identifying granite sources by shrimp u pb zircon geochronology an application to the lachlan foldbelt the potential genetic link between granites and their host sediments can be assessed using zircon age inheritance patterns in the lachlan fold belt southeastern australia granites and associated high grade metasedimentary rocks intrude low grade ordovician country rock this relationship is well exposed in the tallangatta region northeast victoria part of the wagga omeo metamorphic complex in this region granites two i types and two s types have intruded during the mid late silurian between approximately 410 430 ma based on the ages of magmatic zircons the age spectra for inherited zircons from the granites have been compared with those of detrital zircons from the enclosing low and high grade metasediments in broad terms both for detrital zircons in all four sediments and for inherited zircons in three of the four granites the dominant ages are early paleozoic and late precambrian with sporadic older precambrian ages extending up to 3 5 ga the ages of the youngest detrital zircons from the low grade lockhart and talgarno terranes limit the time of sedimentation to ca 466 ma or younger the youngest detrital zircons from two samples of the high grade gundowring terrane are 473 ma making these sediments ordovician or younger not cambrian as originally suggested however the individual age spectra for the four selected metasediments are not well matched when closely examined the age spectra of the inherited zircons in the granites also do not adequately match those in any of the metasediments thus the metasediments might not be representative of the actual source rocks of the granites while the exact source of the granites cannot be identified from the analysed samples the existence of a large population of ca 495 ma inherited zircon grains in the s type granites requires that the granite source contains a significant proportion of cambrian or younger material this does not preclude the existence of a precambrian basement to the lachlan fold belt but indicates that at the level of s type magma generation a cambrian and or younger protolith is required

magmatism in the iberian pyrite belt petrological constraints on a metallogenic model igneous formations associated with massive sulphide deposits in the iberian pyrite belt ipb are essentially composed of basic lavas and dolerites and dacitic to rhyolitic volcanites intermediate lavas are subordinate the basic rocks show variable geochemical characteristics lavas and dolerites comparable to recent within plate alkaline basalts seem restricted to the western and southern parts of the ipb whereas basic rocks comparable to continental tholeiites or arc related basalts occur across the whole belt the felsic rocks are classified as calc alkaline and belong to the low al2o3 and high yb type at given sio2 al2o3 and tio2 contents they show variable zr nb and hree contents heavy rare earth element fractionation decreases from the dacites to the rhyolites gd yb n 1 whereas the negative eu anomaly becomes more pronounced the characteristics of the rhyolites are typical of sulphide fertile volcanic packages trace element modelling suggests that the felsic rocks evolved from a dacitic parent magma through fractional crystallization of hornblende and plagioclase partial melting of an amphibolite protolith which appears as the most probable model for the origin of this dacitic magma requires a high t p gradient in the crust the occurrence of alkaline basalts and continental tholeiites is consistent with formation of the ipb in a tensional tectonic setting however the associated island arc tholeiites suggest a location in a domain of plate convergence emplacement in a fore arc basin over a recently accreted crustal segment is envisaged as a possible hypothesis to account for the geological and petrological constraints a high geothermal gradient and eruption in a submarine tensional basin could have been two key ingredients for the development of massive sulphide deposits within the ipb

the role of hybridization in the genesis of hercynian granitodis in the gredos massif spain inferences from sr nd isotopes the gredos massif is one the better exposed granitoid complexes of the iberian massif it is composed mainly of peraluminous granitoids with subordinate basic and ultrabasic complexes the massif also contains mega enclaves of migmatites with which the granitoids show transitional contacts two major magmatic associations have been distinguished in this study 1 one comprises the granitoids with microgranular enclaves the enclaves and basic rocks 2 the other is formed by leucogranites intrusive into the former series and free of microgranular enclaves field relationships and microstructures indicate that the rocks of the first series are related by a dominant hybridization process the sr nd isotopic study reveals that this process is complex relating different end members of mantle and crustal affinities and occurred around 295 ma ago late with respect to the main deformation phases of the hercynian orogeny the granitoids with microgranular enclaves gme are part of an overall mixing trend involving palaeozoic mantle derived magma and melts of older crustal material amphibole bearing gme in general contain greater proportions of the mantle derived component than the cordierite bearing gme the actual mixing processes took place on a variety of scales sometimes between melts which were themselves hybrids on a local scale this hybridization process can be modelled by simple binary mixing as documented in the case of a composite dyke the isotopic signatures of the basic rocks are probably to a large degree the result of interaction with crustal melts though additionally the presence of an enriched mantle source cannot be elmininated microgranular enclaves and their immediate hosts have differing initial sr and nd isotopic signatures indicating that isotopic equilibrium was not attained this suggests that the enclaves did not reside in their final granitic melt for long before cooling of the whole system the enclaves are considered to have been derived from basaltic melts which had fractionated and hybridised to varying degrees late stage peraluminous leucogranites have similar initial nd isotopic compositions to the evolved gme a crustal source with a radically different nd isotopic composition or age does not need to be invoked in their petrogenesis

reply to comment by m francisco pereira et al on geodynamic evolution of the sw europe variscides the interpretation reported by ribeiro et al 2007 favors the presence of a cadomian basement within the sweurope variscides eventually including older relics considering as well the role of this basement in theevolution of the variscan cycle this particular issue wasdeveloped in a subsequent paper ribeiro et al 2009 thepossibility of grenville inliers is not definitely ruled out bythe arguments of pereira et al 2009 indeed a few nearlyconcordant u pb zircon ages of 0 94 to 1 2 ga are reportedby linnemann et al 2008 several other u pb zircongeochronological data from na gler et al 1995 de la rosaet al 2002 sola etal 2008 and cordani et al 2006 upper intercept date for sample 43b 5 1 assuming lead lossduring variscan metamorphism at 360 ma although notdefinitive also suggest the inheritance of grenville ages inossa morena zone omz rocks furthermore detritalzircons in me rtola formation south portuguese zone proximal greywackes includinghigh grademetamorphiclith oclastssimilar to those rocks found at the omz e vora massif reveal a frequency age distribution peak at 0 94 ga r jorge personal communication 2009 consistent with derivationfrom inliers of a greenville orogen which is well datedbetween 1 2 and 0 95 ga in the north american craton therefore the possibility of pre cadomian cycles remainsdebatable as signed by a question mark in the text of ribeiroet al 2007 as well as the provenance of detrital zircons andtheconnectionsbetweeniberiaandwestafrica amazoniaandlaurentia cratons prior to the assemblage of pannotia super continent further investigations are needed to solve thisproblem within an acceptable range of uncertainty

rise and fall of late pleistocene pluvial lakes in response to reduced evaporation and precipitation evidence from lake surprise california widespread late pleistocene lake systems of the basin and range province indicate substantially greater moisture availability during glacial periods relative to modern times but the climatic factors that drive changes in lake levels are poorly constrained to better constrain these climatic forcing factors we present a new lacustrine paleoclimate record and precipitation estimates for lake surprise a closed basin lake in northeastern california we combine a detailed analysis of lake hydrography and constitutive relationships describing the water balance to determine the influence of precipitation evaporation temperature and seasonal insolation on past lake levels at its maximum extent during the last deglaciation lake surprise covered 1366 km 2 36 of the terminally draining surprise valley watershed using paired radiocarbon and 230 th u analyses we dated shoreline tufa deposits from wave cut lake terraces in surprise valley california to determine the hydrography of the most recent lake cycle this new lake hydrograph places the highest lake level 176 m above the present day playa at 15 19 0 18 calibrated ka 14 c age this significantly postdates the last glacial maximum lgm when lake surprise stood at only moderate levels 65 99 m above modern playa similar to nearby lake lahontan to evaluate the climatic factors associated with lake level changes we use an oxygen isotope mass balance model combined with an analysis of predictions from the paleoclimate model intercomparison project 3 pmip3 climate model ensemble our isotope mass balance model predicts minimal precipitation increases of only 2 18 during the lgm relative to modern compared to an 75 increase in precipitation during the 15 19 ka highstand lgm pmip3 climate model simulations corroborate these findings simulating an average precipitation increase of only 6 5 relative to modern accompanied by a 28 decrease in total evaporation driven by a 7 c decrease in mean annual temperature lgm pmip3 climate model simulations also suggest a seasonal decoupling of runoff and precipitation with peak runoff shifting to the late spring early summer from the late winter early spring our coupled analyses suggest that moderate lake levels during the lgm were a result of reduced evaporation driven by reduced summer insolation and temperatures not by increased precipitation reduced evaporation primed basin and range lake systems particularly smaller isolated basins such as surprise valley to respond rapidly to increased precipitation during late heinrich stadial 1 hs1 post lgm highstands were potentially driven by increased rainfall during hs1 brought by latitudinally extensive and strengthened midlatitude westerly storm tracks the effects of which are recorded in the region9s lacustrine and glacial records these results suggest that seasonal insolation and reduced temperatures have been underinvestigated as long term drivers of moisture availability in the western united states

a 130 ka reconstruction of rainfall on the bolivian altiplano new efforts to link climate reconstructions from shoreline deposits and sediment cores yield an improved and more detailed lake history from the bolivian altiplano on the southern altiplano 10 lake oscillations have been identified from this new unified chronology each coincident with north atlantic cold events such as heinrich events h5 h2 h1 and the younger dryas by coupling this new lake history to a hydrologic budget model we are able to evaluate precipitation variability on the southern bolivian altiplano over the last 130 ka these modeling efforts underscore the relative aridity of the altiplano during the rare and small lake cycles occurring between 80 and 20 ka when colder temperatures combined with little or no change in rainfall produced smaller paleolakes relative aridity between 80 and 20 ka contrasts with the immense tauca lake cycle 18 1 14 1 ka which was six times larger than modern lake titicaca and coincided with heinrich event 1 this improved paleolake record from the southern altiplano reveals a strong link between central andean climate and atlantic sea surface temperature gradients during the late pleistocene even though today rainfall variability is driven mostly by pacific sea surface temperature anomalies associated with el nino southern oscillation however not all heinrich events appear to result in lake expansions most conspicuously during the global cold interval between 80 and 20 ka when the altiplano and amazon basin were relatively arid

isotopic tracers of paleohydrologic change in large lakes of the bolivian altiplano we have developed an 87sr 86sr 234u 238u and 18o data set from carbonates associated with late quaternary paleolake cycles on the southern bolivian altiplano as a tool for tracking and understanding the causes of lake level fluctuations distinctive groupings of 87sr 86sr ratios are observed ratios are highest for the ouki lake cycle 120 95 ka at 0 70932 lowest for coipasa lake cycle 12 8 11 4 ka at 0 70853 and intermediate at 0 70881 to 0 70884 for the salinas 95 80 ka inca huasi 45 ka sajsi 24 20 5 ka and tauca 18 1 14 1 ka lake cycles these sr ratios reflect variable contributions from the eastern and western cordilleras the laca hydrologic divide exerts a primary influence on modern and paleolake 87sr 86sr ratios waters show higher 87sr 86sr ratios north of this divide most lake cycles were sustained by slightly more rainfall north of this divide but with minimal input from lake titicaca the coipasa lake cycle appears to have been sustained mainly by rainfall south of this divide in contrast the ouki lake cycle was an expansive lake deepest in the northern poopo basin and spilling southward these results indicate that regional variability in central andean wet events can be reconstructed using geochemical patterns from this lake system

