u pb monazite ages in amphibolite to granulite facies orthogneiss reflect hydrous mineral breakdown reactions sveconorwegian province of sw norway in the rogaland vest agder terrain of the sveconorwegian province of sw norway two main sveconorwegian metamorphic phases are reported a phase of regional metamorphism linked to orogenic thickening m1 and a phase of low pressure thermal metamorphism associated with the intrusion of the 931 2 ma anorthosite charnockite rogaland igneous complex m2 phase m1 reached granulite facies to the west of the terrane and m2 culminated locally at 800 850 c with the formation of dry osumilite bearing mineral associations monazite and titanite u pb geochronology was conducted on 17 amphibolite to granulite facies orthogneiss samples mainly from a suite of 1050 2 8 ma calc alkaline augen gneisses the feda suite in these rocks prograde negatively discordant monazite crystallized during breakdown of allanite and titanite in upper amphibolite facies at 1012 1006 ma in the feda suite and other charnockitic gneisses concordant to slightly discordant monazite at 1024 997 ma probably reflects breakdown of biotite during granulite facies m1 metamorphism a spread of monazite ages down to 970 ma in biotite hornblende samples possibly corresponds to the waning stage of this first event in the feda suite a well defined monazite growth episode at 930 925 ma in the amphibolite facies domain corresponds to major clinopyroxene formation at the expense of hornblende during m2 growth or resetting of monazite was extremely limited during this phase in the granulite facies domain up to the direct vicinity of the anorthosite complex the m2 event was shortly followed by cooling through ca 610 c as indicated by tightly grouped u pb ages of accessory titanite and titanite relict inclusions at 918 2 ma over the entire region a last generation of u poor monazite formed during regional cooling below 610 c in hornblende rich samples at 912 904 ma this study suggests 1 that monazite formed during the prograde path of high grade metamorphism may be preserved 2 that monazite ages reflect primary or secondary growth of monazite linked to metamorphic reactions involving redistribution of rees and th and or fluid mobilisation 3 that the u pb system in monazite is not affected by thermal events up to 800 850 c provided that conditions were dry during metamorphism

microprobe monazite geochronology understanding geologic processes by integrating composition and chronology abstractmonazite is a light rare earth element lree bearing phosphate mineral that is present in a wide variety of rock types has an extremely variable composition reflecting host rock conditions and is a robust geochronometer that can preserve crystallization ages through a long history of geological events monazite crystals typically contain distinct compositional domains that represent successive generations of monazite which in turn can provide a detailed record of the geologic history of its host rocks the electron microprobe can be used to characterize the geometry of compositional domains analyze the composition of each domain and when carefully configured determine the u th total pb age for domains as small as 5 m in width these data allow the monazite to be linked with and place timing constraints on silicate processes in the host rocks current applications span a broad range of geologic processes in igneous metamorphic hydrothermal and sedimentary rocks

segmentation and coalescence of cenozoic carbonate platforms northwestern great bahama bank seismic profiles over the northwestern great bahama bank reveal that it is formed by the coalescence of three smaller platforms and significant lateral progradation this conclusion is contrary to previous models which assumed that the bank is the result of continuous accumulation on one huge buildup in late cretaceous to early tertiary time a north south trending depression the straits of andros separated an eastern platform andros bank from a western platform bimini bank initially the straits of andros had dimensions similar to the modem tongue of the ocean but was progressively filled from east to west in early to middle tertiary time a second depression the bimini embayment formed within bimini bank by folding it had a maximum depth of about 470 m this depression was also filled from east to west in addition prograding systems built the western margin of bimini bank more than 25 km westward into the straits of florida

partial high grade alteration of monazite using alkali bearing fluids experiment and nature recent advances in the dating of monazite using the electron microprobe indicate that thsio 4 and or cath po 4 2 enriched intergrowths in monazite can give ages younger than the original monazite the morphology of the intergrowths suggests that the original monazite grain has been partly altered by a fluid in which th si and ca are mobile this hypothesis has been tested in the piston cylinder apparatus at 1000 mpa and 900 c utilizing a natural unzoned th and pb bearing monazite ce and na 2 si 2 o 5 h 2 o during the experiment a subset of the monazite grains acquired thsi0 4 enriched areas with sharp compositional boundaries devoid of pb and show evidence of being a pseudomorphic partial replacement of the monazite rather than an overgrowth of the monazite these experiments support the hypothesis that similar th enriched or th depleted patches with sharp compositional boundaries observed in natural monazite could be the result of fluid induced alteration via coupled dissolution reprecipitation if so such altered regions would yield information concerning the nature of the fluid responsible for their formation as well as allow for the dating of single or multiple metasomatic events assuming that all pre existing radiogenic pb is removed during alteration

depositional facies and stratal geometry of an upper carboniferous prograding and aggrading high relief carbonate platform cantabrian mountains n spain seismic scale continuous exposures of an upper carboniferous bashkirian moscovian carbonate platform n spain provide detailed information about the lithofacies and stratal geometries quantified with differential global positioning system measurements of microbial boundstone dominated steep prograding and aggrading platform margins progradational and aggradational platform to slope transects are characterized by distinct lithological features and stratal patterns that can be applied to the understanding of geometrically comparable high relief depositional systems the bashkirian is characterized by rapid progradation at rates of 415 970 m my 1 characteristic outerplatform facies are high energy grainstones with coated intraclasts ooids and pisoids moderate energy algal skeletal grainstones to packstones and lower energy algal packstone and boundstone units the moscovian aggradational phase is characterized by aggradation rates of 108 m my 1 coated grain shoals are less common whereas crinoidal bars nucleated in well circulated settings below wave base boundstones form a belt 30 300 m wide at the platform break and interfinger inwards with massive algal skeletal wackestones mud rich banks the progradational phase has divergent outer platform strata with basinward dips of 12 to 2 steep clinoforms with dips of 20 28 are 650 750 m in relief and possibly sigmoidal to concave in the lower part the basinward dipping outer platform strata might be depositional for less than 6 consistent with lithofacies deepening seaward the basinward dip is attributed to the downward shift of upperslope boundstone forced by late highstand and relative sea level fall and to compaction induced differential subsidence during progradation the aggradational phase is characterized by horizontally layered platform strata clinoforms steepen to 30 45 reaching heights of 850 m and are planar to concave the evolution from progradation to aggradation at the bashkirian moscovian boundary is attributed to increased foreland basin subsidence and decreased boundstone accumulation rates progradation was primarily controlled by boundstone growth rather than by highstand shedding from the platform top within the major phases aggradational progradational increments are produced by third to fourth order relative sea level

