fabric and origin of gneissic layers in anorthositic rocks of the st charles sill ontario the st charles sill is located in the grenville province and consists of rocks of the anorthosite suite the sill is a northwesterly trending body 11 km long and as much as 0 8 km wide and with

u pb evidence for polymetamorphic history of huronian rocks within the grenville front tectonic zone east of sudbury ontario canada abstract this paper presents u pb geochronological data for zircon titanite monazite apatite allanite and rutile from street township east of sudbury that help unravel two main facets of the geology of the grenville front tectonic zone gftz they document the presence of extensive huronian supergroup magmatic ca 2480 2460 ma and sedimentary rocks inside the gftz and highlight the significance of bimodal magmatism in the genesis of the huronian supergroup the data also provide further insights into the complex orogenic evolution of the gftz showing in particular that amphibolite facies metamorphism migmatization and deformation during grenvillian compression at 995 980 ma overprinted but did not totally erase the memory of earlier metamorphic events at ca 1720 and 1470 ma from the point of view of u pb systematics the results emphasize the nearly ubiquitous state of isotopic disequilibrium in these rocks most of the minerals investigated comprise two or more generations and are generally in a state of partial resetting because of the superimposed effects of younger events the preservation of pre grenvillian ages in titanite and allanite demonstrates that processes related to final grenvillian collision and exhumation occurred at great speed only apatite and to some degree rutile yield ages that are clearly post tectonic thereby reflecting the final stages of exhumation and cooling of the region

the development of the central european continental crust since the early archaean based on conventional and ion microprobe dating of up to 3 84 b y old detrital zircons abstract archaean components up to 3 84 b y old have been detected by ion microprobe dating of zircon from various sediments and metasediments of the european hercynides moldanubian of northeastern bavaria and the massif central the early archaean age of 3 84 b y was obtained within the euhedral core of a detrital grain from a moldanubian paragneiss the same detrital zircon grain reveals at least three metamorphic overprints at 2 59 b y b p 1 94 b y b p and 460 m y b p and at least two cycles of mechanical abrasion during surface recycling in the early proterozoic and after the pan african cycle it is thus the crystal with the most complex geological history ever detected further archaean ages for the crystallization of primary magmatic zircons probably in the course of continental crust formation in the first cycle provenances of the two studied areas were found at 3 15 2 9 2 76 and 2 65 b y b p and in a few cases between 2 6 and 2 5 b y b p early proterozoic crust forming events between about 1 75 and 2 15 b y b p play an important role especially for the sediments and metasediments of the montagne noire at the southern rim of the massif central middle and late proterozoic ages cluster around 1 0 b y b p and 600 m y b p for all analysed samples therefore the deposition of the metasedimentary precursors took place after the pan african cycle which is in line with the rb sr whole rock systematics of central european metasediments the ages of metamorphism in the different source areas could be inferred in some cases by lower intercept ages obtained on single detrital grains the following sequence coinciding with times of magmatic events has been so far established 2 59 1 94 and 1 0 b y b p and 600 m y b p thus ion probe dating is also capable of defining secondary effects in the respective provenances which are typical for orogenic cycles the ubiquitous presence of pan african detrital zircons in european metasediments is a strong indication that the detrital material was derived from gondwana however from the presence of metamorphic units including eclogitefacies rocks which probably formed at different continental margins and at various times between about 500 m y and 330 m y ago it is probable that various gondwana derived microcontinental plates contributed to the different basin fillings the gondwana derived detritus is also supported by the observation of two periods of relative quiescence or non activity around 2 3 b y ago and especially around 1 5 b y ago which is a typical feature observed in gondwana cahen et al 1984 but not in laurasia the other possible source of the detritus the lack of zircons crystallized between 3 84 and 3 15 b y ago might have different causes thus the continental crust of the european hercynides probably developed since the early archaean and grew via many crustal accretion events to its present state

evolution of depleted mantle the lead perspective abstract isotopic data have established that compared to estimated bulk earth abundances the sources of oceanic basaltic lavas have been depleted in large ion lithophile elements for at least several billions of years various data on the tertiary mesozoic gorgona komatiite and cretaceous oka carbonatite show that those rocks also sample depleted mantle sources this information is used by analogy to compare pb isotopic data from 2 6 billion year old komatiite and carbonatite from the suomussalmi belt of eastern finland and munro township ontario that are with associated granitic rocks and ores that should contain marked crustal components within experimental error no differences are detected in the isotopic composition of initial pb in either of the rock suites these observations agree closely with sr and nd data from other laboratories showing that depleted mantle could not have originated in those areas more than a few tenths of billions of years before the rocks were emplaced on a world wide basis the pb isotope data are consistent with production of depleted mantle by continuous differentiation processes acting over approximately the past 3 billion years the data show that pb evolution is more complex than the simpler models derived from the rb sr and sm nd systems the nature of the complexity is still poorly understood

a study of soil water movement combining soil water potential with stable isotopes at two sites of shallow groundwater areas in the north china plain in the shallow groundwater areas of the north china plain ncp precipitation infiltration and evapotranspiration in the vertical direction are the main processes of the water cycle in which the unsaturated zone plays an important role in the transformation process between precipitation and groundwater in this paper two typical sites in cangzhou cz and hengshui hs of hebei province with shallow water tables were selected to analyse the relationship among precipitation soil water and groundwater at each site precipitation soil water at depths 10 20 30 50 70 100 150 200 300 cm and groundwater were sampled to analyse the stable isotope compositions of hydrogen and oxygen the soil water potentials at the corresponding depths were observed although the climates at the two sites are similar there are some differences in the infiltration process soil water movement and groundwater recharge sources evaporation occurred at the upper depths which led to the decrease of soil potential and the enrichment of heavy isotopes at the cz site precipitation infiltrated with piston mode and an obvious mixture effect existed during the infiltration process preferential flow may exist in the soil above 100 cm depth however at the hs site soil water moved in piston mode and groundwater was mainly recharged by precipitation when precipitation recharged the groundwater it experienced a strong evaporation effect the results of the soil water movement mechanism provides the transformation relationship among precipitation soil water and groundwater in the middle and eastern ncp copyright 2009 john wiley sons ltd

regional land subsidence simulation in su xi chang area and shanghai city china su xi chang area and shanghai city located in the south of yangtze delta china has subsided due to groundwater overpumping because of the regional scale of the groundwater exploitation cone of depression and land subsidence at present su xi chang area and shanghai city are treated as a single area for land subsidence study to avoid the uncertainty of boundary condition due to the regionalism the characteristics of aquifer system compaction are complex because of the difference in the types compositions and structures of the soils that the hydrostratigraphic units are composed of and in the histories of groundwater level change the hydrostratigraphic units have experienced considering the fact that different hydrostratigraphic units have different kinds of deformation and that an identical unit may also present different deformation characteristics such as elasticity elasto plasticity and visco elasto plasticity at different sites of the cone of depression or in different periods corresponding constitutive laws have been adopted this avoids the shortcomings of the previous research that the same constitutive law was adopted in all the hydrostratigraphic units during the entire time period a coupled flow and subsidence model which includes a three dimensional flow model with variable coefficients and a one dimensional vertical subsidence model is built according to the complicated hydrological condition in the region the simulation model is calibrated using observed data which include compression of individual strata from groups of extensometers and groundwater levels from observation wells from 1995 to 2002 the model reproduced that the primary subsidence layer in shanghai shifts from the shallow aquitard to the fourth confined aquifer because of the groundwater yield variations and the change of exploitation aquifers however the third aquitard was the primary subsidence layer in su xi chang area and the compaction deformation of the sandy aquifers was remarkable the simulation results could provide some reasonable advice about groundwater exploitation in the future

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

upward recharge through groundwater depression cone in piedmont plain of north china plain summary whether a recharge was induced by groundwater depression cones is a crucial issue for water resource management in the north china plain shallow groundwater had been over pumped since 1970s and many groundwater depression cones formed the groundwater depression cone daceying machang occurred even in the piedmont plain in the area water levels of deep and shallow groundwater were observed since 2005 and field survey was conducted in the dry season 2010 the upward recharge induced by the depression cone is verified based on water level records major ions 2 h 18 o and kinds of statistical analyses since august 2006 the water level of the deep groundwater ascended by 1 9 mm d high correlations r 0 86 s 0 67 between the water level series of shallow and deep groundwater were found by two distinct correlation analyses only in the center of depression cone further the reversion of hydraulic gradient of the depression cone occurred in dry seasons since september 2008 hydro geochemical features of the shallow groundwater are consistent with deep groundwater in the center of depression cone which was demonstrated by the fuzzy c means clustering based on principal components and paired t test respectively it is concluded that the deep groundwater recharged the shallow groundwater from the center of the depression cone as a result the groundwater mixture occurred that improves the quality of the shallow groundwater seasonally changed flow of shallow groundwater enhanced the mixture the persistent overpumping of shallow groundwater and the large elevation difference around 1200 m between the recharge zone and the discharge point of deep groundwater facilitate the upward recharge

the armorica microplate fact or fiction critical review of the concept and contradictory palaeobiogeographical data abstract the problem of an armorica microplate detached from gondwana and having had during part of the palaeozoic an independent latitudinal evolution is reconsidered in terms of a critical review of the palaeomagnetic data that were the very roots of this concept and through an alternative approach based on palaeoclimatic and palaeobiogeographical data palaeomagnetic data for the silurian and the devonian of the south european regions supposedly constituting the armorica microplate remain rare and ambiguous those from gondwana are more numerous but contradictory enough to give rise to diverging models regarding the latitudinal evolution of this continent consequently the reality of an armorica microplate cannot be considered as established on the contrary lithological indicators of palaeoclimate and palaeobiogeographical data are in total harmony and indicate that in actual fact the southern european regions remained permanently closely connected with gondwana of which they composed the northern margin although repeatedly maintained for more than 20 years the concept of an armorica microplate can thus be considered a fiction this conclusion should lead to the dismissal of geodynamical models proposed for the variscan belt to which this concept was integrated and are contradicted by inescapable palaeobiogeographical constraints

evolution of palaeoproterozoic mafic intrusions located within the thermal aureole of the sudbury igneous complex canada isotopic geochronological and geochemical evidence abstract impact cratering and their resultant geological phenomena are recognised as significant factors in the lithological and biologic evolution of the earth age dating of impact events is critical in correlating cause and effects for these catastrophic processes the falconbridge and drury township twp intrusions were emplaced at the contact between neoarchaean basement and palaeoproterozoic volcanosedimentary rocks and also lie at the southeast and southwest edges of the sudbury igneous complex sic within its thermal contact aureole the falconbridge twp intrusion is dated at 2441 3 ma by u pb zircon with evidence of archaean inheritance from its host granitoids granitoids from the southernmost abitibi subprovince are dated here between 2670 11 ma for an undeformed algoman granite and 2696 18 ma for a foliated granitoid consistent with existing data from the abitibi greenstone belt and from the wawa subprovince major and trace element geochemical evidence common pb isotopic compositions and end2440 values between 0 and 1 are all consistent with a palaeoproterozoic origin for the falconbridge twp intrusion and support inclusion in the east bull lake type suite of leucogabbroic plutons and sills in contrast the drury twp intrusion gives a u pb zircon age of 1859 13 ma coincident with the date of sic emplacement while the major and trace element compositions are comparable to the falconbridge data the drury displays significant heterogeneity in end2440 with values ranging from 3 7 to 0 1 and contains more radiogenic pb isotopic compositions field geochemical and isotopic evidence clearly distinguishes this intrusion from constituents of the sic itself and indicates that the drury too is a palaeoproterozoic intrusion this requires that apparently unshocked undeformed magmatic looking zircon has been grown or reset in a postmagmatic setting this has significant implications for the identification of mantle derived magmas and crustal remelts associated with large impact craters a resetting mechanism involving aggressive hydrothermal alteration of zircon facilitated by halogen complexing is proposed inducing rapid postshock lead loss and subsequent annealing

