magmatic mn rich garnets in volcanic settings age and longevity of the magmatic plumbing system of the miocene ramadas volcanism nw argentina abstract the miocene corte blanco tuff rhyolite deposit is the product of a large volume and high intensity plinian eruption from the solitary and monogenetic ramadas volcanic centre central andes province of salta nw argentina the corte blanco tuff consists of vitreous tube pumices with rare euhedral sub millimetric mn garnet phenocrysts typically hosting inclusions of u phases as zircon and monazite here we present new textural major and trace elemental analyses of garnet zircon and glass that combined with in situ u th pb zircon and monazite dating are used to reconstruct the thermobaric environment of formation age and longevity of the magmatic plumbing system of the ramadas magma the results indicate to a crystallization path of a peraluminous rhyolitic melt at shallow crustal levels 6 km as sequentially tracked by the initial nucleation of zircon 780 c at 9 16 ma and garnet above or at ca 700 c to the final monazite growth 660 670 c at 8 70 ma in a water saturated h2o 3 5 wt environment shortly before the eruption started these data 1 define for the first time the primary magmatic origin of mn garnet in a rhyolitic volcanic setting 2 provide new partition coefficients of rare earth elements ree between natural garnet zircon and rhyolitic melts and 3 permit reconstruction of the magmatic processes that resulted in the ramadas eruption on a wider scale our results document the spatio temporal p t conditions timing and longevity time scales involved in the petrogenesis of a shallow peraluminous water saturated rhyolitic magmatic plumbing system that is able to generate the conditions for extremely explosive plinian eruptions

high pressure eclogites in northern jiangsu southern shandong province eastern china abstract eclogites are distributed for more than 500 km along a major tectonic boundary between the sino korean and yangtze cratons in central and eastern china these eclogites usually have high p assemblages including omphacite kyanite and or coesite or its pseudomorph and form a high p eclogite terrane they occur as isolated lenses or blocks 10 cm to 300 m long in gneisses type i serpentinized garnet peridotites type ii and marbles type iii type i eclogites were formed by prograde metamorphism and their primary metamorphic mineral assemblage consists mainly of garnet pyrope prp 15 40 mol omphacite jadeite jd 34 64 mol pargasitic amphibole kyanite phengitic muscovite zoisite an sio2 phase apatite rutile and zircon type ii eclogites characteristically contain no sio2 phase and are divided into prograde eclogites and mantle derived eclogites the prograde eclogites of type ii are petrographically similar to type i eclogites the mantle derived eclogites have high mgo feo fe2o3 and cr2o3 compositions in bulk rock and minerals and consist mainly of pyrope rich garnet prp 48 60 mol sodic augite jd 10 27 mol and rutile type iii eclogites have an unusual mineral assemblage of grossular rich grs 57 mol garnet omphacite jd 30 34 mol pargasite rutile pargasitic and taramitic amphiboles calcic plagioclase an68 epidote zoisite k feldspar and paragonite occur as inclusions in garnet and omphacite in the prograde eclogites this suggests that the prograde eclogites were formed by recrystallization of epidote amphibolite and or amphibolite facies rocks with near isothermal compression reflecting crustal thickening during continent continent collision of late proterozoic age equilibrium conditions of the prograde eclogites range from p 26 kbar and t 500 750 c in the western part to p 28 kbar and t 810 880 c in the eastern part of the high p eclogite terrane the prograde eclogites in the eastern part are considered to have been derived from a deeper position than those in the western part subsequent reactions manifested by 1 narrow rims of sodic plagioclase or paragonite on kyanite and 2 symplectites between omphacite and quartz are interpreted as an effect of near isothermal decompression during the retrograde stage the conditions at which symplectites re equilibrated tend to increase from west p 9 kbar and t 680 c equilibrium temperatures of type ii mantle derived eclogites and type iii eclogite are 730 750 c and 680 c respectively

