the coastal terrane of the kaoko belt namibia outboard arc terrane and tectonic significance abstract the coastal terrane or westernmost part of the kaoko belt outboard of the three palms mylonite zone has distinct feldspathic arenite sedimentary sequences no basement distinct nd sediment signatures excluding the archaean and suggesting mesoproterozoic neoproterozoic provenance sources and primitive arc like geochemical signatures for i type granitoids it contains evidence for an older metamorphic event at 650 645 ma not present in any other part of the kaoko belt this metamorphism m1 was of high t low p granulite to upper amphibolite facies and includes migmatisation associated with i type granitic magmatism of arc affinity arc growth at 650 640 ma took place outboard within the adamastor ocean while swakop group facies turbidite sedimentation continued inboard along the passive margin overprinting m1 and m2 fabrics and metamorphic assemblages constrain the docking to have occurred between 650 and 580 ma lack of ophiolite fragments and evidence of intermediate to high p metamorphism along the three palms mylonite zone suggest that it is not a suture and more likely to be part of an arc backarc wrench shear system that developed inboard of an e dipping subduction system where the adamastor ocean was subducted beneath the leading edge of the attenuated congo craton oblique collision between 650 and 580 ma due to docking of this outboard coastal terrane with arc affinities caused i crustal scale oblique overriding of the arc terrane over the passive margin of the congo craton ii crustal scale sinistral shear partitioned into two major ductile shear zones iii high t granulite facies metamorphism in an extruded shear zone bounded core and iv outwards or cratonwards verging basement cored fold nappes

linking the basement geology along the africa south america coasts in the south atlantic the work was financially supported by the czech science foundation 15 05988s we thank martin whitehouse and lev ilyinsky for their support while using the nordsim ion probe in stockholm the nordsim facility is jointly funded by denmark finland norway sweden and iceland this is nordsim publication 446 we appreciate the discussions with stano ulrich and pedro oyhantcabal during various stages of this project the paper benefited from the reviews of cees passchier and an anonymous reviewer cees passchier directed us to the work of abraham ortelius mentioned in the introduction and kindly provided the translation from latin

the stable isotopic composition of modern soil carbonate and its relationship to climate the oxygen isotopic composition of modern soil carbonate is well correlated with the isotopic composition of local meteoric water the carbon isotopic cycle for co 2 in soils can be described in terms of the proportion of biomass using the c 4 photosynthetic pathway and the co 2 respiration rate of the soil at low soil respiration rates significant atmospheric co 2 mixing can occur in general the carbon isotopic composition of soil carbonate is related to the proportion of c 4 biomass present in soil but soils that freeze to the depth of carbonate formation often have a significant atmospheric component this suggests that freezing of the soil solution should be considered as another important mechanism for soil carbonate formation because of these relationships the isotopic composition of soil carbonate may be a paleoclimatic and paleoecologic indicator in cases in which diagenetic alteration has not occurred

40ar 39ar thermochronology of the pan african damara orogen namibia with implications for tectonothermal and geodynamic evolution abstract forty three 40ar 39ar step heating experiments on white mica biotite hornblende and whole rocks from the damara orogen namibia revise and refine the regional perspective on cooling exhumation and tectonic reactivation across the orogen these data also document post orogenic motion on major shear zones and the thermal effects of late syn to post tectonic granite intrusions these data show regional cooling through temperatures of 300 350 c from 545 to 520 ma in the central kaoko zone 530 510 ma in the eastern kaoko zone and 495 480 ma in the southern zone of the damara belt discordant 40ar 39ar age spectra relate to 1 partial resetting of k ar isotopic system in micas by late stage activity in the major kaoko belt shear zones such as the purros and three palms mylonite zones giving apparent ages of 467 6 481 3 and 492 ma and 2 thermal effects from granitic plutions particularly in the ugab domain of the southern kaoko belt 500 475 ma and in the granite dominated central zone of the damara belt 490 460 ma phyllites of the lower grade flank regions of the orogen record apparent 40ar 39ar mica crystallization ages of 517 ma eastern kaoko zone 526 ma northern zone damara belt and 568 553 ma southern foreland zone damara belt the southern foreland samples however are maximum ages due to variable influence of detrital mica rates of cooling vary between zones within the kaoko belt in the central kaoko belt hornblende biotite and white mica apparent 40ar 39ar ages are similar suggestive of rapid cooling through the 300 550 c temperature range whereas in the orogen core or western kaoko belt hornblende and biotite apparent ages are older than the white mica ages because the white micas record the reactivation of the main structures these data support models of diachronous deformation and metamorphism of the component parts of the orogen and suggest that pan african orogenesis in the damara is broadly bracketed between 580 and 500 ma with final movements through 480 ma and spans through the terminal amalgamation of gondwana in the kuunga orogeny through the initiation of subduction under the gondwana continental margin

geochemistry of spanish sepiolite palygorskite deposits genetic considerations based on trace elements and isotopes sepiolite palygorskite deposits in spanish tertiary basins were formed in lacustrine environments the mineral associations present in the mineralised intervals are made up of neoformed phyllosilicates sepiolite palygorskite stevensite detrital silicates quartz feldspars illite interstratified smectite illite al smectite and kaolinite and carbonates calcite dolomite opal a gypsum and halite may also appear sporadically two groups of chemical elements and minerals can be distinguished according to their origin al ti fe mn k ree and transition trace elements are almost exclusively included in the detrital al silicates mg ca cl f and li are concentrated in the minerals formed in the depositional basins si na sr and ba are contained in both detrital and neoformed minerals ree transition trace elements f and li contents can be used to distinguish between phyllosilicates formed by chemical precipitation in the depositional basins detrital phyllosilicates and those formed by transformation of the latter during early diagenetic processes these data together with those on 18o isotopic fractionation indicate the formation of sepiolite and mg smectite as chemical precipitates whereas palygorskite would derive from diagenetic transformation of other inherited clay minerals the values calculated for the sepiolite water and palygorskite water fractionation factors at 20 c are 1 031 and 1 027 respectively

diachronic collision slab break off and long term high thermal flux in the brasiliano pan african orogeny implications for the geodynamic evolution of the mantiqueira province abstract the mantiqueira province in se brazil and uruguay is a complex brasiliano pan african orogenic province that has recently been the topic of conflicting geodynamic models focusing the possibility of multiple collision events during the ediacaran ordovician assembly of the western gondwana supercontinent a thorough compilation of recently obtained high resolution u pb geochronology and geochemistry in the mantiqueira province provided the following results a considering the entire orogenic system the pre syn and post orogenic periods range from 670 595 ma 620 550 ma and 560 490 ma respectively and are coeval with the intrusion of 3 distinct magmatic sequences within each geographic segment of this large orogenic province evolving from i calc alkaline arc related magmatism to ii syn orogenic anatectic magmatism coeval with the metamorphic climax to iii calc alkaline alkaline including shoshonitic and a type affinity magmatism that reveal significant lower crust and mantle contribution b progressively younger ages are present from south to north irrespective of the orogenic period considered implying that the different geographic sectors were from south to north diachronically amalgamated into the mantiqueira province results provide important evidence for the existence of a single diachronic collision event at 630 590 ma followed by the thermal climax at 615 560 ma slab break off asthenospheric upwelling and mantle underplating that sustained long term high heat flux conditions until 540 490 ma when post tectonic calc alkaline alkaline granitoids probably admixed with small amounts of slab melts formed at depth intruded the middle low crust magmatic and metamorphic sequences during the final stages of thermal and orogenic collapse of the mantiqueira province belts

systematic variations in the carbon and oxygen isotopic composition of pedogenic carbonate along elevation transects in the southern great basin united states stable carbon and oxygen isotope variations in holocene soil carbonates that formed in the unsaturated zone were examined along several elevation transects in the southern great basin united states a region with a semi arid climate our intent was to study the relationship between the stable isotopic composition of pedogenic carbonates and climate ecological variations differences in parent material and soil depth 13 c of pedogenic carbonate in three different soil profiles from different elevations decreases with soil depth indicating a decrease in the ratio of atmospheric to plantderived co 2 downprofile pedogenic carbonate at the soil air interface approaches a 13 c value in equilibrium with atmospheric co 2 in all three soils observed 13 c profiles for pedogenic carbonate can be described using a one dimensional model for 12 co 2 and 13 co 2 assuming isotopic equilibrium between soil co 2 and soil carbonate the modeled best fit to observed isotopic profiles suggests that the profile differences in part result from differing soil respiration rates at each site 13 c in deep pedogenic carbonate 50 cm varies by about 12 per mil over a 2 440 m elevation range being enriched in 13 c at the lowest elevations the slope of 13 c for these carbonates versus elevation is very similar for soils developed on carbonate and on noncarbonate parent materials depletion by 4 6 to 4 7 per mil per 1 000 m increase in altitude between 300 to 2 740 m above mean sea level for the localities studied this concordance makes it likely that there has been complete isotopic exchange between hco 3 in solution and soil co 2 prior to carbonate precipitation soil co 2 and soil respiration rates increase systematically with elevation the plantderived component of soil co 2 indicates that c 3 plants dominate the biomass at most measured sites in agreement with plant surveys calculated equilibrium fractionation factors between soil co 2 and soil carbonate are very similar to those observed again indicating complete isotopic exchange between carbon species in all the soil co 2 and soil carbonate data suggest that the 13 c variation with elevation observed in the soil carbonates results from differing soil respiration rates at each site as well as from variations in the proportion of c 3 to c 4 and cam plants in each site9s surface biomass 18 o values in pedogenic carbonates are higher at lower elevations due in part to the more positive 18 o values for meteoric waters at lower elevations the average 18 o value of deep 50 cm pedogenic carbonate at all sites however is enriched 2 4 to 3 7 per mil with respect to values we predict should be in equilibrium with the isotopic composition of local meteoric waters this suggests that evaporative isotopic enrichment of soil waters may have occurred at all elevations prior to precipitation of carbonate or that seasonal differences in the isotopic composition of meteoric waters coupled with differential infiltration may be taking place one or both of these processes also may explain the 18 o decrease in soil carbonate with depth observed in two of three soil profiles

