archean gold mineralization in the wabigoon subprovince a product of crustal accretion evidence from u pb geochronology in the lake of the woods area superior province canada within the lake of the woods greenstone terrain in the wabigoon subprovince of northwest ontario gold is in faults shears and tension veins that developed in response to a late archean northwest directed contraction and emplacement of contemporaneous plutons nine rocks in this area have been dated using u pb analyses of zircon in order to obtain precise time constraints on gold mineralization and to compare its timing with the age of major crustal events in the region a mineralized dike at the duport mine shoal lake is 2716 pm 2 ma old whereas a later dike that cuts mineralized rocks has an age of 2709 pm 2 ma gold mineralization is therefore distinctly younger than calc alkaline and tholeiitic rocks dated at 2724 pm 2 ma in the shoal lake area and 2727 pm 2 ma in the high lake area to the north hydrothermal activity associated with mineralization at the duport mine overprints the amphibolite facies contact metamorphic aureole of the snowshoe bay batholith 2708 pm 7 ma

new constraint on the division of cambrian time a u pb zircon age of 530 7 0 9 ma for a volcanic ash in the lower placentian series of the avalonian terrane in southern new brunswick provides a precise point for calibration of early cambrian biologic evolution the ash is from the middle rusophycus avalonensis zone in saint john new brunswick this date is on subtrilobitic lower cambrian strata that may be approximately correlative with the sub tommotian manykaian stage of siberia and the lowest meishucunian stage of the south china platform this age when combined with published geochronologic data from the cambrian suggests that the manykaian stage may have lasted approximately 13 m y and that the tommotian may have been considerably shorter

time specific black mudstones and global hyperwarming on the cambrian ordovician slope and shelf of the laurentia palaeocontinent abstract the early paleozoic featured nine intervals of strong expansion of an upper slope dysoxic anoxic d a water mass with eustatic rise or epeirogenic transgression strong expansion of this d a water mass led to deposition of time specific macroscale alternations of dark grey black mudstone within oxic green to red mudstone on the middle lower slope this d a facies even onlapped warm carbonate and cool water siliciclastic shelves as in the mesozoic d a muds were deposited in shallow water perhaps tens of metres deep with sea level rise these nine d a macroscale alternations correspond to intervals of global hyperwarming times of very intense greenhouse conditions that resulted from a feedback initiated by higher insolation and heat storage as shallow seas onlap tropical palaeocontinents warm epeiric seas heated the ocean and thermal expansion accelerated eustatic rise ever more extensive epeiric seas heightened oceanic and global temperature as heat storage capacity increased deep ocean circulation intensity fell below that of a greenhouse interval and lead to d a deposition low on the slope and on the platforms to provide the signature of global hyperwarming global hyperwarming differs from a hothouse interval as it does not require co 2 input from large igneous provinces to produce high temperatures and never shows deep sea anoxia late ordovician and late devonian black mudstones that cover much of laurentia record epeirogenic transgressions that led to global hyperwarming and suggest that cold water upwelling or plant terrestrialisation had nothing to do with epeiric sea anoxia global hyperwarming reduced oxygen solubility in these seas and erosion of orogens produced muddy water that limited light penetration and promoted shallow water anoxia the global hyperwarming hypothesis means that relative eustatic and epeirogenic sea levels complement the effect of global pco 2 on climate and sea level must also be regarded as a primary driver of phanerozoic climate

duration of the early cambrian u pb ages of volcanic ashes from avalon and gondwana volcanic zircons from three ashes give a u pb date of 511 1 ma on trilobite bearing upper lower cambrian upper branchian series strata of southern new brunswick that correlate into the siberian middle botomian toyonian stage interval this very young age on the late but not latest early cambrian of avalon is consistent both with a 519 1 ma age on the older caerfai bay shales of south wales that are tentatively correlated into strata with the oldest avalonian trilobites lower branchian and with a 517 1 5 ma age on the antatlasia gutta pluviae zone trilobites of morocco determination of a 522 2 ma zircon age on the moroccan subtrilobitic lower cambrian lie de vin formation is consistent with an earlier reported 521 7 ma age from the lie de vin but suggests that a 526 4 ma age on australian trilobite bearing lower cambrian rocks may be too old a 33 ma duration of the avalonian early cambrian and an 8 ma length of the avalonian trilobite bearing lower cambrian support proposals that

stable isotopic calibration of the earliest skeletal fossil assemblages in eastern siberia precambrian cambrian boundary sections from the eastern margin of the siberian platform provide important reference sections for stable isotopic correlation of the newly defined precambrian cambrian boundary level since they contain some of the earliest pre tommotian shelly fossil assemblages known and are amenable to 13 c stratigraphy stable isotopes are examined from the dvortsy and ulakhan sulugur sections river aldan where sparse pre tommotian assemblages occur and from the uchur river region where the putative anabarites trisulcatus and purella antiqua zones of the nemakit daldynian stage are well developed diagenetic resetting of 18 o and 13 c must be taken into account when comparing such sections simple corrections allow for stable isotopic correlation of basal nemakit daldynian and tommotian strata across eastern siberia

three new upper cambrian stages for north america the existing upper cambrian stages for north america are obsolete three new stratotype defined stages are equivalent to and replace three successive biomeres marjuman stage replaces marjumiid biomere lower boundary at base ptychagnostus atavis zone steptoean stage replaces pterocephaliid biomere lower boundary at base aphelaspis zone sunwaptan stage replaces ptychaspid biomere lower boundary at base irvingella major subzone the base of the overlying ibexian series is placed at the base of the eurekia apopsis subzone

global correlation of the early cambrian of south australia shelly fauna of the dailyatia odyssei zone a lack of well resolved biostratigraphic data has prevented robust regional and global correlation of lower cambrian successions from south australia a new early cambrian biostratigraphy based on data derived from 21 measured stratigraphic sections and drill cores 11 described herein reveals the abundance and diversity of shelly fauna from the arrowie basin and the value of early cambrian small shelly fossils ssf for biostratigraphic studies here we examine shelly fauna associated with the youngest of three recently established biozones the dailyatia odyssei taxon range zone hereafter d odyssei zone and their correlative potential the d odyssei zone features a diverse suite of tommotiids organophosphatic brachiopods bradoriid arthropods molluscs and phosphatic problematica this fauna permits strong correlation often at species level with other major early cambrian terranes particularly antarctica south china and laurentia and suggest a cambrian series 2 stages 3 4 age for the d odyssei zone bradoriids have proven to be useful biostratigraphic tools four new species and three new genera are described herein acutobalteus sinuosus gen et sp nov eozhexiella adnyamathanha gen et sp nov manawarra jonesi gen et sp nov and mongolitubulus descensus sp nov the description of eohadrotreta sp cf zhenbaensis represents the first occurrence of the acrotretoid brachiopod eohadrotreta from australia

early cambrian chronostratigraphy and geochronology of south australia abstract the most successful chronostratigraphic correlation methods enlist multiple proxies such as biostratigraphy and chemostratigraphy to constrain the timing of globally important bio and geo events here we present the first regional high resolution shelly fossil biostratigraphy integrated with 13c chemostratigraphy and corresponding 18o data from the traditional lower cambrian terreneuvian and provisional cambrian series 2 of south australia the global zhuce shice positive excursions ii and iii and the care are captured in lower cambrian successions from the arrowie and stansbury basins the south australian shelly fossil biostratigraphy has a consistent relationship with the 13c results bolstering interpretation identification and correlation of the excursions positive excursion ii straddles the boundary between the kulparina rostrata and micrina etheridgei zones and the care straddles the boundary between the m etheridgei and dailyatia odyssei zones peaking in the lower parts of the latter zone new ca tims zircon dates from the upper hawker group and billy creek formation provide geochronologic calibration points for the upper d odyssei zone and corresponding chemostratigraphic curve embedding the lower cambrian successions from south australia into a global chronostratigraphic context this multi proxy investigation demonstrates the power of integrated methods for developing regional biostratigraphic schemes and facilitating robust global correlation of lower cambrian successions from south australia part of east gondwana with coeval terranes on other cambrian palaeocontinents including south and north china siberia laurentia avalonia and west gondwana

