geochronology at the namew lake ni cu deposit flin flon area manitoba canada a pb pb study of whole rocks and ore minerals pb isotope ratios of whole rock specimens and mineral separates from the gneisses surrounding the namew lake ni cu deposit south of flin flon manitoba are compared with the pb isotope ratios in the ore minerals and with recently reported zircon u pb analyses of samples from the gneisses and the ultramafic host of the ore the pb pb age from the gneisses is 1864 8 ma and there is no evidence for any differences in slopes or initial ratios between individual samples thus there is good agreement between the youngest zircon ages for the gneisses 1862 3 ma and pb pb ages for the same rocks a suite of analyses from a late pegmatite which cuts the gneisses and the ultramafic rock yields a well defined age of 1824 5 ma consistent with the geologic requirement that it be younger than the ultramafic host to the orebody zircon age of 1847 6 ma and similar in age to late intrusive bodies in the flin flon area to the north all these data are in marked contrast to apparent pb pb ages for the sulfp

neoarchean crustal growth by accretionary processes evidence from combined zircon titanite u pb isotope studies on granitoid rocks around the hutti greenstone belt eastern dharwar craton india abstract the neoarchean hutti greenstone belt hosts mesothermal gold deposits and is surrounded by granitoid rocks on all sides combined u pb dating of zircon and titanite from the granitoid rocks constrains their emplacement history and subsequent geologic evolution the golapalli and yelagatti granodiorites occurring to the north of the hutti greenstone belt were emplaced at 2569 17 ma the yelagatti granodiorite yielded a younger titanite age of 2530 6 ma which indicates that it was affected by a post crystallization thermal event that exceeded the titanite closure temperature the western granodiorites from kardikal have identical titanite and zircon ages of 2557 6 ma and 2559 19 ma respectively the eastern kavital granodiorites yielded titanite ages of 2547 6 ma and 2544 24 ma which are identical to the published u pb zircon shrimp ages these ages imply that the granitoid rocks surrounding the hutti greenstone belt were formed as discrete batholiths within a short span of ca 40 ma between 2570 ma and 2530 ma ago they were juxtaposed by horizontal tectonic forces against the supracrustal rocks that had formed in oceanic settings at the end of the archean the first phase of gold mineralization coincided with the last phase of granodiorite intrusion in the hutti area a metamorphic overprint occurred at ca 2300 ma ago that reset the rb sr isotope system in biotites and possibly caused hydrothermal activity and enrichment of au in the ore lodes the eastern dharwar craton consists of quartz monzodiorite granodiorite granite suites of rocks that are younger than the greenstone belts that are older than 2650 ma reported from earlier studies the granitoid magmatism took place between 2650 and 2510 ma ago indicating accretionary growth of the eastern dharwar craton

pb isotopic evidence for u th pb behaviour in a prograde amphibolite to granulite fades transition from the lewisian complex of north west scotland implications for pb pb dating abstract two end member types of u pb fractionation behaviour are recognised type i in which pb isotopic homogenisation occurs and type ii in which pb isotopic homogenisation is incomplete or absent type ii can be further subdivided into three categories depending upon the relationship of the post fractionation 3 238 u 204 pb value to prefractionation 2 in type iia 3 is unrelated to 2 in type iib 3 k 2 where k is a constant in type iic 3 values are uniform each of these types of behaviour results in a predictable effect upon observed uranogenic pb isotope systematics which may lead to an incorrect geochronological interpretation an example of the relationship between these specific types of u pb behaviour and metamorphic grade is provided from the late archaean lewisian complex of n w scotland this complex was metamorphosed during the 2660 ma badcallian event three suites of tonalitic granodioritic grey gneisses have been analysed pyroxene granulite facies from scourie hornblende granulite facies from gruinard bay and amphibolite facies from the northern region the present day pb isotopic composition of these suites can be interpreted as representing a transition from type lib behaviour in the lowest grade rocks resulting in a rotated isochron through type ha in the hornblende granulites which yield a scatter of data points to type i in the pyroxene granulites which yield an isochron corresponding to the badcallian event this transition presumably reflects increasing u depletion with metamorphic grade and ultimately at the highest grade pb mobility and or depletion in addition a consideration of the thorogenic pb isotopic data reveals a systematic variation during prograde metamorphism an attempt to test the model of 2660 ma initial ratio inhomogeneity by analysis of k feldspars from the amphibolite facies gneiss suite reveals 1700 ma laxfordian resetting of their pb isotope systematics this study has implications for pb pb geochronological data particularly in lower grade metamorphic terrains it suggests that reliable isochrons may only be generated by type i behaviour or by the imposition of high 3 values during type iia behaviour at higher grades of metamorphism

late diagenetic redistribution of uranium and disturbance of the u pb whole rock isotope system in a black shale the middle ordovician llandeilo caradoc formation black shales deposited at about 460 ma on the southwest margin of the welsh basin show evidence of a late diagenetic uranium redistribution event evidence for this redistribution event includes a disturbance in the u pb whole rock isotopic system at about 193 ma marked by a large range in the whole rock 238 u 204 pb between 2 and 90 one explanation for this large range is that individual whole rocks have experienced either a gain or a loss of u at the time of the disturbance this hypothesis is supported by the large range in measured u contents from the llandeilo caradoc formation black shales 0 16 to 6 6 ppm upper crust 2 8 ppm post archean average shale paas 3 1 ppm taylor and mclennan 1985 uranium that was likely sequestered during deposition and early diagenesis has been redistributed on a greater than whole rock scale centimeters to meters the u pb isotope systematics identified in this study and other pb isotope data from the welsh basin evans 1996 constrain a reasonable u concentration of 2 91 ppm for an unaltered llandeilo caradoc shale at the time of deposition 460 ma assuming 2 91 ppm to be an average u content at the time of deposition some rocks in the llandeilo caradoc formation have lost as much as 95 of the u likely present at the time of deposition and early diagenesis whereas others exhibit an almost equal enrichment when the u pb isotopic and elemental data from whole rocks and individual u bearing phases i e carbonate apatite monazite organic matter are examined in conjunction with the paragenetic sequence controls on the distribution of u in the llandeilo caradoc whole rocks and a pattern of open system conditions become apparent despite the presence of potential u rich diagenetic phases such as apatite and monazite most of the u in the relatively enriched samples is associated with a non extractable unidentified diagenetic component uranium mass balance considerations and whole rock u pb isotope systematics require that the non extractable component formed at about 193 ma during a late diagenetic uranium redistribution event the redistribution of u within the llandeilo caradoc formation has important implications for the use of u rich sedimentary minerals as geochronometers and for the use of u as a paleo redox indicator specifically a trace element indicator like uranium may in some cases record information about the postdepositional history of a sedimentary rock rather than document the absolute timing and conditions of deposition moreover this unique history can be properly evaluated only through an integrated petrographic geochemical approach that can discern the distribution and source i e depositional versus diagenetic of u in an individual whole rock

evolution of archaean crust in the dharwar craton the nd isotope record abstract a synthesis of the available nd isotope data from the dharwar craton puts important constrains on antiquity of crust extent of basement terranes and events marking juvenile crustal addition and crustal recycling the craton is divided into two blocks namely eastern dharwar craton edc and western dharwar craton wdc nd model ages trace crust extraction as far as 3 5 ga back in both the blocks although rock record of such antiquity is yet to be found in the craton in wdc 3 35 3 0 ga is the most significant period of juvenile crustal addition as well as crustal recycling significantly depleted mantle 1 5 to 6 4 existed below the wdc as early as 3 35 ga which was probably refertilized in a later event from the resultant chondritic to slightly enriched mantle 0 3 to 0 4 juvenile mafic crust was added to the wdc during 2 9 2 6 ga a widespread crustal recycling event at 2 6 ga marks the last major event in this block in edc 3 3 3 0 ga granitoids occur as vestiges and show nd model ages higher than crystallizations ages suggesting recycling of palaeoarchaean crust during 2 7 2 5 ga extensive juvenile magmatism took place in edc from variably depleted mantle 1 4 to 5 6 which is later than the global 2 7 ga peak of crustal growth attendant crustal recycling destroyed most of the earlier crust distribution of nd model ages suggests in contrast to general believe that palaeoarchaean crust was not only abundant in the wdc but also widespread in an area extending from the wdc edc boundary to further 60 150 km east up to the hungund ramagiri kolar hrk belt this belt appears to be a major crustal boundary along which terranes of different nd isotope signatures were amalgamated during the late neoarchaean by horizontal accretion major neoarchaean crustal recycling events documented in the dharwar craton are missing from nd isotope record of the eastern block eb of the north china craton this fact does not support correlation between the two cratons as suggested by some authors

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

the timing of gold mineralization across the eastern yilgarn craton using u pb geochronology of hydrothermal phosphate minerals the highly mineralized eastern goldfields of the eastern yilgarn craton is an amalgamation of dominantly neoarchaean granitoid greenstone terranes and domains that record a history of early rifting followed by westward directed collision with initial arc formation collision and clastic basin formation and final accretion to the western yilgarn proto craton between 2 66 and 2 60 billion years ago the gold deposits that define this region as a world class gold province are the product of orogenic processes that operated during accretion late in the tectonic history after initial compressional deformation d1 d2 and the majority of granitoid magmatism minor gold was also deposited throughout the entire tectonic history in magmatic hydrothermal related systems however such mineralization mostly 0 3 g t gold is nowhere economic unless it overprints or is overprinted by much higher grade orogenic gold lodes robust shrimp u pb geochronology of gold related hydrothermal xenotime and monazite supports structural studies that gold mineralization occurred during late transpressional events d3 d4 shortly before cratonization however westward migration of collision and accretion produced a complementary diachroneity in the timing of gold mineralization of 5 to 20 m y between c 2 65 ma in the east including laverton district kurnalpi terrane to c 2 63 ma in the west including kalgoorlie terrane across the eastern part of the craton the robust geochronology refutes previous suggestions that significant gold mineralization events extended from de to d4 in the evolution of the orogen and that the kalgoorlie gold deposits formed over a period of 45 m y the crustal continuum model is applicable within terranes where orogenic gold depositional events were penecontemporaneous but must be modified to account for diachroneity of orogenic events and gold mineralization across the eastern goldfields

sm nd chronology and petrogenesis of mesosiderites we have obtained sm nd data from four mesosiderite silicate clasts including three clasts with a variety of textures from the vaca muerta type 1a mesosiderite and one gabbroic clast from the mt padbury mesosiderite the gabbroic vaca muerta pebble 12 and basaltic pebble 16 yield identical 147 sm 143 nd ages of 4 48 0 19 ae and 4 48 0 09 ae respectively while the highly recrystallized pebble 5 gives an age of 4 42 0 02 ae mt padbury yields an age of 4 52 0 04 ae all clasts show a correlation of 142 nd 144 nd with 147 sm 144 nd and provide unequivocal evidence for the presence of live 146 sm at the time of their formation calculated initial 146 sm 144 sm values range from 0 004 pebble 5 to 0 006 pebbles 12 16 and mt padbury and are generally consistent with the 147 sm 143 nd ages however discordance of whole rock leach and residue data and some disagreement between 146 sm 142 nd relative ages and 147 sm 143 nd absolute ages indicate small but significant disturbances to the sm nd systematics the ranges of ages and initial 146 sm 144 sm and 143 nd 144 nd values suggest that each of these silicate clasts underwent a separate protracted evolution on its parent body prior to mixing with metal textural and trace element criteria indicate that individual clasts from the same mesosiderite often had very different igneous sources and thermal histories prior to their incorporation in the meteorite pebble 12 is extremely lree depleted probably a result of several melt extraction events whereas pebbles 5 and 16 and mt padbury have nearly chondritic sm nd with bulk ree concentrations higher than chondrites by factors of 5 to 15 in general the sm nd systematics of mesosiderite silicates require formation of the silicates on a parent planet which underwent relatively early and extreme differentiation preservation of diverse old ages and the presence of 146 sm imply that metal silicate mixing did not seriously alter the sm nd isotopic memories of these clasts we present a model for metal silicate mixing which combines the cooling history with isotopic reequilibration for the case of thermal blanketing we show that the total amount of isotopic reequilibration in a sample can be related to the initial temperature depth of burial grain size and diffusion parameters application of this model to the silicate clasts measured in this study indicates that if the metal and silicate were thermally equilibrated above the metal solidus temperature during mixing then the clasts must have been buried no deeper than 1 10 m in regolith during the initial high temperature cooling phase in order to prevent the sm nd systems from being extensively reset in order to reconcile these results with the slow cooling rates at lower temperatures determined from studies of exsolution in the metal phase we infer that heat was transferred rapidly from hot metal to cold silicate material during initial metal silicate mixing and that the deeply buried portions of the mixture then cooled slowly after reaching thermal equilibrium at 600 700 c the data from this study point to the following history for the mesosiderite parent body 1 differentiation of a silicate parent body within the first 50 m y of solar system history to form diverse parent magmas 2 emplacement of primitive and differentiated mafic magmas near the planetary surface and extensive differentiation in the near surface environment 3 formation and reworking of regolith breccias through impact gardening at the near surface of the body 4 mixing of molten fe ni metal with the regolith followed by rapid cooling 100 150 m y after the origin of the solar system 5 slow cooling from temperatures of 700 c to produce the observed nickel diffusion profiles in the iron phase 6 mild impact metamorphism and brecciation to obtain the much younger k ar ages and 7 recent collisions or perturbations sending mesosiderite fragments into earth crossing trajectories

