regional retrograde metamorphism of a high grade terrain the willyama complex broken hill australia abstract retrograde metamorphism has been a major influence in the development of the presently observed lithologies of the willyama complex broken hill two broad types of retrogression are distinguished pseudomorphous and kinematic retrogression the former type of retrogression involves replacement of prograde phases without complete loss of the high grade fabric hence the prograde assemblage can frequently be inferred kinematic retrogression involves the development of a new schistose fabric and like pseudomorphous retrogression is commonly related to f3 deformation retrogression was initiated during the waning stage of prograde metamorphism and was accompanied by an influx of aqueous fluid at similar pressure but lower temperature conditions than prograde metamorphism the source of the water is believed to be the crystallization of cooling partial melts the regional nature of this willyama style of retrogression is attributed to the metasediment rich nature of the sequence the high geothermal gradient and the considerable amount of in situ partial melting

amphibolites from panama anticlockwise p t paths from a pre upper cretaceous metamorphic basement in isthmian central america rocks from the metamorphic basement of the azuero and sona peninsulas panama consist of schistose amphibolites and minor amounts of metasediment in the sona peninsula strongly zoned amphiboles indicate that the amphibolites followed a progressive anticlockwsie p t path prograde from low t low p to medium t high p and are retrograded into the greenschist facies in contrast the amphibolites of the azuero peninsula are affected by a low to medium t low p metamorphism the metamorphic events of the sona amphibolites occurred prior to the intra senonian tectonic phase which affects the mesozoic formations along the pacific coast of costa rica and panama the regional significance of such a basement in isthmian central america is discussed

archaean granulite facies metamorphism of the lewisian of tiree inner hebrides north west scotland the lewisian of tiree north west scotland underwent granulite facies metamorphism prior to 2 4 ga the temperatures and pressures estimated from garnet clinopyroxene garnet orthopyroxene hornblende plagioclase and garnet biotite geothermometers and clinopyroxene plagioclase garnet quartz and orthopyroxene plagioclase garnet quartz geobarometers are 810 50 c and 10 5 1 5 kbar the imprecision of pressure estimates stems largely from uncertainties in garnet activity models calculations of blocking temperatures for fe mg interdiffusion in clinopyroxene and garnet suggest that these temperatures and pressures represent only slightly reset peak metamorphic conditions down temperature re equilibration resulted in chemical zoning over the outer 50 100 m of the mafic minerals p t paths calculated from this mineralogical zoning suggest nearly isobaric cooling however the growth of late sillimanite in metapelites requires that the retrograde p t path had a significant decompression component suggesting that the mineralogical zonation does not define the retrograde p t path the discrepancy between the p t path calculated from mineralogical zonation and that implied by mineral reactions probably results from the net transfer geobarometry reactions closing at higher temperatures than the exchange geothermometers the tiree rocks have a similar history to the mainland scourian complex granulite facies metamorphism accompanied by partial melting occurred prior to the intrusion of the scourie dykes at c 2 4 ga and the rocks underwent retrogression both prior to and after dyke emplacement however peak metamorphic temperatures and pressures on tiree were lower than those recorded in the scourian complex and the tiree rocks may have been at a different crustal level at that time

seamless geological map generation using aster in the broken hill curnamona province of australia abstract the availability of multiple aster image acquisitions enables regional scale geological mapping though instrument irradiance atmospheric and surface scattering effects can cause problems in generating seamless mosaics of geological information products these issues including shortwave infrared swir crosstalk were addressed in producing seamless aster geological maps over the curnamona province associated with the world class pb zn ag broken hill deposit over 35 aster scenes covering an area of approximately 52 000 km2 from 14 different overpass dates were acquired maps of al oh and mg oh carbonate were generated from aster swir data as well as a map of quartz content from the thermal infrared tir data maps of ferrous iron content were also generated from the swir data of individual aster scenes the swir bands also enabled qualitative mapping of the al oh composition though garnet and feldspar rich units were not well mapped using the tir field sampling and spectral measurements together with detailed 1 25 000 mapping and large scale hymap surveying constrained the accuracy of the aster derived geological products

p t time characterization of the trans amazonian orogeny in the imataca complex venezuela metaigneous and metasedimentary rocks from the imataca complex of venezuela record high temperature decompression at granulite facies peak metamorphism preservation of convincing reaction assemblages in many samples allows determination of relative timing of development of observed assemblages sillimanite pseudomorphs after andalusite and absence of kyanite requires a high temperature pressure t p prograde geothermal gradient two pyroxene thermometry and garnet orthopyroxene plagioclase quartz assemblages yield peak t and p in the range of 750 800 c and 8 8 5 kbar one sample of garnet bearing meta norite contains orthopyroxene with approximately 6 al2o3 requiring either higher peak pressures or disequilibrium growth of orthopyroxene from garnet breakdown of garnet sillimanite to hercynite quartz and of garnet sillimanite quartz to cordierite require decompression at high temperature mineral compositions in these assemblages indicate p near 6 kbar for t in the range of 500 700 c coexisting orthopyroxene clinopyroxene plagioclase quartz apparently react to form abundant hornblende and strongly zoned plagioclase with an70 cores and an35 margins garnet is also compositionally zoned in some samples finally alkali feldspar and plagioclase coexist in some samples without perthitic intergrowths and these retrograded feldspars yield temperatures near 550 c rhombohedral oxides are extensively and complexly exsolved in some samples exsolution products include ilmenite hercynite and corundum in other samples exsolution of oxides has apparently been complete and only homogeneous discrete grains of rhombohedral and cubic oxides exist these completely exsolved oxides yield apparent retrograde temperatures below 600 c these observations indicate that the imataca complex reached granulite facies metamorphic conditions along a high t p gradient followed closely by tectonic loading subsequent decompression and slow cooling allowed significant progress along several retrograde reactions as well as equilibration of feldspar compositions and extensive exsolution in oxide phases the observed compositional zoning in garnet and plagioclase developed during retrograde reactions a one dimensional heat flow model constrained by rb sr and 40ar 39ar cooling ages suggests the imataca complex was uplifted from 32 to 16 km at a rate of 0 5 1 km ma1 after which isobaric cooling ensued the p t path given by this model is consistent with the retrograde assemblages observed

