the iberian type of volcano sedimentary massive sulphide deposits the iberian pyrite belt located in the sw iberian peninsula contains many paleozoic giant and supergiant massive sulphide deposits including the largest individual massive sulphide bodies on earth total ore reserves exceed 1500 mt distributed in eight supergiant deposits 100 mt and a number of other smaller deposits commonly with associated stockwork mineralizations and footwall alteration haloes massive sulphide bodies largely consist of pyrite with subordinated sphalerite galena and chalcopyrite and many other minor phases although substantial differences occur between individual deposits both in mineral abundance and spatial distribution these deposits are considered to be volcanogenic roughly similar to volcanic hosted massive sulphides vhms however our major conclusion is that the iberian type of massive sulphides must be considered as a vhms sub type transitional to shms

magmatism in the iberian pyrite belt petrological constraints on a metallogenic model igneous formations associated with massive sulphide deposits in the iberian pyrite belt ipb are essentially composed of basic lavas and dolerites and dacitic to rhyolitic volcanites intermediate lavas are subordinate the basic rocks show variable geochemical characteristics lavas and dolerites comparable to recent within plate alkaline basalts seem restricted to the western and southern parts of the ipb whereas basic rocks comparable to continental tholeiites or arc related basalts occur across the whole belt the felsic rocks are classified as calc alkaline and belong to the low al2o3 and high yb type at given sio2 al2o3 and tio2 contents they show variable zr nb and hree contents heavy rare earth element fractionation decreases from the dacites to the rhyolites gd yb n 1 whereas the negative eu anomaly becomes more pronounced the characteristics of the rhyolites are typical of sulphide fertile volcanic packages trace element modelling suggests that the felsic rocks evolved from a dacitic parent magma through fractional crystallization of hornblende and plagioclase partial melting of an amphibolite protolith which appears as the most probable model for the origin of this dacitic magma requires a high t p gradient in the crust the occurrence of alkaline basalts and continental tholeiites is consistent with formation of the ipb in a tensional tectonic setting however the associated island arc tholeiites suggest a location in a domain of plate convergence emplacement in a fore arc basin over a recently accreted crustal segment is envisaged as a possible hypothesis to account for the geological and petrological constraints a high geothermal gradient and eruption in a submarine tensional basin could have been two key ingredients for the development of massive sulphide deposits within the ipb

rhyolitic globular peperites from the aznalcollar mining district iberian pyrite belt spain physical and chemical controls abstract globular peperites are exposed along the aznalcollar mining district iberian pyrite belt ipb spain these rocks originated by mingling of a low viscosity rhyolitic sill with unlithified wet sediments highly amygdaloidal up to 80 vol millimeters to centimeters across rhyolite globules extend from the upper contact of the sill for several centimeters into fine grained volcaniclastic host rocks the sill is very similar to other rhyolites in the ipb in terms of petrography and chemistry it is aphyric to very sparsely quartz phyric with small quartz phenocrysts vesicularity has been measured in the sill as vesicle area fraction determined by point counting over a rectangular grid and number density per unit area determined by counting every vesicle within the grid area amygdale distribution measured within the sill indicates a process by which vesicles migrated vertically from the lower portions of the sill to the roof zone we suggest that this migration process caused local density changes large enough to allow the rhyolitic magma to become buoyant enough to move through the overlying sediments producing globular peperites globular structures are preserved when rhyolite globules entered the wet sediment

magma sediment mingling in an ordovician rift basin the plouezec plourivo half graben armorican massif france the plouezec plourivo basin ppb armorican massif is an ordovician half graben filled with red bed deposits lying on a cadomian metamorphic basement peperites crop out along its northeast coastal sector in connection with diverse magmatic and hydrothermal products combining field structural observations and geochemical analyses of magmatic rocks leads us to propose a model of emplacement of peperites in an extensional setting the ppb peperites result from mingling between magma and initially wet sediment the main corresponding facies consist of rounded magmatic bodies corrugated lava sediment contacts and a remarkable thin sheet of fluidized sediment extending over about 100 m parallel and c 10 cm above the base of a porphyritic lava flow a few ppb peperites formed under a dominant brittle regime are autoclastic breccias at the base of a lava flow and polymict breccias including both fluidal and blocky volcanic clasts sediment fluidization formation of vapor films magma sediment density contrasts and non explosive fragmentation are the main mechanisms invoked to generate the ppb peperites the c 100 m extended sediment derived sheet lying near the base of a lava flow likely formed by diapiric rise of fluidized sediment through molten lava we show that silicic contamination accompanied these peperitic processes

constitutive relationships and physical basis of fault strength due to flash heating 1 we develop a model of fault strength loss resulting from phase change at asperity contacts due to flash heating that considers a distribution of contact sizes and nonsteady state evolution of fault strength with displacement laboratory faulting experiments conducted at high sliding velocities which show dramatic strength reduction below the threshold for bulk melting are well fit by the model the predicted slip speed for the onset of weakening is in the range of 0 05 to 2 m s qualitatively consistent with the limited published observations for this model earthquake stress drops and effective shear fracture energy should be linearly pressure dependent whereas the onset speed may be pressure independent or weakly pressure dependent on the basis of the theory flash weakening is expected to produce large dynamic stress drops small effective shear fracture energy and undershoot estimates of the threshold slip speed stress drop and fracture energy are uncertain due to poor knowledge of the average contact dimension shear zone thickness and gouge particle size at seismogenic depths

structure and emplacement of a rhyolitic obsidian flow little glass mountain medicine lake highland northern california many rhyolitic obsidian flows show consistent stratigraphic relations among textural units exposed in the flow fronts of undissected flows and in cross sections of older flows the stratigraphy of the holocene little glass mountain rhyolitic obsidian flow consists of from bottom to top air fall tephra deposits basal breccia coarsely vesicular pumice obsidian finely vesicular pumice and surface breccia slightly crystalline rhyolite occurs near the vent areas a model of obsidian flow emplacement based on these textural relations is presented this stratigraphy may reflect the distribution of volatiles within the magma source region with the interlayered contact between coarsely vesicular pumice and overlying obsidian indicating stratification of volatiles in the magma body the distribution of pumiceous and glassy zones along with the orientations of flow banding can be used to map the surface structure of rhyolitic obsidian flows this complex structure as mapped on the little glass mountain flow reflects both the initial flow stratigraphy and subsequent deformational processes during emplacement of the lava three processes disrupt the initial flow configuration the rise of coarse pumice diapirs from the base of the flow the inward propagation of fractures in areas of extension and surface folding in sites of flow parallel compression subvertical flow banding in vent areas indicates that fracturing accompanies the emergence of lava most of the observed upper surface of a dome originates as a fracture plane the structure of domes that form over vent areas may reflect the orientation of dike like conduits as well as the local state of stress during extrusion

