The lower Devonian Kowmung volcaniclastics: A deep‐water succession of mass‐flow origin,northeastern Lachlan Fold Belt,N.S.W.

The Lower to ?Middle Devonian Kowmung Volcaniclastics form the upper part of a succession of Upper Siluran to mid‐Devonian flyschoid rocks in the Yerranderie area of N.S.W., and contain two major facies associations. (1) A mudstone facies association represents the ambient, background sedimentation, comprising predominantly buff mudstone that is host to an assemblage of coarser‐grained sediments, including graded‐bedded to massive siltstone, sandstone, conglomerate, allodapic limestone, and large allochthonous limestone blocks and associated limestone breccia. Bouma sequences are common, sole structures occur and maximum bed thickness is about 3 m. (2) A volcaniclastic facies association intrudes and interrupts the accumulation of the ambient mudstone facies association, and contains massive to partly graded, quartzofeldspathic siltstone, sandstone, breccia and conglomerate. Sedimentation units in the volcaniclastic facies association are up to 120 m thick. The two facies associations interfinger. Stratigraphically, the base of the Kowmung Volcaniclastics is taken as the first sedimentation unit of the volcaniclastic facies association. The mudstone facies association below this level is part of the Siluro‐Devonian Taralga Group.

Both facies associations were deposited in relatively deep‐water. The dominant transport process in both associations was mass‐flow, involving granular mass‐flows (turbidity currents, grain flows), debris flows and avalanches. Massive mudstone is hemipelagic in origin. The volcaniclastic facies association probably represents a submarine volcanic apron around the emergent, volcanic Bindook Complex. Grossly, the succession coarsens upwards, and there is evidence of several sources of sediment, rather than a single point at the head of a submarine fan.

Provenance is diverse. In the mudstone facies association, framework grains in sandstone are microlitic volcanic‐rock fragments with a mafic to intermediate volcanic source. Clasts in conglomerate and breccia are consistent with derivation from the regionally extensive, quartzose Ordovician flyschoid successions. Clasts of ?penecontemporaneous limestone also occur. The volcaniclastic facies association was probably derived largely from the nearby, coeval Bindook Complex, which consists of silicic ash‐flow and ash‐fall tuff, lava, associated sediment and granitoids. Detritus was either derived directly from volcanic eruptions or was worked in fringing littoral and fluvial environments prior to redeposition by mass‐flow. Quartzite boulders mixed with volcanic clasts in the conglomerate suggest that Ordovician quartzarenite was also exposed around the volcanic complex. Tentative provenance correlations have been made between the different rock units in the Kowmung Volcaniclastics and their possible sources in the northern part of the Bindook Complex.     

Stratigraphy and volcanology of the Türkbükü volcanics: products of a stratovolcano in the Bodrum Peninsula,SW Anatolia

The Middle‐Upper Miocene Bodrum magmatic complex of the Aegean region, southwestern Turkey, is mainly represented by intermediate stocks, lavas, pyroclastic and volcaniclastic deposits. Monzonitic stocks and connected porphyry intrusions and extrusions are the first products of the magmatism. These are followed by a volcanic succession consisting of andesitic‐latitic lavas, autobrecciated lavas, pyroclastic and volcaniclastic deposits. The final stage is represented by basaltic and basaltic andesitic flows and dykes intruded into previous units. The volcanic succession crops out in the northern part of the Bodrum peninsula. In the lower part of this succession are widespread pyroclastic deposits, composed of pyroclastic fall and flow units, alternating with epiclastic deposits. Grain size, volume and thickness of the pyroclastic deposits were mainly controlled by the type, magnitude and intensity of the eruption. Further up the section, there are two horizons of debris avalanche deposits forming the coarsest and thickest deposits of the volcaniclastic succession. The debris avalanche deposits indicate at least two different flank collapses coeval with the volcanism. The stratigraphy and map pattern of these volcanic units imply that the northern part of the Bodrum peninsula was the north‐facing flank of a stratovolcano during the mid‐Late Miocene. Copyright © 2006 John Wiley & Sons, Ltd.     