lead isotopes as a provenance tool for quartz examples from plutons and quartzite northeastern minnesota usa abstract the u pb isotope system in plutonic quartz samples approximates the crystallization age of the host granitoid and thus provides a useful provenance tool for detrital quartz grains we present lead isotopic data and u and pb concentrations of quartz from three late archean granitoids and from the early proterozoic pokegama quartzite all in northeastern minnesota lead in granitic quartz is considerably more radiogenic than in its associated whole rock host but it is similar or only slightly more thorogenic than its host pb pb ages obtained from quartz whole rock leached feldspar from the late archean granitic plutons are consistent with existing geochronology suggesting that the u pb system in quartz remained closed since crystallization u pb isotopic data for detrital clear quartz populations in the pokegama quartzite record late archean ages 2550 2650 consistent with a source from the superior province lead isotopic compositions of detrital k feldspar populations and lead and neodymium isotopic compositions of whole rocks samples from the pokegama quartzite also support a superior province source this study highlights the potential of the lead isotope system in detrital quartz k feldspar and whole rocks in provenance studies of sandstones

palaeoproterozoic 1 9 ga extension and breakup along the eastern margin of the eastern dharwar craton se india new sm nd isochron age constraints from anorogenic mafic magmatism in the neoarchean nellore greenstone belt abstract the neoarchean nellore greenstone belt located along the eastern margin of the eastern dharwar craton was the site for younger igneous activity the timing of this magmatism and the host metabasalt critical for constraining extensional events along the deformed southeast margin of the eastern dharwar craton was conducted by the sm nd method samples from the large differentiated gabbroic kandra igneous complex a subset kandra group a define an isochron age of 1926 110 ma nd i 0 51006 12 mswd 0 98 n 5 e nd 1 7 and the kandra group b subset yielded a model 3 isochron age of 1882 120 ma nd i 0 50994 14 mswd 1 8 n 4 e nd 5 1 whereas the three samples from north pamuru gabbro anorthosite yielded an isochron age of 1882 160 nd i 0 51012 18 mswd 0 064 e nd 1 7 the kandra group a and north pamuru samples regressed together yielded a preferred isochron age of 1911 88 ma nd i 0 51008 10 mswd 0 62 n 8 e nd 1 6 for the emplacement of gabbros in contrast host greenstone belt metabasalt from chundi yielded a sm nd isochron age of 2693 94 ma nd i 0 50920 8 mswd 1 4 n 4 e nd 1 2 whereas regressing the chaganam and chundi samples yielded an isochron age of 2654 100 ma nd i 0 50923 9 mswd 1 5 n 11 e nd 0 7 basal metapelite underlying the metabasalt yields old nd model ages between 2888 and 3054 ma the results of this study show that the 2 7 ga nellore greenstone belt was intruded by enriched morb type gabbros at 1911 ma it is proposed that this palaeoproterozoic magmatism formed in response to a major extensional event along the eastern dharwar craton margin that likely initiated breakup from the southeastern margin of the north china craton this exposed craton margin later evolved as the site for collisional events during the late palaeoproterozoic to early mesoproterozoic

rare earth element redistribution and its effects on the neodymium isotope system in the austin glen member of the normanskill formation new york usa abstract three groups of ree patterns are observed for the sandstones and shales of the middle ordovician austin glen member of the normanskill formation one sandstone sample is enriched in middle and light rare earth elements the majority of samples are light rare earth element lree enriched with negative eu anomalies and flat heavy rare earth elements hrees whereas a third group appears to have lost lrees and middle rare earth element mrees homogeneous neodymium isotopic compositions for all samples at about the time of deposition nd 470 ma 8 2 1 1 2sd indicate that the source of the austin glen member was well mixed and that the neodymium had a long term enriched history the differences in the shape and abundances of the ree patterns combined with the neodymium isotope characteristics of the samples lead us to the conclusion that samarium and neodymium were fractionated and that the neodymium isotopic system might have been disturbed at about the time of deposition 470 ma most samples appear to have been affected by this process the two most altered shales lost about half of their neodymium and their sm nd ratios are higher than the sm nd ratios of average shales these two shales give clearly erroneous mantle extraction ages but the average tdm of the provenance can still be deduced 1700 1800 ma from the least disturbed sandstones with normal upper crustal sm nd ratios accordingly this study demonstrates that the rees may be transported and neodymium isotopes may be reequilibrated under certain sedimentary conditions e g diagenesis however in spite of evidence for ree redistribution provenance information tdm original ree patterns may still be inferred

electron microprobe and la icp ms study of monazite hydrothermal alteration implications for u th pb geochronology and nuclear ceramics this paper presents new data that quantify the response of magmatic monazite to three main types of hydrothermal alteration namely sericitization chloritization and greisenization the samples were taken from three paleozoic granites the st nectaire granite massif central france the carnmenellis granite cornwall england and the skiddaw granite lake district england fluid inclusion thermometry and hydrothermal parageneses indicate that alteration took place at temperatures ranging from 260 to 340 c and salinities from 3 to 18 wt nacl equivalent possible monazite alteration mechanisms found in the course of this study using backscattered scanning electron microscopy bse sem electron microprobe laser raman spectroscopy and laser ablation inductively coupled plasma mass spectrometry la icp ms include cationic substitutions monoclinic to hexagonal structure transition accompanied by chemical exchanges selective th removal dissolution reprecipitation and dissolution with replacement by a different mineral these results show that despite the compositional and crystallographic simplicity of monazite it exhibits varied chemical responses to different mineral fluid interactions elemental and isotopic measurements by la icp ms for the unaltered monazites yield statistically significant 232th 208pb and 238u 206pb magmatic crystallization ages in some cases it was possible to give a reasonable estimate of the age of mineral fluid interaction using the altered parts of the monazites or newly precipitated crystals for other minerals 232th 208pb and 238u 206pb systematics were strongly disturbed by variable inputs and or depletions of u th and pb the data suggest that monazite like nuclear waste forms would be good hosts for tetravalent actinides but may release the lanthanides and actinides with lower and higher valencies especially if the fluids are oxidizing or tend to dissolve monazite

lead isotopic compositions of the western dharwar craton southern india evidence for distinct middle archean terranes in a late archean craton abstract lead and sr isotopic compositions of whole rocks and feldspar separates from several fielddefined suites of rocks from the western dharwar craton of southern india elucidate a complex history of the area the evolution of the craton earlier than 3 0 ga is difficult to determine because of thermal and metasomatic overprinting at 3 0 ga many whole rocks were completely reset then but feldspar separates preserve some of the earlier history apparently a series of terranes evolved from the mantle over the period 3 4 to 3 0 ga to form the western dharwar craton in each case metamorphism and anatexis of older terranes accompanied extraction of juvenile material from the mantle the oldest gneisses 3 4 ga had radiogenic pb and sr isotopic compositions when they formed and were derived from older crustal protoliths that were plausibly accreted to the crust at or even before 3 8 ga the old gneisses were greatly depleted in rb probably at 3 4 ga when the protoliths of younger magmatic rocks may have separated from the mantle younger gneisses apparently formed from these mafic intermediate protoliths in the age range of 3 2 to 3 0ga the 3 0 ga event in the central part of the craton was accompanied by intrusion of small diapiric trondhjemites formed by melting of older crust and penecontemporaneous metasomatism of country rocks by hydrous fluids carrying u and other lithophilic elements the resultant deep crust was greatly depleted in heat producing elements and the craton has been inactive since 3 0 ga except for production of minor granites from depleted source regions at 2 5 2 6 ga the lack of activity at 2 5 ga contrasts strongly with extensive magmatism and metamorphism in surrounding areas that had not undergone depletion at 3 0 ga lower crustal depletion also has caused present day reduced heat flow to be very low

emplacement and metamorphism of archaean mafic volcanics at kambalda western australia geochemical and isotopic constraints abstract extensive exploration and mining at kambalda western australia has revealed a conformable mafic ultramafic succession consisting of a sulphide ore bearing ultramafic layer located between metabasalt units geochemical and rb sr u pb isotopic analyses have been carried out on two 70 140 m sections of drill core from the metabasalts and u pb isotopic analyses have been made on sulphide ore samples greenschist metamorphism of the metabasalts is dated at 2610 30 m y and was approximately isochemical except for addition of h 2 o and co 2 and a differential mobilization of k and rb within the units the addition of h 2 o and co 2 is believed to have taken place shortly after extrusion of the metabasalts in a submarine environment resulting in an early greenschist metamorphism the later 2600 m y event is correlated with a high thermal regime also detected at kalgoorlie 50 km to the north a pb pb isochron age of 2720 105 m y may represent the metamorphic event or the time of emplacement of the metabasalts new sm nd data c hauvel et al c laque l ong et al suggest the mafic volcanics are 3200 m y old if true large scale homogenization of pb isotopes is required alternatively the mafic sequence may be only slightly older than 2800 m y and the sm nd data does not have time significance

tectono stratigraphy of late archaean greenstone terranes in the southern eastern goldfields western australia abstract late archaean greenstone terranes in the southern part of the eastern goldfields are regionally extensive faultbounded entities defined on the basis of small but distinct differences in stratigraphy and structure the greenstones comprise volcano sedimentary successions that were all deposited at the same time c 2720 2675 ma on sialic crust hence the terranes are interpreted to represent contemporaneous probably adjacent basins these basins were affected by the same history of granite plutonism and regional deformation c 2675 2620 ma early subhorizontal deformation is poorly understood but includes at least locally significant thrust stacking d1 with a south to north movement direction this was followed by regional east west extension affecting all supracrustal successions above a province wide detachment now lying at depths between 4 and 7 km this detachment may have evolved from the greenstone basement contact but only after extensive reworking clastic basins developed along or above major extensional faults but no volcanism is associated with this extension subsequent east west d2 shortening resulted in regional scale thrusting folding and associated late stage transcurrent fault movements the d2 event is contemporaneous with a major thermal event represented by voluminous granite emplacement c 2660 ma and associated peak regional metamorphism in the greenstone succession predominantly monzo granite derived from older sialic crust was emplaced throughout the period of basin formation and deformation early synvolcanic granite c 2719 2710 ma is a minor component late synvolcanic granite c 2685 2675 ma is widespread and syn d2 granite c 2660 ma is abundant there are a few small late tectonic intrusions c 2640 ma to 2600 ma the lateral lithostratigraphic persistence of the contemporaneous basins the syn d2 low pressure regional metamorphism and the province wide monzo granite magmatism suggest that the greenstone basins possibly developed in a tectonic setting similar to that of failed rifts above aborted spreading centres the basins closed in response to far field compressional stresses

