preface archaean geology of the eastern goldfields of western australia regional overview abstract the eastern goldfields region forms part of the yilgarn craton which after the superior province of canada is one of the largest fragments of archaean granite greenstone terrain exposed at the surface of the earth this craton has much in common with the more intensely studied superior province but there are also significant differences between these two cratons both cratons were assembled from numerous smaller crustal fragments in a relatively brief period during the late archaean the superior province was assembled between 2725 and 2695 ma card 1990 and the yilgarn craton was assembled between c 2670 and 2650 ma myers 1993 both cratons contain fragments of older continental crust and volcanic arcs that formed just prior to collision and cratonisation the superior province contains extensive belts of metasedimentary rocks interpreted as accretionary prisms card 1990 whereas metasedimentary rocks form only a minor component of the yilgarn craton and most appear to postdate accretion both cratons contain significant concentrations of gold and support thriving gold mining industries the most prominent difference between the cratons lies in the nature of the granitoid rocks the superior province is dominated by tonalite and granodiorite whereas the yilgarn craton largely consists of granite this applies to granitoid rocks of all ages in each craton granite studies form a major part of this special volume of precambrian research

geochemistry and nd isotope systematics of archaean granites of the eastern goldfields yilgarn craton australia implications for crustal growth processes abstract the archaean granites and granitic gneisses of the eastern goldfields in the yilgarn craton western australia can be divided into two major groups high ca and low ca and three minor groups high hfse high field strength elements mafic and syenitic the high ca group 68 77 sio 2 with high al 2 o 3 na 2 o and sr and low y shares many features with typical archaean tonalite trondhjemite suites but has higher k 2 o rb and th contents the low ca group 70 76 sio 2 differs from the high ca group in having lower al 2 o 3 cao and na 2 o but higher k 2 o rb th zr y la and ce contents granites of the high hfse mafic and syenitic groups form a minor component 10 20 of the eastern goldfields granites the siliceous 74 77 sio 2 high hfse granites are restricted to a narrow nnw trending zone and are characterised by high tio 2 total feo mgo y zr and ce but only moderate rb th and pb contents the a type syenites 50 68 sio 2 are distinguished by their high total alkalies and mainly occur along tectonic lineaments the mafic group 55 70 sio 2 is lithologically diverse and exhibits a wide range of k 2 o rb th la and ce contents granite emplacement occurred between 2 69 and 2 60 ga contemporaneous with or post dating greenstone formation nd values for the high ca mafic and syenitic groups are similar mostly 0 0 to 2 5 whereas those for the low ca group show a pronounced polarity from 4 5 in the west to 2 0 in the east nd depleted mantle model ages for the low ca granites range from 3 2 ga in the west to 2 75 ga in the east model ages for the high ca group 2 75 2 9 ga overlap with ages for inherited zircons from these granites the high ca granites appear to have been derived at high pressures by partial melting of a mafic to intermediate source subducted oceanic crust crustal underplate or thickened crust or remelting of an older tonalitic source derived by such a process if the high ca granites were largely crust derived then their extensive source rocks must have formed less than 250 ma prior to granite formation the low ca granites appear to be the products of crustal reworking probably from a tonalitic to granodioritic protolith that was progressively younger towards the east perhaps in the order of several hundred million years younger or involved an additional younger component the localisation of the crustally derived high hfse granites suggests either geochemical zonation of the crust or specific tectonic processes such as rifting the mafic granites and possibly the syenites imply a significant mantle contribution and new crustal growth although the extent is equivocal the crust of the eastern goldfields appears to have been generated by both lateral and vertical accretion over a significant period from or before 3 0 ga until the time of granite emplacement

