sedimentology of the bondi main heavy mineral beach placer deposit murray basin southeastern australia abstract drilling and pit wall exposures at the southern end of the bondi main heavy mineral beach placer deposit provided access to the loxton parilla sands which are here divided into five lithofacies the depositional environment of each lithofacies is interpreted as shoaling zone breaker zone surf zone swash zone and backshore dune other units encountered include the duddo limestone loddon river group bookpurnong formation and shepparton formation the interpreted depositional environments for these units are respectively shallow marine fluvial shallow marine shelf and fluvio lacustrine a palaeogeographical model is presented which proposes that the bondi main deposit formed at the southern end of a north trending shoreline this shoreline was truncated to the south by the dundas tableland and is broadly analogous in its geomorphological setting to the modern geographe bay in western australia the deposit therefore appears to have formed in association with a heavy mineral trap at its southern end postdepositional weathering overprints include ferruginous mottling clay eluviation and possibly quartz fracturing and disaggregation the ferruginous mottles formed under spatially and temporally variable redoximorphic conditions with fe being derived from the weathering of fe bearing primary minerals particularly ilmenite postdepositional clays in the loxton parilla sands are interpreted as being derived from the overlying shepparton formation

mobilization of gold as a polymetallic melt during pelite anatexis at the challenger deposit south australia a metamorphosed archean gold deposit the challenger gold deposit in south australia is hosted by pelitic migmatites that underwent peak metamorphism at 7 5 1 5 kbars and at least 800 c nd model ages suggest a protolith age of 2900 ma zircon u pb and garnet sm nd dating indicates that peak metamorphism occurred at 2440 ma and that garnet was present during this event at this time in situ vapor absent melting affected the pelitic host rock indicating that peak metamorphism occurred without introduction of a foreign fluid or melt thus evidence indicating the presence of ore minerals during peak metamorphism implies a prepeak mineralization event this evidence includes the occurrence of invisible gold in lollingite but not in adjacent arsenopyrite and the presence of spherical gold sulfide inclusions in peak metamorphic garnet and other silicates textures developed between pyrrhotite lollingite arsenopyrite and gold the lack of silicate alteration minerals supports this conclusion experimental evidence is presented to support our interpretation that a gold rich polymetallic melt was mobilized into leucosomes synchronously with silicate melt during peak metamorphism visible gold is restricted to migmatitic leucosomes and to a lesser extent melanosomes large inclusions of gold with coexisting arsenopyrite pyrrhotite and bismuth are hosted within peak metamorphic silicate minerals and at grain boundaries trails of micrometric spherical inclusions of predominantly gold and bismuth propagate along annealed fractures from these larger inclusions we show that these gold sulfide inclusions represent the crystallized products of an immiscible polymetallic melt metamorphism of the challenger deposit resulted in partial melting of the pelitic host rock and formation of this gold rich melt these two melts were synchronously redistributed to accumulate as leucosomes during development of a stromatic migmatite formation of a polymetallic melt thereby enabled extensive mobilization of gold producing leucosomes enriched in gold this occurred because the immiscible metallic melt was physically entrained rather than chemically dissolved within the silicate melt gold rich shoots at challenger parallel the plunge of ptygmatically folded leucosomes that are shown to be parasitic to a larger scale fold geometry which appears to be structurally related to the ore shoots it is interpreted that concurrent migration of polymetallic and silicate melt allowed concentration of gold into a series of dilational structures which developed within the larger scale fold geometry challenger represents a new deposit type other deposits around the world such as renco and griffins find are hosted in granulite facies rocks but challenger is the first reported example of leucosome hosted gold in migmatites

zircon u pb chronology of the jianping complex implications for the precambrian crustal evolution history of the northern margin of north china craton abstract the neoarchean to paleoproterozoic jianping complex in the western liaoning province is an important component of the precambrian basement of the north china craton this region connects the neoarchean fengning chengde complex in the west the neoarchean to paleoproterozoic north chaoyang complex in the northeast and the yixian fuxin archean greenstone belt in the east the precambrian jianping complex is dominantly composed of metamorphosed supracrustal sequence and dioritic to granitoid gneisses tonalite trondhjemite granodiorite ttg here we present results from la icpms zircon u pb isotope dating from the various lithological units in this complex which reveal that the magmatic precursors of the metavolcanic rocks associated with the supracrustal sequence were generated during 2555 2550 ma and up to 2615 ma a major magmatic pulse of dioritic to granitic suite occurred during 2538 to 2495 ma this was followed by ca 2485 ma granulite facies metamorphism and a retrograde event at ca 2450 2401 ma our data also reveal a major charnockite emplacement event in this region at ca 1694 ma integrating our new results with the available geologic and previous geochronological data we identify three major growth stages in the crustal evolution history of the northern margin of the eastern continental block of the north china craton at ca 2550 2495 ma ca 2403 2394 ma and ca 1700 ma the la icp ms zircon data provide new insights on the neoarchean to paleoproterozoic tectonothermal evolution history in the northern margin of the eastern continental block of the north china craton

the stratigraphic distribution of ni rich spinels in cretaceous tertiary boundary rocks at el kef tunisia caravaca spain and hole 761c leg 122 abstract ni rich spinels have been found throughhout the world at the cretaceous tertiary boundary these minerals have no counterparts in terrestrial rocks but have been observed in meteoroid ablation material and impact debris suggesting that the cretaceous tertiary boundary spinels result from an extraterrestrial event their stratigraphic distribution which has been observed at the cretaceous tertiary boundary sites of el kef tunisia caravaca spain and hole 761c odp leg 122 indian ocean are markedly different from site to site and are all much less extended than the ir anomalies regional differences in ni rich spinel distibution are entirely explained by a brief event recorded under different bioturbation conditions the wider stratigraphic dispersion of ir is attributed to postdepositional chemical processes at el kef the site which appears to be the least affected by bioturbation and which has the highest sedimentation rate the 1 2 mm thick ni rich spinel spike indicates that the deposition time did not exceed an upper limit of 100 years this upper limit is consistent only with the collision of an asteroid or a comet at the close of the cretaceous

trace fossils after the k t boundary event from the agost section se spain palaeogene trace fossils penetrate the uppermost cretaceous sediments in the agost section betic cordillera se spain chondrites targionii zoophycos isp planolites isp a planolites isp b thalassinoides isp a thalassinoides isp b thalassinoides isp c alcyonidiopsis longobardiae and diplocraterion parallelum have been identified a well developed endobenthic tiering pattern is interpreted the uppermost tier a represents major benthic activity in the shallowest sediments at or just below the seafloor recorded as bioturbated background only discrete rare planolites the next upper tier b displays the highest trace fossil diversity with planolites in the shallow levels and thalassinoides and alcyonidiopsis at slightly deeper ones the intermediate tier c contains randomly distributed thalassinoides and alcyonidiopsis cross cut by zoophycos and large and medium sized chondrites the deepest tier d contains zoophycos cross cut by medium and mainly small sized chondrites two interpretations are proposed 1 vertical partitioning of a singled multi tiered community under steady stable conditions in well oxygenated water bottom reflecting gradual changes deep into the sediment with decreasing oxygen pore water and benthic food and increasing substrate consistency and 2 sequential colonization and community replacement reflecting the work of two successive communities the first community mainly represented by planolites alcyonidiopsis and thalassinoides was produced in a shallow oxygenated soft substrate probably related to decreasing oxygen content and benthic food availability this community was replaced by deeply burrowing organisms that produced zoophycos and chondrites this sequential colonization can be determined by changes in environmental parameters related to the cretaceous palaeogene k t event or may have resulted from normal ecological and early diagenetic processes

processes of selective grain transport and the formation of placers on beaches placers are found on many beaches of the oregon coast in the beach that is the focus of this study the principal placer minerals are ilmenite 22 0 epidote 20 0 garnet 18 7 augite 15 5 hypersthene 8 1 hornblende 5 8 and zircon 2 0 the ilmenite fraction actually being partly chromite a series of beach face sand samples was obtained along a profile when sand was being eroded from the upper beach and transported to offshore bars since the processes of grain selective sorting leading to placer formation were probably most active at such a time the landward most sample contained 96 heavy minerals being part of the placer this concentration systematically decreased offshore to a 6 heavy mineral content in the seaward most sample 54 m offshore determinations of individual heavy mineral concentrations revealed that although they all tended to become concentrated within the placer with light quartz and feldspar dominating the offshore transport the sorting processes were m

the chicxulub asteroid impact and mass extinction at the cretaceous paleogene boundary the cretaceous paleogene boundary similar to 65 5 million years ago marks one of the three largest mass extinctions in the past 500 million years the extinction event coincided with a large asteroid impact at chicxulub mexico and occurred within the time of deccan flood basalt volcanism in india here we synthesize records of the global stratigraphy across this boundary to assess the proposed causes of the mass extinction notably a single ejecta rich deposit compositionally linked to the chicxulub impact is globally distributed at the cretaceous paleogene boundary the temporal match between the ejecta layer and the onset of the extinctions and the agreement of ecological patterns in the fossil record with modeled environmental perturbations for example darkness and cooling lead us to conclude that the chicxulub impact triggered the mass extinction

ichnosedimentologic evidence for a noncatastrophic origin of cretaceous tertiary boundary sands in alabama the clayton sands thin discontinuous sand bodies found at or near the cretaceous tertiary k t boundary at various sites in alabama previously have been attributed to 1 catastrophic tsunami deposition associated with a k t boundary impact or alternatively 2 noncatastrophic transgressive infilling of incised valleys cut during a preceding sea level low stand new observations on the geometry and ichnosedimentologic character of a clayton sand body enveloping strata and associated bounding surfaces exposed along mussel creek central alabama refute the tsunami origin and support the latter mechanism studies of other boundary deposits in the gulf region which may similarly benefit from ichnofossil investigations should consider more rigorously the evidence for sea level change near the k t transition

