the exposed base of a collapsing wedge the nestos shear zone rhodope metamorphic province greece 1 the nestos shear zone nsz in the rhodope metamorphic province is a major high strain zone between two metamorphic terranes microdiamond bearing ultrahigh pressure uhp rocks occur in the nsz which was therefore interpreted as a suture zone where subduction and exhumation of these rocks and terrain accretion occurred during the mesozoic our petrological study of samples from the lower part of the nsz together with monazite dating of a microdiamond bearing garnet schist structural observations already published results from the upper part and other published timing constraints results in a fundamentally different picture the nsz is the base of an eocene age southwestward thrust wedge which included not only the structurally higher parts of the rhodope metamorphic province but also the entire internal hellenides the uhp rocks for the peak pressure of which we derive an age of 200 ma by monazite dating are unrelated to the tectonic processes in the nsz and probably represent slivers of a higher tectonic unit captured by thrusting along the nsz pressure decrease in the footwall and regional extension and basin formation in the hanging wall during the activity of the nsz show that the overlying thrust wedge was collapsing in late eocene times we hypothesize that the transition from subduction to continental collision may start with a pronounced accretion event when the first detachment horizon forms in the middle crust of the downgoing plate such an event could trigger slab retreat an extensional collapse of the internal wedge and subsequent magmatism

40ar 39ar dating of shear zones in the variscan basement of greater kabylia algeria evidence of an eo alpine event at 128 ma hauterivian barremian boundary geodynamic consequences a ductile shearing event is recognized in gneisses and barite veins of the crystalline internal massifs of the alpine maghrebides in algeria dating of shearing movement is carried out by 40ar 39ar laser probe experiments performed on synchronous neoformed micas the proposed age at 128 ma is constrained by a two component mixing model for the saddle shaped age spectra on phengites and by a plateau age at 128 3 0 3 ma on muscovite this hauterivian barremian boundary age records a strong reworking of the alboran plate input of detritals in the flysch basin and surrounding areas it can be correlated with the j magnetic anomaly in the atlantic and with the distensive events in the pyrenees and external alps therefore isotopic ages 80 130 ma obtained on the alpine mediterranean basement could reflect interferences between this event at 128 ma and late alpine stages

u pb dating of monazite and its application to geological problems monazite is an underutilized mineral in u pb geochronological studies of crustal rocks it occurs as an accessory mineral in a wide variety of rocks including granite pegmatite felsic volcanic a

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

electron microprobe and la icp ms study of monazite hydrothermal alteration implications for u th pb geochronology and nuclear ceramics this paper presents new data that quantify the response of magmatic monazite to three main types of hydrothermal alteration namely sericitization chloritization and greisenization the samples were taken from three paleozoic granites the st nectaire granite massif central france the carnmenellis granite cornwall england and the skiddaw granite lake district england fluid inclusion thermometry and hydrothermal parageneses indicate that alteration took place at temperatures ranging from 260 to 340 c and salinities from 3 to 18 wt nacl equivalent possible monazite alteration mechanisms found in the course of this study using backscattered scanning electron microscopy bse sem electron microprobe laser raman spectroscopy and laser ablation inductively coupled plasma mass spectrometry la icp ms include cationic substitutions monoclinic to hexagonal structure transition accompanied by chemical exchanges selective th removal dissolution reprecipitation and dissolution with replacement by a different mineral these results show that despite the compositional and crystallographic simplicity of monazite it exhibits varied chemical responses to different mineral fluid interactions elemental and isotopic measurements by la icp ms for the unaltered monazites yield statistically significant 232th 208pb and 238u 206pb magmatic crystallization ages in some cases it was possible to give a reasonable estimate of the age of mineral fluid interaction using the altered parts of the monazites or newly precipitated crystals for other minerals 232th 208pb and 238u 206pb systematics were strongly disturbed by variable inputs and or depletions of u th and pb the data suggest that monazite like nuclear waste forms would be good hosts for tetravalent actinides but may release the lanthanides and actinides with lower and higher valencies especially if the fluids are oxidizing or tend to dissolve monazite

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

environmental conditions in the donggi cona lake catchment ne tibetan plateau based on factor analysis of geochemical data abstract we present a methodological approach for the characterization of spatial variations of environmental and transport conditions based on geochemical data we analyzed the geochemical characteristics of terrestrial sediments on an alluvial fan and adjacent areas in the eastern part of the donggi cona catchment the geochemical characteristics of the sediment samples were measured using x ray fluorescence spectroscopy a factor model based on nine elements sr rb zn fe mn v ti ca and k gave the best results using factor analysis our results show that factor analysis of geochemical data can explain spatial variations in the catchment even within a relatively small area with more or less constant climatological conditions the four most important factors of our model explain 68 4 of the total variance within the dataset the four factors represent a carbonate weathering and two redox condition signals respectively the spatial distribution of factor loadings points to regions with specific environmental conditions these regions show a certain carbonate input or production factor 1 specific weathering rates factor 2 or specific redox conditions factors 3 and 4 the factor loadings of the first and second factor are used to reclassify the data into four groups being dune loess lake and fluvial gravel sediments this reclassification confirms or improves the prior field classification in 70 of all cases for 23 of the samples the model gave a different interpretation the factor loadings in these cases represent the origin of the material rather than its present sediment type thereby giving valuable information about sediment provenance since the third and fourth factor seem more dependent on location they do not differentiate between different sediment types multivariate statistics of the geochemistry of terrestrial sediments i allows the subdivision of samples into different sediment types ii indicates dominant regions for weathering carbonate production and manganese washout and iii gives suggestions about the provenance of the sediments this is the first detailed study of geochemical parameters on the tibetan plateau not focusing on lake cores resulting in a spatial characterization of the local geochemistry this gives insight in the sediment transport connections between the catchment and the lake illustrating the value of terrestrial sediments as additional indicator of environmental variations

muscovite recrystallization and saddle shaped 40 ar 39 ar age spectra example from the blond granite massif central france muscovites from the blond granite west french massif central were dated by the 40ar 39ar single grain method the 40ar 39ar ages obtained vary from 305 5 0 3 ma to 311 3 0 5 ma and most of the age spectra are slightly saddle shaped the analyzed muscovites show phengitic recrystallization under optical microscope observations sem images and electron microprobe chemical analyses it is proposed that the saddle shaped age spectra result from a partial recrystallization which produced three different isotopic reservoirs in the analyzed white mica single grains domains of early muscovite domains of neocrystallized muscovite formed by phengitic and al fe substitutions and low activation energy sites

mesozoic tertiary structural evolution of an extensional gneiss dome the kesebir kardamos dome eastern rhodope bulgaria greece the tectonic evolution of the rhodope massif involves mid cretaceous contractional deformation and protracted oligocene and miocene extension we present structural kinematic and strain data on the kesebir kardamos dome in eastern rhodope which document early tertiary extension the dome consists of three superposed crustal units bounded by a low angle nne dipping detachment on its northern flank in bulgaria the detachment separates footwall gneiss and migmatite in a lower unit from intermediate metamorphic and overlying upper sedimentary units in the hanging wall the high grade metamorphic rocks of the footwall have recorded isothermal decompression direct juxtaposition of the sedimentary unit onto footwall rocks is due to local extensional omission of the intermediate unit structural analysis and deformational metamorphic relationships give evidence for several events the earliest event corresponds to top to the sse ductile shearing within the intermediate unit interpreted as reflecting mid late cretaceous crustal thickening and nappe stacking late cretaceous palaeocene eocene late tectonic to post tectonic granitoids that intruded into the intermediate unit between 70 and 53 ma constrain at least pre latest late cretaceous age for the crustal stacking event subsequent extension related deformation caused pervasive mylonitisation of the footwall with top to the nne ductile then brittle shear ductile flow was dominated by non coaxial deformation indicated by quartz c axis fabrics but was nearly coaxial in the dome core latest events relate to brittle faulting that accommodated extension at shallow crustal levels on high angle normal faults and additional movement along strike slip faults radiometric and stratigraphic constraints bracket the ductile then brittle extensional events at the kesebir kardamos dome between 55 and 35 ma extension began in paleocene early eocene time and displacement on the detachment led to unroofing of the intermediate unit which supplied material for the syn detachment deposits in supra detachment basin subsequent cooling and exhumation of the footwall unit from beneath the detachment occurred between 42 and 37 ma as indicated by mica cooling ages in footwall rocks and extension proceeded at brittle levels with high angle faulting constrained at 35 ma by the age of hydrothermal adularia crystallized in open spaces created along the faults this was followed by late eocene oligocene post detachment overlap successions and volcanic activity crustal extension described herein is contemporaneous with the closure of the vardar ocean to the southwest it has accommodated an earlier hinterland directed unroofing of the rhodope nappe complex and may be pre cursor of and or make a transition to the aegean back arc extension that further contributed to its exhumation during the late miocene this study underlines the importance of crustal extension at the scale of the rhodope massif in particular in the eastern rhodope region as it recognizes an early tertiary extension that should be considered in future tectonic models of the rhodope and north aegean regions

