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1.
 The Quaternary White Trachytic Tuffs Formation from Roccamonfina Volcano (southern Italy) comprises four non-welded, trachytic, pyroclastic sequences bounded by paleosols, each of which corresponds to small- to intermediate-volume explosive eruptions from central vents. From oldest to youngest they are: White Trachytic Tuff (WTT) Cupa, WTT Aulpi, WTT S. Clemente, and WTT Galluccio. The WTT Galluccio eruption was the largest and emplaced ∼ 4 km3 of magma. The internal stratigraphy of all four WTT eruptive units is a complex association of fallout, surge, and pyroclastic flow deposits. Each eruptive unit is organized into two facies associations, Facies Association A below Facies Association B. The emplacement of the two facies associations may have been separated by short time breaks allowing for limited reworking and erosion. Facies Association A consists of interbedded fallout deposits, surge deposits, and subordinate ignimbrites. This facies association involved the eruption of the most evolved trachytic magma, and pumice clasts are white and well vesiculated. The grain size coarsens upward in Facies Association A, with upward increases of dune bedform wavelengths and a decrease in the proportion of fine ash. These trends could reflect an increase in eruption column height from the onset of the eruption and possibly also in mass eruption rate. Facies Association B comprises massive ignimbrites that are progressively richer in lithic clast content. This association involved the eruption of more mafic magma, and pumice clasts are gray and poorly vesiculated. Facies Association B is interpreted to record the climax of the eruption. Phreatomagmatic deposits occur at different stratigraphic levels in the four WTT and have different facies characteristics. The deposits reflect the style and degree of magma–water interaction and the local hydrogeology. Very fine-grained, lithic-poor phreatomagmatic surge deposits found at the base of WTT Cupa and WTT Galluccio could record the interaction of the erupting magma with a lake that occupied the Roccamonfina summit depression. Renewed magma–water interaction later in the WTT Galluccio eruption is indicated by fine grained, lithic-bearing phreatomagmatic fall and surge deposits occurring at the top of Facies Association A. They could be interpreted to reflect shifts of the magma fragmentation level to highly transmissive, regional aquifers located beneath the Roccamonfina edifice, possibly heralding a caldera collapse event. Received: 26 August 1996 / Accepted: 27 February 1998  相似文献   

2.
The Peperino Albano (approximately 19–36 ka old) is a phreatomagmatic pyroclastic flow deposit, cropping out along the slopes of the associated Albano maar (Colli Albani volcano, Italy). The deposit exhibits lateral and vertical transitions from valley pond to veneer facies, as well as intracrater facies. We present the results of a paleomagnetic study of thermal remanent magnetization (TRM) of the lithic clasts of the Peperino Albano ignimbrite that provide quantitative estimates of the range of emplacement temperatures across the different facies of the ignimbrite. Emplacement temperatures estimated for the Peperino Albano ignimbrite range between 240° and 350°C, with the temperatures defined in the intracrater facies being generally lower than in the valley pond and veneer facies. This is possibly due to the large size of the sampled clasts in the intracrater facies which, when coupled with low temperature at the vent, were not completely heated throughout their volume during emplacement. The emplacement temperatures derived from the paleomagnetic results are in good agreement with the presence of un-burnt plants at the base of the ignimbrite, indicating that the temperature of the pyroclastic flow was lower than the temperature of ignition of wood. Paleomagnetic results from the Peperino Albano confirm the reliability of the paleomagnetic approach in defining the thermal history of pyroclastic flow deposits.  相似文献   

