spaced slope patches spread to form smooth curves in distance elevation space these theoretical results are illustrated with examples from the california king range and the central apennines

the position of the amazonian craton in supercontinents abstract this paper examines the extensive regions of proterozoic accretionary belts that either formed most of the amazonian craton or are marginal to its southeastern border their overall geodynamic significance is considered taking into account the paleogeographic reconstruction of columbia rodinia and gondwana amazonia would be part of columbia together with laurentia north china and baltica forming a continuous continental landmass linked by the paleo to mesoproterozoic mobile belts that constitute large portions of it the rodinia supercontinent was formed in the mesoproterozoic by the agglutination of the existing cratonic fragments such as laurentia and amazonia during contemporary continental collisions worldwide the available paleomagnetic data suggest that laurentia and amazonia remained attached until at least 600 ma since all other cratonic units surrounding laurentia have already rifted away by that time the separation between amazonia and laurentia marks the final break up of rodinia with the opening of the lapetus ocean

late mesozoic magmatism of the jiurui mineralization district in the middle lower yangtze river metallogenic belt eastern china precise u pb ages and geodynamic implications abstract the middle lower yangtze river metallogenic belt mlyrmb extending from daye in hubei province in the west to zhenjiang in jiangsu province in the east hosts a number of large polymetallic cu au mo fe zn pb and ag deposits and constitutes one of the most important metallogenic belts in china the cu au mo deposits in the jiurui district are an important component of the mlyrmb in this study we carried out precise and detailed zircon u pb dating for all types of magmatic rocks from the wushan ore deposit in the jiurui district three samples of cu au mo related porphyries from different ore belts at wushan were analyzed and yielded zircon u pb ages of 148 0 1 0 ma 145 4 0 9 ma and 147 3 0 9 ma respectively a series of dykes were emplaced immediately following the cu au mo related porphyries at wushan a dark colored basic dyke which intruded into the granodiorite porphyry at wujia gold deposit near wushan was dated at 144 5 1 2 ma two lamprophyre dykes taken from the north ore belt at wushan underground mining stops were dated at 143 6 0 9 ma and 144 3 0 9 ma respectively a late stage dyke which was also taken from the wushan north ore belt yielded an age of 142 6 1 0 ma and might represent the end of magmatism in the wushan ore deposit these new geochronological data demonstrate that the time range of magmatism in the wushan ore deposit is approximately between 148 ma and 143 ma showing that the magmatic activity at wushan was rapid and intensive the ages of cu au mo related porphyries from other areas in the jiurui district such as the dongleiwan yangjishan and chengmenba ore deposits were also measured and yielded zircon u pb ages of 141 5 1 7 ma 143 4 1 4 ma and 146 6 1 0 ma respectively combined with those previously reported zircon u pb age results from the jiurui district the present age data set demonstrates that extensive magmatism in the jiurui district was coeval and intensive marked by a magmatic activity in the age range of 148 to 138 ma and peaked between 148 ma and 142 ma according to the statistics of all those precise zircon u pb ages the cu au mo related porphyries in the edong and tongling districts in the mlyrmb show similar ages and they have a slightly younger peak age and a longer duration than that of the jiurui district the geographic shape of the mlyrmb in the cretaceous shows an arcuate structure the jiurui district is located at the transitional point of the arcuate structure and the edong and tongling districts are situated on both sides of the arcuate structure considering that the jiurui district has a slightly older peak age and a shorter duration of magmatic activity than that in the edong and tongling districts it seems that the arcuate structure of the mlyrmb played an important role in the formation of these cu au mo related porphyries consequently we suggest that the genesis of the late mesozoic magmatic rocks along the mlyrmb may have been due to a tectonic activity developed from southeast to northwest which probably has a close relationship with the subduction of the paleo pacific plate beneath the eurasian plate in mesozoic times

time frame of 753 680 ma juvenile accretion during the sao gabriel orogeny southern brazilian shield abstract the time frame of the three main geological events in the neoproterozoic cambai complex juvenile sao gabriel belt in the southern brazilian shield is established by integrating field mapping back scattered electron imaging and sensitive high resolution ion microprobe shrimp ii u pb dating of 96 zircon crystals from nine granitic and metasedimentary rock samples the three events are 1 voluminous flat lying paragneisses cambaizinho complex and orthogneisses vila nova gneisses between 735 and 718 ma 2 tonalite trondhjemite association lagoa da meia lua suite between 710 and 690 ma and 3 late granodiorite intrusions sanga do jobim suite at 680 ma an additional older volcanic event campestre formation was dated at 753 ma these results are most significant for the reconstruction of west gondwana

