Rhyolitic Lava Domes Research Articles

Stratigraphy and eruptive history of Gedemsa caldera volcano, Central Main Ethiopian Rift

Gedemsa caldera is a peralkaline volcanic depression located in the Central sector of the Main Ethiopian Rift. An integrated volcanological study of stratigraphic sections was carried out in order to constrain the eruptive history of Gedemsa caldera volcano (GCV), Ethiopia. Textural analyses on plagioclase crystals together with field observations shed light on magma chamber processes feeding the volcanic complex. A multi-vent eruption is ascribed to the pre-caldera volcanic products, as can be seen from the varying caldera wall sequences comprising lavas and pyroclastic deposits. At least three major caldera-forming eruptions are identified producing: 1) a lower ignimbrite, 2) extensive pumice fall and pyroclastic density current deposits, and 3) an upper ignimbrite exposed in different areas of the caldera, indicating a sector collapse. We identified 20 individual pyroclastic deposits found outside and within the caldera that erupted after the climactic caldera collapse. The most recent volcanic activity at Gedemsa is characterized by rhyolitic lava domes and pyroclastic deposits from eruptions through numerous vents aligned with WNW-ESE pre-existing cross-cutting structures. Macrocryst disequilibrium textures such as fine-scale oscillatory zoning (FOZ), sieve textures, glomerocryst, and synneusis indicate that magma reservoir is characterized by repeated magma injection, convection flows, and mixing.

Amphibole recycling and polybaric crystallization in rhyolitic lava domes from melt inclusion geochemistry at Cerro La Torta, Altiplano-Puna Volcanic Complex

La Torta is a rhyolitic lava dome located in the El Tatio volcanic complex, Región de Antofagasta (northern Chile), to the west of the Cerros de Tocorpuri volcanoes. La Torta is considered to be an important part of the central zone of the Altiplano Puna Magma Body (APMB) based on chemical and petrological signatures that distinguish it from other highly silicic volcanic systems in the area (i.e., Chao dacite). The petrological and geochemical characteristics, along with parental composition and magma evolution, were determined by analyzing amphibole and plagioclase-hosted melt inclusions using electron microprobe (EMP) and laser ablation ICP-MS (LA-ICP-MS). Our results show that La Torta dome was formed from a rhyolitic Ca- and K-rich, calc-alkaline magma, with an average SiO2 content of 76.8 wt % and H2O content of about 5.4 wt %, which was emplaced in the upper continental crust between 3.9 and 7.6 km below the paleosurface. The parental source of La Torta dome was a highly hydrous mush of andesitic composition, with plagioclase + amphibole ± quartz ± pyroxene crystal assemblage.Heating by an influx of magmas ascending from deeper sources to upper crustal depths (i.e., Altiplano-Puna Magma Body) caused the partial melting of amphibole and pyroxene in the source, evidenced by amphibole dissolution textures, enriching the melt in Cr, Ni, HFSE and HREE. As the system starts to cool and ascend, new amphibole crystallized from this mush, trapping the enriched residual melt as enriched melt inclusions in its cores, along with crystallization of biotite and plagioclase. We refer to this process as amphibole recycling. At the same time, the new melt drags the plagioclase and quartz inherited from the parental mush, forming the magma that emplaced at shallower levels in the crust to eventually erupt as La Torta.

Inferring the magmatic roots of volcano-geothermal systems in the Rotorua Caldera and Okataina Volcanic Centre from magnetotelluric models

To investigate the underlying magmatic systems that drive high-temperature geothermal fields and volcanism in the north-central Taupo Volcanic Zone (TVZ), we use a combination of land and lake-bottom magnetotelluric (MT) measurements. In total, MT data from 399 sites (including 34 lake-bottom locations) across 2800 km2 covering the Rotorua Caldera and Okataina Volcanic Centre are inverted to image the 3-D resistivity structure of the crust to 20 km depth. Beneath this region, a singular conductive zone in the mid-crust is shown, inferred to represent a complex of relatively mafic magma, consistent with petrologic and other geophysical data. Discrete low-resistivity (conductive) plumes rise from the margins of this magmatic complex and connect 1-to-1 with the locations of high-temperature geothermal fields at the surface. These plumes are inferred to represent pathways of heat transfer through the crust via magmatic intrusion and exsolved saline fluids, and inform connections with other geothermal areas located nearby to the major fields. An additional conductive plume rises beneath the area where the most voluminous recent volcanism has occurred in the Okataina Volcanic Centre; the summit region (e.g. Makatiti dome) in the Haroharo Volcanic Complex that has erupted 39 km3 (magma equivalent) in the past 9 ka. While hot springs locate around this conductive plume at the base of the rhyolite domes, the plume itself is arrested at 1–2 km depth beneath the resistive (inferred impermeable) young (5 ka old) rhyolite lava domes, and is considered to represent a deep ‘blind’ geothermal system in the TVZ.

