mafic and felsic magma interaction in granites the hercynian karkonosze pluton sudetes bohemian massif the hercynian post collisional karkonosze pluton contains several lithologies equigranular and porphyritic granites hybrid quartz diorites and granodiorites microgranular magmatic enclaves and composite and lamprophyre dykes field relationships mineralogy and majorand trace element geochemistry show that 1 the equigranular granite is differentiated and evolved by small degrees of fractional crystallization and that it is free of contamination by mafic magma 2 all other components are affected by mixing the end members of the mixing process were a porphyritic granite and a mafic lamprophyre the degree of mixing varied widely depending on both place and time all of the processes involved are assessed quantitatively with the following conclusions most of the pluton was affected by mixing implying that huge volumes 475 km of mafic magma were available this mafic magma probably supplied the additional heat necessary to initiate crustal melting part of this heat could have also been released as latent heat of crystallization only a very small part of the karkonosze granite escaped interaction with mafic magma specifically the equigranular granite and a subordinate part of the porphyritic granite minerals from these facies are compositionally homogeneous and or normally zoned which together with geochemical modelling indicates that they evolved by small degrees of fractional crystallization 520 accessory minerals played an important role during magmatic differentiation and thus the fractional crystallization history is better recorded by trace rather than by major elements the interactions between mafic and felsic magmas reflect their viscosity contrast with increasing viscosity contrast the magmatic relationships change from homogeneous hybrid quartz diorites granodiorites to rounded magmatic enclaves to composite dykes and finally to dykes with chilled margins these relationships indicate that injection of mafic magma into the granite took place over the whole crystallization history consequently a long lived mafic source coexisted together with the granite magma mafic magmas were derived either directly from the mantle or via one or more crustal storage reservoirs compatible element abundances e g ni show that the mafic magmas that interacted with the granite were progressively poorer in ni in the order hybrid quartz dioritescgranodioritescenclavesccomposite dykes this indicates that the felsic and mafic magmas evolved independently which in the case of the karkonosze granite favours a deep seated magma chamber rather than a continuous flux from mantle two magma sources mantle and crust coexisted and melted almost contemporaneously the two reservoirs evolved independently by fractional crystallization however mafic magma was continuously being intruded into the crystallizing granite with more or less complete mixing several lines of evidence e g magmatic flux structures incorporation of granite feldspars into mafic magma feldspar zoning with fluctuating trace element patterns reflecting rapid changes in magma composition indicate that during its emplacement and crystallization the granite body was affected by strong internal movements these would favour more complete and efficient mixing the systematic spatial temporal association of lamprophyres with crustal magmas is interpreted as indicating that their mantle source is a fertile peridotite possibly enriched metasomatized by earlier subduction processes

concordant u pb shrimp ages of u rich zircon in granitoids from the muruntau gold district uzbekistan timing of intrusion alteration ages or meaningless numbers abstract for decades the u pb isotope system of zircon is routinely used in geosciences for the determination of intrusion ages of felsic magmatic rocks it is however well known that zircon and the related u pb isotope system may be influenced by secondary hydrothermal alteration and new hydrothermal zircon growth as a result age determination may yield meaningless or alteration ages an example is given here for intensely altered late paleozoic granitoid rocks from the muruntau area central kyzyl kum uzbekistan pb pb single grain evaporation and u pb shrimp isotope systematics of zircon from six granitoid rocks yield roughly consistent results defining ages ranging between 288 and 303 ma at first glance these ages may be understood as intrusion ages of the magmatic rocks hosting the zircon however there is an apparent contradiction between the sequence of the measured u pb shrimp ages and the relative age sequences established in the field evaluation of data from comprehensive mineralogical and petrological studies reveals that the u pb system of magmatic zircon is complicated by inheritance in i type granitoids recrystallization new hydrothermal zircon growth and subsequent secondary alteration widespread albitization of the granitoids led to the formation of new u rich hydrothermal zircon forming overgrowths on older grains or even new whole single crystals later alteration events caused direct and apparent reverse u pb isotope discordances these late disturbances of the u pb system are mainly due to widespread sericitization consequently precise concordant u pb zircon shrimp ages around 290 294 ma defined for u rich largely undisturbed crystal areas constrain the timing of albitization rather than that of the intrusion the reliability of the concordant u pb shrimp ages measured on u rich zircon is confirmed by a u pb shrimp age of hydrothermal monazite 292 8 ma this monazite associates with extremely u rich zircon u pb shrimp age 291 3 ma in a sample of the murun granite recovered beneath the giant muruntau gold deposit by super deep drilling obviously detailed mineralogical studies are important for a correct interpretation of zircon ages when hydrothermal alteration and late re mobilization of zirconium and uranium were significant as often observed in areas with intense ore mineralization the extensive au mineralization found at muruntau is similar in age to albitization but apparently somewhat younger likewise the intrusion age of permian granites is at least somewhat older than the hydrothermal zircon ages confirmed by u pb ages of monazite it is however difficult to constrain further granite intrusion ages based on presently available data

