ndsr isotopic study of proterozoic to triassic sediments from southeastern british columbia abstract sedimentary rocks from the southeastern canadian cordillera varying in age from mid proterozoic to triassic have been analysed for their nd isotopic composition e nd t values range from 1 5 to 17 2 and nd model ages t dm nd vary from 1 00 to 2 32 ga much in excess over their stratigraphic ages the difference between the two ages ranges from 0 43 to 1 93 ga decreasing as the sediments become stratigraphically older nd isotopic compositions of sediments suggest that there are two distinct continental sources of sediments in the cordilleran miogeocline a an older source e nd 0 from 20 3 to 23 8 with apparent crustal residence ages of 2 26 2 32 ga and b a younger source average e nd 0 14 0 with the crustal residence age of about 1 8 ga the older residence ages are interpreted to be due to mixtures of materials from the archean and proterozoic crusts sediments belonging to the laib formation lower cambrian and lower clastic part of the milford group namurian appear to have derived from an archean continental segment with some admixture of sediments from the early proterozoic source the younger crustal source which seems to be the principal supplier of sediments for most of the stratigraphic units aldridge formation middle proterozoic horsethief creek group upper proterozoic hamill and lardeau groups lower cambrian nelway formation middle cambrian active formation ordovician and part of milford group is not the archean canadian shield immediately to the east recent subsurface mapping of the alberta basement beneath the phanerozoic sedimentary cover using u pb zircon dating of igneous lithologies 1 has established the extensive archean and early proterozoic crustal provinces it is proposed here that the early proterozoic province of west central alberta is the most likely source of the easterly derived sedimentary rocks of the cordilleran miogeocline with t cr ages from 1 94 to 1 50 ga on the basis of sedimentologic tectonic and isotopic evidence we suggest that the younger sediments could be to some extent recycled older sediments for the triassic slocan and ymir groups eastern part of quesnel terrane sedimentologic lithologic tectonic and isotopic evidence suggest the source was a western one with about 1 8 ga crustal residence age comparison of the crustal residence ages with stratigraphic ages shows the best fit nd evolution curve for the cordilleran miogeocline approximately coincides with the continental growth curve deduced for the north atlantic province and similar to the growth curve for gondwana during the phanerozoic this curve also passes through the north american composite the levelling off of crustal residence ages for the mid proterozoic to triassic sediments at about 1 8 ga supports the idea of negligible crustal growth since at least 1 7 ga the average crustal residence age of the modern miogeoclinal and cratonic sediments

continental tectonics and continental kinetics abstract we present a model of continental growth which combines the results of geochemical studies and tectonic ideas about the evolution of continents through geological time the process of continental growth is mainly controlled by surface phenomena continental material is extracted from the mantle along subduction zones at the periphery of oceans and is destroyed in collision zones where it is remobilized and made available for subduction we derive an equation for s the portion of the earth s surface occupied by continents which reads as follows ds dt a 1 s b s coefficients a and b depend on the geometry of plates on their number and on their velocities we assume that they decrease exponentially with time with the same time scale this model satisfies both geochemical and tectonic constraints and allows the integration of several current observations in a single framework

geological and isotopic geochemical constraints on the evolution of the fuping complex north china craton abstract the neoarchean to paleoproterozoic fuping complex together with the adjacent hengshan and wutaishan complexes form the middle part of the central zone that separates the north china craton into the eastern and western archean continental blocks the fuping complex comprises three major lithological assemblages the 2 50 ga fuping trondhjemite tonalite granodiorite ttg gneisses the wanzi paragneisses and the 2 08 and 2 02 ga nanying granitic gneisses the ttg gneisses contain abundant enclaves of highly metamorphosed supracrustal rocks including 2 70 ga mafic granulites and amphibolites with endt of 3 29 to 3 96 and tdm of 2 72 2 82 ga and are characterized by tdm of 2 76 3 04 ga and a linear correlation between endt 1 64 to 0 96 and 1 nd indicative of mixing of melts derived from the 2 70 ga metabasic rocks and 3 0 ga crustal materials mafic dikes along the chengnanzhuang shear zone were most likely emplaced at 2 31 ga and have endt of 2 24 to 4 78 and tdm of 2 34 2 59 ga the nanying granitic gneisses with endt of 4 04 to 5 24 were most likely related to partial melting of the 2 70 ga supracrustal rocks and or the 2 50 ga ttg gneisses these geological relationships and isotopic data suggest that the fuping complex probably formed from the following five major tectonomagmatic events 1 mafic magmatism and early crustal growth at 2 7 ga with the possible existence of older 3 0 ga crust s 2 collision related crustal thickening underplating of basaltic magma high grade metamorphism crustal anatexis and ttg magmatism at 2 5 ga 3 extensional deformation and emplacement of the mafic dikes and the nanying granitic plutons along the chengnanzhuang shear zone 2 5 to 2 0 ga 4 s type granitic magmatism and associated amphibolite facies metamorphism 2 0 to 1 80 ga and 5 emplacement of granitic pegmatite dikes at 1 80 ga

the composition and evolution of the continental crust rare earth element evidence from sedimentary rocks the composition of the present day upper crust inferred from the uniformity of sedimentary rock r e e rare earth element patterns is close to that of granodiorite a revised andesite model is used to obtain total crustal composition the lower crust is the composition remaining assuming that the upper crust one third of the total is derived from intracrustal partial melting the upper crustal r e e pattern has pronounced eu depletion eu eu 0 64 the lower crustal pattern has eu enrichment eu eu 1 17 and the total crust has no eu anomaly relative to chondritic abundances the eu depletion in the upper crust is attributed to retention of eu in plagioclase in the lower crust because plagioclase is not stable below 40 km 10 kbar the anomaly is intracrustal in origin the archaean upper crust has a different r e e pattern to that of the present day upper crust being lower in total r e e and la yb ratios and lacking an eu anomaly these data are used to infer the archaean upper crustal composition which resembles that of the present day total crust except that ni and cr contents are higher the archaean crustal composition can be modelled by a mixture of tholeiites and tonalite trondhjemites the latter have steep light r e e enriched heavy r e e depleted patterns consistent with equilibration with garnet and hence probable mantle derivation there is little reason to suppose that the archaean lower crust was different in composition from the upper crust except locally where partial melting episodes occurred the r e e evidence is consistent with isotopic and geological evidence for a low continental growth rate in the early archaean a massive increase to about 70 of the total crust between about 3000 and 2500 ma b p and a slow increase until the present day the change from archaean to post archaean r e e patterns in the upper crust is not isochronous but is reflected in the sedimentary rock r e e patterns at differing times in different continents on the basis of a model composition for the mantle 36 of the potassium 30 of uranium 15 of lanthanum and 3 of ytterbium are concentrated in the present continental crust this enrichment is related to ionic size and valency differences from common mantle cations e g mg fe pre 3 9 ga b p crusts were obliterated by meteorite bombardment no geochemical evidence exists for primordial anorthositic sialic or mafic crusts

nd and sr isotopic systematics of central australian granulites chronology of crustal development and constraints on the evolution of lower continental crust nd and sr isotopic data are reported for a granulite terrain in the proterozoic arunta block of central australia sm nd data from a wide range of rock types define a crust formation age of 2 070 125 ma and provide further evidence for voluminous crustal growth in the proterozoic an nd value of 1 5 0 8 indicates a depleted mantle source for this crustal segment and there is no evidence for a large component of significantly older sialic crust field relationships geochemistry and rb sr data for mafic and felsic granulites indicate that intracrustal differentiation and polyphase deformation were followed by granulite facies metamorphism rb depletion at 1 800 ma rb sr data for strongly retrogressed granulites define an age of 1 700 ma which is interpreted as the time of retrograde biotite growth partial melting at the presently exposed crustal level and anatexis at deeper crustal levels were broadly coeval with the retrograde metamorphism sm nd and rb sr isotopic systematics of minerals indicate that the terrain cooled slowly did not experience significant uplift until 1 000 ma and remained at temperatures above 320 c until the late palaeozoic the mineral data are consistent with geological relationships and petrological evidence for a prolonged period of isobaric cooling followed by uplift late in the metamorphic evolution of the terrain the granulite protoliths appear to have formed in a rift which closed within 280 ma of initial separation deformation and granulite facies metamorphism at 1 800 ma are interpreted to be a consequence of collision between the continental blocks which defined the rift regional retrogression and granitoid magmatism at 1 700 ma are attributed to underthrusting of the granulites by lower grade rocks in the final stages of collision subsequent events in the cooling and uplift history appear to have been controlled by the presence of long lived major faults in the crust and a prolonged history of episodic compression in the continental lithosphere the results of this study suggest that granulite terrains in general cannot be equated with lower continental crust but instead represent assemblages of mainly supracrustal rocks which in some instances have been involved in major collision events

