extensive felsic lavas and rheoignimbrites in the keweenawan midcontinent rift plateau volcanics minnesota petrographic and field recognition the 8 km thick sequence of plateau lavas north shore volcanic group in northeastern minnesota that was erupted during the formation of the 1 1 ga midcontinent rift system includes an unusually large proportion 10 25 of felsic flows rhyolites and icelandites individual flows are now recognized with estimated volumes up to 600 km3 and lateral extents up to 40 km some were probably originally much broader many are anomalously large compared to most silicic lava flows primary structures and textures of these essentially unmetamorphosed rocks indicate eruptive style for most of these large flows in particular the features of the basal meter or so and uppermost several meters show that some flows were emplaced as fluid lavas and others as strongly rheomorphic tuffs it may be essential in interpreting the origin of felsic flows elsewhere to examine petrographically the lowermost few decimeters and the top where the only evidence poorly to strongly welded shards may exist of the pyroclastic origin of a large otherwise lava like flow interior textures of these crystalline flows provide evidence of high temperature eruption especially in the form of abundant paramorphs of magmatic tridymite in the groundmass commonly arranged in a pilotaxitic flow structure based on structures textures field relations and geochemistry we conclude that a all the icelandites and some rhyolites were emplaced as large moderately to very fluid lava flows that spread widely while crystallizing tridymite and that b some large rhyolites which are only sparsely phyric were emplaced as pyroclastic flows subsequently remobilized as rheoignimbrites the mode of emplacement of one of the largest aphyric flows remains enigmatic vent areas or characteristics are unknown the low apparent viscosity of the erupted magmas is probably attributable to compositional factors f content high fe2 as well as to high temperatures and eruption rates estimated temperatures of about 1000 1100 c are associated with the low water content and perhaps great depth of origin of the magmas at the time of massive influx of hot matic melts from the mantle during continental rifting

crystallization and welding variations in a widespread ignimbrite sheet the rattlesnake tuff eastern oregon usa the 7 05 ma rattlesnake tuff covers ca 9000 km2 but the reconstructed original coverage was between 30000 and 40000 km2 thicknesses are remarkably uniform ranging between 15 and 30 m for the most complete sections only 13 of the area is covered with tuff thicker than 30 m to a maximum of 70 m the present day estimated tuff volume is 130 km3 and the reconstructed magma volume of the outflow is 280 km3 dre dense rock equivalent the source area of the tuff is inferred to be in the western harney basin near the center of the tuff distribution based mainly on a radial exponential decrease in average pumice size and is consistent with a general radial decrease in welding and degree of post emplacement crystallization rheomorphic tuff is found to a radius of 40 60 km from the inferred source four facies of welding and four of post emplacement crystallization are distinguishable they are non welded incipiently welded partially welded and densely welded zones and vapor phase pervasively devitrified spherulite and lithophysae zones the vapor phase pervasively devitrified and lithophysae zones are divided into macroscopically distinguishable subzones at constant thickness 20 3 m and over a distance of 1 3 km nonrheomorphic sections can cary between two extremes a entirely vitric sections grading from nonwelded to incipiently welded and b highly zoned sections highly zoned sections have a basal non to densely welded vitric tuff overlain by a spherulite zone that grades upward through a lithophysae dominated zone to a zone of pervasive devitrification which in turn is overlain by a zone of vapor phase crystallization and is capped by partially welded vitric tuff a three dimensional welding and crystallization model has been developed based on integrating local and regional variations of 85 measured sections strong local variations are interpreted to be the result of threshold governed welding and crystallization controlled by residence time above a critical temperature which is achieved through differences in thickness and accumulation rate

widespread lavalike silicic volcanic rocks of trans pecos texas several silicic units of the trans pecos volcanic field have outcrop and thin section scale features of lava flows but areal extents and aspect ratios of ignimbrites these voluminous rocks up to hundreds of cubic kilometres per unit are quartz trachytes to low silica rhyolites 68 to 72 sio 2 lava flow features include flow banding and folding elongated vesicles and autobreccias and vitrophyres at the base and top of units pyroclastic flow features include sheetlike geometry lateral extents up to 70 km aspect ratios as low as 1 700 and areal extents up to 3000 km 2 a few of these units are clearly rheomorphic ignimbrites but others show no unambiguous evidence of a primary pyroclastic origin although no adequate explanation currently exists for the origin of the latter we evaluate two end member hypotheses 1 they are ignimbrites in which extreme rheomorphism has obliterated primary internal features and 2 they are highly viscous lavas with unusually high heat retention or effusion rates that allowed them to spread over great areas either origin requires a rock type and eruptive mechanism not commonly recognized

on estimating crystal shape for crystal size distribution analysis abstract the accurate determination of three dimensional crystal habit long intermediate short axis is a requirement to enable correct stereological conversion of 2d crystal size measurements to true 3d crystal size distributions csds in this contribution we introduce a database and spreadsheet program which provides an objective estimate of true crystal habit from raw 2d measurements the database compares the sample s 2d measurements with 2d shape curves for random sections through 703 different habits the output gives the five best match curves and corresponding crystal habits based on a least squares fit between sample and database the minimum sample size required to give good shape estimates from 2d data is tested using random runs of sections through known shapes with different population densities at minimum 75 crystal sections are required to robustly determine crystal habit for csd measurements if crystals are tabular in shape with more acicular shapes requiring a minimum of 250 sections suggesting a sample size of 250 sections to be used for shape determination in natural examples where the true 3d shape is unknown the crystal habit of the best match shape can then be used to provide a more robust csd analysis which in turn can be used to investigate magmatic processes with more certainty