the pyrope coesite rocks and their country rocks at parigi dora maira massif western alps detailed petrography mineral chemistry and pt path both the coarse and fine grained varieties of the partly coesite bearing pyrope quartzites their interlayered jadeite kyanite rocks and the biotite phengite gneiss country rock common to all of them were subjected to detailed petrographic and textural studies in order to determine the sequence of crystallisation of their mineral constituents which were also studied analytically by microprobe prior to pyrope and coesite growth the mg rich metapelites were talc kyanite chlorite rutile ellenbergerite schists which upon continued prograde metamorphism developed first pyrope megacrysts in silica deficient local environments at the expense of chlorite talc kyanite and subsequently the smaller pyrope crystals with coesite inclusions from reacting talc kyanite based on geobarometrically useful mineral inclusions as well as on experimentally determined phase relations a prograde pt path simplified for water activity 1 is constructed which passes through the approximate pt conditions 16 kbar and 560 c 29 kbar and 720 c and finally up to 37 kbar at about 800 c where the mg rich metapelite was a pyrope coesite rock with phengite kyanite and talc still present during the retrograde path pyrope was altered metasomatically either into phlogopite kyanite quartz or at a later stage to chlorite muscovite quartz both assemblages yield pt constraints the latter about 7 9 kbar 500 600 c the country rock gneisses have also endured high pressures of at least 15 kbar but they provide mostly constraints on the lowest portion of the uplift conditions within the greenschist facies about 5 kbar 450 c microprobe data are presented for the following minerals pyrope ellenbergerite dumortierite unusually mgti rich jadeite vermiculite formed after na phlogopite paragonite and for several generations of phengite chlorite talc phlogopite dravite and glaucophane in the high pressure rocks as well as for biotite chlorite phengites epidote garnet albite and k feldspar in the country rock gneisses an outstanding open problem identified in this study is the preservation of minerals as inclusions within kyanite and pyrope beyond their pt stability limits

decoding the oxygen fugacity of ore forming fluids from garnet chemistry the longgen skarn pb zn deposit tibet abstract the relationship between the compositional zonation of garnet in skarn deposits and the evolution of physicochemical conditions of ore forming fluids is not clear and needs further investigation this study presents the characteristics of trace elements of garnet from the longgen skarn pb zn deposit tibet with an aim to reveal the evolution trend of the physical and chemical conditions fo2 and ph of ore forming fluids and to explore the fingerprint of garnet trace elements in different types of skarn deposits based on robust alteration halo mineral assemblages and petrographic observation the garnets of the longgen skarn pb zn deposit are divided into two types 1 distal exoskarn garnet near limestone with ore bearing and fine oscillatory zoning adr39 9 75 0 grt1 adr represents andradite and 2 proximal exoskarn garnet core adr32 9 70 7 rim adr83 3 99 5 with pb zn rich mineral and coarse oscillatory zoning grt2 according to the total rare earth element ree content in the garnet compositional zone grt1 is further divided into three component zones grt1 1 heavy rare earth element hree enriched chondrite normalized ree distribution patterns and positive eu anomaly eu 1 eu 2 eusample euchondrite smsample smchondrite gdsample gdchondrite grt1 2 hree enriched ree patterns with negative eu anomaly eu 1 similarly five component zones were identified in grt2 grt2 1 ree rich ree patterns with eu 1 and grt2 5 lree enriched ree patterns with eu the fractionation of lree with hree and the behavior of eu indicate that the physical chemical conditions during the formation of grt1 1 grt1 2 and grt2 1 grt2 3 were near neutral ph 6 7 and changed to mildly acidic in grt1 3 and grt2 4 grt2 5 ph

fluid evolution of triassic and jurassic w mineralization in the xitian ore field south china constraints from scheelite geochemistry and microthermometry abstract rare earth element ree enriched scheelite is widely distributed in many ore deposits and has been utilized in a broad range of geochronological and geochemical studies in an effort to decipher skarn and quartz vein hosted w mineralization process scheelite from triassic and jurassic granites skarn ores and quartz vein hosted ores in the xitian ore field south china were analyzed for major and minor element content and fluid inclusion microthermometry all scheelite grains show homogeneous backscatter electron and cathodoluminescence response and uniform w and ca distribution the chondrite normalized ree patterns of scheelite from triassic granites to skarn ores show similar pr sm enrichments and negative eu anomalies but different hree contents in contrast the chondrite normalized ree patterns of scheelite from jurassic granites to quartz vein hosted ores changes from flat with slight lree enrichment to concave with remarkable lree depletion the homogenization temperatures and salinities of fluid inclusions from the scheelite in triassic granites and skarn ores are similar both reduce significantly from the jurassic granites to quartz vein hosted ores combined with pressure ph and mineral assemblage analysis we propose that the ree signatures in the ore forming fluids of skarn type w deposits are mainly controlled by crystallization of ree enriched skarn minerals e g garnet diopside epidote the similarity in lree patterns and y ho ratios between the triassic granite and skarn ore scheelite indicates that the skarn ore forming fluid was derived from the triassic granite while the distinct ree patterns and variable y ho ratios in scheelite from jurassic granites and quartz vein hosted ores imply possible fluid mixing for jurassic w mineralization in light of these results ree rich scheelite could serve to discriminate fluid source and document mineralization processes in a complex ore forming system