microscale trace element distribution across the cretaceous palaeogene ejecta layer at the agost section constraining the recovery of pre impact conditions abstract a high resolution sub mm scale analysis of the distribution of major and trace elements across the ejecta layer marking the cretaceous palaeogene boundary kpgb at the agost section se spain was performed using laser ablation inductivity coupled plasma mass spectrometry la icp ms a kpgb interval 17 mm thick has been selected for this study it includes gray calcareous marl from the uppermost maastrichtian cretaceous 2 02 mm thick red clay the ejecta layer and blackish gray clay boundary clay layer from the lowermost danian palaeogene the unconsolidated sediments were resin embedded under o2 free conditions and analyzed by la icp ms line continuous scan measurements at 20 m increments and a laser beam of 120 m these micron scale analyses show that the anomalous contents of trace and major elements in this boundary are restricted to the ejecta layer which displays a relatively uniform distribution over its 2 mm thickness trace and major elemental ratios such as ca al ti al cr al fe al ni al cu al zn al as al sb al and pb al have similar values below and above the ejecta layer consistent with similar palaeoenvironmental conditions prior to and after the impact event such element distribution points to anomalies exclusively restricted to the ejecta thus a nearly instantaneous re establishment of pre impact conditions right after the impact event is observed

quantitative heavy mineral analysis of a pliocene beach placer deposit in southeastern australia using the autogeosem abstract thirty two heavy mineral separates each comprising approximately 2000 grains were analyzed using the automatic geological scanning electron microscope autogeosem this analysis was undertaken to study the influence of depositional environment on the distribution of heavy minerals throughout a regressive pliocene coastal sequence the majority of these heavy minerals were separated from the loxton parilla sands the host of the bondi main heavy mineral beach placer deposit in the murray basin of southeastern australia the remaining heavy minerals were separated from a number of other neogene units encountered at the deposit the heavy mineral suite is dominated by stable to ultrastable minerals such as zircon rutile and tourmaline along with ilmenite various intermediate titanates and anatase sillimanite andalusite topaz chrome spinel chromite monazite and muscovite also occur these heavy minerals are heterogeneously distributed throughout both the loxton parilla sands and the remaining neogene units the main process controlling the heterogeneous distribution of these heavy minerals throughout the loxton parilla sands appears to have been the hydraulic conditions which are intimately linked with the environment of deposition consequently the dense heavy minerals density 3 5 g cm3 are concentrated in relatively high energy environments such as the breaker zone and swash zone the less dense heavy minerals density 3 5 g cm3 are concentrated in the shoaling zone and shelf environments the erratic distribution of the various heavy minerals in some of the other neogene units is also apparently due to varying depositional conditions postdepositional weathering has altered the heavy mineral assemblages in all units accordingly some primary minerals such as ilmenite appear to have altered to secondary minerals such as pseudorutile and anatase

sequence stratigraphy and plate tectonic significance of the transvaal succession of southern africa and its equivalent in western australia the transvaal succession in the northeastern part of the kaapvaal province of southern africa is the erosional remnant of four 2 7 to 2 1 ga unconformity bounded stratigraphic units or sequences each of which is 102 to 103 m thick and spans 107 to 108 years three of these phanerozoic like cratonic sequences occur in the western part of the kaapvaal province and in the pilbara province of western australia thus the two provinces once were part of the same continent vaalbara the stratigraphically lowest sequence historically assigned to the transvaal succession is part of the ventersdorp supergroup in kaapvaal and the upper part of the fortesque group in pilbara it consists predominantly of arkosic sandstone and basaltic andesite it probably is 2687 2 ma the second sequence contains the quartz arenites dolomites and banded iron formations of the transvaal and griqualand west successions in the kaapvaal and the hamersley group in the pilbara the span of ages 2684 6 to 2432 31 ma lithologies and geographic extent of this sequence imply that the limpopo orogeny which welded the zimbabwe province to vaalbara is 2 47 ga not 2 67 0 05 ga the third sequence the pretoria and postmasburg groups in the kaapvaal and the lower part of the wyloo group in the pilbara probably is the clastic wedge generated by the limpopo orogeny the fourth sequence dominated by the felsites of the rooiberg group is restricted to northeastern kaapvaal sequence stratigraphy lithostratigraphy and lithofacies indicate that the pilbara rifted from the southern edge of western kaapvaal the penge iron formation in northeastern kaapvaal the asbesheuwels iron formation in southwestern kaapvaal and the brockman iron formation in the pilbara are the same formation in which cyclothemic units 0 6 to 15 0 m thick extended 1200 km

proterozoic crustal evolution of the eucla basement australia implications for destruction of oceanic crust during emergence of nuna abstract the crystalline basement beneath the cretaceous to cenozoic bight and eucla basins in western australia has received comparatively little attention even though it lies on the eastern margin of one of the most mineral resource endowed regions on the planet this basement is characterized by a complex geological evolution spanning c 2 billion years but paucity of outcrop and younger basin cover present a daunting challenge to understand the basement geology in this work the composition of the unexposed proterozoic crystalline basement to the bight and eucla basins is investigated through zircon hf isotopes and whole rock geochemistry from new drillcore samples this region includes two geophysically defined basement entities the madura province containing 1 c 1478 ma sleeper camp formation which has variable isotopic signatures including evolved values interpreted to reflect reworking of rare slivers of hyperextended archean crust 2 1415 1389 ma haig cave supersuite with mantle like isotope values interpreted as melting of subduction modified n morb source and 3 1181 1125 ma moodini supersuite with juvenile isotopic signatures interpreted to reflect mixed mafic lower crustal and asthenospheric melts produced at the base of thinned crust the coompana province to the east of the madura province has three major magmatic components 1 c 1610 ma toolgana supersuite with chemical and isotopic characteristics of primitive arc rock 2 c 1490 ma undawidgi supersuite with juvenile isotope values consistent with extensional processes involving asthenospheric input and 3 1192 1140 ma moodini supersuite with strong isotopic similarity to moodini supersuite rocks in the madura province this new isotopic and geochemical data shows that the madura and coompana regions together represent a huge tract of predominantly juvenile material magma sources recognised include 1 depleted mantle producing morb like crust at c 1950 ma but also contributing to younger magmatism 2 recycled c 1950 ma crust reworked in primitive arcs and in intra plate settings and 3 minor evolved material representing fragments of hyperextended continent the observed isotopic evolution pattern is comparable to that of other central australian proterozoic provinces including the musgrave province the northern margin of the gawler craton and components within the rudall province linking these isotopic signatures defines the mirning ocean and its subducted and underplated equivalents in a global context we suggest c 1950 ma crust production reflects the onset of ordered oceanic spreading centres which swept juvenile crustal fragments into nuna

thermobarometric modelling of zircon and monazite growth in melt bearing systems examples using model metapelitic and metapsammitic granulites u pb age data collected from zircon and monazite are used to draw fundamental inferences about tectonic processes in the earth despite the emphasis placed on zircon and monazite ages the understanding of how to relate the timing of growth of zircon and monazite to an evolving rock system remains in its infancy in addition few studies have presented large datasets of geochronological data from zircon and monazite occurring in the same metamorphic rock sample such information is crucial for understanding the growth of zircon relative to monazite in a systematic and predictive manner as per this study the data that exist support the generally held conception that zircon ages tend to be older than monazite ages within the same rock here experimental data for zircon and monazite saturation in melt bearing rocks are integrated with phase diagram calculations the calculations constrain the dissolution and growth behaviour of zircon and monazite with respect to evolving pressure temperature and silicate mineral assemblages in high grade melt bearing metasedimentary rocks several key results emerge from this modelling first that in aluminous metapelitic rocks i e garnet cordierite sillimanite assemblages zircon ages are older than monazite ages in the same rock second that the growth rate of accessory minerals is nonlinear and much higher at and near saturation than at lower temperatures and third that the difference in zircon and monazite ages from the same rock may be ascribed to differences in the temperature s at which zircon and monazite grow rather than differences in closure temperature systematics using our methodology the cooling rate of granulites from the reynolds range central australia have been constrained at 4 c myr 1 this study serves as a first pass template on which further research in applying the technique to a field study can be based

the subglacial geology of wilkes land east antarctica wilkes land is a key region for studying the configuration of gondwana and for appreciating the role of tectonic boundary conditions on east antarctic ice sheet eais behavior despite this importance it remains one of the largest regions on earth where we lack a basic knowledge of geology new magnetic gravity and subglacial topography data allow the region s first comprehensive geological interpretation we map lithospheric domains and their bounding faults including the suture between indo antarctica and australo antarctica furthermore we image subglacial sedimentary basins including the aurora and knox subglacial basins and the previously unknown sabrina subglacial basin commonality of structure in magnetic gravity and topography data suggest that pre eais tectonic features are a primary control on subglacial topography the preservation of this relationship after glaciation suggests that these tectonic features provide topographic and basal boundary conditions that have strongly influenced the structure and evolution of the eais

testing ethological hypotheses of the trace fossil zoophycos based on quaternary material from the greenland and norwegian seas abstract despite the fact that the trace fossil zoophycos has been found in rocks from most of the phanerozoic little consensus has been reached on how and why this intricate burrow was constructed the cenozoic morphotypes of zoophycos typically include a helically coiled spreite arranged around a vertical shaft connected to the sediment surface morphological details and environmental preferences of the zoophycos producer were studied using 156 cores from the norwegian and greenland seas in order to test the different ethological hypothesis proposed for this trace fossil the spreiten were constructed during interglacial or interstadial intervals and consist of a repeated alternation of lamellae consisting of fine grained pelleted material and coarse grained unpelleted material respectively spreiten were encountered in vigorously bioturbated sediment in turbidite layers and in layers dominated by coarse ice rafted debris this indifference to the composition of the substrate effectively rules out ethological models based on different forms of deposit feeding and the large size and wide spacing of the whorls of the spreiten also make the cesspit model unlikely rather the observed features best agree with a cache behaviour where the main purpose of the deep penetration was to store food and to prevent access by other burrowers likely combined with some gardening of microbes however no indubitable evidence of a reworking of the cached material could be found and probably the answer to how the cache was accessed by the producer is to be found in the marginal tube the difference in diameter between marginal tube and spreiten lamellae together with the presence of both open and filled marginal tubes indicate that the marginal tube is the result of a far more complex behaviour than simply a lateral shift through the sediment