geochemistry and provenance of the carboniferous baixo alentejo flysch group south portuguese zone abstract this work is focused on the turbiditic sediments from the carboniferous baixo alentejo flysch group bafg in the south portuguese zone an external zone of the iberian variscides the aim of this work is to constrain the provenance and tectonic setting of these sediments in a context of a complex evolution of sw iberian variscides for this purpose we performed a systematic study of petrographical and geochemical signatures of greywackes and shales from the three bafg formations mertola mira and brejeira major and trace element composition and ratios suggest heterogeneous source area composition for bafg shales and greywackes for the oldest mertola formation greywackes source area is dominated by granitoid rocks with minor mafic input the latter becomes residual in the mira formation the youngest brejeira formation greywackes show clear felsic affiliation associated with an increase in recycled components the shales of all three bafg formations denote a granodioritic affiliation chemical index of alteration cia and plagioclase index of alteration pia values suggest moderate weathering in the source areas of mertola and mira formations these indices together with a cn k relations point out to steady state weathering conditions in the source areas of both formations in contrast both cia and pia values for the brejeira formation indicate variable conditions of palaeoweathering from moderate to intense as a consequence of non steady state conditions probably triggered by tectonic instability in the provenance area compared to the greywackes the shales of all three bafg formations exhibit higher cia and pia values as well as low k2o al2o3 0 2 and index of compositional variability

conventional and ion microprobe u pb dating of detrital zircons of the tentudia group serie negra sw spain implications for zircon systematics stratigraphy tectonics and the precambrian cambrian boundary conventional multi grain and ion microprobe dating of detrital zircons from a very low grade metagraywacke of the tentudia group upper part of the serie negra ossa morena zone sw spain reveals an uppermost vendian age for the deposition of the metagraywacke the youngest detrital zircon grain provides a maximum depositional age of about 565 ma thus these data contradict earlier middle to upper riphean ca 1350 850 ma estimates on the age of the tentudia group and favour a precambrian cambrian boundary falling into the range of 540 to 530 ma the presence of about 20 of pan african detrital zircons ranging from about 700 to 550 ma indicates the derivation from gondwana from the upper intercept ages of the fan shaped data field defined by conventionally determined zircon fractions it can be deduced that 2 1 ga old zircons as well as archean zircons existed in the provenance s of the serie negra sediments this mixing of crustal components of different ages is in line with the nd crustal residence age of 1 9 ga the latter value as well as other model ages of the iberian massif indicates unusually high amounts of ancient crust to be present in the strata this is different to other meta sediments of the european hercynides and suggests that the iberian strata of uppermost precambrian age may contain the detritus of more internal older parts of gondwana than other european strata of comparable ages geochemical data on the analysed sample and further metagraywackes of the tentudia group argue for a deposition in an arc environment such a scenario would conform with the syn to post orogenic shallow marine deposition of the studied sediments furthermore an upper time limit for the pre lower cambrian deformational history including two phases of regional deformation is given by the maximum age of deposition implying a very short time interval for deposition and deformation of the tentudia group concerning the u pb systematics of detrital zircon fractions it is probable that numerous previously published conventional multi grain zircon data on meta sedimentary rocks of the european hercynides readily can be explained by the presence of up to 20 of pan african detrital zircons and later phanerozoic lead loss during metamorphic transformation of the sedimentary protoliths moreover this implies that such metasediments originated from post pan african sedimentary precursors

deglacial changes in dust flux in the eastern equatorial pacific abstract atmospheric dust levels may play important roles in feedbacks linking continental source areas tropical convection marine productivity and global climate these feedbacks appear to be particularly significant in the tropical pacific where variations in local convection and productivity have been demonstrated to have impacts on climate at higher latitudes modeling of past dust levels and related feedbacks has been limited however by a paucity of observational data in this study we present a temporal and spatial survey of dust fluxes to the eastern equatorial pacific over the past 30 kyr glacial and holocene fluxes of 232th a proxy for continental material were calculated by normalization to 230th from a north south transect of cores along 110 w between 3 s and 7 n odp sites 848 853 fluxes were 30 100 higher during the last glacial suggesting increased dustiness in both hemispheres during the glacial period in both time periods dust fluxes decrease towards the south reflecting scavenging of northern hemisphere dust by precipitation at the itcz the holocene meridional dust flux gradient between 7 n and 3 s is characterized by a steep drop in dust levels at the southern edge of the modern range of the itcz while the gradient is shallower and more nearly linear during the last glacial this change may indicate that the glacial itcz in this region was a less effective barrier to inter hemispheric dust transport most likely due to a decrease in convective intensity and precipitation during the last glacial alternatively the change in gradient may be explained by increased variability in the location of the glacial itcz our data do not appear to require a mean southerly displacement of the glacial itcz as suggested by the results of other studies

the missing rheic ocean magmatic arcs provenance analysis of late paleozoic sedimentary clastic rocks of sw iberia abstract early carboniferous turbiditic sedimentary rocks in synorogenic basins located on both sides of the rheic suture in sw iberia were studied for provenance analysis an enigmatic feature of this suture which resulted from closure of the rheic ocean with the amalgamation of pangea in the late carboniferous is that there are no recognizable mid to late devonian subduction related magmatic rocks which should have been generated during the process of subduction on either side of it u pb la icp ms geochronology of detrital zircons from early carboniferous turbidites in the vicinity of the rheic suture in sw iberia where it separates the ossa morena zone with gondwana continental basement to the north from the south portuguese zone with unknown meguma continental basement to the south reveals the abundance of mid to late devonian 51 81 and early carboniferous 13 25 ages the cabrela and mertola turbidites of the ossa morena and south portuguese zones respectively are largely devoid of older zircons differing from the age spectra of detrital zircons in the oldest late devonian strata in the underlying south portuguese zone which contain abundant cambrian and neoproterozoic ages mid to late devonian zircons in the cabrela formation age cluster at c 391 ma eifelian givetian transition and mertola formation age clusters at c 369 ma and at c 387 ma famennian and givetian respectively are attributable to a source terrane made up of magmatic rocks with a simple geological history lacking both multiple tectonic events and older continental basement the terrane capable of sourcing sediments dispersed on both sides of the suture is interpreted to have been completely removed by erosion in sw iberia given that closure of the rheic ocean required subduction of its oceanic lithosphere and the absence of significant arc magmatism on either side of the rheic suture we suggest 1 the source of the zircons in the sw iberia basins was a short lived rheic ocean magmatic arc and 2 given the lack of older zircons in the sw iberia basins this short lived arc was probably developed in an intra oceanic environment

oroclines thick and thin an orocline is a thrust belt or orogen that is curved in map view due to it having been bent or buckled about a vertical axis of rotation two distinct types of oroclines are recognized progressive and secondary progressive oroclines are restricted to the scale of a thrust sheet to thrust belt are thin skinned and develop during thrust sheet emplacement secondary oroclines are larger occurring at the scale of an orogen and are plate scale features that affect crust and lithospheric mantle unlike progressive oroclines which develop during initial orogenesis and in response to the same orogen perpendicular stress responsible for thrust sheet emplacement secondary oroclines are extra orogenic developing after initial orogenesis and in response to an orogen parallel principal compressive stress that is oriented at a high angle to the stress responsible for orogen development we present case studies of the wyoming salient a progressive orocline that characterizes the sevier thrust belt of the western united states and the coupled cantabrian and central iberian oroclines which are linked secondary oroclines affecting the variscan orogen of iberia the vertical axis rotations involved in progressive and secondary orocline formation are most readily quantified through paleomagnetic analysis detailed three dimensional palinspastic restoration that incorporates translation rotation and strain can distinguish the role if any of primary curvature in progressive oroclines the use of tectonic vectors such as paleocurrent directions provides a means of recognizing and characterizing the initial geometry of secondary oroclines because secondary oroclines involve the entire lithosphere detailed studies of coeval metamorphism and magmatism provide a means of constraining the fate of the mantle lithosphere during oroclinal buckling

u pb geochronology of devonian granites in the meguma terrane of nova scotia canada evidence for hotspot melting of a neoproterozoic source abstract u pb isotopic analyses of monazite and zircon from six granitic plutons in the meguma terrane yield nearly concordant ages 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 textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape 373 pm 3 end document ma interpreted as the time of intrusion u pb analyses of euhedral zircons with thick rims overgrowing cores which were abraded to remove all or most of the rim plot on chords between documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage p

high pressure metamorphism in the early variscan subduction complex of the sw iberian massif abstract several units exposed in the boundary area of the ossa morena zone omz and the south portuguese zone spz preserve petrographic and thermobarometric evidence for an early metamorphic episode m1 developed under a high p low to intermediate t gradient related to early variscan subduction in the sw iberian massif in the omz these are the cubito moura unit p min 9 2 kbar and t 395 45 c blueschists from its basal melange p min 12 4 kbar and t 310 11 c and the underlying fuenteheridos group p 10 9 0 4 kbar and t 449 31 c the equivalent units in the spz are the la minilla formation p 8 7 0 4 kbar and t 388 16 c and the lawsonite pseudomorphs bearing pulo do lobo formation all these units formed part of an approximately ne verging orogenic wedge present coordinates developed by the accretion of subducted slabs of the outermost margin of gondwana and other elements of the rheic ocean realm from at least the middle devonian to the lowermost tournaisian high pressure rocks were subsequently emplaced on more internal zones of the omz that only experienced a younger high t low to intermediate p metamorphism m2 this high t event was coeval with magmatic activity from the uppermost devonian to the middle mississippian probably as a consequence of transtensional lithospheric thinning and or delamination of the lower crust and mantle lithosphere of the gondwana margin pre late devonian synorogenic sedimentation in forearc and back arc basins of the subduction complex evolved to a late devonian middle mississippian foreland basin system in early collisional stages finally a new middle late mississippian fold and thrust belt with opposite sw vergence and new foreland basins developed during late collisional stages

sm nd isotopic investigation of the age and origin of the meguma zone metasedimentary rocks the meguma zone of southern nova scotia is a suspect terrane in the eastern appalachians sm nd isotopic data on six samples from the thick sequence of flyschoid metasediments of the meguma group show that these rocks have a mean crustal residence age of tdm 1773 95 ma considerably older than their cambro ordovician depositional age this information should be useful in locating the matching terrane for the meguma and ultimately in determining whether it was derived from one or more precambrian sources with different crustal residence times

