retrograde net transfer reaction insurance for pressure temperature estimates retrograde net transfer reactions significantly affect compositions of metamorphic minerals yet are rarely considered when determining pressure temperature p t conditions two natural amphibolite facies metapelites from the central himalaya of nepal exhibit extremely common compositional patterns including increases in mn and fe fe mg at the rims of garnets which are the result of retrograde garnet dissolution and fe mg exchange with biotite however typical thermobarometric approaches for these rocks result in errors of hundreds of degrees and 3 6 kbar compared with thermobarometry of nearby rocks and petrogenetic grids these large errors result because dissolution of high fe garnet has strongly affected the fe mg ratio of matrix biotite x ray maps help evaluate the extent and chemical effects of retrograde reactions in these samples by identifying mineral regions that retain highest t compositions or through a new data processing approach by permitting correction of mineral compositions to original high t values these approaches ensure against retrograde net transfer reactions and should be applied routinely in thermobarometric studies they ultimately yield p t estimates that are more petrologically reasonable and permit rapid screening of samples for those least affected by retrograde reactions reconsideration of thermobarometry in the central and eastern himalaya indicates that retrograde net transfer reactions are extremely common therefore previous thermobarometric studies based on garnet major element compositions from that region should be reevaluated

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

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

oscillatory and sector zoned garnets record cyclic rapid thrusting in central nepal 1 garnets with oscillatory zoning repeated compositional reversals and sector zoning crystallographically controlled compositional differences were discovered in rocks from near the main central thrust in the langtang region central nepal this zoning results from crystal growth kinetic effects related to extraordinarily rapid possibly cyclic thrusting if cyclic thrusting occurred then timescales of thrust rate inhomogeneities are 25 kyr implying that satellite determination of modern fault movement rates may not accurately reflect slip rates or strain partitioning in orogens on timescales exceeding thousands of years sector zoning in ca but not y mn or mg implies maximum intracrystalline diffusion rates for ca of 10 21 cm2 s at 550 c but faster rates for other elements

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

geochemical isotopic and geochronlologic constraints on the formation of the eagle point basement hosted uranium deposit athabasca basin saskatchewan canada and recent remobilization of primary uraninite in secondary structures the athabasca basin hosts many world class unconformity related uranium deposits recently uranium reserves for the eagle point basement hosted deposit have increased with the discovery of new mineralized zones within secondary structures a paragenetic study of eagle point reveals the presence of three temporally distinct alteration stages a pre athabasca alteration a main alteration and mineralization comprised of three substages and a post main alteration and mineralization stage that culminated in remobilization of uraninite from primary to secondary structures the pre athabasca alteration stage consists of minor amounts of clinochlore followed by dolomite and calcite alteration in the hanging wall of major fault zones and kaolinitization of plagioclase and k feldspar caused by surface weathering the main alteration and uranium mineralization stage is related to three temporally distinct substages all of which were produced by isotopically similar fluids a major early alteration substage characterized by muscovite alteration and by precipitation ca sr lree rich aluminum phosphate sulfate minerals both from basinal fluids at temperatures around 240 c prior to 1 600 ma the mineralization substage involved uraninite and hematite precipitated in primary structures the late alteration substage consists of dravite uranophane beta veins calcite veins and sudoite alteration from mg ca rich chemically modified basinal fluids with temperatures around 180 c the post main alteration and mineralization stage is characterized by remobilization of main stage uraninite from primary to secondary structures at a minimum age of ca 535 ma u pb resetting events recorded on primary and remobilized uraninites are coincident with fluid flow induced by distal orogenies remobilizing radiogenic pb to a distance of at least 225 m above the mineralized zones

