Subvertically- to vertically-dipping Archean strata provide an excellent opportunity to study synvolcanic structures and internal organization of subaqueous volcanic complexes. The Abitibi greenstone belt in Quebec, Canada, hosts a number of these volcanic complexes and specifically the unique Blake River Megacaldera Complex. The Blake River Megacaldera Complex is composed of (i) an initial shield phase known as the Misema Caldera and (ii) two graben-type calderas known as the New Senator and Noranda calderas. The southern portion of the New Senator Caldera, the focus of this study, is of particular interest as the structure hosts the 54 Mt Horne Au-rich volcanogenic massive sulfide deposit. Detailed facies mapping, coupled with geochemical and geochronological analysis at multiple outcrop localities throughout the city of Rouyn-Noranda, Quebec, show that the stratigraphy of the New Senator Caldera is dominated by subaqueous effusive mafic volcanic facies with local felsic effusive and intrusive deposits. With the incorporation of structural data, we have used synvolcanic faults and dyke complexes to divide these facies into specific blocks within this region, here renamed the Rouyn–Pelletier sector. This study focuses on the facies and characteristics of the Pelletier, Senator, and Glenwood blocks and identifies the Évain, Stadacona, and Chadbourne blocks. Geochronological analysis reveals that facies of the Glenwood Block are approximately the same age as those of the felsic flows of the previously identified Horne Block. Together, all extrusive and intrusive volcanic facies from the Stadacona unit north to the Horne Creek fault compose a large caldera complex known as the Rouyn–Pelletier Caldera Complex.

Both intrusive and extrusive volcanic facies of a felsic lava flow complex are exposed as the outcrops of the "Glenwood Rhyolite" within the Blake River Group of the Abitibi greenstone belt, Quebec, Canada. Subvertically-dipping volcanic facies allows for the identification of the internal architecture of an Archean subaqueous felsic flow complex. Detailed facies analyses show volcanic fades and volcanic textures and structures atypical of subaqueous pyroclastic deposits or deposits of dome complexes. Additionally, a complex network of mafic dykes cross-cut the felsic facies. When integrated with facies-specific geochemical analyses, a coherent model for all volcanic and intrusive facies and supplemental information regarding the host volcanic complex has been developed.Detailed mapping reveals extrusive or "exogenous" aphanitic felsic volcanic facies and intrusive or "endogenous" quartz- and feldspar-phyric felsic volcanic facies. Exogenous facies are comprised of massive lobate, in situ brecciated, flow breccia and flow-front breccia facies, whereas the only visible endogenous facies is a quartz- and feldspar-phyric unit that intrudes the brecciated fades on the eastern side of the complex. All felsic facies demonstrate a west to east flow direction. Both exogenous and endogenous facies have similar chemical compositions ranging from dacite to rhyodacite with a transitional affinity. Mafic to intermediate dykes predominantly strike north-south and east-west with occasional east-northeast-trending units. Wispy terminations at some dyke margins and dismembered dykes indicate they were intruded into a still cooling viscoelastic medium. Geochemical analyses further defines that most east-west-trending dykes have a tholeiitic affinity, whereas north-south-trending dykes are dominantly calc-alkaline and east-northeast-trending dykes are transitional.The subaqueous Glenwood felsic flow complex was emplaced in a fault-bounded depression and consists of stacked exogenous dacite/rhyodacite lobes and intruded by a late quartz- and feldspar-phyric endogenous lobe. The intrusion of mafic dykes occurred relatively soon after the cessation of felsic activity, as evidenced by primary textures consistent with intrusion into a viscoelastic medium. Combined, the extrusive and intrusive facies of the complex attests to the complexity of the host volcanic environment. (C) 2014 Elsevier B.V. All rights reserved.

The 2724–2722 Ma Stoughton-Roquemaure Group (SRG) of the Abitibi greenstone belt (the Archean Superior Province, Canada) is a ≤ 2 km thick komatiite–basalt succession intermittently exposed for about 50 km along strike. The ultramafic and mafic rocks occur mainly as pillowed, brecciated, and massive flows with well preserved spinifex textures in the komatiites. Volcanological, comparative stratigraphic and geochemical studies of the group along a volcanic marker horizon at the base of the succession allow the assessment of magma emplacement processes and mantle source rocks. Major feeder channels, secondary distributary tubes surrounded by pillowed flows with minor breccias and hyaloclastites display facies architecture of small volume flow fields (1–2 km3). Within the SRG, Al-depleted (ADK; Barberton-type) and Al-undepleted (AUK; Munro-type) komatiitic lavas are intercalated with tholeiitic basalt flows at a m- to 10s of m scale. Basalts and komatiites are inferred to be mantle plume-related; both rock types form two groups with characteristics of ADK and AUK including Al2O3/TiO2 ~ 9–12 for ADK versus 17–22 for AUK, as well as (Gd/Yb)n with > 1.3 versus ~ 1, respectively. The interdigitation of compositionally different flow units, limited extent of SRG volcanic rocks and facies architecture with the prevalence of small volume flows argue for a relatively small, heterogeneous mantle plume during the incipient stage of the evolution of the Archean Abitibi belt. Assuming that the scale of heterogeneities is comparable to the field expression of compositional changes and stratigraphy, it can be suggested that geochemical plume 'layering' is on 10s to 100s of m-scale. The evolution of this Archean mantle plume from inception to demise compares favorably with the Yellowstone hotspot which is assumed to have developed over 17 m.y. and had a diameter of about 300 km.