welding insights from high temperature analogue experiments abstract the rheological behavior of pyroclastic deposits during welding is incompletely understood and is based on a surprisingly small number of experimental studies here we present results from a new experimental apparatus comprising an automated uniaxial compression load frame that can run constant load up to 1150 kg or constant displacement rate 10 6 to 0 25 cm s tests at elevated temperatures 1100 c deformation experiments were performed on pre fabricated cylinders 4 5 cm diameter 6 cm length of soda lime silica glass beads n 32 sintered rhyolite ash n 7 and cores of pumiceous rhyodacite n 6 experimental runs used strain rates from 10 5 to 10 3 s 1 and stresses of 0 to 5 24 mpa temperatures varied from 535 to 650 c for experiments on soda lime silica glass beads and 825 to 950 c for natural materials in all cases experimental cores showed a strain dependent rheology that is more strongly affected by temperature than by load or strain rate results from these experiments are used to develop a relationship in which the effective viscosity e of the experimental cores is predicted by e o exp 5 3 f 1 f where 0 is melt viscosity and f is sample porosity this rheological model provides a means for exploring the relative roles of emplacement temperature load and glass transition temperature in welding of pyroclastic deposits

voluminous lava like precursor to a major ash flow tuff low column pyroclastic eruption of the pagosa peak dacite san juan volcanic field colorado abstract the pagosa peak dacite is an unusual pyroclastic deposit that immediately predated eruption of the enormous fish canyon tuff 5000 km3 from the la garita caldera at 28 ma the pagosa peak dacite is thick to 1 km voluminous 200 km3 and has a high aspect ratio 1 50 similar to those of silicic lava flows it contains a high proportion 40 60 of juvenile clasts to 3 4 m emplaced as viscous magma that was less vesiculated than typical pumice accidental lithic fragments are absent above the basal 5 10 of the unit thick densely welded proximal deposits flowed rheomorphically due to gravitational spreading despite the very high viscosity of the crystal rich magma resulting in a macroscopic appearance similar to flow layered silicic lava although it is a separate depositional unit the pagosa peak dacite is indistinguishable from the overlying fish canyon tuff in bulk rock chemistry phenocryst compositions and 40ar 39ar age the unusual characteristics of this deposit are interpreted as consequences of eruption by low column pyroclastic fountaining and lateral transport as dense poorly inflated pyroclastic flows the inferred eruptive style may be in part related to synchronous disruption of the southern margin of the fish canyon magma chamber by block faulting the pagosa peak eruptive sources are apparently buried in the southern la garita caldera where northerly extensions of observed syneruptive faults served as fissure vents cumulative vent cross sections were large leading to relatively low emission velocities for a given discharge rate many successive pyroclastic flows accumulated sufficiently rapidly to weld densely as a cooling unit up to 1000 m thick and to retain heat adequately to permit rheomorphic flow explosive potential of the magma may have been reduced by degassing during ascent through fissure conduits leading to fracture dominated magma fragmentation at low vesicularity subsequent collapse of the 75 35 km 2 la garita caldera and eruption of the fish canyon tuff were probably triggered by destabilization of the chamber roof as magma was withdrawn during the pagosa peak eruption

groundmass crystallization of mount st helens dacite 1980 1986 a tool for interpreting shallow magmatic processes the 1980 1986 eruption of mount st helens volcano provides an unprecedented opportunity to observe the evolution of a silicic magma system over a short time scale groundmass plagioclase size measurements are coupled with measured changes in matrix glass plagioclase and fe ti oxide chemistry to document increasing groundmass crystallinity and thus to better constrain proposed physical models of the post may 18 1980 magmatic reservoir measurements of plagioclase microlite and microphenocryst sizes demonstrate that relatively rapid growth approximately 10 9 cm s of groundmass plagioclase occurred immediately subsequent to may 18 relatively rapid plagioclase growth continued through the end of 1980 at an average rate of 3x10 11 cm s plagioclase growth rates then decreased to 1x10 11 cm s through 1986 changes in groundmass crystallinity are reflected in changes in both matrix glass and plagioclase microphenocryst rim chemistry although the matrix glass composition appears to have remained approximately constant from 1981 1986 after a rapid compositional change from may 18 until the end of 1980 plagioclase microphenocrysts show increasingly more complex zoning patterns with time microphenocryst core compositions are commonly positively correlated with crystal size both of these observations indicate continuous groundmass plagioclase growth through 1986 magmatic temperatures estimated from fe ti oxide pairs are approximately constant through 1981 at eruption temperatures of 930 c and at log fo2 of 10 8 by 1985 1986 oxide temperatures decreased to 870 c chemical and textural changes can be explained by 1 rapid degassing and crystallization in response to the intrusion of magma into a shallow 4 5 km reservoir toward the end of the may 18 1980 eruption 2 continued crystallization at a much reduced rate through 1986 due to slow cooling of the shallow magma reservoir growth rates and consequent chemical changes appear to decrease at the end of 1980 this is coincident with the change in eruption style from explosive eruptions sometimes followed by dome growth to solely extrusive dome building events and can be explained by the expected viscosity increase of both degassing and increasing crystallinity the model of twostage crystallization of magma in a shallow reservoir is consistent with conclusions from gas studies casadevall et al 1983 gerlach and casadevall 1986 a b patterns of crater deformation chadwkck et al 1988 and post 1980 seismicity endo et al 1990 although it does not explain the experimental data of hill and rutherford 1989 on the growth rate of amphibole reaction rims textural measurements on mount st helens dacite can also be used to evaluate crystallization kinetics in silicic magmas systems for which experimental data is almost non existent plagioclase growth rates are 5 10 times slower than estimated plagioclase growth rates in basaltic systems a result consistent with the higher viscosity of a more silicic melt furthermore patterns of textural change both average crystal size and number density are similar to those observed during the 1984 mauna loa eruption by lipman and banks 1987 suggesting that the only modification to the crystallization behavior of plagioclase required in extrapolation from basaltic systems is a moderate decrease in rates such that the rate of crystallization scales with the melt viscosity

