Andesitic Lava Flows Research Articles

Illite authigenesis in sandstones of the Guaritas Allogroup (Early Paleozoic): Implications for the depositional age, stratigraphy and evolution of the Camaquã Basin (Southern Brazil)

Several analytical studies performed on alluvial-eolian sandstones of the Early Paleozoic Guaritas Allogroup (Camaquã Basin, southern Brazil) indicate illite to be abundant, showing different morphologies as authigenic grain rims and pore-bridging filaments. Authigenic illite separates of variable grain sizes from distinct stratigraphic intervals of the Guaritas Allogroup yielded 40K–40Ar ages from 521.7 ± 10.3 to 473.7 ± 9.4 Ma. These ages, interpreted to record the timing of illite authigenesis, are coincident with the age of emplacement of the Rodeio Velho andesites (470 ± 19 Ma). Moreover, field structures suggest interaction between hot, andesite lava flows and wet, poorly consolidated sediments of the Pedra Pintada Alloformation (lower strata of the Guaritas Allogroup). This set of data indicates that the Rodeio Velho volcanism could have been responsible for a widespread remobilization of interstitial fluids and consequent authigenic illite precipitation in the sandstones of the Guaritas Allogroup.

Holocene volcanic activity at Koniuji Island, Aleutians

Reconnaissance mapping and 40Ar/ 39Ar age determinations establish an eruptive chronology for Koniuji Island in the central Aleutian island arc. Koniuji is a tiny 0.95 km 2 island that rises only 896 ft above the Bering Sea. Previous accounts describe Koniuji as a mostly submerged, deeply eroded, dormant stratovolcano. However, new 40Ar/ 39Ar ages constrain the duration of subaerial eruptive activity from 15.2 to 3.1 ka. Furnace incremental heating experiments on replicate groundmass separates from two samples of a 30–50 m thick basaltic andesite flow at the southernmost point of the island gave a weighted mean 40Ar/ 39Ar age of 15.2 ± 5.0 (2 σ). The next phase of eruptive activity includes a series of 5.8–4.6 ka basaltic andesitic to andesitic lava flows preserved along the western shoreline. The basal lavas contain numerous mafic enclaves and dioritic cumulates suggesting a major disturbance in the plumbing system during the initial stages of emplacement. The 5.8–4.6 ka lavas are truncated by an andesitic dome complex that includes hornblende-bearing domes, flows and pyroclastics which extruded into the center of the island and comprise the majority of the subaerial eruptive volume. An angular block from within the dome complex yielded 40Ar/ 39Ar age of 3.1 ± 1.9 ka, thereby making it one of the youngest island arc volcanics to be dated using the 40Ar/ 39Ar method. Overall, the 40Ar/ 39Ar data indicate that Koniuji is a nascent stratovolcano that has only recently emerged above sea level, not a glacially-eroded, long-lived volcanic complex like those found on many other central Aleutian Islands.

Volcanic Framework of the Pliocene El Dorado Low-Sulfidation Epithermal Gold District, El Salvador

The Pliocene El Dorado low-sulfidation epithermal Au-Ag vein system, located within an 84-km2 district in north-central El Salvador, is hosted by volcanic rocks formed along the Caribbean plate margin in response to the subduction of the Cocos plate during the Tertiary. The principal volcanic host rocks to epithermal veins, and effective basement in the district, consist of a >400-m-thick sequence of basaltic to andesitic lava flows and volcanogenic sedimentary rocks. Locally, porphyritic basaltic to andesitic domes and dikes intruded this sequence during the late Miocene (10.7 ± 1.9 Ma). The mineralized volcanic basement rocks, outcropping mainly in north and central El Dorado, represent different parts of a complex volcanic sequence composed of multiple superimposed volcanic centers. Vein formation in the district occurred in the Pliocene between 4.7 ± 0.2 Ma and 4.06 ± 0.29 Ma, based on K-Ar and 40Ar-39Ar plateau ages on vein adularia. Vein formation overlapped with Pliocene felsic volcanism (3.94 ± 0.29 Ma to 3.36 ± 0.49 Ma), based on field relationships and 40Ar-39Ar geochronology. Pliocene felsic volcanic rock sequences crop out widely in southern El Dorado at lower elevations relative to central and northern El Dorado. The felsic rocks have sinters intercalated with pyroclastic and sedimentary rocks at the base of the sequence that define the paleosurface of the epithermal vein system, at least in this part of the district. The depth of erosion in central and northern El Dorado, the primary Au-Ag resource, is below the paleosurface, as only the low sulfidation–style epithermal veins are present. Field relationships and geochronology suggest that the onset of extensive felsic volcanism marks the waning stage of hydrothermal activity associated with vein formation and mineralization. From north to south, the El Dorado district represents an oblique cross section through the volcanic and epithermal vein system formed during east-west extension. Northern and central El Dorado, dominated by mineralized mafic to intermediate volcanic basement rocks, represent the deeper parts of the volcanic and hydrothermal system. Southern El Dorado is the shallow level of the volcanic field and epithermal vein system, preserving the surface and shallow subsurface environments, including the sinters and contemporaneous to postmineral felsic volcanic rocks. Although we cannot preclude that the distribution of volcanic rocks represents lateral facies changes in a Pliocene felsic dome field or that the variations in depth within the low-sulfidation epithermal system represent simply lateral flow driven by topography in the geothermal system, the preponderance of evidence suggests that the oblique section is principally due to postmineral deformation associated with formation of large Pliocene and Pleistocene Rio Titihuappa basin lying to the immediate south.

