首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 540 毫秒
1.
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   

2.
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   

3.
 Large volume (100–1000 km3), widespread rhyolitic ignimbrites are the main products of the Taupo volcanic zone (TVZ) of New Zealand, one of the most active silicic volcanic regions on Earth. Several factors have made correlation and the eruptive history of the ignimbrites difficult to resolve, including limited exposure and chronological data, broadly similar lithologies and the lack of stratigraphic successions visible in the field. We have used the isothermal plateau fission track (ITPFT) method on glass shards from the non-welded basal zones to obtain new eruption ages for the widespread units: Ongatiti (1.25±0.12 Ma), Whakamaru group (0.34±0.03 Ma), Matahina (0.34±0.02 Ma), Chimp (0.33±0.02 Ma), Kaingaroa (0.31±0.01 Ma) and Mamaku (0.23±0.01 Ma) ignimbrites. These glasses show little evidence of geochemical alteration and allow the units to be fingerprinted for correlation. The glass ages we have obtained for the late Quaternary units provide an independent check on chronological data obtained from phenocryst phases. The ITPFT method is a useful dating approach for sanidine-poor eruptives which limit the application of 40Ar/39Ar. Errors as limited as 10–30 ka can be obtained from the weighted mean of several age determinations. The thermoremanent magnetic (TRM) direction recorded in the units provides a means of correlation over a wide area of the TVZ, because each ignimbrite can be distinguished by its unique record of palaeosecular variation. These data indicate that the four separately mapped members of the Whakamaru group represent the same phase of activity, occurring within a period of 100 years. The TRM data indicate that the widespread Ahuroa ignimbrite erupted during an excursion in Earth's magnetic field, perhaps associated with the Cobb Mountain subchron (ca. 1.2 Ma). The youngest widespread welded unit, Mamaku ignimbrite (ca. 0.23 Ma), also erupted during an excursion and may represent a southern hemisphere record of the Pringle Falls geomagnetic episode found in the western United States. The palaeomagnetic and ITPFT data for the widespread late Quaternary ignimbrites suggest a major period of caldera formation at 0.34–0.30 Ma. This interval represents the eruption of multiple units from the Whakamaru caldera, followed by the formation of the Okataina and Reporoa calderas in rapid succession. Received: 20 November 1995 / Accepted: 8 May 1996  相似文献   

4.
A new pyroclastic stratigraphy is presented for the island of Ischia, Italy, for the period ∼75–50 ka BP. The data indicate that this period bore witness to the largest eruptions recorded on the island and that it was considerably more volcanically active than previously thought. Numerous vents were probably active during this period. The deposits of at least 10 explosive phonolite to basaltic-trachyandesite eruptions are described and interpreted. They record a diverse range of explosive volcanic activity including voluminous fountain-fed ignimbrite eruptions, fallout from sustained eruption columns, block-and-ash flows, and phreatomagmatic eruptions. Previously unknown eruptions have been recognised for the first time on the island. Several of the eruptions produced pyroclastic density currents that covered the whole island as well as the neighbouring island of Procida and parts of the mainland. The morphology of Ischia was significantly different to that seen today, with edifices to the south and west and a submerged depression in the centre. The largest volcanic event, the Monte Epomeo Green Tuff (MEGT) resulted in caldera collapse across all or part of the island. It is shown to comprise at least two thick intracaldera ignimbrite flow-units, separated by volcaniclastic sediments that were deposited during a pause in the eruption. Extracaldera deposits of the MEGT include a pumice fall deposit emplaced during the opening phases of the eruption, a widespread lithic lag breccia outcropping across much of Ischia and Procida, and a distal ignimbrite in south-west Campi Flegrei. During this period the style and magnitude of volcanism was dictated by the dynamics of a large differentiated magma chamber, which was partially destroyed during the MEGT eruption. This contrasts with the small-volume Holocene and historical effusive and explosive activity on Ischia, the timing and distribution of which has been controlled by the resurgence of the Monte Epomeo block. The new data contribute to a clearer understanding of the long-term volcanic and magmatic evolution of Ischia.  相似文献   