geology and geochemistry of archaean granites in the kalgoorlie region of the eastern goldfields western australia a syn collisional tectonic setting abstract granites in the kalgoorlie region of the archaean yilgarn craton are predominantly biotite granodiorite and monzogranite with some hornblende bearing varieties granites are divided on the basis of field relationships petrography and chemical composition into five supersuites an older supersuite containing four suites pre dates regional folding and four supersuites post date regional folding the potassic composition of the mainly silica rich granites 68 sio2 suggest that the most appropriate source for the majority of supersuites is continental crust with a composition in the range tonalite to granodiorite most supersuites evolved by fractional crystallization mainly of biotite and plagioclase some suites contain synplutonic dykes and microgranite xenoliths that crystallized from enriched mantle derived magmas there was only limited and local chemical interaction between granite and mantle derived magmas geometric and temporal relationships between regional structures and granites indicate that the granites were emplaced during a major orogenic event however s type granites which are widespread in some modern syn collision settings are absent in the kalgoorlie region the predominance of high fo2 i type granites in the study area probably reflects the absence of large graphite bearing metasedimentary accumulations in the lower crustal source regions the distribution of suites and supersuites implies a common source region beneath the kalgoorlie and gindalbie granite greenstone terranes suggesting that they shared a common geological history dating back to late greenstone deposition

archean greenstone belts are not oceanic crust archean greenstone belts with a preponderance of mafic volcanic rocks often preserved in tectonically complex sequences are obvious candidates in the search for remnants of archean ocean crust we review the tectonic setting and stratigraphy of a number of greenstone sequences previously interpreted as archean ophiolites and conclude on the basis of basal unconformities presence of xenocryst zircons geochemical and isotopic evidence for crustal contamination intrusive relationships with older basement and their internal stratigraphy that none of these examples is derived from archean oceanic crust thermal tectonic and isostatic constraints imply that archean oceanic basins did exist and were covered with several kilometers of water we consider it possible indeed probable that relict archean oceanic crust is preserved in granite greenstone terrains or other archean tectonic settings but that an unequivocal archean ophiolite has not yet been recognized

respective roles of the guinea current and local winds on the coastal upwelling in the northern gulf of guinea abstractthe northern gulf of guinea is a part of the eastern tropical atlantic where oceanic conditions due to the presence of coastal upwelling may influence the regional climate and fisheries the dynamics of this coastal upwelling is still poorly understood a sensitivity experiment based on the regional oceanic modeling system roms is carried out to assess the role of the detachment of the guinea current as a potential mechanism for coastal upwelling this idealized experiment is performed by canceling the inertia terms responsible for the advection of momentum in the equations and comparing with a realistic experiment the results exhibit two major differences first the guinea current is found to be highly sensitive to inertia as it is no longer detached from the coast in the idealized experiment the guinea current adjusts on an inertial boundary layer the inertial terms defining its lateral extension second the upwelling east of cape palmas disappears in absence of the guinea current detach

structural setting and geochemistry of archaean i type granites in the bardoc coolgardie area of the norseman wiluna belt western australia abstract regional structures in the bardoc coolgardie area of the archaean norseman wiluna greenstone belt in western australia developed during early large scale thrusting followed by a transpressional regime characterized by enewsw shortening and large scale sinistral strike slip faults structural relationships between granites and greenstone successions indicate emplacement of granitic rocks throughout this deformation history emplacement mechanisms include solid or liquid state diapirism forceful dilation of country rocks and passive stoping i type granites are common and include early and late tectonic intrusions the widespread occurrence of i type granodiorite and monzogranite probably derived by partial melting of lower continental crust or subducted oceanic crust suggests a continental margin setting for the belt alkalic quartz diorite occurs as rounded enclaves in granodiorite and is enriched in ba rb sr ce la p and f which suggests a mantle derived volatile component these quartz diorite melts may have formed by partial melting of metasomatized mantle or alternatively mantle degassing may have lowered solidus temperatures in the source region and thus induced melting emplacement of hot volatilerich quartz diorite melts into the lower crust may have lowered solidus temperatures in the source rocks for granodiorite and monzogranite recent models for archaean gold mineralization suggest that auriferous fluids contained co 2 derived from lower crustal or mantle sources in the bardoc coolgardie area co 2 may have been flushed out during partial melting of the lower continental crust however the origin of the gold in the mineralizing fluids requires some other explanation because gold mineralization is generally a late stage event and took place over a considerably shorter time interval than the formation and emplacement of i type granites