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

nd isotopic anatomy of a pebble conglomerate from the murihiku terrane of new zealand record of a varied provenance along the mesozoic gondwanaland margin abstract the murihiku terrane is a volcano sedimentary terrane of late permian to early cretaceous age that forms part of a collage of accreted terranes in the eastern province of new zealand these terranes record a history of deposition along the gondwanaland margin and include terranes of oceanic character e g brook street terrane and terranes dominated by continental detritus e g torlesse terrane the murihiku terrane is of particular interest because it is relatively large one of the least structurally deformed terranes and preserves a long 120 m y record of sedimentation previous studies have suggested that the murihiku terrane preserves a record of change in provenance from predominantly mafic juvenile sources to dominantly felsic volcanic sources in middle to late triassic time interpreted as containing contributions from precambrian continental crust and modest amounts of detritus supplied directly from this old crust frost c d coombs d s 1989 nd isotope character of new zealand sediments implications for terrane concepts and crustal evolution american journal of science 289 744 770 roser b p coombs d s korsch r j campbell j d 2002 whole rock geochemical variations and evolution of the arc derived murihiku terrane new zealand geological magazine 139 665 685 we present nd isotopic data from a late triassic rhetian otapirian local stage 200 206 ma roundstone cobble conglomerate from near gore south island new zealand the sandstone matrix of this conglomerate has an initial nd of 3 0 and sm nd of 0 219 values that are intermediate between the more radiogenic nd isotopic and higher sm nd ratios of older murihiku siltstones and less radiogenic lower sm nd younger siltstones clasts within this matrix are mainly volcanic we analyzed basaltic andesite andesite and dacite pebbles along with a volcanogenic sandstone pebble and hypabyssal quartz diorite clast these pebbles have initial nd between 3 7 and 2 6 and are probably derived from a late triassic volcanic arc a felsic plutonic pebble with an initial nd of 0 5 and crustal residence age of 1 0 ga may record erosion of gondwanaland proterozoic basement a mudstone rip up clast may be composed of fine grained cratonic detritus it has a nd crustal residence age of 1 1 ga this conglomerate containing both arc derived and older cratonic derived clasts captures in a single sample the contribution of mafic arc felsic arc and craton derived sedimentary rocks to bulk sediment nd budgets recorded in the murihiku terrane

crustal evolution in south india constraints from nd isotopes nd isotope analyses from a range of lithologies including charnockites metasediments and granitoids that comprise the high grade terrain of south india define contrasting age provinces north of the palghat cauvery shear zone the karnataka craton and the granulite blocks of nilgiri and madras are characterized by model nd ages from 3 4 to 2 4 ga with significant crustal growth during the late archean south of the palghat cauvery shear zone a pan african granulite facies event resulted in sm nd fractionation as observed in several incipient charnockite localities model nd age calculations that allow for sm nd fractionation at 550 ma yield ages in the range of 2 9 to 1 3 ga this range compares closely with published model nd ages from the high grade terrain of sri lanka mid proterozoic crustal growth 1 1 ga observed in the vijayan complex of sri lanka may have provided source material for metasediments characterized by young model nd ages in southern peninsular india the palghat cauvery shear z

the history of a continent from upb ages of zircons from orinoco river sand and smnd isotopes in orinoco basin river sediments abstract we report shrimp u pb ages of 49 zircons from a sand sample from the lower orinoco river venezuela and nd model ages of the fine sediment load from the main river and tributaries the u pb ages reflect individual magmatic or metamorphic events the sm nd model ages reflect average crustal residence ages of the sediment sources together they allow delineation of the crust formation history of the basement precursors of the sediments the u pb ages range from 2 83 to 0 15 ga and most are concordant or nearly so discrete age groupings occur at 2 8 2 1 and 1 1 ga the oldest group contains only three samples but is isolated from its closest neighbors by a 600 ma age gap larger age groupings at 2 1 and 1 1 ga make up about a third and a quarter of the total number of analyses respectively the remaining analyses scatter along concordia and most are younger than 1 6 ga the 2 8 and 2 1 ga ages correspond to periods of crust formation of the imataca and trans amazonian provinces of the guyana shield respectively and record intervals of short but intensive continental growth these ages coincide with 2 9 and 2 1 ga nd model ages of sediments from tributaries draining the archean and proterozoic provinces of the guyana shield respectively indicating that the u pb ages record the geological history of the crystalline basement of the orinoco basin zircons with ages corresponding to the major orogenies of the north atlantic continents the superior at 2 7 ga and hudsonian at 1 7 1 9 ga were not found in the orinoco sample the age distribution may indicate that south and north america were separated throughout their history nd model ages of sediments from the lower orinoco river and andean tributaries are 1 9 ga broadly within the range displayed by major rivers and dusts this age does not coincide with known thermal events in the region and reflects mixing of sources with different crust formation ages the igneous and metamorphic history of these sources as recorded by the detrital zircons is that of the orinoco basin basement this implies that despite evidence of fast sedimentary recycling global similarities in nd crustal residence ages and the probability of cross continent mixing through continental drift the sedimentary material carried by individual rivers is mainly derived from the crystalline basement in the basin the global semblance in nd isotope ratios in major river sediments and atmospheric dusts results from the averaging effect of large scale sampling of the continents which are heterogeneous in age on smaller regional scales a large portion of the continental crust in the orinoco basin formed during the trans amazonian orogeny at 2 0 2 1 ga and smaller portions formed both earlier at 2 8 ga and later after 1 6 ga these observations which are consistent with the relative sizes of crustal age provinces in the orinoco basin indicate that sediments from the lower orinoco and andean tributaries contain 25 35 of material added to the crust since trans amazonian times nd model ages of these sediments underestimate the average crust formation age of the basement of the orinoco basin by only about 10 if this relationship holds in other river basins then nd model ages of major rivers and wind blown particulates indicate that the mean age of the continental crust is 1 9 2 ga

sm nd studies of archaean metasediments and metavolcanics from west greenland and their implications for the earth s early history metasedimentary and metavolcanic rocks from the archaean of west greenland have been examined for evidence of crustal components greater than 3 8 ga in age and for their compatibility with the presently adopted bulk earth sm nd parameters sm nd isotopic data have been obtained for the garbenschiefer metagabbro unit metasediments from the isua supracrustal belt gneisses interior to the isua belt and metasediments from the malene supracrustal belt using estimates of emplacement age t of between 3 77 and 3 67 ga for the parental volcanics to the garbenschiefer unit initial143nd 144nd ratios yield positiveendt values between 1 0 and 3 1 relative to the chur parameters for seven out of eight samples model sm nd ages for the isua gneisses and metasediments are only compatible with their estimated stratigraphic ages if their sources were ca 2end relative to chur at those times similarly model sm nd ages for the malene samples are only compatible with stratigraphic age constraints when based on a source evolution with positiveendt implications of these results for the early development of the earth s mantle are discussed

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

the komandorsky basin as a product of spreading behind a transform plate boundary abstract a long term study of the komandorsky basin in particular by the cruises of r vvulkanolog and r vdmitry mendeleev in the summer of 1988 made it possible to describe in detail the main structural features of this marginal basin several ne and n s trending spreading axes were detected in the basin in the rear of the transform sector of the aleutian arc spreading axes are shifted relative to each other along nw striking transform faults there is variation in style from north to south along the strike of the spreading axes and a less well defined symmetry compared to that typical of mid oceanic ridges the tectonic pattern and heat flow reflect several stages starting from the eocene in the history of the opening of the basin caused by the changes in the velocity and direction of the relative motion between the pacific and north american plates

crustal evolution of the hercynian belt of western europe evidence from lower crustal granulitic xenoliths french massif central abstract granulite facies lower crustal xenoliths scavenged by the tertiary basaltic pipes of bournac and roche pointue massif central france have been analysed for major and trace elements and radiogenic sr nd isotope ratios they display wide variations in major elements rare earth elements and isotopes 87 sr 86 sr 0 7041 0 7216 nd 11 9 to 1 5 which tend to correlate with the three different rock types encountered basic and acidic meta igneous and metasediments the basic liquids are of calc alkaline affinity and cumulates are abundant there is a gap in silica content between the basic and acid meta igneous xenoliths the isotopic compositions suggest mixing of old pre existing continental crust represented by the metasediments and mantle derived basic melts which represent juvenile addition to the crust during the hercynian orogeny the basic meta igneous rocks have been contaminated during their emplacement into the crust whereas the surrounding metasediments have suffered partial fusion and contamination by the same liquid producing the hercynian granitoids which have nd and sr initial ratios identical to those of the lower crustal xenoliths the residue from melting may be represented by the acid meta igneous rocks some of which have positive eu anomalies these results indicate that an important basaltic underplating event occurred during the evolution of hercynian orogenic belt although the exact timing of the underplating requires further constraint

some ideas on the rock cycle 1788 1988 abstract hutton s procrastination on the subject of unconformities left his cyclic vision of geology in the shadows while his success in demonstrating the igneous origin of granite led on to classical magmatism a paradigm of secular evolution that reigned practically unchallenged for almost a century when radiometric dating revealed the true extent of geologic time a neo huttonian cyclic view gained strength in europe because of its ability to explain the salt content of the ocean and the sodium fixed in metamorphism of sediments to paragneiss both of which had become embarrassing to the magmatists meantime in north america a quite independent movement was afoot to claim for sediments and cyclic processes their rightful place in geologic theory it led to the acknowledgment that the cyclic and secular views of the earth are complementary and not contradictory and that geologic materials are recycled on varying time scales against a background of slow secular evolution of the earth

sr_nd_pb isotope systematics in amazon and congo river systems constraints about erosion processes abstract 87 sr 86 sr 143 nd 144 nd 206 pb 204 pb 207 pb 204 pb and 208 pb 204 pb isotopic ratios and rb sr sm nd u pb and th concentrations have been measured in the suspended loads of the congo and amazon rivers and their tributaries in the dissolved load 87 sr 86 sr rb sr nd sm u pb and th concentrations are also reported these results show that nd sm th and pb are almost insoluble and that their mass balance is controlled by particulates whereas rb sr and u are fractionated between soluble and particulate phases the 87 sr 86 sr ratios can only be interpreted after computing the amount of carbonate recycling and the partitioning for silicates between soluble and insoluble this paper presents a method based on the lead isotopic system that can be used to test the steady state of erosion which is tacitly assumed in many river and erosion studies the results presented show that the steady state is validated in each river of the congo basin and in the lowland rivers of the amazon basin but in not verified in the rivers from the andes rio solimoes and rio madeira 87 sr 86 sr 206 pb 204 pb and 208 pb 204 pb ratios are positively correlated and negatively correlated with 143 nd 144 nd ratios a number of arguments indicate that these correlations are mixing lines the end members being orogenic and shield components respectively for the amazon basin analysis of the river loads shows that the orogenic zone is favoured some 5 times more than the shield on an equal area basis this leads to a reinterpretation of sm nd data for shales and casts some doubts upon the proposition of secular variations in the sm nd ratio of the continental crust

