dehydration melting of tonalites part i beginning of melting the beginning of dehydration melting in the tonalite system biotite plagioclase quartz is investigated in the pressure range of 2 12 kbar a special method consisting of surrounding a crystal of natural plagioclase an45 with a biotite quartz mixture and observing reactions at the plagioclase margin was employed for precise determination of the solidus for dehydration melting the beginning of dehydration melting was worked out at 5 kbar for a range of compositions of biotite varying from iron free phlogopite to iron rich ann70 with and without titanium fluorine and extra aluminium in the biotite the dehydration melting of phlogopite plagioclase an45 quartz begins between 750 and 770 c at pressures of 2 and 5 kbar at approximately 740 c at 8 kbar and between 700 and 730 c at 10 kbar at 12 kbar the first melts are observed at temperatures as low as 700 c the data indicate an almost vertical dehydration melting solidus curve at low pressures which bends backward to lower temperatures at higher pressures 5 kbar the new phases observed at pressures 10 kbar are melt enstatite clinopyroxene potassium feldspar amphibole in addition to these zoisite was also observed at 12 kbar with increasing temperature phlogopite becomes enriched in aluminium and deficient in potassium substitution of octahedral magnesium by aluminium and titanium in the phlogopite as well as substitution of hydroxyl by fluorine have little effect on the beginning of dehydration melting temperatures in this system the dehydration melting of biotite ann50 plagioclase an45 quartz begins 50 c below that of phlogopite bearing starting composition solid reaction products are orthopyroxene clinopyroxene potassium feldspar amphibole epidote was also observed above 8 kbar and garnet at 12 kbar 750 c the experiments on the iron bearing system performed at 5 kbar were buffered with ninio the f o 2 in high pressure runs lies close to cocoo with the substitution of octahedral magnesium and iron by aluminium and titanium and replacement of hydroxyl by fluorine in biotite the beginning of dehydration melting temperatures in this system increase up to 780 c at 5 kbar which is 70 c above the beginning of dehydration melting of the assemblage containing biotite ann50 of ideal composition the dehydration melting at 5 kbar in the more iron rich ann70 bearing starting composition begins at 730 c and in the ann25 bearing assemblage at 710 c this indicates that quartz biotite plagioclase assemblages with intermediate compositions of biotite ann25 and ann50 melt at lower temperatures as compared to those containing fe richer or mg richer biotites this study shows that the dehydration melting of tonalites may begin at considerably lower temperatures than previously thought especially at high pressures 5 kbar

experimental calibration of the aluminum in hornblende geobarometer with application to long valley caldera california volcanic rocks a new geobarometer based on the al content of igneous hornblendes in equilibrium with melt fluid biotite quartz sanidine plagioclase sphene and magnetite or ilmenite has been calibrated experimentally the calibration was performed by equilibrating the required phase assemblage over the pressure range 2 8 kbar at 740 780c and then analyzing euhedral hornblendes in equilibrium with glass melt experiments were performed on natural samples of both volcanic and plutonic rocks earlier empirical calibrations of this geobarometer relied on analyzing natural hornblendes from plutons with the required phase assemblage and inferring pressure from nearby metamorphic country rocks the experimental calibration differs from the empirical calibrations especially above 5 kbar and shows that the al content of hornblendes in equilibrium with the required phase assemblage is greater for a given total pressure than previously thought the geobarometers uncertainty is dramatically reduced the derived equation is p plus minus 0 5 kbar 3 minus 3 46 plus minus 0 24 4 23 plus minus 0 13 al sup t the geobarometer is applied to post bishop tuff volcanic rocks from long valley caldera california and reveals that most rhyodacites in this complex erupted from depths of about 6 km these eruptions occurred over 500 000 yr suggesting that the rhyodacitic magma reservoir beneath longmore valley had reached a steady p depth t state less

dehydration melting of amphibolite at 10 kbar the effects of temperature and time we have simulated the dehydration melting of a natural low k calcic amphibolite 67 4 hornblende 32 5 anorthite in piston cylinder experiments at 10 kbar and 750 1000 c for 1 9 days the solidus temperature is lower than 750 c garnet appears at 850 c the overall reaction is hb p l cpx al hb ca hb ga opx three stages of reaction are 1 melting dominated by the growth of clinopyroxene and garnet with little change in composition of liquid or garnet 2 a reversal of this reaction between 875 c and 900 c with decreases in the amounts of liquid and garnet and 3 a large increase in liquid along with the loss of hornblende and decrease of plagioclase while clinopyroxene and garnet increase garnet is enriched in pyrope and zoned from fe cores to mg edges range 3 mol pyrope liquid composition is enriched first in an to 950 c and then in ab the liquids are more calcic and aluminous than natural tonalites which is attributed to the plagioclase composition an90 the formation of peraluminous liquid from the metaluminous amphibolite is caused by anorthite not h2o saturated conditions the results are consistent with an amphibolite phase diagram with relatively high solidus temperatures in the garnet absent field 900 1000 c but with a solidus backbend at 7 9 kbar coincident with the garnet in boundary hornblende breakdown due to garnet formation in a closed system must make h2o available for h2o undersaturated melting right down to the h2o saturated solidus below 700 c which defines a large low temperature pt area where hydrous granitoid melts can be generated with residual garnet and hornblende

precise determination of the open ocean 234u 238u composition uranium has a long residence time in the open oceans and therefore its salinity normalized u concentration and 234u 238u activity ratio expressed herein as 234u the deviation from secular equilibrium are assumed to be uniform the marine 234u 238u activity ratio is currently in radioactive disequilibrium and shows a 15 excess of 234u with respect to the secular equilibrium value due to continuous input from riverine sources knowledge of the marine 234u and how it has evolved through the quaternary is important for validating age accuracy in the u series dating of marine carbonates which is increasingly relied upon for providing a chronological basis in paleoclimate research however accurate and precise measurements of 234u are technically difficult thus existing compilations of the open ocean 234u value vary by up to 10 and the assumed uniformity in the oceanic 234u remains to be confirmed using mc icpms techniques and a suite of multiple faraday cups instead of the typical configurations based on a combined faraday cup multiplier array a long term reproducibility of better than 0 3 2 is achieved for 234u measurements applying these very high precision techniques to open ocean seawater samples an average 234u of 146 8 0 1 2 m n 19 is obtained these high precision seawater measurements yield an external reproducibility of better than 0 4 2 and show that the open oceans have a uniform 234u on the sub level these new data constrain the vertical mixing time of the open oceans to less than 1000 years

