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

enrichment of trace elements in garnet amphibolites from a paleo subduction zone catalina schist southern california the abundance p t stability solubility and element partitioning behavior of minerals such as rutile garnet sphene apatite zircon zoisite and allanite are critical variables in models for mass transfer from the slab to the mantle wedge in deep regions of subduction zones the influence of these minerals on the composition of subduction related magmas has been inferred and disputed from inverse modelling of the geochemistry of island arc basalt or by experiment although direct samples of the dehydration partial melting region of a mature subduction zone have not been reported from subduction complexes garnet amphibolites from melanges of circumpacific and caribbean blueschist terranes reflect high t 600 c conditions in shallower regions such rocks record geochemical processes that affected deep seated high t portions of paleo subduction zones in the catalina schist a subduction zone metamorphic terrane of southern california metasomatized and migmatitic garnet amphibolites occur as blocks in a matrix of meta ultramafic rocks this mafic and ultramafic complex may represent either slab derived material accreted to the mantle wedge of a nascent subduction zone or a portion of a shear zone closely related to the slab mantle wedge contact or both the trace element geochemistry of the complex and the distribution of trace elements among the minerals of garnet amphibolites were studied by inaa xrf electron microprobe and sem in order of increasing alteration from a probable metabasalt protolith three common types of garnet amphibolite blocks in the catalina schist are 1 non migmatitic clinopyroxene bearing blocks which are compositionally similar to morb that has lost an albite component 2 garnet amphibolite blocks which have rinds that reflect local interaction between metabasite metaperidotite and fluid and 3 migmatites that are extremely enriched in th hfse lree and other trace elements these trace element enrichments are mineralogically controlled by rutile garnet sphene apatite zircon zoisite and allanite alkali and alkaline earth elements are much less enriched in the solid assemblage and thus appear to be decoupled from the other elements in the inferred metasomatic process es the compositions of migmatitic garnet amphibolite blocks seem to complement that of average island arc tholeiite trace element metasomatism reflects fluid solid rather than melt solid interaction the metasomatic effects indicate that h2o rich fluid perhaps with a significant component of na al silicate and alkalis carried th u sr ree and hfse fractionations of lree in migmatites resemble those of migmatitic metasedimentary rocks underlying the mafic and ultramafic complex exotic lree deposited in allanite in migmatites could have been derived from fluids in equilibrium with subducted sediment if the paleo subduction zone represented by the mafic and ultramafic complex of the catalina schist had continued its thermal and fluid evolution a selvage of similarly enriched rocks might have been generated along the slab mantle wedge contact between 30 and 85 km depth rocks affected by subduction zone metasomatism although rarely recognized at the surface could be volumetrically significant products of the initiation of subduction and may prove to be geochemical probes of convergent margins that approach the significance of xenoliths in the study of other magmatic environments

the timing of sub solidus hydrothermal alteration in the central zone limpopo belt south africa constraints from titanite u pb geochronology and ree partitioning abstract in the central zone of the limpopo belt south africa palaeoproterozoic granulite facies metamorphism was superimposed on an earlier archaean orogenic history previously determined ages of 2030 2020 ma obtained from high temperature chronometers zircon garnet monazite are generally thought to provide the best estimate of the peak of palaeoproterozoic granulite facies metamorphism in the central zone whereas ages as young as 2006 ma from late melt patches suggest that temperatures remained above the wet solidus for an extended period we present a new mc icp ms 207 pb 206 pb age of 2030 9 1 5 ma for titanite found in amphibolite to greenschist facies alteration zones developed adjacent to quartz vein systems and related pegmatites that cut a strongly deformed central zone metabasite this age could potentially date cooling of rocks at this locality to temperatures below the wet solidus alternatively the titanite could be inherited from the metabasite host and the age determined from it date the peak of metamorphism integration of the geochronology with la icp ms trace element data for minerals from the metabasite the hydrothermal vein systems and comparable rocks elsewhere shows that the titanite formed during the amphibolite facies hydrothermal alteration not at the metamorphic peak or during the greenschist facies phase of veining this suggests that high grade rocks in the central zone have cooled differentially through the wet solidus and provides timing constraints on when palaeoproterozoic reworking in the central zone began this study illustrates the potential of combined geochronological and high resolution geochemical studies to accurately match mineral ages to distinct crustal processes

rayleigh fractionation of heavy rare earths and yttrium during metamorphic garnet growth we present laser ablation microprobe inductively coupled mass spectrometry analyses of y yb er dy and gd in garnet crystals from high grade metamorphic and migmatitic rocks from the sierras pampeanas of argentina these rocks contain large garnet porphyroblasts 4 mm that are compositionally zoned rare earth element concentrations in the rims are typically one order of magnitude lower than those in the cores a notable feature of this zonation is the inversion of yb er and yb dy ratios from 1 in the cores to 1 in the rims we show quantitatively that the spatial distribution of these trace elements in garnet can be most simply and effectively explained as arising from rayleigh fractionation during garnet growth we also analyzed a small garnet crystal 2 mm representative of a garnet population that is associated with migmatitic leucosomes this crystal displays uniformly low concentrations of all trace elements that are virtually identical in both absolute and relative magnitudes to those in the rims of the larger garnet porphyroblasts the small crystals and perhaps part of the porphyroblast rims are likely to be peritectic products of incongruent melting reactions we argue that melts formed from garnet bearing sources may generally be unable to equilibrate with metamorphic garnet cores and may thus be more strongly depleted in heavy rare earth elements and y than modeling based on bulk source rock abundances of these elements would indicate