Facies in a Devonian‐Carboniferous volcanic fore‐arc succession,Campwyn Volcanics,Mackay district,central Queensland∗

The Middle Devonian to Early Carboniferous Campwyn Volcanics of coastal central Queensland form part of the fore‐arc basin and eastern flank of the volcanic arc of the northern New England Fold Belt. They consist of a complex association of pyroclastic, hyaloclastic and resedimented, texturally immature volcaniclastic facies associated with shallow intrusions, lavas and minor limestone, non‐volcanic siliciclastics and ignimbrite. Primary igneous rocks indicate a predominantly mafic‐intermediate parentage. Mafic to intermediate pyroclastic rocks within the unit formed from both subaerial and ?submarine to emergent strombolian and phreatomagmatic eruptions. Quench‐fragmented hyaloclastite breccias are widespread and abundant. Shallow marine conditions for much of the succession are indicated by fossil assemblages and intercalated limestone and epiclastic sandstone and conglomerate facies. Volcanism and associated intrusions were widely dispersed in the Campwyn depositional basin in both space and time. The minor component of silicic volcanic products is thought to have been less proximal and derived from eruptive centres to the west, inboard of the basin.     

Volcanic evolution of a long-lived Ordovician island-arc province in the Parkes region of the Lachlan Fold Belt,southeastern Australia

The Ordovician mafic volcanic rocks in the Parkes region of New South Wales occur as three distinct packages of volcaniclastic and coherent volcanic rocks and minor limestone that formed part of an oceanic island arc succession. The oldest package is the Early Ordovician Nelungaloo Volcanics and overlying Yarrimbah Formation. These formations consist of volcanic siltstone, sandstone, polymictic breccia, conglomerate facies interpreted as moderately deep-water turbidites and coarser grained debris-flow deposits emplaced in the medial to distal part of a subaqueous volcaniclastic apron flanking an active volcanic centre(s). Broadly conformable massive to brecciated andesites in the apron deposits are interpreted as synsedimentary sills and/or lava flows. A hiatus in volcanism occurred between the Bendigonian and early Darriwilian (ca 476 – 466 Ma). Deposition of the second package, which produced the Middle to Late Ordovician Goonumbla Volcanics, Billabong Creek Limestone and Gunningbland Formation, commenced with shallow-water limestones and minor volcaniclastic rocks. During an approximately 15 million years period, a thick sequence of bedded volcanic sandstone, limestone and minor siltstone and volcanic breccia were deposited in very shallow to moderate water depths. The top of this package is marked by thick volcanic conglomerate and sandstone mass-flow deposits and approximately coeval basaltic andesite lavas and sills sourced from a nearby volcano. The upper age limit of this package is constrained as approximately 450 Ma by Ea3/4 fossils and monzodiorite that intrudes the Goonumbla Volcanics. The lower limit of the third package, which constitutes the Wombin Volcanics, is poorly constrained and the duration of the hiatus that separates the Goonumbla and Wombin Volcanics is unknown but may be as long as 10 million years. The Wombin Volcanics record development of a thick, proximal volcaniclastic apron flanking a compositionally more evolved volcanic edifice in the immediate Parkes area. Thick crystal-rich turbiditic sandstones of mafic provenance are intercalated with polymictic volcanic breccias and megablock breccias that are interpreted as proximal subaqueous debris-flow and debris-avalanche deposits, respectively. The sequence also includes numerous trachyandesite bodies, many of which were emplaced within the volcaniclastic apron as synsedimentary sills. No evidence was found at Parkes to support the existence of a previously proposed 22 km diameter collapse caldera and the source volcanoes for the Ordovician are envisaged as complex stratovolcanoes.     

Late Palaeozoic arc flank and fore‐arc basin sequence of the New England fold belt in the stanage bay region,central Queensland

Devonian and Carboniferous (Yarrol terrane) rocks, Early Permian strata, and Permian‐(?)Triassic plutons outcrop in the Stanage Bay region of the northern New England Fold Belt. The Early‐(?)Middle Devonian Mt Holly Formation consists mainly of coarse volcaniclastic rocks of intermediate‐silicic provenance, and mafic, intermediate and silicic volcanics. Limestone is abundant in the Duke Island, along with a significant component of quartz sandstone on Hunter Island. Most Carboniferous rocks can be placed in two units, the late Tournaisian‐Namurian Campwyn Volcanics, composed of coarse volcaniclastic sedimentary rocks, silicic ash flow tuff and widespread oolitic limestone, and the conformably overlying Neerkol Formation dominated by volcaniclastic sandstone and siltstone with uncommon pebble conglomerate and scattered silicic ash fall tuff. Strata of uncertain stratigraphic affinity are mapped as ‘undifferentiated Carboniferous’. The Early Permian Youlambie Conglomerate unconformably overlies Carboniferous rocks. It consists of mudstone, sandstone and conglomerate, the last containing clasts of Carboniferous sedimentary rocks, diverse volcanics and rare granitic rocks. Intrusive bodies include the altered and variably strained Tynemouth Diorite of possible Devonian age, and a quartz monzonite mass of likely Late Permian or Triassic age.

