incomplete retention of radiation damage in zircon from sri lanka a suite of 18 zircon gemstones from placers in the highland southwestern complex sri lanka were subjected to a comprehensive study of their radiation damages and ages the investigation included x ray diffraction raman and pl spectroscopy electron microprobe pixe and hrtem analysis as well as u th he and shrimp u th pb age determinations zircon samples described in this study are virtually homogeneous they cover the range from slightly metamict to nearly amorphous generally concordant u th pb ages averaging 555 11 ma were obtained late ordovician zircon u th he ages scattering around 443 9 ma correspond reasonably well with previously determined biotite rb sr ages for rocks from the hswc slightly to moderately metamict zircon has retained the radiogenic he whereas only strongly radiation damaged zircon calculated total fluences exceeding 3 5 10 18 events g has experienced significant he loss when compared to unannealed zircon from other localities sri lanka zircon is about half as metamict as would correspond to complete damage accumulation over a 555 m y lasting self irradiation period suggesting significant annealing of the structural radiation damage insufficient consideration of this has often resulted in significant underestimation of radiation effects in zircon we suggest to estimate effective doses for sri lanka zircon by multiplying total fluences which were calculated using the zircon u th pb age by a correction factor of 0 55 this conversion may be applied to literature data as well because all gem zircon samples from sri lanka this work and previous studies seem to reveal the same general trends of property changes depending on the radiation damage the use of effective doses for sri lanka zircon contributes to more reliable quantitative estimates of radiation effects and makes possible direct comparison between natural and synthetic radiation damaged zircon

amplitude variation with offset and prestack waveform inversion a direct comparison using a real data example from the rock springs uplift wyoming usa abstractrecent advances in seismic data acquisition and processing allow routine extraction of offset angle dependent reflection amplitudes from prestack seismic data for quantifying subsurface lithologic and fluid properties amplitude variation with offset avo inversion is the most commonly used practice for such quantification although quite successful avo has a few shortcomings primarily due to the simplicity in its inherent assumptions and for any quantitative estimation of reservoir properties they are generally interpreted in combination with other information in recent years waveform based inversions have gained popularity in reservoir characterization and depth imaging going beyond the simple assumptions of avo and using wave equation solutions these methods have been effective in accurately predicting the subsurface properties developments of these waveform inversions have so far been along two lines 1 the methods that use a locally 1d model of the subsurface for each common midpo

petrogenesis and tectonic implications of early cretaceous s and a type granites in the northwest of the gan hang rift se china the gan hang rift trending at least 450 km in a ne sw direction is a part of a mesozoic basin and range province in southeastern china detailed shrimp zircon u pb chronology major and trace element and sr nd hf isotope data of three granitic plutons and a diabasic dike in the northwest of the gan hang rift are used to explore the origin of these granites and their relationship to the evolution of the gan hang rift shrimp zircon u pb dating shows that the granitic plutons and diabasic dike were emplaced in the early cretaceous 122 129 ma the tongshan and damaoshan plutons close to the gan hang rift consist mainly of weakly peraluminous granitic rocks which show a2 subtype affinity these granites have initial 87sr 86sr ratios of 0 7080 0 7103 end t values of 1 4 to 5 6 and ehf t in situ zircon values of 3 8 to 1 2 detailed elemental and isotopic data suggest that they were formed by partial melting of granulitized mesoproterozoic metamorphic basement including metasedimentary and metaigneous rocks in the shallow 30 km crust at a high temperature 823 c these pure crustal melts underwent crystal fractionation of biotite plagioclase and k feldspar forming the more felsic end member of granitic rocks the ehu pluton more distant from the gan hang rift is composed of two mica granites which are strongly peraluminous and are s type granites these granites have higher initial 87sr 86sr ratios 0 7162 0 7191 and lower end t 7 1 to 7 4 and ehf t 6 8 in situ zircon values than the tongshan and damaoshan granites detailed elemental and isotopic data suggest that the ehu granites were formed by partial melting of paleoproterozoic metasedimentary basement in the deep 40 50 km crust at a relatively low temperature 757 c with no fractional crystallization and mantle derived magma mixing in their evolution the coeval diabasic dike is calc alkaline and has initial 87sr 86sr ratio of 0 7085 end t value of 0 9 and ehf t in situ zircon value of 7 1 suggesting that it was derived from the asthenospheric mantle wedge that had been metasomatised via addition of subducted sediment derived melt with monazite in the residual phase the association of early cretaceous 122 129 ma s and a type granites in the northwest of the gan hang rift marks the onset of back arc extension or intra arc rift with ongoing extension the crust and lithospheric mantle became progressively thinned the upwelling of asthenosphere triggered partial melting of both metasedimentary and metaigneous rocks in the more thinned crust close to the gan hang rift forming the a type granitic magmas such as tongshan and damaoshan whereas partial melting of metasedimentary rocks in the less thinned crust farther from the gan hang rift formed the s type granitic magmas such as ehu the red sediments with the total thickness more than 10 000 m have been successively deposited in the gan hang rift valley since the late early cretaceous 105 ma suggesting that this region experienced the most back arc extension

self induced fracture generation in zircon the mineral zircon has particular geological significance because of its use in the age determination of rocks and as a structural analogue phase for radioactive waste forms despite its widespread utility however radiation damage of the crystal structure metamictization causes a volume expansion of the crystal lattice and the generation of internal stresses which can induce fractures in the crystal a model has been developed which describes the state of stress in n concentric spherical shells each of which may have different material properties the case of a three shelled composite sphere has been examined explicitly in combination with fracture mechanics theory the model predicts that spatially distinct radial and or concentric fracture sets can be produced and such fracture patterns accurately describe the distribution and types of self induced fractures observed in natural zircons in general the formation of such microfractures in zircon is a function of the degree of metamictization shell thickness and the confining pressure the model predicts that the maximum depths in the earth s crust at which radial versus concentric fractures can form in a crystal are different suggesting that the types of fractures found in a suite of zircons from a particular rock might be potential indicators of the depth of burial because metamictization induced fractures may serve as potential pathways for the rapid leaching of various elements from the zircon crystal this may also have important implications in interpreting the u pb ages of fractured zircons or in evaluating the suitability of related crystalline phases as hosts for nuclear waste

nanoscale evidence for uranium mobility in zircon and the discordance of u pb chronometers scanning transmission electron microscopy stem images coupled with u pb isotopic analysis give new insight into mobility of u and pb in zircon zircon crystals included in xenotime and in plagioclase of a pegmatitic assemblage comprising euxenite correspond to irradiation related alteration of zircon with amorphization microporosity swelling radial cracking and uptake of non formula cations like ca leading to increased discordance of the u pb system fractures and pores represent the vectors for fluid mediated open system behavior stem images demonstrate the presence of u rich clusters 70 nm with uraninite structure within the porosity at high magnification pores are surrounded by a low density halo 100 nm and are coated by a high density wall 50 nm enriched in u or ca or both we interpret the spatial link between u ca and porosity as evidence for u uptake alteration of euxenite crystals into pyrochlore in the rock led to liberation of u and ca these elements were transported by fluids through cracks and incorporated within zircon thanks to porosity by a diffusion reaction process the data on this uncommon sample demonstrate that discordance of the u pb system in zircon affected by radiation damage includes a component of u mobility u gain u loss superimposed on pb mobility pb loss pb gain

