isotope and trace element evidence for late stage intra crustal melting in the high andes abstract purico chascon is an acid igneous complex less than 1 5 ma old rising to 5800 m in the north chilean andes and consisting of andesite dacite cones and dacite domes on an ignimbrite shield the rocks are subdivided into two groups those from chascon appear to exhibit evidence for magma mixing with more basic material now preserved as xenoliths whereas among those at purico no xenoliths have been found 87sr 86sr 0 7095 0 7081 at purico 0 7079 0 7069 at chascon and 0 7061 0 7057 in the xenoliths from the chascon lavas 143nd 144nd 0 51222 0 51236 overall the purico lavas are characterised by higher sio2 rb sr 87sr 86sr and ree abundances and lower sr nd sr ba and143nd 144nd than most andean igneous suites there is no indication ofselective crustal contamination of sr or any systematic change in isotope ratios during differentiation nonetheless the trend of for example high sr nd and sr contents in rocks with low87sr 86sr 0 704 ecuador to low sr nd and sr and high sio2 in rocks with87sr 86sr 0 7081 0 7095 at purico is interpreted as a shift from subduction zone related magmatism to one with greater crustal affinity the formation of the least evolved purico lavas 60 sio2 is discussed in terms of bulk assimilation of crustal material mixing between crustal and mantle derived magmas and partial melting of pre existing crust although such models are still extremely primitive the simplest explanation of the observed chemical variations is that the purico rocks evolved from parental magmas derived by crustal anatexies thermal considerations suggest that such late stage crustal anatexis is a predictable response to crustal thickening which in the andes is thought to have taken place during the cenozoic

eruptive processes and caldera formation in a nested downsagcollapse caldera cerro panizos central andes mountains abstract the cerro panizos ignimbrite center in the central andes mountains produced two ignimbrite sheets and many lava flows the ignimbrite of quebrada cienago was erupted at 7 9 ma and effusive eruptions continued until the two cooling units of the crystal rich dacitic cerro panizos ignimbrite were emplaced at 6 7 ma the lower unit has no laterally continuous flow breaks and was erupted from a single vent or small cluster of vents with limited fluctuation in discharge lithic fragments reach significant concentrations 5 only in the uppermost two meters of this cooling unit where they document vent wall collapse or the opening of a new vent the upper cooling unit contains many flow units with variations in welding thicknesses and lithic fragment concentrations implying an unsteady eruption column the opening of many vents and probable caldera collapse triangulation of anisotropy of magnetic susceptibility flow direction measurements locate a single vent for the lower cooling unit whereas the upper cooling unit had many vents within the present dome cluster a 15 km diameter topographic depression marked by inward dips of 4 8 at the cooling unit contact is centered on the vent area of the lower cooling unit the depression is interpreted as a downsag caldera formed during emplacement of the lower cooling unit collapse began late in the eruption of the lower cooling unit and continued through the emplacement of the upper cooling unit resurgent magmatism occurred as lava flows that mundated the caldera area a ring of dacite domes erupted until at least 6 1 ma in the northern half of the downsag caldera traces the margin of a collapse caldera associated with the upper cooling unit maximum caldera subsidence 353 km 3 is not enough to account for the erupted volume 652 km 3 dre minimum the ash poor crystal rich nature up to 50 crystals in the pumice 75 in the matrix of the cerro panizos pyroclastic flows resulted in poor retention of gases and there is little evidence of fluidization the high crystal content however favored modified grain flow processes in which several particle support mechanisms combine with grain dispersive forces it is postulated that the flows were initially partially fluidized but rapidly lost their gases and grain flow processes grew in relative importance grain flow processes destroyed evidence of fluidization before deposition

the volcanic and magmatic evolution of volcan ollague a high k late quaternary stratovolcano in the andean central volcanic zone abstract volcan ollague is a high k calc alkaline composite volcano constructed upon extremely thick crust in the andean central volcanic zone volcanic activity commenced with the construction of an andesitic to dacitic composite cone composed of numerous lava flows and pyroclastic deposits of the vinta loma series and an overlying coalescing dome and coulee sequence of the chasca orkho series following cone construction the upper western flank of ollague collapsed toward the west leaving a collapse amphitheater about 3 5 km in diameter and a debris avalanche deposit on the lower western flank of the volcano the deposit is similar to the debris avalanche deposit produced during the may 18 1980 eruption of mount st helens u s a and was probably formed in a similar manner it presently covers an area of 100 km2 and extends 16 km from the summit subsequent to the collapse event the upper western flank was reformed via eruption of several small andesitic lava flows from vents located near the western summit and growth of an andesitic dome within the collapse amphitheater additional post collapse activity included construction of a dacitic dome and coulee of the la celosa series on the northwest flank field relations indicate that vents for the vinta loma and post collapse series were located at or near the summit of the cone the vinta loma series is characterized by an anhydrous two pyroxene assemblage vents for the la celosa and chasca orkho series are located on the flanks and strike n55 w radial to the volcano the pattern of flank eruptions coincides with the distribution in the abundance of amphibole and biotite as the main mafic phenocryst phases in the rocks a possible explanation for this coincidence is that an unexposed fracture or fault beneath the volcano served as a conduit for both magma ascent and groundwater circulation in addition to the lava flows at ollague magmas are also present as blobs of vesiculated basaltic andesite and mafic andesite that occur as inclusions in nearly all of the lavas all eruptive activity at ollague predates the last glacial episode 11 000 a b p because post collapse lava flows are overlain by moraine and are incised by glacial valleys present activity is restricted to emission of a persistent 100 m high fumarolic steam plume from a vent located within the summit andesite dome sr and nd isotope ratios for the basaltic andesite and mafic andesite inclusions and lavas suggest that they have assimilated large amounts of crust during crystal fractionation in contrast narrow ranges in 143nd 144nd and 87sr 86sr in the andesitic and dacitic lavas are enigmatic with respect to crustal contamination

strontium neodymium and lead isotopic compositions of deep crustal xenoliths from the snake river plain evidence for archean basement abstract xenoliths of intermediate to felsic granulites found in evolved lavas from the snake river plain have been analyzed for nd pb and sr isotopic compositions and related trace element contents overall they exhibit wide ranges in present day values of 87 sr 86 sr 0 70235 0 83011 143 nd 144 nd 0 51023 0 51148 and 206 pb 204 pb 13 43 24 65 the rb sr u pb th pb and sm nd decay schemes have been variably affected by granulite facies metamorphism and possibly by transport in the host magmas however pb pb and sm nd isochron systematics seemingly are preserved in many of the xenoliths and indicate essentially concordant metamorphic ages of about 2 8 ga nd model ages are significantly older ca 3 1 3 4 ga for many of the xenoliths precambrian metasediments exposed along the southern margin of the snake river plain have sm nd systematics similar to those of the xenoliths these results suggest that at least two significant thermal events occurred during archean evolution of the crust in this region a early ca 3 1 3 4 ga additions of mantle derived magmas to the crust and b regional metamorphism at 2 8 ga accompanied at least locally by magmatism concordance between the sm nd and pb pb ages suggests that these isotopic systems are little affected by high grade metamorphism in this case the distribution of xenoliths in snake river plain lavas supports the presence of archean crust beneath much of southern idaho although such rocks rarely are exposed at the surface thus nd and pb isotopic studies of crustal xenoliths can provide a useful means of determining the extent of crustal age provinces where surface exposures are lacking

