petrogenesis of volcanic rocks that host the world class ag pb navidad district north patagonian massif comparison with the jurassic chon aike volcanic province of patagonia argentina abstract we present the first study of the volcanic rocks of the canadon asfalto formation that host the navidad world class ag pb epithermal district located in the north patagonian massif patagonia argentina these volcanic and sedimentary rocks were deposited in a lacustrine environment during an extensional tectonic regime associated with the breakup of gondwana and represent the mafic to intermediate counterparts of the mainly silicic jurassic chon aike volcanic province lava flows surrounded by autobrecciated carapace were extruded in subaerial conditions whereas hyaloclastite and peperite facies suggest contemporaneous subaqueous volcanism and sedimentation la icpms u pb ages of zircon crystals from the volcanic units yielded middle jurassic ages of 173 9 1 9 ma and 170 8 3 ma in the navidad district volcanic rocks of the canadon asfalto formation show arc like signatures including high k basaltic andesite to high k dacite compositions rb ba and th enrichment relative to the less mobile hfs elements nb ta enrichment in light rare earth elements lree y ti depletion and high zr contents these characteristics could be explained by assimilation of crustal rocks in the jurassic magmas which is also supported by the presence of zircon xenocrysts with permian and middle upper triassic ages 281 3 ma 246 5 218 1 and 201 3 ma and quartz xenocrysts recognized in these volcanic units furthermore sr and nd isotope compositions suggest a contribution of crustal components in these middle jurassic magmas high k basaltic andesite has initial 87sr 86sr ratios of 0 70416 0 70658 and nd t values of 5 3 and 4 high k dacite and andesite have initial 87sr 86sr compositions of 0 70584 0 70601 and nd t values of 4 1 and 3 2 the range of pb isotope values 206pb 204pb 18 28 18 37 207pb 204pb 15 61 15 62 and 208pb 204pb 38 26 38 43 of navidad volcanic rocks and ore minerals suggest mixing pb sources with contributions of mantle and crust 206pb 204pb isotopic ratios of jurassic volcanic rocks of the chon aike volcanic province and sulfides of associated epithermal deposits increase with time from the volcanic event v1 188 178 ma to volcanic events v2 172 162 ma and v3 157 153 ma reflecting variations in the radiogenic pb source as volcanism was migrating towards the proto pacific margin of gondwana

volcanology and petrology of volcanic products from the island of milos and neighbouring islets abstract new geochronological and volcanological data from volcanics of the island group of milos south aegean active volcanic arc allow four main cycles of volcanic activity to be distinguished ranging in age between 3 5 and 0 1 m y b p the oldest volcanic activity consists almost totally of pyroclastics and submarine products followed by subaerial ones the eruptive centers migrated in time and concentrated during the most recent volcanic phase in the central part of milos and on the island of antimilos the oldest products are dominated by andesites and dacites while the most recent ones mainly consist of rhyolites the volcanic evolution and the geochemical characters of the erupted products suggest that the feeding system of the oldest volcanism was located in the deep continental crust where contamination and fractional crystallization jointly occurred the younger cycle of volcanic activity was fed from small and relatively shallow magma chambers where contamination processes played a minor role these magma bodies are considered to responsible for the shallow thermal anomaly giving rise to the high enthalpy field on milos

large scale rhyolite peperites jurassic southern chile development of a jurassic volcano tectonic rift basin in the southern andes created a setting in which thick rhyolitic volcaniclastic sequences accumulated in submarine environments and were penetrated by hypabyssal intrusions during or shortly after deposition in the ultima esperanza district of southern chile extensive masses of peperite were produced when rhyolite magma underwent quenching disruption and commingling with wet unconsolidated sediments during intrusion at shallow levels beneath the sea floor the peperite forms discordant intrusive masses with volumes of up to several cubic kilometers in which large widely spaced coherent rhyolite feeder pods are surrounded by and grade into closely packed and dispersed peperite closely packed peperite consists of tightly fitting clasts separated by sediment filled fractures in dispersed peperite the sediment forms a matrix surrounding large masses of fractured rhyolite and smaller more widely separated rhyolite clasts evidence of in situ quench fragmentation is well preserved on both outcrop and thin section scales thin sections show that clast margins and in some cases entire small clasts underwent cooling contraction granulation releasing shards of quenched rhyolite and fragments of phenocrysts into the adjacent sediment interaction between magma and wet sediment was non explosive and involved fluidization of the host sediments creating space for the intruding magma and causing pervasive injection of highly mobile sediment along thermal contraction cracks in quench fragmented rhyolite the ability of the magma to undergo complex intermixing with large volumes of sediment with widespread preservation of in situ fragmentation textures is interpreted to reflect a relatively low magma viscosity presumably caused by retention of volatiles in the magma at the ambient pressures involved beds of redeposited peperite within the rift basin fill indicate that some of the intrusive peperite masses reached the sea floor undergoing slumping and mass flow the peperites were thus an important local source of coarse grained debris during the evolution of the basin

exogenous and endogenous growth of the unzen lava dome examined by satellite infrared image analysis abstract phase 1 of the most recent eruption of unzen volcano japan occurred between 1991 and 1993 and involved the endogenous and exogenous addition of magma to a growing summit lava dome the temporal variation between endogenous and exogenous styles of dome growth has an important bearing on the hazard posed by dome collapse and explosive events however such data are difficult to acquire and have previously only been obtained in a fully quantitative manner for the mount st helens dome in that case using detailed photogrammetry here we investigate these competing dome growth processes at unzen via analysis of their thermal emission signals present in shortwave infrared swir satellite imagery we find that both the endogenous and exogenous growth styles exhibit identifiable swir signatures whose magnitude can be related to their respective magma supply rates we find that the swir radiant area corresponding to endogenous growth is around four times larger than that corresponding to exogenous growth at the same magma discharge rate analysis of time series imagery indicates that like mount st helens unzen experienced a temporally decreasing magma discharge rate during phase 1 as time progressed and the discharge rate fell the growth style of the unzen dome changed from exogenous only through exogenous decreasing and endogenous increasing to endogenous only thus relative growth rates at the endogenous and exogenous areas of the unzen dome appear to be at least partly controlled by the overall magma discharge rate as with mount st helens the cause of this relationship was most likely the formation and strengthening of a brittle crust at the surface vent which impeded exogenous growth more effectively as time elapsed and the discharge rate fell these results suggest the similarity of the processes operating at the unzen and mount st helens domes though important differences remain in that dome growth was basically continuous at unzen whilst at mount st helens growth episodes were interspersed by significant periods of repose

