Andesitic Lava Flows Research Articles

Mineral assemblages and temperature associated with Cu enrichment in the Seival area (Neoproterozoic Camaquã Basin of Southern Brazil)

The Neoproterozoic sequence of volcanic rocks in the Camaquã Basin included in the Lavras do Sul Shoshonitic Association hosts disseminated Cu deposits. The volcanic sequence in the Seival Mine area includes andesitic lava flows, lapilli tuff, volcanic agglomerate and andesitic dikes with pervasive alteration. The hydrothermal alteration is interpreted to be a product of late–magmatic fluids or the mixing of magmatic with meteoric fluids or basinal brines. The late–magmatic hydrothermal system started after volatile oversaturation and degassing in a magmatic system, which was partially segregated into vesicles. The similar REE contents in the andesine–labradorite and albite indicate that the fluid temperatures ranged from 600 to 250 °C. The crystallization of titanite and Fe–clinochlore to Mg–chamosite followed the albitization process. The temperature range associated with the chloritization process was estimated using a chlorite geothermometer and varied from 251 ± 56 to 183 ± 39 °C. The precipitation of Mg–saponite and smectite–rich chlorite/smectite mixed–layers in some andesitic lava flows and the precipitation of quartz, calcite, barite and hematite in fractures suggest fluid circulation with temperatures lower than the chloritization process. The alteration minerals are represented by chlorite and albite associated with pyrite–chalcopyrite, while chlorite/smectite mixed–layers and barite or hematite are associated with bornite–chalcocite–covellite. Thermodynamic calculations confirm the potential coprecipitation of pyrite–chalcopyrite with chlorite and albite. The occurrence of bornite–chalcocite–covellite and barite seems to be favored by low–temperature chlorite/smectite with neutral to mildly acidic water influx. Consequently, albitization and the process of chloritization at high temperatures (>ca. 251 ± 56 °C) can be used as an exploration guide for primary pyrite–chalcopyrite enrichment, and chlorite/smectite at low temperatures (ca. 250 to 50 °C) can be related to bornite–chalcocite–covellite or the process of Cu enrichment.

Lava flow morphology at an erupting andesitic stratovolcano: A satellite perspective on El Reventador, Ecuador

Lava flows pose a significant hazard to infrastructure and property located close to volcanoes, and understanding how flows advance is necessary to manage volcanic hazard during eruptions. Compared to low-silica basaltic flows, flows of andesitic composition are infrequently erupted and so relatively few studies of their characteristics and behaviour exist. We use El Reventador, Ecuador as a target to investigate andesitic lava flow properties during a 4.5 year period of extrusive eruption between February 2012 and August 2016. We use satellite radar to map the dimensions of 43 lava flows and look at variations in their emplacement behaviour over time. We find that flows descended the north and south flanks of El Reventador, and were mostly emplaced over durations shorter than the satellite repeat interval of 24 days. Flows ranged in length from 0.3 to 1.7 km, and the length of flows generally decreased over the observation period. We measure a decrease in flow volume with time that is correlated with a long-term exponential decrease in effusion rate, and propose that this behaviour is caused by temporary magma storage in the conduit acting as magma capacitor between the magma reservoir and the surface. We use the dimensions of flow levées and widths to estimate the flow yield strengths. We observe that some flows were diverted by topographic obstacles, and compare measurements of decreased channel width and increased flow thickness at the obstacles with observations from laboratory experiments. Radar observations, such as those presented here, could be used to map and measure properties of evolving lava flow fields at other remote or difficult to monitor volcanoes.

New chronostratigraphic constraints on the emplacement of Miocene high-K calc-alkaline igneous rocks from West Edough-Cap de Fer, NE Algeria