geochronology and stratigraphy of the ignimbrites from the 21 30 s to 23 30 s portion of the central andes of northern chile the central volcanic zone cvz of the andes which extends from 14 to 28 s is one of the largest provinces of late tertiary to recent ignimbrite volcanism in the world in the 21 30 s to 23 30 s portion of the cvz in northern chile ignimbrite volcanism was initiated at the beginning of the late miocene 10 4 ma ago and continued until the recent the stratigraphy is dominated by the products of five major dacitic ignimbrite eruptions 100 km3 the artola ignimbrite member 9 4 ma the sifon ignimbrite member 8 3 ma the pelon ignimbrite member 5 5 ma the puripicar ignimbrite member 4 2 ma and the atana ignimbrite member 4 1 ma at least three of these represent only portions of larger ignimbrite sheets erupted from centres of sw bolivia two of the ignimbrites the sifon and atana ignimbrites must represent erupted volumes in excess of 1000 km3 each the major units which represent 90 of the total volume of ignimbrite in this area are intercalated with smaller 50 km3 less extensive volcanic units some of which were erupted from source structures within the study area the upper tertiary to recent volcanic stratigraphy has been revised using the large ignimbrites as the major stratigraphic horizons the ignimbrite formations can be divided into two groups on a geographic basis those in the northern and central portions of the study area are known as the san bartolo group while those in the southern portion belong to the silapeti group these two groups appear to be contemporaneous although the silapeti group may represent a period of ignimbrite volcanism which was initiated later about 6 ma ago the ignimbrite formation of sw bolivia and the punense arancaniano formation of nw argentina are equivalent to the san bartolo and silapeti groups based on correlation of the stratigraphy into adjacent areas in sw bolivia and nw argentina the beginning of the late miocene 10 4 ma marks the onset of the most intense phase ignimbrite volcanism in the cvz this phase of volcanism is suggested to be the result of crustal melting in response to crustal thickening during the quechua phase of deformation and subduction approximately 10 000 km3 of ignimbrite erupted over a period of 9 ma in the 21 30 23 30 s portion and led to the development of a major ignimbrite province

reconstructing middle horizon mobility patterns on the coast of peru through strontium isotope analysis the pre columbian coastal site of ancon peru frequently has been interpreted as an important outpost of the highland wari empire inhabited by individuals from the sierra and the coast in order to test this hypothesis bone tooth pairs from 35 middle horizon 550 ad 1000 ad ancon skeletons were analyzed for strontium isotopes which vary by geologic provenance results indicate that 34 of the 35 individuals have 87 sr 86 sr enamel and bone values higher than ancon s biologically available strontium isotope range nitrogen and carbon isotope data from a subset of the ancon skeletons suggest that the higher than expected 87 sr 86 sr values among the ancon sample likely reflects a diet rich in marine resources rather than migratory activity and highlight the need to use multiple lines of evidence to track residence change at sites where individuals relied on resources other than locally grown terrestrial foods the one remaining individual a young female of elite status has an enamel strontium isotope signature much lower than the available data for local fauna and soil indicating that she was not raised locally her enamel 87 sr 86 sr ratio fits well within the range of strontium isotope values established for the inland wari site of conchopata tung t a knudson k j social identities and geographical origins of wari trophy heads from conchopata peru current anthropology in press and suggests that highland wari immigrants may have been present at ancon during the middle horizon

regional o sr and pb isotope relationships in late cenozoic calc alkaline lavas of the andean cordillera quaternary recent volcanism in the andes has occurred in three regions 45 33 s 28 16 s and 2 s 5 n each of which has a distinct plate tectonic setting and contains volcanic suites with different chemical and isotopic characteristics isotope ratios of o and sr are lowest and those of pb least variable in the southern volcanic zone svz where medium k lavas have isotopic characteristics equivalent to volcanics from intraoceanic arcs where continental crust is absent the svz lavas were probably derived from an asthenospheric mantle source above a shallow benioff zone parental basaltic magmas rose largely unmodified through thin continental crust where differentiation occurred by low pressure crystal fractionation without concurrent modification of isotopic composition the slight enrichment of 206 pb in the northern volcanic zone nvz where the crust is thin and the benioff zone deep suggests a greater subduction zone component for parental medium k magmas the slight 18 o enrichment suggests a small amount of lower crustal interaction isotope ratios of o and sr are highest and those of pb most variable in the central volcanic zone cvz parental magmas in the cvz were probably generated within the same mantle source region as those in the svz and nvz subsequently during transit through the exceptionally thick continental crust of the central andes high k magmas were produced by a combination of bulk contamination in the lower crust and later low pressure fractional crystallization assimilation processes in the upper crust which altered both chemical and isotopic compositions

uplift of the western altiplano plateau evidence from the precordillera between 20 and 21 s northern chile 1 we present a quantitative reconstruction of uplift of the western flank of the altiplano plateau central andes one of the largest monoclines on the earth on the basis of an analysis of tectonic structures syntectonic deposits and geophysical data uplift occurred on a west vergent slowly propagating system of high angle reverse faults merging into a joint detachment that ramps down to midcrustal levels below the plateau edge the upper ramps determine local fold geometries while the lower ramp controls large scale surface tilting and uplift at 20 s this fault system was active between 30 ma and 5 10 ma with maximum shortening rates of 0 22 mm yr between 17 and 10 ma it generated some 2600 m of surface uplift with only minor shortening of 3000 m its activity was largely synchronous to eruption of large volume ignimbrites from a midcrustal source geophysical data indicate that the fault system localized deformation at the boundary between a cool strong forearc crust and a presumably fluid rich and or partially molten zone underneath the plateau area the systematic relation between crustal melting and shortening with uplift at the western plateau margin can be traced along most of the plateau flank with a stepwise decrease in age of deformation and magmatism toward the south indicating discontinuous addition of plateau segments crustal thickening to as much as 70 km from westward underthrusting in the back arc parts of the plateau isostatically compensated the tectonic surface uplift and monocline formation with respect to a stable forearc which only reacted with minor tilting

oxygen isotopic compositions of central andean plutonic and volcanic rocks latitudes 26 29 south abstract oxygen isotope data are reported for 27 igneous rocks of mesozoic to quaternary age from the central andes 26 29 s the plutonic rocks and most of the volcanics have 18 o values between 6 2 and 8 3 the whole rock 18 o values show a weak correlation with initial 87 sr 86 sr data this o sr array differs from documented trends for calc alkaline plutonic suites from california scotland and northern italy but overlaps with data for volcanic and plutonic rocks from ecuador northern chile and southern peru the oxygen isotope results indicate that the magmas evolved without significant contamination from supracrustal rocks e g rocks that experienced 18 o enrichment during surficial weathering the available o sr and pb isotopic data for these rocks are best explained by magma generation in the upper mantle or lower crust from the late mesozoic on the 87 sr 86 sr values were modified at depth by isotopic exchange between the magma and a continually thickening crust of plutonic rocks of late precambrian to early mesozoic age

sm nd studies of archaean metasediments and metavolcanics from west greenland and their implications for the earth s early history metasedimentary and metavolcanic rocks from the archaean of west greenland have been examined for evidence of crustal components greater than 3 8 ga in age and for their compatibility with the presently adopted bulk earth sm nd parameters sm nd isotopic data have been obtained for the garbenschiefer metagabbro unit metasediments from the isua supracrustal belt gneisses interior to the isua belt and metasediments from the malene supracrustal belt using estimates of emplacement age t of between 3 77 and 3 67 ga for the parental volcanics to the garbenschiefer unit initial143nd 144nd ratios yield positiveendt values between 1 0 and 3 1 relative to the chur parameters for seven out of eight samples model sm nd ages for the isua gneisses and metasediments are only compatible with their estimated stratigraphic ages if their sources were ca 2end relative to chur at those times similarly model sm nd ages for the malene samples are only compatible with stratigraphic age constraints when based on a source evolution with positiveendt implications of these results for the early development of the earth s mantle are discussed

mantle convection and early crustal evolution abstract the existence of peridotitic komatiites in the archaean suggests that the archaean mantle was significantly hotter than the modern mantle this evidence is contradicted by estimates of archaean continental geothermal gradients based on the pressure and temperature recorded in metamorphic rocks which suggest that there is no marked difference between archaean and modern continental geothermal gradients numerical modelling shows that small changes in the mantle temperature can have an important influence on convection if the average temperature of the upper mantle is increased by 200 c convection within the mantle becomes chaotic and an upper mantle partial melt zone encircles the globe the crust formed during this period will be komatiitic in composition but will be unstable and will be mixed back into the mantle by subduction later when the mantle temperature falls to 100 c above its present level the upper mantle partial melt zone contracts away from subduction areas it is suggested that the first primitive felsic magmas were generated at subduction zones the appearance of these magmas at 3 8 ga permitted the formation of buoyant continents and eventually led to crustal thickening as a consequence of this thickening the proto continents consisting of a bimodal suite of basalts and sodic granodiorites contained two types of latent energy 1 radioactive energy held in elements such as th k and u and 2 potential energy resulting from the elevation of the continents above sea level the potential energy of the continents led to sedimentation the increase in the rate of sedimentation during the archaean resulted from increased crustal buoyancy at the same time heat released by radioactive elements in the deep crust built up under the insulating blanket of the upper crust this caused a major metamorphic metasomatic and crustal melting event which produced the potassic granites of the late archaean once the radioactive elements had been removed from the lower crust that region of the continent become tectonically stable the proterozoic shelf sediments were deposited at the margins of these stable cratons convection models of the archaean mantle show hot diapirs rising from the boundary layer above the core mantle interface we suggest that these diapirs began to melt at a depth of 450 km giving rise to komatiitic magmas this model requires the average temperature of the archaean upper mantle to be 100 c above that of the modern mantle the similarity between archaean and modern continental geothermal gradients can be explained if archaean continents formed above subduction zones raising the temperature of the archaean mantle by 100 c 1 halves the thickness of the oceanic lithosphere 2 increases the oceanic geothermal gradient at the mid point of a convection cell 3 decreases the viscosity of the mantle by at least an order of magnitude the combination of these effects produces a marked decrease in the strength of the archaean lithosphere and mantle thus the form of archaean tectonics can be expected to have been very different from modern tectonics