ultrahigh pressure metabasaltic garnets as probes into deep subduction zone chemical cycling 1 we demonstrate an approach to examining the metamorphic history of subducting oceanic crust that can complement records of subduction zone chemical cycling derived from studies of igneous rocks produced at volcanic arcs by merging methods utilizing garnet zoning to establish prograde reaction histories with in situ high resolution trace element geochemistry and application to coesite bearing mafic eclogites representing subduction to depths beneath arcs we are able to directly identify geochemical manifestations of reactions contributing to element mobility in the subducting slab that are only inferred in studies of volcanic arcs or theoretical metamorphic models specifically we identify a prograde metamorphic reaction based solely on the zoning of geochemistry and mineral inclusions within garnet and infer that these features are a record of the breakdown of coexisting clinozoisite titanite and probable liberation of trace element laden fluid from the rock during prograde metamorphism we are then able to assign a specific depth interval for the reaction through calculation of the p t dependence of the reaction for these eclogites and comparison with a published p t trajectory because of the robust preservation of records of petrologic and geochemical processes by garnet this methodology is particularly suited for study of ultrahigh pressure uhp eclogites in which severe retrograde alteration generally related to exhumation commonly obscures prograde history

trace element zoning in a metamorphic garnet trace element zoning has been measured in an amphibolite garnet from the tauern window austria by using an ion microprobe humps in the zoning profiles of na sc v y and the heavy rare earth elements mark a period of open system behavior these humps correspond to a part of the major element zoning profile that is interpreted as a p t reversal the source of the mass excesses of these elements remains ambiguous they were derived either on a thin section scale by the breakdown of trace element enriched refractory minerals or externally from unusual trace element enriched fluids p t paths determined from garnet zoning may require modification if open system behavior is important during garnet growth

improved dissolution and chemical separation methods for lu hf garnet chronometry 1 garnet based lu hf geochronology using conventional hf hno3 dissolution methods may be compromised by full or partial digestion of hf rich zircon inclusions this study integrates two complimentary methods to substantially reduce zircon digestion while assuring complete digestion of garnet and sample spike equilibration handpicked garnet fractions are heat treated to 1000 c in an evacuated silica glass ampoule to anneal zircon inclusions and then dissolved with 12 m hcl at 210 c and cold 28 m hf analyses of heat treated garnet and their high temperature breakdown products orthopyroxene spinel quartz from gore mountain new york demonstrate the method is capable of complete dissolution of garnet and routinely achieving sample spike equilibration independent analyses of annealed and unannealed zircons dissolved by hcl cold hf demonstrate the benefit of heat treatment prior to dissolution analyses of zircon rich garnets from peletic paragneisses of labrador canada show the potential of this method by returning lu hf ratios commensurate with the degree of heat treatment finally a complementary chemical separation method for lu and hf is presented that supports analysis of garnet by mc icp ms

u pb garnet chronometry in high grade rocks case studies from the central damara orogen namibia and implications for the interpretation of sm nd garnet ages and the role of high u th inclusions garnets from different migmatites and granites from the damara orogen namibia were dated with the u pb technique after bulk dissolution of the material measured 206pb 204pb ratios are highly variable and range from ca 21 to 613 variations in isotope 208pb 204pb 206pb 204pb and trace element th u u nd sm nd ratios of the different garnets show that some garnets contain significant amounts of monazite and zircon inclusions due to their very low 206pb 204pb ratios garnets from pelitic migmatites from the khan area yield pb pb ages with large errors precluding a detailed evaluation however the 207pb 206pb ages ca 550 500 ma appear to be similar to or older than u pb monazite ages 530 1 517 1 ma and sm nd garnet ages 523 4 512 3 ma from the same sample it is reasonable to assume that the pb pb garnet ages define growth ages because previous studies are consistent with a higher closure temperature for the u pb system in garnet relative to the u pb system in monazite and the sm nd system in garnet for igneous migmatites from oetmoed pb pb garnet ages 483 15 492 16 ma and one sm nd garnet whole rock age 487 8 ma are similar whereas the monazite from the same sample is ca 30 40 ma older 528 1 ma these monazite ages are however similar to monazite ages from nearby unmigmatized granite samples and constrain precisely the intrusion of the precursor granite in this area although there is a notable difference in closure temperature for the u pb and sm nd system in garnet the similarity of both ages indicate that both garnet ages record garnet growth in a migmatitic environment restitic garnet from an unmigmatized granite from omaruru yields similar u pb 493 30 506 30 ma and sm nd 493 6 488 7 ma garnet ages whereas the monazite from this rock is ca 15 25 ma older 516 1 514 1 ma whereas the monazite ages define probably the peak of regional metamorphism in the source of the granite the garnet ages may indicate the time of melt extraction for igneous garnets from granites at oetmoed the similarity between pb pb 483 34 474 17 ma and sm nd 492 5 484 13 ma garnet ages is consistent with fast cooling rates of granitic dykes in the lower crust differences between garnet and monazite u pb ages can be explained by different reactions that produced these minerals at different times and by the empirical observation that monazite seems resistant to later thermal re equilibration in the temperature range between 750 and 900 c e g braun et al 1998 for garnet analyses that have low 206pb 204pb ratios the influence of high inclusions is small however the relatively large errors preclude a detailed evaluation of the relationship between the different chronometers for garnet with higher 206pb 204pb ratios the overall similarity between the pb pb and sm nd garnet ages implies that the inclusions are not significantly older than the garnet and therefore do not induce a premetamorphic pb signature upon the garnet the results presented here show that garnet with low 238u 204pb ratios together with sm nd garnet data and u pb monazite ages from the same rock can be used to extract geologically meaningful ages that can help to better understand tectonometamorphic processes in high grade terranes