age constraints on recycled crustal and supracrustal sources of archaean metasedimentary sequences eastern goldfields province western australia evidence from shrimp zircon dating abstract shrimp u pb zircon dating of sedimentary volcanic and subvolcanic intrusive rocks from unconformity bounded sequences in the eastern goldfields province defines maximum depositional ages for those sequences the kambalda sequence which records a backarc basin was deposited prior to 2700 ma and from at least 2715 ma the spargoville sequence which records an arc adjacent volcano bound basin was deposited between 2700 and 2683 ma the kalgoorlie sequence which records an intra arc rift basin comprises two tectonic stages with depositional ages of 2681 2670 and 2661 2655 ma deposition of the spargoville and kalgoorlie sequences overlapped with the intrusion of granitoids and subvolcanic porphyries thereby indicating that those granitoids were emplaced during crustal extension younger submarine fan kurrawang sequence and braided fluvial merougil and jones creek sequences deposits of a remnant ocean basin were deposited at when coupled to palaeocurrent data provenance dating of detrital zircon suites fingerprints southeast oriented amalgamation of felsic detritus from multiple sources within longitudinal depositional systems and longitudinal reworking of felsic detritus from single sourced transverse systems most felsic detritus was recycled from coeval volcanoplutonic arcs and the orogen to the remnant ocean basin sequences with zircon ages ranging from 2730 to 2660 ma older felsic sources with age ranges of 3570 3130 3030 2900 2870 2770 and 2760 ma could have been structural enclaves within the volcanoplutonic arcs or orogen but were more likely within a distant cratonic basement when coupled with stratigraphic data it is concluded that detrital zircon age distributions preserve evidence for the sedimentary recycling of coeval volcanoplutonic arc or orogenic sources uplifted basin floor and dissected arc successions and cratonic basement virtually all source terrains have apparently vanished presumably through the combination of tectonic and sedimentary recycling and tectonic severance increased use of detrital zircon suites from archaean sedimentary sequences is advocated in order to quantify the balance between sedimentary recycling of felsic rocks from contemporaneous volcanoplutonic sources which themselves document crustal growth in arc environments and from relict volcanoplutonic and crustal basement sources which document the degree of crustal recycling

resetting of neodymium isotopes and redistribution of rees during sedimentary processes the early proterozoic chelmsford formation sudbury basin ontario canada abstract the rees abundances and neodymium isotope compositions of sandstones and siltstones from the early proterozoic chelmsford formation reveal that the rees in some samples were redistributed during the early proterozoic by surficial processes and that prior to redistribution the nd isotope system was partially reset the chelmsford formation is a coarse grained turbidite sequence the youngest of three clastic units that fill the sudbury basin in ontario it is constrained to have been derived from nearby late archean superior province rocks by petrography major and trace element chemistry and pb isotopic compositions of whole rocks and feldspar although ree patterns and nd model ages for samples from around the base of the formation reflect their derivation from undisturbed superior province rocks tdm 2800 ma five samples from higher in the stratigraphic section show evidence that up to 60 of their la has been removed and sm nd has increased by up to 60 these samples also have anomalously high nd model ages tdm 3400 to 4400 ma the presence of pronounced positive ce anomalies in the samples with anomalous tdm indicates the influence of surficial processes on the ree composition the data scatter about a vertical array on a plot of f sm nd vs ndt 1800 nd calculated at 1800 ma approximately the depositional age with values of f sm nd between 0 45 and 0 14 mixing of nd from other sources is difficult to reconcile with the petrography major and trace element geochemistry and nd and pb isotopes removal of an lree enriched phase from the sedimentary system provides a viable mechanism to produce the ree and nd isotopic characteristics of the chelmsford formation samples neodymium isotopes require that this phase was partially isotopically equilibrated with the matrix sediment near the time of sedimentation this study shows the dangers of indiscriminate use of nd model ages in provenance studies and demonstrates that the simultaneous examination of sm nd and nd isotopes on f sm nd vs nd or isochron plots is required for the recognition of sedimentary redistribution of the ree

pb sr and nd isotope systematics of metavolcanic rocks of the hutti greenstone belt eastern dharwar craton constraints on age duration of volcanism and evolution of mantle sources during late archean abstract the hutti greenstone belt hosting major gold mines is made up of tholeiitic basalts felsic volcanic rocks and sediments metamorphosed to amphibolite facies and surrounded by granitoid rocks predominantly of granodiorite in composition the granitoid rocks show intrusive contact relationship at places along the eastern and northern margins of the belt while such relationships could not be observed along the western margin as it is covered by soil the tholeiitic basalts have flat to moderately lree enriched chondrite normalized ree patterns and show depletion in nb relative to morb the hutti metabasalts define a collinear array in the rb sr isotope evolution diagram with a slope corresponding to an age of 2706 130 ma the higher uncertainty is due to scatter caused by mobility of rb and sr during metamorphism and low temperature hydrothermal alteration in the 206 pb 204 pb vs 207 pb 204 pb diagram the metabasalts define a collinear array corresponding to an age of 2637 150 ma with significant scatter indicating that pb was also mobilized by the post crystallization alteration processes the sm nd isotope system in the hutti metabasalts has been least disturbed giving an isochron age of 2662 81 ma which is suggested as the age of formation of their precursors combined with the published u pb shrimp ages on zircons from the felsic volcanic rocks it is inferred that the hutti greenstone belt formed over a period of 86 ma the major greenstone belts of the eastern dharwar craton have evolved coevally at ca 2700 ma ago and are older than the surrounding granitoid rocks the metabasalts have e nd values of 1 4 to 3 0 and e sr values of 23 4 to 2 0 and plot close to the mantle array they were derived from long term lree depleted mantle source the felsic volcanic rocks have e nd values of 1 3 and 1 4 and e sr values of 30 6 and 89 3 respectively their magmas were derived from older continental crustal sources

formation of archean 3600 2500 ma continental crust in the dharwar craton southern india abstract the generation preservation and destruction of continental crust on earth is of wide interest in understanding the formation of continents cratons and supercontinents as well as related mineral deposits in this contribution we integrate the available field petrographic geochronologic elemental nd hf pb isotope data for greenstones ttg gneisses sanukitoids and anatectic granites from the dharwar craton southern india this review allows us to evaluate the accretionary processes of juvenile crust mechanisms of continental growth and secular evolution of geodynamic processes through the 3600 2500 ma window hence providing important insights into building of continents in the early earth the dharwar craton formed by assembly of micro blocks with independent thermal records and accretionary histories the craton can be divided into three crustal blocks western central and eastern separated by major shear zones the western block contains some of the oldest basement rocks with two generations of volcano sedimentary greenstone sequences and discrete potassic plutons whereas the central block consist of older migmatitic ttgs abundant younger transitional ttgs remnants of ancient high grade supracrustal rocks linear volcanic dominated greenstone belts voluminous calc alkaline granitoids of sanukitoid affinity and anatectic granites in contrast the eastern block comprises younger transitional ttgs abundant diatexites thin volcanic sedimentary greenstone belts and calc alkaline plutons published geochronologic data show five major periods of felsic crust formation at ca 3450 3300 ma 3230 3150 ma 3000 2960 ma 2700 2600 ma and 2560 2520 ma which are sub contemporaneous with the episodes of greenstone volcanism u pb ages of inherited zircons in ttgs as well as detrital zircons together with nd pb hf isotope data reveal continental records of 3800 3600 ma the u pb zircon data suggest at least four major reworking events during ca 3200 ma 3000 ma 2620 2600 ma and 2530 2500 ma corresponding to lower crustal melting and spatially linked high grade metamorphic events the ttgs are sub divided into the older 3450 3000 ma ttgs and the younger 2700 2600 ma transitional ttgs the older ttgs can be further sub divided into low al and high al groups elemental and isotopic data suggest that the low al type formed by melting of oceanic island arc crust within plagioclase stability field in contrast the elemental and isotopic features for the high al group suggest derivation of their magmatic precursor by melting of oceanic arc crust at deeper levels 55 65 km with variable garnet and ilmenite in residue the transitional ttgs likely formed by melting of composite sources involving both enriched oceanic arc crust and sub arc mantle with minor contamination of ancient crustal components the geochemical and isotopic compositions of granitoids with sanukitoid affinity suggest derivation from enriched mantle reservoirs finally anatectic granites were produced by reworking of crustal sources with different histories in the light of the data reviewed in this contribution we propose the following scenario for the tectonic evolution of the dharwar craton during 3450 3000 ma ttgs sources oceanic arc crust formed by melting of down going slabs and subsequent melting of such newly formed crust at different depths resulted in ttg magmas on the contrary by 2700 ma the depth of slab melting increased melting of slab at greater depth alongside the detritus results in enriched melts partly modified the overlying mantle wedge subsequent melting of such newly formed enriched oceanic arc crust and surrounding arc mantle generated the magmatic precursor to transitional ttgs finally at ca 2600 2560 ma eventual breakoff of down going slab caused mantle upwelling which induced low degree 10 15 melting of overlying enriched mantle at different depths thereby generating the sanukitoid magmas which upon emplacement into the crust caused high temperature metamorphism reworking and final cratonization the crustal accretion patterns in the dharwar craton share similarities with those in other archean cratons such as the bundelkhand craton in central india pilbara yilgarn craton in western australia southern africa swaziland and limpopo belt north china craton tanzania craton antongil craton ne madagascar

smnd pbpb and rbsr geochronology and petrogenesis of the mafic dyke swarm of mahbubnagar south india implications for paleoproterozoic crustal evolution of the eastern dharwar craton abstract the sm nd rb sr and pb pb isotopic systematics of whole rock and separated mineral fractions from four tholeiitic mafic dykes of the mahbubnagar swarm have been investigated this dyke swarm intrudes the granitic basement of the cuddapah basin in the eastern dharwar craton edc the sm nd data on whole rock samples of four gabbros and one dolerite and 15 mineral fractions yielded a linear array with an isochron age of 2173 64 ma and an initial nd value of 0 4 0 3 the sm nd data of a metapyroxenite and its three mineral fractions plot parallel to this array yielding an isochron age of 2189 123 ma with a negative initial nd value of 1 8 1 4 rb sr and pb pb internal isochron ages of three gabbros and a pb pb internal isochron age of a metapyroxenite dyke range between 2031 and 2284 ma and within the limits of experimental error broadly conform to the sm nd isochron age by contrast rb sr internal isochron ages of three other samples range between 1474 and 1952 ma and one sample yields a pb pb internal isochron age of 3446 ma showing the effects of partial isotopic resetting possibly due to hydrothermal alteration these data suggest that mafic dykes of the mahbubnagar swarm were emplaced at c 2170 ma marking an episode of the crustal extension and fracturing which ultimately resulted in the formation of the cuddapah basin different petrological and geochemical variants of mafic dykes seen in the area most probably represent generation and or crystallization of the progressively evolved batches of magma during a relatively short time interval the initial nd values calculated at 2170 ma range from 0 6 to 2 0 and can be explained as reflecting contamination of depleted mantle derived magma by crustal rocks of edc the initial 87 sr 86 sr ratios 0 7027 0 7064 and model 1 values 8 3 8 5 of apparently undisturbed samples are also consistent with this suggestion

pb isotopic compositions of archean komatiites and sulfides abstract pb isotopic compositions were measured in komatiites and other volcanic rocks from several archean greenstone belts and in magmatic and exhalative sulfides associated with these volcanics thin komatiite flows generally yielded pb pb isochron ages consistent with other geochronological or geological information but thicker basaltic flows gave ages that are too young these results suggest that the thin komatiite flows were hydrothermally altered during eruption and that this event is recorded by the pb pb age thicker basaltic flows resisted alteration during eruption but were affected by later events their pb pb ages probably correspond to the time of metamorphism and not to the time of emplacement magmatic and exhalative sulfides from the abitibi belt in canada fall on the same trend in the 207 pb 204 pb 206 pb 204 pb diagram as komatiites basalts sedimentary rocks and k feldspars from granites the calculated 1 value is 7 8 0 1 a similar suite of rocks and minerals from the kambalda area in australia has distinctly higher 207 pb 204 pb and a higher 1 of 8 3 nd isotopic data demonstrate that some kambalda rocks are highly contaminated with older continental crust the differences in initial pb isotopic compositions are attributed to the presence or absence and the age of underlying granitoid basement kambalda greenstones are underlain and contaminated by old 3 0 ga granitoid crust but significantly older granitoid basement appears absent from the abitibi belt the pb isotopic compositions of kambalda greenstones are dominated by the crustal component and do not represent the composition of the mantle source of these rocks the abitibi greenstones are not highly influenced by crustal contamination and their pb isotopic compositions may reflect those of archean mantle

