horizontal structures in granulite terrains a record of mountain building or mountain collapse in many high temperature 800 c granulite terrains the development of characteristic horizontal structures occurred during the metamorphic culmination and was followed by isobaric cooling the absolute magnitude of isobaric cooling commonly 300 c implies cooling intervals of the order of the thermal time constant of the continental lithosphere 100 m y such prolonged isobaric cooling implies that no significant erosional denudation followed the development of the horizontal structures and thus precludes the prograde deformation being responsible for significant crustal thickening rather the prograde deformation more probably records bulk crustal thinning during extensional collapse of a previously thickened crust possibly triggered by detachment of a thickened thermal boundary layer at the base of the lithosphere

wollastonite and scapolite in precambrian calc silicate granulites from australia and antarctica abstract scapolite wollastonite calcite diopside grossular andradite garnet and sphene occur in calc silicate rocks in the granulite terrain of the arunta block central australia this assemblage buffers the co2 activity at a low value so that any coexisting fluid phase must be h2o rich and co2 poor xco2 0 2 0 3 in contrast the h2o activity in the surrounding felsic and mafic granulites was low thus fluid activities during granulite facies metamorphism were locally buffered in various rock units and fluid flow appears to have been restricted or fluid may have been absent late retrograde rims of garnet and garnet quartz separate phases formed in the high grade stage formation of these rims would have required either an influx of water rich fluid or a decrease in pressure evidence from the surrounding granulites shows that in one locality the calc silicate rocks had undergone late isobaric hydration in another locality minor uplift had occurred soon after peak p t conditions in both scapolite had partly broken down to plagioclase calite a calc silicate rock from the granulite terrain of enderby land antarctica contains scapolite wollastonite calcite diopside quartz and sphene this assemblage also indicates low co2 activities in this rock wollastonite has broken down to calcite quartz to indicate isobaric cooling without influx of hydrous fluid

fate of sediment during plate convergence at the mediterranean ridge accretionary complex volume balance of mud extrusion versus subduction and or accretion drilling results from two mud volcanoes on the mediterranean ridge accretionary complex as well as extensive geophysical surveys have provided new insights about the geometry of these domes at depth mud extrusion is related to buoyancy and plate convergence between africa and eurasia that caused back thrust faulting of accreted strata containing overpressured mud at depth the domes mainly consist of mud breccia formed of as much as 65 polymictic clasts embedded in a clayey matrix of presumed late miocene age volumetric estimates of extruded mud in a well studied area around the olimpi mud volcano field were balanced against sediment input at the deformation front the results demonstrate that only a small fraction of rock mass having entered the subduction zone since the messinian is needed to compensate for the mud expelled in the study area most of the sediment 95 or more is either subducted or incorporated into the accretionary prism the volume of gas expelled with the liquefied overpressured mud was estimated to range between 1 68 10 6 and 2 85 10 7 m 3 yr

proterozoic to mesozoic east gondwana the juxtaposition of india sri lanka and antarctica jurassic and cambro ordovician paleomagnetic pole positions deduced from rocks of sri lanka and antarctica generally conform when sri lanka is juxtaposed with the coast of lutzow holm bay east antarctica in a manner similar to a current gondwana reassembly the highland and southwest groups mostly proterozoic of sri lanka are correlated with the ongul and skallen groups of east antarctica on the basis of similar lithology structural characteristics structural trend tectonothermal history ranging from early proterozoic to mesozoic and the pressure temperature time path of the main metamorphism some neighboring lithotectonic units are also comparable between the two areas fracture lineament systems of sri lanka and antarctica formed after the latest silurian and during or before the jurassic conform in this reconstruction an improved fit of the 2000 m isobaths of india sri lanka antarctica and continuation of the latest archaean mobile belt from enderby land to peninsular india is obtained by fitting sri lanka into lutzow holm bay and enderby land into the embayment in the east coast of peninsular india this juxtaposition of peninsular india to east antarctica involves about 80 km northward movement and about 30 clock wise rotation of sri lanka relative to india the reassembly thus obtained accords well with data supporting the idea that east gondwana existed from the earliest proterozoic to middle mesozoic although some disruption collision events of limited scale are considered possible