crust and mantle evolution in the late archean evidence from a sm nd isotopic study of the north spirit lake greenstone belt northwestern ontario canada sm nd data are presented for volcanic plutonic and sedimentary rocks from the late archean north spirit lake greenstone belt in northwestern ontario the greenstone belt is divided into six lithotectonic assemblages emplaced between 2700 and 3000 ma the earliest assemblage indicative of an island arc setting is characterized by positive initial nd values 0 to 3 whereas shales and granitoids in succeeding assemblages yield increasingly negative nd values as low as 3 this argues strongly that the assemblages formed in close proximity to each other because the younger assemblages likely include recycled crustal material from earlier assemblages through erosion into younger basins and remelting however even the earliest assemblage exhibits decreasing nd values from basaltic to intermediate felsic volcanic strata within the same assemblage this suggests that the initial arc was itself contaminated by either a sedimentary component or remelting of older oceanic crust granitoid samples yield nd values similar to the shales in the belt indicating a common evolutionary history these granitoids may thus represent the roots of the greenstone belt mantle derived rocks such as basalts and komatiites are derived from a variably depleted mantle source with values as high as 3 8 at 2925 ma a 2925 10 8 ma u pb zircon age obtained for a tonalitic pluton on the northern margin of the north spirit lake belt is similar to the age of a volcanic plutonic cycle in the adjacent favourable lake greenstone belt implying that the two belts may have had common orogenic episodes the north spirit lake greenstone belt appears to have nucleated from an initial primitive volcanic arc and through continued volcanism was transformed into a stable platform exhibiting continental features such as intracrustal differentiation and sedimentary recycling to produce the current array of assemblages in this respect the evolution of the north spirit lake greenstone belt resembles the evolution of proterozoic and phanerozoic continents and suggests that archean microcontinents grew in much the same fashion as present day continents

the lower middle cambrian stages and stratotypes from the iberian peninsula a revision lower middle cambrian regional stages of the iberian peninsula cordubian ovetian marianian bilbilian acadoparadoxides and solenopleuropsidae stages are now reconsidered in the light of new stratigraphic and palaeontological trace fossils trilobites and archaeocyaths studies these stages are re described with selected stratotypes and chronostratigraphic boundaries the names leonian and caesaraugustian are formally proposed to replace the provisional stages of acadoparadoxides and solenopleuropsidae respectively a correlation chart of the lower middle cambrian lithostratigraphic units from the iberian peninsula is also presented two regessive transgressive cycles are proposed to explain the lithological and palaeontological peculiarities through the lower middle cambrian sequence in the iberian peninsula

integrated stratigraphic geochemical and paleontological late ediacaran to early cambrian records from southwestern mongolia comment global correlation of sedimentary rock successions requires an integrated approach that uses all fossil based and alternative lithostratigraphic geochemical paleomagnetic geochronologic sequence stratigraphic data e g van couvering and berggren 1977 proposed global correlations and their role in interpreting earth and biotic history require a thorough examination of the utility and temporal resolution allowed by each correlation technique in comparison to the others the article by smith et al 2016 is a welcome contribution to understanding the upper ediacaran lower cambrian succession of the zavkhan terrane in the khasagt khairkhan range of southwestern mongolia their work complements earlier syntheses on the zavkhan terrane completed by soviet mongolian teams e g voronin et al 1982 esakova and zhegallo 1996 and under international geoscience programme igcp project 303 late precambrian cambrian event stratigraphy brasier et al 1996a see associated papers in geological magazine v 133 no 4

the earliest occurrence of trilobites and brachiopods in the cambrian of laurentia abstract the first trilobite to appear in laurentia is a new form resembling the siberian trilobite repinaella the lack of favorable lithologic environment for some distance below suggests that this occurrence is likely to be the oldest in western nevada the trilobites occur on siltstone surfaces and as rare molds in small leached carbonate nodules at parasequence flooding surfaces associated brachiopods are locally abundant these parasequences occur in the lower part of a 50 5 m siliciclastic unit near the top of the andrews mountain member of the campito formation that contains considerable light colored quartzite trace fossils are common and diverse in this interval which is interpreted as a lowstand systems tract below this interval the quartzite is dark gray to dark greenish gray with limited bioturbation of siltstone interbeds suggesting deposition in a dysoxic environment above the trilobite brachiopod bearing interval the siliciclastics are greenish gray also relatively dysoxic and barren of body fossils trilobites reappear suddenly in relative abundance 57 m above first as an eofallotaspis like form marking the base of the montezuman stage then as several species of fallotaspis the occurrence of cf repinaella is approximately correlative with the earliest trilobites in siberia and western gondwana

the middle lower cambrian ovetian lunagraulos n gen from spain and the oldest trilobite records the type material of agraulos antiquus sdzuy 1961 from the la herreria formation northern spain is revised together with additional material and included in the new genus lunagraulos the stratigraphical range of lunagraulos antiquus sdzuy 1961 occurring below that of the trilobite species of the genera lunolenus metadoxides and dolerolenus in the type locality of los barrios de luna in the province of leon northern spain and the accompanying ichnofossil assemblage demonstrate an ovetian age lower part of cambrian stage 3 currently being discussed by the international subcommission on cambrian stratigraphy for this species moreover the trilobite lunagraulos tamamensis n gen n sp is found in the tamames sandstone near the village of la rinconada in the province of salamanca central spain the biostratigraphical position of this new taxon and its accompanying ichnoassemblage is also analysed and assigned to the lowermost ovetian stage the genus lunagraulos is therefore the oldest agraulid found in the fossil record the exceptional presence of lunagraulos in a marine coarse siliciclastic succession a facies rather typical for the ichnofossils cruziana and rusophycus some of the oldest signs of trilobite activity suggests that first trilobite representatives may have inhabited high to middle energy marine environments this hypothesis may also explain both the taxonomic and biostratigraphic heterogeneity of the first trilobite genera appearing across the world due to preservation problems in this type of facies comparison of the lunagraulo s biostratigraphy with other coeval spanish fossil assemblages allows us to propose its intercontinental correlation with the oldest records of currently known trilobites

nd isotopic evidence for early to late archean 3 4 2 7 ga crustal growth in the western superior province ontario canada abstract new sm nd analyses of archean metavolcanic plutonic gneissic and metasedimentary rocks from sachigo and berens river subprovinces north caribou terrane and winnipeg river and wabigoon subprovinces in conjunction with previously published data are used to trace the crustal evolution of the western superior province wsp from ca 3 4 to 2 7 ga the extensive wsp geochronological database which contains 3 17 2 55 ga plutonic rocks allows the calculation of precise e t nd values and the identification of the main periods of new crustal addition at 3 4 0 1 3 02 2 92 and 2 76 2 69 ga mass balance calculations indicate that the western superior province represents a collage of archean crusts comprising 7 44 and 48 wt of crust created at 3 4 3 0 and 2 7 ga respectively although these calculations depend on the isotopic signatures of the mantle and those of the average crust at different times several important observations can be made 1 the importance of pre 2 7 ga terranes recycled in the 2 7 ga orogen 2 the existence of time intervals of 150 300 ma dominated by intra crustal processes resulting in a large range of crystallization ages u pb zircon ages that do not reflect new additions from the mantle 3 much of the new crustal additions at 2 7 ga were contaminated by crustal material subducted into the source region ttg suites or were contaminated during intrusion into the crust sanukitoids

