dynamics of wind forced coherent anticyclones in the open ocean we numerically study the dynamics of coherent anticyclonic eddies in the ocean interior for the hydrostatic rotating stably stratified turbulence we use a high resolution primitive equation model forced by small scale winds in an idealized configuration many properties of the horizontal motions are found to be similar to those of two dimensional and quasi geostrophic turbulence major differences are a strong cyclone anticyclone asymmetry linked to the straining field exerted by vortex rossby waves which is also found in shallow water flows and the complex structure of the vertical velocity field which we analyze in detail locally the motion can become strongly ageostrophic and vertical velocities associated with vortices can reach magnitudes and levels of spatial complexity akin to those reported for frontal regions transport and mixing properties of the flow field are further investigated by analyzing lagrangian trajectories particles released in the pycnocline undergo large vertical excursions because of the vertical velocities associated to the vortices with potentially important consequences for marine ecosystem dynamics

metamorphic evolution of the attic cycladic metamorphic belt on naxos cyclades greece utilizing 40ar 39ar age spectrum measurements 40ar 39ar age spectrum analysis of phengite separates from naxos part of the attic cycladic metamorphic belt in greece indicates that cooling following high pressure low to medium temperature metamorphism m1 occurred about 50 ma ago phengite has 40ar gradients that suggest that part of the scatter observed in conventional k ar ages was caused by diffusion of radiogenic argon from the minerals during a younger metamorphism m2 in central naxos this metamorphism m2 has overprinted the original mineral assemblages completely and is associated with development of a thermal dome excellent 40ar 39ar plateaus at 15 0 0 1 ma 11 8 0 1 ma and 11 4 0 1 ma obtained on hornblende muscovite and biotite respectively from the migmatite zone indicate that relatively rapid cooling followed the m2 event and that no significant thermal overprinting occurred subsequent to m2 toward lower m2 metamorphic grade 40ar 39ar plateau ages of hornblendes increase to 19 8 0 1 ma concomitantly the proportion of excess 40ar in the spectra increases as well we propose that the peak of m2 metamorphism occurred beween 15 0 and 19 8 ma ago k ar ages of biotites from a granodiorite on the west coast are indistinguishable from those found in the metamorphic complex and hornblende k ar ages from the same samples are in the range 12 1 13 6 ma as the latter ages are somewhat younger than most ages obtained from the metamorphic complex intrusion of the granodiorite most likely followed the peak of the m2 metamorphism the metamorphic evolution of naxos is consistent with rapid crustal thickening during the cretaceous or early tertiary causing conditions at which supracrustal rocks experienced pressures in the range 900 1500 mpa transition to normal crustal thicknesses ended the m1 metamorphism about 50 ma ago the m2 metamorphism and granodiorite intrusion occurred during a period of heat input into the crust possibly related to the migration of the hellenic volcanic ar c in a southerly direction through the area

syn amphibolite facies archaean lode gold mineralisation in the mt york district pilbara block western australia the archaean lode gold deposits in the mt york district pilbara block western australia are hosted in banded iron formation main hill breccia hill prospect of the ca 3 33 ga gorge creek group and in amphibolites zakanaka prospect of the ca 3 46 ga warrawoona group gold mineralisation at the main hill breccia hill prospect is associated with breccias comprising quartz clasts in a quartz pyrrhotite matrix and quartz amphibole veins with lollingite being the major host for gold minimum temperatures for gold mineralisation at the prospect are constrained as 455 c to 550 c by arsenopyrite thermometry gold mineralisation at the zakanaka prospect is spatially associated with quartzclinopyroxene calcite microcline calcic amphibole veins and biotite altered wallrock adjacent to the veins temperatures for vein emplacement are estimated as 480 c to 570 c using both plagioclase amphibole thermometry and mineral equilibria with respect to t and xco2 the timing of gold mineralisation relative to the peak of metamorphism is constrained by mineral textures and the relative temperatures of hydrothermal alteration and metamorphism gold mineralisation at both deposits was broadly synchronous with the peak of regional amphibolite facies metamorphism which reached temperatures of 520 c to 640 c based on amphibole plagioclase and garnet biotite thermometry in this respect the deposits are similar to other well documented syn amphibolite facies lode gold deposits from the archaean southern cross greenstone belt in the yilgarn block and represent the deeper section of a crustal continuum of lode gold deposits that includes mesothermal deposits such as those at kalgoorlie at higher crustal levels

