sm nd rb sr and u pb geochronology of a grenville terrane in southern mexico origin and geologic history of the guichicovi complex abstract the guichicovi complex forms the metamorphic basement near the western edge of the maya terrane in southern mexico petrographic field studies show that the guichicovi complex like other high grade metamorphic terranes in southern mexico consists of paragneisses and a igneous suite of rocks metamorphosed under granulite facies conditions the sm nd isotopic results yield depleted mantle model ages of 1 4 1 6 ga for the metaigneous rocks and of 1 5 2 0 ga for the metasediments on a sm nd isochron diagram the isotopic results of the metaigneous suite of rocks define an isochron whose slope corresponds to an age of 1 44 ga this age can be interpreted as the time of a major crust forming event discordant u pb zircon data from a felsic orthogneiss plot on a chord intersecting the concordia at 1231 43 and at 975 35 ma the upper intercept age is interpreted as the time of zircon crystallization hence the age of intrusion of the granitic precursor the lower intercept age reflects the time of peak granulite facies metamorphism concordant u pb isotope ratios of a zircon fraction from a charnockite 991 4 ma gives the time of crystallisation of the charnockite garnet whole rock ages sm nd between 911 and 933 ma and biotite whole rock ages rb sr between 866 and 882 ma document the cooling history of the guichicovi complex age and petrography of the guichicovi complex confirm its connection with other granulite exposures in mexico which comprise the oaxaquia microcontinent the results support the hypothesis that the precambrian crust in mexico together with the colombian andes and the adirondacks originated during the mid proterozoic the grenville orogenic event depressed these terranes into the roots of the orogen our data indicate that the guichicovi complex stayed for 70 ma at a deep crustal level before uplift subsequent breakup of rodinia left oaxaquia including the guichicovi complex together with gondwana further collisions of gondwana and laurentia and repeated transpressional interactions in the paleozoic and early mesozoic led to the present day position of grenville fragments in mexico

geochemical insights into provenance of the middle devonian hamilton group of the central appalachian basin u s a abstract in order to better understand the relationship of clastic influx and organic matter accumulation and preservation in the middle devonian acadian foreland basin we conducted a high resolution provenance analysis of a composite section of the hamilton group the hamilton group includes the organic rich marcellus shale one of the most lucrative unconventional shale gas plays in the world and the overlying mahantango formation geochemical analysis of 121 samples from two nearby wells in monongalia county west virginia reveals subtle changes in clay provenance throughout deposition of the marcellus shale and the mahantango formation major element and trace element geochemistry suggest derivation from a felsic igneous granodioritic upper continental crustal source with additional influx from a recycled quartzose sedimentary source all weathering indices indicate increasing chemical weathering intensities throughout deposition of the marcellus shale followed by consistently moderate chemical weathering associated with accumulation of the mahantango formation sm nd isotopic analysis of ten samples throughout the hamilton group yield end values ranging from 7 06 to 11 75 and nd depleted mantle model ages dm ranging from 1 63 to 1 85 ga with ages becoming younger upsection our results suggest that the extrabasinal detritus of the hamilton group originated from a mixed sediment source with clay influx from both rocks associated with the superior craton to the north and northwest dm 2 7 ga and grenville sourced sediments derived from the adjacent acadian fold thrust belt to the east dm 1 4 1 6 ga older sm nd model ages felsic composition and evidence of sediment recycling suggest little to no contribution from the acadian volcanic arc indicating that volcanic tuff of the tioga ash beds represent isolated episodes of volcanic input into the basin model ages end values and trace element geochemistry indicate increased sediment influx from the acadian fold thrust belt throughout deposition of the hamilton group with the highest sediment influx having occurred during mahantango formation deposition increased clay influx from sediment shed off from the acadian fold thrust belt associated with marginal marine regression increased delivery of clastic detritus diluting the flux of organic matter and likely disrupting basin water stratification that aided in preservation of organic matter in the lower marcellus shale the middle devonian acadian basin serves as an example of the influence of detrital influx on organic matter accumulation and preservation in mud dominated depositional systems

tims u series dating and stable isotopes of the last interglacial event in papua new guinea the extensive flight of uplifted reef terraces which occurs along the vitiaz strait on the northern flank of the huon peninsula in png papua new guinea contains a particularly good record of sea level changes in the last 250 ky the huon terraces were the target of an international expedition which took place in july august 1988 in particular we searched for suitable samples for u series dating in a reef complex designated as vii which is correlated with the last interglacial episode and high sea level stand this complex is composed of a barrier reef viib a lagoon and a fringing reef viia twelve corals from these terraces and two corals from the older reef complex viii were selected for analysis the petrography oxygen and carbon isotope compositions and magnesium and strontium concentrations were determined along with the concentrations and isotopic compositions of uranium and thorium 230 th 234 u ages of the corals with 99 aragonite having primary textures and which show u sr ratios around 0 4 10 3 and initial 234 u values close to that of present seawater appear to be reliable the most reliable ages from complex vii corals fall in two tight groups centered at 118 ky and 134 ky corals with 234 u t values higher than 160 and u sr ratios substantially lower than seawater are assumed to have undergone diagenetic alteration which appears to be common in this area the simplest model for sea level height for terrace vii is a continuous rise between 134 and 118 ky alternatively there may have been two periods of rapid sea level rise in contrast in the bahamas there is evidence that sea level remained rather constant over the time interval 132 to 120 ky the absence of ages between 132 and 120 ky in png could be the result of changes in the local tectonic uplift rates during that time or erosion that disrupted the continuous record in any event we find no basis for accepting a single brief time for the age of the last interglacial and applying this age as a precise chronometer for worldwide correlation or as a test of climatic models the older ages reported here precede the milankovitch solar insolation peak at 128 ky and the younger ages are 10 ky after this peak if the present high precision data are correct then it will be necessary to reassess the validity of the milankovitch theory of climatic changes the fundamental issue which must now be resolved is a means of identifying coral samples that have not been disturbed by diagenetic processes

start of the last interglacial period at 135 ka evidence from a high alpine speleothem a detailed study of growth periods of a flowstone from spannagel cave in the zillertal alps austria at 2500 m above sea level a site highly sensitive to climate changes offers unprecedented new insights into pleistocene climate change in central europe flowstone sample spa 52 has a high u content to 116 ppm analyses of this sample reveal that episodes of calcite deposition started at 204 3 ka 135 1 2 ka and 122 ka suggesting that at these times the mean air temperature at this high alpine site was within 1 5 c of the present day condition the beginning of growth at 135 ka corresponds to the ending of the last glaciation and is concordant with a midpoint age for the penultimate deglaciation at 135 2 5 ka as deduced from the absolutely dated oxygen isotope curve in sediments from the bahamas as well as with recent coral evidence from barbados indicating a high sea level already by 135 8 0 8 ka this set of data supports evidence against northern hemisphere forcing of termination ii because the insolation maximum is at 127 ka

climate and vegetation changes 180 000 to 345 000 years ago recorded in a deep sea core off portugal a new high resolution combined marine proxy pollen sequence from the portuguese margin md01 2443 enables direct comparison of ocean and ice volume changes with vegetation development in south west iberia during marine isotope stages mis 7 8 and 9 changes in sea surface conditions on both orbital and millennial time scales are closely mirrored by forest extent indicating a coherence of climate variability between the north atlantic and south west iberia deglacial warming at the start of mis 7 and 9 was interrupted by incursions of subpolar water masses at the site accompanied by reversals in tree population expansion during mis 7e and 9e the close relationship in timing and amplitude between vegetation and offshore conditions broke down early forest collapse points to pronounced cooling and aridification on land which although not accompanied by a change in water masses at the portuguese margin appears to have coincided with abrupt decreases in atmospheric methane concentration recorded in antarctic ice cores the most extreme glacial conditions of mis 8 occurred during the early part followed by an interval of warmer conditions and tree population expansion similar to 263 ka this has sometimes been considered as indicating an early deglaciation but the data presented here show that it was distinct from termination iii which occurred later in line with milankovitch forcing a sub orbital oscillation in both isotopic and pollen records is observed at the end of mis 7 ca 184 ka which appears to be coeval with a recently identified sea level high stand c 2006 elsevier b v all rights reserved

reply to the comment by a p dickin on lead isotope analyses of leached feldspars constraints on the early crustal history of the grenville orogen

peri laurentian pinwarian age oceanic arc crust preserved in the grenville province insights from the escoumins supracrustal belt abstract the remnants of an island arc system of pinwarian age the escoumins supracrustal belt esb are exposed for over 100 km in the low pressure belt of the central grenville province canada and are part of the quebecia terrane the esb mainly consists of amphibolite facies supracrustal rocks with well preserved primary textures saint simeon group ssg and their plutonic counterpart tadoussac intrusive suite tis that provide a record of the evolution of a mesoproterozoic arc system granodiorite 1502 6 ma and tonalite of the tis have calc alkalic oceanic arc signatures positive end 2 66 with old model age 1672 ma consistent with remelting of previously formed juvenile mafic crust and interaction with an older crustal component and are crosscut by younger within plate granitoids tectonically above the tis the lowest formation of the ssg saint paul du nord consists of sedimentary and minor basalt intruded by syn volcanic juvenile end 5 21 tholeiitic gabbro and is interpreted to represent the remnants of an oceanic basin adjacent to an arc the middle formation moulin a baude records an evolution from a subalkaline tholeiitic to transitional arc basaltic andesite basalt and minor calc alkalic volcanic arc 1492 3 ma dacite with crustal derived signature end 0 77 model age of 1936 ma to b transitional to tholeiitic dacite rhyodacite and 1464 38 32 ma rhyolite and c subalkaline juvenile end 3 91 4 92 tholeiitic basalts and feti basalts of arc rift and back arc affinities this formation is interpreted to represent an oceanic arc undergoing rifting finally the upper formation port aux quilles is composed of epiclastic sedimentary rocks with detrital zircon of laurentian affinities indicating proximity of the arc to laurentia but dominated by 1 5 1 47 ga ages showing a maximum age of deposition of 1476 10 ma the end and nd model ages indicate an association of mixed crustal and mantle derived signatures typical of quebecia crust combined with younger depleted mantle derived basaltic magma the data provide new insights on the organisation of quebecia and show that crust formed in an oceanic arc setting during late geon 15 to geon 14 constitutes a significant component of the central grenville province

differentiation of tholeiitic basalt to a type granite in the sept iles layered intrusion canada the undeformed 564 ma sept iles layered intrusion quebec canada is a large igneous body of c 20 000km3 from the base to the top it consists of a layered series dominated by troctolite and gabbro an anorthositic upper border series and a dominantly granitic upper series the parent magma of the layered series is inferred to be an iron rich tholeiitic basalt 48 wt sio2 15wt feot whole rock compositions from the chilled margin dykes cross cutting the layered series and silicic rocks from the upper series display continuous major and trace element geochemical trends ranging from basalts to ferroan metaluminous a type granites 77 wt sio2 initial 143nd 144nd 0 51201 0 51207 and 87sr 86sr 0 70353 0 70548 indicate a juvenile mantle source and minimal contamination by old crust 1 2 during crystallization geochemical modeling using the melts thermodynamic calculator combined with equations predicting mineral melt equilibria from experiments on tholeiitic basalts indicate that basaltic to monzonitic melt compositions are in equilibrium with the troctolites and gabbros of the layered series fe ti oxides saturate early in the layered series after 14 fractionation of plagioclase olivine cumulates further fractionation of fe ti oxide bearing gabbros drives the residual liquids toward silica enrichment and iron depletion major and trace element modeling indicates that the a type granites from the upper series were produced by protracted fractional crystallization of an iron rich basaltic parent magma at a fraction of residual liquid of only 8 the observed relative volumes of mafic cumulates and silicic rocks in the intrusion are in agreement with the calculations most of the intermediate compositions correspond to magmatic mafic enclave bearing granitoids and display geochemical evidence of hybridization intermediate compositions produced by fractional crystallization are scarce and a daly gap occurs from 57 to 67 wt sio2 this gap could result either from the fractional crystallization process or from silicate liquid immiscibility during that compositional interval

insights into north atlantic deep water formation during the peak interglacial interval of marine isotope stage 9 mis 9 foraminifera abundance and stable isotope records from odp site 984 61 25 n 24 04 w 1648 m in the north atlantic are used to reconstruct surface circulation variations and the relative strength of the north atlantic deep water nadw formation over the period spanning the peak warmth of marine interglacial stage mis 9e 324 336 ka this interval includes the preceding deglaciation termination 4 t4 and the subsequent glacial inception of mis 9d the records indicate a greatly reduced contribution of nadw during t4 as observed in more recent deglaciations in contrast with the most recent deglaciation the lack of a significant nadw signal extended from t4 well into the peak interglacial mis 9e and persisted nearly until the transition to the subsequent glacial stage mis 9d although nadw formation resumed during mis 9e only depths greater than 2000 m appear to have been ventilated the poorly ventilated intermediate depth of site 984 2000 m may have resulted on one hand from a general reduction of deep water ventilation by nadw during the study interval or on the other hand from different pathways of the spread of newly formed nadw that bypassed the study location the intermediate depths may have also been invaded by southern sourced waters as the formation of intermediate depth nadw weakened the absence of any significant nadw signal at the water depth of site 984 during the climatic optimum contrasts sharply with subsequent interglacial peaks mis 5e and the holocene despite the perturbed intermediate depth circulation oceanic heat transport northeastward was not interrupted and may have contributed to the relatively mild interglacial conditions of mis 9e

