thermochronology of the south cyclades shear zone ios greece effects of ductile shear in the argon partial retention zone micas and potassium feldspars from the south cyclades shear zone ios cyclades greece yield varied and complex 40ar 39ar apparent age spectra a correlation exists between 40ar 39ar apparent ages and the relative timing of different episodes of recrystallization and grain growth as indicated by fabric and microstructural analysis the 40ar 39ar apparent age spectra record the effects of variation in the degree of recrystallization and grain growth and partial to complete resetting of argon systematics in potassium bearing minerals during hercynian m0 alpine m1 and late oligocene early miocene m2 metamorphism deformation was strongly partitioned within the shear zone and this led to localized recrystallization and heterogeneous resetting of argon systematics within preexisting minerals modeling suggests the oligo miocene thermal events were of insufficient magnitude and or duration to completely reset the isotopic systematics in these samples our data lead to the concept of the argon partial retention and resetting zone prz defined as that portion of the crust where temperatures are insufficient to completely reset argon systematics within preexisting potassium bearing minerals within the prz some radiogenic argon in preexisting potassium bearing minerals will be outgassed and only partially retained tectonic exhumation of the prz involves movement on crustal scale ductile shear zones accompanied by strongly partitioned deformation and localized recrystallization recrystallization leads to resetting of argon systematics and thus will result in heterogeneous 40ar 39ar age distributions within these ductile shear zones e g in the south cyclades shear zone

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

metamorphic evolution of the attic cycladic metamorphic belt on naxos cyclades greece utilizing 40ar 39ar age spectrum measurements 40ar 39ar age spectrum analysis of phengite separates from naxos part of the attic cycladic metamorphic belt in greece indicates that cooling following high pressure low to medium temperature metamorphism m1 occurred about 50 ma ago phengite has 40ar gradients that suggest that part of the scatter observed in conventional k ar ages was caused by diffusion of radiogenic argon from the minerals during a younger metamorphism m2 in central naxos this metamorphism m2 has overprinted the original mineral assemblages completely and is associated with development of a thermal dome excellent 40ar 39ar plateaus at 15 0 0 1 ma 11 8 0 1 ma and 11 4 0 1 ma obtained on hornblende muscovite and biotite respectively from the migmatite zone indicate that relatively rapid cooling followed the m2 event and that no significant thermal overprinting occurred subsequent to m2 toward lower m2 metamorphic grade 40ar 39ar plateau ages of hornblendes increase to 19 8 0 1 ma concomitantly the proportion of excess 40ar in the spectra increases as well we propose that the peak of m2 metamorphism occurred beween 15 0 and 19 8 ma ago k ar ages of biotites from a granodiorite on the west coast are indistinguishable from those found in the metamorphic complex and hornblende k ar ages from the same samples are in the range 12 1 13 6 ma as the latter ages are somewhat younger than most ages obtained from the metamorphic complex intrusion of the granodiorite most likely followed the peak of the m2 metamorphism the metamorphic evolution of naxos is consistent with rapid crustal thickening during the cretaceous or early tertiary causing conditions at which supracrustal rocks experienced pressures in the range 900 1500 mpa transition to normal crustal thicknesses ended the m1 metamorphism about 50 ma ago the m2 metamorphism and granodiorite intrusion occurred during a period of heat input into the crust possibly related to the migration of the hellenic volcanic ar c in a southerly direction through the area

mechanisms of aggregate stabilization in ultisols from subtropical china abstract the quantification and interpretation of aggregate stability depend on internal soil properties and external factors such as measurement method and aggregate size the objectives of this study were to i determine the aggregate stability in ultisols from subtropical china applying the le bisssonais method ii determine the effect of initial aggregate size on its stability and iii interpret mechanisms of aggregate stabilization in the soils three aggregate size ranges 5 3 3 2 and 2 1 mm were obtained by dry sieving after the wetting treatments the dominant fraction of fragments for each soil was 2 1 mm or 0 63 0 2 mm the mechanisms of aggregate breakdown was in the order slaking mechanical breakdown micro cracking they differed with soil type and composition the normalized mean weight diameter nmwd of the aggregates after fast wetting and wet stirring were more correlated with soil properties such as degree of micro aggregation doa cation exchange capacity cec k 2 o fe 2 o 3 or al 2 o 3 rather than clay and soil organic carbon soc content the binding force by soil organic matter was smaller than the force caused by entrapped air or the force of combination of mechanical stress by stirring and differential swelling of minerals the smaller the aggregate the larger was the aggregate stability according to nmwd the rankings of the soils differed with the soil aggregate sizes and the wetting treatments sandy loams from sandstone sc and sw were the weakest soils while the purple mudstone pp was the strongest all the cultivated soils decreased in aggregate stability compared with the comparable uncultivated soils or parent materials irrespective of the cultivation time and the changes in soc content after cultivation

bombardment history of asteroid 4 vesta recorded by brecciated eucrites large impact event clusters at 4 50 ga and discreet bombardment until 3 47 ga abstract the thermal impact histories of sixteen eucrite meteorites were investigated three monomict eucrites nwa 999 a 87272 87 and stannern five polymict eucrites nwa 1000 nwa 1666 nwa 5601 y 980066 100 and y 980255 100 three quench textured eucrite melt rocks y 981646 21 y 981651 105 and mil 0766214 one eucrite dominantly comprised of quench textured clasts que 99005 11 three unclassified eucrite breccias lap 031316 9 lar 06870 5 que 99799 4 and one unbrecciated eucrite eet 92004 17 included here due to its shock features we have measured fifteen high precision new 40 ar 39 ar plateau ages on plagioclase and matrix for ten of these meteorites with a tight cluster of nine ages obtained from three different polymict breccias these ages range from 4534 56 ma to 4491 16 ma resulting in a concordant age population p 0 16 the fact that such a cluster of ages is recorded in unrelated breccias which are made of a priori unrelated components leads us to propose that those ages recorded a single heating event on a large scale and is interpreted here as a high energy impact event early in the history of vesta at 4500 4 ma we propose that the debris was ejected and isolated from subsequent large impacts in a secondary rubble pile asteroid where the energy of the outgoing shock wave from an impact is significantly reduced as it compacts the target material holsapple et al 2002 and references therein the other analyses define a spread of five plateau ages ranging from 3851 21 ma to 3469 35 ma over 380 ma an additional apparent plateau age of 4288 38 ma but with a diffusion profile of cumulative 39 ar release along with published u pb apatite age of 4 14 ga suggests that the data might either define a true continuum normal background bombardment from 4 5 ga to 3 47 ga or cluster between 3 85 ga and 3 47 ga excavation of a fresh surface at 3 85 ga continuously bombarded until 3 47 ga both scenarios are compatible with a final ejection age of 3 47 ga when a major impact liberates the bulk of the brecciated meteorites into another secondary rubble pile asteroid where the brecciated eucrites stayed relatively well protected from subsequent major impacts based on these results and recent crater counting measurements we propose that the excavation and bulk ejection were caused by the rheasilvia ca 3 47 ga basin forming impact diffusion models on plagioclase crystals with different ar age spectrum signatures from a single breccia nwa1666 4501 7 ma suggest that either 1 the formation of the breccia is very young or 2 different plagioclase crystals have different diffusion characteristics and or 3 the porosity caused heterogeneous temperatures during an impact heating event particularly likely if the 4 5 ga brecciated eucrites were stored in a rubble pile asteroid many or possibly most large asteroids being re accumulated rubble piles with potential large porosity holsapple et al 2002 scenarios 2 and 3 preclude the usage of multi grain aliquots to decipher the time temperature history of most impact breccia using 40 ar 39 ar thermochronology

40ar 39ar thermochronological evidence for formation and mesozoic evolution of the northern central segment of the altyn tagh fault system in the northern tibetan plateau to better constrain the probable timing of formation and evolution of the altyn tagh sinistral strike slip system in the mesozoic a 40ar 39ar thermochronological study has been carried out in the north central segment of the altyn tagh fault system the northern margin of the qaidam basin and the eastern kunlun orogenic belt muscovite biotite and k feldspar separated from mylonite granite pegmatite and metamorphic rocks have been analyzed the range of 40ar 39ar data and structural evidence indicate that a peak metamorphic event in terranes bordering the altyn tagh fault system occurred between 450 and 420 ma at ca 250 230 ma there is evidence for initial sinistral strike slip shearing sinistral strike slip deformation occurred later along the altyn tagh fault system at 165 160 ma and 100 89 ma respectively cooling histories in the northern margin of the qaidam basin and the eastern kunlun orogenic belt show that these areas also experienced rapid cooling ca 250 230 ma as was the case for the early altyn tagh fault system this regional tectonic and cooling process indicates that the initial formation of the altyn tagh sinistral slip fault system occurred in latest permian early triassic time and was coupled with or related to suturing in the northern margin of the qaidam basin and the kunlun orogenic belt cooling events along the altyn tagh fault system between 165 and 160 ma and 100 89 ma were accompanied by differential closure along the bangong lake nujiang suture zone in its eastern and western sectors during the middle late jurassic and early cretaceous respectively zhao et al 2001 1 wang et al 2002 2 1 ref 48 2 ref 41

40ar 39ar thermochronology of the southern gawler craton australia implications for mesoproterozoic and neoproterozoic tectonics of east gondwana and rodinia 40ar 39ar thermochronology of the southern gawler craton australia reveals a protracted history of slow cooling with local crustal heating rapid cooling and fault reactivation between 1700 and 700 ma hornblendes from most of the southern part of the craton including the kalinjala mylonite zone yield ages 1580 1610 ma with younger more discordant samples showing evidence of retrogression these results indicate that the region cooled below 500 c 100 m y after the kimban orogeny and coincident with the gawler range hiltaba suite magmatism to the north perhaps associated with postorogenic extension biotite and k feldspar data indicate that regional cooling after this time was more gradual with a few exceptions locally in the southeastern eyre peninsula rapid cooling at 1425 ma is indicated by nearly concordant hornblende and biotite ages and may record a phase of regional extension other exceptions include areas of lower grade archean rocks that cooled to 350 c by 1700 1640 ma greenschist facies reactivation of shear zones occurred at 1100 ma during the musgravian orogeny on the basis of biotite single grain data and retrogressed amphiboles k feldspar age spectra record cooling at 700 800 ma following regional intrusion of the gairdner dike swarm and or denudation during rifting along the pacific margin of east gondwana and breakup of rodinia tectonothermal events revealed by these results previous geochronological and geological studies and available paleomagnetic data are comparable to other cratons and mobile belts in australia east gondwana and laurentia providing evidence that southeastern australia and north america were adjacent during much of proterozoic time

the interpretation of 40ar 39ar apparent age spectra produced by mixing application of the method of asymptotes and limits abstract a method is presented for the analysis and interpretation of apparent age spectra produced during 40 ar 39 ar step heating experiments application of this method is particularly relevant to complex apparent age spectra produced from minerals taken from exhumed metamorphic tectonites particularly when these rocks once resided in an ancient argon partial retention zone such rocks may have been subject to localised deformation and or recrystallisation during orogenesis yet microstructures relict of earlier episodes of deformation and or metamorphism may retain relatively old apparent ages as a result complex apparent age spectra can be produced during a conventional step heating experiment and these age spectra often cannot be simply interpreted the recognition and the interpretation of a plateau in such spectra is sometimes uncertain and in any case the definition of a plateau is then a concept of limited theoretical validity to avoid these difficulties an alternative strategy is presented based on a theory of mixing gas from different microstructural microchemical reservoirs this method relies on the definition of asymptotes and limits in sequences of apparent ages in apparent age spectra obtained from step heating experiments frequently measured ages fmas in individual datasets can then be recognized using statistical analysis the significance of fmas must be independently assessed

white mica 40ar 39ar age spectra and the timing of multiple episodes of high p metamorphic mineral growth in the cycladic eclogite blueschist belt syros aegean sea greece research support providedby arc discovery projects dp0877274 anddp1201035 54 forster ackno wledges the support ofan australian research council arc australianresearch fellowship

thermochronological analysis of the dynamics of the southern alps new zealand the isotopic ages of material exposed close to the alpine fault in the southern alps new zealand vary in a consistent fashion along the orogen for the k ar system muscovite and biotite ages display two distinct spatial trends that meet in the area of fox glacier where a strong relationship is observed between k ar age and altitude excess argon is present in samples between mount kinnaird and haast pass in the southwest of the southern alps giving unrealistically old inconsistent k ar ages across this region incorporation of excess argon appears to be highly variable however and minimization of its influence on dynamical interpretations of thermochronology is possible through considering only the youngest age from a given area such treatment indicates that excess argon contamination does not play an important role in the systematic k ar age variation identified along the southern alps the observed age distribution is consistent with estimates of the modern rapid uplift and exhumation of the southern alps beginning ca 5 ma k ar mica ages older than this south of the copland valley indicate that substantial late cenozoic exhumation occurred along the southern alps prior to the development of the modern tectonic regime but at rates lower than at present total exhumation during the current phase of tectonism in the southern alps decreases from the whataroa river southward north of fox glacier the southern alps have attained a dynamical steady state such that the observed distribution of exhumation will not change significantly with further deformation and denudation under current conditions

