petedunnite caznsi2o6 stability and phase relations in the system cao zno sio2 the phase relations of petedunnite caznsi2o6 pd were determined experimentally at p t conditions up to 2 5 gpa and 1100 c single phase petedunnite is formed at high pressures p 0 8 gpa reversed experiments show that at lower pressures and temperatures 650 c petedunnite decomposes to willemite zn2sio4 wil hardystonite ca2znsi2o7 har and quartz sio2 qtz according to the reaction 4 pd wil 2 har 3 qtz the boundary curve for this equilibrium reaction is given by p gpa 0 093 0 029 0 0014 0 0003 t c by disregarding the phase transition of quartz the stability field of wil har qtz is restricted toward lower temperatures by zinc feldspar caznsi3o8 zfsp according to the known reaction wil 2 har 7 qtz 4 zfsp these reactions intersect at 650 1 c 0 78 0 01 gpa generating an invariant point i pd additionally petedunnite breakdown reaction is intersected by the low high quartz phase transition curve generating an invariant point i pdqtz at 840 c 1 04 gpa at temperatures 650 c further reactions occur in the system cao zno sio2 including the doubly degenerate reaction zfsp pd qtz and 3 pd har wil zfsp which also intersect the invariant point i pd all reactions involving petedunnite display shallow positive slopes within the p t field indicating that the crystallization of petedunnite is highly pressure sensitive over a wide temperature range this means that an increasing petedunnite component in pyroxene shifts its stability field to higher pressures similar to the effect of a jadeitic component the study of natural clinopyroxene and the correlation of its zinc content with published p t conditions of these mineral assemblages confirmed a significant relationship between extraordinary high zinc concentrations in pyroxene and high metamorphic pressure conditions in addition the petedunnite component is obviously sensitive to the prevailing fluid conditions in terms of the fugacity ratio f s2 f o2 furthermore a distinct temperature dependency of the zinc component was observed in the range of trace element concentration in consequence zn turns out to be a key element with regard to its implementation as a sophisticated petrogenetic indicator of metamorphic conditions therefore routine measurement of zinc in element analyses of clinopyroxenes is strongly recommended

paleomagnetic correlation of late quaternary lava flows in the lower east rift zone of kilauea volcano hawaii paleomagnetic data reflecting paleosecular variation psv of the geomagnetic field are used to correlate individual lava flows in the lower east rift zone of kilauea volcano this area has been recently mapped based on petrography stratigraphy soil development and degree of weathering of the flows eight 14c dates also provide a framework for the relative age assignments site mean directions of remanent magnetization for historic flows in this region indicate that local magnetic anomalies do not mask the psv signal that within flow angular differences are generally 5 and that temporal resolution of the paleomagnetic directions is of the order of 100 years the paleomagnetic correlations do not conflict with the observed stratigraphic relationships and psv reference curves from dated lava flows and lake sediments have also been used to help determine the sequence of directional groups paleomagnetic grouping of flows into eruptive events indicates a different and perhaps more refined eruptive history for the lower east rift zone than implied by the geologic mapping alone the number of individual flows based on petrographic distinctions is likely the same but the frequency of events including petrographically distinct flows is apparently lower than previously thought flows from ridge crest vents bifurcated by the rift indicate formation of the central graben in this region after 300 years ago possibly related to the 1790 eruptions along dual fractures

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

petrology and geochemistry of submarine volcanism in the sicily channel rift abstract submarine magmatism in the sicily channel rift began in the early pliocene and lasted until almost 200 yr ago we present here petrological and geochemical data on volcanic rocks dredged from graham and nameless banks and pantelleria seamounts in the pelagian sector of the sicily channel rift petrological evidence suggests that the ascent of magmas to the surface was relatively rapid probably through channels superimposed over the major tectonic discontinuities of the rift major and trace element data indicate an ocean island basalt affinity for graham and nameless bank alkaline lavas and a depleted tholeiitic signature for one pantelleria seamount which had a shallower mantle source sr nd pb isotopic compositions suggest a heterogeneous mantle source involving both focus zone fozo and high himu components lead isotopic compositions are slightly less radiogenic in the tholeiitic basalt and more radiogenic closer to himu in the alkaline lavas we hypothesize that a decompressional me