Digital geologic map, petrography and U-Pb geochronology of the Río Santiago Shear Zone, southernmost Sierra Madre Occidental, western Mexico

The boundary between the Sierra Madre Occidental (SMO) and the Trans-Mexican Volcanic Belt (TMVB) is well exposed along the Santiago River, on the border of the states of Nayarit and Jalisco. This region includes a polydeformed belt produced by the relative motion between the Jalisco Block and the SMO, here defined as the “Rio Santiago Shear Zone” (RSSZ). In this work, we present detailed geological cartography, petrographic descriptions, and new zircon U-Pb ages that allow us to establish the stratigraphy of the region surrounding the RSSZ, providing temporal constraints to its tectonic evolution. In the mapped area, the base of the SMO stratigraphic column consists of a succession of massive andesitic lava flows intruded by felsic dikes and a felsic subvolcanic body of 27.5 and 27.2 Ma, and it is covered by a series of ignimbrites of 26.8 and 25.3 Ma. A red beds succession separates a second package of ignimbrites of ∼20-19 Ma, in turn capped by rhyolitic lava domes of 19.7 and 18.6 Ma and a rhyodacitic lava flows of 15.6 Ma, this Miocene succession is also intruded by a felsic body of 18.3 Ma west of the La Yesca dam. The SMO succession is covered by rocks belonging to the bimodal volcanism of the TMVB, emplaced between the end of the Miocene and the Pleistocene. The RSSZ developed through several deformation phases that occurred since the end of the Oligocene. The Oligocene succession of the SMO is affected by NE-SW and WNW-ESE trending lateral faults, NW-SE open folds and WNW-ESE and E-W trending reverse oblique faults. Some of the reverse oblique faults were later reactivated as normal faults. WNW-ESE trending normal faults associated with the Tepic-Zacoalco Rift affected the area south of the Santiago River during the Pliocene and Pleistocene. Normal faulting produced an accumulated vertical offset up to 700 m between the northern and southern regions of the Santiago River.

İdrisyayla (Kırka-Eskişehir) Batısında Gözlenen Sferulitik Riyolitlere ait İlk Petrografik Dokusal Veriler

Spherulitic structures are classically defined as concentrically zoned crystal clusters that form in igneous systems (especially in alkaline igneous rocks), and usually occur as different mineral layers or shells.
 The volcano-sedimentary series observed in the Kırka region, south of Eskişehir, within the Kırka-Afyon-Isparta Volcanic Province, Western Anatolia, consists predominantly of Lower Miocene volcanic and pyroclastic sequences with rhyodacitic-rhyolitic in composition, lacustrine limestones, borate-bearing claystones, tuff, stromatolitic limestones and basalts. The formation mechanisms of the spherulitic structures observed in the marginal zone of the sub-alkaline rhyolitic lava dome outcropping within the Neogene series in the west of İdrisyayla in Kırka (Eskişehir) region were described for the first time with this study. The rhyolites are grayish light brown in color, have hypocrystalline hyalopilitic-holohyaline texture and present spherical structures defined as spherulites. These rocks present a completely flowing structure and are composed of volcanic glass containing rarely euhedral plagioclase and quartz microcrystals, The spherical structures observed on the surface of the rhyolites are up to 10-15 cm in size. The spherulites, which are almost round in shape, appear as concentric rings within the lava flow bands. The flow band texture of the volcanic glass phase presents continuity in the inner texture of the spherulitic structures.
 This study suggests that the sferulitic structures in the İdrisyayla (Kırka-Eskişehir) region were formed by the devitrification mechanism as a result of high temperature and rapid cooling. The petro-mineralogical features of the spherulitic rhyolites show that during the early magmatic stage, the oversaturated liquid crystallized in silica-rich fluid phases of relatively different densities, and radial quartz-feldspar crystal growths from a center, caused the formation of the spherulitic structure. It is also suggested that the lithophysae-like structures formed by the crystal growths representing the solid phase in the axial cavities due to the locally folding of the flow bands and the following radial mineral growths representing the liquid phase, were also changed the laminated texture in the marginal phase of the rhyolite.