quantitative compositional analysis of a triassic carbonate platform southern alps italy abstract we have estimated abundance and distribution of automicrite marine cements and skeletal grains in the triassic sella massif an isolated platform flanked by steep 25 35 clinoforms 108 samples were taken at constant intervals from measured sections of the major zones of the platform edifice the platform top margin upper slope and lower slope in a first step carried out in the field and on hand specimen purely detrital deposits were separated from automicrite facies i e beds with automicrite cement filled primary vugs and admixtures of skeletal carbonate and lithoclasts in the second step samples with automicrite facies were thin sectioned and point counted the categories used for point counting were a automicrite b vugs and cement c microspar or neomorphic spar d skeletal grains and e internal sediments at the platform top 46 of samples are pure detrital deposits 27 consist of automicrite facies and 27 are too strongly altered by dolomitization to allow classification at the margin upper slope 68 of samples consist of automicrite facies 22 are pure detrital sediments and 10 are strongly altered at the lower slope 63 are detrital deposits 10 automicrite facies and 27 are extensively dolomitized the most important contributors to the automicrite facies are automicrite 41 on the platform top 29 on the margin upper slope 28 on the lower slope and early marine cement 35 on the platform top 48 on the margin upper slope 27 on the lower slope the amount of skeletal grains is less than 10 the automicrite facies stabilized the platform margin and upper slope automicrite abundant early marine cements and micro organisms such as tubiphytes formed a rigid framework thus substituting for the lack of a metazoan reef on the upper slopes the framework of automicrite facies stabilized the slope but intermittently the automicrite layers are frequently dissected by sediment filled fractures or are broken into clasts we assume that they slid on the layers of loose detritus bigger slides turned into rubbly debris flows that formed metre thick breccias at the lower slope and the proximal basin floor the planar shape and steep angle of the clinoforms indicate that the large scale geometry of the slope was not controlled by the automicrite but rather by non cohesive layers of sand and rubble piled up to the angle of repose the production mode of the sella is comparable of that of a mud mound factory this factory was highly productive in 1 ma the platform aggraded over 300 m and prograded over 2000 m in all directions

geochronology and geochemistry of multiple generations of monazite from the wepawaug schist connecticut usa implications for monazite stability in metamorphic rocks u pb isotope analyses rare earth and trace element analyses and petrographic observations are presented for monazites from the wepawaug schist in southern connecticut usa two samples of kyanite zone schist were collected less than a meter apart each sample contains a different variety of monazite with distinct morphology chemistry and pb isotopic composition one sample preserves a largely amphibolite facies mineralogy including kyanite staurolite garnet biotite and chlorite with little textural evidence of later shearing monazites from this sample are xenoblastic with about 1 wt tho2 0 3 wt cao and a more lree enriched pattern than monazites from the second sample these xenoblastic monazites preserve textural evidence of a retrograde reaction to apatite which involves chlorite indicating that these monazites became unstable during retrograde chloritization of biotite these monazites give strongly discordant u pb ages which fit a chord with an upper intercept age of 411 18 ma interpreted as the minimum growth age of these xenoblastic monazites perhaps during amphibolite facies metamorphism the second sample contains s c banding evidence of dynamic recrystallization and abundant retrograde chlorite this sample contains idioblastic monazites with about 3 wt tho2 0 8 wt cao and with less fractionated ree patterns these monazites give close to concordant u pb ages with a mean 207pb 206pb age of 388 2 ma this age is interpreted as probably representing the time of monazite growth during retrogression of the sample from an amphibolite to greenschist facies mineralogy

neocimmerian events in central and western afghanistan abstract the neocimmerian orogenic belt of afghanistan is about 350 wide and lies between a northern eocimmerican orogenic belt which resulted from the closure of the paleotethys to the north and the alpine himalayan orogenic belt which resulted from the closing of the neotethys to the south this neocimmerian orogenic belt resulted from the closure of the waras penjaw oceanic realm at some time between the neocomian and the cenomano turonian the waras penjaw oceanic realm opened during or shortly after the triassic breaking off from continental asia where the ascent of the northern eocimmerian orogenic belt had recently ended it was located behind an active tectonic and magmatic arc which resulted from the neotethyan subduction during this opening the adjacent continental blocks were subjected to extension at that time the entire region was of the same type as the present japanese archipelago sea of japan eastern continental asia system located behind the western pacific subduction zone when the waras penjaw oceanic realm began to close its southern margin was activated the subsequent collision affected a vast region particularly the remnants of the northern eocimmerian orogenic belt this collision continued to act on the region during the neotethyan subduction shortly after these neocimmerian events the region was of the same type as the present southern andean region located between the south american continent and the eastern pacific subduction zone

microfacies and geochemistry of middle jurassic algal limestones from scotland bathonian algal limestones from the duntulm formation of the great estuarine group western scotland are subdivided into three microfacies cryptalgal laminites thrombolites and porostromate micrites the distribution of early diagenetic carbonate spherulites and cement fringes is fabric selective spherulites are common in the thrombolitic microfacies whereas cement fringes were best developed upon algal nodules with porostromate microstructure syndiagenetic micrites spherulites and cement fringes have stable carbon isotopic ratios indicative of abundant organic derived hco3 incorporation d13c values of 0 05 to 15 enhanced magnesium concentrations in these fabrics relative to neomorphic and burial diagenetic spar cements suggest an original mg calcite mineralogy probably with precipitation under the influence of microbial systems brecciated early diagenetic fabrics and localized pockets of calcite pseudomorphs after gypsum were probably formed during subaerial exposure sulphate supply was probably from storm derived saline lagoon water washed on to supralittoral stromatolitic flats these inundations also allowed the temporary establishment of green algae and encrusting foraminifera the combined geological and geochemical information suggests a palaeoenvironment somewhat similar to recent sub tropical humid zone calcareous algal marshes

formation of monazite via prograde metamorphic reactions among common silicates implications for age determinations three lines of evidence from schists of the great smoky mountains nc indicate that isogradic monazite growth occurred at the staurolite in isograd at 600 c 1 monazite is virtually absent below the staurolite in isograd but is ubiquitous several hundred grains per thin section in staurolite and kyanite grade rocks 2 many monazite grains are spatially associated with biotite coronas around garnets formed via the reaction garnet chlorite muscovite biotite plagioclase staurolite h2o 3 garnets contain high y annuli that result from prograde dissolution of garnet via the staurolite in reaction followed by regrowth and rare monazite inclusions occur immediately outside the annulus and in the matrix but not in the garnet core larger monazite grains also exhibit quasi continuous th zoning with high th cores and low th rims consistent with monazite growth via a single reaction and fractional crystallization during prograde growth common silicates may host sufficient p and lrees that reactions among them can produce observable lree phosphate specifically phosphorus contents of garnet and plagioclase are hundreds of parts per million and dissolution of garnet and recrystallization of plagioclase could form thousands of phosphate grains several micrometers in diameter per thin section lrees may be more limiting but sheet silicates and plagioclase can contain tens to 100 ppm lree so recrystallization of these silicates to lower lree contents could produce hundreds of grains of monazite per thin section monazite ages determined via electron and ion microprobes are 400 ma directly linking prograde barrovian metamorphism of the western blue ridge with the acadian orogeny in contrast to previous interpretations that metamorphism was taconian 450 ma interpretation of ages from metamorphic monazite grains will require prior chemical characterization and identification of relevant monazite forming reactions including reactions previously viewed as involving solely common silicates