The Greek island of Nisyros is the source of tephra found in distal settings throughout the Aegean. In this contribution, we define geochemical fingerprints for the Lower and Upper Pumice eruptions of Nisyros based on micron-beam major (EMPA) and trace (LA-ICP-MS) element data. These two eruptions are compositionally distinct from each other. The high phenocryst content of proximal Lower and Upper Pumice tephra means that the differences between the two magmas are masked in whole-rock analyses. We demonstrate that all previous reports of distal marine and continental tephras correspond to the younger Upper Pumice not the older Lower Pumice. This has important implications for the marine tephrochronology of the Mediterranean and for the age of the Upper and Lower Pumice eruptions. We suggest an age of at least 47ka for the Upper Pumice and an older, unconstrained age for the Lower Pumice.

Subaqueous ponded lavas have been recognized within the structure of the New Senator caldera in the Blake River Group of the Abitibi greenstone belt, Quebec, Canada. Sub-vertical to near-vertical dips of extrusive volcanic facies within the southern sector of the caldera permit the identification of the internal architecture of an Archean subaqueous volcanic complex. Detailed facies analyses of the dominantly mafic sequences have shown two localities with atypical primary volcanic structures and facies inconsistent with those observed at modern subaqueous mafic seamounts. Combined with the use of facies-specific geochemical analyses, a coherent model for the two localities accounting for all observed volcanic structures and features has been developed.Mapping completed at a scale of 1:100 at Localities 1 and 2 reveals a series of sub-parallel to parallel hyaloclastite-rich horizons separating aphanitic to medium grained ponded units. Several primary volcanic structures are observed within the hyaloclastite horizons and are identified as pillowed forms, cigar forms and v-shaped structures. Ponded units range in thickness from 2.5 to 5 m, with the exception of one unit which is approximately 30 m thick. In central portions, this thick unit is medium-grained and has a sub-ophitic texture. Units at Locality 1 strike approximately NW, while those at Locality 2 strike ENE. Several families of mafic dykes cross-cut Locality 1 and trend E-W and NNE. Both volcaniclastic and effusive facies have similar compositions, affinities and trace and rare earth element signatures. All fades have a tholeiitic affinity, range in composition from basalt to andesite and have MORB-type trace and rare earth element signatures. Locality 1 appears slightly more evolved than Locality 2, with later dykes and a late sill being the most evolved facies.Volcanic facies at Localities 1 and 2 are consistent with subaqueous ponded lavas. Bounding synvolcanic structures and volcanic facies validate this interpretation and indicate these ponded lavas are hosted within a large synvolcanic depression at the summit of a mafic shield complex. Primary volcanic structures within hyaloclastite horizons formed via in situ fragmentation when water entered the system as ponded lavas began to cool. Water entered the system through synvolcanic fractures and encountered semi-molten pockets of lava, leading to hydroclastic fragmentation of the lavas. V-shaped and cigar structures formed during more energetic events and changed laterally into pillowed forms as energy dissipated from the system. (C) 2012 Elsevier B.V. All rights reserved.

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 degrees-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 degrees-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. (C) 2012 Elsevier B.V. All rights reserved.

