granite greenstone terranes in the pilbara block australia as coeval volcano plutonic complexes evidence from u pb zircon dating of the mount edgar batholith the mount edgar batholith in the pilbara block western australia consists in part of a gneiss complex that resembles other archean high grade gneiss terranes the complex is extensively intruded by variably foliated granitoids which also intrude the surrounding greenstone succession the warrawoona group the contact between the greenstone belt and the gneiss complex is tectonic field evidence indicates that the banded gneisses are the oldest part of the batholith zircon u pb ages for three samples are 3429 13ma 3443 10ma and3448 8ma all very similar to the age of zircons in the felsic volcanic rocks near the base of the greenstone succession pidgeon 1978 1 two granitoids representing pre and post tectonic phases respectively give zircon ages of 3304 10ma and3314 13ma thereby constraining the major structural events to be contemporary with granite magmatism consistent with a diapiric model for the tectonic evolution of the batholith collins 1989 2 these ages are similar to the zircon age for terminal volcanism in the warrawoona group pidgeon 1984 3 but are 100ma older than the precisely defined rb sr whole rock ages obtained throughout the batholith collins and gray 1989 4 batholith wide resetting or late closure of rb sr isotopic systems is implied possibly associated with late fluid circulation the temporal coincidence between plutonism and volcanism in the mount edgar region strongly suggests that the greenstone succession contains the extrusive equivalents of both the 3300ma granites and the 3440ma gneisses in the batholith the isotopic evidence combined with the structural data indicates that the granites cannot be derived from the surrounding greenstone belt further there is no compelling evidence for a sialic basement to the greenstone succession

fault and stratigraphic controls on volcanogenic massive sulphide deposits in the strelley belt pilbara craton western australia abstract early archaean fe zn cu volcanogenic massive sulphide deposits of the strelley belt pilbara craton occur at the top of a volcanic dominated sequence at the interface of felsic volcanic rocks and siliceous laminites beneath an unconformity overlain by clastic sedimentary rocks the structure of the sulphur springs and kangaroo caves vms deposits is relatively simple with the present morphology reflecting original deposition rather than significant structural modification the rocks have been tilted giving an oblique cross sectional view of discordant high angle deep penetrating faults in the footwall which splay close to the zones of volcanogenic massive sulphide mineralization faults do not extend far into the overlying sedimentary cover indicating their syn volcanic and syn mineralization timing both the sulphur springs and kangaroo caves sulphide deposits are located within elevated grabens in a setting similar to massive sulphide mineralization in modern back are environments mineralization at sulphur springs and kangaroo caves is located at the edge of the grabens at the site of intersecting syn volcanic extensional faults

archaean calc alkaline volcanism in the pilbara block western australia abstract archaean calc alkaline volcanics occur in two distinct settings in the pilbara block within the ca 3500 ma old warrawoona group sequences of intermediate to acid volcanics are interlayered with an extensive sequence of komatiites and tholeiitic basalts in the western pilbara the whim creek group a younger sequence of intermediate to acid volcanics is associated with terrigenous sediments and minor mafic lavas both suites have typical calc alkaline mineralogy and chemical compositions the major and trace element chemistry of the most mafic lavas basalts and basaltic andesites is consistent with derivation by partial melting of the mantle and a spectrum of fractional crystallization trends involving the observed phenocryst phases appears to relate more evolved members to parent magmas the fractionation processes involved in the evolution of the calc alkaline suites operated at higher fluid pressures than those involved in the development of associated komatiitic and tholeiitic lava sequences the setting of volcanism in the pilbara has a number of broad similarities and some differences to modern volcanically active continental margins in which growth of new continental crust appears to be dominated by calc alkaline plutonism with calc alkaline volcanism playing a subordinate role however more detailed information is required before direct comparisons with modern environments can be made with confidence or rejected in favour of non uniformitarian models for archaean crustal evolution

the age and origin of younger granitic plutons of the shaw batholith in the archaean pilbara block western australia the whole rock pb pb method has been used to date four of the younger mainly adamellite late tectonic plutonic phases within the ca 3 5 ga shaw batholith of the archaean east pilbara block three suites give ages within error of 2966 ma porphyritic granites at 2948 50 ma leuco adamellites at 2943 46 ma and garden creek adamellite at 3007 48 ma the post tectonic cooglegong adamellite gives an age of 2847 34 ma the sm nd model isotopic systematics of all four suites indicate derivation from crust ranging between ca 3200 and 3600 ma in age the sources for these four younger plutonic phases were heterogeneous and although exhibiting some isotopic characteristics of the older 3 5 3 3 ga calc alkali plutonic suites were more depleted in the lil elements rb u and th in addition the garden creek adamellite and the cooglegong adamellite lack the very fractionated and hree depleted ree patterns characteristic of both the older calc alkali plutonic rocks and the porphyritic granites and leuco adamellites the crust underlying the shaw batholith at ca 2950 ma must have been both markedly heterogeneous and variably depleted a conclusion consistent with the complex tectonic and plutonic evolution of this region

stratigraphic sequences and their chronostratigraphic correlation abstract the exxon sequence models of posamentier and coworkers incorporate two concepts which i question they suggest that rivers grade to a bayline but evidence from modern major rivers indicates that they do not they also proposed that fluvial sedimentary accomodation space is generated by the seaward shift in equilibrium profiles during sea level fall but this is too simplistic a model based on the use of schematic continental margin configurations which do not incorporate real slope and relief data in most cases a fall in base level leads to deltaic progradation and upstream fluvial incision most coastal plain fluvial wedges result from two processes sea level rise with the drowning and filling of estuaries and active source area tectonism major tectonic events adjacent to foreland basins such as terrane collisions lead to the development of sequences of third order type duration approximately 1 15 m y changes in intraplate stress regimes resulting from changes in regional plate kinematics have a similar episodicity and can be shown to be responsible for some third order cycles on extensional continental margins examination of the chronostratigraphic basis of the mesozoic cenozoic cycle chart of haq and coworkers reveals internal inconsistencies and an unrealistic downplaying of the built in potential for error this chart is not an approved global standard and it should be used with extreme caution

constraints on the age of the warrawoona group eastern pilbara block western australia abstract zircons from mafic and felsic volcanic rocks in the type area of the warrawoona group the basal archaean greenstone succession of the eastern pilbara block have been dated precisely using the ion microprobe shrimp the results allow two alternative time frames for the duration of the warrawoona group dependent on how the dated zircons are considered to relate to the volcanic rocks our favoured interpretation requires a hiatus of 135 5 ma between the duffer formation at 3 46 ga and the overlying felsic volcanic rocks of the wyman formation and a hydrothermal or later magmatic origin for zircons of age 3 33 ga within one duffer formation sample and the underlying metabasalts the alternative time frame requires a short time for deposition of the entire group less than 15 ma at 3 33 ga and a xenocrystic origin for the 3 46 ga zircons of the duffer formation outside the type area of the warrawoona group the age of an intrusive granodiorite requires that greenstones be older than 3 43 ga and the group formed over an interval of 120 ma visibly different zircons within one of the duffer formation samples were found to be palaeozoic in age and presumably constitute hydrothermal growth of new zircon within the rock at low temperature similar zircons were found within samples from other rock units but with a spread of proterozoic ages