Miocene K-rich calc-alkaline magmatic rocks are exposed over a ~ 150 km2 area in Cap de Fer and West Edough, along the NE Algerian Mediterranean coast. They include andesitic pyroclastic and lava flows, small dioritic plutons, and large microgranodioritic intrusions that intrude and/or overlie Miocene sediments. New 40K-40Ar ages obtained on the igneous rocks range from 16.84 ± 0.58 to 12.91 ± 0.31 Ma and define three successive magmatic pulses at ~ 16.5, ~ 15, and ~ 13 Ma. These data are in good agreement with biostratigraphic data obtained on the Miocene marls that yielded Langhian (N8 biozone) to Late Langhian-Serravallian ages (N9–N10 biozones). This consistency suggests that the time span (~ 3.5 m.y.) deduced from K-Ar datings is not due to perturbations of the K-Ar clock; it is more likely related to long-lasting tectono-magmatic processes. According to the regional context (“no-slab” area), these processes result from the uprise of hot asthenospheric mantle through the tear generated by the detachment of the oceanic part of the African slab beneath the Algerian margin that triggered the partial melting of the overlying metasomatized lithospheric mantle of the Kabylides.

Stratigraphy and Geochemistry of the Goedgenoeg and Makwassie Formations, Ventersdorp Supergroup, in the Bothaville area of South Africa

Abstract The Neoarchaean Ventersdorp Supergroup represents a fully evolved bimodal volcano-sedimentary succession on the Kaapvaal Craton. Stratigraphic information was mainly derived from exploration drilling for gold in the underlying Witwatersrand Supergroup. Following the basal flood basalts of the Klipriviersberg Group, the Platberg Group mostly comprises extensive and voluminous intermediate to felsic volcanics of the Goedgenoeg and Makwassie Formations. Detailed stratigraphic and geochemical reports of these two formations were previously mostly restricted to the so-called Bothaville gap, between the Free State and Klerksdorp Gold Fields. The lower part of the Goedgenoeg Formation succession consists of non-porphyritic andesitic lava flows, followed upwards by dacitic phenocryst-rich feldspar porphyries. The basal lava flows are frequently interbedded with sedimentary rocks of the underlying Kameeldoorns Formation. Andesitic lava-flow units persist upwards, occurring interbedded with the Goedgenoeg Formation feldspar porphyries. The feldspar porphyries originated as ash flows; most are highly welded. The upper feldspar porphyries in the succession are absent in large parts of the Bothaville gap. The overlying Makwassie Formation consists of quartz porphyries, also with intermittent interbedded mafic lava units throughout the succession. The upper part of the succession is composed of phenocryst-rich crystal tuffs, only preserved over limited areas of the Bothaville gap. The quartz porphyries are highly welded, high-temperature ash flows with individual flows of tens of metres in thickness, typically separated by thin tuffs and sediments. The Goedgenoeg and Makwassie interbedded andesitic lavas have similar geochemical composition (the geochemical composition of the overlying Rietgat Formation lavas is also similar to that). The Goedgenoeg dacitic feldspar porphyries are separated into two geochemical groups: a lower high-Zr group and an upper low-Zr group. The Makwassie quartz porphyries are of dacitic compositions, while the upper part is rhyolitic. Two geochemical groups can be distinguished in the rhyolites, based on the Zr/Cr ratio. Although each of the two groups occur as distinct stratigraphic units, in some boreholes the low-Zr/Cr group form the lower part of the rhyolite succession overlain by the high-Zr/Cr group, while in other areas the sequence is reversed. A significant hiatus may have existed between eruption of the Goedgenoeg and Makwassie volcanics. The andesitic mafic lavas persistently co-erupted with the Goedgenoeg dacites and with the Makwassie dacites and rhyolites, indicating a stable mafic magma source that was re-mobilised from time to time to initiate eruption of the more viscous dacite and rhyolite crustal melts. The Goedgenoeg and Makwassie dacites are mixed magmas of the mafic andesitic source with crustal melts, while the Makwassie Formation rhyolites are a distinct crustal melt.

Ben Nevis—remnant of a lost volcanic landscape

The summit region of Ben Nevis, Britain's highest mountain, consists of late Silurian to Early Devonian age volcanic rocks originally interpreted as a thick sequence (> 600 m) of andesite lavas and agglomerates that were down‐faulted during caldera subsidence. New digital field mapping of the Ben Nevis area, including both the steep north and south faces of the mountain, has revealed that the volcanic rocks consist largely of volcaniclastic debris flows, and extensive block and ash flow deposits with minor air‐fall tuff units. There is no evidence of any andesite lava flows or a volcanic vent. The volcanic detritus was derived from a volcanic centre situated to the NW of Ben Nevis, perhaps several tens of kilometres away. The rocks forming the summit region of the mountain have been re‐interpreted as a large roof pendant or keel of the former late Silurian to Early Devonian volcanic land surface that once covered much of the SW Highlands of Scotland.

