the mesozoic margin of the maghrebian tethys in the rif belt morocco evidence for polyphase rifting and related magmatic activity

impact of basin structure and evaporite distribution on salt tectonics in the algarve basin southwest iberian margin abstract the sw iberian margin developed as a passive margin during mesozoic times and was later inverted during the mainly cenozoic alpine orogeny the initial syn rift deposits include a lower jurassic evaporite unit of variable thickness in the onshore this unit is observed to thicken basinward i e southward in fault controlled depocenters and salt related structures are only present in areas of thick initial evaporites in the offshore multiple salt structures cored by the lower jurassic evaporites are interpreted on seismic reflection data and from exploratory drilling offshore salt structures include the allochthonous esperanca salt nappe which extends over an area roughly 40 60 km the abundance of salt related structures and their geometry is observed to be controlled by the distribution of evaporite facies which is in turn controlled by the structure of rift related faulting this paper presents a comprehensive study of salt tectonics over the entire onshore and offshore sw iberian passive margin southern portugal and gulf of cadiz covering all aspects from initial evaporite composition and thickness to the evolution of salt related structures through mesozoic extension and cenozoic basin inversion

thermochronology of central ribeira fold belt se brazil petrological and geochronological evidence for long term high temperature maintenance during western gondwana amalgamation abstract the studied sector of the central ribeira fold belt se brazil comprises metatexites diatexites charnockites and blastomylonites this study integrates petrological and thermochronological data in order to constrain the thermotectonic and geodynamic evolution of this neoproterozoic ordovician mobile belt during western gondwana amalgamation new data indicate that after an earlier collision stage at 610 ma zircon u pb age peak metamorphism and lower crust partial melting coeval with the main regional high grade d1 thrust deformation occurred at 572 562 ma zircon u pb ages the overall average cooling rate was low 650 c during sub horizontal d2 shearing until 510 470 ma garnet wr sm nd ages 1 2 c ma being then rapidly exhumed cooled 8 30 c ma during post orogenic d3 deformation with late granite emplacement at 490 ma zircon u pb age cooling rates based on garnet biotite fe mg diffusion are broadly consistent with the geochronological cooling rates a metatexites were cooled faster at high temperatures 6 c ma and slowly at low temperatures 0 1 c ma decreasing cooling rates with time b charnockites show low cooling rates 2 c ma near metamorphic peak conditions and high cooling rates 120 c ma at lower temperatures increasing cooling rates during retrogression the charnockite thermal evolution and the extensive production of granitoid melts in the area imply that high geothermal gradients were sustained for a long period of time 50 90 ma this thermal anomaly most likely reflects upwelling of asthenospheric mantle and magma underplating coupled with long term generation of high hpe heat producing elements granitoids these factors must have sustained elevated crustal geotherms for 100 ma promoting widespread charnockite generation at middle to lower crustal levels

a 700 ma sm nd garnet whole rock age from the granulite facies central kaoko zone namibia evidence for a cryptic high grade polymetamorphic history continental collision of the kalahari and the congo craton in africa and the rio de la plata craton in south america resulted in a structurally complex neoproterozoic belt system the kaoko dom feliciano ribeira belt it is uncertain whether these three cratons collided more or less simultaneously during one single orogenic event at 580 550 ma or whether the belt owe its structural and metamorphic features to several so far poorly constrained events the kaoko belt nw nambia representing the belt system between the southern congo craton and the rio de la plata craton is an ideal object to study these complexities within this belt high grade meta igneous and metasedimentary rocks of the sillimanite k feldspar zone contain large garnet porphyroblasts that grew at ca 730 10 c at 6 7 1 2 kbar during peak metamorphic granulite facies conditions a sm nd garnet whole rock errorchron n 7 mswd 6 0 obtained on a siliceous metasediment yielded an unexpected old age of 692 14 ma which is interpreted as an inherited metamorphic age of an older granulite facies event it is evident that the dated garnets survived the younger high grade granulite facies metamorphism that occurred between ca 570 and 520 ma and preserved their old sm nd isotope systematics implying that the closure temperature for sm nd in garnet is higher than ca 730 c in this case these results imply fast cooling rates at different times during the pan african orogeny that prevent isotopic homogenization at sample scale moreover it is suggestive that trace element ree diffusion in garnet is considerably slower than major element diffusion from a regional point of view it is possible that these specific siliceous metasedimentary rocks may be unrelated to the pan african metamorphic evolution of the kaoko belt between 570 and 520 ma and may represent lithological units that belong to so far unrecognized terranes in the kaoko belt 2007 elsevier b v all rights reserved

zircon trace element characteristics and ages in granulite xenoliths a key to understanding the age and origin of the lower crust arkhangelsk kimberlite province russia garnet granulite and pyroxenite xenoliths from the grib kimberlite pipe arkhangelsk nw russia represent the lower crust beneath russian platform in close vicinity to the cratonic region of the north eastern baltic fennoscandian shield many of the xenoliths have experienced strong interaction with the kimberlite host but in others some primary granulite facies minerals are preserved calculated bulk compositions for the granulites suggest that their protoliths were basic to intermediate igneous rocks pyroxenites were ultrabasic to basic cumulates a few samples are probably metasedimentary in origin zircons are abundant in the xenoliths they exhibit complex zoning in cathodoluminescence with relic cores and various metamorphic rims cores include oscillatory zircon crystallized in magmatic protoliths and metamorphic and magmatic sector zoned zircons recrystallization of older zircons led to the formation of bright homogeneous rims in some samples homogeneous shells are surrounded by darker convoluted overgrowths that were formed by subsolidus growth when a change in mineral association occurred the source of zr was a phase consumed during a reaction which produced garnet late generation zircons in all xenoliths show concordant u pb ages of 1 81 1 84 ga 1 826 11 ma interpreted as the age of last granulite facies metamorphism this event completely resets most zircon cores an earlier metamorphic event at 1 96 1 94 ga is recorded by some rare cores and a few magmatic oscillatory zircons have retained a neoarchaean age of 2 719 14 ma the assemblage of metaigneous and metasedimentary rocks was probably formed before the event at 1 96 ga inherited magmatic zircons indicate the existence of continental crust by the time of intrusion of magmatic protoliths in the late archaean the u pb zircon ages correspond to major events recorded in upper crustal rocks of the region collisional metamorphism and magmatism 2 7 ga ago and reworking of archaean rocks at around 1 95 1 75 ga however formation of the granulitic paragenesis in lower crustal rocks occurred significantly later than the last granulite facies event seen in the upper crust and correlates instead with retrograde metamorphism and small volume magmatism in the upper crust

