multiple foliation development during thrusting and synchronous formation of vertical shear zones abstract three orthogonal foliations developed during large scale n s thrusting in the central portion of the mount isa block australia two of these foliations which are well preserved in their original orientations in porphyroblasts of pelitic schists but considerably affected and commonly destroyed in the schist matrix due to the effects of subsequent deformation and metamorphism formed synchronously and have a common stretching lineation which is near horizontal and trends n s one foliation formed near horizontal and the other near vertical striking n s both developed during n s directed thrusting with the n s vertical foliation accommodating sinistral differential displacement between the thrust belt to the west and that to the east the third foliation which dips steeply to the north and strikes e w formed during n s shortening of the thrust belt and tends to overprint the other foliations formed in the same orogeny the n s vertical foliation is well preserved within a narrow belt of gneissic rocks called the wonga duchess belt that also locally contains the subsequently folded remains of the foliation that formed near horizontal this belt separates multiply thrust repeated beds to the west from large scale apparently less repetitive thrusts to the east the direction of thrusting appears to change across this belt from n to s on the west to s to n on the east the differential deformation to either side of the belt was taken up by sinistral shear along this belt numerous related but smaller scale vertical shear zones occur up to 3 km away to either side of this narrow zone the primary vertical foliation within the wonga duchess belt appears to have formed during transform like faulting by progressive inhomogeneous simple shear with no component of orthogonal shortening

the timing of the cape orogeny new 40ar 39ar age constraints on deformation and cooling of the cape fold belt south africa abstract we conducted a 40ar 39ar thermochronological study of samples from three transects across the cape fold belt of south africa metamorphic conditions along these transects decrease from the greenschist facies in the southern hinterland to the anchizonal foreland to the north nine new 40ar 39ar plateau ages and two mini plateau ages from handpicked muscovite are combined with one earlier published biotite mineral age and reveal two age modes the older population ranges from 261 3 to 276 5 ma n 4 and the younger from 248 2 to 254 6 2 1 ma n 4 the remaining three ages 400 ma are detrital and reflect unreset ages electron microprobe analyses and x ray diffraction studies indicate little mineralogical variation of micaceous material across the belt samples dominantly host muscovite clay minerals such as illite or smectite are absent microstructural characterization of rock fabrics across the belt reveals different mechanisms of strain partitioning and heterogenous deformation of different lithologies specifically pressure solution fracture and limited crystal plasticity are the dominant deformation mechanisms in the shale dominated bokkeveld group and witteberg groups in the foreland region resulting in recrystallization of phyllosilicate minerals strain partitioning into narrow locally deformed horizons of the table mountain group is accompanied by dynamic grain boundary recrystallization in quartz a weak trend of southward younging ages is interpreted as the preservation of deformation ages ca 275 260 ma and younger cooling ages ca 255 245 ma although it remains plausible that these populations represent two deformation events these new age constraints significantly narrow the temporal window of cape fold belt tectonic activity and provide a firm basis for evaluating the depositional patterns in the foreland karoo basin

1600 1500 ma hotspot track in eastern australia implications for mesoproterozoic continental reconstructions mesoproterozoic a type magmatic rocks in the gawler craton curnamona province and eastern mount isa inlier form a palaeo curvilinear belt for reconstructed plate orientations the oldest igneous rocks in the gawler craton are the hiltaba granite suite c 1600 1575 ma the youngest in the mount isa inlier are the williams naraku batholiths c 1545 1500 ma the belt is interpreted as a segment of a hotspot track that evolved between c 1600 and 1500 ma this hotspot track may define a quasilinear part of australia s motion between 1636 and 1500 ma and suggests that australia drifted to high latitudes an implication of this interpretation is that australia and laurentia may not have been fellow travellers leading to the formation of rodinia a hotspot model for a type magmatism in australia differs from geodynamic models for this style of magmatism on other continents this suggests that multiple geologic processes may be responsible for the genesis of proterozoic a type magmas

the 1800 1610 ma stratigraphic and magmatic history of the eastern succession mount isa inlier and correlations with adjacent paleoproterozoic terranes the chronostratigraphic and magmatic framework for the proterozoic mount isa inlier northwest queensland has been substantially revised to match insights provided by isotopic age constraints accumulated over recent years this material and existing chronometric constraints is used to construct a new time space plot for the inlier and to compare eastern succession volcanosedimentary accumulations with temporally equivalent units in the western fold belt the accumulation of volcanosedimentary sequences in the eastern succession occurred in four main periods namely 1890 1850 ma 1800 1750 ma 1710 1690 ma and 1680 1610 ma temporally equivalent units can be identified in the western fold belt periods of sedimentary accumulation were immediately preceded by bimodal igneous activity across the inlier the locus of bimodal volcanism migrates from west to east with time intracontinental rifting is the most likely tectonic setting for the sedimentary basins the majority of the soldiers cap group was deposited between ca 1675 and 1650 ma with the exception of units in the boomarra horst now known to belong to older sequences deposited ca 1775 1750 ma and the llewellyn creek formation at snake creek which may be as old as ca 1690 ma upper amphibolite facies rock units in the southwest e g the gandry dam gneiss are likely to be higher metamorphic grade temporal equivalents of the soldiers cap group five rock units previously assigned to the cover sequence 2 mary kathleen group namely the answer slate kuridala formation staveley formation marimo slate and agate downs siltstone are now known to have been deposited between ca 1675 and 1610 ma these units bar the kuridala formation have been removed from the cover sequence 2 mary kathleen group and given the informal name young australia group the kuridala formation has been included in the soldiers cap group the rock units within the young australia group are likely to be lateral facies equivalents of the younger units within the soldiers cap group rock units within the mount albert group although undated are considered likely to be temporal equivalents of the soldiers cap group and the young australia group the roxmere quartzite is included here in the young australia group that it tectonically overlays the ballara quartzite and mitakoodi quartzite represent a single regionally extensive sequence deposited at ca 1755 ma the double crossing metamorphics previously mapped as basement are likely to correlate with the argylla formation and mitakoodi quartzite most of the eastern succession to the west of the pilgrim fault is likely to have been a topographic high between ca 1780 and 1755 ma prior to the ballara quartzite unconformably overlying the ca 1780 ma argylla formation detrital zircons of argylla and wonga ages indicate that the mary kathleen belt is likely to have been a topographic high at least periodically during accumulation of the soldiers cap and young australia groups soldiers cap group units in the southeast contain numerous detrital zircons giving ages between ca 1730 and 1710 indicating a possible siliciclastic source to the east the host rocks to the pb zn ag deposits are restricted to cover sequence 3 units deposited between ca 1680 and 1590 ma and with the exception of century young from the southeast towards the northwest cu au deposits and prospects are hosted in units from all the major periods of volcanosedimentary accumulation however many of the most significant deposits are found within the younger sequences

vertical extrusion and horizontal channel flow of orogenic lower crust key exhumation mechanisms in large hot orogens a large database of structural geochronological and petrological data combined with a bouguer anomaly map is used to develop a two stage exhumation model of deep seated rocks in the eastern sector of the variscan belt an early sub vertical fabric developed in the orogenic lower and middle crust during intracrustal folding followed by the vertical extrusion of the lower crustal rocks these events were responsible for exhumation of the orogenic lower crust from depths equivalent to 18 20 kbar to depths equivalent to 8 10 kbar and for coeval burial of upper crustal rocks to depths equivalent to 8 9 kbar following the folding and vertical extrusion event sub horizontal fabrics developed at medium to low pressure in the orogenic lower and middle crust during vertical shortening fabrics that record the early vertical extrusion originated between 350 and 340 ma during building of an orogenic root in response to se directed saxothuringian continental subduction fabrics that record the later sub horizontal exhumation event relate to an eastern promontory of the brunia continent indenting into the rheologically weaker rocks of the orogenic root indentation initiated thrusting or flow of the orogenic crust over the brunia continent in a north directed sub horizontal channel this sub horizontal flow operated between 330 and 325 ma and was responsible for a heterogeneous mixing of blocks and boudins of lower and middle crustal rocks and for their progressive thermal re equilibration the erosion depth as well as the degree of reworking decreases from south to north pointing to an outflow of lower crustal material to the surface which was subsequently eroded and deposited in a foreland basin indentation by the brunia continental promontory was highly noncoaxial with respect to the se oriented saxothuringian continental subduction in the early visean suggesting a major switch of plate configuration during the middle to late visean