5.
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   

6.
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   

7.
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   

8.
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   

9.
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   

10.
The 161 ka explosive eruption of the Kos Plateau Tuff (KPT) ejected a minimum of 60 km3 of rhyolitic magma, a minor amount of andesitic magma and incorporated more than 3 km3 of vent- and conduit-derived lithic debris. The source formed a caldera south of Kos, in the Aegean Sea, Greece. Textural and lithofacies characteristics of the KPT units are used to infer eruption dynamics and magma chamber processes, including the timing for the onset of catastrophic caldera collapse.The KPT consists of six units: (A) phreatoplinian fallout at the base; (B, C) stratified pyroclastic-density-current deposits; (D, E) volumetrically dominant, massive, non-welded ignimbrites; and (F) stratified pyroclastic-density-current deposits and ash fallout at the top. The ignimbrite units show increases in mass, grain size, abundance of vent- and conduit-derived lithic clasts, and runout of the pyroclastic density currents from source. Ignimbrite formation also corresponds to a change from phreatomagmatic to dry explosive activity. Textural and lithofacies characteristics of the KPT imply that the mass flux (i.e. eruption intensity) increased to the climax when major caldera collapse was initiated and the most voluminous, widespread, lithic-rich and coarsest ignimbrite was produced, followed by a waning period. During the eruption climax, deep basement lithic clasts were ejected, along with andesitic pumice and variably melted and vesiculated co-magmatic granitoid clasts from the magma chamber. Stratigraphic variations in pumice vesicularity and crystal content, provide evidence for variations in the distribution of crystal components and a subsidiary andesitic magma within the KPT magma chamber. The eruption climax culminated in tapping more coarsely crystal-rich magma. Increases in mass flux during the waxing phase is consistent with theoretical models for moderate-volume explosive eruptions that lead to caldera collapse.  相似文献   

11.
 Coarse, co-ignimbrite lithic breccia, Ebx, occurs at the base of ignimbrite E, the most voluminous and widespread unit of the Kos Plateau Tuff (KPT) in Greece. Similar but generally less coarse-grained basal lithic breccias (Dbx) are also associated with the ignimbrites in the underlying D unit. Ebx shows considerable lateral variations in texture, geometry and contact relationships but is generally less than a few metres thick and comprises lithic clasts that are centimetres to a few metres in diameter in a matrix ranging from fines bearing (F2: 10 wt.%) to fines poor (F2: 0.1 wt.%). Lithic clasts are predominantly vent-derived andesite, although clasts derived locally from the underlying sedimentary formations are also present. There are no proximal exposures of KPT. There is a highly irregular lower erosional contact at the base of ignimbrite E at the closest exposures to the inferred vent, 10–14 km from the centre of the inferred source, but no Ebx was deposited. From 14 to <20 km from source, Ebx is present over a planar erosional contact. At 16 km Ebx is a 3-m-thick, coarse, fines-poor lithic breccia separated from the overlying fines-bearing, pumiceous ignimbrite by a sharp contact. This grades downcurrent into a lithic breccia that comprises a mixture of coarse lithic clasts, pumice and ash, or into a thinner one-clast-thick lithic breccia that grades upward into relatively lithic-poor, pumiceous ignimbrite. Distally, 27 to <36 km from source Ebx is a finer one-clast-thick lithic breccia that overlies a non-erosional base. A downcurrent change from strongly erosional to depositional basal contacts of Ebx dominantly reflects a depletive pyroclastic density current. Initially, the front of the flow was highly energetic and scoured tens of metres into the underlying deposits. Once deposition of the lithic clasts began, local topography influenced the geometry and distribution of Ebx, and in some cases Ebx was deposited only on topographic crests and slopes on the lee-side of ridges. The KPT ignimbrites also contain discontinuous lithic-rich layers within texturally uniform pumiceous ignimbrite. These intra-ignimbrite lithic breccias are finer grained and thinner than the basal lithic breccias and overlie non-erosional basal contacts. The proportion of fine ash within the KPT lithic breccias is heterogeneous and is attributed to a combination of fluidisation within the leading part of the flow, turbulence induced locally by interaction with topography, flushing by steam generated by passage of pyroclastic density currents over and deposition onto wet mud, and to self-fluidisation accompanying the settling of coarse, dense lithic clasts. There are problems in interpreting the KPT lithic breccias as conventional co-ignimbrite lithic breccias. These problems arise in part from the inherent assumption in conventional models that pyroclastic flows are highly concentrated, non-turbulent systems that deposit en masse. The KPT coarse basal lithic breccias are more readily interpreted in terms of aggradation from stratified, waning pyroclastic density currents and from variations in lithic clast supply from source. Received: 21 April 1997 / Accepted: 4 October 1997  相似文献   