archean crustal evolution in china the taishan complex and evidence for juvenile crustal addition from long term depleted mantle abstract the main framework for crustal evolution of the late archean taishan complex in the shandong province of china is constructed using data from field investigation petrographic examination geochemical analysis and isotopic age determination the history of the complex began with emplacement of basic and tonalitic magmas about 2 7 2 75 ga ago which were metamorphosed designated as metamorphism i to amphibolites and grey gneisses shortly after their emplacement these rocks constitute a metamorphic basement termed the wangfushan gneisses the basement gneisses were later intruded by a plutonic association including 1 alkali and light rare earth element lree enriched dioritic rocks and their differentiates 2 6 ga ago 2 granites and trondhjemites collectively termed the hushan granites also 2 6 ga ago and 3 the aolaishan type granites sensu stricto 2 4 2 5 ga ago the plutonic association in conjunction with the older basement gneisses was later subjected to a second period of deformation and metamorphism metamorphism ii 2 ga ago as inferred from published k ar biotite dates the chronological order of all the above events has been determined by a multichronometric approach and the age results in ma with 2 uncertainties are summarized as follows the relatively low i sr initial 87 sr 86 sr ratio and high positive nd t values for the wangfushan gneisses and the diorite group provides strong evidence for continental growth by addition of juvenile material ultimately separated from long term depleted mantle sources by contrast the sr and nd isotopic data for the aolaishan type granites coherently suggest that they are likely to be derived by anatexis of the basement gneisses at shallow depths crustal evolution of the taishan complex offers an additional example supporting the concept of a continental accretion differentiation superevent that during such a worldwide tectonothermal episode particularly in the archean important differentiation of the upper mantle has contributed to the growth of the continental crust by addition of juvenile material

banded amphibolites of the harts range meta igneous complex central australia an early proterozoic basalt tonalite suite abstract a multistage petrogenesis is proposed for banded metabasaltic amphibolite and tonalite tectonically interlayered on a fine centimetre scale within upper and lower amphibolite sequences of the harts range meta igneous complex eastern arunta block central australia the basaltic amphibolites exhibit only slight rare earth element ree fractionation ce n yb n 2 5 and can be related by plagioclase clinopyroxene dominated low pressure fractional crystallisation in the tholeiitic trend early olivine removal from a moderately aluminous trace element impoverished tholeiitic picrite parent magma caused significant ni depletion in the evolved residual liquids that gave rise to the basaltic amphibolites the primary picritic melt was generated by high degrees 30 of hydrous partial melting of a depleted mantle source selectively re enriched in large ion lithophile lil elements chondrite normalised ree patterns for low al 2 o 3 tonalitic layers within the basaltic amphibolite units are moderately fractionated ce n yb n 3 3 7 3 and show significant negative eu anomalies ree modelling indicates that the precursors to the tonalitic rocks could have been generated by small degrees 10 of partial melting of a light rare earth element lree enriched basaltic parent with plagioclase clinopyroxene and minor hornblende comprising coexisting residual phases melting at relatively low p h 2 o the amphibolite sequence is interpreted as a complexly folded and metamorphosed bimodal basalt tonalite suite formed at an early proterozoic rifted continental margin or intra cratonic rift

origin of ree mineralization in the bastnas type fe ree cu mo bi au deposits bergslagen sweden the bastnas type deposits with mineral assemblages of fe oxides ca mg silicates rare earth element ree silicates ree fluorocarbonates and cu fe mo bi sulfides are associated with marble horizons in a strongly na k and or mg altered metavolcanic succession over a distance of at least 80 km in a sw ne trending zone in western bergslagen two subtypes occur 1 enriched relative to the other type in light ree lree and fe exemplified by the bastnas and rodbergsgruvan deposits and 2 enriched in heavy ree hree y mg ca and f represented by deposits in the norberg district bastnasite hosts primary fluid h2o co2 inclusions with salinities of 6 29 eq wt cacl2 and with total homogenization temperatures th tot of ca 300 400 c subtype 2 has late stage fluorite with fluid inclusions that show 1 16 eq wt nacl and th tot of ca 90 150 c molybdenite re os ages obtained from three deposits are 1 904 6 1 863 4 and 1 842 4 ma nd isotopic data from five different ree minerals yielded no defined isochron but a range in end 1 88 ga of 0 2 to 1 6 the oxygen isotope values 18osmow of dolomite and calcite from the associated ree mineralized skarn and recrystallized carbonate assemblages lie in the range 6 1 8 6 overlapping with those of the host marbles carbon isotope values 13cpdb show typical magmatic signatures of 6 7 to 4 4 while the host marbles group around ca 2 4 the sulfur isotope 34scdt values of associated sulfides range between 10 8 and 0 2 the combined evidence suggests ree mineralization beginning at 1 9 ga from mainly svecofennian juvenile magmatic 400 c fluids carrying si f cl s co2 and the ree in addition to other metals mineralization occurred through reactions with dolomitic layers in the supracrustal units coevally with regional metasomatic alteration associated with fluid circulation through an extensive active volcano plutonic complex

