contrasting arc and morb like assemblages in the paleoproterozoic flin flon belt manitoba and the role of intra arc extension in localizing volcanic hosted massive sulphide deposits the flin flon belt trans hudson orogen manitoba and saskatchewan is the largest paleoproterozoic volcanic hosted massive sulphide vms district in the world with 118 7 million tonnes mt of z

coeval sedimentation magmatism and fold thrust development in the trans hudson orogen propagation of deformation into an active continental arc setting wekusko lake area manitoba early structures in the southeastern part of the trans hudson orogen tho wekusko lake area manitoba comprise northwest verging folds and a set of steep southeast dipping faults that occur at a low angle to bedding kinematic indicators are rare however the vergence of the associated folds supports reverse movement on the faults these folds and faults formed prior to the peak of regional metamorphism and therefore must correlate with the widely recognised deformational d2 event which developed pre to syn metamorphism and involved development of a regional south to southwest directed fold thrust belt the d2 folds and reverse faults in the study area are interpreted to have originally formed as gently dipping southwest verging structures i e as inclined folds and thrust faults which were then rotated into a steep northwest verging orientation during later deformation d3 in the southeastern tho regional d2 fold and thrust structures are cut by ca 1839 1830 ma plutons indicating that

southern kermadec submarine caldera arc volcanoes sw pacific caldera formation by effusive and pyroclastic eruption abstract three newly identified submarine caldera volcanoes brothers healy and rumble ii west of the southern kermadec arc within water depths 80 and low density 0 3 0 5 g cm 3 erupted pumiceous clasts these observations are interpreted as evidence of subaqueously quenched pyroclastic eruption s sheathed by ambient water flashing to steam discharging some 5 km3 of pyroclastic material from healy volcano caldera collapse is inferred to be penecontemporeous with the pyroclastic eruption s although it is possible formation as an explosive crater occurred but we consider the latter unlikely given the volume of erupted material and water depth extrapolation of these observations to other submarine arcs indicates pyroclastic eruption of silicic magmas and syn eruptive caldera collapse is possible within water depths of 500 1000 m

submarine silicic caldera at the front of the izu bonin arc japan voluminous seafloor eruptions of rhyolite pumice myojin knoll caldera a submarine rhyolitic center 400 km south of tokyo is one of nine silicic calderas along the northern 600 km of the izu bonin ogasawara arc and the first anywhere to receive detailed submersible based study the caldera slightly smaller than the crater lake structure in oregon is 6 7 km in diameter its inner walls are 500 900 m high and it has a remarkably flat floor at 1400 m below sea level mbsl the caldera collapse volume is 18 km 3 suggesting that more than 40 km 3 of pumiceous tephra may have been erupted at the time the caldera formed precaldera seafloor eruptions built a broad volcanic edifice consisting of overlapping composite volcanoes made of rhyolitic lavas shallow intrusions and a variety of volcaniclastic deposits including thick accumulations of rhyolitic pumice erupted at 900 500 mbsl the caldera forming eruption produced a 150 200 m deposit of nonwelded fines depleted pumice that resembles a colossal layer of popcorn at the top of the caldera wall freshly erupted pumice behaved as sinkers or floaters depending on the environment in which it cooled the pumice clasts deposited proximally and exposed in the caldera wall were likely quenched in eruption columns that remained below sea level this pumice ingested seawater and sank as gases filling its vesicles cooled particularly as steam in its vesicles condensed to liquid water some eruption columns may have broken through the sea surface and entered the air especially during vigorous phases of the caldera forming eruption these pumices had the opportunity to ingest air as they cooled becoming floaters as they fell back to the sea these could have been carried distally on the sea surface by the combined effects of ocean currents and wind the age of the caldera is unknown but it may be as young as several thousand years its magmatic system at depth retains sufficient heat to sustain an actively growing intracaldera kuroko type polymetallic sulfide deposit rich in gold and silver and topped by chimneys emitting fluids as hot as 278 c sufficient time has elapsed however for a 250 m high postcaldera dome to grow on the caldera floor and for the caldera rim to be deeply scalloped by slumping

high precision isotopic characterization of usgs reference materials by tims and mc icp ms the pacific centre for isotopic and geochemical research pcigr at the university of british columbia has undertaken a systematic analysis of the isotopic sr nd and pb compositions and concentrations of a broad compositional range of u s geological survey usgs reference materials including basalt bcr 1 2 bhvo 1 2 andesite agv 1 2 rhyolite rgm 1 2 syenite stm 1 2 granodiorite gsp 2 and granite g 2 3 usgs rock reference materials are geochemically well characterized but there is neither a systematic methodology nor a database for radiogenic isotopic compositions even for the widely used bcr 1 this investigation represents the first comprehensive systematic analysis of the isotopic composition and concentration of usgs reference materials and provides an important database for the isotopic community in addition the range of equipment at the pcigr including a nu instruments plasma mc icp ms a thermo finnigan triton tims and a thermo finnigan element2 hr icp ms permits an assessment and comparison of the precision and accuracy of isotopic analyses determined by both the tims and mc icp ms methods e g nd isotopic compositions for each of the reference materials 5 to 10 complete replicate analyses provide coherent isotopic results all with external precision below 30 ppm 2 sd for sr and nd isotopic compositions 27 and 24 ppm for tims and mc icp ms respectively our results also show that the first and second generation usgs reference materials have homogeneous sr and nd isotopic compositions nd isotopic compositions by mc icp ms and tims agree to within 15 ppm for all reference materials interlaboratory mc icp ms comparisons show excellent agreement for pb isotopic compositions however the reproducibility is not as good as for sr and nd a careful sequential leaching experiment of three first and second generation reference materials bcr bhvo agv indicates that the heterogeneity in pb isotopic compositions and concentrations could be directly related to contamination by the steel mortar pestle used to process the materials contamination also accounts for the high concentrations of certain other trace elements e g li mo cd sn sb w in various usgs reference materials

