an orthopyroxene biotite geothermometer and its application in crustal granulites and mantle derived rocks using the experimental data on fe mg exchange between orthopyroxene and biotite of fonarev konilov 1986 an orthopyroxene biotite geothermometer is developed the thermometer is corrected for mixing of ti and al in octahedral sites in biotite and also for non ideal mixing of fe and mg in orthopyroxene the thermometer is applied to several amphibolite granulite transition facies and granulite facies rocks and also to mantle xenoliths it yields consistent results in rocks of widely varying bulk composition and highly magnesian mantle xenoliths this thermometer removes the difficulty of estimating temperature in garnet free rocks in high grade terrains and also provides independent estimates of temperature in garnet bearing assemblages

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

igneous microstructures in migmatites clear differences between the microstructures of leucosomes and mesosomes occur in some nebulitic migmatites in the proterozoic arunta block central australia the leucosomes show crystal faces of k feldspar microcline microperthite cordierite andalusite and plagio clase against quartz indicating crystallization of a melt in contrast the mesosomes show predominantly polygonal grain shapes modified by 001 crystal faces of biotite and rare crystal faces in some porphyroblasts of garnet cordierite and orthopyroxene indicating re crystallization in the solid state the occurrence of abundant crystal faces in minerals such as feldspar cordierite and andalusite which typically grow crystal faces in magmas but generally not in metamorphic rocks is evidence of former melt therefore igneous microstructures can be preserved in leucosomes that have not undergone penetrative subsolidus deformation and recrystallization the cores of many of the crystals of cordierite and k feldspar have small locally oriented inclusions similar to those in the same minerals in the mesosomes these indicate that the cores remained solid during crystallization of the leucosome which produced inclusion free euhedral rims these crystals provide criteria for recognizing possible restite in granitoid magmas

inherited smnd isotope components preserved in monazite inclusions within garnets in leucogneiss from east antarctica and implications for closure temperature studies abstract application of the sm nd dating technique using garnet can be hampered by the presence of minor microscopic to submicroscopic ree rich inclusions in particular monazite and to a lesser extent zircon an acid leaching method using a mixture of hno 3 hcl has been used to tackle the monazite contamination problem with this leaching technique we demonstrate that inherited monazite inclusions have been preserved in garnet formed under high grade metamorphic conditions even very low levels of monazite contamination 0 001 wt have a profound effect on the sm nd system in the garnet analysis of a garnet from a leucogneiss dyke in the northern prince charles mountains eastern antarctica reveals that unleached garnet concentrates and the monazite inclusions as represented by the leachates define an apparent isochron of 1150 30 ma mswd 1 88 which represents a mixing age of inclusions preserving their inherited sm nd isotopic systematics in contrast analyses of the leached garnet concentrates yield much younger ages of 630 to 555 ma model calculations show that the presence of as little as 0 01 wt of monazite inclusions would swamp the sm nd isotope signature of the garnet concentrates this very low level of contamination is beyond the reach of physical purification processes and without leaching contamination is inevitable in inclusion bearing garnets such contamination can lead to older apparent ages than those of garnet formation and thus has profound implications for sm nd as well as u pb garnet chronometry

sulfide and silicate anatexis in the balmat zinc deposit new york usa and its implications for ore remobilization abstract the cross cutting zone of the fowler ore body is one of the many ore bodies of the balmat ny massive sulfide zinc deposit that underwent remobilization during upper amphibolite metamorphism and associated deformation the kilometer scale remobilization is difficult to reconcile with purely solid state processes given that other polymetallic ores of similar metamorphic grade have been shown to have undergone varying degrees of anatexis sem eds micro petrographic analysis of ores was undertaken to determine if melts were present in balmat ores during metamorphism and deformation low melting temperature micro inclusions of sulfosalt and sulfide assemblages occur predominantly in qtz py and kfs py domains that cross cut peak metamorphic assemblages these results indicate that localized anatexis occurred on the prograde path producing low volumes of melts of varying composition polymetallic sulfide melts were initiated by the prograde breakdown of minor phases containing low melting temperature chalcophile elements including as sb pb and cu alkaline silicate carbonate sulfide melts were fluxed by halogens sulfur and other volatile components released during metamorphism of evaporitic and organic rich horizons in the marble dominated host sequence these low volume melts and residual fluids would have wetted grain boundaries affecting the rheology of the bulk ore and facilitating remobilization of the ores through a combination of solid and liquid state processes

the possibility of a zns bearing sulfide melt at 600 c evidence from the rajpura dariba deposit india supported by laboratory melting experiment abstract inclusions of euhedral pyrite crystals are seen within sphalerite occupying veins and fractures in the rajpura dariba zn pb cu sulfide deposit in rajasthan india sphalerite in the deposit shows wide variation in composition and contains high sulfur with fe s zn varying from 0 49 to 1 06 indicating that a portion of the fe in sphalerite could be expressed as a pyrite fes 2 component and that the pyrite inclusions within sphalerite in the vein ore could have formed due to exsolution we have conducted experiments by the evacuated silica tube method at 600 c pertinent to the peak metamorphic conditions of the rajpura dariba deposit our experiments were aimed at exploring the possibility of a zns bearing sulfide partial melt that could precipitate sufficiently s rich sphalerite which in turn might exsolve pyrite the starting mixture comprised synthetic fes zns pbs cu 2 s and s in the molar proportions of 40 33 9 9 and 9 respectively which approximately corresponded to molar proportions of 27 fes 40 zns 11 pbs and 22 cufes 2 electron probe micro analysis epma of the resulting run product showed the presence of a cu fe zn pb s melt containing 4 61 mol zns in the coexisting sphalerite the estimated fes 2 component was 9 07 mol the eutectic temperature was determined to be 595 c with a melt containing 22 44 mol fes 31 24 mol cu 2 s 5 37 mol zns 35 81 mol pbs and 5 14 mol fes 2 the sphalerite that was in equilibrium with the eutectic melt contained 6 72 mol fes 2 our experimental results suggest that a zns bearing sulfide melt containing as high as 5 mol zns was produced during metamorphism at rajpura dariba by partial melting of the preexisting ore further experiments confirmed that a minimum sulfur fugacity of 2 46 log units is required for initiating melting pyrite and sphalerite containing fe and stoichiometrically excess s are the solidus phases present in all melt bearing experimental run products except one with much higher zns suggesting that these two minerals at rajpura dariba could have simultaneously crystallized from a sulfide partial melt giving rise to the observed poikilitic textural relation between the two