chemical composition genesis and exploration implication of garnet from the hongshan cu mo skarn deposit sw china abstract in this study we present new mineral chemical data on the garnet from the hongshan cu mo deposit one of the largest skarn deposits in the sanjiang tethyan tectonic domain in sw china the aims are to unravel the formation mechanism of the oscillatory zoned garnet at hongshan and to explore possible applications using garnet trace element to distinguish different types of skarn deposits two generations of garnets i e poorly zoned garnet i grt i and the well zoned garnet ii grt ii are identified from the endoskarn and exoskarn at hongshan based on field and petrographic observations epma and la icp ms analyses show that grt i contains a narrow compositional range adr40 07 61 25 grs36 65 57 08 and chondrite normalized hree enriched and lree depleted patterns with minor positive eu anomalies in contrast grt ii shows a wide compositional range adr27 78 67 53 grs30 67 70 92 to almost pure andradite adr94 41 99 99 grs0 00 4 21 with interlayered al rich and fe rich zones in grt ii its al rich zone is chemically similar to grt i whereas its fe rich zone exhibits chondrite normalized lree enriched and hree depleted patterns with distinct positive eu anomalies the positive ree3 vs mg correlations and the poor ree3 vs y and ca correlations altogether indicate that the ree3 incorporated into garnet was co influenced by the menzerite type substitution mechanism c a 2 1 v i i i r e e 3 1 v i i i a l 3 1 v i m g 2 1 v i fluid chemistry and the physicochemical conditions of precipitation the higher u contents and hree enriched patterns of grt i suggest that it was formed under a relatively reducing and near neutral ph condition varying u contents and ree patterns across different oscillatory zones indicate that grt ii was probably formed under alternating physicochemical fluctuations led by repeating crack seal cycles in the hydrothermal system besides chemical data compilation suggests that the garnet from different types of skarn deposits has different compositions and can be used as mineral prospecting tools we suggested that discrimination plots such as eu vs v and ce vs u can distinguish cu skarn deposit from other skarn deposit types whilst the u vs w plot can discriminate between cu w w sn and w mo skarn deposits

crustal kinematics and deep structure of the northern rhine graben germany surface and subsurface geological data unpublished information gleaned from industrial seismic surveys and results from a deep reflection seismic profile across the tertiary northern rhine graben indicate that crustal extension in excess of 6 km was achieved by asymmetric detachment of the upper crust at a depth of about 15 km comparison of a simple geometric kinematic cross sectional model and its depth extrapolation with the dekorp 9 n deep crustal reflection data suggests that extension below the graben was achieved by slip along a gently westward dipping shear zone which truncated and offset lower crustal planar fabrics and which continues into the uppermost mantle the planar lower crustal reflectors probably originated mainly during late variscan permocarboniferous crustal extension and or magmatic underplating

physical explanation of the formation and evolution of inversion zones and marginal troughs inversion zones are elongate structures some tens of kilometers wide and up to hundreds of kilometers long that have deformed in response to compression and produced topography inversion zones in the alpine foreland are mainly associated with mesozoic grabens and troughs and although very important in the geologic picture the conditions of their formation and evolution and their regional geologic significance are not entirely understood the internal structure of inversion zones is variable and depends on details in the pre inversion setting the inversion inducing stress field and the sedimentary fill however on a larger scale most inversion zones share certain principal observational features which sample the physical structure and the rheologic properties of the lithosphere and thereby provide an opportunity to test hypotheses of lithospheric rheology and dynamics the quantitative model presented in this paper explains how inversion zones and the associated marginal troughs are related to lithospheric zones of differential shortening and regional isostatic compensation of the induced topography

rates of li diffusion in garnet coupled transport of li and y rees numerical simulation of stranded diffusion profiles in partially resorbed garnet crystals from the aureole of the makhavinekh lake pluton labrador canada yields quantitative rates of intracrystalline diffusion for li in garnet diffusion coefficients for li at 700 900 c and 0 53 gpa are 0 5 1 5 log10 units lower than those for divalent cations in the same samples and match those for y and yb this correspondence likely stems from coupled substitution of li and y or ree ions in the garnet structure the requirement of local electroneutrality limits li mobility to be no faster than that of y rees because of this coupling li zoning in garnet is retained to comparatively high temperatures making garnet a valuable monitor of the behavior of li in deep crustal systems

