Dacitic Lava Flows Research Articles

Metals, Volatiles, and Lithostratigraphy of Brothers Submarine Volcano

AbstractRelatively fresh volcanic rocks have been sampled by a remotely operated vehicle in situ from the NE caldera wall of Brothers submarine volcano, associated with Seafloor Massive Sulfide‐SMS deposits. Here, we present the first complete stratigraphic column of the NE caldera wall, comprising at least 12 massive dacitic lava flows, up to 80 m‐thick intercalated with multiple volcaniclastic layers associated with tuffaceous sediment layers. Detailed petrographic and geochemical analyses from hand specimen to crystal to silicate melt scale show chemical variability with depth, correlating partly with an increase in pervasive alteration due to volatile degassing. Moreover, while sulfide saturation occurred prior to volatile exsolution—which sequestrated most chalcophile elements as confirmed by the low metal contents of melt inclusions (e.g., Cu ≤ 1.3 μg/g and Au ≤ 7.0 μg/g)—silicate glass records a Cu enrichment and Au loss with differentiation, with interstitial glass accounting for Cu = 4.2 μg/g and Au = 6.6 μg/g and matrix glass for Cu = 6.0 μg/g and Au = 2.8 μg/g, respectively. Our findings suggest multiple sources for metals compensating for the low initial metal contents: (a) from hydrothermal fluids and volatile percolation ensuing interaction with the host rock and thus also replacement and/or dissolution of pre‐existing magmatic sulfides, (b) directly from the magma, consistent with metal release during magma degassing of metal‐ and Cl‐, and S‐ rich volatiles, and (c) from fluid circulation within unusually metal‐rich andesitic volcaniclastic layers (Cu = 40 μg/g, Au = 1.5 ng/g, and Pt = 0.99 ng/g). Our results elucidate the capacity of such hybrid mineralizing submarine volcanic systems to effectively scavenge, transport, and concentrate metals.

Reconstruction of Hualca Hualca volcano (southern Peru) based on geomorphological and geological evidence and 40Ar/39Ar dating

The volcanic history of many of the western Central Andes volcanoes, including Hualca Hualca in the northern Ampato Volcanic Complex, remains poorly constrained. Based on an updated 1:50,000-scale geological map, new cross-sections of the Ampato Complex, a compilation of published dates, new geochemical data and 40Ar/39Ar ages, we present new insights into its volcanic evolution. Our study identifies 7 principal geological units (4 were not reported on the previous geologic map) and 8, mostly constructive, volcanic phases. The 40Ar/39Ar dating supports that Hualca Hualca began its formation during the Early Pleistocene (>1.6 Ma) with the establishment of an ancient stratovolcano. This volcano underwent a massive sector collapse to the north that created a U-shaped large amphitheater. This collapse likely resulted in the temporary damming of the Colca River, though no dates or associated deposits have been identified. Subsequent eruptions led to the formation of a smaller volcano along the amphitheater's scar that emplaced andesitic and dacitic lavas. This volcano dubbed modern Hualca Hualca holds the main summit. One of the longest lavas of this edifice reach the Colca valley and was previously dated at 610 ka. Sometime around 550 ka, volcanic activity migrated to the interior of the amphitheater forming the Nevado de Puye, Cruz del Condor, and Ahuashune domes that produced lavas of andesitic and dacitic composition. Around 164 ka a dacitic lava flow was emitted between Nevado del Puye and the base of the modern Hualca Hualca inside the horseshoe crater. The youngest known collapse of the Hualca Hualca complex involved parts of Nevado de Puye dome and the modern Hualca Hualca volcano, emplacing a debris avalanche that again dammed the Colca River and formed an upstream temporary lake that emplaced lacustrine and volcaniclastic deposits. After this collapse, no other volcanic deposit of the Hualca Hualca complex has been identified. Subsequently, glacial erosion during the local Last Glacial Maximum (17–16 ka) modified the volcanic landscape. Geochemical characteristics of Hualca Hualca suggest that their magmas were produced in a metasomatized mantle wedge and likely underwent crustal contamination during their ascent through the thick continental crust.

Stratigraphy, geochronology, geochemistry and Nd isotopes of the Ouarzazate Group, Anti-Atlas, Morocco: Evidence of a Late Neoproterozoic LIP in the northwestern part of the West African Craton

In the eastern and central Anti-Atlas of Morocco, the Late Neoproterozoic Ouarzazate Group (OG) forms a ∼ 2 km-thick section of volcano-sedimentary rocks deposited during intermittent magmatic activity, spanning a time period between 590 and 540 Ma. In the eastern Anti-Atlas, the stratigraphy of the OG, termed IMS (Imiter Mine succession), includes four units that reflect a gradual change from fluvial to lacustrine depositional sedimentary environments. In the central Anti-Atlas, the deposition of the OG occurs within a caldera environment, consisting of five units referred to as BMS (Bou Azzer Mine succession). UPb dating on zircon from the rhyolite lava flow at the top of the IMS constrains the latest lava flow of the IMS to 570.7 ± 2.1 Ma. In the Bou Azzer inlier, the stratigraphically correlated basal andesite and top dacite lava flows were emplaced at 590.6 ± 4.2 Ma and 555.9 ± 20.4 Ma, respectively, suggesting that the OG was constructed through successive pulses within a long-lived magmatic episode. The magmatic rocks display geochemical signatures typical of continental arc magmas, including high-K calc-alkaline to shoshonite composition, enrichment in LILE, and negative Nb, Sr, Ti and P anomalies. Nd isotopes indicate that magmas supplying the plumbing systems of the IMS and BMS constitute transitional products resulting from a combination of orogenic and intraplate volcanism, involving deep crustal recycling. The compositions of these magmas evolved through processes of crustal contamination, bulk assimilation, and minor fractional crystallization of the parent melt. High εNd values of the BMS rocks argue for a juvenile origin, whereas lower εNdt recorded in the IMS likely reflect old crustal protoliths and an overall less contribution of juvenile material in their source. The magmatic rocks of the OG are interpreted as products of a large igneous province (LIP) developed by a post-collisional delamination model that initiated tens of millions of years after the subduction-collision process stage of the Pan-African orogeny in the northwestern part of the West African Craton (WAC).

