crustal scale block tilting during andean trench parallel extension structural and geo thermochronological insights despite a long history of plate convergence at the western margin of the south american plate that has been ongoing since at least the early paleozoic the southern peruvian fore arc displays little to no evidence of shortening in the light of this observation we assess the deformation history of the southern peruvian fore arc and its geodynamic implications to accomplish this we present a new structural and geo thermochronological data set zircon u pb mica 40ar 39ar apatite and zircon fission track and zircon u th he analyses for samples collected along a 400 km long transect parallel to the trench our results show that the mesoproterozoic gneissic basement was mainly at temperatures 350 c since the neoproterozoic and was later intruded by jurassic volcanic arc plutons along the coast a peculiar apatite fission track age pattern coupled with field observations and a synthesis of available geological maps allows us to identify crustal scale tilted blocks that span the coastal peruvian fore arc these blocks bounded by normal faults that are orthogonal to the trench suggest post 60 ma trench parallel extension that potentially accommodated oroclinal bending in this region block tilting is consistent with the observed and previously described switch in the location of sedimentary sources in the fore arc basin our data set allows us to estimate the cumulative slip on these faults to be less than 2 km and questions the large amount of trench parallel extension suggested to have accommodated this bending

detrital and metamorphic 40 ar 39 ar ages from muscovite and whole rock samples meguma supergroup southern nova scotia 40ar 39ar dates from eight muscovite separates and two whole rock samples of slate siltstone and sandstone provide detrital metamorphic and post metamorphic overprinting ages for the low grade metamorphic rocks of the meguma supergroup in the mahone bay region of southern nova scotia variably reset detrital muscovite ages of 600 550 ma agree with previously obtained u pb dates from detrital zircon separates of foliation parallel muscovite yield precise 40ar 39ar ages of 395 388 ma petrographic and metamorphic data indicate that these are metamorphic mica growth ages the range overlaps with the younger limit of previously reported whole rock ages 410 380 ma but is more tightly constrained an apparent inconsistency between previous 40ar 39ar whole rock ages as old as 410 ma and regional deformation that affected sedimentary rocks as young as 385 ma has been resolved by the younger ages reported here and by recent adjustments to the early paleozoic time scale which suggest that the lochkovian to

spatially correlated anomalous 40ar 39ar age variations in biotites about a lithologic contact near simplon pass switzerland a mechanistic explanation for excess ar forty four biotite samples collected about a lithologic contact between pelite and amphibolite were analyzed for 40ar 39ar and demonstrate the importance of bulk ar diffusivity and system geometry factors not usually considered in the interpretation and collection of 40ar 39ar age data the resulting 40ar 39ar apparent ages range from 11 30 0 05 ma to 17 90 0 10 ma the ages and excess argon contents are spatially and lithologically correlated the pelite samples all yield ages clustering around 12 ma the age expected for cooling through biotite closure 360 c in this region of the alps ages in the amphibolite biotites are older showing a smooth trend between 15 ma at the contact with the pelite to 18 ma 34 cm from the contact this data shows that characterization of the ar closure age for biotite in a given system should not rest on a single sample as otherwise irresolvable differences in age between samples within the same outcrop can exist a generalized mechanistic model for excess argon is presented the presence or absence of excess ar depends on an intrinsic system parameter t the transmissive timescale which is the characteristic time for 40ar to escape through the local intergranular transporting medium itm to some sink for argon to prevent buildup of geochronologically significant excess 40ar t must be very short relative to the true closure age of the mineral a fortran code including radiogenic ar production diffusive loss of ar from biotite and bulk ar diffusion through the itm has been developed application of numerical modeling suggests that the time averaged effective bulk diffusivity deffar in the biotite amphibolite rock during early retrograde cooling is 2 2 1 0 10 8 m2 yr assuming steady state conditions the first such measurement available numerical modeling also provides information about the transmissivity and geologic history specific to the field site including a drop in deffar at 15 5 1 0 ma the timing of this drop is related to coincident rheological changes and the onset of rapid exhumation of the nappe stack

cooling history and tectonic exhumation stages of the south central tibetan plateau china constrained by 40ar 39ar and apatite fission track thermochronology between the qiangtang block and yalung zangpo suture zone in the south central tibetan plateau the following geological units and suture zones have been identified from south to north the gangdese granitic belt the lhasa block the nyainqentanghla shear zone the dangxiong sangxiong tectono granitic belt and the bangong nujiang suture zone to better constrain the tectonic evolution and cooling histories of these units 40 ar 39 ar muscovite biotite and k feldspar as well as apatite fission track dating and thermochronological analysis have been carried out the analytical results indicate that the south central tibetan plateau with the exception of the nyainqentanghla shear zone provides a record of three cooling stages at 165 150 130 110 and 45 35 ma fission track data modelling also indicates that the stages of cooling were different in the different tectonic belts or blocks very different cooling phases occurred in the south central tibetan plateau compared with southern tibet as well as along the yalung zangpo suture zone there is no thermochronological evidence to indicate that the south central part of tibetan plateau was influenced by the underthrusting of indian plate the three stage cooling history and the stages of tectonic exhumation were controlled completely by the closure of the bangong nujiang suture zone along its eastern segment during middle late jurassic 165 150 ma and its western segment in the early late cretaceous 130 110 ma as well as by the collision between the indian and asian plates in the paleogene 45 35 ma 2005 elsevier ltd all rights reserved

geochronological evidence for continuous exhumation through the ductile brittle transition along a crustal scale low angle normal fault simplon fault zone central alps 1 major low angle normal faults juxtapose different structural levels of the crust that record both brittle and ductile deformation field relationships alone cannot establish whether these different responses to deformation represent 1 parts of a single process of exhumation along the detachment or 2 two separate events with the later more discrete brittle detachment exhuming a fossil ductile shear zone from depth these two general models are critically assessed for the low angle normal simplon fault zone sfz in the central alps the sfz shows a spatial transition from a broad ductile mylonitic shear zone to a discrete brittle detachment with identical kinematics the age of the ductile shear zone and ductile brittle transition is controversial we present a detailed geochronological study based on fission track 40ar 39ar and rb sr microsampling dating coupled with structural petrological and chemical analyses that provides tight constraints on sfz timing discontinuous mineral cooling ages over a broad range of temperatures across the fault zone argue for fault activity between 20 and 3 ma on the basis of synkinematic white mica in low temperature shear zones and necks of foliation boudinage the brittle ductile transition in the footwall could be dated at 14 5 10 ma overall the data presented here are consistent with a continuous transition from ductile shearing to a more localized zone of brittle deformation within the same geological framework over a period of 15 ma the sfz is therefore an example of a telescoped crustal section within a single major low angle fault involving a continuous period of exhumation rather than a two stage structure

interpretation and significance of combined trace element and u pb isotopic data of detrital rutile a case study from late ordovician sedimentary rocks of saxo thuringia germany the u pb age and trace element composition of detrital rutile provide information about the metamorphic history of the source region that cannot be constrained by traditional u pb dating of detrital zircon previous provenance investigations focussed on only one of these methods based on a large la icp ms trace element and u pb isotopic dataset of detrital rutile and u pb isotopic data of detrital zircon from late ordovician sedimentary rocks of saxo thuringia germany this paper discusses the application and significance of combining these methods in provenance investigations u pb age spectra from the detrital zircons analysed show multiple age components multimodal age spectra in all samples this is in contrast with the detrital rutile data as only one sample yielded a multimodal u pb age distribution multimodal age spectra of detrital rutile are most likely preserved in sediments from large catchment areas with complex geological histories they may also be related to specific sedimentation events such as glacial washout during the retreat of large ice shields e g the hirnantian glaciation of gondwana unimodal age spectra are however not restricted to small catchment areas if the provenance region is characterized by a pervasive thermal overprint such as the pan african orogeny throughout gondwana unimodal age distributions may further consist of overlapping age cluster detectable by different trace element composition of the detrital rutile grains the combined u pb age and trace element data from detrital rutile grains demonstrate that rutile sourced from metapelitic rocks yield reliable and precise u pb ages in contrast detrital rutile classified to be of metamafic origin generally has too low uranium concentrations to be dated reliably by la icp ms detrital rutile records low to medium grade metamorphic events in the source region and therefore has the potential to better constrain the maximum depositional age of sedimentary rocks in comparison to u pb dating of detrital zircon

