pantasma evidence for a pleistocene circa 14 km diameter impact crater in nicaragua the circa 14 km diameter pantasma circular structure in oligocene volcanic rocks in nicaragua is here studied for the first time to understand its origin geomorphology field mapping petrographic and geochemical investigations all are consistent with an impact origin for the pantasma structure observations supporting an impact origin include outward dipping volcanic flows the presence of former melt bearing polymict breccia impact glass with lechatelierite and low h 2 o 10 km found in the americas

the classification of micrometeorites due to their small size the mineralogical and chemical properties of micrometeorites mms are not representative of their parent bodies on the centimeter to meter scales that are used to define parent body groups through the petrological study of meteorites identifying which groups of mm are derived from the same type of parent body is problematic and requires particles to be rigorously grouped on the basis of mineralogical textural and chemical properties that reflect the fundamental genetic differences between meteorite parent bodies albeit with minimal bias towards preconceived genetic models specifically the interpretation of mms requires a rigorous and meaningful classification scheme at present the classification of mms is however at best ambiguous a unified petrological chemical classification scheme is proposed in the current study and is based on observations of several thousand mms collected from antarctic ice

a tungsten isotope approach to search for meteoritic components in terrestrial impact rocks abstract the identification of meteorite impact structures on earth is based on two main criteria the presence of shock metamorphic effects in the crater rock ejecta and or the confirmation of an extraterrestrial meteoritic component in breccias or melt rocks for the latter both high elemental abundances of siderophile elements especially the platinum group elements with corresponding meteoritic inter element ratios and the osmium os and chromium cr isotopic signatures characteristic of meteorites have been used successfully inspired by earlier suggestions of a meteoritic component in archean rocks based on tungsten w isotope anomalies here we explore the possible use of 182 w which has been produced by the decay of now extinct 182 hf t 1 2 8 9 ma as a tracer of meteoritic component in terrestrial material each group of meteorites has w isotopic compositions that are distinct from each other and from the terrestrial crust 182 w has already been used to try to identify the impactor at the k t boundary by analyzing the sediments and ni rich spinel in the present study we broaden the field of investigation by choosing a different approach namely analyzing a variety of known impact rocks we measured the w isotope composition in four samples from different impact structures gardnos norway morokweng south africa vredefort south africa ries germany as well as in two samples from a distal ejecta layer k t boundary samples from gams austria and berwind canyon usa all these samples are unambiguously impact produced and in several of those materials a meteoritic component has unequivocally been identified by other geochemical proxies in all these samples the isotopic composition of w is identical with analytical error to that of the earth s continental crust and no 182 w anomalies are present even in the samples containing a significant percent level meteoritic component therefore we conclude that in contrast to the cr or os isotopes w isotopes are not suitable for the identification of meteoritic components in terrestrial rocks

alteration and geochemical patterns in the 3 7 3 8 ga isua greenstone belt west greenland abstract volcanic rocks of the 3 7 3 8 ga isua greenstone belt have undergone heterogeneous poly phase post magmatic alteration resulting in mobilisation of many elements including lree and th in contrast nb ta ti zr y and hree were relatively immobile during alteration processes the degree of element mobility appears to have been controlled by chemical composition primary volcanic structures carbonate and silica enrichment and proximity to ultramafic units and shear zones on the basis of the geochemical characteristics of the least altered samples two distinct associations have been recognised in structurally separated lithotectonic sequences these magmatic associations are a low hfse association tio2 0 20 0 40 wt zr 12 30 ppm nb 0 13 0 80 ppm y 6 14 ppm in the central arc sequence resembling tertiary boninites and a high hfse association tio2 0 50 1 14 wt zr 34 77 ppm nb 1 2 2 7 ppm y 11 28 ppm in the outer and inner arc sequences comparable to phanerozoic island arc basaltic to picritic volcanic rocks these two geochemically distinct sequences were likely to have been juxtaposed as a consequence of phanerozoic style plate tectonic processes operating in the early earth the isua volcanic rocks are characterised by sub chondritic nb ta 10 16 ratios post magmatic alteration can be ruled out as the cause of these low ratios rather these ratios reflect a compositional characteristic of the paleoarchean mantle source s the sub chondritic nb ta ratios may have resulted from either preferential sink of nb into the core as a weakly siderophile element or preferential retention of nb in the subducted oceanic crust during subduction zone petrogenetic processes alternatively the building blocks of the earth were not composed of ci like chondritic material

chromium isotope anomaly in an impactite sample from the el gygytgyn structure russia evidence for a ureilite projectile the 3 6 ma 18 km diameter el gygytgyn impact structure arctic russia is unique among the currently known terrestrial impact craters in that it is the only one that was formed in acid volcanic rocks previous analyses of impactites from el gygytgyn showed minor enrichments of the siderophile elements including ir which together with distinct cr enrichments gave rise to speculation that an achondritic projectile was involved we studied the major and trace element composition in samples from the new icdp drill core obtained near the center of the structure as well as the chromium isotopic composition of an impact glass sample collected on the surface several suevitic breccias from the upper part of the suevite sequence in the drill core show higher cr and ni contents compared with felsic volcanic rocks in the lower part of the core and from surface samples however it is difficult to unambiguously establish a meteoritic component from trace element data as input from rare mafic target rocks is a possibility in contrast the cr isotopic composition of the impact glass sample yielded a nonterrestrial e 54 cr value of 0 72 0 31 2 std err this negative e 54 cr is different from known carbonaceous chondrite values e 54 cr of 0 95 to 1 65 but is nearly identical to reported values for ureilites approximately 0 77 the value is however also within analytical error of eucrites approximately 0 38 and ordinary chondrites approximately 0 42 given the chemical signatures found in previous analyses of el gytgytgyn impactites and the similarity of our cr isotopic data to ureilites we suggest that the impacting asteroid could have been an f type asteroid of mixed composition similar to the recent almahata sitta fall in sudan

the problem of inherited 40ar in dating impact glass by the 40ar 39ar method evidence from the tswaing impact crater south africa abstract the tswaing meteorite impact crater is a 1 13 km diameter structure located in the 2 05 ga nebo granite of the bushveld complex the impact age had previously been determined by fission track dating to 220 104 ka 40ar 39ar step heating and total fusion experiments performed on single and multi grain impact glass aliquots gave apparent ages ranging from 1 0 0 3 ma to 204 6 ma these ages indicate that the radiogenic ar derived from the target rocks has not been completely degassed as a result of the impact process despite fusion of the target material results of step heating experiments imply that the 40 ar inherited trapped within the glass is located in two distinct reservoirs thought to be the glass matrix and fluid vapor inclusions or un melted residual clasts calculations assuming an age of 0 2 0 1 ma for tswaing fission track data reveal that the amount of inherited 40ar 40 ar inherited relative to the pre impact concentration varies from 0 015 to 4 15 the spread defined by 40 ar inherited likely reflects the various quench rates experienced by the glass most certainly due to the pre impact position of the sample relative to the center of the crater we compare the influence of 40 ar inherited on the apparent 40ar 39ar age determination of five impact structures our calculations show that the main characteristic controlling the age offset for a given proportion of 40 ar inherited is the age difference between the impact and the target rocks i e the 40ar concentration in the target rock the buffer effect for a given crater structure can be predicted knowing the age of the basement and having a rough estimation of the age of the crater structure itself the occurrence of 40 ar inherited is likely influenced by 1 the degree of polymerization i e silicate structure complexity of the target rock and presumably related to the diffusivity of ar in the melt and glass 2 the ar partial pressure at the grain boundary 3 the quantity of energy involved in the impact and 4 the porosity of the target rocks for glass that inevitably suffers inherited and or excess 40ar the use of the inverse isochron technique can be appropriate but should be applied with careful statistical treatment

an early paleoproterozoic high k intrusive complex in southwestern tarim block nw china age geochemistry and tectonic implications abstract systematic geochronologic geochemical and nd isotopic analyses were carried out for an early paleoproterozoic high k intrusive complex exposed in southwestern tarim nw china the results provide a better understanding of the paleoproterozoic tectonic evolution of the tarim block zircon u pb age dating indicates two paleoproterozoic magmatic episodes occurring at ca 2 41 ga and ca 2 34 ga respectively which were followed by a ca 1 9 ga metamorphic event the 2 41 ga granodiorite adamellite suite shares characteristics of late to post orogenic metaluminous a type granites in its high alkalinity na2o k2o 7 6 9 3 total ree 410 788 ppm zr 370 660 ppm and y 21 7 58 4 ppm contents end t values for the suite range from 3 22 to 4 71 and accordingly the nd modal ages t2dm vary between 3 05 ga and 3 17 ga based on geochemical data the 2 34 ga suite can be subdivided into two sub suites namely a type and s type however both types have comparable nd isotope compositions end t 0 41 to 2 08 and similar narrow t2dm ranges 2 76 2 91 ga geochemical and nd isotopic data for the high k intrusive complex in conjunction with the regional geological setting suggest that both the 2 41 ga suite and the 2 34 ga a type sub suite might have been produced by partial melting of the archean mafic crust in a continental rift environment the s type sub suite is thought to have formed by partial melting of felsic pelites and or metagreywackes recycled from archean crust ttg gabbro enclaves with positive end t value 2 15 have been found to be intermingling within the 2 34 ga suite ca 2 34 2 36 ga gabbroic dykes and adamellites have previously been documented in eastern tarim these observations indicate that the high k intrusions may reflect the emergence of depleted mantle upwelling beneath the tarim block at that time we suggest a three stages model for the precambrian crustal evolution in the tarim block 1 the formation of proto crust ttg by ca 2 5 ga 2 episodes of felsic magmatism possibly occurring in continental rift environments at ca 2 41 ga and ca 2 34 2 36 ga and 3 ca 1 9 ga metamorphism that may represent the solidification of the precambrian basement of the tarim block