anhydrite solubility in differentiated arc magmas abstract the solubility of anhydrite in differentiated arc magmas was experimentally studied at 200 mpa and 800 1000 c over a range of oxygen fugacities from 0 5 log units above the ni nio buffer to the hematite magnetite buffer anhydrite is stable only at oxidizing conditions fo2 re reo2 whereas sulfides only form under reducing conditions the solubility of anhydrite in the melt ultimately regulates the amount of sulfur available to partition between melt and fluid phase during the eruption at oxidizing conditions the solubility product of anhydrite increases with temperature nbo t and melt water content we provide a new calibration of the anhydrite solubility product ksp xcao xso3 which reproduces all available experimental data with greatly improved accuracy ln k sp anhydrite 8 95 146 5 nbo t 2 696 10 4 t k 19 72 nbo t 10 4 t k 0 409 h 2 o wt in this equation the molar fractions xcao and xso3 in the melt as well as the number of non bridging oxygen atoms per tetrahedron nbo t are calculated on an anhydrous basis h2o refers to the melt water content t is temperature in kelvin we apply our model to estimate the sulfur yield of some recent volcanic eruptions and we show that the sulfur yield of the 1991 mt pinatubo dacite eruption was unusually large because only a small fraction of the sulfur was locked up in anhydrite in general high sulfur yields are expected when anhydrite solubility in the melt is high i e for somewhat depolymerized melts for rhyolitic systems most of the available sulfur will be locked up in anhydrite so that even very large eruptions may only have a small effect on global surface temperatures our model therefore allows improved predictions of the environmental impact of explosive volcanic eruptions

the role of ductility contrast and basement architecture in the structural evolution of the crystal creek block mount isa inlier nw queensland australia abstract the leichhardt river fault trough of the mount isa inlier comprises fault blocks which exhibit different local deformational histories and structural orientations many structural patterns in the stratigraphically highest cover sequences do not occur in adjacent fault blocks that expose older and mechanically stronger rocks within the mount isa group sediments of the crystal creek block for example tight e trending upright folds appear to be at odds with the surrounding north south regional trends in this paper it is proposed that an inherited extensional fault geometry played a significant role in controlling the structural patterns developed during later shortening through the integration of detailed surface mapping and forward magnetic modelling an inverted half graben was identified beneath the crystal creek block exposing the underlying origin of local structural complexities structural inversion and buttressing against pre existing faults simultaneously removed extensional displacement across pre rift rocks and induced folds and faults in overlying sag phase sediments this led to the development of two structural levels characterized by different deformational styles and apparently distinct deformational histories

a shortened intraplate rift system in the proterozoic mount isa terrane nw queensland australia the leichhardt river fault trough of the mount isa terrane developed a complex extensional architecture between approximately 1800 and 1600 ma forming the underlying template upon which compressional structures were superimposed during the 1590 to 1500 ma isan orogeny basin fill material accumulated during at least five multiphase periods of rifting and associated postrift subsidence forming a stacked succession of unconformity bounded sequences initial e w extension was associated with a massive magmatic event half graben greater than 50 km in width and of alternating asymmetry localized the extrusion of up to 4 km of continental tholeiites thereafter a period of n s extension resulted in southward tapering north tilted half graben in which synrift basaltic and siliciclastic strata accumulated n s extension was followed by regional postrift subsidence and the deposition of a laterally continuous quartzite carbonate package a multiphase period of e w to nw se extension ensued during which time two unconformity bounded sequences accumulated the stratal architectures of these sequences are strongly asymmetric in cross section exhibiting a pronounced rotational thickening toward the east consistent with their deposition in the hanging walls of east dipping tilt blocks between 15 and 40 km in width finally a period of n s extension resulted in the development of e w trending f1 drag synclines in the highest level cover rocks the association of angular unconformities and block bounding faults e w trending synclines and e w striking faults and the unique internal fold geometries of fault blocks suggest that many fault bounded blocks originated as coherent structural entities during rifting and continued to act as such during subsequent shortening

oxygen and sulfur isotopic composition of volcanic sulfate aerosol at the point of emission volcanic sulfate aerosol is emitted from the vents of many active volcanoes but its origin has not yet been explained we report the first measurements of the isotopic compositions of near vent volcanic aerosol sulfate and use the combined sulfur and oxygen isotope systematics of the aerosol and magma at masaya volcano nicaragua to draw preliminary conclusions about the production mechanism of near source volcanic sulfate the sulfate aerosol does not display a significant mass independent oxygen or sulfur isotopic signature which suggests that primary volcanic aerosol does not contribute to the larger mass independent anomalies found in some ambient atmospheric aerosols and as preserved in ice cores the oxygen isotope composition of the sulfate appeared to become heavier on the particle filters with increasing amount of sample collected and suggests that the 18o value of the sulfate aerosol is 8 the sulfur isotopic composition of the sulfate did not vary with sample loading on the filters and had a mean 34s value of 7 7 0 8 similar to that of the magma 6 6 0 2 the low 17o value the 18o value of the magma 6 6 0 3 and the high 18o value of atmospheric oxygen 23 5 suggest that known atmospheric oxidation pathways at ambient temperatures are not the major routes of production for this sulfate instead the isotopic systematics of aerosol and magma are consistent with sulfate production either by high temperature equilibration of the magmatic gas mixture with small amounts of atmospheric oxygen or by direct emission of so4 2 from the magma

post orogenic 1500 ma thermal history of the proterozoic eastern fold belt mount isa inlier australia the paucity of sedimentary rocks younger than 1500 ma within the mt isa inlier renders it difficult to determine its post orogenic exhumation and tectonic history except by inference from the stratigraphy of the surrounding areas we present hornblende white mica biotite and alkali feldspar 40ar 39ar data from the eastern fold belt of the inlier which reveal a previously unrecognised complex post orogenic non linear cooling history plateau and total fusion 40ar 39ar ages range between 1500 and 1130 ma and record increases in regional cooling rates of up to 18 c my during 1460 1410 and 1280 1050 ma forward modelling of the alkali feldspar 40ar 39ar arrhenius parameters reveals subsequent increases in cooling rates during 740 640 and 600 500 ma the cooling episodes were driven by both erosional exhumation at maximum rates of 0 9 km my and thermal relaxation following magmatic events early mesoproterozoic cooling is synchronous with exhumation and shearing during the anmatjira event in the arunta block and late mesoproterozoic cooling occurred at the same time as magmatic events in the arunta and musgrave blocks as well as grenville aged orogenesis in the albany fraser belt more localised cooling during the middle neoproterozoic may be a far field response to the separation of australia and laurentia whereas latest neoproterozoic cambrian cooling and exhumation was probably driven by collisional tectonics associated with the petermann ranges and delamerian orogenic events major changes in the stress field of the australian plate that are synchronous with the assembly and break up of rodinia and gondwana resulted in the repeated brittle reactivation of the eastern fold belt probably via the displacement of long lived basement faults

continental erosion and large scale relief a worldwide investigation of continental erosion is carried out by the study of large drainage basins on the basis of hydrological data environmental factors and basin relief distribution inside each basin mean geochemical and mechanical denudation rates are defined a multicorrelation analysis shows that the mechanical denudation rates ds are uncorrelated with environmental factors and correlated with mean basin elevation h while chemical denudation rates dd are insensitive to relief but correlated with mean annual precipitation furthermore two linear relationships between h and ds are detected 1 ds m 10 yr 419 10 6 h m 0 245 with v explained variance 95 1 this law concerns basins related to orogenies younger than 250 ma the negative intercept is interpreted as a continental sedimentation rate of 245 m m y an alternative model in which one invokes a critical elevation separating erosion from sedimentation is equally successful and leads to lower sedimentation rates 60 110 m m y for both models one derives from the slope of the adjustments erosion time constants on the order of 2 5 m y 2 ds m 10 yr 61 10 6 h m with v 86 5 this law concerns basins related to older orogenies the null intercept suggests the lack of continental storage because of the more important dispersion of the data the erosion time constant is calculated separately for each basin it ranges from 15 to 360 m y the tectonic implications of these results are discussed in particular the short time constant 2 5 m y agrees with orogenic uplift rates on the order of 1 mm yr observed in active mountain chains