the middle permian hongshanliang manto type copper deposit in the east tianshan constraints from geology geochronology fluid inclusions and h o s isotopes abstract the hongshanliang copper deposit hosted in the tuff of the lower carboniferous yamansu formation is a typical copper deposit in the aqishan yamansu belt east tianshan based on crosscutting relationships of veins textural relationships and mineral assemblages five alteration mineralization stages at hongshanliang have been established chlorite sulfide stage stage i quartz pyrite stage stage ii quartz polymetallic sulfide stage stage iii late veins stage iv and supergene process stage v stage i has mineral assemblage of chlorite pyrite chalcopyrite quartz sericite and is commonly characterized by euhedral pyrite with silicates e g chlorite quartz and sericite and or chalcopyrite as its pressure shadow indicating an early mineralization event with apparent deformation stage ii is characterized by quartz pyrite chalcopyrite or locally quartz gypsum anhydrite pyrite veins cutting stage i chlorite pyrite the main mineralization stage stage iii at hongshanliang has typical mineral assemblages of quartz chalcopyrite chlorite calcite sericite quartz chalcopyrite sphalerite galena pyrite and quartz sphalerite galena chlorite with veins veinlets disseminations and local massive ore types similar to manto type copper deposits regarding mineralization and ore structures detailed fluid inclusion study shows temperature of fluids decreased from stage ii 307 484 c through stage iii peak at 160 180 c consistent with quartz chlorite oxygen isotope geothermometer of 190 c to stage iv 129 169 c with corresponding salinities of 2 7 26 2 wt nacl equiv peak at 8 wt 2 1 12 3 wt nacl equiv peak at 8 wt and 0 5 7 7 wt nacl equiv peaks at 4 6 wt respectively indicating an evolved hydrothermal ore forming system such fluid evolution can also be supported by h o isotopes during water rock reaction from stage ii 18ofluid 6 3 8 3 and dfluid 77 to 64 through stage iii 18ofluid 1 7 to 7 0 and dfluid 82 to 58 to stage iv 18ofluid 2 5 to 2 1 and dfluid 77 to 73 with a magmatic hydrothermal origin furthermore sulfur isotopes suggest that fluids of the two mineralization stages stage i and iii are predominantly magmatic hydrothermal with other influx contribution into the ore forming system i e minor early carboniferous seawater in stage i 34sfluid varying from 3 5 to 6 1 with peak at 0 and minor organic materials in stage iii 34sfluid varying from 6 5 to 4 1 with peak at 0 1 and detection of ch4 and c2h6 from fluid inclusions in combination of alteration paragenesis nature and source of ore forming fluids and comparison with other typical deposits we proposed that the hongshanliang copper deposit underwent two mineralization events with the main mineralization similar to manto type copper deposits 40ar 39ar dating of sericite from a massive chalcopyrite ore indicates the hongshanliang main mineralization formed at 269 0 0 4 ma generally coeval with regional k feldspar granite emplacement ca 272 ma integrating the regional tectonic setting magmatism and metallogenesis of the aqishan yamansu belt and the east tianshan we suggest that the aqishan yamansu belt has potential of mineral prospecting for the middle permian manto type copper deposits

detrital muscovite 40ar 39ar ages from carboniferous sandstones of the british isles provenance and implications for the uplift history of orogenic belts major progradations of clastic sediments are recorded in the sedimentary record of the famennian and the visean namurian of the united kingdom and surrounding waters we have determined 40ar 39ar ages of 162 detrital muscovites from 11 coarse sandstones which were deposited between 370 and 465 ma spanning both progradations detrital mica ages are dominated by a peak at 415 ma with minor peaks at 440 ma and 390 ma the 415 ma muscovites are derived from the unroofing of the scandian nappes during the compressional phase of the caledonian orogeny in scandinavia the 440 ma muscovites record pre scandian orogenic activity which is rarely preserved in the orogenic record thermochronological evidence suggests that episodic postorogenic uplift and exhumation events kept the scandian orogen a major topographic feature and likely a sediment source for over 100 million years after nappe emplacement and implicates tectonic rather than climatic control on the clastic sediment progradations the near total absence of detrital muscovites with ages 415 ma suggests that the scandian nappes had not been entirely eroded despite repeated uplift during the postorogenic extension the river s which supplied the sediments probably ran parallel to the strike of the major scandian thrusts along the length of the caledonian orogen in a manner analogous to the major river systems of contemporary orogenic highlands

dating deformation and cooling in the caledonian thrust nappes of north sutherland scotland insights from 40ar 39ar and rb sr chronology 40 ar 39 ar and rb sr mineral ages have been determined from various lithologies exposed in the caledonian foreland and structurally overlying thrust nappes of north sutherland scotland rb sr muscovite ages of c 428 c 421 and c 413 ma obtained from moine thrust zone mylonites are interpreted to date closely regional thrusting during the late silurian to early devonian 40 ar 39 ar muscovite ages within the lower parts of the moine nappe are mostly anomalously old with respect to rb sr analyses of muscovites from the same samples it is likely that this discrepancy results from a component of extraneous or excess argon 40 ar 39 ar hornblende ages and rb sr and 40 ar 39 ar muscovite ages obtained from structurally higher metamorphic units in the caledonian thrust nappes generally range between c 440 ma and c 410 ma these ages are interpreted to date cooling during and following d 2 regional thrusting and folding within internal sectors of the nappe sequence a possible tectonic model involves the silurian collision of baltica with scottish segments of laurentia resulting in the scandian orogeny and broadly coeval moine thrust zone d 2 structures were superimposed on structures and metamorphic fabrics formed during a regional mid ordovician tectonothermal event dated previously at c 470 460 ma syn d 2 temperatures were generally 600 c and sufficient to achieve more or less complete thermal rejuvenation of rb sr and 40 ar 39 ar isotopic systems in muscovite and hornblende even in areas of low d 2 strain

timing duration and role of magmatism in wide rift systems insights from the jiaodong peninsula china east asia in east asia widespread extensional sedimentary basins together with the close association of metamorphic core complexes and magmatism are prominent features of the large scale wide rift system developed during late mesozoic times this region thus appears as a proper place to study continental extension as well as questions about the link between continental extension and magmatism this paper primarily provides pioneer exhumation time constraints on the linglong mcc jiaodong peninsula including cooling and deformation ages mcc cooling at mid crustal levels occurred at ca 143 ma besides last ductile deformation occurred at ca 134 ma while final exhumation stages under brittle 2 conditions occurred as late as ca 128 ma continuous crustal stretching were then recorded by the emplacement of a synkinematic pluton in the upper crust at ca 128 ma that cooled fast below an intracrustal shear zone crossing the ductile brittle transition at ca 123 ma the ca 120 118 ma age cluster is ascribed to the fast cooling of undeformed plutons marking the end of extension that lasted in the area over a minimum period of ca 30 my combining other mcc exhumation constraints and the onset of subsidence in the sedimentary basins total duration of late mesozoic extension in east asia could be estimated at ca 60 my related with a rather long process for extension from 160 to at least 100 ma east asian continental extension is heterogeneously distributed in space and time as revealed by fundamental differences between two end member classes of migmatite cored mcc already described in other wide rift systems extension seems to have first favoured partial melting which subsequently in turn maintained continental extension

provenance of oligocene synorogenic sediments of the ligurian alps nw italy inferences on belt age and cooling history mineral chemistry 40ar 39ar geochronology on white micas and apatite fission track thermochronology aftt are applied here to study the provenance of the synorogenic molare formation lowermost unit of the tertiary piedmont basin clastic sequence the molare formation was deposited during transgression onto the ligurian alps nappe stack in the early oligocene depositional facies show that clastic distribution remained transversal with local sources located just landward from the coastline phengite mineral chemistry together with 40ar 39ar data clearly shows two distinctive source areas each one mirroring the composition of the basement directly beneath the clastic sequence amphibole mineral chemistry allows second order provenance distinctions within each sector reflecting heterogeneous metamorphic evolution of the bedrock complexes integrated 40ar 39ar dating and aftt suggest that following a fast cooling exhumation episode of the ligurian alps during the oligocene very little net uplift has since occurred this is due to a period of general subsidence from the oligocene late miocene followed by comparable uplift from late miocene pliocene to the present in general our data provide an image of the ligurian alps during the oligocene which is very similar to the present day one

an empirical evaluation of the argon diffusion geometry in muscovite the temperature time significance of 40ar 39ar muscovite ages is difficult to evaluate quantitatively because of ambiguities in the choice of an appropriate model diffusion geometry the effective diffusion dimension and large uncertainties in experimentally determined diffusion parameters laser 40ar 39ar studies of natural 40ar concentration gradients suggest that argon loss in muscovite occurs primarily via cylindrical volume diffusion commonly with an effective diffusion dimension equal to the crystal diameter the present study tests the cylinder model geometry and the control of physical grain size on the effective diffusion dimension by measuring the anisotropy of a natural40ar diffusion gradient parallel and perpendicular to 001 in a single euhedral muscovite phenocryst the new data combined with previous studies clearly indicate loss of 40ar via cylindrical volume diffusion and strongly suggest that loss of 40ar across the 001 sheets slab diffusion geometry is minimal in undeformed crystals

