Ignimbrite Units Research Articles

A revision of the structure and stratigraphy of pre-Green Tuff ignimbrites at Pantelleria (Strait of Sicily)

At Pantelleria, peralkaline silicic magmas were erupted across a range of eruptive typologies and magnitudes: pyroclastic flows, Plinian to strombolian pumice fallout and lava flows. In this paper we focus on the intermediate cycle of eruptive activity which is bracketed by ignimbrite units slightly older than the two caldera collapses which marked the volcanological activity of the island. This age interval (180–85ka) was punctuated by six ignimbrite-forming eruptions (silicic and variably peralkaline) for a cumulative erupted magma volume of approximately 6km3 dense rock equivalent. Based on new 40Ar/39Ar (Na,K)-feldspar ages and petrographic data, we propose an updated volcanostratigraphic scheme for these welded and rheomorphic ignimbrites that can be summarised as follows: (i) the age of the old (‘La Vecchia’) caldera collapse is now tightly constrained between 140 and 146ka and the caldera-forming eruption can be traced to a lithic-rich welded tuff breccia that outcrops in two opposite sectors of the island (south-west and north-east); (ii) four ignimbrite units previously considered unrelated are now merged in two distinct eruptive paroxysmal events at 107 and 85ka. In particular, the 85ka eruptive event is comparable in magnitude to the younger (caldera forming) Green Tuff Plinian eruption; (iii) the recurrence patterns of the 107 and 85ka eruptions, compared to the Green Tuff, allow us to qualitatively assess that the climax in production of low-temperature silicic and peralkaline melt was focused in the age interval 85–45ka.

Volcano-tectonic controls and emplacement kinematics of the Iriga debris avalanches (Philippines)

Mt Iriga in southeastern Luzon is known for its spectacular collapse scar that was possibly created in 1628 ad by a 1.5-km3 debris avalanche. The debris avalanche deposit (DAD) covered 70 km2 and dammed the Barit River to form Lake Buhi. The collapse has been ascribed to a non-volcanic trigger related to a major strike-slip fault under the volcano. Using a combination of fieldwork and remote sensing, we have identified a similar size, older DAD to the southwest of the edifice that originated from a sector oblique to the underlying strike-slip fault. Both deposits cover wide areas of low, waterlogged plains, to a distance of about 16 km for the oldest and 12 km for the youngest. Hundreds of metre-wide and up to 50-m-high hummocks of intact conglomerate, sand and clay units derived from the base of the volcano show that the initial failure planes cut deep into the substrata. In addition, large proportions of both DAD consist of ring-plain sediments that were incorporated by soft-sediment bulking and extensive bulldozing. An ignimbrite unit incorporated into the younger DAD forms small (less than 5 m high) discrete hummocks between the larger ones. Both debris avalanches slid over water-saturated soft sediment or ignimbrite and spread out on a basal shear zone, accommodated by horst and graben formation and strike-slip faults in the main mass. The faults are listric and flatten into a well-developed basal shear zone. This shear zone contains components from the substrate and has a diffuse contact with the intact substrata. Long, transport-normal ridges in the distal parts are evidence of compression related to deceleration and bulldozing. The collapse orientation and structure on both sectors and DAD constituents are consistent with collapse being a response to combined transtensional faulting and gravity spreading. Iriga can serve as a model for other volcanoes, such as Mayon, that stand in sedimentary basins undergoing transtensional strike-slip faulting.

Volcanological evolution and caldera forming eruptions of Mt. Nemrut (Eastern Turkey)

Mt. Nemrut volcano, situated at the west of Lake Van, is one of the historically active volcanoes of the Eastern Anatolia. It has an 8.5×7km diameter summit caldera. Volcanic activity of Mt. Nemrut started ~1Ma ago; the most recent eruptions were in 1441, 1597 and 1692A.D. Among the Eastern Anatolian volcanoes, Mt. Nemrut is the most hazardous volcano for its vicinity. Present day volcanic activity is represented by intra-caldera hydrothermal and fumarolic output and low-level volcano-seismic events.Geological evolution and chronostratigraphy of the volcano is subdivided in three stages: pre-caldera, syn-caldera and post-caldera stages. Pre-caldera products are dominated by felsic lava flows and domes. Trachytic Nemrut and Kantaşı pyroclastics represent the caldera forming activity, of which sequences are composed of fallout units and ignimbrite flows. Both Nemrut and Kantaşı ignimbrite units are low-aspect ratio ignimbrites, they are generally densely welded and present columnar jointed outcrops locally. Extent of Nemrut ignimbrite (volume: 32.6km3) is greater than the Kantaşı ignimbrite (volume: 3.8km3). Post-caldera activity of the volcano is marked by peralkaline rhyolitic (comendite) intra-caldera lava flows and explosive hydrovolcanic activities. Historical activity of the volcano is represented by bimodal basaltic–rhyolitic effusive activity along Nemrut rift zone.