phase equilibrium geobarometers for silicic rocks based on rhyolite melts part 3 application to the peach spring tuff arizona california nevada usa establishing the depths of magma accumulation is critical to understanding how magmas evolve and erupt but developing methods to constrain these pressures is challenging we apply the new rhyolite melts phase equilibria geobarometer based on the equilibrium between melt quartz and two feldspars to matrix glass compositions from peach spring tuff arizona california nevada usa high silica rhyolite we compare the results to those from amphibole geothermobarometry projection of glass compositions onto the haplogranitic ternary and glass sio2 geobarometry quartz 2 feldspar rhyolite melts pressures span a relatively small range 185 230 mpa consistent with nearly homogeneous crystal compositions and are similar to estimates based on projection onto the haplogranitic ternary 250 50 mpa and on glass sio2 255 275 mpa amphibole geothermobarometry gives much wider pressure ranges temperature independent 65 300 mpa temperature dependent 75 295 mpa amphibole only 80 950 mpa average anderson and smith am mineral 80 549 559 1995 blundy and holland contrib miner petrol 104 208 224 1990 or holland and blundy contrib miner petrol 116 433 447 1994 thermometer a b pressures are most similar to phase equilibria results 220 210 190 mpa respectively crystallization temperatures determined previously with rhyolite melts 742 c zr in sphene 769 20 c and zircon saturation 770 780 c geothermometry are similar but temperatures from amphibole geothermometry 450 955 c are notably different the average anderson and smith holland and blundy 1994 thermometer b 710 c temperature is most consistent with previous estimates the rhyolite melts geobarometer effectively culls glass compositions affected by alteration or analytical issues peach spring glass compositions that yield pressure estimates reveal a tight range of plausible na2o and k2o contents suggesting that low na2o and high k2o contents of many peach spring samples are due to alteration use of altered whole pumice compositions in rhyolite melts simulations is likely the cause of the incorrect crystallization sequence reported previously for peach spring compositions using the rhyolite melts geobarometer we estimate a more realistic composition for peach spring tuff high silica rhyolite and the calculated composition finds close matches with some analyzed rocks and results in the expected sequence of crystallization

textural evidence for high grade ignimbrites formed by low explosivity eruptions parana magmatic province southern brazil abstract the parana etendeka province is a lower cretaceous huge bimodal tholeiitic volcanic province 1 million km 3 that predated the gondwana breakup its silicic portion makes up a total volume of at least 20 000 km 3 and in southern brazil it comprises the chapeco porphyritic high ti trachydacites dacites and the palmas microporphyritic aphyric low ti dacites rhyolites the widespread silicic sheets are debated in the literature because they bear similarities between lavas and high grade ignimbrites here we provide new observations and interpretations for flow units with large dark and vesicle poor lens shaped blobs surrounded by a light colored matrix the textural features macro to micro scale of these blobs are different from typical pumice and or fiamme and support a low explosivity pyroclastic origin possibly low column fountain eruptions with discharge rates high enough to produce laterally extensive high grade ignimbrites such an interpretation combined with a conspicuous absence of lithic fragments in the deposits is aligned with a lack of identified calderas in the parana etendeka province maximum timescales of crystallization associated with the juvenile blobs and estimated from csd slopes are on the order of millennia for phenocryst populations and on the order of decades for microphenocryst populations

crystallization of mount st helens 1980 1986 dacite a quantitative textural approach quantitative measurements of crystal size distributions csds have been used to obtain kinetic information on crystallization of industrial compounds randolph and larson 1971 and more recently on hawaiian basalts cashman and marsh 1988 the technique is based on a population balance resulting in a differential equation relating the population densityn of crystals to crystal sizel i e at steady staten no exp l itg whereno is nucleation density g is the average crystal growth rate is the average growth time and the nucleation ratej nog csd inn vsl plots of plagioclase phenocrysts in 12 samples of mount st helens blast dacite and 14 samples of dacite from the 1980 1986 mount st helens dome are similar and averageg 9 6 1 1 10 3 cm andno 1 2 106 cm 4 reproducibility of the measurements was tested by measuring csds of 12 sections cut from a single sample in three mutually perpendicular directions precision of the size distributions is good in terms of relative but not necessarily absolute values 10 growth and nucleation rates for plagioclase have been calculated from these measurements using time brackets of 30 150 years growth ratesg are 3 10 10 12cm s and nucleation ratesj are 5 21 10 6 cm3 s g andj for fe ti oxides calculated from csd data areg 2 13 10 13 cm sec andj 7 33 10 5 cm3 s respectively the higher nucleation rate and lower growth rate of oxides resulted in a smaller average crystal size than for plagioclase sizes of plagioclase microlites 0 01 mm in the blast dacite groundmass have been measured from backscatter sem photographs nucleation of these microlites was probably triggered by intrusion of material into the cone of mount st helens in spring 1980 this residence time of 52 days gives minimum crystallization estimates ofg 1 3 10 11 cm s andj 9 16 1o3 cm3 s the skeletal form of the microlites provides evidence for nucleation and growth at high values of undercooling t relative to the phenocrysts a comparison of nucleation and growth rates for the two crystal populations phenocrysts vs microlites suggests that while growth rate seems to be only slightly affected by changes in t nucleation rate is a very strong function of undercooling a comparison of plagioclase nucleation and growth rates measured in the mount st helens dacite and in basalt from makaopuhi lava lake in hawaii suggests that plagioclase nucleation rates are not as dependent on composition groundmass textures suggest that plagioclase microphenocrysts crystallized at depth rather than in the conduit in the dome or after extrusion onto the surface most of this crystallization appears to be in the form of crystal growth coarsening of groundmass microphenocrysts at small degrees of undercooling rather than extensive nucleation of new crystals this continuous crystallization in a shallow magmatic reservoir may provide the overpressurization needed to drive the continuing periodic domebuilding extrusions which have been the pattern of activity at mount st helens since december 1980