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

pressures and temperatures in precambrian metamorphic rocks modelp t estimates from nearly 100 precambrian rocks have been derived by comparing published descriptions of metamorphic mineral assemblages to a standard set of mineral stability data pressures temperatures and metamorphic geotherms show large variations nevertheless modelp t data reveal two distinct secular trends average metamorphic geotherms have declined through time and average metamorphic pressures have increased through time metamorphic geotherms and pressures from archean rocks average 54 c km and 4 1 kbar respectively corresponding figures for early proterozoic terranes are 47 c km and 4 3 kbar and for late proterozoic terranes 35 c km and 6 kbar these secular trends may reflect gradual cooling and thickening of the earth s crust at least in orogenic belts there are exceptions to both trends found in localities where mineral assemblages indicate high pressures 6 12 kbar and low metamorphic geotherms 20 30 c km these assemblages formed in archean crust that at least locally was thicker than 20 km during metamorphism

saddle shaped 40ar39ar age spectra from young microstructurally complex potassium feldspars abstract a suite of young potassium feldspars show markedly saddle shaped 40 ar 39 ar age spectra as a result of incorporating 10 10 to 10 9 mol g of excess 40ar the minima of these age spectra record reasonable cooling ages based on the known thermal history and geology of the samples acid etching of one sample indicates that excess 40ar is concentrated near grain margins the release of a substantial portion of this excess ar at high temperatures in the laboratory requires that this component be situated in a more retentive site than radiogenic 40ar anion vacancies have been proposed to act in this role in plagioclase and we speculate that this is so in k feldspar as well such a mechanism would explain the observation that relative to radiogenic 40ar excess 40ar is incorporated at low temperatures in nature but is released at high temperatures in the laboratory oxygen diffusion provides an appropriate analogy for this phenomenon being relatively fast under natural hydrothermal conditions but extremely slow in anhydrous environments such as an ar extraction system tem observations made on two of the samples confirm that their effective grain sizes for diffusion are likely to be on the order of ten microns due to the presence of such microstructures as incoherent exsolution lamellae dislocations and stepped twins tem observations also reveal the presence in one sample of orthoclase enclaves in a microcline host

extensional structures during deposition of the 3460 ma warrawoona group in the eastern pilbara craton western australia abstract structures in the granite greenstone terrain of the eastern pilbara were mapped in both the coongan belt greenstones and shaw batholith granitoids important structures that are coeval with an early magmatic event in both the granitoids and the greenstones were identified in the coongan greenstones a brittle extensional fault array is syn kinematic with felsic volcanics of the duffer formation dated at 3452 16 ma pidgeon 1978a in the underlying shaw batholith a major ductile shear zone was mapped at the contact with the coongan belt although the foliation in the shear zone forms part of a domal structure the stretching lineations constantly plunge to the ene the sense of shear as recorded in the shear zone is consistently east up with subsidiary sinistral strike slip movement on the e w trending part and dextral movement on the n s trending part a wedge shaped granitoid intruded into the shear zone during deformation this granitoid is part of the north shaw suite which has been dated at 3467 6 ma mcnaughton et al 1988 the structures in the greenstones and in the batholith have therefore formed during the same event at different crustal levels the structural kinematic magmatic metamorphic and geochronological data are consistent with a model in which these structures represent the downdip part of a dome with a core complex type of geometry in turn this implies that the domal geometry of the shaw batholith was initiated at mid crustal levels at a very early stage namely during deposition of the warrawoona group in the evolution of the pilbara craton as extension occurred during intrusion and extrusion of large volumes of magma the original distribution and thickness of granitoid bodies and greenstones were probably largely determined by the extensional geometry the initial domal shape of the shaw batholith has been enhanced during later deformation phases which caused steepening of n s trending structures

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

west asian climate during the last millennium according to the ec earth model west asia is one of the most vulnerable regions to ongoing climate change but has been poorly investigated therefore it is crucial to understand the impact of anthropogenic greenhouse gas natura

