th u disequilibrium dating of middle and late pleistocene wood and shells from banks and victoria islands arctic canada an attempt has been made at dating middle and late pleistocene deposits from banks and victoria islands using the th u disequilibrium method geological correlations are difficult to establish because of the discontinuous nature of the terrestrial units and the relative absence of datable fossil wood and shells the th u geochemical system likely remained closed for extended periods because of the presence of permafrost which implies very limited or absent water circulation the th u age determinations obtained confirmed the previously established stratigraphic framework and provide chronological information in that samples of sangamonian age are differentiated from those that are younger wisconsinan and holocene or older middle pleistocene the th u disequilibrium method carefully used in areas where water circulation is limited can thus provide chronological control for sediments older than those that can be dated by the radiocarbon method

paleoclimate and location of the border between mediterranean climate region and the saharo arabian desert as revealed by speleothems from the northern negev desert israel speleothem bearing karstic caves of the northern negev desert southern israel provides an ideal site for reconstructing the paleoclimate and paleo location of the border between mediterranean climate region and the saharo arabian desert major periods of speleothem deposition representing humid periods were determined by high resolution 230 th u dating and corresponding studies of stableisotopecompositionwereusedtoidentifythesourceofrainfallduringhumidperiodsandthevegetationtype majorhumidintervals

multiple 3 8 3 1 ga tectono magmatic events in a newly discovered area of ancient rocks the shengousi complex anshan north china craton abstract near anshan city in the north china craton the oldest rocks in asia 3300 ma have been thoroughly documented at two localities baijiafen and dongshan in this paper we report the full geological context for more ancient rocks from a third anshan locality the polyphase migmatite shengousi complex shrimp u pb zircon dating indicates a protracted tectono magmatic history for the shengousi complex the oldest recognised component is banded trondhjemitic gneiss 3773 6 ma which is veined by strongly deformed granitic pegmatite these occur with a second generation of trondhjemitic rocks 3454 8 and 3448 9 ma the next generation of plutonic rocks is a composite suite of iron enriched mafic dykes 3332 6 and 3331 8 ma with broadly coeval felsic veins 3311 4 ma finally there was intrusion of monzogranite 3129 6 ma strong deformation has generally brought these 3773 3129 ma plutonic phases into close concordance to form banded rocks however locally cross cutting relationships are preserved in small lower strain domains that give a relative chronology agreeing with the absolute zircon u pb chronology all of this complex history is recorded in a single some of the the 3450 ma igneous phases discovered in the shengousi complex are new ages for igneous rocks in the anshan area the precambrian geology of the anshan area is thus marked by polyphase eoarchaean orthogneisses the newly recognised 3450 ma phases widespread 3360 3300 magmatic activity found in association with mesoarchaean plutonic rocks and the neoarchaean anshan group and palaeoproterozoic liaohe group metasedimentary rocks this shows that the geology of the anshan area has a strong similarity with the narryer gneiss complex of western australia which contains the polyphase eoarchaean meeberrie gneisses the 3490 3440 ma eurada gneiss association 3300 ma granites and migmatisation and is intercalated with younger precambrian metasedimentary rocks of different ages containing hadean detrital zircons no other ancient gneiss complexes in the world show such a close match in their history with the anshan area rocks this means that the anshan area is prospective for locating new occurrences of hadean crustal components intercalated with younger sedimentary rocks

neoproterozoic granitoid in northwest sulu and its bearing on the north china south china blocks boundary in east china 1 middle neoproterozoic granitoid rocks are found in the wulian region that is located in the northwestern part of the sulu orogen and is approximately bounded by the wulian yantai fault in southeast zircon u pb dating yields concordant ages of 738 10 to 758 5 ma for three granites and one gabbro hornblende ar ar dating on the gabbro gives consistent plateau and isochron ages of 719 1 3 7 and 717 9 7 2 ma respectively this suggests that the wulian granitoids correspond to bimodal magmatism during the mid neoproterozoic in the northern margin of the south china block in response to the breakup of rodinia supercontinent they were tectonic slices scraped off from the upper part of the south china block during the triassic continental subduction this demonstrates that the suture location between the north china block and the south china block lies north of the wulian and penglai groups rather than along the wulian yantai fault that marks the northern margin of ultrahigh pressure metamorphic belt

petrochemistry and sr pb and nd isotopic geochemistry of the paleoproterozoic kuandian complex the eastern liaoning province china abstract a geochemical and isotopic study has been carried out on the paleoproterozoic kuandian complex and some associated granitoids in the eastern liaoning province china the kuandian amphibolite and granite give an imprecise 1 9 0 4 ga all errors are 2 rb sr errorchron with 87 sr 86 sr o 0 711 0 006 a 2 11 0 06 ga pb pb isochron with a first stage growth 8 43 and a 2 31 0 10 ga smnd isochron with 143 nd 144 nd o 0 50970 0 00009 or nd t 1 1 nd depleted mantle model dates tdm of amphibolite are 2 75 to 2 46 ga those of granite are 2 53 to 2 36 ga zircons from the kuandian granite give a minimum 2 14 0 05 ga upper intercept date the kuandian amphibolite is chemically similar to the high mg member of columbia river basalts and the kuandian granite is an anorogenic granite they are genetically related and formed 2 4 to 2 3 ga ago fractional crystallization of olivine pyroxene and plagioclase from precursor magma of the kuandian amphibolite can produce magma with composition of the kuandian granite the paleoproterozoic mantle source in the eastern liaoning province is less depleted than that inferred from the average mantle evolution curve in contrast with the archean mantle in the sino korean craton which is more depleted than the average this study also reveals that the lishan granite of the eastern liaoning province is older than 3 ga and that some granites and dykes previously mapped as proterozoic are of mesozoic age

the relationship between current and paleo groundwater base levels abstract base levels are basically divided into two categories oceanic and marine base levels and continental terminal endorheic ones the marine domain serves as the ultimate base level for both surface and groundwater drainage systems continental inland depressions very often serve as terminal endorheic base levels to both surface drainage and groundwater systems many of the current both marine and inland base levels served also in the past as paleo base levels fluctuations of base level elevations affect the adjoined groundwater system in several ways such as changes in groundwater levels location of divides depths of fresh saline water interfaces and the amount of flow to the base levels this paper provides examples from many different parts of the world that describe the relationship between current and paleo base levels the examples include marine base levels as well as inland endorheic ones both currently above and below sea level close to and distant from the sea

palaeoclimate in the saharan and arabian deserts during the middle palaeolithic and the potential for hominin dispersals to disperse out of sub saharan africa it was necessary for hominins to cross the deserts of either the sahara and or arabia thus understanding the palaeoclimate of the saharo arabian region is central to determining the role these deserts played in the peopling of the planet when did they act as barriers and when were they more humid opening dispersal routes across them to address these questions we have conducted a temporal and spatial evaluation of dated sites from 20 to 350 ka using combined probability density function pdf and geographical information system gis analyses of all sites dated using uranium thorium u th or optically stimulated luminescence osl methods radiocarbon dates were not considered because of contamination problems in this time range the results show that during mis 2 there is little evidence for humidity in arabia as would be expected during the height of the last glacial maximum however the sahara shows a sharp rise in probability at the beginning of mis 2 peaking near the boundary with mis 3 at 29 ka there appear to be brief periods of humidity in mis 3 and 6 though at different times in the sahara ca 37 44 138 154 and 180 ka and arabia ca 40 54 and 163 ka during mis 5 both regions show much evidence for humidity with pdf peaks corresponding to insolation maxima though not all maxima are represented in either the saharan or arabian record this situation can be explained by eccentricity modulated precession when eccentricity is strong insolation is enhanced but also more variable and the desert climate is generally more humid particularly at times of high insolation the opposite happens when eccentricity is low and deserts tend to be more arid but local factors exert more of an influence on climate affecting the timing and strength of the brief humid periods experienced so that they no longer coincide with insolation maxima the spatial distribution of humid sites is compatible with a number of different modern human dispersal theories southern arabia experienced humid periods centred on 54 ka and 125 ka and this could have facilitated dispersal from east africa to southern arabia and beyond via the bab el mandab the sahara shows considerable evidence for humidity during mis 5 and may have had dispersal across its expanse at this time

