the petrology of a large intra oceanic silicic eruption the sandy bay tephra kermadec arc southwest pacific abstract the pumiceous pyroclastic deposits known as the sandy bay tuff on macauley island in the kermadec group represent a medium scale silicic eruption in an oceanic subduction setting the sandy bay eruption occurred about 6310 yr bp forming a large submarine caldera in the summit of the mainly submarine and essentially basaltic macauley volcano the sandy bay magma contained

syenite associated disseminated gold deposits in the abitibi greenstone belt canada a distinct group of gold deposits in the abitibi greenstone belt is spatially associated with quartz monzonite to syenite stocks and dikes the deposits occur mainly along major fault zones in association with preserved slivers of alluvial fluvial timiskaming type sedimentary rocks the deposits consist of disseminated sulfide replacement zones with variably developed stockworks of quartz carbonate k feldspar veinlets within zones of carbonate albite k feldspar and sericite alteration the syenitic intrusions are broadly contemporaneous with deposition of timiskaming sedimentary rocks and together with disseminated gold mineralization they have been overprinted by subsequent regional folding and related penetrative cleavage disseminated gold orebodies occur within composite syenitic stocks or along their margins along satellite dikes and sills and along faults and lithologic contacts away from intrusions orebodies in these different positions are interpreted to represent proximal to distal components of large magmatic hydrothermal systems centered on and possibly genetically related to composite syenitic stocks

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

controls on caldera structure results from analogue sandbox modeling we conducted scaled analogue sandbox models of caldera formation in order to understand the effects of chamber depth and orientation on the spatial and temporal development of calderas dry sand contained in a 1 m diameter cylinder served as a crustal rock analogue and a water filled 0 6 m diameter rubber bladder served as an analogue magma chamber scaling parameters included a length ratio l of 2 5 10 5 and a stress ratio of 1 8 2 4 10 5 in contrast to some previous analogue models the viscosity of the fluid in the chamber and its withdrawal rate were properly scaled generally deformation began with broad sagging followed by an arcuate or linear outward dipping fault that formed on one side of the caldera this fault propagated laterally around the caldera in both directions sometimes joining other faults and typically forming an overall polygonal structure as subsidence continued the caldera grew incrementally outward and progressively formed a series of concentric outward dipping faults lastly a peripheral zone of extension and pronounced sagging and commonly an inward dipping outer fault related to extension developed at the surface as the depth of the chamber increased 1 the area of faulting decreased 2 the symmetry of the caldera was affected and 3 the coherence of the subsiding block decreased tilting the chamber caused highly asymmetric subsidence to occur in this case faults formed first where the bladder was shallowest subsidence then shifted rapidly to where the bladder was deepest producing an elongate trapdoor caldera that was deepest where the bladder was deepest our experiments highlight the roles of sagging and faulting during caldera subsidence surface fault patterns both in our experiments and at natural calderas are frequently not circular the aspect ratio of the block above the magma chamber controls the shape of the caldera which is frequently polygonal the faults at natural calderas determine locations and migration of eruptive vents the degree of subsidence the style of postcal dera resurgent magmatism and the extent of hydrothermal circulation our experiments reveal details of how calderas grow outward incrementally and demonstrate that asymmetric subsidence along linear and arcuate faults is common to many calderas

upb zircon and rutile chronology of archean greenstone formation and gold mineralization in the val d or region quebec abstract u pb geochronology provides an absolute time framework for the evolution of the sigma gold deposit and surrounding rocks at val d or southern abitibi subprovince the bourlamaque batholith the largest pluton in the area gives a 2699 8 1 0 ma u pb zircon age this pluton cuts the val d or formation which hosts the mineralization a u pb zircon age of 2704 9 1 1 ma on a felsic volcanic rock the colombiere rhyolite 13 km east of the mine dates that formation the gold bearing quartz vein system at sigma is hosted by andesites and two generations of porphyry intrusions all metamorphosed to the greenschist facies the oldest porphyry porphyritic diorite shows the same deformation as the volcanic rocks and has a 2703 7 2 5 ma u pb zircon age the porphyritic diorite and volcanic rocks are cut by feldspar porphyry dykes which post date regional folding and have a 2694 0 2 2 ma u pb zircon age regional greenschist metamorphism has been dated directly with a u pb date of 2684 7 ma on rutile in the colombiere rhyolite the mineralization and hydrothermal alteration in the mine are superimposed on the metamorphic minerals hydrothermal rutile from an alteration halo around the veins in andesite has a 2599 9 ma u pb age textural evidence clearly indicates that the wall rock alteration and vein filling are contemporaneous and hence the vein system and gold mineralization appear to have developed at least 80 m y after the formation and metamorphism of host greenstones

a kuroko type polymetallic sulfide deposit in a submarine silicic caldera manned submersible studies have delineated a large and actively growing kuroko type volcanogenic massive sulfide deposit 400 kilometers south of tokyo in myojin knoll submarine caldera the sulfide body is located on the caldera floor at a depth of 1210 to 1360 meters has an area of 400 by 400 by 30 meters and is notably rich in gold and silver the discovery of a large kuroko type polymetallic sulfide deposit in this arc front caldera raises the possibility that the numerous unexplored submarine silicic calderas elsewhere might have similar deposits

the pattern of circumferential and radial eruptive fissures on the volcanoes of fernandina and isabela islands galapagos maps of the eruptive vents on the active shield volcanoes of fernandina and isabela islands galapagos made from aerial photographs display a distinctive pattern that consists of circumferential eruptive fissures around the summit calderas and radial fissures lower on the flanks on some volcano flanks either circumferential or radial eruptions have been dominant in recent time the location of circumferential vents outside the calderas is independent of caldera related normal faults the eruptive fissures are the surface expression of dike emplacement and the dike orientations are interpreted to be controlled by the state of stress in the volcano very few subaerial volcanoes display a pattern of fissures similar to that of the galapagos volcanoes some seamounts and shield volcanoes on mars morphologically resemble the galapagos volcanoes but more specific evidence is needed to determine if they also share common structure and eruptive style

understanding caldera structure and development an overview of analogue models compared to natural calderas abstract understanding the structure and development of calderas is crucial for predicting their behaviour during periods of unrest and to plan geothermal and ore exploitation geological data including that from analysis of deeply eroded examples allow the overall surface setting of calderas to be defined whereas deep drillings and geophysical investigations provide insights on their subsurface structure collation of this information from calderas worldwide has resulted in the recent literature in five main caldera types downsag piston funnel piecemeal trapdoor being viewed as end members despite its importance such a classification does not adequately examine a the structure of calderas particularly the nature of the caldera s bounding faults and b how this is achieved including the genetic relationships among the five caldera types various sets of analogue models specifically devoted to study caldera architecture and development have been recently performed under different conditions apparatus materials scaling parameters stress conditions the first part of this study reviews these experiments which induce collapse as a result of underpressure or overpressure within the chamber analogue the experiments simulating overpressure display consistent results but the experimental depressions require an exceptional amount of doming seldom observed in nature to form therefore these experiments are not appropriate to understand the structure and formation of most natural calderas the experiments simulating underpressure reveal a consistent scenario for caldera structure and development regardless of their different boundary conditions these show that complete collapse proceeds through four main stages proportional to the amount of subsidence progressively characterized by 1 downsag 2 reverse ring fault 3 peripheral downsag 4 peripheral normal ring fault the second part of this study verifies the possibility that these latter calderas constitute a suitable analogue to nature and consists of a comprehensive comparison of the underpressure experiments to natural calderas this shows that all the experimental structures as well as their progressive development are commonly observed at natural calderas highlighting a consistency between models and nature as the shallow structure of experimental calderas corresponds to a precise architecture at depth it provides a unique key to infer the deeper structure of natural calderas recognizing diagnostic surface features within a caldera will thus allow it to be categorized within a precise structural and evolutionary context the general relationship between the evolutionary stage of a caldera and its d s diameter subsidence ratio allows such a quantification with stage 1 calderas characterized by d s 40 stage 2 by 18 the four stages adequately explain the architecture and development of the established caldera end members along a continuum where one or more end members downsag piston funnel piecemeal trapdoor may correspond to a specific stage while such a continuum is controlled by progressive subsidence specific collapse geometries will result from secondary contributory factors roof aspect ratio collapse symmetry pre existing faults these considerations allow proposing an original classification of calderas incorporating their structural and genetic features

archean volcanoes in southwestern abitibi belt ontario and quebec form composition and development southwestern abitibi volcanic belt is composed of numerous deformed volcanoes that were originally circular to subcircular in outline 100 200 km in diameter and 10 16 km in stratigraphic thicknes