the willyama supergroup in the broken hill and euriowie blocks new south wales abstract the willyama supergroup consists of highly deformed metasedimentary schists and gneisses with abundant quartzofeldspathic gneisses lesser basic gneisses and minor lode rocks prograde metamorphism ranged from andalusite through sillimanite to granulite grade radiometric dating in the broken hill block indicates separation of the source of the quartzo feldspathic gneisses from the mantle at 2300 2100 ma a rb sr model source age of 1820 60 ma has been interpreted as the maximum age of deposition deformation d 1 and or d 2 was coincident with high grade metamorphism at 1660 10 ma minor mundi mundi type granitoids and some pegmatites were intruded at 1490 20 ma minor pyroxenites at 561 7 ma dolerite dykes soon after and zoned pegmatites at 500 ma in association with the delamerian orogeny which reactivated retrograde schist zones at 520 40 ma subsequent history mostly comprises faulting weathering uplift and erosion the willyama supergroup has an estimated total thickness of 7 9 km with neither top nor basement exposed no unconformities have been recognized in the sequence the supergroup was deposited in a deepening environment passing upward from sandy facies to more shaly and fine sandy facies and from interpreted bimodal acid basic volcanics to non volcanic facies the metasediments have been interpreted as shelf sediments overlain by deeper water turbidites and minor contourites recently they have been reinterpreted as shallow marine with the quartzo feldspathic gneisses interpreted as fluvio deltaic arkoses in this paper the sequence to the top of thackaringa group is interpreted as mainly fluvio deltaic and lacustrine the broken hill group and sundown group are interpreted as shallow marine the paragon group may have comprised shelf muds overlain by delta front and lacustrine sediments in turn overlain by deeper water fine grained turbidites the broken hill orebody near the top of broken hill group consists of a series of stacked lenses much recent work has concluded that the deposit is volcanic exhalative in origin probably formed from seawater convected through the sequence and driven by heat from high level magma chambers however some features of the deposit are more appropriate to a sedimentary exhalative origin and models have been proposed involving seismic pumping of connate water with ore deposition on or below the seafloor devolatilization of the mantle has been proposed as a source of ore constituents deposition may have occurred in a narrow ne sw trending seafloor depression the willyama supergroup was probably deposited in a rift zone with thin crust the basic gneisses are interpreted as fe rich tholeiites similar to basalts from some oceanic spreading ridges the intense folding post folding leucogranitic intrusions and extensive granulite amphibolite facies metamorphism could have resulted from a collision following subduction however there is no direct evidence of oceanic crust or subduction and the low pressure metamorphic path does not suit a collision model the rocks were probably deposited in a continental rift with some similarities to those interpreted for proterozoic fold belts in northern australia

pre foliation metamorphism in low pressure high temperature terrains abstract porphyroblasts in schists in some middle to upper crustal low pressure regional metamorphic terrains contain equant random inclusions that are much smaller than the average grain size of the matrix which may be strongly foliated the folia typically being deflected around the porphyroblasts these microstructures are consistent with growth of the porphyroblasts either before or during the earliest stages of development of a tectonic foliation although previous brittle deformation may have taken place anatectic leucosomes may delineate bedding and or the earliest tectonic foliations which are folded in subsequent folding events and new leucosomes may occur in axial surfaces of later folds this evidence suggests that high grade metamorphic conditions were in operation before or during the development of the earliest foliation and persisted during subsequent episodes of folding also consistent with this inference is the occurrence of random porphyroblasts boudinaged in the first foliation in many middle to upper crustal low pressure regional metamorphic terrains granitoids appear to be responsible for the development and localization of regional aureole metamorphism when superimposed on a more regional temperature rise the granitoids may also induce and localize folding and the development of foliations owing partly to weakening of the rocks by heat and fluids from the magma and especially to fluids released pervasively from prograde reactions such an interpretation can account for the growth of high grade metamorphic minerals before the development of a tectonic foliation

delamerian aged metamorphism in the southern curnamona province australia implications for the evolution of the mesoproterozoic olarian orogeny monazite electron microprobe u th pb and garnet sm nd isotopic data from metapelitic assemblages in the willyama supergroup in the southern curnamona province south central australia indicate that the terrain underwent regional greenschist to amphibolite grade metamorphism during the c 500 ma delamerian orogeny the delamerian aged mineral assemblages include prograde garnet staurolite and kyanite bearing associations that overprint andalusite and sillimanite bearing assemblages that are interpreted to have developed during the c 1600 ma olarian orogeny importantly the development of secondary kyanite bearing assemblages in the southern curnamona province has been used previously to suggest that the olarian orogeny followed an anticlockwise p t evolution if such assemblages are the product of c 500 ma metamorphism then the anticlockwise p t path is an apparent path due to the overprint of a distinct metamorphic cycle c 1100 ma later making such distinctions is therefore extremely important when using the textural and metamorphic evolution of polycyclic terrains to model the thermal behaviour of the crust during orogeny this study highlights the utility of in situ geochronology linking age data to petrologically important phases and assemblages