12.
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   

13.
The Reporoa Caldera occupies the northern end of the Reporoa Depression, previously described as a tectonic fault-angle depression. Earlier confirmation of the topographic basin as a caldera had been hindered by the lack of an associated young pyroclastic flow deposit of large enough volume to have caused caldera collapse. New exposures on the eastern margin of the Reporoa basin reveal thick lithic lag breccias (>30 m) interbedded within the 0.24 Ma Kaingaroa Ignimbrites. These ignimbrites were previously attributed to the adjacent Okataina Volcanic Centre. Lag breccia thicknesses and maximum clast sizes decrease rapidly outward from the caldera rim, and discrete breccias are absent from ignimbrite sections more than 3 km from the rim. The lithic lag breccias, together with structural and geophysical evidence, confirm Reporoa Caldera as the source of the c. 100 km3 Kaingaroa Ignimbrites, adding another major rhyolitic volcanic centre to the seven previously recognized in the Taupo Volcanic Zone. Other, older, calderas may also be present in the Reporoa Depression.  相似文献   

14.
Non-welded rhyolitic pyroclastic units in the central Snake River Plain are interbedded with the much better exposed, large-volume ‘Snake-River type’ rheomorphic welded rhyolitic ignimbrites and rhyolite lavas. We document one such unit to investigate why it is so different from the interbedded welded ignimbrites. The newly recognised Deadeye Member of southern Idaho is a soil-bounded eruption-unit that comprises ashfall layers and a 4-m-thick ignimbrite that extends for >35 km. The ignimbrite is non-welded, lithic-clast poor and varies from massive to diffuse low-angle cross-bedded. It contains abundant angular clasts of non-vesicular black glass, and upper parts contain accretionary lapilli. The ashfall layers above it contain coated ash pellets and ash clumps, which record moist aggregation of fine ash. The magmas of the Deadeye eruption were closely similar in composition and temperature to those that generated the intensely welded rheomorphic ignimbrites of the central Snake River Plain. We infer that the marked contrast in physical appearance of the Deadeye ignimbrite compared to the other, more typical Snake-River-type welded ignimbrites was the result of emplacement at relatively low temperatures during an eruption in a lacustrine environment. Magmatic volatile-driven fragmentation of the rhyolitic magma was influenced by interaction with lake water that also led to cooling. The Deadeye Member is the first-recorded example of explosive silicic phreatomagmatism in the central Snake River Plain.  相似文献   

15.
The late-seventeenth century BC Minoan eruption of Santorini discharged 30–60 km3 of magma, and caldera collapse deepened and widened the existing 22 ka caldera. A study of juvenile, cognate, and accidental components in the eruption products provides new constraints on vent development during the five eruptive phases, and on the processes that initiated the eruption. The eruption began with subplinian (phase 0) and plinian (phase 1) phases from a vent on a NE–SW fault line that bisects the volcanic field. During phase 1, the magma fragmentation level dropped from the surface to the level of subvolcanic basement and magmatic intrusions. The fragmentation level shallowed again, and the vent migrated northwards (during phase 2) into the flooded 22 ka caldera. The eruption then became strongly phreatomagmatic and discharged low-temperature ignimbrite containing abundant fragments of post-22 ka, pre-Minoan intracaldera lavas (phase 3). Phase 4 discharged hot, fluidized pyroclastic flows from subaerial vents and constructed three main ignimbrite fans (northwestern, eastern, and southern) around the volcano. The first phase-4 flows were discharged from a vent, or vents, in the northern half of the volcanic field, and laid down lithic-block-rich ignimbrite and lag breccias across much of the NW fan. About a tenth of the lithic debris in these flows was subvolcanic basement. New subaerial vents then opened up, probably across much of the volcanic field, and finer-grained ignimbrite was discharged to form the E and S fans. If major caldera collapse took place during the eruption, it probably occurred during phase 4. Three juvenile components were discharged during the eruption—a volumetrically dominant rhyodacitic pumice and two andesitic components: microphenocryst-rich andesitic pumices and quenched andesitic enclaves. The microphenocryst-rich pumices form a textural, mineralogical, chemical, and thermal continuum with co-erupted hornblende diorite nodules, and together they are interpreted as the contents of a small, variably crystallized intrusion that was fragmented and discharged during the eruption, mostly during phases 0 and 1. The microphenocryst-rich pumices, hornblende diorite, andesitic enclaves, and fragments of pre-Minoan intracaldera andesitic lava together form a chemically distinct suite of Ba-rich, Zr-poor andesites that is unique in the products of Santorini since 530 ka. Once the Minoan magma reservoir was primed for eruption by recharge-generated pressurization, the rhyodacite moved upwards by exploiting the plane of weakness offered by the pre-existing andesite–diorite intrusion, dragging some of the crystal-rich contents of the intrusion with it.  相似文献   

