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1.
Although the oldest volcanic rocks exposed at Pantelleria (Strait of Sicily) are older than 300 ka, most of the island is covered by the 45–50 ka Green Tuff ignimbrite, thought to be related to the Cinque Denti caldera, and younger lavas and scoria cones. Pre-50 ka rocks (predominantly rheomorphic ignimbrites) are exposed at isolated sea cliffs, and their stratigraphy and chronology are not completely resolved. Based on volcanic stratigraphy and K/Ar dating, it has been proposed that the older La Vecchia caldera is related to ignimbrite Q (114 ka), and that ignimbrites F, D, and Z (106, 94, and 79 ka, respectively) were erupted after caldera formation. We report here the paleomagnetic directions obtained from 23 sites in ignimbrite P (133 ka) and four younger ignimbrites, and from an uncorrelated (and loosely dated) welded lithic breccia thought to record a caldera-forming eruption. The paleosecular variation of the geomagnetic field recorded by ignimbrites is used as correlative tool, with an estimated time resolution in the order of 100 years. We find that ignimbrites D and Z correspond, in good agreement with recent Ar/Ar ages constraining the D/Z eruption to 87 ka. The welded lithic breccia correlates with a thinner breccia lying just below ignimbrite P at another locality, implying that collapse of the La Vecchia caldera took place at ~130–160 ka. This caldera was subsequently buried by ignimbrites P, Q, F, and D/Z. Paleomagnetic data also show that the northern caldera margin underwent a ~10° west–northwest (outwards) tilting after emplacement of ignimbrite P, possibly recording magma resurgence in the crust.  相似文献   

2.
From detailed fieldwork and biotite 40Ar/39Ar dating correlated with paleomagnetic analyses of lithic clasts, we present a revision of the stratigraphy, areal extent and volume estimates of ignimbrites in the Cerro Galán volcanic complex. We find evidence for nine distinct outflow ignimbrites, including two newly identified ignimbrites in the Toconquis Group (the Pitas and Vega Ignimbrites). Toconquis Group Ignimbrites (~5.60–4.51 Ma biotite ages) have been discovered to the southwest and north of the caldera, increasing their spatial extents from previous estimates. Previously thought to be contemporaneous, we distinguish the Real Grande Ignimbrite (4.68 ± 0.07 Ma biotite age) from the Cueva Negra Ignimbrite (3.77 ± 0.08 Ma biotite age). The form and collapse processes of the Cerro Galán caldera are also reassessed. Based on re-interpretation of the margins of the caldera, we find evidence for a fault-bounded trapdoor collapse hinged along a regional N-S fault on the eastern side of the caldera and accommodated on a N-S fault on the western caldera margin. The collapsed area defines a roughly isosceles trapezoid shape elongated E-W and with maximum dimensions 27 × 16 km. The Cerro Galán Ignimbrite (CGI; 2.08 ± 0.02 Ma sanidine age) outflow sheet extends to 40 km in all directions from the inferred structural margins, with a maximum runout distance of ~80 km to the north of the caldera. New deposit volume estimates confirm an increase in eruptive volume through time, wherein the Toconquis Group Ignimbrites increase in volume from the ~10 km3 Lower Merihuaca Ignimbrite to a maximum of ~390 km3 (Dense Rock Equivalent; DRE) with the Real Grande Ignimbrite. The climactic CGI has a revised volume of ~630 km3 (DRE), approximately two thirds of the commonly quoted value.  相似文献   

3.
Estimates of pyroclastic flow emplacement temperatures in the Cerro Galán ignimbrite and Toconquis Group ignimbrites were determined using thermal remanent magnetization of lithic clasts embedded within the deposits. These ignimbrites belong to the Cerro Galán volcanic system, one of the largest calderas in the world, in the Puna plateau, NW Argentina. Temperature estimates for the 2.08-Ma Cerro Galán 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 Galán 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 Galán 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 Galán 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.  相似文献   

4.
The Donguinyó-Huichapan caldera complex is located 110 km to the NNW of Mexico City, in the central sector of the Mexican Volcanic Belt. It is a 10 km in diameter complex apparently with two overlapping calderas, each one related to an ignimbrite sequence that contrasts in composition, mineralogy, welding, distribution, and physical aspect. The geologic evolution of this complex includes the following phases, 1) A first caldera formed at 5.0 ± 0.3 Ma, with the eruption of several discrete pulses of andesitic to trachydacitic pyroclastic flows that produced a series of densely welded ignimbrites; 2) At 4.6 ± 0.3 Ma, several small shield volcanoes and cinder cones built the rim of this caldera and erupted basaltic-andesite and andesitic lava flows; 3) At 4.2 ± 0.2 Ma, a second caldera was formed associated to the eruption of the Huichapan Tuff, which is a rhyolitic pyroclastic sequence consisting of minor unwelded ignimbrites, pumice fall and surge deposits, and a voluminous welded ignimbrite; 4) Also yielding an age of 4.2 ± 0.2 Ma, several trachydacitic lava domes were extruded along the new ring fracture and formed the rim of the Huichapan caldera, as well as five intra-caldera domes of dacitic and trachydacitic composition. Peripheral volcanism includes a large 2.5 ± 0.1 Ma shield volcano that was emplaced on the Huichapan caldera rim.The two calderas that form the Donguinyó-Huichapan complex have contrasting differences in volcanic styles that were apparently due to their differences in composition. Products erupted by the Donguinyó caldera are basaltic-andesite to trachydacitic in composition, whereas Huichapan caldera products are all high-silica rhyolites.  相似文献   