History of Field Observations on Volcanic Rocks of Western Mexico, Pre-Columbian to Recent

By the time the first detailed reports on western Mexico's volcanic rocks had begun to appear in the 1970s, most of the earlier observations on these rocks and most knowledge of those who made these observations were all but forgotten. A review of previous field observations in this region shows, however, a long history of geologic discovery reflecting or even preceding developments elsewhere.Ethnological studies suggest that the Pre-Columbian inhabitants of the Sierra Madre Occidental (SMO) observed the characteristics of rock formations in their sierra homeland and understood something of the regional stratigraphic relationships. Late sixteenth and early seventeenth century explorers of the Spanish Colonial Period singled out volcanic rock known to them as piedra de malpaís for special recognition, and Padre Kino and his fellow explorers clearly recognized the volcanic origin of piedra de malpaís decades prior to similar observations in Europe. As the Spanish Colonial Period came to a close, Andrés Manuel del Río help organize a state-of-theart mining college in Mexico City where students were instructed in Werner's geognosy prior to their taking positions in Mexico's mining industry, most of it located in western Mexico's volcanic rocks.Although the first part of the tumultuous period between Mexico's revolutions of 1810 and 1910 saw few advances in geological knowledge, the reign of President Porfirio Díaz produced a geologic map of Mexico, the founding of the Instituto de Geología, and an ‘American invasion’ of geologists and mining engineers who locally gathered information on the nature of volcanic rocks of western Mexico. During the same period, Instituto geologist Ezequiel Ordóñez established the general stratigraphic sequence in the SMO, recognized the widespread occurrence of rhyolite there, and applied the petrographic microscope to the study of SMO volcanic rocks. The first identification of ignimbrites in the SMO came as a result of the World War II-era search for strategic minerals, and NASA's push to put a man on the Moon supported a series of student mapping projects producing the SMO's first geologic maps showing individual ignimbrite units and calderas.

High-temperature emplacement of the Cerro Galán and Toconquis Group ignimbrites (Puna plateau, NW Argentina) determined by TRM analyses

Estimates of pyroclastic flow emplacement temperatures in the Cerro Galan ignimbrite and Toconquis Group ignimbrites were determined using thermal remanent magnetization of lithic clasts embedded within the deposits. These ignimbrites belong to the Cerro Galan volcanic system, one of the largest calderas in the world, in the Puna plateau, NW Argentina. Temperature estimates for the 2.08-Ma Cerro Galan ignimbrite are retrieved from 40 sites in 14 localities (176 measured clasts), distributed at different distances from the caldera and different stratigraphic heights. Additionally, temperature estimates were obtained from 27 sample sites (125 measured clasts) from seven ignimbrite units forming the older Toconquis Group (5.60–4.51 Ma), mainly outcropping along a type section at Rio Las Pitas, Vega Real Grande. The paleomagnetic data obtained by progressive thermal demagnetization show that the clasts of the Cerro Galan ignimbrite have one single magnetic component, oriented close to the expected geomagnetic field at the time of emplacement. Results show therefore that most of the clasts acquired a new magnetization oriented parallel to the magnetic field at the moment of the ignimbrite deposition, suggesting that the clasts were heated up to or above the highest blocking temperature (T b) of the magnetic minerals (T b = 580°C for magnetite; T b = 600–630°C for hematite). We obtained similar emplacement temperature estimations for six out of the seven volcanic units belonging to the Toconquis Group, with the exception of one unit (Lower Merihuaca), where we found two distinct magnetic components. The estimation of emplacement temperatures in this latter case is constrained at 580–610°C, which are lower than the other ignimbrites. These estimations are also in agreement with the lowest pre-eruptive magma temperatures calculated for the same unit (i.e., 790°C; hornblende–plagioclase thermometer; Folkes et al. 2011b). We conclude that the Cerro Galan ignimbrite and Toconquis Group ignimbrites were emplaced at temperatures equal to or higher than 620°C, except for Lower Merihuaca unit emplaced at lower temperatures. The homogeneity of high temperatures from proximal to distal facies in the Cerro Galan ignimbrite provides constraints for the emplacement model, marked by a relatively low eruption column, low levels of turbulence, air entrainment, surface–water interaction, and a high level of topographic confinement, all ensuring minimal heat loss.

