synorogenic melting of mafic lower crust constraints from geochronology petrology and sr nd pb and o isotope geochemistry of quartz diorites damara orogen namibia quartz diorites represent the earliest ca 540 ma and most primitive plutonic rocks in the pan african damara belt and they pre date the main phase of high t regional metamorphism two suites of synorogenic quartz diorites are unusual among damaran intrusive rocks in their elemental and isotopic features comparison of the diorite compositions with melts from amphibolite dehydration melting experiments points to a garnet bearing meta tholeiite probably enriched in k2o as a likely source rock partial melting processes generated mafic ca 50 wt sio2 quartz diorites in the deep crust at temperatures of between 1 000 and 1 100 c based on comparison with experimental results and similar temperature estimates based on p2o5 solubility in mafic rocks subsequently the quartz diorites evolved by multistage polybaric differentiation processes including fractional crystallization of mainly hornblende and plagioclase and assimilation of felsic basement gneisses although their chemical characteristics high lile low hfse resemble those of other quartz diorites with calc alkaline affinities they differ in their enriched sr initial 87sr 86sr 0 70943 0 71285 nd initial e nd 9 1 to 15 2 and o 18o 6 8 8 1 isotope compositions neodymium model ages tdm that range from 1 7 to 2 2 ga and large variation in 207pb 204pb relative to 206pb 204pb indicates involvement of ancient crustal material lead 206pb 204pb 17 08 17 23 207pb 204pb 15 53 15 62 208pb 204pb 37 71 38 16 isotope compositions are strongly retarded indicating that the source underwent a pre pan african u pb fractionation and u depletion it is proposed that the quartz diorites originated by synorogenic high temperature melting of mafic lower crust this contrasts with previous suggestions favouring an origin of these rocks by melting of an enriched mantle during pan african times with characteristics modified by subduction of oceanic crust and sedimentary rocks

adakite like and normal arc magmas distinct fractionation paths in the east serbian segment of the balkan carpathian arc are used to assess petrogenetic and regional geodynamic processes associated with late cretaceous subvolcanic intrusions within the sparsely studied timok magmatic complex tmc and ridanj krepoljin zone rkz of eastern serbia the tmc and rkz form part of the apuseni banat timok srednogorie abts magmatic belt a cu au mineralized calc alkaline magmatic arc related to closure of thetethys ocean that extends through romania serbia and bulgaria in se europe zircon ages based on u pb laser ablation inductively coupled plasma mass spectrometry supplemented by existing isotope dilution thermal ionization mass spectrometry data respectively range from 89 to 79 ma and from 76 to 71 ma for the tmc and rkz this age pattern corresponds to cross arc younging away from the european continent adakite like trace element signatures y 18 ppm are linked with samples that extend across the arc these overlap in space and time with samples that conform to a normal arc differentiation trend we performed energy constrained assimilation fractional crystallization ec afc modeling of sr la nd yb concentrations and sr and nd isotopic data results suggest that the two distinct fractionation trends may be explained in terms of a common mantle derived parental magma but distinct fractionation and assimilation paths in the lower and upper crust petrogenesis of the adakite like magmas is consistent with extensive high pressure amphibole fractionation in the lower crust followed by ascent and plagioclase dominant fractionation and assimilation in the upper crust in contrast normal arc signatures appear to have evolved exclusively via an upper crustal differentiation process overall our interpretation supports mantle wedge melting related to weak extension during progressive rollback of a subducting slab

polymetallic sulfide exploration on the deep sea floor the feasibility of the mini moses experiment the recent discoveries of polymetallic sulfide deposits on the deep sea floor have created an interest in geophysical techniques for mapping them a magnetometric resistivity method mini moses has been developed for detecting the distribution of sub sea floor electrical resistivity a vertical long wire bipole feeds electric current into the ocean some of the current enters the sea floor and its magnetic field is measured at various distances by a low frequency induction coil magnetometer the method is sensitive to conductive and resistive zones at depth and information about three dimensional resistivity structures can be obtained the small load resistance of an electric transmitter grounded in the ocean allows high current to be driven by a battery powered instrument the induction coil receiver is more sensitive and less noisy than conventional flux gate magnetometers over the selected frequency band this receiver is therefore well adapted to the almost noise free deep sea floor environment b

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

origin of high potassium talc alkaline i type granitoids many i type granitoid magmas are generated through partial melting of older metaigneous rocks and the compositions of such melts are broadly calc alkaline and metaluminous these melts are granitic to tonalitic and result from thermal extremes in their lower crustal source regions data on the experimental partial melting of common crustal rocks suggest that high k i type granitoid magmas can be derived only from the partial melting of hydrous calc alkaline to high k calc alkaline mafic to intermediate metamorphic rocks in the crust because of their low k2o contents metabasaltic rocks of all kinds are unsuitable as sources and models that propose mixing of mantle derived basaltic magmas and crustal melts are also inadequate there is no requirement that i type calc alkaline magmatism be related in any way to subduction processes