chronology of early cambrian biomineralization data on the first appearances of major animal groups with mineralized skeletons on the siberian platform and worldwide are revised and summarized herein with references to an improved carbon isotope stratigraphy and radiometric dating in order to reconstruct the cambrian radiation popularly known as the cambrian explosion with a higher precision and provide a basis for the definition of cambrian stages 2 to 4 the lophotrochozoa and probably chaetognatha were first among protostomians to achieve biomineralization during the terreneuvian epoch mainly the fortunian age fast evolutionary radiation within the lophotrochozoa was followed by radiation of the sclerotized and biomineralized ecdysozoa during stage 3 the first mineralized skeletons of the deuterostomia represented by echinoderms appeared in the middle of cambrian stage 3 the fossil record of sponges and cnidarians suggests that they acquired biomineralized skeletons in the late neoproterozoic but diversification of both definite sponges and cnidarians was in parallel to that of bilaterians the distribution of calcium carbonate skeletal mineralogies from the upper ediacaran to lower cambrian reflects fluctuations in the global ocean chemistry and shows that the cambrian radiation occurred mainly during a time of aragonite and high magnesium calcite seas

small shelly fossils from the basal emigrant formation cambrian uppermost dyeran stage of split mountain nevada a latest early cambrian fauna of helcionelloid molluscs and small shelly fossils from the basal emigrant formation of nevada is described the fauna is the first of its kind to be described from trilobite bearing strata in the great basin and is well preserved but of limited diversity at the specific level the assemblage is largely endemic but it contains several genera with global distribution in the lower and middle cambrian costipelagiella nevadense n sp in the fauna is the oldest representative of the widespread but little known genus costipelagiella horn 1964 two additional new taxa are described anabarella chelata n sp and parkula esmeraldina n sp

a proposed nomenclature for stages and series for the cambrian of laurentia an internally consistent succession of six named regional stages from youngest to oldest sunwaptan steptoean marjuman delamaran dyeran montezuman with their boundaries defined by regional extinction events within the trilobite record is proposed for the trilobite bearing rocks of laurentia sub trilobite stage s are recommended but unnamed the three youngest stages have been named previously but the lower boundary of the marjuman is shifted downward the three older stages within the trilobite record are new if deemed necessary four regional series are also suggested from youngest to oldest millardan lincolnian waucoban and begadean each of the three youngest series accommodates two stages

neoproterozoic to early cambrian small shelly fossil assemblages and a revised biostratigraphic correlation of the yangtze platform china abstract small shelly fossils ssfs occur widely on the yangtze platform and have great potential for biostratigraphic subdivision of pre trilobitic early cambrian strata based on the ssf record of the shallow water realm five biozones can be recognized for the meishucunian stage in ascending order the biozones are anabarites trisulcatus protohertzina anabarica assemblage zone paragloborilus subglobosus purella squamulosa assemblage zone watsonella crosbyi assemblage zone formerly heraultipegma yunnanensis zone poorly fossiliferous interzone sinosachites flabelliformis tannuolina zhangwentangi assemblage zone in addition one ssf biozone is recognized for the overlying qiongzhusian stage pelagiella subangulata taxon range zone the formerly used siphogonuchites triangularis paragloborilus subglobosus zone and heraultipegma yunnanensis zone are discussed and redefined approximately 80 species of ssfs were screened for their spatial and temporal distribution on the yangtze platform variations in lithofacies and biofacies can be recognized throughout the yangtze platform extending over an area of 2000 900 km in a deeper water shelf setting the first zone is represented by the protohertzina anabarica kaiyangites novilis assemblage zone while younger zones are not recognized in the southern region at the northern platform margin the qiongzhusian is represented by the ninella tarimensis cambroclavus fangxianensis assemblage zone and the rhombocorniculum cancellatum taxon range zone the southeastern shaanxi western hubei region followed a slightly different lithological and faunal development than the rest of yangtze platform indicating a stronger similarity with parts of east gondwana some taxa such as w crosbyi p subangulata r cancellatum microdictyon effusum a trisulcatus protohertzina unguliformis and p anabarica occur nearly worldwide and support an international correlation of early cambrian sequences between the yangtze platform and smaller west gondwanan blocks siberia newfoundland and australia the six investigated zones of the yangtze platform comprise an interval spanning the early nemakit daldynian to the late atdabanian early botoman stage on the siberian platform palaeobiogeographic analysis revealed a strong taxic similarity between the yangtze platform and the tarim platform a smaller number of species are shared with other west gondwanan platform fragments such as india and iran palaeobiogeographic results do not support the previously reported position of the south china block between australia and siberia during the early cambrian

subprovince accretion tectonics in the south central superior province development of tectonic subprovinces as shear bounded granite greenstone and sediment dominated terranes during the late archaean is reviewed and interpreted from relationships between portions of

sm nd and trace element characterization of shales from the abitibi belt labrador trough and appalachian belt consequences for crustal evolution through time nd isotopic compositions and sm nd li k rb sr ba ni and cr abundances are reported for 25 shale samples from the canadian shield late archean abitibi greenstone belt and the mid proterozoi

the link between metazoan diversity and paleo oxygenation in the early cambrian an integrated palaeontological and geochemical record from the eastern three gorges region of south china abstract the link between environmental perturbations especially oceanic redox conditions and evolution during the cambrian biodiversification event is still highly controversial although this issue has attracted considerable attention for decades we here report on the fossil record of the early cambrian biota discovered in the uppermost yanjiahe and shuijingtuo formations that crop out in the luojiacun and qiaojiaping sections eastern three gorges area south china the uppermost member of the yanjiahe formation contains abundant benthic algae based on the fossil record four distinct intervals were recognized in the shuijingtuo formation i few sessile suspension feeders such as sponges were recovered from interval i ii interval ii is dominated by bivalved arthropods living in the water column in this interval are also found a few sponges and eodiscoid trilobites iii no fossils other than few sponge spicules were recovered from interval iii iv diverse benthic communities composed of sponges cnidarians brachiopods as well as bradoriids dominate interval iv comprehensive research on framboidal pyrite morphology whole rock geochemistry as well as in situ la icp ms analyses were carried out on samples from the luojiacun section in order to assess paleo redox conditions the results show that the overall evolution of the paleo redox conditions was characterized by a double episode of sharp anoxic events ae observed at the base of intervals i and iii followed by a gradual oxygenation event oe evidence of oxygenation fluctuations were detected especially within interval iv a case study is presented herein which brings together early cambrian changes in metazoan diversity and changes in environmental conditions it is suggested that periodical oxygenation of the water column increased the biological diversity in the restricted basins of the eastern three gorges thus four intervals of habitat succession can be tentatively reconstructed in the studied area for the cambrian age 2 age 3 interval indicating a transition from uninhabited sediments to well populated habitats finally both the temporal and spatial characteristics of the cambrian aes oes and the fossil record in south china are compared and links between them are discussed in the context of their specific paleo geographical setting

the tectonic evolution of the abitibi greenstone belt of canada based on structural geochemical sedimentological and geochronological studies we have formulated a model for the evolution of the late archaean abitibi greenstone belt of the superior province of canada the southern volcanic zone svz of the belt is dominated by komatiitic to tholeiitic volcanic plateaux and large bimodal mafic felsic volcanic centres these volcanic rocks were erupted between approximately 2710 ma and 2700 ma in a series of rift basins formed as a result of wrench fault tectonics the svz superimposes an older volcanic terrane which is characterized in the northern volcanic zone nvz of the abitibi belt and is approximately 2720 ma or older the nvz comprises basaltic to andesitic and dacitic subaqueous massive volcanics which are cored by comagmatic sill complexes and layered mafic anorthositic plutonic complexes these volcanics are overlain by felsic pyroclastic rocks that were comagmatic with the emplacement of tonalitic plutons at 2717 2 ma the tectonic model envisages the svz to have formed in a series of rift basins which dissected an earlier formed volcanic arc the nvz analogous rift environments have been postulated for the hokuroko basin of japan the taupo volcanic zone of new zealand and the sumatra and nicaragua arcs the difference between rift related submergent volcanism in the svz and emergent volcanism in the nvz resulted in the contrasting metallogenic styles the former being characterized by syngenetic massive sulphide deposits whilst the latter was dominated by epigenetic porphyry type cu au deposits

