on the antarctic peninsula batholith the plutonic rocks of the antarctic peninsula magmatic arc form one of the major batholiths of the circum pacific rim the antarctic peninsula batholith is a 1350 km long by 210 km wide structure which was emplaced over the period 240 to 10 ma with a cretaceous peak of activity that started at 142 ma and waned during the late cretaceous early jurassic and late jurassic early cretaceous gaps in intrusive activity probably correspond to episodes of arc compression in a northern zone of the antarctic peninsula the batholith intrudes palaeozoic mesozoic low grade meta sedimentary rocks and in a central zone it intrudes schists and ortho and paragneisses which have late proterozoic nd model ages and were deformed during triassic to early jurassic compression in a southern zone the oldest exposed rocks are permian sedimentary rocks and deformed jurassic volcanic and sedimentary rocks all these pre batholith rocks formed a belt of relatively immature crust along the gondwana margin with few exceptions jurassic plutons crop out only within the central zone many are peraluminous having s like mineralogies and relatively high 87sr 86sri they are considered to consist largely of partial melts of upper crust schists and gneisses and components of mafic magmas that caused the partial fusion by contrast early cretaceous plutons crop out along the length of the batholith few magma compositions appear to have been affected by upper crust the bulk being compositionally independent of the type of country rock they intrude they are dominated by metaluminous calcic si oversaturated 1 type granitoid rocks with relatively low 87sr 86sri intermediate silicic compositions 5 mgo we interpret these to represent partial melts of basic to intermediate igneous locally garnet bearing lower crust contemporaneous mafic magmas e g syn plutonic dykes form a more alkaline si saturated series having higher 143nd 144nd at the same87sr 86sr than the intermediate silicic series to which they are not petrogenetically related the change from limited partial fusion of upper crust in jurassic times to widespread partial fusion of lower crust in early cretaceous times is considered to be a result of an increasing volume of basaltic intrusion into the crust with time

oxygen isotope evidence for hydrothermal alteration within a quaternary stratovolcano lassen volcanic national park california brokeoff volcano a quaternary stratocone located in the lassen volcanic center in northern california has been deeply eroded exposing a 10 km2 meteoric hydrothermal alteration zone at the core of the volcano portions of the former volcanic edifice are sufficiently well preserved that an unusual opportunity exists wherein the alteration pattern can be correlated with the position of the volcanic cone the 18o analyses of more than 100 whole rock samples consisting primarily of andesitic lavas vary from 9 8 to 0 6 per mil the highest 18o values occur in bleached solfatarically altered rocks that have interacted with low ph fumarolic hot springs associated with the present day hydrothermal system low 18o values are found in propylitically altered rocks that underwent isotopic exchange with meteoric hydrothermal fluids at elevated temperatures mostly during the stratovolcanic stage 650 400ka of the hydrothermal system but probably continuing today at depth electron microprobe analyses of secondary layer silicate minerals in strongly propylitized samples 18o 5 0 revealed the presence of discrete chlorite suggesting that temperatures up to 200 to 250 c were attained in the shallow levels of the system two zones of pervasive meteoric hydrothermal alteration defined by concentric 18o contours that are probably interconnected at depth are located within the original topographic edifice of the volcano the most intensely altered rocks within these equant zones of alteration define nnw trends that coincide with stream valleys and with regional structural patterns a comparison of the characteristics of the 18o depleted zone at brokeoff with those of more deeply eroded volcanic centers such as the comstock lode mining district criss and champion 1991 permits the construction of composite 18o cross sections through a hypothetical intact stratovolcano at both brokeoff and comstock hydrothermal fluids were strongly focused into plumelike zones of intense 18o depletion at comstock these low 18o plumes are associated with faults although major fault displacements are not observed at brokeoff the topographic and alteration patterns are consistent with the presence of a linear array of faults that acted as conduits for fluid flow up into the shallow levels of the volcano

metamorphism of mafic dikes from the central white inyo range eastern california thin mafic dikes possibly correlative with the independence dike swarm of se california transect uppermost proterozoic cambrian metasedimentary strata in the white inyo range textures and bulk rock chemistry indicate that the protoliths were diabases and microdiorites accompanied by ca mg fe ni cr rich hornblende minor augite cumulates analytical data suggest crystal settling and fractionation at shallow depths most of the dikes lie in the mapped aureoles of and were metamorphosed by voluminous late jurassic granitoid plutons however a few metadikes cut these plutons and must have been recrystallized during the emplacement of cretaceous granitic stocks the mafic metadikes thus include members of two or more temporally distinct suites pre late jurassic and latest jurassic cretaceous neoblastic mineral assemblages and element partitioning within these nonfoliated mafic metadikes reflect lower to upper greenschist facies overprints metamorphic parageneses coincident with those developed in the metasedimentary wallrocks are defined by the production of chlorite biotite white mica epidote and actinolite and by albitization of the igneous plagioclase based on analytical and mineralogic data obtained in this study the following conclusions regarding subsolidus recrystallization of the mafic metadikes are advanced 1 newly grown minerals and phase assemblages are systematic in their areal distributions 2 metamorphic grade increases chiefly toward the north and east toward the late jurassic granitoids 3 element fractionation among coexisting neoblastic phases is regular and compatible with a close approach to chemical equilibrium 4 assemblages 3 5 km from the granitic intrusive contacts reflect lowermost greenschist facies physical conditions 5 investigated mafic dikes exhibit mineral parageneses isofacial with the regional contact metamorphic assemblages previously documented for the enclosing pre mesozoic clastic country rocks clearly mafic dikes of several ages of injection and recrystallization are present in the central white inyo range making correlation with the independence dike swarm problematic in any case the dikes record localized contact metamorphism that took place sporadically over portions of an approximately 100 million year interval

characterisation of archaeological mortars and plasters from kyme turkey abstract the present study focuses on the compositional characterisation of roman proto byzantine and medieval mortars sampled in the archaeological site of kyme turkey an aeolian city founded in the middle of the xi century bc by greek populations the mortars were studied through elemental sem eds xrf mineralogical xrpd de astis calcimeter and petrographic polarised optical microscopy analysis the comparison between the samples allowed us to emphasise compositional differences within mortars of the same historical period in particular we detected the prevalent use of cocciopesto which was used as a pozzolanic additive to create hydraulic mortars in the roman and byzantine period the cocciopesto was used to improve the technological properties of an initially slaked lime furthermore compositional similarities allowed us to attribute an unknown sample to the roman period our study also pointed out the fact that the lower quality mortars belonged to the medieval period as a matter of fact the most friable sample belonging to the middle ages turned out to be a mixture of clay lime and volcanic sand

volcanism associated with extension in an oligocene miocene arc southwestern viti levu fiji the wainimala group rocks of southwestern viti levu fiji represent part of an oligocene miocene island arc some of the early volcanism in this arc took place on a substrate of eocene oligocene frontal arc crust mainly andesitic transitional calcalkaline lavas were erupted from major subaerial volcanic edifices while elsewhere tholeiitic magmas formed low lying basaltic lava fields and small felsic volcanic centres on the seafloor the intrusion of a dense bimodal basalt dacite dyke swarm into the underlying frontal arc crust indicates that significant crustal extension accompanied the eruption of the inter edifice lavas geochemical variations within the wainimala volcanic suite are thought to have been generated through fractional crystallisation processes while relatively slow ascent and mixing of magmas took place beneath the major edifices efficient shallow level fractionation and rapid ascent of magmas occurred in the areas between them the contrasting styles of volcanism may reflect the interaction of ascending magmas with pre existing heterogeneities in the frontal arc substrate

the spatial distribution of grains and crystals in rocks characterisation and analysis of the spatial distribution pattern sdp of grains or crystals in rocks is potentially a powerful technique which can be used to constrain the processes which operate in the formation of rocks a method to quantify the sdp of grains in thin section is presented the distance betwen the centre of a grain and the centre of its nearest neighbour is calculated for all the grains in the sample area to produce a distribution of distances that characterises the spatial pattern of grains in the rock this distribution is then normalised to a random distribution of points with the same population density to give a descriptive value r values of r for rock samples are plotted against porosity modal abundance of other phases in igneous and metamorphic rocks to characterise the sdp the sdp of randomly packed distributions of equal size spheres varies systematically with porosity producing a line on a porosity versus r plot termed the random sphere distribution line rsdl rocks which plot above the rsdl have an ordered sdp and those that plot below a clustered sdp the effects of variation in grain packing order grain sorting compaction and random crystallisation overgrowth on determined r values were investigated using a combination of 3 d sphere models and 2 d texture models the maximum possible value of r is 2 148 corresponding to a perfect section through hexagonal cubic close packing of grains the minimum value of r is dependent on the proportion of grains in the sample volume and may be as low as 1 2 for a sample volume with 30 grains which are clustered variations in size sorting can cause r to vary by approximately 0 25 mechanical compaction of a loose framework of grains results in a higher packing order and an increase in r continued compaction creates a fabric in the texture and r decreases as cluster patterns are developed perpendicular to the principal stress the overgrowth of grains in a touching framework alters the location of the geometric grain centre as the crystal shape changes leading to an increase in r as porosity decreases results reveal that grain sorting compaction and grain overgrowth produce different r versus porosity relationships which can be identified in and therefore used to constrain the relative importance of these processes in rock samples

relations among subduction parameters clues to the dynamics of the subduction process are found in the many measurable parameters of modern subduction zones based on a critical appraisal of the geophysical and geological literature 26 parameters are estimated for each of 39 modern subduction zones to isolate causal relationships among these parameters multivariate analysis is applied to this data set this analysis yields empirical quantitative relations that predict strain regime and strike slip faulting in the overriding plate maximum earthquake magnitude benioff zone length slab dip arc trench gap and maximum trench depth excellent correlation is found between length of the benioff zone and the product of convergence rate and age of the downgoing slab this relationship is consistent with the conductive heating model of molnar et al 1979 if the model is modified in one respect the rate of heating of the slab is not constant it is substantially slower during passage of the slab beneath the accretionary prism and overriding plate the structural style in the overriding plate is determined by its stress state though the stress state of overriding plates cannot be quantified one can classify each individual subduction zone into one of seven semiquantitative strain classes that form a continuum from strongly extensional class 1 back arc spreading to strongly compressional class 7 active folding and thrusting this analysis indicates that strain class is probably determined by a linear combination of convergence rate slab age and shallow slab dip interplate coupling controlled by convergence rate and slab age is an important control on strain regime and the primary control on earthquake magnitude arc parallel strike slip faulting is a common feature of convergent margins forming a forearc sliver between the strike slip fault and trench optimum conditions for the development of forearc slivers are oblique convergence a compressional environment and a continental overriding plate the primary factor controlling presence of strike slip faulting is coupling strongly oblique convergence is not required the rate of strike slip faulting is affected by both convergence obliquity and convergence rate maximum trench depth is a response to flexure of the underthrusting plate the amount of flexural deflection at the trench depends on the vertical component of slab pull force which is very sensitive to slab age and shallow slab dip shallow slab dip conforms to the cross sectional shape of the overriding plate which is controlled by width of the accretionary prism and duration of subduction deep slab dip is affected by the mantle trajectory established at shallow depth but may be modified by mantle flow much of the structural complexity of convergent margins is probably attributable to terrane juxtaposition associated with temporal changes in both forearc strike slip faulting and strain regime empirical equations relating subduction parameters can provide both a focus for future theoretical studies and a conceptual and kinematic link between plate tectonics and the geology of subduction zones

the bangudae petroglyph in ulsan korea studies on weathering damage and risk prognosis the prehistoric bangudae petroglyph in the ulsan area represents an outstanding national treasure of the republic of korea since the construction of the sayeon dam the petroglyph with about two hundred carvings is periodically submerged by the sayeon reservoir the danger of increasing damage has resulted in intensive efforts to protect this cultural heritage diagnosis and risk prognosis studies were carried out results of petrographical studies monument mapping and in situ measurements are presented types degree and zonation of damage are evaluated the risk estimation derived from the studies confirms the necessity of preservation measures

scotia sea tectonics from high resolution satellite gravity the release of altimetric data from the geosat geodetic mission by the us navy 1 is leading to a much improved understanding of tectonics in the southern ocean a region in which remoteness and adverse physical conditions have limited the acquisition of geophysical data by research ships the scotia sea is an outstanding example of back arc spreading which is revealed in some detail by free air gravity anomaly maps derived from the latest release of data acquired south of 30 s 2 sea surface height data for this region have been reduced to a grid of free air gravity anomalies and are illustrated here by means of colour shaded relief and contour maps the new data confirm the existence of a number of inactive spreading ridges within the scotia sea and surrounding small basins the amplitudes and wavelengths of gravity anomalies over these ridges conform in general to the expected relationship with spreading rate except in the central scotia sea where a proposed miocene slow spreading ridge appears to have left no clear signature the spreading ridge axis in the east scotia sea comprises seven or more segments separated by small mainly sinistral offsets and exhibits a median valley with depths of 200 1000 m that is reflected in free air lows of 10 40 mgal near both its northern and southern termini the gravity signature of the ridge becomes less distinct with a less pronounced axial low the northernmost segments of the ridge are displaced in a right lateral sense by a feature which appears to represent a southward migrating non transform offset whereas the process of spreading in drake passage and the east scotia sea was comparable to mid ocean ridges that in the central scotia sea may have been disorganized as observed in some western pacific back arc basins

