zircon xenocrysts from the kambalda volcanics age constraints and direct evidence for older continental crust below the kambalda norseman greenstones abstract the hangingwall basalt at kambalda western australia contains zircons that have been shown by ion microprobe analyses to have very high u and th contents and a wide variety of crystallization ages nearly all of these zircons certainly are xenocrysts a few might relate to intrusive veinlets the age of the youngest xenocrysts 2693 50ma 2 shows that the eruptive age of the basalt cannot exceed 2743 ma this confirms that the apparent sm nd isochron giving 3200 ma 1 2 for kambalda mafic and ultramafic rocks is a mixing line 2 between unrelated components enriched and depleted in light rare earth elements mixing probably occurred at depth by erosion of 3200 3500 ma old felsic crust from the walls of the hwb conduits the zircon xenocryst ages are the first direct evidence for the presence of very old felsic crust in the eastern yilgarn block the latter implies that the kalgoorlie norseman greenstone sequences were formed in a continental rather than an oceanic environment

role of crustal contamination in heterogeneous archean volcanics from the leonora region western australia abstract archean supracrustal volcanic successions in the leonora laverton region of the eastern goldfields province western australia are subdivided into two stratigraphic associations 1 an early succession ten km thick containing homogeneous high ti tholeiite basalts displaying nearly flat chondrite normalized incompatible lithophile element patterns with la sm n ratios ranging from 0 88 to 1 12 and 2 an unconformably overlying heterogeneous succession four km thick consisting of a early siliceous low ti tholeiites and associated intermediate calc alkaline volcanics grading to b high ti tholeiite similar to those of association 1 and c late siliceous high mg spinifex textured basalts fractionation models and trace element distribution in high ti basalts suggest that all the flows were derived from similar but not identical parental magmas and that subvolcanic gabbroic fractionation was characteristic basaltic end members of the siliceous low ti suite had identical fractionation patterns but hornblende joined the assemblage in contaminated intermediate calc alkaline members producing low hree fractionates the siliceous basalts exhibit variable enrichment in large ion lithophile lile and light rate earth lree elements with a continuum of la sm n ratios up to 2 75 they also have marked negative anomalies for nb p and ti several samples also exhibit a positive anomaly for zr on normalized distribution diagrams low sr concentrations are inconsistent with mantle enrichment processes and compositions of siliceous volcanics are explained instead by up to 30 contamination of high ti tholeiite liquids by archean crust composed of hornblende tonalite gneiss geochemistry of the volcancis is consistent with eruption in a continental back arc basin tectonic setting

lithophile elements in huronian low ti continental tholeiites from canada and evolution of the precambrian mantle abstract huronian basalts from central ontario canada dated at about 2450 ma and associated with an early rifting episode are classified as siliceous low ti nb tholeiites they display strong enrichment in large ion lithophile lile and light rare earth lree elements compared to modern oceanic lavas the tectonic setting and geochemistry resemble mesozoic rift related low ti flood basalts including the ferrar group of antarctica and the parana and equivalent etendeka volcanics of south brazil and namibia respectively high lile lree ratios are also similar to subduction related island arc tholeiites and it is suggested that enrichment of the huronian lithospheric mantle source occurred through ancient subduction of crustal material probably during formation and consolidation of the archean continental crust melting models suggest that huronian subcontinental mantle source compositions derived from least contaminated aphyric mafic end members had already undergone a complex evolution including withdrawal of archean basalts and hydrous enrichment in incompatible components despite several subsequent melting episodes and a second probably magmatic enrichment event however many aspects of the huronian source signature were preserved and appeared in later basaltic products of this mantle mass keweenawan volcanics for example dated at about 1100 ma preserve low p zr ti and hree abundances