phase equilibrium constraints on pre eruptive conditions of recent felsic explosive volcanism at pantelleria island italy we present experimental phase equilibria carried out on a pantelleritic bulk rock composition with a peralkalinity index pi molar na2o k2o al2o3 1 4 which is representative of the most energetic pumice fall eruption of the resurgent post caldera volcanism on pantelleria island for the explored conditions p 25 150 mpa t 680 800 c h2omelt up to 6 wt fo2 nno where nno is nickel nickel oxide buffer clinopyroxene is the liquidus phase followed by alkali feldspar and then quartz the crystallization of amphibole is limited to temperatures below 700 c aenigmatite crystallizes near the liquidus for p 100 mpa when clinopyroxene is the sole liquidus phase the coexisting melt is characterized by a decrease in peralkalinity index to 1 2 if compared with the starting material approaching comendite composition in contrast when alkali feldspar joins the liquidus phase assemblage residual melts become extremely peralkaline pi 3 2 with feo contents in excess of 14 wt comparison of our experiments with natural phase abundances and compositions constrains the depth of the fastuca pantellerite reservoir to be around 5 1 km or p 120 20 mpa at t 730 10 c with h2omelt around 4 wt and fo2 1 5 log unit below nno such conditions correspond to the dominant phase assemblage of feldspar clinopyroxene and aenigmatite amphibole occurrence requires both higher melt water contents and lower temperatures up to 6 5 wt at 680 c and may reflect provenance from a deeper and wetter portion of the tapped reservoir in contrast quartz is more probably related to crystallization during magma ascent which explains the lower melt water content of its melt inclusions overall the results confirm that pantellerites may be both water rich and reduced magmas these characteristics imply that any coexisting fluid phase in the reservoir will be dominated by water and hydrogen species the latter amounting to a few mole per cent magma unrest at pantelleria may thus be heralded by emissions of hydrous and reduced gases i e co2 poor and h2 rich if the present day reservoir is felsic in composition

the origin of trachyte and pantellerite from pantelleria italy insights from major element trace element and thermodynamic modelling abstract trachyte and peralkaline rhyolite pantellerite and comendite frequently comprise the felsic end member in bimodal suites in continental rift and oceanic island settings in these settings the relationship between the mafic mildly alkaline or transitional basalt and felsic lavas is ambiguous major and trace element models and isotopic data are often consistent with an origin for felsic lavas from either fractional crystallization of transitional basalt or partial melting of alkali gabbro followed by fractional crystallization in this paper we present representative mineral analyses and whole rock analyses from forty samples of a basalt trachyte pantellerite suite collected at pantelleria italy in the strait of sicily rift zone and compare the results of major and trace element modelling with the results of thermodynamic melts modelling from these results we conclude that metaluminous trachyte formed as a result of 70 to 75 low pressure 0 1 gpa fractional crystallization of an assemblage of plagioclase clinopyroxene olivine magnetite and apatite from a hydrous 1 0 1 5 wt h2o transitional basalt magma at relative oxygen fugacities approximately one log unit below the fayalite magnetite quartz buffer fmq 1 the daly gap a lack of intermediate 49 62 wt sio2 volcanic rocks at pantelleria is concluded to be primarily the result of rapid differentiation through that interval relatively rapid crystallization at low pressure may have effected the partial degassing of water saturated 4 wt h2o metaluminous trachyte magma some metaluminous trachyte lavas have positive eu anomalies high k rb ratios high concentrations of ba and low concentrations of incompatible trace elements these are interpreted to be the result of up to 40 accumulation of alkali feldspar comenditic trachyte pantelleritic trachyte and pantellerite formed after an additional 20 to 80 fractional crystallization of an assemblage dominated by alkali feldspar from metaluminous trachyte magma the most evolved pantellerite lavas and tuffs are the result of a total of 95 fractional crystallization of transitional basalt with phenocrysts that equilibrated at low temperatures 4 wt water contents