constraints on the timing of multiple thermal events and re equilibration recorded by high u zircon and xenotime case study of pegmatite from pilawa gorna gory sowie block sw poland abstract the application of zircon and xenotime geochronometers requires knowledge of their potential and limitations related to possible disturbance of the age record the alteration of the intergrown zircon and xenotime in pegmatite from the gory sowie block sw poland was studied using the electron microprobe analysis x ray wds compositional mapping micro raman analysis and la icp ms u pb dating of zircon and xenotime as well as the u th total pb dating of uraninite these microanalytical techniques were applied to understand the formation mechanisms of the secondary textures related to post magmatic processes in the zircon and xenotime intergrowth and to constrain their timing textural and compositional features combined with u pb data indicate that the pegmatite related crystallization of the zircon and xenotime intergrowth occurred ca 2 09 ga 2086 35 ma for zircon and 2093 52 ma for xenotime followed by the re equilibration of zircon and xenotime ca 370 ma 373 18 ma and 368 6 ma respectively during the formation of the younger pegmatite the zircon and xenotime were most likely derived from precambrian basement rocks and emplaced in the pegmatite as a restite the zircon preserved textures related to diffusion reaction processes that affected its high u core up to ca 9 6 wt uo2 which underwent further metamictization and amorphization due to self radiation damage the zircon rim and xenotime were affected by coupled dissolution reprecipitation processes that resulted in patchy zoning age disturbance and sponge like textures xenotime was also partially replaced by fluorapatite or hingganite y and y enriched allanite ce the termination of the low temperature alteration was constrained by the u th total pb age of the uraninite inclusions that crystallized in zircon at 281 2 ma which is consistent with the age of 278 15 ma obtained from the youngest cluster of u pb ages in the re equilibrated high u zircon domains this study demonstrates the importance of the careful examination of compositional microtextural and geochronological data obtained using microanalytical techniques to reconstruct the complex thermal histories recorded by accessory minerals

age of the dahongliutan rare metal pegmatite deposit west kunlun xinjiang nw china constraints from la icp ms u pb dating of columbite fe and cassiterite abstract dahongliutan is a medium size li be ta nb rare metal pegmatite deposit in the tianshuihai terrane west kunlun orogenic belt nw china in this paper la icp ms u pb ages of columbite fe and cassiterite are reported to constrain the metallogenic age columbite fe with high u and low th yielded a late triassic weighted mean 206 pb 238 u age of 211 9 2 4 ma whilst cassiterite yielded a 206 pb 238 u 207 pb 235 u concordia lower intercept age of 218 12 ma and a tera wasserburg lower intercept age of 218 12 ma which is identical to the columbite fe u pb age and thus represents the emplacement age of the pegmatite dikes the ore hosting pegmatite dikes are intimately time space related to the dahongliutan s type granite ca 220 217 ma indicating that they may have been cogenetic integrating new and published geological data we suggest that the dahongliutan pegmatites may have evolved from the granitic magma represented by the dahongliutan s type pluton in a post collisional tectonic setting regionally the tianshuihai terrane is the western extension of the songpan ganzi block and thus the kunlun li mineralization belt may have been connected to the songpan ganzi li mineralization belt which contains eleven lithium deposits with about 9 8 of li 2 o reserves in china implying that the dahongliutan pegmatite deposit may also have favorable ore forming conditions the similar columbite fe and cassiterite ages demonstrate that these minerals can be precisely dated and could provide useful alternatives to accurately constrain the timing and evolution of rare metal mineralization

