identifying granite sources by shrimp u pb zircon geochronology an application to the lachlan foldbelt the potential genetic link between granites and their host sediments can be assessed using zircon age inheritance patterns in the lachlan fold belt southeastern australia granites and associated high grade metasedimentary rocks intrude low grade ordovician country rock this relationship is well exposed in the tallangatta region northeast victoria part of the wagga omeo metamorphic complex in this region granites two i types and two s types have intruded during the mid late silurian between approximately 410 430 ma based on the ages of magmatic zircons the age spectra for inherited zircons from the granites have been compared with those of detrital zircons from the enclosing low and high grade metasediments in broad terms both for detrital zircons in all four sediments and for inherited zircons in three of the four granites the dominant ages are early paleozoic and late precambrian with sporadic older precambrian ages extending up to 3 5 ga the ages of the youngest detrital zircons from the low grade lockhart and talgarno terranes limit the time of sedimentation to ca 466 ma or younger the youngest detrital zircons from two samples of the high grade gundowring terrane are 473 ma making these sediments ordovician or younger not cambrian as originally suggested however the individual age spectra for the four selected metasediments are not well matched when closely examined the age spectra of the inherited zircons in the granites also do not adequately match those in any of the metasediments thus the metasediments might not be representative of the actual source rocks of the granites while the exact source of the granites cannot be identified from the analysed samples the existence of a large population of ca 495 ma inherited zircon grains in the s type granites requires that the granite source contains a significant proportion of cambrian or younger material this does not preclude the existence of a precambrian basement to the lachlan fold belt but indicates that at the level of s type magma generation a cambrian and or younger protolith is required

identification of inherited radiogenic pb in monazite and its implications for u pb systematics the simplest interpretation of uranium lead mineral ages in igneous rocks embodies the assumption that melting of pre existing rocks leads to homogenization of the lead isotopes thus resetting the radiometric clock zircons often violate this assumption by only partially dissolving and preserving a component of lead inherited from before the melting episode but until now such behaviour has not been observed in monazite here we report u pb data for zircons and monazites from a tertiary granite from the himalayas in which zircons and monazites contain inherited radio genic lead an analysis of a single entirely igneous zircon crystal plots on concordia at 19 5 0 4 myr eight monazite fractions lie on a line from an upper intercept of 471 10 myr to the youngest fraction which plots slightly above concordia at 21 myr the presence of excess 206pb in the youngest monazite fractions indi cates that the monazite data represent a mixing line between young magmatic monazite and an older inherited component rather than a resetting of 471 myr monazites at 20 myr these data suggest a closure temperature for pb in monazite significantly higher than previously published estimates

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

chronology of deformation within the turbidite dominated lachlan orogen implications for the tectonic evolution of eastern australia and gondwana ar ar data from fabric forming white mica in slates syntectonic quartz veins and granitic mylonites constrain the timing of metamorphism deformation and exhumation in the lachlan orogen australia these data also help define the tectonic evolution of the tasmanides during paleozoic time the lachlan orogen formed by the progressive accretion of a thick turbidite fan sequence and volcanic terrains to gondwana during the closing of a small marginal ocean or back arc basin this tectonic setting was similar to the present western and southwestern pacific region accretion of the lachlan orogen to gondwana occurred by closing of the basin system by subduction accretion processes and some translation the process is typified in the western lachlan orogen by a major eastward migrating deformation involving chevron folding and faulting over an eastward propagating decollement melange zone and is recorded by ar ar mica ages ranging from 455 ma in the western part to 390 ma in the eastern part in the central lachlan orogen deformation migrated southwestward from 440 430 to 405 ma away from the high temperature wagga omeo metamorphic complex where deformation metamorphism occurred between 440 and 400 ma in the north 400 ma mica ages record deformation and inversion of structures in the cobar basin in the eastern lachlan orogen ar ar mica dates range from 450 to 340 ma ages of 455 445 ma are yielded by the narooma accretionary complex 405 390 ma ages are found along the major thrust faults bounding high grade metamorphic complexes and 360 340 ma cooling ages are found in the inverted extensional basins e g hill end and related structural zones the ar ar results also document periods of reactivation on early formed structures during later deformation elsewhere in the orogen

a weathering related origin of widespread monazite in s type granites the s type granite suites comprising more than a quarter of the extensively developed granites in the lachlan fold belt australia contain monazite which may be related to the chemical weathering of the sedimentary source rocks we report a process whereby chemical weathering fixes mobile rare earth elements ree in hydrous phosphate phases such as florencite and rhabdophane this material contains up to 50 wt lree and occurs as very small particles 3 m dehydration of these hydrous ree phases during anatexis directly yields monazite the low solubility of phosphorus in s type granite melts inhibits dissolution of both monazite and apatite refractory monazite may be thus entrained and transported in s type granites in a manner similar to processes resulting in inherited zircon since both th and the light ree are major components in monazite materials containing this minute phase may be of widespread geochemical significance in both granites and metamorphic rocks