hillslope response to knickpoint migration in the southern appalachians implications for the evolution of post orogenic landscapes the southern appalachians represent a landscape characterized by locally high topographic relief steep slopes and frequent mass movement in the absence of significant tectonic forcing for at least the last 200 ma the fundamental processes responsible for landscape evolution in a post orogenic landscape remain enigmatic the non glaciated cullasaja river basin of south western north carolina with uniform lithology frequent debris flows and the availability of high resolution airborne lidar dems is an ideal natural setting to study landscape evolution in a post orogenic landscape through the lens of hillslope channel coupling this investigation is limited to channels with upslope contributing areas 2 7 km2 a conservative estimate of the transition from fluvial to debris flow dominated channel processes values of normalized hypsometry hypsometric integral and mean slope vs elevation are used for 14 tributary basins and the cullasaja basin as a whole to characterize landscape evolution following upstream knickpoint migration results highlight the existence of a transient spatial relationship between knickpoints present along the fluvial network of the cullasaja basin and adjacent hillslopes metrics of topography relief slope gradient and hillslope activity landslide frequency exhibit significant downstream increases below the current position of major knickpoints the transient effect of knickpoint driven channel incision on basin hillslopes is captured by measuring the relief mean slope steepness and mass movement frequency of tributary basins and comparing these results with the distance from major knickpoints along the cullasaja river a conceptual model of area elevation and slope distributions is presented that may be representative of post orogenic landscape evolution in analogous geologic settings importantly the model explains how knickpoint migration and channel hillslope coupling is an important factor in tectonically inactive i e post orogenic orogens for the maintenance of significant relief steep slopes and weathering limited hillslopes copyright 2011 john wiley sons ltd

internal waves in the clyde sea temperature measurements were made with 15 s and 1 min sampling intervals in the thermocline of the clyde sea to assess the variability associated with internal waves on time scales from min to hours the data were collected during shallow water acoustics experiments which took place near troon during the summer of 1997 the data reveal clear evidence of internal wave activity which was also apparent in a sequence of rapid ctd yo yo profiles intermittent high frequency internal wave events displayed a period of about 10 min which is comparable to the local buoyancy period vertical amplitudes of 3 m were observed corresponding to sound speed fluctuations of about 2 m s 1 vertical displacement spectra calculated from the temperature data suggest that the observed energy levels did not differ significantly from the generic garrett and munk deep water internal wave spectrum a stochastic internal wave model based on the garrett and munk spectrum was used to calculate dynamically consistent internal wave fields from which estimates of acoustic variability were derived three numerical schemes were used to simulate the vertical advection of the density field an eulerian method a lagrangian method and a time varying mode method the eulerian method based on linear dynamics performed as well as the more sophisticated techniques and good agreement was obtained between the observed and simulated variability of the sound channel the observations suggest that the internal wave activity and hence the acoustic variability was phase locked to the surface tide

granitic magmatism basement ages and provenance indicators in the malay peninsula insights from detrital zircon u pb and hf isotope data abstract the malay peninsula lies on two continental blocks sibumasu and east malaya which are intruded by granitoids in two provinces the main range and eastern previous models propose that permian triassic granitoids are subduction related and syn to post collisional we present 752 u pb analyses that were carried out on zircons from river sands in the malay peninsula of these 243 grains were selected for hf isotope analyses our data suggest a more complex sibumasu east malaya collision history 176hf 177hfi ratios reveal that permian triassic zircons were sourced from three magmatic suites a permian crustally derived granitoids b early middle triassic granitoids with mixed mantle crust sources and c late triassic crustally derived granitoids this suggests three permian triassic episodes of magmatism in the malay peninsula two of which occurred in the eastern province although the exact timing of the sibumasu east malaya collision remains unresolved current data suggest that it occurred before the late triassic probably in late permian early triassic our data also indicate that sibumasu and east malaya basements are chronologically heterogeneous but predominantly of proterozoic age some basement may be neoarchaean but there is no evidence for basement older than 2 8 ga finally we show that hf isotope signatures of triassic zircons can be used as provenance indicators

hillslope response to tectonic forcing in threshold landscapes hillslopes are thought to poorly record tectonic signals in threshold landscapes numerous previous studies of steep landscapes suggest that large changes in long term erosion rate lead to little change in mean hillslope angle measured at coarse resolution new lidar derived topography data enables a finer examination of threshold hillslopes here we quantify hillslope response to tectonic forcing in a threshold landscape to do so we use an extensive cosmogenic beryllium 10 10 be based dataset of catchment averaged erosion rates combined with a 500 km 2 lidar derived 1 m digital elevation model to exploit a gradient of tectonic forcing and topographic relief in the san gabriel mountains california we also calibrate a new method of quantifying rock exposure from lidar derived slope measurements using high resolution panoramic photographs two distinct trends in hillslope behavior emerge below catchment mean slopes of 30 modal slopes increase with mean slopes slope distribution skewness decreases with increasing mean slope and bedrock exposure is limited above mean slopes of 30 our rock exposure index increases strongly with mean slope and the prevalence of angle of repose debris wedges keeps modal slopes near 37 resulting in a positive relationship between slope distribution skewness and mean slope we find that both mean slopes and rock exposure increase with erosion rate up to 1 mm a in contrast to previous work based on coarser topographic data we also find that as erosion rates increase the extent of the fluvial network decreases while colluvial channels extend downstream keeping the total drainage density similar across the range our results reveal important textural details lost in 10 or 30 m resolution digital elevation models of steep landscapes and highlight the need for process based studies of threshold hillslopes and colluvial channels