Physical volcanology of the mafic segment of the subaqueous New Senator caldera, Abitibi greenstone belt, Quebec, Canada

Archean calderas provide valuable insight into internal geometries of subaqueous calderas. The New Senator caldera, Abitibi greenstone belt, Canada, is an Archean example of a subaqueous nested caldera with a basal stratigraphy dominated by gabbro-diorite dykes and sills, ponded magmas and basalt and andesite lava flows. The aim of our study is to focus on the use of physical volcanology to differentiate between the various mafic units found at the base of the New Senator caldera. Differentiation between these various mafic units is important from an exploration point of view because in modern subaqueous summit calders (e.g. Axial Seamount) margins of ponded magmas are often sites of VMS formation.

Paleomagnetic and rock magnetic studies of the Sakurajima 1914 and 1946 andesitic lavas from Japan: A comparison of the LTD-DHT Shaw and Thellier paleointensity methods

Paleomagnetic and rock magnetic measurements have been made on samples from the 1914 and 1946 Sakurajima andesitic lava flows in order to investigate the suitability of andesite to record both paleodirection and paleointensity. The lava flows were split in to three sites, each of which consists of two or three sub-sites. A range of rock magnetic experiments have been made on multiple samples per flow and the samples can be split into two broad rock magnetic categories (I and II). Paleodirection was determined by stepwise thermal demagnetization and the mean direction determined from both flows is very close to that expected from IGRF-10. Paleointensity was determined using the LTD-DHT Shaw technique and the Thellier method on sister samples of the same cores. Sixty four out of 72 LTD-DHT Shaw paleointensity experiments were successful and there are no differences between the sub-sites. Seventy out of 71 Thellier paleointensity experiments were successful; however, for sub-sites in two out of the three sites there are large differences between the mean paleointensities. When all paleointensities are normalized to the expected value from IGRF-10, the mean is 0.98 ± 0.11 ( ± σ ) for the LTD-DHT Shaw method and 1.12 ± 0.13 ( ± σ ) for the Thellier method. These means are statistically distinguishable and this is related to the different rock magnetic properties of the samples. If only category I samples are considered, the LTD-DHT Shaw mean is 1.05 ± 0.07 compared with the Thellier mean of 1.08 ± 0.08 . In contrast there is a much larger spread in the means determined from category II samples: 0.94 ± 0.11 from the LTD-DHT Shaw experiments and 1.15 ± 0.15 from the Thellier experiments. This spread could be caused by magnetic interactions between magnetic domains. Assuming high temperature oxidation of titanomagnetite grains during natural cooling, we suggest a scenario that can qualitatively explain the difference between the LTD-DHT Shaw and Thellier paleointensity results.

Late Eocene shoreline volcanism along the continental margin:the volcanic succession at Kabuki Iwa, Oga Peninsula, NE Japan

The Oga Peninsula is situated on the Japan Sea side of the NE Japan Arc (Fig. 1). A Cenozoic succession is well exposed along the coast and has been referred to as a type of Cenozoic succession of northern Japan since Huzioka (1959) established the stratigraphic framework of the succession. Volcanic rocks are extensively exposed in the western part of the peninsula and occupy the lower part of the Cenozoic succession (Huzioka, 1959, 1973). They are believed to represent the initial process of the Japan Sea opening that prevailed during the Early Miocene time, but their volcanological and tectonic aspects remain poorly understood (e.g. Kano et al., 2007a,b, 2008; Kobayashi et al., 2008). This paper describes the occurrence of the volcanic rocks which are exposed at Kabuki Iwa (Rock) near the northern tip of the peninsula (Fig.1), in order to reveal volcanic and associated tectonic activities at that time. The lower volcanic succession in the western Oga Peninsula is divided into the Akashima Formation, Monzen Formation, and Nomuragawa Formation in ascending stratigraphic order and is unconformably overlain by the Middle to Late Miocene marine sediments of the Nishikurosawa and Onnagawa Formations (Figs. 1 and 2). The Nomuragawa Formation is a stratigraphic unit newly identified by Kobayashi et al. (2004). This formation comprises mainly dacite pyroclastic flows, basaltic andesite lava flows and scoriaceous deposits and is correlative with the main part of the Daijima Formation distributed on the immediate eastern side (Fig. 2). The Daijima Formation is accompanied by non-marine clastic sediments at its uppermost part, but radiometric dates indicate both the Nomuragawa and Daijima Formations are Early Miocene in age and mutually correlative (Kobayashi et al., 2004). The Monzen Formation is a mafic to felsic volcanic complex emplaced in a transition area from land to shallow water (Kobayashi et al., 2008) and can be divided into eight units (Fig. 2). Radiometric ages reported for this volcanic succession have been diverse but could span a few million years of Late Eocene to Early Oligocene time (Kano et al., 2007b, 2008). There are many contemporaneous dikes extending mostly in a NE-SW direction, as shown by the geological maps (Huzioka, 1959, 1973), perhaps reflecting an NW-SE extensional stress field of that time (Yamamoto, 1991). The upper most subaqueous rhyolite lava dome of Shinzan Rhyolite is over hundreds of meters thick and suggests the increase of crustal rifting. On the other hand, the overlying Nomuragawa Formation and Daijima Formation are characterized by bimodal volcanism and on-land eruptions. Shallow-water sediments of the uppermost Daijima Formation are likely to record the succeeding subsidence of this area in association with Late Eocene shoreline volcanism along the continental margin: the volcanic succession at Kabuki Iwa, Oga Peninsula, NE Japan