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

a comparison of miocene and archean rhyolite hyaloclastites evidence for a hot and fluid rhyolite lava abstract in this paper we compare the geology and petrography of miocene and archean submarine rhyolite hyaloclastites the hyaloclastites are sparsely 10 or less plagioclase quartz and pyroxene phyric the hyaloclastites consist of a feeder dyke from which branch lava lobes and irregularly shaped lava pods the lava bodies consist of a holocrystalline core with microlitic texture grading outward into a flow layered rim zone and finally into obsidian the proportion of plagioclase and pyroxene microlites decreases outward some layers of the rim zone may be pumiceous vesicularity up to 50 vesicle size 1 mm or less but most of the lava has less than 5 vesicles one or a few cm long the obsidian shows perlitic fracture patterns the lava bodies grade through an in situ breccia into a hyaloclastite composed of angular obsidian granules and in many cases of fragments of lava lobes evidence for alteration at high temperature is as follows in the archean rhyolite hyaloclastites plagioclase microlites are overgrown by quartz albite spherulites furthermore parts of the miocene and archean hyaloclastite have been cemented and granules have been marginally replaced by quartz and albite hyaloclastite cemented at high temperature locally shows columnar joints at low temperatures obsidian has been hydrated and or has been replaced by clay minerals zeolites chlorite or prehnite chess board albite and fibroradial prehnite in archean hyaloclastite is possibly a pseudomorph after zeolites the sparsely porphyritic nature of the lava and the absence of microlites from the quenched glass suggests that the thyolite hyaloclastites extruded at high near liquidus temperature furthermore pumice is present only locally in the flow layered rim zone and in fragments derived from that zone these features suggest that vesiculation was inhibited by the weight of the water column high temperature and possibly the volatile h 2 o content explain the relatively low viscosity and shear strength of the lava and resulted in the flow morphology particular to this type of hyaloclastic rhyolite flows

volcano sedimentary characteristics in the abu treifiya basin cairo suez district egypt example of dynamics and fluidization over sedimentary and volcaniclastic beds by emplacement of syn volcanic basaltic rocks abstract this paper describes the neogene lava sediment mingling from the abu treifiya basin cairo suez district egypt the lava sediment interactions as peperites have been identified for the first time at the study area and can be used as paleoenvironmental indicators the identification of peperite reflects contemporaneous time relationship between volcanism and sedimentation and this finding is of primary importance to address the evolutional reconstruction of the abu treifiya basin characterization of the facies architecture and textural framework of peperites was carried out through detailed description and interpretation of their outcrops the peperites and sedimentary rocks are up to 350 m thick and form a distinct stratigraphic framework of diverse lithology that is widespread over several kilometers at the study area lateral and vertical facies of the peperites vary from sediment intercalated with the extrusive intrusive basaltic rocks forming peperitic breccias to lava sediment contacts at a large to small scales respectively peperites encompass five main facies types ascribed to i carbonate sediments hosted fluidal and blocky peperites ii lava flow hosted blocky peperites iii volcaniclastics hosted fluidal and blocky peperites iv sandstone siltstone rocks hosted blocky peperites and iv debris flows hosted blocky peperites soft sediment deformation structures vesiculated sediments sediments filled vesicles and fractures in lava flows indicate that lava flows mingled with unconsolidated wet sediments all the peperites in this study could be described as blocky or fluidal but mixtures of different clast shapes occur regardless of the host sediment the presence of fluidal and blocky juvenile clasts elucidates different eruptive styles reflecting a ductile and brittle fragmentation the gradual variation from fluidal to blocky peperite texture producing the vertical grading is affected by influencing factors e g the viscosity magma temperature confining pressure sediment fluidization and vapor film at the magma sediment interactions peperites in the study area record deposition within a shallow marine and fluvio lacustrine environment accumulated in a rift related basin developed during pre to syn rift phase respectively the facies transitions peperites in this area resulted from the explosive and sediment depositional processes which were mingled separately by volcanism under contrast geological conditions the development of such contrast in the depositional sequences reflects variation in the accommodation to sediment supply in the same accumulation space inside the depocenters during the rifting of the abu treifiya basin hydrothermal mineralizations comprising quartz and carbonate are restricted to peperites and lava flows

facies analyses and volcanic setting of the giant neves corvo massive sulfide deposit iberian pyrite belt portugal in the iberian pyrite belt volcanic rocks are relatively scarce accounting for approximately only 25 of the geologic record with the remaining 75 consisting of sedimentary units this association is very clear in the host succession to the neves corvo massive sulfide deposit in portugal the neves corvo host succession comprises the products of explosive and effusive rhyolitic eruptions intercalated with mudstone that records a submarine below wave base environment and provides precise biostratigraphic age constraints the first and second volcanic events involved eruptions at local intrabasinal vents the first event generated thick beds of fiamme breccia that are late famennian in age the fiamme were originally pumice clasts produced by explosive eruptions and were subsequently compacted the second event was the late strunian latest famennian effusion of rhyolitic lava that was pervasively quench fragmented the third and final event is younger than the massive sulfide deposits poorly represented in the mine area and minor compared with the two other events the integration of biostratigraphic data with the volcanic facies architecture indicates that the neves corvo ore deposits are similar in age to the late strunian rhyolitic lava although regionally the iberian pyrite belt is essentially a sedimentary succession ore formation at neves corvo can be closely linked to discrete volcanic events that produced a relatively narrow range of volcanic facies

facies architecture and palaeoenvironmental constraints of subaqueous felsic volcanism in the iberian pyrite belt the paymogo volcano sedimentary alignment abstract the paymogo volcano sedimentary alignment pvsa is an essentially igneous sequence associated with vhms deposits of the iberian pyrite belt unlike other areas of this iberian mining district where mafic igneous rocks are abundant in the pvsa these are essentially felsic two facies associations have been distinguished based on petrographic and chemical features a lower dacite facies association and an upper rhyolite facies association the main ore horizons appear related to volcanic breccias of the rhyolite facies association a detailed petrographic study of these breccias suggests they are resedimented hyaloclastite deposits the analysis of volcanic facies indicates that the volcanism developed at water depths deeper than 500 m consistent with the minimum depth at which vhms deposits can form we conclude that detailed volcanological studies provide effective constraints to palaeoenvironmental interpretations which are relevant in assessing regional stratigraphy and area selection for vhms exploration