high pressure granulites at the dawn of the proterozoic abstracthigh pressure metamorphism is uncommon in the ancient geo logical record kanja malai in the salem crustal block southern india contains high pressure kyanite garnet bearing felsic granu lites that equilibrated at 14 16 kbar and 820 860 c laser abla tion inductively coupled plasma mass spectrometry u pb zircon and in situ monazite geochronology indicate that these assemblages grew ca 2490 ma these pressure temperature time constraints pro vide a rare record indicating that thickened crust and low apparent thermal gradient conditions existed during the archean proterozoic transition a period of earth history for which the rock record com monly preserves evidence for comparatively high apparent thermal gradients the thermal regimes required to generate these metamor phic conditions are typical of collisional orogenesis and suggest that the continental lithosphere was capable of supporting crustal thick ening to 45 50 km such crustal thickening provides supporting evi dence that tectonic regimes similar to modern earth style tectonics were in operation at the archean proterozoic transition introduction

rapid marine recovery after the end permian mass extinction event in the absence of marine anoxia a new early triassic marine fauna is described from the central oman mountains the fauna is griesbachian in age on the basis of abundant conodonts and ammonoids and was deposited in an oxygenated seamount setting off the arabian platform margin it is the first griesbachian assemblage from a well oxygenated marine setting and thus provides a test for the hypothesis that widespread anoxia prevented rapid recovery the earliest griesbachian parvus zone contains a low diversity benthic fauna dominated by the bivalves promyalina and claraia a similar level of recovery characterizes the immediate postextinction interval worldwide however the middle upper griesbachian sedimentary rocks isarcica and carinata zones contain an incredibly diverse benthic fauna of bivalves gastropods articulate brachiopods a new undescribed crinoid echinoids and ostracods this fauna is more diverse and ecologically complex than the typical middle to late griesbachian faunas described from oxygen restricted settings worldwide the level of postextinction recovery observed in the oman fauna is not recorded elsewhere until the spathian these data support the hypothesis that the apparent delay in recovery after the end permian extinction event was due to widespread and prolonged benthic oxygen restriction in the absence of anoxia marine recovery is much faster

rapid recovery of life at ground zero of the end cretaceous mass extinction the cretaceous palaeogene mass extinction eradicated 76 of species on earth1 2 it was caused by the impact of an asteroid3 4 on the yucatan carbonate platform in the southern gulf of mexico 66 million years ago 5 forming the chicxulub impact crater6 7 after the mass extinction the recovery of the global marine ecosystem measured as primary productivity was geographically heterogeneous 8 export production in the gulf of mexico and north atlantic western tethys was slower than in most other regions8 11 taking 300 thousand years kyr to return to levels similar to those of the late cretaceous period delayed recovery of marine productivity closer to the crater implies an impact related environmental control such as toxic metal poisoning 12 on recovery times if no such geographic pattern exists the best explanation for the observed heterogeneity is a combination of ecological factors trophic interactions 13 species incumbency and competitive exclusion by opportunists 14 and chance 8 15 16 the question of whether the post impact recovery of marine productivity was delayed closer to the crater has a bearing on the predictability of future patterns of recovery in anthropogenically perturbed ecosystems if there is a relationship between the distance from the impact and the recovery of marine productivity we would expect recovery rates to be slowest in the crater itself here we present a record of foraminifera calcareous nannoplankton trace fossils and elemental abundance data from within the chicxulub crater dated to approximately the first 200 kyr of the palaeocene we show that life reappeared in the basin just years after the impact and a high productivity ecosystem was established within 30 kyr which indicates that proximity to the impact did not delay recovery and that there was therefore no impact related environmental control on recovery ecological processes probably controlled the recovery of productivity after the cretaceous palaeogene mass extinction and are therefore likely to be important for the response of the ocean ecosystem to other rapid extinction events

the record of pliocene sea level change at enewetak atoll abstract detailed seismic stratigraphy lithostratigraphy and chemostratigraphy indicate that atoll wide subaerial exposure surfaces major disconformities developed during major sea level lowstands form prominent seismic reflectors and are coincident with biostratigraphic breaks in the plio pleistocene on enewetak atoll sea level models based on the stratigraphic position and age of major disconformities suggest a maximum sea level highstand elevation of 36 m above present sea level and a maximum sea level lowstand elevation of 63 m below present sea level for the pliocene

importance of pre impact crustal structure for the asymmetry of the chicxulub impact crater the final geometry of an impact crater can be significantly influenced by the geological and geomorphological structure of the impact site therefore it may not on its own provide accurate information regarding the direction and angle of impact

chicxulub crater a possible cretaceous tertiary boundary impact crater on the yucatan peninsula mexico we suggest that a buried 180 km diameter circular structure on the yucatan peninsula mexico is an impact crater its size and shape are revealed by magnetic and gravity field anomalies as well as by oil wells drilled inside and near the structure the stratigraphy of the crater includes a sequence of andesitic igneous rocks and glass interbedded with and overlain by breccias that contain evidence of shock metamorphism the andesitic rocks have chemical and isotopic compositions similar to those of tektites found in cretaceous tertiary k t ejecta a 90 m thick k t boundary breccia also containing evidence of shock metamorphism is present 50 km outside the crater s edge this breccia probably represents the crater s ejecta blanket the age of the crater is not precisely known but a k t boundary age is indicated because the crater is in a thick carbonate sequence shock produced co2 from the impact may have caused a severe greenhouse warming

fe oxide spherules infilling thalassinoides burrows at the cretaceous paleogene k p boundary evidence of a near contemporaneous macrobenthic colonization during the k p event a detailed ichnological analysis focused on thalassinoides structures recorded just below the cretaceous paleogene k p boundary layer of the agost section southeast spain aimed to evaluate macrobenthic colonization during the k p boundary event stereomicroscopy and field emission scanning electron microscopy analyses conducted on material filling in thalassinoides trace fossils reveal the presence of dark infill material that includes spherule like elements similar to those previously differentiated in the k p boundary layer energy dispersive x ray spectrometry indicates a ferruginous composition goethite spherules this represents the first record of spherules related to the k p boundary event within trace fossil infilling material the passive filling of this material into the thalassinoides bioturbation allows interpretation of the gravitational deposition just after thalassinoides trace maker colonization the absence of erosive sedimentary structures or other material deposited between the thalassinoides trace maker colonization and the spherule deposition is interpreted as evidence of rapid colonization during the k p boundary event nearly contemporary with the spherule layer deposition

carbon isotope evidence for the timing of the cretaceous palaeogene macrobenthic colonisation at the agost section southeast spain abstract the cretaceous palaeogene k t boundary section of agost southeast spain one of the most complete sections existing contains a well developed trace fossil assemblage composed of dark and light filled trace fossils registered in the uppermost cretaceous maastrichtian plummerita hantkeninoides biozone sediments the trace fossil assemblage reveals that significant macrobenthic colonisation occurred around the k t boundary interval however difficulties in recognising trace fossils affecting the k t boundary layer as well as in biostratigraphically characterising these trace fossils hindered a conclusive interpretation on the timing of this macrobenthic colonisation with the aim of this characterisation we determined the 13c of the carbonate fraction of dark and light filling materials from the trace fossils since the carbon isotope compositions of uppermost cretaceous and lowermost palaeogene materials are clearly different the data compiled provide evidence for a significant distinction between light filled trace fossils with 13c values higher than 1 6 peedee belemnite pdb and dark filled trace fossils with 13c values between 0 21 and 1 2 pdb most of these being lower than 0 7 pdb the comparison of these values with those from the uppermost maastrichtian to the lowermost danian sediments reveals that the macrobenthic colonisation occurred in different phases across the k t boundary interval

the formation of peak rings in large impact craters large impacts provide a mechanism for resurfacing planets through mixing near surface rocks with deeper material central peaks are formed from the dynamic uplift of rocks during crater formation as crater size increases central peaks transition to peak rings without samples debate surrounds the mechanics of peak ring formation and their depth of origin chicxulub is the only known impact structure on earth with an unequivocal peak ring but it is buried and only accessible through drilling expedition 364 sampled the chicxulub peak ring which we found was formed from uplifted fractured shocked felsic basement rocks the peak ring rocks are cross cut by dikes and shear zones and have an unusually low density and seismic velocity large impacts therefore generate vertical fluxes and increase porosity in planetary crust

heavy mineral beach placers in southeastern australia their nature and genesis southeastern australia has been one of the world s most productive heavy mineral sand provinces concentrations of detrital rutile zircon ilmenite and minor monazite occur as beach placers and in dunes in high stand barriers of quaternary age along the present coast although individual mineral grains are of terrigeneous origin the heavy mineral deposits themselves are not linked to specific source rocks in the hinterland rather they arise under the action of waves during the building of coastal sand barriers a number of fractionating mechanisms that concentrate heavy minerals operate during marine transgressions and under highstand conditions to produce three different types of beach placers type a deposits occur along erosional discontinuities in leaky embayments or between barriers of different age here repeated storm reworking has produced beach face concentrations on drift aligned sectors of coast by a process termed littoral bypassing fractionation type b heavy mineral deposits are found in back barrier washover facies and sometimes as condensed sections at the rear of swash aligned barriers in coastal embayments these were produced by a process of transgressive barrier fractionation that reworked heavy minerals from shelves undergoing erosion during marine transgressions type c heavy mineral deposits occur in prograded barriers and represent episodic influxes of remobilized heavy mineral rich sand that originated elsewhere through the two fractionation mechanisms mentioned above necessary environmental conditions for these fractionating mechanisms to operate include low rates of clastic sediment supply and long periods of weathering and abrasion to create a mature heavy mineral suite an energetic swell wave climate driving large sand fluxes onshore and alongshore and changing sea levels especially marine transgressions that have the effect of moving heavy minerals from the shelf onto the present coast because of the last mentioned effect the mineral prospectivity for the southeastern australian continental shelf is not high

isotopic evidence for the cretaceous tertiary impactor and its type high precision mass spectrometric analysis of chromium in sediment samples from the cretaceous tertiary k t boundary coincident with the extinction of numerous organisms on earth confirms the cosmic origin of the k t phenomenon the isotopic composition of chromium in k t boundary samples from stevns klint denmark and caravaca spain is different from that of earth and indicates its extraterrestrial source the chromium isotopic signature is consistent with a carbonaceous chondrite type impactor the observed differences in the chromium isotopic composition among various meteorite classes can serve as a diagnostic tool for deciphering the nature of impactors that have collided with earth during its history