Genesis of the Loma Galena Pb-Ag Deposit, Navidad District, Patagonia, Argentina: A Unique Epithermal System Capped by an Anoxic Lake

Abstract Loma Galena (978,852 t Pb, 206 Moz Ag) is one of eight epithermal deposits in the world-class Navidad Pb + Ag ± (Zn, Cu) district located in the Cañadón Asfalto continental foreland basin, northern Patagonia, Argentina. This basin formed during the Jurassic in an extensional tectonic regime during the breakup of Gondwana. Host rocks comprise major listric faulted and tilted blocks of K-rich andesite to dacite lava flows (173.9–170.8 Ma; U-Pb ages for zircon) unconformably overlain by mudstone interbedded with stromatolitic and pisolitic limestones, sandstone, coal, and an Sr-rich evaporite layer deposited in a lacustrine environment. The mineralization occurs as disseminations in the organic-rich sedimentary rocks, in veins and hydrothermal breccia dikes in the hanging walls and footwalls of NW- and NE-striking normal faults, in volcanic autobreccias, and in a phreatic breccia at the contact of volcanic and sedimentary rocks. The earliest hydrothermal minerals consist of veins of colloform, crustiform, and cockade calcite 1 (δ13Cfluid –4.7 to 0.8‰; δ18Ofluid 4.8–11.6‰) and siderite. The precipitating fluids were likely basement-exchanged basinal brines having salinities of 9.5 to 16.4 wt % NaCl equiv and temperatures of 154.7° to 212°C. The interaction of these fluids with the host volcanic rocks formed calcite, albite, adularia, and celadonite-glauconite-group minerals followed by chlorite and siderite as fO2 decreased. Fluids intermittently boiled, as evidenced by bladed (platy) texture in calcite 1. Subsequent mineralizing stages contributed to the metal endowment of Loma Galena. The abundance of organic-rich mudstone and δ34S from –15.4 to 12.9‰ for sulfides suggests that the bottom waters of the lake were anoxic and the loci of microbial sulfate reduction (evaporites have δ34S 35‰). Mixing of upflowing metal-rich basinal fluids carrying some S from depth with this H2S-rich connate water efficiently precipitated Ag-bearing framboidal pyrite, colloform pyrite-marcasite, chalcopyrite, bornite, tennantite-tetrahedrite, sphalerite, and galena as veins, breccias, and disseminations in host rocks. The highest grade and tonnage of the ores are found in autobreccias at the junction of the uppermost lava flow and in the overlying mudstone, where the addition of a strong microbial signature is recorded in sulfides. This event also led to partial dissolution of magmatic and hydrothermal feldspar and calcite 1 in the altered volcanic rocks. Mineralization was followed by hydrothermal brecciation and successive precipitation of chalcedony (δ18Ofluid 2.6–4.8‰), barite (δ34S 15.7–22‰; 160.9°–183.8°C; 7.7–9.7 wt % NaCl equiv), calcite 2 (δ18Ofluid –10.2 to –3.7‰, 58°–95°C; 1.9–7.0 wt % NaCl equiv), strontianite, and quartz in brecciated veins and breccias; kaolinite (δ18Ofluid 2–6.2‰), illite-smectite, smectite, and carbonates with minor chalcedony and barite in the volcanic rocks; and calcite, chalcedony, and barite in the sedimentary rocks. A trend of decreasing salinity with decreasing temperature and lowering δ18O of the fluids with time suggests dilution of the basinal fluids by mixing with Jurassic meteoric water (δ18O −9 to −5.2‰). Loma Galena is a unique example of a polymetallic epithermal system formed in a sublacustrine anoxic environment that promoted the efficient deposition and preservation of Ag-bearing sulfides, thereby contributing to the large size and relatively high grade of the deposit.

Early Miocene arc volcanism in the Mexico City Basin: Inception of the Trans-Mexican Volcanic Belt