geochemistry and zircon u pb hf isotopes of the late neoarchean granodiorite monzogranite quartz syenite intrusions in the northern liaoning block north china craton petrogenesis and implications for geodynamic processes abstract late neoarchean weakly foliated to massive granodiorite monzogranite quartz syenite intrusions are widely distributed in the northern liaoning block north china craton la icp ms zircon dating reveals that these rocks were emplaced during 2 56 2 53 ga and underwent regional metamorphism at 2 51 ga these granitoid rocks that contain abundant k feldspar and high k 2 o contents with high k 2 o na 2 o ratios can be subdivided into four lithological types based on their lithological and geochemical characteristics including porphyritic granodiorites medium grained granodiorites medium grained monzogranites and porphyritic quartz syenitic gneisses the porphyritic granodiorites contain biotite as the main mafic mineral and are characterized by high la yb n 16 60 and sr y 46 93 ratios low yb and y contents and weakly positive eu anomalies for most samples they also show depletion of nb ta and ti in the primitive mantle normalized spidergrams with zircon e hf t 2 values of 3 8 to 6 3 suggesting that they were derived from partial melting of juvenile high k mafic rocks at the lower crust the medium grained granodiorites have major mafic minerals of hornblende and biotite they display zircon e hf t 2 values 4 6 to 7 5 similar to the porphyritic granodiorites and have higher cao contents molar cao mgo feo tot ratios and yb y contents with relatively lower la yb n 9 34 and sr y 29 43 ratios for most samples they show weakly negative eu and sr anomalies for most samples with significant nb ta and ti depletion indicating that they might be produced by partial melting of calc alkaline volcanic sources with higher cao contents at relatively higher levels of the lower crust compared with the porphyritic granodiorites the medium grained monzogranites show the highest sio 2 contents 69 71 77 64 wt strongly to moderately fractionated ree patterns la yb n 25 69 for most samples and weakly positive to significantly negative eu and sr anomalies with zircon e hf t 2 values of 2 4 to 4 7 indicating that they may be produced by partial melting of metagreywackes at middle to lower crustal levels the porphyritic quartz syenitic gneisses exhibit the highest k 2 o concentrations of 5 11 7 05 wt and are attributed to the shoshonitic series and they are featured by high la yb ratios 34 45 and positive eu and sr anomalies with zero straddling zircon e hf t 2 values 2 4 to 4 1 we consider that they might be derived from partial melting of a mixed source of juvenile mafic rocks and ancient continental materials in the deep lower crust integrating the available data from the previous studies we conclude that a late neoarchean active continental margin setting can account for the generation of these granodiorite monzogranite quartz syenite intrusions

heterogeneous metamictization of zircon on a microscale the ability of modern raman microprobes to determine the crystallinity of zircon with a lateral resolution of about 1 m and a volume resolution of less than 5 m3 opens up new opportunities for correlating structural chemical and isotopic data on the same micro area within a zircon grain combining this information we confirm by direct measurement the inhomogeneity in the degree of crystallinity of complex zircon grains and the correlation between u th concentration isotopic discordance and degree of metamictization in micro areas of these grains micro areas with a higher degree of metamictization are more chemically heterogeneous than neighbouring more crystalline areas and contain higher concentrations of ca fe u th and y but not always hf which in some cases shows higher concentration in the less metamict areas raman observations that there are no radiation damage halos extending over distances greater than 1 1 5 m preclude alpha and beta radiation as well as fission recoil as viable causes for amorphization and confirm that alpha recoil is the most important mechanism causing zircons to disorder into the metamict state

alpha recoil in u pb geochronology effective sample size matters displacement of the daughter isotope by recoil results in an open system on the nanoscale for a heterogeneous distribution of u and th this redistribution of intermediate and stable daughter isotopes results in subvolumes with a deficit of pb and others with an excess of pb whether such heterogeneities affect the analyzed u pb system depends on 1 the volume of the analyzed sample 2 the degree and scale of heterogeneity in the u and th distribution and 3 the analytical procedure spatial separation of parent and daughter through recoil affects the u pb systematics of leached samples where leaching gives access to domains less than 1 m wide anomalous data patterns originating from recoil induced parent to daughter fractionation are more important if there are strong heterogeneities in the u and th distribution whereby pb excess appears more pronounced than pb deficit fractionation of parent and daughter elements through selective dissolution of u ree rich growth zones in zircon and u inclusions in columbite as well as the presence of u th rich micro inclusions in silicates dated using a step leaching scheme may result in anomalous 207pbrad 206pbrad scattered 206pbrad 238u and 207pbrad 235u and reverse discordance the accumulated structural damage controls the leaching and dissolution behavior but may also influence the non stoichiometric element mobilization during sputtering or ablation in the analysis of u rich samples by shrimp and la mc icp ms

laramide uplift near the ray and resolution porphyry copper deposits southeastern arizona insights into regional shortening style magnitude of uplift and implications for exploration

leaching and differential recrystallization of metamict zircon under experimental hydrothermal conditions we report results of hydrothermal experiments on four alluvial zircons from sri lanka which cover a wide range of radiation damage at 450 c and 1 3 kbar for 744 h with 2 m cacl2 solution as reactive fluid after the hydrothermal treatment the most metamict samples show micrometer thick reaction rims which surround apparently unreacted zircon as revealed by cathodoluminescence cl and nomarski differential interference contrast ndic images these rims have sharp curved and transgressive boundaries with unreacted zircon and are in some cases spread out along cracks the thickness of reaction rims increases with increasing cumulated dosage of the starting materials the reaction rims are strongly enriched in ca up to 7000 ppm and a water species and depleted in radiogenic pb zr and si as revealed by electron microprobe analyses a significant th loss from the reaction rims was detected in the case of the most metamict sample whereas u remained in the structure ft infrared spectrometry and x ray diffraction measurements revealed that the bulk run products were recrystallized using micro raman spectrometry we were able to demonstrate that differential recrystallization took place the reaction rims are strongly recrystallized whereas the unreacted grain interiors underwent only minor recrystallization recrystallization of the rims is accompanied by an enhancement of the integral cl intensity it is suggested that recrystallization in the reaction rims was catalyzed by water infiltration and ion exchange and prevented significant congruent zircon dissolution under the given experimental conditions previous zircon studies have shown that 1 a transgressive morphology 2 a reduced th u ratio and 3 an enhanced cl emission are also characteristics of rims in zircons from high grade metamorphic rocks based on these similarities between natural and experimentally produced rims it is suggested that leaching catalyzed recrystallization is an important alteration process in zircon under wet geological conditions and can account for many complex core rim structures found in natural zircons furthermore the strong enrichment of ca in the reaction rims supports previous assumptions that high ca concentrations in natural zircons are of secondary origin it is suggested that lower u pb concordia intercept ages obtained from single phase zircons with high ca contents date a leaching event

