multiphase u pb geochronology of sintered breccias from the steen river impact structure canada mixed target considerations for a jurassic cretaceous boundary event abstract in situ u pb geochronology has been conducted using laser ablation inductively coupled mass spectrometry la icp ms on shocked and thermally metamorphosed apatite titanite and zircon grains from the steen river impact structure canada the dated relict mineral phases occur within impact melt bearing breccias that underwent post deposition sintering at 450 c t

plate tectonics and magma genesis the framework of plate tectonics with active boundaries and stable plates is the best yet devised for explaining the different types and styles of magmatic activity the dynamic mechanisms of plate tectonics transport rock masses across fusion boundaries in three distinct types of environment and source material associated with plate boundaries these are divergent boundaries where mantle peridotite is transported upwards melting to yield basalt convergent boundaries where oceanic crust is transported downwards melting to yield magma of intermediate sio 2 content and ocean continent convergent boundaries where the lower part of continental margins are melted to yield rhyolite magma hot spots beneath plates possibly generated by mantle plumes yield basaltic magma from mantle and rhyolite magma from overlying continental crust subducted h 2o is involved in the generation of andesites and batholiths and co 2 from uncertain sources is an influential component for the generation of kimberlite and other low sio 2 high alkali magmas below continental plates the chemical differentiation of the earth is accomplished through magmatic processes which are a direct manifestation of convection within the mantle igneous petrology is now a study of processes and products firmly rooted in geophysics and calibrated by laboratory experiments at high pressures and temperatures

u pb dating of monazite and its application to geological problems monazite is an underutilized mineral in u pb geochronological studies of crustal rocks it occurs as an accessory mineral in a wide variety of rocks including granite pegmatite felsic volcanic a

formation and age of sphalerite mineralization in carbonate rocks of bajocian age in the swiss jura mountains evidence of mesozoic hydrothermal activity a combination of petrographic and geochemical techniques was applied to better constrain the origin and evolution of the fluid systems responsible for the formation of disseminated cd rich up to 0 6 wt sphalerite zns mineralization in the northeastern part of the jura mountains switzerland the rb sr ages of sphalerite samples indicate that a main phase of sphalerite formation occurred near the boundary between the late middle and early late jurassic at around 162 ma the negative 34s values 22 3 to 5 3 suggest that biogenic sulfide sulfur was involved in zns precipitation the strontium isotope composition is more radiogenic than that of contemporaneous seawater reflecting the interaction of mineralizing fluids with silicate rocks lead isotope signatures are very uniform 206pb 204pb 18 63 18 67 207pb 204pb 15 63 15 64 208pb 204pb 38 51 38 63 indicating an isotopically well homogenized fluid system the basement rocks underlying the jurassic strata are considered to be the main source of metals for the sphalerite mineralization the migration of deep sourced hydrothermal saline metal bearing fluids into the bajocian host carbonates containing sedimentary reduced sulfur resulted in the precipitation of sulfides the period of sphalerite formation near the middle late jurassic boundary is characterized by enhanced tectonic and hydrothermal activity in europe related to the opening of the central atlantic and tectonic thermal subsidence during spreading of the alpine tethys our study provides evidence that the bajocian carbonate rocks in the jura mountains area were affected by the circulation of deep sourced metal bearing hydrothermal fluids in response to these continent wide tectonothermal events the presence of sphalerite mineralization and associated geochemical anomalies in zn and cd contents in carbonate rocks may also be used to trace basement features

rb sr systematics of permian volcanites in the schwarzwald sw germany from the quartz porphyry occurrences of haigerach and geisberg central schwarzwald which were dated by minerals and whole rock samples the whole rock samples generally yielded lower ages than the minerals this result agrees with other studies on apparently rejuvinated whole rock ages of acid volcanic rocks compare mckerrow et al 1980 petrographic and geochemical observations show that two processes disturbed the rb sr relations 1 fast alterations immediately after extrusion such as sericitization during autohydrothermal bleaching in the pipe area of the volcanoes gain of rb addition of common sr 2 loss of radiogenic 87sr due to long lasting exchange processes with groundwaters

isotope systematics and shock wave metamorphism i u pb in zircon titanite and monazite shocked experimentally up to 59 gpa this study reports the first u pb isotope analyses on experimentally shocked zircon titanite and monazite extracted from proterozoic granitoid rocks in all three types of minerals shock waves produce drastic changes in the crystal lattices causing strong lowering of birefringence turbidization and decolorization of the individual grains moreover x ray patterns indicate transition of the crystals into polycrystalline aggregates of 10 5 mm block size precisely dated grains with concordant or nearly concordant ages were embedded in kbr and shocked at 35 47 5 and 59 gpa u pb isotope analyses on these grains show that shock metamorphism does not fractionate pb isotopes within the analytical precision of 0 1 as far as chemical fractionation is concerned there is no difference in degree of concordancy between shocked and unshocked monazite and small degrees 2 of relative upb fractionation in shocked zircon and titanite are due to time integrated pb loss and not to the shock experiment in consequence the data document that shock wave metamorphism alone does not measurably effect the u pb chronometer questioning the view that lower intercept ages of discordant u pb data reflect shock induced re equilibration of the chronometer in moderately to highly shocked rapidly cooling rocks

deformed monazite yields high temperature tectonic ages the deformation of monazite in the polymetamorphic sandmata granulite complex in india has been investigated by electron backscattered diffraction and sensitive high resolution ion microprobe shrimp quantitative microstructural analyses document the development of deformation twins in 100 001 and graphic 1 orientations low angle 10 boundary development associated with dislocation creep and the development of new grains due to dynamic recrystallization these data represent the first quantitative evidence of crystal plastic deformation of natural monazite shrimp u th pb analysis shows that the host monazite preserves discordant ages as old as 1666 28 ma along a trend from ca 1720 ma to ca 1000 ma with increasingly discordant ages recorded in zones of higher lattice distortion domains of recrystallized new grains within the monazite record a tightly clustered concordia age of 970 14 ma this age is interpreted to represent the timing of monazite dynamic recrystallization associated with deformation of the host protolith and is consistent with partial resetting and pb loss from domains deforming by dislocation creep the complex but systematic relationship between microstructure and age data in monazite provide the first direct evidence of pb isotope resetting during deformation the approach illustrates a new methodology for the dating of deformation events in high grade metamorphic rocks which are typically difficult to constrain 1 embed inline graphic 1 gif

the formation of large neoblasts in shocked zircon and their utility in dating impacts uranium lead u pb geochronology of individual shocked zircon grains has unique potential for dating bolide impact events neoblasts in granular textured zircon have been recognized as the shock related feature most effective at recording the impact age here we report the discovery of large neoblasts 5 100 mu m in dimension in shocked zircon at the sudbury impact structure canada the first report of in situ coarsely granular zircon from a terrestrial impact site other than the vredefort structure south africa the neoblast bearing sample was taken from a heterogeneous lithic clast rich igneous unit associated with the roof rocks of the impact melt sheet making this the first time a crater has been dated using neoblastic zircon from the upper part of its stratigraphy previous in situ discoveries of coarsely granular zircon at vredefort were all in impact generated mafic melt emplaced beneath the impact melt sheet electron backscatter diffraction analysis of the impact aged neoblasts indicates that the high pressure conditions inferred in the formation of many small neoblasts were not necessarily involved in the formation of these large ones their large size internal zonation and occurrence in a slowly cooling environment collectively suggest that large neoblasts at sudbury formed by relatively protracted post impact growth in shocked zircon incorporated into impact related melt based on insight from large neoblast growth in terrestrial settings we suggest that the ca 4 33 ga neoblasts recently reported in lunar zircon may imply a major basin forming event on the moon at that time new knowledge of the cratering environments in which large neoblasts form also raises the prospect of possibly linking ex situ granular zircon in lunar breccias with specific impact structures and thus better calibrating the lunar cratering record with radiometric ages

dating the shock wave and thermal imprint of the giant vredefort impact south africa u pb geochronology of single grains of zircon and monazite has been used to date an episode of intense postimpact metamorphism in the core of the deeply eroded vredefort impact structure of south africa results from two basement units exposed in the uplifted central region indicate that the impact and a later pyroxene hornfels metamorphic event were penecontemporaneous at 2020 3 ma discovery of a synimpact to postimpact dike of norite that intruded at 2019 2 ma is the first recognition of mafic igneous activity related to impact the dike is either derived from a sudbury type impact melt layer since eroded or is the product of decompression melting of kaapvaal mantle in response to the ablation of 15 km of crust at the center of the crater the combined heating effects of the shock wave and impact triggered magmas are thought to have created the 300 km 2 thermal imprint of the asteroid collision with kaapvaal craton and account for the nearly coeval timing relationship between core metamorphism and shock revealed by this study