timing of mesoproterozoic tectonic activity in the northwestern gawler craton constrained by 40ar 39ar geochronology abstract an array of northeast trending shear zones in the north west gawler craton of south australia has been implicated in widely disparate proterozoic continental reconstructions these shear zones are interpreted to have formed in response to sinistral transpression and dissect the north west gawler craton into several geological domains each with contrasting metamorphic histories new 40ar 39ar data provide age constraints for movement along these shear zones and are interpreted to indicate that the karari tallacootra and coorabie shear zones were last active at 1450 ma despite distinct differences in metamorphic grade the various geological domains bounded by shear zones exhibit evidence for commonality in event histories prior to deformation at 1450 ma albeit at different crustal levels this is interpreted to indicate that deformation at 1450 ma was responsible for reshuffling of crustal blocks that were already adjacent rather than amalgamation of exotic terranes the new data also provide additional evidence for a widespread metamorphic and deformational event at 1530 1550 ma across the north western gawler craton the timing of deformation at 1450 is 100 ma younger than suggested in several published tectonic reconstructions and importantly is also significantly older than grenvillian age tectonism in adjacent provinces to the west and north

earliest paleoproterozoic high grade metamorphism and orogenesis in the gawler craton south australia the southern cousin in the rae family neoarchean earliest paleoproterozoic rocks of the gawler craton preserved within the mulgathing complex south australia record metamorphism and deformation during an earliest paleoproterozoic tectonothermal event known as the sleafordian orogeny we present metamorphic data from pelitic gneisses of the mulgathing complex from three locations challenger mt christie and blackfellow hill in order to constrain the pressure temperature time p t t evolution of this orogeny in situ u pb dating via laser ablation inductively coupled plasma mass spectrometry shows that monazites within garnet porphyroblasts have ages ca 2470 ma providing an estimate of the timing peak p t conditions monazite crystallisation from melt occurred at ca 2460 ma in the presence of peritectic garnet either late on the prograde path during biotite dehydration melting or immediately following peak t lobate and patchy rims in leucosome hosted monazite suggest a dissolution reprecipitation process occurred during cooling across the elevated dry solidus at ca 2440 ma monazite younger than ca 2430 2420 ma that is located at grain boundaries between matrix minerals likely reflects monazite crystallisation during cooling across the dry solidus and or subsolidus mineral breakdown during the waning stages of the thermal event phase diagram modelling of pelitic gneisses suggests peak metamorphic conditions at the three localities overlap at 7 kbar and 850 c with apparent thermal gradients in the order of 125 135 c kbar 35 45 c km metamorphism involved a tight anticlockwise p t t path based on mineral assemblage paragenesis and indicates a slow shallow decompressive cooling path sub parallel to the apparent thermal gradient in the order of few degrees per myr a protracted magmatic history involving bimodal magmatism associated with a thinned and underplated extensional basin in the mulgathing complex was likely a thermal precursor to the high thermal gradient metamorphism and convergent transpressive deformation a significant period of post 2 4 ga quiescence in the gawler craton subsequent to basin inversion suggests that this orogenesis may have occurred interior to a plate margin the similarity in timing and style between the sleafordian orogeny in the gawler craton and the 2 5 2 28 ga arrowsmith orogeny of the rae craton supports the notion that these terranes together with the broader gawler adelie craton of the mawson continent the north australia craton elements of the north china sino korean and dharwar cratons formed part of a family of neoarchean terranes described as the rae family of cratons with a distinctive tectono metamorphic record as compared with other archean cratons we suggest the mawson continent is the southern member of this family dominated as it is by terranes now present in the northern hemisphere

sedimentology and ichnology of the cretaceous tertiary boundary in denmark implications for the causes of the terminal cretaceous extinction abstract trace fossils and other sedimentologic evidence of bioturbation in latest cretaceous and earliest tertiary marine deposits in denmark provide important clues to understanding the ecology of the sea bottom immediately before during and after the terminal cretaceous boundary event trace fossil associations at all the onshore danish boundary sites indicate a general upward shoaling of the late maastrichtian chalk sea which was accompanied by an abrupt shift from a carbonate ooze sea floor to a marly clay rich unstable substrate at the boundary although the substrate condition may have become more stable in the early danian water depths never did return to their former deep level there is evidence of localized anoxia during the deposition of some boundary sediments in the fish clay in shallow parts of the danish maastrichtian sea but no clear evidence of anaerobic conditions occurring at the era boundary was observed in boundary strata deposited in deeper water near the center of the basin in fact earliest danian marl there is totally bioturbated a pulse of calcite dissolution in shallow water coincided precisely with the era boundary and this event played a major role in the formation of the fish clay in eastern denmark which is a condensed series of smectitic clay rich layers from which much calcite has dissolved the effects of this dissolution episode however were not so extreme in the marl that was deposited at the same time in slightly deeper water in northwestern denmark evidence from cretaceous tertiary boundary sequences in denmark and elsewhere suggests that no single catastrophe can account for the major biotic extinctions that occurred at the end of the cretaceous period the primary causal factors of the terminal cretaceous extinction event appear to be drastic global sea level regression occurring simultaneously with extensive volcanism on land and a strong pulse of calcite dissolution in ocean surface waters

how complete are cretaceous tertiary boundary sections a chronostratigraphic estimate based on graphic correlation cogent interpretations of data bearing on the cretaceous tertiary k t extinction controversy depend on the existence of accurate chronostratigraphic models for the various k t boundary sections we have employed the graphic correlation technique to summarize biostratigraphic and lithostratigraphic data from 15 intensively sampled k t boundary sections within a common chronostratigraphic model our results indicate that almost all of these sections along with 13 additional boundary sections not used to construct the model contain prolonged and in many cases multiple hiatuses of these 28 boundary sections only six were found to contain a continuous record of sediment accumulation across the k t boundary itself these six k t complete sections are el kef tunisia agost spain caravaca spain and three sections along the brazos river texas a comparative analysis of hiatus distributions among these 28 k t boundary sections also reveals the presence of systematic differences between continental shelf and deep sea depositional environments the lower danian interval immediately following the k t boundary and extending into biochronozones p0 and p1a is typically missing from the deep sea whereas boundary sections deposited in shallower middle neritic to upper slope environments are in most cases complete across the k t boundary these shallow neritic boundary sections however are in many instances disrupted by hiatuses at the p0 p1a boundary and again in the upper part of zone p1a these differential patterns of hiatus distribution between deep sea and continental shelf depositional settings appear to be linked to sea level fluctuations our data suggest that the apparently sudden mass extinction of planktonic foraminifera and anomalies in the occurrence of geochemical tracers that are characteristic of the k t boundary in deep sea sections may be artifacts of a temporally incomplete deep sea stratigraphic record

spike of pyrosynthetic polycyclic aromatic hydrocarbons associated with an abrupt decrease in δ13c of a terrestrial biomarker at the cretaceous tertiary boundary at caravaca spain the first vertical high resolution record of polycyclic aromatic hydrocarbons pahs of pyrosynthetic origin and the corresponding 13 c profile of a terrestrial biomarker across the cretaceous tertiary k t boundary at caravaca spain reveals the following in comparison with adjacent cretaceous marlstones the first thin horizon 0 to 0 5 cm 0 the k t boundary of the boundary clay layer is 1 enriched as much as 112 to 154 fold in typical pyrosynthetic pahs such as coronene benzo g h i perylene and benzo e pyrene and 2 shows an abrupt 13 c decrease of 1 4 1 8 in terrestrial higher plant derived n c 29 alkane the spike of pyrosynthetic pahs associated with an abrupt decrease in 13 c value of a terrestrial biomarker is interpreted to reflect the prevalence of extensive fires with subsequent 13 c decrease in atmospheric co 2 it is estimated that the geologically instantaneous combustion of 18 24 of the terrestrial above ground biomass would be necessary to account for the measured negative isotopic shift at the k t boundary on the basis of carbon mass balance between terrestrial above ground biomass and atmosphere

mesoarchean and paleoproterozoic history of the nimrod complex central transantarctic mountains antarctica stratigraphic revisions and relation to the mawson continent in east gondwana abstract high grade metamorphic and igneous rocks originally mapped as the nimrod group represent the only known crystalline basement of the east antarctic shield exposed in the transantarctic mountains shrimp u pb age data from zircon show that this assemblage preserves multiple geologic events spanning 2 5 b y of archean to early paleozoic history culminating in thermomechanical reworking and active margin magmatism during the ross orogeny new age data from igneous and metamorphic rocks as well as from detrital zircons in metasedimentary quartzites refine the mesoarchean to paleoproterozoic history and indicate the need to abandon the stratigraphic term nimrod group and its five formations in its place we redefine only the igneous and high grade gneissic parts of the assemblage as the nimrod complex and all other metasedimentary rocks exposed in the miller and geologists ranges are included in a newly defined argosy schist within the nimrod complex the oldest layered gneisses represent magmatic mesoarchean crust formed between about 3150 and 3050 ma correspondence of u pb zircon crystallization ages with whole rock sm nd model ages indicates formation from juvenile mantle melts magmatism at ca 3100 ma was followed closely by high temperature metamorphism recorded by zircon crystallization at 2955 2900 ma likely due to high advective heat transfer and or thermal insulation of newly stabilized crust metaigneous units with ages of ca 2500 ma indicate a late neoarchean period of anatexis and or magmatism although the geologic context for these events is uncertain gneissic eclogitic and metaigneous rocks record an important period of deep crustal metamorphism thickening and magmatism between 1730 and 1700 ma referred to as the nimrod orogeny that may have played an early role in assembly of east gondwana cratons a new lithodemic unit argosy schist consists of interlayered mica schist quartzite amphibolite and calc silicate schist quartzites from the schist unit have two distinctive detrital zircon provenance characteristics one type has populations that correlate with dated gneissic and metaigneous units of the nimrod complex indicating a proximal provenance whereas a second type has detrital zircon ages resembling those from neoproterozoic beardmore group sandstones indicating a common broader east antarctic shield provenance their maximum depositional ages range widely from about 2000 to 900 ma leaving unresolved their age and geologic relationship to either the nimrod complex or beardmore group yet they comprise a metasedimentary assemblage that is distinctive from the nimrod complex recent findings of mesoarchean gneisses in the gawler craton geochronology from terre adelie and our new age data from the nimrod complex thus highlight the importance of ca 3 1 2 5 and 1 7 ga events in formation of the composite east antarctic shield adjacent to the modern transantarctic mountains correlative crustal units may extend to the southern prince charles mountains and shackleton range collectively this extensive crustal province referred to as the mawson continent represents an elongate belt of primary mesoarchean crust that experienced major reworking at ca 2 5 and 1 7 ga the latter period includes both high and low p t petrologic signatures providing evidence of crustal thickening and magmatism that may signify collisional processes related to paleoproterozoic cratonal amalgamation during formation of the nuna supercontinent