arkose subarkose quartz sand and associated muds derived from felsic plutonic rocks in glacial to tropical humid climates abstract in the range of climates from high latitude glacial to equatorial tropical the mineral and chemical compositions of over 600 modern nonmarine first cycle sands and muds produced from felsic crystalline basement provenance in various tectonic settings were studied under glacial conditions sediments are developed by mostly mechanical weathering and thus are mineralogically and chemically similar to their source rocks humid tropical climates yield quartz rich sands kaolinite rich muds along with si mg ca na and k in solution between these extremes a wide range of detrital sediment compositions are developed in arid to wet climates in arid climate and semi arid climate sands feldspar is commonly more abundant than quartz and plagioclase k feldspar except in the cases of granite source rocks the muds are feldspathic with smectite dominant in the clays with increasing temperature and moisture in temperate to subtropical climates mafic silicates and feldspars are destroyed so that the sands range from arkoses with plagioclase as the major feldspar to subarkoses with dominant k feldspar through this range of climate muds become more kaolinitic and thus more aluminous with increasingly severe weathering typically feldspathic sands are associated with feldspathic muds with decreasing feldspar in sands there is less feldspar in muds with increasingly intense chemical weathering rare earth elements ree are more fractionated into muds typically sands contain 150 ppm ree with much of the ree in heavy minerals abundance differences are greatest between humid tropical sands and muds in many samples the ree are mostly in monazite allanite sphene and apatite more intensely weathered samples contain more xenotime and zircon evidenced by yb enrichment

how to build an extensional basin in a contractional setting numerical and physical modelling applied to the mejerda basin at the front of the eastern tell of tunisia abstract in tunisia africa eurasia convergence resulted in the tell fold and thrust belt ftb part of the tell rif or maghrebides associated to the south to the atlas intra continental belt the mejerda rift basin is developed over the compressional boundary between the tell and the atlas the main objective of this study is to identify the mechanism responsible for such a basin development the abundance of triassic salt structures in the northern atlas compared to the tell suggests a friction change along the basal decollement formed by the salt level this frictional contrast is expected to produce a heterogeneous deformational response and complex structures on this basis numerical and analogue modelling were used to investigate the impact of a friction change on the deformation style the kinematic approach of the limit analysis theory was applied here via the software optum g2 the onset of faulting in all numerical tests reveals extensional faults appear at the friction boundary beyond the onset of faulting sandbox experiments show extensional features such as collapsing blocks and extensional faults overall the adopted numerical and physical approaches showed evidences of extension in particular the experiment using sandpaper and silicon putty pdms for the decollement transition shows viscous deformation mimicking diapiric movement these observations of numerical and physical model behaviors highlight that compression can produce an extensional basin at the top of a thrust wedge as expression of a strong basal friction change hence the mejerda basin is interpreted as a wedge top basin it developed during the growth of the tell and atlas ftbs the same configuration likely exists in other mountain belts the case of the molasse basin between the jura and the western alps is advocated

extending the krogh legacy development of the ca tims method for zircon u pb geochronologythis article is one of a series of papers published in this special issue on the theme of geochronology in honour of tom krogh tom krogh was without a doubt the most influential u pb geochronologist of the modern era his career was marked by a continuing series of breakthroughs both revolutionary and evolutionary his legacy is such that every lab around the world uses the methods he developed after some fascinating insights into chemical leaching effects in zircon tom largely dropped this approach in favor of his enormously successful air abrasion technique coupled with careful sample selection at the individual grain level here i describe continued experiments with leaching and step wise dissolution techniques and how a series of alternating steps forward and setbacks eventually led to a new chemical abrasion or ca tims method for selectively removing those domains within zircon that have lost pb continuing to strive for improvements in analytical technique is perhaps the best way to honor tom s many contributions and extend his legacy

grenville age a type and related magmatism in southern laurentia texas and new mexico u s a abstract large grenville age plutons are known to be present in the 1340 1370 ma basement in texas and eastern new mexico but their distribution and compositional ranges are poorly known new u pb zircon dates shrimp rg stanford university on subsurface samples have identified widespread presence of compositionally diverse grenville age 1070 1110 ma plutonic rocks in the texas panhandle core from a 175 m thick sill like gabbroic intrusion yielded a date of 1081 8 3 ma in situ differentiation of this tholeiitic magma led to a 7 fold increase in incompatible element concentrations as ree patterns remained essentially flat trace element data suggest an e morb like source to the south alkali feldspar granite with a type affinities was recovered from drill core from the abilene gravity minimum near albany texas it yielded a date of 1078 23 ma similar to undeformed post orogenic granites in the llano uplift further west an anorthosite xenolith from the eocene three sisters intrusion in el paso texas yielded a date of 1068 30 ma which is within error of the 1110 19 ma age determined for the main stage of the nearby a type red bluff granitic suite xenoliths from potrillo maar volcano in the center of the southern rio grande rift include monzonitic xenoliths with granulitic texture which yielded dates of 1072 ma in contrast potrillo maar xenoliths with igneous textures yielded u pb dates of 27 ma development of the granulitic texture is interpreted to reflect granulite facies metamorphism during tertiary time 26 27 ma on the basis of age zoning and u concentrations in the zircon crystals our results show that grenville age magmatism in the texas and new mexico subsurface was widespread and was coeval with syn and post deformation granites in the llano uplift of central texas the compositions of dated samples suggest that mafic magmas were broadly tholeiitic in character and that granitic magmas were a type these magmatic affinities are thought to be associated with crustal extension rather than with subduction moreover the presence of anorthosite xenoliths in the el paso kilbourne hole region the broadly tholeiitic character of mafic rocks and the a type nature of the granitic rocks are broadly similar to amcg anorthosite mangerite charnockite granite suites present in the grenville province of eastern laurentia