three dimensional shear wave velocity structure under the weifang segment of the tanlu fault zone in eastern china inferred from ambient noise tomography with a short period dense seismic array abstract in this study continuous waveforms are observed at a short period dense seismic array which consisted of 302 stations in the weifang segment of the tanlu fault zone from august to october 2017 a total of 122 302 rayleigh wave phase velocity dispersion curves at 1 0 7 5 s periods are extracted to infer a high resolution shear wave tomography model of the shallow crust under the region our model well depicts surface geological structural features and has strong lateral heterogeneities in and around the fault zone both weibei and weifang depressions are imaged as striking low velocity anomalies above 2 3 km depth possibly reflecting the quaternary sedimentary layers but the deeper low velocity anomaly under the weifang depression could indicate a channel for hot material upwelling in the crust wenxian and gongdanshan uplifts are imaged as high velocity anomalies down to 8 km depth possibly indicating the existence of the paleozoic crystalline basement rocks a continuous high velocity anomaly appears under the changle volcano which may be attributed to a magma system solidified in the shallow crust the western and eastern boundaries of the tanlu fault zone where surface mapped faults exist are clearly imaged as low velocity anomalies further demonstrating that the fault zone serves as a channel for hot material upwelling into the shallow crust through the middle and lower crust from the mantle the upwelling processes may be related to mantle dynamics of the big mantle wedge caused by stagnation and dehydration of the subducted pacific slab in the mantle transition zone under the region

experimental determination of trace element partitioning between pargasite and a synthetic hydrous andesitic melt in order to more fully establish a basis for quantifying the role of amphibole in trace element fractionation processes we have measured pargasite silicate melt partitioning of a variety of trace elements rb ba nb ta hf zr ce nd sm yb including the first published values for u th and pb experiments conducted at 1000 c and 1 5 gpa yielded large crystals free of compositional zoning partition coefficients were found to be constant at total concentrations ranging from 1 to 100 ppm indicating henry s law is oparative over this interval comparison of partition coefficients measured in this study with previous determinations yields good agreement for similar compositions at comparable pressure and temperature the compatibility of u th and pb in amphibole decreases in the order pb th u partial melting or fractional crystallization of amphibole bearing assemblages will therefore result in the generation of excesses in238u activity relative to230th similar in magnitude to that produced by clinopyroxene the compatibility of pb in amphibole relative to u or th indicates that melt generation in the presence of residual amphibole will result in the long term enrichment in pb relative to u or th in the residue this process is therefore incapable of producing the depletion in pb relative to u or th inferred from the pb isotopic composition of morb and oib comparison of partition coefficients measured in this study with previous values for clinopyroxene allows some distinction to be made between expected trace element fractionations produced during dry cpx present and wet cpx amphibole present melting rb ba nb and ta are dramatically less compatible in clinopyroxene than in amphibole whereas th u hf and zr have similar compatibilities in both phases interelement fractionations such as dnb dba are also different for clinopyroxene and amphibole changes in certain ratios such as ba nb ba th and nb th within comagmatic suites may therefore offer a means to discern the loss of amphibole from the melting assemblage elastic strain theory is applied to the partitioning data after the approaches of beattie and blundy and wood and is used to predict amphibole melt partition coefficients at conditions of p t and composition other than those employed in this study given values of dca dti and dk from previous partitioning studies this approach yields amphibole melt trace element partition coefficients that reproduce measured values from the literature to within 40 45 this degree of reproducibility is considered reasonable given that model parameters are derived from partitioning relations involving iron and potassium free amphibole

high grade contact metamorphism of calcareous rocks from the oslo rift southern norway shallow level plutons caused extensive contact metamorphism of lower paleozoic shale and carbonate sequences in the permian oslo rift a 500 m long and 100 m wide shale limestone xenolith embedded within monzonites belonging to the skrim plutonic complex experienced high grade contact metamorphism and generation of minerals and mineral assemblages rarely reported from metamorphic rocks the peak metamorphic stage i assemblages in calcite saturated rocks include wollastonite melilites fassaitic pyroxenes phlogopite titanian grossular kalsilite nepheline perovskite cuspidine baghdadite pyrrhotite and occasional graphite mineral reactions involving detrital apatite produced a series of silicate apatites including the new mineral species ca sub 3 5 th u sub 1 5 si 3 o 12 oh this assemblage equilibrated at t 820 870 degrees c with a c rich internally buffered pore fluid 20 40 mol co 2 ch 4 during cooling the shale limestone xenolith experienced infiltration of c poor 2 fluids triggering the formation of retrograde stage ii mineral assemblages comprising monticellite tilleyite vesuvianite grandite garnets diopside and occasional hillebrandite rare potassium iron sulfides rasvumite and djerfisherite formed at the expense of primary pyrrhotite these assemblages probably formed near 700 degrees c formation of diffuse sodalite bearing veinlets was associated with breakdown of nepheline and the replacement of kalsilite and wollastonite by potassium feldspar the sodalite bearing stage iii assemblage formed by the infiltration of saline brines at a maximum temperature of 550 degrees c low temperature stage iv retrogression of the stage i iii assemblage produced scawtite giuseppettite hydrogrossulars phillipsite thomsonite and three hitherto undescribed mineral species