The volcanism of the Jurassic Chon Aike Silicic LIP influenced by Paleozoic inherited crustal structures in the northeastern Deseado Massif, Patagonia

An area of ∼40 km2 located in the northeastern sector of the Deseado Massif is detailed studied, in which Paleozoic metamorphic and Jurassic volcanic rocks crop out. Two genetically linked and compositionally different Jurassic volcanic events of the Chon Aike Silicic LIP are documented by geological mapping and lithofacies and petrographic analyses. The first event was only represented by andesitic lava flows assignable to the intermediate portion of the Bahía Laura Volcanic Complex (Bajo Pobre Formation). The second was represented by the acidic rocks of the Bahía Laura Volcanic Complex (Chon Aike and La Matilde formations) and it was constituted by low-grade and moderate grade ignimbrites, pyroclastic fall deposits (ash tuffs) and several lava bodies (rhyolitic lava domes and lava flows). Firstly, the effusive eruptions dominated during the intermediate magmatic eruptions, whereas the explosives mechanisms dominated during the first stage of the acidic magma extrusion, to end with effusive eruptions of previously degassed magma. According to the morphology of the NNW-elongated lava domes (feeder conduits plugs) and internal measurements carried out on these rhyolitic bodies, fissure conduits are interpreted, which were directly controlled by the Paleozoic inherited structures during the Jurassic extensional tectonic reactivation ascribed to the initial stages of the Gondwana breakup.

Geochemical changes in obsidian outcrops with elevation at Hatis volcano (Armenia) and corresponding Lower Palaeolithic artifacts from Nor Geghi 1

Most descriptions of obsidian-bearing rhyolitic lava flows and domes are largely based on relatively simple cases of tectonic plate subduction in North America, but Armenian geologists proposed since the 1960s that these models are less suitable for describing rhyolitic volcanism in their research area. Obsidian-producing volcanoes that lie in the Armenian Highlands, they argued, are more complex in form and stratification. Hatis volcano in central Armenia is one such example. As we document, Hatis is highly unusual, perhaps unique, in that its obsidian changes in composition with elevation. Prior studies of Hatis obsidian recognized the existence of two different chemical types. Here, though, we report a series of four obsidian chemical types and their spatial distributions across the slopes. Our findings were enabled by the use of portable XRF during our field surveys of Hatis. Additionally, we recognized each of these four chemical types of Hatis obsidian at the Lower Palaeolithic site of Nor Geghi 1, where thousands of obsidian artifacts reflect Pleistocene hominin behaviors from Marine Isotope Stage (MIS) 11 (~424–374 ka) to 9 (~337–300 ka). Thus, all four types of Hatis obsidian are archaeologically significant despite the fact that their outcrops span more than 500 m (from <1600 to greater than 2100 m asl) in elevation on the volcanic slopes, thereby enabling future studies on links between altitude and hominin toolstone acquisition behaviors over hundreds of millennia.

Jurassic volcanism of the Chon Aike Silicic LIP in the northeastern Deseado Massif

Two compositionally different volcanic cycles are documented by geological mapping, lithofacies and petrographic analyses of Jurassic volcanic rocks in the northeastern sector of the Deseado Massif, constituting the western counterpart of the Karoo-Ferrar LIPs in Patagonia. The first cycle is of intermediate composition and is represented by andesitic lava flows, phenoandesitic moderate-grade ignimbrites and volcaniclastic deposits. These rocks constitute a previously unknown large outcrop (~100 km2) of the Bajo Pobre Formation. The second cycle (Chon Aike Formation) is acidic in composition and constituted by several pyroclastic deposits (ignimbrites and co-ignimbrite lag breccias), lavic bodies (rhyolitic lava domes, dykes, and coulees) and rhyolitic pyroclastic dykes. Structural measurements collected in the studied area suggest an extensional tectonic control on the emplacement of, at least, the Chon Aike Formation rocks. We document two major NE-striking (SE-dipping) normal faults, which constitute direct field evidence of the extensional tectonic control on the Jurassic Chon Aike Silicic LIP in the Deseado Massif at the time when southern Gondwana broke up.