magnetic properties of tektites and other related impact glasses we present a comprehensive overview of the magnetic properties of the four known tektite fields and related fully melted impact glasses aouelloul belize darwin libyan desert and wabar glasses irghizites and atacamaites namely magnetic susceptibility and hysteresis properties as well as properties dependent on magnetic grain size tektites appear to be characterized by pure fe2 paramagnetism with ferromagnetic traces below 1 ppm the different tektite fields yield mostly non overlapping narrow susceptibility ranges belize and darwin glasses share similar characteristics on the other hand the other studied glasses have wider susceptibility ranges with median close to paramagnetism fe2 and fe3 but with a high susceptibility population bearing variable amounts of magnetite this signs a fundamental difference between tektites plus belize and darwin glasses and other studied glasses in terms of oxygen fugacity and heterogeneity during formation thus bringing new light to the formation processes of these materials it also appears that selecting the most magnetic glass samples allows to find impactor rich material opening new perspectives to identify the type of impactor responsible for the glass generation

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

chromium isotopic studies of terrestrial impact craters identification of meteoritic components at bosumtwi clearwater east lappajarvi and rochechouart chromium isotopic compositions and elemental abundances in impact melt rock and impact glass samples from four terrestrial impact craters were measured to verify the presence of an extraterrestrial component and to identify the meteorite type of the impactor all meteorite classes have cr isotopic signatures that are different from those of terrestrial rocks thus precise measurements of cr isotopic abundances can unequivocally distinguish terrestrial from extraterrestrial materials for all four studied craters bosumtwi ghana clearwater east canada lappajarvi finland and rochechouart france we found positive 53cr excesses that eliminate carbonaceous chondrite projectiles because those would show apparent negative excesses and enstatite chondrites because of the magnitude of the excess in all four cases ordinary chondrites have been identified as the best fit for the data in the case of lappajarvi interelement correlations together with the cr isotope data make an h chondrite the most likely projectile whereas in the case of clearwater east both l or h chondrites are possible for bosumtwi and rochechouart the high indigenous contents of the siderophile elements and disturbances of the elemental abundances by weathering and hydrothermal alteration respectively do not allow further constraints to be placed on the type of ordinary chondrite involved in the impact

peraluminous i type granites abstract s type granites always contain more al than the amounts of na ca and k in the rock required to form feldspars primarily owing to their derivation from source components that had previously been weathered those rocks are therefore always saturated in al or peraluminous many i type granites are also peraluminous despite i type source rocks typically not being saturated in al it has previously been suggested that this may result from the fractional crystallisation of amphibole however data from compositionally zoned high temperature plutons in the lachlan fold belt show that it is difficult to generate large quantities of peraluminous melt by removal of amphibole most of the i type granites 95 in the lachlan fold belt formed at lower temperatures and almost half of those rocks for which bulk chemical compositions are available are peraluminous among these granites there are 98 separate suites for which there are chemical data for two or more samples with 47 suites that include both metaluminous more mafic and peraluminous more felsic compositions the origin of those peraluminous compositions is fundamental to any understanding of i type granite petrogenesis in this region compositional variations caused by the assimilation or by partial melting of supracrustal rocks are very small as the isotopic variations within these rocks are dominantly between different suites not within suites the partial melting of more mafic source rocks rather than the fractional crystallisation of more mafic magmas is favoured for the origin of these rocks partial melting is the most likely process involved in the petrogenesis of felsic granites where broadly granodioritic monzogranitic batholiths are associated with lesser amounts of tonalite and very minor amounts of mafic rock experimental studies have shown that the melts generated by the partial melting of basaltic to andesitic rocks under crustal conditions are mostly peraluminous during the dehydrational melting of i type granite source rocks at pressures below the garnet stability field biotite and amphibole melt incongruently to yield pyroxenes the excess al is incorporated into the felsic liquid resulting in the generation of peraluminous melts in this instance the excess al in felsic i type granites is a function of the melting process and unrelated to the bulk composition of the source the observed gradation from peraluminous felsic granites to metaluminous compositions in less felsic rocks in largely isotopically closed systems could happen in two ways at higher temperatures of partial melting ca and other components of clinopyroxene could dissolve in the melt with the melt eventually becoming metaluminous alternatively minerals residual from partial melting dominantly pyroxenes and plagioclase could be incorporated in suspension in the melt so that the resulting bulk magma is metaluminous examples of these two extremes and of intermediate cases are well developed among the granites of southeastern australia

shrimp u pb zircon ages of granitoid rocks in the luliang complex implications for the accretion and evolution of the trans north china orogen abstract the luliang complex is situated in the central segment of the trans north china orogen tnco a continent continent collisional belt along which the discrete eastern and western blocks amalgamated to form the basement of the north china craton the complex consists of supracrustal and granitoid rocks of which the latter can be subdivided into pre tectonic ttg gneisses including the yunzhongshan guojiazhuang and chijianling guandishan gneisses syn tectonic gneissic granites represented by the huijiazhuang gneissic granite and post tectonic granites including the luyashan charnockite luchaogou porphyritic granite and tangershang guandishan massive granite the pre tectonic ttg gneisses are mostly calc alkaline and considered to have formed in a magmatic arc environment shrimp u pb analyses reveal that the yunzhongshan gneisses were emplaced at 2499 9 ma representing the earliest arc related magmatic event in the luliang complex this was followed by the intrusion of the guojiazhuang gneisses at 2375 10 ma the most widespread arc related magmatic event in the region was the emplacement of the chijianling guandishan granitoid gneisses of which the tonalitic granodioritic and monzogranitic gneisses were emplaced at 2199 11 ma 2180 7 ma and 2173 7 ma respectively metamorphic zircon overgrowth rims from a tonalitic gneiss yielded a weighted mean 207 pb 206 pb age of 1872 7 ma consistent with the metamorphic age range of 1880 1820 ma defined by metamorphic zircons from various high grade rocks in the tnco thus the evolution of the luliang complex involved emplacement of the yunzhongshan granitoids at 2499 ma the guajiazhuang granitoids at 2375 ma and the chijianling guandishan granitoids at 2199 2173 ma with the final collision between the eastern and western blocks in this area occurring at 1872 7 ma the whole series of magmatic events lasting nearly 650 ma this suggests that the trans north china orogen represents a long lived magmatic arc the new shrimp zircon data combined with structural and petrological studies also reveal the existence of both syn and post collisional granites in the luliang complex the former is represented by the huijiazhuang gneissic granite that was emplaced at 1832 11 ma whereas the latter include the 1815 5 ma luyashan charnockite 1807 10 ma luchaogou porphyritic granite 1798 11 ma guandishan massive granite and 1790 14 ma tangershan massive granite

impact generated hydrothermal systems on earth and mars it has long been suggested that hydrothermal systems might have provided habitats for the origin and evolution of early life on earth and possibly other planets such as mars in this contribution we show that most impact events that result in the formation of complex impact craters i e 2 4 and 5 10 km diameter on earth and mars respectively are potentially capable of generating a hydrothermal system consideration of the impact cratering record on earth suggests that the presence of an impact crater lake is critical for determining the longevity and size of the hydrothermal system we show that there are six main locations within and around impact craters on earth where impact generated hydrothermal deposits can form 1 crater fill impact melt rocks and melt bearing breccias 2 interior of central uplifts 3 outer margin of central uplifts 4 impact ejecta deposits 5 crater rim region and 6 post impact crater lake sediments we suggest that these six locations are applicable to mars as well evidence for impact generated hydrothermal alteration ranges from discrete vugs and veins to pervasive alteration depending on the setting and nature of the system a variety of hydrothermal minerals have been documented in terrestrial impact structures and these can be grouped into three broad categories 1 hydrothermally altered target rock assemblages 2 primary hydrothermal minerals precipitated from solutions and 3 secondary assemblages formed by the alteration of primary hydrothermal minerals target lithology and the origin of the hydrothermal fluids strongly influences the hydrothermal mineral assemblages formed in these post impact hydrothermal systems there is a growing body of evidence for impact generated hydrothermal activity on mars although further detailed studies using high resolution imagery and multispectral information are required such studies have only been done in detail for a handful of martian craters the best example so far is from toro crater marzo g a davila a f tornabene l l dohm j m fairen a g gross c kneissl t bishop j l roush t l mckay c p 2010 icarus 208 667 683 we also present new evidence for impact generated hydrothermal deposits within an unnamed 32 km diameter crater 350 km away from toro and within the larger holden crater synthesizing observations of impact craters on earth and mars we suggest that if there was life on mars early in its history then hydrothermal deposits associated with impact craters may provide the best and most numerous opportunities for finding preserved evidence for life on mars moreover hydrothermally altered and precipitated rocks can provide nutrients and habitats for life long after hydrothermal activity has ceased