u pb geochronology of hydrothermal zircons from the early cretaceous iron skarn deposits in the handan xingtai district north china craton the handan xingtai district in the north china craton is one of the most important concentrations of iron skarn deposits in china with proven reserves of 900 to 1 000 mt at an average of 40 to 55 wt fe the iron mineralization occurs predominantly along contact zones between early cretaceous intermediate silicic intrusions and middle ordovician marine carbonates intercalated with numerous evaporite beds in this paper we present textural features and laser ablation icp ms u pb dating results of hydrothermal zircon from five major iron skarn deposits to place tight constraints on the timing and duration of the district scale iron mineralization zircon grains from the mineralized skarns are anhedral to subhedral crystals and euhedral tetragonal bipyramids they are closely intergrown or texturally associated with diopside garnet epidote calcite albite and phlogopite other common minerals in the skarn assemblages include f rich hornblende wilkeite f f apatite and fluorite zircon grains typically contain abundant inclusions of skarn minerals and daughter mineral rich mostly magnetite halite and sylvite fluid inclusions compositionally these zircon grains have moderately to extremely high th 518 7 477 ppm and u 109 25 610 ppm contents with highly variable th u ratios ranging from 0 01 to 5 23 the morphological textural and geochemical features of the zircons confirm their hydrothermal origin and indicate that they most likely precipitated from high temperature f rich magmatic derived ore forming fluids the hydrothermal zircon grains yield well defined concordant u pb ages for the five studied iron skarn deposits with weighted mean 206 pb 238 u dates ranging from 133 6 0 9 to 128 5 1 4 ma 2 these ages are remarkably consistent with u pb ages 134 1 1 2 to 128 5 0 9 ma 2 of magmatic zircon grains from the ore related intrusions in each deposit demonstrating that iron skarn mineralization was genetically related to the coeval magmatism our new geochronological data when combined with existing isotopic ages indicate that iron mineralization and associated magmatism in the handan xingtai district took place mainly at the ca 137 133 and 131 128 ma intervals iron skarn deposits of similar ages also occur widely in other parts of the eastern north china craton forming the only known giant mesozoic iron skarn province in a cratonic block on the earth the formation of these iron skarn deposits and associated intrusions coincided in time with lithospheric thinning or destruction of the north china craton strongly suggesting a causal link between the two processes

extreme p bi nb sc u and f rich zircon from fractionated perphosphorous granites the peraluminous podlesi granite system czech republic abstract the strongly peraluminous and p rich protolithionite and zinnwaldite leucogranites from podlesi western krusne hory mts czech republic contain accessory zircon with extraordinary enrichment of several elements which constitute trace elements in common zircon elements showing a not yet reported anomalous enrichment include p up to 20 2 wt p2o5 equivalent to 0 60 apfu formula calculated on the basis of 4 oxygen atoms bi up to 9 0 wt bi2o3 0 086 apfu nb up to 6 7 wt nb2o5 0 12 apfu sc up to 3 45 wt sc2o3 0 10 apfu u up to 14 8 wt uo2 0 12 apfu and f up to 3 81 wt f 0 42 apfu strong enrichment of p preferentially involved the berlinite type substitution 2 si4 p5 al3 implying that significant al may enter the si position in zircon incorporation of other exotic elements is primarily governed by the xenotime si4 zr4 p5 y3 pretulite sc3 p5 zr4 si4 brabantite type ca2 u th 4 2p5 2zr4 2si4 and ximengite type bi3 p5 zr4 si4 substitution reactions one part of the anomalous zircons formed late magmatically from a strongly peraluminous p f u rich hydrous residual melt that gave rise to the zinnwaldite granite interaction with aggressive residual fluids and metamictization have further aided in element enrichment or depletion particularly in altered parts of zircon contained in the protolithionite granite most of the zircon from f rich greisens have a composition close to endmember zrsio4 and are chemically distinct from zircon in its granite parent this discrepancy implies that at podlesi granitic zircon became unstable and completely dissolved during greisenization part of the mobilized elements was reprecipitated in newly grown hydrothermal zircon