tephra fountaining rheomorphism and spatter flow during emplacement of the pitigliano tuffs latera caldera italy abstract toward the end of the main ash flow phase ca 156 ka of the latera caldera central italy explosive eruptions near the nw caldera rim produced coarse grained tephra exposed in 35 m high faces of the toscapomice casa collina quarry the pyroclastic sequence fills a narrow sw ne paleovalley and records the near continuous eruption of a compositionally zoned magma batch white phonolitic pumice clasts abruptly grade upward to and partly intrude black spatter like pumice fragments also of phonolite composition this sequence in turn is overlain by densely welded tuff that grades upward to tephriphonolitic scoriae and ash the upper welded part of this sequence referred to as the vulcanite complessa di pitigliano has long been among the more enigmatic deposits in the latera area this sequence displays evidence for both fall and flow emplacement and is here interpreted as an ash fountain fed deposit that locally welded as spatter yet became sufficiently fluidized in distal parts to behave as a spatter rich pyroclastic flow this mechanism requires that the eruption underwent abrupt transition from a vigorously convecting column to a weakly convecting ash fountain and that this period of explosive activity culminated with unusually high accumulation rates the rapidly emplaced tephra retained sufficient heat to weld by compaction and undergo rheomorphism close to the vent yet contained high enough proportions of fluidized dust to have flowed outward along the northern caldera rim and eventually to have welded by vapor phase sintering

the etendeka igneous province magma types and their stratigraphic distribution with implications for the evolution of the parana etendeka flood basalt province detailed geochemical and field data for the volcanic sequence and intrusions of the etendeka igneous province are used to construct a stratigraphic framework for petrogenetic interpretation of the evolution of the etendeka parana continental flood volcanic event geochemical and petrographic characterization of over 1 000 analyzed samples allows 8 mafic and 17 silicic magma types to be recognized both silicic and mafic types can be grouped into high ti and low ti suites on the basis of elevated ti relative to other elements the mafic magmas are khumib high ti tafelberg kuidas horingbaai huab tafelkop albin and esmeralda all low ti amongst the silicic types the goboboseb springbok wereldsend grootberg and beacon low ti quartz latites and the nil desperandum high ti latite have been described previously in addition the hoas low ti nadas sechomib and hoarusib all high ti latites and the fria low ti sarusas ventura khoraseb naude and elliott all high ti quartz latites are described for the first time here there is a marked provinciality in the distribution of the high and low ti suites with the former concentrated in the northern etendeka region and the latter dominant in the southern etendeka stratigraphic distribution of magma types allows two new formations to be defined in the northern etendeka the khumib formation of basaltic flows and the skeleton coast formation dominated by silicic sheets the geochemical provinciality hampers precise correlations between northern and southern etendeka available evidence suggests that the lower part of the awahab formation in the southern etendeka is coeval with the lower part of the khumib formation and that the silicic units in the upper part of the tafelberg formation probably correlate with the skeleton coast formation the paucity of khumib dykes in relation to tafelberg dykes and their field relationships with regard to the volcanic sequence in the northern etendeka suggests that the main khumib eruptive centers lay further north consistent with southward thinning of the khumib basalts in the southern etendeka the doros complex is the eruptive center of the plume derived tafelkop basalts which probably built a shield volcano within the regional flood lava field this work indicates that the etendeka has a greater variety of both silicic and low ti mafic magma types than are known from the se parana detailed comparisons reveal that all the important silicic types in the parana have geochemical equivalents in the etendeka new correlations are santa maria fria ourinhos khoraseb guarapuava sarusas thus extending the previously recognized correlations between southern etendeka quartz latites and the caixas do sul and anita garibaldi rhyolites these correlations emphasize that very large volume silicic systems develop with pronounced lithospheric rifting in continental flood basalt provinces