5.
The giant ignimbrites that erupted from the Cerro Galán caldera complex in the southern Puna of the high Andean plateau are considered to be linked to crustal and mantle melting as a consequence of delamination of gravitationally unstable thickened crust and mantle lithosphere over a steepening subduction zone. Major and trace element analyses of Cerro Galán ignimbrites (68–71% SiO2) that include 75 new analyses can be interpreted as reflecting evolution at three crustal levels. AFC modeling and new fractionation corrected δ18O values from quartz (+7.63–8.85‰) are consistent with the ignimbrite magmas being near 50:50 mixtures of enriched mantle (87Sr/86Sr ~ 0.7055) and crustal melts (87Sr/86Sr near 0.715–0.735). Processes at lower crustal levels are predicated on steep heavy REE patterns (Sm/Yb = 4–7), high Sr contents (>250 ppm) and very low Nb/Ta (9-5) ratios, which are attributed to amphibolite partial melts mixing with fractionating mantle basalts to produce hybrid melts that rise leaving a gravitationally unstable garnet-bearing residue. Processes at mid crustal levels create large negative Eu anomalies (Eu/Eu* = 0.45–0.70) and variable trace element enrichment in a crystallizing mush zone with a temperature near 800–850°C. The mush zone is repeatedly recharged from depth and partially evacuated into upper crustal magma chambers at times of regional contraction. Crystallinity differences in the ignimbrites are attributed to biotite, zoned plagioclase and other antecrysts entering higher level chambers where variable amounts of near-eutectic crystallization occurs at temperatures as low as 680°C just preceding eruption. 40Ar/39Ar single crystal sanidine weighted mean plateau and isochron ages combined with trace element patterns show that the Galán ignimbrite erupted in more than one batch including a ~ 2.13 Ma intracaldera flow and outflows to the west and north at near 2.09 and 2.06 Ma. Episodic delamination of gravitationally unstable lower crust and mantle lithosphere and injection of basaltic magmas, whose changing chemistry reflects their evolution over a steepening subduction zone, could trigger the eruptions of the Cerro Galán ignimbrites.  相似文献   

6.
High spatial resolution U–Pb dates of zircons from two consanguineous ignimbrites of contrasting composition, the high-silica rhyolitic Toconao and the overlying dacitic Atana ignimbrites, erupted from La Pacana caldera, north Chile, are presented in this study. Zircons from Atana and Toconao pumice clasts yield apparent 238U/206Pb ages of 4.11±0.20 Ma and 4.65±0.13 Ma (2σ), respectively. These data combined with previously published geochemical and stratigraphic data, reveal that the two ignimbrites were erupted from a stratified magma chamber. The Atana zircon U–Pb ages closely agree with the eruption age of Atana previously determined by K–Ar dating (4.0±0.1 Ma) and do not support long (>1 Ma) residence times. Xenocrystic zircons were found only in the Toconao bulk ignimbrite, which were probably entrained during eruption and transport. Apparent 238U/206Pb zircon ages of 13 Ma in these xenocrysts provide the first evidence that the onset of felsic magmatism within the Altiplano–Puna ignimbrite province occurred approximately 3 Myr earlier than previously documented.  相似文献   