rapid burial and exhumation during orogeny thickening and synconvergent exhumation of thermally weakened and thinned crust variscan orogen in western europe we present a model demonstrating that the presence of a thinned and thermally weakened crustal domain is a prerequisite for the formation of an orogenic root in mountain belts furthermore this mechanism permits burial to 50 to 70 km in time periods of less than 20 myr and exhumation from these depths in less than 10 myr our example from the variscan belt is based on field geology petrology of metamorphic rocks and detailed geochronology it describes an evolution from i sedimentation and magmatism during back arc extension ii compression leading to burial thickening of the lithosphere and granulite facies metamorphism followed by iii exhumation formation of dehydration melts migmatites and late intrusive granites the existing u pb ar ar data constrain this evolution to no more than 20 to 25 myr thermorheological modelling starts from a hot thinned crust forming a narrow 50 km wide orogenic root within 15 myr at a maximum of 50 km depth and subsequent exhumation by extrusion within 7 myr the model explains exhumation rates of 3 to 4 mm year by compressional extrusion and shows that extension cannot do the same job within the same time extrusion transported the internal thermal structure into the upper crust within a short time followed by rapid cooling whereas exhumation through extension would cause mantle upwelling and the formation of a long lived thermal anomaly the latter would contradict the geochronological and petrological database the model explains orogeny as a thermorheological process without needing to involve an external heat source from the mantle and presents a new explanation of the high temperature and low pressure conditions very typical of the variscan orogenic crust

the rotations of the variscides during the carboniferous collision paleomagnetic constraints from the vosges and the massif central france abstract this study aims at completing and refining results from previous paleomagnetic investigations carried out in various outcrops of the european variscan belt sampling was performed in visean effusive and intrusive rocks of the southern vosges basin and the northern massif central all emplaced in the time range 345 340 ma stepwise thermal demagnetization of 254 specimens from 13 sites of the doller valley and nine sites of the massif central has yielded a very similar paleomagnetic record for both areas with the exception of post carboniferous overprints isolated in the southern vosges two groups of directions with both normal and reversed polarity have been evidenced the first group of directions labelled c occurs in both areas and is characterized by scarce pre tectonic magnetizations two sites and by syn tectonic overprints five sites acquired during the late visean convergence i e in the time range 340 328 ma the mean pole co is 28 s 45 e k 139 95 5 the main group labelled b shows a negative fold test and a positive reversal test indicating that the magnetizations are post tectonic overprints acquired after the main tightening phase i e after 328 ma at a few sites in both areas were evidenced syn tectonic b magnetizations that are in favour of an overprinting in the context of post visean extension tectonics the mean post tectonic b direction of the doller valley southern vosges 62 25 k 52 95 6 n 11 vgp 8 n 127 e is consistent with the mean post tectonic b direction 72 23 k 126 95 5 n 6 vgp 3 n 114 e obtained from the tuffs the grey microgranite and the dyke intrusive into the microgranite of the massif central the occurrence of similar paleomagnetic records in the eastern and central variscides implies that from the massif central to the sudetes the belt has probably moved as one block since early carboniferous the change in inclination of the c components suggests a southward drift of the moldanubian and saxothuringien variscides in the late visean 340 328 ma according to geological features in the same time the rhenohercynian crust was underplating the saxothuringian arc and fore arc zone subsequent to 328 ma the drift turned into a rapid clockwise rotation by about 70 around a pole located on southeastern corner of baltica leading to the closure of the rhenohercynian oceanic basin in the namurian large scale overprinting b components likely related to post rotation extension affected the internal belt the convergence continuing the variscides underwent a second clockwise rotation by about 45 in the late namurian westphalian 320 305 ma stacking of the rhenohercynian sedimentary cover and thrusting over the london brabant basement may be related with this motion

geochronology of the mid german crystalline rise west of the river rhine the mid german crystalline rise has its westernmost exposures at the western margin of the rhine graben in the southern pfalz and the northern alsace the outcrops are made up of granitoid rocks and minor volcano sedimentary sequences radiometric ages obtained by u pb pb pb sm nd and rb sr analyses of the igneous rocks from this area range from 433 to 325 ma thus covering a time span from the silurian to the end of the lower carboniferous because the investigated rocks are according to their chemical composition largely related to subduction zone environments the following three geodynamical scenarios are postulated always taking subduction of oceanic crust beneath the mid german crystalline rise into account a subduction of the rheic ocean during the silurian from the north b subduction of the rhenohercynian ocean during the late devonian 369 ma from the north c subduction of the saxothuringian ocean during the lower carboniferous 334 ma from the south

new zircon shock phenomena and their use for dating and reconstruction of large impact structures revealed by electron nanobeam ebsd cl eds and isotopic u pb and u th he analysis of the vredefort domethis article is one of a series of papers published in this special issue on the theme of geochronology in honour of tom krogh integrated electron nanobeam ebsd cl eds and isotopic measurements u pb u th he of zircon from the collar and centre of the 80 km wide central uplift of the 2020 3 ma vredefort impact structure reveal new shock features in a microstructural progression related to impact basin formation and degree of u pb age resetting 1 planar fractures in 1k0 and 1k2 orientation during initial shock wave compression 2 curviplanar fractures in 1k1 orientation now annealed which host glassy inclusions of partial melt of the host rock 3 microtwin lamellae in an orientation of 65 about 110 attributed to shock wave rarefaction 4 nucleation of impact age crystallites possibly on microtwins during post shock heating by impact melt and 5 crystal plastic deformation linked to crater modification of the core of the central uplift planar fracturing and microtwinning 20 gpa in cold shock zircon in granitoid at a radial distance of 25 km failed to reset zircon age single grain id tims data

light rare earth element enrichments in ureilites a detailed ion microprobe study this paper explores the possible origin of the light rare earth element lree enrichments observed in some ureilites a question that has both petrogenetic and chronologic implications for this group of achondritic meteorites rare earth element and other selected elemental abundances were measured in situ in 14 thin sections representing 11 different ureilites the spatial microdistributions of rees in c rich matrix areas of the three ureilites with the most striking v shaped whole rock ree patterns kenna goalpara and novo urei were investigated using the ion imaging capability of the ion microprobe all olivines and clinopyroxenes measured have lree depleted patterns with little variation in ree abundances despite large differences in their major element compositions from ureilite to ureilite furthermore we searched for but did not find any minor mineral phases that carry lrees the only exception is one ti rich area 20 m in lewis cliff lew 85400 with a major element composition similar to that of titanite ree abundances in this area are high ranging from la 400 ci to lu 40 ci in contrast all ion microprobe analyses of c rich matrix in kenna goalpara and novo urei revealed large lree enrichments in addition c rich matrix areas in the three polymict ureilites elephant moraine eet 83309 eet 87720 and north haig which have less pronounced v shaped whole rock ree patterns show smaller but distinct lree enrichments the c rich matrix in antarctic ureilites tends to have much lower lree concentrations than the matrix in non antarctic ureilites there is no obvious association of the lrees with other major or minor elements in the c rich areas ion images further show that the lree enrichments are homogeneously distributed on a microscale in most c rich matrix areas of kenna goalpara and novo urei these observations suggest that the lrees in ureilites most probably are absorbed on the surface of fine grained amorphous graphite in the c rich matrix it is unlikely that the lree enrichments are due to shock melts or are the products of metasomatism on the ureilite parent body we favor lree introduction by terrestrial contamination

tectonic evolution of the european variscan belt constrained by palaeomagnetic structural and anisotropy of magnetic susceptibility data from the northern vosges magmatic arc eastern france geochronology structural and anisotropy of magnetic susceptibility data from the northern vosges batholith which belongs to the northern vosges mid german crystalline rise arc show contrasting emplacement modes of southern granodiorites and northern granites the ene wsw trending fabrics of granodiorites 346 334 ma are parallel to the metamorphic cleavage affecting the host rocks developed during regional compression the nnw sse trending fabrics of younger granitoids c 330 ma reveal an extensional emplacement mode associated with a normal shear zone separating the two magmatic suites palaeomagnetism shows that the switch from a compressive to an extensional regime coincides with a regional counterclockwise rotation the 330 325 ma extension is further supported by palaeomagnetic and seismic data indicating southeastward tilt of the whole batholith finally the system is rotated clockwise without any structural overprint data from the early carboniferous northern vosges magmatic arc and remote bohemian massif suggest that this evolution is valid for the whole eastern branch of the european variscan belt for which the following model is proposed 1 late devonian early carboniferous east west shortening of the variscides above the rhenohercynian subduction zone 2 axial nnw sse shortening of the assembled variscan orogen associated with 3 internal sinistral rotation of inherited rhenohercynian transform faults and shortening of intervening blocks 4 east west extension and dextral external rotation of blocks between dextrally reactivated transforms supplementary materials analytical procedures for geochronology magnetism anisotropy of magnetic susceptibility and palaeomagnetism are available at www geolsoc org uk sup18635