late pliocene sedimentation in lake baikal implications for climatic and tectonic change in se siberia abstract within the framework of the baikal drilling project bdp a 192 m long sediment core bdp 96 1 was recovered from the academician ridge a submerged topographic high between the north and central basins of lake baikal sedimentological clay mineralogical and geochemical investigations were carried out on the core interval between 90 and 124 m depth corresponding to ca 2 4 3 4 ma the aim was to reconstruct the climatic and tectonic history of the continental region during the intensification of northern hemisphere glaciation in late pliocene time a major climate change occurred in the lake baikal area at about 2 65 ma enhanced physical weathering in the catchment mirrored in the illite to smectite ratio and temporarily reduced bioproduction in the lake reflected by the diatom abundance evidence a change towards a colder and more arid climate probably associated with an intensification of the siberian high in addition the coincident onset of distinct fluctuations in these parameters and in the zr al ratio suggests the beginning of the late cenozoic high amplitude climate cycles at about 2 65 ma fluctuations in the zr al ratio are traced back to changes in the aeolian input with high values in warmer more humid phases due to a weaker siberian high assuming that the sand content in the sediment reflects tectonic pulses the lake baikal area was tectonically active during the entire investigated period but in particular around 2 65 ma tectonic movements have likely led to a gradual catchment change since about 3 15 ma from the western towards the eastern lake surroundings as indicated in the geochemistry and clay mineralogy of the sediments the strong coincidence between tectonic and climatic changes in the baikal area hints at the himalayan uplift being one of the triggers for the northern hemisphere glaciation

thermobarometric modelling of zircon and monazite growth in melt bearing systems examples using model metapelitic and metapsammitic granulites u pb age data collected from zircon and monazite are used to draw fundamental inferences about tectonic processes in the earth despite the emphasis placed on zircon and monazite ages the understanding of how to relate the timing of growth of zircon and monazite to an evolving rock system remains in its infancy in addition few studies have presented large datasets of geochronological data from zircon and monazite occurring in the same metamorphic rock sample such information is crucial for understanding the growth of zircon relative to monazite in a systematic and predictive manner as per this study the data that exist support the generally held conception that zircon ages tend to be older than monazite ages within the same rock here experimental data for zircon and monazite saturation in melt bearing rocks are integrated with phase diagram calculations the calculations constrain the dissolution and growth behaviour of zircon and monazite with respect to evolving pressure temperature and silicate mineral assemblages in high grade melt bearing metasedimentary rocks several key results emerge from this modelling first that in aluminous metapelitic rocks i e garnet cordierite sillimanite assemblages zircon ages are older than monazite ages in the same rock second that the growth rate of accessory minerals is nonlinear and much higher at and near saturation than at lower temperatures and third that the difference in zircon and monazite ages from the same rock may be ascribed to differences in the temperature s at which zircon and monazite grow rather than differences in closure temperature systematics using our methodology the cooling rate of granulites from the reynolds range central australia have been constrained at 4 c myr 1 this study serves as a first pass template on which further research in applying the technique to a field study can be based

age and provenance constraints on seismically determined crustal layers beneath the paleozoic southern central asian orogen inner mongolia china abstract we present 110 ages and 51 in situ 18o values for zircon xenocrysts from a post 99 ma intraplate basaltic rock suite hosted in a subduction accretion complex of the southern central asian orogenic belt in order to constrain a seismic profile across the paleozoic southern orogen of inner mongolia and the northern margin of the north china craton two zircon populations are recognized namely a phanerozoic group of 70 zircons comprising granitoid derived ca 431 99 ma n 31 peak at 256 ma meta granitoid derived ca 449 113 ma n 24 peak at 251 ma and gabbro derived 436 242 ma n 15 peaks at 264 and 244 ma grains each textural type is characterized by a distinct zircon oxygen isotope composition and is thus endowed with a genetic connotation the precambrian population 2605 741 ma n 40 exhibits a prominent age peak at 2520 ma granulite facies metamorphism and four small peaks at ca 1900 1600 and 800 ma our new data together with literature zircon ages significantly constrain models of three seismically determined deep crustal layers beneath the fossil subduction zone forearc along the active northern margin of the north china craton namely 1 an upper arc crust of early to mid paleozoic age intruded by a major permian triassic composite granitoid gabbroic pluton 8 20 km depth 2 a middle crust predominantly consisting of mid meso to neoproterozoic felsic and mafic gneisses and 3 a lower crust composed predominantly of late archean granulite facies rocks we conclude that the paleozoic orogenic crust is limited to the upper crustal level and the middle to lower crust has a north china craton affinity furthermore integrating our data with surface geological petrological and geochronological constraints we present a new conceptual model of orogenic uplift lithospheric delamination and crustal underthrusting for this key ocean continent convergent margin

the 40ar 39ar ages from the west sudetes ne bohemian massif constraints on the variscan polyphase tectonothermal development