sedimentary basin inversion caused by igneous underplating northwest european continental shelf a considerable body of evidence indicates that many of the extensional sedimentary basins in the vicinity of the british isles underwent permanent exhumation during the tertiary the most dramatic indicator of this process is the present day absence of as much as 4 km of anticipated postrift thermal subsidence in basins just north and west of scotland any explanation of this observation must take into account the fact that the entire region has very small long wavelength free air gravity anomalies this important constraint implies either that the crust has been thickened or that low density material has been added to or formed from the lithosphere and rules out models that invoke flexural effects arising from the opening of the north atlantic tertiary epeirogeny is often attributed to compression that is assumed to be related in a general sense to alpine mountain building however to remove 3 km of sedi mentary rock from a basin 100 km wide requires 15 km of shortening minor tertiary compression is observed all over the continental shelf but nowhere is it sufficient to account for the required amount of uplift and erosion in addition exhumation dramatically increases from south to north whereas the observed compression decreases markedly in the same direction at the beginning of the tertiary rifting associated with the initiation of the iceland plume generated substantial volumes of melt inversion of rare earth element concentrations of mgorich igneous rocks suggests that a minimum of 5 km of melt was produced beneath at least part of the continental shelf we infer that much of this melt remains trapped within the lithosphere presumably close to the moho which acted as a density filter such underplating will generate rapid uplift

a refined garnet biotite fe mg exchange geothermometer and its application in amphibolites and granulites a new formulation of garnet biotite fe mg exchange thermometer has been developed through statistical regression of the reversed experimental data of ferry and spear input parameters include available thermo chemical data for quaternary fe mg ca mn garnet solid solution and for excess free energy terms associated with mixing of al and ti in octahedral sites in biotite solid solution the regression indicates that fe mg mixing in biotite approximates a symmetrical regular solution model showing positive deviation from ideality withw femg bi 1073 490 cal mol h r and s r for the garnet biotite exchange equilibrium were derived to be 4301 cal and 1 85 cal respectively the resultant thermometer gives consistent results for rocks with a much wider compositional range than can be accommodated by earlier formulations

plate reorganization a cause of rapid late neogene subsidence and sedimentation around the north atlantic subsidence analysis of wells in the central north sea and labrador grand banks and oft the west greenland scotian shelf and united states atlantic margin shows distinct quantitative stratigraphic correlation patterns of circum north atlantic sites a significant departure from the overall decrease in subsidence for the pliocene occurs in many wells when the rate is found to have increased one or more orders of magnitude from oligocence miocene rates wells were selected along transects from shore to basin to find if relative basin position is influenced by differential basin subsidence although stratigraphic resolution is not detailed more basinward sites experienced up to four times larger subsidence rates in the late neogene than in the oligocene miocene with a peak in the pliocene wells at the basin edge experienced much less subsidence or showed uplift the observations are consistent with a rapid change of intraplate stress at the cause for this observed transition in neogene subsidence major reorganizations of spreading direction and rate occurred during the pliocene along the entire atlantic spreading system possibly in conjunction with more global changes in plate motions we propose that the associated changes in intraplate stress caused the excess margin subsidence relative uplift along basin edges is consistent with this mechanism of relative movement and may explain apparent eustatic changes in sea level

sr bearing zoisite and epidote in ultra high pressure uhp metamorphic rocks from the su lu province eastern china an important sr reservoir under uhp conditions sr bearing zoisite and epidote are common constituents of eclogites and associated paraschists throughout the su lu ultra high pressure uhp province eastern china the sro content of prograde zoisite and epidote reaches 3 2 wt in crystal cores and generally decreases toward crystal margins retrograde epidote is poorer in sro 0 1 wt preliminary rare earth element ree analyses of epidote give la2o3 up to 2 9 wt ce2o3 5 9 wt and nd2o3 3 0 wt ree contents of zoisite are distinctly lower la 2o3 up to 0 16 wt ce2o3 up to 0 26 wt and nd2o3 up to 0 16 wt than coexisting epidote apatite is always more depleted in sro 0 10 0 59 wt on average than coexisting zoisite and epidote and sr ca partition coefficients for zoisite and epidote and apatite sr ca zo ep ap range from 5 to 20 sro content of k white mica 0 012 0 044 wt is an order of magnitude lower than that of apatite an evaluation of the sro content in zoisite and epidote and their modal abundances in seven samples indicates that 70 of the whole rock sro is contained in these minerals apatite and k white mica are only minor reservoirs for sro in these rocks zoisite and epidote are thus regarded as the most important sr reservoirs at uhp conditions where calcic plagioclase and titanite are unstable