a miocene coarse volcaniclastic mass flow deposit in the shimane peninsula sw japan product of a deep submarine eruption a subaqueous volcaniclastic mass flow deposit in the miocene josoji formation shimane peninsula is 15 16 m thick and comprises mainly blocks and lapilli of rhyolite and andesite pumices and non to poorly vesiculated rhyolite it can be divided into four layers in ascending order layer 1 is an inversely to normally graded and poorly sorted lithic breccia 0 3 6 m thick layer 2 is an inversely to normally graded tuff breccia to lapilli tuff 6 11 m thick this layer bifurcates laterally into minor depositional units individually composed of a massive lithic rich lower part and a diffusely stratified pumice rich upper part with inverse to normal grading of both lithic and pumice clasts layer 3 is 2 5 3 m thick and consists of interbedded fines depleted pumice rich and pumice poor layers a few centimeters thick layer 4 is a well stratified and well sorted coarse ash bed 1 5 2 m thick the volcaniclastic deposit shows internal features of high density turbidites and contains no evidence for emplacement at a high temperature the mass flow deposit is extremely coarse grained dominated by traction structures and is interpreted as the product of a deep submarine explosive eruption of vesicular magma or explosive collapse of lava

a special issue on volcanogenic massive sulfide deposits of the trans hudson orogen preface paleoproterozoic volcanogenic massive sulfide vms deposits are hosted in greenstone belts that formed during assembly of earth s first supercontinent nuna at 2 1 to 1 7 ga these deposits account for a significant component of the world s vms tonnage and thus are an important global exploration target among these paleoproterozoic belts the 1 92 to 1 80 ga trans hudson orogen of canada is the largest and best endowed in laurentia with over 29 deposits totaling 320 mt of cu zn au ag pb ore study of its deposits districts and tectonostratigraphic evolution enables greater understanding of metallogenic and geodynamic processes during the paleoproterozoic we briefly overview the trans hudson orogen its vms deposits and their exploration history and illustrate that various distinct types of volcanogenic massive sulfide deposits form in predictive tectonostratigraphic sequences associated with both pre and postaccretion volcanism we present results from a four year program to investigate the deposits and evolution of the trans hudson orogen highlighting various approaches at district and belt scales to improve exploration models for other structurally complex and covered precambrian terranes

apparent welding textures in altered pumice rich rocks fiamme and eutaxitic texture are common in ancient non welded pumice breccias the fiamme are phyllosilicate rich lenses that define a bedding parallel foliation resembling eutaxitic texture in welded ignimbrites pumice breccias in the cambrian mount read volcanics australia are composed of tube pumice clasts bubble wall shards plagioclase crystal fragments and volcanic lithic clasts and contain dark green fiamme and stylolites these sericite or chlorite sericite fiamme are aligned roughly parallel to regional bedding and are enclosed in pale pink feldspar quartz altered domains of uncompacted randomly oriented tube pumice clasts and bubble wall shards similarly pumice breccias in the miocene green tuff belt in the hokuroku basin japan comprise tube pumice clasts fiamme bubble wall shards crystal fragments and non vesicular volcanic clasts the dark green wispy smectite or chlorite fiamme are aligned parallel to bedding and are enclosed in domains of variably clay zeolite and sericite altered uncompacted pumice clasts and shards the fiamme in these examples vary in length 0 5 mm to 1 m length to height ratio 3 1 to 40 1 shape e g flame like feathery branching bow tie wedge and internal texture porphyritic massive vesicular or stylolitic the fiamme define an anastomosing bedding parallel pre tectonic eutaxitic texture delicate feathery terminations and locally preserved vesicles indicate that the fiamme are altered compacted pumice clasts the presence of bedding parallel stylolites and solution seams undeformed shards and pumice clasts adjacent to the fiamme and the overprinting of fiamme by the regional cleavage suggest that the fiamme formed during diagenesis prior to tectonic deformation and that compaction did not involve welding fiamme and eutaxitic texture in these examples resulted from the alteration dissolution and mechanical compaction of cold tube pumice clasts during diagenesis and lithification

alteration and geochemical patterns in the 3 7 3 8 ga isua greenstone belt west greenland abstract volcanic rocks of the 3 7 3 8 ga isua greenstone belt have undergone heterogeneous poly phase post magmatic alteration resulting in mobilisation of many elements including lree and th in contrast nb ta ti zr y and hree were relatively immobile during alteration processes the degree of element mobility appears to have been controlled by chemical composition primary volcanic structures carbonate and silica enrichment and proximity to ultramafic units and shear zones on the basis of the geochemical characteristics of the least altered samples two distinct associations have been recognised in structurally separated lithotectonic sequences these magmatic associations are a low hfse association tio2 0 20 0 40 wt zr 12 30 ppm nb 0 13 0 80 ppm y 6 14 ppm in the central arc sequence resembling tertiary boninites and a high hfse association tio2 0 50 1 14 wt zr 34 77 ppm nb 1 2 2 7 ppm y 11 28 ppm in the outer and inner arc sequences comparable to phanerozoic island arc basaltic to picritic volcanic rocks these two geochemically distinct sequences were likely to have been juxtaposed as a consequence of phanerozoic style plate tectonic processes operating in the early earth the isua volcanic rocks are characterised by sub chondritic nb ta 10 16 ratios post magmatic alteration can be ruled out as the cause of these low ratios rather these ratios reflect a compositional characteristic of the paleoarchean mantle source s the sub chondritic nb ta ratios may have resulted from either preferential sink of nb into the core as a weakly siderophile element or preferential retention of nb in the subducted oceanic crust during subduction zone petrogenetic processes alternatively the building blocks of the earth were not composed of ci like chondritic material

subaqueous eruption fed density currents and their deposits density currents fed directly from subaqueous eruptions can be divided conceptually into three groups based on modes of fragmentation and transport i explosive fragmentation with deposition from a gas supported current ii explosive fragmentation with deposition from a water supported current iii fragmentation of flowing lava with deposition from a water supported current fig 1 group i products include subaqueously emplaced welded ignimbrite and other high temperature emplaced subaqueous pyroclastic flow deposits group ii products are the most varied and include representatives of both high and low concentration turbidity currents grainflows and debris flows and are termed eruption fed aqueous density currents some clasts in such currents are transported and even deposited at high temperature but the transporting currents ranging from grain flows to dilute turbidity currents are water dominated even though steam may be developed along large clasts margins group iii products formed from lava flow fed density currents tend to be weakly dispersed down gradient along the seafloor and generally consist largely of fragments formed by dynamo thermal quenching and spalling bursting of bubbles formed by vapor expansion probably contributes to some group iii beds distinctive column margin fall deposits may form in water excluded zones that developed very locally around vents in association with group i and ii deposits and are distinguished by heat retention structures indicators of gas phase transport and absence of current formed depositional features 2000 elsevier science b v all rights reserved