chemical weathering of volcanic rocks at the island of pantelleria italy information from soil profile and soil solution investigations abstract concentrations of major minor and trace elements were determined in soil profiles and soil solutions from the island of pantelleria sicily channel to evaluate the weathering extent of soils evolved on trachytic and pantelleritic rocks and the aqueous transport of elements by their soil solutions the chemical index of alteration cia indicates a low to moderate degree of weathering consistently the mineralogical and geochemical imprints of the parent rocks are generally preserved the chemical weathering appears to be incongruent owing to primary minerals and glass dissolving to a variable degree while secondary minerals have formed based on the calculated saturation state of primary and secondary minerals with respect to the soil solutions and on mineralogical observations olivine and pyroxene are unstable and dissolving whereas magnetite and hematite are stable the main weathering products are amorphous phases such as ferrihydrite amorphous aluminium hydroxide and silica clay mineral formation appears to be incipient and mainly limited to kaolinite and smectite although for some soil profiles a hydrothermal origin is suggested for these minerals chemical characteristics of soil solutions and elemental mobility are controlled by the element source and water rock interaction

geochronology of the porgera gold deposit papua new guinea resolving the effects of excess argon on k ar and age estimates for magmatism and mineralization abstract mesothermal epithermal gold mineralization at porgera in the highlands of papua new guinea png occurs in structurally controlled veins and disseminations which overprint and cross cut a suite of shallow level comagmatic mafic alkaline stocks and dykes the porgera intrusive complex pic and their sedimentary host rocks conventional k ar apparent ages of twelve hornblende separates from eight different intrusions scatter between 7 and 14 ma but four biotite separates are concordant at 6 02 0 29 ma 2 40 ar 39 ar step heating experiments on six of the hornblende separates reveal saddleshaped age spectra which indicate the presence of excess 40ar one of these samples yields a well defined plateau with an apparent age of 5 96 0 25 ma 2 this age is identical to that of the biotites and suggests an age of 6 0 0 3 ma 2 for emplacement of the pic older apparent ages obtained from conventional k ar and 40 ar 39 ar analyses are explained by excess 40ar contamination conventional k ar analyses of six separates of hydrothermal illite and roscoelite associated with gold mineralization yield apparent ages of between 5 1 and 6 1 ma and indicate that ore deposition occurred within 1 ma of magmatism at porgera evidence for the evolution of a magmatic volatile phase and the presence of excess 40ar both in the intrusives and in hydrothermal fluids associated with the orebody suggest that magmatic fluids may have had some involvement in metallogenesis but the exact nature of this involvement is not yet clear late miocene magmatism and mineralization at porgera are thought to have occurred shortly prior to or during the initiation of continent arc collision and to pre date associated pliocene uplift and foreland deformation in the highlands

stratigraphy and regional significance of the upper jurassic lower cretaceous byers group livingston island antarctica the byers group exposed on byers peninsula western livingston island antarctica comprises a mudstone dominated sequence at least 1 km thick which accumulated in a marginal fore arc environment the basal 105 m thick anchorage formation consists of radiolarian mudstones and tuff rich interbeds of kimmeridgian tithonian age it correlates with upper jurassic organic rich mudstone units throughout the proto south atlantic region the succeeding 244 m thick devils point formation marks the first major pulse of coarse volcaniclastic material into the basin it is in turn followed by the extensive president beaches formation comprising several hundred metres of finely laminated mudstones with at least two major sandstone intercalations molluscan and dinoflagellate cyst taxa indicate a berriasian age and comparatively nearshore depositional environment for this unit an unconformity of late berriasian or early valanginian age separates the three lowest formations from the chester cone formation the fine grained sealer hill member at the base of the latter is dated as valanginian and grades up into several hundred metres of pebbly sandstones and pebble granule conglomerates these mark the second major volcaniclastic pulse and may be of hauterivian or even younger age definition of this major new group will facilitate more precise upper jurassic lower cretaceous stratigraphical correlations within the southern south america scotia arc antarctic peninsula region it will also aid our understanding of the critical palaeogeographical transition in the northern antarctic peninsula from anoxic basin to active magmatic arc

oil reservoirs in grainstone aprons around bryozoan mounds upper harrodsburg limestone mississippian illinois basin several oil pools have been discovered recently in the upper harrodsburg limestone middle mississippian of the illinois basin a depositional model for bryozoan mound complexes has allowed more successful exploration and development in this play in the johnsonville area of wayne county illinois three lithofacies are dominant in the upper harrodsburg 1 bryozoan boundstones 2 bryozoan grainstones and 3 fossiliferous wackestones bryozoan boundstones occur as discontinuous mounds and have low porosity although bryozoan boundstones are not the main reservoir lithofacies they are important because they influenced the distribution of bryozoan grainstones and existing structure bryozoan grainstones have intergranular porosity and are the main reservoir rock bry zoan fragments derived from bryozoan boundstone mounds were concentrated in grainstones around the mounds fossiliferous wackestones are not porous and form vertical and lateral seals for upper harrodsburg grainstones fossiliferous wackestones were deposited in deeper water adjacent to bryozoan grainstone aprons and above grainstones and boundstones after the mounds were drowned upper harrodsburg oil reservoirs occur where grainstone aprons are structurally high the harrodsburg is a good example of a carbonate mound system where boundstone cores are not porous but adjacent grainstones are porous primary recovery in these upper harrodsburg reservoirs is improved by strong pressure support from an aquifer in the lower harrodsburg unfortunately oil production is commonly decreased b water encroaching from that underlying aquifer

soils in the semi arid area of the el melah lagoon ne tunisia variability associated with a closing evolution abstract soils of the semi arid area of the el melah coastal lagoon ne tunisia with a closing evolution dynamic were studied aiming 1 the chemical and mineralogical characterization of surface and subsurface layers of soil profiles from locations previously submerged and of soils developed on dunes 2 to evaluate the dependence on the environment conditions of the geochemical patterns of the soils and 3 to determine chemical and mineralogical variations with the emersion of the sediments resulting from the decrease in the extent of the area permanently covered by water in the lagoon the compositional results obtained showed significant differences depending on the environment littoral plain old dunes and sandy spit but soils of the different environmental zones studied do not appear to be significantly polluted as far as trace elements are concerned among the elements studied high element sc ratios and variations were found for as sb and zn antimony is more concentrated in coarser samples suggesting its adsorption in fe or mn oxides coating quartz grain surfaces arsenic and zinc may be incorporated into the carbonates structure as well as adsorbed on or coprecipitated with iron oxides zinc may also be significantly incorporated in clay minerals iron was found to be more oxidized in the cultivated soil from the old dune strand and more reduced in the sandy spit where ankerite occurs suggesting the reduction of fe3 in oxide hydroxides by microorganisms and incorporation of fe2 in carbonates ree patterns particularly the hree lree are correlated with carbonates indicating preferential incorporation of the hree in carbonates and of the lree in clay minerals iron oxides high ga contents were found in soils and sediments rich in clay minerals al suggesting its incorporation in clay minerals structure therefore ga may be used as an indicator of the clay minerals proportion in sediments and soils carbonates sulphates besides fe and or mn oxides and clay minerals appear to play an important role on the trace elements distribution

the relationship between epithermal and porphyry hydrothermal systems within the tavua caldera fiji the shoshonitic tavua caldera in viti levu fiji defines the volcanic center of the tavua volcano the caldera occurs at the intersection of the northeast trending viti levu lineament and a northwest trending lineament locally manifested by the nasivi shear zone fault related subsidence which contributed to the evolution of the tavua caldera the central inner caldera and the structural preparation for mineralization occurred on these two regional fault systems and local concentric normal faults a continuum of mineralization and alteration styles is evident along the nasivi shear zone caused by a northwesterly evolution and migration of intrusion and hydrothermal activity porphyry style subeconomic copper gold mineralization and associated alteration occurred at depth in the nasivi shear zone at the center of the caldera approximately contemporaneously with the development of the inner caldera acid sulfate style epithermal alteration and minor gold mineralization immediately postdated and were superimposed on this porphyry style alteration farther northwest along the nasivi shear zone transitional ac d sulfate to adularia sericite epithermal alteration and minor gold mineralization occur variation in hydrothermal style is considered to reflect an increasing distance from magmatic source combined with an increasing proportion of meteoric water mixing with the hydrothermal fluid northwest of this system at the intersection of the nasivi shear zone and the viti levu lineament the 4 4 m oz adularia sericite style epithermal emperor gold mine occurs high level epithermal mineralization and alteration occur 4 km northeast of the emperor gold mine in an area which has a demonstrable present day hydraulic connection with the mine variations in fluid inclusion homogenization temperatures from the mine indicate that fluids entered the structurally prepared mine area via the nasivi shear zone emperor epithermal mineralization postdated magmatism by approximately 0 4 ma stable isotope data the presence of albitic alteration beneath mineralization and high salinities within the mine indicate that the emperor northern prospects hydrothermal system is related to an intrusion into the nasivi shear zone beneath the mine this is consistent with a demonstrable northwesterly migration of intrusions with time along the nasivi shear zone albitic phyllic and propylitic assemblages comprise the main porphyry style alteration types in the caldera center calculated temperatures of formation of chlorite in these assemblages indicate that these three alteration facies formed in the temperature range 270 degrees to 340 degrees c in good agreement with available fluid inclusion data and the stability ranges of the mineral assemblages kubler indices of illite likewise imply that these assemblages formed at relatively high temperature epithermal lode alteration mineralization at emperor is shown by fluid inclusion data to have formed around 200 degrees c kubler indices of illite confirm that this alteration formed at significantly lower temperatures than did porphyry alteration and that chlorites in the emperor mine have calculated temperatures of formation mostly below 200 degrees c high level epithermal alteration occurring northeast of emperor is inferred to have formed at even lower temperatures

metamorphic evolution of a subduction complex south shetland islands antarctica a subduction complex composed of ocean floor material mixed with arc derived metasediments crops out in the elephant island group and at smith island south shetland islands antarctica with metamorphic ages of 120 80 ma and 58 47 ma respectively seven metamorphic zones i vii mapped on elephant island delineate a gradual increase in metamorphic grade from the pumpellyite actinolite facies through the crossite epidote blueschist facies to the lower amphibolite facies geothermometry in garnet amphibole and garnet biotite pairs yields temperatures of about 350 c in zone iii to about 525 c in zone vii pressures were estimated on the basis of si content in white mica al2o3 content in alkali amphibole nam4 aliv in sodic calcic and calcic amphibole alvi si in calcic amphibole and jadeite content in clinopyroxene mean values vary from about 6 7 5 kbar in zone ii to about 5 kbar in zone vii results from the other islands of the elephant island group are comparable to those from the main island smith island yielded slightly higher pressures up to 8 kbar with temperatures estimated between 300 and 350 c zoned minerals and other textural indications locally enable inference of p t t trajectories all with a clockwise evolution a reconstruction in space and time of these p t t paths allows an estimate of the thermal structure in the upper crust during the two ductile deformation phases d1 d2 that affected the area this thermal structure is in good agreement with the one expected for a subduction zone the arrival and collision of thickened oceanic crust may have caused the accretion and preservation of the subduction complex in this model d1 represents the subduction movements expressed by the first vector of the clockwise p t t path d2 reflects the collision corresponding to the second vector with increasing temperature and decreasing pressure and d3 corresponds to isostatic uplift accompanied by erosion under circumstances of decreasing temperature and pressure