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

fast cooling following a late triassic metamorphic and magmatic pulse implications for the tectonic evolution of the korean collision belt we discuss the evolution of korea in the context of a relatively short lived tectonically induced magmatic and metamorphic pulse that affected large portions of the crust of the peninsula s southern part during the late triassic recent 40ar 39ar single grain laser step heating dates imply a prolonged metamorphic recrystallization between 243 and 220 ma which occurred in distinct phases that were not coeval throughout the peninsula we obtained identical plateau ages between 231 4 0 8 and 228 9 0 8 ma 1 85 95 39ar release on single grains of detrital muscovite from jurassic sandstones gimpo group a literature review shows that the ages of detrital muscovites are identical to 1 concordant 40ar 39ar ages of biotite 228 ma and amphibole 230 ma in amphibolites of the deokjeongri gneiss formation and the weolhyeonri complex pointing to very rapid cooling of 100 150 c ma and 2 231 229 ma muscovite from the low grade metamorphic mid paleozoic turbidites of the taean formation the efficiency of cooling is further underlined by the near coincidence of these 40ar 39ar ages with 243 229 ma average 234 6 ma zircon u pb ages in the gyeonggi massif and the hongseong belt in the literature it is argued that the late triassic magmatic and metamorphic pulse is superimposed on an earlier tectono metamorphic event possibly related to collision indicated by 1 243 237 ma muscovite ages or age components in age spectra and 2 two generations of folds and associated tectonic foliations truncated by 229 5 ma old syenites and earlier mafic dykes the late triassic thermal pulse could have been the result of post collisional delamination of the lower crust and uppermost mantle and or oceanic slab break off which is also suggested by almost coeval widespread mantle sourced mg rich potassic magmatism continuing ductile deformation is shown by mylonitization of late triassic magmatic rocks an 220 ma muscovite age may be related to this

ages of deformation from k ar and 40ar 39ar dating of white micas a structural and isotopic dating study of white micas from mylonites in the ruby gap duplex off central australia shows that mylonitic deformation occurred during the paleozoic alice springs orogeny deformation of the white micas took place under greenschist facies conditions in the approximate temperature range 250 350 c two distinct populations of white mica have been found 1 porphyroclasts of original muscovite and 2 neocrystallized phengites that define the fabric associated with deformation 40ar 39ar age spectra of neocrystallized phengites yield plateau like spectra of paleozoic age in contrast 40ar 39ar age spectra of muscovite porphyroclasts exhibit proterozoic apparent ages indicating that white micas were not open to significant argon diffusion during formation of the ruby gap duplex we conclude that the ages measured on neocrystallized phengites record the cessation of ductile deformation and migration of tectonic activity toward the internal zones of the thrust system

coherent french range blueschist subduction to exhumation in 2 5 m y coherent oceanic strata in the french range belong to the exotic cache creek terrane of the canadian cordillera they were metamorphosed to blueschist grade tectonically extruded eroded and intruded by plutons perhaps in 2 5 m y sodic amphibole overprint chert as young as late pliensbachian to toarcian age ca 191 to ca 177 ma blueschist mineral assemblages defi ne the early metamorphic fabric along with phengite dated by 40 ar 39 ar as 173 7

preservation of sub microscopic structural relicts in micas from the gran paradiso massif western alps implications for 40ar 39ar geochronology abstract permian igneous biotite and white mica that were re heated at 500 550 c in the middle late eocene were investigated by laser step heating and in situ 40ar 39ar techniques in conjunction with scanning electron microscopy sem electron microprobe emp and transmission electron microscopy tem to assess the influence of short lived metamorphism on k ar systematics large intra and inter grain age variations ranging from the triassic to the late eocene are primarily related to the extent of metamorphic re equilibration brown biotite porphyroclasts from weakly re equilibrated samples are characterized by core to rim zoning in major element composition mineral structure and 40ar 39ar ratios titanium concentration and fe mg fe ratios decrease from crystal cores towards the rims which are compositionally indistinguishable from green biotite aggregates in metamorphic coronas compositional changes are coupled with structural modifications towards the crystal edges where the disordered igneous 1m 2m1 stacks are replaced topotactically by highly ordered metamorphic 1m polytypes apparent ages up to 66 ma were determined in crystal cores by in situ laser probe analyses while ages as young as 45 ma are typical of crystal rims step heating experiments of these zoned biotites yielded a discordant saddle shaped age spectrum with a concordant central segment yielding an error weighted mean age of 44 9 0 3 ma biotite porphyroclasts from a more re equilibrated specimen gave markedly different results with uniform major elements compositional profiles homogeneous argon distribution throughout the crystals and a flat age spectrum at 36 5 0 3 ma tem investigations of these optically homogeneous porphyroclasts revealed that the original igneous structure had been replaced by highly ordered 1m polytypes during alpine metamorphism white mica yielded comparable results with spot ages up to 218 ma in domains characterized by high na na k ratios and low si and ti content representing the best preserved igneous relicts significantly younger apparent ages down to 44 39 ma are typical of domains affected by partial metamorphic re equilibration at high pressure conditions as indicated by increasing si and ti contents or by exhumation related exsolution of amoeboidal quartz these results indicate that different intra grain and inter grain ar concentration patterns arise primarily from the relative amount and distribution of relict igneous and metamorphic crystal chemical domains core to rim age gradients in biotite result from inward directed recrystallization from the crystal edges eventually leading to flat age profiles as metamorphic replacement of pre existing porphyroclasts runs to completion this study calls for caution when estimating thermal histories from apparent age gradients or from age vs grain size correlations unless the presence of pre metamorphic mineral relicts can be ruled out

argon composition of metamorphic fluids implications for 40ar 39ar geochronology the concentration and isotopic composition of argon trapped in fluid inclusions within a mylonitic amphibolite were determined by combining electron microprobe sup 40 ar sup 39 ar laser microprobe and fluid inclusion analyses on the same samples in our study fluid in inclusions formed during ductile deformation was probably derived locally from the paleozoic caledonian sequences whereas brines emplaced in cross cutting fractures were derived from proterozoic footwall rocks several kilometers from the mylonite by utilizing sup 38 ar sub cl to correct for open quotes excess sup 40 ar close quotes we estimate a 370 ma closure age for the amphibole which is distinctly younger than the 440 480 ma plateau ages indicated by step heating spectra the open quotes excess sup 40 ar close quotes responsible for the anomalous plateaus probably resulted from diffusive re equilibration of the amphibole with the fluids present during ductile deformation of the mylonite the significantly younger cooling age determined by our method is concordant with ages determined from more recent geochronological studies in the region these ages indicate that the western gneiss terrain in our study area records deformation and cooling associated with siluro devonian tectonothermal metamorphism rather than an earlier caledonian tectonism as proposed in previous sup 40 ar sup 39 ar investigators 46 refs 8 figs

40ar 39ar dating of white micas from an alpine high pressure metamorphic belt on naxos greece the resetting of the argon isotopic system overprinting of white micas from high pressure low to medium temperature m1 metamorphic assemblages in pelitic schists on naxos during subsequent thermal dome m2 metamorphism ranges from minor in the southeast of the island to complete recrystallization in the amphibolite facies rocks near the migmatites in the centre of the dome the original m1 minerals are phengites si4 6 7 7 0 and the overprinting minerals are muscovites si4 6 0 6 45 40ar 39ar step heating analyses of white mica separates from rocks in the area where phengite and muscovite occur together yield complex age spectra characterized by low apparent ages in the first and the last stages of gas release and high apparent ages in between these upward convex age spectra are shown to be caused by mixing of two generations of micas each of which has a different age spectrum and argon release pattern seemingly good plateaus in some age spectra from white micas of the area must be interpreted as providing meaningless intermediate ages further the upward convex age spectra have been used to trace the isotopic signature of phengites toward increasing m2 metamorphic grade and suggest that as long as phengites can be observed in the rocks upward convex age spectra occur on naxos crystallization of muscovite at the expense of phengite appears to be the main mechanism of resetting argon isotopic ages in white micas however there is also good evidence for argon loss by volume diffusion from phengites simple diffusion calculations suggest that the m2 metamorphism was caused by a shortlived heat source

timing of mineralization and source of fluids in a slate belt auriferous vein system hill end goldfield nsw australia evidence from 40ar39ar dating and o and h isotopes abstract 40 ar 39 ar dating of metamorphic biotite and alteration muscovite from the auriferous veins and host rocks at the hill end goldfield n s w australia has distinguished four major geological events including the timing of gold mineralization the earliest hydrothermal event occurred during the middle devonian tabberabberan orogeny 370 380 ma and resulted in the formation of quartz veins barren of au a second and major episode of vein emplacement occurred in the early carboniferous during the principal phase of metamorphism and deformation at 359 363 ma this was followed by au accumulation in two stages 1 after the major phases of quartz deposition and 2 during and after the development of conspicuous internal vein laminations 357 ma and 343 ma respectively two sources of fluid are proposed for vein and ore formation the first is a local metamorphic fluid characterized by 18 o h 2 o values of 8 9 to 12 5 per mil and d h 2 o values of 87 to 90 per mil the second is a mixed ore fluid with 18 o and d values in the range of 18 o h 2 o 8 4 to 11 per mil and d h 2 o of 49 to 36 per mil progressive entry of this second fluid sourced from trough fill or deeper crustal rocks is linked closely to cycles of gold precipitation at hill end

post orogenic 1500 ma thermal history of the proterozoic eastern fold belt mount isa inlier australia the paucity of sedimentary rocks younger than 1500 ma within the mt isa inlier renders it difficult to determine its post orogenic exhumation and tectonic history except by inference from the stratigraphy of the surrounding areas we present hornblende white mica biotite and alkali feldspar 40ar 39ar data from the eastern fold belt of the inlier which reveal a previously unrecognised complex post orogenic non linear cooling history plateau and total fusion 40ar 39ar ages range between 1500 and 1130 ma and record increases in regional cooling rates of up to 18 c my during 1460 1410 and 1280 1050 ma forward modelling of the alkali feldspar 40ar 39ar arrhenius parameters reveals subsequent increases in cooling rates during 740 640 and 600 500 ma the cooling episodes were driven by both erosional exhumation at maximum rates of 0 9 km my and thermal relaxation following magmatic events early mesoproterozoic cooling is synchronous with exhumation and shearing during the anmatjira event in the arunta block and late mesoproterozoic cooling occurred at the same time as magmatic events in the arunta and musgrave blocks as well as grenville aged orogenesis in the albany fraser belt more localised cooling during the middle neoproterozoic may be a far field response to the separation of australia and laurentia whereas latest neoproterozoic cambrian cooling and exhumation was probably driven by collisional tectonics associated with the petermann ranges and delamerian orogenic events major changes in the stress field of the australian plate that are synchronous with the assembly and break up of rodinia and gondwana resulted in the repeated brittle reactivation of the eastern fold belt probably via the displacement of long lived basement faults