The active submarine NW termination of the South Aegean Active Volcanic Arc: The Submarine Pausanias Volcanic Field

Methana peninsula shows the longest recorded volcanic history at the western end of the South Aegean Active Volcanic Arc, including volcanic products from the Upper Pliocene to recent times. The volcanic rocks comprise widespread dacite domes and andesite lava flows from several small volcanic centers and are only imprecisely dated. In this paper, the integrated analysis of swath bathymetry, side scan sonar data, and high resolution seismic reflection profiles correlated with core samples, has allowed detailed mapping, characterization and precise chronological identification of the Pausanias submarine volcanic field activity offshore northern Methana. Six volcanic cones or domes are recognized, typically 1–3 km in diameter, some elongated NE–SW and some with a small central crater. On their flanks, the acoustically reflective volcanic rocks pass laterally into incoherent transparent seismic facies interpreted as volcaniclastic deposits, possibly including hyaloclastites, that interfinger with the regional basin sediments. A sea-bottom hummocky field, is interpreted as volcanic avalanche and appears to be the submarine continuation of the volcaniclastic apron of northern Methana peninsula. A robust chronostratigraphic framework has been established, based on the recognition of shoreline progradational units and their connection with Quaternary eustatic sea level cycles. Relative dating of the different phases of submarine volcanic activity during the Upper Quaternary has been achieved by correlating the imaged volcaniclastic flows, interlayered within the chronostratigraphically dated sediments. Dating by stratigraphic position, relative to 2D imaged eustatic sea level clinoform wedges appears to be more precise than radiometric methods on land. Three main submarine Volcanic Events (VE) are recognized: VE3 at ~450 ka, a less precisely dated interval at 200–130 ka (VE2), and VE1 at ~14 ka. Based on chronostratigraphic constraints, subsidence rates of 0.16 (±0.008) m/ka in-between Marine Isotopic Stages 6 and 12 and 0.19 (±0.009) m/ka in-between Marine Isotopic Stages 12 and 16 were estimated for the marine basin north of Methana. The morphological similarity to the onshore volcanoes of Methana Peninsula implies that magmatic constructive processes were dominant, regardless of whether in air or in water. The Upper Quaternary submarine volcanic rocks of Methana differ from those known from stratovolcanoes elsewhere in the Mediterranean, (e.g. Kos-Nisyros, Stromboli) and in other volcanic arcs (e.g., Montserrat, St Vincent), in the submarine development of domes or small cones, the paucity of volcano flank failure deposits and the lack of explosive events. Pausanias volcanic products date the onset of NE–SW faulting as well as the following tectonic phase of E-W striking faults, possibly related to basin inversion, caused by a major rifting phase that also affected most of the South Aegean Arc and the adjacent Gulfs of Corinth and Argolikos.

Mechanisms of lava flow emplacement during an effusive eruption of Sinabung Volcano (Sumatra, Indonesia)

The ongoing effusive phase of the eruption of Sinabung Volcano (Sumatra, Indonesia) began in late December 2013, and has produced a 2.9km long andesitic lava flow with two active secondary summit lobes, frequent pyroclastic density currents (PDCs) (with ≤5km runout distance), and associated plumes up to 5km in height. Large intermediate to silicic composition lava flows of the type documented here are common at volcanoes around the world, but they are infrequently observed while active. This eruption provides a special opportunity to observe and study the mechanisms of emplacement and growth of an active andesitic lava flow. We use visible and thermal satellite images to document the flow and describe the dominant processes driving emplacement of the lava over the course of the eruption. Effusion and flow advance rates were at their highest in January–March 2014. A decrease in flow advance rate in late March 2014 from 20 to 70md−1 to <5md−1 was the result of a decrease in effusion rate from ~9m3s−1 to ~3m3s−1. Initial flow emplacement was most likely controlled by the yield strength of the flow crust, which we estimate to have increased in thickness from 1 to 4m during January–June 2014, calculated from average flow surface temperatures that decreased from ~60°C to <30°C during this period. Further decrease in flow advance rate in June 2014 to ~1md−1 suggests that the flow's interior had cooled, and that propagation was limited by the yield strength of the flow's interior (core). Inflation of the flow during this period of core-controlled slow advance eventually caused lava to overtop ridges bounding the flow near the summit, and created significant gravitational instabilities. These instabilities led to collapse of the upper portions of the lava flow and generated PDCs, followed by breakout of new flow lobes from the collapse scars in October 2014 and June 2015. Effusion continues as of June 2017 and presents a significant hazard for collapse and generation of PDCs. This ongoing activity appears to represent a typical eruption of Sinabung, with flow length and area similar to numerous older flows observed around the volcano.

