a new model for the origin of the anomalous radioactivity in niue island south pacific soils abstract the anomalous natural radioactivity of niue island is due to large excesses of both 230 th and 231 pa compared with their parent radionuclides it is suggested that material in niue island soils consisting mainly of goethite and gibbsite partially trapped within porous calcite was exposed to seawater a few hundred thousand years b p and the u content of the seawater adsorbed onto the soil minerals the island then rose and remained essentially above sea level until present the daughter products of the u isotopes grew into radioactive equilibrium with the u parents but then the u parents were leached away some time in the span 100 70 ka b p leaving large amounts of orphan daughters in the soils without supporting parents

effect of submarine groundwater discharge on the coastal ocean inorganic carbon cycle using radium ra isotopes we estimate that the average submarine groundwater discharge sgd flux marine plus terrestrial groundwater into the southwest florida shelf swfs was 20 6 10 3 107 and 18 6 8 3 107 m3 d21 in july and october 2009 respectively the terrestrial groundwater flux was the same order of magnitude as the local river discharge in july 2009 shelf water total alkalinity talk and dissolved inorganic carbon dic concentrations could not be explained by river inputs alone suggesting a groundwater source we estimated sgd fluxes of talk and dic using the sgd flux derived from a shelf water 226ra budget and talk and dic concentration differences between the groundwater and seawater these fluxes were also determined by the observed talk 226ra and dic 226ra relationships in the shelf water and the 226ra flux sustained by sgd these talk and dic fluxes were 11 71 times more than the combined input of local rivers suggesting that sgd was the dominant source of talk and dic to the swfs during 2009 sgd is an important component of the inorganic carbon budget for the coastal ocean

provenance of meso to neoproterozoic cover sediments at the ming tombs beijing north china craton an integrated study of u pb dating and hf isotopic measurement of detrital zircons and whole rock geochemistry abstract the north china craton ncc was subjected to an extensional regime after the luliang movement at 1 8 ga and then was covered by an extensive meso to neoproterozoic sedimentary succession namely the changcheng jixian and qingbaikou groups in ascending order we report age spectra for detrital zircons and monazites hf isotopic systematics of detrital zircons and whole rock chemical and nd isotopic compositions for sediments from the succession in the ming tombs area beijing one of the typical meso to neoproterozoic areas in the ncc detrital zircons of six sedimentary samples have two distinct age peaks at 2 52 ga and 1 85 ga there are some detrital zircons at 2 4 2 0 ga but none at 2 3 ga and only a few 2 7 ga the detrital zircon age spectra change with time sediments in the lower succession changcheng group and in the upper successions jixian and qinbaikou groups are dominated by significant detrital zircon populations of late neoarchean and late paleoproterozoic ages respectively the 2 5 ga detrital zircons of the changcheng group have ehf 2 5 ga values and tdm hf model ages mainly ranging from 2 to 7 and 2 8 to 2 7 ga respectively detrital monazites of a sample from the jixian group exhibit a major age peak between 1 95 and 1 80 ga with some data between 2 0 and 1 95 ga the sedimentary rocks of the changcheng group are characterized by high k2o contents mostly 7 09 15 20 and insignificant eu anomalies eu eu 0 71 1 16 they have tdm nd model ages ranging from 2 70 to 2 43 ga being older than the tdm nd ages 2 11 and 1 99 ga of sedimentary samples from the qingbaikou group based on a comparison with ages for the early precambrian 1 8 ga basement of the ncc it can be concluded that 1 the sediments of the meso to neoproterozoic cover were undoubtedly derived from the ncc itself or once neighboring terranes 2 variations in the detrital zircon age spectra from the lower to the upper successions reflect provenance evolution in that the lower crustal late paleoproterozoic rocks were exposed at the surface after the upper crustal late neoarchean rocks had already been eroded

direct correlation of terrestrial and marine paleoclimatic records from four glacial interglacial cycles dsdp site 594 southwest pacific abstract over the last 350 ka changes in the composition of vegetation on new zealand inferred from pollen analysis of the upper 40 m of dsdp site 594 at 2 4 ka sample intervals reflect regional climatic variations which appear synchronous with implied variations in glacier fluctuation and in global climatostratigraphy described from sedimentary and oxygen isotope records from the same samples nelson et al 1985 pollen assemblages from isotope stages 1 5e 7a 7b and 9 are distinguished by conifer and broadleaf forest taxa which vary in composition between the last four interglacials suggesting significant differences in precipitation temperature and or migration rates glacial pollen assemblages which imply the expansion of herbland and decline in forest components show less variation and generally indicate comparatively cool conditions on the east coast of south island interstadial vegetation is composed of a mosaic of shrubland herbland vegetation the close correspondence between variations in the amplitude and timing of these continuous records of forest development in the changing vegetation of south island new zealand and oxygen isotope climatostratigraphy supports previous suggestions that late quaternary southern and northern hemisphere climatic fluctuations were essentially synchronous

precambrian crustal evolution of the eastern north china craton as revealed by u pb ages and hf isotopes of detrital zircons from the proterozoic jing eryu formation abstract simultaneous in situ u pb dating trace element and hf isotopic analyses have been carried out on detrital zircons obtained from sandstones in the neoproterozoic jing eryu formation from the eastern block north china craton ncc with the aim of constraining the ages of magmatic and metamorphic events and hence the precambrian crustal evolution of the ncc two groups of zircons i e magmatic and metamorphic in origin have been identified based on cathodoluminescence images zircon ti temperatures and th u ratios zircon u pb dating shows two main magmatic events with ages of 2 55 2 47 ga and 1 80 1 70 ga and four metamorphic events with ages of 2 51 2 48 ga 1 96 1 90 ga 1 87 1 85 ga and 1 80 ga in the ncc no zircons with ages of 3 8 ga have been identified in this study indicating either these were covered at the time of deposition or the limited presence of 3 8 ga rocks in the ncc the hf isotopic data show the 2 5 ga magmatic zircons have variable hf t values including both positive and negative values and with crustal model ages of 2 7 2 9 ga which can be interpreted to indicate mixed sources for their parental rocks i e ancient crust 3 8 ga or 3 3 ga and depleted mantle implying significant continental crustal growth at 2 5 ga however paleoproterozoic magmatic zircons all have negative hf t and archean hf model ages suggesting that their parental rocks were formed from recycled archean materials the widespread 2 5 ga metamorphism in the eastern block coupled with extensive 2 5 ga magmatism and crustal growth are consistent with a mantle plume beneath the eastern block of the ncc during the neoarchean whereas a progressive increase in 1 85 ga metamorphic zircon westward across the eastern block toward the trans north china orogen is best explained by collision between the eastern and western blocks at this time

the last glacial maximum in central and southern north island new zealand a paleoenvironmental reconstruction using the kawakawa tephra formation as a chronostratigraphic marker abstract kawakawa tephra formation comprising oruanui ignimbrite flow member and aokautere ash airfall member represents the products of an exceptionally large and widespread volcanic eruption from taupo volcanic centre in the north island of new zealand the eruption occurred during the last glacial maximum and is radiocarbon dated at c 22 6 ka b p thermoluminescence ages are in broad agreement with the radiocarbon age the presence of aokautere ash in loess deposits in alluvial gully fills on river terraces and its absence from unstable sites permits a detailed assessment of geomorphic activity during the last glacial maximum widespread erosion of regolith aggradation of river valleys and deposition of loess particularly in the period following eruption of the kawakawa point to a cold dry variable climate a collation of pollen data for sediments containing aokautere ash and those 14c dated in the range 17 23 ka shows that tall forest was highly restricted in the central and southern parts of the north island an apparently subalpine grassland shrubland was present at sites from present sea level to over 800 m elevation suggesting that factors other than lower temperatures such as exposure to wind and frost fire and reduced rainfall were important in controlling vegetation patterns we conclude that the interval 23 13 ka b p broadly equivalent to oxygen isotope stage 2 represents the period of greatest environmental change in the north island

reassessment of continental growth during the accretionary history of the central asian orogenic belt we argue that the production of mantle derived or juvenile continental crust during the accretionary history of the central asian orogenic belt caob has been grossly overestimated this is because previous assessments only considered the palaeozoic evolution of the belt whereas its accretionary history already began in the latest mesoproterozoic furthermore much of the juvenile growth in central asia occurred in late permian and mesozoic times after completion of caob evolution and perhaps related to major plume activity we demonstrate from zircon ages and nd hf isotopic systematics from selected terranes within the caob that many neoproterozoic to palaeozoic granitoids in the accreted terranes of the belt are derived from melting of heterogeneous precambrian crust or through mixing of old continental crust with juvenile or short lived material most likely in continental arc settings at the same time juvenile growth in the caob occurred during the latest neoproterozoic to palaeozoic in oceanic island arc settings and during accretion of oceanic island arc and precambrian terranes however taking together our data do not support unusually high crust production rates during evolution of the caob significant variations in zircon ehf values at a given magmatic age suggest that granitoid magmas were assembled from small batches of melt that seem to mirror the isotopic characteristics of compositionally and chronologically heterogeneous crustal sources we reiterate that the chemical characteristics of crustally derived granitoids are inherited from their source s and cannot be used to reconstruct tectonic settings and thus many tectonic models solely based on chemical data may need re evaluation crustal evolution in the caob involved both juvenile material and abundant reworking of older crust with varying proportions throughout its accretionary history and we see many similarities with the evolution of the sw pacific and the tasmanides of eastern australia

origin and characteristics of brackish groundwater in abu madi coastal area northern nile delta egypt abstract hydrogeochemical and geoelectrical resistivity investigations were carried out to assess the origin and characteristics of a brackish groundwater in abu madi coastal area twenty six surface water shallow and deep groundwater samples were collected and analyzed for various ionic concentrations as well as oxygen and hydrogen stable isotopic contents in addition a total of 20 vertical electrical sounding sites were conducted to investigate layers thicknesses resistivities and to detect the water bearing layers then 2 d electrical resistivity tomography ert along two profiles in the study area was conducted to get information about the surface water effect on recharge the stable isotopic composition and the chloride concentrations strongly suggest that the deep groundwater in abu madi area is dominated by inland freshwater with a minor seawater component this groundwater was recharged during the first and the third holocene humid climatic cycles where the sea surface was about 125 and 25 m below the current sea level respectively the brackish nature and higher piezometric surface of the groundwater as well as the occurrence of vertical low salinity seawater zone in front of the study area support the possibility of submarine groundwater discharge the geoelectrical resistivity surveying on the other hand revealed a number of geoelectrical groundwater bearing layers the main water bearing layer in the study area is represented by the sixth geoelectrical layer which has relatively high resistivity and a considerable thickness being consistent with the hydrogeochemical observations ert results point to the presence of shallow water bearing layers recharged from the surface water drains with low resistivity and surface rain water of moderate resistivity results from the hydrogeochemical analyses and the different hydrogeological data are consistent with the high resistivity values of this geoelectrical layer however the overall high specific conductance and high sodium hazards indicate that the groundwater in the study area should be carefully treated before irrigation and domestic uses through mixing it with surface irrigation water

kinematic analysis of melange fabrics examples and applications from the mchugh complex kenai peninsula alaska permian to cretaceous melange of the mchugh complex on the kenai peninsula south central alaska includes blocks and belts of graywacke argillite limestone chert basalt gabbro and ultramafic rocks intruded by a variety of igneous rocks an oceanic plate stratigraphy is repeated hundreds of times across the map area but most structures at the outcrop scale extend lithological layering strong rheological units occur as blocks within a matrix that flowed around the competent blocks during deformation forming broken formation and melange deformation was noncoaxial and disruption of primary layering was a consequence of general strain driven by plate convergence in a relatively narrow zone between the overriding accretionary wedge and the downgoing generally thinly sedimented oceanic plate soft sediment deformation processes do not appear to have played a major role in the formation of the melange a model for deformation at the toe of the wedge is proposed in which layers oriented at low angles to s1 are contracted in both the brittle and ductile regimes layers at 30 458 to s1 are extended in the brittle regime and contracted in the ductile regime and layers at angles greater than 458 to s1 are extended in both the brittle and ductile regimes imbrication in thrust duplexes occurs at deeper levels within the wedge many structures within melange of the mchugh complex are asymmetric and record kinematic information consistent with the inferred structural setting in an accretionary wedge a displacement field for the mchugh complex on the lower kenai peninsula includes three belts an inboard belt of late triassic rocks records west to east directed slip of hanging walls a central belt of predominantly early jurassic rocks records north south directed displacements and early cretaceous rocks in an outboard belt preserve southwest northeast directed slip vectors although precise ages of accretion are unknown slip directions are compatible with inferred plate motions during the general time frame of accretion of the mchugh complex the slip vectors are interpreted to preserve the convergence directions between the overriding and underriding plates which became more oblique with time they are not considered indicative of strain partitioning into belts of orogen parallel and orogen perpendicular displacements because the kinematic data are derived from the earliest preserved structures whereas fabrics related to strain partitioning would be expected to be superimposed on earlier accretion related fabrics 1999 elsevier science ltd all rights reserved