the ca 1740 1710 ma leichhardt event inversion of a continental rift and revision of the tectonic evolution of the north australian craton abstract interpretation and modelling of high resolution regional geophysical data of the central leichhardt river fault trough in the mount isa inlier are used to determine the timing of a major basin inversion event following the development of the ca 1780 1740 ma leichhardt superbasin inversion of the leichhardt superbasin formed the regional north south trending leichhardt anticline during east west shortening the limbs of the anticline are overprinted by several east west trending wedge shaped non magnetic sub basins filled with ca 1710 ma calvert and isa superbasin successions these relationships suggest inversion of the leichhardt superbasin occurred between ca 1740 and 1710 ma the event is also known to have affected the northern and eastern north australian craton the scale of the inversion suggests it was a significant event that we have defined as the leichhardt event this event requires a major tectonic driver to the east of the north australian craton possibly the accretion of a micro continental ribbon to the east of the mount isa inlier the results of this study have implication for paleogeographic reconstruction of the australian continent during the formation of nuna because eastern north australian craton faced an ocean at ca 1740 1720 ma the results also challenge the significance and intensity of crustal shortening associated with the ca 1600 1500 ma isan orogeny throughout the western mount isa inlier

nature and origin of proterozoic a type granitic magmatism in the southwestern united states of america the mountain ranges of arizona and adjacent california and nevada contain large areas underlain by proterozoic anorogenic granites comprising the southwesternmost portion of a transcontinental belt of 1 4 1 5 ga old anorogenic complexes that extends across north america northeast into labrador of these a two mica monazite bearing granitic suite resides in central and southeastern arizona as part of a peraluminous subprovince that is bordered on the south southern arizona to sonora and west western arizona and adjacent portions of california and nevada by marginally metaluminous granites bearing biotite sphene hornblende and fluorite all of these 1 4 ga granites are distinctly more potassic iron enriched relative to mg and depleted in ca mg and sr in contrast to typical orogenic granitoids in general the large ion lithophile element enriched composition is a consequence of limited melting of a water deficient crustal source at depths greater than 25 37 km for the peraluminous granites this contrast is less extreme perhaps resulting from a larger degree of melting as a consequence of a greater metasedimentary component and water in its crustal source the anorogenic granitic magmas intruded into the upper crust at depths of 8 17 km or shallower at temperatures up to 790 c the most dramatic variation in the crystallization intensive parameters resides in the oxygen fugacity which spans three orders of magnitude relative to other anorogenic suites all of the magmas crystallized at elevated levels of o2 as reflected in their assignment to the anorogenic magnetite series yet a regionally significant rise in primary o2 levels unmatched elsewhere in the transcontinental belt occurs for plutons in western arizona including the holy moses and hualapai granites the most extreme case is the hualapai granite whose biotite fe fe mg ratios drop due to high o2 to a low of 0 27 down from more typical levels of 0 54 to 0 75 such extreme variations in primary levels of oxygen fugacity must be an indirect imprint of regional changes of the level of oxidation of the lower crust the high fo2 holy moses and hualapai plutons have intruded near the regional boundary between the metaluminous and peraluminous granites and appear to be imaging a major change in the level of oxidation of the lower crust this boundary is also approximately equivalent to significant changes in the nd and pb isotopic compositions of these granites and the metamorphic and magmatic character of the older orogenic terrane on a global scale the crust forming orogenies ended by 1 6 ga ago and the continents entered a long lived era dominated by localized extension and transcontinental intrusion of anorogenic potassic rapakivi granite mafic dike swarms charnockite and anorthosite the absence of orogenic deformation implies that plate consumption became intraoceanic during this time the profuse and widespread nature of the igneous activity has no phanerozoic analogue and is considered to be unique to the proterozoic a crustal overturn model ties the magmatism to heating within a largely undepleted subcontinental mantle the eventual rise of mantle plumes and the transfer of heat into the youthful undifferentiated proterozoic crust subsequent melting and rise of potassic granitic magmas from the lower crust leads to considerable crustal reorganization a process that would continue until both the mantle and crust reached a stable configuration

dehydration melting of tonalites part ii composition of melts and solids dehydration melting of tonalitic compositions phlogopite or biotite plagioclase quartz assemblages is investigated within a temperature range of 700 1000 c and pressure range of 2 15 kbar the solid reaction products in the case of the phlogopite plagioclase an45 quartz starting material are enstatite clinopyroxene and potassium feldspar with amphiboles occurring occasionally at 12 kbar zoisite is observed below 800 c and garnet at 900 c the reaction products of dehydration melting of the biotite ann50 plagioclase an45 quartz assemblage are melt orthopyroxene clinopyroxene amphibole and potassium feldspar at pressures 8 kbar and temperatures below 800 c epidote is also formed almandine rich garnet appears above 10 kbar at temperatures 750 c the composition of melts is granitic to granodioritic hence showing the importance of dehydration melting of tonalites for the formation of granitic melts and granulitic restites at pressure temperature conditions within the continental crust the melt compositions plot close to the cotectic line dividing the liquidus surfaces between quartz and potassium feldspar in the haplogranite system at 5 kbar and ah2o 1 the composition of the melts changes with the composition of the starting material temperature and pressure with increasing temperature the melt becomes enriched in al2o3 and feo mgo potash in the melt is highest just when biotite disappears the amount of cao decreases up to 900 c at 5 kbar whereas at higher temperatures it increases as amphibole clinopyroxene and more an component dissolve in the melt the na2o content of the melt increases slightly with increase in temperature the composition of the melt at temperatures 900 c approaches that of the starting assemblage the melt fraction varies with composition and proportion of hydrous phases in the starting composition as well as temperature and pressure with increasing modal biotite from 20 to 30 wt the melt proportion increases from 19 8 to 22 3 vol 850 c and 5 kbar with increasing temperature from 800 to 950 c at 5 kbar the increase in melt fraction is from 11 to 25 8 vol the effect of pressure on the melt fraction is observed to be relatively small and the melt proportion in the same assemblage decreases at 850 c from 19 8 vol at 5 kbar to 15 3 vol at 15 kbar selected experiments were reversed at 2 and 5 kbar to demonstrate that near equilibrium compositions were obtained in runs of longer duration

