siderophiles in the brachina meteorite impact melting the published high abundances and unusual ratios of siderophiles in the chassignite brachina refs 1 2 and r a schmitt personal communication in ref 1 in comparison with its apparent relatives chassigny shergottites and nakhlites2 11 suggest that brachina might be an impact melt if so the projectile was chemically similar to either some iron meteorites12 or the eagle station trio13 of pallasites as contaminants the latter meteorites would also explain the deviant oxygen isotope ratios14 of brachina without recourse to a separate parent planet we show here that all these meteorites could have formed on mars with brachina ejected as a large tektite like melt blob along with shocked and unshocked target rocks the hypothesis creates predictions which can be tested with data from radiometric ages and dynamic crystallization experiments

low to medium pressure variscan metamorphism in galicia nw spain evolution of a kyanite bearing synform and associated bounding antiformal domains

collapse of the thickened variscan crust in the french massif central mont pilat extensional shear zone and st etienne late carboniferous basin abstract a large scale extensional shear zone is described from the mont pilat area in the northeastern part of the french massif central this shear zone is about 2 km thick and is characterized by well developed n dipping foliation and stretching lineations deformation occurred under conditions of retrograde metamorphism from high temperature conditions to greenschist facies it is superimposed on a composite foliation related to the earlier variscan 380 350 ma compressional events associated with southward emplacement of nappes and hp mt metamorphism regional n s lineations and shear criteria observed at a regional scale indicate a northward displacement of the hanging wall during extension large strains locally recorded in thick mylonitic zones suggest that displacement was substantial some tens of kilometres late superimposed extensional brittle deformation and pseudotachylites show that northward displacement on normal faults continued at a shallower depth a large granitic pluton granite du velay intruded late in the the variscan orogeny was associated with extension some small leucogranitic syntectonic melts were emplaced within the shear zone and are strongly deformed geochronological rb sr data on these leucogranite bodies give an age of 320 ma and 40 ar 39 ar dating of synkinematic biotites from gneissose rocks in the shear zone yields a plateau age of 313 ma these data show that extensional ductile deformation and magmatism were still active during the latest late carboniferous thus extension occurred immediately after the well known late palaeozoic collision type compressional tectonics which had significantly thickened the crust we propose that the extensional tectonism described here is due to the gravitational collapse of the initially thick variscan crust the stephanian intracontinental sedimentary basin of st etienne was formed along the hanging wall of the normal shear zone during the late stages of this post orogenic crustal extension

factors of paleosol formation in a late cretaceous eolian sand sheet paleoenvironment marilia formation southeastern brazil abstract the marilia formation which crops out in southeastern brazil is interpreted as a late cretaceous eolian sand sheet area the sedimentary succession ca 110 m thick is characterized by alternating strata of eolian deposits and paleosols the paleosols constitute more than 66 of the thickness and are an important element in the interpretation of the paleoenvironmental controlling factors which affected the soil formation in this eolian sand sheet in this paper six paleosol profiles are described and assigned to two pedotypes itaja and apore the itaja pedotype is constituted of a sequence of five superimposed polygenetic profiles which show different phases of clay illuviation and carbonate precipitation and it has been classified as aridisol the apore pedotype overlies the itaja pedotype this pedotype shows an increase in leaching cia k ratios reddening and illuvial clay features in respect to itaja and has been classified as alfisol the analysis of the factors which controlled the soil formation revealed that both pedotypes formed on a stable landscape probably covered by a community of low stature plants in which the soils had sufficient time to develop very mature profiles the changes in soil forming processes were driven principally by variations in available soil moisture from precipitation using depth to carbonate functions from bk and cia k proxy from bt horizons of the itaja pedotype mean annual precipitation map estimates range from 240 to 1078 mm yr respectively these contrasting climatic conditions resulted in the superimposition of arid or semi arid with more humid climates and determined the considerable change in the pedogenic features with many horizons showing the interlacing of calcite and clay features in the apore pedotype cia k proxy from bt horizons estimates that averaged map was around 900 mm yr and the prevalent humid conditions can be attested by soil properties and abundance and depth of root traces our results show that the diversity in pedotypes is mainly attributed to differences in paleoclimatic conditions during maastrichtian time and that eolian sedimentation was restricted to periods of harsh arid conditions as in semi arid climates with map estimates around 240 mm yr the formation of soils with well developed calcic horizons was possible

crustal evolution of the ktb drill site from oldest relics to the late hercynian granites the crustal unit penetrated during the german continental deep drilling program ktb the zone of erbendorf vohenstrauss zev comprises an association of metabasites and paragneisses and orthogneisses that underwent high and medium pressure metamorphic cycles during the early palaeozoic in this summary of the structural petrological geochemical and geochronological information from borehole and surface rocks we show that geological models proposed prior to drilling have had to be significantly modified features of the zev such as the dominant nw se trending structures devonian 370 ma medium pressure amphibolite facies metamorphism 0 6 0 8 gpa 720 c and earlier eclogite stage are directly comparable with those of the nearby bohemicum unit in western bohemia intervening units in contrast exhibit ne sw trending structures and carboniferous 315 325 ma low pressure metamorphism all units are cut by predominantly posttectonic granites mostly 320 ma earlier models to explain this contrast by means of a nappe emplacement of the zev in the narrow time interval between the younger regional metamorphism and granite intrusion could not be substantiated because contrary to predictions the same rock association was present in the whole of the drilled 9 1 km and therefore no nappe boundary was drilled instead it is interpreted that shallow crustal levels reached by the zev subsequent to the devonian event shielded most of it from the effects of the carboniferous metamorphism a lack of expected thermal and baric gradients within the drilled metamorphic pile is a result of repeated stacking of an original 3 km depth section in mesozoic times an important result of the geological investigations of the ktb project is the recognition that the present day geometry of the zev is a result of cold shallow level thin skinned tectonics that in the absence of the view into the third dimension offered by the drill hole could not have been accurately predicted from the fossil high grade assemblages and structures seen at the surface

u pb zircon and sm nd model ages of high grade moldanubian metasediments bohemian massif czechoslovakia we report single grain u pb ion microprobe as well as conventional bulk size fraction ages for zircons from 3 metasediment samples of the moldanubian complex bohemian massif one of the largest crystalline complexes of the hercynian foldbelt in europe these are complemented by whole rock sm nd model ages the metasediments are of upper amphibolite to granulite grade and come from the bory massif in moravia nw of brno sample aa 1 and from the varied group aa 2 and monotonous group aa 3 in the ceske budejovice region of sw bohemia

age geodynamic setting and mantle enrichment processes of a k rich intrusion from the meissen massif northern bohemian massif and implications for related occurrences from the mid european hercynian the plutonic complex of the meissen massif northern margin of the bohemian massif comprises dioritic to mainly monzonitic and granitic rocks the diorite to monzonite intrusions show major and trace element patterns typical for shoshonitic series the chemical signatures of less crustally contaminated diorites are similar to arc related shoshonitic rocks derived from continental lithospheric mantle clm sources previously enriched by subduction of altered oceanic crust laser step heating 40ar 39ar analyses on actinolitic to edenitic amphiboles from geographically different occurrences of the monzonitic intrusion yielded concordant plateau ages as well as total gas ages ranging from 329 1 1 4 to 330 4 1 4 ma and from 330 4 2 1 to 330 6 1 8 ma respectively these cooling ages are indistinguishable from sensitive highresolution ion microprobe shrimp 238u 206pb intrusion ages measured on magmatic zircon rims from the monzonite nasdala et al submitted this shows that the monzonite intrusion is probably not related temporally to active subduction because it postdates eclogites of the adjacent saxonian erzgebirge by approximately 20 ma the shoshonitic magmas intruded during strike slip tectonism along the elbe valley zone the enrichment of their mantle sources may be of upper devonian lower carboniferous age or older intrusions of shoshonitic to ultra potassic k rich rocks during the upper visean namurian are widespread in the moldanubian zone based on similar ages and structural relationships a similar post collisional setting to the meissen shoshonitic rocks can be demonstrated most of these occurrences cut high grade nappe units which were subducted during the upper devonian lower carboniferous in contrast to the meissen massif at least the ultra potassic members of the central and the south bohemian batholiths were derived from clm sources enriched by fluids or melts released from subducted oceanic crust and by greater portions of crustal material despite the similar post collisional geodynamic setting of the k rich intrusions different enrichment processes generated mid european hercynian clm sources with heterogeneous major and trace element and isotopic signatures