geochemistry of metatholeiites from the harts range central australia implications for mantle source heterogeneity in a proterozoic mobile belt metatholeiite series from early middle proterozoic basement and cover sequences in the harts range central australia display marked differences in trace element abundances and ratios that reflect the chemical heterogeneity of their mantle sources the trace element data reveal a multistage geochemical evolution for the attendant proterozoic sub continental mantle early formed supracrustal tholeiites from the oonagalabi and entia basement gneiss complexes display slight to moderate light rare earth element lree enrichment ce yb n 1 8 3 9 unfractionated heavy ree distributions and chondritic zr nb and zr y ratios they were generated by variable degrees 15 30 of partial melting of a relatively undepleted mantle source in an extensional ensialic rift environment later formed metabasites comprise numerous sills that intrude the supracrustal sequence within the entia gneiss complex primary basaltic liquids for these rocks possessed more strongly fractionated ree levels ce yb n 3 0 12 3 large ion lithophile lil element enrichment and pronounced depletions in high field strength elements their high la nb ba zr and cr y ratios resemble those of convergent plate margin basaltic magmas derived from mantle sources that have been metasomatised as a result of subduction related processes metatholeiites from the harts range meta igneous complex in the cover sequence display high zr nb ratios reflecting a mantle source that was strongly depleted in incompatible elements during a previous melting event tholeiites from the lower part of the cover sequence show distinct lil element and lree depletion ce yb n 0 8 1 0 features resembling depleted n type mid ocean ridge basalts and some continental tholeiites tholeiites from the upper part of the cover sequence show enrichment of highly incompatible lil elements especially ba and k relative to ree and pronounced decoupling between high field strength elements and ree resulting in high la nb ratios they represent melts derived from a mantle source which has been depleted in a basaltic component and subsequently re enriched in incompatible elements e g via introduction of a subduction related component the inferred mantle evolution is compatible with a tectonic model that involves successive extensional and compressional episodes during repeated ensialic orogenesis as proposed for the harts range basement and cover sequences

water fluxed melting of the continental crust a review abstract water fluxed melting also known as fluid or water present melting is a fundamental process in the differentiation of continents but its importance has been underestimated in the past 20 years during which research efforts focused mostly on dehydration melting reactions involving hydrate phases in the absence of a separate aqueous phase the presence of a free aqueous phase in anatectic terranes influences all major physical and chemical aspects of the melting process from melt volumes viscosity and ability to segregate from rock pores to melt chemical and isotopic composition a review of the literature shows that melting due to the fluxing of aqueous fluids is a widespread process that can take place in diverse tectonic environments active tectono magmatic processes create conditions for the release of aqueous fluids and deformation driven transient high permeability channels capable of fluxing high temperature regions of the crust where they trigger voluminous melting water fluxed melting can be either congruent in regions at the water saturated solidus or incongruent at suprasolidus p t conditions incongruent melting reactions can give rise to peritectic hornblende or to nominally anhydrous minerals such as garnet sillimanite or orthopyroxene in this case the presence of an aqueous phase is indicated by a mismatch between the large melt fraction generated and the much smaller fractions predicted in its absence the relatively small volumes of aqueous fluids compared to that of rocks imply that melting reactions are generally rock buffered fluids tend to move upwards and down temperature however there are cases in which pressure gradients drive fluids up temperature potentially fluxing suprasolidus terranes crustal regions at conditions equivalent to the water saturated solidus represent a natural impediment to the up temperature migration of aqueous fluids because they are consumed in melting reactions in this case continued migration into supra solidus terranes take place through the migration of water rich melts thus melts become the transport agent of water into supra solidus terranes and responsible for water fluxed melting other processes such as the relatively rapid fluid migration through fractures also allow regional aqueous fluids to by pass the water saturated solidus fluid trap and trigger melting above solidus conditions when aqueous fluids or hydrous melts flux rocks at supra solidus conditions they equilibrate with the surroundings through further melting decreasing water activity and giving rise to undersaturated melts it is in these conditions that hornblende or anhydrous peritectic phases are stabilized unlike dehydration melting the melt fraction generated in this case is not limited by the water contained in hydrous minerals but by the volume of water added to the system unlike melting at the water saturated solidus these melts are capable of rising without freezing and do give rise to upper crustal granitic bodies