the composition and formation of miocene tholeiites in the central european cenozoic plume volcanism cecv tholeiites accompanying a majority of alkali basalts are restricted to the highly productive central part of the cecv plume activity in vogelsberg and hessian depression they mainly occur as quartz tholeiites which according to experiments of partial melting and material balances are products of olivine tholeiitic primary melts the differentiation from olivine to quartz tholeiitic melts took place in lower crustal magma chambers where olivine tholeiitic melt intruded due to a density comparable with that of the country rocks the fractionation due to separation of olivine and some clinopyroxene caused contamination of tholeiite magmas by tonalitic partial melts from the wall rocks of the magma chambers the latter process is indicated by relatively high rb k and pb and low nb concentrations and by nd sr and pb isotopes contaminating crustal melts which roughly attained a proportion of 10 contained very low 143nd 144nd ratios from a nd sm fractionation as old as 2 6 ga this is the first evidence from mafic rocks of this high age in the lower crust beneath central europe modelling with incompatible elements allows to recognize olivine tholeiites as products of about 1 partial melting of plume rocks consisting of 35 primitive and 65 depleted mantle materials the production of tholeiites other than alkali basalts is restricted to the highest plume activity and the largest fraction of morb type source rocks

analytical perils and progress in electron microprobe trace element analysis applied to geochronology background acquisition interferences and beam irradiation effects electron probe microanalysis epma of accessory minerals such as monazite xenotime and thorite for minor and trace element concentrations and geochronology requires consideration of beam irradiation effects increasing heat and charge as higher current densities and lengthy counting times are employed and requires careful detailed assessment of background intensities and interferences a carbon coat 250 a thickness is generally inadequate for prevention of absorbed current s uctuation and beam damage when using the high current densities applied for high precision e g 200 na focused beam beam irradiation effects include element mobility in monazite resulting in p loss relative to ree coating materials of higher electrical and thermal conductivity are indicated and use of gold 100 a is strongly suggested systematic compositional and therefore age variability can result simply from analytical effects requiring evaluation of all aspects of data acquisition the spectra relevant to measurement of y th pb and u are complex especially in ree bearing minerals acquisition of detailed spectral wavelength scans allows recognition of background and peak interferences as well as curvature background intensities can be extracted directly from scan data by regression x ray mapping allows delineation of domains guiding background acquisition and detailed quantitative analysis minor substitution or s uorescence of unexpected elements can compromise analyses one documented effect being the s uorescence of k in monazite adjacent to or hosted by k feldspar or micas this effect clearly evident within 10 micrometers from k feldspar can result in erroneous u concentrations leading to misinterpretation of rim ages as younger overgrowths absorption edges associated with th also become relevant to the measurement of u at high th concentrations because background intensity is sensitive to variation in average atomic number backgrounds must be acquired from each identi ed domain with particular attention being paid to th variation misapplication of background intensities can result in large age discrepancies for example application of backgrounds obtained from a high th domain in monazite to measurement of a low th domain 1 3 of the amount in the high th domain results in an overestimation of the um background intensity of 0 008 cps na and an overall increase in age of 70 m y

potential minerals for determining u th pb chemical age using electron microprobe abstract after a decade of studies and development it is now accepted that reliable u th total pb isochron ages can be calculated for monazite using an electron microprobe at m scale either directly on thin sections or on separated grains mounted in polished section the potential for determining u th pb chemical ages from other u and th enriched phases has been investigated compared to chemical monazite dating results for which individual spot age precisions of 20 to 100 ma can be achieved from individual spot analyses using isochron plots for monazite the age homogeneity of a given population of data can be assessed and depending upon the number of analyses n 50 a precision of 5 to 10 ma can be obtained the u content in xenotime widely varies from less than 0 1 wt up to 3 wt but th rarely exceeds 1 wt as a consequence the amount of radiogenic pb produced during a given period remains significantly lower for xenotime than for monazite leading to a lower precision 20 ma on the mean ages xenotime however appears to remain as a closed system but common pb must be carefully checked furthermore the electron microprobe technique epma allows controlling any age discrepancy on xenotime grains as small as 10 20 m that cannot be dated by other isotopic methods such xenotime ages can be useful when studying the monazite xenotime equilibrium the electron microprobe is not the most reliable method for dating zircon since u and th concentrations are generally low and common pb is not negligible nevertheless the spatial resolution of epma coupled with isotope methods allows conclusive in situ studies about radiogenic pb mobility and metamictization thorite does not seem suitable for dating with either isotope methods or epma because of continuous radiogenic pb loss conversely the oxide phases thorianite and baddeleyite are robust minerals with closed systems they are rather rare and seem to incorporate negligible common pb making epma a method of choice for dating them for thorianite the precision on the mean age can be similar as that obtained for monazite or even better while the precision for baddeleyite cannot be significantly better than 20 to 50 ma due to the limited amount of u 0 1 and the lack of th

facies geometry and growth phases of the valdorria carbonate platform pennsylvanian northern spain the 14 km wide valdorria outcrop pennsylvanian northern spain is one of the few examples of entirely exposed flat topped and high relief carbonate platforms reported in the fossil rock record laterally and vertically traceable stratal patterns expose three phases of growth phase i is a 430 m thick platform to slope succession that prograded over 6 km and is dated as early bashkirian akavasian askynbashian phase ii aggraded and prograded exhibiting 180 m thickness of cyclical platform top deposits dated as late bashkirian asatauian phase iii is a mound shape structure that developed over the platform top of phase ii as a new phase of platform nucleation it is 535 m thick and 2 km wide and dated as late bashkirian asatauian transition interval the observed changes of growth styles during platform evolution from a prograding to an aggrading prograding system and a rapid aggradational phase are inferred to be controlled by flexural subsidence in the active cantabrian foreland basin at the variscan orogenic front the metre scale shallowing upward cycles of the platform top are most probably due to glacioeustasy as evidenced by well recorded subaerial exposure surfaces superimposed on subtidal deposits and by a stratal pattern recurrent in a short interval of about 160 kyr observations of outcropping bashkirian cyclothems in an isolated carbonate system devoid of siliciclastic input are relevant for a better understanding of the impact of high frequency sea level fluctuations on the carbonate factory moreover progradation of the platform margin during phase i reaches a rate of 2500 m myr and 1810 m myr during phase ii rates that are high when compared to other pennsylvanian examples the aggradation rate of 447 m myr calculated for late bashkirian transition interval phases i and ii uncorrected for compaction missing beats and erosion is uncommonly high in comparison to coeval pennsylvanian examples the platform exhibits a self nourishing prograding microbial boundstone dominated slope thus the slope shedding model applies well to valdorria however phase ii recorded eustatic variations able to inhibit the slope microbial boundstone factory during low sea level stands this is marked by common slope red stained breccias synchronous to platform top subaerial exposure phases contrarily periods of relative high sea level and rapid subsidence in phase iii registered a greater development of cemented microbial boundstone these observed partly opposing relationships of sea level stands shedding modes and slope architecture provide an improvement of the currently used slope shedding model the overall architecture of the valdorria outcrop compares well with that of other contemporaneous platforms such as sierra del cuera and bolshoi karatau valdorria shares the high relief and flat topped steep slopes cyclothemic patterns and occurrence of karst features with the pricaspian basin platforms tengiz karachaganak and kashagan with minor variations in facies distribution of the internal platform furthermore the continuous seismic scale outcrop of valdorria together with its isolated setting and asymmetrical growth makes it an additional good candidate for potential subsurface analogues of hydrocarbon bearing systems