differential response of zircon u pb isotopic systematics to metamorphism across a lithologic boundary an example from the hope valley shear zone southeastern massachusetts usa a detailed morphological chemical and isotopic study of zircons from a single outcrop of two mineralogically and chemically distinct units of the late precambrian ponaganset gneiss was undertaken to investigate the effects of mylonitization and metamorphism on u pb isotopic systematics late paleozoic amphibolite grade approx 600 c mylonitization of the ponaganset gneiss at this locality is associated with movement along the hope valley shear zone the response of zircon to metamorphism in each gneiss unit is distinct zircons in gray augen gneiss are uncorroded and not overgrown whereas zircons from fluorite bearing pink granitic gneiss are variably corroded and over 50 bear opaque overgrowths the zircon overgrowths are chemically distinct from the primary cores and contain high conentrations of hf u hree and th mylonite derived from the gray gneiss contains only a small population of hf u rich metamorphic zircon but zircons in the pink gneiss derived mylonite are dominated by the hf u rich metamorphic component in terms of their u pb isotopic systematics overgrowth free zircons from both units are markedly discordant gray 10 20 pink 35 but overgrown zircons from the pink gneiss are up to 70 discordant zircons from the mylonites yield younger pb pb and u pb ages than those of the protolith gneisses and isotopic data from each gneiss mylonite pair define a linear array on concordia plots upper intercept ages of the gray gneiss 621 27 ma and the pink gneiss 635 50 ma indicate that the crystallization of both units was coeval and the lower intercept ages gray 270 92 ma pink 285 26 ma fall within the range of other published age estimates for alleghanian metamorphism in southeastern new england e g zartman et al 1988 new growth of zircon suggests that zr was mobile during metamorphism the presence of fluorite in the pink gneiss and a discontinuity in log left f text h text 2 text o mathord left vphantom f text h text 2 text o f text hf right kern nulldelimiterspace f text hf right values obtained from biotite across the pink gneiss gray gneiss contact indicates that dissolution and reprecipitation of zircon may be related to local variations in hf fugacity zircon dissolution reprecipitation in the pink gneiss and the lack of similar features in the contiguous gray gneiss suggests that the degree of isotopic perturbation of zircon during metamorphism is related to bulk chemistry fluid chemistry and or the degree of fluid rock interaction

revealing hidden structures the application of cathodoluminescence and back scattered electron imaging to dating zircons from lower crustal xenoliths abstract the use of cathodoluminescence cl and or back scattered electron bse imaging techniques with in situ ion probe analyses of zircons can help unravel complex crustal histories of metamorphic rocks that otherwise might remain elusive using these techniques we have imaged zircons from three lower crustal xenolith suites that have previously been dated by shrimp sensitive high resolution ion microprobe in all three cases the zircons are featureless in transmitted light but cl and bse reveal internal structures that correlate with distinct growth events generally cl and bse images reveal similar structures with cl showing finer detail neither imaging technique is capable of delineating all growth features in every sample and the best results are obtained using a combination of the two techniques igneous cores in zircons commonly emit a different color cl emission in many cases zoning in the cores is truncated indicating that the zircons either spent time in the supracrustal environment after their initial crystallization and prior to the granulite facies event s or that part of the core zircon was resorbed during the subsequent metamorphic event metamorphic rims when present are commonly 10 to 30 m thick and are nearly always unzoned and featureless igneous cores and metamorphic overgrowths commonly have distinctive cl emission spectra and trace element concentrations however the cl spectra can only be used to qualify chemical differences as a linear relationship has not been shown to exist between cl intensity and trace element concentration in natural zircons in many cases the hf y p u and the heavy rare earth elements hrees concentrations can be correlated to igneous and metamorphic growth using a combination of cl and bse imaging techniques and in situ trace element analyses with either the electron microprobe or pixe particle induced x ray emission

annealing radiation damage and the recovery of cathodoluminescence the structural recovery upon heat treatment of a highly metamict actinide rich zircon u 6000 ppm has been studied in detail using a range of techniques including x ray powder diffraction raman spectroscopy shrimp ion probe electron microprobe transmission electron microscopy and cathodoluminescence analysis the structural regeneration of the amorphous starting material depends on random nucleation it starts between 800 and 950 c when amorphous zrsio4 decomposes to form crystalline zro2 and amorphous sio2 at around 1100 c well crystallised zrsio4 grows at the expense of the oxides u has been retained in the newly grown zircon whereas pb was evaporated during the heat treatment this process is in marked opposition to the reconstitution of moderately metamict minerals which experience a gradual recovery controlled by the epitaxial growth at the crystalline amorphous boundaries both of these recovery processes are not the direct inverse of metamictisation the structural regeneration was found to be connected with a significant increase in the emission of cl in all cases annealing heavily damaged zircon and moderately damaged zircon and monazite we observe that the final wellcrystallised annealing products emit more intense cl than their radiation damaged starting minerals although having almost identical elemental composition our observations are taken as evidence that the cl is not only determined by the chemical composition of the sample but is also strongly controlled by structural parameters such as crystallinity or the presence of defect centres

systematics of internal zircon morphology in major variscan granitoid types the internal morphologies of zircon crystals from different types of granitoids alkaline calcalkaline and anatectic are revealed by cathodoluminescence imaging and are described in terms of growth rates of the crystal faces relative to each other zircons in the alkaline granitoids are characterized by high and constant growth rates of 010 relative to the pyramidal forms and by symmetric grwoth of 011 zircons in the calcalkaline and anatectic granitoids are characterized by fluctuating or gradually decreasing relative growth rates of 010 by asymmetric and highly variable growth of 011 and by a tendency of 110 to become grwoth inhibited corrosion events are interspersed during zircon growth in the calcalkaline magmas in the calcalkaline and anatectic magmas a discontinuity breaks the morphological evolution at late stages of crystallization the discontinuity coincides with a sharp drop in cathodoluminescence the growth behaviour of each crystal form is analysed and compared with predictions made by the periodic bond chain pbc theory it is argued that the relative growth rate of 010 depends on supersaturation that the growth rates of 011 faces are changed in response to different ratios of adsorbing cations na k al and that 110 faces become growth inhibited by the adsorption of h2o or trace elements enriched in the residual liquid morphological and chemical discontinuities at late stages of crystallization are reasonably explained by the formation of larger growth units from smaller ionic entities in the residual liquid important factors controlling the zircon morphology in different types of granitoids are high cooling rates alkaline magmas magma mixing calcalkaline magmas enrichment of h2o and trace elements in residual liquids calcalkaline and anatectic magmas and the major element chemistry of the magma possibly the ratio of na and k to al agpaicity