polyphase growth of accessory minerals during continental collision geochemical evidence from ultrahigh pressure metamorphic gneisses in the sulu orogen abstract an integrated study of petrology geochronology and geochemistry was performed for ultrahigh pressure metamorphic gneisses in the sulu orogen the results are used to elucidate the polyphase growth of such metamorphic minerals as zircon titanite and garnet in response to pressure temperature changes and fluid melt action during continental collision this provides insights into the property of metamorphic fluid melt and their effects on trace element mobility a combined result from ree patterns mineral inclusions and ti in zircon temperatures indicates three stages of zircon growth prograde growth occurred at 237 ma primarily at eclogite facies retrograde growth at 222 ma mostly at eclogite facies and the last growth at 205 ma at granulite facies the three stages of zircon growth are deciphered by distinct ree patterns and trace element compositions reflecting the differences in the property of metamorphic fluid melt the episodic growth of metamorphic zircon is primarily dictated by the episodic concentration of zr and si in metamorphic fluid melt relict domains of magmatic titanite are distinguished from metamorphosed and metamorphic domains by their distinctive ree patterns and trace element compositions the metamorphic titanite exhibit variably elevated nb contents and nb ta ratios suggesting nb ta fractionation due to breakdown of amphibole and or biotite during metamorphism polyphase growth of garnet is suggested by an integrated analysis of mineral inclusions and major and trace elements in large garnet grains trace element abundances vary in different zones of garnet which is ascribed to changes in the paragenesis and composition of matrix minerals involved in garnet forming reactions at different p t conditions therefore the metamorphic growth of zircon titanite and garnet would have occurred not only during prograde subduction but also during retrograde exhumation in the continental collision zones both metamorphic dehydration and partial melting would have taken place episodically during the collisional orogeny the breakdown of hydrous minerals at high to ultrahigh pressure conditions is a key to fluid liberation and element supply for the growth of these accessory minerals

trace element incorporation in titanite constraints from experimentally determined solid liquid partition coefficients titanite liquid partition coefficients for most of the trace elements relevant in petrogenetic studies are provided for titanite saturated liquids equilibrated at 1 5 gpa and 850 c starting from lamproitic compositions the high compatibility observed for ree hfse sr v and sc and the strong incompatibility observed for actinides large ion lithophile and light elements are discussed in terms of available crystal chemical mechanisms for incorporation and crystal structure control the exchange vectors na1ca 1 and al1ti 1 allow local charge balance to be achieved after incorporation of ree3 and r5 at the ca and ti site respectively the significant amounts of h measured are also relevant in this regard the exchange mechanism being oh 1o 1 at the o1 site the incorporation of u4 and th4 at the ca site is scarce and is likely balanced by that of mg2 at the ti site both these substitutions are responsible for strong structural strain titanite can thus be considered an important repository for ree and hfse in metamorphic and igneous rocks and its role must be accounted for when modelling trace element residence during metamorphic reactions and late magmatic crystallisation history due to the measured differences in compatibility titanite crystallisation increases the values of nb ta and lree hree ratios in residual liquids the similar compatibility of u and pb makes titanite suitable for u pb geochronology of igneous rocks only after common pb correction finally this study confirms that the titanite end member is not suitable for radioactive waste disposal due to the discussed crystal chemical constraints and that titanite based waste forms should contain high amounts of na and mg2

high grade metamorphism in the chapleau foleyet area ontario archean rocks superior province kapuskasing structural zone in migmatitic mafic gneiss paragneiss dioritic and tonalitic rocks a lower grade garnet clinopyroxene plagioclase gt cpx pl zone and patchy higher grade orthopyroxene zones are distinguished based on mineral melt equilibria minimum conditions for the gt cpx pl zone are 750 degrees c 6 kbar a h 2 o 0 5 0 7 and for the orthopyroxene zone 770 degrees c 6 kbar a h 2 o 800 degrees c pressure values are 5 4 8 4 kbar and define a nne trending area of relatively high p in the kapuskasing zone supporting the interpretation of a tilted crustal section modified journal abstract

using in situ trace element determinations to monitor partial melting processes in metabasites peak metamorphism 800 850 c 8 10 kbar in the harts range meta igneous complex harts range central australia was associated with localized partial melting by the reaction hornblende plagioclase quartz h2o1 4 garnet clinopyroxene titanite melt in situ trace element determinations of prograde peak and retrograde minerals in migmatitic metabasites and associated tonalitic melts using laser ablation icp ms has allowed monitoring of a range of partial melting processes melting melt segregation and back reaction between crystallizing melt and restitic minerals mass balance calculations indicate that titanite is a major carrier of trace elements such as ti nb ta sm u and th and therefore may be an important accessory phase to control the redistribution of these elements during the partial melting of amphibolites titanite preferentially incorporates ta over nb and hence residual titanite might assist in the formation of melts with high nb ta the fact that single minerals record different rare earth element ree patterns from prograde to peak to retrograde conditions demonstrates that ree diffusion is not significant up to 800 c therefore trace element analysis in minerals can be a powerful tool to investigate high grade metamorphic processes beyond the limits given by major elements