the sticklepath lustleigh fault zone tertiary sinistral reactivation of a variscan dextral strike slip fault dextral strike slip movement on the sticklepath lustleigh fault zone slfz is indicated by displacements of permian and older rocks previous authors have inferred that the main dextral movement which caused these displacements was post permian and noting the presence of small tertiary pull apart basins along the fault zone probably of tertiary age however the geometry of these early tertiary pull apart basins indicates sinistral rather than dextral strike slip movement we present an alternative model for the history of the sticklepath lustleigh fault zone summarized below 1 late variscan strike slip movement with a total displacement of up to 10 km produced the slfz and offset dextrally an earlier variscan thrust 2 extensional reactivation of this thrust led to rapid permo triassic subsidence in the crediton trough and a dextrally offset neighbour the hatherleigh outlier 3 approximately 6 km of early tertiary sinistral movement on the slfz produced small pull apart sedimentary basins and reduced the net dextral offset across the fault 4 in mid tertiary times minor dextral movements on the slfz may have produced reverse faulting on the margins of the tertiary basins

large areal extent of ultrahigh pressure metamorphism in the sulu ultrahigh pressure terrane of east china new implications from coesite and omphacite inclusions in zircon of granitic gneiss abstract coesite and omphacite inclusions have been identified for the first time as minute inclusions in zircon from amphibolite facies granitic orthogneiss in the sulu ultrahigh pressure uhp metamorphic terrane of eastern china by raman spectroscopy and microprobe analyses the occurrences of these minerals in the voluminous granitic gneiss of sulu support a regional and pervasive uhp metamorphic event that predated regional amphibolitic retrogression taking into account the widespread discoveries of coesite in other lithologies we thus conclude that a substantial crustal component in the sulu uhp metamorphic terrane appears to have shared a common history of triassic subduction to mantle depths and later exhumation

ultra high pressure uhp marble and eclogite in the su lu uhp terrane eastern china a large mass of dolomitic marble including many eclogite blocks occurs in orthogneisses of the rongcheng area of the su lu province eastern china the marble consists mainly of dolomite calcite formerly aragonite graphite forsterite diopside talc tremolite and phlogopite aggregates of talc and calcite occur at the boundary between dolomite and diopside tremolite is a reaction product between talc and calcite eclogite blocks are rimmed by dark green amphibolite the primary mineral assemblage in the core of eclogite is na bearing garnet up to 0 2 wt na2o omphacitic pyroxene clintonite and rutile secondary minerals are pargasitic edenitic amphibole plagioclase sodic diopside chlorite zoisite and titanite the peak metamorphic conditions based on stability of the dolomite forsterite aragonite now calcite graphite assemblage under conditions where tremolite is unstable are estimated at t 610 660 c and p 2 5 3 5 gpa for xco 0 001 a reaction between dolomite and diopside to form talc under tremolite unstable conditions indicates a temperature decrease under ultra high pressure conditions p 2 4 gpa xco 0 0013 the formation of secondary tremolite is consistent with a nearly adiabatic pressure decrease post dating the ultra high pressure metamorphism the temperature decrease under ultra high pressure conditions preceding decompression may reflect the underplating of a cold slab and the rapid decompression probably corresponds to the upwelling stage promoted by the delamination of a downwelling lithospheric root the p t conditions of the amphibolitization stage are estimated at 0 9 gpa and 460 c and are similar to conditions recorded by the surrounding orthogneisses