late cadomian crustal tilting and cambrian transtension in the tepla barrandian unit bohemian massif central european variscides the cadomian basement of the tepla barrandian unit is characterized by a classic barrovian type metamorphism the degree of which increases considerably towards the west reaching amphibolite facies conditions in the domaz lice crystalline complex dcc the number and volume of plutons also increases towards the west the emplacement ages of the te s ovice granite and the mrac nice trondhjemite have been determined at 521 7 2 ma and 523 4 5 ma respectively applying conventional u pb analyses of zircons pervasive high temperature prolate fabrics and north northwest dipping dextral oblique slip shear zones within the mrac nice trondhjemite suggest a synkinematic melt emplacement within a lower cambrian transtensional setting transtension is probably related to early stage rifting that introduced the separation of the tepla barrandian unit as part of armorica from gondwana structural and petrological data of the country rocks show that the barrovian type metamorphism and two deformation stages d1 with unknown kinematics and d2 top to the north shearing are older than the melt emplacement and thus can be attributed to the cadomian orogeny the intrusion depth of both plutons is nearly the same ca 7 km although the degree of barrovian type metamorphism differs significantly within the country rocks this suggests late cadomian eastward tilting of the metamorphic isograd planes the weak post plutonic lower greenschist to subgreenschist facies folding and thrusting result from variscan northwest southeast compression

dehydration melting and the granulite transition in metapelites from southern namaqualand s africa in a prograde amphibolite granulite transition zone in the namaqualand metamorphic complex metapelites show an interbanding of the amphibolite facies association biotite sillimanite quartz with the granulite facies association garnet cordierite k feldspar relict graded bedding shows that compositional banding is of sedimentary origin the garnet cordierite k feldspar gneisses contain quartzofeldspathic segregations surrounding garnets and have more fe rich bulk compositions than the biotite sillimanite schists

extending a thickened crustal bulge toward a new geodynamic evolution model of the paleozoic nw bohemian massif german continental deep drilling site se germany abstract fault bounded tectonic metamorphic complexes assembling the nw bohemian massif around the german continental deep drilling ktb site are seen to be extremely heterogeneous in tectonic and metamorphic histories in current models the different complexes were supposed to reflect a puzzle of small pre devonian microplates and the related collision events supposedly lasted until the carboniferous opposed to these models it will be shown that all the boundaries among the complexes were formed by detachment late in a prolonged overall geodynamic history of a thickened crustal bulge during extensional tectonics and associated thermal events that outlasted the onset of collision in the silurian lower devonian by about 70 80 ma micro structures petrological and geochronological data of individual complexes predominantly preserve the late stages rather than the unbroken record of their tectonometamorphic histories such partial histories strongly different among individual complexes depict diverse snapshots taken at different places in the evolving thickened crustal bulge and at different instants in its overall evolution and do not define different precollisional microplates predominantly p t and deformation episodes after terrane juxtaposition are preserved this article presents an integrated view of the structural geology microscopic fabrics p t data and geochronology of such diverse metamorphic complexes this integrated view provides a new understanding of 1 the tectonic evolution during upper silurian devonian collision of the gondwana derived central european lithosphere with laurussia 2 the postaccretionary events that lasted through the upper carboniferous and 3 the earlier lower ordovician metamorphic and magmatic history which is only locally recorded metamorphic complexes occupying the structurally highest position upper tectonic complexes record devonian and earlier tectonometamorphic and magmatic events after the mid devonian such complexes did not experience any metamorphism the recorded devonian events consist of subduction and exhumation of hp rocks and their exhumation involved thrusting and extensional tectonics upper tectonic complexes comprise fragments of both the plate that was subducted in this period and the overriding plate in the footwall carboniferous extension has brought upper tectonic complexes against metamorphic complexes that essentially record lower and upper carboniferous tectonometamorphic and magmatic events lower tectonic complexes in the lower tectonic complexes such events are 1 consecutive extensional stages that created at least three sets of ductile normal detachment systems intersecting each other and various associated thermal pulses as well as 2 predetachment events that are only recorded as petrologic and structural relics transposed during extension inferred as predetachment events are crustal subduction as well as stacking that outlasted thrusting and exhumation of the upper tectonic complexes in the deeper portion of the thickened crustal bulge represented by the lower tectonic complexes spatial variations of p t t d histories during decompression occurred as a result of continued differential exhumation on the three sets of detachment systems and also from various thermal pulses that have affected different parts of this section during progressive shearing and exhumation to various degrees whereas the lower tectonic complexes of the erzgebirge gneiss dome preserve the record of a lower carboniferous history of hp metamorphism and crustal thickening followed by extension in those of the oberpfalz region ktb site a major upper carboniferous thermal pulse mostly erased the pre upper carboniferous strain fabrics and metamorphic record in the oberpfalz region ongoing extension emplaced mid crustal rocks that pervasively equilibrated in the upper carboniferous at high temperatures and low pressures low p t ratio against rocks not exposed to high temperatures at that time in summary a prolonged postaccretionary history concealed large scale structural relationships linked to crustal subduction and thickening associated with former juxtaposition of gondwana lithosphere to laurussia polyphase crustal extension and strike slip created the variety of different metamorphic tectonic complexes observed in the nw bohemian massif

transformation of spinel lherzolite to garnet lherzolite in ultramafic lenses of the austridic crystalline complex northern italy

volcanism on mars more than 15 years of planetary exploration of mars have given insight into the geologic processes that have shaped its surface the newly acquired viking data have shown that volcanism is one of the most important geologic processes operating on mars throughout its history in situ chemical analyses of martian soil by the viking lander spacecraft indicate mafic to ultramafic source rocks this is consistent both with available remote sensing data which indicate the presence of mafic minerals such as pyroxene and olivine and with petrologic modeling based on available geophysical data which suggest that martian lavas are probably iron rich and ultramafic these data strongly suggest that basaltic volcanism is widespread on mars and much of the photogeological data may be studied in this context photogeological analysis of the martian surface has shown two main types of volcanic morphologies the first type is central volcanoes which are volcanic landforms developed by continued and prolonged eruption from a point source vent this category includes 1 shields the classic low profile volcanic mountains of which olympus mons is the most spectacular example 2 domes steep sided constructs such as tharsis tholus that may represent lower rates of eruption than the shields or possibly more silicic lava compositions 3 highland patera radially textured low profile volcanoes that occur in the cratered terrain and are interpreted as ash shields 4 alba patera an apparently unique volcanic landform consisting of a vast volcanic center over 1500 km across with flank slopes of less than a tenth of a degree and 5 various small features such as cinder cones the second major category is volcanic plains which are units recognized by several criteria of which the presence of mare ridges and flow lobes are the most useful volcanic plains are subdivided into four main groups 1 simple flows broad smooth to rolling plains that contain numerous mare type ridges but no flow lobes interpreted as being composed of thick single cooling units 2 complex flows displaying multiple overlapping flow lobes interpreted to be indicative of thin multiple cooling units 3 undifierentiated flows plains that typically lack any morphologic identifying feature but are considered to be volcanic partly on the basis of their association with large volcanic centers and 4 questionable plains volcanic units heavily modified by other processes erosion tectonism etc so that their origins are uncertain when these categories of volcanic morphologies are combined with relative age data provided by crater statistics a volcanic history for mars can be derived as follows early heavy bombardment of mars was accompanied and followed by small scale fluvial channeling extensive flood volcanism the plateau plains and ash shield volcanism in the cratered terrain shortly after this time less extensive flood volcanism continued to resurface the planet during formation of the northern southern hemisphere dichotomy central volcanism became more prominent with the development of the alba patera center as well as the older shields and domes of the northern hemisphere early tharsis and elysium regions the development of the tharsis and elysium uplifts may have triggered the release of large scale catastrophic floods producing large channels continued uplift and lithospheric thinning concentrated volcanic activity in the tharsis region producing large shield volcanoes and extensive lava plains both central vent and plains volcanism have been active throughout martian history but the volumes of extrusion have gradually decreased with time this is consistent with a moonlike thermal history involving a lithosphere of increasing thickness with time gradually turning off the volcanism although many questions remain regarding martian volcanism the viking data have provided a remarkable detailed overview of the probable nature of the volcanic history of mars