Jurassic to Early Cretaceous subduction-related magmatism in the Coastal Cordillera of northern Chile (18°30'-24°S): geochemistry and petrogenesis

Jurassic to Early Cretaceous magmatism in the Coastal Cordillera of northern Chile is represented by thick sequences of mostly basaltic-andesitic to andesitic lava flows and minor sedimentary rocks. The volcanic sucession was intruded by large plutonio bodies and smaller stocks and dikes. New geochemical data, including major and trace elements for a suite of Middle to Upper Jurassic volcanic and plutonio rocks from six localities in the Coastal Cordillera (18°30'-24°S), are presented here. The volcanic rocks are characterized by their petrological and chemical homogeneity; they are highly porphyritic basaltic-andesites and andesites with calc-alkaline to high-K calc-alkaline affinities, higher LILE than HFSE abundances, negative Nb and Ti anomalies, and LREE/HREEfractionation, which are the typical compositional features of subduction-related igneous rocks. No significant differences are observed in rocks from different areas or ages, but the plutonio rocks show subparallel, less and more enriched patterns respectively compared to volcanic rocks. The evolution and differentiation of the parental magmas is mainly due to fractional crystallization dominated by plagioclase, olivine and clinopyroxene. Assimilation of the continental crust was not important, although Th and La contents would indicate increasing sediment contribution or crustal contamination of the magmas with time. The magma source is likely to be a depleted mantle metasomatized by fluids, which originated from dehydration of the subducted oceanic crust. No evidence of slab melting was found in the studied rocks. The extensional tectonic setting that dominated the evolution of the Jurassic-Early Cretaceous arc in northern Chile would have favoured the extrusion of huge amounts of volcanic rocks during a relatively short period of time, avoiding thus a mayor interaction with the continental crust.

Middle to late Cenozoic basin evolution in the western Alborz Mountains: Implications for the onset of collisional deformation in northern Iran

Oligocene‐Miocene strata preserved in synclinal outcrop belts of the western Alborz Mountains record the onset of Arabia‐Eurasia collision‐related deformation in northern Iran. Two stratigraphic intervals, informally named the Gand Ab and Narijan units, represent a former basin system that existed in the Alborz. The Gand Ab unit is composed of marine lagoonal mudstones, fluvial and alluvial‐fan clastic rocks, fossiliferous Rupelian to Burdigalian marine carbonates, and basalt flows yielding 40Ar/39Ar ages of 32.7 ± 0.3 and 32.9 ± 0.2 Ma. The Gand Ab unit is correlated with the Oligocene–lower Miocene Qom Formation of central Iran and is considered a product of thermal subsidence following Eocene extension. The Narijan unit unconformably overlies the Gand Ab unit and is composed of fluvial‐lacustrine and alluvial fan sediments exhibiting contractional growth strata. We correlate the Narijan unit with the middle to upper Miocene Upper Red Formation of central Iran on the basis of lithofacies similarities, stratigraphic position, and an 8.74 ± 0.15 Ma microdiorite dike (40Ar/39Ar) that intruded the basal strata. Deformation timing is constrained by crosscutting relationships and independent thermochronological data. The Parachan thrust system along the eastern edge of the ancestral Taleghan‐Alamut basin is cut by dikes dated at 8.74 ± 0.15 Ma to 6.68 ± 0.07 Ma (40Ar/39Ar). Subhorizontal gravels that unconformably overlie tightly folded and faulted Narijan strata are capped by 2.86 ± 0.83 Ma (40Ar/39Ar) andesitic lava flows. These relationships suggest that Alborz deformation had migrated southward into the Taleghan‐Alamut basin by late Miocene time and shifted to its present location along the active range front by late Pliocene time. Data presented here demonstrate that shortening in the western Alborz Mountains had started by late middle Miocene time. This estimate is consistent with recent thermochronological results that place the onset of rapid exhumation in the western Alborz at ∼12 Ma. Moreover, nearly synchronous Miocene contraction in the Alborz, Zagros Mountains, Turkish‐Iranian plateau, and Anatolia suggests that the Arabia‐Eurasia collision affected a large region simultaneously, without a systematic outward progression of mountain building away from the collision zone.

