generation of plagiogranite by amphibolite anatexis in oceanic shear zones field geochronological and rare earth element ree evidence obtained from the fournier oceanic fragment of the canadian appalachians indicates that its plagiogranite component was generated by the anatexis of amphibolite and not by the fractionation of basic magma we propose that the process occurred in two stages first gabbro was plastically deformed at high temperature to form dry low angle shear zones that subsequently evolved to amphibolite via the addition of light ree enriched hydrothermal solutions second the amphibolite underwent partial melting during shear to yield a migmatite comprising bands of plagiogranite alternating with amphibolite restite the plagiogranite locally coalesced to form pods dikes and lenses that injected the surrounding undeformed gabbro we attribute the development of the fournier plagiogranite to dynamothermal processes occurring in proximity to a spreading center due to asthenosphere induced shear within ocean layer 3 this serves as an important illustration of the dynamic nature of metamorphism and melting that can occur in the ocean crust

evolution of the adriatic carbonate platform palaeogeography main events and depositional dynamics abstract the adriatic carbonate platform adcp is one of the largest mesozoic carbonate platforms of the perimediterranean region its deposits comprise a major part of the entire carbonate succession of the croatian karst external or outer dinarides which is very thick in places more than 8000 m and ranges in age from the middle permian or even upper carboniferous to the eocene however only deposits ranging from the top of the lower jurassic toarcian to the top of the cretaceous can be attributed to the adcp defined as an isolated palaeogeographical entity although the entire carbonate succession of the karst dinarides was deposited within carbonate platform environments there were different types of carbonate platforms located in different palaeogeographical settings carboniferous to middle triassic mixed siliciclastic carbonate deposits were accumulated along the gondwanian margin on a spacious epeiric carbonate platform after tectonic activity culminating by regional middle triassic volcanism recorded throughout adria the african promontory a huge isolated carbonate southern tethyan megaplatform abbreviated as stm was formed with the area of the future adcp located in its inner part tectonic disintegration of the megaplatform during the middle to late early jurassic resulted in the establishment of several carbonate platforms including the adriatic apenninic and apulian separated by newly drowned deeper marine areas including the adriatic basin as a connection between the ionian and belluno basins lagonero basin and the area of the slovenian and bosnian troughs the adcp was characterised by predominantly shallow marine deposition although short or long periods of emergence were numerous as a consequence of the interaction of synsedimentary tectonics and eustatic changes also several events of temporary platform drowning were recorded especially in the late cretaceous when synsedimentary tectonics became stronger leading up to the final disintegration of the adcp the thickness of deposits formed during the 125 my of the adcp s existence is variable between 3500 and 5000 m the end of adcp deposition was marked by regional emergence between the cretaceous and the palaeogene deposition during the palaeogene was mainly controlled by intense synsedimentary tectonic deformation of the former platform area some carbonates mostly eocene in age were deposited on irregular ramp type carbonate platforms surrounding newly formed flysch basins and the final uplift of the dinarides reached its maximum in the oligocene miocene the adriatic carbonate platform represents a part although a relatively large and well preserved one of the broader shallow water carbonate platform that extended from ne italy to turkey although its continuity is somewhat debatable in the area near albanian greece boundary this large carbonate body which was deformed mostly in the cenozoic including a significant reduction of its width needs a specific name and the central mediterranean carbonate platform is proposed abbreviated to cmcp although the local names such as adcp for its nw part should be kept to enable easier communication and to facilitate description of local differences in platform evolution

accessory phase controls on the geochemistry of crustal melts and restites produced during water undersaturated partial melting the profound geochemical conseqences of accessory phase behaviour during partial melting of highgrade metapelites are demonstrated with reference to two geochemically distinct crustal melts produced by biotite dehydration melting reactions under granulite facies kbar 860 cc conditions these two leucogneiss suites from the brattstrand bluffs coastline eastern antarctica have similar field relations transport distances 10 100 s of metres and major element chemistry type 1 leucogneisses have low zr th and lree positive eu anomalies and zr zr and lreet lreet values less than 1 0 i e less than required to saturate the melt mass balance constraints suggest that these melts have been extracted before equilibration with host melanosomes the dry peraluminous nature of vapour undersaturated melts inhibits monazite and zircon solubility and results in concentration of these phases in the residue melts are consequently depleted in lree and hree melanosomes show complementary enrichment in lree while hree patterns are dominated by residual garnet type 2 leucogneisses in contrast have strongly enriched zr th and lree abundances negative eu anomalies and zr zr and lreet lreet 1 resulting from accessory phase entrainment vapour absent partial melting under moderate 6 8 kbar pressure granulite facies conditions of a pelitic source containing monazite is likely to give disequilibrium melts depleted in lree and hree as monazite and garnet are concentrated in the residue if temperatures are high enough 850 870 c to permit relatively large degrees of partial melting then the feldspar component of the source will be removed almost completely giving melts with large positive eu anomalies melts formed under vapour present conditions are unlikely to show such extreme lree and hree depletion or positive eu anomalies even at high degrees of partial melting disequilibrium melting coupled with source entrainment could fortuitously produce ree and trace element signatures similar to those typical of s type granites and usually ascribed to equilibrium melting conditions