a model for garnet and plagioclase growth in pelitic schists implications for thermobarometry and p t path determinations numerical models of the progressive evolution of pelitic schists in the ncmnkfmash system with the assemblage garnet biotite chlorite staurolite plagioclase muscovite quartz h2o are presented with the goal of predicting compositional changes in garnet and plagioclase along different p t paths the numerical models support several conclusions that should prove useful for interpreting the p t paths of natural parageneses i garnet may grow along p t vectors ranging from heating with decompression to cooling with compression p t paths deduced from garnet zoning that are inconsistent with these growth vectors are self contradictory ii there is a systematic relation between garnet and plagioclase composition and growth such that for most p t paths garnet growth requires plagioclase consumption furthermore mass balance in a closed system requires that as plagioclase is consumed the remaining plagioclase becomes increasingly albitic inclusions of plagioclase in the core of garnet should be more anorthitic than those near the rim and zoned matrix plagioclase should have rims that are more albitic than the cores complex plagioclase textures may arise from the local variability of growth and precipitation kinetics iii a decrease of fe fe mg in a garnet zoning profile is a reliable indicator of increasing temperature for the assemblage modelled however there is no single reliable p monitor and inferences about p can only be made by considering plagioclase and garnet together iv consumption of garnet during the production of staurolite removes material from the outer shell of a garnet and may make recovery of peak metamorphic compositions and p t conditions impossible low peak temperatures typically recorded by staurolite bearing assemblages may reflect this phenomenon v diffusional homogenization of garnet affects the computed p t path and results in a clockwise rotation of the computed p t vector relative to the true p t path

strain partitioning during partial melting and crystallizing felsic magmas abstract the rheology of partly molten felsic systems changes dramatically during the onset of melting and through crystallization solid to liquid and liquid to solid transitions are not rheologically identical contradicting the concept of a unique rheological critical melt percentage we use four thresholds separating the successive stages according to the melt fraction for the melting transition 1 the liquid percolation threshold connects the melt pockets at about 8 of melt thus allowing local magma displacement at the metric scale and 2 for greater melt percentages 20 25 a melt escape threshold corresponds to the onset of transport over large distances of the melt and part of the residual solid phase leading ultimately to granitic bodies for the solidification transition 3 the first formation of a rigid framework of crystals which can accumulate stress although the melt can still flow corresponds to the rigid percolation threshold 55 solids and 4 the system becomes totally locked at about 72 75 solid which corresponds to the particle locking threshold the present paper focuses on strain partitioning induced by the coexistence of two contrasting rheological phases between these thresholds on a variety of scales non coaxial deformation is preferentially accommodated by the lower viscosity melt which increases the vorticity in the melt phase and reinforces asymmetries in the flow pattern strain partitioning also increases the scale length of the weakest phase during melting and magma ascent whereas strain partitioning decreases the scale length during crystallizing strain partitioning also affects the migration of melt by raising the particle locking threshold and lowering the melt escape threshold for three dimensional deformations such as transpression the direction of melt segregation e g horizontal may differ from the direction of melt migration e g vertical

gold mineralization in high grade metamorphic shear zones of the renco mine southern zimbabwe the renco gold mine in southern zimbabwe is the only known major gold deposit in the granulite facies terrane of the northern marginal zone of the late archean to mid proterozoic limpopo belt gold mineralization is confined to a system of high temperature mylonite zones characterized by two distinct geometries these include a series of shallow southeasterly dipping anastomosing north northeast to east northeast trending tabular lodes termed shallow reefs and subvertically inclined shallow easterly plunging pipelike lodes termed steep reefs the kinematics and orientation of the mineralized shear zones are consistent with a lateral and frontal thrust zone geometry that formed during the late archean thrusting of the northern marginal zone onto the zimbabwe craton gold is spatially and temporally closely associated with sulfide mineralization including pyrrhotite as the dominant sulfide with minor amounts of chalcopyrite and pyrite associated wall rock alteration comprises a garnet biotite quartz or siderite mineral assemblage mineral textures within the host mylonites as well as garnet biotite thermometry indicate gold deposition at temperatures of about 600 degrees c under mid amphibolite conditions slightly postdating regional peak metamorphic conditions fluid flow in the high temperature shear zones was largely controlled by fracture permeabilities transient episodes of brittle fracturing during conditions of close to lithostatic fluid pressures were promoted by a pronounced strain partitioning within the narrow shear zones into ductile mylonite bands and brittle ductile lithons that contain the bulk of the gold sulfide mineralization the high grade metamorphic ore and alteration mineral assemblages are overprinted by lower greenschist facies parageneses along brittle faults and cataclasites that are related to the mid proterozoic tectonism of the northern marginal zone it is concluded that the mineralization at renco illustrates the rare case of a midcrustal high grade metamorphic gold mineralization in southern africa where the vast majority of late archean lode gold deposits are related to low grade metamorphic granite greenstone terranes

textural evidence for extensive melting of the broken hill orebody galena pyrrhotite and chalcopyrite in samples of zn rich ore from the broken hill orebody show textures similar to those in partially melted metamorphic rocks including strings of low melting phases separating lithons of minerals likely to be restite low dihedral angles between low melting temperature minerals and restitic minerals and enlargement of these dihedral angles due to partial annealing analysis of existing experimental work from the system fe pb zn s indicates that at 1 bar galena and sphalerite coexisting with pyrrhotite of the compositions found at broken hill would have been partially melted at temperatures of about 750 c at 5 kbars this melting would occur at 780 c this temperature is well within the range of metamorphic temperatures reported from the area from these data we conclude that the broken hill orebody was partially molten at peak metamorphism with much of the pb cu ag sb and as in the melt and fe and zn residing mostly in the restite differentiation during cooling enriched the residual melt in sb as ag and other low melting temperature chalcophile elements the main sulfide melt froze after the last penetrative deformation to affect the orebody at temperatures below 720 c and possibly as low as 650 c sulfosalts crystallized out of the remaining melt at temperatures of 600 to 450 c and the final melt crystallized native bismuth at ca 250 c

