application and limitation of fission track geochronology to quarternary tephras abstract zircon and glass are the principal materials used for fission track dating of quaternary tephras this is because zircon is a common accessory mineral in rhyolitic tephras and it usually contains sufficient uranium to produce detectable quantities of spontaneous fission tracks over the time span of the quaternary volcanic glass though uranium poor is typically the dominant component of rhyolitic tephras comprising up to 99 of some distal tephra fall deposits low uranium contents however result in low spontaneous track densities that make the age measurement difficult and laborious a condition exacerbated by very young glasses in addition fission tracks in glass shards are unstable and can partially anneal at ambient temperatures therefore unless the glass age is corrected by plateau or isothermal heating experiments partial annealing causes a reduction in the measured age compared to the true age of the sample fission track dating has a large subjective component that can contribute systematic errors to the analysis microlite inclusions and some structural defects resemble fission tracks and if misidentified and included in the count of spontaneous tracks the calculated age would be too old the most prevalent source of systematic error is contamination in the natural environment for many tephras in the stratigraphic record are reworked tuffaceous sediments these deposits may contain detrital components that could mask the primary age of the tephra however by using the external detector fission track method it is possible to calculate ages of single mineral grains and to distinguish contaminants from primary phases on the basis of their age distribution this grain discrete capability is the major advantage of the fission track method over bulk geochronological systems

a laacher see eruption supplement to tephrabase investigating distal tephra fallout dynamics abstract tephrabase www tephrabase org is an online data repository and research tool for tephrochronological investigations in northern europe this paper presents and discusses an update to tephrabase that contains geographic and as far as available geochemical data on 452 data points for the widespread isochronous laacher see tephra this tephra is dated to c 12 920 calendar years bp and acts as an important chronostratigraphic marker for the lateglacial gi 1a late allerod chronozone in terrestrial and lacustrine palaeoenvironmental archives as well as in archaeological stratigraphies based on these data a revised map of the total tephra distribution is presented suggesting that the north eastern fallout lobe might contain a secondary fallout thickness maximum 230 340 km distant from the eruptive centre in addition new geochemical data from a series of distal tephra deposits spanning most of the north eastern fallout lobe are used to evaluate previous suggestions regarding the distribution of fallout from distinct eruption phases the aim of the study is that future tephrochronological investigations can use this database and the revised distribution map for effectively targeting sites for analysis

millennial scale phase relationships between ice core and mediterranean marine records insights from high precision 40ar 39ar dating of the green tuff of pantelleria sicily strait abstract with the advent of annually resolved polar ice records extending back to 70 ka marine and continental paleoclimate studies have now matured into a discipline where high quality age control is essential for putting on an equal pace layer counted timescale models and late quaternary sedimentary records high resolution u th dating of speleothem records and 40ar 39ar dating of globally recorded geomagnetic excursions have recently improved the time calibration of quaternary archives reflecting the cross disciplinary effort made to synchronize the geologic record at the millennial scale yet tie points with such an absolute age control remain scarce for paleoclimatic time series extending beyond the radiocarbon timescale most notably in the marine record far travelled tephra layers recorded both onland and offshore provide an alternative in such instance to synchronize continental and marine archives via high resolution 40ar 39ar dating of the parent volcanic eruption high resolution 40ar 39ar data are reported herein for one such volcanic marker the green tuff of pantelleria and its y 6 tephra equivalent recorded throughout the central and eastern mediterranean published radiochronometric and 18o orbitally tied ages for this marker horizon scatter widely from about 41 ka up to 56 ka our new 40ar 39ar age at 45 7 1 0 ka 2 reveals that previous estimates are biased by more than their reported errors would suggest including recent orbital tuning of marine records hosting the tephra bed that are reevaluated in the context of this study this improved estimate enables potential phase lags and leads to be studied between deep sea and terrestrial archives with unrivaled near millennial 40ar 39ar precision in the marine record