Neoproterozoic volcanic and sedimentary rocks compose the 750 m thick Koivib Mountains succession, which forms part of the Rosh Pinah Formation of southwest Namibia. Detailed sedimentary facies analysis and physical volcanology enabled determination of four lithofacies and component twelve facies that record subaqueous deposition in a tectonically controlled shallow sea. The mafic volcanic lithofacies is composed of mafic: (1) flow, (2) volcaniclastic, and (3) intrusive facies characteristic of seamount building by effusive volcanic processes. The felsic volcanic lithofacies containing felsic: (4) flow and (5) volcaniclastic facies displays features characteristic of proximal to distal and marginal regions of subaqueous dome-flow complexes. The siliciclastic lithofacies composed of (6) shale, (7) thin- to medium-bedded sandstone, (8) thick-bedded sandstone, and (9) sedimentary breccia facies is consistent with a submarine fan environment in which subaqueous density current and turbidity flow processes were predominant. The silicilastic-carbonate lithofacies, which contains (10) massive bedded carbonate, (11) laminated to cross-bedded calcarenite, and (12) graded bedded calcarenite, formed mainly through erosion of primary carbonate and transport of material from a platform to a slope setting. The overall stratigraphy, combined with geochemical results indicating bimodal volcanism in a within-plate tectonic setting, signifies that the Koivib Mountains succession developed as part of a continental rift sequence associated with Neoproterozoic break-up of the Rodinia supercontinent. The structural geology of the Koivib Mountains revealed two generations of folds, including (i) NW trending tight, isoclinal folds (F1)) and (ii) NNW trending open folds (F2). The second deformation event refolded F-1 folds, producing periclinal structures. The deformation features are consistent with SE transpression associated with the continental collision that occurred during closure of the Adamastor Ocean and formation of Gondwana. (C) 2011 Elsevier B.V. All rights reserved.

A new analytical method using micro-X-ray fluorescence (XRF) is presented for the in situ analysis of major elements in rock samples. This approach has allowed for a separate study of hydrothermal alteration of matrix versus fragments in volcaniclastic material (i.e. flow breccia). This is particularly important for volcanogenic massive sulfide (VMS) exploration in subaqueous felsic dome-flow complexes, where brecciated facies are omnipresent and the imprint of hydrothermal alteration is typically heterogeneous. In this study, eleven elements are measured with a 1.7 by 1.3mm window considered to be representative of each sample, based on replicate analyses. An average is calculated for the analyzed window and yields a nearly complete analysis with the exception of loss of ignition (LOI). Micro-XRF data were validated using whole rock XRF analyses performed on the same sample block. The application of this chemical method has been tested successfully on thin sections from the Cap d'Ours section of the Glenwood rhyolite in the Rouyn-Noranda region of Québec, Canada. With 58 samples spaced at approximately 50m intervals, two styles of alteration zoning were recognized: (1) a lateral and concordant zoning expressed by vent-proximal silicification in the west grading toward vent-distal chlorite–sericite alteration to the east, and (2) vertical and discordant zoning expressed by stronger sericitization in the upper part of later volcanic quartz- and feldspar-phyric endogenous lobes. The former is typical of cooling induced by seawater interaction at the lava–water interface at temperatures greater than 400°C, whereas the latter is related to lower temperature (<300°C) hydrothermal mineralization associated with endogenous lobe emplacement within the volcanic pile. The presented results clearly demonstrate the potential use of the micro-XRF data for characterizing weak to intense hydrothermal alteration in highly fragmented volcanic rocks.

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.

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.

Ancient and modern subaqueous calderas form in deep- to shallow-marine oceanic settings and are primary sites for volcanogenic massive sulphide (VMS) deposits. Caldera structures hosting VMS deposits are concentrated in the Archean Abitibi greenstone belt and in the Wabigoon subprovince. The Hunter Mine and Normetal calderas, two little-known effusive-dominated edifices, are emphasised and Sturgeon Lake is the best known explosive-dominated caldera. Although VMS deposits are linked to calderas, their 5-30 km diameter size, poses problems in pin-pointing highly favourable sites for VMS exploration. The overall caldera geometry is readily recognised based on regional-scale mapping but detailed volcanic facies mapping is required to distinguish caldera subsettings in the Archean. Caldera subenvironments include: (1) the caldera wall featuring (a) chaotic breccias, (b) dyke intrusions, (c) synvolcanic faults and (d) pyroclastic debris; (2) the intracaldera moat or floor with (a) horst and graben structures (synvolcanic faults), (b) superposed dome-flow-hyaloclastite complexes, (c) extensive central dyke swarm and (d) small explosive volcanic edifices formed by magmatic fountaining eruptions and (3) caldera apron deposits showing (a) extensive volcaniclastic debris of pyroclastic and autoclastic origin and (b) local dome-flow-hyaloclastite complexes. All loci are possible sites for VMS formation, but the caldera wall favours large VMS deposits.Based on the studied calderas, a new hydrothermal alteration model is proposed that helps explain chert-Fe carbonate assemblages in Archean volcanic sequences. The early hydrothermal silica seals the volcaniclastic-dominated rocks (cap rock) at or near the edifice-seawater interface. These silicified volcaniclastic turbidite deposits have generally been referred to as chert or exhalites but this interpretation necessitates reconsideration. Overprinting the silicification phase is a pervasive semi-conformable carbonate hydrothermal alteration zone with a discordant focussed root zone along faults. Three distinct carbonate pairings are observed: (1) proximal siderite (side-roplesite) - Fe-ankerite next to the VMS-deposit, (2) an intermediate ankerite-Fe-dolomite zone and (3) a distal calcite-dolomite zone. Transitions between zones are subtle and changes are indicated mineral assemblage overlaps. Our results shed new light on hydrothermal alteration patterns, but also resolve some of the problems associated with chert-iron carbonate formations. The chert and hydrothermal carbonates as well as the VMS deposits are of the replacement type, rather than chemical precipitants and black smoker deposits.