the extent of juvenile crust in the grenville province nd isotope evidence a large nd isotope data set is presented to generate a crustal formation age map for the southwest grenville province placing it in the context of the province as a whole previous studies have shown that most crust in the central and eastern parts of the province is pre grenvillian composed largely of juvenile 1 5 1 7 1 9 and 2 7 ga arc terranes partly reworked by later ensialic arcs and collision zones in contrast new nd data reported here show that juvenile monocyclic crust is restricted to the sw grenville province in part of the central metasedimentary belt interpreted as a failed backarc rift zone there is a clear isotopic distinction between this rift zone and the pre grenvillian crust on the flanks on the flanks of the rift zone there is evidence for two crust forming events comprising an earlier 1 5 ga juvenile pinwarian event and a later 1 4 1 3 ga continental margin arc the elzevirian orogeny based on the geographical distribution of nd model ages in the sw grenville province a revised tectonic model for the grenville orogeny is developed in this model backarc spreading created a rift zone behind a pre grenvillian elzevirian subduction zone that obliquely straddled the edge of a mesoproterozoic continental margin as a result of this geometry the rift zone died out to the north as it propagated into cold archean crust elzevirian subduction was probably halted by the collision of the continental margin with an offshore arc and later with a major continental block giving rise respectively to the shawinigan and the ottawan orogenic events these collisional events can be grouped as parts of the grenville orogenic cycle

age and radiogenic isotopic composition of a late to post tectonic anorthosite in the grenville province the labrieville massif quebec abstract the labrieville anorthosite massif lbv is found in the central granulite terrain of the grenville structural province but it displays no evidence of post emplacement deformation or metamorphism implying intrusion following peak grenvillian metamorphic conditions we report u pb zircon dates of 1008 3 4 ma for border leucogabbro and 1010 5 6 ma for a cogenetic jotunite dike intruding anorthosite we interpret these dates as igneous crystallization ages and regard 1010 ma as a reasonable estimate of the emplacement age for lbv lbv is thus the youngest massif anorthosite yet recognized in north america and its age is consistent with late tectonic emplacement relative to the 1 1 1 0 ga grenville orogeny we also report a u pb date of 1015 1 8 ma for metamorphic zircon in a country rock amphibolite this could reflect the age of grenvillian regional metamorphism or perhaps a later heating episode resulting from the intrusion of numerous late felsic plutons in this area rb sr sm nd and u th pb isotopic compositions for four rock types anorthosite jotunite leucogabbro and a plagioclase megacryst span narrow ranges in each case consistent with comagmatism among these units isr t 1010 ma range from 0 7032 0 7034 and are among the lowest yet reported for anorthosite in the grenville province initial nd values are positive 0 8 to 2 5 like other grenville anorthosites pb isotopic compositions lie near the model mantle evolution curve of zartman and doe 1981 implying no involvement of significantly older crust in the petrogenesis of these rocks collectively these data suggest a source for lbv in the mantle or mafic lower crust lbv is a compositionally extreme anorthosite characterized by alkalic plagioclase an32or12 and high levels of sr 2000 ppm and ba 1000 ppm these properties cannot be attributed to simple crustal contamination of mantle derived basalt we suggest alternatively that lbv s compositional features may be linked with its late tectonic character perhaps reflecting partial melting of mafic lower crust brought about by crustal thickening during the grenville orogeny

mafic and ultrapotassic rocks from the canyon domain central grenville province geochemistry and tectonic implications the canyon domain and the banded complex in the manicouagan area of the grenville province preserve a re cord of magmatic activity from 1 4 to 1 ga this study focuses on 1 4 1 2 ga mafic rocks and 1 ga ultrapotassic dykes geochemistry and sm nd isotopic signatures were used to constrain the origin of these rocks and evaluate the changing role of the mantle with time and tectonic setting from the late evolution of the laurentian margin to the grenvillian orogeny in the manicouagan area the mafic rocks include layers inferred to represent flows homogeneous bodies in mafic migma tite and deformed dykes all of which were recrystallized under granulite facies conditions during the grenvillian orogeny in spite of the complexities inherent in these deformed and metamorphosed mafic rocks we were able to recognize suites with distinctive geochemical and isotopic signatures integration of this data along with available ages is consistent with a 1 4 ga continental arc cut by 1 2 ga non arc basalts derived from depleted asthenospheric mantle with varied degrees of crustal contamination and inferred to represent magmatism in an extensional environment the 1 ga ultrapotassic dykes postdate the grenvillian metamorphism they are extremely enriched in incompatible elements have negative nb anomalies relatively unradiogenic sr isotopic compositions initial 87 sr 86 sr 0 7040 and nd 3t o 15 some dykes have composi tional characteristics consistent with derivation from the mantle ruling out crustal contamination as a major process in their petrogenesis the most likely source region for the ultrapotassic dykes is a metasomatized subcontinental lithospheric man tle with thermal input from the asthenosphere in association with post orogenic delamination

paleozoic peralkaline a type magmatism of the tongbai orogen central china petrogenesis and tectonic implications abstract the precise timing petrogenesis and geodynamic significance of the paleozoic magmatic rocks in the southern qinling tongbai orogenic belt qtob central china are poorly constrained we performed an integrated study of petrology zircon u pb ages geochemistry and hf o nd isotopic compositions of the huangyangshan pluton from the southern qtob to constrain its genesis and tectonic implications a granite sample from the pluton yields a weighted mean zircon 206pb 238u age of 445 6 ma representing the age of the pluton the huangyangshan pluton is dominated by alkaline granite and quartz syenite has a peralkaline signature a nk 0 82 0 96 and belongs to the shoshonite and high k calc alkaline series both the alkaline granite and quartz syenite have geochemical signatures similar to a type granite with high sio2 and total alkali na2o k2o contents high fe numbers and tio2 mgo and ga al ratios and high field strength elements hfses along with low mgo cao and p2o5 contents these samples are enriched in light rare earth elements lrees and have flat heavy rare earth element hree patterns and strongly negative eu anomalies they are enriched in most large ion lithophile elements liles and hfses with significant negative eu ba sr and ti anomalies in primitive mantle normalized trace element diagrams the samples have relatively high zr contents 175 776 ppm yielding minimum magmatic temperatures of 768 c 891 c the huangyangshan pluton has initial zircon ehf t values of 6 6 to 10 0 and whole rock end t values of 2 86 to 3 62 the analyzed zircons yield a mean 18o value of 5 14 0 26 indicating negligible incorporation of hydrothermally altered crustal materials the geochemical signatures of the huangyangshan pluton indicate that the magmas were derived from prompt partial melting of juvenile crust rocks combined with previous results we suggest that the paleozoic huangyangshan peralkaline a type granites resulted from the rifting of a passive continental margin in the northern south china block followed by the opening of the paleotethyan ocean

precise chronology of the last interglacial period 234u 230th data from fossil coral reefs in the bahamas a detailed study of 238 u 234 u 230 th ages was made for different coral species from two bahamian reefs to determine the time scale of the sea level high during the last interglacial period using recently developed mass spectrometric techniques for the measurement of u and th isotopes thirty seven coral samples were analyzed in replicate typical errors at 125 ky are 1 5 ky 2 and 3 ky time intervals appear to be well resolved this high precision permits detailed chronologic study with time resolution adequate to define stages of the reef s history these data demonstrate that the time between transgression and regression of the ocean in the last interglacial was about 12 ky after which sea level fell rapidly at more than 2 m ky the high sea level stand began possibly by 132 ky and certainly by 129 ky ago when sea level reached 6 m above present mean low sea level high sea level was sustained until 120 ky and then fell rapidly this time period covers the milankovitch insolation peak at 65 n but is not sharply defined and the sea level high lasted from about 4 ky before the insolation peak to 8 ky after it we find no evidence for a double peak of sea level rise in the last interglacial episode that is high stands at both 125 and 142 ky there appears to be some discrepancy between this precise chronology for a coral reef on a tectonically stable platform and the chronology assigned to deep sea sediments the present results show that the duration of the last interglacial led to a 12 ky period of high sea level stand which is much longer than the insolation peak the study area consists of two sizeable bank barrier coral reefs of sangamon age that crop out along the coast northwest of cockburn town on san salvador island and at devil s point on great inagua island the initial stage of reef development is not revealed at either site but the main parts of both reefs are well preserved at cockburn town the beginning of the reef was marked by colonization of acropora palmata on hardground areas and possibly other corals at 132 ky ago patch reefs at the southeastern flank of the developing reef were growing by 129 to 126 ky ago by 126 to 123 ky ago the crest of the reef was formed by the frame building coral a palmata and associated patch reefs flourished along its flanks a palmata coral heads grew upward to a mean low sea level 6 m above the present one the reef began to decline as sea level dropped at about 123 to 120 ky ago near two measured profiles at devil s point the top of a rubblestone dominated by a cervicornis and patch reef corals preserved in growth position have been beveled off and represent an ancient wave cut surface 230 th ages on corals below and above this surface indicate that it was cut at about 125 1 ky ago

lead isotope analyses of leached feldspars constraints on the early crustal history of the grenville orogen abstract lead isotopic data on leached k feldspars plagioclases and muscovite provide constraints on the premetamorphic evolution of the grenville orogen in ontario and new york a boundary between reworked archean crust to the northwest and new mantle derived crust generated during mid proterozoic times to the southeast is identified within the northern portion of the gneiss belt of ontario the identification of juvenile mid proterozoic crust at the southeastern margin of the orogen is consistent with previous investigations based on neodymium model ages however near the grenville front lead isotopes indicate the presence of reworked archean crust rather than primary early proterozoic crust postulated on the basis of neodymium model ages this discrepancy between the lead and neodymium isotopic terranes underscores the necessity of using a multidisciplinary approach to identify possible terrane boundaries only through a combination of the lead with neodymium isotopes is it possible to distinguish between crust that formed during a single major orogenic cycle and crust that represents a mixture of old with new mantle derived material the combined neodymium and lead data imply that the grenville orogen in ontario consists essentially of reworked archean crust and juvenile mantle derived material generated around 1 5 ga the grenville orogen involved archean crust that was depleted in thorium and uranium several hundred ma before high grade metamorphism at ca 1 1 ga such a depletion may have a significant impact on the geotherm achieved during orogeny and may explain the lower metamorphic thermal gradients observed for the gneiss belt

coupled uranium isotope and sea level variations in the oceans abstract globally rivers supply uranium to the oceans with excess 234 u relative to secular equilibrium and 234 u taken up by corals can be used for dating in addition the 234 u abundance in sea water at the time the coral was growing can be measured independently the veracity of u series ages used in determining past sea level variations is dependent on selecting pristine corals free from diagenetic alteration a quantitative test for alteration assumes invariant 234 u abundances in the oceans for at least the past half a million years and results from samples outside of a narrow range in modern ocean 234 u abundance are excluded from data sets here we have used previously published data to show that 234 u in the oceans appears to be variable and directly related to changes in sea level not only over long glacial interglacial timescales but also at very short centennial timescales most of the previously discarded data can be used to provide valuable additional sea level information the process permits a unique insight into the interplay between sources and sinks of uranium in the oceans mediated by sea level changes at rates far faster than previously thought possible similar rapid sea level forcing of other trace element abundances in the oceans is likely

reconstruction of quaternary temperature fields by dynamically consistent smoothing conventional methods of palaeoclimate reconstruction provide estimates of climatic parameters using proxy data which have originated from individual sites these reconstructions yield information on the local environment but only limited information on spatial scales that are required for model data intercomparisons we present here a new approach that connects these different scales by an upscaling of the local palaeoinformation together with a dynamically consistent spatial smoothing a probabilistic data based method for local reconstructions is combined with a dynamic constraint on the reconstructed climate parameter which stabilises the reconstruction on the target scale the variational analysis leads to climatological fields being optimised with respect to the proxy data and to the prescribed dynamics in a statistically consistent way this method allows a probabilistic approach of quality control of the palaeodata in terms of their spatial consistency and homogeneity and for an estimation of reconstruction errors the method was applied to palaeobotanical data to reconstruct near surface temperature fields constrained by simple linear dynamics an approximate approach was used to estimate the magnitude of reconstruction errors in terms of standard deviations reconstructed january and july mean temperature of the early eemian 125 000 years bp have errors with a median value of about 1 8 c in january and about 1 1 c in july reconstructions across europe show positive temperature anomalies for scandinavia and near the east coast of the baltic sea in contrast early eemian temperatures were apparently quite similar to those found today in central europe as no drastic differences were reconstructed between the eemian and modern 1961 1990 climate this implies somewhat stronger temperature gradients in the eemian than are observed today