exotic crustal components at the northern margin of the bohemian massif implications from uthpb and hf isotopes of zircon from the saxonian granulite massif abstract the saxonian granulite massif is located at the northern margin of the saxo thuringian zone of the peri gondwana bohemian massif eight felsic and mafic granulites were studied with respect to their geochemistry and u pb zircon geochronology the felsic granulites are interpreted to be derived from continental crust of possible granitoid composition an origin from depleted mantle sources with iat to morb composition can be assumed for the mafic granulites the peak of metamorphism is thought to be timed at about 340 ma while several earlier metamorphic events are supposed to have occurred at about 355 360 370 375 405 and 450 ma they reveal a complex and polyphased geologic evolution of the saxonian granulite massif protolith emplacement likely took place at c 450 and 494 ma hf isotopic data suggest mesoproterozoic crustal ages at least for parts of the massif as these crustal ages are exotic for the bohemian massif their origin has to be searched elsewhere potential source areas could be amazonia and baltica of which the latter is the one preferred furthermore a composite architecture with at least two components the felsic granulites with mesoproterozoic crustal model ages and the mafic granulites of potential island arc origin is hypothesised their amalgamation to the recent appearance of the saxonian granulite massif is likely bracketed between 375 and 340 ma

prestack waveform inversion of three dimensional seismic data an example from the rock springs uplift wyoming usa abstractapplying seismic inversion to estimate subsurface elastic earth properties for reservoir characterization is a challenge in exploration seismology in recent years waveform based seismic inversions have gained popularity but due to high computational costs their applications are limited and amplitude variation with offset angle inversion is still the current state of the art we have developed a genetic algorithm based prestack seismic waveform inversion methodology by parallelizing at multiple levels and assuming a locally 1d structure such that forward computation of wave equation synthetics is computationally efficient this method is capable of inverting 3d prestack seismic data on parallel computers applying this inversion to a real prestack seismic data volume from the rock springs uplift rsu located in wyoming usa we determined that our method is capable of inverting the data in a reasonable runtime and producing much higher quality results than amplitude variation with offset ang

on the dependence of sea surface roughness on wave development abstract the aerodynamic roughness of the sea surface z0 is investigated using data from lake ontario from the north sea near the dutch coast and from an exposed site in the atlantic ocean off the coast of nova scotia scaling z0 by rms wave height gives consistent results for all three datasets except where wave heights in the atlantic ocean are dominated by swell the normalized roughness depends strongly on wave age younger waves traveling slower than the wind are rougher than mature waves alternatively the roughness may be normalized using the friction velocity u of the wind stress again young waves are rougher than mature waves this contradicts some recent deductions in the literature but the contradiction arises from attempts to describe z0 in laboratory tanks and in the field with a single simple parameterization here it is demonstrated that laboratory waves are inappropriate for direct comparison with field data being much smoother than their field equivalents in the open ocea

internal structures and dating of complex zircons from meissen massif monzonites saxony abstract u pb zircon ages were determined for accessory zircons from two monzonites from the meissen massif northern bohemian massif germany by shrimp ion microprobe bse and cl imaging revealed that the monzonite zircons showing oscillatory growth zoning often contain inherited cores raman microprobe measurements showed that zoned rims u 277 1426 ppm th 144 910 ppm are slightly disordered to moderately metamict whereas cores have a wide spectrum of structural states from well crystallized to highly metamict the radionuclide u th content of cores varies from a few hundred to more than 20 000 ppm shrimp ion probe measurements on zoned rims interpreted as zircon growth during crystallization of the monzonite magma gave concordant carboniferous 206 pb 238 u ages of 326 6 ma monzonite from leuben and 330 5 ma monzonite from heidenschanze 95 confidence respectively these ages are consistent with recently published 40 ar 39 ar ages of 329 1 2 8 to 330 4 2 8 ma 2 on amphiboles from the same rocks in contrast shrimp measurements on cores range from slightly discordant proterozoic relic ages in low u and th cores to highly discordant data in the most metamict cores previously obtained 207 pb 206 pb ages single zircon evaporation technique in the range 340 16 to 357 16 ma 1 are probably mixed ages between magmatic and inherited zircon components the u pb ages of the monzonite zircons postdate the upper devonian lower carboniferous subduction and collision events in the area by about 20 ma due to their shoshonitic chemical signature which is typical of rocks from destructive plate boundary settings the meissen massif monzonites might be interpreted as originating during these 350 ma events and the earlier 207 pb 206 pb ages appeared to confirm this interpretation however both the 40 ar 39 ar ages and our current results show that only the enrichment of the lithospheric mantle source but not the monzonite intrusion itself was directly related to these events the monzonite intrusion itself took place 330 ma ago in the framework of strike slip tectonics in the elbe valley zone widely occurring zircon cores are indicative of crustal contamination of the magma causing considerable modification of chemical signatures trace element content of the monzonites

on the valency state of radiogenic lead in zircon and its consequences in zircon u pb systematics extreme robustness up to the temperatures of granulite facies and anatexis contrasts with apparently easy loss of radiogenic pb at low temperatures often without any metamorphic event being in evidence here we propose that this paradoxical behaviour can be understood with the hypothesis that radiogenic pb in zircon is tetravalent we review data and arguments in favour of this hypothesis diffusion profiles calculated for pb sup 2 in a 25 micro m radius zircon xenocryst in a melt at 1000 c combined with the incompatibility of pb sup 2 or for a zircon core inside a younger zircon rim at this temperature show age effects that should have been observed in sims dating further in zircon evaporation as well as in leaching experiments common pb is generally released preferentially to radiogenic pb after removal of less radiogenic pb the evaporation record generally shows pure radiogenic pb during the final evaporation steps the distribution of residual pb in a leached titanite revealed by pixe is similar to that of ti lastly xanes spectra of a 1 ga old titanite predominantly radiogenic pb and an alpine one predominantly common pb are significantly different although the former does not resemble that ofmore pbo sub 2 the arguments why radiogenic pb should be tetravalent are based on analogies with studies relating to the tetravalent state of sup 234 th and the hexavalent state of sup 234 u which show that alpha recoil in silicates generates a strongly oxidizing environment at the site where the recoiling nucleus comes to rest further a zircon grain being small should remain highly oxidizing in its interior by the constant loss of beta particles maintaining the 4 state of radiogenic pb from its effective ion radius similar to that of zr sup 4 and its charge pb sup 4 has to be compatible in the zircon lattice also by analogy with u sup 4 th sup 4 and hf sup 4 its diffusivity should be several orders of magnitude lower than that measured for pb sup 2 in zircon these factors can account for the extreme retentivity of u pb clocks even at high temperatures on the other hand radiogenic pb situated in alpha recoil damaged sites could be leached out by any electrolyte solution that reduces it to the divalent state making it both incompatible and soluble thus discordia can be generated in weathering the curious observation that discordant archaean zircon suites generally define trends to lower intercepts at up to 800 ma instead of zero appears to reflect a c 14 to 28 better average accessibility to leaching of radiogenic sup 207 pb compared to sup 206 pb in accord with the somewhat higher recoil energy of the last branched alpha decays in the sup 235 u chain less