systematic variation in chemistry and nd sr isotopes across a caledonian calc alkaline volcanic arc implications for source materials abstract sr and nd isotope compositions and major and trace element chemistry are reported for 42 primitive calc alkaline lavas from the siluro devonian scottish old red sandstone northwestward increases in concentrations of sr ba k p and lree associated with northwest subduction of iapetus ocean crust 1 are not accompanied by change in rb th or ti group elements these relationships show that the spatial variation cannot be simply the result of change in the degree of partial melting with depth initial 143 nd 144 nd expressed as e nd at 410 ma shows a wide range from 6 4 to 3 7 and in contrast to e sr decreases markedly to the northwest samples from the midland valley show a strong anticorrelation between e nd and e sr with the correlation band offset to more radiogenic compositions than that defined by modern oceanic volcanic rocks the mantle array such an offset is common in modern volcanic arc magmas but cannot be explained by addition to the overlying mantle of a single component enriched in radiogenic sr derived from subducted lithosphere southwest highland lavas lie in a completely separate field on an e sr e nd diagram most fall within the mantle array but samples with highest concentrations of sr etc are offset to less radiogenic compositions consideration of the chemical and isotopic relationships with respect to several models of crustal contamination shows that the additional lree in the northwest cannot be derived from a crustal source the data imply the existence beneath scotland of a large region of mantle with time integrated lree enrichment parts of which may have anomalous nd sr isotopic compositions less radiogenic than the mantle array midland valley lavas are probably derived from a long term lree depleted mantle source modified by interaction with subducted oceanic lithosphere the coexistence of two mantle sources with differing histories is best explained by vertical stratification with deeper more lree enriched layers accessible only to magmas formed at great depth in a subduction zone the spatial variation probably in part reflects this mantle layering and in part the operation of some mantle zone refining process that permits efficient concentration of incompatible elements from large regions of mantle

the periglacial climate and environment in northern eurasia during the last glaciation this paper summarizes the results of studies of the late weichselian periglacial environments carried out in key areas of northern eurasia by several queen teams european science foundation esf programme quaternary environment of the eurasian north the palaeoglaciological boundary conditions are defined by geological data on timing and extent of the last glaciation obtained in the course of the eu funded project eurasian ice sheets these data prove beyond any doubt that with the exception of the northwestern fringe of the taymyr peninsula the rest of the eurasian mainland and severnaya zemlya were not affected by the barents kara sea ice sheet during the last glacial maximum lgm inversed modelling based on these results shows that a progressive cooling which started around 30 ka bp caused ice growth in scandinavia and the northwestern areas of the barents kara sea shelf due to a maritime climate with relatively high precipitation along the western flank of the developing ice sheets in the rest of the eurasian arctic extremely low precipitation rates less than 50 mm yr 1 did not allow ice sheet growth in spite of the very cold temperatures palaeoclimatic and palaeoenvironmental conditions for the time prior to during and after the lgm have been reconstructed for the non glaciated areas around the lgm ice sheet with the use of faunal and vegetation records permafrost eolian sediments alluvial deposits and other evidences the changing environment from interstadial conditions around 30 ka bp to a much colder and drier environment at the culmination of the lgm at 20 15 ka bp and the beginning of warming around 15 ka bp have been elaborated from the field data which fits well with the modelling results

precambrian basement complexes in the north central scandinavian caledonides and their pre caledonian tectonic evolution precambrian basement lithology as exposed in four tectonic windows of the northcentral scandinavian caledonides comprises uniform sequences of metasedimentary mica schists and gneisses borgefjell as well as mica schists dominated by acid metavolcanics bangonaive with minor basic rocks the major part however is made up of intrusive rocks older granites are distributed in the borgefjell area and to the east of it obviously related to the svecofennian revsund granites younger granodiorites quartzsyenites subordinate in the borgefjellet dominate the bangonaive window

the geological history of nili patera mars nili patera is a 50 km diameter caldera at the center of the syrtis major planum volcanic province the caldera is unique among martian volcanic terrains in hosting i evidence of both effusive and explosive volcanism ii hydrothermal silica and iii compositional diversity from olivine rich basalts to silica enriched units we have produced a new geological map using three mosaicked 18 m pixel context camera digital elevation models supplemented by compact remote imaging spectrometer for mars hyperspectral data the map contextualizes these discoveries formulating a stratigraphy in which nili patera formed by trapdoor collapse into a volcanotectonic depression the distinctive bright floor of nili patera formed either as part of a felsic pluton exposed during caldera formation or as remnants of welded ignimbrite s associated with caldera formation both scenarios deriving from melting in the noachian highland basement after caldera collapse there were five magmatic episodes 1 a basaltic unit in the caldera s north 2 a silica enriched unit and the associated nili tholus cone 3 an intrusive event forming a 300 m high elliptical dome 4 an extrusive basaltic unit emplaced from small cones in the east and 5 an extreme olivine bearing unit formed on the western caldera ring fault the mapping together with evidence for hydrated materials implies magmatic interaction with subsurface volatiles this in an area of elevated geothermal gradient presents a possible habitable environment sampled by the hydrothermal deposits additionally similarities to other highland volcanoes imply similar mechanisms and thus astrobiological potential within those edifices

geochemistry of contrasting siliceous magmatic suites in the bushveld complex genetic aspects and implications for tectonic discrimination diagrams abstract the intracratonic bushveld igneous province that formed 2 1 1 9 ga ago contains three contrasting suites of siliceous rocks that are demonstrably of magmatic origin the oldest of these are the high mg hmf and low mg lmf felsites which form interstratified flows in the rooiberg group bushveld granites intrude the rooiberg group and constitute the youngest component of the province well defined interelement variation trends illustrate that the granites do not share the same petrogenetic history as the rooiberg magmas nd isotope measurements indicate that the two eruptive suites probably formed under similar differentiation conditions but from parental magmas that were derived from compositionally different sources on trace element discrimination diagrams the bushveld granites and lmf are usually correctly assigned to a within plate setting but conversely the hmf are generally misclassified as subduction related eruptives it is argued that the trace element signatures of the granites and felsites do not identify their tectonic setting per se but rather point to the melting and crystallization histories of the source regions from which their parent magmas were extracted as such tectonic discrimination diagrams may provide valuable pointers to processes that have affected igneous source materials in much earlier magmatic cycles

chemistry and petrological evolution of the lastarria volcanic complex in the north chilean andes the lastarria volcanic complex along the northern chile argentina border includes three morphostructural components southern spur lastarria the highest cone 5697 m and negriales a geographically associated lava field 5 4 km 3 petrographically the lastarria complex consists of pyroxene andesites and pyroxene amphibole dacites the whole rock geochemistry shows a bimodial silica variation between 57 and 68 with peaks at 59 60 and 61 62 sio 2 petrographie and chemical data indicate different magmatic sources for lastarria and negriales whole rock geochemistry can be explained by crustal contamination and crystal liquid fractionation with differences in storage times in magma chambers being a major controlling factor strong textural mineralogical and chemical evidence for magma mixing shortly before explosive eruptions at lastarria suggests that this process may have triggered the violent eruptive volcanic activity which characterizes the latest stages of the main cone abundant bombs of banded clear pumice and dark scoria in pyroclastic flow deposits are the texturally heterogeneous products resulting from incomplete mixing homogenization