palaeoproterozoic assembly of the napier complex southern india and western australia implications for the evolution of the cuddapah basin abstract a detailed examination of lithostratigraphic tectonothermal geochronologic and palaeomagnetic database of the dharwar bastar cratons of south india the yilgarn craton of western australia and the east antarctic shield has shown broad similarity two palaeomagnetic key poles of 2400 ma age from the dharwar and yilgarn cratons suggest near neighbor positions matching patterns of craton outlines orientation of mafic dyke swarms and palaeo north direction for 2400 ma have established a continental assembly of siwa an acronym for south india si and western australia wa at 2400 ma in this assembly the yilgarn craton fits against the bastar craton and the singhbhum craton the available geological data from the napier complex of the eastern antarctica and the dharwar craton of south india were used to prepare the barcode style reconstruction of archaean tectonostratigraphic events of both the cratons the method of matching continental outlines the internal features of two continental blocks and the palaeomagnetic data demonstrate that the napier complex was also a part of siwa during the period 2200 1900 ma and was located at the position of the cuddapah basin of india adjacent to the dharwar craton and the yilgarn craton the separation of the napier complex from the south indian block led to the development of the cuddapah basin at 1950 ma

the timescale and mechanism of granulite formation at kurunegala sri lanka incipient charnockite formation at kurunegala in sri lanka is characterized by the growth of orthopyroxene at the expense of amphibole and biotite in an originally homogeneous gneiss mineral equilibria in the charnockite assemblage record pressure temperature p t conditions of 738 60 c and 6 9 1 2 kbar at 17 0 1 2 log fo2 and ah2o 0 18 0 16 wholerock trace element and isotopic measurements show that charnockite formation was accompanied by a systematic depletion of sm rb pb u sr nd with a fractionation of rb sr sm nd and th u ratios and crystallization of the charnockite assemblage at 535 5 ma major element fe mg ca and sm nd equilibration between minerals occurred at 524 9 ma whereas pb and rb sr underwent continued exchange to 501 5 ma and 486 1 ma respectively trace element data for both amphibolite and charnockite minerals show that depletion on a whole rock scale can be accounted for either by changes in mineral modes or trace element abundances within the immediate area of dehydration the fractionation of sm nd on a whole rock scale is controlled by the breakdown of amphibole without the growth of a major new host phase for sm in the charnockite rubidium and sr are dependent on the relative behaviour of biotite plagioclase and alkali feldspar modelling of dehydration melting involving the breakdown of amphibole biotite and alkali feldspar reproduces the observed sm nd and rb sr fractionation and indicates the loss of small melt fractions on a cm scale from the charnockite these observations suggest that partial melting is the most plausible means of effecting both the dehydration and depletion that accompanies charnockite formation

pb and nd isotope constraints on the origin of high mg and tholeiitic amphibolites kolar schist belt south india the late archean north south trending kolar schist belt in south india 4 km wide by 80 km long is thought to be a suture between two gneiss terranes krogstad et al 1989 within this volcanics dominated belt are recognized both tholeiitic and high mg komatiitic and picritic amphibolites which make up some 70 and 5 respectively of the exposed outcrops a massive tholeiitic amphibolite separates the belt into western and eastern parts a pb pb whole rock age of 2732 155 ma on samples from a single outcrop of massive tholeiite is a minimum age for this rock samples of this rock have nd values at 2700 ma that range between 3 8 and 6 8 1 initial 238u 204pb of 7 5 and k1 initial 232th 238u of about 4 two different types of high mg amphibolites are recognized from the western part of the belt a picritic or p type and a komatiitic or k type the p type have highly variable ce al ratios all greater than chondritic nd yb ratios greater than chondritic nd at 2700 ma of 1 5 to 8 and pb isotope compositions variable in 207pb 204pb with 1 of about 8 0 and k1 of about 4 the trace element data suggest that the light ree enrichment is a character of the mantle source and is not due to residual garnet the k type amphibolites have near chondritic ce al and nd yb ratios nd at 2700 ma of 1 5 to 8 and 1 of about 8 and k1 of about 4 although the p type is light ree enriched compared to the k type both types have similar ce nd ratios as well as initial pb and nd isotopes if the 2696 136 ma age for the sm nd isochron which includes both types of high mg amphibolite has any significance it dates the time of light ree enrichment of the mantle source for the p type komatiitic amphibolites the high mg amphibolites in the eastern part of the belt are light ree enriched have pb isotopic compositions that are variable in 207pb 204pb with a 1 about 8 5 and nd at 2700 ma of 1 8 to 4 5 hydrothermal fluids associated with metamorphism and shearing prior to about 2400 ma ago were responsible for the introduction of gold quartz carbonate veins into the kolar schist belt the pb isotope composition of galena in these veins suggests that these fluids may have also introduced extraneous pb from adjacent older granitoid gneisses into the amphibolites which could be responsible for the variability in the 207pb 204pb ratios of the samples this extraneous pb probably is not responsible for the distinct pb isotope character of each type of amphibolite

lead lead age of komatiitic lavas and limitations on the structure and evolution of the precambrian mantle abstract using lead isotopes we have studied komatiites and associated sulfides from three complexes at barberton the lead lead isochron gives an age of 3 46 0 07gy in good agreement with the sm nd age at munro township three distinct komatiitic flows yield significantly different ages pyke hill is 2 72 0 02gy theo s flow is 2 47 0 13gy and fred s flow is 2 58 0 02gy old 2 72 gy is the emplacement age while the alignment obtained on theo s flow and fred s flow clearly points out the effect of a metamorphism posterior to 2 7 gy at cape smith the age obtained by the lead lead method is 1 6 0 13gy this age is younger than that obtained by other methods in contrast the isochron obtained on sulfide gives an age of 1 88 0 17gy in agreement with sm nd methods these results together with others from the literature are used to describe the evolution of pb in the mantle the important result inferred is that the mantle 2 70 gy ago was very different from the primitive mantle according to both th u and u pb values indeed the latter are lower than the planetary values this evolution is the result of the formation of the continental crust

u pb and hf isotope records in detrital and magmatic zircon from eastern and western dharwar craton southern india evidence for coeval archaean crustal evolution abstract u pb and lu hf isotopic records in detrital and magmatic zircons from the eastern and the western dharwar craton edc and wdc suggest presence of 3 4 ga crustal components in both the blocks magmatic zircon from the western block has ages ranging between 3 1 and 3 2 ga while those from the eastern block show a bi modal age distribution with records of 3 0 3 2 ga protolith components as well as 2 7 and 2 5 ga metamorphic overgrowths detrital and magmatic zircons from granitoid conglomerate clasts of the western dharwar block show initial hf values consistent with models for the early archaean mantle suggesting their juvenile character lower hf values in younger as well as detrital zircons from eastern dharwar block reflect importance of reworking of 3 2 ga crust the combined u pb and hf isotope study strengthens suggestions for contemporaneous crust formation processes in both the western and eastern parts of the dharwar craton

continental lithospheric evolution constraints from the geochemistry of felsic volcanic rocks in the dharwar craton india abstract felsic magmatism associated with ocean ocean and ocean continent subduction processes provide important evidence for distinct episodes of crust generation and continental lithospheric evolution rhyolites constitute an integral component of the tholeiitic to calc alkaline basalt andesite dacite rhyolite badr association and contribute to crustal growth processes at convergent plate margins the evolution of the dharwar craton of southern peninsular india during meso to neoarchean times was marked by extensive development of greenstone belts these granite greenstone terranes have distinct volcano sedimentary associations consistent with their geodynamic setting the present study deals with geochemistry of rhyolites from the chitradurga shimoga greenstone belts of western wdc and the gadwal kadiri greenstone belts of eastern edc sectors of dharwar craton to compare and evaluate their petrogenesis and geodynamic setting and their control on the continental lithospheric evolution of the dharwar craton at a similar range of sio2 al2o3 fe2o3 the rhyolites of wdc are more potassic whereas the edc rhyolites are more sodic and less magnesian with slight increase in tio2 minor increase in mgo content of wdc rhyolites reflects their ferromagnesian trace elements which are comparatively lower in the rhyolites of edc the relative enrichment in lile k rb and depletion in hfse nb ta zr hf marked by negative nb ta zr hf and ti anomalies endorse the convergent margin processes for the generation of rhyolites of both the sectors of dharwar craton the high silica potassic rhyolites of shimoga and chitradurga greenstone belts of wdc showing prominent negative eu and ti anomalies flat hree patterns correspond to type 3 rhyolites and clearly point towards their generation and emplacement in an active continental margin environment the geochemical characteristics of gadwal and kadiri rhyolites from eastern dharwar craton marked by aluminous compositions with low and fractionated hree patterns and minor negative eu anomalies are in conformity with type 1 rhyolites and suggest that they were erupted in an intraoceanic island arc system the overall geochemical systematics of the rhyolites from both the sectors of dharwar craton suggest a change in the geodynamic conditions from intraoceanic island arc of eastern dharwar craton and an active continental margin of western dharwar marked by ocean ocean subduction and migration of oceanic arc towards a continent followed by arc continent collision that contributed for the evolution of continental lithosphere in the dharwar craton

gold mineralisation and alteration of penakacherla schist belt india constraints on archaean subduction and fluid processes abstract in the dharwar craton southern india gold deposits are found mostly along the six arcuate shear zones passing through late archaean greenstone belts 2 7 ga one such shear zone complex extends for about 400 km within and along the ramagiri hungund schist belt the penakacherla sector of this shear zone is excellently exposed enabling a detailed investigation of synorogenic gold mineralisation and its relationship to associated hydrothermal alteration metamorphism and deformation under ne sw compression associated with archaean subduction processes converted mafic volcanic rocks into amphibolites and intermediate to felsic volcanic rocks into quartz mica schists continued compression generated a 50 100 m wide shear zone complex consisting of mafic phyllonites advection of hydrothermal fluids through this shear zone and reaction between fluids and the mafic phyllonites resulted in a silicified k metasomatic assemblage mainly consisting of chlorite amphibole k mica plagioclase ankerite quartz fe oxides pyrite chalcopyrite and arsenopyrite networks of quartz and carbonate veinlets a few millimeters to a few centimeters thick formed along the foliation planes giving rise to microscopic alteration envelope in which individual veinlet systems are merged into one another to form a composite alteration system gold is found within these quartz veinlets mafic phyllonites and at their mutual contacts hydrothermal fluids have modified the primary major minor trace and lree compositions of host rocks such that their mutual behaviour became non systematic some hfse and hree also show minor mobility but the overall ree pattern generally resembles that of the precursor mafic volcanic rocks mass and volume loss gain by si and ca has made significant impact on al ti and zr abundances which are generally immobile during hydrothermal alteration however element pairs such as zr hf v sc and nb ta maintain primary inter element ratios although their absolute abundances are drastically diluted similarly ree in highly silicified and carbonatised samples are reduced but patterns remain similar to those of relatively least altered mafic phyllonites with la n yb n between 1 and 3 in some samples lree enrichment is observed elevating in la n yb n from 3 to 11 pathfinder elements and base metals such as as cd cu pb zn and sb have been added along with the au and ag 13 c of carbon varies from 16 to 21 suggesting a biogenic origin whereas coexisting pyrite 34 s ranges from 1 to 3 pointing towards the involvement of magmatic or average crustal sulphur overall concentrations of k rb sr ba nb ta ti cs cr co v y and sc and many of the ratios such as k rb la sc la yb indicate that metamorphism devolatilisation and dehydration of an oceanic subducting slab might have partially contributed the mineralising fluids and generated the alteration assemblage observed in the host rocks fluid sources were mantle and greenstone belt dehydration and devolatilisation generating observed compositional and alteration diversity