snc meteorites evidence against an asteroidal origin the 1 3 b y crystallization ages and other features of the nine snc meteorites shergottites nakhlites and chassignites distinguish them from all other meteorites which apparently formed more than 3 b y earlier proposed models for the origin of the young snc meteorites include internal melting either within a large planet such as mars or within a large asteroid and impact melting on one or more asteroids although an asteroidal source is dynamically more reasonable we discuss petrologic geochemical and isotopic observations that strongly argue against an asteroidal origin for snc meteorites either as impact melts or as endogenically derived melts cumulate textures as displayed by the snc meteorites have never been observed in terrestrial or lunar impact melts even the largest impact melt sheets cooled relatively quickly by thorough mixing with cool clastic debris and the resulting rocks show obvious lithic and or mineral clasts derived from the target materials no clasts or remnants of lasts are present in snc meteorites the chemical and isotopic homogeneity of impact melt rocks is not displayed by snc meteorites differences in initial 87sr 86sr among some of the shergottites which are otherwise chemically similar preclude an origin in the same impact melt sheet these relationships indicate that the snc meteorites did not form within an impact melt sheet either on an asteroid or anywhere else instead their parental magmas probably formed by internal melting within their parent body although thermal models of large asteroids 800 km diameter can be devised to allow endogenic melting as young as 1 5 b y ago isotopic and geochemical data for snc meteorites preclude them from having formed in this way specifically the isotopic signature of the ancient 4 6 b y large scale heating event which established the shergottite sources is inconsistent with such asteroidal thermal models which require cold accretion of materials with initially low thermal conductivities the thermal models further require that the young melts were restricted to the asteroid s core total fragmentation of such asteroids would be required to liberate these rocks into earth crossing orbits destruction of an 800 km asteroid would have produced an immense swarm of asteroids with similar orbits no hirayama family with sufficient mass has been identified in the asteroidal belt if retention of residual garnet in the shergottite sources is a requirement as indicated by geochemical modeling this would directly imply a parent body larger than the largest asteroid ceres diameter 1020 km a planet sized parent body such as mars still remains the most likely candidate for the snc meteorites

high pressure granulites from the sudetes south west poland evidence of crustal subduction and collisional thickening in the variscan belt two high grade gneissic complexes of the western sudetes the gory sowie block and the nieznik area complex contain small predominantly felsic granulitic inliers with minor cpx bearing intercalations the p t conditions of the granulite facies events and of the subsequent re equilibration are estimated using the ternary feldspar thermometer and the geo calc computer program version twq jan 92 in the gory sowie granulites the peak granulitic event occurred at c 18 20 kbar and 900 c and the late decompressive re equilibration within a range of 4 10 kbar and temperatures decreasing to 600 700 c the latter event is thought to have coincided with the main metamorphic phase in the surrounding gneisses the p t estimates are more scattered in the nieznik granulites but the peak conditions for the granulitic event are estimated at pressure over 22 kbar possibly around 30 kbar and temperature exceeding 900 c the analysed samples from the nieznik area bear no significant evidence of lower pressure re equilibration integrating the thermobarometric data and some age constraints indicates that the gory sowie granulites belong to the early stage type i granulites of the variscan belt c 400 ma old which are interpreted as fragments of continental crustal materials subducted to mantle depths in the earliest stages of the variscan orogeny the nieznik granulites are more problematic they may belong to a younger high p suite c 350 ma old widespread in the southern and eastern parts of the bohemian massif and possibly related to the climax of the variscan continent continent collision

partially retrograded eclogites of the munchberg massif germany records of a multi stage variscan uplift history in the bohemian massif eclogites with a wide range in bulk composition are present in the munchberg massif part of the variscan basement of the bohemian massif in north east bavaria new analyses of the primary phases garnet omphacite phengite and amphibole as well as the secondary phases clinopyroxene ii various amphiboles biotite phlogopite plagioclase margarite paragonite prehnite and pumpellyite reveal a complex uplift history new discoveries were made of samples with very jadeite rich primary omphacite as well as a secondary omphacite in a symplectite with albite various geothermobarometric techniques together with thermodynamic databases incorporating separately determined activity composition values and experimental data have clustered the minimum conditions for the primary assemblages to the p t range 650 60 c 14 3 1 kbar however jadeite in omphacite kyanite paragonite in phengite and zoisite grossular in garnet kyanite quartz relationships suggests pressures of 25 28 kbar at the same temperatures the fact that the secondary omphacite plagioclase assemblage yields pressures within a few hundred bars of the minimum pressures for the plagioclase free assemblages strongly suggests that the minimum values are serious underestimates zoning inclusion suites and breakdown reactions of primary phases in addition to new minerals formed during uplift define a polyphase metamorphic evolution which from geochronological evidence occurred solely within the variscan cycle the complex breakdown in other bohemian massif eclogites and the distinct variation in their temperatures during uplift suggest a multi stage thrusting model for the regional evolution of the eclogites such an evolution has significance with respect to incorporation of mantle slices into crustal sequences and fluid derivation from successively subducted units possibly driving the breakdown reactions

petrogenesis of the snc shergottites nakhlites chassignites meteorites implications for their origin from a large dynamic planet possibly mars abundances of major minor and trace elements were determined in the whole rock and mineral separates of the two distinct lithologies present in the antarctic achondrite elephant moraine a eeta 79001 via sequential instrumental inaa and radiochemical neutron activation analysis rnaa additional chalcophile siderophile and volatile trace elements are reported for the shergottites alha 77005 shergotty and zagami the rare earth element ree abundances of each of the lithologies of eeta 79001 a and b exhibit chondritic normalized patterns similar to alha 77005 the close relationship of the antarctic shergottites indicates that alha 77005 is a residual source produced by incongruent melting of a source similar in bulk composition to eeta 79001a and that eeta 79001b and the interstitial phases in eeta 79001a are the melts produced by such melting episodes incorporating previous isotopic studies and our chemical data we suggest that either fractionation of or contamination by feldspar could account for the different shergottite sources the large ion lithophile lil trace element abundances of the shergottites require variable but extensive degrees of nonmodal melting of isotopically constrained parent sources the extreme extent of such melting is hypothesized to be represented by alha 77005 as a residual composition derivation of a pre 1 3 ae 109yr fractionation sequence is suggested in which ol ol opx opx cpx and opx cpx plag sequentially produce residual liquids capable of accounting for the precursor sources for the snc meteorites on the basis of chemical isotopic and petrologic considerations we conclude that the snc sources are consistent with their derivation by extensive fractionation of a primitive magma initially produced from a source having chondritic refractory lil trace element abundances fe fe fe mg atomic 0 3 and a mineralogy primarily of ol and pyx our model dependent deductions suggest that this magma could be derived without plagioclase or garnet left in the residuum petrogenetic and age relationships among snc meteorites suggest a single complex provenance on a dynamic planet not unlike the earth considering that each of these meteorites possesses chemical isotopic and petrologic features consistent with data presently available from mars we conclude that a martian origin is quite probable