The rhyolitic–andesitic eruptive history of Cotopaxi volcano, Ecuador

At Cotopaxi volcano, Ecuador, rhyolitic and andesitic bimodal magmatism has occurred periodically during the past 0.5 Ma. The sequential eruption of rhyolitic (70–75% SiO2) and andesitic (56–62% SiO2) magmas from the same volcanic vent over short time spans and without significant intermingling is characteristic of Cotopaxi’s Holocene behavior. This study documents the eruptive history of Cotopaxi volcano, presenting its stratigraphy and geologic field relations, along with the relevant mineralogical and chemical nature of the eruptive products, in order to determine the temporal and spatial relations of this bimodal alternation. Cotopaxi’s history begins with the Barrancas rhyolite series, dominated by pumiceous ash flows and regional ash falls between 0.4 and 0.5 Ma, which was followed by occasional andesitic activity, the most important being the ample andesitic lava flows (∼4.1 km3) that descended the N and NW sides of the edifice. Following a ∼400 ka long repose without silicic activity, Cotopaxi began a new eruptive phase about 13 ka ago that consisted of seven rhyolitic episodes belonging to the Holocene F and Colorado Canyon series; the onset of each episode occurred at intervals of 300–3,600 years and each produced ash flows and regional tephra falls with DRE volumes of 0.2–3.6 km3. Andesitic tephras and lavas are interbedded in the rhyolite sequence. The Colorado Canyon episode (4,500 years BP) also witnessed dome and sector collapses on Cotopaxi’s NE flank which, with associated ash flows, generated one of the largest cohesive debris flows on record, the Chillos Valley lahar. A thin pumice lapilli fall represents the final rhyolitic outburst which occurred at 2,100 years BP. The pumices of these Holocene rhyolitic eruptions are chemically similar to those of older rhyolites of the Barrancas series, with the exception of the initial eruptive products of the Colorado Canyon series whose chemistry is similar to that of the 211 ka ignimbrite of neighboring Chalupas volcano. Since the Colorado Canyon episode, andesitic magmatism has dominated Cotopaxi’s last 4,400 years, characterized by scoria bomb and lithic-rich pyroclastic flows, infrequent lava flows that reached the base of the cone, andesitic lapilli and ash falls that were carried chiefly to the W, and large debris flows. Andesitic magma emission rates are estimated at 1.65 km3 (DRE)/ka for the period from 4,200 to 2,100 years BP and 1.85 km3 (DRE)/ka for the past 2,100 years, resulting in the present large stratocone.

The Early Mesozoic volcanic arc of western North America in northeastern Mexico

Volcanic successions underlying clastic and carbonate marine rocks of the Oxfordian–Kimmeridgian Zuloaga Group in northeastern Mexico have been attributed to magmatic arcs of Permo–Triassic and Early Jurassic ages. This work provides stratigraphic, petrographic geochronological, and geochemical data to characterize pre-Oxfordian volcanic rocks outcropping in seven localities in northeastern Mexico. Field observations show that the volcanic units overlie Paleozoic metamorphic rocks (Granjeno schist) or Triassic marine strata (Zacatecas Formation) and intrude Triassic redbeds or are partly interbedded with Lower Jurassic redbeds (Huizachal Group). The volcanic rocks include rhyolitic and rhyodacitic domes and dikes, basaltic to andesitic lava flows and breccias, and andesitic to rhyolitic pyroclastic rocks, including breccias, lapilli, and ashflow tuffs that range from welded to unwelded. Lower–Middle Jurassic ages (U/Pb in zircon) have been reported from only two studied localities (Huizachal Valley, Sierra de Catorce), and other reported ages (Ar/Ar and K–Ar in whole-rock or feldspar) are often reset. This work reports a new U/Pb age in zircon that confirms a Lower Jurassic (193 Ma) age for volcanic rocks exposed in the Aramberri area. The major and trace element contents of samples from the seven localities are typical of calc-alkaline, subduction-related rocks. The new geochronological and geochemical data, coupled with the lithological features and stratigraphic positions, indicate volcanic rocks are part of a continental arc, similar to that represented by the Lower–Middle Jurassic Nazas Formation of Durango and northern Zacatecas. On that basis, the studied volcanic sequences are assigned to the Early Jurassic volcanic arc of western North America.