stability of high al titanite from low pressure calcsilicates in light of fluid and host rock composition titanite of variable al and f content was found in granulite to amphibolite facies calcsilicates in central dronning maud land antarctica the highest observed al content corresponds to an xal 5 al al 1 ti of 0 53 previously such high values of xal were reported from high pressure rocks but the titanite of this study is from a lowpressure terrain the compositional variations in titanite can be described for all samples by a set of three linearly independent exchange vectors added to the catisio5 endmember titanite in most rocks these vectors are al 1f1ti21o21 t i 20 25m0 25o21oh1 and oh1f21 in one sample the ti20 25m0 25o21oh1 vector is replaced by a si20 25m0 25o21oh1 vector the actual amount of exchange along these vectors and therefore the amount of al in titanite depends on p and t on the composition of the coexisting fluid phase in terms of its h2o hf fugacity ratio and on host rock composition in terms of al2o3 tio2 activity ratio it is inferred that in suitable chemical environments high al titanite is stable over a wide p t range therefore the al content of titanite should not be used in geothermobarometry even qualitatively additionally because of the coupled substitutions al1f1ti21o21 and al1oh1ti21o21 the concentration of f in titanite is strongly dependent on the host rock chemistry this rules out the easy use of titanite as a monitor of fluid composition

the palaeozoogeography of oligocene to recent marine ostracoda from the neotropics mid and south america and antarctica abstract classic biogeographical research has shown that the continent of south america supports a diverse and priceless biota of which ostracods are an important component the distribution patterns of oligocene to recent shelf ostracods from the neotropics to antarctica are explained in terms of dispersal and vicariant events the quantitative examination of a newly constructed database containing over 140 genera has allowed the measurement of generic similarity and endemicity between biotas of different geographical regions the measurement of these parameters has aided the construction of a series of palaeoendemicity and communality maps these maps emphasise changes in the spatio temporal distribution of mid to late tertiary ostracods and can aid in the recognition of abiotic mechanisms that modify genera distribution it has been demonstrated that changes in the oceanic currents and water mass temperature are significant in the formation and maintenance of zoogeographical domains in the oligocene recent of the neotropics and antarctica south america was an important centre of origin for ostracods during the oligocene however few genera appear able to disperse northwards towards the caribbean the migratory success or failure of benthonic ostracods is closely linked to oceanographical and climatic conditions and their physiology within the meso american region filter and corridor pathways have allowed rapid dispersal of shallow water ostracods which has lead to decreased endemism although a distinctive ostracod assemblage was established in the oligocene of antarctica the expansion of the drake passage permitted a new suite of cryophilic genera to emerge on the continent during the mio pliocene within the meso american region the alteration of oceanic circulation patterns subsequent to the closure of the panamanian portal may have initiated the development of a proto panamanian province in the early pliocene

late proterozoic to early palaeozoic tectonism in dronning maud land antarctica supercontinental fragmentation and amalgamation the mid proterozoic early palaeozoic tectonic development of western dronning maud land is examined in the light of recent proposals that suggest geological continuity between laurentia and gondwana during the middle proterozoic the sweat hypothesis gondwana amalgamation from the remaining constituents followed fragmentation and rifting of laurentia from this supercontinent culminating in a late cambrian orogeny at c 500 ma the mid proterozoic early palaeozoic tectonic development of western dronning maud land was significantly different to that observed in the transantarctic mountains and the c 500 ma event in particular is characterized by 1 a regional isotopic resetting event at c 470 ma 2 scarce coeval magmatism comprising only one major intrusion of alkaline affinity and volumetrically small s type granitic sheets derived from local anatexis of gneisses 3 structural deformation considerably less intense than during the preceding kibaran 1100 ma orogeny comprising relatively open to gentle folding 4 reactivation of thrusting initiated during the kibaran orogeny with associated metasomatism it is concluded that the effects of the late cambrian c 500 ma event are consistent with crustal thickening through large scale underplating rather than subduction related processes as seen in the ross orogeny of the transantarctic mountains it is also concluded that gondwana amalgamation occurred along suture lines not currently exposed in outcrop in dronning maud land these data neither contradict nor support the sweat hypothesis as currently argued