dynamics of the substorm expansive phase meridian scanning photometer msp and magnetometer data from the canadian auroral network for the open program unified study canopus ground based array have been used to study the dynamics of the substorm expansive phase the relative latitudinal motions of the msp 630 0 557 7 and 486 1 nm emissions for eight isolated events have been studied the data show the expansive phase to comprise three stages an explosive tens of seconds onset rapid poleward motion of 557 7 nm emissions of the order of a few minutes and a period of slower tens of minutes poleward moving 630 0 nm emissions we interpret the rapid poleward motion of the 557 7 nm data in terms of a region of instability expanding rapidly down the magnetotail possibly accelerating plasma sheet electrons as it proceeds all events show that lobe flux reconnection occurs after near earth onset with typical time delays between 1 and 5 min the extremely short time interval between near earth onset and the beginning of lobe flux reconnection presents a severe observational constraint on any substorm model which attempts to explain the substorm expansive phase magnetometer data from the canopus churchill line have been used to study the dynamics of the substorm electrojets during the growth and early expansive phases of the substorm the borders of these electrojets closely follow the motion of the msp data during the growth and expansive phases at the instant of expansive phase onset the growth phase electrojets as observed along the churchill meridian line disappear being replaced by the substorm westward electrojet associated with the current wedge the equatorward border of the substorm westward electrojet is embedded within the proton aurora emissions and appears at expansive phase onset 2 equatorward of the growth phase electrojets further showing the near earth proximity of expansive phase onset no indication of reconnection on closed field lines prior to expansive phase onset was observed in either the magnetometer or the photometer data

evaluation of low latitude pi2 pulsations as indicators of substorm onset using polar ultraviolet imagery impulsive pi2 pulsations have long been recognized as one of the key signatures of magnetic activity during substorm periods due to their wide observable range both in latitudes and longitudes it is well documented that there is usually more than one pi2 wave burst associated with a substorm and only one of them corresponds to the onset of the substorm this observational fact poses obstacles to determining substorm onsets with pi2 signals although the pi2 have become one of the most popular indicators for substorm onsets the reliability of using the pi2 in this fashion has not been seriously investigated in this paper we address this question with a statistical approach by using 650 auroral substorm onsets identified with polar ultraviolet images for a time interval from april 1996 to may 1997 a comparison of the low latitude pi2 pulsation onsets from kakioka l 1 07 with the auroral breakups indicates that identifying substorm onset with the pi2 alone is often ambiguous of a total of 119 isolated defined as 10 min of quiet time preceding the onset pi2 bursts seen within 10 min from a magnetic positive bay there were 65 events 55 taking place within 3 min from breakups and 34 events 29 indicating no sign of an auroral breakup within 10 min of the pi2 burst this result suggests that pi2 may not be as a good indicator of the substorm onset as it was thought to be interestingly it is always possible to associate pi2 pulsations with some forms of auroral intensification when compared to auroral breakups pi2 onsets are subject to a small delay of 1 3 min with a peak around l min delays of pi2 onsets are revealed to be a function of location relative to auroral breakup this dependence is found to be consistent with the time of flight for a fast mode wave in a plasmapause cavity mode model propagating in the magnetosphere

on relative timing in substorm onset signatures we investigate the timing of popular substorm onset signatures to understand their temporal relationship proxies for substorm onsets include auroral breakups high latitude magnetic bays low latitude pi2 bursts dispersionless injections at geostationary orbits and auroral kilometric radiation we use the auroral breakup identified with polar uvi images as a common reference time frame to calibrate the others results illustrated by two well defined auroral substorms unambiguously indicate that none of the four frequently used substorm onset proxies can provide a consistent timing of substorm onset this inconsistency in substorm onset timing is attributed as a consequence of temporal and spatial limitations on each observational technique a delay between the proxy identifiers and the auroral breakup is found to be typical it is therefore strongly suggested from this study result that a common reference time frame for substorm onset is necessary and we propose it should be auroral breakups we argue that there is a need for an intercalibration of magnetospheric substorm phenomenology by using a unified definition of the substorm onset

late cenozoic tectonism collapse caldera and plateau formation in the central andes abstract the evolution of andean volcanism including the formation of late miocene to recent collapse calderas on the puna plateau is generally interpreted in terms of the kinematic framework of the nazca and south american plates we present evidence that caldera dynamics and associated ignimbrite volcanism are genetically linked to the activity of first order nw se striking zones of left lateral transtension on the local and regional scales consequently ages of collapse calderas indicate activity of these fault zones which initiated at about 10 ma on the puna plateau the onset of such faulting points to a change in the deformation regime from dominantly vertical thickening to orogen parallel stretching upon reaching maximum crustal thickness and critical surface elevation horizontal magma sheets that formed at mid crustal level possibly due to heat advection by volume increase of asthenospheric mantle below thickened crust were tapped by sub vertical faults this accounts well for the observed tectono magmatic phenomena at surface it follows that formation of collapse calderas and eruption of voluminous ignimbrites appear to be related to the mechanical evolution of the andean plateau rather than to changes in the geometry of the wadati benioff zone or plate boundary kinematics

calderas and caldera structures a review abstract calderas are important features in all volcanic environments and are commonly the sites of geothermal activity and mineralisation yet it is only in the last 25 years that a thorough three dimensional study of calderas has been carried out utilising studies of eroded calderas geophysical analysis of their structures and analogue modelling of caldera formation as more data has become available on calderas their individuality has become apparent a distinction between caldera caldera complex cauldron and ring structure is necessary and new definitions are given in this paper descriptions of calderas based on dominant composition of eruptives basaltic peralkaline andesitic dacitic rhyolitic can be used and characteristics of each broad group are given styles of eruption may be effusive or explosive with the former dominant in basaltic calderas and the latter dominant in andesitic dacitic rhyolitic and peralkaline calderas four end member collapse styles occur plate or piston piecemeal trapdoor and downsag but many calderas have multiple styles features of so called funnel and chaotic calderas proposed in the literature can be explained by other collapse styles and the terms are considered unnecessary ground deformation comprises subsidence or collapse essential characteristics of a caldera and uplifting doming and fracturing due to tumescence and or resurgence frequent but not essential collapse may occur on pre existing structures such as regional faults or on faults created during the caldera formation and the shape of the collapse area will be influenced by depth size and shape of the magma chamber the final morphology of a caldera will depend on how the caldera floor breaks up whether collapse takes place in one event or multiple events whether vertical movement is spasmodic or continuous throughout the eruptive sequence and whether blocks subside uniformly or chaotically at one or more collapse centres a meaningful description of any caldera should therefore include number of collapse events presence or absence of resurgence caldera floor coherency caldera floor collapse geometry and dominant composition of eruptives

the development of the substorm expansive phase the eye of the substorm the explosive onset and development of a clearly defined auroral substorm has been analysed using data from the ultraviolet imager aboard the viking satellite these data permit the evolution of the substorm to be traced with a time resolution as high as 40 seconds the envelope of the disturbed region the eye expands both poleward and azimuthally to the east and west reaching an azimuthal extent of almost six hours of magnetic local time after about 15 minutes the poleward expansion involves irregular poleward motion accompanying intensifications within the eye with the maximum poleward displacement being at the meridian of the center of the eye within the eye discrete regions of enhanced luminosity develop with a tendency for each newly disturbed region to be to the west of the previous ones it is however observed that a newly excited region can be to the east of the pre existing activated region each new intensification is accompanied by a brief further poleward expansion of the high latitude edge of the eye each region of enhanced luminosity within the eye remains relatively stationary and fluctuates in intensity over its lifetime we identify these regions of enhanced luminosity with westward traveling surges as defined from ground observations in the earlier literature our uv imager data demonstrate that these surges do not necessarily propagate although the overall envelope of the substorm disturbed region expands both eastward and westward during the lifetime of the event