a palaeomagnetic study and age determinations of tertiary rocks in nicaragua central america summary palaeomagnetic k ar and ar40 ar39 measurements have been made to elucidate nicaragua s plate tectonic history these show that the nicaraguan highland rotated 90 counterclockwise between 30 and 18 myr ago but the pacific coastal plain has not rotated a lack of data has prevented a direct comparison with the tectonic history of adjacent areas in particular the chortis block which is the part of the caribbean plate that is geologically most closely related to the nicaraguan highland however our data suggest that the caribbean plate in this part of central america was not consolidated until c 15 ma

petrogenesis of the red mountain pluton laramie anorthosite complex wyoming implications for the origin of a type granite abstract the red mountain pluton the youngest intrusion in the laramie anorthosite complex is one of five monzonitic to granitic a type plutons intruded at 1 43 ga in the laramie mountains of southeastern wyoming the red mountain pluton ranges in composition from fayalite monzonite clinopyroxene quartz monzonite and biotite hornblende quartz syenite to granite the red mountain pluton is petrologically and geochemically distinct from the northern and southern sherman batholith the sybille intrusion and the maloin ranch pluton it has higher feo t feo t mgo and higher k 2 o is more strongly enriched in rees and at any given silica content it has lower abundances of tio 2 feo mgo cao and p 2 o 5 the fayalite monzonite in the red mountain pluton crystallized at temperatures around 1000 c and oxygen fugacities 1 2 log units below that of the fmq buffer similar to the crystallization conditions of the sybille and maloin ranch plutons the biotite hornblende quartz syenite crystallized at oxygen fugacities near or slightly below those of fmq most of the units in the pluton carry mantle like nd sr and pb isotopic compositions similar to the least contaminated anorthositic and ferrodioritic rocks of the lac indicating that the pluton evolved mainly via differentiation substantial crustal assimilation occurred only in the late dikes and the granite we suggest that all of the 1 43 ga a type intrusions in southeastern wyoming are ultimately derived from tholeiites extreme differentiation or partial melting produced monzonitic magmas that have assimilated varying amounts of siliceous crust the differences between the red mountain pluton and other a type granitoids in southeastern wyoming are related to the extent of differentiation prior to crustal assimilation the compositional ranges of a type granitoids in southeastern wyoming suggest that the geochemical spectrum of a type granites worldwide similarly are produced from evolved mantle derived melt that has assimilated varying proportions of crustal material

earthquake precursory studies in india scenario and future perspectives earthquakes are the worst natural calamities that strike without any notice and cause immediate loss of life and property internationally serious and scientifically acceptable earthquake prediction studies started in 1970s and short term prediction of the heicheng earthquake of february 4 1975 in china is a land mark in india a successful medium term prediction of august 6 1988 earthquake m 7 5 in northeast indian region encouraged to intensify such studies in the country these predictions were based mainly on seismological precursors initially the precursory studies were taken in isolated manner but after validation of specific geophysical parameters efforts were made to adopt an integrated approach involving collection analysis and interpretation of various precursory observations in a comprehensive and coordinated manner accordingly a few multi parametric geophysical observatories mpgos have been established at the selected locations in seismically active areas in the country this approach led to successful short term prediction of some moderate earthquakes m p 4 in koyna region famous for reservoir triggered seismicity in western india simultaneously efforts have been made to generate long term multi parametric observations from these observatories as a basic scientific input required for future earthquake prediction related studies the real time analysis of these data sets would help to understand the earthquake generation process and attaining the predictive capabilities by developing models for short term earthquake forecasting to facilitate direct observations and test the hypothesis of reservoir triggered seismicity rts as well as to understand the earthquake generation processes it is planned to undertake deep borehole 6 8 km investigations in koyna region of western india the paper highlights the efforts made so far in india in the area of earthquake precursory studies as well as the future road map

reworked old crust derived shoshonitic magma the guarany pluton northeastern brazil abstract the 572 ma guarany stock consists of magmatic epidote bearing hornblende monzodiorite to biotite granite that intruded paleoproterozoic orthogneisses about 10 km inland from the coast in northeastern brazil co magmatic diorite enclaves and dikes are abundant throughout the pluton the monzodiorite granite pluton and diorite enclaves are shoshonitic and display continuous trends in variation diagrams they display chemical and isotopic characteristics of crustal melts such as enrichment in incompatible elements high back calculated initial 87 sr 86 sr ratios avg 0 71253 negative end 0 57ga values avg 14 58 as well as high and variable 9 1 to 11 1 vsmow 18 o zircon values correlations between o isotope and whole rock silica contents as well as initial 87 sr 86 sr ratios with 1 sr concentrations suggest hybridization of a lower continental crustal melt with more felsic crustal rocks concomitant with fractional crystallization amphibole chemistry and whole rock zr tio 2 and p 2 o 5 contents suggest magma solidification at a pressure 7 kbar and near liquidus temperature 900 c the parental magma was likely formed by partial melting of old tdm 2 0 ga amphibolitic lower continental crustal rocks in a post collisional setting probably triggered by underplating of mantle derived mafic magma during the period of relaxation after collision

geochronology and geochemistry of igneous rocks from the kongling terrane implications for mesoarchean to paleoproterozoic crustal evolution of the yangtze block abstract the kongling terrane is a high grade metamorphic complex exposed in the northern part of the yangtze block it mainly consists of ttg gneisses amphibolites migmatites and metasedimentary rocks the amphibolites occur as enclaves or layers in the ttg gneisses and both the amphibolites and the ttg gneisses are intruded by granitic and diabasic dykes u pb ages of zircons from the amphibolite migmatite granite and diabase are determined by la icp ms igneous zircons from an amphibolite have an upper intercept age of 2945 13 ma mswd 1 2 while metamorphic zircons yield an upper intercept age of 2870 13 ma mswd 0 9 igneous zircons from a migmatite and a granite have weighted average 207pb 206pb age and upper intercept age of 1987 5 ma mswd 1 7 and 1985 6 ma mswd 1 3 respectively igneous zircons from a diabase dyke yield an upper intercept age of 1783 23 ma mswd 0 86 the protoliths of the amphibolites are tholeiitic basalts and exhibit slightly depleted to variably enriched lree patterns with moderate enrichment of rb and ba and depletion in nb and zr similar to oceanic island arc volcanics they have high cr co ni mgo and feot content with negative nd t values indicating that the tholeiitic basalts resulted from high degree batch melting of enriched mantle wedge the granites are strongly peraluminous with a cnk 1 1 and they have relatively low hree and y and negative nd t values detailed elemental and isotopic data suggest that they formed by partial melting of recycled products of the ttg gneisses amphibolites and older crustal material in a continental collisional setting the diabases are characterized by relatively high al2o3 feot and tio2 contents and low zr ba ratios their high initial 87sr 86sr ratios and negative nd t values indicate that they were derived from enriched lithospheric mantle in extensional setting therefore the kongling terrane occurred as an oceanic island arc at ca 2 95 ga and experienced earliest metamorphism at ca 2 87 ga which probably contributed to the formation of the high grade metamorphic terrane this terrane experienced a strong collisional event at 2 0 ga which may have led to cratonization over all of the yangtze block and also experienced lithospheric extension during 1 85 1 78 ga the global 2 0 ga paleoproterozoic collisional events were related to the assembly of supercontinent columbia