an electron microprobe study of the u th pb systematics of metamorphosed monazite the role of pb diffusion versus overgrowth and recrystallization abstract an electron microprobe emp was used to obtain u th pb chemical ages from monazite that underwent an amphibolite facies metamorphic overprint in the monashee complex southern canadian cordillera u pb isotope dilution dating of single grains yielded discordant ages that plot in linear arrays on a concordia diagram upper intercepts of the arrays indicate primary growth in the proterozoic and cretaceous and lower intercepts are coeval with a tertiary metamorphic event the microprobe s high spatial resolution 2 m and ability to quickly differentiate between domains with greatly contrasting u th and pb concentrations made it the ideal tool for investigating the cause of the u pb isotopic discordance the microprobe dating shows that proterozoic monazite lies directly adjacent to generally small

low temperature replacement of monazite in the ireteba granite southern nevada geochronological implications the ireteba pluton is a relatively homogeneous 64 ma zircon ion probe age two mica granite that was intruded by two 16 ma miocene plutons at depths ranging from 5 to 13 km deeper levels of the ireteba and miocene plutons were ductilely deformed at 15 16 ma at shallow levels remote from the miocene plutons the ireteba granite appears to have experienced little miocene heating and deformation monazites from different portions of the pluton reflect the different histories experienced by the host rock irregularly shaped patchy zones with high huttonite component thsio4 are widespread in monazite at deep levels adjacent to miocene plutons but less common in shallow level rock monazite grains with extensive replacement generally have irregular embayed surfaces in undeformed rocks distant from the miocene plutons monazites are less modified and more nearly euhedral though fine networks of replacement veins are common and irregular rims are evident in some grains secondary monazite from these samples is poorer in huttonite ion probe th pb dating yields 60 65 ma ages for magmatic and some replacement zones in monazite from the shallow samples and veins yield apparent ages as young as mid tertiary monazites from deep samples yield a few 55 65 ma ages for remnant magmatic zones and abundant miocene ages for replacement zones 14 18 ma these data demonstrate extensive miocene replacement of magmatic monazite especially at deep levels near miocene plutons and they suggest an early replacement episode as well both events were probably related to influxes of fluid the first may have been associated with initial solidification of the ireteba pluton and the second with the miocene plutons and or extensional deformation ambient temperatures at the time of replacement indicate that secondary monazite growth occurred at t as low as 400 c or less

age of the dahongliutan rare metal pegmatite deposit west kunlun xinjiang nw china constraints from la icp ms u pb dating of columbite fe and cassiterite abstract dahongliutan is a medium size li be ta nb rare metal pegmatite deposit in the tianshuihai terrane west kunlun orogenic belt nw china in this paper la icp ms u pb ages of columbite fe and cassiterite are reported to constrain the metallogenic age columbite fe with high u and low th yielded a late triassic weighted mean 206 pb 238 u age of 211 9 2 4 ma whilst cassiterite yielded a 206 pb 238 u 207 pb 235 u concordia lower intercept age of 218 12 ma and a tera wasserburg lower intercept age of 218 12 ma which is identical to the columbite fe u pb age and thus represents the emplacement age of the pegmatite dikes the ore hosting pegmatite dikes are intimately time space related to the dahongliutan s type granite ca 220 217 ma indicating that they may have been cogenetic integrating new and published geological data we suggest that the dahongliutan pegmatites may have evolved from the granitic magma represented by the dahongliutan s type pluton in a post collisional tectonic setting regionally the tianshuihai terrane is the western extension of the songpan ganzi block and thus the kunlun li mineralization belt may have been connected to the songpan ganzi li mineralization belt which contains eleven lithium deposits with about 9 8 of li 2 o reserves in china implying that the dahongliutan pegmatite deposit may also have favorable ore forming conditions the similar columbite fe and cassiterite ages demonstrate that these minerals can be precisely dated and could provide useful alternatives to accurately constrain the timing and evolution of rare metal mineralization

monazite chemical composition some implications for monazite geochronology an investigation of the chemical composition of monazite from a number of localities has been carried out samples used include monazites from metamorphic rocks granitic rocks and a hydrothermal ore deposit the ree distribution pattern of monazite varies greatly in accordance with its geological environment a remarkable feature of the monazites studied is that their chondrite normalised ree distribution patterns are mostly uniform between grains within the same sample but differ significantly from sample to sample this characteristic apparently indicates that there is an important effect on the ree distribution of monazite exerted by the host rock or source material from which monazite crystallised another important feature shown by the monazites studied is that monazites in rocks containing garnet as a major mineral show extreme depletion of hree whereas monazites in rocks without garnet or monazite that formed after the garnet breakdown contain significantly higher amounts of y and hree this suggests that the phase assemblage especially garnet plays an important role in the ree distribution of monazites in these rocks the value of ree distribution in monazite is exemplified with regard to the origin of monazite in the lewisian metamorphic rocks which is a fundamental issue in monazite geochronology

preservation of old prograde metamorphic u th pb ages in unshielded monazite from the high pressure paragneisses of the variscan ulten zone italy abstract three samples of garnet kyanite paragneiss from the variscan ulten zone northern italy were studied in detail for u th pb monazite dating monazite in these gneisses is abundant shows highly variable grain size and occupies different textural positions within the matrix as inclusion in garnet and kyanite within apatite aggregates monazite shows different deformation features as a function of the textural position enclosed shielded monazite is generally more fractured than matrix unshielded monazite the integration of textural information with deformation features and in situ u th pb analyses by la icp ms indicates that there is no direct correlation between textural site and monazite ages old ages of 351 343 ma determined on portions of large matrix unshielded monazite and on rare domains of monazite shielded by garnet have been related to a prograde stage of the variscan metamorphic evolution of the ulten zone ages of 330 326 ma which are related to the thermal peak are recorded by small matrix monazite external domains of large matrix monazite and by domains of fractured monazite enclosed in garnet and kyanite large old unshielded grains formed as blasts during the prograde metamorphic history and survived the peak metamorphism during which crystallisation re crystallisation partially occurred