An early Jurassic graben caldera of Chon Aike silicic LIP at the southernmost massif of the world: The Deseado caldera, Patagonia, Argentina

Detailed geological mapping of 900 km2, newly defined volcanic and volcaniclastic lithofacies, multiple stratigraphic profiles, and a new zircon U/Pb eruption age (178.7 ± 1.9 Ma) describe an early Jurassic extensional-related piece-meal collapsed graben caldera in the eastern sector of Deseado Massif, at southern Patagonia. This volcano-tectonic structure, here referred to as The Deseado Caldera, collapsed at least twice controlled by two major NW-striking normal faults, during which a large acidic ignimbrite (predominantly moderate to extremely high-grade) and co-ignimbrite lag breccias were extruded through multiple NW-fissure vents. During the two post-collapse periods, alluvial fans, lagoons and laharic sedimentary environments re-worked the intra-caldera volcaniclastic deposits. Finally, numerous elongated and aligned rhyolitic lava domes and dykes were emplaced during the final magmatic resurgence stage. This caldera represents the first clear evidence of the extensional tectonic regime influence, associated with the inception of Gondwana breakup, on the eruptive mechanisms of the Jurassic Chon Aike Silicic LIP in the Deseado Massif.

The enigmatic origin and emplacement of the Samosir Island lava domes, Toba Caldera, Sumatra, Indonesia

The 60 × 20-km Samosir resurgent dome within the 74-ka Toba Caldera features several clusters of rhyolitic lava domes. Because the previously known Tuk-Tuk and Samosir Fault lava domes occur near the base of a major resurgent dome fault, they appear associated with resurgent uplift, and dating them could provide constraints on the timing of resurgence. Towards that goal, we mapped and sampled the Tuk-Tuk and Samosir Fault lava domes, discovered two additional lava dome clusters in the uplifted interior of Samosir Island, then determined modal mineralogy, major and trace element geochemistry, and 40Ar/39Ar sanidine ages on several samples from each cluster. These quartz-bearing rhyolite lava domes have SiO2 contents ranging from ~ 70-76 wt%, crystallinities of 28-54%, and 40Ar/39Ar ages (~ 74-76 ka) that overlap with those of the Youngest Toba Tuff (YTT), ~ 74-75 ka. Our field observations and the remarkable similarity in mineralogy, geochemistry, and ages among the dome clusters and YTT support the interpretation that these domes represent remnant YTT magma that erupted shortly after the climactic YTT eruption. Later, during resurgence, these pre-resurgent lava domes were uplifted to their present locations, some near current lake level, others up to 440 m above lake level.

Structural control of mass movements on slopes formed of magmatic and metamorphic rocks: the case study of Wielisławka Mt. (SW Poland, Sudetes Mts.)

We indicate the structural controls on, and provide an evolutionary model of, mass movements which developed on the slopes of a rhyolitic lava dome built of massive, sub-intrusive Permian rhyolites and its low-grade metamorphic cover, comprising Ordovician and Silurian sericite schists and metacherts (greenschist facies). The phenomena studied occur on the low-altitude, dome-like Wielislawka Mt. (370 m a.s.l.) in the Western Sudetes, SW Poland. A multidisciplinary approach involving geological and geomorphological fieldwork, LiDAR-based geomorphometric analyses, as well as analyses based on data obtained from terrestrial laser scanning (TLS), performed within old adits and shafts in the landslide area, have allowed determination of the origin and recent extent of the landslide phenomena. The geometry and development of the slip surface are closely linked with measured, existing discontinuities within the massif. As they enable observation of the initial stages of mass movement in the excavations within the cover rocks of the rhyolitic massif, the old adits and shafts are unique objects for the observation and reconstruction of landslide processes.