partial melting of the broken hill galena sphalerite ore experimental studies in the system pbs fes zns ag2s for many years the genesis of the broken hill pb zn ag deposit has been and continues to be hotly debated mostly because many of the original characteristics of the deposit have been obscured by high grade metamorphism and deformation metamorphism not only overprinted the deposit but fundamentally modified the sulfides through metamorphic remobilization based on experimental studies it had also been speculated that partial melting of the sulfides may have occurred the lack of experimental data at elevated pressures in the pbs fes zns system has limited this suggestion to mere speculation we present the results of an experimental study of the melting behavior of sulfides which show that broken hill ores must have been partially molten under peak metamorphic conditions

sm nd dating and cooling history of scourian granulites sutherland the scourian gneisses of the lewisian complex of north west scotland have been the subject of many geochronological investigations sm nd whole rock measurements suggest that the protoliths of the lewisian complex differentiated 2 92 0 05 gyr ago from an approximately chrondritic mantle1 rb sr and pb pb whole rock ages between 2 8 and 2 6 gyr reflect subsequent depletion in rb and u refs 2 3 which has generally been associated with granulite facies metamorphism zircons which are thought to have crystallized during this metamorphism have also given u pb ages of 2 66 gyr ref 4 this granulite facies metamorphism involving pressures in excess of 10 kbar and peak temperatures estimated between 820 50 c ref 5 and 1 250 c refs 6 7 affects a terrain including supracrustal rocks7 and thus implies a major tectonic event we report here the first geochronological data measured on mineral assemblages that have also provided estimates of the conditions of metamorphism such measurements are possible because the major minerals of granulite facies assemblages show an unusually favourable spread in sm nd ratios garnet especially is strongly enriched in sm ref 8 data for three garnetiferous basic gneiss assemblages suggest that closure of the sm nd system occurred significantly later than the peak of the metamorphism

p t paths derived from garnet growth zoning in an extensional setting an example from the tormes gneiss dome iberian massif spain the tormes gneiss dome nw iberian massif variscan belt of spain comprises a metamorphic core complex lower unit bounded by a major extensional detachment despite metamorphic temperatures in the upper amphibolite facies 700 740 c metapelites from the highest levels of the lower unit contain garnet with preserved growth zoning these rocks were used for reconstruction of quantitative p t paths based upon interpretation of microfabrics and thermodynamic modelling of garnet zoning the results are consistent with a two stage tectonothermal evolution under high grade conditions 1 an early compressional phase of deformation that led to upper amphibolite facies barrovian type metamorphism and to p increase and t rise to approximately 9 kbar and 700 725 c 2 a subsequent major extensional phase of deformation that led to quasi isothermal decompression from 8 9 to 3 kbar at t conditions between 700 and 740 c several lines of structural textural and petrological evidence suggest that up to 15 20 km of overburden was removed from the lower unit by tectonic exhumation while these rocks were still at upper amphibolite facies conditions a final stage of quasi isobaric cooling to greenschist facies conditions is locally recorded in late low grade detachments

ore petrology chemistry and timing of electrum in the archean hypozonal transvaal lode gold deposit western australia the transvaal lode gold deposit is hosted in ultramafic schists and graphite bearing pelites that are metamorphosed to lower and mid amphibolite facies assemblages the orebody is structurally controlled by the transvaal shear zone which overprinted and reactivated the penetrative regional fabric the main tabular orebody is formed of a massive quartz sulfide vein and related splays and adjacent hydrothermally altered wall rock prograde alteration in the ultramafic rocks consists of a proximal diopside actinolite quartz zone which grades outward into a distal amphibole hornblende actinolite biotite plagioclase quartz schist in pelitic metasedimentary rocks a proximal biotite muscovite quartz graphite zone grades outward into a distal cordierite quartz biotite graphite schist retrograde chlorite talc and zoisite replaced prograde hydrothermal alteration minerals and exhibit generally postkinematic fabrics the dominant sulfide assemblages in the hydrothermal alteration zones at transvaal are pyrrhotite loellingite pyrrhotite arsenopyrite 1 and loellingite arsenopyrite 1 other assemblages include chalcopyrite pyrrhotite pyrite loellingite and pyrrhotite cubanite these assemblages are interpreted to consist of a mixture of equilibrium assemblages that relate to a prograde hydrothermal alteration event and disequilibrium assemblages that formed during cooling and reequilibration of the rocks arsenopyrite 1 is interpreted to have formed both during the prograde alteration event and during high temperature retrogressive replacement of loellingite average compositions of arsenopyrite 1 in equilibrium with loellingite and pyrrhotite indicate paleotemperatures of about 510 degrees or 20 degrees c about 40 degrees c lower than estimates of the peak metamorphic temperatures low and high fe pyrrhotite and arsenopyrite 2 for instance would not be in equilibrium with other sulfides and are interpreted to be retrograde both electrum and gold occur in the transvaal gold deposits but electrum is the more abundant gold bearing phase and contains between 24 and 56 wt percent silver the compositions of electrum grains correspond to gold finenesses of about 431 to 732 with silver the dominant impurity electrum occurs mostly at the interface between arsenopyrite 1 loellingite or as inclusions in loellingite pyrrhotite and arsenopyrite 1 more rarely it occurs as as fracture fill within arsenopyrite 1 the differing silver contents of electrum can be explained by differing fluid rock reactions low ag electrum about 24 wt ag occurs where the lode is entirely hosted by ultramafic schists whereas high ag electrum is at the ultramafic schist graphitic pelite contact it is proposed that fluid interaction with reducing pelites produced ag rich electrum through destabilization of gold bisulfide and silver chloride complexes whereas fluid that reacted with more oxidized ultramafic rocks formed au rich electrum as silver chloride complexes were not destabilized the main phase of hydrothermal alteration which included the crystallization of prograde silicates pyrrhotite and loellingite was syntectonic with respect to the transvaal shear zone and at approximately peak metamorphic conditions electrum in equilibrium with pyrrhotite and or loellingite was thus deposited at synpeak metamorphic conditions however the majority of electrum grains coexist with arsenopyrite 1 and composite loellingite arsenopyrite 1 grains and are interpreted to have formed from submicroscopic gold released from loellingite during high temperature retrogressive replacement by arsenopyrite these are also considered to be part of the main gold bearing hydrothermal event retrograde low temperature postmetamorphic silicate and sulfide minerals such as chlorite muscovite pyrite and cubanite are evidence for processes that occurred after the main gold depositing hydrothermal activity took place renewed flux of hydrothermal fluids during the protracted metamorphic history of the terrane could have produced the low temperature retrogression the occurrence of low temperature sulfides is likely the result of in situ reequilibration during cooling of the rocks