The exhumed Eocene Sultepec-Goleta Volcanic Center of southern Mexico: record of partial collapse and ignimbritic volcanism fed by wide pyroclastic dike complexes

The Sultepec-Goleta area in southern Mexico hosts one of the major Eocene silicic volcanic centers that make up the Cenozoic volcanic record of the north-central part of the Sierra Madre del Sur. This center is represented by a partially exhumed NNE-SSW trending ignimbritic field that covers an area of ∼400 km2 with a preserved volume of ∼200 km3. A remarkable feature of volcanic center is the exposure of pyroclastic dike complexes up to 1 km wide that extend almost continuously along the western and southern flanks of the volcanic field. The ignimbritic record comprises four units with different textural features as well as variable proportions of the main components (i.e. phenocrysts, pumice and lithic clasts). The most extensive unit is the basal Goleta Ignimbrite, of which the middle and upper members are richer in phenocrysts than the lower member. It has a thickness that ranges from ∼200 m in the north to at least 600 m in the south of the volcanic center. The overlying ignimbrite units (Cienega, Lobera and Potrero ignimbrites) are phenocryst-poor and are distributed in the northern sector of the volcanic field. The semi-curvilinear trend of the pyroclastic dikes along the southern flank of the Goleta Range, coupled with the greater thickness of the Goleta Ignimbrite encircled by the dikes, is indicative of the development of a partial-collapse caldera. Based on spatial relationships as well as analogies in the nature and abundances of the components, it has been recognized that the central and southern dike complexes fed the relatively phenocryst-rich Goleta Ignimbrite, whereas the phenocryst-poor units of the northern sector were extruded through the pyroclastic conduits distributed around Sultepec. We suggest that the stirring produced by the partial collapse of the magma chamber roof beneath the southern sector of the study area triggered the tapping of phenocryst-rich portions of the zoned magma chamber, resulting in an increase of phenocryst content in the upper members of the Goleta Ignimbrite. The presence of breccias and lithic-rich facies dominated by wallrock fragments at the borders of the pyroclastic dikes, including those of the northern extra-caldera sector, indicate that erosion of the conduit walls above the fragmentation level contributed to widening of the conduits and the construction of wide dike complexes.

The Holocene Pucón eruption of Volcán Villarrica, Chile: deposit architecture and eruption chronology

The Pucon eruption was the largest Holocene explosive outburst of Volcan Villarrica, Chile. It discharged >1.0 km3 of basaltic-andesite magma and >0.8 km3 of pre-existing rock, forming a thin scoria-fall deposit overlain by voluminous ignimbrite intercalated with pyroclastic surge beds. The deposits are up to 70 m thick and are preserved up to 21 km from the present-day summit, post-eruptive lahar deposits extending farther. Two ignimbrite units are distinguished: a lower one (P1) in which all accidental lithic clasts are of volcanic origin and an upper unit (P2) in which basement granitoids also occur, both as free clasts and as xenoliths in scoria. P2 accounts for ∼80% of the erupted products. Following the initial scoria fallout phase, P1 pyroclastic flows swept down the northern and western flanks of the volcano, magma fragmentation during this phase being confined to within the volcanic edifice. Following a pause of at least a couple of days sufficient for wood devolatilization, eruption recommenced, the fragmentation level dropped to within the granitoid basement, and the pyroclastic flows of P2 were erupted. The first P2 flow had a highly turbulent front, laid down ignimbrite with large-scale cross-stratification and regressive bedforms, and sheared the ground; flow then waned and became confined to the southeastern flank. Following emplacement of pyroclastic surge deposits all across the volcano, the eruption terminated with pyroclastic flows down the northern flank. Multiple lahars were generated prior to the onset of a new eruptive cycle. Charcoal samples yield a probable eruption age of 3,510 ± 60 14C years BP.

U–Pb dating of zircon in subsurface, hydrothermally altered pyroclastic deposits and implications for subsidence in a magmatically active rift: Taupo Volcanic Zone, New Zealand