The rocks of the Yarrol terrane accumulated in shallow (Mt Holly, Campwyn) and deeper (Neerkol) marine conditions proximal to an active magmatic arc which was probably of continental margin type. The Youlambie Conglomerate was deposited unconformably above the Yarrol terrane in a rift basin. Late Permian regional deformation, which involved east‐west horizontal shortening achieved by folding, cleavage formation and east‐over‐west thrusting, increases in intensity towards the east.     

The Cerro Morado Andesites: Volcanic history and eruptive styles of a mafic volcanic field from northern Puna, Argentina

The Upper Miocene Cerro Morado Andesites constitutes a mafic volcanic field (100 km²) composed of andesite to basaltic andesite rocks that crop out 75 km to the east from the current arc, in the northern Puna of Argentina. The volcanic field comprises lavas and scoria cones resulting from three different eruptive phases developed without long interruptions between each other. Lavas and pyroclastic rocks are thought to be sourced from the same vents, located where orogen-parallel north-south faults crosscut transverse structures.The first eruptive phase involved the effusion of extensive andesitic flows, and minor Hawaiian-style fountaining which formed subordinate clastogenic lavas. The second phase represents the eruption of slightly less evolved andesite lavas and pyroclastic deposits, only distributed to the north and central sectors of the volcanic field. The third phase represents the discharge of basaltic andesite magmas which occurred as both pyroclastic eruptions and lava effusion from scattered vents distributed throughout the entire volcanic field. The interpreted facies model for scoria cones fits well with products of typical Strombolian-type activity, with minor fountaining episodes to the final stages of eruptions.Petrographic and chemical features suggest that the andesitic units (SiO₂ > 57%) evolved by crystal fractionation. In contrast, characteristics of basaltic andesite rocks are inconsistent with residence in upper-crustal chambers, suggesting that batches of magmas with different origins or evolutive histories arrived at the surface and erupted coevally.Based on the eruptive styles and lack of volcanic quiescence gaps between eruptions, the Cerro Morado Andesites can be classified as a mafic volcanic field constructed from the concurrent activity of several small, probably short-lived, monogenetic centers.     

The ordovician (caradoc) volcanic rocks of montgomery,Powys, N. Wales

The Ordovician (Caradoc, Soudleyan) rocks of Montgomery, Powys are shales interbedded with locally conglomeratic volcaniclastic sediments composed of andesitic detritus. New formal lithostratigraphic units are proposed: Montgomery Volcanic Group comprising in ascending order: Castle Hill Shale Formation, Castle Hill Conglomerate Formation and Quarry Sandstone and Shale Formation. The volcaniclastic strata are reinterpreted as deposits of a submarine volcaniclastic fan system sourced by contemporaneous andesitic island volcanism. The observed diagenetic sequence is typical of marine volcanic sandstones and was dominated by hydration reactions related to the degradation of abundant unstable volcanic detritus. Diagenesis has resulted in the virtual destruction of original porosity in the volcaniclastic rocks.     

Shallow‐water microbialite‐volcaniclastic facies association in the Cambro‐Ordovician Mt Windsor Subprovince,Australia

The Trooper Creek Formation is a mineralised submarine volcano‐sedimentary sequence in the Cambro‐Ordovician Seventy Mile Range Group, Queensland. Most of the Trooper Creek Formation accumulated in a below‐storm‐wave‐base setting. However, microbialites and fossiliferous quartz‐hematite ± magnetite lenses provide evidence for local shoaling to above fairweather wave‐base (typically 5–15 m). The microbialites comprise biogenic (oncolites, stromatolites) and volcanogenic (pumice, shards, crystal fragments) components. Microstructural elements of the bioherms and biostromes include upwardly branching stromatolites, which suggest that photosynthetic microorganisms were important in constructing the microbialites. Because the microbialites are restricted to a thin stratigraphic interval in the Trooper Creek area, shallow‐water environments are interpreted to have been spatially and temporarily restricted. The circumstances that led to local shoaling are recorded by the enclosing volcanic and sedimentary lithofacies. The microbialites are hosted by felsic syneruptive pumiceous turbidites and water‐settled fall deposits generated by explosive eruptions. The microbialite host rocks overlie a thick association (≤?300 m) of andesitic lithofacies that includes four main facies: coherent andesite and associated autoclastic breccia and peperite; graded andesitic scoria breccia (scoriaceous sediment gravity‐flow deposits); fluidal clast‐rich andesitic breccia (water‐settled fall and sediment gravity‐flow deposits); and cross‐stratified andesitic sandstone and breccia (traction‐current deposits). The latter three facies consist of poorly vesicular blocky fragments, scoriaceous clasts (10–90%), and up to 10% fluidally shaped clasts. The fluidal clasts are interpreted as volcanic bombs. Clast shapes and textures in the andesitic volcaniclastic facies association imply that fragmentation occurred through a combination of fire fountaining and Strombolian activity, and a large proportion of the pyroclasts disintegrated due to quenching and impacts. Rapid syneruptive, near‐vent aggradation of bombs, scoria, and quench‐fragmented clasts probably led to temporary shoaling, so that subsequent felsic volcaniclastic facies and microbialites were deposited in shallow water. When subsidence outpaced aggradation, the depositional setting at Trooper Creek returned to being relatively deep marine.     