facies architecture of a silicic intrusion dominated volcanic centre at highway reward queensland australia abstract the highway reward massive sulphide deposit is hosted by a silicic volcanic succession in the cambro ordovician seventy mile range group northeastern australia three principal lithofacies associations have been identified in the host succession the volcanogenic sedimentary facies association the primary volcanic facies association and the resedimented syn eruptive facies association the volcanogenic sedimentary facies association comprises volcanic and non volcanic siltstone and sandstone turbidites that indicate submarine settings below storm wave base lithofacies of the primary volcanic facies association include coherent rhyolite rhyodacite and dacite and associated non stratified breccia facies autoclastic breccia and peperite the resedimented volcaniclastic facies association contains clasts that were initially formed and deposited by volcanic processes but then redeposited by mass flow processes resedimentation was more or less syn eruptive so that the deposits are essentially monomictic and clast shapes are unmodified this facies association includes monomictic rhyolitic to dacitic breccia resedimented autoclastic facies siltstone matrix rhyolitic to dacitic breccia resedimented intrusive hyaloclastite or resedimented peperite and graded lithic crystal pumice breccia and sandstone pumiceous and crystal rich turbidites the graded lithic crystal pumice breccia and sandstone facies is the submarine record of a volcanic centre s that is not preserved or is located outside the study area pumice shards and crystals are pyroclasts that reflect the importance of explosive magmatic and or phreatomagmatic eruptions and suggest that the source vents were in shallow water or subaerial settings the lithofacies associations at highway reward collectively define a submarine shallow intrusion dominated volcanic centre contact relationships and phenocryst populations indicate the presence of more than 13 distinct porphyritic units with a collective volume of 0 5 km 3 single porphyritic units vary from the size and shape of the lavas and intrusions reflect a restricted supply of magma during eruption intrusion the style of emplacement and the subaqueous emplacement environment due to rapid quenching and mixing with unconsolidated clastic facies the sills and cryptodomes did not spread far from their conduits the shape and distribution of the lavas and intrusions were further influenced by the positions of previously or concurrently emplaced units magma preferentially invaded the sediment avoiding the older units or conforming to their margins large intrusions and their dewatered envelope may have formed a barrier to the lateral progression and ascent of subsequent batches of magma

themis observations of substorms on 26 february 2008 initiated by magnetotail reconnection 1 on 26 february 2008 the themis satellites observed two substorms that occurred at about 0405 and 0455 ut angelopoulos et al 2008 made a comprehensive study of the second event in this paper we display detailed features of the two substorms with emphasis on the first in both substorms a distinct auroral intensification occurred during the earliest stage of onset about 1 to 2 min after midtail reconnection began this initial intensification was weak and localized and thus had the signatures of a pseudobreakup in both substorms a second major intensification occurred next in the substorm onset sequence followed by rapid and extensive poleward expansion this second intensification had the features of the major expansion onset and was nearly coincident with observations of earthward flows and magnetic dipolarization in the near earth tail during the growth phase of the two substorms open magnetic flux accumulated in the polar cap in the expansion recovery phase the polar cap open flux was quickly reduced these observations are in agreement with the assertion that tail reconnection initiates the initial pseudobreakup and the ensuing major expansion and releases and transports energy to eventually cause near earth dipolarization and the expansion phase onset of these two substorms

intrusive and extrusive growth of the mount st helens lava dome lava domes grow by a combination of endogenous intrusive and exogenous extrusive addition of magma either as single continuous events1 2 or through a series of relatively small pulses which together build a larger construct3 evaluating the hazards that attend growth of a dome requires an assessment of the processes controlling whether lava is added to its interior or surface because many of these dangers depend on how volatiles are admitted to or released from hot magma if the volatile pressure inside an inflating dome becomes high enough sudden exposure provided by flow front slumping can lead to explosive decompression and the generation of pyroclastic flows4 5 on the other hand lava erupted on the oversteepened surface of a dome may collapse fragment and mix with snow or ice to form pyroclastic flows surges and mudflows6 7 here we provide a quantitative assessment of the partitioning of magma into endogenous intrusions and exogenous lobes using high resolution digital topographic maps of the mount st helens dome derived from aerial photographs taken periodically between 1980 and 1986 endogenous exogenous and total volume production rates follow distinct trends which provide important clues about the nature of eruption mechanisms calculating endogenous and exogenous components for active domes like those recently formed at redoubt volcano may help quantify magma chamber processes and provide another tool for understanding the re equilibration of shallow magmatic systems

distinguishing peperite from other sediment matrix igneous breccias lessons from the iberian pyrite belt abstract breccias composed of coarse monomictic porphyritic igneous clasts and fine grained matrix are common in subaqueous volcanic successions we use the descriptive name sediment matrix igneous breccia for this facies and have recognized at least five different origins among examples in the iberian pyrite belt 1 peperite 2 sediment infill volcanic breccia 3 mud matrix resedimented hyaloclastite 4 mud rich water settled fiamme breccia and 5 apparent sediment matrix igneous breccia because the components and textures are similar discriminating among the different origins can be very difficult both peperite and sediment infill volcanic breccia can occur along top contacts of thick intervals of felsic coherent and monomictic breccia facies the presence of peperite indicates that the contact is intrusive whereas the presence of sediment infill volcanic breccia indicates that the contact is depositional hence correct distinction between peperite and sediment infill volcanic breccia is an important means of discriminating felsic intrusions from felsic lavas and domes the distinction underpins the reconstruction of volcanic centers and facies architecture and the ordering of volcanic intrusive sedimentary and mineralizing events in ancient submarine volcanic successions in addition in volcanic hosted massive sulfide districts such as the iberian pyrite belt paleoseafloor positions are considered highly prospective for massive sulfide ore bodies correct interpretation of sediment matrix igneous breccias is also important in this context because sediment infill volcanic breccia mud matrix resedimented hyaloclastite and mud rich water settled fiamme breccia all form or are deposited at the seafloor and hence delineate paleoseafloor positions in contrast peperite and apparent sediment matrix igneous breccias form subsurface and do not delineate paleoseafloor positions

