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

halogen bearing minerals in syenites and high grade marbles of dronning maud land antarctica monitors of fluid compositional changes during late magmatic fluid rock interaction processes meta sedimentary rocks including marbles and calcsilicates in central dronning maud land cdml in east antarctica experienced a pan african granulite facies metamorphism with peak metamorphic conditions around 830 20 c at 6 8 0 5 kbar which was accompanied by the post kinematic intrusion of huge amounts of syenitic charnockitic magmas at 4 5 0 7 kbar the marbles and calcsilicates may represent meta evaporites as indicated by the occurrence of metamorphic gypsum anhydrite and cl rich scapolite that formed in the presence of saline fluids with x nacl in the range 0 15 0 27 the marbles and calcsilicates bear biotite tremolite and or hornblende and humite group minerals clinohumite chondrodite and humite which are inferred to have crystallized at about 650 c and 4 5 kbar the syenitic intrusives contain late magmatic biotite and amphibole formed between 750 and 800 c as well as relictic magmatic fayalite orthopyroxene and clinopyroxene two syenite and two calcsilicate samples contain fluorite corona textures in the marbles and calcsilicates suggest very low fluid rock ratios during the formation of the retrograde 650 c assemblages biotite in all but two syenite samples crystallized at log f h 2 o f hf ratios of 2 9 0 4 while in the calcsilicates both biotite and humite group minerals indicate generally higher log f h 2 o f hf values of up to 5 2 a few samples though overlap with the syenite values log f h 2 o f hcl derived from biotite covers the range 0 5 2 6 in all rock types within a single sample the calculated values for both parameters vary typically by 0 1 to 0 8 log units water and halogen acid fugacities calculated from biotite olivine orthopyroxene feldspar quartz equilibria and the above fugacity ratios are 1510 2790 bars for h2o 1 3 5 3 bars for hf and 7 600 bars for hcl the results are interpreted to reflect the reaction of relatively homogeneous magmatic fluids in terms of log f h 2 o f hf derived from the late magmatic stages of the syenites with both earlier crystallized still hotter parts of the syenites and with adjacent country rocks during down temperature fluid flow fluorine is successively removed from the fluid and incorporated into f bearing minerals close to the syenite into metamorphic fluorite in the course of this process log f h 2 o f hf increases significantly chlorine preferably partitions into the fluid and hence log f h 2 o f hcl does not change markedly during fluid rock interaction

magmatic and metamorphic controls on chemical variations within the eiksunddal eclogite complex sunnmore western norway the eiksunddal eclogite complex is one of the larger group b country rock eclogites within the western gneiss region wgr of norway whole rock chemical data suggest a magmatic origin for the rocks within the eclogite complex the chemical variations among the eclogitic rocks were controlled by accumulation of olivine fo80 and plagioclase an70 and probably magnetite the composition of the interstitial trapped liquid is derived by linear regression methods and is a high al orthopyroxene normative basalt the most likely conditions of crystallization are t 1150 c p 10 kbar the al rich nature of the interstitial liquid may have implications for the formation of anorthosites and leucogabbros associated with similar mafic rocks elsewhere in the wgr trace element characteristics are compatible with a continental origin of these rocks during the subsequent metamorphic history the marginal parts of the complex have suffered variable degrees of contamination abundant amphibole inclusions in large garnets in garnet websterites suggest local hydration at pre eclogitic stages this may be associated with introduction of radiogenic sr as previously reported by w l griffin and h k brueckner during prograde metamorphism alkali exchange occurred during growth of garnet clinopyroxene and biotite at the expense of pargasitic amphibole and orthopyroxene the most extensive metasomatism however was probably associated with the introduction of fluid during amphibolitization retrogression of the eclogitic rocks three model reactions eqs 5 7 qualitatively explain the relation between degree of retrogression and the observed metasomatic effects whether si and ca are gained or lost during retrogression seems to be controlled by the extent of amphibole growth in diopside plagioclase symplectites initially formed by breakdown of omphacite alumina has behaved in a more or less immobile way during metasomatism low mobility of al is also reflected by the formation of subsilic al rich amphiboles and al rich plagioclase during garnet breakdown this demonstrates the importance of kinetic factors in controlling mineral chemistry