contrasting response of monazite and zircon to a high t thermal overprint abstract contact metamorphism in the aureole of the 1322 ma makhavinekh lake pluton northern labrador affected monazite and zircon in the adjacent 1850 ma metapelitic gneisses transformation of regional garnet and sillimanite to lower pressure symplectitic intergrowths of cordierite orthopyroxene and spinel was accompanied by resorption of inherited monazite inclusions in garnet coupled with the appearance of coronitic high y monazite rims in situ ion microprobe dating is used to show that high y rims formed during contact metamorphism liberation of y and hree from garnet also gave rise to new xenotime growth the coronitic nature of monazite overgrowths reflects the diffusion controlled nature of net transfer reactions whereas its higher y composition reflects equilibration with xenotime at peak t 800 c conditions in the inner aureole very thin overgrowths on inherited zircon were also encountered but only where zircon is surrounded by the symplectitic assemblage reflecting liberation of zr from garnet although these overgrowths are too thin to date using conventional ion microprobe techniques well developed triple junctions between zircon and orthopyroxene suggests that they grew in textural equilibrium with the contact metamorphic assemblage in contrast to monazite inherited zircon remained intact during contact metamorphism exhibiting no change in morphology other than the growth of thin rims or internal zoning throughout the aureole however inherited sector zoned zircons of anatectic origin display evidence for intracrystalline pb redistribution in the inner aureole in these samples ion microprobe analyses encountered heterogeneous pb signals and a dispersion of 207 pb 206 pb dates away from the well constrained 1850 ma age of regional metamorphism whereas analyses from the outer aureole faithfully record the age of regional metamorphism those from the inner aureole are normally and reversely discordant and distributed along a line collinear with a 1850 to 1322 ma discordia this disturbance is correlated with proximity to the pluton implying that pb was mobile in the zircon lattice during contact metamorphism most grains are characterized by apparent pb loss from low u domains and apparent pb gain in higher u domains these data are interpreted to reflect recovery of strained crystalline domains leading to expulsion of pb that was able to efficiently diffuse into higher u domains that were partly amorphous prior to rapid reheating in the inner aureole

contrasted monazite hydrothermal alteration mechanisms and their geochemical implications in spite of the major importance of monazite as a repository for the rare earths and th in the continental crust for u th pb geochronology and as a possible form for high level nuclear waste very little work has been carried out so far on the behaviour of this mineral during fluid rock events this contribution describes two contrasting examples of the hydrothermal alteration of monazite the first case comes from a sample of the carnmenellis granite cornwall southwest england chloritized at 284 16 c whereas the other occurs in the skiddaw granite lake district northwest england which underwent greisenization at 200 30 c an integrated study involving backscattered scanning electron microscopy electron microprobe analyses and laser ablation inductively coupled plasma mass spectrometry la icp ms reveals that the chloritization event was characterized by the coupled substitution 2ree3 th4 ca2 in the altered parts of the monazite thus leaving the p o framework of the crystal untouched in contrast greisenization led to the coupled substitution ree3 p5 th4 si4 and therefore involved a partial destruction of the phosphate framework the resulting rare earth element patterns are quite different for these two examples with a maximum depletion for dy and er in the altered parts of the carnmenellis monazite whereas the skiddaw monazite shows a light rare earth depletion but an yb and er enrichment during alteration this latter enrichment accompanied by an increase in u but roughly unchanged pb concentrations probably resulted from a decrease in the size of the 9 coordinated site in monazite thereby favouring the smaller rare earths these contrasted styles of monazite alteration show that the conditions of fluid rock interaction will not only affect the aqueous geochemistry of the lanthanides actinides and lead and the relative stability of the different minerals holding these elements variations in these conditions will also lead to various possible chemical exchanges between the crystalline phases and the hydrothermal fluids the occurrence of common lead along penetrative cracks in the carnmenellis monazite shows that only a leaching prior to the u pb analyses of the whole grain will permit an accurate determination of the magmatic crystallization age in contrast for the skiddaw case it may be possible to date the fluid rock event by in situ 207pb 206pb geochronology the observation that the altered parts of both monazite examples display nd leaching and no significant sm nd fractionation indicates that they should not affect the host whole rock nd isotopic signatures finally it appears that monazite like ceramics designed for the containment of high level nuclear wastes will retain th and the geochemically equivalent transuranic elements during fluid rock events similar to those documented in this study but may release nd u and the corresponding radionuclides to the environment

disconnected runoff contributing areas evidence provided by ancient watershed management systems in arid north eastern marmarica nw egypt abstract this study presents the importance of disconnectivity in dryland area runoff demonstrated by manmade water harvesting structures dated to greco roman times located on the coastal strip of some 20 km width along the mediterranean coast of modern northwestern egypt covering the north eastern part of the region known in antiquity as marmarica the area receives winterly rainfalls of up to 140 mm further south precipitation decreases quickly and desert conditions become more pronounced bedrocks are predominantly calcareous soils are loamy stony calcareous and shallow except in relief sinks with sedimentary deposits the land rises from the coast up to 230 m a s l on the marmarica plateau in a sequence of zonal northsloping plains and scarps the northern parts of which are dissected and drained by wadis agriculturally suitable areas comprise some 9 of the coastal zone and adjacent tablelands overland flow controls the discharge dynamics and is the main source of wadi runoff and hence agricultural water supply the land use pattern is scattered because cropping areas depend mainly on suitability of soils and the generation of runoff harvest which are closely interrelated because of the arid water and sediment regime the patchiness of runoff generation increases further south where aridity is higher and topography inhibits greater drainage patterns the abundance of cisterns many of them originally greco roman is strong evidence that tableland overland flows occur and are frequently disconnected from larger drainage systems abandoned ancient rainwater harvesting and watershed management systems are abundant all over the region and incorporate the hydrological experience of the former inhabitants having been dated with osl and pottery mainly from greco roman times while the region prospered some two millenia ago the ancient water management schemes are generally abandoned today one of the most outstanding features of the ancient systems was the systematic harvesting of tableland and valley slope overland flow above the wadi beds part of this strategy was to separate the remote parts of large natural watersheds from the local parts by shallow stone bunds on the tableland the collected tableland overland flow irrigated terraced fields on the tableland kurum the remaining water harvesting areas mainly valley slopes were obviously sufficient to supply water for small tributary wadis large side terraces and the floodplains of large wadis the artificial disconnection of local runoff areas from remote parts of the northern watersheds is hydrologically well adapted to the runoff dynamics in the flat landscape of the northern marmarica because it protected valleys from low frequency high magnitude runoff events which are generated mainly on the remote parts of large catchments but which are inefficient in terms of water harvesting terracing and water harvesting structures are frequently buried in well sorted fine grained sediment

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

transmission electron microscope study of polyphase and discordant monazites site specific specimen preparation using the focused ion beam technique electron microprobe emp u th pb dating on polyphase and discordant monazites from polymetamorphic granulites of the andriamena unit north central madagascar reveals inconsistent chemical ages to explain these drastic variations transmission electron microscopy tem foils were prepared directly from thin sections by using the focused ion beam technique the most important result of the tem study is the demonstration of the presence of small 50 nm pb rich domains where large variations in emp ages occur we suggest that radiogenic pb was partially reincorporated in monazite during the recrystallization at 790 ma because the excited volume of emp is 4 m3 u th pb dating yielded various apparent older ages without geological significance in addition tem analysis of the foils revealed the presence of an 150 nm wide amorphous zone along the grain boundary of monazite and its host quartz this fe si al rich phase may have formed as a result of fluid activity at 500 ma and the phase s amorphous state may be due to the irradiation from u and th decay in the monazite this demonstrates for the first time the enormous potential of the tem investigations on site specific specimens prepared with the focused ion beam technique for the interpretation of geochronological data