accretionary tectonics of back arc oceanic basins in the south tianshan insights from structural geochronological and geochemical studies of the wuwamen ophiolite melange

mesozoic large scale magmatism and mineralization in south china preface abstract south china is famous for its polymetallic deposits with more than 50 of the world s w and sb reserves and 20 of the sn reserves as well as abundant cu mo pb zn au ag ta nb hree and u this special issue reports recent progress on the tectonic evolution magmatism and ore deposits of south china which leads to a more comprehensive understanding of the mesozoic large scale magmatism and mineralization events 1 south china has been located in backarc settings during multiple subductions since the paleozoic which may have contributed to pre enrichments of the ore forming materials 2 the pacific plate subduction was a principle factor that controlled the late mesozoic large scale magmatism and mineralization in south china through flat subduction ridge subduction slab tearing and slab rollbacks

highly differentiated fluorine rich alkaline granitic magma linked to rare metal mineralization a case study from the boziguo er rare metal granitic pluton in south tianshan terrane xinjiang nw china abstract rare metal bearing alkaline granites have attracted extensive interests because of their economic significance the early permian 290 ma boziguo er alkaline granitic pluton exposed in the middle segment of the south tianshan terrane xinjiang nw china contain nb ta zr th u and ree bearing accessory minerals and host a large scale rare metal deposit the unalterated alkaline granites are mainly composed of alkali feldspars 40 vol orthoclase microcline and 30 vol albite quartz 20 vol arfvedsonite 5 vol and biotite 5 vol with accessory minerals at least including zircon monazite pyrochlore fe ti oxides xenotime bastnasite and astrophyllite the existence of two types of alkaline feldspars which respectively show nearly pure ab and or end member compositions suggest that the boziguo er pluton can be classified as subsolvus granites rare metal elements in the pluton are mainly contained by pyrochlore nb ta zircon zr hf th u and hree and monazite lrees grains mineral chemistry suggests that the pyrochlore and monazite are of primary igneous mineral based on morphology internal texture and mineral chemistry four types of zircons have been identified type i zircon may have crystallized from volatile undersaturated early stage magmas while type iia zircon crystallized from volatile fluorine saturated magmas the type iib and type iii zircons could have grown in an lree depleted silicate melt after the crystallization of the lree rich minerals e g monazite or an early aqueous fluid during the transition from a magmatic to hydrothermal system the type iv zircons can be regarded as low temperature hydrothermal ones a semiquantitative calculation based on mineral compositions of biotites yields 0 51 2 14 wt for f contents in granitic melts the high contents of fluorine in the hydrous minerals suggest a fluorine enriched characteristic for melt which would not only prolong the duration of the magmatic differentiation but can also enhance the solubility of ore forming elements in magma the magmatic differentiation was mainly dominated by fractional crystallization of alkali feldspar and amphibole extensive rare metal mineralization in the pluton likely occurred within a limited temperature interval and was related to the crystallization of a highly evolved intergranular melt at the latest magmatic stage

hydrous species in zircon a survey of hydrous species in zircon from a number of localities with a range of occurrences and structural states was carried out using infrared spectroscopy and electron microprobe analysis stepwise heating experiments on a crystalline and a nearly metamict zircon were also performed the region of ir spectra of oh bearing crystalline zircon from 4000 to 2500 cm 1 exhibits anisotropic oh bands resulting from at least three oh sites one at vacant tetrahedra and two that may be at si occupied tetrahedra one site associated with si occupied tetrahedra is the most important producing ir absorption bands at 3420cm 1 e c and 3385cm 1 e c oh maybe introduced during crystal growth by a coupled substitution of the form m 3 h 4 zr 4 the sites at vacant tetrahedra account for considerably less oh producing relatively weak ir absorption bands at 3510 cm 1 e c e c oh may be introduced during crystal growth by the hydrogrossular substitution of the form 4 oh sio 4 4 an additional oh band at 3270 cm 1 appears to be associated with an occupied tetrahedron heating experiments produced two more oh environments and indicate that h can migrate among the sites at high temperatures some crystalline samples are entirely free of oh ir spectra of metamict zircon exhibit broad asymmetric isotropic oh bands similar to h 2o bands but the absence of the h 2o bending mode band indicates that oh is the dominant hydrous species present ir spectra of partially metamict zircon samples retain some anisotropy the breadth of their oh bands is a function of degree of metamictizatron the maximum oh observed in any metamict or partially metamict zircon sample was approximately 0 1wt expressed as h 2o sucho h is probably secondary the maximum primary oh observed is 0 05 wt partially metamict zircon from a hydrothermal vug crystalline crustal zircon contains more than an order of magnitude less oh the maximum oh content observed in crystalline zircon 0 01 wt i s in a mantle zircon sample from a kimberlite suggesting high activity of h 2o in the mantle additional oh is present in some metamict and partially metamict zircon samples in the form of inclusions of hydrous silicates such as kaolinite fluid inclusions containing h 2o occur in some samples this may be a partial explanation of the high h 2o contents reported for some radiation damaged zircon based on loss on heating measurements

pb u and th diffusion in natural zircon zircon zrsio4 is one of the most widely used minerals for determining the age origin and thermal history of rocks by u th pb geochronology but the parameters describing the solid state volume diffusion rates of these elements in natural zircon which are themselves important for establishing the limits on the applicability of zircon for geochronological studies have remained poorly quantified1 this is because of the measurement difficulties associated with the low p p m concentrations and low diffusion rates of these elements in natural zircon and the chemical and physical heterogeneity present in most crystals here we present direct measurements of the uranium thorium and lead loss from a thermally treated gem quality natural zircon and show that lead diffuses much faster than uranium or thorium we find that the u th pb isotopic system in natural zircon will typically have a closure temperature greater than 900 c which explains why zircon is apparently such a robust geochrometer and is capable of remaining isotopically closed through extended periods of high grade metamorphism and partial melting of the host rock