iron speciation in volcanic topsoils from fogo island cape verde iron oxide nanoparticles and trace elements concentrations abstract iron contents its compounds and forms in topsoils of the volcanic fogo island cape verde were studied by neutron activation analysis mossbauer spectroscopy and x ray diffraction the topsoils were collected from all units of the island in diverse geological formations carbonatite nephelinites limburgite pyroclasts deposits pre historic lavas and historic lavas fine materials from mota gomes volcanic crater the most recent eruption which occurred in 1995 were also sampled the total iron content varies significantly from 1 7 fe2o3 weight in acid sulfate topsoils ph 3 9 up to between 9 9 and 20 6 in the other geological contexts ph 5 9 to 9 3 the most oxidized topsoils from the pre caldera formations are the oldest ones as expected while the reverse is observed for the post caldera topsoils in the latter case fast weathering of the lava outer layers which have a higher glass component may explain this observation after erosion periods of 50 200 years significant amounts of the highly oxidized small particles are removed the inner part of the lava less oxidized is then exposed and more slowly weathered than the former outer layers topsoils from pre caldera formations derived from the more weather resistant inner layers are also strongly altered since they have been exposed for periods of time several orders of magnitude longer than the post caldera ones in topsoils fe2 mainly occurs in pyroxenes phyllosilicates and pure or slightly oxidized magnetite with increasing age the fe2 total fe ratio of the pre caldera topsoils gradually decreases down to 4 indicating that the iron that is still incorporated in the silicates structures is increasingly oxidized maghemite is detected instead of magnetite and hematite appears becoming the main fe containing phase in the oldest materials these results suggest that in a first weathering step fe2 is oxidized within the silicates structure and within magnetite which gradually evolves to maghemite with the eventual decomposition of silicates fe3 is remobilized in hematite as weathering proceeds maghemite is also replaced by hematite goethite was not detected by mossbauer spectroscopy which may be explained by the aridity of the island iron in jarosite was identified by mossbauer spectroscopy in topsoils developed on volcanic conduits the presence of this mineral in alkaline surficial environments may be explained by a hypogene origin and stability over at least a few hundred years in the surficial environments of the semi arid fogo island contents of cr and as that are potentially harmful to health increase with the concentration of nanosized fe oxides this correlation may be related to their high specific surface area

timescales of orogeny jurassic construction of the klamath mountains an electronic supplement of this material may be obtained on a diskette or anonymous ftp from kosmos agu org login to agu s ftp account using anonymous as the username and guest as the password go to the right directory by typing cd apend type ls to see what files are available type get and the name of the file to get it finally type exit to leave the system paper 94ycj2454 timescales of orogeny jurassic construction of the klamath mountains b r hacker m m donato c g barnes m o mcwilliams and w g ernst diskette may be ordered from american geophysical union 2000 florida avenue n w washington dc 20009 15 00 payment must accompany order classical interpretations of orogeny were based on relatively imprecise biostratigraphic and isotopic age determinations that necessitated grouping apparently related features that may in reality have been greatly diachronous isotopic age techniques now have the precision required to resolve the timing of orogenic events on a scale much smaller than that of entire mountain belts forty five new 40ar 39ar ages from the klamath mountains illuminate the deformation metamorphism magmatism and sedimentation involved in the jurassic construction of that orogen leading to a new level of understanding regarding how preserved orogenic features relate to ancient plate tectonic processes the new geochronologic relationships show that many jurassic units of the klamath mountains had 200 ma or older volcanoplutonic basement subsequent formation of a large 170 ma arc was followed by contractional collapse of the arc collision with a spreading ridge may have led to large scale nw se extension in the central and northern klamaths from 167 to 155 ma coincident with the crystallization of voluminous plutonic and volcanic suites marked cooling of a large region of the central klamath mountains to below 350 c at 150 ma may have occurred as the igneous belt was extinguished by subduction of colder material at deeper structural levels these data demonstrate that the klamath mountains and perhaps other similar orogens were constructed during areally and temporally variant episodes of contraction extension and magmatism that do not fit classical definitions of orogeny

the age and stratigraphy of fore arc magmatism on alexander island antarctica fore arc magmatic sequences associated with high mg number andesite lavas unconformably overlie lemay group accretionary complex in alexander island high resolution 40ar 39ar u pb zircon fission track and k ar ages demonstrate that subduction related fore arc magmatism migrated northwards along the length of alexander island between c 80 ma and c 46 ma the magmatic rocks represent a third of the western margin of the antarctic peninsula magmatic arc and are critical to the understanding of the final phase of subduction along the southern antarctic peninsula margin the onset of late cretaceous magmatism is recorded by poorly exposed volcanic rocks on monteverdi peninsula 79 7 2 5 ma in central and northern alexander island the magmatic rocks can be distinguished by the proportion range and types of lithofacies present and by the periods of magmatism represented the volcanic rocks of the colbert mountains range in age from c 69 62 ma and are dominated by large volume dacitic and rhyolitic crystal rich ignimbrites interpreted as caldera fill deposits elgar uplands sequences range in age from c 55 50 ma and contain approximately equal proportions of pyroclastic deposits and less evolved basaltic andesite and andesite lavas including high mg number andesite lavas near the base of three sequences the volcanic rocks of finlandia foothills probably represent the youngest calc alkaline units on alexander island 48 2 ma the sequence is lithologically similar to the elgar uplands and also contains high mg number andesite lavas but it is dominated by polymict conglomerates with minor lavas which were deposited in a graben associated with regional extension plutonic rocks exposed in the rouen mountains adjacent to the elgar uplands yielded a u pb age of 56 3 ma which is in discordance with a previously published rb sr age 46 3 ma probably due to hydrothermal perturbation of the rb sr system northwards migration of magmatism was caused by the progressive collision and subduction of three ridge segments prior to the previously reported ridge crest trench collisions that occurred c 20 30 ma later and following which subduction ceased

genesis and evolution of a permian jurassic magmatic arc accretionary wedge and reevaluation of terranes in the central klamath mountains investigation of key areas of the central klamath mountains demonstrates that genetic relationships connect several regionally significant lithologic units that were previously considered to be separate tectonostratigraphic terranes the rocks embody an intraoceanic tholeiitic to alkaline magmatic arc and contiguous accretionary wedge formed in permian to middle jurassic time the eastern hayfork and st claire creek units comprise permian jurassic oceanic sedimentary rocks deposited nearby and coeval with eruption of the north fork salmon river volcanoplutonic arc the more inboard st claire creek unit consists of stratigraphically coherent chert and minor argillite probably deposited in a back arc setting whereas the more outboard eastern hayfork sedimentary rocks were disrupted during formation of an accretionary wedge the volcanoplutonic north fork salmon river arc extends for 200 km in the central klamath mountains and was active episodically or continuously from permian through middle jurassic time alkalic members of the mafic igneous suite may have erupted following subduction of a spreading center a varied heating history has produced an amalgam of submarine and regional low pressure moderate temperature metamorphism overprinted locally by contact metamorphism geochronology of weakly metamorphosed mafic dikes sills 10 new 40ar 39ar analyses suggests that significant heat was retained regionally into early cretaceous time

geochronology of the larderello geothermal field new data and the closure temperature issue the larderello geothermal field is generally accepted to have been produced by a granite intrusion at 4 9 km depth hydrothermal parageneses and fluid inclusions always formed at temperatures greater than or equal to the current ones which implies that the field has always undergone a roughly monotonic cooling history fluctuations 40 k since intrusion of the granite at 4 ma the heat required to maintain the thermal anomaly over such a long period is supplied by a seismically anomalous body of 32000 km3 rooted in the mantle borehole minerals from larderello are thus a unique well calibrated natural example of thermally induced ar and sr loss under geological conditions and time spans the observations biotites retain ar above 450 c agree well with other albeit less precise geological determinations but contrast with laboratory determinations of diffusivity from the literature we therefore performed a hydrothermal experiment on two larderello biotites and derived a diffusivity dlab 370 c 5 3 10 18 cm2s 1 in agreement with published estimates of diffusivity in annite from dlab and the rejuvenation of the k ar ages we calculate maximum survival times at the present in hole temperatures they trend smoothly over almost two orders of magnitude from 352 ka to 5 3 ka anticorrelating with depth laboratory diffusivities are inconsistent not only with geological facts but also among themselves from the geologically constrained lifetime of the thermal anomaly we derive a diffusivity dg 370 c 3 81 1021 cm2s 1 3 1 orders of magnitude lower than dlab the cause of these discrepancies must be sought among various laboratory artefacts overstepping of a critical temperature t enhanced diffusivities in wet experiments presence of fast pathway dislocation and pipe diffusion and of dissolution reprecipitation reactions which we imaged by scanning electron microscopy these phenomena are minor in geological settings in the absence of mineral transformation reactions complete or near complete resetting is achieved only by volume diffusion therefore laboratory determinations will necessarily result in apparent diffusivities that are too high compared to those actually effecting the resetting of natural geochronometers

late cretaceous to early tertiary subduction history of the antarctic peninsula quantitative estimates of the rates and azimuths of phoenix plate convergence with the antarctic peninsula have been derived from plate rotation calculations for two periods in the late cretaceous and early tertiary 83 0 67 7 and 61 1 53 4 ma published marine magnetic anomaly identifications and flow lines interpreted from gravity anomaly maps were used in simple spherical geometry calculations to derive phoenix pacific stage rotations these were combined with published pacific antarctic rotation data to determine contemporaneous phoenix antarctic stage rotations the results indicate a significant change in azimuths of relative motion between the late cretaceous and early tertiary late cretaceous and perhaps earlier oblique subduction probably caused migration of fore arc slivers along the margin resulting in variations in width of the accretionary prism comparison between synthetic magnetic profiles and a 900 km long magnetic profile across ocean floor produced at the antarctic phoenix ridge during the early tertiary establishes the time of a major decrease in spreading rate and hence also in convergence rate as chron c23r 52 3 ma the associated change in subduction dynamics may have caused the dextral transtensional deformation observed in the george vi sound region and initiated uplift of blueschist facies rocks now exposed on smith island the calculated convergence history does not provide a simple explanation for the occurrence of high mg andesite lavas on alexander island which were erupted more than 20 ma before the antarctic phoenix ridge arrived at the margin the existence of these lavas implies either earlier subduction of another ridge or slab break up beneath alexander island

forearc tectonic evolution of the south shetland margin antarctic peninsula the main provinces of the south shetland margin antarctic peninsula characterized on the basis of multichannel seismic long range side scan sonar and swath bathymetry data include from northwest to southeast 1 the oceanic crust of the former phoenix plate flexed down toward the margin and affected by normal faulting in the upper crust 2 a narrow trench with a horizontally layered sediment fill which onlaps the thin sedimentary cover on the oceanic crust and shows incipient deformation near its landward edge 3 an accretionary prism with a complex internal structure the toe of which is overthrust above the youngest trench deposits 4 a midslope forearc basin with an asymmetric synformal structure and 5 the continental shelf which includes two distinct tectonic provinces calculations of the late cenozoic convergence history at the trench indicate a rapid decrease in convergence rate after 6 7 ma from about 60 mm yr resulting from the slowing and eventual cessation of spreading on the antarctic phoenix ridge once spreading had completely ceased 3 5 2 4 ma the convergence rate at the trench equalled the rate of extension in bransfield strait which was probably less than 10 mm yr at first but may have increased since 1 3 ma and this in turn would imply a corresponding increase in convergence rate above the basal detachment the toe of the accretionary prism is composed of a stack of thrust fault bounded wedges laterally and vertically segmented by normal faults most of the trench fill sediments are overthrust by the toe of the accretionary prism and subducted beneath it for as far as they can be traced on the seismic profiles this suggests that there may be tectonic erosion of the forearc since a large part of the trench sediment appears to be derived from erosion on the forearc slope the forearc basin records a long history of subsidence during which the depocenter migrated landward paralleling retrogradational erosion of the distal margin of the basin subduction of the hero fracture zone at approximately the same location since 10 ma may have enhanced tectonic erosion of the accretionary prism in the forearc basin a synform is coincident with a bathymetric reentrant but its axis is oblique to the trends of both the basin and the fracture zone the origin of this synform and the final phase of uplift of the blueschist facies terrane on which smith island stands are probably related to their position above the subducting fracture zone ridge the present elevation of smith island is thought to result from uplift which followed a ridge crest trench collision directly southwest of the hero fracture zone at 5 5 ma the northwestern edge of the mesozoic to early cenozoic arc batholith trending northeast southwest on the shelf is proposed to have acted as a backstop to the accretionary prism during cenozoic subduction

post nevadan detachment faulting in the klamath mountains california a group of faults surrounding the trinity terrane in the core of the klamath mountains province of northern california displays a commonality in physical features and in age that sets them apart from other faults in the klamath mountains the faults dip shallowly outward from the central trinity terrane and feature younger on older detachment fault geometry several exhibit shallowly south southwest or south southeast plunging slickenside striations distinctive muscovite bearing dacite muscovite bearing rhyolite and plagioclase porphyry dikes are common along these faults as are epithermal mineralization and associated small but economic gold and or copper massive sulfide deposits two of the faults crosscut the triassic siskiyou thrust in addition the cecilville fault cuts the late jurassic soap creek ridge fault so extensional faulting postdates the widely developed nevadan deformational event in the klamath mountains a 126 3 ma k ar whole rock age obtained on a muscovite bearing rhyolite dike intruding the fault zone at menzel gulch on the southern margin of the trinity terrane gives a minimum age of early cretaceous for this fault evidence for tertiary extensional faulting on the la grange fault previously reported by schweickert and irwin records a later episode of extension on the southern margin of the trinity terrane