Geochemical and tectonic evolution of Late Cretaceous to early Paleocene magmatism along the Southern Central Andes (35-36°S)

Late Cretaceous-early Paleocene magmatism at Southern Central Andes is represented by incomplete and not well-preserved outcrops. A recent U-Pb age from a volcano-sedimentary sequence in the southern Andes of Mendoza province (35°30′S) led to identify Late Mesozoic-Early Cenozoic magmatism, which was not previously registered. We focus on the petrological and geochemical characterization of this sequence, informally named as Los Ángeles Unit (67 Ma). This magmatic association comprises basaltic and andesitic lava flows interbedded with volcanic breccias and lithic sandstones; all of them intruded by subvertical basaltic dykes. Geochemically, the volcanic rocks are mainly tholeiitic with an arc-like signature and high LILE relative to HFSE content. A general enrichment in incompatible elements is seen towards the upper and younger lava flows, associated with a more alkaline character in the youngest ones. A regional comparison among Los Ángeles Unit and other Late Cretaceous-Paleocene arc magmatic series along the Southern Central Andes (35°-41°S) shows that the studied sequence has a weaker arc-like signature and less slab-fluid influence than in coeval southern units. This contrasting geochemical behavior is proposed to be a consequence of the subduction of the Farallon-Aluk spreading ridge beneath the studied latitudes (35°30′S) at ∼70 Ma. Thus, the enrichment seen along the volcanic sequence together with the alkaline-like signature seen in the younger volcanic levels of Los Ángeles Unit would be related to the subduction of anhydrous oceanic crust and potentially the incipient development of a slab window. Similar geochemical features are seen to the south in the magmatic expressions of the northern Eocene Pilcaniyeu Belt outcrops (∼40°S; 44 Ma), as they show a limited arc-like signature and an intraplate-like composition. The segmented geometry of the Farallon-Aluk spreading ridge and its progressive migration from north to south is considered to be the main cause of the development of contrasting magmatic associations during time and space.

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.

Late Eocene volcanism in North Patagonia (42°30′–43°S): Arc resumption after a stage of within-plate magmatism

Mid-Cenozoic widespread arc magmatism in North Patagonia extends from the forearc to the retroarc zones, representing an anomalous large volume when compared to the present-day arc zone and even other past arc configurations. It represents a crucial stage in Andean arc evolution as was developed after a period of arc waning and within plate magmatism. Controversies exist regarding the origin of these volcanic sequences, with scarce integrated field, geochemical and geochronological analyses. We focused our study on the El Maitén Belt, located in the present-day retroarc zone, particularly on a poorly studied section corresponding to the southern outcrops of this volcanic belt. This volcanism consists of basaltic and andesitic lava flows and interbedded pyroclastic deposits, whose emplacement was controlled by extensional tectonics as indicated by the occurrence of wedge-like strata associated with normal faults. A U-Pb age on the basal part of this section shows that magmatic activity started by 37Ma, earlier than previous studies that considered this volcanism as Oligocene. Geochemically, these rocks are part of the subalkaline and particularly tholeiitic series. All samples show trace element enrichments, depletions and ratios characteristic of arc magmas, though fluids and sediment imprint seem limited. On these bases, we propose decompression melting as the main process associated with the genesis of this volcanism. Therefore, this magmatic association constrained to the late Eocene represents the earliest evidence of arc volcanism in the Patagonian Andes, under an extensional regime, after a Paleogene waning of arc activity.