igneous processes and closed system evolution of the tharsis region of mars a quantitative petrologic model for the formation of the tharsis rise of mars has been developed that is consistent with global gravity and topography data partial melting of the mantle underlying the tharsis plateau results in the depletion of basaltic chemical components in the source region iron is partitioned preferentially into the basaltic liquid so that the density of the source region is lowered relative to surrounding unmodified mantle basalt is erupted to the surface and or intruded at shallow levels in the crust where it solidifies to low pressure mineral assemblages or perhaps glass these low pressure materials are less dense than their mantle equivalents hence the net volume of the tharsis crust mantle system is increased resulting in the prominent topographic rise with no net gain or loss of mass the distribution of densities with depth is quantitatively consistent with the isostatic models of sleep and phillips 1979 1985 typical values of parameters that are constrained by the gravity and topography data are 150 km crustal thickness 250 km elastic mantle thickness 26 partial melting in the mantle source region and 32 addition of intrusive material to the crust this closed system petrologic process is strictly constructional in nature but shallow intrusion of magma into the crust can cause uplift of overlying strata as advocated by r j phillips and n h sleep submitted manuscript 1987

p t evolution and fluid inclusion characteristics of retrograded eclogites munchberg gneiss complex germany kyanite eclogites occur as part of the munchberger nappe pile in ne bavaria west germany eclogites are overprinted by subsequent amphibolite facies metamorphism the preservation of primary eclogitic textures as well as symplectitic textures are indicative of rapid decompression eclogite formation is estimated to have occurred under conditions of high h2o activities at pressures between 20 and 26 kbar and temperatures ranging between 590 and 660 c as is shown by the coexistence of omphacite jd 50 kyanite zoisite and quartz minimum pressure estimates independent of the water activity range between 9 and 16 kbar at the relevant temperatures detailed studies of fluid inclusion reveal two predominant groups of aqueous brine inclusions high salinity 14 17 wt nacl equiv and low salinity 0 8 wt nacl equiv inclusions fluid compositions of both groups of inclusions yield isochores passing close to the estimated amphibolite facies pt field the compositions of these fluids are in good agreement with fluid compositions considered from mineral equilibria none of the fluid inclusions has densities appropriate for eclogite facies metamorphism but probably reflect later amphibolite facies metamorphism

40ar39ar chronology of variscan tectono metamorphic events in an exhumed crustal nappe the monts du lyonnais complex massif central france abstract the monts du lyonnais metamorphic complex is part of the high grade nappes of the massif central france retrogression of metamorphic conditions occurred synchronously with evolution of deformation from penetrative thrust structures at high t conditions stage p1 2 700 50 c to strike slip ductile shear at lower p t conditions stage p3 t 600 c this evolution distinguishes progressive exhumation and cooling of this previously buried variscan segment in this study 40ar 39ar chronologic investigations are addressed to representative samples of each tectono metamorphic stage associated with the exhumation of the nappe the two mineral generations yield indistinguishable dates of 350 340 ma which are interpreted as post metamorphic cooling ages however the minerals which crystallized during the late intra continental stage p3 yield concordant plateau dates while the minerals which grew during the p1 2 stage and escaped later retrogressive deformation systematically display disturbed age spectra these heterogeneous profiles could result from the presence of a minor excess ar component incorporated at the time of the p3 event when intense deformation took place under decreasing metamorphic temperature and synkinematic granite emplaced along ductile wrench faults this tectono metamorphic event could have been responsible for a local redistribution of ar from the deformed rock volumes into the less or not deformed ones or for a redistribution of ar at the grain scale in the monts du lyonnais complex the effects of intense deformation and dynamic crystallization were probably associated with retrograde cooling to control the contrasting behaviour of the two mineral generations as a regional consequence these radiometric data imply that complete retrogression of the monts du lyonnais complex was achieved 340 ma ago the sharp cluster of ages obtained for the amphiboles muscovites and biotites used in this study suggests relatively rapid cooling at least 50 c ma 1 of the complex after attainment of metamorphic conditions associated with the p3 event the younger values of 325 315 and 250 ma produced by k feldspar and plagioclase respectively may reflect that 1 the metamorphic complex experienced slow cooling at most 10 c ma 1 between 325 and 315 ma ago and 2 a late weak thermal event

chronology of late paleozoic tectonothermal activity in the southeastern bohemian massif austria moldanubian and moravo silesian zones 40ar 39ar mineral age controls abstract new 40ar 39ar hornblende and muscovite ages from the moldanubian and moravian zones of the southeastern bohemian massif indicate rapid cooling associated with continental underplating during early carboniferous plate collision and closure of the moravo silesian foredeep 40ar 39ar ages have been determined for hornblende concentrates from 1 amphibolite from the moldanubian nappe complex including the dobra gneiss rehberg amphibolite and loosdorf complex all part of the variegated unit 2 micaschist structurally overlying the bittesch gneiss and 3 amphibolite within the bittesch gneiss moravian zone the five hornblende concentrates display variably discordant 40ar 39ar age spectra because of contamination with extraneous argon components however 36ar 40ar vs 39ar 40ar isotope correlations are well defined for the five hornblende concentrates mswd the hornblende and muscovite ages are interpreted to date post variscan metamorphic cooling through ca 500 c and ca 400 c respectively no record of pre variscan thermal events is preserved in either the hornblende or muscovite argon systems however the widespread occurrence of extraneous argon components suggest a pre variscan stage of tectonothermal development the near concordancy of the hornblende and muscovite cooling ages suggest relatively rapid cooling through the contrasting argon closure temperatures and indicate that relatively high crustal levels had likely been attained during the early carboniferous

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

trace metal concentrations in modern marine sabkhas abstract trace metal data from a number of hydrologically different types of modern marine sabkhas are presented indicating that these environments do not have elevated trace metal concentrations compared to recent near shore marine sediments and therefore are not active loci of trace metal enrichment today

petrology structure and geotectonic implications of glaucophane bearing eclogites and related rocks from the malpica tuy mt unit galicia northwest spain abstract the malpica tuy mt unit is a narrow band of basement in galicia northwest spain composed of mica schists paragneisses and calc alkaline to peralkaline orthogneisses with minor amounts of metamorphosed basic rocks garnet omphacite glaucophane barroisite rutile phengite paragonite kyanite zoisite developed in eclogites s s and eclogitic impure sandstones garnet phengite rutile zoisite in the calc alkaline orthogeneisses and garnet phengite rutile chloritoid kyanite in the metasediments biotite breakdown in the orthogneisses and related hornfelses produced garnet with development of coronitic textures plagioclase porphyroblasts with aligned inclusions of rutile and phengite are a common feature of many paragneisses the minimum p t conditions for the equilibrium of garnet omphacite parageneses at the peak of the metamorphism were 600 c 12 2 kbar the mineral chemistry of the orthogneisses and metasediments is compatible with these conditions furthermore the structural data also suggest an in situ origin for the eclogites i e the same as the metamorphism of the enclosing rocks the crystalline area surrounding the mt unit is mainly composed of metasediments and anatectic granites the t at the peak of the metamorphism was higher than 700 c the p being of the order of 5 kbar the two domains represent different parts of the hercynian foldbelt tectonically juxtaposed the juxtaposition is the result of complex tangential tectonics three main phases of compressive deformation having been recognized a first stage of subduction of a thinned crust the materials of the mt unit and subsequent obduction development of hydrated minerals in eclogitic rocks with emplacement of these materials over an already metamorphosed crustal area is suggested the mt unit would represent a remnant of a hercynian thrust nappe