minto block superior province missing link in deciphering assembly of the craton at 2 7 ga plate tectonic models of the superior province are rooted in the granite greenstone and metasedimentary belts of the southern part of the craton north striking domains of the minto block in the northeastern part of the province evolved at similar times in 3 1 2 8 2 725 and 2 69 ga events requiring expansion of models of late archean assembly to accommodate minto geology western and eastern protocratons 3 1 2 8 ga rifted at 2 79 ga to produce an ocean basin that was mostly consumed by subduction at 2 725 ga the leaf river plutonic suite of cale alkalic hornblende biotite orthopyroxene clinopyroxene granodiorite represents magmatic arcs built on the protocratons whereas the intervening goudalie domain containing fault bounded fragments of rifted continental crust rift volcanics primitive oceanic crust 2724 ma island arc rocks and a 2718 ma back arc assemblage marks the suture terminal collision at 2 7 ga led to thickening and crustally derived granitoid magmatism the southern superior province also experienced vigorous activity between 2 725 and 2 69 ga as island arcs oceanic plateaus continental fragments and accretionary prisms were amalgamated progressively from north to south in a regime of dextral transpression then stitched by granites a northern proto superior craton had continental magmatic arcs built on its eastern and southern flanks in response to west northwest directed subduction orthogonal convergence in the east produced wide plutonic arcs in contrast to terrane accretion tectonics in the more oblique regime along the southern margin

the red lake greenstone belt superior province evidence of plume related magmatism at 3 ga and evidence of an older enriched source abstract the red lake greenstone belt of the superior province contains a number of volcano sedimentary assemblages that range in age from 2 99 ga to 2 74 ga the oldest of these is the 2 99 2 96 ga balmer assemblage which is part of the north caribou terrane one of the oldest portions of the superior province the balmer assemblage dominates the red lake greenstone belt and is significant in that it contains evidence for plume generated magmatism the assemblage is dominated by mafic volcanic flows with intercalated komatiitic lavas mafic and ultramafic sills and shallow water clastic and chemical sediments the basalts have 5 12 mgo and mantle normalized signatures which range from slightly light rare earth element ree depleted through essentially flat to light ree enriched some lavas display slight negative nb and ti anomalies indicating selective crustal contamination the komatiites have 12 14 mgo are the al undeplated type and are strongly depleted in light ree they have e nd values of ca 2 the geochemistry combined with sm nd isotope data indicates that the komatiites came from a long term enriched source that was probably generated at ca 3 4 ga that source underwent a melting event that depleted it in light ree prior to komatiite generation the presence of komatiites from a long term enriched source and the strong contrast in the ree profiles of the basalts and the komatiites are consistent with a plume model for the generation of the balmer assemblage the komatiites having originated in the plume core and the basalts from entrained upper mantle or mantle melted by the plume

differential response of u pb systems in coexisting accessory minerals winnipeg river subprovince canadian shield implications for archean crustal growth and stabilization the u pb isotopic systems of zircon monazite titanite and some apatite and the pb isotopic composition of k feldspar have been investigated in three areas of the winnipeg river subprovince wrs of the superior province canada in order to define the timing of magmatic and metamorphic processes in this archean gneissic granitoid terrain

precambrian cambrian boundary global stratotype ratified and a new perspective of cambrian time the precambrian cambrian boundary global stratotype is the base of the phycodes pedum zone at fortune head southeastern newfoundland the initial two stage radiation of the phanerozoic fauna the appearance of phanerozoic aspect trace producers and subsequent origin of diverse skeletalized metazoans was earliest cambrian fossil assemblages with deep complex burrows or originally mineralized skeletal fossils with exception of cloudina are early cambrian or younger correlation into avalon indicates that strata once referred to the proterozoic e g manykaian or nemakit daldynian and rovno stages are lowest cambrian and that the siberian tommotian stage is middle lower cambrian the thick sequence and complex epeirogenic history of the placentian series as compared with the trilobite bearing branchian series suggest that most early cambrian time is represented by the subtrilobitic lower cambrian

cambrian small skeletal fossil and carbon isotope records of the southern huangling anticline hubei china and implications for chemostratigraphy of the yangtze platform abstract rich small skeletal fossil assemblages are described from the yanjiahe formation and tianzhushan member of southern huangling anticline the anabarites trisulcatus protohertzina anabarica assemblage zone small skeletal fossil zone 1 is recognized in the middle to upper yanjiahe formation while the thin topmost conglomeratic limestone unit yielded a mixed assemblage resulting from reworking and redeposition of source material of small skeletal fossil zones 1 to 3 including anabarites trisulcatus protohertzina anabarica p unguliformis watsonella crosbyi aldanella attleborensis anabarella plana our revised biostratigraphic correlation indicates the existence of longer hiatuses in the yanjiahe formation and between the yanjiahe and shuijingtuo formations the obtained 13ccarb values in our profile deviate in some details from those of some previous reports based on the new biostratigraphic correlation the 13ccarb trend records values oscillating between 3 to 4 4 in the upper anabarites trisulcatus protohertzina anabarica assemblage zone which is unusual for the yangtze platform this interval of positive 13ccarb values in the upper yanjiahe formation and the dahai member of eastern yunnan are here interpreted to reflect the impact of a local aquafacies moreover it is here argued that the selection of a carbon isotope peak as a primary index tool in the laolin section for defining the cambrian stage 2 must be treated with caution as this isotope excursion is influenced by a regional water mass and does not reflect an open marine seawater signature underlining other technical and theoretic concerns on the general application of chemostratigraphy in cambrian shallow water carbonate platforms overall we suggest that chemostratigraphic correlations need to be supported by biostratigraphic tools

geochemical oxygen and neodymium isotope compositions of metasediments from the abitibi greenstone belt and pontiac subprovince canada evidence for ancient crust and archean terrane juxtaposition abstract the abitibi greenstone belt agb and pontiac subprovince ps in the southwestern superior province are adjacent greenstone plutonic and metasedimentary dominated terranes respectively separated by a major fault zone metasediments from these two contrasting terranes are compared in terms of major and trace element and o and nd isotope compositions and detrital zircon ages the following two compositional populations of metasediments are present in the low grade abitibi southern volcanic zone 1 1 a mafic element enriched population mep characterized by flat depleted ree patterns enhanced mg cr co ni and sc low incompatible element contents and minor or absent normalized negative troughs at nb ta and ti and 2 2 a low mafic element population lmep featuring lree enriched patterns enhanced rb cs ba th and u contents and pronounced normalized negative troughs at nb ta and ti these geochemical features are interpreted to indicate that the mep sediments were derived from an ultramafic and mafic dominated oceanic provenance whereas the lmep sediments represent mixtures of mafic and felsic arc source rocks the ps metasediments are essentially indistinguishable from abitibi lmep on the basis of majorelement and transition metal abundances suggesting comparable types of source rocks and degrees of maturity but are distinct in terms of some trace elements and o isotope compositions the pontiac metasediments are depleted in 18o and enriched in cs ba pb th u nb ta hf zr and total ree and also have higher ratios of rb k cs rb ba rb ta nb th la and ba la relative to the abitibi lmep two subtypes of ree patterns have been identified in ps metasediments both have moderately fractionated distributions with la yb n 3 9 17 and 7 8 13 6 respectively but the first has minor eu anomalies eu eu 0 89 1 16 whereas the second features pronounced negative anomalies eu eu 0 52 0 81 coupled with high concentrations of incompatible elements such as k rb th u nb and ta the first subtype is interpreted to be derived from provenances of mixed mafic and felsic volcanic rocks whereas the eu depleted type has geochemical features that are typical of post archean sediments or archean k rich granites and volcanic equivalents abitibi metasediments have nd 2 7 ga values of 0 71 to 1 83 corresponding to model tchur 2 82 2 55 ga with the majority around 2 65 ga close to the depositional age of 2 7 ga in contrast ps metasediments are characterized by nd 2 7 ga 0 95 to 3 51 corresponding to t dhur 2 7 to 3 1 ga consistent with a detrital zircon age spectrum of 2 69 ga to 3 1 ga pontiac subprovince and abitibi metasediments have contrasting provenances suggesting their separate initial geodynamic settings and later tectonic juxtaposition major and trace elements o and nd isotopic compositions and detrital zircon ages collectively suggest that some of the ps metasediments were derived mainly from juvenile rocks whereas others require various degrees of contribution from older 3 0 3 5 ga evolved crust mixed with juvenile 2 7 ga rocks older components were derived either directly from erosion of an ancient continental arc or from contemporaneous volcanic rocks which may contain recycled older components if the ps represents in part an accretionary prism to the abitibi southern volcanic zone then oblique subduction may have been involved which allowed the pontiac subprovince to juxtapose or sample exotic source rocks