7.
The 35 × 20 km Cerro Galán resurgent caldera is the largest post-Miocene caldera so far identified in the Andes. The Cerro Galán complex developed on a late pre-Cambrian to late Palaeozoic basement of gneisses, amphibolites, mica schists and deformed phyllites and quartzites. The basement was uplifted in the early Miocene along large north-south reverse faults, producing a horst-and-graben topography. Volcanism began in the area prior to 15 Ma with the formation of several andesite to dacite composite volcanoes. The Cerro Galán complex developed along two prominent north-south regional faults about 20 km apart. Dacitic to rhyodacitic magma ascended along these faults and caused at least nine ignimbrite eruptions in the period 7-4 Ma (K-Ar determinations). These ignimbrites are named the Toconquis Ignimbrite Formation. They are characterised by the presence of basal plinian deposits, many individual flow units and proximal co-ignimbrite lag breccias. The ignimbrites also have moderate to high macroscopic pumice and lithic contents and moderate to low crystal contents. Compositionally banded pumice occurs near the top of some units. Many of the Toconquis eruptions occurred from vents along a north-south line on the western rim of the young caldera. However, two of the ignimbrites erupted from vents on the eastern margin. Lava extrusions occurred contemporaneously along these north-south lines. The total D.R.E. volume of Toconquis ignimbrite exceeds 500 km3.Following a 2-Ma dormant period a single major eruption of rhyodacitic magma formed the 1000-km3 Cerro Galán ignimbrite and the caldera. The ignimbrite (age 2.1 Ma on Rb-Sr determination) forms a 30–200-m-thick outflow sheet extending up to 100 km in all directions from the caldera rim. At least 1.4 km of welded intracaldera ignimbrite also accumulated. The ignimbrite is a pumice-poor, crystal-rich deposit which contains few lithic clasts. No basal plinian deposit has been identified and proximal lag breccias are absent. The composition of pumice clasts is a very uniform rhyodacite which has a higher SiO2 content but a lower K2O content than the Toconquis ignimbrites. Preliminary data indicate no evidence for compositional zonation in the magma chamber. The eruption is considered to have been caused by the catastrophic foundering of a cauldron block into the magma chamber.Post-caldera extrusions occurred shortly after eruption along both the northern extension of the eastern boundary fault and the western caldera margin. Resurgence also occurred, doming up the intracaldera ignimbrite and sedimentary fill to form the central mountain range. Resurgent doming was centred along the eastern fault and resulted in radial tilting of the ignimbrite and overlying lake sediments.  相似文献   

8.
The Latera caldera is a well-exposed volcano where more than 8 km3 of mafic silica-undersaturated potassic lavas, scoria and felsic ignimbrites were emplaced between 380 and 150 ka. Isotopic ages obtained by 40Ar/39Ar analysis of single sanidine crystals indicate at least four periods of explosive eruptions from the caldera. The initial period of caldera eruptions began at 232 ka with emplacement of trachytic pumice fallout and ignimbrite. They were closely followed by eruption of evolved phonolitic magma. After roughly 25 ky, several phonolitic ignimbrites were deposited, and they were followed by phreatomagmatic eruptions that produced trachytic ignimbrites and several smaller ash-flow units at 191 ka. Compositionally zoned magma then erupted from the northern caldera rim to produce widespread phonolitic tuffs, tephriphonolitic spatter, and scoria-bearing ignimbrites. After 40 ky of mafic surge deposit and scoria cone development around the caldera rim, a compositionally zoned pumice sequence was emplaced around a vent immediately northwest of the Latera caldera. This activity marks the end of large-scale explosive eruptions from the Latera volcano at 156 ka.  相似文献   

9.
Taupo volcanic centre is one of two active rhyolite centres in the Taupo Volcanic Zone (TVZ), and has been sporadically active over the past ca. 300 ka. At least four large-scale ignimbrites have erupted from the centre, including the well documented 26.5 ka Oruanui ignimbrite and 1.8 ka Taupo ignimbrite. Because stratigraphy of earlier ignimbrites and their sources are masked by later volcanism, disrupted by regional tectonics and obscured by poor exposure, indirect methods must be applied in order to determine their source regions. In this paper detailed componentry, density and petrology of lithic fragments from three ignimbrites (Rangatira Point, Oruanui, Taupo) are used to reveal aspects of the sub-Taupo caldera geology, including the evolution of the Taupo volcanic centre, to assist in ignimbrite correlation and to evaluate structures within the Taupo caldera complex. Lithic fragments identify a complex subsurface geology. The Rangatira Point ignimbrite sampled dominantly rhyolite lavas, plus a variety of welded ignimbrites, rare high-silica dacites and a single dolerite. Most lithic fragments in the Oruanui ignimbrite are andesite with minor rhyolite, welded ignimbrite, dacite and rounded greywacke, while in the Taupo ignimbrite, rhyolite is again the dominant lithic component with subordinate welded ignimbrites, andesite, and greywacke. The densities of lithic fragments indicate similar ranges of values for all lava types, and thus density is a poor indicator of lithology. Care must, therefore, be taken before interpreting subcrustal stratigraphy using density as the sole criterion. The petrography and geochemistry of lithic types are more specific, and the variation can be used to identify sources for the ignimbrites. Both pumice chemistry and rhyolite lithic fragments from the Rangatira Point ignimbrite are comparable to domes exposed at the southern end of the Western Dome Complex and, combined with limited outcrop information, suggest the most likely source for this unit is in the northern part of the Taupo caldera complex. The dominance of andesite lithic fragments in the Oruanui ignimbrite suggests a major andesite cone existed beneath the source area, and the different lithic suites between Oruanui and Taupo ignimbrites suggest these ignimbrites came, at least in part, from mutually exclusive collapse structures. We believe that the Oruanui caldera is sited principally in the northwestern part of present-day Lake Taupo and the Taupo caldera in the northeastern part. Identification of abundant ignimbrite lithics in the Taupo ignimbrite, which are considered to represent an intracaldera facies of an earlier ignimbrite, that is not exposed at the surface, suggest there was a further (pre-Oruanui) ignimbrite caldera in the Taupo ignimbrite eruptive vent region.  相似文献   