Sedimentary processes caused by felsic caldera-forming volcanism in the Late Miocene to Early Pliocene intra-arc Aizu basin, NE Japan arc

Six large Late Miocene to Quaternary calderas, > 10 km in diameter, cluster together with several medium to small calderas and stratovolcanoes in a 60 × 30 km area of the Aizu volcanic field, southern NE Japan arc. These caldera volcanoes were built on a WNW–ESE trending highland coincident with a local uplifted swell since Late Miocene. The flare-up of felsic volcanism occurred synchronously along the NE Japan arc. Pyroclastic flow sheets from the calderas spread over the surrounding intra-arc basins and are interstratified with various sediments. Geochronological data indicates that the large-caldera eruptions have occurred six times since 8 Ma, at intervals of 1 to 2 million years. Late Miocene to Early Pliocene extra-caldera successions in the basin consist of nine sedimentary facies associations: (1) primary pyroclastics, (2) lahars, (3) gravelly fluvial channels, (4) sandy fluvial channels, (5) floodplains, (6) tidal flats, (7) delta fronts, (8) pro-delta slopes, and (9) pro-delta turbidites. The distribution of facies associations show westward prograding of volcaniclastic aprons, made up of braid delta, braidplain, pyroclastic flow sheet, and incised braided river deposits. The extra-caldera successions record: 1) an increase in felsic volcanism with an associated high rate of volcaniclastic sediment supply at about 10 Ma, prior to catastrophic caldera-forming eruptions; and 2) progradation of volcaniclastic aprons toward the back-arc side in response to the succeeding caldera-forming eruptions and sea-level changes, until about 3 Ma.     

Contrasting compositional trends of rocks and olivine-hosted melt inclusions from Cerro Negro volcano (Central America): implications for decompression-driven fractionation of hydrous magmas

Melt inclusions in olivine Fo_83–72 from tephras of 1867, 1971 and 1992 eruptions of Cerro Negro volcano represent a series of basaltic to andesitic melts of narrow range of MgO (5.6–8 wt %) formed by ~46 wt % fractional crystallization of olivine (~6 wt %), plagioclase (~27 wt %), pyroxene (~13 wt %) and magnetite (<1 wt %) from primitive basaltic melt (average SiO₂ = 49 wt %, MgO = 7.6 wt %, H₂O = 6 wt %) as it ascended to the surface from the depth of about 14 km. The crystallization occurred at increasing liquidus temperature from 1,050 to 1,090 °C in the pressure range from 400 to 50 MPa and was induced by release of mixed H₂O–CO₂ fluid from the melt at decreasing pressure. Matrix glass compositions fall at the high-Si end of the melt inclusion trend and represent the final stage of melt crystallization during and after eruption. The bulk compositions of erupted Cerro Negro magmas (tephras and lavas) range from high- to low-MgO (3–10 wt %) basalts, which form a compositional array crossing the trend of melt inclusions so that virtually no rock from Cerro Negro has composition akin to true melt represented by the inclusions. The variations of the bulk magma (rocks) and melt (melt inclusions) compositions can be generated in a dyke connecting a deep primitive magma reservoir with the Cerro Negro edifice. While the melt inclusions represent the compositional trend of instantaneous melts along the magma pathway at decreasing pressure and H₂O content, occurrence of low-Mg to high-Mg basalts reflects the process of phenocryst re-distribution in progressively evolving melt. The crystallization scenario is anticipated to operate everywhere in dykes feeding basaltic volcanoes and can explain the predominance of plagioclase-rich high-Al basalts in island arc as well as typical compositional variations of magmas during single eruptions.     