metamorphic evolution of the precambrian south badakhshan block based on mineral reactions in metapelites and metabasites associated with whiteschists from sare sang western hindu kush afghanistan abstract textural relations and metamorphic fabrics of metapelites and associated metabasites from the precambrian sare sang series which is known to contain lapis lazuli deposits and whiteschists indicate an increase in metamorphic pressures and temperatures to the transition of eclogite amphibolite facies boundary and a subsequent retrograde p t trajectory to amphibolite facies conditions the metapelites are characterized by the presence of garnet biotite plagioclase k feldspar and al2sio5 phases showing the p t path involved change by compression from sillimanite to kyanite stability fields and subsequent decompression along an overall clockwise loop the metabasites contain garnet hornblende and plagioclase biotite and clinopyroxene plagioclase symplectites that might represent a replacement product after omphacite maximum p t conditions of 12 13 kbar at 700 750 c were deduced from thermobarometric calculations peak metamorphism was accompanied by dehydration melting of muscovite but not biotite followed by minor back reaction of garnet k feldspar h2o al2sio5 biotite plagioclase retrograde phases in metapelites and metabasites are muscovite calcite and fluorine and or chlorine rich amphiboles and biotite which probably resulted from infiltration of fluid that had equilibrated with surrounding metacarbonates and metaevaporites

the margins of laurussia in central and southeast europe and southwest asia the continuation of the laurussian margin into central and southeast europe and southwest asia is defined by the rheic suture which becomes harder to trace to the southeast because of complex post variscan strike slip displacements and tectonic stacking linked to the cimmerian and alpine cycles the uncertain extent of the neoproterozoic baikalian orogeny fringing baltica ambiguous early paleozoic paleobiogeographic data and strike slip activity complicate the distinction of the tornquist suture from the transeuropean suture zone there is growing evidence for the incorporation of the brunovistulian malopolska and west moesian terranes into the late neoproterozoic cambrian baltican margin the istanbul zonguldak terrane along with the bitesh and east moesia terranes may have been part of the avalonian terrane assemblage although an arabian nubian shield or baltica provenance cannot be excluded the terranes of the armorica terrane assemblage bordering the rheic suture include the outer carpathian tatra getic and balkan terranes in southeast europe and sakarya and the anatolide tauride terrane in turkey further east the rheic suture may be present in the caucasus in all probability between the greater caucasus and the scythian platform or alternatively between the greater caucasus and transcaucasia alborz in northern iran is interpreted as a distal part of transcaucasia the location of the rheic suture in the southeast part of the turan domain of central asia is poorly constrained

monazite paragenesis and u pb systematics in rocks of the eastern mojave desert california u s a implications for thermochronometry abstract studies of the paragenesis and u pb systematics of monazite in rocks from the eastern mojave desert california corroborate its potential usefulness as a prograde thermochronometer and in dating granite inheritance unmetamorphosed latham shale and its equivalents at grades ranging from greenschist to upper amphibolite facies are virtually identical in composition monazite is absent in the shale and low grade schists but it is abundant in schists at staurolite and higher grades lower grade schists instead include minute th and ce oxides and unidentified ce poor lree phosphates that apparently are lower temperature precursors to monazite thus monazite originates when the pelite passes through lower amphibolite facies conditions monazites from three upper cretaceous granites yield ages that are strongly discordant upper intercepts of 1 6 1 7 ga are similar to those defined by u pb data for coexisting zircons and coincide with a period of copious magmatism in the mojave crust as the host upper cretaceous granitic magmas were all above 700 c effective closure of the restitic monazites to pb loss must be well in excess of this temperature u pb compositions of monazite from proterozoic granitoids and schist also indicate high pb retentivity taken together these studies support the suggestion that monazite can be an effective prograde thermochronometer at least in pelites it is not usually retained as a detrital mineral but rather forms during moderate temperature metamorphism its u pb system should not be reset by subsequent higher grade metamorphism

electron microprobe dating of monazite because monazite is extremely rich in u and th radiogenic pb pb accumulates very quickly and reaches in about 100 ma a level where it is possible to analyse it with the electron probe assuming that common pb is negligible and that partial loss of pb has not occurred the simultaneous measurement of u th and pb allows to obtain a geologically meaningful age from a single electron probe analysis here we present the results of two years of systematical investigations aiming to define both the limits and potential of this method a specific statistical method to deal with the large number of data which can be obtained on a single sample is described and several guidelines illustrated by examples are suggested to optimize the method electron probe measurements carried out on samples of known age from 200 ma to 3 1 ga yield ages that always fall inside the confidence interval of the isotopically determined age demonstrating that this method is reliable the younger age limit is approximately 100 ma although it can be younger in some favourable cases in old monazites extremely high pb contents have been found up to 5 wt indicating that monazite can tolerate high radiation doses without experiencing lead loss the final precision on the age for a normal monazite is 30 50 ma for a total counting time of 600 s a complete dating procedure can be completed in less than 1 h first results indicate that old ages can be preserved in monazite either in small relict cores in crystals or by the coexistence of several generations of monazites in a sample this method has all the advantages of the electron probe it is non destructive has an excellent spatial resolution monazites as small as 5 i m can be dated and because it is possible to work on normal polished thin sections the petrographical position of the dated crystal is known this method offers a large number of geologists access to an in situ dating technique at moderate cost

five generations of monazite in langtang gneisses implications for chronology of the himalayan metamorphic core monazite grains from greater himalayan sequence gneisses langtang valley nepal were chemically mapped and then dated in situ via th pb ion microprobe analysis correlation of ages and chemistry reveals at least five different generations of monazite ranging from c 9 to 300 ma petrological models of monazite chemistry provide a link between these generations and the thermal evolution of these rocks yielding an age for the melting of greater himalayan rocks within the main central thrust sheet c 16 ma and for the timing of thrust sheet emplacement that are younger than commonly viewed chemical characterization of monazite is vital prior to chronological microanalysis and many ages previously reported for monazite from the greater himalayan sequence are interpretationally ambiguous

calcium diffusivity in alumino silicate garnets an experimental and atem study concentration gradients in calcium are common in metamorphic or magmatic garnets and can be used to determine the timescales of geological processes however the kinetics of ca diffusion in garnet is poorly constrained and experimental studies have to date yielded widely varying diffusion coefficients in this paper we describe a new method for generating diffusion profiles in garnet we incorporated polished and compositionally homogeneous garnet seeds in a finely ground powder of clinopyroxene and garnet during the experiments 1 3 gpa 1 050 1 250 c and o2 the graphite o2 buffer the mineral powder partially melted recrystallized and formed a 10 50 m wide overgrowth zone of compositionally distinct garnet around the seeds long duration experiments generated measurable relaxation profiles at these seed overgrowth interfaces we performed analytical transmission electron microscope traverses across the interfaces in each experiment thirteen usable compositional profiles were obtained with characteristic distances of diffusion ranging from 300 to 1 000 nm from these profiles ca fe mg interdiffusion coefficients were retrieved using an analytical solution for the diffusion equation and the data were cast in an arrhenius relation linear regression of the data yields an activation energy q ca fe mg equal to 188 48 kj mol 1 and a frequency factor d 0 equal to 6 6 10 14 m2 s 1 within the compositional range studied the composition of garnet has no major effect on the ca fe mg interdiffusion coefficient the very slow diffusion rate of ca is in agreement with natural observations indicating that ca diffuses more slowly than fe and mg the ca diffusion coefficients derived from this study are not model dependent and can be used to determine the durations of geological events from ca relaxation profiles in natural garnets