late pleistocene and holocene ice wedge activity on the blackstone plateau central yukon canada ice wedge activity can be used to reconstruct past environmental conditions we investigated the moisture source and timing of ice wedge formation on the blackstone plateau a section of permafrost exposed ice wedges that developed at two distinct depths the first set formed syngenetically and penetrated alluvial silts from the top of permafrost the second set truncated by an erosional or thaw contact was found solely in icy muddy gravels 3 1 m depth the 18o and d excess records of the ice wedges suggest that they formed from freezing of snow meltwater whose isotopic composition evolved during meltout the 14cdoc results suggest that climate was favorable to ice wedge growth between 32 000 30 000 and 14 000 12 500 cal yr bp but there was likely a hiatus during the last glacial maximum due to climate being too dry during the early to mid holocene ice wedges were inactive as a result of warmer and wetter climate ice wedge re initiated around 6360 cal yr bp with a peak in activity between 3980 and 920 cal yr bp a period characterized by cool and moist climate overall timing of ice wedge activity was broadly consistent with the climate and vegetation evolution in the western arctic

petrological evolution of the crust and mantle in its simplest form crust mantle evolution can be thought of as a study of the processes by which material is lost from the mantle and forms new crust the overall goal is achievement of mass balance between the average composition of material known to have been added to the crust and the estimated average composition of the entire crust attainment of this goal for the earth has been elusive primarily because the materials available for study have generally been involved in several stages of evolution including fractionation within both the crust and mantle and recycling of material from crust to mantle thereby complicating the interpretation of available data although the overall composition of the continental crust generally has been modelled as andesitic taylor and mclennan 1981a the present upper continental crust is granodioritic and requires a more mafic lower crust it has been proposed taylor and mclennan 1981a that originally the archean crust had an andesitic composition throughout its volume and that partial melting in the lower crust developed more felsic melts that rose to enrich the upper crust leaving a more mafic residuum of lower crust this concept has been challenged by weaver and tarney 1981a who argue that the granulites of presumed lower crustal origin do not have the correct compositions for this model if one considers more recent additions to the crust from the mantle over the past several hundred million years it appears that these latest additions are basaltic with very little if any granitic additions in fact barker 1981 in his introduction to a special issue of jgr on granites and rhyolites states to date no one has demonstrated that such magmas may form in the mantle indeed much experimental and geochemical work indicates that such siliceous liquids cannot originate there therefore such magmas must be generated in the crust in island arc environments where the average composition of added material normally is thought to be andesitic a anderson 1982 has shown that the parent melt is basaltic not andesitic in addition kay 1980 has shown that the k content of the newly added material is less than the average k content of the previously existing crust in the oceanic crust the additions from the mantle are basaltic and according to o hara 1982 probably are derived from very mafic picritic melts as parents continental flood basalts which have also added material from mantle to crust also are clearly derived from basaltic parental melts basaltic volcanism study project 1981 thus the evidence for the nature of material subtracted from the mantle and added to the crust for the past several hundred million or perhaps a billion years points to very mafic melts this contrasts significantly with the overall andesitic composition of continental crust it implies furthermore that if the overall archean crustal composition was andesitic as estimated by taylor and mclennan 1981a then additions to the crust have become more mafic with time a concept in direct conflict with most models in which secular trends are from mafic to felsic with time veizer and jansen 1979 and mclennan et al 198o a question which is beginning to appear either explicitly or implicitly is whether crustal volcanic rocks or the crust as a whole really represents the parent melt that formed in the mantle a anderson 1982 o hara 1982 and nisbet and walker 1982

genesis of granitoid batholiths of peninsular malaysia and implications for models of crustal evolution evidence from a ndsr isotopic and upb zircon study nd sr and u pb isotopic data for the late triassic west coast province batholiths and permian to triassic east coast province batholiths of peninsular malaysia allow estimates of the ages of the crustal fragments comprising the peninsula to be made initial nd and sr values for granitoids from the west coast province range from 6 to 10 and 160 0 716 to 660 0 751 respectively nd model ages calculated based on a depleted mantle evolution model tdmnd range from 1300 ma to 1800 ma and are in general agreement with the mid proterozoic upper intersection ages of u pb zircon reverse discordia 1500 1700 ma initial nd and sr values for granitoids from the east coast province range from 0 8 to 6 and 10 0 705 to 130 0 714 respectively calculated tdmnd ages of 900 1400 ma for these granitoids are comparable to two u pb zircon reverse discordia intercepts that yield 800 ma and 1350 ma the general agreement of u pb zircon inheritance ages and tdmnd ages are interpreted to correspond to the proterozoic crust formation ages of the continental fragments represented by the west coast and east coast batholithic provinces mid proterozoic 1300 1900 ma crust formation ages are commonly shown by other phanerozoic continental margin plutonic and volcanic belts the ubiquitous mid proterozoic crust formation ages and the absence of archaean signatures suggest voluminous juvenile additions to the continental crust in the mid proterozoic such ages at continental margins would imply that many continental blocks had achieved very much their present day extent by the mid proterozoic

combined sr o isotope relationships and petrogenesis of andean volcanics of south america active andesitic type volcanism in the southern andes lat 42 36 s is characterized by low sr and o isotopic compositions which fall in the range 87sr 86sr 0 7036 0 7043 18o 5 2 6 8 in contrast volcanic centres in the central andes lat 24 21 s are significantly enriched in both 87sr and 18o we present the first combined sr and o isotope study of the southern volcanic zone svz in chile crustal thickness 40 km 1 and of the southern part of the central volcanic zone cvz in the borderlands of bolivia chile and argentina crustal thickness 60 70 km 1 only one of 13 volcanic centres studied lascar exhibits a correlated increase in both 87sr and 18o parallel with the basalt andesite rhyolite compositional change indicative of contamination by upper crust through a fractional crystallization assimilation mechanism the other centres of the cvz for which there is an overall sr o correlation document the variable contamination of mantle derived magmas at depth in the lower crust which had been thickened by mesozoic and earlier plutonism

the origin and significance of crystal rich inclusions in pumices from two chilean ignimbrites crystal rich 70 98 phenocrysts magmatic inclusions in pumices for compositionally heterogeneous ignimbrites from the central andes of northern chile are interpreted as the products of crystal accretion at the sidewalls of the magma chambers the inclusions are typically andesitic in composition and are found as peppery textured lenses and bands or as discrete ovoid blobs within dacitic pumices from the early erupted portions of the ignimbrites the inclusions have a bimodal gain size with large phenocrysts 1 mm typical of those of the host pumice set in a dominant finegrained framework these inclusions may be an important link between the experimental and geochemical models for the origin of compositional layering in magma chambers by sidewall crystallization the presence of similar inclusions in other ignimbrites and volcanics as well as plutonics suggest that they may be a common feature of silicic magmas

small scale disequilibrium in a magmatic inclusion and its more silicic host basaltic andesite inclusions and their host dacite from the purico chascon complex in northern chile are isotopically distinct textural characteristics of the inclusions are typical of those resulting from magma mingling serial sectioning across the interface of an inclusion and its host dacite complemented by microdrill sampling and detailed microprobe work has enabled us to examine the scale of mixing and chemical isotopic and trace element disequilibrium the results of this work show that 1 the composition of the inclusion is relatively homogeneous 2 the dacite host is generally higher in 87sr 86sr and lower in 143nd 144nd than the enclave but it is heterogeneous on a small scale and probably a hybrid 3 the isotopic composition in the marginal zone apparently on both host and inclusion sides of the weakly chilled interface actually shows the highest 87sr 86sr and lowest 143nd 144nd 4 large plagioclase crystals in the inclusions and host are xenocrystic with higher 87sr 86sr than any of the other samples these observations are reconciled with a model of magma evolution in a crustal magma chamber in such a scenario the mafic magma is overlain by a cap of rhyolite a partial melt of the dacitic ignimbrites which now underlie the purico chascon complex overturn of such a magma system gives rise to a hybrid dacite containing discrete mafic inclusions