new 230th u and 14c ages from lake lahontan carbonates nevada usa and a discussion of the origin of initial thorium five sets of coeval lacustrine carbonate samples from pleistocene lake lahontan in western nevada were dated by both the ams 14c and 230th u isochron methods all five groups of samples were analyzed for u th isotopes by alpha spectrometry and one of the groups was additionally measured by thermal and secondary ionization mass spectrometry tims and sims for comparison the 14c ages were corrected to calendar years using the calibration curve recommended by bard et al 1992 without local reservoir correction on the 14c ages mean 230th u isochron ages of some sets are apparently older than their calendar corrected 14c ages by up to 2300 years modern carbon contamination of these carbonate samples through recrystallization or deposition of secondary calcite is likely to be responsible for part of the age discrepancies we explored additional biases associated with the isochron ages maybe produced by the presence of initial th coprecipitated from the lake water it can be shown that if dissolved hydrogenous th is directly incorporated into the pure carbonates then the three component mixing among 1 detrital th 2 hydrogenous th adsorbed on detritus and 3 hydrogenous th incorporated by the carbonate can introduce a positive age bias we have developed an approach to estimate the magnitude of this bias of the lake lahontan carbonates the preliminary estimates suggest a positive age bias of 1000 to 2000 years for two sets of the samples

superimposed thermal histories in the southern limit of the ossa morena zone portugal the mississippian volcano sedimentary complex in the toca da moura cabrela areas represents remnants of intra volcanic marine sedimentary basins formed during the collision between the ossa morena zone with the south portuguese zone these rock units are unconformably overlain by the pennsylvanian intramontane coal bearing santa susana basin vitrinite reflectance determinations from rocks of these two basins indicate two episodes of thermal maturation during the first episode the toca da moura cabrela volcano sedimentary complexes attained high maturation levels equivalent to anthracite coal rank 3 0 3 5 r oran which pre dates the middle moscovian santa susana basin the santa susana basin attained moderate maturation levels equivalent to bituminous coal rank 1 35 1 5 r oran recording a second episode of thermal maturation here peak thermal conditions did not overprint the first maturation episode the observed effects of magmatic intrusion on the thermal maturity and the lack of any increase in vitrinite reflectance with depth through c 400 m of section in borehole sdj 1 indicate high geothermal gradients during the first maturation episode a contemporaneous magmatic event associated with the c 335 320 ma cuba alvito gabbros diorites of the beja massif was the possible cause for the high geothermal gradients postulated for the first maturation episode burial under a post upper moscovian sedimentary cover was the most likely process to account for the maturation levels determined for the santa susana basin and for the second episode of thermal maturation

the transition from pangea amalgamation to fragmentation constraints from detrital zircon geochronology on west iberia paleogeography and sediment sources abstract detrital zircon u pb geochronology data from late carboniferous to triassic clastic sedimentary rocks in sw iberia were used to investigate the regional paleogeography during the transition from pangea amalgamation to break up the major u pb zircon age peaks are middle devonian to carboniferous 390 300 ma cambrian ordovician 530 440 ma cryogenian ediacaran 750 540 ma stenian tonian 1 2 0 9 ga and paleoproterozoic 2 3 1 8 ga rapid exhumation of variscan crystalline rocks at the contact between the south portuguese zone and ossa morena zone explains the abundance of late paleozoic ages in the upper carboniferous lower permian continental successions the u pb zircon data constrain the maximum depositional age of the santa susana basin to c 304 ma and the viar basin to c 297 ma the triassic sequences despite being c 100 ma younger than the variscan tectonothermal events contain low proportions of late paleozoic zircon the major peaks in all zircon spectra closely resemble those found in the adjacent basement rocks indicating small source areas mainly located near the rift shoulders longer travelled fluvial systems are postulated for the eastern portions of the algarve basin which was closer to the westward advancing tethys ocean than the rift basins of west iberia sequences that contain significant proportions of 1 2 0 9 ga zircon are probably recycled from post collisional carboniferous permian continental deposits that were more extensive than those found today

the southern iberian shear zone a major boundary in the hercynian folded belt a long ductile shear zone extending across the northern parts of sevilla and huelva provinces southwest spain and very probably stretching further west into portugal is characterized by a conspicuously developed l s fabric and the existence of various mylonitic textures the shear zone closely associated with amphibolites of tholeiitic affinity represents an important oceanic suture of the hercynian belt in europe the sense of displacement inferred from microstructures is consistent with southwest directed upthrusting evolving to a left lateral shearing movement

variscan accretionary complex of northwest iberia terrane correlation and succession of tectonothermal events the allochthonous terranes of northwest iberia can be correlated with specific paleogeographic realms of the continental masses and intervening oceans involved in the variscan collision assuming that the existing ophiolites represent the suture formed by the closure of the rheic ocean the units in the footwall to the suture correspond to the outer edge of the gondwana continental margin and the units in the hanging wall are interpreted as fragments of the conjugate margin represented by the meguma terrane this correlation establishes a precise link between circum atlantic terranes and makes it possible to draw a relatively simple scenario of the successive tectonothermal events recorded following the amalgamation of avalon to laurentia the remaining outboard terranes underwent a progressive accretion to this continent that ended with the collision between laurentia and gondwana

the oldest zircons in the solar system abstract here we report the occurrence chemistry and u th pb isotopic systematics of three meteoritic zircon assemblages two from the vaca muerta mesosiderite and one from the simmern h5 chondrite one of the vaca muerta zircons occurs in the mesosiderite proper the other in a eucritic clast associated with chromite ilmenite and tridymite whereas the simmern zircon occurs in a chondrule composed predominantly of chromite and feldspar like terrestrial zircons the meteoritic zircons are enriched in the heavy rare earth elements but unlike terrestrial zircons they do not show a positive ce anomaly this feature is also absent in one lunar zircon analyzed and probably reflects the oxidation state of the formation environment under oxidising conditions ce4 which can substitute for zr4 in the zircon structure is stabilized whereas under relatively reducing conditions ce3 is stable the zircon from simmern has depletions in the relative abundances of tm and yb a characteristic of volatility fractionation of the ree the u concentrations of the two vaca muerta zircons are quite different at 1 and 50 ppm respectively both zircons have extremely low initial lead and give radiogenic ages that are the same albeit with relatively large errors on vm 1 zircon because of its much lower u concentration and hence radiogenic pb concentration the mean of the two analyses made on vm 2 zircon is concordant with a207pb 206pb age of 4563 15 ma 2 the simmern zircon has an exceptionally low u concentration of around 180 ppb and only a poorly constrained 207pb 206pb age of 4100 700 ma could be obtained

cambrian ensialic rift related magmatism in the ossa morena zone evora aracena metamorphic belt sw iberian massif sm nd isotopes and shrimp zircon u th pb geochronology abstract the late ediacaran c 560 550 ma serie negra sediments of the evora aracena metamorphic belt ossa morena zone sw iberian massif preserve a record of the erosion of an avalonian cadomian magmatic arc and subsequent related turbiditic sedimentation detrital zircon from the serie negra is characterized by predominantly ediacaran and cryogenian ages with few paleoproterozoic and archean cores and a marked lack of grenvillian ages these features when combined with the metasediments enrichment in lree la yb 14 negative eu anomalies low 147 sm 144 nd values 0 121 and negative e nd 550 5 5 indicate that the protolith serie negra sediments were derived from a continental magmatic arc a period of late cadomian ca 560 540 ma tectonism was followed by an extended episode of widespread bimodal magmatism related to cambrian ca 540 500 ma rifting this tectonic inversion is expressed in the geological record by a regional early cambrian unconformity shrimp zircon u th pb ages from four felsic orthogneisses from the evora massif record cambrian 527 10 ma 522 5 ma 517 6 ma and 505 5 ma crystallization ages for their igneous protoliths this confirms the existence of widespread lower paleozoic igneous activity in the ossa morena zone i a lower cambrian ca 535 515 ma igneous felsic dominated sedimentary complex with calc alkaline signature and associated carbonate and siliciclastic deposition and ii a middle cambrian ordovician ca 515 490 ma igneous bimodal sedimentary complex with calc alkaline and tholeiitic signatures and associated dominant siliciclastic deposition but also carbonate sediments the cambrian felsic magmatism was characterized by negative eu anomalies la lu n 0 8 11 147 sm 144 nd 0 1289 0 1447 and e nd 500 ranging from 1 5 to 0 8 a tendency towards peraluminous compositions suggests late fractionation low degrees of partial melting or the mixing of crustal and mantle derived material in the magma source region some felsic rocks possibly represent the last residual melts of high temperature zircon undersaturated mafic magmas later affected by crustal contamination while others indicate partial melting of crustal metasediments variably contaminated by basaltic liquids the transition from early felsic dominated to later more mafic magmatism suggests the gradual opening of the system to tholeiitic n e morb products th n ta n 147 sm 144 nd 0 1478 0 1797 and e nd 500 values ranging from 6 4 to 7 3 while the n morb amphibolites have 147 sm 144 nd 0 1818 0 1979 and e nd 500 values of 5 8 and 7 0 reaching a maximum of 9 1 in contrast other amphibolites have a negative ta anomaly 1 35 n ta n 147 sm 144 nd values ranging from 0 1639 to 0 1946 and e nd 500 values of 3 5 to 5 2 suggesting derivation by crustal assimilation processes the sub alkaline igneous precursors of the amphibolites were most likely generated in a rift setting by asthenospheric upwelling these results strengthen the proposed geodynamic scenarios for the sw iberian massif by which cadomian accretion gave rise to an ensialic rift that developed into a proto oceanic basin and incipient spreading opening of the rheic ocean a similar transition from a convergent to a divergent plate boundary during the ediacaran to cambrian ordovician has also been reported in other segments of the northern gondwana margin

mass spectrometric 230th 234u 238u dating of the devils hole calcite vein the devils hole calcite vein contains a long term climatic record but requires accurate chronologic control for its interpretation mass spectrometric u series ages for samples from core dh 11 yielded 230 th ages with precisions ranging from less than 1 000 years 2 for samples younger than 140 ka thousands of years ago to less than 50 000 years for the oldest samples 566 ka the 234 u 238 u ages could be determined to a precision of 20 000 years for all ages calcite accumulated continuously from 566 ka until 60 ka at an average rate of 0 7 millimeter per 10 3 years the precise agreement between replicte analyses and the concordance of the 230 th 238 u 234 u 238 u ages for the oldest samples indicate that the dh 11 samples were closed systems and validate the dating technique in general

erratum to early to mid holocene aridity in central chile and the southern westerlies the laguna aculeo record 34 s quaternary res 58 2002 160 170

chemical characterization of micronesian ceramics through instrumental neutron activation analysis a preliminary provenance study abstract the development of exchange models from pacific islands archaeological data is a major task facing oceanic archaeologists chemical characterization of ceramics can assist provenance studies required for modelling past human interactions this regional instrumental neutron activation analysis inaa conducted on ceramics collected from the micronesian ceramic producing islands of belau chuuk guam kosrae pohnpei and yap allows us to form compositional groups and provenance ceramics from the coral islands of fais satawal and ulithi different island contexts which cause post depositional chemical transformations in ceramic sherds do not hinder the provenance assignments