resetting of ree and nd and sr isotopes during carbonitization of a komatiite flow from finland abstract new isotopic nd sr c and o and trace element data are reported for a 10 m thick komatiite flow unit from the 2660 ma tipasjarvi greenstone belt of eastern finland this unit was selected because of its anomalously variable ree distribution patterns more specifically while the lower cumulate zone of the flow exhibits nearly flat lree patterns la sm n 0 93 1 10 the spinifex textured upper zone shows strongly depleted lree patterns la sm n 0 40 such a fractionation in ree cannot result from low pressure differentiation because the flow has mgo contents ranging from 25 to 30 and only olivine a phase which does not fractionate the ree should have crystallized in previous studies the ree patterns have been interpreted by the assumption that the flow had a composite origin a lree depleted parental magma was inferred for the upper part the lower part was thought to be derived from a parental magma with a chondritic distribution of lree in fact in the present study a reassessment of field and petrographic data indicates that the lower part of the flow is rich in secondary carbonate 10 20 wt determination of the chemical and isotopic composition of the carbonate and further evaluation of the chemical and isotopic patterns of the flow in the light of the new data clearly show that the addition of carbonate is responsible for the variation in ree patterns the tipasjarvi flow is not a magmatically composite flow but a single and uniformly depleted flow whose lower portion has experienced severe alteration of primary chemical and isotopic signatures by invasion of secondary carbonate

smnd ages of archaean metavolcanics of the dharwar craton south india abstract well constrained sm nd arrays of whole rocks are reported for the first time from three important stratigraphic units of the dharwar volcano sedimentary sequence of south india if interpreted as isochrons these arrays will correspond to the following ages and initial nd ratios relative to chur 2 911 0 049 ga and 0 20 0 40 for the kalasapura mafic basal metavolcanics of the bababudan greenstone belt 2 848 0 070 ga and 0 88 0 82 for the santaveri formation comprising mafic to felsic metavolcanics overlying the kalasapura rocks and 2 747 0 015 ga and 1 72 0 10 for the ingaldhal mafic felsic metavolcanic suite of the chitradurga greenstone belt the possibility that these sm nd arrays are mixing lines between unrelated components depleted and enriched respectively in light rare earth elements has been discussed leading to the conclusion that the ages closely correspond to their time of eruption the older suites from the bababudan belt from a mantle source of chur like composition the younger suite from the chitradurga belt from one with a prior history of lree depletion

accretion rate of extraterrestrial particles determined from osmium isotope systematics of pacific pelagic clay and manganese nodules abstract pelagic clay and mn nodules from domes sites in the north pacific and a varved glacial lake deposit from connecticut were analysed for os concentration and isotopic composition by isotope dilution secondary ion mass spectrometry after treatment by nis fusion or oxalic acid leaching bulk pelagic clay from domes site c has 187 os 186 os 6 5 and os 0 14 ng g oxalic acid leaches of this same sediment and of mn nodules from domes sites a and c have more radiogenic 187 os 186 os ratios which average 8 3 bulk glacial lake hitchcock sediment has 187 os 186 os 12 5 and os 0 06 ng g the total os flux to north pacific pelagic clay is 7 to 10 ng os cm2 106 y lake hitchcock sediment provides an integrated value for the local crustal 187 os 186 os ratio the oxalic acid leaches are assumed to attack hydrogenous phases selectively a simple model in which the only sources of os to the ocean are continental crust with the isotopic composition of lake hitchcock 187 os 186 os 12 5 and extraterrestrial particles with 187 os 186 os 1 1 results in a cosmic flux of osmium to the sediment of 4 9 ngos cm2 106 y of which 20 is hydrogenous a model in which the sources of os to the ocean are continental crust with an 187os 186os ratio of 30 from the model of palmer and turekian 1986 oceanic mantle or crust with 187 os 186 os 1 04 and extraterrestrial particles with 187 os 186 os 1 1 results in a cosmic flux of os to the sediment of 5 7 ngos cm2 106 y of which none is hydrogenous these extraterrestrial os fluxes correspond to maximum c 1 chondrite accretion rates of between 4 9 104 and 5 6 104 tonnes y

loss of isotopic nd o and chemical ree memory during metamorphism of komatiites new evidence from eastern finland komatiites are often considered to depict the chemical and isotopic composition of their source rocks in the archean mantle however a weakness of these rocks in tracking the initial compositional heterogeneity of the earth s mantle is the ubiquitous presence of metamorphic recrystallization which casts some doubt about the preservation of primary chemical and isotopic characteristics two spinifex textured komatiite flows from the 2 75 ga old kuhmo greenstone belt siivikkovaara area of eastern finland document this weakness both flows have experienced low to medium grade metamorphism t 450 50 c and now consist entirely of secondary metamorphic assemblages of amphibole chlorite plagioclase with minor proportions of magnetite and ilmenite mgo contents range from 25 to 8 which suggests that low pressure differentiation was likely controlled by olivine and clinopyroxene fractional crystallization however neither major nor trace elements fall on olivine and or clinopyroxene control lines this is particularly well illustrated by the ree as there is an overall 60 variation of ce sm n ratios 0 38 to 0 91 which far exceeds that expected from olivine and clinopyroxene fractionation alone in fact careful evaluation of petrographic including mineral composition data and chemical characteristics shows that most elements of geological interest including the reputedly immobile ree were mobile on a whole rock scale during metamorphic recrystallization of these two flows this view is fully supported by sm nd isotopic data since both whole rock and mineral amphibole and plagioclase samples lie on a single isochron relationship at t 1800 ma an age which corresponds to the time of regional metamorphism thus the meta komatiite flows from siivikkovaara document a case of komatiite flow units in which metamorphism has induced whole rock scale resetting of primary ree patterns and sm nd isotope systematics as regards the nature of the mechanism responsible for this resetting it is suggested here that the secondary mineralogy played an important role as there are correlations between whole rock 147sm 144nd ratios major and trace element chemistry oxygen isotopic ratios and modal proportions of secondary minerals consideration of oxygen isotopic data as well as previous results from a komatiite flow from the nearby tipasjarvi belt further enable us to propose that the ree carrier was likely a co2 rich fluid the process of secondary ree redistribution prevents estimation of the true initial nd isotopic compositions of the two flows taken as a whole the results presented in this paper show that great care should be be exercised in the use of meta komatiites as probes of archean mantle composition

pb isotope composition of ni cu pb ore deposits in an archean greenstone belt evidence for proterozoic remobilization in the pontiac subprovince of canada

boninites from the neoarchaean gadwal greenstone belt eastern dharwar craton india implications for archaean subduction processes abstract boninites are rare in archaean arcs all over the world whilst there are some reports from the phanerozoic convergent margins in this paper boninites are reported for the first time from an archaean greenstone belt of india which have not been reported from any other indian greenstone belt as yet the greenstone belts of western dharwar craton are characterized by high mg basalts and komatiites whereas in the greenstone belts of eastern dharwar craton boninites represent the high mg lavas these boninites are characterized by 1 high mgo 12 24 wt 2 enriched in transition elements such as ni 423 1184 ppm cr 943 2765 ppm 3 low tio2 0 22 0 36 wt 4 high al2o3 tio2 ratio 29 50 5 gd ybn less than one 6 slight enrichment in lree depletion in mree compared to lree and hree resulting in u shaped ree patterns 7 negative nb ta zr hf ti and v anomalies despite the depletion in mree these characteristics differentiate them from deep mantle plume related magmas these boninites are compositionally similar to those found in modern and fossil island arcs from intraoceanic subduction settings and provide evidence to propose that two stage melting of the archaean mantle wedge generated this type of high mg magma the other high mg type magma komatiitic suite found in the greenstone belts of western dharwar craton has an enriched deep mantle plume source and its dry melting whereas boninites are generated by wet melting of the refractory depleted mantle wedge under high temperature and fluid pressure generated by dehydration of the slab the geological setting hfse and ree data show that possibly the 2 7 ga peak of geological activity had transitional characteristics between those of paleoarchaean and phanerozoic high thermal and fluid flux is inferred to generate these mg enriched and mree hfse lfse depleted suite this discovery of boninites from gadwal greenstone belt indicate that even in archaean subduction of small young and hot oceanic plates produced magmatic assemblage resembling to those of some of the cenozoic subduction complexes

nonlinear acoustic scattering from a gassy poroelastic seabed a model for difference frequency backscatter from trapped bubbles in sandy sediments was developed a nonlinear volume scattering coefficient was computed via a technique similar to that of ostrovsky and sutin nonlinear sound scattering from subsurface bubble layers in natural physical sources of underwater sound edited by b r kerman kluwer dordrecht 1993 pp 363 373 which treats the case of bubbles surrounded by water biot s poroelastic theory is incorporated to model the acoustics of the sediment biot fast and slow waves are included by modeling the pore fluid as a superposition of two acoustic fluids with effective densities that differ from the pore fluid s actual density and account for its confinement within sediment pores the principle of acoustic reciprocity is employed to develop an expression for the backscattering strength model behavior is consistent with expectations based on the known behavior of bubbles in simpler fluid media

nd isotopic systematics and chemistry of central australian sapphirine granulites an example of rare earth element mobility abstract lower proterozoic sapphirine bearing and associated granulites from central australia exhibit the greatest range of present day 143 nd 144 nd ratios nd o 26 5 to 112 3 yet reported for rocks believed to be cogenetic the nd isotopic data and ree abundances of these rocks demonstrate extreme fractionation of the rare earths during the formation of stratiform cu pb zn sulfide deposits with which they are closely associated field relationships petrography and chemistry of the sapphirine granulites suggest that their protoliths comprised chlorite rich rocks which were generated by hydrothermal alteration of a range of rock types prior to metamorphism calculations employing ree abundances of the sapphirine granulites and associated rocks combined with bulk solid fluid distribution coefficient data yield high fluid rock ratios consistent with a pre metamorphic hydrothermal origin for the unusual ree patterns the sm nd data for these rocks define an age of 1760 75ma which is significantly younger than the crust formation age of the terrain 2070 125ma but indistinguishable from the rb sr whole rock age for granulite facies metamorphism 1790 35ma these data are interpreted in terms of major hydrothermal fractionation of the rare earths shortly perhaps tens of millions of years before granulite facies metamorphism followed by redistribution of nd isotopes or small fractionations of the sm nd ratio during the granulite facies event and possibly also during intense retrogression which reset rb sr whole rock and u pb zircon and monazite systematics at about 1700 ma

rare earth mobility in young arc type volcanic rocks from northern new zealand a comment to a paper by e kuschel and i e m smith

subduction related tectonic evolution of the neoarchean eastern dharwar craton southern india new geochemical and isotopic constraints the neoarchean eastern dharwar craton edc is distinct from the mesoarchean western dharwar craton wdc in many aspects of its geology the important distinguishing features of the edc are the predominance of younger granitoids abundance of gold mineralization and the exposure at lower crustal depths no consensus exists on evolutionary models for edc mutually exclusive plume and subduction derived tectonic models have been proposed geochemical and radiogenic isotopic studies on the granitoids and volcanic rocks of three greenstone belts along a cross section in the northern part of edc are presented herein the evolved nd isotopic signatures radiogenic pb isotopic ratios and arc like geochemical signatures are suggestive of a subduction regime and the involvement of recycled older crust in the derivation of these rocks the proposed petrogenetic mechanism involves multi stage processes in a supra subduction regime involving slab dehydration formation of hydrous basaltic melts and re melting and interaction with sub arc basaltic crust at low pressures where amphibole plagioclase is the dominant residual phase there is a notable systematic decrease in the extent of older crustal involvement from west to east in the edc this is in concurrence with the younging of dharwar craton from west to east and eastward subduction the proposed petrogenetic model can efficiently explain the variations in older crustal involvement which is very common in other archean cratons c 2012 elsevier b v all rights reserved

pb and nd isotopic study of two archean komatiitic flows from alexo ontario isotopic analysis of two archean komatiitic flows from alexo ontario gives a pb pb isochron age of 2690 15 ma and a sm nd isochron age of 2752 87 ma these ages agree well with u pb zircon ages from underlying and overlying volcanics the variations in element ratios that define the isochrons were not produced during crystallization of the lavas the spread in u pb was caused by submarine alteration soon after eruption and the spread in sm nd resulted from a differences in the composition of the residue of melting and b contamination of the upper komatiite flow through thermal erosion of the lower flow the 147sm 144nd ratio of uncontaminated komatiite is 0 25 which reflects the depleted nature of its mantle source the th u ratio of about 3 4 is probably also representative of depleted mantle the initial nd of 2 44 0 51 indicates that the mantle depletion took place long before magma formation