hercynian and eocimmerian events in afghanistan and adjoining regions abstract the area centered around afghanistan is a collage of continental blocks derived from gondwanaland this collage comprises among others 1 1 a hercynian orogenic belt widely represented in afghanistan and also by some sparse outcrops in the northern iran it is a subduction type orogen constructed at the margin between the paleotethys and the turan turkestan tarim block 2 2 a northern eocimmerian orogenic belt widely represented in afghanistan and the north of iran it is a collision type orogen constructed in the same place as the hercynian orogenic belt and resulted from the closure of the paleotethys and the collision of the central iran and central afghanistan blocks with the turan turkestan tarim block 3 3 a southern eocimmerian orogenic belt of which outcrops are now confined to southern iran but which may project beneath the neogene and quaternary formations of southern afghanistan it is a subduction type orogen constructed at the margin between the neotethys and the central iran central afghanistan block which itself was at that time subjected to regional extension this belt is now incorporated in the alpine himalayan orogen the geology of these belts suggests that the opening of the neotethys became effective only from the permian or triassic and was in a red sea type phase during the upper devonian its initial rifting possibly occurred during the lower paleozoic much like the paleotethys in which initial rifting also occurred in lower paleozoic time and whose opening became effective from the ordovician thus each of the oceanic realms in this region appears to have possibly resulted from the same extension of gondwanaland during the lower paleozoic after which they followed more or less different paths of evolution moreover the proposed geodynamic interpretation implies that during the permian and triassic there was in this region a certain coexistence of the vanishing paleotethys with the relatively young neotethys which was beginning to subduct

titaniferous magnetite ilmenite thermometry and titaniferous magnetite ilmenite orthopyroxene quartz oxygen barometry in granulite facies gneisses bamble sector se norway implications for the role of high grade co 2 rich fluids during granulite genesis oxygen fugacities have been estimated for a wide distribution of samples from the granulite facies terrane region c of the bamble sector se norway using both the titaniferous magnetite ilmenite and orthopyroxene titaniferous magnetite quartz oxygen barometers these oxygen fugacities are estimated using temperatures calculated from the titaniferous magnetite ilmenite thermometer of ghiorso and sack 1991 and are both internally consistent with each other as well with the thermometer in samples for which the estimated temperature is high the two oxygen barometers show good agreement whereas agreement is poor for low temperature samples in these low temperature samples oxygen fugacities estimated from titaniferous magnetite ilmenite are considerably less than those estimated from orthopyroxene titaniferous magnetite quartz an increase in this discrepancy with decrease in temperature appears to reflect preferential resetting of the hematite component in the ilmenite grains without significant alteration of the more numerous titaniferous magnetite grains this is due in part to greater re equilibration of the ilmenite grains during retrograde interoxide resetting between the ilmenite grains and the titaniferous magnetite grains the mean temperature for the non reset samples 791 17 c 1 is in good agreement with temperatures obtained from garnet orthopyroxene kd exchange thermometry in the same region 785 795 c 1 harlov 1992 2000a most non reset oxygen fugacities range from log10 f o2 14 to 11 8 or approximately 0 5 1 5 log units above quartz fayalite magnetite at 7 5 kbar both these temperatures and the range of oxygen fugacities are in good agreement with those estimated using the titaniferous magnetite ilmenite thermometer oxygen barometer of andersen et al 1991 the quilp equilibrium quartz ulvospinel ilmenite pyroxene is used to project self consistent equilibrium temperatures and oxygen fugacities for samples reset due to hematite loss from the ilmenite grains these projected temperatures and oxygen fugacities agree reasonably well with the non reset samples the mean projected quilp temperature is 823 6 c 1 this result supports the conclusion that low titaniferous magnetite ilmenite temperatures down to 489 c and accompanying low oxygen fugacities are the result of hematite loss from the ilmenite grains non reset oxygen fugacities lie approximately 1 5 log10 units above the upper graphite stability curve indicating that the stable c o h fluid phase interacting with these gneisses whether regionally or locally was co2 this is borne out by the presence of numerous co2 rich fluid inclusions in these rocks

non ideal mixing in the phlogopite annite binary constraints from experimental data on mg fe partitioning and a reformulation of the biotite garnet geothermometer the existing experimental data ferry and spear 1978 perchuk and lavrent eva 1983 on mg fe partitioning between garnet and biotite are disparate the underlying assumption of ideal mg fe exchange between the minerals has been examined on the basis of recently available thermochemical data using the updated mixing parameters for the pyrope almandine asymmetric regular solution as inputs ganguly and saxena 1984 hackler and wood 1984 thermodynamic analysis points to non ideal mixing in the phlogopite annite binary in the temperature range of 550 c 950 c the non ideality can be approximated by a temperature independent one constant margules parameter the retrieved values for enthalpy of mixing for mg fe biotites and the standard state enthalpy and entropy changes of the exchange reaction were combined with existing thermochemical data on grossular pyrope and grossular almandine binaries to obtain geothermometric expressions for mg fe fractionation between biotite and garnet t in k begin gathered text t hw 20286 0 text 0193p 2080 x text mg text gt text text 2 text 6350 x text fe text gt text text 2 hfill text 13807 x text ca text gt text 1 x text mn text gt text 8540 x text fe text gt text x text mg text gt text 1 x text mn text gt text hfill text 4215 x text ca text gt text x text mg text gt text x text fe text gt text 4441 text 2x text mg text bt text 1 mathord left vphantom text 2x text mg text bt text 1 text 13 text 138 right kern nulldelimiterspace text 13 text 138 hfill text 8 text 3143 ink text d text 6 text 276 x text ca text gt text 1 x text mn text gt hfill text t gs 13538 0 text 0193p 837 x text mg text gt text 2 text 10460 x text fe text gt 2 hfill text 13807 x text ca text gt 1 text x text mn text gt text 19246 x text fe text gt text x text mg text gt text 1 x text mn text gt hfill text text 5649 x text ca text gt text x text mg text gt text x text fe text gt text 7972 2x text mg text bt text 1 mathord left vphantom text 5649 x text ca text gt text x text mg text gt text x text fe text gt text 7972 2x text mg text bt text 1 text 6 text 778 right kern nulldelimiterspace text 6 text 778 hfill text 8 text 3143ink text d text 6 text 276 x text ca text gt 1 text x text mn text gt hfill end gathered the reformulated geothermometer is an improvement over existing biotite garnet geothermometers because it reconciles the experimental data sets on fe mg partitioning between the two phases and is based on updated activity composition relationship in fe mg ca garnet solid solutions