calculation of co 2 activities using scapolite equilibria constraints on the presence and composition of a fluid phase during high grade metamorphism thermodynamic and phase equilibrium data for scapolite have been used to calculate co2 activities aco2 and to evaluate the presence or absence of a fluid phase in high grade scapolite bearing meta anorthosite granulites calc silicates and mafix xenoliths the assemblage scapolite plagioclase garnet quartz may be used to calculate or limit aco2 by the reaction meionite quartz grossular anorthite co2 granulites from four high grade terranes grenville province canada sargut belt india furua complex tanzania bergen arcs norway yield aco2 0 4 1 with most 0 7 for scapolite bearing granulites from the furua complex in which aco2 0 9 calculated h2o activities ah2o based on phlogopite dehydration equilibria are uniformly low 0 1 0 2 the aco2 calculated for meta anorthosite from the grenville province ontario ranges from 0 2 to 0 8 for grenville meta anorthosite also containing epidote the ah2o calculated from clinozoisite dehydration ranges from 0 2 to 0 6 calc silicates from the grenville sargur and furua terranes mostly yield aco2 1 the calculated fluid activities are consistent with metamorphism 1 in the presence of a mixed co2 h2o fluid phase in which co2 is the dominant fluid species but other c o h s species are minor 2 in the absence of a bulk fluid phase fluid absent metamorphism or 3 in the presence of a fluid bearing melt phase the results for many granulites and grenville meta anorthosite are consistent with the presence of a co2 rich mixed co2 h2o fluid phase in contrast the relatively restricted and low values of aco2 for calc silicates require an h2o rich fluid or absence of a fluid phase during metamorphism the range of values for xenoliths are most consistent with absence of a fluid phase the primary implication of these results is that a co2 rich fluid accounts for the reduced ah2o in scapolite bearing granulites however scapolite may be stable with a wide range of fluid compositions or in the absence of a fluid phase and the presence of scapolite is not a priori evidence of a co2 rich fluid phase in addition close association of scapolite free mafic granulites with scapolite bearing granulites having identical mineral compositions in the furua complex and the absence of scapolite from most granulite terranes implies that a co2 rich fluid phase is not pervasive on an outcrop scale or common to all granulite terranes

thermal and baric evolution of garnet granulites from sri lanka abstract garnet granulites from sri lanka preserve textural and chemical evidence for prograde equilibration at temperatures of at least 700 750 c and pressures in the vicinity of 6 8 kbar associated strain patterns suggest prograde metamorphism occurred during and immediately following an episode of crustal thickening with the prograde p t conditions probably reflecting a combination of the conductive and advective transport of heat at the mid levels of tectonically thickened crust the occurrence of prograde wollastonite provides evidence for internally buffered fluid compositions or fluid absent conditions during peak metamorphism and precludes pervasive advection of a co2 rich fluid the advective heat component is therefore likely to have been provided by the transport of silicate melt intricate symplectitic textures record partial re equilibration of the garnet granulites to lower pressures 4 6 kbar at high temperatures 600 750 c and testify either to the erosional denudation of the overthick crust prior to significant cooling i e quasi isothermal decompression or to a subsequent static heating possibly of early palaeozoic age pan african the metamorphic history of the sri lankan granulites is compared with high grade terrains in the neighbouring fragments of gondwana with the emphasis on similarities with proterozoic granulites of the east antarctic craton