pb diffusion in zircon abstract chemical diffusion of ti under anhydrous conditions at 1 atm and under fluid present elevated pressure 1 1 1 2 gpa conditions has been measured in natural zircon the source of diffusant for 1 atm experiments was a zro2 tio2 zircon mixture with experiments run in crimped pt capsules diffusion experiments done in the presence of h2o co2 fluid were run in a piston cylinder apparatus using a source of ground tio2 zrsio4 and sio2 with oxalic acid added to produce h2o co2 vapor and partially melt the solid source material yielding an assemblage of rutile zircon melt vapor nuclear reaction analysis nra with the resonant nuclear reaction 48ti p 49v was used to measure diffusion profiles for both sets of experiments the following arrhenius parameters were obtained for ti diffusion normal to cover the temperature range 1350 1550 c at 1 atm d ti 3 33 10 2 exp 754 56 kj mol 1 r t m 2 s 1 ti diffusivities were found to be similar for experiments run under fluid present conditions a fit to all of the data yields the arrhenius relation d 1 34 102 exp 741 46 kj mol 1 rt m2 s 1 these data suggest that zircon should be extremely retentive of ti chemical signatures indicating that the recently developed ti in zircon crystallization geothermometer watson e b harrison t m 2005 zircon thermometer reveals minimum melting conditions on earliest earth science 308 841 844 will be quite robust in preserving temperatures of zircon crystallization titanium diffuses somewhat faster in zircon than larger tetravalent cations u th and hf but considerably more slowly than pb the ree and oxygen hence ti crystallization temperatures may be retained under circumstances when radiometric ages or other types of geochemical information are lost

high precision lu hf geochronology of eocene eclogite facies rocks from syros cyclades greece abstract garnet bearing high pressure low temperature hp lt metamorphic rocks of the cycladic blueschist unit have been investigated by lu hf geochronology eclogites from syros island yield precise ages of 52 2 0 3 ma 51 4 0 4 ma and 50 2 ma preserved major and trace element growth zoning in garnet suggests that the closure temperature of the lu hf system in garnet was higher than the estimated peak metamorphic temperature of 500 c hence lu hf ages most likely reflect garnet growth in the studied samples our new lu hf grt ages are in excellent agreement with previously published u pb shrimp ages of metamorphic zircon and the maximum reported white mica rb sr and ar ar ages from syros garnet growth in all samples apparently took place over a narrow time interval because the lu hf ages cluster tightly even though the distribution of lu between cores and rims varies among samples we find no evidence for garnet generations that are significantly older or younger than the given isochron ages this in conjunction with the agreement among the ages of metamorphic zircon the maximum white mica ages and the lu hf ages strongly suggests that the 52 ma age dates a single hp event that affected these rocks