the role of inclusions in u pb and sm nd garnet geochronology stepwise dissolution experiments and trace uranium mapping by fission track analysis the u pb and sm nd dating of garnet are important tools for understanding rates of tectonometamorphic processes and have been widely applied in studies of metamorphic terranes however the budgets of uranium lead samarium and neodymium in garnet separates from metamorphic rocks may be dominated by contributions from inclusions of monazite or zircon a combined fission track and stepwise dissolution technique is proposed for evaluating the role of inclusions of monazite and zircon in the budgets of uranium lead samarium and neodymium in garnets used for u pb and sm nd chronology variations of th u u nd and sm nd ratios between successive dissolution steps reveal the contributions of monazite and zircon inclusions the stepwise dissolution procedure does not induce any apparent artifacts on 207pb 206pb and sm nd ages the technique has been applied to garnets from three metamorphic terranes almandine garnet samples from the high grade pikwitonei granulite domain manitoba canada and the wind river range wyoming usa have the majority of their uranium samarium neodymium and radiogenic lead pb hosted by micrometer scale inclusions of monazite fission track densities reveal that uranium is 108 times more abundant in inclusions than in garnets stepwise dissolution of the samples shows that neodymium and uranium are associated with the dissolution of monazite in contrast grossularandradite garnets from cascade slide adirondack highlands ny usa show little variation in fission track density and similar isotopic ratios between dissolution steps indicating that the budgets of uranium samarium neodymium and pb are not significantly influenced by inclusions the demonstrated success of these techniques leads us to recommend similar procedures as a routine matter in u pb and sm nd garnet geochronology of metasedimentary rocks

mobility and h2o loss from fluid inclusions in natural quartz crystals petrographic observations on quartz crystals from the mole granite australia and other localities shed new light on the mechanisms of post entrapment modification of fluid inclusions these modifications include migration away from pseudosecondary trails changes in fluid salinity and density shape distortion and the formation of sweat haloes around strongly deformed inclusions increases in fluid salinity which usually are associated with inclusion migration indicate water losses of up to 50 however la icp ms analysis of unmobilized and mobilized inclusions of the same trail reveals basically unchanged ratios of major and trace element cations with the exception of li which seems to be incorporated into the crystal lattice during migration despite the fact that all these modifications are closely related to deformation processes they occur not only in mechanically deformed quartz but also in free standing crystals in the latter samples stress has been generated internally as a result of brazil twinned growth and compositional zonation these observations and their interpretation leads to a list of practical criteria that should help in differentiating between reliable and suspect fluid inclusions in other samples

nd and sr systematics in an eclogite xenolith from tanzania evidence for frozen mineral equilibria in the continental lithosphere combined isotopic nd sr and petrographic analyses are reported for an eclogitic xenolith from the vissury mine tanzania an exceedingly large difference in measured nd values in garnet and clinopyroxene 200 units suggest a major mineral equilibration at 1 75 0 014 ga nd isotopic heterogeneities are demonstrated in different generations of garnet and zonation in sr and nd isotope compositions is present within the clinopyroxene the variation in measured nd isotope compositions in different generations of garnet corresponds to 16 units exsolution of garnet from clinopyroxene occurred in an essentially closed system with respect to sr and nd petrographically different generations of garnet are present one garnet with rutile inclusions as well as the rims of clinopyroxene are clearly off the isochron features like this can originate either if a system is not closed e g affected by a migrating fluid or if a remanent isotopic heterogeneity survived the homogenisation at 1 75 ga the subsolidus partitioning of sm and nd between garnet and clinopyroxene results in higher fractionation of the smnd ratio in garnet than in the liquid crystal partitioning since the closure temperature for the sm nd exchange between garnet and clinopyroxene may be around 850 c in a dry system many of the lithospheric ultramafic rocks may exhibit frozen mineral equilibria the isotopic data suggest that this eclogite was emplaced at 1 75 ga into the lithosphere mantle at temperatures of about 1040 c and then cooled as a closed system at a rate of 6 50 c my down to 840 c where all the exchange reactions were efficiently frozen in at 1 75 ga ago the coincidence of the emplacement of granites 1 7 1 8 ga into the central part of the tanzanian craton with the age of the eclogite suggests that lower crustal melting was triggered by a thermal event in the upper lithosphere

gold bearing iron formation in a granulite terrane of the canadian shield a possible deep level expression of an archean gold mineralizing system gold occurs in metamorphosed iron formation within a migmatized paragneiss terrane in the eastern superior province of the canadian shield the main gold prospect which is located 100 m from a tonalite pluton has undergone skarn like metamorphism that occurred late during the granulite grade metamorphism despite its occurrence in a high grade metamorphic host rock the mineralization shares many of the characteristics of lowergrade archean greenstone belt gold mineralization such as 1 enrichment in au as s sb w u and feo 2 a low base metal gold ratio 3 the inclusion of gold in arsenopyrite 4 the proximity to a felsic pluton 5 an iron rich fractured host rock and 6 a similar mineralization time around 2650 ma this gold occurrence occurs near the metamorphic peak and represents a deep level expression of the widespread crustal scale mineralizing system documented for the late archean the main conclusion of this work is that the principal characteristics of archean gold mineralization are present in the deep crust and do not come from the leaching of upper level rock piles such as greenstone belts