high temperature metamorphic imprint on calc silicate granulites of rayagada eastern ghats india implication for the isobaric cooling path calc silicate granulites from rayagada north central sector of eastern ghats granulite belt show a wide range of mineral assemblages and chemical compositions which can be grouped as gr i grossular rich garnet wollastonite scapolite calcite clinopyroxene gr ii andradite rich garnet scapolite calcite clinopyr oxene and gr iii scapolite calcite clinopyroxene plagioclase assemblages petrographic features suggest the following several reactions in the cao al2o3 sio2 vapor system mei 4wo cal 3grs qtz 2co2 mei 3wo 2cal 3grs co2 mei 3an cal wo co2 cal qtz mei 5wo 3grs 2qtz co2 an wo grs qtz mei 5cal 3qtz 3grs 6co2 and the following reactions in the cao feo mgo al2o3 sio2 vapor system cpxss scp wo grtss qtz co2 4hd 2cal o2 2adr 2qtz 2co2 cpxss scp grtss cal qtz these reactions have been used to estimate peak t xco2 condition for these granulites a maximum temperature of 920 c has been calculated at an estimated pressure of 9 kbar a t xco2 diagram shows an isobaric cooling from 920 c to 815 c a range of xco2 0 50 at 920 c to 0 25 at 815 c has been observed for gr i calc silicate granulites based on the reaction sequences including coronal garnet forming reactions this sequence is suggestive of internal fluid buffering rather than external fluid influx and the differences in xco2 conditions has been thought to be due to local buffering of fluid phases group ii and gr iii calc silicate granulites on the other hand exhibit relatively lower temperature conditions

the origins of granulites a metamorphic perspective although many recent reviews emphasize a uniformity in granulite pressure temperature p t conditions and paths granulites in reality preserve a spectrum of important petrogenetic features which indicate diversity in their modes of formation a thorough survey of over 90 granulite terranes or occurrences reveals that over 50 of them record p t conditions outside the 7 5 1 kbar and 800 50 c average granulite regime preferred by many authors in particular an increasing number of very high temperature 900 1000 c terranes are being recognized both on the basis of distinctive mineral assemblages and geothermobarometry petrogenetic grid and geothermobarometric approaches to the determination and interpretation of p t histories are both evaluated within the context of reaction textures to demonstrate that the large range in p t conditions is indeed real and that both near isothermal decompression itd and near isobaric cooling ibc p t paths are important amphibolite granulite transitions promoted by the passage of co 2 rich fluids as observed in southern india and sri lanka are exceptional and not representative of fluid related processes in the majority of terranes it is considered on the contrary that fluid absent conditions are typical of most granulites at or near the time of their recorded thermal maxima itd granulites are interpreted to have formed in crust thickened by collision with magmatic additions being an important extra heat source erosion alone is not however considered to be the dominant post collisional thinning process instead the itd paths are generated during more rapid thinning 1 2 mm yr exposure related to tectonic exhumation during moderate rate or waning extension ibc granulites may have formed in a variety of settings those which show anticlockwise p t histories are interpreted to have formed in and beneath areas of voluminous magmatic accretion with or without additional crustal extension ibc granulites at shallow levels very rapid 5 mm yr extensional thinning subsequent to collision it is suggested that the preservation of ibc paths rather than itd paths in many granulites is primarily related to the rate and timescale of extensional thinning of thickened crust and that hybrid itd to ibc paths should also be observed most ibc granulites and probably many itd granulites have not been exposed at the earth s surface as a result of the tectonic episodes which produced them but have resided in the middle and lower crust for long periods of time 100 2000 ma following these events the eventual exhumation of most granulite terranes only occur through their incorporation in later tectonic and magmatic events unrelated to their formation