identity and emplacement of domical structures in the western arcadia planitia mars 1 morphologic morphometric and thermophysical aspects of domical structures in a restricted part of the mars northern plains were characterized using mars odyssey and mars global surveyor data the majority of domical structures consist of relatively high thermal inertia subcircular to irregularly shaped core bodies surrounded by adjacent relatively low albedo low thermal inertia annuli and dispersed aureoles of apparently disaggregated material core profiles vary considerably from low to high aspect ratios elongate arms of material extend from the central summit regions in some of the structures core features include apparent blocky surface texture summit fissures and pits apparent summit and flank ruptures suggestive of eruption of material and concentric and straight summit ridges core heights range from 30 to 400 m mean 160 m diameters range from 620 to 3090 m mean 1550 m aspect ratios range from 0 02 to 0 36 mean 0 11 and volumes range from 0 1 to 1 8 km3 an additional morphologic type consists of subcircular domical mounds that are apparently covered by plains materials we propose that the majority of the structures are partially extrusive cryptodomes and lava domes whereas the mounds are entirely intrusive cryptodomes

a reassessment of lake and wetland feedbacks on the north african holocene climate 1 large parts of the sahara were vegetated during the early to mid holocene several positive feedbacks most notably related to vegetation have been shown to have favored the northward migration of the desert boundary during this period numerous lakes and wetlands existed in the sahara region and might have acted as a local moisture source however earlier model studies of the effects of open water surfaces on the mid holocene north african climate suggested that these were weak and did not contribute significantly to this northward migration of the north african climate zones using a state of the art climate model we suggest that the effect of open water surfaces on the mid holocene north african climate might have been much stronger than previously estimated regionally more than doubling the simulated precipitation rates it is thus possible that this effect combined to other known positive feedbacks favored the appearance of the green sahara

reconstruction of the tectonic volcanic and sedimentary setting of strongly deformed zn cu massive sulfide deposits at benambra victoria the wilga and currawong massive sulfide deposits at benambra are hosted by a strongly deformed upper silurian rhyolite dacite andesite basalt sediment succession dominated by silicic volcanics and sedimentary rocks this host succession developed in an ensialic back arc or intra arc basin of limited extensional origin on the active gondwana continental margin strong deformation is related to compressional and transcurrent crustal movements during closing of the depositional basin at the end of the silurian and to subsequent steep faulting the massive sulfide deposits are essentially synvolcanic because 1 they have the same three generations of tectonic structures and the same intensity of deformation as their host rocks suggesting that the ores predate the earliest compressional deformation 2 the ores are constrained to a late silurian age similar to their host rocks because both the host rocks and this earliest deformation are late silurian in age 3 there is a tight stratigraphic control on the location of the massive sulfides and 4 the stratigraphy depositional setting and alteration are similar to many other less deformed volcanic associated massive sulfide districts deformation and alteration inhibit detailed stratigraphic correlation and modify rock textures but do not destroy facies relationships which allow sedimentological and volcanological interpretation sedimentary facies associations indicate that mineralization occurred in the basin center after fault controlled subsidence from a mixed subaerial and subaqueous environment with active rhyolitic volcanism through a marine shelf environment with mixed limestone volcaniclastic sedimentation to a moderate to deep water environment with mudstone and turbidite sedimentation and rhyolitic to basaltic volcanism juvenile elastic and nonelastic volcanic rocks are closely associated subequal in volume and comprise 50 to 70 percent of the volcano sedimentary sequence evaluation of fragmentation and emplacement processes indicates that the juvenile volcaniclastics are mainly in situ resedimented and intrusive hyaloclastites rather than pyroelastics classification of primary contact relationships of the volcanics into passive slightly disruptive and highly disruptive types and the internal facies organization of the volcanic units indicate that the volcanics form extrusive domes extrusive tabular flows partially emergent cryptodomes and sills and entirely intrusive sills shallow sills and cryptodomes emplaced into wet subsea floor sediments are texturally similar to but more abundant than lavas in the environment of mineralization these shallow intrusions are distinguished by disruptive upper contacts including sediment veined and sediment matrix hyaloclastite their recognition is critical because the mineralized stratigraphic interval can only be correlated using volcanics and sediments emplaced on the sea floor based on extrusive units the massive sulfides occur within 100 m above the last major extrusive rhyolites and within 50 m below the first major basalt lava integrating the tectonic structural sedimentological and volcanological data shows that the massive sulfides formed in siltstone in the proximal near vent facies association of a low profile mainly nonexplosive moderate to deep water submarine volcano composed of turbiditic sediments interleaved with numerous rhyolitic to basaltic sills lavas and associated hyaloclastites the ores can be related in space and time to the advanced stage of extension and subsidence in a continental margin incipient rift when relatively deep marine conditions were attained generation of voluminous rhyolitic crustal melts waned and basaltic magma started to penetrate up deep extensional faults to the sea floor the spatial association of the last rhyolite extrusions the massive sulfides and the first basalt extrusions suggests that these rocks are genetically related and that all were fed via the same extensional fault system this setting of silicic volcanic associated massive sulfides at benambra provides a model for the many deposits related to mainly nonexplosive volcanism and an alternative to the recently popular submarine pyroclastic caldera model