Detailed 40Ar/39Ar dating of geologic events associated with the Mantos Blancos copper deposit, northern Chile

The 40Ar/39Ar geochronological method was applied to date magmatic and hydrothermal alteration events in the Mantos Blancos mining district in the Coastal Cordillera of northern Chile, allowing the distinction of two separate mineralization events. The Late Jurassic Mantos Blancos orebody, hosted in Jurassic volcanic rocks, is a magmatic-hydrothermal breccia-style Cu deposit. Two superimposed mineralization events have been recently proposed. The first event is accompanied by a phyllic hydrothermal alteration affecting a rhyolitic dome. The second mineralization event is related to the intrusion of bimodal stocks and sills inside the deposit. Because of the superposition of several magmatic and hydrothermal events, the obtained 40Ar/39Ar age data are complex; however, with a careful interpretation of the age spectra, it is possible to detect complex histories of successive emplacement, alteration, mineralization, and thermal resetting. The extrusion of Jurassic basic to intermediate volcanic rocks of the La Negra Formation is dated at 156.3 ± 1.4 Ma (2σ) using plagioclase from an andesitic lava flow. The first mineralization event and associated phyllic alteration affecting the rhyolitic dome occurred around 155–156 Ma. A younger bimodal intrusive event, supposed to be equivalent to the bimodal stock and sill system inside the deposit, is probably responsible for the second mineralization event dated at ca. 142 Ma. Other low-temperature alteration events have been dated on sericitized plagioclase at ca. 145–146, 125, and 101 Ma. This is the first time that two distinct mineralization events have been documented from radiometric data for a copper deposit in the metallogenic belt of the Coastal Cordillera of northern Chile.

The Favona Epithermal Gold-Silver Deposit, Waihi, New Zealand

The Favona deposit is a new epithermal vein discovery (>595,000 oz Au, >2,367,500 oz Ag resource), located less than 2 km east of the world-class Martha deposit (>6.7 million oz Au, >42.1 million oz (Moz) Ag), in the Hauraki goldfield of New Zealand. The concealed Favona vein is hosted in late Miocene andesitic lava flows and occurs to the east of the nearby historically mined Silverton, Amaranth, Mascotte, and Union veins. Hydrothermal alteration is characterized by a variety of alteration minerals that display distinct zonation along four subsurface cross sections (800 m long × 450 m deep) that transect the Favona, Cowshed, and Amaranth veins. Hydrothermal quartz and pyrite are ubiquitous. Adularia is widespread and envelopes veins; it locally coexists with hydrothermal albite adjacent to the Amaranth vein at depth. Both adularia and albite are variably replaced by illite, which, together with broadly coextensive chlorite, are generally restricted to the foot-wall of the Favona and Cowshed veins, although both minerals appear locally at depth in the hanging wall. Interstratified illite-smectite containing 10 to 70 percent smectite and rare discrete smectite are mostly confined to the hanging wall of the Favona and Cowshed veins. Late-stage kaolinite, cristobalite, and rare alunite locally overprint the above minerals at shallow levels, whereas late-stage overprinting calcite occurs at depth near the Amaranth, Cowshed, and southern portion of the Favona veins. Aeromagnetic data show that the Favona and Martha deposits both fit within a broad zone of hydrothermally induced demagnetization that is approximately 3.7 × 3.0 km. Alteration is broadly similar at each deposit, although albite and calcite are more abundant at Martha. Fluid inclusion data indicate that both the Favona and Martha deposits formed at similar levels with veins developed over extensive vertical intervals exceeding 400 and 600 m, respectively. Based on these data, we propose that the veins comprising the Favona and Martha deposits formed at similar shallow levels (<1 km) in a single paleogeothermal system. It is unclear if the Martha and Favona veins systems formed within a single upflow zone, a migrating upflow, or within two separate upflow zones. Regardless, widespread alteration surrounding the veins includes quartz, adularia, albite, chlorite, illite plus pyrite, and reflects formation from alkali chloride waters that contained moderate concentrations of aqueous CO2. Focused and episodic boiling of these chloride waters along extensive open conduits resulted in the formation of gold- and silver-bearing colloform- and crustiform-banded quartz veins. Steam-heated waters formed above and on the margins of the paleogeothermal system, and during the eventual thermal collapse resulted in overprinting by kaolinite, cristobalite, and alunite from descending steam-heated acid-sulfate waters and late calcite from localized steam-heated CO2-rich waters. The discovery of the Favona deposit next to the large Martha deposit highlights the importance of determining the physical limits of hydrothermal alteration in a system in order to identify the limits of paleogeothermal activity and define the area that may be explored for veins. The veins at Favona, Martha, and Golden Cross (Empire vein), combined with a resource at Karangahake, collectively total more than 3.5 Moz Au and should encourage the exploration of known epithermal deposits throughout the Hauraki goldfield for additional resources.