humite and scapolite bearing assemblages in marbles and calcsilicates of dronning maud land antarctica new data for gondwana reconstructions this study investigates marbles and calcsilicates in central dronning maud land cdml east antarctica the paleogeographic positioning of cdml as part of gondwana is still unclear however rock types mineral assemblages textures and p t conditions observed in this study are remarkably similar to the kerala khondalite belt in india the cdml marbles and calcsilicates experienced a pan african granulite facies metamorphism at c 570 ma and an amphibolite facies retrogression at c 520 ma the highest grade assemblage in marbles is forsterite spinel calcite dolomite in calcsilicates the assemblages are diopside spinel diopside garnet scapolite wollastonite clinopyroxene quartz scapolite anorthite calcite clinopyroxene wollastonite these assemblages constrain the peak metamorphic conditions to 830 20 c 6 8 0 5 kbar and xco2 0 46 during retrogression highly fluoric humite group minerals humite clinohumite chondrodite replaced forsterite and garnet rims formed at the expense of scapolite during reactions with wollastonite calcite or clinopyroxene but without involvement of anorthite metamorphic conditions were about 650 c 4 5 0 7 kbar 0 2 xco2fluid 0 36 and the co existence of garnet clinopyroxene wollastonite and quartz constrains fo2 to fmq 1 5 log units mineral textures indicate a very limited influx of h2o rich fluid during amphibolite facies retrogression and point to significant variations of fluid composition in mm sized areas of the rock gypsum was observed in two samples it probably replaced metamorphic anhydrite which appears to have formed under amphibolite facies conditions the observed extensive anorogenic magmatism anorthosites a type granitoids and the character of metamorphism between 610 and 510 ma suggest that the crustal thermal structure was characterized by a long lived 50 100 ma rise of the crustal geotherm probably caused by magmatic underplating

the pan african event in east antarctica a view from sri lanka and the mozambique belt abstract late pan african 580 550 ma high grade deformation of the sri lankan basement gave rise to a fold and thrust belt the resulting axial planes to upright folds and earlier stretching lineations define a large scale syntaxial bend this syntaxis is attributed to e w shortening and simultaneous sinistral strike slip along a n s trending transcurrent zone located west of sri lanka prior to gondwana break up the resulting geometry is consistent with a corner effect brun and burg 1982 produced by westward indenting of the basement underneath eastern sri lanka during collision in gondwana reconstructions the sinistral strike slip zone fits into the late kinematics of the mozambique belt shackleton 1986 these new structural data constrain the position of sri lanka in gondwana and favour reconstructions in which sri lanka is located near the lutzow holm bay in east antarctica a review of data from adjacent gondwana fragments suggests that the tectonometamorphic evolution of the sri lankan basement is closely related to the interplay between two orogenic belts the mozambique belt and the lutzow rayner belt it is argued that a late event of n s shortening gondwana coordinates in the lutzow rayner belt can be attributed to a pan african tectonometamorphic event in this interpretation the lutzow rayner belt developed in part simultaneously with the mozambique belt reworking an older granulitic basement in the rayner complex

granulites and garnet cordierite gneisses from dronning maud land antarctica abstract the central sector of muhlig hofmannfjellet 3 e 71 s in western dronning maud land east antarctic shield is dominated by large intrusive bodies of predominantly orthopyroxene bearing quartz syenites charnockites metasedimentary rocks are rare however two distinct areas with banded gneiss marble quartzite sequences of sedimentary origin were found during the norwegian antarctic research expedition nare 1989 90 cordierite bearing metapelitic gneisses from two different localities contain the characteristic mineral assemblage cordierite garnet biotite k feldspar plagioclase quartz sillimanite spinel thermobarometry indicates equilibration conditions of about 650 c and 4 kbar associated orthopyroxene garnet granulites on the other hand revealed pressures of about 8 kbar and temperatures of 750 c the earlier granulite facies metamorphism is not well preserved in the cordierite gneisses as a result of excess k feldspar combined with interaction with an h2o rich fluid phase probably released by the cooling intrusives these two features allowed the original high grade k feldspar garnet assemblages to recrystallize as cordierite biotite sillimanite gneisses completely re equilibrating them phase relationships indicate that the younger metamorphic event occurred in the presence of a fluid phase that varied in composition between the lithologies