geochronology and nd sr systematics of lusatian granitoids significance for the evolution of the variscan orogen in east central europe a variety of pre variscan granitoids and two variscan monzogranites occurring in the central and western parts of the lusatian granodiorite complex lgc saxonia were dated by the single zircon evaporation method complemented by whole rock nd isotopic data and rb sr whole rock and mineral ages the virtually undeformed pre variscan granitoids constitute a genetically related mostly peraluminous magmatic suite ranging in composition from two mica granodiorite muscovitebearing biotite quartz diorite tonalite and granodiorite to biotite granodiorite and monozogranite

different age response of zircon and monazite during the tectono metamorphic evolution of a high grade paragneiss from the ruhla crystalline complex central germany results of tims sims and sem analyses show that zircon and monazite in a high grade paragneiss of the ruhla crystalline complex central germany were formed and or altered during different stages of a tectono metamorphic history between early devonian and permian times detrital zircon cores of 460 ma place an older limit on the age of anatexis and show that the paragneiss sequence contains rocks at least as young as early cambrian metamorphic zircon growth commenced at 365 ma peaking at 360 355 ma at the same time that granite dykes were emplaced in contrast monazite in the paragneiss preserves little record of the metamorphic peak most monazite grains grew or were recrystallised in the lower carboniferous at 339 ma contemporaneous with the emplacement of voluminous diorite and granite bodies these intrusions and related tectonics caused some of the high u zircon overgrowths to undergo moderate to severe pb loss a second pb loss event between 300 and 280 ma can be related to late carboniferous early permian large scale block faulting

complicated secondary textures in zircon record evolution of the host granitic rocks studies from western tauern window and otztal stubai crystalline complex eastern alps western austria abstract samples of metamorphosed and deformed granitic rocks were collected from two alpine complexes with well constrained metamorphic history western tauern window and otztal stubai crystalline complex zircon grains from these samples were investigated in situ by a combination of scanning electron microscope techniques cathodoluminescence cl imaging and raman spectroscopy the aims were to describe and interpret complicated secondary textures and microstructures in zircon based on cross cutting relationships between secondary microstructures reconstruct the sequence of processes affecting zircon crystals link the evolution of zircon with the history of the host rocks the results indicate that zircon in the sampled granitic rocks forms growth twins and multi grain aggregates which are unusual for this mineral moreover various secondary textures have been found in the sampled zircon often cross cutting each other in a single crystal these include distorted oscillatory cl zoning with inner zones forming inward penetrating cl bright embayments which are the evidence of dry recrystallization via annealing lattice recovery cl mosaicism with no preservation of growth zoning but abundant nano and micro scale pores and mineral inclusions which are the evidence of recrystallization by coupled dissolution reprecipitation and or leaching embayed zircon boundaries filled with apatite monazite epidote and mylonitic matrix indicating mineral fluid reactions resulting in zircon dissolution and fragmentation overgrowth cl dark rims which contain nano pores and point to transport and precipitation of dissolved zircon matter we conclude that zircon in our meta granites is sensitive to metamorphism deformation events and was reactive with metamorphic fluids additionally we have found evidence of crystal plastic deformation in the form of low angle boundaries and bent grain tips which is a result of shearing and ductile deformation of the host rock we suggest that the observed complicated secondary textures in zircon can be linked to the evolutionary stages of the host rocks such as magmatic crystallization prograde metamorphism peak of amphibolite facies metamorphism post peak cooling and exhumation formation of ductile shear zones and final cooling to 250 c

magmatic structures in the krkonose jizera plutonic complex bohemian massif evidence for localized multiphase flow and small scale thermal mechanical instabilities in a granitic magma chamber abstract the present paper examines magmatic structures in the jizera and liberec granites of the krkonose jizera plutonic complex bohemian massif the magmatic structures are here interpreted to preserve direct field evidence for highly localized magma flow and other processes in crystal rich mushes and to capture the evolution of physical processes in an ancient granitic magma chamber we propose that after chamber wide mixing and hybridization as suggested by recent petrological studies laminar magma flow became highly localized to weaker channel like domains within the higher strength crystal framework mafic schlieren formed at flow rims and their formation presumably involved gravitational settling and velocity gradient flow sorting coupled with interstitial melt escape local thermal or compositional convection may have resulted in the formation of vertical schlieren tubes and ladder dikes whereas subhorizontal tubes or channels formed during flow driven by lateral gradients in magma pressure after the cessation or deceleration of channel flow gravity driven processes settling of crystals and enclaves gravitational differentiation development of downward dripping instabilities accompanied by compaction filter pressing and melt segregation dominated in the crystal mush within the flow channels subsequently magmatic folds developed in schlieren layers and the magma chamber recorded complex late magmatic strains at high magma crystallinities late stage magma pulsing into localized submagmatic cracks represents the latest events of magmatic history of the chamber prior to its final crystallization we emphasize that the most favorable environments for the formation and preservation of magmatic structures such as those hosted in the jizera and liberec granites are slowly cooling crystal rich mushes therefore where preserved in plutons these structures may lend strong support for a mush model of magmatic systems