nested calderas and trapdoor uplift at pantelleria strait of sicily pantelleria is dominated by a caldera 6 km in diameter that formed 55 000 yr ago during eruption of a rheomorphic fall deposit that is zoned from pantellerite to trachyte trachytic lavas then built a shield that nearly filled the caldera magma subsequently reinflated the shallow reservoir and raised the northwest half of the trachyte shield 275 m creating montagna grande by trapdoor uplift the lid eventually failed and pantellerite magma erupted as domes flows and pumice cones along the trapdoor faults and their hingeline we have identified a second caldera 93 000 yr old within which the younger caldera is nested a still older structure may encircle them both replenishment of magma reservoirs beneath calderas can promote dilatant reactivation of older subsidence structures resulting in multicyclic collapses that reuse these structures and produce nested calderas

downsag and extension at calderas new perspectives on collapse geometries from ice melt mining and volcanic subsidence structures at calderas may form as a result of precursory tumescence subsidence due withdrawal of magmatic support resurgence and regional tectonism structural reactivation and overprinting are common to explore which types of structures may derive directly from subsidence without other factors evidence is reviewed from pits caused by the melting of buried ice blocks mining subsidence scaled subsidence models and from over 50 calderas this review suggests that complex patterns of peripheral deformation with multiple ring and arcuate fractures both inside and outside caldera rims topographic embayments arcuate graben and concentric zones of extension and compression may form as a direct result of subsidence and do not require a complex subsidence and inflation history downsag is a feature of many calderas and it does not indicate subsidence on an inward dipping ring fault as has been inferred previously where magmatic inflation is absent or slight initial arcuate faults formed during collapse are likely to be multiple and dip outwards to vertical associated downsag causes the peripheries of calderas undergo radial centripetal extension and this accounts for some of the complex peripheral fractures arcuate crevasses graben and some topographic moats the structural boundary of a caldera defined here as the outermost limits of subsidence and related deformation including downsag commonly lies outside ring faults and outside the embayed topographic wall it is likely to be funnel shaped i e inward dipping even though ring and arcuate fractures within it may dip outward inward dipping arcuate normal faults at shallow levels and steep inward dipping contacts between a caldera s fill and walls may both occur at a caldera that has initially subsided on outward dipping ring faults they arise due to peripheral surficial extension gravitational spreading and scarp collapse topographic enlargement at some calderas and the formation of embayments may reflect general progressive downsag and localized downsag respectively these processes may occur in addition to surficial degradation of oversteep ring fault scarps

palaeomagnetic rock magnetic and microscopy studies of historic lava flows from the paricutin volcano mexico implications for the deflection of palaeomagnetic directions summary we report the results of a detailed rock magnetic study of basaltic andesites for one of the best documented cases of the first occurrence of a new volcano the 1943 1952 eruption of paricutin in central mexico 102 oriented standard palaeomagnetic cores corresponding to 12 sites were collected from seven different lava effusion episodes which cover the interval between 1943 september december and 1946 march august thermomagnetic investigations reveal that remanence is carried in most cases by ti poor titanomagnetite resulting from oxy exsolution that probably occurred during the initial flow cooling unblocking temperature and coercivity point to small pseudo single domain magnetic grains for these titano magnetites thermal and alternating field demagnetization indicates that the lavas are characterized by univectorial or two component magnetizations site mean declinations and inclinations range from 331 9 to 32 3 and from 17 6 to 51 3 respectively the overall mean direction for 12 sites is dec 1 8 inc 37 5 k 26 and 95 8 7 23 samples were pre selected for thellier palaeointensity experiments because of their stable remanent magnetization and relatively low within site dispersion thellier palaeointensity experiments have yielded widely dispersed palaeointensity values from the historically observed geomagnetic intensity although individual specimens yielded technically high quality palaeointensity results the values obtained are significantly different from the expected value of approximately 45 t the palaeomagnetic record is dominantly characterized by shallower than expected sitemean inclinations as compared with data from the teoloyucan geomagnetic observatory and estimates based on the international geomagnetic reference field 1945 and 1950 models several alternative interpretations are examined but no single factor resulting in abnormal shallow inclinations can be isolated direct field observations suggest that shallow inclinations and angular scatter can be associated with lava emplacement in terms of internal viscous deformation and or movement of parts of lava after cooling below the blocking temperatures during lava advance other factors analysed that could contribute to the shallow inclinations and angular scatter are high magnetic refraction effects and outcrop scale to regional magnetic field anomalies