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

interactions between andesitic magma and poorly consolidated sediments examples in the neogene shirahama group south izu japan abstract the upper miocene lower pliocene shirahama group of south izu japan is a sequence of volcanielastic shallow sea sediments and subaqueous lava flows it is gently warped with indistinct preferred orientation and is cross cut by many andesitic intrusive bodies which range in size from less than a meter to several hundred meters the intrusive bodies exhibit various shapes and weak preferred orientation and often have hyaloclastic and peperitic textures along their peripheries hydrothermally altered and chaotically disturbed zones of host rocks also occur along the contacts the areas where the intrusive bodies are abundant are almost coincident with the areas where the more inclined beds and synsedimentary faults are developed the intrusive bodies dragged the surrounding beds with or without synsedimentary faults some intrusions occurred along the faults conjugate sets of the faults indicate that they were produced under unstable stress conditions these relationships between the beds and the intrusive bodies are assumed to have originated mostly by interaction between hot magma and poorly consolidated wet sediments at a shallow depth beneath the sea floor when the intrusions took place the stress conditions in the beds were irregular and unstable judging from the shapes and orientation of the intrusive bodies and also from the deformation characteristics of the enclosing beds some of the intrusive bodies are probably feeder dikes which supplied lave to the shirahama group

comparison of rio tinto spain and guaymas basin gulf of california an explanation of a supergiant massive sulfide deposit in an ancient sill sediment complex the volcano sedimentary unit that hosts the stratiform massive sulfide mineralization at rio tinto is reinterpreted as a sill sediment complex that in many respects is analogous to the example that is currently forming in the guaymas basin gulf of california however rio tinto is distinctive because 1 it is a product of bimodal magmatism 2 it has a much higher sill sediment ratio 4 1 lessening the extent of ore forming metal depletion during interaction between hydrothermal fluid and wet sediment and 3 most of the sills were emplaced penecontemporaneously as shown by the ubiquitous development of peperitic sill margins despite the narrowness of the sedimentary screens these data together with evidence for local extrusive behavior imply rapid formation of the complex at exceptionally high levels of emplacement it follows that in the complex variations in thickness of 400 m were directly expressed as basin floor deformation and that troughs in the resultant topography were able to trap exhalative hydrothermal plumes thus preserving the sulfides from dispersion also crucial to the mineralization process was the sheetlike form of the sill complex which by acting as a cap prevented regular convective cell patterns from forming and focused considerable discharge at areas of weakness the intense magmatic pulse the creation of pronounced topography and the capping of the hydrothermal system combined to make a 500 mt supergiant deposit

mud volcanoes indicators of stress orientation and tectonic controls abstract this study examines the use of specific mud volcano features i e elongated calderas aligned vents and elongated volcanoes as potential indicators of tectonic stress orientation the stress indicator principles widely recognised for magmatic systems have been discussed and applied to mud volcano settings such as in the northern apennines and the azerbaijan greater caucasus as well as in other instances where the analysis was fully based on a remote sensing study the results of these applications are promising the obtained maximum horizontal stress sh directions generally showing a good correlation with those determined in the upper crust by classical methods i e earthquake focal mechanism solutions well bore breakouts therefore stress information from mud volcanoes could be used as a proxy for stress orientation 1 where stress data is lacking 2 where settings are inaccessible i e underwater or the surface of planets or simply 3 as supplementary stress indicators this study also pays special attention to structural elements that may control fluid expulsion at various length scales and pathways that should have spawned the mud volcanoes and controlled their paroxysmal events and eruptions different types of sub planar brittle elements have been found to focus fluid flow rising up through fold cores where the vertical zonation of stresses may take part in this process by creating distinctive feeder fracture fault sets on a regional scale mud volcanoes in active fold and thrust belts may occur over wider areas such as the prolific mud volcanism in azerbaijan or may cluster along discrete structures like the steep pede apennine thrust in the northern apennines where the generation of overpressures is expected to establish a positive feedback loop allowing for fault movement and mud volcanism

variation in peperite textures associated with differing host sediment properties peperites formed by mixing of magma and wet sediment are well exposed along punta china baja california mexico where two sills intrude a section of lava flows limestones and volcaniclastic rocks irregular lobes and dikes extend from the sills several meters into host sediments including highly comminuted flow top breccias lithic lapilli tuff breccias and shelly micrites whereas intrusive contacts with lava flows are sharp and planar where one sill intruded both coarse grained volcaniclastic rock and fine grained limestone textural differences between the hosts produced strikingly different styles of peperite blocky masses of the basaltic intrusions up to 1 m in size were dispersed for distances up to 3 m into host lithic lapilli tuff breccias the blocks consequently underwent in situ fragmentation as they were rapidly quenched the high degree of dispersion resulted from steam explosions as the magma enveloped pockets of water in the coarse grained permeable host elutriation of fine grained material from vertical pipes in tuff breccia above the lower sill provides evidence for meter scale fluidization of the host the contact zone between the basaltic magma and the shelly micrite host resembles a mixture of two viscous immiscible fluids fluidal peperite intrusion occurred behind a stable vapor film which entrained lime mud particles and carried them off grain by grain as magma advanced into the host thin section scale elutriation pipes formed microglobular peperite represents a frozen example of a fuel coolant interaction fci between basaltic magma and fluidized micrite host the intimate intermixing of magma and host at the submillimeter level is attributed to fluid instabilities developed along the magma vapor host interface such intimate intermixing of magma and water bearing fragmental debris is commonly a precursory step toward explosive hydrovolcanism