the archean proterozoic boundary in the kaapvaal province of southern africa abstract the kaapvaal province has long been considered unusual in that archean cover sequences developed over granite greenstone gneiss terranes 3060 25 ga whereas in other provinces the oldest cover sequences are generally 2500 ga however two unconformity bounded sequences 2 5 ga and of greater inter regional extent than the unconformably underlying archean sequences can now be recognized in the kaapvaal province the two basal unconformities of these sequences are candidates for the temporally defined 2 5 ga archean proterozoic boundary the older but geologically more discordant unconformity occurs at the base of the uppermost group of the ventersdorp supergroup the pniel group and in at least five other erosional remnants in central transvaal and adjacent botswana these other remnants include parts of the buffelsfontein and wolkberg groups which heretofore have been considered to be conformable protobasinal phases of the younger transvaal succession the pniel and the other remnants are truncated by the locally well known unconformity at the base of the transvaal succession on a world wide basis cratonization and the evolution of mineralogically mature cover sequences were and continue to be diachronous the kaapvaal province is unusual only in that it also contains archean cover sequences however with major conformities at about 2 5 ga and the presence of proterozoic unconformity bounded sequences the kaapvaal province seems little different than most other provinces that contain the archean proterozoic boundary

sea level changes an integrated approach sea level changes an integrated approach in october 1985 sepm sponsored a four day conference entitled sea level changes an integrated approach the purpose of the conference was to provide a forum for an interdisciplinary exchange of ideas on sea level changes and to provide an opportunity for integrating various types of evidence in approaching unresolved issues the conference was successful in bringing together scientists from industry academia and government representing all of the major geosciences disciplines presentations of many new papers plus significant releases of data that were previously held proprietary provided fertile ground for discussion this much cited volume represents the best of the material presented at the conference includes the early vail chart

tectonic evolution of the late archaean to early proterozoic mount bruce megasequence set western australia the mount bruce megasequence set formerly the mount bruce supergroup was deposited on the pilbara craton during the late archaean and early proterozoic it comprises two megasequences the chichester range megasequence and the overlying hamersley range megasequence each megasequence comprises three supersequences or supersequence packages whose tectonic history can be explained in terms of phanerozoic style tectonics the basal nullagine supersequence which comprises mostly subaerial basalt and terrigenous sedimentary rock was deposited during an episode of wnw ese directed crustal extension that was initiated at about 2770 ma the overlying mount jope supersequence which comprises mostly basalt and tuff was deposited during the subsequent development of a wnw ese trending rift before about 2690 ma mudrock carbonate sedimentary rock and iron formation in the overlying circa 2690 to circa 2600 ma marra mamba supersequence package were deposited on the ensuing divergent craton margin an unconformity or condensed succession covering a time span of as much as 130 m y separates the top of the chichester range megasequence from the hamersley range megasequence during which time an oceanic island arc is interpreted to have collided with the southern pilbara craton margin subsequently the subducting oceanic plate flipped from southwest dipping to northeast dipping and convergence resulted in the formation of a marginal subduction related orogen with associated arc and backarc volcanism iron formation and distal arc derived tuffs of the brockman supersequence package were deposited on a siliciclastic starved platform in a neutral continental backarc setting at around 2470 ma the overlying circa 2440 ma woongarra supersequence was also deposited in a backarc continental setting and comprises iron formation felsic tuffs and probably coeval mafic rocks the latter two rocks types were derived from a backarc magmatic belt shortening of the southern pilbara craton margin resulted in the development of a backarc compressive cratonic basin in which iron formation and siliciclastic and carbonate sedimentary rock of the turee creek supersequence were deposited and ultimately resulted in a continent continent collision the mount bruce megasequence set and its contained iron formations are preserved in the relatively distal hinterland of this collisional orogen

a 3500 ma plutonic and volcanic calc alkaline province in the archaean east pilbara block variably foliated predominantly granodioritic plutonic rocks from the northern part of the shaw batholith in the east pilbara archaean craton are dated at 3 499 22 ma 2 errors by a whole rock pb pb isochron these rocks intrude the surrounding greenstone sequence and their age is indistinguishable from that sequence high strain grey gneisses which occupy much of the western and southern shaw batholith are chemically and isotopically similar to the north shaw suite and are inferred to have been derived from this suite by tectonic processes felsic volcanics within the greenstones together with a major portion of the granitic batholiths apparently formed in a calc alkaline volcanic and plutonic province at 3 500 ma this volcanic and plutonic suite is similar to modern calc alkaline suites on the basis of major element rare earh element and most other trace element contents the archaean suite contrasts with modern equivalents only in having lower concentrations of hree and higher concentrations of ni and cr

gliding and overthrust nappe tectonics in the barberton greenstone belt abstract regional recumbent folds inverted stratigraphy nappes and olistostromes are described from the southern part of the 3 3 3 5 ga barberton greenstone belt overthrusting of thin rigid silicified slabs with minimum dimensions of 25 km2 and up to 500 m in thickness occurred over minimum distances of 86 km more ductile and coherent units were overfolded up to at least 2 km during their emplacement the glide planes on which these nappes travelled were zones of high fluid pressures related to hydrothermal fluid circulation patterns driven by heat sources from igneous intrusions the upwelling areas of the geothermal convection cells were sites of mud pools and hydrothermal vents which may mark the trailing edges pull aparts of the overthrust units progressive silica and carbonate precipitation due to decreasing temperatures within the zones of fluid migration distant from the areas of high heat flow probably acted as built in braking systems below the travelling slabs active sedimentation and metasomatism during this tectonism indicates a protracted history for the evolution of the greenstone belt the recognition of nappe and overthrust tectonics in the barberton belt processes which may have been commonplace in archaean terrains necessitates a re evaluation of the stratigraphy of this belt

continental rifting and the archean proterozoic transition in the western australian shield the norseman wiluna belt dominated by lowermost sequences of deformed and metamorphosed basalts and komatiites and the fortescue group with successions of gently dipping little metamorphosed basalts and rare komatiites are typical examples of archean style greenstones and proterozoic style platform sequences respectively isotopic evidence however indicates that they are broadly coeval and both archean ca 2 7 0 05 ga and geologic evidence suggests that both relate to continental rifting their contrasting volcanism and subsequent deformation are inferred to result from their positions relative to rift axes and the state of the basement the norseman wiluna belt representing a failed rift in stretched relatively hot crust and the fortescue group representing a continental basalt succession on thick relatively cold crust the fortescue group immediately follows a major period of crustal reworking and stabilization whereas eruption of the greenstones of the norseman wiluna belt was closely followed by such an event thus the diachronous change in tectonic style from greenstones to platforms the archean proterozoic transition in this case appears to relate to a finite period of stabilization of continental crust rather than to a fundamental change in tectonic style or process with time