geochemical and isotopic characteristics of blueschist facies rocks from the ile de groix armorican massif northwest france abstract nine samples of blueschist facies metabasic rocks and four samples of associated metasedimentary lithologies were analysed for ree contents and nd isotopic composition sampling includes the main rock types exposed on groix concentrating on those metabasic rocks with least dispersed rb sr systems the ree abundance patterns appear not to have suffered significant alteration since emplacement of the protolith magma despite subsequent blueschist facies metamorphism the data when integrated into a wider geochemical data base allow the identification and characterization of both tholeiitic and alkaline magma types in the original igneous suite which was differentiated from depleted mantle sources the basalts probably originated in an oceanic context some sediments are predominantly volcaniclastic in which a continental crustal component may be recognized other sediments are closer in composition to continental crust with variable additions of tholeiitic or alkali basalt debris the volcanic sedimentary assemblage on groix is suggestive of ocean island magmatic activity associated with a sedimentary sequence derived from a nearby continent

are strongly peraluminous magmas derived from pelitic sedimentary sources igneous compositions that require the presence of a phase more aluminous than biotite are here designated as strongly peraluminous ps such compositions are common but subordinate in felsic rocks and exceedingly rare in more mafic rocks the term s type has been applied to many igneous rocks based largely or even solely upon their ps compositions neglecting or placing less emphasis on other criteria for sedimentary parentage in chappell and white s i and s type classification system the s type designation was in fact initially restricted to granites which were thought to have been derived from a chemically mature strongly weathered pelitic shaly source application of chappell s and white s criteria together with others based upon isotopic and phase equilibria considerations leads to the following conclusions about ps igneous rocks worldwide 1 very few ps rocks are derived entirely from pelitic sources 2 a majority are derived largely from intermediate to felsic crustal sources including

geochemistry and isotopic characteristics of mafic phulad ophiolite and related rocks in the delhi supergroup rajasthan india implications for rifting in the proterozoic abstract regionally metamorphosed mafic rocks phulad ophiolite in the delhi supergroup n w india have tholeiitic compositions the data indicate that there are two independent rock suites phulad barr jetgarh pbj meta basalts with element abundances and ratios resembling midocean ridge basalts morb and ranakpur desuri r d meta basalts and meta gabbros with relatively high al2o3 and low tio2 for similar mgo and high lil element rb ba sr coupled with low hfs zr nb and ree sm nd abundances the compositions of these rocks are characteristic of ocean arc basalts dioritic rocks at ranakpur exhibit low k tholeiitic differentiation trends and have close chemical affinity with the spatially associated meta basalts ranakpur diorites define a sm nd whole rock isochron corresponding to an age of 1012 78 ma with an initial nd isotopic composition that is similar to bulk earth sm nd mineral isochrons for a meta gabbro diorite and quartz diorite at ranakpur correspond to ages of 838 36 835 43 and 791 43 ma respectively these younger ages reflect metamorphic re equilibration of isotope compositions during widespread granite and granodiorite emplacement in this region during the late proterozoic model sm nd ages for the meta basalts calculated relative to undepleted mantle vary between 1300 1700 ma the chemistry of both mafic rock suites indicates derivation from source regions which had been depleted during previous melting episodes so the model ages are a maximum age constraint ranakpur desuri mafic rocks have negative initial nd 1 0 to 4 5 and low sm nd ratios suggesting crustal contamination or reworking with a few exceptions the pbj meta basalts have positive initial nd 1 5 to 4 8 indicating that they were derived from heterogeneously enriched although longterm depleted mantle the chemical data support the hypothesis that the pbj meta basalts are fragments of proterozoic ocean crust

lead isotopic composition of hercynian granitic k feldspars constrains continental genesis lead isotope compositions have been determined in many k feldspars from granitic rocks emplaced during the variscan hercynian orogeny sensu lato around 300 myr in various parts of europe and the eastern us the ratios show a regular geographical pattern with contours arched around southern france and parallel to the atlantic coast of the us this distribution parallels predriftpalaeogeographical zones during the palaeozoic the lead isotope pattern of variscan granitic rocks is similar to that of some orogenic volcanic suites but is better explained by crustal melting than by an hercynian mantle crust material mixing the continental crust from these areas must be older than 3 000 myr large scale fluid circulation in the crust during orogenic processes apparently explains the observed geographical patterns of lead isotopes

rates of generation and destruction of the continental crust implications for continental growth less than 25 of the volume of the juvenile continental crust preserved today is older than 3 ga there are no known rocks older than approximately 4 ga and yet a number of recent models of continental growth suggest that at least 60 80 of the present volume of the continental crust had been generated by 3 ga such models require that large volumes of pre 3 ga crust were destroyed and replaced by younger crust since the late archaean to address this issue we evaluate the influence on the rock record of changing the rates of generation and destruction of the continental crust at different times in earth9s history we adopted a box model approach in a numerical model constrained by the estimated volumes of continental crust at 3 ga and the present day and by the distribution of crust formation ages in the present day crust the data generated by the model suggest that new continental crust was generated continuously but with a marked decrease in the net growth rate at approximately 3 ga resulting in a temporary reduction in the volume of continental crust at that time destruction rates increased dramatically around 3 billion years ago which may be linked to the widespread development of subduction zones the volume of continental crust may have exceeded its present value by the mid late proterozoic in this model about 2 6 2 3 times of the present volume of continental crust has been generated since earth9s formation and approximately 1 6 1 3 times of this volume has been destroyed and recycled back into the mantle this article is part of a discussion meeting issue earth dynamics and the development of plate tectonics9

oldest rocks from peninsular india evidence for hadean to neoarchean crustal evolution abstract the evolution of continental crust during hadean and archean and related geodynamic processes provides important clues to understand the early earth history here we report evidence for hadean and eoarchean crust from the fringe of the coorg block one of the oldest crustal blocks in peninsular india we present geological petrological and geochemical data together with zircon u pb ages and lu hf isotopes from a suite of metaigneous granitoids diorite charnockite metavolcanics and metasedimentary quartz mica schist calcareous schist ferruginous quartzite and bif rocks petrological and geochemical studies indicate that the igneous suite formed from subduction related arc magmatism and that the sedimentary suite represents an imbricated accretionary package of continental shelf sequence and pelagic components mineral thermometry suggests metamorphism under temperatures of 710 730 c and pressures up to 8 kbar magmatic zircons in the metaigneous suite show oscillatory zoning and high th u contents up to 3 72 and record multiple pulses of magmatism at ca 3 5 3 2 2 7 and 2 5 2 4 ga the metasedimentary rocks accreted along the margins of the coorg block show multiple zircon population with mean 207pb 206pb ages at 3 4 3 2 3 1 2 9 2 7 2 6 2 5 2 2 2 0 and 1 3 ga and overprinted by younger thermal event at ca 0 8 0 7 ga zircons in the 3 5 ga dioritic gneiss show positive ehf t values ranging from 0 0 to 4 2 and hf crustal model ages tdmc of 3517 to 3658 ma suggesting that the parent magma was derived from eoarchean juvenile sources the zircons in the 3 2 ga charnockite display ehf t values in the range of 3 0 to 2 9 and hf crustal model ages tdmc of 3345 to 3699 ma the neoarchean metagranites diorites and felsic tuff show both positive and negative ehf t values and a range of tdmc values from 2904 to 3609 ma suggesting magma derivation from meso to eoarchean juvenile and reworked components the tdmc values of the dominant zircon population in the metasedimentary suite range from 3126 to 3786 ma with the oldest value 4031 ma recorded by zircon grain in a ferruginous quartzite the ehf t values of detrital zircons also show both positive and negative values with a dominant crustal source our zircon u pb and lu hf data suggest vestiges of neohadean primordial continental crust in peninsular india with episodic crustal growth during eoarchean mesoarchean and neoarchean building the continental nuclei the results contribute to the understanding of crustal evolution in the early history of the earth

geochemical and isotopic studies of the sedimentary and granitic rocks of the altai orogen of northwest china and their tectonic implications the altai orogen northwest china represents the southwestern margin of the central asian orogenic belt geochemical and nd sr isotope analyses were carried out on the palaeozoic sedimentary and granitic rocks in order to trace their sources and to evaluate the pattern of continental growth of the orogen nd isotopic data for both the granites and sediments suggest a significant proportion of middle proterozoic crust beneath the altai orogen however addition of juvenile material arc back arc oceanic crust during palaeozoic times is also significant trace elements and isotopic data of sediments suggest their sources were immature they represent mixtures between a palaeozoic juvenile component and an evolved continental crust the early palaeozoic sediments show e nd t 3 4 to 5 0 t dm 1 5 1 8 ga and i sr 0 710 0 712 they represent a passive margin setting with a predominance of evolved crustal material in the source the devonian sequences however might have been deposited in a back arc basin setting produced by subduction of the junggar oceanic crust along the irtysh fault a significant addition of arc material into the sedimentary basin is responsible for the highly variable e nd values 6 to 0 and i sr 0 711 0 706 the carboniferous rocks were also deposited in a back arc basin setting but with predominantly arc material in the source as suggested by an abrupt increase in e nd t 6 to 3 and decrease in i sr 0 7045 0 7051 voluminous syn orogenic granitoids have e nd t 2 1 to 4 3 i sr 0 705 0 714 and t dm 0 7 1 6 ga they were not derived by melting of local metasedimentary rocks as suggested by previous workers but by melting of a more juvenile source at depth post orogenic granites have higher e nd t 4 4 than the syn orogenic granitoids indicating their derivation from a deeper crustal level where juvenile crust may predominate