reversed determination of the reaction phlogopite quartz enstatite potassium feldspar h 2 o in the ranges 750 875 degrees c and 2 12 kbar at low h 2 o activity with concentrated kcl solutions the reaction of phlogopite plus quartz to enstatite potassium feldspar and aqueous fluid in the system kmash kcl was reversed at 2 12 kbar and 750 875 degrees c and at low h 2 o activities by reversal of the h 2 o content of concentrated kcl solutions equilibrated with product and reactant assemblages synthetic 1m phlogopite kmg 3 alsi 3 o 10 oh 2 and enstatite mgsio 3 maintained end member stoichiometry throughout and the potassium feldspar kalsi 3 o 8 was a high sanidine based on unit cell refinements the broad p t xh 2 o and narrow reversal ranges of this investigation were possible because of the low and well defined h 2 o activity yet powerful fluxing action of concentrated kcl solutions solubility experiments on quartz and potassium feldspar in our p t xh 2 o range showed that fluid phase solution of silicate constituents was too small to have affected the h 2 o activity in the experiments the new determinations are more definitive than previous work done at very low pressures with pure h 2 o or in co 2 h 2 o mixtures they establish the standard free energy of the reaction in the experimental range with an uncertainty of about 1 kj and indicate that the synthetic phlogopite has maximal al si disorder under our experimental conditions the standard enthalpy of reaction at 298 k is 106 54 or 2 0 kj 2sigma based on our reversals a value 6 kj less positive than that currently used by many workers in calculations of biotite stability and h 2 o activity in the petrogenesis of high grade metamorphic rocks the lower thermal stability that we find for phlogopite requires revision in estimates of h 2 o activity of granulite facies metamorphism typical values for the natural assemblage orthopyroxene biotite garnet potassium feldspar plagioclase quartz at deep crustal metamorphic conditions 750 850 degrees c 5 10 kbar are alpha h2o 0 4 0 6 compared with values of 0 15 0 30 which would have been estimated with previously available data on phlogopite stability an important consequence of the expanded h 2 o activity range of granulites is that alkali chloride solutions of only moderate concentration xh 2 o 0 5 0 7 which are the values observed in actual fluid inclusions in many kinds of igneous and metamorphic rocks are a feasible alternative to the vapor absent conditions considered necessary by many workers based on previous low estimates of h 2 o participation of concentrated brines in deep crust upper mantle metamorphic processes enables alkali metasomatism and other kinds of chemical transport in an aqueous fluid without large scale melting of the crust

the influence of deformation on the formation of axial planar leucosomes and the segregation of small melt bodies within the migmatitic napperby gneiss central australia in the napperby gneiss central australia a distinctive group of elongate leucosomes occurs along the axial planes of mesoscopic crenulations that have harmonic two dimensional kink band like geometries the leucosomes truncate a fabric defined by biotite quartz k feldspar plagioclase sillimanite and are chiefly composed of k feldspar and quartz with or without plagioclase they are characterized by accessory garnet or aggregates of intergrown ilmenite and magnetite produced during the melting reactions bi plag q sill g ksp melt and bi plag q ru ilm mt ksp melt the absence of garnet and intergrown ilmenitemagnetite aggregates in the gneiss surrounding the crenulation zones indicates that these melting reactions preferentially occurred in the deformation zones in spite of a homogeneous distribution of reactant phases in the gneiss this means that strain partitioning and partial melting are coupled processes in which the garnet and ilmenite magnetite producing melt reactions initially occurred in high strain zones where the melting reaction activation energy was first overstepped this was a consequence of an additional strain induced matrix free energy component on top of the temperature effect once reaction product nucleation had occurred the deformation zones became the sites for continued melting which was energetically favoured over nucleating garnet and ilmenite magnetite elsewhere the melt reactions producing garnet and ilmenite magnetite resulted in the formation of grain supported leucosomes with a higher melt component than the surrounding gneiss and the leucosomes were therefore relatively incompetent in a differential stress field diffusional processes will cause elongation of the leucosome in a plane normal to 1 parallel to the axial planes of folds if sufficient melting has occurred an interconnected leucosome network will result creating a potentially important buoyancy driven melt drainage system in the napperby gneiss interconnected leucosome bearing crenulation zones are mostly vertical which is an ideal orientation for buoyancy driven diffusive melt migration however melt segregations remained small due to a melt impermeable lithological boundary overlying the gneiss

modeling of radial microcracks at corners of inclusions in garnet using fracture mechanics radial microcracks around inclusions in garnet are important indicators of ultrahigh pressure metamorphism and have been used to interpret changes in pressure as a function of temperature during decompression of metamorphic terrains we investigate the growth of microcracks from inclusion corners using a two dimensional fracture mechanics based boundary element method code and propose that microcracks emanating from inclusions are primarily controlled by inclusion shape and size and do not require high confining lithostatic pressure or significant decompression to initiate results suggest that inclusions 100 m diameter develop microcracks during modest decompression the magnitude of decompression required to initiate microcracks decreases with increased degree of faceting and increased inclusion size the potential for crack growth at each corner depends on the corner sharpness and the overall shape of the inclusion we integrate model results with petrographic observations to understand mechanisms and conditions of garnet microcracking during regional metamorphism

magnetotelluric constraints on the tectonic setting of grenville aged orogenesis in central australia abstract magnetotelluric mt data were collected over a 480 km long profile in central australia extending from the mesoarchaean palaeoproterozoic gawler craton into the mesoproterozoic neoproterozoic musgrave province the two regions have contrasting geological histories and many reconstruction models of proterozoic australia propose that they were juxtaposed during the mesoproterozoic despite the significant differences in geological background between the gawler craton and the musgrave province the mt data did not image an electrical structure or any change in electrical character between them mt models suggest that southern central australia is electrically separated into two parts the first comprising the northernmost gawler craton and the musgrave province has a 5 km thick layer of moderate resistivity overlying highly resistive crust the second comprising the central northern gawler craton displays moderate resistivity to the base of the crust the lithospheric mantle underlying both electrical regions is uniformly highly resistive our results suggest that the musgrave province and gawler craton are more closely related than is currently recognized we propose that the gawler craton forms basement to the musgrave province and that the grenvillian musgrave orogeny was an intracratonic reworking event that overprinted the older basement

the fluid regime of high temperature metamorphism during granitoid magma genesis the strong negative correlation between the temperatures and melt h2o contents of granitoid magmas implies that the crustal melting reactions that produced the magmas were strongly buffered with t and ah2o co varying within a narrow band this observation can only be explained if the partial melting reactions that created the magmas were either fluid absent from the outset or evolved toward this condition as melting progressed since these melting reactions occur during upper amphibolite to granulite facies metamorphism it is reasonable to conclude that metamorphic events responsible for the generation of granitoid magmas generally occur in the absence of excess pervasive fluid