dissolution solution transfer diffusion versus fluid flow and volume loss during deformation metamorphism dissolution and solution transfer during deformation metamorphism are controlled by the partitioning of deformation into progressive shearing and shortening components progressive shearing is readily accommodated by slip on the planar crystal structure of phyllosilicates and graphite without accumulating dislocation density gradients across grain boundaries progressive shortening is accommodated by the cores of most other minerals including sulphides these minerals develop strain and hence dislocation density gradients on their rims due to progressive shearing along grain boundaries these gradients are particularly large when the mineral abuts phyllosilicate or graphite the resulting chemical potential gradients between the core and rim drive dissolution causing removal of the highly strained grain margins removal of dissolved material by solution transfer is aided by the geometry of shearing of phyllosilicates and graphite around other grains in an active anastomosing foliation interlayers and interfaces on boundaries lying at a low angle to the direction of shearing and oriented relative to the sense of shear such that they can open gape by small amounts water present in these interlayer spaces becomes destructured considerably enhancing diffusion rates along the foliation penetrative volume loss especially in deforming metamorphosing pelitic rocks is large at all metamorphic grades increasing and becoming more penetrative with depth to at least the transition into granulite and eclogite facies transference of material by fluid flow from deep to high levels in the earth s crust is precluded because thousands to tens of thousands of rock volumes of fluid are required necessitating continual recirculation of fluid from shallow to deep crustal levels in one large or several small sets of cells unless some extremely large scale form of fluid channelling is possible reassessment of diffusion mechanisms and hence rates during deformation and pervasive foliation generation in large volumes of rock where fluid channeling cannot provide enough fluid indicates that diffusion can proceed with sufficient rapidity that massive recirculation of fluid is no longer required the amount of fluid can be reduced sufficiently to allow large volume losses by a one way flow of fluid to the earth s surface in deforming metamorphosing environments where the fluid pressure equals or exceeds the hydrostatic pressure deformation partitioning controlled dissolution progressively changes the bulk chemistry of a rock containing phyllosilicates or graphite during deformation metamorphism because matrix minerals other than phyllosilicates and graphite are preferentially removed the large size of porphyroblasts if present tends to preserve them from dissolution hence the bulk chemistry operative during subsequent porphyroblast growth can have changed considerably from that operative when the first porphyroblasts grew in rocks in which bedding is still well preserved

an example of the use of veins to establish a cover fold history irregully formation western australia abstract structural correlation using fold overprinting and style plus the timing and orientation of possible syntectonic veins are used to interpret three minor folding events in part of the irregully formation the formation lies near the base of the middle proterozoic bangemall group in the bangemall basin of western australia it comprises interbedded well laminated dolostones and orthoquartzites metamorphosed under lower greenschist facies conditions bedding in the formation is folded about a gently nw plunging axis most mesofolds are consistent with the bedding distribution exhibiting a fold axis maximum plunging gently nw and vertical axial planes folds with ne e and nne trending axial planes are common in the area but most cannot be explained by reorientation of the dominant nw trending folds a deformation history accounting for different fold geometries is established using fold overprinting in conjunction with dilational offsets between fold related quartz and dolomite veins a number of approaches to determine the history of folding are possible fold overprinting is the most valuable criterion but in weakly deformed terrains it may not be easily recognised so that alternative methods should be examined fold directions in the irregully formation are parallel to trends in the underlying ashburton formation suggesting a degree of basement reactivation as a fold chronology in the cover has been established accounting for mesofolds and macrofolds the interpreted reactivation is believed to have involved compression rather than passive drape over earlier structures in the irregully formation differently oriented folds probably developed through a degree of cover shortening associated with reactivation of basement faults or folds

laser 40ar39ar ages of tephra from indian ocean deep sea sediments tie points for the astronomical and geomagnetic polarity time scales tephra layers within deep sea sediment cores are valuable stratigraphic markers which can be radiometrically dated with high precision by the laser 40ar39ar technique two neogene ash layers from ocean drilling program odp site 758 have been dated these layers are located within detailed oxygen isotope and paleomagnetic stratigraphies which have previously been assigned ages based on the astronomically derived geomagnetic polarity time scale adgpts 7 10 28 we use the radiometric dates from the ash layers to independently evaluate the ages assigned to oxygen isotope stage 19 1 and the paleomagnetic reversals marking the brunhes matuyama boundary and the termination of the nunivak event ash d 20 21 yields an age of 800 20 ka which is consistent with an adgpts age of 780 ka for the overlying brunhes matuyama transition and with the astronomical age for oxygen isotope stage 19 1 ash i located near the top of the nunivak subchron bears evidence of excess 40ar an error weighted mean of five plateau ages gives an apparent age of 4 519 012 ma but an errorchron through a subset of the data suggests a possible eruption age of 4 43 03 ma the plateau age is in agreement with the adgpts 7 10 28 and the errorchron age is in agreement with the time scale of baksi 29 the laser 40ar39ar ages of both ashes are significantly older than the ages from the radiometrically derived time scale rdgpts of mankinen and dalrymple 3 which is primarily based upon a large dataset of k ar dated paleomagnetic poles it is essential to reconcile the adgpts with the rdgpts because radiometric age tie points are the best hope of extending the adgpts to times before 5 ma an analysis of the expected and actual variance for the astronomical and mankinen and dalrymple 3 time scales suggests that only a handful of low k ar ages may have biased the original gpts it is not necessary to invoke general systematic errors in the ar dating system to explain the discrepancies between the rdgpts and adgpts

interpretation of a set of faults across the hinge and a limb of a large scale flexure in the mount isa district queensland australia in terms of fractures related to the folding process abstract a revised interpretation of a number of faults across the hinge and western limb of a large scale anticlinal flexure in the mount isa district has been made in terms of the faults following earlier formed be joints such joints often develop in weakly or moderately folded competent sediments as a result of either tensile stresses that were active at a late stage during folding or the influence of residual stresses generated during tectonic uplift the joints are oriented such that on a stereographic projection their poles plot parallel to the a axis of a fabric cross and at 90 to the fold axis b bc joints are thus approximately normal to bedding and contain the fold axis and hence they fan around the axial plane of the fold containing them across the hinge and western limb of a steeply n plunging large scale f 2 flexure in the mount isa district a number of faults at high angles to bedding fan about the axial plane making use of the fold geometry and local bedding orientation it is possible to predict the orientation of ideal bc fractures at locations within the fold these predictions fit well with the observed fault pattern the movement on the faults although apparently complex appears consistent with continued shortening perpendicular to an axial plane cleavage during the d 2 deformation or as part of a later d 2 deformation