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

u pb geochronology of the eastern abitibi subprovince part 2 noranda kirkland lake area u pb zircon ages for 15 volcanic and plutonic units in the noranda and kirkland lake areas help constrain the history of volcanism plutonism sedimentation and deformation in the south central part of the abitibi belt volcanism occurred over an interval of at least 50 ma beginning with the deposition of the volcanic and volcaniclastic units within the pacaud structural complex at 2747 ma following a period of apparent quiescence magmatism resumed at 2730 2725 ma with the eruption of volcanic rocks in the normetal and lac abitibi area from 2715 until about 2698 ma volcanism occurred sporadically throughout much of the area culminating in the eruption of the blake river group from 2703 to 2698 ma several large intrusive bodies yield ages that indicate that they are plutonic equivalents of the blake river group plutons that are considered to have been emplaced during the kenoran orogeny give ages that are only slightly younger than the youngest volcanic units of the blake river group emphasizing

igneous evolution of a complex laccolith caldera the solitario trans pecos texas implications for calderas and subjacent plutons the solitario is a large combination laccolith and caldera herein termed laccocaldera with a 16 km diameter dome over which developed a 6 2 km caldera this laccocaldera underwent a complex sequence of predoming sill laccolith and dike intrusion and concurrent volcanism doming with emplacement of a main laccolith ash flow eruption and caldera collapse intracaldera sedimentation and volcanism and late intrusion detailed geologic mapping and 40ar 39ar dating reveal that the solitario evolved over an interval of approximately 1 m y in three distinct pulses at 36 0 35 4 and 35 0 ma the size duration and episodicity of solitario magmatism are more typical of large ash flow calderas than of most previously described laccoliths small volumes of magma intruded as abundant rhyolitic to trachytic sills and small laccoliths and extruded as lavas and tuffs during the first pulse at 36 0 ma emplacement of the main laccolith doming ash flow eruption and caldera collapse occurred at 35 4 ma during the most voluminous pulse a complex sequence of debris flow and debris avalanche deposits megabreccia trachyte lava and minor ash flow tuff subsequently filled the caldera the final magmatic pulse at 35 0 ma consisted of several small laccoliths or stocks and numerous dikes in caldera fill and along the ring fracture solitario rocks appear to be part of a broadly cogenetic metaluminous suite peralkaline rhyolite lava domes were emplaced north and west of the solitario at approximately 35 4 ma contemporaneous with laccolith emplacement and the main pulse in the solitario the spatial and temporal relation along with sparse geochemical data suggest that the peralkaline rhyolites are crustal melts related to the magmatic thermal flux represented by the main pulse of solitario magmatism current models of laccolith emplacement and evolution suggest a continuum from initial sill emplacement through growth of the main laccolith although the solitario laccocaldera followed this sequence of events our field and 40ar 39ar data demonstrate that it developed through repeated episodic magma injections separated by 0 4 to 0 6 m y intervals of little or no activity this evolution requires a deep long lived magma source well below the main laccolith laccoliths are commonly thought to be small shallow features that are not representative of major silicic magmatic systems such as calderas and batholiths in contrast we suggest that magma chambers beneath many ash flow calderas are tabular floored intrusions including laccoliths evidence for this conclusion includes the following 1 many large plutons are recognized to be laccoliths or at least tabular 2 the solitario and several larger calderas are known to have developed over laccoliths and 3 magma chambers beneath calderas which are as much as 80 km in diameter cannot be as deep as they are wide or some would extend into the upper mantle the solitario formed during a tectonically neutral period following laramide deformation and preceding basin and range extension therefore space for the main laccolith was made by uplift of its roof and possibly subsidence of the floor not by concurrent faulting laccolith type injection is probably a common way that space is made for magma bodies of appreciable areal extent in the upper crust

deep structure of an archaean greenstone terrane more than half of the continental crust was formed before about 2 500 million years ago1 yet there is still considerable uncertainty regarding the structure and evolution of the ancient archaean cratons2 7 modern analogues proposed for the setting of archaean greenstone belts include aborted intra cratonic rifts continental margins and various oceanic settings2 7 alternatively archaean tectonics may have been quite different from the plate tectonics we see today2 4 6 the abitibi belt in canada is the largest archaean greenstone ternane3 and has been a touchstone for many models because its surface geology is relatively well known8 10 it contains significant gold and base metal deposits10 and it is less deformed and has suffered less erosion than most archaean ter ranes here we present deep seismic reflection data from part of the abitibi belt which reveal extensive subhorizontal and gently dipping reflection zones truncated by steeply inclined discontinuities which project to major fault zones at the surface our preliminary model of the abitibi belt comprises a mostly juvenile magmatic crust similar to that exposed in the nearby kapuskasing uplift11 12 dissected and imbricated by deep penetrating discontinuities associated with thrust and strike slipe faulting in view of the similarity of greenstone belts worldwide2 7 13 our seismic sections provide new constraints on the general processes that have affected archaean metavolcanic terranes

equatorward and poleward expansion of the auroras during auroral substorms using all sky tv auroral data with high spatial and temporal resolution the formation of the auroral bulge is investigated we show how the discrete auroral structures within the poleward expanding bulge develop systematically toward the west the east and also equatorward from a localized breakup region auroral structure at the western end of the bulge a surge develops with clockwise rotation as viewed along the magnetic field direction at the eastern part of the bulge thin auroral features propagate eastward from the breakup region around the central meridian of the bulge auroral features expand equatorward and become north south aligned the n s aurora the n s aurora and the eastward propagating aurora develop into diffuse and pulsating aurora after the expansion we suggest that these discrete auroral structures in the bulge develop along the plasma streamlines in a localized distorted two cell equipotential distribution the negative potential peak is concentrated at the western part of the bulge and the positive potential region is spread poleward of the eastward propagating aurora equatorward expansion of auroral structures would be attributed in this scenario to the subsequent earthward transport of plasma from the onset region energetic particles would also be injected at the n s auroral region as well as to the east of the region and would trigger the pulsating auroral activity poleward expansion on the other hand would be produced by the tailward motion of the onset region in accordance with the tailward moving current disruption region

cdaw 9 analysis of magnetospheric events on may 3 1986 event c the ninth coordinated data analysis workshop cdaw 9 focused upon several intervals within the promis period march june 1986 event interval c comprised the period 0000 1200 ut on may 3 1986 which was a highly disturbed time near the end of a geomagnetic storm interval a very large substorm early in the period commenced at 0111 ut and had a peak ae index value of 1500 nt subsequent activity was lower but at least three other substorms occurred at 2 3 hour intervals the substorms on may 3 were well observed by a variety of satellites including isee 1 2 and imp 8 in the magnetotail plus scatha goes gms and lanl spacecraft at or near geostationary orbit a particularly important feature of the 0111 ut substorm was the simultaneous imaging of the southern auroral oval by de 1 and of the northern auroral oval by viking the excellent constellation of spacecraft near local midnight in the radial range 5 9 re made it possible to study the strong cross tail current development during the bstorm growth phase and the current disruption and current wedge development during the expansion phase we use a time evolving magnetic field model to map observed auroral features out into the magnetospheric equatorial plane there was both a dominant eastward and a weaker westward progression of activity following the expansion phase a clear latitudinal separation 10 of the initial region of auroral brightening and the region of intense westward electrojet current was identified the combined ground near tail and imaging data for this event provided an unprecedented opportunity to investigate tail current development field line mapping and substorm onset mechanisms we find evidence for strong current diversion within the near tail plasma sheet during the late growth phase and strong current disruption and field aligned current formation from deeper in the tail at substorm onset we conclude that these results are consistent with a model of magnetic neutral line formation in the late growth phase which causes plasma sheet current diversion before the substorm onset the expansion phase onset occurs considerably later due to reconnection of lobelike magnetic field lines and roughly concurrent cross tail disruption in the inner plasma sheet region

caldera morphology in the western galapagos and implications for volcano eruptive behavior and mechanisms of caldera formation abstract caldera morphology on the six historically active shield volcanoes that comprise isabela and fernandina islands the two westernmost islands in the galapagos archipelago is linked to the dynamics of magma supply to and withdrawal from the magma chamber beneath each volcano caldera size e g volumes 2 9 times that of the caldera of kilauea hawai i the absence of well developed rift zones and the inability to sustain prolonged low volumetric flow rate flank eruptions suggest that magma storage occurs predominantly within centrally located chambers at the expense of storage within the flanks the calderas play an important role in the formation of distinctive arcuate fissures in the central part of the volcano repeated inward collapse of the caldera walls along with floor subsidence provide mechanisms for sustaining radially oriented least compressive stresses that favor the formation of arcuate fissures within 1 2 km outboard of the caldera rim variations in caldera shape depth to diameter ratio intra caldera bench location and the extent of talus slope development provide insight into the most recent events of caldera modification which may be modulated by the episodic supply of magma to each volcano a lack of correlation between the volume of the single historical collapse event and its associated volume of erupted lava precludes a model of caldera formation linked directly to magma withdrawal rather caldera collapse is probably the result of accumulated loss from the central storage system without sufficient recharge and as has been suggested for kilauea may be aided by the downward drag of dense cumulates and intrusives