rate and historical change of direct recharge from precipitation constrained by unsaturated zone profiles of chloride and oxygen 18 in dry river bed of north china plain quantifying of direct recharge derived from precipitation is crucial for assessing sustainability of well irrigated agriculture in the north china plain the land use is dominated by groundwater irrigated farmland where the direct recharge derived from precipitation and irrigation to characterize the mean rate and historical variance of direct recharge derived from precipitation unsaturated zone profiles of chloride and d18o in the dry river bed of the beiyishui river were employed the results show that archival time scale of the profile covers the duration from 1980 to 2002 corresponding to depths from 5 to 2 m which is indicated by matching the d18o peaks in the isotope profile with the aridity indexes gained by instrumental records of annual precipitation and annual potential evaporation using the chloride mass balance method the mean rate of the direct recharge corresponding to the archival time scale is estimated to be 3 8 0 8 mm year 1 which accounts for about 0 7 of the long term average annual precipitation further the direct recharge rates vary from 2 1 to 6 8 mm year 1 since 1980 despite the subhumid climate the estimate of recharge rates is in line with other findings in semiarid regions the low rate of direct recharge is considered as a result of the relative dry climate in recent decades in dry river bed unsaturated zone profiles of chloride and d18o combined with instrumental records could offer valuable information about the direct recharge derived from precipitation during droughts copyright c 2011 john wiley sons ltd

evolution of the sublayer of the sudbury igneous complex geochemical sm nd isotopic and petrologic evidence abstract the mineralized sublayer at the base of the sudbury igneous complex sic consists of two variants the noritic contact sublayer and radial and concentric quartz dioritic offset dykes both are characterized by the presence of significant quantities of ni cu pge sulphides and by a prominent population of recrystallized diabasic textured and melanocratic to ultramafic fragments the two variants of the sublayer contain compositionally distinct inclusion populations and inclusion bearing matrices contact sublayer and offset dykes hosted by north range granitoid footwall can be distinguished from those hosted by south range basaltic and metasedimentary footwall environments the compositional variation in sic rocks can be described in terms of contributions from exposed crustal rocks and differentiation of the resultant melt s the basaltic inclusion population is characterized by hornfels recrystallization of the plagioclase and is geochemically and isotopically identified with huronian basalts which comprise the south range footwall with ce yb n ratios of around 2 5 and e nd 1850 between 2 to 5 the melanocratic inclusions in the sublayer are typically coarse grained and undeformed with incompatible element contents and radiogenic isotopic compositions intermediate between those of the basaltic inclusions and those of the melt sheet which has ce yb n ratios of around 10 and e nd 1850 around 9 calculated crystallization models are consistent with derivation of the ultramafic inclusions by crystallization from a magma produced by mixing of molten basaltic footwall with basal melt sheet it is proposed that the sublayer appeared as the marginal facies of a meteorite impact melt sheet as a result of footwall melting following the impact this basal layer was progressively enriched in sulphides and mafic cumulates from above through differentiation during cooling offset dykes were emplaced and the magmas effectively removed from the system subsequently continued evolution of the marginal facies produced the more mafic inclusions in the contact sublayer no extracrustal e g mantle component is envisioned in this model to explain the silicate compositional distributions in the sic and mafic crustal rocks in the target zone are implicated as the metal source for the sic deposits

delimitation of ground failure zones due to land subsidence using gravity data and finite element modeling in the queretaro valley mexico abstract sedimentary basins of arid and semiarid zones are often subject to regimes of intense ground water withdrawal as it is normally the only source of water for development of communities an associated phenomenon is land subsidence which can develop to ground failures and consequently damage to infrastructure aquifer deformation can be analyzed using a stress strain or a flux force approach depending on the aquifer material compact or loose and on whether the water withdrawal forms a predominant flow direction toward a cone of depression geometry of the aquifer system also plays an important role as uneven thickness induces differential compaction and hence tensional and shear stresses on the ground mass in this work we present a stress strain approach to analyze subsidence for an unconfined aquifer of varying thickness this is done in two steps namely when the aquifer is in equilibrium and when it is totally depleted using this scheme in a region where ground failure is evident a portion of the aquifer system of the queretaro valley is analyzed the geometry of the hydrologic basement is first modeled using gravity measurements properly correlated with wells and field data then a stress analysis is implemented using the finite element method in order to identify probable zones of ground weakness which are calibrated with known ground failures the results indicate that both tensional and shear stress are present which induce ground failure in the form of surface faults

gondwana break up and the northern margin of the saxothuringian belt variscides of central europe at the northwestern edge of the hercynian bohemian massif saxothuringian belt new u pb zircon age data from rift related magmatic rocks indicate that the initiation of gondwana break up in this area started during the middle to upper cambrian magmatic rocks from a bimodal morb to within plate volcanic sequence in the vesser area are dated between ca 517 and 501 ma the volcaniclastic sequences analysed exhibit basal layers of conglomerates and mature sandstones which can be correlated with a widespread gondwana derived onlap horizon of an uppermost cambrian tremadocian age that links the vesser area with the saxothuringian continental basin the association of the vesser rocks with the saxothuringian terrane as part of the armorican terrane assemblage is further demonstrated by a coeval magmatic development and by identical detrital components which are derived from a common cadomian basement white mica with a ca 539 ma k ar minimum age and inherited zircon signatures the vesser unit situated between the nw margin of the saxothuringian zone and the mid german crystalline zone probably represents a n facing remnant of an ocean continent transition of the or within the armorican terrane assemblage and involves sections of the early break up process at the peri gondwanan shelf south of the rheic ocean

neoarchean trondhjemitic and tonalitic orthogneisses identified within the northern grenville province in ontario by precise upb dating and petrologic studies abstract integrated field petrologic and geochronological studies reveal the presence of heretofore unrecognized neoarchean trondhjemitic and tonalitic orthogneisses north and northwest of lake nipissing in the northern grenville province of ontario the high al type trondhjemitic orthogneiss termed warren trondhjemite forms several intrusions with a total exposed area of 300 km 2 the tonalitic orthogneiss including the tilden tonalite forms a discontinuous zone of intrusions about 20 km long and up to 1 6 km wide both trondhjemitic and tonalitic orthogneisses are geochemically distinct from paragneiss that forms the country rocks precise u pb zircon and titanite dating of the warren trondhjemite at six locations and of the tilden tonalite at one location indicates that the former was emplaced at 2679 2 ma whereas the latter was intruded at 2669 4 ma a 996 to 975 ma metamorphism shown by the lower intercept ages of concordia plots of all samples caused partial pb loss from primary neoarchean zircons and titanites and also induced new growth of zircons in a local pegmatitic patch and of titanite in orthogneisses intrusive relationships between the orthogneiss and the country rock paragneiss indicate that the paragneiss is also archean in age the orthogneiss and the paragneiss together constitute an archean crustal sequence southeast of the grenville front boundary fault within the ontario segment of the grenville province this archean sequence resembles the pontiac subprovince of the superior province in western quebec in several important ways including the dominance of compositionally similar metasediments comparable depositional ages of the metasediments and contemporaneous granitoid plutonism at about 2680 to 2670 ma differences also exist between the two for example the major granitoids in the pontiac subprovince are generally more k rich

changes in flow and chemistry of groundwater heavily affected by human impacts in the baiyangdian catchment of the north china plain to identify impacts of air pollution sewage drainage agricultural production over pumping and reservoir storage on groundwater a field survey was conducted in the baiyangdian catchment of the north china plain major ions and water isotopes were measured results show that hydrological processes and hydrogeochemical evolution of shallow groundwater were greatly disturbed by human activities excessive pumping resulted in significant declines of groundwater levels over the study area this also induced infiltration of surface water into groundwater a groundwater depression cone was the conflux center of groundwater surrounded by recharge zones including alluvial fans and surface water in alluvial plain pumping almost was the only way to discharge groundwater emission of so x and no x contributed at least 11 of rock weathering by dissolving into infiltrating precipitation surface waters containing sewage replenished ambient groundwater with an average mixing ratio of 74 17 due to groundwater level drawdown as a result groundwater had elevated concentrations of na and so4 2 with na exchanged into aquifer sediments about 29 16 of na was exchanged from groundwater into soil matrix agriculture nitrate was high only in the recharge zones the most important result is that the transformation of the study area from a place rich in water resource into an area lack of water just took several decades with the joint action of the heavily human activities our study also indicates that shallow groundwater could sensitively respond to and record environmental changes

the late variscan control on the location and asymmetry of the upper rhine graben the nne trending upper rhine graben urg of the european cenozoic rift system developed from c 47 ma onwards in response to changing lithospheric stresses in the northwestern foreland of the alps the composite graben structure consists of three segments each c 100 km long and 30 40 km wide but flares to c 60 km near its southern and to c 80 km near its northern termination normal faulting induced a total extension of 5 8 km of the 1 2 km thick mesozoic sedimentary franconian platform and underlying variscan basement rocks however distribution of an up to 3 5 km thick sedimentary graben fill and cumulative displacements near eastern and western main border fault systems suggest that subsidence of the graben floor and shoulder uplift created strong cross sectional asymmetries cumulative w down displacements 3 km along strongly segmented transfer faults in the east contrast with e down displacements 600 km long zone that delimits the trailing edge of a sw moving lithospheric block in the urg area ne sw oriented seismic anisotropy at sublithospheric depths of c 60 80 km suggest active mantle flow in this direction as a possible driving force for the reactivation of pre graben lithospheric shear zones

the hydrothermal stability of zircon preliminary experimental and isotopic studies experimental investigations of the stability of the u pb isotopic system in nonmetamict zircons show that appreciable losses of pb and u can be induced at amphibolite grade conditions 400 c to 600 c 4 to 6 kb in 2 m nacl and 2 hno sub 3 solutions the severity of u loss and to a lesser extent pb loss varies with solution composition in this case the 2 m nacl solution induced more pb and u loss than the 2 hno sub 3 solution at the same p t conditions scanning electron microscopy of the run products also revealed a range of corrosion related surface features which suggests that some of the observed trends in pb and u loss must be attributed to zircon dissolution backscattered electron bse imaging of the run products further suggests that partial homogenization of chemical zoning patterns occurred during the experiments microprobe analyses of treated and untreated grain show that both populations have a similar range of hf contents thus the apparent loss of sharp well defined zoning features is most likely due to small scale smearing out of formerly sharp chemical gradients and is perhaps related to the annealing of lattice defects caused by alpha recoil damage thus experimentally induced u pb isotopicmore discordance in zircon is a complex function of zircon stability and annealing effects less

polyphase zircon in ultrahigh temperature granulites rogaland sw norway constraints for pb diffusion in zircon shrimp u pb ages have been obtained for zircon in granitic gneisses from the aureole of the rogaland anorthosite norite intrusive complex both from the ultrahigh temperature uht 900 c pigeonite in zone and from outside the hypersthene in isograd magmatic and metamorphic segments of composite zircon were characterised on the basis of electron backscattered electron and cathodoluminescence images plus trace element analysis a sample from outside the uht zone has magmatic cores with an age of 1034 7 ma 2 n 8 and 1052 5 ma 1 n 1 overgrown by m1 metamorphic rims giving ages between 1020 7 and 1007 5 ma in contrast samples from the uht zone exhibit four major age groups 1 magmatic cores yielding ages over 1500 ma 2 magmatic cores giving ages of 1034 13 ma 2 n 4 and 1056 10 ma 1 n 1 3 metamorphic overgrowths ranging in age between 1017 6 ma and 992 7 ma 1 corresponding to the regional m1 sveconorwegian granulite facies metamorphism and 4 overgrowths corresponding to m2 uht contact metamorphism giving values of 922 14 ma 2 n 6 recrystallized areas in zircon from both areas define a further age group at 974 13 ma 2 n 4 this study presents the first evidence from rogaland for new growth of zircon resulting from uht contact metamorphism more importantly it shows the survival of magmatic and regional metamorphic zircon relics in rocks that experienced a thermal overprint of c 950 c for at least 1 myr magmatic and different metamorphic zones in the same zircon are sharply bounded and preserve original crystallization age information a result inconsistent with some experimental data on pb diffusion in zircon which predict measurable pb diffusion under such conditions the implication is that resetting of zircon ages by diffusion during m2 was negligible in these dry granulite facies rocks imaging and th u y systematics indicate that the main processes affecting zircon were dissolution reprecipitation in a closed system and solid state recrystallization during and soon after m1