variscan vs alpine tectonothermal evolution of the southern carpathian orogen constraints from 40ar 39ar ages abstract 40 ar 39 ar hornblende and muscovite ages indicate that basement units exposed within all major alpine nappes of the southern carpathian orogen were penetratively deformed and metamorphosed during late variscan carboniferous tectonothermal events subsequent exhumation resulted in cooling from ca 500 c hornblende k ar retention temperature to ca 375 c muscovite k ar retention temperature between ca 320 and 295 ma late variscan exhumation of middle crustal levels is interpreted to have resulted from intracontinental contraction and regional uplift subsequent to variscan continental collision between gondwana derived tectonic elements e g africa and laurussia muscovite ages within mylonite zones suggest that late variscan shear zones were probably associated with a regional shear system that developed along the southern margins of laurussia alpine metamorphic effects within the southern carpathians are generally nonpenetrative and are only significant within narrow zones of retrogression that are interpreted to have formed during initial attenuation of variscan crust associated with formation of the jurassic severin rift

nd isotopic mapping of the sri lanka basement update and additional constraints from sr isotopes abstract the high grade crystalline basement of sri lanka can be subdivided into three provinces distinguished by distinct isotopic model ages this confirms the conclusions of milisenda et al 1988 made on the basis of much more limited data high grade ortho and paragneisses from the highland complex in central sri lanka yield palaeoproterozoic to archaean nd model ages from 2 0 to 3 4 ga and reflect an extended premetamorphic crustal history the wanni complex in the western and northwestern part of the island is characterized by meso to neoproterozoic nd model ages ranging from 1 0 to 2 0 ga the vijayan complex in the southeast is occupied by neoproterozoic i type granitoids with nd model ages from 1 0 to 1 8 ga this distinct crustal province appears to fit best a detached continental margin plutonic terrane the isotopic data preclude a common premetamorphic history of the wanni and vijayan complexes with the ancient highland complex the combined nd sr and pb isotope data together with recent geochronological studies strongly suggest that the distinct crustal age provinces are separated by tectonic discontinuities and that they represent terranes that were accreted just prior to the high grade metamorphic event in neoproterozoic time

pre alpine evolution of a segment of the north gondwanan margin geochronological and geochemical evidence from the central serbo macedonian massif abstract the serbo macedonian massif smm represents a composite crystalline belt within the eastern european alpine orogen outcropping from the pannonian basin in the north to the aegean sea in the south the central parts of the massif i e southeastern serbia southwestern bulgaria eastern macedonia consist of the medium to high grade lower complex and the low grade vlasina unit new results of u pb la icp ms analyses coupled with geochemical analyses of hf isotopes on magmatic and detrital zircons and main and trace element concentrations in whole rock samples suggest that the central smm and the basement of the adjacent units i e eastern veles series and struma unit originated in the central parts of the northern margin of gondwana these data provided a basis for a revised tectonic model of the evolution of the smm from the late ediacaran to the early triassic the earliest magmatism in the lower complex vlasina unit and the basement of struma unit is related to the activity along the late cadomian magmatic arc 562 522 ma subsequent stage of early palaeozoic igneous activity is associated with the reactivation of subduction below the lower complex and the eastern veles series during the early ordovician 490 478 ma emplacement of mafic dykes in the lower complex due to aborted rifting in the middle ordovician 472 456 ma and felsic within plate magmatism in the early silurian 439 2 ma the third magmatic stage is represented by carboniferous late to post collisional granites 328 304 ma these granites intrude the gneisses of the lower complex in which the youngest deformed igneous rocks are of early silurian age thus constraining the high strain deformation and peak metamorphism to the variscan orogeny the permian triassic 255 253 ma stage of late to post collisional and within plate felsic magmatism is related to the opening of the mesozoic tethys

timing of k alkaline magmatism in the balkan segment of southeast european variscan edifice id tims and la icp ms study the variscan orogen in southeast europe is exposed in isolated remnants affected by a subsequent alpine tectono magmatic overprint unlike the central european variscides in se europe the juxtaposition and correlation of the events and products are impeded by the scarcity of variscan domains with preserved magmatic metamorphic sedimentological and structural characteristics to reveal the particular evolution of the variscan orogen in balkan mts we present the results of id tims and la icp ms dating of three potassic alkaline intrusions svidnya buhovo seslavtsi and shipka the age determinations from the plutons do not permit to establish their unequivocal ages but they bracket the time interval of emplacements based on geochronological tectonic and stratigraphic evidence the emplacement interval for plutons could be 317 310 ma for svidnya 330 310 ma for buhovo seslavtsi and 320 303 ma for shipka these results show that the generation of potassic alkaline magmas was post visean and is contemporaneous with the adjacent numerous calc alkaline granitoid plutons thus the variscan orogen in the balkan mts is not characterized by a time dependent geochemical evolution of magma generation hence the observed differences in the rocks compositions can be interpreted solely by distinction between the magma sources the available data for both potassic alkaline and calc alkaline rocks indicate that the major episodes of crustal stacking and shearing in the balkan part of the variscan edifice are pre visean 330 ma the present study reveals that the potassic alkaline rocks from the balkan mts are younger than the central european potassic granitoids durbachites it suggests that melting of enriched mantle source took place at different times throughout the variscan orogen in spite of the alkaline character of the magmas the studied zircons show a complex nature with inherited cores and magmatic overgrowths the observed heterogeneities in the zircons imply the presence of much older materials in the petrogenesis of the rocks from the potassic alkaline plutons