mineralogy and sr nd isotopes of spm and sediment from the mandovi and zuari estuaries influence of weathering and anthropogenic contribution clay minerals and sr nd isotopes of suspended particulate matter spm and bottom sediment were investigated along transect stations of the mandovi and zuari estuaries western india to determine the provenance and role of estuarine processes on their distribution kaolinite and illite followed by minor goethite gibbsite and chlorite were present in spm and bottom sediment at all stations both during monsoon and pre monsoon smectite occurred in traces at river end stations but its contents increased downstream in both estuaries smectite contents were much higher in zuari than in mandovi estuary the 87 sr 86 sr ratios and nd of spm were higher than those in hinterland rocks and laterite soils the sr ratios were highest at river end stations of both estuaries and decreased sharply seaward the sm nd ratios of spm and sediment were close to that of iron ore material flushed into the estuaries the mean nd of spm and sediment were similar in both estuaries it is suggested that the smectite is formed in coastal plains and its distribution downstream is controlled by lithology and drainage basin of rivers abundant kaolinite and high sr ratios reflect chemical weathering and lateritization of source rocks sr isotopic ratios along transects are influenced by changes in salinity organic matter and turbidity high and near identical nd values along transect stations of both estuaries suggest that the nd isotopic compositions are influenced by the lateritization of source rocks and anthropogenic contribution of ore material

two high levels of continental waters in the southern tunisian chotts at about 90 and 150 ka uranium and thorium content and activity ratios were measured on 20 samples 19 molluscs and 1 calcareous concretion from the great chotts in southern tunisia results were studied by the isochron method and by age frequency histograms they suggest that two major flood episodes took place about 150 and 90 ka uranium activity ratios indicate a ground water supply of continental origin in agreement with biological indicators which show large variations in salinity from fresh to marine like conditions the existence of a lacustrine phase radiocarbon dated to 17 40 ka as previously suggested for the northern sahara is highly questionable

late archean sm nd isochron age for mafic ultramafic supracrustal amphibolites from the northeastern sino korean craton china isotopic data published by previous workers document the presence of early archean 3 5 ga metabasaltic enclaves in grey gneisses west of bohai bay eastern hebei province p r c metabasaltic and metakomatiitic amphibolites crop out as lower supracrustal members of the archean section northeast of bohai bay in southern jilin eastern liaoning provinces six analyzed specimens of the latter unit contain more magnesium on analysis averaging approximately 13 vs 7 wt percent mgo but although they exhibit a wider range of ree concentrations they average a similar range of ree abundance patterns la lu 24 7x chondrite as the former 24 9 chondrite new sm nd isotopic and ree analyses combined with available petrologic geologic information indicate that metakomatiitic amphibolites from the sino korean craton northeast of bohai bay are late archean in emplacement age a wholerock sm nd isochron age of 2660 75 2 ma was obtained with nd t 4 4 0 5 as in many parts of the sino korean craton the highly positive nd t value suggests a prior extended history 1 0 ga of depletion of subcontinental mantle sources in some incompatible elements and in lree relative to hree a seventh analyzed sample a mafic inclusion in tonalitic gneiss of unknown age has an unusual chemical composition with strong enrichment in lree sr and zr and severe depletion in ni and cr judging from isotopic data which yield a much younger depleted mantle model age of 1 67 ga it could represent a cognate xenolith in a mid proterozoic tonalitic intrusion shown on present maps as archean in age the protolith might have been an evolved alkali basalt

the palaeoproterozoic boron deposits in eastern liaoning china a metamorphosed evaporite abstract we have carried out a field study of the palaeoproterozoic borate deposits of eastern liaoning province china the borates are hosted within mg silicate and mg carbonate rocks and have previously been interpreted as either skarns or metamorphosed volcanogenic exhalative deposits the deposits contain textures that are compatible with a sedimentary origin of the borates and there is a lack of the associated mineralization that is characteristic of magnesian skarns in addition preliminary boron isotope data indicate that the boron within the borates was not derived from the footwall rocks that have been proposed as the source of the skarn fluids a purely exhalative model can be dismissed on the basis of geochemical arguments and comparison with modern environments which show that borates are too soluble to precipitate directly from hydrothermal fluids the geology and field relationships show a number of similarities with recent non marine evaporite settings and young continental rifts in arid environments we conclude therefore that the liaoning deposits are metamorphosed evaporites in which the boron was originally present as primary evaporite borate minerals this model places the liaoning borates in the same category as the majority of other economic borate deposits in the world

a middle late pleistocene palustrine lacustrine travertine system borj edouane unit nw tunisia sedimentology stable isotopes and palaeohydrological implications the carbonate borj edouane unit represents the northernmost documented middle late pleistocene continental carbonate deposits in tunisia the unit consists of two main facies associations asymmetrically distributed 1 calcrete palustrine laminar crust 2 microbialites the first accumulated carbonate formed in the distal zone of alluvial fans calcretes and in shallow lakes subjected to periodic exposure palustrine facies that occupied the westernmost strip of the basin the second association formed in permanent lakes stromatolites oncoids and planar stromatolites located in the central areas of the basin fed by springs associated with a concentration of vertical faults in the eastern area of the basin travertine facies at time of deposition these facies communicated with the lake via a transitional lacustrine travertine environment transitional travertine stromatolite facies all facies show a restricted range of 18o values around 5 8 0 5 suggesting formation from paleoprecipitation largely of atlantic derivation brought about by westerly winds as for the 13c values these show a wide range 6 4 to 11 7 overall relatively high values for all facies with the lower values corresponding to the calcrete palustrine association and the higher ones linked to the microbialite facies the trend in travertines probably mimics an increasing influence of carbon derived from dissolution of carbonate bedrock of the cretaceous substrate while that of palustrine samples records increasing respired c contribution the sedimentological and isotopic characteristics of the borj edouane unit reflect wetter conditions at the time of deposition and point to a humid period at the middle late pleistocene transition

deformation history of the paleoproterozoic liaohe assemblage in the eastern block of the north china craton the paleoproterozoic liaohe assemblage and associated liaoji granitoids represent the youngest basement in the eastern block of the north china craton various structural elements and metamorphic reaction relations indicate that the liaohe assemblage has experienced three distinct deformational events d1 to d3 and four episodes of metamorphism m1 to m4 the earliest greenschist facies event m1 is recognized in undeformed or weakly deformed domains wrapped by the s1 schistosity suggesting that m1 occurred before d1 the d1 deformation produced small mostly meter scale isoclinal and recumbent folds f1 an associated penetrative axial planar schistosity s1 a mineral stretching lineation l1 and regional scale ductile shear zones concurrent with d1 was m2 metamorphism which occurred before d2 and produced low to medium pressure amphibolite facies assemblages regionally divergent motion senses reflected by the asymmetric f1 folds and other sense of shear indicators together with the radial distribution of the l1 lineation surrounding the liaoji granitoids imply that d1 represents an extensional event the d2 deformation produced open to tight f2 folds of varying scales s2 axial crenulation cleavages and ene ne striking thrust faults involving broadly nw se compression following d2 was m3 metamorphism that led to the formation of sillimanite and cordierite in low pressure type rocks and kyanite in medium pressure rocks the last deformational event d3 formed nw wnw trending folds f3 axial planar kink bands spaced cleavages s3 and strike slip and thrust faults which deflect the earlier d1 and d2 structures d3 occurred at a shallow crustal level and was associated with or followed by a greenschist facies retrograde metamorphic event m4 the liaohe assemblage and associated liaoji granitoids are considered to have formed in a paleoproterozoic rift the late spreading of which led to the occurrence of the early extensional deformation d1 and the m1 and m2 metamorphism and the final closing of which was associated with the d2 and d3 phases of deformation and m3 and m4 metamorphism

u series measurements in tyrrhenian deposits from mallorca further evidence for two last interglacial high sea levels in the balearic islands the campo de tiro type section for the tyrrhenian ecostratigraphic beds of the balearic islands shows four indurated littoral conglomerates and beach rocks unconformably superimposed the lower two units 1 and 2 are separated by a thin layer of reddish continental silts both contain a typical eutyrrhenian fauna whereas a neotyrrhenian fauna characterizes the overlying units 3 and 4 thu measurements by thermal ionisation mass spectrometry on mollusk shells from these deposits yielded ages of 135 ka unit 1 117 ka units 2 and 3 and a scatter of ages around 100 ka unit 4 the stratigraphic relationships and thu data indicate i that uranium was uptaken by mollusks shells but arca sp each time during a relatively short early diagenetic interval before cementation ensured a fair closure of the radioactive system ii two high sea stands characterized the last interglacial isotopic substage 5e of the balearic islands area iii the duration of this episode was 17 ka and iv the change in faunal assemblages and disconformity observed between units 2 and 3 are due to fluctuations in sea level and surface water conditions which occurred during the second high sea level episode of the last interglacial

