an evaluation of the single grain zircon evaporation method in highly discordant samples comparison of results from the isotope dilution and the single grain zircon evaporation methods on highly discordant samples shows that the evaporation method can yield accurate crystallization ages on some zircon populations that display complex discordant patterns three granite gneiss samples from the western gneiss region norway which crystallized at approximately 1660 ma and experienced profound lead loss in zircon during caledonian metamorphism at 400 ma were analyzed by both the isotope dilution tucker et al 1990 and evaporation methods 207ph 206pb evaporation ages on one sample agree with the isotope dilution crystallization age two other samples produced a range of anomalously young 207pb 206pb evaporation ages analyses at the highest evaporation temperatures from one of these two samples were consistent with crystallization ages determined by the isotope dilution method but it would be difficult to interpret them correctly without independent age information the anomalously young results are generally easy to recognize by the pattern of increasing 207ph 206ph ages with increasing evaporation temperature a more serious problem however is that some grains produced 207ph 206pb age plateaus over a range of increasing evaporation temperatures that cannot be correlated with any documented geologic event in the region the evaporation analyses yielding the youngest 207pb 206pb ages also display the most common lead suggesting that common lead may diffuse into the zircon grains during loss of radiogenic lead a pegmatite from massachusetts usa contains zircons with visible xenocrystic cores the 207ph 206pb evaporation ages at lower temperatures agree well with the 299 ma crystallization age determined by the isotope dilution method on the zircon rims r d tucker unpubl data at higher evaporation temperatures a range of older 207pb 206pb ages are produced by mixing pb evaporated from both the zircon rims and xenocrystic cores large changes in the 206pb 208pb ratio accompany the increase in 207pb 206pb ages and indicate mixing of two distinct zircon domains thus in addition to providing useful 207pb 206pb age information the evaporation method may help distinguish between the discordance produced by diffusional lead loss and the mixing of two domains of differing age

the incidence and role of gullies after logging in a coastal redwood forest the distribution and morphological characteristics of channels were mapped in a redwood forest at caspar creek california usa to evaluate the extent to which recent logging has influenced channel conditions in the area in the north fork caspar creek watershed second cycle logging of the early 1990s appears to have triggered increased coalescence of discontinuous gullies within clearcut tributary watersheds and upstream channel limits in logged watersheds are now located significantly farther upslope than in control watersheds the magnitudes of observed increases in peakflow after logging are consistent with the change in drainage density relations between channel morphological variables and indices of stream power are less well defined in logged watersheds than in controls suggesting that logging may have led to disruption of previously established channel forms correlations between suspended sediment yields and indices of gully erosion suggest that in channel erosion associated with hydrologic change is an important source of post logging sediment at caspar creek common sediment control measures such as use of riparian buffer strips and reduction of road surface erosion would not be effective for reducing sediment input from this source

geology and geomorphology control suspended sediment yield and modulate increases following timber harvest in temperate headwater streams abstract suspended sediment transport is an important contributor to ecologic and geomorphic functions of streams however it is challenging to generalize predictions of sediment yield because it is influenced by many factors in this study we quantified the relevance of natural controls e g geology catchment physiography on suspended sediment yield ssy in headwater streams managed for timber harvest we collected and analyzed six years of data from 10 sites five headwater sub catchments and five watershed outlets in the trask river watershed western oregon united states we used generalized least squares regression models to investigate how the parameters of the ssy rating curve varied as a function of catchment setting and whether the setting modulated the ssy response to forest harvesting results indicated that the highest intercepts of the power relation between unit discharge and ssy were associated with sites underlain primarily by friable rocks e g sedimentary formations the greatest increases in ssy after forest harvesting up to an order of magnitude also occurred at sites underlain by the more friable lithologies in contrast basins underlain by resistant lithologies intrusive rocks had lower ssy and were more resilient to management related increases in ssy as such the impact of forest management activities e g use of forested buffers building of new roads on the variability in ssy was primarily contingent on catchment lithology sites with higher ssy or harvest related increases in ssy also generally had a lower mean elevation and slope b greater landscape roughness and c lower sediment connectivity we used principal component analysis pca to further explore the relationship between ssy and several basin physiographic variables the pca clearly separated sites underlain by friable geologic units from those underlain by resistant lithologies results are consistent with greater rates of weathering and supply of sediment to headwater streams in catchments with more friable lithologies and limited sediment supply in catchments underlain by resistant lithologies we hypothesize that a similar framework may aid in predicting the overall ssy of a catchment as well as its susceptibility to increases in ssy following forest harvesting

nondestructive chemical dating of young monazite using xrf 1 design of a mini probe age data for samples from the central alps and comparison to u pb tims data the analytical capabilities of an improved mini xrf instrument for trace element analysis are explored to obtain u th pb ages for single monazite grains as small as 50 m the detection limits for u th and pb with counting times of 10 min are 10 ppm counts collected for 40 min are sufficient to date single monazite grains as young as 15 ma the precision of the age data depends essentially on the counting statistics of the main x ray peak npb l 1 2 used to analyze for lead the age uncertainty is proportional to the age of the grain the total contents of th and u and the squared integration time the reliability of the chemical dating method is tested using monazite separates from amphibolite facies metamorphic monazite samples from the central alps for which the chemical ages are compared to mass spectrometric u pb tims ages we show that populations of analyses obtained using the new instrument on single grains as young as 25 30 ma agree within error with the tims data for monazite separates with complex age patterns such as polymetamorphic samples yielding discordant isotopic ages the statistical analysis of chemical age data obtained from populations of 26 50 grains is advantageous in that it may provide additional insight into the reasons for the discordancy chemical th u pb dating of monazite by the xrf method offers considerably improved accuracy and precision compared with chemical dating by electron microprobe which typically becomes impractical for samples less than 100 200 ma in age conversely the minimum grain size required for the new instrument 50 m is well above the spatial resolution of the electron microprobe 2 m a companion paper scherrer et al this volume describes further developments towards microanalysis by xrf with improved spatial resolution

40ar 39ar laser probe age determination confirms the ries impact crater as the source of glass particles in graupensand sediments grimmelfingen formation north alpine foreland basin mineralogical and chemical characteristics of reworked suevite material recovered from graupensand deposits grimmelfingen formation north alpine foreland basin middle miocene are identical to those recorded in ries suevite 40ar 39ar laser probe analyses of two glass particles from graupensand components and of a glass fragment from the ries suevite yielded ages ranging between 14 3 and 14 4 ma identical within analytical uncertainties hitherto the ries impact event was dated at 15 ma conventional k ar and 40ar 39ar methods whereas the graupensande have been placed into the ottnangian around 18 5 ma on the basis of mammal biostratigraphy small sample masses involved in laser probe technique allowed rigorous preselection of highly pure glass particles thus reducing possible age determination errors through partly molten or even unmolten mineral fragments from the variscan crystalline basement our results reconfirm previous statements considering the graupensande as reworked products of the ries impact ejecta blanket thus placing the age of the grimmelfingen formation into the badenian of middle miocene rather than into the ottnangian early miocene

high pressure behavior of kyanite decomposition of kyanite into stishovite and corundum the pressure stability of kyanite was experimentally reversed with the use of a multi anvil apparatus kyanite was found to decompose into its oxides stishovite and corundum between 14 or 0 5 gpa at 1000 degrees c and 17 5 or 1 0 gpa at 2000 degrees c reliable thermodynamic calculations can be performed to temperatures of approximately 1500 degrees c up to this temperature the location of the equilibrium kyanite corundum stishovite determined in this study constrains the equilibrium coesite stishovite a set of thermodynamic data was calculated by linear programming from the kyanite breakdown reaction and the coesite stishovite equilibrium feasible values for the fitted thermo dynamic properties are 28 5 to 26 3 mpa k for the temperature derivative of the bulk modulus dk dt p of kyanite 815254 to 813635 j mol for g 0 sub 1298 of stishovite and 24 6 to 26 3 j mol k for s 0 sub 1298 of stishovite the experimental results indicate 1 that in peraluminous eclogites of basaltic or sedimentary origin stishovite may coexist stably with corundum at a depth greater than 420 450 km and 2 that in an inhomogeneous al enriched mantle corundum could be a minor constituent in the lower mantle

single zircon dating constraining the maximum age of the barberton greenstone belt southern africa one of the central issues in early precambrian crustal evolution is the age and genetic relationship between greenstone belt supracrustal assemblages and nearby high grade gneiss terranes the crucial question in most cases is which of the two is older we report zircon ages using the new single grain evaporation technique from a metaquartzite and a felsic volcanic flow of the onverwacht group in the barberton greenstone belt bgb south africa and swaziland that constrain the age of this basal part of the greenstone sequence between 3451 15 and 3438 6 ma this is almost 100 ma younger than previous age estimates assigned to the onverwacht and agrees well with recent sm nd pb pb and zircon ion microprobe dating tonalitic orthogneisses of the ancient gneiss complex agc adjacent to the bgb have ages up to 3644 4 ma and are thus demonstrably older than the greenstones also they cannot be derived from them by partial melting as has been proposed previously this age relationship as well as the presence of metaquartzites and thrust slices of pregreenstone tonalite in the onverwacht tectonic sequence rule out an entirely oceanic evolution for the early history of the bgb and suggest that continental crust was either below or nearby

proximal and distal styles of pegmatite related metasomatic emerald mineralization at ianapera southern madagascar the ianapera emerald deposit is located in the neoproterozoic vohibory block of southern madagascar the local geology consists of intercalated migmatitic gneissic units and calcareous metasedimentary rocks containing boudinaged metamorphosed mafic ultramafic lenses all intruded by pegmatite veins these units occur near the hinge of the tightly folded ianapera antiform within a few kilometers of the ampanihy shear zone emerald mineralization is hosted by metasomatic phlogopite veins and bodies developed within the mafic ultramafic rocks based on field and textural relationships we distinguish proximal and distal styles of mineralization proximal mineralization occurs at the contact of pegmatite veins with mafic ultramafic units in the distal style pegmatites are not observed three types of emeralds could be distinguished mainly on the basis of color and mineral zoning some of these emeralds have the most al depleted and cr rich composition ever recorded another characteristic feature to the ianapera deposit and to our knowledge yet unreported is the association of some emeralds with scapolite in metasomatised mafic rocks mineral inclusions are common in most emeralds and include phlogopite carbonates barite k feldspar quartz pyrite zircon monazite bastnaesite phenakite plus fe and cr oxides however feldspar and rare earth element bearing minerals occur predominantly in proximal emeralds which also have a more incompatible trace element signature than distal emeralds we propose a model related to syn to post tectonic magmatic hydrothermal activity pegmatitic bodies intruded units of the ianapera antiform probably during tectonic relaxation exsolution of fluids rich in halogens and incompatible elements from the cooling pegmatites caused hydrothermal metasomatism of cr bearing mafic ultramafic rocks in direct contact with the pegmatites local fracturing favored fluid infiltration permitting the formation of distal mineralization emerald composition was controlled by the chemistry of the host rock the presence of carbonate mineral inclusions in the emeralds and the high f activity indicated by elevated f contents in newly formed minerals suggest transport of be as a fluoride carbonate complex it seems likely that beryl formation was triggered by precipitation of f rich phlogopite which removed the complexing ligand from the fluid