felsic fire fountaining beneath archean seas pyroclastic deposits of the 2730 ma hunter mine group quebec canada abstract the lapilli tuff breccias ltb 1 and ltb 2 of the archean hunter mine group in the south central part of the abitibi greenstone belt are inferred to be the product of subaqueous lava fountaining intercalated sub wave base iron formations interstratified turbiditic tuffs the absence of wave induced sedimentary structures and the stratigraphic position of lapilli tuff breccias beneath basaltic komatiites support this contention a complete eruptive sequence shows a tripartite division into a massive breccia b stratified lapilli tuff and c turbiditic tuff lapilli tuff division the massive breccia division is characterized by clusters of isolated and compressed irregular shaped clasts inferred to be deposited directly from the hot magmatic lava fountain abundant vesicular pyroclasts with a vesicle content of up to 60 exhibit locally coalescing vesicles indicating bubble nucleation prior to eruption the prevalence of irregular to amoeboid clast shapes suggests transport from the vent in a steamy rich high density current to the site under a self generated steam cupola ubiquitous subequant lapilli size pyroclasts of the stratified lapilli tuff division suggest that significant ingress of water into the fountain changed the prevalent fragmentation process from magmatic to hydrovolcanic the turbiditic tuff lapilli tuff division composed of pumice lithic fragments and vitric ash is envisaged to have formed by gravitational collapse of a subaqueous turbulent eruptive plume this type of eruptive mechanism constituted a minor but important process of volcanic construction on the ocean floor during the archean and possibly during incipient arc and backarc formation in modern day settings

a miocene basanite peperitic dyke at stanley northwestern tasmania australia abstract a miocene basanite dyke at stanley northwestern tasmania australia displays well preserved peperite texture the dyke is 2 m wide and has intruded basaltic breccia host sediment one contact of the dyke is fluidally shaped and amoeboid apophyses 10 25 cm long extend into the host sediment whereas the other contact is characterized by blocky peperite texture comprising tabular to wedge shaped clasts up to 30 cm across separated by host sediment the clasts have internal spherical fractures and some show splinter texture vesicles are common in the clasts and those intersected by clast margins have been filled with sediment the interior of the dyke comprises close packed blocky peperite consisting of tabular wedge shaped and polyhedral clasts tens of centimetres across separated by host sediment these clasts show well developed jigsaw fit texture the textures and structures in the basanite dyke are inferred to have formed in two stages an earlier hotter apophysis forming stage and a later cooler angular clast forming stage both of which occurred during the intrusion of magma into wet poorly consolidated sediment in a shallow marine environment during the apophysis forming stage the magma had relatively low viscosity and progressively displaced wet sediment the wet sediment around the dyke was partly fluidized by vaporization of pore water the angular clast forming stage reflects a change in the rheological behaviour of the magma from ductile to brittle most likely in response to decreasing temperature the chilled parts of the dyke were subject to stress arising from cooling contraction and also from continued pulsatory movement of hotter still ductile magma in the interior of the dyke resulting in brittle fragmentation brittle fragmentation was accompanied by movement of host sediment into the newly created open spaces forming blocky peperite spherical fractures splinter texture and sediment filled vesicles formed during the angular clast forming stage because the host sediment is texturally identical either side of the dyke increasing magma viscosity as temperature decreased combined with pulsatory intrusion were evidently important in the production of the blocky peperite

postvolcanic paleozoic of the iberian pyrite belt an example of basin morphologic control on sediment distribution in a turbidite basin abstract the iberian pyrite belt south portuguese zone hercynian chain an important metallogenic province has a volcanic sedimentary origin overlying this volcanic succession is a thick sedimentary unit the culm group which represents postvolcanic paleozoic sediments of the iberian pyrite belt it consists of three stratigraphic units the basal shaly formation bsf the culm facies turbiditic formation cftf and the shallow platform sandy unit spsu the bsf is represented by finegrained sediments of volcanic to nonvolcanic origin that mark the end of volcanism in the region the reworking of volcanic products in a shallow marine basin and the beginning of autochthonous sedimentation of pelagic clay it constitutes a depositional sequence sensu mitchum et al 1977 the d stribution facies and facies associations of the turbidites of the cftf are related to the configuration of the basin which controlled the mechanisms of deposition and distribution of the detritus from different source areas the spsu represents sediments eroded from the volcanic upland and redistributed over the shelf detailed study of these three units and their relationships provides the means to define the geometry and evolution of the postvolcanic carboniferous basin of the iberian pyrite belt the basin was roughly subdivided by the paymogo and the puebla de guzman paleoridges together with the allochthonous ossa morena zone north of the study area they formed three topographic barriers that delineated two interconnected subbasins sedimentation in the postvolcanic basin of the iberian pyrite belt can be related to a model of oscillating sea level synorogenic characteristics of the culm group sediments support a tectonic origin for the sea level oscillations particularly since the sediments had already been affected by the first pulses of the hercynian orogeny

peperite a review of magma sediment mingling the study of peperite is important for understanding magma water interaction and explosive hydrovolcanic hazards this paper reviews the processes and products of peperite genesis peperite is common in arc related and other volcano sedimentary sequences where it can be voluminous and dispersed widely from the parent intrusions it also occurs in phreatomagmatic vent filling deposits and along contacts between sediment and intrusions lavas and hot volcaniclastic deposits in many environments peperite can often be described on the basis of juvenile clast morphology as blocky or fluidal but other shapes occur and mixtures of different clast shapes are also found magma is dominantly fragmented by quenching hydromagmatic explosions magma sediment density contrasts and mechanical stress as a consequence of inflation or movement of magma or lava magma fragmentation by fluid fluid shearing and surface tension effects is probably also important in fluidal peperite fluidisation of host sediment hydromagmatic explosions forceful intrusion of magma and sediment liquefaction and shear liquification are probably the most important mechanisms by which juvenile clasts and host sediment are mingled and dispersed factors which could influence fragmentation and mingling processes include magma host sediment and peperite rheologies magma injection velocity volatile content of magma total volumes of magma and sediment involved total volume of pore water heated presence or absence of shock waves confining pressure and the nature of local and regional stress fields sediment rheology may be affected by dewatering compaction cementation vesiculation fracturing fragmentation fluidisation liquefaction shear liquification and melting during magma intrusion and peperite formation the presence of peperite intraclasts within peperite and single juvenile clasts with both sub planar and fluidal margins imply that peperite formation can be a multi stage process that varies both spatially and temporally mingling of juvenile clast populations formed under different thermal and mechanical conditions complicates the interpretation of magma fragmentation and mingling mechanisms