mega lake in the kalahari a late pleistocene record of the palaeolake makgadikgadi system abstract a distinct series of beach ridges marking the former shorelines of large inter connected lacustrine basins in the kalahari can be clearly identified from landsat imagery and shuttle radar topography mission srtm data these basins which form the terminal sump of the okavango system in northern botswana are now almost completely dry during the quaternary they were intermittently occupied by large stable lake bodies and are thought to have periodically filled to a point of coalescence inundating an area that at its largest extent encompassed 66 000 km 2 poor chronological control has previously limited the utility of this important palaeo archive as part of a region wide lake palaeo shoreline research programme a systematic optically stimulated luminescence osl dating programme has utilised a lightweight hydraulic auger to take samples at depth from relict shoreline features twenty drill sites have generated 140 samples for dating establishing a firm chronology for multiple lake full phases in all three component basins ngami mababe and makgadikgadi of this mega lake this paper presents the final set of ages in the programme derived from four cores from the western and north eastern shorelines of makgadikgadi and uses these ages to establish a chronology of mega lake high stands during the last 300 ka providing a rare directly dated long terrestrial record of positive hydrological excursions within the southern hemisphere

nd isotopic constraints on crustal formation in the north china craton abstract recent tectonic analysis suggests that the north china craton consists of two archean continental blocks called the eastern and western blocks separated by the paleoproterozoic trans north china orogen although the published geochronological data are not sufficient to constrain the detailed tectonothermal evolution of the craton the available nd isotopic data show some important differences in nd model ages between the tectonic units the eastern block shows two main nd model age peaks one between 3 6 and 3 2 ga and the other between 3 0 and 2 6 ga limited nd isotopic data from the western block show a large range of model ages between 3 2 and 2 4 ga these differences are consistent with the recently proposed model the nd isotopic data from mantle derived mafic rocks indicate that the mantle beneath the north china craton was depleted in the archean consistent with major crustal growth during this period in the paleoproterozoic however the mantle derived mafic rocks show negative end t values implying crustal contamination this may have resulted from subduction and collision between the eastern and western block implying that the mechanisms of crustal formation and evolution may have been different between the archean and paleoproterozoic the north china craton was re activated by addition of mantle derived magma into the lower crust in the late mesozoic resulting in rejuvenation of the lower crust this indicates that underplating is also an important mechanism for continental addition although in this case it may not equate to crustal growth since it was preceded by removal of lithospheric mantle and possible some lower crust

u series isochron dating a generalized method employing total sample dissolution the age of a sedimentary deposit can be determined by measuring 238u 234u 232th and 230th in two or more samples of the deposit with the requirement that these samples have different u th ratios but an identical 230th232th ratio when deposited this paper presents the theoretical and experimental aspects of the dating method the latter involves techniques for securing samples of varied u th ratios from the same deposit plotting isochrons of the radiochemical data obtained on totally dissolved samples for age analysis and estimating the errors involved although applications to impure carbonates and lacustrine evaporite sediments are given as examples in this paper the method potentially can be generalized for dating a variety of authigenic deposits such as rock varnish peat opal sulfate minerals and marine phosphorites

uranium series dated authigenic carbonates and acheulian sites in southern egypt field investigations in southern egypt have yielded acheulian artifacts in situ in authigenic carbonate deposits caco 3 cemented alluvium along the edges of nowaggraded paleovalleys wadi arid and wadi safsaf uranium series dating of 25 carbonate samples from various localities as far apart as 70 kilometers indicates that widespread carbonate deposition occurred about 45 141 and 212 ka thousand years ago most of the carbonate appears to have been precipitated from groundwater which suggests that these three episodes of deposition may be related to late pleistocene humid climates that facilitated human settlement in this now hyperarid region carbonate cements from sediments containing acheulian artifacts provide a minimum age of 212 ka for early occupation of the paleovalleys

palhydaf palaeohydrology in africa program objectives methods major results abstract the palhydaf program aims to reconstruct the late quaternary 130 ka b p to present environmental and hydrological changes of northwest africa and to interpret them in terms of palaeoclimate for testing coupled atmospheric ocean circulation models lacustrine sequences were obtained from closed basins which lie along two meridional transects southern tunisia southern niger and algeria mali particular attention is given to groundwater storage and periods of recharge of the aquifers the chronological framework is based on 14c dates conventional and ams methods and or the th u disequilibrium method in favourable cases e g mali correlations can be made between the chronology of surface palaeohydrology and that of aquifer recharge various environmental markers mineralogy elemental chemistry stable isotopes biological remains are used for palaeoenvironmental purposes quantitative estimates of palaeoclimatic variables e g air humidity may be attempted through the combination of geochemical and palaeobiological data the major results already obtained are summarized for the following individual sectors 1 southern tunisia 1 northern algeria 2 southern algeria 3 northern niger 3 mali 4 southern niger at the present stage of investigation the following new inferences can be drown one or several humid episode s occurred in the northern sahara between 150 and 75 ka b p several factors suggest that these humid conditions also affected lower latitudes although their southern extent cannot be given precisely at present the previously postulated existence of large lakes at 20 35 ka b p is not confirmed this time interval seems to coincide with raised water tables but not to a major humid phase in all cases the 14c chronologies obtained for the late pleistocene have to be checked by comparison with other dating methods e g th u disequilibrium after the arid period of the last glacial maximum the return of humid conditions occurred in two major steps at 12 and 9 5 9 ka b p for both the southern and the northern sahara

humidity changes in southern tunisia during the late pleistocene inferred from u th dating of mollusc shells abstract calcareous deposits mainly consisting of mollusc shell accumulations which have been dated by the u th disequilibrium method mark the shorelines of paleolake highstands in the great chotts area of southern tunisia the 5 sites studied consist of discontinuous accumulations of fossils of marine like organisms e g cerastoderma glaucum melania tuberculata melanopsis praemorsa cerithium rupestre u th isochron plots and age frequency histograms for 39 shell samples are reported here limited variations for u content and 234u 238u activity ratios ar of shells support the hypothesis of closure of the geochemical system with respect to this element it is remarkable that 234u 238u ar of shells collected in chott fejej or chott jerid are clustered around different values reflecting probably different groundwater recharge from the continental intercalaire ci or complexe terminal ct aquifers furthermore waters collected near wadi el akarit show 234u 238u ar values comparable to those observed for shells 14c determinations made on aliquots of some of these samples suggested an age distribution between 18 and 34 ka bp the u th data of these 39 shell samples imply that 4 distinct flood episodes of these lakes occurred at about 30 95 100 130 150 and 180 200 ka for the episode centred around 30 ka the frequency histogram of ages shows a multimodal age group that could represent the existence of several humid pulses rather than a unique event moreover the comparison of 13c and 18o with those of older humid pleistocene phases when very large palaeolakes have been recorded suggests that these young carbonate shells are not related to a true highstand lake it is suggested that they represent a period of less humid climatic conditions with carbonate accumulation in minor water ponds in which intensive biological activity could have taken place it should be noted that this period was less arid than the present

megalakes in the sahara a review the sahara was wetter and greener during multiple interglacial periods of the quaternary when some have suggested it featured very large mega lakes ranging in surface area from 30 000 to 350 000 km2 in this paper we review the physical and biological evidence for these large lakes especially during the african humid period ahp 11 5 ka megalake systems from around the world provide a checklist of diagnostic features such as multiple well defined shoreline benches wave rounded beach gravels where coarse material is present landscape smoothing by lacustrine sediment large scale deltaic deposits and in places tufas encrusting shorelines our survey reveals no clear evidence of these features in the sahara except in the chad basin hydrologic modeling of the proposed megalakes requires mean annual rainfall 1 2 m yr and a northward displacement of tropical rainfall belts by 1000 km such a profound displacement is not supported by other paleo climate proxies and comprehensive climate models challenging the existence of megalakes in the sahara rather than megalakes isolated wetlands and small lakes are more consistent with the sahelo sudanian paleoenvironment that prevailed in the sahara during the ahp a pale green and discontinuously wet sahara is the likelier context for human migrations out of africa during the late quaternary

recharge and paleo recharge of groundwater in different basins in tunisia abstract assessment of the recharge is of strategic importance for a sustainable development of the aquifer resource especially in arid and semi arid areas this study synthesizes and compares the results of isotope hydrology investigations obtained for different aquifer systems in tunisia the 2h 18o values and 14c and 3h contents are distinct in unconfined and confined groundwater and vary from northern to southern parts of tunisia reflecting different recharge conditions low 2h and 18o values of deep confined groundwater in the southern and central parts agrees with groundwater residence times that suggest an important phase of recharge during the holocene and pleistocene under paleo climatic conditions that are quite humid from the current the study of paleo groundwater signals of these aquifers indicates that the transition from the end of the pleistocene to the holocene is associated with a much humid climate to give a widespread age signal in southern and central deep groundwater in tunisia furthermore the current climatic conditions are the determining factors in the recharge process of tunisia aquifers so they allow only very low recharge rates of most aquifers as a result paleo water is dominating in many groundwater basins particularly where the extraction of water exceeds the present recharge