pan african extension and near isothermal exhumation of a granulite facies terrain dronning maud land antarctica t he m uhlig hofmann and filchnerfjella in central dronning maud land antarctica consist of series of granitoid igneous rocks emplaced in granulite and upper amphibolite facies meta morphic rocks the area has experienced high temperature metamorphism followed by near isothermal decompression partial crustal melting voluminous magmatism and extensional exhumation during the later phase of the late neoproterozoic to cambrian pan african event remnants of kyanite garnet ferritschermakite rutile assemblages indicate an early higher pressure metamorphism and crustal overthickening the gneisses experienced peak granulite facies temperatures of 800 900 ca t intermediate pressures breakdown of garnet sillimanite spinel bearing assemblages to cordierite shows subsequent re equilibration to lower pressures an e w foliation dominating the gneisses illustrates transposition of migmatites and leucocratic melts which evolved during the near isothermal decompression occurrence of extensional shear bands and shear zones evolving from the ductile partial melting stage through semiductile towards brittle conditions shows that the uplift persisted towards brittle crustal conditions under tectonic w sw vergent extension late orogenic pan african quartz syenites intruded after formation of the main gneiss fabric contain narrow semiductile to brittle shear zones illustrating that the extensional exhumation continued also after their emplacement the latest record of the pan african event is late magmatic fluid infiltration around 350 400 c and 2 kbar at this stage the pan african crust had undergone 15 20 km exhumation from the peak granulite facies conditions we conclude that the later phase of the pan african event in central dronning maud land is characterized by a near isothermal decompression p t path and extensional structures indicating tectonic exhumation which is most likely related to a late orogenic collapsing phase of the pan african orogen

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

fluid rock interaction in crustal shear zones a directed percolation approach the interplay among mechnical chemical hydrological and thermal processes in the evolution of crustal shear zones makes an analytical approach to their study difficult as an alternative a stochastic description using percolation theory is used to gain insight into reaction softening and volume loss in ductfle deformation zones directed percolation is preferred to ordinary percolation as a model because in common with natural shear zones directed percolation clusters have high length width ratios and anisotropic permeability in addition transport along the cluster length for p p c is linear with time modeling fluid advection the process of strain softening is modeled by subjecting a critical cluster to variable amounts of simple shear resulting in a geometry similar to that of natural shear zones a stochastic model for the evolution of porosity with time on a directed lattice displays fixed point behavior for a range of pore collapse rates the observation that natural shear zones from a variety of tectonic settings display mean volume losses of 60 70 suggests that the system naturally evolves toward a critical state this can be explained by assuming that pore collapse is regulated by the tendency for fluid pressure to remain close to lithostatic volume loss in crustal shear zones appears to be an example of hydromechanical chemical self organized critical behavior

precipitation of rutile and ilmenite needles in garnet implications for extreme metamorphic conditions in the acadian orogen u s a we report the discovery of oriented needles of rutile and ilmenite in garnet crystals from granulite facies metapelitic rocks of the merrimack synclinorium connecticut and present a precipitation model for their origin the rocks were strongly metamorphosed and deformed during the devonian acadian orogeny the needles are primarily elongated parallel to in garnet rutile has anomalous extinction angles as great as 35 cf griffin et al 1971 rutile and ilmenite needles are typically a few hundred nanometers to several micrometers in diameter and are several tens of micrometers to nearly a millimeter long other oxide inclusions that may be present include submicrometer to micrometer scale twinned rutile bicrystals as well as srilankite and a crichtonite group mineral some garnet cores have unusual box shaped quartz inclusions which coexist with ti fe oxide needles and commonly contain micrometer scale rods of f oh cl apatite negative garnet crystal pores are also widespread ti fe oxide needles are restricted to garnet core regions rims have a distinctly different inclusion population dominated by granulite facies minerals including sillimanite spinel cordierite and k feldspar consequently the garnet core regions represent an earlier distinct period of growth relative to the rims garnet cores contain 25 35 pyrope and a host of minor and trace constituents including tio2 0 07 0 6 wt cr2o3 0 01 0 10 wt na2o 0 01 0 03 wt p2o5 0 01 0 09 wt and zro2 up to 150 ppm na2o and zro2 correlate positively with tio2 titanium zoning is preserved in some garnets zoning profiles and two dimensional chemical mapping show that ti and to a lesser degree cr are depleted around ti fe oxide inclusions therefore we conclude that the needles are precipitates that formed from ti bearing garnet during exhumation and cooling garnet contained sufficient ti to form precipitates no ti source external to garnet was necessary titanium bearing garnets that contain oriented ti fe oxide needles are known primarily from ultrahigh pressure metamorphic rocks mantle peridotites and pyroxenites and high pressure granulites thus the presence of needle bearing garnets in connecticut strongly suggests that a previously unrecognized domain of extreme pressure and or temperature metamorphism exists in the acadian orogen

