variations in the surface texture of suspended quartz grains in the loire river an sem study statistical size distribution and scanning electron microscopic studies of suspended sand grains in the loire river at montjean france were carried out over a period of a year hydrological cycle to discern seasonal variations the sand fraction in suspension is better sorted during winter average mean 0 69 mm median 0 65 0 95 mm sorting value 1 1 1 35 and is dominated by quartzo feldspathic minerals during summer on the contrary this fraction is rich in mica minerals and is poorly sorted average mean 1 21 mm median 0 33 0 95 mm sorting value 1 4 2 0 the grain size of the coarser fraction shows a tendency to increase with river discharge up to an optimum discharge of about 1000 m3 s 1 and thereafter decreases scanning electron photomicroscopy of quartz grains from the suspended sand population indicates that both mechanical and chemical features occur the former conchoidal fractures mechanical fractures and breakage dominate in the samples collected near the water surface during winter floods and the latter solution pits vermicular features silica flowers neogene silica and diatoms in the samples from the bottom during low summer flows during summer chemical action takes place on sediments prior to or upon their deposition sediments that are resuspended during winter floods undergo mainly physical processes attrition abrasion mixed surface features are therefore observed during average and low river discharge inheritance of these surface features from the source area is however not completely excluded thus the history of quartz grains in suspension can be reconstructed from sem exoscopic studies therefore it can be proposed that the fluvial quartz grain surface textures result from a combination of alternating chemical and physical processes

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

imprints in silica grains induced during an open channel flow experiment determination of microtextural signatures during aqueous transport abstract the aim of this work is to identify and characterize microtextural signatures in silica glass grains used as analogous to quartz that are produced during aqueous transport at different flow velocities with variable sediment concentrations transport distances and time intervals to achieve this an open channel flow experiment was conducted with a mixture of sand and silica glass microspheres in varying conditions velocity from 0 67 to 1 4 m s duration 1 or 10 minutes distance 0 to 2 5 m and sediment concentration 60 or 80 experimental conditions were used to replicate natural phenomena such as river superficial velocity or coastal swash processes before the experiment the microsphere surfaces were imaged and clear of any microtextural imprint increasing velocity distance and sediment concentration exhibited a strong correlation with higher numbers of surfaces abundantly covered with microtextures of mechanical origin i e craters abrasion marks and v shaped percussion marks sem microphotographs of silica were analyzed and classified to provide examples of the specific microtextures produced during the open channel flow experiment the purpose of the experiment was to characterize surface microscopic signatures in quartz grains replicating hydrodynamic conditions of coastal and fluvial environments the results demonstrated a strong correlation between higher velocities and higher sediment concentrations and a larger presence of microtextural mechanical imprints in the grains analyzed thus demonstrating a clear relation between microtextural imprints and water flow modes these results have important implications for future microtextural works analyzing grain imprints and their relation to sediment transport types an example demonstrated here is that the higher presence of v marks could be used as an indicator of supercritical flow conditions

assembling and reactivating the proterozoic capricorn orogen lithotectonic elements orogenies and significance abstract the capricorn orogen was initiated during palaeoproterozoic suturing events that brought together the archaean yilgarn and pilbara cratons to form the west australian craton the orogen comprises palaeoproterozoic plutonic and medium to high grade metamorphic rocks of the gascoyne complex a series of palaeoproterozoic volcano sedimentary and sedimentary basins including the ashburton blair yerrida bryah padbury and earaheedy basins and the deformed margins of the pilbara and yilgarn cratons major pulses of deformation and metamorphism took place during the ca 2200 ma ophthalmian orogeny the 2000 1960 ma glenburgh orogeny the 1830 1780 ma capricorn orogeny and a more localized unnamed event at the end of the palaeoproterozoic 1670 1620 ma the orogen has been the site of repeated intracratonic reactivation with renewed basin formation magmatism and orogeny during the mesoproterozoic and the neoproterozoic the exhumed palaeoproterozoic components of the orogen are unconformably overlain by the intracratonic mesoproterozoic edmund and collier basins bangemall supergroup which have been deformed during the neoproterozoic edmundian orogeny the ophthalmian and glenburgh orogenies affected the northern and southern margins of the orogen respectively the former resulted in the development of a foreland basin along the pilbara craton margin whereas the latter is associated with accretion of an allochthonous element the glenburgh terrane of the gascoyne complex to the northwestern margin of the yilgarn craton effects of the capricorn orogeny extended across the entire orogen and provide a younger limit for amalgamation of the yilgarn and pilbara cratons a number of basins developed along both the northern and southern margin of the orogen during this event and were filled with sediment eroded from the yilgarn craton the gascoyne complex and a presently unexposed early palaeoproterozoic terrane the capricorn orogeny corresponds with a major phase of continental collision both in australia and world wide which appears to mark a major phase of supercontinent assembly mineralization within the orogen includes a wide variety of deposit types that can be related to collisional settings including the world class iron orebodies of the hamersley basin

implications of quartz grain microtextures for onset eocene oligocene glaciation in prydz bay odp site 1166 antarctica abstract this paper presents the results of the scanning electron microscopic sem analysis of quartz grains from a selection of samples at site 1166 ocean drilling program leg 188 drilled site 1166 on the prydz bay continental shelf antarctica to document onset and fluctuations of east antarctic glaciation this site recovered upper pliocene holocene glacial sediments directly above cretaceous through lower oligocene sediments recording the transition from preglacial to early glacial conditions sem analysis of quartz grains at site 1166 was used to characterize the glacial and preglacial sediments by their diagnostic textures angular edges edge abrasion as well as arcuate to straight steps are the most frequent features in glacial deposits the highest frequency of grains with round edges is present in middle late eocene fluvio deltaic sands however angular outlines fractured plates with subparallel linear fractures and edge abrasion indicating glacier influence are also present preglacial carbonaceous mudstone and laminated gray claystone show distinctive high relief quartz grains and some chemical weathering on grain surfaces the results of the microtextural analysis of quartz grains are used to verify some critical periods of ice sheet evolution such as the transition from the east antarctic preglacial to glacial conditions on the continental shelf from middle late eocene to late eocene early oligocene time

transformation of an archean craton margin during proterozoic basin formation and magmatism the albany fraser orogen western australia abstract the albany fraser orogen is a well preserved example of proterozoic modification of an archean craton margin the formation of two successive basin systems accompanied by magmatism along the southern and southeastern yilgarn craton margin in western australia reflect distinct changes in tectonic regimes resulting in significant transformations of the archean craton crust provenance analysis from detrital zircons indicates that the first basin system the c 1815 1600 ma barren basin was dominantly filled with neoarchean detritus derived from the yilgarn craton and paleoproterozoic detritus derived from coeval voluminous mostly felsic magmatism the abundance of locally derived sediment deposited onto a reworked archean yilgarn craton substrate coupled with the progressively more juvenile isotopic signature of magmas indicates a largely extensional tectonic setting consistent with either a continental rift basin or a long lived backarc basin system this extensional regime led to the formation of a passive margin and oceanic basin along the craton edge the c 1600 1305 ma arid basin and formation of an ocean continent transition in the most heavily modified outboard region of the craton margin the eastern nornalup zone a change in tectonic regime is denoted by the formation of an oceanic arc the c 1410 ma loongana arc preserved in the adjoining madura province oceanic subduction led to accretion of the loongana arc onto the craton edge recorded by the presence of exotic detritus derived from this arc deposited into a foreland basin uppermost arid basin this accretion event triggered crustal thickening and the earliest phase of magmatism of stage i of the albany fraser orogeny at c 1330 ma a change to west dipping subduction beneath the accreted portion of the loongana arc placed the orogen into a backarc setting into which the remainder of the recherche supersuite mafic and felsic magmas were intruded thus the albany fraser orogen is regarded as an archean craton margin that preserves a long history of proterozoic transformation dominated by extensional processes that resulted in the formation of orogen wide basin systems accompanied by magmatism the recognition that the albany fraser orogen has always been part of the west australia craton and the tectonic regimes that have affected it provide important constraints on reconstructions of the nuna and rodinia supercontinents