the geochemistry and petrogenesis of the agnew intrusion canada a product of s undersaturated high al and low ti tholeiitic magmas

recharge history and controls on groundwater quality in the yuncheng basin north china environmental isotopes and water quality indicators e g tds and no 3 contents were used to characterize the age recharge history and controls on the quality of groundwater resources in the yuncheng basin north china where extensive extraction occurs for agriculture and domestic supply delta 18 o and delta 2 h values as low as 10 6 and 73 parts per thousand respectively together with low radiocarbon activities 70 pmc indicating a significant component of modern post 1950s recharge the shallow groundwater has higher delta 18 o and delta 2 h values up to 8 1 and 54 parts per thousand that are similar to those in modern summer monsoon rainfall indicating that current recharge is via direct infiltration and or leakage of ephemeral streams during heavy rain events historic recharge rates estimated using radiocarbon ages are between 1 and 10 mm year corresponding to 20 mg l in some deep wells 180 m preferential leakage has occurred near the linyi fault in the northern sushui river basin salinisation nitrate contamination and excessive drawdown of deep groundwater are major concerns for the future of the groundwater resources in this region c 2010 elsevier b v all rights reserved

provenance deposition and age of gneiss units from the ktb drill hole germany evidence from shrimp and conventional u pb zircon age determinations in order to investigate the provenance deposition age and metamorphism of metasedimentary gneisses from the pilot and the main holes of the german continental deep drilling program ktb zircons from volcano metasedimentary v unit and monotonous greywacke g unit gneisses were dated by ion microprobe shrimp and conventional u pb zircon methods the zircons were found to consist of anhedral detrital or subhedral cores surrounded by one or two metamorphic rims analysis of growth zones in 37 zircons indicated ages for the detrital cores between 2 25 and 2 05 ga in a v unit gneiss sample k52 and 1 8 2 35 ga in two g unit gneisses k747 and k798 neo proterozoic 650 545 ma subhedral igneous cores were identified in all investigated ktb gneiss samples and an additional two zircon growth episodes in the source area of the sediments at 712 15 and 525 11 ma were also recognised zircon overgrowths dated at 487 13 ma are interpreted to have formed during an early metamorphic episode following incorporation of the zircons into their host metasediments a second metamorphic overprint indicated by a date of 381 22 11 ma was determined for zircon overgrowth in the g unit gneisses the dates determined for zircon growth phases suggest that the ktb metasediments were derived from northern gondwana hypothetical provenance areas and a new model of terrane positions that is consistent with the geochronological geodynamic and palaeo geographic preconditions at the northern border of gondwana in late precambrian to early ordovician times are proposed

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

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

controls on the formation of contact type platinum group element mineralization in the east bull lake intrusion the east bull lake intrusion is the type example of a series of 2 48 to 2 49 ga layered mafic intrusions that developed within the rifted southern margin of the superior craton central ontario the east bull lake intrusion contains contact type pge mineralization e g strata bound pge rich disseminated sulfide mineralization occurring within 200 to 300 m of the contact with footwall and sidewall units although the mineralization locally occurs in massive plagioclase cumulates anorthosite zone most of the sulfides are concentrated within an underlying heterolithic inclusion rich unit inclusion bearing zone that grades into an intrusive breccia disseminated sulfides also occur in fine grained gabbroic veins within brecciated footwall rocks border zone geochemical characteristics of the contact type sulfide mineralization and its host rocks include 1 high average pd abundances throughout the east bull lake intrusion including an average of ca 200 ppb in the lower series 2 low average s contents typically contact type pge mineralization in the east bull lake intrusion reflects flow enhanced gravitational settling of a cu rich sulfide liquid followed by local fractionation of ni rich monosulfide solid solution to produce minor amounts of cu and pt rich residual sulfide liquid most of the observed compositional variation in the sulfides can be modeled using variable silicate liquid sulfide liquid mass ratios field and geochemical observations suggest that the parental magmas to the mineralized parts of the east bull lake intrusion were s saturated and pge rich at the time of their emplacement these fertile parent magmas most likely originated as buoyant feldspathic residual magmas that escaped from a deeper more primitive subchamber although first described from the east bull lake intrusion contact type pge mineralization is now recognized in all of the major east bull lake suite intrusions similar styles of magmatic sulfide mineralization are developed in the contact environments of the bushveld igneous complex platreef deposits and the duluth complex minnesota e g dunka road deposit these contact type deposits are part of the family of basal accumulation magmatic sulfide deposits the economics of low sulfide tenor contact type pge cu ni mineralization such as that developed in the east bull lake intrusion will be most favorable in systems in which 1 the parent magmas possessed high initial pge tenors and reached s saturation prior to emplacement 2 the parent magmas experienced vigorous convection in the early stages of chamber development and 3 segregated sulfide was not diluted by coprecipitating silicate minerals a scenario that is most likely to occur in feldspathic magmas in which plagioclase is the principal high temperature liquidus mineral

protracted coeval crust and mantle melting during variscan late orogenic evolution u pb dating in the eastern french massif central the late stages of the variscan orogeny are characterized by middle to lower crustal melting and intrusion of voluminous granitoids throughout the belt which makes it akin to hot orogens these processes resulted in the development of large granite migmatite complexes the largest of which being the 305 300 ma old velay dome in the eastern french massif central fmc this area also hosts a wide range of late variscan plutonic rocks that can be subdivided into four groups i cordierite bearing peraluminous granites cpg ii muscovite bearing peraluminous granites mpg iii k feldspar porphyritic calc alkaline granitoids kcg and iv mg k rich monzo diorites and lamprophyres vaugnerites new results of la sf icp ms u pb zircon and monazite dating on 33 samples from all groups indicate that both granites and mafic rocks emplaced together over a long period of 40 million years throughout the carboniferous as shown by intrusion ages between 337 4 1 0 and 298 9 1 8 ma for the granitoids and between 335 7 2 1 and 299 1 1 3 ma for the vaugnerites low zircon saturation temperatures and abundant inherited zircons with predominant late ediacaran to early cambrian ages indicate that the cpg and mpg formed through muscovite or biotite dehydration melting of ortho and paragneisses from the lower gneiss unit the kcg and vaugnerites contain very few inherited zircons if any suggesting higher magma temperatures and consistent with a metasomatized lithospheric mantle source for the vaugnerites the kcg can be explained by interactions between the cpg mpg and the vaugnerites or extensive differentiation of the latter the new dataset provides clear evidence that the eastern fmc was affected by a long lived magmatic episode characterized by coeval melting of both crustal and mantle sources this feature is suggested here to result from a lithospheric scale thermal anomaly triggered by the removal of the lithospheric mantle root the spatial distribution of the dated samples points to a progressive southward delamination of the lithospheric mantle perhaps in response to rollback following continental subduction or to retro delamination owing to the retreat of a south verging subduction zone

deducing the ancestry of terranes shrimp evidence for south america derived gondwana fragments in central europe we present here an example of how the sensitive high resolution ion microprobe shrimp zircon dating method can provide a terrane specific geochronological fingerprint for a rock and thus help to reveal major tectonic boundaries within orogens this method applied to inherited zircons in a ca 580 ma metagranitoid rock from the eastern bohemian massif has provided for the first time in the central european variscan basement unequivocal evidence for mesoproterozoic and late paleoproterozoic geologic events ca 1 2 ga 1 5 ga and 1 65 1 8 ga the recognition of such zircon ages has important consequences because it implies that parts of the precambrian section of variscan central europe were originally derived from a grenvillian cratonic province as opposed to the common assumption of an african connection a comparison with previously published shrimp data suggests however that these mesoproterozoic and late paleoproterozoic zircon ages may be restricted to the moravo silesian unit in the eastern variscides whereas the saxothuringian and moldanubian zones appear to contain a typical north african i e neoproterozoic plus eburnian inherited zircon age spectrum this finding supports new tectonic concepts according to which variscan europe is composed of a number of completely unrelated terranes with extremely different paleogeographic origins the moravo silesian unit can be best interpreted as a peri gondwana terrane which was situated in the realm of the amazonian cratonic province by the late precambrian comparable to the avalonian terranes of north america and the united kingdom

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

zircon ages and nd isotopic and chemical compositions of orthogneisses from the black forest germany evidence for a cambrian magmatic arc single zircon u pb dating combined with 207pb 206pb ages obtained by the evaporation method constrains the emplacement of tonalitic trondhjemitic and granodioritic orthogneisses of the moldanubian zone in the black forest between 500 and 510 ma two detrital zircon populations of 1 9 and 1 6 ga indicate early middle proterozoic material in the former setting of the basement the initial end values range from 0 1 to 3 4 and mean crustal residence ages of 1 0 1 4 ga are consistent with involvement of early middle proterozoic crust and a subordinate juvenile component probably originating from subduction related melting of the mantle the orthogneisses have fractionated ree patterns and slightly higher k2o na2o ratios than typical low k tonalite trondhjemite granite suites the chemical data are interpreted as evidence for melting of amphibolite and contributions from evolved crust the emplacement of the orthogneisses was superceded by a high temperature metamorphic event at 480 ma which we interpret as a result of lithospheric thinning in a marginal basin behind a cambrian magmatic arc

gondwana derived microcontinents the constituents of the variscan and alpine collisional orogens abstract the european variscan and alpine mountain chains are collisional orogens and are built up of pre variscan building blocks which in most cases originated at the gondwana margin such pre variscan elements were part of a pre ordovician archipelago like continental ribbon in the former eastern prolongation of avalonia and their present day distribution resulted from juxtaposition through variscan and or alpine tectonic evolution the well known nomenclatures applied to these mountain chains are the mirror of variscan resp alpine organization it is the aim of this paper to present a terminology taking into account their pre variscan evolution at the gondwana margin they may contain relics of volcanic islands with pieces of cadomian crust relics of volcanic arc settings and accretionary wedges which were separated from gondwana by initial stages of rheic ocean opening after a short lived ordovician orogenic event and amalgamation of these elements at the gondwanan margin the still continuing gondwana directed subduction triggered the formation of ordovician al rich granitoids and the latest ordovician opening of palaeo tethys an example from the alps external massifs illustrates the gradual reworking of gondwana derived pre variscan elements during the variscan and alpine tertiary orogenic cycles

numerical evaluation of volumetric weighted mean transmissivity estimates in laterally heterogeneous aquifers summary most previous investigations to evaluate the effective or average transmissivity in heterogeneous environments have used calculations based on areas to weight the effects of each heterogeneity analysis of spatial volumetric variations within the cone of depression expressed at the potentiometric surface offers a more general solution to evaluate the meaning of transmissivity t and storativity s values derived from aquifer tests in these environments the cooper jr h h jacob c e 1946 a generalized graphical method for evaluating formation constants and summarizing well field history eos trans agu 27 4 526 534 method is used to demonstrate that t variations reflected in slope changes in plots of the pumping well drawdown data correspond to changes in the volumetric weighted mean transmissivity vwmt over time lognormal distributions of transmissivity are represented by block heterogeneities within two simulated aquifers for both spatially random and spatially correlated data sets by analyzing the volumetric evolution of the cone of depression observed in the potentiometric surface the nature of t averaging within the cone of depression as a function of time is illustrated volumetric analysis shows that the average aquifer t varies with time as the cone of depression progressively envelops different heterogeneities the initial trend is controlled primarily by the heterogeneities directly surrounding the pumping center if steady shape conditions are not achieved late time vwmt values do not approach an asymptotic limit