a felsic dyke swarm formed under the sea the archean hunter mine group south central abitibi belt quebec canada an extensive rhyolitic dyke swarm has intruded subaqueous pyroclastic deposits iron formations hyaloclastite breccias and lava flows of the 2730 ma hunter mine group hmg in the south central part of the archean abitibi belt quebec the dyke swarm has a minimum width of 500 m and can be traced perpendicular to the section for 2 4 km based on crosscutting relationships chilled margins quartz content and colour five distinct dyke generations have been established each dyke generation has several magmatic pulses as indicated by parallel rows of columnar joints absence of brecciation between parallel rows suggests extremely brief intervals between magma pulses the central parts of most dykes display inverted v shaped patterns of columnar joint convergence inferred to indicate differential cooling during the late stages of dyke propagation the dykes commonly display delicate spherulites suggesting rapid cooling solidification temperatures between 400 and 600 c and penecontemporaneous devitrification quartz feldspar aggregates in the groundmass have locally developed microgranophyric textures large spherulites near the chilled margins probably formed at temperatures below 400 c percolation of abundant water throughout the dyke complex is suggested by ubiquitous prominent chilled dyke margins development of a chilled margin 500 m along one dyke suggests that water percolated at least 500 m below the water rock interface because the dykes intruded subaqueous pyroclastic deposits of similar composition dyke emplacement below the sea floor is inferred interstratification of pillowed flows and brecciated pillowed flows containing rhyolite fragments at the top of the 4 5 km thick sequence indicates that the central felsic complex probably never emerged during its evolutionary history supporting the contention that the felsic dyke complex was emplaced beneath the archean sea floor

volcanism and related slope to shallow marine volcaniclastic sedimentation an archean example near chibougamau quebec canada abstract volcaniclastic sedimentary rocks and k rich lava flows of the archean hauy formation are discontinuously exposed in a 850 m thick sequence in the waconichi synclinorium northwest of chibougamau quebec the interstratified sedimentary rocks comprise two major sedimentary facies associations the graded bedded facies association wbfa indicative of deposition by sediment gravity flows and the wavy bedded facies association wbfa defined by wave induced structures the gbfa is near the base of the stratigraphic section and is interstratified up section with the wbfa in the gbfa the abundance of synsedimentary folding truncated slump folds load casts and low angle erosional surfaces as well as sediments deposited by turbidity currents and debris flows support designation as a volcanic slope deposit in the wbfa trough crossbeds wavy bedded gravels and plane bedded volcaniclastic sandstones are indicative of a nearshore setting the abundance of angular to subangular lithic volcanic fragments and broken and euhedral pyroxene crystals in the sediments is suggestive of a pyroclastic origin some beds may represent primary pyroclastic material but unequivocal petrographic recognition of pumice or glass shards is not possible because of metamorphic overprint composition suggests an initial magmatic or phreatomagmatic eruption subsequently reworked along the shoreline and transported downslope lava flows comprising massive to lobate andesites and massive to blocky basalts constitute most of the stratigraphic sequence pillowed flows are strikingly absent from the section both compositional types have an increase in vesicularity and change in flow form up section from massive to lobate or blocky suggesting progressively shallower water depths the composite character represented by hauy lava flows and associated volcaniclastic sediments in characteristic of stratovolcanoes the studied section is an integral part of an evolved arc with an adjacent backarc basin the volcano sedimentary facies indicate the development of a volcanic island in one of these basins

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

archean terrane docking upper crust collision tectonics abitibi greenstone belt quebec canada abstract the northern nvz and southern volcanic zones svz of the abitibi greenstone belt are separated by the major e trending destor porcupine manneville fault zone dpmfz the dpmfz is interpreted to be the locus of archean terrane docking between the older diffuse volcanic arc of the nvz 2730 2710 ma and the younger arc segments of the svz 2705 2698 ma two distinct evolutionary phases can be documented along the dpmfz of the abitibi greenstone belt and include 1 arc arc collision occurring between 2697 and 2690 ma and 2 arc fragmentation between 2689 and 2680 ma identification of these two events along the dpmfz is based on detailed structural studies sedimentary basin analysis and precise u pb age determinations the thrusting event representative of the arc arc collision phase is characterized by shallow north dipping foliations 20 40 and dip parallel stretching lineations in the eastern manneville segment of the dpmfz local overturned mafic pillowed units suggest recumbent folding late strike slip or transcurrent movement displayed in the late orogenic sedimentary duparquet basin records the arc fragmentation phase basin geometry e trending en echelon folds shallow e plunging stretching lineations and a late ne striking cleavage cross cutting the folds support a dextral shear sense along the western destor porcupine segment of the dpmfz the sedimentary facies observed in the basin are consistent with those of modern strike slip basins located along the east anatolian fault turkey hazar lake and the hope fault new zealand hanmer basin precise u pb zircon age determinations from porphyry stocks located at the northern and southern limits of the duparquet basin yielded 2681 1 ma and 2689 3 2 2 9 ma respectively these ages constrain the rapid change from thrusting to transcurrent movement it is apparent that once thrusting ceased the response to oblique subduction continued in the form of strike slip displacement modern fold and thrust belts commonly show this evolution the deformation pattern is the result of oblique convergence the abitibi greenstone belt is considered to be an archean analogue of modern subducting oceanic plates such as those found in the western pacific

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

physical volcanology and structural development of cerro azul volcano isabela island galapagos implications for the development of galapagos type shield volcanoes erupted from circumferential vents near the summit these flows are nearly an order of magnitude smaller in volume than the predominantly aa flows erupted from radial eruptive fissures near the break in slope 0 06 0 1 km3 the differences in volume and flow morphology with altitude are due to slower eruption rates from summit vents than from flank vents which in turn are attributable to the different heights the magmas must ascend from shallow reservoirs these observations support the contention that the steep upper flanks were constructed by the buildup of short lava flows rather than by the structural deformation of originally gently dipping flanks in addition to the higher eruption rates a subdued lower flank geometry is promoted by the deposition of lava deltas onto the shallow galapagos platform on the western northern and eastern flanks of the volcano 40ar 39ar geochronology and volume estimates show that despite their morphologic differences the growth of the western galapagos shields has been nearly synchronous precluding an evolutionary model for their development the wide variations in elevation volume area and the distribution of slope angles among the western volcanoes can be linked instead to different long term eruption rates and to a lesser degree the position of each volcano relative to the edge of the galapagos platform

mineral chemistry and k ar ages of plutons across the karakoram fault in the shyok nubra confluence of northern ladakh himalaya india abstract petrographic analyses of plutons across the karakoram fault in the shyok nubra confluence of northern ladakh area revealed that the rocks consist of diorite granodiorite granite and leucogranite pyroxene and biotite chemistries by epma indicate that most of the diorite granodiorite and granite are calc alkaline and formed in an arc related tectonic setting these calc alkaline plutons yield k ar biotite ages from 49 3 1 1 to 51 2 1 1 for ladakh batholith in khardung la and from 60 8 1 3 to 65 8 1 4 ma for granites and diorites in hundar area the k ar ages of the hundar igneous complex are within the age range of 46 70 ma of the ladakh batholith near leh town and the neighbor sites indicating the hundar igneous complex as a part of the ladakh arc their ages are also coincident with the khardung volcanics 67 4 and 60 5 ma the calc alkaline granites from panamik of karakoram block yield k ar biotite ages of 95 7 2 1 and 96 7 2 1 ma suggesting that the granites are member of the plutons 90 120 ma occurring in northern pakistan tangtse in ladakh and western tibet that emplaced during subduction of the neo tethyan oceanic plate under the karakoram block the leucogranite and granodiorite in the panamik region yield k ar biotite and muscovite ages ranging from 9 18 0 21 to 9 45 0 21 ma the leucogranites are the s type related to post collision tectonic setting and are considered to have emplaced in relating to activation of the karakoram fault