coeval sedimentation magmatism and fold thrust belt development in the trans hudson orogen geochronological evidence from the wekusko lake area manitoba canada lithological and structural mapping in the east wekusko lake area of the flin flon belt trans hudson orogen suggested an intimate relationship between magmatism fluvial sedimentation and initiation of fold and thrust belt deformation conventional u pb geochronology of volcanic rocks in fault bounded assemblages provides a minimum age of 1876 2 ma for mccafferty liftover back arc basalts and ages of between 1833 and 1836 ma for the herb lake volcanic rocks a rhyolite which unconformably overlies western missi group fluvial sedimentary rocks has complex zircon systematics this rock may be as old as about 1856 ma or as young as 1830 ma the sedimentary rocks overlying this rhyolite are locally intercalated with 1834 ma felsic volcanic rocks and yield sensitive high resolution ion microprobe shrimp u pb and pb evaporation detrital zircon ages ranging from 1834 to 2004 ma the eastern missi group is cut by an 1826 4 ma felsic dyke and contains 1832 1911 ma detrital zircons the dominant source

explosive deep water basalt in the sumisu backarc rift eruption of 1 million year old tholeiitic basalt 1800 meters below sea level 18 megapascals in a backarc rift behind the bonin arc produced a scoriaceous breccia similar in some respects to that formed during subaerial eruptions explosion of the magma is thought to have produced frothy agglutinate which welded either on the sea floor or in a submarine eruption column the resulting 135 meter thick pyroclastic deposit has paleomagnetic inclinations that are random at a scale of 2 5 meters high magmatic water content which is about 1 3 percent by weight after vesiculation contributed to the explosivity

structural development of the snow lake allochthon and its role in the evolution of the southeastern trans hudson orogen in manitoba central canada the snow lake allochthon is a zone of tectonic interleaving of sedimentary rocks of an inverted marginal basin kisseynew domain with island arc and oceanic rocks it is located in the southeaster

trace element geochemistry and petrogenesis of felsic volcanic rocks associated with volcanogenic massive cu zn pb sulfide deposits volcanogenic massive cu zn pb sulfide vms deposits occur primarily in subaqueous rift related environments e g oceanic fore arc arc back arc continental margin or continental are hosted primarily by bimodal mafic felsic volcanic successions and are typically associated with felsic volcanic rocks with specific geochemical characteristics fi alkalic dacites and rhyodacites despite being abundant in the rock record are typically barren some fii calc alkalic rhyodacites and rhyolites host vms deposits but most are barren fiii tholeiitic and fiv depleted rhyolites and high silica rhyolites are much less abundant in the rock record but commonly host vms deposits regardless of age and fiii rhyolites appear to host the largest deposits most petrogenetic models proposed for the formation of fii and fiii fiv felsic volcanic rocks link felsic magma genesis to fractionation processes in high level magma chambers now represented by associated subvolcanic intrusions where the magma is also interpreted to have supplied the heat and or metals required to generate and sustain the vms forming convective hydrothermal system however the relatively constant compositions of fii and fiii fiv felsic volcanic rocks within individual areas the high eruptive temperatures at or above liquidus of fiii rhyolites and the bimodality of vms hosting volcanic successions indicate that fractional crystallization within subvolcanic intrusions could not have generated or significantly modified the compositions of fii and fiii fiv magmas this coupled with detailed geological geochemical and geochronological studies indicates that many of these subvolcanic intrusions were emplaced in multiple phases and that the later most voluminous phases often cut ore associated hydrothermally altered rocks a reassessment of the physical conditions responsible for producing the geochemistry of ore associated fii and fiii fiv felsic volcanic rocks and a review of the compositions of felsic volcanic rocks associated with vms deposits that range in age from mesoarchean to cenozoic provide important constraints on models for vms associated felsic volcanic rocks and their relationship to mineralization the compositions of felsic volcanic rocks may be explained by low to moderate degrees of partial melting of mafic sources at a range of depths within rift environments where the mineralogy and composition of the source regions modes and degrees of partial melting pressure and temperature of melting and to a lesser extent subsequent fractionation processes account for the compositional variations from fi through fii to fiii fiv long lived enhanced heat flow and structural permeability of rift environments that allows partial melting to form some fii rhyolites at midcrustal levels 10 15 km and fiii fiv rhyolites at shallow crustal levels 10 km both within the zone of brittle fracture permeability are essential to sustain the high temperature convective hydrothermal systems that are required to form large vms deposits and camps rift environments contain long lived thermal magmatic and structural corridors that focus magma ascent heat flow high temperature convective hydrothermal systems and emplacement of subvolcanic intrusions that are favorable environments for the formation of vms deposits and fii and fiii fiv felsic volcanic rocks

fiamme textures in volcanic successions flaming issues of definition and interpretation abstract fiamme are aligned flame like lenses found in welded ignimbrite fiamme also occur in welded pyroclastic fall deposits secondary welded pumice rich facies diagenetically altered and compacted non welded pumiceous volcaniclastic facies and lavas fiamme can be formed in a variety of ways the common genetic use of the term fiamme for pumice clasts that have undergone welding compaction is too narrow fiamme is best used as a descriptive term for elongate lenses or domains of the same mineralogy texture and composition which define a pre tectonic foliation and are separated by domains of different mineralogy texture or composition this descriptive term can be used regardless of the origin of the texture and remains appropriate for flattened pumice clasts in welded ignimbrite

shear wave splitting and anisotropy in the charlevoix seismic zone quebec during the month of august 1984 an experiment was undertaken in the charlevoix seismic zone to test the hypothesis that shear wave splitting could be observed in a seismically active region seismograms recorded by digital 3 component seismographs located very close to the epicentres of three earthquakes showed shear wave splitting the amount of splitting was estimated to be 1 6 to 2 of the shear wave velocity from particle motion diagrams the average azimuth of the direction of polarization was 30 these observations can be explained by the presence of vertical cracks striking 30 east of north with a minimum crack density of e 0 03