component migration patterns during the formation of a metamorphic layering mount franks area willyama complex n s w australia abstract mineral shapes sizes and proportions in mica rich m and quartz mica qm domains of an s 4 metamorphic layering have been compared in two mutually perpendicular orientations with shapes sizes and proportions in mildly crenulated s 2 enclaves preserved within a rock from part of the willyama complex n s w australia the study has shown that data from two orientations are necessary to place constraints on the movement of mineral components dissolved in solution during formation of the layering comparison of mineral data has shown that sio 2 has been dissolved from quartz grains in both m and qm domains the sio 2 lost from m domains has not migrated into qm domains measurements have also shown that biotite has undergone less solution than quartz most of the biotite components have probably undergone reaction to form probable syn s 4 muscovite and chlorite the muscovite forming reaction requires some silica on the lefthand side of the reaction probably more than can be supplied by the chlorite forming reaction some of this extra silica was supplied from dissolved quartz grains however only a small amount is needed for this purpose and most of the sio 2 has left the system some syn s 4 muscovite may have formed in m domains where it lies within the crenulated s 2 direct evidence for its growth is hard to find

low pressure regional metamorphism in the omeo metamorphic complex victoria australia the omeo metamorphic complex forms the southern end of the wagga metamorphic belt which is the main locus of palaeozoic low pressure metamorphism in the lachlan fold belt south eastern australia it comprises metamorphosed ordovician quartz rich turbidites originally derived from precambrian cratonic rocks prograde regional metamorphism occurred in the early silurian very soon after sedimentation had ceased the sequence of metamorphic zones with increasing grade is chlorite biotite cordierite andalusite k feldspar and sillimanite k feldspar migmatites occur in the sillimanite k feldspar zone but large bodies of s type granite were derived from rocks underlying the exposed ordovician sequence p and t estimates for the highest grade rocks are t 700 c and p 3 5 kbar indicating a very high p t gradient of 65 c km the high heat flow during prograde metamorphism probably resulted from a combination of a thermal anomaly persisting from a pre metamorphic back arc basin environment and intrusion of hot mantle derived magmas into the lower and middle crust regional retrograde metamorphism coincided with a general reheating of the crust in the siluro devonian accompanied by intrusion of many i type plutons and resetting of the k ar dates of some earlier plutons the omeo metamorphic complex was exposed to erosion at this time

fluctuations in a magmatic sulphur isotope signature from the pinnacles mine new south wales australia sulphur isotopic compositions of 29 sulphide samples from the broken hill type pinnacles deposit nsw are found to cluster at 0 mean 0 8 the restricted range of the 34s values between 3 5 and 3 7 with a mean of 0 8 is interpreted as reflecting partial oxidation of a dominantly magmatic sulphur source 34s data for galena samples fall into two groups 1 isotopically heavier galenas range 0 7 to 0 0 mean 0 4 which come mainly from the footwall zn lode and 2 isotopically lighter galenas range 3 5 to 0 8 mean 2 2 which are from the main pb lode sphalerite pyrrhotite and chalcopyrite have slightly heavier isotopic compositions range 1 6 to 3 7 mean 0 3 but exhibit the same stratigraphic differentiation these data are interpreted as representing fluctuating conditions at the site of ore deposition in which upwelling hydrothermal fluids were subject to increasing fo2 and decreasing temperature with time

crystallization of melts pegmatite intrusion and the inverian retrogression of the scourian complex north west scotland partial melting of tonalitic gneisses in the 2 7 ga badcallian granulite facies metamorphic episode in the scourian complex of north west scotland produced a suite of granitic to trondhjemitic liquids on cooling and excavation of the complex these melts underwent fractional crystallization and the residual liquids eventually became water saturated comparison with experimental data suggests that water saturation would have occurred in these melts at around 620 700 c from the retrograde p t time path followed by the complex it is estimated that h2o dominated fluids were exsolved from these melts at c 2 5 ga it is proposed that these fluids were the cause of the 2 5 ga inverian retrogression of the scourian complex and that water saturated melts formed during the crystallization of the leucogneisses were intruded as a suite of pegmatites the timing of pegmatite intrusion is consistent with this proposition as are the temperature estimates timing distribution and nature of the inverian phase of metamorphism it is likely that the crystallization of melts is an important process in bringing about hydrous retrogressive metamorphic episodes in a number of other basement terrains such as west greenland and australia