recognizing metasedimentary sequences potentially hosting concealed massive sulfide accumulations in the iberian pyrite belt using geochemical fingerprints abstract promising advances in the development of new greenfield and brownfield exploration methods and methodologies for massive sulfide ore systems in the iberian pyrite belt ipb have been made over the last two decades however the effects of lateral and vertical facies variations in metasedimentary piles forming the two main lithostratigraphic units that potentially host massive sulfide deposits the phyllite quartzite group pqg and the volcanic sedimentary complex vsc on mineral exploration remain unknown herein we report on results of a multi element geochemical study of 133 samples representing pqg and vsc metasedimentary rocks from the albernoa area 713 km2 this dataset is complemented by whole rock sr nd and pb isotopic data for a subset of 12 vsc metasedimentary rock samples the prevalent siliciclastic component in the analyzed rocks derives from an evolved felsic source as suggested by the zr sc th sc eu eu relationships that according to cia values should have been affected by significant weathering before they were deposited in the basin from middle devonian to visean time the siliciclastic component comprises clay and fine grained sand whose relative abundance can be distinguished on the basis of al2o3 tio2 sc tio2 ga tio2 zr tio2 and sio2 al2o3 ratios these terrigenous components are in places variably mixed with volcanic derived fractions leading to the formation of tuffaceous metapelites typical of the vsc sequences but also present in the upper stratigraphic sections of the pqg in general vsc samples display the following parameters 2 5 sio2 al2o3 6 5 0 02 tio2 al2o3 0 06 and 0 2 cao na2o k2o al2o3 0 25 whereas pqg samples display 2 5 sio2 al2o3 12 0 08 tio2 al2o3 0 58 and cao na2o k2o al2o3 0 3 fe2o3 tio2 ratios 10 and al2o3 al2o3 fe2o3 mno ratios 0 6 indicate the strong influence of an early developed prior to variscan metamorphism and deformation hydrothermal component on the prevalent siliciclastic component sometimes mixed with a variable volcanic derived fraction moreover values of as sb sc cu zn pb sc and 5 fe2o3 mgo mno al2o3 ratios between 1 and 10 indicate the influence of seafloor metasomatism processes and ratios 10 indicate close proximity to hydrothermal discharge potentially associated with massive sulfide mineralization the sr nd pb isotopic data show that vsc metasedimentary rocks are mostly derived from old reworked upper continental crust isotope ratios characterizing the main siliciclastic component are somewhat disturbed by the significant presence of volcanic derived components and or mineral changes due to early hydrothermal alteration mineralization processes which should have involved isotopically distinct fluids

first direct dating of late pleistocene ice wedges by ams we present the first direct dating by 14 c accelerator mass spectrometry of three late pleistocene syngenetic icewedges from the seyaha cross section they are representative of permafrost with multistage ice wedges from the north of western siberia the most important result is the clear vertical age stratification of the ice i e the old ice is located beneath the young this shows that a timescale can be assigned to these ice wedges penetrating down into the permafrost the age of the ice shows a depth of not more than 3 5 m for frost cracking water penetrated into the icewedges at that depth the lower part of the ice wedges from the seyaha cross section has been dated between 21 000 and 14 000 bp fl 2000 elsevier science b v all rights reserved

type of volcanoes hosting the massive sulfide deposits of the iberian pyrite belt the volcanic sedimentary complex vsc of the iberian pyrite belt ipb in southern portugal and spain comprises an upper devonian to lower carboniferous submarine succession with a variety of felsic volcanic lithofacies the architecture of the felsic volcanic centres includes felsic lavas domes pyroclastic units intrusions and minor mafic units that define lava cryptodome pumice cone volcanoes the diversity of volcanic lithofacies recognized in different areas of the ipb mainly reflects variations in proximity to source but also differences in the eruption style the ipb volcanoes are intrabasinal range in length from 2 km to 8 km and their thickest sections vary from 400 m to 800 m these volcanoes are dominated by felsic lavas domes that occur at several stratigraphic positions within the volcanic centre however the pyroclastic units are also abundant and are spatially related to the lavas domes the intrusions are minor and define cryptodomes and partly extrusive cryptodomes the hydrothermal systems that formed the neves corvo and lousal massive sulfide ore deposits are associated with effusive units of felsic volcanic centres at neves corvo the massive sulfide orebodies are associated to rhyolitic lavas that overlie relatively thick fiamme rich pyroclastic unit in several other locations within the belt pyroclastic units contain sulfide clasts that may have been derived from yet to be discovered coeval massive sulfide deposits at or below the sea floor which enhances the exploration potential of these pyroclastic units and demonstrates the need for volcanic facies analysis in exploration