u pb systematics of monazite and xenotime case studies from the paleoproterozoic of the grand canyon arizona monazite is accepted widely as an important u pb geochronometer in metamorphic terranes because it potentially preserves prograde crystallization ages however recent studies have shown that the u pb isotopic system in monazite can be influenced by a variety of processes that partially obscure the early growth history in this paper we attempt to interpret complex monazite and xenotime u pb data from three paleoproterozoic granite dikes exposed in the grand canyon single crystal monazite analyses from an unfoliated granite dike spread out along concordia from the crystallization age of the dike defined by u pb zircon data to be 1685 1 ma to 1659 2 ma a span of 26 million years back scattered electron bse imaging reveals that magmatic domains within most crystals from this sample are truncated by secondary domains associated with prominent embayments at the grain margin fragments of a single crystal yield contrasting concordant dates and fragments from the edges and tips of crystals yield the youngest dates based on these observations we suggest that the secondary domains formed at least 26 million years after the crystal formed monazite and xenotime dates from the second sample a sheared dike that cross cuts the previous dike spread out along concordia over 16 million years and range up to 2 4 normally discordant again bse imaging reveals secondary domains that truncate both magmatic zoning and xenocrystic cores fragments sliced from specific domains of a previously imaged monazite crystal demonstrate that the secondary domain is 13 million years younger than the core domain textures revealed in bse images suggest that the secondary domains formed by fluid mineral interaction normal discordance appears to result from both radiation damage accumulated at temperatures below 300 c and water mineral interaction monazite data from the third sample exhibit dispersion in both the 207pb 206pb dates 1677 1690 ma and discordance 1 6 to 3 1 reverse discordance in these monazites cannot be explained by incomplete dissolution or excess thorogenic 206pb sliced fragments from several crystals reveal dramatic intragrain u pb disequilibrium that does not correlate with either th or u concentration or position within the crystal we suggest that reverse discordance resulted from mechanisms that involve exchange or fractionation of elemental u or elemental pb and that neither the u pb dates nor the 207pb 206pb dates are reliable indicators of the rock s crystallization age given the large number of processes proposed in the recent literature to explain monazite u pb systematics from rocks of all ages our results can be viewed as another cautionary note for single crystal and multi crystal monazite geochronometry however we suggest that because individual crystals can preserve a temporal record of primary and secondary monazite growth micro sampling of individual monazite crystals may provide precise absolute ages on a variety of processes that operate during the prograde peak and or retrograde history of metamorphic terranes

two stage partial melting during the variscan extensional tectonics montagne noire france one of the striking features that characterise the late stages of the variscan orogeny is the development of gneiss and migmatite domes as well a s extensional late carboniferous and permian sedimentary basins it remains a matter of debate whether the formation of domes was related to the well documented late orogenic extension or to the contractional tectonics that preceded migmatization and magmatism are expected to predate extension if the domes are compression related regional anticlines but they must both precede and be contemporaneous with extension if they are extensional core complexes in the montagne noire area s outhern french massif central where migmatization magmatism and the deformation framework are well documented the age of the extensional event was unequivocally constrained to 300 0 ma therefore dating migmatization in this area is a key point for discriminating between the two hypotheses and understanding the late palaeozoic evolution of this part of the variscan belt for this purpose a migmatite and an associated anatectic granite from the montagne noire dome were dated by la icp ms u th pb on zircon and monazite and laser probe 40 ar 39 ar k ar on muscovite although zircon did not record any variscan age unequivocally related to compression 380 330ma two age groups were identified from the monazite crystals a first event at ca 319 ma u th pb on monazite is interpreted as a first stage of migmatization and as the emplacement age of the granite respectively a second event at ca 298 295 ma recorded by monazite u th pb and by the muscovite 40 ar 39 ar system in the migmatite and in th e granite could be interpreted as a fluid induced event probably related to a second melting event identified through the syn extensional emplacement of the nearby montalet leucogranite ca 295 ma ago the age s of these two events post date the variscan compression and agree with an overall extensional context for the development of the montagne noire dome shaped massif comparison of these results with published chemical epma dating of monazite from the same rocks demonstrates that the type of statistical treatment applied to epma data is crucial in order to resolve different monazite age populations

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

regional scale cretaceous albitization in the pyrenees evidence from in situ u th pb dating of monazite titanite and zircon abstract albitization is a common metasomatic process active in various geodynamic contexts in the northern pyrenees there are several occurrences of albitites but until now only one occurrence has been dated 117 ma ar ar dating this paper presents new u th pb ages for several albitite occurrences throughout the pyrenees to test whether they are contemporaneous and if so to specify the regional extent of the albitization event ages obtained from large euhedral titanite and monazite grains from distinct albitites are 110 8 and 98 2 ma respectively the zircon u th pb isotopic system did not record this cretaceous metasomatic event even when grains were selected in metasomatically zr enriched rocks or in hydrothermal structures millimetre sized veins cross cutting granitoids we argue that the total time span of 20 ma recorded by albitites corresponds to a long lived hydrothermal system that was active during the rotation of iberia around europe along the north pyrenean fault because albitization and talc mineralization have the same spatial and temporal distribution in the pyrenees we argue that these two metasomatic phenomena are two independent records of this single regional scale long lived hydrothermal system

multiple generations of extensional detachments in the rhodope mountains northern greece evidence of episodic exhumation of high pressure rocks abstract an integrated structural and petrological study shows that exhumation of high rocks of the rhodope occurred in several pulses the structurally uppermost kimi complex recording an early cretaceous high p metamorphism was exhumed between about 65 and 42 ma the sidironero kardamos and kechros complexes that record early tertiary high metamorphism at least 19 kbar at 700 c were exhumed at 42 30 ma exhumation occurred with isothermal decompression strain episodes depict thrusting of medium high grade above lower grade high p rocks syn thrusting extension and post thrusting extension polyphase extension created several generations of detachment zones which in sum excise about 20 km of material within the crustal profile this reduction in crustal thickness is consistent with a reduction of the present crustal thickness from more than 40 km to less than 30 km in the eastern rhodope we mapped the post thrusting xanthi low angle detachment system over 100km from its break up zone above the sidironero complex central rhodope into the eastern rhodope this detachment shows an overall ese dip with a ramp and flat geometry cutting across the earlier thrust structures the kimi complex is the hanging wall of all synand post thrusting extensional systems on top of the kimi complex marine basins were formed from the lutetian c 48 43 ma through the oligocene during extension successively at c 26 to 8 ma the thasos pangeon metamorphic core complexes were exhumed in these times representing the early stages of aegean back arc extension the strymon and thasos detachment systems caused crustal thinning in the western rhodope renewed heating of the lithosphere associated with magmatism and exhumation of hot middle crust from beneath the sidironero complex occurred we focusoxn the geometry timing and kinematics of extension and contraction structures related to the 42 30 ma interval and how these exhumed high p rocks we interpret high rocks exhumed in this interval as a window of the apulian plate beneath the earlier in the cretaceous accreted kimi complex

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

zonation of monazite in metamorphic rocks and its implications for high temperature thermochronology a case study from the lewisian terrain abstract monazite is a valuable chronometer for many geological processes and a proper interpretation for the age derived from monazite requires an adequate understanding of the behaviour of this phase and its chemical and isotopic systems during geological processes this contribution reports the results of a detailed study on chemical and isotopic zonation of monazite in lewisian granulites which is of general significance for monazite geochronology in high grade metamorphic rocks detailed investigations on two lewisian granulite samples using sem epma and sims have revealed that lewisian monazites frequently show petrographic chemical and age zonation the petrographical zonation which reflects the chemical compositional variation can be characterised as follows on the basis of the bse images 1 concentric zoning 2 patchy zoning and 3 intergrowth like zoning the chondrite normalised ree distribution patterns are remarkably similar between different zones of single monazite grains this rules out the possibility that any portion of a zoned monazite in these two samples is detrital in origin some possible mechanisms are proposed for the formation of monazite zonation these include 1 intergrowth of monazite crystals with different composition 2 episodic growth or regrowth of monazite in response to the change of environment conditions during thermal event s which may involve recrystallisation or replacement of the original crystals in short the diversity of zoning patterns in monazites observed in this study cannot be explained assuming a single mechanism accordingly interpretation of the chemical and age information from any zoned monazite requires an adequate understanding of its formation mechanism line scans of th u pb la ce nd sm ca and si across monazite zones have been performed on two monazite grains which experienced granulite facies metamorphism the very sharp boundaries in these line scans indicate that the diffusive transport of these elements in monazite is low even at the temperature of granulite facies metamorphism the effective closure of monazite grains may also depend upon other factors such as the armouring effects of the host phase and recrystallisation of the mineral itself thus great care needs to be taken for geological application of the closure temperature concept