source constraints on the genesis of danubian granites in the south carpathians alpine belt romania abstract the pre alpine basement of the lower danubian nappes in the south carpathians is made up of two precambrian terranes dr gsan and lainici p ius that were intruded by pan african cadomian and variscan granitoid massifs we focus on the major and trace element geochemistry 1 in the dr gsan terrane of the variscan retezat and par ng intrusions 2 in the lainici p ius terrane of the variscan furc tura petreanu and frumosu intrusions and of the pan african v rful pietrii usi a and olte granites and granitic leucosomes of migmatites and 3 in the upper danubian nappes basement of the variscan muntele mic sf rdin cherbelezu and ogradena intrusions for each intrusion in which a range of composition is observed we decipher the differentiation mechanisms fractional crystallization hybridization melt laden with restite minerals etc in order to define the parental liquid compositions the latter are calc alkaline to alkali calcic except olte that is calcic and medium to high k in composition with la yb n 10 and sr y 15 most melts display the so called continental adakite affinities the parental melt compositions are compared with experimental data to determine the melting conditions and the nature of the source rock when the p t conditions can be estimated the temperatures range between 850 c and 875 c and the pressure between 5 and 15 kbar regardless of the ages of the granites and the terrane in which they have intruded the source rock composition is dominated by a variety of mafic igneous compositions or metasediments rich in volcanic components clay rich pelitic protoliths have not been identified we confirm a variscan age c 300 ma for the frumosu intrusion granite and inherited precambrian ages c 1 7 1 9 and 2 6 2 9 ga for the motru dyke swarm thus both dr gsan and lainici p ius together with the upper danubian basement terranes were affected by variscan post collisional granitic plutonism in the south carpathians both pan african and variscan granites were generated in a crust thickened by stacking of terranes the source of the pan african granites in lainici p ius is different from that of the variscan granites from lainici p ius and dr gsan terranes and from the upper danubian nappe basement but all these sources were notably depleted in metapelite component

one million year old groundwater in the sahara revealed by krypton 81 and chlorine 36 measurements of 81 kr kr in deep groundwater from the nubian aquifer egypt were performed by a new laser based atom counting method 81 kr ages range from 2 10 5 to 1 x 10 6 yr correlate with 36 cl cl ratios and are consistent with lateral flow of groundwater from a recharge area near the uweinat uplift in sw egypt low 2 h values of the 81 kr dated groundwater reveal a recurrent atlantic moisture source during pleistocene pluvial periods these results indicate that the 81 kr method for dating old groundwater is robust and such measurements can now be applied to a wide range of hydrolugic problems

dehydration melting and water saturated melting of basaltic and andesitic greenstones and amphibolites at 1 3 and 6 9 kb

the fast evolution of a crustal hot zone at the end of a transpressional regime the saint tropez peninsula granites and related dykes maures massif se france granite and dolerite occurring in the eastern unit of the maures massif southern france variscan belt were emplaced in the waning stage of the evolution of a dextral shear zone along the gondwana border with laurasia we present a geochronological and petro geochemical study major and trace elements sr nd isotopes of 1 the foliated moulin blanc cordierite granite and related foliated dykes 2 the equant camarat granite and dyke and 3 the andesite basalt dolerite dykes frequently mingled with leucogranite material at pinet and capon tips and cross cutting leucogranitic dykes zircon and monazite dating of the moulin blanc granite gives a crystallization age of 301 2 ma shrimp dating on zircons from the leucogranite component of a pinet composite dyke only reveals zircon core ages of mainly 310 10 ma age of migmatization of the host gneiss undulating contacts of dolerite dykes within the c 300 ma old camarat granite suggest penecontemporary relationships between the two intrusions the age interval between the moulin blanc granite and the dolerite dykes would thus be very short the moulin blanc pluton is formed by mingling and mixing of melts of granitic and dioritic composition the latter occurring mainly as microgranular mafic enclaves two other types of diorites are also identified in the pluton the foliated dykes are similar in composition to the moulin blanc granite which confirms their relationship to the main pluton the moulin blanc granite is a typical example of a pluton formed by repeated recharges coming from a deeper magma chamber formation of the camarat dykes is controlled by hydrothermal fluids that are responsible for the non charac behavior of trace elements the pinet and capon leucogranite components of the dolerite dykes have distinct geochemical signatures different from the camarat granite and dykes the cross cutting leucogranitic dykes have much geochemical similarities with the capon granite the age of similar dykes in the tanneron massif supports the contemporaneity of camarat granite and dolerites the dolerite geochemistry was deeply modified by alteration processes but an andesite basalt composition is still recognizable by using immobile element ratios in a end vs sri diagram all rocks plot close to a mixing hyperbola between a depleted mantle component and a crustal component characteristic of the variscan crust in the french massif central there is a broad correlation between the ndtdm 2 stage model ages and the degree of fractionation of the rocks modeling of dehydration melting in a range of p t conditions reveals that a variety of lithologies were the source of the saint tropez magmas high alumina and alkali basalt amphibolites metagraywackes metapelites and meta arkoses these sources were melted almost simultaneously at different levels in the crust the calculated critical dyke width required to transport the granitic melt by buoyancy is far too large compared to the observed granitic dyke thicknesses therefore tectonic processes and or melt overpressure must have played a role in the emplacement mechanism thermo mechanical constraints on mixing between mafic and felsic melts suggest that in the pinet composite dykes both melts should have mixed the observed lack of mixing is interpreted as due to quick cooling the region was affected by a hot zone at the end of a transpressional regime