neoproterozoic tectono thermal evolution of the gariep belt and its basement namibia and south africa abstract within the pan african gariep belt in southwestern africa a para autochthonous predominantly sedimentary sequence deposited in a rift graben and subsequently on a passive continental margin with a kibaran basement is distinguished from an allochthonous predominantly oceanic terrane that was thrust on top of the former during collision of the rio de la plata and the kalahari plates 40ar 39ar mineral cooling ages presented here set the following constraints after kibaran high grade metamorphism the consolidated basement cooled through 300 c at 1006 ma rifting between 781 and 741 ma was followed by the formation of oceanic crust the lower parts of which cooled through 500 c sometime between 630 and 600 ma after inversion from extension to compression some of the oceanic crustal material was shredded off and incorporated into an accretionary wedge a process that is indirectly dated between 573 576 ma by hornblende plateau ages continent continent collision and thrusting of the internal onto the external zones of the tectonic belt culminated between 547 543 ma as indicated by further hornblende plateau ages muscovite cooling ages for the internal zone are between 526 and 529 ma those for the tectonically deeper external zone are 483 ma in the north and between 495 506 ma in the south of the belt in conjunction with published data these results suggest successive closure of first the northern adamastor ocean kaoko belt followed by the khomas sea intracontinental damara belt and finally the southern adamastor ocean gariep belt

oxygen isotope evidence of climate change from pedogenic clay minerals in the himalayan molasse abstract in this study we examine the oxygen isotope ratios of smectite and kaolinite in a stratigraphic sequence of paleosols in the siwalik himalayan molasse on the potwar plateau northern pakistan there is a 3 4 o increase in the smectite 18 o values synchronous with a 3 5 o increase in 18 o values of pedogenic calcite at 8 5 6 5 ma previously observed by quade et al 1989 the covariation of the 18 o values of smectite with calcite gd 18 o values through time strongly suggests a pedogenic origin of smectite and that its 18 o values reflect the oxygen isotope ratio of the soil water present during pedogenesis these data show that the 18 o values of clay minerals in paleosols in aggrading sedimentary sequences are viable paleoclimate indicators kaolinite 18 o values are similar to smectite 18 o values as would be expected for pedogenic kaolinite but there is greater variation in these data which we attribute to incomplete mineral separation the measured permil oxygen isotope fractionation between calcite and smectite within individual paleosols is on average 3 o higher than the equilibrium fractionation this isotopic disequilibrium can be explained by the calcite forming predominantly during the dry winter season and the smectite forming predominantly during the wet summer season the synchronous 3 5 o increase in oxygen isotope ratios of smectite and calcite at 8 5 6 5 ma indicates that this 18 o 16 o increase is not due to diagenesis but rather results from a climate change the most likely causes of the 18 o 16 o increase in smectite and calcite are either increased aridity resulting in increased soil water evaporation thus 18 o enrichment and or a change from more continental to more marine sourced precipitation the tibetan plateau which is suggested to have undergone rapid uplift around 8 ma e g harrison et al 1992 may have acted as an effective orographic barrier to low 18 o precipitation originating from central asia causing higher 18 o values of the precipitation falling on the potwar plateau currently it is impossible to distinguish between these two potential causes of the 18 o 16 o increase at 8 5 6 5 ma but these clay mineral 18 o values support that there was a significant climate change in this region

varied crystallization conditions for neogene sepiolite and associated mg clays from madrid basin spain traced by oxygen and hydrogen isotope geochemistry abstract 18 o d measurements were made on sepiolite and associated saponite and kerolite stevensite mixed layers from miocene sediments of the madrid basin in spain to evaluate the relative impact of the isotopic composition of the interacting fluids and the crystallization temperature on these minerals in specific continental crystallization environments such as alluvial palustrine and mudflat areas preliminary experiments of progressive heating of pure sepiolite and saponite under vacuum allowed to set a pretreatment at 200 c to remove the non bonded water of both sepiolite and smectite before analysis of the structural oxygen and hydrogen in combining crystallization temperatures of the different mg clays with potential isotopic compositions of the interacting fluids it can be stated that the studied clay minerals precipitated during sedimentation in connection with various fluids of meteoric origin for saponite and stevensite and with an evaporated more saline water for sepiolite the calculated crystallization temperatures ranged from as low as 7 5 to 35 5 c for sepiolite and from about 22 5 to 48 5 c for saponite in the case of the kerolite stevensite mixed layer it is likely that the stevensite domains crystallized first at a calculated temperature of about 10 5 c being progressively replaced by kerolite domains at a significantly higher temperature towards 55 5 c the calculated crystallization temperature of sepiolite appears also to increase from palustrine mudflat mudflat palustrine to alluvial environments the increasing trend goes from palustrine sepiolite formed at as low as 7 5 c from a parent water that interacted also with saponite to sepiolite crystallized from evaporated waters at higher temperatures in mudflat palustrine and alluvial facies if such mineral formation occurred quasi continuously rather than episodically sepiolite and saponite formed at any given locality over a range of temperatures from waters with varied isotopic compositions any given mg mineral might then result from a mixture of crystals with different isotopic compositions consisting of end members that interacted with the initial parental water of the smectite 18 o 7 25 and d 48 at low temperature of about 10 5 c and evaporated water 18 o 4 00 and d 32 at temperatures up to 40 5 c

the role of biomineralization in the origin of sepiolite and dolomite sepiolite is an important industrial mineral whose mechanism of formation and detailed origin are still not clear sepiolite is frequently found with dolomite mg smectites or palygorskite in the intermediate unit of miocene sediments of the madrid basin herein we describe mineralogy texture and mineral chemistry of sepiolite rich layers up to 90 concentration in marl clay sediments either with gypsum chert micro crystalline quartz or dolomite that are up to 2 to 3 m thick the materials studied were formed in shallow lake environments or mudflats these sediments contain dolomite aggregates resembling mineralized microorganisms biomorphs with ovoid morphologies and spherical to tubular internal voids the regular size and relatively uniform shape of the aggregates are likely controlled by the shape and size of the microorganisms the paper describes the presence of organic debris silicification processes and potassium silicate depletion related to the basal sediments that preceded sepiolite rich layers potassium depletion as a nutrient in detrital components and the occurrence of organic and reduced sediment u and sulfide concentration at the interface with sepiolite also can be linked to biosignatures related to the origin of sepiolite in addition the occurrence of dolomite with carbon that has 13c pdb values of

the transpressional connection between dom feliciano and kaoko belts at 580 550 ma a new u pb shrimp age of 551 4 ma on a mylonitic porphyry that intruded into the sierra ballena shear zone southernmost dom feliciano belt uruguay and a review of relevant published data make possible a more refined correlation and reconstruction of brasiliano pan african transpressional events paleogeographic reconstruction kinematics and timing of events indicate a connection between the shear systems of the dom feliciano and kaoko belts at 580 550 ma sinistral transpression recorded in shear zones accommodates deformation subsequent to collision between the congo and rio de la plata cratons the correlation is strengthened by the similarity of magmatic and metamorphic ages in the coastal terrane of the kaoko belt and the punta del este terrane of the dom feliciano belt this post collisional sinistral transpression brought these units near to their final position in gondwana and explains the different evolution at 550 530 ma while in the kaoko belt an extensional episode resulted in exhumation as a consequence of collision in the damara belt in the dom feliciano belt sinistral transpression occurred associated with the closure of the southern adamastor ocean due to kalahari rio de la plata collision