physical volcanology and geochemistry of a late archaean volcanic arc kurnalpi and gindalbie terranes eastern goldfields superterrane western australia abstract late archaean 2715 2704 ma calc alkaline volcanic complexes in the kurnalpi terrane are identical in facies and geochemistry to modern intra arc volcanic complexes preserved volcanic rocks and associated feldspathic sedimentary rocks represent proximal to medial submarine facies of submarine to locally emergent intra oceanic arc related volcanoes the least altered volcanic rocks are dominantly andesite but range from basalt and basaltic andesite to rare rhyolite and are the product of magmas similar in composition to those of modern arcs intermediate magmas were most likely derived from large ion lithophile element lile enriched mafic magmas by fractional crystallisation with or without assimilation processes the kurnalpi terrane thus provides an example of a late archaean volcanic arc in which hydrated mantle wedge melting was the main mechanism for magma genesis ttd suite volcanism derived from slab melting was either absent or played a subordinate role the adjacent gindalbie terrane contains younger 2692 2680 ma bimodal basalt rhyolite volcanic complexes with associated quartz rich sedimentary rocks and calc alkaline intermediate silicic volcanic complexes volcanosedimentary basement to the gindalbie volcanic sequences is deeply eroded equivalents of the calc alkaline volcanic complexes their associated sedimentary rocks and younger basaltic sequences found in the kurnalpi terrane gindalbie basalt rhyolite successions locally contain volcanic hosted massive cu zn mineral deposits rhyolites derived by low pressure fractional crystallisation or melting of andesites are more abundant than in the kurnalpi volcanic complexes facies associations and the local occurrence of distinctive incompatible element enriched mildly peralkaline a type rhyolites are consistent with rifting of a mature arc system the two terranes most likely represent volcanic evolution of the same late archaean volcanic arc but are different tectonic fragments that were welded together by 2660 ma

incompatible element enrichment in archean basalts a consequence of contamination by older sialic crust rather than mantle heterogeneity high abundances of incompatible elements in mgo rich archean basalts from western australia which were thought to have been derived from an enriched mantle source are probably the result of contamination of primitive magmas by small amounts of older sialic crust because these lavas represent some of the more extreme cases of enrichment in incompatible elements observed in archean basalts it may be possible to explain much of the trace element variation previously ascribed to a heterogenous archean mantle by variable crustal contamination of mantle derived magmas recent calculations of thermal erosion rates predict that high temperature high mgo magmas abundant in the archean are most susceptible to contamination therefore a cautious reappraisal of isotopic and trace element data from archean greenstone belts is required before the real extent of archean mantle heterogeneity can be established

asymmetric structures in a simulated landfalling hurricane abstract highly asymmetric structures in a landfalling hurricane can lead to the formation of heavy rains wind gusts and tornados at prefered locations relative to the center of the hurricane in this study the development of asymmetric structures in an explicitly simulated idealized hurricane during landfall was investigated it was found that the boundary layer friction and its associated convection produce a low level positive potential vorticity pv band ahead of the hurricane the interaction between the pv band and the eyewall pv ring leads to a temporary weakening and reintensifying cycle asymmetric structures arise from the near discontinuity of the surface friction and the latent heat flux the breaking of the eyewall in the rear quadrants is favorable for the intrusion of the low moist entropy air into the core consequently pv increases significantly in the core in and just above the boundary layer due to the stabilization after the hurricane makes landfall the diabatic heating in the

dual aircraft investigation of the inner core of hurricane norbert part i kinematic structure abstract on 24 25 september 1984 air motions and physical processes in the eyewall region of hurricane norbert were investigated with two national oceanic and atmospheric administration noaa aircraft operations center aoc wp 3d research aircraft one aircraft equipped with the airborne doppler radar flew repeated radial penetrations of the eyewall mapping the three dimensional wind field throughout a 40 km radius of the storm center these data provide the first complete documentation of the three dimensional wind field of the inner core of a hurricane on spatial scales of 10 25 km in the horizontal and 1 km in the vertical and temporal scales of 1 2 h the wind field was asymmetric and the asymmetry varied with altitude as altitude increased the location of the maximum tangential wind sloped upwind from left of the storm track at 1 km altitude to right of the storm track above 3 km the radial wind at 1 km altitude was inward in front of the storm and outward behind this pattern in the radial