classic and new dating methods for assessing the temporal occurrence of mass movements abstract establishing landslide chronologies is essential both for understanding causes of mass movements as part of fundamental geomorphological research and for the assessment of landslide hazards as part of applied geomorphology we discuss the applicability of different methods for determining landslide chronologies frequencies and dynamics in relation to the type of material being dated sampling location and type of mass movement the emphasis is on the possibilities and limitations of the different dating methods established methods such as radiocarbon dating lichenometry and dendrochronology are briefly discussed the paper focuses however on 1 new analytical techniques like accelerator mass spectrometry ams thermally ionising mass spectrometry tims and laser fusion which have led to significant improvements of established dating methods and on 2 new dating methods like optically stimulated luminescence osl dating and alpha recoil track art dating

mass transfer and preferred orientation development during extensional microcracking in slate belt folds elura mine australia microstructures in slate belt rocks at the elura mine near cobar south eastern australia indicate that volume loss by syntectonic dissolution is coupled with mass accretion by reprecipitation of the dissolved material in dilational sites the mass accretion is sustained primarily by repetitive tensile microfracturing at high pore fluid pressures oriented growth in the inter and intragranular microcracks is locally host controlled creating lattice and shape preferred orientations the grain scale crack seal features throughout the rock reflect rhythmic fluid pressure fluctuations a balance is achieved between the fracture induced permeability and consequent flushing rates and the rate of fluid build up in a relatively sealed environment instability in the balancing factors can lead to localization and intensification of tensile failure and hence tension vein formation in the grain aggregate growth of veins by crack seal also reflects a steady state but with more localized fluctuations of fluid flow on the aggregate scale still larger imbalances between flushing and fluid accumulation i e pressure variations induce breccia veining the larger pressure gradients over greater distances associated with dilation localization from pervasive microfracturing to spaced breccia domains allow fluid channelling with an increased potential for chemical fluid rock disequilibrium therefore large breccia vein systems tend to be sites of extensive fluid rock interaction and replacement as spectacularly illustrated by the syntectonic sulphide orebodies at elura the huge amounts of silicate carbonate and sulphide accumulated during folding at elura illustrate the large scale of source and sink couples possible in solute mass transfer

solution transfer syn s2 an inferred means of deriving fault fill in the lake moondarra area mt isa queensland australia based on oxygen isotope results many of the faults within the lake moondarra area cropout as wide zones which are filled with massive or fibrous milky quartz additionally a variety of rock fragments from the surrounding metasediments are present in the fault zones the second regional slaty cleavage s2 is locally observed through the quartz and country rock fragments within some of the zones indicating that these fault fills originated pre or early syn s2 the absence of the first regional slaty cleavage s1 suggests that the fault fills developed post or late syn s1 evidence supporting an early syn s2 timing for the development of these fills is provided by the results of oxygen isotope analysis carried out on quartz specimens collected from the faults and also by the nature of the quartz specimens of quartz taken from the various fault zones have 18o values between 12 4 and 14 5 this suggests that metamorphic water isotopically equilibrated with connate formation water was the aqueous fluid which transported the silica 18o values for the silica in the quartz rich sometimes dolomitic metasediments of the mount isa and haslingden groups within the lake moondarra area are similar to the quartz within the fault fills the lack of any metamorphic event between the first and second deformation together with the sometimes fibrous nature of the fault filling material with fibres parallel to the mineral elongation in s2 i e l22 suggests that the quartz in the faults was derived syntectonically at grain to grain contacts in the metasediments within the lake moondarra area early in the development of s2 the quartz is believed to have moved by the process of solution transfer to the faults as they underwent dilation during d2 subsequently the quartz was precipitated as a result of a chemical potential gradient

single grain argon laser probe dating of phengites from the blueschist to greenschist transition on sifnos cyclades greece measurements of 40ar 39ar age spectra on single white mica crystals of tertiary age is a new means of obtaining detailed geochronological information of alpine blueschists internal consistency of the data set as well as excellent agreement with previous conventional k ar results demonstrates that this new technique can be used with confidence to obtain information on the thermal evolution of young metamorphic belts