geochronological constraint on the cambrian chengjiang biota south china the cambrian chengjiang biota of south china provides compelling fossil evidence for the rapid appearance of metazoan phyla in earth history the cambrian explosion however the timing of the chengjiang biota is poorly constrained owing to lack of datable rock materials within the maotianshan shale that yields the fossils here we integrate secondary ion mass spectrometry sims and chemical ablation isotope dilution thermal ionization mass spectrometry ca id tims u pb analyses of detrital zircons from the maotianshan shale to provide high precision geochronological constraint on the chengjiang biota the youngest group of sims u pb detrital zircon dates yields an age peak at 520 ma six zircons from this group are further dated by the ca id tims u pb technique but suggesting that they were not formed from a single zircon growth event thereby neither the age peak nor the weighted mean age defined by the youngest sims u pb dates could represent the maximum depositional age of the maotianshan shale instead the youngest ca id tims u pb date 518 03 0 69 0 71 ma 2 analytical uncertainty incorporates u pb tracer calibration uncertainty provides the first robust maximum age of the chengjiang biota this new geochronological constraint on the chengjiang biota indicates that the cambrian explosion reached its major phase around 518 03 0 69 0 71 ma demonstrating a protracted process 22 myr for the cambrian explosion supplementary material sims and ca id tims zircon u pb data are available at https doi org 10 6084 m9 figshare c 4024849

cambrian chronostratigraphy current state and future plans abstract important steps have been made by the international subcommission on cambrian stratigraphy iscs toward global chronostratigraphic subdivision of the cambrian the consensus of opinion is that the system should be subdivided into four series representing subequal spans of time the lower two series will correspond approximately to the traditional lower cambrian the third series will correspond approximately to the traditional middle cambrian and the fourth series corresponds approximately to the traditional upper cambrian the fourth series called the furongian series was ratified by the international commission on stratigraphy ics and the international union of geological sciences iugs in 2003 selection of horizons representing evolutionary events that can be correlated on a global scale and which can be used as stage or series boundaries is much more difficult for the lower two series intervals of the cambrian than it is for the upper two series intervals of the cambrian the most practical concept for subdividing the system into stages involves the establishment of two stages each for the first and second series and the establishment of three stages each for the third and fourth series the lowermost stage of the furongian called the paibian stage has been ratified by the ics and the iugs

geochronological constraints on the timing of magmatism deformation and gold mineralization in the red lake greenstone belt northwestern ontario the paper presents u pb ages on zircon baddeleyite titanite rutile and monazite from volcanic and plutonic rocks of the red lake greenstone belt in the northern superior province the purpose of the study was to refine a previously outlined time frame for the volcanic evolution of the belt define the time of major plutonism and deformation and constrain the age of formation of gold deposits volcanism spanned at least 270 ma between 3000 and 2730 ma new ages of 2989 3 and 2894 2 ma for units of a lower volcanic sequence and and 2744 1 ma for units of an upper volcanic sequence corroborate and refine the previously established relationships local gabbroic and felsic intrusions cutting through the lower sequence yield ages of 2870 15 and respectively volcanism was succeeded by major plutonism deformation alteration and metamorphism between about 2730 and 2700 ma these processes were interrelated and progressed as a relatively continuous sequence of events accentuated by major activity d

pre tommotian age of the lower pestrotsvet formation in the selinde section on the siberian platform carbon isotopic evidence carbon isotopic data from the selinde section in the southeastern part of the siberian platform area are correlated with the reference isotopic profile from the lower cambrian stratotype sections of the lena aldan region but also show additional 13 c excursions unrecognized there the chemostratigraphic correlation suggests that the geological and fossil record of the lower pestrotsvet formation in the selinde section has a deeper history than the stratotype region this conclusion is important for both constraining the age of the earliest cambrian marine transgression on the siberian platform and providing a clearer understanding of the pace and order of early cambrian geochemical and biological events

u pb zircon ages for magmatism in the red lake greenstone belt northwestern ontario u pb dating was carried out on nine volcanic rocks and two felsic intrusions from the red lake greenstone belt in order to establish an absolute time framework for the magmatic evolution of the area and yield first indications on the time of deformation and gold mineralization the data indicate a protracted period of igneous activity spanning at least 270 ma felsic volcanic rocks near the top of the tholeiitic to komatiitic sequence in the eastern part of the belt yield ages of and a third unit dated at contains inherited zircons older than 2982 ma which casts some uncertainty on the validity of the inferred intercept age rocks in the western part of the belt previously believed to form a relatively young calc alkalic sequence but now known to be dominantly tholeiitic are shown to be relatively old with ages of and these two dates also bracket the age of stromatolites occurring in chemical sediments that are under and overlain by the dated units another volcanic horizon in the centre of the b

geochronological constraints on the tectonic evolution of a late archaean greenstone belt wabigoon subprovince northwest ontario canada abstract u pb isotopic age analyses have been carried out on zircon and titanite from rocks near the contact between the kakagi lake savant lake volcanic belt ksvb within the wabigoon subprovince and the english river subprovince in northwest ontario canada two tonalite gneisses intruded by mafic dykes at tannis lake and caribou lake have minimum zircon ages of 3051 ma and 3075 ma respectively this contrasts with a 2775 1 ma age on zircon from a felsic tuff located within a lower predominantly mafic volcanic sequence in the interior of ksvb these data combined with previous studies indicate that ksvb is presently flanked by older sialic crust in the sioux lookout and kashaweogama lake areas lower mafic volcanic sequences are unconformably overlain by sedimentary units that are in sheared contact with overlying calc alkaline volcanic units titanite and zircon analyses from clasts within a basal conglomerate indicate a maximum age for deposition of 2698 4 ma this late age is confirmed by a 2703 2 ma zircon age for an unconformably underlying porphyry intrusion one clast gives a zircon age of 2903 16 ma suggesting that the sediments were at least partly derived from older material in the english river subprovince data from previous studies show that the structurally overlying calc alkaline volcanic sequences are at least 30 ma older than the sediments the absence of zircon inheritance in all but the youngest ksvb sequences as well as geological and geochemical arguments suggest that the greenstone belt evolved largely in an ensimatic environment in isolation from older sialic material the intervening oceanic crust must therefore have been consumed by subduction which led to arc magmatism and eventual collision between the arc ksvb and continental material english river subprovince the sediments unconformably overlying the lowermost mafic volcanic sequences were deposited in a partly subaerial continental environment since chemical and textural evidence suggests the lower mafic volcanic sequences were deposited in an oceanic environment they must have been obducted onto the edge of a continental block this was perhaps followed by nappe and thrust emplacement on top of the sediments of calc alkaline sequences derived from earlier arc magmatism continued compression resulted in isoclinal folding of the supracrustal sequences into their present vertical positions