local to regional scale structural controls on mineralisation and the importance of a major lineament in the eastern mount isa inlier australia review and analysis with autocorrelation and weights of evidence although major crustal lineaments may play an important role in mineralisation the relationship between lineaments and mineral deposits can be quite cryptic and structural controls may vary as a function of scale along lineaments major lineaments alone may be of limited use for detailed target generation the cloncurry lineament in the eastern part of the mount isa inlier is a crustal scale structure defined by potential field derived worms weights of evidence quantifies the association between mineral occurrences and this lineament autocorrelation is used to recognise structural controls on mineralisation at different scales by progressively limiting the lengths of the vectors between mineral occurrence points in the autocorrelation plot the weights of evidence analysis shows that au au cu cu au and cu deposits have a positive spatial correlation to the cloncurry lineament which suggests it that acted as a primary crustal scale control on the localisation of cu and au through focussing mineralisation systems on a broad scale however autocorrelation defines a variety of local structural controls which can be interpreted as shear zones variably oriented fault sets en echelon fault arrays and potentially the orientation of bedding and or iron formations which localise fluid flow and mineral deposition at finer scales the results suggest that major lineaments defined by geophysical contrasts can be used in conjunction with techniques of spatial analysis for targeting structurally controlled mineralisation in areas under thin cover adjacent to mineralised terrains such as the mt isa inlier

experimental evidence for high temperature felsic melts formed during basaltic intrusion of the deep crust basaltic intrusion into the continental crust was simulated with melting experiments on layered charges of high alumina olivine tholeiite haot and metapelite at 11 kbar rapid diffusion of cao and na 2 o into the metapelite and h 2 o k 2 o sio 2 and al 2 o 3 into the basalt resulted in spectacular changes in the phase assemblage garnet megacrysts crystallized profusely across the basalt layer cumulate assemblages closely resembled rocks found in underplating localities and glass compositions graded from dacite to rhyolite even at temperatures as high as 1150 c silicic melts can be generated during interaction of basalt with metapelite at temperatures well above the h 2 o saturated granite minimum calculations based on these results and comparison with natural occurrences suggest that high temperature felsic melts with 2 o can form and segregate at rates of 1 cm yr

neodymium isotopic constraints for the origin of mesoproterozoic felsic magmatism gawler craton south australia mesoproterozoic felsic magmatism of the gawler range volcanics and hiltaba suite granites occurred at 1585 1595 ma across much of the gawler craton south australia nd isotopic analysis of this felsic magmatism combined with petrological and geochemical arguments suggest derivation by partial melting of both paleoproterozoic and archean crust the majority of samples analyzed have nd isotopic and geochemical characteristics compatible with the involvement of paleoproterozoic crust stabilized during the 1 85 1 71 ga kimban orogeny as sources for the mesoproterozoic magmatism others require derivation from sources dominated by archean rocks this cycle of paleoproterozoic crustal stabilization followed by involvement of this crust mesoproterozoic felsic magmatism is one previously documented from many parts of mesoproterozoic laurentia on the basis of models proposing east australia antarctica to be the conjugate landmass at the rifted margin of western north america it appears that the voluminous mag

a model for low pressure facies metamorphism during crustal thickening low pressure metamorphic facies i e high t p ratios are widespread in a wide range of tectonic settings explanations offered for the occurrence of these facies include extensional and or magmatic models however these fail to explain that the low p facies metamorphism is commonly coeval with a phase of pervasive crustal thickening with t p ratios increasing during or slightly lagging behind the thickening we propose an alternative explanation based on the approximate synchroneity of crustal thickening and erosion thinning of the mantle lithosphere

yardea dacite large volume high temperature felsic volcanism from the middle proterozoic of south australia the yardea dacite is a large volume felsic volcanic unit from the middle proterozoic gawler range volcanics of south australia it has been previously described as an ignimbrite however some samples contain no petrographic evidence for a pyroclastic origin but have characteristics compatible with final crystallization from a nonfragmented magma these samples may have erupted as lavas but others are likely to be extremely densely welded ignimbrites suggesting a compound nature for the unit geothermometry and phase equilibria indicate that the yardea dacite originated from a high temperature 1000 c felsic magma with a low water content 2 the yardea dacite is not associated with a known caldera of the valles type and shares many characteristics of recently described cenozoic felsic volcanic rocks from the western united states interpreted as rheoignimbrites or as unusually extensive lavas