unravelling textural heterogeneity in obsidian shear induced outgassing in the rocche rosse flow abstract obsidian flow emplacement is a complex and understudied aspect of silicic volcanism of particular importance is the question of how highly viscous magma can lose sufficient gas in order to erupt effusively as a lava flow using an array of methods we study the extreme textural heterogeneity of the rocche rosse obsidian flow in lipari a 2 km long 100 m thick 800 year old lava flow with respect to outgassing and emplacement mechanisms 2d and 3d vesicle analyses and density measurements are used to classify the lava into four textural types glassy obsidian 40 vesicles high aspect ratio shear banded lava 20 40 vesicles and low aspect ratio frothy obsidian with 30 60 vesicles textural heterogeneity is observed on all scales m to m and occurs as the result of strongly localised strain magnetic fabric described by oblate and prolate susceptibility ellipsoids records high and variable degrees of shearing throughout the flow total water contents are derived using both thermogravimetry and infrared spectroscopy to quantify primary magmatic and secondary meteoric water glass water contents are between 0 08 0 25 wt water analysis also reveals an increase in water content from glassy obsidian bands towards frothy bands of 0 06 0 08 wt reflecting preferential vesiculation of higher water bands and an extreme sensitivity of obsidian degassing to water content we present an outgassing model that reconciles textural volatile and magnetic data to indicate that obsidian is generated from multiple shear induced outgassing cycles whereby vesicular magma outgasses and densifies through bubble collapse and fracture healing to form obsidian which then re vesiculates to produce dry vesicular magma repetition of this cycle throughout magma ascent results in the low water contents of the rocche rosse lavas and the final stage in the degassing cycle determines final lava porosity heterogeneities in lava rheology vesicularity water content microlite content viscosity play a vital role in the structural evolution of an obsidian flow and overprint flow scale morphology post emplacement hydration also depends heavily on local strain whereby connectivity of vesicles as a result of shear deformation governs sample rehydration by meteoric water a process previously correlated to lava vesicularity alone

a hybrid origin for the qianshan a type granite northeast china geochemical and sr nd hf isotopic evidence abstract major and trace element whole rock sr and nd isotope and zircon hf isotope data are reported for a suite of a type granites and mafic microgranular enclaves from the early cretaceous 126 2 ma qianshan pluton liaodong peninsula northeast china with the aim of investigating the sources and petrogenesis of a type granites the qianshan pluton includes hornblende alkali feldspar granite graphic biotite granite and mafic microgranular enclaves the hornblende alkali feldspar granites have high sio 2 fe 2 o 3t mgo k 2 o na 2 o rb zr and lree contents and low ba and sr concentrations with strongly negative eu anomalies their high rb sr 87 rb 86 sr 16 76 24 15 and initial 87 sr 86 sr ratios 0 7215 to 0 7283 negative e nd t values 14 1 to 16 5 and zircon e hf t values 18 9 to 11 5 indicate they were mainly derived from a crustal source but with involvement of high e nd t and e hf t materials graphic biotite granites have similar geochemical features and sr nd hf isotopic compositions to enclaves indicating they were the result of crystal fractionation of evolved mafic magmas but with involvement of low e nd t and e hf t materials the mafic enclaves have an igneous texture and contain acicular apatite suggesting quenching of mafic magmas that have co mingled with the host granites they have low initial 87 sr 86 sr ratios 0 7097 0 7148 negative e nd t 14 5 to 11 9 and zircon e hf t 17 1 to 6 9 values and are enriched in liles and lrees and depleted in hfses when coupled with the high mgo mg up to 54 this indicates derivation from an enriched lithospheric mantle source but contaminated by crustal materials geochemical and sr nd and zircon hf isotopic compositions rule out simple crystal liquid fractionation or restite unmixing as the major genetic link between enclaves and host rocks instead magma mixing of mantle derived mafic and crustal derived magmas coupled with crystal fractionation is compatible with the data this example shows that at least some a type granites formed through a complex process involving mantle and crustal derived magma mixing crystal fractionation and infracrustal melting

early paleoproterozoic magmatism in the yangtze block evidence from zircon u pb ages sr nd hf isotopes and geochemistry of ca 2 3 ga and 2 1 ga granitic rocks in the phan si pan complex north vietnam abstract our understanding of the early evolution of the yangtze block is limited by the sparsely dispersed nature of pre neoproterozoic exposures new integrated petrographic zircon u pb age and hf nd isotope analyses and whole rock geochemical data for early paleoproterozoic granites in the phan si pan complex provides new insights into the evolution of the yangtze block as well as its role in the pre nuna supercontinent la icp ms zircon u pb dating of magmatic zircons from quartz monzonite and gneissic granite yielded 207pb 206pb ages of 2306 12 ma and 2096 15 ma respectively zircons from the quartz monzonite have ehf t values ranging from 4 1 to 2 1 corresponding to tdm2 model ages of 3002 2890 ma whereas zircons in the gneissic granite have ehf t values between 0 95 and 1 72 and corresponding tdm2 model ages of 2660 2516 ma which are consistent with their whole rock nd isotope values geochemically the quartz monzonites are i type granites combined with their relatively high sr y ratios and low y concentrations as well as fractionated ree patterns with relatively high lree but low hree concentrations they were probably generated by partial melting of the thickened middle lower crust under elevated temperature geochemical and isotopic signatures suggest that the ca 2 1 ga gneissic granites are high k calc alkaline ferroan a type granites formed by partial melting of juvenile crustal source at high temperature and low pressure with little involvement of ancient crustal material the phan si pan complex has a distinct early paleoproterozoic crustal evolution history compared with the other crustal provinces of the yangtze block suggesting independent histories that were not unified until the late paleoproterozoic during the assembly of nuna moreover the magmatism and tectonic evolution of the north vietnam region is broadly similar to that of the arrowsmith orogen of the rae craton in laurentia suggesting a potential spatial linkage the geologic record of the yangtze block does not support an early paleoproterozoic shutdown of plate tectonics

generation of sodium rich magmas from newly underplated basaltic crust sodium rich rocks of trondhjemite tonalite dacite ttd or granodiorite ttg suites form much of precambrian continental crust1 they are thought to have formed by partial melting of subducted oceanic crust2 3 a process that would have been much more widespread early in earth history than at present owing to the higher thermal gradients prevailing at that time4 phanerozoic ttd suites do exist however and seem also to relate to subduction zones5 defant and drummond6 proposed that these suites form where young 25 myr hot oceanic lithosphere is subducted and melts thus locally simulating the conditions that led to widespread crustal growth in the archaean here we describe plutonic and volcanic rocks from the cordillera blanca complex in peru which have characteristics of the high al ttd suite but which were produced above a subduction zone containing a 60 myr old slab we present evidence that the complex formed by partial melting of newly underplated basaltic crust and argue that this mechanism should be considered more generally as an additional way of generating sodium rich arc magmas

chromium isotopic homogeneity between the moon the earth and enstatite chondrites abstract among the elements exhibiting non mass dependent isotopic variations in meteorites chromium cr has been central in arguing for an isotopic homogeneity between the earth and the moon thus questioning physical models of moon formation however the cr isotopic composition of the moon relies on two samples only which define an average value that is slightly different from the terrestrial standard here by determining the cr isotopic composition of 17 lunar 9 terrestrial and 5 enstatite chondrite samples we re assess the isotopic similarity between these different planetary bodies and provide the first robust estimate for the moon in average terrestrial and enstatite samples show similar e 54 cr on the other hand lunar samples show variables excesses of 53cr and 54cr compared to terrestrial and enstatite chondrites samples with correlated e 53 cr and e 54 cr per 10 000 deviation of the 53cr 52cr and 54cr 52cr ratios normalized to the 50cr 52cr ratio from the nist srm 3112a cr standard unlike previous suggestions we show for the first time that cosmic irradiation can affect significantly the cr isotopic composition of lunar materials moreover we also suggest that rather than spallation reactions neutron capture effects are the dominant process controlling the cr isotope composition of lunar igneous rocks this is supported by the correlation between e 53 cr and e 54 cr and 150sm 152sm ratios after correction of these effects the average e 54 cr of the moon is indistinguishable from the terrestrial and enstatite chondrite materials reinforcing the idea of an earth moon enstatite chondrite system homogeneity this is compatible with the most recent scenarios of moon formation suggesting an efficient physical homogenization after a high energy impact on a fast spinning earth and or with an impactor originating from the same reservoir in the inner proto planetary disk as the earth and enstatite chondrites and having similar composition

new pliocene pleistocene 40ar 39ar ages fill in temporal gaps in the nicaraguan volcanic record abstract the volcanic record of western nicaragua documents a significant lull in volcanic activity that has persisted from the late miocene 7 ma to the formation of the modern volcanic front around 350 ka this study fills this gap for the first time with samples collected in northwest nicaragua between cosiguina and san cristobal volcanoes and with samples collected from the nicaraguan depression we found two previously unknown volcanic units ranging from 3 6 to 1 3 ma and the improved volcanic record allows us to reconstruct the geochemical evolution of the nicaraguan arc u th values increased by nearly threefold since the miocene following the carbonate crash at 10 ma when dominantly carbonate sediment deposition shifted toward hemipelagic sediment deposition this transition was thought to be abrupt however our new data show that it took place gradually over the last 7 ma northwest nicaragua is a particularly interesting case study because it contains middle miocene volcanism on either side of the nicaraguan depression the coyol formation 25 7 ma to the east and the tamarindo formation 14 7 11 7 ma to the west the presence of mid miocene volcanism on either side of the nicaraguan depression has led to the hypothesis that the two coeval units currently separated by 100 km were once connected and have since been separated by extension here we present data suggesting that the tamarindo and coyol are geochemically distinct and therefore cannot be considered part of the same unit