16.
Glass-bearing plutonic fragments occur as rare accessory lithics within the ca. 64 ka Rotoiti and Earthquake Flat ignimbrites that were erupted from Okataina caldera complex, Taupo Volcanic Zone, New Zealand. Granitoid lithic fragments are only found in the Rotoiti ignimbrite and fall into two groups. Group 1 granitoids have textures consistent with a period of slow cooling followed by rapid quenching, and were excavated by the Rotoiti eruption from a single incompletely solidified magma body. Although isotopic ratios for the Group 1 granitoids are similar to the host ignimbrite, they are not cognate, having different chemistry, mineralogy, mineral chemistry and crystallisation history. It is more likely that they represent fragments of a separate incompletely solidified magma chamber that was intercepted by the erupting Rotoiti ignimbrite magma. Low LILE and high HFSE abundances favour a comagmatic link with the ca. 0.28 Ma Matahina ignimbrite and it is suggested they are derived from an isolated cupola of the Matahina magma chamber that remained at depth (between 3.5 and 5 kbar pressure) after eruption of the Matahina ignimbrite. Migration toward the surface probably accompanied development of the Rotoiti magma system in the upper crust. Most geochemical variation in Group 1 granitoids is related to the abundance of biotite, the concentration of which is controlled by differential shear. REE abundance is controlled by light REE-enriched accessory minerals preferentially included within biotite. Although Eun remains constant in the Group 1 granitoids, Eu/Eu* varies systematically with (La/Yb)n and is controlled by variations in Sm and Gd rather than in Eu. Group 2 granitoid fragments have a wide range of composition, comparable to many Okataina rhyolites, including those found as lithic fragments in the Rotoiti ignimbrite. Rare microdiorite fragments occur in both Rotoiti and Earthquake Flat ignimbrites and typically contain vesicular interstitial glass indicating that they were incompletely solidified prior to eruption. Those from the Rotoiti ignimbrite are comparable to the (>64 ka) Matahi basaltic tephra and probably represent part of the same magmatic event which generated the Matahi tephra.  相似文献   

17.
A model is presented for the emplacement of intermediate volume ignimbrites based on a study of two 6 km3 volume ignimbrites on Roccamonfina Volcano, Italy. The model considers that the flows were slow moving, and quickly deflated from turbulent to non-turbulent conditions. Yield strength and density increased whereas fluidisation decreased with time and runout of the pyroclastic flows. In proximal locations, on the caldera rim, heterogeneous exposures including discontinuous lithic breccias, stratified and cross-stratified units interbedded with massive ignimbrite suggest deposition from turbulent flows. In medial locations thick, massive ignimbrite occurs associated with three types of co-ignimbrite lithic breccia which we interpret as being emplaced by non-turbulent flows. Multiple grading of different breccia/lithic concentration types within single flow units indicates that internal shear occurred producing overriding or overlapping of the rear of the flow onto the slower-moving front part. This overriding of different parts of non-turbulent pyroclastic flows could be caused by at least two different mechanisms: (1) changes in flow regime, such as hydraulic jumps that may occur at breaks in slope; and (2) periods of increased discharge rate, possibly associated with caldera collapse, producing fresh pulses of lithic-rich material that sheared onto the slower-moving part of the flow in front.We propose that ground surge deposits enriched in pumice compared with their associated ignimbrite probably formed by a flow separation mechanism from the top and front of the pyroclastic flow. These turbulent clouds moved ahead of the non-turbulent lower part of the flow to form stratified pumice-rich deposits. In distal regions well-developed coarse, often clast-supported, pumice concentrations zones and coarse intra-flow-unit lithic concentrations occur within the massive ignimbrite. We suggest that the flows were non-turbulent, possessed a relatively high yield strength and may have moved by plug flow prior to emplacement.  相似文献   