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

trace element rich brines in eclogitic veins implications for fluid composition and transport during subduction primary and pseudosecondary fluid inclusions occur in oscillatory and sector zoned omphacite in eclogitic veins from the monviso ophiolitic complex in the western alps the inclusions contain aqueous brines and daughter crystals of halite sylvite calcite dolomite albite anhydrite and or gypsum barite baddeleyite rutile sphene fe oxides pyrite and monazite this daughter mineral suite indicates high solubilites of na k ca mg fe zr ti p ba ce la th and s species and provides direct evidence for transport of high fieldstrenght large ion lithophile and light rare earth elements as dissolved species during subduction fluid inclusion heterogeneities preserved within and between adjacent grains in the veins however suggest that the scale of fluid equilibration was small a crack seal geometry in some of the veins implies that fluid release in pulses rather than steady flow controlled mineral deposition and growth in the veins from these observations we develop a model of fluid release and entrapment in which pulses of fluid are associated in time with increments of shear and tensile failure the rate of fluid release and the reduction in porosity both depend on the rate of plastic flow vein fluids may initially be derived from decreptitation of early fluid inclusions in the host eclogites small scale fluid heterogeneities implied by the fluid inclusions in the veins are best interpreted in terms of limited fluid flow and hence limited metasomatism we conclude that element recycling into the mantle wedge during subduction will depend at least as strongly on fluid transport mechanisms as on element solubilities in the fluid phase at monviso despite evidence for high trace element solubilities in saline brines the elements were not removed from the downgoing slab prior to teaching depths of 40 km

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

deep sea fan pinch out geometries and their relationship to fan architecture tanqua karoo basin south africa exceptional exposures of permian basin floor fans fans 3 4 and a slope fan fan 5 in the tanqua karoo foreland basin of south africa have enabled an investigation of the relation between the pinch out geometries and fan architecture the pinch out geometry of fan 3 is characterized by the down dip transition from thin to medium bedded sheet deposits to pinch out fingers which are overlain by younger prograding sheet deposits this geometry reflects the progradational stacking pattern of the fan in contrast the fan 4 pinch out fingers consist of stacked channel fills in the same conduit this pinch out configuration relates to the dominant aggradational style observed on the mid and distal parts of fan 4 fan 5 represents a slope fan comprising an axial channel conduit which branches down slope into three distributary channels the distal fan is characterized by larger channel fills which may represent bypass channels to other basin floor fans the very thick bedded nature of the youngest channel fill unit suggests early bypass followed by retrogradation as indicated by the presence of thinner bedded heterolithic channel fill deposits along the axial conduit although some of the massive pinch out channels exhibit basal scour their depositional morphology suggests that they mainly originated due to the infill of subtle topographic depressions by low concentration turbidity currents instead of describing these features as channel fills the use of the term pinch out fingers is preferred

archaean cambrian crustal development of east antarctica metamorphic characteristics and tectonic implications abstract the east antarctic shield consists of a variety of archaean and proterozoic cambrian high grade terranes that have distinct crustal histories and were amalgamated at various times in the precambrian cambrian high grade pan african tectonism at 600 500ma is recognized from four distinct belts the dronning maud land lutzow holm bay prydz bay and denman glacier belts these high grade belts juxtapose distinct mesoproterozoic and neoproterozoic crustal provinces maud rayner and wilkes the rauer terrane and have also marginally affected archaean cratonic remnants in the napier complex and southern prince charles mountains the wilkes province experienced its principal tectonothermal events prior to 1130ma and was not affected by the younger events that characterize the maud province 1150 and 1030 990ma the rayner province 990 920ma and the rauer terrane 1030 990ma these differences between the isotopic event records of the basement provinces now separated by the pan african belts require that the older provinces were not formerly parts of a continuous grenville belt as proposed in the sw us east antartic model east antarctica was not a single unified crustal block within either east gondwana or rodinia until the cambrian which is now demonstrated to be the key phase of high grade and ultrahigh temperature uht metamorphism associated with supercontinent assembly the high grade pan african tectonism is characterized by extensive infracrustal melting clock wise p t paths rapid post peak exhumation along isothermal decompression paths to shallow or mid crustal levels by 500 ma and the generation at least locally of uht conditions a significant flux of heat from the mantle into the deep and initially overthickened crust is required to produce these observed metamorphic effects whilst the thermal evolution can be explained by models that invoke the removal of most of the lithospheric mantle following crustal thickening and prior to rapid extension of the remaining crust these one dimensional models are inconsistent with present crustal thicknesses of 25 35km in the pan african domains of the east antarctic shield

