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

an immense shield volcano within the shatsky rise oceanic plateau northwest pacific ocean most oceanic plateaux are massive basaltic volcanoes however the structure of these volcanoes and how they erupt and evolve is unclear because they are remote and submerged beneath the oceans here we use multichannel seismic profiles and rock samples taken from integrated ocean drilling program core sites to analyse the structure of the tamu massif the oldest and largest edifice of the shatsky rise oceanic plateau in the north western pacific ocean we show that the tamu massif is a single immense volcano constructed from massive lava flows that emanated from the volcano centre to form a broad shield like shape the volcano has anomalously low slopes probably due to the high effusion rates of the erupting lavas we suggest that the tamu massif could be the largest single volcano on earth and that it is comparable in size to the largest volcano in the solar system olympus mons on mars our data document a class of oceanic volcanoes that is distinguished by its size and morphology from the thousands of seamounts found throughout the oceans

discovery of a recurrent lava lake on saunders island south sandwich islands using avhrr imagery abstract monitoring of the remote south sandwich islands volcanic arc using advanced very high resolution radiometer avhrr data has identified a radiant pixel on channels 3 3 55 3 93 m and rarely 4 10 3 11 3 m the position of the pixel is coincident with mount michael saunders island the active summit crater of a snow covered basaltic stratovolcano with a persistent steam plume the radiant pixel was continuously present in successive avhrr images acquired during intervals of several months in the period examined march 1995 february 1998 although apparently disappearing for similar time intervals more than 5000 images were examined during this study the radiant pixel is interpreted to indicate that the crater has contained a lava lake the first to be recorded in the south sandwich islands the lake appears to persist in the crater for several months at a time at least it may have drained completely at times and was probably absent when the crater was viewed briefly during an overflight in january 1997 persistent or recurring lava lakes are very uncommon world wide and that at mount michael is one of only two recorded in the antarctic region

a comparison of the horne volcanogenic massive sulfide deposit and intracauldron deposits of the mine sequence noranda quebec most of the massive sulfide deposits of the rouyn noranda area are associated with rhyolite and andesite flows of the mine sequence and occur within the noranda cauldron a volcanic subsidence structure related to partial evacuation of an underlying magma chamber the flavrian pluton the horne massive sulfide orebodies lie within a sequence of rhyolitic breccias located just south of the inferred margin of the subsidence structure and are separated from the mine sequence by the horne creek fault traditionally the horne deposits and the intracauldron deposits have been interpreted to occur within the same stratigraphic interval even though they share some characteristics such as mineral zoning patterns alteration assemblages and an association with synvolcanic faults the two deposit groups are clearly distinguishable intracauldron massive sulfide deposits are small 5 mt cu zn rich concordant mounds associated with interflow horizons and are closely linked to vent zones where synvolcanic faulting provided hydrothermal fluids with cross stratal structural permeability the three major sulfide deposits at the horne mine are much larger 20 150 mt and formed in a fault bounded sediment filled graben on the flank of a rhyolitic vent complex the orebodies are au cu rich but zn poor and have bowl shaped morphologies which resulted from extensive subsea floor sulfide precipitation whereas the intracauldron sulfide deposits lie within a bimodal flow dominated succession consisting mainly of andesite basalt and high silica rhyolite with volumetrically insignificant pyroclastic rocks the horne orebodies occur in felsic fragmental rocks and rhyolite flows which are part of a stratigraphic sequence which contains virtually no mafic volcanic rocks also compared to all intracauldron rhyolites the horne rhyolites show a fundamental depletion in incompatible trace elements such as the high field strength and rare earth elements which implies that the horne rhyolites may have followed a different magmatic evolutionary path such differences in deposit characteristics their paleoenvironments of formation and the physical volcanology and geochemistry of their host volcanic successions suggest that contrary to previous interpretations the massive sulfide deposits of the intracauldron mine sequence and those of the horne sequence may have formed at different stages in the evolution of the noranda cauldron

effects of water depth on pumice formation in submarine domes at sumisu izu bonin arc western pacific domes of the sumisu volcanic complex western pacific having summits at ocean depths of 1100 600 245 and 95 m are mantled with compositionally identical rhyolitic pumice that has similar vesicularity but that varies systematically in distribution size and surface texture suggesting that facies and morphology can serve as useful indicators of eruption depth at depths 500 m the pumice formed a thick carapace on dense rhyolite and disintegrated by quench fracture and mechanical failure into a jumble of giant meters to tens of meters polyhedral blocks with smooth curviplanar surfaces that display a single quenched margin vesiculation was arrested on eruption in seawater in all but the interior of the thickest carapace at 500 m depths the pumice occurs as an apron of blocky giant and smaller rough textured clasts enclosed by quenched margins and pockmarked by coarse cm vesicles no carapace pumice occurs and the summit is composed of craggy dense dome rock these shallower water pumice clasts resemble those spalled from historic submarine dome forming eruptions that buoyed to the sea surface we interpret spalling to result from vent derived weak volatile driven explosions that take place at water depths 500 m our study shows that an increase in hydrostatic pressures over a range of 12 mpa reduces volatile driven explosivity for subaqueous rhyolitic dome forming eruptions but does not affect vesicularity we conclude that meter size highly vesicular pumice is diagnostic of subaqueous dome eruptions in water depths of at least 1300 m and its morphology can be used to distinguish between explosive and effusive origins

subaqueous calderas in the archean abitibi greenstone belt an overview and new ideas abstract the 300 700 km abitibi greenstone belt in canada contains numerous world class archean volcanogenic massive sulfide vms deposits yet documentation of archean subaqueous calderas hosting such deposits is lacking the modern sunrise deposit in the myojin knoll caldera of the izu bonin arc shows that submarine calderas are first order sites for vms the hunter mine and normetal calderas as well as the blake river megacaldera complex are reviewed with respect to geometry regional geology physical volcanology dyke emplacement and hydrothermal carbonate alteration these subaqueous calderas are placed into the geodynamic context of the abitibi greenstone belt the abitibi belt displays a complex history of oceanic arc formation evolution collision and fragmentation with numerous types of volcanic edifices and sedimentary basins controlled by oblique subduction and mantle plumes analogue models yielding new insights into the sequence of caldera forming events and their synvolcanic fault patterns are applied to explain archean examples underpressure experiments develop a consistent sequence of fault patterns that compare favourably with natural caldera events during subsidence two sets of faults propagate from the magma chamber margin 1 early reverse outward dipping faults and 2 normal inward dipping faults the largest massive sulfide deposits form preferentially at the caldera margin based on experimental studies and detailed facies mapping the hunter mine caldera is of the segmented piecemeal variety and the normetal caldera is akin to a piston structure the blake river megacaldera contains 1 the e w trending misema 2 the nw trending new senator and 3 the ne trending noranda calderas with the latter two being nested graben type calderas consistent with oblique archean subduction the poorly documented hydrothermal carbonate alteration is discussed and a new exploration model for calderas is presented the alteration study was evaluated at three levels microprobe analysis geochemical staining of drill core and whole rock analyses there were two principal stages of alteration related to vms formation an early silicification phase developed either at depth and or at the seawater interface whereby silicification was bedding parallel in fine grained volcaniclastic deposits near the water surface a pervasive and extensive semi conformable carbonate alteration zone with a discordant focused root zone along faults evolved subsequently three distinct carbonate pairings were observed 1 proximal siderite sideroplesite fe ankerite next to the vms deposit 2 an intermediate ankerite fe dolomite zone and 3 a distal calcite dolomite zone the carbonate phase is associated with the mineralizing event transitions between carbonate species are subtle and changes are indicated by mineral assemblage overlap the silica carbonate hydrothermal alteration pattern is a replacement product and is far more widespread than has been documented this alteration can be easily confused with primary carbonates and chemical precipitation deposits e g banded iron formation if detailed volcano sedimentary facies mapping with alteration has not been conducted and if outcrop is insufficient