18.
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   

19.
The small- to moderate-volume, Quaternary, Siwi pyroclastic sequence was erupted during formation of a 4 km-wide caldera on the eastern margin of Tanna, an island arc volcano in southern Vanuatu. This high-potassium, andesitic eruption followed a period of effusive basaltic andesite volcanism and represents the most felsic magma erupted from the volcano. The sequence is up to 13 m thick and can be traced in near-continuous outcrop over 11 km. Facies grade laterally from lithic-rich, partly welded spatter agglomerate along the caldera rim to two medial, pumiceous, non-welded ignimbrites that are separated by a layer of lithic-rich, spatter agglomerate. Juvenile clasts comprise a wide range of densities and grain sizes. They vary between black, incipiently vesicular, highly elongate spatter clasts that have breadcrusted pumiceous rinds and reach several metres across to silky, grey pumice lapilli. The pumice lapilli range from highly vesicular clasts with tube or coalesced spherical vesicles to denser finely vesicular clasts that include lithic fragments.Textural and lithofacies characteristics of the Siwi pyroclastic sequence suggest that the first phase of the eruption produced a base surge deposit and spatter-poor pumiceous ignimbrite. A voluminous eruption of spatter and lithic pyroclasts coincided with a relatively deep withdrawal of magma presumably driven by a catastrophic collapse of the magma chamber roof. During this phase, spatter clasts rapidly accumulated in the proximal zone largely as fallout, creating a variably welded and lithic-rich agglomerate. This phase was followed by the eruption of moderately to highly vesiculated magma that generated the most widespread, upper pumiceous ignimbrite. The combination of spatter and pumice in pyroclastic deposits from a single eruption appears to be related to highly explosive, magmatic eruptions involving low-viscosity magmas. The combination also indicates the coexistence of a spatter fountain and explosive eruption plume for much of the eruption.Editorial responsibility: R. Cioni  相似文献   

20.
Three voluminous rhyolitic ignimbrites have been identified along the southern margin of the central Snake River Plain. As a result of wide-scale correlations, new volume estimates can be made for these deposits: ~350 km3 for the Steer Basin Tuff and Cougar Point Tuff XI, and ~1,000 km3 for Cougar Point Tuff XIII. These volumes exclude any associated regional ashfalls and correlation across to the north side of the plain, which has yet to be attempted. Each correlation was achieved using a combination of methods including field logging, whole rock and mineral chemistry, magnetic polarity, oxygen isotope signature and high-precision 40Ar/39Ar geochronology. The Steer Basin Tuff, Cougar Point Tuff XI and Cougar Point Tuff XIII have deposit characteristics typical of ‘Snake River (SR)-type’ volcanism: they are very dense, intensely welded and rheomorphic, unusually well sorted with scarce pumice and lithic lapilli. These features differ significantly from those of deposits from the better-known younger eruptions of Yellowstone. The ignimbrites also exhibit marked depletion in δ18O, which is known to characterise the SR-type rhyolites of the central Snake River Plain, and cumulatively represent ~1,700 km3 of low δ18O rhyolitic magma (feldspar values 2.3–2.9‰) erupted within 800,000 years. Our work reduces the total number of ignimbrites recognised in the central Snake River Plain by 6, improves the link with the ashfall record of Yellowstone hotspot volcanism and suggests that more large-volume ignimbrites await discovery through detailed correlation work amidst the vast ignimbrite record of volcanism in this bimodal large igneous province.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号