metamorphic evolution mineral chemistry and thermobarometry of schists and orthogneisses hosting ultra high pressure eclogites in the dabieshan of central china as is typical of ultra high pressure uhp terrains the regional extent of the uhp terrain in the dabieshan of central china is highly speculative since the volume of eclogites and paragneisses preserving unequivocal evidence of coesite and or diamond stability is very small by contrast the common garnet xmn 0 18 0 45 phengite si 3 2 3 35 zoned epidote ps38 97 biotite titanite two feldspars quartz assemblages in the more extensive orthogneisses have been previously thought to have formed under low p t conditions of ca 400 50 c at 4 kbar however certain orthogneiss samples preserve garnets with xca up to 0 50 rutile inclusions within titanite or epidote and relict phengite inclusions within epidote with si contents p f u of up to 3 49 overlapping with the highest values 3 49 3 62 recorded for phengites in samples of undoubted uhp schists these and other mineral composition features such as a site deficiencies in the highest si phengites na in garnets linked to y yb substitution and al f ti 1 o 1 substitution in titanites are taken to be pointers towards the orthogneisses having experienced a similar metamorphic evolution to the associated uhp schists and eclogites re evaluated garnet phengite and garnet biotite fe mg exchange thermometry and calculated 5 rutile 3 grossular 2sio2 h2o 5 titanite 2 zoisite equilibria indicate that the orthogneisses may indeed have followed a common subduction related clockwise p t path with the uhp paragneisses and eclogites through conditions of pmax at ca 690 c 715 c and 36 kbar to tmax at ca 710 c 755 c and 18 kbar prior to extensive re crystallisation and re equilibration of these ductile orthogneisses at ca 400 c 450 c and 6 kbar the consequential conclusion that it is no longer necessary to resort to models of tectonic juxtapositioning to explain the spatial association of these dabieshan orthogneisses with undoubted uhp lithologies has far reaching implications for the interpretation of controversial gneiss eclogite relationships in other uhp metamorphic terrains

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

in situ la icp ms study of garnets in the makeng fe skarn deposit eastern china fluctuating fluid flow ore forming conditions and implication for mineral exploration abstract most garnets in fe cu skarn deposits have simple petrographic textures lree enriched geochemical patterns are enriched in fe and are distinguished from garnets of typical w or w mo skarn deposits by their extremely low w sn and mo contents however little is known about the garnets in fe skarn systems that have more complex textures and high concentrations of w sn and mo here we describe diverse textures and la icp ms data for garnets in the largest fe skarn deposit makeng in eastern china and we discuss the dynamics of the mineralizing fluids and the significance of garnet for w mo exploration garnets from the makeng deposit fall into four main types corresponding to four stage garnet growth based on their textural characteristics and trace element contents stage 1 al rich relatively homogeneous garnets adr51 55 with a portion adr17 48 and hree enriched patterns and weak negative eu anomalies stage 2 fe rich garnet with lree enriched or flat patterns and weak negative or positive eu anomalies represented by oscillatory zoned rims adr50 90 with dodecahedral faces and oscillatory zones cores adr54 68 rims adr65 80 with composite dodecahedral trapezohedral faces stage 3 nearly pure andradite garnets with significantly lree enriched patterns and strong positive eu anomalies as represented by irregular zones adr70 99 in oscillatory zoned rims and relatively homogeneous garnets adr95 100 with extremely weak zoning and stage 4 al rich garnet with hree enriched patterns and weak negative or weak positive eu anomalies as represented by garnet veinlets adr45 77 with weak zoning these four stages of garnet were formed by periodic fluctuations in the fluid flux which can be explained by a kinetic dispersion model a low flux responsible for the al rich garnet a high flux responsible for the fe rich garnets the highest flux responsible for the nearly pure andradite garnet and a low flux responsible for the veins of al rich garnet relative to garnet in fe cu skarn deposits all garnet types in the makeng fe skarn deposit are rich in trace elements such as w sn and mo especially the pure andradite garnet in stage 3 which has the highest contents of w sn and mo these elemental enrichments are closely correlated with fe lree hree and fo2 signatures the w and mo signatures are identical to those of garnets in typical w or w mo skarn deposits and they are useful geochemical indicators during exploration for w mo ore deposits

tio2 exsolution from garnet by open system precipitation evidence from crystallographic and shape preferred orientation of rutile inclusions we investigated rutile needles with a clear shape preferred orientation in garnet from ultra high pressure metapelites from the kimi complex of the greek rhodope by electron microprobe electron backscatter diffraction and tem techniques a definite though complex crystallographic orientation relationship between the garnet host and rutile was identified in that rt 001 is either parallel to grt or describes cones with opening angle 27 6 around grt each rt 001 small circle representing a cone on the pole figure displays six maxima in the density plots this evidence together with microchemical observations in tem when compared to various possible mechanisms of formation corroborates a precipitate origin a review of exchange vectors for ti substitution in garnet indicates that rutile formation from garnet cannot occur in a closed system it requires that components are exchanged between the garnet interior and the rock matrix by solid state diffusion a process we refer to as open system precipitation osp the kinetically most feasible reaction of this type will dominate the overall process the perhaps most efficient reaction involves internal oxidation of fe2 to fe3 and transfer from the dodecahedral to the octahedral site just vacated by text ti 4 6 text m 2 3 text tial left text alsi 2 right text o 1 2 6 text m 2 2 5 text tialsi 3 text o 1 2 10 text m 2 3 0 text al 1 8 text fe 0 2 text si 3 text o 1 2 text m 2 2 text e 1 2 text tio 2 osp is likely to occur at conditions where the transition of natural systems to open system behaviour becomes apparent as in the granulite and high temperature eclogite facies