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

geochronological constraints on the legs lake shear zone with implications for regional exhumation of lower continental crust western churchill province canadian shield the legs lake shear zone marks the southeastern boundary of an extensive region 20 000 km2 of high pressure 0 8 1 5 gpa granulite facies rocks in the western churchill province canada the shear zone is one of the largest exhumation related structures in the canadian shield and coincides with the central segment of the 2 800 km long snowbird tectonic zone the movement history of this shear zone is critical for the development of models for the exhumation history of the high pressure region we used electron microprobe u th pb dating of monazite with supplemental id tims u pb geochronology to place constraints on the timing of shear zone activity combining these and other data we suggest that regional exhumation occurred during at least three distinct phases over an 150 million year period the first phase involved high temperature decompression from 1 0 to 0 8 0 7 gpa shortly following 1 9 ga peak metamorphism possibly under an extensional regime the second phase involved rock uplift and decompression of the hanging wall to 0 5 0 4 gpa during east vergent thrusting across the legs lake shear zone at ca 1 85 ga this phase was likely driven by early collision related convergence in the trans hudson orogen the final phase of regional exhumation involving the removal of 15 20 km of overburden from both footwall and hanging wall likely occurred after 1 78 ga and may have been related to regional extensional faulting

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

monazite occurrence chemistry and chronology in the granitoid rocks of the lachlan fold belt australia an electron microprobe study in situ electron microprobe dating of monazite holds the promise of being an effective technique for obtaining chronologic data our research focuses on i and s type granitoids of the lachlan fold belt australia whose petrology and zircon chronology have been thoroughly characterized this study documents the textural relationships morphology zoning and ages of monazite in these granitoid rocks the i type granitoids that lack monazite usually contain other mineral phases enriched in rare earth elements such as allanite titanite and bastnasite only silica rich highly evolved i type granitoids contain monazite this preference in i type rocks for phases other than monazite to host the ree and th limit the applicability of monazite dating for this group of igneous rocks on the other hand monazite is ubiquitous in s type rocks both as interstitial and included grains high resolution x ray maps of individual monazites reveal complex patterns of chemical zoning weighted averages of multiple analyses for individual chemical domains show small but systematic differences in age these weighted averages of the chemical domains are considered the best estimate of the age of the monazite monazites from 8 different s type samples range in age from 405 to 759 ma with the majority being 490 ma and older these are premagmatic ages for these granitoids which have crystallization ages of 400 to 430 ma these premagmatic ages are similar to cambro ordovician ages obtained from inherited zircon cores in the same granitoids indicating that monazite can survive anatexis in peraluminous rocks thus monazite dating in peraluminous rocks may illuminate characteristics composition and age of source rocks and anatectic processes

thermal regimes anatexis and orogenesis relations in the western lachlan fold belt southeastern australia abstract crustal evolution of the lachlan fold belt of southeastern australia has involved the mobilisation of significant melt volumes now preserved as granites suggesting that large parts of the lower crust were at t 800 c for part of the thermal evolution this anomalous thermal activity has been variously related to continental extension magmatic underplating and to thinning of the mantle lithosphere overriding considerations reflected in the surface geology must include the tectonic setting the thermal regime and the state of stress in the crust at the time of anatexis one dimensional geothermal modelling suggests that no one mechanism can generate the required thermal regime necessary to produce the volume of granite observed in the crust the observed geological relationships do not match those predicted for lithospheric thinning due to continental extension magmatic underplating can cause melting but must involve extensive source zones and allow continual feeding of the magma to higher crustal levels the crustal thickening lithospheric thinning mechanism involves upwelling of hot asthenospheric mantle to the base of the crust but the melt products are confined to a very thin layer at the crust mantle boundary mechanisms for melt extraction and emplacement to higher crustal levels must be explained in the western lachlan fold belt the presence of a major mid crustal detachment fault ductile shear zones in the lower crust and marked internal shortening of the upper and possibly lower crustal blocks suggests that shear heating may also provide a significant component to the heat budget of the evolving crust melting will not occur however unless thermal equilibration of both the upper and lower plates takes place prior to shear heating such equilibration will involve time scales in the order of 30 50 ma unless extra heat is added into the lower crust e g advective input inferred flexural rigidity of the lithosphere related to basinal subsidence during the period of orogenesis in central victoria suggests that thermal effects related to granitoid intrusion must be largely post tectonic and that felsic intrusive activity is a consequence of the crustal thickening rather than the cause thermal regimes during orogenesis cannot be restricted to a single mechanism upwelling of hot asthenosphere causing significantly higher heat input at the base of the crust coupled with shear heating and internal strain heating of the crust during the crustal thickening stage as well as thermal effects related to magma intrusion into the lower crust result in a complex thermal history during collisional orogenesis