metamorphic rocks seek meaningful cooling rate interpreting 40ar 39ar ages in an exhumed ultra high pressure terrane 40ar 39ar dating of metamorphic white mica is commonly used to elucidate the timing of cooling and or exhumation of metamorphic terranes in theory so long as the dated mineral originally crystallised above its closure temperature the yielded 40ar 39ar ages should not vary significantly within or between samples and should vary only due to grain size 3 ma variation depending on the estimated cooling rate infra red laser single grain fusion and ultra violet laser spot muscovite and or phengite data from mafic eclogite bodies and their host gneisses in the nordfjord region of the western gneiss complex wgc norway show that the ages yielded both within and between white micas from the same mafic eclogite sample vary by up to 284 ma ages yielded within the same pelitic gneiss sample vary by up to 28 ma overall the 40ar 39ar phengite dates from mafic eclogites range from 750 to 440 ma and are significantly older than the previously reported 405 400 ma u pb zircon age constraining peak metamorphism in the nordfjord wgc the felsic host gneiss samples which recrystalised during a post peak exhumation related amphibolite facies overprint yield single grain fusion and laser spot 40ar 39ar white mica dates spanning 420 to 385 ma encompassing previous estimates of the timing of the entire metamorphic cycle these data suggest that ar is heterogeneously distributed both within grains and between grains on the cm to sub mm scale in all samples and imply restricted ar transport and inefficient removal during the metamorphic cycle open system zero grain boundary ar concentration behaviour is an inherent assumption in the commonly applied closure temperature formulation but unless this can be demonstrated linking 40ar 39ar ages to temperature is no longer trivial the high spatial and chronological precision afforded by uv laser ablation mean that intra grain ar concentration distributions may be used to assess open vs closed system behaviour the data presented here suggest that open system behaviour cannot be assumed in metamorphic systems even those in which the pressure temperature and time conditions in theory allow for efficient diffusion during the metamorphic cycle high spatial resolution 40ar 39ar data and robust comparison with numerical models of open system behaviour provide the greatest future potential for unravelling the complexities of metamorphic exhumation timescales

geochemistry of late metamorphic hydrothermal alteration and graphitisation of host rock macraes gold mine otago schist new zealand abstract the macraes gold deposit in the otago schist new zealand formed during late metamorphic fluid flow through a lower greenschist facies shear zone mineralisation occurred near to the brittle ductile transition at about 300 c large volumes of host rock in a shear zone up to 120 m thick have been hydrothermally altered by this fluid activity most alteration is not structurally controlled apart from proximity to the shear zone ductile and brittle microshears traverse the most mineralised rocks and some structural control of fluid flow occurred as well fluid flow was slow similar to that in metamorphic rocks mm year and diffusion through interconnected fluid was a significant chemical process localised extensional hydrofractures m scale are filled with mineralised quartz most alteration of the host rocks was isochemical with respect to the lithophile elements and mineralised rocks have been variably enriched in as au sb w mo and bi but not co or cd addition of sulphur has occurred to both host rocks and mineralised rocks up to 1 wt above a background of 0 1 wt host rock sulphur is mainly pyritic and is not structurally controlled mineralised rocks have pyrite and arsenopyrite along microshears pyrite chalcopyrite sphalerite and galena have formed from sulphidation of silicates with no addition of metals graphite has been added to mineralised rocks along microshears up to 3 wt locally above a background of 0 1 wt noncarbonate carbon graphite deposition may have occurred as a result of mixing of two fluids water methane and water carbon dioxide graphitisation and sulphidation reactions released low d water which accumulated in the slow moving mineralising fluid distinction between this low d reaction water and meteoric water incursion is difficult

the p t t history of blocks in serpentinite matrix melange west central baja california thermochronologic petrologic and geochemical analyses of epidote amphibolite amphilbolite eclogite and blueschist facies blocks in serpentinite matrix melanges from east san benito island and cedros island west central baja california provide constraints on the p t t history of this disrupted terrane results of sup 40 ar sup 39 ar step heating experiments on minerals separated from these blocks vary according to metamorphic grade and indicate different p t t histories mid jurassic 160 170 ma epidote amphibolite and amphibolite facies blocks were probably derived from oceanic crust and associated sediments that were metamorphosed during limitation of subduction coarse grained blueschist blocks were likely metamorphosed during continued subduction in late early cretaceous time that is 115 100 ma some epidote amphibolite blocks are partially overprinted by blueschist facies mineral assemblages and apparently record both metamorphic events fission track analyses of apatites indicate that the blocks underwent significantly different post metamorphic cooling histories epidote amphilbolite and amphibolite blocks cooled below approximately 100 degrees c form mid jurassic to paleocene time blueschist blocks cooled below approximately 100 degrees c in oligocene miocene time in general mafic blocks have trace element concentrations and ree patterns characteristic of ocean flood basalts this study demonstrates that samples with very different p t t histories can evidently occur over relatively small length scales 1 km in serpentinite matrix melanges 67 refs 16 figs 4 tabs

geochronology of blueschists from west central baja california and the timing of uplift in subduction complexes a geochronologic study of blueschists from west central baja california has provided constraints on the timing of high pressure low temperature metamorphism and the subsequent uplift of this subduction complex mineral separates from coherent blueschists and blueschist blocks enclosed in serpentinite melange were analyzed by 40 ar 39 ar fission track and rb sr methods 40 ar 39 ar age spectra for white mica separated from four blueschist blocks and an overprinted eclogite block in serpentinite matrix melange show slow cooling gradients between 115 95 ma rb sr apparent ages for one block are concordant with these results 40 ar 39 ar ages of blue amphibole are primarily the result of the degassing of white mica intergrowths as confirmed by microprobe analyses apatite fission track ages indicate a blueschist block and an eclogite block cooled to a temperature below 100 c at 22 ma and 32 ma respectively subduction related metamorphism of the coherent blueschists occurred in la

determining the extent and nature of mazatzal related overprinting of the penokean orogenic belt in the southern lake superior region north central usa abstract twenty one hornblende and mica 40ar 39ar dates from central and northwest wisconsin usa provide important information on the timing spatial extent and intensity of mazatzal age metamorphism and deformation which overprinted the paleoproterozoic 1870 1820 ma penokean orogenic belt in the southern lake superior region 1760 1750 ma mica plateau ages from bedrock beneath undeformed 1750 1630 ma quartzites are interpreted as the time of lower temperature 300 350 c cooling and crustal stabilization after the penokean orogeny six mica ages from bedrock underlying deformed paleoproterozoic quartzites cluster around 1600 ma 1576 1614 ma the complete absence of 1760 1750 ma mica ages beneath regions of deformed quartzites suggests widespread heating to temperatures above 300 350 c during mazatzal related deformation and metamorphism the 1600 ma mica ages are interpreted to date the cooling phase of this metamorphism two anomalously young biotite dates are interpreted to indicate partial resetting associated with mesoproterozoic rifting at 1100 ma ten hornblende 40ar 39ar dates obtained in this study address the higher temperature overprinting effects in the southern lake superior region one latest archean age of 2503 18 ma and two ages of 1853 and 1830 ma are interpreted as remnant evidence of archean and penokean age amphibolite metamorphic events respectively the majority of hornblende ages are younger than the penokean orogeny scattering between 1796 and 1638 ma microtextural analysis indicates that similar microstructures exist in samples yielding highly discordant hornblende ages this suggests that shearing and recrystallization did not play an important role in the retention or loss of argon the 1638 ma hornblende age is concordant with the mazatzal orogeny to the south and is interpreted as representing complete thermal or fluid related resetting associated with that event six other post penokean ages scatter over a 70 million year interval 1796 1723 ma and probably reflect variable retention of radiogenic argon they are interpreted to indicate variable degrees of partial intermediate temperature 350 500 c resetting of hornblende argon systematics at 1650 1630 ma collectively these data suggest that the effects of the mazatzal orogeny in the southern lake superior region involved 350 500 c metamorphism and penetrative deformation

the approach to steady state thermochronological distribution following orogenic development in the southern alps of new zealand a diachronous sequence of isotopic ages along the southern alps of new zealand illustrates details of the development of the modern tectonic regime of this orogen at about 5 ma coupled with the rapid cooling rates experienced in the southern alps which ensure negligible residence time at temperatures allowing partial radiogenic product retention during exhumation and thus effectively instantaneous thermochronological closure this record presents important general insight into the transient physical and thermal effects of changes in tectonic conditions the tectono thermal response of the southern alps to the change in dynamical conditions at 5 ma is resolved into two evolutionary stages that are observed with progressive exhumation the removal by erosion of material that had cooled below the relevant closure temperature prior to the change in dynamics at 5 ma results in an initial decrease in age this is followed by a sharp drop to younger ages as material subjected to thermal re equilibration associated with the modern orogenic regime is exposed culminating in the eventual exposure of time invariant ages reflecting the new steady state dynamics of the region this two step response is a direct result of the relationship between cooling and exhumation and illustrates the care needed in reconstructing physical histories from isotopic ages and cooling rates in tectonically active regions

mesozoic and cenozoic exhumation history and magmatic hydrothermal events of the central tianshan mt range nw china evidence from u th he and 40ar 39ar dating abstract the thermal history of rocks in the awulale area central chinese tianshan is significant to understanding the formation and tectonothermal evolution of the tianshan orogen k feldspar 40ar 39ar and apatite u th he age data reveal at least four mesozoic cenozoic tectonothermal events in the central tianshan which occurred during the middle late triassic 244 4 216 5 ma early jurassic ca 180 3 ma early cretaceous 99 1 98 1 ma and early miocene 18 6 13 7 ma this thermal history is consistent with that throughout the tianshan orogenic belt and surrounding blocks modeling of the thermal history shows that intrusive rocks in the central tianshan have experienced four stages of cooling in the mesozoic cenozoic stage i occurred from ca 310 to 230 ma after magma emplacement and resulted in cooling to the 40ar 39ar closure temperature of k feldspar 200 180 c during this time tectonism in the central tianshan was not intense and the rapid cooling was due mainly to thermal exchange with a maximum cooling rate of 7 1 c myr stage ii occurred from ca 230 to 180 ma collision between the qiangtang microcontinent and the eurasia continental margin resulted in active tectonism in the central tianshan and cooling was caused mainly by tectonic uplift with the maximum uplift of intrusive rocks exceeding 9 km stage iii occurred from ca 180 to 45 ma when regional tectonism was weak and cooling was gradual during this stage the apatites in the syenite porphyry remained in the he partial retention zone for a long time this indicates that during this stage the central tianshan was only weakly influenced by the far field collisions of lassa and karakoram pamir with the southern margin of eurasia stage iv occurred from 45 ma to the present day and was related to the far field influence of the middle late himalayan orogeny during which tectonism in the central tianshan became more intense in this final stage many types of intrusive rock were exhumed to the surface with the maximum uplift exceeding 2 km furthermore the cooling history and regional structural trends indicate that since the mesozoic under the influence of tectonic compression there has been differential uplift on the northern and southern sides of the east awulale area the cooling and exhumation rates of intrusive rocks are much higher on the northern side of the area than the southern side this was related to reverse movement on the yuximolegai fault as exhumation rate of the hanging wall northern region was higher than that of the footwall southern region under n s compression these findings combined with the inversion model results indicate that this differential uplift occurred mainly from the late triassic to early jurassic