simulations of a time varying non dipole field during geomagnetic reversals and excursions abstract following brown et al 2007 reversals and excursions have been simulated by decreasing the axial dipole field to zero while leaving up a time varying non dipolar field similar to the cal7k model korte m constable c g 2005 continuous geomagnetic models for the past 7 millennia ii cals7k geochem geophys geosyst 6 2 doi 10 1029 2004gc000801 we also envisaged the situation in which the total field changes sign we first investigated the relationship between the non axial dipole structure of the historical and present field snapshots and the virtual geomagnetic poles vgps the accumulation of vgps in the vicinity of large flux concentrations in south atlantic and australia is constrained by the equatorial dipole only a few sites directly above the anomalies are associated with vgps in their vicinity therefore it seems delicate to associate recurring clusters of vgps during reversals with magnetic flux concentrations at the cmb the individual vgps resulting from simulations of reversals show all kinds of trajectories some being very simple with longitudinal confinement while others are complex with alternances of slow and fast changes as well as loopings of vgps preference for a specific longitude frequently happens in individual records but this is not systematic a direct consequence is that records of different reversals from a global array of sites are effectively dominated by preferred longitudinal bands thus a time varying non dipolar transitional field can generate anisotropy of the vgp distribution smearing of the signal by sedimentary records considerably amplifies the longitudinal confinement of the vgps overall these simulations account for most characteristics reported in detailed reversal records and thus strongly support the concept of a non dipolar transitional field we also simulated field excursions by assuming either a vanishing dipole followed by its return to initial polarity or by a short episode with the opposite polarity preceding its restoration the model accounts for the different characteristics inherent to the volcanic and sedimentary records of the recent excursions the simple structure of sedimentary vgp paths dominated by a large loop between northern and southern latitudes partly results from smearing of the signal therefore these records do not fully describe the complexity of the excursional field and remain exclusively sensitive to very low degree harmonics most detailed studies of excursions exhibit reversed directions which cannot exist without a very short dipolar episode of opposite polarity

jurassic granitoid magmatism in the dinaride neotethys geochronological constraints from detrital minerals three independent single grain geochronometers applied to detrital minerals from central dinaride sediments constrain the timing of felsic magmatism that associated the jurassic evolution of the neotethys the lower cretaceous clastic wedge of the bosnian flysch sourced from the dinaride ophiolitic thrust complex yields magmatic monazite and zircon grains with dominant age components of 164 3 and 152 10 ma respectively a unique tephra horizon within the adriatic carbonate platform was dated at 148 11 ma by apatite fission track analysis these consistent results suggest that leucocractic melt generation in the central dinaride segment of the neotethys culminated in middle to late jurassic times coeval with and slightly post dating subophiolitic sole metamorphism growth of magmatic monazite and explosive volcanism call for supra subduction zone processes at the convergent neotethyan margin new compilation of geochronological data demonstrates that such jurassic felsic rocks are widespread in the entire dinaride hellenide orogen

calcareous nannofossil age constraints on miocene flysch sedimentation in the outer dinarides slovenia croatia bosnia herzegovina and montenegro flysch deposits are associated with the outer dinaride nappe front they overlie eocene platform carbonate to bathyal marl successions that subsequently cover cretaceous plat form carbonates of apulia and the dinaride nappes planktonic foraminifer biostratigraphy indi cates eocene age of flysch sedimentation new calcareous nannofossil data reveal that several assemblages are present besides the dominant mid eocene species cretaceous paleocene oligocene and miocene taxa were also identified throughout the entire flysch belt widespread occurrence of nannofossil species of zone nn4 6 indicates that flysch deposition lasted up to at least the mid miocene ubiquitous occurrence of various pre miocene taxa demonstrates that extensive possibly submarine sediment recycling has occurred in the cenozoic as flysch remnants are typically sandwiched between thrust sheets these new stratigraphic ages give a lower bracket on deformation age of the coastal range the data provide a link between cretaceous compression in the bosnian flysch and recent deformation in the adriatic offshore area