3.
 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   

4.
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   

5.
The Rio Caliente ignimbrite is a multi-flow unit orcompound ignimbrite formed during a major late Quaternary explosive rhyolitic eruption of La Primavera volcano, Mexico. The eruption sequence of the ignimbrite is complex and it occurs between lower and upper plinian air-fall deposits. It is, therefore, anintraplinian ignimbrite. Air-fall layers, pyroclastic surge, mudflow and fluviatile reworked pumice deposits also occur interbedded between ignimbrite flow units. A chaotic near-vent facies of the ignimbrite includes co-ignimbrite lag breccias segregated from proximal pumice flows. The facies locates a central vent but one which could not have been associated with a well defined edifice. Many of the lithics in the exposed lag breccias and near-vent facies of the ignimbrite appear to be fragments of welded Rio Caliente ignimbrite, and indicate considerable vent widening, or migration, during the eruption. Nearer vent the ignimbrite is thickest and composed of the largest number of flow units. Here it is welded and is a simple cooling unit. Evidence suggests that it was only the larger thicker pumice flows that escaped to the outer parts of the sheet. Detailed analysis of four flow units indicates that the pumice flows were generally poorly expanded, less mobile flows which would be produced by collapse of low eruption columns. The analogy of a compound ignimbrite with a compound lava flow is, therefore, good — a compound lava flow forms instead of a simple one when the volumetric discharge rate (or intensity) is low, and in explosive eruptions this predicts lower eruption column heights. A corollary is that the ignimbrite has a high aspect ratio. The complex eruption sequence shows the reinstatement of plinian activity several times during the eruption after column collapse occurred. This, together with erosional breaks and evidence that solidified fragments of already welded ignimbrite were re-ejected, all suggest the eruption lasted a relatively significant time period. Nearly 90 km3 of tephra were erupted. The associated plinian pumice fall is one of the largest known having a volume of 50 km3 and the ignimbrite, plus a co-ignimbrite ash-fall, have a volume of nearly 40 km3. Published welding models applied to the reejected welded blocks indicate an eruption duration of 15-20d, and a maximum average magma-discharge rate of 1.4 × 104 m3/s for the ignimbrite. This is low intensity when compared with available data from other ignimbrite-forming eruptions, and concurs with all the geological evidence presented. The total eruption duration was perhaps 15-31d, which is consistent with other estimates of the duration of large magnitude explosive silicic eruptions.  相似文献   

6.
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   

7.
 Rheomorphic ignimbrite D (13.4 Ma, Upper Mogán Formation on Gran Canaria), a multiple flow–single cooling unit, is divided into four major structural zones that differ in fabric and finite strain of deformed pyroclasts. Their structural characteristics indicate contrasting deformation mechanisms during rheomorphic flow. The zones are: (a) a basal zone (vitrophyre) with pure uniaxial flattening perpendicular to the foliation; (b) an overlying shear zone characterized by asymmetric fabrics and a significantly higher finite strain, with an ellipsoid geometry similar to stretched oblate bodies; (c) a central zone with a finite strain geometry similar to that of the underlying shear zone but without evidence of a rotational strain component; and (d) a slightly deformed to non-deformed top zone where the almost random orientation of subspherical pyroclasts suggests preservation of original, syn-depositional clast shapes. Rheomorphic flow in D is the result of syn- to post-depositional remobilization of a hot pyroclastic flow as shown by kinematic modeling based on: (a) the overall vertical structural zonation suggested by finite strain and fabric analysis; (b) the relation of shear sense to topography; (c) the interrelationship of the calculated vertical cooling progression at the base of the flow (formation of vitrophyre) and the related vertical changes in strain geometry; (d) the complex lithification history; and (e) the consequent mechanisms of deformational flow. Rheomorphic flow was caused by load pressure due to an increase in the vertical accumulation of pyroclastic material on a slope of generally 6–8°. We suggest that every level of newly deposited pyroclastic flow material of D first passed through a welding process that was dominated by compaction (pure flattening) before rheomorphic deformation started. Received: 25 June 1997 / Accepted: 28 October 1998  相似文献   

8.
 Ignimbrites of the 13-ka Upper Laacher See Tephra were deposited from small, highly concentrated, moderately fluidized pyroclastic flows. Their unconsolidated nature, and the prominence of accidental Devonian slate fragments, make these ignimbrites ideal for clast fabric studies. The upper flow unit of ignimbrite M14 has characteristics typical of a type-2 ignimbrite. Layer 2a and the lower part of layer 2b of the flow unit have strong, upstream-inclined a[p] fabrics (a[p] means long particle axes parallel to flow direction). Only clasts with a/b axial ratios of 2.5 or greater preserve good a[p] fabrics, whereas the a–b planes of flat fragments dip upstream irrespective of axial ratio. The a-axis fabric becomes weaker, flatter, and more girdle-like in the upper half of layer 2b. At one locality the a-axis fabric appears to rotate 40° up through the flow unit, suggesting either shear decoupling of different levels in the moving flow or unsteadiness effects in a flow depositing progressively at its base. The existence of similarly strong a[p] fabrics in layer 2a and the lower half of layer 2b appears inconsistent with the common interpretation that ignimbrite flow units are emplaced as a plug of essentially non-shearing material (layer 2b) on a thin shear layer (layer 2a), and that the entire flow freezes en masse to form the deposit. The data suggest that, if the flow froze en masse, it was shearing pervasively through at least half its thickness. Another possibility is that the flow unit aggraded progressively from the base up, and that the fabrics record the integrated history of shear directions and intensities immediately above the bed throughout the duration of deposition. Received: 13 February 1997 / Accepted: 4 April 1998  相似文献   