geoavailability of lithogenic trace elements of environmental concern and supergene enrichment in soils of the sierra de aracena natural park sw spain abstract the chestnut grove soils of the sierra de aracena natural park sw spain are a striking example of geoavailability and pedochemical enrichment with geologically sourced trace elements notably zn pb ag and cd but also as sb cu and tl the data from this study provide evidence that such metal loid s were originally bound to reactive sulphides pyrite sphalerite and galena and ag sb pb sulfosalts residing in the soil parent rock at the early stages of weathering these geoavailable elements were easily liberated from the mineralised bedrock through oxidative dissolution reactions and then transferred to the soil thus reaching anomalously high concentrations the most extreme amounts of zn 12 26 g kg 1 pb 5 g kg 1 as 346 mg kg 1 cd 319 mg kg 1 cu 197 mg kg 1 ag 154 mg kg 1 sb 109 mg kg 1 and tl 82 mg kg 1 were measured in soils developed over carbonate hosted sulphide occurrences and their related gossan outcrops when compared to the regional pedogeochemical baseline the soils are largely enriched with all the elements analysed except co cr and ni yielding concentration factors particularly high for zn 50 8 cd 49 3 ag 30 6 sb 22 5 and pb 20 8 and geo accumulation indices that consistently point to strong geogenic contamination the pedochemical enrichment may be reasonably attributed to the combined result of precipitation as a solid phase e g zn vermiculite hemimorphite hydrozincite and plumbogummite and specific adsorption by soil constituents mainly organic matter and iron oxy hydroxides the survey area can be considered as a hot spot in terms of both valuable metal prospecting and environmental concern although the soil acted as a natural sink for lithogenic trace elements it could eventually turn into a source of potentially hazardous contaminants in response to future land use changes and biogeochemical alterations

argon loss by alkali feldspars abstract ar loss is generally thought to preclude the use of alkali feldspars for dating old rocks by the k ar method and has been thought to occur via structural features such as the boundaries of perthite lamellae or twins we have done a 40 ar 39 ar step heating study of a suite of alkali feldspars from the klokken syenite intrusion u pb age 1166 4 ma which have been fully characterized by transmission electron microscopy all are low albite low microcline intergrowths but some are non turbid strain controlled cryptoperthites while others are turbid coarse microperthites geological and textural evidence suggests that the turbidity developed by fluid rock interactions during the initial cooling of the pluton in the stability field of low microcline 5 a after crystallization the finest cryptoperthites have numerous fully coherent perthite lamellar boundaries lamellar periodicity 40 nm and twin composition planes the have retained radiogenic 40 ar and yield total degassing ages as high as 1125 16 ma and maximum ages on the age spectra of 1162 16 ma the coarse turbid microperthites with patchy intergrowths and twins on the mm scale gave total degassing ages as low as 650 ma maximum age 750 ma argon leakage occurs least in feldspars with the most lamellar interfaces but in proportion to the degree of turbidity which is thought to be caused by large numbers of micropores strain controlled intergrowths in perthites allow the recognition of volumes of alkali feldspar crystals which have not interacted with deuteric fluids these retain argon and can give reliable k ar ages in old rocks saddle shaped age spectra appear to result from the preferential degassing at 1000 c of part of the feldspar structure which has lost more of its argon in nature the laboratory degassing does not occur in a manner analogous to that which has occurred naturally

geochemical fingerprints in topsoils of the volcanic brava island cape verde abstract topsoils developed on different lithologies of all the geological units of the volcanic brava island cape verde archipelago were studied by neutron activation analysis and mossbauer spectroscopy sampling was performed according to the recommendations of the igcp 259 international geochemical mapping significant chemical contents variations were found even within the same volcano stratigraphic unit inherited from the parent rock composition under the semi arid climate of the island the chemical heterogeneity of topsoils within the same geological unit particularly in the upper unit was evidenced by a multivariate statistical analysis some differences found can be related with the geographical location underlying basement high contents of mn co ga ba la ce nd sm eu tb ta w th and u were observed in soils related to carbonatites and phonolites ree and w can be used to trace outcrops of extrusive carbonatites in general iron is strongly oxidized occurring in the silicates structure as well as in hematite oxidized magnetite and maghemite the global iron oxidation degree the fraction of fe 3 in silicates and the fraction of nanosized oxides which are higher in topsoils of the older units may be an indicator of weathering degree as br and sb contents were also found to increase with weathering topsoils developed on extrusive carbonatites are clearly distinguished due to the absence of fe 2 in the silicate phases and the remarkable predominance of low oxidized magnetite

mafic dike swarms in the south shetland islands volcanic arc unravelling multiepisodic magmatism related to subduction and continental rifting eight groups of mafic dikes and related high level stocks cut triassic accretionary complex and mesozoic magmatic arc formations on livingston island some are affected by silicic sericitic alteration related to cretaceous hydrothermal activity and propylitic epidosite alteration analogous to that in ocean floor sheeted dikes alteration was accompanied by major and trace element metasomatism ar ar analysis of the freshest rocks indicates five intrusive events some of which are unexpectedly young groups 1 3 were intruded in the mid to late cretaceous 108 74 ma and were coeval with the calc alkaline arc between 70 and 50 ma relatively rapid and oblique plate convergence led to strike slip tectonism and a pause in magmatism at 52 ma orthogonal slow convergence resulted in extensional faulting and emplacement of calc alkaline group 2 and primitive tholeiitic dikes groups 4 6 between 51 and 45 ma extension of antarctic peninsula southern south american crust culminated in emplacement of mafic to intermediate medium grained plutons and group c porphyries between 44 and 36 ma localized hydrothermal flow along fault zones resulted in partial to complete argon loss from nearby cretaceous lavas and ar ar reset ages of 40 ma in mid cretaceous hydrothermal k feldspar primitive olivine basalts group d and epithermal carbonate veins 31 29 ma were emplaced during along arc extension accompanying the opening of drake passage and powell basin excess argon occurs in two forms strongly held in melt inclusions in the primitive tholeiites and weakly held in some secondary alteration there is no radiometric evidence in the area studied for magmatism related to late cenozoic subduction nor to the pleistocene recent opening of the back arc bransfield rift

excess argon crustal fluids and apparent isochrons from crushing k feldspar using a combination ofin vacuo crushing and stepped heating the40ar39ar technique has been applied to two k feldspar bearing sedimentary rocks from the proterozoic of north china with a view to studying ancient fluids trapped in these rocks and their chronology correlations between natural ar isotopes and those produced by neutron irradiation permitted a clear distinction to be made between different components of natural argon crushing released cl correlated excess40ar and palaeoatmospheric36ar trapped in moderately saline fluid inclusions in quartz and possibly the k feldspar the very high40ar cl ratios of these fluids 2 7 10 4 and1 7 10 4 is interpreted as indicating a metamorphic source for the40ar more surprisingly crushing also released from the k feldspar a k correlated component with a very well defined40ar k ratio which for both samples corresponded formally to an apparent age comparable to or slightly less than the stratigraphic age in contrast stepped heating yielded40ar k ratios corresponding to significantly younger ages which are interpreted as low grade metamorphic ages the explanation of the k correlated component released on crushing and the significance if any of the distinct40ar k ratio is obscure although several effects involving microporosity and a combination of39ar recoil and diffusion are suggested there is a contribution of this component to the stepped heating release pattern which may have implications for attempts to use k feldspar as a40ar39ar thermochronometer

an 40ar 39ar geochronology on a mid eocene igneous event on the barton and weaver peninsulas implications for the dynamic setting of the antarctic peninsula the genesis of basaltic to andesitic lavas mafic dikes and granitoid plutons composing the subaerial cover on the barton and weaver peninsulas antarctica is related to arc formation and subduction processes precise dating of these polar rocks using conventional 40ar 39ar techniques is compromised by the high degree of alteration with loss on ignition as high as 8 in order to minimize the alteration effects we have followed a sample preparation process that includes repeated acid leaching acetone washing and hand picking followed by an overnight bake at 250 c after this procedure groundmass samples can yield accurate age plateaus consisting of 70 100 of the total 39ark released using high resolution heating schedules the different rock types studied on the barton and weaver peninsulas yielded almost coeval ages suggesting a giant igneous event in the weaver and barton peninsulas at 44 5 ma a compilation of newly published ages indicate that this event took place throughout the whole south shetland islands suggesting a dynamic incident occurred at this stage during the arc evolution history we related this igneous event to a mantle delamination mechanism during eocene times the delamination process began at 52 ma and the resultant upwelling of asthenosphere baffled the subduction of phoenix plate causing an abrupt decrease in convergence rate then multiple magmatic sources were triggered resulting in a culminating igneous activity during 50 40 ma with a peak at 45 ma along the archipelago the delamination also caused the extension regime indicated by the dike swarm plugs and sills all over the archipelago and the uplift of smith metamorphic complex and livingston island delamination process may have finished at some time during 40 30 ma leaving a weak igneous activity at that stage and thereafter the convergence rate then recovered gradually as indicated by the magnetic anomaly identifications this model is supported by seismic observation of deep velocity anomalies beneath the antarctic peninsula

tectonic setting of primitive magmas in volcanic arcs an example from the antarctic peninsula abstract primitive magmas representing mantle partial melts minimally affected by fractionation and assimilation are rare in the magmatic arc environment most examples are either associated with high rates of arc parallel extension or occur along faults and dykes perpendicular to the trend of the arc and related to arc compression in two cases the vanuatu and solomon islands arcs such arc compression is being caused by collision of seamounts in the antarctic peninsula primitive mafic dykes were emplaced perpendicular to the continental arc ar ar and k ar data suggest intrusion of the dykes at c 126 106 ma possibly during mid cretaceous regional compression of the arc the dykes form two compositional groups one group has low la n yb n ratios 0 31 0 49 lower nb yb and higher th nb than n morb age corrected nd values of 7 3 to 7 9 and are interpreted as melts of subduction modified sub arc asthenosphere the other has high la n yb n ratios 3 86 8 92 higher nb yb and th nb than n morb age corrected nd values of 2 8 to 3 4 and are interpreted as melts of sub arc lithosphere the absence of dykes compositionally between these groups suggests that the primitive magmas avoided storage and mixing in magma chambers

non destructive microtomography based imaging and measuring laboratory induced degradation of travertine a random heterogeneous geomaterial used in urban heritage the aim of this study is to apply x ray microfocus computed microtomography xct a promising non destructive 3d microscopy imaging technique based on measurements of x ray linear path attenuation coefficient in order to study a portuguese travertine a random heterogeneous geomaterial used in urban heritage constructions this study evaluates the impact of soluble sulphate salt induced decay phenomena on texture characteristics at a micrometric scale this is done to better describing imaging measuring and understanding the impact of an artificially induced sulphate decay process on petrographic petrophysical properties a portuguese travertine was chosen as the object of our study its laboratory induced changes were systematically monitored using non destructive techniques to determine voids pores fractures matrix fractions and size spectrum evolution based on 3 d images this investigation demonstrates that the xct potential constitutes a valid complementary tool when analysing decay processes of complex natural materials in different environmental conditions it clearly provides suggestive and important qualitative and or quantitative estimates at different spatial scales of environmentally induced stone voids matrix network spatial structure texture evolution correlations further qualitative quantitative parameterization assessment and statistical validation will be the next step to be taken on this study

stratigraphy and petrology of petroleum producing waulsortian type carbonate mounds in fort payne formation lower mississippian of north central tennessee the petroleum producing subsurface mounds of the fort payne formation lower mississippian in north central tennessee represent a facies that is generally absent or poorly developed in surface sections to the west near the cincinnati arch and to the east in the appalachian fold belt the cross section and isopach maps of the fort payne mound unit and submound unit and a structural map of the underlying chattanooga shale provide evidence for predicting undiscovered mounds by interpolating and extrapolating along several northeast southwest mound trends a petrologic study of cores and well cuttings indicates the character of facies microfacies sedimentary structures porosity and morphology of mounds which provide a basis for interpreting sedimentary environments and the history of mound development interpretations are based on subsurface data and they are reinforced by a comparison with the analogous waulsortian mounds and lenses of the same age in europe waulsortian type mounds are widespread on the surface of western europe and north america however differences in morphology porosity development and the emplacement of petroleum in fort payne mounds are related to local paleogeography in a shallow cratonic shelf sea subjected to cyclic regression and transgression due to regional tectono eustatic events fort payne mounds produced more than 5 5 million bbl of oil through 1980 although the source beds have not been identified petroleum may have been derived from the submound or mound units of the fort payne formation or from the underlying chattanooga shale