mesozoic plate tectonic reconstruction of the carpathian region abstract palaeomagnetic palaeobiogeographic and structural comparisons of different parts of the alpine carpathian region suggest that four terranes comprise this area the alcapa tisza dacia and adria terranes these terranes are composed of different mesozoic continental and oceanic fragments that were each assembled during a complex late jurassic cretaceous palaeogene history palaeomagnetic and tectonic data suggest that the carpathians are built up by two major oroclinal bends the alcapa bend has the meliata oceanic unit correlated with the dinaric vardar ophiolite in its core it is composed of the western carpathians eastern alps and southern alcapa units transdanubian range bukk this terrane finds its continuation in the high karst margin of the dinarides further elements of the alcapa terrane are thought to be derived from collided microcontinents czorsztyn in the n and a carbonate unit tisza in the se the tisza dacia bend has the vardar oceanic unit in its core it is composed of the bihor and getic microcontinents this terrane finds its continuation in the serbo macedonian massif of the balkans the bihor getic microcontinent originally laid east of the western carpathians and filled the present carpathian embayment in the late palaeozoic early mesozoic the vardar ocean occupied an intermediate position between the western carpathian austroalpine transdanubian high karst margin and the bihor getic serbo macedonian microcontinent the vardar and pindos oceans were opened in the heart of the mediterranean adriatic microcontinent in the late permian middle triassic vardar subducted by the end of jurassic causing the bihor getic serbo macedonian microcontinent to collide with the internal dinaric western carpathian margin an external penninic vahic ocean tract began opening in the early jurassic separating the austroalpine western carpathian microcontinent and its fauna from the european shelf further east the severin ceahlau magura also began opening in the early jurassic but final separation of the bihor getic ribbon and its fauna from the european shelf did not take place until the late middle jurassic the alcapa and the tisza dacia were bending during the albian maastrichtian the two oroclinal bends were finally opposed and pushed into the gates of the carpathian embayment during the palaeogene and neogene at that time the main n s shortening in distant alpine and hellenic sectors was linked by a broader right lateral shear zone along the former vardar suture

metamorphic soles from the albanian ophiolites petrology 40ar 39ar geochronology and geodynamic evolution in the eastern mediterranean region the albanian ophiolites are remnants of one branch of the tethyan oceanic basin obducted on carbonate margins during the mesozoic pressure temperature time and structural data in the metamorphic soles of these ophiolites place new constraints on the closure of this basin and the emplacement history of oceanic rocks the petrological and structural work documents an apparently inverted metamorphic gradient with granulite facies at the top decreasing downward through amphibolite then greenschist facies until unmetamorphosed rocks thermobarometry indicates that the granulite facies formed at peak temperatures of 800 860 c for pressures close to 1 gpa these conditions point to the subduction of the ophiolites beneath young oceanic lithosphere to a depth of 30 40 km a large set of 40ar 39ar data has been obtained on the metamorphic soles and some ophiolite intrusives using single grains and bulk mineral populations ages range from 160 to 174 ma corresponding to the middle jurassic bajocian bathonian within the same sole the convergence of results using single grains and mineral populations of amphibole and muscovite and the lack of internal age gradient within muscovite are interpreted to result from fast cooling of metamorphic soles the main result documented by this study is a systematic younging of metamorphic sole ages from south to north with a difference of 14 ma along the 140 km length of the belt the age of igneous rocks such as plagiogranites and mafic dikes in the ophiolites is equivalent to that of metamorphic soles which indicates that the ophiolite was still hot and young at the time of metamorphic sole formation the emplacement of the albanian ophiolites is the consequence of a complex tectonic evolution continental rifting slow mid ocean spreading followed by west dipping intraoceanic subduction initiated at the ridge axis symmetric intraoceanic thrusting with opposite vergence and at last emplacement onto the continental margin the geochronological and stratigraphic data suggest that this evolution lasted only a few million years moreover they indicate that this evolution stopped earlier to the south with the closure of a narrow basin than to the north where the basin was wider due to faster oceanic spreading