9.
The Tiribí Tuff covered much of the Valle Central of Costa Rica, currently the most densely populated area in the country (∼2.4 million inhabitants). Underlying the tuff, there is a related well-sorted pumice deposit, the Tibás Pumice Layer. Based on macroscopic characteristics of the rocks, we distinguish two main facies in the Tiribí Tuff in correlation to the differences in welding, devitrification, grain size, and abundance of pumice and lithic fragments. The Valle Central facies consists of an ignimbritic plateau of non-welded to welded deposits within the Valle Central basin and the Orotina facies is a gray to light-bluish gray, densely to partially welded rock, with yellowish and black pumice fragments cropping out mainly at the Grande de Tárcoles River Gorge and Orotina plain. This high-aspect ratio ignimbrite (1:920 or 1.1×10−3) covered an area of at least 820 km2 with a long runout of 80 km and a minimum volume outflow of 25 km3 (15 km3 DRE). Geochemically, the tuff shows a wide range of compositions from basaltic-andesites to rhyolites, but trachyandesites are predominant. Replicate new 40Ar/39Ar age determinations indicate that widespread exposures of this tuff represent a single ignimbrite that was erupted 322±2 ka. The inferred source is the Barva Caldera, as interpreted from isopach and isopleth maps, contours of the ignimbrite top and geochemical correlation (∼10 km in diameter). The Tiribí Tuff caldera-forming eruption is interpreted as having evolved from a plinian eruption, during which the widespread basal pumice fall was deposited, followed by fountaining pyroclastic flows. In the SW part of the Valle Central, the ignimbrite flowed into a narrow canyon, which might have acted as a pseudo-barrier, reflecting the flow back towards the source and thus thickening the deposits that were filling the Valle Central depression. The variable welding patterns are interpreted to be a result of the lithostatic load and the influence of the content and size of lithic fragments.  相似文献   

10.
The Cana Creek Tuff is one of four rhyolitic ignimbrite members of the Late Carboniferous Currabubula Formation, a volcanogenic conglomeratic braidplain sequence exposed along the western margin of the New England Orogen in northeastern New South Wales. The source is not exposed but was probably located tens of kilometres to the west of existing outcrops. The medial to distal parts of the tuff average about 70 m in thickness, are widespread (minimum present area 1400 km2), and comprise a primary pyroclastic facies (ignimbrite, ash-fall tuff) and a redeposited volcaniclastic facies (sandstone, conglomerate). Both facies are composed of differing proportions of crystal fragments (quartz, plagioclase, K-feldspar), pumiceous clasts (pumice, shards, fine ash), and accidental lithics. The eruption responsible for this unit was explosive and of large magnitude (dense rock equivalent volume about 100 km3). That it was also phreatomagmatic in character is proposed on the basis of: the intimate association of primary and redeposited facies; the presence of accretionary lapilli both in ignimbrite and in ash-fall tuff; the fine grain size of juvenile pyroclasts; the low grade of the ignimbrite; and the close similarity in facies, composition and magnitude to the deposits from the 20,000y. B.P. phreatomagmatic eruption at Taupo, New Zealand (the Wairakei and parts of the Hinuera Formations). The eruption began and ended from a vent with excess water available, possibly submersed in a caldera lake, and generated volcaniclastic sheet floods and debris flows. The emplacement of the primary pyroclastic facies is correlated with an intervening stage when the water:magma mass ratio was lower. The deposits from a large-magnitude, phreatomagmatic eruption are predicted to show systematic lateral variations in facies. Primary pyroclastic facies predominate near the source although the preserved stratigraphy is an incomplete record because of widespread contemporaneous erosion. Volcaniclastic facies, redeposited from proximal sites by floods, dominate at medial and distal locations. In areas hundreds of kilometres from the source, the eruption is registered by thin layers of fine-grained airfall ash.  相似文献   