controls on intra plate seismicity in southwestern australia abstract although the southwest seismic zone swsz located about 150 km to the east of perth in southwestern australia is one of the most seismically active areas in australia there is little understanding as to why the earthquakes are occurring an analysis of geophysical geological and geodetic data from the area suggests that the swsz coincides with a precambrian terrane boundary seismic data show that the terrane boundary zone dips at a shallow angle in a northeasterly direction reactivation of this zone of weakness in the contemporary stress field east west maximum horizontal stress is interpreted to be the first order control on seismicity in the region gravity data show that the terrane boundary is offset by near orthogonal structures which are interpreted as faults at least one of these trends corresponds with a linear zone of epicentres the temporal and spatial distributions of epicentres associated with the 1968 meckering earthquake ml 6 9 suggest that the second order distribution of seismicity in the swsz can be explained by the intersection model whereby stresses are amplified by space problems associated with displacements on crosscutting faults it is speculated that a zone of high density and high seismic velocity in the lower crust may also be a second order control on the local seismicity however confirmation awaits better delineation of the extent of this zone

uplift structure of the southern kapuskasing zone from 2 45 ga dike swarm displacement the ca 2 ga kapuskasing zone a 500 km long belt of uplifted crust cuts across the regional structural trend of the archean superior province in canada and disrupts the continuity of 2 45 ga matachewan dikes that form a major swarm in its vicinity as expressed by positive gravity and magnetic anomalies it appears to end about 100 km east of lake superior until now the nature of this termination has remained obscure regional variations in paleomagnetic polarity and in the intensity of feldspar clouding caused by magnetite exsolution are two physical properties of the dike swarm that depend upon postemplacement erosion level they show that the kapuskasing zone as a narrow fault bounded crustal uplift continues after sinistral offset for a further 60 km to the southwest before terminating in a major cross fault dual magnetic polarity and variable levels of feldspar clouding are features found in many other proterozoic dike swarms worldwide and therefore in principle may be used to document other examples of craton response to tectonism

microtextural characteristics of quartz grains transported and deposited by tsunamis and storms article i nfo the complex transport and depositional processes associated with tsunamis and storms and the peculiarities of local inundation present questions associated with the recognition and differentiation of the sedimentary signature of these events this work presents a study of quartz grains transported and deposited by tsunami and storm waves with the objective of identifying specific microtextural signatures caused by high energy marine inundations and to correlate them with their principal sedimentary sources in this empirical study 1150 quartz grains 78 samples and their microtextural signatures were observed analyzed and classified using scanning electron microscope photomicrographs the results suggest that although no specific microtextural signature is associated with high energy inundations there are strong increases in the percent age of fresh surfaces and percussion marks when compared with the potential source material moreover tsunami and storm grains present the greatest microtextural variance among all the grains analyzed never theless specific local conditions and sediment concentrations constrain the microtextural implications on tsunami or storm grains one laboratory experiment was designed to test microtextural implications in grains subjected to variable velocities sediment concentration and time the surface microscopic signature in quartz grains of high energy events further contributes to the development of more efficient sedimentological criteria to identify deposits associated with tsunami and storm events

delamination and detachment of a lithospheric root abstract orogens can be formed by shortening of the lithosphere on time scales much shorter than that of thermal diffusion for time scales of a few ma the major part of the uplift of an orogen which may occur during the transition from crustal shortening to extension may be due to delamination of the mantle lithosphere rather than to convective thinning the negative buoyancy forces of a lithospheric root may cause lateral shortening while the lithospheric root delaminates peels off from the crust detaches and is replaced by hot upwelling asthenosphere this provides a large input of heat into the crust geological observations show that the late orogenic phase may be characterized by volcanic activity high temperature low pressure metamorphism extension high heat flow and a rapid within a few ma increase of the topography we present dynamically and thermally consistent numerical models of the evolution of an orogenic root mantle rheology is based on laboratory data of olivine while models with the rheology of olivine do not allow for delamination due to its very high viscosity the incorporation of a quasi brittle rheology is necessary for modelling delamination of the mantle lithosphere successful delamination models are always associated with a rapid change in topography we examined the parameter range of our models and found different regimes of behaviour of the orogenic root of the mantle lithosphere no delamination delamination delamination and full detachment for a fixed viscosity of the upper crust of 1023 pa s delamination occurs for a range of viscosities of 1023 pa s to 1020 pa s for the lower crust depending on the thickness of the lithospheric root but full detachment of the delaminated lithospheric slab only occurs if the viscosity of the lower crust is greater than approximately 1021 pa s hot asthenosphere is welling up into the gap opening during delamination further implications of our models are lithospheric roots or unsupported slabs of at least 100 170 km depth extent are needed to provide sufficient negative buoyancy to allow delamination and detachment in all delamination models significant amounts of lower crustal material are subducted into the mantle in applying our models to selected orogens we argue that mantle delamination may have played an important role in the variscan and the himalayan orogenies

warakurna large igneous province a new mesoproterozoic large igneous province in west central australia coeval mafic igneous rocks emplaced rapidly over 1 5 3 10 6 km 2 in western and central australia represent the erosional rem nants of a late mesoproterozoic large igneous province named here the warakurna large igneous province shrimp u pb dating of rocks separated by as much as 1500 km indicates that the main episode of magmatism occurred between 1078 and ca 1070 ma the warakurna large igneous province includes layered mafic ultramafic intrusions and mafic to felsic volcanic rocks and dikes in central australia a 1000 km long mafic sill province in western australia and several swarms of mafic dikes the large areal ex tent and short duration imply emplacement above a mantle plume head despite their wide separation the mafic rocks have similar mid oceanic ridge basalt normalized trace element patterns and rare earth element characteristics west directed paleocurrents westward radiating dike swarms and the occurrence of high mg rocks indicate that the center of the plume head was located be neath central australia other late mesoproterozoic large igneous provinces in the laurentia and kalahari cratons appear to be significantly older than the warakurna large igneous province in australia and thus are unlikely to be related to the same mantle plume head

evolution of the western margin of australia during the rodinian and gondwanan supercontinent cycles abstract the proto darling fault zone and its successor the darling fault extend for 1 000 km along the western continental margin of australia and appear to have been active at several periods during the geological past deformation commenced at 2 570 ma and affected late archaean granitoids along the western margin of the yilgarn craton much of the later activity reflects events related to the accretion and breakup associated with the rodinia and gondwanaland supercontinent cycles in the north rocks of the northampton and mullingarra complexes form part of a high grade grenvillian orogenic belt lying to the west of the darling fault referred to as the pinjarra orogen they underwent granulite facies metamorphism 1080 ma ago and form part of the global collisional event that resulted in the amalgamation of rodinia these rocks extend southward beneath phanerozoic sedimentary cover the perth basin where they are constrained to the east by the darling fault and to the west by the dunsborough fault the latter marking the eastern boundary of the leeuwin complex the leeuwin complex is a fragment of pan african crust that has traditionally been considered part of the pinjarra orogen it is composed predominantly of upper amphibolite to granulite facies felsic orthogneisses derived from a type anorogenic granitoids conventional and shrimp u pb zircon geochronology has established that the granitoids evolved between 780 ma and 520 ma and were metamorphosed at 615 ma these events are equated with rifting associated with the breakup of rodinia sm nd whole rock data support the juvenile nature of the crust and provide no evidence for the involvement of pre existing archaean continental material during the phanerozoic the dunsborough and darling faults were reactivated as normal faults defining the inner arm of a major rift system within eastern gondwanaland and controlling sedimentation in the perth basin that now overlies the grenvillian terrane major normal movement on the darling fault ceased by the late jurassic and it appears that continental breakup in the early cretaceous occurred along fractures closely related to the western boundary of the leeuwin complex that defined the eastern margin of the outer arm of the rift system breakup between australia and greater india commenced at 132 ma and was followed by eruption of the bunbury basalt at 130 ma and 123 ma this possibly resulted from hot spot activity beneath eastern gondwanaland and may have been a reflection of the kerguelen plume though the evidence is equivocal it is argued from the petrographic geochemical and isotopic characteristics together with the likely contiguity of the eastern gondwanaland continents since the assembly of rodinia that the leeuwin complex evolved within an intracrustal rift and is not an exotic terrane it is distinct from adjacent portions of the pinjarra orogen and should be considered a separate terrane it is recommended that use of the term pinjarra orogen be confined to rocks recording the grenvillian events thereby excluding those rocks the leeuwin complex that evolved during the later pan african orogeny