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

excess argon crustal fluids and apparent isochrons from crushing k feldspar using a combination ofin vacuo crushing and stepped heating the40ar39ar technique has been applied to two k feldspar bearing sedimentary rocks from the proterozoic of north china with a view to studying ancient fluids trapped in these rocks and their chronology correlations between natural ar isotopes and those produced by neutron irradiation permitted a clear distinction to be made between different components of natural argon crushing released cl correlated excess40ar and palaeoatmospheric36ar trapped in moderately saline fluid inclusions in quartz and possibly the k feldspar the very high40ar cl ratios of these fluids 2 7 10 4 and1 7 10 4 is interpreted as indicating a metamorphic source for the40ar more surprisingly crushing also released from the k feldspar a k correlated component with a very well defined40ar k ratio which for both samples corresponded formally to an apparent age comparable to or slightly less than the stratigraphic age in contrast stepped heating yielded40ar k ratios corresponding to significantly younger ages which are interpreted as low grade metamorphic ages the explanation of the k correlated component released on crushing and the significance if any of the distinct40ar k ratio is obscure although several effects involving microporosity and a combination of39ar recoil and diffusion are suggested there is a contribution of this component to the stepped heating release pattern which may have implications for attempts to use k feldspar as a40ar39ar thermochronometer

rb sr and 40ar 39ar ages in blueschists from the ile de groix armorican massif france implications for closure mechanisms in isotopic systems rb sr and 40ar 39ar data mainly ranging from 345 5 to 361 5 ma and from 348 4 1 4 to 364 6 1 4 ma respectively have been obtained on phengite from the hp lt rocks from the ile de groix armorican massif france which represents one of the few occurrences of variscan blueschists petrological data show two blueschist facies units recording distinct peak p t conditions about 16 20 kbar 500 c and 12 16 kbar 450 c in the upper and lower units respectively followed by a nearly isothermal decompression in the greenschist facies rb sr and 40ar 39ar results defining two groups of ages mainly around 345 353 ma and 358 366 ma are consistent in well preserved blueschist facies rocks as well as in strongly overprinted samples from both units one sample excepted there is no evidence of inherited or excess argon and resetting of the isotopic systems oldest rb sr and 40ar 39ar ages are found in rocks with well preserved blueschist facies parageneses while younger ages are obtained on samples with a strong greenschist facies overprint this suggests that the closure of the k ar system is diachronous in samples which record the same p t evolution a closure of the k ar clock at around 450 500 c in phengite from rocks left nearly undeformed after the blueschist facies metamorphism is in agreement with petrological geochemical and structural data thus in these samples the oldest rb sr and 40ar 39ar ages around 360 370 ma represent the age of the hp metamorphism in the overprinted samples later closure of the k ar and rb sr clocks is the result of retrogression and continuing ductile deformation the small age difference between the oldest and the youngest rb sr and 40ar 39ar ages around 10 20 ma argues in favour of a relatively fast cooling

40ar 39ar analyses of artificially mixed micas and the treatment of complex age spectra from samples with multiple mica populations abstract artificial mixtures of two age populations of muscovite and of biotite were prepared at relative weight percents of 3 1 1 1 and 1 3 and analyzed by the 40ar 39ar vacuum furnace step heating method the original micas are late jurassic and late cretaceous samples that yield flat age spectra with plateau ages defined by 97 100 of the gas release and no evidence for excess argon the age spectra from mica mixtures yield patterns that systematically decrease with decreasing jurassic mica content and increasing cretaceous component however the mixed muscovite spectra are relatively flat consistent with the two original samples showing similar degassing patterns during vacuum furnace heating in contrast the mixed biotite spectra are highly discordant with an overall decrease in age during the step heat analysis this is consistent with the degassing patterns of the original mica samples which indicate the jurassic biotite high fe mg outgases at lower temperatures and the cretaceous biotite low fe mg remains retentive until the higher temperature steps the individual degassing patterns and compositions show consistencies with known compositional controls on argon diffusivity indicating that micas may maintain argon retention characteristics rooted in crystal chemistry during vacuum furnace heating all but one of the mixed samples failed to yield ages reflecting the age of the original micas these results imply that complex age spectra obtained for multiply deformed rocks with multiple mica populations may define geologically meaningless ages as a result of simultaneous degassing i e mixing of sample reservoirs during heating comparison of laboratory degassing rates and crystal chemistry indicates compositional controls on argon retention are likely preserved during vacuum furnace step heating and micas may degas retain argon in a predictable manner these conclusions support assertions that recovery of fossil age gradients in slowly cooled samples may be possible using furnace step heating

on the thermomechanical evolution of compressional orogens summary we present results from a newly developed fully coupled thermomechanical model of the continental crust in which crustal shortening at a convergent plate boundary is driven by a basal velocity discontinuity which represents delamination and subduction in the underlying mantle this new dynamical model incorporates complex rheologies elasticity thermally activated creep and brittle frictional behaviour allows for extremely large deformation and is coupled along its top boundary to a complex erosion sedimentation model the model is based on the dynamic lagrangian remeshing dlr method which uses lagrangian spatial discretization of the crust and therefore allows for an accurate tracking of rock particles as they travel through the deforming orogen this information is in turn used to produce synthetic ptt paths we have used this model to predict the distribution of apparent ages for a wide range of isotopic systems at the surface of an actively deforming orogen we have also predicted other geophysical and geological observables such as the metamorphic grade of exposed rocks regarded here as a first order approximation for total denudation topography surface heat flux and the thickness of sediment deposited in the adjacent foreland basins results clearly demonstrate that the highest exhumation rates and thus the youngest isotopic ages are found in regions of maximum topography near the centre of the orogen but that the most deeply exhumed rocks are found on the side of the orogen in the vicinity of the retro shear zone a crustal scale fault which accommodates most of the crustal shortening within the orogen it is also in this region that the isotopic systems characterized by the greatest closure temperatures display the youngest ages and the highest grade metamorphic rocks are found these conclusions are derived from the assumption of uniform erosion across the orogen in cases where rainfall and thus erosion is orographically controlled the tectonic style of the orogen is different from the uniform erosion case as is the distribution of isotopic ages and metamorphic grades across the orogen

40ar 39ar impact ages and time temperature argon diffusion history of the bunburra rockhole anomalous basaltic achondrite abstract the bunburra rockhole meteorite is a brecciated anomalous basaltic achondrite containing coarse medium and fine grained lithologies petrographic observations constrain the limited shock pressure to between ca 10 gpa and 20 gpa in this study we carried out nine 40 ar 39 ar step heating experiments on distinct single grain fragments extracted from the coarse and fine lithologies we obtained six plateau ages and three mini plateau ages these ages fall into two internally concordant populations with mean ages of 3640 21 ma n 7 p 0 53 and 3544 26 ma n 2 p 0 54 respectively based on these results additional 40 ar 39 ar data of fusion crust fragments argon diffusion modelling and petrographic observations we conclude that the principal components of the bunburra rockhole basaltic achondrite are from a melt rock formed at 3 64 ga by a medium to large impact event the data imply that this impact generated high enough energy to completely melt the basaltic target rock and reset the ar systematics but only partially reset the pb pb age we also conclude that a complete 40 ar resetting of pyroxene and plagioclase at this time could not have been achieved at solid state conditions comparison with a terrestrial analog lonar crater shows that the time temperature conditions required to melt basaltic target rocks upon impact are relatively easy to achieve ar data also suggest that a second medium size impact event occurred on a neighbouring part of the same target rock at 3 54 ga concordant low temperature step ages of the nine aliquots suggest that at 3 42 ga a third smaller impact excavated parts of the 3 64 ga and 3 54 ga melt rocks and brought the fragments together the lack of significant impact activity after 3 5 ga as recorded by the bunburra rockhole suggests that 1 either the meteorite was ejected in a small secondary parent body where it resided untouched by large impacts or 2 it was covered by a porous heat absorbing regolith blanket which when combined with the diminishing frequency of large impacts in the solar system protected bunburra from subsequent major heating events finally we note that the total k ar resetting impact event history recorded by some of the brecciated eucrites peak at 3 8 3 5 ga is similar to the large impact history recorded by the bunburra rockhole parent body ca 3 64 3 54 ga this study and could indicate a similar position in the asteroid belt at that time

core complex related extension of the aegean lithosphere initiated at the eocene oligocene transition 1 core complex related stretching of the aegean lithosphere was accommodated in part by the south cyclades shear zone scsz which outcrops on the islands of ios and thera aegean sea greece this paper reports new microstructurally focused 40ar 39ar geochronology that suggests the scsz initiated just before the time of the eocene oligocene transition at 35 ma therefore it is not only miocene extension that led to the tethyan metamorphic core complexes exposed in the cycladic archipelago it is shown here that pervasive top to the south directed ductile shear took place in the scsz for 5 ma during the early oligocene followed by initiation of overprinting north sense shear zones at the time of the oligocene miocene transition at 25 ma this means that modern core complex related extension in the aegean started much earlier than has been previously recognized and that stretching of the aegean lithosphere took place in a distinctly episodic fashion not in a manner that can be described as continuous or progressive the data also mean that the complementary shear sense geometry of the scsz and the north cyclades shear zone ncsz cannot be interpreted as crustal scale boudinage initiated during crustal extension in miocene time since the opposing sense shear zones operated at distinctly different times the paper illustrates the difficulties involved in systematically determining the age of deformation fabrics using 40ar 39ar geochronology and provides a case study that shows the value of carefully designed step heating experiments on microstructurally controlled samples in samples such as these from complex metamorphic tectonites laser fusion or step heating experiments with relatively few steps would have led to erroneous conclusions

chronology and tectonic framework of turbidite hosted gold deposits in the western lachlan fold belt victoria results abstract 40 ar 39 ar data constrain the history of metamorphism deformation and mineralization in the western subprovince of the lachlan fold belt rocks in the mount stavely volcanic complex and the bushy creek pluton give 40 ar 39 ar ages of about 500 ma for hornblende and biotite the dates from the mount stavely volcanic complex limit the age of mineralization in the metavolcanic rocks of the stavely belt whereas the date for the bushy creek pluton extends the area of cambrian ordovician plutons of the delamerian orogeny to east of the grampians mountains and helps define the surface expression of the western border of the paleozoic lachlan fold belt 40 ar 39 ar dating of metamorphic mica growth phengite in cleaved slates and phyllites within the landsborough fault avoca fault zone heathcote fault zone and mount wellington fault zone gives ages of 453 2 440 2 426 4 and 410 390 ma respectively these data indicate a progression of deformation from west to east starting in late ordovician early silurian time in the stawell and bendigo ballarat structural zones the major phase of deformation in the western subprovince of the lachlan fold belt is therefore silurian and not devonian sericites from major gold deposits in the stawell zone and bendigo ballarat zone give 40 ar 39 ar dates of about 440 ma that are coincident with folding and thrusting a second episode of mineralization and fault reactivation at about 420 to 410 ma is found in some deposits in these zones based on data from a previous study in the melbourne zone including the woods point dike swarm and locally in the bendigo ballarat zone some gold deposits give sericite dates of about 380 360 ma these ages are consistent with the close association of many melbourne zone gold deposits with devonian dikes and plutons the results indicate that gold mineralization in the stawell zone bendigo ballarat zone and melbourne zone was episodic and both temporally and spatially associated with crustal heating linked to regional metamorphism and plutonism initial stages of mineralization in the province comprising many of the large deposits in western and central victoria e g bendigo ballarat stawell wattle gully tarnagulla accompanied metamorphism folding and thrusting subsequent mineralization is widespread but produced mainly smaller deposits

thermochronology of the torlesse accretionary complex wellington region new zealand the torlesse complex comprises several mesozoic accretionary prism complexes together forming continental basement over large parts of new zealand this study focuses on the thermal history of relatively low grade graywacke rocks exposed in a transect in southern north island that crosses the structural grain of the torlesse complex including its older and younger parts zircon fission track ft ages for the late triassic rakaia terrane which is the most inboard of the accretionary complexes are partially annealed some possibly reset and may indicate early cretaceous 134 10 ma cooling from maximum temperatures tmax probably related to imbrication of younger complexes of the pahau terrane numerical modeling of the zircon ft ages and published 40ara 39ar muscovite and biotite ages for the rakaia terrane suggest tmax values of 265 310 c and exhumation from depths of 10 12 km the rocks underlying the aorangi range and involving the youngest accretionary complex have experienced much lower tmax values of 210 and 110 c bracketed by reset apatite ft ages and detrital zircon ft ages the occurrence of a circa 100 ma component of zircon ft ages in both the weakly and highly indurated rocks beneath the aorangi range as well as in remnants of an overlying albian accretionary slope basin whatarangi formation imply multistorey accretion and incorporation of sediment into the youngest prism this circa 100 ma zircon ft age component also places a maximum age on the termination of mesozoic subduction beneath the new zealand region the occurrence of reset apatite ft ages across the whole of the wellington transect indicates that at least 4 km of exhumation occurred during the late miocene