sm nd dating of spatially controlled domains of garnet single crystals a new method of high temperature thermochronology ganguly and tirone meteorit planet sci 36 2001 167 175 recently presented a method of determining the cooling rates of rocks from the difference between the core and bulk ages of a crystal as determined by a single decay system here we present the first application of the method using the core and bulk ages of garnet single crystals according to the sm nd decay system in two rock samples with contrasting cooling rates which can be constrained independently the samples belong to the metamorphic core complex valhalla british columbia and the mid crustal magmatic arc exposure of the salinian terrane california we have micro sampled the garnet crystals over specific radial dimensions and measured the nd isotopes of these small sample masses as ndo via solid source mass spectrometry to determine the sm nd age difference between the core and bulk crystals using a peak metamorphic p t condition of 8 1 kbar 820 30 c spear and parrish j petrol 37 1996 733 765 the core 67 3 2 3 ma and bulk 60 9 2 1 ma ages of the british columbian garnet sample yield a cooling rate of 2 13 c myr which is in very good agreement with the cooling rates that we have derived by modeling the retrograde fe mg zoning in the same garnet and assuming the same peak metamorphic p t condition considering earlier cooling rate data derived from closure temperature vs age relation of multiple geochronological systems spear and parrish j petrol 37 1996 733 765 a cooling rate of v15 20 c myr seems most reasonable for the valhalla complex diffusion kinetic analysis shows that the sm nd core age of the selected garnet crystal could not have been disturbed during cooling consequently the core age of the garnet crystal 67 3 2 3 ma corresponds to the peak metamorphic age of the valhalla complex the salinian sample on the other hand yields indistinguishable core 78 2 2 7 ma and bulk 77 9 2 9 ma ages as expected from its fast cooling history which can be constrained by the results of earlier studies the sm nd decay system in garnet has relatively high closure temperature usually s 650 c therefore the technique developed in this paper fills an important gap in thermochronology since the commonly used thermochronometers are applicable only at lower temperatures simultaneous modeling of the retrograde fe mg zoning in garnet spatially resolved sm nd ages of garnet single crystals and resetting of the bulk garnet sm nd age

prograde temperature time evolution in the barrovian type locality constrained by sm nd garnet ages from glen clova scotland abstract the timing of garnet growth during metamorphism associated with the grampian orogeny in the sillimanite zone of the barrovian type locality in glen clova scotland was determined by sm nd geochronology two high precision garnet whole rock ages were achieved by employing hf partial dissolution of garnet separates to optimize purity multiple garnet growth generations were identified on the basis of the geochronology and detailed textural and chemical data an early stage at 472 9 2 9 ma during d 2 deformation under garnet zone conditions c 500 550 c and a later stage at 464 8 2 7 ma during or slightly after d 3 deformation mostly under sillimanite zone conditions peak temperature of c 660 c but possibly including some growth during kyanite zone conditions when combined with recently published garnet ages from the kyanite and garnet zones the data suggest that peak metamorphic temperatures in at least these three of barrow9s zones were achieved roughly contemporaneously the difference between garnet zone and sillimanite zone peak temperature attainment is 2 8 3 7 ma the near contemporaneity of peak temperature attainment in different metamorphic zones requires an additional source of heat beyond thermal relaxation of a variably over thickened crust we suggest that local igneous intrusions with synmetamorphic ages provided that additional heat

petrology of basement dominated terranes i regional metamorphic t t path from u pb monazite and sm nd garnet geochronology central damara orogen namibia abstract mineral chemistry combined with sm nd garnet whole rock and u pb monazite ages provides important constraints on the geological history of metapelites migmatites and granites from the high grade basement dominated part of the damara orogen namibia based on thermobarometric data the peak of regional metamorphism reached c 700 750 c and c 5 kbar u pb monazite and sm nd garnet mineral ages obtained from unmigmatized garnet and cordierite bearing metapelites indicate that these peak metamorphic conditions were reached between 525 2 and 504 3 ma some migmatites with restitic melanosomes record sm nd garnet ages of 518 3 ma indicating that high grade regional metamorphism also culminated in partial melting some other migmatites and granites give older sm nd garnet whole rock ages between 530 3 and 540 4 ma different initial nd isotope compositions of granites leucosomes and melanosomes indicate that in this case migmatization is intrusion related crust derived granites intruded at 515 2 512 3 and 509 5 ma and are probably related to the second peak of regional metamorphism the youngest granites intruded at 469 3 ma indicating that intrusion of melts is superimposed upon the regional metamorphic history age constraints for the period between c 540 and 510 ma provide substantial evidence for crustal thickening together with igneous activity whereas the high temperature event which occurred at c 470 ma must be related to extensional tectonics the concordance between the monazite and garnet ages suggest that monazite grew at high grade metamorphic conditions it is further demonstrated that combined monazite and garnet chronometry are useful in identifying the timing and nature of particular tectonometamorphic and intrusive events in polymetamorphic high grade terranes