shock induced growth and metastability of stishovite and coesite in lithic clasts from suevite of the ries impact crater germany the microtextures of stishovite and coesite in shocked non porous lithic clasts from suevite of the ries impact structure were studied in transmitted light and under the scanning electron microscope both high pressure silica phases were identified in situ by laser raman spectroscopy they formed from silica melt as well as by solid state transformation in weakly shocked rocks stage i fine grained stishovite 1 8 m occurs in thin pseudotachylite veins of quartz rich rocks where it obviously nucleated from high pressure frictional melts generally no stishovite was found in planar deformation features pdfs within grains of rock forming quartz the single exception is a highly shocked quartz grain trapped between a pseudotachylite vein and a large ilmenite grain in which stishovite occurs within two sets of lamellae it is assumed that in this case the small stishovite grains formed by the interplay of conductive heating and shock reverberation in strongly shocked rocks stages ib iii above 30 gpa grains of former quartz typically contain abundant and variably sized stishovite 6 m embedded within a dense amorphous silica phase in the interstices between pdfs the formation of transparent diaplectic glass in adjacent domains results from the breakdown of stishovite and the transformation of the dense amorphous phase and pdfs to diaplectic glass in the solid state coesite formed during unloading occurs in two textural varieties granular micrometre sized coesite occurs embedded in silica melt glass along former fractures and grain boundaries these former high pressure melt pockets are surrounded by diaplectic glass or by domains consisting of microcrystalline coesite and earlier formed stishovite the latter is mostly replaced by amorphous silica

deformation induced polymorphic transformation experimental deformation of kyanite andalusite and sillimanite abstract torsion experiments were performed on the al 2 sio 5 polymorphs in the sillimanite stability field to determine basic rheological characteristics and the effect of deformation on polymorphic transformation the experiments resulted in extensive transformation of andalusite and kyanite to sillimanite no transformation occurred during the hot press no deformation stage of sample preparation which was carried out at similar p t conditions and duration as the torsion experiments experiments were conducted on fine grained 4 s to a maximum shear strain of 400 electron back scattered diffraction ebsd analysis of the experiments revealed development of lattice preferred orientations with alignment of sillimanite and andalusite 001 slightly oblique to the shear plane the kyanite experiment could not be analyzed using ebsd because of near complete transformation to sillimanite very little strain 30 is required to produce widespread transformation in kyanite and andalusite polymorphic transformation in andalusite and kyanite experiments occurred primarily along 500 m wide shear bands oriented slightly oblique and antithetic to the shear plane and dominated by sub m 100 150 nm fibrolitic sillimanite shear bands are observed across the entire strain field preserved in the torsion samples scanning transmission electron microscope imaging shows evidence for transformation away from shear bands e g fibrolitic rims on relict andalusite or kyanite relict grains typically have an asymmetry that is consistent with shear direction these experimental results show that sillimanite is by far the weakest of the polymorphs but no distinction can yet be made on the relative strengths of kyanite and andalusite these observations also suggest that attaining high bulk strain energy in strong materials such as the al 2 sio 5 polymorphs is not necessary for triggering transformation strain energy is concentrated along grain boundaries and transformation occurs by a dynamic recrystallization type process these experiments also illustrate the importance of grain size sensitive creep at high strains in a system with simultaneous reaction and deformation

u pb single zircon pb evaporation and sm nd isotopic study of a granulite domain in se madagascar madagascar is a poorly known precambrian domain at the eastern border of the mozambique belt strongly affected by the pan african orogeny this large island represents a key part of gondwanaland and the accurate dating of the major metamorphic event is critical for the reconstruction of that supercontinent in the southeastern zone of the island metamorphism has reached high temperature and low pressure conditions 850 c and 5 kbar with moderate retrogression a u pb conventional multigrain single zircon stepwise pb evaporation and sm nd geochronological study on meta felsic rocks has shown that basement of at least lower proterozoic age was strongly metamorphosed around 570 580 ma this tectometamorphic event nearly obliterated any pre pan african zircon isotopic memory the granulite facies metamorphism is associated with the syn metamorphic emplacement of the anosyan charnock ites and granites a strong channelized fluid contribution at 545 ma has produced the re crystallization of biotite apatite

growth of early archaean crust in the ancient gneiss complex of swaziland as revealed by single zircon dating abstract we present new single grain zircon ages derived from ion microprobe and evaporation analysis for rocks of the ancient gneiss complex agc of swaziland southern africa that enable us to document a more extended history of early archaean crustal growth in the eastern kaapvaal craton than previously documented the oldest rocks appear to be strongly flattened tonalitic trondhjemitic orthogneisses dated as 3644 4 ma compston and kroner 1988 which are in faulted contact with rocks of the barberton greenstone belt bgb in an agc enclave in northwest swaziland and which also record subsequent thermal metamorphic plutonic events at 3580 3500 3430 3200 and 3000 ma these rocks are tectonically interbanded with 3200 ma old granodiorites and k rich leucogranites thereby providing evidence for a severe and important post 3 2 ga deformation event that is also inferred for 3500 to 3200 ma old tonalitic to trondhjemitic gneisses in the agc of northeast swaziland in central swaziland zircons from the oldest tonalitic gneiss of the agc have an age of 3560 ma whereas younger additions of granitoid which also intrude the dwalile greenstone enclave in the agc record ages of 3450 3460 ma intense ductile deformation has repeatedly obliterated original intrusive contacts so that preserved field relationships often cannot resolve the above chronology chemical and whole rock isotopic data are consistent with derivation of some agc gneisses from mantle sources and thus imply very short crustal residence times for their granitoid precursors but this is not generally true for the entire agc we suggest that at least some gneisses resulted from intracrustal melting perhaps during crustal thickening events as recorded in granulite parageneses within the agc documentation of 3 5 ga or older granitoid crust in all major regions of the agc now exposed suggests the presence of continental terrains of up to at least 100 km in diameter before the barberton greenstone succession was deposited the combined geochronologic and structural metamorphic data for the agc and bgb reveal a complex history of growth and deformation for the eastern kaapvaal craton that lasted for at least 600 ma from 3 64 to about 3 ga ago the magmatic and tectonic evolution of the agc was broadly similar to that of the bgb and suggests that early archaean greenstone and gneiss terrains were coeval but formed and evolved in different settings and at different crustal levels our age data and the structural metamorphic evolution of the eastern kaapvaal craton are compatible with models suggesting crustal growth through vertical magmatic underplating and early crustal thickening uplift and erosion as a result of tectonic interleaving due to repeated horizontal shortening

ca rich majorite derived from high temperature melt and thermally stressed hornblende in shock veins of crustal rocks from the ries impact crater germany shock veins up to 1 1 mm thick were found within non porous lithic clasts from suevite breccia of the nordlinger ries impact structure these veins were studied by optical microscopy in transmitted and reflected light and by scanning electron microscopy in shocked amphibolites two types of ca rich majorite occur within and adjacent to the veins the first type crystallized from shock induced melts within the veins si contents of these majorites suggest dynamic pressure of 15 17 gpa implying minimum temperatures in the range of 2 150 2 230 c the second type of majorite was formed adjacent to the shock veins within pargasitic hornblende this majorite contains significant amounts of h2o 0 7 0 9 wt based on the textural setting the shrinkage cracks and the chemical compositions of both phases a solid state mechanism is deduced for the hornblende to majorite phase transition both genetic types of ca rich majorite are described for the first time from a terrestrial impact crater along with stishovite majorite constitutes the second silicate mineral displaying sixfold coordination of si at ries using micro raman spectroscopy jadeite coesite and jadeite grossular were identified within local melt glasses of alkali feldspar and plagioclase composition respectively stishovite aggregates produced by solid state reaction along with shock induced high pressure melt glasses of almandine composition were also detected in shock veins of a garnet cordierite sillimanite restite the quenched homogeneous almandine glasses point to melting temperatures of more than 2 500 c for the veins our findings demonstrate that terrestrial shock veins can give valuable information on shock induced mineral transformations and transient high pressures of host rocks during a natural impact event

precise determinations of the equilibria kyanite sillimanite and kyanite andalusite and a revised triple point for al 2 sio 5 polymorphs

plutonism deformation and metamorphism in the proterozoic flin flon greenstone belt canada limits on timing provided by the single zircon pb evaporation technique the authors have used the single zircon pb evaporation technique to determine the age of granitoids in the western flin flon domain of the trans hudson orogen in canada and to constrain the timing of molasse sedimentation deformation metamorphism and mesothermal gold mineralization the plutons were intruded between 1 860 and 1 834 ma about 30 m y after volcanism and provide further evidence of a major period of intrusive activity throughout the trans hudson orogen at that time the older plutons 1 860 to 1 848 ma are synchronous with p2 deformation and the early stages of peak metamorphism whereas the younger plutons are synchronous with or postdate the p3 deformation event molasse sedimentary units are deformed and metamorphosed thus deposition must have begun at about 1 860 ma mesothermal gold mineralization postdates all plutons and may be related to high grade peak metamorphism in adjacent terranes at about 1 815 ma this study also provides further support for the use of the pb evaporation technique as a suitable method for dating single zircons and indicates that the sup 208 pb sup 206 pb ratio of zircons may be a useful tool in detecting the presence and source of xenocrystic zircons