auroral radar measurements at 16 cm wavelength with high range and time resolution auroral radar measurements performed with the eiscat facility are presented backscatter cross sections of the irregularities produced by the two stream farley buneman or gradient drift plasma instabilities have been recorded with a range separation of 1 5 km corresponding to a spacing of successive values in height of about 0 4 km the apparent height profiles of the backscatter have a width of about 5 6 km and occur between 95 and 112 km altitude with a mean at 104 km very often fast motions of the backscatter layers are observed which can be explained as fast moving ionospheric structures controlled by magnetospheric convection the maximal time resolution of the measurements is 12 5 ms the statistics of the backscatter amplitudes at this time resolution is close to a rice distribution with a rice parameter a 3 7 the observed backscatter spectra do not change significantly in shape when the integration time is reduced from 5 s to 100 ms

variations of spectral width with aspect angle for type ii vhf echoes from the eastward electrojet continuous wave radar data from the university of saskatchewan summer 1981 radar auroral campaign were examined to study the variation of spectral width with aspect angle in order to reduce the uncertainties caused by refraction from strong optical forms the data were restricted to stable eastward electrojet events a consistent and sharp increase of spectral width with aspect angle was observed in qualitative agreement with theory

origin of hybrid ferrolatite lavas from magic reservoir eruptive center snake river plain idaho the mineralogy and geochemical characteristics of intermediate composition ferrolatites and related lavas from the magic reservoir eruptive center central snake river plain have been investigated to evaluate the origin and petrologic significance of these hybrid lavas the ferrolatites are chemically uniform but contain a disequilibrium phenocryst xenocryst assemblage derived in part from mixed rhyolitic and basaltic magmas that are closely represented by extrusive units in the area the hybrid lavas also contain xenoliths of archean granulites and have high 87sr 86sr and low 143nd 144nd ratios all of which suggest significant magma crust interaction quantitative models including magma mixing minor crystal fractionation and crustal contamination very closely reproduce the observed compositions of these ferrolatites closed system fractionation and or simple bulk contamination models are not as successful and can be ruled out it appears that preexisting mafic and silicic magmas from distinct sources e g mantle and crust encounter one another in crustal level magma chambers under conditions where intimate mixing may occur despite wide differences in the physical properties of these liquids

external influences on hurricane intensity part iii potential vorticity structure abstract the interaction of hurricane elena 1985 with a baroclinic wave was reexamined using both potential vorticity pv and a formulation for eliassen palm fluxes in cylindrical coordinates the hurricane began to deepen rapidly as a narrow upper level positive pv anomaly became nearly superposed over the low level center the intensification appeared to represent an evaporation wind feedback activated by constructive interference of the pv anomalies enhanced convection associated with this process eroded the upper pv anomaly and prevented it from crossing the hurricane and reversing the intensification the eliassen palm flux divergence showed that maximum eddy activity remained in the upper troposphere prior to the reintensification this activity was produced in part because the action of the outflow anticyclone of the hurricane contributed to synoptic scale wave breaking the upper level pv anomaly that approached the center was much narrower in extent than the original synoptic scale trough th

reflection and refraction by tilted layers an explanation for vhf auroral backscatter at large aspect angles auroral structures are often accompanied by tilts to the electron density contours on the underside of the e region by including the effects of these tilts on radio wave refraction and reflection it is possible to understand many of the previously puzzling observations of vhf auroral backscatter at large angles away from perpendicularity to the earth s magnetic field it is shown that many of the characteristics of echoes from the vhf canadian bistatic auroral radar system bars are consistent with this process for bars observations for which the tilt hypothesis is correct the resulting doppler velocities can be interpreted in the same way as those from stare and used to give electric fields and convection patterns over the bars field of view