subaqueous rhyolitic dome top tuff cones a model based on the devonian bunga beds southeastern australia and a modern analogue the relics of two late devonian subaqueous rhyolitic dome top tuff and pumice cone successions are preserved in the bunga beds outlier of the boyd volcanic complex southeastern australia these cone successions and other rhyolitic volcanics of the bunga beds are associated with turbidite and other deep water massflow sedimentary rocks the two cone successions have a generally similar stratigraphy at the base flow banded variably autobrecciated and quench fragmented rhyolite representing an intrusive to extrusive dome is overlain by rhyolitesediment breccia representing extrusion of the dome through the deep water sediment pile and resedimentation down its flanks in the northern cone succession an overlying succession of bedded pumiceous crystal rich to crystal poor tuffs represents the onset of pyroclastic activity and growth of a tuff cone an overyling debris flow deposit represents degradation of part of the cone the topmost unit a stratified pumice succession is thought to represent another cone building eruptive phase and is separated from the underlying strata by a major slide surface the southern cone succession contains less tuff and abundant pumice and is also terminated by a debris flow deposit indicating cone degradation a modern analogue for the inferred eruptive style and sequence is the 1953 1957 rhyolite eruption that formed the tuluman island lava tuff cone complex in the bismarck sea the eruptions were often cyclical consisting of an initial inferred submarine lava forming stage passing into a pumicecone forming stage in some cases a subaeriallava forming stage and a final stage following the cessation of volcanism during which the cones collapsed gravitationally or were destroyed by wave erosion using observations from both the tuluman island eruptions and the preserved stratigraphies of the devonian tuff cones a dynamic model is proposed for the formation of subaqueous rhyolitic dome top tuff and pumice cones

endogenous growth of a miocene submarine dacite cryptodome rebun island hokkaido japan momo iwa rebun island hokkaido japan is a dacite cryptodome 200 300 m across and 190 m high the dome is inferred to have intruded wet poorly consolidated sediment in a shallow marine environment the internal structure of the dome is concentric with a massive core banded rim and narrow brecciated border all of which are composed of compositionally uniform feldspar phyric dacite boundaries between each of the zones are distinct but gradational the massive core consists of homogeneous coherent unfractured dacite and is characterized by radial columnar joints 60 200 cm across the banded rim encircles the massive core and is 40 m wide it is characterized by large scale flow banding parallel to the dome surface the flow banding comprises alternating partly crystalline and more glassy bands 80 150 cm thick the outermost brecciated border is up to 80 cm thick and consists of in situ breccia and blocky peperite the in situ breccia comprises polyhedral dacite clasts 5 20 cm across and a cogenetic granular matrix the blocky peperite consists of polyhedral dacite clasts 0 5 2 cm across separated by the host sediment mudstone the internal structures of the dome suggest endogenous growth involving a continuous magma supply during a single intrusive phase and simple expansion from the interior although much larger the internal structures of momo iwa closely resemble those of lobes in subaqueous felsic lobe hyaloclastite lavas

morphology and growth style of a miocene submarine dacite lava dome at atsumi northeast japan abstract a miocene submarine lava dome at tate iwa atsumi yamagata japan displays well preserved primary morphological features the dome currently rises above a wave cut coastal platform and is 55 m high and 90 180 m across it comprises a massive core and a lava lobe hyaloclastite rim both of which are composed of compositionally uniform feldspar phyric dacite sio2 64 wt the boundary between these two zones is distinct but gradational the massive core consists of homogeneous coherent dacite and is characterized by flow banding along the margin and by columnar joints radiating from the centre to the margin the lava lobe hyaloclastite rim encircles the massive core and consists of a complex of 80 90 dacitic lava lobes and 10 20 hyaloclastite the lava lobes are 1 6 m thick 3 12 m wide and more than 5 m long each lobe consists of a radially columnar jointed core and a glassy rim 10 30 cm thick the hyaloclastite comprises polyhedral dacite clasts 5 100 cm across in a matrix of dacite fragments up to 5 mm across the lava lobes and hyaloclastite have gradational contacts in places paleobathymetric studies based on foraminifera in sediment beneath the dome and the k ar age of the dome suggest that it was extruded in a middle bathyal environment 1500 2000 m below sea level at 12 9 0 6 ma the internal structures of the dome suggest that it formed by a combination of exogenous growth involving extrusions of small dacite lobes and endogenous growth involving a continuous magma supply and simple expansion from the interior the presence of small lava lobes along the rim suggests that the magma had relatively low viscosity at the time of extrusion in spite of its high silica content magma temperatures calculated by two pyroxene and fe ti oxide geothermometers were 999 1042 and 957 1005 c respectively the inferred low viscosity may be attributed to the high temperature of the magma and or a high confining pressure resulting from the deep sea environment

facies architecture and origin of a submarine rhyolitic lava flow dome complex ponza italy the islands of ponza and gavi western pontine archipelago italy preserve parts of a subaqueously emplaced high silica 75 77 sio2 rhyolitic lava flow that overlies the margins of three older domes exposures of the lava alone cover 24 km2 and with a thickness of 150 m exposed in the cliffs the preserved volume of the lava flow is 3 6 km3 and the original volume is likely to have exceeded 7 km3 the lava flow consists of coherent obsidian and several hyaloclastite facies including in situ and clast rotated breccias and sandstones which all exhibit gradational relationships the coherent obsidian occurs as separate domains and is often flow banded although flow folds occur the banding is generally subhorizontal dipping gently to the nne the hyaloclastite representing 90 of the lava by volume has a diffuse layering varying from 0 5 to 2 m in thickness this layering also dips gently to the nne and is marked in places by alternation of some coherent horizons coarse and finely fragmented in situ hyaloclastite pumiceous hyaloclastite and clast rotated hyaloclastite the layering is laterally continuous and consistent in orientation over most of the 13 km length of ponza the subhorizontal orientation of the flow banding and presence of resedimented deposits in the upper parts of the sequence indicate that the unit is a lava flow the resedimented deposits comprise both proximal and distal deposits of debris flows and turbidity currents initiated by gravitational instability of the upper and marginal parts of the lava flow and include stratified monomictic obsidian breccias and bedded vitric siltstones and sandstones the lava flow has been intruded and crosscut by rhyolitic bodies of various ages including discordant coherent to internally fractured contemporaneous rhyolite bodies and late penecontemporaneous dikes with glassy margins and alteration haloes