textural variations and fragmentation processes in peperite formed between felsic lava flow and wet substrate an example from the cretaceous buan volcanics southwest korea abstract multiple exposures of peperite within the cretaceous buan volcanics southwest korea have been examined in order to determine variations in their textural characteristics and to investigate their mode of formation along undulating boundaries between rhyolite lava flow and deformed host sediment expressed as a series of load and flame structures exposures commonly contain two distinct types of peperite type 1 peperites are composed mostly of rounded juvenile clasts at their base and polyhedral juvenile clasts at their upper levels interpreted to have formed via a two stage process firstly abrasion of juvenile clasts occurred after their fragmentation due to shear stress imparted by the overlying and still moving lava flow forming rounded juvenile clasts subsequent in situ quenching fragmentation of the lava flow produced clasts with platy to polyhedral shapes immediately after emplacement of the lava flow type 2 peperites laterally extend into the interior of featureless rhyolite as layers that decrease in thickness with increasing distance away from the flame zone these layers exhibit horizontal textural variations ranging from poorly sorted mixtures of ash to block sized angular juvenile clasts in the proximal zone to closely packed polyhedral and tabular juvenile clasts with jigsaw crack textures in the middle and distal zones type 2 peperite are inferred to have formed due to internal steam explosions that resulted from an expansion of heated pore water leading to an increase in pore fluid pressure that had been vertically injected into the interior of the rhyolite from the flame zone the proximal zone composed mainly of poorly sorted mixtures of juvenile clasts represents the explosion sites juvenile clasts in the middle and distal zones are interpreted to have formed due to three separate processes the development of fractures in the rhyolite during the internal steam explosions injection of the host sediment through the fractures and in situ quenching fragmentation deformation of the host sediment exerted an important control on peperite forming processes with the internal steam explosions suggested to have formed the closely packed juvenile clasts with a jigsaw crack texture rather than the clasts that are widely dispersed

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

volcanic architecture in the odiel river area and the volcanic environment in the riotinto nerva unit iberian pyrite belt spain abstract in order to assess the volcanic architecture of the vsc in the iberian pyrite belt spain a study of the volcanic and sedimentary lithofacies has been undertaken on a section up to 1000 m thick in the odiel river considered to be also representative of other areas of the riotinto nerva unit among other mineral deposits this unit includes the world class volcanic hosted massive sulfide deposits of the riotinto mining district several conclusions have been obtained a a complex thick volcanic succession as well as mafic and felsic sills and thin sedimentary intervals predated the genesis of the sulfide deposits which did not form in a dominantly sedimentary environment b two successive felsic volcanic sequences can be distinguished showing textural and chemical variations from bottom to top c mafic lithofacies decrease upwards in the sequence and d volcanic structures eruption style and internal organization suggest a deep subaqueous depositional environment which agrees with the depositional setting for massive sulfide deposits

the fox permafrost tunnel a late quaternary geologic record in central alaska the fox permafrost tunnel which penetrates 110 m into frozen sediments of gold stream valley provides a continuous exposure of fossiliferous silt and alluvium above schistose bedrock deposition of fluvial gravel was followed by a long interval of loess accretion and permafrost aggradation that was punctuated by episodes of thaw and of gullying and redeposition of silt imbricated sandy gravel above the bedrock contains lenses of finer alluvium that contain wood fragments and some rooted stumps radiocarbon dates indicate that the gravel is older than 40 ka but absence of mature soil and weathering profiles at its upper contact indicates that fluvial activity must have continued until shortly before loess accretion began at the tunnel site silt is the most widespread depositional unit in the tunnel this deposit is of eolian origin loess but some has been redeposited by slope processes the silt units contain abundant ground ice as pore filings lenses wedges and buried pond ice loess accretion was interrupted by a period when little loess accumulated and when large ice wedges formed in the lower loess unit and subsequently were truncated by thaw loess began forming sometime before 40 ka and was rapidly accreting by 39 ka under xeric conditions with open vegetation a sharply decreased rate of loess accretion associated with local erosion and thaw between about 36 and 30 ka is marked by anomalous cation concentration values lenses of buried sod fossils indicative of moist to wet substrates and truncated ice wedges beneath small frozen ponds or streamlets that occupied ice wedge troughs a later episode of rapid loess influx under drier conditions began after 30 ka and coincided with glacial advances of late wisconsin age in the adjoining alaska range large ice wedges also formed in the upper loess unit but only their bases are exposed in the tunnel and their history of development is uncertain fanlike deposits of poorly sorted debris near the tunnel portal formed between about 12 5 and 11 ka during deep erosion of loess slopes under moister conditions the deposits locally form two subunits the younger over whelmed a stand of tall willows on the floor of goldstream valley between about 11 3 and 11 1 ka the older may have formed about 1 000 yr earlier stratigraphic records elsewhere in central alaska indicate variable middle wisconsin environments followed by colder and drier conditions that began between 30 and 25 ka and persisted until perhaps 12 5 ka widespread loess erosion and redeposition subsequently occurred under moister and probably warmer conditions renewed early holocene loess deposition may have been widespread but its exact environmental controls are uncertain our data challenge three generally accepted concepts of late quaternary periglacial processes in central alaska we contend that 1 many ice wedge systems may have formed under interstadial conditions rather than full glacial conditions 2 episodes of rapid loess influx may have been partly out of phase with episodes of glacier expansion and 3 redeposition of loess by solifluction sheetwash and gully formation may have been episodic and required conditions moister than those under which the loess initially accreted

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

submarine silicic volcanism and associated sedimentary and tectonic processes ramsey island sw wales lower ordovician marine strata formed in an ensialic marginal basin environment record early episodes of distant explosive silicic volcanism and a later volcanotectonic episode which was a precursor to the development of a wide variety of near to source submarine silicic volcanic phenomena the early episodes are represented by well bedded turbiditic tuffs these and older strata at least 770 m thick in normal stratigraphical succession are allochthonous and large scale wet sediment sliding with reworking of strata in cohesive debris flows reflects major disturbance of the sea floor during the emplacement of silicic magmas volcanotectonic uplift along a n s fault exceeded 1 km and led to the emergence and erosion of a rhyolitic volcanic island this was followed closely by submergence and in water depths perhaps of about 500 m there accumulated welded and non welded rhyolitic ash flow tuffs rhyolitic turbiditic tuffs rhyolite lavas and a wide variety of debris flow deposits shallow intrusions of 9low9 viscosity rhyolite were emplaced contemporaneously the more or less simultaneous emplacement of rhyolitic tuff with both 9low9 viscosity and high viscosity rhyolites is attributed to variable water content related perhaps to volatile fluxing in a magma body the location of the near to source extrusive rocks and consanguineous intrusions along the contemporary n s fault is considered to reflect channelling of magma initially along a deep crustal fracture