the nature of the kt impactor a 54cr reappraisal according to the variations observed in the various meteorite classes 54cr represents an isotopic tool for planetary body discrimination in the search for the nature of the cretaceous tertiary kt impactor cr isotopic measurements were already performed at kt boundary the current work is aimed to modernize cr isotopic data with high precision measurements to confirm a carbonaceous chondrite type infall the contribution of which is also estimated isotopic signatures of two marine clays caravaca stevens klint exhibit an isotopic ratio which would represent a mixing of a carbonaceous chondrite of cm2 type with terrestrial material in a ratio 6 to 19 a single impactor may account for both marine and continental cr isotopic signatures

a possible buried paleoproterozoic collisional orogen beneath central south china evidence from seismic reflection profiling abstract the south china craton consisting of the yangtze and cathaysia blocks has figured importantly in the neoproterozoic supercontinent of rodinia however its lack of grenville age structures and metamorphism and the presence of paleoproterozoic high grade metamorphism and s type magmatism raise questions about the timing and nature of the yangtze cathaysia collision here we present results of a 400 km long high resolution seismic reflection profile across the purported suture between the yangtze and cathaysia blocks the seismic profile reveals folded and thrust imbricated seismic reflectors which we interpret to represent the relics of the yangtze cathaysia collisional orogen the inferred orogen was extended in the neoproterozoic resulting in its burial by neoproterozoic flysch strata geochronological data suggest that this buried orogen was formed in the paleoproterozoic 2 0 1 9 ga likely associated with the assembling of the columbia supercontinent these results call for major revision of the models for the formation of the south china craton and its role in the assembling of the rodinia and the columbia supercontinents

first evidence for archean continental crust in northern vietnam and its implications for crustal and tectonic evolution in southeast asia southeast asia or in a more strict sense the indochinese continent was previously considered to be composed entirely of proterozoic to phanerozoic rocks and reliable evidence for archean crust was lacking gneisses from the cavinh complex south of the red river shear zone northern vietnam however show archean nd model ages of 3 4 3 1 ga zircon separates from the rocks yielded u pb dates of 2 8 2 5 ga the first convincing evidence for the presence of a late archean complex in southeast asia given that the red river shear zone was propagating in the south china block with a left lateral offset of 600 km the cavinh complex can be correlated with the late archean kangding complex in the western margin of the yangtze craton southwestern china the cavinh complex therefore represents one of the oldest crustal nuclei of the south china block

where have all the craters gone earth s bombardment history and the expected terrestrial cratering record here we make the first quantitative estimates of the maximum total number of large terrestrial impact craters that can survive until the current era on earth our estimates of crater survival are made using the age of earth s crust and the expected flux of large extraterrestrial bodies striking earth this analysis neglects effects such as weathering and burial which tend to preferentially erase smaller craters thus our results represent the maximum number of craters we expect to exist on earth today although the constant recycling of crustal material creates a strong bias toward younger ages our model suggests that the terrestrial cratering record can be used to determine the size frequency distribution of the impactors that bombarded earth over the past 3 5 g y however our model suggests that the cratering record cannot be used to distinguish between scenarios with a constant flux of impactors over time and scenarios where the bombardment rate was considerably higher in earth s ancient history luckily layers of distal impact ejecta may act as records of impacts even when the source craters have been tectonically recycled we suggest that searches for these distal impact ejecta layers will be more fruitful for constraining the bombardment history of earth than searches for large craters

mn cr systematics in primitive meteorites insights from mineral separation and partial dissolution abstract cr isotopic compositions have been measured on carbonaceous chondrites cc tafassasset paris niger i nwa 5958 nwa 8157 and jbilet winselwan in bulk samples the 54 cr 52 cr ratios expressed as e 54 cr range from 0 93 to 1 58 e units these values are in agreement with values characteristic for distinct petrologic types despite this 54 cr heterogeneity the variability in the 53 cr 52 cr ratios expressed as e 53 cr of 0 2 e units and the mn cr ratios is consistent with the previous finding of an isochron in the mn cr evolution diagram the mn cr ratio in cc corresponds to variable abundances of high t condensate formed and separated at the beginning of the solar system thus the canonical 53 mn 55 mn ratio can be defined based on a consistent chronology for u pb and mn cr between the earliest objects formed in the solar nebula and the d orbigny angrite we define a canonical 53 mn 55 mn ratio and e 53 cr i of 6 8 10 6 and 0 177 respectively the internal mn cr systematics in tafassasset and paris were studied by two approaches leaching technique and mineral separation despite variable e 54 cr values up to 30 e linear co variations were found between e 53 cr and mn cr ratio the mineral separates as well as the leachates of tafassasset fall on a common isochron indicating that 1 cooling of the tafassasset s parent body occurred at 4563 5 0 25 ma and that 2 54 cr is decoupled from the other isotopes even though temperatures 900 c have been reached during metamorphism in the case of paris the leachates form an alignment with a 53 mn 55 mn ratio higher than the canonical value this alignment is not an isochron but rather a mixing line based on leachates from various cm and ci we propose the occurrence of three distinct cr reservoirs in meteoritic material pure54 high53 and low53 characterized by a e 53 cr and e 54 cr of 0 and 25 000 2 17 and 8 and 0 5 and 151 respectively pure54 has already been described and is carried by highly refractory nano spinel high53 is mn rich and most probably carried by sulfides in the matrix whereas low53 is characterized by low mn cr ratios and it is sensitive to metamorphism this component could correspond to mineral phases such as refractory oxides and carbide variable mixing proportions of high53 and low53 would explain the larger than expected uncertainty mswd of 5 5 on the cc bulk regression line a monte carlo simulation allows us to evaluate the impact of the dispersion of the initial cr isotopic ratios as a function of variable high53 the co variation of the mn cr ratio and the e 53 cr defined by the mineral separates from paris corresponds to an age of 4566 44 0 66 0 75 ma while their e 54 cr still differ by at least 0 42 e this age is likely to date the segregation of forsteritic olivines most probably from type i chondrules from fayalitic olivines from type ii chondrules and given the sampling procedure by handpicking of hundreds of grains corresponds to the average age of chondrule formation

petrography of impact glasses and melt breccias from the el gygytgyn impact structure russia the el gygytgyn impact structure 18 km in diameter and 3 6 ma old in arctic siberia russia is the only impact structure on earth mostly excavated in acidic volcanic rocks the late cretaceous volcanic target includes lavas tuffs and ignimbrites of rhyolitic dacitic and andesitic composition and local occurrence of basalt although the ejecta blanket around the crater is nearly completely eroded bomb shaped impact glasses redeposited after the impact event occur in lacustrine terraces within the crater here we present detailed petrographic descriptions of newly collected impact glass bearing samples the observed features contribute to constrain the formation of the melt and its cooling history within the framework of the impact process the collected samples can be grouped into two types characterized by specific features 1 pure glasses containing very few clasts or new crystals and which were likely formed during the early stages of cratering and 2 a second type which represents composite samples with impact melt breccia lenses embedded in silicate glass these mixed samples probably resulted from inclusion of unmelted impact debris during ejection and deposition after deposition the glassy portions continued to deform whereas the impact melt breccia inclusions that probably had already cooled down behaved as rigid bodies in the flow

isotopic evidence for the cretaceous tertiary impactor and its type high precision mass spectrometric analysis of chromium in sediment samples from the cretaceous tertiary k t boundary coincident with the extinction of numerous organisms on earth confirms the cosmic origin of the k t phenomenon the isotopic composition of chromium in k t boundary samples from stevns klint denmark and caravaca spain is different from that of earth and indicates its extraterrestrial source the chromium isotopic signature is consistent with a carbonaceous chondrite type impactor the observed differences in the chromium isotopic composition among various meteorite classes can serve as a diagnostic tool for deciphering the nature of impactors that have collided with earth during its history

chromium isotope evidence in ejecta deposits for the nature of paleoproterozoic impactors abstract non mass dependent chromium isotopic signatures have been successfully used to determine the presence and identification of extra terrestrial materials in terrestrial impact rocks paleoproterozoic spherule layers from greenland graenseso and russia zaonega as well as some distal ejecta deposits lake superior region from the sudbury impact 1849 0 3 ma event have been analyzed for their cr isotope compositions our results suggest that 1 these distal ejecta deposits are all of impact origin 2 the graenseso and zaonega spherule layers contain a distinct carbonaceous chondrite component and are possibly related to the same impact event which could be vredefort 2023 4 ma or another not yet identified large impact event from that of similar age and 3 the sudbury ejecta record a complex meteoritic signature which is different from the graenseso and zaonega spherule layers and could indicate the impact of a heterogeneous chondritic body