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

isotopic dating and structural relationships of granitoids and greenstones in the east pilbara western australia a structural rb sr isotopic study has been made on two small areas in the eastern pilbara block western australia the sites were chosen because they showed good interrelationships of granitoid and greenstone units at warrery gap on the western side of the corunna granitic dome the acid volcanics of the duffer formation at the top of the lower talga talga subgroup of the warrawoona group show some updating but a good 3506 62 ma isochron with initial ratio ir of 0 7006 0 0011 was recognized consistent with the 3452 16 ma zircon measurement of pidgeon 1978b both a penetrative d1 deformation and the doming of d2 preceded intrusion of late tectonic granitoids dated at 3270 22 ma ir 0 7015 0 0003 into both the duffer formation and the overlying pillow lavas of the salgash subgroup thus the salgash subgroup is much older than suggested by glikson 1979 and the stratigraphic succession cannot contain a hiatus between the apparently conformable talga talga and salgash subgroups of the magnitude he proposed the granitic domes clearly owe their form to the d2 deformation rather than to batholitic intrusion but near horizontal structural lineations suggest that they were not formed by diapiric movements a granodiorite and pink feldspar granite from just within the corunna granitic dome are slightly deformed pooled isochrons indicate an age of 3232 27 ma but different irs of 0 7032 and 0 7009 respectively at tambourah in the eastern shaw granitic dome local d2 also deforms an intrusive microadamellite of age 3087 34 ma and ir 0 7103 0 0057 there is therefore a real spread in ages of d2 granitoids and d2 deformation between about 3300 and 3100 ma layered megacrystic gneiss at tambourah also intruded by the microadamellite contains a nebulous foliation argued to be local s1 layered and homogeneous megacrystic gneiss produce updated rb sr total rock isochrons of 2995 95 and 2779 38 ma respectively the primary age of these d1 gneisses is clearly greater than that of the d2 granitoids and is probably indicated by pidgeon s 1978c zircon age of 3417 40 ma from the shaw granitic dome if so hickman s 1975 migmatite suite contains both d1 gneisses and d2 granitoids separated in age at tambourah by ca 300 ma although neither appears to be older than the lower part of the preserved layered sequence the protocrust on which that sequence was deposited has yet to be directly identified

exsolution in hornblende and its consequences for 40ar39ar age spectra and closure temperature abstract step heating studies of hornblende and biotite which coexist in an amphibolite near north walpole vermont reveal dramatically different 40 ar 39 ar age spectra the biotite mg 59 is characterized by a plateau over 98 percent 39ar release at 343 5 ma in strong contrast to the amphibole which exhibits both a complex age spectrum and a k ar age significantly younger than the biotite the amphibole release pattern contains three complete age cycles which range between extremes of 199 ma and 408 ma interestingly the k ca ratio calculated from reactor produced isotopes also varies with ar release but follows a trend which is antipathetic to the 40 ar 39 ar age pattern detailed microstructural examination including transmission electron microscopy tem reveals a complex exsolution structure that is consistent with both the complexity of the release pattern as well as the low degree of radiogenic 40ar 40ar retentivity in the amphibole this result underscores the variability of closure temperature tc for 40ar in metamorphic amphiboles and should warn against accepting a single universal value of tc

40ar39ar laserprobe ages of metamorphic hornblendes from the coongan belt pilbara western australia abstract 40 ar 39 ar laser step heating analyses of metamorphic amphiboles and a whole rock sample from metavolcanic rocks of the archaean coongan greenstone belt in the south eastern pilbara in western australia provide evidence that these rocks with no previous age constraints belong to the ca 3460 ma old warrawoona group the western margin of the belt is formed by an east dipping ductile shear zone the eastern margin is undeformed and shows a narrow contact metamorphic zone the coongan belt shows a metamorphic gradient from amphibolite facies at the western margin to greenschist facies in the centre of the belt the rocks in the centre of the belt are transected by north south striking retrograde shear zones one of which has been constrained at ca 2940 ma and may be related to the ca 2940 ma intrusive event in the eastern pilbara foliated granodiorites from the ductile shear zone along the western margin of the belt yield dates of ca 3170 ma which are interpreted as the age of recrystallisation after deformation deformed blue green hornblendes from the adjacent foliated amphibolites however give younger and inconsistent dates of ca 3090 ma and ca 3020 ma these dates cannot be interpreted as meaningful ages and indicate an inhomogeneous distribution of argon in the shear zone age spectra from blue green hornblendes from the western part of the coongan belt constrain the age of a regional metamorphic event at 3240 ma which compares well with the metamorphic hornblende ages of ca 3200 ma obtained in the western shaw belt by wijbrans and mcdougall the same event is indicated by complex age spectra of partially metamorphosed mafic volcanic rocks which yield ca 3260 ma maximum ages in the higher temperature steps the age spectrum of contact metamorphic hornblendes from the eastern margin indicates that the cooling age of the undeformed granite in the south western corunna downs batholith is ca 3400 ma which is 70 90 ma younger than the north shaw granodiorites in the shaw batholith but considerably older than previously assumed that is ca 3300 ma all obtained ages fall within the expected range of ca 3 5 2 94 ga and generally increase from west to east through the coongan belt the effects of excess argon on the age spectra seems to be rather considerable in some of the experiments