a review of the superior province of the canadian shield a product of archean accretion abstract the superior province consists of northern and southern high grade gneiss subprovinces and a central region of alternating granite greenstone and metasedimentary belts subprovinces are commonly fault bounded and display contrasting lithological assemblages metamorphic and structural styles geophysical characteristics and ages they are comparable to phanerozoic suspect terranes the middle and late archean rocks of the superior province are of mantle and juvenile crustal origin with little evidence of inheritance of components older than ca 3 1 ga high grade gneiss subprovinces are mainly deeper level equivalents of the supracrustal rich belts but one minnesota river valley is part of an older ca 3 6 ga terrane that was juxtaposed late in the tectonic history granite greenstone terranes are characterized by low grade volcano sedimentary sequences tholeiitic komatiitic submarine lava plain tholeiitic calc alkalic submarine to subaerial central complexes early platform type quartz arenite stromatolitic carbonate late timiskaming type shoshonitic alkalic volcanic fluvial sediment that range in age from ca 3 0 ga to 2 7 ga the volcanics represent oceanic island arc and continental arc volcanism the quartz arenite carbonate sequences deposition on relatively stable early formed ca 3 0 ga crust and the timiskaming type sequences deposition in pull apart basins developed along strike slip faults late archean ca 2 75 2 70 ga greenstone sequences occur throughout the superior province middle archean ca 3 0 2 8 ga sequences are common in northern and central superior province rare or absent in the south volcanism was accompanied by mafic to trondhjemitic plutonism the metasedimentary belts consist of variably metamorphosed volcanogenic turbidites with detrital zircons ranging in age from ca 3 0 ga to 2 7 ga the metasediments were deposited after major volcanism and probably represent accretionary prisms late archean orogenesis accompanied and followed by calc alkalic plutonism youngs southward from about 2 73 2 72 ga to 2 70 2 68 ga it involved large scale north south compression and dextral transpression and displays thin to thick skinned transitions the contemporaneity of events along the lengths of the belts and their southward younging coupled with the evidence of major compression are consistent with subduction driven oblique accretion of oceanic and continental volcanic arcs accretionary sedimentary wedges older microcontinental fragments etc in a convergent margin setting analogous to parts of the pacific rim differences eg abundance of komatiites and tonalitic plutonic suites are attributable to a hotter archean mantle

late archean convergent margin tectonics and gold mineralization a new look at the norseman wiluna belt western australia on a world scale the most important hydrothermal gold deposits occur either in late archean ca 2700 ma greenstone belts or at late paleozoic to quaternary convergent plate boundaries in the latter environments epithermal and porphyry hosted deposits form during subduction related magmatism in volcanic arcs and deep level meso thermal deposits form during deformation in continental margin orogenic belts the mesothermal deposits are similar to archean gold deposits in greenstone belts such as the norseman wiluna belt in western australia we here suggest that this similarity exists because the norseman wiluna belt is an orogenic belt with a tectonic history similar to that of younger mineralized convergent margins such as the north american cordillera it is most likely that the norseman wiluna belt resulted from the interaction of lithospheric plates during the major period of continental growth and stabilization that occurred in the late archean the distribution of gold mineralization being controlled by convergent margin tectonics

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

late archaean felsic alkaline igneous rocks in the eastern goldfields yilgarn craton western australia a result of lower crustal delamination numerous felsic alkaline intrusions were emplaced in the granite greenstone terrain of the eastern goldfields yilgarn craton between c 2650 and 2630 ma intrusion coincided with the waning stages of voluminous regional granitic magmatism the alkaline rocks are localized along and adjacent to major crustal structures that separate lithostratigraphic greenstone terranes and are divided into four geographically and compositionally distinct suites viz the mount monger emu claypan and ninnis suites the rocks have features in common with a type granites including high total alkalis an anhydrous primary mineralogy an apparent anorogenic setting and enrichment in some hfse although they also have low concentrations of nb and y they show a range of features that together point to a crustal source e g the alkaline rocks are not associated with voluminous mafic magmatism they have high lile concentrations they carry zircon xenocrysts of similar age to whole rock sm nd model ages they show evidence for regional source heterogeneity suites have constant and distinct la y and nb y ratios and the rocks show many compositional similarities to the slightly older voluminous crust derived granites extreme enrichments in sr and ba in the mount monger suite however suggests that their source may have been metasomatized before or during melting intrusion of the alkaline rocks heralds a poorly recognized third tectonothermal event in the evolution of the eastern part of the yilgarn craton and was broadly synchronous with the intrusion of voluminous felsic fractionated granites crustal extension and widespread au mineralization the late archaean geology of the eastern goldfields shows many features in common with phanerozoic collisional belts thought to be profoundly influenced by processes of lithospheric delamination or convective thinning this newly recognized third tectonothermal event in the eastern goldfields may relate to the delamination or convective thinning of a dense garnet rich lower crust that resulted from extraction of earlier voluminous granite magma and it is possible that such processes have had a greater influence on late archaean crustal evolution than previously recognized