stratigraphy and volcanic facies architecture of the torres syncline southern brazil and its role in understanding the parana etendeka continental flood basalt province abstract the torres syncline is a large structure that constitutes the eastmost outcrop of the parana etendeka cfb in south american side and this work focuses the stratigraphy and facies architecture of the volcanic pile in the syncline the volcanic sequence along the study area permits the division of three regions main valley intermediate zone and south hinge each of them with distinct stratigraphy which probably reflects the structural evolution of the syncline the stratigraphy of the torres syncline is composed by 1 botucatu palaeoerg 2 basic volcanic episode i 3 basic volcanic episode ii 4 acidic volcanic i 5 basic volcanic episode iii and 6 acidic volcanic episode ii the five volcanic episodes recognized in study area can be related to five volcanic facies architecture compound braided tabular classic tabular lobate escoriaceous dome field acidic lavas and tabular flows acidic lavas the basic episode i is composed by pahoehoe flows with a compound braided facies architecture that covered the botucatu palaeoerg the basic episode ii is a tabular classic facies architecture predominantly composed by simple flows 10 20 m thick reaching the total thickness of 500 m in main valley the acidic episode i is exposed in main valley and south hinge and is composed by acidic lavas forming lava dome field facies architecture with a thickness of 150 m the basic episode iii is predominantly constituted by a flows with tabular lobate escoriaceous facies architecture the acidic episode ii is constituted by tabular flow volcanic facies acidic flows and outcrops all along the study area the torres syncline constitute the eastmost on shore exposures of the parana etendeka cfb in south american side and detailed stratigraphic volcanological and structural studies in these area coupled with correlation with huab basin nw namibia africa will aim the understanding of the gondwana breakup process and the early stages of the south atlantic margin opening

compaction and gas loss in welded pyroclastic deposits as revealed by porosity permeability and electrical conductivity measurements of the shevlin park tuff pyroclastic flows produced by large volcanic eruptions commonly densify after emplacement processes of gas escape compaction and welding in pyroclastic flow deposits are controlled by the physical and thermal properties of constituent material through measurements of matrix porosity permeability and electrical conductivity we provide a framework for understanding the evolution of pore structure during these processes using data from the shevlin park tuff in central oregon united states and from the literature we find that over a porosity range of 0 70 matrix permeability varies by almost 10 orders of magnitude from 10 20 to 10 11 m 2 with over three orders of magnitude variation at any given porosity part of the variation at a given porosity is due to permeability anisotropy where oriented core samples indicate higher permeabilities parallel to foliation horizontally than perpendicular to foliation vertically this suggests that pore space is flattened during compaction creating anisotropic crack like networks a geometry that is supported by electrical conductivity measurements we find that the power law equation k 1 1 3 10 21 5 2 provides the best approximation of dominant horizontal gas loss where k 1 permeability and porosity application of kozeny carman fluid flow approximations suggests that permeability in the shevlin park tuff is controlled by crack or disk like pore apertures with minimum widths of 0 3 and 7 5 m we find that matrix permeability limits compaction over short times but deformation is then controlled by competition among cooling compaction water resorption and permeable gas escape these competing processes control the potential for development of overpressure and secondary explosions and the degree of welding in the deposit processes that are applicable to viscous densification of volcanic deposits in general further the general relationships among porosity permeability and pore geometry are relevant for flow of any fluid through an ignimbritic host

snake river sr type volcanism at the yellowstone hotspot track distinctive products from unusual high temperature silicic super eruptions a new category of large scale volcanism here termed snake river sr type volcanism is defined with reference to a distinctive volcanic facies association displayed by miocene rocks in the central snake river plain area of southern idaho and northern nevada usa the facies association contrasts with those typical of silicic volcanism elsewhere and records unusual voluminous and particularly environmentally devastating styles of eruption that remain poorly understood it includes 1 large volume lithic poor rhyolitic ignimbrites with scarce pumice lapilli 2 extensive parallel laminated medium to coarse grained ashfall deposits with large cuspate shards crystals and a paucity of pumice lapilli many are fused to black vitrophyre 3 unusually extensive large volume rhyolite lavas 4 unusually intense welding rheomorphism and widespread development of lava like facies in the ignimbrites 5 extensive fines rich ash deposits with abundant ash aggregates pellets and accretionary lapilli 6 the ashfall layers and ignimbrites contain abundant clasts of dense obsidian and vitrophyre 7 a bimodal association between the rhyolitic rocks and numerous coalescing low profile basalt lava shields and 8 widespread evidence of emplacement in lacustrine alluvial environments as revealed by intercalated lake sediments ignimbrite peperites rhyolitic and basaltic hyaloclastites basalt pillow lava deltas rhyolitic and basaltic phreatomagmatic tuffs alluvial sands and palaeosols many rhyolitic eruptions were high mass flux large volume and explosive vei 6 8 and involved h2o poor low 18o metaluminous rhyolite magmas with unusually low viscosities partly due to high magmatic temperatures 900 1 050 c sr type volcanism contrasts with silicic volcanism at many other volcanic fields where the fall deposits are typically plinian with pumice lapilli the ignimbrites are low to medium grade non welded to eutaxitic with abundant pumice lapilli or fiamme and the rhyolite extrusions are small volume silicic domes and coulees sr type volcanism seems to have occurred at numerous times in earth history because elements of the facies association occur within some other volcanic fields including trans pecos texas etendeka parana lebombo the english lake district the proterozoic keewanawan volcanics of minnesota and the yardea dacite of australia

rheomorphism of welded tuffs abstract peralkaline welded tuffs from the islands of gran canaria canary islands and pantelleria italy show abundant evidence for post depositional flow it is demonstrated that rheomorphism or secondary mass flowage can occur in welded tuffs of ignimbrite and air fall origin the presence of a linear fabric is taken as the diagnostic criterion for the recognition of the process deposition on a slope is an essential condition for the development of rheomorphism after compaction and welding internal structures produced during rheomorphic flow can be studied by the methods of structural geology and show similar dispositions to comparable features in sedimentary slump sheets it is shown that secondary flowage can occur in welded tuffs emplaced on gentle slopes provided that the apparent viscosity of the magma is sufficiently low compositional factors favor the development of rheomorphism in densely welded tuffs of peralkaline type