global tectonics and eustasy for the past 2 billion years abstract continental freeboard and eustasy as gauged by the relative position of the world shelf break with respect to sea level have varied by 250 m from today s ice free shelf break depth of 200 m during the past 600 ma assuming constant or uniformly accreting continental crust and ocean water volume in an ice free world sea level fluctuations can be attributed to variation in the world ocean basin volume caused by changes in either its area or its depth relative to the world shelf break an increase in volume and lowering of sea level occur as 1 the world ocean floor ages cools and subsides 2 accreting continents collide thicken and decrease in area and 3 poorly conductive continental platforms become thermally elevated due to a size induced stasis over the mantle conversely a decrease in the age of the world ocean floor attenuation of continental crust during rifting and an increase in continent number and mobility will reduce the world ocean basin volume and raise sea level theoretical sea level calculated from these principles correlates well with calibrated first order cycles of eustatic sea level change for the phanerozoic the record closely fits a simple model of retardation and acceleration of terrestrial heat loss during alternating periods of supercontinent accretion and fragmentation calibrated to sea level highstands successive first order marine transgressions and orogenic pangea regressions characterize a self sustaining 440 ma plate tectonic cycle for the late precambrian and phanerozoic the cycle can be recognized as far back as 2 ga from the tectonic evidence of continental collision and rifting recorded in global orogenic peaks and mafic dike swarms and may be related to major episodes of glaciation and evolutional biogenesis

upb zircon geochronology of archaean felsic units in the marble bar region pilbara craton western australia abstract archaean supracrustal rocks that are well exposed in the marble bar region pilbara craton have been assigned to the warrawoona group and younger sequences the predominantly volcanic warrawoona group previously dated at 3300 to 3500 ma is largely basaltic with locally intercalated thick felsic volcanic units the supracrustal rocks have been folded and are distributed around the margins of large roughly circular to ovoid granitic gneissic batholiths a u pb zircon geochronology study was undertaken to obtain precise age constraints for some of the ore deposits in the area especially the big stubby north pole barite and miralga creek zn pb cu au deposits in support of efforts to improve archaean lead isotope models the results also help significantly in interpreting stratigraphic relationships and crustal evolution in the area the new conventional zircon u pb data indicate that most of the warrawoona group from the duffer formation upwards was deposited over a period of 20 ma from 3470 ma to 3450 ma the age of the duffer formation is closely constrained by results of 3471 5 ma and 3465 3 ma and the age of the upper salgash subgroup appears to be established at 3454 1 ma the panorama formation at the southern margin of the north pole dome is 3458 1 9 ma unless the duffer formation was deposited over a period of at least 10 ma this casts doubt on a previously suggested panorama duffer correlation the age of the lower part of the north pole succession is greater than 3458 ma consistent with the correlation of the basal greenstones at north pole with the talga talga subgroup the north pole chert barite unit which is well known for its preservation of early archean stromatolites microfossils and evaporite sediments is clearly older than 3458 ma the 3325 ma age now established for the wyman formation shows that this unit must be excluded from the warrawoona group the age of the talga talga subgroup remains uncertain a felsic schist in the mount ada basalt which has been dated at 3449 3 ma is interpreted to be a sill probably related to the 3450 ma granitoid bodies an age 3724 ma for a zircon xenocryst in the panorama formation is the oldest obtained so far for zircon from the pilbara granite greenstone terrane and indicates the existence of pre pilbara supergroup sialic basement ages of 3465 3 ma and 3449 2 ma applied to previous pb isotope data for the big stubby and miralga creek deposits respectively in combination with comparable data for other syngenetic archaean sulphide deposits yield a general single stage pb evolution model with the parameters t0 4560 ma a0 9 0818 b0 9 9002 and c0 29 343 this gives model ages in accord with known constraints for many archean deposits a model lead age of 3490 ma for the north pole deposit which is older than 3457 ma may be too great but attempts to directly date this deposit have not yet succeeded there are striking parallels between the warrawoona group ages reported here and those recently obtained for the barberton region kaapvaal craton south africa the talga talga subgroup and lower onverwacht tjakastad subgroup sequences have approximately equivalent constraints and an age for the upper salgash subgroup agrees closely with those for the hoogenoeg formation of the upper onverwacht group geluk subgroup

archean foreland basin tectonics in the witwatersrand south africa the witwatersrand basin of south africa is the best known of archean sedimentary basins and contains some of the largest gold reserves in the world sediments in the basin include a lower flysch type sequence and an upper molassic facies both of which contain abundant silicic volcanic detritus the strata are thicker and more proximal on the northwestern side of the basin which is at least locally bound by thrust faults these features indicate that the witwatersrand strata may have been deposited in a foreland basin and a regional geologic synthesis suggests that this basin developed initially on the cratonward side of an andean type arc remarkably similar phanerozoic basins may be found in the southern andes we suggest that the continental collision between the kaapvaal and zimbabwe cratons at about 2 7 ga caused further subsidence and deposition in the witwatersrand basin regional uplift during this later phase of development placed the basin on the cratonward edge of a collision related plateau now represented by the limpopo province similarities are seen between this phase of witwatersrand basin evolution and that of active basins north of the tibetan plateau e g the tarim and tsaidam basins the geologic evidence does not appear so compatible with earlier suggestions that the witwatersrand strata were deposited in a rift or half graben

isotope and rare earth element evidence for a late archaean terrane boundary in the southeastern pilbara craton western australia abstract rb sr sm nd and ree data have been obtained from granitic rocks exposed in archaean granite greenstone terrain in the southeastern part of the pilbara craton samples were collected from the kurrana batholith the cooninia billinooka and sylvania inliers and from the bottom of a borehole drilled through the hamersley basin succession in the fortescue valley almost all the samples have 87 rb 86 sr 144 nd and sm nd model ages tdm in the range 3 1 to 3 3 ga these characteristics together with ree patterns differ from published data for granitic rocks of the northern pilbara granite greenstone terrain that have similar geological histories rb sr isotopic systematics have been variably disturbed with data arrays on isochron plots all indicating ages younger than the accepted age of the base of the hamersley basin succession 2 76 ga this is attributed to pervasive burial metamorphism beneath the hamersley basin and to later hydrothermal events some rock units appear to be younger than their northern pilbara equivalents a shear zone separating the mosquito creek synclinorium from the kurrana batholith has the characteristics of a suture zone and is postulated to be the boundary between two distinct terranes that joined to form the pilbara craton between 3 0 and 2 76 ga