review of archean supracrustal assemblages of the southern abitibi greenstone belt in ontario canada products of microplate interaction within a large scale plate tectonic setting abstract previously formulated plate convergent models account for many aspects of the large scale regularity and parallelism between different subprovinces of the archean superior province such models do not however appear to adequately explain the within subprovince evolution of 2 75 2 68 ga supracrustal assemblages of the late archean southern abitibi greenstone belt principal supracrustal assemblage classes include l assemblages dominated by tholeiitic metabasalt and metakomatiite with varying amounts of tholeiitic intermediate to felsic metavolcanic rocks these assemblages probably formed in extensional settings the ratio of basalt to komatiite and the ratio of basalt komatiite to intermediate felsic metavolcanic rocks may be a function of mantle temperature and crustal thickness at the site of extension respectively 2 assemblages dominated by intermediate to felsic fragmental and or effusive rocks are generally of calcalkalic affinity and likely formed in island arc settings 3 assemblages containing a substantial component of iron formation may represent older supracrustal assemblages that form structural and or stratigraphic basement to some of the younger supracrustal assemblages 4 assemblages consisting almost wholly of turbidite facies metasedimentary rocks appear to postdate volcanic activity related to the previous assemblages deposition of these sediments broadly coincides with the onset of regional deformation these assemblages may represent distal deposits related to an emerging collisional orogen 5 assemblages consisting of alluvial fluvial metasedimentary rocks with or without spatially associated alkalic metavolcanic rocks are associated with crustal scale shear zones and are the youngest assemblages within the greenstone belt alluvial fluvial assemblages likely represent the final emergence of the consolidating orogen geochronological data and assemblage distribution suggest that the supracrustal assemblages of the southern abitibi greenstone belt may represent the amalgamation of products of diverse and intricate microplate tectonic settings repeated in time and space prior to the impingement of the large scale plate tectonic regime that dictated the final large scale configuration of the archean superior province

gneiss domes and extensional deformation in the highly mineralised archaean eastern goldfields province western australia abstract the recognition of large areas of gneiss within granitic complexes of the northeastern part of the eastern goldfields province of western australia based on analysis of regional aeromagnetic data has resulted in a re evaluation of the structural data for that region five magnetic domains have been established in the granitic complexes which are marginal to the main greenstone belts at least two of these domains have a distinctive domal form and also have internal domeshaped regions the ballard and laverton domains have extensive areas of layered gneissic and migmatitic rocks which flank the internal domes boundary relationships at the laverton and ballard domains show that the domes are concordant sheets and the metamorphic conditions of the boundary rocks imply that the sheets were intruded at a middle crustal level strain conditions in the boundary zones indicate that emplacement of the gneiss domains to the crustal level of the surrounding lower grade greenstones was initially along relatively gently dipping shear zones which were active during uplift structural relationships around both the domal granitic complexes and the later granite batholiths however show that regional shortening deformation events were not caused directly by the emplacement of any of the granitic rocks the tectonic hypothesis presented for the eastern goldfields is similar to that proposed for the formation of cordilleran metamorphic core complexes the uplift event responsible for the crustal extension required to allow the emplacement of the gneiss may have been driven by the same thermal input responsible for the generation of the voluminous granites intrusive into both the gneiss complexes and the surrounding greenstone belts