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

age mapping and dating of monazite on the electron microprobe deconvoluting multistage tectonic histories high resolution x ray mapping and dating of monazite on the electron microprobe are powerful geochronological tools for structural metamorphic and tectonic analysis x ray maps commonly show complex th u and pb zoning that reflects monazite growth and overgrowth events age maps constructed from the x ray maps simplify the zoning and highlight age domains microprobe dating offers a rapid in situ method for estimating ages of mapped domains application of these techniques has placed new constraints on the tectonic history of three areas in western canada age mapping has revealed multiphase monazite with older cores and younger rims included in syntectonic garnet microprobe ages show that tectonism occurred ca 1 9 ga 700 m y later than mylonitization in the adjacent snowbird tectonic zone in new mexico age mapping and dating show that the dominant fabric and triple point metamorphism occurred during a 1 4 ga reactivation not during the 1 7 ga yavapai mazatzal orogeny in norway monazite inclusions in garnet constrain high pressure metamorphism to ca 405 ma and older cores indicate a previously unrecognized component of ca 1 0 ga monazite in all three areas microprobe dating and age mapping have provided a critical textural context for geochronologic data and a better understanding of the complex age spectra of these multistage orogenic belts

electron microprobe dating of monazites from high grade gneisses and pegmatites of the kerala khondalite belt southern india monazites of five samples one leptynitic garnet biotite gneiss one khondalite one augen gneiss and two pegmatites from the central and northern part ponmudi unit of the kerala khondalite belt kkb in southern india were analyzed with the electron microprobe dating technique monazites in the augen gneiss and the pegmatites yield grain sizes between 200 800 m th abundances are rather low 10 wt and the distribution of th u and pb within single grains is fairly homogeneous contrasting to this monazites in the leptynitic gneiss and the khondalite are small 150 m they often display very complex th u pb patterns and contain high th concentrations up to 20 wt the statistical treatment of individual ages from the investigated samples revealed three populations of lower proterozoic 1 9 ga upper proterozoic 580 ma and ordovician age 470 ma as well as mid proterozoic ages between 0 8 1 7 ga which are not regarded to be of geological significance lower proterozoic ages are preserved in the cores of monazites from leptynitic gneisses and khondalites they fairly agree with sm nd model ages for similar rocks of the kkb and give a minimum age for first monazite growth or complete homogenization the prominent pan african population with mean values between 540 and 580 ma is present in the leptynitic gneiss the khondalite and the augen gneiss and in line with published isotope ages for the kkb the ordovician population finally marks the emplacement of granitic pegmatites subsequent to the pan african high grade metamorphic event there is an obvious discrepancy between khondalites and leptynitic gneisses on the one hand and augen gneisses on the other concerning the presence of lower proterozoic ages while these are abundant in the former often rimmed by upper proterozoic ages they are completely absent in the latter it appears unlikely that lower proterozoic ages were completely reset during a pan african event exclusively in the augen gneisses while they were preserved in leptynitic gneisses and khondalites it is further concluded that the augen gneisses are of magmatic origin and were derived from porphyritic granites thus the upper proterozoic age of 605 37 ma calculated for the investigated sample approximates the time of magma emplacement which slightly precedes the peak stage of pan african high grade metamorphism in the kkb and of monazite crystallization from the granitic melt a characteristic feature of the investigated monazites is the resetting of lower proterozoic and pan african ages to significantly younger values due to partial pb loss monazites not shielded by other minerals e g garnet suffered selective mobilization of pb along fractures or at their rims while th and u concentrations remained almost unchanged the results presented in this study indicate that this was mainly due to fluid rock interaction it is concluded that thermal diffusion of pb even at the suggested temperatures of 900 c only had minor influence on the th u pb composition in monazite and that the closure temperature for this system must be significantly higher than previously assumed 750 c