geochemistry of calcretes calcic palaeosols and hardpan coimbatore southern india formation and paleoenvironment abstract there is no major chemical variation in the late quaternary calcic nodules formed within vertisols in the foot hill regions and hardpan calcretes greater than 200 ka formed over the precambrian substrate that occur as duricrust horizons in the plains around coimbatore region tamil nadu india the bulk chemistry of calcic nodules and the hardpan calcretes show very little variability in cao sio2 al2o3 and fe2o3 content the trend of trace metal content in calcic nodules and in the hardpan calcrete is mn zn ni cr pb co cu and zn mn ni co pb cu cr respectively paas normalised ree data of the calcretes calcic nodules and the hardpan demonstrate a positive euanomaly this could be attributed to feldspar apatite mineral alteration and soil digenetic processes differential weathering leaving behind plagioclase phenocrysts and apatite grains which are enriched in eu the stable isotope values of the calcic nodules do not exhibit a wide range 18o 3 39 to 5 84 and 13c 3 01 to 6 64 compared to the hardpan calcretes 18o 2 91 to 12 98 and 13c 0 05 to 7 4 the palaeoenvironment during the formation of the calcretes nodules was dominated by the c4 plants with sparse vegetation cover and that the parent material bedrock supported a thin veneer of soil column with sparse vegetation cover as present day calcic nodule accumulations in the soils has resulted from differential weathering and chemical histories within the soil profiles while the hardpan calcretes formed by complex pedogenic and chemical groundwater processes in arid semi arid conditions over along geological period during the late mis3 to the lgm period the south west monsoon was weak with mean annual rainfall mar between 300 and 500 mm yr the sea level was low and the rainfall at the time of carbonate formation would have remained damp enough to allow silicate weathering of the soil sediments

a new lithostratigraphic subdivision and geodynamic model for the pan african western saldania belt south africa abstract a provenance study involving u pb and lu hf isotope analyses of detrital zircon grains as well as whole rock geochemical rb sr sm nd and pb isotope analyses of siliciclastic metasedimentary rocks from the principal tectonostratigraphic units of the pan african western branch of the saldania belt in south africa revealed age relationships and source domains that are not compatible with current stratigraphic schemes and geodynamic models for the evolution of this belt the previous subdivision into three terranes cannot be upheld the sediments in both the former tygerberg and swartland terranes seemingly have the same provenance but differ from those in the former boland terrane the latter is considered a para autochthonous zone on top of basement rocks of the late mesoproterozoic namaqua natal belt on the basis of our new data it is recommended to unify the former tygerberg and swartland terranes into a single terrane for which the name malmesbury terrane is proposed the colenso fault previously regarded as a terrane boundary is reinterpreted as reactivated old structure inherited from the basement within the malmesbury terrane whereas the major terrane bounding fault is the piketberg wellington fault which can be linked most likely with the schakalsberge thrust further north in the gariep belt the age of sedimentation is constrained within a narrow time window between 557 and 552 ma in the malmesbury terrane and between 609 and 532 ma in the boland zone no evidence of older cryogenian sedimentary rocks could be found except for the volcanic dominated bridgetown formation which is presumably a tectonic sliver the cambrian klipheuwel group is undoubtedly younger than and unrelated with regard to its provenance to the other saldanian sediments all geochemical evidence points to an active continental margin as source for the sediments in the malmesbury terrane and boland zone while the ages of the detritus in the former are strongly dominated by neoproterozoic especially cryogenian ages those in the boland zone are mainly early tonian and late mesoproterozoic the most likely source of the cryogenian detritus is the dionisio cuchilla pelotas magmatic arc in southeastern brazil and uruguay which formed within a splinter of kalahari craton i e arachania a back arc position relative to that arc is inferred for the depo basin in which the redefined malmesbury group making up the neoproterozoic succession of the malmesbury terrane was deposited consequently our new data support the hypothesis that the neoproterozoic successions of the western saldania belt and their northern equivalents in the western gariep belt do not reflect a full wilson cycle but only the waning stages of an ediacaran back arc basin the main suture between the kalahari and rio de la plata cratons must be located therefore to the west of the dionisio cuchilla pelotas magmatic arc probably along the major gercino sierra balena lineament within southeastern south america this implies that the opening of the modern south atlantic followed to a large extent the axis of the postulated former ediacaran back arc basin

deciphering polymetamorphic episodes in high grade metamorphic orogens constraints from pbsl sm nd and lu hf garnet dating of low to high grade metasedimentary rocks from the kaoko belt namibia abstract three dating techniques for metamorphic minerals using the sm nd lu hf and pb isotope systems are combined and interpreted in context with detailed petrologic data from crustal segments in nw namibia the combination of isochron ages using these different approaches is a valuable tool to testify for the validity of metamorphic mineral dating here pbsl lu hf and sm nd garnet ages obtained on low to medium grade metasedimentary rocks from the central kaoko zone of the neoproterozoic kaoko belt nw namibia indicate that these samples were metamorphosed at around 550 560 ma on the other hand granulite facies metasedimentary rocks from the western kaoko zone underwent two phases of high grade metamorphism one at ca 660 625 ma and another at ca 550 ma providing substantial evidence that the 660 625 ma event was indeed a major tectonothermal episode in the kaoko belt our age data suggest that interpreting metamorphic ages by applying a single dating method only is not reliable enough when studying complex metamorphic systems however a combination of all three dating techniques used here provides a reliable basis for geochronological age interpretation

oxygen isotope studies of diagenesis in the basal belly river sandstone pembina i pool alberta abstract the oxygen isotope compositions of authigenic clay and carbonate minerals from the pembina i pool basal belly river sandstone provide useful constraints on the diagenesis of this unit early diagenetic chlorite has 18o values 4 2 to 6 3 smow compatible with low temperature crystallization from brackish water the 18o values of diagenetic calcite 11 8 to 14 7 smow and kaolinite 10 4 to 12 8 smow are best explained by crystallization at temperatures of at least 45 to 70 c from pore fluids similar in 18o to the existing formation water 9 smow the implicatio of such temperatures is that the sandstone was much more deeply buried in the past than it is at present at least 1 300 1 400 m of overlying material may have been eroded since maximum burial

precise determination of phanerozoic zircon pb pb age by multicollector sims without external standardization 1 zircon has long been recognized as the best geochronometer and the most important timekeeper in geosciences modern microbeam techniques such as sims and la icpms have been successfully applied to in situ u pb zircon age determinations at spatial resolutions of 20 30 m or less matrix matched calibration by external standardization of well characterized natural zircon references is a principal requirement for precise microbeam u pb zircon age determination due to fractionation effects between pb and u which usually result in an external age error exceeding 1 alternatively zircons with a closed u pb system can be directly dated by measurement of 207pb 206pb isotopic ratio without external standardization which has been a common practice for zircons older than 1 0 ga but not for relatively young 1 0 ga and particularly phanerozoic ones because of limitations of analytical precision we describe in this paper a method of 207pb 206pb measurement on phanerozoic zircons using a new generation of large radius magnetic sector multicollector cameca ims 1280 sims in combination with multicollector mode a nuclear magnetic resonance nmr magnet controller and oxygen flooding techniques we achieve precisions of 207pb 206pb ratio of 0 1 and 0 1 0 2 propagating to pb pb age errors 0 4 and 1 3 excluding u decay constant uncertainties for zircons of latest neoproterozoic and late paleozoic to mesozoic age respectively therefore the multicollector sims is capable of direct determination of zircon pb pb ages as young as mesozoic age with uncertainties of geological significance this technique is useful for direct dating of zircons in thin sections moreover it has significance for dating of some other u rich minerals i e baddeleyite and zirconolite that are not suitable for sims u pb dating by external standardization

soil mineral genesis and distribution in a saline lake landscape of the pantanal wetland brazil abstract the origin of the saline lakes in the pantanal wetland has been classically attributed to processes occurring in past periods however recent studies have suggested that saline water is currently forming from evaporative concentration of fresh water which is provided annually by seasonal floods major elements ca mg k and alkalinity appear to be geochemically controlled during the concentration of waters and may be involved in the formation of carbonates and clay minerals around the saline lakes the mineralogy of soils associated with a representative saline lake was investigated using xrd tem eds and icp ms in order to identify the composition and genesis of the secondary minerals suspected to be involved in the control of major elements the results showed that ca mg and k effectively undergo oversaturation and precipitation as the waters become more saline these elements are incorporated in the authigenically formed carbonates smectites and micas surrounding the saline lake the control of ca occurs by precipitation of calcite and dolomite in nodules while mg and k are mainly involved in the neoformation of mg smectites stevensitic and saponitic minerals and probably iron enriched micas ferric illite in surface and subsurface horizons therefore our study confirms that the salinity of pantanal historically attributed to inheritance from former regimes has a contribution of current processes

provenance of the late ediacaran rocha formation cuchilla dionisio terrane uruguay tectonic implications on the assembly of gondwana abstract a provenance study on the rocha formation cuchilla dionisio terrane uruguay based on elemental and isotope geochemistry and geochronology revealed characteristics typical of turbidites linked to orogenic arcs textural and mineralogical immaturity and low weathering and sedimentary recycling indicated by low cia values th u and zr sc ratios we separate data into two groups namely northern and southern sectors even though the whole unit shows a provenance of upper continental crust denoted by th sc zr sc th u ratios around ucc averages ree patterns parallel to paas and negative eu anomalies a more depleted component was detected mainly in the northern sector ns showing lower th sc and th u ratios negative eu anomalies of up to 0 9 high abundances of compatible elements and lower end t and 87sr 86sr ratios than the southern sector ss u pb detrital zircon ages indicate 569 10 ma as the maximum sedimentation age the most prominent age peak is neoproterozoic and while the ns shows dominance of tonian and a broad data spread for paleoproterozoic ages the southern sector comprises both the youngest and oldest zircon and displays a pattern with a more pronounced stenian population whereas paleoproterozoic ages are restrained to orosirian a k s test indicates that the change in provenance shown by detrital zircon patterns is reflecting variations of the sediment supply linked to basinal geomorphic heterogeneities within an active margin given detailed whole rock lithogeochemistry nd pb and sr isotopic analyses as well as detrital zircon age patterns correlations with the oranjemund group of the marmora terrane and with the tygerberg formation and associated units of the malmesbury terrane western saldania belt raised from previous interpretations are reassessed source areas were most likely located within the cuchilla dionisio terrane and kalahari craton and therefore deposition within a foredeep basin occurred before tangential accretion to the rio de la plata craton along the sierra ballena shear zone