atmospheric radiocarbon calibration to 45 000 yr b p late glacial fluctuations and cosmogenic isotope production more than 250 carbon 14 accelerator mass spectrometry dates of terrestrial macrofossils from annually laminated sediments from lake suigetsu japan provide a first atmospheric calibration for almost the total range of the radiocarbon method 45 000 years before the present the results confirm the recently revised floating german pine chronology and are consistent with data from european and marine varved sediments and combined uranium thorium and carbon 14 dating of corals up to the last glacial maximum the data during the glacial show large fluctuations in the atmospheric carbon 14 content related to changes in global environment and in cosmogenic isotope production

the 1 75 1 68ga anorthosite mangerite alkali granitoid rapakivi granite suite from the northern north china craton magmatism related to a paleoproterozoic orogen abstract the amgrs is a suite of anorthosite mangerite alkali granitoid rapakivi granite in the northern north china craton ncc new zircon shrimp and laser ablation inductively coupled plasma mass spectrometry la icpms u pb dating of the damiao anorthosite changsaoying k feldspar granite lanying anorthosite and quartz syenite and the gubeikou k feldspar granite yielded emplacement ages of 1726 9 ma 1753 23 ma 1739 43 ma 1712 15 ma and 1692 19 ma respectively combined with previous geochronological data from the damiao norite and mangerite and shachang rapakivi granite it indicates that the whole amgrs was intruded between 1750 ma and 1680 ma the damiao anorthosites and norites exhibit a high mg and this decreases from mafic up to 63 to felsic t dm c ages ranging from 2 32 ga to 2 43 ga and 2 70 ga to 2 88 ga respectively this suggests that these rocks are the fractional crystallization products of one parental magma zircon hf and whole rock sr nd pb isotopic systematics combined with high la yb 10 and intermediate th ta ratios suggest that the parental magma of the amgrs could have been derived from em i lithosphere mantle enriched by recycled archean continent crustal materials accompanied by some assimilation of lower crustal components the mafic to felsic compositions of the amgrs are different from typical bimodal rocks formed in an anorogenic intra continent rift setting but resemble those emplaced during post collisional post orogenic extensional tectonic setting this is compatible with their age of

a reevaluation of tectonic discrimination diagrams and a new probabilistic approach using large geochemical databases moving beyond binary and ternary plots 1 statistically rigorous confidence intervals calculated from analyses of basaltic volcanic samples obtained from two large geochemical databases petdb and georoc demonstrate that published binary and ternary discrimination diagrams seldom correctly classify samples from mid ocean ridges island arcs and ocean islands with better than 60 accuracy the confidence intervals provide a measure of certainty that was previously absent from these diagrams allowing for a more robust analysis of results a new probabilistic method for geochemical discrimination is developed using the geochemical databases the new method is n dimensional and uses a priori data to construct probability distribution functions from which a posteriori probabilities are generated tests of the new method demonstrate single analysis classification success rates for volcanic rocks from island arcs to be 83 from ocean islands to be 75 and from mid ocean ridges to be 76

late neoarchean or late paleoproterozoic high pressure granulite facies metamorphism from the east hebei terrane north china craton abstract it is debated whether there are neoarchean high pressure hp granulites in the east hebei terrane north china craton three mafic granulite samples from the saheqiao belt are investigated here for their petrography p t evolution and zircon dating all the samples contain the typical hp granulite assemblages involving garnet clinopyroxene plagioclase and amphibole these assemblages commonly overprint on an early phase assemblage dominated by coarse grained clinopyroxene these hp granulites share clockwise p t paths with the peak conditions constrained to be 900 c 12 5 kbar and 880 c 12 8 kbar for two samples using the isopleths of the maximum ti content from amphibole and the minimum anorthite from plagioclase in p t psueudosections the post peak p t paths are inferred to be dominant of decompression with cooling based on the core rim zoning with increasing anorthite in plagioclase plagioclase coronas around garnet and decreasing ti from core to rim in amphibole zircon dating yields one metamorphic age of 2 48 ga for sample j1850 and two metamorphic ages of 2 47 and 1 81 ga for sample yc223 the older zircons show enriched hree patterns indicating no garnet involved whereas the younger zircons have flat hree patterns reflecting the presence of garnet moreover the older ages are consistent with the late neoarchean granulites that have anticlockwise p t paths and the younger ages correspond to the hp granulites from both metabasic dykes and supracrustal rocks constrained already thus the studied hp granulites are considered to be formed in the late paleoproterozoic related to a crust thickening orogenic event

vegetation history from the kaihinu last interglacial to the present west coast south island new zealand abstract an outline of the vegetation history of hokitika westport area west coast south island since the beginning of the kaihinu interglacial is based on pollen analytical data from 20 sites interpretation of the pollen diagrams is constrained by the geology of the sites and by the unreliability of radiocarbon dates caused by widespread sample contamination by modern carbon accumulation of the pollen bearing materials took place within the framework of deposition and erosion resulting from kaihinu last interglacial and aranui postglacial high sea levels and intervening glacier advances of the otira last glaciation the principal groups of sites are associated with 1 high sea level deposits 2 fluvial deposits 3 loessic terrace cover deposits and 4 wet hollows together with spot samples mostly previously described from late otiran full glacial sites the sites exemplify a wide variety of vegetation types and climatic environments ranging from warm temperate podocarp rain forest through beech nothofagus forest and shrubland to full glacial grassland based on the patterns of climatic change and geological events tentative correlations are made with deep sea oxygen isotope stages of late stage 6 through the substages of stage 5 followed by stages 4 to 1 forest prevailed throughout the kaihinu interglacial stage 5 being initially dominated by the podocarp dacrvdium cupressinum and then by nothofagus individual sites cover estimated time ranges up to ca 35 000 years in apparently unbroken sequences in the terrace coverbeds none more than 2 m thick severe mixing of pollen is at least partly related to breaks in the sequences a widespread early aranuian break latest stage 2 and another in the mid otiran interstadial stage 3 are both in transitional periods between fully glacial and fully interglacial conditions and probably required periods of seasonally dry and windy conditions to strip the loessic deposits from wide areas

tectonic attribution of the langshan area in western inner mongolia and implications for the neoarchean paleoproterozoic evolution of the western north china craton evidence from la icp ms zircon u pb dating of the langshan basement abstract the langshan area in western inner mongolia china is situated on the conjunction of the alxa block in the west the northern north china craton ncc in the east and the central asian orogenic belt in the north but its tectonic attribution has long been in debate because the alxa block may be an independent precambrian block from the ncc and the boundary between the two blocks may lie along the bayanwulashan to the west of the langshan instead of around the helanshan area to the east of the langshan in this study new la icp ms zircon u pb ages of four gneisses one amphibolite and three granites combined with previously published age data from langshan basement rocks indicate that the langshan area may be subdivided into the main langshan to the north of a series of nnw trending faults and the southern langshan in the south the main langshan basement rocks were mainly formed during neoarchean paleoproterozoic with five major stages of 2 7 ga 2686 ma 2 6 ga 2619 2563 ma 2 5 ga 2534 2491 ma 1 95 ga 1969 1938 ma and 1 7 ga 1672 ma these stages are largely consistent with the major tectonothermal events of the northern ncc during neoarchean paleoproterozoic but significantly different from the principal formation period ca 2 5 1 9 ga of alxa basement rocks the southern langshan is characterized by the presence of late paleoproterozoic 1 9 ga and neoproterozoic 0 9 ga rocks and the latter is consistent with the neoproterozoic magmatism in central alxa block which might be formed in a orogenesis related compressional environment in contrast to contemporaneous rift related extensional tectonic setting within the ncc therefore the main langshan and the southern langshan are most likely closely related to the northern ncc and the alxa block respectively

palaeoenvironmental reconstruction from the last interglacial using phytolith analysis southeastern north island new zealand phytoliths extracted from loess resting conformably on lacustrine sediments in southeastern north island new zealand provide a nearly continuous vegetation climate record spanning the time period from the last interglacial marine oxygen isotope stage 5 to the present phytoliths reveal changes in vegetation patterns following changes in climate correlation between tree shrub phytolith fluctuations and the specmap oxygen isotope curve between marine oxygen isotope stages 1 and 5 suggest that changes in the ratio of arboreal to non arboreal phytoliths directly result from changes in climate these data generally support the existing pollen and diatom record this study confirms the usefulness of phytolith fossils for providing environmental and climate information from the quaternary sedimentary record especially in cases where the deposits contain no fossil pollen or diatoms copyright 2000 john wiley sons ltd

ophiolite genesis and global tectonics geochemical and tectonic fingerprinting of ancient oceanic lithosphere ophiolites and discussions on their origin and significance in earth s history have been instrumental in the formulation testing and establishment of hypotheses and theories in earth sciences the definition tectonic origin and emplacement mechanisms of ophiolites have been the subject of a dynamic and continually evolving concept since the nineteenth century here we present a review of these ideas as well as a new classification of ophiolites incorporating the diversity in their structural architecture and geochemical signatures that results from variations in petrological geochemical and tectonic processes during formation in different geodynamic settings we define ophiolites as suites of temporally and spatially associated ultramafic to felsic rocks related to separate melting episodes and processes of magmatic differentiation in particular tectonic environments their geochemical characteristics internal structure and thickness vary with spreading rate proximity to plumes or trenches mantle temperature mantle fertility and the availability of fluids subduction related ophiolites include suprasubduction zone and volcanic arc types the evolution of which is governed by slab dehydration and accompanying metasomatism of the mantle melting of the subducting sediments and repeated episodes of partial melting of metasomatized peridotites subduction unrelated ophiolites include continental margin mid ocean ridge plume proximal plume distal and trench distal and plume type plume proximal ridge and oceanic plateau ophiolites that generally have mid ocean ridge basalt morb compositions subduction related lithosphere and ophiolites develop during the closure of ocean basins whereas subduction unrelated types evolve during rift drift and seafloor spreading the peak times of ophiolite genesis and emplacement in earth history coincided with collisional events leading to the construction of supercontinents continental breakup and plume related supermagmatic events geochemical and tectonic fingerprinting of phanerozoic ophiolites within the framework of this new ophiolite classification is an effective tool for identification of the geodynamic settings of oceanic crust formation in earth history and it can be extended into precambrian greenstone belts in order to investigate the ways in which oceanic crust formed in the archean

environmental change in northland new zealand during the last glacial and holocene abstract the quaternary vegetation and climatic history of the northernmost region of new zealand northland has long been the subject of speculation although the coverage is incomplete and individual records are punctuated by gaps or marked changes in sedimentation rates poor resolution and dating uncertainties recent work points to environmental changes that differ significantly from those at higher latitudes in new zealand conifer angiosperm forest has been important in northland during most of the last glacial cycle with agathis australis and nothofagus truncata dominated communities prominent during interstadials and stadials respectively fire and effective precipitation appear to have exerted greater influence on vegetation developments than temperature although the latter may control the competitive balance between agathis and nothofagus and will influence the environmental response to precipitation changes with the exception of human impacts during the last millenium the most striking vegetation changes are associated with widespread fires towards the end of the last glacial and the moist preceding interstadial in contrast pre human era holocene environments have been comparably stable although there is some evidence for increasing natural disturbance and a drier slightly cooler climate during the late holocene in several key aspects a glacial maximum to lateglacial dry phase very moist interstadial preceding the last glacial maximum no widespread loss of forest and the importance of fire and precipitation change in vegetation development during the last glacial cycle generally northland shows stronger affinities with south eastern australia than with other regions in new zealand

shallow subsurface diagenesis of pleistocene periplatform ooze northern bahamas previous studies on early submarine diagenesis of periplatform carbonates have implied that these originally polymineralic aragonite magnesian calcite calcite sediments are susceptible to early diagenesis only in current swept open seaways or where surficially exposed by erosion on the seafloor it has also been proposed that while in the shallow subsurface periplatform oozes retain their original mineralogy for at least 200 000 400 000 yr and remain unlithified for tens of millions of years evidence is reported here for extensive calcitization and selective lithification of periplatform oozes of late pleistocene age in two piston cores collected from water depths of 1 000 m north of little bahama bank it is shown that shallow 30 m subsurface diagenesis can significantly alter the original mineralogy of periplatform oozes to predominantly calcite in less than 440 000 yr and that cementation by calcite can produce chalk ooze sequences within the same time frame periplatform oozes that originally contain a high percentage of bank derived magnesian calcite appear to have a higher diagenetic potential than those originally low in magnesian calcite shallow subsurface calcitization and fithification greatly reduce the diagenetic potential of periplatform carbonates and chalk ooze sequences apparently can persist for tens of millions of years and to burial depths of at least 300 m shallow subsurface diagenesis at water depths 1 000 m proceeds via dissolution of magnesian calcite and aragonite and reprecipitation of calcite as allochem fillings exterior overgrowths and cement it is speculated that density driven kohout convection where seawaters under saturated with respect to magnesian calcite and aragonite and saturated supersaturated with respect to calcite flow through the margins of carbonate platforms is the primary driving mechanism for shallow subsurface diagenesis removal of mg during early stages of deep seafloor and shallow subsurface diagenesis should increase the mg content of interstitial waters which is likely to increase the dolomitizing potential of kohout convection fluid flow