direct dating of plio quaternary pumices by 40ar 39ar step heating and single grain laser fusion methods the example of the monts dore massif massif central france abstract k feldspars separated from plio quaternary pumice flows of the monts dore massif massif central france give 40 ar 39 ar saddle shaped age spectra laser probe analysis of hand picked single grains gives ages in agreement with those of overlying and underlying lava flows previously dated by the conventional k ar method the laser probe ages are lower than the minimum ages measured on the populations of grains by the step heating method as previously suggested by lo bello and co workers for the pumice flow of neschers belonging to the same volcanic massif the saddle shaped age spectra are ascribed to the different ar release patterns of two populations of k feldspars 1 young sanidine phenocrysts of the pumices and 2 old k feldspars plucked from the hercynian basement during the explosive eruption measurements of both the granitic rocks from surrounding areas and the granitic xenoliths included in pumices give ages around 330 ma and show that most of the xenocrysts included in the pumices did not lose significant amounts of argon during the eruption with the 40 ar 39 ar step heating method we were able to detect contamination of quaternary k feldspars by hercynian k feldspars as low as 0 25 because pumice flows of the monts dore massif crop out over large areas these new ages will be useful for establishing a precise stratigraphy of the european plio quaternary

incremental strain analysis using two generations of syntectonic coaxial fibres an example from the monte marguareis brianconnais cover nappe ligurian alps italy abstract composite calcite quartz fibres developed in strain fringes on pyrites in the monte marguareis cover nappe ligurian brianconnais northern italy are utilised for an incremental strain analysis classical methods require non coaxial fibres to reconstruct incremental strain histories as an innovation this contribution shows that two generations of coaxial fibres grew independently on different pyrite sites during the two major deformation phases d1 and d2 which affected the monte marguareis structural unit rare curved fibres are shown by meso and microstructural analyses to have grown during d1 and been deformed by d2 folds this peculiarity provides a way to estimate quantitatively the strain associated with these deformation phases without the uncertainties related to classical methods d1 strain shows a marked strain partitioning among different lithologies and within the same lithology the measured elongations ranging between 1 84 and 8 45 measurements of d2 incremental elongation are available only for one lithology and indicate that d2 stretching is approximately half that of d1 and its contribution to the finite strain ellipsoid cannot be neglected this finding is consistent with qualitative inferences on strain partitioning between d1 and d2 phases measured d2 elongations display higher homogeneity throughout the area both l1 fibre orientation generally ne sw and d1 fold asymmetry suggest d1 deformation occurred in a ductile shear zone possibly induced by the south westward motion of nappes consistently with regional tectonics the l2 lineation orientation approximately ne sw and the d2 folds asymmetry suggest that the d2 phase is likely to be the product of a regionally recognised backthrusting event

thrusting and duplex formation at mount isa queensland australia large scale repetition of mount isa and haslingden group rocks in the anticline north of mount isa is interpreted as the core of a thrust duplex which has been subsequently folded parallel to the direction of thrusting displacement of mount isa group rocks above the duplex is extremely significant to the regional geology and exploration for more mount isa style pb zn and cu orebodies interpretation of the geometry of the thrust belt indicates thrusting on different stratigraphical levels below and above the duplex during its formation horses formed from a heelwards protruding footwall ridge instead of a trough and the complex effects on stratigraphical cross sections through duplexes of sloping lateral footwall ramps on horses

40 ar 39 ar ages and stratigraphy of the latera caldera italy the latera caldera is a well exposed volcano where more than 8 km3 of mafic silica undersaturated potassic lavas scoria and felsic ignimbrites were emplaced between 380 and 150 ka isotopic ages obtained by 40ar 39ar analysis of single sanidine crystals indicate at least four periods of explosive eruptions from the caldera the initial period of caldera eruptions began at 232 ka with emplacement of trachytic pumice fallout and ignimbrite they were closely followed by eruption of evolved phonolitic magma after roughly 25 ky several phonolitic ignimbrites were deposited and they were followed by phreatomagmatic eruptions that produced trachytic ignimbrites and several smaller ash flow units at 191 ka compositionally zoned magma then erupted from the northern caldera rim to produce widespread phonolitic tuffs tephriphonolitic spatter and scoria bearing ignimbrites after 40 ky of mafic surge deposit and scoria cone development around the caldera rim a compositionally zoned pumice sequence was emplaced around a vent immediately northwest of the latera caldera this activity marks the end of large scale explosive eruptions from the latera volcano at 156 ka

40ar 39ar laser dating of single grains ages of quaternary tephra from the east eifel volcanic field frg four late pleistocene tephra deposits from the east eifel volcanic field have been dated by 40ar 39ar single grain laser fusion of k feldspar and phlogopite crystals this is a high speed and high precision technique which because it is grain specific can directly detect the presence of older contaminating material our results indicate that the huttenberg tephra from wehr volcano erupted 213 4 ka bp and that the rieden volcanic complex erupted at least over the period 410 to 470 ka bp essential rieden phonolite lapilli contain abundant sanidine with apparent ages older than the eruption age