experimental resetting of the u th pb systems in monazite abraded fragments 200 400 m of a large chemically homogeneous and non metamict brazilian monazite crystal characterised by a concordant u pb ages of 474 1 ma 208pb 206pb 19 5 were hydrothermally treated at varying temperatures with solutions of different compositions product monazites were analysed with scanning electron microscope sem electron microprobe emp secondary ion mass spectrometer sims and isotope dilution thermal ionisation mass spectrometer id tims experiments with pure water over a temperature range of 800 1200 c at 700 mpa and durations ranging from 5 to 60 days showed that even at 1200 c any dissolution and recrystallization of new monazite is confined to the outermost surface of the grain neither pb diffusion at the emp scale nor significant discordancy were observed we performed experiments at 800 and 1000 c for different durations using different fluid compositions at quartz saturation a 10 wt cacl2 fluid a 10 wt srcl2 fluid a 10 wt nacl fluid and a fluid containing nbs 982 pb standard 208pb 206pb 1 for all runs emp traverses revealed no pb diffusion profiles significant overgrowths of newly formed monazite are documented by sem analyses they occurred only in the 1000 c experiments when either cacl2 or pb bearing fluids were present in the cacl2 experiment two zones could be distinguished within the crystal a core possessing the initial monazite composition and a rim consisting of newly formed monazite produced by dissolution precipitation which was enriched in ca and pb free id tims dating of single grains treated with srcl2 and cacl2 solutions at 1000 c are significantly discordant experiments employing the nbs pb standard produced sub concordant monazite for which the 207pb 206pb apparent age has become older than prior to the experiment 544 ma at 800 c and 495 ma at 1000 c the newly grown monazite rim had obviously incorporated pb from the fluid none of our reaction products contained a detectable diffusion profile the only resetting mechanism we detected involved dissolution precipitation the extent of the dissolution precipitation process depends on fluid composition and is a more efficient mechanism than diffusion for controlling the resetting of monazite in natural rocks

format and philosophy for collecting compiling and reporting microprobe monazite ages microprobe monazite dating has been increasingly used to constrain the timing of deformation and metamorphism because of the potential to date very small monazite domains down to 5 m or less in structural and petrologic context this paper presents an analytical strategy presentation format and error considerations for microprobe monazite dating the strategy involves high resolution compositional mapping to delineate compositional domains within monazite crystals then for each compositional domain a series of th u and pb analyses are made and a single date and error are calculated the number of analyses in each domain is determined by the desired statistical precision of the date results from several monazite grains are typically combined and along with textural relationships are used to build an argument that the dates constrain the age of a deformation or metamorphic event the total error involves three components short term random error dominated by counting statistical uncertainty short term systematic error uncertainty in background correction conductive coating variation and calibration and long term systematic error uncertainty in standard composition mass absorption factors decay constants etc in homogeneous compositional domains short term random errors 2 of less than 10 m y can be obtained from five to ten analyses however short term systematic error mainly background estimation uncertainty would typically result in a doubling of the magnitude of random error microprobe dates are presented as a single gaussian probability distribution for each domain along with representative compositional maps it is recommended that a consistency standard be analyzed during each analytical session and the results be reported along with those from the unknown this proposed strategy and format are compatible with those of other geochronological techniques they incorporate analytical limitations associated with trace as opposed to major element microprobe analysis and will allow better comparisons to be made between labs and between different geochronological techniques

partial resetting of the upb isotope system in monazite through hydrothermal experiments an sem and upb isotope study abstract natural monazites of different grain sizes were treated experimentally under hydrothermal conditions and their u pb isotope systematics measured before and after the runs runs were carried out with monazite grain size fractions ranging from 40 to 125 m in diameter and on 207 pb 206 pb ages close to 380 ma u concentrations remained always unchanged all recrystallized monazites from the powdered starting material define a discordia with intercept ages of 379 3 2 3 and 7 18 ma the latter indicating the hydrothermally induced pb loss at the time of the experiment sem observations of monazite grains from hydrothermally treated grain size fractions exhibited only minor dissolution features confined to the grain surfaces run products from monazite powders showed recrystallization and grain growth induced by a dissolution precipitation mechanism increasing temperature resulted in a distinct enlargement of grain sizes at 650 and 750 c a dissolution precipitation process in presence of a fluid phase rather than pb volume diffusion acts as a very efficient mechanism for the resetting of the u pb isotope system of monazites even at low temperatures this process is most important in shear zones where channelized fluid flow may occur