conditions and timing of metamorphism in the southern abitibi greenstone belt quebec regional metamorphism ranging in grade from the subgreenschist facies to the greenschist amphibolite facies transition affects all archean supracrustal rocks 2677 ma in the rouyn noranda area contact metamorphic minerals associated with the posttectonic preissac lacorne batholith 2643 ma show no evidence of a regional retrograde event accordingly the age of regional metamorphism can be bracketed between 2677 and 2643 ma three reaction isograds were mapped in subgreenschist facies metabasites dividing the low grade rocks into three metamorphic zones the pumpellyite actinolite zone the prehnite pumpellyite zone and the prehnite epidote zone in addition the pumpellyite actinolite epidote quartz bathograd corresponding to a pressure of approximately 200 mpa occurs on both sides of the porcupine destor fault low pressure regional metamorphism is also indicated both by the occurrence of an actinolite oligoclase zone and the persistence of pre regional metamorphic andalusite the coincidence

growth and mass wasting of volcanic centers in the northern south sandwich arc south atlantic revealed by new multibeam mapping new multibeam swath bathymetric sonar data acquired using an em120 system on the rrs james clark ross supplemented by sub bottom profiling reveals the underwater morphology of a not vert similar 12 000 km2 area in the northern part of the mainly submarine south sandwich volcanic arc the new data extend between 55 45 s and 57 20 s and include protector shoal and the areas around zavodovski visokoi and the candlemas islands groups each of these areas is a discrete volcanic center the entirely submarine protector shoal area close to the northern limit of the arc forms a 55 km long east west trending seamount chain that is at least partly of silicic composition the seamounts are comparable to small subaerial stratovolcanoes in size with volumes up to 83 km3 indicating that they are the product of multiple eruptions over extended periods zavodovski visokoi and the candlemas island group are the summits of three 3 3 5 km high volcanic edifices the bathymetric data show evidence for relationships between constructional volcanic features including migrating volcanic centers structurally controlled constructional ridges satellite lava flows and domes and mass wasting of the edifices mass wasting takes place mainly by strong erosion at sea level and dispersal of this material along chutes probably as turbidity currents and other mass flows that deposit in extensive sediment wave fields large scale mass wasting structures include movement of unconsolidated debris in slides slumps and debris avalanches volcanism is migrating westward relative to the underlying plate and major volcanoes are asymmetrical being steep with abundant recent volcanism on their western flanks and gently sloping with extinct eroded volcanic sequences to their east this is consistent with the calculated rate of subduction erosion of the fore arc

an archean megacaldera complex the blake river group abitibi greenstone belt abstract the archean blake river group of the southern abitibi greenstone belt is defined as a subaqueous megacaldera compelling evidence include 1 a radial and concentric organization of synvolcanic mafic to intermediate dykes 2 an overall dome geometry defined by the volcanic strata 3 a peripheral distribution of subaqueous volcaniclastic units 4 a zonal distribution of carbonate alteration and 5 a distinct annular synvolcanic inner and outer ring fault pattern three caldera forming events have been identified 1 the early misema caldera 2 the new senator caldera and 3 the well known noranda caldera the multi vent brg mafic volcanic complex developed on a monotonous sequence of tholeiitic basalts forming a submarine plain and experienced a first major collapse that created the giant misema caldera 80 km in diameter an endogenic dyke swarm intruded the synvolcanic fractures and an underlying magma chamber developed major volcaniclastic units were generated by local volcanic centers and summit calderas formed along the outer and inner ring faults this fault system was used as a conduit for co2 rich hydrothermal activity renewed volcanic activity was associated with a resurgent central dome inside the misema caldera this second collapse event created the 35 km by 14 km nw trending new senator caldera this caldera was produced by multi step sagging after the underlying magmatic chamber migrated to the se and formed the flavrian powel plutons the final collapse resulted in the formation of the noranda caldera that generated a well developed 070 trending fracture pattern associated with several vms deposits the multiple caldera setting provides an effective model to explain the presence of vms mineralization along the synvolcanic fractures associated with the three collapse episodes however the importance of the new senator caldera fracture pattern is emphasized because of its role in the positioning of the giant horne mine deposit

oblique archean subduction accretion and exhumation of an oceanic arc during dextral transpression southern volcanic zone abitibi subprovince canada abstract the archean abitibi subprovince divided into northern nvz and southern svz volcanic zones is characterized by the crustal scale destor porcupine manneville fault zone dpmfz which links the svz and nvz and the cadillac larder lake fault zone cllfz which separates the abitibi and pontiac subprovinces the dpmfz and cllfz represent major deformation zones that record over 60 million years of archean deformation the dextral transpressional phase in the nvz which represents incipient svz deformation occurred between 2700 and 2692 ma and is characterized by southeast se trending dextral faults e g macamic fault synorogenic flysch sedimentation is focussed at the interface between the svz nvz and pontiac abitibi subprovinces the se trending parfouru fault linking the two major trench fault systems is interpreted as a reactivated leaky transform fault and is associated with synorogenic flysch sedimentation the early monzodioritic suite of the preissac lacorne batholith was related to dpmfz thrusting ca 2692 2690 ma the change from early thrusting to transcurrent motion is recorded in the duparquet strike slip basin which formed between 2690 and 2680 ma along the dpmfz and the granada pull apart basin which evolved between 2680 and 2670 ma along the cllfz the timing of these basins indicates a diachronous evolution renewed thrusting affected the granada basin but not the northern duparquet strike slip basin so that a southward migrating deformation front is inferred late exhumation resulted in extension along both fault zones and was responsible for the juxtaposition of medium and low grade metamorphic rocks the monzogranitic suite of the lamotte pluton which occurred between 2660 and 2642 ma is linked to exhumation final dextral transpression post dating exhumation produced shearing and folding the svz of the abitibi subprovince shows the salient attributes of modern oblique orogenic collisions with alternating phases of thrusting and strike slip movement along crustal scale faults during dextral transpression