chronology of late paleozoic tectonothermal activity in the southeastern bohemian massif austria moldanubian and moravo silesian zones 40ar 39ar mineral age controls abstract new 40ar 39ar hornblende and muscovite ages from the moldanubian and moravian zones of the southeastern bohemian massif indicate rapid cooling associated with continental underplating during early carboniferous plate collision and closure of the moravo silesian foredeep 40ar 39ar ages have been determined for hornblende concentrates from 1 amphibolite from the moldanubian nappe complex including the dobra gneiss rehberg amphibolite and loosdorf complex all part of the variegated unit 2 micaschist structurally overlying the bittesch gneiss and 3 amphibolite within the bittesch gneiss moravian zone the five hornblende concentrates display variably discordant 40ar 39ar age spectra because of contamination with extraneous argon components however 36ar 40ar vs 39ar 40ar isotope correlations are well defined for the five hornblende concentrates mswd the hornblende and muscovite ages are interpreted to date post variscan metamorphic cooling through ca 500 c and ca 400 c respectively no record of pre variscan thermal events is preserved in either the hornblende or muscovite argon systems however the widespread occurrence of extraneous argon components suggest a pre variscan stage of tectonothermal development the near concordancy of the hornblende and muscovite cooling ages suggest relatively rapid cooling through the contrasting argon closure temperatures and indicate that relatively high crustal levels had likely been attained during the early carboniferous

igneous charnockites and related rocks from the bjerkreim sokndal layered intrusion southwest norway a jotunite hypersthene monzodiorite derived a type granitoid suite the upper part of the bjerkreim sokndal layered intrusion bksk introduction comprises a suite of charnockitic granitoids mangerite quartz charnockitic granitoids are a major component of mangerite and charnockite with some jotunites the petrology and anorthosite mangerite charnockite rapakivi granite geochemistry major and trace elements of eight main rock types amcg complexes and therefore play a crucial role in 88 samples are considered the rocks form a suite of k2o rich the formation and evolution of the proterozoic crust alkali calcic granitoids of a type deriving from jotunitic magma their origin has been long debated though intimately two trends of inferred liquids are distinguished the main liquid associated with massif anorthosites they are generally line of descent which passes from jotunites through two pyroxene considered to be derived from distinct parental magmas quartz mangerites to amphibole charnockites the second one which generated in different sources emslie 1978 1991 morse encompasses olivine bearing quartz mangerites and charnockites and 1982 duchesne 1984 wiebe 1992 ashwal 1993 it is rooted into mangeritic liquids comparison with the tellnes rock has however been recognized that some charnockitic series reveals that contamination by leucogranitic material has taken rocks could also be derived from an intermediate monplace simultaneously with fractional crystallization in both bksk zodioritic to ferrodioritic jotunitic magma duchesne trends zr rb cs and th are particularly sensitive to assimilation et al 1989 wilmart et al 1989 owens et al 1993 and the higher water content in bksk zircon and apatite have whose origin is also controversial the jotunitic magma saturated at the liquidus of quartz mangerite around 900 c close could either be derived from a crustal source by the to the fayalite magnetite quartz fmq buffer the two pyroxene partial melting of basic rocks duchesne et al 1989 quartz mangerites cannot have been equilibrated with the mangerite duchesne 1990 or be residual magmas after the forcumulates on which they rest a major influx of magma took place mation of massif anorthosites which have been conwhen mangeritic cumulates were being deposited it mingled and partly mixed with the resident contaminated magma taminated to various degrees by crustal material emslie et al 1994 the latter hypothesis seems to explain a large number of amcg complexes in the grenville and nain provinces and elsewhere accepting that some charnockites are derived from jotunites there is a logical