melting of archaean sialic crust under high ah2o conditions genesis of 3300 ma na rich granitoids in the mount edgar batholith pilbara block western australia the voluminous 3300 ma old granitoids of the mount edgar batholith pilbara block western australia are typically trondhjemites and silicic tonalites with sio2 ranging from 65 75 wt they have high sr average 425 ppm at 70 6 sio2 and na2o 3 5 5 0 wt contents with k2o rapidly increasing 1 0 4 5 wt with increasing silica so that the most fractionated are granodioritic a lack of restite minerals calcic plagioclase cores and mafic microgranitoid enclaves suggests that the granitoids were produced from chemically evolved sources and that they crystallised from melts approximating the bulk composition of the batholith 70 6 wt sio2 based on experimental data mass balance and trace element modelling these silicic melts are considered to have been produced by high degrees of partial melting involving complete plagioclase dissolution of tonalitic dacitic precursors similar in composition to the dacites of the 3450 ma greenstone succession the data are consistent with field and isotopic evidence from the gneiss complex of the batholith where orthogneisses have formed virtually in situ from older 3450 ma banded tonalitic gneisses that resemble coeval metamorphosed felsic volcanic rocks of the duffer formation or deeper level equivalents a number of granitoids from other batholiths of the pilbara block are geochemically similar to the mount edgar batholith and probably were derived from similar sources the following three stage model of crustal growth is envisaged 1 production of a thick primitive mafic crust at 360 ma remnants of which exist as gabbroic anorthosite xenoliths in some granitoids mcnaughton et al 1988 2 partial melting of the mafic crust at 3500 3450 ma to produce tonalitic dacitic rocks at shallow mantle depths 60 km associated with formation of the major greenstone belts of the pilbara block 3 infracrustal melting of the tonalitic dacitic rocks at 3300 ma to produce the abundant silicic tonalitic trondhjemitic sodic granitoids uniformitarian and non uniformitarian tectonic models involving arc magmatism rifting mantle plumes and delamination are discussed for the three stage crustal growth process but the available geological evidence does not favour any particular model the data indicate that archaean granitoids are similar to those produced in modern subduction related environments but the process of archaean magmatism might have been quite different

controls of the eruption of mafic and ultramafic lavas ruth well ni cu prospect west pilbara the markedly bimodal early archean volcanic suite exposed near the ruth well ni cu prospect west pilbara shows the intimate interfingering and interlayering of spinilex textured komatiite mgo 30 20 and komatiitic basalt mgo 14 10 implying closely spaced and or closely timed eruptions of these two different extrusive products the favored hypothesis involves derivation of basalt from komatiite in transient magma chambers from which only the lower density basalt can be erupted parental komatiite is erupted directly from depth during periods of higher regional stress or in areas lacking magma chambers although metamorphic alteration of these magma products renders compositional constraints less firm derivation of basalt from komatiite in this particular case by fractional crystallization is supported by data for mgo ni ti and zr rare pyroclastic ultramafic rocks are not part of this sequence but may represent catastrophic mixing of parental and daughter products

stratigraphy and sedimentology of an early archean felsic volcanic sequence eastern pilbara block western australia with special reference to the duffer formation and implications for crustal evolution abstract a sequence of early archean felsic volcanic and volcaniclastic rocks and chert is present in the upper half of the warrawoona group throughout greenstone belts in the eastern pilbara block western australia it includes three formations the duffer formation which makes up the bulk of the sequence consists of up to 5 km of metamorphosed dacitic to rhyolitic volcaniclastic breccia and lava and lesser amounts of tuffaceous rocks and mafic lava it is overlain in several sections by the sandy panorama formation and both are capped by silicified orthochemical and biogenic deposits of the towers formation including both the strelley pool and marble bar cherts these units are identifiable in at least six greenstone belts in the eastern pilbara block the duffer formation is divided into four facies a structureless and graded breccia b complexly organized conglomerate c turbiditic tuff breccia and tuff and d lava these facies represent deposition on predominantly subaqueous volcaniclastic aprons flanking felsic volcanic centers a three stage model of the evolution of these aprons includes 1 an early stage of felsic and mafic eruptions and subaqueous deposition 2 a middle stage during which the bulk of the sequence was deposited involving shallow water to subaerial felsic volcanism and subaqueous deposition and 3 a terminal stage of subaerial felsic eruptions and subaerial deposition post duffer felsic volcanism was marked by explosive pyroclastic activity which resulted in widespread deposition of ash in surrounding shallow water and subaerial systems this phase of activity is represented by the panorama formation after the cessation of volcanism erosion of the emergent vents and regional subsidence led to an interval of orthochemical and biogenic sedimentation represented by the towers formation paleocurrent and facies patterns in the duffer and panorama formations support earlier proposals that the intrusion of granitoid rocks in the eastern pilbara block was coeval with felsic volcanism it seems likely that this volcanism was the surficial expression of plutonism that represented early stages of cratonization in the eastern pilbara the calc alkaline volcanism and depositional styles represented by this sequence are similar to those associated with many phanerozoic arcs the abundance of shallow water facies closely spaced and possibly non linear distribution of felsic centers close association of felsic units with thick sequences of basalt erupted at or near sea level sparse syn volcanic deformation and lack of granitic detritus suggest however that the setting of this archean felsic volcanism was in many details unlike that of most modern arcs

turbidites and iron formations beardmore geraldton ontario application of a combined ramp fan model to archaean clastic and chemical sedimentation sandy turbidites grain flows conglomeratic mass flows and oxide facies iron formation are present in the late archaean beardmore geraldton terrain a metasedimentary belt which extends for at least 80 km in an e w direction the marine portion of this basin contains four lithofacies associations la 1 thinbedded iron formation clastic sediment association this association represents a continuum of deposit types containing iron formation subtypes are defined on the basis of bedding attributes and the proportion of iron formation to sand silt 2 thin bedded turbidite dominated association these sediments consist mostly of silt sand beds which either show no vertical trends or thin and fine upwards over a few metres 3 medium bedded turbidite dominated association most of these sediments are medium to coarsegrained vertically unstructured sand sequences with occasional structured intervals 4 thick bedded association this is dominated by poorly graded sands up to 7 8 m in thickness sand beds are characterised by a thin basal zone of coarse sand and pebbles a large central interval containing a mixture of medium and coarse sand and a thin upper zone of fine sand silt the overall depositional system was initiated by transport of sediment by braided streams to the strand area where it accumulated in distributary mouth bars we infer a nearshore break in slope locally with large channels la 4 extending from close to the strand line across deltaic surfaces to the deeper portions of submarine fans structured portions of la 3 however many deltaic surfaces probably were not tapped by major channels but merged downslope into a submarine ramp sediment was transported across the ramp by slump events and sheet like grain flows unstructured portions of la 3 iron formation and la 2 sediments probably accumulated both in upper mid ramp areas with low sediment delivery rates and distal to fan ramp successions as major streams on the braid plain changed position associated submarine channels and slump fed ramp deposits also would have shifted laterally this produced overlap of different facies associations in both the fan and ramp environments which may explain why observed vertical trends in bedding are limited to several metres we suggest that on narrow active archaean cratonic margins additional non regular processes such as variations in sediment supply related to periods of heightened pyroclastic activity and seismic activity associated with the arc also contributed to the lack of vertically structured sequences turbidite sequences in such environments in general may contain important contributions from both submarine ramp and submarine fan sedimentation