archean gravity driven tectonics on hot and flooded continents controls on long lived mineralised hydrothermal systems away from continental margins abstract we present the results of two dimensional numerical modelling experiments on the thermal evolution of archean greenstones as they sink into a less dense hot and weak felsic crust we compare this thermal evolution to that obtained via the analysis of isotopic data and fluid inclusion microthermometry data obtained in the paleoarchean to mesoarchean warrawoona synform eastern pilbara craton western australia our numerical experiments reveal a two stage evolution in the first stage cooling affects zones of downwelling as greenstone belts are advected downward whereas adjacent domes become warmer as deep and hot material is advected upward we show that this is consistent with stable isotopes data from the warrawoona synform which reveal an early episode of seafloor like alteration 90 160 c strongly focused along steeply dipping shear zones in a second long lived stage lateral heat exchanges between domes and basins dominate the system as domes cool down while downwelling zones become increasingly warmer in the warrawoona greenstone belt stable isotopes in gold bearing quartz veins post dating the sagduction related vertical fabrics reveal that rock fluid interaction occurred at much higher temperatures 234 372 c than seafloor like alteration we propose that emplacement of thick and dense continental flood basalts on flooded hot and weak continental plates led to conditions particularly favourable to hydrothermal processes and the formation of mineral deposits we further argue that sagduction was able to drive crustal scale deformation in the interior of continents away from plate margins on largely flooded continents sagduction related shear zones acted as fluid pathways promoting gold mineralisation far away from active plate boundaries continental rift zones or collisional mountain belts

metamorphism denudation and sea level in the archean and cooling of the earth abstract over long enough time scales an antagonistic balance acting between tectonics uplift and rifting and the sedimentary cycle erosion and deposition probably keeps continental surfaces close to sea level we argue that this balance constitutes the primary mechanism for keeping long term continental freeboard constant by regulating the continental thickness an important implication is that the long term uplift or down warp history of ancient continental segments provides direct information on secular changes in the isostasy of the adjacent ocean basins through time the amount of uplift can be estimated from the regional metamorphic grade the burial pressures at present exposure of ten undisturbed archean granite greenstone segments are estimated to be 1 5 0 5 kb from this we infer a maximum long term uplift and erosion of 5 2 km for stable continent since 3 0 ga implying a mean continental thickness of 46 km at the time of crustal stabilization for a constant mean ocean depth isostasy requires a palaeo oceanic crust 14 2 km thick in the middle archean alternatively if little continent existed then the oceans would have been shallower and the palaeo oceanic crust 21 2 km thick assuming plate spreading did occur in the archean potential temperatures in the shallow mantle at 3 ga were 90 30 c or 170 20 c for no continent hotter than those prevailing today these temperatures translate into time averaged cooling rates for the shallow mantle of 57 c ga 1 and are comparable to estimates of the present day cooling rate for the whole earth the low burial pressures are a strong argument against the existence of catastrophic mantle flush events in the geological past which have been predicted by some mantle convection models taken together these constraints suggest a plume related geological setting i e oceanic plateau like flood basalt for the initial stage of greenstone belt evolution

continental thermal and tectonic regimes during the archaean metamorphic pressures estimated from mineral assemblages in archaean high grade terrains may be used to constrain geothermal regimes in the archaean continental crust in two ways first they may be combined with temperature estimates to yield bounds on the heat flux during the thickening events that produced the metamorphism the pressure temperature data do not require any greater supply of heat to the base of the continental crust in the archaean than is supplied at the present day second the pressures alone may be used to infer the height of archaean mountains these mountains appear to have had elevations comparable with those of present day mountain belts it seems probable that the elevation of present day belts is limited by the creep strength of the lithosphere and in view of the strong temperature dependence of creep in earth materials this second observation also implies that archaean continental thermal regimes were similar to present day ones these observations together with the common