The Cerro Rajón Formation—a new lithostratigraphic unit proposed for a Cambrian (Terreneuvian) volcano-sedimentary succession from the Caborca region,northwest Mexico

A new mappable rock unit, the Cerro Rajón Formation, is proposed for the Cambrian succession of the Caborca region, Sonora, México. Formerly Unit 1 of the Puerto Blanco Formation, the Cerro Rajón Formation is interpreted as a volcano-sedimentary succession deposited along the coast of a passive margin that was impacted by rift-related volcanism. At its proposed type locality, in Cerro Rajón, the Cerro Rajón Formation consists of 270–285 m of tuffaceous conglomerate, metabasalt, mafic tuff, mafic lapillistone, mafic agglomerate, and quartzite with minor siltstone, limestone, and dolostone- and quartzite-dominated conglomerate. The unit contains a major disconformity near its base, where m-to dm-thick conglomerate locally replaces the fine-grained clastics that make up the base of the Cerro Rajón Formation. δ¹³C chemostratigraphy and biostratigraphy of the Rajón and its bounding strata limits Rajón deposition to the Fortunian Stage of the Terreneuvian Series. Volcanic rocks in the Cerro Rajón Formation are represented by mafic to ultramafic flows, including picrobasalts and metabasalts with hydrothermal alteration characteristics, evidenced by replacement of clinopyroxenes by chlorite, actinolite, and epidote. The mineral paragenesis of these volcanic rocks suggests the succession experienced greenschist grade metamorphism. Basalt geochemistry is consistent with low silica (34.32–48.21%) magmatism with high TiO₂ concentrations (3.63–7.52%), related to continental rift volcanism with OIB characteristics. This volcanism could represent the last southern evidence of the rifting process that occurred along the western margin of Laurentia or could be related to volcanic rift deposits further afield.     

Volcaniclastic resedimentation in distal fluvial basins induced by large-volume explosive volcanism: the Ebisutoge–Fukuda tephra, Plio-Pleistocene boundary, central Japan

The Ebisutoge–Fukuda tephra (Plio‐Pleistocene boundary, central Japan) has a well‐recorded eruptive style, history, magnitude and resedimentation styles, despite the absence of a correlative volcanic edifice. This tephra was ejected by an extremely large‐magnitude and complex volcanic eruption producing more than 400 km³ total volume of volcanic materials (volcanic explosivity index=7), which extended more than 300 km away from the probable eruption centre. Remobilization of these ejecta occurred progressively after the completion of a series of eruptions, resulting in thick resedimented volcaniclastic deposits in spatially separated fluvial basins, more than 100 km from the source. Facies analysis of resedimented volcaniclastic deposits was carried out in distal fluvial basins. The distal tephra (≈100–300 km from the source) comprises two different lithofacies, primary pyroclastic‐fall deposits and reworked volcaniclastic deposits. The resedimented volcaniclastic succession shows five distinct sedimentary facies, interpreted as debris‐flow deposits (facies A), hyperconcentrated flow deposits (facies B), channel‐fill deposits (facies C), floodplain deposits with abundant flood‐flow deposits (facies D) and floodplain deposits with rare flood deposits (facies E). Resedimented volcaniclastic materials at distal locations originated from unconsolidated deposits of a climactic, large ignimbrite‐forming eruption. Factors controlling inter‐ and intrabasinal facies changes are (1) temporal change of introduced volcaniclastic materials into the basin; (2) proximal–distal relationship; and (3) distribution pattern of pyroclastic‐flow deposits relative to drainage basins. Thus, studies of the Ebisutoge–Fukuda tephra have led to a depositional model of volcaniclastic resedimentation in distal areas after extremely large‐magnitude eruptions, an aspect of volcaniclastic deposits that has often been ignored or poorly understood.     

Discussion: History of coastal dunes at triangle Cliff,Fraser Island,Queensland

The Drummond Basin represents a major, backarc extensional system located at the inboard margin of the northern New England Orogen. Its synrift (cycle 1) infill is distinctively volcanic and volcani‐clastic in character and displays complex facies relationships and considerable variations in thickness controlled by the history and fabric of extensional faulting and the distribution of coeval volcanic centres. Subtle inheritance signatures in the age spectra obtained by SHRIMP (II) Pb‐U dating of zircons from volcanic units have impeded age assignment. New geochronologic data indicate that basinal subsidence was initiated in the north in latest Devonian (Famennian) time but was delayed until the Early Carboniferous (Tournaisian) in the south. Northern successions are dominated by volcaniclastic strata that accumulated distal to the loci of contemporary volcanism, whereas southern successions are dominated by silicic flows and ash‐flow tuffs and associated hypabyssal intrusive suites proximal to, or coincident with, volcanic loci. The Burdekin, Clarke River and Bundock Creek Basins located north of the Drummond Basin are broadly coeval features with comparable Infill. They likewise represent backarc basins developed inboard of the northern New England Orogen which trends offshore at latitude 20°S and appears to be represented in basement cores recovered from the Coral Sea. Calc‐alkaline magmatism of Late Devonian‐Early Carboniferous age extended at least 400 km inboard of the Gondwanan plate margin now represented in Queensland and related to an acute angle of subduction along the active margin at that time.     