pb sr and nd isotope systematics of uranium mineralised stromatolitic dolomites from the proterozoic cuddapah supergroup south india constraints on age and provenance abstract the pb sr and nd isotopic compositions were determined on uranium mineralised and barren stromatolitic dolomite samples from the vempalle and tadpatri formations of the cuddapah supergroup in southern india subsamples from individual handspecimens were analyzed to minimize the potential of heterogeneous initial pb compositions all of the u mineralised samples gave pb pb ages close to 1800 ma but with high mswd values presumably due to local redistribution of u and pb in the samples the best pb pb age of 1756 29 ma is interpreted as the time of u mineralisation and as a minimum age for carbonate sedimentation and dolomitisation within the cuddapah supergroup the sr isotopic compositions of the samples vary with the mineralised samples being more radiogenic even the lowest 87 sr 86 sr of the barren sample exceeds the seawater 87 sr 86 sr about 1800 ma ago on the other hand the nd systematics of the samples does not appear to have been disturbed by processes subsequent to carbonate sedimentation the calculated initial nd isotopic compositions suggest a dominant continental crustal source that was at least 600 ma old at the time of deposition of the carbonate sediment

assessment of the u th pb system in two archean metabasalts deciphering the complex histories of sulphides and silicates using acid leaching methods abstract a u th pb study of archean metabasalts from two greenstone belts in the eastern wawa subprovince of the canadian shield indicated variable disturbances had occurred in both whole rock systems changes in the pb content appear to predominate over loss of parent elements and meaningful pb isochron ages could not be obtained from either of the metabasalts detailed leaching experiments on the rocks and analysis of associated disseminated sulphides indicate that the pb isotopic compositions of the whole rocks are dominated by updated but for the most part cogenetic sulphides model ages for pb released by sulphides and metabasalt acid leaches suggest that the sulphide pb has been updated or remobilized during discrete thermal episodes the validity of the inferred events is supported by ages indicated by other isotopic systems for nearby rocks the silicate residues of the acid leached volcanics give well defined pb isochron ages the gamitagama belt metabasalt has a pb isochron age of 2694 54 ma zircons from this rock unit have been dated by u pb at 2691 ma demonstrating that the acid leaching technique on whole rocks can isolate residues which give meaningful ages sulphides in this metabasalt appear to be affected by an event at 2 55 ga a similar rock from the michipicoten belt has undergone a multistage history and yields a pb isochron age of 2761 36 ma consistent with a zircon u pb date for overlying felsic metavolcanic rocks later events affecting this basalt occurred at approximately 2 4 2 2 and 1 6 ga the results show that through acid leaching the primary ages of metabasalts and their later overprinting histories can be approximated initial pb ratios for these metabasalts have been estimated it appears that both depleted and enriched mantle reservoirs contributed to volcanism in this area acid leaching combined with measurement of u and th contents appears to be a useful tool for better understanding the pb isotope systematics of archean metabasalts

geochemistry of the neoarchaean high mg basalts boninites and adakites from the kushtagi hungund greenstone belt of the eastern dharwar craton edc implications for the tectonic setting abstract volcanic rocks of the kushtagi hungund greenstone belt from the eastern dharwar craton edc show extreme diversity due to the presence of high mg high fe high mg fe basalts high mg dacites and andesites boninitic affinities and sodic plagioclase quartz rich felsic rocks adakites with na2o 4 6 wt the adakites drummond and defant 1990 martin 1999 and high mg dacites andesites are being reported for the first time from greenstone belts of the dharwar craton trace elements in these different types of volcanic rocks have distinct characteristics for example the basalts have 1 al2o3 tio2 16 pm value of 17 2 la yb n 1 3 3 low order negative anomalies of nb nb zr zr hf hf and ti ti 4 zr nb 4 10 and 5 ce nb 3 4 two samples of andesites having mg enrichment and al2o3 tio2 ratios of 31 35 are interpreted to have boninitic affinities other dacites and andesites are characterized by 1 high mg 8 10 wt 2 high al2o3 tio2 22 27 3 zr nb 8 14 4 la yb n 2 7 5 negative nb nb zr zr hf hf and ti ti anomalies and 6 sc lu 67 172 the adakites are characterized by 1 a narrow range of sio2 67 70 wt 2 enriched na2o 4 6 wt with very high na2o k2o 2 5 7 7 3 low mg 0 38 0 48 4 high la yb n 13 31 with fractionated patterns no eu anomaly and gd yb n 2 4 5 very high al2o3 tio2 50 80 and 6 enriched ni cr and co total 50 ppm these adakites resemble 3 0 ga tonalite trondhjemite gneisses of the dharwar craton which are interpreted as products of partial melting of the basaltic slab the available data suggests that partial melting of the different mantle and slab sources generated this collage of compositionally different rocks within an island arc setting

postmagmatic thermal activity in the abitibi greenstone belt noranda and matagami districts evidence from whole rock pb isotope data the pb isotope compositions of the footwall sequences of the noranda and matagami volcanogenic massive sulfide deposits in the abitibi greenstone belt have been examined at noranda whole rock pb isotope data for the flavrian northwest rusty ridge and amulet formations and the subvolcanic flavrian intrusion scatter about a line that corresponds to a pb pb age of 2638 or 14 ma pb pb ages calculated for the individual units are all within error of this age but are significantly younger than the 2700 or 2 ma u pb zircon age for the flavrian intrusion the pb pb array passes above a tightly constrained field for the galena pb from ore deposits within the mine sequence the younger pb pb ages and the lack of collinearity with the sulfide ores reflect rehomogenization of the pb isotope compositions in these rocks approximately 60 m y after their formation at matagami whole rock pb isotope data fall along two arrays the first which includes the porphyritic metarhyolite of the watson lake group from the footwall and the key tuffite corresponds to a pb pb age of 2584 super 19 sub 20 ma the second array includes the bell river igneous complex and the overlying wabassee group basalts and corresponds to an age of 2647 or 14 ma these pb pb ages are approximately 75 and 140 m y younger than corresponding zircon u pb ages of 2725 or 2 and 2725 super 3 sub 2 ma both arrays pass above a tightly constrained field for galena pb from several matagami ore deposits and are probably the result of differences in the initial pb isotope compositions of the respective rocks the apparent initial pb values most likely represent heterogeneities in the mantle source regions for these rocks the younger pb pb ages for the two arrays indicate pb isotope homogenization in these rocks approximately 100 m y after their formation the cause of the different ages for the two arrays remains controversial but may be the result of either incomplete isotopic homogenization of pb or an earlier closure of the u pb systems in the rocks constituting the older array the pb homogenization implied by the young pb pb ages may be the result of the pervasive circulation of hydrothermal solutions that may have been derived from the dewatering of the lower parts of a newly formed thickened archean crust such large scale processes apparently occurred 50 to 150 m y after this crust was formed and assembled

pan african charnockite formation in kerala south india sm nd mineral ages of gneisses and associated granulites from the ponmudi incipient charnockite locality south india indicate that granulite metamorphism occurred at or shortly after 558 ma proterozoic ages recorded by garnet separates reflect a detrital age or an earlier metamorphic event preserved by inclusions within garnet the age of post metamorphic uplift 440 460 ma is constrained by sr isotope equilibration between biotite and plagioclase since charnockite formation and subsequent uplift north of the palghat cauvery shear zone had terminated by earliest proterozoic time these results confirm two distinct periods of granulite formation in south india and suggest that the palghat cauvery shear zone represents the boundary between two blocks of strongly contrasting geological histories both incipient charnockite formation and subsequent uplift at ponmudi may be correlated with the tectonothermal evolution of the highlands group in sri lanka the similarity between nd and sr model ages for charnockites and gneisses from ponmudi indicates that no significant rb sr fractionation has occurred during the crustal history of these incipient charnockites pb isotopic ratios suggest that th u ratios were fractionated during charnockite formation at about 500 ma in contrast to charnockites found north of the palghat cauvery shear zone fractionation of u pb during the archaean did not occur in the ponmudi granulites

multi stage crustal growth and neoarchean geodynamics in the eastern dharwar craton southern india abstract the dharwar craton is a composite archean cratonic collage that preserves important records of crustal evolution on the early earth here we present results from a multidisciplinary study involving field investigations petrology zircon shrimp u pb geochronology with in situ hf isotope analyses and whole rock geochemistry including nd isotope data on migmatitic ttg tonalite trondhjemite granodiorite gneisses dark grey banded gneisses calc alkaline and anatectic granitoids together with synplutonic mafic dykes along a wide northwest southeast corridor forming a wide time window in the central and eastern blocks of the dharwar craton the dark grey banded gneisses are transitional between ttgs and calc alkaline granitoids and are referred to as transitional ttgs whereas the calc alkaline granitoids show sanukitoid affinity our zircon u pb data together with published results reveal four major periods of crustal growth ca 3360 3200 ma 3000 2960 ma 2700 2600 ma and 2570 2520 ma in this region the first two periods correspond to ttg generation and accretion that is confined to the western part of the corridor whereas widespread 2670 2600 ma transitional ttg together with a major outburst of 2570 2520 ma juvenile calc alkaline magmatism of sanukitoid affinity contributed to peak continental growth the transitional ttgs were preceded by greenstone volcanism between 2746 ma and 2700 ma whereas the calc alkaline magmatism was contemporaneous with 2570 2545 ma felsic volcanism the terminal stage of all four major accretion events was marked by thermal events reflected by amphibolite to granulite facies metamorphism at ca 3200 ma 2960 ma 2620 ma and 2520 ma elemental ratios la yb n sr y nb ta hf sm and hf nd isotope data suggest that the magmatic protoliths of the ttgs emplaced at different time periods formed by melting of thickened oceanic arc crust at different depths with plagioclase amphibole garnet titanite ilmenite in the source residue whereas the elemental ba sr la yb n sr y nb ta hf sm and hf nd isotope data ehf t 0 67 to 5 61 end t 0 52 to 4 23 of the transitional ttgs suggest that their protoliths formed by melting of composite sources involving mantle and overlying arc crust with amphibole garnet clinopyroxene plagioclase ilmenite in the residue the highly incompatible and compatible element contents ree k ba sr mg ni cr together with hf and nd isotope data ehf t 4 5 to 3 2 end t 1 93 to 1 26 of the sanukitoids and synplutonic dykes suggest their derivation from enriched mantle reservoirs with minor crustal contamination field elemental and isotope data ehf t 4 3 to 15 0 end t 0 5 to 7 0 of the anatectic granites suggest their derivation through reworking of ancient as well as newly formed juvenile crust secular increase in incompatible as well as compatible element contents in the transitional ttgs to sanukitoids imply progressive enrichment of neoarchean mantle reservoirs possibly through melting of continent derived detritus in a subduction zone setting resulting in the establishment of a sizable continental mass by 2700 ma which in turn is linked to the evolving earth the neoarchean geodynamic evolution is attributed to westward convergence of hot oceanic lithosphere with continued convergence resulted in the assembly of micro blocks with eventual slab break off leading to asthenosphere upwelling caused extensive mantle melting and hot juvenile magma additions to the crust this led to lateral flow of hot ductile crust and 3d mass distribution and formation of an orogenic plateaux with subdued topography as indicated by strain fabric data and strong seismic reflectivity along an e w crustal profile in the central and eastern blocks of the dharwar craton

plate tectonics 2 5 billion years ago evidence at kolar south india the archean kolar schist belt south india is a suture zone where two gneiss terranes and at least two amphibolite terranes with distinct histories were accrted amphibolites from the eastern and western sides of the schist belt have distinct incompatible element and isotopic characteristics sugesting that their volcanic protoliths were derived from dint mantle sources the amphibolite and gneiss terranes were juxtaposed by horizontal compression and shearing between 2530 and 2420 million years ago ma along a zone marked by the kolar schist belt this history of accretion of discrete crustal terranes resembles those of phanerozoic convergent margins and thus suggests that plate tectonics operated on earth by 2500 ma