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

mineralogical and physiochemical evolution of the los santos scheelite skarn salamanca nw spain the los santos scheelite skarn deposit located in the province of salamanca northwest spain occurs in meter thick strata bound but irregular lenses of 1 km in length and contains a current total resource of 3 09 million metric tons mt with an average grade of 0 54 percent wo3 it was produced by the metasomatic alteration of upper vendian lower cambrian metasedimentary rocks and granitic rocks located in the western part of the avila batholith plutonic complex magmatic activity in the los santos zone comprises two intrusions including biotite monzogranite 281 280 6 ma granite i and porphyritic biotite granodiorite monzogranite 270 269 6 ma granite ii the mineralized skarn distributed close to the intrusions occurred in two main stages i prograde skarn and ii retrograde alteration of the stage i skarn the prograde exoskarn is made up of a thick almost monomineral mass of hedenbergite hd82 87 accompanied by grossular grs53 73 and scheelite rich in mo sch i early mineralization is associated with granite i thus it is possible to recognize an endoskarn of calcic plagioclase and clinopyroxene developed over aplitic granite the stage i skarn formed from h2o rich fluids xco2 0 1 with an upper temperature limit of 630 c at 0 18 to 0 2 gpa a log f o2 of 21 and log f s2 ranging from 9 7 to 5 0 after the emplacement of granite ii there was a late event characterized by pegmatites dike swarms breccia pipes and intense fracturing associated with the retrograde alteration which was superimposed on massive skarn the retrograde skarn has a complex paragenesis that can be divided into two substages in the first subcalcic garnet formed associated with the leaching of early scheelite and the precipitation of late scheelite poor in mo sch ii there is also ferroactinolite together with important amounts of anorthite zoisite apatite titanite quartz and sulfides mainly pyrrhotite the first substage skarn formed from a complex h2o co2 ch4 n2 c3h8 nacl cacl2 mixture with f o2 values of 23 and log f s2 ranging from 10 to 7 the data on fluid inclusions indicate that fluid boiling and immiscibility at temperatures between 439 and 405 c and pressures ranging from 0 1 to 0 042 gpa was a possible mechanism in the sch ii and sulfide deposition during the first substage of the retrograde alteration of the skarn the second substage is characterized by the formation of clinozoisite prehnite quartz calcite chlorite white mica zeolite and mineral phases belonging to the bi te ag s au system the mineral assemblages indicate that the f o2 values are between 25 9 and 33 4 while log f s2 ranges from 10 6 to 6 3 for t 300 c all the data suggest that the development of the los santos skarn represents the evolution of a magmatic hydrothermal system in this model carbon aqueous fluids would have been exsolved from the plutonic complex during its emplacement and crystallization the gradual interaction of magmatic fluids at high temperatures with the graphitic rocks of the schist greywacke complex resulted in a reduction in ore fluids and higher ch4 co2 ratios the fluid associated with the last substage of the retrograde alteration of the deposit belongs to the h2o nacl system and suggests an influx of a surface derived fluid

coesite in clinopyroxene in the caledonides and its implications for geodynamics coesite is reported for the first time from the caledonide orogen it occurs as inclusions in clinopyroxene in the dolomite eclogite at grytting norway and provides valuable new evidence to support the hypothesis of extremely high pressure 30 kbar in certain norwegian eclogites this finding has implications for the geodynamics and kinetics of the concepts of microscale intracrystalline tectonic overpressure and of macroscale local or regional tectonic overpressure and for the hypotheses of subduction and obduction involving a more than 100km vertical component of transport