In central Mexico, remnants of the early volcanic activity in the Trans-Mexican Volcanic Belt (TMVB) are exposed at Sierra de Guadalupe in the northern part of the Mexico City Basin and to the south in the Tepoztlán, Malinalco, Tenancingo, and Chiltepec areas. A few published studies indicate a general early to middle Miocene age for some of these volcanic centers. We present new geologic, geochronologic and geochemical data that refine the timing of this early volcanic activity and its geochemical character, shedding light on the geodynamic evolution during this period. The Sierra de Guadalupe is a volcanic complex with effusive and explosive activity that generated andesitic lavas with plagioclase, ortho- and clinopyroxene plus amphibole phenocrysts, as well as dacitic lava flows and domes with plagioclase, orthopyroxene, amphibole and/or biotite phenocrysts. Pyroclastic and epiclastic (lahars) deposits are also observed in the northern slopes of this volcanic complex. The chemical composition of the Sierra de Guadalupe rocks is calc-alkaline andesitic to dacitic. New 40Ar/39Ar and U-Pb zircon ages demonstrate that volcanic activity at Sierra de Guadalupe lasted ca. 5.0 Ma from ca. 20.1 Ma to 14.8 Ma. Remnants of volcanic centers found in the Tepoztlán, Malinalco, Tenancingo, and Chiltepec areas ~80 km to the south show similar compositions and eruptive styles, with 40Ar/39Ar ages ranging between 21 and 16 Ma. Rocks with similar ages and lithologies were also described in deep wells drilled in the Mexico City Basin. As a whole, the age distribution shows that early to middle Miocene volcanism, making up the initial activity of the TMVB, migrated toward the north from Tepoztlán (310 km from the trench) up to Pachuca (445 km from the trench), and is likely associated with a shallowing the subducting Cocos slab underneath this region. Our results confirm that magmatism in the TMVB began in the early Miocene by dehydration of the Cocos slab as attested by high Ba/La and Th/Yb ratios.

Dynamics of caldera collapse during the Coranzulí eruption (6.6 Ma) (Central Andes, Argentina)

The Coranzulí caldera (23°00′ S – 66°15′ W) is one of the least known caldera complexes in the eastern part of the Argentinian Altiplano-Puna plateau (Central Andes). It lies at the intersection of N-S, NW-SE and NE-SW fault systems and was formed about 6.6 Ma ago; during the eruption, four main crystal-rich dacitic ignimbrites were emplaced in different directions around the caldera. Caldera collapse was not homogeneous, rather it occurred along different sectors of the ring fault as subsidence progressed. The location of co-ignimbrite lag breccias and the composition of the dominant lithic fragments within the different ignimbrite flow units reveal how the caldera collapse developed. According to the succession of deposits, in particular the lack of an initial fallout and the presence of co-ignimbrite lag breccias associated with the different ignimbrite units, we interpret this caldera-forming eruption as a pulsating boiling-over event in which the caldera collapse developed immediately after the onset of the eruption, favored by a transtensive tectonic system. Within the central part of the caldera, there is Cerro Coranzulí, a resurgent dome that exposes a thick intracaldera ignimbrite succession covered by ~ 100-m-thick dacite lava flows that erupted at the end of caldera formation.

Eruptive chronology and tectonic context of the late Pleistocene Tres Vírgenes volcanic complex, Baja California Sur (México)

Based on a detailed geological map and stratigraphy aided by new 230Th/238U and 206P/238U age determinations we established the eruptive chronology of the Tres Vírgenes Volcanic Complex (TVVC). The complex consists of three northeast-southwest aligned stratovolcanoes, which from older to younger are El Viejo, El Azufre, and La Virgen and associated cinder cones and domes. The alignment was fed from fissures that mark the southern trace of the left-lateral Cimarron Fault. The TVVC was built upon the basement rocks of the Peninsular Ranges Batholith (~99 Ma), the Santa Lucia Formation (21.6 Ma), the Esperanza basalt (7.6 Ma), and the 1.1 Ma ignimbrite of El Aguajito caldera. The TVVC began its formation at ca. 300 ka mainly with dacitic lava flows and domes that formed El Viejo volcano (4.2 km3). At around 173 ka, activity migrated to the southwest to build El Azufre volcano (3.8 km3) with the emission of dacitic lavas and several phases of dome construction and destruction with the generation of pyroclastic density currents. El Azufre's activity ended ~128 ka with the extrusion of the summit dome. At about 112 ka, volcanism migrated 4.2 km to the southwest to begin the construction of La Virgen Volcano through the emission of andesitic and dacitic lavas that built the largest cone of the complex until ~22 ka (31.2 km3). This volcano was built upon several stages of cone construction with the emission of lava flows and peripheral dacitic domes and cinder cones. Between 128 and 112 ka six cinder cones vented along the Cimarron Fault between the La Virgen and El Azufre volcanoes. By assuming a constant emission of magma the total volume of the TVVC (39 km3) was erupted at an average rate of 0.13 km3/kyr. Of this volume, effusive eruptions dominated the evolution of the complex (89%) of which 62% are dacites and 38% andesites. The TVVC rocks have compositions of 50–67 wt% in silica, and low-medium K (0.5–2.5 wt% K2O). Trace element signatures of the TVVC rocks (Sr/Y <40, La/Yb <10, Nb-Ti negative anomalies and relative enrichments of LILE and LREE) suggest a post-subduction mixture between adakitic and calc-alkaline magmas. Xenoliths of granodiorites hosted in lavas and La Virgen tephra suggest that magma chambers feeding the complex have stagnated at depths >2 km within the Peninsular Ranges Batholith as confirmed by geobarometry, aeromagnetic data, and the drill-holes of the geothermal field. Today, the Tres Vírgenes geothermal field generates 10 MW of electricity although future exploration of several nearby areas may increase such capacity.