metamorphic rocks seek meaningful cooling rate interpreting 40ar 39ar ages in an exhumed ultra high pressure terrane 40ar 39ar dating of metamorphic white mica is commonly used to elucidate the timing of cooling and or exhumation of metamorphic terranes in theory so long as the dated mineral originally crystallised above its closure temperature the yielded 40ar 39ar ages should not vary significantly within or between samples and should vary only due to grain size 3 ma variation depending on the estimated cooling rate infra red laser single grain fusion and ultra violet laser spot muscovite and or phengite data from mafic eclogite bodies and their host gneisses in the nordfjord region of the western gneiss complex wgc norway show that the ages yielded both within and between white micas from the same mafic eclogite sample vary by up to 284 ma ages yielded within the same pelitic gneiss sample vary by up to 28 ma overall the 40ar 39ar phengite dates from mafic eclogites range from 750 to 440 ma and are significantly older than the previously reported 405 400 ma u pb zircon age constraining peak metamorphism in the nordfjord wgc the felsic host gneiss samples which recrystalised during a post peak exhumation related amphibolite facies overprint yield single grain fusion and laser spot 40ar 39ar white mica dates spanning 420 to 385 ma encompassing previous estimates of the timing of the entire metamorphic cycle these data suggest that ar is heterogeneously distributed both within grains and between grains on the cm to sub mm scale in all samples and imply restricted ar transport and inefficient removal during the metamorphic cycle open system zero grain boundary ar concentration behaviour is an inherent assumption in the commonly applied closure temperature formulation but unless this can be demonstrated linking 40ar 39ar ages to temperature is no longer trivial the high spatial and chronological precision afforded by uv laser ablation mean that intra grain ar concentration distributions may be used to assess open vs closed system behaviour the data presented here suggest that open system behaviour cannot be assumed in metamorphic systems even those in which the pressure temperature and time conditions in theory allow for efficient diffusion during the metamorphic cycle high spatial resolution 40ar 39ar data and robust comparison with numerical models of open system behaviour provide the greatest future potential for unravelling the complexities of metamorphic exhumation timescales

rapid cooling and exhumation in the western part of the mesoproterozoic albany fraser orogen western australia abstract the albany fraser orogen of southwestern australia is an understudied orogenic belt which is interpreted to record the mesoproterozoic suturing of the yilgarn craton of western australia to the mawson craton of east antarctica during rodinia assembly previous u pb geochronology has dated peak amphibolite to granulite facies metamorphism in the orogen at ca 1180 ma here we report the first 40ar 39ar thermochronology of hornblende biotite and muscovite grains from a 360 km transect across the western albany fraser orogen and uncover a record of strikingly fast syn orogenic cooling and exhumation to the north muscovites from the northern foreland record cooling at ca 1159 ma in the central and southern domains of the orogen the biranup and nornalup zones hornblende yields ca 1169 ma cooling ages and biotite yields ca 1172 1144 ma cooling ages the new cooling ages imply that the three domains were exhumed rapidly following peak metamorphism at ca 1180 ma attained a similar structural level by ca 1159 ma and have experienced a uniform exhumation history since that time to constrain mineral closure temperatures and post peak metamorphic cooling rates we conducted a monte carlo simulation which fully propagates uncertainty and minimises error correlations modelling of cooling from hornblende to biotite closure temperatures ca 585 c and 365 c respectively in the nornalup and biranup zones yields fast cooling rates of 33 9 17 c ma and 22 5 7 c ma respectively these fast cooling rates imply rapid exhumation in an active tectonic setting undergoing peak metamorphism although the structural evolution of the albany fraser orogen remains poorly constrained the transpressional tectonic activity associated with deformation in the western part of the albany fraser orogen may have been an active driver of this fast exhumation this is distinctly different from exhumation models for granulite facies domains in other mesoproterozoic orogens which typically experience post orogenic slow 1 5 c ma cooling driven by mechanisms such as orogenic collapse and erosion we consider that the observed differences reflect the interpreted syn tectonic transpressional exhumation history of the albany fraser orogen which is an underrepresented tectonic regime in the mesoproterozoic cooling record

excess argon in k ar and ar ar geochronology the k ar and ar ar dating techniques occasionally produce anomalously old ages attributed to excess argon and such data is often rejected as not offering any insight into the age thermal history or geochemistry of the rock however improvements in the quantification of argon geochemistry now provide a framework to model excess argon in both open and closed systems solubility data for argon in hydrous fluids melts and emerging data for minerals can be used to understand the behaviour of excess argon and provide valuable insights into the environment in which the samples cooled to their argon retention or closure temperature treating excess argon as a trace element also throws light on its behaviour in minerals above the closure temperature in deeply buried dry systems such as eclogites blueschists granulites and even in the lithospheric mantle extremely low partition coefficients between k feldspar and hydrous fluid phases predict lower excess argon susceptibility than micas and this is observed in fluid poor systems variation of partition coefficients can lead to excess argon in fluids being introduced into minerals or removed from minerals as grain boundary fluids change during flow through a rock however excess argon can also be introduced or removed from minerals by varying temperature without the need for fluid flow high mineral melt and mineral fluid partition coefficients are also the reason why excess argon is often concentrated in inclusions within minerals partition coefficients between minerals and hydrous fluids as low as 10 6 lead fluid inclusions to dominate the radiogenic argon budget particularly in low potassium minerals melt inclusions are less dominant but become critical in dating younger samples

geochemistry of post 1540 ma granites in the cloncurry district northwest queensland proterozoic fe oxide cu au co mineralization in the cloncuriy district is spatially and temporally related to intrusion of voluminous postpeak metamorphic granites that form most of the williams and naraku batholiths detailed mapping and geochemical study of these granites in the southern part of the district have identified two groups of intrusions that are referred to here as the cloncurry supersuite and eureka supersuite the cloncurry supersuite is composed mainly of monzogranite and syenogranite which consist of quartz plagioclase alkali feldspar hornblende biotite with accessory magnetite titanite apatite and zircon the timing of intrusion of the wiley igneous complex is interpreted to be pre or syn d 3 based on overprinting relationships while hornblende geobarometry suggests an emplacement depth of approximately 14 km the granites are subalkaline high k metaluminous rocks and range from 59 to 74 wt percent sio 2 increasing sio 2 is accompanied by increasing k 2 o rb and th and decreasing mgo fe 2 o 3 feo tio 2 cao p 2 o 5 sr eu zr cu zn ni v and sc this is consistent with fractionation of observed phases including plagioclase amphibole biotite magnetite titanite and apatite the eureka supersuite ranges from diorite to syenogranite in composition more mafic varieties contain pyroxene and hornblende while more felsic varieties contain biotite as the only ferromagnesian silicate sodiccalcic alteration events are closely linked in space and time to intrusion of eureka supersuite magmas the timing of intrusion is interpreted to be pre and post d 3 based on overprinting relationships between different plutons and mylonitic fabrics hornblende geobarometry suggests an emplacement depth of approximately 10 km for the mount angelay igneous complex eureka supersuite granites in the mount angelay igneous complex range from 51 to 76 wt percent sio 2 the granites are alkaline and k 2 o rich leading to their classification as a shoshonite series with increasing sio 2 content tio 2 al 2 o 3 fe 2 o 3 t mno mgo cao p 2 o 5 sr eu cu zn ni v and sc decrease indicating fractionation of observed minerals including apatite magnetite pyroxene hornblende plagioclase and biotite some samples are characterized by very high abundances of k 2 o p 2 o 5 sr ba and light rare earth elements lree compared to other eureka supersuite samples this may reflect their derivation from a source that was variably enriched in incompatible elements compared with many other shoshonitic rocks high k eureka supersuite samples are strongly depleted in sr and to a lesser extent eu and enriched in y and heavy rare earth elements hree this suggests that garnet was not a stable phase in the source region that the source contained plagioclase as a stable phase and or the granites evolved by extensive plagioclase fractionation partial melting in the lower crust to generate post 1540 ma granites was most likely caused by intrusion of basic magma into the crust eureka supersuite magmas were probably derived from partial melting of variably incompatible element enriched gabbroic rocks while cloncurry supersuite magmas were most likely derived from partial melting of more felsic material of dioritic to tonalitic composition