10.
The Jemez Mountains volcanic field (JMVF), located in north-central New Mexico, has been a site of basaltic to rhyolitic volcanism since the mid-Miocene with major caldera forming eruptions occurring in the Pleistocene. Eruption of the upper Bandelier Tuff (UBT) is associated with collapse of the Valles Caldera, whereas eruption of the lower Bandelier Tuff (LBT) resulted in formation of the Toledo Caldera. These events were previously dated by K-Ar at 1.12 ± 0.03 Ma and 1.45 ± 0.06 Ma, respectively. Pre-Bandelier explosive eruptions produced the San Diego Canyon (SDC) ignimbrites. SDC ignimbrite “B” has been dated at 2.84 ± 0.07 Ma, whereas SDC ignimbrite “A”, which underlies “B”, has been dated at 3.64 ± 1.64 Ma. Both of these dates are based on single K-Ar analyses.40Ar/39Ar dating of single sanidine crystals from these units indicates revision of the previously reported dates. Isochron analysis of 26 crystals from the UBT gives a common trapped 40Ar/36Ar component of 304.5, indicating the presence of excess 40Ar in this unit, and defines an age of 1.14 ± 0.02 Ma. Isochron analysis of 26 crystals from the LBT indicates an atmospheric trapped component and an age of 1.51 ± 0.03 Ma. An age of 1.78 ± 0.04 Ma, based on the weighted mean of 5 individual analyses, is indicated for SDC ignimbrite “B”, whereas 3 analyses from SDC ignimbrite “A” give a weighted mean age of 1.78 ± 0.07 Ma. Evidence for xenocrystic contamination in the SDC ignimbrites comes from analyses of a correlative air-fall pumice unit in the Puye Formation alluvial fan giving ages of 1.75 ± 0.08 and 3.50 ± 0.09 Ma. The presence of xenocrysts in bulk separates used for the original K-Ar analyses could account for the significantly older ages reported.Geochemical data indicate that SDC ignimbrites are early eruptions from the magma chamber which evolved to produce the LBT, as compositions of SDC ignimbrite “B” are virtually identical to least evolved LBT samples. Differentiation during the 270-ka interval between eruption of SDC ignimbrite “B” and the LBT produced an array of high-silica rhyolite compositions which were erupted to form the LBT. Mixed pumices associated with eruption of the LBT indicated an influx of more mafic magma into the system which produced shifts in some incompatible trace-element ratios. Lavas and tephras of the Cerro Toledo Rhyolite record the geochemical evolution of the Bandelier magma system during the 370-ka interval between eruption of the LBT and the UBT.The combined geochronologic and geochemical data place the establishment and evolution of the Bandelier silicic magma system within a precise temporal framework, beginning with eruption of the SDC ignimbrites at 1.78 Ma, and define a periodicity of 270–370 ka to ash-flow eruptions in the JMVF. These intervals are comparable to those in other multicyclic caldera complexes and are a measure of the timescales over which substantial fractionation of large silicic magma bodies occur.  相似文献   

11.
40Ar/39Ar ages and paleomagnetic correlations using characteristic remanent magnetizations (ChRM) show that two main ignimbrite sheets were deposited at 4.86 ± 0.07 Ma (La Joya Ignimbrite: LJI) and at 1.63 ± 0.07 Ma (Arequipa Airport Ignimbrite: AAI) in the Arequipa area, southern Peru. The AAI is a 20–100 m-thick ignimbrite that fills in the Arequipa depression to the west of the city of Arequipa. The AAI is made up of two cooling units: an underlying white unit and an overlying weakly consolidated pink unit. Radiometric data provide the same age for the two units. As both units record exactly the same well-defined paleomagnetic direction (16 sites in the white unit of AAI: Dec = 173.7; Inc = 31.2; α95 = 0.7; k = 2749; and 10 sites in the pink unit of AAI; Dec = 173.6; Inc = 30.3; α95 = 1.2; k = 1634), showing no evidence of secular variation, the time gap between emplacement of the two units is unlikely to exceed a few years. The >50 m thick well-consolidated white underlying unit of the Arequipa airport ignimbrite provides a very specific magnetic zonation with low magnetic susceptibilities, high coercivities and unblocking temperatures of NRM above 580°C indicating a Ti-poor titanohematite signature. The Anisotropy of Magnetic Susceptibility (AMS) is strongly enhanced in this layer with anisotropy values up to 1.25. The fabric delineated by AMS was not recognized neither in the field nor in thin sections, because most of the AAI consists in a massive and isotrope deposit with no visible textural fabric. Pumices deformation due to welding is only observed at the base of the thickest sections. AMS within the AAI ignimbrite show a very well defined pattern of apparent imbrications correlated to the paleotopography, with planes of foliation and lineation dipping often at more than 20° toward the expected vent, buried beneath the Nevado Chachani volcanic complex. In contrast with the relatively small extent of the thick AAI, the La Joya ignimbrite covers large areas from the Altipano down the Piedmont. Ti-poor titanomagnetites are the dominant magnetic carriers and AMS values are generally lower than 1.05. Magnetic foliations are sub horizontal and lineations directions are scattered in the LJI. The AMS fabrics are probably controlled by post-depositional compaction and welding of the deposit rather than transport dynamics. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