subaqueous eruption and emplacement of the shinjima pumice shinjima moeshima island kagoshima bay sw japan abstract the shinjima pumice is a fines depleted pumice lapilli tuff emplaced several thousands years ago at about 100 140 m below sea level this 40 m thick deposit comprises many poorly defined flow units which are 1 10 m thick diffusely stratified and showing upward coarsening of pumice clasts with a sharp to transitional base parallel to wavy diffuse stratifications are commonly represented by alignment of pumice clasts especially in the lower half of the flow units pumice clasts of block to coarse lapilli size commonly have thermal contraction cracks best developed on the surfaces demonstrating that they were hot but cooled down to the ambient temperatures prior to their emplacement these features are suggestive of the direct origin of the shinjima pumice from subaqueous eruptions a theoretical consideration on the behavior of subaqueous eruption plumes and hot and cold pumice clasts suggests that subaqueous eruption plumes commonly collapse by turbulent mixing with the ambient water and are transformed into water logged mass flows

hydrothermal alteration of felsic volcanic rocks at the helen siderite deposit wawa ontario within the footwall volcanic sequence five chemically and mineralogically distinct alteration types have been defined least altered sericite chlorite chloritoid and ankerite an alteration model is proposed in which shallow circulating seawater 1 3 km heated by a subvolcanic intrusion encountered rocks of felsic composition within the upper part of the volcanic succession seawater rock reactions evolved an acidic potassium rich solution which migrated upward reacting with overlying felsic rocks converting plagioclase to sericite and quartz porosity of the altered rocks decreased and a widespread sericitic alteration zone formed at depths greater than 3 km circulating seawater came into contact with basaltic rocks and ensuing reactions enriched the seawater in fe mg mn ca and co 2 this second hydrothermal solution moved upward and encountered sericite rich rocks near the sea floor boiling of the solution and or mixing with relatively cold seawater rapidly decreased co 2 solubility causing mass precipitation of iron carbonate this led to the formation of the ankerite assemblage in the immediate footwall rocks and the siderite iron formation on the sea floor modified journal abstract

paleoproterozoic 1 90 1 86 ga arc volcanism in the flin flon belt trans hudson orogen canada geochemical and isotopic nd sr data are reported on paleoproterozoic 1904 1864 ma maficintermediate 1870 ma collision and subsequent intra arc deformation

an enhanced equivalent continuum model for layered rock mass incorporating bedding structure and stress dependence abstract layered rock masses exhibit significant anisotropy in both the deformability and strength properties of this rock mass are largely affected by its bedding structure and also by the stress state change for the purpose of characterizing these salient features using the equivalent continuum concept an enhanced model is proposed in this paper based on the ubiquitous joint model in the enhanced model the intact bedded rock is assumed to behave as a transversely isotropic elastic body a modified anisotropic strength criterion is adopted to describe the direction dependence of the strength of intact bedded rock in addition stress sensitive parameters are utilized for both the intact rock and bedding plane among which the dilation angle and strength parameters of intact rock are influenced by confining pressure and loading history whereas the stiffnesses of bedding planes are closely related to the current normal stress the effect of layer thickness on the mechanical behavior of rock mass is reflected by continuously updating the global stiffness matrix after failure of bedding plane and by conditions for conducting bedding plane related calculations determined by the relative scales between element size and layer thickness preliminary validation of this model is conducted by comparing with the closed form solutions and with laboratory tests generally good agreements can be achieved between numerical simulations and theoretical experimental results which indicate that applicability of the enhanced model on underground engineering issues related to layered rock mass is promising

intraoceanic tectonics and the development of continental crust 1 92 1 84 ga evolution of the flin flon belt canada the flin flon belt is a complex amisk collage of distinct tectono stratigraphic assemblages that was brought together at a relatively early stage in the tectonic evolution of the paleoproterozoic trans hudson orogen four main tectono stratigraphic assemblage types are recognized isotopically juvenile oceanic arc 1 90 1 88 ga ocean floor ca 1 90 ga oceanic plateau ocean island and isotopically evolved arc 1 92 1 90 ga oceanic arc assemblages include tholeiitic calc alkaline and lesser shoshonitic volcanic and volcaniclastic rocks as well as turbidites and arc rift volcanic rocks the tectono stratigraphic assemblages were juxtaposed in an accretionary complex amisk collage at ca 1 88 1 87 ga probably as a result of arc arc collision collisions accretionary collage structures are largely obliterated by subsequent deformation and metamorphic events but can be inferred where cut by calc alkaline plutons related to a 1 866 1 838 ga arc coeval subaerial volcanism is recorded in ca 1 87 1 85 ga calc alkaline to shoshonitic volcaniclastic sequences unroofing of the accretionary collage development of a paleosol and deposition of continental alluvial fluvial sedimentary rocks occurred ca 1 85 1 84 ga coeval with the waning stages of post accretion arc magmatism stabilization of the flin flon belt by 1 85 1 84 ga as part of a philippines or japan like microcontinent is attributed to both tectonic and magmatic thickening of the lithosphere

detailed morphology and structure of an active submarine arc caldera brothers volcano kermadec arc a survey of the brothers caldera volcano kermadec arc with the autonomous underwater vehicle abe has revealed new details of the morphology and structure of this submarine frontal arc caldera and the geologic setting of its hydrothermal activity brothers volcano has formed between major sw netrending faults within the extensional field of the havre trough brothers may be unique among known submarine calderas in that it has four active hydrothermal systems two high temperature sulfide depositing sites associated with faulting on the northwestern and western walls i e the nw caldera and w caldera hydrothermal sites respectively and gas rich sites on the summits of the constructional cones that fill most of the southern part of the caldera i e the upper and lower cone sites the 3 0 3 4 km caldera is well defined by a topographic rim encompassing 320 of its circumference and which lies between the bounds of two outer half grabenshaped faults in the northwest and southeast sectors there is not a morphologically well defined continuous ring fault at the map resolution although near vertical scarps are present discontinuously at the base of sections of the wall the width of the wall varies from 200 m at its southwest portion to 750 m on its northern section the widest part of the wall is its northwest sector which also has the largest documented area of hydrothermal alteration and where sea floor magnetization is lowest in addition to primary northwest southeast elongation and southwest northeast structures caused by faulting within the regional back arc strain field there are also less well developed west southwestnorth northeast regional structures intersecting the volcano that is appar ent on the abe bathymetry and at outcrop scale from submersible observations asymmetrical trap doorstyle caldera collapse is considered a possible mechanism for the formation of the brothers caldera