did the giant broken hill australia zn pb ag deposit melt abstract recent published research has proposed that some metamorphosed massive sulfide deposits including the broken hill deposit australia have partially melted here we discuss the evidence for and against this process the broken hill deposit is located in the curnamona craton within a rift in an apparent upward coarsening sequence of clastic metasediments into which paleoproterozoic mafic and felsic melts intruded the deposit was metamorphosed to granulite facies conditions 750 800 c and 5 6 kbar and was subjected to at least five periods of deformation at peak p t conditions a melt phase was produced in silicate rocks especially granitoids and psammopelites and in ore characterized by the systems cu as s ag pb s ag sb as s cu sb pb s sb as s cu sb s and fe as s however the proportion of minerals in these s bearing systems is insignificant 2 feo mno cao al 2 o 3 p 2 o 5 co 2 zns pbs fes feass feas 2 which is the most relevant system to broken hill ores p t conditions were never high enough to produce a melt phase in this system authors arguing for sulfide rock melting suggest that mn rich lithologies garnetite and quartz garnetite which are intimately associated with the pb rich ores at the broken hill deposit were produced by a reaction between mn rich sphalerite and aluminous wall rocks however to produce such rocks would require mn contents of sphalerite compositions that are unrealistically high and yet to be found in nature moreover the implication of the melt model is that wherever mn rich quartz garnetite and garnetite are found sphalerite should be located next to them this is clearly not the case the presence of polyphase sulfide inclusions within garnet in garnetite has also been considered by some in the literature to be sulfide melt inclusions with the implication that the enclosing garnet was a product of melting however we consider these sulfides to be products of hydrothermal processes during retrograde metamorphism it is impossible to form garnetite and quartz garnetite in rocks within the curnamona craton that formed at upper greenschist lower amphibolite facies conditions by partial melting p t conditions were too low we consider that the fe and mn component of garnetite and quartz garnetite are products of exhalation and inhalation at or near the sea floor and that these rocks are meta exhalites or meta inhalites some garnet rich rocks also formed by metasomatic processes throughout the protracted metamorphic history that affected the curnamona craton

boron isotope evidence for the involvement of non marine evaporites in the origin of the broken hill ore deposits identifying the palaeogeographic setting and mode of origin of stratabound ore deposits can be difficult in high grade metamorphic terranes where the effects of metamorphism may obscure the nature of the protoliths here we report boron isotope data for tourmalines from the early proterozoic broken hill block in australia which hosts giant lead zinc silver sulphide deposits with one exception the 11b 10b ratios are lower than those for all other tourmalines from massive sulphide deposits and tour malinites elsewhere in the world we propose that these low ratios reflect leaching of boron from non marine evaporitic borates by convecting hydrothermal fluids associated with early proterozoic continental rifting a possible modern analogue is the salton sea geothermal field in california

metamorphism of the kisseynew gneisses and related rocks of the reindeer zone trans hudson orogen northern saskatchewan in the reindeer zone of saskatchewan the mostly metasedimentary kisseynew gneiss crops out in a 300 km wide belt extending from the tabbernor fault to the manitoba border metamorphic grade varies

the canada basin 1989 1995 upstream events and far field effects of the barents sea 1 the effects of upstream events on southern canada basin waters were examined using physical and geochemical data collected at one location between 1989 and 1995 these events included atlantic layer warming relocation of the atlantic pacific water mass boundary and increased ventilation of boundary current waters early signals of change in the canada basin first appeared in 1993 along the continental margin and by 1995 were evident in the basin interior and farther downstream differences in physical and geochemical properties nutrients oxygen 129i and cfcs were observed between 150 and 1600 m of the water column by 1995 the boundary between pacific and atlantic origin water was shallower and atlantic origin water occupied more of the canada basin water column atlantic origin lower halocline water was colder and more ventilated fram strait branch water was colder fresher and more ventilated and barents sea branch water was warmer fresher more ventilated and occupied a larger volume of the water column than in previous years water mass analysis showed that the composition of atlantic origin water in the canada basin in 1995 included 20 more water from the barents sea two related events upstream appear to be the source of this change an atmospheric regime shift in 1989 that increased cyclonic circulation a transition unprecedented within the latter half of the 20th century in magnitude geographic reach and apparent oceanographic impacts and a large volume of dense water flowed from the barents sea to the arctic ocean between 1988 and 1989 these two phenomena illustrate the close relationship between atmospheric and oceanic events whose combined influence was evident 6 years later and 5100 km downstream in the canada basin at depths 1000 m

granulite facies metamorphism and retrogressive evolution of the monts du lyonnais metabasites massif central france abstract garnet bearing pyriclasites have been found within the monts du lyonnais formation near st andrela cote in the eastern part of the massif central the rocks appear to have differentiated from a tholeiitic magma of continental affinity and they may be genetically related to quartzofeldspathic granulites considered to be the products of crustal melts by means of electron microprobe analyses the consistent pressure and temperature estimates based essentially on pyroxene garnet equilibria show that the peak of metamorphism was at about 860 c and 10 kbar furthermore this study tends to confirm the affiliation of monts du lyonnais granulites to the early variscan granulite eclogite facies metamorphism which extends from bohemia to spain through central europe this event may be expected in the vicinity of an ancient suture zone where a continental collision followed a subduction

water activity changes across an amphibolite granulite facies transition broken hill australia phase compositions in pelitic and mafic gneisses place tight constraints on pressure ranging from 3 up to 6 kb and to a lesser extent on temperature 500 up to 800 c during prograde regional metamorphism of the willyama complex broken hill se australia these limits allow an evaluation of water activity across the terrain using various equilibria in pelitic and mafic gneisses the stability of cummingtonite and biotite over much of the terrain places upper limits on temperature and the presence of syn metamorphic partial melts in the metasediments places lower limits on a h2o garnet biotitesillimanite k feldspar quartz relations combined with the partial melting data suggest a decrease in water activity from near 1 0 in the lower grade zones to 0 5 0 2 in the broken hill little broken hill part of the two pyroxene zone this result is compatible with less precise hornblende orthopyroxene clinopyroxene quartz relations