40Ar/39Ar geochronology and palynology of the Cerro Negro Formation,South Shetland Islands,Antarctica: A new radiometric tie for Cretaceous terrestrial biostratigraphy in the Southern Hemisphere

The upper part of the Jurassic‐Cretaceous Byers Group, exposed on Byers Peninsula in the South Shetland Islands, Antarctica, consists of 1.4 km of non‐marine strata assigned to the Cerro Negro Formation. Silicic pyroclastic units close to the base of the formation have yielded new ⁴⁰Ar/³⁹Ar ages of 120.3 ± 2.2 Ma on plagioclase from one horizon, and 119.4 ± 0.6 and 119.1 ± 0.8 Ma on biotite and plagioclase from a second horizon. Plagioclase from a welded ignimbrite close to the topmost exposed part of the formation has given an ⁴⁰Ar/³⁹Ar age of 119 ± 3.0 Ma. These ages indicate that the Cerro Negro Formation was deposited during a relatively short period in early Aptian times. The identification of palynomorph taxa has enabled us to propose correlations for the Cerro Negro Formation with spore/pollen zonations of South America and Australia. The presence of Interulobites pseudoreticulatus, Appendicisporites and F. wonthaggiensis in the Cerro Negro Formation supports correlation with the Interulobites‐Foraminisporis and the lower part of the tectifera‐corrugatus zones in southern South America. The presence of Foraminisporis asymmetricus and other palynomorphs suggests correlation with the Cyclosporites hughesii Interval Zone of early to late Aptian age in Australia. These data represent a valuable addition to the few radiometric ties available for Mesozoic terrestrial palynostratigraphy in the Southern Hemisphere.     

Indian Continental Crust Recovered from Elan Bank, Kerguelen Plateau (ODP Leg 183, Site 1137)

At Site 1137 on Elan Bank of the Kerguelen Plateau, a largeigneous province in the southern Indian Ocean, a fluvial, volcaniclastic,polymict conglomerate and a fluvial sandstone are intercalatedwith subaerially erupted tholeiitic basalt flows. Clasts recoveredfrom the conglomerate have highly variable lithologies, includingalkali basalt, rhyolite, trachyte, granitoid and gneiss. Majorand trace element abundances and whole-rock isotopic data forthe sandstones, the conglomerate matrix and representative clastsfrom the conglomerate are used to infer the origin of thesediverse rock types. The gneiss clasts show an affinity to crustalrocks from India, particularly those of the Eastern Ghats Beltand its possible East Antarctic corollary, the Rayner Complex.The felsic volcanic clasts are not genetically related to theintercalated basalt flows, despite being erupted contemporaneouslywith these basaltic magmas. These felsic volcanic clasts probablyformed from partial melting of evolved upper continental crust.The granitoid also probably formed by partial melting of continentalcrust and could be an intrusive equivalent of the felsic volcanicrocks. In contrast, the alkali basalt clasts have isotopic compositionsthat are more similar to those of the tholeiitic basalt flowsrecovered at Site 1137; however, these clasts are highly alkalic(tephrite to phonotephrite) and have a distinct petrogenesisfrom the tholeiitic basalt flow units. KEY WORDS: geochemistry; Indian Ocean; Kerguelen Plateau; large igneous provinces; Ocean Drilling Program     

Geochemistry,dispersal, volumes and chronology of Holocene silicic tephra layers from the Katla volcanic system,Iceland