Recognising and correlating hydrothermally altered rock units within buried volcanic sequences in the Taupo Volcanic Zone (TVZ) in New Zealand is difficult. This is because of broad similarities in the lithologies of many major ignimbrite units, and the destruction by hydrothermal alteration of distinctive chemical and mineralogical characteristics. However, magmatic zircons are commonly present, are highly resistant to hydrothermal alteration and yield crystallisation ages in intensely altered rocks. Crystallisation-age spectra have been obtained by SIMS techniques (SHRIMP-RG) on zircons extracted from cores from altered ignimbrites penetrated by drillholes at the Waiotapu, Te Kopia and Orakei Korako geothermal fields in the central TVZ. At Waiotapu, the thick (up to 350 m) densely welded Waiotapu Ignimbrite returned a zircon age spectrum with a probability density function (pdf) peak of 0.79 Ma, consistent with an eruption age (from 40Ar/ 39Ar techniques) of 0.71 ± 0.06 (1 s.d.) Ma. Three older ignimbrite sheets yielded age spectra that were consistent stratigraphically. The shallowest of the three yielded sparse zircons that gave a pdf peak of 1.24 Ma and it may correlate with the 1.18 ± 0.02 Ma Ahuroa ignimbrite. The middle sheet, although 220 m thick, yielded an age spectrum identical to that obtained from pumice in the widespread 1.21 ± 0.04 Ma Ongatiti ignimbrite, extending earlier estimates of the likely volume of this large deposit. The deepest sheet has a spectrum consistent with an eruption age of 1.45 ± 0.05 Ma; it has no surficial correlative, but its likely coeruptive ash forms part of a concentrated group of primary or secondary tephra in sediments on the ocean floor east of New Zealand and in sedimentary basins across the North Island. These three ignimbrites were previously correlated with either major ignimbrites exposed on the Paeroa Fault scarp, 10 km to the west, or the Akatarewa Ignimbrite that occurs in drillholes at Te Kopia and Orakei Korako, but these correlations are disproved from our age data. Drillholes at Te Kopia (TK) and Orakei Korako (OK), reach, but do not penetrate the base of, one or more deposits collectively termed the Akatarewa Ignimbrite. Samples from 62 (TK) and 165 m (OK) below the top contact of this unit yielded closely similar age spectra and pdf maxima of 1.00 (TK) and 1.02 Ma (OK), implying that the same deposit was penetrated at both fields. This ignimbrite is not correlated with a specific surface equivalent; several candidate deposits exist, but all are areally restricted. The Akatarewa Ignimbrite at TK and OK is, however, not the same deposit as those earlier correlated with it at Waiotapu. The age data are consistent with there being two zones of subsidence on both sides of a horst aligned with the three geothermal fields sampled. The eastern zone is the Reporoa Basin, which has high heat flow but little surface faulting, and the western one is the axis of the Taupo Rift, currently with low heat flow but abundant surface faulting. Overall subsidence rates at the three geothermal fields studied vary between 2 and < 0.2 mm/yr and imply that subsidence is uneven in time and with position along and across the strike of the TVZ. Such complex patterns will complicate distinctions between background regional subsidence and more localised effects due to exploitation in the TVZ.

Aquifer investigations in the León‐Chinandega plains, Nicaragua, using electromagnetic and electrical methods

ABSTRACTThe Leon‐Chinandega plains are located in the north‐western part of Nicaragua and represent the most important groundwater reservoir of the country. The aquifer is comprised of three hydrogeological units. The top unit is an unconfined alluvial and below a more consolidated volcanic aquifer is found. These aquifers rest on an ignimbrite unit that acts as the basement of the aquifer. The plains are mainly used for agricultural purposes and most of the irrigation comes from upper aquifer wells (not deeper than 70 m) allocated within the shallow unconfined aquifer.Transient electromagnetic soundings (TEM) and continuous vertical electrical soundings (CVES) were carried out in order to obtain information regarding the geology of the study area. Two TEM profiles extending from the Pacific coast towards the volcanic chain were carried out with an approximate separation between soundings of 250 metres. A grid of TEM soundings was also performed in an area located between the towns of Posoltega and Quezalguaque. CVES were carried out in different areas of the plains in order to obtain detailed information on the geology and distribution of the shallow aquifer. This information was later correlated with information available from wells in the area.One of the objectives of this investigation was to test and evaluate the applicability of the TEMfast48 equipment in the study area. In the plains area the TEM method shows very good results where a general geophysical model of mostly a three‐layered earth can be obtained. In the coastal areas the models are more irregular and complex. The results from the CVES generally agree with the TEM models but present more details, especially in areas of complex geology. The penetration depth for the TEM soundings reached almost 100 m at most, depending on the depth to a low resistivity layer. In some areas the depth to the basement was estimated with long‐layout CVES.In general, it was possible to obtain consistent geoelectrical models of the area and the methods complement each other well. The geoelectrical models are an excellent addition to other investigation methods as they provide an overview of the aquifer system and can serve as a basis for refining the conceptual and numerical models of the aquifer system. This information is expected to become very valuable for the exploration, management and protection of the groundwater resources in the León‐Chinandega plains to ensure sustainability of the resource.