petrogenesis of cambro ordovician subduction related granitic magmas of the quebec appalachians canada abstract in the quebec appalachians cambro ordovician subduction related granitic rocks are found as intrusive bodies aci in the ascot complex volcanic arc and as eroded fragments mgc in the adjacent magog group sedimentary basin major and trace element geochemistry indicates that aci and mgc rocks are typical peraluminous granitoids both suites are lree enriched with minor negative eu anomalies aci granitoids displaying higher mree and hree abundances than mgc ree and trace element data indicate that aci and mgc granitoids are chemically similar but differences in their respective fractionation trends and trace element ratios demonstrate a significant petrological decoupling 143 nd 144 nd are higher in the mgc granitoids than in the aci both suites display negative nd ratios 10 to 3 implying limited juvenile material input sm nd isotopes from both suites suggest that they originated from the anatectic partial melting of a compositionally zoned grenville like continental crust

origin of archean ferropicrites geochemical constraints from the boston creek flow abitibi greenstone belt ontario canada abstract new major element trace element and nd isotope data for non cumulate samples from the fe enriched boston creek flow bcf have permitted the development of a petrogenetic model for the origin of this geochemically unusual flow the data reveal high contents of nb ta lree ti ni co and ir as well as in fe and low contents of al al 2 o 3 tio 2 6 hree y v sc pd and pt compared to komatiitic rocks and typical tholeiitic rocks of similar magnesian content primitive mantlenormalized incompatible trace element profiles for the bcf peak in the ta nb la region la sm n 2 0 la yb n 5 4 th ta n 0 7 and bottom in the al sc region la sc n 11 0 la al n 9 7 and reveal slight depletions in zr and hf relative to mree sm zr n 1 3 and ti nd t 2720 ma values for the flow are 2 5 indicating derivation of the parental melt from a long term depleted mantle source these geochemical characteristics are somewhat similar to those of modern oceanic within plate basalts these unusual geochemical characteristics and comparisons to proterozoic ferropicrite flows at pechenga russia indicate that the bcf is ferropicritic rather than komatiitic in affinity alteration assimilation and magma mixing and crystal fractionation processes cannot explain the bcf therefore the distinctive geochemistry of the bcf is concluded to have been a feature of its mantle source and is attributed to mantle processes models involving melting of core material in the mantle source may explain the enrichment of fe and high ni in the bcf but they are contradicted by the relatively low pt and pd models involving olivine equilibration in the mantle are consistent with the high ni and co similar composition of assumed bcf parental melt equilibrium olivine at 25 kbar and that of modern fe rich lherzolite olivine fo85 and comparison of the mgo and feo content of the melt with those of experimentally derived melts that are saturated in olivine at high pressure models involving garnet equilibration in the mantle could explain the low al hree y v and sc but are contradicted by the very low al2o3 content of the assumed parental melt compared with those of experimentally derived melts of similar feo and mgo content that are saturated in garnet at high pressure alternatively the unusual geochemistry of the bcf can be explained by a two source component mixing model somewhat like those commonly accepted for oceanic within plate basalts petrogenesis the first source component is peridotite depleted by extraction of melt s prior to generation of the bcf parental melt which explains the relatively high compatible element contents and low al hree sc pd and pt contents the second source component is highly enriched small degree melt fractions formed at mantle depths sufficient to stabilize garnet majorite which explains the relative enrichments in fe ti and the highly incompatible elements mixing of the two source components must have occurred immediately prior to melting to maintain the depleted nd isotope composition and may have been facilitated by metasomatic amphibole stabilization at relatively shallow 100 km depths in the mantle

geochemical and srnd isotopic study of charnockites and related rocks in the northern prince charles mountains east antarctica implications for charnockite petrogenesis and proterozoic crustal evolution charnockite plutons were intruded into meso neoproterozoic 1000 ma high grade metamorphic zone in the northern prince charles mountains pcm east antarctica immediately after peak granulite metamorphism in the region detailed geochemical and sr nd isotopic studies were carried out on these plutons and related rocks which enables important constraints to be placed on the regional tectonic setting as well as the origin of igneous charnockites the pcm charnockites are geochemically distinctive characterised by having much higher tio2 p2o5 k2o k2ona2o zr nb y pb la ce and ba and lower mgo cao mg th u srba and rbba than for example i type granites from the lachlan foldbelt the decrease in zr nb y and ce with increasing sio2 sharply contrast with those of the i type granites isotopically the pcm charnockites are relatively uniform and evolved characterised by limited ranges of initial 87sr86sr ratios 0 7063 to 0 7100 initial nd values mainly 4 0 to 5 9 and nd depleted mantle model ages 1 60 to 1 98 ga implying derivation from pre existing crustal sources the geochemical and isotopic features are most consistent with crystal fractionation of dry hot magmas with pyroxene k feldspar plagioclase apatite zircon ilmenite and magnetite as early crystallizing phases although the involvement of a mantle derived magma via afc process cannot be ruled out we consider that the charnockitic magmas were mainly derived from pre existing subduction related crustal sources geochemically and isotopically similar to those of the i type granites the partial melting probably occurred under dry granulitic conditions at elevated temperatures 950 1050 c with orthopyroxene plagioclase and magnetite being residual phases under such conditions elements including k rb ba la ce nd zr nb and y will be strongly incompatible and partition into the melt the relatively low u and th values in the charnockites are probably due to u th depletion in the sources which may have been caused by dehydration and u th removal during amphibolite to granulite transition of the sources we consider that the pcm charnockites and related regional metamorphism resulted from meso to neoproterozoic continental collision between archaean and palaeoproterozoic cratonic blocks in east antarctica the meso neoproterozoic collision was probably a global event possibly related to construction of the rodinia supercontinent during this collisional period or earlier orogenic events in the region e g arc continent collision in the palaeo mesoproterozoic calcareous sediments formed at plate margins or back arc basins would have been tectonically transported to depth release of co2 rich fluids upon tectonic burial may have been responsible for amphibolite to granulite transition without causing dehydration melting to generate i type granites instead subsequent uplift of the dehydrated but fertile granulite crust during a period of crustal rebound may have facilitated decompression melting to produce high temperature water deficient charnockitic melts syn to post collision lithosphere delamination asthenosphere upwelling and magma underplating may have also aided in heating the lower crust above its water deficient solidus temperature

coral indicators of past sea level change a global repository of u series dated benchmarks fossil corals provide valuable data for reconstructing past sea levels as they are often well preserved in the fossil record and can be dated with u series methods here we present a global and internally consistent database of usingle bondth dated fossil coral sea level indicators including full consideration of all known associated uncertainties both vertical and chronological we include carefully determined taxon specific depth distributions rather than blanket depth uncertainty terms as used in most previous work this is based on a synthesis of extensive modern ecological information on depth ranges these ranges are found to be spatially variable between ocean basins between regions and on sub regional scales because depth itself is not limiting instead depth distributions arise from complex physical chemical and biological interactions with coral reef growth distribution and composition one of the main causes for recognition of the greater depth variability of coral taxa has been the routine inclusion of deep diving and rov surveys in coral ecological studies over the past few decades which has broken through the shallow water bias of early surveys by adding frequent observations on deeper occurrences although more are needed it is also clear from our assessment that coral habitat depth distributions must be determined on the species level to reduce uncertainties in reconstructions of past sea levels and that application to sea level studies then requires these studies also to identify fossil corals to the species level samples identified only to the genus level give rise to wide uncertainties in habitat depth and hence sea level our database contains extensive metadata to assist evaluations of dating quality as well as geomorphic and stratigraphic metadata we demonstrate with examples how such metadata can help to evaluate sea level reconstructions for example by identifying outlier points one example discusses the last interglacial lig where we use the available data with their uncertainties to assess probabilistically the time at which local sea levels exceed that of the present which yields a mean age of 124 6 ka with 95 probability bounds at 118 5 and 129 5 ka we conclude with identification of key outstanding issues relating to i current incomplete understanding of tectonic setting including the current lack of independent verification of uplift subsidence rates and reliance of somewhat unsatisfactory and circular use of the elevation of last interglacial deposits ii the depth distributions of coral taxa and iii the complete documentation of stratigraphic geomorphological and other contextual information with suggestions for strategies to address these issues

rediscovery of the mattawa anorthosite massif grenville province quebec we present new field observations and laboratory data confirming the presence of the mattawa anorthosite massif mat whose existence in south central quebec was hinted at more than 35 years ago mat thus represents a newly recognized member of the late to post tectonic 1060 1010 ma andesine anorthosite belt that includes the ch teau richer st urbain and labrieville massifs the dominant rock type at mat is foliated andesine anorthosite or leuconorite and orientations of foliations indicate that the pluton has the shape of a dome mat contains separate core and border zones which are distinguished on the basis of plagioclase composition and concentrations of ga rb sr and ba xenoliths of labradorite anorthosite having ga sr and ba concentrations different from those of the host andesine anorthosites occur sporadically throughout the pluton as well lastly rocks enriched in fe ti and p jotunite oxide apatite gabbronorite nelsonite ilmenitite also occur at mat primarily near the core b

evidence for paleoproterozoic grenvillian and pan african age gondwanan crust beneath northeastern mexico abstract a number of previous studies including two recent reconstructions of rodinia have considered the grenvillian basement of mexico to be continuous with the laurentian basement of texas isotopic studies and u pb dating of zircons from precambrian granitoids that occur as cobbles and boulders in a paleozoic conglomerate exposed in the state of coahuila northeastern mexico indicate the region is underlain by gondwanan rather than laurentian basement essentially nothing was known of the basement before this study because the region is blanketed with phanerozoic sedimentary and volcanic deposits nine out of ten analyzed samples have grenvillian crystallization ages 1232 7 to 1214 2 ma this phase of magmatic activity in northeastern mexico was coeval with the construction of volcanic arcs in east central and southern mexico 1220 1150 ma but was later than the arc magmatism of laurentia e g elzevirian arcs 1230 ma in texas and the grenville province a subset of the grenvillian samples contains xenocrystic 1 85 ga zircons which are the first evidence of paleoproterozoic crust south of the paleozoic ouachita suture in mexico the remaining sample has a pan african crystallization age 580 4 ma and this is the first sample of pan african age basement found in mexico the pan african orogeny marked the assembly of gondwana and laurentia was unaffected by that event two lines of evidence argue that some of the grenvillian rocks of northeastern mexico were affected by a pan african age event first two of the grenvillian clasts have u pb lower intercept ages within error of 580 ma second common pb and nd isotopic compositions of the pan african granite strongly suggest that it was derived either by melting of grenvillian basement or by assimilating massive amounts of that basement thus far no other grenvillian terrane is known to have the exact combination of nd pb isotopic compositions and absolute ages as the coahuila samples although there are some similarities with the santa marta massif of colombia and the cordillera de merida of venezuela

evidence from u th dating against northern hemisphere forcing of the penultimate deglaciation milankovitch proposed that summer insolation at mid latitudes in the northern hemisphere directly causes the ice age climate cycles1 this would imply that times of ice sheet collapse should correspond to peaks in northern hemisphere june insolation but the penultimate deglaciation has proved controversial because june insolation peaks 127 kyr ago whereas several records of past climate suggest that change may have occurred up to 15 kyr earlier2 3 4 5 6 7 8 there is a clear signature of the penultimate deglaciation in marine oxygen isotope records but dating this event which is significantly before the 14c age range has not been possible here we date the penultimate deglaciation in a record from the bahamas using a new u th isochron technique after the necessary corrections for recoil mobility of 234u and 230th and a small age correction for sediment mixing the midpoint age for the penultimate deglaciation is determined to be 135 2 5 kyr ago this age is consistent with some coral based sea level estimates but it is difficult to reconcile with june northern hemisphere insolation as the trigger for the ice age cycles potential alternative driving mechanisms for the ice age cycles that are consistent with such an early date for the penultimate deglaciation are either the variability of the tropical ocean atmosphere system or changes in atmospheric co2 concentration controlled by a process in the southern hemisphere

u th ages obtained by mass spectrometry in corals from barbados sea level during the past 130 000 years the study of the sea level record during the last glacial cycle has primarily proceeded indirectly by means of oxygen isotope measurements on foraminifera from deep sea sediments1 2 the direct approach of dating sea level indicators stagnated during the past decade mainly because the samples required to complete our knowledge of the past glaciations are below the present day sea level3 7 using the 14c ages of acropora palmata samples collected by drilling offshore the island of barbados fairbanks8 presented the first detailed chronology for the last deglaciation this radiocarbon chronology is limited to the past 30 kyr because of the short half life of 14c 5 730 yr we must therefore rely on other dating methods to obtain information for the whole last glacial cycle during the past four years 230th 234u dating of corals by thermal ionization mass spectrometry has been shown to be significantly more precise and accurate than the classical counting method9 14 we have used this technique to measure u th ages in coral samples from the barbados collection and from subaerially exposed outcrops see also ref 15 here we present results bearing on the sea level record for the past 130 000 years we conclude that the last deglaciation started 3 000 years earlier than previously thought and confirm that there were two surges in melt water8 at about 11 kyr and 14 kyr bp before present

insolation values for the climate of the last 10 million years new values for the astronomical parameters of the earth s orbit and rotation eccentricity obliquity and precession are proposed for paleoclimatic research related to the late miocene the pliocene and the quaternary they have been obtained from a numerical solution of the lagrangian system of the planetary point masses and from an analytical solution of the poisson equations of the earth moon system the analytical expansion allows the direct determination of the main frequencies with their phase and amplitude numerical and analytical comparisons with the former astronomical solution ber78 are performed so that the accuracy and the interval of time over which the new solution is valid can be estimated the corresponding insolation values have also been computed this analysis leads to the conclusion that the new values are expected to be reliable over the last 5 ma in the time domain and at least over the last 10 ma in the frequency domain