geochronology of mesoproterozoic hybrid intrusions in the konkiep terrane namibia from passive to active continental margin in the namaqua natal wilson cycle abstract this work investigates a number of gabbro granite hybrid rocks exposed in the great escarpment between the coastal namib desert and the inland plateau of namibia they form part of the tectonostratigraphic konkiep terrane which lies between the mesoproterozoic namaqua natal province and largely palaeoproterozoic rehoboth province in southern africa microbeam u pb and lu hf analyses of zircon were used to investigate the geochronology of these rocks which have previously been assigned to the mooirivier metamorphic complex or the kairab formation the oldest hybrid rocks at neuhof valley are u pb dated at 1359 6 ma and cut by 1347 8 ma unfoliated granite dykes but have the youngest lu hf crustal residence ages between 1690 and 1430 ma together with nearby volcanic rocks of the 1333 6 ma neuhof and 1327 10 ma welverdiend formations which we dated they are thought to reflect rifting of the rehoboth province followed by the ocean development by seafloor spreading phase of the namaqua wilson cycle the hybrid rocks at goede hoop foothills are u pb dated at 1230 6 ma with a number of xenocrysts up to 1532 ma similar hybrid rocks at hauchabfontein have 20 30 ma younger u pb ages of 1211 19 ma and 1202 7 ma two samples they formed by gabbro intruding an 1195 5 ma granite within error but older from field relations and contain many xenocrysts with u pb ages 1755 29 ma n 7 1367 and 1260 17 ma n 24 all the goede hoop foothills and hauchabfontein hybrid rocks have old lu hf crustal residence ages from 1935 to 2000 ma and one sample up to 2320 ma reflecting involvement of palaeoproterozoic rehoboth province crust this 1230 to 1200 ma group of hybrid rocks corresponds in age to the arc magmatism recognised in the namaqua natal province to the west and south of the konkiep terrane the hybrid rocks probably formed during the closure of the namaqua ocean by eastward subduction under the rehoboth province corresponding to the previously proposed rehoboth magmatic arc and forming the konkiep terrane by massive intrusions into the margin of the rehoboth province the hybrid rocks we investigated in the konkiep terrane are thus related to the rifting phase and the pre collisional subduction phase of the namaqua wilson cycle the arc related magmatism ended abruptly at 1200 ma as the namaqua natal terranes were assembled by collisions and subduction ceased we did not find evidence for the 1200 to 1150 ma post collisional metamorphic cycle and magmatism which prevailed in the adjacent grunau terrane of the namaqua province suggesting that the konkiep terrane did not experience crustal thickening during the namaqua collision and terrane assembly an undeformed felsic lava at sukses which we dated at 1105 10 ma corresponds in age to the langberg formation in the rehoboth terrane it is coeval with the keimoes suite of post tectonic intrusions and younger koras lavas in the namaqua natal province and these 1100 ma rocks all extend the area of the plume related umkomdo large igneous province documented on the kaapvaal craton this part of the konkiep terrane originated in the rehoboth province and was then modified by magmatic intrusions coupled to the evolution of the namaqua natal province

geochronological constraints on the evolution of the southern dom feliciano belt uruguay abstract new u pb sensitive high resolution ion microprobe k ar and ar ar data from the southernmost dom feliciano belt allow the identification of four major events orthogneisses from the punta del este terrane indicate a magmatic episode at c 770 ma and high grade metamorphism at 641 17 ma granitoid emplacement at 627 23 ma was roughly coeval with peak metamorphism volcaniclastic rocks of the las ventanas formation dated at 573 11 ma can be correlated with the peripheral foreland basin 571 8 ma transpression and coeval high k calc alkaline magmatism is recorded in the maldonado granite dated at 564 7 ma the following events are postulated 1 magmatism at 850 750 ma related to rifting 2 metamorphism and granite emplacement at 650 600 ma 3 molasse sequences and foreland basins at c 573 ma 4 late magmatism at 580 560 ma associated with transpression the data indicate that 1 the punta del este terrane could be a portion of the coastal terrane of the kaoko belt 2 granitoid emplacement at 650 600 ma in the punta del este and nico perez terranes favours westward subduction and 3 widespread post collisional synkinematic magmatism occurred in the dom feliciano and kaoko belts between 580 and 550 ma supplementary material analytical methods and data are available at http www geolsoc org uk sup18369

the sierra ballena shear zone in the southernmost dom feliciano belt uruguay evolution kinematics and deformation conditions the sierra ballena shear zone sbsz is part of a high strain transcurrent system that divides the neoproterozoic dom feliciano belt of south america into two different domains the basement on both sides of the sbsz shows a deformation stage preceding that of the transcurrent deformation recognized as a high temperature mylonitic foliation associated with migmatization grain boundary migration and fluid assisted grain boundary diffusion enhanced by partial melting were the main deformation mechanisms associated with this foliation age estimate of this episode is 658 ma the second stage corresponds to the start of transpressional deformation and the nucleation and development of the sbsz during this stage pure shear dominates the deformation and is characterized by the development of conjugate dextral and sinistral shear zones and the emplacement of syntectonic granites this event dates to 658 600 ma based on the age of these intrusions the third stage was a second transpressional event at about 586 to 560 ma that was associated with the emplacement of porphyry dikes and granites that show evidence of flattening deformation in the sbsz took place during the late stages under regional low grade conditions as indicated by the metamorphic paragenesis in the supracrustals of the country rocks granitic mylonites show plastic deformation of quartz and brittle behavior of feldspar a transition from magmatic to solid state microstructures is also frequently observed in syntectonic granites mylonitic porphyries and quartz mylonites resulted from the deformation of alkaline porphyries and quartz veins emplaced in the shear zone quartz veins reflect the release of silica associated with the breakdown of feldspar to white mica during the evolution of the granitic mylonites to phyllonites which resulted in shear zone weakening quartz microstructures characteristic of the transition between regime 2 and regime 3 grain boundary migration and incipient recrystallization in feldspar indicate deformation under lower amphibolite to upper greenschist conditions 550 400 c on the other hand the mylonitic porphyries display evidence of feldspar recrystallization suggesting magmatic or high t solid state deformation during cooling of the dikes

mapping using the tsyganenko long magnetospheric model and its relationship to viking auroral images the tsyganenko long magnetospheric model 1987 has been used in conjunction with ultra violet images taken by the viking spacecraft to investigate the relationship of the auroral distribution to different magnetospheric regions the model describes the large scale structure of the magnetosphere reasonably well for dipole tilt angles near zero but it appears to break down at higher tilt angles even so a wide variety of auroral configurations can be accurately described by the model it appears that the open closed field line boundary is a poor indicator of auroral arc systems with the possible exception of high latitude polar arcs the auroral distribution typically called the oval maps to a region in the equatorial plane quite close to the earth and can be approximately located by mapping the model current density maximum from the equatorial plane into the ionosphere although the model may break down along the flanks of the magnetotail the large scale auroral distribution generally reflects variations in the near earth region and can be modeled quite effectively

neoproterozoic magmatic and metamorphic events in the cuchilla dionisio terrane uruguay and possible correlations across the south atlantic abstract the cuchilla dionisio or punta del este terrane of the southern dom feliciano belt eastern uruguay experienced a prolonged neoproterozoic to cambrian tectonothermal evolution zircon and monazite age data from orthogneisses of the basement of the terrane cerro olivo complex provide evidence of widespread magmatism at c 780 ma and high grade up to granulite facies metamorphism between 655 and 632 ma isotope data imply that the orthogneiss protolithic melts formed in a within plate rift related setting by neoproterozoic re melting of lower crustal material that was extracted from the mantle during the palaeoproterozoic we show that the terrane extends further north than previously thought the presence of inherited meso and palaeoproterozoic zircon grains indicates the existence of older crustal components in the terrane the ages of the detrital zircon grains and their hf isotope signatures cannot be explained by a rio de la plata craton provenance but are consistent with a southern african kalahari craton origin ar ar ages of 628 and 625 ma were obtained on magnesiohornblende in amphibolite from the northern cuchilla dionisio terrane whose protolith formed in an ediacaran back arc basin these ages are interpreted as the time of amphibolite facies metamorphism thus crustal convergence between the kalahari or congo and the rio de la plata craton and subduction of the intervening oceanic basin must have been underway between 655 and 625 ma the early neoproterozoic basement of the cuchilla dionisio terrane is part of the southern african namaqua province that was separated from the kalahari craton during rodinia break up and opening of the southern adamastor ocean in the tonian closure of this ocean in the late cryogenian to early ediacaran was associated with arc magmatism and high temperature metamorphism of the cuchilla dionisio terrane basement a back arc basin formed and was closed between this arc and the western african margin relics of this basin were obducted onto the cuchilla dionisio terrane at c 625 ma late neoproterozoic intrusive igneous rocks from the cuchilla dionisio and the coastal terranes were emplaced in a supra subduction setting at least in uruguay and have the same hf isotope signature