At least 12 silicic tephra layers (SILK tephras) erupted between ca. 6600 and ca. 1675 yr BP from the Katla volcanic system, have been identified in southern Iceland. In addition to providing significant new knowledge on the Holocene volcanism of the Katla system which typically produces basaltic tephra, the SILK tephras form distinct and precise isochronous marker horizons in a climatically sensitive location close to both the atmospheric and marine polar fronts. With one exception the SILK tephras have a narrow compositional range, with SiO₂ between 63 and 67%. Geochemically they are indistinguishable from ocean transported pumice found on beaches in the North Atlantic region, although they differ significantly from the silicic component of the North Atlantic Ash Zone One (NAAZO). Volumes of airborne SILK tephra range from 0.05 to 0.3 km³. We present new isopach maps of the six largest layers and demonstrate that they originate within the Katla caldera. The apparently stable magma system conditions that produced the SILK tephras may have been established as a consequence of the eruption of the silicic component of NAAZO (ca. 10.3 ka) and disrupted by another large‐scale event, the tenth century ad Eldgjá eruption (ca. 1 ka). Despite the current long repose, silicic activity of this type may occur again in the future, presenting hitherto unknown hazards. Copyright © 2001 John Wiley & Sons, Ltd.     

Geochemical constraints on the origin of mafic and silicic magmas at Cordón El Guadal, Tatara-San Pedro Complex, central Chile

The aim of this study is to quantify the crustal differentiation processes and sources responsible for the origin of basaltic to dacitic volcanic rocks present on Cordón El Guadal in the Tatara-San Pedro Complex (TSPC). This suite is important for understanding the origin of evolved magmas in the southern Andes because it exhibits the widest compositional range of any unconformity-bound sequence of lavas in the TSPC. Major element, trace element, and Sr-isotopic data for the Guadal volcanic rocks provide evidence for complex crustal magmatic histories involving up to six differentiation mechanisms. The petrogenetic processes for andesitic and dacitic lavas containing undercooled inclusions of basaltic andesitic and andesitic magma include: (1) assimilation of garnet-bearing, possibly mafic lower continental crust by primary mantle-derived basaltic magmas; (2) fractionation of olivine + clinopyroxene + Ca-rich plagioclase + Fe-oxides in present non-modal proportions from basaltic magmas at ∼4–8 kbar to produce high-Al basalt and basaltic andesitic magmas; (3) vapor-undersaturated (i.e., P _H2O<P _TOTAL) partial melting of gabbroic crustal rocks at ∼3–7 kbar to produce dacitic magmas; (4) crystallization of plagioclase-rich phenocryst assemblages from dacitic magmas in shallow reservoirs; (5) intrusion of basaltic andesitic magmas into shallow reservoirs containing crystal-rich dacitic magmas and subsequent mixing to produce hybrid basaltic andesitic and andesitic magmas; and (6)␣formation and disaggregation of undercooled basaltic andesitic and andesitic inclusions during eruption from shallow chambers to form commingled, mafic inclusion-bearing andesitic and dacitic lavas flows. Collectively, the geochemical and petrographic features of the Guadal volcanic rocks are interpreted to reflect the development of shallow silicic reservoirs within a region characterized by high crustal temperatures due to focused basaltic activity and high magma supply rates. On the periphery of the silicic system where magma supply rates and crustal temperatures were lower, cooling and crystallization were more important than bulk crustal melting or assimilation. Received: 2 July 1997 / Accepted: 25 November 1997     

Evolution of the early devonian bindook volcanic complex,wollondilly basin,eastern lachlan fold belt

The Early Devonian Bindook Volcanic Complex consists of a thick silicic volcanic and associated sedimentary succession filling the extensional Wollondilly Basin in the northeastern Lachlan Fold Belt. The basal part of the succession (Tangerang Formation) is exposed in the central and southeastern Wollondilly Basin where it unconformably overlies Ordovician rocks or conformably overlies the Late Silurian to Early Devonian Bungonia Limestone. Six volcanic members, including three new members, are now recognised in the Tangerang Formation and three major facies have been delineated in the associated sedimentary sequence. The oldest part of the sequence near Windellama consists of a quartz turbidite facies deposited at moderate water depths together with the shallow‐marine shelf Windellama Limestone and Brooklyn Conglomerate Members deposited close to the eastern margin of the basin. Farther north the shelf facies consists of marine shale and sandstone which become progressively more tuffaceous northwards towards Marulan. The Devils Pulpit Member (new unit) is a shallow‐marine volcaniclastic unit marking the first major volcanic eruptions in the region. The overlying shallow‐marine sedimentary facies is tuffaceous in the north, contains a central Ordovician‐derived quartzose (?deltaic) facies and a predominantly mixed facies farther south. The initial volcanism occurred in an undefined area north of Marulan. A period of non‐marine exposure, erosion and later deposition of quartzose rocks marked a considerable break in volcanic activity. Volcanism recommenced with the widespread emplacement of the Kerillon Tuff Member (new unit), a thick, non‐welded rhyolitic ignimbrite followed by dacitic welded ignimbrite and air‐fall tuff produced by a large magnitude eruption leading to caldera collapse in the central part of the Bindook Volcanic Complex, together with an additional small eruptive centre near Lumley Park. The overlying Kerrawarra Dacite Member (new unit) is lava‐like in character but it also has the dimensions of an ignimbrite and covers a large part of the central Bindook Volcanic Complex. The Carne Dacite Member is interpreted as a series of subvolcanic intrusions including laccoliths, cryptodomes and sills. The Tangerang Formation is overlain by the extensive crystal‐rich Joaramin Ignimbrite (new unit) that was erupted from an undefined centre in the central or northern Bindook Volcanic Complex. The volcanic units at Wombeyan and the Kowmung Volcaniclastics in the northwestern part of the complex are probably lateral time‐equivalents of the Tangerang Formation and Joaramin Ignimbrite. All three successions pre‐date the major subaerial volcanic plateau‐forming eruptions represented by the Barrallier Ignimbrite (new unit). The latter post‐dated folding and an extensive erosional phase, and unconformably overlies many of the older units in the Bindook Volcanic Complex. This ignimbrite was probably erupted from a large caldera in the northern part of the complex and probably represents surface expressions of part of the intruding Marulan Batholith. The final volcanic episode is represented by the volcanic units at Yerranderie which formed around a crater at the northern end of the exposed Bindook Volcanic Complex.     