40ar 39ar thermochronology of the kampa dome southern tibet implications for tectonic evolution of the north himalayan gneiss domes structural and thermochronological studies of the kampa dome provide constraints on timing and mechanisms of gneiss dome formation in southern tibet the core of kampa dome contains the kampa granite a cambrian orthogneiss that was deformed under high temperature sub solidus conditions during himalayan orogenesis the kampa granite is intruded by syn tectonic leucogranite dikes and sills of probable oligocene to miocene age overlying paleozoic to mesozoic metasedimentary rocks decrease in peak metamorphic grade from kyanite staurolite grade at the base of the sequence to unmetamorphosed at the top the kampa shear zone traverses the kampa granite metasediment contact and contains evidence for high temperature to lowtemperature ductile deformation and brittle faulting the shear zone is interpreted to represent an exhumed portion of the south tibetan detachment system biotite and muscovite 40 ar 39 ar thermochronology from the metasedimentary sequence yields disturbed spectra with 14 22 0 18 to 15 54 0 39 ma cooling ages and concordant spectra with 14 64 0 15 to 14 68 0 07 ma cooling ages petrographic investigations suggest disturbed samples are associated with excess argon intracrystalline deformation mineral and fluid inclusions and or chloritization that led to variations in argon systematics we conclude that the entire metasedimentary sequence cooled rapidly through mica closure temperatures at 14 6 ma the kampa granite yields the youngest biotite 40 ar 39 ar ages of 13 7 ma immediately below the granite metasediment contact we suggest that this age variation reflects either varying mica closure temperatures re heating of the kampa granite biotites above closure temperatures between 14 6 ma and 13 7 ma or juxtaposition of rocks with different thermal histories our data do not corroborate the inverse mica cooling gradient observed in adjacent north himalayan gneiss domes instead we infer that mica cooling occurred in response to exhumation and conduction related to top to north normal faulting in the overlying sequence top to south thrusting at depth and coeval surface denudation

using white mica 40ar 39ar data as a tracer for fluid flow and permeability under high p conditions tauern window eastern alps new single grain fusion muscovite and paragonite 40ar 39ar data from eclogite and blueschist units exposed in the tauern window eastern alps yield a range of apparent ages from 90 to 23 ma these apparent ages are generally older than expected for 40ar 39ar cooling ages given constraints from other geochronological systems such as rb sr and u pb numerical ar in muscovite diffusion models for tauern window nappe p t paths in an open system suggest that 40ar 39ar ages should lie between 29 and 24 ma and that they should constrain cooling and decompression following the post high pressure barrovian overprint the measured ranges of apparent 40ar 39ar dates suggest that the assumption of open system behaviour is not valid for this region the local and or regional generation of fluid during exhumation promoted pervasive recrystallization of high pressure lithologies throughout the tauern window to greenschist and amphibolite facies assemblages the old apparent 40ar 39ar white mica dates in all lithologies are therefore interpreted as being due to inefficient removal of grain boundary ar by the grain boundary fluids during the barrovian overprint due to high ar concentrations or limited connectivity or both this caused spatially mm scale and temporally variable fluxes of ar out of and probably into white mica in both metasedimentary and metabasic lithologies

fundamentals of noble gas thermochronometry the ideal geochronometer would be a universally stable phase that quantitatively retains both parent and daughter isotopes though a few mineral systems such as zircon u pb dating come reasonably close to this ideal most minerals are incompletely retentive of daughter product nuclides under crustal conditions the mechanisms by which the daughter product can be lost from minerals include dissolution reprecipitation reactions e g salt obradovich et al 1982 recrystallization e g micas undergoing deformation chopin and maluski 1980 and diffusive loss e g 40ar degassing of k feldspar foland 1974 the latter mechanism is perhaps the most common source of discrepancy between a radiometric mineral date and the age of the rock from which it formed geochronologists have learned to turn this non ideal behavior to their advantage and we now understand that most mineral ages from exhumed crustal rocks act in effect as kinetic thermometers sensitive to geologically induced thermal effects such apparent ages are a measure of the temperature range over which daughter product ceased to be lost from a crystal with intracrystalline diffusion usually acting as the rate limiting process consider the case in which a mineral sample containing a radioactive parent element experiences a complex thermal evolution possibly involving heating as well as cooling within the sample daughter product is continually produced by radioactive decay and lost by diffusion at natural boundaries although random at the scale of an individual atom both diffusion and radioactive decay are highly predictable processes over longer and larger scales involving many particles coupled with the strong temperature dependence of diffusion the elegant mathematics of the production diffusion relationship make it possible to recover information about the thermal history experienced by such a sample simply by knowing the amount of daughter product remaining in the mineral following cooling or even better by knowing the distribution of daughter product within the

laboratory experiments on crust development and rainsplash erosion of loess soils china abstract loess soil samples obtained from wangjiagou lishi china were subjected to laboratory simulated rainfall at intensities of 72 mm h 1 for a range of durations from 1 to 30 minutes measured splash loss was found to relate to a soil strength index 1 p 20 2 in this index p 20 is defined as penetration by a standard fall cone penetrometer at a moisture content of 20 and estimated from penetration soil moisture regression equations similarly subsequent splash loss of crusted samples over a 10 minute period is significantly related to the antecedent soil strength index as a result of crust development strength of the surface soil is increased by as much as 11 times while splash loss is reduced by a maximum of 76 data on splash loss and crust stength were used to infer that crust and seal development while tending towards increasing stability as has been suggested previously appear to follow a cyclic process of crust formation and disruption this inference is supported by micromorphological observations

continental rifting and metamorphic core complex formation ahead of the woodlark spreading ridge d entrecasteaux islands papua new guinea 1 we evaluate the role of a metamorphic core complex mcc on normanby island in the woodlark rift located 1 km thickness of blueschist derived mylonites formed in a midcrustal shear zone during the pliocene at 400 500 c this top to the north zone appears to have reactivated the gently dipping base of the papuan ophiolite papuan ultramafic body pub and its continued activity appears to control the north dipping asymmetry of active half grabens to the north of the mcc and rapid subsidence of the woodlark rise mylonites in the mcc s lower plate have been exhumed along a detachment as a result of 50 km of slip at rates of 12 mm yr the inactive back tilted detachment preserves fault surface megamullions and mylonitic lineations parallel to the plio pleistocene plate motion a second se vergent detachment has been established on the opposite flank of this rolling hinge style mcc probably since 0 8 at depth and provide a sufficient mechanism for activating low angle normal faults in the rift mcc inception was not localized to the tip of the woodlark mor instead extreme crustal thinning near the mcc preconditioned later continental breakup the lower crust appears to be weak thickening beneath unloaded footwalls to uplift mccs above sea level and flowing laterally to even out regional crustal thickness contrasts on a 1 6 m y timescale deep seated transforms separate rheologically distinct domains in which extension has been localized along the weak pub to cause mcc formation vs those in which slip is distributed across an imbricate zone of more uniform strength normal faults the trobriand fault connects in the eastern woodlark rift to the owen stanley fault in the papuan ranges which is probably moving at nearly the full plate velocity

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

new insight into the dynamic development of the southern alps new zealand from detailed thermochronological investigation of the mataketake range pegmatites

transition from subduction to exhumation related fabrics in glaucophane bearing eclogites oman evidence from relative fabric chronology and 40ar 39ar ages abstract controversy over the age of peak metamorphism and therefore the tectonic evolution of the arabian margin relates to the polydeformed and polymetamorphosed nature of glaucophane bearing eclogites from the saih hatat window beneath the allochthonous samail ophiolite in ne oman these eclogites contain relicts of earlier fabrics structures and metamorphic assemblages and provide a record of change from subduction to exhumation the eclogites are part of a mafic layer that was disrupted into boudins up to 0 5 km in length within a lower plate shear zone as sifah shear zone the megaboudins not only preserve the relicts of the highest grade of metamorphism but also an early ene trending lineation and sheathlike isoclines enveloped by the flat lying schistosity the boudin bearing layer is isoclinally folded with calc schist mafic schist and quartz mica schist where the regional folds have axes parallel to the ne trending stretching lineation a type folds textural evidence suggests multiple growth events for garnet and clinopyroxene requiring polymetamorphism of the mafic layers that formed the eclogite megaboudins the surrounding calc schist and quartz mica schist are both intensely deformed with transposition foliation containing an ne trending lineation in phengite and asymmetric shear indicators such as c type shear bands and asymmetric pressure shadows around garnets that give top to the ne sense of shear although consistent ene trending lineations in all the boudins suggest that they have largely acted as passive nonrotating rigid bodies the presence of ne vergent asymmetric mesofolds extensive dynamic recrystallisation multiple generations of phengites and a range of 40ar 39ar apparent ages within the megaboudins suggest however that they have not acted entirely passively during the later deformation phengites isolated from the high p low t fabrics show groupings in 40ar 39ar apparent ages interpreted as distinct metamorphic cooling intervals at 140 135 120 98 and 92 80 ma microstructural relations suggest that age groupings younger than 100 ma reflect phengite growth during exhumation with the top to the ne shearing the older ages 120 110 ma from fabrics that give top to the s shear sense may reflect growth during the subduction phase the combination of groupings of apparent argon ages older than the crystallisation age of the samail ophiolite the suggestion of different geothermal gradients and superposed metamorphism suggest that the eclogites and garnet blueschists formed as a result of underthrusting along a break that was not directly related to the metamorphic sole of the ophiolite the glaucophane eclogites are interpreted as having formed at different times under varying pressure temperature conditions during underthrusting with variations in the rate of underthrusting allowing thermal equilibration and or rapid cooling at different crustal levels

when can muscovite 40ar 39ar dating constrain the timing of metamorphic exhumation abstract metamorphic cooling rates determined using muscovite 40 ar 39 ar ages underpin many tectonic models but are based on several simplifying approximations we demonstrate that a number of these approximations especially those relating to the assignment of a single closure temperature might not hold for certain metamorphic scenarios and particularly for metamorphism during rapid orogenic cycles using numerical diffusion models that include a recently reported significant pressure dependence of ar diffusion in muscovite we systematically interrogate the approximations associated with linking 40 ar 39 ar dates to pressure temperature histories the results of the simulations are presented in a simple graphic form allowing evaluation of the pressure temperature pt regions in which ar diffusion is efficient and hence those in which the muscovite may yield true 40 ar 39 ar cooling ages we suggest that a robust method for determining whether metamorphic muscovite 40 ar 39 ar ages relate to the timing of cooling involves 1 the determination of the pt conditions and relative timing of muscovite crystallisation during the metamorphic cycle 2 the collection of high precision and high spatial resolution 40 ar 39 ar data from muscovite grains and 3 the comparison between analytical data and numerical diffusion models which test different post crystallisation pressure temperature histories