three metamorphic monazite generations in a high pressure rock from the bohemian massif and the potentially important role of apatite in stimulating polyphase monazite growth along a pt loop abstract based on microstructural and compositional criteria three generations of monazite can be identified in a peraluminous garnet rich high pressure metamorphic rock from the bohemian massif the first monazite generation m1 formed probably on the prograde part of the clockwise pt loop a second m2 during the high pressure stage 26 3 kbar and 830 30 c and a third generation m3 during subsequent near isothermal decompression of the rock to 8 2 kbar the high pressure m2 monazites are volumetrically dominant and form 100 300 m grains they are characterised by an unusual sr brabantite component sro up to 2 wt and grew together with accessory sr rich fluorapatites sro up to 33 wt the sr contents of both minerals are indicative of high pressure conditions and result from the breakdown of plagioclase and the release of sr that occurred during this process m3 monazites occur exclusively as small grains 10 100 m within and at the rims of apatite microstructures show that they grew at the expense of the apatite extracting lrees from the latter under retrograde conditions these parasite monazites in apatite are characterised by very low th contents which is consistent with having been derived primarily from an apatite source m1 monazites are represented by resorbed irregular shaped cores which are occasionally preserved within the sr brabantite rich high pressure monazites m2 these m1 cores are generally sr poor indicating that they should have formed prior to the high pressure stage in the presence of plagioclase the dissolution of m1 monazite and the reprecipitation of a th u enriched m2 monazite during the high pressure stage implies that lrees were taken up by the high pressure sr rich fluorapatite to some extent and not completely processed for the formation of the m2 monazite this would also explain the widespread retrograde growth of m3 monazite within the apatite the ability of apatite to act as an interim host for lrees at peak pt conditions could be one of the reasons why monazite growth during regional metamorphism is often periodic

electron microprobe age mapping of monazite high resolution x ray maps of th u pb and y in monazite can be used to construct age maps which reveal the continuous spatial distribution of ages in a single grain of monazite the age mapping algorithm and three examples are presented to illustrate the capabilities and applications of this mapping technique the insights it can provide into monazite geochronology in general and its limitations compared to electron microprobe emp quantitative dating and other in situ geochronologic techniques age maps offer critical information for unraveling metamorphic and tectonic histories and for interpreting results from other geochronologic techniques and they are a valuable aid for rigorously locating in situ analytical points age mapping also can be used to better understand the behavior of the u th pb system in monazite during metamorphism deformation and s uid circulation events age maps presented in this paper reveal unsuspected age heterogeneities on the micrometer scale like a now healed fracture not visible in back scattered electron bse images or young domains less than 5 m in width located inside an older core in both cases using age maps as a template for locating in situ analysis points will minimize the peril of age mixing and erroneous geological interpretations in addition to providing critical information for illustrating and interpreting the history of complex polygenetic monazite age mapping may ultimately lead to a better understanding of the processes involved with monazite growth and recrystallization and thus even more powerful applications of the monazite geochronometer the agemap program is available in a windows version and can be downloaded from the internet at the following address http www geo umass edu probe agemap

multiproxy record of late quaternary climate change and middle stone age human occupation at wonderkrater south africa here we provide a multiproxy record of climate change and human occupation at wonderkrater a spring and peat mound site situated in the interior of southern africa recently extracted sediment cores yielded a number of middle stone age msa artefacts prompting exploratory excavation of the sediments to understand better the geomorphology of the site age of the sediments cultural lithic sequence vegetation and faunal remains and to try to establish whether human use of the site was to some extent climatically driven excavations yielded late pleistocene mammal fauna and flora and three small msa lithic assemblages with age estimates of 30 ka 45 ka and 138 01 7 7 ka the upper layers comprise peat that preserves macrobotanical and faunal remains implying local fen conditions in acacia savanna woodland at 12 ka below the upper peat layers a 1 m thick layer of white sand yielded two msa lithic assemblages in association with faunal remains dated to between 30 8 0 7 ka and 45 ka clay underlying the sand has an osl age of 63 1 5 8 ka and sandy peat below it has an infrared stimulated luminescence irsl age of 70 10 ka faunal remains in the lower sand levels and dental stable carbon isotope analysis of herbivores indicate a substantial grassland component in the landscape during late mis 3 45 ka charcoal phytolith and pollen data show a change from moderately warm and dry grassy savanna woodland in the lower sand levels to cooler and wetter grassland with woody shrubs in the uppermost levels by 30 ka the conditions that resulted in the deposition of the sand also attracted people to the site but whether it served as an oasis in an arid landscape or was occupied during wet phases is unclear the composition of the lithic assemblages which include many tools suitable for cutting suggest that the peat mound may have been used as a place to harvest reeds process plant materials and butcher animals that were either deliberately or accidentally trapped in mud or peat