a 3 1 billion year old marble and the 87sr 86sr of late archean seawater a marble band in the 2 75 gyr old ramagiri schist belt in the dharwar craton of south india gave a pb pb age of 3 075 0 095 gyr the geochemical data including high sr and low ba and mn indicate seawater origin for the parent rock and that there was insignificant geochemical exchange between the marble and the surrounding rocks the calculated initial nd isotopic composition and 1 indicate an older continental crustal source for the nd and pb the initial 87sr 86sr of the marble is 0 70128 which is higher than the calculated mantle value at 3 ga although pre 3 gyr old marine carbonate rocks are thought to be buffered by mantle sr the ramagiri marble contains evolved crustal sr despite this the marble has the lowest measured 87sr 86sr among carbonates and represents one of the least radiogenic periods in seawater sr isotope composition

postcrystallization disturbance in the neodymium and lead isotope systems of metabasalts from the ramagiri schist belt southern india archean metabasaltic rocks from three separate blocks that constitute the north south trending ramagiri schist belt of the eastern dharwar craton have large variations in nd2750 even samples of the same rock type from one outcrop that are less than 50 cm apart from each other show a range of up to 5 5 nd2750 units the trends of the fields defined by the three suites in smnd vs nd2750 space intersect at smnd 0 1 to 0 2 and nd2750 8 to 12 suggesting that these rocks have lost lrees and have interacted with and exchanged at least nd with a common extraneous phase the isotopic composition of this phase suggest derivation of its neodymium from sources which had a long term history of lree enrichment this phase may have derived its isotopic composition from older continental crust one of the unaltered dikes cutting the three blocks in the belt and the surrounding gneisses gives a 147sm144nd 143nd144nd age of 2454 100 ma a minimum for the time of block assembly and alteration of rees including nd while a set of samples from the central block lie along an isochron with a pb pb age of 2746 64 ma the samples from the eastern and western blocks show large uncorrelated variations in 207pb204pb with 206pb204pb residues after acid leaching of sample powders have lead isotope ratios that are distinct for samples from each of the block and are compatible with their derivation from archean mantle the lead isotope composition of the acid leaches suggest disturbance of the metabasalts by an isotopically extraneous phase that had variable and high 207pb204pb and 208pb204pb ratios for a given 206ph204pb relative to expected archean mantle values both lead and neodymium isotope disturbances are best explained as a result of alteration of these rocks by an extraneous phase which had derived its isotopic composition from ca 3 5 ga continental crust

palaeoproterozoic indian shield in the global continental assembly evidence from the palaeomagnetism of mafic dyke swarms abstract palaeomagnetic study of palaeoproterozoic mafic dykes in the basement along the margins of the cuddapah basin the largest precambrian sedimentary basin in south india is presented in detail for a general discussion of palaeoproterozoic igneous activity in india the results are compared with all earlier published data on mafic dykes in india and are integrated with recently published high precision u pb baddeleyite ages to provide a comprehensive account of palaeoproterozoic igneous activity in india the analysis consolidates palaeomagnetic poles for six age divisions between 2 45 and 1 85 ga with robust statistical criteria our best estimates of overall mean poles from 241 dykes are situated at 1 17 7 n 106 0 e a95 9 0 7n 24 at c 2 45 ga 2 7 1 n 57 2 e a95 4 5 n 69 at c 2 37 ga 3 41 6 s 5 5 e a95 5 1 n 34 at c 2 22 ga 4 4 7 n 343 0 e a95 4 4 n 31 at 2 18 ga 5 49 2 n 332 9 e a95 4 8 n 24 at 1 99 1 89 ga and 6 73 7 n 282 6 e a95 2 9 n 39 at 1 86 ga the data permit us to construct an apparent polar wander path for the indian shield for an 600 ma interval of the palaeoproterozoic eon 2 45 1 85 ga testing and evaluation of continental reconstructions for this interval which are mostly based on geological correlations reveal many inconsistencies between 2 45 and 2 37 ga the indian shield was situated at higher latitudes similar to the yilgarn craton of australia it was subsequently located near the equator at 2 22 2 18 1 99 and 1 86 ga thus an india australia connection is supported during these times but a proposed australia kaapvaal link in zimvaalbara and a dharwar india slave connection in sclavia or a superior zimbabwe india connection in superia are inconsistent with indian data in addition the close palaeomagnetic comparison between the palaeoproterozoic dykes of dharwar bastar bundelkhand cratons in india indicates an age of 2 45 ga for orogenic activity along the central indian tectonic zone hence matching this zone with the 2 0 1 8 ga trans north china orogenic belt or positioning north china adjacent to india or juxtaposition of the indian shield along the western margin of laurentia in the columbia reconstructions is not supported the indian data appear to be in accord with the essential features of the refined protopangaea model and the original ur configuration finally the results are interpreted in terms of four dyke emplacement events in the age range 2 45 2 18 ga linked to short lived 5 10 ma lips developed over mantle plumes the dykes of 1 99 1 89 ga age probably relate to continued long lived igneous activity while the 1 86 ga dykes are relatively fewer in number and may represent waning stage of a large igneous event related to a major mantle plume

rare earth mobility in young arc type volcanic rocks from northern new zealand abstract samples of late cenozoic arc type volcanic rocks from the hauraki volcanic region in northern new zealand show evidence for enrichment of the rare earth elements rees these samples have variable to high la and y abundances together with ree abundance patterns which are up to an order of magnitude higher than those considered typical of the suite as a whole scanning electron microscope imaging and analysis shows the presence of micron scale veins and patches containing ree bearing phases these observations are explained as the result of cryptic alteration processes which enrich the rees but not other chemical components and which are probably the result of low temperature

the dharwar craton southern india interpreted as the result of late archaean oblique convergence abstract the dharwar craton comprises two distinct parts separated by a steep n s sinistral shear zone in the western part a pre 2900 ma complex of orthogneisses granodiorites and narrow tracts of supracrustal rocks sargur group forms the basement to volcanic and sedimentary basins of the dharwar supergroup ca 2800 2550 ma late archaean deformation is characterised by ne sw crustal shortening and steep n s sinistral shear zones the eastern part is underlain by parallel steep n s or nw se linear belts of calc alkaline anatectic and juvenile granites and granodiorites dharwar batholith ca 2750 2510 ma with intervening volcanic and sedimentary schist belts ca 2800 2550 ma the plutonic belts are 15 25 km wide up to 150 km long and bounded by steep nw se high strain zones up to 2 km wide with sinistral shear sense except one which is dextral magmatic state fabrics and structures in the plutonic rocks are parallel to solid state sinistral shear fabrics in the high strain zones but diffuse magmatic banding is commonly oblique to these zones and coincides with the plane of instantaneous shortening in sinistral shear magmatic state structures swarms of vertical nw se dykes of granite and the vertical wedge shape of the linear belts are consistent with emplacement of the batholith during sinistral shear when magma pressure exceeded regional horizontal compressive stress upright folds and schistosity steep reverse faults and effects of regional ht lp metamorphism show that deformation was partitioned into ne sw shortening in the schist belts during emplacement of the plutonic belts in the sinistral shear regime the western part of the craton is interpreted as the foreland to an accretionary arc represented by the batholith and schist belts intra arc basins in the east ne sw shortening and sinistral transcurrent displacements in the foreland and arc are consistent with arc normal and arc parallel displacements during oblique convergence analogous to mesozoic cenozoic convergent settings

bizarre geochemistry of komatiites from the crixas greenstone belt brazil komatiite lava flows in the crixas greenstone belt goias brazil have textures and volcanic structures typical of archean komatiites but are geochemically most unusual the flows are porphyritic and massive or layered with spinifex upper parts and olivine cumulate lower parts mgo contents range from 18 to 40 in such lavas only olivine and minor chromite can have crystallized but neither major nor trace elements fall on olivine control lines in mgo variation diagrams cao and sr fall on lines with slopes steeper than olivine control lines sio2 feo na2o k2o and y show little systematic variation zr shows a large variation that does not correlate with mgo and al2o3 decreases markedly with decreasing mgo the aberrant behaviour is highlighted by the ree rare earth elements in spinifex and olivine cumulate layers from three flows in the spinifex layers chondrite normalized ree patterns are hump shaped with maxima at nd or sm la sm n 0 6 gd yb n 1 6 2 3 whereas cumulate zones in the same flows have steadily sloping patterns with lree enriched relative to hree la sm n 1 3 gd yb n 1 4 neither normal magmatic processes acting during emplacement of the komatiites nor thermal erosion and wall rock assimilation can explain these effects and we speculate that elements commonly thought of as immobile e g al zr ree migrated during hydrothermal alteration or metamorphism a pb pb whole rock isochron gave an age of 2 728 140 ma and selected sm nd analyses an apparent isochron age of 2 825 98 ma nd 0 the pb pb age is believed to be the approximate time of emplacement interpretation of the sm nd data is complicated by the evidence of mobility of ree

3 35ga komatiite volcanism in the western dharwar craton southern india constraints from nd isotopes and whole rock geochemistry abstract we present field petrographic sm nd whole rock isochron and whole rock geochemical data for komatiites from sargur group greenstone belts of the western dharwar craton field evidence such as pillow structure indicates their eruption in a marine environment petrographic data reveal that the igneous mineralogy has been altered during post magmatic hydrothermal alteration processes corresponding to greenschist to lower amphibolite facies conditions with rarely preserved primary olivine and orthopyroxene a 16 point sm nd whole rock isochron gives an age of 3352 110 ma for the timing of eruption of komatiite lavas about 60 of the studied komatiite samples show al depletion whilst the remaining are al undepleted the al depleted komatiites are characterised by high cao al2o3 ratios 1 01 1 34 and low al2o3 tio2 5 16 whereas al undepleted komatiites show lower cao al2o3 ratios 0 59 0 99 and higher al2o3 tio2 17 26 trace element distribution patterns of komatiites suggest that most of the primary geochemical and nd isotopic compositions are preserved with only minor influence of post magmatic alteration processes and negligible crustal contamination the chemical characteristics of al depleted komatiites such as high gd yb n together with lower hree y zr and hf imply their derivation from deeper upper mantle with garnet majorite involvement whereas lower gd yb n slightly higher hree y zr and hf suggest derivation from shallower upper mantle without garnet involvement the observed chemical characteristics cao al2o3 al2o3 tio2 mgo ni cr nb zr y hf ree indicate derivation of the komatiite magmas from different depths in a plume setting whereas sub contemporaneous felsic volcanism and ttg accretion can be attributed to an arc setting in order to explain the spatial association of komatiite volcanism with contemporaneous mafic felsic volcanism and ttg accretion we propose a combined plume arc setting nd isotope data of the studied komatiites indicate depleted mantle reservoirs which may have evolved by early 4 53 ga global differentiation of the silicate earth as suggested by boyet and carlson boyet m carlson r w 2005 142nd evidence for early 4 53 ga global differentiation of silicate earth science 309 577 581 or extraction of continental crust during the early archaean

nd sr isotope constraints on the age and origin of unconformity type uranium deposits in the alligator rivers uranium field northern territory australia the results of an sm nd and rb sr isotope study on the large and rich unconformity type uranium gold platinum group elements deposits in northern australia indicate identical sm nd ages for primary uranium mineralization at nabarlek 1 616 or 50 m y and jabiluka ii 1 614 or 132 m y in excellent agreement with new and previously published rb sr data on u ores and altered host rocks in contrast the sm nd ages are older than published u pb ages which range from 900 to 1 437 m y sm nd data on uraninites from koongarra are strongly scattered but the geologic setting and nd model ages for the least disturbed samples suggest an age between 1 600 to 1 650 m y identical to the age of primary mineralization at nabarlek and jabiluka ii the narrow range of sm nd ages for these three deposits 1 600 1 650 m y indicates that mineralization took place soon after the deposition of unconformably overlying sandstones and volcanics of the 1 650 m y old kombolgie formation relatively high initial epsilon nd values and low 87 sr 86 sr ratios in the ores indicate that ore fluids derived their metal content from two main sources a preunconformity source comprising the local wall rocks lower proterozoic metasediments and amphibolites and a postunconformity source including the volcano sedimentary rocks of the kombolgie formation which form part of the evaporite bearing mcarthur group at nabarlek the kombolgie formation isotopic signature is obscured by the intensive wall rock alteration involving isotopically similar amphibolites and dolerites but mass balance calculations indicate major contributions of u and heavy rare earth elements from sources outside the alteration zone the most likely external metal sources are again the postunconformity volcanics and sandstones in contrast isotopically more evolved sources such as u rich late archean basement gneisses and post tectonic high level granites are less probable metal sources on the basis of their much lower epsilon nd and higher 87 sr 86 sr values suggesting that the ore forming hydrothermal activity did not penetrate the metamorphic basement except in the mineralized shear zones the new data lend support to models of ore deposition that involve reaction of hot oxidized u bearing formation meteoric waters whose source is the postunconformity rocks e g the kombolgie formation with reducing metasediments of the lower proterozoic basement highly altered volcanics within the kombolgie formation are favored as major u sources the origin of the mineralization at ranger may have involved different processes recent u pb data indicate a 1 737 m y age for mineralization at this deposit significantly earlier than that of the other deposits and most importantly earlier than that of deposition of the kombolgie formation the sm nd data for ranger uraninites do not form an isochron but indicate a possible age range from 1 600 to asymp 1 750 m y data resolution is insufficient to allow reliable distinction between a pre or post kombolgie age however nd sr initial ratios for ranger point to unreasonably high epsilon nd and high 87 sr 86 sr value sources when calculated for any age younger than 1 650 m y the age of kombolgie sedimentation and basin formation in contrast uraninite nd sr initial ratios at 1 737 m y are easily explained as mixtures involving nd and sr leached from various lower proterozoic and late archean sources the data are thus more compatible with a pre kombolgie origin for uranium mineralization at ranger