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

false pyroclastic textures in altered silicic lavas with implications for volcanic associated mineralization there is strong emphasis in recent literature that zn cu pb massive sulfide deposits associated with felsic volcanism the kuroko type deposits are directly related to submarine calderas formed by large submarine pyroclastic eruptions altered silicic volcanic rocks with the appearance of welded and nonwelded pyroclastic flow deposits volcaniclastic debris flows and massive to thin bedded tuffaceous rocks form a major component of a silurian succession that hosts zn cu pb massive sulfide deposits at benambra southeastern australia however critical evaluation of rock textures indicates that these silicic volcanic rocks are mainly lavas and associated autoclastic facies with remarkably deceptive false pyroclastic and volcaniclastic textures including apparent pumice fiamme and glass shards intercalated sediments indicate that most of the silicic volcanics were emplaced in subaqueous settings ranging from relatively shallow water to deep marine the false textures were produced mainly in originally glassy lava by the combined effects of devitrification perlitic fracture and pervasive hydrothermal alteration these processes are related to the emplacement and cooling history of individual lavas and to a regional essentially synvolcanic hydrothermal system tectonic foliation of the more altered and mechanically weakened rocks has led to partial dismembering of phenocrysts and groundmass fabrics thereby further enhancing clastic appearance subsequent to recognition of widespread false pyroclastic textures at benambra similar features have been identified in three other australian massive sulfide districts these results suggest that a reinvestigation of host rocks is warranted at other deposits where submarine pyroclastics are documented noncaldera settings combining subsidence in an extensional volcano tectonic basin passive submarine lava effusion within the basin and ambient sedimentation derived from basin margins including resedimented pyroclastic debris may be more relevant models in a number of massive sulfide districts

variations in large synvolcanic alteration zones at snow lake manitoba canada with proximity to associated volcanogenic massive sulfide deposits juvenile 1 89 ga oceanic arc volcanic rocks of the flin flon volcanic belt at snow lake are characterized by extensive zones with anomalous 1 82 ga metamorphic mineral assemblages including porphyroblasts of garnet staurolite amphibole biotite gahnite and or kyanite they were produced from altered rocks created during premetamorphic 1 89 ga synvolcanic hydrothermal fluid rock interaction three separate episodes of hydrothermal alteration are recognized that span evolution of the host volcanic rocks from a primitive nascent arc to mature arc geotectonic setting the geologic geochemical mineralogical and isotopic attributes of the zones indicate that they include volcanogenic massive sulfide vms related alteration and they were produced at high and low temperatures and formed in seafloor near seafloor and subseafloor intrastratal environments large scale alteration zones at snow lake are up to 20 km in strike length and 0 8 km wide their large exploration footprint which is 36 times greater in areal extent compared to associated pipe like alteration zones means that such zones provide a useful target to vector in exploration to vms depositional settings within volcanic belts the vms related large scale alteration zones at snow lake display diagnostic variations in intensity and style of alteration along strike toward vms deposits are stratigraphically underlain by altered portions of synvolcanic intrusions are crossed by discordant zones of more intensely altered rocks and can be demonstrated to have formed by interaction with high temperature 350 c hydrothermal fluids cu zn rich vms deposits at snow lake formed in flow dominated sequences are hosted by large rhyolite flow complexes and comprise lensoid orebodies in contrast zn cu rich vms deposits although also rhyolite associated formed in volcaniclastic dominated sequences are spatially related to a district aquifer and comprise stratiform laterally continuous orebodies this suggests that exploration for cu zn vms deposits could selectively target flow dominated sequences and focus on rhyolite flow complexes that display significant alteration similarly exploration for zn cu vms deposits could focus on volcaniclastic dominated sequences in which a potential aquifer has been identified

water settling and resedimentation of submarine rhyolitic pumice at yali eastern aegean greece abstract the yali pumice breccia is a very thick 150 m quaternary succession of submarine pumice that has been uplifted and exposed in the southern part of yali island in the eastern aegean greece the pumice breccia comprises moderately to well sorted 0 03 3 m thick beds of loose pumice clasts that are poor in fine matrix predominantly the larger pumice clasts cobbles and boulders are prismatic with quenched margins and internal polyhedral joints in comparison the smaller pumice clasts are polyhedral angular to subrounded blocky pebbles and granules these smaller clasts lack quenched margins and have sharp curviplanar surfaces that cross cut vesicle boundaries we interpret the larger pumice clasts to be the products of spalling and explosive fragmentation of a small extrusion of submarine pumiceous lava the smaller pumice clasts were generated by a combination of 1 passive and explosive disintegration of the larger pumice clasts and 2 phreatomagmatic explosions phreatomagmatic explosions also generated a minor component of non or poorly vesicular juvenile and basement derived lithic clasts the yali pumice breccia includes four major facies cobble boulder pebble mixed cobble dominant and mixed pebble dominant the cobble boulder facies occurs in well sorted massive tabular beds of large 64 mm 1 5 m pumice up to 3 m thick that lack both lithic clasts and fine matrix this facies has textural and lithological features consistent with deposition of large pumice clasts by means of water settling from suspension the three remaining facies are comparatively less well sorted although still fines poor and include rare lithic clasts they exhibit massive or internally diffusely stratified wedging 0 1 2 m thick beds of coarser grained medium pebble to cobble pumice clasts that onlap or are separated by wedge shaped thinner

subaqueous intermediate to silicic composition explosive volcanism a review abstract mass flow deposits of pyroclastic debris commonly consist of two parts a lower coarse grained basal section and an upper finer grained bedded section subaqueous pyroclastic flow deposits particularly those derived from a vertically directed eruption appear better sorted than their subaerial counterparts because of the efficient sorting characteristics of water compared to air subaqueous deposits frequently are depleted in bubble wall pumice that floats off and vitric ash that remains in suspension hence the deposits show a relative enrichment of crystals especially at the bases from 1 flotation and suspension of the vitric material and 2 ingestion of water at the head of the flow a subaqueous volcaniclastic flow deposit may contain evidence indicative of both turbulent and non turbulent movement the flow may have had a turbulent head and non turbulent body or flow transformations may have occurred from 1 collapse of the eruption column 2 changes in slope and in the velocity of the flow and or 3 the amount of water mixed into the flow retention of heat in the subaqueous environment may be enhanced by lateral eruptions high eruption rates large eruptive volumes and little or no mixing of pyroclastic debris with water the lack of welded subaqueous deposits in the geologic record may be a result of 1 effusive rather than explosive eruptions in a deep water environment 2 explosive phreatic disintegration of pyroclastic flows in a shallow subaqueous environment and or 3 the inability of subaerial pumiceous pyroclastic flows to penetrate the land water interface characteristics of explosive intermediate to silicic composition subaqueous volcanic eruptions also have been summarized in terms of 1 environment of the eruption 2 orientation of the eruption and 3 composition of the juvenile debris these three aspects of explosive subaqueous volcanism are ideal end members and gradations no doubt occur