a regional archeomagnetic model for europe for the last 3000 years scha dif 3k applications to archeomagnetic dating 1 the available european database of archeomagnetic field values and instrumental data has been used to produce a regional model for the geomagnetic field in europe for the last 3000 years from 1000 b c to 1900 a d connecting with the epoch covered by the igrf models this new model scha dif 3k constitutes an improvement with respect to the previous regional archeomagnetic model scha di 00 f which used relocated values and was only valid for the last 2000 years the new model has been obtained by least sums of absolute deviation inversion of paleomagnetic data using spherical cap harmonics for the spatial representation of the field and sliding windows in time an algorithm has been developed to jointly model the three archeomagnetic elements declination inclination and intensity the resulting model provides the direction and intensity of the earth s magnetic field over the european continent northern africa and western asia for the last 3000 years the fit to the european archeomagnetic database is more accurate than that provided by global archeomagnetic models in addition this model represents a step forward in archeomagnetic dating studies since the relocation error is avoided and can also be used to study the rapid changes of the geomagnetic field archeomagnetic jerks that have been recently proposed

paleomagnetism of pleistocene volcanic rocks from pantelleria island sicily channel italy abstract a paleomagnetic investigation has been carried out on pleistocene volcanic rocks from pantelleria in the sicily channel this island is characterised by a bimodal volcanism of peralkaline rhyolite to trachyte and basalt composition the radiometric data indicate that volcanic activity started around 324 ka bp samples have been collected from 30 sites in 16 volcanic units spanning activity during the last 150 ka magnetic properties vary systematically with lithotype and indicate high ti titanomagnetite to magnetite as the main carriers of magnetisation stable characteristic remanent magnetisation chrm directions isolated by alternating fields af demagnetisation show normal polarity consistent with emplacement during the brunhes chron and vary within the paleosecular variation psv range however the inclinations are low as compared with the geocentric axial dipole gad inclination at pantelleria especially for volcanic units younger than 50 ka the mean chrm direction computed from 27 sites is d 358 8 i 46 9 k 30 95 5 2 and the inclination anomaly is i 8 1 both tectonic movements and the presence of large magnetic anomalies around the island of pantelleria fail to explain this low inclination which may therefore be related to a significant long lived non dipolar field contribution in the area over the past 150 kyr

palaeomagnetic data from the borrowdale volcanic group volcano tectonics and late ordovician palaeolatitudes nineteen palaeomagnetic sampling sites from the borrowdale volcanic group between borrowdale and langdale english lake district yield high unblocking temperature magnetization components which postdate the structural tilt and therefore postdate the formation of the broad block faulted syncline which characterizes this part of the central lake district the mean magnetization direction before tilt correction dec 347 2 inc 61 9 95 6 9 corresponds to a palaeolatitude of 43 s 5 7 the magnetization is associated with pervasive late ordovician hydrothermal alteration and is certainly pre devonian based on comparison with available palaeomagnetic data from southern britain this implies that the block faulted synclinal structure of the borrowdale langdale region is pre acadian in age consistent with recent interpretations of late ordovician volcano tectonics based on mapping and volcanological studies

the pantelleria graben sicily channel central mediterranean an example of intraplate passive rift abstract we present new high resolution swath bathymetric data and multichannel seismic profiles acquired in the pantelleria graben one of the three main tectonic depressions forming the sicily channel rift zone this region experienced a late miocene early pliocene continental extension with the development of nw trending fault bounded troughs later accompanied by widespread volcanic manifestations data support the interpretation that the pantelleria graben evolution was dominated by two tectonic phases a lithospheric scale continental rifting early pliocene in which the whole graben was formed and a successive phase late pliocene pleistocene characterized by a magma assisted extensional mechanism ascending magmas within the graben floor seem to migrate from the s e sector of the depression toward the n w sector which is almost entirely floored by igneous material and where the volcanic edifice of the pantelleria island is emerged the volcanic activity is presently concentrated north of the pantelleria island the tectonic evolution of the pantelleria graben characterized by the chronological sequence of events rifting doming volcanism can be referred to as a passive rifting model crustal stretching and subsequent fault development and rifting within the pelagian block may have been controlled by slab pull forces of the northward subducting african slab