upper ordovician palaeogeography of the bohemian massif implications for armorica summary the palaeogeographic position of armorica in the upper ordovician is still rather unclear due mainly to the lack of reliable palaeomagnetic data to help resolve this a palaeomagnetic study of caradocian and ashgillian sediments and volcanics of the barrandian basin in the central bohemian massif czech republic has been carried out a total of 29 sites were sampled including four from a diabase sill intruded into hirnantian age sediments thus providing a baked contact test three components of magnetization labelled a b and c have been resolved in addition to a low temperature component which corresponds to the present day direction of the local geomagnetic field d i 360 67 component a southerly and shallow before bedding correction is only sporadically present and corresponds to a remagnetization event of permo carboniferous age which is well known throughout hercynian europe component b present as stable endpoints in the sill and as a secondary component in the host rock sandstones yields an overall mean direction of 360 39 95 9 3 k 52 3 in situ and 355 21 95 9 3 k 52 3 after bedding correction component c identified as stable endpoints in sedimentary and volcanic rocks passes both a fold test and a baked contact test and is interpreted as being the primary direction of magnetization the overall mean direction for c is 183 59 95 9 5 k 30 3 nine sites after bedding correction which corresponds to a palaeopole position of 80 s 360 e this translates into palaeolatitudes of 40 s for the area studied in latest ordovician times but indicates large amounts of rotation up to 170 of the bohemian massif before final consolidation of hercynian europe although such large amounts of rotation are difficult to accommodate they are consistent with palaeomagnetic results obtained from lower ordovician and upper silurian rocks elsewhere in the barrandian basin if the bohemian massif is interpreted as being an integral part of the armorican microplate then these results indicate that prior to caradoc times armorica rifted away from the northern margin of gondwana and that by ashgillian times it was located at intermediate palaeolatitudes

determining sedimentation ages of high grade metamorphic gneisses by their palynological record a case study in the northern schwarzwald variscan belt germany in the northern schwarzwald moldanubian zone of the variscan belt paragneisses drilled from approximately 2500 m depth at the buhl 1 drilling site have experienced amphibolite facies metamorphism and were later partly transformed to mylonites under greenschist facies conditions completely graphitised acritarchs and chitinozoans were recovered from these paragneisses and mylonitic gneisses whereas both groups of palynomorphs give evidence of a marine depositional environment only the chitinozoans are stratigraphically useful in this case and indicate a paleozoic age previous findings of acritarchs in a different high grade metamorphic gneiss unit within the schwarzwald indicate an upper proterozoic marine sedimentation age the spectrum of palynomorphs found thus far in the schwarzwald in combination with petrological data suggests that within the moldanubian domain of the armorican microplate rocks with different sedimentary records later experienced diverging prograde metamorphic histories in general the results reveal that palynological studies in combination with petrological data are a promising tool for reconstructing pre orogenic settings in collisional belts

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

using major ions and stable isotopes to characterize recharge regime of a fault influenced aquifer in beiyishui river watershed north china plain a thorough understanding of recharge is usually a prerequisite for effective groundwater management the recharge regime remains unrevealed in the piedmont plain of the north china plain where is considered as the main recharge zone of the quaternary aquifer to characterize the recharge regime a case study is presented in the piedmont plain of the beiyishui river watershed the piedmont plain consists of proluvial fan and alluvial fan plain and groundwater quality in the two parts is distinct total dissolved solids and isotopic compositions are higher in the groundwater of proluvial fan than the groundwater of the alluvial fan plain local precipitation only recharges the groundwater in the proluvial fan fracture water ascending along a buried normal fault and lateral inflow from the proluvial fan feed the unconfined aquifer in the alluvial fan plain the runoff of the beiyishui river seeps in the streambed of upper reaches and then overflows seasonally in middle reach the recharge from middle reach to the aquifer is negligible in addition the fracture water originates from precipitation in the mountainous area with an average elevation about 500 m and discharges to the overlaying aquifer the contribution of fracture water to the aquifer was estimated to be 77 9 in the alluvial fan plain area due to the mixture in the aquifer major ions leached from soil of the proluvial fan are diluted and the hydrochemical pattern is changed from ca mg hco3 so4 to ca mg hco3 it is considered that fracture water is the major recharge source of the unconfined aquifer and the variation of precipitation in mountainous area would primarily affect the recharge this paper also shows the hydrological efficiency of buried normal fault as preferential flow

river valley pluton ontario a late archean early proterozoic anorthositic intrusion in the grenville province abstract the river valley pluton is a ca 100 km2 body of anorthositic and gabbroic rocks located about 50 km northeast of sudbury ontario the pluton is situated entirely within the grenville province but its western margin is a series of imbricate thrust faults associated with the grenville front tectonic zone it is dominated by coarse leuconorite and leucogabbro with lesser anorthosite gabbro and rare ultramafics igneous textured rocks are abundant and consist of plagioclase an60 70 charged with fe ti oxide inclusions low ca pyroxene orthopyroxene and or inverted pigeonite and augite the most unfractionated rocks are minor olivine gabbros with fo70 80 a variety of deformed and recrystallized equivalents of the igneous textured rocks is also present and these are composed largely of calcic plagioclase and hornblende ten samples including both igneous and deformed lithologies give a pb pb whole rock isochron of 2560 155ma which is our best estimate of the time of primary crystallization the river valley pluton is thus the oldest anorthositic intrusive yet reported from the grenville province but is more calcic and augitic than typical massifs and lacks their characteristic fe ti oxide ore deposits the river valley body may be more akin to similar gabbro anorthosite bodies situated at the boundary between the archean superior province and huronian supracrustal belt of the southern province west of the grenville front an sm nd isochron from 3 igneous textured leucogabbros and an augite mineral separate gives 2377 68 ma implying slight disturbance of the sm nd whole rock mineral system during later metamorphism the rb sr system has been substantially disturbed giving an age of 2185 105 ma which is similar to internal pb pb isochron ages of 2165 130 ma and 2100 35 ma for two igneous textured rocks it is uncertain whether these ages correspond to a discrete event at this time or represent a partial resetting of the rb sr and pb pb systems from a younger event such as the grenvillian orogeny of ca 1 0 ga none of the isotopic systems we investigated however gives an age near 1 0 ga suggesting that neither the river valley pluton nor the immediately surrounding gneisses were strongly affected by metamorphism associated with the grenvillian orogeny initial isotopic ratios for the river valley pluton correspond to single stage model parameters of 8 06 nd 0 to 3 and isr 0 7015 to 0 7021 collectively these suggest either an enriched mantle source or crustal contamination of a mantle derived magma the crustal component involved must have been older and more radiogenic than the majority of rocks exposed at the surface in the nearby superior province

the amazon laurentian connection as viewed from the middle proterozoic rocks in the central andes western bolivia and northern chile middle proterozoic rocks underlying the andes in western bolivia western argentina and northern chile and early proterozoic rocks of the arequipa massif in southern peru form the arequipa antofalla craton these rocks are discontinuously exposed beneath mesozoic and cenozoic rocks but abundant crystalline clasts in tertiary sedimentary rocks in the western altiplano allow indirect samples of the craton near berenguela western bolivia the oligocene and miocene mauri formation contains boulders of granodiorite augen gneiss 1171 20 ma and 1158 12 ma u pb zircon quartzose gneiss and granofels that are inferred to have arkosic protoliths 1100 ma source region u pb zircon quartzofeldspathic and mafic orthogneisses that have amphibolite and granulite facies metamorphic mineral assemblages 1080 ma metamorphism u pb zircon and undeformed granitic rocks of phanerozoic age the middle proterozoic crystalline rocks from berenguela and elsewhere in western bolivia and from the middle proterozoic belen schist in northern chile generally have present day low 206pb 204pb 15 57 and elevated 208pb 204pb 37 2 to 50 7 indicative of high time averaged th u values the middle proterozoic rocks in general have higher present day 206pb 204pb values than those of the early proterozoic rocks of the arequipa massif 206pb 204pb between 16 1 and 17 1 but lower than rocks of the southern arequipa antofalla craton 206pb 204pb 18 5 a difference inferred to reflect grenvillian granulite metamorphism the pb isotopic compositions for the various proterozoic rocks lie on common pb isotopic growth curves implying that pb incorporated in rocks composing the arequipa antofalla craton was extracted from a similar evolving pb isotopic reservoir evidently the craton has been a coherent terrane since the middle proterozoic moreover the pb isotopic compositions for the arequipa antofalla craton overlap those of the amazon craton thereby supporting a link between these cratons and seemingly precluding part of the arequipa antofalla craton from being a detached fragment of another craton such as eastern laurentia which has been characterized by a different u pb history pb isotopic compositions for the arequipa antofalla craton are furthermore distinct from those of the proterozoic basement in the precordillera terrane western argentina indicating a pb isotopic and presumably a tectonic boundary between them the pb isotopic compositions for the precordillera basement are similar to those of eastern laurentia and support other data indicating that these rocks are a detached fragment of north america finally the distinct pb isotopic evolution history of the arequipa antofalla craton and eastern laurentia require minor modification to tectonic models linking eastern north america scotland to the oroclinal bend in western south america

sr and nd isotope geochronology geologic history and origin of the adirondack anorthosite abstract we have analyzed samples from the adirondack marcy massif for rb sr and sm nd isotopes in an attempt to determine directly the primary crystallization age of a proterozoic massif type anorthosite rock suite the oldest age obtained 1288 36ma is from a 4 point sm nd isochron defined by igneous textured whole rock and mineral separate data from a local layered sequence gradational from oxiderich pyroxenite to leuconorite this age is older than silver s 1969 1113 ma zircon age of associated charnockites but is within the window of permissible anorthosite ages based on previous geochronology and field relationships as such 1288 ma may represent the time of crystallization of the massif for the most part however both sm nd and rb sr isotopic systems did not survive granulite facies metamorphism internal isochrons based on whole rocks and minerals yield ages between 995 and 919 ma these isotopic data suggest that the granulite fades metamorphism experienced by the massif was a prograde event that occurred a minimum of 100 ma and as much as 350 ma after crystallization of the massif the relatively large range in rb abundance and in calculated initial 87 sr 86 sr 0 7039 0 7050 and 143 nd 144 nd ratios among anorthosite suite rocks particularly those at or near the contacts of the marcy massif is explicable by variable contamination with crustal materials and or fluids derived from surrounding acidic metaplutonic rocks paragneisses and marbles despite uncertainies caused by crustal contamination and metamorphic resetting of primary ages marcy samples have epsilon nd values between 0 44 and 5 08 implying a source for the massif with long term depletion in light rare earth elements a probable source material would be depleted mantle

40ar 39ar thermochronology of paleoproterozoic granitoids of northeast burkina faso west african craton implications for regional tectonics abstract the greenstone belts of northeast burkina faso in the west african craton wac comprise birimian meta sedimentary and mafic ultramafic complexes that are intruded by granitoids and late dolerite dykes the granitoid greenstone belt assemblages underwent at least two phases of deformation that have resulted in their principal nw and ne trends we performed a detailed 40ar 39ar dating together with zircon u pb ages in order to unravel the thermo chronological history of the studied granitoids our previous zircon u pb data indicated that the granitoids were emplaced between 2138 7 and 2181 7 ma this period is well known for crustal growth in the wac the emplacement of granitoids was concomitant with d1 or the tangaean event 2130 2170 ma amphibole and micas yield two 40ar 39ar age groups the first group 2046 12 2132 13 ma constrained the d2 deformation event and the second group 1918 24 to 1984 12 ma constrained the d3 or wabo tempelse event considering closure temperatures of the isotopic systems the u pb ages and the present 40ar 39ar dating results suggest that after emplacement the granitoids cooled to 315 c at ca 2020 ma with a mean cooling rate of 6 5 c ma corresponding to a mean uplift rate of 0 65 km ma the additional k feldspar 40ar 39ar data indicate a later slow cooling stage with a rate of 0 54 c ma corresponding to a mean uplift rate of 0 054 km ma subsequent to the end of the major crustal growth in the wac at 2000 ma