archaean basic volcanism in the eastern goldfields province yilgarn block western australia abstract petrographic petrological and geochemical studies have demonstrated the presence of three distinctive basic volcanic suites in the eastern goldfields province yilgarn block western australia these are termed the high magnesian series basalts hmsb the low magnesian series basalts lmsb and the siliceous high magnesian series basalts shmsb the hmsb and shmsb constitute differentiation series which contain both high mgo 9 5 14 wt and low mgo the hmsb and lmsb were derived from source mantle regions which were variably depleted in the incompatible elements archaean komatiites were derived from similarly depleted source regions and it is argued that the main petrogenetic difference between these three volcanic suites was the degree of partial melting from which they were derived the depleted nature of the source regions may have been induced by earlier small degree source mantle regions of the shmsb were variably enriched in the incompatible elements and water and parental magmas of the shmsb were derived from moderately hydrous conditions of partial melting the relative proportions of each basalt suite varies considerably between the layered successions examined for example the basic volcanics overlying the komatiites at kambalda are shmsb while the footwall volcanics consist predominantly of hmsb with subordinate lmsb however the norseman succession where no ultramafic volcanics are known to occur is comprised mainly of lmsb with a smaller proportion of hmsb

the tectonic evolution of the abitibi greenstone belt of canada based on structural geochemical sedimentological and geochronological studies we have formulated a model for the evolution of the late archaean abitibi greenstone belt of the superior province of canada the southern volcanic zone svz of the belt is dominated by komatiitic to tholeiitic volcanic plateaux and large bimodal mafic felsic volcanic centres these volcanic rocks were erupted between approximately 2710 ma and 2700 ma in a series of rift basins formed as a result of wrench fault tectonics the svz superimposes an older volcanic terrane which is characterized in the northern volcanic zone nvz of the abitibi belt and is approximately 2720 ma or older the nvz comprises basaltic to andesitic and dacitic subaqueous massive volcanics which are cored by comagmatic sill complexes and layered mafic anorthositic plutonic complexes these volcanics are overlain by felsic pyroclastic rocks that were comagmatic with the emplacement of tonalitic plutons at 2717 2 ma the tectonic model envisages the svz to have formed in a series of rift basins which dissected an earlier formed volcanic arc the nvz analogous rift environments have been postulated for the hokuroko basin of japan the taupo volcanic zone of new zealand and the sumatra and nicaragua arcs the difference between rift related submergent volcanism in the svz and emergent volcanism in the nvz resulted in the contrasting metallogenic styles the former being characterized by syngenetic massive sulphide deposits whilst the latter was dominated by epigenetic porphyry type cu au deposits

simultaneous chatanika radar and s3 2 satellite measurements of ionospheric electrodynamics in the diffuse aurora simultaneous chatanika radar and s3 2 satellite measurements have been used to investigate the spatial variation of the auroral zone ionosphere near the equatorward edge of the diffuse aurora during early evening satellite passes on 2 days in february 1976 the equatorward boundary of the diffuse aurora was nearly overhead at chatanika the configurations of e region ionization and conductivity were nearly identical in both passes but the latitudinal variations of f region ionization were quite dissimilar during both passes the ionospheric electric field in the diffuse aurora was directed northward as a result the closing current of the field aligned current system was primarily a pedersen current and the eastward electrojet was larger than the northward ionospheric current although the morphology of ionospheric conductivity was very similar on the 2 days different patterns were observed for the latitudinal distribution of electric field and field aligned current