Extensive young silicic volcanism produces large deep submarine lava flows in the NE Lau Basin

New field observations reveal that extensive (up to ~ 402 km2) aphyric, glassy dacite lavas were erupted at multiple sites in the recent past in the NE Lau basin, located about 200 km southwest of Samoa. This discovery of volumetrically significant and widespread submarine dacite lava flows extends the domain for siliceous effusive volcanism into the deep seafloor. Although several lava flow fields were discovered on the flank of a large silicic seamount, Niuatahi, two of the largest lava fields and several smaller ones (“northern lava flow fields”) were found well north of the seamount. The most distal portion of the northernmost of these fields is 60 km north of the center of Niuatahi caldera. We estimate that lava flow lengths from probable eruptive vents to the distal ends of flows range from a few km to more than 10 km. Camera tows on the shallower, near-vent areas show complex lava morphology that includes anastomosing tube-like pillow flows and ropey surfaces, endogenous domes and/or ridges, some with “crease-like” extrusion ridges, and inflated lobes with extrusion structures. A 2 × 1.5 km, 30-m deep depression could be an eruption center for one of the lava flow fields. The Lau lava flow fields appear to have erupted at presumptive high effusion rates and possibly reduced viscosity induced by presumptive high magmatic water content and/or a high eruption temperature, consistent with both erupted composition (~ 66% SiO2) and glassy low crystallinity groundmass textures. The large areal extent (236 km2) and relatively small range of compositional variation (σ = 0.60 for wt% Si02%) within the northern lava flow fields imply the existence of large, eruptible batches of differentiated melt in the upper mantle or lower crust of the NE Lau basin. At this site, the volcanism could be controlled by deep crustal fractures caused by the long-term extension in this rear-arc region. Submarine dacite flows exhibiting similar morphology have been described in ancient sequences from the Archaean through the Miocene and in small batches on present-day seafloor spreading centers. This study shows that extensive siliceous lavas can erupt on the modern seafloor under the right conditions.

Hydrogeological Modelling of Some Geothermal Waters of Ivrindi, Havran and Gönen in the Province Capital of Balikesir, Western Anatolia, Turkey

In this study, hydrogeological, hydrogeochemical and isotope geochemical features of Havran, Gönen and Ivrindi within the province capital of Balıkesir, Turkey were investigated in detail. The Early Triassic Karakaya formation in the study area of Havran forms the oldest rocks consisting of spilitic basalts, diabases, gabbros, mudstones, cherts and radiolarites. There are limestone blocks in this formation with intercalations with sandstones and with feldspar contents, quartzite, conglomerates and siltstones. Oligocene to Miocene granodiorite intrusions were generated in association with intensively volcanic events in the area. Between Upper Oligocene and Early Miocene, andesitic and dacitic pyroclastic rocks cropped out due to intensively volcanism. Later, conglomerates, sandstones, claystones, marls and limestones as lacustrine sediments formed from Middle to Upper Miocene in the study area. In the study area of Gönen, the Lower Triassic Karakaya formation consists of basalts, diabases, gabbros, mudstones, cherts and radiolarites and forms the basement rocks overlain by Upper Jurassic to Lower Cretaceous sandy limestones. Upper and Middle Miocene volcanics which can be considered intensive Biga Peninsula volcanos outcrop in the area. These andesitic lava flows are of black, gray and red color with intensive fissures. Neogene lacustrine sediments consist of conglomerates, sandstones, marl, claystone and clayey limestones. Upper Miocene to Pliocene rhyolitic pyroclastics and dacitic lava flows are the volcanic rocks which are overlain by Pliocene conglomerates, sandstones and claystones. In the study area of Ivrindi, the Çaldağ limestones are the oldest formation in Permian age. Çavdartepe metamorphic rocks are of Lower Triassic in which can be observed marbles sporadically. The Kınık formation consisting of conglomerates, sandstones, siltstones and limestones are of Lower Triassic age and display a lateral Stratigraphic progress with volcanic rocks. Upper Miocene to Pliocene Yürekli formation consists of dacites and rhyodacites. Upper Miocene to Pliocene Soma formation is composed of clayey limestone, marl, siltstone, intercalations of sandstone, agglomerate and andesitic gravels and blocks cemented by tuffs. Quaternary alluvium is the youngest formation. The samples of geothermal waters in the area of Havran can be considered as Na-Ca-(SO4)-HCO3, Na-(SO4)-HCO3 and Na-SO4 type waters. In comparison, the geothermal waters in Gönen are of Na-(SO4)-HCO3 and Na-HCO3 type waters. The geothermal waters of Ivrindi are considered as Na-Ca-HCO3 type waters. In the area, a groundwater sample is of Ca-Mg-HCO3 type water.The geothermal waters belong to the cations of Na+K>Ca>Mg in Havran, Gönen and Ivrindi and to the anions of SO4>HCO3>Cl in Havran, HCO3>SO4>Cl in Gönen and SO4>HCO3>Cl in Ivrindi. In the diagram of Na-K-Mg1/2, the geothermal waters in Havran, Gönen and Ivrindi of the province capital of Balıkesir can be classified as immature waters.

The Quaternary history of effusive volcanism of the Nevado de Toluca area, Central Mexico

Andesite and dacite lava flows and domes, and intermediate-mafic cones from the Nevado de Toluca area were classified into five groups using field data and 40Ar/39Ar geochronology constraints. Thirty-four lava units of diverse mineralogy and whole-rock major-element geochemistry, distributed between the groups, were identified. These effusive products were produced between ∼1.5 and ∼0.05 Ma, indicating a mid-Pleistocene older-age for Nevado de Toluca volcano, coexisting with explosive products that suggest a complex history for this volcano. A ∼0.96 Ma pyroclastic deposit attests for the co-existence of effusive and explosive episodes in the mid-Pleistocene history. Nevado de Toluca initiated as a composite volcano with multiple vents until ∼1.0 Ma, when the activity began to centralize in an area close to the present-day crater. The modern main edifice reached its maximum height at ca. 50 ka after bulky, spiny domes erupted in the current summit of the crater. Distribution and geochemical behavior in major elements of lavas indicate a co-magmatic relationship between different andesite and dacite domes and flows, although unrelated to the magmatism of the monogenetic volcanism. Mafic-intermediate magma likely replenished the system at Nevado de Toluca since ca. ∼1.0 Ma and contributed to the eruption of new domes, cones, as well as effusive-explosive activity. Altogether, field and laboratory data suggest that a large volume of magma was ejected around 1 Ma in and around the Nevado de Toluca.