pan african granulite formation in the kabye massif of northern togo west africa pb pb zircon ages metamorphic zircons from seven granulite facies orthogneisses of tonalitic composition enderbites collected from four different nappes of the kabye massif in the dahomeyide belt of northern togo were dated by the pb pb evaporation method they yielded consistent neoproterozoic ages with a mean of 612 5 0 8 ma interpreted to reflect the peak of regional granulite facies metamorphism following pan african continental collision between the west african and benin nigerian plates these results support previous ages obtained by various chronometers on high grade rocks from the same suture zone and from surrounding units in togo benin and ghana they are also similar to zircon ages from granulites in the mozambique belt of souteastern africa these pan african metamorphic ages reflect continental amalgamation resulting in the formation of the gondwana supercontinent towards the end of the neoproterozoic

majoritic garnet grains within shock induced melt veins in amphibolites from the ries impact crater suggest ultrahigh crystallization pressures between 18 and 9 gpa shock induced melt veins in amphibolites from the nordlinger ries often have chemical compositions that are similar to bulk rock i e basaltic but there are other veins that are confined to chlorite rich cracks that formed before the impact and these are poor in ca and na majoritic garnets within the shock veins show a broad chemical variation between three endmembers 1 text viii text m 2 3 text vi text al 2 text iv text sio 4 3 normal garnet grt 2 text viii text m 2 3 text vi text m 2 text si ti text iv text sio 4 3 majorite maj and 3 text viii text na text m 2 2 text vi text si ti text al text iv text sio 4 3 na majorite50grt50 whereby m2 mg2 fe2 mn2 ca2 in particular we observed a broad variation in vi si ti which ranges from 0 12 to 0 58 cations per formula unit cpfu all these majoritic garnets crystallized during shock pressure release at different ultrahigh pressures those with high vi si ti 0 36 0 58 cpfu formed at high pressures and temperatures from amphibole rich melts while majoritic garnets with lower vi si ti of 0 12 0 27 cpfu formed at lower pressures and temperatures from chlorite rich melts furthermore majoritic garnets with intermediate values of vi si ti 0 24 0 39 crystallized from melts with intermediate contents of ca and na to the best of our knowledge the morb type ca na rich majoritic garnets with maximum contents of 2 99 wt na2o and calculated crystallisation pressures of 16 18 gpa are the most extreme representatives ever found in terrestrial shocked materials at the ries the duration of the initial contact and compression stage at the central location of impact is estimated to only 0 1 s we used a 200 m thick shock induced vein in a moderately shocked amphibolite to model its pressure temperature time p t t path the graphic model manifests a peak temperature of 2600 c for the vein continuum pressure lasting for 0 02 s a quench duration of 0 02 s and a shock pulse of 0 038 s the small difference between the continuum pressure and the pressure of majoritic garnet crystallization underlines the usefulness of applying crystallisation pressures of majoritic garnets from metabasites for calculation of dynamic shock pressures of host rocks majoritic garnets of chlorite provenance however are not suitable for the determination of continuum pressure since they crystallized relatively late during shock release an extraordinary glass and majorite bearing amphibole fragment in a shock vein of one amphibolite documents the whole unloading path

an insight into the breakup of gondwana identifying events through low temperature thermochronology from the basement rocks of madagascar 1 fission track analysis was applied to the precambrian suites of madagascar in order to identify the lower temperature cooling histories and their relationships to the phanerozoic events that affected the island apatite ages range from 431 to 68 ma and zircon ages range from 452 to 238 ma thermochronologically the island can be divided into a southern central and northern region each with a subdivision on an east west basis the southern region is sharply separated from the central region by strongly contrasting apparent apatite ages over the northwest southeast striking ranotsara shear zone rsz the change in apparent ages over the rsz is indicative of later reactivation along younger brittle faults the central region has the oldest ages of the island and has a diffuse contact to the third region northward along the entire western margin of the precambrian basement initial paleozoic exhumation was followed by heating burial by sediments during jurassic and cretaceous times a decrease in ages along the eastern margin from 119 to 68 ma coincides with the predicted positions of the marion hot spot after effects of erosion are considered on the other hand these ages may represent progressive opening of the margin in a southward direction together with associated denudation of the rift shoulder the eastern part of the central region has remained very stable since at least devonian times undergoing only long term very slow exhumation at rates of 1 5 m myr

sediment production from forest road surfaces erosion on roads is an important source of fine grained sediment in streams draining logged basins of the pacific northwest runoff rates and sediment concentrations from 10 road segments subject to a variety of traffic levels were monitored to produce sediment rating curves and unit hydrographs for different use levels and types of surfaces these relationships are combined with a continuous rainfall record to calculate mean annual sediment yields from road segments of each use level a heavily used road segment in the field area contributes 130 times as much sediment as an abandoned road a paved road segment along which cut slopes and ditches are the only sources of sediment yields less than 1 as much sediment as a heavily used road with a gravel surface

ries crater and suevite revisited observations and modeling part i observations we report results of an interdisciplinary project devoted to the 26 km diameter ries crater and to the genesis of suevite recent laboratory analyses of crater suevite occurring within the central crater basin and of outer suevite on top of the continuous ejecta blanket as well as data accumulated during the past 50 years are interpreted within the boundary conditions imposed by a comprehensive new effort to model the crater formation and its ejecta deposits by computer code calculations artemieva et al 2013 the properties of suevite are considered on all scales from megascopic to submicroscopic in the context of its geological setting in a new approach we reconstruct the minimum maximum volumes of all allochthonous impact formations 108 116 km3 of suevite 14 22 km3 and the total volume of impact melt 4 9 8 0 km3 produced by the ries impact event prior to erosion these volumes are reasonably compatible with corresponding values obtained by numerical modeling taking all data on modal composition texture chemistry and shock metamorphism of suevite and the results of modeling into account we arrive at a new empirical model implying five main consecutive phases of crater formation and ejecta emplacement numerical modeling indicates that only a very small fraction of suevite can be derived from the primary ejecta plume which is possibly represented by the fine grained basal layer of outer suevite the main mass of suevite was deposited from a secondary plume induced by an explosive reaction fuel coolant interaction of impact melt with water and volatile rich sedimentary rocks within a clast laden temporary melt pool both melt pool and plume appear to be heterogeneous in space and time outer suevite appears to be derived from an early formed melt rich and clast poor plume region rich in strongly shocked components melt clasts and originating from an upper more marginal zone of the melt pool crater suevite is obviously deposited from later formed clast rich and melt poor plumes dominated by unshocked and weakly shocked clasts and derived from a deeper central zone of the melt pool genetically we distinguish between primary suevite which includes dike suevite the lower sublayer of crater suevite and possibly a basal layer of outer suevite and secondary suevite represented by the massive upper sublayer of crater suevite and the main mass of outer suevite

cooling of a late syn orogenic pluton evidence from laser k feldspar modelling of the carion granite madagascar since the late 1980s it has been hypothesized that the wide range of apparent argon ages seen within single k feldspar samples might be due to a distribution of diffusion domain sizes within the mineral to test and apply this idea an analytical technique that combines conventional laboratory degassing experiments resistance heating with numerical inversion procedures has been developed to extract cooling history information from feldspars a key part of the method involves careful control of temperature in the laboratory to constrain the diffusion parameters of the feldspar samples in our study we have k feldspar data from single crystals that mimic the types of data seen in classic resistance heater fusion experiments our step heating data are based on using a continuous argon ion laser with no direct control on temperature however with only a single added free parameter in the model we show that it is possible to analyze this data in the multi domain style and make some simple inferences on the nature of the cooling history of the carion pluton in central madagascar the carion granitic pluton in central madagascar was intruded into warm continental crust following orogenic events related to the final amalgamation of gondwana u pb shrimp dating of the pluton yields an emplacement age of 532 1 5 2 ma followed by relatively slow cooling as constrained by 40ar 39ar ages on hornblende biotite and k feldspar four hornblende samples yielded a mean 40ar 39ar age of 512 7 2 6 ma a biotite sample yielded an age of 478 9 1 0 ma and modeled k feldspar ages show cooling from 350 c at 466 ma to 100 c by 410 ma collectively the data suggest that the pluton cooled from 850 c at 532 1 5 2 u pb zircon ma to 500 c at 512 7 2 6 ma 40ar 39ar hornblende or approximately 18 c ma slowing to 4 c ma between 512 ma and 478 ma and finally to about 3 c ma between 478 and 410 ma

impact induced phase transformations at 50 60 gpa in continental crust an epma and atem study strongly shocked gneiss fragments from the allochthonous polymict breccia of the 23 ma old haughton impact crater were studied by optical microscopy electron probe microanalysis epma and analytical transmission electron microscopy atem the study is focused on the substantiation of shock induced phase transformations in rock forming minerals and their implications for post shock thermal regimes and impact age dating in the gneiss fragments the original layering is largerly preserved although all the minerals have experienced dramatic phase transformations chemically the various shock generated decomposition regions still reflect the pre shock mineralogy however significant heterogeneities exist on the 10 100 m scale four different regions of decomposition products can be distinguished epma 1 shocked biotite rich regions with strong variations in si al k fe and mg on the 10 m scale 2 shocked quartz with about 3 al and k and traces of fe and mg 3 a region with al si 1 which does not correspond to any pre shock mineral and 4 shocked k feldspar showing a significant deficit in k and traces of fe and mg on the nanometer scale a uniform bimodal reaction pattern emerges all minerals are transformed to new crystals embedded in a silica rich glass the shock generated crystals are 1 spinel in shocked biotite 2 quartz cristobalite and coesite in shocked quartz 3 an unknown polymorph of corundum al2o3 in feldspar and in the al si 1 region and 4 mullite in association with sio2 glass in shocked sillimanite grains all crystals range in size between 0 05 and 1 0 m taking into account estimated durations and pressure temperature conditions for shock wave passage and post shock temperatures the reactions observed can be ascribed to high temperatures during and after pressure release the different mineral glass parageneses suggest temperatures in excess of 1400 c moreover given the small size of the newly formed minerals very rapid cooling down to 1200 c is required