quartz latite rheoignimbrite flows of the etendeka formation north western namibia the etendeka formation of north western namibia consists of a sequence of interbedded quartz latites and tholeiitic basalts and forms part of the karoo igneous province in southern africa the age of the etendeka formation is approximately 130 135 ma the quartz latites make up a significant proportion of the stratigraphic succession 25 of the total stratigraphic thickness and form as much as 60 of the outcrop area in the southern etendeka apart from some systematic differences between pitchstones and devitrified quartz latite largely explained by alteration processes individual quartz latite units exhibit remarkably uniform compositions with no significant vertical or lateral variation geochemistry can be used as a primary criterion for the correlation of major quartz latite units over much of the southern etendeka area enabling the reconstruction of the etendeka formation stratigraphy in this region individual quartz latite units occur as voluminous 400 2600 km3 widespread up to 8800 km2 sheet like deposits typically between 40 and 300 m thick each unit consists of basal main and upper zones the main zone generally constitutes over 70 of the thickness of the unit and typically consists of texturally featureless devitrified quartz latite in contrast the basal and upper zones of the flow are characterised by flow banding pitchstone lenses and breccia with rare occurrences of pyroclastic textures the quartz latites are sparsely porphyritic 10 phenocrysts with glassy or devitrified groundmass textures the phenocrysts consist of plagioclase pyroxene titanomagnetite and rare ilmenite pyroxene geothermometry indicates high 1000 1100 c temperatures of crystallisation which coupled with the absence or primary hydrous phases indicates that the quartz latites were relatively hot h2o undersaturated magmas the quartz latites display features common to both rhyolite lavas and ignimbrites and are clearly the products of an unusual eruption style the local preservation of pyroclastic textures and the broad areal extent of these units lead to the conclusion that the quartz latites are high temperature rheomorphic ignimbrites i e rheoignimbrites a combination of high eruption temperature and relatively low viscosity helps to explain the often completely welded and homogeneous textures observed in most quartz latite outcrops in the etendeka area

emplacement history of a low viscosity fountain fed pantelleritic lava flow abstract the strongly peralkaline 8 ka flow mayor island new zealand was emplaced as a low viscosity fountain fed lava flow the flow stratigraphy comprises from top to base a finely vesicular pumiceous carapace an upper obsidian layer central crystalline rhyolite a lower obsidian layer incorporating a crumble breccia above a fall lapilli unit rare relict clasts welded within the lowermost 0 5 m of the lower obsidian layer support a fountain fed origin for the 8 ka flow five physical parameters density porosity microlite size void and autoclast aspect ratios and water content were measured throughout the thickness of the lava flow and show that prolonged vesiculation occurred in the upper 33 of the flow thickness and that autoclast and pyroclast shapes and microlite lengths were influenced by the thermal conditions of the lava after eruption crystallisation of the flow interior commenced before the flow stopped moving compared with calc alkaline rhyolites the high flow temperature 950 c and peralkaline chemistry contributed to a less polymerised low viscosity melt log 6 4 pa s these data and structural observations of the 8 ka flow suggest that near complete homogenisation and remobilisation of spatter from a lava fountain occurred on the caldera rim producing a distally tapering lava sheet that also spilled back down the caldera wall towards the vent

distribution and significance of crystalline perlitic and vesicular textures in the ordovician garth tuff wales diverse spherulitic and granular crystalline fabrics perlitic textures and fabrics related to the growth and migration of vesicles occur in the garth tuff a largely welded ordovician ignimbrite defining the distribution of such textures helps to constrain the ignimbrite s cooling and degassing history suites of spherulitic and perlitic textures closely reflect variation in cooling rates seven facies are defined based on the style and intensity of crystallisation 1 a medium to coarsely crystalline equigranular facies 2 an intensely spherulitic facies 3 a sparsely spherulitic facies 4 a pectinate facies 5 a microcrystalline to cryptocrystalline equigranular facies 6 a lithophysal facies and 7 a transitional perlite pectinate facies textural changes from facies 1 to 5 reflect progressively higher cooling rates facies 1 occurs in proximal settings in the ignimbrite s core facies 2 to 5 successively envelop facies 1 with facies 2 becoming the dominant fabric in the ignimbrite s core in medial settings facies 5 is typically developed in the originally glassy perlitic zones at the ignimbrite s welded margins crystallisation under hydrous conditions is reflected by second boiling textures in the sporadically developed lithophysal facies the seventh facies reflects a subtle interplay between cooling hydration and crystallisation which locally prevented perlitic fracturing the distribution of amygdales reflects patterns of volatile migration and entrapment in the lower levels of the ignimbrite amygdales occur in irregular concentrations or rare subvertical pipe like structures pipe like structures attest to fumarolic activity while the ignimbrite was in a rheomorphic state amygdales are widespread and evenly distributed in the upper levels of the ignimbrite however the top of the welding profile is characterised by a thin poorly vesiculated originally vitrophyric horizon that abruptly caps an intense concentration of amygdales ductile and brittle fabrics developed during the upward migration of gas microscopic drag folds occur around some amygdales jigsaw fit to clast rotated breccias originated through both late stage pneumatic fracturing and autobrecciation vaporisation of water at the flow base provided a significant source of volatiles in addition to gas released during cooling and crystallisation secondary alteration has enhanced or modified some fabrics perlitic zones were susceptible to patchy chlorite sericite carbonate diagenetic alteration diagenesis and metamorphism have contributed to the infilling of vesicles