paleomagnetic secular variation at vulcano aeolian islands during the last 135 kyr abstract paleosecular variation psv of the earth s magnetic field during the last 135 kyr has been investigated in lavas scoriae and pyroclastic rocks of vulcano aeolian islands about 1000 samples have been collected at 77 sites from 25 distinct volcanic units whose age is either known from published isotopical data or constrained on the grounds of statigraphical relationships magnetic mineralogy investigation shows that ti magnetite is the main ferromagnetic mineral at most sites secondary magnetization components are either absent or easily removed by stepwise thermal or alternating field demagnetization the mean site direction of the characteristic remanent magnetization is usually well defined since the semi angle of confidence is greater than 5 at only four sites the mean paleomagnetic direction over the last 135 kyr d 9 4 i 53 2 95 3 5 differs from the geocentric axial dipole gad at vulcano d 0 i 57 8 and might be interpreted as the effect of a long term non axial dipolar component the psv record from vulcano agrees well with those from the lacustrine sediments of lago grande di monticchio 100 kyr bp and lago di mezzano 30 kyr bp located in the italian peninsula brandt et al quat sci rev 18 1999 961 976 the inclination anomaly i found at vulcano corresponds to about half of the shallowing observed in the sediments of the two lakes and the declination anomaly d may be used to tie the declination values derived from azimuthally unoriented cores to the geographical reference system in order to find the optimum site to be used as reference for psv studies in italy the angular values of the earth s magnetic field measured at the 113 repeat stations of the italian geomagnetic network coticchia et al boll geod sci aff 40 2001 277 291 have been analyzed with the relocation via pole method noel and batt geophys j int 102 1990 753 756 the viterbo station lat 42 27 n long 12 02 e proved the best since the mean error is 0 3 for both declination and inclination wherever the original psv site is located in italy a preliminary composite psv curve for the last 30 kyr bp is thus proposed merging and relocating to viterbo the data from vulcano and the curve from lago di mezzano corrected for the gad deviation found at vulcano

the eruptive history of pantelleria sicily channel in the last 50 ka six silicic eruptive cycles have been recognized in the last 50 ka at pantelleria the products of each cycle exhibit a compositional variation from pantellerite to less peralkaline rhyolite or to trachyte the relationships between the range of chemical variation the erupted volume and the time of eruptions allow us to estimate an average differentiation rate of 5 crystal fractionation per 1000 years and a constant long term rate of magma discharge of 0 1 km3 per 1000 years pressure increase in the magma chamber caused by the addition of new magma accumulation of highly differentiated volatile rich magma in the roof zone and a concomitant build up of a vapour phase is postulated as a possible triggering mechanism for eruptions

recent volcanic history of pantelleria a new interpretation abstract the island of pantelleria is located in the pantelleria rift between sicily and africa this rift is floored by a 20 km thick continental type crust that is characterized by horsts and grabens formed by nw se tensional faults and ne sw shear faults several faults and fractures that intersect the island follow these regional trends prominent volcano tectonic features on the island include two calderas the younger being related to the eruption of the green ignimbrite and the uplifted montagna grande block pantelleria mostly consists of peralkaline silicic rocks with minor trachytes and basalts volcanism on the island is characterized by both explosive and effusive activity reconstruction of the volcanic history of the island since the eruption of the green ignimbrite is based on geological and geochronological data the eruption of the green ignimbrite 50 000 y b p was followed by collapse of the monastero caldera that has been the site of most of the subsequent acid volcanism the montagna grande volcanic complex 35 000 y b p fills the central part while the lower pantelleritic lava flows 16 000 y b p erupted along its rim very probably the young mt gelfiser and khaggiar domes are also located on the caldera rim the montagna grande block rose within the caldera at an estimated average rate between 1 0 and 1 5 cm y for about 20 000 y subsequent pumice cones 9 000 y b p endogenous domes and lava flows also erupted inside this caldera among the main acid volcanic units younger than the green ignimbrite only the mt gelkhamar endogenous dome is not linked to this caldera in the last 50 000 years basaltic activity only occurred in the northern part of the island the basaltic vents are aligned parallel with the main tensional trends of the rift at least two phases have been recognized the older is about 29 000 y b p and the younger is a few thousands of years old a change occurred in the eruptive style after the eruption of the green ignimbrite and collapse of the monastero caldera explosive activity diminished and extrusion of lava became dominant