freshwater to marine like environments from holocene lakes in northern sahara continuous swamp and lacustrine sequences from the northern edge of the great western erg are dated at 9 300 3 000 yr bp diatoms ostracods molluscs and foraminifera show great changes in salinity ranging from freshwater to marine like environments the stable isotope composition of carbonates indicates that the evaporation of groundwater supplies is responsible for these fluctuations in salt contents until now evidence for the occurrence of a wet climatic phase of early and middle holocene age in the north north west sahara was based on a restricted number of 14c dates from scattered lacustrine deposits and paleosols1 8 the present study complements this knowledge and highlights the importance of local hydrological factors in interpreting palaeolimnological changes

in situ zircon u pb dating and whole rock geochemistry of metasedimentary rocks from south liaohe group jiao liao ji orogenic belt constraints on the depositional and metamorphic ages and implications for tectonic setting abstract the liaohe group is an important paleoproterozoic lithostratigraphic unit of the jiao liao ji orogenic belt and traditionally subdivided into the north and south liaohe groups however their tectonic setting and evolution are still a matter of debate the precise depositional and metamorphic data of the studied metasedimentary rocks from the south liaohe group provide important information on the controversial discussion of the tectonic setting and evolution based on cathodoluminescence cl imaging and connecting la icp ms u pb geochronology zircon grains separated from metasedimentary rocks of different formations of the south liaohe group were subdivided into detrital and metamorphic zircons or zircon domains igneous zircon cores of detrital zircons of the li eryu gaojiayu dashiqiao and gaixian formations yield u pb age peaks at 2130 ma 2520 ma 2160 ma 2500 ma and 2040 ma indicating that their sediments were transported from the liaoji granitoids the archean basement rocks of the adjacent complex in addition deposition age of the sedimentary precursors was investigated which must have taken place at some time after 2 05 ga 2 07 ga 2 04 ga and 1 92 ga respectively these new geochronological data can be well explained by 1 a continent arc collision model which suggests that the south liaohe group formed an intervening island arc and foreland basin sandwiched between the longgang and nangrim complexes and 2 a northward subduction of the nangrim complex source rocks of the south liaohe group these source rocks were successively deposited along the margin of the two complexes

structural deformation and mineralization in the early proterozoic liaojitite suite eastern liaoning province china abstract the liaojitite suite located in eastern liaoning province china formed in the eugeosyncline of an early proterozoic mobile zone structural deformation occurred during several orogenic episodes especially during the second deformation of the third episode the luliang orogeny 1900 2000 ma ago superposed dome shaped structures and their unevenly distributed groups formed during the luliang orogeny constitute the principal tectonic style in the mobile zone the tectonic style may be considered transitional in the evolution from an oval style in archean granite greenstone terrains to a linear style in phanerozoic terrains the superposed dome shaped structures contain the liaoji granite in the cores ringed successively by the boron bearing series and the outer turbidite series the simple appearances of the dome shaped structures mask their complicated multiply deformed nature which is revealed by tracing their formation and evolution from primary volcanic domes to granitic diapirs and finally to superposed dome shaped structures the evolution of the superposed dome shaped structures represents a change in crustal activity from simple shear to horizontal compression mineralization in the liaojitite suite was closely related to the three deformations of the luliang orogeny metallogenic elements were concentrated during the first deformation physical re emplacement of orebodies occurred during the second and third deformations

sm nd and shrimp u pb zircon geochronology of high pressure granulites in the sanggan area north china craton timing of paleoproterozoic continental collision abstract high pressure hp mafic granulites are widely distributed as sheet like and lenticular bodies within highly deformed tonalitic gneisses in the sanggan area north china craton they have clockwise isothermal decompression and isobaric cooling p t paths implying continental collision tectonics whole rock analyses of 15 samples define a sm nd isochron yielding an age of 2547 87 ma with e nd t 2 7 corresponding nd depleted mantle model ages show a similar range and are interpreted as the age of the magmatic protolith to the high pressure granulites a combined sm nd mineral whole rock isochron from one of these samples yields an age of 1813 23 ma taken to represent the metamorphic age at an early stage on the cooling path shrimp u pb zircon analyses of three hp mafic granulite samples give uniform weighted mean 207 pb 206 pb ages of 1817 12 1817 12 and 1819 16 ma these ages are interpreted to date metamorphism of the hp granulites and with the sm nd mineral whole rock result strongly suggest that the time of continental collision was at ca 1 82 ga an additional sample records two age populations one at 1803 9 ma and an older one at 1872 16 ma the latter may indicate the timing of either an earlier metamorphic event or possibly the earliest stage of a single prolonged metamorphic episode that peaked at 1 82 ga these results confirm the paleoproterozoic age for assembly of the north china craton

u series dating of impure carbonates an isochron technique using total sample dissolution abstract u series dating is a well established technique for age determination of late quaternary carbonates materials of sufficient purity for nominal dating however are not as common as materials with mechanically inseparable aluminosilicate detritus detritus contaminates the sample with extraneous th we propose that correction for contamination is best accomplished with the isochron technique using total sample dissolution tsd experiments were conducted on artificial mixtures of natural detritus and carbonate and on an impure carbonate of known age results show that significant and unpredictable transfer of radionuclides occur from the detritus to the leachate in commonly used selective leaching procedures the effects of correcting via leachate residue pairs and isochron plots were assessed isochrons using tsd gave best results followed by isochron plots of leachates only

la icp ms u pb zircon ages of the liaohe group in the eastern block of the north china craton constraints on the evolution of the jiao liao ji belt abstract as a major lithostratigraphic unit of the palaeoproterozoic jiao liao ji belt in the eastern block of the north china craton the liaohe group consists of an assemblage that is transitional from a lower arkose and volcanic rich sequence langzishan li eryu and gaojiayu formations through a middle carbonate rich sequence dashiqiao formation to an upper argillaceous sequence gaixian formation tectonically associated with the liaohe group are large volumes of granitoid rocks named the liaoji granite whose emplacement age has been dated at about 2 1 2 2 ga regionally the liaohe group and the associated liaoji granite extend along a nee trending belt that is flanked by 2 45 2 55 ga basement gneisses u pb zircon data and cathodoluminescence cl imaging are used to constrain the maximum depositional and metamorphic ages of the liaohe group which are critical in understanding the tectonic setting and history of the jiao liao ji belt cathodoluminescence images reveal that most detrital zircon grains from the liaohe group are characterized by concentric oscillatory zoning with comparatively low luminescence and high th u ratios suggesting magmatic origin but minor zircon grains have structureless overgrowth or recrystallization rims that are characterized by low th u ratio indicating metamorphic origin results of this study show that the detrital zircon grains of the langzishan formation the lowest sequence of the liaohe group have concordant u pb ages ranging from 2 05 to 2 24 ga suggesting that the liaohe group was deposited at some time after 2 05 ga although most overgrowth or recrystallization rims are too narrow to be analyzed some rims yield concordant or upper intercept ages around 1 93 ga interpreted as the peak metamorphic age of the liaohe group detrital zircon grains from the dashiqiao formation in the middle sequence of the liaohe group have concordant u pb ages ranging from 2 25 to 2 53 ga some of which are similar to those of the basement gneisses in the area this study indicates that the liaohe group was deposited after the emplacement of the liaoji granite and the latter may be a major component of the provenance for the langzishan formation in contrast the protoliths of the middle sequence of the liaohe group were most probably derived from the basement gneisses these new zircon ages for the liaohe group and recent data for the liaoji granite combined with lithological structural and metamorphic considerations support a rifting model for the evolution of the jiao liao ji belt