12.
By applying a number of analytical techniques across a spectrum of spatial scales (centimeter to micrometer) in juvenile components, we show that the Cerro Galán volcanic system has repeatedly erupted magmas with nearly identical geochemistries over >3.5 Myr. The Cerro Galán system produced nine ignimbrites (∼5.6 to 2 Ma) with a cumulative volume of >1,200 km3 (DRE; dense rock equivalent) of calc-alkaline, high-K rhyodacitic magmas (68–71 wt.% SiO2). The mineralogy is broadly constant throughout the eruptive sequence, comprising plagioclase, quartz, biotite, Fe–Ti oxides, apatite, and titanite. Early ignimbrite magmas also contained amphibole, while the final eruption, the most voluminous Cerro Galán ignimbrite (CGI; 2.08 ± 0.02 Ma) erupted a magma containing rare amphibole, but significant sanidine. Each ignimbrite contains two main juvenile clast types; dominant “white” pumice and ubiquitous but subordinate “grey” pumice. Fe–Ti oxide and amphibole-plagioclase thermometry coupled with amphibole barometry suggest that the grey pumice originated from potentially hotter and deeper magmas (800–840°C, 3–5 kbar) than the more voluminous white pumice (770–810°C, 1.5–2.5 kbar). The grey pumice is interpreted to represent the parental magmas to the Galán system emplaced into the upper crust from a deeper storage zone. Most inter-ignimbrite variations can be accounted for by differences in modal mineralogy and crystal contents that vary from 40 to 55 vol.% on a vesicle-free basis. Geochemical modeling shows that subtle bulk-rock variations in Ta, Y, Nb, Dy, and Yb between the Galán ignimbrites can be reconciled with differences in amounts of crystal fractionation from the “grey” parent magma. The amount of fractionation is inversely correlated with volume; the CGI (∼630 km3) and Real Grande Ignimbrite (∼390 km3) return higher F values (proportion of liquid remaining) than the older Toconquis Group ignimbrites (<50 km3), implying less crystal fractionation took place during the upper-crustal evolution of these larger volume magmas. We attribute this relationship to variations in magma chamber geometry; the younger, largest volume ignimbrites came from flat sill-like magma chambers, reducing the relative proportion of sidewall crystallization and fractionation compared to the older, smaller-volume ignimbrite eruptions. The grey pumice clasts also show evidence of silicic recharge throughout the history of the Cerro Galán system, and recharge days prior to eruption has previously been suggested based on reversely zoned (OH and Cl) apatite phenocrysts. A rare population of plagioclase phenocrysts with thin An-rich rims in juvenile clasts in many ignimbrites supports the importance of recharge in the evolution and potential triggering of eruptions. This study extends the notion that large volumes of nearly identical silicic magmas can be generated repeatedly, producing prolonged geochemical homogeneity from a long-lived magma source in a subduction zone volcanic setting. At Cerro Galán, we propose that there is a zone between mantle magma input and upper crustal chambers, where magmas are geochemically “buffered”, producing the underlying geochemical and isotopic signatures. This produces the same parental magmas that are delivered repeatedly to the upper crust. A lower-crustal MASH (melting, assimilation, storage, and homogenization) zone is proposed to act as this buffer zone. Subsequent upper crustal magmatic processes serve only to slightly modify the geochemistry of the magmas.  相似文献   

13.
Sources of large-volume ignimbrites in the Central Andes are difficult to identify by conventional means. MSS band 7 LANDSAT imagery of the region was obtained with the specific objective of using the synoptic view to identify large ignimbrites and their sources. Two are described. The Guataquina ignimbrite covers some 2300 km2 and probably has a volume of some 70 km3. It appears to have a source in Cerro Guacha, a complex caldera-graben structure 25 km across. The Cerro Galan ignimbrite covers an area of some 2000 km' on the flanks of a major resurgent caldera some 30 km by 20 km across. Younger volcanic rocks have been erupted from two points on the caldera wall, and the structure appears to have had a geological history broadly similar to that of the Valles caldera.  相似文献   