the musgrave province stitching north west and south australia abstract the musgrave province in central australia is an east west trending 120 000 km2 region composed of lithologic units that range in age from earliest mesoproterozoic to neoproterozoic it is one of the few terrains in australia that records significant grenvillian aged tectonism and has a distinctly younger geological history than the surrounding fragments of proterozoic australia compared to other proterozoic terranes of australia the evolution of the musgrave province is poorly understood the region contains evidence for the development of earliest mesoproterozoic arc related magmatism with major juvenile crustal additions comprising intercalated felsic and mafic gneisses with protolith ages ranging from ca 1 60 to 1 54 ga this period of arc magmatism was followed by closure of the ocean separating the north australian craton nac from the south australian craton sac at ca 1 54 ga arc magmatism and collision was followed by a period of extension during which protoliths to the metasedimentary rocks of the eastern musgrave province were deposited at ca 1 4 ga the combined juvenile nd isotopic signature and detrital zircon ages from these metasediments suggests provenance from non australian sources with possible sources originating from laurentia granitic magmatic events occurred at ca 1 3 and 1 20 1 14 ga the latter comprising the voluminous a type granites of the pitjantjatjara supersuite the intrusion of these magmatic units provide key temporal constraints on deformational events and are coeval with the voluminous a type suites of laurentia which would invite correlation of australia with laurentia at ca 1 30 1 14 ga these magmatic episodes are accompanied by deformation at granulite facies at ca 1 30 ga unnamed event and 1 23 1 15 ga musgrave orogeny these deformation metamorphic events have completely recrystallised any primary layering within the pre 1 30 ga rocks forcing interpretations on their genesis to be inferred from geochemical and isotopic data the tectonic significance of these events is not clear however they may form part of a larger grenville aged deformational belt extending into the western australian albany fraser province and antarctica emplacement of the voluminous mafic ultramafic giles complex and associated granulite facies metamorphism of the giles event occurred at ca 1 078 ga the layered intrusions of the giles complex are generally fault bound and are considered to represent discrete bodies emplaced in an extensional regime associated with generation of the warakurna large igneous province the ca 1 08 ga alcurra dolerite unnamed ca 1 0 ga olivine dolerites and ca 0 8 ga amata dolerite dissect much of the musgrave province the alcurra dolerite and unnamed olivine dolerite dykes have been proposed as originating from a modified sub continental lithospheric mantle whereas the amata dolerite is proposed to be plume related coinciding with the initiation of rifting creating the centralian superbasin an earliest cambrian intraplate transpressional event is recorded in the ca 550 ma petermann orogeny this resulted in deep crustal exhumation of much of the northern musgrave province from beneath the centralian superbasin with mylonitic deformation recorded at pressures of 12 kbar and temperatures of 700 750 c

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

amphibolite dehydration melting sorting out the solidus amphibolite melts under vapour absent conditions by dehydration melting during which h 2o released from hornblende is transferred directly into h 2o undersaturated silicate liquid five independent 1991 studies on amphibolite dehydration melting differ from each other and from earlier conceptual treatments five solidi between 7 and 10 kb vary from 975 c to 740 c two solidi above 10 kb are near vertical near 975 c and 850 c some differences are due to different amphibolite compositions and mineralogy we use the results of earlier experiments involving garnet supported by our own new results to construct a closed system phase diagram for a simple fully hydra ted amphibolite hornblende plagioclase showing the solidus and the melting interval for hornblende garnet other minerals liquid the amphibolite vapour absent solidus is defined by the beginnings of two multivariant reactions 1 a near vertical curve large positive d p dt where the formation of augite dominates and 2 a near horizontal curve a t higher pressures where the formation of garnet dominates the solidus curve bends back estimated near 900 c 9 kb with slope changing to low positive dp dt the new phase diagram expands the field for liquid generation with garnet amphibolite residues to much lower temperatures and pressures than the other recent experimental results increasing the range from which small amounts of hydrous granitoid melts may be segregated by compaction the reversed near horizontal slope of the solidus near 10 kb has interesting petrological consequences

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

a framework of overprinting orogens based on interpretation of the mount isa deep seismic transect a deep seismic reflection transect and coincident refraction study reveal the crustal structure developed during mesoproterozoic orogenesis preserved in the mount isa inlier of northwest queensland australia geologic interpretation of the reflection data indicates a two stage evolution for east west shortening the first stage involved west directed folding and thrusting above a decollement in the eastern part of the terrane this decollement system was the forehand of an orogenic system rooted east of the present day inlier the second stage is characterized by a network of basement cutting reverse and strike slip faults that dissected the decollement this overprinting system was linked to crustal thickening within the mount isa terrane the refraction profile indicates the crust beneath the inlier is currently approximately 55 km thick rocks with transitional velocities 7 km s in the lower 15 km are interpreted as mafic underplate formed during the pre orogenic rift history of the inlier the refraction profile also reveals two west dipping high velocity layers within the crust evaluating the origin of these layers in light of the kinematics of crustal thickening suggests they are portions of the underplated material thrust to midcrustal levels during the second stage of shortening the overprinting orogenic scenario has implications for the geodynamic evolution of the inlier that should have a direct bearing on mineralizing systems localized by reverse faults developed during the second stage of shortening this basement cutting stage established a linked fault network able to tap deeper crustal circulation during the intrusion of late to postorogenic granitoids