evaluating the source and seasonality of submarine groundwater discharge using a radon 222 pore water transport model article i nfo pore water radon 222 rn distributions from indian river lagoon florida are characterized by three zones a lower zone where pore water 222 rn and sediment bound radium 226 ra are in equilibrium and concentration gradients are vertical a middle zone where 222 rn is in excess of sediment bound 226 ra and concentration gradients are concave downward and an upper zone where 222 rn concentration gradients are

alpha emitting isotopes and chromium in a coastal california aquifer abstract the unadjusted 72 h gross alpha activities in water from two wells completed in marine and alluvial deposits in a coastal southern california aquifer 40 km north of san diego were 15 and 25 picocuries per liter pci l although activities were below the maximum contaminant level mcl of 15 pci l when adjusted for uranium activity there is concern that new wells in the area may exceed mcls or that future regulations may limit water use from the wells coupled well bore flow and depth dependent water quality data collected from the wells in 2011 with analyses for isotopes within the uranium actinium and thorium decay chains show gross alpha activity in marine deposits is associated with decay of naturally occurring 238 u and its daughter 234 u radon activities in marine deposits were as high as 2230 pci l in contrast gross alpha activities in overlying alluvium within the piedra de lumbre watershed eroded from the nearby san onofre hills were associated with decay of 232 th including its daughter 224 ra radon activities in alluvium from piedra de lumbre of 450 pci l were lower than in marine deposits chromium vi concentrations in marine deposits were less than the california mcl of 10 g l effective july 1 2014 but 53 cr compositions were near zero and within reported ranges for anthropogenic chromium alluvial deposits from the nearby las flores watershed which drains a larger area having diverse geology has low alpha activities and chromium as a result of geologic and geochemical conditions and may be more promising for future water supply development

zircon the metamorphic mineral a mineral that forms under conditions as variable as diagenesis to deep subduction melt crystallization to low temperature alteration and that retains information on time temperature trace element and isotopic signatures is bound to be a useful petrogenetic tool the variety of conditions under which zircon forms and reacts during metamorphism is a great asset but also a challenge as interpretation of any geochemical data obtained from zircon must be placed in pressure temperature deformation fluid context under which condition and by which process zircon forms in metamorphic rocks remains a crucial question to answer for the correct interpretation of its precious geochemical information in the last 20 years there has been a dramatic evolution in the use of zircon in metamorphic petrology with the advent of in situ dating techniques zircon became relevant as a mineral for age determinations in high grade metamorphic rocks since then there has been incredible progress in our understanding of metamorphic zircon with the documentation of growth and alteration textures its capacity to protect mineral inclusions zircon thermometry trace element patterns and their relation to main mineral assemblages solubility of zircon in melt and fluids and isotopic systematics in single domains that go beyond u pb age determinations metamorphic zircon is no longer an impediment to precise geochronology of protolith rocks but has become a truly indispensable mineral in reconstructing pressure temperature time fluid paths over a wide range of settings an obvious consequence of its wide use is the rapid increase of literature on metamorphic zircon and any attempt to summarize it can only be partial in this chapter reference to published works are intended as examples and not as a compilation this chapter approaches zircon as a metamorphic mineral reporting on its petrography and texture deformation structure and mineral chemistry including trace element and isotopic systematics linking this information together highlights the potential of zircon as a key mineral in petrochronology

radium isotopes in saline seepages south western yilgarn western australia abstract water samples from saline seepages in the south western yilgarn block of western australia contain high activities of the four naturally occurring radium isotopes activities of up to 310 pci l for 226ra and 1720 pci l for 228ra were measured and the 228 ra 226 ra ratio averaged 6 1 activities of the two short lived radium isotopes were also high 223ra activities of up to 94 pci l were found with an average 226 ra 223 ra ratio of 3 3 considerably lower than the natural abundance ratio of 21 4 activities of up to 23 pci l 227ac the long lived t 1 2 22 years grandparent of 223ra were also measured the analysis of surface granite samples the probable source rocks of the radium gave th u activity ratios of around 1 5 the higher 228 ra 226 ra ratios of the waters were attributed to readily leached 228ra in the weathered granites as a result of thorium remaining after weathering leach experiments on u th ore by nacl solutions showed that all four radium isotopes were equally leached sulphate anions reduced the 226ra and 228ra leaching to a greater extent than for 223ra and 224ra suggesting that the latter isotopes were being supported in solution by parent isotopes in particular this suggested 227ac was leached into the sulphate solution but this does not fully account for the amount of 227ac seen in the seepage waters