spatial distribution of 1950 1800 ma metamorphic events in the north china craton implications for tectonic subdivision of the craton abstract great progress has been made in the last twenty years in understanding the unification of the north china craton according to a prevailing model it developed by amalgamation of four sub blocks by three spatially and temporally separate orogenic belts the first at 1950 ma formed the western block the second at 1900 ma created the eastern block and the third at 1850 ma unified the eastern and western blocks however new data show that the 1950 ma orogen was affected by granulite facies reequilibration at 1800 ma and parts of the 1850 ma orogen have evidence of metamorphism at 1950 1900 ma new zircon u pb ages enable the paleoproterozoic metamorphic events to be statistically sub divided into four at 1955 ma m 1 1920 ma m 2 1885 ma m 3 and 1850 ma m 4 the m 1 2 events always appear together mainly in two belt areas including the helanshan qianlishan northern ordos yinshan liangcheng huai an lushan jiaobei and liaodong regions whereas the m 3 4 events are distributed over larger areas and were superimposed on the m 1 2 activities this age distribution is inconsistent with previous models of three separate orogens it should also be noted that the igneous rocks contemporary with the regional ultra high temperature high pressure granulite facies metamorphisms m 1 2 were previously described as arc related series i e the gabbro norites i s type granites and trimodal volcanics based on the spatial distribution of the paleoproterozoic metamorphic events m 1 4 and of the coeval igneous rocks we speculate that the assembly of the ncc was created by one orogen which formed between the eastern and western cratons in two tectonic stages two marginal arcs the korean and xuwujia developed against both eastern sides of the two sub cratons and were both accreted and deformed at 1965 1900 ma m 1 2 subsequently the amalgamation of the two sub cratons resulted in metamorphism on the western side of the eastern craton and in metamorphism that was superimposed on both accreted arcs at 1900 1790 ma m 3 4

the occurrence and chemical implications of geothermal convection of seawater in continental shelves 1 geothermal convection is an important driving force for fluid flow and diagenesis in carbonate platforms and the similar shape of continental shelves in general suggests that geothermal convection could occur in continental shelves around the world numerical models based on seven transects of the shelf off the east coast of north america were developed to bracket possible fluxes simulations assuming relatively permeable sediments medium grained carbonates failed to produce significant thermal variations suggesting that chemical tracers will be required to detect this flow in the field fluxes calculated assuming lower permeabilities fine grained carbonates between cape fear and the savannah river were approximately 3 orders of magnitude smaller than river discharge for the area a difference similar to that between global river discharge and hydrothermal convection at mid ocean ridges sediment diagenesis during geothermal convection in continental shelves thus has the potential to affect ocean chemical budgets including potassium calcium and magnesium

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

an integrated latest quaternary stage 3 to present paleoclimatic and paleoceanographic record from offshore northern new zealand abstract paleoceanographic and onshore paleoclimatic changes during the last 59 000 yr are established from three deep sea sediment cores off northeast new zealand using an integrated log of sediment texture caco3 content palynology and planktonic and benthic foraminiferal 18o and 13c data together with dated silicic tephras these records from the isotopic stage 4 3 boundary to the present record northern new zealand vegetation history changes in a subsidiary equatorward flow of circumpolar deep water and sea surface temperatures ssts for subtropical water stw between latitudes 36 42 and 35 51 s relative to the holocene isotopically derived ssts record average changes of 2 c 2 c and 2 c for the 59 000 43 000 43 000 24 000 and 24 000 12 000 yr time slices respectively the apparent 2 c warming for the 59 000 43 000 yr period is interpreted to reflect changes in the dominant depth habitat of globigerina bulloides in response to upwelling a 2 c cooling of ssts during isotope stage 2 is interpreted in part to reflect upwelling of cool subsurface water resulting from strong and persistent westerly airflow across new zealand with the concomitant enhanced surface water production of caco3 onshore vegetation consistent with these changes are recorded with full conifer hardwood forest prior to 43 000 yr followed by a change to vegetation implying cooler and drier conditions between 43 000 and 12 000 yr and a subsequent return to full forest during the holocene the sequence of biopelagic and hemipelagic sedimentation observed within these cores reflect oscillation of sea level about a threshold eustatic level that controls the transport of terrigenous detritus offshore local variations and interplay of the regional oceanography and morphology and tectonism of the continental shelf will dictate that relative to present sea level this threshold eustatic sea level will vary in depth and hence age along a continental margin data from the new zealand region reveal an extremely steep meridional thermal gradient across the southern and central new zealand region during the last glaciation with minor cooling of stw to the north apart from localized nearshore upwelling zones but pronounced cooling of subantarctic water to the south of the subtropical convergence zone

discovery of sapphirine bearing mg al granulites in the north china craton implications for paleoproterozoic ultrahigh temperature metamorphism abstract we report here for the first time the occurrence of mg al granulites within the khondalite belt of the north china craton and provide robust evidence for extreme crustal metamorphism at ultrahigh temperature uht conditions in this region the uht indicator assemblages in these rocks include equilibrium sapphirine quartz high alumina orthopyroxene sillimanite quartz and high temperature perthites the surrounding khondalites also carry spinel quartz equilibrium assemblage the sapphirines occurring in various microstructural associations are markedly magnesian x mg 0 69 0 77 with a composition varying from 7 9 3 to 2 2 1 end members spinel is a solid solution of mg spinel and hercynite x mg 0 41 0 54 with zno content varying from 0 48 to 1 79 wt garnet shows a compositional range between pyrope and almandine x mg 0 39 0 48 with minor grossular 2 6 3 8 mol and spessartine 0 5 1 8 mol the orthopyroxenes display marked compositional variations in terms of al 2 o 3 content with the core of the medium grained orthopyroxene adjacent to sillimanite in the matrix showing the highest al 2 o 3 content 8 4 wt the history of prograde to peak metamorphism of these rocks is indicated by inclusions of sapphirine spinel quartz and sillimanite within garnet sapphirine formed during the peak stage through the reaction spl qtz sil spr occurrence of sapphirine and quartz inclusions trapped in sillimanite suggests the reaction spr qtz grt opx sil a series of retrograde reactions are observed in these rocks as corona and symplectite textures around early coarse grained minerals such as opx sil qtz crd grt sil qtz crd grt qtz crd opx grt o 2 crd qtz fe 2 o 3 in mag or ilm and finally opx kfs h 2 o bt qtz whereas the stability of sapphirine quartz and orthopyroxene sillimanite quartz in the matrix assemblage suggests t 1000 c and p 10 kbar peak metamorphism conventional geothermobarometric estimates confirm the ultrahigh temperature nature and also trace the retrograde conditions 930 970 c at p 10 kbar al in opx 900 1000 c perthite 930 990 c grt opx and 910 940 c sapphirine spinel electron microprobe dating of monazites in textural association with the uht assemblages yield a sharply defined isochron age of 1917 48 ma which correlates with the 1927 11 ma obtained in a previous study from monazites in the surrounding khondalites carrying spinel quartz assemblage an evaluation of the assemblages and textures using appropriate petrogenetic grids indicate that following peak ultrahigh temperature conditions the rocks underwent initial isobaric cooling and subsequent isothermal decompression and we interpret these trajectories to be part of an overall anti clockwise p t evolution the study area located within the northern margin of the north china craton defines a collisional orogenic belt where continental collision and deep subduction was possibly followed by extensional collapse of the orogen evidence for the extensional phase is provided by the several mafic magmatic intrusives of paleoproterozoic age occurring in the region which suggest a possible link between the thermal anomaly associated with extreme crustal metamorphism in this region with mantle derived magmas the uht metamorphism in north china craton coincided with the assembly of the paleoproterozoic supercontinent columbia

mid latitude trans pacific reconstructions and comparisons of coupled glacial interglacial climate cycles based on soil stratigraphy of cover beds abstract south westland new zealand and southern chile are two narrow continental corridors effectively confined between the pacific ocean in the west and high mountain ranges in the east which impart significant influence over regional climate vegetation and soils in both these southern mid latitude regions evidence for extensive and repeated glaciations during cold phases of the quaternary is manifested by arrays of successively older glacial drift deposits with corresponding outwash plain remnants in south westland these variably aged glacial landforms are mantled by layered multisequal soils characterised by slow loess accretion and pedogenesis in an extreme leaching and weathering environment these cover bed successions have undergone repeated coupled phases of topdown and upbuilding soil formation that have been related to fluctuating cycles of interglacial warm and glacial cold climate during the quaternary in this study we recognise multisequal soils overlying glacial landforms in southern continental chile but unlike the spodic podzolic soil sequences of south westland these are of dominantly volcanigenic andic provenance and are very similar to multisequal soils of andic provenance that predominate in and adjacent to areas of rhyolitic to andesitic volcanism in north island new zealand here we develop a soil stratigraphic model to explain the observed occurrence of multisequal soils mantling dominantly glacial landforms of southern continental chile based on proxy data from southern chile we propose that persistent vegetation cover and high precipitation on the western side of the andes during colder than present episodes tended to suppress the widespread production of glacially derived loessial materials despite the pervasive occurrence of glacial and glacio fluvial deposits that have frequently inundated large tracts of this landscape during the quaternary given the lack of loess cover beds that have traditionally assisted in the relative dating of glacial episodes prior to the late quaternary surface exposure dating techniques could provide another chronological alternative to address this issue however there have been two main obstacles to successfully apply this dating technique in patagonia first minimum exposure ages may be obtained on moraines older than the last glacial cycle due to erosion although dating outwash plains is more robust second on the wet western side adjacent to the andes persistent vegetation cover during both glacial and post glacial times as well as widespread inundation by volcanic mass flows appear preventive we make a case that soil genesis within this region appears to be dominated by a constant flux of intermittently erupted andean sourced tephra which has continued to upbuild soils at the ground surface separated by intervals where topdown weathering processes are intensified as already demonstrated by new zealand studies multisequal soil successions have a clear implied connection to coupled glacial and interglacial climate cycles of the quaternary on this basis similar sequences in northwest patagonia provide a relatively untapped archive to enable quaternary glacial and environmental changes in this pervasively glaciated volcanic region to be constructed

multiple sources for the origin of granites geochemical and nd sr isotopic evidence from the gudaoling granite and its mafic enclaves northeast china geochemical and sr and nd isotopic data have been determined for mafic to intermediate microgranular enclaves and host granitoids from the early cretaceous gudaoling batholith in the liaodong peninsula ne china the rocks include monzogranite porphyric granodiorite and quartz diorite monzogranites have relatively high 87rb 86sr ratios 0 672 0 853 low initial 87sr 86sr ratios 0 7052 0 7086 and e nd t values 18 5 to 20 9 indicating that they were mainly derived from a newly underplated crustal source with a short crustal residence time quartz diorites have high initial 87sr 86sr ratios 0 7118 0 7120 and negative e nd t values 13 2 to 18 1 coupled with high al2o3 and mgo contents indicating they were derived from enriched lithospheric mantle with contributions of radiogenic sr from plagioclase rich metagreywackes or meta igneous rocks i e ancient lower crust two groups of enclaves with igneous textures and abundant acicular apatites are distinguished dioritic enclaves and biotite monzonitic enclaves dioritic enclaves have low al2o3 13 5 16 4 wt and high mgo mg 72 3 concentrations low initial 87sr 86sr ratios 0 7058 0 7073 and negative e nd t values 7 2 and are enriched in liles and lrees and depleted in hfses suggesting they were derived from an enriched lithospheric mantle source biotite monzonitic enclaves have sr and nd isotopic compositions similar to the monzogranites indicating they were crystal cumulates of the parental magmas of these monzogranites granodiorites have transitional geochemistry and nd and sr isotopic compositions intermediate between the monzogranites quartz diorites and the enclaves geochemical and sr and nd isotopic compositions rule out simple crystal liquid fractionation or restite unmixing as the major genetic link between enclaves and host rocks instead magma mixing of mafic mantle derived and juvenile crustal derived magmas coupled with crystal fractionation and assimilation of ancient lower crust is compatible with the data this example shows that at least some calc alkaline granitoids are not produced by pure intracrustal melting but formed through a complex multi stage hybridization process involving mantle and crustal derived magmas and several concomitant magmatic processes crystal fractionation crustal assimilation and crustal anatexis