transport modeling applied to the interpretation of groundwater 36cl age 1 we use reactive transport modeling to consider how diffusion and hydrodynamic dispersion cross formational flow and subsurface production affect the steady state distribution in flow regimes of the radioactive isotope 36cl and the relationship of the isotope distribution to groundwater residence time or age the isotope forms naturally in the atmosphere dissolves in rainwater and then decays in the subsurface with a half life of 301 000 years hence it is important for age dating very old groundwater in a simple flow regime composed of an aquifer confined above and below by aquitards isotopic age may correspond rather well with a groundwater s piston flow age this correspondence is favored where the aquifer is thick cross formational flow is insignificant salinity is low and the diffusion coefficient within the aquitards is small the maximum dateable age however is somewhat smaller than expected from the isotope s half life owing to the effect of dead chloride dating based on isotope abundance the 36cl cl ratio may be less accurate than that based on 36cl concentration cross formational flow can strongly affect the 36cl distribution and abundance preventing the rates and even direction of flow within an aquifer from being interpreted using the piston flow model where salinity is moderate or high the isotope distribution is controlled by subsurface production and dating on the basis of the decay of atmospheric 36cl is not possible in models of simple flow regimes the 36cl method fails to predict groundwater age accurately where groundwater chlorinity exceeds 75 150 mg kg 1 reactive transport models hold considerable promise for improving interpretation of the rates and patterns of groundwater flow from radioisotope distributions

isotope characteristics of meteoric water and groundwater in the sahelo sudanese zone the stable isotopic concentration 18o and 2h of water in phreatic aquifers in the sahelo sudanese zone of africa is representative of the original mean rainfall contributing to ground water recharge the isotopic decrease of 18o in the rainwater and groundwater 0 084 100 km 1 showed an east west gradient this suggests a supply of water vapor transported by the zonal flows east african jet tropical easterly jet and easterly waves thus increasing the contribution from the evaporating surfaces of the indian ocean in relation to that of the guinean monsoon the gradient also indicates the importance of atmospheric recycling of continental water through evapotranspiration a model of conservation of the isotopic masses is presented which takes into account the return of continental vapor toward the rain giving clouds the remaining fraction of water vapor during the east west transit between djibouti and dakar is thus determined by evapotranspiration

u pb zircon and monazite geochronology of post collisional hercynian granitoids from the central iberian zone northern portugal abstract in the central iberian zone ciz of the iberian massif large volumes of granitoids were emplaced during the post collisional stage of the hercynian orogeny syn to post d3 the last ductile deformation phase twelve granitic units and a quartz monzodiorite were selected for a u pb zircon and monazite geochronological study they represent successive stages of the d3 event the ucanha vilar lamego sameiro and refoios do lima plutons are coeval 313 2 ma 319 4 ma 316 2 ma and 314 2 ma respectively and belong to the earliest stage later on the braga massif was emplaced its different units yielding the same age 309 3 ma for the braga granite 309 1 ma for the gonca granite and 311 5 ma for a related quartz monzodiorite the braga massif is subcontemporaneous with the agrela and celeiros plutons 307 3 5 ma and 306 2 ma respectively in agreement with field data the briteiros granite is younger 300 1 ma followed by the emplacement of the peneda geres massif geres paufito illa and carris granites the geres granite emplaced at 296 2 ma seems to represent a first magmatic pulse immediately followed by the intrusion of the paufito granite at 290 2 5 ma for the carris granite a minimum emplacement age of 280 5 ma was obtained based on these results the following chronology is proposed 1 syn d3 biotite granitoids 313 319 ma 2 late d3 biotite dominant granitoids 306 311 ma 3 late to post d3 granitoids ca 300 ma 4 post d3 granitoids 290 296 ma these chronological data indicate that successive granitic intrusions were emplaced in the ciz during a short time span of about 30 ma that corresponds to the latest stages of the hercynian orogeny a rapid and drastic change occurred at about 300 ma between a compressive ductile tectonic regime d3 ca 300 320 ma associated to calc alkaline monzonitic and aluminopotassic plutonism and a fragile phase of deformation d4 which controlled the emplacement of the subalkaline ferro potassic plutonism at 290 296 ma

slab breakoff a model of lithosphere detachment and its test in the magmatism and deformation of collisional orogens we present a model proposing that oceanic lithosphere detaches from continental lithosphere during continental collision slab breakoff allowing an explanation of syn to post collisional magmatism and metamorphism continental collisions are preceded by subduction of dense oceanic lithosphere and followed by attempted subduction of buoyant continental lithosphere this situation of opposing buoyancy forces leads to extensional deformation in the subducted slab a narrow rifting mode of deformation will result if there is strain localization slab breakoff results we have assessed the plausiblity of this process by quantitatively evaluating an upper bound for the strength of the lithosphere and have compared it with the change in buoyancy force during continental sbduction whether breakoff will occur and the depth at which it will occur is a strong function of temperature and hence the subduction velocity for a subduction velocity of 1 cm yr breakoff could occur at depths of between 50 and 120 km while at higher velocities it is still likely to occur but at deeper depths as a result of the rifting during breakoff the asthenosphere upwells into the narrow rift and following breakoff it impinges on the mechanical lithosphere of the overriding plate the resulting conducted thermal perturbation leads to melting of the metasomatised overriding mantle lithosphere producing basaltic magmatism that leads to granitic magmatism in the crust dry asthenospheric mantle will melt only if breakoff occurs at a depth shallower than ca 50 km breakoff removes the force at the downdip side of the continental crust while the enhanced heating leads to a reduction of the strength of the underlying crust both effects facilitate the freeing of buoyant crustal sheets which can then rise towards the surface leading to the rapid exhumation of eclogite facies continental crust the cessation of subduction and replacement of the cold oceanic lithosphere by asthenosphere leads to rapid uplift of the orogen we have tested the variety of predicted expressions of slab breakoff in the geological record of the alps the aegean islands and the dabie shan a comparison of the various belts highlights 1 that the magmatism and metamorphism are found near the suture and the centre of the orogen demonstrating the general steepening of suture during collision 2 that a vague inverse correlation exists between maximum depth of metamorphism and volume of syn orogenic magmatism and 3 that the melts can be emplaced in both compressive and extensional environments we suggest that slab breakoff is an important step in the evolution of many orogenic belts and it allows an explanation for the combined presence in collisional orogens of magmatism with a mantle parentage and ultra high pressure metamorphics