crustal architecture of basin inversion during the proterozoic isan orogeny eastern mount isa inlier australia abstract the folded and metamorphosed supracrustal rocks of the eastern margin of the mount isa inlier nw queensland australia provide a record of basin inversion processes at high temperatures from the foreland toward the core of a developing orogen deep water turbiditic metasedimentary rocks of the lower maronan supergroup ca 1 71 1 66 ga occupy the hanging wall of a crustal scale thrust the cloncurry overthrust which separates them from calc silicate and meta evaporitic rocks of the doherty formation ca 1 74 1 72 ga in the footwall during the early stages of the isan orogeny ca 1 60 1 58 ga the rocks of the maronan supergroup were expelled from their basin and thrust to the north northwest shortening was restricted to the hanging wall and produced a 10 km scale north to northwest verging nappe with a crumpled frontal zone of upright east west trending folds this morphology is consistent with a push from the rear mechanism of nappe emplacement with the simple shear component of deformation increasingly accommodated within the hanging wall down dip toward the core of the orogen metamorphism associated with this folding reached upper amphibolite facies temperatures at moderate to low pressures 4 6 kbar metamorphic zones in the hanging wall of the cloncurry overthrust were antiformally folded with continuing deformation the relationship between faulting folding and metamorphic zoning across the cloncurry overthrust are consistent with a thrust system despite that fact that younger rocks are juxtaposed over older basin inversion provides the best explanation for this phenomenon within this context metamorphism during the early stages of orogenesis can be explained by the inheritance of elevated geothermal gradients produced by lithospheric thinning and sediment loading during formation of the basin the overall orogenic architecture was controlled by a combination of thermal softening which focussed early deformation within the basin and weak zones at the base of the basin and the base of the crust this resulted in crustal scale thrusts of opposite vergence that partitioned upper crustal and lower crustal deformation

nd isotope and petrogenetic constraints for the origin of the mount angelay igneous complex implications for the origin of intrusions in the cloncurry district ne australia abstract the mesoproterozoic mount angelay igneous complex contained intrusions that were emplaced into amphibolite facies metasedimentary rocks during two periods of 1550 and post 1540 ma magmatism sm nd isotopic analysis together with mineralogical and chemical considerations suggest that the intrusions were produced from a paleoproterozoic crustal source with a t 2 model age 2200 ma on geochemical and petrological grounds the 1550 ma trondhjemitic intrusions are interpreted to have been produced by melting of amphibolite under garnet stable conditions 8 10 kbar the late syn to post peak metamorphic timing of these intrusions suggested that they were associated with the tectono thermal event that produced regional peak metamorphic mineral assemblages the post 1540 ma intrusions are k rich and consist of two groups of synchronously emplaced intrusions 1 a high k monzodiorite and monzogranite suite that range between 51 and 77 wt sio 2 and 2 a high k na enriched hornblende monzonite the chemistry and mineralogy of these intrusions suggested that they were derived via plagioclase stable and garnet unstable melting 2 o p 2 o 5 cr v and zn compared with slightly younger high k monzonite which is interpreted to have formed via one of two mechanisms 1 melting of a low k amphibole and plagioclase rich source or 2 melting of residual material that produces a potassic and incompatible element rich melt these magmas likely contained mantle derived material particularly the k rich intrusions of mafic composition the heat required for the production of post 1540 ma intrusions appears to have been generated by the intrusion of high t mantle derived mafic material into the crust 25 30 km 8 10 kbar this model is consistent with the synchronous emplacement of mafic and felsic magma and the lack of a consanguineous regional metamorphic association and suggests high t high degree partial melting in localised pockets within fertile source regions in the crust an increase in sm nd model source age and decrease in end with increasing sio 2 in the k rich intrusions suggests the incorporation of juvenile material in the more mafic rocks the origin of this component is unknown but it may represent either the incorporation of mantle derived material during melting or the partial melting of crust with a younger mafic component on a district scale the 30 million year period over which the k rich post 1540 ma intrusions were emplaced suggested that mantle derived material continued to be injected into the crust a mantle component to these rocks and the global distribution of proterozoic intrusions with similar geochemical affinities strongly suggests a world wide period of mantle induced crustal melting at that time the dominant paleoproterozoic isotopic composition of most of these intrusions suggests melting of similarly composed and matured source rocks

thrusting and duplex formation at mount isa queensland australia large scale repetition of mount isa and haslingden group rocks in the anticline north of mount isa is interpreted as the core of a thrust duplex which has been subsequently folded parallel to the direction of thrusting displacement of mount isa group rocks above the duplex is extremely significant to the regional geology and exploration for more mount isa style pb zn and cu orebodies interpretation of the geometry of the thrust belt indicates thrusting on different stratigraphical levels below and above the duplex during its formation horses formed from a heelwards protruding footwall ridge instead of a trough and the complex effects on stratigraphical cross sections through duplexes of sloping lateral footwall ramps on horses

crustal flow modes in large hot orogens abstract crustal scale channel flow numerical models support recent interpretations of himalayan tibetan tectonics proposing that gravitationally driven channel flows of low viscosity melt weakened middle crust can explain both outward growth of the tibetan plateau and ductile extrusion of the greater himalayan sequence we broaden the numerical model investigation to explore three flow modes homogeneous channel flow involving laterally homogeneous crust heterogeneous channel flow involving laterally heterogeneous lower crust that is expelled and incorporated into the mid crustal channel flow and the hot fold nappes style of flow in which mid lower crust is forcibly expelled outward over a lower crustal indentor to create fold nappes that are inserted into the mid crust the three flow modes are members of a continuum in which the homogeneous mode is driven by gravitational forces but requires very weak channel material the hot fold nappe mode is driven tectonically by for example collision with a strong crustal indentor and can occur in crust that is subcritical for homogeneous flows the heterogeneous mode combines tectonic and gravitationally driven flows preliminary results also demonstrate the existence and behaviour of mid crustal channels during advancing and retreating dynamical mantle lithosphere subduction an orogen temperature magnitude t m diagram is proposed and the positions of orogens in t m space that may exhibit the flow modes are described together with the characteristic positions of a range of other orogen types

metasomatic albitites and related biotite rich schists from a low pressure polymetamorphic terrane snake creek anticline mount isa inlier north eastern australia microstructures and p t d paths rocks of the snake creek anticline are mainly pelitic schists psammitic schists and quartzites that were metamorphosed during multiple high t low p events extending from d1 to d5 with the metamorphic peak occurring late to post d3 albitites are widespread but are concentrated in five areas they are typically fine to medium grained and consist of albite with or without combinations of quartz biotite staurolite cordierite garnet andalusite sillimanite kyanite gedrite and tourmaline from the presence or absence of albite inclusions in porphyroblasts the albitites are interpreted as forming early in the d3 event as a result of infiltration of external fluids psammitic schists and quartzites were preferentially altered but pelitic schists were also albitized in localities where the alteration was more extreme with the replacement of muscovite total and the replacement of quartz and biotite variable structural controls on albitization include fracturing and syn d3 shear zones in fold hinges biotite schists with abundant porphyroblasts combinations of staurolite garnet andalusite and cordierite occur adjacent to albitites and it is argued that they formed by the addition of fe and mg sourced from the albitites in several albitite rich areas cordierite grew early in d3 and was partly or entirely replaced during or after d3 by combinations of biotite andalusite tourmaline staurolite and sillimanite a postulated p t d path involved an increase in pressure with or without a decrease in temperature subsequent to early d3 albitization followed by an increase in temperature up to the metamorphic peak late d3 to early d4 the metamorphism was contemporary in part with the emplacement of the williams batholith c 1550 1500 ma which probably supplied the na rich fluids