226ra and 228ra in the mixing zones of the pee dee river winyah bay yangtze river and delaware bay estuaries 226ra and 228ra have non conservative excess concentrations in the mixing zones of the pee dee river winyah bay estuary the yangtze river estuary and the delaware bay estuary laboratory experiments using pee dee river sediment indicate desorption of 226ra to increase with increasing salinities up to 20 in winyah bay desorption from river borne sediments could contribute almost all of the increases for both isotopes desorption adds only a portion of the excess 228ra measured in the yangtse river and adjacent shelf waters and delaware bay in the yangtze river the mixing zone extends over a considerable portion of the continental shelf where 228ra is added to the water column by diffusion from bottom sediments while 226ra concentrations decrease from dilution diffusion of 228ra from bottom sediments in delaware bay primarily occurs in the upper part of the bay 22 water where fine grained sediments predominate a diffusive flux for 228ra of 0 33 dpm cm 2 year was determined for delaware bay

an mis 5a to mis 4 or early mis 3 environmental and climatic reconstruction from the northwest south island new zealand using beetle fossils a fossil beetle study is presented from the northwest coast south island new zealand the samples were recovered from overbank deposits on a marine terrace situated 4 km east of westport new zealand 41 45 s 171 36 e the age of the site is constrained to between 74 and 40 ka but discrepancies between luminescence and radiocarbon age results prevent more precise age control the older of two samples indicates a closed canopy nothofagus southern beech forest with deep litter and dead wood on the forest floor the younger sample yields a similar nothofagus forest environment with subalpine affinities and nearby tussock grassland for the older sample mean summer and winter minimum temperatures were similar to modern values mean summer temperature was ca 3 0 c cooler than modern during the younger phase while mean minimum winter temperature was ca 5 0 c cooler than present indicating both cooling and increased seasonality both samples suggest a 30 40 decrease in mean annual precipitation from modern values persistence of closed canopy nothofagus forest during the period of 74 40 ka contrasts with regional pollen studies which imply widespread shrub land at these times this implies that this site may have served as a glacial refugium for nothofagus forest copyright 2007 john wiley sons ltd

mobilization of radiogenic pb in zircon revealed by ion imaging implications for early earth geochronology zircon is arguably the most commonly used geochronometer but the reliability of ages obtained requires a full understanding of processes that might compromise the integrity of its u pb systematics here we present the results of a multifaceted ion microprobe study of zircon grains from the napier complex east antarctica a region affected by pervasive high temperature metamorphism at 2 5 ga and from which previous zircon geochronological interpretations have been problematic both u pb spot analysis 15 m and high spatial resolution 2 m scanning ion imaging of pb isotopes have been applied in an attempt to quantify the effects of metamorphism spot analyses spread along concordia yielding 207 pb 206 pb ages from 2 5 ga to 3 9 ga with the oldest grains reversely discordant ion images of uranogenic pb reveal a surprising micrometer scale patchy distribution that is unrelated to crystal morphology or damage the 207 pb 206 pb ratios within these subdomains correspond to apparent zircon ages as old as 4 2 ga these are interpreted as artifacts of ancient redistribution of radiogenic pb a process that can generate meaningless ages and are not relicts of ancient including hadean zircon scanning ion imaging thus facilitates identification of unsupported radiogenic pb and enables testing of the validity of old ages from zircon known to have a long and complicated history

an improved alpha scintillation counting method for determination of th u ra 226 th 230 excess and pa 231 excess in marine sediments in a previous paper it was shown that thick source counting is a simple means for determining th u th 230 excess and pa 231 excess concentrations and hence sedimentation rates for deep sea sediments here it is shown that radon escape can lead to inaccurate results if powdered samples are used and that this problem can be overcome by preparing samples as a borate glass before measurement glassed samples also permit a novel measurement of the ra 226 content by measurement of the post fusion buildup of rn 222 a determination shown to be necessary for near surface sediments that have a deficit of ra 226 it is also shown that po 210 is lost during the fusion and that this loss can be allowed for in the calculations the above observations are found on comparisons of measured and calculated count rates for several th and u standards and on comparisons with spectrometry results from four sets of deep sea core samples

diverse sources for igneous blocks in franciscan melanges california coast ranges igneous blocks in franciscan melanges are of three chemical petrologic types 1 tholeiitic basalts of both arc and spreading center origin with depletions in light relative to heavy rare earth elements 3 tio 2 1 high y zr and y ti ratios and relict augites that generally have low al and ti and well defined iron enrichment trends 2 basalts of probable seamount origin with marked enrichments in light relative to heavy rare earth elements 5 tio 2 1 lower y zr and y ti than 1 and ti al rich augites showing little if any iron enrichment trends and 3 hypabyssal intrusives having sio 2 52 tio 2 1 flat or only slightly fractionated rare earth abundance patterns and diopsidic augites that are very low in ti and al and show no iron enrichment trends all of the blocks are metamorphosed most are undeformed pumpellyite bearing greenstones and a few contain sodic amphibole lawsonite sodic pyroxene the melanges are probably olistostromal in origin deriving the

dynamic hydrology and diagenesis of a submerged pleistocene fringing reef discovery bay jamaica abstract in the discovery bay area of north jamaica a pleistocene fringing reef grew and was subsequently planed off as sea level rose to and fell from a highstand approximately 5 to 10 m below modern sea level between 132 000 and 147 000 yrs bp dynamic movement of seawater through the reef today evidenced by flow of water in and out of open core holes has caused precipitation of cements which overprint previous diagenetic fabrics in certain zones in the reef flat and distal fore reef facies where porosity is diffuse modern water movement is slow and has had little effect on pre existing diagenetic fabrics which are dominantly vadose in texture in facies with conduit porosity as in the reef crest and proximal portions of the shallow fore reef seawater is channelized and high flow rates have resulted in the precipitation of substantial amounts of holocene sparry high magnesian calcite cements 15 18 mol mgco 3 up to 250 m thick resulting in incorporation of significant amounts of 14 c in the pleistocene rocks smaller mg calcite crystals having lower magnesium content are characteristic of zones with diffuse porosity the dynamic marine system apparently also favors the precipitation of dolomite in zones of diffuse porosity water motion within the reef results in the slow movement out of the man made conduits in zones of conduit porosity such as in facies having highly localized permeability water is sucked into open core holes in accordance with bernoulli s principle it is not known to what depths such channelized flow occurs data from a recording current meter demonstrate that the driving force responsible for the dynamic hydrology has a strong tidal component the tidal component is augmented by wind driven waves and is probably superimposed on another unidentified current which contributes to the flow of water in the reef at all tidal stages

the stranded beach dune sequence of south east south australia a test of thermoluminescence dating 0 800 ka the remarkable sequence of stranded beach dunes in south east south australia deposited during the past 800 ka has provided us with a rare set of samples for testing thermoluminescence sediment dating methods here we report thermoluminescence ages obtained from quartz separated from the dunes and compare them with accepted ages for the high sea levels responsible for the dunes the independent ages of the dunes were obtained from earlier work by schwebel which has been extended and refined by modelling the dune formation using sea level variations derived from the oceanic 18o record the work is unusual in extending thermoluminescence dating to an 800 ka sequence with modest geological control of the age the agreement between thermoluminescence and geological ages is satisfactory for eight dunes with ages in the range 120 800 ka quartz is not usually thought to be useful for thermoluminescence dating in this time span two factors contribute to our success the first is that the dose rates are low about 0 5 gy ka 1 or one quarter of that typical of sediments the second is that the thermoluminescence vs dose response continues to rise monotonically above the saturation region at doses above 300 gy

neoarchean to paleoproterozoic continental evolution and tectonic history of the north china craton a review rocks of archean age are widely distributed in the north china craton ncc and can be divided into high grade regions and granite greenstone belts even though most underwent amphibolite to granulite facies metamorphism quartzo feldspathic gneisses occupy 70 to 85 of the total exposure of archean rocks in the craton supracrustal rocks commonly consist of intercalated volcano sedimentary rocks including metabasites meta intermediate to acid rocks banded iron formations metapelites and minor marbles the oldest rocks outcrop over a small area of 4 km2 and are a quartzite amphibolite sequence that yield 3 8 3 5 ga isotopic ages chemical and geochronological data constrain the earliest main crust forming episode at ca 2 9 2 7 ga when most rocks were metamorphosed to upper amphibolite or granulite facies at moderate pressures mafic granulites can be classified into two textural types referred to as near isobaric cooling ibc and near isothermal decompression itd types high pressure granulites and retrograded eclogites occur mainly near the junction of the provinces of hebei shanxi and inner mongolia in the central ncc and in eastern shandong province of the eastern ncc as well as locally in henan province in the southern ncc these high pressure rocks show clockwise p t paths indicating decompression from 14 kb to 5 6 kb an exposed section interpreted as representing precambrian continental lower crust has also been identified in the northern part of the craton although some mafic ultramafic metamorphic rocks have been interpreted as remnants of ancient oceanic crust and mantle by a number of geologists geochemical and petrological evidence demonstrates that many supracrustal orthogneiss associations have island arc affinities therefore several tectonic models have been proposed including continent continent collision and island arc terrain accretion achieved by arc arc or arc microcontinent collision the ncc underwent multiple high grade metamorphic events of which the two most important took place during the intervals 2600 2450 ma and 1950 1750 ma which are considered to be related to early precambrian supercontinental cycles

palaeomagnetism of precambrian dyke swarms in the north china shield the 1 8 ga lip event and crustal consolidation in late palaeoproterozoic times abstract the north china shield ncs is cut by a laterally extensive dyke swarm emplaced at 1 78 1 76 ga when an extensional regime succeeded regional metamorphism and completion of cratonisation by 1 85 ga palaeomagnetic study of these dykes and adjoining metamorphic country rocks identifies a dominant shallow axis comprising a contiguous population with ne to n declinations and rare opposite polarity dykes with ne shallow magnetic declination a1 d i 36 1 recognised from previous study and emplaced in granulite terranes in the north are displaced by more northerly declinations a2 d i 8 2 in lower grade metamorphic terranes to the south contact tests indicate a primary cooling related origin to these magnetisations although tests are in part ambiguous because magnetisations in the granulite basement are comparable petrologic and rock magnetic considerations imply that magnetisation of the dykes occurred during uplift from depths as deep as 20 km following the peak of metamorphism at 1 85 ga a temporal migration a2 a1 is implied by the higher crustal level and earlier acquisition of the former and the deeper source and later acquisition of the latter a third population of dyke magnetisations a3 d i 18 43 is distributed towards steeper inclinations and close to the mesozoic recent palaeofield these are either partial or complete overprints of a1 a2 magnetisations with greater degrees of alteration indicated by demagnetisation and thermomagenetic spectra or are much younger dykes of mesozoic tertiary age a minority fourth later precambrian but presently undated dual polarity population has a magnetisation 11 dykes d i 108 7 with contact tests indicating a primary cooling related origin the 1 78 1 76 ga time of emplacement of the dominant dyke swarms in this study is widely represented by contemporaneous igneous rocks in other major shields linked to major large igneous province lip related events the new definition of a 1 83 1 76 ga apw swathe from the north china shield permits a comparison with other shields and yields a constraint to continental configurations during the late palaeoproterozoic a quasi integral reconstruction of palaeopangaea is tested here and supported by conformity of predominantly of uplift related palaeopoles from the 1 90 1 70 ga tectono thermal belts and from sw ne trending apw implied by the distribution of poles from the 1 80 ga igneous suites including the lip events this trend incorporates the a2 a1 migration and the granulite terrane cooling polar swathe from north china the reconstruction indicates that continental crust consolidated in palaeoproterozoic times by accretion of 2 3 1 7 ga orogenic belts around a hemispheric and crescent shape core already established by late archaean times the north china shield is interpreted to have bordered the western cratonic margin of the indian shield in a proximity supported by correlation of geological features and suggested by a number of previous workers the central orogenic zone of the north china shield characterised by tectono thermal activity prior to 1 85 ga was then contiguous with a comparable zone running through the centre of the indian shield and continuing into the capricorn belt of western australia the 1 78 1 76 ga dykes in north china continue into dyke swarms in the south india shield and may have been sourced in a plume related lip focussed near the continental margin in the xiong er aulacogen