palaeohydrological corridors for hominin dispersals in the middle east 250 70 000 years ago abstract the timing and extent of palaeoenvironmental connections between northeast africa the levant and the arabian peninsula during the middle and late pleistocene are critical to debates surrounding dispersals of hominins including movements of homo sapiens out of africa although there is evidence that synchronous episodes of climatic amelioration during the late middle and late pleistocene may have allowed connections to form between northern africa and western asia a number of palaeoclimate models indicate the continued existence of an arid barrier between northern arabia and the levant here we evaluate the palaeoenvironmental setting for hominin dispersals between and within northeast africa and southwest asia during marine isotope stages mis 7 5 using reconstructions of surface freshwater availability as an environmental proxy we use remotely sensed data to map palaeohydrological features lakes wetlands and rivers across the presently hyper arid areas of northern arabia and surrounding regions integrating these results with palaeoclimate models palaeoenvironmental proxy data and absolute dating to determine when these features were active our analyses suggest limited potential for dispersals during mis 7 and 6 but indicate the formation of a palaeohydrological corridor the tabuk corridor between the levant and the arabian interior during the mis 6 5e glacial interglacial transition and during mis 5e a recurrence of this corridor following a slightly different route also occurred during mis 5a these palaeohydrological and terrestrial data can be used to establish when proposed routes for hominin dispersals became viable furthermore the distribution of arabian archaeological sites with affinities to levantine assemblages some of which are associated with homo sapiens fossils and the relative density of middle palaeolithic assemblages within the tabuk corridor are consistent with it being utilised for dispersals at various times

shrimp u pb zircon geochronology of the liaoji granitoids constraints on the evolution of the paleoproterozoic jiao liao ji belt in the eastern block of the north china craton abstract the paleoproterozoic jiao liao ji belt lies at the eastern margin of the eastern block of the north china craton and is composed mainly of the liaohe group and the liaoji granitoids the liaoji granitoids are divisible into pre tectonic magnetite and hornblende biotite monzogranitic gneisses and post tectonic or anorogenic porphyritic monzogranites and alkaline syenites shrimp u pb zircon geochronology combined with cathodoluminescence cl imaging has enabled for resolution of magmatic and metamorphic events that can be directed towards understanding the history of the paleoproterozoic jiao liao ji belt shrimp u pb zircon analyses reveal that the pre tectonic magnetite monzogranitic gneisses were emplaced in the period 2176 2166 ma and metamorphosed at 1914 ma and the pre tectonic hornblende biotite monzogranitic gneisses were emplaced at 2150 2143 ma these data demonstrate that the pre tectonic monzogranitic gneisses of the liaoji granitoids were not younger than the deposition of the liaohe group as previously considered since the latter contains a large amount of 2100 2000 ma detrital zircons in contrast the pre tectonic monzogranitic gneisses of the liaoji granitoids or similar aged granitoids may have been an important component of the provenance for the liaohe group shrimp u pb zircon analyses also reveal that the post tectonic porphyritic monzogranites and granites in the jiao liao ji belt were emplaced at 1875 10 and 1856 31 ma respectively simultaneously with the emplacement of the alkaline syenites that was dated at 1857 20 and 1843 23 ma similar post tecotnic or anorogenic granites are not only limited to the jiao liao ji belt but are also found in the north and south korea suggesting that the 1 85 ga post tectonic or anorogenic granitic magmatic event may have occurred in both the korean peninsula and the eastern block of the north china craton the shrimp zircon ages of this study combined with lithological structural metamorphic and geochemical data is consistent with a rift closure model which suggests that the archean complexes on the northern and southern sides of the jiao liao ji belt were originally situated on a single continental block that underwent paleoproterozoic rifting associated with the formation of the liaohe group and liaoji granitoids and closed upon itself at 1 9 ga

chronologie th u des concretions calcaires des varves du lac glaciaire de deschaillons wisconsinien inferieur late pleistocene lake deschaillons varves from the central st lawrence lowlands represent a lithostratigraphic unit whose upper limit is defined by the overlying gentilly till which was laid down

shorelines in the sahara geomorphological evidence for an enhanced monsoon from palaeolake megachad the sahara desert is the most extensive desert on earth but during the holocene it was home to some of the largest freshwater lakes on earth of these palaeolake megachad was the biggest landsat tm images and shuttle radar topography mission srtm digital topographic data reveal numerous shorelines around palaeolake megachad at its peak sometime before 7000 years ago the lake was over 173 m deep with an area of at least 400 000 km2 bigger than the caspian sea the biggest lake on earth today the morphology of the shorelines indicates two dominant winds one northeasterly that is consistent with the present day winds in the region the other originated from the southwest we attribute it to an enhanced monsoon caused by a precessionally driven increase in northern hemisphere insolation subsequent desiccation of the palaeolake is recorded by numerous regressive shorelines in the sahara desert

a dynamic explanation for the origin of the western mediterranean organic rich layers the eastern mediterranean sapropels are among the most intensively investigated phenomena in the paleoceanographic record but relatively little has been written regarding the origin of the equivalent of the sapropels in the western mediterranean the organic rich layers orls orls are recognized as sediment layers containing enhanced total organic carbon that extend throughout the deep basins of the western mediterranean and are associated with enhanced total barium concentration and a reduced diversity dysoxic but not anoxic benthic foraminiferal assemblage consequently it has been suggested that orls represent periods of enhanced productivity coupled with reduced deep ventilation presumably related to increased continental runoff in close analogy to the sapropels we demonstrate that despite their superficial similarity the timing of the deposition of the most recent orl in the alboran sea is different than that of the approximately coincident sapropel indicating that there are important differences between their modes of formation we go on to demonstrate through physical arguments that a likely explanation for the origin of the alboran orls lies in the response of the western mediterranean basin to a strong reduction in surface water density and a shoaling of the interface between intermediate and deep water during the deglacial period furthermore we provide evidence that deep convection had already slowed by the time of heinrich event 1 and explore this event as a potential agent for preconditioning deep convection collapse important differences between heinrich like and deglacial like influences are highlighted giving new insights into the response of the western mediterranean system to external forcing

tectonothermal history of the basement rocks in the western zone of the north china craton and its tectonic implications the basement of the north china craton can be divided into the eastern central and western zones based on lithological structural metamorphic and geochronological data the western zone comprises two different petrotectonic units archaean tonalitic trondhjemitic granodioritic ttg grey gneisses and metamorphic mafic rocks and palaeoproterozoic khondalite series the former is characterized by isobaric cooling ibc type anticlockwise p t paths in the north northwestern part of the zone and near isothermal decompression itd type clockwise p t paths in the eastern part adjacent to the central zone on the other hand the tectonothermal evolution of palaeoproterozoic khondalite series rocks is characterized exclusively by nearly isothermal decompression following the peak of metamorphism and then cooling defining clockwise p t paths the archaean ttg gneisses and associated mafic rocks with anticlockwise metamorphic p t paths reflects an origin related to underplating and intrusion of mantle derived magmas which may be derived from mantle plumes they represent a late archaean continental block in the western part of the north china craton the palaeoproterozoic khondalite series rocks represent passive continental margin deposits they were metamorphosed and deformed in the late palaeoproterozoic during the amalgamation of the western continental block with another continental block in the east part of the north china craton the itd type clockwise p t t paths of the palaeoproterozoic khondalite series rocks record the tectonothermal histories of the collision of the western and eastern continental blocks which resulted in the final assembly of the north china craton at c 1800 ma

deformation history of the hengshan wutai fuping complexes implications for the evolution of the trans north china orogen abstract the trans north china orogen separates the north china craton into two small continental blocks the eastern and western blocks as one of the largest exposure in the central part of the orogen the hengshan wutai fuping complexes consist of four lithotectonic units the wutai hengshan and fuping complexes and the hutuo group the hengshan complex contains high pressure mafic granulites and retrograded eclogites structural analysis indicates that most of the rocks in these complexes underwent three distinct episodes of folding d 1 to d 3 and two stages of ductile thrust shearing stz 1 between d 1 and d 2 and stz 2 after d 3 the d 1 deformation formed penetrative axial planar foliations s 1 mineral stretching lineations l 1 and rarely preserved small isoclinal folds f 1 in the hengshan and fuping complexes in the wutai complex however large scale f 1 recumbent folds with sw vergence are displayed by sedimentary compositional layers penetrative transposition resulted in stacking of thrust sheets which are separated by ductile shear zones stz 1 the kinematic indicators of stz 1 in the hengshan and wutai complexes show top to the s230 w thrusting likely related to northeastward oblique pre collisional subduction d 1 resulted in crustal thickening with resultant prograde peak metamorphism the hutuo group did not undergo the d 1 deformation either because sedimentation was coeval with the d 1 deformation or because it was at a high structural level and was not influenced directly by the early deformation the d 2 deformation produced nw verging asymmetric and recumbent folds the d 2 deformation is interpreted to have resulted from collision between the eastern and western blocks of the north china craton in the hutuo group and the fuping complex the development of ese verging asymmetric tight folds is associated with d 2 the structural pattern resulting from superimposition of d 1 and d 2 is a composite synform in the hengshan wutai fuping complexes all four lithotectonic units were superposed during the later d 3 deformation the d 3 deformation developed nw trending open upright folds ongoing collision led to development of transpressional ductile shearing stz 2 forming the transpressional zhujiafang dextral ductile shear zone between the northern hengshan complex and the southern hengshan complex and generating the sinistral longquanguan ductile shear zone between the fuping complex and the wutai complex respectively the stz 1 and d 2 deformation were possibly responsible for fast syn collisional exhumation of the high pressure mafic granulites and retrograded eclogites the structural patterns and elucidation of the deformation history of the hengshan wutai fuping complexes places important constraints on the tectonic model suggesting that an oceanic lithosphere between the eastern and western blocks underwent northeastward directed oblique subduction beneath the western margin of the eastern block and that the final closure of this ocean led to collision between the two blocks to form the coherent basement of the north china craton