40ar 39ar laser probe dating by step heating and spot fusion of phengites from the dora maira nappe of the western alps italy 40ar 39ar laser probe dating of phengitic micas has been carried out by step heating and spot fusion procedures these micas represent successive deformation stages in the structural evolution of the internal dora maira nappe western alps single phengites from a gneiss affected by a single ductile strain under retrogressive conditions sample 99 1 display complete isotopic resetting with nearly homogeneous intracrystalline ar distribution and yield plateau ages of about 40 ma small clusters of phengites from an earlier foliation were selected from a polydeformed mica schist sample ptx3 they show a partial isotopic resetting in response to overprinting during the retrogressive deformation stage with a concentric age zoning from 68 ma on the rim to 87 ma in the core on one cleavage plane this zonation is fully consistent with the laser derived discordant age spectrum which ranges from 40 to 90 ma from low to high temperatures according to the deformation history of both samples these preliminary data suggest a deformation control on ar migration during recrystallization processes and they are consistent with the timing of the collisional evolution previously reported for southern dora maira units this study shows that the 40ar 39 ar continuous laser probe dating technique produces data accurate enough to discriminate several tectonometamorphic episodes recorded in single hand samples

argon behaviour in an inverted barrovian sequence sikkim himalaya the consequences of temperature and timescale on 40ar 39ar mica geochronology abstract 40 ar 39 ar dating of metamorphic rocks sometimes yields complicated datasets which are difficult to interpret in terms of timescales of the metamorphic cycle single grain fusion and step heating data were obtained for rocks sampled through a major thrust sense shear zone the main central thrust and the associated inverted metamorphic zone in the sikkim region of the eastern himalaya this transect provides a natural laboratory to explore factors influencing apparent 40 ar 39 ar ages in similar lithologies at a variety of metamorphic pressure and temperature p t conditions the 40 ar 39 ar dataset records progressively younger apparent age populations and a decrease in within sample dispersion with increasing temperature through the sequence the white mica populations span 2 9 ma within each sample in the structurally lower levels garnet grade but only 0 3 ma at structurally higher levels kyanite sillimanite grade mean white mica single grain fusion population ages vary from 16 2 3 9 ma 2 to 13 2 1 3 ma 2 from lowest to highest levels white mica step heating data from the same samples yields plateau ages from 14 27 0 13 ma to 12 96 0 05 ma biotite yield older apparent age populations with mean single grain fusion dates varying from 74 7 11 8 ma 2 at the lowest structural levels to 18 6 4 7 ma 2 at the highest structural levels the step heating plateaux are commonly disturbed temperatures 600 c at pressures of 0 4 0 8 gpa sustained over 5 ma appear to be required for white mica and biotite ages to be consistent with diffusive open system cooling at lower temperatures and or over shorter metamorphic timescales more 40 ar is retained than results from simple diffusion models suggest diffusion modelling of ar in white mica from the highest structural levels suggests that the high temperature rocks cooled at a rate of 50 80 c ma 1 consistent with rapid thrusting extrusion and exhumation along the main central thrust during the mid miocene

listric normal faulting during postorogenic extension revealed by 40ar 39ar thermochronology near the robertson lake shear zone grenville orogen canada the robertson lake shear zone is a major plastic to brittle extensional shear zone in the grenville orogen that bounds the mazinaw and sharbot lake domains and provides information on the style of late extension and the unroofing history of the orogen argon isotope data were collected from hornblende and micas to determine 40ar 39ar ages constrain the temperature time histories of these two domains and infer the unroofing history of the region hornblende cooling ages across the mazinaw domain footwall show little variation indicating uniform unroofing of the footwall since 950 ma phlogopite muscovite and biotite cooling ages of the footwall are 924 to 890 ma the cooling history of the mazinaw domain is characterized by slow cooling after peak metamorphism circa 1000 ma accelerated cooling 4 5 c m y from 950 ma to 890 ma and an average cooling rate of 1 c m y to circa 590 ma when these rocks were at or near the surface the cooling history of sharbot lake hanging wall domain is drastically different than that of the mazinaw domain hornblende and biotite cooling ages in the central portion of the domain are 1009 and 969 ma respectively indicating a cooling rate of 5 c m y after slow cooling from metamorphic temperatures biotite and phlogopite cooling ages determined from samples located at different distances from the shear zone do not lie along the same cooling curve indicating that the cooling history varied across the domain cooling rates in the hanging wall adjacent to the shear zone are low 2 c m y a biotite cooling age 1029 ma and preservation of an amphibole growth age 1205 ma in the hanging wall adjacent to the shear zone reflect shallow crustal levels for this sample since 1205 ma these data indicate that the hanging wall away from the shear zone was unroofed from deeper crustal levels faster and much later than the hanging wall adjacent to the shear zone the varied cooling histories across the region are resolved by listric normal faulting that lead to uniform unroofing of the footwall and differential unroofing across the hanging wall due to rotation during fault displacement

constraining deformation stages in brittle ductile shear zones from combined field mapping and 40ar 39ar dating the structural evolution of the grimsel pass area aar massif swiss alps abstract 40 ar 39 ar dating of syn kinematic white mica phengite and biotite is used to constrain the age of shear zones within the granodiorites of the grimsel pass area aar massif central alps three sets of steeply dipping shear zones have been mapped stage 1 shear zones are very broad 200 m wide zones of biotite bearing rocks with a progressive increase in intensity of ductile deformation from rim to core and have dip slip kinematics subsequent stage 2 shear zones are narrower contain phengite rich mylonites and show a gradual change from dextral strike slip to dip slip kinematics across strike strain localisation during the formation of stage 2 shear zones is interpreted to have been aided by reaction weakening during extensive fluid rock interaction brittle precursors of stage 2 shear zones are marked by biotite rich cataclasites preserved along the northern contact of the overprinting main stage 2 shear zone biotite from a stage 1 shear zone yields an age of 21 1 0 2 ma phengites in stage 2 mylonites in the grimsel pass area have a very narrow age range 13 8 12 2 ma which is interpreted to bracket the duration of ductile deformation during stage 2 deformation at mid crustal levels however the biotite rich cataclasites at the margin of a stage 2 shear zone may have localised fluid flow up to 2 ma prior to the onset of main stage 2 ductile deformation stage 3 shear zones are brittle strike slip faults containing cataclasites breccias and clay fault gouges earlier studies have established that these late faults formed at

molybdenite re os and muscovite 40ar 39ar dating of the xihuashan tungsten deposit central nanling district south china abstract the xihuashan tungsten deposit in the central nanling region south china is an important vein type ore deposit hosted in cambrian strata and mesozoic granitic intrusions wolframite and molybdenite are the principal ore minerals the gangue minerals are mainly quartz and muscovite wolframite and molybdenite are products of the first stage hydrothermal activity whereas muscovite formed dominantly at the second stage molybdenite re os and muscovite 40ar 39ar dating have been carried out to investigate the age of mineralization re os isotopic dating for molybdenite associated with wolframite yield a precise well constrained isochron age of 157 8 0 9 ma mswd 1 5 ar ar isotopic analyses of muscovite yield a plateau age of 152 8 1 6 ma in agreement with an inverse isochron age of 152 8 1 6 ma which is 5 mys younger than the re os age the molybdenite re os age is interpreted as the age of tungsten mineralization this age coincides well with the zircon u pb age of the host granitic intrusion reported previously the 5 mys difference between molybdenite re os and muscovite 40ar 39ar ages probably represents the duration of hydrothermal activity the results show that the xihuashan tungsten deposit is one of many important tungsten tin deposits formed during 150 to 160 ma associated with large scale lithospheric extension in south china

the tectonic evolution of western central iran seen through detrital white mica abstract a first order survey of 40ar 39ar dating of detrital white mica from jurassic to pliocene sandstones has been carried out in order to reveal the tectonic evolution of blocks in central iran the central iran block was believed to represent a stable precambrian block our results indicate that 1 only a very small proportion of precambrian but abundant paleozoic and mesozoic detrital white mica indicate the phanerozoic mostly mesozoic age of metamorphic crust exposed in central iran the oldest but scarce detrital white mica grains have ages ranging from 524 to 826 ma heralding a late precambrian and cambrian crystalline basement or cannibalism from older clastic successions 2 jurassic and cretaceous sandstones from the west and east of the chapedony fault yield different age spectra with a dominance of variscan ages ca 308 385 ma in the biabanak unit west of the chapedony fault compared to coeval sandstones from the block east of the chapedony fault where variscan ages are subordinate and cimmerian ages predominate the micas from the biabanak unit are most likely derived from the variscan accretionary complex exposed in the anarak jandaq areas further northwest this result underlines the importance of a major block boundary identified as the chapedony fault which is in extension of a fault previously proposed 3 two stages of cimmerian events are visible in our data set from cretaceous and paleogene sandstones a cluster around 170 ma and at ca 205 ma these clusters suggest a two stage cimmerian evolution of the largely amphibolite grade metamorphic posht e badam and boneh shurow complexes 4 the youngest micas in paleogene conglomerates have an age of ca 100 ma and are most likely derived from the base of the posht e badam complex no record of the uplifted eocene chapedony metamorphic core complex has been found in eocene and pliocene clastic rocks