40ar39ar geochronology of rhyolites erupted following collapse of the yellowstone caldera yellowstone plateau volcanic field implications for crustal contamination abstract single crystal laser probe 40 ar 39 ar dating of 133 grains of sanidine and plagioclase has enabled us to resolve the eruption ages of the upper basin member rhyolites the lava flows and related tuffs that erupted within the yellowstone caldera shortly after its collapse 630 ky ago on eruption of the lava creek tuff two lavas and a tuff that erupted from the eastern ring fracture zone yield an eruptive age of 481 8 ka whereas two flows from the western ring fracture zone yield eruptive ages of 516 7 and 198 8 ka most of the units contain old xenocrysts explaining why previous attempts at dating these earliest post caldera units by the conventional k ar method yielded poorly resolved and in some cases anomalous ages the tuff shows the most severe contamination grains from a single pumice lapilli in the tuff show as large an age range as those from bulk vitrophyre indicating that the xenocrysts were incorporated in the magma prior to its near surface explosive fragmentation diffusion calculations indicate that the xenocrysts could not have remained in the magma for more than a few years without degassing and giving ages indistinguishable from the phenocrysts thus the contamination represented by the xenocrysts probably occurred during fracturing and conduit propagation rather than during caldera collapse which took place more than 100 ky earlier the apparent ages of xenocrysts and their compositions as determined by electron microprobe suggest that the eocene absaroka volcanics are the main contaminant with a single xenocryst probably coming from precambrian basement rocks most of the xenocrysts are difficult to distinguish optically or chemically from feldspar phenocrysts illustrating the necessity of single crystal to date many young volcanic rocks accurately

fluid inclusions in crack seal veins at dugald river mount isa inlier implications for palaeostress states and deformation conditions during orogenesis abstract an integrated geometrical investigation and microthermometric analysis of fluid inclusion trails in fibrous crack seal veins at dugald river mount isa inlier allowed distinction of two general types of fluid inclusions a co 2 ch 4 rich variety and a h 2 o rich variety at least three stages of fluid percolation are identified to be related to the vein formation trails of pseudosecondary inclusions have a constant orientation relative to the direction of the regional maximum principal shortening direction 3 this relationship allows the trails to be regarded as potential structural markers however trails of secondary inclusions generally show a deviation from the established bulk shortening direction and possibly reflect the overprinting of a later deformation event microthermometric data combined with knowledge of metamorphic reactions suggest that the p t path of crackseal deformation started at about 450 c and 2 8 kbar then crossed 340 c and 1 2 kbar and possibly terminated around 130 c and at near surface pressure corresponding to the regional crustal uplift this study shows the importance of combined geometrical and microthermometric studies of fluid inclusions for understanding the physical conditions of crack seal deformation a common phenomenon in low grade metamorphic terrains

proterozoic low pressure high temperature metamorphism and an anticlockwise p t t path for the hazeldene area mount isa inlier queensland australia in the hazeldene area situated in the mount isa inlier queensland the metamorphic grade changes from chlorite zone through biotite and cordierite zones to sillimanite k feldspar zone microstructural studies of rocks near the sillimanite isograd demonstrate that cordierite grew early during the development of a steep foliation s2 was replaced by biotite andalusite and sillimanite at the metamorphic peak late in s2 and in turn by kyanite chlorite adjacent to localized small post d2 shear zones although the anticlockwise p t t path is well defined the precise p t conditions are uncertain because of problems with experimental and thermodynamic data the best estimate for the metamorphic peak for rocks close to the sillimanite isograd is around 600 c at 4 kbar the metamorphism has been dated at 1544 ma and was synchronous with a major crustal shortening event because proposed extensional events occurred more than 60 ma earlier their contribution to the peak metamorphic thermal perturbation would have been insignificant the syn metamorphic mica creek pegmatites the abundance of high heat producing elements in the nearby pre d2 sybella granite and advective heat by fluids which caused considerable metasomatism in the hazeldene area may have each contributed to the thermal budget however the metamorphic thermal gradient may be 80 c km 1 or higher strongly suggesting a local magmatic control as none are known in the area such syn metamorphic plutons would have to lie beneath the exposed high grade rocks

rheology of the lithosphere selected topics we review recent results concerning the rheology of the lithosphere with special attention to the following topics 1 the flexure of the oceanic lithosphere 2 deformation of the continental lithosphere resulting from vertical surface loads and forces applied at plate margins 3 the rheological stratification of the continents 4 strain localization and shear zone development and 5 strain induced crystallographic preferred orientations and anisotropies in body wave velocities we conclude with a section citing the 1983 1986 rock mechanics literature by category

isotopic and structural responses of granite to successive deformation and metamorphism within the sybella granite west of mount isa in northern australia differential response of u pb and rb sr isotopic systems to deformation metamorphism enables ages of emplacement and three discrete deformation metamorphic events to be determined based on zircon u pb systems two of the plutons were emplaced at 1671 pm 8 ma and 1668 pm 26 21 ma and the third at 1610 pm 10 ma a separate microgranite body gives aberrantly old u pb ages of up to 1780 ma due to partial inheritance of older zircon the three discrete deformation metamorphic events d 1 d 2 and d 3 occurred at 1610 pm 13 ma 1544 pm 12 ma and 1510 pm 13 ma respectively based on whole rock rb sr isotopic systems the youngest body was emplaced syntectonically and has possible significance to the origin of the mount isa ag pb zn orebodies homogenization of rb and sr isotopes via extensive syntectonic fluid migration associated with deformation partitioning and resultant solution transfer appears to have been