234u 238u mass spectrometry of corals how accurate is the uth age of the last interglacial period direct measurement of u and th by mass spectrometry recently provided a spectacular improvement in the precision of coral dating by the uth disequilibrium method 1 mass spectrometric data also provide a much better resolution for the examination of subtle diagenetic effects revealed by small variations of the 234u 238u ratio such perturbations may strongly affect the accuracy of the uth chronometer a compilation of all the corals analyzed to date by mass spectrometry shows that most of the corals from terraces of the last interglacial have initial 234u 238u ratios higher than present day seawater in contrast to modern holocene and last glacial corals some samples that have very high 234u 238u initial ratios up to 1 2 and high u concentrations up to 4 ppm were probably contaminated by continental groundwaters however even apparently pristine samples have 234u 238u initial ratios which are still slightly higher than present day seawater mean value 1 160 compared to 1 140 1 150 2 with little overlap between the two distributions this difference in the u initial ratio raises some uncertainty about the accuracy of the uth age determinations of these corals in spite of the fact that the 234u230th ages cluster in a narrow range between 122 and 133 kyr the data could also be interpreted as resulting from slight contamination of corals that are significantly older than 125 kyr two possible explanations may explain these data 1 all these samples may have been diagenetically altered since they all come from surface outcrops which have been directly exposed to precipitations or soil waters for 125 kyr in this case the true age of these corals remains uncertain depending on the timing of the alteration process i e initial late or continuous 2 the second possibility is that some of the differences in 234u 238u ratios measured in 125 kyr cold corals compared to modern seawater may be due to a higher 234u 238u ratio in seawater 125 kyr ago this could result from temporal variations in the weathering regimes of continental land masses

234u 238u ratios and230th ages for hateruma atoll corals implications for coral diagenesis and seawater234u 238u ratios published results for corals that are more than ca 50 ka old from for example barbados and vanuata often yield initial 234u 238u activity ratios that are significantly greater than 1 144 0 004 the value for modern seawater this study reports results for corals from hateruma atoll japan and investigates the effects of diagenesis on the initial 234u 238u ratios of the corals hateruma consists of over 95 reef material of less than 500 ka in age because groundwaters associated with recent carbonates tend not to have elevated 234u 238u ratios the scope for increasing the 234u 238u ratios of hateruma corals through groundwater interaction should be minimal the corals examined range from those which show no petrographic or x ray evidence for diagenetic alteration and are pristine to those containing secondary cements of either aragonite or high or low mg calcite these various phases have been mechanically separated and analysed in agreement with previous assertions the analyses of these separates demonstrates that the presence of calcite should be a cause for rejection of coral samples it is also shown that aragonite cement can form later than coral death and may bias estimates of age and initial 234u 238u ratios of the six pristine corals three have anomalous 230th 238u ratios requiring u loss and or230th addition the remaining fifteen of the eighteen total analyses reported here yield ages from 1 6 ka to 260 ka no evidence for an increase in 234u 238u ratios in past seawater from this period has been found the best estimates for 234u 238u ratios are close to the current seawater value

mass spectrometric 230th 234u 238u dating of the devils hole calcite vein the devils hole calcite vein contains a long term climatic record but requires accurate chronologic control for its interpretation mass spectrometric u series ages for samples from core dh 11 yielded 230 th ages with precisions ranging from less than 1 000 years 2 for samples younger than 140 ka thousands of years ago to less than 50 000 years for the oldest samples 566 ka the 234 u 238 u ages could be determined to a precision of 20 000 years for all ages calcite accumulated continuously from 566 ka until 60 ka at an average rate of 0 7 millimeter per 10 3 years the precise agreement between replicte analyses and the concordance of the 230 th 238 u 234 u 238 u ages for the oldest samples indicate that the dh 11 samples were closed systems and validate the dating technique in general

u and th concentrations and isotope ratios in modern carbonates and waters from the bahamas the short residence times of th and pa in seawater make them very responsive to changes in the ocean environment we use a new multi ion counting technique to make th and pa isotope measurements in seawaters from a near shore environment in which oceanic chemical tracers are not overwhelmed by terrestrial inputs the bahamas an unusual feature of the bahamas setting is the shallow depth of water residing on the bank tops these waters have significantly lower 232th 230th 10 000 than those immediately adjacent to the banks 24 000 31 000 and a 231pa 230th near the production ratio 0 1 the change in 232th 230th and 231pa 230th on the bank tops is explained by almost quantitative removal of th and pa by scavenging and their replacement with a mixture of 230th and 231pa alpha recoiled from the underlying carbonates together with th from dust dissolution analysis of a water profile in the tongue of the ocean which separates the great and little bahama banks allows us to trace the movement of bank top water to depth a distinct minimum in both 232th 230th 13 000 and 231pa 230th 0 5 is observed at 430 m and is interpreted to reflect density cascading of bank top water with entrained carbonate sediment these results suggest that th and pa can be used as water mass tracers in near shore environments uranium concentration measurements on the same waters demonstrate that u is conservative across a range in salinity of 2 psu with a concentration of 3 33 ppb at a salinity of 35 the incorporation of u and th isotopes into marine carbonates has also been assessed by analyzing carbonate samples from the same location as these bahamas waters such incorporation is critical for u th geochronology u isotope analyses demonstrate that seawater 234u averages 146 6 and does not vary by more than 2 5 o and that carbonates capture this value additional high precision measurements 1 o on modern carbonates confirm that all oceans have identical 234u modern marine carbonates are shown to have 232th 230th ratios that reflect the local seawater in which they formed

composition and smnd isotopic data of the lower crust beneath san luis potosi central mexico evidence from a granulite facies xenolith suite quaternary volcanics from san luis potosi slp in central mexico sampled a xenolith suite comprising uppermantle nodules spinell lherzolites hornblende pyroxenites spinel pyroxenites and lower crustal xenoliths the latter consist of granulite facies mafic to intermediate metaigneous rocks and high grade metasediments precursors of the metaigneous suite range from gabbroic to tonalitic compositions whole rock ree patterns permit classification of metaigneous rocks into cumulates and fractionated melts mafic xenoliths consist of plagioclase clinopyroxene orthopyroxene garnet amphibole apatite rutile scapolite garnet has reacted extensively with clinopyroxene forming corona textures of secondary plagioclase orthopyroxene and amphibole intermediate xenoliths are garnet free and quartz bearing all samples display similar ca al zoning patterns documented in orthopyroxene clinopyroxene and plagioclase which are along with the presence of coronas around garnet interpreted as representing partial re equilibration stages from 940 60 c and 7 11 kbar down to lower temperatures and pressures there is no indication of a secondary heating event in these xenoliths sm nd isotopic compositions of the spinel lherzolites indicate a heterogeneous composition of the mantle segment beneath slp similar sr and nd isotopic signatures reveal that the hornblende pyroxenites are high pressure cumulates linked to the host basanites four mainly intermediate metaigneous granulites from a single vent which are free of secondary amphibole and two spinel pyroxenites regarded as the most primitive member of the metaigneous suite define a regression line in a sm nd isochron diagram yielding an age value of 1248 69 ma and an initial nd value of 2 3 metaigneous xenoliths containing secondary amphibole plot significantly above the regression line which is probably due to the introduction of externally derived nd during amphibole formation under retrograde p t conditions discussing the geological significance of the calculated age value there are some arguments for interpreting this age as representing the intrusion age of the magmatic precursors this gives evidence for formation of lower crust of basic to intermediate compositions during the mid proterozoic now situated beneath slp

the geon 14 arc related mafic rocks from the central grenville province the late paleoproterozoic to mesoproterozoic ca 1 7 1 2 ga evolution of the active southeastern margin of laurentia terminated with the grenvillian continental collision and the development of a large hot long duration orogen at ca 1 09 0 98 ga as a result much of the hinterland of the grenville province consists of paleoproterozoic and mesoproterozoic rocks mostly preserved as an imbricate stack of high grade gneisses that represent a potential repository of active margin processes this study presents geochronologic geochemical and isotopic analyses of two granulite facies suites of ca 1 45 1 40 ga mafic tholeiites from the canyon domain manicouagan area central grenville province one suite consists of 1439 76 68 ma high feti mafic sills with end values of 0 4 tdm 2 57 2 72 ga indicate derivation from variably depleted to enriched morb type mantle sources probably in an extensional back arc setting before intrusion in a ca 1 5 ga supracrustal metasedimentary sequence the other

nd isotope mapping of a pinwarian age composite arc belt in the quebecia terrane of the central grenville province canada abstract there continues to be confusion in the geological community concerning the ages of crustal formation and crustal accretion within the central grenville province of quebec to clarify the geological history of crustal growth in the eastern part of the quebecia terrane sixty new nd isotope analyses are presented from a 60 000 km2 area northwest of baie comeau most samples yield pinwarian nd model ages that suggest them to be derived from juvenile oceanic arc terranes however two large slivers with paleoproterozoic tdm ages attest to the existence of hidden blocks of old crust which divide the area of juvenile pinwarian crust into a composite arc belt of smaller accreted terranes the old crustal blocks probably do not reach the present surface but represent an igneous crustal protolith of younger i type magmatism by comparison with a mesozoic tectonic analogue it is proposed that accreted juvenile arc crust with attached slivers of older crust were rifted away from an active margin to the west and translated eastwards until blocked by the previously accreted labradoria terrane although the quebecia composite arc belt consists of over 95 of pinwarian juvenile arc crust the age of arc accretion was post pinwarian the proposed oblique nature of the accretion process may explain the lack of a major collisional orogen at that time this paper shows the usefulness of large scale nd isotope mapping of precambrian crustal terranes to clarify complex processes of crustal formation and arc accretion

geochemical characterization ree nd and pb isotopes of atmospheric mineral dust deposited in two maritime peat bogs from the st lawrence north shore eastern canada dust deposited on two ombrotrophic peat bogs baie and idh bogs of the st lawrence gulf and estuary north shore quebec was geochemically characterized using rare earth element ree concentrations nd and pb isotopes along with particle grain size both cores display similar nd values which suggests either a common source or sources with similar signatures in both regions combining nd isotope data with ree patterns and particle size allowed for better insights into the source of deposited dust and the inference of past environmental and climatic conditions in both regions rees nd and grain size distribution suggest that over the last 2000 years the baie bog received more local dust due to increased local storminess in response to greater regional hydroclimatic variability the same phenomenon occurred in the idh bog since 620 cal a bp i e during the little ice age where hydroclimatic and paleoecological changes have been previously documented while the dust reconstructions and regional climatic records agree relatively well the discrepancies between paleodust records highlight the complex and variable structure of late holocene changes in paleoclimate and more particularly past dust deposition in eastern canada

u pb geochronology geochemistry and provenance of the grenvillian huiznopala gneiss of eastern mexico abstract the huiznopala gneiss is the smallest 25 km2 of four exposures of grenvillian granulites in eastern and southern mexico the gneiss comprises three major lithologic units 1 a main series of orthogneisses 2 an anorthosite gabbro complex and 3 a layered paragneiss sequence thirty one u pb zircon ages from seven samples all lie within a restricted range of 200 million years and the geochronological results are interpreted to reflect principally a two stage history the earliest stage was arc magmatism that extended from 1200 to 1150 ma the second stage included granulite facies metamorphism which peaked at about 725 50 c and 7 2 1 0 kbar and perhaps emplacement of the anorthosite gabbro complex at 1000 ma a pegmatite with an age of 988 3 ma post dates ductile deformation the timing of ductile deformation and granulite facies metamorphism of the huiznopala gneiss corresponds closely to that of final thrusting and deformation in the grenville province thus the huiznopala gneiss may be in the core of the orogen formed during the final thrusting event recognized in the grenville orogeny gondwanan faunal assemblages are found in lower paleozoic sedimentary rocks that overlie grenvillian granulites elsewhere in mexico however the pb isotope compositions of samples from the huiznopala gneiss overlap with those of the basement from laurentia adirondack highlands and lowlands texas and they are distinct from those of the gondwanan arequipa antofalla craton of southern bolivia and northern chile the magmas parental to the huiznopala arc and those parental to the laurentian crust may have shared a common isotope reservoir and the pb isotope compositions are consistent with the paleomagnetic results that place southern mexico adjacent to ontario quebec at about 950 ma