a pliocene shoaling basaltic seamount ba volcanic group at rakiraki fiji abstract at rakiraki in northeastern viti levu the pliocene ba volcanic group comprises gently dipping pyroxene phyric basaltic lavas including pillow lava and texturally diverse volcanic breccia interbedded with conglomerate and sandstone three main facies associations have been identified 1 the primary volcanic facies association includes massive basalt flows and sills pillow lava and related in situ breccia pillow fragment breccia autobreccia in situ hyaloclastite peperite 2 the resedimented volcaniclastic facies association consists of bedded monomict volcanic breccia and scoria lapilli rich breccia 3 the volcanogenic sedimentary facies association is composed of bedded polymict conglomerate and breccia together with volcanic sandstone and siltstone mudstone facies pillow lava and coarse hyaloclastite breccia indicate a submarine depositional setting for most of the sequence thick massive to graded beds of polymict breccia and conglomerate are interpreted as volcaniclastic mass flow deposits emplaced below wave base well rounded clasts in conglomerate were reworked during subaerial transport and or temporary storage in shoreline or shallow water environments prior to redeposition red oxidised lava and scoria clasts in bedded breccia and conglomerate also imply that the source was partly subaerial the facies assemblage is consistent with a setting on the submerged flanks of a shoaling basaltic seamount the coarse grade and large volume of conglomerate and breccia reflect the high supply rate of clasts and the propensity for collapse and redeposition on steep palaeoslopes the clast supply may have been boosted by vigorous fragmentation processes accompanying transition of lava from subaerial to submarine settings the greater proportion of primary volcanic facies compared with resedimented volcaniclastic and volcanogenic sedimentary facies in central and northwestern exposures near rakiraki indicates they are more proximal than those in the southeast towards viti levu bay the proximal area coincides with one of two zones where nw se trending mafic dykes are especially abundant and it is close to several small dome like intrusions of intermediate and felsic igneous rocks the original surface morphology of the volcano is no longer preserved though the partial fan of bedding dip azimuths in the south and east and the wide diameter exceeding 20 km are consistent with a broad shield

exogenous and endogenous construction of the subaqueous glenwood felsic lava flow complex abitibi greenstone belt quebec canada abstract both intrusive and extrusive volcanic facies of a felsic lava flow complex are exposed as the outcrops of the glenwood rhyolite within the blake river group of the abitibi greenstone belt quebec canada sub vertically dipping volcanic facies allows for the identification of the internal architecture of an archean subaqueous felsic flow complex detailed facies analyses show volcanic facies and volcanic textures and structures atypical of subaqueous pyroclastic deposits or deposits of dome complexes additionally a complex network of mafic dykes cross cut the felsic facies when integrated with facies specific geochemical analyses a coherent model for all volcanic and intrusive facies and supplemental information regarding the host volcanic complex has been developed detailed mapping reveals extrusive or exogenous aphanitic felsic volcanic facies and intrusive or endogenous quartz and feldspar phyric felsic volcanic facies exogenous facies are comprised of massive lobate in situ brecciated flow breccia and flow front breccia facies whereas the only visible endogenous facies is a quartz and feldspar phyric unit that intrudes the brecciated facies on the eastern side of the complex all felsic facies demonstrate a west to east flow direction both exogenous and endogenous facies have similar chemical compositions ranging from dacite to rhyodacite with a transitional affinity mafic to intermediate dykes predominantly strike north south and east west with occasional east northeast trending units wispy terminations at some dyke margins and dismembered dykes indicate they were intruded into a still cooling viscoelastic medium geochemical analyses further defines that most east west trending dykes have a tholeiitic affinity whereas north south trending dykes are dominantly calc alkaline and east northeast trending dykes are transitional the subaqueous glenwood felsic flow complex was emplaced in a fault bounded depression and consists of stacked exogenous dacite rhyodacite lobes and intruded by a late quartz and feldspar phyric endogenous lobe the intrusion of mafic dykes occurred relatively soon after the cessation of felsic activity as evidenced by primary textures consistent with intrusion into a viscoelastic medium combined the extrusive and intrusive facies of the complex attests to the complexity of the host volcanic environment

the early development of an archean submarine caldera complex with emphasis on the mattabi ash flow tuff and its relationship to the mattabi massive sulfide deposit the south sturgeon lake area of northwestern ontario is underlain by an archean submarine caldera complex which is host to six massive sulfide deposits the complex is approximately 30 km in strike length and is partly filled by mesobreccias subaqueous debris flow deposits and other epiclastic rocks as well as five major ash flow tuff units the most voluminous of these the mattabi ash flow tuff is up to 800 m in thickness and can be traced for more than 15 km across the caldera complex based on flow morphology and composition the mattabi ash flow tuff is divisible into two distinct units a bedded quartz crystal and pumice rich unit 150 650 m thick and a massive to poorly bedded ash unit 20 150 m thick the bedded unit is host to the mattabi massive sulfide deposit as well as numerous other massive sulfide occurrences in addition this unit exhibits a pronounced stratigraphic and geochemical cyclicity individual beds flow units can be subdivided into basal quartz and pumice rich lower sections which are overlain by bedded ash tops thicknesses of individual beds vary from 14 to 155 m each bed also exhibits a pronounced zonation of trace elements with high concentrations of zr y and nb at and or near the base gradually decreasing upward into the ash tops the overlying massive ash unit contains no known massive sulfide mineralization and lacks any pronounced geochemical zoning the bedded nature and geochemical zonation of individual flow units can be explained by periodic eruptions from a zoned or layered recharged siliceous magma chamber the difference in flow morphology between the bedded unit and the overlying massive ash flow tuff most likely reflects the deeper submarine nature of the eruptive vents associated with the bedded deposits the periodic and subaqueous nature of the mattabi eruptions is indicated by the presence of exhalites and massive sulfide lenses between flow units the ore lenses which comprise the mattabi deposit formed between eruptive events from high temperature fluids whose origin may be related to processes within the magma chamber

late variscan magmatism revisited new implications from pb evaporation zircon ages on the emplacement of redwitzites and granites in ne bavaria pb evaporation zircon analyses coupled with a detailed cathodoluminescence cl study on the complete series of granitoids from the northern oberpfalz ne bavaria provide new evidence for the commencement and timing of late variscan magmatism all granitoids analysed in this study were dated before by rb sr and or k ar methods investigated samples comprise medium grained i type dioritic rocks redwitzites i s type granites leuchtenberg marktredwitz g1 zainhammer and s type granites falkenberg liebenstein mitterteich friedenfels steinwald flossenburg barnau the zircon evaporation technique reveals three groups of 207pb 206pb ages which are interpreted to represent magmatic crystallisation 1 ages of 324 321 ma are found in all analysed redwitzites and in almost all i s type granites 2 the granites of falkenberg and liebenstein yield ages of 315 ma 3 ages between 312 and 310 ma are recorded in the mitterteich friedenfels steinwald and flossenburg granites titanite crystals from different redwitzite bodies yield conventional u pb ages of 325 322 ma identical to the pb evaporation zircon data of these rocks the s type granites of groups 2 and 3 contain zircons with relict cores but only a small number of them yield older ages indicating that some of the cores must have lost their radiogenic pb from the geochronological data we infer that metamorphic conditions of the variscan crust produced different granite types at different times the data support a model involving an early period of mantle related magmatism which postdates the final convergence stage of the variscan orogen this magmatic activity was at the same time as the thermal peak of regional metamorphism and is followed by a late period of crustal anatexis which is probably related to post collisional extension of the thickened variscan crust