use of joint growth directions and rock textures to infer thermal regimes during solidification of basaltic lava flows abstract thermal contraction joints form in the upper and lower solidifying crusts of basaltic lava flows and grow toward the interior as the crusts thicken lava flows are thus divided by vertical joints that by changes in joint spacing and form define horizontal intraflow layers known as tiers entablatures are tiers with joint spacings less than about 40 cm whereas colonnades have larger joint spacings we use structural and petrographic methods to infer heat transfer processes and to constrain environmental conditions that produce these contrasting tiers joint surface morphology indicates overall joint growth direction and thus identifies the level in a flow where the upper and lower crusts met rock texture provides information on relative cooling rates in the tiers of a flow lava flows without entablature have textures that develop by relatively slow cooling and two joint sets that usually meet near their middles which indicate mostly conductive cooling entablature bearing flows have two main joint sets that meet well below their middles and textures that indicate fast cooling of entablatures and slow cooling of colonnades entablatures always occur in the upper joint sets and sometimes alternate several times with colonnades solidification times of entablature bearing flows constrained by lower joint set thicknesses are much less than those predicted by a purely conductive cooling model these results are best explained by a cooling model based on conductive heat transfer near a flow base and water steam convection in the upper part of an entablature bearing flow calculated solidification rates in the upper parts of such flows exceed that of the upper crust of kilauea iki lava lake where water steam convection is documented use of the solidification rates in an available model of water steam convection yields permeability values that agree with measured values for fractured crystalline rock we conclude therefore that an entablature forms when part of a flow cools very rapidly by water steam convection flooding of the flow top by surface drainage most likely induces the convection colonnades form under conditions of slower cooling by conductive heat transfer in the absence of water

internal structure and emplacement of an upper pliocene dacite cryptodome milos island greece the upper pliocene kalogeros cryptodome is exposed on the northeastern part of milos greece the dacite cryptodome is 800 1300 m across and at least 120 m high it is inferred to have intruded wet unconsolidated pumiceous sediments in a shallow marine environment the kalogeros cryptodome includes five facies the coherent dacite facies 80 vol consists of massive non vesicular dacite and is characterised by radial columnar joints 20 250 cm across the banded dacite facies 15 vol encircles the coherent core and is up to 40 m thick it comprises alternating bands 0 5 4 m thick of pale grey and black dacite the fractured dacite facies less than 1 vol forms an irregular zone 1 3 m thick at the outer margin of the cryptodome the outermost massive dacite breccia facies less than 2 vol consists of blocky to polyhedral dacite clasts 1 40 cm in diameter and is characterised by domains of jigsawfit and clast rotated breccia the stratified dacite breccia 2vol is clast to matrix supported monomictic poorly sorted and composed of dacite clasts up to several metres in diameter the facies association collectively records endogenous growth of a cryptodome that involved a continuous magma supply during a single intrusive phase and simple expansion inflation during emplacement the margins of the kalogeros cryptodome were quench fragmented forming an outer domain of intrusive hyaloclastite and intensely fractured dacite the near solid outer carapace insulated the hotter less viscous interior laminar shear accompanied inflation generating large scale flow banding around the outer part of the core once stagnant concentric isotherms were established within the cryptodome and controlled the orientation of columnar joints cryptodomes are characterised by a well developed internal concentric distribution of distinctive textural domains modest autoclastic breccia and the absence of redeposited autoclastic facies

spherulitic aphyric pillow lobe metatholeiitic dacite lava of the timmins area ontario canada a new archean facies formed from superheated melts fragmental rocks of the v10 units of the vipond formation of the tisdale assemblage previously have been identified as pillow basalts but many samples are shown to be intermediate to felsic in character likely tholeiitic dacite in composition specifically the v10b unit is mapped as a pillow lobe dacite aside from being more geochemically evolved in terms of their immobile trace elements these rocks differ from typical pillow basalts in that they have more abundant primary breccia and hyaloclastite the pillow lobes are contorted having been folded in a plastic state and are zoned typically having a spherulite rich core moreover the flows are aphyric interpreted to mean that they were erupted in a superheated state this along with their pillow lobe nature demonstrates that they were erupted as relatively low viscosity melts for such silicic compositions interaction with water quenched the outer pillow lobe and contributed to the formation of the abundant breccia the fact that the melt was crystal and microlite free inhibited crystal growth such that the bulk of the lobes were quenched to crystal free glass nucleation occurred only in the cores where cooling rates were lower in comparison to the medial and exterior areas of the pillow lobes although in the cores crystal growth rates were high so that abundant spherulite formation took place the flows are exposed over a strike length of more than 10 km and are interpreted to be continuous and to have resulted from fissure eruptions the resulting porous permeable high fe mg high surface area glassy rocks may have been ideal for channeling and reacting with gold bearing hydrothermal solutions descriptions of most other archean subaqueous felsic lobes document that the lavas were phyric and that spherulites formed near the external lobe margin the distinctly different zoning of the v10b pillow lobes suggests that they constitute a new archean facies formed from superheated melts

textures formed during emplacement and cooling of a palaeoproterozoic small volume rhyolitic sill the 52 m thick rhyolitic sill at onedin in northwestern australia intruded wet unconsolidated sediment of the koongie park formation during the palaeoproterozoic the sill is composed of five main zones a thin 1 2 m thick basal peperitic contact zone with sparse large 61 cm amygdales and abundant spherulites grades up over 1 m into the lower zone the lower zone is 10m thick and made up of radiating feldspar laths surrounded by granophyric quartz and feldspar and has less than 5 amygdales in the overlying middle zone 20m thick amygdales increase in abundance from 5 near the base to 20 near the top this zone is dominated by spherulitic rhyolite the 18 m thick upper zone is complex with alternating 10 cm thick layers containing abundant spherulites lithophysae amygdales and perlite amygdales make up to 40 of the rock a pumiceous 2 3 cm thick layer occurs at the upper contact fluidal and blocky peperite marks the 1 2 m thick upper contact zone the five zones formed in response to different cooling rates within the sill the lower zone experienced the slowest cooling rate whereas the upper and basal contact zones with abundant glass cooled rapidly there is an overall upward increase in the amygdale abundance in many cases early formed bubbles became nucleation sites for lithophysae spherical bubbles formed in melt above the glass transition temperature as crystallisation proceeded irregularly shaped bubbles formed relatively late in crystalline zones textural zones in the rhyolitic sill at onedin are most similar to those in large volume rhyolitic lavas even though it is a small volume intrusion this similarity may be related to efficient heat retention beneath covering sediments for the sill or beneath a thick crust for large volume lavas the presence of the crystalline lower zone immediately above the base and a high proportion of crystalline rhyolite and the lack of autobreccia and sub vertical flow banding near the top distinguish this rhyolite as a sill rather than a lava