crystal size distribution csd in rocks and the kinetics and dynamics of crystallization ii makaopuhi lava lake crystal size distribution csd theory has been applied to drill core samples from makaopuhi lava lake kilauea volcano hawaii plagioclase and fe ti oxide size distribution spectra were measured and population densities n were calculated and analyzed using a steady state crystal population balance equation n n0 exp l g slopes on ln n versus crystal size l plots determine the parameter g a product of average crystal growth rate g and average crystal growth time the intercept is j g where j is nucleation rate known temperature depth distributions for the lava lake provide an estimate of effective growth time allowing nucleation and growth rates to be determined that are independent of any kinetic model plagioclase growth rates decrease with increasing crystallinity 9 9 5 4 10 11 cm s as do plagioclase nucleation rates 33 9 1 6 10 3 cm3 s ilmenite growth and nucleation rates also decrease with increasing crystallinity 4 9 3 4 10 10 cm s and 15 2 2 10 3 cm3 s respectively magnetite growth and nucleation rates are also estimated from the one sample collected below the magnetite liquidus g 2 9 10 10 cm s j 7 6 10 2 cm3 s moments of the population density function were used to examine the change in crystallization rates with time preliminary results suggest that total crystal volume increases approximately linearly with time after 50 crystallization a more complete set of samples is needed for material with 50 crystals to define the entire crystallization history comparisons of calculated crystallization rates with experimental data suggests that crystallization in the lava lake occurred at very small values of undercooling this interpretation is consistent with proposed thermal models of magmatic cooling where heat loss is balanced by latent heat production to maintain equilibrium cooling

magmatism and continental break up in the south atlantic high precision40ar 39ar geochronology abstract a detailed ar ar study of the parana etendeka continental flood basalts cfb has been undertaken using laser spot analysis data provide information not only on the age of the samples but also on the variability of the non radiogenic ar component and state of alteration the results indicate the parana etendeka cfb were erupted over 10 million years between 137 and 127 ma providing a minimum overall eruption rate of 0 1 km3 yr 1 this is an order of magnitude less than that previously proposed for this the deccan and siberian cfb provinces but it is similar to estimates for hawaii iceland and the columbia river cfb in detail the new ar ar analyses indicate that magmatism within the parana etendeka province migrated from nw to se prior to and during the opening of the south atlantic providing an explanation for the asymmetry of the cfb lavas about the south atlantic moreover chemically defined magma types were erupted at different times in different places and so within the paranathey may not be used as reliable chronostratigraphic units rather such magmatic units may reflect the extent of compositionally distinct source regions in the uppermost mantle and indicate that partial melting took place over a wide area under the paranabasin the nw se migration of the onset of magmatism might be interpreted as a plume trace but the inferred rate of movement is 3 times faster than that inferred from subsequent magmatism on the rio grande rise it is argued that the onset of magmatism reflected extensional tectonics normal to the ponta grossa dyke swarm given that magmatism preceded the main phase of rifting and that extension across the ponta grossa was moderate current plume models have difficulty in predicting sufficient melting within the asthenosphere

age of the ponta grossa dike swarm brazil and implications to parana flood volcanism the ponta grossa dike swarm pgds occurs in a nw trending 200 km wide zone exposed just east of the parana basin in southeastern brazil the predominantly basaltic dikes intrude crystalline basement paleozoic mesozoic sediments and rarely flows of the parana etendeka flood volcanic province pep the pgds resembles the failed arm of a rift rift rift triple junction related to the separation of south america and africa detailed geochemical studies of the dikes including major minor trace element and sr nd isotopic analyses indicate that they probably represent feeders for the voluminous phase of flood volcanism represented by relatively uncontaminated predominantly high tio2 lavas of the northern pep where lava accumulations reach 1700 m thick 40ar 39ar stepwise degassing data using both laser and radiofrequency induction furnace on plagioclase separates from eighteen dikes and one sill yield seventeen plateau ages three are between 120 7 1 3 ma and 125 8 0 6 ma and fourteen are clustered between 129 2 0 4 ma and 131 4 0 5 ma isochron ages are not significantly different from the plateau ages and plateau ages are adopted in all but two cases as being the most accurate age estimates the age probability distribution for the dominant pulse 131 4 0 4 to 129 2 0 4 ma shows a pronounced peak at 130 5 ma this distribution probably reflects the magma production history in the region the new geochronologic data are consistent with conclusions based on paleomagnetic and chemical stratigraphic data that the pgds is younger than the volumetrically dominant majority of volcanism in the southern pep which occurred at 133 132 ma the younger commonly ne trending dikes may reflect the initiation of full drift which was coincident with major basin development in the adjacent continental borderland at 125 120 ma the pgds may indeed represent the failed arm of a rift rift rift triple junction but the triple junction did not coincide exactly in time or space with the site that would be inferred for plume impact