an overview of the nature distribution and inferred tectonic setting of granitoids in the late archaean norseman wiluna belt abstract the highly mineralized late archaean norseman wiluna belt yilgarn block western australia has lithostratigraphic associations and a structural style and history that has recently been interpreted in terms of a subduction related tectonic setting it is important to investigate whether the geochemistry and petrology of the granitoids in the belt are compatible with such a model the granitoid suite within the belt can be subdivided into association 1 large composite batholiths of calc alkaline affinity spatially and temporally associated with calc alkaline volcanic centres association 2 plutons ranging from quartz diorite and tonalite to monzogranite and syenogranite spatially related to dike swarms of felsic to intermediate porphyries and calc alkaline lamprophyres and association 3 peralkaline to alkaline batholiths and stocks which are spatially and temporally associated with rhyolitic volcanic centres and major lineaments the calc alkaline associations 1 and 2 have characteristics comparable with suites of variable fractionated i type granitoids in phanerozoic continental margin are settings including 1 a wide range in sio 2 2 meta to peraluminous compositions 3 a progression from granodiorite to granite with plutons becoming more k 2 o rich and hornblende poor with time 4 lile enrichment and relative depletion of ta nb and ti in contrast to most phanerozoic granitoids the norseman wiluna belt granitoids have highly fractionated ree patterns and low contents of yb and y with respect to average post archaean equivalents ree data and trace element characteristics suggest derivation from a mafic source leaving hornblende and or garnet as residual phases peralkaline granitoids of association 3 exhibit enrichments on sio 2 na 2 o k 2 o zr y nb and ta with respect to the calc alkaline granitoids they post date associations 1 and 2 and are consistent with the concept that they represent a continuation of magmatism in a post orogenic environment thus the nature and distribution of the granitoids of the norseman wiluna belt are consistent with interpretations that the belt formed in a subduction related convergent margin setting

dehydration melting and water saturated melting of basaltic and andesitic greenstones and amphibolites at 1 3 and 6 9 kb

paleoseismology the application of multiple parameters in four case studies in sweden abstract many different primary and secondary effects of an earthquake can be used in order to reconstruct and determine a paleoseismic event in this case we advocate the application of multiple criteria four case studies in sweden are examined two late glacial and two late holocene events special accounts are given of bedrock fracturing away from the primary fault liquefaction with multiple phases and structureless sand layers tsunami distribution of seabed turbidites dating by varves methane venting tectonics and magnetic grain rotation the multitude of observational data related to all four cases are combined into simple tables where the environmental and spatial effects can be explicitly assessed in terms of documentation intensity and magnitude

evolution of the archaean granite greenstone terranes of the eastern goldfields western australia shrimp upb zircon constraints abstract thirty five high precision shrimp u pb zircon dates with an average 95 confidence error of 6 ma have been obtained from the granite greenstone terranes in the southern part of the eastern goldfields of western australia these dates establish the timing of major deformation metamorphism and granite intrusion events and test inferred stratigraphic relationships within the greenstones these new data reveal that felsic volcanic rocks were erupted throughout the southern part of the eastern goldfields between c 2713 and c 2672 ma analytically indistinguishable dates of c 2705 ma were obtained for felsic volcanic rocks interbedded with komatiitic lavas at three sites dates on felsic volcanic rocks within tectonostratigraphic packages that include komatiites are also consistent with an inferred eruption age for these komatiites of c 2705 ma komatiitic lavas in the southern part of the eastern goldfields were probably emplaced during a single voluminous eruption that resulted in the flooding of a vast area of the late archaean landscape filling the topographic lows around older felsic volcanic vents the stratigraphic complexity within tectonostratigraphic domains is attributed to the existence of an undulating topography during the eruption of the mafic and ultramafic lavas and to the rapid and localized deposition of felsic volcanic rocks close to a number of isolated volcanic centres prior to and after the komatiite lava eruption event between c 2685 and c 2675 ma early pre d2 intrusive equivalents of the felsic volcanic rocks were emplaced as thick granite sheets into the base of the greenstone sequences regional east west compression between c 2675 and c 2657 ma reactivated early extensional structures and resulted in the stacking of the greenstones this was accompanied by further intrusion of granite within developing antiforms burial of greenstone slices by stacking and the effect of hydrothermal fluids emanating from both the granites and buried greenstones resulted in regional metamorphism to greenschist facies prior to c 2640 ma monzogranite and granodiorite emplaced near the western southern and eastern margins of the greenstones were deformed and metamorphosed to upper amphibolite facies and converted into gneisses during d2 and d3 compression events movement along large scale transcurrent d3 shear zones had ceased by c 2635 ma remnants of a c 2958 to c 2930 ma greenstone sequence preserved near norseman and ravensthorpe in the southeastern yilgarn craton and xenocryst zircons ranging in age from c 2910 2730 ma in the c 2713 2672 ma felsic volcanic rocks indicate that the exposed greenstone sequences were deposited on continental basement the eastern goldfields granite greenstone terranes were formed by the closure of a series of narrow back arc rift basins that developed along a continental margin and above an active subduction zone located to the east of the rift basins