triassic mineralization with cretaceous overprint in the dahu au mo deposit xiaoqinling gold province constraints from shrimp monazite u th pb geochronology abstract the xiaoqinling terrane in central china is the second largest gold producing area in china previous application of age constraints has yielded ages for the gold mineralization ranging from paleoproterozoic through triassic indosinian to jurassic and cretaceous yanshanian shrimp u th pb dating of hydrothermal monazite in a mo mineralized quartz vein from the dahu au mo deposit in the northern xiaoqinling terrane identifies two mineralization episodes hydrothermal monazite intergrown with molybdenite yields an initial precipitation age of 216 5 ma which is identical to but more precise than the molybdenite re os isochron age of 218 41 ma a younger generation of monazite is distinguished by data falling on a mixing array it is inferred that the younger monazite was formed at the expense of the older ones during the yanshanian probably during a single event that occurred at 125 ma combining these results with available geochronological data and geological evidence we conclude that molybdenum mineralization accompanied by monazite deposition took place in the indosinian followed by a yanshanian tectono hydrothermal event that resulted in large scale gold mineralization in the xiaoqinling terrane our observation demonstrates that monazite is sensitive to hydrothermal overprint and thus is useful for dating fluid infiltration event

geochronological constraints on post collisional shear zones in the variscides of sardinia italy terra nova 24 42 51 2012 abstract the post collisional tectonics in the inner zone of the variscan belt of corsica sardinia recorded a dextral transpression developed with an orogen parallel tectonic transport recent works documented the presence of nearly parallel dextral and sinistral crustal scale shear zones responsible for the exhumation of the metamorphic complexes to constrain for the first time the timing of both shear zones and the timing of the transpressional shearing zircons and monazites were collected for u th pb ages results indicate that the shear zones have been active at c 320 ma in a transpressional setting widespread in the southern european variscan belt

partial resetting of the u th pb systems in experimentally altered monazite nanoscale evidence of incomplete replacement alteration experiments on natural monazite crystals manangotry standard madagascar under alkali conditions at 300 400 500 and 600 c and 200 mpa were conducted to clarify mechanisms behind incomplete resetting of u th pb geochronological systems in monazite replaced by dissolution and precipitation processes above 400 c experimental products show typical replacement textures a compositionally distinct monazite rim referred as altered rim surrounds the primary monazite mnz1 isotopic and electron microprobe u th pb in situ dating of the altered rim yields intermediate ages between pristine monazite 555 ma and complete experimental resetting 0 ma lead is systematically detected in altered rims with concentration decreasing from 400 c to 600 c the origin of incomplete resetting is elucidated with transmission electron microscope images that reveal an incomplete replacement of mnz1 by a secondary monazite mnz2 within the altered rim with increasing temperature the size and volume of the mnz2 within the altered rim become more important because no structural pb or pb nanoinclusions were observed pb in the altered rim is attributed to the mnz1 component partial resetting of u th pb systems depends on the nanomixture of different mnz1 proportions in the analyzed volume and explains the higher rejuvenation at 600 c than at lower temperatures although microanalytical techniques have the spatial resolution to date micrometer sized rims they are unable to resolve a nanoscale mixture of pristine and secondary monazite that could occur in altered rims formed by fluid driven replacement especially at low temperatures porosity and or inclusions and complex age scattering in zoned monazite are significant markers that can indicate a possible nano sized partial replacement

metamorphozed hercynian granitoids in the alpine structures of the central rhodope bulgaria geotectonic position and geochemistry abstract orthogneisses of late hercynian protolith age crop out in the central rhodope high grade metamorphic complex which is part of the alpine orogen in south eastern europe they compose a tectonic unit bordered by late alpine extensional shear zones these rocks reflect eocene amphibolite facies migmatization

deformation of granitic rocks across the brittle ductile transition abstract a microstructural analysis has been carried out on mylonites and mylonitic gneisses of the eastern peninsular ranges mylonite zone which were formed over a range of metamorphic conditions from lower greenschist to amphibolite facies composite planar fabrics in the form of c and s planes are found at all metamorphic grades fractured feldspars kinked biotites and ductile deformation of quartz characterize the lower greenschist facies mylonites at mid upper greenschist grade orthoclase grains show dynamic recrystallization textures whereas plagioclase exhibits low temperature plasticity with only minor recovery biotite ribbons form by progressive rotation and coalescence of kink band segments to produce chevron fold patterns at epidote amphibolite grade and above recovery processes and annealing recrystallization predominate in all minerals residual orthoclase porphyroclasts show strain related myrmekite formation along those sides of the grains that face the instantaneous shortening direction myrmekite formation due to replacement reactions cannot explain this geometry it is proposed that the myrmekites formed due to a combination of exsolution replacement and strain enhanced diffusion

implications of shear sense criteria for the tectonic evolution of the central rhodope massif southern bulgaria a systematic record of sense of shear criteria associated with peak conditions of amphibolite facies metamorphism from the central rhodope massif bulgaria gives evidence for superposition of two distinct terranes the lower terrane is characterized by south vergent flat lying thrusting that involved eclogites the upper terrane is characterized by east vergent flat lying shearing possibly associated with crustal extension we suggest that both regional metamorphism and deformation may be cretaceous in age there was erosion prior to emplacement of several crystalline sheets at shallow depths during the early cenozoic this was followed by oligocene extension and rhyolitic volcanism the rhodope massif can no longer be regarded as a stable paleozoic or older microcontinent

pressure temperature evolution of eclogites from the kechros complex in the eastern rhodope ne greece the rhodope domain in ne greece consists of different tectonometamorphic complexes involved in the alpine collisional history between the eurasian and african plates in the kechros complex which is the lowermost tectonic unit in the east rhodope a lense of kyanite eclogite occurs within orthogneiss and common eclogites are found between serpentinized peridotite and underlying pelitic gneisses in kyanite eclogite the high pressure hp mineral assemblage is grt omp jd35 55 ky ph qz rt indirectly inferred tlc law a na rich tremolite and zoisite formed at or near peak metamorphic conditions in common eclogites the hp mineral assemblage is grt omp jd29 41 rt and with less certainty amp gln rich brs wnc hbl czo the inclusions in garnet are glaucophane actinolite barroisite hornblende omphacite clinozoisite titanite rutile and rarely paragonite and albite in kyanite eclogite peak p t conditions are constrained at 2 2 gpa and 615 c using garnet omphacite phengite geothermobarometry and very similar values of 585 32 c and 2 17 0 11 gpa with the average p t method by which conditions of formation could also be narrowed down for the common eclogite 619 53 c and 1 69 0 17 gpa and for a retrogressed eclogite 534 36 c and 0 77 0 11 gpa ages for the hp metamorphism in the kechros complex are not yet available a rb sr white mica age of 37 ma from orthogneiss records a stage of the exhumation the hp event may be coeval with the eocene hp metamorphism 49 55 ma recorded in the nestos shear zone in central rhodope and in the attic cycladic crystalline belt where it is interpreted as the result of subduction and final closure of the axios vardar ocean and subsequent subduction of the apulian continental crust a promontory of the africa continent under the southern margin of the european continent in the late cretaceous and early tertiary

interpretation of monazite ages obtained via in situ analysis abstract monazite grains from nepal and vietnam were compositionally analyzed with an electron microprobe and dated th pb with an ion microprobe five sources of uncertainty explain age distributions from single samples that appear inconsistent with a single population 1 pb loss due to diffusion 2 dissolution reprecipitation reactions along a retrograde path 3 analytical uncertainties 4 analyses of overlapping age domains and 5 episodic monazite growth the influence of these factors is sample dependent but can be evaluated 1 using peak metamorphic conditions and x ray element maps to assess potential polymetamorphism or retrogression 2 obtaining other geochronologic data including previous work or dating other minerals in the sample 3 evaluating any method related uncertainty including counting statistics for electron microprobe analyses or calibration reproducibility for ion microprobe analyses and 4 ascertaining the potential growth mechanism of the monazite grain including dissolution of detrital grains or production from rare earth element ree oxide or allanite chemical contents of monazite grains analyzed in this study fail to reflect timing information or mineral growth mechanisms instead of relying on monazite chemical composition major mn fe mg ca and minor y element garnet zoning patterns and peak p t conditions should be used to facilitate age interpretation this thermobarometric data records the sample s thermal history changes in garnet growth rate and mechanisms and accessory mineral breakdown