evolution of the archean continental crust some 20 years of plate tectonic theory combined with new insights into the fine structure of the lithosphere the application of multielement geo chemical and isotopic studies paleomagnetism and geophysical modeling of mantle processes have profoundly influenced present thinking on the origin and evolution of the earth s early continental crust previously our knowledge of the continental crust was based almost exclusively on field geological observations although there is now general agreement on how the earth worked for the last 200 m y because of observable evidence in the oceans and continents e g bird 1980 condie 1982 it has proved difficult to extend this history into more ancient times in view of the lost oceanic record and the ambiguity and complexity of the pre mesozoic rock relationships in the continents dewey 1982 however preserved characteristic rock as semblages uniquely identifying modern type wilson cycle processes i e opening and closure of oceans underlain by oceanic crust have now been recognized in continental terranes as old as 900 m y and these assemblages provide strong evidence for the conclusion that the present global tectonic regime has governed the evolution of the lithosphere at least since the late precambrian kroner 1977 1981a b goodwin 1981 profound disagreement on the older crustal history however prevails to the present day since typical features characterizing phanerozoic accretionary terranes such as obducted ophiolites blueschists and franciscan type melanges have not been found in more ancient regions thus two types of evolutionary models have been developed one type postulates uniformitarian development back to the earliest archean burke

sm nd evidence for high grade ordovician metamorphism in the arunta block central australia sm nd ages from the harts range in the south eastern arunta inlier in central australia indicate that regional metamorphism up to granulite facies occurred in the early ordovician c 475 ma this represents a radical departure from previous tectonic models for the region and identifies a previously unrecognized intraplate event in central australia peak metamorphic assemblages 800 c and 10 5 kbar formed at around 476 14 ma and underwent approximately 4 kbar of near isothermal decompression at 475 4 ma a coarse grained unfoliated garnet clinopyroxene bearing marble inferred to have recrystallized late in the decompressional evolution gives an age of 469 7 ma two lines of evidence suggest the early ordovician tectonism occurred in an extensional setting first the timing of the high grade lower crustal deformation coincides with a period of marine sedimentation in the amadeus and georgina basins that was associated with a seaway that developed across central australia second isothermal decompression of lower crustal rocks was associated with the formation of a regional sub horizontal mid crustal foliation in the entia gneiss complex which forms the structurally lowest part of the harts range upper amphibolite facies metamorphism c 700 c 8 9 kbar occurred at 479 15 ma there is no evidence that p t conditions in the entia gneiss complex were as high as in the overlying units this implies that the extensional system was reworked during a later compressional event sm nd data from the mid amphibolite facies c 650 c and 6 kbar detachment zone that separates the irindina supracrustal assemblage and entia gneiss complex give an age of 449 10 ma this age corresponds to the timing of a change in the pattern and style of sedimentation in the amadeus and georgina basins and indicates that the change in basin dynamics was associated with mid crustal deformation it also suggests that compressional deformation culminating in the devonian to carboniferous 400 300 ma alice springs orogeny may have begun as early as c 450 ma at present the extent of early ordovician tectonism in central australia is unknown however granulite facies metamorphism and associated intense deformation imply an event of regional extent an implication of this work is that high grade lower crustal metamorphism and intense deformation occurred during the development of a broad shallow slowly subsiding intraplate basin

garnet growth during amphibolite anatexis implications of a garnetiferous restite formation of a garnet clinopyroxenite restite may have important physicochemical consequences for deep crustal and mantle processes we have experimentally simulated the dehydration melting of amphibolite and report the modal and compositional evoltuion of the mafic granulitic mineral assemblage the restite with an emphasis on garnet a natural low k calcic amphibolite was powdered and run in sealed gold capsules in a piston cylinder apparatus at 10 kbar 750 1000 c 1 to 9 day runs and fo 2 ni nio garnet first appears in 8 day runs at 825 25 c above 850 c clinopyroxene and poikilitic garnet comprise most of the restite garnet peaks at 950 c at 32 vol of the material within the capsule or 41 vol of the restite assemblage and disappears between 975 1000 c there is no cross over of fe mg partitioning between coexisting liquids and garnets liquids remain mg poorer than garnets throughout the melting interval but do increase in mg with increasing temperature thus the hotter liquids may be less likely to react with mantle wedge peridotite the modes are consistent with ree patterns of archean tonalitic trondhjemitic rocks generated by the partial melting of garnet bearing mafic crust a garnet clinopyroxenite with 41 vol garnet has a density of 3 5g cm 3 which may induce lower crustal delamination in hot nonthickened amphibolitic crust this crust is seismically indistinguishable from lithosphere or mantle because garnet growth traps water bearing hornblende inclusions the restite may contain up to 0 3 wt water thus delivery of poikilitic garnet to the mantle possibly by delamination or by subduction may be an important mechanism for adding water to the mantle