sr z sbnd nd isotopic record of multi stage interactions between mantle derived magmas and crustal components in a collision context the ultramafic granitoid association from vivero hercynian belt nw spain late stage epizonal calc alkaline granitoids of the hercynian belt are often associated with coeval more basic rocks such granitoids have been the focus of special attention when studying the role of mantle derived magmas in granitoid genesis these studies conclude that basic magmas are involved in granitoid production but their contribution to the chemical composition of granitoids is often controversial the vivero massif nw spain includes calc alkaline granitoids and coeval ultramafic and mafic rocks but these granitoids are syntectonic and intruded into middle crustal levels during an early stage scattered major and trace element chemistry together with heterogeneous sr nd isotopic results preclude an origin of this ultramafic to granitoid association by any closed system fractionation process ultramafic rocks include peridotites pyroxenites and homblendites with endi from 3 5 to 1 3 and 87sr86sr i ratio from 0 7059 to 0 7080 they are interpreted as cumulates formed by in situ fractional crystallization from contaminated basaltic magmas particularly by interaction with 87sr enriched fluids mafic rocks comprise biotite amphibole diorites quartz diorites and tonalites with ndi from 3 8 to 0 0 and 87sr86sr i ratio from 0 7058 to 0 7073 they represent either cumulates or more evolved magmas of calc alkaline affinity these rocks display an inverse correlation between the degree of differentiation and the degree of contamination inferred from isotope data this points to wall rock assimilation by thermal erosion during ascent of basic magmas up to the middle crust and supports the participation of mantle sources with positive end and 87sr86sr i 0 7058 granitoids range from amphibole biotite tonalites through biotite granodiorites to monzonitic granites they show an isotopic trend which diverges from that of ultramafic and mafic rocks endi 6 0 to 1 9 decreases while 87sr86sr i ratio 0 7061 to 0 7082 increases from tonalites to monzonitic granites reflecting an increasing contribution of crustal components in more differentiated rocks assimilation and fractional crystallization mixing included account for such an evolution contamination of basic magmas by this process occurring at deeper crustal level produced hybrid tonalites and related mafic magmatic enclaves only monzonitic granites are interpreted as pure crustal melts their hree fractionated patterns gd yb 4 5 suggest equilibrium with a garnet rich residuum partial melting of moderate time integrated rb sr protolith such as greywackes and or meta igneous rocks of intermediate acid composition could account for the isotopic composition of these monozogranites this example shows that at least some calc alkaline granitoids of collisional setting are not produced by pure intra crustal melting they are formed through a complex multi stage hybridization process involving mantle derived magmas crustal components and several concomitant magmatic processes

crustal sources in lower palaeozoic rocks from nw iberia insights from laser ablation u pb ages of detrital zircons detrital zircons from greywackes belonging to an ordovician volcanosedimentary igneous complex in nw spain were dated using the laser ablation inductively coupled plasma mass spectrometry technique u pb results indicate the existence of three pre cambrian crustal sources in nw iberia the age grouping dated at c 590 620 ma confirms the presence of a cadomian avalonian basement in this zone the second age group c 1 1 1 2 ga is the first evidence for a grenvillean age crustal component in nw iberia the age grouping dated at c 1 9 2 0 ga corresponds to the age of the icartian igneous basement exposed in other areas of the european variscides

the high pressure iberian czech belt in the variscan orogen extrusion into the upper gondwanan plate a siluro devonian high pressure hp belt the iberian czech or ic belt extends from iberia cordoba coimbra shear zone through armorica and the massif central to the bohemian massif iberian czech ic belt and includes arc and periarc rocks morb and supra subduction ophiolites and passive margin sequences it has generally been interpreted to be either a a suture of the galician south brittany massif central and moldanubian ocean b a nappe rooted to the north in the rheic ocean or c the result of post collisional strike slip shuffling of the gondwanan margin during which a slice of the rheic ocean was inserted into the gondwanan margin we agree with a rheic ocean origin but propose that the ic belt represents a pre collisional part of the gondwanan continental oceanic margin that was removed by subduction erosion underwent hp metamorphism and was then extruded into the overlying gondwanan plate this model explains i the lack of paleolatitudinal and faunal differences across the ic belt ii the juxtaposition of active margin tectonics within synchronous passive margin sequences and iii the pre collisional age of most of the hp metamorphism the complete range of hp rocks in the ic belt from blueschist to hot eclogites indicates that previous correlations with b and a type subduction respectively cannot be sustained

inaa of pre contact basalt quarries on the samoan island of tutuila a preliminary baseline for an artifact centered provenance study abstract this project presents a material centered instrumental neutron activation analysis inaa characterization of 120 geologic samples selected from four fine grained basalt quarries on the samoan island of tutuila previous attempts at definitive differentiation of these tutuilan quarries have utilized x ray fluorescence xrf in this study clear differentiation of each analyzed quarry was achieved using inaa biplots of canonical discriminant analysis cda scores for the inaa data illustrate clear separation based on the variation in chemical composition between each quarry the samples analyzed not only define quarry separation but also provide the core group for a preliminary baseline necessary for future artifact centered provenance studies inclusion of these core group samples in the baseline was confirmed by stepwise discriminant analysis these findings suggest the ability to determine artifact quarry of origin on the island of tutuila which can elucidate the importance of individual tutuilan quarries in the production distribution and consumption of fine grained basalt artifacts in polynesia

u series isochron dating a generalized method employing total sample dissolution the age of a sedimentary deposit can be determined by measuring 238u 234u 232th and 230th in two or more samples of the deposit with the requirement that these samples have different u th ratios but an identical 230th232th ratio when deposited this paper presents the theoretical and experimental aspects of the dating method the latter involves techniques for securing samples of varied u th ratios from the same deposit plotting isochrons of the radiochemical data obtained on totally dissolved samples for age analysis and estimating the errors involved although applications to impure carbonates and lacustrine evaporite sediments are given as examples in this paper the method potentially can be generalized for dating a variety of authigenic deposits such as rock varnish peat opal sulfate minerals and marine phosphorites

complexity of saharan paleoclimate reconstruction and implications for modern human migration abstract chronological and geochemical data from scarp and scarp foot depression sediments ssds have been used for deciphering saharan paleoclimates and patterns of migration of anatomically modern humans amh u th dating of thick accumulations of ssds indicates prevalent deposition during long lasting cool glacial periods marine isotope stage mis 6 8 10 and 12 and limited deposition during warm interglacial periods mis 5 7 and 13 in contrast quaternary sediments associated with short residence hydrologic systems in sinai yielded mis 2 osl ages of 27 7 to 10 2 ka the lack of ssds of mis 2 ages and the wide range of warm and cool stages in eastern sahara is attributed to 1 lengthy travel times of groundwater prior to discharge and deposition of ssds from deep groundwater originating from distant sources and 2 sampling and dating of ssds from depressions where natural discharge has been apparently continuous through wet and dry periods given that the previously dated depression ssds could have been deposited during dry periods and that the ages of ssds reflect the timing of groundwater discharge rather than the ages of the wet periods during which recharge occurred we suggest that earlier interpretations of saharan paleoclimate and amh migrations that heavily relied on dating of ssds should be revisited

u pb geochronology and petrology of the late paleozoic gil marquez pluton magmatism in the variscan suture zone southern iberia during continental collision and the amalgamation of pangea the origin of plutonic complexes that stitch suture zones developed during collision is not well understood in southern iberia the pulo du lobo suture zone pdlz is intruded by the syn to postcollisional gil marquez pluton gmp thought to be part of the sierra norte batholith u pb la icpms zircon data on various phases of the gmp yield from oldest to youngest 1 a 354 4 7 6 ma unfoliated gabbro 2 a 345 6 2 5 ma foliated intermediate phase 3 a 346 5 5 4 ma unfoliated porphyritic granite 4 a 335 1 2 8 ma unfoliated biotite granite this sequence is consistent with cross cutting relationships observed in the field the range in ages is consistent with interpretations that the gmp is part of the composite ca 350 308 ma snb inherited ages preserved in the gmp intermediate and felsic phases indicate that its magmas traversed through south portuguese zone and pdlz crust during emplacement the ca 345 ma emplacement of the late kinematic foliated intermediate phase constrains the age of late stage strike slip deformation within the pdlz and the lack of a foliation in the older gabbro indicates that is was not proximal to a shear zone neither at the time of emplacement nor during its subsequent history the unfoliated porphyritic granite and unfoliated biotite granite cut the foliation of the intermediate phase indicating emplacement during the waning stages of collision while the ca 335 ma biotite granite intrudes the santa ira flysch thereby providing a tight constraint for the latest stage of deformation in the pdlz

superposed hercynian and cadomian orogenic cycles in the ossa morena zone and related areas of the iberian massif polyphase ductile deformation and metamorphism in the ossa morena zone of the iberian massif are related to a complex evolution including two major tectonothermal episodes of cadomian late proterozoic to early cambrian and hercynian age middle to late paleozoic petrological structural and geochronological data attest to a number of tectonic episodes in the 620 480 ma interval the model presented for this orogeny comprises stages of volcanic arc generation backarc extension tectonic inversion crustal thickening and cratonization in an andean type continental margin the hercynian cycle began in the early paleozoic with episodes of continental rifting orogenic events occurred in the 390 300 ma interval and affected both the cadomian basement which was heterogeneously reworked and overprinted and the paleozoic cover hercynian regional metamorphism was generally of low grade high pressure assemblages and thermal domes developed locally and were related to subduction and late extensional events the geodynamic model proposed for the cadomian and hercynian orogenies and the correlation with comparable areas from pre mesozoic massifs elsewhere in western europe account for the geodynamic scenarios that drove the fate of cadomian terranes

age of detrital zircon and titanite in the meguma group southern nova scotia canada clues to the origin of the meguma terrane abstract precise u pb isotopic analysis of seventeen selected high quality laboratory abraded zircon single grains seven multigrain fractions of similar material and four paramagnetic titanite fractions record the ages of the various detrital grain types from two turbidite samples collected from the top and about 5 km below the top of the goldenville formation of the meguma group the results show that the source of both sediments contained 600 ma and 2000 ma old volcanic plutonic components whereas a third 3000 ma old component occurs exclusively in the upper sample and the youngest zircons and titanite are found in the lower sample lead lead ages of bulk fractions at 2300 ma and two nearly concordant single grains at 2040 ma and 2140 ma upper sample together with 2157 ma bulk fraction and 2060 ma and 2080 ma single grain fractions lower sample indicate that the 2000 ma component dominates the pink purple euhedral grain types detrital titanite gives ages of 605 ma upper sample and 552 ma lower sample with the latter sample containing a minor 1975 ma component colourless zircons record this same age difference the lower sample contained the youngest concordant single zircon grain 566 ma and 580 605 ma bulk grain fraction even though two concordant single grains have ages of 632 and 643 ma estimated bulk fraction ages for certain zircon grain types from the upper sample fall in the range of 605 to 675 ma these data suggest that while the youngest rocks in the source region were removed by erosion during the early stages of deposition of the goldenville formation the oldest source rocks were not unroofed until later in the depositional history the 566 8 ma age of the youngest concordant zircon from the stratigraphically lower sample provides a maximum age for the deposition of this grain with the base of the cambrian placed between 545 and 530 ma the goldenville formation may straddle the precambrian cambrian boundary graptolites from the immediately overlying halifax formation are of early ordovician tremadocian age the lower sample contains spherical zircons that may have been derived from mafic granulite with an estimated age of 2000 2200 ma with this one possible exception all of the zircons analyzed appear to be igneous in origin and all are relatively euhedral indicating that rocks of the ages referred to above probably outcrop in the source area anomalous discordance patterns imply that part of the 2960 ma area was metamorphosed by the 2000 ma event and that both the 2960 and 2000 ma regions were metamorphosed during the 600 ma event the complete lack of detritus with ages between 680 and 2040 ma virtually eliminates a northern north american or baltic shield provenance however the available data are not sufficient to distinguish between a west african or guyana shield source