Robust 24 ± 6 ka 40Ar/ 39Ar age of a low-potassium tholeiitic basalt in the Lassen region of NE California

40Ar/ 39Ar ages on the Hat Creek Basalt (HCB) and stratigraphically related lava flows show that latest Pleistocene tholeiitic basalt with very low K 2O can be dated reliably. The HCB underlies ∼ 15 ka glacial gravel and overlies four andesite and basaltic andesite lava flows that yield 40Ar/ 39Ar ages of 38 ± 7 ka (Cinder Butte; 1.65% K 2O), 46 ± 7 ka (Sugarloaf Peak; 1.85% K 2O), 67 ± 4 ka (Little Potato Butte; 1.42% K 2O) and 77 ± 11 ka (Potato Butte; 1.62% K 2O). Given these firm age brackets, we then dated the HCB directly. One sample (0.19% K 2O) clearly failed the criteria for plateau-age interpretation, but the inverse isochron age of 26 ± 6 ka is seductively appealing. A second sample (0.17% K 2O) yielded concordant plateau, integrated (total fusion), and inverse isochron ages of 26 ± 18, 30 ± 20 and 24 ± 6 ka, all within the time bracket determined by stratigraphic relations; the inverse isochron age of 24 ± 6 ka is preferred. As with all isotopically determined ages, confidence in the results is significantly enhanced when additional constraints imposed by other isotopic ages within a stratigraphic context are taken into account.

Clay minerals related to the hydrothermal activity of the Bouillante geothermal field (Guadeloupe)

The geothermal field of Bouillante (Guadeloupe, FWI) is a high-enthalpy hydrothermal system emplaced in submarine volcanoclastic formations (hyaloclastites, scarce lava flows) and subaerial formations (andesitic lava flows, pyroclastites, lahars) which belong to the Lesser Antilles arc. Three directional wells were drilled in 2001 to optimize the productivity of the geothermal field up to 15 MWe and to investigate the vertical distribution of clay alteration from the surface area down to a depth of 1000 m where temperatures exceed 250 °C. Special attention has been paid to the “clay signature” of the fractured zones which channel the present geothermal fluids. Three successive zones, dominated, respectively by dioctahedral smectite, illite and chlorite were identified at increasing depths. Alteration petrography indicates that these mineralogical clay zones result from the spatial superimposition of at least two successive alteration stages. The first one, assimilated to a propylitic alteration stage, affected all parts of the system and consisted of crystallization of trioctahedral phyllosilicates (chlorite or corrensite), Ca-silicates (heulandite-clinoptilolite, prehnite, pumpelleyite, wairakite and epidote), quartz and minor calcite in replacement of most of the primary minerals of the intersected volcanic or volcanoclastic formations. The later stage of alteration is related to the circulation of the present geothermal fluids and is assimilated to argillic or phyllic alteration. It consists of a more or less intense argillization which results from the crystallization of aluminous dioctahedral clay phases (smectite, illite ± I–S mixed layers, and accessory kaolinite) associated with quartz, calcite, hematite or pyrite. The permeable zones which channel most of the present geothermal fluids are fracture controlled and do not contain specific clay parageneses. However the illite ± I–S mixed layers minerals differ from those of the surroundings by specific properties including both crystal structure and texture. These specific properties (decrease in the expandable component of the illitic material, increase of the illite crystallinity) can be controlled by the nucleation/growth rates operating in zones of active flow regime. Being mainly a product of the earlier propylitic alteration stage, chlorites are much less informative on the fracture controlled permeable levels. However, the compositional variations of chlorites recorded within the shallower fractured zone suggest a significant change in fO 2 conditions related to early circulation of fluids along the major near west striking normal faults (Plateau fault).

Evolution and hydrological conditions of a maar volcano (Atexcac crater, Eastern Mexico)