garnet orthopyroxene plagioclase quartz barometry refinement and application to the english river subprovince and the minnesota river valley recent experimental theoretical and thermodynamic studies permit better calibration of two reactions for geobarometry grossular pyrope quartz anorthite enstatite grossular almandine quartz anorthite ferrosilite if both reactions are applied using the same thermodynamic data and activity models they should yield the same pressure for a given garnet pyroxene plagioclase quartz assemblage application to a variety of high grade terrains generally yielded excellent results however poor results have been obtained for high fe rocks which can be traced to errors in activity models for garnet and or pyroxene either a two site ideal mixing model for orthopyroxene cf wood and banno 1973 underestimates enstatite activity for high fe orthopyroxenes or the ganguly and saxena 1984 model overestimates pyrope activity in low mg garnets

melt segregation in the continental crust distribution and movement of melt in anatectic rocks the grain and outcrop scale distribution of melt has been mapped in anatectic rocks from regional and contact metamorphic environments and used to infer melt movement paths at the grain scale anatectic melt is pervasively distributed in the grain boundaries and in small pools consequently most melt is located parallel to the principal fabric in the rock typically a foliation short branched arrays of linked melt bearing grain boundaries connect melt depleted parts of the matrix to diffuse zones of melt accumulation protoleucosomes where magmatic flow and alignment of euhedral crystals grown from the melt developed the distribution of melt leucosome and residual rocks normally melanocratic in outcrop provides different but complementary information the residual rocks show where the melt came from and the leucosomes preserve some of the channels through which the melt moved or sites where it pooled different stages of the melt segregation process are recorded in the leucosome melanosome arrays regions where melting and segregation had just begun when crystallization occurred are characterized by short arrays of thin branching leucosomes with little melanosome a more advanced stage of melting and segregation is marked by the development of residual rocks around extensive branched leucosome arrays generally oriented along the foliation or melting layer places where melting had stopped or slowed down before crystallization began are marked by a high ratio of melanosome to leucosome because most of the melt has drained away very few leucosomes remain to mark the melt escape path this is common in melt depleted granulite terranes many migmatites contain abundant leucosomes oriented parallel to the foliation mostly these represent places where foliation planes dilated and melt drained from the matrix via the branched grain boundary and larger branched melt channel leucosome arrays collected melt collected in the foliation planes was partially or fully expelled later when discordant leucosomes formed leucosomes or veins oriented at high angles to the foliation layering formed last and commonly lack melanocratic borders hence they were not involved in draining the matrix of the melting layer discordant leucosomes represent the channels through which melt flowed out of the melting layer

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

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

synmetamorphic lode gold deposits in high grade archean settings lode gold deposits that formed under conditions of the middle to upper amphibolite and granulite facies have been identified in the yilgarn block of western australia these deposits which form a continuum with lower metamorphic grade mesothermal gold depos its show that at least some of the fluids responsible for mesothermal gold deposits originated in or below the middle crust studies of 18 o suggest that fluid compositions were controlled by the wall rocks at least for some elements these more proximal gold deposits provide more information about deep fluid sources than do investigations of higher level mineralization deposited from fluids strongly modified by wall rock interaction during the early part of their ascent

rheology and deformation mechanisms of an isotropic mica schist we have investigated the transitional semibrittle deformation of a mica schist 75 biotite by shortening cylinders cored at 0 45 and 90 to foliation to varying strains at confining pressures pc to 500 mpa constant strain rates e from 1 5 10 7 to 1 6 10 4 s 1 and temperatures t from 25 to 400 c deformation is concentrated within one or more throughgoing millimeter wide shear zones at all conditions these localize at low strains e 2 through the nucleation and coalescence of dense sets of intragranular microkink bands mkbs despite distinct differences in the relative number of mica grains oriented favorably for slip and kinking in different loading directions the differential stresses required for shear zone development vary little with fabric orientation biotite schist undergoes a transition from strain softening to steady strength mechanical response at confining pressures in the range 75 to 150 mpa the pressure sensitivity of strength characterized by the slope of the mohr envelope decreases from 0 5 at pc 100 mpa to 0 1 at pressures greater than 200 mpa reflecting the increasing contribution of glide and kinking in biotite at higher pressures however dilatancy associated with microcracking and void formation along mkb boundaries persists to at least 500 mpa within the pressure insensitive regime 200 pc 500 mpa temperature and strain rate dependencies of strength determined in stepping tests reveal a strong history dependence to flow that cannot strictly be described by a steady state constitutive law samples deformed in steps from low to high temperatures or fast to slow strain rates consistently exhibit stronger temperature and strain rate sensitivities than those deformed along t decreasing or e increasing paths path dependent effects may reflect differences in the degree to which inherited dislocation substructures are utilized or overprinted during later deformation increments by assuming an exponential relationship between differential stress d and strain rate e of the form e c exp d exp q rt we fit the data with two end member flow laws with a single activation energy q 89 kj mol and exponential constants ss 0 15 0 01 mpa 1 and ws 0 55 0 04 mpa 1 that account for the different responses observed along stepping paths that are strongly sensitive or weakly sensitive to t and e respectively application of the results to crustal deformation suggests that mica rich aggregates are weaker than other common rock types throughout a broad midcrustal depth range supporting the inference that retrograde reaction to phyllosilicates may be important in localizing crustal deformations within large faults and shear zones

archean and proterozoic mineralization and tectonics at the renco mine northern marginal zone limpopo belt zimbabwe gold mineralization at the renco mine occurs within the northern marginal zone of the limpopo belt along east northeast trending structures that formed in the late arehean 2 6 ga pb pb step leach age on syntectonic garnet under granulite facies the mineralization is hosted by a massive enderbite which intruded at 2 57 ga u pb zircon age north northeast trending pyroxenite dikes were intruded around 2 4 ga pb pb step leach age of garnet and pyroxene probably associated with the intrusion of the great dyke and satellites the northern marginal zone had been exhumed to crustal levels corresponding to greenschist facies by this time the major gold bearing pyrrhotite chalcopyrite or bismuth mineralization occurred under greenschist facies conditions 400 degrees c this occurred at approximately 2 0 ga as deduced from cooling of biotite associated with mineralization 1 88 ga rb sr age this ca 2 0 ga age of the mineralization implies a link with the important early proterozoic tectono thermal event in the limpopo belt during which shearing occurred under greenschist facies conditions in the triangle shear zone the reneo deposit is not as formerly suggested a high grade mineralization although there is some evidence for a precursory high grade phase of mineralization in the late archean its genesis is quite distinct from most of the greenstone associated deposits of the zimbabwe craton which are known or inferred to be late archean in age