a new lower cambrian shelly fossil biostratigraphy for south australia abstract definition of early cambrian chronostratigraphic boundaries is problematic with many subdivisions still awaiting ratification integrated multi proxy data from well resolved regional scale schemes are ultimately the key to resolving broader issues of global correlation within the cambrian in australia early cambrian biostratigraphy has been based predominantly on trilobites phosphatic shelly fauna have great potential as biostratigraphic tools especially in pre trilobitic strata because they are widespread and readily preserved but they have remained underutilised here we demonstrate their value in a new biostratigraphic scheme for the early cambrian of south australia using a diverse shelly fauna including tommotiids brachiopods molluscs and bradoriids biostratigraphic data are derived from ten measured stratigraphic sections across the arrowie basin targeting hawker group carbonates including the wilkawillina wirrapowie and ajax limestones and the mernmerna formation the stratigraphic ranges of shelly fossils are predictable and repeatable across the arrowie basin allowing three discrete shelly biozones to be identified spanning terreneuvian stage 2 to series 2 stages 3 4 the kulparina rostrata zone new and part of the overlying micrina etheridgei zone new are pre trilobitic predominantly terreneuvian the cambrian series 2 stage 3 dailyatia odyssei zone new features a very diverse shelly fauna and will be described in detail in a separate publication these zones provide robust means to correlate terreneuvian series 2 successions in neighbouring coeval basins in australia particularly the stansbury basin wider correlation is possible throughout east gondwana and especially with south china

challenges in defining the base of cambrian series 2 and stage 3 formal subdivision of the cambrian system into four series and ten stages is in progress the base of cambrian stage 3 provisional which is conterminous with the base of cambrian series 2 provisional is expected to be placed at a horizon close to the first appearance of trilobites which marks the onset of the largest phase of the cambrian explosion conceptually an ideal boundary position would be marked by a significant and globally recognizable bioevent that divides the lower part of the cambrian system into a sub trilobitic terreneuvian series and a trilobite dominated series 2 if the level is to be identified principally through biostratigraphic means its position also needs to be recognizable using non biostratigraphic means and its correlation potential should be tested through detailed investigation of several continuous successions embracing the critical interval major difficulties in identifying a suitable horizon include strong biotic provincialism a plethora of regional unconformities and facies changes on different paleocontinents in the cambrian terreneuvian series 2 boundary interval levels that provide potential for intercontinental correlation include the first appearance position of trilobites and the approximately equivalent first appearance positions of certain small shelly fossil and acritarch species in addition a non biostratigraphic marker such as a stable carbon isotopic excursion may be useful as a correlation level cambrian series 2 and stage 3 are provisional stratigraphic units and their conterminous base was previously suggested to coincide to the appearance of trilobites compared to other potential chronostratigraphic indicators the first appearance of trilobites still has advantages for recognizing and correlating the base of stage 3 and should remain as the provisional primary marker for the boundary position

further shrimp geochronology on the early cambrian of south china zircon 206pb 238u ages are reported for bentonites from two cambrian sections in south china the meishucun and the fandian the analyzed zircons vary widely in age because of the presence of detrital and altered grains spot ages were corrected for calibration drift during analysis and the age of reference zircon sl13 was calibrated as 580 ma against other standards using sims use of mixture modeling was essential to resolve the different age components and to identify the volcanic ages two age groups occur within the meishucun bed 5 bentonite of which the larger at 518 7 2 5 ma is interpreted as a post depositional zircon growth event and the other at 539 4 2 9 ma as the volcanic age bentonites from the meishucun bed 9 and its fandian equivalent have two age groups 526 5 1 1 ma and 540 2 1 0 ma we interpret the younger of these as the tuff magmatic age and the older as detrital small shelly fossils indicate meischucunian bed 5 is an equivalent of the late nemakit daldynian and its 539 4 2 9 ma age both closely times the base of the cambrian and is a maximum for the basal tommotian the 526 5 1 1 ma for bed 9 and its equivalents approximates the close of the tommotian

chronostratigraphic correlations their importance for the definition of geochronologic units abstract the present article resumes the philosophy underlying the re definition of chronostratigraphic geochronologic units through their boundaries by working groups of the international commission on stratigraphy ics following the ics guidelines with respect to the historical approach this philosophy implies a change in the way how stages are defined stages originally defined by their contents are now defined by their lower boundary only the concept of the global stratotype standard section and point gssp as before stages may be characterized by their contents but precise definitions of stages and their scope can only be attained via boundary definitions the following subjects are dealt with in detail 1 the choice of the level of a chronostratigraphic geochronologic boundary which is to be defined by a gssp this is determined by practical considerations concerning the correlation of the envisaged stratigraphic level 2 if traditional names are to be preserved some kind of compromise is always necessary because none of them is clearly and unequivocally defined this goal can be attained by a democratic vote 3 because of gaps and condensation even the more favorable historical stratotypes are unsuitable for the redefinition of classical stages by their lower boundaries

formation of an archaean continent about 30 of the earth is covered by continents but only about 10 small kernels of these continentsae known as archaean cratonsae are continental fragments formed before 2 5 gyr ago the kaapvaal craton of south africa which formed and stabilized between 3 7 and 2 7 gyr ago is one of the oldest reasonably sized examples of these continental fragments it consists of a mosaic of subdomains that have been welded together by processes similar to those of modern day plate tectonics the earliest subdomains may have owed their origin to the onset of efficient recycling from the earth s hydrosphere into the mantle

the age and provenance of metasedimentary rocks in the quetico subprovince ontario from single zircon analyses implications for archean sedimentation and tectonics in the superior province abstract u pb isotopic analyses have been performed on single detrital zircon grains from greywacke samples collected along the northern boundary of the quetico subprovince in northwest ontario secondary lead loss was virtually eliminated by choosing crack free grains and removing the outer surface using air abrasion thus the 207 pb 206 pb age can be considered a good approximation to the age of crystallization of each grain zircons in the quetico metasediments show a range of ages from 2698 3 ma to 3009 4 ma deposition and metamorphism of the metasediments is bracketed within a 10 5 ma time span between crystallization of the youngest detrital zircon grain and a late tectonic intrusion the blalock pluton which cuts the sediments and has a zircon age of 2688 4 ma the most easterly metasediment sample contains zircons with ages which cluster around 3007 6 ma which is within error of the age of a batholith dated in the adjacent marmion lake area of the wabigoon subprovince this along with the fact that most detrital zircons were only slightly abraded by sedimentary transport suggests that the source of many zircons may have been proximal from the north there is also a significant cluster of young zircon ages in the range 2698 2709 ma surprisingly only a few ages correspond to the time of most intense arc type felsic volcanism 2710 2745 ma in the main greenstone belt of the western wabigoon subprovince sedimentary deposition occurred at a time of extensive deformation in the adjacent wabigoon subprovince the pattern of detrital grain ages indicates that this involved uplift of a complex older sialic landmass petrographic evidence for the presence of substantial felsic volcanic detritus as well as the fact that many of the zircons only slightly pre date deposition indicates that much of the sediment was probably derived from erosion of active felsic volcanic centers these data combined with regional geochronological and field studies favour deposition in a basin formed by late orogenic collisional events

erosional and depositional characteristics of regional overwash deposits caused by multiple hurricanes regional scale washover deposits along the florida gulf and atlantic coasts induced by multiple hurricanes in 2004 and 2005 were studied through coring trenching ground penetrating radar imaging aerial photography and prestorm and poststorm beach profile surveys erosional and depositional characteristics in different barrier island sub environments including dune field interior wetland and back barrier bay were examined over the eroded dune fields the washover deposits are characterized by an extensive horizontal basal erosional surface truncating the old dune deposits and horizontal to slightly landward dipping stratification over the marshes in the barrier island interior the washover deposits are characterized by steep tabular bedding with no erosion at the bottom overwash into the back barrier bay produced the thickest deposits characterized by steep prograding sigmoidal bedding no significant erosional feature was observed at the bottom washover deposits within the dense interior mangrove swamp demonstrate both normal and reversed graded bedding the washover deposits caused by hurricanes frances 2004 and jeanne 2004 along the southern florida atlantic coast barrier islands are substantially different from those along the northern florida barrier islands caused by ivan 2004 and dennis 2005 in terms of regional extension erosional features and sedimentary structures these differences are controlled by different overall barrier island morphology vegetation type and density and sediment properties the homogeneity of sediment along the northern florida coast makes distinguishing between washover deposits from ivan and dennis difficult in contrast along the atlantic coast barrier islands the two overwash events as demonstrated by two phases of graded bedding of the bimodal sediments are easily distinguishable