an evaluation of uranium series dating of fossil echinoids from southern california pleistocene marine terraces abstract fossil sea urchins strongylocentrotus from pleistocene marine terraces on the southern california channel islands have been dated by the uranium series method in order to test the suitability of echinoids for dating marine terraces results indicate that urchin plates and spines do not behave as closed systems with respect to both uranium and thorium calculated ages based on these data do not agree with uranium series ages 120 000 and 127 000 yrs obtained previously from corals from the same localities thus fossil sea urchins strongylocentrotus are not considered suitable for uraniumseries dating of pleistocene marine terrace deposits

triple junction migration during paleozoic plate convergence the aracena metamorphic belt hercynian massif spain new data on the petrology and structure of the aracena metamorphic belt shows that this is a subduction related low pressure high temperature complex developed by plate convergence at the north margin of gondwana during the paleozoic the low pressure inverted metamorphic gradient in morb derived amphibolites resulted from heating from the continental hanging wall during subduction this implies that the previous heating of the continental rocks was related to subduction of an oceanic ridge and the creation of a slab window beneath the continental margin this slab window brought the asthenosphere in contact with the continental margin inducing a shallow thermal anomaly and partial melting of the lithospheric mantle resulting in boninite magmatism

isotopic fractionation during leaching of impure carbonates and their effect on uranium series dating abstract experiments were designed to evaluate the behaviour of detritus during the leaching of impure carbonates by applying the u th isochron technique we used one natural detritus and a pure well dated caco 3 alone and in artificial mixtures one set of experiments was designed to study the effects of sample pre treatment on the detritus and a second set to examine the effect of acid attacks on both the carbonate and the detritus fractions in a mixture of constant proportions for both sets of data the extracted fraction from the detritus was calculated for 238 u 234 u 230 th and 232 th and their constancy established these conditions are necessary to apply the l l method schwarcz h p latham a g 1989 dirty calcites 1 uranium series dating of contaminated calcite using leachates alone chemical geology isotopes geoscience section 80 35 43 for u th dating finally the l l method was applied to the artificial impure carbonate and the resulting age is found in agreement with the age of the pure carbonate sample we concluded that conditions of the l l method to be applied for ageing impure carbonates hold

lake level chronology on the southern bolivian altiplano 18 23 s during late glacial time and the early holocene stratigraphic analyses of outcrops shorelines and diatoms from the southern bolivian altiplano uyuni coipasa basin reveal two major lacustrine phases during the late glacial period and the early holocene based on a chronology established by radiocarbon and u th control a comparison of14c and230th 234u ages shows that during times of high lake level radiocarbon ages are valid however during low water periods 14c ages must be corrected for a reservoir effect the lacustrine tauca phase started a little before 16 00014c yr b p and the lake level reached its maximum between 13 000 and 12 00014c yr b p a dry event ticana occurred after ca 12 000 and before 950014c yr b p a moderate lacustrine oscillation coipasa event occurred between ca 9500 and 850014c yr b p using a reservoir corrected conventional14c chronology comparisons between the lake level chronology in the uyuni coipasa basin and data from other southern tropical areas of south america suggest that the lacustrine evolution may reflect large scale climatic changes

protactinium 231 dating of carbonates by thermal ionization mass spectrometry implications for quaternary climate change measurement of protactinium 231 231pa in carbonates by thermal ionization mass spectroscopy yields 231pa ages that are more than 10 times more precise than those determined by decay counting carbonates between 10 and 250 000 years old can now be dated with 231pa methods barbados corals that have identical 231pa and thorium 230 230th ages indicate that the timing of sea level change over parts of the last glacial cycle is consistent with the predictions of the astronomical theory two devils hole calcite subsamples record identical 231pa and 230th ages suggesting that the chronology of this climate record is accurate

lake highstands on the altiplano tropical andes contemporaneous with heinrich 1 and the younger dryas new insights from 14 c u th dating and δ 18 o of carbonates this study provides new geochronological and stable isotope constraints on late pleistocene fluctuations in lake level that occurred in the closed watershed of the central altiplano between 25 and 12 ka u series isochrons and 14 c ages from carbonates are used to confirm and refine the previous chronology published placzek et al 2006b our new data support three successive lake highstands during the late pleistocene i the lake sajsi cycle from 25 to 19 ka that culminated at 3670 m at about 22 ka almost synchronously with the global last glacial maximum ii the lake tauca cycle that lasted from 18 to 14 5 ka and was characterized by the highest water level reached at least 3770 m from 16 5 to 15 ka iii the lake coipasa cycle from 12 5 to 11 9 ka that reached an elevation of 3700 m 42 m above the elevation of the salar de uyuni 3658 m these high amplitude lake level fluctuations are in phase with the cold warm oscillations that occurred in the north atlantic and greenland during the late pleistocene heinrich 1 bolling allerod younger dryas such temporal coincidence supports the hypothesis that wet events recorded in the central altiplano are controlled by the northesouth displacement of the inter tropical convergence zone resulting from changes in the meridional temperature gradient finally the oxygen isotope ratios measured in these lacustrine carbonates allows for calculation of the 18 o value of paleolake waters estimates of water 18 o v smow are 2 8 0 7 for lake tauca and 1 6 0 9 for lake coipasa these data are used to constrain changes in lake hydrology and can be interpreted to indicate that the proportion of precipitation arising from local water recycling was less than 50

age constraints to the relationships between magmatism metamorphism and tectonism in the aracena metamorphic belt southern spain the aracena metamorphic belt amb southwest iberian peninsula is characterized by the following geological elements a a high temperature low pressure ht lp metamorphic belt a few kilometres wide and more than 200 km long b a linear belt of oceanic amphibolites with a low pressure inverted metamorphic gradient c crustal scale ductile shear zones and d mafic noritic intrusions of high mg andesite boninite composition the relationships between these elements led to the proposal of a model of ridge subduction for this sector of the hercynian belt of europe this interpretation is supported by the age relationships displayed between the main rock units considered representative of the main tectonic and petrological processes responsible for the geological elements mentioned previously the results of a geochronological study ar ar rb sr and sm nd clearly support a late paleozoic tectonic evolution at an active continental margin the time evolution of the metamorphism in the oceanic domain ranging from 342 6 0 6 ma in the west to 328 4 1 2 ma in the east over a distance of 70 km along the metamorphic belt support a tectonic model of triple junction migration responsible for the creation at depth of a slab free window with decisive consequences for the thermal evolution of the region the origin of the linear metamorphic belt of ht lp regime may be explained by the migration along a continental margin of a punctual thermal anomaly induced by the creation of a triple junction at the continental margin

birth and demise of the rheic ocean magmatic arc s combined u pb and hf isotope analyses in detrital zircon from sw iberia siliciclastic strata paleozoic continental reconstructions indicate that subduction of rheic oceanic lithosphere led to collision between laurussia and gondwana which was a major event in the formation of the ouachita appalachian variscan orogenic belt and the amalgamation of pangea however arc systems which record rheic ocean subduction are poorly preserved the preservation of devonian detrital zircon in late devonian early carboniferous siliciclastic rocks of sw iberia rather than arc related igneous rocks indicates that direct evidence of the arc system may have been largely destroyed by erosion here we report in situ detrital zircon u pb isotopic analyses of late devonian early carboniferous siliciclastic rocks from the pulo do lobo zone which is a reworked late paleozoic suture zone located between laurussia and gondwana detrital zircon age spectra from the pulo do lobo zone frasnian formations show striking similarities revealing a wide range of ages dominated by neoproterozoic and paleoproterozoic grains sourced from rocks typical of peri gondwanan terranes such as avalonia the meguma terrane and the ossa morena zone pulo do lobo rocks also include representative populations of mesoproterozoic and early silurian zircons that are typical of avalonia and the meguma terrane which are absent in the ossa morena zone the famennian tournaisian formations from the pulo do lobo zone however contain more abundant middle late devonian zircon indicating the contribution from a previously unrecognized source probably related to the rheic ocean magmatic arc s the middle late devonian to early carboniferous zircon ages from the siliciclastic rocks of sw iberia south portuguese pulo do lobo and ossa morena zones have a wide range in ehft values 8 2 to 8 3 indicating the likely crystallization from magmas formed in a convergent setting the missing rheic ocean arc was probably built on a meguma avalonia type basement we propose for the pulo do lobo zone that the frasnian sedimentation occurred through the opening of a back arc basin formed along the laurussian active margin during rheic ocean subduction as has been recently proposed for the rhenohercynian zone in central europe detrital zircon ages in the frasnian siliciclastic rocks indicate provenance in the meguma terrane avalonia and devonian rheic ocean arc s as a result of back arc basin inversion the frasnian formations underwent deformation metamorphism and denudation and were unconformably overlain by famennian to visean siliciclastic strata including the phyllite quartzite formation of the south portuguese zone the latest devonian early carboniferous detritus were probably shed to the pulo do lobo zone represa and santa iria formations by recycling of devonian siliciclastic rocks from the south portuguese zone meguma terrane and from a new distinct source with baltica laurentia derivation preserved in the horta da torre formation and alajar melange

geochronology and stratigraphy of late pleistocene lake cycles on the southern bolivian altiplano implications for causes of tropical climate change large paleolakes 33 000 60 000 km2 that once occupied the high altitude poopo coipasa and uyuni basins in southern bolivia 18 22 s provide evidence of major changes in low latitude moisture in these now dry or oligosaline basins extensive natural exposure reveals evidence for two deep lake and several minor lake cycles over the past 120 k y fifty three new u th and 87 new 14c dates provide a chronologic framework for changes in lake level deposits from the ouki deep lake cycle are extensively exposed in the poopo basin but no deep lakes are apparent in the record between 98 and 18 1 ka the ouki lake cycle was 80 m deep and nineteen u th dates place this deep lake cycle between 120 and 98 ka shallow lakes were present in the terminal uyuni basin between 95 and 80 ka salinas lake cycle at ca 46 inca huasi lake cycle and between 24 and 20 5 ka sajsi lake cycle the tauca deep lake cycle occurred between 18 1 and 14 1 ka resulting in the deepest 140 m and largest lake in the basin over the past 120 ka multiple 14c and u th dates constrain the highest stand of lake tauca along a topographically conspicuous shoreline between 16 4 and 14 1 ka a probable post tauca lake cycle the coipasa produced a 55 m deep lake that is tentatively dated between 13 and 11 ka we suggest that paleolakes on the bolivian altiplano expanded in response to increased moisture in the amazon and enhanced transport of that moisture onto the altiplano by strengthened trade winds or southward displacement of the intertropical convergence zone itcz pole to equator sea surface temperature sst and atmospheric gradients may have influenced the position of the itcz affecting moisture balance over the altiplano and at other locations in the amazon basin links between the position of the itcz and the ca 23 ka precessional solar cycle have been postulated march insolation over the altiplano is a relatively good fit to our lake record but no single season or latitude of solar cycling has yet to emerge as the primary driver of climate over the entire amazon basin temperature may influence altiplano lake levels indirectly as potentially dry glacial periods in the amazon basin are linked to dry conditions on the altiplano intensification of the trade winds associated with la nina like conditions currently brings increased precipitation on the altiplano and deep lake development during the tauca lake cycle coincided with apparently intense and persistent la nina like conditions in the central pacific this suggests that sst gradients in the pacific are also a major influence on deep lake development on the altiplano