constraints on long lived mesoproterozoic and palaeozoic deformational events and crustal architecture in the northern mount painter province australia abstract meta sedimentary and igneous rocks preserved in the hanging wall of the paralana fault northern mount painter province record a protracted ca 1600 myr multi staged metamorphic and poly deformational evolution related to mesoproterozoic and palaeozoic orogenic events the mount painter province was located near the margin of both the south australian craton and the north australian craton which allows comparisons of the tectonic evolution of these margins and provides context for the evolution of the eastern proterozoic australia for both the columbia and gondwana supercontinents within the mount painter province meta sedimentary rocks form part of a basin system developed adjacent to the margins of the gawler craton and curnamona province following the olarian wartakan orogenic system sedimentation was followed by ca 1590 1552 ma ductile deformation d 1 d 3 at mid to upper crustal levels preserving a rapid ca 6 myr burial and exhumation cycle initial deformation is characterised by the development of a layer parallel fabric s 1 s 1 is overprinted by recumbent north verging folds f 2 formed during mid to upper amphibolite facies peak metamorphism m 2 which are overprinted by tight upright shallow ne sw plunging folds f 3 d 3 folds developed contemporaneous with sinistral oblique shearing and the development of proto mylonitic to ultra mylonitic fabrics along the paralana fault zone the timing of d 3 deformation is constrained by the development of a penetrative steeply dipping foliation s 3 in the ca 1585 1569 ma shallowly emplaced mount neill granite and relatively undeformed granites dated at ca 1552 ma further constraint on the absolute timing of mesoproterozoic tectonism is provided through the dating of zircon growth from samples of the meta sedimentary radium creek metamorphics this dating reveals a 1555 16 ma metamorphic event interpreted to be the consequence of m 3 amphibolite facies regional metamorphism and accompanying metasomatism the transition between d 2 and d 3 is interpreted to reflect a shift from ca 1591 1585 ma n s shortening to ca 1585 1552 ma nw se shortening this episode of orogenesis is correlated with the ca 1585 1540 ma kararan orogeny in the northern gawler craton which distinctly postdates the ca 1600 1590 ma olarian orogeny as previously interpreted the mesoproterozoic structural architecture was reactivated during neoproterozoic extension d 4 and the ca 500 ma delamerian orogeny delamerian structures are formed in a predominantly brittle regime early thin skinned thrusting was followed by thick skinned deformation d 5 d 6 during which open f 5 folds developed above oblique lateral ramps and produced type 0 overprinting patterns with f 3 mesoproterozoic folds steeply plunging f 6 buckle folds with se trending axial traces are the last phase of deformation recorded both mesoproterozoic ductile deformation phases and the palaeozoic ductile brittle to brittle overprints were focussed along the paralana fault emphasising the repeated long lived evolution of this crustal scale fault system

structural evolution of a 1 6 ga orogeny related to the final assembly of the supercontinent nuna coupling of episodic and progressive deformation

petrology geochronology and isotope geochemistry of the post 1820 ma granites of the mount isa inlier mechanisms for the generation of proterozoic anorogenic granites four distinct episodes of granitic intrusion occurred in the mount isa inlier at 1820 1800 ma 1760 1740 ma 1700 1670 ma and 1560 1480 ma combined these granites cover at least 4700 km2 and they contrast in composition and intrusive style with an earlier major felsic magmatic event which occurred between 1870 and 1840 ma post 1820 ma granite batholiths are of two types large major coarse grained anorogenic i type intrusives and small minor microgranites which are probably s type the i type granites are all pink to red colour and most contain fluorite rapakivi textures are common they have a high ratio of fe mg and are more enriched in tio2 zr nb y and u than the 1870 1840 ma felsic igneous rocks the post 1820 ma i types are interpreted to have formed by small degrees of partial melting of a mafic underplate the smaller microgranites are relatively k2o enriched and are sometimes muscovite bearing they are interpreted to be derived from proterozoic feldspathic metasediments the first three episodes of granite magmatism are coeval with major extensional sedimentary sequences but they are generally spatially displaced from the maximum area of surface rift development this separation is predicted by the asymmetric detachment model of continental extension the tectonic relationship of the youngest granite episode is not clearly defined the 143nd 144nd initial ratios of the post 1820 ma granites fall into two groups one having positive relatively primitive nd values of 1 8 to 3 5 the other having negative nd values ranging from 2 9 to 3 7 samples with positive nd values are interpreted as partial melts of mafic sources formed post 1870 ma whilst the group with negative nd values are derived from significantly older sources formed probably around 2300 2000 ma the sources of these granites are produced by significant underplating events that may occur during either extension or compression

constraints on melting and magma production in the crust major intrusions of granitic rocks are found in several tectonic settings and in all cases crustal melts may contribute to the volumes of granitic magma high grade metamorphism and partial melting of the crust take place predominantly under fluid absent conditions we present a model for calculating the amounts of melt that may be formed by fluid absent breakdown of micas and amphiboles in common crustal rock types pelitic quartzofeldspathic intermediate and mafic melt proportions depend mainly on the kind of source rock the pressure at which melting takes place the temperature and the hydrous mineral h2o content of the source as a consequence of the pressure dependence of water solubility in silicate melts any given source rock will produce more melt by a given fluid absent reaction at lower pressure at a given pressure higher temperature reactions can produce more melt from a given source rock based on a survey of the compositions of common rock types we show that the amounts of melt can vary from 50vol thus crustal rocks vary widely in their fertility as magma sources depending on the types and amounts of hydrous phases they contain in general muscovite breakdown in pelites will yield only small quantities of melt and lead to migmatite formation biotite breakdown in pelites occurs at higher temperature and because most high grade pelites below granulite grade are biotite rich can yield up to about 50 vol of granitoid melt rocks of intermediate composition and hornblende rich mafic rocks are potentially highly fertile magma sources also provided that the high temperatures necessary for biotite and hornblende breakdown are realized pyroxene rich mafic rocks and quartzofeldspathic rocks are much less potentially fertile data suggest that mechanisms exist for the efficient segregation of melt and restite in systems with 30and probably 20vol melt the pressure temperature history of a region can greatly influence crustal source fertility through its control over the occurrence of subsolidus dehydration and melting equilibria