the mechanisms of petrogenesis of the archaean continental crust comparison with modern processes the petrographic and chemical composition of magmatic rocks generated during the archaean appears to be different from that of post archaean rocks komatiites are widespread before 2 5 ga and rarely occur afterwards in addition the archaean continental crust is primarily ttg tonalitic trondhjemitic and granodioritic in composition exhibiting typical trondhjemitic differentiation trends whereas modern equivalents are granodioritic to granitic following classical calc alkaline differentiation trends this distinction becomes more prominent when rare earth elements ree are taken into account archaean ttg are yb poor ybn 8 5 and have high layb ratios 5 layb n 150 in comparison the post 2 5 ga granitoids emplaced in subduction zone geodynamic environments have high yb content 4 5 ybn 20 with very low layb n ratios 20 theoretical calculations and experimental petrology have shown that the ttg can be produced by partial melting of an archaean tholeiite transformed into garnet bearing amphibolite consequently the low heavy ree content of the ttg is explained by the influence of both residual garnet and hornblende in their source after 2 5 ga the role of these minerals in calc alkaline magma genesis becomes progressively less important which is interpreted in terms of a cooling earth model in modern subduction zone environments the subducted oceanic slab is relatively old and cold and the geothermal gradient along the benioff plane in low ca 10 c km consequently the down going lithosphere undergoes dehydration before partial melting is able to occur the liberated fluids are light ree and lile enriched and ascend into the overlying mantle wedge where they induced partial fusion the produced magmas separate from their mantle source region leaving a residue mainly composed of olivine and pyroxenes mantle derived magmas typically exhibit high yb contents due to low kdyb values for olivine and pyroxenes during the archaean the subducted lithosphere was relatively young and hot providing high geothermal gradients along the benioff zone thus partial melting of the subducted slab was possible at lower temperatures before dehydration would take place garnet and hornblende are the main residual phases accounting for the low yb contents of the archaean ttg this model can be tested using a modern analogue of archaean like subduction processes in south chile an oceanic ridge has subducted and all thermodynamic calculations indicate that this creates locally high geothermal gradients along the benioff zone thus in very small areas archaean like environments may be simulated in modern subduction zones the modern andesites produced in this environment show archaean geochemical characteristics with low ybn 5 whereas the majority of andesites along the andean arc have modern patterns with ybn ranging from 8 to more than 17 this conclusion was generalised to all young subducted lithospheres all over the world in conclusion it appears that since the archaean there has been a change in the site of continental crust genesis the location of calc alkaline magma source in subduction zone environments has migrated through time from the subducted slab to the mantle wedge this is a direct consequence of the progressive cooling of the earth

yttrium zoning in garnet coupling of major and accessory phases during metamorphic reactions the concentration of yttrium in pelitic garnets as a function of metamorphic grade has been examined in relation to the distribution of xenotime ypo 4 in samples from new england and british columbia samples with xenotime present only as inclusions in garnet generally possess high y cores and concentrations that drop off discontinuously along zoning shoulders of variable width to low y outboard regions samples with matrix xenotime are restricted to the garnet zone y concentration of these garnets generally decreases smoothly from core to rim xenotime may also be present in reaction zones around garnet in xenotime bearing samples y g r t is strongly temperature dependent and ranges from 5000 ppm in the garnet zone to 150 ppm in the sillimanite zone measured yttrium zoning profiles in xenotime absent samples are reproduced with both rayleigh fractionation and diffusion models but p t histories of the samples examined favor the rayleigh model with garnet volume bulk rock yttrium and mode of y hree accessory phases controlling the profile shape high yttrium annuli in staurolite zone samples may form by garnet overgrowth of proximal matrix enriched in yttrium due to garnet consumption during discontinuous staurolite forming reactions an increase in y g r t and hree g r t in garnet from anatectic samples is related to dissolution of phosphates in vapor absent peraluminous melt with partitioning of highly compatible y and hree into garnet grown during anatexis textural analysis reveals that phosphates are absent from regions of garnet grown in equilibrium with melt a main result of this study is identification of an intimate coupling between major pelite phases and accessory phases during reaction progress this coupling is of great advantage in that it may be used to 1 calibrate sensitive geothermometers and geobarometers 2 identify particular regions of garnet grown in different garnet producing reactions over a range of grades and 3 reveal portions of pelite reaction history invisible to major elements

localized rotation of principal stress around faults and fractures determined from borehole breakouts in hole b of the taiwan chelungpu fault drilling project tcdp abstract to reveal details of stress perturbations associated with faults and fractures we investigated the faults and large fractures accompanied by stress induced borehole breakouts or drilling induced tensile fractures in hole b of the taiwan chelungpu fault drilling project tcdp then we determined the relationship between the faults and fractures and stress orientation changes we identified faults and fractures from electrical images of the borehole wall obtained by downhole logging but also from photographs and descriptions of retrieved core samples and measured the variations in the principal horizontal stress orientation ascertained from borehole breakouts observed on the electrical images in the vicinity of the faults and fractures identification of geological structures faults fractures and lithologic boundaries by electrical images only is difficult and may sometimes yield incorrect results in a novel approach therefore we used both the electrical images and core photographs to identify geological structures we found four patterns of stress orientation change or no change in the vicinity of faults and fractures in tcdp hole b i abrupt discontinuous rotation in the vicinity of faults or fractures ii gradual rotation iii suppression of breakouts at faults fractures or lithologic boundaries and iv no change in the stress orientation we recognized stress fluctuations that is heterogeneous mesoscale 10 cm stress distributions with respect to both stress orientation and magnitude in addition we found that stress state changes occurred frequently in the vicinity of faults fractures and lithologic boundaries