Late Quaternary tephra layers around Raoul and Macauley Islands,Kermadec Arc: implications for volcanic sources,explosive volcanism and tephrochronology

A suite of deep‐sea cores were collected along transects up to 100 km across the fore‐arc and back‐arc regions of the predominantly submarine Kermadec arc near Raoul and Macauley islands, southwest Pacific. The cores reveal a macroscopic tephra record extending back >50 ka. This is a significant addition to the dated record of volcanism, previously restricted to fragmented late Holocene records exposed on the two islands. The 27 macroscopic tephra layers display a wide compositional diversity in glass (～50–78 wt% SiO₂). Many tephra layers comprise silicic shards with a subordinate mafic shard population. This could arise from magma mingling and may reflect mafic triggering of the silicic eruptions. Broadly, the glass compositions can be distinguished on diverging high‐K and low‐K trends, most likely arising from different source volcanoes. This distinction is also reflected in the tephra records exposed on Raoul (low‐K) and Macauley (high‐K) islands, the likely source areas. Heterogeneous tephra comprising shards of both high‐ and low‐K affinity, silicic and mafic compositions, and more homogeneous tephra with subordinate outlier shard compositions, are best explained by post‐depositional mixing of separate eruption deposits or contemporaneous eruptions. Evidently, the slow sedimentation rates of the calcareous oozes (～10¹–10² mm ka^?1) were insufficient to adequately separate and preserve closely spaced eruption deposits. This exemplifies the difficulty in assessing eruption frequencies and magmatic trends, and erecting a tephrostratigraphy, using geochemical fingerprinting in such environments. Despite these difficulties, the ca. 5.7 ka Sandy Bay Tephra erupted from Macauley Island can be correlated over a distance of >100 km, extending east and west of the island, showing that the mostly submerged volcanoes are capable of wide tephra dispersal. Hence there is potential for developing chronostratigraphies for the southwest Pacific beyond the region covered by the extensive rhyolite marker beds from the Taupo Volcanic Zone. Copyright © 2011 John Wiley & Sons, Ltd.     

松辽盆地徐家围子断陷营城组火山岩相和火山机构分析

徐家围子断陷营城组火山岩极为发育。文章通过对火山岩的岩芯观察、薄片鉴定、岩芯测试及测井资料、二维、三维地震资料的综合分析 ,将营城组火山岩盆地分为 3大相区 ,即火山喷发区、过渡区和沉积区。火山喷发区岩性由各种熔岩、火山碎屑岩和少量砂砾岩组成 ;过渡区岩性以火山碎屑岩与沉积岩互层为特征 ,夹少量火山熔岩 ;沉积区岩性包括砂砾岩、砂岩、粉砂岩和泥岩。火山喷发区内可识别出 8种类型的火山岩相 (空落相、溢流相、基底涌流相、火山碎屑流相、火山泥石流相、火山沉积相、次火山相、隐爆角砾岩相 )和 3种类型的火山机构 (层火山、微型盾火山和渣锥火山 )。不同类型的火山机构具有不同火山作用、岩相分布特征和含油气性。因此火山岩相及火山机构分析对火山作用研究和油气勘探均有重要意义。