Magmatic evolution of the Boku Volcanic Complex, Main Ethiopian Rift

The Boku volcanic complex is a Quaternary center situated on the axial segment of the Main Ethiopian Rift (MER), located 92 km South-east from Addis Ababa. The main objective of this study is to understand the magmatic evolution of the volcanic complex and to develop a model to answer some outstanding questions related to bimodal products of rift related volcanism, using geological mapping, petrographic and geochemical approaches. The Boku complex is characterized by two main phases of activity: pre-caldera/caldera forming eruptive activity and post-caldera eruptive activity. The volcanic stratigraphy consists from bottom to top of a sequence of rhyolitic lava flows, pumice flows, welded ignimbrite, pumice fall, rhyolitic lava dome, obsidian flow, lower basaltic lava flow, ash flow, basaltic scoria and upper basaltic lava flows. The lithologic varieties together with the geochemical results indicate that the Boku eruptive products are bimodal in composition; no intermediate compositions are found. The mafic rocks are transitional to weakly subalkaline basalts while the silicic rocks are predominantly peralkaline rhyolites. These two groups of rocks are co-genetic and related to each other by fractional crystallization processes starting from mantle-derived basaltic magma with a small component of crustal contamination. The available geophysical, geochemical and field data suggest that the evolution of the evolved silicic center which hosts a bimodal rock distribution can be explained as a result of prolonged stagnation of transitional basaltic melt (sourced from the mantle) at relatively high pressures, where these evolve to intermediate compositions. The transitional basaltic melts can occasionally erupt to the surface along weakness lines such as faults that infrequently cut the lower part of the shallow reservoir, generating basalts from the intra-caldera and lateral eruptive centers. We consider the intermediate magma is mechanically trapped at mid-crustal depths because of its higher crystal load (ca. 50%). Silicic magma is formed at shallow depth by prolonged fractional crystallization of the intermediate magma and minor assimilation of crustal material. These silicic magmas generate both effusive and explosive eruption products in the overall stratigraphy of the volcanic complex.

Characteristics and consequences of lava dome collapse at Ruawahia, Taupo Volcanic Zone, New Zealand

The eruption of large, rhyolitic lava domes may be accompanied by the formation of large block and ash flows. This may be linked to the style of dome extrusion—whether it forms a series of individual lobes, flows or spines (exogenous) or grows by internal inflation (endogenous). Lava domes can transition from one extrusive style to another as a result of a change in extrusion rate or the formation of facilitating structures such as shear zones. How this change can affect large rhyolitic lava domes is unclear as there are few historically recorded rhyolitic dome eruptions. Here, we present structures at Ruawahia lava dome (a well exposed ~ 700-year-old lava dome), how these facilitating structures enable exogenous extrusion at Ruawahia dome, and link this to collapse episodes along the fringes of the dome during growth. Ruawahia dome is part of the Tarawera dome complex, a chain of domes running parallel to regional structures across the Okataina caldera complex in the Taupo Volcanic Zone, New Zealand. Ruawahia dome consists of (1) a high porosity (44–52%), crystalline (65% DRE), locally brecciated carapace facies with rare bread-crusting and ‘ropey’ flow textures; (2) a core facies of dominantly low to moderate porosity (20–25%) with elongate vesicles that mark weak flow bands; and (3) thin (< 5 m thick) interior breccia zones. Flow bands at Ruawahia are complex and do not fit with hypothesised flow band orientations attributed to a single phase of exogenous or endogenous dome growth. Inward dipping flow bands on ramp structures on the flow surface suggest a flow-like (coulee) morphology; however, steeply dipping and multidirectional flow bands on the edges of the dome challenge this hypothesis. Widespread block and ash flow deposits have been sourced from the leading dome fronts to the NW and SE; these collapse events left behind inflated and bread-crusted outcrops on these dome fronts, suggesting syn-eruption collapse events that led to expansion of a hot, pressurised dome interior. We consider Ruawahia erupted from multiple, aligned vents, either as lobes confined within the crater of a pyroclastic cone formed during the initial Plinian phase of the eruption or those able to flow down the cone flank. The confined lobes formed steep internal breccia zones as individual dome lobes extruded past one another. Lobes that were able to overcome the pyroclastic cone rim (or where the vent was outside the crater) were able to flow down the flanks as bulldozing, thickening flows with dominantly ductile interiors and brittle exteriors; these flows collapsed as the front thickened, possibly due to a decrease in gradient, producing widespread block and ash flows. The removal of lava associated with collapse generated a decompression event which resulted in fragmentation, cracking and vesiculation in the hot interior of the lava flows. These events left behind a re-vesiculated and bread-crusted lava flow front and produced block and ash flows with abundant breadcrust bombs that reached the base of Tarawera.