locking of southward extrusion in favour of rapid crustal scale buckling of the greater himalayan sequence nar valley central nepal the south tibetan detachment system stds bounds the upper limit of the greater himalayan sequence ghs which consists of the exhumed middle crust of the himalaya in the annapurna range of central nepal the ghs comprises a sequence of amphibolite grade augen gneisses with a 3 5 km thick leucogranite at the higher structural levels manaslu granite two major low angle normal sense shear zones have been mapped the chame detachment has similar grade metamorphic rocks above and below and is interpreted as a ductile shear zone wholly within the ghs the phu detachment is a ductile brittle normal fault which wraps around the top of the manaslu leucogranite and defines the uppermost youngest strand of the stds placing folded unmetamorphosed palaeozoic rocks of the tethyan sedimentary sequence above the ghs our data indicate that ductile flow and southward extrusion of the ghs terminated with cessation of movement on the brittle upper strand of the phu detachment at c 19 ma which was followed almost immediately by crustal scale buckling argon thermochronology reveals that the bulk of the metamorphic rocks and lower portions of the tethyan sedimentary sequence in the nar valley cooled through the hornblende biotite and muscovite closure temperatures at c 16 ma suggesting very rapid cooling rates the thermochronology results indicate that this cooling occurred 2 3 million years earlier than in the frontal part of the extruded ghs although the extrusion in the frontal part of the ghs must have locked at the same time as in the nar valley the exhumation there was slower and most probably only assisted by erosion rather than by rapid folding as is the case in the nar valley this buckling indicates a step northward in deformation within the himalayan belt which may be a response to a lower deforming taper geometry in the foreland

petrogenesis of jurassic a type granites associated with cu mo and w sn deposits in the central nanling region south china relation to mantle upwelling and intra continental extension abstract the central nanling region in south china was the site of uplift extensional deformation granite magmatism and hydrothermal activity from 180 to 150 ma information on the age petrological characteristics major and trace element contents whole rock sr and nd and zircon hf isotopic compositions of granitic plutons were synthesized to constrain the sources evolutionary processes and uplift cooling rates of intrusions that host cu mo and w sn deposits in this region the older 180 160 ma cu mo related granitoids are less differentiated a1 type alkaline rocks with faster cooling and uplift rates they have younger nd and hf two stage model ages and higher end t and ehf t values that indicate mantle derived magmas were moderately contaminated by upper crust the younger 160 150 ma w sn related granites are more differentiated a2 type aluminous rocks with slower cooling and uplift rates they have older nd and hf two stage model ages and lower end t and ehf t values that require more upper crustal contamination the time space distribution and characteristics of these granitoids and ore deposits are indicative of centralized mantle upwelling and gradational crustal extension the older rapidly uplifted less contaminated granitoids with cu mo mineralization occur near the center of an intra continental extension bulge produced by mantle upwelling whereas the younger slowly uplifted more contaminated granitoids with w sn mineralization occur on the periphery the results suggest that metals in the cu mo deposits may have come from the mantle whereas those in the world class nanling w sn polymetallic deposits were derived from early mesoproterozoic 1 4 1 6 ga crustal basement of the cathaysia block in south china

the cambrian initiation of intra oceanic subduction in the southern paleo asian ocean further evidence from the barleik subduction related metamorphic complex in the west junggar region nw china abstract in this study we present new evidence from the barleik subduction related metamorphic complex in the southern west junggar region northwestern china for the cambrian initiation of intra oceanic subduction in the southern paleo asian ocean the barleik metamorphic complex is mainly composed of blueschist and amphibolite blocks within an ophiolitic melange and their protoliths are calc alkaline andesite and alkali and tholeiitic basalts the calc alkaline andesite has a zircon u pb age of 502 2 ma obtained from magmatic cores of zircon grains and shares geochemical features similar to the 515 485 ma intra oceanic arc magmatic rocks in the west junggar region by contrast the alkali and tholeiitic basalts have trace element features similar to ocean island and enriched mid ocean ridge basalts respectively rutile and sodic calcic amphibole from the amphibolite have a u pb age of 502 25 ma and a 40 ar 39 ar age of 504 ma respectively which are in good agreement within errors with a 40 ar 39 ar age of 492 4 ma for phengite from the blueschist these metamorphic ages of 500 ma are interpreted to represent the timing of pacific type subduction related metamorphism and are also compatible with ages of the oldest supra subduction zone ophiolites 531 512 ma and intra oceanic arc plutons 515 485 ma in the southern west junggar region being one of the oldest subduction related metamorphic complexes 509 490 ma in the southern central asian orogenic belt the barleik metamorphic complex together with the oldest arc plutons definitely indicate the initial intra oceanic subduction in the southern paleo asian ocean at least in the early cambrian

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

thermal history of the eastern gulf of suez ii reconstruction from apatite fission track and k feldspar measurements abstract apatite fission track aft measurements from miocene graben fill sediments of the eastern gulf of suez in three deep boreholes belayim 113 m 2 6 ras garra m 1 and alma 2 yield a wide range of ages from 125 to 320 ma and mean track lengths 10 1 12 7 m this range is similar to that recorded from precambrian crystalline basement flanking the graben which is the major source for the rift fill since the aft ages exceed the age of the host strata which are presently at their highest post depositional temperatures the rift related thermal regime is one of only moderate heating downhole aft data in the belayim 113 m 2 6 borehole attest to a higher rift temperature than in the ras garra m 1 borehole some 85 km to the south where little or no thermal overprinting is evident these findings are consistent with previously reported bottom hole temperatures and vitrinite reflectance data in the study area despite the higher syn rift thermal regime indicated for belayim 113 m 2 6 apatite provenance ages in ras garra m 1 are considerably younger thus the aft data in the ras garra m 1 area do not record significant rift related thermal effects but rather they mainly retain a pre rift provenance signature which reflects the order and depth of erosion at the uplifted flanks the younger aft ages in ras garra m 1 despite the weaker rift related thermal effect suggest a deeper level proportional to an additional 5 10 c of exhumation of the uplifted crystalline basement southwards along the eastern rift flank by mid miocene time this result is consistent with earlier findings which indicate both increased extension and heat flow southwards in the gulf of suez and earlier exhumation of the rift flank immediately preceding extension and opening of the gulf of suez the most deeply exhumed basement rocks presently exposed on the rift flanks were heated to temperatures 110 c total annealing of apatite but 40 ar 39 ar data and non resetting of zircon ft clocks in sinai 40 ar 39 ar data from granite penetrated in graben drilling at 3 89 km further corroborates pre rift palaeotemperatures

disturbed 40ar39ar systematics in hydrothermal biotite and hornblende at the scotia gold mine western australia evidence for argon loss associated with post mineralisation fluid movement hornblende and biotite that formed during gold mineralisation at the scotia mine western australia have erratic 4oar 3gar release spectra and total gas ages that are 200 900 million year younger than the ca 2600 2620 ma minimum age of gold mineralisation as given by 4oar 3gar plateau muscovite ages of crosscutting pegmatite dykes analysed homblendes are dominated by magnesio hornblende but also contain small domains of ferro actinolitic hornblende actinolitic hornblende and actinolite biotite also appears to be substantially altered to chlorite along cleavage planes relatively young apparent ages and high k ca ratios of argon released from homblendes at tempera tures less than 1000 c are interpreted to be the result of degassing of contaminant biotite however this cannot totally explain the young ages of homblendes gas fractions released at furnace temperatures above looo c where the effect of biotite degassing is demonstrably negligible still have apparent ages that are 200 900 million years younger than the age of muscovite from post gold pegmatite dykes the close proximity of disturbed hydrothermal hornblende samples to apparently undisturbed pegmatite muscovite samples less than a few metres in some cases is difficult to reconcile with argon loss in hydrothermal hornblende being the product of thermally driven volume diffusion given a suitable thermal history argon loss could occur preferentially in homblendes if 1 the closure for slow cooling and blocking for reheating temperatures of hydrothermal homblendes were lower than published estimates as has been observed in structurally complex metamorphic homblendes and or 2 the closure and blocking temperature of pegmatite muscovite were higher than commonly estimated however neither of these interpretations can easily explain the large variation in hornblende ages it is instead suggested that argon loss occurred during mineral fluid interaction during movement of a retrograde fluid along the mineralised lode structures and that this occurred at ambient temperatures below the blocking temperature of pegmatite muscovite there is abundant geological evidence for the passage of such a fluid at the scotia mine including the presence of numerous late brittle fractures containing retrogressive low temperature mineral assemblages late fluid movement is probably related to proterozoic erogenic activity along the nearby southeastern margin of the yilgarn craton the difference in argon systematics between hydrothermal minerals and pegmatite muscovite is largely ascribed to the relatively low permeability of the more massive pegmatite dykes with respect to ore zones preventing fluid egress to muscovite samples the variations in ages of hydrothermal minerals are probably related to the extent of fluid mineral interaction as this is a function of parameters such as fluid rock ratio fluid p t x conditions permeability and mineral microstructures that may vary on short time and length scales recognition of possible argon loss in hornblende via fluid interaction is important for the interpretation of 4oar 3gar systematics in environments such as many hydrothermal ore deposits where minerals may be exposed to fluids after crystallisation copyright 0 1997 elsevier science ltd

tectonic and metamorphic architecture of the hp belt of new caledonia abstract new caledonia includes a well exposed example of high pressure orogenic belt formed as a result of cenozoic plate tectonic reorganizations at the australia pacific plate boundary the metamorphic belt is relatively well accessible and has deserved extensive attention mainly for its metamorphic and petrological features the architecture of this belt however still lacks general consensus the aim of this paper is i to review the nature and origin of the main tectonic units of the belts and ii to revisit the available structural models describing its architecture and geodynamic evolution for that we integrate the available field petrological geochemical and geochronological data together with new results from large scale field survey and focused ar ar geochronological data all together these data allow proposing a new model for the tectonic and metamorphic architecture of the belt also comprising a newly established tectonostratigraphic unit this architecture resembles that characterizing alpine type metamorphic belts formed in non pacific settings and clearly differs from cordilleran type belts found in eastern pacific regions moreover the collected data also allow refining our understanding of the oceanic and transitional oceanic lithosphere involved in the eocene subduction and plausible analogies with hyper extended margins known in present day ocean continent transition zones

timing of exhumation of the ereendavaa metamorphic core complex north eastern mongolia u pb and 40ar 39ar constraints abstract the occurrence of numerous ne sw trending rift basins that locally bound metamorphic core complexes in transbaikalia northern mongolia and north eastern china indicates that the eastern domain of the central asian orogenic belt was affected by a large scale nw se extensional tectonic event during late jurassic to early cretaceous times available geochronological information about the formation of metamorphic core complexes in the region define partly synchronous periods in transbaikalia 134 121 ma and north china 136 110 ma suggesting dome exhumation during the early cretaceous in north eastern mongolia the ereendavaa range lined by the onon shear zone was recently interpreted as one of these metamorphic core complexes of late jurassic to early cretaceous age on the basis of u pb zircons data from two synkinematic pegmatite dykes however this wide time range remains to be discussed the present work constrains the tectono thermal evolution of the range by providing new mc icp ms u pb zircons ages from pegmatite dykes and metamorphic igneous rocks and mica and amphibole 40 ar 39 ar laser step heating ages from metamorphic and magmatic rocks that crop out within the range the results show that 1 the ereendavaa range was the locus of a widespread magmatic activity prior to its exhumation from late jurassic times 2 cooling of the range began at least at 137 9 0 3 ma as revealed by a 40 ar 39 ar plateau ages obtained from an hornblende sampled within the range core and 3 exhumation along the onon shear zone occurred from 130 3 1 9 ma down to 124 8 0 3 ma as shown by ages of most of micas and amphiboles from the shear zone these more accurate ages are consistent with metamorphic core complex ages from adjacent areas furthermore they argue for an extensional dynamic paroxysm at the scale of eastern asia during a narrow time range in the early cretaceous between ca 130 ma and ca 120 ma