origin and evolution of granulites in normal and thickened crusts rocks buried in the upper part of a doubly thickened crust during orogeny can show prograde reactions with transitional features from lower grade rocks up to granulite facies they can be exposed at the earth9s surface within this one erogenic cycle for example the granulites exposed in british columbia have concordant u pb zircon ages of 65 85 ma in contrast rocks buried in the lower part of a thickened crust will crystallize granulites that typically show only retrograde relations with younger lower grade rocks at the cessation of orogeny such rocks will undergo near isobaric cooling from the peak metamorphism and can have very long residence times at the base of a crust of normal thickness these granulites require a second orogeny to uplift and expose them at the surface as an example the archean granulites of enderby land antarctica were metamorphosed at 3070 ma at 1000 c at 8 10 kbar pressure they then followed at least a 400 c isobaric cooling path they also had a prolonged residence time of 2000 m y near the base of the crust before uplift no doubt a continuum exists between these extremes but it is proposed that many intermediate and high pressure granulites form in the lower parts of thickened crust and therefore require a two stage cycle of tectonism to be uplifted and exposed at the earth9s surface although most granulites probably form in thickened crust at active plate margins underplating of normal 30 40 km thickness crust by mantle melts during extension can also produce granulites these granulites would also undergo isobaric cooling and require a later orogeny to expose them an example is the paleozoic lachlan fold belt of eastern australia which has undergone only minor uplift since the major ultrametamorphism at about 400 ma however isotopic petrologic and geochemical differences are to be expected between these extremes of granulite formation

precambrian tectonics and crustal evolution in south india about 3 4 ga ago voluminous calc alkaline magmas represented by the granitoid gneisses of southern india were emplaced into a now poorly preserved non continental crust unstable ensialic basins initiated at about 3 0 ga were filled with volcanic and sedimentary rocks up to about 2 6 ga this basement cover sequence was deformed at the close of the archaean first by northward accretion and thickening of several crustal slabs formerly separated by prisms of stable shelf sediments structures produced by this episode were refolded and dislocated by large n s strike slip shear belts to impart intense steep planar fabrics to large volumes of the crust fluids rich in co possibly derived from sedimentary material driven beneath the crustal slabs by the thickening mechanism purged the deep crust of h o to form granulites upward migration of hot h o rich fluids encouraged partial melting at intermediate crustal levels these late archaean tectonic and thermal events began soon after the close of basin f

reaction textures in calc silicate granulites from the bolingen islands prydz bay east antarctica implications for the retrograde p t path calc silicate granulites from the bolingen islands prydz bay east antarctica exhibit a sequence of reaction textures that have been used to elucidate their retrograde p t path the highest temperature recorded in the calc silicates is represented by the wollastonite and scapolite bearing assemblages which yield at least 760 c at 6 kbar based on experimental results the calc silicates have partially re equilibrated at lower temperatures down to 450 c as evidenced by the successive reactions 1 wollastonite scapolite calcite garnet co2 2 wollastonite co2 calcite quartz 3 wollastonite plagioclase garnet quartz 4 scapolite plagioclase calcite quartz 5 garnet co2 h2o epidote calcite quartz and 6 clinopyroxene co2 h2o tremolite calcite quartz the reaction sequence observed indicates that aco2 was relatively low in the wollastonite bearing rocks during peak metamorphic conditions and may have been further lowered by local infiltration of h2o from the surrounding migmatitic gneisses on cooling fluid activities in the bolingen calc silicates were probably locally variable during the granulite facies metamorphism and large scale co2 advection did not occur a retrograde p t path from the sillimanite stability field c 760 c at 6 kbar into the andalusite stability field c 450 c at 3 kbar is suggested by the occurrence of secondary andalusite in an adjacent cordierite sillimanite gneiss in which sillimanite occurs as inclusions in cordierite