14.
Post-10 ka rhyolitic eruptions from the Haroharo linear vent zone, Okataina Volcanic Centre, have occurred from several simultaneously active vents spread over 12 km. Two of the three eruption episodes have tapped multiple compositionally distinct homogeneous magma batches. Three magmas totalling ~8 km3 were erupted during the 9.5 ka Rotoma episode. The most evolved Rotoma magma (SiO2=76.5–77.9 wt%, Sr=96–112 ppm) erupted from a southeastern vent, and is characterised by a cummingtonite-dominant mineralogy, a temperature of 739±14°C, and fO2 of NNO+0.52±0.11. The least evolved (SiO2=75.0–76.4 wt%, Sr=128–138 ppm, orthopyroxene+ hornblende-dominant) Rotoma magma erupted from several vents, and was hotter (764±18°C) and more reduced (NNO+0.40±0.13). The ~11 km3 Whakatane episode occurred at 5.6 ka and also erupted three magmas, each from a separate vent. The most evolved (SiO2=73.3–76.2 wt%, Sr=88–100 ppm) Whakatane magma erupted from the southwestern (Makatiti) vent and is cummingtonite-dominant, cool (745±11°C), and reduced (NNO+0.34±0.08). The least evolved (SiO2=72.8–74.1 wt%, Sr=132–134 ppm) magma was erupted from the northeastern (Pararoa) vent and is characterised by an orthopyroxene+ hornblende-dominant mineralogy, temperature of 764±18°C, and fO2 of NNO+0.40±0.13. Compositionally intermediate magmas were erupted during the Rotoma and Whakatane episodes are likely to be hybrids. A single ~13 km3 magma erupted during the intervening 8.1 ka Mamaku episode was relatively homogeneous in composition (SiO2=76.1–76.8 wt%, Sr=104–112 ppm), temperature (736±18°C), and oxygen fugacity (NNO+0.19±0.12). Some of the vents tapped a single magma while others tapped several. Deposit stratigraphy suggests that the eruptions alternated between magmas, which were often simultaneously erupted from separate vents. Both effusive and explosive activity alternated, but was predominantly effusive (>75% erupted as lava domes and flows). The plumbing systems which fed the vents are inferred to be complex, with magma experiencing different conditions in the conduits. As the eruption of several magmas was essentially concurrent, the episodes were likely triggered by a common event such as magmatic intrusion or seismic disturbance.  相似文献   

15.
Tofua Island is the largest emergent mafic volcano within the Tofua arc, Tonga, southwest Pacific. The volcano is dominated by a distinctive caldera averaging 4 km in diameter, containing a freshwater lake in the south and east. The latest paroxysmal (VEI 5–6) explosive volcanism includes two phases of activity, each emplacing a high-grade ignimbrite. The products are basaltic andesites with between 52 wt.% and 57 wt.% SiO2. The first and largest eruption caused the inward collapse of a stratovolcano and produced the ‘Tofua’ ignimbrite and a sub-circular caldera located slightly northwest of the island’s centre. This ignimbrite was deposited in a radial fashion over the entire island, with associated Plinian fall deposits up to 0.5 m thick on islands >40 km away. Common sub-rounded and frequently cauliform scoria bombs throughout the ignimbrite attest to a small degree of marginal magma–water interaction. The common intense welding of the coarse-grained eruptive products, however, suggests that the majority of the erupted magma was hot, water-undersaturated and supplied at high rates with moderately low fragmentation efficiency and low levels of interaction with external water. We propose that the development of a water-saturated dacite body at shallow (<6 km) depth resulted in failure of the chamber roof to cause sudden evacuation of material, producing a Plinian eruption column. Following a brief period of quiescence, large-scale faulting in the southeast of the island produced a second explosive phase believed to result from recharge of a chemically distinct magma depleted in incompatible elements. This similar, but smaller eruption, emplaced the ‘Hokula’ Ignimbrite sheet in the northeast of the island. A maximum total volume of 8 km3 of juvenile material was erupted by these events. The main eruption column is estimated to have reached a height of ∼12 km, and to have produced a major atmospheric injection of gas, and tephra recorded in the widespread series of fall deposits found on coral islands 40–80 km to the east (in the direction of regional upper-tropospheric winds). Radiocarbon dating of charcoal below the Tofua ignimbrite and organic material below the related fall units imply this eruption sequence occurred post 1,000 years BP. We estimate an eruption magnitude of 2.24 × 1013 kg, sulphur release of 12 Tg and tentatively assign this eruption to the AD 1030 volcanic sulphate spike recorded in Antarctic ice sheet records.  相似文献   