conductive cooling of lava columnar joint diameter and stria width as functions of cooling rate and thermal gradient abstract much evidence exists that column diameter and stria width of columnar joints in lava flows vary inversely with cooling rate t t and thermal gradient t x at the point of fracture as the flow cools some evidence suggests that lava flows cool conductively a conductive cooling model yields cooling rate and thermal gradient through space and time for cooling igneous bodies both for extrusive and intrusive settings thermal gradient and cooling rate are then linked to stria width and column diameter with faster cooling leading to narrower columns and steeper gradients to narrower striae this model shows large regions within a body where the ratio of thermal gradient to cooling rate is nearly constant and predicts that the ratio of stria width to column diameter is also nearly constant this prediction is consistent with field observations both in this and other works the model also suggests that if conductive cooling dominates then joint spacing i e column diameter should increase inward from the margins of the body as should stria width

the facies architecture of a paleoproterozoic basaltic andesite synvolcanic intrusion flin flon manitoba canada the paleoproterozic 1920 unit occurs at the base of the hidden formation overlying greater than 90 million tons of volcanogenic massive sulfide deposits at flin flon manitoba canada the 1920 unit is a laterally discontinuous fe ti p rich basaltic andesite fe2o3 12 wt tio2 1 1 wt and p2o5 0 30 wt consisting of massive pillowed and in situ brecciated basaltic andesite the 1920 unit is characterised by facies textures and structures similar to those associated with subaqueous basaltic lavas however the presence of blocky and fluidal peperite along the upper contact and lens shaped inclusions of tuff within and along its basal contact as well as the discordancy of the upper contact suggest that the predominantly massive basaltic andesite was emplaced as a high level synvolcanic sill into wet unconsolidated volcaniclastic deposits the presence of a breccia containing clasts of the 1920 unit within the package of volcaniclastic rocks overlying the 1920 unit and a pillowed facies within the 1920 unit suggest that locally the 1920 unit breached the seafloor an abundance of synvolcanic dikes within the footwall to the 1920 unit including the presence of a basaltic andesite dike that merges into the 1920 unit indicates the possible location of the feeder for the 1920 unit in the northwestern portion of the area

fluidization of wet sediments during the emplacement and cooling of various igneous bodies during the emplacement and cooling of igneous bodies fluidization of wet sediment by water vapour may occur by heating or by pressure relief during the opening of fractures momentary fluidization causes sediment reconstitution and local transport continuous fluidization can result in substantial sediment displacement theoretical considerations show that fluidization due to heating is not likely to occur at depths where pressure is much greater than 312 bars studies of the emplacement of sills of andesite ayrshire fluid rhyolite ramsey island and viscous rhyolite moelwyn hills recognize a wide range of phenomena attributable to wet sediment fluidization and in places modify existing interpretations the transgressive bases of the subaqueously welded ash flow tuffs of the capel curig volcanic formation are consistent with emplacement attended by fluidization of the subjacent sediment evidently fluidization is a means by which large volumes of wet sediment can be replaced by igneous material with minimal disturbance of the remaining host also fluidization can result in exceptionally complex contact relationships between sedimentary host and intrusive rocks and can cause fractures formed during cooling to be thoroughly pervaded by sediment fluidization is commonly associated with the formation of peperites

magma water interactions in subaqueous and emergent basaltic in the subaqueous growth and emergence of a basaltic volcano clasts are formed by one or a combination of 1 explosive release of magmatic volatiles 2 explosive expansion and collapse of steam formed at magma water contact surfaces 3 explosive expansion of steam following enclosure of water in magma or entrapment of water close to magma and 4 cooling contraction these processes named respectivelymagmatic explosivity contact surface steam explosivity bulk interaction steam explosivity andcooling contraction granulation can be enhanced by mutual interaction and feedback the first three explosive processes are limited at certain water depths hydrostatic pressures and become increasingly vigorous at shallower levels the depth of onset of magmatic explosivity depends largely on juvenile volatile content it is up to 200 m for tholeiitic magmas and up to 1 km for alkalic magmas at the depth where formation of clastic deposits becomes predominant over effusion of lavas magmatic explosivity is subordinate to steam explosivity as a clast forming process the upward transition to accumulation of dominantly clastic deposits is not simply related to the onset of substantial exsolution of magmatic volatiles and can occur without it contact surface explosivity commonly requires initiation by a vigorous impact between magma and water and although no certain depth limit is known likelihood of such explosivity decreases rapidly with depth clast generation by bulk interaction explosivity appears to be restricted to depths much shallower than that of the critical pressure of water which in sea water is at about 3 km cooling contraction granulation can occur in any depth of water but at shallow levels may be replaced by contact surface explosivity during continuous eruption under water tephra can be ejected and deposited within a cupola of steam such that rapid quenching does not occur emergent volcanoes are characterized by distinctive steam explosive activity that results primarily from a bulk interaction between rapidly ascending magma and a highly mobile slurry of clastic material water and steam the water gets into the vent by flooding across or through the top of the tephra pile and violent explosions cease when this access is sealed the eruptions during emergence of surtsey and capelinhos typify the distinctive explosive activity the style and controls of which are different from those of maar volcanoes