variability in the uranium isotopic composition of the oceans over glacial interglacial timescales uranium series mass spectrometric analyses of corals from the uplifted last glacial terraces at huon peninsula papua new guinea that grew from 50 000 years ago to 30 000 years ago show systematically low values of 234u 238u at the time of coral growth compared with modern corals when combined with coral data from other studies a systematic trend emerges indicating shifts in the 234u 238u ratio at times of major glacial interglacial transitions that involve large variations in sea levels from last glacial to holocene the rate of change in 234u is approximately 1 per thousand years the variations in the u budget of the oceans appear to be due to accumulation of excess 234u in near shore areas in anoxic and suboxic sediments in salt marshes and mangroves in estuaries and in continental margins during periods of warm climate and high sea levels these near shore areas are exposed during periods of low sea level resulting in rapid oxidation of u into highly soluble phases the subsequent release of 234u enriched uranium into the oceans occurs over a sustained period in step with rising sea levels

the southern limit of archean crust and significance of rocks with paleoproterozoic model ages nd model age mapping in the grenville province of western quebec abstract detailed nd model age mapping in the grenville province of western quebec has identified biotite gneisses with crustal formation age intervals of 2 5 2 9 ga and 1 8 2 0 ga although the presence of archean crust in the grenville parautochthonous belt has been known for some time the results of nd model age mapping combined with geochemical and aeromagnetic data indicate that the archean parautochthon extends more than 20 km to the south of the renzy shear belt this belt which was previously defined as the southern limit of archean crust corresponds to an isolated allochthon with nd model ages ranging from 2 0 to 1 4 ga and does not demarcate the allochthon boundary thrust rocks with model ages of 1 8 2 0 ga exposed to the south of the archean crust geochemically possess an arc affinity and are interpreted as a paleoproterozoic arc a model age boundary between archean crust and this arc has been defined by this study the age boundary probably represents a suture along which the southern margin of laurentia and the arc converged

timing and duration of the last interglacial evidence for a restricted interval of widespread coral reef growth abstract we report new mass spectrometric u series ages for eight last interglacial fossil reefs along the continental margin of western australia corals were selected in growth position from localities that are characterized by apparently low levels of diagenesis and relative tectonic stability so that the fossil reefs provide critical information on last interglacial sea levels without requiring corrections for tectonic movements in addition we have improved the constraint on the timing of onset of reef growth by recovering drill core coral from the base of the reefs uranium and thorium isotopes were measured with high levels of precision leading to improvements in age resolution and allowing samples which have undergone diagenetic exchange of uranium and thorium to be more easily identified and discarded these data supplement our previous results for rottnest island and leander point leading to more than seventy mass spectrometric u series ages from which constraints can be placed on the timing duration and character of the last interglacial sea level highstand reliable ages show that reef growth started contemporaneously at 128 1 ka along the entire western australian coastline while relative sea levels were at least 3 m above the present level because western australia is located far from the former penultimate glacial maximum ice sheets and are not significantly effected by glacial unloading these data constrain the timing of onset of the last interglacial period to 128 1 ka assuming reef growth started soon after sea level approached interglacial levels a unique regressive reef sequence at mangrove bay constrains the timing of termination of the last interglacial period to 116 1 ka the major episode of reef building however both globally and locally along the western australian coast is restricted to a very narrow interval occurring from 128 ka and 121 ka suggesting that global ocean surface temperatures were warm and or sea levels were stable enough to allow prolific reef growth only during the earlier part of the last interglacial

history of the greenland ice sheet paleoclimatic insights abstract paleoclimatic records show that the greenland ice sheet consistently has lost mass in response to warming and grown in response to cooling such changes have occurred even at times of slow or zero sea level change so changing sea level cannot have been the cause of at least some of the ice sheet changes in contrast there are no documented major ice sheet changes that occurred independent of temperature changes moreover snowfall has increased when the climate warmed but the ice sheet lost mass nonetheless increased accumulation in the ice sheet s center has not been sufficient to counteract increased melting and flow near the edges most documented forcings and ice sheet responses spanned periods of several thousand years but limited data also show rapid response to rapid forcings in particular regions near the ice margin have responded within decades however major changes of central regions of the ice sheet are thought to require centuries to millennia the paleoclimatic record does not yet strongly constrain how rapidly a major shrinkage or nearly complete loss of the ice sheet could occur the evidence suggests nearly total ice sheet loss may result from warming of more than a few degrees above mean 20th century values but this threshold is poorly defined perhaps as little as 2 c or more than 7 c paleoclimatic records are sufficiently sketchy that the ice sheet may have grown temporarily in response to warming or changes may have been induced by factors other than temperature without having been recorded

foredeep tectonics and carbonate platform dynamics in the huon gulf papua new guinea hawaii mr1 side scan sonar and six channel seismic reflection data reveal a history of carbonate platform growth drowning and back stepping in the huon gulf papua new guinea this is one of the few modern sites where active carbonate platform development and foredeep subsidence are linked 230 th methods date aragonitic shallow water corals recovered from a modern depth of 2000 m at 348 10 ka this documents rapid subsidence of the huon gulf in response to the encroaching finisterre mountains at an average rate of 5 7 mm yr for the past 348 k y the highest subsidence rate reported from any foredeep setting carbonate deposition has moved toward the foreland at an average rate of 110 mm yr over the same period comparisons of the measured age with sea level history derived from the oxygen isotope record suggest that the reefs may have formed during sea level lowstands and drowned during rapid rates of sea level rise

pleistocene sea levels and tectonic uplift based on dating of corals from sumba island indonesia we collected and dated fossil corals from the uplifted reef tracts of sumba island mass spectrometry tims and ams was used to obtain reliable and precise radiochronological results it is possible to identify outcrops corresponding to major sea level high stands isotope stage 1 starting at 7 1 kyr bp 5a c a 86 kyr bp 5c c a 105 kyr bp 5e between 119 and 132 kyr bp and 9 c a 305 kyr bp the radiochronological data are thus broadly consistent with the timing of sea level fluctuations predicted by the astronomical theory of paleoclimates comparison with similar data from other terrace sequences and from benthic 18o records indicates that the average tectonic uplift rate at cape laundi sumba is on the order of 0 2 0 5m kyr the lowest values 0 3m kyr lead to the build up of polycyclic topographic terraces in which it is possible to find corals grown during different interstadial or even interglacial times

neogene quaternary slow coastal uplift of western europe through the perspective of sequences of strandlines from the cotentin peninsula normandy france abstract the cotentin peninsula normandy france displays sequences of marine terraces and rasas the latter being wide late cenozoic coastal erosion surfaces that are typical of western european coasts in portugal spain france and southern england remote sensing imagery and field mapping enabled reappraisal of the cotentin coastal sequences from bottom to top the n cotentin sequence includes four previously recognized pleistocene marine terraces t1 to t4 at elevations

comparison of changes in vegetation in northeast greece with records of climate variability on orbital and suborbital frequencies over the last 450 000 years abstract a new direct pollen orbital tuning procedure based on a correspondence between changes in certain vegetation elements and march and june perihelion configurations is applied to the tenaghi philippon record northeast greece the development of a refined chronological scheme allows comparisons to be made with records of climate variability from the north atlantic as well as of global sea level ice volume and atmospheric co2 content on orbital frequencies the comparison reveals a close correspondence between relative ice volume extent and tree population size during glacial intervals during interglacial and interstadial periods the degree of forest development is more closely associated with high latitude insolation and related climate regimes rather than extent of residual ice volume on suborbital frequencies the greek record shows similar repeat times in peaks of steppe vegetation with north atlantic ice rafting events but the amplitude of this variability in the two records is not always proportional overall what emerges is that the major shifts in the relative abundance of forest v open vegetation communities at tenaghi philippon on orbital and suborbital frequencies over the last 450 000 years are coherent with high latitude changes affecting atmospheric and oceanic circulation glacial decreases in atmospheric co2 content contributed to the elimination of tree populations by exacerbating water stress conditions during arid intervals but do not appear to be the primary driver of the observed vegetation changes at this latitude variations in co2 levels in the order of 40 ppmv between different interglacial interstadials periods with adequate moisture availability appear to have had a limited effect on the nature and size of tree populations

uranium series dating of fossil coral reefs extending the sea level record beyond the last glacial cycle abstract absolutely dated records of past sea level change are extremely important for understanding the advance and retreat of the large ice sheets when combined with other complementary climate archives and climate models such records offer the potential to gain an improved understanding of earth s natural climate cycles providing a firmer basis for assessing the role of anthropogenic effects such as greenhouse gas emissions in modifying climate the u series dating of fossil coral reefs has been widely utilized to provide absolutely dated records of past sea level change and numerous observations now exist for the past 130 000 years spanning the last glacial cycle despite this controversies still exist regarding the exact timing and character of sea level events within this time interval and extending the sea level history further back in time on the basis of robust and independent age constraints for older fossil reefs remains very elusive this is primarily due to a progressive loss in the resolution of the u series chronometer as one goes further back in time coupled to a lack of well preserved dateable coral in older fossil reefs to overcome these limitations the primary challenges are three fold first new analytical protocols are required to improve the resolution of the u series chronometer enhanced analytical precision must be coupled to accuracy through continued refinement of the u series decay constant determinations and via the implementation of rigorous inter laboratory calibration exercises second efforts should continue to be focussed on gaining an improved understanding of the mechanisms controlling open system exchange of the u series isotopes in fossil reef systems this will allow the number of reliable u series observations to be extended third alternative dateable archives of past sea level change must continue to be emphasized to further complement the coral reef database these limitations are discussed in the context of current developments that further advance the application of u series chronology to older fossil reef systems formed prior to the last glacial cycle

crustal formation in the grenville province nd isotope evidence ninety eight new nd isotope analyses are presented for gneissic rocks from the grenville province when combined with over 250 published nd analyses and some unpublished analyses these data can be used to establish a first order crustal formation age map for most of the grenville parautochthon and allochthonous polycyclic belt in particular the geographic extents of a juvenile labradorian arc labradoria and a juvenile 1 5 ga arc quebecia are defined each with an area probably in excess of 100 000 km2 in addition pre 1 75 ga arcs are identified in ontario barilia and labrador makkovikia this work largely fills the last major gap in the continent wide crustal formation age map of the precambrian shield of north america

high precision u series dating of corals from western australia and implications for the timing and duration of the last interglacial abstract u series ages using methods of thermal ionisation mass spectrometry tims are reported for last interglacial fossil reefs along the stable coastal margin of western australia thorium isotope ratios were measured with superior precision using methods of charge collection high levels of precision in the measurement of both uranium and thorium isotopes has reduced the age uncertainty due to analytical errors excluding the uncertainty in the decay constants by a factor of four over the precisions reported by many earlier tims workers uncertainties in 234u t determined from both 230th 238u and 234u 238u are also significantly smaller than previously reported allowing samples which have undergone diagenetic exchange of uranium and thorium to be more easily identified strict criteria were adopted to screen the new western australian data reliable ages range from 127 to 122 ka published tims observations from other localities have been assessed using the same strict criteria when these are combined with glacio hydro isostatic sea level models they indicate that the last interglacial period occurred from at least 130 to 117 ka however these age constraints are largely determined from single data points and need to be verified with additional ages before considering them to be robust estimates for the timing of onset and termination of the last interglacial globally the main episode of reef growth appears to be confined to a narrow interval occurring from 127 to 122 ka in direct agreement with the narrow range in ages obtained from the western australian sites this may indicate that the last interglacial was of short duration extending from 127 to 122 ka only alternatively this interval may reflect a major reef building event in the middle of a longer duration 130 117 ka interglacial interval

uranium series dating of marine and lacustrine carbonates of the possible uranium series dating schemes the most important and most widely applied to marine carbonates is 230th dating with 231pa dating playing an increasingly important role for this reason this review will focus on these two methods 230th dating also referred to as u th dating or 238u 234u 230th dating involves calculating ages from radioactive decay and ingrowth relationships among 238u 234u and 230th 232th is also typically measured as a long lived essentially stable index isotope over the time scales relevant to 230th dating at present 230th dating can in principle be used to date materials as young as 3 years and in excess of 600 000 years edwards et al 1987a 1993 edwards 1988 see stirling et al 2001 for an example of dating corals in excess of 600 000 years old 231pa dating also referred to as u pa dating involves calculating ages from the ingrowth of 231pa from its grandparent 235u at present 231pa dating can be used to date materials as young as 10 years and as old 250 000 years edwards et al 1997 230th dating covers all of the 231pa time range and more with somewhat higher precision and is therefore the method of choice if a single method is applied however the combination of 231pa and 230th dating is of great importance in assessing possible diagenetic mobilization of the pertinent nuclides and thereby the accuracy of the ages allegre 1964 ku 1968 even if the primary age exceeds the 250 000 year limit of 231pa dating the combined methods can be used to assess the degree to which the samples have remained closed over the past 250 000 years e g edwards et al 1997 thus

orbital forcing of the marine isotope stage 9 interglacial milankovitch orbital forcing theory has been used to assign time scales to many paleoclimate records however the validity of this theory remains uncertain and independent sea level chronologies used to test its applicability have been restricted largely to the past approximately 135 000 years here we report u series ages for coral reefs formed on henderson island during sea level high stands occurring at approximately 630 000 and approximately 330 000 years ago these data are consistent with the hypothesis that interglacial climates are forced by northern hemisphere summer solar insolation centered at 65 degrees n latitude as predicted by milankovitch theory