kinematics and ar 40 ar 39 geochronology of the gaoligong and chongshan shear systems western yunnan china implications for early oligocene tectonic extrusion of se asia the gaoligong and chongshan shear systems glss and csss in western yunnan china have similar tectonic significance to the ailaoshan red river shear system asrrss during the cenozoic tectonic development of the southeastern tibetan syntaxis to better understand their kinematics and the cenozoic tectonic evolution of se asia this paper presents new kinematic and 40 ar 39 ar geochronological data for these shear systems all the structural and microstructural evidence indicate that the glss is a dextral strike slip shear system while the csss is a sinistral strike slip shear system and both were developed under amphibolite to greenschist grade conditions the 40 ar 39 ar dating of synkinematic minerals revealed that the strike slip shearing on the glss and csss at least began at 32ma possibly coeval with the onset of other major shear systems in se asia the latestage shearing on the glss and csss is dated at 27 29ma by the biotite 40 ar 39 ar ages consistent with that of the wang chao shear zone wcsz but 10ma earlier than that of the asrrss the dextral gaoligong shear zone within the glss may have separated the india plate from the indochina block during early oligocene combined with other data in western yunnan we propose that the baoshan southern indochina block escaped faster southeastward along the csss to the east and the glss to the west than the northern indochina block along the asrrss accompanying with the obliquely northward motion of the india plate during early oligocene 28 36ma during 28 17ma the northern indochina block was rotationally extruded along the asrrss relative to the south china block as a result of continuously impinging of the india plate

and mg fe si al2 compositional control on argon behaviour in high pressure white micas a continuous laser probe study from the dora maira nappe of the internal western alps italy abstract high pressure white micas from a restricted area in the dora maira nappe of the western alps italy were analyzed by the conventional 40 ar 39 ar bulk sample step heating method and by the step heating and the spot fusion 40 ar 39 ar continuous laser probe techniques on single grains high si ferich phengites from fine grained gneisses display homogeneous 35 40 ma plateau and spot fusion ages consistent with the upper eocene 40 ar 39 ar phengite ages documented over the whole internal western alps in contrast phengite separates and single grains of high si and mg rich to intermediate composition selected from metapelitic and mg rich mica schist samples yield older discordant apparent ages even when sampled in the same outcrop as the fine grained gneisses these local age variations are interpreted to reflect primarily an increasing ar retentivity with increasing mg content in phengites in metapelite derived white micas the isotopic discordance is marked by 1 1 a characteristic bulk sample upward convex spectrum shape 2 2 single grain disturbed age spectra 3 3 a systematic radially outward decrease in spot ages across individual mica cleavage planes and 4 4 strong grain to grain spot age variations in rock sections 1 1 in mg rich mica schists the isotopic discordance is observed as a reproducible upward convex bulk sample age spectrum with high 140 ma intermediate apparent ages 2 2 upward convex single grain age spectra and 3 3 irregular intra and intergrain age gradients on geochemical grounds it is argued that the isotopic discordance occurring in the metapelites is mainly controlled by the compositional changes introduced by the mg fe si al2 celadonitic re equilibration of the high pressure phengites during greenschist overprinting in mg rich phengites this chemical re equilibration may induce ar loss below their presumed higher blocking temperature a model of ar behaviour in high pressure white micas is proposed which explains the age variations recorded throughout the different lithologies in terms of a simple interaction between the mg fe compositional control on ar retentivity and the mg fe si al2 modifications occurring in phengites during progressive overprinting

first u pb shrimp zircon and 40ar 39ar ages of metarhyolites from the afyon bolkardag zone sw turkey implications for the rifting and closure of the neo tethys abstract the afyon bolkardag zone abz is one of the most important metamorphic zones in turkey this zone consists of metasedimentary and metavolcanic rocks which have undergone regional greenschist facies metamorphism the metavolcanic rocks are dominated by rhyolite dacite and trachyandesite in the ilgin konya area the low grade metavolcanics were originally rhyolitic lavas and tuffs we present new precise sensitive high resolution ion microprobe shrimp 206 pb 238 u age analyses of zircons and 40 ar 39 ar age data of phengite from metarhyolites that enable a more precise reconstruction of the abz shrimp analyses of euhedral or sub euhedral magmatic zircons from two metarhyolite samples yield weighted means 230 2 ma and 229 2 ma these ages are interpreted as formation age of rhyolitic volcanics implying they were formed in the late triassic time carnian we suggest that these upper triassic magmatic rocks were formed in an extensional tectonic setting on the northern active margin of gondwana which led to the development of the northern branch of the neo tethys 40 ar 39 ar dating of two phengite samples from metarhyolites gave plateau ages of between 63 7 0 1 ma and 62 6 0 1 ma these dates are interpreted as the metamorphic age of these volcanic rocks and as the age of metamorphism that affected the abz consequently these earliest tertiary ages also constrain the timing of the closure of the neo tethys

precise dating of low temperature deformation strain fringe analysis by 40ar 39ar laser microprobe pyritized graptolites from the welsh basin united kingdom slate belt acted as rigid bodies during cleavage formation and epizonal white micas formed within the resulting strain shadows orthogonal to the principal stress orientation although the quantities of mica are small they are a pure synkinematic mineral and have been dated by ar 40 ar 39 infrared laser microprobe as a means to dating cleavage four samples of strain fringe mica from different hand samples yielded ages ranging from 394 4 3 1 to 397 8 1 8 ma 2sigma with a mean age of 396 1 1 4 ma 2sigma by focusing on minerals that are unequivocally synkinematic this technique provides a novel solution to the problems of isotopically dating slaty cleavage previous studies have predominantly relied on dating whole rock slate samples or separated illite grains by ar 40 ar 39 techniques problems encountered included 1 separating the effects of isotopic contamination by detrital phases 2 ar 39 loss during the irradiation of illite mineral separates and 3 thermally induced ar 40 loss in nature from fine grained minerals by circumventing these problems this new method provides the first unequivocal and high precision age data for acadian deformation in the well characterized welsh basin slate belt with such precision the method may afford geologists the opportunity to track tectonic fronts across orogens and assess the rates of accretion processes in areas that are peripheral to sites of continent continent collision

40ar 39ar geochronologic constraints on the timing of massive sulfide and vein type pb zn mineralization in the western meseta of morocco the meseta domain of morocco is located on the southern margin of the late paleozoic hercynian orogenic zone 40ar 39ar ages of igneous and hydrothermal minerals biotite and illite were analyzed from a representative massive sulfide deposit at hajar 1 5 mt zn 0 45 mt pb 0 11 mt cu and 900 t ag and a vein type deposit at tighza 1 0 mt pb 0 3 mt zn and 1 000 t ag in the western meseta a weighted mean age of hydrothermal biotite of the hajar deposit is 300 9 2 6 ma 2 uncertainty identical to a hydrothermal illite age of 305 4 4 8 ma these two ages are calculated from about 93 percent of the released 39ar another illite age calculated from 74 percent of the released 39ar 313 4 9 1 ma overlaps with or is slightly older than these concordant ages these 40ar 39ar results indicate that the biotite and illite alteration of the hajar deposit formed almost simultaneously at 304 to 300 ma this age range is younger than the visean age previously assumed on the basis of geologic setting these 40ar 39ar ages are interpreted as the age of hydrothermal activity subsequent to massive sulfide mineralization a magmatic intrusion beneath the deposit may have triggered this hydrothermal activity three 40ar 39ar weighted mean ages and one plateau age from the tighza deposit range from 280 6 to 267 7 ma igneous biotite of the kaolin granite and hydrothermal illite of the ighrem aouss and north veins in the deposit indicate ages of 280 6 5 5 ma 280 6 3 1 ma and 275 5 3 4 ma respectively these three ages are identical within 2 uncertainties the age of the hydrothermal biotite from the mispickel granite 267 7 2 3 ma is slightly younger this younger age may have resulted from two periods of magmatic hydrothermal activity or from resetting of hydrothermal biotite by ar loss regardless these ages indicate that granite intrusion and hydrothermal alteration associated with galena and sphalerite bearing veins occurred mainly at 280 to 275 ma in the tighza deposit the identical ages of the magmatic intrusion and hydrothermal alteration suggest that the vein type pb zn mineralization as well as w mineralization was associated with the magmatic intrusion the influence of the late deformation and metamorphic overprint may be related to the lowest temperature steps of the analytical data the dates obtained from the lowest temperature steps from both the tighza and hajar deposits concentrate between 235 ma and 220 ma this time range is compatible with the geologic constraints on the deformation and these dates are thus attributed to the post visean regional metamorphism