empirical constraints on shock features in monazite using shocked zircon inclusions shock deformation microstructures in monazite have been systematically characterized for the first time in grains from the vredefort impact structure in south africa electron backscatter diffraction mapping has identified 12 unique orientations of monazite deformation twins including 7 orientations that have not previously been described in experiments or nature other shock features include planar deformation bands and strain free neoblasts which have been shown to date deformation shock twinned zircon inclusions within the deformed monazite require pressures of 20 gpa thus providing critical empirical constraints on formation conditions confirming a hypervelocity impact origin of the monazite microstructures the vredefort monazite grains described here represent the first case of using shocked mineral inclusions to empirically calibrate shock microstructures formed in the host mineral these results conclusively establish monazite as a recorder of shock deformation and highlight its use in identifying and dating impact structures

age of variscan magmatism from the balkan sector of the orogen central bulgaria abstract new high resolution secondary ion mass spectrometry u th pb data obtained from zircon and monazite from representative variscan intrusions in the balkan sector of the variscan orogen central bulgaria constrain the ages of magmatism to the interval 315 285 ma the samples investigated are granitoids from two contrasting intrusive suites calc alkaline granitoids of the first suite include the san nikola granite 311 9 4 1 ma and the petrohan granodiorite 304 6 4 0 ma from the balkan terrane and the smilovene 304 1 5 5 ma and hisara 303 5 3 3 ma granites from the sredna gora terrane zircons from these intrusions predominantly lack inherited cores the second suite consists of two mica leucocratic granites and includes the koprivshtitsa 312 0 5 4 ma and strelcha 289 5 7 8 ma plutons zircons from intrusions of the second suite are 80 90 xenocrystic apparent ages of inherited cores are the magmatic age and slightly older 300 400 ma to ordovician 450 ma ages and to neoproterozoic 600 900 ma ages gabbroic magmas related to the first suite are found only in the more northerly balkan terrane while the leucogranites occur only in the southerly sredna gora terrane although coeval the two suites of intrusions are not genetically related and furthermore were likely derived from different source regions via different melt generating mechanisms we propose a tectonic and petrogenetic model whereby following juxtaposition of the balkan and sredna gora terranes during variscan collision the two suites of magmas were generated by continued subduction under eastern europe due to the influx of mafic magmas into the base of the crust and by influx of water rich fluids into high grade metapsammitic rocks of the sredna gora metamorphic series the new ages are somewhat younger than ages for post collisional intrusions in the central european variscan massifs i e bohemian black forest and vosges where ages of intrusions are predominantly 340 320 ma although some younger intrusions may also occur within the intra alpine massifs both older 340 320 ma and younger 310 290 ma granitoids are present farther west intrusions in the iberian massif appear to be predominantly younger 325 290 ma similar to the intrusions of the balkans this may indicate that collision of adjacent crustal blocks culminated at earlier times in the central and more northerly portions of the orogen and moved both east west and south as the evolution of the orogen progressed

a pressure temperature phase diagram for zircon at extreme conditions abstract hypervelocity impact processes are uniquely capable of generating shock metamorphism which causes mineralogical transformations and deformation that register pressure p and temperature t conditions far beyond even the most extreme conditions created by terrestrial tectonics the mineral zircon zrsio4 responds to shock deformation in various ways including crystal plasticity twinning polymorphism e g transformation to the isochemical mineral reidite formation of granular texture and dissociation to zro2 sio2 which provide robust thermobarometers that record different extreme conditions the importance of understanding these material processes is two fold first these processes can mobilize and redistribute trace elements and thus be accompanied by variable degrees of resetting of the u pb system which is significant for the use of zircon as a geochronometer second some features described herein form exclusively during shock events and are diagnostic criteria that can be used to confirm the hypervelocity origin of suspected impact structures we present new p t diagrams showing the phase relations of zrsio4 polymorphs and associated dissociation products under extreme conditions using available empirical and theoretical constraints we present case studies to illustrate zircon microstructures formed in extreme environments and present electron backscatter diffraction data for grains from three impact structures mistastin lake of canada ries of germany and acraman of australia that preserve different minerals and microstructures associated with different shock conditions for each locality we demonstrate how systematic crystallographic orientation relationships within and between minerals can be used in conjunction with the new phase diagrams to constrain the p t history we outline a conceptual framework for a zircon based approach to extreme thermobarometry that incorporates both direct observation of high p and high t phases as well as inferences for the former existence of phases from orientation relationships in recrystallised products a concept we refer to here as phase heritage this new approach can be used to unravel the pressure temperature history of zircon bearing samples that have experienced extreme conditions such as rocks that originated in the earth s mantle and those shocked during impact events on earth and other planetary bodies

a terrestrial perspective on using ex situ shocked zircons to date lunar impacts deformed lunar zircons yielding u pb ages from 4333 ma to 1407 ma have been interpreted as dating discrete impacts on the moon however the cause of age resetting in lunar zircons is equivocal as ex situ grains in breccias they lack lithologic context and most do not contain microstructures diagnostic of shock that are found in terrestrial zircons detrital shocked zircons provide a terrestrial analog to ex situ lunar grains for both identifying diagnostic shock evidence and also evaluating the feasibility of dating impacts with ex situ zircons electron backscatter diffraction and sensitive high resolution ion microprobe u pb analysis of zircons eroded from the ca 2020 ma vredefort impact structure south africa show that complete impact age resetting did not occur in microstructural domains characterized by microtwins planar fractures and low angle boundaries which record ages from 2890 ma to 2645 ma an impact age of 1975 39 ma was detected in neoblasts within a granular zircon that also contains shock microtwins which link neoblast formation to the impact however we show that granular texture can form during regional metamorphism and thus is not unique to impact environments these results demonstrate that dating an impact with ex situ shocked zircon requires identifying diagnostic shock evidence to establish impact provenance and then targeting specific age reset microstructures with the recognition that zircon can deform plastically in both impact and magmatic environments age resetting in lunar zircons that lack diagnostic shock deformation may record magmatic processes rather than discrete impacts identifying shock microstructures that record complete age resetting for geochronological analysis is thus crucial for constructing accurate zircon based impact chronologies for the moon earth or other planetary bodies

structural characteristics of the sudbury impact structure canada impact induced versus orogenic deformation a review orogenic deformation both preceding and following the impact event at sudbury strongly hinders a straightforward assessment of impact induced geological processes that generated the sudbury impact structure central to understanding these processes is the state of strain of the sudbury igneous complex the solidified impact melt sheet its underlying target rocks overlying impact breccias and post impact sedimentary rocks this review addresses 1 major structural metamorphic and magmatic characteristics of the impact melt sheet and associated dikes 2 attempts that have been made to constrain the primary geometry of the igneous complex 3 modes of impact induced deformation as well as 4 mechanisms of pre and post impact orogenic deformation the latter have important consequences for estimating parameters such as magnitude of structural uplift tilting of pre impact huronian strata and displacement on major discontinuities which collectively have not yet been considered in impact models in this regard a mechanism for the emplacement of offset dikes is suggested that accounts for the geometry of the dikes and magmatic characteristics as well as the occurrence of sulfides in the dikes moreover re interpretation of published paleomagnetic data suggests that orogenic folding of the solidified melt sheet commenced shortly after the impact uncertainties still exist as to whether the sudbury impact structure was a peak ring or a multi ring basin and the deformation mechanisms of rock flow during transient cavity formation and crater modification

magma pulses in the central variscan belt episodic melt generation and emplacement during lithospheric thinning magmatism in the central part of the variscan orogenic belt in europe has occurred in several distinct episodes each episode spanned a few million years and was characterized by a distinct geochemical signature and geodynamic setting i between 340 and 330 ma visean high k diorites and granitoids within a zone extending parallel the orogen axis possibly tracing a palaeosuture ii at 310 307 ma westphalian calc alkaline granitoids in the intra alpine domain iii at 304 295 ma stephanian subalkaline granitoids in intra alpine and pennine domains and finally iv at 280 270 ma permian granitoids in pennine austroalpine and south alpine domains the generation and emplacement of granitoid melts in the variscan crust are suggested to be short term pulses that are controlled by extensional processes in the lithosphere such as thermal erosion and relaxation of the lithospheric mantle and decompressional melting

campaign style titanite u pb dating by laser ablation icp implications for crustal flow phase transformations and titanite closure abstract u pb dates of titanite from 150 samples of chiefly quartzofeldspathic gneiss and leucosomes were measured across the western gneiss region of norway to understand deformation and metamorphism of typical crustal rocks during ultrahigh pressure uhp subduction and exhumation titanite is unstable at these high temperatures and pressures and indeed most of the titanite yielded post uhp dates a modest number of titanites sampled across large areas however have pre uhp u pb dates indicating that they survived their excursion to and return from mantle depths metastably this has three important implications 1 titanite grains can remain closed to complete pb loss during regional metamorphism at temperatures as high as 750 c and pressures as high as 3 gpa 2 phase transformations in quartzofeldspathic rocks can be inhibited at the same conditions 3 quartz bearing rocks can remain undeformed even at high temperature and pressure both of the latter were previously recognized the present study simply presents a new method for evaluating both using titanite u pb dates