monazite and xenotime u th pb geochronology by ion microprobe dating highly fractionated granites at xihuashan tungsten mine se china zircon monazite and xenotime have proven to be valuable chronometers for various geological processes due to their commonly high u th and low common pb contents however zircons that have crystallized in highly fractionated granites often have such high u contents that radiation damage can lead to scattered u pb ages when measured with secondary ion mass spectrometry sims in this study monazite and xenotime were separated from a number of highly fractionated granites at the xihuashan tungsten mine southeast china for alternative dating methods by sims for monazite analysis obvious excess 204 pb signal mainly from interference of 232 th 144 nd 16 o 2 was observed in high th 2 wt monazite which hinders 204 pb based common pb correc tions a 207 pb based common pb correction method was used instead by employing power law relationships between pb u versus uo2 u p b th versus tho2 th and suitable exponentials monazites with tho2 contents in the range of 3 19 do not exhibit this matrix effect inde pendent sims u pb ages and th pb ages of three phases of xihuashan granite samples were consistent with each other and yielded dates of 158 7 0 7 158 0 0 7 and 156 9 0 7 ma respectively xenotime does show marked matrix effects due to variations of u th and y or total rare earth element ree referred as rree hereafter contents suitable correction factors require end member standards with extremely high or low u th and y or rree con tents no excess 204 pb was observed indicating that the 204 pb based common pb correction method is feasible independent 207 pb 206 pb ages can be obtained although multi collector mode is necessary to improve precision the main difficulties with dating xenotime are when high th u mineral inclusions are ablated we can identify when this occurs however by comparing the measured uo2 u and tho2 th with those in xenotime standards three xeno time samples from the first phase of xihuashan granite yielded a weighted mean 207 pb 206 pb date of 159 5 4 4 ma mswd 1 0 and a 206 pb 238 u date of 159 4 0 9 ma mswd 1 6 which are consistent with mona zite u pb and th pb ages from the same granites this study demonstrates that monazite and xenotime are better sims chronometers for highly fractionated granites than zircon which can yield doubtful ages due to high u contents

the hellenic subduction system high pressure metamorphism exhumation normal faulting and large scale extension the cenozoic history of the retreating hellenic subduction system in the eastern mediterranean involves subduction accretion arc magmatism exhumation normal faulting and large scale continental extension from 60 mya until the recent ages for high pressure metamorphism in the central aegean sea region range from 53 ma in the north the cyclades islands to 25 20 ma in the south crete younging of high pressure metamorphism reflects the southward retreat of the hellenic subduction zone the shape of pressure temperature time paths of high pressure rocks is remarkably similar across all tectonic units suggesting a steady state thermal profile of the subduction system and persistence of deformation and exhumation styles the high pressure metamorphic events were caused by the underthrusting of fragments of continental crust that were superimposed on slab retreat most of the exhumation of high pressure units occurred in extrusion wedges during ongoing lithospheric convergence at 23 19 mya large s

the sperrgebiet domain aurus mountains sw namibia a 2020 850ma window within the pan african gariep orogen abstract the aurus mountains in sw namibia form a tectonic inlier sperrgebiet domain within the neoproterozoic gariep belt we document the geological history from field mapping whole rock geochemical metamorphic and u pb zircon isotope data the calc alkaline granodioritic orthogneisses of the wasserkuppe suite dated at 2020 ma are the predominant oldest rocks which are interpreted to form part of a 2 ga sperrgebiet arc the tectono magmatic equivalent of the 250 km wide richtersveld arc to the southeast but 100 ma older 2 ga detritus is common throughout the western namaqua belt but its source was hitherto unknown the suite intruded synchronously with amphibolite facies metamorphism and deformation considered an early phase of the palaeoproterozoic orange river orogeny the wasserkuppe suite is intruded by alkali granite and microgranodiorite dated at 1885 ma interpreted as equivalents of the vioolsdrif suite richtersveld arc the sperrgebiet domain is thus seen as an early phase of a process of palaeoproterozoic island arc development at the margins of the archaean kaapvaal craton rehoboth province it was cannibalised by the later richtersveld arc which dominated the palaeogeography of this part of southern africa 100 m y later and which forms the crustal infrastructure of much of western namaqualand the palaeoproterozoic arc rocks are overlain by a clastic metasedimentary sequence the aurus schist detrital zircon geochronology indicates that these rocks were predominantly derived from both arcs with minor archean and younger 1800 ma detritus of unknown source s bodies of latest mesoproterozoic 1000 ma warmbad granite and pegmatite are mapped throughout the area and represent the only namaquan rocks definitively identified evidence for namaquan metamorphic or tectonic imprints are lacking apart from a garnet clinopyroxene metabasic inclusion in a quartz feldspar leucogneiss which gave a typical namaquan p t estimate of 700 750 c at 6 7 kbar the leucogneisses distributed throughout the area were dated at 840 ma and are interpreted as metamorphosed felsic igneous rocks of the richtersveld suite formed during early rifting of the proto gariep basin the sperrgebiet rocks are variably deformed by the pan african gariep orogeny that produced regionally pervasive fabrics predominantly partitioned into discrete high strain shear zones in a nw trending sinistral transpressive tectonic regime

adamastor an ocean that never existed abstract existing models of tectonic evolution of the neoproterozoic orogenic system rimming the shores of the south atlantic ocean the aracuai ribeira congo and dom feliciano kaoko gariep belts interpret the belts as subduction related orogens and emphasize the role of the adamastor ocean in their pre collisional evolution a critical problem in such an interpretation is the confined nature of the northern termination of the orogenic system as well as a very short time span between the end of rifting and onset of convergence recognized in its southern part in this contribution we review the data for the pre and synorogenic evolution of this system of orogens here collectively called the south atlantic neoproterozoic orogenic system and show that the data speak against the presence of a large oceanic domain before the onset of its orogenic evolution we propose a new and simple intracontinental model suggesting that neoproterozoic oceanic crust played only a minor role in the development of the south atlantic neoproterozoic orogenic system and that its overall architecture and thermal evolution is the result of inversion of large scale rift structures with a protracted and probably episodic extensional history true oceanic crust probably developed only in the southern part of the rift system but it must have been narrow akin to the red sea gulf of aden stage of the adamastor rift evolution just before the onset of convergent thickening

an isotopic study of paleosol carbonates from olduvai gorge the isotopic composition of paleosol carbonates from olduvai gorge can be used to give estimates of the paleoclimatic history of east africa during the plio pleistocene oxygen isotopes are indicative of major changes in the isotopic composition of meteoric water while carbon isotopes are indicative of major changes in the fraction of biomass using the hatch slack c4 photosynthetic pathway three major periods of climatic change in this terrestrial setting occurred at about 1 67 1 3 and 0 6 myr these involved major changes in flora as well as major changes in the isotopic composition of meteoric water overall the climate has become warmer and more arid over the past 2 2 myr

the cuchilla dionisio terrane uruguay an allochthonous block accreted in the cambrian to sw gondwana abstract the cuchilla dionisio terrane cdt of uruguay is a tectonostratigraphic unit defined as the block to the east of the sierra ballena shear zone sbsz it is composed of a palaeo to mesoproterozoic metamorphic basement cerro olivo and chafalote complexes intensely reworked and intruded by syncollisional to late orogenic granites between 680 and 550 ma dacitic and rhyolitic volcanism is recorded around 570 575 ma cerros de aguirre and sierra de rios formations the cdt is correlated with the pelotas terrane of southern brazil the western boundary of the block in brazil is defined by the cangucu and major gercino shear zones which are the northward extension of the sbsz the present position of the cdt is not a consequence of orthogonal collision after completion of a wilson cycle but of lateral sinistral accretion along the above mentioned megashear zones the allochthonous nature of the cdt is postulated on the grounds that a magmatism in the cdt is ca 120 ma younger than metamorphism of neighbouring volcanosedimentary successions e g porongos group in brazil and b while intense volcanism occurred in the cdt in the late vendian represented by the cerros aguirre and sierra de rios formations a passive continental margin deepening to the e existed on the eastern edge of the rio de la plata craton represented by the arroyo del soldado group therefore the cdt and rio del la plata craton were separated hundreds or thousands of kilometers by late vendian times accretion of the cdt took place at 530 ma by tangential collision and was one of the last events in the complicated amalgamation of w gondwana in uruguay the cdt collided with the nico perez terrane generating the sbsz and sinistrally reactivating the sarandi del yi piriapolis shear zone in brazil the cdt pelotas terrane collided with the parana block and a series of parautochthonous terranes curitiba apiai characterized by a transamazonian basement strongly reworked in the neoproterozoic the provenance of the cdt is still uncertain considering that both transamazonian and namaqua natal kibaran ages have been reported from its basement previously reported nd model ages tdm strongly suggest an african affinity the dom feliciano belt is thus a collage of diachronous units formed in different geotectonic settings and not the product of a single orogenic cycle terrane docking at different stages mainly by lateral accretion was the dominant process therefore usage of the name dom feliciano belt should be restricted only to descriptive purposes