nature and evolution of metamorphic fluids in the precambrian khondalites of kerala south india abstract the precambrian charnockite khondalite groups of rocks in the kerala region constitute a significant part of the granulite facies terrain of south india the khondalites comprise interlayered garnet biotite and garnet sillimanite gneisses analyses of coexisting mineral pairs from these rocks give temperatures of 869 50 c garnet biotite and 862 75 c garnet cordierite a garnet orthopyroxene thermometer records 644 c for charnockite intercalated with the khondalites fluid inclusion studies show that the early fluids were high density co 2 rich type 0 97 g cm 3 the graphite bearing garnet sillimanite gneisses show traces of ch 4 also the early high density regime was followed by moderate density co 2 0 73 0 75 g cm 3 and mixed co 2 h 2 o 0 70 g cm 3 fluids a dominant aqueous regime with h 2 o densities in the range 0 79 0 57 g cm 3 manifested during the waning stage a combination of data from solid and fluid phases denote that the high density co 2 rich inclusions entrapped at a pressure of 6 1 kbar represent the fluid phase present at or near the peak of metamorphism the delineated fluid evolution path corresponds to the piezothermic array of khondalites and is similar in form to that of the charnockites the array is characterised by high convexity towards the temperature axis which depicts a fast rate of uplift that exceeds the rate of heat transfer

garnet coronas in scapolite wollastonite calc silicates from east antarctica the application and limitations of activity corrected grids calc silicate boudins within proterozoic granulite facies gneisses of the northern prince charles mountains east antarctica preserve a number of reaction textures including garnet coronas between calcite and scapolite garnet quartz coronas between scapolite and wollastonite and between plagioclase and wollastonite calcite quartz intergrowths in wollastonite and calcite plagioclase symplectites in scapolite these textures have been modelled using petrogenetic grids for reactions in the cao al2 o3 sio2 co2 system but with reduced mineral activities to account for additional components in real mineral compositions such fixed composition reduced activity grids are strictly valid only at the point in p t aco2 space where an assemblage last equilibrated and do not show the true positions of reactions away from this point because mineral compositions change with reaction progress in this case however mineral compositions close to end member values and low extents of reaction progress mean that compositional change was limited and the grids are good approximations to true pseudosections over the entire p t aco2 range of interest the grids show that the textures are consistent with near isobaric cooling from about 850 to 700d c at 7 kbar a p t path compatible with thermobarometric studies of other lithologies from the area phase relationships indicate that co2 activities were buffered by the local mineral assemblage during peak and retrograde metamorphism either under fluid absent conditions or within a non pervasive fluid phase previous studies of garnet coronas in scapolite wollastonite calc silicates have used qualitative grids based on limited experimental data to invoke garnet growth during water infiltration at high temperature but the grids used here show that garnet coronas can form on cooling without any need for water influx

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

determining the direction of contact metamorphic fluid flow an assessment of mineralogical and stable isotope criteria one dimensional fluid advection dispersion models predict differences in the patterns of mineralogical and oxygen isotope resetting during up and down temperature metamorphic fluid flow that may in theory be used to determine the fluid flow direction with respect to the palaeotemperature gradient under equilibrium conditions down temperature fluid flow is predicted to produce sharp reaction fronts that separate rocks with isobarically divariant mineral assemblages in contrast up temperature fluid flow may produce extensive zones of isobarically univariant mineral assemblages without sharp reaction fronts however during contact metamorphism mineral reaction rates are probably relatively slow compared with fluid velocities and distended reaction fronts may also form during down temperature fluid flow in addition uncertainties in the timing of fluid flow with respect to the thermal peak of metamorphism and the increase in the variance of mineral assemblages due to solid solutions introduce uncertainties in determining fluid flow directions equilibrium down temperature flow of magmatic fluids in contact aureoles is also predicted to produce sharp 18o fronts whereas up temperature flow of fluids derived by metamorphic devolatilization may produce gradational 18o vs distance profiles however if fluids are channelled significant kinematic dispersion occurs or isotopic equilibrium is not maintained the patterns of isotopic resetting may be difficult to interpret the one dimensional models provide a framework in which to study fluid rock interaction however when some of the complexities inherent in fluid flow systems are taken into account they may not uniquely distinguish between up and down temperature fluid flow it is probably not possible to determine the fluid flow direction using any single criterion and a range of data is required