basaltic to andesitic volcaniclastic rocks in the blake river group abitibi greenstone belt 2 origin geochemistry and geochronology the archean blake river group brg of ontario and quebec is dominated by submarine mafic to intermediate lavas with more restricted felsic volcanic rocks given the good quality of outcrop and high level of preservation of some brg rocks the mafic to intermediate lavas were used in the 1970s and 1980s to better understand the evolution of massive and pillowed submarine flows and their associated fragmental facies pillow breccias hyaloclastite potentially the brg could also represent a useful volcanic succession for the study of explosive submarine eruption products in the ancient record before this is possible however a regional inventory of the mafic to intermediate volcaniclastic units is needed in order to clarify their characteristics and origins in this paper we compare and contrast volcaniclastic rocks from three areas within the same formation of the northern brg in quebec the monsabrais area the lac duparquet area and the d alembert tuff area close examination reveals pronounced differences in terms of lateral continuity thickness grading bedding clast shapes textures etc in the volcaniclastic rocks these differences are interpreted to reflect vastly different emplacement processes ranging from hyaloclastite generation as a result of self fragmentation and lava contact with water dominant in the monsabrais and lac duparquet areas to aqueous density currents likely fed directly by explosive submarine eruptions dominant in the d alembert tuff

archean metavolcanics from the rouyn noranda district abitibi greenstone belt quebec 2 mobility of trace elements and petrogenetic constraints qualitative evaluation of altered mafic pillows from the rouyn noranda district of the abitibi metavolcanic belt reveals that for low grade metamorphism the high field strength elements hfs zr y ti and the rare earth elements ree are immobile however where alteration is extreme the ree elements are leached and redeposited by carbonate rich and k rich metasomatic fluids whilst zr y and zr ti remain unchanged rubidium ba and to a lesser extent sr are mobile at all levels of alteration however provided certain filters are applied the original geochemical values of the lavas for the alkaline elements the hfs elements and the ree can be defined the geochemical variations thus identified for the volcanic series of the rouyn noranda district are ascribed to a combination of contamination and fractional crystallization of a primitive tholeiitic magma

dioritic intrusions of the slavkovský les kaiserwald western bohemia their origin and significance in late variscan granitoid magmatism mafic and intermediate intrusions occur in the slavkovsk les as dykes sills and minor tabular bodies emplaced in metamorphic rocks or enclosed in late variscan granites near the sw contact of the western krusne hory erzgebirge granite pluton they are similar in composition and textures to the redwitzites defined in ne bavaria single zircon pb evaporation analyses constrain the age of a quartz monzodiorite at 323 4 4 4 ma and of a granodiorite at 326 1 5 6 ma the p t range of magma crystallization is estimated at 1 4 2 2 kbar and 730 870 c and it accords with a shallow intrusion level of late variscan granites but provides lower crystallization temperatures compared to the bavarian redwitzites we explain the heterogeneous composition of dioritic intrusions in the slavkovsk les by mixing between mafic and felsic magmas with a minor effect of fractional crystallization increased k ba rb sr and ree contents compared to tholeiitic basalts suggest that the parental mafic magma was probably produced by melting of a metasomatised mantle the melts being close to lamprophyre or alkali basalt composition diorites and granodiorites originated from mixed magmas derived by addition of about 25 35 and 50 vol respectively of the acid end member granite to lamprophyre or alkali basalt magma our data stress an important role of mafic magmas in the origin of late variscan granitoids in nw bohemian massif and emphasize the effect of mantle metasomatism on the origin of k rich mafic igneous rocks

pb pb and u pb zircon dating of subvolcanic rhyolite as a time marker for hercynian granite magmatism and sn mineralisation in the eibenstock granite erzgebirge germany considering effects of zircon alteration tin mineralisation is closely related to rhyolite stocks and dykes which occur in the endo and exocontact of the eibenstock granite erzgebirge germany the same structures which cut the granite control the rhyolite emplacement and the location of ore bearing greisen bodies albitisation and greisenisation related to tin mineralisation as well as sericitisation and argillic alteration may be traced by changes in chemical and mineralogical composition of both rhyolite and granite wall rock comprehensive zircon studies by scanning electron microscopy secondary and backscattered electron as well as cathodoluminescence imaging edx measurements reveal that zircon from rhyolite and from the enclosing granite shows significant changes in chemical composition and crystallinity including distortion of the u pb isotope system when affected by greisenisation single evaporation analysis of zircon from rhyolite with little or no greisenisation gave a 207pb 206pb age of 290 5 ma whereas zircon from little altered granite gave a 207pb 206pb age of 320 8 ma the single evaporation age of zircon from rhyolite is confirmed by 238u 206pb shrimp ages of 297 8 ma the significant time gap of at least 20 ma between granite intrusion and rhyolite formation suggests that the late magmatic evolution of the eibenstock granite cannot be regarded as a source for tin ore forming fluids as previously assumed

in situ hydroclastic fragmentation of subaqueous ponded lavas new senator caldera abitibi greenstone belt quebec canada abstract subaqueous ponded lavas have been recognized within the structure of the new senator caldera in the blake river group of the abitibi greenstone belt quebec canada sub vertical to near vertical dips of extrusive volcanic facies within the southern sector of the caldera permit the identification of the internal architecture of an archean subaqueous volcanic complex detailed facies analyses of the dominantly mafic sequences have shown two localities with atypical primary volcanic structures and facies inconsistent with those observed at modern subaqueous mafic seamounts combined with the use of facies specific geochemical analyses a coherent model for the two localities accounting for all observed volcanic structures and features has been developed mapping completed at a scale of 1 100 at localities 1 and 2 reveals a series of sub parallel to parallel hyaloclastite rich horizons separating aphanitic to medium grained ponded units several primary volcanic structures are observed within the hyaloclastite horizons and are identified as pillowed forms cigar forms and v shaped structures ponded units range in thickness from 2 5 to 5 m with the exception of one unit which is approximately 30 m thick in central portions this thick unit is medium grained and has a sub ophitic texture units at locality 1 strike approximately nw while those at locality 2 strike ene several families of mafic dykes cross cut locality 1 and trend e w and nne both volcaniclastic and effusive facies have similar compositions affinities and trace and rare earth element signatures all facies have a tholeiitic affinity range in composition from basalt to andesite and have morb type trace and rare earth element signatures locality 1 appears slightly more evolved than locality 2 with later dykes and a late sill being the most evolved facies volcanic facies at localities 1 and 2 are consistent with subaqueous ponded lavas bounding synvolcanic structures and volcanic facies validate this interpretation and indicate these ponded lavas are hosted within a large synvolcanic depression at the summit of a mafic shield complex primary volcanic structures within hyaloclastite horizons formed via in situ fragmentation when water entered the system as ponded lavas began to cool water entered the system through synvolcanic fractures and encountered semi molten pockets of lava leading to hydroclastic fragmentation of the lavas v shaped and cigar structures formed during more energetic events and changed laterally into pillowed forms as energy dissipated from the system