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

crustal scale structures in the proterozoic mount isa inlier of north australia their seismic response and influence on mineralisation abstract the proterozoic mount isa inlier in northern australia is prospective for base metals and gold it contains a western fold belt including the leichhardt river fault trough and an eastern fold belt separated by older basement rocks of the kalkadoon leichhardt belt sediments and volcanics in both fold belts were deposited in rifts which were subsequently shortened by up to 50 mineralisation appears to be partitioned large tonnage lead and zinc deposits are more prevalent in the leichhardt river fault trough and most gold and copper occurrences are in the eastern fold belt cross sections of the inlier derived from coincident seismic reflection and refraction profiling are dominated by the youngest tectonic events the refraction data imply a west dipping lens of high velocity intermediate to mafic rock in the middle to upper crust in the east of the inlier it is collinear with another lens in the lower crust in the west of the inlier the lenses form a belt of high velocity rock cutting the crust from top to bottom and from east to west the reflection data reveal different styles of compression related structures in the east and west of the inlier thin skinned tectonics dominate in the eastern fold belt the sediments and volcanics are thrust to the west along a number of shallowly east dipping upper crustal detachments the detachments in turn are cut by steeply east dipping reverse faults which link into the zone of high velocity rocks defined by the refraction data in contrast faults in the western fold belt are steep and penetrate to mid crustal levels and probably also link into the belt of high velocity material deeper in the crust the partitioning into different tectonic styles occurs across the kalkadoon leichhardt belt which appears to have acted as a buttress during the crustal shortening published mineralisation models attribute the lead zinc mineralisation to circulating fluids at shallow crustal levels within the leichhardt river fault trough high reflectivity of faults with a close spatial association with mineralisation is attributed to alteration along the fault caused by migrating fluids copper gold mineralisation in the eastern fold belt is scattered but known major deposits lie along trend from a thrust fault shown in the seismic data to be highly reflective this fault links via the upper crustal detachments and the high velocity lenses into the middle to lower crust and is seen as a likely control on fluid migration pathways from lower crustal levels into the supracrustal eastern fold belt the partitioning of the tectonic styles seen in the seismic data and the mode of linking of faults into the middle to lower crust are seen as primary factors in the partitioning of mineralisation in the region

structure of the isan orogeny under cover to the east of the mount isa inlier revealed by multiscale edge analysis and forward and inverse modelling of aeromagnetic data aeromagnetic data reveals the presence of three north south orientated tens of kilometre scale elliptical structures under cover to the east of the mount isa inlier in queensland australia the holy joe creek boot creek and nora creek structures are typically defined by three narrow continuous magnetic ridges highly magnetically susceptible amphibolite bodies similar to those exposed in the inlier are probably the source of most of these magnetic anomalies multiscale edge analysis 2d forward modelling and 3d inverse modelling methods have been used to resolve the geometry and near surface dip of the structures these techniques indicate that the structures dip moderately to steeply around their edges the holy joe creek structure exhibits sub vertical to steeply west dipping sides and a moderate plunge around the northern closure the boot creek and nora creek structures are more complex magnetically but their eastern edges also have sub vertical to steep westerly dips diapirism and domes in an egg box style type 1 fold interference pattern are considered as possible origins for the elliptical structures however disparity between the shape of the structures and possible diapiric bodies and the lack of complementary basin structures to the domes makes these two explanations unlikely a more plausible alternative is that the structures are the result of deformation in a single phase producing folds with a variable hinge line plunge these results indicate that east west shortening during the d2 event of the isan orogeny was the major deformation in the buried rocks of the study area suggesting a shared structural history with the mount isa inlier to the west

the high grade mo re merlin deposit cloncurry district australia paragenesis and geochronology of hydrothermal alteration and ore formation the merlin deposit located in the cloncurry district mt isa inlier is the world s highest grade mo re deposit although the district is renowned for iron oxide copper gold iocg type mineralization merlin and other mo rich occurrences in the district cannot be classified as iocg type deposits the merlin deposit is hosted by the kuridala group 1690 1650 ma which comprises interbedded phyllite and carbonaceous slate calcsilicate rocks and silicified metasiltstone the mount dore granite part of the williams naraku batholith ca 1540 1490 ma is locally thrust over the metasedimentary rock package an early cu pb zn sulphide stage mount dore deposit mostly overlies with only partial spatial overlap the younger mo re mineralization merlin deposit cu pb zn is mainly hosted by carbonaceous slate as infill of hydrothermal coast supported carbonate breccias the younger mo re mineralization merlin deposit occurs as molybdenite infill of hydrothermal matrix supported breccias along or near the contact between carbonaceous slate and calc silicate rocks and locally cuts the cu pb zn mineralization based on detailed petrography and radiometric age dating u pb zircon and titanite ages re os molybdenite ages we define four main stages of hydrothermal alteration stage 1 represents the earliest hydrothermal stage dated here by u pb titanite geochronology at 1551 12 ma and is characterized by na ca alteration with the formation of hematite dusted albite amphibole carbonates titanite apatite quartz magnetite epidote stage 2 is characterized by the formation of k feldspar tourmaline carbonates quartz apatite and cu pb zn mineralization with chalcopyrite sphalerite pyrite galena stage 3 is dated at 1535 6 ma re os molybdenite and consists of the main mo re event that formed intensely mineralised matrix supported breccias accompanied by k feldspar chlorite alteration stage 4 dated here at 1521 3 ma re os molybdenite is of an age similar to that of the emplacement of the mount dore granite 1517 7 ma u pb zircon and is interpreted as either a remobilization event or a later addition of mo re in veins and disseminations that extends several meters below the mo re breccias with formation of chlorite k feldspar apatite rutile the mount dore granite was thrust above the metasedimentary package at ca 1500 ma which coincided with the waning of the hydrothermal activity in the cloncurry district this event is interpreted to be manifested at the merlin deposit as minor millimeter scale mo re vein formation collectively our data indicate that the merlin and the mount dore mineralization formed via a complex and protracted hydrothermal history that includes a time period ca 1560 1540 that has previously been regarded as unprospective for ore deposit formation in the region