rb sr and u pb isotope studies on migmatites from the schwarzwald germany constraints on isotopic resetting during variscan high temperature metamorphism mineral and isotope studies were undertaken on migmatites from the schwarzwald moldanubian zone of the variscan belt the aims of the study were to date the migmatite formation and to determine the processes involved in migmatization in order to evaluate their influence on isotopic resetting textural evidence and the comparison of mineral compositions from leucosomes and mesosomes of two centimetre scale migmatite profiles respectively suggest that migmatitic textures and mineral assemblages were formed by metamorphic segregation deformation enhanced mass transport rather than by partial melting anatexis the results of rb sr thin slab dating on these profiles indicate that sr isotopes were not completely reset during migmatization no true isochron ages but ages of approximate isotopic homogenization were obtained on the thin slabs by calculating 87sr 86sr ratios back to various stages in their evolution the coincidence of these rb sr data with u pb ages of monazites from migmatites and non migmatitic gneisses shows that gneisses and migmatites were formed during the same high temperature event in the carboniferous 330 335 ma the observation that high temperature metamorphism failed to equilibrate sr isotopes on the centimetre scale imposes limitations on the use of conventional whole rock isochron techniques in dating migmatites

accretion and exhumation at a variscan active margin recorded in the saxothuringian flysch the saxothuringian flysch basin on the north flank of the central european variscides was fed and eventually overthrust by the northwestern active margin of the tepla barrandian terrane clast spectra mineral composition and isotopic ages of detrital mica and zircon have been analyzed in order to constrain accretion and exhumation of rocks in the orogenic wedge the earliest clastic sediments preserved are of early famennian age ca 370 ma they are exposed immediately to the nw of the suture and belong to the par autochthon of the foreland besides ultramafic ophiolite material these rocks contain clasts derived from early paleozoic continental slope sediments originally deposited at the nw margin of the saxothuringian basin these findings together with the paleogeographic position of the famennian clastics debris on the northwestern passive margin indicate that the saxothuringian narrow ocean had been closed by that time microprobe analyses of detrital hornblendes suggest derivation from the randamphibolit unit now present in the middle part of the saxothuringian allochthon munchberg nappes detrital zircons of metamorphic rocks formed a little earlier ca 380 ma indicate rapid recycling at the tectonic front the middle part of the flysch sequence ca early to middle visean both in the par autochthon and in the allochthon contains abundant clasts of paleozoic rocks derived from the northwestern slope and rise together with debris of cadomian basement 500 ma granitoids and 380 ma early variscan crystalline rocks all of these source rocks were still available in the youngest part of the flysch c middle to late visean but some clasts record in addition accretion of the northwestern shelf our findings permit deduction of minimum rates of tectonic shortening well in excess of 10 30 mm per year and rates of exhumation of ca 3 mm a and possibly more

a discussion of isotopic systematics and mineral zoning in the shergottites evidence for a 180 m y igneous crystallization age abstract proposed igneous and metamorphic ages of the shergottites a suite of achondritic basaltic meteorites are examined in terms of the shergottites petrography and mineral chemistry no metamorphic or hydrothermal event appears capable of resetting the refractory sr and nd isotopic systems of the shergottites since the time of their igneous crystallization the approximately concordant rb sr 180 20 m y and u th pb 190 30 m y internal isochrons are thus interpreted as representing the time of igneous crystallization the sm nd internal isochrons which yield older apparent ages also show large scatter and are regarded as artifacts of the shergottites complex igneous history rubidium sr sm nd and pb pb wholerock isochrons are interpreted as mixing lines and are reasonably attributed to igneous processes such as wall rock assimilation and magma mixing if the shergottites are indeed less than 200 m y old they represent the youngest known extraterrestrial basalts this conclusion is supportive of the hypothesis that the snc meteorites are samples of the planet mars

the metamorphic signature of contemporaneous extension and shortening in the central himalayan orogen data from the nyalam transect southern tibet abstract geological relationships and geochronological data suggest that in miocene time the metamorphic core of the central himalayan orogen was a wedge shaped body bounded below by the n dipping main central thrust system and above the n dipping south tibetan detachment system we infer that synchronous movement on these fault systems expelled the metamorphic core southward toward the indian foreland thereby moderating the extreme topographic gradient at the southern margin of the tibetan plateau reaction textures thermobarometric data and thermodynamic modelling of pelitic schists and gneisses from the nyalam transect in southern tibet 28 n 86 e imply that gravitational collapse of the orogen produced a complex thermal structure in the metamorphic core amphibolite facies metamorphism and anatexis at temperatures of 950 k and depths of at least 30 km accompanied the early stages of displacement on the main central thrust system our findings suggest that the late metamorphic history of these rocks was characterized by high t decompression associated with roughly 15 km of unroofing by movement on the south tibetan detachment system in the middle of the metamorphic core roughly 7 8 km below the basal detachment of the south tibetan system the decompression was essentially isothermal near the base of the metamorphic core roughly 4 6 km above the main central thrust the decompression was accompanied by about 150 k of cooling we attribute the disparity between the p t paths of these two structural levels to cooling of the lower part of the metamorphic core as a consequence of continued and probably accelerated underthrusting of cooler rocks in the footwall of the main central thrust at the same time as movement on the south tibetan detachment system

kinematics of the variscan thrusting in the eastern moldanubicum bohemian massif czechoslovakia evidence from the naměst granulite massif abstract the tectonic history of the nam st granulite massif eastern part of the bohemian massif moldanubian zone czechoslovakia is interpreted in terms of polyphased generally north directed variscan nappe stacking original acid granulites formed during a presumed phase of lithospheric subduction 750 c 11 14 kbar 370 350 ma were converted into ht mylonites during the dominant deformation phase d 1 as a consequence of complex thrust wrench processes related to intra moldanubian deep crustal stacking 700 c 9 6 kbar 350 330 ma bringing granulites and the surrounding gfohl gneiss on top of the moldanubian nappe sequence this phase was followed by a northeastward superposition of the already stacked moldanubian nappe over the moravian zone phases d 2 d 3 650 400 c 5 3 kbar 335 325 ma which also marginally affected the cadomian basement of the brunovistulicum the variscan tectonic history was terminated by a namurian to lower permian phase of sinistral strike slip movements along nne ssw trending faults

u pb zircon and sm nd dating of moldanubian hp ht granulites from south ohemia czech republic conventional multigrain u pb systematics of zircon and single grain evaporation data of zircons in combination with nd model ages are presented for pyroxene free felsic garnetiferous granulites and orthopyroxene bearing granulites of felsic mafic and intermediate composition from the blansky les mountains granulite complex s bohemia zircons from mafic and intermediate granulites yield concordia intercept ages between 346 5 and 351 6 ma as well as a single grain evaporation age of 346 12 ma these ages define the time of variscan high grade metamorphism in s bohemia and are supported by an internal sm nd garnet zircon plagioclase whole rock isochron age of 343 21 ma euhedral zircons from felsic granulites of probable granitic derivation yield u pb ages of 366 5 ma and 373 12 ma interpreted as intrusion ages of the granitic precursor an inherited proterozoic crustal component is documented by poorly defined concordia upper intercept ages around 1700 ma and nd model ages between 1 3 and 2 1 ga