extensive soft sediment deformation and peperite formation at the base of a rhyolite lava owyhee mountains sw idaho usa in the northern owyhee mountains sw idaho a 200 m thick flow of the miocene jump creek rhyolite was erupted on to a sequence of tuffs lapilli tuffs breccias and lacustrine siltstones of the sucker creek formation the rhyolite lava flowed over steep palaeotopography resulting in the forceful emplacement of lava into poorly consolidated sediments the lava invaded this sequence liquefying and mobilising the sediment propagating sediment subvertically in large metre scale fluidal diapirs and sediment injectites the heat and the overlying pressure of the thick jump creek rhyolite extensively liquefied and mobilised the sediment resulting in the homogenization of the sucker creek formation units and the formation of metre scale loading structures simple and pendulous load casts detached pseudonodules density contrasts between the semi molten rhyolite and liquefied sediment produced highly fluidal rayleigh taylor structures local fluidisation formed peperite at the margins of the lava and elutriation structures in the disrupted sediment the result is a 30 40 m zone beneath the rhyolite lava of extremely deformed stratigraphy brittle failure and folding is recorded in more consolidated sediments indicating a differential response to loading due to the consolidation state of the sediments the lava sediment interaction is interpreted as being a function of 1 the poorly consolidated nature of the sediments 2 the thickness and heat retention of the rhyolite lava 3 the density contrast between the lava and the sediment and 4 the forceful emplacement of the lava this study demonstrates how large lava bodies have the potential to extensively disrupt sediments and form significant lateral and vertical discontinuities that complicate volcanic facies architecture

submarine volcanism eruption styles products and relevance to understanding the host rock successions to volcanic hosted massive sulfide deposits the increasing hydrostatic pressure of the water column with increasing water depth in subaqueous environments limits the ability of superheated volatiles to expand instantaneously against the ambient pressure explosive submarine eruptions are only likely in water depths less than 1 km and generally less than 500 m thus refuting models of highly explosive very deep water calderas popularly proposed as the host volcanic centers for the kuroko volcanic hosted massive sulfide vhms deposits of japan large volumes of blocky breccias can be produced by quench fragmentation during submarine lava eruptions and such deposits may have been mistaken for pyroclastic deposits in many cases in the past pyroclastic debris may occur in the host rock successions of some vhms deposits and may even be very voluminous however facies characteristics and theoretical constraints indicate that these are usually the products of mass flow deposition from shallow water or basin margin volcanic centers the maximum water depths for submarine explosive eruptions coincide approximately with the minimum pressures and water depths required to prevent boiling of mineralizing hydrothermal fluids in the stockwork before the fluids reach the sea floor the key elements in evaluating the prospectivity of ancient volcanic successions for vhms deposits appear to be deep water sediments and lavas or shallow intrusions in an extensional basin setting pyroclastic debris in many cases at least appears to be an accidental externally introduced component there is little evidence that explosive submarine calderas are essential as host volcanic centers to vhms deposits

pumiceous peperite in a submarine volcanic succession at mount chalmers queensland australia pumiceous peperite comprising irregularly shaped apophyses of feldspar phyric rhyolitic tube pumice and siltstone occurs within well bedded volcaniclastic sandstone and siltstone facies of the early permian berserker beds at mount chalmers australia the tube pumice structure is preserved where sericite or silica have replaced the glass of vesicle walls and vesicles have been infilled by silica in some instances the peperite occurs gradationally above or below intervals of coherent feldspar phyric rhyolite that are also predominantly pumiceous the siltstone in the pumiceous peperite is texturally homogeneous locally vesicular and contains shards and crystals derived from disintegration of the pumiceous rhyolite pumiceous rhyolite and peperite occur at various positions in the stratigraphy and may represent interconnected intrusive digits or lobes intrusion of the lobes was accommodated by expansion of the pore water and possible fluidisation of the host sediment resulting in local destruction of bedding the lobes developed chilled margins at contacts with wet sediment and inflated in response to vesiculation and the supply of new magma cooling of the lobes was possibly accompanied by development of microfractures in the glassy vesicle walls rupture of the chilled margin and propagation of fractures into the interior could have temporarily and locally depressurised the lobes resulting in failure disintegration and mixing with the adjacent wet and or steam rich sediment hot pumiceous rhyolite in lobe interiors may have interacted directly with the wet sediment and been dismembered by quench fragmentation and or steam explosions bubbles of magmatic gas and or steam were trapped in the sediment that mixed with the pumiceous rhyolite the development of pumiceous texture in the sills was favoured by emplacement beneath a thin cover of wet sediment in a relatively shallow submarine shelf setting in which the confining pressure was sufficiently low to permit vesiculation this setting was also important in limiting the extent of degassing of the pumiceous rhyolite during cooling

contrasting interactions of two magmas with the same water saturated hosts big bend national park texas abstract rhyolitic and mafic dikes cut lower tertiary sediments tuffs and mafic lavas of the eocene oligocene chisos formation in big bend national park texas two contrasting magmas injected into the same hosts under the same conditions provide a controlled experiment on magma host interaction peralkaline rhyolite dikes are extensive and stand in bold topographic relief on contact surfaces against mudstones close packed spheroids of chilled rhyolite indicate vigorous movement of magma within envelopes of fluid against sandstones the rhyolite dike contacts are more irregular and locally protrude as blunt fingers into the wall rock against mafic lava flows dike contacts are planar and the rhyolite is less altered mafic dikes cut the rhyolite dikes some contacts of these mafic dikes are peperitic indicating intrusion into soft wet sediment and tuff the same hosts of the older rhyolitic dikes must also have been water saturated and unconsolidated higher viscosity and lower temperature prevented rhyolite magma from forming detached bodies or engulfing its host to form peperites estimated pressures and contact temperatures approximate or exceed the critical point of water indicating that the fluid envelopes at rhyolite dike contacts were very unstable because even small temperature fluctuations would have produced wide variations in fluid density around the critical point the resulting volume instability triggered rapid pulsations in the fluid envelope deforming the contacts where permeability of the host was low hot fluid accumulated at contacts but where permeability was high fluid could escape before reaching high temperature