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

structural evolution of the warrawoona greenstone belt and adjoining granitoid complexes pilbara craton australia implications for archaean tectonic processes abstract the pilbara granite greenstone terrain 3 6 2 8 ga australia is quoted as the key area formed by passive gravity driven solid state diapirism but contrasting views have been reported recently which led us to re examine the key area the warrawoona greenstone belt and the adjoining mt edgar and corunna downs granitoid complexes we found i an asymmetric non synclinal distribution of metamorphic grade and deformation intensity across the warrawoona greenstone belt ii a major shear zone on the margin of the mt edgar granitoid complex with a unidirectional stretching lineation pattern iii a ne trending transcurrent shear zone within the central gneiss complex of the mt edgar granitoid complex iv coeval magmatic activity and doming of the granitoid complexes and v the loss of about 9 km of stratigraphic units in the warrawoona greenstone belt the area has had a prolonged history with at least four deformation phases between ca 3 47 and 2 8 ga the main domal geometry of the mt edgar granitoid complex was obtained during active ne sw extension at 3 31 ga d x 2 which appears to been in response to e w compression possibly the e w compression resulted in crustal thickening and partial melting probably of a 3 46 3 43 ga granitic basement causing crustal weakening collapse and extrusion of the 3 31 ga granitoids extension at 3 31 ga was accommodated by the mt edgar shear zone a mid crustal detachment and by the associated beaton well zone which acted as a transfer zone within the gneissic basement doming of the mt edgar granitoid complex is interpreted to have occurred in an active extensional core complex type mode an indication for deformation prior to 3 32 ga comes from syntectonically intruded gabbro diorite and dolerite sills within the mt edgar shear zone which intruded possibly as early as 3 46 3 45 ga d x 1 ultramafic schistose xenoliths within the gabbro diorite intrusions are the oldest observed deformed rocks formed during d x post doming deformation involved ne sw compression d x 3 followed by dextral transcurrent deformation d x 4 during nw se compression the latter may be related to the regional phase of deformation at 2 93 ga during which the eastern and western pilbara were amalgamated comparison of this deformation sequence to others within the marble bar domain suggests a common tectonic history from 3 46 ga onwards our structural and kinematic data especially the 3 31 ga unidirectional stretching lineation pattern lead us to reject solid state diapirism due to convectional overturn as the main cause for doming of the mt edgar granitoid complex we suggest that the cause lies in a plate tectonic process

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

vaalbara earth s oldest assembled continent a combined structural geochronological and palaeomagnetic test the only remaining areas of pristine 3 6 2 7 ga crust on earth are parts of the kaapvaal and pilbara cratons general similarities of their rock records especially of the overlying late archean sequences suggest that they were once part of a larger vaalbara supercontinent here we show that the present geochronological structural and palaeomagnetic data support such a vaalbara model at least as far back as 3 1 ga and possibly further back to 3 6 ga vaalbara fragmented prior to 2 1 ga and possibly as early as 2 7 ga suggesting supercontinent stability of at least 400 myr consistent with neoproterozoic and phanerozoic analogues

oldest reliable 40ar 39ar ages for terrestrial rocks barberton mountain komatiites we report here the first 40ar 39ar ages for komatiites and komatiitic basalts from the barberton mountain greenstone belt on the transvaal swaziland border in southern africa1 both rock types display remarkable argon retentivity while some variation is found among the samples the best argon age estimate for the time of metamorphism is in the 3 450 3 490 myr range this age is only slightly less than that found in komatiites from the same area by sm nd dating 3 540 15 myr 2 within the uncertainties of the decay constants involved the two age estimates are not different the results show that the principal pervasive greenschist metamorphism1 in the area must have occurred within 100 myr of the eruption of the komatiite analytically precise ages were obtained despite potassium contents as low as 80 p p m in the presence of high calcium concentrations 3 5 ca to our knowledge the results represent by far the oldest reliable ages obtained for terrestrial rocks using the k ar system

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