internal structures of zircons from archaean granites from the darling range batholith implications for zircon stability and the interpretation of zircon u pb ages internal structures in zircons from granitoids from the late archaean darling range batholith show secondary features revealed by hf etching which record reconstitution of the zircons and modification of the distribution of trace elements during post crystallisation cooling of the granitoid zircons from the granites commonly contain unzoned to weakly zoned cores surrounded by rims showing oscillatory zoning which has been modified by recrystallisation the most striking feature is the development of high trace element concentration areas found in zircons from a number of granites these structures range from enhanced trace element concentrations in primary zones to a single accumulation of most trace elements in one band about half way between the outer edge and the centre of the zircon in any zircon the extent of the concentration of trace elements towards the formation of a single trace element band appears to be inversely related to the fading and broadening of primary oscillatory zones in the outer rim this suggests that the trace element bands formed by migration of trace elements from the outer primary zones to new concentration sites on an inner set of primary zones this explanation is supported by the formation of multiple curved trace element bands that transgress primary zoning and the determination of younger shrimp ages on depleted zircon outer rims compared to remnant primary oscillatory zoned areas of the zircon and unzoned centres also observed in some granite zircons is a finely convoluted zoning which overprints oscillatory zoning in parts of a zoned zircon and in rare cases occurs throughout the zircon this structure is explained in terms of secondary migration and reconcentration of trace elements in curved bands all structures can be transgressed by generally rounded lobes and patches of low u weakly nebulously zoned zircon this is interpreted as a late stage interaction between the zircon and fluids formed during cooling and crystallisation of the granitoid resulting in recrystallisation of affected parts of the zircon with accompanying loss of trace elements from the zircon

time calibration of a p t path from a variscan high temperature low pressure metamorphic complex bayerische wald germany and the detection of inherited monazite a temperature time path was constructed for high temperature low pressure ht lp migmatites of the bayerische wald internal zone of the variscan belt germany the migmatites are characterised by prograde biotite dehydration melting peak metamorphic conditions of approximately 850 c and 0 5 0 7 gpa and retrograde melt crystallisation at 800 c the time calibration of the pressure temperature path is based on u pb dating of single zircon and monazite grains and titanite separates on 40ar 39ar ages obtained by incremental heating experiments on hornblende separates single grains of biotite and k feldspar and on 40ar 39ar spot fusion ages of biotite determined in situ from sample sections additionally crude estimates of the duration of peak metamorphism were derived from garnet zoning patterns suggesting that peak temperatures of 850 c cannot have prevailed much longer than 2 5 ma the temperature time paths obtained for two areas approximately 30 km apart do not differ from each other considerably u pb zircon ages reflect crystallisation from melt at 850 800 c at 323 ma southeastern area and 326 ma northwestern area the u pb ages of monazite mainly coincide with those from zircon but are complicated by variable degrees of inheritance the preservation of inherited monazite and the presence of excess 206pb resulting from the incorporation of excess 230th in monazite formed during ht lp metamorphism suggest that monazite ages in the migmatites of the bayerische wald reflect crystallisation from melt at 850 800 c and persistence of older grains at these temperatures during a comparatively short thermal peak the u pb ages of titanite 321 ma and 40ar 39ar ages of hornblende 322 316 ma and biotite 313 309 ma reflect cooling through the respective closure temperatures of approximately 700 570 500 and 345 310 c published in the literature most of the feldspars ages 305 296 ma probably record cooling below 150 300 c while two grains most likely have higher closure temperatures the temperature time paths are characterised by a short thermal peak by moderate average cooling rates and by a decrease in cooling rates from 100 c my at temperatures between 850 800 and 700 c to 11 16 c my at temperatures down to 345 310 c further cooling to feldspar closure for ar was probably even slower the lack of decompressional features the moderate average cooling rates and the decline of cooling rates with time are not easily reconciled with a model of asthenospheric heating rapid uplift and extension due to lithospheric delamination as proposed elsewhere instead the high peak temperatures at comparatively shallow crustal levels along with the short thermal peak require external advective heating by hot mafic or ultramafic material

geochronology of high pressure mafic granulite dykes in sw sweden tracking the p t t path of metamorphism using hf isotopes in zircon and baddeleyite although the u pb zircon chronometer has been widely used for dating metamorphism in moderate to high grade rocks it is generally difficult to link the u pb age of zircon to specific metamorphic reactions in this study the initial hf isotopic composition of secondary zircon is compared with the evolution of hf isotopic composition of the bulk sample back projected from the measured value through time this approach may enhance the interpretation of radiometric ages performed on metamorphic mineral assemblages here u pb sm nd and lu hf geochronology and thermobarometry have been integrated and applied to two metamorphosed diabase dykes in the sveconorwegian orogen sw sweden the dykes are located 5 km east of the nne trending gota alv deformation zone in the idefjorden terrane and trend parallel to this zone the lunden dyke is recrystallized into a coronitic granulite facies assemblage u pb isotopic analyses of baddeleyite in this dyke indicate an emplacement age of 1300 ma thermobarometric techniques applied to garnet and omphacitic clinopyroxene coronas indicate high pressure metamorphism at c 15 kbar and c 740 c growth of polycrystalline zircon at the expense of baddeleyite occurred at 1046 6 ma identical hf isotopic composition of polycrystalline zircon and baddeleyite shows that the baddeleyite to zircon transition took place before hf equilibration among the other metamorphic minerals and hence the c 1046 ma age of polycrystalline zircon sets an upper age limit of metamorphism of this sample the haregarden dyke is recrystallized into a granoblastic transitional upper amphibolite to granulite facies assemblage the estimated p t conditions are c 10 kbar and c 700 c analyses of small 30 m clear and round zircon in this sample yield a concordia u pb age of 1026 4 ma which is indistinguishable from the lu hf and sm nd mineral isochron ages of 1027 9 ma and 1022 34 ma this type of secondary zircon plots at the lower end of the lu hf isochron and indicates simultaneous growth with garnet at c 1026 ma a time when hf isotopic equilibrium among minerals must have been reached

a partisan review of proterozoic anorthosites most anorthosites of the massif type crystallized in the episode 1 7 1 2 gyr with a pronounced peak of the age distribution near 1 4 gyr they were emplaced anorogenically at depths as shallow as 7 km where the ambient temperature of country rocks was probably less than 250 c depths of emplacement may have been as great as 25 km or more in rare cases the greater depths of equilibration estimated from granulite facies metamorphism may be incorrectly interpreted as emplacement depths but in any case they are demonstrably not required or characteristic of anorthosite emplacement penetrative deformation and metamorphism of anorthosites are post emplacement accidents of the local geologic history and are not directly caused by the presence of anorthosites granitic rocks mangerite charnockite suite associated with anorthosite are in general later or contemporaneous products of crustal anatexis with chemical and isotopic signatures distinct from the anorthosites and their residua such granitic rocks should not therefore be summed with the anorthositic rocks to obtain bulk compositions the magmas that produced most anorthosites were dry as shown by high temperature mineralogy and anhydrous mineral assemblages in contact aureoles residua from their crystallization are ferrodiorites to ferrosyenites typical of closed system fractionation these residua were locally and frequently ejected into contemporaneously molten granite where they formed pillows and cooled rapidly the overall chemistry of anorthosites and residua is broadly tholeiitic and consistent with derivation from the mantle olivine bearing magmas locally ranged from leucotroctolite anorthosite to later but coexisting picrite or melatroctolite in the same pluton confirming a wide spectrum of magma types a signal feature of troctolitic and noritic magmas is their low augite content implying high content of spinel component large anorthosite complexes such as nain and harp lake consist of many plutons representing repeated injections of separate magma batches with varying chemistry the abundant true anorthosites richer in plagioclase than magmas cosaturated with a mafic phase must represent plagioclase enrichment by either mechanical or chemical processes or both the role of kinetics in nucleation and solidification of such rocks may be centrally important it is proposed that hyperfeldspathic plagioclase supersaturated liquids were generated by quasi isothermal extraction of mafic minerals from tholeiitic magma enroute to and at the site of emplacement and that such a kinetic process was uniquely permitted in an environment of aborted continental rifting anorthositic rocks may have much to say about the episodic versus continuous geochemical evolution of the earth s mantle

geochemistry of metasediments in the hercynian and pre hercynian crust of the schwarzwald the vosges and northern switzerland abstract several units in the pre hercynian gneiss basement and hercynian schists of the schwarzwald southwestern germany and northern switzerland and the vosges france were characterized geochemically the samples are attributed to possible sedimentary protoliths by means of their main constituents groups of analyses belonging to the defined protoliths are then compared by means of statistical procedures the mann whitney test and multiple discriminant analysis which reveal significant differences so that every area is characterized by a specific pattern of major and trace elements furthermore correlations between some units of the schwarzwald and the vosges are possible the gneisses of ste marie aux mines vosges can be closely correlated with those of the central schwarzwald whereas there is a distinct contrast between gneisses of the central schwarzwald and gneisses of the southern schwarzwald the schists of baden baden schwarzwald most probably have their counterpart in the schists of ville in the vosges

the origin of proterozoic massif type anorthosites evidence from interactions between crustal xenoliths and basaltic magma plagioclase rich reaction zones occur around numerous aluminous crustal xenoliths within a suite of palaeogene sub volcanic basic sheets on the isle of mull nw scotland the xenoliths consist of a glassy core containing mullite needles generated from the melting of pelitic source rocks thick plagioclase mantles grew at the interface between the aluminous liquid and the enclosing basaltic magma and provide a high level analogue for the petrogenesis of proterozoic massif type anorthosites similar interactions between mantle derived basic magmas ponded at the base of the crust and relatively al rich lower crustal lithologies would result in the precipitation of large volumes of plagioclase anorthosite massifs were then emplaced at higher crustal levels as crystal rich mushes within relatively juvenile proterozoic crust the model negates the need to crystallize large volumes of mafic minerals prior to the production of plagioclase saturated liquids and also accounts for the significant influence of crustal sources on the isotopic signatures of all members of the anorthosite suite

characteristics of aquifer system deformation in the southern yangtse delta china excessive groundwater pumpage has caused regional cones of depression and severe land subsidence in the southern yangtse delta china the characteristics of aquifer system compaction are complex because of the difference in the types compositions and structures of the soils that the hydrostratigraphic units are composed of and in the histories of groundwater level change the hydrostratigraphic units have experienced based on the data measured from extensometer groups and observation wells the characteristics of deformation for individual hydrostratigraphic units are analyzed the results show that different hydrostratigraphic units have different kinds of deformation and that an identical unit may also present different deformation characteristics such as elasticity elasto plasticity and visco elasto plasticity at different sites of the cone of depression or in different periods if the groundwater level rises continuously and remains constant later the aquifers and the aquitard units that consist of hard clay primarily exhibit elastic behaviour but the aquitard units that consist of soft clay deform plastically and by creep and exhibit visco elasto plastic behaviour if the groundwater level falls but is much higher than the previous lowest value aquifers and aquitards consisting of hard clay exhibit elasto plastic behaviour if the groundwater level falls below the previous lowest value aquifers and aquitards consisting of hard clay deform plastically and by creep and exhibit visco elasto plastic behaviour

mobility of arsenic in west bengal aquifers conducting low and high groundwater arsenic part i comparative hydrochemical and hydrogeological characteristics abstract the present study demonstrates the importance of hydrogeochemical characteristics groundwater flow and recharge of an aquifer in the release of as to groundwater the study area 20 km 2 is located in chakdaha block nadia district west bengal which hosts groundwaters of variable as content the spatial distribution pattern of as is patchy with areas containing groundwater that is high in as 200 g l 1 found in close vicinity to low as 1 groundwaters within 100 m the concentration of groundwater as is found to decrease with depth in addition the data shows that there is no conspicuous relationship between high groundwater as concentration and high groundwater abstraction although the central cone of depression has enlarged over 2 a and is extending towards the se of the study area the river hooghly which forms the nw boundary of the study site shows dual behaviour effluent and influent during pre and post monsoon periods respectively complicating the site hydrogeology the observed groundwater flow lines tend to be deflected away from the high as portion of the aquifer indicating that groundwater movement is very sluggish in the as rich area this leads to a high residence time for this groundwater package prolonging sediment water interaction and hence facilitating groundwater as release

a u pb age for mineralized nipissing diabase gowganda ontario concordant baddeleyite and rutile analyses from mineralized nipissing diabase at gowganda s castle mine yield a u pb age of baddeleyite is a primary phase of the diabase therefore the age dates the magmatic emplacement the genesis of rutile is less certain rutile could have formed during an episode of alteration veining and ag mineralization in which case the u pb age would also date the secondary event and demonstrate that ag mineralization was related to the nipissing magmatism however a primary magmatic origin of the rutile cannot be completely excluded for now thus the u pb age may not constrain the time of ag mineralization further research is needed to clarify this point