microstructure of 24 1928 ma concordant monazites implications for geochronology and nuclear waste deposits the microstructure of monazite was studied using scanning electron microscopy sem electron microprobe analysis emp x ray diffraction patterns xrd and transmission electron microscopy tem four well characterized monazites were investigated having very different concordant u pb ages 24 to 1928 ma and up to 15wt tho2 and 0 94wt uo2 the sem and emp analyses of polished single crystal fragments reveal the absence of significant chemical zoning xrd and tem investigations show that the monazites are not metamict despite their old ages very high abundances of radionuclides and hence high time integrated radiation doses except for the youngest one the monazite crystals are composed of a mosaic of crystalline but slightly distorted domains this structure is responsible for the presence of 1 mottled diffraction contrasts on the tem and 2 a second structural phase b with very broad reflections in the xrd patterns older monazites receive higher self irradiation doses and hence they contain higher amounts of this b phase for the 1928 ma monazite xrd reveals only the broad reflections of phase b implying that the whole monazite was affected by radiation damage that resulted in total distortion of the lattice it is concluded that radiation damage in the form of amorphous domains does not accumulate in monazite because selfannealing heals the defects as they are produced by alpha decay damage the only memory of irradiation induced defects is the presence of distorted domains as the diffusion rate of pb in an undisturbed monazite lattice is extremely low pb loss due to volume diffusion out of the monazite lattice is virtually impossible this is considered as one reason why almost all monazites have concordant u th pb ages moreover as long term self irradiation effects are limited in monazite we consider this phase as a good candidate for the storage of high level nuclear waste under the aspect of its high resistance to irradiation

monazite xenotime thermochronometry methodology and an example from the nepalese himalaya monazite xenotime thermochronometry involves the integration of petrographic geochronological and geochemical techniques to explore the thermal evolution of igneous and metamorphic rocks containing these accessory minerals the method is illustrated in this paper by application to an orthogneiss sample from the everest region of the nepalese himalaya that contains leucogranitic segregations produced by in situ anatexis observations of phase relationships and the internal structure of accessory minerals made using both transmitted light and electron microscopy revealed the existence of multiple generations of monazite and xenotime and guided microsampling efforts to isolate grain fragments of himalayan tertiary and pre himalayan age nearly concordant u pb isotopic ratios for 13 single monazite and xenotime grains ranged in age from 28 37 to 17 598 ma making determination of the timing of anatexis difficult without additional information presuming that monazite and xenotime were in equilibrium over that entire interval temperatures estimated from the yttrium contents of dated monazites range from 677 535 c only the highest temperatures are consistent with experimental constraints on the conditions necessary to produce anatectic melts of appropriate composition implying that the 25 4 24 8 ma dates for the grains with high apparent equilibration temperatures provide the best estimates for the age of anatexis two monazite crystals yielded 207pb 235u dates that are statistically indistinguishable from the 207pb 235u dates of coexisting xenotime crystals permitting the application of both quantitative y partitioning and semi quantitative nd partitioning thermometers as a cross check for internal consistency one of these sub populations of accessory minerals with a mean 207pb 235u date of 22 364 0 097 ma provides inconsistent y partitioning 641 39 c and nd partitioning 515 560 c temperatures we suspect the discrepancy may be caused by the high th concentration 6 12 wt tho2 in this subpopulation s monazite the y partitioning thermometer was derived from experimental data for the ce y po4 binary and may be inappropriate for application to high th monazites for the other subpopulation mean 207pb 235u date 22 11 0 22 ma the y and nd partitioning temperatures are indistinguishable 535 49 and 525 550 c respectively this consistency strongly suggests that the sample experienced a temperature of 535 c at 22 11 ma this finding is tectonically important because temperatures at higher structural levels were much higher by 100 c at the same time lending support to earlier suggestions of a major structural discontinuity within the upper part of the himalayan metamorphic core at this longitude an additional finding of uncertain importance is that inherited monazite and xenotime yielded u pb discordia with indistinguishable upper intercept ages 465 5 8 7 and 470 11 ma respectively and application of the y partitioning thermometer to the inherited monazites produced a restricted range of model temperatures averaging 470 c whether or not these temperatures are geologically meaningful is unclear without independent corroboration of the assumption of equilibrium between the inherited monazites and xenotimes but it appears that monazite xenotime thermochronometry may be useful for seeing through high grade metamorphism to extract temperature time information about inherited mineral suites