Specific mineral associations of hydrothermal shale (South Kamchatka)

The sequence of hydrothermal shale from the East Pauzhet thermal field within the Pauzhet hydrothermal system (South Kamchatka) was studied in detail. It was established that the formation of shale resulted from argillization of an andesitic lava flow under the influence of an acidic sulfate vapor condensate. The horizons with radically different compositions and physical properties compared to those of the overlying homogeneous plastic shale were distinguished at the base of the sequence. These horizons are characterized by high (up to two orders of magnitude in comparison with average values in hydrothermal shale) concentrations of F, P, Na, Mg, K, Ca, Sc, Ti, V, Cr, Cu, and Zn. We suggested a geological–geochemical model, according to which a deep metal-bearing chloride–hydrocarbonate solution infiltrated into the permeable zone formed at the root of the andesitic lava flow beneath plastic shale at a certain stage of evolution of the hydrothermal system.

Quantitative geometric description of fracture systems in an andesite lava flow using terrestrial laser scanner data

Permeability hosted in andesitic lava flows is dominantly controlled by fracture systems, with geometries that are often poorly constrained. This paper explores the fracture system geometry of an andesitic lava flow formed during its emplacement and cooling over gentle paleo-topography, on the active Ruapehu volcano, New Zealand. The fracture system comprises column-forming and platy fractures within the blocky interior of the lava flow, bounded by autobreccias partially observed at the base and top of the outcrop. We use a terrestrial laser scanner (TLS) dataset to extract column-forming fractures directly from the point-cloud shape over an outcrop area of ∼3090 m2. Fracture processing is validated using manual scanlines and high-resolution panoramic photographs. Column-forming fractures are either steeply or gently dipping with no preferred strike orientation. Geometric analysis of fractures derived from the TLS, in combination with virtual scanlines and trace maps, reveals that: (1) steeply dipping column-forming fracture lengths follow a scale-dependent exponential or log-normal distribution rather than a scale-independent power-law; (2) fracture intensities (combining density and size) vary throughout the blocky zone but have similar mean values up and along the lava flow; and (3) the areal fracture intensity is higher in the autobreccia than in the blocky zone. The inter-connected fracture network has a connected porosity of ∼0.5 % that promote fluid flow vertically and laterally within the blocky zone, and is partially connected to the autobreccias. Autobreccias may act either as lateral permeability connections or barriers in reservoirs, depending on burial and alteration history. A discrete fracture network model generated from these geometrical parameters yields a highly connected fracture network, consistent with outcrop observations.

Geochemical study of pyroclastic rocks in Maninjau Lake, West Sumatra

Maninjau area situated in Barisan Mountain range was a central of volcanic eruption during Plistocene-Holocene. Eruption of Maninjau volcano resulted in andesitic lava flows, and pyroclastic flows. Maninjau deposites accummulated around Maninjau Lake are covered by andesitic rocks. This Plistocene - Holocene volcano might have a close link with Semangko fault (Sumatra Fault). Pyroclastic rocks in the study area predominantly consist of pumice deposites with some crystals of plagioclase ± quartz ± pyroxene + hornblende + andesitic, lithics and glass shards. These deposites are slightly unwelded and mostly weathered. On the basis of its chemical composistion, Maninjau deposite can be divided into Ryolitic Pumiceous Tuff (Ignembrite Maninjau) and Andesitic Pumiceous Tuff (Tephra) deposites. Ignembrite Maninjau deposite is characterized by the occurrence of high-K rhyolite, whereas Tephra deposite is able to be grouped into calc-alkaline andesite. In general, these deposites have major elements with various silisic contents; this sugests that Maninjau deposites originated from heterogeneous magmas, and was erupted from a magma chamber. The relationship between SiO2 and another oxide shows negative correlation except Na2O. Pumice deposites contain various high trace elements, suggesting that the source of magma was likely from the subducted slab of oceanic plate underneath an active continental margin.