u series dating of impure carbonates an isochron technique using total sample dissolution abstract u series dating is a well established technique for age determination of late quaternary carbonates materials of sufficient purity for nominal dating however are not as common as materials with mechanically inseparable aluminosilicate detritus detritus contaminates the sample with extraneous th we propose that correction for contamination is best accomplished with the isochron technique using total sample dissolution tsd experiments were conducted on artificial mixtures of natural detritus and carbonate and on an impure carbonate of known age results show that significant and unpredictable transfer of radionuclides occur from the detritus to the leachate in commonly used selective leaching procedures the effects of correcting via leachate residue pairs and isochron plots were assessed isochrons using tsd gave best results followed by isochron plots of leachates only

tectonometamorphic evolution of the aracena metamorphic belt sw spain resulting from ridge trench interaction during variscan plate convergence 1 the aracena metamorphic belt is a high temperature low pressure ht lp band located at the southern end of the european variscan chain it marks a suture between armorica and avalonia this belt is characterized by 1 the presence of mid ocean ridge basalt derived metabasites that were affected by an inverted ht lp metamorphism related to sw verging thrusting whose thermal peak diachronously migrated eastward 2 the occurrence of a ht lp metamorphism related to an extensional event affecting continental rocks belonging to the ossa morena zone with peak temperatures that are 150 c higher than that recorded in the oceanic metabasites and 3 the presence of near trench magmatism with high mg andesite composition these and other characteristics can be interpreted in terms of a ridge trench trench triple junction that migrated along the armorican margin during variscan oblique convergence

glacial interglacial changes in moisture balance and the impact on vegetation in the southern hemisphere tropical andes bolivia peru abstract a palynological investigation of the last glacial interglacial cycle in the southern hemisphere tropical andes reveals changes in the moisture balance as the main driver in vegetation change thirty accelerator mass spectrometry radiocarbon dates biostratigraphy and tephra correlation reveal that a 119 m sediment core recovered from the huinaimarca sub basin of lake titicaca 16 0 to 17 5 s 68 5 to 70 w 3810 masl contains sediments covering 151 000 years correlation of aridity indicators with precessional variations in insolation is used to fine tune the structure of the age depth curve within this period variations in isoetes concentration above below 10 000 grains cm3 identify the extent of shallow water environments examination of another palaeolimnological indicator pediastrum and consideration of the bathymetry of the huinaimarca sub basin allow the reconstruction of lake level fluctuations these data indicate five wet dry cycles between c 151 000 and 14 200 cal yr bp high stands are suggested during the transition into c 134 000 cal yr bp and out of c 114 000 and 92 000 cal yr bp the last interglacial and during full glacial conditions c 70 000 and 45 000 cal yr bp these cycles are superimposed on a general trend of deepening lake levels through the glacial period this interpretation is supported by correlation with sediments from salar de uyuni 20 s 68 w 3653 masl the youngest wet episode is concurrent with palaeolake minchin c 45 000 cal yr bp with further evidence for an additional wet period commencing c 28 000 cal yr bp concomitant with palaeolake tauca the timing of lake level fluctuations is also supported by palaeoshoreline reconstructions from the uyuni poopo region however our data do not suggest a major peak in lake level in huinaimarca during the ouki lake cycle c 120 000 98 000 cal yr bp as inferred from u th ages obtained from palaeoshorelines around lago poopo the most extreme dry event occurs during the last interglacial period and resulted in a sedimentary hiatus tentatively dated to c 121 000 129 000 cal yr bp the observed wet dry cycles are shown to have a marked and rapid impact on the vegetation the aridity of the last interglacial promoted a community dominated by chenopodiaceae amaranthacae with no modern andean analogue polylepis acaena pollen is also shown to fluctuate markedly 0 20 particularly during the transitions into and out of the last interglacial it is probable that this pollen taxon is primarily representative of the high altitude arboreal genus polylepis which is a key component of highly biodiverse andean woodlands today rapid fluctuations indicate the sensitivity of this ecosystem to natural environmental pressure and potential vulnerability to future human impact and climate change the 100 000 year eccentricity solar cycle is shown to be the major controlling factor in moisture balance and vegetation over the last glacial interglacial cycle however significant fluctuations in moisture balance are also evident on timescales considerably shorter than the full glacial interglacial cycle we have linked these to precessional 21 000 year forcing nevertheless precise independent dating during the full glacial cycle is required to confirm the importance of this forcing mechanism

unstable flow magma mixing and magma rock deformation in a deep seated conduit the gil marquez complex south west spain the gil marquez complex is an exceptional outcrop of plutonic rocks ranging in composition from diorites to granites emplaced into devonian terrigenous metasediments of the southernmost part of the hercynian basement of iberia a combined study of this complex including field geology petrology structural geology and geochemistry reveals that it represents an ancient conduit of magma transport through the continental crust this conduit allowed the intrusion of magmas of contrasted compositions two end members and several hybrids are identified the first end member is a biotite granite and the second is a basaltic magma generated by partial melting of a depletedmantle source both magmas rose through a common channel in which favorable conditions for unstable flow and magma mixing occurred the observed relations in the gil marquez complex show that mixing in conduits may be an important mechanism for producing homogeneous hybrid magmas

zircon u pb geochronology of paragneisses and biotite granites from the sw iberian massif portugal evidence for a palaeogeographical link between the ossa morena ediacaran basins and the west african craton

chemical and isotopic composition of the lower crust beneath the meguma lithotectonic zone nova scotia evidence from granulite facies xenoliths granulite facies xenoliths from an upper devonian lamprophyre dyke near tangier nova scotia provide new information about the lower crust in the meguma zone two mineralogically and chemically distinet groups of xenoliths occur both groups contain quartz feldspar biotite fe ti oxides sulfides in addition group a contains garnet graphite aluminosilicates spinel sapphirine hight al2o3 subgroups a1 and a2 or clinopyroxene sphene apatite high cao sub group a3 group b has highly variable proportions of orthopyroxene b1 clinopyroxene b2 and amphibole b3 trace element contents of the highly aluminous xenoliths compare closely with average to upper crustal model compositions and are similar in many aspects to other undepleted granulite facies rocks lowp t sedimentary assemblages of quartz chlorite clay minerals calcite albite or paragonite can account for the compositions of group a xenoliths within group b a high mgo mgo 13 wt subgroup with high transition metal contents and low mgo mgo 9 wt sub groups with higher lil large ion lithophile element contents exist although the rare earth and high field strength elements indicate a tholeiitic or low k calc alkaline chemistry the lil elements are as high as those from high k calc alkaline volcanic are rocks isotopically group a ranges from endt 2 56 to 0 80 and87sr 86srt 0 7046 to 0 7182 fort 370 ma for group b these values are 1 45 to 5 33 and 0 7028 to 0 7048 respectively model ages tchur are correspondingly low and are tightly constrained 544 52 ma these young ages contrast with the middle proterozoic nd model ages of the overlying meguma zone low grade flysch this time inverted stratigraphy appears to be the product of a tectonic break between a younger autochthonous tangier lower crust avalon and an older allochthonous meguma group upper crust

seismic crustal structure in the southwest of the iberian peninsula and the gulf of cadiz abstract the crust under the southwestern iberian peninsula and the gulf of cadiz has been sampled by 1200 km of deep seismic refraction wide angle reflection profiles together with many seismic reflection lines and bore holes wide angle seismic data were collected during the last three decades commercial multichannel data provide a detailed image of the uppermost crust improving the confidence about the models of the deeper structures p wave velocities within the thick column of sediments in the gulf of cadiz range from 2 0 to 3 8 km s while the algarve and the sines areas have higher velocities of 4 3 to 4 8 km s the top of the palaeozoic basement rises to the northwest outcropping in the south portuguese zone of the iberian massif and is characterized by p wave velocities of 5 7 5 9 km s high velocities of 6 4 km s have been found at shallow depths of 7 to 10 km in the south portuguese zone that could be related to the mafic and ultramafic rocks in the beja acebuches zone lower crustal velocities are in the range of 6 7 6 9 km s the crustal thickness shows important lateral changes from 29 km beneath the guadalquivir basin iberian massif contact to 35 km in the southeastern part of the south portuguese zone from the interpretation of these seismic data a geodynamic model of the evolution of the crust in southwestern iberia and the gulf of cadiz is proposed the guadalquivir basin and its continuation at sea the gulf of cadiz is a flexure area of the crust that could be related to the overloading due to the overthrusting of the alboran domain over the iberian plate

depositional and tectonic setting of the paleoproterozoic lower aillik group makkovik province canada evolution of a passive margin foredeep sequence based on petrochemistry and u pb tims and lam icp ms geochronology abstract the paleoproterozoic lower aillik group is a deformed metasedimentary metavolcanic succession located in the makkovik province of labrador eastern canada the group is situated near the boundary between reworked archaean gneiss of the nain north atlantic craton and juvenile paleoproterozoic crust that was both tectonically accreted and formed on or adjacent to this craton during the ca 1 9 1 78 ma makkovikian orogeny the lower aillik group is structurally underlain by archaean gneiss and structurally overlain by ca 1860 1807 ma bimodal dominantly felsic volcanic and volcaniclastic rocks of the upper aillik group we present geochemical data from metavolcanic rocks and u pb geochronological data from several units of the lower aillik group in order to address the depositional and tectonic history of this group u pb data were obtained using both thermal ionization mass spectrometry tims and laser ablation microprobe inductively coupled plasma mass spectrometry lam icp ms two quartzite units near the structural base of the lower aillik group contain detrital zircons only of archaean age and are interpreted to have been deposited on the nain craton during post 2235 ma rifting and initiation of a passive continental margin overlying mafic metavolcanic rocks contain thin horizons of intermediate tuff one of which is dated at 2178 4 ma this relatively old age and an inferred stratigraphic relationship with underlying sedimentary units suggest that the volcanic rocks represent transitional oceanic crust consistent with their geochemical similarity to tholeiitic rifted margin sequences of mesozoic age in eastern north america a package of interlayered psammitic and semipelitic metasedimentary rocks that appears to stratigraphically overlie the mafic volcanic unit is dominated by paleoproterozoic detrital zircons but also contains archaean grains this package was deposited after 2013 ma the age of the youngest concordant zircon the u pb data imply a minimum 165 m y time gap between mafic volcanism and sedimentation and are consistent with deposition of the psammite semipelite unit in an evolving foredeep that heralded the approach of a paleoproterozoic arc terrane accretion of this terrane to the nain cratonic margin at ca 1 9 ga initiated the makkovikian orogeny although the lower aillik group is highly deformed and may contain internal tectonic boundaries or be incomplete the u pb and geochemical data allow quantitative assessment of a prolonged rift drift basin closure cycle that characterized the early paleoproterozoic evolution of the southern nain cratonic margin