unraveling the boundary between turbidites of the kisseynew belt and volcano plutonic rocks of the flin flon belt trans hudson orogen canada the flin flon kisseynew boundary in the eastern trans hudson orogen is interpreted here as an early thrust fault that places 1 86 1 84 ga kisseynew belt turbidites over previously deformed 1 91 1 88 ga arc and ocean floor assemblages of the flin flon belt the basin in which sedimentary rocks of the kisseynew belt were deposited has been interpreted to have formed partly within the flin flon belt the fault that juxtaposes the two belts is interpreted to have been localized near the ancestral basin margin resulting in development of a major ramp zone during basin closure this interpreted ramp zone provides an explanation for the steep to shallow structural transition that corresponds to increasing metamorphic grade collapse of the kisseynew sedimentary basin and juxtaposition of the two belts are attributed to southwest verging folding and thrusting that initiated prior to emplacement of 1 83 ga plutons this magmatism was followed by regional greenschist to upper amphibolite grade metamorphism 1 8

submarine silicic volcanism of the healy caldera southern kermadec arc sw pacific i volcanology and eruption mechanisms the submarine healy volcano southern kermadec arc with a 2 2 5 km wide caldera is pervasively mantled with highly vesicular silicic pumice within a water depth of 1 150 1 800 m pumices comprise type 1 white light grey pumice with 30 mm vesicles and weak moderate foliation type 2 grey pumice with millimetre scale laminae flow banded foliation including stretched vesicles 55 mm in length and a minor finely vesicular type 3 pumice all types are sparsely porphyritic with undevitrified glassy groundmass 68 70 sio2 which is microlite and lithic free coexisting pyroxenes yield magma temperatures of 950 c pumice density is 0 5 g cm 3 and vesicularity is 78 83 vesicle size distributions for types 1 and 2 pumice range from 20 m to 20 mm with a strong power law relation with d 2 5 0 4 for vesicles 9 mpa will limit vesicularity to less than the observed 78 83 whereas pressure 6 mpa require a more shallower reconstruction of the edifice and larger volume syn eruptive collapse uniformly high vesicularity is interpreted as evidence of insulation within an eruption column comprising steam and hot pyroclasts most pyroclasts cool condensing and ingesting water into steam inflated vesicles and then sink progression into pyroclastic mode would expand the eruption column displace ambient water reduce the hydrostatic load and further promote vesiculation and fragmentation pyroclasts within the column would quench at these reduced pressures we argue that healy eruptions deeper than 1 000 m cannot be pyroclastic volumes for the lower and upper bounds of edifice size are 2 36 and 3 58 km3 respectively but do not account for intra caldera pumice fill these volumes are considered to be predominantly primary eruption output as shown by a dearth of accessory lithics in all pumice yielding at an average 81 vesicularity eruptive pumice volumes of between 10 and 15 km3 some pyroclasts may have risen to the sea surface and be a correlative of the sea rafted loisels pumice the latter occurs in some new zealand holocene beach sequences and has a estimated age of 590 80 calendar years

evolution of the paleoproterozoic snow lake arc assemblage and geodynamic setting for associated volcanic hosted massive sulphide deposits flin flon belt manitoba canada the majority of volcanogenic massive sulphide vms deposits at the east end of the paleoproterozoic flin flon greenstone belt occur in the 1 89 ga snow lake arc assemblage vms deposits in this

evidence for voluminous bimodal pyroclastic volcanism during rifting of a paleoproterozoic arc at snow lake manitoba the thrust bounded mcleod road birch lake mb sequence occurs within the paleoproterozoic snow lake arc sla assemblage of the flin flon belt stratigraphic correlation of volcanic strata of the mb sequence with strata of the thrust bounded chisel sequence indicates that distinctive submarine eruption fed pyroclastic flow deposits are more extensive and voluminous than previously recognized 10 km3 these voluminous felsic pyroclastic deposits define a distinct magmatic and explosive volcanic event during bimodal volcanism that accompanied rifting of the sla the felsic pyroclastic deposits define the remnants of a basin or of nested basins that formed during arc rifting and subsidence and their eruption immediately preceded formation of the chisel sequence volcanogenic massive sulfide vms deposits although the chisel sequence ore interval is recognized in the mb sequence the lack of vms related alteration indicates that vms hydrothermal activity was restricted to the chisel portion of the

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

subaqueous strombolian eruptions and eruption fed pyroclastic deposits in a paleoproterozoic rifted arc hidden formation flin flon canada abstract the hidden formation of the paleoproterozoic flin flon arc assemblage comprises the stratigraphic hanging wall to the volcanogenic massive sulfide vms deposits at flin flon manitoba canada it represents a basaltic volcanic edifice erupted within a large subsidence structure developed in a rifted arc environment it is predominately pillow lavas with subordinated amounts volcaniclastic rocks rocks that stratigraphically overly the schist and mandy vms deposits 5 km southeast of the main flin flon deposits have been correlated through litho and chemo stratigraphy with the hidden formation they display evidence of pyroclastic volcanism and concomitant subsidence pillow lavas and hyaloclastite are unequivocal evidence of subaqueous eruption the hidden formation in the schist mandy area comprises mafic tuff mafic megabreccia basalt lava fluidal clast breccia and plagioclase crystal rich volcaniclastic lithofacies intruded by synvolcanic dikes and sills the megabreccia lithofacies is characterized by angular basalt clasts 1 20 m in a matrix of finer basalt clasts and is interpreted to have been deposited by mass flows due to slumping of the caldera wall a within flow vertical transition in the lavas from massive to pillowed flows suggests a slowing of emplacement velocity decrease in temperature and an increase in the viscosity during eruption based on the monolithic nature of the fluidal clast breccias and the fluidal shapes of clasts with delicate tails and intact glassy margins they are interpreted to be the result of strombolian eruptions an increase in the ingestion of water in the vent s lead to a change from magmatic to phreatomagmatic fragmentation planar and cross laminations graded beds and the abundance of broken plagioclase crystals in combination with scoria and lithic clasts suggest the mafic tuff and plagioclase crystal rich volcaniclastic lithofacies were deposited by subaqueous eruption fed density currents derived from a phreatomagmatic eruption detailed mapping of the volcanic lithofacies in the schist mandy area has facilitated the identification of synvolcanic faults defining inner caldera margins and volcanic vents these structures when traced downwards through stratigraphy define structural corridors corresponding with the schist and mandy vms deposits indicating they represent long lived structures that also acted as magmas and fluid pathways