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

large scale crustal structure of the northwestern yilgarn craton western australia evidence from nd isotopic data and zircon geochronology u pb zircon geochronology and nd isotope geochemistry have been used in the northwestern yilgarn craton region of western australia to map gneiss units of different ages and to provide a model for late archean crustal evolution despite the inherent difficulties of minimal exposure and variations in the appearance of units brought about by heterogeneous strain the 3300 3730 ma gneisses and intercalated metasedimentary units crop out in a 100 km wide tract bounded on both sides by areas containing 2920 3000 ma gneisses between 2750 and 2620 ma several generations of granitoids were emplaced throughout the region regardless of whether the granitoids intruded the 3300 3730 ma or 2920 3000 ma gneisses modeling of their initial nd isotopic compositions shows that they were most likely formed by partial melting of predominantly 2920 3000 ma gneisses only a minor input from early archean sources is indicated for granitoids cutting the 3300 3730 ma gneisses and intercalated metasediments some of the granitoids that intrude the 3300 3730 ma gneisses contain 2920 ma inherited zircons supporting the nd evidence that their source is dominated by 2920 3000 ma gneisses the 3300 3730 ma gneisses are interpreted as belonging to an allochthonous terrain emplaced over a younger terrain comprising the 2920 3000 ma gneisses subsequently partial melting concentrated in the underlying 2920 3000 ma terrain gave rise to the late archean granitoids

precambrian large igneous provinces lips and their dyke swarms new insights from high precision geochronology integrated with paleomagnetism and geochemistry

single zircon ages for granitoid gneisses in the central zone of the limpopo belt southern africa and geodynamic significance abstract ion microprobe evaporation and vapour digestion single zircon emplacement ages for granitoid rocks throughout the central zone cz of the limpopo mobile belt south africa define three broad age groups namely at 3188 3314 2637 2734 and ca 2500 ma respectively all these granitoids including the youngest group display polydeformational features suggesting that much of the tectonic history of the cz occurred in the late archaean and early proterozoic the oldest group of predominantly tonalitic trondhjemitic granodioritic gneisses was found in relatively small domains from the extreme w to the e of the cz and probably represents remnants of a once more widespread early archaean terrain the most abundant granitoid gneisses belong to the 2 6 2 7 ga age group and were emplaced into already ductilely deformed gneisses of the oldest group several of these rocks were derived from intracrustal melting rare metamorphic zircons with ages of 2560 2575 ma reflect a high grade metamorphic event in the cz that has also been postulated by other workers a strongly deformed orthogneiss with a protolith emplacement age of 2510 ma proves beyond doubt that at least part of the structural history in the cz occurred in the early proterozoic an anatectic granite in the extreme w of the cz intruded at 2022 ma and is probably related to the peak of granulite facies metamorphism previously dated at 2027 6 ma in the messina area the second period of granitoid magmatism is broadly contemporaneous with charnockite and enderbite emplacement and granulite facies metamorphism in the northern and southern marginal zones of the limpopo belt broad contemporaneity of extensive granitoid magmatism in all three zones raises doubts about the exotic nature of the cz and suggests a revised evolutionary model for the limpopo belt

scale integrated architecture of a world class gold mineral system the archaean eastern yilgarn craton western australia abstract world class mineral systems such as those found in the archaean eastern yilgarn craton are the product of enormous energy and mass flux systems driven by lithospheric scale processes these processes can create big footprints or signatures in the lithosphere which can be observed at a range of scales and via a variety of methods including geophysics isotopes tectonostratigraphy and geochemistry we use these datasets to describe both the architecture structure of world class gold systems of the yilgarn craton and the signatures of their formation by applying an understanding of the most critical elements of the process and their signatures new areas especially undercover may be targeted more predictably than before knowledge of the major architectural elements of the yilgarn craton has increased greatly over the last decade through the collection of a wide range of geophysical geochemical and isotopic data these data range in scale from 1 lithospheric scale which provide information on the entire craton and their underlying volumes down to depths in excess of 350 km through 2 regional scale which provide information at the province scale and to depths of 30 40 km to 3 mine and camp scale which provide information on the top few kilometres of the crust the architecture structure in the upper mantle and lower crust can be inferred from broadband seismic tomography and analysis of long wavelength gravity data geophysical data such as magnetotellurics and deep crustal reflection seismic also provide evidence for the signatures of the flow of fluids through this architecture the pathways and they illustrate that the scale of the system is many orders of magnitude larger than the immediate deposit itself of particular importance is the role of deep crustal penetrating shear zones or faults which link the mantle with domes in the upper crust together these data provide first order insights into the physical and chemical properties of the yilgarn craton these insights provide geodynamic clues and constraints as to why the yilgarn is so well endowed in metals such as gold groves et al 1989 the intersection of favourable architectural features provides connections between the mantle and the upper crust in the kalgoorlie region possibly explaining why this area is so well endowed in comparison to others

the distribution of 3 0 ga and 2 7 ga volcanic episodes in the yilgarn craton of western australia abstract the results of a conventional zircon u pb investigation of the ages of felsic volcanic rocks in a number of greenstone belts in the yilgarn craton of western australia indicate that volcanism took place during two distinct episodes at ca 3 0 and ca 2 7 ga each episode spans a time period of about 75 ma and the two ages of volcanism are spatially separated 3 0 ga volcanic rocks are found in greenstone belts in the northern part of the western gneiss terrain and in the murchison province whereas 2 7 ga volcanic rocks are present in the southern part of the western gneiss terrain and in the southern cross and eastern goldfields provinces there is no evidence of inheritance in the zircon u pb systems which would support an older crustal origin for these rocks present results together with previous studies of the ages of granites and gneisses suggest that the volcanic episodes were accompanied by broadly contemporaneous granite plutonism it is suggested that the evolution of the yilgarn craton took place in three main stages the first was the development of a stable 3 4 ga granitic nucleus which incorporated earlier generations of granitoid a 3 7 ga layered complex and possibly 4 2 ga primitive crust this was followed by the two separate granite greenstone episodes at ca 3 0 and ca 2 7 ga involving the addition of significant amounts of new crust each episode was characterised by the accretion of extensive granite greenstone terranes at the margins of the stable craton the intrusion of granite plutons into the evolving craton and the reworking of older cratonic rocks to form granites and gneisses with inherited isotopic systems

the application of u pb geochronology to mafic ultramafic and alkaline rocks an evaluation of three mineral standards abstract a key to proving the accuracy of u pb age determinations by the various techniques available is routine analysis of an internal well characterized matrix matched mineral standard of known age in general there are very few well studied natural mineral standards available to the geological community in this study the nature geochemistry and u pb systematics of baddeleyite titanite and perovskite from three potential mineral standards are evaluated a compilation of concordant to near concordant analyses of a single phalaborwa baddeleyite crystal yields a precise u pb age of 2059 60 0 35 ma a large variation in uranium 51 2124 ppm thorium 0 5 73 ppm and lead 19 782 ppm concentrations and the presence of some u bearing mineral inclusions zircon monazite and baddeleyite zircon intergrowths require some caution when using aliquots of this phalaborwa baddeleyite crystal as a mineral standard the khan titanite crystal investigated here overall has a relatively uniform chemical composition e g u 584 95 ppm th 473 73 ppm pb 57 8 ppm th u 0 81 0 05 but parts of the crystal contain abundant mineral inclusions the model 1 upper intercept age of 522 2 2 2 ma for nine khan titanite analyses is considered the current best estimate for its crystallization age some variability in the u pb systematics with 207pb 206pb ages that vary between 508 and 560 ma was observed and could be related to the presence of tiny u bearing mineral inclusions such as rutile or allanite so careful selection of material from this titanite crystal is also required a single perovskite crystal from the ice river alkaline intrusion has a relatively uniform chemical composition uranium 141 25 ppm thorium 1954 270 ppm lead 41 5 ppm and th u 14 5 1 3 the u pb results for eight perovskite fractions yield a weighted average 206pb 238u age of 356 5 1 0 ma although there are always some limitations to using natural mineral crystals as u pb standards such as the presence of mineral inclusions alteration and geochemical heterogeneity the three crystals investigated here are considered suitable to be used as internal u pb standards for any of the currently used u pb dating techniques