cordierite bearing schists and gneisses from timor eastern indonesia p t conditions of metamorphism and tectonic implications in the boi massif of western timor the mutis complex which is equivalent to the lolotoi complex of east timor is composed of two lithostratigraphical components various basement schists and gneisses and the dismembered remnants of an ophiolite cordierite bearing pelitic schists and gneisses carry an early mineral assemblage of biotite garnet plagioclase al silicate but contain no prograde muscovite sillimanite occurs in a textural mode which suggests that it replaced and pseudomorphed kyanite at an early stage and some specimens of pelitic schist contain tiny kyanite relics in plagioclase textural relations between and mineral chemistries of ferro magnesian phases in these pelitic chists and gneisses suggest that two discontinuous reactions and additional continuous compositional changes have been overstepped possibly with concomitant anatexis as a result of decrease in pload during high temperature metamorphism the simplified reactions are garnet and or biotite sillimanite quartz cordierite hercynite ilmenite excess components p t conditions during the development of the early mineral assemblage in the pelitic gneisses are estimated to have been p 10 kbar and t 750 c based upon the plagioclase garnet al silicate quartz geobarometer and the garnet biotite geothermometer p t conditions during the subsequent development of cordierite bearing mineral assemblages in the pelitic gneisses are estimated to have been p 5 kbar and t 700 c with xh2o 0 5 based upon the fe content of cordierite occurring in the assemblage quartz plagioclase sillimanite biotite garnet cordierite coexisting with melt final equilibration between some of the phases suggests that conditions dropped to p 2 3 kbar and t 600 c a similar exhumation p t path is suggested for the pelitic schists with early metamorphic conditions of p 6 2 kbar and t 745 c and subsequent development of cordierite under conditions in the range p 3 4 kbar and t 600 700 c the tectonic implications of these p t estimates are discussed and it is concluded that the p t path followed by these rocks was caused by decompression during rifting and synmetamorphic ophiolite emplacement resulting from processes during the initiation and development of a convergent plate junction located in southeast asia during late jurassic to cretaceous time

an extended episode of early mesoproterozoic metamorphic fluid flow in the reynolds range central australia the products of metamorphic fluid flow are preserved in zones within the marbles and metamorphosed semipelites of the upper calcsilicate unit in the granulite portion of the late palaeoproterozoic reynolds range group northern arunta block central australia the zones of retrogression characterized by minerals such as wollastonite grossular and clinohumite local resetting of oxygen isotopic compositions and local major element metasomatism were channelways for water rich fluids derived from granulite facies metapelites u th pb isotopic ages measured by the shrimp ion microprobe on zircon and monazite from a granulite facies semipelite an early semiconcordant aluminous quartz rich fluid flow segregation and a late discordant quartz rich segregation record some of the extended thermal history of the area zircon cores from the semipelite show its likely protolith to be an igneous rock 1812 11 ma old itself derived from a source containing zircon as old as 2 2 ga low th u overgrowths on the zircon grew during granulite facies metamorphism at 1594 6 ma monazite cooled to its blocking temperature at 1576 8 ma zircon cores from the semiconcordant segregation are dominantly 2 3 ga old indicating that the source of the fluids was not the particular metamorphosed semipelite studied two generations of low th u overgrowths on the zircon give indistinguishable ages for the older and younger of 1589 8 and 1582 8 ma respectively the monazite age is the same 1576 12 ma zircon from the late discordant segregation gave 1568 4 ma fluid flow occurred for at least 18 3 ma and ended 26 3 ma after the peak of metamorphism suggesting a very slow cooling rate of 3 c ma 1 the last regional high grade metamorphism in the reynolds range occurred at 1 6 ga not 1 78 ga as previously thought the high grade event at 1 78 ga is a separate event that affected only the basement to the reynolds range group

pressure temperature time paths and a tectonic model for the evolution of granulites petrologic studies and application of well calibrated mineralogic thermometers and barometers reveal several important features common to many granulite terrains 1 peak metamorphic pressure and temperature conditions cluster around values of 7 5 pm 1 kbar and 800 pm 50 c implying average geothermal gradients of 30 35 c km 2 in comparison with upper amphibolite facies rocks especially in paired amphibolite granulite terrains it is evident that granulites are distinguished from amphibolites by the former s higher metamorphic temperatures but not higher pressure 3 initial cooling of granulite terrains from peak conditions is nearly isobaric for garnet closure temperatures of 600 pm 50 c the dp dt retrograde paths inferred from compositionally zoned garnet rims are 2 8 b c in most granulites 4 primary coarse sillimanite is found in most granulites and retrograde kyanite is known in a few these data suggest that granulites form as a result of anomalous thermal gradients