zirconium abundance in granulite facies minerals with implications for zircon geochronology in high grade rocks zircon growth in high grade metamorphic rocks may be triggered by net transfer reactions involving the breakdown of a phase bearing zirconium zr we have measured the concentration of zr in the major minerals in granulite facies rocks of differing bulk composition both garnet and hornblende contain tens of parts per million zr and no other major phase contains significant zr simple calculations show that reaction of either garnet or hornblende to form non zr bearing phases will release sufficient zr to account for at least some new zircon growth u pb ages from new zircon grown as a result of either hornblende or garnet breakdown are not expected to record the time of peak metamorphism but rather will record the time of particular metamorphic reactions allowing direct correlation of zircon ages with petrologically derived pressure temperature time paths this approach offers the potential for more rigorous interpretation of the metamorphic significance of zircon ages than has previously been possible

study of strain and partial melt transfer in a banded migmatite abstract an attempt is made to analyse in detail the strain and area change around the tip of a foliation parallel leucosome strain is determined in the mesosomes above and below the leucosome and the melanosome in both the xz foliation and lineation parallel and yz foliation parallel lineation perpendicular planes using the following methods march analysis of biotite orientation fabric fry method of zircon position fabric and r f method using cordierite grain outlines in addition march analysis of the orientation of sillimanite inclusions within the cordierite grains helps confirm the last method area change is measured by determining the population density of zircon around the leucosome in the xz plane the amount of material lost can account for the area gain in the leucosome and we suggest the sample at the observed scale was closed in terms of melt movement the results are used to construct tentative strain paths for the various components of the migmatite evidence suggests the leucosome was filled by true melt fluid deformation of the leucosome melt is demonstrated by the orientations of zircons

timing of gold mineralization in the mt york district pilgangoora greenstone belt and implications for the tectonic and metamorphic evolution of an area linking the western and eastern pilbara craton abstract hydrothermal gold mineralization occurs in banded iron formation and amphibolite host rocks respectively in the main hill and zakanaka deposits in the northern sector of the pilgangoora greenstone belt of the pilbara craton the supracrustal sequences of the greenstone belt have been folded into n s trending megascale isoclinal f2a folds which have been refolded into open to tight f2b folds d2 to d3 strike slip shear zones developed preferentially along the limbs of f2a isoclinal folds and along the granitoid greenstone belt contact gold mineralization is controlled by f2b folds and shear zones were interpreted to have been activated or reactivated during d2b deformation in both deposits gold associated hydrothermal alteration comprises zoned quartz calc silicate veins and pyrrhotite pyrite at the zakanaka deposit and quartz amphibole biotite diopside and pyrrhotite lollingite arsenopyrite at the main hill deposit thermobarometric and textural studies indicate that gold mineralization occurred synchronous with the peak of amphibolite facies metamorphism in both deposits lead isotope studies on gold associated hydrothermal microcline titanite and pyrrhotite in the zakanaka deposit yield a pb pb isochron with an age of 2888 2 6 1 ma for the alteration and hence gold mineralization this age is confirmed by a concordant pb pb age using shrimp on hydrothermal titanite from the zakanaka deposit of 2904 70 ma the initial isotopic ratio in the ore system estimated using the isotopic composition of hydrothermal microcline is much more primitive than 2888 ma pb from accepted growth curves lead isotope modeling using the possible ages of rock formation and metamorphic events in the eastern pilbara craton shows that the primitive pb isotope signature is consistent with the 2888 ma age of alteration if the pb was derived from old crust which underwent an early high grade depletion event the age of hydrothermal gold associated alteration at zakanaka indirectly constrains the age of the f2b folding event and the peak of amphibolite facies metamorphism in the pilgangoora greenstone belt to ca 2 89 ga the special position of the pilgangoora greenstone belt in the central pilbara permits the interpretation of the age of gold mineralization in the light of the tectonic evolution in both parts of the craton and indicates that mineralization and the associated structural and metamorphic events post dated the main magmatic evolution in both the east and the west pilbara craton but coincided with late deformational events along strike slip shear zones in the west pilbara and late syn to post tectonic granitoids and associated metamorphism in the east pilbara

evolution of ptygmatic folds in migmatites from the type area s finland abstract it is proposed that ptygmatic folds in migmatites form during leucosome melanosome differentiation volume loss by depletion of quartz and feldspar takes place in the interlimb domains of folds leaving a residual melanosome buckling leucosomes increase in volume due to this process and are protected from substantial thinning concurrent with tectonic segregation albeit at a lower rate the leucosome expands by replacing the melanosome this process is induced by fluid activity at the leucosome boundary as suggested by the occurrence of secondary offshoot veinlets segregation and growth causes the most typical aspect of ptygmatic folds preservation of an overall buckle shape during the stage when the folds become isoclinal and devoid of interlimb matrix