assembling north china craton within the columbia supercontinent the role of double sided subduction abstract an evaluation of recent s wave receiver functions s wave velocities and two versions of p wave tomographic images along various transects in the north china craton provides some clues on the subduction collision history of the different crustal blocks and their final amalgamation within the paleoproterozoic columbia supercontinent interpretation of a n s seismic section of the craton suggests thick slab debris sinking to various depths in the mantle the w e seismic corridors show the preservation of a thick 200 km lithospheric root tectosphere beneath the ordos block and its variable and extensive erosion towards the yanliao block eastern block this zone is characterized by layers with marked velocity contrast and suggests repeated stacking of the remnants of underplated and accreted paleoproterozoic oceanic lithosphere the present day lithosphere asthenosphere boundary beneath this region probably marks the erosional plane along which decratonization occurred through subduction erosion from the east and thermal and material erosion by upwelling asthenosphere from below resulting in the partial destruction of the tectosphere and its thinning towards the east within the asthenosphere below the yanliao block younger and thinner slabs predominate in the absence of any prominent thick high velocity layers these younger slabs define a westward polarity and constitute a mega scale duplex formed by underplating through phanerozoic subduction process particularly the pacific plate subduction from the east the lithologic associations within the inner mongolia suture zone dividing the yinshan block to the north and ordos block to the south correspond to an accreted ocean plate stratigraphic sequence with the tonalite trondhjemite granodiorite ttg gneisses charnockites and calc alkaline granites representing a continental arc built up through subduction from the north the seismic transects bring out a contrasting polarity in the subduction regime with an oblique east to southward subduction of the yinshan block and a westward subduction of the yanliao block here i propose a double sided subduction history for the ncc similar to the ongoing subduction process in the western pacific such double sided subduction is considered to promote rapid amalgamation of continental fragments within supercontinents and the subduction polarities and mantle dynamics of ncc are therefore considered to be critical in evaluating the final assembly of the paleoproterozoic supercontinent columbia

multi chronometric ages and origin of archaean tonalitic gneisses in finnish lapland a case for long crustal residence time the tojottamanselka gneisses of the koitelainen region northern finland have been dated by the sm nd and the common pb methods the sm nd data of seven samples from a small area 100 m 100 m define an isochron oft 3 06 0 12 2 ga with correspondingind 0 50848 9 2 orend t 3 7 1 8 this age is in good agreement with the zircon u pb discordia age 3 1 ga reported by kroner et al 1981 and is interpreted as the time of magmatic emplacement the distinctly negativeend t value is found for the first time for archean tonalitic gneisses and implies derivation of these magmas by remelting of continental material with a long 200 500 ma crustal residence time a few samples on the other hand possessend t values close to zero hence they are thought to be derived by partial melting of basaltic sources with nearchondritic ree distribution patterns

environmental and sea level changes on banks peninsula canterbury new zealand through three glaciation interglaciation cycles abstract a greater than 200 ka record of marine transgressions and regressions is recorded from a 75 m core from banks peninsula canterbury new zealand this record comprises thick suites of muddy sediments attributed to back barrier lake and lagoonal environments alternating with thin soil and loess complexes these deposits have been dated using radiocarbon and thermoluminescence tl techniques supported by proxy data diatoms phytoliths pollen and sedimentology the aqueous deposits are attributed to three interglacials and an interstadial marine isotope stages 1 5a 5c 6 and 7 the loesses and paleosols date to the intervening stadials isotope stages 2 5d or 6 and probably 8 on the basis of transgressive beach facies back barrier swamps and barrier blocked lake deposits a partial sea level curve including data from isotope stage 5 is presented our data indicate that banks peninsula has been tectonically stable over that period and we provide sea level points that support the existing isotope curve during stages 5 and 6 detailed diatom records are limited to isotope stage 1 and the latter part of stage 5 diatom histories recorded from these stages are remarkably consistent both indicate a progressive floral change from marine types through freshwater colonising species to freshwater planktonic assemblages these reflect parallel histories of coastal evolution during the two interglacials in both cases marine transgression in the early part of the isotope phase was followed by lagoon development implying that a gravel spit extended across the embayment from the west this was succeeded by lake development when the lagoon was cut off by the juncture of the spit with banks peninsula this lake deepened as the coast rotated into swash alignment and the spit was converted into a gravel barrier the vegetation history of the site indicates that mixed podocarp broadleaf forests similar to the pre european flora of banks peninsula occupied the region during isotope stages 1 and 7 this contrasts with the palynological interpretation of a marine record dsdp site 594 from off the canterbury coast which suggested that isotope stage 7 was markedly cooler than the holocene during glacial periods forest was eliminated and replaced by a tall shrubland of mixed montane and coastal affinities

eolian evidence for spatial variability of late quaternary climates in tropical africa abstract study of the eolian fraction of late quaternary sediments from the tropical atlantic reveals that two modes of long term climate variability have existed in tropical africa during the last 150 000 yr tropical northwest africa i e the southwestern sahara and sahel was driest during glaciations and stades but wetter than at present during interglaciations and interstades this may be a response to ice sheets at higher latitudes via equatorward displacement of the westerlies and the subtropical high in contrast central equatorial africa southeast of the sahara was most arid during interstades and times of ice growth and most humid during deglaciation wet periods in this area correspond to insolation maxima in northern hemisphere summer a 23 000 yr precessional rhythm is suggested supporting a direct link between african monsoon intensity and orbitally modulated insolation the late holocene is the only time observed when both areas are arid during an interglacial episode this may reflect in part anthropogenic disturbance of late holocene climates

age and genesis of the neoarchean algoma type banded iron formations from the dengfeng greenstone belt southern north china craton geochronological geochemical and sm nd isotopic constraints abstract banded iron formations bifs are important for understanding the composition and redox condition of paleo oceans as well as the evolution of the precambrian crust the neoarchean dengfeng greenstone belt ca 2 54 2 50 ga in the southern north china craton ncc consists of metamorphosed igneous and sedimentary assemblages including some bifs here we present mineralogical geochronological geochemical and sm nd isotopic data of the dengfeng bifs for the first time to constrain their age origins and source characteristics and then discuss their paleo environmental implications the bifs are mainly associated with metabasaltic rocks and quartz rich schists and can be classified as algoma type bifs zircon u pb dating of quartz rich schists interbedded with the bifs coupled with ages of wall rock metabasites demonstrate that the dengfeng bifs formed at 2 53 2 51 ga most bif samples have high contents of sio2 and fe2o3t and extremely low contents of al2o3 tio2 and high field strength elements e g zr hf th u indicating that they are detrital free deposits in the post archean australia shale normalized multi element diagram the detritus free bif samples are characterized by depleted light rare earth elements positive la and y anomalies and suprachondritic y ho ratios resembling geochemical features of modern seawater the absence of negative ce anomaly indicates an anoxic condition positive eu anomalies and relatively high concentrations of transition metal elements suggest that high temperature hydrothermal fluids contributed to the source of bifs isotopically all bif samples have positive end t values 0 12 to 4 53 consistent with the range of those 0 10 to 4 31 of ca 2 66 2 50 ga dengfeng metabasites and ttg gneisses suggesting input of possible juvenile crustal fluxes the elemental and isotopic characteristics of the dengfeng bifs collectively suggest that they precipitated from anoxic seawater with various input of hydrothermal fluids and oceanic crust derived fluxes

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

the vegetation and climate during the last glacial cold period northern south island new zealand abstract pollen assemblages from howard valley south island new zealand were used to reconstruct the palaeovegetation and infer past climate during the period ca 38 21 cal ka which encompasses the marine isotope stage mis 3 2 transition and last glacial cold period lgcp a glacier occupied the upper howard valley during the last glacial whilst extensive glaciofluvial outwash surfaces were constructed in the lower valley episodic periods of fluvial aggradation and incision have produced a complex sequence of terraces flanking the main howard river and its tributaries sedimentary sequences from three exposed valley fills sampled for palynological analysis and radiocarbon dating consist of a complex vertical and lateral arrangement of coarse textured cobbly sandy gravels interbedded with organic rich silt deposits palynology of these organic rich horizons was directly compared to an existing beetle record from these same horizons during late mis 3 the site was dominated by marshy shrubland vegetation interspersed with mixed beech forest indicating temperatures 2 3 c cooler than present climate cooling began as early as 35 7 cal ka and coincides with evidence of cooling from other sites in new zealand south america and with an antarctic cooling signature a three phase vegetation and inferred climate pattern occurs at the site during the lgcp beginning with a transition to an alpine sub alpine grassland comparable to communities growing near treeline today marking the change to glacial conditions before 31 cal ka a small increase in tree abundance between ca 25 8 and 22 7 cal ka suggests minor climate amelioration during the mid lgcp during this phase a possible volcanically induced vegetation disruption caused by the deposition of the kawakawa tephra at 25 cal ka is evident in the pollen record this is followed by a further decline in tree pollen and increase in alpine grassland and herb pollen indicating further deterioration of conditions and a period of maximum cooling 4 5 c ca 22 5 20 9 cal ka this corresponds with the timing of greatest ice advance based on the geomorphic evidence in the region contrary to previous studies in new zealand the pollen and beetle records are in close agreement both indicate the site was forested during late mis 3 with progressive reduction of trees during the lgcp the consistent presence of beech pollen nevertheless confirms small refugia of trees persisted in the region

age and evolution of a late archean to paleoproterozoic upper to lower crustal section in the wutaishan hengshan fuping terrain of northern china abstract the taihangshan wutaishan area forms part of the central north china craton and consists of three main components the hengshan and fuping complexes containing predominantly ductilely deformed late archean to paleoproterozoic high grade granitoid orthogneisses intruded by mafic dykes of gabbroic composition and the low grade late archean wutai greenstone belt developed between the high grade terrains and consisting of bimodal volcanic rocks and metasediments associated with coeval granitoids zircon dating shows that both the hengshan and fuping complexes were intruded by major granitoid bodies between 2 52 and 2 48 ga with rare occurrences of 2 7 ga gneisses wutai granitoids adjacent to the fuping complex also contain 2 7 ga xenocrysts all possibly reflecting the same older basement rare granitic gneisses in the hengshan have emplacement ages of 2 35 ga and show the same structural features as the older rocks indicating that the main deformation occurred after 2 3 ga deposition of supracrustal rocks and emplacement of mafic dykes occurred in the paleoproterozoic together with intrusion of granitic bodies at 2 1 2 0 ga and all underwent regional metamorphism at 1 88 1 85 ga we interpret the hengshan and fuping granitoid gneisses as the lower plutonic part of a late archean to early paleoproterozoic japan type magmatic arc with the upper volcanic part represented by the wutai complex components of this arc may have evolved at a continental margin as indicated by the 2 7 ga zircons virtually all magmatic events are recorded in all three complexes however the 1 85 ga metamorphic event is only recorded in the higher grade portions of the wutai since low grade and presumably high crustal level precluded metamorphic zircon growth shear zones in the hengshan are interpreted as major lower crustal discontinuities post dating the peak of high pressure granulite metamorphism we suggest that they formed during orogenic collapse and uplift of the hengshan complex in the late paleoproterozoic 1800 ma

the distribution and relationship between palynomorphs in aerosols and deep sea sediments off the coast of northwest africa abstract palynomorphs in eolian dust and bottom sediments from the northeastern atlantic off the coast of northwest africa were studied primary examination was for terrestrial plant detritus pollen spores opal phytoliths and freshwater diatoms but observations about marine entities were also made three distinct geographic palynofloras were detected a northern palynoflora consisting of pollen derived primarily from the mediterranean basin a central or saharan palynoflora characterized by pollen derived from desert plants and a southern tropical equatorial palynoflora composed of pollen derived from tropical plants but including up to 50 allochthonous pollen from the drier interior of west africa palynomorph distributions are related closely to both source vegetation and to atmospheric and oceanic transport mechanisms and in many cases distribution patterns in bottom sediments and aerosols are complementary the quantity of pollen and spores per gram of bottom sediment ranges from greater than 2000 off the saharan coast in mauritania to less than 50 in deep ocean basins pollen and spores in mediterranean aerosols may exceed 40 per cubic meter of air during the summer and range between 4 to 6 for tropical aerosols during the winter tropical aerosols and bottom sediments contained the greatest abundance of fungal spores the distribution of opal phytoliths and freshwater diatoms in both aerosols and bottom sediments indicates that dust storms are a major transporting agent for these entities from the interior of west africa to the gulf of guinea these storms are also an important agent for the transport of pollen to the tropical atmosphere distances of transport may exceed 5000 km for mediterranean pollen and even greater distances for palynomorphs in eolian dust from interior west africa the abundance of dinoflagellates and microforaminifera in bottom sediments is directly related to the area of upwelling off the west african coast

interaction of weathering solutions with oxygen and u pb isotopic systems of radiation damaged zircon from an archean granite darling range batholith western australia zircons from archean granites from the darling range batholith in the yilgarn craton of western australia have been shown to have complexly discordant u pb systems with a strong component of zero age disturbance the only geological event that has affected the granites in recent times is the pervasive regional weathering our aim in this study was to investigate the effects of weathering on the u pb and oxygen systems of the zircons and to this end we report secondary ion mass spectrometry sims results of oh oxygen and u pb isotope systems of six typical zircons from a sample of the granite these results confirmed the presence of oh in highly radiation damaged parts of the zircons demonstrating fluid interaction within grains the presence of oh was accompanied by significant changes in the 18o 16o ratios the data suggest trends where 18o values in individual grains both increase and decrease with increasing oh sims measurements showed the u pb systems are variably and unsystematically discordant in radiation damaged parts of the zircons particularly those with elevated oh contents the complex u pb systems are interpreted in terms of multiple disturbance events between 450 and 0 ma involving low temperature fluid induced movement of radiogenic pb decoupled from parent u and th within the radiation damaged zircons together with some pb loss