genesis and transformations of monazite florencite and rhabdophane during medium grade metamorphism examples from the sopron hills eastern alps electron microprobe studies on the age mineral chemistry and alteration on accessory lree phosphate minerals have been carried out in medium grade metamorphic rocks of the sopron hills belonging to the lower austroalpine tectonic unit monazite and xenotime is relatively common whereas rhabdophane and florencite are restricted to certain rock types a first generation of monazite was formed in mica schists during the pre alpine hercynian metamorphism at 575 700 jc and 1 8 3 8 kbar as evidenced by p t data from the literature their mineral paragenetic and textural characteristics and supported by th u total pb ages of ca 300 ma in orthogneisses monazite is rare and of igneous origin kyanite quartzites and leucophyllites that were formed by mg metasomatism contain inherited monazite from the precursor rocks a new generation of monazite was also formed during the alpine metamorphism at v550 jc 13 kbar according to the literature data giving ages around 75 ma pronounced negative eu anomalies were found in the igneous monazites eu eu 0 4 small differences have been observed in yand hree contents whereas the lree sections of the rare earth element ree patterns nearly coincide th and ca enter the monazite structure at the expense of ree nearly according to the brabantitic replacement 2ree 3 xth 4 ca 2 in some mica schists monazite is altered to rhabdophane rhabdophane distinguished from monazite by quantitative electron microprobe analysis by low oxide total is found in many mica schists and orthogneisses it forms fine grained aggregates often attached to apatite or monazite it usually has higher yand ca contents and a less pronounced negative eu anomaly than that of coexisting monazite it may have been formed either by crystallization from ree containing hydrous solutions or from monazite reacting with y ca containing solutions florencite appears only in some leuchtenbergite bearing leucophyllites kyanite quartzites and ree rich clasts it is often idioblastic and may be grown on apatite or monazite it is chemically close to its ideal composition but ca sr and th may replace ree in minor amounts in some grains tho2 may reach 10 wt the data indicate that the charge balance is maintained by different mechanisms in low and high thorian florencite no yor hree above gd could be measured in florencite no fractionation was observed between coexisting monazite and florencite however monazite inclusions in florencite are depleted in la ce and enriched in hree d 2002 elsevier science b v all rights reserved

the relative stability of monazite and huttonite at 300 900 c and 200 1000 mpa metasomatism and the propagation of metastable mineral phases monazite is both partially replaced and overgrown by a thsio4 phase along grain rims in a series of experiments from 300 to 900 c and 200 to 1000 mpa all experiments consisted of 10 mg of 100 500 m size euhedral to subhedral crystals of a natural th free monazite ce 5 mg of th no3 4 5h2o 2 5 mg of sio2 and 5 mg of h2o loaded into 3 mm wide 1 or 1 3 cm long platinum capsules that were arc welded shut experimental conditions were 300 c at 200 and 500 mpa 300 400 500 600 and 700 c at 500 mpa cold seal hydrothermal autoclave and 900 c at 1000 mpa catz assembly piston cylinder press back scattered electron bse imaging electron back scattered diffraction ebsd analysis and transmission electron microscopy tem indicates that in the experiments from 500 to 900 c the thsio4 phase took the form of monoclinic huttonite implying that huttonite associated with monazite could exist metastably over a much greater p t range than previously thought tem analysis of a foil cut perpendicular to the monazite huttonite interface from the 600 c 500 mpa experiment using a focused ion beam fib indicates that the huttonite as well as the interface between the huttonite and monazite is characterized by fluid inclusions high resolution tem analysis indicates that the huttonite monazite interface is coherent in the case of replacement of monazite by huttonite fluid aided dissolution reprecipitation is proposed as the most likely mechanism responsible