ams record of brittle dilation viscous stretching and gravity driven magma ascent in area of magma rich crustal extension vosges mts ne france orogenic compression related fabrics 340 335 ma were reworked during regional extensional deformation 328 325 ma in a large anatectic crustal domain of the central vosges ne france the extension was first accommodated by brittle dilation affecting vertically anisotropic high grade rocks associated with emplacement of subvertical granitic sheets the ams fabric of granitoids is consistent with highly partitioned transtensional deformation marked by alternations of flat and steep foliations and development of orthogonal lineations this deformation passes to top to the southwest ductile shearing expressed in southerly migmatitic middle crust the ams fabric revealed moderately west dipping foliations bearing subhorizontal nnw sse trending lineations and predominantly plane strain to prolate shapes this fabric pattern is interpreted as a viscous response of stretched partially molten crust during continuous ductile extension vertical ascent of voluminous granites and stoping of the upper crust occurs further south this gravity ascent triggered by extension leads to development of south dipping ams foliations south plunging lineations and oblate fabrics in various crustal granites vertical shortening related to ascent of these 325 ma granitoids and persistent n s stretching is responsible for reworking and remelting of originally vertical compression related fabric in roof supracrustal granites 340 ma and development of highly prolate fabrics in these rocks this work shows that the finite shape of ams fabric ellipsoid is highly sensitive to both strain regime and superpositions of orthogonal deformation events

u pb investigations on zircons from pre variscan gneisses i a study from the schwarzwald west germany abstract field observations in the schwarzwald indicate that pre variscan rocks were formed by the following events 1 1 intrusion of granitic to granodioritic magma into sediments 2 2 a sequence of two anatectic events with an intervening regional metamorphism and development of a gneissic schistosity the absolute chronology of these older gneisses and migmatites has not yet been satisfactorily resolved we collected several samples of a coarse grained homogeneous orthogneiss of granodioritic composition and one sample of a metasedimentary source rock related to this orthogneiss body in the einetwald forest north of steinach kinzig valley the metasedimentary rock corresponds to the biotitepoor paragneisses in the schwarzwald the zircons in the latter sample are detrital and only very small specimens 45 m are euhedral the zircons from the orthogneiss samples are euhedral some are zoned and some have dark inclusions analyses of the three coarser fractions of the paragneiss form a linear array on a concordia diagram with upper and lower intercepts of about 2000 myr and 524 20 myr respectively the 2000 myr value is interpreted as the average age of the sedimentary source rocks for this material the 524 myr intercept defines an event responsible for most of the lead loss in the zircons as well as new zircon growth analyses of the zircons from the orthogneiss samples do not show a linear array on the concordia diagram because of inclusions of older inherited zircons stepwise leaching of two zircon fractions shows newly grown u rich zircon rims around old cores a 207 pb 206 pb age of 517 15 myr was found for the first dissolution steps about 50 of the zircons dissolved in subsequent steps with progressive dissolution of the residue 207 pb 206 pb age increased to 950 myr the u pb zircon data for the paragneiss and the orthogneiss establish an age of 520 15 myr for the magma intrusion

hydrocarbon geochemistry of the puget sound region ii sedimentary diterpenoid steroid and triterpenoid hydrocarbons cyclic components of the aliphatic hydrocarbon mixtures extracted from puget sound sediment cores include a suite of c19 and c20 diterpenoid hydrocarbons of which fichtelite sandaracopimaradiene and isopimaradiene have been identified although apparently also derived from vascular plants these diterpenoid hydrocarbons have relative abundances distinctly different from the co existing plant wax n alkane suite five c27 c28 and c29 diasteranes and four c29 c30 and c31 17 h 21 h hopanes occur in relatively constant proportion as components of a highly weathered fossil hydrocarbon assemblage these chromatographically resolved cycloalkanes along with the strongly covarying unresolved complex mixture have been introduced to puget sound sediments from adjacent urban centres at increasing levels over the last 100 yr in the absence of any major oil spill naturally occurring triterpenoid hydrocarbons including hop 22 29 ene diploptene are also present a new group of c30 polyenes has been detected which contains compounds apparently structurally related to a co existing bicyclic c25 diene and to c20 and c25 acyclic multibranched hydrocarbons described in a previous paper barrick et al 1980

the application of u pb geochronology to mafic ultramafic and alkaline rocks an evaluation of three mineral standards abstract a key to proving the accuracy of u pb age determinations by the various techniques available is routine analysis of an internal well characterized matrix matched mineral standard of known age in general there are very few well studied natural mineral standards available to the geological community in this study the nature geochemistry and u pb systematics of baddeleyite titanite and perovskite from three potential mineral standards are evaluated a compilation of concordant to near concordant analyses of a single phalaborwa baddeleyite crystal yields a precise u pb age of 2059 60 0 35 ma a large variation in uranium 51 2124 ppm thorium 0 5 73 ppm and lead 19 782 ppm concentrations and the presence of some u bearing mineral inclusions zircon monazite and baddeleyite zircon intergrowths require some caution when using aliquots of this phalaborwa baddeleyite crystal as a mineral standard the khan titanite crystal investigated here overall has a relatively uniform chemical composition e g u 584 95 ppm th 473 73 ppm pb 57 8 ppm th u 0 81 0 05 but parts of the crystal contain abundant mineral inclusions the model 1 upper intercept age of 522 2 2 2 ma for nine khan titanite analyses is considered the current best estimate for its crystallization age some variability in the u pb systematics with 207pb 206pb ages that vary between 508 and 560 ma was observed and could be related to the presence of tiny u bearing mineral inclusions such as rutile or allanite so careful selection of material from this titanite crystal is also required a single perovskite crystal from the ice river alkaline intrusion has a relatively uniform chemical composition uranium 141 25 ppm thorium 1954 270 ppm lead 41 5 ppm and th u 14 5 1 3 the u pb results for eight perovskite fractions yield a weighted average 206pb 238u age of 356 5 1 0 ma although there are always some limitations to using natural mineral crystals as u pb standards such as the presence of mineral inclusions alteration and geochemical heterogeneity the three crystals investigated here are considered suitable to be used as internal u pb standards for any of the currently used u pb dating techniques

lead isotopic composition of hercynian granitic k feldspars constrains continental genesis lead isotope compositions have been determined in many k feldspars from granitic rocks emplaced during the variscan hercynian orogeny sensu lato around 300 myr in various parts of europe and the eastern us the ratios show a regular geographical pattern with contours arched around southern france and parallel to the atlantic coast of the us this distribution parallels predriftpalaeogeographical zones during the palaeozoic the lead isotope pattern of variscan granitic rocks is similar to that of some orogenic volcanic suites but is better explained by crustal melting than by an hercynian mantle crust material mixing the continental crust from these areas must be older than 3 000 myr large scale fluid circulation in the crust during orogenic processes apparently explains the observed geographical patterns of lead isotopes

dating shear zones with plastically deformed titanite new insights into the orogenic evolution of the sudbury impact structure ontario canada abstract the sudbury structure is a mineralized impact crater that hosts different families of ore controlling shear zones with poorly known orogenic affinities discriminating whether these deformation events relate to the 1 85 ga crater modification stage or later regional tectonism that collapsed the impact structure is important both for crustal and mineral exploration studies we have combined underground mapping with isotopic and microstructural analysis of titanite and host minerals in a benchmark ore controlling mylonitic shear zone of the mining camp the six shaft shear zone from the creighton mine three growth stages of chemically and microstructurally characterised titanite grains were identified related with the pre syn and late deformation stages in situ u pb age dating of the syndeformational grains demonstrates that a shearing event took place at 1645 54 ma during the mazatzalian labradorian orogeny 1 7 1 6 ga this event led to the plastic deformation and local scale remobilization of primary ni cu pge sulphides in creighton mine sudbury south range the adopted novel petrochronological approach can reveal the age significance of syn deformational processes and holds promise for the untangling of complex syn orogenic processes in precambrian terranes globally

polyorogenic reworking of ore controlling shear zones at the south range of the sudbury impact structure a telltale story from in situ u pb titanite geochronology

combined oxygen isotope and u pb zoning studies of titanite new criteria for age preservation this material is based upon work supported by the u s national science foundation award ear 0838058 the u s department of energy office of basic energy sciences under award number de fg02 93er14389 and geological society of america student research grant 9224 10 wiscsims is partially supported by nsf ear 1053466 and 1355590

microscopic planar deformation features in quartz of the vredefort structure anomalous but still suggestive of an impact origin abstract the orientation and distribution of diagnostic shock produced planar features in quartz has been examined in over 80 samples from both the core and collar rocks of the vredefort structure in south africa these features are widespread occurring in all but four samples the majority of the planar features have a basal 0001 orientation with a few higher pressure so called features and three features occurring near the center of the structure their relative distribution and preservation are however anomalous compared to those at other large terrestrial impact structures and recorded shock pressures calculated on the basis of planar feature orientation do not show a regular decrease radially outward as observed at other known impact structures the apparently anomalous development of planar features is interpreted as a result of post shock recrystallization if the effect of recrystallization of quartz which generally increases inward toward the core of the structure is considered and only the highest pressure type planar feature present in any one sample is taken as a measure of shock pressure then higher pressures are recorded in the core compared to the collar rocks no evidence of a previously reported additional post recrystallization shock event is found in this study although previous researchers report that pseudotachylite which may or may not be associated with the shock event cuts recrystallized quartzites in is concluded that although anomalous the evidence from planar features in quartz is still consistent with the vredefort structure being an erosional remnant of a large complex impact structure