burrowing below the basal cambrian gssp fortune head newfoundland the range of treptichnus pedum the index trace fossil for the treptichnus pedum zone extends some 4 m below the global standard stratotype section and point for the base of the cambrian period at fortune head on the burin peninsula in southeastern newfoundland the identification of zigzag traces of treptichnus isp even further below the gssp than t pedum in the fortune head section and in other terminal proterozoic successions around the globe supports the concept of a gradational onset of three dimensional burrowing across the proterozoic cambrian boundary although t pedum remains a reasonable indicator for the base of the cambrian period greater precision in the stratotype section can be achieved by a detailed re evaluation of the stratigraphic ranges and the morphological variation of ichnotaxa included in the t pedum zone

cambrian evolutionary radiation context correlation and chronostratigraphy overcoming deficiencies of the first appearance datum fad concept abstract use of the first appearance datum fad of a fossil to define a global chronostratigraphic unit s base can lead to intractable correlation and stability problems fads are diachronous they reflect species evolutionary history dispersal biofacies preservation collection and taxonomy the cambrian evolutionary radiation is characterised by diachronous fads biofacies controls and provincialism of taxa and ecological communities that confound a stable lower cambrian chronostratigraphy cambrian series and stage definitions require greater attention to assemblage zone successions and non biostratigraphic particularly carbon isotope correlation techniques such as those that define the ediacaran system base a redefined basal cambrian trichophycus pedum assemblage zone lies above the highest ediacaran type biotas vendobionts putative metazoans and calcareous problematica such as cloudina and the basal asteridium tornatum comasphaeridium velvetum zone acritarchs this definition and the likely close correspondence of evolutionary origin and local fad of t pedum preserves the fortune head newfoundland gssp of the cambrian base and allows the presence of sub cambrian branched ichnofossils the sub tommotian equivalent base of stage 2 a suggested laolinian stage should be defined by the i l4 zhuce 13 c positive peak bracketed by the lower ranges of watsonella crosbyi and aldanella attleborensis molluscs and the skiagia ornata fimbrioglomerella membranacea zone acritarchs the w crosbyi and a attleborensis fads cannot define a stage 2 base as they are diachronous even in the newfoundland type w crosbyi zone the series 2 base cannot be based on a species fad owing to the provincialism of skeletalised metazoans in the terreneuvian series 2 boundary interval and global heterochrony of the oldest trilobites a series 2 and stage 3 a suggested lenaldanian series and zhurinskyan stage new gssp base is proposed at the siberian lower atdabanian 13 c iv peak which correlates into south china avalonia and morocco and assigns the oldest trilobites to the terminal terreneuvian series

samarium neodymium elemental and isotopic systematics in sedimentary rocks abstract trace element sm nd th sc and nd isotopic data for sedimentary rocks are examined in order to evaluate 1 1 possible secular trends in the composition of the sedimentary mass and upper continental crust 2 2 the role of tectonic setting in controlling the chemical and nd isotopic composition of the sedimentary mass when plotted against either sedimentation age or average provenance age determined mainly from nd model ages sm nd in sedimentary rocks decreases abruptly from about 0 21 to 0 19 at the archean proterozoic transition consistent with a change towards a more felsic upper crustal composition the change is only somewhat less pronounced for turbidite sediments than for those of continental affinity a trend toward increasing sm nd approaching or exceeding archean values is observed for phanerozoic turbidites this latter change is interpreted as indicating undifferentiated young crust as a component in the sediment which has been lost from the older post archean record due to sedimentary and igneous intracrustal recycling processes when plotted against sedimentation age th sc ratios exhibit a trend directly comparable to that of sm nd in this case th sc increases abruptly at the archean proterozoic boundary from about 0 5 to 1 0 and then decreases to about 0 7 during the phanerozoic nd isotopic composition of modern sediments is affected by tectonic association with those deposited at active margin settings tending to have higher nd reflecting younger provenance and higher and more scattered sm nd reflecting less homogenized and differentiated provenance it is likely that these distinctions can be traced back for about 1 2 ga but with the current data base are not readily resolved in older sedimentary rocks the degree to which the preserved sedimentary mass is open to new additions from the mantle either through replacement during sedimentary recycling or sediment mass growth is highly variable but averages about 15 25 during the phanerozoic

acritarchs from the duolbagaisa formation cambrian series 2 miaolingian on the digermulen peninsula finnmark arctic norway towards a high resolution cambrian chronostratigraphy

oxygen and carbon isotope stratigraphy of early cambrian carbonates in southeastern newfoundland and england carbonate rocks have been sampled through predominantly siliciclastic sediments above the precambrian cambrian global stratotype level in southeastern newfoundland to assess their potential for oxygen and carbon isotope stratigraphy comparable successions were sampled at nuneaton and comley in england greatly depleted 18 o signals are attributed to widespread thermal alteration during deep burial and granitic intrusion including within the stratotype region carbon isotope ratios appear to have been less affected and these are described from nine sections a provisional composite 13 c curve is based on non ferroan pink nodular and bedded micrites several 13 c excursions occur in the fossiliferous bonavista group and allow the position of the tommotian atdabanian boundary to be identified chemostratigraphic correlation of the new precambrian cambrian boundary stratotype may however prove difficult because of the lack of suitable well preserved carbonates the search must begin for a comparable reference section allowing global correlation of the boundary level using chemostratigraphy as well as biostratigraphy

age structure and orogenic significance of the berens river composite batholiths western superior province the berens river area of northwestern ontario is underlain mainly by archean felsic plutonic rocks which enclose minor supracrustal and gneissic enclaves and merge with the greenstone belt rich uchi subprovince to the south u pb geochronology using zircon and monazite shows that the batholiths evolved mainly between 2750 and 2690 ma by sequential and essentially continuous intrusive activity into an older substratum composed of 3000 2800 ma volcanic and tonalitic crust there is a broad but not strict compositional transition from early biotite tonalite and hornblende tonalite progressing with time towards a greater abundance of hornblende granodiorite to granite and finally to late biotite granite rare peraluminous granites and sanukitoid dioritic monzodioritic to granitic plutons the tonalite suites were predominantly synvolcanic the late granitic intrusions postdated volcanism but were largely synchronous with the main compressional events that caused widespread sedimentation deformation

u pb systemstics in single zircons from the pontiac sediments abitibi greenstone belt twenty six detrital zircon grains from the late archaean pontiac sediments of the abitibi greenstone belt were dated individually by the u pb method in order to identify possible elements of an ancient pre 2 7 ga shield the results show that at least two components were present at the source areas of these sediments 1 materials of about 2700 2725 ma which were most likely derived from contemporaneous acid igneous rocks and 2 significantly older materials parts of which are at least 2940 ma old which were either part of an uplifted sedimentary basin or a sialic basement complex these results can be used to suggest a close link between igneous activity and basin subsidence and that the development of the abitibi volcanic belt was closely associated with the presence of some older sialic blocks

multiple δ13c excursions spanning the cambrian explosion to the botomian crisis in siberia new high resolution 13 c data through the lower cambrian of siberia reveal multiple positive excursions coincident with phases in the explosion of invertebrate phyla nemakit daldynian to middle botomian stages comparison of the 18 o and trace element mg fe mu sr chemistry indicate that six new 13 c cycles are primary rather than diagenetic features with potential for global correlation positive 13 c excursions up to 3 indicate that fractional organic carbon burial rates were high but variable through the cambrian explosion values for 13 c dropped sharply from around 2 2 to 1 6 in botomian time coincident with mass extinction of the archaeocyathan reef biota the rapid fluctuation of the 13 c signal and the temporal coincidence of the pronounced negative shift with the extinction event hint that the 13 c record may record productivity variations during the early cambrian radiation