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

high energy coralgal stromatolite frameworks from holocene reefs tahiti french polynesia drill cores from holocene reefs on tahiti french polynesia reveal a framework composed of massive branching acroporids encrusted by coralline algae associated with sessile vermetid gastropods and arborescent foraminifers laminated micritic crusts form coatings over coral branches or more commonly over related encrusting organisms throughout the cored reef sections these crusts appear as a major structural and volumetric component of the reef framework the microbial nature of these micritic crusts is inferred from their typical organic growth forms and geometry the occurrence of microbial remains and stable isotope measurements the reef communities accumulated at depths less than 5 m below mean sea level in a high energy environment throughout vertical growth from 7140 170 yr bp to the present the nature of the involved benthic communities stable isotope data and high calcification rates of microbially encrusted corals strongly suggest that local environmental conditions have been optimal for reef development for the last 7000 years the causes of the predominance of microbial communities over actual encrusters red algae foraminifers remain problematic and could be related to short term fluctuations in ecological parameters microbial micritic crusts seemingly played a prominent role in protecting the coralgal colonies from bioeroders and grazers and possibly in strengthening the framework due to rapid lithification the record of similar microbial crusts in other quaternary reef tracts suggests that microbial communities may have played a more prominent role in quaternary reefs than presently recognized

nanosims study of seismically deformed zircon evidence of y yb ce and p redistribution and resetting of radiogenic pb lattice defects in zircon can cause trace elements redistribution and disturbance of isotopic systems this study investigates in detail how seismically induced deformation microstructures in zircon correlate with trace element and isotope re equilibration felsic mylonites with pseudotachylyte veins from the ivrea verbano zone in northern italy were studied with special focus on deformed zircon we have revealed the following post growth deformation microstructures planar deformation bands pdbs planar fractures pfs non planar fractures both healed and open and finite strain patterns pdbs are planar portions of crystal lattice that are strictly parallel to 100 crystallographic planes and are rotated to up to 3 with respect to the host grain they are from 0 5 to 1 m wide and have average spacing of 5 m pdbs originate in seismically active environment at elevated differential stress strain rate and temperature several grains in which pdbs are observed were analyzed with ion microprobe ion maps indicate redistribution of radiogenic pb isotopes associated with pdb formation isotopic redistribution preferably occurs in pdbs with larger crystallographic misorientation profiling demonstrated clear spatial correlation of pdbs with variations of ree abundances both gain and loss and possible correlations with increased and decreased hf ti and p abundances trace elements can be depleted or enriched compared to the abundance in surrounding matrix in deformed domains depending on the spacing of pdbs and the proximity of the analyzed volume to grain boundary or to detrital core 207 pb 206 pb ratio demonstrates systematic pb loss in the pdb bearing lattice domains with respect to pdb free domains in some cases pb gain is observed where the pdbs source radiogenic pb from older detrital cores our study has important implications for geochronology and microchemistry of zircon from seismically deformed sections of ivrea verbano zone and from other paleo seismic zones of the world zircon found in seismically deformed rocks near pseudotachylyte veins may demonstrate distorted and even reset isotopic ages and altered trace element abundances enhanced trace element exchange between deformed zircon and host mylonite can influence mass balance calculations for the bulk rock