paleomagnetism of spatter lavas from stromboli volcano aeolian islands italy implications for the age of paroxysmal eruptions 1 we report on 270 paleomagnetic directions retrieved from 17 different spatter deposits spread over the northern and western flanks of the stromboli volcano aeolian islands italy the spatter were emplaced during previously undated high energy strombolian eruptions occurring after the third seventh century ad our paleomagnetic directions were superimposed over the paleosecular variation curve of the geomagnetic field for the last two millennia obtained by reducing to the coordinates of stromboli the archeomagnetic and magnetic observatory data from france and italy this comparison suggests that most of the spatter exposed in the northern flank of the volcano were emplaced during several eruptions occurring between ca 1400 and 1600 ad while the western flank of stromboli was diffusely spread by spatter during the powerful xx century eruptions conversely there is no evidence for paroxysmal fire fountain activity occurring before 1400 ad and between 1600 and 1907 ad

the manganese deposits of griqualand west south africa some mineralogic aspects iron content of the manganese sesquioxides as an indication of the temperature of formation of the ores in various localities temperatures of formation postmasburg field and kalahari field lateral zoning manganese ores in both fields were precipitated by solutions which moved from west to east the main mineralization episode dates from the proterozoic era modified journal abstract

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

fission track analysis of apatite and zircon defines a burial depth of 4 to 7 km for lowermost upper devonian catskill mountains new york apatite and zircon grains separated from a sandstone layer of earliest late devonian age catskill mountains have been subjected to fission track analysis a 125 m y age obtained on the apatite grains requires a temperature for the sediment of less than 120 c during the past 125 m y at some time prior to 125 m y ago temperatures were above 120 c long enough to cause complete fading of tracks analysis of zircon grains resulted in a fission track age of 320 m y zircon data indicate that the temperature of the sediment layer enclosing the grains did not exceed 175 to 200 c over a 235 rn y period time between sedimentation and 125 m y ago if one assumes a typical geothermal gradient of 25 c km a burial depth of between 4 and 7 km is indicated for the lowermost upper devonian atskill mountains

magnetic fabric welding texture and strain fabric in the nuraxi tuff sardinia italy anisotropy of magnetic susceptibility ams has been used to interpret flow directions in ignimbrites but no study has demonstrated that the ams fabric corresponds to the flow fabric in this paper we show that the ams and strain fabric coincide in a high grade ignimbrite the nuraxi tuff a miocene rhyolitic ignimbrite displaying a wide variability of rheomorphic features and a well defined magnetic fabric natural remanent magnetization nrm data indicate that the magnetization of the tuff is homogeneous and was acquired at high temperatures by ti magnetite crystals comparison between the magnetic fabric and the deformation features along a representative section shows that ams and anisotropy of isothermal remanent magnetization airm fabric are coaxial with and reproduce the shape of the strain ellipsoid magnetic tests and scanning electron microscopy observations indicate that the fabric is due to trails of micrometer size pseudo single domain magnetically interacting magnetite crystals microlites formed along discontinuities such as shard rims and vesicle walls mimicking the petrofabric of the tuff the fabric was thus acquired after deposition before late rheomorphic processes and accurately mimics homogeneous deformation features of the shards during welding processes and mass flow