thermodynamics of the amphiboles ca mg fe2 quadrilateral a thermodynamic model has been developed and calibrated for monoclinic and orthorhombic amphiboles compositionally contained within the ca mg fe 2 amphibole quadrilateral the model incorporates the energetic consequences of cation ordering of fe 2 and mg over nonequivalent sites in the crystal structure and accounts for the temperature pressure and compositional dependence of the orthorhombic monoclinic phase transition calibration is based on previously published work on the thermodynamic properties of fe 2 mg amphiboles and tremolite and experimental cation ordering data along with solvus width and tieline orientations of natural coexisting amphiboles in the quadrilateral among derived parameters is the enthalpy of formation of end member ferroactinolite 10534 966 kj mol a calculated femg 1 isopotential solvus at mid composition in the quadrangle agrees well with a revised calibration of experimental data from cameron 1975 the solvus is not strongly asymmetric and narrows with increasing fe mg ratio and temperature phase relations along the mg and fe sides of the quadrilateral are compared and contrasted cummingtonite anthophyllite phase relations are shown in the t x f e plane with and without coexisting calcic amphibole the breakdown reactions of quadrilateral amphiboles to assemblages of two pyroxenes olivine quartz and h 2 o are depicted as functions of temperature pressure composition h 2 o activity and oxygen fugacity some highlights of their relevance to metamorphic and igneous systems are discussed

mixed carbonate siliciclastic sedimentation on a miocene fault block gulf of suez egypt abstract the miocene sediments at abu shaar red sea egypt include both carbonates and detrital silicates whose distribution is closely related to a pre miocene basement fault block as such they demonstrate the relationships between local distensional tectonics and sedimentation because of the vertical bathymetric relief of the basement block the carbonates include both a spectacular talus and massive platform facies in spite of the presence of scattered corals sedimentation is not truly reefal as deposits consist essentially of muddy bioclastic and silicate sands the most typical ecological elements are numerous spectacular stromatolites rigidity of the sedimentary mass was mainly induced by early marine cementation carbonate siliciclastic transitions on and around the abu shaar platform were determined mainly by basement morphology and related pre miocene tectonics

shallow level processes in ocean island magmatism editorial surface some of the most detailed and comprehensive magmatism at ocean islands typically has been associated recent studies of individual ocean island magma systems with deep rooted mantle plumes rarely these are cosuggest however that many of the important criteria incident with oceanic spreading centres e g iceland that we use to infer mantle sources may be compromised but more commonly they form intra plate hotspots e g by interaction between magmas and the oceanic and hawaii canary islands if magmatism at ocean islands island crust through which they ascend the occurrence is indeed intimately linked with mantle plumes and of contamination at ocean islands has been deemed less these plumes are derived from great depths 670 km likely than in continental settings where magmas pass discontinuity or core mantle boundary effectively conthrough thicker lower density crust this contention is stituting the major mode of deep upwelling in the global apparently supported by the numerous case studies of mantle convection system then their surface manicontamination of continental magmas the difference in festation should provide some of our best constraints on the observable chemical consequences of contamination the composition and nature of the deep mantle in in the two environments must be appreciated in the contrast the dominant volume of present day volcanic context of the compositional distinctions of the crust activity which is concentrated along plate boundaries the compositional contrast between continental crust provides little definitive information on the deep mantle and mantle derived magma is commonly large and over the past 20 years considerable progress in our therefore the effects of contamination in terms of standunderstanding of the composition of the upper mantle ard indicators such as isotope ratios are readily aphas been inextricably linked to advances in analytical preciated in contrast basaltic magmas erupted on ocean geochemistry particularly the use of isotope and inislands pass through dense basaltic crust and any comcompatible trace element ratios in oceanic basalts as positional differences between magma and contaminant tracers of their mantle sources early studies have now may be subtle it is important to point out however that been followed up by a number of very detailed volthe potentially subtle effects of contamination do not canological petrological geochemical studies of inimply that such effects are irrelevant in the grand picture dividual oceanic islands although hawaii has of mantle geodynamics which itself is dependent on traditionally served as our de facto model of ocean island compositional subtlety magmatism by virtue of the sheer volume of geochemical given the large volume of high quality data and the and geophysical data accumulated there we now realize large number of studies of individual ocean islands that that a spectrum of volcano types and compositions exists now exist the time has come to ask what effects shallowand these have been used to map mantle domains to infer level processes at ocean islands may have in modifying processes of partial melting and to calculate recycling magma compositions this question was addressed at budgets between crust and mantle a chapman conference sponsored by the american the use of ocean island basalt oib geochemistry geophysical union from 9 to 16 november 1996 in to define mantle sources and partial melting processes tenerife canary islands the conference was attended depends implicitly on the assumption that the primary by 45 scientists from seven nations representing interests magmas are not significantly modified other than by closed system fractional crystallization en route to the varying from isotope geochemistry to geophysics this

the importance of microbial binding in neogene quaternary steep slopes abstract the geometry and inclination of carbonate platform slopes are typically described as the result of the sediment texture components and grain size occurring in the slope three carbonate slopes from the holocene of the tongue of the ocean bahamas and three slopes from an upper miocene carbonate platform in se spain are investigated to identify the factors controlling dip angles although platforms differ in size and elevation with regard to the surrounding basin both locations present slopes with linear profiles and steep angles over 35 the slope facies in both locations are very similar with the matrix and some bioclasts presenting microfabrics that evidence microbial activity during deposition these include 1 clotted micrite patches locally connecting bioclasts or infilling primary pores 2 porostromate structures in the micrite 3 dense micritic masses 4 binding structures 5 micritic crusts and 6 peloidal textures the holocene and miocene depositional geometries and facies distribution are the response to various sedimentary processes including rockfalls gravity flows and in situ carbonate production extensive microbial binding is identified as the controlling factor for early stabilization of the slope which in combination with subsequent rapid cementation produces and maintains the steep slope angles consequently microbial induced stabilization of modern and neogene slopes is as significant as in paleozoic and triassic examples

major intrusive stages in afghanistan typology age and geodynamic setting afghanistan straddles the gondwana eurasia boundary most of its major structural areas comprise large plutonic belts based on some 700 major element analyses the typology of the eleven main plutonic belts has been determined these belts differ from each other on the whole they display a very broad range of petrographic types from nepheline syenites and gabbros to leucocratic muscovite tourmaline granites and a wide diversity of magmatic associations from cafemic or alumino cafemic associations of various types tholeiitic calc alkaline subalkaline i e monzonitic alkaline peralkaline to various aluminous ones taking into account the typology of the belts and chronological data four major intrusive stages have been recognized and related to the geodynamic evolution of afghanistan and central asia 1 aluminous lower palaeozoic ca 500 ma stage related to linear crustal thinning at the gondwana scale 2 cafemic and alumino cafemic triassic ca 230 ma stage related to southward dipping subduction of oceanic crust beneath an island arc involving a large continental strip detached from the southern asia margin 3 cafemic and alumino cafemic cretaceous ca 100 ma stage dominantly related to subduction of oceanic crust under marginal gondwana fragments 4 eocene oligocene stage ca 35 ma induced by the india eurasia collision including two quite different groups of intrusives a a cafemic and alumino cafemic group related to extensional phenomena dominantly associated with the westward extrusion of central afghanistan along the herat fault these phenomena occurred on both sides of the gondwana palaeozoic asia boundary b an aluminous group resulting from crustal thickening in marginal gondwana fragments