cinder cone clustering in the transmexican volcanic belt implications for structural and petrologic models cinder cone distributions have most often been characterized using univariate statistics here a new technique to volcano distribution studies cluster analysis is applied to cinder cone distribution in the central transmexican volcanic belt tmvb a total of 1016 cinder cones are identified over an area of approximately 60 000 km2 application of cluster analysis reveals structure in cinder cone distribution using a search radius parameter of 16 km 75 of the cinder cones within the central tmvb are found within eight clusters of 45 159 cones each these clusters are each 2000 to 5000 km2 in area only 22 cones are found within clusters of three or fewer cinder cones indicating that clustering is a pervasive phenomenon some petrologic variation is evident among clusters low mg alkaline cinder cones are found within a single cluster 360 400 km from the trench application of alignment analysis techniques including the two point azimuth method the hough transform and two dimensional fourier analysis demonstrates that cinder cone alignments have common orientations on a regional scale within three clusters all located in the southernmost part of the michoacan guanajuato volcanic field in the western portion of the study area these alignments consist of tens of cinder cones are 20 50 km long and are all oriented with azimuths of 020 040 parallel to the direction of plate convergence high mg lavas which last fractionated at pressures in excess of 8 kbar are only found associated with these alignments indicating that magma transport is significantly enhanced in these areas although local alignments of three to six cinder cones occur within mapped fault zones that transect the area regional cinder cone alignment patterns and the distribution of clusters themselves do not appear to be affected by the presence of these fault zones the cluster analysis the alignment analysis and some petrologic data support the hypothesis that the tmvb is segmented near 101 w

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

detection of inherited monazite in the manaslu leucogranite by 208pb232th ion microprobe dating crystallization age and tectonic implications abstract although leucogranites are among the least petrologically variable of all igneous rocks ironically they are among the most difficult to reliably date the high himalayan leucogranites have been the subject of numerous geochronological investigations because of their interrelationship with the most significant tectonic features of that mountain belt for a variety of reasons linked to the minimum melt composition of these leucogranites these dating studies have not been entirely successful we report results of a new ion microprobe dating method based on the decay of 232 th to 208 pb in monazite that has directly revealed the presence of inherited pb in monazite from the manalsu granite casting doubt on its previously accepted age monazite ages obtained from this leucogranite yield two distinct populations a large number of ages with a normal distribution and mean age of 22 4 0 5 ma 2 s e that we interpret to be the crystallization age and a smaller inherited fraction with an age of ca 600 ma because formation of the granite is thought to be related to slip on the main central thrust both this date and a second less precise result from a structurally similar pluton near mt everest indicate that the main central thrust was active in the interval 24 22 ma but do not constrain its initiation together with crosscutting relationships these data require that movement on the north himalayan fault occurred prior to 22 ma at both locations

microprobe monazite geochronology putting absolute time into microstructural analysis abstract high resolution compositional mapping and dating of monazite on the electron microprobe is a powerful addition to microstructural analysis and an increasingly important tool for tectonic analysis microprobe monazite geochronology can be an efficient reconnaissance tool for evaluating metamorphic and deformational age domains but more importantly its in situ nature and high spatial resolution offer an entirely new level of structurally and texturally specific geochronologic data that can be used to put absolute time constraints on p t d paths constrain the rates of metamorphic and deformational processes and provide new links between metamorphism and deformation microprobe geochronology is particularly applicable to three persistent microstructural microtextural problem areas 1 constraining the chronology of metamorphic assemblages 2 constraining the timing of deformational fabrics and 3 interpreting other geochronological results although some monazite generations can be directly tied to metamorphism or deformation at present the most common constraints rely on monazite inclusion relations in porphyroblasts that in turn can be tied to the deformation and or metamorphic history microprobe mapping and dating allow geochronology to be incorporated into the routine microstructural analytical process resulting in a new level of integration of time t into p t d histories