u pb geochronology of archean metasedimentary rocks in the pontiac and abitibi subprovinces quebec constraints on timing provenance and regional tectonics abstract u pb isotopic ages of single detrital zircons have been obtained from metasandstones in the northern part of the pontiac subprovince and from the cadillac kewagama and lac caste groups within the southern abitibi subprovince youngest detrital zircon ages from these sequences are 2685 2686 2687 and 2693 ma respectively the lac fourniere pluton which intrudes pontiac group metasedimentary rocks is dated at 2682 1 ma a similar age was previously measured from a pluton that cuts the kewagama group these data bracket deposition of greywacke in the pontiac and abitibi subprovinces to 2685 3 ma in contrast the youngest zircon found in a volcaniclastic unit interbedded with timiskaming group metawacke is 2673 2 ma while the age of a pluton that intrudes the timiskaming is 2672 1 ma thus the timiskaming group is distinctly younger than the other metasedimentary rocks the normalized probability distributions of detrital zircon ages from the pontiac and the abitibi metasedimentary rocks as well as of dated igneous rocks from the southern abitibi subprovince all show that their major zircon populations formed in the age range 2685 2710 ma the distributions for the abitibi metaturbidites and dated igneous rocks are very similar suggesting that metasedimentary rocks in the abitibi belt were mostly derived from the surrounding granite greenstone terrain the largest peak of the pontiac distribution is a few million years younger than that from the abitibi metasedimentary rocks but there is a correlation between major peaks and both groups show similar age ranges for older pre abitibi components 2750 2850 and 3020 3030 ma deposition of the ca 2685 ma sediments was virtually syn deformational based on previous field geochemical and isotopic studies as well as the detrital zircon age data the pontiac group may represent a foreland basin that may have developed from an accretionary prism the source of much of the sediment may have been an uplifted extinct continental arc to the north the arc was mostly eroded into the sediment basin and perhaps tectonically overridden by a southward advancing nappe complex comprised of its back arc basin now partly preserved in the southern volcanic zone of the abitibi subprovince metaturbidites exposed between metavolcanic assemblages within the greenstone belt may represent overturned unconformities at the structural base of the greenstones these were brought up by high angle faults which also localized deposition of non marine timiskaming type sediments it is tentatively suggested that pontiac type metasedimentary rocks extend most of the way beneath the abitibi subprovince the oldest part of the basin emerging as the quetico group in the western superior province

the las canadas caldera tenerife canary islands an overlapping collapse caldera generated by magma chamber migration abstract the las canadas caldera is one of the most important geological structures of tenerife stratigraphic structural volcanological petrological geochronological and geophysical data suggest that the las canadas caldera resulted from multiple vertical collapse episodes that occurred during the construction of the las canadas edifice upper group three long term 200 ka cycles of phonolitic explosive activity each culminating with a caldera collapse have been identified in the upper group during the construction of the upper group the focus of felsic volcanism migrated from west to east using the results of field observations experimental analogue models and numerical studies we propose that the formation of the overlapping las canadas collapse caldera is related to the migration of the associated magma chamber our model implies that each collapse of this overlapping caldera partly or completely destroyed the feeding magma chamber this destruction led to changes in the local stress field that favoured the formation of a new chamber at one side of the previous one resulting in magma chamber migration the proposed model accounts for the formation of the las canadas caldera in particular it explains the geometrical relationships stratigraphy and chronology of the caldera wall deposits comparison with other overlapping collapse calderas suggests that our model may apply to other overlapping calderas

late orogenic basins in the archaean superior province canada characteristics and inferences abstract the late orogenic archaean duparquet kirkland and stormy basins of the canadian superior province are characterized by bounding crustal scale faults and abundant porphyry stock emplacement lava flows and pyroclastic deposits are restricted to the kirkland and stormy basins and coarse clastic detritus characterizes the duparquet basin seven distinct lithofacies are identified 1 mafic volcanic 2 felsic volcanic 3 pyroclastic 4 volcaniclastic 5 conglomerate sandstone 6 sandstone argillite conglomerate and 7 argillite sandstone tuffaceous sandstone the mafic and felsic volcanic lithofacies represent effusive lava flows the pyroclastic lithofacies is formed of subaerial surge and airfall deposits and the volcaniclastic lithofacies is composed of reworked volcanic debris the conglomerate sandstone lithofacies is interpreted as alluvial fan fan delta or proximal braided stream deposits whereas the sandstone argillite lithofacies is consistent with sandy dominated flood or braidplain deposits a dominantly shallow water lacustrine setting is inferred for the argillite sandstone lithofacies these different lithofacies record the basin history and can be used to identify basin forming processes lithofacies stacking and rapid lateral changes of lithological units in conjunction with interformational unconformities and basin margin faults suggest tectonically induced sedimentation volcanism can also influence basin evolution and the delicate balance between erosion sedimentation and prevalent transport processes is affected by volcanic input catastrophic influx of pyroclastic material facilitated mass wasting processes and formation of non confined hyperconcentrated flood flow deposits account for local congestion of alluvial or fluvial dispersal patterns confined stream flow processes govern sedimentation during intravolcanic phases or prominent tectonic uplift in addition climate which controls the weathering processes and vegetation which stabilizes unconsolidated material affects the transport and depositional process a co 2 rich aggressive weathering humid archaean atmosphere favours traction current deposits and an absence of vegetation promotes rapid denudation although tectonism is the prevalent long term controlling factor in restricted basins the effects of volcanism climate and lack of vegetation can also be detected

precise age of the platiniferous merensky reef bushveld complex south africa by the u pb zircon chemical abrasion id tims technique the bushveld complex south africa is the world s largest layered intrusion and host to the majority of the known resources of platinum group elements chromium and vanadium despite intensive study determining the precise crystallization age of the bushveld complex has proven to be difficult in this study zircon and rutile were identified in thin section and separated from a sample of pegmatitic feldspathic orthopyroxenite from the pge rich merensky reef west mine townlands shaft rustenburg section low u 21 105 ppm zircon occurs with interstitial biotite and is locally in direct contact with sulfide rutile occurs mainly as inclusions within or overgrowths on chromite especially in the chromitite stringers that bound the reef and also as acicular grains with biotite a u pb crystallization age of 2054 4 1 3 ma was determined from single crystal chemical abrasion id tims u pb dating of zircon and multigrain fractions of rutile yielded a u pb cooling age of 2055 0 3 9 ma integration of the merensky reef crystallization age with existing u pb age determinations for intrusions that pre and postdate emplacement and crystallization of the layered rocks of the bushveld complex indicate that magmatism occurred at ca 2054 ma and is contemporaneous with related magmatic activity across the northern kaapvaal craton of south africa and botswana

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

near ridge seamount chains in the northeastern pacific ocean high resolution bathymetry and side scan data of the vance president jackson and taney near ridge seamount chains in the northeast pacific were collected with a hull mounted 30 khz sonar the central volcanoes in each chain consist of truncated cone shaped volcanoes with steep sides and nearly flat tops several areas are characterized by frequent small eruptions that result in disorganized volcanic regions with numerous small cones and volcanic ridges but no organized truncated conical structure several volcanoes are crosscut by ridge parallel faults showing that they formed within 30 40 km of the ridge axis where ridge parallel faulting is still active magmas that built the volcanoes were probably transported through the crust along active ridge parallel faults the volcanoes range in volume from 11 to 187 km3 and most have one or more multiple craters and calderas that modify their summits and flanks the craters 1 km diameter range from small pit craters to calderas as large as 6 5 8 5 km although most are 2 4 km across crosscutting relationships commonly show a sequence of calderas stepping toward the ridge axis the calderas overlie crustal magma chambers at least as large as those that underlie kilauea and mauna loa volcanoes in hawaii perhaps 4 5 km in diameter and 1 3 km below the surface the nearly flat tops of many of the volcanoes have remnants of centrally located summit shields suggesting that their flat tops did not form from eruptions along circumferential ring faults but instead form by filling and overflowing of earlier large calderas the lavas retain their primitive character by residing in such chambers for only short time periods prior to eruption stored magmas are withdrawn probably as dikes intruded into the adjacent ocean crust along active ridge parallel faults triggering caldera collapse or solidified before the next batch of magma is intruded into the volcano probably 1000 10 000 years later the chains are oriented parallel to subaxial asthenospheric flow rather than absolute or relative plate motion vectors the subaxial asthenospheric flow model yields rates of volcanic migration of 3 4 3 3 and 5 9 cm yr 1 for the vance president jackson and taney seamounts respectively the modeled lifespans of the individual volcanoes in the three chains vary from 75 to 95 kyr these lifespans coupled with the geologic observations based on the bathymetry allow us to construct models of magma supply through time for the volcanoes in the three chains

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

rift zone reorganization through flank instability in ocean island volcanoes an example from tenerife canary islands the relationship between rift zones and flank instability in ocean island volcanoes is often inferred but rarely documented our field data aerial image analysis and 40ar 39ar chronology from anaga basaltic shield volcano on tenerife canary islands support a rift zone flank instability relationship a single rift zone dominated the early stage of the anaga edifice 6 4 5 ma destabilization of the northern sector led to partial seaward collapse at about 4 5 ma resulting in a giant landslide the remnant highly fractured northern flank is part of the destabilized sector a curved rift zone developed within and around this unstable sector between 4 5 and 3 5 ma induced by the dilatation of the curved rift a further rift arm developed to the south generating a three armed rift system this evolutionary sequence is supported by elastic dislocation models that illustrate how a curved rift zone accelerates flank instability on one side of a rift and facilitates dike intrusions on the opposite side our study demonstrates a feedback relationship between flank instability and intrusive development a scenario probably common in ocean island volcanoes we therefore propose that ocean island rift zones represent geologically unsteady structures that migrate and reorganize in response to volcano flank instability

a study of a magnetospheric substorm recorded by the viking auroral imager a substorm recorded at 80 second time resolution in images from the viking auroral imager has been studied using data from ground observations from a geosynchronous satellite and from isee 1 in the magnetotail plasma sheet substorm onset marked by negative bays at the ground particle injection at synchronous orbit and plasma sheet dropout at isee 1 appeared to produce only local brightening of the night side oval a rapid poleward expansion of the auroras twenty minutes later was accompanied by a major reduction of tail energy density and swelling of the inner plasma sheet recovery 15 minutes later of auroral zone bays and the plasma sheet was accompanied by fading of the equatorward auroras that left a bright poleward arc as the primary auroral emission