volcanic sedimentary and tectonostratigraphic environments of the 3 46 ga warrawoona megasequence a review abstract the warrawoona megasequence comprises a 3 46 ga assemblage of tholeiitic and calc alkaline volcanic rocks interlayered with cherty sedimentary rocks deposition occurred in a range of shallow to deeper water environments ranging from the shoreline to distal deposits in sediment starved basins volcanism provided the dominant or only source of clastic sediment sedimentary rocks also contain evaporites probable stromatolites other evidence for the existence of microbial life and possible evidence of meteorite impact the tectonostratigraphic assemblage and its contained facies associations are most similar to those developed in younger volcanic arc or near arc settings the compositions of volcanic rocks are also comparable with those of younger tholeiitic and calc alkaline suites developed above subduction zones the calc alkaline suite has a complex petrogenesis which probably involves mantle derived basaltic melts intermediate to silicic melts derived from subducted mafic crust magma mixing and fractional crystallization intense hydrothermal alteration and a distinctive association of metal deposits are also compatible with the interpretation that the warrawoona megasequence contains examples of early archaean volcanic arc and near arc assemblages

3 1 ga tuff from the sholl belt in the west pilbara further evidence for d diachronous volcanism in the pilbara craton of western australia abstract an age of 3112 6 2 ma determined by conventional techniques on single zircons from a felsic volcanic rock from the sholl belt in the western part of the archaean pilbara craton of western australia is interpreted as the age of felsic volcanism this is about 100 ma older than felsic volcanic rocks in the nearby but unconnected whim creek belt and is significantly younger than felsic volcanism in the east pilbara which took place during two distinct episodes at 3450 ma and 3300 ma the present results rule out previous correlations between the felsic volcanic rocks of the sholt belt and the 3452 16 ma old duffer formation in the east pilbara

horizontal tectonics pre dating upper gorge creek group sedimentation pilbara block western australia abstract the greenstone belt stratigraphy of the c 3500 ma old pilbara block has been subdivided into the lower dominantly volcanic warrawoona group and the upper mainly sedimentary gorge creek group regional mapping indicated that these units evolved as a near continuous succession with only minor internal unconformities and that both groups suffered their most intense tectonic imprint together as a result of granite diapirism however an upper gorge creek conglomerate a lalla rookh correlate with a basal angular unconformity contains felsic schist clasts derived from highly strained portions of the warrawoona group the folds in the conglomerate are upright as expected for diapirically generated structures and are as complex as any attributed to this process the schist clasts were therefore probably generated in a pre doming event high strain and high grade metamorphism are confined to the lower parts of the greenstone stratigraphy where a polyphase deformation history may be clearly defined the low strain low grade areas are generally regarded as having a simpler tectonic history but some examples contain very tight minor folds that pre date a widespread subhorizontal cleavage and both events pre date the doming in other low grade areas thrust structures are truncated by the angular unconformity at the base of the lalla rookh correlate and the thrust geometry suggests they formed in already folded rocks

unusually large subsidence and sea level events during middle paleozoic time new evidence supporting mantle convection models for supercontinent assembly new evidence from basin modeling documents a nearly synchronous episode of rapid subsidence between middle devonian and earliest mississippian time in almost all preexisting margins and interior basins of north america the subsidence events appear to have been accompanied by a large sea level rise relative to the stable cratonic platform herein we suggest that the timing and magnitude of basin subsidence and sea level change are compatible with predictions by recent mantle convection models for the assembly of supercontinents if so the sea level and subsidence events were consequences of the onset of accretion of pangea above a major zone of mantle downwelling

early archean foredeep sedimentation related to crustal shortening a reinterpretation of the barberton sequence southern africa abstract the upper dominantly terrigenous sedimentary successions of the early archean barberton sequence kaapvaal province record an evolution similar to that of phanerozoic foredeeps and may represent the oldest known foredeep sequence rather than a passive continental margin sequence as previously proposed pre foredeep ocean crust the 3 5 ga old onverwacht group consists of bimodal volcanics with intercalated shallow water sediments gravity modeling and boundary shear zones indicate that deformed onverwacht crust was obducted northward onto sialic continental crust immediately before during or after burial by the synorogenic terrigenous clastic fill foredeep basin deepening due to crustal loading from the south is recorded by basinal iron formation chert and mudstone of the basal fig tree group which accumulated paraconformably on the onverwacht overlying sediments of the fig tree group consist of 2 km of submarine fan greywackes and mudstones shoaling of the basin is indicated by the transition to the overlying braided alluvial and shallow marine sediments of the ca 3 3 ga old moodies group 3 km thick changing ree patterns and relative abundances of ni cr sr th and ur in the sediments of the fig tree and moodies groups indicate a significant ultramafic mafic volcanic component diluted upward in the stratigraphic sequence by an increasing granitic contribution a similar trend is reflected in sandstone petrography in which the ratio of extrusive to intrusive rock fragments decreases upwards in the fig tree group whereas the moodies group sediments are enriched in quartz and potassium feldspar a persistent chert component in both units suggests that the tectonic setting of the basin is analogous to that of phanerozoic recycled orogens the distribution of facies in the fig tree and moodies groups and paleocurrent petrographic and geochemical data indicate progressive unroofing of a southerly source terrane consisting of the onverwacht volcanics and intercalated sediments and the 3 5 3 2 ga old ancient gneiss complex agc in addition older components of the fig tree group were recycled into overlying stratigraphie units the agc consists of mantle derived 3 5 ga old bimodal metavolcanics and metasediments a tonalite gneiss batholith metabasite dikes and meta anorthosites its structures record progressive northwestward uplift beginning around 3 4 ga ago intense homogeneous ductile strains were overprinted by northwest verging major folds rotated by large scale northwestward simple shear followed by imbricate faulting and pseudotachylite formation by 3 3 ga ago the fig tree and moodies groups had also shortened with similar kinematics to those of the agc by partly syndepositional folding and volume loss during the formation of transecting cleavage further folding and slide formation followed during subsolidus diapiric rise of the initially tabular kaap valley tonalite by 3 2 ga and emplacement of trondhjemite plutons from 3 3 to 3 0 ga ago