40ar 39ar age gradients in micas from a high temperature low pressure metamorphic terrain evidence for very slow cooling and implications for the interpretation of age spectra estimates of the time temperature history of geologic samples are often based on 40 ar 39 ar ages obtained through incremental heating experiments on minerals with different ar retentivities laser incre mental heating of single crystals of muscovite and biotite from an 1700 ma monzogranite from a low pressure high temperature metamorphic terrain in central arizona suggested an essentially uniform distribution of radiogenic 40 ar in each crystal and yielded ages of 1412 5 ma for the muscovite and 1410 10 ma for the biotite the similarity in these ages implies rapid 25 k m y cooling of the monzogranite after a reheating episode slightly before 1412 ma in contrast detailed age mapping of single crystals from the same samples with the laser microprobe revealed large radiogenic 40 ar gradients indicative of very slow 400 m y evidence for such low cooling rates has important implications for the longevity of high thermal gradients in the early middle proterozoic lithosphere in addition inconsistency between the laser mapping and incremental heating results for these micas suggests caution when interpreting incremental heating data for slowly cooled samples

the exhumation history of collision related mineralizing systems in tibet insights from thermal studies of the sharang and yaguila deposits central lhasa abstract the large newly discovered sharang porphyry mo deposit and nearby yaguila skarn pb zn ag mo deposit reside in the central lhasa terrane northern gangdese metallogenic belt tibet multiple mineral chronometers zircon u pb sericite 40 ar 39 ar and zircon and apatite u th he reveal that ore forming porphyritic intrusions experienced rapid cooling 100 c ma during a monotonic magmatic hydrothermal evolution the magmatic hydrothermal ore forming event at sharang lasted 6 0 myr 1 8 myr for cooling from 900 to 350 c and 4 0 myr for cooling from 350 to 200 c whereas cooling was more prolonged during ore formation at yaguila 1 8 myr from 900 to 500 c and a maximum of 16 myr from 900 to 350 c all porphyritic intrusions in the ore district experienced exhumation at a rate of 0 07 0 09 mm yr apatite he ages between 37 and 30 ma combined with previous studies this work implies that uplift of the eastern section of the lhasa terrane expanded from central lhasa 37 30 ma to southern lhasa 15 12 ma at an increasing exhumation rate all available geochronologic data reveal that magmatic hydrothermal exhumation activities in the sharang yaguila ore district occurred within four periods of magmatism with related mineralization significant porphyry type mo mineralization was associated with late cretaceous eocene felsic porphyritic intrusions in the central lhasa terrane resulting from neotethyan oceanic subduction and india asia continental collision

the effect of the instability of muscovite during in vacuo heating on 40ar 39ar step heating spectra abstract the goal of this study was to ascertain the nature of the ar release mechanism s for a single well characterized muscovite heated incrementally in vacuo 1 at temperatures and time intervals typical of 40ar 39ar step heating experiments and 2 isothermally at 500 c and 700 c for time intervals much longer than typical 40ar 39ar experiments the rate of delamination was monitored by sem and the rate of dehydroxylation was determined by xrd analyses the rate of in vacuo ar loss was compared to that for muscovite heated at 700 c and 2 kbar h2o pressure for all experiments the rate of ar loss was found to be consistent with the rates of dehydroxylation and delamination at 500 c ar loss behavior for a 3000 m diameter muscovite grain was approximated by volume diffusion but the ar release pattern became increasingly non fickian with decreasing grain size we propose that the cause of the non fickian behavior is delamination the portions of the isothermal runs exhibiting diffusion like profiles also yielded discordant 40ar 39ar ages and low atmospheric ar contamination we attribute this phenomenon to the preferential concentration of atmospheric ar in interlayer extended defects and of 40ar in interlayer vacancies whereas 39ar is uniformly distributed throughout the muscovite structure by recoil the 40ar 39ar age spectrum of the muscovite heated under hydrothermal conditions has a staircase shape comparable to that usually interpreted as due to 40ar diffusive loss whereas 40ar 39ar laser spot analyses did not allow this interpretation the in vacuo ar release rate for this sample however indicates that it had been altered during the experiment therefore we propose that this 40ar 39ar age spectrum reflects the differential release of ar from the alteration phases vs muscovite for our sample dehydroxylation and delamination control the in vacuo ar release at temperatures less than 900 c whereas ar diffusion from thin dehydroxylated slabs may dominate the ar release mechanism for temperatures greater than 900 c this model would predict that the recovery of fossil 40ar concentration gradients from muscovite by typical incremental heating schedules is not possible the rates of delamination in vacuo and the rate of ar diffusion for muscovite in nature however both depend upon its the chemical composition structural integrity and grain size consequently for samples with either a naturally occurring broad range of grain sizes such as detrital muscovite and authigenic clay from sedimentary rocks or a compositionally or structurally heterogeneous population of grains such as muscovite from shear zones 40ar 39ar incremental heating may be able to detect intergranular variations in the k ar age

thick skinned versus thin skinned thrusting rheology controlled thrust propagation in the variscan collisional belt the southeastern bohemian massif czech republic austria the variscan nappe assembly within the southeastern bohemian massif includes 1 crystalline nappes which were transported from initial granulite to amphibolite facies conditions to uppermost crustal levels 2 nappe assembly of footwall units under very low to low grade metamorphic conditions and 3 frontal thin skinned imbrication of flysch sequences which were deposited within a deep and narrow foreland basin during loading of the lower plate thick skinned thrusting of hinterland tectonic units resulted in an inverted metamorphic zonation and contrasts with the thin skinned tectonic character of foreland units 40ar 39ar hornblende and muscovite plateau ages from hanging wall units reflect complete rejuvenation of intracrystalline argon isotopic systems during late variscan thrusting between 340 ma and 325 ma the 40ar 39ar mineral ages together with previously published 40ar 39ar and sm nd data record systematically decreasing variscan mineral ages structurally downward into footwall units this is explained by rapid exhumation and cooling synchronously with thrusting along crustal scale ramps during foreland directed nappe propagation the location of decollement zones within specific structural levels was apparently controlled by crustal scale rheological con trasts late variscan cooling ages contrast with cadomian 40ar 39ar hornblende and muscovite ages recorded within the structural basement immediately below the major variscan thrust plane where intracrystalline isotopic systems were only partially rejuvenated during variscan thrusting these record a sequence of cadomian magmatic and metamorphic events in foreland units including 1 pre intrusive metamorphism and deformation under amphibolite facies metamorphic conditions 610 ma 2 greisen formation during a cadomian collisional event 600 ma and 3 intrusion of calcalkaline granitoid bodies with i type affinities 590 ma

post eocene coupled oroclines in the talesh nw iran paleomagnetic constraints abstract the talesh mountains nw iran witnessed a long deformation history from the triassic cimmerian orogeny to the ongoing arabia eurasia collision this protracted multi stage deformation has generated a remarkably curved orogen with a puzzling kinematic and deformational history in this study we investigate the origin of the talesh curvature through paleomagnetic analyses on rocks of paleozoic mesozoic and cenozoic age our results indicate that at least two major large scale vertical axis rotations took place since the late cretaceous 1 a pre eocene 73 17 clockwise rotation and 2 post eocene differential rotations that formed the z shaped mountain belt within a crustal scale shear zone the latter involved an increasing amount of clockwise cw rotation from south 16 to north 48 the orocline formation likely started during the oligocene where an approximately east west oriented mountain belt was buckled by the arabia eurasia collision with arabia acting as a rigid indenter and the south caspian basin as a rigid backstop we hypothesise that the ne sw oriented aras and lahijan fault zones inherited from transform faults related to the mesozoic opening of the caspian black sea back arc accommodated the coupled orocline formation

time calibration of a pt path from the western tauern window eastern alps the problem of closure temperatures new hornblende k ar and 39ar 40ar and mica rb sr and k ar ages are used to place specific timemarks on a well constrained pressure temperature path for the late alpine metamorphism in the western tauern window after identification of excess 40ar the closure behavior of ar in hornblende is compared with that of sr and ar in phengite and biotite samples were collected in three locations whose maximum temperatures were 570 c zemmgrund 550 c pfitscher joch and 500 540 c landshuter hutte

timing and processes of late mesozoic mid lower crustal extension in continental ne asia and implications for the tectonic setting of the destruction of the north china craton mainly constrained by zircon u pb ages from metamorphic core complexes abstract late mesozoic large scale crustal extension occurred across the northeastern section of continental asia and metamorphic core complexes mccs are the major features in this region this paper presents new zircon u pb and ar ar ages of biotite and hornblende for pre or syn mylonitized and post kinematic unmylonitized intrusions from these mccs and attempts to determine the timing of initiation duration and termination of the mid lower crustal ductile extension by integrating with geochronological data from previous studies these mccs have similar 40 ar 39 ar biotite and hornblende ages 130 to 110 ma mainly from 130 to 120 ma which are interpreted as the cooling ages and represent the timing of the late stage formation of the mccs zircon u pb ages for syn to post kinematic intrusions from some mccs suggest that the mid lower crustal extension probably initiated at ca 150 145 ma with a peak occurring 145 130 ma and continued to 130 120 ma which is older than that suggested by 40 ar 39 ar ages by 10 20 ma the early ductile extension most likely initiated in mongolia and northern china during the late jurassic 150 145 ma at a deeper level then propagated southwards 140 130 ma and finally developed throughout the entire continental ne asia 130 120 ma based on the timing and processes of the extension a geodynamic model and tectonic setting of the destruction of the north china craton ncc are proposed

laser 40ar 39ar evaluation of slow cooling and episodic loss of 40ar from a sample of polymetamorphic muscovite volume diffusion models predict that crystals with large diffusion dimensions can record a wide range of thermal conditions in the earth s crust direct measurements of the zoning of radiogenic argon 40 in single muscovite porphyroblasts from a complex terrain in the vermont appalachians record multiple crustal events that span 150 million years the crystal radius was the effective dimension for argon diffusion approximately 1000 micrometers late deformation features inside the crystals locally decreased the diffusion dimension and promoted loss of argon 40 zoning patterns of radiogenic isotopes as observed in this study are an increasingly important diagnostic tool for studying the thermal record of tectonic processes