a sm nd isochron on pelites 1 ga in excess of their depositional age and its possible significance fine grained tremadocian 490 ma sediments of the almaden syncline central iberian zone spain were analyzed for sm nd and rb sr isotopes in order to get information on the crustal evolution of the iberian peninsula these shallow water platform sediments consist mainly of 2 m illite muscovite and quartz plus kaolinite chlorite and minor quantities of plagioclase neodymium contents fall into a range of 11 76 ppm end 490ma range from 1 to 4 and 147sm144nd ratios from 0 1135 0 1504 the rocks yield an isochron age of 1524 173 ma 95 confidence level end init 7 1 more than three times older than the sedimentation age but in line with the mean neodymium crustal residence age of the samples 1 51 ga a comparison of crustal residence ages calculated in a two step model with those calculated in a single step model indicate that syn or post sedimentary fractionation of 147sm144nd ratios is not very likely rather the data seem to indicate an inherited isochron relationship reflecting the composition and mean age of the provenance thus the sm nd system was not disturbed during erosion sedimentation or diagenesis a rb sr errorchron of the tremadocian samples indicates a postdepositional age of 335 15 ma it probably reflects the main folding between lower namurian and upper westphalian or the deepest burial of the strata

where have all the craters gone earth s bombardment history and the expected terrestrial cratering record here we make the first quantitative estimates of the maximum total number of large terrestrial impact craters that can survive until the current era on earth our estimates of crater survival are made using the age of earth s crust and the expected flux of large extraterrestrial bodies striking earth this analysis neglects effects such as weathering and burial which tend to preferentially erase smaller craters thus our results represent the maximum number of craters we expect to exist on earth today although the constant recycling of crustal material creates a strong bias toward younger ages our model suggests that the terrestrial cratering record can be used to determine the size frequency distribution of the impactors that bombarded earth over the past 3 5 g y however our model suggests that the cratering record cannot be used to distinguish between scenarios with a constant flux of impactors over time and scenarios where the bombardment rate was considerably higher in earth s ancient history luckily layers of distal impact ejecta may act as records of impacts even when the source craters have been tectonically recycled we suggest that searches for these distal impact ejecta layers will be more fruitful for constraining the bombardment history of earth than searches for large craters

the recommended cambrian ordovician global boundary stratotype of the xiaoyangqiao section dayangcha jilin province china this paper provides a brief outline of the current detailed inter disciplinary work on the xiaoyangquiao section trying to expose all the aspects for reference tied to the global single stratigraphic point gssp concept for defining the cambrian ordovician boundary the 45 m critical interval of this section outcrops very well along the steep bank of a stream and is free from folding faulting intrusions and has not been affected by weathering colour alternation of conodonts and acritarchs and crystallinity indices of illite all indicate a maximum thermal grade of 100 c the lithofacies being of great lateral persistence for a hundred kilometres consists mainly of a rhythmical sequence of lime mudstone and shales deposited in a moderately deep outer shelf environment of quiet water well below the normal storm wave base chemical investigation of the rocks demonstrates strong positive correlation between a12o3 content and those of k2o mgo fe2o3 tio2 be cr co ni zn and ba indicating these components are tied to the clay fractions a stable depositional environment is demonstrated by the uniform chemistries through the boundary interval close to the boundary itself p2o5 contents are low indicating continuous sedimentation at fairly substantial rates the major biological events the biostratigraphic framework and the stratigraphic range of conodont graptolite trilobite and acritarch taxa are illustrated briefly with diagrams following the majority views of the calgary plenary session the boundary level is to be chosen at a point marked by the first appearance datum fad of the selected conodont taxon or taxa in the vicinity of the level close to but below the first influx of nematophorous graptolites the following four points marked by the incoming of conodont taxon or taxa are recommended for consideration of the golden spike 1 fad of cordylodus intermedius at 5 28 m below the first influx of nematophorous graptolites 2 fad of hirsutodontus simpler cordylodus drucei albiconus postcostatus at 5 23 m 3 fad of semiacontiodus lavadamensis utahconus utahensis monocostodus sevierensis at 3 85 m 4 fad of cordylodus lindstromi at 2 23 m for the following reasons the fad of h simplex c drucei a postcostatus is favoured 1 the taxa are all distinct and widely dispersed 2 intensive evolutionary change took place in conodonts graptolites trilobites and acritarchs prior to or after this point all the fossil groups occur together providing correlation with many regions throughout the world 3 the point is in a position between the previously widely accepted boundary levels based on graptolites and trilobites 4 the proposed point lies within a thin laterally persistent rhythmical sequence the fad of cordylodus lindstromi is also a favourable point sharing many advantages mentioned above but this point is less satisfactory in being defined by the fad of a single taxon c lindstromi which also has an extremely small population size an isochron age of 500 7 7 4 ma is determined from clay fractions of mudstones 8 5 m below the proposed point by means of the rubidium strontium method the nd signature determined from conodonts trilobites hyolithids and acrotretid brachiopods has a mean value of 6 7 comparable with that of the coeval oceanic water mass occupying southeastern north america and europe and indicating that northeastern china bordered the same ocean the mean tdm model age determined was 1 26 ga at the time of sedimentation compatible with the mean tdm model age of approximately 1 1 ga for the pacific ocean today the relatively low value of the tdm model age indicates a substantial input from young orogenic volcanic island arcs and terranes

nd isotopes of siliciclastic rocks from tibet western china constraints on provenance and pre cenozoic tectonic evolution nd isotope data from pre cenozoic siliciclastic rocks from tibet are used to determine variations of source regions with time and hence tectonic history on average the tdm and tchur ages of the tibetan samples are up to 1 83 0 36 ga and 1 25 0 45 ga respectively indicating that tibet is underlain by lower proterozoic basement and has close affinity with the himalayas an nd isotope crisis occurred at the early permian with older samples defining a trend of slight decrease in end values with time while the lower permian rocks sharply increasing 6 5 end units this is related to the coeval widespread basaltic magmatism in tibet which could be responsible for tibet rifting away from northern gondwanaland the central qiangtang meta sedimentary samples have distinct nd isotope characteristics end 0 19 87 1 51 tdm 2 07 0 13 ga tchur 1 78 0 21 ga from the songpan ganzi samples end 0 12 77 2 87 tdm 1 49 0 18 ga tchur 1 08 0 23 ga suggestive that these two groups were derived from differentsourcesandthatthe former werenot theequivalents ofthe latterthat wasunderthrust fromthe jinsasuture andmayrepresent subducted continental crust the mid jurassic flysch rocks in northern lhasa have typical upper crust nd isotopic signatures and could have formed under a passive margin environment their bangong counterparts averagely 6 35 units higher in end values unveil derivations from a likely magmatic arc along the northern bangong margin basaltic pebblewith low nd content and endvalue from the berriasian valanginian rocks in central tibet along with the samples with generally high end values shows that the bangong ophiolites could have been a main source of sediments during the earliest cretaceous time a significant transition of sediment sources occurred at the beginning of the hauterivian stage 120 ma sedimentary basaltic tuff end t 2 27 in the hauterivian lower cenomanian samples in central tibet and common nd signatures for input of mantle like materials in these rocks indicate that mantle derived magmatic rocks could have constituted a significant source the huge hauterivian lower cenomanian sediments in central tibet are thus inferred to have been accumulated under a back arc rifting setting 2007 elsevier b v all rights reserved

episodic mafic crust formation from 4 5 to 2 8 ga new evidence from detrital zircons slave craton canada the hf and 18o values in detrital zircons from the slave craton canada indicate three episodes of crust formation between ca 4 5 and 2 8 ga namely at ca 4 4 4 5 ga ca 3 8 ga and ca 3 4 ga most of the juvenile crust appears to have been mafic in composition and there is no clear evidence for initial granitic protocrust in the hadean of the slave craton the range of initial hf values in zircons increases from 3 9 to 2 8 ga indicating that both extraction of new material from mantle and reworking of the older crust are important for the secular evolution of the continental crust a preliminary review of available hf data in zircons suggests that the three episodes of crust generation may have been of global importance the mafic crust formed in the archean and the hadean was then reworked for at least 0 5 1 5 b y as indicated by data from the slave craton gondwana and the limpopo belt of africa

petrology of the archean pontiac and kewagama sediments and implications for the stratigraphy of the southern abitibi belt we present an interpretation of the petrology of two archean sedimentary groups in the southeastern portion of the abitibi greenstone belt 1 the pontiac sediments which represent an assemblage of quartz rich sandstones and 2 the kewagama sediments which are quartz poor and rich in volcanic fragments the pontiac sediments display a north south decrease in bed thickness sand shale ratio and grain size suggesting derivation from a northern source relative to their present position petrographic and rare earth element ree analysis indicates that three provenances contributed to the pontiac 1 a mature craton 2 light heavy ree fractionated igneous rocks probably of tonalite trondhjemite composition and 3 ultramafic mafic volcanics in contrast to the pontiac sediments the kewagama sediments were derived from acid to mafic volcanic rocks ratio of 3 1 with minor contributions from plutonic rocks and from graded sedimentary rocks possibly the pontiac the ree data indicate a source frac

processes that control mineral and element abundances in shales abstract a shale s composition can be used to infer physical chemical and biological processes that have acted on it over its life history and this knowledge is used to surmise conditions at the time when each process was active this review lists the most important processes that control shale mineral and element abundances during each of the five stages of its life history 1 during weathering transport and deposition the most important processes are chemical weathering hydraulic sorting and mixing adsorption reverse weathering and incorporation of organic matter and biominerals 2 during early diagenesis the most important processes are exchange across the water sediment interface compaction microbial respiration mineral transformations and concretion formation 3 during burial diagenesis the most important processes are compaction catagenesis and metagenesis and mineral transformations 4 during exhumation the most important processes are decompaction brine invasion groundwater penetration and microbial respiration 5 during weathering the most important processes are organic matter oxidation mineral oxidation and hydrolysis groundwater transport and disaggregation this classification of processes provides a way to envision how concurrent and consecutive processes that occurred over a shale s life history have affected its mineralogy and chemical composition