felsic igneous rocks within the 3 3 to 3 5 ga barberton greenstone belt high crustal level equivalents of the surrounding tonalite trondhjemite terrain emplaced during thrusting felsic igneous rocks within the circa 3 5 ga ophiolitic onverwacht group of the barberton greenstone belt have traditionally been mapped as recurring volcanic units within a continuous stratigraphic succession these felsic units have been interpreted as part of several mafic to felsic volcanic cycles making up this succession our field and geochemical data show that the felsic igneous rocks are not closely related to the mafic ultramafic rocks of the onverwacht group rather the data indicate that these felsic units are high level equivalents of the widespread gneissic to isotropic trondhjemite tonalite plutons which surround and intrude the greenstone belt the felsic igneous rocks are predominantly shallow level intrusions and subsurface felsic domes associated with minor volcanics and volcanoclastics and postdate the onverwacht group by 40 50 ma the felsic intrusions were preferentially emplaced along thrust zones cutting the ophiolite basement and its overlying sedimentary cover structural and chemical data imply that the hydrated ophiolitic rocks of the barberton greenstone belt are allochthonous and were thrust over similar simatic rocks from which the felsic igneous rocks were syntectonically derived by partial melting such structural imbrication of simatic crust can be best incorporated in a model advocating archean intraoceanic obduction associated with shallow angle subduction similar to that documented for the neotectonics of the western aleutians

secular variation in greenstone sequence development emphasizing superior province canada abstract evidence from the superior province of the canadian shield suggests the existence of several distinct lithostratigraphic associations in greenstone belts 1 quartz arenite and carbonate bearing sequences 2 mafic ultramafic volcanic sequences 3 mafic to felsic volcanic cycles and 4 timiskaming type sequences of fluviatile sediments and calc alkaline to alkaline volcanics recently recognized quartz arenite bearing sequences in the sachigo and wabigoon subprovinces are from base to top quartz arenite stromatolite bearing carbonate oxide facies iron formation and komatiite or tholeiite primary structures in the quartz arenites indicate shallow water deposition and represent mature sedimentation granitoid provenance of these sediments is postulated based upon petrography of coarse arenites and clasts of associated conglomerates the few dated superior province quartz arenite bearing sequences are 2 85 ga the shallow water origin of the primary structures and biota and the areal extent of the quartz arenite and carbonate sequences invite comparison with platform sediments at passive margins platform sequences unconformably overlie 3 ga granitoid rocks and older mafic volcanics where original lower contacts are preserved in two localities mafic volcanic sequences contain sheet flows iron formations and deep water argillites based on physical volcanology and sedimentology these sequences are compared to modern deep water mafic plain and hawaiian volcanism they generally underlie mafic to felsic volcanic cycles and overlie platform sequences mafic sequences are 2740 2700 ma in age mafic to felsic volcanic cycles consist of deep water tholeiitic flows overlain by shallow water to subaerial intermediate to felsic pyroclastics and flows the facies style of volcanism petrography and geochemistry of the mafic to felsic cycles most closely resemble the characteristics of crust in continental magmatic arc systems this type of lithostratigraphic association varies in age from 2775 to 2700 ma in the superior province with minor exceptions timiskaming type sequences consist of fluviatile conglomerate immature arenites and wackes with subaqueous to subaerial volcanics of variable chemistry longitudinal and lateral variation within fault bounded basins is extreme and the sequences have generally been affected by only late stages of deformation the combination of rock types facies distribution the fault bounded nature of the repositories and the locus of sedimentation along zones of transcurrent displacement suggest timiskaming sequences are best compared with modern pull apart basins superior province timiskaming sequences are within the superior province platform sequences are the earliest postorogenic lithostratigraphic unit in greenstone belts followed by mafic plain sequences arc volcanics and pull apart basins we suggest that platform sequences based upon limited evidence are preceded by an orogeny and that pull apart basins represent a subsequent orogeny the lithostratigraphic units above have distinct metallogenic associations 1 platform sequences based upon compilation of several shields may contain detrital au and u and oxide facies iron formations associated felsic volcanics contain no recognized volcanogenic cu zn deposits in the superior province the following metallogenic associations are noted 2 mafic plain sequences contain komatiite flow related ni deposits cu ni and platinum group deposits are associated with flows and coeval intrusions 3 arc volcanic sequences contain volcanogenic cu zn sulphide deposits and scarce deposits of porphyry type cu mo and 4 timiskaming sequences and related plutonism are related to lode au

origin of the 3500 3300 ma calc alkaline rocks in the pilbara archaean isotopic and geochemical constraints from the shaw batholith abstract calc alkaline plutonic rocks intruded at 3450ma comprise a major component of the shaw batholith in the archaean east pilbara block western australia new whole rock pb isotopic geochronology confirms the extent of these rocks but a minor plutonic phase is dated at 3338 52 ma and represents a second plutonic event of the same age as much of the nearby mt edgar batholith the sm nd isotopic systematics of the 3450ma rocks imply their derivation from a heterogeneous source which probably included a slightly older crustal component as well as a depleted mantle component the 3338 52 ma pluton includes components derived from crustal sources older than 3600 ma the geochemistry and sm nd isotopic systematics of these rocks are consistent with crustal growth in the early archaean from upper mantle sources as depleted as the modern upper mantle the shaw batholith calc alkaline suites exhibit very similar chemical trends on variation diagrams to modern calc alkaline plutonic rocks which can be modelled by a combination of mixing and fractionation a suite collected from outcrops displaying prominent igneous layering shows distinct geochemical trends which can be modelled by differentiation into a component enriched in ferromagnesian minerals principally hornblende and possibly sphene magnetite and epidote and into a leucocratic component containing quartz plagioclase and k feld par these archaean calc alkaline plutonic rocks in common with rocks from many other archaean calc alkaline provinces exhibit very fractionated ree patterns with depleted hree contents a feature considered to result from equilibrium with garnet at depth in lower crustal regions the geochemistry of the pilbara archaean calc alkaline rocks is identical to the subset of modern continental margin calc alkaline plutonic rocks with fractionated ree patterns such as those from the central and eastern peninsular ranges batholith western usa the tectonic setting in which the archaean calc alkaline rocks formed is still not known this reflects both uncertainty associated with the petrogenesis and environments of modern calc alkaline rocks as well as the limited knowledge of the precise timing and relationships of plutonic depositional and tectonic events in the pilbara archaean

on the metamorphic history of an archaean granitoid greenstone terrane east pilbara western australia using the40ar 39ar age spectrum technique abstract age spectrum analyses of blue green hornblendes from amphibolites from the western shaw belt east pilbara western australia indicate an age of at least 3200 ma for early regional metamorphism ages on hornblende and muscovite from the narrow contact zone with the adjacent yule batholith probably date updoming of the granitoid gneiss terranes at 2950 ma hornblendes from within the shaw batholith and from a contact zone of a post tectonic granitoid yield ages of 2840 2900 ma indicating either prolonged high temperatures within the granitoid gneiss terranes or a separate thermal pulse associated with the intrusion of post tectonic granitoids the preservation of very old hornblendes in a narrow greenstone belt surrounded by massive granitoid gneiss domes indicates that remarkable contrasts in metamorphic geotherms existed over short distances during the late archaean suggesting that updoming occurred during a period of rapid tectonism