dating errors in high resolution stratigraphy a detailed x ray radiograph and ams 14 c study of zoophycos burrows abstract bioturbation in oxic deep sea settings is often considered if considered at all to consist of a 2 10 cm thick homogenized layer and some easily recognizable distinct burrows which could be avoided when sub sampling the sediment cores however there is one trace that is often difficult to discern in unlithified sediments that manifests transport of considerable amounts of sediment over large vertical distances and that is widely spread in deep sea sediments in all oceans this trace zoophycos is therefore predestined to cause errors and even age reversals in high resolution dating in order to better estimate the size of these age errors we have performed ams 14 c dating of the trace itself and of directly surrounding material in two gravity cores from the portuguese continental slope the results show an age falsification of 1110 2525 years at sedimentation rates of 10 20 cm ka 1 since zoophycos burrows are often poorly or not at all visible in fresh cores all observations of zoophycos were made on x ray radiographs to avoid dating errors of this kind an x ray radiograph study prior to sub sampling is strongly recommended

full fit palinspastic reconstruction of the conjugate australian antarctic margins 1 despite decades of study the prerift configuration and early rifting history between australia and antarctica is not well established the plate boundary system during the cretaceous includes the evolving kerguelen broken ridge large igneous province in the west as well as the conjugate passive and transform margin segments of the australian and antarctic continents previous rigid plate reconstruction models have highlighted the difficulty in satisfying all the available observations within a single coherent reconstruction history we investigate a range of scenarios for the early rifting history of these plates by developing a deforming plate model for this conjugate margin pair potential field data are used to define the boundaries of stretched continental crust on a regional scale integrating crustal thickness along tectonic flow lines provides an estimate of the prerift location of the continental plate boundary we then use the prerift plate boundary positions along with additional constraints from geological structures and large igneous provinces within the same australian and antarctic plate system to compute full fit poles of rotation for australia relative to antarctica our preferred model implies that the leeuwin and vincennes fracture zones are conjugate features within gondwana but that the direction of initial opening between australia and antarctica does not follow the orientation of these features rather the geometry of these features is likely related to the earlier rifting of india away from australia antarctica previous full fit reconstructions based on qualitative estimates of continental margin overlaps generally yield a tighter fit than our preferred reconstruction based on palinspastic margin restoration

felsic metavolcanic rocks related to the broken hill pb zn ag orebody australia geology depositional age and timing of high grade metamorphism two key issues related to the origin of the massive broken hill pb zn ag orebody are the genesis of its host rocks and their depositional age these granulite facies layered gneisses have been ascribed alternately to volcanic and nonvolcanic regimes determination of the depositional age of the sequence has never been seriously addressed north of broken hill amphibolite facies precursors to the granulite facies gneisses show compelling evidence of a volcanic origin in their texture chemistry and zircon morphology these felsic metavolcanic rocks are in the upper part of the broken hill group stratigraphically equivalent to the broken hill orebody the present work traces textural zircon morphological and isotopic changes across the amphibolite to granulite facies transition in order to determine the primary depositional age of this supracrustal sequence and the timing of highgrade regional metamorphism ion microprobe shrimp u pb data on zircons from the lower grade amphibolite facies felsic metavolcanic rocks define a major magmatic population at 1690 or 5 ma this is considered to be the age of zircon crystallization and eruption of felsic volcanics in this part of the broken hill group these shrimp data clarify inheritance and discordance problems that had plagued conventional multigrain u pb analyses zircon results from granulite facies stratigraphic equivalents of these rocks near southern cross and at broken hill have patterns similar to those of the amphibolite facies rocks including igneous relics that provide the same primary age additionally zircon in the highest grade granulite facies rocks approximately 800 degrees c forms metamorphic overgrowths surrounding the magmatic relics these younger zircon selvedges have a pooled age of 1600 or 8 ma which determines the timing of this granulite facies event consistent u pb monazite results yield this same age 1599 or 9 ma suggesting that the monazite blocking temperature is close to that of zircon or that cooling to 725 degrees c was achieved relatively rapidly this is some 60 m y younger than the previously accepted age for high grade metamorphism at broken hill irrespective of metamorphic grade in the gneisses all biotite rb sr ages have been reset to approximately 480 ma a reflection of early paleozoic delamerian orogenesis inherited zircon grains interpreted as xenocrysts and in some cases mantied by 1690 ma igneous rims have older proterozoic to late archcan ages 1780 2700 ma the presence of xenocrystic zircon components points to an ensialic tectonic setting for the broken hill block and reflects accidental incorporation in the felsic volcanic protoliths of lithic fragments and or complex igneous sources such as found in crust exposed in the gawler craton to the west the 1690 or 5 ma depositional age determined here provides a firm chronologic tie between the early proterozoic sequences enclosing major stratiform pb zn ag orebodies at broken hill mount isa 1670 or 19 ma and mcarthur river 1690 or 27 ma in northern australia in light of the uncertainties over timing and mode of origin of these deposits the similar ages of their host sequences have important implications for early proterozoic tectonics and metallogeny

structural setting of the neoarchean terrains in the commonwealth bay area 143 145 e terre adelie craton east antarctica lithological and structural maps from the east commonwealth bay unit ecb terre adelie and georges v land antarctica are presented with a summary of the main structural and metamorphic data obtained in the area the ecb unit is structured during the late archean lower proterozoic at 2 4 2 5 ga with i granulite metamorphism at 9 6 kbar and 850 50 c of lower mid crust and amphibolite metamorphism p 5 kbar t 750 c of the upper crustal levels ii the lower crustal granulites are uplifted and suffered local partial melting and then retrogression into the amphibolite facies at 550 50 c 5 kbar granulites are extruded in the core of a crustal scale anticlinal fold granulites being retrogressed only on the rims of the anticline crustal scale folding along with other structural features results from intense ne sw shortening that prevailed during the late archean orogenic cycle strike slip and extensive motions are only minor in that process iii top to the east thrusting and nappe piling has at least locally occurred under lower amphibolite to greenschist facies conditions finally it seems that iv the paleoproterozoic 1 7 ga structural imprint may have only affected the rims of the archean units the tectonic context observed in the 1 7 ga cape hunter phyllites features mainly e w shortening component and vertical extrusion the eastern mertz and western port martin parts of the archean block were reactivated by localized dextral shearing

ichnological analysis of the cretaceous palaeogene boundary interval at the caravaca section se spain abstract ichnological analysis of the cretaceous palaeogene k pg boundary at caravaca betic cordillera southeast spain reveals significant data for the interpretation of the k pg boundary event the uppermost maastrichtian marls below the cretaceous palaeogene k pg boundary at caravaca contains the dark coloured trace fossils chondrites targionii planolites thalassinoides zoophycos and alcyonidiopsis the composition of trace fossil associations in the upper maastrichtian and lower danian marls is generally the same yet much more poorly preserved in the latter showing in general no ichnological evidence of a severe macroinfaunal crisis at the k pg boundary the dark coloured trace fossils penetrate from two bioturbated horizons in the dark boundary layer separated by two horizons with primary lamination the ichnofauna rapidly recovered after the k pg event following deposition of the lower parallel laminated bed 14 mm thick that occurs in the dark boundary layer above the k pg boundary zoophycos is the deepest trace fossil penetrating as far as 90 cm below the k pg boundary probably in stiff or partly firm substrate which was dewatered to some extent during the slow accumulation of the danian dark boundary layer the firmness probably prevented the chondrites trace maker from deeper burrowing the maastrichtian sediments may be contaminated with danian microfossils due to bioturbation

the first day of the cenozoic highly expanded cretaceous paleogene k pg boundary section from the chicxulub peak ring recovered by international ocean discovery program iodp international continental scientific drilling program icdp expedition 364 provides an unprecedented window into the immediate aftermath of the impact site m0077 includes 130 m of impact melt rock and suevite deposited the first day of the cenozoic covered by

partial collapse of the marine carbon pump after the cretaceous paleogene boundary the impact of an asteroid at the end of the cretaceous caused mass extinctions in the oceans a rapid collapse in surface to deepocean carbon isotope gradients suggests that transfer of organic matter to the deep sea via the biological pump was severely perturbed however this view has been challenged by the survival of deep sea benthic organisms dependent on surface derived food and uncertainties regarding isotopic fractionation in planktic foraminifera used as tracers here we present new stable carbon d 13 c and oxygen d 18 o isotope data measured on carefully selected planktic and benthic foraminifera from an orbitally dated deep sea sequence in the southeast atlantic our approach uniquely combines d 18 o evidence for habitat depth of foraminiferal tracer species with species specific d 13 c eco adjustments and compares isotopic patterns with corresponding benthic assemblage data our results show that changes in ocean circulation and foraminiferal vital effects contribute to but cannot explain all of the observed collapse in surface to deep ocean foraminiferal d 13 c

occurrence and formation of water laid placers a placer is a deposit of residual or detrital mineral grains in which a valuable mineral has been concentrated by a mechanical agent the agent is usually running water and the valuable mineral is usually denser than quartz for example gold diamond cassiterite ilmenite or chromite it is difficult to overemphasize the importance of placers as sources of mineral wealth the witwatersrand south africa paleoplacers discovered in 1886 alone have provided over half of all the gold ever mined in the world pretorius 1976 and alluvial placer deposits of cassiterite in southeast asia presently are the world s major source of tin toh 1978 the earliest evidence of placer mining comes from stone carvings in egypt dated at 2500 b c macdonald 1983 and as early as 7 b c strabo pointed out that in turdetania spain more gold was procured from washing sand in the rivers than by digging in the mines given the economic importance of water laid placers and the long history oftheir exploitation it is remarkable that they have received so little attention in sedimentary research mining and exploration practices for the most part have been guided by empirical rules of thumb that are locally useful but lack any firm physical basis of understanding see bateman