paleomagnetism and u pb geochronology of easterly trending dykes in the dharwar craton india feldspar clouding radiating dyke swarms and the position of india at 2 37 ga abstract a u pb baddeleyite age of 2367 1 ma from a diabase dyke together with previously published age data suggests that a major early proterozoic dyke swarm cuts across the structural grain of the archean dharwar craton in india paleomagnetic data suggest the swarm is at least 300 km wide and 300 km long and has a fan angle of at least 30 with convergence to the west it was originally emplaced at high latitudes and together with the widgiemooltha dykes of the yilgarn block of australia may have been a segment of a larger radiating swarm related to a long lived plume event that was active for about 50 my from 2418 to 2367 ma a regional change in the intensity of brown feldspar clouding in the dykes suggests that the dharwar craton was tilted northwards in harmony with previous observations on the structure and metamorphism of the archean rocks towards the south the brown feldspar clouding becomes more intense and locally assumes a blacker more sooty appearance the black clouding whose precise origins remain unknown is accompanied by a remagnetization and appears to coincide closely with a region of carbonatite magmatism at 800 ma and with a shear zone and change in structural trend related to pan african deformation at 550 ma paleomagnetic studies suggest that i the high coercivity part of the remanent magnetization is carried by magnetite exsolved within either brown or black clouded feldspars and ii as a more general observation diabase with brown clouded feldspar can carry a primary magnetization but not if the clouding is black

late archaean synorogenic basins of the eastern goldfields superterrane yilgarn craton western australia part iii signatures of tectonic escape in an arc continent collision zone abstract siliciclastic supersequences in the eastern goldfields superterrane are remnants of basins that were linear zones of subsidence between intra terrane faults fluvial deposits record proximal to medial braid plains deep marine canyon and fan deposits were linked across intra terrane compartments only by mud rich deposits axial depositional systems dominate with only minor evidence for slope aprons although transverse and axial systems are preserved in some fluvial deposits basin development occurred during two stages separated by an inversion event with the kalgoorlie terrane recording change from a fluvial to a deep marine basin and the kurnalpi terrane change from distal to proximal deep marine facies tracts sparse palaeocurrents establish that uplift to the north controlled sediment dispersal lithofacies stacking is aggradational whereas abrupt upwards change in facies tracts is back stepping within depositional sequences and between depositional sequence sets but forward stepping between depositional sequences the stratigraphic architecture is atypical of synorogenic basins but consistent with a strike slip regime characterised by pulsed uplift subsidence steady state basin lengthening and subsidence punctuating catastrophic subsidence and lengthening and tectonic anchoring the basins are interpreted to have been similar to graben and half graben of the western anatolian extensional province of turkey with linear zones of subsidence developing adjacent to strike slip oblique slip or normal dip slip faults controls on stratigraphic patterns are considered to have been regional rather than at the scale of each basin the envisaged tectonic setting is similar to projected collision between the philippine archipelago west of the philippine fault and continental crust of the south china sea but the orogenic architecture is similar to that of accreted superterranes in the canadian cordillera interpreted north or northwest convergence between the burtville terrane to the east and the ancestral yilgarn continent to the northwest produced a south propagating orogen with the kalgoorlie gindalbie and kurnalpi terranes trapped between in a tectonic escape corridor tectonic escape to the south between bounding strike slip faults produced back stepping of facies tracts that simulated basin lengthening whereas forward stepping of facies tracts was the response to south propagating uplift

provenance of proterozoic basal aravalli mafic volcanic rocks from rajasthan northwestern india nd isotopes evidence for enriched mantle reservoirs abstract komatiites komatiitic basalts and tholeiites from nathdwara rajasthan have been investigated for their nd isotopic characteristics 143nd 144nd vary broadly in the komatiites 0 51131 0 51252 and show a restricted range in komatiitic basalts and tholeiites 0 51120 0 51179 and 0 51141 0 51202 respectively the corresponding epsilon nd nd values indicate an enriched source for these rocks the model ages of komatiitic basalts and tholeiites cluster between 2 2 and 3 0 ga the komatiites show older model ages between 3 5 and 3 8 ga if the correlations found between nd and sm isotopic ratios can be interpreted as isochrones then the komatiites point to the age of 2300 ma and the komatiitic basalts and tholeiites constrain the age of 1800 2000 ma the age of 2300 ma for the komatiite samples is similar to the range estimated for the komatiitic basalt and tholeiite samples 1800 2000 ma considering the errors involved collectively the data suggest an age of 2300 1800 ma for the basal aravalli volcanism the variation of nd isotopic ratios between the komatiites and the associated basalts is too large to reflect analytical bias or open system behavior for sm and nd it may be due to significant mantle heterogeneity in the source region isotopically distinct sources for the komatiites and komatiitic basalts are possible but simple mixing between a depleted mantle component and a crustal component in the manner of magmatic assimilation of wall rock material is inconsistent with the observed isotopic ratios the inference of an enriched mantle component appears inevitable as the alternative model of mixing would have generated more siliceous high magnesian boninitic rocks with much lower fe and ti than are observed this enriched material may have been part of the sub continental lithospheric mantle sclm under the aravalli craton which could have generated the basal aravalli magmatic rocks we suggest that during early middle proterozoic the sclm under cratonic regions consisted of relatively fertile mantle capable of generating komatiites tholeiites and alkalic magmas

geological setting and shrimp u pb geochronological evidence for ca 2680 2660 ma lode gold mineralization at jundee nimary in the yilgarn craton western australia the 7 million oz jundee nimary lode gold deposit occurs in the northern portion of the yandal greenstone belt in the northeastern part of the archean yilgarn craton of western australia gold mineralization at jundee nimary is similar in structural style mineralogy geochemistry and relative timing with respect to deformation and metamorphism to other western australian archean greenstone hosted gold deposits but is unusual in the fact that mineralized structures are crosscut by structurally late intermediate to felsic dykes within the deakin south open cut gold mineralization is hosted in brittle ductile shear zones primarily developed within the dacitic mitchell porphyry the moore porphyry a broad dyke of porphyritic granodiorite intrudes the mitchell porphyry crosscutting and post dating gold mineralization analytically indistinguishable shrimp u pb zircon ages of 2678 5 ma for the mitchell porphyry and 2669 7 ma for the moore porphyry require that gold mineralization at jundee nimary occurred at ca 2680 2660 ma approximately 40 million years earlier than the majority of published robust ages for gold mineralization in the yilgarn craton which mostly overlap at ca 2640 2630 ma the close spatial and temporal relationship between gold mineralization and felsic to intermediate magmatism at jundee nimary also raises the possibility of a genetic link between hydrothermal and igneous activity however additional work is required to establish a firm connection current research on lode gold mineralization in archean paleozoic and phanerozoic terranes suggests a model which postulates that these deposits formed during transpressional to compressional deformation in accretionary and collisional orogens and that their formation is intimately related to orogenic processes consequently mineralization and regional metamorphism are expected to be diachronous as terranes are accreted and the front of orogenesis migrates consideration of the new data presented in this paper in conjunction with previously published dates supports the hypothesis that gold mineralization along with regional metamorphism was generally diachronous from northeast to southwest across the yilgarn craton over a period of approximately 40 million years from ca 2680 2660 ma to ca 2640 2630 ma this is directly analogous to the accepted model for the timing of orogenic lode gold mineralization in other provinces and therefore provides further support for a unified model for this style of deposit through geological time

40ar 39ar age constraints on timing of a late hydrothermal event in the hutti greenstone belt karnataka india abstract obtaining constraints on the timing of orogenic gold mineralization hosted in shear zones in greenstone belts is often difficult because of the lack of easily datable minerals and problems of overprinting of earlier events of mineralization by later hydrothermal events in the orogenic hutti gold deposit the u pb shrimp date obtained from a monazite grain within arsenopyrite that gave an age of 2547 10 ma is generally accepted as the age of early hydrothermal mineralization nevertheless other work has indicated that hydrothermal mineralization may have occurred in later episodes as well this study reports new 40ar 39ar ages for hydrothermal biotite amphibolite and hornblende in granites that occur along the northern and southeastern boundary of the greenstone belt the 40ar 39ar ages of biotite samples are inferred to be the result of re setting of the ages of alteration biotite by the last phase of hydrothermal activity in this area at 2332 ma the study indicates that the effects of the thermal event are more significant in areas of greater hydrothermal access such as within the hutti shear zone this late hydrothermal alteration event may be related to the d3 deformational event of roy 1991 the results of the present work are important because it demonstrates a significant hydrothermal event much after the major 2 58 2 52 ga accretionary event and accompanying ttg and felsic magmatism in the eastern dharwar craton

neoarchean greenstone volcanism and continental growth dharwar craton southern india constraints from sims u pb zircon geochronology and nd isotopes abstract we present sims u pb zircon ages and nd isotope data for the felsic volcanic rocks from seven neoarchean greenstone belts of the eastern dharwar craton edc and from the chitradurga greenstone belt in the western dharwar craton wdc zircon ages show bimodal age distribution of felsic volcanism the ca 2 70 2 65 ga felsic volcanic event is contemporaneous with 2 7 ga mafic greenstone volcanism and emplacement of juvenile tonalitic to granodioritic crust while 2 58 2 54 ga felsic volcanics are coeval and spatially and probably genetically in some cases linked to the major episode calc alkaline magmatic accretion in the edc the chitradurga and veligallu greenstone belts host felsic volcanics of the first generation the latter showing inheritance at ca 2 95 ga four of the five greenstone belts hosting the second generation of felsic volcanics chitradurga kolar kadiri hutti show crustal inheritance at ca 2 6 2 7 2 9 3 0 3 1 and 3 3 ga ndt indicate derivation of the felsic volcanics from juvenile sources or short lived crustal sources with minor influence of older crust the new and existing data are consistent with two stage growth of the dharwar craton in the neoarchean first stage accretion led to the growth of a 2 7 2 6 ga juvenile crustal province of mafic volcanics and felsic plutons along the eastern margin of the wdc second stage accretion 2 58 2 52 ga led to the emplacement of ttg and calc alkaline plutons and felsic volcanics throughout the eastern dharwar craton an active margin context could apply for the two stage accretion scenario considering a west dipping subducting slab beneath the craton in the framework of long lived ultra hot accretionary orogen but the great width of influence of magmatic accretion and or reworking particularly during the second stage would suggest large scale mantle flow reorganization that would have generated large plume s

u pb ages for zircon and titanite from the ramagiri area southern india evidence for accretionary origin of the eastern dharwar craton during the late archean abstract the north south trending ramagiri schist belt is trident shaped with three prongs separating three granitic terranes whereas the western and eastern prongs contain metatholeiites and banded ferruginous quartzite the central prong includes felsic metavolcanics and volcanogenic metasediments the u pb zircon age for the pyroclastics in the central prong of the ramagiri schist belt is documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape 2707 pm 18 end document ma considered as the time of emplacement of the felsic volcan