geochronology and geochemistry of metamorphic rocks in the jiaobei terrane constraints on its tectonic affinity in the sulu orogen abstract tectonic affinity of tectono lithological units close to ultrahigh pressure metamorphic belt is a key issue for understanding the geodynamics of continental collision this is particularly so for the jiaobei terrane northeast of the dabie sulu orogenic belt in china new data from la icpms zircon u pb dating whole rock elements and nd sr isotopes and mineral o isotopes are presented for metamorphic rocks from this terrane the results place geochronological and geochemical constraints on their protolith nature and metamorphic timing and thus on its tectonic affinity to one of the two triassic collided continents the north and the south china blocks protolith ages for ttg gneiss amphibolite and mafic granulite are 2 7 2 5 and 2 4 ga respectively regional metamorphism took place extensively at 1 76 ga protolith of the ttg gneiss was generated by partial melting of mantle derived rocks at the root of a thickened crust protolith of the amphibolite was probably a product of arc like magmatism protolith of the mafic granulite was derived from a depleted mantle source both of them were locally contaminated by supracrustal materials protoliths of paragneiss and schist in the fenzishan group were mostly derived from supracrustal sources but protolith of amphibolite in the fenzishan group is of mantle derived signature unlike the uhp metaigneous rocks in the dabie sulu orogenic belt that show unusual 18 o depletion the jiaobei metamorphic rocks have basically preserved their original mantle like o isotope compositions in general the nature and timing of geological events recorded in the metamorphic rocks from the jiaobei terrane are comparable with those from the north china block rather than the south china block thus the jiaobei terrane is concluded to have tectonic affinity to the former but behave like a micro continent during the triassic continental collision the 1 76 ga regional metamorphism in the jiaobei terrane is likely related to reworking of the arc continent collisional orogen in the periphery of the north china block rather than the 1 85 ga collision event between the eastern and western north china blocks the present study lends support to the common assumption that the suture boundary between the north and south china blocks in the sulu orogen is located along the wulian yantai fault tectonic mingling along the wulian yantai fault is probably related to subduction erosion during the continental collision

calibration of the c 14 timescale over the past 30 000 years using mass spectrometric u th ages from barbados corals uranium thorium ages obtained by mass spectrometry from corals raised off the island of barbados confirm the high precision of this technique over at least the past 30 000 years comparison of the u th ages with 14c ages obtained on the holocene samples shows that the u th ages are accurate because they accord with the dendrochronological calibration before 9 000 yr bp the 14c ages are systematically younger than the u th ages with a maximum difference of 3 500 yr at 20 000 yrbp the u th technique thus provides a way of calibrating the radiocarbon timescale beyond the range of dendrochronological calibration

uranium series age estimates and paleoclimatic significance of pleistocene tufas from the lahontan basin california and nevada abstract an extended chronology of lahontan basin lake levels based on uranium series age estimates correlates with the global ice volume record lake highstands occur at or shortly after times of maximum ice sheet size moderate size lakes occur when the global ice volume is about 80 of its maximum the data indicate that lake levels rise and fall relative to the proximity of the mean position of the jetstream when the continental ice sheet is above some threshold size or shape it appears that the large scale circulation and climatic conditions are right for producing lakes of moderate to large size within the lahontan basin

the inner zone of the liaoji paleorift its early structural styles and structural evolution abstract a suite of eugeosynclinal formations together with common occurrence of extensional granitic rocks constitutes the major part of the inner zone of the liaoji paleorift in liaodong peninsula china two characteristic associations of deformation structures formed under different crustal conditions have been recognized in the liaoji paleorift zone from macro meso and micro structural studies and tectono thermal analyses magmatic core complex mcc a special structural style of extensional regime inherited from early rifting is constituted by the spatial temporal and genetic associations of magmatic cores bedding parallel ductile shear zones and extensional nappes such complexes are widely developed in the rift zone superposed on the mccs thrust nappes and thrusting related brittle ductile shear zones dominate a compressional event in the later evolution of the rift zone early structures were strongly deformed and transformed by later structures and meanwhile also exerted control on the latter e g the thrust nappes are often related to early folds and represented by fold nappes with extensive reactivation and reemplacement of granitic rocks in the cores a schematic evolutionary model for the early structural evolution of the early proterozoic in the inner zone is presented which indicates a different evolution of early rift zones from those of younger ones

uranium isotopes as tracers of groundwater evolution in the complexe terminal aquifer of southern tunisia abstract the complexe terminal ct multi layer aquifer is formed by neogene paleogene sand deposits upper senonian campanian maastrichtian limestones and turonian carbonates the chemical composition and isotopes of carbon and uranium were investigated in groundwater sampled from the main hydrogeological units of the ct aquifer in southern tunisia we paid special attention to the variability of uranium contents and isotopes ratio 234u 238u to provide a better understanding of the evolution of the groundwater system uranium concentrations range from 1 5 to 19 5 ppb typical of oxic or mildly reducing conditions in groundwaters the lowest concentrations are found southeast of the study area where active recharge is supposed to take place when looking at the isotope composition it appears that all the samples including those from carbonate levels are in radioactive disequilibrium with significant 234u excess a clear cut distinction is observed between turonian and senonian carbonate aquifers on the one hand with 234u 238u activity ratios between 1 1 and 1 8 and the sandy aquifer on the other hand showing higher ratios from 1 8 to 3 2 the distribution of uranium in this complex aquifer system seems to be in agreement with the lithological variability and are ultimately a function of a number of physical and chemical factors including the uranium content of the hosting geological formation water rock interaction and mixing between waters having different isotopic signatures significant relationships also appear when comparing the uranium distribution with the major ions composition it is noticeable that uranium is better correlated with sulfate calcium and magnesium than with other major ions as chloride or bicarbonate the 14c activities and 13c values of dic cover a wide range of values from 1 1 pmc to 30 2 pmc and from 3 6 to 10 7 respectively 14c model ages estimated by the fontes and garnier model are all younger than 22 ka and indicate that the recharge of ct groundwater occurred mainly during the end of the last glacial and throughout the holocene

polyphase deformation of the fuping complex trans north china orogen structures shrimp u pb zircon ages and tectonic implications abstract the fuping complex is located in the central part of the trans north china orogen tnco a paleoproterozoic collisional belt along which the eastern and western blocks were assembled to form the north china craton three stages of deformation d 1 d 3 have been identified in the fuping complex d 1 formed tight to isoclinal folds f 1 penetrative foliation s 1 and mineral lineation l 1 which resulted from the early nw se orientated thrusting and shearing d 2 progressively overprinted d 1 and was dominated by mostly nw se shortening and top to the se e thrusting under a compressive environment d 1 and d 2 occurred in association with the peak metamorphism m 1 d 3 occurred during the exhumation after the crustal thickening producing regional scale nww see trending open folds f 3 and associated low angle detachment faults and was responsible for the isothermal decompression m 2 metamorphism of the complex following the d 3 deformation the complex was exhumed to shallow crustal levels where the rocks underwent the retrogressive cooling m 3 metamorphism syn tectonic leucocratic dykes are widespread in the fuping complex two leucocratic dykes that did not experience d 1 but underwent d 2 yield shrimp zircon ages of 1843 12 ma and 1844 18 ma which indicate that d 1 must have occurred at some time earlier than 1844 ma whereas d 2 must have occurred at some time later than this age two weakly deformed post d 2 leucocratic dykes yield shrimp zircon ages of 1817 14 ma and 1815 45 ma suggesting that the d 2 deformation occurred in the period 1843 1815 ma the new shrimp data indicate that the collision related deformation of the fuping complex must have occurred in the paleoproterozoic not in the late archean structural and geochronological data presented in this study in association with previous lithological and geochemical data suggest that the fuping complex may have been located in the hinterland of an eastward subduction zone and underwent intensive deformation and metamorphism involved in the final collision between the western and eastern blocks at 1 85 ga