phase equilibria involving humite minerals in impure dolomitic limestones part ii calculated stability of chondrodite and norbergite

fluid induced retrogression of granulites in the bergen arcs caledonides of w norway fluid inclusion evidence from amphibolite facies shear zones abstract at holsnoy island bergen arcs caledonides of w norway amphibolite facies shear zones transect grenvillian granulite facies rocks within the shear zones the orthopyroxene bearing mineral assemblages of the protolith are replaced by biotite and amphibolite bearing parageneses the shear zones are assumed to have formed at t 600 75 c and within the kyanite field but below the stability of omphacite quartz veins central in the shear zones contain saline aqueous fluid inclusions representing a fluid phase with 2 16 wt nacl and a variable na ca ratio in areas with only static retrogression aqueous fluid inclusions with variable salinity from 5 to ca 50 wt nacl are associated with co2 inclusions with minor n2 contents xn2 4 6 mole the aqueous fluid was introduced during amphibolite facies metamorphism the carbonic fluid was inherited from the protolith the composition and density properties of the aqueous fluids are compatible with introduction at the assumed amphibolite facies conditions and suggest an uplift path sub parallel to their isochores the carbonic fluid inclusions have been re equilibrated during late stages of post metamorphic uplift the data presented highlight the differences between the fluid regimes in the granulite facies co2 eclogite facies n2 rich fluids and amphibolite facies h2o salt metamorphic events at holsnoy

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

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

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

fluid rock history of granulite facies humite marbles from ambasamudram southern india an extensive humite bearing marble horizon within a supracrustal sequence at ambasamudram southern india was studied using petrological and stable isotopic techniques to define its metamorphic history and fluid characteristics at peak metamorphic temperatures of 775 73 c based on calcite graphite carbon isotope thermometry the mineral assemblages suggest layer by layer control of fluid compositions clinohumite calcite bearing assemblages suggest xco2 0 9 at 790 c and local wollastonite scapolite grossular bearing zones formed at xco2 of c 0 3 retrograde reaction textures such as scapolite quartz symplectites after feldspar and calcite and replacement of dolomite diopside or tremolite dolomite after calcite forsterite or calcite clinohumite are indicative of retrogression under high xco2 conditions calcite preserves late proterozoic carbon and oxygen isotopic signatures and the marble lacks evidence for extensive retrograde fluid infiltration while during prograde metamorphism the possible infiltration of aqueous fluids did not produce significant isotopic resetting isotopic zonation of calcite and graphite grains was likely produced by localized co2 fluid infiltration during retrogression contrary to the widespread occurrence of humite marbles related to retrograde aqueous fluid infiltration the ambasamudram humite marbles record a prograde to peak metamorphic humite formation and retrogression under conditions of low xh2o

fluid channelling during ductile shearing transformation of granodiorite into aluminous schist in the tauern window eastern alps ductile shearing in the core of the tauern window austria transformed metagranodiorite into si undersaturated garnet chlorite staurolite schist at a depth of c 35 40 km during the alpine orogeny four distinct zones have been recognized extending from the wallrock into the centre of the shear zone zone i unaltered metagranodiorite with subordinate amphibolite zone ii biotite white mica garnet schist zone iii biotite phengite schist zone iv quartz absent garnet chlorite staurolite schist with garnets up to 10 cm across whole rock analyses show a dramatic decrease in sio2 from 65 wt in zone i to 35 wt in zone iv ca na and sr also decrease across the shear zone whereas al ti fe mg p cr ni zn and rb all increase towards zone iv mass balance calculations indicate that shearing was accompanied by up to 60 volume loss near the centre of the shear zone comparison of the tauern window samples with other shear zones in granitic hosts indicates that silica loss accompanied by gains in mg fe and ti is typical for volume loss shear zones but is distinctly different from the element behaviour exhibited in shear zones that are thought to represent approximately isovolumetric behaviour in the samples studied here volume loss appears to have resulted from channellized fluid flow during shearing producing time integrated fluid fluxes of 108 cm3 cm 2 in zone iv this large volume of fluid may have originated in part from dehydration of flysch carried beneath the metagranodiorites during eocene movement on the north penninic subduction zone development of an inverted thermal gradient during subduction would have allowed the fluid to scavenge large amounts of silica from the shear zone during ascent and heating