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

kulanaokuaiki tephra ca a d 400 1000 newly recognized evidence for highly explosive eruptions at kilauea volcano hawai i k lauea may be one of the world9s most intensively monitored volcanoes but its eruptive history over the past several thousand years remains rather poorly known our study has revealed the vestiges of thin basaltic tephra deposits overlooked by previous workers that originally blanketed wide near summit areas and extended more than 17 km to the south coast of hawai i these deposits correlative with parts of tephra units at the summit and at sites farther north and northwest show that k lauea commonly regarded as a gentle volcano was the site of energetic pyroclastic eruptions and indicate the volcano is significantly more hazardous than previously realized seventeen new calibrated accelerator mass spectrometry ams radiocarbon ages suggest these deposits here named the kulanaokuaiki tephra were emplaced ca a d 400 1000 a time of no previously known pyroclastic activity at the volcano tephra correlations are based chiefly on a marker unit that contains unusually high values of tio 2 and k 2 o and on paleomagnetic signatures of associated lava flows which show that the kulanaokuaiki deposits are the time stratigraphic equivalent of the upper part of a newly exhumed section of the uw kahuna ash in the volcano9s northwest caldera wall this section thought to have been permanently buried by rockfalls in 1983 is thicker and more complete than the previously accepted type uw kahuna at the base of the caldera wall collectively these findings justify the elevation of the uw kahuna ash to formation status the newly recognized kulanaokuaiki tephra to the south the chief focus of this study is defined as a member of the uw kahuna ash the kulanaokuaiki tephra is the product of energetic pyroclastic falls no surge or pyroclastic flow deposits were identified with certainty despite recent interpretations that uw kahuna surges extended 10 20 km from k lauea9s summit

archean mafic dyke swarms near the cameron river and beaulieu river volcanic belts and their implications for tectonic modelling of the slave province northwest territories three dense swarms of archean mafic dykes which intrude the beaulieu river and cameron river volcanic belts and adjacent granitoid terranes of the sleepy dragon complex and meander lake complex in the southern slave province all contain clusters of multiple parallel sided sheeted dykes none of the swarms can be convincingly modelled as being part of an ophiolite assemblage the 3 km by 20 km step9nduck dyke complex comprising about 350 mainly metabasaltic to meta gabbroic dykes separated by screens of granitoid gneiss 2750 ma represents profuse mafic magmatism in the meander lake complex dense dyke swarms within the cameron river belt fed the lavas and are an integral part of the volcanic stratigraphy all basaltic magmas represented by the volcanic belts and the dyke sets are predominantly mantle derived fractionated tholeiites nd isotope geochemistry indicates that they were all contaminated by archean sialic material isotopic systematics are satisfied by mixing of depleted mantle and varying proportions of sleepy dragon type crust the dyke swarms in the sleepy dragon complex and the step9nduck swarm record extensional events in archean continental crust possibly they are subsurface equivalents of now eroded flood basalts that signalled the activity of mantle plumes during archean times this hypothesis is consistent with magmatic underplating of continental lithosphere and with extensive involvement of continental material

experimental study of caldera formation scaled experiments have been carried out on caldera collapse mechanisms using silicone as analogue magma and dry sand as analogue rock experiments were carried out in two and three dimensions using a range of roof aspect ratios thickness width 0 2 to 4 5 appropriate for caldera collapse they reveal a general mechanism of collapse only weakly dependent on the shape of the reservoir for low roof aspect ratios 1 subsidence starts by flexure of the roof and the formation of outward dipping reverse ring faults which in turn trigger formation of peripheral inward dipping normal ring faults the subsidence always occurs asymmetrically in cross section the reverse faults delimit a coherent piston bounded on each side by an annular zone of inwardly tilted strata located between the reverse and normal ring fault sets the surface depression consists of a nondeformed area piston surrounded by an annular extensional zone tilted strata for high aspect ratios 1 multiple reverse faults break up the roof into large pieces and subsidence occurred as a series of nested wedges 2 d or cones 3 d the extensional zone dominates the surface depression in the case where preexisting regional faults do not play a major role the collapse mechanics of calderas probably depends strongly on the roof aspect ratio calderas with low roof aspect ratios are predicted to collapse as coherent pistons along reverse faults the annular extensional zone might be the source of the large landslides that generate intracaldera megabreccias collapse into magma reservoirs with high roof aspect ratios may be the origin of some funnel calderas where explosive reaming is not dominant

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

formation of dykes feeder dykes and the intrusion of dykes from magma chambers the paper discussed the formation of dykes and applies the results to iceland it is postulated that dykes follow the pathway of least work and of least tensile strength and thereby intrude the subvertical joints in lava flows it is suggested that dykes form in magma pulses where each dyke is split in two by the next magma pulse and so on in iceland the estimated time between successive magma pulses is of the order of several hundred days statistical considerations indicate that in iceland the probability of seeing dykes end upwards is only 1 2 which agrees with observations it is concluded that many and probably the majority of dykes are non feeders this together with the low probability of finding the connection between feeder and lava flow explains the scarcity of observed feeder dykes it is concluded that overpressure in shallow magma chambers needed to drive magma through crustal fractures in iceland is usually smaller than the tensile strength of the host rock several mpa which thereby is the critical factor in dyke intrusion

detailed analysis of a substorm event on 6 and 7 june 1989 2 stepwise auroral bulge evolution during expansion phase 1 we have analyzed in detail the auroral bulge evolution during the expansion phase of an isolated substorm which was observed by the uv imager aboard the akebono satellite it was found that there were three distinct stages in the evolution stage 1 was characterized by rapid poleward and azimuthal predominantly westward expansions in a short time about 2 min stage 2 was characterized by a very slow poleward and slower and continuous azimuthal expansions there was a certain period for transition between stage 1 and stage 2 and it was characterized by a very slow poleward and rapid eastward expansions stage 3 started about 11 min after the onset and was characterized by a sudden reactivation of the rapid poleward and azimuthal expansions the reactivation started around the initial onset meridian and then spread both eastward and westward at the azimuthal front the expansion first occurred at the lowest latitudes spread poleward to around the highest latitudes of stage 1 and then spread further poleward after a brief interval hence the local expansion also had three distinct stages similar to the global one the ground based observations showed that the highest latitude of the local first stage was very close to the latitude of auroral activity that appeared near the ionospheric plasma sheet boundary layer psbl region a few minutes before the onset the further poleward expansion during the local third stage started with a significant intensification of the poleward most auroral activity during the local third stage the bright electron auroral region was bifurcated into a poleward expanding part and an equatorward moving part the proton auroral emission coexisted in the bulge during the local first and second stages and almost disappeared soon after the bifurcation during the local third stage based on these observations we discuss the evolution in the magnetosphere during the expansion phase

late archaean thrusting in the northwestern pontiac subprovince canadian shield abstract in the northwestern archaean pontiac subprovince south verging structures such as drag folds with subhorizontal axes stretched objects down dip mineral lineations low angle thrusts and allochthonous metavolcanic slabs were mainly produced by thrusting in response to n s regional compression deformation of the dominantly metasedimentary sequences occurred under barrovian type metamorphism and the horizontal shortening was accompanied by downdip stretching in the regional foliation s1 two nappes are assumed to be present their boundary being marked by an abrupt change in structural style and metamorphic facies two phases of close to isoclinal folding are evident in the upper nappe but are absent from the lower nappe in the upper nappe d2 gives and e w tectonic trend to the rock units f2 folds range from upright to recumbent but are most typically moderately southward inclined folds that plunge gently towards the east or west besides mostly retrograde recrystallization within d2 fault zones s2 is a crenulation cleavage that did not develop sufficiently to obliterate the regional schistosity s1 and its associated stretching and mineral lineations this contrasts with the regionally penetrative nature of the main deformation event d1 and suggests a narrowing or partitioning of high strain zones with time possibly due to a decrease in the regional temperature syn orogenic sedimentation in the northwestern pontiac subprovince was probably enhanced by tectonically controlled uplift resulting from early regional kenoran compression the sedimentary basin was subsequently deformed by further n s compression which led to internal stacking nappe type displacements barrovian metamorphism and thrusting of the southern abitibi over the pontiac subprovince the common style of folding and the same orientation of various structural elements such as cleavage and lineation in the transition zone between the abitibi and pontiac subprovinces indicate continuity in deformation style regional scale transcurrent displacement along the northern subprovince boundary appears as a late localized response to regional compression