the cooling history of the late pliocene eldzhurtinskiy granite caucasus russia and the thermochronological potential of grain size age relationships abstract isotopic age investigations have been carried out on samples from the late pliocene eldzhurtinskiy granite great caucasus russia in order to elucidate the cooling history of this kilometere sized plutonic body the samples were taken from a horizontal profile from the edge to the centre of the granite and from a vertical profile from the top to a depth of 4570 m the 40ar 39ar results on biotites ranging from 2 5 to 1 2 ma indicate a vertical age zoning of the granite caused by regional uplift an uplift rate of 4 mm a is calculated from the age differences together with the results of fission track dating on apatite and zircon and the present day rock temperatures the biotite ages can be used to evaluate cooling rates below 400 c the cooling rates of about 180 c ma are relatively high and spatially and temporarily fairly uniform throughout the granite a geothermal gradient of 45 c km is derived from uplift and cooling rates this is identical to the present day borehole gradient the 40ar 39ar ages of distinct biotite grain fractions from one and the same sample specimen are discordant generally they show clear age grain size correlation as expected from ar diffusion theory for cooling systems thus the biotite results support the additional detailed thermochronological information the cooling rates in the temperature range of biotite closure based on the grain size age correlations of the biotites are close to 180 c ma they provide evidence that within a certain temperature interval the time temperature path of a rock may be determined exclusively by isotopic dating of different grain size fractions of only one type of mineral

two stage erosion and deposition in a continental margin setting an 40ar 39ar laserprobe study of offshore detrital white micas in the norwegian sea detrital white micas from a restricted region of the norwegian sea have been analysed by the 40ar 39ar laserprobe examining the ages of individual white mica grains from offshore triassic jurassic and cretaceous sandstones reveals a narrow range of detrital white mica ages 369 424 ma moreover there is an inverse relationship between sediment age and detrital white mica age with the triassic sands containing a larger proportion of younger white micas and the cretaceous sands containing the oldest white micas in addition residence times of the detritus which is the age of the sediment subtracted from the age of the detrital white mica range from c 150 to c 300 ma for the triassic and cretaceous sands respectively the inverse age relationships combined with the large residence times for these stratigraphic levels can best be explained by an erosion deposition model involving at least two stages taking into account previous provenance studies in the lofoten region of the norwegian sea these new data suggest that detritus shed initially from the exhuming caledonian nappes of nw norway in late silurian early devonian times was deposited in intramontane basins that were themselves subsequently unroofed and re eroded from permo triassic times this caused the recycling of the caledonian detritus from the devono carboniferous basins and led to the inverted detrital age distribution ovserved in the triassic cretaceous sediments examined in this study while sedimentary recycling is not a surprising feature for mesozoic basins in the north and norwegian seas this study documents a previously untapped method 40ar 39ar laserprobe provenance studies by which to identify its effect and extent

compositional controls on 40 ar 39 ar ages of zoned mica from a rare element pegmatite a high spatial resolution ar ar dating study of compositionally zoned micas using uv laser extraction has been used to investigate the effect of composition and compositional boundaries upon argon diffusion in mica the crystals are characterised by muscovite cores and li f rb cs rich lepidolite rims produced by the interaction of a residual melt vapour phase with the original muscovite during the late stages of pegmatite crystallisation single crystals exhibit dramatic intra grain age variations with a maximum range of 2 880 2 117 ma backscattered electron images combined with uv laser analyses reveal that apparent ages younger than 2 600 ma are directly associated with the compositionally distinct rims suggesting that ar diffusion was faster in the lepidolite than in muscovite although it is not possible to be fully quantitative using the present data it seems that argon diffusion rates in lepidolite are similar to those in biotite major rearrangements of the crystal structure needed to accommodate high concentrations of lithium fluorine and large interlayer cations rb cs in muscovite account for lower argon retentivity of the crystal rims in most cases the age profiles show no disturbance at the compositional boundary despite the transition from a di octahedral to tri octahedral mica indicating that in general the boundaries do not act as fast diffusion pathways however in some cases there is a clear drop in ages at the compositional boundary which acts as an effective sub grain boundary the difference between the two types may be related to the internal structure of the compositional boundaries or their degree of contact with the grain boundary network of the rock

postglacial marine diatom record of the palmer deep antarctic peninsula odp leg 178 site 1098 1 total diatom abundance 1 late pleistocene to holocene climate variability in the antarctic peninsula is described from ocean drilling program odp leg 178 site 1098 palmer deep using total diatom abundance valves per gram dry sediment as a paleoproductivity proxy diatom frustules are absent in the diamicton at the base of the core which was deposited prior to age calibrated circa 13 2 ka immediately above the diamicton frustules are in low abundance as rare fragments and become increasingly more abundant and well preserved up core a rapid and major increase in productivity after circa 12 8 ka is evident from distinctly laminated sediments that overlie the diamicton the strongly laminated section was deposited during a deglaciation phase that followed the retreat of grounded ice after the last glacial maximum diatom productivity decreased during a climatic cooling and or change in meltwater supply until circa 9 0 ka between 9 0 and 4 4 ka productivity increased steadily again with diatom abundance maxima reached during a mid holocene climatic optimum the optimum is interrupted by a brief cold event of 1000 years duration 6 0 5 0 ka after 4 4 ka productivity generally decreased and the last 500 years have experienced the lowest diatom productivity of the postglacial period the palmer deep sediment is compared to ice core and lake sediment records from other regions around the antarctic peninsula these diverse data sets confirm that the antarctic peninsula has been a region of rapid and variable climate fluctuation since the last glacial maximum

timing of iron oxide cu au u hydrothermal activity and nd isotope constraints on metal sources in the gawler craton south australia the eastern gawler craton hosts australia s premier uranium bearing iron oxide copper gold iocg belt the 500 km long olympic cu au u province in addition to the olympic dam cu u au and prominent hill cu au deposits numerous barren and weakly mineralized iocg prospects are present in the province new geochronological data for hydrothermal minerals combined with constraints from host rock ages demonstrate that alteration and associated iocg mineralization formed between 1570 and 1600 ma in three districts of the province this iocg hydrothermal activity temporally overlaps with magmatism of the hiltaba suite and gawler range volcanics titanites in paragenetically early magnetite bearing alteration in the olympic dam and prominent hill districts yield u pb ion probe ages of 1576 5 and 1567 10 ma respectively molybdenite in veins crosscutting magnetite biotite and albitic alteration in the moonta wallaroo district have re os ages of 1574 6 and 1599 6 ma respectively these represent minimum ages of the iocg alteration assemblages in this district a muscovite 40ar 39ar age of 1575 11 ma provides a minimum age of paragenetically later hematitic alteration in the olympic dam district neodymium isotope compositions were determined for 44 whole rock samples from barren and weakly mineralized cu au prospects and host rocks in the olympic dam and prominent hill districts the new geochronological framework enables comparison of the nd isotope data across two iocg districts at the time of formation of the olympic dam deposit ca 1590 ma magnetite rich weakly cu mineralized alteration from five prospects yields a relatively narrow range end 1590 values of 5 8 to 4 1 both hematite and magnetite rich alteration yield generally similar end 1590 values that match values from fresh and weakly altered paleoproterozoic metasedimentary and metagranitic rocks 6 6 to 3 5 as well as from most felsic hiltaba suite intrusions and gawler range volcanics in the eastern gawler craton ca 6 to 4 these data are consistent with crustal sources for ree and by implication for associated copper in the barren and weakly mineralized prospects mineralization and alteration in these minor iocg systems can be geochemically discriminated from the giant olympic dam deposit where greater inputs of mantle derived ree and other ore components are evident 2007 society of economic geologists inc

high pressure metamorphism in the aegean eastern mediterranean underplating and exhumation from the late cretaceous until the miocene to recent above the retreating hellenic subduction zone 1 we report 40ar 39ar ages from various tectonic units in the aegean and westernmost turkey on the basis of published geochronologic data and our 40ar 39ar ages we propose that the aegean is made up of several high pressure units which were successively underplated from the late cretaceous until the miocene ages for high pressure metamorphism range from 80 83 ma in parts of the vardar izmir ankara suture zone in the north to 21 24 ma for the basal unit in the cyclades and the external high pressure belt on crete in the south published seismic data suggest that high pressure metamorphism is currently occurring underneath crete younging of high pressure metamorphism in a southerly direction mimics the southward retreat of the hellenic subduction zone we propose that distinct stages of high pressure metamorphism were controlled by the underthrusting of fragments of mainly thinned continental crust and that these punctuated events were superposed on progressive slab retreat by far most of the exhumation of the high pressure units occurred early during the orogenic history in a forearc position