continental mafic dyke swarms as tectonic indicators an example from the vestfold hills east antarctica abstract the two main types of magma that were emplaced as mafic dyke swarms in the vestfold hills are high mg tholeiite hmt and fe rich tholeiite frt the former magma type is less dense than the vestfold hills felsic crust whereas the latter is more dense at deep to intermediate crustal levels therefore frt dyke emplacement requires a higher deviatoric extensional stress as is reflected in a more constant orientation palaeoproterozoic hmt frt bimodal dyke swarms were emplaced around 2 4 ga and 2240 ma the geometry and orientation of 2240 ma hmt dykes imply a low deviatoric extensional stress field inconsistent with a mantle plume origin for magmatism current petrogenetic models for hmt magmatism suggest generation at active convergent plate boundaries although this is broadly consistent with emplacement of the bimodal dyke swarms subsequent to a crustal thickening event at 2 5 ga the exact mechanisms for melting and the generation of crustal extensional stress are unknown a possible scenario involves the sinking of gravitationally unstable oceanic crust a major mesoproterozoic magmatic extensional event in the vestfold hills started with normal faulting followed by lamprophyric dyke emplacement and by two distinct episodes of frt dyke emplacement at 1380 and 1245 ma a consistent direction of the stress field during this event suggests control of a linear belt of crustal uplift by comparison with other mafic dyke swarms this is interpreted to be due to a sheet of upwelling asthenosphere elongated plume around 1 4 ga rather than a low angle detachment this upwelling led to thermal erosion of the base of the lithosphere that channelled a subsequent magma pulse at 1245 ma erosion during a long period of uplift prior to 1380 ma was followed by sedimentation during thermal subsidence as suggested by segmentation of 1245 ma dykes the lithospheric structure inherited from the mesoproterozoic magmatic extensional event was probably reactivated during two high grade tectono metamorphic events around 1 0 and 0 5 ga the vestfold hills formed part of a relatively stable craton peripheral to the mobile belt that was formed during these events

micromorphological changes as an indicator of the transition from glacial to glaciofluvial quartz grains evidence from svalbard abstract the micromorphology of quartz grains in a sedimentary environment is determined by the transport mechanism and the nature of weathering both these aspects change during the transport history of grains thus important questions include how are quartz grains affected by possible changes in the transport medium and how quickly do the different micromorphological features develop or disappear the main goal of this study was to characterize the changes in the micromorphological features of quartz grains during the transition from a glacial to a glaciofluvial environment and to identify a set of diagnostic microtextures that can be used to distinguish between glacial and glaciofluvial quartz grains the samples came from the moraines of the bertilbreen and horbyebreen glaciers svalbard and from the sediments of glacier fed rivers in the forelands of these glaciers a total of 30 different micromorphological features was observed on 800 different quartz grains from 13 samples of glaciofluvial sediment and 3 samples of glacial sediment it was found that the frequency of rounded grains v shaped pits meandering ridges and cemented microblocks on glaciofluvial grains increased significantly with increasing length of fluvial transport whereas the frequency of angular grains straight steps straight and curved grooves adhering particles pitting and oriented etch pits decreased significantly different types of micromorphological features of quartz grains change with fluvial transport at different rates adhering particles after the first kilometer of fluvial transport straight steps and meandering ridges after the second kilometer of fluvial transport and v shape pits angular shape and straight grooves after the third kilometer of fluvial transport are reliable mechanical micromorphological features for distinguishing between glacial and glaciofluvial quartz grains

analysis of barrier island surface sediments by scanning electron microscopy abstract the analysis of quartz grain surface microtextures from a sedimentary environment where grain transport and depositional pathways were closely interconnected allowed insight into subtle microfeature alteration during movement between subenvironments four fire island new york and one montauk point beach sample were analysed principally through use of a checklist approach using 36 common surface features beach overwash floodtidal delta dune and inlet grains displayed similar surface features only the latter two samples displaying distinctive microtextural patterns the pattern of development of inlet grains was related to the application of higher shear stresses on the grains during transport in the shallow channel the results from dune grains testify to the effectiveness of aeolian transport mechanisms in rapidly altering existing features glacially derived microtextures altered by subsequent subaqueous action were prevalent but whether these glacial microfeatures are derived from the headland region montauk point or from a closer offshore glacial lobe source is not determined comparison of offshore lobe and adjacent sediments revealed contrasting surface and morphological differences statistical treatment of checklist data in binary form 1 0 using stepwise discriminant analysis proved workable but more rigorous testing is required to prove its relevance and use in deciphering raw textural data

unraveling the record of successive high grade events in the central zone of the limpopo belt using pb single phase dating of metamorphic minerals abstract dating of relic metamorphic assemblages can provide important information about the timing and character metamorphic grade and or pt evolution of early high grade episodes in polymetamorphic provinces using pb stepwise leaching of metamorphic silicates we have dated multiple granulite facies metamorphic episodes in the central zone cz of the limpopo belt ages of 2 52 ga were obtained from sillimanite and cogenetic garnet and ages of about 2 01 ga from titanite garnet and clinopyroxene together with new and published conventional age data from accessory phases and in the context of combined petrological and structural data these results lead us to a reinterpretation of the tectono metamorphic history of the cz three distinct high grade events at about 3 2 3 1 ga 2 65 2 52 ga and 2 0 0 05 ga are recognized each of these is suggested to correspond to a tectonic episode of distinct character a for the 3 2 ga event magmatic activity can mainly be identified best represented for example by the sand river gneisses or the messina layered intrusion the field relationships concerning the tectonometamorphic history of this early archean event are largely erased by at least two high grade metamorphic overprints b late archean 2 6 2 52 ga low pressure granulite facies metamorphism was associated with voluminous granitic and charnockitic plutonism the anticlockwise p t evolution of these granulites probably reflects deep crustal processes associated with magmatic underplating or in plating contemporaneous with vertical crustal growth of the zimbabwe craton around 2 6 ga c during the proterozoic event 2 05 1 95 ga tectonic thickening was caused by the collision of the kaapvaal and zimbabwe cratons the cz was squeezed between these two cratons and as a consequence underwent high pressure granulite facies metamorphism with a clockwise p t evolution the structural metamorphic and geochronological data can be best explained with a tectonic model that describes this final event as a dextral transpressive orogeny

cosmogenic 21ne analysis of individual detrital grains opportunities and limitations we use a numerical model describing cosmogenic nuclide acquisition in sediment moving through the upper gaub river catchment to evaluate the extent to which aspects of source area geomorphology and geomorphological processes can be inferred from frequency distributions of cosmogenic 21ne 21nec concentrations in individual detrital grains the numerical model predicts the pathways of sediment grains from their source to the outlet of the catchment and calculates the total 21nec concentration that each grain acquires along its pathway the model fully accounts for variations in nuclide production due to changes in latitude altitude and topographic shielding and allows for spatially variable erosion and sediment transport rates model results show that the form of the frequency distribution of 21nec concentrations in exported sediment is sensitive to the range and spatial distribution of processes operating in the sediment s source areas and that this distribution can be used to infer the range and spatial distribution of erosion rates that characterise the catchment the results also show that lithology can affect the form of the 21nec concentration distribution indirectly by exerting control on the spatial pattern of denudation in a catchment model results further indicate that the form of the distribution of 21nec concentrations in the exported sediment can also be affected by the acquisition of 21nec after detachment from bedrock in the diffusive hillslope and or advective fluvial domains however for such post detachment nuclide acquisition to be important this effect needs to at least equal the nuclide acquisition prior to detachment from bedrock copyright 2009 john wiley and sons ltd

a comparative sem study on the micromorphology of glacial and nonglacial clasts with varying age and lithology preliminary research on striated clasts from a variety of depositional environments suggests that scanning electron microscopy sem of striated clasts of varying lithology in diamictons may prove useful in interpreting a glacial origin evaluating whether sem analysis of clasts from diamictons is an applicable technique to define a glacial origin requires a better understanding of the microfeatures occurring on glacial and nonglacial clasts we describe microtextures and surface characteristics for samples of quartzite granite limestone basalt chert pillow basalt and quartz pebbles collected from a variety of depositional environments our study suggests that it is possible to differentiate between glacial and nonglacial deposits based on frequency and morphology of diagnostic surface microfeatures observed on entrained clasts these microfeatures are best preserved and identified on competent and monomineralic samples e g quartzite chert limestone and poorly preserved on less resistant and p