16.
In Anatolia (Turkey), extensive calc-alkaline volcanism has developed along discontinuous provinces from Neogene to Quaternary times as a consequence of plate convergence and continental collision. In the Nevsehir plateau, which is located in the Central Anatolian Volcanic Province, volcanism consists of numerous monogenetic centres, several large stratovolcanoes and an extensive, mainly Neogene, rhyolitic ignimbrite field. Vent and caldera locations for the Neogene ignimbrites were not well known based on previous studies.In the Neogene ignimbrite sequence of the Nevsehir plateau, we have identified an old group of ignimbrites (Kavak ignimbrites) followed by five major ignimbrite units (Zelve, Sarimaden Tepe, Cemilköy, Gördeles, Kizilkaya) and two smaller, less extensive ones (Tahar, Sofular). Other ignimbrite units at the margin of the plateau occur as outliers of larger ignimbrites whose main distributions are beyond the plateau. Excellent exposure and physical continuity of the units over large areas have allowed establishment of the stratigraphic succession of the ignimbrites as, from bottom to top: Kavak, Zelve, Sarimaden Tepe, Cemilköy, Tahar, Gördeles, Sofular, Kizilkaya. Our stratigraphic scheme refines previous ones by the identification of the Zelve ignimbrite and the correlation of the previously defined ‘Akköy’ ignimbrite with the Sarimaden Tepe ignimbrite. Correlations of distant ignimbrite remnants have been achieved by using a combination a field criteria: (1) sedimentological characterisitics; (2) phenocryst assemblage; (3) pumice vesiculation texture; (4) presence and characteristics of associated plinian fallout deposits; and (5) lithic types. The correlations significantly enlarge the estimates of the original extent and volume of most ignimbrites: volumes range between 80 km3 and 300 km3 for the major ignimbrites, corresponding to 2500–10,000 km3 in areal extent.The major ignimbrites of the Nevsehir plateau have an inferred source area in the Derinkuyu tectonic basin which extends mainly between Nevsehir and the Melendiz Dag volcanic complex. The Kavak ignimbrites and the Zelve ignimbrite have inferred sources located between Nevsehir and Derinkuyu, coincident with a negative gravity anomaly. The younger ignimbrites (Sarimaden Tepe, Cemilköy, Gördeles, Kizilkaya) have inferred sources clustered to the south between the Erdas Dag and the Melendiz Dag volcanic complex. We found evidence of collapse structures on the northern and southern flanks of the Erdas Dag volcanic massif, and of a large updoming structure in the Sahinkalesi Tepe massif. The present-day Derinkuyu tectonic basin is mostly covered with Quaternary sediments and volcanics. The fault system which bounds the basin to the east provides evidence that the ignimbrite volcanism and inferred caldera formation took place in a locally extensional environment while the basin was already subsiding. Drilling and geophysical prospecting are necessary to decipher in detail the presently unknown internal structure of the basin and the inferred, probably coalesced or nested, calderas within it.  相似文献   

17.
Calderas worldwide have been classified according to their dominant collapse styles, although there is a good deal of speculation about the processes involved. Recent laboratory experiments have tried to constrain these processes by modelling magma withdrawal and observing the effects on overlying materials. However, many other factors also contribute to final caldera morphology. Rotorua Caldera formed during the eruption of the Mamaku Ignimbrite. Collapse structure and evolution of Rotorua Caldera is interpreted based its geophysical response, geology and geomorphology, and the stratigraphy of the Mamaku Ignimbrite. Rotorua Caldera is situated at the edge of the extensional Taupo Volcanic Zone, in which major faults strike NE-SW. A second, less dominant fault set strikes NW-SE. These two fault sets have a strong influence on the morphology of Rotorua Caldera. No one style of collapse can be applied to Rotorua Caldera; it was formed during a single eruption, but subsided as many blocks and shows features of trapdoor, piecemeal and downsag types of collapse. Here Rotorua Caldera is described, according to its composition, activity and geometry, as a rhyolitic, single event, asymmetric, multiple-block, single locus collapse structure. The Mamaku Ignimbrite is the only ignimbrite to have erupted from Rotorua Caldera. Extracaldera thickness of the Mamaku Ignimbrite is up to 145 m, whereas inside the caldera it may be greater than 1 km thick. The Mamaku Ignimbrite can be separated into a basal tephra sequence and main ignimbrite sequence. The main ignimbrite sequence contains no observable flow unit boundaries but can be split into lower, middle and upper parts (LMI, mMI, uMI respectively) based on crystal content, welding, jointing, devitrification and vapour phase alteration. Juvenile clasts within the ignimbrite comprise three consanguineous silicic pumice types and andesitic fragments. Only the most evolved pumice type occurs in the basal tephra sequence. All three pumice types occur together throughout the main ignimbrite sequence, whereas the andesitic fragments are only present in uMI. Lithic lag breccias in uMI indicate a late stage of caldera collapse. Concentration of lithic fragments increases towards the middle of the ignimbrite, and may also reflect increased subsidence rate during an earlier stage. Collapse of Rotorua Caldera is thought to have occurred throughout the eruption of the main ignimbrite sequence of the Mamaku Ignimbrite, allowing simultaneous eruption of all the different pumice types and causing the abrupt transition from deposition of the basal tephra sequence to the main ignimbrite sequence.  相似文献   