fluid induced disturbance of the monazite th pb chronometer in situ dating and element mapping in pegmatites from the rhodope greece bulgaria abstract the monazite th pb chronometer is widely used in a variety of geological contexts but may be affected by interaction with fluids in the central part of the rhodope massif greece bulgaria synfolial pegmatite veins were emplaced within two major synmetamorphic shear zones in the chepelare shear zone czs which displays no evidence of post metamorphic fluid flow consistent monazite th pb ages of 36 ma are interpreted as dating vein emplacement higher in the metamorphic pile zircon and monazite give u th pb ages of 42 ma for vein emplacement in the nestos shear zone nsz pegmatites show evidence of post emplacement interaction with fluids during greenschist facies ductile deformation evidence for this includes the precipitation of calcite in microscale tension gashes within feldspar porphyroclasts and the oxygen isotope disequilibrium between quartz and feldspar aqueous carbonic fluids deriving from carbonate lithologies in the footwall of the nsz are identifiable in the pegmatites by the high 18 o value of quartz up to 21 8 and the presence of calcium in calcite veinlets in these samples some of the monazite grains show large intragrain scattering of th pb age spanning up to 12 ma with intergrain scattering reaching 16 ma from 39 to 55 ma within individual grains age domains correlate with chemical heterogeneities and some show a characteristic ca excess these chemical and isotopic alterations are interpreted to be caused by interaction with fluids derived from carbonate lithologies complementary u pb data on zircons from the nsz pegmatites yield mostly mesozoic ages related to an older metamorphic cycle and an age of 48 ma for one grain with typical magmatic zoning the th pb ages of 49 3 1 6 to 54 9 1 7 ma probably relate to the emplacement of the pegmatites the spread of younger th pb ages from 38 6 1 1 to 46 2 1 6 ma probably reflects the period of fluid circulation during progressive cooling to greenschist facies conditions or variable perturbations of the monazite isotope system during fluid assisted greenschist facies deformation this is consistent with 39 ar 40 ar mica ages of 32 to 34 ma interpreted as a tighter constraint for the timing of the greenschist facies metamorphism this study illustrates the capacity of monazite to record distinct events in a single rock and highlights the need for identifying the potential involvement of fluids in order to interpret monazite ages

time constraints for low angle shear zones in the central rhodopes bulgaria and their significance for the exhumation of high pressure rocks in the central rhodopes of southern bulgaria an eclogite bearing rock sheet belonging to the middle allochthon starcevo unit is over and underlain by eclogite free amphibolite facies rock units along low angle shear zones the borovica shear zone at the top and the starcevo ardino shear zone at the base the age of these shear zones is determined by u pb zircon dating of pre syn and posttectonic magmatic rocks mostly pegmatite veins using la sf icp ms zircons from pre to syntectonic pegmatites within the borovica shear zone yielded ages of ca 45 43 ma indicating that the shear zone was active at that time and zircons from a pretectonic pegmatite and a posttectonic granitoid body within the starcevo ardino shear zone yielded ages of ca 45 and ca 36 ma respectively giving a time frame for the activity of that shear zone which probably rather postdated the activity of the borovica shear zone by combining the ages with the kinematics of the shear zones and the metamorphic history of the rock units the following scenario is sketched soon after the starcevo unit reached peak pressure eclogite facies it was exhumed to a mid crustal level by top to the north west extensional unroofing along the borovica shear zone in a kinematic framework of orogen parallel extension beginning at ca 40 ma the partly exhumed starcevo unit was underthrust from the south west by continental crust of the foreland apulia forming the lower allochthon of the rhodopes along the starcevo ardino shear zone these results underline the significance of orogen parallel extension for the exhumation of high pressure rocks with respect to regional geology of the hellenides and the aegean it is found that the tectonic architecture of the rhodopes is essentially of tertiary age cretaceous syn metamorphic shear zones do exist but are largely restricted to higher levels of the nappe stack upper allochthon the rhodopes do not represent an older essentially mesozoic core of the hellenides but are formed by the internal higher metamorphic portions of the same major nappe systems as occur in the hellenides

high resolution 5 μm u th pb isotope dating of monazite with excimer laser ablation ela icpms abstract monazite lree po 4 a common accessory mineral in magmatic and metamorphic rocks is complementary to zircon in u th pb geochronology because the mineral can record successive growth phases it is useful for unravelling complex geological histories a high spatial resolution is required to identify contrasted age domains that may occur at the crystal scale bulk mineral techniques such as id tims applied to single monazite grains recording multiple overgrowths or isotope resetting can result in partly scattered discordant analytical points that produce inaccurate intercept ages laser ablation la icpms has been demonstrated to be a useful technique for u th pb dating of zircons and this study tests its analytical capabilities for dating monazite a sector field high resolution icpms coupled with a 193 nm arf excimer laser ablation microprobe is capable of achieving a high spatial resolution and producing stable and reliable isotope measurements the u th pb systematic was applied to monazite grains from several samples a lower palaeozoic lens from high grade terrains in southern madagascar neogene hydrothermal crystals from the western alps a palaeoproterozoic very high temperature granulite from central madagascar and a variscan leucogranite from spain directly on a polished thin section the major aim was to compare and or reproduce tims and emp ages of monazite from a variety of settings and ages the three independent 206 pb 238 u 207 pb 235 u and 208 pb 232 th ratios and ages were calculated isotope fractionation effects mass bias laser induced fractionation were corrected using a chemically homogeneous and u pb concordant monazite as external standard this study demonstrates that excimer laser ablation ela icpms allows u th pb dating of monazite with a high level of repeatability accuracy and precision as well as rapidity of analysis a spatial resolution almost comparable to that of emp in terms of crater width 5 m produced precise 208 pb 232 th 206 pb 238 u and 207 pb 235 u ratios for dating palaeozoic to precambrian monazites the advantages of ela icpms isotope dating are precision accuracy and the ability to detect discordance in the case of late miocene hydrothermal monazites from the alps a larger spot size of 25 m diameter is required and precise and accurate ages were obtained only for 208 pb 232 th systematics results from the variscan granite show that in situ u th pb dating of monazites with ela icpms is possible using a 5 m spot directly on thin sections so that age data can be placed in a textural context

constraining the prograde and retrograde p t t path of eocene hp rocks by shrimp dating of different zircon domains inferred rates of heating burial cooling and exhumation for central rhodope northern greece ion microprobe shrimp dating was carried out on different zircon domains from metamorphic rocks of the hp ht terrane of central rhodope northern greece to constrain the timing of prograde and retrograde stages within a single tectono metamorphic cycle a well determined p t t relative path for the metamorphic rocks of this terrane was used as a petrological basis for the geochronological investigations ion microprobe work was assisted by cathodoluminescence cl images of the zircon crystals the geochronological results revealed that hercynian continental crust was subducted during the eocene several stages of the eocene tectono metamorphic cycle including both the prograde and retrograde parts of the p t path above ca 300 c 0 3 gpa were dated using zircons from the following rock types 1 a deformed quartz vein probably formed at ca 300 c 0 3 gpa zircons in this vein precipitated from a hydrothermal fluid they yielded an age of 45 3 0 9 ma which corresponds to the time of a low t prograde stage of metamorphism 2 in kyanite eclogites zircons were entirely reset during eclogite facies metamorphism resetting was very probably enhanced by the presence of fluids derived by h2o liberating reactions close to the p peak they yielded an age of 42 2 0 9 ma 3 orthogneisses surrounding the kyanite eclogites contained zircons with magmatic oscillatory zoned cores which yielded hercynian ages of 294 8 ma age of granitic protolith formation whereas cl bright metamorphic rims yielded like the eclogite zircons ages of 42 0 1 1 ma therefore both the eclogites and orthogneisses are interpreted to have approached maximum depth at around 42 ma 4 in a leucosome of a migmatized orthogneiss oscillatory zoned zircons yielded an age of 40 0 1 0 ma at this time the rocks reached maximum temperatures during early decompression 5 a late pegmatite crosscutting the schistosity of amphibolites contained oscillatory zoned zircons that yielded a crystallization age of 36 1 1 2 ma thus the whole tectono metamorphic cycle above ca 300 c 0 3 gpa lasted from 45 3 0 9 ma to 36 1 1 2 ma that is 9 2 ma with an extreme error value of 2 1 ma based on combined shrimp and petrological data the average rates of heating and burial during subduction above ca 300 c 0 3 gpa are 94 c ma and 15 mm a respectively rates of cooling and exhumation also above 300 c 0 3 gpa are calculated to be 128 c ma and 7 7 mm a the eocene age of metamorphism in central rhodope implies that the terrane of at least central rhodope and the cyclades very probably was part of the same continental crust