magmatic and metamorphic history of paleoarchean tonalite trondhjemite granodiorite ttg suite from the singhbhum craton eastern india texturally controlled dating of zircon from paleoarchean tonalite trondhjemite granodiorites of the older metamorphic tonalitic gneisses and the singhbhum granite batholith phases i ii and iii from the singhbhum craton in eastern india reveals a polycyclic evolution of the archean crust the granitoid suites were emplaced in two pulses at 3 45 3 44 ga and 3 35 3 32 ga tonalites and trondhjemites of the older metamorphic tonalitic gneisses were emplaced at ca 3 45 3 44 ga together with phase iii of the singhbhum granite pluton while granites belonging to the older metamorphic tonalitic gneisses were emplaced at ca 3 35 3 32 together with phase i and phase ii of the singhbhum granite pluton both crustal units underwent an early phase of relatively high grade metamorphism at 3 30 3 28 ga followed by extensive fluid induced alteration during low grade metamorphism at 3 19 3 12 ga and 3 02 2 96 ga the two units have also been marginally affected at ca 2 52 ga and 1 06 ga by major metamorphic events in the north singhbhum mobile belt and the singhbhum shear zone at the northern margin of the craton the zircon grains in granites have inherited cores with ages of ca 3 61 ga and 3 46 3 41 ga and with well developed oscillatory growth zonation which suggests the granitic magmas were derived by partial melting of an igneous precursor or sedimentary rocks derived from an igneous source the emplacement of the expansive granitoids belonging to the older metamorphic tonalitic gneisses and the singhbhum granite was synchronous with the amphibolite facies metamorphism ca 3 32 ga of older meta igneous and metasedimentary rocks belonging to the older metamorphic group major felsic crust formation in the craton occurred in a narrow time interval between 3 46 and 3 32 ma with minor contributions of material as old as 3 6 ga the complex polycyclic evolution of the paleoarchean crust in the singhbhum craton can account for the wide range of often disparate ages obtained using whole rock isochron dating techniques with some of the isochron dates being geologically meaningful while others representing mixing lines or disturbance of the isotopic systems during metamorphism

grain scale variations in trace element composition of fluid altered zircon acasta gneiss complex northwestern canada sem back scattered electron images of zircon separates from a 2 9 ga granitic dyke from the acasta gneiss complex slave province reveal primary igneous oscillatory zoning which is transgressively overprinted by irregular alteration domains electron microprobe analyses show that altered zircon domains are depleted in zr and si relative to unaltered zircon while retaining a constant zr si ratio sims trace element analyses indicate that lree are preferentially enriched in the altered domains altered zircon contains elevated concentrations of ba up to 580 ppm ca up to 2 weight of the element and common pb 50 ppb the presence of lree ca ba and common pb in the zircons is interpreted to be the result of secondary alteration by an aqueous fluid and not by primary incorporation during crystallization we propose that the alteration related elements are located in amorphous zircon domains

the difficulties of dating mafic dykes an antarctic example archaen gneisses of the vestfold hills of east antarctica are transected by several compositionally discrete suites of tholeiitic dykes a representative of one of those suites which has been dated in the present study shows that not only rb sr whole rock isochrons but also u pb zircon techniques if not properly applied can produce erroneous crystallisation ages two zircon populations were recovered from the mafic dyke itself one of which is 2 483 9 ma in age and clearly of xenocrystic origin the other yields an age of 1 025 56 ma which is not ascribed to the magmatic crystallisation of the dyke but rather to the time that it underwent metamorphic metasomatic alteration as a response to high grade tectonism in the adjacent mobile belt it is presumed that the zircon in question formed by the breakdown of another mineral or minerals possibly magmatic baddeleyite due either to ingress of a siliceous fluid or more probably by the release of silica from the breakdown of pyroxene to amphibole a cogenetic 1 2 cm wide felsic vein of late magmatic early hydrothermal origin also contains two zircon populations again most of the grains therein which are interpreted as of xenocrystic origin grew at 2 483 9 ma however a few euhedral zircons with very high u and th contents grew at 1 248 4 ma which is taken to be the formation age of both the felsic vein and the enclosing mafic dyke

chaotic three dimensional distribution of ba rb and sr in feldspar megacrysts grown in an open magmatic system as has been demonstrated in recent years the heterogeneities of coeval magmas can be more successfully revealed by zoned megacrysts rather than by analysis of the whole rocks hosting them here the geochemical heterogeneities of feldspar megacrysts from the karkonosze granite poland are investigated by la icp ms the crystals are the product of migration and growth in regions of poorly mixed magmas 3d modeling of the ba sr and rb distributions emphasizes the importance of micro domain growth morphologies two models of element behavior a relative concentration model and a composition gradient model provide a potentially effective tool for tracking the mixing process on a microscale measured concentrations of elements of different mobilities do not agree with what might be expected from the mixing of two end member magmas if mixing was the only process occurring linear correlations between the concentrations of any two elements should be observed this however is not the case for combinations of any two of the three elements modeling reveals differing non linear correlations between concentrations the megacryst heterogeneities provide an insight into how mixing magmas are chaotically advected to growing crystals and the degree of inter magma element exchange between the magmas