18.
 Analogue experiments in part I led to the conclusion that pyroclastic flows depositing very high-grade ignimbrite move as dilute suspension currents. In the thermo–fluid–dynamical model developed, the degree of cooling of expanded turbulent pyroclastic flows dynamically evolves in response to entrainment of air and mass loss to sedimentation. Initial conditions of the currents are derived from column-collapse modeling for magmas with an initial H2O content of 1–3 wt.% erupting through circular vents and caldera ring-fissures. The flows spread either longitudinally or radially from source up to a runout distance that increases with higher mass flux but decreases with higher gas content, temperature, bottom slope and coarser initial grain size. Progressive dilution by entrainment and sedimentation causes pyroclastic currents to transform into buoyant ash plumes at the runout distance. The ash plumes reach stratospheric heights and distribute 30–80% of the erupted material as widespread co-ignimbrite ash. Pyroclastic suspension currents with initial mass fluxes of 107-1012 kg/s can spread for tens of kilometers with only limited cooling, although they move as supercritical, strongly entraining currents for the eruption conditions considered here. With increasing eruption mass flux, cooling during passage through the fountain diminishes while cooling during flow transport increases. The net effect is that eruption temperature exerts the prime control on emplacement temperature. Pyroclastic suspension currents can form welded ignimbrite across their entire extent if eruption temperature is To>1.3.Tmw, the minimum welding temperature. High eruption rates, a large fraction of fine ash, and a ring-fissure vent favor the formation of extensive high-grade ignimbrite. For very hot eruptions producing sticky, partially molten pyroclasts, analysis of particle aggregation systematics shows that factors favoring longer runout also favor more efficient aggregation, which reduces runout. As a result, very high-grade ignimbrites cannot spread more than a few tens of kilometers from their source. In cooler pyroclastic currents, particles do not aggregate, and the sedimentation process may involve re-entrainment of particles, which potentially leads to more extensive cooling and longer runout; such effects, however, are only significant when net erosion of substrate occurs. Model results can be employed to estimate mass flux and duration of ignimbrite eruptions from measured ignimbrite masses and aspect ratios. The model also provides an alternative explanation of the observed decrease in H/Lratios with ignimbrite mass. Received: 10 May 1998 / Accepted: 21 October 1998  相似文献   

19.
 Outflow sheets of the Hiko tuff and the Racer Canyon tuff, which together extend over approximately 16 000 km2 around the Caliente caldera complex in southeastern Nevada, have long been considered to be products of simultaneous or near-simultaneous eruptions from inset calderas in the west and east ends, respectively, of the caldera complex. New high-precision 40Ar/39Ar geochronology and paleomagnetic data demonstrate that emplacement of the uppermost part of the Racer Canyon tuff at 18.33±0.03 Ma was nearly synchronous with emplacement of the single outflow cooling unit of the much larger overlying Hiko tuff at 18.32±0.04 Ma. Based on comparison with the geomagnetic polarity time scale derived from the sea-floor spreading record, we conclude that emplacement of the first of several outflow cooling units of the Racer Canyon tuff commenced approximately 0.5 m.y. earlier. Only one paleomagnetic polarity is found in the Hiko tuff, but at least two paleomagnetic reversals have been found in the Racer Canyon tuff. The two formations overlap in only one place, at and near Panaca Summit northeast of the center of the Caliente caldera complex; here the Hiko tuff is stratigraphically above the Racer Canyon tuff. This study demonstrates the power of combining 40Ar/39Ar and paleomagnetic data in conjunction with phenocryst compositional modes to resolve problematic stratigraphic correlations in complex ash-flow sequences where use of one method alone might not eliminate ambiguities. Received: 13 January 1997 / Accepted: 7 May 1997  相似文献   

20.
The Campanian Ignimbrite (36000 years B.P.) was produced by the explosive eruption of at least 80 km3 DRE of trachytic ash and pumice which covered most of the southern Italian peninsula and the eastern Mediterranean region. The eruption has been related to the 12-x15-km-diameter caldera located in the Phlegraean Fields, west of Naples. Proximal deposits on the periphery of the Phlegraean Fields comprise the following pyroclastic sequence from base to top: densely welded ignimbrite and lithic-rich breccias (unit A); sintered ignimbrite, low-grade ignimbrite and lithic-rich breccia (unit B); lithic-rich breccia and spatter agglutinate (unit C); and low-grade ignimbrite (unit D). Stratigraphic and componentry data, as well as distribution of accidental lithic types and the composition of pumice clasts of different units, indicate that coarse, lithic-rich breccias were emplaced at different stages during the eruption. Lower breccias are associated with fines-rich ignimbrites and are interpreted as co-ignimbrite lag breccia deposits. The main breccia unit (C) does not grade into a fines-rich ignimbrite, and therefore is interpreted as formed from a distinct lithic-rich flow. Units A and B exhibit a similar pattern of accidental lithic types, indicating that they were erupted from the same area, probably in the E of the caldera. Units C and D display a distinct pattern of lithics indicating expulsion from vent(s) that cut different areas. We suggest that unit C was ejected from several vents during the main stage of caldera collapse. Field relationships between spatter agglutinate and the breccia support the possibility that these deposits were erupted contemporaneously from vents with different eruptive style. The breccia may have resulted from a combination of magmatic and hydrothermal explosive activity that accompanied extensive fracturing and subsidence of the magma-chamber roof. The spatter rags probably derived from sustained and vigorous pyroclastic fountains. We propose that the association lithic-rich breccia and spatter agglutinate records the occurrence of catastrophic piecemeal collapse.  相似文献   

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