subaqueous pyroclastic flows and ignimbrites an assessment an assessment of the literature on subaqueous pyroclastic flows and their deposits shows that the term pyroclastic flow is frequently used loosely to describe primary hot gas rich pyroclastic flows mass flows which resulted from the transformation of gassupported flows into water supported ones and secondary mass flows carrying redeposited pyroclastic debris based on subaerial pyroclastic flows the term pyroclastic flow should be restricted to demonstrably hot gas rich mass flows of pyroclastic debris using this definition very few examples of subaqueous pyroclastic deposits with evidence for hot emplacement and of having been wholly submerged have been described in the majority of these cases the evidence for a hot state of emplacement and for the subaqueous nature of the host depositional environment is inadequate the only unequivocal cases of hot pyroclastic flow deposits with adequate supporting evidence are the ordovician nearshore shallow marine ignimbrites of ireland and wales and miocene ignimbrites of southwest japan resulting from the passage of subaerially erupted pyroclastic flows into shallow water other possible examples are near vent dense clast deposits in the donzurobo formation of japan possible submarine intra caldera ponded ignimbrite successions in california and wales and near vent pumiceous deposits of ramsay island wales all other purported cases are either clearly the result of water supported mass flow transportation and deposition debris avalanches debris flows turbidity currents or lack adequate supporting evidence regarding the heat state or the palaeoenvironment only the shallow marine ignimbrites of ireland and wales show adequate evidence of welding but even these could have been nearly wholly exposed above sea level when welding occurred we conclude that when pyroclastic flows enter water they are generally disrupted explosively and or ingest water and transform into water supported mass flows and we suggest the various scenarios in which this occurs there is no evidence to suggest that welding in wholly subaqueous environments is common

submarine eruption fed and resedimented pumice rich facies the dogashima formation izu peninsula japan in the izu peninsula japan the pliocene pumice rich dogashima formation 4 55 0 87 ma displays exceptional preservation of volcaniclastic facies that were erupted and deposited in a below wave base marine setting it includes high concentration density current deposits that contain clasts that were emplaced hot indicating an eruption fed origin the lower part of the dogashima 2 unit consists of a very thick sequence 1 m pumice and ash rich bed overlying the breccias may be derived from delayed settling of pyroclasts from suspension in dogashima 1 and 2 thick cross and planar bedded facies composed of sub rounded pumice clasts are intercalated with eruption fed facies implying inter eruptive mass wasting on the flank of a submarine volcano and reworking and resedimentation by high energy tractional currents in a below wave base environment

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

geochemistry of 1 9 ga morb and oib like basalts from the amisk collage flin flon belt canada evidence for an intra oceanic origin subaqueously erupted basalts that occur in kilometre scale allochthons within the 1 9 ga flin flon belt canada appear to have been generated at oceanic ridges and possibly oceanic plateaus remote from archean cratons the ocean floor basalts fall into two categories 1 n type resembling n morbs and mariana type back arc basin basalts depleted to flat ree patterns high zrnb variable thnb and initial nd 3 3 to 5 4 2 e type resembling transitional and plume morbs slightly enriched ree patterns lower zrnb initial nd 3 1 to 4 5 in the largest and best studied allochthon the elbow athapapuskow assemblage mixing between depleted n morb and enriched oib sources or melts coupled with variable addition of a subduction lile component can explain the chemical variations in the basalts zircon u pb dates of 1904 ma for a syn volcanic diabase sill and 1901 6 5 ma for a gabbro peridotite cumulate complex demonstrate that crystallization of the ocean floor basalts overlapped with in part eruption of the tectonically juxtaposed 1 90 1 88 ga arc volcanic rocks the elbow athapapuskow allochthon is interpreted as back arc basin crust that developed simultaneously with flin flon arc magmatism subaerially erupted basalts that chemically resemble tholeiitic oibs 8 14 mgo relative hree depletion initial nd 2 2 to 3 4 occur in tectonic contact with the elbow athapapuskow assemblage the oibs may have been generated by deeper garnet residue melting of enriched mantle tapped during extension in the elbow athapapuskow back arc basin and were possibly erupted onto a remnant arc deeper mantle melting is also indicated by the presence of the lree enriched oceanic plateau like basalts of the sandy bay assemblage the back arc 01b and plateau volcanic assemblages were jux taposed against ca 1 9 ga arc assemblages in a philippines like intraoceanic accretionary complex by 1 87 ga

development of a transverse to orogen parallel extension lineation in a complex collisional setting trans hudson orogen manitoba canada abstract in the southeastern trans hudson orogen tho the wide dispersion in orientation of the l3 extension lineation from orogen parallel to transverse is considered primary as it cannot be accounted for by simple overprinting of a single precursor orientation interpretation of the origin of this extension lineation is complicated by the fact that its gently northeast plunging orientation at the north end of the study area is inconsistent with the evidence for sinistral east side up movement of the steep oblique slip d3 faults the hypothesis presented here to explain the complicated lineation pattern is based on evidence that the preceding period d2 southwest directed transport overlapped temporally and spatially with d3 northwest southeast transpressional shortening d2 and d3 are attributed to successive but overlapping collisional events d3 transpressional shortening is interpreted to have propagated westward through the tho resulting in a prolonged and complex transition between d2 and d3 deformation regimes the dispersion of the l3 extension lineation is interpreted to reflect the competing effects of both deformation regimes which appear to have been partitioned in the crust with d2 effects best developed in higher metamorphic grade rocks and d3 effects best developed in lower metamorphic grade rocks