neotethys closure history of anatolia insights from 40ar 39ar geochronology and p t estimation in high pressure metasedimentary rocks the multiple high pressure hp low temperature lt metamorphic units of western and central anatolia offer a great opportunity to investigate the subduction and continental accretion related evolution of the eastern limb of the long lived aegean subduction system recent reports of the hp lt index mineral fe mg carpholite in three metasedimentary units of the gondwana derived anatolide tauride continental block namely the afyon zone the oren unit and the southern menderes massif suggest a more complicated scenario than the single continental accretion model generally put forward in previous studies this study presents the first isotopic dates white mica 40ar 39ar geochronology and where possible are combined with p t estimates chlorite thermometry phengite barometry multi equilibrium thermobarometry on carpholite bearing rocks from these three hp lt metasedimentary units it is shown that in the afyon zone carpholite bearing assemblages were retrogressed through greenschist facies conditions at c 67 62 ma early retrograde stages in the oren unit are dated to 63 59 ma in the kurudere nebiler unit hp mesozoic cover of the southern menderes massif hp retrograde stages are dated to c 45 ma and post collisional cooling to c 26 ma these new results support that the oren unit represents the westernmost continuation of the afyon zone whereas the kurudere nebiler unit correlates with the cycladic blueschist unit of the aegean domain in western anatolia three successive hp lt metamorphic belts thus formed the northernmost tavsanli zone c 88 82 ma the oren afyon zone between 70 and 65 ma and the kurudere nebiler unit c 52 45 ma the southward younging trend of the hp lt metamorphism from the upper and internal to the deeper and more external structural units as in the aegean domain points to the persistence of subduction in western anatolia between 93 90 and c 35 ma after the accretion of the menderes tauride terrane in eocene times subduction stopped leading to continental collision and associated barrovian type metamorphism because by contrast the aegean subduction did remain active due to slab roll back and trench migration the eastern limb below southwestern anatolia of the hellenic slab was dramatically curved and consequently teared it therefore is suggested that the possibility for subduction to continue after the accretion of buoyant e g continental terranes probably depends much on palaeogeography

the thermal response of a metamorphic belt to extension constraints from laser ar data on metamorphic micas abstract metamorphic rocks may be exhumed to the surface either by isostatic rebound uplift and erosion or tectonically via thrusting or extension the latter mechanism is particularly important in the himalayas where a large extensional detachment system appears to be responsible for very fast unroofing at 20 ma here we present the results of a laser argon study of micas and quartz aimed at investigating the thermal response of an orogenic belt to such extension the data when combined with existing constraints on the prograde evolution lead to a uniquely complete thermal history for a metamorphic belt laser argon data are reported for quartz and micas from semi pelitic metamorphic rocks from the zanskar himalayas nw india the samples derive from three traverses extending from low grade rocks near the detachment to high grade rocks in the core of the belt most samples demonstrably contain no excess component and the ages of muscovite and biotite show a correlation with distance from the extensional detachment the ages themselves as well as the difference between the muscovite and biotite ages both increase as the detachment is approached muscovite and biotite ages from within the shear zone date deformation associated with extension at 20 25 ma the remaining data are most readily interpreted in terms of early slow cooling of the lower grade rocks near the detachment in response to erosional denudation before the detachment became active rocks in the core of the orogen were still heating at this time later fast cooling of the high grade rocks in the core of the belt ensued when the extensional regime became active simple heat transfer calculations based on the distance of the high grade rocks from the fault and the argon ages of their micas provide further constraints on the timing of extension and suggest that the fault first moved 23 25 ma ago and that it abruptly terminated metamorphism in the core of the metamorphic belt our analysis of the thermal history suggests that cooling and exhumation rates determined from metamorphic belts beneath extensional detachments will often be minima

mesozoic cooling across the yidun arc central eastern tibetan plateau a reconnaissance 40ar 39ar study abstract 40 ar 39 ar dating of hydrous minerals from igneous and metamorphic rocks was undertaken with the aim of characterizing the thermal history of the yidun arc eastern tibetan plateau regional metamorphism of palaeozoic rocks of the western yidun arc occurred during the early triassic collision with the qiangtang block along the jinsha jiang suture hornblende muscovite and biotite 40 ar 39 ar ages from syn tectonic igneous rocks within this collision zone suggest that the region had cooled to below 300 c by the late triassic muscovite samples from mica schists within this collision zone yield variably discordant age spectra with generally triassic 40 ar 39 ar ages consistent with cooling of the region to below muscovite closure temperatures by the close of the triassic hornblende and biotite 40 ar 39 ar ages from granite and granodiorite plutons in the eastern yidun arc suggest post emplacement cooling of these plutons occurred rapidly consistent with these granites being high level intrusions in sum these results suggest that by the beginning of the jurassic the yidun arc as a whole had cooled to below 300 c two cretaceous granites in the central yidun arc also experienced rapid cooling suggesting they too were emplaced at shallow crustal depths post triassic deformation of the region must have occurred at or below the brittle ductile transition within the crustal levels now exposed

thermochronologic constraints on the tectonic evolution of active metamorphic core complexes d entrecasteaux islands papua new guinea metamorphic core complexes in the d entrecasteaux islands papua new guinea formed as the result of active extension at the western end of the propagating woodlark basin spreading center domes of high grade metamorphic rocks i e amphibolites eclogites and migmatites intruded by large granodiorite bodies comprise the lower plate of the d entrecasteaux metamorphic core complexes the domes are transected by kilometer scale shear zones a thermochronologic study of the d entrecasteaux islands utilizing k ar 40ar 39ar and fission track techniques has documented the unroofing history of these active metamorphic core complexes gneisses in the cores of the domes cooled rapidly 100 c m y as indicated by hornblende and biotite 40ar 39ar apparent ages of 2 7 to 3 0 ma and 1 6 to 1 7 ma respectively and apatite fission track ages of 0 4 to 0 9 ma 40ar 39ar apparent ages on white mica biotite and potassium feldspar and fission track ages on apatites from shear zone gneisses indicate extremely rapid cooling in some cases 500 c m y and suggest shear zones were active from 4 0 to 3 5 ma and 1 9 to 1 4 ma in general 40ar 39ar mineral ages for retrogressed core zone gneisses shear zone gneisses and granodiorites are 2 0 to 3 0 ma amphiboles 1 5 to 1 7 ma muscovites and 1 4 to 1 8 ma biotites and 1 0 to 2 0 ma k feldspars apatite fission track ages from core zone gneisses shear zone gneisses and granodiorites range from 0 4 to 1 0 ma thermochronologic results indicate that emplacement of granodiorites closely coincided with retrogression of the metamorphic basement and movement on the outer shear zones bounding the gneiss domes the granodiorite bodies associated with the d entrecasteaux islands domes represent syn kinematically emplaced granitoids intruded into an area of active continental extension

40ar 39ar dates on hornblende muscovite and biotite from the mica creek area british columbia regional metamorphic and tectonic implications a metamorphic high the selkirk monashee cariboo complex smc is a 200 km 60 km area cored by k feldspar sillimanite rocks in the mica creek area there appear to be no discontinuities in metamorphic isograds structural trends or the late precambrian stratigraphic units u pb dates suggested that the continuities may only be apparent in this paper we report 40ar 39ar dates on muscovite biotite and hornblende from some of the same samples dated by u pb and samples that crop out very close to the dated samples the goal was to set constraints on the alternative interpretations of the metamorphic structural history these interpretations are as follows 1 u pb ages record different times of metamorphic crystallization and deformation and there are significant tectonic displacements the mappable continuities are only apparent 2 the structural and stratigraphic continuity is real and the range is due to localized heating and deformation events spanning 100 ma the relative uniformity of the

repeated thermal resetting of phengites in the mulhacen complex betic zone southeastern spain shown by40ar 39ar step heating and single grain laser probe dating abstract this study reports the results of the first 40 ar 39 ar combined induction furnace and laser probe dating of phengites from the mulhacen hp lt metamorphic complex in the betic cordilleras southern spain laser step heating and spot fusion analyses on different halves of a split single grain were made with a continuous laser probe spot fusion analysis resulted in ages of about 30 31 ma in the core and ages as low as 25 26 ma in the rim laser step heating on the other half of the grain gave a spectrum with apparent ages increasing from about 25 ma to 29 5 ma the age spectrum and the decreasing ages towards the rim of the grain may imply that resetting essentially occurred by volume diffusion of radiogenic 40 ar due to late stage reheating resulting from extensional tectonics ages around 30 ma in the core of the grain are interpreted as minimum estimates of the cooling age of the main tectono metamorphic phase d 2 induction furnace step heating on phengite separates from mica schists and one gneiss resulted in two types of age spectra type i spectra show monotonously rising apparent ages from 14 5 1 9 ma to 20 7 0 2 ma and in a second sample from 16 9 0 7 to 29 7 0 2 ma type ii spectra are characterized by plateaus of 14 4 0 1 ma the gneiss sample 17 3 0 1 ma and 17 6 0 1 ma type ii spectra show low temperature apparent ages significantly below the plateau age implying resetting subsequent to initial cooling modelling of the age spectra demonstrated that the plateau ages are possibly the result of strong resetting 75 85 of argon loss of an older isotope system total fusion of a number of phengite single grains from marbles taken close to type ii mica schists yielded ages of 15 4 1 2 ma and 17 0 0 7 ma the observed repeated resetting is coeval with major volcanic activity in basins adjacent to the metamorphic ranges pointing to a resetting by advective fluid transport related to volcanism