thermobarometry and geotectonic significance of high pressure granulites examples from the moldanubian zone of the bohemian massif in lower austria

geological constraints on the paleozoic tectonic evolution of tectonostratigraphic terranes in the iberian massif abstract the presence of oceanic paleozoic sequences in the iberian variscan fold belt is used to identify and separate suspected exotic terranes from a reference iberian autochthon terrane which constitutes the greater part of it in this paper the relevant data characterizing the pre accretionary evolution of the paleozoic iberian terranes are presented as well as the available evidence concerning the accretionary process itself emphasis is placed on the analysis of the sedimentary igneous and structural expression of the major events which affected the iberian terranes before during and after their amalgamation the main conclusions of such an analysis may be summarized as follows 1 a recognition of rift and passive margin stages in all the iberian massif terranes during the early paleozoic 2 b late paleozoic accretion of the exotic terranes to the iberian autochthon via a long and complex process variscan orogeny which involved initial subduction underneath the latter and eventual obduction followed by oblique collision of a promontory the iberian autochthon northwestern margin with a northwestern continental block this latter process could account for the large scale crustal imbrication shown by the northern branch of the orogen coeval with predominant strike slip faulting in the southern branch as well as with the formation of the typical arcuate shape of this orogenic belt major present uncertainties relate to the palinspastic provenance of continental suspect terranes and their correlation with similar units in other circum atlantic paleozoic orogens a discussion and speculations on this subject are dealt with in the final section of this paper

thermobarometry and granite genesis the hercynian low p high t velay anatectic dome french massif central the velay dome french massif central offers a quasi continuous section across an anatectic domain comprising low to high grade schists gneisses and granites two main tectonometamorphic events and their related generation of granitic material were recognized in addition to a major barrrovian tangential event d2 attributed to intracontinental collision tectonics i a medium to low p high t event d3 which gave rise to migmatites and syntectonic monzonitic granites and granodiorites and ii a widespread melting event d4 which led to the generation of migmatities the velay granite and post anatectic granites thermobarometry on samples collected from both the metamorphic envelope and the granitic core distinguishes two distinct geotherms i a first associated with the d3 event characterized by p 5 kbar t 750 c and water present melting biotite remains stable which led to large scale migmatization but minor amount of granites ii a second associated with the d4 event and characterized by vapour absent melting p 4 5 kbar t 760 850 c which gave rise to the velay granites and late migmatitic granites the temperature increase during the d4 event is attributed to the intrusion of hot mafic magmas within the crust the time integrated features of the different granitic rocks in the velay dome can be directly related to ah2o in the source region and illustrate the progressive dehydration of a middle to lower crustal segment over 60 ma

geochronology and geochemistry of eclogites from the marianske lazně complex czech republic implications for variscan orogenesis the marianske lazn complex mlc is located in the bohemian massif along the north western margin of the tepla barrandian microplate and consists of metagabbro amphibolite and eclogite with subordinate amounts of serpentinite felsic gneiss and calcsilicate rocks the mlc is interpreted as a metaophiolite complex that marks the suture zone between the saxothuringian rocks to the north west and the tepla barrandian microplate to the south east sm nd geochronology of garnet omphacite pairs from two eclogite samples yields ages of 377 7 and 367 4 ma samples of eclogite and amphibolite do not define a whole rock sm nd isochron even though there is a large range in sm nd ratio implying that the suite of samples may not be cogenetic eclogites do not have correlated nd values and initial 87sr 86sr ratios five of the eight eclogite samples have high nd values 10 2 to 7 1 consistent with derivation from a morb like source but variable 87sr 86sr ratios 0 7033 to 0 7059 which probably reflect hydrothermal seawater alteration three other eclogite samples have lower nd values 5 4 to 0 8 and widely variable 87sr 86sr ratios 0 7033 to 0 7096 such low nd values are inconsistent with derivation from a morb source and may reflect a subduction or oceanic island basalt component in their source the mlc is an important petrotectonic element in the bohemian massif providing evidence for cambro ordovician formation of oceanic crust and interaction with seawater late devonian frasnian famennian high and medium pressure metamorphism related to closure of a saxothuringian ocean basin early carboniferous visean thrusting of the tepla terrane over saxothuringian rocks and late visean extension

faunal evidence for oceanic separations in the palaeozoic of britain the faunal criteria for the recognition of the relative separation of old continents are 1 the contrast between shallow not deep water faunas off opposite shorelines 2 the recognition of deeper water facies surrounding the edges of continents and 3 the recognition of the disposition of ancient climatic belts these criteria are defined and used to assess the relative position of the various parts of what is now britain in the palaeozoic the iapetus ocean is well characterized by faunal differences from the cambrian until the lower ordovician but from caradoc times onwards these differences dwindle until by the latest silurian only the distribution of the ostracods separates the two sides of the ocean on faunal grounds in the early ordovician s british faunas are comparable with those from bohemia france and elsewhere to the s indicating connection with gondwanaland and these faunas differ from those in the baltic area a distribution which we attribute to a true ocean tornquist9s sea between n and s europe tornquist9s sea probably closed in the upper ordovician and from ashgill times onwards the s british and baltic faunas remained similar during the middle and late silurian there is evidence of increasing faunal distinction between britain the baltic area and north america on the one hand and southern europe and gondwanaland on the other indicating an opening rheic ocean which appears to have persisted until late carboniferous times

zircon dating of north bohemian granulites czech republic further evidence for the lower carboniferous high pressure event in the bohemian massif u pb zircon and rutile multigrain ages and 207pb 206pb zircon evaporation ages are reported from high pressure felsic and metapelitic granulites from northern bohemia czech republic the granulites in contrast to those from other occurrences in the bohemian massif do not show evidence of successive ht mplp overprints multigrain size fractions of nearly spherical multifaceted metamorphic zircons from three samples are slightly discordant and yield a u pb concordia intercept age of 348 10 ma whereas single zircon evaporation of two samples resulted in 207pb 206pb ages of 339 1 5 and 339 1 4 ma respectively a rutile fraction from one sample has a u pb concordia intercept age of 346 14 ma all ages are identical within error and a mean age of 342 5 ma was adopted to reflect the peak of hp metamorphism because rutile has a lower closing temperature for the u pb isotopic system than zircon the results and the p t data imply rapid uplift and cooling after peak metamorphism the above age is identical to ages for high grade metamorphism reported from the southern bohemian massif and the granulite massif in saxony it can be speculated that all these granulites were part of the same lower crustal unit in early carboniferous being separated later due to crustal stacking and subsequent late variscan orogenic collapse

thermobarometry diffusion modelling and cooling rates of crustal garnet peridotites two examples from the moldanubian zone of the bohemian massif abstract different origins for crustal garnet peridotites are illustrated by two variscan bodies in adjacent tectonically juxtaposed terranes in the moldanubicum of the bohemian massif eastern moravia the nove dvory garnet peridotite contains lenses of eclogite and is allochthonous and allofacial with respect to surrounding gfohl gneiss the mohelno spinel garnet peridotite is devoid of eclogite and has concordant contacts with surrounding felsic granulite the bulk of the mohelno body is spinel peridotite and garnet occurs only along its margins where it has formed at the expense of spinel thermobarometry of mineral cores from garnet bearing assemblages yields values for nove dvory from 1195 c 54 6 kbar to 970 c 41 8 kbar and for mohelno from 1265 c 27 9 kbar to 1070 c 23 4 kbar the compositional zoning profiles in garnets at both localities are matched closely by diffusion modelling in which bilinear cooling rates are utilized at nove dvory initial cooling rates of 50 200 c ma are followed by slower rates of 10 50 c ma in contrast at mohelno an extremely rapid initial cooling rate of 1 c year was followed by one of 500 c ma an idealized tectonic model is proposed for the emplacement of the mohelno peridotite to account for its cooling history the predicted garnet zoning profiles that result from conductive cooling agree closely with the observed zoning profiles both peridotites were derived from the upper mantle the nove dvory body was emplaced from a deep fault zone into the crust as garnet peridotite and the mohelno body was emplaced as a hot slab of spinel peridotite in which garnet formed only along contacts as the body cooled into the garnet stability field