palaeozoic evolution of the variscan vosges mountains a geological synthesis of the palaeozoic vosges mountains ne france is presented using existing observations and new data the geodynamic evolution involves 1 early palaeozoic sedimentation and magmatism 2 late devonian subduction triggering back arc spreading 3 early lower carboniferous continental subduction continent continent collision and polyphase deformation and metamorphism of the orogenic root and 4 late lower carboniferous orogenic collapse driven by thermal weakening of the middle crust the evolution is integrated within the framework of the european variscan belt the northern vosges comprise sediments of rhenohercynian affinity separated from tepla barrandian metasediments by a lower carboniferous magmatic arc the latter is correlated with the mid german crystalline rise and is ascribed to the south directed subduction of the rhenohercynian basin the saxothuringian moldanubian suture is thought to be obliterated by the magmatic arc while the lalaye lubine fault is interpreted as the tepla barrandian moldanubian boundary the central vosges are paralleled with the moldanubian domain of the bohemian massif where identical lithologies record the devonian carboniferous se directed subduction of the saxothuringian passive margin below the moldanubian upper plate the southern vosges represent the upper moldanubian crust and are linked to the southern black forest the presence of an oceanic domain to the south of the vosges black forest remains unclear

variable microstructural response of baddeleyite to shock metamorphism in young basaltic shergottite nwa 5298 and improved u pb dating of solar system events abstract the accurate dating of igneous and impact events is vital for the understanding of solar system evolution but has been hampered by limited knowledge of how shock metamorphism affects mineral and whole rock isotopic systems used for geochronology baddeleyite monoclinic zro2 is a refractory mineral chronometer of great potential to date these processes due to its widespread occurrence in achondrites and robust u pb isotopic systematics but there is little understanding of shock effects on this phase here we present new nano structural measurements of baddeleyite grains in a thin section of the highly shocked basaltic shergottite northwest africa nwa 5298 using high resolution electron backscattered diffraction ebsd and scanning transmission electron microscopy stem techniques to investigate shock effects and their linkage with u pb isotopic disturbance that has previously been documented by in situ u pb isotopic analyses the shock altered state of originally igneous baddeleyite grains is highly variable across the thin section and often within single grains analyzed grains range from those that preserve primary magmatic twinning and trace element zonation baddeleyite shock group 1 to quasi amorphous zro2 group 2 and to recrystallized micro granular domains of baddeleyite group 3 these groups correlate closely with measured u pb isotope compositions primary igneous features in group 1 baddeleyites n 5 are retained in high shock impedance grain environments and an average of these grains yields a revised late amazonian magmatic crystallization age of 175 30 ma for this shergottite the youngest u pb dates occur from group 3 recrystallized nano to micro granular baddeleyite grains indicating that it is post shock heating and new mineral growth that drives much of the isotopic disturbance rather than just shock deformation and phase transitions our data demonstrate that a systematic multi stage microstructural evolution in baddeleyite results from a single cycle of shock loading heating and cooling during transit to space and that this leads to variable disturbance of the u pb isotope system furthermore by linking in situ u pb isotopic measurements with detailed micro to nano structural analyses it is possible to resolve the timing of both endogenic crustal processes and impact events in highly shocked planetary materials using baddeleyite this opens up new opportunities to refine the timing of major events across the solar system

lacustrine organic facies a biomarker study using multivariate statistical analysis abstract four distinct depositional settings in the devonian orcadian basin of scotland have been characterised by their minerology and organic geochemistry with the help of multivariate statistical analysis the data visualise the organic matter source input the depositional environment and the level of thermal maturity principal component analysis proved invaluable for extracting information from a data set composed of a large number of parameters the thermal maturity of the majority of the samples lies within a restricted range in the oil window and biomarker composition reflects predominantly organic input and depositional environment basin margin deposits are clearly differentiated from deeper water central lake deposits salinity and anoxicity variations are seen within the latter type of deposits

saturated and olefinic hydrocarbons in recent sediments from the baltic sea abstract saturated and olefinic hydrocarbons in the top sections of contemporary sediments from four stations in the baltic sea teili 1 f 81 eb 1 and xv 1 were analysed by capillary gas chromatography and gc ms techniques the 0 2 2 4 4 5 and 10 11 cm sections showed a small unresolved complex mixture ucm however in all these sections a strong predominance of odd over even carbon numbered compounds exists indicating an insignificant petroleum hydrocarbon input in the areas studied the resolved hydrocarbons are of mixed marine autochthonous and terrigenous origin the 0 5 cm sections contained the highest proportions of short chain and branched chain hydrocarbons derived mostly from algae and bacteria in general an increase was observed in the ratios of saturated to unsaturated and of longer chain to short chain hydrocarbons with increasing depth because of the facile diagenesis of unsaturated components and faster biodegradation of short chain hydrocarbons in addition between one and seven hydrocarbons the exact structure of which could not be assigned were detected in the samples most of these components were cyclic and unsaturated and a number were triterpenes

integrated geophysical modelling of a giant complex impact structure anatomy of the vredefort structure south africa abstract only three very large confirmed impact structures are known on earth the chicxulub crater mexico 65 ma ca 180 km wide the sudbury structure canada 1 85 ga 200 km in diameter and the vredefort structure in south africa 2 02 ga while extensive data on large impact structures have been obtained by remote sensing studies of such features on other planetary bodies only this small number of large terrestrial impact structures can provide data crucial to understanding these catastrophic impact processes on earth integrated modelling of gravity and magnetic data constrained by geological as well as refraction and reflection seismic data accomplished the reconstruction of the vredefort impact structure in south africa approximately 250 km wide the original vredefort impact structure covered the whole extent of the archaean witwatersrand basin distinguished by enormous gold resources as it is structurally preserved today in fact it is clear that the preservation of vast volumes of economically important witwatersrand strata is the direct result of the formation of the ring basin around the central uplift the vredefort dome of the impact structure this study is the first attempt to create an integrated and geophysically well constrained model of this very large complex impact structure

the age of the kagenfels granite northern vosges and its bearing on the intrusion scheme of late variscan granitoids in the saxothuringian part of the vosges france a first series of variscan plutonic rocks diorites to granites has been intruded by several younger granites rocks of both the older generations have been cross cut by the late orogenic kagenfels granite the averages of the hitherto published mineral ages of the earlier rock generations are 331 and 334 ma respectively whereas rb sr and k ar dates around 290 ma have been reported for the kagenfels granite because of the unlikely large age hiatus a redetermination of the intrusion age of the kagenfels granite formation appeared to be irrevocable the newly obtained mineral ages on the kagenfels granite k ar and 40ar 39ar biotite ages as well as single zircon radiogenic 207pb 206pb data 331 5 ma are about 40 ma older than the previous results they are interpreted as giving the time of emplacement of the kagenfels granite during the latest viscan the mineral ages of the earlier plutonic rocks in this part of the variscan orogeny in all probability are not significantly different from their ages of intrusion therefore the age concordance of all three granitoid generations constrains a rather narrow time interval of orogenic magmatism close to the lower upper carboniferous boundary

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

devonian permian magmatic pulses in the northern vosges mountains ne france result of continuous subduction of the rhenohercynian ocean and avalonian passive margin

u th he dating of terrestrial impact structures the manicouagan example the accurate dating of meteorite impact structures on earth has proven to be challenging melt sheets are amenable to high precision dating by the u pb and 40ar 39ar methods but many impact events do not produce them or they are not preserved in cases where high temperature shock metamorphism of the target materials has occurred without widespread melting these isotopic chronometers may be partially reset and yield dates that are difficult to interpret unambiguously as the age of impact however the u th he chronometer is sensitive to thermal resetting and can provide a powerful new tool for dating impactites we report u th he dates for accessory minerals from the manicouagan impact structure in quebec canada nine zircons from a melt sheet sample yield a weighted mean age of 213 2 5 4 ma 2se indistinguishable from the published 214 1 ma 2 u pb zircon age for the impact in contrast five apatites from this sample yield dates between 205 9 6 5 and 162 0 5 3 ma 2 indicating variable postimpact helium loss due to low temperature thermal disturbance preimpact titanite crystals from a shocked meta anorthosite sample yield two dates consistent with the impact age at 212 27 and 214 13 ma 2 and two younger dates of 189 6 6 9 and 192 2 9 8 ma 2 suggestive of postimpact helium loss these results indicate that u th he chronometry is a suitable method for dating impact events although interpretation of the results requires recognition of possible 4he loss related to reheating subsequent to impact