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

loess in italy genesis characteristics and occurrence abstract there is currently a renewed interest in loess genesis and occurrence in italy well known loess profiles in northern italy have been re examined and previously unknown loess deposits in central and southern italy have been reported this work combines a meta analysis of published data with new data presented here for the first time to provide the state of the art on the spatial distribution characteristics genesis and deposition ages of loess in italy a database of 98 soil horizons from 91 soil profiles was created and made available it stores information on soils formed from loess or containing layers that show admixture of loess soil data include the source of information the topographic geomorphological and geological setting kind of parent material and land use main soil horizon properties as described in the field soil classification particle size distribution and chemical data loess is reported from almost all regions of italy it is generally pedogenized throughout the entire loess body and forms the parent material of deep and complex soil profiles the thickness of the loess deposits varies from a few decimeters to a few meters pedogenesis and local reworking partially change the original loess characteristics in this study we propose a set of parameters that help recognizing loess in soils particle size distribution and sorting geomorphological setting pedostratigraphic position shape of the grains under optical and scanning electron microscope sem and soil micromorphological characteristics the ages of the loess deposits range between 70 ka bp and 18 ka bp there are also a number of samples that have been dated to the holocene the meta analysis suggests that climate is not the only driver of loess deposition in italy geomorphological instability and human disturbance as well as the influence of saharan dust most likely played major roles too this study demonstrates that loess is much more widespread in italy than previously estimated yet further research on the spatial and temporal distribution of loess deposition across the mediterranean region is needed to better understand its genesis sources and trajectories periods of enhanced loess formation and the role of loess deposits in ecosystem functioning and resilience

scanning electron microscope study of zircons and rutiles from a podzol chronosequence at cooloola queensland australia summary a chronosequence of four soils derived from aeolian deposited sand at cooloola along the eastern coast of southern queensland has been studied to document predepositional and pedogenic changes in zircon and rutile grains in the fine sand fraction 53 125 m with increasing soil age the soils range in age from holocene typic quartzipsamment presently drifting sand to mid pleistocene spodic quartzipsamment 90 ka bp parent materials are moderately stratified sands with irregular depositional layers in places having high concentration of heavy mineral grains scanning electron microscopy was used to characterize grain surface microtexture by a semi quantitative method based on frequency of occurrence of ten classes of surface microtextures pedogenic changes were quantified and differences between horizons in the same profile between different profiles within the same horizon and between minerals were tested for significance using the 2 distribution zircon grains are less chemically weathered during pedogenesis than rutile possibly due to the higher iron content of the latter however zircons tend to show more features which can be associated with physical alteration rutile grains in the illuviated b2hs horizons of the troporthod and spodic quartzipsamments are more thickly coated with poorly crystalline iron and alumino silicate compounds compared with zircon grains and the presence of grain coatings does not seem to have prevented the surfaces of grains from developing solution pits with increasing age there is a preponderance of weathering features on grains of both minerals the presence of various shapes and surface microtextures on grains from the different soils suggests a multiple grain source for zircon and rutile and a complex predepositional history for these minerals nevertheless microtextural features of zircons and rutiles are valuable for the interpretation of the extent and nature of post depositional weathering

timing of late archaean granulite facies metamorphism in the southwestern yilgarn craton of western australia evidence from upb ages of zircons fro mafic granulites abstract conventional single and multigrain zircon u pb ages were determined from six samples of mafic granulites from locations widely distributed throughout the southwestern yilgarn craton zircons interpreted to have formed during the granulite facies event record ages within the narrow time range of 2649 2640 ma interpretation of this as the age of the granulite facies metamorphism is supported by an identical age on metamorphic titanite from one of the granulite samples zircons in two of the samples contained remnant euhedral zoning interpreted to have been inherited from the igneous protolith however u pb ages of these zircons were not significantly different from the metamorphic age suggesting that emplacement of the parent rocks was coeval with metamorphism or that the zircons were completely reset during the metamorphic event a third sample of mafic granulite contains rounded zircons with complex forms which record an age of 2798 ma indicating that these are inherited from an older protolith at present there is no geochronological evidence for rocks of this age in greenstone belts in the yilgarn craton placing in doubt suggestions that all the mafic granulites are high grade metamorphic equivalents of greenstone belts found elsewhere in the craton the granulite facies metamorphism is significantly younger than the age of 2670 ma for lower grade greenstones to the west supporting the hypothesis that the western limit of granulite facies rocks marks the boundary between two terranes that accreted after the peak of metamorphism

late archean collision and delamination in the southwest yilgarn craton the driving force for archean orogenic lode gold mineralization orogenic lode gold deposits formed within a range of crustal environments over most of the yilgarn craton from 2640 to 2630 ma but there is little evidence for a causative widespread tectonomagmatic event at this time in the most highly mineralized greenstone terranes in the exposed deeper crustal levels of the southwest yilgarn craton however field geochemical pb isotope and shrimp u pb zircon studies define a continent continent collisional event with a widespread thermal anomaly probably due to lithospheric delamination at this time it is proposed that this event was of sufficient scale to drive the giant upper crustal fluid circulation systems with lateral and vertical flow at the scale of hundreds and tens of kilometers respectively that produced the widespread gold mineralization

some key issues in reconstructions of proterozoic supercontinents abstract supercontinents containing most of the earth s continental crust are considered to have existed at least twice in proterozoic time the younger one rodinia formed at 1 0 ga by accretion and collision of fragments produced by breakup of the older supercontinent columbia which was assembled by global scale 2 0 1 8 ga collisional events little consensus has been reached regarding configurations of these supercontinents because of some unresolved issues concerning continental fits one of these issues concerns how siberia was related to laurentia previous reconstructions that consider the aldan shield of siberia as a continuation of the wyoming province of laurentia have been largely abandoned in favor of models connecting siberia to northern laurentia but it remains controversial which part of siberia is contiguous with northern laurentia also at issue is the western laurentia australia east antarctica connection most rodinia reconstructions place australia together with east antarctica adjacent to either western canada the sweat hypothesis or the western united states the auswus hypothesis however recent studies combining paleomagnetic and isotopic age data have called into question the validity of sweat auswus and other variants another issue is the position of north china in rodinia columbia limited paleomagnetic data seem to be consistent with the paleo mesoproterozoic north china siberia baltica connection whereas geological data support the recently proposed archean to mesoproterozoic north china india connection controversial issues have also been raised about the timing and history of the amalgamation and fragmentation of south america and west africa both geological and paleomagnetic data suggest that south america sao francisco and amazonia cratons and west africa congo and west african cratons coalesced into a single landmass along the 2 1 2 0 ga transamazonian eburnean orogens however whether they were divorced and then re married to form part of gondwana or remained largely coherent from their amalgamation at 2 1 2 0 ga until their incorporation into gondwana is unclear also little known is the position of amazonia west africa in the proposed supercontinents with some workers believing that they existed as a separate landmass whereas others place amazonia west africa adjacent to baltica in summary although geological and paleomagnetic data are supportive of the existence of proterozoic supercontinents rodinia and columbia they are insufficient to determine their exact geometries

geodynamics of the eastern yilgarn craton abstract over the last decade there have been significant advances in our understanding of the stratigraphy magmatism deformation metamorphism and timing of mineralisation in the eastern yilgarn craton eyc of western australia the integration of these disciplines has enabled a holistic review of the tectonic history of the eyc which favours a paraautochthonous tectonic model a significant advance has been the recognition of a 2810 ma rifting event off the eastern margin of the youanmi terrane which set up the north northwest trending architecture of the eyc rifting was followed by a 100 myr long phase of tectonic activity initiated by the establishment of a convergent margin at 2715 ma characterised by a west dipping subduction zone to the east of the eyc subduction was associated with the deposition of 2715 2670 ma volcanic stratigraphy and the emplacement of voluminous tonalite trondhjemite granodiorite magmatism which resulted in magmatic thickening of the crust volcanism was terminated by a 5 myr pulse of east northeast contraction followed by mid orogenic extension possibly linked to lithospheric and lower crustal delamination the lack of ultra high pressure metamorphism and the presence of high geothermal gradients preclude this contractional event from recording continent continent collision mid orogenic extension at 2665 ma resulted in the introduction of metasomatised mantle melts mafic granites and syenites deposition of siliciclastic basins which record metamorphic anticlockwise ptt paths and the start of significant economic gold mineralisation in the eyc high stretching factors and or delamination associated with this extension event resulted in significant heat input into the base of the crust which eventually led to the emplacement of low ca crustal melt granites and cratonisation of the eyc major gold mineralisation postdates mid orogenic extension and was associated with renewed compression transpression 2650 ma and the development of steep predominantly north northwest striking sinistral and later north to north northeast striking dextral strike slip faults during low ca granite emplacement