the composite archaean grey gneisses petrological significance and evidence for a non unique tectonic setting for archaean crustal growth abstract the geodynamic context of formation of the archaean continental crust is a matter of debate the crust is largely made of grey gneiss complexes a composite rock assemblage dominated by granitoids that are generally regarded as belonging to the ttg tonalite trondhjemite granodiorite series using a large database of published ttg and grey gneiss compositions it is possible to show that the granitoids forming grey gneiss complexes actually belong to at least four main geochemical groups i a potassic component made of granodiorites and formed by melting of existing crustal lithologies and ii three sodic groups ttg proper that comprise low medium and high pressure groups the geochemistry of the low pressure group is consistent with derivation from a plagioclase and garnet amphibolite the medium pressure group was formed in equilibrium with a garnet rich plagioclase poor amphibolite whereas the high pressure group derived from a rutile bearing eclogite as the temperature of melting of metamafic rocks is largely independent from pressure this corresponds to melting along a range of contrasting geothermal gradients in turn reflecting a range of tectonic sites for the formation of the archaean continental crust

the location and evolution of the tectonic boundary between the paleoproterozoic jiao liao ji belt and the longgang block northeast china abstract in this paper we discuss the location nature and evolution of the tectonic boundary between the jiao liao ji belt and the longgang block northeast china the boundary in terms of paleogeography which involves a study of the liaohe group or tectonics employing non seismic geophysical data e g gravity data and magnetotelluric soundings existing geological data and the differences in physical properties between the jiao liao ji belt and the longgang block we clarified the spatial distribution of the tectonic boundary and characterized the geological bodies at different depths on either side of the boundary we analyzed the most recently updated bouguer gravity anomaly gravity magnetic and electrical profile data as well as comprehensive data on the physical properties of the rocks in the research area thus providing information on the geological nature of the boundary at deep levels the results indicate that during the early period of formation of the jiao liao ji belt a basin controlling boundary fault developed that constituted the original tectonic boundary between the jiao liao ji belt and the longgang block this fault extended from shuangtaling to sandaoling lian shanguan dawanggou balidianzi and erhulai to guaimozi following a roughly east west trend most previous studies have considered that the liaohe group which extends from benxi city to benxi county and huanren county is the paleo geographic boundary of the jiao liao ji belt

a new perspective of the alboran upwelling system reconstruction during the marine isotope stage 11 a high resolution coccolithophore record abstract a high resolution study of the mis 12 mis 11 transition and the mis 11 430 376 kyr coccolithophore assemblages at ocean drilling program site 977 was conducted to reconstruct the palaeoceanographic and climatic changes in the alboran sea from the variability in surface water conditions the nannofossil record was integrated with the planktonic oxygen and carbon stable isotopes as well as the uk 37 sea surface temperature sst at the studied site during the investigated interval the coccolithophore primary productivity reconstructed from the ppp primary productivity proxy absolute values of gephyrocapsa caribbeanica small gephyrocapsa group revealed pronounced fluctuations that were strongly associated with variations in the intensity of the regional alboran upwelling system the comparison of the nannoplankton record with opal phytolith content for the studied site and the already available pollen record at the nearby integrated ocean drilling program site u1385 suggests an association of the upwelling dynamics with the variability of the north atlantic oscillation like nao like phase high ppp during positive nao like phases is the result of intensified upwelling owing to the complete development of the surface hydrological structures at the alboran sea this scenario was identified during the mis 12 mis 11 transition 428 422 kyr the late mis 11c 405 397 kyr and mis11 b to mis 11a 397 376 kyr two short term minima in the ppp and sst were observed during mis 11 b and were coeval with the north atlantic heinrich type ht events ht3 390 kyr and ht2 384 kyr increased abundance of the subpolar coccolithus pelagicus subsp pelagicus and gephyrocapsa muellerae was consistent with the inflow of cold surface waters into the mediterranean sea during the ht events lowered ppp during negative nao like phases is the result of moderate upwelling by the incomplete development of surface hydrological structures at the alboran sea this scenario is expressed during the early mis 11c 422 405 kyr overall the results of our study provide evidence of the important role of atmospheric circulation patterns in the north atlantic region for controlling phytoplankton primary production and oceanographic circulation dynamics in the western mediterranean during mis 11

a late archean foreland fold and thrust belt in the north china craton implications for early collisional tectonics abstract the archean north china craton is divided into the western and eastern blocks along the central orogenic belt a 1600 km long archean foreland basin and thrust belt fringes the eastern side of the central orogenic belt rocks in the orogen form tectonically stacked east vergent fold and thrust sheets including foreland basin sediments 2 50 ga ophiolitic melange and an island arc complex foreland basin sediments overlie a passive margin sequence and include a 2 50 ga deep water turbidite sequence that grades upward and westward into shallow water molasse now disposed in structurally imbricated east verging thrusts and asymmetric folds that gradually migrated craton ward with deformation uplift and erosion of the orogen there is a strong linked relationship of the formation of the foreland basin to collision of the east and west blocks of the north china craton along the central orogenic belt at 2 50 ga the qinglong foreland basin and central orogenic belt of the north china craton represents one of the best preserved archean orogen to craton transitions in the world its classic internal to external zonation and flexural response to loading demonstrate that convergent tectonics in the archean were broadly similar to phanerozoic convergent margin processes

melanges through time life cycle of the world s largest archean melange compared with mesozoic and paleozoic subduction accretion collision melanges abstract melanges represent a significant component of collisional and accretionary orogenic belts we present a comparison of subduction accretion melanges of different ages to test how melange forming processes at convergent margins changed through time we describe the mesozoic mchugh melange in alaska and the paleozoic proto tethyan laohushan melange of the qilian orogenic belt of the alpine himalayan chain together with documentation of a neoarchean melange that marks the suture between the eastern block and central orogenic belt of the north china craton ncc the mchugh is a classic accretionary melange that has not yet experienced a collision and the laohushan is an accretionary collisional melange with structures related to both stages of its evolution we compare the characteristics of these melanges with the world s largest archean melange the newly documented circa 2 5 ga 1600 km long jianping zunhua zanhuang dengfeng jzzd melange of the north china craton the jzzd is a classical melange in terms of map patterns lithological relations structural characteristics including mixed rock types of different ages origins and metamorphic histories and preserves both paired metamorphic features and local blocks preserving ultra high pressure uhp assemblages it shows high stratal disruption with both block in matrix and highly mixed thrust bounded tectonic slices types of map patterns the melange is part of an arc accretionary complex that was emplaced over a 2 7 2 5 ga passive margin sequence on the western edge of the eastern block of the ncc at 2 5 ga the frontal zone of the accretionary complex marks the suture between the eastern block and an oceanic arc terrane in the central orogenic belt cob and exhibits significant along strike n s variations in the north the jianping pyroxenite melange contains blocks of 2 52 ga metapyroxenite metabasalt tonalite trondhjemite granodiorite diorite ttgd gneisses in a metasedimentary and serpentinite matrix intruded by 2 49 2 47 ga plutons metamorphosed to amphibolite granulite conditions at 2 47 ga the zunhua shangying ophiolitic melange contains relatively intact 2 5 ga ophiolitic sheets in a 2 5 ga metasedimentary melange intruded by 2 48 ga granitic dikes the zunhua ultra high pressure uhp podiform chromite melange has blocks of pillow basalt bif dikes gabbros pyroxenite harzburgite some with 2 55 ga podiform chromites in dunite envelopes enclosed in a 2 52 2 46 ga metasedimentary matrix metamorphosed and intruded by granitic dikes at 2 46 ga the chromites and dunite pods formed through reaction of a boninitic melt and harzburgite during subduction initiation bewtween 2 6 2 55 ga uhp conditions are documented by tio2 ii inclusions in chromites indicating subduction to at least 270 km depth the zanhuang melange includes blocks of metapsammites metagabbros metabasalts and ultramafic rocks in a metapelitic matrix intruded by 2 49 2 54 ga granites then imbricated with 2 65 ga marble metapelite quartzite of the passive margin of the eastern block the dengfeng melange includes upper plate blocks of metagabbro metabasalt bif and felsic rocks in a sandy to pelitic matrix thrust over a turbiditic broken formation melange unit in the east circa 2 5 ga metamorphic conditions record a relatively high t p anatexis 950 c gpa in the west whereas the central part of the belt records intermediate t p metamorphism

plio pleistocene macroplant fossil remains and phytoliths from lowermost bed ii in the eastern palaeolake margin of olduvai gorge tanzania abstract fossil macroplants and phytoliths occur in the sediments of the palaeoanthropological site of olduvai gorge northern tanzania current research on hominin land use in the basin during the plio pleistocene has a strong palaeoenvironmental emphasis on the palaeovegetation in particular the occurrence of macroplants and phytoliths at olduvai and their usefulness and reliability for palaeovegetation interpretation are assessed here for the first time modern reference collections from analogous environments have been established and used to identify the fossil taxa from the eastern palaeolake margin of lowermost bed ii as the test case for very detailed spatially and temporally constrained vegetation reconstruction fourier transform infrared spectrometry ftir analysis was used to identify the minerals of the fossil sediments modern plants produce more phytoliths than are preserved in the fossil soils but there were sufficient numbers to show diverse and changing vegetation over short time periods silicified macroplant fragments of sedge culms and dicot stems occur with phytoliths but do not always represent the same plant taxa a more complex flora was shown when using both forms of fossil plants during lowermost bed ii times at the locality of flkn monocots and dicots occurred indicating the presence of marshland and grassland after the deposition of tuff if at vek the phytoliths indicate a shift in dominance from dicots to monocots to a mixture of the two and including palms at hwkee initially palms dominated then there was a mixture of palms grasses and dicots the vegetation at mck comprised dicots monocots sedges and grasses this detailed vegetation data complements the more broad scale vegetation interpretations that have been deduced from the pollen isotopes and faunal data sets

timing of metamorphism in the paleoproterozoic jiao liao ji belt new shrimp u pb zircon dating of granulites gneisses and marbles of the jiaobei massif in the north china craton abstract the paleoproterozoic jiao liao ji belt lies in the eastern block of the north china craton with its southern segment extending across the bohai sea into the jiaobei massif high pressure pelitic and mafic granulites have been recently recognized in the paleoproterozoic jingshan group jiaobei massif new shrimp u th pb geochronology combined with cathodoluminescence cl imaging of zircon has been applied to the determination of the timing of the metamorphism of the high temperature and high pressure granulites and associated gneisses and marbles metamorphic zircons in these high pressure granulites gneisses and marbles occur as either single grains or overgrowth or recrystallization rims surrounding and truncating oscillatory zoned magmatic zircon cores metamorphic zircons are all characterized by nebulous zoning or being structureless with high luminescence and relatively low th u values metamorphic zircons from two high pressure mafic granulites yielded 207pb 206pb ages of 1956 41 ma and 1884 24 ma one metamorphic zircon from a garnet sillimanite gneiss also gave an apparent 207pb 206pb age of 1939 15 ma these results are consistent with interval of ages of c 1 93 1 90 ga already obtained by previous studies for the north and south liaohe groups and the laoling group in the northern segment of the jiao liao ji belt metamorphic zircons from a high pressure pelitic granulite and two pelitic gneisses yielded weighted mean 207pb 206pb ages of 1837 8 ma 1821 8 ma and 1836 8 ma respectively two diopside olivine phlogopite marbles yielded weighted mean 207pb 206pb ages of 1817 9 ma and 1790 6 ma these paleoproterozoic metamorphic ages are largely in accordance with metamorphic ages of c 1 85 ga produced from the ji an group in the northern segment of the jiao liao ji belt and c 1 86 1 80 ga obtained for the high pressure pelitic granulites from the jingshan group in the southern segment as this metamorphic event was coeval with the emplacement of a type granites in the jiao liao ji belt and its adjacent areas it is interpreted as having resulted from a post orogenic or anorogenic extensional event

structural relationships along a neoarchean arc continent collision zone north china craton the archean north china craton is composed of the western block eastern block and the intervening central orogenic belt a 4 10 km wide and 85 km long tectonic melange belt informally called the zanhuang tectonic melange is documented in the zanhuang massif of the central orogenic belt separating the eastern block from an archean arc terrane in the central orogenic belt the melange belt contains a structurally complex tectonic mixture of metapelites metapsammites marbles and quartzites mixed with exotic tectonic blocks of volcanic mafic and ultramafic rocks metabasalts that locally include relict pillow structures and tonalite trondhjemite granodiorite ttg gneisses the zanhuang tectonic melange marks the suture of an arc continent collisional zone between the western zanhuang massif in the central orogenic belt and the eastern block of the north china craton and it is one of the best preserved archean tectonic melanges in the world here using zircon u pb dating of various types of blocks from the zanhuang melange we show that the formation and associated deformation of the zanhuang melange occurred in the neoarchean ca 2 5 ga high precision 1 20 1 200 lithostructural mapping of three key outcrops reveals details of the internal fabrics and kinematics of the melange and regional structural relationships along the arc continent collisional zone a synthesis of studies on the tectonic evolution of the north china craton coupled with our new fabric and kinematic analysis of the zanhuang melange further constrains the initial amalgamation timing and geometry of the arc continent collision between the fuping arc terrane in the central orogenic belt and the eastern block with a northwest dipping subduction polarity the asymmetric structures and mixture of different blocks and matrices with folding and thrusting events in the zanhuang melange record kinematic information that is consistent with the tectonic setting of an accretionary wedge that was thrust over the passive margin of the eastern block by 2 5 ga lithostructural mapping shows that the classic melange and fold and thrust structures along the neoarchean arc continent collisional zone are broadly similar to phanerozoic collisional belts