shrimp dating of zircons from high grade metasediments of the schwarzwald sw germany and implications for the evolution of the moldanubian basement four metasedimentary zircon populations from different tectonometamorphic units of the central and the northern schwarzwald variscan belt sw germany were investigated using sem cathodoluminescence and shrimp dating despite partially strong modifications of primary internal morphologies during variscan metamorphism at amphibolite 750 c 0 4 0 6 gpa and granulite facies conditions 950 1 000 c 1 4 1 8 gpa many grains show well preserved protolith ages the detritus indicates a northern gondwana origin and different palaeozoic episodes of sediment deposition and consolidation two of the studied sediments were deposited in cambrian early ordovician times and consolidated in positions close to northern gondwana late ordovician and rare devonian detritus from sediments of two other tectonometamorphic units indicates much later sedimentation close to the leading edge of gondwana or a terrane assemblage during northern drift towards laurussia subsolidus growth of new zircon due to variscan granulite facies metamorphism of one of the tectonometamorphic units is precisely dated at 335 2 ma

presence of palaeoproterozoic and archean components in the granulite facies rocks of central iberia the hf isotopic evidence abstract hf isotope analyses in previously dated zircon crystals from four variscan granulite facies rocks from central iberia have been studied to examine crustal evolution processes two samples are from exposed migmatite terranes the other two are granulite xenoliths from lower crustal levels hf model ages show that lower crustal granulites display a bigger contribution of archean components than exposed high grade rocks 14 of the zircon population instead of 6 reinforcing ideas of preservation of ancient crustal material in the lower crust the distribution of age hf isotope data from zircon suggests two juvenile crust formation events in central iberia the main one during the neoproterozoic 540 620 ma and a possible second mesoproterozoic event around 1 0 1 2 ga magmatism appears in the central iberian zone exclusively related to that younger juvenile addition hf data of granulite zircons agrees with recent palaeogeographical reconstruction models which present central iberia as a peri gondwanan terrane located between the west african craton and the arabian shield far from the amazonia microplate

nd model age mapping of the southeast margin of the archean foreland in the grenville province of ontario nd model ages are used to map the southeast margin of superior southern province basement within the grenville province of ontario this boundary located 60 km southeast of the grenville front is tentatively identified as a collisional suture between an archean foreland to the northwest and a proterozoic island are of penokean age to the southeast

p t evolution of a variscan lower crustal segment a study of granulites from the schwarzwald germany pressure temperature time p t t paths of orogenic granulites introduction provide important information on the thermal and chemical structure general background of the lower continental crust through time and constraints on tectonic granulites are typical rocks of the earth s middle to processes we present the first detailed petrological investigation of lower crust under high temperature conditions they are granulites from the variscan schwarzwald pelitic granulites from the found as xenoliths in basaltic volcanic rocks mainly central schwarzwald gneiss complex csgc are characterized by within continental rifts but most granulites occur as the peak assemblage garnet rutile kyanite antiperthite complexes or terranes in orogenic settings orogenic quartz felsic to intermediate granulites from the southern granulites display a wide compositional range harley schwarzwald gneiss complex ssgc exhibit different peak as1989 1998 and are known from a variety of collisional semblages with clinopyroxene orthopyroxene ternary feldspar garnet belts that formed during different episodes since the quartz and sillimanite and manifold retrograde reaction textures archaean e g harley 1998 clarke et al 2000 moller peak p t conditions were calculated by two feldspar thermometry et al 2000 determination of bulk rock and mineral garnet orthopyroxene thermometry and various geobarometers minicompositions calculation of peak equilibration conmum estimates for peak conditions are 950 1010 c and 1 4 1 8 ditions and dating of orogenic granulites thus provide us gpa for the granulites of the csgc which followed a clockwise with important constraints on the thermal and chemical p t path the retrograde path is characterized by initial isothermal structure of the earth s continental crust at different decompression associated with partial melting followed by isobaric geological times cooling peak conditions for the ssgc are 1015 c and 1 5 gpa harley 1989 showed that equilibration conditions minimum temperature maximum pressure no prograde relics are deduced from natural orogenic granulites cover a wide preserved and isothermal decompression was less pronounced than range in particular pressures are highly variable and in the csgc other variscan hp ht granulites from central the granulite field may be divided into a low pressure europe show similar lithologies equilibration temperatures and lp a medium pressure mp and a high pressure hp ages 340 335 ma the heat for widespread high temperature facies according to green ringwood 1967 peak metamorphism in the variscan lower crust could have been supplied temperatures of many granulite terranes scatter around by repeated intrusion of subduction related basic magmas rapid 800 c bohlen 1987 but an increasing number of near isothermal decompression of the granulites may have been ultra high temperature uht granulite complexes facilitated by considerable volumes of partial melt and by orogenic extension 900 1100 c and 0 7 1 3 gpa harley 1998 are being recognized e g harley hensen 1990 dasgupta et al 1995 klemd brocker 1999 hokada 2001 regarding retrograde p t paths harley 1989 distinguished two

from cadomian subduction to early palaeozoic rifting the evolution of saxo thuringia at the margin of gondwana in the light of single zircon geochronology and basin development central european variscides germany

geodynamic evolution of the european variscan fold belt palaeomagnetic and geological constraints the variscan fold belt of europe resulted from the collision of africa baltica laurentia and the intervening microplates in early paleozoic times over the past few years many geological palaeobiogeographic and palaeomagnetic studies have led to significant improvements in our understanding of this orogenic belt whereas it is now fairly well established that avalonia drifted from the northern margin of gondwana in early ordovician times and collided with baltica in the late ordovician early silurian the nature of the gondwana derived armorican microplate is more enigmatic geological and new palaeomagnetic data suggest armorica comprises an assemblage of terranes or microblocks palaeobiogeographic data indicate that these terranes had similar drift histories and the rheic ocean separating avalonia from the armorican terrane assemblage closed in late silurian early devonian times an early to mid devonian phase of extensional tectonics along this suture zone resulted in formation of the relatively narrow rhenohercynian basin which closed progressively between the late devonian and early carboniferous in this contribution we review the constraints provided by palaeomagnetic data compare these with geological and palaeobiogeographic evidence and present a sequence of palaeogeographic reconstructions for these circum atlantic plates and microplates from ordovician through to devonian times

sm nd isotopic evidence for crustal contamination in the ca 1750 ma wanapitei complex western grenville province ontario the wanapitei complex consists of a variably metamorphosed gabbronorite lying immediately adjacent to the grenville front boundary fault u pb zircon geochronology indicates a crystallization age of for a noritic component with both grenville aged ca 1000 ma metamorphism and minor older inheritance indicated geochemical evidence is consistent with plagioclase pyroxene fractionation but indicates additional open system behaviour data indicate contamination of a depleted mantle by light rare earth element enriched material during the penokean this was followed during emplacement by extensive contamination of the then isotopically near chondritic magma with variable amounts of evolved archean crustal material on the order of 40 in extreme cases generating end 1 75 values between 0 and 7 5

neoproterozoic to early cambrian history of an active plate margin in the tepla barrandian unit a correlation of u pb isotopic dilution tims ages bohemia czech republic abstract the tepla barrandian unit tbu of the bohemian massif shared a common geological history throughout the neoproterozoic and cambrian with the avalonian cadomian terranes the neoproterozoic evolution of an active plate margin in the tepla barrandian is similar to avalonian rocks in newfoundland whereas the cambrian transtension and related calc alkaline plutons are reminiscent of the cadomian ossa morena zone and the armorican massif in western europe the neoproterozoic evolution of the tepla barrandian unit fits well with that of the lausitz area saxothuringian unit but is significantly distinct from the history of the moravo silesian unit the oldest volcanic activity in the bohemian massif is dated at 609 17 19 ma u pb upper intercept subduction related volcanic rocks have been dated from 585 7 to 568 3 ma lower intercept rhyolite boulders which pre dates the age of sedimentation of the cadomian flysch st chovice group accretion uplift and erosion of the volcanic arc is documented by the neoproterozoic dob is conglomerate of the upper part of the flysch the intrusion age of 541 7 8 ma from the zgorzelec granodiorite is interpreted as a minimum age of the neoproterozoic sequence the neoproterozoic crust was tilted and subsequently early cambrian intrusions dated at 522 2 ma t sovice granite 524 3 ma vsepadly granodiorite 523 3 ma smr ovice tonalite 523 1 ma smr ovice gabbro and 524 0 8 ma orlovice gabbro were emplaced into transtensive shear zones

silurian paleogeography of armorica new paleomagnetic data from central bohemia due to the lack of reliable paleomagnetic data from armorica for mid ordovician to late devonian times the drift history for this tectonic element is poorly understood in order to help rectify this problem a paleomagnetic study of upper silurian volcanic intrusive and sedimentary rocks from the barrandian basin of the central bohemian massif czech republic has been carried out this basin comprises a sequence of unmetamorphosed lower paleozoic rocks the folding of which has an uppermost age limit of early carboniferous stepwise thermal demagnetization of the samples collected 182 samples from 21 sites and principal component analysis of both great circle and stable endpoint data reveal a total of three directions of magnetization the first labelled a is in the direction of the present day local earth s magnetic field d i 360 67 and is thus interpreted as being of recent origin the other two directions termed b and c comprise the intermediate and high blocking temperature directions respectively the overall in situ mean direction obtained for b identified between 150 and 320 c is 195 8 d i k 18 5 95 13 2 eight sites it fails the fold test and yields an in situ paleo south pole position of 42 s 354 e this coincides with the late carboniferous segment of the apparent polar wander apw path for stable europe and is considered to be a secondary overprint of this age the third magnetization termed c is identified between 320 and 600 c gives an overall mean direction of 208 40 k 96 8 95 5 7 eight sites after bedding correction it passes fold and reversal tests on both local and regional levels and is interpreted as representative of the late silurian paleofield direction in the bohemian massif it yields a paleo pole position of 55 s 325 e assuming a normal polarity the inclination value obtained translates into paleolatitudes of 23 s for this part of armorica and implies a position adjacent to the southern margin of baltica this therefore indicates closure of the intervening ocean by late silurian times however adopting this polarity also implies large scale up to 140 anticlockwise rotations of the bohemian massif prior to deformation in the early carboniferous

34s and 18o in dissolved sulfate as tracers of hydrogeochemical evolution of the triassic carbonate aquifer exposed to intense groundwater exploitation olkusz zawiercie region southern poland abstract the objective of this study was to determine the sources of so 4 2 in groundwater of the olkusz zawiercie major groundwater body the quality of groundwater was relatively good in the past but fluctuations of the water table level have caused degradation of water quality variations in the water table level and the formation of the depression cone have resulted from both groundwater withdrawal and zn pb mine dewatering as a result within the extended vadose zone of the aquifer weathering of pyrite and accompanying sulfides has taken place since 1992 the water table has risen and this process has been associated with an increase in concentrations of so 4 2 ca and mg at the same time the ph has decreased and periodically high fe concentrations have been detected high concentrations of mg and sr have been observed and since gypsum layers are known to be present a de dolomitisation process has been hypothesized the phreeqc program for windows was used to estimate saturation indices for calcite dolomite gypsum and epsomite isotopic data for so 4 2 dissolved in the groundwater and archival data on isotopic composition of ore sulfides were used to solve the isotope balance equation and to estimate the fraction of dissolved so 4 2 that originated from pyrite oxidation and gypsum dissolution the results have shown that dissolution of pyrite oxidation products has a significant influence on chemical composition of groundwater especially in the southern part of the cone of depression by solving the additional combined mass transfer and isotope balance equations it was inferred that a variation in isotopic composition of weathered sulfides must also occur