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

ree geochemistry and isotopic data of archean silicic volcanics and granitoids from the pilbara block western australia implications for the early crustal evolution eighteen silicic volcanic rocks of the warrawoona group and ten associated plutonic rocks from the pilbara block western australia have been chosen for geochemical and isotopic studies silicic volcanics of the unsb upper member of north star basalt are dated at 3 56 3 57 by both the rb sr and the sm nd methods the respective 1 initial isotopic composition values are 0 7005 5 sr and 0 50810 39 nd this age is consistent with the stratigraphic interpretation that the talgatalga subgroup in which the north star basalt occupies the lowermost position is overlain by the duffer formation whose age was earlier established at 3 45 by the zircon u pb method the new rb sr data on six silicic lava samples from the duffer formation yield an isochron of 3 23 0 28 2v though imprecise this age agrees with the zircon age within error limits rb sr ages of 2 3 2 4 obtained for the panorama rocks and the wyman formation do not correspond to their initial eruption ages chemical arguments suggest that these ages represent the time of metasomatism associated with the widespread thermal event in this region about 2 3 2 4 ago geochemically most of these analyzed rocks volcanic and plutonic are of tonalite trondhjemitegranodiorite ttg composition a typical feature found in many other archean terrains they generally show fractionated ree patterns except the panorama formation rocks furthermore the wyman formation rhyolites and the post tectonic adamellites show significant negative eu anomalies suggesting a similar mode of magma generation and a probable genetic link theoretical considerations suggest that most of these ttg rocks could have been generated by partial melting of amphibolitic or basaltic sources followed by fractional crystallization although the archean granitic gneisses often possess mantle like isr values the trace element data indicate that they could not have been derived by direct melting of upper mantle materials the immediate tectonic implication is that in any archean terrain the formation of na rich continental crust of ttg suite must be preceded by the presence of basaltic crust the occurrence of this basaltic crust is a matter of controversy such crust might have been totally destroyed by repeated melting processes or its remnants are now represented by some of the mafic ultramafic enclaves within the tonalite trondhjemite batholiths

distinct basement terranes identified from granite geochemistry in late archaean granite greenstones yilgarn craton western australia abstract hidden basement terranes have been identified by a geochemical study of granites that intrude deformed greenstones in the eastern goldfields of the yilgarn craton the granites are calc alkaline rocks with a high average silica content 72 5 wt lie mainly in the compositional range of granodiorite to monzogranite and are derived from melting of pre existing sialic crust the greenstones comprise several tectono stratigraphic terranes that were essentially assembled before intrusion of the granites the distribution of granite types generally shows no spatial relationship to these terranes or to terrane boundary faults the emu fault which separates the kurnalpi terrane to the east from the kalgoorlie and gindalbie terranes to the west provides an exception granites to the east of this fault have lower tio 2 feo 2 and mgo and generally higher na 2 o compared to granites to the west these contrasting compositions result from differences within the source regions and are interpreted to reflect two distinct basement terranes the basement terrane beneath the kurnalpi region had a higher proportion of tonalite than was present beneath the kalgoorlie and gindalbie terranes we suggest that a regional tonalite trondhjemite granodiorite basement was accreted prior to complete accumulation of the greenstones and was later recycled to form the granites the emu fault is inferred to reflect a suture in the underlying basement of the greenstones