Shallow crustal velocities and volcanism suggested from ambient noise studies using a dense broadband seismic network in the Tatun Volcano Group of Taiwan

The Tatun Volcano Group (TVG) is situated adjacent to the Taipei metropolis and was active predominantly around 0.8–0.2Ma (Pleistocene). Various recent lines of evidence suggest that the TVG is a potentially active volcano and that future volcanic eruptions cannot be ruled out. Geothermal activities are largely constrained to faults, but the relationship between volcanism and detailed velocity structures is not well understood. We analyzed ambient seismic noise of daily vertical components from 2014 using a dense seismic network of 40 broadband stations. We selected a 0.02° grid spacing to construct 2D and 3D shallow crustal phase velocity maps in the 0.5–3s period band. Two S-wave velocity profiles transect Chishingshan (Mt. CS) in the shallow 3km crust are further derived.The footwall of the Shanchiao Fault is dominated by low velocity, which may relate to Tertiary bedrock buried under andesitic lava flows dozens to hundreds of meters thick. The hanging wall of the Shanchiao Fault is the location of recent major volcanic activities. Low velocity zones in the southeast of Dayoukeng (DYK) may be interpreted as hydrothermal reservoirs or water-saturated Tertiary bedrock related to Cenozoic structures in the shallow crust. High velocities conspicuously dominate the east of the TVG, where the earliest stages of volcanism in the TVG are located, but where surface hydro-geothermal activities were absent in recent times. Between the Shanchiao Fault and Kanchiao Fault high velocities were detected, which converge below Mt. CS and may be related to early stages of magma conduits that gradually consolidated. These two faults may play a significant role with the TVG. The submarine volcanism adjacent to the Keelung coastline also requires further attention.

The influence of tectonic subsidence on volcaniclastic sedimentation: The Cretaceous upper Daeri Member, Wido Island, Korea

AbstractThe Daeri Member, a Cretaceous volcanic–sedimentary succession, can be divided into lower, middle, and upper parts based on vertical changes in its lithologic characters. The lower Daeri Member is composed of siliciclastic deposits formed in a semi‐arid floodplain environment, which is overlain by the middle Daeri Member consisting mainly of andesite lava flow. After the emplacement of the andesite, activities of intrabasinal normal faults created accommodation on hanging wall blocks together with the development of intrabasinal topographic relief. The upper Daeri Member occurs only in hanging wall blocks and is composed of rhyolitic volcaniclastic sediments formed during an explosive volcanic eruption. Following the eruption, owing to semi‐arid climatic conditions and the destruction of vegetation, the eruptive materials were easily remobilized and deposited by episodic sediment gravity flows, resulting in deposition of the resedimented volcaniclastic deposits with sheet‐like geometry. Away from the intrabasinal normal faults, the resedimented volcaniclastic deposits show a decrease in grain size together with changes in inferred depositional processes from debris flows to hyperconcentrated flows and supercritical sheetfloods. This suggests that the resedimented volcaniclastic deposits were stacked on alluvial fan environments induced by intrabasinal topographic relief associated with normal fault activities. In addition, episodic movement of the faults gave rise to periodic fluctuation of the accommodation and an increase in gradient of the alluvial fan surface, resulting in the development of coarsening‐upward trends in the resedimented volcaniclastic deposits. The development of the alluvial fan and the coarsening‐upward trends indicate that dynamic tectonic subsidence and concomitant changes in the intrabasinal physiographic relief influenced the depositional processes and sizes of the transported volcaniclastic sediments of the upper Daeri Member. Thus, it is necessary to carefully observe tectonic signatures in volcaniclastic successions, particularly the syneruptive lithofacies, in order to reconstruct the tectonic and volcanic histories of receiving basins.

SHRIMP U–Pb zircon ages and whole-rock geochemistry for the Şapçı volcanic rocks, Biga Peninsula, Northwest Turkey: implications for pre-eruption crystallization conditions and source characteristics