Folding, thrusting and development of push-up structures during the Miocene tectonic inversion of the Austral Basin, Southern Patagonian Andes (50°S)

Rift and post-rift sections of the Austral Basin in the Southern Patagonian fold and thrust belt were examined in outcrop and in 2D seismic reflection sections to evaluate geometric and kinematic aspects of the extension and subsequent inversion. The syn-rift section is composed of dacitic lava flows of the El Quemado Complex (Jurassic), the lowermost unit of the basin. The coastal sandstones of the Springhill Formation (Tithonian-Berriasian) and marine shales of the Río Mayer Formation (Berriasian-Albian) are thought to be sag units. However, field data suggest that these units are also part of the syn-rift successions, as evidenced by extensional growth strata in the lower levels of the Río Mayer Formation. A dacitic flow that is texturally and compositionally similar to those of the syn-rift phase and that is termed here the Río Guanaco dacite is interfingered with the basal strata of the Río Mayer Formation and has a U-Pb zircon crystallization age of c. 141Ma (Berriasian). The outcropping section was inverted by compression in the early Miocene, according to structural and isotopic data of adjacent areas, producing a broad anticline with inverted and fossil extensional faults as well as newly generated thrusts. At the seismic scale, most of the extensional master faults present negligible reverse slip, and shortening was accommodated in hanging wall push-up structures. This situation is interpreted as a result of the competent metamorphic basement and volcanic syn-rift section, inducing frictional lock-up of the master faults.

Paleoproterozoic andesitic volcanism in the southern Amazonian craton, the Sobreiro Formation: New insights from lithofacies analysis of the volcaniclastic sequences

The Sobreiro Formation (SF) records one excellent and well-preserved example of subaerial Precambrian (ca. 1.88Ga) volcanism on earth. It is located in the São Felix do Xingu region (SFX), in the eastern part of Pará State, southern Amazonian craton, northern of Brazil. The high-K calc-alkaline composition of the Sobreiro rocks indicates that this formation likely generated in an ocean-continent convergent margin. This paper documents the architecture of a series of basaltic-andesite to andesite and minor dacite lava flows and associated volcaniclastic rocks. These rocks are divided into primary and secondary lithotypes, depending if they resulted from a direct volcanic activity (pyroclastic) or reworked processes and massive to stratified, depending on the different transport and emplacement mechanisms. Primary lithofacies is formed of pyroclastic flow and surge deposits, volcanic breccias and welded ignimbrites; secondary volcaniclastic lithofacies consist of reworked debris. Mass-flows, hyperconcentrated flows and stream floods are interpreted as fluvial/alluvial deposits representing periods of stream and river reworking and re-establishment after an eruptive phase or an edifice failure event. These different lithofacies record the volcanic history of the Sobreiro Formation. A complex volcanic environment is thought to have existed with emission of large lava flows and explosive eruptions. The modern volcanological approach used here can serve as a model for the evolution of Precambrian volcano-sedimentary basins. Our approach sheds new light on the different processes operating on volcanic edifices and to constrain the depositional environment and thus geodynamic setting of Precambrian continental volcanic belts.

Antisana volcano: A representative andesitic volcano of the eastern cordillera of Ecuador: Petrography, chemistry, tephra and glacial stratigraphy

Antisana volcano is representative of many active andesitic strato-volcanoes of Pleistocene age in Ecuador's Eastern Cordillera. This study represents the first modern geological and volcanological investigation of Antisana since the late 1890's; it also summarizes the present geochemical understanding of its genesis. The volcano's development includes the formation and destruction of two older edifices (Antisana I and II) during some 400 + ka. Antisana II suffered a sector collapse about 15,000 years ago which was followed by the birth and growth of Antisana III. During its short life Antisana III has generated ≥50 eruptions of small to medium intensity, often associated with andesitic to dacitic lava flows and tephra, as well as with late Pleistocene and Holocene glacial advances. Throughout its long history Antisana's lavas have been characterized by a persistent mineral assemblage, consisting of 30–40 vol% phenocrysts of plagioclase, both clino- and orthopyroxene, and Fe-Ti oxides, with rare occurrences of olivine or amphibole, frequently in a microcrystalline to glassy matrix. This uniformity occurs despite the magma's progressive chemical evolution over ≥400 ka from early basic andesites (53–58 wt% SiO2) to intermediate and Si-rich andesites (58–62% SiO2), and recently to dacites (63–67% SiO2). Chemical diagrams suggest that crystal fractionation was the most likely magmatic process of evolution. The exception to this slowly evolving history was the short-lived emission at ∼210 ka of the Cuyuja lavas from Antisana II that generated a 73 km long andesitic lava flow. Contrasting with Antisana's general magmatic trend, Cuyuja lava (∼11 km3) is a high-Mg andesite with unusually high concentrations of incompatible elements. Antisana developed within the Chacana caldera complex, a large active siliceous center that began ∼3 Ma ago, however its lavas are chemically distinct from coeval lavas of Chacana.