Giant rhyolite lava dome formation after 7.3 ka supereruption at Kikai caldera, SW Japan

Kikai submarine caldera to the south of the Kyushu Island, SW Japan, collapsed at 7.3 ka during the latest supereruption (>500 km3 of magma) in the Japanese Archipelago. Multi functional research surveys of the T/S Fukae Maru in this caldera, including multi-beam echosounder mapping, remotely operated vehicle observation, multi-channel seismic reflection survey, and rock sampling by dredging and diving, provided lines of evidence for creation of a giant rhyolite lava dome (~32 km3) after the caldera collapse. This dome is still active as water column anomalies accompanied by bubbling from its surface are observed. Chemical characteristics of dome-forming rhyolites akin to those of presently active small volcanic cones are different from those of supereruption. The voluminous post-caldera activity is thus not caused simply by squeezing the remnant of syn-caldera magma but may tap a magma system that has evolved both chemically and physically since the 7.3-ka supereruption.

長野県大門峠地域に分布する流紋岩質溶岩ドームのK-Ar年代 : 諏訪-八ヶ岳火山地域の流紋岩類の活動時期について

長野県大門峠地域に分布する流紋岩質溶岩ドームのK-Ar年代 : 諏訪-八ヶ岳火山地域の流紋岩類の活動時期について

A27 1934-35 年昭和硫黄島噴火における流紋岩質溶岩ドームの形成プロセス

A27 1934-35 年昭和硫黄島噴火における流紋岩質溶岩ドームの形成プロセス

Giant rafted pumice blocks from the most recent eruption of Taupo volcano, New Zealand: Insights from palaeomagnetic and textural data

Giant blocks of pumice lie strewn along a former shoreline of intracaldera Lake Taupo, New Zealand, and are the sole subaerial evidence of the most recent volcanism at the Taupo supervolcano. Geochemically they are identical to material erupted during the complex and multiphase 1.8ka Taupo eruption, which they post-date by one to two decades. The blocks, some of which are >10m long, show complex jointing patterns indicative of both surface chilling and continued interior expansion, as well as heterogeneous vesicularity, with dense rims (mean density 917kg/m3) grading via an intervening transition zone (mean density 844kg/m3) into a more highly vesicular interior (mean density 815kg/m3). Analysis of thermal demagnetisation data indicates significant reorientation of the blocks as they cooled through a series of blocking temperatures. Some parts of block rims cooled to below 580°C well before emplacement on the shore, whereas other parts in the interior and transition zones, which cooled more slowly, acquired different orientations before stranding. Some block interiors cooled after blocks were finally deposited, and record the direction of the 1.8ka field. The blocks are believed to be derived from one or both of a pair of rhyolitic lava domes that developed on the bed of Lake Taupo several decades after the climactic Taupo eruption over the inferred vent area.These, and similar giant rafted pumice blocks in other marine and lacustrine settings raise a number of questions about how volatile-rich felsic magma can be erupted underwater with only limited thermal fragmentation. Furthermore, the prolonged flotation of out-sized fragments of vesiculated magma formed during subaqueous dome-growth contrasts with the rapid sinking of smaller pieces of hot plinian pumice under laboratory conditions. The genesis of pumice forming the blocks is not entirely clear. Most simply the blocks may represent part of a vesiculated carapace of a growing lava dome, broken loose as the dome grew and deformed then rising buoyantly to the surface. Parts of the carapace could also be released by local magma-water explosions. Some textures of the pumice, however, suggest fresher magma released from beneath the carapace. This may suggest that silicic dikes and pillows/pods intruded into a growing mound of silicic hyaloclastite, itself formed by quench fragmentation and thermal granulation of the dike margins. This fragmental cover would have inhibited cooling of a still-hot and actively vesiculating interior, which was then released to float to the surface by gravitational destabilisation and collapse of the growing pile. Following their formation, the large fragments of pumice floated to the lake's surface, where they were blown ashore to become embedded in accumulating transgressive shoreface sediments and continue cooling.