mid cenozoic stress evolution and magmatism in the southern cordillera texas and mexico transition from continental arc to intraplate extension orientations of dikes veins faults and slickenlines reveal the evolution of stress during eocene to miocene magmatism in the southern cordillera where most thoroughly studied by us in trans pecos texas magmatism began at about 48 ma shortly after the cessation of laramide folding dikes and veins that formed from then until about 32 ma strike dominantly east northeast this indicates that the least principal stress 3 was north northwest additional data suggest that the maximum principal stress 1 was east northeast the stress field changed to 1 vertical and 3 east northeast i e east northeast extension at least by 28 ma and probably by 31 ma dikes and veins that formed between 31 and 17 ma when all magmatism ceased in texas strike north northwest this change marks the beginning of regional basin and range extension however major normal faulting exclusively of high angle type did not begin until about 24 ma a similar stress change marked by a similar change in dike and vein orientations occurred throughout the southwestern united states and northern mexico the time of change is not well constrained in texas but available information allows it to have occurred at the same time thoughout the southern cordillera we suggest the earlier stress field is related to east northeast convergence between the farallon and north american plates the change in stress is approximately coincident with collision of the east pacific rise and paleotrench extension may be related to the change from a convergent to a transform margin along the western edge of north america the changes in the stress field are accompanied by changes in the sources and compositions of magmas erupted in texas contemporaneity of the changes in stress and magmatism indicates that they are related combined with regional age patterns paleostress and geochemical data indicate that pre 31 ma magmatism in the southern cordillera occurred in a subduction related continental volcanic arc subsequent magmatism occurred in an environment of intraplate extension of the basin and range province

chemical and mineralogical characterization of historic mortars from the santa eulalia de boveda temple nw spain bulk samples of mortars from the santa eulalia de boveda temple nw spain have been characterized from a chemical and mineralogical point of view with the aims of contributing technological and provenance inferences and evaluating possible correlations with the various identified stages along historical periods of construction mortar samples were analyzed by x ray diffraction and neutron activation analysis mineralogical associations found are similar quartz alkali feldspars calcite and mica for all samples only varying in their proportions mainly calcite statistical and geochemical studies allowed the differentiation of two main groups enhancing definition of the granite nature of the aggregate and of a significant variation in calcite proportion lime binder two outlying samples were identified one with higher calcite proportion and other corresponded to a plaster with a different raw material source a clear correlation between the chemical groups and the independently defined chronological stages could not be found in this work however mortar samples from the earlier stage were associated with a higher aggregate binder proportion

rare earth and other trace and major elemental distribution in a pedogenic calcrete profile slimene ne tunisia a calcrete profile developed on the top of a calcareous consolidated dune located in a coastal area of ne tunisia semi arid climate was studied with the aim to investigate the behavior of the chemical elements rare earth elements ree and other trace and major elements during the processes associated with calcrete formation particularly dissolution and precipitation of carbonates in the vadose zone the profile shows a vertical sequence with clear zonations from the surface downwards a reddish soil at the land surface a nodular horizon a laminar structured level and the consolidated old dune chemical and mineralogical analyses of samples from all levels were performed by neutron activation analysis and x ray diffraction bulk samples and 2 m fraction detailed studies of iron speciation and iron minerals were done by mossbauer spectroscopy microfauna mineralogical and chemical variations in the slimene weathering profile point to a long term aerial exposure of the old dune in a semi arid carbonated environment leading to the development of a pedogenic calcrete profile the mineralogical variations with depth indicate carbonate dissolution at the surface and downward leaching of calcium secondary carbonates overgrow primary ones and precipitate as coating or concretions below minerals found in the upper levels correspond to original materials of the old dune and also weathering phases and atmospheric inputs phyllosilicates decrease while calcite and k feldspars increase with depth kaolinite was the only clay mineral found in the old dune illite and chlorite were also found in all samples with calcrete authigenic smectite formation occurs above the laminar structured calcrete due to restricted drainage conditions the most significant chemical variations associated with the calcrete formation are i enrichment of co u br and ree in calcrete ii depletion of middle ree in the upper levels particularly eu and enrichment of middle ree and heavy ree in calcrete and iii hf zr cr th cs ta ga rb and k appear to be retained in the upper levels where calcite has been dissolved here iron is more reduced fe3 occurs in iron oxides goethite and hematite and clay minerals the ratio goethite hematite appears to increase down the profile and fe2 is mainly present in clay minerals

the use of volcanic materials for the manufacture of pozzolanic plasters in the maya lowlands a preliminary report this paper focuses on the description and characterization of the nature of isotropic and silicon rich particles observed in plaster samples from the maya archaeological sites of calakmul and lamanai based on the composition and morphology of these particles it is proposed that volcanic ash and glass were incorporated into some of the ancient maya plasters in order to produce hydraulic reactions from a technological point of view the intent on the part of the maya to produce such reactions is relevant because it would reflect resource planning and complex knowledge of materials results also confirm previous reports of the presence of volcanic ash layers at the site of calakmul and suggest that there were considerable ash falls in the maya lowlands during prehispanic times which may have made the material available in many areas of the central lowlands if as we propose here volcanic ash was employed in lowland maya plasters then the use of volcanic materials by the lowland maya was more intensive than has been recognized our paper also reviews the literature about the use of volcanic materials in lowland maya materials and suggests future lines of research

submarine and superimposed contact metamorphic oxygen isotopic exchange in an oceanic arc sawyers bar area central klamath mountains california usa new bulk rock oxygen isotope data indicate a complicated history of fluid rock interactions in the upper few kilometers of a basaltic arc attending its mesozoic accretion to the western margin of north america folded multiply recrystallized weakly metasomatized mafic volcanics and interstratified sediments of the sawyers bar terrane an eastern segment of the western triassic and paleozoic belt were investigated the following scenario can now be reconstructed 1 island arc tholeiites iats ocean island basalts oibs and distal turbidites were deposited in a subsea environment during permian and early mesozoic time 170 245 ma basaltic rocks underwent low temperature alteration by seawater recrystallization occurred at 100 200 c and 1 kbar alkali exchange and variable mg enrichment were accompanied by increases in bulk rock 18o values of the greenstones from 6 to approximately 10 o preceeding initial stages of island arc formation 2 middle jurassic 165 170 ma suturing of the seaward oceanic arc structurally beneath a landward 227 ma blueschist terrane resulted in regional deformation and greenschist facies metamorphism pervasive overprinting took place without important chemical or isotopic exchange under conditions of 300 425 c 3 1 kbar 3 granitoid plutons emplaced during late middle jurassic time 160 165 ma heated adjacent wallrocks to 500 600 c at pressures of approximately 2 3 kbar thermal upgrading resulted in devolatilization of isotopically heavy metasediments and in the exchange of high 18o fluids with intercalated greenstones 18o values in iat and oib metavolcanics increased from 9 to 10 along axial portions of ns trending folds to more than 15 o where metabasalts are intimately interlayered with the metasediments construction of this sector of the continental margin through the assembly of a weakly deformed and polymetamorphosed oceanic arc was attended by a sequence of aqueous fluid rock isotopic exchanges reflecting the p t time evolution of the complex

waulsortian like mounds of the mississippian pekisko formation northwestern alberta petrographic and chemical attributes abstract the tournasian age pekisko carbonates in the normandville field northwestern alberta form waulsortian like bryozoan crinoid mounds that developed in fairly deep low energy cool water systems close to the ramp margin three main depositional environments occur 1 crinoidal apron with wackestone grainstone and floatstone facies 2 mound flank with grainstone wackestone packstone and floatstone facies dipping 35 and 3 bryozoan mound core composed of rudstone and floatstone facies with fenestrate bryozoa minor crinoids and carbonate mud local highs due to fault bounded blocks created from the collapse of the devonian peace river high may have controlled the location of mound nucleation diagenesis of the bryozoan crinoid mounds included calcite cementation compaction dolomitization silicification and hydrocarbon emplacement events the mound core facies contains submarine fascicular optic calcite and bladed prismatic calcite cements and later ferroan brightly luminescent pore filling blocky spar cement the crinoid apron facies contains syntaxial cement associated with crinoids and the ferroan blocky spar cement the mounds are dominantly limestone however in one well dolomite dominates the lower section four types of dolomite have been identified partial replacive chemical compaction related pervasive dolomite and saddle dolomite cement all dolomites are non stoichiometric caco3 mole 56 6 62 6 the partial zoned replacive dolomite replaces micrite and syntaxial rim calcite in mound flank and crinoid apron facies the chemical compaction related dolomite is found along dissolution seams and stylolites and has similar cl characteristics to the replacive dolomite the pervasive dolomite is fabric destructive and has dull cores and bright rims in cl saddle dolomite 0 15 mm has brightly luminescent concentric zoning and occurs in vugs and fossil pore spaces chemical and isotopic analysis of the bryozoan crinoid mounds indicate that the original marine signatures in micrite early cements some crinoids and brachiopods have been preserved however carbon isotopic values for some crinoids matrix and dolomite show more positive values compared to known mississippian carbonate values recrystallization during shallow burial has reset the oxygen isotopic composition of some crinoids and micrite oxygen and carbon isotopic compositions of most dolomites overlap with altered crinoids and early calcite cements however saddle dolomites have lighter 18o values similar to saddle dolomites from the devonian wabamun group in this area the isotopic variations in later ferroan calcite cements show an inverted j trend possibly due to variable amounts of water rock interaction while the sr isotopic ratio of submarine calcite cement coincides with that of mississippian seawater the later ferroan calcite cement is more radiogenic indicating a different source of fluids

ridge trench collision induced switching of arc tectonics and magma sources clues from antarctic peninsula mafic dykes compositions and distributions of mafic dykes in the antarctic peninsula continental arc show that tapping of several mantle sources was tectonically controlled in the cretaceous to tertiary between 135 ma and 55 ma calc alkaline dykes intruded the arc in the late cretaceous however between 95 ma and 65 ma there was a pulse of compositionally diverse magmatism this change resulted from collision of an ocean spreading centre with the trench as a consequence non partitioned dextral transtensional shear in the overriding plate became partitioned into strike slip and extensional domains calc alkaline magmatism was therefore replaced by strike slip related shoshonitic magmatism towards the rear arc and extensionrelated tholeiitic magmatism towards the fore arc oib like dykes were emplaced because of the break in otherwise continuous subduction during the early tertiary subduction continued but ceased after a late tertiary ridge trench collision

sequence development of a mixed carbonate siliciclastic high relief ramp mississippian kentucky u s a abstract the discovery of the supergiant oil fields of the caspian region has focused attention on eustatic tectonic and climatic controls on lower carboniferous facies this paper documents the development of a highly complex mixed carbonate siliciclastic ramp succession of the lower mississippian upper osagean lower meramecian fort payne salem interval kentucky that formed peripheral to an abandoned delta system borden formation following drowning of the paleoshelf during a time of arid climate and tectonic quiescence the antecedent borden deltaic marine paleoshelf stood 100 m above the starved basin plain to the southwest and had marginal slopes of less than 0 5 the succession is a supersequence 0 to 150 m thick approximately 5 to 10 my duration composed of two upper osagean fort payne and two lower meramecian warsaw salem depositional sequences which correlate with the global sea level cycles of ross and ross 1988 the supersequence lowstand systems tract on the basin floor consists of the lower part of a unit of green shale and mounds mound growth appears to have continued during supersequence transgression culminating in some mounds up to 60 m thick on the borden paleoshelf the supersequence transgressive systems tract is a condensed glauconite horizon s the supersequence highstand tract consists of marine quartz sandstone and shale peritidal carbonates high energy crinoidal bryozoan grainstone banks and deep ramp and slope siliceous calcisiltite third order sequences have local lowstand transgressive sand bodies those in sequence 2 are clinoformed strike sand lobes up to 50 m thick while those in the younger sequences are discontinuous sheets dip oriented lobes the siliciclastic units appear to be associated with a paleodrainage system off the borden paleoshelf in upper osagean sequences 1 and 2 third order highstand systems tracts are aggradational on the paleoshelf and prograded to the southwest as a series of clinoforms into the starved basin lower meramecian sequences 3 and 4 highstand systems tracts are relatively layer cake reflecting loss of accommodation during late osagean basin filling and they prograded more to the west into the subsiding reelfoot graben illinois basin the extremely complex facies relations reflect the interaction of the antecedent shelf slope basin topography the mixed carbonate and siliciclastic systems mounded basin floor topography the highly channeled margin morphology eustasy and relatively stable tectonics

hydrocarbon potential of the mississippian carbonate platform bechar basin algerian sahara the western bechar basin contains a 1500 2000 m thick mississippian succession of carbonates consisting of superposed shoaling upward sequences whose porosity or diagenesis have never been investigated to establish the reservoir potential of these carbonates in this paper we present a diagenetic and porosity evolution of the visean part of the mississippian carbonate platform that may lead to the development of new exploration fairways in the region a diagenetic model is proposed in which ooid and crinoid grainstones may have a reservoir potential if hydrocarbon migration occurred before the end of the serpukhovian prior to complete occlusion of porosity