A high-resolution 40Ar/39Ar lava chronology and edifice construction history for Ruapehu volcano, New Zealand

Ruapehu is an active ~150km3 andesite-dacite composite volcano located at the southern end of the Taupo Volcanic Zone, New Zealand. The growth of the present-day edifice has occurred throughout coeval eruptive and glacial histories since ~200ka. We present high-precision 40Ar/39Ar eruption ages for 46 samples and whole-rock major element geochemical data for 238 samples from lava flows. These new and existing data are interpreted in the context of geomorphologic and geologic mapping, volcano-ice interaction processes and glacier reconstructions to present an improved chronostratigraphic framework and new edifice evolution history for Ruapehu. Sub-glacial to ice-marginal effusive eruption of medium-K basaltic-andesites and andesites constructed the northern portion of the exposed edifice between ~200 and 150ka, and a wide southeast planèze as well as parts of the northern, eastern and western flanks between ~160 and 80ka. None of the dated lava flows have ages in the range of 80–50ka, which may reflect an eruptive hiatus. Alternatively the lack of ages may be the result of erosion and burial of lavas and syn-eruptive glacial conveyance of lava flows to the ring-plain during glacial advance at 70–60ka. From ~50–15ka edifice growth occurred via effusive eruptions onto the glaciated flanks of the volcano, resulting in construction of ice-bounded planèzes and ridges. The distribution of dated ice-marginal lavas indicates a general decrease in glacier thicknesses over this time, except for a short-lived period centred at ~31ka when peak ice cover was reached. The compositional ranges of medium- to high-K andesite and dacite lava flows within 50–35ka eruptive packages define broadly bimodal high- and low-MgO trends. Lavas erupted at ~35–22ka have compositions that fill a transitional geochemical field between older dacites and younger andesites. Large-scale retreat of flank glaciers since ~15ka has resulted in intra-valley lava flow emplacement at elevations below ~1500m on the edifice. Between 15 and 10ka unstable cones were constructed through effusive activity in the presence of remnant summit and upper flank glaciers and the emplacement of eruptive deposits on top of hydrothermally altered and fragmental sub-glacial and ice-marginal deposits. Debuttressing of two northern summit cones and a southern summit cone as ice underwent continued post-glacial retreat pre-conditioned the edifice for two major sector collapses. Ultimate triggering of the collapse events and deposition of debris avalanche deposits on the northern and south-eastern flanks may have been the result of tectonic or volcanic activity. The northern collapse scar is infilled by a new cone comprising <10ka lava flows that form the upper northern and eastern flanks of the modern edifice. Late Holocene-Recent eruptive activity has occurred through Crater Lake, which occupies the site of the collapsed southern cone. Deglaciation did not induce enhanced magma flux rates at Ruapehu as indicated by calculated eruptive volumes and their chronologies. However, volcano-ice interactions have had a fundamental influence on eruptive styles, sector collapse events, and the shape of the edifice by modifying the distribution, morphology and preservation of eruptive deposits for at least 200kyr. Our approach to integrating field and geochronological datasets from Ruapehu is directly relevant to the evolution of glaciated Quaternary composite cones worldwide, particularly those along the circum-Pacific continental volcanic arcs.

Submarine Volcanism: a Review of the Constraints, Processes and Products, and Relevance to the Cabo de Gata Volcanic Succession

Understanding of submarine volcanism still lags behind understandingof subaerial volcanism for the obvious reason that eruptionsare usually not, or only partially visible, and deposits are largelyinaccessible. In addition, our understanding of the effects of theambient water mass and the way in which erupting magma andwater interact, and the ways in which the physical properties of thewater mass control and influence eruption styles, dispersal processesand deposit characteristics, is still at a relatively early stage. In particular,in the past there has been a very simplistic approach to assessingconstraints on eruption styles using only ambient hydrostaticpressure, whereas equally important properties such as bulk modulus,compressibility, deformability, attenuation properties, thermalconductivity, heat capacity, have not been adequately considered.This review summarises our understanding on these issues, andbriefly summarises varying eruption conditions, styles and depositcharacteristics.The origin of pumice deposits preserved in subaqueous settingsneeds to be interpreted with care. They could represent local explosiveevents, but could also represent pumice sourced from distantexplosive events and vents, even subaerial, and deposited by falloutthrough water, the passage of pyroclastic flows into water fromsubaerial vents, long distance rafting of buoyant pumice by currents,reworking and resedimentation, and even non-explosive subaqueousvesiculation and quench fragmentation of subaqueously eruptedlavas in shallow or even deep water.The Miocene Cabo de Gata volcanic succession of southeasternSpain was deposited in shallow marine environments based on interbeddedfossiliferous limestones, which represent periods of volcanichiatus. The volcanic facies are consistent with a shallow marine setting,involving eruption of low volatile bearing magmas to formandesite and dacite lava flows, domes and associated hyaloclastites,as well as eruptions of volatile rich rhyolitic magmas that producedpumice deposits erupted from subaerial vents or vents at waterdepths that were too shallow to suppress explosive eruptionsthrough the effects of hydrostatic pressure, but were then reworkedand resedimented.