palaeogeographic and tectonic evolution of the pilbara craton northwestern australia abstract two regional unconformities have been confirmed and followed throughout the greenstone belts of the pilbara craton the unconformities are usually low angle ones but each was preceded by considerable erosion in some areas granitoids are truncated at the younger unconformity a thick development of cherty banded iron formation bif and fine grained sediment has been used by most previous authors for correlations between greenstone belts when this unit and both unconformities which sandwich it are used as marker horizons considerable changes are required to the distribution of the major stratigraphic groupings as shown in most syntheses and regional maps for instance where volcanic units are in continuity of deposition with the bifs and sediments they are now grouped together the terms warrawoona gorge creek and whim creek groups are retained but they vary significantly from the accepted correlations this has engendered a new tectonic framework and history of evolution for the craton the warrawoona group is restricted to a possible protocontinent in the southeast overlapped by the gorge creek group but the whim creek group is developed essentially frontally to the protocontinent the observed distribution of tin nickel and some copper and pge mineralisation is consistent with the major tectonic provinces of the pilbara craton

sequence stratigraphy of the archaean supracrustal belts of the pilbara block western australia abstract the stratigraphic succession of the 3580 2770 m y old granite greenstone terrain of the pilbara block is re assessed by the first and second order sequence stratigraphic techniques that are the standard for phanerozic terrains with the view to testing the conformity of stratigraphic cause and response a holistic tectonostratigraphic framework is presented that pairs the supracrustal and crustal rock records the resulting sequence scheme is tested against the cause of first order sequence cycles viz global sea level responses to supercontinent breakup and assembly three first order inter regional unconformities divide the stratigraphic succession of the supracrustal belts into four units of first order megasequence status in order of decreasing age these are named the warrawoona gorge creek roebourne and mount negri megasequences a total of 17 second order supersequences are also named although some may represent supersequence sets the megasequences and or their genetic crustal equivalents represent the preserved rock record of four megacycles of forearc arc and or back arc geotectonic evolution associated with the convergent margins of a pilbara continent individual supersequences are the rock record of separate second order basins or basin phases and reflect the different geotectonic components of each megasequence the four megacycles had respective longevities from oldest to youngest of 150 m y 230 m y 160 m y and 180 m y they are identical in their stratigraphic successions and predicted first order sea level cycles to phanerozoic first order tectono eustatic cycles and are interpreted as the same response to global tectonic change the warrawoona and gorge creek megacycles and the roebourne and mount negri megacycles are tectonogenetic pairs of the east pilbara and the west pilbara megacycle sets respectively the east pilbara megacycle set evolved over a period of 380 m y from 3490 ma to 3110 ma whereas the west pilbara megacycle set evolved over a period of 340 m y from 3110 ma to 2770 ma the two megacycle sets are identical in their longevity geotectonic components and predicted sea level cycles to convergent margin wilson and supercontinent cycles and are thereby interpreted individually as the oldest established records of the supercontinent cycle the implication of this interpretation is that a steady state global tectonic regime has been in operation from early in earth s history which has important consequences for theoretical modelling of the earth s early tectonic style and secular changes in its geochemical regime

late archaean crustal extension sedimentary basin formation flood basalt volcanism and continental rifting the nullagine and mount jope supersequences western australia abstract the mount bruce megasequence set formerly the mount bruce supergroup is a succession of volcanic and sedimentary rocks that was deposited on the pilbara craton of western australia in the late archaean to early proterozoic its history can be described and explained in terms of modern style sequence stratigraphy and of phanerozoic style plate tectonics the megasequence set comprises the chichester range megasequence that was deposited on a rifted to divergent continental margin and the overlying hamersley range megasequence that was deposited on a convergent continental margin the chichester range megasequence comprises in order of superposition the nullagine and mount jope supersequences and the marra mamba supersequence package the nullagine and mount jope supersequences are each the rock record of one phase of a protracted two phase late archaean 2770 2700 ma continental break up the nullagine supersequence consists of the mount roe sequence which comprises mostly subaerial basalt and the overlying hardey sequence package which comprises mostly terrigenous sedimentary rocks that were deposited in extensional sedimentary basins the mount jope supersequence consists of in order of superposition the kylena sequence which comprises mostly basalt the tumbiana sequence which comprises mostly tuff and tuffaceous rocks and the maddina sequence package which comprises mostly basalt the nullagine supersequence was deposited during an episode of wnw ese directed crustal extension that possibly led to the development of a rift to the west of the present pilbara craton the mount jope supersequence was deposited during the subsequent development of a wnw ese trending rift the position of which approximates the present southern craton margin regional scale uplift was associated with the development of both supersequences particularly with the nullagine supersequence basement strike slip faults were reactivated as dip slip structures during crustal extension and controlled the locations of some hardey sequence package extensional sedimentary basins nullagine supersequence and mount jope supersequence rifting led to a divergent continental margin bordering an ocean along the south and probably the far west of the craton post rift thermal subsidence was accompanied by the accumulation of mudrock carbonate sedimentary rock and iron formation of the marra mamba supersequence package in a shelf setting the implications of these interpretations are that at 2800 ma the pilbara craton was significantly larger than at present and that it had a sufficiently thick subcontinental lithosphere that divided it in a manner analogous to the break up of phanerozoic continents the nullagine and mount jope supersequences contain three stacked flood basalt successions which were erupted over a time period of between of 55 and 80 m y stacked flood basalt successions covering such a long time period are not known in the phanerozoic and are tangible evidence of a hotter late archaean mantle

structural evolution of the middle archean coastal pilbara terrane western australia 1 the middle archean coastal pilbara terrane is composed of two lithotectonic units the karratha and cleaverville roebourne supercomplexes the karratha supercomplex lies tectonically beneath the cleaverville roebourne supercomplex and consists of 3270 3250 ma granitic batholiths and metamorphosed terrigenous sediments it is identified as a juvenile continental crust with tonalite trondhjemite granodiorite type granites the cleaverville roebourne supercomplex is interpreted as a dismembered immature island arc which has a restored crustal thickness of 20 km this supercomplex consists of 3195 ma bimodal volcanic chemical sedimentary sequences metabasite low k granitic gneiss and peridotite regionally distributed granite porphyries circa 3020 3000 ma have intruded all of the coastal pilbara terrane detailed structural analyses and zircon u pb dating suggest that the coastal pilbara terrane records two phases of deformation the first phase is an immature island arc and micro continental crust collision and is represented by top to the northwest thrusting d1 circa 3100 3020 ma this phase of deformation an arc continent collision appears to represent an important transition in the formation of continental crust the 3020 ma granite porphyry event may be related to syn post collision igneous activity and may reflect a second crustal thickening associated with magmatic underplating the second phase of deformation is characterized by regional scale left lateral strike slip faulting d2 circa 3020 2925 ma and by more localized right lateral strike slip faulting d3 circa 2925 2770 ma this phase of craton scale strike slip faulting may represent stabilization of the newly formed continental crust