The Atexcac maar is located in the central part of the Serdán–Oriental lacustrine/playa basin in the eastern Mexican Volcanic Belt. It is part of a dispersed and isolated monogenetic field consisting of maar volcanoes, basaltic cinder cones and rhyolitic domes. Atexac is a maar volcano excavated into pyroclastic deposits, basaltic lava flows and the flanks of a cinder cone cluster, which itself was built on a topographic high consisting of limestone. It has an ENE-trending elliptical shape with beds, mostly unconsolidated deposits that dip outward at 16–22°. The Atexcac crater was formed from vigorous phreatomagmatic explosions in which fluctuations in the availability of external water, temporal migration of the locus of the explosion, and periodic injection of new magma were important controls on the evolution of the maar crater. Variations in grain sizes and component proportions of correlated deposits from the different sections suggest a migration of the locus of explosions, producing different eruptive conditions with fluctuating water–magma interactions. Deposits rich in large intrusive and limestone blocks are associated with a matrix enriched in small andesitic lapilli. This could suggest differential degrees of fragmentation due to inherited (previously acquired) fragmentation and/or relative distance to the locus of explosions. Initial short-lived phreatic explosions started at the southwest part of the crater and were followed by an ephemeral vertical column and the influx of external water that led to relatively shallow explosive interactions with the ascending basaltic magma. Drier explosions progressed downward and/or laterally northward, sampling subsurface rock types, particularly intrusive, limestone and andesitic zones as well as localized altered zones (N-NE), caused by repetitive injection of basaltic magma. A final explosive phase involved a new injection of magma and a new influx of external water producing wetter conditions at the end of the maar formation. We infer the aquifer was formed by fractured rocks, predominantly andesitic lava flows and limestone rocks. Andesitic accessory clasts dominate in all stratigraphic levels but these rocks are not exposed in the nearby area. These local hydrogeological conditions contrast with those at nearby maar volcanoes, where the water for the magma/water interactions apparently mostly came from a dominantly unconsolidated tuffaceous aquifer, producing tuff rings with a much lower profile than Atexcac.

Base surge deposits, eruption history, and depositional processes of a wet phreatomagmatic volcano in Central Anatolia (Cora Maar)

Cora Maar is a Quaternary volcano located to the 20 km northwest of Mount Erciyes, the largest of the 19 polygenetic volcanic complexes of the Cappadocian Volcanic Province in central Anatolia. Cora Maar is a typical example of a maar-diatreme volcano with a nearly circular crater with a mean diameter of c.1.2 km, and a well-bedded base surge-dominated maar rim tephra sequence up to 40 m in thickness. Having a diameter/depth ratio ( D/ d) of 12, Cora is a relatively “mature” maar compared to recent maar craters in the world. Cora crater is excavated within the andesitic lava flows of Quaternary age. The tephra sequence is not indurated, and consists of juvenile clasts up to 70 cm, non-juvenile clasts up to 130 cm, accretionary lapilli up to 1.2 cm in diameter, and ash to lapilli-sized tephra. Base surge layers display well-developed antidune structures indicating the direction of the transport. Both progressive and regressive dune structures are present within the tephra sequence. Wavelength values increase with increasing wave height, and with large wavelength and height values. Cora tephra display similarities to Taal and Laacher See base surge deposits. Impact sags and small channel structures are also common. Lateral and vertical facies changes are observed for the dune bedded and planar bedsets. According to granulometric analyses, Cora Maar tephra samples display a bimodal distribution with a wide range of Md φ values, characteristic for the surge deposits. Very poorly sorted, bimodal ash deposits generally vary from coarse tail to fine tail grading depending on the grain size distribution while very poorly sorted lapilli and block-rich deposits display a positive skewness due to fine tail grading.

Collapse mechanism of the Paleogene Sakurae cauldron, SW Japan

The Paleogene Sakurae cauldron of SW Japan is characterized by a nested structure with a polygonal outline (21×13 km2) including a circular collapsed part (5 km in diameter). Total thickness of the caldera infill amounts to 2,000 m. The lower member of the infill consists mainly of felsic crystal tuff and lesser intercalated andesitic lava flows, whereas the upper member is composed of high-grade ignimbrite capped with a large rhyolitic lava dome. These members represent the first and second stage eruptions, respectively. Faults bounding the cauldron rim comprise intersecting radial and concentric faults, producing the polygonal outline of this cauldron. The primary collapse of this cauldron thus occurred as a polygonal caldera basin where products of the first stage eruption accumulated. In contrast, the inner collapse part is defined by a ring fracture system. This sector subsided concurrently with accumulation of the high-grade ignimbrite of the second stage eruption. This inner circular collapse thus represents syn-eruptional subsidence concurrent with the climactic eruption. Magma drainage during the first stage probably induced outward-dipping ring fractures in the chamber roof. Opening of the ring fractures following subsidence of the central bell-jar block caused rapid evacuation of magma as voluminous pumice flows, even though magma pressure may have decreased to some degree.

Snow-contact volcanic facies and their use in determining past eruptive environments at Nevados de Chillán volcano, Chile

Studies of the eruptive products from volcanoes with variable ice and snow cover and a long history of activity enable reconstruction of erupted palaeoenvironments, as well as highlighting the hazards associated with meltwater production, such as jokulhlaups and magma-water interaction. Existing difficulties include estimation of ice/snow thicknesses and discrimination between ice- and snow-contact lithofacies. We present field evidence from the Cerro Blanco subcomplex of Nevados de Chillan stratovolcano, central Chile, which has erupted numerous times in glacial and non-glacial periods and most recently produced andesitic lava flows in the 1861–1865 eruption from the Santa Gertrudis cone on the northwest flank of the volcano. The main period of lava effusion occurred during the winter of 1861 when the upper flanks of the volcano were reportedly covered in snow and ice. The bases and margins of the first lava flows produced are cut by arcuate fractures, which are interpreted as snow-contact features formed when steam generated from the melting of snow entered tensional fractures at the flow base. In contrast, the interiors and upper parts of these flows, as well as the overlying flow units, have autobrecciated and blocky textures typical of subaerial conditions, due to insulation by the underlying lava. Similar textures found in a lava flow dated at 90.0±0.6 ka that was emplaced on the northwest flank of Cerro Blanco, are also inferred to be ice and snow-contact features. These textures have been used to infer that a small valley glacier, overlain by snow, existed in the Santa Gertrudis Valley at the time of the eruption. Such reconstructions are important for determining the long-term evolution of the volcano as well as assessing future hazards at seasonally snow-covered volcanoes.