Eruption and emplacement of a laterally extensive, crystal-rich, and pumice-free ignimbrite (the Cretaceous Kusandong Tuff, Korea)

The Cretaceous Kusandong Tuff, Korea, is a thin (1–5 m thick) but laterally extensive (~ 200 km) silicic ignimbrite emplaced in a fluviolacustrine basin adjacent to a continental volcanic arc. The tuff has been used as an excellent key bed because of its great lateral continuity and unique lithology, characterized by the virtual absence of juvenile clasts and an abundance of quartz and feldspar crystals (up to 55–73 vol.%). The tuff is mostly massive and ungraded and locally shows crude internal layering, basal inverse grading and near-top normal grading of crystals, either erosional or non-erosional lower surfaces, and flat-lying to imbricated grain fabrics. Fragile intraformational clasts of mudstone and tuff are also included. These features provide only ambiguous information on the properties of the responsible pyroclastic density currents: i.e. whether they were dense and laminar or dilute and turbulent. The overall lateral continuity and sheet-like geometry of the tuff suggests, however, that the transport system of the currents was highly expanded, dilute, and turbulent. A plug-flow or slab-flow model cannot explain the origin of crude internal layering, imbricated grain fabrics, and the high crystal content, which is most likely the result of vigorous sorting processes within a dilute and turbulent current. Features indicative of deposition from a dense and laminar transporting medium are locally present, suggesting that a dense and laminar depositional system could develop locally at the base of the dilute and turbulent transport system. The virtual absence of juvenile clasts in the tuff is interpreted to be due to rapid ascent, sudden decompression, and full fragmentation of silicic magma into fine glass shards and crystals. Scarcity of basement-derived accidental components together with the absence of pumiceous fallout deposits beneath the tuff is interpreted to be due to shallow-level fragmentation of magma followed by immediate generation of pyroclastic density currents from shallow-level blasts at the onset of eruption. The eruption occurred through multiple vent sites in a short period of time, producing a seemingly single but actually composite ignimbrite unit. Such an eruption was probably possible because of a regional tectonic event within the basin or in its vicinity. It is proposed that a composite ignimbrite with the characteristics of the Kusandong Tuff can be an exemplary product of syntectonic volcanism that can provide an insight into the interpretation of structural and stratigraphic evolution of a sedimentary basin.

Http://www.socgeol.it/368/58/products/a_simplified_scheme_for_the_description_of_textural_features_in_welded_ignimbrites_the_example_of_san_pietro_island_sardinia_italy.html

Welded ignimbrites show a large variability of welding intensity and of post-depositional crystallization. According to existing literature,a complete characterization of welded ignimbritic units involves: 1) a classification scheme which includes up to 5-6 categoriesof welding intensity; 2) the definition of the style of post depositional crystallization. This classification can be very time-consumingand well beyond the standards required for a general description of ignimbrite units for regional geological mapping purposes.During the geological survey of the San Pietro Island (Sardinia, Italy) in the framework of the new geological map of Italy on a scale1:50000 (CARG Project), a large number of samples from different ignimbritic units has been collected. The geology of the area is characterizedby Oligocene-Miocene volcanic deposits, which testify for the intense explosive activity with the deposition of many variablywelded ignimbritic units and, subordinately, local effusive activity, mainly characterized by the emplacement of lava coulees. All theproducts derive from the eruption of highly evolved magmas of both calc-alkaline and peralkaline affinity.We present here a new simplified scheme for the facies classification of these ignimbritic deposits. The scheme is based on threemain textural features: the aspect ratio of the fiammae, the degree of glass shards alignment and the style of crystallization of the matrix.All these features can be used to describe the ignimbrite units for the scope of their lithological characterization in mapping projects.We suggest that the scheme can have a general interest for the geological mapping of areas dominated by the presence of weldedignimbritic units.

Ignimbrites of the Pliocene Bagaces Formation near Canas (Guanacaste, Costa Rica)

By this study, four units within the so far mostly undivided Bagaces Formation - a major ignimbrite unit of the Plio- to Pleistocene magmatic succession in northern Costa Rica - were identified and their ignimbrite sequences stratigraphically distinguished. The basal welded Montenegro ignimbrite of mid-Pliocene age forms the lowermost stratigraphic member. It is followed by three non-welded ignimbrite units of Upper Pliocene age, Sandillal (4.1-4.15 ± 0.02 Ma) and Canas (2.06 ± 0.02 Ma) and the locally exposed Rio Tenorio ignimbrite (estimated age 2 Ma). The ignimbrite units of the Bagaces Formation show decreasing thicknesses from NE to SW. For the non-welded Sandillal and Canas units, distal lobes could be defined. Lateral pumice and lithics size distribution, as well as isopach data indicate a paleotopographical control for the deposition of the ignimbrites and allowed defining the source area of the ignimbrites. Ar-Ar-ages and compositions analysis of the pyroclastic units refer to the eruptive evolution of the Neogene volcanic arc.