comparative analysis of coatings on granitic substrates from urban and natural settings nw spain abstract rock coatings are frequent on natural rock outcrops and even more on stone surfaces of historical buildings and monuments they are interesting as some of them have a protective effect against erosion in the last 30 years research on rock coatings has been performed separately on either buildings or natural outcrops this paper studies and compares thin coatings found on granitic rock surfaces of historical buildings with coatings found on natural granite outcrops in a same climatic area nw spain by analysing their surface and cross sections as rock surfaces of buildings have known age they can be used to assess rates of weathering and coating growth this is the first comparative study of formation of coatings on natural and built surfaces in the same region both endogenous rock characteristics and exogenous factors climatic conditions air pollution that could affect the coatings formation are considered to assess the origin and formation of coatings x ray diffraction xrd and x ray fluorescence xrf analyses of the underlying rocks xrd analyses of the coatings and scanning electron microscopy sem observations of surface and cross sections of the coatings performing energy dispersive spectrometry eds and x ray maps were carried out to establish their composition fieldwork and results indicate that natural outcrops are widely covered by biological coatings or coatings formed due to rock weathering while coatings found on rock ashlars are highly variable with air pollution other building materials organic droppings and micro environmental factors contributing to coatings on rock ashlars in a more significant way than substrate characteristics and exposure time

laser probe40ar39ar measurements of loss profiles within individual hornblende grains from the giants range granite northern minnesota usa abstract a nd glass laser has been used as a probe mapping natural argon diffusion in partially outgassed hornblende grains at a resolution of around 50 m the hornblendes are from the giants range granite 2700 ma minnesota taken approximately 2 km from the contact with the duluth gabbro complex 1100 ma the full range of ages can be discerned within individual hornblende grains using the laser extraction technique but more importantly diffusion profiles up to 500 m in length have been detected the profiles reflect large variations in domain size within single grains diffusive argon loss was controlled not only by lattice dominated volume diffusion but a network of fast diffusion paths now defined by biotite inclusions and also indirectly by mineral reactions associated with the contact metamorphism three natural argon loss profiles from subgrains of different sizes yield good fits to theoretical volume diffusion loss curves and a mean time integrated diffusion parameter d t dt of 7 1 0 9 10 5 cm 2 this parameter constrains the possible heat loss models for the duluth gabbro

morphology seismic structure and tectonic development of the powell basin antarctica the powell basin located near the scotia antarctica strike slip plate boundary is bounded on three sides by continental crustal blocks and it separates the south orkney microcontinent from the northeastern antarctic peninsula on the basis of the entire geophysical data set available the main structural and geological elements of the basin are identified the northern domain is characterized by an oceanic like environment with a ridge axis in the centre of the basin two conjugate deep restricted sedimentary basins are located at the base of the northeastern and northwestern margins and filled by thick sedimentary sequences the southern domain is mainly floored by transitional crust and presents rift related basement graben the margins surrounding the basin are different with the northern and southern margins mainly transcurrent whereas most of the eastern and western margins show passive margin characteristics the sedimentary sequences filling the basin have been correlated to the main tectonic processes responsible for powell basin opening and partially dated using marine magnetic anomaly identifications three main evolutionary phases are recognized that explain the geological setting of the powell basin an early phase of separation between the antarctic peninsula and the south orkney microcontinent produced stretching and progressive continental lithospheric thinning with the creation of restricted pull apart basins proto powell phase this phase was followed by a northward movement possibly accomplished with a clockwise rotation of the south orkney microcontinent drift phase with consequent oceanic crustal accretion in the northern sector of the basin and development of rift related structures in the southern sector phase spanning 27 18 ma as determined from the marine magnetic anomalies a successive clockwise rotation of the south orkney microcontinent determined the complete development of the northern sector of the basin with spreading and final structuration of the margins encircling the basin plain

orthomagmatic quartz and post magmatic carbonate veins in a reported porphyry copper deposit andean intrusive suite livingston island south shetland islands a previously reported porphyry cu mo deposit in an eocene pluton within the south shetland island magmatic arc has been re interpreted as three distinct hydrothermal assemblages the oldest assemblage 1 exsolved under confinement from the deep 6 km cooling magma whereas assemblages 2 and 3 formed during tectonic magmatic episodes at depths of 1 5 km in the late cenozoic the three assemblages occur over the 5 11 km mapped in barnard point tonalite pluton

anticorrelated rb sr and k ar age discordances leuchtenberg granite ne bavaria germany the leuchtenberg granite oberpfalz ne bavaria displays a continuous differentiation trend ranging from mildy peraluminous coarse grained porphyritic biotite granites bg to strongly peraluminous medium to fine grained garnet bearing muscovite granites gmg the rb sr and k ar age determinations of whole rock and mineral samples from the granite and associated intermediate rocks redwitzites have revealed two divergent age gradients rb sr wholerock dates decrease and initial 87sr 86sr ratios increase for successively more evolved subsets of the granite all bg samples 87rb 86sr 2 16 yield a date of 326 2 ma with a low initial 87sr 86sr ratio of 0 70778 0 00013 1 while all gmg samples 87rb 86sr 70 to 1000 yield a younger date of 317 2 ma with an enhanced initial 87sr 86sr ratio of 0 7146 0 0039 the k ar measurements on biotites and muscovites give closely concordant dates for the gmg 326 323 ma and the southern lobe of the bg 324 320 ma the northern lobe of the bg including the redwitzites shows a well defined trend of decreasing k ar dates from 320 ma to 300 ma towards the northwest critical consideration of both isotope systems leads to the conclusion that the rb sr system of the gmg was disturbed by a later hydrothermal event the ca 326 ma whole rock rb sr date for the bg is not in conflict with any of the k ar mineral dates and is taken as approaching the crystallization age of the leuchtenberg granite the k ar age progression within the northern lobe of the bg indicates that this part either cooled down over a protracted period of some 20 ma or experienced reheating at ca 300 ma the study highlights the potential of combined rb sr and k ar dating in deciphering detailed chronology on the scale of a single igneous intrusion

evolution of the pacific margin of the northern antarctic peninsula an overview the sector of the northern antarctic peninsula between the tula and shackleton fracture zones provides evidence for the subduction of south east pacific oceanic crust under antarctic continental crust during late mesozoic through miocene times the pre subduction depositional history of this sector includes the formation of a marine siliciclastic turbidite wedge permian triassic deposited in a marginal basin setting it was folded and thrust retroarc before the middle jurassic to form the trinity accretion foldbelt which extended for several hundred kilometres along the pacific margin of gondwanaland the foldbelt was deeply eroded and levelled under subaerial conditions then unconformably covered either by middle upper jurassic alluvial to lacustrine deposits in the north or by early cretaceous basic lavas in the south

successive geothermal volcanic hydrothermal and contact metasomatic events in cenozoic volcanic arc basalts south shetland islands antarctica hydrothermal alteration in volcanic arcs occurs in many settings and may involve magmatic marine lacustrine or groundwaters driven by magmatic tectonic or thermal events king george island part of the south shetland island cenozoic volcanic arc contains an 80 km long zone of propylitized volcanic rocks with numerous occurrences of quartz veining silicic sericitic argillic and advanced argillic alteration on barton peninsula a basaltic lava sequence 49 44 ma intruded by a small high level granodiorite pluton 42 ma contains these alteration types previously interpreted as a single porphyry copper system in this study we report three possibly four distinct fossil hydrothermal episodes 1 banded chalcedonic quartz quartz sericite and propylitic alteration occurs along ese faults and as reworked clasts in nearby tuffs drusy quartz calcite veins with silicic sericitic argillic and propylitic wallrocks may represent feeders to the near surface silicification these characteristics and anomalous ag pb sb au plus te se zn as suggest a neutralph geothermal system that was active during volcanism 2 the lavas and banded quartz rocks were brecciated veined and replaced by alunite native sulphur pyrite and pyrophyllite quartz pyrite zunyite diaspore assemblages with anomalous hg se as bi au tl sb cu such advanced argillic alteration is diagnostic of degassing of a felsic magma into shallow 500 m meteoric groundwaters rhyolite tuffs previously not reported on king george island may represent leakage of this magma to the surface 3 subsequent burial to 3 km was followed by emplacement of a granodiorite pluton and formation of a silicic contact metasomatic aureole containing muscovite biotite actinolite magnetite k feldspar and tourmaline disseminated andalusite corundum also formed in areas previously affected by the advanced argillic alteration iron copper sulphide veinlets are locally abundant but a porphyry style geochemical signature is not present early cretaceous ar ar ages near the intrusive contact indicate flow of an excess ar bearing hydrothermal plume up the contact finally isolated areas of propylitic alteration in the lavas nearby may be related either to quartz veins of episode 1 at depth or to 4 continued circulation of heated groundwaters around the cooling pluton

geodynamic evolution of the antarctic peninsula during mesozoic times and its bearing on weddell sea history this review of the tectonic evolution of the antarctic peninsula during mesozoic times highlights four main events 1 late triassic late jurassic extension 2 late jurassic early cretaceous dextral transpression 3 early cretaceous extension and 4 mid cretaceous compression magmatism was virtually continuous during much of this period with the exception of possible breaks in the known record in early jurassic and late jurassic early cretaceous times the second of these breaks corresponded to the first compressional event there was no apparent hiatus in the magmatic record during the mid cretaceous compressional event although there was a significant change in the pattern of sedimentation in the larsen basin on the eastern margin of the weddell sea at about that time the tectonic evolution of the peninsula is compared to and puts some constraints on existing weddell sea models the late triassic late jurassic arc extension correlates with initial rifting in the weddell sea region during sinistral motion between east and west gondwana however there is no known record of large scale pre mid jurassic transcurrent deformation in the antarctic peninsula that would have been consistent with rotation of west antarctic crustal blocks in the initial rifting period the peninsula wide late jurassic early cretaceous compressional event may correlate with the major change in gondwana plate motions from e w to n s african reference frame separation of east and west gondwana this change probably resulted in formation of an ocean continent boundary along the northern margin of the weddell sea embayment and initial seafloor spreading geological data do not seem to support subduction of southwestern proto weddell sea oceanic lithosphere beneath the eastern margin of the peninsula at that time cretaceous arc extension was coeval with the initial seafloor spreading phase in the weddell sea mid cretaceous arc compression linked to a global increase in ocean floor spreading rates and a superplume event correlates with a change from ne sw to nw se spreading in the weddell sea

trace element geochemistry in topsoil from east china the concentrations of 23 trace elements in 50 topsoil samples collected from sites ranging between 18 19 n and 49 13 n in east china were analyzed sc ti v cr mn co ni cu zn ga and ta have mean contents more than two times higher than in the continental upper crust three elements rb sr and ba are present at lower concentrations than in the continental upper crust finally a group of elements consisting of ge y zr nb sc hf pb th and u are present at concentrations 1 2 times higher than in the continental upper crust however concentrations of trace elements are mainly affected by parent rock the contents of sc ti v cr mn co and cu for 29 soils from basalt were found to increase from north to south whereas rb sr and ba contents were found to decrease in addition element concentration shows a close relationship with annual average temperature aat as well as annual average precipitation aap since the 29 soils are all from basalt the trends of the elemental contents should reflect the influence of climate which determines the intensity of weathering these elemental trends suggest that the content of certain elements may indicate the intensity of basalt weathering ba nb and sr nb ratios were both found to have good correlations with aat and aap in this study which means that these ratios can also indicate the intensity of chemical weathering of basalt

provenance of glass shards in archaeological lime plasters by la icp ms implications for the ancient routes from the gulf of mexico to teotihuacan in central mexico abstract we present the chemical characteristics of samples collected from the central courtyard of teopancazco a neighbourhood centre in teotihuacan mexico the most important classic mesoamerican city the rhyolitic glass shards used as aggregates have a composition comparable to the glass material sourced from the magmatic system of altotonga located along the route to the gulf coast people travelling from the gulf coast may have transported the glass shards to teotihuacan during the xolalpan phase 350 550 ad however the reasons for using glass shards as a construction material remain unknown these new data confirm the close relationship between inhabitants of teopancazco and the gulf coast already revealed by previous archaeological and archaeometric studies