Origin and evolution of hydrothermal fluids in epithermal Pb-Zn-Cu ± Au ± Ag deposits at Koru and Tesbihdere mining districts, Çanakkale, Biga Peninsula, NW Turkey

The Koru and Tesbihdere mining districts in Biga Peninsula, Northwestern Turkey, consist of twelve deposits covering approximately 12km2. The epithermal Au-Ag enriched base metal veins and associated low-grade breccia and stockwork at Koru and Tesbihdere are hosted by Oligocene subaerial and calc-alkaline volcanic rocks including basaltic andesite lavas, dacitic lava-tuffs, rhyolitic lava-domes and tuffs. NW- to N-trending strike-slip faults and E- and NE-trending faults constitute the most important ore-controlling structures in the Koru and Tesbihdere districts respectively. In the Koru mining district, galena is the dominant ore mineral in barite-quartz veins containing sphalerite, chalcopyrite, pyrite, bornite, enargite and tennantite. According to base metal content, the Tesbihdere mining district can be subdivided into sphalerite-galena dominated Tesbihdere mineralization and chalcopyrite-pyrite dominated Bakır and Kuyu Zones mineralization. Gold is present in small quantities with maximum 3.14g/t Au values either as free grains in quartz or as micro inclusions in pyrite and galena. The most widespread silver minerals are polybasite, pearceite, argentite and native silver which commonly occur as replacements of galena, sphalerite and pyrite, and other sulfides, or as fillings of microfractures in sulfides and quartz.Microthermometric measurements of primary liquid-rich fluid inclusions in sphalerite, barite and quartz in Koru indicate that the veins were formed at temperatures between 407 and 146°C from fluids with salinities between 0.7 and 12.5wt.% equiv. NaCl. Barite from the Tahtalıkuyu, Kuyutaşı and 5th Viraj mineralization show the highest homogenization temperatures. Fluid inclusion data for ore-stage quartz and sphalerite from the Tesbihdere mining district, indicate that these minerals were deposited at temperatures between 387 and 232°C from more diluted fluids with moderate salinities between 0.2 and 10.6wt.% NaCl equiv. Tahtalıkuyu and 5th Viraj mineralization show only boiling trends while Kuyutaşı, Tesbihdere, Bakır and Kuyu Zones mineralization show both boiling and isothermal mixing trends. The O and H isotope compositions of ore fluids from the Tahtalıkuyu (δ18O=−1.40 to 0.25‰; δD=−72.49 to −52.68‰) and Kuyutaşı (δ18O=−2.29 to 3.59‰; δD=−90.70 to −70.93‰) mineralization indicate that there was a major contribution from a magmatic component to ore genesis. Based on 9 quartz samples associated with orebodies at the Tesbihdere mining district, the relatively higher δ18O and lower δD isotope compositions from hydrothermal fluids could be attributed to a relatively dilute fluid derived by the mixing with meteoric water. The Pb isotope compositions also reveal that most of the lead in both mining districts is derived from the Oligocene-Miocene magmatic rocks, possibly with smaller contributions from the Eocene magmatic rocks.

U–Pb zircon ages from volcanic and sedimentary rocks of the Ediacaran Bas Draâ inlier (Anti-Atlas Morocco): Chronostratigraphic and provenance implications