the emplacement history of a remarkable heterogeneous chemically zoned rheomorphic and locally lava like ignimbrite tl on gran canaria abstract ignimbrite tl on gran canaria is a complex compositionally zoned rheomorphic tuff that locally exhibits features previously considered to be diagnostic of lavas it is made up of two locally overlapping lobes of ignimbrite that were emplaced during a single eruptive episode the eastern lobe is high grade with rheomorphic zones and localised patches that are lava like the western lobe is extremely high grade more extensively lava like and welded to its top surface both parts are zoned with a basal comendite rich zone grading up through a mixed zone into an upper trachyte rich zone lithic contents and the relative proportions of comendite and trachyte pyroclasts vary with height each comendite rich zone is vitroclastic whereas each trachyte rich zone is partly lava like with local gradations into vitroclastic ignimbrite mixed zones are intermediate in character and locally show compositional banding gradational zoning in massive ignimbrite best seen in lower strain zones and welding fabrics that are pervasively lineated and oblique to bedding suggest that deposition was sustained agglutination was rapid and rheomorphic deformation began during the sustained deposition the viscosity and porosity of the agglutinate varied with height because successively deposited pyroclast populations varied in grainsize composition and temperature the hot agglutinate continued to compact and shear downslope after the density currents had dissipated causing further rheomorphic folding thrusting attenuation and autobrecciation the western lobe locally overlies the partly welded top of the eastern lobe in part because it advanced rheomorphically across it for at least 300 m hot state loading and auto intrusion occurred due to unstable density layering in the chemically zoned agglutinate deformation behaviour changed during cooling and degassing and because of heat transfer between juxtaposed agglutinates and localised retention of dissolved volatiles where there was an overlying impermeable cap

the formation of high grade ignimbrites i experiments on high and low concentration transport systems containing sticky particles high grade ignimbrites are thought to be deposited by pyroclastic flows at temperatures exceeding minimum welding temperature or even solidus temperature corresponding pyroclastic flow particles range from plastic to partially liquid and are able to aggregate or coalesce this contrasts with particles in pyroclastic flows producing unwelded ignimbrite which are capable of elastic grain interactions the low aspect ratio and great areal extent of high grade ignimbrites requires transport in a particulate state either by a high concentration mass flow facilitated by fluidizing gas reducing internal friction or by b expanded turbulent flow of low but downward increasing concentration this paper presents experiments designed to investigate the effects of plastic to liquid particles on these two contrasting transport mechanisms gas fluidization experiments using polyethyleneglycole peg powders heated above minimum sintering tms and melting tm temperatures cover a wide range of fluidization velocities umf ua 0 6 ut but are always in the bubbly fluidization regime similar to fluidized ignimbrite ash where particle volume concentration outside the bubbles is high 10 1 when the powders reach a critical temperature tm t tms defluidization by catastrophic particle aggregation immediately commences in both stationary and laterally moving fluidized beds as well as in experiments using mixtures of high and low tm 30 wt peg powders when t tms of the lower tm powder this indicates that extended particulate transport at t tms is not possible at such high particle concentrations in the turbulent flow experiments liquid sprays of molten peg or water vertically injected into a high re 104 horizontal air flow form a low concentration 10 5 to 10 4 turbulent suspension current proximal formation of partially coalesced aggregates which settle faster than individual particles causes the measured downstream decay of sedimentation rate to be steeper than predicted by theory of single solid particle sedimentation from turbulent suspensions as particles become finer downstream and coalescence efficiency decreases in response to cooling more distally formed aggregates become too small and rare to modify sedimentation rate decay from that of suspension flows containing solid particles the key difference between the two transport systems is particle concentration c since particle collision rate rcoll c2 collision rates in fluidized beds are so high that all particles immediately aggregate when coalescence efficiency 1 ecoal 0 is larger than 10 3 low concentration suspensions on the other hand require much higher values of ecoal for significant aggregation to occur dilute pyroclastic flows will have higher particle volume fractions 10 3 than the experimental currents but then viscous pyroclasts should have lower coalescence efficiencies than peg droplets experimental results thus support an expanded turbulent transport mechanism of pyroclastic flows generating extensive high grade ignimbrite sheets

geology of the peralkaline volcano at pantelleria strait of sicily situated in a submerged continental rift pantelleria is a volcanic island with a subaerial eruptive history longer than 300 ka its eruptive behavior edifice morphologies and complex multiunit geologic history are representative of strongly peralkaline centers it is dominated by the 6 km wide cinque denti caldera which formed ca 45 ka ago during eruption of the green tuff a strongly rheomorphic unit zoned from pantellerite to trachyte and consisting of falls surges and pyroclastic flows soon after collapse trachyte lava flows from an intracaldera central vent built a broad cone that compensated isostatically for the volume of the caldera and nearly filled it progressive chemical evolution of the chamber between 45 and 18 ka ago is recorded in the increasing peralkalinity of the youngest lava of the intracaldera trachyte cone and the few lavas erupted northwest of the caldera beginning about 18 ka ago inflation of the chamber opened old ring fractures and new radial fractures along which recently differentiated pantellerite constructed more than 25 pumice cones and shields continued uplift raised the northwest half of the intracaldera trachyte cone 275 m creating the island s present summit montagna grande by trapdoor uplift pantellerite erupted along the trapdoor faults and their hingeline forming numerous pumice cones and agglutinate sheets as well as five lava domes degassing and drawdown of the upper pantelleritic part of a compositionally and thermally stratified magma chamber during this 18 3 ka episode led to entrainment of subjacent crystal rich pantelleritic trachyte magma as crenulate inclusions progressive mixing between host and inclusions resulted in a secular decrease in the degree of evolution of the 0 82 km3 of magma erupted during the episode