age of the barramundi orogeny in northern australia by means of ion microprobe and conventional u pb zircon studies abstract u pb zircon geochronological studies are used to quantify the tectonostratigraphic history of a major crust forming event in early proterozoic terranes of northern australia magmatism and tectonic processes associated with this event the barramundi orogeny appear similar over very wide areas about one third of the australian continent the barramundi event marks the deformation metamorphism and cratonization of the earliest proterozoic 2 0 1 9 ga basinal successions and it was followed by and closely linked to extensive orogenic felsic magmatism the yaringa metamorphics are an inlier of paragneiss and schist that pre dates 1865 3 ma leichhardt felsic magmatism and younger basin formation in the mount isa inlier from an ion microprobe u pb zircon study of massive and migmatitic gneiss the time of yaringa metamorphism is established at 1890 8 ma recognized younger proterozoic deformations in the interval 1610 1550 ma are generally not evident in zircon u pb systems but have totally reset local rb sr whole rock values conventional u pb work on multi grain zircon samples of yaringa gneiss gives a geologically meaningless age 2090 40 ma that is an artifact of complex zircon growth part of which was initially inherited from a 2 55 to 2 2 ga provenance and the other major part of which was formed in situ during the 1890 ma high grade metamorphism predictably for simple zircon systems in low grade metavolcanic terranes conventional and ion microprobe zircon ages are in close agreement dating of volcaniclastic sequences above and below the barramundi tectonic break in the pine creek inlier brackets the orogeny in this terrane to a similar time frame 1885 1870 ma some zircons which crystallized at this time form selvedges around older cores whose age of 2100 2000 ma may be relics of an earlier deep crustal underplating the uniform chemistry of post barramundi felsic magmatism and the similar tectonic setting of the areally extensive barramundi event are mirrored by their temporal coherency cycles of geosynclinal deposition burial metamorphism and uplift took place during intervals of several million years or less implying comparable rates for early proterozoic orogenesis to those established for tertiary and quaternary settings

proterozoic low pressure high temperature metamorphism and an anticlockwise p t t path for the hazeldene area mount isa inlier queensland australia in the hazeldene area situated in the mount isa inlier queensland the metamorphic grade changes from chlorite zone through biotite and cordierite zones to sillimanite k feldspar zone microstructural studies of rocks near the sillimanite isograd demonstrate that cordierite grew early during the development of a steep foliation s2 was replaced by biotite andalusite and sillimanite at the metamorphic peak late in s2 and in turn by kyanite chlorite adjacent to localized small post d2 shear zones although the anticlockwise p t t path is well defined the precise p t conditions are uncertain because of problems with experimental and thermodynamic data the best estimate for the metamorphic peak for rocks close to the sillimanite isograd is around 600 c at 4 kbar the metamorphism has been dated at 1544 ma and was synchronous with a major crustal shortening event because proposed extensional events occurred more than 60 ma earlier their contribution to the peak metamorphic thermal perturbation would have been insignificant the syn metamorphic mica creek pegmatites the abundance of high heat producing elements in the nearby pre d2 sybella granite and advective heat by fluids which caused considerable metasomatism in the hazeldene area may have each contributed to the thermal budget however the metamorphic thermal gradient may be 80 c km 1 or higher strongly suggesting a local magmatic control as none are known in the area such syn metamorphic plutons would have to lie beneath the exposed high grade rocks

jarosite and hematite at meridiani planum from opportunity s mossbauer spectrometer mossbauer spectra measured by the opportunity rover revealed four mineralogical components in meridiani planum at eagle crater jarosite and hematite rich outcrop hematite rich soil olivine bearing basaltic soil and a pyroxene bearing basaltic rock bounce rock spherules interpreted to be concretions are hematite rich and dispersed throughout the outcrop hematitic soils both within and outside eagle crater are dominated by spherules and their fragments olivine bearing basaltic soil is present throughout the region bounce rock is probably an impact erratic because jarosite is a hydroxide sulfate mineral its presence at meridiani planum is mineralogical evidence for aqueous processes on mars probably under acid sulfate conditions

the synchronism of crustal thickening and high t low p metamorphism in the mount isa inlier australia 1 an example the central soldiers cap belt abstract in the soldiers cap group of the central eastern part of the early to middle proterozoic mount isa inlier tracing of mesoscopic structures has led to the recognition of an early nw closing fold nappe with an e w trending coaxially shortened frontal zone d1 folded around a n s trending upright syncline d2 an andalusite sillimanite prograde metamorphic event indicating a paleo geothermal gradient of approximately 50 c km occurred during early d2 times the coexistence of such moderately high paleo geothermal gradients and crustal thickening is common in the early to middle proterozoic mobile belts of northern australia crust mantle delamination has been suggested by etheridge et al 1987 to explain this coexistence here the thermal implications of this model and the additional thermal effects of waning crustal extension magmatic events and of the radioactive selfheating of the thickened crust are qualitatively discussed

geometry of a middle proterozoic extensional decollement in northeastern australia abstract an 80 km long belt of gneissic and mylonitic rocks is exposed in the core of a major anticlinorium in the middle proterozoic mt isa inlier of northeastern australia these rocks in the mary kathleen fold belt represent an early originally horizontal mid crustal shear zone which is interpreted as an extensional decollement between a lower ductile sheet and an upper more brittle plate the decollement was subhorizontal over a distance of at least 80 km and its depth was perhaps as shallow as 7 km shearing was accompanied by the syn tectonic intrusion of a bimodal complex of granitic and basic rocks over a period of 70 90 m y moderate to extreme ductile shearing was pervasive throughout the 1 1 5 km thickness of the lower plate that is now exposed massive dilation accompanied shearing and approximately 70 of the volume of the shear zone is occupied by igneous intrusive rocks which have been variably deformed to gneissic granite mylonite and amphibolite strain softening driven by this high thermal input is responsible for the pervasive ductile response at such shallow depths peak metamorphic conditions at the decollement were approximately 625 c to 675 c at 2 kbar intense ductile shearing in the upper plate is restricted to the lower few hundred metres above this zone brittle processes dominate although evidence is presented that the sheet as a whole has undergone a component of ductile shearing probably heterogeneously distributed large sheet like granite and dolerite bodies were introduced into dilation sites associated with synthetic faults in this plate but unlike their synchronous counterparts in the lower plate about 1 km below these are internally undeformed the middle proterozoic history of the area involves a cycle of extension and shortening with the shortening axis perpendicular to the earlier extension axis we speculate that the driving mechanism for both extension and shortening is mantle convection with switches in stress related to mantle upwelling

high radiogenic heat producing granites and metamorphism an example from the western mount isa inlier australia comment and reply extract we welcome the paper of mclaren et al 1999 because it makes an important contribution to our understanding of the source of thermal anomalies in low pressure metamorphic belts however we argue that high radiogenic heat producing granites such as the sybella batholith although locally important in the overall thermal budget were not the primary cause of low pressure metamorphism in the mount isa inlier and that synmetamorphic intrusions were a significant additional factor

chronology of magmatism skarn formation and uranium mineralization mary kathleen queensland australia uranium mineralization is hosted in a metasedimentary sequence intruded by felsic and mafic bodies in which the zircon u pb systems are 1 730 to 1 740 m y old felsic dikes characterized by exceptionally high uranium contents crystallized at essentially the same time 1 737 or 15 m y primary rb sr systematics in these igneous rocks are profoundly disturbed u pb discordia establish age of uranium mineralization as 1 550 or 15 m y derivation of uranium protore in the host rocks prior to regional metamorphism and 1 550 m y uraninite formation was probably by ca 1 740 m y hydrothermal activity modified journal abstract

age constraints on the tectonothermal evolution of the selwyn zone eastern fold belt mount isa inlier the selwyn zone is characterized by multiple ductile deformation and metamorphic events there is general consensus that early in the isan orogeny there were both extensional and thrust events that produced bedding parallel foliation sbp we propose that these were followed by the d1 event that resulted in a regional steep east west foliation and that the snake creek anticline and the dominant s2a regional foliation formed as upright north south structures local overturning of d2 structures was due to subsequent localized deformation d2b that resulted in structures with flat lying axial planes d3 and d4 produced steep upright folds and crenulations with north northwest and northeast trending axial surfaces metamorphism largely controlled by the emplacement of mafic rocks is dominantly low pressure but the p t t path is complex early growth of cordierite m1 followed by andalusite was succeeded by a clockwise medium pressure loop of ca 500 600 mpa in the northern snake creek anticline as indicated by early d2a kyanite growth the metamorphic peak during d2 was around 580 c and 400 mpa at the sillimanite isograd and reached a maximum of around 670 c and 400 500 mpa in the migmatitic sillimanite k feldspar zone at the cannington and osborne mines previous age determinations for the metamorphic peak of 1580 1600 ma are confirmed with new electron microprobe dating of monazite whereas an age grouping of 1600 1630 ma may represent the d1 m2 event additional ages in the range 1640 1680 ma are recorded in albitites from the soldiers cap group and it is suggested that they formed by infiltration of hydrothermal fluids during extension and the emplacement of mafic sills this is supported by shrimp dating of zircon from a trondhjemite lens in a largely mafic sill complex in the snake creek anticline that provided an age of 1686 8 ma also indicating that at least part of the soldiers cap group is older than previously indicated a zircon shrimp age of 1527 4 ma was obtained for a sample of albitized saxby granite this age also represents the d4 m6 event and is the timing of the extensive brecciation and albitization of calc silicate rocks of the corella formation in the selwyn zone