pb diffusion in monazite a combined rbs sims study abstract we report measurements of pb diffusion in both synthetic cepo4 and natural monazites run under dry 1 atm conditions powdered mixtures of prereacted cepo4 and pbzro3 were used as the source of pb diffusant for in diffusion experiments conducted in sealed pt capsules for durations ranging from a few hours to several weeks following the diffusion anneals pb concentration profiles were measured with rutherford backscattering spectroscopy rbs and supplemented by measurements with secondary ion mass spectrometry sims in order to evaluate potential compositional effects upon pb diffusivity and simulate diffusional pb loss that might occur in natural systems we also conducted out diffusion experiments on pb bearing natural monazites in these experiments monazite grains were surrounded by a synthetic zircon powder to act as a sink monazites from these experiments were analyzed with sims over the temperature range 1100 to 1350 c the arrhenius relation determined for in diffusion experiments on synthetic monazite is given by d 0 94 exp 592 39 kj mol 1 rt m 2 s 1 diffusivities for synthetic and natural monazites are similar as are results of measurements made on the same samples using both rbs and sims the activation energy for pb diffusion we determined is more than three times that for natural monazite reported previously with pb diffusivities at

diamond former coesite and supersilicic garnet in metasedimentary rocks from the greek rhodope a new ultrahigh pressure metamorphic province established abstract we report here the first discovery of ultrahigh pressure uhp indicator minerals and textures from the rhodope metamorphic province rmp northern greece in particular we document the exsolution of quartz rutile and apatite in sodic garnet from metapelites garnet biotite kyanite gneisses which attests to the presence of a si ti na p rich precursor garnet phase similar textures in garnet have been reported in the literature only once before for eclogites from the su lu uhp metamorphic province china we also document the presence of microdiamonds and multicrystalline quartz pseudomorphs after coesite included in garnet from both eclogites and metapelites we then argue that these rocks had once been transported to depths exceeding 220 km well into the upper mantle thus establishing the rmp as another important uhp metamorphic belt in the world

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

late cenozoic extension of the alpine collisional orogen northeastern greece origin of the north aegean basin the rhodope metamorphic core complex exposed beneath the strymon valley detachment in northeastern greece comprises a platform carbonate sequence 5000 m thick intruded by tertiary calc alkaline plutons the final thickening of the north aegean alpine collisional orogen in paleogene time and its neogene quaternary extensional dismemberment in the backarc of the hellenic subduction zone produced structures within and above the rhodope core complex that are related here to four successive deformations in early middle eocene time d1 the falakron marble series was subducted northeastward beneath the serbo macedonian west thracian gneiss complex a heterogeneous ophiolite bearing high grade metamorphic terrane that was accreted to southeastern europe in cretaceous time the orogen began to extend on a northeast southwest axis in earliest miocene time evidenced in the rhodope core complex by the emplacement of the symvolon granodiorite ca 21 ma within a northwest trending midcrustal coaxial rupture of the falakron slab d2 the strymon valley detachment system succeeded the d2 symvolon rupture and related structures facilitating unroofing of the core complex and a transition from ductile to brittle deformation ca 16 3 5 ma d3 the serbo macedonian gneiss complex the island of thasos and a supradetachment basin were translated relatively southwestward as much as 80 km in the d3 hanging wall balanced reconstructions of d2 and d3 displacements predicate two new hypotheses concerning the cenozoic tectonic evolution of northeastern greece first the southwest vergent thasos detachment and northeast vergent low angle normal faults in the olympos region may both have rooted in the coaxial symvolon rupture zone to form a bivergent early miocene extensional system second the olympos and falakron carbonate platforms may be correlative remnants of the subducted eastern margin of the apulian microcontinent implying that the vardar zone the putative alpine suture is a rootless ophiolite belt and that segments of the alpine suture are actually exposed over a zone as wide as 200 km including the rhodope province cumulative d2 and d3 stretching of greater than or equal to 100 created the north aegean basin the north anatolian fault which accommodates the westward escape of anatolia from the pontide suture propagated into the north aegean region in late pliocene time its offshore continuation the north aegean trough transfers dextral strike slip displacement into extension to the north principally expressed in northern greece by the thermaikos strymon and drama half grabens d4

in situ shrimp u pb dating of monazite integrated with petrology and textures does bulk composition control whether monazite forms in low ca pelitic rocks during amphibolite facies metamorphism abstract bulk composition and specific reaction history among common silicate minerals have been proposed as controls on monazite growth in metapelitic rocks during amphibolite facies metamorphism it has also been implied that monazite that formed during greenschist facies metamorphism may be preserved unchanged under upper amphibolite facies conditions if correct this would make the interpretation of monazite ages in polymetamorphic rocks exceedingly difficult because isotopic dates could vary significantly in rocks that have experienced identical metamorphic conditions but differ only slightly in whole rock composition low ca pelitic schists from the mount barren group in southwestern australia display a range of whole rock compositions in afm space and different peak mineral assemblages resulting from amphibolite facies metamorphism 8 kb 650 c in this study we test whether bulk composition controls the formation of monazite through geochronology and textural evidence linking monazite growth with deformation and peak metamorphism x ray element mapping of monazite from the metapelitic rocks reveals concentric zoning in many grains with compositionally distinct cores and rims in situ shrimp u pb geochronology of monazite yields two 207 pb 206 pb age populations the cores and texturally early monazite give an age of 1209 10 ma interpreted to record prograde metamorphism whereas the rims and late monazite define a single population of 1186 6 ma which is considered the likely age of peak thermal metamorphism the growth of monazite was widespread in low ca pelitic schists representing a broad range of compositions in afm space indicating that variations in bulk composition in afm space did not control the formation of monazite during amphibolite facies metamorphism in the mount barren group

solubility of cepo4 monazite and ypo4 xenotime in h2o and h2o nacl at 800 c and 1 gpa implications for ree and y transport during high grade metamorphism abstract monazite and xenotime are important accessory minerals in metasediments because they host ree and are useful for geochronology and geothermometry it is therefore essential to understand their behavior during metasomatic processes that attend metamorphism and subduction in order to constrain the solubility of monazite and ypo 4 xenotime at high pressure and temperature we carried out weight loss experiments on synthetic single crystals of cepo 4 monazite and ypo 4 xenotime using hydrothermal piston cylinder methods at 800 c and 1 gpa in h 2 o and h 2 o nacl fluids results indicate that cepo 4 and ypo 4 dissolved congruently and that their solubilities in pure h 2 o are very low 0 04 0 04 and 0 25 millimolal respectively the solubility of cepo 4 rises to an increasing extent with added nacl to 7 94 0 07 millimolal at 50 mol nacl in contrast the solubility of ypo 4 rises to a decreasing degree with increasing nacl to 4 36 0 08 millimolal at 50 mol nacl best fit equations for the solubilities of the two phases are m cepo 4 3 56 10 5 5 82 10 3 x nacl 1 97 10 2 x 2 nacl and m ypo 4 2 48 10 4 1 26 10 2 x nacl 8 89 10 3 x 2 nacl the solubility of ypo 4 is greater than that of cepo 4 at nacl mole fractions x nacl of 0 00 0 27 the solubility enhancement behavior implies that ce dissolves as anhydrous chloride complexes whereas y forms mixed cl oh solutes the results provide a simple mechanism for redistributing ree and y in deep crustal and upper mantle environments the h 2 o ce ratio inferred for subduction zone melts and silicate rich fluids can also be produced by a cepo 4 saturated fluid with x nacl 0 1 in addition neutral ph h 2 o nacl fluids can transport substantial ree and y obviating the need to invoke highly acid solutions in environments where they are unlikely