40ar39ar dating of the skaergaard intrusion we report the first 40ar 39ar data for the skaergaard intrusion based on analyses of hornblende and biotite from a transgressive granophyre from upper zone a uza of the layered series hornblende separates heated in a resistance furnace and laser heated single crystals of biotite from the granophyre give plateau ages that are consistent with isochron ages of 55 48 0 30 and 55 40 0 14 ma respectively 2 internal uncertainty with initial 40ar39ar ratios of 292 4 19 0 and 291 1 7 2 if uncertainties in the ages of 40ar 39ar standard fluence monitors are considered external uncertainties 2 errors are 0 76 and 0 72 ma respectively resistance heating analysis of biotite separated from a gabbro from lower zone a lza of the skaergaard layered series indicates that the sample has been extensively contaminated by ar inherited from old crust and does not constrain the intrusion age apart from indicating that it is less than approximately 57 ma old the ar cooling ages are consistent within uncertainty with both pluton cooling models presented by norton and taylor 1 but the data agree best with their model s2 which assumes a smaller permeability 10 13 cm2 combination of the new ages and the norton and taylor cooling models constrains the emplacement and final crystallization of the skaergaard magma chamber to be at 55 75 0 30 ma and 55 65 0 30 ma respectively evidence for late crystallizing quartz and tridymite near the sandwich horizon sh combined with phase equilibria constraints on the near solidus crystallization temperature of the sh 975 25 c require the pressure of sh solidification to be 60 15 mpa this means that the tertiary basalts and sediments overlying the kangerdlugssuaq region were 3 2 0 8 km thick at 55 65 0 30 ma assuming that the 6 7 km thick tertiary section exposed further east along the east greenland coast is representative of that which accumulated above the skaergaard we infer that the skaergaard emplacement occurred during the main stage of the east greenland flood basalt event after approximately half of the tertiary section had erupted the onset of coastal flexure in the kangerdlugssuaq region was after 55 48 0 76 ma

kinetically limited isotope exchange in a shallow level normal fault mineral mountains utah we have conducted a stable isotope and geochemical study of the corral canyon fault zone a shallow level low angle normal fault on the west flank of the mineral mountains in southwest utah our stable isotope and geochemical data document the infiltration of local meteoric water during brittle deformation and hydrothermal alteration of the shear zone during tertiary basin and range extensional faulting within the fault zone 18o depletion correlates with progressive hydrothermal alteration and cataclasis demonstrating that the processes of fluid infiltration hydrothermal alteration fracturing comminution of grains and 18o exchange took place concurrently interpretation of our data using a model of combined fluid flow and kinetically limited isotope exchange leads us to conclude that the exposed fault zone was infiltrated by isotopically unevolved fluids from the wall rock adjacent to the fault zone and that an important component of fluid flow in this fault was between the wall rock and the fault zone rather than strictly within the fault plane we propose that the rate of isotopic alteration in shallow level fault zones is enhanced by comminution and recrystallization of grains during deformation so that fluids in the wall rock because of the relatively coarser grain size remain relatively unevolved until they infiltrate the fault zone where they rapidly become isotopically evolved due to exchange with cataclasized grains we present a general model for calculating the duration of isotopic alteration in flow systems characterized by kinetically limited isotope exchange calculated exchange durations for the corral canyon fault zone are short on the order of thousands to a few hundreds of thousands of years compared to the likely duration of deformation of millions of years the geologically short duration of isotope exchange leads us to suggest that the fluid flow responsible for the isotopic alteration was episodic of short duration and possibly related to discrete seismic events this result corroborates diverse lines of evidence that support a model of alternating fluid flow and permeability sealing by hydrothermal precipitation within fault zones

crustal growth of the antarctic peninsula by accretion magmatism and extension a subduction accretion model incorporating new geophysical data is presented to explain the geology of the antarctic peninsula from late palaeozoic to cenozoic time according to the model the peninsula consists of overlapping accretionary magmatic and extensional regimes that were diachronous across the peninsula and have built the crust to its present form the crust which contains a small proportion of sialic basement was mainly formed by accretionary and magmatic processes and modified to its present shape by extension the gondwanide orogeny for the antarctic peninsula is interpreted in terms of the accretionary processes

disturbed 40ar39ar systematics in hydrothermal biotite and hornblende at the scotia gold mine western australia evidence for argon loss associated with post mineralisation fluid movement hornblende and biotite that formed during gold mineralisation at the scotia mine western australia have erratic 4oar 3gar release spectra and total gas ages that are 200 900 million year younger than the ca 2600 2620 ma minimum age of gold mineralisation as given by 4oar 3gar plateau muscovite ages of crosscutting pegmatite dykes analysed homblendes are dominated by magnesio hornblende but also contain small domains of ferro actinolitic hornblende actinolitic hornblende and actinolite biotite also appears to be substantially altered to chlorite along cleavage planes relatively young apparent ages and high k ca ratios of argon released from homblendes at tempera tures less than 1000 c are interpreted to be the result of degassing of contaminant biotite however this cannot totally explain the young ages of homblendes gas fractions released at furnace temperatures above looo c where the effect of biotite degassing is demonstrably negligible still have apparent ages that are 200 900 million years younger than the age of muscovite from post gold pegmatite dykes the close proximity of disturbed hydrothermal hornblende samples to apparently undisturbed pegmatite muscovite samples less than a few metres in some cases is difficult to reconcile with argon loss in hydrothermal hornblende being the product of thermally driven volume diffusion given a suitable thermal history argon loss could occur preferentially in homblendes if 1 the closure for slow cooling and blocking for reheating temperatures of hydrothermal homblendes were lower than published estimates as has been observed in structurally complex metamorphic homblendes and or 2 the closure and blocking temperature of pegmatite muscovite were higher than commonly estimated however neither of these interpretations can easily explain the large variation in hornblende ages it is instead suggested that argon loss occurred during mineral fluid interaction during movement of a retrograde fluid along the mineralised lode structures and that this occurred at ambient temperatures below the blocking temperature of pegmatite muscovite there is abundant geological evidence for the passage of such a fluid at the scotia mine including the presence of numerous late brittle fractures containing retrogressive low temperature mineral assemblages late fluid movement is probably related to proterozoic erogenic activity along the nearby southeastern margin of the yilgarn craton the difference in argon systematics between hydrothermal minerals and pegmatite muscovite is largely ascribed to the relatively low permeability of the more massive pegmatite dykes with respect to ore zones preventing fluid egress to muscovite samples the variations in ages of hydrothermal minerals are probably related to the extent of fluid mineral interaction as this is a function of parameters such as fluid rock ratio fluid p t x conditions permeability and mineral microstructures that may vary on short time and length scales recognition of possible argon loss in hornblende via fluid interaction is important for the interpretation of 4oar 3gar systematics in environments such as many hydrothermal ore deposits where minerals may be exposed to fluids after crystallisation copyright 0 1997 elsevier science ltd

microscopic calcite dendrites in cold water tufa implications for nucleation of micrite and cement dendritic calcite forms in an active cold water tufa system in association with extracellular polymeric substances eps that discontinuously coat bryophytes and cyanobacteria dendrites consist of 100 200 nm thick calcite fibres that form 3d lattice like domains in each dendrite domain fibres have three structurally equal orientations which correspond in disposition to radii from the centre of a calcite unit cell to the convex triple face junctions on its surface fibres do not form in the orientation of the c axis the external form of each dendrite has the shape of half of a shortened octahedron with an upper triangular surface parallel to the substrate dendrite nucleation takes place on or in microbial eps whether microbial cells are present or not and is probably effected by attraction of ca 2 cations to negatively charged eps together with co2 degassing and concomitant ph increase of supersaturated spring water in stream splash zones ensuing dendrite growth is abiogenic and controlled by diffusion dendrite c axes are perpendicular to the substrate probably because the negative charge of eps forces the orientation of ca 2 and co 2 planes within the developing dendrite crystal to be parallel to the eps film surface dendrites are eventually filled and overgrown by solid syntaxial calcite which gradually and completely obliterates the dendrites as more familiar calcite crystal forms develop no trace of the dendritic nucleus remains in the rock record calcite crystal nucleation may take place by this mechanism in many marine and meteoric settings given that microbial eps is now assumed to be virtually ubiquitous in these environments this phenomenon could contribute to the development of familiar fabrics such as marine micrite cement and fibrous calcite cement radial ooids peloids abiogenic stromatolites sea floor precipitates microbialites tufa travertine speleothems and some meteoric cements it may also contribute to the substrate normal orientation of c axes of common cement fabrics

k ar geochronology of the south shetland islands lesser antarctica apparent lateral migration of jurassic to quaternary island arc volcanism abstract approximately 70 k ar whole rock ages for low k tholeiitic and andesitic volcanic and intrusive rocks from the south shetland islands antarctica including about 50 not previously published are reviewed activity mainly spanned the range 130 to 30 ma jurassic cretaceous to oligocene miocene with a very recent 2 ma more alkaline renewal throughout the main period of activity magmatism or perhaps its cessation migrated continuously northeastwards along the length of the island chain

comparative study of deterioration forms on nearby granitic bridges from an urban setting in the nw iberian peninsula abstract stone coatings biological colonization efflorescence and detachments are frequent on granitic surfaces of our cultural heritage infrastructure in order to find the different origins for each deterioration form this paper studies and compares several coatings efflorescence and flakes found on granitic surfaces of bridges from different ages built in the same area ourense in nw iberian peninsula at 90 km from the sea under the same environmental and traffic conditions the most recent bridge was the new bridge built in the beginning of 20th century and the oldest one was the roman bridge with the 1st century ad foundations but its current structure was built in 17th century the new bridge has exhibited a greater variety of deterioration forms such as black crusts carbonate crusts efflorescence and biological colonization on the roman bridge black crusts efflorescence to a greater extent and flakes associated with salt crystallization processes have been identified scanning electron microscopy with energy dispersive x ray spectroscopy x ray diffraction and fourier transform infrared ftir spectroscopy studies were carried out in order to establish their composition and optical and electronic microscopies and x ray microtomography were used to study their morphologies as a result anthropogenic factors industrial activities traffic pollution and fertilizers are considered to be the main contributors to crusts efflorescence and flake formation on both studied substrates granite ashlars and mortars to a lesser extent natural factors marine influence and bird droppings should be taken into account

crustal structure of the antarctic peninsula sector of the gondwana margin around anvers island from geophysical data abstract we investigated the crustal structure of the northern antarctic peninsula ap a composite magmatic arc terrane at the pacific margin of gondwana it was formed in mesozoic cenozoic by collision accretion of allochthonous terranes to the gondwana margin during subduction and progressive ridge collision of former phoenix plate under the ap the ap is distinguished by belts of high amplitude gravity and magnetic anomalies inferred to indicate the presence of plutons of mafic composition comprising the huge ap batholith for a better understanding of the lithosphere structure and tectonic processes of the ap continental margin we performed 2d gravity and magnetic modeling along line iii iii which crosses the margin south of anvers island our model incorporates data on seismic refraction dss 13 and reflection line 8545 03 profiles and physical property data our modeling shows that the crust of the ap margin near anvers island is of continental type and comprises two crustal domains which were welded together during mid cretaceous collision on the east there is a thick 40 km high velocity density continental crust of the gondwana margin which occupies the ap inner shelf and mainland and larsen ice shelf the space between the gondwanian block and oceanic crust of the bellingshausen sea is occupied by thinner 26 km crust of lower densities of supposed accreted terrane characterized by gravity anomalies caused probably by accretional tectonics as a result of cretaceous subduction two belts of magnetic anomalies forming the pma pacific margin anomaly probably belong to different crustal domains the western belt of the pma relates to accretional block while the eastern belt of the pma corresponds to the gondwana margin block this may represent an evidence that the pma relates to a tectono magmatic event occurred after collision of gondwana margin and accreted terrane in mid cretaceous and had been imprinted on already formed continental margin

rare earth mobility in young arc type volcanic rocks from northern new zealand abstract samples of late cenozoic arc type volcanic rocks from the hauraki volcanic region in northern new zealand show evidence for enrichment of the rare earth elements rees these samples have variable to high la and y abundances together with ree abundance patterns which are up to an order of magnitude higher than those considered typical of the suite as a whole scanning electron microscope imaging and analysis shows the presence of micron scale veins and patches containing ree bearing phases these observations are explained as the result of cryptic alteration processes which enrich the rees but not other chemical components and which are probably the result of low temperature

paleomagnetism and tectonics of the south shetland islands and the northern antarctic peninsula new paleomagnetic data presented here from 61 sites in paleozoic mesozoic and cenozoic igneous and sedimentary rocks from the antarctic peninsula and the south shetland islands constrain the relative motion of the antarctic peninsula since the mid cretaceous and allow the quantification of tectonic rotation between the different blocks recognized within the area paleozoic and jurassic results failed the fold test and suggest an important remagnetization in the area the similarity between these results and those obtained from cretaceous intrusives indicates a mid cretaceous age for the remagnetization the paleopoles obtained for the different blocks and for different ages suggest that there is no relative rotation among them these combined results allow us to obtain a cretaceous 90 ma and paleocene 60 ma paleopole these paleopoles document a very low apparent polar wandering of the antarctic peninsula for the last 100 ma

melt instabilities in an intraplate lithosphere and implications for volcanism in the harrat ash shaam volcanic field nw arabia we investigate melt generation in a slowly extending lithosphere with the aim of understanding the spatial and temporal relationships between magmatism and preexisting rift systems we present numerical models that consider feedback between melt generation and lithospheric deformation and we incorporate three different damage mechanisms brittle damage creep damage and melt damage melt conditions are calculated with a helmholtz free energy minimization method and the energy equation is solved self consistently for latent heat and shear heating effects using a case of a slowly extending 1 1 5mm yr continental lithosphere with a relatively low surface heat flow similar to 50mw m 2 we show that melt rich shear bands are nucleated at the bottom of the lithosphere as a result of shear heating and damage mechanisms upon further deformation melt zones intersect creep damage zones thus forming channels that may be used for the melt to migrate upward if a preexisting structure resides only in the brittle crust it does not control the path of melt migration to the surface and melt filled channels propagate from the bottom upwards independently of upper crustal structures in contrast a preexisting weak structure that reaches a critical depth of 20km allows fast similar to 2ma propagation of melt filled channels that link melt damage from the bottom of the lithosphere to near surface structures our model results may explain the short time scale volume and magma extraction from the asthenosphere through a low surface heat flow lithosphere such as observed for example in the harrat ash shaam volcanic field northwestern arabia which developed in the arabian plate and is spatially linked to the azraq sirhan graben