high precision 40ar 39ar sanidine geochronology of ignimbrites in the mogollon datil volcanic field southwestern new mexico 40ar 39ar age spectra have been obtained from 85 sanidine separates from 36 ignimbrites and one rhyolitic lava in the latest eocene oligocene mogollon datil volcanic field of southwestern new mexico of the 97 measured age spectra 94 yield weighted mean plateau ages each giving single spectrum 1 precision of 0 25 0 4 0 07 0 14 ma replicate plateau age determinations for eight different samples show within sample 1 precisions averaging 0 25 plateau ages from multiple n 3 8 samples of individual ignimbrites show 1 within unit precision of 0 1 0 4 0 04 0 13 ma this within unit precision represents a several fold improvement over published k ar data for the same ignimbrites and is similar to the range of precisions reported from single crystal laser fusion studies a further indication of the high precision of unit mean 40ar 30ar ages is their close agreement with independently established stratigraphic order two samples failed to meet plateau criteria apparently due to geologic contamination by older feldspars effects of minor contamination are shown by six other samples which yielded slightly anomalous plateau ages 40ar 39ar plateau ages permit resolution of units differing in age by 0 5 0 15 ma or less this high resolution combined with paleomagnetic studies has helped to correlate ignimbrites among isolated ranges and has allowed development of an integrated timestratigraphic framework for the volcanic field mogollon datil ignimbrites range in age from 36 2 to 24 3 ma ignimbrite activity was strongly episodic being confined to four brief 2 6 m y eruptive episodes separated by 1 3 m y gaps ignimbrite activity generally tended to migrate from the southeast toward the north and west

electrical resistivity structure of the eastern moroccan atlas system and its tectonic implications the electrical resistivity structure of the crust and upper mantle of the atlas mountain system was studied using magnetotelluric and geomagnetic deep soundings field experiments were done in eastern morocco along a traverse from the anti atlas to the rides rifaines in two campaigns in 1983 and 1988

the use of glass for dating volcanic ash by thermoluminescence the development of a new physical dating method for late pleistocene tephrochronometry is described here tests of laboratory procedures applied to glass from independently dated samples show that the thermoluminescence tl additive dose dating method can yield accurate ages for both proximal and distal tephra from a few hundred years to at least 400 ka the technique involves purification of 4 to 11 m glass by heavy liquid centrifugation pre tl readout heating at 50 75 c for 8 days and careful regression analysis of tl growth curves several unknown age tephra are dated directly by this method for the first time these include five ash beds from summer lake oregon 67 200 ka a sheep creek valley ash from alaska 81 9 ka an ash from nevada 200 50 ka correlated to the wadsworth tephra an unknown ash 210 30 ka from rye patch dam in nevada previously correlated to the 400 to 600 ka dibekulewe tephra and ash 9m from washtucna in eastern washington 250 30 ka

regional slaty cleavage formation and fold axis rotation by re use and reactivation of pre existing foliations the fiery creek slate belt north queensland abstract a deformation history comprised of six separate deformation events of differing intensity has affected the rocks of the south palmer river region of the the hodgkinson province north queensland within this region a zone of pervasive slaty cleavage herein termed the fiery creek slate belt has developed as a result of the superposition of fabrics formed during several of these events the most important processes in the formation of this composite cleavage were the re use and reactivation of the favourably oriented steep n s trending s2 foliation by the intense fourth deformation event d4 this produced micro meso and macroscopic folds in an originally shallow s3 foliation produced during the intervening d3 deformation with an axial planar s2 s4 foliation the d4 stretching lineation l44 plunges subvertically to steeply north and indicates that shear during d4 was oriented steeply north south in the fiery creek slate belt d2 fold axes are interpreted to have formed in much shallower orientations than their present moderately n s plunging to subvertical orientations we consider this to be a result of d4 shear which caused variable degrees of rotation of d2 fold axes toward the d4 stretching lineation due to subparallelism of the bulk shortening directions of the d2 and d4 events near total destruction of the pre d4 foliations during slaty cleavage formation has produced a misleading impression of a simple deformation history there is no relationship between metamorphic grade and intensity of slaty cleavage development