radial expansion of the tail current disruption during substorms a new approach to the substorm onset region the substorm onset region and the radial development of the tail current disruption are examined from a new viewpoint the reconfiguration of the magnetotail field at substorm onset can be understood in terms of a sudden decrease disruption in tail current intensity the north south component bz is very sensitive to whether the spacecraft position is earthward or tailward of the disruption region while the change in the sun earth component bx is most sensitive to the change in tail current intensity near the spacecraft if the current disruption starts in a localized range of radial distance and expands radially a distinctive phase relationship between the changes in bx and bz is expected to be observed this phase relationship depends on whether the current disruption starts on the earthward side or the tailward side of the spacecraft thus it is possible to infer the direction of the radial expansion of the current disruption from magnetic field data of a single spacecraft this method is applied to isee observations of a tail reconfiguration event that occurred on march 6 1979 the phase relationship indicates that the disruption region expanded tailward from the earthward side of the spacecraft during the event this model prediction is consistent with the time lag of magnetic signatures observed by the two isee spacecraft the expansion velocity is estimated at 2 re min 200 km s for this event furthermore it is found that the observed magnetic signatures can be reproduced to a good approximation by a simple geometrical model of the current disruption the method is used statistically for 13 events selected from the isee magnetometer data it is found that the current disruption usually starts in the near earth magnetotail x 20re and often within 15 re of the earth the phase relationship between bx and bz is discussed in terms of the near earth neutral line model and the result is compared with previous studies on the substorm onset region

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

paleogeographic and paleotectonic evolution of a basement controlled archean supracrustal sequence chibougamau caopatina quebec the 7 14 km thick archean volcano sedimentary sequence in the chibougamau caopatina region ccr composed of two mafic to felsic cycles and two thick essentially epiclastic successions is divided into a southern caopatina segment scs and northern chibougamau segment ncs the sedimentary units which evolved after cycle 1 volcanic rocks in the scs and after the combined cycles 1 and 2 in the ncs document a diachronous sedimentary basin evolution this criterion has been employed to distinguish an early evolutionary stage in the south and a late evolutionary stage in the north in each segment the basin episode represents a unique extensional event volcanism preceded the major sedimentary rifting event in both segments composition of sediments and sedimentary facies correlated with the interpreted basin margin faults support the interpretation of two phases of crustal extension volcanic construction and sill intrusion were the prevalent factors governing emergence of volcanic islands in the cc

regional scale hydrothermal alteration in the central blake river group western abitibi subprovince canada implications for vms prospectivity the late archean blake river group is a thick succession of predominantly mafic volcanic rocks within the southern zone of the abitibi greenstone belt it contains a number of silicic volcanic centers of different size including the large noranda volcanic complex which is host to 17 past producing volcanogenic massive sulfide deposits the noranda complex consists of a 7 to 9 km thick succession of bimodal mafic and felsic volcanic rocks erupted during five major cycles of volcanism massive sulfide formation coincided with a period of intense magmatic activity cycle iii and the formation of the noranda cauldron hydrothermal alteration in these rocks is interpreted to reflect large scale hydrothermal fluid flow associated with rapid crustal extension and rifting of the volcanic complex the alteration includes abundant albite chlorite epidote and quartz silicification which exhibit broad stratigraphic and structural control and correlate with previously mapped whole rock oxygen isotope zonation the mine sequence volcanic rocks are characterized by abundant iron rich chlorite fe fe mg 0 5 hydrothermal amphibole ferroactinolite and coarse grained epidote of clinozoisite composition 10 wt fe2o3 lacking the clinozoisite solid solution alteration in the mine sequence volcanic rocks persists along strike well beyond the limits of the main ore deposits as far as several tens of kilometers and can be readily distinguished from greenschist facies metamorphic assemblages at a regional scale the lack of similar alteration in the pre cauldron sequences is consistent with limited 18o depletion and suggests that the early history of the volcanic complex did not support large scale high temperature fluid flow in these rocks comparisons with a much smaller barren volcanic complex in nearby ben nevis township reveal important differences in the alteration mineralogy between volcanoes of different size with implications for area selection during regional scale mineral exploration the ben nevis complex consists of a 3 to 4 km thick succession of mafic intermediate and felsic volcanic rocks centered on a small subvolcanic intrusion alteration of the volcanic rocks comprises mainly low temperature assemblages of prehnite pumpellyite magnesium rich chlorite fe fe mg 10 wt fe2o3 and calcite actinolite magnetite alteration occurs proximal to the intrusive core of the complex but the limited extent of this alteration indicates only local high temperature fluid circulation adjacent to the intrusion a distal zone of carbonate alteration is located 4 6 km from the center of the volcano although iron bearing carbonates are present locally within this zone the absence of siderite argues against a high temperature origin for this alteration these observations do not offer positive encouragement for the existence of a fossil geothermal system of sufficient size or intensity to have produced a large massive sulfide deposit

volcaniclastic sedimentation in and around an ordovician subaqueous caldera lower rhyolitic tuff formation north wales the lower rhyolitic tuff formation of ordovician age in north wales records the collapse infilling and subsequent resurgence of a volcanic caldera with an original diameter of about 15 km this volcanic center controlled patterns of volcaniclastic sedimentation providing enough topographic relief for both a shallow lagoon depositional basin within the caldera and a source of sediment derived from the rim within the caldera sediment consists of tuffaceous laminated siltstone and immature coarse grained volcaniclastic sandstone containing plane beds ripple cross laminations symmetrical wave ripples and hummocky cross stratification coarse grained matrix supported conglomerate layers and layers of ash flow tuffs are also present these sediments accumulated in shallow water above fair weather wave base conglomerate units represent debris flows from the caldera rim a nearby shoreline and elevated areas associated with resurgent domes sedimentation outside of the caldera consisted of deposition of background suspension and volcanic ash suspension and turbidite deposition on a pyroclastic apron the outer margin of the apron was dominated by fine grained suspension and turbidite deposition whereas the inner margin of the apron contains hummocky cross stratification and other evidence of reworking by episodic storm waves local highs with associated shallow water sedimentation existed outside the caldera even though deposited in high energy marine environments all sedimentary rocks are both texturally and mineralogically very immature this textural immaturity differs from the typical very mature marine sediments and was caused by rapid depositional rates and a local volcanic sediment source

caldera collapse during the 2000 eruption of miyakejima volcano japan a collapsed caldera 1 6 km in diameter and 450 m in depth was formed at the summit of miyakejima volcano during the 2000 eruption the collapsed caldera appeared on 8 july with a minor phreatic eruption 12 days after seismic activity and magma intrusion occurred northwest of the volcano growth of the caldera took from 8 july to the middle of august with seismic swarms associated with the continuous intrusion of magma northwest of the volcano the growth rate of the caldera was about 1 4 107 m3 day and the final volume of the collapsed caldera was about 6 108 m3 major phreatomagmatic eruptions produced a total of about 1 6 1010 kg 1 1 107 m3 of volcanic ash after caldera growth the caldera structure and the nature of the eruptive materials of the first collapse on 8 july suggest that the surface subsidence was caused by the upward migration of a steam filled cavity with stoping of the roof rock above the magma reservoir the diameter of the stoping column was estimated to be 600 700 m from circumferential faults that developed in the caldera floor and the collapse of the caldera wall enlarged the diameter of the caldera to 1 6 km the total volume of the caldera and the horizontal diameter of the stoping column gave a subsidence of the caldera floor of 1 6 2 1 km

u pb systemstics in single zircons from the pontiac sediments abitibi greenstone belt twenty six detrital zircon grains from the late archaean pontiac sediments of the abitibi greenstone belt were dated individually by the u pb method in order to identify possible elements of an ancient pre 2 7 ga shield the results show that at least two components were present at the source areas of these sediments 1 materials of about 2700 2725 ma which were most likely derived from contemporaneous acid igneous rocks and 2 significantly older materials parts of which are at least 2940 ma old which were either part of an uplifted sedimentary basin or a sialic basement complex these results can be used to suggest a close link between igneous activity and basin subsidence and that the development of the abitibi volcanic belt was closely associated with the presence of some older sialic blocks