chlorine 36 dating of very old groundwater 1 the great artesian basin australia chlorine 36 has many advantages as a dating tool for very old groundwater these advantages include a suitable half life 3 01 x 10 sup 5 years simple geochemistry conservative behavior in groundwater and a general absence of subsurface sources at levels comparable to the atmospheric input recent advances in tandem accelerator mass spectrometry have permitted the analysis of sup 36 cl at low abundance expected following residence in the subsurface for 10 sup 6 years or more in order to test the suitability of sup 36 cl for dating very old groundwater the sup 36 cl cl ratios of 26 groundwater samples from the great artesian basin of australia have been measured groundwater ages calculated from the sup 36 cl data compare favorably with ages computed independently from hydrodynamic simulations

cosmogenic radiogenic and stable isotopic constraints on groundwater residence time in the nubian aquifer western desert of egypt measurements of radiochlorine sup 36 cl radiogenic noble gases sup 4 he and sup 40 ar and stable chlorine isotope ratios were obtained to assess the residence time of groundwater in the nubian aquifer of the western desert of egypt measured sup 36 cl cl ratios yield apparent residence times from approx 0 2 to 1 2 x 10 sup 6 years in the deep 600 1200 m groundwater assuming constant cl and le 0 16 x 10 sup 6 years in the shallow 600 m groundwater values of delta sup 37 cl in the groundwater strengthen the application of the sup 36 cl dating method by constraining cl sources and identifying groundwater mixing dissolved gases were measured in some of the deep groundwater samples measured sup 4 he concentrations indicate accumulation of radiogenic sup 4 he that is qualitatively consistent with the age progression indicated by the sup 36 cl cl ratios but the flux of external sup 4 he from the underlying crust has not been quantified and is not constant throughout the aquifer concentrations of sup 40 ar range from 3 3 to 6 7 x 10 sup 4 ccstp g and indicate excess air incorporation at recharge measured sup 40 ar sup 36 ar ratios do not exceed the atmospheric ratio a two dimensional numerical hydrodynamic transect of the aquifer was modeled from the areamore of the uweinat uplift to the northern bahariya oasis predicted groundwater velocities in the deep portion of the aquifer are 0 5 3 5 m yr with groundwater residence times up to 9 x 10 sup 5 years residence times up to 1 3 x 10 sup 6 years are predicted in the confining shale aquifer properties are estimated by using the model to fit the measured sup 36 cl cl ratios under these conditions hydrodynamic residence times are within about 30 percent of those calculated from sup 36 cl when mixing of cl sup is accounted for in the highest cl sup deep groundwaters by mutually calibrating multiple methods hydrodynamic sup 36 cl and sup 4 he a consistent picture of the nubian aquifer has emerged in which lateral flow from a southern recharge area dominates the deep horizons while shallow horizons contain younger autochthonous recharge less

partial melting of two amphibolites contrasting experimental results under fluid absent conditions a large portion of the lower continental crust may be amphibolitic in composition and without a free fluid phase as a consequence h2o undersaturated or fluid absent melting of amphibolites may be responsible for the formation of some granites and migmatites produced during major orogenic events in an attempt to determine the systematics of melting under fluid absent conditions a series of piston cylinder experiments was conducted on two natural amphibolites one a meta alkali basalt aba with a total water content of wt contained in hornblende and the other a meta island arc tholeiite iat which has 1 1 3 wt water contained in hornblende cummingtonite and biotite the experimentally determined melting ranges of the two amphibolites showed that the solidus temperatures and sta temperature interval over which amphibole was stable were controlled by the amphibolites different bulk compositions and their resulting metamorphic assemblages the volume of melt produced by melting of the two amphibolites were compared with estimated amounts based on burnham s 1979 water melt solubility model and the fluid absent melting model presented by clemens and vielzeuf 1987 the observed melt volumes were greater than estimated as the water content of melt largely detemines the volume of melt produced independent measurements of the water content of the glass formed during partial melting in the aba were made by thermogravimetric analyses the water content of the aba glass is 2 wt which is less than the assumed melt water content water content of the melt used in previous modeling of fluidabsent anatexis in mafic lithologies as a consequence more melt can be expected during fluid absent partial melting of mafic lower crust as is observed in the experiments a modification of the clemens and vielzeuf 1987 fluid absent melting model for mafic compositions has been made using the experimental data available on melting in basaltic systems and is presented here for pressures of 5 8 and 10 kbar tectonic scenarios in which the crust is thickened i e by collision then undergoes extension or where a previously thinned crust is later rethickened provide enough heat so that amphibolite melting under fluid absent conditions can become importan and hence responsible for some melts produced during post collisional magmatism the results may also have applications to melting in hydrated oceanic crust in subduction zones and in island arc terains