zircon reaction and stability of the u pb isotope system during interaction with carbonate fluid experimental hydrothermal study the interpretation of metamorphically induced u pb isotopic discordance requires a thorough understanding of zircon fluid interactions with this aim we have studied the behaviour of metamict and crystalline zircon phases and their u pb systems by cathodoluminescence after treatment by 2m na2co3 solution at t 200 800 c and p 1 5 kbar for 3 14 days x ray diffraction microprobe and isotope dilution analysis the data indicate that zircon transformation under hydrothermal conditions depends on the experimental conditions and the degree of structural damage reconstitution of defective and impurity enriched zones of metamict zircon homogenization of impure element concentrations and increase of crystallinity was observed at 400 c and p 1 kbar considerable lead and uranium loss occurred under these conditions as a result of zircon dissolution newly formed baddeleyite accommodating u from 2m na2co3 solution and zr na silicate were recognized this process intensified with increasing pressure study of crystalline zircon indicates that migration of u and pb took place only during dissolution of zircon at t above 650 c in the presence of carbonate ions essential u and pb amounts are lost from metamict zircon at a lower p t than is typical for greenschist facies metamorphism

structure of the bighorn mountain region wyoming from teleseismic receiver function analysis implications for the kinematics of laramide shortening basement cored uplifts are observed globally and remain an enigmatic feature of plate tectonics due to the fact that in many cases they occur distant from a plate boundary the laramide bighorn arch in wyoming is an archetypal basement involved foreland arch and provides an excellent setting for the investigation of such structures previous studies proposed diverse arch formation models each of which predicts a unique crustal geometry we use high resolution crustal imaging from teleseismic p wave receiver functions to test these models we obtained our data from 239 three component seismometers deployed as part of the bighorns arch seismic experiment as well as coeval regional transportable array stations a sequential two layer thickness vp vs h stacking algorithm constrains sediment and crustal structure receiver function common conversion point stacking results in 2 d transect images across the arch our results define an upwarp of the crust beneath the central and northern arch that extends into the powder river basin north northeast of the arch the lack of moho cutting faults or a moho geometry mirroring the arch rules out most shortening models except a crustal detachment model where shortening was accomplished by fault propagation folding on a thrust splay ramping off a midcrustal detachment fault the mismatch between gentle symmetric moho and asymmetric laramide arch geometries and their trends suggests a pre laramide origin for at least a part of the moho high this high perhaps in combination with a lesser degree of laramide lithospheric buckling may have caused emergent laramide thrusting and thus nucleated the bighorn arch our results suggest that midcrustal detachment can form basement involved foreland arches and suggest the hypothesis that preexisting undulations in the moho may have nucleated individual arches

tectonic evolution of a laramide transverse structural zone sweetwater arch south central wyoming structural anisotropy of magnetic susceptibility ams and paleomagnetic data record patterns of layer parallel shortening lps vertical axis rotation and regional fault fold evolution across the sweetwater arch a major west to wnw trending basement cored laramide uplift in wyoming the southern arch flank is bounded by a wnw striking reverse fault zone that imbricated basement and cover rocks the northern flank is bounded by a west striking fault zone with a component of strike slip and nw trending en echelon folds and the eastern plunge transitions into an area of multiple trending faults and folds synorogenic strata record major arch uplift from maastrichtian to early eocene time followed by arch collapse lps with development of systematic minor fault sets and ams lineations preceded large scale folding lps directions estimated from both minor fault and ams data were oriented wsw along the northern flank subparallel to laramide regional shortening but were refracted to the ssw along the southern flank and to the west along the eastern arch plunge additional minor faults developed along steep fold limbs during continued shortening with directions remaining ssw along the southern flank but becoming more variable along the eastern plunge where an increasingly heterogeneous stress field developed as additional faults were activated along basement heterogeneities vertical axis rotation was limited along the arch flanks whereas the eastern plunge underwent counterclockwise rotation deflections in shortening directions were partly related to basement heterogeneities including weak supracrustal belts on the arch flanks a strong granitic core and local reactivation of precambrian shear zones

transport of uranium thorium and lead in metamict zircon under low temperature hydrothermal conditions abstract understanding the stability of radiation damaged zircon under low temperature hydrothermal conditions is crucial to the application of zircon for u pb geochronology and as a host phase for the disposal of plutonium waste we report the results of an investigation of the stability of partially metamict zircon by leaching experiments at 175 c with a 2 m alcl 3 and a 1 m hcl cacl 2 solution as hydrothermal fluids for 1340 h cathodoluminescence cl and backscattered electron bse images show that the zircon grains have developed a reaction rim several micrometers thick or deeply penetrating reticulated alteration zones with sharp boundaries to unaltered metamict zircon these zones have experienced severe loss of si u th and pb and gain of al or ca and a water species as revealed by electron microprobe sensitive high resolution ion microprobe shrimp analyses and infrared spectroscopy micro raman and infrared measurements on the altered areas show that disordered crystalline remnants of the partially metamict zircon structure were partially recovered whereas recrystallization of the embedding amorphous phase was not observed no detectable structural or chemical changes were detected inside the unaltered areas intensive fracturing which was most intense in the hcl cacl 2 experiment occurred inside the altered areas due to the volume reduction associated with the recovery of the disordered crystalline material and probably with the leaching reactions we explain the formation of deep penetrating alteration zones by a percolation type diffusion model which is based on the assumption that percolating interfaces or areas of low atomic density between crystalline and amorphous regions as well as between amorphous domains exist along which fast chemical transport is possible the idea of the existence of fast diffusion interfaces is supported by the sharp chemical gradients at the margin to unreacted zircon the model was used to estimate for the first time diffusion coefficients for u th and pb diffusion in amorphous zircon at 175 c by assuming that volume diffusion inside the amorphous domains is the loss rate limiting process these estimates show that volume diffusion in metamict zircon can cause significant loss of u th and especially loss of radiogenic pb over geological time scales even at temperatures as low as 175 c our results show that recent pb loss discordias can be generated 1 by predominate pb loss from metamict zircon through volume diffusion at low temperatures where thermal healing of the structure is insignificant and 2 by leaching of pb and u and th from metamict zircon through an external fluid