isotopic and structural responses of granite to successive deformation and metamorphism within the sybella granite west of mount isa in northern australia differential response of u pb and rb sr isotopic systems to deformation metamorphism enables ages of emplacement and three discrete deformation metamorphic events to be determined based on zircon u pb systems two of the plutons were emplaced at 1671 pm 8 ma and 1668 pm 26 21 ma and the third at 1610 pm 10 ma a separate microgranite body gives aberrantly old u pb ages of up to 1780 ma due to partial inheritance of older zircon the three discrete deformation metamorphic events d 1 d 2 and d 3 occurred at 1610 pm 13 ma 1544 pm 12 ma and 1510 pm 13 ma respectively based on whole rock rb sr isotopic systems the youngest body was emplaced syntectonically and has possible significance to the origin of the mount isa ag pb zn orebodies homogenization of rb and sr isotopes via extensive syntectonic fluid migration associated with deformation partitioning and resultant solution transfer appears to have been

high cu concentrations in vapor type fluid inclusions an artifact abstract studies of natural fluid inclusions have shown that cu commonly occurs at higher concentrations in vapor type inclusions than in coexisting brine inclusions a phenomenon which has been interpreted to arise from copper partitioning into the vapor phase in the first part of this study we attempted to experimentally reproduce this behavior in hydrothermal experiments covering a range of p t conditions 600 800 c 70 130 mpa fluid compositions nacl kcl fecl 2 0 5 6 6 wt s fluid acidities quench fluid ph 0 3 10 and sulfur speciation h 2 s dominated to so 2 dominated however as in several other studies we did not succeed in reproducing conditions under which cu partitions into the vapor phase in view of recent observations that quartz hosted fluid inclusions can diffusively loose or gain cu after entrapment we set out to determine if the evidence from natural fluid inclusions could be compromised for this purpose we synthesized vapor and brine inclusions from cu h 2 o nacl s fluids at 800 c 130 mpa and re equilibrated them in slightly different fluids at 800 c 70 mpa measuring some inclusions by la icp ms after each step vapor inclusions indeed experienced a dramatic increase in cu from 0 3 0 03 to 5 7 3 3 wt while brine inclusions remained largely unmodified leading to a change in the partition coefficient d cu vap brine from a true value i e before re equilibration of 0 4 0 05 to an apparent value of 8 3 4 9 the requirements for substantial diffusional gain of cu in fluid inclusions are a change in the ph of the surrounding fluid from 1 to more neutral and the presence of s in the pre existing fluid inclusions these requirements are also fulfilled in nature cooling magmatic hydrothermal fluids experience a change from acidic to more neutral ph due to buffering along the feldspar mica join and natural vapor inclusions typically contain significant amounts of sulfur a reversal experiment performed on natural quartz hosted fluid inclusions from the erongo granite namibia showed that this process can be reversed with the measured d cu vap brine value of 11 9 3 being modified to 0 06 0 04 thus d cu vap brine values 1 measured on natural fluid inclusions in quartz are likely a secondary feature caused by post entrapment copper diffusion realistic d cu vap brine values in porphyry cu environments are between 0 11 and 0 15 and reconstructed vapor brine mass ratios are in the order of 4 9 this suggests that the main transporting agent of cu at the porphyry level are brines and that models based on copper into the vapor phase are incorrect

asymmetric extension of the middle proterozoic lithosphere mount isa terrane queensland australia abstract the middle proterozoic mount isa basin of the mount isa terrane hosts several large pb zn ag deposits and is arguably one of the richest mineral provinces in the world the deformed remnants of this basin extend from the eastern margin of the leichhardt river fault trough through to the murphy tectonic ridge in the far north of the terrane the mount isa basin initially evolved in response to nw se directed extension during the mount isa rift event this event began before 1708 ma and had ceased by 1653 ma a sag basin continued to evolve thereafter until 1595 ma regional analysis of the highest level cover rocks of the mount isa basin reveals a notable difference in the locus of syn rift sedimentation syn rift magmatism and post rift subsidence although crustal extension was widespread across the mount isa basin tectonic subsidence was focussed along the n s oriented mount isa rift approximately 3 5 km of fluvial to shallow marine clastic sediments were deposited into isolated rift basins bimodal volcanism and emplacement of shallow level plutons occurred along the western and northwestern margins of the mount isa rift magmatic provinces mark the locus of significant subcrustal lithospheric thinning asthenospheric upwelling and mafic underplating within these magmatic provinces the syn rift sequences are fewer and thinner 750 2000 m and were dominantly deposited in subaerial environments suggesting a relatively stable uplift and subsidence history the position of maximum subcrustal lithospheric extension is determined by the position of greatest post rift subsidence this occurred beneath the northern mount isa terrane where the thickest post rift sequences are preserved and the depositional history is more protracted we propose that the evolution of the mount isa basin is a consequence of asymmetric extension of the middle proterozoic lithosphere

regional estimation of electric fields and currents in the polar ionosphere a new technique is presented to estimate electric fields and currents in a localized region of the high latitude ionosphere by combining two magnetogram inversion algorithms this paper describes the concept and practical procedures of the method as well as the first results of our efforts in which this new scheme is applied to northern scandinavia computing the ionospheric parameters on a small scale examining latitudinal profiles of these parameters and precipitating particles it is found that the region of the most intense precipitation in the morning sector is located equatorward of the region of the strongest electric field to evaluate the relative importance of ionospheric and magnetospheric effects the field aligned current is divided into two components e and e these two components give often the opposite directions in the resultant field aligned currents the relative strength of the two components appears to vary considerably with latitude

the oxidation state of sulfur in magmatic fluids abstract sulfur compounds in volcanic gases are responsible for the global cooling after explosive eruptions and they probably controlled the early evolution of the earth s atmosphere we have therefore studied the oxidation state of sulfur in aqueous fluids under the pressure and temperature conditions and oxygen fugacities typical for magma chambers 0 5 3 kbar 650 950 c ni nio to re reo 2 buffer conditions sulfur speciation was determined by raman spectroscopy of quenched fluids trapped as inclusions in quartz our results show that sulfur in hydrothermal fluids and volcanic gases is much more oxidized than previously thought and in particular some explosive eruptions may release a significant fraction of sulfur as so 3 or its hydrated forms in the pressure range from 500 to 2000 bar the equilibrium constant k 1 of the reaction 2h 2 s 3o 2 2so 2 2h 2 o in aqueous fluids can be described by lnk 1 57 1 7 1 173 480 7592 t 1 where t is temperature in kelvin the equilibrium constant k 2 for the reaction so 2 o 2 so 3 in aqueous fluids where so 3 may include hydrated forms such as h 2 so 4 was found to be strongly pressure dependent with lnk 2 5 2 5 7 19 243 5993 t 1 at 1500 bar lnk 2 11 1 1 3 25 383 1371 t 1 at 2000 bar and lnk 2 22 1 2 2 37 082 2248 t 1 at 2500 bar our data imply that volcanoes may directly inject hexavalent sulfur in the form of h 2 so 4 into the atmosphere not only on earth but possibly also on venus and on mars when it was still tectonically active remote measurements from satellites may have underestimated the sulfur yield of some recent eruptions moreover the mechanisms of the interaction of volcanic gases with the stratosphere need to be reconsidered