post collisional melting of crustal sources constraints from geochronology petrology and sr nd isotope geochemistry of the variscan sichevita and poniasca granitoid plutons south carpathians romania the sichevita and poniasca plutons belong to an alignment of granites cutting across the metamorphic basement of the getic nappe in the south carpathians the present work provides shrimp age data for the zircon population from a poniasca biotite diorite and geochemical analyses major and trace elements sr nd isotopes of representative rock types from the two intrusions grading from biotite diorite to biotite k feldspar porphyritic monzogranite u pb zircon data yielded 311 2 ma for the intrusion of the biotite diorite granites are mostly high k leucogranites and biotite diorites are magnesian and calcic to calc alkaline sr and nd isotope and trace element data ree th ta cr ba and rb permit distinguishing five different groups of rocks corresponding to several magma batches the poniasca biotite diorite p1 shows a clear crustal character while the poniasca granite p2 is more juvenile conversely sichevita biotite diorite s1 and a granite s2 are more juvenile than the other sichevita granites s2 geochemical modelling of major elements and ree suggests that fractional crystallization can account for variations within p1 and s1 groups dehydration melting of a number of protoliths may be the source of these magma batches the variscan basement a subduction accretion wedge could correspond to such a heterogeneous source the intrusion of the sichevita poniasca plutons took place in the final stages of the variscan orogeny as is the case for a series of european granites around 310 ma ago especially in bulgaria and in iberia no alleghenian granitoids late carboniferous early permian times being known in the getic nappe the geodynamical environment of sichevita poniasca was typically post collisional of the variscan orogenic phase

thermal constraints on crustal rare gas release and migration evidence from alpine fluid inclusions the palfris marl of the helvetic alpine nappes contains four distinct vein fill generations ch4 rich gas is found in abundant fluid inclusions within these carbonate veins while free ch4 gas has also been produced from exploratory boreholes through this formation the stable isotope and helium neon and argon isotopic composition of these fluids has been determined a constant radiogenic 40ar concentration of 1 25 0 13 1 ppm in these differently sited fluids requires an intimate association between the 40arrad source and the hydrocarbon phase this can only be reasonably explained if the 40arrad was input into the hydrocarbon phase during hydrocarbon generation migration or storage prior to entrapment in the fluid inclusions stable isotope results constrain the maturity of hydrocarbon production while fluid inclusion formation pressures and temperatures record values of up to 2 5 kbars and 250 c these values place limits on the range of thermal conditions in which the hydrocarbon 40arrad relationship was established all fluids within inclusions also contain radiogenic 4he40ar values at predicted crustal production ratios these observations provide the first evidence that both 4herad and 40arrad can be quantitatively released on a regional scale bounded by the thermal conditions required to produce the hydrocarbon phase and the conditions under which the fluid inclusions were formed t 190 250 c these results require that negligible quantities of excess 40arrad decoupled from 4herad have been released into this system given the wide array of mechanisms which can potentially cause decoupling of these two species this result provides an important constraint on the role of these processes within the sedimentary fluid regime in contrast the free borehole gas contains excess radiogenic 4he and 21ne relative to 40arrad in proportions which can be accounted for by local production and subsequent diffusion from the surrounding marl the latter pattern is consistent with rare gas migration in lower temperature environments a conceptual model which considers both diffusional and metamorphic release of helium and argon and the ability of the surrounding fluid regime to transport the rare gases from their respective mineral production sites is consistent with both these results and data from regional rare gas studies

alpine polyphase tectono metamorphic evolution of the south carpathians a new overview abstract the main terrains involved in the cretaceous tertiary tectonism in the south carpathians segment of the european alpine orogen are the getic supragetic and danubian continental crust fragments separated by the severin oceanic crust floored basin during the early middle cretaceous times the danubian microplate acted initially as a foreland unit strongly involved in the south carpathians nappe stacking multistage folding thrusting events uplift erosion and extensional stages and the development of associated sedimentary basins characterize the south carpathians during cretaceous to tertiary convergence and collision events the main cretaceous tectogenetic events responsible for contraction and crustal thickening processes in the south carpathians are mid cretaceous austrian phase and latest cretaceous laramide or getic phase in age the architecture of the south carpathians suggests polyphase tectonic evolution and mountain building and includes from top to bottom the getic supragetic basement cover nappes the severin and arjana cover nappes and danubian basement cover nappes all tectonically overriding the moesian platform the severin nappe complex including obarsia and severin nappes with late jurassic early cretaceous ophiolites and turbidites is squeezed between the danubian and getic supragetic basement nappes as a result of successive thrusting of dismembered units during the inferred mid to late cretaceous subduction collision followed by tectonic inversion processes early cretaceous thick skinned tectonics was replaced by thin skinned tectonics in late cretaceous thus the former middle cretaceous austrian nappe stack and its albian lower senonian cover got incorporated in the intra senonian laramide getic stacking of the getic supragetic severin arjana nappes onto the danubian nappe duplex the two contraction events are separated by an extensional tectonic phase in the upper plate recorded by the intrusion of the banatitic magmas 84 73 ma the overthrusting of the entire south carpathian cretaceous nappe stack onto the fold thrust foredeep units and to the moesian platform took place in the late miocene intra sarmatian times and was followed by extensional events and sedimentary basin formation