new multibeam mapping and geochemistry of the 30 35 s sector and overview of southern kermadec arc volcanism new multibeam mapping and whole rock geochemistry establish the first order definition of the modem submarine kermadec arc between 30 degrees and 35 degrees s twenty two volcanoes with basal diameters 5 km are newly discovered or fully mapped for the first time giggenbacb macauley havre haungaroa kuiwai ngatoroirangi sonne kibblewhite and yokosuka for each large volcano edifice morphology and structure surficial deposits lava fields distribution of sector collapses and lava compositions are determined macauley and havre are large silicic intra oceanic caldera complexes for both concentric ridges on the outer flanks are interpreted as recording mega bedforms associated with pyroclastic density flows and edifice foundering other stratovolcanoes reveal complex histories with repeated cycles of tectonically controlled construction and sector collapse extensive basaltic flow fields and the development of summit craters and or small nested calderas combined with existing data for the southernmost arc segment we provide an overview of the spatial distribution and magmatic heterogeneity along similar to 780 km of the kermadec arc at 30 degrees 36 degrees 30 s coincident changes in arc elevation and lava composition define three volcano tectonic segments a central deeper segment at 32 degrees 20 34 degrees 10 s has basement elevations of 3200 m water depth and relatively simple stratovolcanoes dominated by low k series basalt basaltic andesite in contrast the adjoining arc segments have higher basement elevations typically 3 km coincide with the shallower arc segments the dominant mode of large caldera formation is interpreted as mass discharge pyroclastic eruption with syn eruptive collapse hence the shallower arc segments are characterized by both the generation of volatile enriched magmas from crustal melting and a reduced hydrostatic load allowing magma vesiculation and fragmentation to initiate and sustain pyroclastic eruptions proposed initiation parameters for submarine pyroclastic eruptions are water depths 70 wt sio2 and a high discharge rate c 2005 elsevier b v all rights reserved

three hawaiian calderas an origin through loading by shallow intrusions the calderas of kilauea and mauna loa are highly dynamic structures and in the 200 year historic period have varied in volume by a factor of 2 and gained or lost 1 km3 per century the deeply eroded caldera of the extinct koolau volcano in oahu is wider than active hawaiian calderas and its lavas have a strong centripetal dip and funnel structure not evident at kilauea or mauna loa the differences can be attributed to the different erosion depths and the time integrated subsidence profile of kilauea is also a stepped funnel having its apex at halemaumau koolau caldera is the focus of an extraordinarily intense dike complex and an intriguing feature is the great diminution in dike concentration into the caldera it is thought that dike injection in any part of the complex generally continued until it reached 50 to 65 in outer parts of the caldera the complex was maintained at or rebuilt to this value despite subsidence in the center of the caldera where the positive bouguer anomaly is centered subsidence evidently greatly outpaced the capacity of dike injections to rebuild the complex assuming the same dike injection rate as kilauea and mauna loa yields a volumetric subsidence rate in koolau caldera exceeding 1 km3 per century hawaiian calderas are much more dynamic than calderas of silicic volcanoes shaped by frequent small events instead of a few great ones the temporal and volumetric correspondence of historical subsidence events with eruptions is poor and this and the high subsidence rates argue for a caldera forming mechanism that consumes the subsided rocks it is suggested that subsidence is caused by the great localized excess load of intrusive rocks carrying the center of the volcano into the thermally weakened lithosphere above the hawaiian hot spot it is envisaged that under steady state conditions the magma chamber rises as the injection of intrusions causes the level of neutral buoyancy at which the chamber is located to ascend and keeps pace with subsidence

reconstruction and evolution of archean intracaldera facies the rouyn pelletier caldera complex of the blake river group abitibi greenstone belt canada subvertically to vertically dipping archean strata provide an excellent opportunity to study synvolcanic structures and internal organization of subaqueous volcanic complexes the abitibi greenstone belt in quebec canada hosts a number of these volcanic complexes and specifically the unique blake river megacaldera complex the blake river megacaldera complex is composed of i an initial shield phase known as the misema caldera and ii two graben type calderas known as the new senator and noranda calderas the southern portion of the new senator caldera the focus of this study is of particular interest as the structure hosts the 54 mt horne au rich volcanogenic massive sulfide deposit detailed facies mapping coupled with geochemical and geochronological analysis at multiple outcrop localities throughout the city of rouyn noranda quebec show that the stratigraphy of the new senator caldera is dominated by subaqueous effusive mafic volcanic facies with local felsic effusive and intrusive deposits with the incorporation of structural data we have used synvolcanic faults and dyke complexes to divide these facies into specific blocks within this region here renamed the rouyn pelletier sector this study focuses on the facies and characteristics of the pelletier senator and glenwood blocks and identifies the evain stadacona and chadbourne blocks geochronological analysis reveals that facies of the glenwood block are approximately the same age as those of the felsic flows of the previously identified horne block together all extrusive and intrusive volcanic facies from the stadacona unit north to the horne creek fault compose a large caldera complex known as the rouyn pelletier caldera complex

using submarine lava pillars to record mid ocean ridge eruption dynamics abstract submarine lava pillars are hollow glass lined basaltic cylinders that occur at the axis of the mid ocean ridge and within the summit calderas of some seamounts typically pillars are 1 20 m tall and 0 25 2 0 m in diameter with subhorizontal to horizontal glassy selvages on their exterior walls lava pillars form gradually during a single eruption and are composed of lava emplaced at the eruption onset as well as the last lava remaining after the lava pond has drained on the deep sea floor the surface of a basaltic lava flow quenches to glass within 1 s thereby preserving information about eruption dynamics as well as chemical and physical properties of lava within a single eruption investigation of different lava pillars collected from a single eruption allows us to distinguish surficial lava pond or lava lake geochemical processes from those operating in the magma chamber morphologic major element petrographic and helium analyses were performed on portions of three lava pillars formed during the april 1991 eruption near 9 50 n at the axis of the east pacific rise modeling results indicate that the collected portions of pillars formed in 2 5 h suggesting a total eruption duration of 8 20 h these values are consistent with observed homogeneity in the glass helium concentrations and helium diffusion rates major element compositions of most pillar glasses are homogeneous and identical to the 1991 flow but slight chemical variations measured in the outermost portions of some pillars may reflect post eruptive processes rather than those occurring in subaxial magma bodies because lava pillars are common at mid ocean ridges mors the concepts and techniques we present here may have important application to the study of mor eruptions thereby providing a basis for quantitative comparisons of volcanic eruptions in geographically and tectonically diverse settings more research is needed to thoroughly test the hypotheses presented here

accessory minerals and the geochemical evolution of crustal magmatic systems a summary and prospectus of experimental approaches it has recently been recognized that accessory minerals in continental rocks may contain a substantial fraction of the whole rock complement of trace elements and geochemically important isotopes this observation is in itself interesting but its importance is magnified many fold when one considers the possible consequences to the geochemical rearrangement that accompanies crustal melting if for example an accessory phase is insoluble in early formed partial melts it may effectively retain in the residue many incompatible elements e g ree and daughter isotopes e g radiogenic pb that would otherwise be partitioned into the melt in addition should an unmelted accessory mineral e g zircon be entrained in a departing melt fraction it may carry with it radiogenic components pb nd143 hf176 characteristic of the source region the effectiveness of phases such as zircon apatite sphene allanite epidote and monazite in the roles noted above can only be gauged from knowledge of the three fundamental accessory phase parameters i e 1 the solubilities of the accessory minerals in crustal melts 2 the equilibrium mineral liquid partition coefficients for the trace element and isotopes of interest and 3 the diffusivities that govern the rates at which equilibrium will be approached in general these parameters can only be measured in laboratory experiments and relatively few data have been accumulated to date it does seem clear however that apatite generally is residual insoluble during crustal melting and zircon can be depending on melting conditions and abundance of zircon in the source petrographic and geochemical studies indicate that monazite allanite and epidote may also be highly insoluble in some cases but the specific circumstances that promote insolubility remain unknown the few experimentally determined partition coefficients now available for apatite and zircon agree qualitatively with natural rock data on mineral separates however preliminary diffusion measurements and inferences from natural systems suggest that trace element and isotopic equilibrium between undissolved accessories and crustal melts is in fact rarely approached ironically it is this very disequilibrium in the form of preserved diffusion profiles that may afford the most information on time temperature histories of melting events continued experimental interest in the fundamental accessory phase parameters will eventually provide the tools that will enable rigorous assessment not only of the timing and duration of crustal melting episodes but also of the extent of melting and re distribution of trace elements and radiogenic isotopes