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

shrimp u pb geochronological constraints on archean volcanic hosted massive sulfide and lode gold mineralization at mount gibson yilgarn craton western australia the base metal rich mount gibson gold deposits are hosted within the ductile mount gibson shear zone in the retaliation greenstone belt of the murchison province of the archean yilgarn craton the deposits are unique in the context of the yilgarn as they exhibit two main styles of hydrothermal alteration and sulfide mineralization consistent with overprinting of synvolcanic volcanic hosted massive sulfide style mineralization by synorogenic shear hosted lode gold mineralization on the basis of model ages based on sulfide pb isotope ratios these two events are separated by approximately 300 m y subvolcanic intrusive rocks from within the host sequence to mineralization at the orion two and hornet deposits have shrimp ii zircon u pb ages of crystallization of 2929 or 3 and 2934 or 6 ma respectively these ages are within error and provide a minimum age for the host sequence and consequently the volcanic hosted massive sulfide mineralizing event at mount gibson on the basis of geochemical evidence the intrusions are interpreted to have been derived from the same parent magma as the rocks they intrude and consequently ca 2930 ma is probably the approximate age of the host rocks to mineralization the regional granitoid which intrudes the base of the greenstone belt has a crystallization age of 2935 or 3 ma a post tectonic granite porphyry which crops out approximately 25 km north northeast of the mount gibson mine site has a shrimp ii zircon u pb crystallization age of 2623 or 7 ma this granitoid has identical field relationships similar whole rock geochemistry and similar pb isotope ratios to those of an internal granitoid which cuts gold mineralization in the deep south pit and is consequently interpreted to be cogenetic and contemporaneous the 2623 or 7 ma date is therefore interpreted to be a minimum age for synorogenic gold mineralization at mount gibson a morphologically and chemically distinctive population of zircons extracted from the sample of 2934 ma altered felsic schist from the hornet deposit gives a shrimp ii zircon u pb age of 2627 or 13 ma these zircons have characteristics which are consistent with a hydrothermal origin and the age is interpreted to represent the timing of synorogenic gold mineralization at mount gibson the interpreted mineralization age is within error of the inferred age of the postmineralization post tectonic granitoid the age is also within error of the majority of absolute ages for gold mineralization from across the yilgarn craton which generally fall in the range 2630 or 10 ma shrimp zircon u pb geochronology thus provides strong support for the two stage model proposed for mineralization at mount gibson synvolcanic volcanic hosted massive sulfide style mineralization occurred at ca 2930 ma and was overprinted by lode gold mineralization some 300 m y later at ca 2630 ma

precambrian tectonic history of the west australian craton and adjacent orogens this review is a synthesis of the precambrian geology of the west aus tralian craton and its fringing orogens it describes the evidence that has recently been reinvestigated and reinterpreted to reveal a long dynamic history of repeated aggregation and dispersal of crustal fragments cratons are large composite geological units of continental crust that have survived intact for significant periods of geological time they are generally tabular bodies with great areal extent millions of square kilo meters and lithospheric thickness 200 400 km orogens are narrow steep zones of tectonic and metamorphic activity associated with the deposition of sediments and the intrusion of plutonic rocks they occur within or along the margins of continents and are generally a few hundred kilometers wide thousands of kilometers long and extend to the base of the litho sphere most orogens are initially associated with compressive tectonics crustal thickening and the transient formation of mountain belts cratons and orogens may survive for very long periods of time but as part of a thin crust on a dynamic mantle they are inherently mobile and travel as rafts for great distances around the surface of the earth they may be torn apart dispersed and reassembled with other continental fragments the boundaries of these crustal fragments once established remain zones of crustal weakness that may be reactivated as intra continental mobile belts or lines along which cratons are assembled and dismembered cf hoffman 1 988

hydrodynamic evolution from quartz microtextures of a beach ridge in qinghai lake china abstract beach ridges are landforms that exist on beach shorelines and can supply abundant information regarding palaeoenvironments to better understand the hydrodynamic processes of beach ridges a quartz microtexture analysis was carried out in this study the microtextural features of a total of 1063 quartz grains from seven beach ridge samples and eight reference samples from rivers a dune and a nearby beach were observed and analysed by scanning electron microscopy the qualitative and quantitative results suggest that the quartz microtextures of the beach ridge samples present more fresh surfaces and percussion marks than the river samples but less angularity the upper sequence samples from the beach ridge present higher values of fresh surfaces and percussion marks and lower dissolution than the lower sequence samples a cluster analysis shows that the hydrodynamic conditions of qinghai lake underwent pronounced changes from weak to strong and then to weak at the present time furthermore quantitative analyses indicate that the beach ridge and river sedimentary environments can be distinguished through the microtextural features of the 250 125 m size fraction however more grain size fractions should be observed and analysed to interpret the hydrodynamic conditions the quartz microtextural analysis of beach ridges may be an effective method reflecting the palaeohydrodynamic conditions

terrane accretion in the southwestern yilgarn craton evidence from a deep seismic crustal profile abstract interpretation of a seismic gravity and magnetic traverse across the western margin of the yilgarn craton undertaken approximately 110 km north of perth together with new rb sr and u pb geochronological data from the southwestern portion of the craton have led to a re evaluation of the geological relationships and to the development of a terrane accretion model for the region the geophysical data established the three layered nature of the crust with e dipping reflectors present throughout its thickness of 32 40 km reflectors in the upper crust correlate with shear zones mapped at the surface and are interpreted as major thrusts that have transported crustal slices from the east these sole out at a depth of about 10 km along a well defined decollement and indicate thin skinned tectonism the shear zones define the boundaries of three terranes which from west to east are herein defined as the balingup boddington and lake grace terranes the balingup terrane is composed of trough facies metasediments believed to have been deposited at 2 8 ga and subsequently metamorphosed under medium pressure amphibolite facies conditions this terrane contains some of the youngest granitoids in the yilgarn craton 2612 and 2527 ma and these have been extensively sheared by late archaean movement along the darling fault zone the boundary with the boddington terrane to the east is not precisely defined partly due to lack of exposure it is marked in the south by a narrow 100 km long strip of tectonised quartz monzonite and further north by a change in the rb sr biotite ages of granitoids from 400 500 ma in the west to 2300 2600 ma in the east the boddington terrane contains a sequence of archaean shelf facies metasediments part of the jimperding metamorphic belt believed to have been deposited at 3 1 ga and metamorphosed up to amphibolite granulite transition facies under low pressure conditions these sequences occur mainly in flat lying nappes which may have been tectonically transported from the east during assembly with the lake grace terrane the boddington terrane also contains two low grade greenstone belts formed between 2650 and 2670 ma and intruded by 2640 ma granitoids the eastern boundary of the boddington terrane is marked by a major change in gravity which also corresponds with a series of shear zones at the surface a change in structural style and a major change in the nature of the granitoids east of this boundary in the lake grace terrane granitoids of two ages have been recognised charnockites formed at 2640 ma and post tectonic granodiorites formed at 2580 ma metasedimentary rocks metamorphosed to granulite facies occur as rafts and lenses with steep upright folds and are enclosed in leucocratic garnetiferous granitic gneiss greenstones also metamorphosed to granulite facies and recording ages of around 2790 ma are also enclosed within the charnockites and are thus substantially older than the low grade greenstones in the boddington terrane to the west assembly of the balingup and boddington terranes appears to have been completed prior to the emplacement of the logue brook granite at 2612 ma the boddington and lake grace terranes came together after the 2640 ma granulite facies metamorphic event but the precise timing of assembly is not currently constrained it was certainly completed by 2411 ma the time of emplacement of a major set of e w trending mafic dykes the widgemooltha dyke suite which extend for over 500 km across the yilgarn craton although the widgemooltha dyke suite appears to be absent from the balingup terrane this is possibly related to extensive nw shearing during the proterozoic which also led to extensional re activation of the decollement that marks the base of the upper crust in the region