geochemistry and provenance of sediments from the pongola supergroup south africa evidence for a 3 0 ga old continental craton abstract the 3 0 ga pongola supergroup of south africa is one of the oldest well preserved supracrustal successions the sequence is divided into the nsuze chiefly volcanics quartzites and pelites and the overlying mozaan groups chiefly quartzites and pelites relative to north american shale composite nasc most pongola pelites have similar la yb and eu eu ratios and large ion lithophile elements lile high field strength elements hfse ree v and sc contents ni 30 300 ppm and cr 100 800 ppm contents are greater than those of nasc while cr ni ratios are generally higher than shales and pelites of all ages these trends may reflect residual cr enrichment during intense chemical weathering of komatiitic source rocks local populations of nsuze pelites based on major and trace element data may result from source heterogeneities and or contemporaneous volcanic input into the sedimentary basin pongola quartzite compositions and detrital modes are similar to those of sandstones from phanerozoic passive margin and basement uplift tectonic settings ree lile and hfse contents in pelites suggest that pongola source rocks were remarkably similar in composition to average phanerozoic upper continental crust trace element geographic and isotopic constraints favor 3 2 to 3 0 ga hood granites as a major source for pongola detritus moderate contributions from mafic sources are also required whereas relatively minor contributions from tonalite and komatiite are allowed these results indicate the existence of a chemically evolved felsic craton in southern africa by 3 0 ga this craton appears to have been more evolved than sources for either older moodies 3 3 ga or most of the younger witwatersrand sediments 2 8 2 7 ga pelites from the kaapvaal craton record at least two mafic to felsic source area compositional cycles in pelites deposited from the mid to late archean additional cycles may be masked by a depositional or erosional hiatus between the moodies and pongola sequences these sedimentary patterns suggest that crustal development in southern africa was episodic in nature during the early precambrian

u pb geochronology of the central uchi subprovince superior province u pb zircon and titanite ages for rocks of the central uchi subprovince in northwestern ontario indicate a late archean magmatic and tectonic development spanning over 200 ma an early period at 2900 2800 ma formed volcano plutonic complexes presumably linked to 3 1 2 8 ga terrains of the northwestern superior province a later period of southward growth by magmatic and tectonic accretion occurred at 2750 2710 ma and was concluded by large scale compression and plutonism at 2700 ma the oldest 2890 2860 and 2840 2820 ma components occur in the pickle lake and meen dempster greenstone belts and as gneisses in the seach achapi and the lake st joseph batholiths in northern and central sectors of the region together with distinct 2750 2740 ma volcano plutonic complexes they form a collage assembled by multiple episodes of tectonic juxtaposition and magmatic accretion plutons of 2730 2710 ma age are intrusive into these older northern domains whereas their volcanic counterparts compose the lake st joseph and miminiska fort hope greenstone belts to the south late tectonic to posttectonic granitoid rocks intruded a region extending from the northern berens river subprovince to the southern english river subprovince at 2700 ma these plutons were cut by regional scale faults formed by residual north northwest directed shortening the timing of this movement seems to be recorded by titanite ages of 2690 2670 ma reactivation of the same faults may account for proterozoic pb loss observed in some of the zircon populations the age patterns are consistent with crustal growth along a continental margin in a north dipping subduction environment

a new early cambrian bradoriid arthropoda assemblage from the northern flinders ranges south australia abstract a new assemblage of early cambrian bivalved arthropods bradoriida is described from the arrowie syncline in the northern flinders ranges south australia the well preserved largely endemic fauna comprises a total of six taxa including five new species jiucunella phaseloa sp nov jixinlingella daimonikoa sp nov mongolitubulus anthelios sp nov neokunmingella moroensis sp nov phasoia cf spicata opik 1968 and sinskolutella cuspidata sp nov this assemblage is derived from a carbonate sedimentary package representing a high energy shallow water archaeocyath renalcis biohermal facies of terreneuvian stage 2 age which transitions up section to a more restricted low energy intra shelf lagoonal environment that correlates with a cambrian series 2 stage 3 age the new taxon j phaseloa sp nov has a first appearance datum fad in shallow water biohermal facies of the hideaway well member of the wilkawillina limestone at a level 47 m below the fad of pelagiella subangulata which is taken to approximate the base of series 2 stage 3 in south australia along with liangshanella circumbolina this makes j phaseloa sp nov amongst the oldest bivalved arthropods in south australia and potentially greater gondwana the presence of 25 bradoriid taxa from the early cambrian of south australia suggests east gondwana represents a major centre of origin for the bradoriida

the growth of the continent through geological time studied by nd isotope analysis of shales abstract we determined the nd isotopic composition and the sm nd ratios in a series of australian shales ranging from 0 2 gy to 3 3 gy the first result of this study is the constancy of the sm nd ratio in these shales as in granitoids secondly the initial 143nd 144nd ratio gives a regular curve decreasing through geological time both results confirm that shales are representative samples of the continental crust when insoluble elements like ree are studied we calculated their nd model ages of crustal differentiation the model ages regularly decrease with the stratigraphic ages and after 2 gy the curve flattens and tends to an asymptotic value at around 1 8 gy the significance of the shales is that they represent a mixture of continental materials and we consider the model age of such a mixture directly linked with the mean age of the continental portion feeding the sedimentary basin from these results we deduce a quantitative model of the growth curve of the continental source of the shales taking into account the effects of erosion which selectively sample recent mountains relative to shield areas we propose that the results obtained here are representative of the whole gondwana continent having studied the case of the australian shales we try to extend our study first we applied our quantitative approach to the recent results obtained by o nions et al on north atlantic provinces the continental growth curves obtained by our inversion procedure are quite distinct from the australian shales showing the regional character of shales with these two cases studied we try to compare our results with the already developed model for continental growth the consideration of the surface conservative versus surface non conservative models clearly show the non uniqueness of the geological interpretation of the growth curve on the other hand we have calculated for each case the recycling rate versus geological time by comparing the growth curve with the hurley rand province age curve such recycling increases with time in agreement with the data obtained on nd and sr initial ratios on granitoids

contrasting types of late archean plutonic rocks in northwestern ontario implications for crustal evolution in the superior province abstract the late archean aulneau and lount lake batholiths have contrasting geochemical characteristics that are interpreted to be a consequence of derivation from different sources the aulneau batholith which was intruded into low grade metavolcanic rocks of the wabigoon subprovince is characterized by high na2o and low k2o rb and y together with enriched light rare earth elements lree near chondritic heavy rare earth elements hree negligible eu anomalies and mantle type initial sr and nd isotopic compositions these geochemical characteristics are consistent with derivation from the partial melting of metamorphosed tholeiitic basalt in contrast the lount lake batholith which was intruded into older 2 83 3 17 ga tonalitic plutonic rocks of the winnipeg river subprovince has relatively high k2o rb y and ree low na2o and prominent eu anomalies the initial sr and nd isotopic compositions of the lount lake batholith are consistent with a crustal source at least 200 400 ma older than the crystallization age of the batholith these geochemical characteristics indicate that the lount lake batholith originated from the partial melting of deeper level equivalents of the tonalitic metaplutonic rocks 2 83 3 17 ga which they intrude these results suggest that reworked 3 0 ga crust may be extensive in the winnipeg river subprovince and that the geochemical and isotopic signature of late archean plutonic rocks may be a useful reconnaissance tool in the identification of areas of extensive older crust the results can be interpreted within a uniformitarian model diachronous ensimatic magmatic arcs represented by the winnipeg river and western wabigoon subprovinces developed independently collision of these arcs at 2707 8 ma marks the cessation of subduction and related magmatic activity and the initiation of high grade metamorphism and melting in tectonically thickened portions of the crust

carbon isotope stratigraphy of the precambrian cambrian sukharikha river section northwestern siberian platform a high resolution carbon isotope profile through the uppermost neoproterozoic lower cambrian part of the sukharikha section at the northwestern margin of the siberian platform shows prominent secul