in situ multiphase u pb geochronology and shock analysis of apatite titanite and zircon from the lac la moinerie impact structure canada in situ u pb geochronology has been performed via laser ablation inductively coupled mass spectrometry on shocked apatite titanite and zircon from the 8 km diameter lac la moinerie impact structure canada the three analyzed phases are inherited from target rocks that were transformed to impact melt bearing breccias and clast laden impact melts apatite yields an array of u pb ratios populating a region between a younger regression with a refined age of 453 5 ma and an older regression with a lower intercept age of 1708 10 ma titanite defines a younger regression with an age of 444 15 ma and an older array with a lower intercept at 1844 24 ma zircon yields an upper intercept age of 1810 13 ma and a lower intercept age of 433 21 ma within error of apatite and titanite the oldest ages from all three phases define paleoproterozoic igneous metamorphic events in granitoid target rocks of the de pas suite isotopic resetting in apatite and titanite with relatively high pb diffusion rates was controlled by a combination of proximity to impact generated superheated melt and the formation of fast diffusion pathways facilitated by shock effects and dynamic recrystallization zircon resetting was controlled by shock effects and metamictization which resulted in significant pb loss we outline an approach for discriminating true impact ages from metamict zircons with high common pb that have experienced post impact pb loss lower intercept ages of all phases constrain the age of formation for the lac la moinerie impact structure with apatite yielding the most precise age of 453 5 ma with an age of 453 ma lac la moinerie joins a growing cluster of more than 12 terrestrial impact structures formed in the middle to late ordovician with potential connection to the l chondrite parent body breakup event and a period of enhanced bombardment on earth

kinetically induced compositional zoning in titanite implications for accessory phase melt partitioning of trace elements usually it is assumed that the partitioning of trace elements into titanite in metaluminous granitoid plutonic environments takes place under equilibrium conditions and that compositional zoning is due solely to progressive changes in melt chemistry and or mineral melt partition coefficients examination of titanites from a variety of caledonian metaluminous granitoids and related rocks has revealed that sector zoning is present indicating disequilibrium partitioning the sector zoning in titanites is defined principally by the distribution of the rare earth elements ree y nb al and fe the ree y and nb preferentially occur within the minor 100 sectors relative to the morphologically important 111 sectors the reverse is true of al and fe which preferentially occur within the 111 sectors relative to the 100 sectors the patterns of sector zoning are complicated by the fact that the relative growth rates of the various crystal faces fluctuated during growth sector zoning indicates that crystal interface kinetics are responsible for the observed patterns of element partitioning it is concluded that differences in the lateral layerspreading rates of crystal faces bring about the sector zoning the results have implications for the use of trace element partition coefficients in the modelling of fractionation processes

geology of the giant sudbury polymetallic mining camp ontario canada a new 1 100 000 scale compilation bedrock geology map provided with this issue presents a geologic synthesis of canada s most prolific mining camp the world class sudbury structure with total past production and current reserves of over 1 7 billion tonnes of ni cu co pt pd au ag ore lydon 2007 the polymetallic ore is hosted within one of earth s largest preserved impact craters the new map of the ni cu pge sudbury district after ames et al 2005 is included in a pocket at the back of this issue map 1 and is linked to mineral deposit commodity and geochronology data in tables 1 and 2 of this paper view this table table 1 list of mineral deposits and occurrences of the sudbury map area map 1 view this table table 2 u pb isotope data across the map area since 1891 six compilation maps of the geology of sudbury have been published bell 1891 collins 1937 cooke 1946 card 1969 dressler 1984 ames et al 2005 fig 1 some of the ni cu pge mines have operated for over a century whereas new ore deposits discovered as recently as 2004 are already in production or are in the process of being developed i e advanced prospects in the last 15 years sudbury has been the training ground for 60 postgraduate geoscience students in north america fig 1 from 1903 to 1990 only 23 graduate theses were completed on sudbury related topics eight of them in the 1970s after the discovery of shatter cones dietz 1964 fig 1 number of graduate theses of north american geoscientists trained on sudbury geology per decade showing the minor peak in training after the discovery of shatter cones and the recent education boom with over 50 geoscience graduate theses completed since 1990 the dashed lines represent the regional geologic maps of the sudbury area

shocked titanite records chicxulub hydrothermal alteration and impact age abstract hydrothermal activity is a common phenomenon in the wake of impact events yet identifying and dating impact hydrothermal systems can be challenging this study provides the first detailed assessment of the effects of shock microstructures and impact related alteration on the u pb systematics and trace elements of titanite catisio5 focusing on shocked granite target rocks from the peak ring of the chicxulub impact structure mexico a 1 mm long shock twinned titanite grain preserves a dense network of irregular microcracks some of which exploit shock twin interfaces secondary microcrystalline anatase and pyrite are heterogeneously distributed along some microcracks in situ laser ablation multi collector inductively coupled plasma mass spectrometry la mc icpms analysis reveals a mixture of three end member pb components the pb components are 1 common pb consistent with the pb isotopic signature of adjacent alkali feldspar 2 radiogenic pb accumulated since magmatic crystallization and 3 a secondary younger pb signature due to impact related complete radiogenic pb loss the youngest derived ages define a regression from common pb that intersects concordia at 67 4 ma in agreement with the established age of 66 04 0 05 ma for the chicxulub impact event contour maps of la mc icpms data reveal that the young ages are spatially restricted to microstructurally complex domains that correlate with significant depletion in trace elements ree y zr nb mo sn th and reduction in magnitude of the eu eu anomaly mapping by time of flight secondary ion mass spectrometry tof sims show that patterns of localised element depletion in titanite are spatially related to microcracks which are enriched in al the spatial correlation of ages and trace element abundance is consistent with localised removal of pb and other trace elements from a pervasive network of fast fluid pathways in fractured domains via a fluid mediated element transport process associated with the impact event here we interpret the 67 4 ma u pb age to represent hydrothermal pb loss in the chicxulub peak ring in the wake of the impact event these results highlight the potential of our analytical approach using titanite geochronology and geochemistry for dating post impact hydrothermal activity in impact structures elsewhere

pb diffusion in zircon abstract chemical diffusion of ti under anhydrous conditions at 1 atm and under fluid present elevated pressure 1 1 1 2 gpa conditions has been measured in natural zircon the source of diffusant for 1 atm experiments was a zro2 tio2 zircon mixture with experiments run in crimped pt capsules diffusion experiments done in the presence of h2o co2 fluid were run in a piston cylinder apparatus using a source of ground tio2 zrsio4 and sio2 with oxalic acid added to produce h2o co2 vapor and partially melt the solid source material yielding an assemblage of rutile zircon melt vapor nuclear reaction analysis nra with the resonant nuclear reaction 48ti p 49v was used to measure diffusion profiles for both sets of experiments the following arrhenius parameters were obtained for ti diffusion normal to cover the temperature range 1350 1550 c at 1 atm d ti 3 33 10 2 exp 754 56 kj mol 1 r t m 2 s 1 ti diffusivities were found to be similar for experiments run under fluid present conditions a fit to all of the data yields the arrhenius relation d 1 34 102 exp 741 46 kj mol 1 rt m2 s 1 these data suggest that zircon should be extremely retentive of ti chemical signatures indicating that the recently developed ti in zircon crystallization geothermometer watson e b harrison t m 2005 zircon thermometer reveals minimum melting conditions on earliest earth science 308 841 844 will be quite robust in preserving temperatures of zircon crystallization titanium diffuses somewhat faster in zircon than larger tetravalent cations u th and hf but considerably more slowly than pb the ree and oxygen hence ti crystallization temperatures may be retained under circumstances when radiometric ages or other types of geochemical information are lost

possible origin of n alkanes with a remarkable even to odd predominance in recent marine sediments abstract n alkane distributions with a remarkable even to odd predominance c16 c24 were found in marine surface sediments the previously proposed diagenetic reduction of the corresponding n fatty acids could not be considered as a source for these n alkane based on a comparison of compositional features of n alkane n fatty acid and n alcohol distributions carbon isotope analyses and other geochemical parameters the data indicate that the even predominant n alkanes were derived directly from marine bacteria

hydrocarbon geochemistry of siliciclastic microbial laminated deposits from puck bay poland abstract samples of siliciclastic organic rich microbial laminated deposits from the brackish water environment of the puck bay southern baltic sea were analyzed by means of gas chromatography and mass spectrometry the distribution of extractable hydrocarbon fraction was examined gc ms investigations revealed that there occur n alkanes from c 16 to c 35 hopanoids and isoprenoids pristane and phytane in spite of a number of subfossilized cyanobacteria and microalgae in the composition of the n alkane fraction long chain alkanes predominate high contents of c 22 and c 24 alkanes relative abundance and variety of hopanoids two of which have not been described previously seems to be better indicators of the presence of cyanobacteria and microalgae and of their biodegraded remains