jarosite argon diffusion and dating aqueous mineralization on earth and mars abstract jarosite 40 ar 39 ar ages can be used to date surface processes such as weathering and environmental transitions i e aridification on earth and mars to better interpret jarosite ages from a thermochronological perspective the diffusion kinetics of argon in jarosite were determined incremental fractional loss measurements indicate an activation energy e of 37 8 1 5 kcal mol and a log d o a 2 of 5 68 0 63 s 1 corresponding to a closure temperature of 143 28 c assuming a cooling rate 100 c ma downward extrapolation of these parameters to martian surface temperatures 22 c predicts 40 ar 39 ar age spectra using the least retentive e d o a 2 pairs predict that if held at 22 c or less for 4 0 ga supergene jarosite would preserve original growth ages manifest as plateau ages consisting of 95 of the gas release because of its susceptibility to mineralogical breakdown 40 ar 39 ar ages on preserved martian jarosite will reflect the time since water was present at a location that has since undergone aridification and remained hydrologically inactive and thermally quiescent

a field based estimate of the zircon fission track closure temperature abstract a field based estimate of the zircon fission track closure temperature is proposed from 13 zircon fission track cooling ages of the gold butte block se nevada using previously published and new thermochronological data the thermal history of the gold butte block is re evaluated resulting in a geothermal gradient estimate of 20 3 c km and a slow cooling rate of 0 55 0 02 c m y from the late jurassic to the early miocene before onset of rapid miocene cooling the rapidly cooled part of the gold butte block provides a slip rate of 10 5 km m y of the hanging wall that caused the miocene exhumation of this crustal section assuming these thermal conditions are correct the closure temperature of the zircon fission track system is determined at 205 18 c which is in good agreement with previous field based estimates because of the variation in zircon fission track annealing behavior with radiation damage field studies are a suitable way to calibrate the closure temperature of the zircon fission track system

mineralogy 40ar 39ar dating and apatite fission track dating of rocks along the castle mountain fault alaska abstract the castle mountain fault is a 200 km long right lateral fault that forms the northern boundary of the cook inlet basin and matanuska valley alaska fault gouge and fault rock at six localities contain the clay minerals illite smectite chlorite and interstratified illite smectite at one locality gouge contains deformed illite smectite with very little wall rock chlorite contamination fine 36 24 0 08 ma for fine to coarse sizes respectively argon retention ages obtained from 40ar and 39ar retained in the three size fractions of illite at room temperature during neutron irradiation are 37 36 0 15 42 11 0 14 and 47 20 0 10 respectively apatite fission track ages were measured in arkose at a locality on the fault 60 km west of the gouge locality three samples of arkose were dated one within 10 m of the fault core one 170 m from the fault and one 335 m from the fault the sample nearest to the fault yielded an age of 29 3 2 8 ma but it only had four track lengths at 10 13 m two apatite grains from the intermediate sample yielded a pooled age of 34 3 6 1 ma the distant sample 25 grains counted 101 track lengths yielded an age of 32 0 2 9 ma this sample has a broad distribution of track lengths and a broad distribution of individual grain ages ranging from 14 8 5 1 to 67 8 8 8 ma monte carlo modeling of the apatite age and track length data is consistent with hydrothermal mineralization at 37 39 ma followed by rapid uplift and cooling after 10 ma the 40ar 39ar total gas ages k ar are minimum ages and the argon retention ages are maximum ages the thermal model derived from the fission track data and the argon retention age for the finest illite fraction of 37 ma date a hydrothermal mineralization event on the fault

the tectonometamorphic and magmatic evolution of the uppermost unit in central crete melambes area constraints on a late cretaceous magmatic arc in the internal hellenides greece abstract crete consists of a nappe pile that formed during alpine subduction and collision the lower nappes belong to the external hellenides whereas the uppermost nappe is ascribed to the pelagonian zone of the internal hellenides the uppermost unit consists of several subunits including the asterousia crystalline complex acc which comprises metasedimentary rocks meta granitoids and serpentinite the protolith age and the tectonometamorphic evolution of which are largely unknown in the present study we present new structural microfabric and geochronological data from the uppermost unit in the melambes area central crete 206 pb 238 u zircon ages la icp ms and id tims indicate granitic and dioritic intrusions between 71 9 0 6 and 76 9 0 3 ma identical ages have previously been obtained from comparable intrusions in eastern crete and on anafi the composition and geochemical signature suggest an extended magmatic arc along the southern active margin of the pelagonian lycian block post intrusive shearing transformed granite into orthogneiss whereas diorite remained free from foliation because of the lower amount of mechanically weak phases deformation microfabrics suggest top to the se shearing under amphibolite facies conditions of the acc and at greenschist facies conditions of rocks at the base of the acc referred to as akoumianos greenschist the akoumianos greenschist is considered as the northern part of the pindos realm that was subducted underneath the pelagonian lycian active margin based on our new and on published data the following orogenic stages are suggested to have contributed to the evolution of the hellenides during the late cretaceous to eocene 1 pre middle campanian collision and subduction of the pindos lithosphere underneath the southern margin of the pelagonian lycian terrane led to obduction and offscraping of serpentinized ocean floor and stacking of the acc during amphibolite facies top to the se thrusting 2 formation of a campanian magmatic arc along the pelagonian lycian active margin 3 maastrichtian collision and stacking of the magmatic arc during top to the se mylonitic shearing 4 palaeocene top to the se greenschist facies shearing of the acc on top of the akoumianos greenschist 5 late eocene thrusting of the uppermost unit on top of the arvi and pindos units thus top to the se was the dominant shear sense in the southern aegean from at least the mid late cretaceous until the eocene

intracratonic strike slip partitioned transpression and the formation and exhumation of eclogite facies rocks an example from the musgrave block central australia mountain belts developed in an intracratonic setting during the neoproterozoic in the center of australia mesoproterozoic granulite and amphibolite facies gneisses of the musgrave block were overprinted during the petermann orogeny 550 ma with zones of high strain concentrated along broadly east west trending shear zones these zones formed under eclogite facies t 650 c and p 12 kbars to greenschist facies conditions and may form part of a strike slip related crustal scale flower type structure granulite facies gneisses preserve ages older than 700 ma suggesting that these rocks did not experience temperatures greater than 350 c at 550 ma for these rocks not to have been thermally equilibrated to temperatures of 350 c at a depth of 12 kbars requires either an extraordinarily low geothermal gradient 9 c km 1 or if the geothermal gradient was more typical 20 c km 1 then transport to that depth and subsequent exhumation occurring in a short time interval 40 myr

argon diffusion domains in k feldspar i microstructures in mh 10 samples of mh 10 a k feldspar thought to contain discrete diffusion domains for argon have been examined by light and tem microsopy to search for domain boudaries we examined both unheated k feldspar separates and samples heated in vacuum between 750 and 1150 c in order to characterize the initial microstructure as well as any changes resulting from laboratory heating the mh 10 rock shows almost no evidence of plastic deformation and the k feldspar does not possess networks of dislocations which might define a subgrain structure derived from external stresses three classes of substructure are present in the k feldspar but only the third appears to have been affected by laboratory heating 1 cross hatched extinction in light microscope is common but variably developed tem reveals almost no albite pericline twinning but tweed microstructure is ubiquitous 2 turbid zones occur with about 5 volume abundance micropores 1 to 2 m in diameter characterize these regions which often also contain bleds of albite up to 40 m indiameter tem shows these turbid zones to be very complex with intricate twin and tweed structures at the sub micron scale and numerous dislocation and strain features 3 albitic exsolution lamellae 1 m thick 20 m long and separated by 1 m are rarely seen in the light microscope but tem shows that 0 01 m thick by 0 2 1 m long lamellae separated by 0 1 to 0 5 m occur in about 20 of the sample these lamellae are disk shaped have a semi coherent or coherent relationship to the host k feldspar and seem to disappear upon heating to 750 c extensive diffusion studies have been conducted on hm 10 and the results analyzed in terms of distribution of sizes of diffusion domains the preferred result of the analyses was a distribution consisting of three distinct sizes the largest domain size identified in this way is approximately 50 to 100 m and this corresponds to blocks of k feldspar defined by the network of fractured turbid zones the smallest domain size inferred from diffusion analysis is 0 1 m which corresponds to the spacing between albite exsolution lamellae that may represent fast diffusion pathways alternatively the modulated argon distribution accompanying the lamellar variation in feldspar composition may result in an overestimation of the initial diffusion coefficients even though a promising agreement exists between domain analyses based upon diffusional and microstructural analyses at least two questions remain unanswered firstly no clear candidate has been identified for a domain of intermediate size secondly the diffusion analysis has been formulated with independent gas release from all domains whereas the observed structures are likely to be nested i e small domains exist inside bigger domains with the network of semi coherent lamellar boundaries having connectivity and permeability which can provide only a modest increase in argon diffusivity over bulk values

modelling the effect of arbitrary p t t histories on argon diffusion in minerals using the macargon program for the apple macintosh argon diffusion in mineral grains has been numerically modelled using p t t histories that may be relevant to multiply metamorphosed orogenic terranes and for rocks that have resided at high ambient temperatures in the earth s crust for long durations the macargon program1 generates argon concentration profiles in minerals assuming argon loss occurs via volume diffusion it can be run on an apple macintosh computer with arbitrary p t t histories used as input finite difference equations are used in the calculation of 40ar concentration profiles across individual diffusion domains the associated macspectrometer generates model spectra after a p t t history has been specified the form of model 40ar39ar apparent age spectra suggests that considerable caution needs to be exercised in the use of the closure temperature concept and in the interpretation of the significance of plateaux observed in many 40ar39ar apparent age spectra particularly in cases involving metamorphic rocks where complex p t t histories might apply although modelled spectra cannot be directly compared to experimentally determined 40ar39ar age spectra especially when hydrous phases are involved or in cases where loss of argon has not occurred via volume diffusion they do provide insight into theoretically expected age spectra for samples that have experienced complex p t t histories

phanerozoic history of the mahanadi region india the mahanadi basin is a nw se stretching gondwana basin perpendicular to the east coast of india the metamorphic basement to the basin mainly comprises rocks of the eastern ghats mobile belt egmb which experienced a complex thermotectonic evolution involving several episodes of deformation and magmatism it can be inferred from 40ar 39ar ages on hornblende 845 ma and biotite 622 and 593 ma from basement rocks that the last high grade metamorphism in the western part of the egmb i e the western khondalite zone occurred during the late grenvillian with any pan african overprint only reaching anchimetamorphic conditions only pseudotachylite intrusions and their related thermal imprint with ages around 515 ma display a direct pan african signature fission track analyses on nine zircon 419 313 ma and 49 apatite 443 119 ma samples indicate a rift related formation of the mahanadi basin as an asymmetrical half graben in two stages during the permo carboniferous and the middle triassic moreover the gondwana breakup in eastern india is signified by dyke intrusions and the thermal overprint of thrusts and faults in the mahanadi region and the onset of ocean floor production in the mahanadi offshore basin at around 120 ma