a two stage model for the formation of the granite greenstone terrains of the kalgoorlie norseman area western australia abstract accurate dating of granites and greenstones from the kalgoorlie norseman area shows that there is considerable overlap in time between the formation of granites and greenstones however within any given area the oldest granites are always younger than the oldest greenstones it is suggested that the long lived 100 ma thermal anomaly in the uppermost asthenosphere that gave rise to the greenstones is also responsible for regional metamorphism and widespread crustal melting leading to the production of granites the time lag between greenstone and granite formation reflects the time taken to conduct heat from the mantle to the lower crust anatexis of the lower crust produces a layer of light granitic magma that may temporarily stop the ascent of basaltic magma where this happens the basaltic magma ponds at the base of the crust supplying additional heat and accelerating the melting process eventually the granitic melt layer becomes unstable and rises within the crust areas intruded by granite become topographic highs with topographic lows forming above areas where light melt has been withdrawn from the lower crust when the topographic highs protrude above sea level they are eroded to shed sediments into the topographic lows which contain the basaltic greenstones thus the change from a basalt dominated lower greenstone succession to a sediment and felsic volcanic dominated upper whitestone sequence seen in many granite greenstone terrains results from the intrusion of granitic magma into the upper crust the crust of the kalgoorlie norseman area formed in two distinct stages separated by a time break of up to 700 ma the first stage involved the formation of primitive crust probably above subduction zones this primitive crust was characterized by low rb sr and high sm nd ratios the second stage resulted from a major change in mantle convection that produced thermal upwellings in the mantle below the continents this led to the eruption of basaltic magmas and to metamorphism and anatexis of the lower crust to produce granitic magmas melting reset the rb sr sm nd and u pb systems giving the impression that the second event which occurred at 2 7 ga was a major crust forming event it was in fact a crustal reworking process although some basalt was added to the crust at this time although the two stage history of the kalgoorlie norseman area is clearly seen in the lead isotopes and in zircons it is not readily discernible from the rb sr and sm nd systematics due to mantle like ratios of those elements in the primitive crust

constraints on interpretations of lower crustal structure tectonic setting and metallogeny of the eastern goldfields and southern cross provinces provided by granite geochemistry new trace element geochemical data on granites fromthe eastern southern cross province and the western eastern goldfields province of the yilgarn block provide additional constraints on tectonic and metallogenic syntheses the new data also enables estimates of the nature and composition of the lower crust to be made all granites were derived by partial melting of the lower crust and all appear to have had a significant crustal prehistory using multi element primordial mantle normalised trace element abundance patterns the granites can be divided into four broad groups three of which are predominantly sr undepleted and y depleted whilst the fourth group is always sr depleted and y undepleted the first three groups are believed to have had garnet but not plagioclase stable at some stage of their prehistory whilst the fourth group had plagioclase but not garnet seismic data does not show any evidence of a significant garnet bearing layer within the crust as the present day trace element signatures of the granites were probably derived from earlier melting events interpretations of tectonic setting existing at the time of their emplacement based solely on their geochemistry are questionable the ultimate source of granites of groups 1 2 and 3 may have been derived by partial melting associated with subduction group 4 granites which have never had garnet as a significant component in their source regions closely resemble the rift related proterozoic anorogenic suites the geochemistry of the granites also predicts the sharp crustmantle boundary shown by seismic data some plutons are enriched in rb u and y and also have decreasing krb ratios with increasing sio2 similar trends are found in granites which have associated sn w and au mineralisation in other australian provinces alteration is common in some parts of plutons and regional mapping of these altered granites could define regional fluid circulation systems which may be related to mineralisation

age and origin of granitic rocks in the kalgoorlie norseman region of western australia implications for the origin of archaean crust three granites from the norseman area at stennet rocks buldania rocks and east of lake johnston have u pb zircon ages of 2691 8 95 c l 2689 22 and 2661 8 ma respectively one stennet rocks contains 3100 3200 ma cores providing support for oversby s 1975 argument that all three plutons were derived by anatexis of much older basement common leads within the zircons show evidence of having been derived from within the rock and represent mixtures of initial rock leads and radiogenic leads the unusually 208pb rich compositions of the common leads suggest the breakdown of a th rich phase perhaps uranothorite as a major contributor to the radiogenic component available zircon geochemical and isotopic data are used to develop a two stage model for archaean crustal development in this region with formation of primitive crust of broadly andesitic composition from depleted mantle or basalt beginning at 3400 ma followed by extensive thermal reworking and possibly continued growth over the period 3000 3300 ma in the second stage at 2700 ma anatexis resulted in large scale intra crustal differentiation giving a thick upper granitic layer and a lower crust residual from anatexis it is this second major thermal event that is recorded by most geochronometers implicit in the andesitic crustal growth model is that because the sm nd and rb sr ratios of newly formed archaean crust are close to mantle values this process is transparent to the sm nd and rb sr isotope systems until recycling of incompatible element rich sediment becomes important following the late archaean production of a wide spread differentiated upper crust