archean blocks and their boundaries in the north china craton lithological geochemical structural and p t path constraints and tectonic evolution an examination of lithological geochemical geochronological structural and metamorphic p t path data suggests that the basement of the north china craton can be divided into eastern and western blocks separated by major crustal boundaries that roughly correspond with the limits of a 300 km wide zone called the trans north china orogen the eastern block consists predominantly of late archean domiform tonalitic trondhjemitic granodioritic ttg batholiths surrounded by anastomosing networks and linear belts of open to tight synforms of minor volcanic and sedimentary rocks metamorphosed from greenschist to granulite facies at 2 5 ga with anticlockwise p t paths some early to middle archean rocks are locally present in the eastern block but their tectonic history is unclear due to reworking by the 2 5 ga tectonothermal event the western block has a late archean assemblage structural style and metamorphic history similar to that of the eastern block but it differs in the absence of early to middle archean assemblages and in being overlain by and interleaved with paleoproterozoic khondalites which were affected by a 1 8 ga metamorphic event involving clockwise p t paths a mantle plume model is proposed for the formation and evolution of late archean basement rocks in the eastern and western blocks based on a combination of extensive exposure of ttg gneisses affinities of mafic rocks to continental tholeiitic basalts presence of voluminous komatiitic rocks dominant diaprism related domiform structures anticlockwise p t paths and a short time span from the primary emplacement of ttg and ultramafic to mafic rocks until the onset of regional metamorphism between the two blocks is the trans north china orogen which is bounded by two major fault systems and is composed of late archean to paleoproterozoic ttg gneisses and granitoids interleaved with abundant sedimentary and volcanic rocks that are geochemically interpreted as having developed in magmatic arc and intra arc basin environments these rocks underwent multiple phases of compressional deformation and peak high pressure metamorphism followed by rapid exhumation during the late paleoproterozoic at 1 8 ga as a result of collision between the eastern and western blocks resulting in the amalgamation of the north china craton

tectonic implications of new shrimp and tims u pb geochronology of rocks from the sask craton peter lake domain and hearne margin trans hudson orogen saskatchewan this study reports new sensitive high resolution ion microprobe shrimp and thermal ionization mass spectrometry tims u pb geochronological data for thirteen rocks from the archean to paleoproterozic sask craton the peter lake domain and hearne margin saskatchewan seven samples from the exposed sask craton in both the glennie domain and pelican window record zircon growth between 2425 and 2525 ma and at 1800 ma older precursor material has been identified most notably a refined crystallization age of 3117 ma for a sample of quartzofeldspatic gneiss q gneiss in the pelican window which also experienced ca 2450 ma zircon growth three samples from the peter lake domain and a sample from the linn island inlier of the wollaston domain yield ages of 2575 ma one sample from the peter lake domain is distinctly older yielding an age of 2640 ma but also displays zircon growth at 2575 ma the simplest interpretation of these data is that the peter lake domain is likely a part of the hearne margin

refined proterozoic evolution of the gawler craton south australia through u pb zircon geochronology abstract through the application of both conventional u pb zircon analyses and small sample u pb isotopic analyses the nature and timing of tectonic events leading to the formation of the gawler craton have been defined more precisely constraints on deposition of early proterozoic iron formation bearing sediments have been narrowed down to the period 1960 1847 ma deformed acid volcanics including the economically important moonta porphyry have zircon ages of 1790 and 1740 ma the voluminous acid gawler range volcanics and correlatives to the east were erupted over a short interval at 1592 2 ma and were intruded by anorogenic granites at 1575 ma small sample zircon analyses proved to be an extremely valuable adjunct to conventional analyses generally yielding more concordant data which forced a curved discordia through an upper intercept slightly younger than from a conventional straight line discordia

sedimentary provenance and age of metamorphism of the vestfold hills east antarctica evidence for a piece of chinese antarctica abstract the vestfold hills terrane in east antarctica is a granulite facies terrane that preserves a record of sedimentation magmatism and metamorphism that differs significantly from the other archaean terranes exposed in east antarctica sensitive high resolution ion mass spectrometry shrimp u pb isotopic data from detrital zircon cores metamorphic rims and metamorphic zircon and monazite from the granulite facies chelnok and taynaya paragneisses that demonstrate archaean ages that are incompatible with the established ages of other east antarctic source regions the original sediments to the chelnok and taynaya paragneisses were deposited between c 2575 and c 2520 ma and underwent a protracted period of high grade granulite facies metamorphism from c 2520 to c 2450 ma that is synchronous with the emplacement of the tonalitic mossel gneiss and the granitic to gabbroic rocks of the crooked lake gneiss group the distinctive detrital zircon age populations and the period metamorphism recorded by the vestfold hills paragneisses lead us to speculate that 1 these rocks were sourced from and deposited on the margin of the crystalline basement to the north china craton at the end of the archaean and 2 they underwent a period of high grade metamorphism related to orogenesis driven by the collision of the north china craton with the archaean nucleus of south india during the late archaean to early paleoproterozoic

the hydraulic equivalence of mica abstract settling experiments performed on silt to fine sand sized mica flakes with a holographic micro velocimeter revealed that mica is the hydraulic equivalent of quartz spheres having diameters a factor of 4 to 12 times smaller mica in the very fine to fine sand sizes has been traditionally used by sedimentologists to delineate areas of deposition or nondeposition and potential winnowing of fines and is here found to be the hydraulic equivalent of silt sized particles but not of clay experiments also showed that mica flakes tend to settle at orientations which are neither perpendicular nor parallel to the gravitational vector and tend to generally maintain their orientation throughout equations for the settling of a disc in lerman and others 1974 and that developed by komar and reimers 1978 are shown to be mathematically similar for the coarse silt to fine sand ranges of discs and are adequate predictors of settling rates of mica flakes a comparison of the hydraulic equivalency of quartz spheres to coarse silt through fine sand sized mica flakes is presented

magmatic zircon lu hf isotopic record of juvenile addition and crustal reworking in the gawler craton australia abstract new in situ zircon lu hf isotopic data are presented from magmatic rocks distributed across the gawler craton australia these rocks range in composition from granite to gabbro with the majority being granite or granodiorite and moderately peraluminous in composition the new lu hf isotopic data together with previously published data provide insight into the magmatic evolution of the craton and crust and mantle interaction through time increased juvenile content of magmatic rocks correlate with periods of extensional tectonism in particular basin formation and associated magmatism during the neoarchean to earliest paleoproterozoic c 2555 2480 ma middle paleoproterozoic c 2020 1710 ma and late paleoproterozoic c 1630 160 ma in contrast magmatic rocks associated with periods of orogenic activity show greater proportions of crustal derivation particularly the magmatic rocks generated during the c 1730 1690 ma kimban orogeny the final two major magmatic events of the gawler craton at c 1630 1604 ma and c 1595 1575 ma both represent periods of juvenile input into the gawler craton with e hf t values extending to as positive as 8 however widespread crustal melting at this time is also indicated by the presence of more evolved e hf t values to 6 5 the mixing between crust and mantle sources during these two youngest magmatic events is also indicated by the range in two stage depleted mantle model ages t dm c between 1 76 ga and 2 51 ga significant mantle input into the crust particularly during formation of the c 1595 1575 ma hiltaba suite and gawler range volcanics likely facilitated the widespread crustal magmatism of this time period viewed spatially average e hf t and t dm c values highlight three of the major shear zones within the gawler craton as potentially being isotopic as well as structural boundaries differences in isotopic composition across the coorabbie shear zone in the western gawler craton the middle bore fault in the northern gawler craton and to a lesser extent the kalinjala shear zone in the southern gawler craton broadly correspond to crustal and even lithospheric scale discontinuities evident in geophysical studies therefore these shear zones may approximate some of the first order crustal domains within the gawler craton

the late archean record a puzzle in ca 35 pieces abstract the global archean record preserves ca 35 large cratonic fragments and a less well defined number of smaller slivers most archean cratons display rifted margins of proterozoic age and therefore are mere fragments of supercratons which are defined herein as large ancestral landmasses of archean age with a stabilized core that on break up spawned several independently drifting cratons the tectonic evolution of individual archean cratons such as the slave craton of north america or the kaapvaal craton of southern africa should therefore always be considered in the context of their ancestral supercratons this is particularly true for many of the smaller cratons which are too limited in size to preserve the complete tectonic systems that led to their formation these limitations not only apply to the crustal geology of archean cratons but also to their underlying lithospheric mantle keels if these keels are archean in age as their broad correlation with ancient surface rocks suggests they also are rifted and drifted remains of larger keels that initially formed below ancestral supercratons the study of archean cratons and their lithospheric keels should thus be global in scope in the search for late archean supercratons a craton like the slave with three to four rifted margins has a ca 10 maximum probability of correlating with any of the ca 35 remaining cratons around the globe assuming wholesale recycling of archean cratons has been limited since ca 2 0 ga alternatively if the original number of independently drifting cratons was significantly larger the probability of successful correlations is much less due to repeated cycles of break up and plate tectonic dispersal since ca 2 0 ga the probability of correlation is probably independent of present day proximity unless there is independent evidence that two neighbouring cratons were only separated by a narrow ocean nevertheless most previously proposed craton correlations rely erroneously on present day proximity implicitly extrapolating relatively recent paleogeography back to the archean considering the fundamental differences between some of the better known cratons such as the slave superior and kaapvaal it seems likely that these cratons originated from independent supercratons sclavia superia vaalbara with distinct amalgamation and break up histories this is contrary to widely held opinion based largely on an idealized view of the supercontinent cycle that all cratons shared a common history in a single late archean supercontinent in contrast to the superior and kaapvaal cratons which are radically different from the slave craton and each other the dharwar craton of peninsular india the zimbabwe craton of southern africa and the wyoming craton of north america all show significant similarities to the slave craton it thus seems likely that at least some of these were nearest neighbours to the slave craton in the ca 2 6 2 2 ga sclavia supercraton