The emergence and growth of a submarine volcano: The Kameni islands, Santorini (Greece)

The morphology of a volcanic edifice reflects the integrated eruptive and evolutionary history of that system, and can be used to reconstruct the time-series of prior eruptions. We present a new high-resolution merged LiDAR-bathymetry grid, which has enabled detailed mapping of both onshore and offshore historic lava flows of the Kameni islands, emplaced in the centre of the Santorini caldera since at least AD 46. We identify three new submarine lava flows: two flows, of unknown age, lie to the east of Nea Kameni and a third submarine flow, located north of Nea Kameni appears to predate the 1925–1928 lava flows but was emplaced subsequent to the 1707–1711 lava flows. Yield strength estimates derived from the morphology of the 1570/1573 lobe suggest that submarine lava strengths are approximately two times greater than those derived from the onshore flows. To our knowledge this is the first documented yield strength estimate for submarine flows. This increase in strength is likely related to cooling and thickening of the dacite lava flows as they displace sea water. Improved lava volume estimates derived from the merged LiDAR-Bathymetry grid suggest typical lava extrusion rates of ∼2–3m3s−1 during four of the historic eruptions on Nea Kameni (1707–1711, 1866–1870, 1925–1928 and 1939–1941). They also reveal a linear relationship between the pre-eruption interval and the volume of extruded lava. These observations may be used to estimate the size of future dome-building eruptions at Santorini volcano, based on the time interval since the last significant eruption.

Magmatic controls on eruption dynamics of the 1950 yr B.P. eruption of San Antonio Volcano, Tacaná Volcanic Complex, Mexico–Guatemala

San Antonio Volcano, in the Tacaná Volcanic Complex, erupted ~1950yr. B.P., with a Pelean type eruption that produced andesitic pyroclastic surges and block-and-ash flows destroying part of the volcano summit and producing a horse-shoe shaped crater open to the SW. Between 1950 and 800yrB.P. the eruption continued with effusive andesites followed by a dacite lava flow and a summit dome, all from a single magma batch. All products consist of phenocrysts and microphenocrysts of zoned plagioclase, amphibole, pyroxene, magnetite±ilmenite, set in partially crystallized groundmass of glass and microlites of the same mineral phases, except for the lack of amphibole. Included in the andesitic blocks of the block-and-ash flow deposit are basaltic andesite enclaves with elongated and ellipsoidal forms and chilled margins. The enclaves have intersertal textures with brown glass between microphenocrysts of plagioclase, hornblende, pyroxene, and olivine, and minor proportions of phenocrysts of plagioclase, hornblende, and pyroxene. A compositional range obtained of blocks and enclaves resulted from mixing between andesite (866°C±22) and basaltic andesite (enclaves, 932°C±22), which may have triggered the explosive Pelean eruption. Vestiges of that mixing are preserved as complex compositional zones in plagioclase and clinopyroxene-rich reaction rims in amphibole in the andesite. Whole-rock chemistry, geothermometry, experimental petrology and modeling results suggest that after the mixing event the eruption tapped hybrid andesitic magma (≤900°C) and ended with effusive dacitic magma (~825°C), all of which were stored at ~200MPa water pressure. A complex open-system evolution that involved crustal end-members best explains the generation of effusive dacite from the hybrid andesite. Amphibole in the dacite is rimmed by reaction products of plagioclase, orthopyroxene, and Fe–Ti oxides produced by decompression during ascent. Amphibole in the andesite, however, lacks such rims. Because the andesite was at 866±22°C and the dacite was at ~825°C, the reaction rims indicate that the andesitic magma ascended at 0.023ms−1 during the explosive phase of the eruption, whereas the dacitic magma rose more slowly at ~0.002–0.004ms−1.

Geochronology and magmatic evolution of the Huautla volcanic field: last stages of the extinct Sierra Madre del Sur igneous province of southern Mexico

The Huautla volcanic field (HVF), in the Sierra Madre del Sur (SMS), is part of an extensive record of Palaeogene magmatism reflecting subduction of the Farallon plate along the western edge of North America. Igneous activity resulting from Farallon subduction is also exposed to the north, in the Sierra Madre Occidental (SMO) and Mesa Central (MC) provinces. We present the results of a stratigraphic and K–Ar, Ar–Ar, and U–Pb geochronological study of the Huautla volcanic successions, in order to refine our knowledge on the petrologic and temporal evolution of the northern SMS and gain insights on magmatic–tectonic contrasts between the SMS and the SMO–MC provinces. The HVF is made up of lava flows and pyroclastic successions that overlie marine Cretaceous sequences and post-orogenic continental deposits of Palaeogene age. In the study area, the main Oligocene succession is pre-dated by the 36.7 million years its caldera west of the Sierra de Huautla. The HVF succession ranges in age from ∼33.6 to 28.1 Ma and comprises a lower group of andesitic–dacitic lava flows, an intermediate sequence of ignimbrites and dacitic lavas, and an upper group of andesitic units. The silicic succession comprises a crystal-poor ignimbrite unit (i.e. the Maravillas ignimbrite; 31.4 ± 0.6, 32.0 ± 0.4 Ma; ∼260 km3), overlain by a thick succession of dacitic lavas (i.e. the Agua Fría dacite; 30.5 ± 1.9, 31.0 ± 1.1 Ma). Integration of the new stratigraphic and geochronological data with prior information from other explosive centres of the north-central SMS allows us to constrain the temporal evolution of a silicic flare-up episode, indicating that it occurred between 37–32 Ma; it consisted of three major ignimbrite pulses at ∼36.5, ∼34.5, and ∼33–32 Ma and probably resulted from a progressive, mantle flux-driven thermomechanical maturation of the continental crust, as suggested in the HVF by the transition from andesitic to voluminous siliceous volcanism. The information now available for the north-central sector of the SMS also allows recognition of differences between the temporal and spatial evolution of magmatism in this region, and of that documented in the southern SMO and MC provinces, suggesting that such contrasts are probably related to local differences in configuration of the subduction system. At ∼28 Ma, the MC and southern SMO provinces experienced a trenchward migration of volcanism, associated with slab rollback; on the other hand, the broad, more stable distribution of Oligocene magmatism in the central and north oceanic plate was subducting at a low angle.