further evidence for an early carboniferous 340 ma age of high grade metamorphism in the saxonian granulite complex u pb and pb pb zircon ages for metamorphic zircons from granulites in the saxonian granulite complex are reported using the shrimp ion microprobe conventional multigrain and single gain techniques and the evaporation method this is complemented by a pb pb evaporation age for a post granulite granite emplaced into the schist mantle around the granulites during uplift of the complex we also demonstrate that zircon ages are not reset during high grade metamorphism as commonly argued but have a very high closure temperature and usually preserve the isotopic composition reflecting the time of their formation multifaceted zircons from four granulite samples that probably grew close to the peak of high grade metamorphism yielded identical u pb and pb pb ages of 340 ma which support previously published data and unambiguously show that the granulites formed during a lower carboniferous event and not in the early palaeozoic or precambrian as previously suggested older cores in some of the metamorphic zircons reveal early palaeozoic components at 470 485 ma that we interpret as ages reflecting magmatic crystallization of the granulite precursors one sample suggests an inherited component as old as 1700 ma the post granulite granite has a pb pb evaporation age of 333 1 1 0 ma and the short time interval between granulite metamorphism and granite intrusion implies that uplift crustal extension and cooling of the granulite complex occurred rapidly after peak metamorphic conditions

mixed seawater and hydrothermal sources of nodular chert in middle permian limestone on the eastern paleo tethys margin south china abstract middle permian limestone and marl of the maokou formation deposited along the eastern paleo tethys margin south china host abundant chert nodules that formed in association with the global permian chert event pce an enhanced understanding of the origin of the chert may help elucidate middle permian oceanic and diagenetic conditions however the mechanisms responsible for the formation of chert especially the source of silica remain controversial maokou chert nodules which are made up of quartz and subordinate calcite hematite pyrite and fluorite and rimmed by f bearing sepiolite laminae appear to have formed mainly by the replacement of precursor nodular limestone by silica the nodular limestone may have formed by differential compaction of interbedded limestone and marl after which carbonate was replaced gradually outward from nodule centers by dissolved silica released from the dissolution of siliceous skeletal grains including spicules and radiolarians in host marl during early diagenesis the introduction of high t hydrothermal fluids into ocean water is evidenced by the presence of f bearing minerals fluorite and sepiolite a hybrid rare earth element ree y pattern and a wide range of 30si values 0 4 to 0 7 documented from the maokou nodular chert conservative estimation indicated that middle permian seawater may have had a 30si composition 0 7 to 1 8 similar to that of modern open ocean water suggesting that seawater was the dominant silica source of nodular chert mixing calculations based on ree y geochemistry suggest additional contributions of ca 0 05 0 2 high t hydrothermal fluid and ca 1 10 of continental derived freshwater to ocean water such increased influx of silica to seawater may have been related to the early rifting induced by emeishan volcanism and intensified chemical weathering during an interglacial period which played an important role in promoting the flourishing of siliceous organisms and subsequent chert enrichment during the middle permian

numerical simulation of temperature effects at fissures due to shock loading the localized appearance of specific shock features in target rocks and meteorites such as melt veins and high pressure polymorphs suggests that regions with a local increase in pressure and temperature exist as a shock wave propagates through an inhomogeneous rock in this paper we investigate the effect of planar fissures on the local temperature distribution using numerical simulations time dependent parameters such as temperature pressure and displacement are evaluated the simulation model is based on a shock equation of state for the involved materials dunite and quartzite and simulates geometries that were also used in shock loading experiments an artificial gap between the materials simulates an open fissure at the interface a strong temperature increase occurs at a gap size of 0 1 mm which potentially can cause melting in a thin layer at the interfaces the temperature decreases with decreasing gap size temperature and pressure excursions at the interface are induced by the closure of the gap which causes a second shock wave to superpose the primary wave open fissures and fractures which occur ubiquitously in shallow buried target rocks and projectiles thus act as local pressure and temperature amplyfiers and may be responsible for thin melt vein formation in shocked rocks

oxygen isotope systematics of gem corundum deposits in madagascar relevance for their geological origin the oxygen isotopic composition of gem corundum was measured from 22 deposits and occurrences in madagascar to provide a gemstone geological identification and characterization primary corundum deposits in madagascar are hosted in magmatic syenite and alkali basalt and metamorphic rocks gneiss cordieritite mafic and ultramafic rocks marble and calc silicate rocks in both domains the circulation of fluids especially along shear zones for metamorphic deposits provoked in situ transformation of the corundum host rocks with the formation of metasomatites such as phlogopite sakenite and corundumite secondary deposits placers are the most important economically and are contained in detrital basins and karsts the oxygen isotopic ratios 18o 16o of ruby and sapphire from primary deposits are a good indicator of their geological origin and reveal a wide range of 18o vienna standard mean ocean water between 1 3 and 15 6 metamorphic rubies are defined by two groups of 18o values in the range of 1 7 to 2 9 cordieritite and 3 8 to 6 1 amphibolite magmatic rubies from pyroxenitic xenoliths contained in the alkali basalt of soamiakatra have 18o values ranging between 1 3 and 4 7 sapphires are classified into two main groups with 18o in the range of 4 7 to 9 0 pyroxenite and feldspathic gneiss and 10 7 to 15 6 skarn in marble from andranondambo the 18o values for gem corundum from secondary deposits have a wide spread between 0 3 and 16 5 the ruby and sapphire found in placers linked to alkali basalt environments in the northern and central regions of madagascar have consistent 18o values between 3 5 and 6 9 ruby from the placers of vatomandry and andilamena has 18o values of 5 9 and between 0 5 and 4 0 respectively the placers of the ilakaka area are characterized by a huge variety of colored sapphires and rubies with 18o values between 0 3 and 16 5 and their origin is debated a comparison with oxygen isotope data obtained on gem corundum from eastern africa india and sri lanka is presented giant placer deposits from sri lanka madagascar and tanzania have a large variety of colored sapphires and rubies with a large variation in 18o due to mingling of corundum of different origin mafic and ultramafic rocks for ruby desilicated pegmatites for blue sapphire syenite for yellow green and blue sapphire and skarn in marbles for blue sapphire

a natural shock induced dense polymorph of rutile with α pbo2 structure in the suevite from the ries crater in germany a dense post rutile titanium dioxide tio2 phase was discovered in shocked garnet gneisses from the ries crater by reflected light microscopy laser rnicroraman spectroscopy and micro beam x ray diffraction the raman spectrum consists of nine bands at wave numbers 152 175 285 315 340 358 428 532 and 575 cm 1 these bands are identical to those of the alpha pbo2 structured polymorph of tio2 synthesized in a dynamic laboratory experiment the diffraction pattern of the natural mineral revealed an orthorhombic lattice similar to the alpha pbo2 polymorph with the cell parameters a 4 535 2 angstrom b 5 499 2 angstrom c 4 900 2 angstrom space group phen columbite type structure density rho 4 34g cm 2 where the numbers in parentheses are standard deviations in the last significant digits this new polymorph is 2 denser than rutile the rutile alpha pbo2 polymorph phase transformation occurs exclusively at the grain boundaries between rutile and the shock compacted host biotite and advances inwards in rutile this textural relation establishes phase boundaries as the preferable fabric settings for dynamic induced high pressure phase transitions ab initio calculations negates the formation of the alpha pbo2 polymorph by inversion of a parental fluorite structured polymorph during decompression heating of the experimentally produced alpha pbo2 polymorph above 500 degreesc shows that it inverts back in a laboratory time scale to rutile hence the survival of the alpha pbo2 structured polymorph in naturally shocked rocks constrains the post shock temperature of the tio2 bearing assemblage at an upper bound of 500 degreesc the presence of this dense phase is expected in the earth s upper mantle below 123 km the rutile alpha pbo2 structured polymorph phase transition in subducted crustal limbs in the upper mantle should then be accompanied by considerable changes in the partitioning and fractionation of nb and ta between this tio2 polymorph and the coexisting dense silicates c 2001 elsevier science b v all rights reserved

time of formation and peak of variscan hp ht metamorphism of quartz feldspar rocks in the central erzgebirge saxony germany the variscan erzgebirge represents an antiform with a core of gneisses and mica schists surrounded by a phyllitic mantle the gneiss eclogite unit geu in the central part is a composite tectonometamorphic assemblage characterized by a hp ht imprint and comprises migmatitic para and orthogneisses ht mylonites hp granulites eclogites and garnet peridotites it is tectonically sandwiched between two major units with distinctly lower pt histories the geu experienced a characteristic kinked retrograde pt path after hp ht equilibration with 1 strong near isothermal decompression at high temperatures 2 extensive re equilibration at medium pressures followed 3 by rapid cooling during continued uplift we dated zircons pb pb evaporation from granitoid orthogneisses and metapelites of the geu the orthogneisses contain euhedral long prismatic zircons of igneous origin that provided protolith ages between 470 and 524 ma metapelites retain well preserved granulite facies mineral assemblages and contain spherical multifaceted metamorphic zircons that grew near the peak of hp ht metamorphism inclusions of prograde hp phengite 15 kbar and rutile are included in one such zircon metamorphic zircons of three samples from different localities yielded identical 207pb 206pb ages of 340 5 0 7 ma 341 2 0 5 ma and 341 6 0 5 ma respectively consideration of these zircon ages with published 39ar 40ar white mica ages suggests fast cooling and uplift rates in excess of 50 c ma and 4 km ma this is typical for large scale extensional tectonic unroofing of the ultra deep part of a fossil thickened variscan continental crust 60 km during continuing continental collision and orogenic collapse

shock induced formation of kyanite al2sio5 from sillimanite within a dense metamorphic rock from the ries crater germany a dense 3 34 g cm 3 garnet sillimanite rich metamorphic rock from the suevite breccia of the ries impact crater was studied by scanning electron microscopy and raman microprobe spectroscopy in the strongly shocked rock clast kyanite was formed from sillimanite under momentary high pressures of natural shock waves kyanite aggregates were found as thin 0 3 2 0 m seams along grain boundaries between and fractures within sillimanite grains within these seams kyanite c axes are oriented perpendicular to original grain boundaries and fractures in addition larger up to 10 m isolated kyanite grains were rarely found within host sillimanite filamentary kyanite aggregates and isolated crystals typically show shrinkage cracks due to volume decrease 10 locally broad interstices between sillimanite crystals are filled with aluminosilicate glass containing a high volume fraction of sub micrometer sized euhedral crystals the silica rich glass suggests incongruent melting of sillimanite at local post shock temperatures significantly higher than 1 300 c the edges of adjacent sillimanite grains are thermally and chemically altered the local generation of temperature spikes is attributed to strong shock wave interactions due to very high shock impedance contrasts