trace element and nd isotopes in shales as indexes of provenance and crustal growth the early paleozoic from the brabant massif belgium abstract the cambrian ordovician shales from the brabant massif show an increase in th 6 7 17 7 ppm and la 16 6 71 2 ppm contents in lree hree ratio la n yb n 3 8 13 9 and in tnd values 3 3 to 8 9 with younger age of the sedimentary rocks this evolution is related to a change in the dominant source rock from basic metavolcanics in the early cambrian to granite gneiss schist rocks in the early ordovician the ta content appears to be controlled by the provenance area as well the modification in the origin of detritus is not reflected in the abundance of sc cr and v but it is slightly recorded in the co and ni concentrations tnd responds more sensitively to source changes than sm nd ratios and crustal residence ages and it is confirmed to be a more powerful parameter to detect new addition of mantle material to the crust the evolution of tnd with stratigraphic age tnd vs t for the brabant shales parallels the trends for coeval clastic rocks from brittany france and southern britain michard et al 1985 davies et al 1985 and is considered to be a characteristic of the armorica plate the tnd vs t path of sediments deriving from crystalline rocks first cycle sediments turns out to be a convenient function to delineate the boundaries of fossil lithospheric plates on the contrary such first cycle sediments are not very useful to investigate the long term continental growth because their tnd vs t path represents a second order pattern episodic evolution that disturbs the first order trend of the sedimentary nd isotope record secular evolution

genesis of granitoid batholiths of peninsular malaysia and implications for models of crustal evolution evidence from a ndsr isotopic and upb zircon study nd sr and u pb isotopic data for the late triassic west coast province batholiths and permian to triassic east coast province batholiths of peninsular malaysia allow estimates of the ages of the crustal fragments comprising the peninsula to be made initial nd and sr values for granitoids from the west coast province range from 6 to 10 and 160 0 716 to 660 0 751 respectively nd model ages calculated based on a depleted mantle evolution model tdmnd range from 1300 ma to 1800 ma and are in general agreement with the mid proterozoic upper intersection ages of u pb zircon reverse discordia 1500 1700 ma initial nd and sr values for granitoids from the east coast province range from 0 8 to 6 and 10 0 705 to 130 0 714 respectively calculated tdmnd ages of 900 1400 ma for these granitoids are comparable to two u pb zircon reverse discordia intercepts that yield 800 ma and 1350 ma the general agreement of u pb zircon inheritance ages and tdmnd ages are interpreted to correspond to the proterozoic crust formation ages of the continental fragments represented by the west coast and east coast batholithic provinces mid proterozoic 1300 1900 ma crust formation ages are commonly shown by other phanerozoic continental margin plutonic and volcanic belts the ubiquitous mid proterozoic crust formation ages and the absence of archaean signatures suggest voluminous juvenile additions to the continental crust in the mid proterozoic such ages at continental margins would imply that many continental blocks had achieved very much their present day extent by the mid proterozoic

rare earth and trace element geochemistry of termite mounds in central and northeastern namibia mechanisms for micro nutrient accumulation abstract rare earth element ree and trace element concentrations of ten termite mounds and adjacent topsoil from central and northeastern namibia were used to investigate the processes underlying the alteration of soil chemical and physical properties by termites macrotermes spp accumulation of micro nutrients in the mounds was of particular interest because of the ecological implications of the enhanced availability of these scarce elements the absolute concentrations of ree and trace elements including nine micro nutrients b fe mn ni cu zn se mo and cd were greater in termite mounds compared to topsoil suggesting a possible external supply of enriched materials or accumulation of in situ weathering products of the underlying bedrock similarities between the chondrite normalized ree patterns of the samples and those of the average upper continental crust ucc and post archaean average australian shale compositions indicated that the mounds and topsoil at each site originated from the same host bedrock coherent behavior between two incompatible elements th and la in both topsoil and mound samples suggested that the two sets of samples were exposed to two different weathering processes the relatively low zr sc ratios fine particle sizes and the persistence of positive eu anomalies of the mounds relative to ucc were interpreted as reflecting 1 a lack of grain sorting effects and 2 active transport of materials rich in primary clay minerals and trace elements from the deeper regolith to the surface by termites lower permeability of mound materials may also prevent leaching of primary minerals and trace elements titanium and zr enrichment in most mound samples 8 samples was attributed to the decomposition of biotite by macrotermes with the concomitant release of ti and zr into the mounds suggesting the termites ability to transform soil minerals and enhance chemical weathering this was further corroborated by a substantial increase in th u ratios in most mounds relative to topsoil because of the oxidizing conditions of the mounds ti and zr are likely to be adsorbed onto fe oxides positive ce anomalies relative to ucc and the high ph values in the mounds indicate calcrete aggradation or mound aeration as a result of burrowing activities of the termites and evaporation from mounds the impacts of the changes in environmental conditions and the termite activities on the mound geochemistry are highlighted by heavy ree hree as well as fe and mn retentions in the mounds heavy ree and light ree lree are likely to coprecipitate with carbonate complexes and fe and mn oxyhydroxides and in the process scavenge a series of micro nutrients such as zn cu cd and co a substantial enrichment of se in mounds despite the oxidizing conditions of the mounds could be attributed to active transport of se enriched materials from deep soil horizons the study demonstrated the suitability of ree and trace element geochemistry for assessing the influence of macrotermes on the physico chemical properties of semi arid soils

the influence of sediment recycling and basement composition on evolution of mudrock chemistry in the southwestern united states this paper reports systematic changes in mudrock composition through time on a single con tinental cmstal block the changes reflect both sediment recycling processes and changes through time in the composition of crystalline material being added to the sedimentary system and are related to tectonic evolution as the block matures from a series of accreted arc terranes to a stable craton the major and trace element distributions reflect different aspects of the provenance of the mudrocks in this study major elements record sediment recycling processes as well as changing proportions of sedi mentary and first cycle source rocks with the exceptions of kzo which tends to increase and sioz and a o3 which show no trend most major oxides tend to decline in relative abundance in younger mud rocks patterns shown by the index of compositional variability fe03 kro naao cao mgo mno tiojialro and by k20 a1203 indicate that the major oxide trends are due to decreasing proportions of nonclay silicate minerals and a concomitant increase in the proportion of clay minerals probably due to decreasing input of first cycle detritus coupled with recycling of sedimentary material excursions from progressive trends marked by increases in mgo k20 and cao reflect episodes of large scale input of nonclay first cycle minerals from crystalline source rocks due to large scale basement uplift the chemistry of low solubility trace elements in contrast is not sensitive to recycling effects and reflects the composition of first cycle input incompatible elements are progressively enriched relative to compatible elements in younger mudrocks and values for chondrite normalised rare earth elements also increase in addition the eu anomaly becomes systematically more negative in younger samples these trends cmnot be explained by diagenetic or weathering processes and therefore indicate that the proportion of fnc tionated granitic first cycle detritus being added to the sedimentary system becomes greater with time these results confirm the importance of tectonic setting in controlling mudrock chemistry and also demonstrate that there is a dynamic relationship between the tectonic evolution of a continental block and the composition of its sedimentary mantle

radiogenic isotopes systematics and applications to earth surface processes and chemical stratigraphy radiogenic isotopes have wide application to chemical stratigraphy geochronology provenance studies and studies of temporal changes in earth surface processes this paper briefly reviews the principles of radiogenic isotope geochemistry and the distribution of a number of elements of interest in the environment and then uses this information to explore the range of applications to chemical stratigraphy and other fundamental subjects of sedimentary geology many of these applications center on the reconstruction of secular variations in paleoseawater isotope signals these seawater isotope curves can be used for stratigraphic correlation and to assess changes through earth history in processes such as tectonic uplift climate change biogeochemical cycling ocean circulation crustal growth and surface water evolution understanding the modern biogeochemical cycle of the element isotope system of interest is essential for gaining the maximum information regarding the operation of ancient cycles the main sources of uncertainty for reconstructing seawater isotope curves involve diagenetic alteration sample impurities and errors in age assignments rigorous sample selection criteria and pretreatment methods are keys to reconstructing the most accurate seawater isotope curves applications are discussed for the rb sr sm nd la ce lu hf re os u th pb k ar and cosmogenic isotope systems d 2003 elsevier b v all rights reserved

contrasting ore styles and their role in understanding the evolution of the altaids abstract the altaids is a paleozoic orogenic collage in central asia it occupies 10 of the eurasian continent and it is one of the richest concentrations of mineral deposits in the world comparable to those of the circum pacific and tethyan orogenic belts in particular it contains many types of hydrothermal ore deposits that are sensitive to the nature and origin of their host terranes including porphyry copper gold molybdenum volcanogenic massive sulfide copper lead zinc and orogenic gold deposits which are all diagnostic of convergent continental margins extensional arc related submarine basins and transpressional terrane boundaries within the altaids porphyry copper deposits are mostly concentrated within the kazakhstan and tuva mongolia micro continents where they formed almost continuously from the early ordovician to permian only a few are cambrian and triassic and most are late carboniferous most volcanogenic massive sulfide ore deposits are distributed along the south urals and rudny altai in the western altaids where they formed in a short time window from the early to middle devonian important orogenic gold deposits formed along regional strike slip faults between craton island arc or micro continent boundaries from the neoproterozoic to permian mostly in the early permian in limited key areas volcanogenic massive sulfide deposits formed upon much older 100 ma basement rocks than those of coeval porphyry deposits indicating that the altaids could not have developed from a single arc furthermore the porphyry deposits outline the distribution of several continental fragments the combined area of which is far less than that of the altaids which suggests that extensive new juvenile material has been added into island arc systems which may have been accreted to surrounding continental margins these metallogenic distributions in localized areas were a result of the mechanisms of crustal growth within the overall tectonic evolution of the altaids and they are consistent with the interpretation that the altaids is a multiple subduction accretion collage