u pb geochronology of apatite and zircon from the brent impact structure canada a late ordovician sandbian katian boundary event associated with l chondrite parent body disruption in situ la icp ms u pb geochronology has been performed on apatite and zircon within thermally recrystallized clast laden and clast poor impact melt rocks from the brent impact structure a total of 377 laser analyses on 120 impact melt grown n 9 and impact recrystallized zircon grains n 111 were obtained from which a concordia age of 452 8 2 7 ma mswd 0 57 n 11 and a weighted average mean 206pb 238u age of 453 2 2 9 ma mswd 0 60 n 11 are calculated a total of 300 laser analyses from 100 relict apatite grains were obtained with an unanchored regression through all data yielding a lower intercept age of 453 2 6 0 ma mswd 5 8 n 300 that overlaps within error of zircon 207pb 206pb ratios obtained from feldspar clasts within clast laden impact melt retain the same initial pb composition as the target rocks from which they are derived while feldspars that crystallized from impact melt have 207pb 206pb ratios indicative of isotopic re equilibration between basement lithologies of two different ages a similar variability in 207pb 206pb is recorded by apatite this provides evidence for the involvement of neoproterozoic lake nipissing alkaline suite as well as mesoproterozoic grenville gneisses in the production of impact melt at brent recrystallized apatite grains exhibit enrichments in light rare earth elements lrees along neoblast grain boundaries indicative of trace element substitution and phase precipitation during impact induced recrystallization an age of 452 8 2 7 ma from zircon and 453 2 6 0 ma from apatite places the impact event in the late ordovician at or near the sandbian katian boundary confirming brent s involvement in the middle to late ordovician crater cluster event a period of enhanced impactor flux to earth related to the l chondrite parent body disruption

seismic images and three dimensional architecture of a proterozoic shear zone in the sudbury structure superior province canada a broad zone of intense ductile to brittle deformation transects the paleo proterozoic sudbury igneous complex and likely formed during the final stages of the penokean orogeny on the southern margin of the superior province herein we present a three dimensional image of this deformation zone derived from four seismic reflection profiles additional information from rock property measurements drill results and published structural mapping suggests that the seismic data define the three dimensional disposition of listric shear zones the seismic image of the deformation is best developed within the onaping formation however the images of the deformation zone envelope are imperfect being most clear where truncating known contacts within the sudbury igneous complex and its contact with huronian host rocks the southern edge of the seismically imaged shear zones does not extend significantly beyond the exposed sudbury igneous complex suggesting limits to the extent of deformation in the sudbury structure

intragrain oxygen isotope zoning in titanite by sims cooling rates and fluid infiltration along the carthage colton mylonite zone adirondack mountains ny usa oxygen isotopes are an attractive target for zoning studies because of the ubiquity of oxygen bearing minerals and the dependence of mineral 18 o 16 o ratios on temperature and fluid composi tion in this study subtle intragrain oxygen isotope zoning in titanite is resolved at the 10 lm scale by secondary ion mass spectrometry the patterns of d 18 o zoning differ depending on microstructural context of individual grains and reflect multiple processes including diffusive oxy gen exchange partial recrystallization grain size reduction and grain growth using the chrono logical framework provided by structural relations these processes can be related to specific events during the grenville orogeny titanite was sampled from two outcrops within the car thage colton mylonite zone ccmz a long lived shear zone that ultimately accommodated exhumation of the adirondack highlands from beneath the adirondack lowlands during the ott awan phase 1090 1020 ma of the grenville orogeny titanite is hosted in the diana metasyenite complex which preserves three sequentially developed fabrics an early nw dipping protomylonitic fabric s1 is crosscut by near vertical ultramylonitic shear zones s2 which are locally reoriented by a nnw dipping mylonitic fabric s3 texturally early titanite pre s2 shows diffusion domi nated d 18 o zoning that records cooling from peak ottawan granulite facies conditions numerical diffusion models in the program fast grain boundary yield good fits to observed d 18 o profiles for cooling rates of 50 20 cm a 1 which are considerably faster than the 1 5 cm a 1 cooling rates previously inferred for the adirondack highlands from regional thermochronology high cool ing rates are consistent with an episode of rapid shearing and exhumation along the ccmz during gravitational collapse of the ottawan orogen at c 1050 ma texturally later titanite syn s2 h as higher overall d 18 o and shows a transition from diffusion dominated to recrystallization dominated d 18 o zoning indicating infiltration of elevated d 18 o fluids along s2 shear zones and continued shear ing below the blocking temperature for oxygen 500 c for grain sizes at the study site the tex turally latest titanite post s3 h as growth dominatedd 18 o zoning consistent with porphyroblastic grain growth following cessation of shearing along the harrisville segment of the ccmz

u pb isotopic dating of titanite microstructures potential implications for the chronology and identification of large impact structures identifying and dating large impact structures is challenging as many of the traditional shock indicator phases can be modified by post impact processes refractory accessory phases such as zircon while faithful recorders of shock wave passage commonly respond with partial u pb age resetting during impact events titanite is an accessory phase with lower pb closure temperature than many other robust chronometers but its potential as indicator and chronometer of impact related processes remains poorly constrained in this study we examined titanite grains from the sudbury ontario canada and vredefort south africa impact structures combining quantitative microstructural and u pb dating techniques titanite grains from both craters host planar microstructures and microtwins that show a common twin host disorientation relationship of 74 about in the vredefort impact structure the microtwins deformed internally and developed high and low angle grain boundaries that resulted in the growth of neoblastic crystallites u pb isotopic dating of magmatic titanite grains with deformation microtwins from the sudbury impact structure yielded a 207pb 206pb age of 1851 12 ma that records either the shock heating or the crater modification stage of the impact event the titanite grains from the vredefort impact structure yielded primarily pre impact ages recording the cooling of the ultra high temperature ventersdorp event but domains with microtwins or planar microstructures show evidence of u pb isotopic disturbance despite that the identified microtwins are not diagnostic of shock metamorphic processes our contribution demonstrates that titanite has great potential to inform studies of the terrestrial impact crater record

metallogenic provinces and the remobilization process studied by lead isotopes lead zinc ore deposits from the southern massif central france the two main problems of ore genesis are the origin of metals and the process es of emplacement lead isotopes have proven to be a powerful tool for studying these questions we have studied pb zn mineralizations from montagne noire and cevennes southern massif central france by means of lead isotopes this study shows the existence of two populations of ore deposits defined by 17 76 206 pb 204 pb 17 94 and 18 25 206 pb 204 pb 18 63 a general model of lead evolution in these deposits is proposed after evolving in the earth s mantle until approximately 3 b y ago the lead became incorporated into a segment of continental crust then the less radiogenic population of ore deposits was emplaced contemporaneously with the formation of cambrian carbonate host rocks the second population is related to the emplacement of hercynian granitoids or in more general terms to fluid circulations associated with the hercynian orogeny it can also be shown that some of the deposits are remobilized from preexisting ore deposits

pre hercynian mantle lead transfer to basement rocks as indicated by lead isotopes of the schwarzwald crystalline sw germany ii lead isotope evolution of the european hercynides

origin of large volume pseudotachylite in terrestrial impact structures large volume pseudotachylite bodies in impact structures are dike like and consist of angular and rounded wall rock fragments enveloped by a microcrystalline and sporadically glassy matrix that crystallized from a melt knowledge of the formation of pseudotachylite bodies is important for understanding mechanics of complex crater formation most current hypotheses of pseudotachylite formation inherently assume that fragmentation and melt generation occur during a single process based on the structure of pseudotachylite bodies at sudbury canada and vredefort south africa we show that these processes differ in time and space we demonstrate that the centimeter to kilometer scale bodies are effectively fragment and melt fi lled tension fractures that formed by differential rotation of target rock during cratering highly variable pseudotachylite characteristics can be accounted for by a single process i e drainage of initially superheated impact melt into tension fractures of the crater fl oor

shocked monazite chronometry integrating microstructural and in situ isotopic age data for determining precise impact ages monazite is a robust geochronometer and occurs in a wide range of rock types monazite also records shock deformation from meteorite impact but the effects of impact related microstructures on the u th pb systematics remain poorly constrained we have therefore analyzed shock deformed monazite grains from the central uplift of the vredefort impact structure south africa and impact melt from the araguainha impact structure brazil using electron backscatter diffraction electron microprobe elemental mapping and secondary ion mass spectrometry sims crystallographic orientation mapping of monazite grains from both impact structures reveals a similar combination of crystal plastic deformation features including shock twins planar deformation bands and neoblasts shock twins were documented in up to four different orientations within individual monazite grains occurring as compound and or type one twins in 001 100 left 10 bar 1 right 110 left 212 right and type two irrational twin planes with rational shear directions in 0 bar 1 bar 1 and bar 1 bar 1 0 sims u th pb analyses of the plastically deformed parent domains reveal discordant age arrays where discordance scales with increasing plastic strain the correlation between discordance and strain is likely a result of the formation of fast diffusion pathways during the shock event neoblasts in granular monazite domains are strain free having grown during the impact events via consumption of strained parent grains neoblastic monazite from the inlandsee leucogranofels at vredefort records a 207pb 206pb age of 2010 15 ma 2 n 9 consistent with previous impact age estimates of 2020 ma neoblastic monazite from araguainha impact melt yield a concordia age of 259 5 ma 2 n 7 which is consistent with previous impact age estimates of 255 3 ma our results demonstrate that targeting discrete microstructural domains in shocked monazite as identified through orientation mapping for in situ u th pb analysis can date impact related deformation monazite is therefore one of the few high temperature geochronometers that can be used for accurate and precise dating of meteorite impacts