11.
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   

12.
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   

13.
Facies variations east-northeast of Mount St. Helens preserve a record of depositional processes in the 18 May 1980 lateral blast cloud. This paper reports new field, grain-size and component data from the ENE sector of the timber-blowdown zone and presents a model for blast flow and sedimentation. The first-erupted ejecta was rich in juvenile components and extends to the distal blowdown limit. The last-erupted ejecta was rich in accidental lithics and reached no further than a few kilometres from the mountain due to waning discharge. The blast cloud was a turbulent stratified flow which transported and deposited sediment in the manner of a high-density turbidity current. The possibility that the blast was emplaced as a giant shearing fluidised bed is not favoured by compositional zoning patterns. Depositional conditions were strongly influenced by the rate of suspended-load fallout from the blast. Within about 8 km from vent rapid sedimentation caused deposition under moderate to high concentration conditions and formation of a basal hindered-settling zone able to detach gravitationally and drain into local depressions. The resulting proximal facies resembles a low-aspect-ratio ignimbrite. Fines depletion in the proximal facies is attributed to a combination of residual turbulence and rapid gas escape during particle settling and compaction through the hindered-settling zone. Component data suggest that the blast head played no significant role in the generation of fines depletion in the blast deposit as suggested by previous workers. With increasing distance from vent the rate of particle fallout declined and sedimentation took place under increasingly dilute and tractional conditions, building up antidune-like bedforms. Wavelengths of these bedforms range from 20 to <1 m, and decrease away from vent. There is a systematic relationship between antidune migration direction and depositional slope. The transition from proximal (ignimbrite-like) to distal (surge-like) facies suggests a possible gradation in transport and deposition processes between conventional pyroclastic surges and high-velocity pyroclastic flows.  相似文献   

14.
Low temperature eclogite facies metamorphism in Western Tianshan, Xinjiang   总被引：3，自引：0，他引：3  
According to the field occurrences and petrological study, the low temperature eclogite facies metamorphic rocks in Western Tianshan of Xinjiang can be divided into five types: (i) massive glaucophane-epidote eclogites and glaucophane-paragonite eclogites; (ii) schistose or gneissic mica eclogites; (iii) banded calcite eclogites; (iv) pillow glaucophane eclogites; (v) garnet-omphacite quartzites. Their eclogite facies metamorphism has undergone four stages of evolution: (i) pre-peak lawsonite-blueschist facies stage,T = 350–4000°C,P = 0.7–0.9 GPa; (ii) peak eclogite facies stage,T = 530 ± 20°C,P = 1.6–1.9 GPa; (iii) retrograde epidote-blueschist facies stage, T=500–530°C,P = 0.9–1.2 GPa and (iv) retrograde blueschist-greenschist facies stage,T= 450–550°C,P= 0.7–0.8 GPa. The metamorphic PT path of Western Tianshan eclogites is characterized by clockwise ITD resulting from the subduction of Tarim plate northward to Yili-Central Tianshan plate followed by fast uplift to the surface. But there were at least two stages of blueschist facies retrograde metamorphism overprinted during their uplift.  相似文献   