petrochronology based on ree minerals monazite allanite xenotime apatite ree minerals monazite xenotime and allanite are ree1 minerals sensu stricto because lanthanides la lu and yttrium are critical constituents in them apatite does not require ree but because it contains substantial ree in many rocks it is included in this review all four minerals also host unusually high radionuclide concentrations notably th and u forming the basis of their utility as geochronometers this quartet of accessory minerals is playing an increasingly important role in petrochronology because they provide ways to link robust spot ages to petrogenetic p t conditions so can lend petrogenetic context to chronology based on other minerals part i of this review assembles the basic requisites prior to integrative petrochronologic analysis individual characteristics of the four ree minerals are addressed first i e their crystal chemistry and stability relations thermobarometers and trace element geochemistry used for tracing petrogenesis are discussed next and finally their chronology is summarized part ii presents case studies to highlight the specific strengths of ree minerals used to resolve the dynamics of a broad range of processes from diagenetic to magmatic conditions finally a brief section at the end outlines a few of the current challenges and promising perspectives for future work to introduce the four ree minerals in style let us recall the origins of their names the three phosphates have well respected greek grandparents and allanite has solid scottish roots yet of all four of them show idiosyncracies in etymology or type material apatite had long puzzled naturalists as it shows great chemical and physical variability and can resemble other minerals once properly identified abraham gottlieb werner named it apatite his reasoning referred to the greek root and giving the precise latin translation decipio taken literally both mean i deceive or i mislead which sounds like an apt confession from this mineral for having fooled

mesozoic ophiolites sutures and arge scale tectonic movements in afghanistan abstract the tectonic history of afghanistan appears to be the result of successive accretion of fragments of gondwana to the active margin of laurasia since the end of the paleozoic the margin in afghanistan lies along the present herat and panjshir faults south of hindu kush swings around the central pamirs and can presumably be traced along the present western altyn tagh and kunlun faults in tibet north of this boundary paleozoic rocks have been deformed in the upper paleozoic whereas south of it there is no trace of the hercynian orogeny the first collision of gondwanian fragments with laurasia probably occurred in the early mesozoic along the hindu kush and kunlun to the south ophiolites along the panjao and pangong nu chiang sutures respectively in central afghanistan and central tibet testify for another suturing event in the upper jurassic or lower cretaceous the indus tsangpo suture between india and tibet corresponds in eastern afghanistan to two ophiolite subbelts near kabul and khost both ophiolite complexes have been emplaced between maestrichtian and lower eocene by choking of two northward dipping subduction zones after complete contact between the indian and asian continents was achieved presumably in the end of eocene the penetration of india into asia caused large scale intra continental shortening a large part of the shortening was accommodated by strike slip faulting along mesozoic and more ancient sutures central afghanistan in particular was extruded to the west along the herat suture by the protrusion of the pamir wedge it subsequently collided with the lut block

complexity in the behavior and recrystallization of monazite during high t metamorphism and fluid infiltration abstract a detailed study of monazite grains in silica undersaturated and quartz bearing mg al metapelite from the oygarden group of islands east antarctica reveals a complex history of growth and recrystallization during two separate events in the neoproterozoic and earliest cambrian monazite grains from garnet poor and garnet rich metapelite preserve core domains that have ages corresponding to growth and or recrystallization at granulite facies p 9 1 0 gpa t 850 900 c conditions during the rayner structural episode between 930 and 890 ma high th rims 22 wt tho 2 that are in textural equilibrium with sapphirine orthopyroxene symplectites formed after garnet during late rayner decompression occur on monazite grains in garnet rich assemblages and give electron microprobe ages 883 18 ma within error of core domains 903 14 ma these high th rim domains are interpreted to have formed through recrystallization of liberated inclusions via a process dominated by coupled dissolution reprecipitation reactions facilitated by transient fluid films on the large surface areas in the symplectite and not by new growth in garnet poor metapelite monazite grains that have grain boundaries in textural equilibrium with the granulite facies assemblage also show alteration to higher th compositions on rims and along vein like fractures this compositional shift is accompanied by partial to complete resetting of ages to 500 ma along with minor modification of core domain ages textures and patterns of chemical age resetting are interpreted to also be the result of a coupled dissolution reprecipitation reaction process but in contrast to garnet bearing assemblages this alteration occurred during a fluid influx at lower temperatures during regional pan african tectonism this study highlights the susceptibility of monazite to resetting with fluid present conditions dramatically increasing the potential for recrystallization

evolution of the eastern karakoram metamorphic complex ladakh nw india and its relationship to magmatism and regional tectonics abstract the eastern karakoram terrane nw india records crustal evolution in the core of the himalayan tibetan orogen recent debate has centred on whether prograde metamorphism anatexis and leucogranite emplacement were the result of localised shear heating and magma advection within the karakoram fault zone kfz or instead predate the kfz and were the result of regional tectonometamophic events inclusions within andalusite porphyroblasts that grew during 15 7 ma leucogranite emplacement have fabrics that are random or discordant to the kfz matrix foliation indicating that the kfz initiated after this time therefore earlier anatexis and metamorphism are the result of regional metamorphic events amphibole plagioclase thermobarometry on a c 17 ma migmatite melanosome later exhumed within a transpressional zone of the kfz shows that melting occurred at 688 c and 522 mpa amphibolites record an older kyanite grade metamorphic event that occurred at 677 736 c and 875 1059 mpa metapelites also record a kyanite grade event which is constrained by ti in biotite thermometry to have occurred at 622 c and 650 mpa the tectonometamorphic history of the eastern karakoram correlates closely with that of the central karakoram away from the kfz this correlation supports the interpretation that metamorphism and anatexis were regional in extent and also indicates a limited offset of

open system processes in the differentiation of mafic magma in the teide pico viejo succession tenerife oceanic island basalts are commonly thought to differentiate by fractional crystallization yet closed system fractionation models have so far failed to reproduce major and trace element variations observed in mafic lavas from the teide pico viejo stratovolcano complex on tenerife here new high precision plagioclase trace element data are fed into such a fractionation model the results confirm that fractionation of phenocrysts found in the lavas does not reproduce trace element variations in particular enrichment of sr and zr observed in the teide pico viejo mafic suite this enrichment of sr and zr is tested by an energy constrained recharge assimilation and fractional crystallization ec rafc model at high t and low t intervals consistent with previously determined magma storage beneath tenerife at sub moho depths published mineral melt equilibrium relations using the plagioclase anorthite content 0 4 x an supplementary material detailed average mineral compositions of low and high sr zr lavas composition of the bulk extracts and full graphs of the recalculated closed system model are available at www geolsoc org uk sup18604