oxygen isotopes in meteoric calcite cements as indicators of continental paleoclimate because meteoric water 18 o values decrease with decreasing ambient temperature and increasing latitude 18 o values of meteoric calcite cement should exhibit a similar relation with paleolatitude and be an indicator of continental paleotemperatures to test this we compiled isotopic and paleolatitude data for 20 meteoric cements and nine speleothems ranging in age from devonian to modern and in paleolatitude from 3 5 to 83 mean 18 o values for meteoric cements and speleothems both show the same negative correlation with paleolatitude the 18 o vs latitude trend for these carbonates is almost identical to that predicted for modern inland environments but differs from the trend for coastal environments this suggests that the ground water controlling the ultimate composition of meteoric cement is derived predominantly from inland recharge if it is assumed that the modern meteoric water 18 o vs temperature relation is valid for the past and that insignificant evaporation occurred prior to carbonate precipitation then coastal and inland paleotemperatures can be calculated from the 18 o values of meteoric calcite 18 o mcl and seawater 18 o sw by using the equations t coastal 13 3 32 6 0 231 0 0613 18 o mcl 18 o sw and t inland 17 8 16 2 0 572 0 1233 18 o mcl 18 o sw where t is temperature in c calcite precipitated from coastal meteoric water at temperatures between 0 and 25 c will exhibit a narrow range in 18 o 6 to 4 where 18 o sw 0 the 18 o of calcite precipitated from inland meteoric water will be sensitive to paleotemperature ranging from 14 to 5 where 18 o sw 0 for temperatures of 0 to 25 c

characteristics genesis and parameters controlling the development of a large stratabound htd body at matienzo ramales platform basque cantabrian basin northern spain abstract at matienzo basque cantabrian basin northern spain a large stratabound htd body 4 by 2 km 2 and 80 400 m thick delimited by two parallel sinistral strike slip faults is exposed in aptian carbonates the margins of the htd body are characterised by dolomite tongues indicating that some limestone beds were more prone to dolomitisation however no clear relationship between htd occurrence and precursor limestone facies can be established massive limestone beds as found at the top of the htd body act as barriers to hydrothermal processes since no dolomite is present in or above these beds three types of dolomites have been differentiated i e 1 matrix 2 coarse crystalline and 3 zebra dolomite the distribution of the dolomite types is attributed to ascending fluid flow and changing degree of dolomite oversaturation the dolomite body was formed by two dolomitisation phases under burial conditions no indications for a synsedimentary early diagenetic dolomitisation have been observed the first dolomitisation phase is characterized by ferroan dolomite and the second by non ferroan dolomite the two htd phases are characterised by depleted 18 o values ranging between 10 and 16 v pdb 13 c values similar to the aptian albian marine signature and homogenisation temperatures of primary fluid inclusions between 120 c and 150 c the dolomitising fluid was enriched in 87 sr compared to aptian seawater excluding the latter as an unmodified fluid source for dolomitisation microthermometry of primary fluid inclusions indicates that the dolomitising fluid evolved from a moderate saline 9 7 14 0 wt nacl to a more saline 10 9 21 0 wt nacl h 2 o nacl brine the dolomitising fluid likely originated from evaporated seawater fluid circulation through the dolomitised strata is inferred to have taken place during the tectonically active period of the late albian throughout which important sinistral strike slip movements along basement faults occurred

timing of deformation in the sarandi del yi shear zone uruguay implications for the amalgamation of western gondwana during the neoproterozoic brasiliano pan african orogeny u pb and hf zircon sensitive high resolution ion microprobe shrimp and laser ablation inductively coupled plasma mass spectrometry la icp ms ar ar hornblende and muscovite and rb sr whole rock muscovite isochron data from the mylonites of the sarandi del yi shear zone uruguay were obtained in order to assess the tectonothermal evolution of this crustal scale structure integration of these results with available kinematic structural and microstructural data of the shear zone as well as with geochronological data from the adjacent blocks allowed to constrain the onset of deformation along the shear zone at 630 625 ma during the collision of the nico perez terrane and the rio de la plata craton the shear zone underwent dextral shearing up to 596 ma under upper to middle amphibolite facies conditions which was succeeded by sinistral shearing under lower amphibolite to upper greenschist facies conditions until at least 584 ma after emplacement of the cerro caperuza granite at 570 ma the shear zone underwent only cataclastic deformation between the late ediacaran and the cambrian the sarandi del yi shear zone is thus related to the syncollisional to postcollisional evolution of the amalgamation of the rio de la plata craton and the nico perez terrane furthermore the obtained data reveal that strain partitioning and localization with time magmatism emplacement and fluid circulation are key processes affecting the isotopic systems in mylonitic belts revealing the complexity in assessing the age of deformation of long lived shear zones

high resolution 40ar 39ar dating using a mechanical sample transfer system combined with a high temperature cell for step heating experiments and a multicollector argus noble gas mass spectrometer 40ar 39ar dating of young 5 10 16 mol 36ar is better than 0 5 1 0 1 n 4 8 we illustrate the system performance by 40ar 39ar dating of whole rock samples and mineral separates from the oman ophiolite as well as from the siebengebirge heldburg and rhon volcanic provinces in central germany

upper cretaceous feldspars in the cenozoic limagne basin a key argument in reconstructing the palaeocover of the massif central france abstract the northern part of the n s trending basin of the limagne graben located in the northern part of the massif central france contains oligocene and miocene calcareous lacustrine deposits the area is known for its stromatolite reefs surrounded by marl clay and uncemented carbonate sand with oncolites because the origin of the carbonate accumulated by cyanobacteria has long been debated the clayey deposits associated with the stromatolites were analysed using xrd sem and k ar dating methods the results show a monotonous assemblage of carbonates dioctahedral smectite illite smectite reichweite r 0 glauconitic illite and abundant orthoclase detrital minerals such as quartz and detrital illite are found near the basin s borders but are very scarce in its central part indicating a lack of detrital input from the regional basement hence the abundant orthoclase found in all the clayey deposits of the limagne basin cannot be linked to erosion of the surrounding hercynian basement the sem analysis showed the orthoclase crystals to have a tabular morphology with sharp boundaries and to be embedded within a clay matrix whilst the k ar dating of the orthoclase and glauconitic illite yielded ages ranging from 90 to 66 2 ma i e turonian to maastrichtian these two results clearly indicate that the fill of the limagne basin composed mainly of carbonate clay and orthoclase must have been partly due to the erosion of an upper cretaceous sedimentary blanket once covering the massif central basement this cover was probably constituted of chalk dioctahedral smectite i s and glauconitic illite associated with authigenic orthoclase and flint flints are found reworked in the alluvial formations of the late pliocene lower bourbonnais sands and clays sables et argiles du bourbonnais and in the pleistocene terraces moreover relicts of the upper cretaceous cover were preserved during the sedimentary filling of the cenozoic limagne basin authigenic orthoclase of similar shape has been described in the chalk group of the paris basin and from localities in northern france and belgium the mineral is rare in the turonian chalk but increases rapidly at the base of the coniacian to campanian chalk it has also been noted that the number of orthoclase crystals increases as the quantity of clastic components decreases similar orthoclase crystals were found associated with dioctahedral smectite in the decarbonated residues of the coniacian to santonian chalk from fecamp and etretat northern france moreover our results are consistent with other data from within and around the paris basin such as the palaeogeography and facies distribution of the chalk formations the residual flints in the clay with flints of the southern paris basin and around the morvan and apatite fission track thermochronology data from the hercynian basement of the massif central and the morvan all these data indicate a major connection between the paris basin and the tethys and an extensive palaeocover on the massif central and morvan basement