ree geochemistry and isotopic data of archean silicic volcanics and granitoids from the pilbara block western australia implications for the early crustal evolution eighteen silicic volcanic rocks of the warrawoona group and ten associated plutonic rocks from the pilbara block western australia have been chosen for geochemical and isotopic studies silicic volcanics of the unsb upper member of north star basalt are dated at 3 56 3 57 by both the rb sr and the sm nd methods the respective 1 initial isotopic composition values are 0 7005 5 sr and 0 50810 39 nd this age is consistent with the stratigraphic interpretation that the talgatalga subgroup in which the north star basalt occupies the lowermost position is overlain by the duffer formation whose age was earlier established at 3 45 by the zircon u pb method the new rb sr data on six silicic lava samples from the duffer formation yield an isochron of 3 23 0 28 2v though imprecise this age agrees with the zircon age within error limits rb sr ages of 2 3 2 4 obtained for the panorama rocks and the wyman formation do not correspond to their initial eruption ages chemical arguments suggest that these ages represent the time of metasomatism associated with the widespread thermal event in this region about 2 3 2 4 ago geochemically most of these analyzed rocks volcanic and plutonic are of tonalite trondhjemitegranodiorite ttg composition a typical feature found in many other archean terrains they generally show fractionated ree patterns except the panorama formation rocks furthermore the wyman formation rhyolites and the post tectonic adamellites show significant negative eu anomalies suggesting a similar mode of magma generation and a probable genetic link theoretical considerations suggest that most of these ttg rocks could have been generated by partial melting of amphibolitic or basaltic sources followed by fractional crystallization although the archean granitic gneisses often possess mantle like isr values the trace element data indicate that they could not have been derived by direct melting of upper mantle materials the immediate tectonic implication is that in any archean terrain the formation of na rich continental crust of ttg suite must be preceded by the presence of basaltic crust the occurrence of this basaltic crust is a matter of controversy such crust might have been totally destroyed by repeated melting processes or its remnants are now represented by some of the mafic ultramafic enclaves within the tonalite trondhjemite batholiths

absence of silicic volcanism on mars implications for crustal composition and volatile abundance on earth eruptions of silicic magmas with volumes of 1 100 km3 are generally associated with stratovolcanoes or composite cones whereas larger 100 1000 km3 ignimbrite deposits are erupted from resurgent calderas and related structures small volumes of silicic rocks are generated by differentiation within basic magma chambers associated with stratovolcanoes but large silicic deposits are largely confined to areas where the granitic crust is thicker than 25 km as mantle derived magmas rise through the thick granitic crust fractional crystallization and assimilation of hydrous crustal rocks concentrate volatiles and sio2 in the magma yielding explosive silicic eruptions on mars under similar chemical conditions ignimbrite eruptions could take place over a broader range of physical conditions than on earth but no large volume ignimbrite deposits can be identified positively explosive basic to ultrabasic eruptions may be responsible for small cinder conelike features on mars and perhaps for the ancient flank scoured paterae such eruptions may have been driven by near surface water ice magma interactions or volatile rich magmas the paterae represent a unique style of volcanism that stopped 2 b y ago we suggest that the volatiles associated with these features were derived from mantle sources and that the cessation of patera formation may coincide with the termination of the period of maximum planetary degassing volcanism since 2 b y ago has been dominantly effusive except for minor explosive ash events such as those which blanketed part of the summit of hecates tholus we propose that such eruptions were driven by volatiles generated by differentiation of a magma chamber similar to icelandic explosive eruptions the apparent lack of large scale silicic volcanism on mars is circumstantial evidence that mars is deficient in volatiles and lacks a granitic crust

petrogenesis of andean andesites from combined o sr isotope relationships the descent of the oceanic nazca plate below the western margin of south america is responsible for the seismicity tectonism and magmatism characteristic of andean type continental margins within this geological setting there is much debate concerning the petrogenesis of the calc alkaline volcanic association a thermal model for the descent of oceanic below continental lithosphere suggests that melting of subducted oceanic crust the asthenospheric mantle wedge and the lower crust might occur1 and hence calc alkaline volcanic rocks erupted at the surface in such regions might be derived from one or more of these sources in the andes for example pichler and zeil2 have invoked partial melting of lower continental crust while others3 5 envisage petrogenesis of andesites from sub crustal mantle sources possibly in conjunction with crustal contamination6 9 we present here new o isotope data for volcanic rocks from three contrasted volcanic settings in ecuador north chile and north west argentina we then consider the significance of observed o sr isotope relationships and attempt to evaluate the important factors in the petrogenesis of andean andesitic lavas

first direct dating of late pleistocene ice wedges by ams we present the first direct dating by 14 c accelerator mass spectrometry of three late pleistocene syngenetic icewedges from the seyaha cross section they are representative of permafrost with multistage ice wedges from the north of western siberia the most important result is the clear vertical age stratification of the ice i e the old ice is located beneath the young this shows that a timescale can be assigned to these ice wedges penetrating down into the permafrost the age of the ice shows a depth of not more than 3 5 m for frost cracking water penetrated into the icewedges at that depth the lower part of the ice wedges from the seyaha cross section has been dated between 21 000 and 14 000 bp fl 2000 elsevier science b v all rights reserved

two overlapping plates subducting beneath central japan as revealed by strontium isotope data abstract in central japan the pacific plate subducts westward beneath the eurasian plate and the philippine sea plate subducts northwestward into the mantle wedge between the eurasian plate and the subducted pacific slab there the northeast japan arc is joined to the izu ogasawara arc we determined 87 sr 86 sr ratios and rb and sr contents for 47 volcanic rock samples from 15 quaternary volcanoes in central japan and summarized the geographical distribution of the ratios the general trend of slowly increasing 87 sr 86 sr ratio from the back arc side toward the volcanic front in the northeast japan arc is broken by a marked high ratio above 0 7060 centered around akagi volcano located at the southernmost region of the arc elsewhere the ratio along the volcanic front in this arc varies within the range 0 7038 to 0 7045 the marked high 87 sr 86 sr ratio is considered to be due to the addition of slab derived components transported by the philippine sea plate to the magma generating region in the mantle wedge beneath central japan therefore the geographical distribution of the high ratio may correspond to that of the philippine sea slab derived components in the mantle wedge and we may draw the underground outline of the philippine sea plate this outline implies that an aseismic portion of the philippine sea plate continues a few tens km ahead of the seismic one a belt of low 87 sr 86 sr ratios from the izu peninsula northwestward along the northern end of the izu ogasawara arc coincides with the zone where the subducting philippine sea plate is not observed seismologically while it is detected seismologically on both sides of the belt

neogene magmatism and its possible causal relationship with hydrocarbon generation in sw colombia the cretaceous oil bearing source and reservoir sedimentary succession in the putumayo basin sw colombia was intruded by gabbroic dykes and sills the petrological and geochemical character of the magmatic rocks shows calc alkaline tendency pointing to a subduction related magmatic event k ar dating of amphibole indicates a late miocene to pliocene age 6 1 0 7 ma for the igneous episode in the basin therefore we assume the intrusions to be part of the andean magmatism of the northern volcanic zone nvz the age of the intrusions has significant tectonic and economic implications because it coincides with two regional events 1 the late miocene pliocene andean orogenic uplift of most of the sub andean regions in peru ecuador and colombia and 2 a pulse of hydrocarbon generation and expulsion that has reached the gas window high la yb k nb and la nb ratios and the obtained sr nd pb isotopic compositions suggest the involvement of subducted sediments and or the assimilation of oceanic crust of the subducting slab we discuss the possibility that magma chamber s west of the basin below the cordillera did increase the heat flow in the basin causing generation and expulsion of hydrocarbons and co2

recharge in volcanic systems evidence from isotope profiles of phenocrysts strontium isotope ratios measured from core to rim across plagioclase feldspar crystals can be used to monitor changes in the isotope composition of the magma from which they grew in samples from three magma systems from convergent margin volcanoes sudden changes in major element composition petrographic features and strontium isotope composition were found to correspond to discrete magmatic events most likely repeated recharge of more mafic magma with lower ratios of strontium 87 to strontium 86 into a crustally contaminated magma

old model nd ages in namibian pan african rocks the nd isotope analytical technique is a powerful tool for studying many geological processes particularly the evolution of the upper mantle as seen through the isotope and trace element geochemistry of mantle derived volcanic rocks1 3 and magmatic processes along destructive plate margins4 5 however previous studies of continental crust6 8 have been largely restricted to archaean examples and concerned primarily with dating their time of derivation from the upper mantle in the present study we investigated the nd and sr isotope characteristics of rocks involved in a relatively young 800 450 myr orogenic event such events represent critical stages in the evolution of most of the earth s continental crust new material is added from the mantle and pre existing crust is remobilized by erosion and sedimentation deformation and magmatic activity nd and sr isotopes were used to outline the range in age and geochemical characteristics of rock sequences and provinces within both the upper mantle and the pre existing crust which were sampled during the orogeny and hence provided the major components of what is now a stable segment of continental crust