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

massive and brecciated dikes in the mcdougall and despina faults noranda quebec canada abstract the mcdougall and despina faults of the central noranda volcanic complex cut subaqueous volcanic rocks in the archean abitibi greenstone belt rhyodacitic dikes occupy the faults along with lesser amounts of andesitic dioritic and a mixed basaltic rhyodacitic dike there are two types of rhyodacitic dikes one massive the other brecciated massive dikes are homogeneous and spherulitic brecciated dikes are dominated by curved angular fragments with a few vesicles both occur either alone or together in the faults where the two occur together they are commonly interlayered in concentric layered lobes the faults are interpreted as fissures for pulses of nonexplosive rhyodacitic lava many intrusive pulses interacted with an external fluid which occupied the faults this interaction resulted in brecciated glassy margins and massive crystalline pulse interiors magma fluid interaction is thus invoked as the mechanism responsible both for dike brecciation and the concentric layering the dikes are considered as intrusive analogs of extrusive rhyolitic lobe lava observed in iceland and in noranda

age constraints and characteristics of subaqueous volcanic construction the archean hunter mine group abitibi greenstone belt the archean hunter mine group hmg is a 6 7 km thick south facing volcanic sequence at the southern limit of the northern volcanic zone abitibi greenstone belt the hmg is composed of 1 felsic lava flows and endogenous and exogenous domes with autoclastic breccia deposits 2 pyroclastic fire fountain products and high to low concentration volcaniclastic sediment gravity flows of autoclastic and pyroclastic origin interstratified with iron formation 3 mafic pillowed and brecciated flows 4 a prominent felsic dyke swarm and 5 abundant mafic intrusions the stratigraphy is complex as inferred by detailed volcano sedimentary facies mapping a 5 7 km wide n trending felsic dyke swarm coupled with abundant synvolcanic and synsedimentary faults chaotic breccias and mineralization suggest a central volcanic depression which is consistent with a caldera structure voluminous effusive activity and explosive fire fountaining eruptions contributed to caldera development rifting of this arc edifice is supported by the presence of felsic dykes and mafic tholeiitic sills the lithofacies and overlying komatiites flows are consistent with a deep water marine setting u pb age determinations help constrain the hmg caldera formation to 6 million years the sampled quartz feldspar phyric dykes yielded u pb zircon ages of 2728 3 4 3 4 and 2728 9 0 8 ma and together with a previously published dyke age of 2729 6 1 4 ma from the same dyke swarm a case is made that the swarm evolved over 1 million years 2728 6 2729 6 ma a lobate flow and flow breccia revealed similar ages of 2724 6 4 6 1 7 and 2727 6 4 2 2 0 ma whereas the up to 1 km thick gabbro diorite roquemaure sill precedes both felsic dykes and flows as indicated by the 2731 8 2 2 2 0 ma age the 2706 4 5 7 4 3 ma feldspar quartz phyric dykes occurred late in the history of the hmg and were not associated with subaqueous volcanic construction the 207pb 206pb ages of analyzed zircon fractions have an inherited component within error of the oldest crystallization ages obtained for the hmg complex collectively these age determinations and inheritance as well as stratigraphy support a tripartite division into a 2734 2730 ma lower formational stage a 2732 ma middle formational stage and a 2730 2728 ma upper formational stage the study shows that age constraints without detailed facies mapping may lead to erroneous conclusions on volcanic construction and that felsic volcanic dykes and flows may inherit an earlier component of their own volcanic edifice

geodynamic settings of mineral deposit systems mineral deposits represent extraordinary metal concentrations that form by magmatic magmatic hydrothermal or hydrothermal processes in geodynamic environments typified by anomalously high thermal and or mechanical energy near plate boundaries as they require the conjunction of specific environmental conditions to form particular mineral deposit types tend to occupy specific geodynamic niches the temporal distributions of mineral deposit types reflect both formational and preservational processes in the archaean and palaeoproterozoic these were linked because of preservation in continental crust connected to thick buoyant subcontinental lithospheric mantle sclm but were decoupled by the neoproterozoic and phanerozoic as a result of evolution to thinner increasingly dense sclm the transition marks a change from mantle plume influenced plate tectonics to modern style plate tectonics with broadly coincident environmental changes and a major impact on the nature and abundance of preserved mineral deposit types as mineral deposits represent an integral part of tectonic process they are essential indicators of that process and geodynamic settings and should be incorporated into any holistic tectonic terrane analysis their distribution also provides a particularly critical test on ancient continental reconstructions derived from palaeomagnetic data conversely such reconstructions provide a first order targeting tool for the conceptual exploration required to discover new mineral provinces and deposits under cover

time transgressive archean unconformities underlying molasse basin fill successions of dissected oceanic arcs superior province canada abstract late archean diachronous unconformities underlying tectonically controlled coarse clastic successions are located throughout the superior province of canada examples studied in detail include hiatuses at the base of the timmins and kirkland basins in the abitibi greenstone belt and the crowduck and stormy basins in the wabigoon subprovince the timmins molasse basin fill succession overlies a high angle subaerial unconformity separating deepwater turbiditic deposits from fluvial conglomerates a subaerial erosional unconformity defines the contact between mafic flows and overlying felsic volcaniclastic deposits and fluvial conglomerates of the kirkland basin the crowduck and stormy successions are characterized by shallow water and subaerial unconformities between quartz feldspar porphyry stocks and clastic deposits the pronounced characteristics of the molasse basins which include 1 basal unconformities with older sedimentary volcanic or plutonic rocks 2 bounding fault systems 3 ba

u pb zircon geochronology in the southwestern abitibi greenstone belt superior province the abitibi belt is the largest continuous greenstone belt in the superior province of the canadian shield it comprises several composite komatiitic tholeiitic calc alkalic and sedimentary sequences that are folded transected by major faults and intruded by various generations of plutonic rocks precise u pb geochronology has been carried out in the belt for the past decade to solve chronostratigraphic and metallogenetic problems this paper presents new zircon ages and reassesses previously published ones now refined by the addition of abraded and concordant zircon analyses volcanic and subvolcanic units of the timmins area yield the following ages 2727 1 5 ma for a tuff of the upper deloro formation 2703 1 5 ma for a tuff of the upper tisdale formation and 2698 4 ma for the krist fragmental assigned to the top of the tisdale formation the age of a dunite intrusive into the upper deloro formation is revised at 2707 3 ma whereas the aquarius diorite east of timmins yields a poorly define

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

synvolcanic and younger plutonic rocks from the blake river group implications for regional metallogenesis the blake river group of the abitibi greenstone belt is host to the well known volcanogenic massive sulfide vms deposits of the noranda camp quebec canada in the noranda camp massive sulfide deposits are spatially associated with the coeval flavrian powell intrusive complex which is interpreted to have driven hydrothermal circulation that formed the deposits in the camp in the western blake river group in ontario there are also abundant intrusive rocks yet relatively insignificant accumulations of vms deposits these intrusive rocks are younger 2686 9 1 2 and 2688 5 2 3 ma and are associated with porphyry style cu mo au mineralization with re os molybdenite ages of 2682 4 5 ma the intrusions are also smaller in areal extent and do not have a temporally extended polyphase history like the flavrian powell intrusive complex the intrusions of the western blake river group can also be distinguished by lithogeochemical attributes and a distinct petrological history compared to the flavrian powell intrusive complex they have lower total y yb th hfse and ree with higher zr y la yb sm yb al2o3 yb and la smucn ratios and lower zr nb zr th zr la zr tio2 and nb thucn ratios petrologic modeling is consistent with the western blake river group intrusive rocks being generated as relatively low temperature melts 40 km in the crust where garnet is stable in the residue e g garnet amphibolite residue in contrast modeling of the flavrian powell intrusive complex is consistent with melting at shallower levels in the crust e g 800o to 850oc the occurrence of high temperature magmatism at high levels within this crust that is coeval with volcanic activity explains the occurrence of numerous vms deposits with the flavrian powell intrusive complex as this intrusive complex would have generated abundant heat at high levels in the crust to drive hydrothermal circulation in contrast the intrusions of the western blake river group were generated at depth and at lower temperature increasing the probability of heat loss upon emplacement from depth hence decreasing the probability of hydrothermal circulation at high levels in the crust and likely explaining the paucity of vms mineralization associated with these intrusive rocks the petrological attributes of the flavrian powell intrusive complex and western blake river group may be useful in delineating potentially fertile versus less prospective intrusive complexes in greenfields exploration areas notably the western blake river group intrusions have geochemical features similar to some phanerozoic porphyry cu mo au associated intrusive systems consistent with the observed mineralization and highlighting the potential of porphyry cu mo au systems of late archean age elsewhere in the abitibi