models of garnet differential geochronology abstract rayleigh distillation models are developed to describe theoretical growth zoning of lu hf rb sr sm and nd in typical garnet crystals from metapelites and metabasites effects of diffusion limited transport within the matrix and intracrystalline diffusion are also considered qualitatively theoretical zoning profiles show strong depletions of lu in garnet rims compared to cores but virtually invariant hf rb sr sm and nd profiles generally consistent with natural profiles for lu and hf and previously published models theoretical isochron diagrams for lu hf exhibit distinctive arcuate distributions and high mswds consistent both with himalayan data and with expectations that garnet growth durations exceed chronologic resolution by as much as an order of magnitude predicted isochron diagrams for sm nd and rb sr exhibit vertical arrays for garnet and high mswds that are generally lower than for lu hf in metapelites inherent chronologic resolution for bulk separates is best for lu hf in metapelites and rb sr but analytical considerations favor rb sr or sm nd for chronologic zoning studies diffusion limited transport in the rock matrix strongly influences zoning profiles but does not change the main trends on isochron diagrams intracrystalline diffusion will initially rotate lu hf isochrons to steeper slopes giving older apparent ages the natural himalayan data indicate growth of garnet in one rock from the greater himalayan sequence at 34 ma consistent with previously measured monazite ages from the same rock data from another himalayan rock suggest polymetamorphism that includes a paleozoic component

rare earth diffusion kinetics in garnet experimental studies and applications abstract we determined the diffusion coefficient of sm in almandine garnet as function of temperature at 1 bar and fo 2 corresponding to that of wustite iron buffer and to a limited extent that of a few other selected rare earth elements in almandine and pyrope garnets both garnets were demonstrated to have metastably survived the diffusion annealing at conditions beyond their stability fields the experimental diffusion profiles were analyzed by secondary ion mass spectrometry and in addition by rutherford back scattering spectroscopy for two samples transmission electron microscopic study of an almandine crystal that was diffusion annealed did not reveal any near surface fast diffusion path using reasonable approximations and theoretical analysis of vacancy diffusion the experimental data were used to develop an expression of rare earth element ree diffusion coefficient in garnet as a function of temperature pressure fo 2 ionic radius and matrix composition calculation of the closure temperature for the sm nd decay system in almandine garnet in a metamorphic terrain shows very good agreement with that constrained independently modeling of the ree evolution in melt and residual garnet suggests that for dry melting condition the ree pattern in the melt should commonly conform closely to that expected for equilibrium melting however for much lower solidus temperatures that would prevail in the presence of a h 2 o co 2 fluid the concentration of light ree in the melt could be significantly lower than that under equilibrium melting condition a reported core and rim differences in the ree content of a garnet crystal in a mantle xenolith in kimberlite have been reproduced by assuming that the ree zoning was a consequence of entrapment in a magma derived from an external source for 32 000 yr before the eruption