high temperature retrogression of granulite facies marbles form the reynolds range group central australia phase equilibria isotopic resetting and fluid fluxes reynolds range group rocks underwent granulite facies metaintroduction morphism m2 at 1 6 ga 5 kbar 750 800 c and were the uplift history of high grade metamorphic belts comsubsequently retrogressed in narrow strike parallel zones at 1 59 monly involves partial re equilibration of mineral as1 57 ga within these zones metacarbonates that initially equisemblages to lower temperature and pressure conditions librated at x 2 0 8 during m2 were mineralogically reset this re equilibration is facilitated by fluid rock interby the infiltration of water rich fluids x 2 0 02 0 3 at action and new mineral assemblages are often best 650 700 c and 3 4 kbar do carb values of the retrogressed developed in discrete zones of extensive retrogression metacarbonates were variably reset during fluid infiltration with the e g van reenen 1986 cartwright 1988 peters lowest values 10 13 suggesting that the fluids that caused wickham 1994 cartwright buick 1995 petrological retrogression were exsolved from segregated partial melts themselves studies of retrograde mineral assemblages may provide derived from the underlying granulite facies metapelites minimportant information about the tectonics and hydroeralogical and isotopic resetting were locally accompanied by silica geology of the lower and middle crust metasomatism the mineralogically reset marbles record time inhowever although there have been many recent quantegrated fluid fluxes of typically 10 10 m m and metasomatic titative studies of metamorphic fluid rock interaction in rocks record time integrated fluid fluxes as high as 10 10 m contact aureoles cartwright valley 1991 ferry m for upward flow of high temperature fluids through the marbles dipple 1991 young morrison 1992 jamveit et al over a distance of 200 m in 18 ma the observed mineralogical 1992 davis ferry 1994 and during prograde regional and isotopic resetting and metasomatism require intrinsic permetamorphism bickle baker 1990a 1990b ferry meabilities between 10 and 10 m that vary across strike 1992 stern et al 1992 leger ferry 1993 cartwright on a centimetre to metre scale indicating that fluid flow was strongly et al 1995 buick cartwright 1996 there have been channelled relatively few studies of high temperature retrogression vennemann stuart smith 1992 peters wickham 1994 cartwright buick 1995 therefore the origins of the retrograde fluids and the absolute timing of

clastic metasediments of the early proterozoic broken hill group new south wales australia geochemistry provenance and metallogenic significance whole rock analyses of samples of pelite psammite and psammopelite from the early proterozoic broken hill group willyama supergroup in the broken hill block new south wales australia reveal distinctive geochemical signatures major element data show high al2o3 and k2o low mgo and na2o and relatively high fe2o3tmgo ratios compared to average early proterozoic clastic metasediments high field strength elements hfse are especially abundant including nb most 15 27 ppm ta most 1 0 2 2 ppm th 17 36 ppm hf 4 15 ppm and zr most 170 400 ppm y 33 74 ppm is also high concentrations of ferromagnesian elements are generally low sc 20 ppm ni 62 ppm co 26 ppm cr most 100 ppm data for rare earth elements rees show high abundances of light rees lacn 116 250 chondrite lacn 437 in one sample high lacnybcn ratios 5 6 13 9 and large negative eu anomalies eueu 0 32 0 57 the geochemical data indicate derivation of the metasedimentary rocks of the broken hill group by the erosion mainly of felsic igneous or meta igneous rocks high concentrations of hfse y and rees in the metasediments suggest a provenance dominanted by anorogenic granites and or rhyolites including those with a type chemistry likely sources of the metasediments were the rhyolitic to rhyodacitic protoliths of local quartz feldspar biotite garnet gneisses e g potosi type gneiss that occur within the lower part of the willyama supergroup or chemically similar basement rocks in the region alternative sources may have included early proterozoic anorogenic granites and or rhyolites in the mount isa and or pine creek blocks of northern australia or in the gawler craton of south australia metallogenic considerations suggest that the metasediments of the broken hill block formed enriched source rocks during the generation of pegmatite hosted deposits and concentrations of la ce nb ta th and sn in the region li be b w and u in pegmatite minerals of the district may have been acquired during granulite facies metamorphism of the local metasediments

excess40ar in metamorphic rocks from broken hill new south wales implications for40ar 39ar age spectra and the thermal history of the region 40ar 39ar age spectrum analyses of samples from broken hill new south wales indicate that the region has experienced a complex thermal history following high grade metamorphism 1660 ma ago the terrain cooled slowly 3 c ma 1 until about 1570 ma ago when the temperature fell below about 500 c following granitoid emplacement 1500 ma ago the region remained relatively cold until affected by a thermal pulse 520 40ma ago causing temperatures to rise to 350 c in some places during this event accumulated40ar was released from minerals causing a significant ar partial pressure to develop laboratory ar solubility data combined with the40ar 39ar age spectra gives a local estimate of this partial pressure of 10 4atm the region finally cooled below 100 c about 280 ma ago 40ar 39ar age spectrum analyses of hornblende plagioclase and clinopyroxene containing excess40ar are characterized by saddle shaped age spectra detailed analysis of plagioclase samples reveals a complex diffusion behaviour which is controlled by exsolution structures this effect in conjunction with the presumed different lattice occupancy of excess40ar with respect to radiogenic40ar appears to be responsible for the saddle shaped age spectra

timing and nature of fluid flow and alteration during mesoproterozoic shear zone formation olary domain south australia the development of shear zones at mid crustal levels in the proterozoic willyama supergroup was synchronous with widespread fluid flow resulting in albitization and calcsilicate alteration monazite dating of shear zone fabrics reveal that they formed at 1582 22 ma at the end of the olarian d3 deformational event and immediately prior to the emplacement of regional s type granites two stages of fluid flow are identified in the area first an albitizing event which involved the addition of na and loss of si k and fe and a second phase of calcsilicate alteration with additions of ca fe mg and si and removal of na fluid fluxes calculated for albitization and calcsilicate alteration were 5 56 109 to 1 02 1010 mol m 2 and 2 57 108 5 20 109 mol m 2 respectively these fluxes are consistent with estimates for fluid flow through mid crustal shear zones in other terranes the fluids associated with shearing and alteration are calculated to have 18o and d values ranging between 8 and 11 and 33 and 42 respectively and nd values between 2 24 and 8 11 our results indicate that fluids were derived from metamorphic dehydration of the willyama supergroup metasediments fluid generation occurred during prograde metamorphism of deeper crustal rocks at or near peak pressure conditions shear zones acted as conduits for major crustal fluid flow to shallow levels where peak metamorphic conditions had been attained earlier leading to the apparent retrograde fluid flow event thus the peak metamorphism conditions at upper and lower crustal levels were achieved at differing times prior to regional granite formation during the same orogenic cycle leading to the formation of retrograde mineral assemblages during shearing