dating sedimentary rocks using in situ u pb geochronology of syneruptive zircon in ash fall tuffs 1 mm thick absolute ages for sedimentary rocks are required to construct a temporal framework in which to decipher earth9s history the most reliable method for dating precambrian sedimentary rocks is u pb geochronology of zircon in intercalated volcanic rocks however extracting sufficient zircons involves destructive mineral separation procedures often requiring several kilograms of sample we have dated felsic tuffs 207 pb 206 pb dates between ca 2600 ma and ca 2680 ma all consistent with their stratigraphic position and available age constraints the dates represent the timing of syndepositional volcanism and provide reliable estimates for the age of deposition zircons in a 0 5 mm tuff band 15 mm above the jeerinah impact layer provide an age of 2632 7 ma which represents a close approximation for the timing of a major asteroid impact the strewn field of which is distributed across two archean cratons in situ geochronology of zircon in felsic tuffs requires only small volumes of sample and is especially useful for drill core samples where material is limited it avoids the destruction of samples associated with mineral separation retaining textural information and avoiding potential contamination this approach adds significantly to the number of sedimentary rocks that can be dated accurately

diversity within a unified model for archaean gold mineralization in the yilgarn craton of western australia an overview of the late orogenic structurally controlled gold deposits abstract the archaean yilgarn craton has produced 3000 tonnes of gold mainly from structurally controlled deposits that formed during the latest stages of an orogenic event that affected the entire craton and culminated in the period 2 66 2 63 ga as a group these late orogenic deposits encompass a wide range of host rocks structural settings and structural styles and alteration types however several consistent features justify grouping the deposits into a single class of mineralization these features include timing relative to orogenesis high gold to base metal ratios an association with potassium metasomatism and carbonation of host rocks low sulfide contents and a low salinity h 2 o co 2 ch 4 ore fluid most deposits formed in domains of low mean stress during east west regional compression and preserve evidence in the form of quartz vein arrays and hydraulic breccias for high fluid pressures and rock dilation the range of ore depositional conditions implied by alteration assemblages at various deposits makes the term mesothermal an inappropriate description for this class of mineralization the term late orogenic structurally controlled gold deposits is proposed as one which more completely encompasses the class of mineralization described in this overview late orogenic structurally controlled gold deposits formed in thermal equilibrium with high grade metamorphic rocks at the margins of the greenstone belts but were retrograde with respect to lower grade host rocks in the cores of greenstone belts these relationships probably reflect diachronous metamorphism and a single gold mineralization event which was contemporaneous with the later high grade metamorphism geochronological data indicate gold deposition at ca 2 63 ga in the western part of the yilgarn craton isotopic data suggest similar depositional ages in the eastern part of the craton eastern goldfields superterrane but this timing is difficult to reconcile with field relationships that suggest broad contemporaneity between regional metamorphism and gold mineralization at ca 2 66 ga the main competing genetic models for late orogenic structurally controlled gold mineralization in the yilgarn craton propose an orthomagmatic deeply sourced or metamorphogenic ore fluid none of these models can account satisfactorily for all the features of the late orogenic structurally controlled gold deposits

first evidence of archean mafic dykes at 2 62 ga in the yilgarn craton western australia links to cratonisation and the zimbabwe craton abstract the archean yilgarn craton in western australia hosts at least five generations of proterozoic mafic dykes the oldest previously identified dykes belonging to the ca 2408 2401 ma widgiemooltha supersuite we report here the first known archean mafic dyke dated at 2615 6 ma by the id tims u pb method on baddeleyite and at 2610 25 ma using in situ shrimp u pb dating of baddeleyite aeromagnetic data suggest that the dyke is part of a series of ne trending intrusions that potentially extend hundreds of kilometres in the southwestern part of the craton here named the yandinilling dyke swarm mafic magmatism at 2615 ma was possibly related to delamination of the lower crust during the final stages of assembly and cratonisation and was coeval with the formation of late stage gold deposit at boddington paleogeographic reconstructions suggest that the yilgarn and zimbabwe cratons may have been neighbours from ca 2690 ma to 2401 ma and if the zimbabwe and kaapvaal cratons amalgamated at 2660 2610 ma the 2615 ma mafic magmatism in the southwestern yilgarn craton may be associated with the same tectonic event that produced the ca 2607 2604 ma stockford dykes in the central zone of the limpopo belt paleomagnetic evidence and a similar tectonothermal evolution including coeval low pressure high temperature metamorphism voluminous magmatism and emplacement of mafic dykes support a configuration where the northern part of the zimbabwe craton was adjacent to the western margin of the yilgarn craton during the neoarchean worldwide reliably dated mafic dykes of this age have so far been reported from the yilgarn craton the limpopo belt and the sao francisco craton

u th pb and lu hf systematics of zircon from ttg s leucosomes meta anorthosites and quartzites of the limpopo belt south africa constraints for the formation recycling and metamorphism of palaeoarchaean crust abstract results of more than 400 in situ u th pb and lu hf zircon isotope analyses combined with cathodoluminescence images and field relationships reveal that the oldest magmatic rocks of the limpopo belt musina area formed during the palaeoarchaean between 3 40 and 3 27 ga experienced a first anatexis at 3 24 3 07 ga m1 and were overlain by sediments mainly between 3 24 and 3 0 ga subsequently the infra and supracrustal rocks together underwent two high grade metamorphic overprints a first during the neoarchaean at 2 65 ga m2 and a second at 2 025 ga m3 hafnium isotope data additionally reveal that the palaeoarchaean magmatic rocks result from substantial recycling of much older crust this is well reflected by nearly chondritic hf int obtained from primary magmatic zircon domains of four sand river ttg gneiss samples hf 3 27 3 28 ga 1 3 to 1 8 from leucosomes within the sand river gneisses hf 3 40 ga 2 0 hf 3 28 ga 1 5 and from two meta anorthosite samples of the messina layered intrusion hf 3 35 ga 0 1 1 4 crustal recycling is also supported by hafnium isotope data of detrital zircon grains from two quartzite samples these data reflect a general increase of the hf int between 3 65 and 3 2 ga from ca 4 0 to 0 0 indicating that an eo to palaeoarchaean crust was mixed with juvenile mantel material in the hinterland of the limpopo belt the present study also demonstrates that hf isotope data provide a powerful tool which makes it possible to distinguish zircon domains formed during distinct magmatic or metamorphic events from such affected by a pseudomorphic alteration even if the investigated zircon grain underwent multiple alteration processes

composition and mineralogy of suspended sediment in the fluvio estuarine zone of the loire river france the fluvio tidal transition of suspended sediment in terms of mineralogy and composition in the loire river drainage basin the largest french river basin was investigated in the fluvial zone at montjean and in the tidal zone at mauves thouare for a complete seasonal cycle at montjean where the river experiences unidirectional flow the composition and mineralogy especially clays and clay minerals of river suspended material rsm are governed by the river discharge upstream contributions climatic conditions and microbiological activities however due to reversing tidal and river currents at thouare in the zone of tidal dynamics these relations are changed in the downstream direction sand and clay content in the rsm decrease while the silt content increases among clay minerals between these two observation stations montmorillonite remains constant kaolinite diminishes and the other minerals increase downstream combustible material organic and nitrate no3 contents in the rsm increase whereas the phosphate po43 and caco3 contents decline considerably during transport at the head of the tidal zone where the river flow encounters the tidal influence there is a relatively stationary water mass tidal slack where sands clays phosphate carbonate and silica are deposited by physical and chemical processes physical sedimentation takes place by simple gravitational deposition sands and by sorting and complicated differential settling clays chemical sedimentation takes place by precipitation calcite caco3 apatite ca5 po4 3 oh f cl coagulation of dissolved silica sio2 in connection with seasonal algal bloom and eutrophic events resume le bassin de la loire est le plus vaste bassin fluvial franqais il constitue un domaine de choix pour l etude des apports a l ocean la composition des matieres en suspension mes a eteetudiee sur un cycle saisonnier complet en deux endroits choisis afin de mettre en evidence les effets du principal changement de regime hydrologiyue du fleuve au passage du milieu fluvial au milieu fluvio estuarien soumis a la maree dynamique et donc depourvu d effet de salinite les variations des mes sont regies par les debits liquides les apports du bassin versant les conditions climatiques et les activites biologiques et microbiologiques dans la zone ou s exerce la maree dynamique le lien de ces variations avec celles de debit liquide fluvial est cependant tres perturbe entre les deux stations de mesure les teneurs en sables et argiles diminuent et ies silts augumentent la kaolinite diminue la montmorillonite se maintient constante les interstratifies et i illite croissent les matieres organiques les substances colloidales ainsi que les nitrates augmentent phosphates et carbonates decroissent ces phenomenes peuvent s interpreter ainsi le debit fluvial est contrecarre par le flot ou on observe aussi une zone d imrnobilite relative des masses deau l etale qui produit deux types de resultats dynamiques decantation accelerie des silts aprks attrition des sables triagregation sorting et sedimentation differentiklle des argiles chimiques precipitation de calcite d apatite de silice sous confrole biogeochimique saisonnier etiages algual bloom d ete succedant aux crues et aux apports detritiques d hiver