the fox permafrost tunnel a late quaternary geologic record in central alaska the fox permafrost tunnel which penetrates 110 m into frozen sediments of gold stream valley provides a continuous exposure of fossiliferous silt and alluvium above schistose bedrock deposition of fluvial gravel was followed by a long interval of loess accretion and permafrost aggradation that was punctuated by episodes of thaw and of gullying and redeposition of silt imbricated sandy gravel above the bedrock contains lenses of finer alluvium that contain wood fragments and some rooted stumps radiocarbon dates indicate that the gravel is older than 40 ka but absence of mature soil and weathering profiles at its upper contact indicates that fluvial activity must have continued until shortly before loess accretion began at the tunnel site silt is the most widespread depositional unit in the tunnel this deposit is of eolian origin loess but some has been redeposited by slope processes the silt units contain abundant ground ice as pore filings lenses wedges and buried pond ice loess accretion was interrupted by a period when little loess accumulated and when large ice wedges formed in the lower loess unit and subsequently were truncated by thaw loess began forming sometime before 40 ka and was rapidly accreting by 39 ka under xeric conditions with open vegetation a sharply decreased rate of loess accretion associated with local erosion and thaw between about 36 and 30 ka is marked by anomalous cation concentration values lenses of buried sod fossils indicative of moist to wet substrates and truncated ice wedges beneath small frozen ponds or streamlets that occupied ice wedge troughs a later episode of rapid loess influx under drier conditions began after 30 ka and coincided with glacial advances of late wisconsin age in the adjoining alaska range large ice wedges also formed in the upper loess unit but only their bases are exposed in the tunnel and their history of development is uncertain fanlike deposits of poorly sorted debris near the tunnel portal formed between about 12 5 and 11 ka during deep erosion of loess slopes under moister conditions the deposits locally form two subunits the younger over whelmed a stand of tall willows on the floor of goldstream valley between about 11 3 and 11 1 ka the older may have formed about 1 000 yr earlier stratigraphic records elsewhere in central alaska indicate variable middle wisconsin environments followed by colder and drier conditions that began between 30 and 25 ka and persisted until perhaps 12 5 ka widespread loess erosion and redeposition subsequently occurred under moister and probably warmer conditions renewed early holocene loess deposition may have been widespread but its exact environmental controls are uncertain our data challenge three generally accepted concepts of late quaternary periglacial processes in central alaska we contend that 1 many ice wedge systems may have formed under interstadial conditions rather than full glacial conditions 2 episodes of rapid loess influx may have been partly out of phase with episodes of glacier expansion and 3 redeposition of loess by solifluction sheetwash and gully formation may have been episodic and required conditions moister than those under which the loess initially accreted

ratio of 36cl cl in ground ice of east siberia and its application for chronometry abundance of the cosmogenic nuclide chlorine 36 36cl was measured together with the chloride cl concentration in different horizons of quaternary permafrost samples collected from various types of ground ice in the northeastern part of siberia the 36cl cl in 32 samples ranged in value from 2 4 10 14 to 1 4 10 12 nonetheless after a few extreme values were excluded these 36cl cl ratios provided a local permafrost chronometry the general concordance of the modeled ages with geological expectations and other chronological methods supports the potential power of the proposed dating method however the large observed change in ratios from higher to lower values during the transition from last glacial maximum to holocene climatic conditions remains unexplained an attempt to make use of the corresponding beryllium 10 10be absolute concentrations in the same samples failed because input of 10be attached to particulate matter into permafrost is unknown further 36cl cl serial measurements of modern precipitation and fossil ground ice are needed to refine this dating method into a practical tool with a clear protocol

common pb isotopic compositions of the lima arequipa and toquepala segments in the coastal batholith peru implications for magmagenesis abstract common pb isotopic compositions for 65 feldspar samples from the 188 37 ma old gabbroic to monzogranitic plutons of the peruvian coastal batholith show changes along strike that can be related to variable contamination of mantle derived magmas by the local precambrian basement results have the following isotopic ranges 206 pb 204 pb 17 580 20 803 207 pb 204 pb 15 555 15 709 and 208 pb 204 pb 38 104 41 177 with averages of 206 pb 204 pb 18 630 207 pb 204 pb 15 610 and 208 pb 204 pb 38 500 the gabbroic dioritic and tonalitic plutons north and east of lima the lima segment have ratios similar to the isotopically homogeneous reservoir identified by tilton 1979 tilton and barreiro 1980 and barreiro and stern 1982 for rocks in central and southern chile the homogeneous reservoir has been suggested to be enriched subcontinental mantle more siliceous rocks granodiorites and monzogranites in the lima segment have considerably higher ratios 206 pb 204 pb 18 680 20 803 207 pb 204 pb 15 610 15 709 and 208 pb 204 pb 38 500 41 177 nine of ten monzogranites from the ring dike complexes in the lima segment define a linear array with a slope of 0 1143 correlation coefficient 0 9398 and a pb isochron age of 1 9 ga this isochron age has no simple chronological meaning although it may represent a component of sedimentary debris derived from 1 9 ga crustal materials feldspars from plutons in the arequipa and toquepala segments south of lima tend to have low pb isotopic ratios 206 pb 204 pb 17 580 18 603 207 pb 204 pb 15 555 15 633 and 208 pb 204 pb 38 104 38 749 the ratios plot between the isotopic signatures of local precambrian granulites and gneisses and the chile type enriched subcontinental magmatic reservoir indicating that these end members were the principal magma sources the oldest plutonic rocks in the segments early to mid jurassic are closest to the pb isotopic ratios of the precambrian basement contamination of plutons in the arequipa and toquepala segments by unradiogenic pb from the precambrian basement and lack of it in the lima segment supports crustal models by couch et al 1981 and jones 1981 which show a thick precambrian basement layer throughout southern peru and an extremely thin layer beneath the lima segment

mesoproterozoic dyke swarms in foreland and nappes of the central scandinavian caledonides structure magnetic fabric and geochemistry as an example of microstructural and magnetic fabric evolution and geochemistry of mafic dykes during a subsequent orogenic overprint a major mesoproterozoic dyke complex in scandinavia the vasterbotten complex of the central scandinavian dolerite group is traced westwards into the crystalline nappes of the early phanerozoic caledonian orogen using geophysical field microscopic magnetic and geochemical information dykes and sills are characterized and their overprint during caledonian orogeny documented the vasterbotten complex is composed of sets of dykes trending ne sw nw se and wnw ese respectively similar dykes are exposed in allochthonous positions lower and middle allochthons in the caledonian fold and thrust belt the autochthonous dykes are generally undeformed and retain both their primary texture and mineralogy chilled margins are well preserved in the caledonian lower and middle allochthons similar dykes in crystalline basement rocks are progressively faulted and sheared when proceeding from the marginal to the interior parts of the orogen dyke margins are more likely to be sheared than the interior parts of dykes in the lower allochthon under very low and low grade metamorphic conditions dykes are distinctly less competent than granitic host rocks thick dykes are more competent than gneisses thin dykes do not show such competence contrasts in the middle allochthon metre scale dykes with patches of altered plagioclase phenocrysts can still be discerned in low strain domains highly sheared dykes are drawn out to thin layers of centimetre thickness dykes are deformed together with the crystalline country rocks under greenschist grade metamorphic conditions without major competence contrasts magnetic fabrics show an evolution similar to the silicate mineral fabrics the magnetic fabrics in the dykes are transformed successively from ferromagnetic magmatic in the autochthon to ferromagnetic deformational in the lower allochthon and finally paramagnetic deformational in the middle allochthon as a consequence the magnetic susceptibility decreases for several orders of magnitude geochemically the dykes are dominantly sub alkaline basalts typical for continental tholeiites and can be distinguished from the neoproterozoic dykes in the sarv nappe equivalents highest part of the middle allochthon which show a more morb like e morb magmatic signature preliminary age information from a dyke in the lower allochthon of the borgefjell area and the middle allochthon is consistent with the assumption that these dykes are time equivalent with the central scandinavian dolerite group therefore the studied dykes may represent an extension of the vasterbotten complex or a new complex of the central scandinavian dolerite group according to section restorations the caledonian allochthons were situated further wnw relative to their present position and originally the mafic dykes cut across all of the fennoscandian lithosphere at least to the present atlantic margin and the earlier passive margin of the baltica terrane as a consequence these dykes may provide a link for pre caledonian and pre grenvillian plate reconstructions