the archean proterozoic transition evidence from the geochemistry of metasedimentary rocks of guyana and montana abstract chemical and isotopic compositions of ancient sedimentary rocks have in some cases provided evidence of first order changes yin the composition and extent of continental crust from the archean to the proterozoic we reversed the typical sampling pattern comparing metapelites and carbonates from proterozoic greenstone belts with those from an archean continental margin sequence and found the reverse of the typical age composition relations metapelites and volcaniclastic metagreywackes from the early proterozoic greenstone belts of northern guyana resemble those of many archean greenstone belts in ree trace and major element compositions mean ratios la sm 5 0 la th 5 1 eu eu 0 80 th u 3 4 median k 2 o na 2 o 0 8 schists from an archean continental marginal sequence in montana have compositions like many post archean metapelites derived from continental sources mean ratios la sm 6 8 la th 2 7 eu eu 0 67 th u 5 2 median k 2 o na 2 o 1 7 other archean continental basin sediments also resemble their post archean counterparts implying that upper continental crusts of these eras may not have differed significantly in these respects however temporal change may have occurred in upper crustal contents of ni and cr many archean clastic metasedimentary rocks of both continental and greenstone belt provenance have contents of these compatible elements higher than in their post archean equivalents dolomitic limestone from a greenstone belt of northern guyana has a very low 87 sr 86 sr ratio of 0 7007 such low values may reflect the local influence of mantle derived volcanic rocks during carbonate deposition diagenesis or metamorphism and they may not be reliable indicators of the evolution of sr in seawater archean dolomitic marbles from the continental marginal sequence of montana have 87 sr 86 sr ratios of 0 7065 and higher significantly more radiogenic than many accepted values for carbonates of archean greenstone belts

singularity analysis of global zircon u pb age series and implication of continental crust evolution abstract recently compiled global databases of igneous and detrital zircon u pb ages have been integrated with other types of isotope data e g neodymium hafnium and oxygen to characterize the episodic growth of the continental crust and the development of supercontinents however to what extent do the age peaks reflect the rate of continent crust growth or the differential preservation due to supercontinent settings is a long standing question instead of analyzing amplitudes and periodicity of the age series here the analysis focuses on shapes of the individual age peaks described by a power law model for measuring the scale invariant fractality and singularity of time series records the results indicate that zircon age distributions around peaks follow power law distribution regardless of the bin size used to measure the age distribution based on the commonly accepted mechanisms phase transition self organized criticality and multiplicative cascade processes for generation of power law distributions one can relate the nonlinearity of the age peaks to short spurts of accelerated magmatic activities due to avalanches superplumes slab breakoff etc occurred during episodic convection of the mantle the exponents of the power law age distributions estimated from the age peaks can be used as an index to quantify the intensity of a singularity the values of exponents calculated at all major age peaks from five global databases exhibit an episodic nature with a mean duration of approximately 600 800 myr both intensity of the zircon episodes and their duration for the interval from 3 ga to 0 5 ga depict a descending trend which may signify mantle cooling

secular boron isotope variations in the continental crust an ion microprobe study abstract tourmalines of various chemical compositions hosted in magmatic rocks with ages ranging from 3 8 ga to the present were analysed for 11 b by ion microprobe a large range of 11 b values from 2 to 30 1 5 was observed the 11 b values are correlated with the chemical composition of the tourmalines the li rich tourmalines being enriched in 11 b compared to fe and or mg rich tourmalines this may reflect mixing in the continental crust between a marine reservoir characterized by high li contents and 11 b values and a continental reservoir characterized by low li contents and 11 b values no relationship between the age and the 11 b values is observed for the fe or for the mg rich tourmalines in contrast the 11 b values of the li rich tourmalines show a tendency to regularly decrease with increasing age this pattern is consistent with a simple model of constant boron isotopic fractionation between seawater and the continental crust during the progressive extraction of the continents a variation in the mean 11 b value of the continental crust from 20 ga ago to 7 at present is predicted by the model the largest variation occurred during the first 2 5 ga

the alkaline peralkaline granitic post collisional tin zebane dyke swarm pan african tuareg shield algeria prevalent mantle signature and late agpaitic differentiation abstract the tin zebane dyke swarm was emplaced at the end of the pan african orogeny along a mega shear zone separating two contrasting terranes of the tuareg shield it is located along the western boundary of the archaean in ouzzal rigid terrane but inside the adjacent tassendjanet terrane strongly remobilized at the end of the precambrian the tin zebane swarm was emplaced during post collisional sinistral movements along the shear zone at 592 2 5 8 ma 19wr rb sr isochron it is a dyke on dyke system consisting of dykes and stocks of gabbros and dykes of metaluminous and peralkaline granites all rock types have sr and nd isotopic initial ratios sr i 0 7028 and e nd 6 2 typical of a depleted mantle source similar to the prevalent mantle prema at that period no crustal contamination occurred in the genesis of the tin zebane swarm even the samples showing evidence of fluid interaction essentially alkali mobility have the same isotopic signature the peralkaline granites have peculiar geochemical characteristics that mimic subduction related granites this geochemical signature is interpreted in terms of extensive differentiation effects due to late cumulates comprising aegirine zircon titanite allanite and possibly fergusonite separated from the liquid in the swarm itself due to magmatic flow turbulence the tin zebane dyke swarm is thus of paramount importance for constraining the differentiation of mantle products to generate highly evolved alkaline granites without continental crust participation in a post collisional setting

the erosion of the alps nd isotopic and geochemical constraints on the sources of the peri alpine molasse sediments abstract sm nd data from molasse sedimentary deposits from eastern france and switzerland are used to quantify the erosion of the alps during the oligocene and miocene the average present day nd value of the continental sedimentary deposits increases from 11 for cretaceous and eocene substratum to 9 for the first oligocene molasse sediments this increase requires the erosion of mesozoic marine sediments to explain the average nd value 9 1 of the rupelian and lower chattian sediments then the average nd value 9 7 and the chemical compositions of the upper chattian and aquitanian sediments are consistent with the erosion of granitic rocks of the variscan crust this change in source allows us to define a second cycle in the peri alpine molasse which began at 24 5 ma with the deposition of the calcaires et dolomies this basin wide unit represents the sedimentary record of a halt in the input of alpine detritus in the western part of the molasse basin and we suggest that the limit between lower and upper chattian sediments at 24 5 ma corresponds to a major tectonic event in the alps a further increase of 1 nd unit recorded by the burdigalian marine sandstones average of 8 defines a third molasse cycle which resulted from the erosion of late variscan alkaline granites having high nd values between 1 8 and 5 2 this study concludes that the erosion of the alps increased the nd values of the sedimentary mass in two stages 1 during the rupelian and lower chattian by recycling of marine chemical sediments having nd values similar to that of tethys seawater nd 8 and which represent 90 of the eroded materials and 2 during the burdigalian by the erosion of a variscan crust representing at least 20 of the eroded material half of this variscan material was composed of alkaline granites with high nd values suggesting the addition of mantle derived material to the crust during late variscan events

provenance and tectonic settings of the late paleozoic sandstones in central inner mongolia ne china constraints on the evolution of the southeastern central asian orogenic belt abstract late paleozoic sedimentary strata outcrop extensively in central inner mongolia and are a key to understanding the tectonic evolution of the southeastern central orogenic belt a combined analysis of petrography whole rock major and trace element and nd isotope is carried out on representative sandstones from the late paleozoic sedimentary strata 420 270 ma the sandstones are mainly wackes and litharenites in lithology with low sio2 al2o3 of 2 85 9 47 averagely 5 22 and poor textural and compositional maturities implying short sediment transportation between the depositional basins and provenances the trace element compositions are generally comparable to that of the average upper continent crust ucc with negatively sloping chondrite normalized rare earth element distribution patterns la yb n 3 43 11 averagely 6 94 and flat ucc normalized trace element distribution patterns the nd isotopic compositions show great variation nd t 5 01 to 5 35 with depositional time of the sandstones and coincide well with the arc magmatic phases in central inner mongolia the geochemical signatures of the sandstones indicate that the dominant provenances are intermediate to felsic arc magmatic rocks that have ages approximating the deposition although old recycled sediments may have made a minor contribution an active continental arc setting during the late paleozoic in central inner mongolia controlled by the northward subduction of the paleo asian oceanic slab was the most likely depositional tectonic setting of the sandstones this active continental arc setting continued to at least 270 ma implying that the final closure of the paleo asian ocean along the solonker suture zone most likely occurred sometime during the late permian to early triassic the northward subduction of the paleo asian ocean is likely of west pacific style in which the present day baolidao arc has a close genetic link with the south mongolian microcontinent and likely the former originally formed as the arc margin of the latter

rare earth element variations in mid archean banded iron formations implications for the chemistry of ocean and continent and plate tectonics abstract abundances of major and rare earth elements rees are reported for mid archean 3 3 3 2 gyr sedimentary rocks including banded iron formations bifs and ferruginous siliceous mudstone from the cleaverville area in the pilbara craton western australia geological lithological and geochemical lines of evidence indicate that these sedimentary rocks preserve a continuous record of depositional environments ranging from that typical of mid oceanic spreading centers to convergent plate boundary settings a range of environments most likely caused by plate movements except for the mudstone the ree content of these sedimentary rocks changes gradually from the lower to upper stratigraphic horizons europium anomalies decrease up section eu eu values normalized to nasc change from 3 5 to 1 1 as the ree contents and lree hree ratios increase the striking similarity in these ree signatures of bifs and modern hydrothermal sediments leads us to propose that the bifs were in situ hydrothermal precipitates near a mid ocean ridge significant amounts of terrigenous materials contributed to the siliceous and ferruginous mudstone of the uppermost horizon the observation that the source of the sediments shifted from proximal hydrothermal through distal hydrothermal to terrigenous suggests that plate tectonics dominated by horizontal movement was already operating in the mid archean distal hydrothermal sediments without a eu anomaly when normalized to nasc suggest that mid archean seawater had already been strongly influenced by a riverine flux from an upper continental crust and that this component bore no eu anomaly i e it had a negative eu anomaly when normalized to chondrite in addition to an absence of a eu anomaly mid archean seawater did not have a ce anomaly suggesting less oxic conditions in the mid archean than in the modern ocean