40ar 39ar laser probe dating of the central european tektite producing impact event a new 40ar 39ar data set is presented for tektites from the central european strewn field moldavites this is the only strewn field that is entirely situated in a continental environment and still characterized by scattered ages 14 15 3 myr the main objectives of the study were to define more precisely the moldavite formation age and provide a good calibration for a glass standard proposed for fission track dating the laser total fusion ages obtained on chips from 7 individual specimens from the southern bohemian and moravian subfields are restricted to a narrow interval of time with an average of 14 34 0 08 myr relative to the 27 95 0 09 myr of the fish canyon tuff biotite this result gives a more precise age not only for the tektite field but also for its producing impact if the genetic link between the moldavites and the nordlinger ries impact crater is maintained then this new age has to be considered a reliable estimate for the ries crater also this new value places the formation of central european tektites within the lower serravallian period in the latest geologic timescales evidence of their impact products such as glass spherules or shocked minerals can therefore be sought in sedimentary marine formations in a more precisely defined age interval

pbpb dating of apatite by a stepwise dissolution technique abstract apatites from different geologic settings have been dated by a new sequential dissolution method the procedure is based on stepwise exposure of apatite to weak intermediate acids a pegmatite sample from the kerala khondalite belt in southern india yielded an age of 509 25 ma apatites from an enderbite sample from the northern marginal zone of the limpopo belt in southern africa yielded an age of 1970 26 ma which is in agreement with the high grade metamorphism at around 2 0 ga reported from the triangle shear zone and amphibolite facies metamorphism in the transition zone apatites from an amphibolite facies gneiss from the transition zone south of the nmz yielded an age of 1878 18 ma this age is somewhat younger than a sphene age on the same rock of 1971 17 ma and is believed to reflect the time of retrograde metamorphism in the presence of a fluid phase the sequential dissolution method is a new tool that allows dating of apatites which have relatively unradiogenic pb ratios because the spread can be significantly increased and no correction for common lead is needed this study further demonstrates that apatites from magmatic and high grade metamorphic rocks yield ages very close to those of monazite zircon or garnet unless the rock subsequently interacted with a fluid phase

book review the al 2 sio 5 polymorphs derrill m kerrick reviews in mineralogy volume 22 mineralogical society of america 1991 xii 406 p us 20 00 isbn 0 939950 27 8

u pb and sm nd dating of high pressure granulite and upper amphibolite facies rocks from sw sweden abstract geochronological data from different radiometric systems on high grade metamorphic rocks in the southwest swedish granulite region southwestern sweden are generally analytically precise but sometimes in conflict with each other age data from seven samples on which a variety of radiometric methods have been applied are presented the age of peak metamorphism is determined by metamorphic zircons from two garnet amphibolites at c 975 ma using the conventional u pb isotope and pb pb evaporation methods titanites from the same type of rocks give ages of c 923 3 ma and 945 2 ma the sm nd data on mafic granulites and garnet amphibolites yield surprisingly young ages between 945 and 900 ma similar to previous sm nd ages these ages are comparable to or even younger than the 40 ar 39 ar hornblende ages that range between 1030 and 930 ma assuming that the sm nd systematics are entirely controlled by volume diffusion the closure temperature of the sm nd system in minerals from these rocks would be comparable to or lower than that of ar diffusion in hornblende i e around 500 c

cadomian lower crustal contributions to variscan granite petrogenesis south bohemian pluton austria constraints from zircon typology and geochronology whole rock and feldspar pb sr isotope systematics zircons show a mean pb pb evaporation age of 355 a hybrid pyroxene bearing weinsberg type granitoid of the south 9 ma interpreted as the age of zircon growth during a carboniferous bohemian batholith austria consists of two independent mineral partial melting event in the lower crust granite emplacement at assemblages that were formed during two different magmatic events 345 5 ma is inferred from u pb analysis of the anhedral the older inherited assemblage forms unevenly distributed millimetrezircon population the comparably low radiogenic common pb sized multi grain patches of quartz mesoperthitic alkali feldspar isotope composition of megacrystic alkali feldspars suggests that at andesine bytownite clinopyroxene xmg 0 50 0 54 least some domains of these crystals are inherited from the older orthopyroxene xmg 0 35 0 42 ilmenite accessories pyroxene bearing mineral assemblage rb sr whole rock dating is it is interpreted to represent remnants of a mangeritic igneous thus severely jeopardized by the presence of the inherited alkali rock with a superimposed granulite facies re equilibration texture feldspar crystals leading to widely scattering data points and characterized by unzoned pyroxenes and plagioclase the enclosing errorchron ages of no geological significance younger assemblage with alkali feldspar oligoclase andesine quartz biotite accessories crystallized from a biotite bearing granitic melt with feldspars exhibiting typical magmatic zoning

zircon ages of the metavolcanic rocks and metagranites of the ollo de sapo domain in central spain implications for the neoproterozoic to early palaeozoic evolution of iberia dating the pre middle ordovician metavolcanic rocks and metagranites of the ollo de sapo domain has historically been difficult because of the small compositional variation the effects of the variscan orogeny and as revealed in this paper the unusually high fraction of inherited zircon components the first reliable zircon data u pb ion microprobe and pb pb stepwise evaporation indicate that the ollo de sapo volcanism spanned 495 5 ma to 483 3 ma and was followed by the intrusion of high level granites from 483 3 ma to 474 4 ma in both metavolcanic rocks and metagranites no less than 70 80 of zircon grains are either totally precambrian or contain a precambrian core overgrown by a cambro ordovician rim about 80 90 of inherited zircons are early ediacaran 602 614 ma and derived from calc alkaline intermediate to felsic igneous rocks generated at the end of the pan african arc continent collision in the villadepera region located to the west both the metagranites and metavolcanic rocks also contain meso archaean zircons 3 0 3 2 ga which ultimately originated from the west african craton in the hiendelaencina region located to the east both the metagranites and metavolcanic rocks lack meso archaean zircons but they have two different inherited zircon populations one cryogenian 650 700 ma and the other tonian 850 900 ma which suggest older than ediacaran additional island arc components the different proportion of source components and the marked variation of the 87 sr 86 sr init suggest at least tentatively that the across arc polarity of the remnants of the pan african arc of iberia trended east west with respect to the current coordinates during cambro ordovician times and that the passive margin was situated to the west

two distinctive granite suites in the sw bohemian massif and their record of emplacement constraints from geochemistry and zircon 207pb 206pb chronology

shock veins in the central uplift of the manicouagan impact structure context and genesis we describe the development of shock veins that penetrate the anorthositic central uplift of the manicouagan impact structure they occur as thin 2 5 mm wide linear micro fault systems that can be traced for several meters in length and which predominantly trend radially from the point of impact the shock veins are distinguished by the development of maskelynite along vein margins and stishovite in vein matrices which are otherwise absent in non veined regions these phases define a shock excursion of up to 30 gpa in contrast to bulk shock effects of 12 gpa defined by development of shatter cones planar fractures and planar deformation features in various minerals the shock veins at manicouagan share many similarities with vein systems developed in meteorites they also provide an in situ context with which to better understand meteoroid source and lofting conditions in addition to containing high pressure phases the shock veins exhibit evidence for high temperature partial melting of host silicate clasts with the generation of flow textured fragments and glasses the formation of microcrystallites and dendrites from some melts indicates rapid cooling we propose a two stage generation mechanism comprising an initial high pressure shock excursion estimated to last 0 5 s based on projectile size considerations followed by a longer high temperature pulse of a few seconds duration we suggest that the shock excursion is initiated by target heterogeneities that cause distortions in the hemispherically propagating shock front this results in radially oriented tearing and vein formation with shock amplification occurring via intra vein shock reverberation high speed displacement along the veins is driven by stress release on rarefaction which results in frictional melting via adiabatic heating

age and evolution of a late archean to paleoproterozoic upper to lower crustal section in the wutaishan hengshan fuping terrain of northern china abstract the taihangshan wutaishan area forms part of the central north china craton and consists of three main components the hengshan and fuping complexes containing predominantly ductilely deformed late archean to paleoproterozoic high grade granitoid orthogneisses intruded by mafic dykes of gabbroic composition and the low grade late archean wutai greenstone belt developed between the high grade terrains and consisting of bimodal volcanic rocks and metasediments associated with coeval granitoids zircon dating shows that both the hengshan and fuping complexes were intruded by major granitoid bodies between 2 52 and 2 48 ga with rare occurrences of 2 7 ga gneisses wutai granitoids adjacent to the fuping complex also contain 2 7 ga xenocrysts all possibly reflecting the same older basement rare granitic gneisses in the hengshan have emplacement ages of 2 35 ga and show the same structural features as the older rocks indicating that the main deformation occurred after 2 3 ga deposition of supracrustal rocks and emplacement of mafic dykes occurred in the paleoproterozoic together with intrusion of granitic bodies at 2 1 2 0 ga and all underwent regional metamorphism at 1 88 1 85 ga we interpret the hengshan and fuping granitoid gneisses as the lower plutonic part of a late archean to early paleoproterozoic japan type magmatic arc with the upper volcanic part represented by the wutai complex components of this arc may have evolved at a continental margin as indicated by the 2 7 ga zircons virtually all magmatic events are recorded in all three complexes however the 1 85 ga metamorphic event is only recorded in the higher grade portions of the wutai since low grade and presumably high crustal level precluded metamorphic zircon growth shear zones in the hengshan are interpreted as major lower crustal discontinuities post dating the peak of high pressure granulite metamorphism we suggest that they formed during orogenic collapse and uplift of the hengshan complex in the late paleoproterozoic 1800 ma