argillization processes at the el berrocal analogue granitic system spain mineralogy isotopic study and implications for the performance assessment of radwaste geological disposal abstract the el berrocal granite u bearing quartz vein uqv system has been studied as a natural analogue of a high level radioactive waste repository the main objective was to understand the geochemical behaviour of natural nuclides under different physicochemical conditions within this framework the argillization processes related to fracturing and formation of the uranium quartz vein were studied from a mineralogical and isotopic standpoint in order to establish their temperatures of formation and thus complete the geothermal history of the system for this purpose 18 o values were determined for pure mineral from the unaltered granite and quartz from the uranium quartz vein as well as for mixture samples from the hydrothermally altered granite sericitised granite and clayey samples from fracture fillings including the clayey walls of the uranium quartz vein the isotopic signature of quartz from the uranium quartz vein and the monophasic nature of its fluid inclusions led us to conclude that the isotopic signature of water in equilibrium with quartz was approximately in the range from 8 3 to 5 7 v smov its temperature of formation being around 85 120 c the 18 o values of pure sericite from the hydrothermally altered granite calculated by means of the oxygen fraction molar method indicate that its temperature of formation in equilibrium with the aforementioned waters is also in the range from 70 c to approximately 120 c clays from fracture fillings and clayey walls of the uranium quartz vein are usually mixtures in different proportions of illite approximately formed between 70 and 125 c two generations of kaolinite formed at approximately 90 130 c and at around 25 c respectively smectite formed at 25 c and occasionally palygorskite formed either between 30 and 45 c or 19 and 32 c depending on the fractionation equation used these data suggest that sericite from the hydrothermally altered granite quartz from the uranium quartz vein illite and the first generation of kaolinite from the fracture fillings resulted from the same hydrothermal process affecting the el berrocal granite in relation to fracturing under certain physicochemical conditions t 100 c ph 8 and log h 4 sio 4 between 4 and 3 illite and kaolinite can be paragenetic as a result of weathering processes smectite was formed from hydrothermal illite and inherited albite under alkaline weathering while the second generation of kaolinite was formed from smectite under acid conditions and close to the sulphide rich uranium quartz vein palygorskite is an occasional mineral formed probably either during the thermal tail of the above described hydrothermal process or during weathering processes in both cases palygorskite must have formed from alkaline si mg rich solutions finally these data and processes are discussed in terms of natural analogue processes drawing some implications for the performance assessment of a deep geological radwaste repository dgrr

controls on the genesis of some high fluoride groundwaters in india india has an increasing incidence of fluorosis dental and skeletal with some 62 million people at risk high fluoride groundwaters are present especially in the hard rock areas south of the ganges valley and in the arid north western part of the country the phenomenon is related to groundwater with residual alkalinity ca2 hco3 fluoride concentrations are governed by adsorption equilibria and by fluorite solubility evapotranspiration leads to a precipitation of calcite a lowering of ca activity and increase in na ca ratios and this allows an increase in f levels in southern india mg seems to be controlled by dolomite while sepiolite and palygorskite are mg sinks in rajasthan but may then release f under alkaline conditions the latter two minerals are probably also important sources and sinks for f in the hydroxy positions the increase in the extent of sodic soils as a result of irrigation is a contributing factor to the increasing incidence of fluorosis remedial measures including addition of gypsum and rainwater harvesting are needed even in areas where the sodicity does not cause structural problems in the soil

geology of the skorpion supergene zinc deposit southern namibia the skorpion zinc deposit in southwestern namibia comprises a significant nonsulfide orebody 24 6 mt at 10 6 zn and subordinate amounts of primary base metal sulfide mineralization which structurally underlies the nonsulfide ores at depth the host rocks of the blind nonsulfide orebody and relict sulfide mineralization consist of a mixed volcano sedimentary sequence of neoproterozoic age that has formed in an initial continental rift system between the kalahari cratonic province and the rio de la plata cratonic province the metavolcanic and subvolcanic rocks display a distinct bimodal geochemical composition of within plate affinity and have been emplaced under an extensional crustal regime zircons from a rhyolitic vesicular flow extruded approximately contemporaneously with the volcanic hosted sulfide mineralization have been dated by shrimp u pb analysis and yield an age of 751 9 5 5 ma the basin that hosts the skorpion deposit has been shaped by horst and half graben tectonics which also controlled the sedimentary deposition of siliciclastic and carbonate sediments within both shallow and deeper water environments the rock sequence was affected by a major tectono metamorphic event the pan african brasiliano orogeny at approximately 550 to 545 ma metamorphism reached uppermost greenschist to lowermost amphibolite facies and the complex deformational style of the region is the result of intense faulting folding and thrusting the skorpion orebody is hosted by arkosic metaarenites and subordinately by volcaniclastic metasedimentary rocks it is irregularly shaped transgressive to sedimentary layering and has a relatively flat top which is covered by a blanket of unmineralized overburden consisting of calcrete boulder beds and recent sand dunes at depth the orebody has a relatively sharp contact with a barren marble unit on the east and south and a more gradational boundary within the siliciclastic host rocks on the west and north the nonsulfide ore minerals comprise predominantly sauconite zn smectite substantial amounts of hemimorphite and smithsonite and subordinate amounts of scholzite tarbuttite atacamite hydrozincite and hetearolite the ore minerals are mainly euhedral and subhedral crystals and occur as open space fillings in intergranular voids fractures and breccias and as replacements of feldspar and mica the metasedimentary host rocks of the nonsulfide ore contain abundant iron hydroxide pseudomorphs after disseminated sulfides and stringers and after rare semimassive stratiform layers of sulfides gossanous manganese and iron hydroxide rich siliciclastic metasedimentary rocks are common in the western and northwestern vicinity of the nonsulfide orebody alteration of the supergene mineralized host rocks includes removal of calcite cement and the breakdown of detrital albite rich plagioclase and of minor orthoclase local alteration phenomena also include silicification and rare secondary barite both a result of the alteration and breakdown of feldspar minor base metal sulfide mineral deposits occur in metarhyolitic flows and felsic hyaloclastic rocks structurally below the nonsulfide orebody the primary sulfide minerals comprise pyrite pyrrhotite sphalerite minor chalcopyrite and galena as well as secondary chalcocite and colloidal sphalerite brunckite the volcanic hosted sulfide ore minerals are deformed and partly recrystallized and thus of premetamorphic and predeformation origin in contrast the skorpion nonsulfide zinc deposit is of postmetamorphic origin as shown by two hydrous low p low t ore assemblages the secondary assemblages occur at least partly as delicate euhedral and subhedral crystals in secondary pore space open fractures veins and breccias the nonsulfide ore contains pseudomorphs from in situ oxidation and replacement of preexisting sulfides and ore and also from the remobilization or precipitation of base metals the skorpion nonsulfide zinc deposit is of supergene origin and formed from the oxidation of sulfide protore by circulating meteoric water as suggested by textural evidence and by initial stable isotope data the timing of ore formation is still undefined but lies within the large age bracket between 545 ma peak metamorphism and subsequent partial exhumation and the deposition of the nonmineralized overburden of namib desert gravels 21 ma and dune sands

timing and sources of pre collisional neoproterozoic sedimentation along the sw margin of the congo craton kaoko belt nw namibia abstract isotopic dating of detrital zircon populations from metasediments and of magmatic zircon from intercalated metavolcanic layers in the medium to high grade part of the kaoko belt in namibia provides first robust constraints on the age of neoproterozoic sedimentation along the southwestern margin of the congo craton dating of detrital zircons from metasediments directly overlying the cratonic basement shows maximum sedimentary ages of c 1 00 and c 1 45 ga and age populations comparable with known protolith ages from the gneisses of the congo kalahari cratons dating of zircon from associated metavolcanics constrains the age of the earliest preserved sediments at c 740 710 ma detrital zircon populations from the samples collected from the upper parts of the metasedimentary succession contain only small proportion of grains with ages similar to those from the congo craton these samples show dominance of c 1 00 ga c 750 ma and c 650 ma old zircon grains that are probably derived from the punta del este coastal terrane dom feliciano and kaoko belts that acted as an arc back arc domain at c 650 630 ma neodymium model ages for the samples of metasedimentary cover of the cratonic basement provide another evidence that the youngest preserved sediments could not have been derived from the congo craton recognition of the punta del este coastal terrane crust as a source region for the youngest pre collisional sediments of the kaoko belt suggests that this terrane must have been in close proximity to the congo craton passive margin already some time prior to their mutual collision at c 570 550 ma this is in accord with an interpretation that the c 650 630 ma arc back arc punta del este coastal terrane has developed directly on top of or very close to the attenuated congo craton passive margin

neoproterozoic igneous complex emplaced along major tectonic boundary in the kaoko belt nw namibia ion probe and la icp ms dating of magmatic and metamorphic zircons granitoid intrusions of the boundary igneous complex separate segments with different ages of high grade metamorphism in the kaoko belt nw namibia two granitoids of this complex were dated at 575 10 ma secondary ionization mass spectrometry sims or 571 9 ma laser ablation inductively coupled plasma mass spectrometry la icp ms and 562 11 ma sims or 572 4 ma la icp ms respectively the age of granulite facies metamorphism in the eastern part of the western kaoko zone was established at 549 5 ma sims by analysing metamorphic overgrowths of older c 1850 1000 ma zircons from melt segregations in amphibolites the coastal part of the western kaoko zone consists of horizons of migmatitic metasedimentary rocks that are intercalated with fine grained orthogneisses and amphibolites resembling metamorphosed sequences of bimodal volcanic rocks zircons from felsic members of two bimodal suites have sims ages of 805 4 ma and 810 840 ma respectively that are interpreted as dating their respective igneous protoliths melt segregations in the mafic member of the lower bimodal suite contain two populations of zircon dated at 650 5 ma sims or 645 5 ma la icp ms and 629 6 ma sims or 630 5 ma la icp ms respectively the later age is indistinguishable from the age of 630 4 ma sims or 625 10 ma la icp ms obtained from melt patches present in overlying metagreywackes the available age data suggest that the boundary igneous complex masks the suture between the coastal terrane and the rest of the kaoko belt ages of granitoid intrusions in this igneous complex are indicative of magmatic activity between 580 and 550 ma