two neoarchean supercontinents revisited the case for a rae family of cratons abstract a variety of early paleoproterozoic supercontinents or supercratons have been proposed including kenorland superia and vaalbara improved geologic knowledge of the churchill province canadian shield suggests that the classical kenorland configuration with linked rae and hearne cratons is invalid and represents a younger ca 1 9 ga nuna configuration we review the early paleoproterozoic tectonic history of the rae craton and propose that the ca 2 55 2 50 ga macquoid and 2 50 2 28 ga arrowsmith orogens represent critical piercing points for supercontinent reconstruction utilizing the stratdb and dateview databases we have identified nearly two dozen regions worldwide that show effects of these orogenic events including the rae gawler mawson peninsular india west africa amazonia north china and sask crustal blocks we propose that these cratonic blocks form a distinct rae family lineage utilizing new time space plots for the rae family and components of the superia and vaalbara supercratons we demonstrate that the rae family lacks the distinctive early paleoproterozoic depositional and glaciogenic record characteristic of superia and vaalbara but has an important contemporaneous record of juvenile crust formation and contractional orogenesis we propose the collective history of the rae family is best reconciled if it represents the now dispersed blocks of a third earliest paleoproterozoic supercraton that we term nunavutia in this model the magmatic and tectonometamorphic record of these cratons includes an early 2 55 2 45 ga period of interior collisional orogenesis in which the supercraton amalgamated and a later phase of long lived peripheral orogenesis 2 45 2 28 ga prior to its partial break up at ca 2 05 ga forming within an otherwise globally quiescent period the history of the rae family is important for assessment of constrasting models of early paleoproterozoic geodynamic activity

hydraulic sorting of heavy mineral grains by swash on a medium sand beach abstract hydraulic sorting of detrital mineral grains in the swash zone was investigated using data on sedimentology and flow dynamics obtained from fishermans beach on the east coast of australia the beach is characteristically reflective displaying a steep beach face composed of medium sand and virtually no surf zone samples were taken from two beds enriched in heavy minerals and from the adjacent beds above and below the mineralogy of the samples was determined and measurements of grain size and settling velocity were obtained the heavy mineral grains had both a smaller intermediate diameter and a smaller settling velocity than the light mineral grains the difference between the two mineral types was typically in the range 0 25 0 50 and 2 0 2 5 cm s 1 respectively on the basis of these sedimentological measurements alone any one of three mineral sorting mechanisms could provide a feasible explanation for the development of the heavy mineral enriched beds suspension sorting entrainment sorting or shear sorting a numerical model for swash based on the nonlinear shallow water wave theory is used to provide a quantitative description of flow dynamics in the swash zone at the time the heavy mineral enriched beds were formed for the probable wave height responsible for producing the enriched beds modeled maximum flow velocity and bed shear stress at the mid swash position reached 3 5 m s 1 and 79 51 dynes cm 2 respectively given the shallow water depths and the lack of sorting in the horizontal direction of flow suspension sorting does not seem to be of overriding importance at fishermans beach the large bed shear stresses seem to also preclude entrainment sorting because for most of the time all mineral types are predicted to be in motion this contrasts with previous studies on finer grained beaches where entrainment sorting was favored and suggests that beach type may play a role in determining the sorting mechanism operating at a particular location shear sorting was found to be feasible within the modeled flow constraints at fishermans beach but it does not provide an entirely satisfactory explanation for the observed mineral sorting patterns we contend that the inability to unequivocally identify the true sorting mechanism is more a reflection of our poor understanding of the operation of these sorting mechanisms particularly within the swash zone rather than an indication that any of these mechanisms are physically inoperative in the swash zone

contrasting styles of phanerozoic and precambrian continental collision abstract there are differences in the style of collisional orogens between the phanerozoic and the precambrian most notably the appearance of blueschists and ultrahigh pressure metamorphic uhpm rocks in the geological record since the late neoproterozoic whereas these rocks are absent from older orogens understanding collisional orogenesis in the context of present day values for ambient upper mantle temperature and radiogenic heat production provides a reference from which to extrapolate back to conditions in the precambrian to evaluate differences in the way phanerozoic and precambrian collisional orogens develop a series of experiments was run using a 2 d petrological thermomechanical numerical model in which the collision of spontaneously moving continental plates was simulated for values of ambient upper mantle temperature and radiogenic heat production increasing from those appropriate to the present day thus models of modern collisional orogens involving different modes of exhumation of uhpm rocks were extrapolated back to conditions appropriate for the precambrian based on these experiments an increase of the ambient upper mantle temperature to 80 100 k above the present day value leads to two distinct modes of collision that are different from the modern collision regime and for which the terms truncated hot collision regime strong mafic lower continental crust and two sided hot collision regime weak felsic lower continental crust are proposed some proterozoic orogens record post extension thickening to generate counter clockwise metamorphic p t paths followed by slow close to isobaric retrograde cooling such as occurred in the paleoproterozoic khondalite belt in the north china craton and the late mesoproterozoic early neoproterozoic eastern ghats province part of the eastern ghats belt of peninsular india these orogens have similarities with the truncated hot collision regime in the numerical models assuming subsequent shortening and thickening of the resulting hot lithosphere other proterozoic orogens are characterized by clockwise looping metamorphic p t paths and extensive granite magmatism derived from diverse crustal and subcontinental lithospheric mantle sources these orogens have similarities with the two sided hot collision regime in the numerical models both regimes are associated with shallow slab breakoff that precludes the formation of uhpm rocks the temperature of the ambient upper mantle where this transition in geodynamic regimes occurs corresponds broadly to the neoproterozoic era

geochronology and tectonic evolution of karimnagar and bhopalpatnam granulite belts central india abstract the karimnagar granulite belt kgb and the bhopalpatnam granulite belt bgb occur along both flanks of the pranhita godavari pg rift basin we present a state of the art overview on the geochronological and tectonic aspects of these belts and surrounding geologic domains and report new age data on zircon monazite and uraninite recovered from granulite facies assemblages from kgb and bgb based on electron microprobe analyses epma zircons from kgb charnockites show core ages of up to 3 1 ga mantled by rims of 2 6 ga zircons from bgb have 1 9 ga cores mantled by 1 7 ga rims zircons with core ages of 1 6 to 1 7 ga in bgb rocks suggest new growth at this time monazites and uranitite from kgb show clear peaks with well defined ages in the narrow range between 2 42 0 08 ga and 2 47 0 03 ga rims of monazite show mean age of 2 21 0 08 ga monazites from bgb define sharp linear trend in pbo vs tho2 diagram delineating a clear isochron with age of 1 59 0 03 ga age data from kgb and bgb presented in this report negate current models linking these terrains to godavari granulite belt and considering them as single and contemporaneous entity the mid archaean to early palaeoproterozoic signature recognized from kgb is totally missing in bgb on the other hand kgb rocks do not record any evidence for major mesoproterozoic thermal regime the two granulite belts shouldering the pg rift basin have therefore evolved in different times under distinct p t conditions and thermal regimes our results have important implications in evaluating models of supercontinent assemblies particularly the older assemblies of ur columbia and rodinia while tectonothermal events in kgb broadly match with those of east dharwar we propose that bgb represents a 1 6 ga collisional mobile belt between the bastar and the dharwar cratons the 1 6 ga collisional mobile belt at the southern margin of the bastar craton was superposed by rift activity along the pg basin at 1 5 ga this sequence of events goes against the existence of a 3 0 ga old contiguous assembly of ur but closely matches with the history of accretion and break up of the columbia further parts of the pg basin located away from the influence of the eastern ghats mobile belt neither recorded any grenville ages 1 0 ga corresponding to the rodinia accretion nor late pan african ages ca 550 ma relating to the gondwana amalgamation indicating that the region did not witness any of these younger tectonic events

the arrowsmith orogeny geochronological and thermobarometric constraints on its extent and tectonic setting in the rae craton with implications for pre nuna supercontinent reconstruction abstract geochronological and thermobarometric data for two widely spaced and previously unstudied areas of the rae craton canada provide important temporal and spatial links between previously investigated areas in the northern and southern rae craton that had been interpreted to have been reworked during the ca 2 5 2 3 ga arrowsmith orogeny on southern boothia peninsula upper amphibolite to granulite facies metamorphism dated by in situ shrimp u pb analysis of monazite occurred at ca 2 64 2 60 2 54 2 50 2 49 2 44 and 2 37 2 34 ga with retrograde monazite growth at 2 27 ga shrimp u pb ages of zircon in two metasedimentary rocks provide a further indication of metamorphism at ca 2 54 and 2 50 ga which also corresponds to the age determined for a k feldspar augen granite textural relationships indicate that the regional foliation formed between ca 2 54 ga and 2 50 2 49 ga peak metamorphic conditions of 700 c and 5 6 kbar are interpreted to have been reached during the 2 37 2 34 ga event in the mccann mcarthur lake region of the southwestern rae craton upper amphibolite to granulite facies metamorphism is dated by in situ shrimp monazite ages of ca 2 53 2 47 and 2 37 2 28 ga textural relationships indicate deformation initiated by ca 2 37 ga and terminated by ca 2 30 ga metamorphic conditions of 5 kbar and 700 735 c were quenched at ca 2 3 ga following peak conditions of 7 kbar and 800 c the evidence for regional deformation at 2 54 2 49 ga on southern boothia peninsula supports an andean type accretionary margin setting for the arrowsmith orogeny a collisional event may have terminated high grade metamorphism in the northern region by ca 2 34 ga while metamorphism associated with continental arc magmatism continued in the south until at least ca 2 28 ga these combined data demonstrate the continuity and broad extent of the arrowsmith orogen along the western flank of the rae craton extending 1500 km from northern saskatchewan to northern baffin island the broad extent of this globally rare orogenic event provides an important piercing point for supercontinent reconstructions which are discussed in a companion paper the inferred convergent tectonic activity between ca 2 54 and 2 28 ga is interpreted to have occurred on the flank of a growing supercontinent a setting that could account for an apparent slowdown but not shutdown in global tectonics