sumisu volcano izu bonin arc japan site of a silicic caldera forming eruption from a small open ocean island sumisu volcano was the site of an eruption during 30 60 ka that introduced 48 50 km3 of rhyolite tephra into the open ocean environment at the front of the izu bonin arc the resulting caldera is 8 10 km in diameter has steep inner walls 550 780 m high and a floor averaging 900 m below sea level in the course of five research cruises to the sumisu area a manned submersible two rovs a deep tow camera sled and dredge samples were used to study the caldera and surrounding areas these studies were augmented by newly acquired single channel seismic profiles and multi beam seafloor swath mapping caldera wall traverses show that pre caldera eruptions built a complex of overlapping dacitic and basaltic edifices that eventually grew above sea level to form an island about 200 m high the caldera forming eruption began on the island and probably produced a large eruption column we interpret that prodigious rates of tephra fallback overwhelmed the sumisu area forming huge rafts of floating pumice choking the nearby water column with hyperconcentrations of slowly settling tephra and generating pyroclastic gravity currents of water saturated pumice that traveled downslope along the sea floor thick compositionally similar pumice deposits encountered in odp leg 126 cores 70 km to the south could have been deposited by these gravity currents the caldera rim presently at ocean depths of 100 400 m is mantled by an extensive layer of coarse dense lithic clasts but syn caldera pumice deposits are only thin and locally preserved the paucity of syn caldera pumice could be due to the combined effects of proximal non deposition and later erosion by strong ocean currents post caldera edifice instability resulted in the collapse of a 15 sector of the eastern caldera rim and the formation of bathymetrically conspicuous wavy slump structures that disturb much of the volcano s surface

tectonic evolution of the northern volcanic zone abitibi belt quebec the archean abitibi subprovince has been divided formally into a northern volcanic zone nvz including the entire northern part of the subprovince and a southern volcanic zone svz on the basis of distinct volcano sedimentary successions related plutonic suites and precise u pb age determinations the nvz has been further formally subdivided into i a monocyclic volcanic segment mvs composed of an extensive subaqueous basalt plain with scattered felsic volcanic complexes 2730 2725 ma interstratified with or overlain by linear volcaniclastic sedimentary basins and ii a polycyclic volcanic segment pvs comprising a second mafic felsic volcanic cycle 2722 2711 ma and a sedimentary assemblage with local shoshonitic volcanic rocks a sequence of deformational events d1 d6 over a period of 25 ma in the nvz is consistent with a major compressional event north south shortening was first accommodated by near vertical east trending folds and with continued deformation was concentrated along ma

theoretical analysis of snow dam decay snow dams have been observed in many stream channels in the arctic and the sub arctic but there is no theoretical analysis of their decay processes these processes include snowmelt seepage erosion down cutting snow slope failure and flotation of the snow dam snowmelt can be determined by the energy balance seepage of water will cause serious erosion at the point where the stream or pond level in front of the dam intersects the dam face erosion by water overflowing the dam is due to down cutting under cutting and thermal erosion down cutting increases with discharge but decreases with the shear strength of the snow where a hydraulic jump occurs downstream of the dam under cutting can accelerate dam decay thermal erosion depending mainly on water temperature may be less significant than the previous two processes slope failure occurs when the driving forces exceed the resisting forces and these are affected by snow property and snow load when the stream or pond level downstream of the dam rises rapidly the dam is prone to float as peak flow often occurs during the break up period results of the theoretical study of the mechanisms of snow dam decay will improve flood forecasting for arctic streams

volcanic evolution of the south sandwich volcanic arc south atlantic from multibeam bathymetry abstract new multibeam bathymetry data are presented for the south sandwich intra oceanic arc which occupies the small sandwich plate in the south atlantic and is widely considered to be a simple end member in the range of intra oceanic arc types the images show for the first time the distribution of submarine volcanic tectonic and erosional depositional features along the whole length of the 540 km long volcanic arc allowing systematic investigation of along arc variations the data confirm that the volcanic arc has a simple structure composed of large volcanoes which form a well defined volcanic front but with three parallel cross cutting seamount chains extending 38 60 km from near the volcanic front into the rear arc there is no evidence for intra arc rifting or extinct volcanic lines topographic evidence for faulting is generally absent except near the northern and southern plate boundaries most of the volcanic arc appears to be built on ocean crust formed at the associated back arc spreading centre as previously proposed from magnetic data but the southern part of the arc appears to be underlain by older arc or continental crust whose west facing rifted margin facing the back arc basin is defined by the new bathymetry the new survey shows nine main volcanic edifices along the volcanic front and ca 20 main seamounts the main volcanoes form largely glaciated islands with summits 3 0 3 5 km above base levels which are 2500 3000 m deep in the north and shallower at 2000 2500 m deep in the south some of the component seamounts are interpreted to have been active since the last glacial maximum and so are approximately contemporaneous with the volcanic front volcanism seven calderas all either submarine or ice filled have been identified adventure volcano a newly discovered submarine volcanic front caldera volcano is described for the first time all but one of the calderas are situated on summits of large volcanoes in the southern part of the arc and most are associated with current or historic volcanic or hydrothermal activity shallow shelves around the islands are generally 1 10 km wide submerged banks up to 1100 m deep are interpreted as subsided erosional surfaces seamounts and emergent volcanoes experienced a range of mass wasting processes including by landsliding and smaller mass flows

the complex facies architecture and emplacement sequence of a miocene submarine mega pillow lava flow system muriwai north island new zealand the early miocene waiatarua formation at maori bay muriwai north island new zealand consists of a complex association of basaltic andesite volcanic facies including entablature jointed thick massive facies colonnade jointed thin massive sheet facies mega pillow facies normal pillow facies and minor associated fragmental facies including vitric sandstone and breccia interpreted as hyaloclastite and peperite field observations suggest that the facies which form the waiatarua formation lava were emplaced as multiple flow lobes in a single lava flow from one sustained eruption magma discharge rate at the vent was high however magma supply rate to the more distal and lateral portions of the flow as its surface area increased varied considerably higher magma supply rates produced thick massive and thin sheet facies whereas mega pillow facies and normal pillow facies were produced contemporaneously with the thick and thin massive sheet facies but were restricted to portions of the distal flow subjected to lower magma supply rates the evolution of the waiatarua formation lava flow at maori bay has been reconstructed from the complex facies architecture this suggests that the propagation of the lava involved 7 discrete lobes that were emplaced successively