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

experimentally determined activity composition relations for ca rich scapolite in the system caal2si2o8 naalsi3o8 caco3 at 7 kbar

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

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

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

origin of gem corundum in calcite marble the revelstoke occurrence in the canadian cordillera of british columbia abstract the calcite marble hosted gem corundum ruby sapphire occurrence near revelstoke british columbia canada occurs in the monashee complex of the omineca belt of the canadian cordillera corundum occurs in thin folded and stretched layers with green muscovite ba bearing k feldspar anorthite an0 85 1 phlogopite na poor scapolite other silicate layers within the marble are composed of 1 diopside tremolite quartz and 2 garnet alm0 7 0 5grs0 2 0 4 na rich scapolite diopside tremolite na k amphiboles non silicate layers in the marble are either magnetite or graphite bearing predominantly pink locally red or purple opaque to transparent corundum crystals have elevated cr2o3 0 21 wt and variable amounts of tio2 rare blue rims on the corundum crystals contain higher amounts of tio2 0 53 wt and fe2o3 0 07 wt the associated micas have elevated cr v ti and ba contents petrography of the silicate layers show that corundum formed from muscovite at the peak of metamorphism 650 700 c at 8 5 9 kbar because the marble is almost pure calcite dolomite is very rare the corundum was preserved because it did not react with dolomite to spinel calcite during decompression the scapolite bearing assemblages formed during or after decompression of the rock at 650 c and 4 6 kbar gem quality corundum crystals formed especially on borders of the mica feldspar layers in an assemblage with calcite whole rock geochemistry data show that the corundum bearing silicate mica feldspar layers formed by mechanical mixing of carbonate with the host gneiss protolith the bulk composition of the silicate layers was modified by si and fe depletion during prograde metamorphism high element mobility is supported by the homogenization of 18o and 13c values in carbonates and silicates for the marble and silicate layers the silicate layers and the gneiss contain elevated contents of cr and v due to the volcanoclastic component of their protolith

mid crustal metasomatic reaction veins ina spinel peridotite in central dronning maud land east antarctica rare metresized lenses of spinel peridotite are enclosed in high grade metamorphic rocks the rocks experienced a medium p granulite facies metamorphism at 575ma and a low p amphibolite facies overprint at 530ma the latter is probably related to extensive granitoid magmatism between 530 and 500ma which produced large volumes about half of the outcrops today of granitic to syenitic rocks as well as abundant k feldspar quartz pegmatites one of the spinel peridotite lenses in the schirmacher oasis of central dronning maud land is crosscut by several small up to 10 cm wide veins with a characteristic zoned sequence of mineral assemblages which was formed by reaction of a hydrous sio2 saturated fluid or pegmatitic melt with the peridotite the zoned sequence consists of the following mineral assemblages from the centre of the vein towards the outer margin zone 0 plagioclase quartz zone 1 green biotite intergrown with zircon clinoamphibole zone 2 cummingtonite dark brown biotite intergrown with rutile clinoamphibole zone 3 cummingtonite light brown biotite spinel zone 4 olivine orthopyroxene spinel clinopyroxene unaltered peridotite this sequence was investigated with respect to its conditions of formation modal mineralogy mineral chemistry fluid inclusions and oxygen and hydrogen isotope compositions of selected minerals based on the stability of cummingtonite and on equilibrium calculations in the mgo sio2 h2o system and on quartz biotite oxygen isotope thermometry the reaction vein formed at 650 c which is in accord with typical pegmatite crystallization temperatures the pegmatite of zone 0 is interpreted to have formed in an open fissure whereas on textural grounds zone 3 replaces former peridotite on the basis of mass balance constraints the boundary between zones 1 and 2 is interpreted to approximately represent the former boundary between peridotite and the open fissure before reaction oxygen isotope systematics show that the infiltrating fluid had an isotopic composition of 9 101 2 smow all minerals of the reaction vein with the exception of the inherited spinel and olivine in the adjacent peridotite are in equilibrium with such a fluid spinel in the peridotite is depleted in o compared with coexisting olivine which suggests isotopic disequilibrium spinel in zone 3 has a distinctly different isotopic composition compared with that in the peridotite apparently approaching but not reaching equilibrium the combination of mineral chemistry and mass balance constraints of the modal mineralogy constrains the volume change during metasomatism and the direction of elemental diffusion it is indicated that mg cr and ni always diffused towards the vein whereas si al k na h2o and possibly fe diffused into the peridotite