tectonic framework and evolution of the gawler craton southern australia the gawler craton is the major crustal province in the southern australian proterozoic and is pivotal in models seeking to describe the evolution of proterozoic australia the craton is host to the olympic dam iron oxide cu au u ree deposit as well as a number of other iron oxide copper gold iocg au and iron ore deposits the evolution of the gawler craton is dominated by two major phases of tectonic activity late archean and late paleoproterozoic early mesoproterozoic in total spanning ca 1 billion years the late archean 2560 2500 ma basin development was coeval with arclike felsic magmatism and mafic ultramafic magmas including komatiites collisional deformation between 2480 and 2420 ma led to ca 400 m y of tectonic quiescence conceivably within an early paleoproterozoic continental interior the second major phase of tectonic activity was in the middle to late paleoproterozoic and early mesoproterozoic 2000 1450 ma paleoproterozoic tectonism was initially dominated by rift related events which produced a series of basins over the interval ca 1900 to 1730 ma some of which contain significant iron ore reserves transient contractional deformation at ca 1850 ma led to batholith scale granitic magmatism in part derived from melting of the late archean basement major basin development was finally terminated by the 1730 to 1690 ma kimban orogeny the effects of which are widespread and include the formation of crustal scale shear systems granitic magmatism and low to high grade metamorphism the kimban orogeny was followed by a renewed period of extension between ca 1680 and 1640 ma leading to local magmatism and sedimentation which appears in part to have been coeval with the high grade ca 1650 ma ooldean event in the western gawler craton at ca 1620 to 1615 ma the generation of the arc related st peter suite in the southern gawler craton implies that craton components were located at an active plate margin the arc environment was superseded by a transition to a continental interior regime which coincided with the voluminous gawler range volcanics 1595 1590 ma and hiltaba suite granitoids 1595 1575 ma the hiltaba suite is dominated by high t fractionated felsic rocks with coeval mafic magmatism confirming a mantle involvement emplacement of the hiltaba suite coincided with the major mineralizing interval in the gawler craton regionally two distinct mineral systems have been recognized the olympic iocg province in the eastern part of the craton and the gold dominated systems within the central gawler gold province the spatial distribution of iocg versus au dominated mineral systems appears to reflect regional variations in crustal composition and hiltaba suite petrogeneses hiltaba aged granites in the olympic iocg province are isotopically more evolved richer in u and th and oxidized compared to similar aged granites in the au dominated province modern day heat flow in the iocg province is significantly higher 90 10 mwm 2 compared to the au dominated province 54 5 mwm 2 suggesting there are important lithospheric compositional differences between the two metallogenic provinces which may reflect an older phase of craton assembly widespread northwest southeast contractional deformation coeval with the emplacement of the hiltaba suite is expressed by the formation and or reactivation of shear zones ranging up to crustal scale within this regime northwest trending structures such as those in the vicinity of the olympic dam deposit are likely to have accommodated dilation associated with strike slip movements intersection between these structures and northeast trending contractional faults may have formed suitable structural traps for 1590 to 1580 ma mineralization the timing of syn hiltaba deformation overlaps with transpression and medium to high grade metamorphism associated with the ca 1570 to 1540 ma kararan orogeny which dominates the geophysical architecture of the central northern part of the craton the youngest phase of deformation in the craton is expressed by reactivation of shear zones between ca 1470 and 1450 ma and regional cooling

provenance of eastern magallanes foreland basin sediments heavy mineral analysis reveals paleogene tectonic unroofing of the fuegian andes hinterland abstract documenting variations in the composition of foreland basin detrital sediments is a useful tool for reconstructing the kinematic history of associated orogenic systems herein we report the results of our sem eds heavy mineral analysis of the late cretaceous to early miocene eastern magallanes basin of southernmost argentina our data indicate that campanian to middle eocene sediments had a mafic ophiolitic provenance which we interpret as being derived from the patagonian fuegian magmatic arc and the mafic floor of the preceding rocas verdes marginal basin upper middle eocene to lower miocene heavy minerals on the other hand indicate a metamorphic metasedimentary provenance which we interpret as being derived from the cordillera darwin metamorphic complex together these data indicate an abrupt shift in sediment provenance in middle to late eocene time thereby corroborating recent interpretations of the basin s detrital zircon geochrononology and providing further support for temporal and possibly genetic relationships between development of the patagonian orocline the opening of drake passage and the oi 1 glaciation of antarctica

evolution and architecture of a large felsic igneous province in western laurentia the 1 6 ga gawler range volcanics south australia the gawler range volcanics grv of south australia are an intracontinental subaerial large igneous province lip formed during supercontinent assembly in the mesoproterozoic like many lips the grv are intraplate were erupted over a geologically short time 2 million years and are dominated by voluminous lavas however the grv are overwhelmingly dominated by felsic units the grv are essentially undeformed and have not been deeply buried so that their original textures are exceptionally well preserved furthermore they are well exposed in very gently northward and eastward dipping sections up to 4 km thick allowing assessment of the facies architecture and evolution of this felsic volcanic lip the evolution of the grv can be clearly separated into two main stages initial eruptions at numerous volcanic centres produced small to moderate volume geochemically distinct felsic lavas and lava domes together with ignimbrites and minor mafic and intermediate lavas forming a sequence 0 5 to 3 km thick volcanic activity in this lower sequence varied from effusive to explosive and was not much different in style or products from phanerozoic felsic volcanic provinces however the second stage produced at least three voluminous felsic units each of which represents about 1000 3000 km3 of magma moreover the dominance of evenly porphyritic textures and lack of pyroclastic textures pumice shards broken crystal lithic clasts in these units suggests that they were erupted effusively and flowed as lavas each of these felsic lavas are generally dominated by a single uniform composition and commonly mingled with a subordinate and compositionally distinct lava

protolith heterogeneity as a factor controlling the feedback between deformation metamorphism and melting in a granulite hosted gold deposit abstract structures and alteration associated with mineralizing pre anatectic hydrothermal alteration at the challenger gold mine in south australia strongly influenced and preferentially focused subsequent metamorphism partial melting and deformation this event resulted in efficient deformation enhanced melt extraction from the host rocks immediately adjacent to mineralization leaving restitic garnet cordierite k feldspar rich mineral assemblages progressive reintegration of silicate melt into the highly restitic proximal gneiss bulk composition using mineral equilibrium modelling in the system nckfmashto indicates that hydrothermal alteration stabilized a higher proportion of hydrous minerals leading to increased fertility and thus melting in the proximal gneiss further modelling at lower temperatures provides an insight into mineral assemblages that may have been associated with mineralization temperature composition modelling provides evidence that composite gold sulphide melts were armoured by quartz and as such were immobile during peak metamorphism at challenger melt permeability of the mineralized zones was enhanced via the formation of a structural fabric folding boudinage fracturing and the formation of shear zones giving consideration to the relationships and orientations of these structures a model for preferential deformation and melt migration within the mineralized shear zone under transpression is developed where melt is essentially pumped out of the system parallel to the maximum principal elongation direction

authigenic monazite and detrital zircon dating from the proterozoic rocky cape group tasmania links to the belt purcell supergroup north america abstract the oldest known rocks in tasmania occur in the proterozoic rocky cape group a 10 km thick quartzarenite siltstone pelite dominated succession previously constrained to have been deposited between 1450 ma and 750 ma the rocky cape group contains the enigmatic fossil horodyskia string of beads and has the potential to place tasmania within supercontinent reconstructions detrital zircon and authigenic monazite grains dated via u pb laser ablation inductively coupled plasma mass spectrometry la icpms analysis yield a deposition window between c 1450 ma youngest zircon populations and c 1330 ma oldest authigenic monazite population for the 9 km thick lower middle units pedder river siltstone lagoon river quartzite balfour subgroup which hosts horodyskia detention subgroup the upper units 1 km include the irby siltstone which is younger than c 1310 ma this unit is likely separated from both the lower middle units and the overlying c 1900 ma grains were identified authigenic monazite age distributions are complex with multiple age domains within most samples the common pb corrected 206u 238pb ages defined by oldest grains in each sample identify three statistically significant groups 1 1358 1292 ma inclusive of two sigma errors lagoon river quartzite and pedder river siltstone 2 1283 1239 ma cowrie siltstone and balfour subgroup and 3 1085 9 ma detention subgroup we suggest monazite was precipitated during episodic fluid flow events at these three stages in various parts of the basin the original source for ree bearing fluids could be detrital monazite which is rarely preserved and or organic matter from the interbedded carbonaceous shales the rocky cape group has a shared provenance with the higher grade metasediments surprise bay and fraser formations of nearby king island the newly derived depositional ages also overlap and support the correlation of these rock associations on the basis of current datasets there are no obvious correlations that can be made with mesoproterozoic basins preserved in mainland australia instead an overlap in the timing of deposition similarities in detrital zircon signatures and analogous depositional environment suggests the c 1 45 1 37 ga upper belt purcell supergroup missoula and lemhi groups of western north america constitutes a plausible correlation with the tasmanian mesoproterozoic succession if the unexposed palaeoproterozoic basement of tasmania correlates with the transantarctic mountains region of east antarctica as previously proposed we suggest that the overlying mesoproterozoic sequences were deposited during rifting of the supercontinent nuna between proto australia including the mawson craton of antarctica and laurentia as predicted by the most recent palaeogeographic reconstructions both the tasmanian and western laurentian packages were affected by episodic post depositional fluid flow events between c 1 35 and 1 05 ga possible thermotectonic imprints of the subsequent assembly of rodinia

superimposed neoarchaean and paleoproterozoic tectonics in the terre adelie craton east antarctica evidence from th u pb ages on monazite and 40ar 39ar ages in order to understand the tectonic behaviour of a stabilized neoarchaean continental crust during subsequent tectonic activity we investigated the compositemetamorphic basement along the terre adelie and george vth land coastline also known as the terre adelie craton east antarctica two domains are recognized 1 a neoarchaean basement composed of granulite rocks to the east and overlain to thewest by amphibolites and 2 two paleoproterozoic detrital basins overly theneoarchaean crust and extend further west newgeochronological data fromthe terre adelie craton define a tectonic evolution with threemajor peaks activity th u pb electron probe analyses of monazite from the neoarchaean granulites constrain the main structuration event at ca 2 45 ga in agreement with zircon ages from throughout the neoarchaean domain local resetting together with low temperature recrystallization of monazites occurred at ca 1 7ga along hydrated anastomozing metre scale shear zones new 40ar 39ar ages obtained by stepwise heating techniques on amphibole biotite and muscovite from both the neoarchaean basement and the paleoproterozoic basins illustrate the differential evolutions of basement and its sedimentary cover during the major 1 7ga transpressive event a final event at ca 1 55 1 50 ga is only recognized close to the mertz shear zone 145 e bounding the terre adelie craton to the east the new data allow us to propose detailed geological pictures of the terre adelie craton geodynamic evolution from the neoarchaean to the mesoproterozoic