melt production during granulite facies anatexis experimental data from primitive metasedimentary protoliths we conducted fluid absent partial melting experiments at 0 5 and 1 0 gpa in the temperature range 750 to 1000 c to investigate the influence of bulk rock mg 100mg mg fe and the effects of additional tio2 on the granulite grade anatectic evolution of relatively magnesian metapelites and metagreywackes in these experiments melting began between 780 and 830 c by the incongruent breakdown of biotite to produce quartz saturated granulite facies residual mineral assemblages in equilibrium with h2o undersaturated granitic melt the glass quenched melt compositions produced in this study vary little generally the glasses have compositions similar to those of many natural strongly peraluminous leucogranites the solidus temperatures in both rock types increase with increasing mg but are unaffected by the presence or absence of a tio2 component at 0 5 gpa the metapelites melted at temperatures up to 50 c lower than the equivalent metagreywackes but at 1 gpa there was no discernible difference this study suggests that the fluid absent solidus has a steep positive dp dt slope in metapelites and steep negative dp dt slope in metagreywackes the pattern of melt production with increasing temperature is strongly controlled by the upper limit of biotite stability in tio2 free compositions this was found to increase by 15 to 20 c in the metapelites and by 30 to 40 c in the metagreywackes as a function of increasing mg from 49 to 81 the presence of a tio2 component increases the upper limit of biotite stability by 50 c in the metapelites and by 80 c in the metagreywackes over that observed in the equivalent tio2 free compositions in consequence in the tio2 free samples large pulses of melt up to 35 wt are produced over narrow temperature ranges as little as 15 c in these experiments between 830 and 875 c in the tio2 bearing samples the major pulse of melt production occurs more gradually between 830 and 900 c

relationships between precipitation soil water and groundwater at chongling catchment with the typical vegetation cover in the taihang mountainous region china vegetation cover plays an important role in the process of evaporation and infiltration to explore the relationships between precipitation soil water and groundwater in taihang mountainous region china precipitation soil water and water table were observed from 2004 to 2006 and precipitation soil water and groundwater were sampled in 2004 and 2005 for oxygen 18 and deuterium analysis at chongling catchment the soil water was sampled at three sites covered by grass carex humilis and carex lanceolata acacia and arborvitae respectively precipitation is mainly concentrated in rainy seasons and has no significant spatial variance in study area the stable isotopic compositions are enriched in precipitation and soil water due to the evaporation the analysis of soil water potential and isotopic profiles shows that evaporation of soil water under arborvitae cover is weaker than under grass and acacia while soil water evaporation under grass and acacia showed no significant difference both delta o 18 profiles and soil water potential dynamics reveal that the soil under acacia allows the most rapid infiltration rate which may be related to preferential flow in the process of infiltration after a rainstorm antecedent water still takes up over 30 of water in the topsoil the soil water between depths of 0 115 cm under grass has a residence time of about 20 days in the rainy season groundwater recharge from precipitation mainly occurs in the rainy season especially when rainstorms or successive heavy rain events happen

the age and provenance of metasedimentary rocks in the quetico subprovince ontario from single zircon analyses implications for archean sedimentation and tectonics in the superior province abstract u pb isotopic analyses have been performed on single detrital zircon grains from greywacke samples collected along the northern boundary of the quetico subprovince in northwest ontario secondary lead loss was virtually eliminated by choosing crack free grains and removing the outer surface using air abrasion thus the 207 pb 206 pb age can be considered a good approximation to the age of crystallization of each grain zircons in the quetico metasediments show a range of ages from 2698 3 ma to 3009 4 ma deposition and metamorphism of the metasediments is bracketed within a 10 5 ma time span between crystallization of the youngest detrital zircon grain and a late tectonic intrusion the blalock pluton which cuts the sediments and has a zircon age of 2688 4 ma the most easterly metasediment sample contains zircons with ages which cluster around 3007 6 ma which is within error of the age of a batholith dated in the adjacent marmion lake area of the wabigoon subprovince this along with the fact that most detrital zircons were only slightly abraded by sedimentary transport suggests that the source of many zircons may have been proximal from the north there is also a significant cluster of young zircon ages in the range 2698 2709 ma surprisingly only a few ages correspond to the time of most intense arc type felsic volcanism 2710 2745 ma in the main greenstone belt of the western wabigoon subprovince sedimentary deposition occurred at a time of extensive deformation in the adjacent wabigoon subprovince the pattern of detrital grain ages indicates that this involved uplift of a complex older sialic landmass petrographic evidence for the presence of substantial felsic volcanic detritus as well as the fact that many of the zircons only slightly pre date deposition indicates that much of the sediment was probably derived from erosion of active felsic volcanic centers these data combined with regional geochronological and field studies favour deposition in a basin formed by late orogenic collisional events

assessment of the impact of anthropogenic activities on the groundwater hydrology and chemistry in tarsus coastal plain mersin se turkey using fuzzy clustering multivariate statistics and gis techniques summary tarsus coastal plain tcp is an economically and ecologically important area situated in between the fertile fluvio deltaic plains of two rivers delicay and tarsus mersin se turkey where anthropogenic activities agricultural industrial and domestic are very intense twenty four water quality parameters were surveyed at 193 groundwater and 10 surface water sites during august 2008 the objective was to characterize the physico chemical properties of groundwaters in tcp assess the impact of anthropogenic activities on the groundwater hydrology and chemistry and identify the major hydrogeochemical processes occurring in the area groundwater samples were grouped into hydrochemically distinct and spatially continuous four water classes i e c1 c2 c3 and c4 using the fuzzy c means fcm clustering method where membership values were interpolated using the ordinary kriging technique principal components analysis pca was used to decipher various underlying natural and anthropogenic processes creating these distinct water classes four principal components pcs were extracted in pca which explained more than 73 of the total variance in water quality major factors responsible for the variations in chemistries of water classes are identified as 1 water rock interaction and nitrate contamination 2 salinization by seawater intrusion and evaporite dissolution 3 geogenic anthropogenic cr fe and mn and 4 anthropogenic zn pollution overexploitation of the aquifer is clearly evident especially at settlements located near the coastal zone where the water table is lowered 2 5 m below the sea level salinization is well known in the area and is attributed not only to seawater intrusion but also to dissolution of evaporitic series from the handere formation hydrochemical evidence also suggest that in the area subsurface paleo river channels and the deposits infilling the ancient lagoon area within quaternary recent alluvial deposits act as significant hydrological features where preferential groundwater flow occurs along them

isotopic evidence from the eastern canadian shield for geochemical discontinuity in the proterozoic mantle most workers agree that proterozoic anorthosite massifs represent the crystallization products of mantle derived magmas1 2 although the composition of the parental melts is a major unsolved petrological problem3 as mantle derived rocks the massifs can be used as geochemical probes of their late precambrian upper mantle sources we report here nd and sr isotopic compositions of anorthosites and related rocks from the grenville and nain provinces of the eastern canadian shield here 75 of the earth s known anorthosite is found in a 1 600 km belt from the adirondack mountains of northern new york state to the eastern coast of labrador4 fig 1 the results indicate that the massifs were derived from at least two distinct mantle source regions which were established before 1 650 myr ago and were episodically involved in magmatism over 500 myr one reservoir below the grenville province and probably below much of the eastern superior province was isotopically similar to the depleted modern day mid ocean ridge basalt morb source the other reservoir was chondritic to moderately enriched and is most easily identified in the nain province but may have occurred scattered throughout the superior province

review of nd isotopic data and xenocrystic and detrital zircon ages from the pre variscan basement in the eastern bohemian massif speculations on palinspastic reconstructions

cyclic deformation and chemical transport in the folson lake fault zone east bull lake anorthosite gabbro complex evidence for seismic pumping abstract the east bull lake anorthosite gabbro a late archean 2472 24ma magmatically layered lopolith at the southern margin of the superior province has experienced regional retrograde metamorphism and is transected by the folson lake fault zone a structure of regional extent the fault zone displays repeated episodes of ductile brittle deformation and vein development it has been extensively metasomatized in domains marked by the sequential development of quartz adularia epidote chlorite laumontite calcite and clay fe oxyhydroxide downhole geochemical profiles through the fault zone show coherent patterns for zr hf zr th ta nb and cr co ni which are interpreted to reflect variations in primary igneous compositions by alteration insensitive elements metasomatism is marked by peaks of sio2 k2o or na2o with al2o3 fe mn and mg and ca plus volatiles corresponding to domains of quartz adularia or albite chlorite and laumontite plus calcite respectively the analogue elements th and u are present at uniformly low levels averaging 1 1 and 0 4 ppm respectively thorium covaries with zr such that variations of abundance reflect primary differences whereas th u ratios scatter between 1 5 to 10 indicating minor remobilization of u strontium has been extensively redistributed within the fault zone and cs covaries with k retrograded areas of the pluton have whole rock 18 values ranging between 4 5 and 7 2 per mil both below and above primary values of 5 7 per mil indicating open system behaviour during regional retrograde metamorphism within the fault zone hydrothermal reservoirs involved in later k metasomatism were at temperatures of 150 250 c where 18 o h2o 2 6 to 1 7 per mil likely representing isotopically and chemically evolved deep pore fluids expelled during crustal activity related to tectonic processes in the nearby grenville province calcite and laumontite developed by the advection through the fault zone of ca rich thermal brines at temperatures around 50 200 c where 18oh2o 15 to 0 per mil such fluids were evolved formation brines with a major component of meteoric h2o expelled in the lower paleozoic the multiple deformation metasomatism events are interpreted in terms of episodic seismic pumping on a fault that was intermittently active over a time scale in excess of 1 ga

the east bull lake intrusive suite remnants of a 2 48 ga large igneous and metallogenic province in the sudbury area of the canadian shield the 2 49 to 2 475 ga intrusions of the east bull lake intrusive suite occur in an east northeast trending discontinuous belt along the presently exposed boundary between the archean superior and the proterozoic southern provinces of the canadian shield near sudbury ontario the east bull lake intrusive suite is part of a regional paleoproterozoic magmatic event extending from 2 49 to 2 44 ga which also includes bimodal volcanic rocks felsic plutons and the regionally extensive hearst and matachewan dike swarms this regional magmatic event is thought to be the result of a mantle plume driven intracontinental rifting event that led to the development of a major basin to the south that was subsequently filled by sedimentary rocks of the huronian supergroup the east bull lake intrusive suite appears to have been emplaced along a major axial rift fault related to this rifting event compelling field and geochemical evidence summarized here indicates that the three largest east bull lake suite intrusions the east bull lake agnew lake and river valley intrusions crystallized from similar low ti high al tholeiitic parent magmas that originated in deeper more basic chambers it is proposed that the primary magmas for the east bull lake suite intrusions were second stage melts derived from a sublithospheric depleted mantle source that had been modified by neoarchean subduction and accretion events most of the petrological characteristics of the east bull lake suite intrusions reflect plagioclase dominated fractional crystallization that generated a pronounced fe enrichment trend in the residual magmas orthopyroxene is the high temperature pyroxene and occurs as a cumulus phase through most of the stratigraphy of each intrusion of the suite local olivine rich cumulates present in all of the major east bull lake suite plutons in some instances appear to be comagmatic with associated plagioclase rich cumulates a distinctive marginal facies comprising brecciated and locally thermally recrystallized and or partially melted footwall rocks is a common feature of the east bull lake suite plutons and indicates that a high energy flow regime existed during the initial stages of emplacement this marginal unit typically grades into a heterolithic inclusion rich gabbronorite that contains erratically distributed mafic autoliths and footwall derived xenoliths chemical contamination of the mineralized matrix of these inclusion bearing rocks can be extreme at a local scale the inclusion rich zone is overlain by a thick interval of undifferentiated plagioclase rich cumulates that locally display spectacular glomerocrystic textures rhythmically and irregularly modally layered leucogabbronorite and gabbronorite make up much of the middle parts of the stratigraphy massive to crudely layered ferrogabbro and ferrosyenite only occur in the uppermost part of the agnew lake intrusion varitextured gabbro and pegmatitic gabbro occur as irregular layers and local veins and pods throughout the stratigraphy the east bull lake suite is currently being explored for contact type platinum group element pge cu ni mineralization that has a clear spatial association with orthopyroxene rich cumulates which are otherwise poorly represented in these intrusions disseminated pge and cu rich sulfide mineralization is concentrated within the inclusion bearing zones but extends upward into overlying plagioclase cumulates and appears to have collected along the lower margins of these intrusions through the combined action of vigorous convection of the resident magma and downward percolation of dense mafic residual liquid the resultant narrow zones of sulfide mineralization were thus derived from a much larger volume of parent magma feeder dikes to the mineralized parts of the intrusions appear to have been saturated in sulfide upon emplacement and had relatively high background pge contents compared to those for other known magmatic pge deposits however the economic potential of the contact type mineralization hinges on the efficacy of the physical concentration process which upgraded the metal content and size of the original sulfide particles inside the magma chambers based on the proposed basal accumulation model the best prospective mineralization is most likely to occur in embayments along the sidewall and floor of the intrusions