episodic dike intrusions in the northwestern sierra nevada california implications for multistage evolution of a jurassic arc terrane in the northwestern sierra nevada california volcanic and plutonic rocks of the smartville and slate creek complexes both fragments of a jurassic arc terrane are tectonically juxtaposed against ophiolitic and marine rocks that represent late paleozoic early mesozoic oceanic basement this oceanic basement is intruded by early jurassic dikes that are coeval with hypabyssal and plutonic rocks within the smartville and slate creek complexes these dikes have geochemical characteristics reflecting a depleted and metasomatized source as commonly observed in modern fore arc settings and incipient volcanic arcs and are interpreted to be the conduits for the early jurassic arc volcanism which was built on and across the disrupted oceanic basement late jurassic sheeted dikes intruding the smartville complex have basaltic compositions compatible with an intra arc or back arc origin and indicate that a spreading event occurred within the arc in early late jurassic time these interpretations support models for a complex multistage evolution via episodic magmatism and deformation within a singly ensimatic jurassic arc terrane west of the north american continent

geological mineralogical and geochemical characteristics of zeolite deposits associated with borates in the bigadic emet and kirka neogene lacustrine basins western turkey the bigadic emet and kirka lacustrine basins of western turkey may be considered as tibet type graben structures that were developed during the miocene within the izmir ankara suture zone complex the volcanic sedimentary successions of these basins are made up of mudstone carbonate limestone and dolomite and detrital rocks and also of crystal or vitric tuffs about 135 to 200 m thick the degirmenli bigadic emirler bigadic kopenez emet and karaoren kirka tuffs constituting the zeolite deposits are situated beneath four borate deposits colemanite ulexite borax the most abundant diagenetic silicate minerals are k and ca clinoptilolites in the zeolite deposits and li rich trioctahedral smectites stevensite saponite and hectorite and k feldspar in the borate deposits in the degirmenli emirler kopenez and karaoren deposits the following diagenetic facies were developed from rhyolitic glasses rich in k and poor in na glass smectite k clinoptilolite opal ct caclinoptilolite k feldspar analcime quartz and k feldspar analcime quartz k feldspar which is also rarely associated with phillipsite karaoren and heulandite degirmenli and karaoren succeeds clinoptilolite and precedes analcime in these diagenetic facies where dioctahedral smectites opal ct and quartz are the latest minerals no diagenetic transformations exist between clinoptilolite k feldspar and analcime that were formed directly from glass the lateral facies distributions resulted from the differences in salinity and ph of pore water trapped during deposition of the tuffs but vertical distributions in vitric tuffs seem to have been controlled by the glass liquid ratio of the reacting system and the permeability or diffusion rate of alkali elements the bigadic emet and kirka zeolite deposits which were formed in saline basins rich in ca and mg ions show similar chemical changes i e loss of alkalis and gain in alkaline earth elements that have taken place during the diagenetic transformation of rhyolitic glasses to dioctahedral smectites or clinoptilolite the absence of sodic zeolites such as mordenite erionite chabazite and silica rich phillipsite is mainly due to the very high k na ratio of the starting materials rather than initial alkaline conditions or high na content in lake waters

soiling of limestone in an urban environment characterized by heavy vehicular exhaust emissions the inorganic and organic chemical composition of weathering sulphate patinas on the limestone walls of emmanuel college cambridge uk have been analysed by ft ir spectroscopy pyrolysis gas chromatography mass spectrometry and scanning electron microscopy the college location is characterized by heavy vehicular exhaust emissions including those from diesel powered public transport vehicles due to its proximity to the main cambridge coach station a complex mixture of gaseous and particulate air pollutants are found deposited in the weathering patinas including organic compounds such as pahs which represent markers for present day vehicular pollution slight differences in composition between patinas collected at different heights on the building facades with in particular more evidence for a biogenic overprint in samples collected at roof level as opposed to ground street level are found analytical results confirm how building stones in urban areas acted in the past and still do today as passive repositories for any kind of gaseous and particulate air pollutant present in the surrounding atmosphere accordingly weathering patinas are slowly changing their composition to accommodate new classes of present day air pollutants

fluid disturbed hornblende k ar ages from the dalradian rocks of connemara western ireland the results of k ar age determinations and hydrogen and oxygen stable isotope measurements on hornblendes from the poly metamorphic dalradian schists of connemara western ireland indicate that the hornblende k ar systems have been significantly disturbed by low temperature fluid reaction correlations between k ar ages d values structural water contents and the occupancy of the hornblende structural a sites suggest that excess water as h 3 o occupies vacant a site positions during stable isotope exchange and displaces ar causing lowering of the k ar ages this process occurs below the normally accepted closure temperature for ar in hornblende and is not detectable by optical or x ray investigation of the mineral this process may not be unique to connemara hornblendes and so is of potential importance to geochronology in other metamorphic terranes

sequence stratigraphy and depositional dynamics of carbonate buildups and associated facies from the lower mississippian fort payne formation of southern kentucky u s a abstract carbonate buildups are a common feature in mississippian sections worldwide yet despite intense study questions remain about depositional processes that form such structures and their interrelations if any to each other carbonate buildups located in siliciclastic basins are particularly understudied and poorly understood this study focuses on a new exposure of buildups in the siliciclastic dominated fort payne basin osagean of south central kentucky it represents a first attempt to combine field and petrographic descriptions of facies in the buildup interval of the fort payne with the concepts of sequence stratigraphy to gain better insight into their depositional histories the buildup interval is composed of five distinct facies two of these the fossiliferous green shale and the tabular crinoidal packstone grainstone represent the background sedimentation during buildup nucleation and growth the remaining three facies make up the buildups themselves the shaly packstone marks the inception of buildup growth the massive wackestone represents the core of the buildups and the crinoidal grainstone is a buildup flank facies deposited near the end of buildup growth the buildups of the fort payne formation grew during the transgressive phase of one third order sequence in the early osagean the time at which buildup growth ceased cannot be determined precisely with available data but likely occurred either at the maximum flooding surface or during the early highstand three commonly used depositional models abbreviated as the lithoherm model the baffling model and the microbial model are evaluated with respect to the fort payne buildups none of the models is deemed adequate to explain all of the features of the buildups rather a combination of all three seems to be required to account fully for all of the characteristics noted in this study this multi faceted depositional model compares favorably with other carboniferous buildups worldwide suggesting commonalities that transcend the properties of the basins in which they grew

rates of processes in magmatic arcs implications for the timing and nature of pluton emplacement and wall rock deformation abstract the construction of arcs and in many cases the emplacement of plutons occur in tectonically active regions it is critical therefore to evaluate the rates of structural and magmatic processes when trying to understand the evolution of arcs and the associated pluton wall rock systems our best estimates of average rates or durations of processes in shallow to moderate level arc environments are the following 1 crystal growth rates in magma of 10 4 cm year 1 2 growth rates of metamorphic porphyroblasts between 10 5 and 10 2 cm year 1 3 long term magma supply rates of 10 1 km 3 year 1 and short term rates of up to 350 km 3 year 1 4 diapiric ascent rates for mafic plutons of 1 3 m year 1 5 cooling of plutons to ambient wall rock temperatures in 10 5 10 6 years 6 final crystallization of plutons in a small fraction of the time needed for complete cooling 7 fault displacements of 3 cm year 1 8 development of cleavages in fault zones in less than 10 6 years at strain rates of 10 13 s 1 or higher and 9 the development of regional cleavages in 10 6 years at strain rates of 10 14 s 1 these rates indicate that processes operating in magmatic arcs are relatively rapid pluton emplacement cleavage development etc occur over time spans of tens of thousands to no more than a few million years however the rate of ascent and crystallization of plutons at shallow levels is generally shorter than that needed to get large displacements on faults or widespread cleavages developed at deeper levels or in zones undergoing faster strain rates the time spans of the various processes approach one another thus plutons with otherwise similar characteristics emplaced in regions undergoing fast strain rates or at deeper crustal levels may appear quite different structurally from those emplaced at shallow levels or in regions undergoing slower strain rates comparison of these data also suggests that the rate at which wall rock deforms is the limiting factor controlling the rates of other structural processes during emplacement of plutons unless fast strain rates or multiple deformation mechanisms are considered thus emplacement mechanisms that rely on the transport of magma over 10 5 to 10 6 years are favored and need further consideration finally we argue that the structural and other characteristics of pluton wall rock systems will depend on rates of various processes involved and that these rates at the very least influence and sometimes invalidate the timing criteria previously published by us and others

geochemistry of mafic dykes in the antarctic peninsula continental margin batholith a record of arc evolution mafic dykes of the antarctic peninsula continental margin arc are compositionally diverse comprising calc alkaline dominant shoshonite tholeiite and oib like varieties their compositions give information about different mafic magma sources tapped during arc evolution the compositional groups represent partial melts of at least five distinct mantle sources a low nd subduction modified garnet bearing lithospheric mantle older calc alkaline a high nd subduction modified garnet bearing lithospheric mantle shoshonites a high nd subduction modified spinel bearing asthenospheric mantle younger calc alkaline e morb like spinel bearing asthenosphere depleted by a previous melting event tholeiites and within plate non subduction modified garnet and spinel bearing asthenosphere oib like slab derived fluids subducted sediment and arc crust also contributed to the magmas consideration of previous work in the light of our new compositional and geochronological data enables presentation of a summary of arc evolution for most of the cretaceous and tertiary the tectonic regime of the antarctic peninsula arc was transtensional and calc alkaline magmas intruded an oceanic spreading centre collided with the trench during the late cretaceous and induced tectonic changes which caused tapping of different magma sources a pulse of shoshonitic tholeiitic and oib like mafic magmatism resulted three ridge trench collisions are now recognized during the history of the arc in mid late jurassic late cretaceous and early mid tertiary times

global geochemical mapping and its implementation in the asia pacific region abstract igcp projects 259 international geochemical mapping and 360 global geochemical baseline under the aegis of unesco s international geochemical correlation program made recommendations for standardizing geochemical mapping protocols and for conducting a wide spaced sampling of the earth s land surface based on a global reference network of approximately 5000 160 160 km cells a pilot study has been conducted throughout china during which these recommendations were implemented during the pilot study 500 floodplain samples representing drainage basins ranging in area from 1000 to 6000 km2 were collected throughout china and analyzed the widely spaced sampling used for the pilot study was compared to the very dense sampling million samplings of sediment for all of china used for china s regional geochemical national reconnaissance program the geochemical maps generated from the wide spaced sampling are strikingly similar to those generated from data based on the detailed sampling such low density floodplain sampling presents a relatively low cost way to get a quick overview of the geochemistry of a large area of the earth s surface the implementation of this project in the asia pacific region and the problems encountered are discussed in the present paper

structural setting and timing of hydrothermal veins and breccias on hurd peninsula south shetland islands a possible volcanic related epithermal system in deformed turbidites quartz veins and vein breccias in a greywacke shale sequence of carboniferous triassic age were previously regarded as mesothermal silicified fault breccias and related to an adjacent eocene granodiorite pluton new mapping of vein assemblages and textures and their structural and cross cutting relationships demonstrates that the steeply dipping sheeted epithermal textured vein array was hydraulic in origin and possibly cretaceous in age the main vein and breccia swarm trends for 14 km nne along strike and 2 km across strike cutting large irregular areas of silicified and brecciated sandstone and patchy areas of pyritic propylitic and k feldspar alteration angular vein fabrics and hydraulic disruption textures indicate wedging by hydrothermal solutions hydraulic rupture brecciation and fragment transport followed by open space precipitation in veins generally 15 cm thick and breccias up to a few metres thick hydrothermal quartz chlorite calcite and chalcedony predominate with variable amounts of chalcopyrite galena sphalerite and pyrite epidote arsenopyrite k feldspar and andradite garnet are conspicuous in places breccias were pre and syn mineralization whereas mineral precipitation was pre syn and post breccia formation hydrothermal activity was simultaneous with extensional faulting striking nne and accompanied by intrusion of dacitic dykes there followed conjugate shearing on east and ese striking faults intrusion of high level tonalite stocks and several phases of basaltic andesite dyke intrusion these hypabyssal rocks were probably coeval with the antarctic peninsula volcanic group dominating livingston island dated between 130 and 75 ma minor copper and iron sulphide bearing veins occur in adjacent volcanic and hypabyssal intrusive rocks the hurd peninsula veins may therefore form part of a volcanic epithermal hydrothermal system adularia sericite quartz type of cretaceous age rather than a porphyry related system of eocene age