eclogites in the french palaeozoic orogen geodynamic significance abstract eclogites are widespread in the devonian internal zone of the french palaeozoic orogen they occur in different tectonic settings either in high pressure low temperature or in high pressure high temperature environments they are tectonically incorporated into several major allochthonous units as demonstrated either by their blastomylonitic texture or by their being in evident disequilibrium with their country rocks the protoliths of the presently well identified eclogite bearing series present different typical associations depending on their present location the igneous series include within plate basalts normal and transitional morbs and even calc alkaline rocks they were erupted mostly during the palaeozoic although they sometimes contain inherited precambrian zircons further some of them may be remnants of old precambrian oceanic crust most of the eclogitic parageneses were generated during the earliest silurian tectonometamorphic climax either in low temperature or in high temperature conditions later they followed various retrograde courses in still geographically distinct low temperature or high temperature associations the basic rocks in the leptynolitic amphibolitic complex and particularly the eclogites thus provide sufficiently clear evidence to support a new geodynamic scenario implying one or several subordinate narrow tensional zones in a thinned but well developed continental crust rather than in a true oceanic domain

post collisional variscan lamprophyres black forest germany 40ar 39ar phlogopite dating nd pb sr isotope and trace element characteristics abstract subsequent to high pressure and temperature metamorphism of the axial zone of the variscan foldbelt in central europe at ca 340 to 330 ma formation of lamprophyre dikes during transtensional tectonics may be viewed as the beginning of the post collisional stage of the orogeny we report the results of 40 ar 39 ar mica dating major and trace element data and isotope compositions for lamprophyre and rhyodacite samples from the southern black forest the chemical compositions of these rocks shed light on the upper mantle and crust at the end of the variscan orogeny 40 ar 39 ar plateau ages for four phlogopite biotite separates from lamprophyres indicate emplacement at 332 to 314 ma this event coincides with melting of the crust as indicated by 40 ar 39 ar biotite plateau ages of ca 332 ma for rhyodacite dikes which are probably related to coeval undeformed granites incompatible trace element patterns of the lamprophyre samples reveal the characteristics of evolved continental crust and are interpreted as evidence for melting of sediment in a subduction modified mantle nd sr and pb isotope compositions indicate an enriched mantle source with end values of 1 5 to 6 8 which is similar to variscan crust significant contamination of the lamprophyric melts by variscan crust can be ruled out as mantle derived phlogopite phenocrysts have similar end values as in the whole rock samples we propose that the isotope compositions and incompatible trace element characteristics of the lamprophyres were predominantly inherited from melted sediment the isotope compositions of variscan lamprophyres from western europe suggest that enriched upper mantle was only partly delaminated when ascending hot mantle triggered melting of the lower crust as has been invoked for the origin of post collisional granites the isotope compositions of tertiary basalts and mantle xenoliths indicate a depleted upper mantle under western europe implying that the enriched variscan material was efficiently removed and mixed into the convecting mantle

the meteorite asteroid connection two olivine rich asteroids two asteroids have clear indications of olivine rich surface petrology without any indication of pyroxene or plagioclase suggesting that the olivine may be quite pure they provide probable examples of mantles of differentiated parent asteroids exposed by fragmentation and are good candidates for the parent bodies of the unusual olivine meteorite brachina or the olivine iron alloy meteorites called pallasites

glaucophane bearing assemblage overprinted by greenschist facies metamorphism in the variscan kaczawa complex sudetes poland an early palaeozoic ordovician metamudstone sequence near wojcieszow kaczawa mts western sudetes poland contains numerous metabasite sills up to 50 m thick these subvolcanic rocks are of within plate alkali basalt type primary igneous phases in the metabasites clinopyroxene salite and kaersutite are veined and partly replaced by complex metamorphic mineral assemblages particularly the kaersutite is corroded and rimmed by zoned sodic sodic calcic and calcic amphiboles the matrix is composed of actinolite pycnochlorite albite an 0 5 epidote ps 27 33 titanite calcite opaques and occasionally biotite phengite and stilpnomelane the sodic amphiboles are glaucophane to crossite in composition with nab from 1 9 to 1 6 they are rimmed successively by sodic calcic and calcic amphiboles with compositions ranging from magnesioferri winchite to actinolite no compositions between nab 0 92 and nab 1 56 have been ascertained the textures may be interpreted as representing a greenschist facies overprint on an earlier blueschist or blueschist greenschist transitional assemblage the presence of glaucophane and no traces of a jadeitic pyroxene quartz association indicate pressures between 6 and 12 kbar during the high pressure episode temperature is difficult to assess in this metamorphic event the replacement of glaucophane by actinolite chlorite albite with associated epidote allows restriction of the upper pressure limit of the greenschist recrystallization to 8 kbar between 350 and 450 c the mineral assemblage representing the greenschist episode suggests the p t conditions of the high pressure part of the chlorite or lower biotite zone the latest metamorphic recrystallization under the greenschist facies may have taken place in the visean

variscan oceans ages origins and geodynamic implications inferred from geochemical and radiometric data abstract a review of the magmatic associations providing evidence for the existence of extensional regimes during the palaeozoic evolution of variscan europe is presented the available geochronological data allow recognition of two major rifting events one in the early ordovician and the other in the middle late devonian scarce relicts of ophiolitic complexes indicate that both extensional episodes reached a true ocean spreading stage it is suggested that the variscan belt resulted from a complex two stage collision process through successive suturing in the silurian and the early carboniferous of these oceanic domains the superposition of two distinct opening closure cycles involving geographically separate oceanic basins may partly account for the unusual width of the variscan orogen

k ar geochronology of different tectonic units at the northwestern margin of the bohemian massif abstract the polymetamorphic moldanubian mo of the northeasthern margin of the bohemian massif has been thrust to the north onto the mainly paleozoic sedimentary saxothuringian of the fichtelgebirge fg these two units have undergone polyphase deformation and the last regional event to affect both units was a low pressure metamorphism in which temperatures decreased towards the north in contrast the nappe units of the erbendorf vohenstrauss zone zev and the erbendorf greenschist zone egz which partly cover the border of the moldanubian and the saxothuringian and the munchberg nappe pile mm which lies on the saxothuringian were in parts subjected to a late medium pressure metamorphic event the zev the egz the mo and the fg are intruded by late carboniferous granites conventional k ar analyses mainly of hornblendes and muscovites from the autochthonous fg and mo the units beneath the nappes have yielded exclusively carboniferous dates the oldest dates point to a regional cooling of the rocks which outcrop at the present day surface at about 330 320 ma i e at the early late carboniferous boundary the late carboniferous cooling history was largely governed by the thermal influence of the post kinematic granites 320 295 ma especially in the fg and the northern mo the high grade metamorphic rocks in the western part of the zev and in the upper three nappes of the mm mostly yield dates around 380 ma i e early devonian the results show a relatively wide scatter moreover biotites frequently appear to be older than the coexisting muscovites both observations indicate that the rocks underwent a later thermal influence whether some groups of older dates e g 400 ma are due to excess argon or to inherited argon is still open to discussion slightly scattered muscovite dates around 366 ma were obtained for the prasinite phyllite series one of the lower nappes of the mm a single hornblende from the egz gave the same age these two nappes have therefore probably been affected by a late devonian thermal and or tectonic event the muscovite dates obtained from the paleozoic bavarian lithofacies the lowermost nappe of the mm and the hornblende dates from the eastern part of the zev are indistinguishable from those of the autochthonous units fg and mo