40ar 39ar geochronology of post valles caldera rhyolites jemez volcanic field new mexico the40ar 39ar dating of postcollapse rhyolites in the valles caldera 1140 kyr defines five intervals during which rhyolite domes flows and pyroclastic rocks were emplaced from ring fracture vents previously ages were based on limited k ar dating and stratigraphic relations the 40ar 39ar dates are concordant with the previous dates for about half of the samples others have ages significantly older 7 25 despite evidence for small amounts of xenocrystic material eruptive activity occurred during relatively brief periods at 1133 973 915 800 787 557 521 and 300 170 kyr separated by quiescent intervals of 115 300 kyr isochrons defined by single crystal sanidine fusions indicate trapped 40ar 39ar ranging from 282 to 326 sanidine analyses yield reliable and consistent results whereas biotite and glass data may be ambiguous biotite may have low radiogenic yields contain variable amounts of extraneous argon or define apparent isochrons discordant with cogenetic sanidine unaltered unhydrated glass in one case yields an isochron concordant with coexisting sanidine whereas in another is undateable due to large amounts of non radiogenic argon the revised chronology has implications for the evolution of the jemez magmatic system as well as for the geomagnetic polarity timescale the discrete periods of volcanism relations among rhyolites erupted within these intervals and the duration of the intervening quiescent intervals are consistent with the generation evolution and eruption of several distinct magmas following caldera collapse in addition the 40ar 39ar data suggest revision of the brunhes matuyama polarity reversal boundary from the currently accepted 730 kyr to 780 kyr

isotope and chemical microsampling constraints on the history of an s type rhyolite san vincenzo tuscany italy fine scale heterogeneities in strontium isotope ratios 40ar 39ar ages and chemical com sition have been determined for individual mineral grains and enclaves in the rhyolites of san vincenzo italy the rhyolites are peraluminous with phenocrysts of alkali feldspar piagioclase biotite and cordierite and have previously been divided into two groups on the basis of whole rock major element chemistry group b lavas are distinguished from group a in having higher mgo and cao contents as well as containing two pyroxenes and chilled latite enclaves the group b lavas show textural evidence for d sequilib um such as resorption and sieve zones in feldspars laser fusion 40ar 39ar dating of individual alkali feldspar crystals suggests that the eruption of these rhyolites took place 4 38 0 04 ma though small amounts of excess argon are present in subhedral and cloudy crystals the groundmass in the group a sample has a higher initial r7sr 86sr 0 725 than that of the group b 0 7 13 the sr isotope compositions of chemically and texturally characterized individual grains reveal marked disequilibrium between minerals and the host glass as well as within grain zonation the feldspars and coarse grained biotites have initial 87sr 8 sr that are intermediate between those of the group a and b glasses with alkali feldspar generally having higher initial 87sr 86sr ratios than plagioclase chilled latite enclaves which are present in only the group b lavas have relatively low initial 8 sr 86sr ratios in the range 0 7082 0 7088 the mineral chemistry and isotope data indicate that restite does not form a significant portion of the crystalline assemblage and therefore cannot be the source of the strontium isotope dis equilibrium xenocrysts mixed into the rhyolite from a mafic magma were identified as clinopyroxene megacrysts clinopyroxene orthopyroxene clots orthopyroxene plagioclase clots and plagioclase with extensive sieve textures these grains all have relatively low initial 87sr 86sr ratios other grains that may be xenocrysts are cordierites with cores containing inclusions of biotite spinel and sillimanite and alkali feldspars with excess 4 ar the strontium isotope di quilib um in phenocrysts of plagioclase alkali feldspar and biotite is attributed to a changing melt chemistry that was too rapid for strontium diffusive equilibration a mode1 for the san vincenzo magma chamber is proposed in which the sr isotope evolution of the group b rhyolites is due to magma mixing of a crustal melt with latite magma containing unra diogenic strontium the variable initial strontium isotope compositions of group a phenocrysts may be due to assimilation or crystallization along the interface between mingled group a and b magmas

evolution of the quaternary melitite nephelinite herchenberg volcano east eifel the quaternary herchenberg composite tephra cone east eifel fr germany with an original bulk volume of 1 17 107 m3 dre of 8 2 106 m3 and dimensions of ca 900 600 90 m length width height erupted in three main stages a initial eruptions along a nw trending 500 m long fissure were dominantly vulcanian in the northwest and strombolian in the southeast removal of the unstable underlying 20 m thick tertiary clays resulted in major collapse and repeated lateral caving of the crater the northwestern lower cone 1 lc1 was constructed by alternating vulcanian and strombolian eruptions b cone building mainly strombolian eruptions resulted in two major scoria cones beginning initially in the northwest cone 1 and terminating in the southeast cones 2 and 3 following a period of simultaneous activity of cones 1 and 2 lapilli deposits are subdivided by thin phreatomagmatic marker beds rich in tertiary clays in the early stages and devonian clasts in the later stages three dikes intruded radially into the flanks of cone 1 c the eruption and deposition of fine grained uppermost layers phreatomagmatic tuffs accretionary lapilli and strombolian fallout lapilli presumably from the northwestern center cone 1 terminated the activity of herchenberg volcano the herchenberg volcano is distinguished from most strombolian scoria cones in the eifel by 1 small volume of agglutinates in central craters 2 scarcity of scoria bomb breccias 3 well bedded tephra deposits even in the proximal facies 4 moderate fragmentation of tephra small proportions of both ash and coarse lapilli bomb size fraction 5 abundance of dense ellipsoidal juvenile lapilli and 6 characteristic depositional cycles in the early eruptive stages beginning with laterally emplaced fine grained xenolith rich tephra and ending with fallout scoria lapilli herchenberg tephra is distinguished from maar deposits by 1 paucity of xenoliths 2 higher depositional temperatures 3 coarser grain size and thicker bedding 4 absence of glassy quenched clasts except in the initial stages and late phreatomagmatic marker beds and 5 predominance of strombolian cone building activity the characteristics of herchenberg deposits are interpreted as due to a high proportion of magmatic volatiles dominantly co2 relative to low viscosity magma during most of the eruptive activity