sulfur sources of sedimentary buckshot pyrite in the auriferous conglomerates of the mesoarchean witwatersrand and ventersdorp supergroups kaapvaal craton south africa large rounded pyrite grains 1 mm commonly referred to as buckshot pyrite grains are a characteristic feature of the auriferous conglomerates reefs in the witwatersrand and ventersdorp supergroups kaapvaal craton south africa detailed petrographic analyses of the reefs indicated that the vast majority of the buckshot pyrite grains are of reworked sedimentary origin i e that the pyrite grains originally formed in the sedimentary environment during sedimentation and diagenesis forty one of these reworked sedimentary pyrite grains from the main vaal basal kalkoenkrans beatrix and ventersdorp contact reefs were analyzed for their multiple sulfur isotope compositions 34s 33s and 36s to determine the source of the pyrite sulfur in addition five epigenetic pyrite samples pyrite formed after sedimentation and lithification from the middelvlei and the ventersdorp contact reefs were measured for comparison the 34s 33s and 36s values of all 41 reworked sedimentary pyrite grains indicate clear signatures of mass dependent and mass independent fractionation and range from 6 8 to 13 8 1 7 to 1 7 and 3 9 to 0 9 respectively in contrast the five epigenetic pyrite samples display a very limited range of 34s 33s and 36s values 0 7 to 4 0 0 3 to 0 0 and 0 3 to 0 1 respectively despite the clear signatures of mass independent sulfur isotope fractionation very few data points plot along the primary archean photochemical array suggesting a weak photolytic control over the data set instead other factors command a greater degree of influence such as pyrite paragenesis the prevailing depositional environment and non photolytic sulfur sources in relation to pyrite paragenesis reworked syngenetic sedimentary pyrite grains pyrite originally precipitated along the sediment water interface are characterized by negative 34s and 33s values suggesting open system conditions with respect to sulfate supply and the presence of microbial sulfate reducers on the contrary most reworked diagenetic sedimentary pyrite grains pyrite originally precipitated below the sediment water interface show positive 34s and negative 33s values suggesting closed system conditions negligible 33s anomalies from epigenetic pyrite suggest that the sulfur was sourced from a mass dependent or isotopically homogenous metamorphic hydrothermal fluid contrasting sulfur isotope compositions were also observed from different depositional environments namely fluvial conglomerates and marine modified fluvial conglomerates the bulk of the pyrite grains from fluvial conglomerates are characterized by a wide range of 34s values 6 2 to 4 8 and small 33s values 0 3 this signature likely represents a crustal sulfate reservoir derived from either volcanic degassing or from weathering of sulfide minerals in the hinterland reworked sedimentary pyrite grains from marine modified fluvial conglomerates share similar isotope compositions but also produce a positive 33s 34s array that overlaps with the composition of archean barite suggesting the introduction of marine sulfur these results demonstrate the presence of multiple sources of sulfur which include atmospheric crustal and marine reservoirs the prevalence of the mass dependent crustal sulfur isotope signature in fluvial conglomerates suggests that sulfate concentrations were probably much higher in terrestrial settings in comparison to marine environments which were sulfate deficient however the optimum conditions for forming terrestrial sedimentary pyrite were probably not during fluvial progradation but rather during the early phases of flooding of low angle unconformities i e during retrogradational fluvial deposition coupled in some cases with marine transgressions immediately following inflection points of maximum rate of relative sea level fall

comparing 1800 1600 ma accretionary and basin processes in australia and laurentia possible geographic connections in columbia abstract we focus on potential pinning points on the southern and eastern margins of australia and the southern and western margins of laurentia and interpret a convergent plate boundary along the southern margin of the proto australian continent during the palaeo to mesoproterozoic 1800 1600 ma specific geological events within the southern australian margin can be correlated with events preserved along the medicine bow yavapai mazatzal belt in southwestern laurentia these belts were characterised by southward continental growth with varying degrees of crustal reworking and accretion of juvenile material in australia crustal reworking appears to have been the dominant process from 1800 to 1670 ma with more juvenile material accreted at 1620 ma in contrast the southern laurentian terranes were dominated by accretion of juvenile material from 1800 to 1620 ma although reworked continental crust was also accreted to the margin in the interior of both continents basins developed with comparable histories from 1800 to 1670 ma thereafter the evolution of the interior basins diverged with sag phase processes dominant in northern australia and a protracted depositional hiatus in laurentia the geologic evolution is consistent with palaeomagnetic data and provides further evidence for geographic links between eastern australia and western laurentia from 1800 to 1600 ma

structurally bound s2 s1 s4 s6 in terrestrial apatite the redox evolution of hydrothermal fluids at the phillips mine new york usa abstract the oxidation state of sulfur s plays a critical role in the formation of igneous and hydrothermal mineral systems constraining the oxidation state of s during mineralization and alteration is a valuable tool for understanding primary versus secondary processes that affect mineral systems recent experimental studies demonstrate that apatite which is a ubiquitous accessory mineral in igneous and hydrothermal ore forming systems structurally incorporates multiple oxidation states of s i e s6 s4 s2 and that the abundance of reduced versus oxidized s in apatite is systematically related to oxygen fugacity in this study we used micro x ray absorption near edge structure xanes spectroscopy at the s k edge to measure the oxidation states of s in natural apatite from the phillips mine magnetite sulfide mineral deposit putnam county new york micro xanes transects were collected within two apatite grains starting near the edge of 1 a pyrrhotite inclusion and 2 an inclusion assemblage consisting of pyrite ferroan carbonate pyroxene and magnetite significant compositional and textural variations within the apatite were observed by electron probe micro analysis epma wavelength dispersive wds spectroscopy element mapping and cathodoluminescence cl imaging and used in combination with the xanes data to discuss the formation of the phillips mine apatite the xanes analyses reveal that apatite contains variable proportions of s6 s4 s1 and s2 with corresponding peak absorption energies of 2481 7 0 3 ev 2477 9 0 4 ev 2471 8 0 1 ev and 2469 8 0 04 ev respectively notably this marks the first observation of reduced s species s2 s1 in terrestrial apatite peak areas ratios s6 s demonstrate systematic variations in the oxidation state of s within the apatite grains elevated s6 s peak area ratios typically coincide with higher concentrations of s and rare earth elements within the apatite grains several observations including the presence of multiple oxidation states of s in apatite and monazite inclusions that record secondary fluid mediated dissolution reprecipitation of apatite indicate differences in the oxidation of s thus oxygen fugacity during primary mineralization and secondary alteration i e metasomatism we propose a model for the formation of the phillips mine apatite wherein the primary apatite grains crystallized from a reduced s bearing hydrothermal fluid characterized by a low so2 h2s ratio subsequently metasomatism of apatite in the presence of an oxidized fluid which contained an elevated so2 h2s ratio resulted in the exsolution of rare earth elements from apatite and concomitant growth of monazite and the structural incorporation of oxidized s s6 and s4 in apatite this study demonstrates that the oxidation states of s in apatite provide valuable geochemical information regarding the redox evolution of mineralized systems

geochronological and geochemical results from mesozoic basalts in southern south china block support the flat slab subduction model the debate on the mesozoic tectonics of the south china block scb featuring a broad orogenic belt and a large continental magmatic province has been rejuvenated in the past years there are a number of competing tectonic models each predicting different time space and compositional evolutional trends for the mesozoic igneous rocks including sporadic basalts in this paper we report high precision 40 ar 39 ar ages and geochemical and sr nd isotopic data for basaltic rocks in central eastern scb these results together with a data compilation indicate three evolutionary stages for the basalts stage 1 195 160 ma basalts occur only in inland scb they exhibit low initial 87 sr 86 sr ratios 0 7038 to 0 7078 high e nd t values 1 5 to 6 0 and ocean island basalt oib like geochemical characteristics such as low la nb ratios 0 6 to 1 4 implying negligible to minor lithosphere contamination stage 2 160 110 ma basalts mostly from the same inland region with minor occurrence closer to the coast are characterized by variable initial 87 sr 86 sr ratios 0 7053 to 0 7102 end t values 6 0 to 5 0 and low la nb ratios 0 9 to 1 8 indicating an enhanced asthenosphere lithosphere interaction stage 3 110 to 80 ma basalts can be further divided into two sub groups based on their geographic and geochemical variations basalts in the coastal regions are characterized by low end t values 8 1 to 3 7 and variably high la nb ratios up to 4 8 indicating a strong arc signature on the other hand basalts in the inland regions have high e nd t values 1 9 to 6 8 and low la nb ratios 0 6 to 1 2 that are similar to the stage 1 basalts we interpret these features as results of geodynamics processes related to the break up foundering and retreating of an early mesozoic flat subducted oceanic slab