single zircon grains record two paleoproterozoic collisional events in the north china craton abstract the western block of the north china craton consists of the yinshan block in the north and the ordos block in the south which were amalgamated along the east west trending khondalite belt at 1 95 ga the western block then collided with the eastern block to form the coherent basement of the north china craton along the north south trending trans north china orogen at 1 85 ga the huaian complex a high grade terrrane located at the conjunction of the khondalite belt and trans north china orogen records metamorphic events associated with both collisions the complex consists of lithologies from both the khondalite belt and trans north china orogen of which the former consist of graphite garnet sillimanite gneiss garnet quartzite felsic paragneiss calc silicate rock and marble together called the khondalite series zircons in the graphite garnet sillimanite gneiss can be divided into three types 1 spherical grains without internal structures 2 grains with a core and rim structure and 3 grains with a dark core surrounded by double rims except for the dark core in type 3 all other types of zircon domains are structureless and highly luminescent with very low th u ratios typical of a metamorphic origin analyses on the cores of type 2 and the inner rims of type 3 from two samples yield upper intercept ages of 1946 26 and 1947 22 ma similar to previously determined metamorphic ages from the khondalite belt and thus interpreted as the time of collision between the yinshan and ordos blocks analyses on type 1 zircons rims of type 2 and the outer rims of type 3 from the same two samples give ages of 1850 15 and 1857 16 ma interpreted as the time of collision between the eastern and western blocks thus zircons in the graphite garnet sillimanite gneiss of the huaian complex record both of the paleoproterozoic collisional events in the north china craton

recharge and paleorecharge of the deep groundwater aquifer system in the zeroud basin kairouan plain central tunisia abstract the multilayered aquifer of zeroud basin is characterized by the hydrodynamic complexity due to natural geology and anthropogenic management features stable and radioactive isotope data have been used to investigate the origin of the groundwater and its recharge area and to understand the deep groundwater system within the southern kairouan sedimentary basin most of the stable isotopic data indicate that most deep groundwater samples derived either from meteoric water or from the zeroud river and were not affected by any significant degree of evaporation during recharge current tritium concentration in the groundwater is very low however it proved useful in the qualitative identification of modern recharge and mixing of recent and old groundwater radiocarbon deduced ages range from more than 30 000 years to modern a modern recharge characterizes the deep aquifer upstream of the basin the groundwater with ages range from more than 30 000 years and depleted of heavy isotopes are found in the deeper aquifer aquifer b downstream of the basin and could be inherited from paleorecharge

archean blocks and their boundaries in the north china craton lithological geochemical structural and p t path constraints and tectonic evolution an examination of lithological geochemical geochronological structural and metamorphic p t path data suggests that the basement of the north china craton can be divided into eastern and western blocks separated by major crustal boundaries that roughly correspond with the limits of a 300 km wide zone called the trans north china orogen the eastern block consists predominantly of late archean domiform tonalitic trondhjemitic granodioritic ttg batholiths surrounded by anastomosing networks and linear belts of open to tight synforms of minor volcanic and sedimentary rocks metamorphosed from greenschist to granulite facies at 2 5 ga with anticlockwise p t paths some early to middle archean rocks are locally present in the eastern block but their tectonic history is unclear due to reworking by the 2 5 ga tectonothermal event the western block has a late archean assemblage structural style and metamorphic history similar to that of the eastern block but it differs in the absence of early to middle archean assemblages and in being overlain by and interleaved with paleoproterozoic khondalites which were affected by a 1 8 ga metamorphic event involving clockwise p t paths a mantle plume model is proposed for the formation and evolution of late archean basement rocks in the eastern and western blocks based on a combination of extensive exposure of ttg gneisses affinities of mafic rocks to continental tholeiitic basalts presence of voluminous komatiitic rocks dominant diaprism related domiform structures anticlockwise p t paths and a short time span from the primary emplacement of ttg and ultramafic to mafic rocks until the onset of regional metamorphism between the two blocks is the trans north china orogen which is bounded by two major fault systems and is composed of late archean to paleoproterozoic ttg gneisses and granitoids interleaved with abundant sedimentary and volcanic rocks that are geochemically interpreted as having developed in magmatic arc and intra arc basin environments these rocks underwent multiple phases of compressional deformation and peak high pressure metamorphism followed by rapid exhumation during the late paleoproterozoic at 1 8 ga as a result of collision between the eastern and western blocks resulting in the amalgamation of the north china craton

palaeoproterozoic assembly of the north china craton the basement of the north china craton consists of the eastern and western blocks separated by the central zone both the eastern and western blocks are dominated by late archaean tonalitic trondhjemitic granodioritic gneiss complexes interdigitated with minor supracrustal rocks metamorphosed at 2 5 ga with anticlockwise p t paths the central zone is composed of reworked late archaean components and palaeoproterozoic juvenile crustal materials that underwent regional metamorphism at 1 85 ga with clockwise p t paths involving isothermal decompression as a result of collision between the eastern and western blocks which resulted in the final assembly of the north china craton

marine deposits 35 000 25 000 years old in the chott el djerid southern tunisia the chott el djerid is a large salt covered depression in southern tunisia fig 1 the late nineteenth century idea that the basin had been invaded by the sea1 2 has been superseded by the view that any quaternary flooding of the chott must have resulted from climatic changes as the fossils found in its sediments are not exclusively marine and there is no indication of the fluctuations in sea level or of the crustal movements that would be required to fill and then empty the basin2 3 we report here evidence for a marine connection 35 000 25 000 years ago and show that it implies subsequent uplift by some 80 metres in an area long regarded as either stable2 or prone to subsidence throughout the quaternary4

zircon geochronology and metamorphic evolution of mafic dykes in the hengshan complex of northern china evidence for late palaeoproterozoic extension and subsequent high pressure metamorphism in the north china craton abstract magmatic and metamorphic zircons have been dated from ductilely deformed gabbroic dykes defining a dyke swarm and signifying crustal extension in the northern part of the hengshan complex of the north china craton these dykes now occur as boudins and deformed sheets within migmatitic tonalitic trondhjemitic granodioritic and granitic gneisses and are conspicuous due to relics of high pressure granulite or even former eclogite facies garnet pyroxene bearing assemblages shrimp ages for magmatic zircons from two dykes reflect the time of dyke emplacement at 1915 ma whereas metamorphic zircons dated by both shrimp and evaporation techniques are consistently in the range 1848 1888 ma the youngest granitoid gneiss yet dated in the hengshan has an emplacement age of 1872 17 ma these results complement recent geochronological studies from the neighbouring wutai and fuping complexes to the se of the hengshan showing that a crustal extension event occurred in the late palaeoproterozoic this preceded a major high pressure collision type metamorphic event in the central part of the north china craton that occurred in the palaeoproterozoic and not in the late archaean as previously thought our data support recent suggestions that the north china craton experienced a major craton wide orogenic event in the late palaeoproterozoic after which it became cratonized and acted as a stable block

a pleistocene lacustrine episode in southeastern libya fossiliferous lacustrine deposits of middle pleistocene age have been found in central fezzan libya 1 on each side of the east west depressions of the wadi shati south of the precambrian qarqaf anticline1 8 they represent the shoreline of a former brackish water lake fig 1 which some 130 000 yr bp covered an area about 100km long and 10 20 km wide at present the area may be classified as hyperarid since 1978 a collaborative study of the fossil assemblages has been undertaken it included macro and micropalaeontological determination th u dating stable isotope and chemical analyses in parallel with sedimentological and archaeological studies of the sites here we consider the th u dating and the isotopic palaeohydrology of the lacustrine episode