Dynamics and deposits generated by the Kos Plateau Tuff eruption: Controls of basal particle loss on pyroclastic flow transport

The explosive eruption of voluminous silicic magmas often produces widespread and massive deposits formed from pyroclastic density currents. While these punctuated events dramatically alter the landscape and have potential climate‐altering impact, our understanding of the internal structure and transport dynamics of these eruptions is hampered by a lack of direct observations. We utilize the natural boundary conditions encountered by the eruption of the Kos Plateau Tuff to probe its internal structure as well as constrain the neotectonic activity in the region and eruption duration of this moderate to large (&gt;60 km3) event. At the time of the eruption, 161 ka, the lower sea level in the Mediterranean may have resulted in flows that traversed mostly land to the north of the eruptive vent, while flows to the south may have encountered an expanse of water. Steep topography and overwater transport can impede the transport of the dense basal portions of the flow where particles make multiple or sustained contact with the bed. We use an Eulerian‐Eulerian‐Lagrangian computational approach coupled with overwater and overland boundary conditions, including topography, to determine the role of bed load versus suspended load in the transport of these flows. We find that a ring vent structure and eruptive fluxes greater than ∼2 × 106 m3/s are required for the spatial distribution of the KPT. The maximum grain size and deposit locations of the first voluminous ignimbrite unit (D) can be explained by suspended flow to the south, consistent with overwater transport, and bed load and suspended load transport to the north, consistent with overland transport. However, the maximum lithic size for the largest and last ignimbrite unit (E) requires some bed load transport in all directions. We propose that the boundary conditions were significantly altered during the course of the eruption, through either the in‐filling of a shallow sea to the south or the development of a thick pumice raft to aid saltation. On the basis of the inferred eruptive flux, we estimate that the duration of the eruption climax, in which most of the material was erupted, likely only lasted from a few hours to a day.

An integrated geophysical and geological study of the Monturaqui impact crater, Chile

Abstract— –The Monturaqui impact crater (350–370 m in diameter and 0.1 Ma old), located in a remote area in northern Chile, was surveyed in December 2003 with detailed geophysics (gravity and magnetics), topography, petrophysics, and geology. The geology of the Monturaqui area is characterized by a basement of Paleozoic granites overlain by Pliocene ignimbrite units. No impact breccia was found in the area. The granites are the main lithology affected by the impact. Although the granite samples analyzed did not show evidence of shock metamorphism, quartz, and to a lesser extent feldspar and biotite grains from impactite samples exhibit different degrees of shock, ranging from planar microdeformation and cleavage to the development of intense planar deformation features (PDFs) and diaplectic glasses in some grains. The differential GPS survey allowed the creation of a detailed digital elevation model of the crater. Its dimensions are 370 m along the east‐west direction, 350 m along the north‐south direction, and ∼∼34 m deep. The crater exhibits a circular morphology with a preferred northwest‐southeast elongation that coincides with the steepest slopes (∼∼35°) on the southeast edge. The newly acquired gravity data shows a negative anomaly of ∼∼1 mGal at the center and allowed the creation of a 3‐D model with a RMS error of &lt;0.1 mGal, which supports the predictions of a fracturing‐induced low‐density granitic layer on top of the unfractured basement.

The influence of paleogeographic setting and crustal subsidence on the architecture of ignimbrites in the Bay of Naples (Italy)

The Margin of Campania (including Campania Plain and the Bay of Naples) featured recurrent large-scale ignimbrite eruptions between 270 and 15 ka. These ignimbrite deposits are exposed in the valleys of the Apennines and in limited areas along the Margin of Campania, whereas are covered by younger deposits in the Campania Plain and the Bay of Naples. Of all these eruptions, the Campania Ignimbrite (39 ka) and the Neapolitan Yellow Tuff (15 ka) are the best studied and are considered to be the most important. Numerous papers suggest that these two eruptions originated from one or two nested calderas located in Campi Flegrei and the Bay of Pozzuoli, in the northern part of the Bay of Naples. The recognition of the way, the site and the timing of a volcano-tectonic collapse is of paramount importance to establish which of the structures were active during the eruption. Our study demonstrates that the recognition of the areas involved in volcano-tectonic collapses is possible by means of a three-dimensional basin analysis. In the Bay of Naples the sequence stratigraphy interpretation of more than 4000 km of single-channel and multi-channel seismic reflection profiles made it possible to reconstruct the three-dimensional architecture of the ignimbrite units. Basin analysis reveals that before ignimbrite eruptions the physiography of the Bay of Naples was characterised by a lateral variation in depositional environment ranging from shelf to deep marine. Our structural and stratigraphic analysis documents volcano-tectonic deformations that occurred during the eruptions of the Pre-Campania Ignimbrite tuffs and of the Campania Ignimbrite: these deformations were controlled by normal faults with planar geometry. The overall features of the Campania Ignimbrites and the quantitative evaluation of crustal subsidence support a model of fissural emission associated to an asymmetrical crustal subsidence of the Campania Margin.