time calibration of a p t path from a variscan high temperature low pressure metamorphic complex bayerische wald germany and the detection of inherited monazite a temperature time path was constructed for high temperature low pressure ht lp migmatites of the bayerische wald internal zone of the variscan belt germany the migmatites are characterised by prograde biotite dehydration melting peak metamorphic conditions of approximately 850 c and 0 5 0 7 gpa and retrograde melt crystallisation at 800 c the time calibration of the pressure temperature path is based on u pb dating of single zircon and monazite grains and titanite separates on 40ar 39ar ages obtained by incremental heating experiments on hornblende separates single grains of biotite and k feldspar and on 40ar 39ar spot fusion ages of biotite determined in situ from sample sections additionally crude estimates of the duration of peak metamorphism were derived from garnet zoning patterns suggesting that peak temperatures of 850 c cannot have prevailed much longer than 2 5 ma the temperature time paths obtained for two areas approximately 30 km apart do not differ from each other considerably u pb zircon ages reflect crystallisation from melt at 850 800 c at 323 ma southeastern area and 326 ma northwestern area the u pb ages of monazite mainly coincide with those from zircon but are complicated by variable degrees of inheritance the preservation of inherited monazite and the presence of excess 206pb resulting from the incorporation of excess 230th in monazite formed during ht lp metamorphism suggest that monazite ages in the migmatites of the bayerische wald reflect crystallisation from melt at 850 800 c and persistence of older grains at these temperatures during a comparatively short thermal peak the u pb ages of titanite 321 ma and 40ar 39ar ages of hornblende 322 316 ma and biotite 313 309 ma reflect cooling through the respective closure temperatures of approximately 700 570 500 and 345 310 c published in the literature most of the feldspars ages 305 296 ma probably record cooling below 150 300 c while two grains most likely have higher closure temperatures the temperature time paths are characterised by a short thermal peak by moderate average cooling rates and by a decrease in cooling rates from 100 c my at temperatures between 850 800 and 700 c to 11 16 c my at temperatures down to 345 310 c further cooling to feldspar closure for ar was probably even slower the lack of decompressional features the moderate average cooling rates and the decline of cooling rates with time are not easily reconciled with a model of asthenospheric heating rapid uplift and extension due to lithospheric delamination as proposed elsewhere instead the high peak temperatures at comparatively shallow crustal levels along with the short thermal peak require external advective heating by hot mafic or ultramafic material

electron microprobe observations of pb diffusion in metamorphosed detrital monazites electron microprobe analyses have been made on monazite grains from paragneiss samples in the andalusite sillimanite transition 620 15 c and sillimanite orthoclase 680 15 c zones of the cretaceous ryoke metamorphic belt southwest japan monazites from pelitic gneisses are of metamorphic origin euhedral to subhedral and chronologically homogeneous giving chemical th u total pb isochron chime ages of 98 8 3 3 98 0 3 2 ma two psammitic gneisses of individual metamorphic grade contain both metamorphic monazite grains and detrital ones as old as ca 1700 ma most detrital monazite grains are heterogeneous in the tho2 and uo2 concentrations and have multiple or single rims as young as ca 100 ma several detrital monazite grains are well rounded in form exhibit homogeneous th and u distributions and show a pb diffusion profile in the margin the width of the diffusion zones is approximately constant throughout grains from each psammitic gneiss 18 22 m for 620 c and 48 58 m for 680 c assuming the isothermal diffusion of pb from homogeneous monazite spheres during a 5 ma duration of peak metamorphism we obtain diffusion coefficients of 1 9 0 3 10 21 and 1 5 0 3 10 20 cm2 s at 620 c and 680 c respectively these data derive an activation energy of 2 44 2 85 1 26 105 j mol and a frequency factor of 3 4 10 7 8 5 10 12 2 2 107 cm2 s taking account of uncertainties of 15 c in the temperatures and of 20 in the diffusion coefficients the diffusion parameters obtained from natural samples in this study provide a reliable insight into the closure temperature for pb in monazite that has been poorly understood so far

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

reply to comment by brian stevens on evidence and timing of crustal extension versus shortening in the early tectonothermal evolution of a proterozoic continental rift sequence at broken hill australia

hot orogens tectonic switching and creation of continental crust many granulite terrains were too hot to have formed during continental collision rather along with many high grade metamorphic terrains that typify continental crust most formed in accretionary orogens during tectonic switching when prolonged lithospheric extension was interrupted by intermittent transient contraction based on modern and ancient examples tectonic switching occurs when slab retreat induces upper plate extension causing arc splitting formation of microcontinent slivers and backarc basins then intermittent arrival of buoyant oceanic plateaus induces transient flat subduction or slab flip and crustal thickening during slab retreat basaltic magmas produced from decompressed asthenosphere advect into the extending orogen causing granulite facies metamorphism and granite generation but subsequent thickening during flat subduction cools the region thickening is focused in the thermally softened backarc region and if sediment filled a hot short lived 10 m y narrow 50 100 km orogenic belt forms such thickening is often misleadingly ascribed to arc or microcontinent collision once slab retreat mode is reestablished lithospheric extension recommences and a new arc backarc system forms generally outboard arrival of another plateau will reverse the procedure and another short lived hot orogen will form within the orogenic system cycles of tectonic switching efficiently produce continental crust

overthrust terranes in the lachlan fold belt southeastern australia the paleozoic history of the lachlan fold belt involves terrane translation and accretion of discrete allochthonous fragments four major terranes developed adjacent to the gondwana continental margin and were overthrust in the middle devonian to form a partly composite crust for southeastern australia