role of crustal contribution in the early stage of the damara orogen namibia new constraints from combined u pb and lu hf isotopes from the goas magmatic complex abstract the tholeiitic to calc alkaline goas complex represents the earliest magmatic activity of the inland branch of the damara orogen and reflects the pan african convergence between the congo and kalahari cratons new results from laser ablation inductively coupled plasma mass spectrometry la icp ms zircon u pb geochronology coupled with single zircon lu hf isotopic data constrain the age of emplacement and allow the identification of a crustal contribution to the magma sources no temporal gap is registered between the first tholeiitic intrusion at ca 575 ma and the main diorite to granite episodes invariably sub chondritic ehf t values 3 8 to 34 4 attest to a significant crustal residence time with long term reworking of multiple and mixed crustal components no significant juvenile magma was involved in the magmatogenesis hf model ages coupled with an exhaustive dataset of u pb geochronological data suggest that the crustal components which fed the goas sources might have formed during specific orogenic events with a major contribution of material derived from the central western african paleoproterozoic eburnean orogen geochemistry u pb geochronology and hf isotope systematics however do not unequivocally clarify the long debate regarding whether or not the pan african orogenic cycle included ocean closure and subduction as the involvement of crustal melting processes during the early stage of the damaran event seems to have played a major role

hf isotopic compositions of the standard zircons and baddeleyites used in u pb geochronology abstract zircon and baddeleyite u pb geochronological dating is widely used in solid earth sciences and the advent of rapid in situ methods of analysis such sims and icp ms has re emphasized the importance of having uniform standards recently it has been shown that hf isotopic data can provide important information on these minerals since they contain high concentrations of hf but have low lu hf ratios which results in negligible age correction however the complex internal structures that result from multiple thermal events such as inherited cores and metamorphic overgrowths require that the hf isotopic data be measured with high spatial resolution however the isobaric interferences of 176 yb and 176 lu on 176 hf hamper the precise determination of the 176 hf 177 hf ratio during in situ laser ablation mc icpms analysis it is shown here that mass biases of yb yb and hf hf change with time during analyses and behave differently for solutions and solid material therefore it is suggested that the mean yb value of the individual spot be used to obtain the precise isotopic composition for in situ zircon and baddeleyite hf isotopic analyses for low yb hf 176 yb 177 hf yb value have little effect on the accuracy of the hf isotopic composition however the appropriate yb isotopic abundance is necessary for high yb hf 176 yb 177 hf 0 001 zircons and baddeleyites since the interference of 176 yb on 176 hf is significant using the mean yb value of the individual spot and newly published yb isotopic abundance data six standard zircons and two standard baddeleyites have been investigated using a neptune mc icpms with 193 nm laser for zircons the obtained 176 hf 177 hf ratios are 0 282307 31 2sd for 91500 0 282680 31 2sd for temora 0 281729 21 2sd for cz3 282177 17 2sd for cn92 1 0 282983 17 2sd for fm0411 and 281234 11 2sd for phalaborwa the baddeleyites from phalaborwa and sk10 2 have 176 hf 177 hf ratios of 0 281238 12 and 0 282738 13 2sd these results agree well with the values obtained by the solution method and indicate that these standards have different hf isotopic compositions in which the extremely low 176 lu 177 hf and 176 yb 177 hf values of cz3 zircon and phalaborwa baddeleyite make them excellent standards for machine calibration during in situ zircon hf isotopic measurement with the other standards being more suitable for the development of the correction method

the influence of authigenic clay formation on the mineralogy and stable isotopic record of lacustrine carbonates abstract the mineralogical compositional and stable isotopic variability of lacustrine carbonates are frequently used as proxies for ancient paleoenvironmental change in continental settings under the assumption that precipitated carbonates reflect conditions and chemistry of ancient lake waters in some saline and alkaline lake systems however authigenic clay minerals forming at or near the sediment water interface are a major sedimentary component often these clays are rich in mg influencing the geochemical budget of lake waters and are therefore expected to influence the properties of contemporaneous authigenic carbonate precipitates which may also contain mg this paper documents evidence for a systematic feedback between clay mineral and carbonate authigenesis through multiple precessionally driven m scale sedimentary cycles in lacustrine oil shale deposits of the eocene green river formation from the uinta basin ne utah in the studied section authigenic mg rich trioctahedral smectite content varies cyclically between 9 and 39 wt the highest concentrations occur in oil shales and calcareous mudstones deposited during high lake level intervals that favored sedimentary condensation lengthening the time available for clay diagenesis and reducing dilution by other siliciclastic phases an inverse relation between dolomite percentage of carbonate and trioctahedral smectite abundance suggests the mg uptake during clay authigenesis provides a first order control on carbonate mineralogy that better explains carbonate mineralogical trends than the possible alternative controls of 1 variable mg ca ratios in lake water and 2 degree of microbial activity in sediments we also observe that cyclical change in carbonate mineralogy believed to be induced by clay authigenesis also causes isotopic covariation between 13cpdb and 18opdb of bulk sediments because of differences in the equilibrium fractionation factors of dolomite and calcite 2 and 2 6 respectively this provides an alternative mechanism for the common pattern of isotopic covariation which is typically attributed to the effect of simultaneous changes in water balance and biological activity on the carbon and oxygen isotopic composition of lake waters these findings may help improve paleoenvironmental reconstructions based on lacustrine carbonate records by adding to the factors known to influence the mineralogical compositional and stable isotopic signals recorded by lacustrine carbonates

dating recurrent shear zone activity and the transition from ductile to brittle deformation white mica geochronology applied to the neoproterozoic dom feliciano belt in south brazil abstract argon geochronology of white mica has a good potential of recording recurrent dynamic recrystallization in shear zones in the transition from ductile to brittle deformation as temperature ranges for mineral crystallization and isotopic closure of different grain size fractions overlap between ca 275 425 c this study presents over 40 k ar ages of coarse grained muscovite and multiple clay sized white mica fractions extracted from mylonites and fault gouges in southernmost brazil complemented by detailed structural and xrd characterization the widespread obtained ages are best explained in the context of recurrent deformation under retrograde conditions during progressive strain localization and transition from low grade ductile deformation into brittle faulting the onset of the ibare shear zone is constrained in the tonian followed by reactivation during the ediacaran evolution of the adjacent dom feliciano belt which records a remarkable long term history of recurrent deformation for over 100 myr new data extends the geochronological constraints of the dorsal do cangucu shear zone into low temperature conditions estimating the onset of brittle activity at 525 470 ma by dating early fault gouges formed shortly after final syn kinematic white mica crystallization in the hosting mylonites dating of recurrent faulting events during the paleo mesozoic complement the regional sedimentary and thermochronological record

a connection between the neoproterozoic dom feliciano brazil uruguay and gariep namibia south africa orogenic belts evidence from a reconnaissance provenance study abstract a provenance study of neoproterozoic siliciclastic successions in the stratigraphically and tectonically lowermost and uppermost parts of the pan african gariep belt stinkfontein subgroup and oranjemund group respectively in southwestern africa as well as in the rocha group of the punta del este terrane dom feliciano belt in uruguay revealed that the oranjemund and rocha groups can be correlated and most likely formed in the same basin thus the rocha group is considered to represent the fill of the westernmost part of a re activated ediacaran gariep basin the lower parts of the oranjemund and rocha groups reflect erosion of mafic rocks whereas the upper parts are derived from a predominantly felsic source area first major and trace element geochemical data are consistent with oceanic islands of within plate geochemistry in the immediate vicinity as the most likely source of the mafic input into the lower part of the oranjemund group whereas a combination of continental margin and island arc is indicated as source for the bulk of the younger sediments in the upper oranjemund group age spectra obtained by shrimp u pb analyses of detrital zircon grains from the stinkfontein subgroup port nolloth group the oranjemund group and the rocha group are very similar except for a lack of the youngest age group around 600 ma in the stinkfontein subgroup in all three units zircon grains of 1000 1200 ma dominate with a further peak in the age distribution between 1700 and 2000 ma these ages compare well with the pre gariep basement geology in southwestern africa where the former age range corresponds to magmatic and high grade metamorphic activity in the mesoproterozoic namaqua natal belt and the latter to an extensive palaeoproterozoic andean type volcanic arc richtersveld terrane comparable ages are conspicuously absent in the basement of the rio de la plata craton in south america derivation of the rocha group sediments from a similar source as the contemporaneous oranjemund group sediments is therefore suggested the most likely source of the youngest detrital zircon grains in these two stratigraphic units is the 640 590 ma magmatic arc of the dom feliciano belt this interpretation explains the continental island arc component in the sediment provenance and takes into account the absence of comparable magmatism in the gariep belt two stages of basin evolution are distinguished for the gariep basin an early failed continental rift that subsequently became re activated as back arc basin related to the cryogenian dom feliciano arc consequently the gariep basin does not represent the main suture between the rio de la plata and the kalahari cratons this suture is now suspected to be west of the dom feliciano volcanic arc and might be connected with the sierra ballena major gercino lineament