chilling and brecciation of a devonian rhyolite sill intruded into wet sediments northern sierra nevada california localized pockets of peperite breccia occur near the base of a well exposed hypabyssal rhyolite sill within the upper devonian volcanic sequence of the northern sierra nevada the peperites resulted from chilling fragmentation and commingling of rhyolite with wet poorly consolidated sediment zones of hyaloclastite breccia extend into the sill above the peperites and represent areas where chilled rhyolite underwent fragmentation but was not intermixed with sediment a dense spherulitic intergrowth developed rapidly within the chilled rhyolite immediately prior to its fragmentation formation of this quench texture probably reflects a reduction in viscosity of the magma due to retention of volatiles in the melt under conditions of relatively high confining pressure

processes of magma wet sediment interaction in a large scale jurassic andesitic peperite complex northern sierra nevada california the middle jurassic tuttle lake formation in the northern sierra nevada california comprises a thick volcaniclastic sequence deposited in a submarine island arc setting and penetrated by numerous related hypabyssal intrusions a composite andesite diorite intrusive complex 4 5 km long and 1 5 km thick was emplaced while the host tuttle lake sediments were still wet and unconsolidated large parts of the intrusive complex consist of peperite formed where andesitic magma intruded and intermixed with tuff lapilli tuff and tuff breccia the southern half of the complex consists of augite phyric andesite containing peperite in numerous small isolated pockets and in more extensive laterally continuous zones the peperites comprise three main types recognized previously in other peperite studies fluidal peperite consists of small 30 cm closely spaced at least partly interconnected globular to amoeboid andesite bodies enclosed by tuff this peperite type developed during intrusion of magma into fine grained wet sediment along unstable interfaces and fluidization of the sediment facilitated development of complex intrusive geometries blocky peperite and mixed blocky and fluidal peperite formed where magma intruded coarser sediment and underwent variable degrees of brittle fragmentation by quenching and dynamic stressing of rigid margins possibly aided by small steam explosions the northern half of the intrusive complex consists predominantly of a different type of peperite in which decimetre scale plagioclase phyric andesite clasts with ellipsoidal elongate or angular polyhedral shapes are closely packed to widely dispersed within disrupted host sediment textural features suggest the andesite clasts were derived from conduits through which magma was flowing and preserved remnants of the conduits are represented by elongate sinuous bodies up to 30 m or more in length disruption and dispersal of the andesite clasts are inferred to have occurred at least partly by steam explosions that ripped apart a network of interconnected feeder conduits penetrating the host sediments closely packed peperite is present adjacent to mappable intrusions of coherent andesite and along the margin of a large mass of coarse grained diorite these coherent intrusions are considered to be major feeders for this part of the complex examples of magma wet sediment interaction similar in scale to the extensive peperites described here occur elsewhere in ancient island arc strata in the northern sierra nevada based on these and other published examples large scale peperites probably are more common than generally realized and are likely to be important in settings where thick sediment sequences accumulate during active volcanism careful mapping in well exposed terrains may be required to recognize large scale peperite complexes of this type

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

the postglacial pleistocene of the northern russian mainland abstract the paper summarizes the results of recent studies of late pleistocene postglacial geological events which occurred in the flatlands of northern russia stretching from the timan ridge to the laptev sea coast the available sedimentary record includes thawed and perennially frozen subaerial sediments and in places lacustrine and fluvial formations hundreds of geochronometric measurements by radiocarbon and luminescence techniques place this record within the time span of 50 11 ka bp which confirms an early mid weichselian age of the underlying glacial complex first established by the queen programme the spectrum of proxies includes sedimentary facies pollen data plant macrofossils oxygen isotope values megafauna remains insect assemblages traces of dna paleosols and permafrost properties together they indicate a continental progressively deteriorating climate with very cold winters and warmer than present summers through the second half of the late pleistocene minor climatic fluctuations are largely due to slightly changing humidity during mis 3 these changes are more pronounced in periglacial european russia in the vicinity of the barents kara ice sheet where the mid weichselian biotic maximum contrasts with conditions in the late weichselian polar desert in siberia the periglacial environments were more stable and beneficial for biota inspite of the increased continentality the recent data accumulated by various researchers are incompatible with the conventional correlation of the karginsky interglacial strata of siberia with mis 3 or any part of the european pleniglacial

magmatic evolution and tectonic setting of the iberian pyrite belt volcanism rocks has been carried out to determine the primary magmatic aynity tectonics iberian pyrite belt and tectonic setting of the iberian pyrite belt volcanism most of the basaltic rocks are continental tholeiites but a few samples show an alkaline aynity the origin of the basaltic rocks and their diversity of compositions are explained by a single mixing model between e and n morb mid ocean ridge basalt and as introduction similation of crustal material calc alkaline intermediate and silicic volcanic hosted massive sulphide deposits are mainly rocks include basaltic andesites andesites dacites and rhyolites associated world wide with calc alkaline submarine volvolumetrically dacites and rhyolites are the most abundant inter canism petrological and geochemical data together with mediate and silicic rocks are not related by fractional crystallization stratigraphic and structural studies have been of major nor is there a relationship between the basaltic and calc alkaline importance in constraining geological models of the volrocks by the same process we suggest that in the iberian pyrite canism associated with massive sulphide deposits inbelt silicic calc alkaline magmas were generated on a large scale tegrated studies in areas such as the kuroko province in by the invasion of continental crust by mafic magmas generated in japan ohmoto 1983 the mount read volcanics in the underlying upper mantle the diversity of compositions shown tasmania crawford et al 1992 and the mount windsor by dacites and rhyolites can mainly be explained either by diverences volcanics in northwestern australia stolz 1995 have in the composition of the source rocks or by diverent degrees of revealed that this volcanism may be developed during partial melting of upper crust rocks andesites however formed by diverent stages of the subduction process always being mixing between basaltic magmas and upper crust material the new located on the overriding plate geochemical data agree with previously published tectonostratigraphic this paper documents the petrology and geochemistry data which suggest that the iberian pyrite belt volcanism formed of the volcanism of the iberian pyrite belt an early on the south portuguese plate owing to strike slip tectonics this carboniferous metallogenic province that extends from portugal to spain in southwest iberia and constitutes local extensional tectonic setting was related to transtension as a