structures textures and cooling histories of columbia river basalt flows flood basalt flows of the columbia river basalt group commonly exhibit well developed colonnade and entablature structures formed during cooling of individual flows the colonnade refers to the well defined columnar structure of many flows which appears in the lowermost 10 to 30 and in places the uppermost 10 to 20 entablature refers to those parts of a flow that have smaller column diameters and more irregular pattern fractures than those of colonnade entablature commonly occupies the central 60 to 70 of a flow in some cases however the entablature is interrupted by colonnade in such cases the entablature and colonnade may be repeated once or twice within a single flow in other cases flows may lack an entablature entirely petrographic examination reveals a pronounced correlation between structural and textural features thin sections of colonnade basalt have relatively coarse cruciform or octahedral fe ti oxide crystals and 20 glassy mesostasis the central entablature basalt in contrast has much more mesostasis as much as 60 and has textural features such as feathery fe ti oxide crystals indicative of more rapid cooling these features are the reverse of what might be expected for normal conductive cooling because cooling rates at the flow margins should be greater than those in the flow interior we reconciled the observed basalt textures with thermal models by taking into account rainfall and deranged drainage during crystallization of the flow downward migration of water through cracks would cause convective cooling of underlying unsolidified lava this effect and its consequences have been modeled numerically in order to clarify the physical constraints on the mechanism by which the central part of the lava can be quenched we have found that a water infiltration front can explain the presence of a thick quenched flow interior provided that the infiltration front moves downward just behind the lava solidification front despite heavy rainfall on hawaii for example 250 cm yr at kilauea iki exposed prehistoric lava flows on that island exhibit only colonnades we deduce from this that heavy rainfall alone is inadequate to quench flow interiors extensive flooding by deranged drainage or perhaps extremely heavy rainfall 250 cm yr is apparently required to produce entablatures the model also can be used to explain the occurrence of repeated entablature and colonnade by considering the effects of intermittent inundation the imposition of boundary conditions that mimic the removal and reoccurrence of flooding yields cooling histories that are consistent with the development of multiple colonnades and entablatures

setting of zn cu au ag massive sulfide deposits in the evolution and facies architecture of a 1 9 ga marine volcanic arc skellefte district sweden the skellefte mining district occurs in an early proterozoic mainly 1 90 1 87 ga svecofennian magmatic province of low to medium metamorphic grade in the baltic shield in northern sweden the district contains over 85 pyritic zn cu au ag massive sulfide deposits and a few vein au deposits and subeconomic porphyry cu au mo deposits the massive sulfide deposits mainly occur within and especially along the top of a regional felsic dominant volcanic unit attributed to a stage of intense extensional continental margin arc volcanism from facies analysis we interpret the paleogeography of this stage to have comprised many scattered islands and shallow water areas surrounded by deeper seas all the major massive sulfide ores occur in below wave base facies associations however some ores occur close to stratigraphic intervals of above wave base facies associations and the summits of some volcanoes that host massive sulfides emerged above sea level intense marine volcanism was superceded at different times in different parts of the district by a stage of reduced volcanism uplift resulting in subregional disconformities and then differential uplift and subsidence resulting in a complex horst and graben paleogeography uplift of the arc is attributed to the relaxation of crustal extension and the emplacement of granitoids to shallow crustal levels a few massive sulfide ores formed within the basal strata of this second stage the horst and graben system was filled by prograding fluvial deltaic sediments and mainly mafic lavas and during this stage the skellefte district was a transitional area between renewed arc volcanism of more continental character to the north and subsidence and basinal mudstone turbidite sedimentation to the south this whole volcanotectonic cycle occurred within 10 to 15 m y we define 26 main volcanic sedimentary and intrusive facies in the skellefte district the most abundant facies are 1 normal graded pumiceous breccias which are interpreted as syneruptive subaqueous mass flow units of pyroclastic debris 2 porphyritic intrusions and 3 mudstone and sandstone turbidites facies associations define seven main volcano types which range from basaltic shields to andesite cones and rhyolite calderas despite this diversity of volcano types most massive sulfide ores are associated with one volcano type subaqueous rhyolite cryptodome tuff volcanoes these rhyolite volcanoes are 2 to 10 km in diameter 250 to 1 200 m thick at the center and are characterized by a small to moderate volume rhyolitic pyroclastic unit intruded by rhyolite cryptodomes sills and dikes massive sulfide ores occur near the top of the proximal near vent facies association the remarkable coincidence in space and time between the ores and this volcano type indicates an intimate genetic relationship between the ores and the magmatic evolution of the volcanoes many of the massive sulfide ores occur within rapidly emplaced volcaniclastic facies and are interpreted to have formed by infiltration and replacement of these facies some of the ore deposits have characteristics of both marine massive sulfides and subaerial epithermal deposits we suggest that massive sulfides in the skellefte district span a range in ore deposit style from deep water sea floor ores to subsea floor replacements to shallow water and possibly subaerial synvolcanic replacements facies models are provided for the mineralized rhyolite volcanoes and volcanological guides are provided for exploration for blind ores within these volcanoes

origin and metamorphism of peperite and associated rocks in the devonian elwell formation northern sierra nevada california the late devonian elwell formation is part of a late paleozoic island arc assemblage exposed in the northern sierra nevada california in the 10 km interval from mount elwell plumas county to packer lake sierra county it consists largely of numerous andesitic sills interleaved with phosphatic radiolarian chert and turbidity current and debris flow deposits peperite formed by interaction of intrusive andesitic magma and still moist incompletely lithified chert not far beneath the sea floor occurs sparingly the whole assemblage was thoroughly recrystallized greenschist facies regional metamorphism during the late jurassic nevadan orogeny the phosphatic chert is composed of white fluorapatite rich lenses and nodules enclosed in a black matrix consisting of microgranular quartz and carbonaceous material probably graphite radiolaria family entactiniidae are so abundant that the chert is thought to have originated as radiolarian ooze the phosphatic lenses and nodules probably formed by direct inorganic precipitation of fluorapatite from pore water within decaying radiolarians at the sediment water interface the andesitic sills are generally aphyric and aphanitic original textures varied from hyalopilitic in sill interiors to pilotaxitic at sill margins the andesite was charged with plagioclase now albite microlites and quench morphologies occur where sills intruded the wettest chert relatively thick sills commonly display columnar structure whereas a few originally more viscous sills are flow laminated the top of a sill emplaced essentially at the sediment water interface is pillowed many sills bear xenoliths of phosphatic chert and scarce quartz megacrysts evidently derived from the underlying dacitic sierra buttes formation appear nearly everywhere concentrations of major element oxides and immobile trace elements indicate that the andesite originated as part of the tholeiitic rock association of island arcs rare earth abundance patterns are particularly diagnostic of this paleotectonic setting most peperite has poorly sorted coarse ash to coarse block andesitic fragments uniformly distributed through a voluminous black chert matrix where invaded chert retained the most pore water quenched andesitic magma formed irregularly shaped lapilli and small pillows that initially exhibited ductile behavior rapid cooling below the softening point of glass resulted in brittle fracture of the lapilli during dispersal through the chert by steam explosions steam explosions also incorporated into the peperite rigid clasts of locally dewatered chert and shattered already formed phosphatic nodules where the invaded chert was better lithified freezing sill margins simply granulated quietly in situ by thermal contraction cracking of microlitic glass fluid chert injected widely spaced contraction joints deep within the sills in all cases there was sufficient heated connate water to cause palagonitization and chloritization of andesitic fragments greenschist facies metamorphism produced two mineral assemblages in the interiors of andesitic sills the second perhaps reflecting locally higher pco 2 ph 2 o during metamorphism 1 albite quartz epidote actinolite chlorite ti rich mineral calcite stilpnomelane white mica and 2 albite quartz chlorite ti rich mineral calcite stilpnomelane white mica epidote rich specks occurring sporadically in the sills represent metasomatism on a scale of millimetres to centimetres such specks are mineralogically zoned in andesitic fragments in intensely recrystallized peperite additional examples of local meta somatic activity include syntectonic crystallization of calcite rhombohedra across original vesicle walls in a sill cleavage developed locally during regional metamorphism is parallel to axial planes of closely appressed folds that deformed andesitic fragments in peperite and flow laminae in sills