transformation of enstatite diopside jadeite pyroxenes to garnet the high pressure stability of enstatite en diopside di jadeite jd pyroxenes has been investigated experimentally with a split sphere anvil apparatus ussa 2000 on the enstatite pyrope join the compositions of garnet coexisting with enstatite were determined at 100 165 kbar and 1450 1850 c the results indicate complete solubility between enstatite and pyrope in the system cao mgo al2o3 sio2 cmas the compositions of coexisting pyroxenes and garnet were determined at 100 165 kbar and 1250 1750 c at 157 kbar 1650 c garnet with the composition en79di21 mol forms on the en di join in the system na2o mgo al2o3 sio2 nmas the compositions of coexisting pyroxenes and garnet were determined at 60 160 kbar and 1200 1850 c on the en jd join the first garnet has the composition en48jd52 at 135 kbar 1650 c and en53jd47 at 140 kbar 1500 c on the di jd join the first garnet with the composition di63jd37 forms around 170 kbar 1650 c in the en di jd system the first appearance of garnet with the composition en42di9jd49 is estimated at 133 kbar 1650 c the new pyroxene with the composition namg0 5si2 5o6 napx transforms to garnet at 154 kbar 1650 c the experimental results indicate that the transformation of a twopyroxene assemblage to garnet and residual pyroxene in the en di jd system could occur at pressures consistent with the 400 km seismic discontinuity and in a pressure interval of 0 3 kbar

evidence for intracontinental thrust related exhumation of the ultra high pressure rocks in china coesite and diamond bearing ultra high pressure metamorphic rocks produced during the triassic continental collision between the sino korean and yangtze plates are offset by about 530 km along the sinistral tanlu fault in eastern china the tanlu fault terminates abruptly south of the ultra high pressure metamorphic terrain in dabie shan and does not extend into the foreland fold belt along the yangtze river this suggests that in this region the mesozoic major sinistral displacement along the tanlu fault was transferred to southward directed thrusting which led to the exhumation of the ultra high pressure metamorphic rocks formed earlier postcollisional himalayan type intracontinental thrusting and concomitant erosion appear to be the main agents in the exhumation of the very high pressure metamorphic rocks in china

a new approach to the relief of great britain iii derivation of the contribution of neotectonic movements and exceptional regional denudation to the present relief abstract this paper builds on the contribution of relative rock resistance to contrasts in the relief of britain to establish the contribution of other factors these include the slope of the major rivers towards the sea itself in part a function of the size of material eroded from the headwater uplands and thus of their lithology and the steepness of their valleyside slopes the pattern of denudational unloading and thus of isostatic uplift is mapped and compared with actual mean altitude it is shown that the sum of predicted isostatic uplift and local base level is closely related to the current mean altitude of our uplands the predicted mean height based on the six equations for each rock resistance class relating mean altitude to river distance can also be compared with actual mean elevation this shows positive departures which are most readily explained by neotectonic uplift notable in the scottish highlands the alston block of the northern pennines and fforest fawr in south wales negative departures may be due to relative neotectonic subsidence as in buchan where there is coincidence with positive gravity anomalies however most negative departures can be linked with severe glacial erosion especially in such areas as westernmost scotland the lake district the vale of belvoir and the wash fens basin where evidence for deep glacial erosion is already strong regional height differences related to the major structural regions of britain are calculated they make a smaller contribution to differences in average elevation and thus to the overall relief of britain

uplift driven valley incision and river terrace formation in southern england outside the limits of middle pleistocene glaciation the river basins of southern england contain long cenozoic terrestrial sedimentary records of greatest importance are the river terrace sequences which contain biostratigraphical and sedimentary evidence that testifies to the high amplitude climatic changes of the quaternary however trends in valley development cannot easily be accommodated within the framework of quaternary climatic change it is argued in this paper that episodic incision and terrace development result from uplift a consequence of the interplay between continuing alpine orogenic movements and erosion driven isostasy using a simple linear model height age an uplift rate of ca 7 cm ka 1 is estimated for the terrace sequence in the upper thames valley this preliminary model is evaluated using the records of adjacent basins 1997 john wiley sons ltd