geochemical stratigraphy of the huronian continental volcanics at thessalon ontario contributions of two stage crustal fusion the 1500 m thick sequence of huronian continental volcanics at thessalon ontario is subdivided into 4 volcanic cycles each of which includes abundant early mafic end members central intermediate flows and late rhyolite units major and trace element concentrations are dominated by extensive gabbroic fractionation trends that ultimately produced two types of felsic flows 1 rhyolites with high light rare earth element lree and relatively low large ion lithophile element lile concentrations high lree low lile rhyolites and 2 following late separation of ree rich accessory phases rhyolites depleted in lree low lree high lile rhyolites mafic end members of individual volcanic cycle are progressively less siliceous and less enriched in lile and lree with height in the stratigraphic section ti zr ratios gradually rise from 35 in early mafic flows to stabilize at about 85 in late units while average sio2 contents decrease from 56 to about 50 and mg rises from about 48 to 52 nd values are consistently negative indicating variable degrees of pre fractionation crustal contamination of the end member magmas during their uprise through the crust mixing models are consistent with up to 50 contamination by crustal material of tonalitic hornblende gneiss composition a progressive increase in nd from about 5 0 to 0 5 upward in the volcanic succession reflects a decreasing degree of crustal contamination due to development of insulating layers along margins of the feeder system detailed stratigraphic variations suggest that successive magmas batches were intercepted by a progressively fractionating periodically replenished magma source giving rise to open system magmatism despite the prevalence of crustal assimilation in the huronian lavas la sr n ratios are too low in least contaminated end members to be explained by contamination of tholeiitic magmas the late basalts resemble instead modern island are basalts and it is suggested that the subcontinental mantle source was enriched by subduction related processes during crustal formation within individual volcanic cycles gabbroic fractionation trends systematically deviate from calculated factors toward compositions characteristic of hornblende gneiss such relations suggest that further crustal contamination of the magmas occurred simultaneous with crystal fractionation probably within undulating sills at upper crustal levels quantitative analysis suggests assimilation fractional crystallization a fc ratios of about 0 45 as a result of extensive two stage contamination rhyolites from the initial volcanic cycle incorporate a total of over 60 of crust

tectonic setting of late archean bimodal volcanism in the michipicoten wawa greenstone belt ontario the tectono stratigraphic relationships depositional environments rock associations and major and trace element compositions of the late archean 2744 2696 ma bimodal basalt rhyolite volcanic rocks of the michipicoten wawa greenstone belt ontario are compatible with an origin along a convergent plate margin that varied laterally from an immature island arc built on oceanic crust to a more mature arc underlain by continental crust this environment is similar to that of the cenozoic taupo kermadec tonga volcanic zone michipicoten basaltic rocks most of which are proximal deposits compositionally similar la yb n 0 63 1 18 to modern oceanic island arc tholeiites are interpreted as having formed along the largely submerged island arc voluminous michipicoten rhyolitic pyroclastic rocks la yb n 4 3 18 7 ybn 5 7 15 9 probably erupted subaerially from the continental arc with distal facies deposited subaqueously on the adjacent oceanic island arc and proximal facies deposited in subaeri

dual radar investigation of e region plasma waves in the southern polar cap origins and characteristics of small scale plasma irregularities in the polar ionosphere are investigated using a dual radar setup in which the e region is probed from opposite directions by two super dual auroral radar network superdarn facilities at the mcmurdo and dome concordia antarctic stations in certain time intervals velocity agreement is observed when velocities are compared at the same physical location in the horizontal plane such an agreement is widely expected if velocity at a given location is largely controlled by the convection electric field in other cases however velocity agreement is unexpectedly observed when measurements are considered at the same slant range distance along the radar beam for both radars this implies that it is not the electric field at a given location that is a controlling factor raytracing results show that the same range agreement may be explained for certain e region density conditions when echo altitude increases with radar range backscatter observations under generally unfavorable conditions for irregularity generation and the critical role of propagation conditions in the polar cap are discussed the observed e region velocity in the polar cap is demonstrated to depend indirectly on the plasma density distribution which is important for establishing the fundamental dependence on the convection electric field

spatial relationship between large aspect angle vhf radio aurora and 557 7 nm emissions evidence for refraction a recently proposed explanation for large aspect angle vhf backscatter observations based on refraction through auroral ionization structures in the e region moorcroft 1989 has been confirmed by an experimental comparison of observations of auroral e region coherent backscatter at 48 5 mhz made with the bistatic auroral radar system or bars with simultaneous 557 7 nm emissions detected with a ccd based all sky imager the apparently large aspect angle radio auroral echoes are found to occur predominantly on the side of the visual aurora away from the radar for both the premidnight and the postmidnight sectors for the red lake bars radar this means that the strongest echoes occur to the north of the visual form in sharp contrast with previous comparisons particularly for the premidnight sector which found that radar echoes occur predominantly southward of the visual aurora these results are consistent with the proposed explanation based on refraction in the auroral e region