the significance of titanite and apatite u pb ages constraints for the post magmatic thermal hydrothermal evolution of a batholithic complex berens river area northwestern superior province canada abstract titanite apatite and in one case rutile from a suite of twenty seven samples representing the major phases of an extensive and well dated archean plutonic domain of the northwestern superior province have been dated by u pb to determine the temporal progress of thermal and hydrothermal events accompanying and following major plutonism and to assess the isotopic behaviour of these geochronometers most samples contain 1 dark brown titanite with 350 20 ppm u showing shapes and textural relationships indicative of a primary magmatic origin and yielding ages corresponding either to the time of magmatic crystallization or to that of secondary metamorphic or magmatic events known in the area 2 pale titanite with 30 2 ppm u showing textural evidence for a secondary origin by breakdown of biotite hornblende oxide or older titanite and yielding ages that in general post date the major plutonic and metamorphic events in the area 3 apatite containing 30 5 ppm u occasionally up to 150 ppm u showing igneous shapes and textural relationships but yielding post magmatic ages similar to those of coexisting pale titanite 4 rutile in one sample postdating the ages of coexisting titanite and apatite the common coexistence of two noncoeval titanite generations demonstrates that the often invoked mechanism of pb diffusion during slow cooling in this case has no general validity the distribution of primary dark titanite ages however could be explained by late closure to diffusion or alternatively as the result of resetting by the thermal or hydrothermal effects caused by younger plutonic activity the ages of the secondary pale titanites probably reflect crystallization during discrete retrogressive events promoted by moderate heating or enhanced fluid activity the correspondence between pale titanite and apatite ages suggests that the same processes also caused the isotopic resetting of apatite on a regional scale the data pattern permits the identification of domains characterized by differences in the length and intensity of the thermal and hydrothermal processes the most protracted activity is recorded in northern domains of the berens river area whereas the gold mineralized red lake area in the south underwent a very rapid decline of the thermal hydrothermal activity these data also highlight important differences between the red lake and other gold camps of the superior province such as hemlo and val d or where hydrothermal activity locally associated with mineralization occurred as much as 30 80 m y after the major orogenic developments

archean age for the granulite facies metamorphism near the center of the vredefort structure south africa granulite facies metamorphic assemblages in rocks exposed near the center of the 2 02 ga vredefort impact structure previously have been interpreted either as early proterozoic genetically related to the 2060 ma bushveld complex or as archean and representative of lower crust that rebounded to upper crustal levels following an impact event zircon and monazite recovered from the granulite facies rocks record high grade metamorphism at 3107 9 ma and a primary age of 3425 ma for detrital zircon a shock deformed but otherwise pristine dolerite dike that intrudes the granulite terrane yields a u pb zircon age of 2560 ma providing a minimum age for the metamorphism these isotopic age data are difficult to reconcile with a regional high grade metamorphic event in the crust beneath vredefort at 2060 ma instead the preimpact high temperature metamorphic history observed in the vredefort lower crustal rocks indicates an enigmatic high temperature event during the stabilization of diamondiferous archean tectosphere

pre hercynian mantle lead transfer to basement rocks as indicated by lead isotopes of the schwarzwald crystalline sw germany lead isotopes of k feldspars from five granites of the se schwarzwald and from metamorphites are positively correlated in 207 204 as well as 208 204 vs 206 204 diagrams

paleomagnetic investigations and k ar dating on the variscan plutonic massif of the champ du feu and its volcanic sedimentary environment northern vosges france abstract paleomagnetic investigations have been carried out on samples from the plutonic series of the champ du feu massif and the devonian ordovician host rocks in order to date the different phases of magnetization a series of samples has been selected for k ar analysis on micas amphiboles and feldspars the diorites granodiorites diabases and hornfelses are characterized by high susceptibilities of up to 10 1 si thermal demagnetization reveals several groups of directions of characteristic magnetization the a direction d 202 i 23 49 n 153 e is consistent with the permian virtual geomagnetic pole for europe the b 1 direction d 59 i 22 11 n 128 5 e was obtained from the effusive parts of the kagenfels granite dated at 284 ma diorites microdiorites and volcanics emplaced a short time after the intrusion of the natzwiller granite dated at 344 and 351 ma display three groups of directions the c 1 direction d 328 i 15 27 n 223 5 e the c 2 d 323 i 38 13 s 222 e and the c 3 d 272 i 41 16 s 260 e the c 1 and c 2 directions agree well with those obtained previously for visean volcanics from the southern vosges in the host rocks we have identified b and c overprintings despite the lack of tectonic control the paleomagnetic data demonstrate at least two important motions of the vosges first a rotation of undetermined direction relative to stable europe which affected the variscan belt between the late visean and 320 ma and second a clockwise rotation which occurred during the stephanian autunian in late variscan time

the moldanubian zone in the french massif central vosges schwarzwald and bohemian massif revisited differences and similarities in order to portray the main differences and similarities between the northeastern variscan segments french massif central fmc vosges black forest and bohemian massif bm we review their crustal scale architectures the specific rock associations and lithotectonic sequences as well as the ages of the main magmatic and metamorphic events this review demonstrates significant differences between the moldanubian domains in thebmand the fmc on this basis we propose distinguishing between the eastern and western moldanubian zones while the vosges black forest mountains are an intermediate section between the bm and the fmc the observed differences are the result of first the presence in the french segment of an early large scale accretionary system prior to the main variscan collision and second the duration of saxothuringian armorican subduction which generated long lived magmatic arc and back arc systems in the bohemian segment while the magmatic activity in the fmc was comparably short lived

precise upb chronometry of 345 340 ma old magmatism related to syn convergence extension in the southern vosges central variscan belt abstract the southern vosges host a large volcano sedimentary basin which contains clastic sediments of famennian late devonian ca 365 ma to late visean ca 340 335 ma age the basin is bordered by an exhumed high grade basement complex and intruded by several granitoid plutons a chronology for basin evolution and magmatism was established dating volcanic rocks of the basin as well as cross cutting granitoid rocks by the u pb technique applied to zircon and titanite the basin evolution started with olistolithic deposits and overlying marine shales that have a famennian fauna ca 365 ma and contain detrital zircons with an age of 386 ma the lowermost rhyolite flows interlayered with the basin sediments and tholeiitic basalts yielded an age of 345 2 ma following a tectonic event at the bottom of the upper visean the rates of sedimentation and volcanism were strongly enhanced leading to basin emersion and were concluded by the extrusion of the uppermost ignimbrite molkenrain rhyolite emplaced at 340 2 ma the whole volcano sedimentary association was subsequently intruded by melts of the ballons and cre tes granites as well as by small monzodioritic and dioritic satellite intrusions along the border of the ballons granite within 3 2 myr 342 1 to 339 5 2 5 ma the zircons of most of the volcanic and plutonic rocks investigated contain inherited lead of 0 6 and 2 1 ga age the differences in initial nd isotopic composition of the ballons and cre tes granites can be explained by differential uptake of crustal material during magma genesis the precise age data prove that magmatism and basin formation in the southern vosges were extremely short lived and coeval to the exhumation of adjacent high grade gneiss terrains revealing an episode of extension at the end of the lower carboniferous between 345 and 340 ma visean the tectonic regime in the internal part of the variscan orogen is that of extension and strike slip while convergent tectonics were still ongoing in the external part of the variscan orogen

new shock microstructures in titanite catisio5 from the peak ring of the chicxulub impact structure mexico accessory mineral geochronometers such as apatite baddeleyite monazite xenotime and zircon are increasingly being recognized for their ability to preserve diagnostic microstructural evidence of hypervelocity impact processes to date little is known about the response of titanite to shock metamorphism even though it is a widespread accessory phase and a u pb geochronometer here we report two new mechanical twin modes in titanite within shocked granitoid from the chicxulub impact structure mexico titanite grains in the newly acquired core from the international ocean discovery program hole m0077a preserve multiple sets of polysynthetic twins most commonly with composition planes k1 1 11 and shear direction 1 and less commonly with the mode k1 130 1 in some grains 130 deformation bands have formed concurrently with the deformation twins indicating dislocation slip with burgers vector b can be active during impact metamorphism titanite twins in the modes described here have not been reported from endogenically deformed rocks we therefore propose this newly identified twin form as a result of shock deformation formation conditions of the twins have not been experimentally calibrated and are here empirically constrained by the presence of planar deformation features in quartz 12 5 and 17 5 gpa and the absence of shock twins in zircon 20 gpa while the lower threshold of titanite twin formation remains poorly constrained identification of these twins highlight the utility of titanite as a shock indicator over the pressure range between 12 and 17 gpa given the challenges to find diagnostic indicators of shock metamorphism to identify both ancient and recent impact evidence on earth microstructural analysis of titanite is here demonstrated to provide a new tool for recognizing impact deformation in rocks where other impact evidence may be erased altered or did not manifest due to generally low 20 gpa shock pressure