wolf volcano galapagos archipelago melting and magmatic evolution at the margins of a mantle plume wolf volcano an active shield volcano on northern isabela island in the galapagos archipelago has undergone two major stages of caldera collapse with a phase of partial caldera refilling between wolf is a typical galapagos shield volcano with circumferential vents on the steep upper carapace and radial vents distributed in diffuse rift zones on the shallower sloping lower flanks the radial fissures continue into the submarine environment where they form more tightly focused rift zones wolf s magmas are strikingly monotonous estimated eruptive temperatures of the majority of lavas span a total of only 22 c this homogeneity is attributed to buffering of magmas as they ascend through a thick column of olivine gabbroic mush that has been deposited from a thin shallow 2 km deep subcaldera sill that is in a thermochemical steady state wolf s lavas have the most depleted isotopic compositions of any historically active intraplate ocean island volcano on the planet and have isotopic compositions except for he he indistinguishable from midocean ridge basalt erupted from the galapagos spreading center gsc 250 410 km away from the peak of influence of the galapagos plume wolf s lavas are enriched in incompatible trace elements and have systematic major element differences relative to gsc lavas however wolf s magmas result from lower extents of melting deeper melt extraction and a greater influence of garnet compared with gsc magmas but wolf and the gsc share the same sources these melt generation conditions are attributed to melting in a thermal and mechanical boundary layer of depleted asthenosphere at the margins of the galapagos plume the lower degrees of melting and extraction from deeper levels result from a thicker lithospheric cap at wolf than exists at the gsc

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

synthesis of the structural evolution and associated gold mineralization of the cadillac fault abitibi canada abstract the cadillac fault zone cfz is a 250 km long east trending crustal discontinuity associated with numerous world class gold deposits in the archean abitibi subprovince in quebec the fault follows the boundary between the abitibi and the pontiac subprovinces and can be divided into four segments 1 rouyn 2 joannes 3 malartic and 4 val d or each displays different attitudes and structural styles the malartic segment strikes se whereas the other segments are e trending abrupt changes in strike or smaller scale faults represent limits between the segments structural styles include four generations of foliation with numerous shear sense indicators and folding specifically related to each foliation structural features can be correlated from one segment to another but display clear variations in attitude or expression a stretching lineation signature is definitely associated with each segment in the malartic segment the predominant e trending schistosity displays an angular relationship with the contact between groups and bedding suggesting that the se trending architecture of this segment was already emplaced prior to ductile deformation the evidence suggests a model where the cfz originates from several independent structures inherited from volcanism prior to timiskaming deformation and abitibi pontiac accretion the proto cfz was then linked up during the timiskaming deposition forming high strain zones inherited structures form the frame on which each deformation event was overprinted with different responses depending on segmentation the structural styles of each segment can be integrated in a structural evolution model for the cfz the n s shortening led to the development of a dominantly e striking penetrative foliation and folding during the subsequent extensional stage the cfz was reactivated with normal kinematics during the pontiac uplift a third deformation event is represented by the local development of nw trending cleavage axial planar to the s shaped minor folds finally a late nw se shortening event produced a dextral strike slip movement with ne striking vertical foliation and development of z shaped folds on bedding surfaces and anterior fabrics linkage of the cfz had a major impact on hydrothermal activity in the southern abitibi as it channeled fluids along large scale pathways distribution of gold bearing intrusions can also be related to segmentation and inherited structures of the cfz this association is expressed by gold mineralization styles present in several longitudinal fields reflecting fault segmentation

towed camera investigations of shallow intermediate water depth submarine stratovolcanoes of the southern kermadec arc new zealand southern kermadec arc stratovolcanoes of predominantly basaltic and andesitic composition provide a depth transect of the transition between effusive and explosive submarine volcanism observations along 4 6 km of towed camera track from the crests and upper flanks of the clark and rumble iii volcanoes reveal a consistent pattern of substrate types that are interpreted to record effusive and explosive eruption processes below 700 m water depth substrates are dominated by massive blocky flows pillow lavas pillow and angular block talus localized sheet flows and minor granule sand volcaniclastic detritus the latter typically forms a substrate mode of 5 20 above 450 m sand lapilli that is in part winnowed and rippled have a substrate mode of 50 100 with minor components of massive flows pillow lavas and coarse talus this difference in abundance of sand lapilli between 450 and 700 m is interpreted to record the transition between effusive and explosive both phreatic and phreatomagmatic eruptions between 600 and 700 m a varied distribution of sand lapilli abundance is interpreted as recording a mix of effusive and explosive eruptions consistent with possible differences in the physical interaction of magma water and known compositional magma heterogeneity

depositional gaps in abitibi greenstone belt stratigraphy a key to exploration for syngenetic mineralization models of greenstone belt development are crucial for exploration allochthonous models predict belts to be a collage of unrelated fragments whereas autochthonous models allow for prediction of syngenetic mineral deposits within specific stratigraphic intervals superior province greenstone belts consist of mainly volcanic units unconformably overlain by largely sedimentary timiskaming style assemblages and field and geochronological data indicate that the abitibi greenstone belt developed autochthonously we describe major revisions to stratigraphy of the abitibi greenstone belt and the implications of an autochthonous development of the volcanic stratigraphy for exploration for syngenetic mineralization the abitibi greenstone belt is subdivided into seven discrete volcanic stratigraphic episodes on the basis of groupings of numerous u pb zircon ages of pre 2750 2750 to 2735 2734 to 2724 2723 to 2720 2719 to 2711 2710 to 2704 and 2704 to 2695 ma we present revised lithotectonic and or stratigraphic nomenclature using these time intervals including 1 isotopic inheritance in younger episodes which indicates that the older episodes 2750 2735 and 2734 2724 ma had greater extent than is presently seen 2 dikes feeding younger volcanic episodes 2706 ma cutting older volcanic units 2734 2724 ma and 3 2710 to 2704 ma mafic to ultramafic sills intruding the 2719 to 2711 ma episode changes to the nomenclature include the identification of pre 2750 ma volcanic episode supracrustal fragments in the northern and southern abitibi greenstone belt and subdivision of the 2719 to 2711 ma 2710 to 2704 ma and 2704 to 2695 ma episodes into lower and upper parts we present the results of this lithostratigraphic subdivision as the first geochronologically constrained stratigraphic and or lithotectonic map of the abitibi greenstone belt many of the volcanic episodes are intercalated with and capped by a relatively thin sedimentary interface zone dominated by chemical sedimentary rocks stratigraphic and geochronological analysis of these zones indicates discontinuous deposition with localized gaps of 2 to 27 m y between volcanic episodes the zones consist of up to 200 m of iron formation chert breccia heterolithic debris flows of volcanic provenance sandstone and or argillite and conglomerate modeling of the time required for deposition of the volcanic units based on rates of magma production in modern arc and plume environments is on the order of 103 to 104 years whereas the time interval between basalt rhyolite cycles is 106 years the sedimentary interface zones are therefore interpreted as condensed sections zones with very low rates of sedimentation in a basinal setting or zones with negligible rates of sedimentation marked by silicification of existing rock types the sedimentary interface zones are therefore considered submarine correlative conformities disconformities or unconformities separating the equivalent of group level stratigraphic and lithotectonic units the unconformity bounded stratigraphic model provides a new regional to deposit scale interpretive model for use in exploration for syngenetic mineralization