the number of impact craters on earth any room for further discoveries abstract only 128 impact craters exposed at earth s surface have been found so far while new craters are discovered occasionally taking into account the permanent consumption of craters by erosion we present the first estimate on the number of impact craters that should be present at earth s surface our study yields no evidence for any systematic incompleteness in the available inventory of the craters larger than about 6 km in diameter exposed at the surface in contrast more than 90 craters in the diameter range from 1 km to 6 km should still be waiting to be discovered and even more than 250 between 0 25 km and 1 km diameter the transition from a probably complete inventory above 6 km to a strongly incomplete record at smaller sizes may be related to the difference between simple and complex craters beyond these results on the terrestrial crater record our findings tentatively suggest that erosion rates on the 10 to 100 million year scale may be closer to present day erosion rates than previously assumed

zircon u pb ages and hf isotopic compositions of alkaline silicic magmatic rocks in the phan si pan tu le region northern vietnam identification of a displaced western extension of the emeishan large igneous province abstract in situ zircon u pb and hf isotope analyses were carried out for alkaline silicic magmatic rocks from the phan si pan tu le region in northern vietnam to constrain their possible sources and to determine their petrogenetic relationship with the emeishan large igneous province elip sw china nine granites and nine rhyolites yield zircon 206pb 238u ages from 262 ma to 249 ma coinciding with the timing of silicic magmatism in the panxi area of the elip the zircon ehf t values 14 to 3 of these granites and rhyolites suggest a moderately depleted mantle source and overlap with those of peralkaline and metaluminous granites in the panxi area i e ehf t 14 to 4 the zircon hf isotope ratios show that the zircons probably record the original source characteristics whereas whole rock nd isotope data indicate an evidence for crustal contamination that may have occurred at lower temperatures during magma emplacement the synchroneity coupled with petrological and geochemical similarities indicate that silicic rocks from the phan si pan tu le region are cogenetic with the panxi silicic plutonic rocks and that they are likely derived by similar petrogenetic processes i e fractionation of mafic magmas or partial melting of mafic rocks therefore we propose that the phan si pan tu le region represents a displaced portion of the elip inner zone

filling in the juvenile magmatic gap evidence for uninterrupted paleoproterozoic plate tectonics abstract despite several decades of research on growth of the continental crust it remains unclear whether the production of juvenile continental crust has been continuous or episodic throughout the precambrian models for episodic crustal growth have gained traction recently through compilations of global u pb zircon age frequency distributions interpreted to delineate peaks and lulls in crustal growth through geologic time one such apparent trough in zircon age frequency distributions between 2 45 and 2 22 ga is thought to represent a pause in crustal addition resulting from a global shutdown of magmatic and tectonic processes the 2 45 2 22 ga magmatic shutdown model envisions a causal relationship between the cessation of plate tectonics and accumulation of atmospheric oxygen over the same period here we present new coupled u pb hf and o isotope data for detrital and magmatic zircon from the western churchill province and trans hudson orogen of canada covering an area of approximately 1 3 million km2 that demonstrate significant juvenile crustal production during the 2 45 2 22 ga time interval and thereby argue against the magmatic shutdown hypothesis our data is corroborated by literature data showing an extensive 2 22 2 45 ga record in both detrital and magmatic rocks on every continent and suggests that the operation of plate tectonics continued throughout the early paleoproterozoic while atmospheric oxygen rose over the same time interval we argue that uninterrupted plate tectonics between 2 45 and 2 22 ga would have contributed to efficient burial of organic matter and sedimentary pyrite and the consequent rise in atmospheric oxygen documented for this time interval

a new global database of mars impact craters 1 km 1 database creation properties and parameters 1 impact craters have been used as a standard metric for a plethora of planetary applications for many decades including age dating geologic mapping and stratigraphic relationships as tracers for surface processes and as locations for sampling lower crust and upper mantle material utilizing craters for these and other investigations is significantly aided by a uniform catalog of craters across the surface of interest consequently catalogs of craters have been developed for decades for the moon and other planets we present a new global catalog of martian craters statistically complete to diameters d 1 km it contains 384 343 craters and for each crater it lists detailed positional interior morphologic ejecta morphologic and morphometric data and modification state information if it could be determined in this paper we detail how the database was created the different fields assigned and statistical uncertainties and checks in our companion paper robbins and hynek 2012 we discuss the first broad science applications and results of this work

transformations to granular zircon revealed twinning reidite and zro2 in shocked zircon from meteor crater arizona usa granular zircon in impact environments has long been recognized but remains poorly understood due to lack of experimental data to identify mechanisms involved in its genesis meteor crater in arizona usa contains abundant evidence of shock metamorphism including shocked quartz the high pressure polymorphs coesite and stishovite diaplectic sio2 glass and lechatelierite fused sio2 here we report the presence of granular zircon a new shocked mineral discovery at meteor crater that preserve critical orientation evidence of specific transformations that occurred during formation at extreme impact conditions the zircon grains occur as aggregates of sub micrometer neoblasts in highly shocked coconino sandstone cs comprised of lechatelierite electron backscatter diffraction shows that each grain consists of multiple domains some with boundaries disoriented by 65 around a known 112 shock twin orientation other domains have 001 in alignment with 110 of neighboring domains consistent with the former presence of the high pressure zrsio4 polymorph reidite additionally nearly all zircon preserve zro2 sio2 providing evidence of partial dissociation the genesis of cs granular zircon started with detrital zircon that experienced shock twinning and reidite formation at pressures from 20 to 30 gpa ultimately yielding a phase that retained crystallographic memory this phase subsequently recrystallized to systematically oriented zircon neoblasts and in some areas partially dissociated to zro2 the lechatelierite matrix experimentally constrained to form at 2000 c provided the ultrahigh temperature environment for zircon dissociation 1670 c and neoblast formation the capacity of granular zircon to preserve a cumulative pressure temperature record has not been recognized previously and provides a new method for investigating histories of impact related mineral transformations in the crust at conditions far beyond those at which most rocks melt

the 2 65 ga a type granite in the northeastern yangtze craton petrogenesis and geological implications abstract the presence of old residual continental crustal rocks in cratons is crucial for investigation of their early evolution due to the poor exposure of archean rocks the early evolution of the yangtze craton has not been well constrained yet in this study a combined study of zircon u pb and hf o isotopes as well as whole rock major and trace elements and nd isotopes have been conducted for a granite pluton at huji town in the northern part of the yangtze craton u pb dating for magmatic zircons yielded a weighted mean age of 2656 6 ma revealing another archean outcrop in the yangtze craton besides the kongling terrane the granite samples show relatively evaluated k 2 o na 2 o cao tfeo mgo ratios and zr nb ce ga and y contents sharing the same features as the a type granite they have pronounced eu sr and ba negative anomalies on the primitive mantle normalized trace element spidergram implying the residual of plagioclase in the magma sources their high zr contents of 440 529 ppm correspond to initial magma temperatures of 930 962 c the zircons have negative hf t values of 2 7 to 0 1 and hf model ages of ca 3 2 ga respectively the 18 o values of the zircons range between 4 94 and 5 72 and give an average of 5 26 0 14 suggesting no contribution of supracrustal rock into the host magma therefore the hf model ages indicate the generation of the huji granite by reworking of ca 3 2 ga juvenile crustal rocks combined with previous studies the finding of the neoarchean huji granites with ca 3 2 ga hf model ages suggests the widespread of archean basement in the yangtze craton the occurrence of the ca 2 65 ga huji granites likely records the thermally stable lithosphere and thus heralds the craton stabilization of the eastern part of the yangtze block

the 2 40ga ringvassoy mafic dykes west troms basement complex norway the concluding act of early palaeoproterozoic continental breakup abstract a north to northwest trending mafic dyke swarm of gabbronoritic and gabbroic composition makes up a significant part of the archean basement on the island of ringvassoy in northern norway u pb geochronology of zircon and baddeleyite in a gabbronorite provides an age of emplacement of 2403 3 ma metamict zircon in a plagioclase phyric dyke yield data that are discordant but consistent with the age of the gabbronoritic dyke titanite indicates a metamorphic overprint at 1768 4 ma the two types of dyke show some distinct chemical characteristics they are both tholeiitic enriched in lrees and lile elements but depleted in hfs elements including nb their nd isotopic composition yields nd values of 1 5 to 1 8 for gabbronorites and 0 4 to 1 3 for the plagioclase phyric dykes the chemical and isotopic constraints are typical of continental basalts the ringvassoy mafic dykes correlate broadly with a global palaeoproterozoic magmatic event that formed extensive bimodal intrusive and extrusive suites in most archaean cratons including the northeastern fennoscandian shield in detail the 2403 3 ma ringvassoy dykes postdated most episodes of magmatism at this time on the regional scale there is a distinct trend from a 2505 2490 ma suite present in the kola peninsula over a second 2460 2440 ma suite present both in kola and further south in karelia to the 2403 ma dykes on ringvassoy this variation suggests that the locus of maximum extension and magmatic activity may have been shifting with time