formation of authigenic monazite ce to monazite nd from upper carboniferous graywackes of the drahany upland roles of the chemical composition of host rock and burial temperature abstract irregular aggregates of monazite ce to monazite nd up to 150 m in diameter have been found in graywackes from the protivanov formation drahany upland czech republic monazite aggregates enclose a variety of detrital and authigenic minerals textural evidences suggest that they are of authigenic origin and grew during very low grade metamorphism at a maximum temperature 200 300 c in relation to a deep burial rarely they enclose relics of detrital monazite with a chemical composition typical for high grade monazite intensive fluid related ree remobilization occurred in relation to this very low grade metamorphism authigenic monazite appears to form only in ca poor rocks in ca rich graywackes which underwent the same low temperature metamorphic overprint authigenic synchysite ce or parisite ce are common instead in the overlaying rozstani and myslejovice formations t 150 200 c remobilization of rees was low and authigenic monazite did not form even in the ca poor graywackes authigenic monazite shows low concentrations of th u ca y hree and a low eu anomaly the central parts of the aggregates are enriched in nd mree whereas the outer parts are enriched in la and ce zoning pattern suggests an evolution in fluid chemistry during diagenesis very low grade metamorphism or it relates to different solubility of the individual ree phosphates the major source of rees in the pore fluids are probably detrital minerals mainly monazite other minor sources of rees probably include fe oxide hydroxide and or clay minerals which could have contributed to the nd mree rich cores in the authigenic monazite

summary and discussion of p t estimates from garnet pyroxene plagioclase quartz bearing granulite facies rocks from sri lanka abstract in granulite facies rocks of granitic to basic composition from the highland complex hc of sri lanka maximum p t estimates for about 115 locations have been obtained from core compositions of garnet orthopyroxene and plagioclase their distribution shows a decrease in pressure from the east and southeast where pressures exceed 9 kbar towards the west and southwest where pressures are about 5 kbar the accompanying temperature decrease in the order of about 200 c is less pronounced the regional distribution pattern of garnet quartz clinopyroxene reaction textures around orthopyroxene and plagioclase indicates that the lower crustal rocks of the eastern and southeastern parts of the hc have experienced an episode of near isobaric cooling subsequent uplift resulted in the breakdown of garnet and clinopyroxene to orthopyroxene plagioclase this reaction texture is found in all parts of the hc with the exception of the west and southwest where the higher crustal levels are exposed comparisons of the array of calculated maximum p t data with published continental geotherms for normal to elevated mantle heat flows 30 50 70 and 90 10 3 w m 2 at the base of a 36 km crust show that the depth versus temperature gradient is not a simple geothermal one the p t array in which the gradient is steeper than the geotherms would be consistent with magmatic heating this model would suggest a scenario in which intrusions that were generated in the lower crust were emplaced at higher crustal levels and heated up the surrounding rocks however since the number of known slightly pre to synmetamorphic intrusions in sri lanka is very limited other mechanisms of the heat supply seem to be required prior to uplift and exposure of this partial cross section through the lower to upper crust

widespread assimilation of a seawater derived component at loihi seamount hawaii many tholeiitic and transitional pillow rim and fragmental glasses from loihi seamount hawaii have high cl contents and cl k 2o ratios and ratios of cl to other incompatible components such as p 2o 5 h 2o etc relative to other hawaiian subaerial volcanoes e g mauna loa mauna kea and kilauea we suggest that this results from widespread contamination of loihi magmas by a cl rich seawater derived component assimilation of high cl phases such as saline brine or cl rich minerals halite or iron hydroxychlorides with high cl h 2o ratios can explain the range and magnitude of cl contents in loihi glasses as well as the variations in the ratios of cl to other incompatible elements brines and cl rich minerals are thought to form from seawater within the hydrothermal systems associated with submarine volcanoes and loihi magmas could plausibly have assimilated such materials from the hydrothermal envelope adjacent to the magma chamber our model can also explain semiquantitatively the observed contamination of loihi glasses with atmospheric derived noble gases provided the assimilant has concentrations of ne and ar comparable to or slightly less than seawater this is more likely for brines than for cl rich minerals leading us to favor brines as the major assimilant cl br ratios for a limited number of loihi samples are also seawater like and show no indication of the higher values expected to be associated with the assimilation of cl rich hydrothermal minerals although cl enrichment is a common feature of lavas from loihi submarine glasses from other hawaiian volcanoes show little kilauea or no mauna loa mauna kea evidence of this process suggesting that assimilation of seawater derived components is more likely to occur in the early stages of growth of oceanic volcanoes summit collapse events such as the one observed at loihi in october 1996 provide a ready mechanism for depositing brine bearing rocks from the volcanic edifice into the top of a submarine summit magma chamber

rates of fe mg mn and ca diffusion in garnet extraction of the thermal and chemical histories preserved by modified compositional zoning in garnet is compromised by large uncertainties in experimentally determined diffusion coefficients for principal divalent cations some of this uncertainty derives from long down temperature extrapolations but much of it arises from an inability to account accurately for large variations in diffusivity as a function of garnet composition numerical simulation of stranded diffusion profiles in partially resorbed natural garnets constrains diffusivities at low temperature and reveals a systematic dependence of diffusion rates on host garnet composition a comprehensive model incorporating a simple compositional dependence allows accurate prediction of diffusion rates for fe mg mn and ca for nearly all garnet compositions across the full range of geologically relevant temperatures pressures and oxygen fugacities

textural chemical and isotopic insights into the nature and behaviour of metamorphic monazite monazite is a mineral of choice for dating metamorphism in amphibolite and granulite grade metapelites however there exist a number of difficulties that complicate the interpretation of monazite geochronological data and prevent its application to many geological problems the two main obstacles addressed in this contribution are firstly the minor but significant e g 1 30 ma dispersal in duplicate isotope dilution thermal ionisation mass spectrometry id tims u pb age data commonly recorded from a single rock and secondly the difficulty of attaching monazite age data to pressure and temperature information through a multidisciplinary approach utilising tims and laser ablation multicollector inductively coupled plasma mass spectrometry la mc icpms isotope data quantitative and qualitative emp chemical analyses of monazite and textural studies we assess the significance of pb loss older components and continuous and episodic monazite growth in the generation of dispersed age data three samples from the canadian cordillera and one sample from the himalaya of pakistan are examined each sample exhibits an age dispersion of between 1 and 12 ma for single crystal and multi grain tims u pb monazite age determinations consideration of the closure temperature for pb diffusion in monazite and the metamorphic temperatures experienced by these samples suggests diffusive pb loss did not play a significant part in generating this age dispersal the la mc icpms study indicates that an older component 100 ma older than the tims ages contributed to the age dispersal in three of the four samples in all the samples however chemical analyses identified that the majority of monazites examined exhibited significant intra crystalline zoning in y content the la mc icpms analysis of one sample that was constrained to zones of distinct y content indicates that these zones are of distinct age we suggest that monazite grown before the appearance of garnet and during garnet breakdown is relatively rich in y whereas monazite grown after garnet is relatively poor in y a combination of these chemical data with textural observations suggests that once monazite had entered the mineral assemblage it grew or recrystallised episodically throughout the prograde and retrograde paths of the metamorphic event this behaviour contributes to and in one of the samples controls the observed age dispersal this recognition allows the generation of pressure temperature time points by combining textural and chemical information of monazite with in situ age determinations and pressure temperature information from garnet thus the episodic growth of compositionally distinct monazite throughout a metamorphic event provides the geochronologist with a very valuable chronological tool