oxygen isotope disequilibrium between quartz and sanidine from the bandelier tuff new mexico consistent with a short residence time of phenocrysts in rhyolitic magma abstract oxygen isotope analyses are reported from co existing quartz and feldspar from the bandelier tuff and cerro toledo high silica rhyolitic pyroclastic deposits erupted from the valles caldera new mexico quartz shows little variation outside analytical error but 18 o in feldspar varies over 1 in most samples 18 o 16 o fractionation between quartz and feldspar is significantly less than is predicted for equilibrium at temperatures appropriate for rhyolitic magma in the tshirege upper member of the bandelier tuff isotopic fractionation between mineral pairs is close to equilibrium in the later erupted ignimbrite but non equilibrium in the initial plinian deposit these relationships are interpreted in terms of a model where most phenocrysts are derived from a highly porphyritic carapace around the magma chamber that was disrupted by eruption thus scattering crystals throughout the magma carapace quartz and feldspar are initially isotopically lighter than the bulk aphyric magma due to chemical communication with low 18 o country rock in the meteoric hydrothermal system surrounding the chamber we assume that quartz and feldspar were initially in isotopic equilibrium diffusive re equilibration of crystals begins when the carapace disintegrates and the minerals are immersed in the bulk magma just prior to and during eruption feldspar is isotopically lighter than quartz at equilibrium but responds more rapidly than quartz to an external change due to a higher diffusion coefficient for oxygen hence immersion in the isotopically heavier bulk magma causes feldspar and quartz 18 o values to initially converge over 10 2 years and then diverge over 10 3 10 4 years as first feldspar and then quartz re equilibrate with the new magma higher 18 o variability of feldspar than quartz indicates that the shorter timescale applies to the bandelier and cerro toledo rhyolites two important implications of this interpretation are 1 that the bandelier magmas developed in an aphyric condition and their porphyritic character is an artifact of eruption and 2 that a protective mechanically rigid cognate carapace around a silicic magma chamber may limit interaction with low 18 o hydrothermally altered crust thus hindering the development of significant volumes of low 18 o silicic magma

syntectonic vein and fibre growth associated with multiple slaty cleavage development in the lake moondarra area mount isa australia abstract a structural interpretation of syntectonic quartz veins has been made within a group of low grade metasediments four vein sets were distinguished using the orientation of veins and the fibres they contained dilational relationships indicate the existence of two further sub parallel vein sets the six vein sets developed within the area can be assigned to two groups of three vein sets that are geometrically related the veins belonging to one of the groups display consistent dilational relationships indicating that they predate the veins of the other group the three vein sets of each group are related to regional slaty cleavages which are n s and nnw trending time relationships between veins of the two groups allows an overprinting relationship to be established between the cleavages and associated folds the two earliest vein sets developed orthogonal to the s 2 cleavage and lie perpendicular and parallel to the mineral elongation lineation l 2 2 they both contain quartz fibres which grew respectively parallel and perpendicular to l 2 2 within the s 2 plane a third set of veins developed parallel to s 2 and post date the earlier vein sets these latter veins are believed to be a result of relaxation of the d 2 stress field the same geometrical relationship as that described above is apparent between the second group of three vein sets and the d 3 strain ellipsoid cross cutting relationships indicate that extension occurred at the same time in the two vein sets with fibres parallel to the x and y axes of the s 3 strain ellipsoid fibre curvature of a syntaxial nature observed within some d 2 veins is believed to be a result of later deformation effects or further dilation of d 2 veins during d 3

vein geometry in the hilton area mount isa queensland implications for fluid behaviour during deformation abstract folding and shear zone movement occurred synchronously in rocks of the mid proterozoic mount isa group in the hilton area northwest queensland syndeformational veins developed in shear zones and in sets spatially associated with shear zone intersections veins can be divided into the following two types 1 1 fibrous and non fibrous quartz dolomite veins which formed in orientations consistent with the regional e w subhorizontal shortening direction 2 2 fibrous and non fibrous quartz dolomite veins which formed parallel to shear planes s 3 cleavage and bedding indicating local syndeformational extension parallel to the regional shortening direction the complex geometry of the shear zones suggests that shear parallel veins formed as a result of movement over asperities during shear zone movement s 2 parallel veins on the other hand appear to have formed during restricted episodes of fluid overpressuring during post failure stress drops shear zones are divisible into highly graphitic and weakly graphitic end member types highly graphitic shear zones show abundant dissolution and little veining while weakly graphitic shear zones show abundant veining and little dissolution this indicates that highly graphitic shear zones were syndeformational fluid pathways while weakly graphitic shear zones were syndeformational fluid sinks