reaction softening by dissolution precipitation creep in a retrograde greenschist facies ductile shear zone new hampshire usa we describe strain localization by a mixed process of reaction and microstructural softening in a lower greenschist facies ductile fault zone that transposes and replaces middle to upper amphibolite facies fabrics and mineral assemblages in the host littleton schist near claremont new hampshire here na poor muscovite and chlorite progressively replace first staurolite then garnet and finally biotite porphyroblasts as the core of the fault zone in approached across the transect higher grade fabric forming na rich muscovite is also progressively replaced by fabric forming na poor muscovite the mineralogy of the new phyllonitic fault rock produced is dominated by na poor muscovite and chlorite together with late albite porphyroblasts the replacement of the amphibolite facies porphyroblasts by muscovite and chlorite is pseudomorphic in some samples and shows that the chemical metastability of the porphyroblasts is sufficient to drive replacement in contrast element mapping shows that fabric forming na rich muscovite is selectively replaced at high strain microstructural sites indicating that strain energy played an important role in activating the dissolution of the compositionally metastable muscovite the replacement of strong high grade porphyroblasts by weaker na poor muscovite and chlorite constitutes reaction softening the crystallization of parallel and contiguous mica in the retrograde foliation at the expense of the earlier and locally crenulated na rich muscovite defined foliation destroys not only the metastable high grade mineralogy but also its stronger geometry this process constitutes both reaction and microstructural softening the deformation mechanism here was thus one of dissolution precipitation creep activated at considerably lower stresses than might be predicted in quartzo feldspathic rocks at the same lower greenschist facies conditions this article is protected by copyright all rights reserved

granulite facies metamorphism in the mallee bore area northern harts range implications for the thermal evolution of the eastern arunta inlier central australia abstract the mallee bore area in the northern harts range of central australia underwent high temperature medium to high pressure granulite facies metamorphism individual geothermometers and geobarometers and average p t calculations using the program thermocalc suggest that peak metamorphic conditions were 705 810c and 8 12 kbar partial melting of both metasedimentary and meta igneous rocks forming garnet bearing restites occurred under peak metamorphic conditions comparison with partial melting experiments suggests that vapour absent melting in metabasic and metapelitic rocks with compositions close to those of rocks in the mallee bore area occurs at 800 875c and 9 10 kbar the lower temperatures obtained from geothermometry imply that mineral compositions were reset during cooling following the metamorphic peak the rocks underwent local mylonitization at 680 730c and 5 8 7 7 kbar after mylonitization ceased garnet retrogressed locally to biotite which was probably caused by fluids exsolving from crystallizing melts these three events are interpreted as different stages of a single continuous clockwise p t path the metamorphism at mallee bore probably occurred during the 1745 1730 ma late strangways orogeny and the area escaped significant crustal reworking during the anmatjira and alice springs events that locally reached amphibolite facies conditions elsewhere in the harts ranges

the stability of a laminated voussoir beam back analysis of a historic roof collapse using dda abstract the stability of a horizontally bedded and vertically jointed roof referred to here as a laminated voussoir beam is studied using careful documentation of a historic roof collapse which occurred in an ancient underground water storage reservoir dated back to ca 1000 b c the roof of the opening failed immediately after the excavation leaving a dome shaped loosened zone with a span of 7 m and height of 2 5 m consisting of horizontal beds and vertical joints with mean spacing of 50 and 25 cm respectively the ancient engineers erected a massive pillar at the center of the dome in order to passively support the failed roof and the opening remained in service for several hundred years following the failure analysis of the roof using iterative voussoir beam procedure beer g and meek j l design curves for roofs and hanging walls in bedded rock based on voussoir beam and plate solutions trans inst min metall 1982 91 a18 22 shows that the roof was more sensitive to failure by shear along the abutments rather than by crushing at hinge zones and that the required friction angle req for stability would have been 36 the available friction angle is estimated between 38 6 and 46 4 and therefore the result of the iterative solution is considered unconservatively wrong results of dda shi g h and goodman r e two dimensional discontinuous deformation analysis int j numer anal methods geomech 1989 13 359 380 shi g h block system modeling by discontinuous deformation analysis computational mechanics publications southampton 1993 pp 209 however indicate that req must have been greater than 60 a shear strength which was not available at the time of construction thus the immediate failure using dda we further demonstrate that req is related to joint spacing or block length in a parabolic function with increasing joint spacing req decreases to a minimum value and than increases with further increase in joint spacing this result is attributed to the interaction of two competing forces one is the stabilizing axial thrust which increases with increasing moment arm in individual blocks a function of joint spacing or block length the other is the destabilizing vertical load which increases with increasing weight of overlying blocks