mid crustal fluid mixing in a proterozoic fe oxide cu au deposit ernest henry australia evidence from ar kr xe cl br and i fluid inclusions in six quartz veins associated with cu au mineralisation at the giant ernest henry iron oxide copper gold deposit 167 mt 1 1 cu 0 54 ppm au in northwest queensland have been analysed for naturally occurring and neutron produced noble gas isotopes of ar kr and xe a combination of thermal and mechanical decrepitation methods enables distinction between four types of fluid inclusion ultra high salinity 30 to 70 wt nacl eq fluid inclusions have compositions that define two end members that are variably mixed in different samples the first end member has a 40 ar 36 ar value of 29 000 a 40 are cl value of 3 10 3 and mantle like br cl and i cl values of 1 2 10 3 and 11 10 6 respectively the second end member has a much lower 40 ar 36 ar value of less than 2500 a 40 are cl value of 10 6 low br cl values of 0 4 10 3 and i cl values of 1 2 10 6 all ratios are molar carbon dioxide and later lower salinity liquid vapour fluid inclusions have similar 40 ar 36 ar values of less than 2500 in all samples these data are compatible with genetic models in which cu au mineralisation formed at a depth of 6 10 km from circulation of magmatic fluids derived from regionally abundant a type granites and a high salinity halite dissolution brine generated from sedimentary formation waters in the upper crust the largest source of co2 was probably carbonate rich lithologies in the midcrust later lower salinity fluids with a surficial origin diluted the mineralising brines and are preserved in the latest secondary fluid inclusions these data provide insight on the composition of crustal fluids during the proterozoic furthermore the magmatic fluid endmember derived from melts generated by re melting lower crustal paleoproterozoic igneous rocks with a mantle source preserves mantle like br cl and i cl these geochemical characteristics are interpreted to provide insight on i recycling at subduction zones and the composition of seawater in the paleoproterozoic 2007 elsevier b v all rights reserved

accessory minerals and the geochemical evolution of crustal magmatic systems a summary and prospectus of experimental approaches it has recently been recognized that accessory minerals in continental rocks may contain a substantial fraction of the whole rock complement of trace elements and geochemically important isotopes this observation is in itself interesting but its importance is magnified many fold when one considers the possible consequences to the geochemical rearrangement that accompanies crustal melting if for example an accessory phase is insoluble in early formed partial melts it may effectively retain in the residue many incompatible elements e g ree and daughter isotopes e g radiogenic pb that would otherwise be partitioned into the melt in addition should an unmelted accessory mineral e g zircon be entrained in a departing melt fraction it may carry with it radiogenic components pb nd143 hf176 characteristic of the source region the effectiveness of phases such as zircon apatite sphene allanite epidote and monazite in the roles noted above can only be gauged from knowledge of the three fundamental accessory phase parameters i e 1 the solubilities of the accessory minerals in crustal melts 2 the equilibrium mineral liquid partition coefficients for the trace element and isotopes of interest and 3 the diffusivities that govern the rates at which equilibrium will be approached in general these parameters can only be measured in laboratory experiments and relatively few data have been accumulated to date it does seem clear however that apatite generally is residual insoluble during crustal melting and zircon can be depending on melting conditions and abundance of zircon in the source petrographic and geochemical studies indicate that monazite allanite and epidote may also be highly insoluble in some cases but the specific circumstances that promote insolubility remain unknown the few experimentally determined partition coefficients now available for apatite and zircon agree qualitatively with natural rock data on mineral separates however preliminary diffusion measurements and inferences from natural systems suggest that trace element and isotopic equilibrium between undissolved accessories and crustal melts is in fact rarely approached ironically it is this very disequilibrium in the form of preserved diffusion profiles that may afford the most information on time temperature histories of melting events continued experimental interest in the fundamental accessory phase parameters will eventually provide the tools that will enable rigorous assessment not only of the timing and duration of crustal melting episodes but also of the extent of melting and re distribution of trace elements and radiogenic isotopes

the noble gas systematics of late orogenic h2o co2 fluids mt isa australia abstract the noble gases he ne ar kr and xe are powerful geochemical tracers because they have distinctive isotopic compositions in the atmosphere crust and mantle this study illustrates how noble gases can be used to trace fluid origins in high temperature metamorphic and mineralising environments and at the same time provides new information on the composition of noble gases in deeper parts of the crust than have been sampled previously we report data for h2o and co2 fluid inclusions trapped at greenschist to amphibolite facies metamorphic conditions associated with three different styles of mineralisation and alteration in the proterozoic mt isa inlier of australia sulphide fluid inclusions are dominated by crustal 4he however co variations in fluid inclusion 20ne 22ne 21ne 22ne 40ar 36ar and 136xe 130xe indicate noble gases were derived from three or more reservoirs in most cases the fluid inclusions elemental noble gas ratios e g ne xe are close to the ranges expected in sedimentary and crystalline rocks however the elemental ratios have been modified in some of the samples providing evidence for independent pulses of co2 and interaction of co2 with high salinity aqueous fluids compositional variation is attributed to mixing of i magmatic fluids or deeply sourced metamorphic fluids characterised by basement derived noble gases with 20ne 22ne 8 4 21ne 22ne 0 4 40ar 36ar 40 000 and 136xe 130xe 8 ii basinal metamorphic fluids with a narrow range of compositions including near atmospheric values and iii noble gases derived from the meta sedimentary host rocks with 20ne 22ne 8 9 8 21ne 22ne these data provide the strongest geochemical evidence available for the involvement of fluids from two distinct geochemical reservoirs in mt isa s largest ore deposits in addition the data show how noble gases in fluid inclusions can provide information on fluid origins the composition of the crust s major lithologies fluid rock interactions and fluid fluid mixing or immiscibility processes

thermoluminescence dating of aceolianites from lord howe island and south west western australian abstract views differ as to how the deposition of aeolianites correlate with glacial interglacial cycles this study evaluates the potential of thermoluminescence tl dating to determine the timing of aeolianite deposition at two australian sites the study areas are rottnest island and the fremantle coastline in western australia and lord howe island in the tasman sea the chronostratigraphy of aeolianite on lord howe island has been comprehensively examined by brooke 1999 whereas the depositional chronology of western australian aeolianite is known only from dating at one or two key sites tl dating provides a chronological framework for both sites and reveals that the timing of deposition of these aeolianites is probably related to sediment supply dune and minor associated marine deposits of lord howe island appear to have been emplaced mainly during sea level highstands when the shelf surrounding the island was flooded and beach sediments accumulated in contrast the aeolianites upon rottnest island and along the adjacent western australian coastline seem to have been deposited during periods of both high and low sea level indicating a more continuous supply of shallow marine sediments