helium isotope fluxes and groundwater ages in the dogger aquifer paris basin the dogger aquifer of the paris basin has been shown to be first order homogeneous in its helium concentration and its helium isotopic ratio although slight differences are noted in relation to specific chemical water types the helium isotopic composition of the dogger aquifer contains a small component of mantle helium that is associated with a magmatic input although the heat accompanying this magma is small constraints on the age of the overlying albien aquifer are used to calculate the flux of helium out of the dogger this constraint allows a calculation of the residence time of waters in the dogger aquifer of 4 myr 50 which is significantly longer than estimates derived from hydrologic models and hydrologic testing it is possible that the hydrologic ages and the helium ages can be reconciled by a vertically layered dogger aquifer with gradients in he salinity and fluid flow vertical profiles of fluid composition helium concentration and permeability are required in the paris basin to resolve this question

1 57 ga magmatism in the south carpathians implications for the pre alpine basement and evolution of the mantle under the european continent abstract chemical and isotopic analyses of amphibolites and gneisses from the oldest lithostratigraphic group in the south carpathians produce new insights regarding the chronology and evolution of the mobile proterozoic crust in central eastern europe these results date an episode of magmatism related to oceanic crust consumption reveal a period of continental stabilization and put constraints on the underlying mantle composition amphibolites from the cump na group have their origin in basalts chemically similar to island arc tholeiites a whole rock isochron age of documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand

geochemical constraints on crustal anatexis a case study from the pan african damara granitoids of namibia major and trace element models of recently published vapour absent mica dehydration melting experiments are used to identify granitoids generated by muscovite and biotite dehydration melting and to distinguish between plagioclase limited and biotite limited biotite dehydration melting in the case of granitoids from the pan african damara mobile belt namibia many of the leucogranites and salem type granitoids may be modelled by biotite dehydration melting the low rb sr granitoids e g donkerhuk onanis salem onanis donkerhuk nomatsaus salem goas probably reflect feldspar limited biotite dehydration melting a pelitic source whereas the high rb sr suites e g bloedkoppie leucogranite stinkbank leucogranite salem swakopmund leucocratic stink bank granite reflect biotite limited biotite dehydration melting a greywacke source alaskites from the damara belt have major element compositions which are consistent with muscovite dehydration melting and their positive eu anomalies are linked to high k2o reflecting k feldspar entrainment combined zr and lree light rare earth element solubility models indicate that insufficient time probably less than 104 years had elapsed between melt generation and melt extraction to ensure that the alaskite melts attained their equilibrium concentrations of zr and the lrees in contrast the leucogranites and salem type granites have attained their equilibrium inventories of these trace elements combined fe2o3 and mgo contents in some samples from two granitoids the salem goas and donkerhuk onanis intrusions are higher than those readily attainable by biotite dehydration melting indicating either 1 that they contain a contribution from melts generated by incipient garnet breakdown or 2 that they contain small amounts of an entrained ferromagnesian phase

petrogenesis of archaean trondhjemites tonalites and granodiorites from eastern finland major and trace element geochemistry

variscan collisional magmatism and deformation in nw iberia constraints from u pb geochronology of granitoids u pb geochronology of variscan granitoid rocks from the west asturian leonese zone of the nw iberian belt documents the episodic nature of magmatism in this section of the western european variscides each magmatic episode is characterized by granitoids with distinct features and has a duration on the scale of several millions of years the ages of these granitoids place new constraints on the age and duration of magmatic and tectonic events that are consistent with previous structural studies and proposed models for the tectonic evolution and migration of deformation in the nw iberian variscan belt granitoid rocks in this zone belong to two main magmatic episodes syn and post tectonic relative to the variscan orogeny and are broadly representative of the granitoid types found in the nw iberian variscan belt the syntectonic association is formed by i tonalite granodiorite monzogranite intrusions emplaced synchronously with the main phase of crustal deformation d 2 at c 325 ma and ii younger leucogranite intrusions emplaced synchronously with syn convergence extensional structures at c 320 310 ma the post tectonic association is composed of i volumetrically dominant tonalite granodiorite monzogranite intrusions and associated minor mafic intermediate rocks emplaced at c 295 290 ma and ii scarce leucogranite intrusions emplaced at c 290 285 ma