u pb garnet chronometry in high grade rocks case studies from the central damara orogen namibia and implications for the interpretation of sm nd garnet ages and the role of high u th inclusions garnets from different migmatites and granites from the damara orogen namibia were dated with the u pb technique after bulk dissolution of the material measured 206pb 204pb ratios are highly variable and range from ca 21 to 613 variations in isotope 208pb 204pb 206pb 204pb and trace element th u u nd sm nd ratios of the different garnets show that some garnets contain significant amounts of monazite and zircon inclusions due to their very low 206pb 204pb ratios garnets from pelitic migmatites from the khan area yield pb pb ages with large errors precluding a detailed evaluation however the 207pb 206pb ages ca 550 500 ma appear to be similar to or older than u pb monazite ages 530 1 517 1 ma and sm nd garnet ages 523 4 512 3 ma from the same sample it is reasonable to assume that the pb pb garnet ages define growth ages because previous studies are consistent with a higher closure temperature for the u pb system in garnet relative to the u pb system in monazite and the sm nd system in garnet for igneous migmatites from oetmoed pb pb garnet ages 483 15 492 16 ma and one sm nd garnet whole rock age 487 8 ma are similar whereas the monazite from the same sample is ca 30 40 ma older 528 1 ma these monazite ages are however similar to monazite ages from nearby unmigmatized granite samples and constrain precisely the intrusion of the precursor granite in this area although there is a notable difference in closure temperature for the u pb and sm nd system in garnet the similarity of both ages indicate that both garnet ages record garnet growth in a migmatitic environment restitic garnet from an unmigmatized granite from omaruru yields similar u pb 493 30 506 30 ma and sm nd 493 6 488 7 ma garnet ages whereas the monazite from this rock is ca 15 25 ma older 516 1 514 1 ma whereas the monazite ages define probably the peak of regional metamorphism in the source of the granite the garnet ages may indicate the time of melt extraction for igneous garnets from granites at oetmoed the similarity between pb pb 483 34 474 17 ma and sm nd 492 5 484 13 ma garnet ages is consistent with fast cooling rates of granitic dykes in the lower crust differences between garnet and monazite u pb ages can be explained by different reactions that produced these minerals at different times and by the empirical observation that monazite seems resistant to later thermal re equilibration in the temperature range between 750 and 900 c e g braun et al 1998 for garnet analyses that have low 206pb 204pb ratios the influence of high inclusions is small however the relatively large errors preclude a detailed evaluation of the relationship between the different chronometers for garnet with higher 206pb 204pb ratios the overall similarity between the pb pb and sm nd garnet ages implies that the inclusions are not significantly older than the garnet and therefore do not induce a premetamorphic pb signature upon the garnet the results presented here show that garnet with low 238u 204pb ratios together with sm nd garnet data and u pb monazite ages from the same rock can be used to extract geologically meaningful ages that can help to better understand tectonometamorphic processes in high grade terranes

seawater 234 u 238 u during the last 800 thousand years abstract constraining the history of seawater 234u 238u is important because this ratio is used to assess the validity of u th ages and because it provides information about the past rate of physical weathering on the continents this study makes use of u rich slope sediments from the bahamas in an attempt to reconstruct seawater 234u 238u for the last 800 kyr for the last 360 kyr u th dating of these sediments provides ages and initial 234u 238u values sixty seven samples largely from marine highstands have initial 234u 238u which scatter somewhat about the modern seawater value 1 145 but neither this scatter nor the average value increases with age of sample these data contrast with published coral data and suggest that seawater 234u 238u has remained within 15 of the modern value for the last 360 kyr this confirms the rejection of coral u th ages where the initial 234u 238u is significantly different from modern seawater data from older highstands dated with 18o stratigraphy or by the presence of the brunhes matuyama b m reversal at 780 kyr allow seawater 234u 238u to be assessed prior to the range of the 230th chronometer unfortunately diagenetic scatter in the data between the b m reversal and 360 kyr is rather large probably relating to low u concentrations for these samples but there is no indication of a trend in seawater 234u 238u with age high u samples from close to the b m reversal show less diagenetic scatter and an initial 234u 238u that averages 1 102 this lower value can be explained by lower seawater 234u 238u at the time of the b m reversal or by progressive loss of 234u from the sediment by recoil a simple box model is presented to illustrate the response of seawater 234u 238u to variations in riverine input such as might be caused by changes in continental weathering comparison of the bahamas 234u 238u data with model results indicates that riverine 234u 238u has not varied by more than 65 for any 100 kyr period during the last 360 kyr it also indicates that the ratio of physical to chemical weathering on the continents has not been higher than at present for any extended period during the last 800 kyr

radionuclides in ground water of the carson river basin western nevada and eastern california u s a abstract ground water is the main source of domestic and public supply in the carson river basin ground water originates as precipitation primarily in the sierra nevada in the western part of carson and eagle valleys and flows down gradient in the direction of the carson river through dayton and churchill valleys to a terminal sink in the carson desert because radionuclides dissolved in ground water can pose a threat to human health the distribution and sources of several naturally occurring radionuclides that contribute to gross alpha and gross beta activities in the study area were investigated generally alpha and beta activities and u concentration increase from the up gradient to down gradient hydrographic areas of the carson river basin whereas222rn concentration decreases both226ra and228ra concentrations are similar throughout the study area alpha and beta activities and u concentration commonly exceed 100 pci l in the carson desert at the distal end of the flow system radon 222 commonly exceeds 2 000 pci l in the western part of carson and eagle valleys adjacent to the sierra nevada radium 226 and228ra concentrations are alpha emitting radionuclides in the ground water originated from the dissolution of u rich granitic rocks in the sierra nevada by co2 oxygenated water dissolution of primary minerals mainly titanite sphene in the granitic rocks releases u to the water dissolved u is probably removed from the water by adsorption on fe and mn oxide coatings on fracture surfaces and fine grained sediment by adsorption on organic matter and by coprecipitation with fe and mn oxides these coated sediments are transported throughout the basin by fluvial processes thus u is transported as dissolved and adsorbed species a rise in the water table in the carson desert because of irrigation has resulted in the oxidation of u rich organic matter and dissolution of u bearing coatings on sediments producing unusually high u concentration in the ground water alpha activity in the ground water is almost entirely from the decay of u dissolved in the water beta activity in ground water samples is primarily from the decay of40k dissolved in the water and ingrowth of238u progeny in the sample before analysis approximately one half of the measured beta activity may not be present in ground water in the aquifer but instead is produced in the sample after collection and before analysis potassium 40 is primarily from the dissolution of k containing minerals probably k feldspar and biotite radon 222 is primarily from the decay of226ra in the aquifer materials radium in the ground water is thought to be mainly from alpha recoil associated with the decay of th in the aquifer material some ra may be from dissolution or desorption or ra rich coatings on sediments