the sm nd age of kambalda volcanics is 500 ma too old although sm nd isotopic analyses of volcanics from kambalda western australia form a 3 2 ga linear array pb pb analyses of the same suite give an isochron age of 2 73 0 03 ga and a 2 7 ga model age for the associated sulphides we suggest that the 3 2 ga age is incorrect and that the sm nd array results from mixing between depleted mantle and either older continental crust or enriched mantle

chemical composition of archaean komatiites implications for early history of the earth and mantle evolution abstract estimates of the chemical composition of the archaean mantle derived from elemental abundance ratios in komatiites combined with ultramafic xenolith data support a model of a multistage heterogeneous accretion history of the earth and synchronous core formation 4 6 ga ago most refractory lithophile element abundance ratios in komatiites and xenoliths are close to chondritic except for v ti and ca al depletion of vanadium is likely due to its partial incorporation into the iron core whereas fractionation of ca al observed in archaean al undepleted komatiites 1 20 times chondrites and in some modern fertile spinel lherzolite xenoliths 1 15 times chondrites could be due to small amounts of garnet rich in al but poor in ca segregation into the lower mantle during partial or complete melting of the upper mantle in the very early history of the earth however this process may have had only a small effect on the overall chemical composition of the upper mantle simultaneous occurrence of early archaean al undepleted al ti chondrites and al depleted al ti 0 5 chondrites and depletion of sc and heavy ree peridotitic komatiites in the barberton area s africa and late archaean newton township canada argue against derivation of peridotitic komatiites from a circum global magma ocean garnet separation from a mantle diapir which intersects the solidus at great depth 200 km in a hotter early archaean mantle could explain the chemical characteristics of al depleted komatiites alternatively these two types of komatiites could have been derived from different layers in a fractionated mantle a limited amount of hf isotope data for archaean komatiites seems to suggest that both mechanisms are important this chemically and minerallogically layered mantle if it existed was homogenized by mantle convection after early archaean times constant p 2 o 5 tio 2 ni mg co mg fe mg ratios siderophile lithophile and pge abundances estimated for the mantle sources of komatiites from archaean to modern times strongly argue against continuous growth of the earth s core since the early archaean extensive crustal contamination might have been involved in the generation of archaean early proterozoic siliceous high magnesian basalts with boninite affinity however involvement of chemically modified ancient continental lithosphere may also be important in the generation of these basalts

diurnal radio aurora variations at 50 mhz measured by the bistatic auroral radar system radars mean doppler velocity and intensity data obtained from the normal operating mode alternating single pulse double pulse mode as also used by stare of the bars radars from selected intervals between march 1986 and july 1987 have been used to describe typical morphological characteristics and diurnal variations in the backscatter comparisons with the maria magnetometers at gillam and eskimo point verify that the bars drift velocities obtained by standard merging for which one assumes that the radar doppler shifts depend on the cosine of the angle between the electron hall drift and the radar look direction are in qualitative agreement with the equivalent currents three basic time intervals could be identified 1 an evening interval between 2100 and 0200 ut 1500 to 2000 mlt with westward e b drift which occurs only under disturbed conditions and for which the nipawin backscatter intensity is very low most of the time 2 a breakup interval dominated by eastward drift but including westward turnings which under disturbed conditions can extend from 0200 to 1000 ut 2000 to 0400 mlt but which occurs even under relatively quiet conditions when no backscatter precedes or follows it in which case it lasts about 2 hours around 0600 ut 3 an interval with oscillations around a mainly eastward drift backscatter in this interval does not seem to depend on the occurrence of the previous two types and lasts from 1000 up to 1500 ut 0400 to 0900 mlt no backscatter has yet been measured during the rest of the day 1500 to 2100 ut or 0900 to 1500 mlt the period from 2130 ut may 28 to 1430 ut may 29 1987 is used as a sample day during which all three backscatter types occurred