nd and sr isotopic constraints on the source of alleghanian granites in the raleigh metamorphic belt and eastern slate belt southern appalachians u s a abstract pennsylvanian permian granites that intrude the raleigh metamorphic belt and the eastern slate belt of the southern appalachian orogen account for 40 of the total volume of granite produced during the alleghanian orogeny we report major and trace element concentrations and initial nd and sr isotopic compositions from 31 samples of allegahanian granites that intrude these belts to further constrain the nature of the sources of the granites nd isotopic compositions of three raleigh metamorphic belt orthogneisses and two eastern slate belt metavolcanic rocks are also presented to further constrain the nature of the crust in this region of the orogen major and trace element concentrations of raleigh metamorphic belt and eastern slate belt alleghanian granites are similar for every element examined major element concentrations of the alleghanian granites indicate that they are high k 2 o 3 5 wt slightly peraluminous al 2 o 3 cao na 2 o k 2 o 0 96 1 12 trace element compositions of the alleghanian granites are similar to those of granites produced in both volcanic arc and continental collision settings initial nd isotopic compositions of raleigh metamorphic belt granites range from e nd 300 ma 4 4 to 3 0 and initial 87 sr 86 sr ratios from 0 703 to 0 708 eastern slate belt granites have nd 300ma between 2 7 and 2 2 and initial 87 sr 86 sr ratios range between 0 703 and 0 706 depleted mantle model ages of granites from both belts range from 600 to 1070 ma gneisses from the raleigh metamorphic belt and volcanic rocks of the eastern slate belt have e nd 300 ma values of 4 4 to 4 4 similar to those of the alleghanian granites suggesting that anatexis of the crust in these belts could have produced the granites the neodymium isotopic compositions of the gneisses from the raleigh metamorphic belt are significantly more radiogenic than that of grenville crust the granites that intrude the raleigh metamorphic belt also show no evidence of interaction with grenvillian crust suggesting that neither the surface rocks nor unexposed portions of the raleigh metamorphic belt are predominantly composed of grenville crust thus in contrast to previous suggestions the raleigh metamorphic belt is significantly different than the adjoining goochland terrane but is similar to the nearby carolina terrane

the timing of sea level high stands during marine isotope stages 7 5 and 9 constraints from the uranium series dating of fossil corals from henderson island direct dating of fossil coral reefs using the u series chronometer provides an important independent test of the milankovitch orbital forcing theory of climate change however well dated fossil corals pre dating the last interglacial period 130 thousand years ago ka are scarce due to 1 a lack of sampling localities 2 insufficient analytical precision in u series dating methods and 3 diagenesis which acts to violate the assumption of closed system u series isotopic decay in fossil corals here we present 50 new high precision u series age determinations for fossil corals from henderson island an emergent coral atoll in the central south pacific u series age determinations associated with the marine isotope stage mis 9 interglacial and mis 7 5 interstadial periods are reported the fossil corals show relatively little open system u series behaviour in comparison to other localities with fossil coral reefs formed prior to the last glacial cycle however open system u series behaviour is still evident in most of the dated corals in particular percent level shifts in the 230th 238u act composition are observed leading to conventional u series ages that are significantly younger or older than the true sample age this open system u series behaviour is not accounted for by any of the open system u series models indicating that new models should be derived the new u series ages reported here support and extend earlier findings reported in stirling et al 2001 providing evidence of prolific coral reef development on henderson island at not vert similar320 ka most likely correlated with mis 9 3 and subsequent reef development at not vert similar307 ka during mis 9 1 while relative sea level was potentially not vert similar20 m lower than during mis 9 3 the u series ages for additional well preserved fossil corals are suggestive of minor reef development on henderson island during mis 7 5 245 230 ka at 240 3 0 8 and 234 7 1 3 ka all u series observations are consistent with the milankovitch theory of climate change in terms of the timing of onset and termination of the dated interglacial and interstadial periods the best preserved samples also suggest that the oceanic 234u 238u during mis 9 and mis 7 5 was within five permil of the modern open ocean composition

terminations and their correlation with solar insolation in the northern hemisphere a record from a loess section in northwest china a high resolution loess section in northwest china is analyzed for the major climate terminations and their correlation with solar insolation over the past 0 8 ma based on age controls and a grain size age model proposed by porter and an porter s c an z s 1995 correlation between climate events in the north atlantic and china during the last glaciation nature 375 305 308 a timescale for the shagou loess section is constructed which is independent of orbital tuned results the timescale is similar to other loess sections and the mid point ages of the boundary between a paleosol and its underlying loess layer are very close to that of terminations recorded in the marine record shagou section records indicate that grain size changed sharply during the termination process terminations occurred at the time of the maximum or increase of july radiation in the northern hemisphere at high latitudes suggesting that insolation might be a factor affecting terminations in the chinese loess this result supports the milankovitch hypothesis however variations in the amplitude of the insolation and grain size records are not proportionally matched so that the mechanism remains unsolved

quaternary marine terraces on sal island cape verde archipelago abstract the quaternary sedimentary record of sal island includes marine and related aeolian and alluvial fan deposits the substratum of the island is volcanic with ages between 25 and 0 6 ma quaternary marine units generally occur as raised marine terraces forming a broad staircase between elevations of 55 60 m and present sea level terraces include a basal conglomerate overlaid by calcarenite both host corals algae and molluscs a chronostratigraphic framework for the middle pleistocene to holocene units has been generated based on a geomorphologic map of the quaternary landforms and associated deposits and morphosedimentary analysis with support of laboratory dating u series by tims in corals 14 c analyses palaeomagnetic measurements and k ar ages from other literature u series dating of corals from marine terraces provides benchmarks for the last interglacial oxygen isotope substage 5e and holocene deposits the present elevation of the marine terraces and their staircase arrangement suggest a change in vertical movement trend around 330 ka from an uplift to either subsidence or stabilization

a radiometric calibration of the specmap timescale the astronomical theory of climate change asserts that earth s climate is affected by changes in its orbit which vary the seasonal and latitudinal distribution of solar radiation this theory is the basis of the orbitally tuned specmap timescale a key constraint for this important chronology was the mid point of the penultimate deglaciation initially dated to 127 000 years ago recent work suggests this event may be considerably older casting doubt on the astronomical theory the specmap timescale and the accuracy of orbitally tuned chronologies difficulties with u th coral dating of sea level events have impeded progress on this problem because most corals are not closed systems here we use a new approach to u th dating that corrects for open system behavior and produces a sea level curve of sufficient resolution to confidently correlate with specmap over the last 240 000 years permitting a reassessment of both this critical chronology and a central tenet of climate change theory high precision ages for 24 oxygen isotope events provide a 240 000 year chronology for marine 18o records that is independent of orbital tuning assumptions although there appear to be significant differences between the radiometric and orbitally tuned timescales near the lastglacial maximum and at the marine isotope stage 7 6 boundary a comparison of radiometric and specmap ages for identical isotope events suggest that the specmap timescale is quite accurate and that its errors were in general overestimated despite suborbital complexity orbital cyclicity is clearly evident in our record high amplitude sea level oscillations at periods greater than 20 000 years are very close in phase to summer insolation in the northern hemisphere although sea level changes cannot be uniquely tied to a specific season or latitude of insolation forcing the simplest explanation is that long period high amplitude sea level change is linked to northern hemisphere insolation forcing these results validate the principles of orbital tuning and suggest such timescales are generally robust

dating open systems by the 238 u 234 u 230 th method application to quaternary reef terraces abstract the sedimentation conditions and diagenetic processes of biogenic and inorganic carbonates are such that the assumptions of a closed system evolution and the absence of initial th invoked in classical 238 u 234 u 230 th dating methods are very often not fulfilled for such material this problem is of interest for all studies requiring accurate dating of corals or aragonite rich sediments as in palaeoclimatology or neotectonics extending the approach of henderson and slowey nature 404 2000 61 66 we propose a consistent model that takes into account possible initial 230 th excess and where continuous selective redistribution gain or loss of 234 u 234 th and 230 th is controlled by recoil processes this model applied to quaternary marine terraces in barbados marie galante lesser antilles arc and pacific islands is able to explain the large scatter in isotopic compositions observed in the series of cogenetic corals or sediments we show that recoil processes generally tend to increase the 234 u 238 u and 230 th 234 u ratios with time leading to an overestimation of the apparent ages calculated using classical methods on the basis of simple assumptions identical marine initial 234 u 238 u 0 and 230 th 232 th 0 ratios we propose a method for calculating a best estimate of the age of cogenetic series our model offers an alternative way to date old reef terraces where corals are almost systematically altered and where classical dating methods do not apply it may also explain the apparent variations of the 234 u 238 u seawater initial ratio estimated from corals formed in the last 300 ka and the apparent uncoupling between sea level highstands and high latitude northern hemisphere summer insolation our set of equations can be applied to all weathering processes where the mobility of u series isotopes is controlled by recoil

rapid fluctuations in sea level recorded at huon peninsula during the penultimate deglaciation about 140 000 years ago the breakup of large continental ice sheets initiated the last interglacial period sea level rose and peaked around 135 000 years ago about 14 meters below present levels a record of last interglacial sea levels between 116 000 years to 136 000 years ago is preserved at reef vii of the uplifted coral terraces of huon peninsula in papua new guinea however corals from a cave situated about 90 meters below the crest of reef vii are 130 000 2000 years old and appear to have grown in conditions that were 6 c cooler than those at present these observations imply a drop in sea level of 60 to 80 meters after 130 000 years sea level began rising again in response to the major insolation maximum at 126 000 to 128 000 years ago the early about 140 000 years ago start of the penultimate deglaciation well before the peak in insolation is consistent with the devils hole chronology

the timing of high sea levels over the past 200 000 years the 230 th ages and 234 u 238 u ratios were determined for barbados corals that grew during periods of high sea level within the last 200 000 years the similarity of the initial 234 u 238 u ratios of some of the corals to the modern marine value suggests that these samples are pristine and that the marine 234 u 238 u ratio 83 000 and 200 000 years ago was within 2 per mil of the modern value the accuracies of the 230 th ages are evaluated on the basis of the 234 u 238 u values and a model of the behavior of uranium and thorium isotopes during diagenesis for the last three interglacial and two intervening interstadial periods sea level peaked at or after peaks in summer insolation in the northern hemisphere this overall pattern supports the idea that glacial interglacial cycles are caused by changes in earth9s orbital geometry the sea level drop at the end of the penultimate interglacial the last interglacial and a subsequent interstadial period lagged behind the decrease in insolation by 5 000 to 10 000 years

rapid sea level fall and deep ocean temperature change since the last interglacial period we have dated huon peninsula papua new guinea and barbados corals that formed at times since the last interglacial period applying both 230 th and 231 pa dating techniques as a test of age accuracy we show that marine isotope stage mis 5e ended prior to 113 1 8 0 7 kyr when sea level was 319 m during mis 5b sea level was 357 m at 92 6 8 0 5 kyr having dropped about 40 m in approximately 10 kyr during the mis 5c 5b transition sea level then rose more than 40 m during the mis 5b 5a transition also in about 10 kyr mis 5a lasted until at least 76 2 8 0 4 kyr at a level of 324 m at that time combined with earlier data that places mis 4 sea level at 381 m at 70 8 kyr our late mis 5a data indicate that sea level fell almost 60 m in less than 6 kyr 10 6 m kyr during the mis 5 4 transition the magnitude of the drop is half that of the glacial interglacial amplitude and approximatelyequivalent to the volume of the present dayantarctic ice sheet during this interval the minimum average rate of net continental ice accumulation was 18 cm yr likely facilitated by efficient moisture transport from lower latitudes at three specific times 60 6 8 0 3 50 8 8 0 3 and 36 8 0 2 kyr during mis 3 sea level was between 385 and 374 m sea level then dropped to 3107 m at 23 7 8 0 1 kyr early in mis 2 before dropping further to last glacial maximum lgm values and then rising to present values during the last deglaciation times of rapid sea level drop correspond to times of high winter insolation at low northern latitudes and high winter latitudinal gradients in northern hemisphere insolation supporting the idea that these factors mayhave resulted in high water vapor pressure in moisture sources and efficient moisture transport to high latitude glaciers therebycontributing to glacial buildup we combined our sea level results with deep sea n 18 o records as a means of estimating the temperature and ice volume components in the marine n 18 o record this analysis confirms large deep ocean temperature shifts following mis 5e and during termination i deep ocean temperatures changed bymuch smaller amounts between mis 5c and 2 maximum temperature shift in the deep pacific is about 2 whereas the shift at a site in the atlantic is 4 under glacial conditions temperatures at both sites are near the freezing point the shift in the atlantic is likelycaused bya combination of changing proportions of northern and southern source waters as well as changing temperature at the sites where these deep waters form