15.
The Zaragoza ignimbrite and two enclosing rhyodacite pumice fall layers were emplaced during the 15 km3 (DRE), ∼0.1 Ma Zaragoza eruption from Los Humeros volcanic centre, 180 km east of Mexico City. The ignimbrite comprises several massive flow-units, the largest of which locally exceeds 20 m in thickness and is regionally traceable. It comprises massive lapilli-ash with vertical elutriation pipes, and has a fine-grained inverse-graded base and a pumice concentration zone at the top. It also exhibits an unusual gradational ‘double’ vertical compositional zonation that is widely traceable. A basal rhyodacitic (67.6–69 wt% SiO2) zone grades up via a mixed zone into a central andesitic (58–62 wt% SiO2) zone, which, in turn, grades up into an upper rhyodacitic (67.6–69 wt% SiO2) zone. Zoning is also defined by vertical variations in lithic clast populations. We infer that pyroclastic fountaining fed initially rhyodacite pumice clasts to a sustained granular fluid-based pyroclastic density current. The composition of the pumice clasts supplied to the current then gradually changed, first to andesite and then back to rhyodacite. Inverse grading at the base of the massive layer may reflect initial waxing flow competence. The pumice concentration at the top of the massive layer is entirely rhyodacitic and was probably deposited during waning stages of the current, when the supply of andesitic pumice clasts had ceased. The return to rhyodacitic composition may have been the result of eruption-conduit modification during collapse of Los Potreros caldera, marked in the ignimbrite by a widespread influx of hydrothermally altered lithic blocks, and/or a decrease in draw-up depth from a compositionally stratified magma chamber as the eruptive mass flux waned. The massive layer of ignimbrite thins locally to less than 2 m, yet it still shows the double zonation. Correlation of the zoning suggests that the thin massive layer is stratigraphically condensed, and aggraded relatively slowly during the same time interval as did the much thicker (≤50 m) massive layer.Editorial responsibility: J McPhie  相似文献   

16.
The young non-welded Taupo ignimbrite shows remarkable lateral variations which are documented by granulometric and component analyses, and studies of maximum clast size and density. The grain size spans practically the entire known ignimbrite field, the coarser proximal ignimbrite having a median diameter 100 times greater than the finest distal ignimbrite. The content and maximum size of lithic fragments decrease also by a factor of 100 between proximal and distal parts. The content of free crystals first rises to reach a peak, but thereafter decreases to attain a very low value in far-distal exposures. The pumice maximum size decreases by a factor of about 10, and the most conspicuously coarse pumice rocks occur in a girdle nearly halfway out from vent to distal limit. The pumice in each grain size class decreases in density to half of its near-source value in distal ignimbrite. The overall outward trend is towards an ignimbrite which consists wholly of fine vitric ash; some distal exposures closely approach this condition.These variations are accounted for by a combination of processes operating in the moving ash flow. One is a continuous fragmentation of pumice leading to a rounding of the clasts, a progressive decrease in maximum size, the generation of much vitric dust, and the liberation of crystals. Another is a continuous sedimentation of heavy constituents (lithics and crystals), and an antipathetic rise of lighter coarse pumice towards the top of the flow. These processes operated in a moving flow whose upper layers travelled progressively farther from source; it is the topmost layers, strongly depleted in heavy constituents and enriched in light pumice, which have travelled the farthest and constitute the far-distal parts of the ignimbrite.A number of ignimbrite facies are characterized: the ignimbrite proper, with its proximal, distal, and pumice concentration zone facies; the deposits which form in the head and are then over-ridden by the body of the flow, including the fines-depleted ignimbrite variant and the heavies-enriched ground layer; and the ignimbrite veneer deposits which are left behind by the flow, which differ little from the ignimbrite except in their landscape-mantling form and the occurrence in them of lee-side coarse pumice lenses.  相似文献   

17.
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   

18.
The Tosu pyroclastic flow deposit, a low-aspect-ratio ignimbrite (LARI), has widely distributed breccia facies around Aso caldera, Japan. The proximal facies, 9–34 km away from the source, consists of 3 different lithofacies, from bottom to top: a lithic-enriched and fines-depleted (FD) facies, a lithic-enriched (LI) facies with an ash matrix, and a fines- and pumice-enriched (NI) facies. Modes of emplacement of FD, LI, and NI are interpreted as ground layer, 2b-lithic-concentration zone, and normal ignimbrite, respectively. These stratigraphic components in the Tosu originated from the flow head (FD) and the flow body (LI and NI), and were generated by a single column collapse event. Remarkably thick FD and LI, in contrast to thin NI, suggest that due to high mobility most ash and punice fragments in the Tosu were carried and deposited as NI in the distal area. Heavier components were selectively deposited as FD and LI in the proximal area. The rate of falloff of lithic-clast size in the Tosu shows an inflection at 20 km from the source. In a survey of well-documented pyroclastic flows, the inflection distance of a LARI is generally greater than that of a high-aspect-ratio ignimbrite, so that the eruption of the former is probably more intense than the latter.  相似文献   

19.
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20.
 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   

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