petrogenesis of a mesoproterozoic quartz latite granitoid suite from the roxby downs area south australia abstract mesoproterozoic granitoids from the roxby downs area south australia show a range in composition from mafic quartz monzodiorites 56 sio 2 to felsic granites 71 sio 2 and are interpreted as consanguineous on the basis of similar age isotopic composition and mineralogical and geochemical characteristics these granitoids have geochemical similarities with other mesoproterozoic granitoids the world over including enrichments in k 2 o rb ba th u rare earth elements ree nb y zr and depletions in mgo cao sr cr ni v sc relative to typical phanerozoic calc alkaline granitoids the mafic granitoids have mineralogical textural and geochemical features consistent with an origin as cumulates from a more felsic primary magma and the felsic granites likely represent the fractionated residual melt to this accumulation process none of the granitoids from the batholith preserve the primary magma composition which is deduced to be similar to abundant quartz latite volcanic rocks of the region it is estimated by geochemical modeling that about 27 fractionation of observed phenocrysts from a primary quartz latite is required to produce a typical felsic granite 71 sio 2 production of the primary quartz latite magma was likely the result of partial melting of existing paleoproterozoic crust in response to intrusion into the crust of mantle derived magmas the combination of widespread crustal melting contemporaneous granulite facies metamorphism and mantle derived magmatism suggests involvement of large scale tectonothermal processes in the origin of these granitoids the petrogenesis of these rocks and similar occurrences in laurentia and baltica indicates that processes involving reworking of existing paleoproterozoic crust were similar throughout many continental areas during the mesoproterozoic

noble gas and halogen constraints on regionally extensive mid crustal na ca metasomatism the proterozoic eastern mount isa block australia fluid inclusions in late isan quartz veins associated with regional na ca alteration albitisation in the mary kathleen fold belt and the cloncurry district of the eastern mt isa block have been analysed for naturally occurring and neutron produced isotopes of ar kr and xe the noble gases have been extracted using a thermal decrepitation procedure that enables partial deconvolution of the different fluid inclusion types including variably saline aqueous liquid carbon dioxide and mixed aqueous carbonic varieties the variably saline 5 65 wt aqueous fluid inclusions dominate and have 40ar 36ar values of less than 2700 in most of the samples from across the region these fluid inclusions have extremely variable molar br cl values of 0 3 4 10 3 and i cl values of 0 2 35 10 6 and the fluids are interpreted to represent sedimentary formation waters derived from the upper crust that have dissolved variable quantities of halite or scapolite to achieve their ultra high salinity fluid inclusions in a sample from the snake creek anticline in the cloncurry district have the highest 40ar 36ar value of 25 000 demonstrating a deep magmatic or metamorphic fluid origin in this case a magmatic origin is favoured because this fluid is very similar to a fluid involved in iron oxide copper gold iocg mineralization at ernest henry that was also interpreted to have a magmatic origin and because the aqueous fluid inclusions have br cl of 1 2 10 3 and i cl of 10 10 6 that are similar to mantle derived igneous rocks carbon dioxide fluid inclusions dominate samples from the knobby quarry in the mary kathleen fold belt and have a maximum 40ar 36ar value of 6000 7000 these fluid inclusions are estimated to have a 36ar concentration of 1 4 ppb that is similar to the range determined for aqueous fluid inclusions in all the other samples in addition aqueous and carbonic fluid inclusions in all the samples have similar 129xe 36ar plus 84kr 36ar values that are unfractionated and close to the air air saturated water asw range these data are interpreted to indicate an independent and dominantly metamorphic origin for co2 and the presence of mixed aqueous carbonic fluid inclusions are attributed to fluid mixing and or mingling rather than fluid unmixing however the data do not preclude the presence of a minor magmatic co2 component in the samples from the knobby quarry fluid inclusions in most of the samples from the mary kathleen fold belt have higher br cl and i cl values higher 36ar concentrations and lower maximum 40ar 36ar values than fluid inclusions in samples from the cloncurry district this suggests na ca alteration in these different parts of the eastern mt isa block occurred independently however fluid inclusions associated with na ca alteration in the cloncurry district have a very similar composition to fluid inclusions in iocg mineralization stage quartz veins from ernest henry these data are therefore compatible with a genetic relationship between regional na ca alteration and iocg mineralization in the cloncurry district in both cases the ultra saline hydrothermal fluids had a dominant origin from sedimentary formation water but are interpreted to contain a magmatic component sourced from the late isan williams naraku batholiths which may have driven fluid convection

discrete element modelling applied to mineral prospectivity analysis in the eastern mount isa inlier numerical modelling using a discrete element technique is employed here to examine the response of a fracture system in the eastern mount isa inlier to an applied stress regime model scale parameters and boundary conditions were varied to test which combinations of geometry and material properties produce the best correspondence between the known mineral deposit distribution and zones of anomalous model stress modelled areas of combined low minimum principal stress 3 and low mean stress m show the best correlation with deposits but these areas do not clearly correspond to specific fault orientations or configurations rather the models produce complex zoning of stress anomalies in response to the partitioning of stress across complex fault blocks and the interaction between more competent granitoid bodies less competent meta sedimentary rocks and the fault and rock boundary complexities adding fluid to the models and identifying an optimum orientation of the stress field with 1 in an ese direction produces the highest degree of visual correspondence with the known mineral deposit distribution and the previous empirical prospectivity analysis and also identifies several potentially prospective areas that may not have been previously tested by explorers the models are consistent with mineralisation occurring or being remobilised from earlier concentrations during a major phase of regional fluid flow facilitated by a complex fault array late during the evolution of the isan orogeny and synchronous with the waning stages of the emplacement of the williams batholith

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