origin and evolution of granitoids associated with the kadiri greenstone belt eastern dharwar craton a history of orogenic to anorogenic magmatism abstract different tectonic models have been proposed to explain the neoarchaean evolution of the eastern dharwar craton edc here we present whole rock elemental and nd isotopic compositions coupled with zircon u pb age and trace elemental data on granitoids associated with the neoarchaean kadiri greenstone belt edc to track the tectonic evolutionary history of the terrain earlier works on volcanic rocks of the kadiri belt tholeiitic oceanic plateau basalt high mg andesite and potassic adakite envisaged continental margin arc oceanic plateau collision in this work the oldest recognized granitoid is a 2 56 ga syntectonic sanukitoid like hornblende granodiorite enriched in large ion lithophile as well as ferromagnesian elements it was probably derived from a metasomatized mantle wedge above a subduction zone this is followed by syntectonic emplacement of 2 54 ga transitional ttgs and potassic adakitic volcanic rocks intrusion of 2 51 2 52 ga crustally derived granites mostly k rich nd 0 9 to 3 7 marks the end of orogenesis in the area a convergent orogenic setting characterized by assembly of diverse terranes repeated slab break off and crustal reworking is proposed to interpret the association of various granitoids and volcanic rocks finally intrusion of crustally derived highly silicic k rich a type granite nd 0 6 to 4 8 and mantle derived syenite occurred in a post subduction non compressive environment during palaeoproterozoic lithospheric delamination and attendant mantle melting and crustal reworking are suggested as possible mechanism for generation of these rocks the negative nd values and mesoarchaean nd depleted mantle model ages of some neoarchaean to palaeoproterozoic kadiri granitoids attest to the presence of older crust in the eastern part of the edc the final cratonization of the edc 2 52 2 50 ga occurred later than in most of the other cratons

the archaean ravensthorpe terrane western australia synvolcanic cu au mineralization in a deformed island arc complex abstract the archaean 2 95 3 00 ga ravensthorpe greenstone belt in the southern yilgarn craton western australia can be subdivided into three tectonostratigraphic units the ravensthorpe terrane the carlingup terrane and the cocanarup greenstones the ravensthorpe terrane comprising the manyutup tonalite and the annabelle volcanics is a low k calc alkaline intrusive extrusive complex that is similar to modern island arc associations geochemical data and a close timing and spatial relationship suggest a co magmatic relationship between manyutup tonalite and annabelle volcanics these rocks have lower k 2 o and less fractionated rare earth element patterns than typical granitoids and acid to intermediate volcanic rocks of the yilgarn craton widespread propylitic alteration of the manyutup tonalite does not obscure magmatic trends the ravensthorpe terrane was thrust eastwards over the adjoining carlingup terrane during an accretionary event that was accompanied by regional metamorphism volcanic and volcaniclastic metasedimentary rocks that comprise the annabelle volcanics contain cu zn cu au and au mineralization in various states of deformation and metamorphism from upper greenschist to upper amphibolite facies conditions strata bound cu zn sulphide mineralization in upper amphibolite facies volcanic rocks at west river are associated with a broad area of cordierite orthoamphibole rock cu au mineralized sulphide rich shears in a greenschist facies environment at kundip are associated with intense chloritization of volcanic wallrocks alteration of volcanic rocks at both localities involved enrichment of fe and mg and depletion of ca and na and is interpreted as the product of sea floor alteration followed by metamorphism these and other base metal rich deposits in the ravensthorpe terrane are interpreted as deformed and metamorphosed stringer zones that formed within an archaean synvolcanic submarine hydrothermal system of the type that is commonly linked to massive sulphide deposits sulphide and base metal poor gold quartz vein deposits are also present in the ravensthorpe terrane these probably formed at a later stage during regional deformation low k calc alkaline rocks such as those that comprise the ravensthorpe terrane are uncommon in the yilgarn craton as is synvolcanic cu au mineralization