long term exhumation of an aegean metamorphic core complex granitoids in the northern menderes massif western turkey the egrigoz koyunoba and alacam plutons are located in the northern portion of the menderes massif a region characterized today by large scale extension to gain a better understanding of their exhumation history we acquired zircon ages geochemical analyses and cathodoluminescence cl images to search for evidence of micro to macro scales of deformation the combination is a powerful means to decipher their tectonomagmatic history in situ ion microprobe 238u 206pb zircon ages of the granitoids range from 30 0 3 9 ma to 14 7 2 6 ma 1 and indicate the plutons crystallized over 15 m y the dated zircons show cl zoning consistent with igneous crystallization and are only located adjacent to or as inclusions in biotite grains the youngest ages are dominated by a blue color in cl whereas a majority of older zircons are yellow higher th u contents are typically shown by zircons with black or dark green colors in cl most of the samples of these granitoids indicate they are magnesian calc alkalic and peraluminous granite to granodiorites but variations exist likely reflecting heterogeneity caused by magma mixing partial melting crustal contamination and post emplacement fluid interactions as evidenced by the cl images cl images show that all samples experienced magma mixing multiple episodes of brittle deformation and fluid mediated alteration sources for the generation of northern menderes massif granitoids include simple slab induced upwelling from the subduction of the african plate along the hellenic arc adiabatic decompression as the northern edge of turkey9s anatolide tauride block delaminates and upwelling asthenosphere as the continental lithosphere thins during extension we present a model in which western turkey is an amalgamation of stacked subduction zones that transition from north to south over time northern menderes massif granitoids document their complex geologic history in their ages chemistry and textures

magmatic zircons from i s and a type granitoids in tibet trace element characteristics and their application to detrital zircon provenance study abstract this study reports a dataset of 21 trace elements of magmatic zircons from typical i type 93 analyses and s type 73 analyses granitoids from the lhasa and himalayan terranes southern tibet and a type granitoids 21 analyses from the songpan ganzi terrane eastern tibet our results indicate that magmatic zircons from the i type granitoids are characterized by relatively lower pb concentrations and higher nb pb n ratios distinct from those of the s type granitoids that reveal higher pb lower nb pb n and significant eu negative anomalies eu eu 0 3 while these values are transitional in zircons from the a type granitoids such differences most likely governed by compositional variations in the host magmas are considered as a useful tool in addition to u pb and hf isotopes for tracing the source provenance of detrital zircons consequently the trace element compositions of detrital zircons with a diagnostic age peak 1170 ma from the lhasa terrane indicate the presence of coeval s type magmatism in their source region this study exemplifies the usefulness of zircon trace element geochemistry in investigating sedimentary source provenance and paleogeographic reconstruction

an overview of the late cretaceous eocene positive inversions and oligo miocene subsidence events in the foreland of the tunisian atlas structural style and implications for the tectonic agenda of the maghrebian atlas system abstract structural interpretation of surface and subsurface data in the eastern foreland basins of tunisia allows us to recognize positive inversion structures i e related to compressional events expressed and recorded in the paleogene sedimentary pile of the atlas domain these episodes are followed by a period of relative tectonic quiescence during oligocene early miocene with development of extensional structures with slightly tilted panels grabens and locally the development of listric normal faults branched at depth on decollement levels comparison of the seismic interpretations and field data collected along the south atlas front as well as in the atlas belt allows us to propose a tectonic scenario at the scale of eastern tunisia atlas in particular we emphasise the role of the so called atlas event middle late eocene which was initially defined in algeria but poorly outlined in tunisia we will show that the tectonic agenda defined in eastern tunisia is consistent with the one proposed elsewhere in the maghreb allowing us to propose a unified view of the geodynamic evolution of the whole atlas system during this period the negative inversion recorded during oligocene early miocene along with the occurrence of synsedimentary normal faults are related to an increase of the subsidence rate in the frame of continuous shortening coeval to basin formation in the front of the atlas the whole oligo miocene evolution results from two different mechanisms flexuration in the front of the atlas and the onset of the extension of the pantelleria linosa malta rift system of central mediterranean which attained a climax stage later during pliocene quaternary

chromium chromium isotopes and selected trace elements western mojave desert usa abstract chromium vi concentrations in excess of the california maximum contaminant level mcl of 50 g l occur naturally in alkaline oxic ground water in alluvial aquifers in the western mojave desert southern california the highest concentrations were measured in aquifers eroded from mafic rock but cr vi as high as 27 g l was measured in aquifers eroded from granitic rock chromium vi concentrations did not exceed 5 g l at ph

an extended episode of early mesoproterozoic metamorphic fluid flow in the reynolds range central australia the products of metamorphic fluid flow are preserved in zones within the marbles and metamorphosed semipelites of the upper calcsilicate unit in the granulite portion of the late palaeoproterozoic reynolds range group northern arunta block central australia the zones of retrogression characterized by minerals such as wollastonite grossular and clinohumite local resetting of oxygen isotopic compositions and local major element metasomatism were channelways for water rich fluids derived from granulite facies metapelites u th pb isotopic ages measured by the shrimp ion microprobe on zircon and monazite from a granulite facies semipelite an early semiconcordant aluminous quartz rich fluid flow segregation and a late discordant quartz rich segregation record some of the extended thermal history of the area zircon cores from the semipelite show its likely protolith to be an igneous rock 1812 11 ma old itself derived from a source containing zircon as old as 2 2 ga low th u overgrowths on the zircon grew during granulite facies metamorphism at 1594 6 ma monazite cooled to its blocking temperature at 1576 8 ma zircon cores from the semiconcordant segregation are dominantly 2 3 ga old indicating that the source of the fluids was not the particular metamorphosed semipelite studied two generations of low th u overgrowths on the zircon give indistinguishable ages for the older and younger of 1589 8 and 1582 8 ma respectively the monazite age is the same 1576 12 ma zircon from the late discordant segregation gave 1568 4 ma fluid flow occurred for at least 18 3 ma and ended 26 3 ma after the peak of metamorphism suggesting a very slow cooling rate of 3 c ma 1 the last regional high grade metamorphism in the reynolds range occurred at 1 6 ga not 1 78 ga as previously thought the high grade event at 1 78 ga is a separate event that affected only the basement to the reynolds range group