ABSTRACTPalaeogene and Neogene volcanic rocks are widespread on the Biga Peninsula of Northwest Turkey. These rocks were formed during the Eocene, Oligocene–Miocene, and late Miocene, and the early Miocene Şapçı volcanic rocks in the Balıkesir area consist of andesitic lava flows and associated pyroclastics. Temperatures, pressures, and oxygen fugacities calculated for the hornblendes in these andesitic rocks are 903–930°C, 3.3–4.8 kbar, and –9.91 to –11.88, respectively, and for the biotites they are 755–788°C, 1.30–1.74 kbar, and –14.88 to –13.98, respectively. SHRIMP U–Pb dating of zircons from three andesite samples gave ages of 22.72 ± 0.19, 22.97 ± 0.23, and 18.72 ± 0.17 Ma (early Miocene), and these are regarded as crystallization ages. Geochemical analyses show that the volcanic rocks are mainly high-K and calc-alkaline, and have high contents of large-ion lithophile elements and low contents of high-field strength elements, revealing that they evolved from parental magmas that were derived from an enriched subcontinental lithospheric mantle source. The chondrite-normalized rare earth element patterns of the rocks are concave upwards with LaCN/LuCN = 11.9–21.2 and EuCN/Eu* = 0.84–0.92, implying significant fractional crystallization of hornblende during their evolution. According to the petrological data with regional geology, Neogene magmatic activity on the Biga Peninsula has a post-collisional feature, and was closely related to slab break-off geodynamic model after collision of Tauride–Anatolide Block and Sakarya continent.

B3-03 Investigation on the lava tube cave and lava flows in Medicine Lake Volcano : Why lava tube caves cannot be found in the andesite lava flow?

B3-03 Investigation on the lava tube cave and lava flows in Medicine Lake Volcano : Why lava tube caves cannot be found in the andesite lava flow?

Characteristic of volcanic deposits in the southern part of Gunung Karang, Pandeglang - West Java

In the southern part slope of Gunung Karang in Pandeglang, there are many well-outcrops of volcanic materials that developed during Quaternary. The deposits represent various lithofacies both vertically and laterally. In general the lithologic characteristics that are found in the study area can be grouped into three units, (1). Tertiary sedimentary rock, which is consists of tuffaceous mudstones interbedded with fine-grained tuffaceous sandstone and siltstone, (2) primary volcanic product consist of andesite lava flow, volcanic breccia/agglomerate, and pyroclastic rocks (ash tuff, lapilli tuff, bomb/block, and lapilli tuff breccia, and (3) secondary volcanic products, which is consisting of laharic breccia and coarse-grained thick-bedded sandstone. In general the deposits become younger to the top of topography (Gunung Karang), but locally the deposits are most likely overlapping each other’s. Keywords: Volcanic deposits, Gunung Karang

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.

Geophysical image of the hydrothermal system of Merapi volcano

We present an image of the hydrothermal system of Merapi volcano based on results from electrical resistivity tomography (ERT), self-potential, and CO2 flux mappings.The ERT models identify two distinct low-resistivity bodies interpreted as two parts of a probably interconnected hydrothermal system: at the base of the south flank and in the summit area. In the summit area, a sharp resistivity contrast at ancient crater rim Pasar-Bubar separates a conductive hydrothermal system (20–50Ωm) from the resistive andesite lava flows and pyroclastic deposits (2000–50,000Ωm). The existence of preferential fluid circulation along this ancient crater rim is also evidenced by self-potential data. The significative diffuse CO2 degassing (with a median value of 400gm−2d−1) is observed in a narrow vicinity of the active crater rim and close to the ancient rim of Pasar-Bubar. The total CO2 degassing across the accessible summital area with a surface of 1.4 ⋅ 105m2 is around 20td−1. Before the 2010 eruption, Toutain et al. (2009) estimated a higher value of the total diffuse degassing from the summit area (about 200–230td−1). This drop in the diffuse degassing from the summit area can be related to the decrease in the magmatic activity, to the change of the summit morphology, to the approximations used by Toutain et al. (2009), or, more likely, to a combination of these factors.On the south flank of Merapi, the resistivity model shows spectacular stratification. While surficial recent andesite lava flows are characterized by resistivity exceeding 100,000Ωm, resistivity as low as 10Ωm has been encountered at a depth of 200m at the base of the south flank and was interpreted as a presence of the hydrothermal system. No evidence of the hydrothermal system is found on the basis of the north flank at the same depth. This asymmetry might be caused by the asymmetry of the heat supply source of Merapi whose activity is moving south or/and to the asymmetry in topography caused by the presence of Merbabu volcano in the north. On the basis of our results we suggest that stratified pyroclastic deposits on the south flank of Merapi screen and separate the flow of hydrothermal fluids with the gaseous part rising through the crater rims, while the liquid part is flowing downwards to the base of the edifice.