pb pb and u pb zircon dating of subvolcanic rhyolite as a time marker for hercynian granite magmatism and sn mineralisation in the eibenstock granite erzgebirge germany considering effects of zircon alteration tin mineralisation is closely related to rhyolite stocks and dykes which occur in the endo and exocontact of the eibenstock granite erzgebirge germany the same structures which cut the granite control the rhyolite emplacement and the location of ore bearing greisen bodies albitisation and greisenisation related to tin mineralisation as well as sericitisation and argillic alteration may be traced by changes in chemical and mineralogical composition of both rhyolite and granite wall rock comprehensive zircon studies by scanning electron microscopy secondary and backscattered electron as well as cathodoluminescence imaging edx measurements reveal that zircon from rhyolite and from the enclosing granite shows significant changes in chemical composition and crystallinity including distortion of the u pb isotope system when affected by greisenisation single evaporation analysis of zircon from rhyolite with little or no greisenisation gave a 207pb 206pb age of 290 5 ma whereas zircon from little altered granite gave a 207pb 206pb age of 320 8 ma the single evaporation age of zircon from rhyolite is confirmed by 238u 206pb shrimp ages of 297 8 ma the significant time gap of at least 20 ma between granite intrusion and rhyolite formation suggests that the late magmatic evolution of the eibenstock granite cannot be regarded as a source for tin ore forming fluids as previously assumed

chronological constraints on the pre orogenic history burial and exhumation of deep seated rocks along the eastern margin of the variscan orogen bohemian massif czech republic key lithological units of the high grade eastern margin of the bohemian massif were dated using the u pb and pb pb methods on zircons in order to establish a chronological framework for the geodynamic evolution of the variscan orogenic root the protolith ages for metagranitoids orthogneisses and granulites of thickened lower and middle crust reveal the existence of magmatic activity that occurred over a 100 million year time interval from cambro ordovician to early devonian times probably related to discontinuous intracontinental rifting of neoproterozoic crust our geochronological data suggest that the eastern part of the orogenic root represents thermally softened and rifted neoproterozoic crust preserved farther to the east as the brunia microcontinent zircon ages for felsic granulites high grade gneisses of the lower crust and of a syn convergence granodioritic intrusion in the upper crust indicate that thickening and exhumation of the crust occurred during a narrow time interval between 370 and 340 ma exhumation of the lower crust to mid crustal levels was a localized process that occurred at 340 ma and was associated with crustal scale folding in the internal part of the root as well as orogenic channel flow along the eastern collisional margin both types of exhumation mechanisms were driven by deep level wedging indentation of the easterly brunia continent followed by deposition of heavy minerals and pebbles derived from high pressure rocks in the adjacent foreland basin final orogenic development was characterized by ne sw dextral transpressive shearing parallel to the brunia margin as well as dextral trans tension associated with activity along the elbe lithospheric fault these processes affected the marginal parts of the orogenic root and were accompanied by 330 to 325 ma old syntectonic granitoid intrusions along reactivated lithotectonic boundaries rotation of the assembled orogenic belt accompanied by lithospheric faulting driven by westerly subduction roll back may be the most plausible model to explain late deformation of the orogenic root

origin of the high variability of water mineral content in the bedrock aquifers of southern madagascar this study assesses the causes of the high spatial variability of the mineral content of groundwater in crystalline bedrock of southern madagascar although many kilometres from the coast and at a mean altitude of 400 m a s l wells drilled in this area produce water with electrical conductivities in the range of 300 30 000 mu s cm 1 with a high spatial variability chemical and isotopic data are used to identify the processes involved in the groundwater mineralization it is shown that the chemical composition of the groundwater in this region has its origin in i normal silicate and carbonate weathering reactions and ii input of marine salts probably via rainfall recharge modified by evapo concentrative processes probably including precipitation and re dissolution of secondary evaporites in the unsaturated zone to obtain a better understanding of the spatial salinity distribution well parameters such as yields weathered zone thickness weathered materials and morphological positions upper slope mid slope lower slope or valley bottom are scrutinized a correlation was found between high salinity and low flow shallow groundwater environments flat hill tops valley bottoms weakly developed and clayey weathered zones and between low salinity and high flow environments granular well developed weathered zones and situation on valley slopes

importance of late magmatic and hydrothermal fluids on the sm nd isotope mineral systematics of hypersolvus granites abstract despite the significant impact of the sm nd radiometric system in various fields of isotope geochemistry very few attempts have been made to use it at the mineral scale in granite studies this approach has been used on two granites from the anorogenic province of corsica se france two statistically acceptable alignments were obtained in an isochron diagram for the hypersolvus fayalite bearing granite of mantelluccio alkali feldspar amphibole inclusion free zircon fluorite and fergusonite give an age of 331 10 ma whereas allanite whole rock amphibole and inclusion free zircon yield a significantly younger age at 291 13 ma the latter is validated by a zircon u pb age of 283 1 ma as the magmatic emplacement date of the granite the other alignment giving a 50 ma older age can be interpreted either as being inherited or more likely as the result of hydrothermal convection interacting with the country rocks at the end of granite crystallization the sm nd systematics of epidote records a second fluid rock event which probably occurred at least 200 ma after the emplacement of the granite extremely low 143 nd 144 nd ratio indicates that the epidote crystallized from a fluid that carried lanthanides in the crust over a distance of at least 1 km zircons with inclusions have a lower initial 143 nd 144 nd ratio than inclusion free zircons this feature is attributed to apatite and or zircon core inheritance through contamination during the ascent of the granitic melt in the crust two isochrons have been also obtained for the hypersolvus peralkaline granite of evisa amphibole fluorite sphene and fergusonite yield an age of 259 6 ma whereas the whole rock alkali feldspar and metamict zircon give an age at 209 14 ma in this case the former value is roughly in agreement with a possibly slightly rejuvenated whole rock rb sr date of 249 3 ma suggesting that it documents the magmatic crystallization age of the granite sm nd isotope systematics of the alkali feldspar metamict zircon and possibly allanite were re equilibrated during an important hydrothermal event that affected western europe in early jurassic times in both examples however it appears that the whole rock nd isotopic signatures were not strongly affected by the various fluid rock events indicating that they can still be used for petrogenetic investigations in contrast these results show that like k ar and rb sr the sm nd systematics of minerals with weak resistance to post magmatic processes can be used to trace and date hydrothermal events

interpreting titanite u pb dates and zr thermobarometry in high grade rocks empirical constraints on elemental diffusivities of pb al fe zr nb and ce length scales of compositional heterogeneity in titanite from 750 to 1000 c metamorphic rocks from southern madagascar were measured to provide empirical constraints on elemental diffusivities the calculated pb diffusivity is comparable to experimental estimates of sr diffusivity because of this u pb dates from rocks that reached peak temperatures 900 c thus zr in titanite thermobarometry should not be reset by diffusion in all but the smallest grains in the hottest rocks al and nb diffuse at similar rates to zr ce and fe diffuse slower than pb but faster than zr differences in empirical and experimental estimates of elemental diffusivities might be related to the complexity of most natural titanite solid solutions compared to the near end member titanite used in experiments

applications of single grain zircon evaporation analyses to detrital grain studies and age discrimination in igneous suites abstract 207 pb 206 pb ages determined by evaporation of single zircon grains from rocks of the new england appalachians in the usa permit an assessment of the applicability of this method to rocks that have been strongly disturbed by orogenesis tests conducted on highly metamorphosed and deformed orthogneisses of known protolith age whose u pb upper intercept zircon ages obtained through isotope dilution methods show profound post crystallization discordance demonstrate that the evaporation method is successful in recovering protolith ages uncertainties are 10 15 my for rocks of paleozoic age or older preliminary results suggest that the method is particularly useful for provenance studies of sedimentary rocks and in the discrimination among groups of rocks that are known to differ in age but are otherwise difficult to separate on the basis of field or petrographic characteristics 207 pb 206 pb ages of detrital zircon grains from two metasedimentary units from opposite sides of the new england appalachians reveal the distinct provenances of these rocks the missisquoi formation in southeastern vermont usa is comprised in part of detritus derived from the 1 0 1 2 ga grenvillian province of laurentia whereas this component is conspicuously absent from the plainfield formation in easternmost connecticut the detrital zircon age data suggest that the missisquoi formation accumulated peripheral to the laurentian craton during the early paleozoic whereas the plainfield formation detritus was supplied by non laurentian ossibly west african sources during the late proterozoic or early paleozoic in southern vermont the evaporation method successfully distinguishes between two groups of augen gneisses with ages of 960 ma and 1120 ma which were first recognized through isotope dilution methods collectively these results indicate that the dating of single zircon grains by the evaporation method can be widely exploited in the study of complexly deformed and strongly metamorphosed rocks

geochemistry zircon ages and whole rock nd isotopic systematics for palaeoproterozoic a type granitoids in the northern part of the delhi belt rajasthan nw india implications for late palaeoproterozoic crustal evolution of the aravalli craton determination of zircon ages as well as geochemical and sm nd isotope systematics of granitoids in the khetri copper belt of the aravalli mountains nw india constrain the late palaeoproterozoic crustal evolution of the aravalli craton the plutons are typical a type within plate granites derived from melts generated in an extensional tectonic environment they display ree and multi element patterns characterized by steep lree enriched and almost flat hree profiles and distinct negative anomalies for sr p and ti initial e nd values range from 1 3 to 6 2 and correspond to crustal sources with mean crustal residence ages of 2 5 to 2 1 ga a lower mafic crustal anatectic origin is envisaged for these granitoids and the heterogeneous e nd t values are inferred to have been acquired from the magma source region zircon pb pb evaporation and u pb ages indicate widespread rift related a type magmatism at 1711 1660 ma in the northern delhi belt and also suggest a discrete older magmatic event at around 1800 ma the emplacement ages of the compositionally distinct a type granitoid plutons and virtually coeval granulite metamorphism and exhumation in another segment of the aravalli mountains further signify that part of the aravalli crust evolved during a widespread extensional event in late palaeoproterozoic time

age constraints on late paleozoic evolution of continental crust from electron microprobe dating of monazite in the peloritani mountains southern italy another example of resetting of monazite ages in high grade rocks in situ monazite microprobe dating has been performed for the first time on trondhjemite and amphibolite facies metasediments from the peloritani mountains in order to obtain information about the age of metamorphism and intrusive magmatism within this still poorly known sector of the hercynian belt all samples show single stage monazite growth of hercynian age one migmatite and one biotitic paragneiss yielded monazite ages of 311 4 and 298 6 ma respectively these ages fit with previous age determinations in similar rocks from southern calabria indicating a thermal metamorphic peak at about 300 ma at the same time as widespread granitoid magmatism the older of the two ages might represent a slightly earlier event possibly associated with the emplacement of an adjacent trondhjemite pluton previously dated by shrimp at 314 ma no evidence for pre hercynian events and only a little indication for some monazite crystallization starting from ca 360 ma were obtained from monazite dating of the metasediments suggesting either a single stage metamorphic evolution or a significant resetting of the monazite isotope system during the main hercynian event ca 300 ma rare monazite from a trondhjemite sample yields evidence for a late hercynian age of about 275 ma this age is interpreted as representing a post magmatic stage of metasomatic monazite crystallization which significantly postdates the emplacement of the original magmatic body