ca 2 5 billion year old coeval ultramafic mafic and syenitic dykes in eastern hebei implications for cratonization of the north china craton abstract a group of extremely rare coeval ultramafic mafic and syenitic dykes was discovered in the eastern hebei region of the north china craton these dykes intrude the 3 8 2 55 ga old caozhuang complex an olivine gabbro dyke and syenite dyke yield respectively shrimp zircon u pb ages of 2516 26 ma and 2504 11 ma interpreted as the magmatic crystallization age their zircons have single stage hf model ages 2677 ma and 2705 ma the olivine gabbros have mg values of 59 63 similar to high magnesian tholeiitic basalt they show relatively lree enriched patterns without eu anomalies la yb n 9 28 9 78 gd yb n 2 8 eu 0 96 1 00 with enrichment in liles and depletion in hfses they also contain high cr and ni and have zr hf and nb ta ratios similar to those of the primitive mantle the syenites are alkaline in composition with 8 67 8 88 wt na 2 o k 2 o and show high total ree contents 543 854 ppm and strong lree enriched patterns with minor eu negative anomalies la yb n 50 101 gd yb n 5 2 5 3 eu 0 80 0 82 trace element diagrams show strong lile enrichment relative to hfse zr hf and nb ta ratios are 47 49 and 26 76 respectively which are much higher than those of primitive mantle and higher than the average nb ta ratio of post archean continental crust the petrological and geochemical features indicate that these dykes were derived from a deep subcontinental lithospheric mantle source which implies that the ncc probably had continental crust of considerable thickness at this time combined with evidence for 2 5 ga granite intrusion and metamorphism in the eastern hebei region and adjacent areas we propose that the ncc has been a present scale craton at the end of archean

microtextural analysis of quartz grains of tsunami and non tsunami deposits a case study from tirua chile abstract in order to estimate the tsunami hazard it is essential to reliably identify and differentiate tsunami deposits from other high energy events like storms recently the microtextural analysis of quartz grain surfaces was introduced as a method to differentiate between tsunami and other deposits using tsunami deposits sampled from a bank profile of the tirua river central chile an area that was significantly affected by the 2010 and 1960 chile tsunamis we tested the microtextural analysis method for its capability to identify tsunami deposits a total of 815 quartz grain surfaces of two tsunami layers two non tsunami marsh sediment samples and three reference samples from nearby beach dune and river were analyzed using scanning electron microscopy sem we grouped the detected microtexture features into five microtextural families angularity fresh surfaces percussion marks adhering particles and dissolution features both the tsunami deposits and reference samples reveal high numbers of fresh surfaces and percussion marks thus there are no statistically significant differences between tsunami beach dune and river deposits in characteristics and abundances in all microtextural families our study indicates that the microtextural analysis of quartz grains may not be a suitable method to identify tsunami deposits in tirua chile due to local factors such as high numbers of inherited microtextures and the possible effects of the high amount of heavy minerals

the crustal continuum model for late archaean lode gold deposits of the yilgarn block western australia most archaean gold ores belong to a coherent genetic group of structurally controlled lode deposits that are characteristically enriched in au with variable enrichments in ag as w sb bi te b and pb but rarely cu or zn and are surrounded by wallrock alteration haloes enriched in k lile and co2 with variable na and or ca addition evidence from the yilgarn block of western australia combined with similar evidence from canada and elsewhere indicates that such deposits represent a crustal continuum that formed under a variety of crustal regimes over at least a 15 km crustal profile at pt conditions ranging from 180 c at 1 kb to 700 c at 5 kb individual deposits separated by tens to hundreds of kilometres collectively show transitional variations in structural style of mineralisation vein textures and mineralogy of wallrock alteration that relate to the pt conditions of their formation at varying crustal depths specific transitions within the total spectrum may be shown also by deposits within gold camps although nowhere is the entire continuum of deposits recorded from a single gold camp or even greenstone belt

timing relationships and structural controls on the location of au cu mineralization at the boddington gold mine western australia the boddington gold mine is hosted in archean volcanic volcaniclastic and shallow level intrusive rocks that form the northern part of the saddleback greenstone belt a fault bounded sliver of greenstones located in the southwestern corner of the yilgarn craton western australia total au content of the boddington gold mine past production plus in situ resource exceeds 400 metric tons making the boddington gold mine one of the largest au mines currently operating in australia geologic mapping and radiometric dating indicate that five phases of igneous activity occurred during development of the saddleback greenstone belt basaltic intermediate and minor felsic volcanism occurred between approximately 2714 and 2696 ma and again at approximately 2675 ma an older suite of ultramafic dikes was emplaced between approximately 2696 and 2675 ma and a younger suite was emplaced between approximately 2675 and 2611 ma granitoid plutons crystallized at approximately 2611 ma and cut all the other archean rocks in the saddleback greenstone belt regional upper greenschist facies metamorphism accompanied the earliest phase of ductile deformation d 1 sericite quartz or arsenopyrite altered shear zones developed during subsequent ductile deformation d 2 crosscutting relationships indicate that d 1 and d 2 predate approximately 2675 ma further ductile shear zones characterized by quartz albite sericite or pyrite alteration developed during d 3 after approximately 2675 ma narrow brittle faults d 4 with biotite or clinozoisite alteration halos active between approximately 2675 and 2611 ma cut the three generations of ductile shear zones rare quartz albite fluorite molybdenite or chalcopyrite or pyrrhotite veins developed prior to d 1 and the regional metamorphism these veins are not associated with any au mineralization or significant cu quartz or pyrite or molybdenite or au veins and crosscutting clinozoisite biotite or actinolite or quartz chalcopyrite pyrrhotite or galena or molybdenite or scheelite au veins developed during movement on the d 4 faults between approximately 2675 and 2611 ma mineralized veins crosscut the three generations of ductile shear zones but are not foliated movement on the d 4 faults controlled the location of mineralization within the boddington gold mine higher grade mineralization occurs along the d 4 faults and coplanar pyroxenite dikes and where the faults intersect older shear zones and quartz veins widespread lower grade stockwork mineralization is concentrated in the general vicinity of the d 4 faults the orientation of veins within stockworks is consistent with vein development during sinistral strike slip movement on the d 4 faults au cu or mo or w mineralization at the boddington gold mine therefore occurred late in the tectonic evolution of the saddleback greenstone belt the timing of mineralization at the boddington gold mine is analogous to many other structurally late au deposits in the yilgarn craton e g mount magnet mount charlotte and wiluna movement on the d 4 faults and mineralization may have been coeval with the emplacement of granitoid intrusions at approximately 2611 ma whereas these granitoids are unaltered and therefore unlikely to have been the source of significant volumes of hydrothermal fluids they may have provided the thermal energy necessary to drive circulation of auriferous hydrothermal fluids through d 4 faults that may also have accommodated their intrusion previous workers at the boddington gold mine have inferred that mineralization is genetically linked to subvolcanic intrusions emplaced between approximately 2714 and 2696 ma however this inference is inconsistent with the crosscutting relationships of structures and mineralized veins which indicate that mineralization occurred between approximately 30 and 80 ma after emplacement of these rocks