slow cooling of deep crustal granulites and pb loss in zircon abstract we attempted to obtain the magmatic crystallization age of a metamorphosed granulite facies t 800 90 c p 8 1 kbar massif type anorthosite from the ankafotia body of southwest madagascar the sample studied is a coarse grained leuconorite with good preservation of igneous texture and mineralogy although plagioclase which contains abundant rutile and zircon inclusions has been slightly recrystallized thirty three isotope dilution u pb analyses of zircons representing single grain fragments 29 analyses and multi grain fractions 4 analyses yield a spectrum of concordant ages from 631 to 549 ma a time span of more than 80 myr back scattered electron and cathodoluminescence images show that most grains are either homogeneous structureless fragments 35 or are permeated to a variable degree by anastamosing cracks occupied by relatively u and or th enriched zircon 45 a smaller percentage of grains show relict magmatic zoning 20 thin u and or th rich overgrowths occur on about 25 of grains raman spot analyses demonstrate that all fragments are highly crystalline and non metamict there are marked correlations between zircon grain size and internal features such that the oldest grains are larger and show relict magmatic zoning the youngest grains are small fragments containing high u crack networks ion microprobe spot analyses show that each zircon grain preserves a distinct trace element signature rare earth element patterns show heavy ree enrichment with negative eu anomalies and positive ce anomalies we suggest that the ca 80 myr spread in concordant u pb ages in this sample is indicative of high temperature pb loss during one or more protracted periods of granulite facies metamorphism with only minor episodic or continuous metamorphic zircon growth volume diffusion and or fracture assisted diffusion seems to be the dominant mechanism of pb loss cooling curves calculated using recently measured pb diffusion parameters conform to the age size relationship and imply very slow cooling rates 1 2 c myr or less as might be expected for a terrane in which granulite conditions were maintained for an extended period of time our results therefore suggest a note of caution for interpretation of concordant zircon ages in meta igneous rocks affected by high grade metamorphism of long duration

the origin of intraplate deformation in the atlas system of western and central algeria from jurassic rifting to cenozoic quaternary inversion abstract analysis of petroleum exploration data supplemented by paleostress data enabled discussion of the origin of the deformation in the western and central saharan atlas algeria this intraplate area has recorded the breakup of pangea late triassic the opening of the maghrebian tethys since the dogger and subsequently its closure oligocene to present however the two periods of strong coupling between europe and africa late lutetian and pleistocene which correspond to rapid uplifts of the atlas system and important deformations are not collision related they can be correlated to the beginning and the end of the development of the western mediterranean sea

ree u th and hf distribution in zircon from western carpathian variscan granitoids a combined cathodoluminescence and ion microprobe study a cathodoluminescence cl investigation on individual zircon crystals from variscan s type granites from the western carpathians has been combined with ion microprobe rare earth element ree and trace element analyses to explore possible connections between the cl emission of natural zircon crystals and their chemical composition the work was supported by single zircon u pb dating both by thermal ionization mass spectrometry tims and ion microprobe which helped to identify inherited zircon cores derived from related protoliths and to constrain the ages of inheritance and anatexis neither the ree nor trace elements such as y or th were found to have any direct correlation with the cl brightness the intensity of the cl signals was not directly reflected in the ree concentrations or the shape of the ree patterns uranium is probably the element responsible for quenching or suppressing the cl signals zircon cores inherited from different protoliths were distinguished by their ree patterns this enabled within a particular geological area the identification of inherited cores which possibly originate from the same protolith and therefore might lie on the same discordance line thereby improving the accuracy of inheritance ages inferred from tims single zircon analyses

precambrian to early cretaceous rocks of the strandja massif northwestern turkey evolution of a long lasting magmatic arc1 the strandja massif northwestern turkey forms a link between the balkan zone of bulgaria which is correlated with the variscan orogen in europe and the pontides where cimmerian structures are

provenance of the lunz formation carnian in the western carpathians slovakia heavy mineral study and in situ la icp ms u pb detrital zircon dating abstract the middle carnian julian detritic sediments of the lunz formation were deposited during of the so called carnian pluvial event they are the only siliciclastic sediments embedded in purely carbonatic formations the paper brings the first provenance analysis of this formation from the western carpathians slovakia where it occurs in the hronic fatric and veporic zones sandstones of the lunz formation belong to feldspatic greywackes and less to lithic greywackes modal analyses indicate that the sand material was derived from dissected arc and less from a recycled orogen the heavy mineral assemblage is dominated by zircon and by apatite slightly lesser amount of garnet cr spinels and tourmaline analysis of tourmalines schorl and dravite shows their primary provenance mainly from the metapelites and metapsammites coexisting or not coexisting with an al saturating phase some were also derived from fe3 rich quartz tourmaline rocks calc silicate rocks and metapelites and a slightly smaller group of grains was derived from li poor granitoid rocks and their associated pegmatites and aplites most of the garnet grains belong to almandine less to spessartine all of them have a wide spectrum of potential primary source rocks from low to medium grade metamorphic rocks and magmatites the analysed spinel grains cr rich spinel chromite and magnesiochromite match mostly the podiform chromitites harzburgite and cumulate fields or supra subduction zone peridotites field whereas al depleted spinel best match the arc volcanic field some spinels were affected by alteration or metamorphism the in situ la icp ms isotopic u pb dating shows a wide age interval of detrital zircon crystals in the lunz formation concordant zircon data show a wide interval of late triassic carnian to early proterozoic ages 230 2500 ma majority of zircons shows paleozoic pre variscan and variscan ages whereas neoproterozoic to paleoproterozoic age populations are relatively scarce age populations and chemical composition of zircon reveal their dominant primary provenance from various suites of magmatic rocks including variscan granitic rocks and pre variscan metaigneous lithologies characteristic is big scatter in zircon ages and the dominance of ultrastable minerals as zircon tourmaline rutile and cr spinels and quite a good correlation between zircons and cr spinels this indicates that most of the heavy minerals were not directly derived from some distinct primary magmatic or metamorphic sources but as recycled particles from older sediments or metasediments in the context with the previously published palaeogeographic reconstructions the provenance analysis indicates that the source area was most likely situated outside the alpine carpathian shelf they are most likely related to the coeval mostly fluvial stuttgart formation in the central european basin germanic triassic basin

mobility of trace elements and oxygen in zircon during metamorphism consequences for geochemical tracing this study addresses the mobility of the rare earth elements ree o u and pb during the formation of metamorphic zircon by overgrowth and coupled dissolution reprecipitation for this purpose the texture and ree 18o and u pb compositions of zircon crystals from a metabasite an anatectic granite and a metapelite sampled on naxos island greece have been characterised the behaviour of o and ree during the formation of the zircon metamorphic domains shows that two end members can be distinguished i closed or partially closed system behaviour for oxygen and trace elements in zircon formed by coupled dissolution reprecipitation metamorphic domains partially preserve the textural and geochemical characteristics of the precursor zircon chemical or isotopic equilibrium between the metamorphic zircon and the matrix is not completely reached for all the elements such zircons have to be used with caution for geochemical tracing when re equilibration is not complete the u pb system is also not completely reset and the metamorphic domains yield old scattered ages ii open system behaviour for oxygen and trace elements in zircon formed by overgrowth these metamorphic zircons do not show an inherited shape and are devoid of relic magmatic zoning their composition reflects growth in an open system at the scale of the zircon grain with the fluid phase and the surrounding minerals because equilibrium between the metamorphic zircon and the matrix was thus favoured zircon crystallisation can be more confidently related to the p t history and fluid circulation events by geochemical tracing