experimental hydrothermal alteration of partially metamict zircon we present the results of a series of hydrothermal experiments on grains from two partially metamict zircon samples from sri lanka in the temperature range 350 to 650 c and with different solutions 2 m alcl 3 2 m cacl 2 pure h 2 o and a multi cation solution under these conditions sharply bounded reaction fronts penetrated into the zircon grains and developed complex lobate and rim structures that resemble structures found in natural zircon systems the reaction zones are characterized by a marked increase in the cathodoluminescence intensity a decrease of the back scattered electron emission and an increased degree of structural order as revealed by micro raman and infrared spectroscopy sensitive high resolution ion microprobe and electron microprobe measurements revealed that the altered areas gained solvent cations e g ca 2 ba 2 mg 2 al 3 from the solution and lost variable amounts of zr si hf the ree u th as well as radiogenic pb a comparison between dry and hydrothermal annealing trends shows that the kinetics of structural recovery including recrystallization of the amorphous phase in metamict zircon is strongly enhanced under hydrothermal conditions this finding suggests that water catalyzes structural recovery processes in metamict zircon we found that the structure of the reacted areas does not resemble that of crystalline zircon i e is still characterized by a temperature dependent degree of disorder which would not be expected if the reaction is controlled by a coupled dissolution and re precipitation process instead the alteration process can be described best by a diffusion reaction recrystallization model in this model it is postulated that the diffusion of water into the metamict structure is the driving force for moving recrystallization fronts we found that the rate and the extent of solid state recrystallization of the amorphous phase is an important factor in determining the mobility of trace elements this interpretation is indicated by the observation that trace elements including u and th were preferentially lost during the reaction with a fluid at low temperatures where recrystallization of the amorphous material was slow or not activated at all the observed chemical alteration patterns are believed to reflect a competition between the kinetics of long range diffusion and ion exchange and the kinetics of the short range diffusion necessary for the recrystallization process

the effect of chemical and physical imperfections in zircon grains in influencing the u pb age analyses insights from zircon fission track etching abstract understanding the chemical etching effects in the crystalline structure of zircon grains is crucial to know their chemical composition morphology and isotopic geochemistry after chemical etching the zircon grain can be classified as homogeneous heterogeneous hybrid and anomalous depending on the surface fission track density observed under an optical microscope the complementary techniques such as optical microscopy micro raman spectroscopy scanning electron microscopy imaging by cathodoluminescence and secondary electrons and electron microprobe were employed to characterize the zircon grain surfaces from igneous metamorphic and sedimentary rocks before and after chemical etching except for raman spectroscopy measurements of u pb age are concordant in areas on the grain surface with uniform fission track density the chemical and isotopic analyses show that the etching is capable of revealing the different layer imperfections of the zircon crystalline structure which can be inherent in its crystallization or acquired during its geological history therefore identification of crystallographic preserved areas on the zircon surface which are associated with uniform fission track density helps to get highly concordant u pb ages in general the results showed that the u pb system depends significantly on the amorphization degree of the zircon crystalline structure which in most cases can only be revealed by etching

reverse age zonation of zircon formed by metamictisation and hydrothermal fluid leaching abstract understanding the stability of radiation damaged zircon is crucial to the application of zircon to u pb geochronology especially for zircons rich in radioactive nuclides electron microprobe and laser raman spectral analyses show that zircons from the ejinao sodalite syenite in southeast china are composed of cores without zonation and oscillatory rims the cores are rich in uranium and have experienced increased metamictisation an internal sieved structure and radial cracks were formed by radiation damage and radiation induced volume expansion infrared spectral analyses show that zircon from the ejinao sodalite syenite contains large amounts of oh and h 2 o this finding suggests that the zircon crystals experienced hydrothermal alteration the pb loss of zircons is ascribed to their interaction with a hydrothermal fluid derived from the ejinao syenite radiogenic pb 4 situated in recoil damaged sites has been significantly reduced to pb 2 by the hydrothermal fluid becoming incompatible in the crystal lattice and highly mobile in solution consequently it has been leached out from the zircon this pb loss leads to diverse 206 pb 238 u ages from 134 ma to 147 ma and severe pb loss from the inner core even produces reverse age zonations with cores approximately 5 ma younger than the rims the real crystallisation age for zircons from the ejinao sodalite syenite should be approximately 145 ma considering the entire range of analytical data obtained by both the shrimp and la icpms methods

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

the basement revealed tectonic insight from a digital elevation model of the great unconformity usa cratonic platform across much of north america the contact between precambrian basement and paleozoic strata is the great unconformity a surface that represents a 0 4 b y long hiatus a digital elevation model dem of this surface visually highlights regional scale variability in the character of basement topography across the united states cratonic platform specifically it delineates phanerozoic tectonic domains each characterized by a distinct structural wavelength horizontal distance between adjacent highs and or structural amplitude vertical distance between adjacent lows and highs the largest domain the midcontinent domain includes long wavelength epeirogenic basins and domes as well as fault controlled steps the pronounced change in land surface elevation at the rocky mountain front coincides with the western edge of the midcontinent domain on the basement dem in the rocky mountain and colorado plateau domains west of the rocky mountain front structural wavelength is significantly shorter and structural amplitude significantly higher than in the midcontinent domain the bordering basins domain outlines the southern and eastern edges of the midcontinent domain as emphasized by the basement dem several kilometers of structural relief occur across the boundary between these two domains even though this boundary does not stand out on ground surface topography a plot of epicenters on the basement dem supports models associating intraplate seismicity with the midcontinent domain edge notably certain changes in crustal thickness also coincide with distinct changes in basement depth

the influence of recent lead loss on the interpretation of disturbed upb systems in zircons from igneous rocks in east greenland abstract a series of caledonian igneous rocks from east greenland have been dated by means of u pb isotope determinations on zircons all discordias obtained give too low intercept ages in one case more than 200 ma younger than the rb sr mineral isochron age from the same sample leach experiments show that recent or subrecent loss of lead is responsible for the geologically meaningless ages obtained further it is shown that the recent lead loss is not caused by laboratory treatment but must be related to natural processes x ray investigations combined with transmission electron microscope analyses show a correlation between recent lead loss and the chemistry of the investigated zircons

a review of deformation pattern templates in foreland basin systems and fold and thrust belts implications for the state of stress in the frontal regions of thrust wedges aesthetically appealing thrust systems and related large scale anticlines in both active and fossil foreland fold and thrust belts and the economic potential associated with the m have captured the interest of structural geologists for many decades as a consequence a large amount of data on sub seismic deformation patterns from thrust related an ticlines is available in the literature we provide a review of deformation pattern templates from fi eld data in foreland fold and thrust belts and show that the most frequent tre nds of sub seismic syn orogenic deformation structures hosted in km scale thrust related folds frequently and pa radoxically indicate a syn th rusting strike slip stress fi eld con fi guration with a near vertical 2 and a sub horizontal 3 rather than a contractional one where the latter is expected to be the vertical principal axis of the stress ellipsoid this a pparent inconsistency between sub seismic syn orogenic deformation structures and stress fi eld orientation is here named the 2 paradox field data support a possible expla nation of the paradox provided by the major role played by i nherited early orogenic extensional deformation struc tures on thrust fault nucleation nucleation of major thru sts and their propagation is facilitated and driven by the positive inversion and linkage of the early orogenic sub s eismic extensional inheritance developed in the foreland basin this process eventually leads to the development of l argereversefaultzonesandcanoccurbothincontractional and strike slip stress fi eld con fi gurations