evolution of the darling range batholith yilgarn craton western australia a shrimp zircon study ma and slow cooling indicated by marginal recrystallization and primary and secondary internal structures observed in zircons from continued pb loss from the zircons until 2628 2616 ma granites from the late archaean darling range batholith record stages in the evolutionary history of the granites and provide a basis for a shrimp u pb study on the timing of granite evolution many granite zircons contain cores a few low u cores retain concordant to slightly discordant u pb ages of 2 8 ga some show discordant intermediate ages but most have nearly concordant

characterizing the recharge regime of the strongly exploited aquifers of the north china plain by environmental tracers 1 we employed environmental tracers 3h 3he noble gases stable isotopes 18o and 2h to study groundwater recharge and residence times in the strongly exploited north china plain aquifer system in the area of shijiazhuang the capital of the hebei province groundwater in the unconfined parts of the piedmont plain contains tritium down to depths of about 100 m and exhibits 3h 3he ages of less than 40 years 3h 3he ages correlate well with sampling depth but less so with distance along a transect from the mountains in the west through the recharge area in the piedmont plain indicating a minor role of lateral inflow compared to spatially distributed recharge the increase of groundwater age with increasing thickness of the saturated zone combined with the steady groundwater table descent over the last decades implies an effective recharge rate of around 0 3 m yr mostly due to infiltration of precipitation and irrigation water despite the recycling of irrigation water a water balance deficit remains causing depletion of the aquifers anthropogenic modifications of the natural recharge regime appear to be reflected by a trend of increasing stable isotope ratios in groundwater of the last decades

using shrimp zircon dating to unravel tectonothermal events in arc environments the early palaeozoic arc of nw iberia revisited dating of zircon cores and rims from granulites developed in a shear zone provides insights into the complex relationship between magmatism and metamorphism in the deep roots of arc environments the granulites belong to the uppermost allochthonous terrane of the nw iberian massif which forms part of a cambro ordovician magmatic arc developed in the peri gondwanan realm the obtained zircon ages confirm that voluminous calc alkaline magmatism peaked around 500 ma and was shortly followed by granulite facies metamorphism accompanied by deformation at c 480 ma giving a time framework for crustal heating regional metamorphism deformation and partial melting the main processes that control the tectonothermal evolution of arc systems traces of this arc can be discontinuously followed in different massifs throughout the european variscan belt and we propose that the uppermost allochthonous units of the nw iberian massif together with the related terranes in europe constitute an independent and coherent terrane that drifted away from northern gondwana prior to the variscan collisional orogenesis

four zircon ages from one rock the history of a 3930 ma old granulite from mount sones enderby land antarctica ion microprobe u th pb analyses of zircons from a granulite grade orthogneiss from mount sones enderby land antarctica record the ages of four principal events in the history of the gneiss three of which already have been recognized through previous isotopic dating of other samples the structure of the zircons indicates at least four different stages of growth the several zircon ages were obtained by grouping the analyses according to the stage they represented in the observed stratigraphic succession of growth and thereby defining separate u pb discordance patterns for each stage the stratigraphically oldest zircon rare discrete cores is indistinguishable in age from the most common euhedrally zoned zircon both crystallized when the tonalitic precursor of the orthogneiss was emplaced into the crust 3927 10 ma ago making the orthogneiss currently the oldest known terrestrial rock the outer parts of most grains and some whole grains recrystallized at 2948 31 17 ma during or immediately after possibly 100 ma of high granulite grade metamorphism the recrystallized zircon was isotopically disturbed by tectonism associated with reactivation of the southern margin of the napier complex at 1000 ma in the intervening time at 2479 23 ma the cores and zoned zircon suffered a major isotopic disturbance involving movement of radiogenic pb which left most of the crystals with radiogenic pb deficiencies but produced local radiogenic pb excesses in others a new generation of zircon characterized by very high th u and low u grew at that time that event deformation and possibly a minor rise in temperature produced widespread perturbations of other isotopic systems throughout the napier complex

u pb and 39ar 40ar data constraining the ages of the source emplacement and recrystallization cooling events from late to post d3 variscan granites of the gouveia area central portugal abstract late carboniferous u pb ages have been measured on igneous zircon from late and post d 3 sn bearing two mica granites from the gouveia area in the central iberian zone inherited zircon from these granites is mainly of neoproterozoic age a small amount is paleoproterozoic and cambrian and none is mesoproterozoic the most likely source of the granites is gondwanan sediments derived from the west african craton and surrounding pan african belts rare inherited zircon of ordovician silurian devonian and carboniferous age is of similar age to zircon in synorogenic greywackes and conglomerates of the nw iberian massif ordovician granitic rocks might have contributed to the source igneous zircon 310 1 4 3 ma from a late d 3 granite is older than coexisting monazite 301 4 2 6 ma the monazite re crystallized and is of similar age to zircon from a post d 3 granite 297 3 3 1 ma that surrounds this granite monazite from post d 3 granites is of similar age 289 ma but slightly younger than the zircon from the same granites 297 3 3 1 and 302 6 6 7 ma the 39 ar 40 ar age of muscovite from variscan late and post d 3 granites of the gouveia area is the same 286 ma showing that the area was heated above 350 c in the late carboniferous early permian muscovite from variscan late d 3 granites suffered ar loss and muscovite from variscan post d 3 granites records cooling after emplacement

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

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

sr nd and pb isotopes in proterozoic intrusives astride the grenville front in labrador implications for crustal contamination and basement mapping abstract we report sr nd and pb isotopic compositions of mid proterozoic anorthosites and related rocks 1 45 1 65 ga and of younger olivine diabase dikes 1 4 ga from two complexes on either side of the grenville front in labrador anorthositic or diabasic samples from the mealy mountains grenville province and harp lake nain churchill provinces complexes have very similar major minor and trace element compositions but distinctly different isotopic signatures all mealy mountains samples have isr 0 7025 0 7033 end 0 6 to 5 6 and pb isotopic compositions consistent with derivation from a mantle source depleted with respect to nd sm and rb sr pb isotopic compositions for the mealy mountains samples are slightly more radiogenic than model mantle compositions all harp lake samples have isr 0 7032 0 7066 end 0 3 to 4 4 and variable but generally unradiogenic 207pb 204pb and 206pb 204pb compared to model mantle suggesting mixing between a mantle derived component and a u depleted crustal contaminant crustal contaminants are probably a variety of archean high grade quartzofeldspathic gneisses with low u pb ratios and include a component that must be isotopically similar to the early archean 3 6 ga uivak gneisses of labrador or the amitsoq gneisses of west greenland this would imply that the ancient gneiss complex of coastal labrador and greenland is larger than indicated by present surface exposure and may extend in the subsurface as far west as the labrador trough if harp lake and mealy mountains samples were subjected to the same degree of contamination as suggested by their chemical similarities then the mealy contaminants must be much younger probably early or middle proterozoic in age the labrador segment of the grenville front therefore appears to coincide with the southern margin of the archean north atlantic craton and may represent a pre mid proterozoic suture

unravelling the pre variscan evolution of the habach terrane tauern window austria by u pb shrimp zircon data the u pb shrimp age determinations of zircons from the habach terrane tauern window austria reveal a complex evolution of this basement unit which is exposed in the penninic domain of the alpine orogen the oldest components are found in zircons of a metamorphosed granitoid clast of a migmatitic leucosome and of a meta rhyolitic variscan tuff which bear cores of archean age the u pb ages of discordant zircon cores of the same rocks range between 540 and 520 ma it is assumed that the latter zircons were originally also of archean origin and suffered severe lead loss whilst being incorporated into early cambrian volcanic arc magmas the provenance region of the archean 2 64 2 06 ga zircons is assumed to be a terrane of gondwana affinity i e the west african craton hoggar shield reguibat shield the caledonian metamorphism left a pervasive structural imprint in amphibolite facies on rocks of the habach terrane it is postdated by discordant zircons of a migmatitic leucosome at 440 ma presumably ca 420 ma alpine and variscan upper greenschist to amphibolite facies conditions caused partial lead loss in zircons of a muscovite gneiss white schist only where extensive fluid flow and brittle deformation due to its position near a nappe sole thrust enhanced the grains susceptibility to isotopic disturbance the habach terrane an active continental margin with ensialic back arc development showed subduction induced magmatic activity approx between 550 and 507 ma back arc diorites and arc basalts were intruded by ultramafic sills and subsequently by small patches of mantle dominated unaltered and in the vicinity of a major tungsten deposit altered granitoids fore arc shales and back arc greywackes cherts basin sediments as well as arc and back arc magmatites were not only nappe stacked by the caledonian compressional regime closing the presumably narrow oceanic back arc basin and squeezing mafic to ultramafic cumulates out of high level magma chambers 496 482 ma it also induced uplift and erosion of deeply rooted crystalline complexes and triggered the development of a successor basin filled with predominantly clastic greywacke arkosic sediments the study demonstrates that the basement rocks exposed in the habach terrane might be the missing link between similar units of the more westerly positioned external domain i e aar aiguilles rouges mont blanc and the austroalpine domain to the east oetztal silvretta

palaeozoic polymetamorphism in the north qinling orogenic belt central china insights from petrology and in situ titanite and zircon u pb geochronology abstract the qinling orogenic belt experienced multiple phases of orogenesis during the palaeozoic unraveling the timing and p t conditions of these events is the key to understanding the convergence processes between the south china and the north china blocks the songshugou complex located in the southern part of the north qinling orogenic belt has registered multistage metamorphism in palaeozoic and thus potentially provides insights into the tectonic evolution of the qinling orogenic belt in this study three metabasic rocks a garnet pyroxenite a garnet amphibolite and a gneissic amphibolite from the songshugou complex were selected for petrological study and zircon and titanite u pb dating our results show that the metabasic rocks experienced three metamorphic events during the palaeozoic the first metamorphic event m1 is characterized by high pressure conditions two zircon grains in equilibrium with garnet and in absence of plagioclase were recognized from the garnet pyroxenite sample they yielded ti in zircon temperatures of 660 851 c at 12 0 kbar and a weighted mean age of 498 15 ma providing the constraints on the temperature and timing of prograde or peak metamorphism m1 1 zircons that are inequilibrium with garnet from the garnet pyroxenite and the garnet amphibolite gave u pb ages of 494 9 ma and 484 4 ma and ti in zircon temperatures of 793 33 c and 738 18 c respectively thus these zircons were formed on the retrograde amphibolite facies conditions at 8 0 kbar m1 2 titanite inclusions were found in actinolite cores of zoned amphibole from the garnet amphibolite they yielded a u pb age of 470 ma and zr in titanite temperature of 676 23 c at pressure of 7 0 kbar suggesting that the amphibolite facies retrogression perhaps persisted to 470 ma weakly zoned zircons from the garnet amphibolite and inclusion free titanites from the garnet pyroxenite gave consistent u pb ages of 418 5 ma and 423 10 ma and ti in zircon temperature of 742 26 c and zr in titanite temperature of 764 18 c at 7 0 kbar respectively it is suggested that a heating event m2 is registered by a subsequent phase of amphibolite facies metamorphism the ilmenite bearing titanite crystals from the garnet pyroxenite yielded a u pb age of 352 4 ma recording a late thermal event m3 on the basis of combined petrological and geochronological results we propose a revised tectonic model for the north qinling orogeny in palaeozoic the high pressure granulites were formed by the northward subduction of the shangdan oceanic slab and the arc continent collision at ca 500 ma their exhumation happened at ca 494 484 ma as a result of slab breakoff subsequent amphibolite facies metamorphism dated at ca 440 420 ma are coeval with the widespread magmatism in the north qinling terrane which are likely caused by the reinitiation northward subducted of shangdan oceanic slab at ca 350 ma the north qinling terrane was likely affected by another thermal overprinting event