possible distinguishing characteristics of very deepwater explosive and effusive silicic volcanism recent seafloor exploration has shown that volcanic hosted massive sulfides vhms occur in modern silicic calderas formed by highly explosive eruptions at substantial water depths sampling of these has so far been restricted to surficial deposits ancient analogs provide a time integrated view of the structure and fill of deepwater calderas but constraints on paleo water depths have previously been lacking the ordovician bald mountain vhms northern maine usa is unique in its degree of preservation not only permitting detailed textural and structural analysis of the enclosing volcanic rocks but also allowing fluid inclusion analysis to determine a paleo water depth of eruption of greater than 1 45 km the deposits that result from very deepwater explosive eruptions have distinctive textural characteristics recognized in other ancient examples where paleo water depths are less well constrained i propose that hydrostatic pressure suppresses the explosivity of the eruption sufficiently to retard the formation of fine ash producing ignimbrite composed largely of pumice lapilli it also inhibits exsolution of dissolved water in the magma producing fluidal rhyolite eruptions and intrusions these include silicic fire fountaining to produce proximal cones of hyalotuff or spatter accumulations and intrusions of globular peperite

products of neptunian eruptions a common pyroclastic facies in subaqueous volcanic successions comprises massive to graded very thick several to tens of meters laterally extensive several kilometers beds of nonwelded pumice lapilli with volumes ranging to tens of cubic kilometers this facies may be overlain by laminated ash or bimodal ash and giant 1 m pumice clasts and underlain by coarse lithic breccia the association is inferred to be the typical product of sustained magmatic volatile driven explosive eruptions from vents at water depths of 1300 200 m we propose the term neptunian for such eruptions and their products the eruption column rapidly mixes with the surrounding water cools increases in density and collapses while remaining under water lithic clasts that are too heavy to be entrained in the column are deposited close to the source forming a neptunian lithic breccia pumice lapilli are rapidly waterlogged and form the dominant component in the collapsing column and in eruption fed water supported density currents neptunian density currents hot buoyant giant pumice clasts continue to rise and may reach the water surface before being waterlogged and settling along with temporarily suspended ash forming neptunian suspension deposits eruption magnitude fragmentation mechanisms and juvenile pyroclast characteristics especially vesicularity are very similar in neptunian and plinian style eruptions but column behavior differs primarily because of the contrasting physical properties of the ambient fluid water versus air

volcanic reconstruction of the paleoproterozoic stroud breccia understanding the transition from a primitive to mature arc and its impact on vms ore formation snow lake manitoba canada abstract the stroud breccia is a 400 m thick package of dominantly felsic volcaniclastic rocks within the paleoproterozoic snow lake arc assemblage it stratigraphically overlies the ore interval for the anderson stall and rod volcanogenic massive sulfide vms deposits at snow lake manitoba canada and occupies the stratigraphic interval between the underlying primitive arc anderson sequence and the overlying mature arc chisel sequence the stroud breccia is interpreted to be a product of subaqueous pyroclastic and effusive dominantly rhyolitic volcanism and concomitant subsidence it consists of felsic tuff to lapilli tuff crystal rich felsic tuff felsic lapillistone felsic tuff breccia mafic tuff to lapilli tuff heterolithic tuff breccia to breccia and rhyolite the finer felsic volcaniclastic units are characterised by angular or broken quartz crystals angular lithic rhyolite lapilli and pumice fine ash broken crystals and pumice suggest they were deposited by eruption fed density currents associated with a phreatomagmatic eruption the coarser felsic volcaniclastic units are characterized by blocks of strongly quartz feldspar porphyritic rhyolite and pumice and are interpreted to have been deposited during the reworking of pyroclastic debris flow deposits on the seafloor during mass wasting events at fault scarps planar and cross laminations graded beds and bomb sag structures suggest the mafic tuff to lapilli tuff unit was deposited by a combination of subaqueous eruption fed density currents and eruption driven ballistically emplaced ejecta likely through subaqueous tephra jetting lithofacies analyses and geochemistry has allowed comparison between the felsic volcanic rocks of the stroud breccia and those that host the anderson stall and rod vms deposits detailed mapping has led to the identification of synvolcanic faults defining basins and volcanic vents that indicate that the process of incipient rifting of the anderson sequence primitive arc continued during emplacement of the stroud breccia these structures also define magma and fluid pathways critical in controlling the formation of the anderson vms deposits meaning they represent deep seated and long lived structures that can be used as vectors to mineralization at depth

textural and chemical characteristics of diagenetic and hydrothermal alteration in glassy volcanic rocks examples from the mount read volcanics tasmania the mount black and sterling valley volcanics comprise a complex cambrian succession of originally glassy felsic to mafic submarine volcanic facies in western tasmania the mount black and sterling valley volcanics have been modified by the combined effects of diagenesis hydrothermal alteration and regional metamorphism metamorphosed diagenetic and hydrothermal alteration facies have been distinguished using a combination of alteration mineralogy overprinting relationships texture distribution intensity and whole rock geochemistry alteration facies that are interpreted to be the regionally metamorphosed equivalents of diagenetic clays and zeolites are regionally extensive and overprinted by the regional cleavage although widespread their distribution is patchy and their intensities are highly variable reflecting the complexity of the original textures in glassy volcanic rocks thin films of sericite carbonate and chlorite sericite hematite replace clays that coated original glassy surfaces at the onset of diagenesis prior to compaction feldspar quartz sericite chlorite sericite and chlorite sericite hematite alteration facies replace zeolites and clays that filled pore space and altered glass prior to and synchronous with compaction the final diagenetic alteration facies chlorite epidote began after compaction and continued during metamorphism diagenetic alteration involved significant mineralogical and textural changes but only minor changes in chemistry the chemical changes associated with diagenetic alteration facies are consistent with the interaction of rhyolitic and basaltic glass with seawater during burial alteration facies that are interpreted to be hydrothermal are local in distribution crosscut stratigraphic boundaries and involve significant changes in texture mineralogy and composition early hydrothermal alteration facies quartz sericite and calcite quartz hematite predated compaction and were synchronous with diagenesis chlorite calcite magnetite and chlorite pyrite alteration facies were synchronous with regional deformation and are associated with tectonic breccias faults and shear zones chlorite quartz calcite alteration facies postdated regional deformation