in situ discovery of shock induced graphite diamond phase transition in gneisses from the ries crater germany reflected light microscopy and fine scale lasts microraman spectroscopy of shocked garnet cordierite sillimanite gneisses in suevites of the ries meteorite impact crater germany led to the discovery of impact diamonds in their pristine graphite diamond assemblages graphite diamond textural relations permit a clear determination of the solid state nature of the formation of diamond from graphite which is estimated to have occurred at a peak shock pressure between 30 and 40 gpa shock induced transformations were promoted only in unkinked and undeformed graphite booklets at the graphite garnet graphite sillimanite or graphite rutile interfaces where the difference in shock impedance is very high reverberations of shock waves with short wavelengths similar to the grain sizes at the phase boundaries are probably important constraints for dynamic graphite diamond phase transformation raman spectroscopic investigations of hard transparent carbon platelets intercalated between fine grained diamond and deformed graphite revealed the platelets to be raman inactive the platelets are either dense amorphous carbon or an unknown dense crystalline carbon phase that is raman inactive

zircon geochronology and metamorphic evolution of mafic dykes in the hengshan complex of northern china evidence for late palaeoproterozoic extension and subsequent high pressure metamorphism in the north china craton abstract magmatic and metamorphic zircons have been dated from ductilely deformed gabbroic dykes defining a dyke swarm and signifying crustal extension in the northern part of the hengshan complex of the north china craton these dykes now occur as boudins and deformed sheets within migmatitic tonalitic trondhjemitic granodioritic and granitic gneisses and are conspicuous due to relics of high pressure granulite or even former eclogite facies garnet pyroxene bearing assemblages shrimp ages for magmatic zircons from two dykes reflect the time of dyke emplacement at 1915 ma whereas metamorphic zircons dated by both shrimp and evaporation techniques are consistently in the range 1848 1888 ma the youngest granitoid gneiss yet dated in the hengshan has an emplacement age of 1872 17 ma these results complement recent geochronological studies from the neighbouring wutai and fuping complexes to the se of the hengshan showing that a crustal extension event occurred in the late palaeoproterozoic this preceded a major high pressure collision type metamorphic event in the central part of the north china craton that occurred in the palaeoproterozoic and not in the late archaean as previously thought our data support recent suggestions that the north china craton experienced a major craton wide orogenic event in the late palaeoproterozoic after which it became cratonized and acted as a stable block

experimental generation of shock induced pseudotachylites along lithological interfaces to understand the mechanism of formation of shock induced pseudotachylites and particularly the role that rock heterogeneities and interfaces play in their formation shock recovery experiments were carried out on samples consisting of two distinct lithologies dunite and quartzite it was possible to generate melt veins of 1 6 m width along lithological interfaces at moderate shock pressures 6 to 34 gpa the magnitudes of displacement along the interface strain rate and the kinetic heat production indicate that friction is an important heat source that largely contributes to the energy budget of the melt veins the experimentally produced veins resemble natural s type pseudotachylites the geometry of the veins depends on the orientation of the interface with respect to the shock front and includes strong variations in thickness formation of melt pockets and injection veins sudden changes in vein orientation and sharp vein margins two types of melt were observed vesicle free and vesicular melts dense vesicle free melt rock is likely to represent high pressure melts vesicular melts were also generated during shock compression but they remained in a molten state during pressure release and continued shearing intermingling of comminuted olivine and melt suggests that ultracataclasis of olivine induced by a dynamic tensile failure is a precursor stage to frictional melting shock wave interferences at the lithological interface provide the necessary stress conditions to start dynamic failure of olivine the composition of the frictional melts ranges from olivine normative to enstatite normative and is thus largely determined by olivine melting the validity of the sequence of friction melting susceptibilities of rock forming minerals inferred from tectonically produced pseudotachylites is confirmed and can now be applied to ultra high strain rates during shock compression

in situ observation of the decomposition of kyanite at high pressures and high temperatures in situ observations of the decomposition of kyanite al 2 sio 5 were carried out in a multi anvil high pressure apparatus using synchrotron radiation where the phase change from kyanite to stishovite corundum was observed at high pressures and high temperatures the phase boundary of this decomposition at t 1200 1900 k and p 5 15 gpa was determined to be p gpa 10 2 0 0016 t k previous studies using the quench method showed a discrepancy in the transition pressure of the decomposition our results using in situ observations resolve this dispute

landslide inventory in a rugged forested watershed a comparison between air photo and field survey data abstract landslide inventories are routinely compiled by means of aerial photo interpretation api when examining photo pairs the forest canopy notably in old growth forest hides a population of not visible landslides in the present study we attempt to estimate how important is the contribution of landslides not detectable from aerial photographs to the global mass of sediment production from mass failures on forested terrain of the capilano basin coastal british columbia api was coupled with intensive fieldwork for identification and measurement of all landslides a 30 year framework was adopted we show that not visible landslides can represent up to 85 of the total number of failures and account for 30 of the volume of debris mobilised such percentages display high sub basin variability with rates of sediment production varying by one order of magnitude between two sub basins of the study area this is explained qualitatively by gis based analysis of slope frequency distributions drainage density and spatial distribution of surficial materials such observations find further support in the definitions of transport limited and supply limited basins as a practical consideration to land managers we envisage that supplementary fieldwork for landslide identification is mandatory in transport limited systems only fieldwork has demonstrated that gully related failures have a greater importance than one could expect from api

id tims using the step wise dissolution technique versus ion microprobe u pb dating of metamict archean zircons from ne madagascar abstract a geochronological study using the step wise dissolution technique and in situ ion microprobe analysis is applied to late archean metamict zircons of granitoids from northeastern madagascar basement both techniques provide concordant u pb ages additionally the ion microprobe study provided in situ measurements on the pb isotopic composition of the metamict domains finally pb pb evaporation ages were found to be slightly but systematically younger than the dating obtained by the two other methods based on these data granites from the antongil belt were emplaced during at least three episodes separated by periods of magmatic inactivity of about 10 ma at 2513 2523 and 2532 ma respectively these results are consistent with previous whole rock rb sr and u pb zircon dating from this part of madagascar the occurrence of this late archean basement in northeastern madagascar can be related to southern india where large volumes of contemporaneous granitoids were emplaced

suevite breccia of the ries impact crater germany petrography chemistry and shock metamorphism of crystalline rock clasts clasts of deep seated crystalline basement rocks in suevites of the ries crater germany were catalogued lithologically and classified with regard to their degree of shock metamorphism the sample suite consisted of 806 clasts from 10 outcrops in fallout suevites and 447 clasts from drill cores encountering crater suevite in the crater interior these clasts can be grouped into seven types of metamorphic and nine types of igneous rocks one hundred forty three clasts representing these lithologies were analyzed for major element bulk composition the fallout suevite contains on average 4 vol of crystalline basement clasts 0 4 vol of sedimentary rocks 16 vol of glass bodies some of them aerodynamically shaped and 79 vol of groundmass on average 52 of all crystalline clasts are from metamorphic sources and 42 are of igneous origin using the shock classification of stoffler 1974 8 of all crystalline clasts appear unshocked 10 gpa and 34 30 and 27 of clasts are shocked to stages i 10 35 gpa ii 35 45 gpa and iii 45 60 gpa respectively the bulk composition of suevite glasses is consistent with the modal proportions of crystalline rock types observed in the clast populations this indicates that the glasses originate by shock fusion of a similarly composed basement the crater suevite contains the same crystalline rock types that occur in the fallout suevites the bore hole nordlingen 1973 yields an average of 62 vol metamorphic and 38 vol igneous rocks the crater suevite differs from fallout suevites by a higher clast glass ratio by preponderance 65 95 of clasts shocked to stage i only and by the absence of aerodynamically shaped glass bodies the source of crystalline clasts and melt particles of suevites is a volume of rocks located deep in the crystalline basement to which the projectile transmittted most of its energy so that only rocks of the basement were shocked by pressures exceeding 10 gpa deep burst impact model fallout suevites were ejected propelled by an expanding plume of vaporized rock and withdrew preferentially from this volume melt and highly shocked clasts leaving in the transient cavity the crater suevite with more clasts of modest shock levels and less melt

juxtaposition of india and madagascar a perspective abstract proterozoic terrains in south india and madagascar provide important clues in understanding the gondwanaland tectonics especially the assembly of this mega continent during the pan african period the archaean terrains in both madagascar and india are characterized by n s trending greenstone belts occurring within gneissose granitic rocks in the northern part extensive development of k rich granitic rocks of ca 2 5 ga is also characteristic in both areas such a broad age zonation of younger dharwar ca 2 6 3 0 ga in the north and the older sargur ca 3 0 3 4 ga in the south as in south india remains to be identified in future studies from madagascar the occurrence of greenschist facies rocks in the northeastern part and higher grade rocks in most of other parts in the north central terrain of madagascar is comparable with the general tendency of increasing metamorphic grade from northwestern to southern areas ranging from greenschist to granulite facies in south india the proterozoic crystalline rocks in both continents show pronounced lithological similarity with the wide occurrence of graphite bearing khondalite in association with charnockitic rocks while the archaean proterozoic boundary is well defined in southern india by the palghat cauvery or the kkpt shear zones as recently identified this boundary is ill defined in madagascar due to extensive pan african overprinting as well as the development of the proterozoic cover sequence the itremo group there is also a possible general correlation between the mesoproterozoic cover sequences in madagascar and india such as between the itremo group of west central madagascar and the kaladgi and cuddapah sequences of south india the pan african granulite facies metamorphism of ca 0 5 ga extensively developed in both india and madagascar is generally comparable in intensity and extent p t conditions and p t t paths also appear comparable with the general range of ca 700 1000 c and 6 9 kb and near isothermal decompressional paths a type granite plutons and alkaline rocks including anorthosites and mafic plutonic rocks of ca 500 800 ma develop in both terrains provide strong basis for the correlation of both terrains and define a pan african igneous province within east gondwanaland major shear zones in both continents are expected to play a critical role in the correlation albeit are still poorly constrained detailed elucidation of the tectonic history of the shear zones and the timing of various events along the shear zones would provide important constraints on the correlation of the two continental fragments