nd and sr isotope systematics of clastic metasediments from isua west greenland identification of pre 3 8 ga differentiated crustal components recent field work on the isua metasediments has established that there are two lithological sequences a and b which are divided into a number of distinct formations each clastic metasedimentary unit has a relatively unique trace element signature the depleted mantle sm nd model ages of samples from all major units range from 3 59 to 3 98 ga whereas rb sr model ages cover a very wide range 1 9 4 6 ga the samples from the sequence a felsic gneiss formation a6 yield a relatively good rb sr isochron age of t 3 71 0 07 ga the sequence b samples do not yield a rb sr isochron however some of the samples plot close to the a6 rb sr isochron and sequences a and b appear to have essentially the same deposition age the sm nd data for both sequences yield initial end values in the range 1 to 3 for a deposition age tstrat 3 64 ga if tstrat 3 81 ga the range is 0 4 this range appears to reflect real variability of initial end values corresponding to the time of deposition data trends in end versus 1 nd diagrams suggest at least three distinct components in each of sequences a and b model age calculations suggest that some of these components must be derived from continental crustal sources that predate the time of deposition of these sediments by as much as 0 4 b y other components appear to be derived from volcanic sources that are roughly coeval with sediment deposition the very high initial end values implied for early depleted mantle sources 2 to 4 suggest a very large pre 3 8 ga continental crust alternatively these high values could reflect an early terrestrial mantle with highly fractionated cumulate layers that formed in a 4 5 ga magma ocean

clay mineralogy of the middle mingshui formation upper campanian to lower maastrichtian from the skin borehole in the songliao basin ne china implications for palaeoclimate and provenance abstract clay mineralogy of a 60 m thick section of the middle mingshui formation upper campanian to lower maastrichtian from the skin borehole songliao basin in ne china has been studied to assess the palaeoclimate and provenance the sedimentary succession is composed of shore to shallow lacustrine mudstones and siltstones in the lower part and meandering fluvial sandstone mudstone sequences in the upper part the clay mineral assemblage is characterized by the predominance of smectitic minerals with some exceptions of illite rich layers and trace amounts of kaolinite and chlorite the high smectite content 90 on average in lacustrine sediments could source from erosion of smetitic soils and volcanic materials in source areas the lesser xing an zhangguangcai range was the potential provenance area for the sediments with volcanic activity occurring at 70 ma when the temperate semi humid palaeoclimate was favorable for the clay formation smectite presented in meandering fluvial sandstones is of early diagenetic origin formed by hydrolysis and alteration of primary minerals after deposition illite in overbank deposits is interpreted as a result of erosion of granitic rocks and sorting in sedimentation processes the clay mineralogy of the studied strata contains information on palaeoclimate and provenance although early diagenetic processes may have influenced the mineralogical composition in some intervals of sandstones

hf isotope systematics in granitoids from the central and southern alps first initial hf isotopic compositions for samples from the alpine domain are presented and discussed the results are mainly based on zircons and a few whole rocks with ages between 30 and 450 ma of those so far analyzed the present day hf isotopic compositions of zircons from non metamorphic and metamorphic granitoid rocks vary between 0 2824 and 0 2829 zircon populations with concordant u pb ages have much higher initial 176hf 177hf than inversely discordant populations which have been contaminated with older zircons containing less radiogenic hf correlated nd hf crustal residence ages have been found involving model parameters of ehf end f lu hf f sm nd 1 6 for the depleted mantle and f lu hf f sm nd 1 2 for elemental fractionations in the crust the model implies 176lu 177hf of 0 017 for the bulk crust it is suggested that the granitoid rocks are the result of mixing of subcontinental mantle derived magmas with 1 7 ga old recycled and partially molten crustal material the continental mantle component mass ratio values for the granitoids range between 0 3 and 2

episodic growth of the gondwana supercontinent from hafnium and oxygen isotopes in zircon it is thought that continental crust existed as early as 150 million years after planetary accretion but assessing the rates and processes of subsequent crustal growth requires linking the apparently contradictory information from the igneous and sedimentary rock records for example the striking global peaks in juvenile igneous activity 2 7 1 9 and 1 2 gyr ago imply rapid crustal generation in response to the emplacement of mantle super plumes rather than by the continuous process of subduction yet uncertainties persist over whether these age peaks are artefacts of selective preservation and over how to reconcile episodic crust formation with the smooth crustal evolution curves inferred from neodymium isotope variations of sedimentary rocks detrital zircons encapsulate a more representative record of igneous events than the exposed geology and their hafnium isotope ratios reflect the time since the source of the parental magmas separated from the mantle these model ages are only meaningful if the host magma lacked a mixed or sedimentary source component but the latter can be diagnosed by oxygen isotopes which are strongly fractionated by rock hydrosphere interactions here we report the first study that integrates hafnium and oxygen isotopes all measured in situ on the same precisely dated detrital zircon grains the data reveal that crust generation in part of gondwana was limited to major pulses at 1 9 and 3 3 gyr ago and that the zircons crystallized during repeated reworking of crust formed at these times the implication is that the mechanisms of crust formation differed from those of crustal differentiation in ancient orogenic belts

the osmium isotopic composition of the continental crust osmium isotopic compositions and concentrations have been determined for deltaic and continental shelf sediments from three major rivers the amazon changjiang and mississippi and for two loesses from the upper mississippi river valley the loesses have heavy mineral and clay compositions characteristic of mississippi river and delta sediment concentrations range from 15 to 120 pg os g and 187os 186os ratios range from 8 2 to 10 9 sediments which are polluted surface sediment from the changjiang estuary or which have scavenged metals from seawater gulf of mexico slope surface sediment and distal amazon shelf sediment have high osmium concentrations and less radiogenic isotopic compositions purely terrigenous sediments have 15 90 pg os g and 187os 186os 10 0 10 9 north american loess and mississippi delta sediment osmium isotopic compositions are indistinguishable we believe that 187os 186os 10 11 is a good average for currently eroding upper continental crust these sediments have neodymium isotopic compositions close to the global mean for river sediment terrigenous 187os 186os is significantly more radiogenic than seawater 187os 186os 8 6 requiring a nonriverine source of osmium to seawater terrigenous osmium is also significantly more radiogenic than bulk pelagic clay osmium we use the improved estimate of terrigenous 187os 186os to recalculate the flux of extraterrestrial osmium to pacific marine sediment if the residence age of osmium in an upper crustal region is similar to the neodymium model age of a sediment derived from that region a model dependent upper crustal 187re 186os ratio can be calculated for the model ages of the sediments analyzed in this study tdm 1 5 1 7 ga model 187re 186os ratios are 340 360 and can be used to correct for bias resulting from the preferential erosion of young crust the 187os 186os ratio of 2 2 ga crust with 187re 186os 400 is 14 2 we have also used the data from this study to estimate upper crustal osmium and rhenium concentrations of 50 pg g and 390 pg g respectively

archaean plate tectonics revisited 2 paleo sea level changes continental area oceanic heat loss and the area age distribution of the ocean basins in a previous paper we derived plate tectonic models for continental accretion from the early archaean 3800 m y b p until the present the models are dependent upon the number of continental masses the seafloor creation rate and the continental surface area the models can be tested by examining their predictions for three key geological indicators sea level changes stable isotopic evolution e g continental surface area and oceanic heat loss models of paleo sea level changes produced by the accretion of the continents reproduce the following features of earth history 1 greater continental emergence lower sea level during the archaean than the proterozoic 2 maximum continental emergence about 3000 m y b p and 3 maximum continental submergence high sea level from 30 to 125 m y b p the high sea level stand between 380 525 m y b p is only weakly reproduced probably due to the simplified nature of the model changes in the number of continental masses can result in tectonic erosion or accretion of the continents with resulting changes in sea level the two major transgressions in the phanerozoic although still requiring some increase in the total terrestrial heat loss can be sucessfully explained by a combination of increases in continental surface area and in seafloor creation rate changes in the total heat loss of the ocean basins predicted by our plate tectonic models closely parallel the changes in terrestrial heat production predicted by wasserburg et al 1964 this result is consistent with thermal history models which assume whole mantle convection the history of changes in continental surface area predicted by our best continental accretion models lies within the ranges of estimated continental surface area derived from independent geochemical models of isotope evolution

sr nd isotopic and geochemical constraints on provenance of late paleozoic to early cretaceous sedimentary rocks in the western hills of beijing north china implications for the uplift of the northern north china craton late paleozoic to mesozoic strata which were deposited in the western hills of beijing north china recorded the contemporary tectonic evolution of the northern part of the north china craton ncc fine grained sandstones and siltstones of these strata were analyzed for their major and trace element concentrations and sr nd isotope compositions their la sc th sc la co th co and cr th ratios are similar to those of sediments derived from felsic source rocks indicating that the original source materials were mostly intermediate to acidic rocks their end 0 values show dramatic shifts implying changes in provenances from the late paleozoic to the early cretaceous the end 0 values of the late paleozoic and the late jurassic to early cretaceous formations are very low whereas the end 0 values of the xingshikou and the xiaoyaopo formations are much higher indicating that a juvenile component has been added into their sources based on the sr nd isotope compositions the late paleozoic and the late jurassic to early cretaceous samples fall in the paleozoic to mesozoic magmatic rocks distributed in the northern part of the ncc however the sr nd isotope compositions of the xingshikou and the xiayaopo formations vary between the xing meng orogenic belt xmob and the north part of the ncc components these changes can be interpreted by the complex uplift and exhumation history of the northern ncc from the late paleozoic to the early cretaceous during the late carboniferous the uplift of the inner mongolia paleo uplift impu which probably commenced at the late carboniferous might supply detrital materials to the late paleozoic strata in the late triassic the uplift of the xmob and impu was responsible for the mixing source of the xingshikou formation after that the northern xmob became the main source of the xiayaopo formation resulting from the uplift of the xmob due to the collision of siberia and north china mongolia plates during the middle to late jurassic finally the uplift of the yftb which occurred from the late jurassic to early cretaceous provided the source materials and blocked the input of the xmob components

a clearer view of crustal evolution u pb sm nd and lu hf isotope systematics in five detrital minerals unravel the tectonothermal history of northern china