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

titanian clinohumite in the carbonatites of the jacupiranga complex brazil mineral chemistry and comparison with titanian clinohumite from other environments

diamond inclusions in garnets from metamorphic rocks a new environment for diamond formation diamonds commonly occur in kimberlites lamproites and alluvial sediments derived from these rocks more recently diamonds or their graphite pseudomorphs have been discovered in ultramafic massifs1 and picrites2 here we report the occurrence of diamonds in situ in crustal rocks highly retrograded high pressure metamorphic garnet pyroxene and pyroxene carbonate garnet rocks biotite gneisses and schists from the kokchetav massif northern kazakhstan ussr the diamonds are cubo octahedral averaging 12 m in size and occur in zircons and with euhedral graphite as inclusions in unzoned garnets we believe that the zircon and garnet matrices protected these diamonds from retrogressive transformation to graphite mica rutile titanite clinopyroxene kyanite and zircon also occur as inclusions in garnet often intergrown with the diamonds equilibration relations of inclusions and host garnets indicate that both diamonds and graphite crystallized from a fluid phase under static conditions at pressures of 40 kbar and temperatures 900 1 000 c

evolution of metamorphic volatiles during exhumation of microdiamond bearing granulites in the western gneiss region norway fluid inclusions in garnet kyanite and quartz from microdiamond bearing granulites in the western gneiss region norway document a conspicuous fluid evolution as the rocks were exhumed following caledonian high and ultrahigh pressure hp uhp metamorphism the most important of the various fluid mixtures and daughter minerals in these rocks are n2 co2 magnesian calcite n2 co2 ch4 graphite magnesian calcite n2 ch4 n2 ch4 h2o co2 and h2o nacl cacl2 nahcolite rutile also occurs in the n2 co2 inclusions as a product of titanium diffusion from the garnet host into the fluid inclusions volatiles composed of n2 co2 magnesian calcite characterise the ambient metamorphic environment between hp uhp peak and early retrograde metamorphism during progressive decompression the mole fraction of n2 increased in the fluid mixtures as amphibolite facies conditions were reached ch4 and later h2o appeared in the fluids concomitant with the disappearance of co2 and magnesian calcite graphite is ubiquitous in the host lithologies and fluid inclusions thermodynamic modelling of the metamorphic volatiles in a graphite buffered c o h system demonstrates that the observed metamorphic volatile evolution was attainable only if the fo2 increased from c 3 5 0 3 to 0 8 0 3 log units relative to the fmq oxygen buffer external introduction of oxidising aqueous solutions along a system of interconnected ductile shear zones adequately explains the dramatic increase in the fo2 the oxidising fluids introduced during exhumation were likely derived from dehydration of oceanic crust and continental sediments previously subducted during an extended period of continental collision in conjunction with the caledonian orogeny

nitrogen bearing aqueous fluid inclusions in some eclogites from the western gneiss region of the norwegian caledonides minerals in eclogites from different localities in the western gneiss region of the norwegian caledonides age 425 ma contain a variety of fluid inclusions the earliest inclusions recognized are contained in undeformed quartz grains protected by garnet and consist of h2o n2 with x text h text 2 text o geqslant 0 8 the reconstructed p v t x properties of these fluid inclusions are compatible with peak or early retrograde metamorphic conditions matrix minerals quartz garnet apatite plagioclase contain a complex pattern of mostly truly secondary inclusions dominated by co2 and n2 the textural patterns and p v t x properties of these inclusions are incompatible with the high pressures of the eclogite forming metamorphic event but suggest that they were formed during uplift by a combination of remobilization of preexisting inclusions and influx of external fluids the fluid introduced at a late stage was dominated by co2 and did not contain n2 the present data agree with theoretical predictions of eclogite fluids from mineral equilibria and highlight the differences between granulite co2 and eclogite h2o n2 fluid regimes the provenance of the nitrogen in the eclogite fluid inclusions represents an important but unsolved question in the petrology of high pressure metamorphic rocks