coeval argon 40 argon 39 ages of moldavites from the bohemian and lusatian strewn fields 40ar 39ar ages of four tektites moldavites from southern bohemia near cesk budejovice czech republic and a tektite from lusatia near dresden germany have been determined by 11 step degassing experiments the purpose of the study was to enlarge the 40ar 39ar data base of moldavites and to check the age relations of the bohemian and lusatian samples the mean plateau age of the bohemian samples which range from 14 42 to 14 70 ma is 14 50 0 16 0 42 2 ma errors in parentheses include age error and uncertainty of standard monitor age the plateau age of the lusatian sample of 14 38 0 26 0 44 2 ma confirms the previously published 40ar 39ar age of 14 52 0 08 0 40 2 ma and demonstrates that the fall of lusatian and bohemian tektites were contemporaneous because of their geochemistry and their ages there is no doubt that the lusatian tektites are moldavites accepting that moldavites are ejecta from the nrdlinger ries impact the new ages also date the impact event this age is slightly younger about 0 2 0 3 ma than the age suggested by earlier k ar determinations

whole grain evaporation for 207 pb 206 pb age investigations on single zircons using a double filament thermal ion source a technique has been developed and tested to analyse 207pb 206pb apparent ages by thermal evaporation of radiogenic lead directly from untreated whole zircon grains 0 3 mm the evaporation analyses are performed in the double filament arrangement of a thermal ion mass spectrometer thims the method is a powerful tool to distinguish between different lead components occurring in the same grain because differing activation energies of the competing lead components cause their sequential evaporation from the zircons the evaporation of test samples results in 207pb 206pb apparent ages in good agreement with u pb ages known from literature single zircons from a granite of the marble mountains california yield an age of crystallization of 1 410 30 ma ceylon zircons from heavy mineral bearing gravels yield 560 40 ma as age of crystallization of the pegmatitic gravel sources individuals from a heterogeneous zircon population of a diatexite from the southern schwarzwald sw germany indicate metamorphic zircon formation around 500 ma and the existence of middle proterozoic relics 1 95 0 05 ga

deformed a type granites in northern malawi east central africa pre or syntectonic metamorphosed and deformed a type granites ranging in age from 1087 to 1119 ma occur in strike slip and reverse slip shear zones of the ubendian and irumide belts of northern malawi peak metamorphic conditions in the granites and their wallrocks reached about 12 13 kbar and 680 740 c microstructures reveal that the granites experienced a seemingly progressive deformation from magmatic to solid state magmatic deformation is manifested by the coexistence of aligned magmatic potassium feldspar plagioclase amphibole biotite and ilmenite with undeformed quartz and schlieren structures high temperature solid state deformation is characterized by widespread grain boundary migration dynamic recrystallization and myrmekitic potassium feldspar replacement the tectonic foliation is generally subparallel to the magmatic foliation in the granites and also subparallel to the borders of the plutons and to the foliation in the wallrock kinematic indicators in both granite and wallrock reveal sinistral movement in the ubendian belt shear zones and top to the ese thrusting in the irumide belt shear zones the overall parallelism of magmatic and tectonic foliations would suggest syntectonic emplacement of the granites with respect to a c 1100 ma irumide orogeny however similar deformation patterns of granite related dykes and some pan african pegmatites and geochronologic work suggests that the first deformation of the granites their related dykes and some of the pegmatites occurred during the pan african orogeny this study implies that a purely structural approach to distinguish syn and pretectonic granites is rendered difficult when post emplacement deformation occurred under high grade metamorphic conditions strongly heterogeneous deformation with a pronounced degree of coaxial flattening in the granites allows magmatic structures to be preserved in low strain zones and to be passively rotated into parallelism with penetrative deformation structures our work also implies that there was no irumide orogen in northern malawi

constraints on the role of impact heating and melting in asteroids received 1996 august 30 accepted in revised form 1997 january 30 abstract imaging of asteroids gaspra and ida and laboratory studies of asteroidal meteorites show that im pacts undoubtedly played an important role in the histories of asteroids and resulted in shock metamorphism and the formation of breccias and melt rocks however in recent years impact has also been called upon by numerous authors as the heat source for some of the major geological processes that took place on asteroids such as global thermal metamorphism of chondrite parent bodies and a variety of melting and igneous events the latter were proposed to explain the origin of ureilites aubrites mesosiderites the eagle station pallasites acapulcoites lodranites and the iab iiicd and iie irons we considered fundamental observations from terrestrial impact craters combined with results from laboratory shock experiments and theoretical consider ations to evaluate the efficiency of impact heating and melting of asteroids studies of terrestrial impact craters and relevant shock experiments suggest that impact heating of aster oids will produce two types of impact melts 1 large scale whole rock melts total melts not partial melts at high shock pressure and 2 localized melts formed at the scale of the mineral constituents mineral specific or grain boundary melting at intermediate shock pressures the localized melts form minuscule amounts of melt that quench and solidify in sifu thus preventing them from pooling into larger melt bodies partial melting as defined in petrology has not been observed in natural and experimental shock metamor phism and is thermodynamically impossible in a shock wave induced transient compression of rocks the total impact melts produced represent a minuscule portion of the displaced rock volume of the parent crater internal differentiation by fractional crystallization is absent in impact melt sheets of craters of sizes that can be tolerated by asteroids and impact melt rocks are usually clast laden thermal metamorphism of country rocks by impact is extremely minor experimental and theoretical considerations suggest that 1 single dis ruptive impacts cannot raise the average global temperature of strength or gravity dominated asteroids by more than a few degrees 2 cumulative global heating of asteroids by multiple impacts is ineffective for asteroids less than a few hundred kilometers in diameter 3 small crater size low gravity and low impact velocity suggest that impact melt volume in single asteroidal impacts is a very small 0 014 1 fraction of the total displaced crater volume 4 total impact melt volume formed during the typical lifetime of an asteroid is a small fraction 0 001 of the volume of impact generated debris and 5 much of the impact melt generated on asteroidal targets is ejected from craters with velocities greater than escape velocity and thus not retained on the asteroid the inescapable conclusion from these observations and calculations is that impacts cannot have been the heat source for the origin of the meteorite types listed above and we must turn to processes other than impact such as decay of short lived radionuclides or electromagnetic induction during an early t tauri phase of the sun to explain heating and melting of the parent bodies of these meteorites

unraveling sedimentary provenance and tectonothermal history of high temperature metapelites using zircon and monazite chemistry a case study from the eastern ghats belt india abstract the geochemical behavior of detrital zircon and monazite during granulite facies anatexis in metapelites from the eastern ghats belt egb india is explored using u pb geochronology hf isotopes and trace elements in a metapelite from the ongole domain detrital zircon reequilibrated by coupled dissolution reprecipitation and diffusion reaction during ultrahigh temperature metamorphism at 1 63 ga the event completely reset the u pb systems but hf isotopes and trace elements were only partially reequilibrated overgrowths on the altered cores date migmatization at 1 61 ga monazite yields metamorphic ages similar to those of zircon in metapelites from the eastern ghats province egp detrital zircon grains give 2 44 1 40 ga ages and metamorphic ones 1 2 0 5 ga ages metamorphic components include detrital grains reequilibrated by coupled dissolution reprecipitation in the presence of anatectic melt and newly crystallized overgrowths and grains in reequilibrated domains the u pb system wa

shock metamorphism in sillimanite from the haughton impact structure devon island canada clasts of shocked garnet sillimanite gneisses comprise a minor fraction of the allochthonous breccia at the haughton impact structure refractive indices of the diaplectic and fused components of the gneisses and reduced specific gravity indicate shock pressures from 35 to 55 5 gpa and effective post shock temperatures from 500 to 1 000 c in a suite of selected samples

conventional u pb dating versus single grain pb evaporation dating of complex zircons from a pegmatite in the high grade gneisses of southwestern sweden u pb multi grain dating of zircons from a pegmatite dyke in southwestern sweden yield analyses which define a discordia with upper and lower intercept ages of 1510 32 ma and 631 64 ma respectively dating by the single zircon evaporation method on prismatic zircons from the same dyke indicates a primary crystallization age of 1409 20 ma in addition four oval zircon grains yield a 207pb206pb age of 969 27 ma the older single zircon evaporation age is interpreted as representative of pegmatite emplacement whereas the younger age is suggested to correspond to isotopic disturbance and new zircon growth during a late sveconorwegian metamorphic event to explain the false intercept ages in the u pb concordia diagram this study suggests a three stage model including 1 crystallization of zircons during pegmatite intrusion 2 episodic lead loss and new zircon growth during high grade metamorphism and 3 a recent lead loss event as the dykes are strongly deformed the protolith age of the pegmatite provides an upper age limit for the gneiss forming event the single zircon age of 969 27 ma is interpreted to be associated with this event which took place during high grade metamorphic conditions

logging and turbidity in the coastal watersheds of northern california continuous turbidity data for the 2004 2005 winter runoff seasons were used to assess water quality characteristics in 28 coastal watersheds in northern california turbidity probes collected data during the winter period typically spanning the months of october may stream biota such as salmonids suffer not only from turbidity extremes but also from chronic turbidity we used turbidity at the 10 exceedence level to index chronic turbidity in the 28 streams watersheds draining to the streams spanned disturbance categories from pristine redwood forest to intensive commercial timber harvest grouping the sites by timber harvest history showed that the pristine unharvested or background group mean was 8 fnu formazin nephelometric units at the 10 exceedence level in water year 2005 wy2005 while the legacy older harvest low and high harvest rate group means were 16 32 and 61 fnu respectively regression analyses of turbidity on watershed natural physiographic characteristics and land use histories logging and roads showed the rate of recent logging mean annual percent of watershed area explained the greatest amount of variability in turbidity at the 10 exceedence level drainage area was also significant but was secondary to harvest rate none of the other watershed variables was found to improve the regression models despite much improved best management practices contemporary timber harvest can trigger serious cumulative watershed effects when too much of a watershed is harvested over too short a time period