the isotopic composition of yb and the precise and accurate determination of lu concentrations and lu hf ratios by isotope dilution using mc icpms 1 we report yb isotopic data for the purpose of accurately correcting measurements of lu and lu hf ratios during multicollector inductively coupled plasma mass spectrometer mc icpms analysis the yb isotopic ratios agree with recent results of chu et al 2002 and amelin and davis 2004 when corrected for mass fractionation to the same reference values protocols for lu measurement involving simultaneous subtraction of yb interference and application of a fractionation correction based on yb are described it is shown that lu concentrations and lu hf ratios determined by mc icpms using this methodology are more precise and potentially more accurate than results based on analysis of lu by thermal ionization mass spectrometry tims a number of mc icpms lu hf isotopic analyses of standard rocks and other geological samples previously analyzed by tims are presented these document reproducibility of lu hf within 0 2 where most discrepancies can be assigned to the unknown lu mass fractionation during tims analysis

trace element distributions in silicates during prograde metamorphic reactions implications for monazite formation to assess the petrogenetic relationship between monazite and major silicates during prograde metamorphism ree were measured across coexisting zoned silicates in garnet through kyanite grade pelitic schists from the great smoky mountains western blue ridge terrane southern appalachians to establish ree concentrations and distributions before and after the monazite in isograd and to identify the role major silicates play in the formation of monazite results indicate significant scavenging of light rare earth elements lree from silicates during the monazite in isograd reaction however the absolute concentration of lree hosted in the silicates was insufficient to produce monazite in the quantity observed in these schists monazite must have formed mainly from either the dissolution of allanite or some other source of concentrated lree possibly adsorbed onto grain boundaries even though direct evidence for allanite is lacking in a majority of the samples laser ablation icp ms analyses and theoretical thermodynamic calculations show that monazite may have formed as a result of contributions from both allanite and major silicates allanite breakdown initially formed monazite and monazite production drew lree liberated from allanite major silicates and possibly from crystal boundaries in many rocks the reaction was further promoted by the staurolite in reaction allowing for rapid isogradic monazite growth

lu hf geochronology and trace element distribution in garnet implications for uplift and exhumation of ultra high pressure granulites in the sudetes sw poland combining lu hf garnet geochronology with in situ trace element analyses in garnet allowed its to gain new insight into the metamorphic evolution of uhp uht rocks in the stary gieraltow region in the polish sudetes prograde garnet growth recorded by rayleigh type heavy ree hree zoning in the felsic granulites indicates that the obtained 386 6 4 9 ma lu hf age represents the time of garnet crystallization on a prograde uhp metamorphic path the surrounding rocks were metamorphosed at the same time as indicated by 381 2 6 7 ma sm nd garnet age obtained for the mid crustal metapelites the second metamorphic episode which affected most of the lower crust in the orlica snieznik massif osm occurred at ca 340 ma as determined by u pb zircon and sm nd garnet dating of granulites in this and previous studies is interpreted as a high temperature event which took place on a retrograde path trace element distribution in garnets from the layered granulites showed significant differences in distribution of medium and hree in garnets from mafic and felsic protoliths over the course of the metamorphic evolution this had strong impact on the isotopic dating results and led to decoupling of the sm nd and lu hf clocks which recorded timing of the two different metamorphic episodes separated by as much as 40 ma moreover the preservation of the hree growth zonation profile in garnets from the felsic granulites whose minimum metamorphic temperature was established at 900 degrees c implies that the lu hf system under relatively dry conditions does not undergo significant diffusional re equilibration even at such extreme temperatures and therefore it sill provides the age of prograde garnet growth under hydrous conditions at least some resetting will take place as documented by the partially relaxed hree zonation profile in the amphibolitised mafic granulite which yielded a 10 ma younger age the hree distribution study appeared to be a particularly valuable and essential tool which allowed us to distinguish garnet growth from post growth complexities and hence provide improved age interpretation medium ree on the other hand did not show any obvious correlation with the isotopic signature of garnet two distinct metamorphic episodes recorded in the stary gieraltow region show that buoyancy driven uplift of uhp rocks can be arrested at the base of a continental crust if not supported by any additional force in our case study the uhp rocks would have never reached the surface if their uplift had not been resumed after a long pause under a different tectonic regime the multistage discontinuous uplift revealed by the uhp rocks of the osm provides a new scenario for the exhumation of continental crust from mantle depths distinct from the fast track exhumation histories recognized in uhp terranes elsewhere c 2006 elsevier b v all rights reserved