the origin of retrograde shear zones in the napier complex implications for the tectonic evolution of enderby land antarctica abstract retrograde shear zones rszs in granulites at fyfe hills in the napier complex east antarctica formed during a late proterozoic or early palaeozoic reactivation event these rszs preserve a structural and metamorphic record of excavation from depth equivalents of 6 8 kbar 20 26 km to depth equivalents of 3 5 kbar 10 17 km excavation was essentially isothermal suggesting that the rszs formed in an allochthonous terrain during crustal thickening by low angle thrusting rehydration of the rszs was accompanied in part by large ion lithophile element enrichment and reduction in k rb features which are attributed to 1 the retrograde fluid having originated in continental crust that had not previously undergone granulite facies metamorphism and 2 the fluid having been introduced into the rszs by the intrusion of granitic dykes and pegmatites the saturation of these rsz pegmatites in components characteristically depleted in deep crustal and mantle sources such as boron and beryllium supports a low grade source for the retrograde fluid furthermore a rehydration mechanism involving intrusion of pegmatites suggests that the fyfe hills granulites are underlain by a significantly lower grade hydrous terrain

on the formation of granulites the tectonic settings for the formation and evolution of regional granulite terranes and the lowermost continental crust can be deduced from pressure temperature time p t time paths and constrained by petrological and geophysical considerations p t conditions deduced for regional granulites require transient average geothermal gradients of greater than 35 c km 1 implying minimum heat flow in excess of 100 mwm 2 such high heat flow is probably caused by magmatic heating tectonic settings wherein such conditions are found include convergent plate margins continental rifts hot spots and at the margins of large deep seated batholiths however particular p t time paths do not allow specific tectonic settings to be distinguished at this time under different conditions both clockwise cw pmax attained before tmax and anticlockwise acw pmax attained slightly after tmax paths are possible in the same tectonic setting both cw and acw end member paths can yield nearly isobaric cooling ibc paths such cooling paths are clearly not an artefact of thermobarometry but can be constrained by solid solid and devolatilization equilibria and geophysical modelling in terms of understanding the evolution of the deep crust a potentially significant group of regional granulite terranes are those that show evidence for acw ibc paths such paths are the likely result of i episodic igneous activity resulting in intrusions within all levels of the crust ii thickening of the crust by magmatic underplating iii slow uplift as a result of the formation of a deep garnet rich crustal root and iv excavation resulting from a later tectonic event unrelated to that resulting in the formation of the granulites the later event might be triggered by the delamination of the garnet rich lowermost crust

retracted structural complexity in the curnamona province south australia polyphase strain partitioning and reactivation in this paper we examine the apparent timing relationships between the structural and metamorphic events of the amphibolite facies sequence in the walter outalpa shear zone wosz area in the southwestern part of the palaeo to mesoproterozoic curnamona province the local structural geometry and style are largely due to heterogeneous non coaxial flow in which foliation development is related to the superposition of two main independent structural events d2 and d3 strain was progressively partitioned into high strain zones in the southern part of the study area in contrast to the low strain areas in the north of the study area d2 deformation developed under amphibolite facies conditions and is partitioned between zones of thrusting and flattening folding related to nappe formation d3 deformation developed under greenschist facies conditions and is partitioned between strike slip faulting and folding d3 high strain zones form discontinuous nw trending anastomosing strands that contain relics of the d2 thrust related foliation these relicts preserve the higher grade mineral assemblages that are partially overprinted by d3 retrograde assemblages strike slip movement along the wosz is therefore interpreted to have re used the pre existing d2 thrust such that d2 strain partitioning influenced renewed d3 strain partitioning the high grade metamorphism and phase of pervasive deformation d2 m2 are ascribed to the d2 mesoproterozoic olarian orogeny that occurred in the curnamona province between ca 1600 and 1590 ma the tectonic context of the d3 m3 shearing event is highly uncertain recent geochronological sm nd isotope ages from garnet from within the wosz shear zone yield ca 520 490 ma delamerian ages dutch r a hand m clark c 2005 cambrian reworking of the southern australian proterozoic curnamona province constraints from regional shear zone systems geol soc london 162 5 763 775 however the d3 m3 shear foliation are truncated by the neoproterozoic unconformity flooring the adelaide rift complex stratigraphy suggesting pre delamerian orogeny activity along the shear zone it is thus likely that the d3 movement along the shear zone records a combination of late olarian orogenic movement with a component of reactivation during the delamerian orogeny 2005 elsevier b v all rights reserved

timing and mechanisms of carbon isotope exchange in granulite facies calc silicate boudins rauer group east antarctica mineralogically zoned decimeter diameter calc silicate boudins enclosed within paragneisses from little italy island rauer group east antarctica underwent granulite facies metamorphism p 5 700 900 mpa t 5 840 6 40 8c followed by near isothermal decompression to 200 400 mpa during decompression several mineral reactions occurred in the presence of a pore fluid 0 4 the calc silicate boudins show a general xco2 increase in calcite d 13 c values from core as low as 217 to rim 2 9t o 2 10 by contrast d 18 o values show little variation across the boudins the d 13 c profiles are similar to those predicted to result from diffusion within a sphere that is surrounded by a homogeneous reservoir diffusion of carbon isotopes probably occurred synchronous with the post peak metamorphic mineral reactions at that time centimeter scale diffusion would have been facilitated by the presence of the fluid while isotopic exchange between the minerals and the fluid would have been promoted by recrystallization for metamorphic porosities of 10 25 2 10 23 the d 13 c profiles could have formed in a few thousand to hundreds of thousands of years small millimeter to centimeter scale variations in d 13 c values that may have been initially present within the boudins would have been homogenized on much shorter timescales than those required to form the profiles the calculated timescales may reflect the time over which metamorphic recrystallization occurred and a reactionenhanced porosity was present graphite formed locally in the paragneisses only at margins of the boudins probably reflects the local escape of co 2 bearing fluids from the boudins into relatively low rocks fo2