the genesis of the archaean welcome well volcanic complex western australia the welcome well volcanic complex east of leonora western australia is interpreted to be the eroded remnant of an archaean stratovolcano andesitic flows and intercalated mudflow deposits comprising the volcanic centre give way to coarse poorly sorted lithic wackes that were deposited in alluvial fans skirting the lower slopes or base of the subaerial volcanic edifice these deposits are succeeded both laterally and vertically by fine grained subaqueous turbiditic sediments that are intercalated with pillowed tholeiitic basalts

archaean basin development and crustal extension in the northeastern yilgarn block western australia abstract supracrustal sequences in the northeastern yilgarn block western australia form two contrasting lithotectonic associations most sequences were deposited in subaqueous to subaerial environments in a basin 120 km in width the preserved succession reaches a maximum of e 10 km in thickness in the central part of the basin although there is no record of the basement on which it was deposited in the later stages of its development the basin was intruded by calc alkaline granitoids that are in part comagmatic with calc alkaline volcanic rocks in the upper part of the stratigraphic succession brittle fracture normal strike faults and conjugate cross faults open upright folds and peak low strain metamorphism were coincident with granitoid emplacement the eastern and western margins of the basin were greatly modified during a period of increased crustal extension marked initially by the generation of a mildly peralkaline volcanic plutonic suite the volcanic members of which were deposited in linear faultbounded depressions a major growth fault indicated by thin persistent units of oligomictic and polymictic conglomerate and localized intense metasomatism was probably contemporaneous with felsic volcanic activity subsequent sediments derived largely from granitic source areas were deposited in narrow grabens the youngest felsic plutonic rocks in the area include small bodies of alkali feldspar syenite many of which are found near the margins of the late sediment filled grabens tectonism at the eastern basin margin extended into the proterozoic in contrast to sequences at the centre of the basin those at its margin have undergone isoclinal folding ductile deformation and high strain metamorphism the geological framework of the northeastern yilgarn block emphasizes the importance of crustal extension in the development of younger archaean supracrustal sequences and provides valuable support for extension dependent greenstone belt evolutionary models e g groves and batt

generation of archean tonalites thermal evolution models of young subducting oceanic plates under archean conditions suggest that tholeiitic basalts in the subducting wet crust melted at shallow depths 50 to 80 km and produced archean tonalites the latent heat of fusion consumed by the crust kept the overlying peridotitic mantle wedge below the melting temperature of peridotite this prevented both fusion in the wedge and production of cale alkaline island are volcanic rocks similar to those observed in modern subduction zones

lewisian gneiss geochemistry and archaean crustal development models abstract the geochemistry of lewisian amphibolite facies gneisses from northwest scotland is described with particular reference to the rare earth elements ree and compared with the geochemistry of lewisian granulite facies gneisses the results show that there are no significant differences between laxfordian amphibolite facies and scourian granulite facies gneisses in terms of ree and other immobile trace elements at equivalent silica levels although the mobile radioactive heat producing elements k rb th u are significantly lower in the granulites in both types the basic gneisses have moderately fractionated ree patterns while the intermediate and acid gneisses have strongly fractionated ree patterns with low heavy ree abundances and decreasing levels of total ree with increasing sio 2 the most silicic gneisses develop large positive europium anomalies these gross chemical similarities between gneisses from intermediate amphibolite facies and lower granulite facies crustal levels constrain models for the evolution of the archaean crust the depletion of k rb cs th and u in granulites but not other incompatible trace elements cannot be explained by magmatic processes the positive eu anomaly in the more siliceous gneisses of both facies is a function of the primary processes of crustal generation and not secondary processes such as intracrustal melting or fractional crystallisation fractionation of radioactive heat producing elements from other trace elements is a result of granulite facies metamorphism with these elements being removed by an active fluid phase the apparent lack of partial melting in lower crustal granulites suggests a model for archaean crustal growth largely through underplating by primary tonalitic magmas