Twenty-five million years of silicic volcanism in the southern central volcanic zone of the Andes: Geochemistry and magma genesis of ignimbrites from 25 to 27 °S, 67 to 72 °W

Silicic volcanism in the Andean Central Volcanic Zone (CVZ) produced one of the world's largest Neogene ignimbrite provinces. The largest and best-known CVZ ignimbrites are located on the Altiplano-Puna plateau north of 24 °S. Their compositions and huge erupted volumes suggest an origin by large-scale crustal melting, and present-day geophysical anomalies in this region suggest still active zones of partial melting in the middle crust. Farther south in the CVZ, the Cerro Galán complex erupted ignimbrites in the late Miocene and Pliocene that are quite similar in volume and composition to those from north of 24 °S and they have a similar origin. However, there are a great many other, smaller ignimbrites in the southern CVZ whose compositions and geodynamic significance are poorly known. These are the subject of this paper. We present a geochemical study of 28 ignimbrite units from the southern CVZ at 25 °S to 27 °S, whose ages cover the full span of arc activity in this area, from about 25 Ma to 1 Ma. The small to medium volume ignimbrites (< 10 km 3) form valley-fill or sheet-like deposits, many of which are chemically zoned. Notwithstanding individual differences, there are several common characteristic features within the group of southern CVZ ignimbrites. By far the dominant composition is metaluminous, crystal-poor rhyolite (mean values from 236 samples: SiO 2 = 73.5 wt.%, A/CNK = 1.02, K 2O/Na 2O = 1.4). Regular major and trace element differentiation trends suggest fractionation from intermediate arc magmas and this is supported by radiogenic isotopic ratios of Sr, Nd and Pb, which show complete overlap between the silicic ignimbrites and andesite–dacites from contemporary stratovolcanoes. There are no major changes in composition of the silicic ignimbrites over the 25 Ma span of activity. We attribute minor but significant differences in isotopic composition and Nb–Ta concentration according to location in the West Cordillera (Chile) or the southern Puna plateau (Argentina) to variations in local basement compositions. The major conclusion of this study is that silicic magmas of crustal origin are rare in the CVZ south of 24 °S, and the only known example, Cerro Galán, is considerably younger than the ignimbrite “flare-up” in the Altiplano-Puna plateau region. The thermal conditions for widespread crustal melting in the Altiplano-Puna north of 24 °S are related to increased mantle heat flux caused by slab steepening and/or delamination at the base of overthickened lithosphere. These conditions have not (yet) been achieved in the southern CVZ because of the southward shallowing of slab dip and younging of compressive deformation and plateau growth.

Discrimination of fluvial and eolian features on large ignimbrite sheets around La Pacana Caldera, Chile, using Landsat and SRTM-derived DEM

Satellite images and ground-based observations were used to investigate the morphology and origin of valleys and ridges across large ignimbrite provinces around La Pacana Caldera, N. Chile. We studied 12 separate sites that included five different ignimbrite units that range in age from ∼ 5.6 ± 0.5 Ma to 1.35 ± 0.15 Ma. Over 400 features from within these study sites were quantified. Study sites typically have dimensions of 25 × 35 km. Measurements were made using Landsat TM, ETM+, and SRTM-derived DEM and Shaded Relief images. Morphological measurements were made of the ridges and channels, including the direction of regional slope, length, orientation and gradient of each feature. A classification of the different ignimbrite erosion influences, resulting in varying landscape morphologies identified 4 landscape types; Fluvial, Eolian, Modified and Complex. We conclude that the most prevalent features are fluvial-derived channels but the most distinctive are linear, wind-derived ridges. The orientation of these ridges suggests that strong uni-directional winds from the northwest have been constant over a long period of time (> 1 My).