crustal origin of early proterozoic syenites in the congo craton ntem complex south cameroon two petrographically different syenite suites can be distinguished in the mostly archean ntem complex of cameroon a suite of two pyroxene syenites and a suite of clinopyroxene syenites the syenites occur as discrete intrusions surrounded by archean gneisses based on structural relationships the syenites are of late to post orogenic origin the pb pb zircon evaporation ages indicate that the syenites intruded at ca 2 3 ga and thus postdate significantly the archean episode of crust formation and differentiation that generated the surrounding gneiss terrane the isotope signatures and the trace elements provide unambiguous evidence that the two syenite suites are of crustal origin the syenite suites do not show simple chemical trends have highly variable trace element contents and display only a small variation in their major element contents particularly in their sio2 contents the isotope characteristics including 143nd 144nd 87sr 86sr and common pb point to a heterogeneous source or multiple sources for the syenites the sr isotope compositions calculated for the time of intrusion vary over a wide range from an unrealistically low 87sr 86sr ratio of 0 690 to high values of 0 713 in contrast the initial 143nd 144nd ratios are more homogenous with end 2320 from 5 8 to 7 5 the pb isotopes from leached k feldspars indicate that the sources of the syenites had a history of u depletion and th u enrichment the syenites show the same isotope characteristics as the surrounding gneiss terrane thus the deep continental crust of the ntem complex itself is the most likely source for the syenite melts this conclusion is also supported by the heterogeneity of the trace elements in the syenites that exclude a homogenous source in addition inherited zircons from the syenites yield pb pb evaporation ages that are consistent with ages obtained for zircons extracted from the surrounding gneisses in contrast to many other syenites worldwide the syenites in the ntem complex do not contain juvenile mantle material but are the sole products of crustal reworking thus the syenites did not contribute to the crustal growth in the ntem complex however this conclusion may not preclude the possibility that the syenites were generated in the deep crust during a time of addition of mantle material to the continental crust the cause for the generation of these large syenite bodies in the ntem complex is most likely an episode of deep seated thermal activity that was so far not known from this part of the congo craton

response to the comment by w h schwarz et al on joint determination of 40k decay constants and 40ar 40k for the fish canyon sanidine standard and improved accuracy for 40ar 39ar geochronology by p r renne et al 2010

terrestrial impact melt rocks and glasses abstract the effects of meteorite and comet impact on earth are rock brecciation the formation of shock metamorphic features rock melting and the formation of impact structures i e simple craters complex craters and multi ring basins large events such as the 65 ma chicxulub impact are believed to have had catastrophic environmental effects that profoundly influenced the development of life on earth in this review an attempt is made to summarize some of the voluminous literature on impact melting one important aspect of planetary impact provide some comments on this process and to make suggestions for future research the products of impact melting are glasses impact melt rocks and pseudotachylites our treatise deals mainly with the geological setting petrography and major element chemistry of melt rocks and glasses impact glasses in several petrographic aspects are similar to volcanic glasses but they are associated with shock metamorphosed mineral and rock fragments and in places with siderophile element anomalies suggestive of meteoritic contamination they are found in allogenic breccia deposits within fall back suevite and outside fall out suevite impact craters and as spherules in distal ejecta large events such as the k t boundary chicxulub impact are responsible for the formation of worldwide ejecta horizons which are associated with siderophile element anomalies and shock metamorphosed mineral and rock debris impact glasses have a bulk chemical composition that is homogeneous but exemptions to this rule are common on a microscopic scale however impact glasses are commonly strikingly heterogeneous tektites are glasses ejected from craters over large distances they are characterized by very low water and volatile contents and element abundances and ratios that are evidence that tektites formed by melting of upper crustal sedimentary rocks four tektite strewn fields are known three of which can be tied to specific impact craters impact melt rocks form sheets lenses and dike like bodies within or beneath allogenic fallback breccia deposits in the impact crater and possibly on crater terraces and flanks dikes of impact melt rocks also intrude the rocks of the crater floor they commonly contain shock metamorphosed target rock and mineral fragments in various stages of assimilation and are glassy or fine to coarse grained chemically they are strikingly homogeneous but as with impact glasses exemptions to this rule do exist large and thick melt bodies such as the sudbury igneous complex sic are differentiated or may represent a combination of impact melt rocks sensu strictu and impact triggered deep crustal melts a concerted multidisciplinary approach to future research on impact melting and on other aspects of meteorite and comet impact is advocated impact models are models only and uncritical reliance on their validity will not lead to a better understanding of impact processes especially of melting excavation and deposition of allogenic breccias and the spatial position of breccias in relation to sheets and lenses of melt rocks within the crater impact triggered pressure release melting of target rocks beneath the excavation cavity may be responsible for the existence of melt rocks beneath the impact melt rocks sensu strictu this controversial idea needs to be tested by a re evaluation of existing data and models be they based on field or laboratory research only a relatively small number of terrestrial impact structures has been investigated in sufficient detail as it relates to geological and geophysical mapping in this review we summarize observations made on impact melt rocks and impact glasses in a number of north american brent haughton manicouagan new quebec sudbury wanapitei all in canada asian popigai russia zhamanshin kazakhstan two south african structures morokweng and vredefort the henbury crater field of australia and one european crater ries germany our tables listing major element chemical compositions of impact glasses and melt rocks however include also data from structures not dealt with in further detail

chromium isotopes in late eocene impact spherules indicate a likely asteroid belt provenance abstract spherules and magnetic fractions extracted from two late eocene impact deposits massignano italy and ocean drilling program hole 709c western indian ocean have anomalous concentrations of ir and high concentrations of cr ni co and fe an excess of 53 cr relative to 53 cr 52 cr ratios in terrestrial samples shows that much of the cr in these samples is extraterrestrial the absence of relatively large 54 cr excesses as found in carbonaceous chondrites precludes a carbonaceous chondrite source and the data best fit ordinary chondrites the two localities produced samples with somewhat different elemental and isotopic compositions but these are likely different components of the same impact rather than from two distinct impacts the multiple impacts and dust deposition in the late eocene were more likely caused by disruption of a large asteroid in the inner asteroid belt than by a comet shower the brangane asteroid family may be the result of this disruption the source of the dust and impactors in the late eocene and its members may be composed of h chondrite lithologies based upon the coeval 35 ma peak in h chondrite cosmic ray exposure ages

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

a new stratigraphy for the tertiary volcanic rocks of the nicaraguan highland three major volcanic events have occurred in nicaragua since middle tertiary time first the oligocene was dominated by rhyolite shield volcanism producing the highland ignimbrite second creation of northeast and later northwest striking dilatational fracture zones allowed the extrusion of basaltic to andesitic magmas along the pacific coast constructing the northwest oriented coyol volcanic arc this miocene arc can be followed all along the pacific coast of the chortis block third a pliocene pleistocene southwestward shift of arc volcanism created the modern volcanic arc this volcanic history is supported by a new stratigraphy based on volcanic centers and related facies the old cyclic layer cake stratigraphy of the coyol group has been abandoned

the arrowsmith orogeny geochronological and thermobarometric constraints on its extent and tectonic setting in the rae craton with implications for pre nuna supercontinent reconstruction abstract geochronological and thermobarometric data for two widely spaced and previously unstudied areas of the rae craton canada provide important temporal and spatial links between previously investigated areas in the northern and southern rae craton that had been interpreted to have been reworked during the ca 2 5 2 3 ga arrowsmith orogeny on southern boothia peninsula upper amphibolite to granulite facies metamorphism dated by in situ shrimp u pb analysis of monazite occurred at ca 2 64 2 60 2 54 2 50 2 49 2 44 and 2 37 2 34 ga with retrograde monazite growth at 2 27 ga shrimp u pb ages of zircon in two metasedimentary rocks provide a further indication of metamorphism at ca 2 54 and 2 50 ga which also corresponds to the age determined for a k feldspar augen granite textural relationships indicate that the regional foliation formed between ca 2 54 ga and 2 50 2 49 ga peak metamorphic conditions of 700 c and 5 6 kbar are interpreted to have been reached during the 2 37 2 34 ga event in the mccann mcarthur lake region of the southwestern rae craton upper amphibolite to granulite facies metamorphism is dated by in situ shrimp monazite ages of ca 2 53 2 47 and 2 37 2 28 ga textural relationships indicate deformation initiated by ca 2 37 ga and terminated by ca 2 30 ga metamorphic conditions of 5 kbar and 700 735 c were quenched at ca 2 3 ga following peak conditions of 7 kbar and 800 c the evidence for regional deformation at 2 54 2 49 ga on southern boothia peninsula supports an andean type accretionary margin setting for the arrowsmith orogeny a collisional event may have terminated high grade metamorphism in the northern region by ca 2 34 ga while metamorphism associated with continental arc magmatism continued in the south until at least ca 2 28 ga these combined data demonstrate the continuity and broad extent of the arrowsmith orogen along the western flank of the rae craton extending 1500 km from northern saskatchewan to northern baffin island the broad extent of this globally rare orogenic event provides an important piercing point for supercontinent reconstructions which are discussed in a companion paper the inferred convergent tectonic activity between ca 2 54 and 2 28 ga is interpreted to have occurred on the flank of a growing supercontinent a setting that could account for an apparent slowdown but not shutdown in global tectonics