open system modelling for u series and esr dating of teeth abstract the study of the constituents of teeth dentine and enamel by a combination of esr and u series techniques offer unique constraints on the history of uranium migration into these materials our results imply that it is inappropriate to assume a simplified u uptake model for neither the components of a single tooth nor for the faunal remains of a given site however the combination of both methods may provide a new technique for establishing reliable chronologies for archaeological quaternary sites of up to several hundred thousand years

sw grenville province canada the case against post 1 4 ga accretionary tectonics abstract seven accretionary sutures formed between 1 16 and 1 03 ga have been identified by different authors in the ontario quebec adirondack oqa segment of the mesoproterozoic grenville orogen in canada with one exception the inferred accretionary terrane boundaries lie within or at the margins of the central metasedimentary belt cmb located between the central gneiss belt and the adirondack highlands central granulite terrane however geological geochronological and petrological data suggest that the grenville orogen on both sides of the proposed terrane boundaries sutures preserves a common 1 4 1 03 ga tectonomagmatic history inconsistent with its origin as a post 1 4 ga collage of exotic tectonic blocks features which straddle the proposed 1 16 1 03 ga sutures from the central gneiss belt via the adirondack highlands to the mauricie area include 1 mesoproterozoic continental crust 1 5 1 4 ga forming the host and or basement to younger magmatic and supracrustal suites 2 a 1 35 1 3 ga continental arc remnants of which occur from the cmb boundary zone cmbbz in ontario to the appalachians in the united states built on the 1 5 1 4 ga continental crust 3 intrusions of 1 17 1 13 ga age in the central gneiss belt mafic suite and the adirondack highlands and their quebec extension amcg suite i e anorthosite massifs and related granitoids 4 relics of 1 18 1 14 ga sedimentary basins in the northwestern cmb and the mauricie area we propose that an alternative model can adequately account for the observed geology of this part of the grenville orogen wherein the rocks of the oqa segment were part of an andean type margin between 1 4 and 1 2 ga at 1 35 1 3 ga a continental magmatic arc was built upon the southeastern margin of laurentia represented by the 1 5 1 4 ga mesoproterozoic continental crust the arc split at 1 3 ga forming an ensialic back arc basin relics of which now occur in the northwestern part of the cmb and the back arc basin was flanked to the southeast by an active 1 28 1 25 ga arc collision between the laurentian margin and another continent amazonia occurred at 1 2 ga resulting in closure of the back arc basin and initiation of thrusting along the cmbbz post collisional lithospheric shortening led to convective removal of thickened subcontinental lithosphere upper mantle melting and extension of the overlying crust resulting in widespread magmatic activity at 1 17 1 13 ga including emplacement of the amcg massifs crustal extension generated sedimentary basins now represented by the st boniface sediments in the mauricie area 1 18 to between 1 15 and 1 09 ga and the penecontemporaneous flinton group in the northwestern cmb renewed post collisional granulite facies shortening commenced at 1 12 ga manifested as nappes in the central gneiss belt and thrusting in the mauricie area continued post collisional shortening at 1 08 1 05 ga was more localised resulting in reactivation of thrusting in the cmbbz and initiation of the kinematically compatible tawachiche shear zone along the eastern border of the quebec extension of the adirondack highlands the characteristics of the oqa segment of the grenville orogen can all be accounted for in the context of 1 a 1 4 1 2 ga southeast facing andean type margin to a laurentian upper plate associated with northwest dipping subduction 2 continental collision at 1 2 ga and 3 subsequent continued post collisional shortening without invoking accretion of exotic terranes between 1 4 and 1 0 ga

sm nd evidence for a major 1 5 ga crust forming event in the central grenville province the majority of gray gneisses from an 80 000 km 2 area in the central quebec grenville province define a sm nd isochron that yields an age of 1 53 0 07 ga major element trace element and initial nd isotopic data all point to a subduction related orogenic source for this terrane it is attributed to a major crustal extraction event that formed a mid proterozoic island arc the age of the terrane agrees with published data from the eastern and western grenville province in newfoundland and ontario respectively we suggest that 1 5 ga are fragments were accreted along much of the length of the laurentian foreland within 100 m y of their formation in an event marked by widespread granitoid plutonism

enriched grenvillian lithospheric mantle as a consequence of long lived subduction beneath laurentia geochemical and nd isotopic data from mafic and newly discovered ultramafic rocks in the adirondack lowlands suggest widespread enrichment of the lithospheric mantle under the grenville province incompatible element abundances and previously published hf t dm zircon depleted mantle model age and nd t dm ages from rocks of the anorthosite mangerite charnockite granite suite in the adirondack highlands document similar enrichment in the lower crust and its strong influence on subsequent magmatic events throughout the ontario quebec adirondack segment of the grenville province likely the consequence of long lived ca 1 4 1 2 ga northwest directed subduction along the southeast edge of laurentia previously proposed andean margin this enrichment is similar to that associated with the vast 240 000 km 2 ultrapotassic province of the western churchill province enrichment of the lithospheric mantle beneath orogenic belts is a predictable and important differentiation process that has operated on earth for at least the past 3 b y

sea level during the penultimate interglacial period based on a submerged stalagmite from argentarola cave italy accurate constraints are presented for the absolute age duration and sea level for the highstand corresponding to marine isotope stage mis 7 1 our work is based on a stalagmite collected in a currently submerged cave from italy argentarola cave flooding of this coastal cave is clearly marked within the stalagmite by a thick marine overgrowth of serpulid worms precise uranium thorium u th ages measured by thermal ionization mass spectrometry suggest that the stalagmite growth varied in phase with climate during mis 7 2 and mis 6 the penultimate glacial period in addition the period of marine transgression is bracketed between 202 and 190 kyr bp defining a mis 7 1 duration of 11 2 kyr comparison with previous studies provides a new constraint on the sea level during mis 7 1 that has approximately remained between 318 and 39 m relative to present sea level the timing of the mis 7 1 highstand is in full agreement with the specmap chronology which contrasts with the growing body of evidence showing a significant phase difference for mis 5 5 a tempting conclusion is that second order sea level highstands such as mis 7 1 may be purely driven by astronomical changes in contrast with major terminations such as termination ii atmospheric co2 levels may partly explain this contrast since the two sea level transitions mis 7 2 7 1 and mis 6 5 5 are characterized by very different co2 rises 20 vs 100 ppm fl 2002 elsevier science b v all rights reserved

nd isotopic evidence for juvenile crust in the carolina terrane southern appalachians nd isotopic analyses of whole rock samples from the older portion of the carolina terrane one of the largest terranes in the appalachian orogen demonstrate that part of this terrane is composed of juvenile mantle derived crust these data suggest that the terrane may not have originally been built upon old evolved basement material but rather may have been built upon oceanic crust a recent study by other workers demonstrates a more crustally evolved nd isotopic signature for younger components of the carolina terrane these data may indicate that the terrane interacted with evolved crust at a later time possibly by amalgamation with a more evolved crustal fragment before final accretion to laurentia rather than indicating a primary old basement a juvenile nature for the older portion of the terrane contrasts with models that suggest it is an evolved crustal fragment that formed in a continental margin setting a scenario proposed to explain the high proportion of felsic volcanic rocks within the terrane it is herein suggested that carolina is a chemically evolved but isotopically juvenile crustal fragment because it remained in an oceanic setting for an unusually long time in this regard the carolina terrane is similar to some of the large accreted terranes in the canadian cordillera such as wrangellia and alexander the presence of juvenile crust in the carolina terrane documents that at least part of the southern appalachian orogen is not composed solely of reactivated pre existing continental crust the importance of this part of the orogen in terms of the volume of juvenile phanerozoic crustal material in north america may be larger than previously thought however until additional major appalachian terranes have been isotopically characterized the volume of juvenile crust in the whole orogen remains unknown the isotopic make up of a terrane can be an important aspect of terrane analysis as different terranes may have significantly different isotopic compositions while even widespread pieces of a single terrane should have very similar isotopic characteristics the nd isotopic data for the carolina terrane form the beginning of an isotope database for terranes in the southern appalachians

40ar 39ar and nd sr isotopic characteristics of mid ordovician north american k bentonites a test of early paleozoic laurentia gondwana interactions highly altered tephra layers k bentonites of mid ordovician age are widespread throughout the midcontinent region of the united states and the appalachian taconic foreland basin the numerous ash layers their wide areal extent and their large volume provide evidence that extensive volcanic activity accompanied taconic orogenesis the mineralogical and geochemical characteristics of these widespread ordovician tephra layers can be used to constrain the nature of their source region and thereby help test recent hypotheses that gondwana collided with laurentia in mid ordovician time initial 143nd 144nd and 87sr 86sr ratios of two k bentonites that occur traced throughout the midcontinent are 0 51183 0 51187 and 0 7101 0 7120 respectively isotopic ratios for k bentonites within the taconic basin are 0 51178 0 51201 and 0 70564 0 70905 the isotopic composition of the tephra layers and the ubiquitous presence of proterozoic zircon xenocrysts within the k bentonites is consistent with an origin by crustal anatexis the cooling history of the source is constrained by 842 996 ma 40ar 39ar apparent ages of hornblende xenocrysts contained within two taconic basin k bentonites a possible south american source region of the k bentonites is the arequipa massif of peru this region contains early paleozoic arc related rocks many of which contain mid proterozoic zircon xenocrysts and early proterozoic gneisses which were affected by grenville age granulite grade metamorphism gneisses within the adirondack highlands of the grenville province have similar end 450 ma values and hornblende cooling ages to the k bentonites consistent with this region as a source caradocian k bentonites in sweden have sr isotopic ratios similar to the north american beds if the swedish and north american k bentonites were produced from the same source then baltica and laurentia were proximal in ordovician time precluding a gondwana laurentia link if caradocian k bentonites with north american characteristics are found in western south america support for a laurentia gondwana link would be established if such k bentonites cannot be found considerable doubt would be cast on this model

origin and provenance of spherules and magnetic grains at the younger dryas boundary abstract one or more bolide impacts are hypothesized to have triggered the younger dryas cooling at 12 9 ka in support of this hypothesis varying peak abundances of magnetic grains with iridium and magnetic microspherules have been reported at the younger dryas boundary ydb we show that bulk sediment and or magnetic grains microspherules collected from the ydb sites in arizona michigan new mexico new jersey and ohio have 187os 188os ratios 1 0 similar to average upper continental crust 1 3 indicating a terrestrial origin of osmium os in these samples in contrast bulk sediments from ydb sites in belgium and pennsylvania exhibit 187os 188os ratios 1 0 and at face value suggest mixing with extraterrestrial os with 187os 188os of 0 13 however the os concentration in bulk sample and magnetic grains from belgium is 2 8 pg g and 15 pg g respectively much lower than that in average upper continental crust 31 pg g indicating no meteoritic contribution the ydb site in pennsylvania is remarkable in yielding 2 to 5 mm diameter spherules containing minerals such as suessite fe ni silicide that form at temperatures in excess of 2000 c gross texture mineralogy and age of the spherules appear consistent with their formation as ejecta from an impact 12 9 ka ago the 187os 188os ratios of the spherules and their leachates are often low but os in these objects is likely terrestrially derived the rare earth element patterns and sr and nd isotopes of the spherules indicate that their source lies in 1 5 ga quebecia terrain in the grenville province of northeastern north america

a comparison of nd model ages and u pb zircon ages of grenville granitoids constraints on the evolution of the laurentian margin from 1 5 to 1 0 ga terra nova 24 7 15 2012 abstract concordance and discordance between nd model ages and u pb ages is still widely misunderstood in the geological community in the sw grenville province plutonic orthogneisses often yield tdm ages significantly older than u pb crystallization ages but this can be understood in a model where a series of crustal nd isotope evolution lines departs at intervals from the depleted mantle dm evolution line seven samples with concordant u pb and nd model ages have initial e nd on the dm line attributed to melting of juvenile crust generated in an elzevirian back arc rift zone in contrast plutons with discordant u pb and nd model ages lie near crustal evolution lines defined by suites of basement gneisses on the flanks of the rift zone this study shows that nd data from large suites of geographically constrained undated grey gneiss provide a crustal evolution framework for orogen scale interpretation of more scattered but precisely dated plutons

a global compilation of coral sea level benchmarks implications and new challenges i present a quality controlled compilation of sea level data from u th dated corals encompassing 30 studies of 13 locations around the world the compilation contains relative sea level rsl data from each location based on both conventional and open system u th ages i have applied a commonly used age quality control criterion based on the initial 234u 238u activity ratios of corals in order to select reliable ages and to reconstruct sea level histories for the last 150 000 yr this analysis reveals scatter of rsl estimates among coeval coral benchmarks both within individual locations and between locations particularly during marine isotope stage mis 5a and the glacial inception following the last interglacial the character of data scatter during these time intervals imply that uncertainties still exist regarding tectonics glacio isostacy u series dating and or coral position to elucidate robust underlying patterns with confidence limits i performed a monte carlo style statistical analysis of the compiled coral data considering appropriate age and sea level uncertainties by its nature such an analysis has the tendency to smooth obscure millennial scale and finer details that may be important in individual datasets and favour the major underlying patterns that are supported by all datasets this statistical analysis is thus functional to illustrate major trends that are statistically robust what we know trends that are suggested but still are supported by few data what we might know subject to addition of more supporting data and improved corrections and which patterns data are clear outliers unlikely to be realistic given the rest of the global data and possibly needing further adjustments prior to the last glacial maximum and with the possible exception of the 130 120 ka period available coral data generally have insufficient temporal resolution and unexplained scatter which hinders identification of a well defined pattern with usefully narrow confidence limits this analysis thus provides a framework that objectively identifies critical targets for new data collection improved corrections and integration of coral data with independent stratigraphically continuous methods of sea level reconstruction