U–Pb LA-ICP-MS analyses on zircon extracted from felsic ignimbrites and sandstones of the Bas Draâ succession (BDS) constrain the post Pan-African evolution in the Anti-Atlas (AA) at the northern margin of Gondwana. The 2000-m-thick Ediacaran BDS consists of four volcanic units including: (i) rhyolitic ignimbrite sheets (AO I); (ii) high-silica andesitic and rhyolitic lavas (AO II); (iii) silica-poor lava and pyroclastics (AO III); and (iv) rhyolitic lava dome complexes (AO IV). AO III is dominated by basaltic and andesitic lava and it contains at least three sandstone lenses. The BDS is capped by an angular unconformity, which is defined by sedimentary rocks of the Adoudou Formation of the Taroudant Group.The U–Pb data set (99 zircon grains with 90–110% concordance) from the ignimbrite yields an age of 565±6Ma (564.8±5.6Ma), interpreted as the crystallization age, which represents the early volcanic activity in the BDS. For the upper BDS, maximum ages of ca. 552 and ca. 541Ma are indicated by U–Pb ages of detrital zircons extracted from sandstone. These age constraints confine the BDS to the Upper Ouarzazate Supergroup.Most detrital-zircon ages (three sandstone samples with 367 zircon grains analyzed) fall in the range of ca. 606−541Ma. This main Ediacaran peak is interpreted to have been derived from the post Pan-African magmatic complexes surrounding the West African Craton (WAC). Secondary age peaks at ca. 652Ma and ca. 655Ma are inferred to originate from Early to Late Pan-African magmatism of the Anti-Atlas region. Due to the complete absence of pre-Neoproterozoic zircon ages, our data set is interpreted to indicate a large voluminous volcanic episode that covered the Palaeoproterozoic and Cryogenian basement or the river system was confined to the BDS basin. Facies and provenance analyses of BDS sandstones indicate that the detrital input was mostly locally derived, with a minor amount from an external source. In addition, large peaks of older zircon ages are recognized only on the lower sandstone lens (BD 86, AOIII). Thus, the drainage system was large within the lower part and became more localized at the end of BDS evolution.The maximum depositional age for the zircons separated from the uppermost AO III sandstone lens (541Ma) shifts the Precambrian–Cambrian boundary down from the Lower Adoudou Formation, as proposed by previous studies, to the uppermost Ouarzazate Supergroup or to the hiatus between both units.The volcanic activity within the BDS is linked to a transtensional regime that operated along the Gondwana margin during the Ediacaran. The BDS volcanic activity is contemporaneous with the widespread volcanic, volcanoclastic and siliciclastic rocks of Upper Ouarzazate Supergroup.

The Chaitén rhyolite lava dome: Eruption sequence, lava dome volumes, rapid effusion rates and source of the rhyolite magma

We use geologic field mapping and sampling, photogrammetric analysis of oblique aerial photographs, and digital elevation models to document the 2008-2009 eruptive sequence at Chaiten Volcano and to estimate volumes and effusion rates for the lava dome. We also present geochemical and petrologic data that contribute to understanding the source of the rhyolite and its unusually rapid effusion rates. The eruption consisted of five major phases: 1. An explosive phase (1-11 May 2008); 2. A transitional phase (11-31 May 2008) in which low-altitude tephra columns and simultaneous lava extrusion took place; 3. An exogenous lava flow phase (June-September 2008); 4. A spine extrusion and endogenous growth phase (October 2008-February 2009); and 5. A mainly endogenous growth phase that began after the collapse of a prominent Peleean spine on 19 February 2009 and continued until the end of the eruption (late 2009 or possibly earliest 2010). The 2008-2009 rhyolite lava dome has a total volume of approximately 0.8 km3. The effusion rate averaged 66 m3s-1 during the first two weeks and averaged 45 m3s-1 for the first four months of the eruption, during which 0.5 km3 of rhyolite lava was erupted. These are among the highest rates measured world-wide for historical eruptions of silicic lava. Chaiten’s 2008-2009 lava is phenocryst-poor obsidian and microcrystalline rhyolite with 75.3±0.3% SiO2. The lava was erupted at relatively high temperature and is remarkably similar in composition and petrography to Chaiten’s pre-historic rhyolite. The rhyolite’s normative composition plots close to that of low pressure (100-200 MPa) minimum melts in the granite system, consistent with estimates of approximately 5 to 10 km source depths based on phase equilibria and geodetic studies. Calcic plagioclase, magnesian orthopyroxene and aluminous amphibole among the sparse phenocrysts suggest derivation of the rhyolite by melt extraction from a more mafic magmatic mush. High temperature and relatively low viscosity enabled rapid magma ascent and high effusion rates during the dome-forming phases of the 2008-2009 eruption.