a type granites in northeastern china age and geochemical constraints on their petrogenesis abstract a type granites are widely distributed in northeastern china ne china they were emplaced during three major episodes the permian late triassic to early jurassic and early cretaceous and evolved in different tectonic regimes according to their mineralogical and geochemical characteristics two subgroups of a type granites aluminous and peralkaline can be recognized the peralkaline subgroup contains alkali mafic minerals such as riebeckite arfvedsonite and sodic pyroxene while the aluminous subgroup contains annite and fe rich calcic or sodic calcic amphibole with respect to the aluminous subgroup the peralkaline granites contain higher rb ga and total rare earth elements ree but lower mgo cao al 2 o 3 ba and sr based on the discrimination criteria of eby geology 20 1992 641 the permian and late triassic to early jurassic a type granites belong to the a 2 post orogenic type whereas the early cretaceous granites are of the a 1 anorogenic type nd isotopic compositions of these a type granites indicate their derivation from a dominantly juvenile crustal source their origin is thought to have involved partial melting of an underplated lower crustal source because the generation of a type granites requires high melting temperature we propose models involving slab break off lithospheric delamination and extension the permian a type granites have the same age range as those in eastern junggar southern mongolia and central inner mongolia they occur along a major suture and form a narrow belt between the north china and siberian cratons we suggest that their formation was associated with post collisional slab break off the late triassic to early jurassic a type granites are likely to be the product of lithospheric delamination after the final collision of the major crustal blocks in the late paleozoic to early triassic the early cretaceous a type granites have an anorogenic affinity and were possibly associated with rifting in eastern china at this time associated with the onset of paleo pacific subduction consequently we conclude that the a type granites in ne china were generated at three different times involving multiple processes operative in different tectonic environments

genesis of the two main types of peraluminous granitoids two mica monzogranites to leucogranites and biotite rich cordierite bearing tonalites to monzogranites form two distinct groups of peraluminous granitoids they can be distinguished by mineral and rock associations and by the variation of their peraluminosity during differentiation except for the rare muscovite bearing granitoids produced by extreme fractionation or local contamination of metaluminous magmas the majority of peraluminous granitoids are produced by partial melting of crustal rocks production of either muscovite bearing granites or biotite rich cordierite bearing granitoids does not depend only on the nature of the sources but is also controlled by the physical parameters of partial melting and consequently by the way anatexis of a thickened crust is enhanced biotite rich cordierite bearing granitoids are generated where mantle derived magmas are injected into or have underplated crustal rocks two mica granites are generated where thickened crust is affected by major crustal shears or thrusts correlations between differentiation and peraluminosity indicate the dominant role of either restite unmixing or fractional crystallization in the production and evolution of the various types of peraluminous granitoids

shock metamorphosed zircon in terrestrial impact craters to ascertain the progressive stages of shock metamorphism of zircon samples from three well studied impact craters were analyzed by optical microscopy scanning electron microscopy sem and raman spectroscopy in thin section and grain separates these samples are comprised of well preserved rapidly quenched impactites from the ries crater germany strongly annealed impactites from the popigai crater siberia and altered variably quenched impactites from the chicxulub crater mexico the natural samples were compared with samples of experimentally shock metamorphosed zircon below 20 gpa zircon exhibits no distinct shock features above 20 gpa optically resolvable planar microstructures occur together with the high pressure polymorph reidite which was only retained in the ries samples decomposition of zircon to zro2 only occurs in shock stage iv melt fragments that were rapidly quenched this is not only a result of post shock temperatures in excess of 1700 c but could also be shock pressure induced which is indicated by possible relics of a high pressure polymorph of zro2 however zro2 was found to revert to zircon with a granular texture during devitrification of impact melts other granular textures represent recrystallized amorphous zrsio4 and reidite that reverted to zircon this requires annealing temperatures 1100 c a systematic study of zircons from a continuous impactite sequence of the chicxulub impact structure yields implications for the post shock temperature history of suevite like rocks until cooling below 600 c

estimating peak currents at ground lightning impacts using remanent magnetization 1 the impact of lightning is permanently recorded in struck material by a secondary magnetization called lightning induced remanent magnetization lirm we introduce an original rock magnetic method based on alternating field demagnetization to define both the geometry and the intensity of the peak lightning magnetic field it also provides positive evidence than a material has effectively been struck on a schoolyard tree struck by lightning we were able to locate the impact to within a few centimeters to prove that it was a vertical negative current and to estimate a peak current intensity of 99 7 ka this value is 14 ka higher than the one provided by meteorage lightning detection network and offers an independent calibration

shocked rocks and impact glasses from the el gygytgyn impact structure russia the elgygytgyn impact structure is about 18 km in diameter and is located in the central part of chukotka arctic russia the crater was formed in volcanic rock strata of cretaceous age which include lava and tuffs of rhyolites dacites and andesites a mid pliocene age of the crater was previously determined by fission track 3 45 0 15 ma and 40ar 39ar dating 3 58 0 04 ma the ejecta layer around the crater is completely eroded shock metamorphosed volcanic rocks impact melt rocks and bomb shaped impact glasses occur in lacustrine terraces but have been redeposited after the impact event clasts of volcanic rocks which range in composition from rhyolite to dacite represent all stages of shock metamorphism including selective melting and formation of homogeneous impact melt four stages of shocked volcanic rocks were identified stage i 35 gpa lava and tuff contain weakly to strongly shocked quartz and feldspar clasts with abundant pfs and pdfs coesite and stishovite occur as well stage ii 35 45 gpa quartz and feldspar are converted to diaplectic glass coesite but no stishovite stage iii 45 55 gpa partly melted volcanic rocks common diaplectic quartz glass feldspar is melted and stage iv 55 gpa melt rocks and glasses two main types of impact melt rocks occur in the crater 1 impact melt rocks and impact melt breccias containing abundant fragments of shocked volcanic rocks that were probably derived from now eroded impact melt flows on the crater walls and 2 aerodynamically shaped impact melt glass bombs composed of homogeneous glass the composition of the glasses is almost identical to that of rhyolites from the uppermost part of the target cobalt ni and ir abundances in the impact glasses and melt rocks are not or only slightly enriched compared to the volcanic target rocks only the cr abundances show a distinct enrichment which points toward an achondritic projectile however the present data do not allow one to unambiguously identify a meteoritic component in the elgygytgyn impact melt rocks

an international and multidisciplinary drilling project into a young complex impact structure the 2004 icdp bosumtwi crater drilling project an overview the bosumtwi impact crater in ghana arguably the best preserved complex young impact structure known on earth displays a pronounced rim and is almost completely filled by lake bosumtwi a hydrologically closed basin it is the source crater of the ivory coast tektites the structure was excavated in 2 1 2 2 gyr old metasediments and metavolcanics of the birimian supergroup a drilling project was conceived that would combine two major scientific interests in this crater 1 to obtain a complete paleoenvironmental record from the time of crater formation about one million years ago at a near equatorial location in africa for which very few data are available so far and 2 to obtain a complete record of impactites at the central uplift and in the crater moat for ground truthing and comparison with other structures within the framework of an international and multidisciplinary drilling project led by the international continental scientific drilling program icdp 16 drill cores were obtained from june to october 2004 at six locations within lake bosumtwi which is 8 5 km in diameter the 14 sediment cores are currently being investigated for paleoenvironmental indicators the two impactite cores lb 07a and lb 08a were drilled into the deepest section of the annular moat 540 m and the flank of the central uplift 450 m respectively they are the main subject of this special issue of meteoritics planetary science which represents the first detailed presentations of results from the deep drilling into the bosumtwi impactite sequence drilling progressed in both cases through the impact breccia layer into fractured bedrock lb 07a comprises lithic in the uppermost part and suevitic impact breccias with appreciable amounts of impact melt fragments the lithic clast content is dominated by graywacke besides various metapelites quartzite and a carbonate target component shock deformation in the form of quartz grains with planar microdeformations is abundant first chemical results indicate a number of suevite samples that are strongly enriched in siderophile elements and mg but the presence of a definite meteoritic component in these samples cannot be confirmed due to high indigenous values core lb 08a comprises suevitic breccia in the uppermost part followed with depth by a thick sequence of graywacke dominated metasediment with suevite and a few granitoid dike intercalations it is assumed that the metasediment package represents bedrock intersected in the