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

geochemistry of the ediacaran early cambrian transition in central iberia tectonic setting and isotopic sources abstract a complete ediacaran early cambrian stratigraphic transition can be observed in the southern part of the central iberian zone iberian massif two different stratigraphic units underlying ordovician series display geochemical and sm nd isotopic features in agreement with an evolving geodynamic setting pusa shales early cambrian rest unconformably on greywackes of the lower alcudian formation late ediacaran both sequences present minor compositional variations for major and trace element contents and similar ree patterns close to those of paas post archean australian shale trace element contents and element ratios suggest mixed sources with intermediate to felsic igneous contributions for both units tectonic setting discrimination diagrams for the ediacaran greywackes indicate that these turbiditic series were deposited in a sedimentary basin associated with a mature active margin volcanic arc however the compositions of the cambrian shales fit better with a more stable context a back arc or retro arc setting end t and tdm ages are compatible with dominance of a similar cratonic source for both sequences probably the west africa craton end 565 values for the ediacaran greywackes 3 0 to 1 4 along with tdm ages 1256 1334 ma imply a significant contribution of juvenile material probably derived from the erosion of the volcanic arc however end 530 values in the cambrian shales 5 2 to 4 0 together with older tdm ages 1444 1657 ma suggest a higher contribution of cratonic isotopic sources probably derived from erosion of the adjacent mainland coeval with the progressive cessation of arc volcanism along the peri gondwanan realm during the cambrian there was a period of more tectonic stability and increasing arrival of sediments from cratonic areas the geochemistry of the ediacaran cambrian transition in central iberia documents a tectonic switch in the periphery of gondwana from an active margin to a more stable context related to the onset of a passive margin

the evolution of strontium isotopes in the upper continental crust radiogenic isotopes in clastic sediments record how the average age and composition of the exposed continental crust has changed with time most of the continental crust is 1 5 gyr old and the upper crust is characterized by high rb sr ratios critically however late precambrian and younger sediments have surprisingly low depositional 87sr86sr ratios given their old source ages and high measured rb sr refs 1 2 this paradox has prompted suggestions that the 87sr86 ratio in clastic sediments is buffered by a significant flux of new low 87sr86 material from the upper mantle with the implication that relatively large quantities of 87sr were generated in the sedimentary reservoir and subsequently recycled into the upper mantle1 2 we argue here that the strontium isotope paradox instead largely reflects the gradual increase in rb sr with time in the upper continental crust the rb sr ratio of the upper crust is increased by intracrustal melting and fractional crystallization in plagioclase bearing systems and by weathering erosion and sedimentation the longer material resides in the continental crust the more likely it is to be reworked by magmatic and sedimentary processes and so to acquire high rb sr ratios one corollary is that 50 less87sr has been generated in clastic sediments in the past 2 0 gyr than in models that assume constant crustal rb sr fig 3 of ref 2

intracrustal recycling and upper crustal evolution a case study from the pan african damara mobile belt central namibia abstract clastic sediments from the pan african damara mobile belt show a wide range in depositional nd nd 0 18 to 1 and several key trace element ratios notably th nd and la sm increase from 0 2 to 0 7 and 4 0 to 8 0 respectively with decreasing nd 0 such trace element ratio nd 0 data arrays are attributed to binary mixing within the sedimentary mass and the data array defined by the damara sediments is consistent with data from other regions the low nd 0 end member is characterised by high th nd th y la sm and hf zr ratios and reflects highly differentiated upper crust dominated by granitoids pb isotope data suggest that the low nd 0 end member also had relatively high time integrated th u 4 by contrast the high nd 0 end member reflects juvenile upper crust but this end member has la sm th nd and hf zr ratios 4 0 0 2 and 0 02 respectively slightly lower than those of taylor and mclennan s preferred model for bulk crust and significantly lower than those of the andesite model intracrustal differentiation is discussed in terms of bulk crust present day average upper crust post archaean shale and a highly differentiated low nd 0 end member trace element ratios which are not significantly fractionated during erosion and sedimentation e g th nd th y and la sm show relatively smooth increases between these reservoirs whereas other trace element ratios which are fractionated both by sedimentary and magmatic processes e g rb sr and rb zr increase dramatically in the more differentiated crustal reservoirs

book review geosounding principles 2 time varying geoelectric soundings h p patra and k mallick methods in geochemistry and geophysics 14b elsevier amsterdam 1980 xvi 424 pp dfl 180 00 us 87 75

on the effect of continents on mantle convection at the earth s surface continents and oceans impose different thermal boundary conditions at the top of the mantle laboratory experiments are used to investigate the consequences of this for mantle convection the upper boundary of the experimental tank was made of copper plates enforcing a fixed temperature and had a conductive lid of finite width in the middle beneath this lid the thermal boundary condition was of the mixed type with a biot number depending on the dimensions and thermal conductivity of the lid experimental values of the biot number were scaled to earth values experiments were run for a large range of rayleigh numbers from 104 to 107 and for several lid widths the effects of temperature dependent viscosity and of the shape of the lid were investigated at steady state in all cases there is an upwelling beneath the conductive lid which feeds two symmetrical and elongated convective cells three different dynamic regimes were identified as a function of rayleigh number independently of the lid width at rayleigh numbers lower than 1 2 105 the upwelling is steady both in geometry and temperature structure at rayleigh numbers between 1 2 105 and 2 106 this central upwelling is intermittent at larger values of the rayleigh number there is no longer a simple upwelling structure but a set of small plumes rising together and distorted by a cellular circulation of large horizontal extent thus the conductive lid always imposes a large scale flow pattern the length of these convective cells is a function of lid width it is equal to the lid width at large values and decreases to the rayleigh benard value as the lid width decreases to zero a fluid loop model explains the most important features of this form of convection the cell length is such that the upwelling temperature is minimized for a given rayleigh number and lid width and is an increasing function of lid width and a decreasing function of rayleigh number transient experiments demonstrate that the large scale flow structure develops rapidly with even small horizontal temperature differences implications for the earth are that large scale convection cells exist in conditions which in the absence of continents would probably lead to a chaotic convection pattern dominated by plumes at high rayleigh number continental breakup is effected by a large scale line upwelling structure which includes a number of individual plumes

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

integrated nd isotopic and u pb detrital zircon systematics of clastic sedimentary rocks from the slave province canada evidence for extensive crustal reworking in the early to mid archean abstract a combined u pb geochronology and nd isotopic analysis of clastic metasedimentary rocks from the archean slave province northwest territories canada is presented a series of clastic sedimentary rocks with deposition age of 3 13 ga to 2 58 ga was collected from the west central slave province these samples include conglomerates as well as finer sedimentary rocks such as greywacke arkose and quartzite although it is generally agreed that the nd model age t dm of clastic sedimentary rocks represents the average sedimentary provenance age t dm of samples studied here is generally older than the average u pb age of detrital zircons extracted from the same sample rather there seems to be a better 1 1 relationship between the t dm and u pb age of the oldest zircon from each sample a simple model relationship between u pb geochronology and t dm suggests that such correlation is a result of archean crustal evolution of the slave province being dominated by crustal reworking i e intracrustal processes rather than addition of juvenile materials from the mantle one exception is the period between 3 1 and 3 2 ga during which period a significant portion of detrital zircons have u pb ages exceeding the t dm indicating a large input of juvenile material and or decreasing level of crustal reworking an alternative explanation for the u pb vs t dm trend may be a mixing of older mafic juvenile crust containing no zircon and younger felsic juvenile crust however the general lack of extensive pre 2 8 ga mafic crust in the slave province makes this scenario unlikely the results of this study cannot constrain the change in the volume of continental crust at any given period of time since the addition of juvenile crust may always be counterbalanced by recycling of crustal material back into the mantle however a dominance of crustal reworking during the early to mid archean strongly implies that a large part of ancient crust was lost during the younger tectonomagmatic events since the majority of zircon may be lost during crustal reworking processes unless they are preserved as an inherited core absence of a large quantity of ancient detrital zircon cannot be used as evidence for the lack of large sialic crust existing on the early earth

a nd isotope investigation of sediments related to crustal development in the british isles the times of original fractionation of the sm and nd component of clastic sediments from a mantle source crustal residence age have been estimated from sm nd model ages calculated relative to a depleted mantle evolution in this way the provenance and evolution of selected precambrian and phanerozoic sediments and metasediments from the british isles have been examined whereas some archaean and early proterozoic sediments have sm nd model ages that are close to their stratigraphic age the phanerozoic sediments analysed have model ages as much as 2 0 ga in excess of their stratigraphic age a more detailed study of lower palaeozoic sediments deposited on the northern margin of the iapetus ocean provides evidence for a marked change of provenance in the ordovician after the deposition of the dalradian supergroup a component with comparatively high143nd 144nd and sm nd ratio presumably basaltic is present in the sediments throughout the accretionary prism crustal residence age estimates average about 1 5 ga for both these lower palaeozoic sediments and modern pelagic clays and collectively fail to provide any evidence for significant continental growth during the phanerozoic