Kinematic link between episodic trapdoor collapse of the Negra Muerta Caldera and motion on the Olacapato-El Toro Fault Zone, southern central Andes

A combined geochronological and structural analysis of the Miocene Negra Muerta Caldera was designed to better understand caldera formation associated with prominent faults on the central Andean plateau. Rb–Sr ages of the caldera outflow facies indicate that caldera formation occurred in two volcano-tectonic episodes. The first episode commenced with explosive eruption of the 9.0±0.1 Ma andesitic Acay Ignimbrite followed by a period of volcanic quiescence and moderate tectonic activity. Dominant volcanic and tectonic activity occurred during the second episode, which is bracketed by eruption of the 7.6±0.1 Ma rhyolitic Toba 1 Ignimbrite and effusive discharge of the 7.3±0.1 Ma rhyodacitic to andesitic lava flows. Structural relationships between rocks of the Negra Muerta Volcanic Complex and collapse-induced normal faults, notably NE-striking normal faults, agree with simultaneous volcanic activity and floor subsidence of the caldera during the second episode. Floor subsidence was achieved by tilting on an outward dipping reverse fault to the northwest of the caldera floor around a hinge zone located south of the caldera floor. This induced horizontal extension of the caldera floor and was accomplished by fragmentation of, and intrusion of dikes into, the floor. Collapse-induced and post-collapse fault populations of the caldera do not differ significantly in the directions of their axes of maximum extension and are in this respect kinematically compatible with left-lateral slip on the nearby Olacapato-El Toro Fault Zone. This furnishes evidence for a kinematic control by prominent faults on the formation of collapse calderas in the central Andes. The structural analysis of the Negra Muerta Caldera shows that collapse calderas can serve as deformation markers that contribute in elucidating the regional kinematic regime and the time of activity of prominent dislocations genetically related to collapse calderas.

Geotechnical characterisation of stratocone crater wall sequences, White Island Volcano, New Zealand

Geotechnical characterisation is undertaken for 3 broad units comprising the bulk of the stratigraphy identified on White Island Volcano, Bay of Plenty, New Zealand, an active island stratovolcano. Field and laboratory measurements were used to describe rock mass characteristics for jointed lava flow units, and ring shear tests were undertaken to derive residual strength parameters for joint infilling materials within the lavas. Rock Mass Rating (RMR) and Geological Strength Index (GSI) values were calculated and converted to Mohr–Coulomb strength parameters using the Hoek–Brown criterion. Backanalysis of known landslide scarps was used to derive strength parameters for brecciated rock masses and hydrothermally altered rock masses. Andesite lava flows have high intact strength ( σ ci = 184 ± 50 MN m − 2 ; γ = 24.7 ± 0.3 kN m − 3 ) and typically 3 wide, infilled joint sets, one parallel to flow direction and two steeply inclined, with spacings of 0.3–1.7 m. Joints are rough, with estimated friction angles for clean joints of ϕ j = 42–47°. Joint infill materials are clayey silts derived from weathering of wall rocks and primary volcanic sources; they have low plastic (54%) and liquid (84%) limits and residual strength values of c r = 0 kN m − 2 and ϕ r = 23.9 ± 3.1°. RMR values range from 70 to 73, giving calculated strength parameters of c′ = 1161–3391 kN m − 2 and ϕ′ = 50.5–62.3°. Backanalysis suggests brecciated rock masses have c′ = 0 kN m − 2 and ϕ′ = 35.4°, whereas GSI observations in the field suggest higher cohesion ( c′ = 306–719 kN m − 2 ) and a range of friction angles bracketing the backanalysed result ( ϕ′ = 30.6–41.7°). Hydrothermally altered rock masses have c′ = 369 kN m − 2 and ϕ′ = 14.9°, indicating considerable loss of strength, especially frictional resistance, compared with the fresh lava units. Values measured at outcrop scale in this study are in keeping with other published values for similar volcanic edifices; backanalysed data suggest weaker rock mass properties than those determined at outcrop. This is interpreted as a scale issue, whereby rock mass characteristics of a large rock mass (crater wall scale) are weaker than those of small outcrops, due in part to the overestimation of friction angle from measurements on small exposures.