late carboniferous high pressure metamorphism of the kassan metamorphic complex kyrgyz tianshan and assembly of the sw central asian orogenic belt abstract high pressure hp metamorphism of the kassan metamorphic complex kmc in the western kyrgyz tianshan has been related to either late ordovician or late carboniferous permian subduction processes we report sm nd ages for retrogressed eclogite samples and 40ar 39ar cooling ages for enclosing garnet muscovite samples from the kmc as new age constraints on hp metamorphism and rock exhumation these data will be used for an upgraded paleogeographic model for late paleozoic crustal consolidation in the southwestern central asian orogenic belt the retrogressed eclogite samples have transitional alkaline to tholeiitic affinity and trace element patterns consistent with protoliths derived from garnet bearing mantle sources at rifting plate margins geothermobarometric data for a retrogressed eclogite sample indicate peak metamorphic conditions of 540 30 c at 1 6 0 1 gpa samples from different lithotectonic units of the kmc provide coherent sm nd garnet whole rock ages of 317 4 ma and 316 3 ma 2 the prograde major element zoning in the mm sized garnets in combination with the moderate peak metamorphic temperature support our interpretation of the sm nd garnet ages as unambiguous evidence for late carboniferous hp metamorphism the sm nd garnet growth ages overlap within error with the 40ar 39ar mica cooling ages of 314 2 ma and 313 2 ma 2 indicating rapid uplift of the subduction complex after peak metamorphism the ca 317 313 ma hp exhumation event of the kmc is contemporaneous with those of the atbashi and akeyazi ca 500 km east in nw china hp complexes and implies similar collision histories at the south tianshan suture to the east and west of the talas fergana fault tff the exhumation of the kmc and atbashi hp complexes overlaps with the initiation of the tff rolland et al 2013 suggesting incipient separation of the chatkal and atbashi complexes during rock exhumation and early plate collision

fault gouge dating in the southern appalachians usa illite age analysis iaa the method of comparing radiometric ages of successive size fractions with varying percentages of detrital illite has been successfully applied to several rock types including fault gouge shales and argillaceous limestones iaa results are presented for fi ve fault rocks including four clay gouges and one cataclasite from the exhumed southern appalachian foreland fold thrust belt eastern united states determining detrital versus authigenic illite is now an established procedure utilizing x ray analysis to quantify illite polytypes less established however is how to interpret 40 ar 39 ar ages as a function of diagenetic grade both a total gas age incorporating the recoiled argon fraction after irradiation and a retention age omitting the recoiled fraction are obtained for a sample we relate their respective use to diagenetic grade and specifi cally the crystallite thickness of illite when measured crystallite sizes are equal to or smaller than that calculated from ar recoil we use total gas ages here 5 nm while for crystallites greater than 10 nm we use retention ages the four clay gouge ages are all the same within error and much younger than that of the cataclasite we conclude that a major period of frontal appalachian fault activity occurred during the early permian cisuralian 276 280 ma and that these foreland faults were active simultaneously as part of an internally deforming regional thrust wedge

evaluation of radioisotope dating of carlin type deposits in the great basin western north america and implications for deposit genesis a variety of techniques have been used in attempts to date the mineralization in carlin type deposits in the great basin with highly variable results these techniques and results are reviewed in this paper along with presentation of two new dates for the rodeo deposit on the carlin trend and for the barneys canyon deposit in utah complete resetting of sericite by hydrothermal fluids of the temperature and duration of hydrothermal activity that form carlin type deposits is considered highly unlikely therefore dates from sericite are generally of questionable value unless that sericite can be shown to have formed during the same hydrothermal event during which au was deposited for the deposits that have been investigated to date sericite dates rarely if ever record the age of au mineralization however sericite ages do appear to record pre au events in some districts such events may have contributed to ground preparation and to a much lesser extent to the tenor of the ore fission track and u th he techniques provide important age constraints on mineralization in some districts but also suffer from a less than clear association with au rb sr dating of galkhaite an hg sulfosalt provides the only direct age of mineralization but galkhaite has been recognized in only a few locations and dated in only two deposits in a similar manner adularia is contemporaneous with au at twin creeks but twin creeks is the only carlin type deposit where adularia has been reported several other techniques u pb reos sm nd have been used in attempts to date the deposits but with limited success the best available ages are those from preore and postore igneous events and those from galkhaite galkhaite from getchell indicates a mineralization age of 39 ma and that from rodeo carlin trend indicates a mineralization age of 40 ma a new u th he age 34 ma for apatite at barneys canyon suggests contemporaneity with the nearby bingham porphyry cu system the most reliable data from several deposits and districts in the great basin indicate that carlin type mineralization occurred in a relatively narrow time interval between 33 and 42 ma this interval coincides with the transition from a compressional to an extensional tectonic environment in the region although similar tectonic environments have existed at different times and places in western north america no other carlin type mineralization is known to exist in the region therefore although the age and tectonic environment of these deposits is now reasonably well constrained it is clear that tectonic environment alone is insufficient to explain the genesis of carlin type deposits additional perhaps unique factors such as host rock geochemistry must have contributed to the development of these exceptional deposits

kinematics and timing of continental block deformation from margins to interiors during the deformation of continental blocks the magnitude of tectonic stress generally decreases with increasing distance from the margin of the block however the timing and kinematics of stress transmission from the margins to the interiors of continents are poorly resolved even though this information is critical to our understanding of the dynamics of continental deformation here we present a case study of mesozoic deformation of the north china craton ncc field investigations of mesozoic thrust faults and folds granitic intrusions and dykes combined with zircon shrimp and la icp ms dating and muscovite 40ar 39ar plateau ages reveal the age of the ne sw trending tectonic belts as 180 155 ma where the deformation of the craton margin occurred 10 20 ma earlier than that of the craton s interior although the kinematics of deformation are similar for the interior and the margin of the ncc strain decreases with increasing distance from the margin notably the bulk of the strain in the cratonic interior was focused in zones of pre existing weakness overall we determined that the ncc deformed under conditions of uniaxial compression a conclusion that is compatible with simple rheological models and that the stress magnitude attained in the cratonic interior was much less than that along its margin

structure kinematics and ages of transpression during strain partitioning in the chongshan shear zone western yunnan china abstract the chongshan shear zone extends from the eastern himalayan syntaxis to the lincang granitic pluton in yunnan province china the structure and kinematics show that the shear zone comprises mainly of mylonitic gneiss migmatite and schist with a dextral dominated strike slip motion in an n s trending northern segment and a sinistral strike slip shear in nw se trending middle and southern segments both were developed under a bulk regional scale sinistral transpression shrimp and la icpms u pb and 40 ar 39 ar dating reveal two tertiary magmatic events along the zone followed by younger sinistral strike slip shear the eocene magmatic event c 55 38 ma followed by metamorphism at c 36 ma happened before the strike slip motion the strike slip shear along the zone began c 32 ma which generated shear heating from 32 to 22 ma the 40 ar 39 ar ages of syn kinematic micas range from 19 to 14 ma and indicate that the strike slip shear continued to this time with coeval transpressional exhumation and uplift of the metamorphic rocks along the biluoxueshan chongshan chain the chongshan zone is thus a cenozoic shear zone which was contemporaneous with motion on the left lateral ailao shan red river shear zone and the right lateral gaoligong shear zone and should be important in accommodating the northwards movement of india during collision during oligocene to miocene times the continental block that was extruded between the ailao shan red river and gaoligong shear zones was dismembered into at least two major fragments by the chongshan shear zone

thermal history of the giant qulong cu mo deposit gangdese metallogenic belt tibet constraints on magmatic hydrothermal evolution and exhumation abstract a complete thermal history for the qulong porphyry cu mo deposit tibet is presented zircon u pb geochronology indicates that the mineralization at qulong resulted from brecciation veining events associated with the emplacement of a series of intermediate felsic intrusions combined with previously published ages our results reveal a whole intrusive history of the qulong composite pluton causative porphyries were emplaced at 16 0 ma as revealed by 40ar 39ar dating of hydrothermal biotite 15 7 0 2 ma and sericite 15 7 0 2 ma zircon and apatite u th he zhe and ahe dating of qulong revealed that both followed similar monotonic thermal trajectories from 900 c u pb ages 17 5 15 9 ma to 200 c zhe 15 7 14 0 ma and that the causative porphyries experienced faster cooling at a maximum rate of greater than 200 c myr the qulong deposit was exhumed between 13 6 ma and 12 4 ma ahe at an estimated rate of 0 16 0 24 mm y which is consistent with previous estimates for other gangdese miocene porphyry deposits our ahe thermochronology results suggest that neither the gangdese thrust system nor the yadong gulu graben affected or accelerated exhumation at the qulong deposit