raman micro spectroscopy on diamond graphite and other carbon polymorphs from the ultrahigh pressure metamorphic kimi complex of the rhodope metamorphic province ne greece raman micro spectroscopy was applied on carbon inclusions in garnet porphyroblasts from kyanite biotite garnet schists of the rhodope metamorphic province rmp ne greece diamond and cuboids of poorly to highly ordered graphite were identified either as single phase inclusions or as polyphase inclusions along with co2 and or carbonates calcite magnesian calcite questionable raman bands that may be assigned to other c phases nanodiamond lonsdaleite a different c polymorph have been observed the presence of diamond confirms beyond any doubt the ultrahigh pressure uhp metamorphism reported by mposkos and kostopoulos 1 e mposkos d kostopoulos diamond former coesite and supersilisic garnet in metasedimentary rocks from the greek rhodope a new ultrahigh pressure metamorphic province established earth planet sci lett 192 2001 497 506 in the rmp cuboid graphite showing variable degree of disordering most probably formed after diamond the possible involvement of co2 and or c o h fluids in the formation of diamond is discussed

comparison of 40ar 39ar conventional and laser dating of biotites from the north tregor batholith bulk samples and single grains of biotites from the north tregor batholite armorican massif france were analysed by the 40ar39ar method apparently pure single biotite grains analysed by the step heating procedure with the laser probe displayed plateau ages or slightly disturbed age spectra one strongly chloritized single biotite grain gave a very disturbed age spectrum with younger ages the bulk samples of biotites of the same rocks displayed disturbed age spectra characterized by younger low temperature ages and a hump at intermediate temperature steps these disturbances are explained by 39ar recoil from the biotite into the chlorite which is clearly evidenced by hrtem observations and not by excess 40ar the more concordant results obtained on single biotites are probably due to a more careful selection of the minerals in most cases we obtain good concordance of the ages 600 610 ma measured on the apparently pure single biotite grains and the high temperature ages of both the bulk samples of amphiboles and biotites from the same rocks

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

nanosims mapping and la icp ms chemical and u th pb data in monazite from a xenolith enclosed in andesite central slovakia volcanic field in this study we use nanosims element and isotope ratio mapping and la icp ms trace element measurements to elucidate the origins of monazites from a restitic xenolith enclosed in a 13 5 0 3 ma andesitic lava slovakia the xenolith lava interaction is mainly characterized by the growth of a plagioclase bearing corona around the xenolith and magmatic garnet overgrowths on primary metamorphic garnets within the xenolith nanosims images 89y 139la 208pb 232th and 238u and trace element analyses indicate that variations of hree y and eu contents in the monazite are correlated with the resorption and the following overgrowth of garnet and plagioclase in the xenolith three domains are distinguished in the monazite grains the inherited variscan core at ca 310 ma m1 domain characterized by low y and hree contents and a weak negative eu anomaly the inner rim m2 domain crystallized during the growth of the plagioclase magmatic corona large negative eu anomaly and the resorption of metamorphic garnet high hree and y contents and the external rim m3 domain crystallized during the growth of the plagioclase corona large negative eu anomaly and during the crystallization of magmatic garnet low y hree contents at 13 ma i e the age of the andesitic lava the age and chemical zonation of the monazites attest to the preservation of primary monazite in the xenolith despite the interaction with the andesite lava nanosims imaging provides high quality sub m scale images of the monazite that reveals chemical domains that were not distinguishable on wds x ray maps especially for depleted elements such as u and pb owing to its small size the m2 domain could not be accurately dated by the la icp ms method however nanosims isotopic maps reveal that the m2 domain has similar 208pb 232th isotope ratios to the m3 domain and thus similar ages these results support the hypothesis that melt assisted partial dissolution precipitation in monazite efficiently records chemical and mineralogical changes during xenolith lava interaction

apatite monazite and xenotime in metamorphic rocks this chapter focuses on phosphates that are significant in metamorphic rocks a quick survey of phosphate mineral descriptions at a commercial mineral web site revealed over 500 phosphate mineral names remarkably only three are common in metamorphic rocks apatite monazite and xenotime and this chapter is restricted to discussion of these minerals apatite monazite and to a lesser extent xenotime have enjoyed intensive study during the previous half century following world war ii considerable study was made of minerals that contain fissionable materials and the sometimes large concentrations of u and th in monazite culminated in a number of seminal papers on the occurrence of that mineral e g overstreet 1967 during the 1970s and early 1980s the study of apatite and monazite turned towards their role as a sink for rees and other trace elements in rocks e g watson 1980 watson and capobianco 1981 throughout this period monazite and to some extent xenotime enjoyed attention as a geochronometer e g parrish 1990 and through today this attention has continued to grow accessory minerals such as monazite xenotime and apatite have come to the forefront of research in metamorphic petrology in recent years for two reasons first it has recently been recognized that trace elements in metamorphic rocks and especially trace element zoning in garnet and other major phases contain considerable detailed information about the reaction history a rock has experienced coupled with the fact that diffusion of many trace elements in metamorphic minerals is in general considerably slower than diffusion of major elements in those same minerals the possibility has recently emerged of recovering details of metamorphic petrogenesis that were previously unattainable inasmuch as accessory minerals play a dominant role in the mass budget of many trace elements attention has naturally turned to their paragenesis it has also recently been discovered that

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

lateral growth of the continental crust through deep level subduction accretion a re evaluation of central greek rhodope abstract the rhodope massif of northern greece and southern bulgaria has been variably interpreted as a fragment of pre alpine possibly hercynian or precambrian age continental crust that was actively involved in european margin convergent tectonics during the mesozoic and tertiary alternatively we propose that for central greek rhodope the lithological associations and tectonothermal history are consistent with progressive south directed mesozoic crustal growth through deep level subduction accretion a process which involved the near continuous transferral of material of oceanic and continental margin affinity from footwall to hanging wall within a north directed palaeotethyan subduction system central greek rhodope comprises a sequence of schists gneisses and migmatites of wide compositional variation that have undergone intense tectonic intermixing extensive occurrences of marble are associated with amphibolites variably retrogressed eclogites meta gabbros and ultramafic rocks quartzo feldspathic gneisses are abundant throughout the area and are associated with psammitic meta greywacke sequences pelites and quartzites the presence of intercalated marble slivers and thin manganiferous and iron rich siliceous bands is significant the various components exposed in the subduction accretion complex were first underthrust northwards to deep crustal levels subjected to high pressure eclogite facies metamorphism and underplated to the base of the overriding eurasian plate this latter accretion event was characterised by intense southwest directed ductile thrusting and pervasive amphibolite facies metamorphic overprinting it was accompanied by migmatisation and intracrustal melt generation and followed by subduction related magmatism deep level subduction accretion leads to southward growth of the continental margin which in turn leads to subduction retreat allowing the encroachment of the magmatic arc from the north subsequent extensional tectonic exhumation brought the complex up to its present position within the southern margin of eurasia this new interpretation may have implications for other high grade terrains within convergent and collisional systems that have been assumed to represent ancient or pre existing continental crust if so then the importance of deep level subduction accretion as a mechanism of continental crustal growth has hitherto been underestimated