heterogeneous exhumation of the mount isa orogen in ne australia after 1 6 ga nuna assembly new high precision 40ar 39ar thermochronological constraints

response to the comment by w h schwarz et al on joint determination of 40k decay constants and 40ar 40k for the fish canyon sanidine standard and improved accuracy for 40ar 39ar geochronology by p r renne et al 2010

sequential porphyroblast growth and crenulation cleavage development during progressive deformation abstract six stages of crenulation cleavage development during a single deformation event d2 can be recognised in both matrix and porphyroblast inclusion trails in the robertson river formation n e australia these stages progress from undeformed to crenulation and differentiated crenulation of s1 through development of differentiated crenulation cleavage s2 and differentiated schistosity s2 to homogeneous foliation s2 these rocks underwent prograde metamorphism during d2 and chloritoid garnet staurolite andalusite and sillimanite isograds have been mapped most rocks are rich in porphyroblasts which commonly contain well defined inclusion trails the geometry of these trails varies from one mineral type to the next depending on the timing of porphyroblast growth relative to the stage of crenulation cleavage development in the schistose matrix the deformation history involved progressive bulk inhomogeneous shortening and the strain is very heterogeneous on all scales partly as a consequence of this hence each of the six stages of crenulation cleavage development formed locally at various times during d2 therefore if a porphyroblast grew late in the deformation it could overgrow all stages of crenulation cleavage development including the very early ones the temperature increased during metamorphism in any one locality and consequently some porphyroblasts overgrew others that were unstable in the new conditions preserving them from destruction hence the precise timing of mineralogical reactions deduced from isograds and mineral chemistry relative to the stages in schistosity development can also be determined in the andalusite zone the sequence of porphyroblasts in decreasing order of age is biotite garnet biotite staurolite andalusite dissolution of garnet occurred during growth of staurolite and especially andalusite and dissolution of staurolite accompanied andalusite growth

anhydrite stability and the effect of ca on the behavior of sulfur in felsic magmas the distribution of sulfur in the system na2o k2o cao al2o3 sio2 h2o s was investigated at 2 kbar 750 950 c and oxygen fugacities ranging from the co coo to the re reo2 buffer anhydrite caso4 crystallized in all the experiments conducted at oxidizing conditions ni nio 0 5 to 1 and above under otherwise equal conditions an inverse relationship between sulfur and cao concentration was observed in melts coexisting with anhydrite with the solubility product k cao so3 being constant where cao and so3 are the molar fraction of cao and so3 in the quenched glasses this suggests that anhydrite dissociates upon dissolution in the melt according to caso4 anhydrite ca2 melt so42 melt the solubility product strongly depends on temperature with lnk 28 573 917 t 11 26 0 80 this corresponds to an enthalpy of dissolution of h r 237 5 7 6 kj mol under reducing conditions co coo and ni nio buffer cao has no effect on the fluid melt partition coefficient of sulfur d sfluid melt at 850 c 2 kbar partition coefficients were 519 30 at the ni nio buffer and 516 11 at the co coo buffer for cao contents in the melt up to 1 wt these data are virtually identical to those measured in the cao free haplogranite system under reducing conditions however under more oxidizing conditions the fluid melt partition coefficient of sulfur appeared to have increased somewhat in the presence of cao this increase may however also be related to the fact that the final melt compositions in these runs were distinctly peraluminous our data show that calcium has no effect on the degassing of sulfur at reducing conditions but it greatly reduces the amount of sulfur available for rapid degassing under oxidizing conditions by stabilizing anhydrite

polyphase deformation in the western mount isa inlier australia episodic or continuous deformation abstract the polyphase deformation history of the mount novit ranges western mount isa inlier australia involved the development of four generations of n s to nw plunging folds development of which alternated with movement on n s striking faults shear zones with west over east displacement the spatial relationships between successive structures and the distinct similarities in the kinematics style and asymmetry are all consistent with continuous deformation during e w to ne sw shortening additional support for this hypothesis is based on the development of the three major fold generations and the intervening faults and shear zones during low p high t metamorphism although the duration of this metamorphic event cannot be constrained recent studies have linked low p metamorphism to heat advection by migrating melts thermal modelling indicates that such metamorphic events will be short lived in comparison to those associated with thermal relaxation of over thickened crust low p metamorphism is unlikely to persist through two or more separate tectonic events therefore syn metamorphic polyphase deformation in the mount novit ranges apparently formed during a single continuous tectonic episode this structural history differs from both the regional scheme and that of previous workers in this area the main differences are i no evidence was found for early thrust faulting prior to n s folding ii there are four generations of n s to nw se folds as opposed to two and iii this study favors a single consistent tectonic regime in contrast to the interpretation of three discrete isolated tectonic events

the crustal scale architecture of the eastern succession mount isa the influence of inversion the three dimensional crustal architecture of the eastern part of the mount isa inlier is investigated from serial cross sections constructed using geological map data revised chronostratigraphy gravity magnetics worms multiscale wavelet edges of potential field data and seismic data the top part of the crust consists of rift and platform type metasediments that were deposited in three cover sequences from 1850 to 1610 ma these rocks constitute the mount isa eastern succession and they were intruded by mafic felsic plutons dykes and sills of various ages before and during the isan orogeny ca 1 6 1 5 ga the eastern succession overlies a felsic metamorphic basement which in turn sits on a tonalitic gabbroic lower crust the depositional basin architecture for the eastern succession was controlled by major n s trending structures that penetrated the lower crust and accommodated e w extension these structures also underlie major upper crustal structures such as the mitakoodi culmination and snake creek anticline that were formed by contraction in the isan orogeny positive inversion may therefore have been a key process in the evolution of the eastern part of the inlier and governs its architecture at the crustal scale inversion involved reactivation of basement penetrating structures which localised contractional structures in the cover sequences above as well as influencing pluton emplacement the felsic metamorphic basement may have been penetratively deformed during inversion the spatial association between the basin controlling and contractional structures suggests that either early extensional displacements were completely reversed by later contraction or that much of the eastern succession has remained essentially parauthochthonous relative to the basement

photochemical and climate consequences of sulfur outgassing on early mars abstract ancient mars might have been warm and wet compared to today but climate models have trouble reproducing this warmth partly because of the faintness of the young sun and partly because of inherent limitations to co2 h2o greenhouse warming in particular rayleigh scattering of incoming sunlight by a dense co2 rich atmosphere leads to a high planetary albedo thereby reducing the amount of sunlight absorbed by the planet it has been recently suggested that the presence of 1 100 ppmv so2 in mars early atmosphere might have provided enough additional greenhouse warming to maintain a warm wet early mars we show however that this warming should have been more than offset by cooling from sulfate and sulfur aerosols in early martian atmosphere hence the paradox of mars early climate remains unresolved

volatile sulphur and chlorine major and trace element geochemistry of mafic to intermediate tephras from the chilean southern volcanic zone 33 43 s here we present the first systematic investigation of volatile geochemistry along the southern volcanic zone svz of chile holocene olivine hosted melt inclusions in the most mafic tephras sampled from 16 volcanoes along the volcanic front of the svz between 33 s and 43 s were analysed for pre eruptive sulphur chlorine and major element contents these results are combined with trace element compositions of the host whole rocks the highest fractionation corrected gas contents occur in the least degassed melt inclusions from small monogenetic cones of los hornitos cabeza de vaca and apagado from both the transitional and the southern central svz reaching 3 000 g g s and 1 400 g g cl while the lowest abundances of 1 100 g g s and 600 g g cl were found in the central svz at volcan lonquimay volcan llaima and volcan villarrica chlorine co varies with trace element indicators for the degree of melting and or source enrichment such that the lowest cl contents are found in high degree melts from the most depleted mantle sources the size of the volcanic edifices correlates inversely with cl abundances in the melt this could reflect more extensive degassing during ascent through the complex magma plumbing systems beneath the stratovolcanoes or greater dilution during larger degrees of melting of more depleted sources or a combination of these factors compared to other subduction zones the svz melt inclusions exhibit cl and s abundances in the same range as most of those from the central american and those from the marianas arcs