structural and microstructural evolution of the rattlesnake mountain anticline wyoming usa new insights into the sevier and laramide orogenic stress build up in the bighorn basin the rocky mountains in western us provide among the best examples of thick skinned tectonics following a period of thin skinned tectonics related to the sevier orogeny the compressional reactivation of basement faults gave birth to the so called laramide uplifts arches the bighorn basin located in wyoming is therefore a key place to study the transition from thin to thick skinned tectonics in orogenic forelands especially in terms of microstructural and stress strain evolution our study focuses on a classic laramide structure the rattlesnake mountain anticline rma wyoming usa a basement cored anticline located in the western part of the bighorn basin stress and strain evolution analysis in folded sedimentary layers and underlying faulted basement rocks were performed on the basis of combined analyses of fractures fault slip data and calcite twinning paleopiezometry most of the fractures are related to three main tectonic events the sevier thin skinned contraction the laramide thick skinned contraction and the basin and range extension serial balanced cross sections of rma and displacement profiles suggest that all thrust faults were coeval evidencing strain distribution in the basement during faulting the comparison of rma with another structure located in the eastern edge of the bighorn basin i e the sheep mountain anticline sma allows to propose a conceptual model for the geometric and kinematic evolution of laramide related basement cored anticlines finally the stress evolution is reconstructed at both the fold scale and the basin scale we show that the evolution of stress trends and magnitudes was quite similar in both structures rma and sma during laramide times thick skinned tectonics in spite of different stress regimes during sevier thin skinned tectonics and post laramide times stress trends and fracture patterns were different in these two structures these results suggest that the distance to the orogenic front influenced the fracture patterns but not the foreland stress magnitudes which were likely controlled by the structural style

relict proterozoic basement in the nanling mountains se china and its tectonothermal overprinting 1 the basement beneath the caledonian belt in se china has been overprinted by indosinian and yanshanian events and is poorly known in situ u pb and hf isotope analysis of zircons from granites and surrounding rocks in the nanling mountains area of the fold belt reveal that the lanhe felsic gneiss is an outcrop of mesoproterozoic basement probably with a partly late archean protolith this is consistent with nd model ages of granites in this region and exposures of cathaysia basement in the coastal area of se china in the nanling mountains this basement was reactivated to generate granitoid rocks during the caledonian indosinian and yanshanian tectonothermal events the caledonian overprint is documented in extensive exposures of the zhuguang migmatites 425 470 ma and by previous u pb zircon dating of adjacent granites the jiangnan granite may be of similar age but strongly reworked the indosinian overprint resulted in the formation of granites such as the luxi and xiazhuang bodies 233 246 ma in the guidong complex the yanshanian overprint produced the most widely distributed granites circa 160 ma such as the jiufeng and fogang bodies and parts of the guidong complex hf isotope data show that all of the phanerozoic magmatic rocks were derived from the mesoproterozoic basement and minor archean components without obvious contributions from juvenile sources the presence of zircons with archean age 2 5 2 7 ga in both the lanhe gneiss and devonian sediments indicates that the badu and mayuan groups may not be the oldest basement of the cathaysia block some archean basement circa 2 5 2 7 ga may still be exposed but unrecognized in this region and cathaysia may include archean microcontinental fragments

geomorphic expression of the southern central andes forebulge 37 s argentina abstract we present a geomorphologic analysis of an east west transect located east of the southern andes of argentina 37 s we observe a succession of zones that underwent erosion and deposition during the pleistocene if the proximal andean foothills are incised a proximal depozone receives sediments feeding the megafan of the rio colorado on the chadileuvu plain more distally the abandoned palaeo valleys and bending of the valley floors reflect a localized uplift further to the east another depozone corresponds to the pampa deprimida lowland this pattern is consistent with the presence of a classical flexural geometry of the lithosphere the distal uplift of the foreland corresponds in terms of location length 150 km and amplitude 240 m to the andean forebulge modelled by a geophysical approach in this study we identify the morphological imprint of this bulge and show its effect on the fluvial activity

azimuthal anisotropy analysis of p wave seismic data and estimation of the orientation of the in situ stress fields an example from the rock springs uplift wyoming usa abstractestimating the orientation and magnitude of maximum and minimum horizontal in situ stress is important for characterizing naturally fractured unconventional and carbon sequestered reservoirs for naturally fractured reservoirs they are needed to guide directional drilling for unconventional reservoirs they are used for optimal placements of hydraulic fractures and for carbon sequestered reservoirs they are used to avoid fracturing of overlying seal rocks in addition a knowledge of stress fields can be used to induce fractures within the target reservoirs and enhance additional storage for carbon sequestration experiments the orientation and magnitude of in situ stress can be calculated at the well locations for locations away from the wells analysis of the azimuthal dependence of the amplitude variation with angle gradient or azimuthal angle stacks are used to quantify anisotropy which are then related with well data and other geologic information for stress estimation such azimutha

lead diffusion in apatite and zircon using ion implantation and rutherford backscattering techniques abstract ion implantation was used to introduce pb into the minerals zircon and apatite diffusion profiles were obtained using rutherford backscattering and the results were fitted with a model to determine the diffusion coefficients this approach is both simple and useful in studying diffusion over a temperature range of geologic interest without inordinate annealing times results for apatite over the temperature range 600 900 c are in good agreement with earlier results obtained for higher temperatures watson et al 1985 and are described by the following arrhenius law d 1 27 10 4 cm 2 s 1 exp 54 6 1 7 kcl mol 1 rt this suggests that radiation damage induced by ion implantation has little effect on pb diffusion in this case due to rapid annealing of induced damage at low temperatures the activation energy stated above is somewhat smaller than that previously determined 70 kcal mol 1 reflecting the larger range in 1 t sampled by the present results closure temperatures calculated with these diffusion parameters are in good agreement with those inferred from geochronologic data diffusion coefficients for pb in zircon determined in this study are greater than those obtained from a single experimental measurement and estimates of pb diffusion based on geochronologic data and u pb zoning in natural zircons the differing results for zircon and apatite obtained in this study appear to be related to each mineral s ability to anneal the radiation damage induced by ion implantation

ancient geologic events on mars revealed by zircons and apatites from the martian regolith breccia nwa 7034

effects of wind trend and gustiness on the sea drag lake george study 1 the air sea coupling is usually parameterized in terms of the drag coefficient cd but the scatter of experimental data around such dependences is very significant and has not improved noticeably over some 30 years in the paper which is meant to be the first in a series a complex approach to the problem is suggested multiple mechanisms contributing into the sea drag are to be singled out studied separately evaluated and then reunited in a joint parameterization for cd dependences of the drag coefficient on the wind speed and sea state and effects of wind trends and gustiness are investigated in detail gustiness is found to be responsible for the most distant outliers our approach also combines an experimental lake george study with theoretical investigations conducted by means of the wowc wind over waves coupling model overall agreement of the model with measured wind stresses is quite good within 20 for the bulk of the data lower envelopes of the drag dependences are an important result of the paper they provide some lake george ideal relationships for the sea drag almost any deviation from such conditions causes the drag to increase it is suggested that decrease of the drag with respect to the ideal conditions which exhibits itself in a number of known open ocean data sets would be caused by a momentum flux back from the waves to the wind due to long waves outrunning the wind behavior of the charnock parameter in terms of wave age is also considered