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

calculation and application of clinopyroxene garnet plagioclase quartz geobarometers recently published thermodynamic and experimental data in a variety of chemical systems have been evaluated to derive gibbs free energies for hedenbergite and pyrope these were used to calculate the geobarometric equilibria

influence of preexisting low metamorphic grade sedimentary successions on the distribution of iron oxide copper gold mineralization in the olympic cu au province gawler craton the primary driver for mineralization in the olympic cu au province of the eastern gawler craton south australia was a major early mesoproterozoic thermal event partly manifest as extensive a type i type and mafic magmatism magmatism of this age is present across the gawler craton and the curnamona province to the east yet hematite rich iron oxide copper gold iocg deposits are apparently restricted to a north south belt along the eastern margin of the gawler craton whereas geochemical differences between the early mesoproterozoic granites in the eastern versus western gawler craton appear to influence the type of mineralization in these regions there are also stark differences in the nature of the rocks into which these magmas intruded metamorphic data from the gawler craton and curnamona province show that most of the western southern and northern gawler craton underwent amphibolite to granulite facies metamorphism either during a ca 2465 to 2410 ma event sleafordian orogeny a ca 1730 to 1690 ma event kimban orogeny or both in contrast across the eastern gawler craton and curnamona province potentially evaporate bearing sedimentary successions including the ca 1750 ma wallaroo group and ca 1710 to 1640 ma willyama supergroup were largely unmetamorphosed prior to the early mesoproterozoic magmatic and metallogenic event thus a clear spatial partitioning of paleoproterozoic orogenesis exists in the western southern and northern gawler craton including a broad spatial correlation between rocks of low metamorphic grade and the region of iocg mineralization multiple fluid sources e g sedimentary metamorphic and magmatic were important in generating the chemistry suitable to precipitate cu au u ree bearing minerals in the olympic cu au province therefore we suggest that the preservation of rocks at low metamorphic grade containing formation waters and porosity provided a fertile geochemical environment into which high temperature felsic and mafic magmas and their associated hydrothermal cells were emplaced and developed this example from the eastern gawler craton and curnamona province suggests that prior orogenic events can operate as a first order control on the location of large mineral provinces

rates of grain boundary diffusion through enstatite and forsterite reaction rims the growth rates of enstatite rims produced by reaction of fo92 and sio2 were determined at 250 1500 mpa and 900 1100 c for a wide range of water contents growth rates were also determined for forsterite rims between mgo and mg2si2o6 and between mgo and sio2 rim growth rates are parabolic indicating diffusion controlled growth of the polycrystalline rims which are composed of 2 m diameter grains rim growth rates were used to calculate the product of the grain boundary diffusion coefficient d a times the effective grain boundary thickness assuming in turn that mgo sio2 and mg2si 1 are the diffusing components coupled diffusion of a cation and oxygen or interdiffusion of mg and si the values for d mgo d and d for enstatite at 1000 c and 700 mpa confining pressure with about 0 1 wt water are about five times larger than the corresponding d a values for samples initially vacuum dried at 250 c most of the increase in d a occurs with the first 0 1 wt water the activation energy for diffusion through the enstatite rims 1100 950 c is 162 30 kj mole the diffusion rate through enstatite rims is essentially unchanged for confining pressures from 210 1400 mpa but the nucleation rate is greatly reduced at low confining pressure for 1 0 wt water present and limits the conditions at which rim growth can be measured the corresponding values for d a through forsterite rims are essentially identical for the two forsterite producing reactions when 0 1 wt water is added and similar to the d a values for enstatite at the same conditions the d a values for forsterite are 28 times larger for samples starting with 0 1 wt water compared to samples that were first vacuum dried thus water enhances these grain boundary diffusion rates by a factor of 5 30 depending on the mineralogy but the total range in d a is only slightly more than an order of magnitude for as wide a range of water contents as expected for most crustal conditions

textural evolution of synthetic fluid inclusions in quartz during reequilibration with applications to tectonic reconstruction experimental reequilibration of synthetic 10 wt nacl h2o inclusions in natural quartz reveals that reequilibration textures show distinct differences depending upon the p t path followed by the inclusion after formation these differences combined with other geological information may be used to determine whether the sample rock followed a dominantly isothermal or isobaricp t path following entrapment the intensity and style of inclusion reequilibration features is related to the direction and magnitude of the departure of thep t path from the original isochore for the inclusion thus fluid inclusion reequilibration textures not only permit inclusionists to determine whether the rocks followed an isothermal or isobaric retrogradep t path but also the magnitude of departure of this path from one that is isochoric

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

internally consistent recalibrations of mineral equilibria for geothermobarometry involving garnet orthopyroxene plagioclase quartz assemblages and their application to the south indian granulites abstract three reactions are calibrated as geothermobarometers for garnet orthopyroxene plagioclase quartz assemblages namely 1 2 ferrosilite 1 3 pyrope 1 2 enstatite 1 3 almandine a ferrosilite anorthite 2 3 almandine 1 3 grossularite quartz b and enstatite anorthite 2 3 pyrope 1 3 grossularite quartz c the internally consistent geothermobarometers based on reactions a b and c are calibrated from experimental data only the thermodynamic parameters of reaction a are derived from published experimental data in the fmas system n 104 in the range 700 1400 c and 5 50 kbar while those for reaction b are derived by summation of the existing reversed experimental data of the mineral equilibria ferrosilite fayalite quartz d and anorthite fayalite 2 3 almandine 1 3 grossularite e the retrieved thermodynamic parameters for reactions a b and c are respectively h0 cal 3367 209 2749 350 and 3985 545 s0 cal k 1 1 634 0 163 8 644 0 298 and 5 376 0 391 and v01 298 cal bar 1 0 024 0 60946 and 0 5614 on a one cation basis the derived margules parameters of the ternary ca fe mg in garnet are wfe mg 1256 1 0 0 23 t k wmg fe 2880 1 7 0 13 t k wca mg 4047 77 1 5 t k wmg ca 1000 77 1 5 t k wca fe 723 0 332 0 02 t k wfe ca 1090 cal and the ternary constant c123 4498 1 516 0 265 t k cal subregular solution model of non ideal mixing and fe mg al in orthopyroxene wfe mg 948 200 0 34 0 10 t k wfe al 1950 500 and wmg al 0 cal regular solution model of non ideal mixing the anorthite activity in plagioclase is calculated by the al avoidance model of subregular ca na mixing commonly used for geobarometry based on reactions b and c when the geothermobarometers are applied to garnet orthopyroxene plagioclase quartz assemblages n 45 of wide compositional range from the precambrian south indian granulites temperature ranges of 690 860 c x 760 45 c and pressure ranges of 5 10 kbar were obtained the p t values were estimated simultaneously and there is no difference in the pressure calculated from pmg reaction c and pfe reaction b in the existing calibrations this difference is 1 kbar or more furthermore there is no compositional dependence of the ln k of the experimental data in the fmas n 104 and the cfmas n 78 systems at different temperatures and the estimated temperatures of the south indian granulites