Eruptive history of Mount Katmai, Alaska

Mount Katmai has long been recognized for its caldera collapse during the great pyroclastic eruption of 1912 (which vented 10 km away at Novarupta in the Valley of Ten Thousand Smokes), but little has previously been reported about the geology of the remote ice-clad stratovolcano itself. Over several seasons, we reconnoitered all parts of the edifice and sampled most of the lava flows exposed on its flanks and caldera rim. The precipitous inner walls of the 1912 caldera remain too unstable for systematic sampling; so we provide instead a photographic and interpretive record of the wall sequences exposed. In contrast to the several andesite-dacite stratovolcanoes nearby, products of Mount Katmai range from basalt to rhyolite. Before collapse in 1912, there were two overlapping cones with separate vent complexes and craters; their products are here divided into eight sequences of lava flows, agglutinates, and phreatomagmatic ejecta. Latest Pleistocene and Holocene eruptive units include rhyodacite and rhyolite lava flows along the south rim; a major 22.8-ka rhyolitic plinian fall and ignimbrite deposit; a dacite-andesite zoned scoria fall; a thick sheet of dacite agglutinate that filled a paleocrater and draped the west side of the edifice; unglaciated leveed dacite lava flows on the southeast slope; and the Horseshoe Island dacite dome that extruded on the caldera floor after collapse. Pre-collapse volume of the glaciated Katmai edifice was ∼30 km3, and eruptive volume is estimated to have been 57 ± 13 km3. The latter figure includes ∼40 ± 6 km3 for the edifice, 5 ± 2 km3 for off-edifice dacite pyroclastic deposits, and 12 ± 5 km3 for the 22.8-ka rhyolitic pyroclastic deposits. To these can be added 13.5 km3 of magma that erupted at Novarupta in 1912, all or much of which is inferred to have been withdrawn from beneath Mount Katmai. The oldest part of the edifice exposed is a basaltic cone, which gave a 40Ar/39Ar plateau age of 89 ± 25 ka.

Geochronology of miocene volcanism in the northern part of the Lesser Caucasus (Erusheti Highland, Georgia)

Study of epochs of tectonomagmatic activity of the Earth in different parts of the planet and under various geodynamic environments suggests as one of the main tasks of such investigations establishment of their age, duration, evolution dynamics, spatial–age regularities of volcanic focus migration, and other characteristics allowing one to reconstruct the history of magma gen� esis for concrete regions on the level of individual time lags of the geochronological scale. In this paper we discuss the results of the study of geochronology of Miocene volcanism in the northern part of neovolcanic province in the Lesser Caucasus, which allows us to determine the time of beginning and total duration of the Late Cenozoic epoch of tec� tonomagmatic activization in this sector of the Cauca� sian–Anatolian segment of the Alpine foldbelt. Products of the earliest impulses of Neogene– Quaternary volcanism within the northern end of the mountain system of the Lesser Caucasus, as is evident from the results of geological and stratigraphic investi� gations [1], are abundant in the upper reaches of the Kura River on its left bank, where they form the exten� sive (60 × 30 km) Erusheti volcanic highland on the territory of Georgia and Turkey (Fig. 1). They are united in the Goderdzi Formation in the Georgian part of the region. The lower parts of sections of the formation are composed of the pyroclastic series with a thickness of up to 700 m represented by welded and loose layered tuff, breccia, and agglutinate, in which thin lava flows of dacitic or more basic composition are sometimes observed. Numerous volcanic edifices (Gumbati, Shalosheti, Digra, and others) significantly destroyed and smoothed by erosion, their dacite lava flows forming a sheet with a thickness of up to several hundred meters, and overlying pyroclastic formations on most of the region are located on the surface of the Erusheti highland. The maximal heights within the highland reach 3000 m (on the territory of Turkey); its basement is entirely composed of sedimentary and volcanogenic series of Eocene age. Effusive rocks of the Goderdzi Suite in the east in valleys of the Kura River and its right tributary Paravani are overlain by mainly basic Pliocene lavas of the Dzhavakheti high� land (3.7–1.6 Ma [2]) and further, in the latitudinal direction most likely entirely thin out in sections within the area of western ends of the Samsari ridge, where Quaternary lavas occur directly on rocks of the Early Cenozoic–Mesozoic basement [3]. The available concepts on the age of rocks of the Goderdzi Formation were mainly based on the results of investigations of plant and fossil remains in horizons of diatomic clays, which are known and were previ� ously mined in some locations at the bottom of the young effusive series in the Erusheti Highland [1, 4]. These data allowed us to establish the possible age range of the formation of pyroclastic rocks and lavas of the formation as Late Miocene–Early Pliocene. !"# !"$ !"% !"" !"& $# $$ $% !"’ !"( !&) !&* !&! + , .