recognizing distinct portions of seamounts using volcanic facies analysis examples from the archean slave province nwt canada abstract archean volcanic rocks in the mafic dominated ca 2 66 2 69 ga point lake and beaulieu river belts slave province northwest territories are significant in demonstrating the facies that characterize specific portions of pillow volcanoes or seamounts irrespective of tectonic setting three distinct localities mapped in detail display facies consistent with 1 proximal deep water 2 medial to distal deep water and 3 medial shallow water seamount settings the proximal facies in the point lake belt include a 55 m thick non vesicular pillowed sequence cut by numerous mafic dykes and sills dykes contain multiple chilled margins indicating successive magma pulses which contributed to edifice construction abundant feeder conduits in addition to the absence of fragmental facies and vesicles are typical of the central deep water portion of seamounts where growth is initiated the medial to distal deep water facies in the point lake belt are represented by a 30 80 m thick assemblage of disorganized pillow breccia and pillowed and massive flows with 5 27 vesicularity massive non vesicular hyaloclastite intermingled with sedimentary material fluidal peperite in addition to thin shale units interstratified with pillow breccia and hyaloclastite indicate that sedimentation and volcanism were contemporaneous an increase in fragmental units and vesicularity relative to the proximal deep water facies is suggestive of the medial to distal part of a seamount in shallower water bedded tuffs laterally along strike with massive flows are the results of turbidity current deposition immediately following localized subaqueous eruptions a medial shallow water seamount setting is represented in the beaulieu river belt by a 5 85 m thick sequence of vesicular lobate pillowed and massive flows stratified pillow breccia and hyaloclastite and mafic dykes vesicularity ranges from 21 49 in pillowed flows 5 40 in massive flows and 20 35 in pillow breccia and hyaloclastite stratified pillow breccia developed along steep flow fronts in shallow water whereas bedded hyaloclastite formed during reworking and redeposition of autoclastic hyaloclastite on seamount flanks in shallow water the volcanic facies associations in the study areas are analogous to those of modern seamounts associated with the mid atlantic ridge and east pacific rise as well as mesozoic cenozoic seamounts in the canary islands fiji southwest japan the sea of japan and cyprus volcanological studies in the point lake and beaulieu river volcanic belts and subsequent comparisons with phanerozoic analogues demonstrate the manner in which distinct portions of ancient seamounts can be recognized in similar archean terranes

evolution of the south central part of the archean abitibi belt quebec part i stratigraphy and paleogeographic model the region comprises parts of the southern zone of the abitibi greenstone belt and of the bellecombe gneiss belt and the belleterre angliers greenstone belt the stratigraphic sequence of the abitibi belt consists of 1 a lower ultramafic division 2 a middle tholeiitic division and 3 an upper diverse division these divisions lap onto an older volcanic sequence to the north and west in the bellecombe belt sediments overlie the lower ultramafic division these sediments coarsen toward the abitibi belt and toward the stratigraphic top the volcanic rocks of the belleterre angliers belt overlie the sediments of the bellecombe belt five paleogeographic phases have been recognized during phase i a deep marine ultramafic lava plain covered the south part of the abitibi belt and the bellecombe belt faulting took place at the duparquet destor break during phase ii tholeiitic basalts formed a deep marine lava plain in the west of the abitibi belt whereas flows erupted mainly from central volcanic c

evolution of the southern abitibi greenstone belt based on u pb geochronology autochthonous volcanic construction followed by plutonism regional deformation and sedimentation abstract new mapping and u pb zircon geochronological results in the southern abitibi greenstone belt sagb support an autochthonous regional stratigraphy comprised of nine supracrustal assemblages rather than the collage of allochthonous terranes proposed in recent publications based on lithological and geochronological criteria these supracrustal assemblages also encompass the swayze shining tree and montcalm greenstone belts indicating that despite separation by large granitic intrusions they are in fact part of one extensive greenstone belt in conjunction with the mapping the geochronological results support coherent upward facing stratigraphic sections with about 20 of the samples containing inherited zircons with ages similar to those found in underlying assemblages the seven oldest assemblages represent semi continuous volcanism from 2750 to 2697 ma the volcanism is compositionally diverse ranging from komatiite and tholeiitic basalt to calc alkaline mafic to felsic lavas intimate intermingling of the different magma clans occurs throughout much of the stratigraphic section regional fault control on the distribution of the volcanic assemblages provides evidence for early dip slip movement associated with volcanic extension dating back to at least 2725 ma the two youngest assemblages are dominantly sedimentary and were unconformably deposited on the volcanic assemblages in close proximity to regional faults the earlier sedimentary assemblage consists of turbidites and minor iron formation deposited from 2696 to 2692 ma and the youngest one of subaerial conglomerates fluvial sandstones and alkalic to shoshonitic volcanic rocks ranging in age from 2687 to 2675 ma the sedimentary assemblages are broadly contemporaneous with the emplacement of syntectonic granitic plutons regional folding and reactivated movement on the regional faults probably related to accretion of the abitibi to the superior province craton autochthonous repetition of different geodynamic environments over the 50 million years of volcanic activity with five dominantly tholeiitic komatiitic assemblages and two dominantly calc alkaline assemblages as well as the extensive intermingling of these different magma clans in a number of the assemblages appears to be unique to the archean it suggests complex large scale and long lived interaction between mantle plumes and subduction zone magmas the nd isotope data indicate a derivation of the diverse volcanic magmas from homogeneously depleted sources with end 2 5 0 5 demonstrating a lack of contamination by ancient enriched components

timing and characteristics of the archean subaqueous blake river megacaldera complex abitibi greenstone belt canada abstract the archean blake river group of the abitibi greenstone belt represents a megacaldera complex which evolved over 8 11 m y from approximately 2704 to 2696 ma the early misema caldera developed from a series of amalgamated shield volcano complexes identified by the remnants of mafic dyke and sill systems these remnants are found in the jevis clericy montsabrais renault clifford tannahill and colnet regions where summit calderas are delineated by circular ring dyke structures the secondary 330 trending new senator caldera formed within the envelope of the misema caldera and exhibits a box work graben type structure finally the felsic dominated 070 striking noranda caldera well known for its vms endowment represents the final collapse of the megacaldera complex deformation has overprinted the calderas but structural patterns can be used in order to reconstruct the pre existing volcanic architecture structures such as mafic ring dyke complexes and rhyolitic dome flow complexes have nucleated fold geometries and synvolcanic fractures were reactivated within zones of ductile deformation precise u pb geochronological analyses were conducted in selected areas with specific emphasis on the misema and new senator calderas felsic volcanism progressed throughout the evolution of the megacaldera complex and intermediate to felsic units have been dated to limit this evolution the misema caldera formed between 2704 and 2702 ma via the amalgamation of shield volcanoes that were probably active prior to 2704 ma the new senator caldera was generated between 2702 and 2700 ma during paroxysmal felsic volcanism followed by the collapse of the noranda caldera culminating between 2700 2696 ma the misema caldera was generated by a gravitational stress field consistent with the formation of ring and radial dyke architecture whereas the se trending new senator caldera is more compatible with a sw trending principal compression direction related to oblique convergence in the abitibi belt the noranda caldera is interpreted as a ne rift structure that formed in the final stages of megacaldera evolution