correlation of upper pleistocene sediments in northern west siberia abstract a geochronometric database comprising 121 optically stimulated luminescence and 59 radiocarbon dates plus u th date on peat from 24 key sections in northern west siberia is presented and discussed these data have been obtained during russian norwegian joint research for the past 15 years and are augmented by reports on radiocarbon dated mammoth carcasses in west siberia together they provide the basis for revising the regional stratigraphic scheme and for correlation to the european quaternary chronostratigraphy and marine oxygen isotope record the geochronologic correlation of sedimentary formations across west siberia involves classical stratotypes of fluvial shallow limnic and temperate marine sediments of the karginsky horizon ca 130 ka which was previously attributed to mis 3 the older kazantsevo marine formation is thus assigned a middle pleistocene age the uppermost glacial sedimentary complex containing glacial ice was deposited by shelf based ice sheets between ca 100 and 60 ka bp the postglacial pleistocene sediments are mostly ice bound loess like deposits and sink hole silts containing numerous mammoth carcasses that are radiocarbon dated to between ca 42 and 25 ka bp there are no traces of glacial activity on the siberian plains during mis 2 a period for which palaeoclimatic proxies suggest a cold and dry environment this synthesis indicates that the prior usage of the siberian correlation horizons of kazantsevo and karginsky as equivalents of the eemian and middle pleniglacial is stratigraphically invalid

helium strontium isotope constraints on mantle evolution beneath the roman comagmatic province italy european social fund scottish universities carnegie trust for the universities of scotland

geochemical variations in andean basaltic and silicic lavas from the villarrica lanin volcanic chain 39 5 s an evaluation of source heterogeneity fractional crystallization and crustal assimilation at 39 5 s in the southern volcanic zone of the andes three pleistocene recent stratovolcanoes villarrica quetrupillan and lanin form a trend perpendicular to the strike of the andes 275 to 325 km from the peru chile trench basalts from villarrica and lanin are geochemically distinct the latter have higher incompatible element abundances and la sm but lower ba la and alkali metal la ratios these differences are consistent with our previously proposed models involving a a west to east decrease in an alkali metal rich high ba la slab derived component which causes an across strike decrease in degree of melting or b a west to east increase in the contamination of subduction related magma by enriched subcontinental lithospheric mantle silicic and mafic lavas from the stratovolcanoes have overlapping sr nd and o isotopic ratios silicic lavas also have geochemical differences that parallel those of their associated basalts e g rhyolite from villarrica has lower la sm and incompatible element contents than high sio2 andesite from lanin at each volcano the most silicic lavas can be modelled by closed system fractional crystallization while andesites are best explained by magma mixing apparently crustal contamination was not an important process in deriving the evolved lavas basaltic flows from small scoria cones 20 35 km from villarrica volcano have high incompatible element contents and low ba la like lanin basalts but trend to higher k rb 356 855 and lower 87sr 86sr 0 70361 0 70400 than basalts from either stratovolcano however all basalts have similar nd pb and o isotope ratios the best explanation for the unique features of the cones is that the sources of svz magmas e g slab derived fluids or melts of the subcontinental lithospheric mantle have varying alkali metal and radiogenic sr contents these heterogeneities are not manifested in stratovolcano basalts because of extensive subcrustal pooling and mixing this model is preferable to one involving crustal contamination because it can account for variable sr isotope ratios and uniform nd and pb isotope ratios among the basalts and the divergence of the cones from across strike geochemical trends defined by the stratovolcanoes

a geochemical traverse across the north chilean andes evidence for crust generation from the mantle wedge major and trace element and sr and nd isotope analyses are presented on 186 0 ma magmatic rocks along an east west traverse across north chile at 22 s esr ranges from 25 to 100 and end from 6 to 9 but the low end and high esr values are in rocks generated in the last 15 ma it is argued that previous discussions of petrogenesis in north chile have been hampered because the changes in magma chemistry in this area of unusually thick crust reflect not one but two processes one results in a progressive shift of esr from 25 to 20 and end from 6 to 6 in jurassic to recent rocks which is accompanied by increasing ta sm and sr and decreasing th ta the second is largely confined to the younger rocks and it is characterised by esr increasing up to 100 with increasing sio2 and decreasing sr and it results in relatively shallow trends on an end esr diagram the preferred interpretation is that trend 1 is due to the mobilisation of old late proterozoic mantle lithosphere as magmatism migrated eastwards and that trend 2 is due to crustal melting and contamination with differentiation in this area of thickened continental crust it follows that the mantle wedge is the principal site of crust generation and it is argued that 20 of the sr in the recent northern chile rocks is derived from the subducted ocean crust

link between archaean continent formation and anomalous sub continental mantle the petrology and geochemistry of the subcontrnental mantle from 70 to 160 km depth under archaean cratons as sampled by kimberlites differs from the mantle sources of mid ocean ridge basalts morb and hotspot volcanics here i propose that both the formation of this anomalous mantle and the origin of large masses of tonalitic granodioritic crust in the early archaean can be explained by a single open system fractional crystallization model if an extensive partially molten layer existed at shallow levels in the pre archaean due to the production of large amounts of morb type magma above sites of mantle upwelling the magma at loci of fractional crystallization would reach a steady state in major and trace element compositions which would be similar to that of archaean granodiorites and tonalites while this fractional crystallization process operated the cumulate would chemically resemble a hydrated tholeiite pressure induced breakdown of amphibole could cause partial re melting of this cumulate and this could lead to depletion resulting in a sub continental mantle keel with chemical characteristics similar to those observed

growth of the continental crust a planetary mantle perspective abstract this paper addresses the net growth through time of the total continental crust cc not to be confused with sporadic growth of individual continental masses e g by accretion of island arcs a process that does not necessarily require net growth geochemists tend to emphasize differences between cc and oceanic crust oc but from a whole planet perspective cc and oc are remarkably similar in composition and density the cc oc distinction is canonically defined in terms of elevation however the earth s bimodal elevation pattern clearly reflects a genetic distinction the oc participates directly in mantle convection whereas the cc is the remaining relatively stable portion of the crust the earth s advective cooling mechanism requires massive subduction of lithosphere which due to the inherent buoyancy of crust tends to require a low crust total lithosphere thickness ratio given relatively constant lithosphere thickness except near thermally buoyant mid ocean ridges the earth responds by heaping most of its crust into a few thick continents allowing subduction to freely operate over a large area of relatively thin crust this crustal thickness bimodality engenders the bimodal elevation pattern prevailing wisdom tends to hold that the rate of cc growth has been slow and ongoing typical models show cc volume roughly proportional to 1 h where h is the total heat generation by the earth s k th and u however such models are generally vague as to the mechanism that supposedly holds back growth even after 4 5 ga the strong t dependence of mantle rheology probably ensured that the mantle took only a few million years to cool within a few hundred degrees of its modern t gradual compositional evolution of the cc from a plausible initial composition e g basaltic anorthositic might have slightly lowered the mean cc density but this effect is of second order importance in comparison to the general increase in crust total lithosphere thickness ratio for any given crustal thickness engendered by temporal cooling of the mantle intense cratering occurring coevally with early intense crust mantle recycling probably reconstituted virtually all of the cc extant prior to 3 9 ga but cc was probably not recycled into the mantle any faster than it was regenerated lunar samples and meteorites provide strong circumstantial evidence that in primordial times the outer few hundred kilometers of all terrestrial planets were largely molten outcomes of primordial magmatism must have been strongly dependent on the pressure regimes within the planetary interiors in general the far greater dt dp of silicate melting curves vs adiabats probably tended to foster a layered magmasphere with a partially molten or paracumulate layer constrained by dt dp relationships to a p range of roughly 3 gpa surmounted by fully molten magma ocean layer also limited in thickness by efficient convective cooling in the earth p stabilized garnet and al rich pyroxene were key factors limiting ultimate yield of crustal feldspar this effect has been examined with quantitative models of high p fractional crystallization including in some models periodic replenishment of the magma ocean from below results indicate that a plausibly deep 300 400 km magma ocean would produce a crust comparable in both composition and thickness to that of the modern earth mildly worrisome discrepancies involve cao for which high p pyroxene compositions are poorly constrained and sio 2 the crust probably evolved to its present sio 2 rich composition through differentiation catalyzed by h 2 o which presumably was not as abundant in the magma ocean as in the crust soon afterward nominal model results for mean crustal thickness are typically 1 2 1 3 the modern global mean but for this parameter the model is designed to give conservatively high results so the discrepancy is probably not significant after the chaotic first few million years of solar system history i suggest that planetary size and composition for more than time govern crustal volume