u pb zircon geochemical and sr nd hf isotopic constraints on age and origin of jurassic i and a type granites from central guangdong se china a major igneous event in response to foundering of a subducted flat slab abstract the mesozoic geology of se china is characterized by widespread jurassic to cretaceous igneous rocks consisting predominantly of granites and rhyolites and subordinate mafic lithologies however the tectonic regime responsible for the inland jurassic granites remains controversial we report here u pb zircon ages geochemical and sr nd hf isotopic data for the nankunshan alkaline granite and the fogang granitic batholith in central guangdong mineralogical and geochemical features suggest that the fogang and nankunshan rocks are i and aluminous a type granites respectively shrimp u pb zircon analyses yield consistent ages ranging from 159 3 ma to 165 2 ma for four samples from the fogang batholith and an age of 158 5 ma for the nankunshan granite the fogang granites having i sr 0 7098 0 7136 e nd t 4 3 to 12 2 and e hf t 11 5 to 3 1 for the magmatic zircons were derived from paleoproterozoic mafic intermediate igneous protolith with minor addition of mantle derived magmas the nankunshan rocks have relatively lower i sr 0 706 0 708 higher e nd t 0 3 to 2 4 and e hf t 5 7 to 1 1 for the magmatic zircons and some oib like trace element ratios they were likely generated through extensive fractional crystallization of mantle derived alkaline parental magma associated with crustal assimilation these 160 ma i and a type granites in central guangdong were emplaced coeval with the widespread 165 155 ma i and a type granites and syenites following the initiation of intraplate basaltic and or bimodal igneous magmatism at 180 170 ma in the adjacent regions we interpret these jurassic igneous rocks as anorogenic magmatism formed during a major igneous event in response to foundering of an early mesozoic subducted flat slab beneath se china continent

zircon morphology geochronology and trace element geochemistry of the granites from the huangshaping polymetallic deposit south china implications for the magmatic evolution and mineralization processes abstract the huangshaping polymetallic deposit is a large scale deposit located in the central nanling region south china different types of mineralization have been classified in the area which are attributed to three types of granitoids from the composite granitic pluton quartz porphyry granophyre and granite porphyry in order to study the magmatic genesis and evolutionary process as well as constrain the crystallization and mineralization events 100 zircon separates from different intrusive stages of the pluton have been analyzed for trace element contents and u pb isotopic concentrations the results show that zircons from the huangshaping granitoids are enriched in th u hf ti ree and other trace elements indicating hydrothermal fluid alteration and transformation additionally there is a large variation between different types of granitoids in terms of trace element concentrations compared to the zircons from the quartz porphyry and the granophyre zircons from the granite porphyry are more enriched in th u hf ti and p but strongly depleted in eu this suggests that the magma that formed the granite porphyry possessed higher temperatures possibly experienced stronger crystal fractionation and originated from a more evolved complex magmatic source region with higher degree of crustal maturity combined with characteristics of crystal morphology and internal texture hydrothermal zircons were distinguished from magmatic zircons in each granitoid to reveal mineralization processes the u pb geochronology results show that there are two major stages of mineralization 190 ma weak cu pb zn mineralization related to the quartz porphyry and the granophyre and 155 ma strong w sn mo pb zn mineralization associated with the granite porphyry compositions of these hydrothermal zircons suggest that the earlier stage mineralization could be close to single relatively cold reduced and quickly ascended fluids whereas the later stage might be related to hot oxidized strongly fractionated and highly evolved mixed fluids this study also indicates that hydrothermal zircons from typical composite granitic plutons could be a good tool to reveal the multi stage magmatic and mineralization process in middle to late jurassic south china

layer parallel shortening across the sevier fold thrust belt and laramide foreland of wyoming spatial and temporal evolution of a complex geodynamic system abstract varying patterns of layer parallel shortening lps and vertical axis rotations from the thin skin sevier fold thrust belt to the thick skin laramide foreland of wyoming are quantified from integrated structural anisotropy of magnetic susceptibility ams and paleomagnetic analyses within the sevier belt widespread early lps was accommodated by spaced cleavage fracture sets minor folds and minor faults lps directions are subperpendicular to structural trends of systematically curved thrust sheets of the wyoming salient reflecting a combination of primary dispersion and secondary rotation during thrusting within the laramide foreland limited lps was accommodated mostly by minor faults with conjugate wedge and strike slip geometries lps directions in gentler fold limbs vary from perpendicular to acute with structural trends of variably oriented anastomosing basement cored arches steep forelimbs display more complex relations including younger fault sets that developed during evolving stress states and localized vertical axis rotations although internal strain is limited weak ams lineations defined by kinked and rotated phyllosilicates are widely developed and consistently oriented perpendicular to measured lps directions palinspastically restored lps directions corrected for paleomagnetically determined vertical axis rotations vary on average from w e in the sevier belt to wsw ene in the laramide foreland in detail lps directions display deflections related to primary sedimentary wedge geometry and basement fabrics lps in the sevier belt is interpreted to partly reflect stress transmitted from the hinterland through the growing orogenic wedge and topographic stress along the front of the wedge lps in the laramide foreland is interpreted to partly reflect basal traction during flat slab subduction beneath thick cratonic lithosphere with spatial temporal variations in stress trajectories related to basement heterogeneities and evolving fault systems evidence for significant late n s laramide shortening as proposed by multi stage shortening models is not observed

neoproterozoic amorphous ekanite ca2th0 9u0 1si8o20 from okkampitiya sri lanka a metamict gemstone with excellent lead retention performance a rare gemstone form of ekanite from okkampitiya eastern sri lanka has a chemical composition of ca2th0 9u0 1si8o20 corresponding to that of the tetragonal mineral ekanite the okkampitiya material however has undergone amorphization and is found in a fully metamict state which is ascribed to the long term accumulation of high levels of self irradiation damage 1 38 x 10 20 alpha decay events per gram the ekanite nevertheless does not show any post growth chemical alteration and the radiogenic pb has been retained the okkampitiya ekanite yielded a neoproterozoic pb 207 pb 206 age of 562 1 0 8 ma uncertainty quoted at the 95 confidence level that is concordant within the uncertainties of decay constants the remarkable pb retention performance of ekanite contradicts the common hypothesis that metamictization in minerals results in u pb discordance the exceptional chemical durability of the heavily radiation damaged geological material described here has also implications for materials science research high radiation resistance a key criterion in the search for advanced nuclear waste forms may not be a prerequisite for high aqueous durability of a solid