a mechanism for na incorporation in garnet an example from garnet in orthogneiss from the su lu terrane eastern china sodian and yttrian na y garnet occurs in orthogneisses of the su lu terrane of eastern china with major quartz microcline and albite and subordinate biotite titanite hematite and zircon acmite rich pyroxene muscovite epidote allanite and hastingsitic amphibole occur in a few of the samples the na y garnet can be divided into almandine spessartinegrossular ai 1 6 per formula unit pfu for 0 12 hereafter denoted as ai garnet and andradite spessartine feh 1 3 pfu hereafter denoted as feh garnet groups average composition of the ai garnet in each sample ranges from alm3ssps13prp grs49adr2 to aim2lsps67prpogrssadr7 the feh garnet contains a calderite component cdr and its composition is alm4 7spslo 2 prp 2adrs9 8 cdr3 12 na y and yb contents of the ai rich garnet reach 0 37 1 46 and 0 42 oxide wt respectively and na y yb 0 99 t 0 16 the feh garnet contains up to 0 11 2 1 and 0 48 wt of na20 y203 and yb203 respectively and its na y yb value 0 17 t 0 1 is lower than that of the ai garnet we conclude that na y and yb are incorporated in the ai rich garnet by the 8jna 8j y yb ca mn 2substitution whereas in the feh garnet y and yb are dominantly incorporated by the 8j y yb l4j ai feh ca mn sl substitution with up to 3 mol y yb for ca mn comparison ofna y yb of garnets in the orthogneisses and in some pegmatites implies that increasing pressure favors incorporation of na with y and yb and that under the albite epidote amphibolite facies or higher pressure conditions y and yb probably substitute into the garnet structure by 8jna 8j y yb ca mn 2 instead of the 8j y yb 4j al feh ca mn sl type substitution effective under lower pressure conditions

origin of retrograde fluid in ultrahigh pressure metamorphic rocks constraints from mineral hydrogen isotope and water content changes in eclogite gneiss transitions in the sulu orogen abstract by taking advantage of having depth profiles between contrasting lithologies from core samples of the chinese continental scientific drilling ccsd project a combined study was carried out to examine changes in mineral h isotope total water and hydroxyl contents in garnet and omphacite across the contacts between ultrahigh pressure uhp eclogite and gneiss in the sulu orogen east central china the samples of interest were from two continuous core segments from the ccsd main hole at depths of 734 21 737 16 and 929 67 932 86 m respectively the results show d values of 116 to 64 for garnet and 104 to 82 for omphacite consistent with incorporation of meteoric water into protoliths of uhp metamorphic rocks by high t alteration both equilibrium and disequilibrium h isotope fractionations were observed between garnet and omphacite suggesting fluid assisted h isotope exchange at local scales during amphibolite facies retrogression while bulk water analysis gave total h2o concentrations of 522 1584 ppm for garnet and 1170 20745 ppm for omphacite structural hydroxyl analysis yielded h2o contents of 80 413 ppm for garnet and 228 412 ppm for omphacite it appears that significant amounts of molecular h2o are present in the minerals pointing to enhanced capacity of water storage in the uhp eclogite minerals hydrogen isotope variations in the transition between eclogite and gneiss show correlations with variations in their water contents petrographically the degree of retrograde metamorphism generally increases with decreasing distance from the eclogite gneiss boundary thus retrograde metamorphism results in mineral reactions and h isotope variation because hydroxyl solubility in nominally anhydrous minerals decreases with dropping pressure significant amounts of water are expected to be released from the minerals during decompression exhumation decompression exsolution of structural hydroxyl from 1 m3 volume of eclogite composed of only garnet and omphacite results in release of a quantitative estimate of 3 07 3 44 kg water that can form 140 156 kg amphibole during exhumation therefore it is concluded that fluid for retrogression of the eclogites away from the eclogite gneiss boundary was derived from the decompression exsolution of structural hydroxyl and molecular h2o in nominally anhydrous minerals for the eclogites adjacent to gneiss in contrast the retrograde metamorphism was principally caused by aqueous fluid from the gneiss which is relatively rich in water consequently both the origin and availability of metamorphic fluid during exhumation of deeply subducted continental crust are deciphered by this combined study focusing on the transitions and the retrograde processes between the felsic and mafic uhp rocks