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

age and tectonic setting of late archean greenstone gneiss terrain in henan province china as revealed by single grain zircon dating the authors report precise u pb zircon ages for single grains of a metarhyodacite from the late archean dengfeng greenstone belt in henan province china near the southern margin of the north china craton most grains belong to an igneous population whose u pb isotopic systematics define a straight line intersecting concordia at 2512 12 ma and this is interpreted as the time of crystallization of the original greenstone volcanics several grains are distinctly older between 2576 9 and 2945 44 ma and the authors interpret the older grains as xenocrysts of pre greenstone continental crust that provide evidence for crustal derivation or crustal contamination of the original rhyodacitic lava the xenocrysts suggest evolution of the dengfeng greenstone belt in a continental environment that may be represented by the taihua high grade gneisses bordering the dengfeng greenstones and for which the authors obtained ages of 2806 7 and 2841 6 ma the data add evidence to the now widely held concept that most archean greenstones developed on or near older continental crust and were therefore prone to crustal contamination in such cases conventional zircon dating may not always record the precise age of rock formation

the paleoproterozoic north hebei orogen north china craton s collisional suture with the columbia supercontinent abstract understanding the geologic history and position of the north china craton in the paleoproterozoic columbia supercontinent has proven elusive paleoproterozoic orogenic episodes 2 00 1 85 ga are temporally associated with ultimate stabilization of the north china craton ncc followed by the development of extensive craton wide rift systems at 1 85 1 80 ga the age difference between the sedimentary cover and the metamorphic basement is up to 500 700 ma suggesting that uplift and doming of cratonic basement occurred in the latest paleoproterozoic mafic dike swarms 1 80 1 77 ga and anorogenic magmatism 1 80 1 70 ga record the extensional breakup and dispersal of the north china craton during this stage the late paleoproterozoic tectonic framework and geological events documented provide important constraints for reconstruction of the ncc within the late paleoproterozoic supercontinent of columbia an east west striking thousand kilometer long belt of khondalites granulite facies metapelites stretches along the northern margin of the north china craton on the cratonward side of the northern hebei orogenic belt this granulite belt includes mg al sapphirine bearing granulites that reached ultrahigh temperature peak metamorphic conditions of 1000 c at 10 kbars at 1927 11 ma following peak ultrahigh temperature conditions the rocks underwent initial isobaric cooling and subsequent isothermal decompression and these trajectories are interpreted to be part of an overall anti clockwise p t evolution indicating that the northern margin of the craton experienced continental collision at 1 93 1 92 ga the position of the khondalite belt south of the northern hebei orogenic belt makes it analogous to tibet a continental collision related plateau characterized by double crustal thicknesses and granulite facies metamorphism at depth we suggest that the tectonic evolution of the ncc during this period was closely related to the assembly and break up of the columbia supercontinent and that the ncc was adjacent to the baltic and amazonian cratons in the period 2 00 1 70 ga craton wide extension occurred within 100 150 ma of collision along the northern margin of the craton at 1 93 1 92 ga it is concluded that mantle upwellings are chiefly responsible for the breakup of the ncc from the paleoproterozoic supercontinent

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

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

last interglacial beetle fauna from new zealand abstract fossil beetles from two last interglacial lake deposits from southern wairarapa central new zealand are provisionally ascribed to marine oxygen isotope stages mis 5a e both assemblages represent ecological successions from lake margins to forest the lower sample mis 5e is characterized by species found today in northern new zealand these species including lorelus crassicornis dasytes laticeps cryptobius nitidius stenomalium sulcithorax psilocnaeia nana and microbrontes lineatus represent a southward displacement from modern distributions by up to 700 km climate reconstruction indicates that temperatures at the time of deposition were 1 6 2 5 c warmer in the summer january and 2 3 3 2 c warmer in the winter july than at present these results match local and regional pollen and phytolith findings of warmer wetter conditions at the thermal maximum of the last interglaciation in contrast the upper sample is characterized by species that have widespread modern day distributions this indicates that modern conditions were attained later in mis5 after the mis 5e thermal maximum

neoarchean seafloor hydrothermal metamorphism of basalts in the zanhuang ophiolitic melange north china craton abstract epidosites are suggested to form in up flow zones located at the base of ore forming oceanic hydrothermal systems and are commonly preserved in ophiolitic sequences however occurrences of epidosites in archean tectonic melanges are rarely documented in this study we report the presence of late neoarchean epidosites in the zanhuang ophiolitic melange in the north china craton using meter scale litho structural and petrologic analysis zircon u pb dating major and trace element and oxygen and strontium isotopic geochemistry field studies show that the epidosites are mainly preserved as the cores of meta pillow lavas and are distributed as blocks in different matrices within the tectonic melange the pillow rims are used as protolith to estimate the element mobility during the alteration process the epidosites display strong depletion in na2o 0 01 0 27 wt mgo 0 43 3 01 wt depletion in large ion lithophile elements lile e g rb ba cs u and transition metals e g co sc cu ni zn and show enrichment of cao 13 35 21 0 wt sr and pb the high field strength elements hfse e g zr hf ta th of epidosites are varied but rare earth elements ree are unchanged the amphibolitic pillow rims are characterized by enrichment of lile flat heavy rare earth elements hree patterns and depletion in hfse with negative ti anomalies and positive pb and ba anomalies oxygen and strontium isotopic ratios of epidosites are 5 0 8 6 and 0 70284 0 70551 respectively both of which are greater than those of fresh basalts the major and trace elements oxygen and strontium isotopic values and extensive element migration indicate that the protolith was altered by a submarine hydrothermal system in the absence of continental influence magmatic zircons from two samples of epidosites yield 207pb 206pb ages of 2512 13 ma mswd 0 51 and 2542 13 ma mswd 1 4 confirming that the epidosites were generated at ca 2 51 2 54 ga based on geochemical data and field observations we suggest that the protolith of the epidosites formed in an arc related tectonic setting during the late neoarchean this study shows that hydrothermal activity operating in the late neoarchean was similar to that of the modern ocean floor

a late archean tectonic melange in the central orogenic belt north china craton abstract laterally extensive belts of melange characterize phanerozoic convergent plate margins but are rare in archean terranes we document a late archean melange in the zanhuang massif of the north china craton ncc the zanhuang melange separates a passive margin to foreland basin sequence developed on the western edge of the eastern block of the ncc from an arc terrane consisting of trondhjemitic tonalitic and granodioritic ttg gneisses in the central orogenic belt cob of the ncc the melange belt contains a structurally complex tectonic mixture of metapelites metapsammites marbles and quartzites mixed with exotic tectonic blocks of ultramafic and metagabbroic rocks metabasalts that locally include relict pillow structures and ttg gneisses all units in the melange have been intruded by mafic dikes that were subsequently deformed and are now preserved as garnet amphibolite boudins we interpret the melange to mark the suture zone between the eastern block and the arc terrane in the cob field relationships and geochemistry suggest that the exotic ultramafic metagabbroic metabasaltic blocks are possible slivers of an intra oceanic arc or fore arc ophiolite incorporated into the melange during the arc continent collision process a circa 2 5 ga granitic pluton intrudes the melange and undeformed circa 2 5 ga pegmatites cut the melange tectonic models for the evolution of the cob are varied but include models that favor collision at 2 5 ga 2 1 ga and 1 8 ga this work shows clearly from field structural relationships and geochronology that the first collision must have occurred prior to 2 5 ga consistent with late archean suturing of the western margin of the eastern block with an arc terrane fuping terrane during an arc continent collision the presence of an archean melange with exotic blocks in a suture zone between an archean arc and continental margin is clear evidence for the operation of plate tectonics at circa 2 5 ga

the southern hemisphere westerlies in the australasian sector over the last glacial cycle a synthesis abstract the southern hemisphere westerlies in the southwest pacific are known to have waxed and waned numerous times during the last two glacial cycles though even semi continuous histories of the westerlies extend back no more than about 20 000 years we have good evidence for at least three scales of events a westerly maximum occurs at the last glacial maximum there is less conclusive evidence for another westerly maximum in the late holocene and for a minimum at ca 11 ka it is too early to ascribe even a cycle to these data but there are grounds to suggest that milankovitch precessional forcing may underlie the observed pattern there is also a quasi 2600 year cycle present in antarctic ice cores that appears to correlate to variation in westerly flow there is strong centennial scale variability in historical times the little ice age lia ca 1400 1850 was associated with a poleward shift in the circumpolar trough in the southern ocean strengthened westerly circulation over tasmania and a strengthening of southwesterly circulation and neoglaciation in southern new zealand while the preceding period 800 1400 ad was less certainly marked by reduced westerly flow from modern records we know that decadal and inter annual variability is important e g pacific decadal oscillation high latitude mode also known as the antarctic oscillation el nino southern oscillation only a minority of the proxies examined can identify changes on these temporal scales but data from tree rings ice cores and laminated lake sediments do indicate systematic changes in these phenomena through time rossby wave patterns are shown to play a critical role in long duration events as well as at the synoptic scale we conclude that westerly circulation is as strong now as at any time in the last glacial cycle in addition changes in latitudinal boundaries in the westerlies may be nearly as large in inter annual zonal shifts ca 2 maximum as in glaciation interglaciation movements ca 3 4 there is however reasonable evidence of strengthening weakening across much of the westerly belt at westerly maxima minima such as the lia early holocene

early paleozoic to devonian multiple accretionary model for the qilian shan nw china abstract the qilian shan located in the northern tibetan plateau of nw china has an excellent record of early to middle paleozoic subduction accretion that resulted from convergence between the alax and qaidam blocks but there is no consensus about its detailed tectonic history this paper summarizes its tectonic divisions and discusses its tectonic evolution from the cambrian to devonian the belt has the following tectonic divisions in the far north the southern passive margin of the alax block is juxtaposed against an early cambrian to ordovician marianan type intra oceanic arc north qilian the north qilian arc is separated from a japanese type arc central qilian to the south by a high pressure metamorphic belt composed mostly of ophiolitic slices and oceanic crusts the central qilian is bounded to the south by a wide melange zone south qilian consisting of ophiolitic slices and continental margin sequences farther to the south lies the oulongbuluk microcontinent that is separated from the qaidam block and farther south by an ultrahigh pressure metamorphic belt tectonostratigraphic analysis together with geochemical geochronological and geophysical data indicates a complex evolution by subduction accretion processes from the cambrian to the devonian before final amalgamation and docking to the northern alax block this model solves the long lasting discussion on the polarity of subduction in paleozoic time this multiple subduction accretion history sheds light on the continuity of paleozoic sutures along the qilian shan and the nature of the altyn tagh fault and thus contributes to an improved understanding of the tectonic architecture of central asia