comment on how to build an extensional basin in a contractional setting numerical and physical modelling applied to the mejerda basin at the front of the eastern tell of tunisia by mannoubi khelil et al this study was supported by research projects cgl2015 67130 c2 1 r erasmus mundus external cooperation window and by scientific cooperation agreement 0534 between the office national des mines onm the tunis el manar university and the group for relief and active processes analysis arpa from the university of granada

state of disequilibrium between 238u 234u 226ra and 222rn in groundwater from bedrock abstract approximately one thousand drilled wells were investigated for their natural radioactivity the determinations of 238u 234u 226ra and 222rn from 310 samples showed a high state of radioactive disequilibrium between the members of the uranium series present in water the 238 u 226 ra activity ratio usually fell in the range 1 20 and the 238 u 222 rn activity ratio in the range 1 20 10 4 the highest activity ratios being from samples with an elevated uranium content the 234 u 238 u activity ratio varied between 0 76 and 4 67 the most frequent values showing a 60 excess of 234u in the samples most of the 234 u 238 u activity ratios near unity were found in samples with a high uranium content several drilled wells with anomalously high uranium contents were found in southern finland the average 226ra and 222rn contents of these wells were not exceptionally high which suggests high mobility of uranium in groundwater from the areas involved

stratal extension in thrust footwalls makran accretionary prism implications for thrust tectonics small scale stratal extension is commonly associated with thrust faults in the makran accretionary prism of southwest pakistan low angle normal faults dipping in the direction of transport relative to the thrust are common in gouge zones and duplexes and may also cut the footwall and hanging wall where they cause stratal extension these faults are probably riedel shears we have studied a broad zone of more complex extensional deformation beneath one bedding parallel thrust the zone disrupts earlier compressional structures and affects a significant volume of the footwall normal faults initiating in a rearward sequence branch off the thrust and cut down into the footwall merging at a lower stratigraphic level part of the thrust displacement was thereby transfe red to that level and fragments of the footwall were detached and transferred southward beneath the hanging wall the latter remained undeformed and accommodation structures are confined to the footwall these effects may have been caused by variations in sliding resistance along the fault if a thrust transfers part or all of its displacement down section along normal faults in this way before eventually cutting back up toward the surface it will detach a rootless fragment of footwall rocks from a position forward of the initial ramp through that horizon this process can be referred to as footwall plucking it can explain rootless basement slices along thrusts for example and it has important implications for section restoration techniques and the interpretation of thrust belts

relationships between very high pressure subduction complex assemblages and intrusive granitoids in the tavsanli zone sivrihisar massif central anatolia abstract the sivrihisar massif of central anatolia exposes blueschist and eclogite facies metasedimentary and metabasaltic rocks in close association with granitoid plutons the massif is located within the tavsanli zone the subducted and exhumed northern continental margin of the anatolide tauride block the relationship of the very high pressure assemblages to co existing granitoid plutonic bodies kaymaz and sivrihisar is unclear as the region can be considered the type locality for the entrainment of excess argon in k bearing minerals samples from its granitoids and a subduction complex assemblage were collected imaged with cathodoluminescence cl and dated using zircon u pb in situ in thin section ion microprobe methods the granitoids are heterogeneous as evidenced by cl images and geochemical variations finer grained sivrihisar samples are syenite and show plagioclase replacing k feldspar whereas coarser grained rocks are monzonite and show the opposite reaction our samples of the kaymaz granitoids are extremely si rich 82 97 wt sio 2 and have experienced sericitization cl images of both granitoids show evidence for fluid interactions at both the subsolidus and lower temperature stages in their tectonic history subduction along the tavsanli zone was ongoing during the early to late cretaceous and entrained zircons with igneous zonation in cl that crystallized as early as the paleoproterozoic sivrihisar massif granitoids record zircon crystallization from the late cretaceous to early oligocene here we present a model in which the sivrihisar and kaymaz melts source from a subducting slab along the afyon zone further to the south and mix with an igneous component generated during decompression due to break off of the tavsanli slab the model speculates that the tavsanli zone is the northernmost segment of an amalgamation of stacked subduction zones that transition in activity from north to south over time

reply to comment on how to build an extensional basin in a contractional setting numerical and physical modeling applied to the mejerda basin at the front of the eastern tell of tunisia

delta chromium 53 52 isotopic composition of native and contaminated groundwater mojave desert usa abstract chromium vi concentrations in groundwater sampled from three contaminant plumes in aquifers in the mojave desert near hinkley topock and el mirage california usa were as high as 2600 5800 and 330 g l respectively 53 52cr compositions from more than 50 samples collected within these plumes ranged from near 0 to almost 4 near the plume margins assuming only reductive fractionation of cr vi to cr iii within the plume apparent fractionation factors for 53 52cr isotopes ranged from eapp 0 3 to 0 4 within the hinkley and topock plumes respectively and only the el mirage plume had a fractionation factor similar to the laboratory derived value of e 3 5 one possible explanation for the difference between field and laboratory fractionation factors at the hinkley and topock sites is localized reductive fractionation of cr vi to cr iii with subsequent advective mixing of native and contaminated water near the plume margin chromium vi concentrations and 53 52cr isotopic compositions did not uniquely define the source of cr near the plume margin or the extent of reductive fractionation within the plume however cr vi and 53 52cr data contribute to understanding of the interaction between reductive and mixing processes that occur within and near the margins of cr contamination plumes reductive fractionation of cr vi predominates in plumes having higher eapp these plumes may be suitable for monitored natural attenuation in contrast advective mixing predominates in plumes having lower eapp the highly dispersed margins of these plumes may be difficult to define and manage