punctuated aggradational cycles a general hypothesis of episodic stratigraphic accumulation according to the hypothesis of punctuated aggradational cycles presented here the stratigraphic record consists of small scale 1 5 m thick shallowing upward cycles pacs separated by surfaces marked by abrupt change to deeper facies this motif pervasive in time and environment is produced by relatively long periods tens of thousands of years of base level stability punctuated by geologically instantaneous relative base level rises of at least basin wide extent as basin wide lithologic time stratigraphic units pacs are fundamental to all aspects of stratigraphic analysis including correlation paleoenvironmental interpretation and paleogeographic reconstruction of several possible allogenic mechanisms for a pervasively cyclic stratigraphic record glacial eustasy driven by orbital perturbations is preferred the pac hypothesis is presented as a comprehensive model to be tested and modified by examination of the stratigraphic record from an episodic perspective

the archaean whim creek belt an ensialic faultbounded basin in the pilbara block australia abstract the whim creek belt in the western pilbara block australia is an elongate structural basin 80 km long and up to 15 km wide it contains a c 2950 ma old sequence of bimodal volcanics volcaniclastics and terrigenous sediments which unconformably overlies granitoids and older deformed and metamorphosed supracrustal sequences the volcano sedimentary sequence was deposited in terrestrial to marine environments in a fault bounded basin with approximately the same configuration as the present whim creek belt alluvial and subaqueous fanglomerate facies were deposited along the northern margin of the basin with submarine fan and basinal facies deposited longitudinally in the centre of the basin the basin margin faults are now part of a regional pattern of strike slip faults which were active both prior to and after basin formation and probably had a significant lateral component during basin evolution the whim creek belt is thus interpreted as an archaean rift or pull apart basin similar basins containing bimodal volcanics and terrigenous sediments occur at the head of the present gulf of california e g salton trough although a direct tectonic analogy cannot be assumed this interpretation implies that the archaean tectonic pattern in the western pilbara involved externally imposed deformation and was not solely the result of gravity driven uprise of granitic batholiths

a comparative study of the geochemical and isotopic systematics of late archaean flood basalts from the pilbara and kaapvaal cratons geochemical and sm nd isotopic data are reported for igneous rocks of the 2 76 to 2 69 ga fortescue group of the pilbara craton western australia and the 2 70 ga ventersdorp supergroup of the kaapvaal craton southern africa the metamorphic history of the fortescue group has also been investigated using whole rock rb sr and pb pb isochron techniques igneous of both sequences have mixed tholeiitic and calc alkaline affinities and show immobile element correlations indicating derivation from chemically heterogeneous lree enriched sources both sequences have similar negative nd values between 1 5 and 4 4 for the fortescue group and 0 and 3 4 for the ventersdorp supergroup which are not correlated with mg mg fe2 the negative nd values of the fortescue group rocks probably do not result from crustal assimilation within crustal level magma chambers during or following differentiation as the fortescue group felsic differentiates have nd values within the ranges of values found in associated mafic rocks and zircon xenocrysts are absent in the felsic differentiates of the fortescue group instead these results are consistent with either extensive crustal contamination of primitive komatiitic or picritic fortescue group parent magmas prior to their differentiation or derivation from negative nd or enriched mantle sources the presence of zircon xenocrysts in some ventersdorp samples provides compelling evidence of some crustal contamination although correlations between immobile element ratios such as la yb ti zr ti sc and v zr and nd indicate derivation from chemically heterogeneous enriched mantle sources a whole rock sm nd isochron date of 3308 138 ma 600 ma older than the time of eruption indicated by u pb zircon data and within 300 ma of the formation of the kaapvaal granite greenstone terrane may date an sm nd fractionation event within the subcontinental lithospheric mantle sources of the ventersdorp rocks rb sr and pb pb isochrons obtained for several flows from the fortescue group register hydrothermal events occurring between 2 45 and 2 0 ga and probably associated with burial metamorphism similarities in the field geology geochemistry and isotopic characteristics of the fortescue and ventersdorp sequences and phanerozoic examples of continental flood basalt volcanism suggest a common mode of origin possibly involving the interaction of asthenospheric mantle plumes with subcontinental lithospheric mantle sources which have been modified by subduction processes

timing of gold mineralization in the mt york district pilgangoora greenstone belt and implications for the tectonic and metamorphic evolution of an area linking the western and eastern pilbara craton abstract hydrothermal gold mineralization occurs in banded iron formation and amphibolite host rocks respectively in the main hill and zakanaka deposits in the northern sector of the pilgangoora greenstone belt of the pilbara craton the supracrustal sequences of the greenstone belt have been folded into n s trending megascale isoclinal f2a folds which have been refolded into open to tight f2b folds d2 to d3 strike slip shear zones developed preferentially along the limbs of f2a isoclinal folds and along the granitoid greenstone belt contact gold mineralization is controlled by f2b folds and shear zones were interpreted to have been activated or reactivated during d2b deformation in both deposits gold associated hydrothermal alteration comprises zoned quartz calc silicate veins and pyrrhotite pyrite at the zakanaka deposit and quartz amphibole biotite diopside and pyrrhotite lollingite arsenopyrite at the main hill deposit thermobarometric and textural studies indicate that gold mineralization occurred synchronous with the peak of amphibolite facies metamorphism in both deposits lead isotope studies on gold associated hydrothermal microcline titanite and pyrrhotite in the zakanaka deposit yield a pb pb isochron with an age of 2888 2 6 1 ma for the alteration and hence gold mineralization this age is confirmed by a concordant pb pb age using shrimp on hydrothermal titanite from the zakanaka deposit of 2904 70 ma the initial isotopic ratio in the ore system estimated using the isotopic composition of hydrothermal microcline is much more primitive than 2888 ma pb from accepted growth curves lead isotope modeling using the possible ages of rock formation and metamorphic events in the eastern pilbara craton shows that the primitive pb isotope signature is consistent with the 2888 ma age of alteration if the pb was derived from old crust which underwent an early high grade depletion event the age of hydrothermal gold associated alteration at zakanaka indirectly constrains the age of the f2b folding event and the peak of amphibolite facies metamorphism in the pilgangoora greenstone belt to ca 2 89 ga the special position of the pilgangoora greenstone belt in the central pilbara permits the interpretation of the age of gold mineralization in the light of the tectonic evolution in both parts of the craton and indicates that mineralization and the associated structural and metamorphic events post dated the main magmatic evolution in both the east and the west pilbara craton but coincided with late deformational events along strike slip shear zones in the west pilbara and late syn to post tectonic granitoids and associated metamorphism in the east pilbara