on lava dome growth with application to the 1979 lava extrusion of the soufriere of st vincent abstract a theoretical analysis is presented for the spread of a viscous liquid flowing under its own hydrostatic pressure on a horizontal surface in order to model lava dome formation two situations are considered in detail the spreading of a constant volume of liquid and the case where the amount of liquid is continually increased experiments with silicone liquids show close agreement with theory the formation of a basaltic andesite lava extrusion in 1979 on the crater floor of the soufriere of st vincent west indies provided the motivation for and an application of the model the extrusion reached a diameter of 868 m and a height of 133 m over a period of 150 days over the first 90 days the growth relationships were consistent with those predicted by theory application of the theory to the soufriere dome suggests an effective viscosity of 2 x 10 12 poise for the basaltic andesite lava the large effective viscosity calculated for the lava may be attributed to the dominant influence of a high viscosity skin which forms at the margins of the flow as it cools after 70 days the rate of growth of the extrusion markedly decreased because a substantial collar of rubble accumulated at the flow front due to this collar the growth of the extrusion ceased after 150 days from approximately two weeks after the initiation of the extrusion the discharge rate of lava decreased approximately linearly with increasing dome height this observation suggests that the lava ascended under a decreasing hydrostatic driving pressure and that extrusion ceased when the lava column reached hydrostatic equilibrium

morphological features of miocene submarine coherent lavas from the green tuff basins examples from basaltic and andesitic rocks from the shimokita peninsula northern japan basaltic and andesitic volcanic rocks of miocene age exposed in the shimokita peninsula northern japan illustrate morphological features of typical submarine coherent lavas of the green tuff basins in japan they are pillow lobes with surface structures such as ropey wrinkles corrugations spreading cracks and tensional cracks and lava lobes composed of a lithic core and glassy border zone or rim with an in situ breccia zone grading outward into surrounding hyaloclastite in addition they include massive lavas with columnar joints and jointed dykes the submarine coherent lavas and dykes are commonly associated with hyaloclastic breccias such as pillow fragment breccia and angular fragment breccia the descriptions of the miocene volcanic rocks in the shimokita peninsula provide good criteria for recognition of submarine coherent lavas of basalt and andesite

an upper pliocene coarse pumice breccia generated by a shallow submarine explosive eruption milos greece the filakopi pumice breccia fpb is a very well exposed pliocene volcaniclastic unit on milos greece and has a minimum bulk volume of 1 km3 it consists of three main units a basal lithic breccia 4 8 m mainly composed of angular to subangular andesitic and dacitic clasts up to 2 6 m in diameter b very thickly bedded poorly sorted pumice breccia 16 17 m and c very thick reversely graded grain supported coarse pumice breccia 6 5 20 m at the top the depositional setting is well constrained as shallow marine up to a few hundred metres by overlying fossiliferous and bioturbated mudstone this large volume of fine pumice clasts is interpreted to be the product of an explosive eruption from a submarine vent because 1 pumice clasts are the dominant component 2 the coarse pumice clasts 64 mm have complete quenched margins 3 very large 1 m pumice clasts are common 4 overall the formation shows good hydraulic sorting and 5 a significant volume of ash was deposited together with the coarsest pyroclasts

paleozoic fluidization folding and peperite formation northern sierra nevada california recognition of peperite is important in establishing broad contemporaneity of magmatism and sedimentation at an occurrence of peperite discovered recently among late paleozoic island arc deposits in the northern sierra nevada it is possible to reconstruct the peperite forming process largely from field observations alone the generally concordant upper margin of a basalt intrusion adjoins thinly laminated basaltic tuff in the taylor formation that was water saturated and unlithified at the time of intrusion the water saturated ash initially was heated by the intrusion at approximately constant essentially hydrostatic pressure concurrently the top of the intrusion was quenched and divided by shrinkage fractures pressure momentarily approached zero as the shrinkage fractures opened abruptly causing vaporization of the pore water and substantial expansion and fluidizing flow of the resulting steam the unlithified ash was swept into the shrinkage fractures and where the downward propagating fractu

active tectonics of the aegean region the aegean sea and its surrounding regions comprise one of the most rapidly deforming parts of the alpine himalayan mountain belt though the deformation of the belt as a whole is related to thc northward move ment of africa arabia and india relative to eurasia the tectonics of the aegean region itself is dominated by strike slip and extensional motions the actively deforming part of the aegean region is not large about 700 x 700 km2 compared with that in asia yet it has had an influence on our views of continental tectonics disproportionate to its size and comparable perhaps only to tibet fundamental contributions from the aegean have been made to many topics including