paleohydrologie isotopique de l oued el akarit sud tunisie au pleistocene superieur et a l holocene previous data from gobert and howe coque and page on the upper quaternary deposits from wadi el akarit are expanded through radiocarbon and stable isotope measurements 18c and 13c from carbonates 18o and 34s from sulphates these new results report on detailed stratigraphic profiles and indicate fluviatile and palustrine environments during the late pleistocene vegetal dams linked to spring activity caused water stagnation fine grained sediments were deposited with appreciable amounts of organic matter which locally developed into some peaty levels in the upper part of the typical cross section located on the left bank about 100 m upstream of the railway bridge pleistocene deposits show a palustrine carbonate crust this formation is attributed to the drainage induced by the major marine regression at glacial maximum during the holocene stagnation periods still occurred interrupted by flood episodes marked by bottom transport and coarse deposits in the typical cross section deposits are well drained and oxidized however downstream a stagnation episode takes place with a rich malacofauna dominated by cerastoderma cardium the uneven distribution of rainfall is documented by the alternation of fine and coarse deposits and it is accentuated by the climatic evolution toward aridity which results in the formation of a gypsum crust after 4000 yr bp later exceptional floods removed sediments from the wadi channel cutting the upper quaternary deposits for about 13 m along the central course of the wadi radiocarbon ages on carbonates are discussed according to various possible origins of total dissolved carbon atmospheric co2 organic activity in the basins and aquifer supply a simple exchange mixing model between aquifer bicarbonate and atmospheric co2 is used for calculations of initial carbonate activities chemical isotopic and hydraulic steady states are assumed in this treatment present day 14c and 13c contents of aquifer bicarbonate are thus adopted for past values the model leads to acceptable corrected ages for pleistocene carbonate samples corrected ages are younger than apparent ages by a few thousands of years and become consistent with ages given by organic matter from nearby levels however holocene samples appear overcorrected their crystallization appears to be controlled by modern organogenic co2 holocene 14c ages are probably correct 18o contents of carbonate indicate a continental origin for environmental water nodules and carbonate concretions crystallized in bottom sediments through upward leakage of confined aquifers they show 18o contents compatible with present day stable isotope compositions of artesian springs this supports the concept of a good time constancy in stable isotope contents shell carbonate values indicate a variable evaporation rate from the same initial waters 18o and 34s contents of gypsums are also compatible with aqueous sulphates from springs the data set falls within the range of permo triassic sulphate and suggests deep hydraulic connections within aquifer systems the entire formation appears to be of continental origin the occurence of cardium rich basins can only be attributed to the initial salinity of springs waters 7 9 g 1 1 sightly increased by evaporation this signifies that one can not deduce any tectonic uplift from their location with respect to sea level

initial constraints on the timing of granitic magmatism in north korea using u pb zircon geochronology abstract the geology of north korea is very poorly known in this paper twenty seven granitic samples from the tumangang belt and the rangnim massif including diorite granodiorite and monzogranite were selected for in situ zircon u pb isotopic dating in the tumangang belt granites formed mainly in the jurassic with one sample giving a triassic age this is contrary to previous views that considered the granites to be permian to triassic in age in the rangnim massif precambrian granitic magmatism mainly took place at 2 1 and 1 9 1 8 ga no archean emplacement ages have been documented in this study although some inherited discordant zircon may be of this age mesozoic magmatism in this area can be divided into three stages with ages of 234 213 173 and 114 109 ma the triassic granites are mostly exposed in the north and the early cretaceous ones in the west the distribution of jurassic granites is less well documented and only one sample was identified in this study however previous data indicate they are more widely distributed in the area the 2 1 and 1 9 1 8 ga precambrian ages are comparable to paleoproterozoic magmatism in northeastern china indicating that the rangnim massif was most likely part of the north china craton during that time however the absence of 2 5 ga granites and the occurrence of 1 2 ga granites indicate that the rangnim massif was probably at the margin of the north china craton and affected by mesoproterozoic magmatism the similarity in age patterns of the mesozoic granites in north korea to surrounding areas of south korea and the liaodong peninsula of northeastern china indicate that they were controlled by subduction of the pacific ocean under eastern asia however roll back or retreat of the subduction zone resulted in a younging trend from northwest to southeast during the cretaceous

on the ages of humid holocene and late pleistocene phases in north africa remarks on late quaternary climatic reconstruction for the maghreb north africa by p rognon discussion sur l article de rognon p paru en 1987 dans palaeogeogr palaeoclimatol palaeoecol vol 58 p 11 34

a comparison of u pb and hf isotopic compositions of detrital zircons from the north and south liaohe groups constraints on the evolution of the jiao liao ji belt north china craton the jiao liao ji orogenic belt is one of the important paleoproterozoic mobile belts in the north china craton ncc and its central segment consists of the north and south liaohe groups and the liaoji granitoids the north and south liaohe groups consist of sedimentary and volcanic successions metamorphosed from greenschist to lower amphibolite facies and the liaoji granitoids are divisible into 2 2 2 1 ga pre tectonic monzogranitic gneisses and 1 88 1 85 ga post tectonic porphyritic monzogranites and alkaline syenites combined with cathodoluminescence cl imaging we report la icp ms laser ablation inductively coupled plasma mass spectrometry u pb and hf isotopic data for detrital zircons from the north and south liaohe groups zircons grains from both groups have igneous zircon cores that yielded two u pb age populations at 2 0 2 2 ga major and 2 5 ga minor indicating that their provenances were similarly dominated by paleoproterozoic liaoji granitoids and late archean basement rocks some structureless zircon rims in both groups yield concordant or upper intercept ages 1 9 ga interpreted as the peak metamorphic age of the two groups these results suggest that both groups were deposited in the period 2 0 1 9 ga shortly after the emplacement of the liaoji granitoids moreover zircons from both groups have similar hf model age peaks t c dm peaks at 2 66 2 75 ga and 2 80 3 00 ga

deformation history of the hengshan complex implications for the tectonic evolution of the trans north china orogen structural analysis indicates that the hengshan complex underwent five distinct episodes of deformation d1 d5 the d1 episode formed small isoclinal folds f1 penetrative axial planar foliations s1 and mineral stretching lineations l1 d1 fabrics were reworked by following d2 deformation associated with d2 was the development of nw verging asymmetric folds and accompanying thrust faults both d1 and d2 resulted from crustal thickening and coherent with prograde and peak metamorphism d2 deformation was followed by transpressional dextral shearing d3 including top to nw oblique slip shearing and nne ssw dextral strike slip shearing ongoing collision led to development of the zhujiafang ductile shear zone d4 a near e w trending high strain belt across the hengshan complex d5 deformation is characterized by f5 open folds and associated normal faults probably related to exhumation of the complex structural patterns of the hengshan complex can place important insights into a recently proposed tectonic model suggesting that an ocean between the eastern and western blocks underwent eastward directed subduction beneath the western margin of the eastern block and closure of this ocean finally led to collision between the two blocks to form the coherent north china craton

quantifying paleorecharge in the continental intercalaire ci aquifer by a monte carlo inversion approach of 36cl cl data abstract past variations of the recharge is estimated in the atlas mountains the main recharge area of the continental intercalaire ci aquifer one of the major saharan aquifers over the last 775 kyr in this mediterranean climatic context continental archives generally record single humid events grouped in hypothetical time interval and do not offer continuous chronicles of precipitation we propose to use spatially distributed 36 cl cl data as a temporal constraint to infer the past recharge in the atlas area based on a simplified but robust climatic scenario assuming a piston model we apply a markov chain monte carlo mcmc inversion approach to attribute a specific recharge to the last nine interglacial periods and an undifferentiated recharge to the glacial periods the interglacial recharge values vary from a few mm yr 1 to more than 60 mm yr 1 glacial recharge is less than 1 mm yr 1 these values are then analyzed in terms of intensity and allow questioning some initial hypothesis especially the generally accepted value of the initial 36 cl cl value around 133 10 15 at at 1 our analysis suggests a higher value around 175 10 15 at at 1 this approach allows us to bring out paleoclimatic information retained in these continental archives especially computed recharges provide reliable evidence that marine isotope stage mis 5 was more humid than the holocene period in agreement with marine archives that document wet condition in the north sahara during that period

u pb and hf isotopic study of detrital zircons from the yejishan group of the luliang complex constraints on the timing of collision between the eastern and western blocks north china craton abstract the trans north china orogen is a recently recognized continent continent collisional belt along which the discrete eastern and western blocks amalgamated to form the coherent basement of the north china craton however there is a hot debate on the timing of the collision between the two blocks with one school of thought arguing the collision occurred at 2 5 ga whereas others believe it took place at 1 85 ga u pb ages and hf isotopic data of detrital zircons from foreland basins in the trans north china orogen can provide rigorous constraints on this controversial issue one of such foreland basins is represented by the yejishan group in the luliang complex in the middle sector of the western margin of the trans north china orogen the group consists of conglomerates quartz wackes sandstones and minor limestones most of which contain large amounts of detrital zircons cl images reveal that nearly all detrital zircons from the yejishan group possess oscillatory zoning with th u ratios of 0 2 2 34 typical of an igneous origin the 207 pb 206 pb ages of detrital zircons range between 3 24 and 1 81 ga with most having paleoproterozoic ages of which the youngest group ages are around 1 87 1 81 ga this indicates that the foreland basin in which the yejishan group was deposited must have formed not earlier than 1 87 1 81 ga thus supporting the collision between the eastern and western blocks at 1 85 ga not at 2 5 ga