lead isotopic evidence for evolutionary changes in magma crust interaction central andes southern peru abstract lead isotopic measurements were made on andean igneous rocks of jurassic to recent age in moquegua and tacna departments southernmost peru to clarify the petrogenesis of the rocks and in particular to investigate the effect of crustal thickness on rock composition this location in the cordillera occidental is ideal for such a study because the ca 2 ga precambrian basement rocks arequipa massif have a distinct pb isotopic signature which is an excellent tracer of crustal interaction and because geomorphological research has shown that the continental crust was here thickened drastically in the later tertiary seven samples of quartz diorites and granodiorites from the ilo and toquepala intrusive complexes and seven samples of toquepala group subaerial volcanics were analyzed for pb isotopic compositions the plutonic rocks range in age from jurassic to eocene the volcanic rocks are all late cretaceous to eocene with one exception the pb isotopic ratios are in the ranges 206 pb 204 pb 18 52 18 75 207 pb 204 pb 15 58 15 65 and 208 pb 204 pb 38 53 38 74 the data reflect very little or no interaction with old continental material of the arequipa massif type lead from four miocene huaylillas formation ash flow tuffs two pliocene capillune formation andesites and five quaternary barroso group andesites has lower 206 pb 204 pb than that in the pre miocene rocks but relatively high 207 pb 204 pb and 208 pb 204 pb 206 pb 204 pb 18 16 18 30 207 pb 204 pb 15 55 15 63 208 pb 204 pb 38 45 38 90 tilton and barreiro 9 have shown that contamination by arequipa massif granulites can explain the isotopic composition of the barosso group lavas and the new data demonstrate that this effect is evident to varying degrees in all the analysed neogene volcanic rocks the initial incorporation of such basement material into the magma coincided with the early miocene uplift of this segment of the cordillera occidental 32 and thus with the creation of a thick crustal root the data strongly imply a relationship between crustal thickness and degree of crustal contamination of magmas in this area but a rigorous relationship is not yet established

the postglacial pleistocene of the northern russian mainland abstract the paper summarizes the results of recent studies of late pleistocene postglacial geological events which occurred in the flatlands of northern russia stretching from the timan ridge to the laptev sea coast the available sedimentary record includes thawed and perennially frozen subaerial sediments and in places lacustrine and fluvial formations hundreds of geochronometric measurements by radiocarbon and luminescence techniques place this record within the time span of 50 11 ka bp which confirms an early mid weichselian age of the underlying glacial complex first established by the queen programme the spectrum of proxies includes sedimentary facies pollen data plant macrofossils oxygen isotope values megafauna remains insect assemblages traces of dna paleosols and permafrost properties together they indicate a continental progressively deteriorating climate with very cold winters and warmer than present summers through the second half of the late pleistocene minor climatic fluctuations are largely due to slightly changing humidity during mis 3 these changes are more pronounced in periglacial european russia in the vicinity of the barents kara ice sheet where the mid weichselian biotic maximum contrasts with conditions in the late weichselian polar desert in siberia the periglacial environments were more stable and beneficial for biota inspite of the increased continentality the recent data accumulated by various researchers are incompatible with the conventional correlation of the karginsky interglacial strata of siberia with mis 3 or any part of the european pleniglacial

an open system fractional crystallization model for very early continental crust formation abstract a model is discussed which seeks to explain the origin of archaean tonalitic trondhjemitic granodioritic ttg crust by an open system fractional crystallization mechanism following the assumption that in the pre and early archaean the melt which formed above sites of large scale mantle upwelling was too voluminous and too hot to solidify an almost global near surface partially molten zone could form the magma in this layer would converge above mantle sinks and undergo fractional crystallization recharged fractionating reservoirs would form at sites of convergence the major element composition of the magma in such reservoirs would evolve towards one resembling that of ttg rocks provided that amphibole was a significant fractionating phase from such reservoirs their incompatible as well as compatible trace element abundances would also become similar to those of ttg terrains it is proposed that such terrains resulted from the consolidation of these recharged fractionating reservoirs it is shown that the reservoirs would remain indistinguishable from the primitive mantle in terms of radiogenic isotopes so that model ages cannot be greater than the age of their consolidation consideration of the main aspects of greenstone belts and archaean high grade terrains indicates no contradictions with the model the intrusion of late archaean adamellites and granites is interpreted as an indirect effect of pressure induced breakdown of amphibole in the cumulate underlying the ttg crust which in turn resulted from crustal shortening cratonization is seen as the result of consolidation not only of the crust but also of the underlying cumulate as subcontinental lithosphere

volcanological and petrological evolution of vulcano island aeolian arc southern tyrrhenian sea petrological and geochemical data are reported for volcanic rocks from vulcano island the subaerial volcanism 120 ka to present built up a nw se elongated composite structure affected by two intersecting multistage calderas volcanics older than 20 ka consist mostly of high k calc alkaline hkca to shoshonitic sho mafic rocks these magmas interacted significantly with the continental crust which generated variable sr isotopic ratios 0 70412 0 70520 however a major role was also played by input of parental liquids into the magma chamber which prevented further evolution of the magmas hkca sho and potassic ks rocks formed from 20 to 8 ka display a much larger range of sio2 from shoshonites to rhyolites and higher concentrations of incompatible elements with respect to the previous stage sr isotopic ratios show small variations 0 70448 0 70486 mixing of silicic and mafic liquids and fractional crystallization processes fc were the main evolutionary processes during this stage volcanics younger than 8 ka consist of sho and leucite bearing ks mafic rocks with abundant intermediate and silicic products mafic and intermediate rocks display similar incompatible element abundances and sr isotopic ratios as the previous stage volcanics whereas higher 87sr 86sr 0 70494 0 70583 are observed in some rhyolites these products originated from a complex interplay of fc crustal assimilation and magma mixing processes the most mafic rocks show increasing incompatible element abundances rb sr rb ba mg al mg ca and a decrease in large ion lithophile to high field strength element ratios passing from older hkca sho to the younger sho ks volcanics these variations suggest a shifting of magma sources from a slightly metasomatized asthenosphere fertile peridotite to a more strongly metasomatized lithospheric mantle residual peridotite time related petrological and geochemical variations have been used to develop a model for the evolution of the vulcano plumbing system

end member models for andean plateau uplift abstract diverse techniques have been applied over the past decade to quantify the uplift history of the central andean plateau ap in this study opposing models for surface uplift are evaluated including a rapid rise of 2 5 km 10 6 ma and a slow and steady rise since 40 ma these end member models are evaluated by synthesizing observations of the ap lithosphere and the history of deformation sedimentation exhumation magmatism uplift and fluvial incision structural and geophysical studies estimate variable shortening magnitudes 530 150 km involving cover to basement rocks an isostatically compensated thick crust 80 65 km high heat flow and zones of variable velocity and attenuation in the crust and mantle these observations have invoked interpretations such as a hot weak lithosphere partial melt crustal flow and perhaps current localized delamination but do not provide strong support for massive delamination required by the rapid uplift model deformation and associated exhumation began 60 40 ma and generally migrated eastward with consistent long term average shortening rates 12 8 mm yr in bolivia favoring the slow uplift model volcanic and helium isotope evidence show an ap wide zone of shallow mantle melting and thin lithosphere that has existed since 25 ma which is inconsistent with the rapid rise model that suggests lithospheric thinning occurred 10 6 ma paleoaltimetry data suggest a rapid 2 5 km elevation gain 10 to 6 ma but are equally consistent within error with a linear rise since 25 ma widespread fluvial incision 2 5 1 km occurred along the western flank since 11 8 ma and may be associated with surface uplift as proposed by the rapid rise model however the paleoaltimetry and incision data can also be explained by regional climate change associated with plateau uplift implications of these results for reconstructions of ap evolution are that 1 substantial deformation of a weak lithosphere is essential 2 ap growth has taken significantly longer 40 myr and was more uniform along strike 1500 km than previously appreciated and 3 the slow and steady uplift model is most consistent with available constraints we conclude that the rapid uplift model may be an overestimate and that a more protracted cenozoic uplift history is tenable