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) basin margin porphyry stocks; (4) coarse clastic facies; and (5) locally interstratified tholeiitic, calc‐alkaline, and ultrapotassic volcanic rocks, can be compared with several examples from other subprovinces within the Superior craton. The basal unconformities preserved along basin margins are time transgressive throughout the Superior Province, as indicated by a successive age decrease in molasse successions from north to south. These hiatuses mark a significant change in tectonic regime from compression, during which deepwater turbidite and pillowed flow deposits were uplifted, to extension and transcurrent (strike‐slip) movement, during which the molasse basins formed. This major change in deformation was possibly associated with the development of a forebulge during oblique convergence and accretion of multiple oceanic arcs.

The 20‐ to 1000‐m‐thick >2.8 Ga Bell Lake group is a siliciclastic‐dominated succession in the Slave craton that contains features consistent with continental rifting. The Archean succession is composed of four principal lithofacies: (1) conglomerate, (2) quartz arenite, (3) sandstone‐argillite, and (4) argillite‐sandstone. An unconformity with 2.8–4.0 Ga basement indicates an autochthonous craton‐cover relationship, and pillowed flows in depositional contact with the argillite‐sandstone lithofacies reflect contemporaneous sedimentation and volcanism. The basement, composed of layered intrusions, granitic plutons, and gneissic rocks, served as the source for the quartz arenite with detrital chromite. Mesoarchean Slave stratigraphic sections contain fining‐upward (FU) and coarsening‐upward (CU) sequences, with the former predominant at craton‐cover contacts. The CU sequences are considered to be the results of rifting that produced a coastline featuring estuaries and embayments affected by tides and sporadic terrestrial floods. Successions from the Superior, Dharwar, and Zimbabwe cratons compare favorably with those of the Slave craton, wherein all examples contain quartz arenite, quartz pebble conglomerate, and basaltic‐komatiitic volcanic rocks. The Slave craton, interpreted as a rifted continental margin, underwent significant crustal attenuation, which facilitated the ascent of mantle‐derived magma. Modern analogues include a passive margin or shelf adjacent to an orogenic collision zone. The Bell Lake group and other documented craton‐cover successions exhibit a recurring theme of early quartz‐rich sedimentation and subsequent basaltic‐komatiitic volcanism. This is consistent with continental breakup successions that are commonly induced by mantle plumes.

The >2.8 Ga siliciclastic-dominated, craton-cover succession with subordinate mafic volcanic rocks at Beniah Lake in the Slave craton has been informally referred to as the Bell Lake group. The Beniah Lake succession, up to 1 km thick, is composed of 7–120 m thick coarsening-upward (CU) sequences arranged with the following lithofacies architecture: (1a) argillite–sandstone, (1b) mafic volcanic lithofacies, (2) sandstone–argillite, (3a) quartz arenite and (3b) conglomerate lithofacies. A pan-Slave unconformity, indicating an autochthonous 2.8 to 4.013 Ga basement–cover relationship is present and locally well exposed. The lithofacies and their arrangement are consistent with a tide-influenced coastal setting, whereby the change from argillite–sandstone to quartz arenite lithofacies represents an initial estuary system, defining transgression with a maximum flooding surface, evolving into a fan-delta, defining progradation with a maximum regressive surface. The mafic volcaniclastic lithofacies and associated pillowed flow units, suggesting drowning of the siliciclastic succession, coincide with a change from rift to drift tectonics, as crustal attenuation facilitated magma ascent. The Slave-wide unconformities and lithofacies contacts permit a sequence stratigraphic approach, with the major Slave basement–quartz arenite unconformity suggestive of a 1st order sea level change coinciding with the proposed (super) continent break-up or rifting. The consistent stacking of CU-sequences with capping subaqueous volcaniclastic-volcanic units are considered the expression of 2nd order sea level cycles probably caused by changes of spreading rates at oceanic ridges. The CU-sequences and their thicknesses constitute 3rd order sea level changes and reflect the immediate response to crustal thinning. Time constraints for all these orders remain elusive but the hierarchy is consistent with sequence stratigraphic events. The observed relative sea level change is attributed primarily to tectono-eustasy where mantle dynamics caused rifting, basin subsidence and mafic volcanism. The latter occurred as the crust was sufficiently attenuated.