use of uranium thorium dating to determine past 14c reservoir effects in lakes examples from antarctica the chronologies of many lacustrine records suffer from radiocarbon reservoir effects due to the presence of dissolved dead carbon or to slow air water exchange here we use the tims uranium thorium disequilibrium method in conjunction with ams radiocarbon dating to determine the age of lacustrine carbonates and to quantify the past radiocarbon reservoir effect in two antarctic lakes with differing characteristics by correcting a single sample u th age for detrital contamination a 14 c offset of v18 000 yr was obtained for carbonates from the former grounding line of the ross sea ice sheet in glacial lake trowbridge this large reservoir effect is believed to result from the direct input of old co2 from glacial meltwater in the second example an isochron approach on coeval samples formed at the bottom of lake vida now exposed due to lower lake level yielded an age of 9550 340 yr b p and a radiocarbon reservoir age of 3600 yr this offset was probably the result of lack of aeration due to perennial ice cover and or strong density stratification this evidence for long term isolation of the lake bottom indicates another level of hardship for life in the dry valley lacustrine environment an environment studied as an analogue for extreme periods of earth history as well as for exobiological implications the success of the u th technique on these two examples indicates that tims u th dating will be of widespread use in dating the important climate information recorded in the dry valleys both within and beyond the 14 c age range fl 2001 elsevier science b v all rights reserved

uranium series age estimates and paleoclimatic significance of pleistocene tufas from the lahontan basin california and nevada abstract an extended chronology of lahontan basin lake levels based on uranium series age estimates correlates with the global ice volume record lake highstands occur at or shortly after times of maximum ice sheet size moderate size lakes occur when the global ice volume is about 80 of its maximum the data indicate that lake levels rise and fall relative to the proximity of the mean position of the jetstream when the continental ice sheet is above some threshold size or shape it appears that the large scale circulation and climatic conditions are right for producing lakes of moderate to large size within the lahontan basin

geological constraints on the paleozoic tectonic evolution of tectonostratigraphic terranes in the iberian massif abstract the presence of oceanic paleozoic sequences in the iberian variscan fold belt is used to identify and separate suspected exotic terranes from a reference iberian autochthon terrane which constitutes the greater part of it in this paper the relevant data characterizing the pre accretionary evolution of the paleozoic iberian terranes are presented as well as the available evidence concerning the accretionary process itself emphasis is placed on the analysis of the sedimentary igneous and structural expression of the major events which affected the iberian terranes before during and after their amalgamation the main conclusions of such an analysis may be summarized as follows 1 a recognition of rift and passive margin stages in all the iberian massif terranes during the early paleozoic 2 b late paleozoic accretion of the exotic terranes to the iberian autochthon via a long and complex process variscan orogeny which involved initial subduction underneath the latter and eventual obduction followed by oblique collision of a promontory the iberian autochthon northwestern margin with a northwestern continental block this latter process could account for the large scale crustal imbrication shown by the northern branch of the orogen coeval with predominant strike slip faulting in the southern branch as well as with the formation of the typical arcuate shape of this orogenic belt major present uncertainties relate to the palinspastic provenance of continental suspect terranes and their correlation with similar units in other circum atlantic paleozoic orogens a discussion and speculations on this subject are dealt with in the final section of this paper

geochemical and isotopic disequilibrium in crustal melting an insight from the anatectic granitoids from toledo spain in the hercynian anatectic complex of toledo act the anatectic granitoids include leucogranitic leucosomes leucogranitic massifs and restite rich granites they show a broad range of initial sr and nd isotope ratios from 0 711 to 0 720 and 0 51164 to 0 51203 respectively which clearly indicate the absence of isotopic homogenization in the melts broadly the ranges reflect the isotopic variation of the metapelitic protoliths if crustal melting occurs under water undersaturated conditions as is the case of the act the generated melts do not isotopically and chemically equilibrate with the granulitic residuum the preservation of heterogeneities could arise through a number of processes 1 duration of the process in which the presence of melts with disequilibrium features and the high solid content of several of the granites in the act point to a very short lived magmatic system 2 limited diffusion in which poorly segregated and volatile undersaturated granitoids have a very restricted opportunity for isotopic and chemical homogenization and 3 magma dynamics in which the low melting rates the high restite and low water contents and the silica rich composition of these granites together with the small volume and high viscosity values of the melts seriously restrict the physical processes leading to homogenization

direct u th dating of organic and carbonate rich lake sediments from southern scandinavia u th ages of holocene lacustrine sediments from southern sweden have been determined to assess the reliability of this technique for dating climatic events from the last interglacial glacial interglacial cycle and for calibration of the14c dating method the sediments are organic and carbonate rich and they are shown to be very well suited to high precision238u 234u 230th dating by thermal ionisation mass spectrometry tims twenty one samples from 2 58 m of sediment from lake igelsjon southern sweden yield ages between 3058 89 and 9426 262 yr 2 error the sediments consist of hydrogenous calcium carbonate 50 90 organic matter 5 50 and siliceous residue 5 the high u contents of the lake sediments between 6 and 76 ppm are in contrast to the low concentrations in the lake and groudwater 1 9 and 3 2 ppb respectively and mass balance calculations based on u content of water and sediments can be used to calculate a lakewter residence time of 24 days which is close to the residence time calculated using hydrologic data recent ground and lakewater 234u 238u 0 920 is enriched in 234u compared to soils from the catchment area 234u 238u 0 587 and 234u 238u in the lake sediments shows a marked decrease with time from 0 958 at 9426 yr bp to 0 919 at 3058 yr bp this is attributed to progressive leaching of u from soils which have experienced a previous history of recoil related u loss and it indicates that the assumption of constant initial 234u 238u ratios may be a source of significant error in the calculation of 234u 238u ages for continental deposits

probing the composition of unexposed basement south portuguese zone southern iberia implications for the connections between the appalachian and variscan orogens geochemistry and sm nd and u pb magmatic zircon isotope data from a postcollisional batholith that crosscuts the allochthonous south portuguese zone spz of southern iberia suggest that the base

palaeozoic within plate volcanic rocks in nova scotia canada reinterpreted isotopic constraints on magmatic source and palaeocontinental reconstructions palaeozoic volcanism in the avalon terrane of northern nova scotia occurred during three time intervals cambrian early ordovician late ordovician early silurian and middle late devonian in the meguma terrane of southern nova scotia palaeozoic volcanism is limited to the middle ordovician geochemical data show that most of these volcanic rocks are bimodal within plate suites initial e nd signa tures range from 5 4 to 1 9 in the rhyolites and 6 8 to 2 7 in the basalts a difference attributable to the absence or presence respectively of a significant crustal component the data and regional tectonic settings of the avalon and meguma terranes suggest that the volcanism was generated in three different within plate settings 1 cambrian early ordovician volcanism related to thermal decay of late proterozoic arc magma tism during transtensional deformation 2 middle ordovician early silurian volcanism during sinistral tele scoping between laurentia and gondwana where extensional bends in the appalachians produced rifting and 3 devonian volcanism resulting from lithospheric delamination during dextral transpression and tele scoping in each setting active faults served as conduits for the magmas nd isotopic data indicate that the source of the palaeozoic felsic volcanic rocks is isotopically indistinguishable beneath southern and northern nova scotia and did not substantially change with time this crustal source appears to have separated from the mantle during the proterozoic a conclusion consistent with the hypothesis that the palaeozoic rocks in nova scotia were deposited upon a late proterozoic oceanic cratonic volcanic arc terrane the nd data when combined with published faunal palaeomagnetic and u pb isotopic data suggest that the avalon terrane was peripheral to gondwana off northwestern south america during neoproterozoic and early palaeozoic times

cosmogenic 3he production rate in the high tropical andes 3800 m 20 s implications for the local last glacial maximum abstract to improve both precision and accuracy of cosmic ray exposure dating methods there is a crucial need of calibration sites constraining the production rate of cosmogenic isotopes this is particularly true in the high tropical area where existing scaling models present significant discrepancies this study presents a new calibration site for cosmogenic 3he located at 3800 m in the tropical altiplano 19 9 s 67 6 w on the southern flank of the tunupa volcano in the vicinity of the salar de uyuni it consists in a fluvio glacial outwash that is stratigraphically bracketed by two successive lacustrine shorelines these shorelines are well dated by 14c and u series dating n 57 allowing determination of the age of the boulder field at the surface of the delta at 15 3 0 5 ka 1 eleven andesitic boulders were sampled on this fluvio glacial surface and the cosmogenic 3he contents of their pyroxene phenocrysts and amphiboles was analyzed the nucleogenic contribution from 6li capture is well constrained by determining the u th sm 4he age of these rocks this correction is minimal combined with the absolute dating of the delta these 3hec data yield a local production rate of 999 38 1 at g 1 yr 1 at 3800 m and 19 89 s after scaling with the time dependent scaling scheme of stone 2000 this result yields a rate of 121 5 1 at g 1 yr 1 at high latitude and sea level this new calibration is the highest among the existing global dataset it will thus permit the establishment of new robust glacial chronologies in the high tropics with an uncertainty lower than 5 at 1 such precision may have important implications in paleoclimatology notably because it will allow comparison with other well dated paleoclimatic archives including this new data the updated global 3hec production rate is 122 15 at g 1 yr 1 using the time dependent scaling of stone 2000 the new site specific 3hec production rate is used here to refine glacier fluctuations on cerro tunupa confirming that the local last glacial maximum was synchronous with the lake tauca highstand 15 5 ka data also suggest that a dramatic glacial retreat occurred at about 15 ka few hundred years before the lake tauca regression synchronously with the onset of the bolling allerod

a study of inherited zircons in granitoid rocks from the south portuguese and ossa morena zones iberian massif support for the exotic origin of the south portuguese zone abstract trace element and u pb isotopic analyses of inherited zircon cores from a sample of gil marquez granodiorite south portuguese zone spz and almonaster nebulite ossa morena zone omz in the aracena metamorphic belt have been obtained using laser ablation inductively coupled plasma mass spectrometry these data reveal differences in the age of deep continental crust in these two zones inherited zircon cores from the ossa morena zone range at 600 100 ma 1 7 2 ga and 2 65 2 95 ga while those from the south portuguese zone range at 400 500 and 700 800 ma these data support the exotic origin of the south portuguese zone basement relative to the rest of iberian massif the young ages of inherited zircon cores and nd model ages of magmatic rocks of the south portuguese zone are comparable to results from granulite facies xenoliths and granitic rocks from the meguma terrane and avalonia and support a correlation between the basement of the southernmost part of the iberian massif and the northern appalachians