Fayalite rhyolites and a zoned magma chamber of the Paleocene Yakutinskaya volcanic depression in Primorye, Russia

During the Late Paleocene, at least five ignimbrite units were emplaced from the Yakutinskaya caldera complex in Primorye, Russia. The erupted ignimbrites show two distinct chemical cycles, believed to represent the “high”-silica and “low”-silica parts of the compositionally zoned magma chamber. Two petrographically distinctive types of rhyolites are distinguishable in each chemical cycle, based on their phenocryst chemistry and silica content: (1) “low”-silica rhyolites with mineral assemblages of quartz, sanidine, plagioclase, ferrohypersthene, ferroaugite (Ca41Mg21Fe38), biotite, and hornblende, and (2) “high”-silica rhyolites with a similar mineral assemblage to “low”-silica, but containing more Fe-rich clinopyroxene (Ca44Mg2Fe54) and biotite, and a with lower phenocryst abundance. This difference is related to the variation in chemical composition and temperature of the magma in the zoned magma chamber for each eruption cycle. Rb-Sr mineral-rock isochron ages show that the ignimbrites erupted between 59.7 ± 1.6 and 54.8 ± 2.6 (2σ) Ma (Late Paleocene), and initial 87Sr/86Sr ratios are distinct in the different ignimbrite units. The “high”-silica rhyolites show the highest 87Sr/86SrI ratios (0.70810-0.70738), whereas “low”-silica rhyolites show lower 87Sr/86SrI ratios (0.70659-0.70724). The compositional zoning of the single magma chamber can be explained by the large-scale mass transport in the liquid phase due to roofward migration and concentration of volatile species.

Correlation and petrogenesis of silicic pyroclastic rocks in the Northern Pannonian Basin, Eastern-Central Europe: In situ trace element data of glass shards and mineral chemical constraints

The Neogene volcanism of the Carpathian–Pannonian Region, Eastern-Central Europe started with repeated explosive eruption of silicic magmas. Volcanic products consist mostly of non-welded and partially to densely welded ignimbrites, which cover large areas in the Pannonian Basin. Since this volcanism occurred during a long time interval, from 21 Ma to 13.5 Ma ago, these pyroclastic deposits have great stratigraphic importance, as well as providing valuable information about petrogenetic processes during the formation of the back-arc basin area. In this study, we used in situ trace element data of glasses (glass shards, pumice and fiamme glass) determined by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), combined with microprobe geochemical data of the main mineral phases (plagioclase, biotite), to correlate the scattered outcrops of the ignimbrite units. Based on these data, we distinguished four ignimbrite units in the Bükkalja Volcanic Field, Northern Pannonian Basin. Each of these units is characterized by specific geochemical fingerprints. Thorium, Nb, Y and the rare earth elements are the most effective discriminator elements in glasses. The modal composition of mineral phases (occurrence or lack of certain minerals) and chemistry of plagioclases and biotites are also good correlation tools, especially the Fe, Mg and Ti contents of biotites. We suggest that these correlation criteria, particularly the grain-specific LA-ICP-MS data, could be effectively used also in other ignimbrite fields and in wider tephrochronological studies. The in situ trace element composition of glasses, representing the liquid part of the erupted magma, can be also used to constrain the petrogenesis of the rhyolitic magmas. Trace element ratios such as La/Nb, La/Y and Th/Nb suggest the importance of minor (e.g., hornblende and ilmenite) and accessory (e.g., zircon, allanite) minerals controlling the composition of the erupted melt. Rhyolitic magmas probably evolved from metaluminous andesitic parental melts via fractional crystallization. Syn-eruptive magma mingling was detected in the genesis of the Middle Ignimbrite Unit, based on the strong intra-sample geochemical variation both in the glasses and in the phenocrysts.

Transport and emplacement of ignimbrites and resedimented volcaniclastics from Gutai Mts., Eastern Carpathians, Romania

Gutâi Mts. had started to be built up in Middle Miocene, ca. 15.4 Ma ago. A series of explosive events developed starting with a major magmatic explosion and caldera collapse responsible for large volumes of ignimbrites. Successive explosions followed caldera collapse triggering a series of pyroclastic currents that underwent subsequent reworking. Mass flow has been the main transport mechanism recorded by the sedimentary structures of either ignimbrites or post-ignimbrites volcaniclastics. Multiple ignimbrite units resulted from subaerial mass flows, successively emplaced by progressive aggradation from the basal layer of a density-stratified pyroclastic current. The overlying sequence is composed of different volcaniclastics of pyroclastic origin interlayered with mudstones. They preserve the original composition of ignimbrites, but lack the evidence of hot-state deposition, recording the emplacement from more or less dilute mass flows. A syn-eruptive stage of resedimentation is suggested prior to emplacement in submarine conditions, determined by the transformation of gas-supported pyroclastic currents into water-supported mass flows after transition from subaerial to submarine conditions. The syn-eruptive resedimented volcaniclastics may be correlated with the ignimbrite-type subaerial pyroclastic flows, but they show different degrees of fluidization due to the impact of submarine environment.