glass transition temperatures of natural hydrous melts a relationship with shear viscosity and implications for the welding process abstract glass transition temperatures tg have been determined for natural multicomponent melts using differential scanning calorimetry trachytic dacitic phonolitic and basaltic base compositions have been analysed over a range of water contents up to 3 75 wt for each sample tg has been obtained over a range of cooling heating rates using the extrapolated onset and the peak temperatures in heat capacity temperature curves tg of all compositions are strongly reduced by increasing water content particularly for the first 1 wt added base composition also has an effect with the lowest tg occurring in the peralkaline phonolite suite for all samples a clear dependence on the cooling heating rate has been recorded these results have been compared with rheological investigations on the same samples on the basis of the equivalence of the shear and enthalpic relaxation process timescales we provide a method to predict the shear viscosity at the glass transition for all the melts investigated in this study both dry and hydrous our determinations of tg provide a lower limit for the time temperature envelope that gives rise to densely welded deposits and constraints on their emplacement temperature furthermore by using the viscosity values predicted at the glass transition we suggest that welding processes may occur over timescales on the order of tens of seconds to tens of minutes at tg

correlation of welded ignimbrites on pantelleria strait of sicily using paleomagnetism although the oldest volcanic rocks exposed at pantelleria strait of sicily are older than 300 ka most of the island is covered by the 45 50 ka green tuff ignimbrite thought to be related to the cinque denti caldera and younger lavas and scoria cones pre 50 ka rocks predominantly rheomorphic ignimbrites are exposed at isolated sea cliffs and their stratigraphy and chronology are not completely resolved based on volcanic stratigraphy and k ar dating it has been proposed that the older la vecchia caldera is related to ignimbrite q 114 ka and that ignimbrites f d and z 106 94 and 79 ka respectively were erupted after caldera formation we report here the paleomagnetic directions obtained from 23 sites in ignimbrite p 133 ka and four younger ignimbrites and from an uncorrelated and loosely dated welded lithic breccia thought to record a caldera forming eruption the paleosecular variation of the geomagnetic field recorded by ignimbrites is used as correlative tool with an estimated time resolution in the order of 100 years we find that ignimbrites d and z correspond in good agreement with recent ar ar ages constraining the d z eruption to 87 ka the welded lithic breccia correlates with a thinner breccia lying just below ignimbrite p at another locality implying that collapse of the la vecchia caldera took place at 130 160 ka this caldera was subsequently buried by ignimbrites p q f and d z paleomagnetic data also show that the northern caldera margin underwent a 10 west northwest outwards tilting after emplacement of ignimbrite p possibly recording magma resurgence in the crust

modes and times of caldera resurgence the 10 ka evolution of ischia caldera italy from high precision archaeomagnetic dating abstract ischia is a well exposed and densely populated late quaternary caldera in the campanian magmatic province of italy ischia caldera experienced an average uplift rate of 3 3 cm year in the last ca 30 ka and is still actively resurging during the last 10 ka coeval with the resurgence a volcanic field of alkali trachytic to trachyandesitic lava domes lava flows tuff and scoria rings and pumice cones developed mainly on the eastern sector of the caldera along both resurgence related faults and regional nnw and ne striking faults in order to improve both our understanding of the recent volcanic history and the evaluation of future risks on ischia island a high precision archaeomagnetic dating method was applied to the products of 12 volcanic centres with probable age 14 c ages archaeomagnetic and other geochronological data as well as stratigraphic constraints show that during the studied time interval ischia volcanism occurred in five periods separated by phases of quiescence and coeval with earthquake and landslide events this fact suggests a pulsating mode of uplifting and deformations of the ischia resurgence during the two oldest periods of activity 7200 6800 bc and 4100 2300 bc resurgence probably produced a dome shaped structure location and geometry of vents suggest the occurrence of magma uprise along the fractures produced by bending of the overburden crustal block most of magma was emplaced as intrusions at the interior of the resurgent block whereas volcanism was represented by very viscous differentiated and crystallized lavas that emplaced as domes and high aspect ratio flows the resurgent dome caused recurrent lateral collapses that removed about 2 5 km 3 of rocks during the three youngest periods of activity 1800 1000 bc 650 bc 355 ad and 1302 ad resurgence affected a fault bounded asymmetric block this resulted from both a hydrostatic rebound of the crustal block after removal of material involved into huge slope instability triggering an increase in uplift rate and b new influxes of less evolved magma batches into the shallow reservoir that in turn favoured the intense volcanism of the last 4000 years