millennial scale variability to 735 ka high resolution climate records from santa barbara basin ca 1 determining the ultimate cause and effect of millennial scale climate variability remains an outstanding problem in paleoceanography partly due to the lack of high resolution records predating the last glaciation recent cores from santa barbara basin provide 2500 5700 year windows of climate with 10 50 year resolution ages for three cores determined by seismic stratigraphic correlation oxygen isotope stratigraphy and biostratigraphy date to 293 ka mis 8 450 ka mis 12 and 735 ka mis 18 these records sample the late pleistocene during which the 100 kyr cycle strengthened and the magnitude of glacial interglacial cyclicity increased thus these records provide a test of the dependence of millennial scale behavior on variations in glacial interglacial cyclicity the stable isotopic 18o composition of planktonic foraminifera shows millennial scale variability in all three intervals with similar characteristics duration cyclicity to those previously documented during mis 3 at this site stadial g bulloides 18o values are 2 75 1 75 average 2 25 and interstadial values are 1 75 0 5 average 1 with rapid decadal scale interstadial and stadial initiations of 1 2 as in mis 3 interstadials lasted 250 1600 years and occurred every 650 1900 years stadial paleotemperatures were 3 5 9 5 c and interstadial paleotemperatures were 7 5 13 c upwelling evidenced by planktonic foraminiferal assemblages and 13c increased during interstadials similar to mis 3 high productivity during some stadials was reminiscent of the last glacial maximum this study builds upon previous records in showing that millennial scale shifts were an inherent feature of northern hemisphere glacial climates since 735 ka and they remained remarkably constant in the details of their amplitude cyclicity and temperature variability

styles of proterozoic crustal growth on the southeast margin of laurentia evidence from the central grenville province northwest of lac st jean quebec the southeast margin of laurentia was a very long lived active continental margin part of whose history is recorded in the grenville province of the canadian shield within this province nd isotope mapping can be used to define the boundaries between terranes with a variety of crustal formation ages and can also distinguish between crustal growth by oceanic and continental arc magmatism the former gives rise to large terranes with homogeneous nd isotope signatures and well defined boundaries whereas the latter leads to areas with heterogeneous nd isotope signatures one of the best examples of continental arc magmatism in the grenville province is provided by the region northwest of lac st jean quebec eighty new nd isotope analyses are used along with aeromagnetic data to divide this area into three blocks bounded by abrupt changes in nd model age the western block consists almost exclusively of tonalitic grey gneisses with archean model ages the eastern block is composed almost exclusively of

centennial scale cycles in middle holocene sea level along the southeastern swedish baltic coast submerged sea grasses and stoneworts growing in the immediate nearshore of the baltic sea are sensitive indicators of sea level the alternating changes in concentrations of sea grass and stonewort macrofossils obtained from an ancient lagoon of the baltic sea are ascribed to middle holocene sea level fluctuations along the southeastern swedish coast abundances of stonewort and sea grass macrofossils between 7500 and 6000 cal calibrated yr b p were lower and less variable indicating high and relatively stable sea level low frequency fluctuations of sea level with a 1470 yr period during this time may have been forced by tidal action in the millennial band after 6000 cal yr b p sea level was lower and exhibited strong fluctuations at centennial time scales as evident in the stonewort and sea grass macrofossil records such a high frequency oscillatory mode of sea level may have been regulated by centennial scale solar activities or tidal actions or both the modest variation in solar irradiance is unlikely to have resulted in large scale oscillations of sea level by directly changing the steric component instead it governed sea level fluctuations by changing the regional storminess through a north atlantic oscillation nao like system storms embedded in the nao induced southwesterly winds played a major role in the centennial scale fluctuations of the baltic sea level by pushing saltwater from the north sea into the baltic basin less

paleoproterozoic crustal history of the southwestern grenville province evidence from nd isotopic mapping nd isotopic mapping in the north bay area of the central gneiss belt southwestern grenville province has revealed the precise trend of a tdm model age line developed between the uplifted southern margin of the archean superior craton tdm 2 7 ga and a paleoproterozoic allochthon tdm 1 9 ga separating these two crustal blocks is a narrow zone of gneisses with intermediate tdm ages these transitional gneisses are interpreted to reflect a remnant fault or ductile shear zone of uncertain age along which crustal material from both blocks mechanically mixed during their juxtaposition accordingly the nature of the tdm line in the north bay area is interpreted to be tectonic in the temiscaming area widespread exposures of mature metasedimentary gneisses are shown by their tdm ages to be dominantly of paleoproterozoic provenance these results are consistent with the existing detrital zircon geochronology inferring a maximum depositional age of 1 7 ga the anorogenic chemistry of the north bay o

proterozoic orogens of the northeastern canadian shield new information from the lithoprobe ecsoot crustal reflection seismic survey as part of the eastern canadian shield onshore offshore transect ecsoot lithoprobe acquired 1250 km of deep seismic reflection data along the coast of labrador and across ungava bay to image evidence of proterozoic crustal accretion to the archean nuclei of the nain and superior provinces of the canadian shield the relatively pristine archean crust of the nain province has low reflectivity and generally lacks systematic reflector orientations reworking of archean crust on the margins of the makkovik province has little effect on this weak signature in contrast the archean crust in the eastern churchill rae province appears to have been overprinted by a strongly developed whole crustal easterly dipping reflection fabric interpreted to result from proterozoic collision of the nain and superior provinces in the paired new quebec and torngat orogens juvenile proterozoic crust in the makkovik and grenville provinces also shows strong whole crustal dipping reflection fabrics interpretable as outw

u series evidence for widespread reef development in shark bay during the last interglacial field observations and u series ages reveal that shark bay western australia wa has been inundated by the sea on at least three occasions during the late pleistocene holocene resulting in a succession of marine deposits around the bay the exact age of these deposits has until now been problematic due to a lack of reliable and accurate age data this study reports 16 new u series coral dates from emergent reef deposits around shark bay and point to an extended period of coral reef development during the peak of the last interglacial marine isotope stage mis 5e this is attributed to enhancement of marine circulation within the reaches and basins a result of higher sea levels and an absence of major sill and bank structures stromatolites are absent from the geological record within shark bay until the late holocene suggesting that sea levels and marine sedimentary processes that have operated during the present sea level highstand are unique to this period there is little direct evidence of fossil reef development occurring during interglacials of the middle late pleistocene mis 9 11

petrology and isotopic composition of a grenvillian basement fragment in the northern appalachian orogen blair river inlier nova scotia canada mesoproterozoic metaplutonic rocks in northern cape breton island introduction nova scotia occur in a tectonic inlier within the appalachian in eastern north america mesoproterozoic laurentian orogen although they have been multiply metamorphosed and basement rocks and their neoproterozoic to mid paleovariably deformed the petrology and geochemistry of these rocks zoic volcanic and sedimentary cover preserve a record provide insight into the tectonomagmatic evolution of easternmost of the proterozoic to mid paleozoic geologic history of laurentia anorthosite syenite and granitoid plutons eastern laurentia the record includes mesoproterozoic 1100 980 ma intruded the sailor brook and polletts cove river orogeny on the ancient north american craton breakgneisses new nd isotopic data are presented from a biotite rich up of a neoproterozoic supercontinent development of part of the sailor brook gneiss ndi 0 7 two anorthosite a passive continental margin and mid paleozoic orogeny samples ndi 2 1 and 2 8 and a charnockite unit exposures of mesoproterozoic laurentian basement in ndi 0 4 new u pb zircon data from the anorthosite yield easternmost canada and the usa occur in a dispb pb ages between 975 and 1095 ma disrupted u pb continuous band of inliers in the appalachian orogen systematics preclude a unique age interpretation but the 1095 ma from newfoundland to alabama fig 1 they consist single grain date is a minimum age for the anorthosite field relations mostly of rocks formed or deformed during major and trace element geochemistry and isotopic characteristics 1190 980 ma grenvillian tectonic and magmatic indicate that the anorthosite and charnockite were probably coexisting events and are distinct from neoproterozoic and melts but not differentiates of a single parent magma the lithological paleozoic units of accreted appalachian terranes in that and chemical characteristics of the lowland brook syenite are they include granulite facies metamorphic rocks and similar to those of a contemporaneous belt of ultrapotassic plutons in many cases anorthosite charnockite mangerite suites in the southern grenville province and both have the chemical bartholomew 1984 and papers therein pettingill et al characteristics of certain modern continental margin arc magmas 1984 rankin et al 1989 1993 owen erdmer 1990 these data indicate that the mesoproterozoic units in the blair hughes et al 1997 river inlier were juvenile crustal additions to eastern laurentia the sinuous trend of the band of basement inliers during grenville related orogenic events broadly reflects promontories and re entrants that formed

u pb geochronologic nd isotopic and geochemical evidence for the correlation of the chopawamsic and milton terranes piedmont zone southern appalachian orogen abstract we report u pb crystallization ages from four metavolcanic rocks and two granitic gneiss samples as well as whole rock chemical analyses and sm nd isotopic ratios from 25 metaigneous and metasedimentary rocks from the chopawamsic and milton terranes southern appalachian orogen a metarhyolite sample from the chopawamsic formation and a metabasalt sample from the ta river formation in the chopawamsic terrane have indistinguishable u pb crystallization ages of documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape 471 4 pm 1 3

architecture of a 1 38 1 34 ga granite rhyolite complex as revealed by geochronology and isotopic and elemental geochemistry of subsurface samples from west texas usa abstract during the evolution of laurentia a mesoproterozoic felsic igneous belt extended from fennoscandia through canada to the southwestern united states this belt referred to as the granite rhyolite province in north america forms much of the west texas and eastern new mexico basement we present data from 41 closely spaced wells in west texas that penetrated several hundred meters into the southern granite rhyolite province and provide the first opportunity to develop a three dimensional view of the basement from subsurface samples the felsic rock types include hornblende bearing quartz monzonite ignimbritic rhyolite and comagmatic granite with eutectic textures these rocks plot in the high k to ultra high k field quartz syenite which plots exclusively in the ultra high k field is the final felsic phase of magmatism all of the felsic magmas were variably oxidizing based on biotite compositions u pb chronology supports the intrusive relations established by petrography the quartz monzonite is oldest 1380 ma and is followed by the granite rhyolite sequence 1360 ma and the quartz syenite 1340 ma nd model ages range from 1520 to 1740 ma indicating involvement of older proterozoic crust in many wells thick mafic sills intrude the granite rhyolite sequence nd model ages for the mafic rocks range from 1560 to 1440 ma implying that they are mesoproterozoic in age the mafic rocks include an alkaline oib like suite which is not supportive of a subduction origin u pb ages are also reported for the mescalero well 1 in eastern new mexico in this well metasedimentary and metavolcanic rocks of the debaca sequence unconformably overlie a quartz syenite similar to the quartz syenite from the west texas wells detrital zircons from the basal meta arkose of the sequence are 1690 and 1320 ma indicating a maximum age of 1300 ma for this sequence

mesoproterozoic lamprophyres in the labrieville massif quebec clues to the origin of alkalic anorthosites mesoproterozoic lamprophyre dikes occur at four localities within the 1010 ma labrieville anorthosite in the grenville province of southern quebec all dikes are thin 20 cm wide oriented n57 w and steeply dipping three are biotite lamprophyres but the fourth is dominated by amphibole laser total fusion 40ar 39ar analyses on amphibole or biotite yield minimum ages for the dikes of 945 5 8 ma 907 0 5 9 ma 892 0 6 2 ma and 889 2 9 8 ma a crystallization age of 1 ga for all dikes is consistent with these data and rb sr isotopic systematics all lamprophyres contain high levels of cr 284 393 ppm ni 101 180 ppm ba 1380 5580 ppm sr 2359 3109 ppm and rare earth elements ree lan 191 288 with average lan lun 26 and no eu anomalies at 1 ga all lamprophyres have a similar initial sr isotopic ratio of 0 7034 similar to bulk earth in contrast e nd values vary considerably from 4 to 10 2 indicating sources with variable long term light ree enrichment negative correlations between 143nd 144nd and la nb and k suggest involvement of at least two source components probably asthenosphere and metasomatized subcontinental lithosphere the lamprophyres are only slightly younger than 1010 ma labrieville massif which is also alkalic and enriched in sr and ba relative to most other anorthosites the lamprophyres provide unequivocal evidence for a sr and ba rich component in the lithospheric mantle that may have contributed to the parental magmas of labrieville and other andesine anorthosites in this part of quebec