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

reconstructing late holocene palaeoenvironments in bangladesh phytolith analysis of archaeological soils from somapura mahavihara site in the paharpur area badalgacchi upazila naogaon district bangladesh palaeoenvironmental reconstruction based on phytolith analysis of late holocene aged soils in and around the somapura mahavihara archaeological monastery site in the paharpur area badalgacchi upazila of naogaon district in northwestern bangladesh was undertaken results indicate five climate zones marked by alternatively cool and temperate events the phytolith assemblages of zones 1 3 and 5 are clearly characterized by a higher proportion of pooideae phytoliths the climate indices for these zones are all greater than 50 indicating cooler climatic conditions in contrast the phytolith assemblages of zones 2 and 4 are typified by a higher proportion of panicoideae phytoliths their climate indices are 42 and 44 respectively indicating the presence of a warmer more temperate climate in general the dominance of grasses over broad leaved trees throughout the sequences suggests that generally cool to temperate and dry conditions with some cyclical variability persisted around paharpur and the surrounding region throughout the pala dynasty from ad 730 to 1080 the question of whether these shifts in vegetation might be linked with el nino southern oscillation events and whether these might in turn have cultural responses is raised

extreme crustal metamorphism during columbia supercontinent assembly evidence from north china craton abstract a vast supracrustal belt of khondalites granulite facies metapelites occur along the northern margin of the north china craton we report here for the first time spinel quartz equilibrium assemblage from these rocks in two textural settings 1 high zno up to 14 47 wt spinel with quartz as inclusions within garnet porphyroblasts defining pressure above 12 kbar and temperature of 900 c and 2 low zno down to 1 2 wt spinel in association with quartz in the matrix assemblage formed during peak ultrahigh temperature conditions ca 975 c and 9 kbar we present a unique case of decompression where the metamorphic conditions of the rocks traversed mostly through the spinel quartz extended stability field monazite grains in textural association with both types of spinel quartz textures were analysed for age determination and the data define two age peaks at 1927 11 ma and 1819 11 ma since the peak thermal regime of the khondalites was close to or exceeded the theoretical closure temperature of pb in monazite we infer the 1819 11 ma age as the timing of ultrahigh temperature event in this craton our data lend support to the idea of ca 1 9 1 8 ga e w collisional orogen at the northern margin of the north china craton we correlate the extreme crustal metamorphism with tectonics associated with the assembly of the north china craton within the columbia supercontinent

geophysical and geological tests of tectonic models of the north china craton abstract the geometry and timing of amalgamation of the north china craton have been controversial with three main models offering significantly different interpretations of regional structure geochronology and geological relationships one model suggests that the eastern and western blocks of the ncc formed separately in the archean and an active margin was developed on the eastern block between 2 5 and 1 85 ga when the two blocks collided above an east dipping subduction zone a second presumes the eastern block rifted from an unknown larger continent at circa 2 7 ga and experienced a collision with an arc perhaps attached to the western block above a west dipping subduction zone at 2 5 ga and the 1 85 ga metamorphism is related to a collision along the northern margin of the craton when the ncc joined the columbia supercontinent a third model suggests two collisions in the central orogenic belt at 2 1 and 1 88 ga but recognizes an early undated deformation event recent seismic results reveal details of the deep crustal and lithospheric structure that support both the second and third models showing that subduction beneath the central orogenic belt was west directed and that there is a second west dipping paleosubduction zone located to the east of the cob dipping beneath the western block ordos craton the boundaries identified through geophysics do not correlate with the boundaries of the trans north china orogen suggested in the first model and the subduction polarity is opposite that predicted by that model high pressure granulite facies metamorphism at 1 85 ga is not restricted to the tnco as suggested by the first model but is documented across the ncc as predicted by the second model suggesting a major continent continent collision along the north margin of the craton at 1 85 ga further it has recently been shown that in the southern tnco there is no record of metamorphism at circa 1 85 ga but only at 2 7 2 5 ga showing that the tnco as defined as a circa 1 85 ga orogen does not exist this is further confirmed by recent re os isotopic studies which show that the subcontinental lithospheric mantle beneath the southern cob is late archean in age and that a province in the northern ncc is circa 1 8 ga correlating with the proposed collision belt of the ncc with the columbia supercontinent across the entire ncc the cob is an archean convergent belt re worked in the paleoproterozoic and the paleoproterozoic tectonism is widespread across the ncc as predicted by the model whereby the previously amalgamated eastern and western blocks experienced a continental collision with columbia at circa 1 85 ga but uplift exhumation rates are slow necessitating a re evaluation of the tectonic models of the ncc

is the ordos basin floored by a trapped oceanic plateau abstract the ordos basin in china has about 10 km of neoarchean to quaternary sediments covering an enigmatic basement of uncertain origin the basement is tectonically stable has a thick mantle root low heat flow few earthquakes and has been slowly subsiding for billions of years the basement has geophysical signatures that indicate it is dominantly intermediate to mafic in composition and is similar to some other cratons world wide and also to several major oceanic plateaus it was accreted to the amalgamated eastern block and central orogenic belt of the north china craton ncc in the paleoproterozoic then involved in several proterozoic tectonic events including being over thrust by an accretionary orogen and intruded by andean arc related magmas and then involved in a continent continent collision during amalgamation with the columbia supercontinent thus the basement rocks are deformed metamorphosed to granulite facies and determining their initial origin is difficult we suggest that the data is consistent with an origin as an oceanic plateau that accreted to the ncc and later experienced different episodes of differentiation associated with later subduction and collisions formation of cratonic lithosphere by accretion of oceanic plateaus may be one mechanism to create stable cratons other cratons that apparently formed by partial melting of underplated and imbricated oceanic slabs are stable in some cases but also re activated and de cratonized in some cases in asia where they have been affected by younger subduction hydration slab roll back and melt peridotite reactions this suggests that the initial mode of craton formation may be a factor in the preservation of stable cratons and de cratonization is not only influenced by younger tectonic activity

the potential of chironomid insecta diptera larvae in archaeological investigations of floodplain and lake settlements the head capsules of chironomids insecta diptera are well preserved in sediments and can be recognised to a high taxonomic level usually genus and sometimes species work on lake sediments has shown that they can be accurate indicators of water temperature oxygen regimes and nutrient status particularly total phosphorus and chlorophyll a as well as heavy metal pollution no systematic chironomid analyses however have previously been undertaken on archaeological deposits in order to address this we assessed the use of chironomids in three types of archaeological deposit firstly a lake core at the edge of a lake settlement crannog secondly a palaeochannel infill adjacent to a multi period settlement site and lastly a roman well deposit from a floodplain environment the chironomid assemblages are shown to vary significantly both between and within the sites and reflect the immediate environment and the adjacent area the lake sediment assemblage reflects the construction of the crannog through elevated levels of organic detritus wood and woody debris the palaeochannel assemblage reveals changing natural conditions and nutrient enrichment probably associated with settlement during the saxon period the well assemblage is taxonomically restricted and indicative of organic debris dead plant material animal dung and possibly human effluent deposited after abandonment of the well dry storage of the palaeochannel and well sediments for several years did not appear to affect the concentration or state of preservation of the head capsules the chironomid reconstructions are shown to provide an additional indicator of human activity which has the potential to provide quantitative data on the character of aquatic environments associated with archaeological sites

geothermal springs of the west florida continental shelf evidence for dolomitization and radionuclide enrichment abstract on the sea bed of the west florida continental shelf about 45 km ssw of ft myers florida an 85 km 2 area has been discovered in which six thermal springs discharge warm chemically altered seawater from vents and seepage zones the spring water apparently originates in the subsurface ocean around the florida platform and penetrates the highly porous strata of the platform about 500 1000 meters below sea level it percolates toward the interior of the platform and is geothermally heated to about 40 c en route then it rises along more vertical flow channels and is discharged in warm submarine springs beneath the platform several chemical processes alter the percolating seawater one process seems to be a secondary dolomitization of the limestone of the platform because in the discharging seawater magnesium is lower by 2 7 mmole kg and calcium higher by 3 6 mmole kg than in normal seawater with the same chlorinity other reactions within the sediments of the platform enrich the spring effluents 1000 fold in 226 ra 10 000 fold in 222 rn and 90 fold in 228 ra compared to the seawater surrounding the platform thus the springs may be important sources of radionuclides for the gulf of mexico the percolating seawater also loses all of its oxygen and nitrate to reduction processes loses most of its phosphate and 40 of its 238 u and roughly quadruples its silica content coastal carbonate platforms are fairly common geological features thus processes like those beneath the west florida shelf may function on a world wide basis to play an important role in the diagenesis of carbonate sediments

reactive transport modeling of early burial dolomitization of carbonate platforms by geothermal convection reactive transport models rtms permit quantitative investigation of diagenesis and its effects on reservoir quality the rtm toughreact is used to investigate diagenesis in an isolated platform driven by geothermal kohout convection of seawater which has been invoked to explain dolomitization during early burial previous short 0 1 m y rtm simulations suggested that convection can drive dolomitization mostly at greater than 50c and anhydritization but complete dolomitization requires greater than 30 60 m y our more extended rtm simulations 30 m y indicate significant nonlinearities in the system consistent with high temperature experiments with parts of the platform completely dolomitized within 10 15 m y as dolomitization proceeds the process becomes predominantly flux controlled with development of a wedge shaped dolomite body which thins from the margin to the interior at considerably shallower depth and cooler temperatures 20 30c than suggested by short simulations dolomitization is relatively insensitive to boundary conditions such as relative sea level and platform geometry but is significantly slower in circular than elongate platforms sediment permeability and reactive surface area commonly inversely related are key controls dolomitization is limited to the margin of low permeability muddy platforms despite a high reactive surface area dolomitization of more permeable grainy platforms is limited by a lower reactive surface area occurring only in the platform core due to widespread cooling sedimentary layering produces a complex diagenetic stratigraphy dolomitization favoring more reactive beds at shallow depth where permeability is not limiting but switching to more permeable beds at depth bank marginal fracturing limits dolomitization of the platform interior whether the fractures are baffles or conduits for flow

subducted sedimentary serpentinite melanges record of multiple burial exhumation cycles and subduction erosion abstract serpentinite matrix melanges give insight into large scale convergent plate margin processes particularly because of the derivation of the serpentinite from oceanic mantle similar to shale matrix melanges a field geologist may easily recognize the sedimentary origins of little deformed serpentinite matrix melanges but melanges within accretionary prisms have undergone significant deformation and recrystallization of matrix serpentinite melanges of the franciscan subduction complex of california have a seemingly intact and foliated matrix such exposures contrast sharply with the granular undeformed sedimentary serpentinite melanges of the coeval great valley group gvg forearc basin deposits that depositionally overlie coast range ophiolite that structurally overlies the franciscan nonetheless franciscan serpentinite melanges display evidence of sedimentary origins including sedimentary breccia composed of exotic block material tolay ridge sedimentary serpentinite breccia panoche pass road basal serpentinite conglomerate with exotic clasts sunol regional wilderness and serpentinite sandstones and conglomerates including a basal conglomerate overlying coherent metagraywacke tiburon peninsula these examples record two burial exhumation cycles to blueschist facies depths in addition a melange breccia in the panoche pass area may have components that record three burial exhumation cycles to blueschist or greater depth exhumation rates for various cycles ranged from about 1 2 to 10 mm year the tiburon peninsula serpentinite melange occupies the structurally highest horizon in the franciscan of the san francisco bay area and regional field relationships indicate deposition at ca 100 ma apparently about 65 ma of subduction erosion non accretion followed initiation of franciscan subduction in this region the oldest franciscan serpentinite melanges are at least 35 ma younger than sedimentary serpentinites of the gvg subduction erosion may have facilitated reworking of forearc sedimentary serpentinite deposits into the trench evidence of multiple burial exhumation cycles indicates reworking of subduction complex material consistent with existence of clastic units that have fossils indicative of significantly older ages than detrital zircon ages

optical dating studies of postglacial aeolian deposits from the south central interior of british columbia canada abstract tests of the validity of optical dating of the fine grained 4 11 m component of aeolian deposits from six sites in south central british columbia using 1 4 ev infrared excitation of potassium feldspars were made making use of known age tephra beds at one site on the western side of the fraser valley samples bracketing the mazama 7 5 7 6 ka cal yrs and bridge river tephras 2 3 ka cal yrs yielded optical ages in accordance with the known ages when a correction for thermal transfer was used the same method was applied to aeolian deposits on the fraser plateau known to have been deposited between 14 and 2 3 ka cal yrs the ages obtained from five samples from four separate exposures spanning 10 km varied from 30 to 94 ka clearly indicating that this sediment was not exposed to sufficient sunlight prior to burial as a result of a detailed investigation it is suggested that the reason is that some grains had been transported over short distances

age dating and the orbital theory of the ice ages development of a high resolution 0 to 300 000 year chronostratigraphy using the concept of orbital tuning a continuous high resolution deep sea chronostratigraphy has been developed spanning the last 300 000 yr the chronology is developed using a stacked oxygen isotope stratigraphy and four different orbital tuning approaches each of which is based upon a different assumption concerning the response of the orbital signal recorded in the data each approach yields a separate chronology the error measured by the standard deviation about the average of these four results which represents the best chronology has an average magnitude of only 2500 yr this small value indicates that the chronology produced is insensitive to the specific orbital tuning technique used excellent convergence between chronologies developed using each of five different paleoclimatological indicators from a single core is also obtained the resultant chronology is also insensitive to the specific indicator used the error associated with each tuning approach is estimated independently and propagated through to the average result the resulting error estimate is independent of that associated with the degree of convergence and has an average magnitude of 3500 yr in excellent agreement with the 2500 yr estimate transfer of the final chronology to the stacked record leads to an estimated error of 1500 yr thus the final chronology has an average error of 5000 yr