a large drop in atmospheric 14c 12c and reduced melting in the younger dryas documented with 230th ages of corals paired carbon 14 14 c and thorium 230 230 th ages were determined on fossil corals from the huon peninsula papua new guinea the ages were used to calibrate part of the 14 c time scale and to estimate rates of sea level rise during the last deglaciation an abrupt offset between the 14 c and 230 th ages suggests that the atmospheric 14 c 12 c ratio dropped by 15 percent during the latter part of and after the younger dryas yd this prominent drop coincides with greatly reduced rates of sea level rise reduction of melting because of cooler conditions during the yd may have caused an increase in the rate of ocean ventilation which caused the atmospheric 14 c 12 c ratio to fall the record of sea level rise also shows that globally averaged rates of melting were relatively high at the beginning of the yd thus these measurements satisfy one of the conditions required by the hypothesis that the diversion of meltwater from the mississippi to the st lawrence river triggered the yd event

sampling for provenance tailings from prehistoric stone quarries in the south pacific stone is a principal component in archaeological assemblages across the pacific it provided a major element in architecture used as trade goods and was fashioned into implements and ornaments monolithic monuments statues money and emblematic symbols of victory status and ethnicity in short stone is a pervasive element in the economic social and political fabric of pacific cultural traditions however provenance of rock quarries the source of this raw material has received limited attention in oceania provenance studies have been sidestepped or short changed in the rush to analyse artifactual materials here we establish a foundation for more robust provenance studies and field sampling strategies through a discussion of 1 quarry identification and 2 sampling within prospective sources a model of quarry identification sampling and characterization is drawn from work on rapa nui easter island and the fundamental precepts are outlined for future sampling experiments and provenance work in stone quarries generally

u th dating of lake lisan late pleistocene dead sea aragonite and implications for glacial east mediterranean climate change u series dating can be an effective means to obtain accurate and precise ages on quaternary carbonates however most samples require a correction for u and th in admixed detritus this complication is often addressed through generation of u th isochrons requiring analyses of several coeval samples in addition presence of water derived hydrogenous th in the carbonate can cause inaccuracies in isochron ages this study reports a high resolution u series chronology of sediments deposited by lake lisan the last glacial precursor of the dead sea the strategy employed combines multiple measurements from a few stratigraphic heights and fewer analyses from many heights in a single described and measured section the resulting chronology is based on ages at 22 heights in a 40 m thick section covering the interval of 70 14 calendar ka bp the effects of admixed detritus are evaluated using trace elements nearly pure aragonite samples indicated by very low abundances of insoluble elements such as nb and zr were found to contain hydrogenous th which causes the uncorrected u 230th age of a modern sample to be 2 5 ka nevertheless accurate ages have been obtained by correcting for the detrital and aqueous interferences the resulting ages are in stratigraphic order and their accuracy is evidenced by consistency of lisan formation u series and 14c ages with the coral based calendar radiocarbon age calibration the u th ages provide a context to unravel the limnological history of lake lisan boundaries between the lower middle and upper stratigraphic units correspond to the mis 4 3 and 3 2 transitions respectively during mis 2 and 4 the lake generally showed a stable two layer configuration and a positive fresh water balance reflected by deposition of laminated aragonite detritus dry intervals during mis 2 and 4 are indicated by thick gypsum layers and an inferred depositional hiatus which are temporally associated with heinrich events h1 at 17 ka and h6 at 65 ka respectively during mis 3 the lake level was unstable with intermittent dry periods indicated by abundant clastic layers and a significant hiatus between 43 49 ka clastic layers are associated with dansgaard oeschger events during mis 3 and indicate lake level declines during abrupt northern hemisphere warmings overall the climate of the eastern mediterranean region shows a strong linkage to the northern hemisphere climate with increasing lake size and stability during cold periods and fluctuations and dessication during warmings and heinrich events

u th dating of carbonate platform and slope sediments absolute chronology of marine sediment beyond the 14c age range provides a test for models of climate change and has many other applications u th techniques have been used for such chronology by dating corals but extending these techniques to marine sediment is complicated by the presence of significant initial 230th both in detrital material and scavenged from seawater in this study we investigate four methods of solving the initial 230th problem for a particular type of marine sediment the aragonite rich sediments of carbonate platforms and slopes bulk sediment u th analyses can be corrected for initial th to yield ages with 2 to 3 kyr precision for highstand periods when sediment aragonite contents are particularly high uncertainty on the corrections causes inadequate precision for sediment from other periods however removal of scavenged th before analysis would enable a dramatic increase in this precision but has not proved successful despite a range of chemical leach approaches using heavy liquids to separate the various carbonate minerals found in bahamas sediment enables an isochron approach to correct for initial th but the presence of initial th from two sources requires correction or removal of one source of initial th before the other is deconvolved by the isochron quantitative removal of detrital material before isochron analysis proves a successful approach such isochron data demonstrate that although sediment remains closed to u th on a centimetre scale nuclides are moved from grain to grain by recoil such intergrain exchange is expected to be observed in all sediments containing mineral grains with different u concentrations measured 234u 238u allows the recoil movement to be corrected and results in isochron ages with precision sometimes as low as 3 kyr the accuracy of this approach has been proved by dating samples within the 14c age range sediments spanning the penultimate deglaciation have also been dated after a small correction for bioturbation the age for this event is found to be 135 2 3 5 ka this date is 8 kyr before the peak in northern hemisphere insolation and suggests that deglaciation is initiated by a mechanism in the southern hemisphere or tropics this isochron approach shows considerable promise for dating of sediments older than this event which will provide further information about the timing and mechanisms of global climate change

investigating the production and distribution of plain ware pottery in the samoan archipelago with laser ablation inductively coupled plasma mass spectrometry la icp ms this paper presents a provenance study of 170 ceramic artifacts and 21 ceramic tiles from three islands in the samoan archipelago using laser ablation inductively coupled plasma mass spectrometry la icp ms our analyses confirm that la icp ms can be used to differentiate between clay formations on a single island we identify different distribution patterns for pottery recovered from lowland and highland sites on tutuila island we also examine evidence for movement of pottery between islands and find only limited evidence for such movement our findings suggest dynamic patterns of prehistoric interaction and site use that need to be evaluated with further data from across the archipelago

highly depleted oceanic lithosphere in the rheic ocean implications for paleozoic plate reconstructions the rheic ocean formed at ca 500 ma when several peri gondwanan terranes e g avalonia and carolinia drifted from the northern margin of gondwana and were consumed during the late carboniferous collision between laurussia and gondwana a key event in the formation of pangea several mafic complexes ranging in age from ca 400 330 ma preserve many of the lithotectonic and or chemical characteristics of ophiolites they are characterized by anomalously high end values that are typically either between or above the widely accepted model depleted mantle curves these data indicate derivation from a highly depleted hd mantle and imply that i the mantle source of these complexes displays time integrated depletion in nd relative to sm and ii depletion is the result of an earlier melting event in the mantle from which basalt was extracted the extent of mantle depletion indicates that this melting event occurred in the neoproterozoic possibly up to 500 million years before the rheic ocean formed if so the mantle lithosphere that gave rise to the rheic ocean mafic complexes must have been captured from an adjacent older oceanic tract the transfer of this captured lithosphere to the upper plate enabled it to become preferentially preserved possible mesozoic cenozoic analogues include the capture of the caribbean plate or the scotia plate from the pacific to the atlantic oceanic realm our model implies that virtually all of the oceanic lithosphere generated during the opening phase of the rheic ocean was consumed by subduction during laurentia gondwana convergence

a reassessment of u th and14c ages for late glacial high frequency hydrological events at searles lake california u th isochron ages of tufas formed on shorelines suggest that the last pluvial event in lake lahontan and searles lake was synchronous at about 16 500 cal yr b p equivalent to a radiocarbon age of between 14 000 and 13 500 yr b p whereas the timing of this pluvial event determined by radiocarbon dating is on the order of 1000 yr younger the timing of seven distinct periods of near desiccation in searles lake during late glacial time has been reinvestigated for u th age determination by mass spectrometry u th dating of evaporite layers in the interbedded mud and salt unit called the lower salt in searles lake was hampered by the uncertainty in assessing the initial230th 232th of the samples the resulting ages corrected by a conservative range of initial230th 232th ratios suggest close correlation of the abrupt changes recorded in greenland ice cores dansgaard oeschger events and wet dry conditions in searles lake between 35 000 and 24 000 cal yr b p

strategies for successful u th dating of paleolake carbonates an example from the bolivian altiplano we report over 90 u th dates from carbonates deposited around paleolakes on the bolivian altiplano petrographic and chemical data for tufas and the siliciclastic detritus contained within them allow 1 assessment of possible diagenetic effects 2 a development of a strategy for selection of carbonate samples with low initial th contents and 3 assessment of the uncertainty due to initial th this strategy allows us to produce precise u th dates from lacustrine carbonates the principal consideration in dating such carbonates is the composition and quantity of initial th incorporated into the carbonate and additional uncertainty is introduced because this initial th may have two sources in lacustrine deposits isochron plots measured 230th 232th and x ray diffraction and trace element chemistry of silicic residues all favor regional soil and dust as the sole source of initial th in carbonates from the bolivian altiplano a situation that simplifies single sample dating of altiplano carbonates

magmatic evolution and tectonic setting of the iberian pyrite belt volcanism rocks has been carried out to determine the primary magmatic aynity tectonics iberian pyrite belt and tectonic setting of the iberian pyrite belt volcanism most of the basaltic rocks are continental tholeiites but a few samples show an alkaline aynity the origin of the basaltic rocks and their diversity of compositions are explained by a single mixing model between e and n morb mid ocean ridge basalt and as introduction similation of crustal material calc alkaline intermediate and silicic volcanic hosted massive sulphide deposits are mainly rocks include basaltic andesites andesites dacites and rhyolites associated world wide with calc alkaline submarine volvolumetrically dacites and rhyolites are the most abundant inter canism petrological and geochemical data together with mediate and silicic rocks are not related by fractional crystallization stratigraphic and structural studies have been of major nor is there a relationship between the basaltic and calc alkaline importance in constraining geological models of the volrocks by the same process we suggest that in the iberian pyrite canism associated with massive sulphide deposits inbelt silicic calc alkaline magmas were generated on a large scale tegrated studies in areas such as the kuroko province in by the invasion of continental crust by mafic magmas generated in japan ohmoto 1983 the mount read volcanics in the underlying upper mantle the diversity of compositions shown tasmania crawford et al 1992 and the mount windsor by dacites and rhyolites can mainly be explained either by diverences volcanics in northwestern australia stolz 1995 have in the composition of the source rocks or by diverent degrees of revealed that this volcanism may be developed during partial melting of upper crust rocks andesites however formed by diverent stages of the subduction process always being mixing between basaltic magmas and upper crust material the new located on the overriding plate geochemical data agree with previously published tectonostratigraphic this paper documents the petrology and geochemistry data which suggest that the iberian pyrite belt volcanism formed of the volcanism of the iberian pyrite belt an early on the south portuguese plate owing to strike slip tectonics this carboniferous metallogenic province that extends from portugal to spain in southwest iberia and constitutes local extensional tectonic setting was related to transtension as a