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

adakite like volcanism of ecuador lower crust magmatic evolution and recycling in the northern andes of ecuador a broad quaternary volcanic arc with significant across arc geochemical changes sits upon continental crust consisting of accreted oceanic and continental terranes quaternary volcanic centers occur from west to east along the western cordillera frontal arc in the inter andean depression and along the eastern cordillera main arc and in the sub andean zone back arc the adakite like signatures of the frontal and main arc volcanoes have been interpreted either as the result of slab melting plus subsequent slab melt mantle interactions or of lower crustal melting fractional crystallization and assimilation processes in this paper we present petrographic geochemical and isotopic sr nd pb data on dominantly andesitic to dacitic volcanic rocks as well as crustal xenolith and cumulate samples from five volcanic centers pululagua pichincha ilalo chacana sumaco forming a nw se transect at about 0 latitude and encompassing the frontal pululagua pichincha main ilalo chacana and back arc sumaco chains all rocks display typical subduction related geochemical signatures such as nb and ta negative anomalies and lile enrichment they show a relative depletion of fluid mobile elements and a general increase in incompatible elements from the front to the back arc suggesting derivation from progressively lower degrees of partial melting of the mantle wedge induced by decreasing amounts of fluids released from the slab we observe widespread petrographic evidence of interaction of primary melts with mafic xenoliths as well as with clinopyroxene and or amphibole bearing cumulates and of magma mixing at all frontal and main arc volcanic centers within each volcanic center rocks display correlations between evolution indices and radiogenic isotopes although absolute variations of radiogenic isotopes are small and their values are overall rather primitive e g end 1 5 to 6 87sr 86sr 0 7040 0 70435 rare earth element patterns are characterized by variably fractionated light to heavy ree la ybn 5 7 34 and by the absence of eu negative anomalies suggesting evolution of these rocks with limited plagioclase fractionation we interpret the petrographic geochemical and isotopic data as indicating open system evolution at all volcanic centers characterized by fractional crystallization and magma mixing processes at different lower to mid crustal levels as well as by assimilation of mafic lower crust and or its partial melts thus we propose that the adakite like signatures of ecuadorian rocks e g high sr y and la yb values are primarily the result of lower to mid crustal processing of mantle derived melts rather than of slab melts and slab melt mantle interactions the isotopic signatures of the least evolved adakite like rocks of the active and recent volcanoes are the same as those of tertiary normal calc alkaline magmatic rocks of ecuador suggesting that the source of the magma did not change through time what changed was the depth of magmatic evolution probably as a consequence of increased compression induced by the stronger coupling between the subducting and overriding plates associated with subduction of the aseismic carnegie ridge

ordovician high grade metamorphism of a newly recognised late neoproterozoic terrane in the northern harts range central australia granulite facies rocks from the northernmost harts range complex arunta inlier central australia have previously been interpreted as recording a single clockwise cycle of presumed palaeoproterozoic metamorphism 800 875 c and 9 10 kbar and subsequent decompression in a kilometre scale e w striking zone of noncoaxial high grade c 700 735 c and 5 8 6 4 kbar deformation however new shrimp u pb age determinations of zircon monazite and titanite from partially melted metabasites and metapelites indicate that granulite facies metamorphism occurred not in the proterozoic but in the ordovician c 470 ma the youngest metamorphic zircon overgrowths from two metabasites probably meta volcaniclastics yield 206pb 238u ages of 478 4 ma and 471 7 ma whereas those from two metapelites yield ages of 463 5 ma and 461 4 ma monazite from the two metapelites gave ages equal within error to those from metamorphic zircon rims in the same rock 457 5 ma and 462 5 ma respectively zircon and possibly monazite ages are interpreted as dating precipitation of these minerals from crystallizing melt within leucosomes in contrast titanite from the two metabasites yield 206pb 238u ages that are much younger 411 5 ma 417 7 ma respectively than those of coexisting zircon which might indicate that the terrane cooled slowly following final melt crystallization one metabasite has a second titanite population with an age of 384 7 ma which reflects titanite growth and or recrystallization during the 400 300 ma alice springs orogeny the c 380 ma titanite age is indistinguishable from the age of magmatic zircon from a small late and weakly deformed plug of biotite granite that intruded the granulites at 387 4 ma these data suggest that the northern harts range has been subject to at least two periods of reworking 475 460 ma 400 300 ma during the palaeozoic detrital zircon from the metapelites and metabasites and inherited zircon from the granite yield similar ranges of proterozoic ages with distinct age clusters at c 1300 1000 and c 650 ma these data imply that the deposition ages of the protoliths to the harts range complex are late neoproterozoic or early palaeozoic not palaeoproterozoic as previously assumed

petrogenesis of archaean trondhjemites tonalites and granodiorites from eastern finland major and trace element geochemistry