effects of wetting rate and simulated rain duration on soil crust formation of red loam the characteristics of crusts are closely related to soil erosion in red loam under a rainfall condition to better manage crust prone soils and to accurately predict hydrological processes it is necessary to understand the mechanism of crust formation wetting rate is an important factor influencing soil aggregate stability and crust formation in this study we considered two factors wetting rate slow 2 mm h 1 fast 50 mm h 1 and rainfall quantity intensity of 60 mm h 1 with every 5 min as a treatment from 0 to 60 min the results show that soil in the fast wetting treatment had higher bulk density stronger crust strength and lower infiltration rate therefore the fast wetting treatment had higher ability to resist the damage caused by raindrops and to resist soil erosion and higher runoff rate than slow wetting treatment crust formation destruction formation process appeared sharp in fast wetting treatment but gentle in slow wetting treatment surface morphology and profile microstructure show that crusts were formed earlier in fast wetting treatment the aggregates in slow wetting treatment had higher stability than those in fast wetting treatment the crust strength of the soil pretreated by slow wetting was weaker than that in fast wetting case due to the persistent presence of stable aggregates albeit microaggregates and consequently reduced levels of compaction

thermochronologic evaluation of postcollision extension in the anatolide orogen western turkey 1 to better understand the driving mechanisms behind the transition from collision to extension in a convergent orogen data from multiple low temperature thermochronometers were obtained from the simav detachment fault sdf the earliest developed major extensional structure recognized in the western anatolide orogen of western turkey twenty two zircon fission track ft 26 apatite ft 12 apatite u th he ages and 26 apatite ft track length analyses are presented the data establish that the sdf was a major extensional fault active between 25 and 19 ma the coincidence in timing of magmatism with cessation of sdf activity implies the detachment became locked owing to doming induced by magma emplacement zircon ft ages away from the influence of miocene magmatism record rapid footwall cooling between 25 and 21 ma at a slip rate of up to 15 km myr and demonstrate that active ductile deformation along the sdf migrated northward with time apatite ft ages from the footwall of the sdf are spatially invariant over 100 km and consistently 2 3 myr younger than the zircon fission track ages from the same samples these data are consistent with a regional but relatively rapid erosion linked cooling phase that removed 2 3 km of overburden following cessation of sdf activity but before deposition of sedimentary rocks on the detachment surface at 16 4 ma postcollision extension of the anatolide orogen with development of the sdf can be best explained as the result of postcollision magmatism and thermal weakening of the crust inducing the extensional reactivation of an earlier major out of sequence thrust

sediment concentration and vertical mixing under breaking waves abstract field measurements of near bed sediment concentrations and sediment diffusivity in the breaker zone and in the inner surf zone of natural beaches were obtained using stacks of optical and fiber optical backscatter sensors under conditions with strongly plunging breakers as well as with spilling surf bores the field data suggest that significant differences exist between these wave types in terms of near bed sediment concentration levels and vertical mixing of sediment from the bed upward into the water column when plotting mean sediment concentration profiles in log c z space profiles are upward concave for spilling surf bores and sediment diffusivity increases approximately linearly upward from the bed for plunging waves however mean sediment concentration profiles are linear in a log c z plot indicating vertically constant sediment diffusivity this suggests that under surf bores sediment suspension from the bed occurs through diffusion processes while vertical mixing is mainly a convective process under plunging waves under surf bores sediment is largely confined to the lower layers of the water column while it is lifted to significantly higher elevations under plunging breakers

geology geochronology and fluid characteristics of the pingqiu gold deposit southeastern guizhou province china abstract the junction of the southeastern guizhou the southwestern hunan and the northern guangxi regions is located within the southwestern jiangnan orogen and forms a ne trending 250 km gold belt containing more than 100 gold deposits and occurrences the pingqiu gold deposit is one of the numerous lode gold deposits in the southeastern guizhou district gold mineralization is hosted in neoproterozoic lower greenschist facies metamorphic rocks and controlled by fold related structures vein types present at pingqiu include bedding parallel and discordant types with saddle reefs and their down limb extensions dominating but with lesser discordant types the major sulfide minerals are arsenopyrite and pyrite with minor sphalerite galena chalcopyrite and rare pyrrhotite marcasite and tetrahedrite much of the gold is m to mm sized grains and occurs as fracture controlled isolated grains or filaments in quartz galena sphalerite pyrite and wallrock three types of fluid inclusions are distinguished in hydrothermal minerals type 1 aqueous inclusions have homogenization temperatures of 171 396 c and salinities of 1 4 9 8 wt nacl equiv type 2 aqueous carbonic inclusions yield final homogenization temperatures of 187 350 c with salinities of 0 2 7 7 wt nacl equiv type 3 inclusions are carbonic inclusions with variable relative content of co2 and ch4 and minor amounts of n2 and h2o the close association of co2 rich inclusions and h2o rich inclusions in groups and along the same trail suggests the presence of fluid immiscibility the calculated 18oh2o values range from 4 3 to 8 3 and dh2o values of fluid inclusions vary from 55 8 to 46 9 a metamorphic origin is preferred on the basis of geological background and analogies with other similar deposit types two ore related sericite samples yield well defined 40ar 39ar plateau ages of 425 7 1 7 ma and 425 2 1 3 ma respectively these data overlap the duration of the caledonian gold mineralization along the jiangnan orogen and suggest that gold mineralization was post peak regional metamorphism and occurred during the later stages of the caledonian orogeny overall the pingqiu gold deposit displays many of the principal characteristics of the bendigo gold mines in the western lachlan orogen se australia and the dufferin gold deposit in the meguma terrane nova scotia canada but also some important differences which may lead to the disparity in gold endowment however the structural make up at deposit scale and the shallow mining depth at present indicate that the pingqiu gold deposit may have considerable gold potential at depth

differential exhumation in response to episodic thrusting along the eastern margin of the tibetan plateau abstract thermochronological data from the songpan ganze fold belt and longmen mountains thrust nappe belt on the eastern margin of the tibetan plateau in central china reveal several phases of differential cooling across major listric thrust faults since early cretaceous times differential cooling indicated by distinct breaks in age data across discrete compressional structures was superimposed upon a regional cooling pattern following the late triassic indosinian orogeny 40ar 39ar data from muscovite from the central and southern longmen mountains thrust nappe belt suggest a phase of differential cooling across the wenchuan maouwen shear zone during the early cretaceous the zircon fission track data also indicate differential cooling across a zone of brittle re activation on the eastern margin of the wenchuan maouwen shear zone during the mid tertiary between 38 and 10 ma apatite fission track data from the central and southern longmen mountains thrust nappe belt reveal differential cooling across the yingxiu beichuan and erwangmiao faults during the miocene forward modelling of apatite fission track data from the northern longmen mountains thrust nappe belt suggests relatively slow regional cooling through the mesozoic and early tertiary followed by accelerated cooling during the miocene beginning at ca 20 ma to present day regional cooling is attributed to erosion during exhumation of the evolving longmen mountains thrust nappe belt lmtnb following the indosinian orogeny differential cooling across the wenchuan maouwen shear zone and the yingxiu beichuan and erwangmiao faults is attributed to exhumation of the hanging walls of active listric thrust faults thermochronological data from the longmen mountains thrust nappe belt reveal a greater amount of differential exhumation across thrust faults from north to south this observation is in accord with the prevalence of proterozoic and sinian basement in the hanging walls of thrust faults in the central and southern longmen mountains the two most recent phases of reactivation occurred following the initial collision of india with eurasia suggesting that lateral extrusion of crustal material in response to this collision was focused along discrete structures in the lmtnb

interpretation of u th he single grain ages from slowly cooled crustal terranes a case study from the transantarctic mountains of southern victoria land low temperature thermochronologic techniques e g apatite fission track aft thermochronology and u th he dating constrain near surface t t paths and are often applied to uplift denudation and landscape evolution studies samples collected in vertical profiles from granitic walls on either side of the ferrar glacier southern victoria land antarctica were analyzed using aft thermochronology and apatite u th he dating to further constrain the lowest temperature thermal history of this portion of the transantarctic mountains aft central ages vary systematically with elevation and together with track length information define a multi stage cooling denudation history in the cretaceous and early tertiary apatite u th he single grain age variation with elevation is not as systematic with considerable intra sample age variation although many complicating factors e g u and th rich micro inclusions fluid inclusions variation in crystal size particle ejection correction zonation and particle ejection correction implantation of he into a crystal or impediment of he diffusion out of a crystal and 147sm derived particles may contribute to age dispersion we found that variation in single grain ages correlated with cooling rate samples that cooled relatively quickly have less variation in single grain ages whereas samples that cooled relatively slowly 3 c m y or resided within an u th he partial retention zone heprz prior to more rapid cooling have a comparatively greater variation in ages decay of u and th via particle emission creates a 4he concentration profile dependent upon the initial parent u th within a crystal variation of single grain ages for samples with non homogeneous u th distributions will be enhanced with long residence time in the partial retention zone i e slow cooling because of the relative importance of loss via volume diffusion and loss via particle ejection with respect to the u th zonation and the grain boundary correction of ages for particle ejection ft correction factor typically assumes uniform u and th distribution within the crystal and when applied to a population of crystals with different u and th distributions will enhance the variation in ages most complicating factors listed above for apatite u th he ages result in ages that are too old we propose that if considerable variation in u th he single grain ages exists that a weighted mean age is determined once outlier single crystal ages are excluded using the criterion of chauvenet or a similar approach we suggest that the true age or most representative age for that age population lies between the minimum u th he age and the weighted mean age we apply this approach coupled with composite age profiles to better constrain the t t history of the profiles along the ferrar glacier significant intra sample variation in single crystal apatite u th he ages and other minerals dated by the u th he method should be expected especially when the cooling rate is slow the variation of u th he single crystal ages is therefore another parameter that can be used to constrain low temperature thermal histories

k ar and ar ar dating the aim of this chapter is to present the k ar and ar ar dating techniques in the context of noble gas studies since there are already several recent texts on k ar and ar ar dating dickin 1995 mcdougall and harrison 1999 the focus of this section will be aspects of argon transport and storage in the crust which affect k ar and ar ar dating including ar loss from minerals by diffusion and ar gain by minerals or excess argon the k ar dating technique was one of the earliest isotope dating techniques developed soon after the discovery of radioactive potassium and provided an important adjunct to u pb and u he dating techniques the ease of measurement and ideal half life 1250 million years see table 2 below for dating geological events has made this the most popular of isotopic dating techniques aldrich and nier 1948 first demonstrated that 40ar was the product of the decay of 40k and soon after k ar ages were being measured in several laboratories most often using an absolute method such as a mcleod gauge to measure argon concentrations the first published k ar results by such a technique were those of smits and gentner 1950 who analyzed sylvite from the buggingen oligocene evaporite deposits obtaining an age of 20 million years mass spectrometers which simultaneously measured very small amounts of gas and the isotope ratios necessary to make corrections for atmospheric contamination quickly replaced manometric techniques crucially the use of static vacuum techniques pioneered by john reynolds at the university of california berkeley meant that mass spectrometers were sufficiently sensitive to analyse the small amounts of gas released from common rocks and minerals although the earliest mass spectrometers were built in house the introduction of the commercially available ms10 farrar et al 1964 a small 180 metal mass spectrometer built for

apparent partial loss age spectra of neoarchean hornblende murmansk terrane kola peninsula russia the role of biotite inclusions revealed by 40ar 39ar laserprobe analysis metamorphic hornblende frequently yields spectra with progressively increasing 40ar 39ar age steps often interpreted as caused by partial resetting due to thermally activated radiogenic argon loss by solid state diffusion yet in many cases rising ca k ratio spectra for such samples imply the presence of minor inclusions of k contaminant minerals to avoid parts of grains with mineral inclusions or compositional zoning we drilled tiny discs from thin sections under a petrographic microscope laser step heating of drilled biotite free hornblende discs yielded flat age and ratio spectra in contrast furnace step heated hornblende separates from the same samples produced apparent loss age spectra moreover biotite free samples yielded flat spectra by laser and furnace dating consequently apparent loss spectra result from degassing of included substantially younger biotite before its hornblende host during laboratory step heating c 2640 ma hornblende ages constrain the murmansk terrane s cooling