a note against a small body origin for shergottites nakhlites and chassignites with the possible exception of brachina shergottites nakhlites and chassignites all crystallized about 1 3 b y ago on the same planet and they were probably ejected from it together about 180 m y ago no breccias from the parent planet have yet been identified thus a dominantly impact processed surface or a small body internal melt scenario is virtually untenable these meteorites must have formed in a magmatic complex and on a planet from which it was difficult to remove material as far as we know it only happened once everything we know about shergottites nakhlites and chassigny is consistent with an igneous origin on mars

geochemical and geochronological cross section of the deep variscan crust the cabo ortegal high pressure nappe northwestern spain the high pressure rocks of the cabo ortegal nappe and related massifs are represented by several tectonometamorphic units which underwent separate histories before regional overthrusting and amphibolite facies recrystallization at 380 ma the occurrence of final cooling is recorded by 40ar 39ar and rb sr mica ages of up to 350 ma u pb zircon dating and sm nd and rb sr isotope data together with ree and major and trace element geochemistry provide constraints on the timing of earlier tectonothermal events and the nature of the protoliths the earliest tectonothermal event occurred at 490 480 ma and corresponds to the emplacement of lree enriched magmas of probable calc alkaline affinity and their metamorphism in the granulite facies this occurred at approximately the same time as the eclogite facies metamorphism of morb like protholiths associated with basic to intermediate granulites together this suggests formation of syn accretion granulites in an active margin setting a second high grade metamorphic event is recorded at 420 ma in metasedimentary formations associated with eclogite like assemblages garnet omphacite plagioclase this event is possibly related to a collisional plate tectonic setting in a back arc basin the age and origin of the associated ultrabasic rocks remains unclear but a late pyroxenite vein has yielded an emplacement age of 390 ma

tectonometamorphic evolution of the bohemian massif evidence from high pressure metamorphic rocks the variscan orogenic belt of which the bohemian massif is a part is typically recognized for its characteristic low pressure high temperature metamorphism and a large volume of granites however there are also bodies of high pressure rocks eclogites garnet peridotites and high pressure granulites which are small in size but widely distributed throughtout the massif initially the high pressure rocks were considered to be relicts of a much older orogenic event but the increasing data derived from isotopic and geochronological investigations show that many of these rocks have palaeozoic protoliths metamorphic ages from the high pressure rocks define no single event instead a number of discrete clusters of ages are found between about 430 ma and the time of the dominant low pressure event at around 320 330 ma

rates of grain boundary diffusion through enstatite and forsterite reaction rims the growth rates of enstatite rims produced by reaction of fo92 and sio2 were determined at 250 1500 mpa and 900 1100 c for a wide range of water contents growth rates were also determined for forsterite rims between mgo and mg2si2o6 and between mgo and sio2 rim growth rates are parabolic indicating diffusion controlled growth of the polycrystalline rims which are composed of 2 m diameter grains rim growth rates were used to calculate the product of the grain boundary diffusion coefficient d a times the effective grain boundary thickness assuming in turn that mgo sio2 and mg2si 1 are the diffusing components coupled diffusion of a cation and oxygen or interdiffusion of mg and si the values for d mgo d and d for enstatite at 1000 c and 700 mpa confining pressure with about 0 1 wt water are about five times larger than the corresponding d a values for samples initially vacuum dried at 250 c most of the increase in d a occurs with the first 0 1 wt water the activation energy for diffusion through the enstatite rims 1100 950 c is 162 30 kj mole the diffusion rate through enstatite rims is essentially unchanged for confining pressures from 210 1400 mpa but the nucleation rate is greatly reduced at low confining pressure for 1 0 wt water present and limits the conditions at which rim growth can be measured the corresponding values for d a through forsterite rims are essentially identical for the two forsterite producing reactions when 0 1 wt water is added and similar to the d a values for enstatite at the same conditions the d a values for forsterite are 28 times larger for samples starting with 0 1 wt water compared to samples that were first vacuum dried thus water enhances these grain boundary diffusion rates by a factor of 5 30 depending on the mineralogy but the total range in d a is only slightly more than an order of magnitude for as wide a range of water contents as expected for most crustal conditions

snc meteorites clues to martian petrologic evolution the shergottites nakhlites and chassigny snc meteorites are apparently cumulate mafic and ultramafic rocks that crystallized at shallow levels in the crust of their parent body the mineralogy and chemistry of these meteorites are remarkably like equivalent terrestrial rocks although their ratios of fe fe mg and certain incompatible elements and their oxygen isotopic compositions are distinctive all have crystallization ages of 1 3 b y or younger and formed from magmas produced by partial melting of previously fractionated source regions isotope systematics suggest that the snc parent body had a complex and protracted thermal history spanning most of geologic time some meteorites have been severely shock metamorphosed and all were ejected from their parent body at relatively recent times possibly in several impact events late crystallization ages complex petrogenesis and possible evidence for a large gravitational field suggest that these meteorites are derived from a large planet trapped gases in shergottite shock melts have compositions similar to the composition measured in the martian atmosphere ejection of martian meteorites may have been accomplished by acceleration of near surface spalls or other mechanisms not fully understood if snc meteorites are of martian origin they provide important information on planetary composition and evolution the bulk composition and redox state of the martian mantle as constrained by shergottite phase equilibria must be more earthlike than most current models planetary thermal models should benefit from data on the abundances of radioactive heat sources the melting behavior of the mantle and the timing of planetary differentiation calculated depletion of chalcophile elements in source regions indicates a core dominated by sulfides and paleomagnetic measurements suggest the presence of a weak magnetic field within the last several hundred thousand years concentrations of volatile elements indicate that the snc parent body was not volatile depleted and trapped atmospheric components measured in shock melts may be useful in understanding planetary degassing by providing comparisons for spectral reflectance data and viking soil analyses these meteorites may also constrain surface mineralogy and weathering mechanisms

garnet zoning and reaction textures in overprinted eclogites bohemian massif european variscides a record of their thermal history during exhumation eclogites from different parts of the variscan basement of the bohemian massif show a variation in temperatures recorded at peak pressures as well as during subsequent exhumation these differences are reflected in the extent of modification of pre syn and post eclogite facies garnet compositions and by the types of reactions taking place at garnet rims munchberg massif eclogites formed at conditions of 600 650 c 20 kbar were mainly overprinted in the amphibolite facies field and contain garnets with an old homogeneous core and often well preserved idiomorphic growth zoning to mg rich rims in the marianske lazn complex the eclogite facies stage 640 715 c 15 kbar many eclogites show well developed granulite facies overprints with widespread orthopyroxene growth in garnet and omphacite domains spinel and or sapphirine after kyanite and in some cases even a low pressure high temperature overprint that produced fe rich olivine the latter rocks clearly endured higher temperatures during breakdown but remarkably strong growth zoning and sharp overgrowth features are preserved in some cases diffusion modeling shows that the preservation of zoning in garnet of rocks that have experienced granulite facies temperatures is only possible if the time spent at these conditions is short 1 ma this time scale is inconsistent with a smooth p t t path as expected from thermal modeling for erosion or extension driven exhumation and implies that these variscan rocks experienced short lived thermal events during their exhumation history