temporal relationship between granite cooling and hydrothermal uranium mineralization at dalongshan in china a combined radiometric and oxygen isotopic study abstract a combined study using multi radiometric dating and oxygen isotopic geothermometry was carried out for mesozoic quartz syenite alkali feldspar granite and associated hydrothermal uranium mineralization at dalongshan in the middle lower yangtze valley of east central china radiometric dating of the quartz syenite yields a whole rock rb sr isochron age of 135 6 4 3 ma a zircon u pb isochron age of 132 9 2 2 ma and k ar ages of 126 2 118 3 and 94 4 ma for hornblende biotite and orthoclase respectively the alkali feldspar granite yields a whole rock rb sr isochron age of 117 3 3 3 ma a zircon u pb isochron age of 114 7 2 1 ma and k ar ages of 112 2 109 3 and 88 4 ma for hornblende biotite and orthoclase respectively oxygen isotope thermometry for both granites gives temperatures of 685 to 720 555 to 580 435 to 460 and 320 to 330 c for hornblende magnetite biotite and orthoclase respectively when paired with quartz the systematic differences among the ages by the different techniques on the different minerals are used to reconstruct the cooling history of the granite the results yield rapid cooling rates of 27 4 to 58 6 c ma from 800 to 300 c in the early stage but slow cooling rates of 6 3 to 7 2 c ma from 300 to 150 c in the late stage the regular sequence of oxygen isotope temperatures for the different quartz mineral pairs demonstrates that diffusion is a dominant factor controlling the closure of both radiometric and o isotopic systems during granite cooling pitchblende u pb isochron dating yields an uranium mineralization age of 106 4 2 9 ma which is younger than the age of the granite emplacement and thus considerably postdates the time of magma crystallization but is close to the closure time of the k ar system in the biotite this points to a close relationship between granite cooling and ore forming process it appears that hydrothermal mineralization took place in the stage of slow cooling of the granite whereas the rapid cooling of the granite was concurrent with the migration of hydrothermal fluids along fault structures therefore the activity of the ore forming hydrothermal system is temporally dictated by the cooling rates of the granite and may lag about 25 to 30 ma behind the crystallization timing of associated granite

sensitive high resolution ion microprobe analysis of zircon reequilibrated by late magmatic fluids in a hybridized pluton zircon from a microgranular enclave in the ca 315 ma postcollisional karkonosze pluton western sudetes northeastern bohemian massif is characterized by unusual morphologies and reequilibration textures blocky clustered and skeletal th u rich zircon grains are internally corroded along discrete boundary zones and subsequently replaced by porous microgranular aggregates of zircon and various other minerals including thorite the boundary zones have textures and compositions characteristic of diffusion controlled chemical reaction fronts including enrichment in ca ba and light rare earth elements whereas microgranular domains are typical of zircon replacement and regrowth by coupled dissolution and precipitation initial zircon crystallization occurred with the mingling of mafic magma into a cooler granitic melt whereas zircon modification is attributed to the reaction of late magmatic fluids from the host granite with the enclave precise dating of reequilibrated zircon as 304 2 ma indicates that fluid activity which is also responsible for scheelite mineralization postdates the emplacement of the main part of the pluton by several millions of years

radiation damage in zircon and monazite monazite and zircon respond differently to ion irradiation and to thermal and irradiation enhanced annealing monazite cannot be amorphized by 800 kev kr ions at temperatures greater than 175 c whereas zircon can be amorphized at temperatures up to 740 c the damage process i e elastic interactions leading to amorphization in radioactive minerals metamictization is basically the same as for the ion beam irradiated samples with the exception of the dose rate which is much lower in the case of natural samples the crystalline to metamict transition in natural samples with different degrees of damage from almost fully crystalline to completely metamict is compared to the sequence of microstructures observed for ion beam irradiated monazite and zircon the damage accumulation process representing the competing effects of radiation induced structural disorder and subsequent annealing mechanisms irradiation enhanced and thermal occurs at much higher temperatures for zircon than for monazite the amorphization dose expressed as displacements per atom is considerably higher in the natural samples and the atomic scale process leading to metamictization appears to develop differently ion beam induced amorphization data were used to calculate the decay event dose required for amorphization in terms of a critical radionuclide concentration i e the concentration above which a sample of a given age will become metamict at a specific temperature this equation was applied to estimate the reliability of u pb ages to provide a qualitative estimate of the thermal history of high u natural zircons and to predict whether actinide bearing zircon or monazite nuclear waste forms will become amorphous metamict over long timescales